Micro-alloyed steels, which are also sometimes called high strength low alloy (HSLA) steels, are a group of low carbon (C) steels which utilize small amounts of alloying elements to attain yield strengths higher than 275 MPa in the as-rolled or normalized condition. These steels have better mechanical properties and sometimes better corrosion resistance than as-rolled C steels. Also, since the higher strength of the micro-alloyed steels can be achieved at lower C contents, the weldability of several micro-alloyed steels is comparable to or better than that of the mild steel.
The technology of micro-alloying involves the addition of a fraction of a percent of the micro-alloying elements to simple low C steel. The use of ‘micro’ alloy concentrations, which produce remarkable changes in mechanical properties, distinguishes the technology from ‘alloying’ in the conventional sense (low alloy steels family) where concentration of the alloying elements can range from 0.25 % to one or two or possibly several percent. Micro-alloyed steels are designed to provide better mechanical properties and / or higher resistance to atmospheric corrosion than conventional C steels. They are not considered to be alloy steels in the normal sense since they are designed to meet specific mechanical properties rather than a chemical composition.
Micro-alloyed steels are low or medium C steels or low alloy steels with the addition of elements such as niobium (Nb), titanium (Ti), vanadium (V), and zirconium (Zr). These additions increase the strength of the steel, individually and also in different combinations. Controlled additions of sulphur (S), and occasionally tellurium (Te), are also added to improve the machinability. Strengthening by micro-alloying elements permits a dramatic reduction in C content which greatly improves weldability and notch toughness.
Micro-alloyed steels are one of the core alloy steels in the development of modern advanced high-strength steels. Owing to their superior mechanical properties, micro-alloyed steels allow a more efficient design, with improved performance, even under difficult environmental conditions. In addition, they permit reductions in component weight and manufacturing cost. Present developments are mostly focused on the optimization of the chemical composition and process parameters of the micro-alloyed steels to achieve the microstructures needed to fulfill the most challenging mechanical properties and performance requirements.
Though the work of strengthening steels through addition of small percentage of alloying elements started as early as 1916 in USA, the term ‘micro-alloying’ , as applied to steels, is normally accepted as emanating from the paper by Beiser published in 1959, which reported the results of small additions of Nb to commercial heats of a C steel. The term micro-alloying was adopted by Prof. TM Noren-Brandel in 1962 and became pervasive as a result of the landmark conference ‘Micro-alloying 75’. The main development of micro-alloyed steels has taken place over the past 50 -60 years, and was initially concentrated on Nb additions. Micro-alloyed steels have been developed originally for large diameter oil and gas pipelines.
Micro-alloyed steels before the 1980s contained typically, 0.07 % to 0.12 % C, upto 2 % manganese (Mn) and small additions of Nb, V, and Ti (all normally 0.1 % maximum) in various combinations. Other elements which can be present include Zr, molybdenum (Mo), boron (B), aluminum (Al), nitrogen (N2) and rare earth elem3nts (REEs). The micro-alloying elements are used to refine the grain microstructure and / or facilitate dispersion strengthening through precipitation. They are normally regarded as having a low hardenability effect.
The original aim in the development of the micro-alloyed steels was to develop high strength and toughness in the steels having ferrite-pearlite (FP) microstructure in the as-rolled condition. While the earlier micro-alloyed steels sought to avoid transformation to acicular ferrite and bainite, modern micro-alloyed steels, built upon a sounder understanding of the processing routes and the development of microstructure than was available prior to the 1980s, embrace these phases.
Micro-alloyed steels are primarily hot rolled into the normal wrought product forms (sheet, strip, bar, plate, and structural sections) and are normally furnished in the ‘as hot rolled’ condition. However, the production of hot rolled micro-alloyed steel products can also involve special hot mill processing which further improves the mechanical properties of some micro-alloyed steels and product forms. The usefulness and cost effectiveness of these processing methods are highly dependent on product form and the content of the alloying method. These processing methods include (i) controlled rolling, (ii) accelerated cooling of, preferably, controlled-rolled micro-alloyed steels, and (iii) quenching or accelerated air or water cooling of low C steels, (iv) normalizing, and (v) inter-critical annealing. These processing methods are described later in the article.
In addition to hot rolled products, micro-alloyed steels are also furnished as cold rolled sheet and forgings. The main advantage of micro-alloyed forgings (like ‘as hot rolled’ micro-alloyed steel products) is that yield strengths in the range of 275 MPa to 485 MPa or perhaps higher can be achieved without heat treatment. Base compositions of these micro-alloyed FP forgings are typically 0.3 % to 0.5 % C and 1.4 % to 1.6 % Mn (manganese). Low C bainitic micro-alloyed steel forgings have also been developed.
Micro-alloyed steel is produced preferably by a thermo-mechanical rolling process, also known as controlled rolling, and possibly with accelerated cooling, which maximize grain refinement as a basis for improved mechanical properties. Prior to thermo-mechanical processing, the micro-alloyed steel was heated into the austenite temperature range for all of the precipitates to be taken into solution after forming, and then the material was quickly cooled to 600 deg C to 540 deg C. Medium C directly quenched (DC) micro-alloyed steels, avoiding FP microstructures, can also be forged.
Micro-alloyed steels lie, in terms of performance and cost, between C steel, or mild steels and low alloy steels. Until around 1980, low alloy steels were designed to have yield strength between 500 MPa to 750 MPa without heat treatment. The weldability of micro-alloyed is at least equal to that of mild steel, and can be improved by reducing C content while maintaining strength. Fatigue life and wear resistance are superior to similar heat-treated steels. The disadvantages are that ductility and toughness are not as good as quenched and tempered (Q&T) steels.
Cold-worked micro-alloyed steels do not need as much cold working to achieve the same strength as other C steels. This also leads to higher ductility. Hot-worked micro-alloyed steels can be used from the air-cooled state. If controlled cooling is used, the material can produce mechanical properties similar to Q&T steels. Their machinability is better than Q&T steels because of their more uniform hardness and their FP microstructure. Since micro-alloyed steels with FP micro-structure are not quenched and tempered, they are not susceptible to quench cracking, nor they are required to be straightened or stress relieved. However, because of this, they are through-hardened and do not have a softer and tougher core like Q&T steels.
Micro-alloyed steels include several standard and proprietary grades designed to provide specific desirable combinations of properties such as strength, toughness, formability, weldability, and atmospheric-corrosion resistance. These steels are not considered alloy steels, even though their desired properties are achieved by the use of small alloy additions. Instead, micro-alloyed steels are classified as a separate steel category, which is similar to as-rolled low C steel with improved mechanical properties achieved by the judicious (small) addition of alloying elements and, perhaps, special processing techniques such as controlled rolling.
Although micro-alloyed steels are available in numerous standard and proprietary grades, they can be divided into seven categories namely (i) weathering steels which contain small amounts of alloying elements such as copper (Cu) and phosphorus (P) for improved atmospheric corrosion resistance and solid-solution strengthening, (ii) micro-alloyed FP steels which contain very small (normally, less than 0.1 %) additions of strong carbide or carbo-nitride forming elements such as Nb, V, and / or Ti for precipitation strengthening, grain refinement, and possibly transformation temperature control, (iii) as-rolled pearlitic steels which can include C-Mn steels but which can also have small additions of other alloying elements to improve strength, toughness, formability, and weldability, (iv) acicular ferrite (low C bainite) steels which are low C (less than 0.08 % C) steels with an excellent combination of high yield strengths, weldability, formability, and good toughness, (v) dual-phase steels which have a microstructure of martensite dispersed in a ferritic matrix and provide a good combination of ductility and high tensile strength, (vi) inclusion shape controlled steels which provide improved ductility and through-thickness toughness by the small additions of Zr, Ti, or calcium (Ca),or perhaps REEs so that the shape of the sulphide inclusions are changed from elongated stringers to small, dispersed, almost spherical globules, and (vii) hydrogen-induced cracking (HIC) resistant steels which are with low C, low S, inclusion shape control, and limited Mn segregation, plus Cu contents higher than 0.26 %.
These seven categories are not necessarily distinct groupings, in that micro-alloyed steel can have characteristics from more than one grouping. For example, all the above types of steels can be inclusion shape controlled. Micro-alloyed FP steel can also have additional alloys for corrosion resistance and solid-solution strengthening.
Weathering steels – The first micro-alloyed steels developed were the weathering steels. These steels contain Cu and other elements which improve corrosion resistance, solid-solution strengthening, and some grain refinement of the ferritic microstructure. The solid-solution strengthening effect of several alloying elements is shown in Fig 1. Of these, Cu, P, and silicon (Si) provide corrosion resistance in addition to solid-solution strengthening.
Fig 1 Solid solution strengthening and corrosion resistance of weathering steels
These steels reduce corrosion by forming their own protective oxide surface film. Although these steels initially corrode at the same rate as plain C steel, they soon show a decreasing corrosion rate, and after a few years, continuation of corrosion is practically non-existent. The protective oxide coating is fine textured, tightly adherent, and a barrier to moisture and oxygen (O2), effectively preventing further corrosion. Plain C steel, on the other hand, forms a coarse-textured flaky oxide which does not prevent moisture or O2 from reaching the underlying non-corroded steel base.
The atmospheric-corrosion resistance of the low P weathering steels is two to six times that of C structural steel, while the high P weathering steels can have atmospheric corrosion resistance of five to eight times higher than that of C steel (Fig 1), depending on the environment. The high P weathering steel has minimum yield strength of 345 MPa in section thicknesses upto 13 mm.
Micro-alloying with V and / or Nb can improve the yield strength of weathering steels. The addition of Nb also improves toughness. Normalizing or controlled rolling-cooling can also refine the grain size (and thus improve toughness and yield strength). However, if normalizing or accelerated cooling is used to refine grain size, the effect of C content and alloy additions on hardenability and the potential for undesirable transformations to upper bainite and Widmanstatten ferrite is tobe considered.
Micro-alloyed ferrite-pearlite steels – These steels use additions of alloying elements such as Nb and V to increase strength (and thereby increase load-carrying ability) of hot rolled steel without increasing C and / or Mn contents. Extensive studies during the 1960s on the effects of Nb and V on the properties of structural grade steels resulted in the discovery that very small amounts of Nb and V (less than 0.1 % each) strengthen the standard C- Mn steels without interfering with subsequent processing. Hence, the C can be reduced to improve both weldability and toughness since the strengthening effects of Nb and V compensated for the reduction in strength due to the reduction in the C content.
The mechanical properties of the micro-alloyed steels result, however, from more than just the mere presence of micro-alloying elements. Austenite conditioning, which depends on the complex effects of alloy design and rolling techniques, is also an important factor in the grain refinement of hot rolled micro-alloyed steels. Grain refinement by austenite conditioning with controlled rolling methods has resulted in improved toughness and high yield strengths in the range of 345 MPa to 620 MPa.
The development of controlled-rolling processes coupled with alloy design has produced increasing yield strength levels accompanied by a gradual lowering of the C content. Many of the micro-alloyed steels have C contents as low as 0.06 % or even lower, yet are still able to develop yield strengths of 485 MPa. The high yield strength is achieved by the combined effects of fine grain size developed during controlled hot rolling and precipitation strengthening that is due to the presence of V, Nb, and Ti.
The different types of micro-alloyed FP steels include (i) V micro-alloyed steels, (ii) Nb micro-alloyed steels, (iii) Nb-Mo steels, (iv) V-Nb micro-alloyed steels, (v) V-N2 micro-alloyed steels, (vi) Ti micro-alloyed steels, (vii) Nb-Ti micro-alloyed steels, and (viii) V-Ti micro alloyed steels. These steels can also include other elements for improved corrosion resistance and solid-solution strengthening, or improved hardenability (if transformation products other than ferrite-pearlite are desired).
Some specifications of micro-alloyed steels do not specify the range of micro-alloying additions needed to achieve the desired strength level. These steels are frequently specified in terms of mechanical properties, with the amounts of micro-alloying elements left to the discretion of the steel producer.
Vanadium micro-alloyed steels – The development of V containing steels occurred shortly after the development of the weathering steels, and flat rolled products with upto 0.1 % V are widely used in the hot rolled condition. V containing steels are also used in the controlled-rolled, normalized, or Q&T condition. V contributes to strengthening by forming fine precipitate particles of V(CN) (vanadium carbo nitride) in ferrite having diameter 5 nano meters (nm) to 100 nm during cooling after hot rolling. These V precipitates, which are not as stable as Nb precipitates, are in the solution at all normal rolling temperatures and hence are very dependent on the cooling rate for their formation. Nb precipitates, however, are stable at higher temperatures, which is beneficial for achieving fine grain ferrite.
The strengthening from V averages between 5 MPa and 15 MPa per 0.01 % V, depending on the C content and rate of cooling from hot rolling (and hence section thickness). The cooling rate, which is determined by the hot rolling temperature and the section thickness, affects the level of precipitation strengthening in a 0.15 % V steel, as shown in Fig 2. An optimum level of precipitation strengthening occurs at a cooling rate of around 170 deg C/minute (min). At cooling rates lower than 170 deg C/min, the V(CN) precipitates coarsen and are less effective for strengthening. At higher cooling rates, more V(CN) remains in solution, and hence a smaller fraction of V(CN) particles precipitate and strengthening is reduced. For a given section thickness and cooling medium, cooling rates can be increased or decreased by increasing or decreasing, respectively, the temperature before cooling. Increasing the temperature results in larger austenite grain sizes, while decreasing the temperature makes rolling more difficult.
Fig 2 Effect of cooling rate and niobium carbide on yield strength
Mn content and ferrite grain size also affect the strengthening of V micro-alloyed steels. It is seen that the 0.9 % increase in the Mn content increases the strength of the matrix by 34 MPa because of solid-solution strengthening. The precipitation strengthening by V is also improved since Mn lowers the austenite-to-ferrite transformation temperature, thereby resulting in a finer precipitate dispersion. This effect of Mn on precipitation strengthening is greater than its effect in Nb steels. However, the absolute strength in a Nb steel with 1.2 % Mn is only around 50 MPa less than that of V steel but at a much lower level o the alloying element(that is, 0.06 % Nb versus 0.14 % V).
The third factor affecting the strength of V steels is the ferrite grain size produced after cooling from the austenitizing temperature. Finer ferrite grain sizes (which result in not only higher yield strengths but also improved toughness and ductility) can be produced by either lower austenite-to-ferrite transformation temperatures or by the formation of finer austenite grain sizes prior to transformation. Lowering the transformation temperature, which affects the level of precipitation strengthening as mentioned above, can be achieved by the alloying element additions and / or increased cooling rates. For a given cooling rate, further refinement of ferrite grain size is achieved by the refinement of the austenite grain size during rolling.
The austenite grain size of hot rolled steels is determined by the recrystallization and grain growth of austenite during rolling. V hot rolled steels normally undergo conventional rolling but are also produced by recrystallization controlled rolling. With conventional rolling, V micro-alloyed steels provide moderate precipitation strengthening and relatively little strengthening from grain refinement. The maximum yield strength of conventionally hot rolled V micro-alloyed steels with 0.25 % C and 0.08 % V is around 450 MPa. The practical limit of yield strengths for hot rolled V micro-alloyed steel is around 415 MPa, even when controlled rolling techniques are used. V micro-alloyed steels subjected to recrystallization controlled rolling need a Ti addition so that a fine precipitate of TiN (titanium nitride) is formed which restricts austenite grain growth after recrystallization. Yield strengths from conventional controlled rolling are limited to a practical limit of around 415 MPa because of the lack of retardation of recrystallization. When both strength and impact toughness are important factors, controlled-rolled low C- Nb steel is preferable.
Nb micro-alloyed steels – Nb increases yield strength by precipitation hardening like V. The magnitude of the increase depends on the size and amount of precipitated NbC (niobium carbide). However, Nb is also a more effective grain refiner than V. Hence, the combined effect of precipitation strengthening and ferrite grain refinement makes Nb a more effective strengthening agent than V. The normal Nb addition is 0.02 % to 0.04 %, which is around one-third of the optimum V addition. Fig 2 shows effect of NbC on the yield strength for various sizes of NbC particles, Strengthening by Nb is 35 MPa to 40 MPa per 0.01 % addition. This strengthening is accompanied by a considerable impairment of notch toughness until special rolling procedures are developed and C contents are lowered to avoid formation of upper bainite. In general, high finishing temperatures and light deformation passes are to be avoided with the Nb steels since this can result in mixed grain sizes or Widmanstatten ferrite, which impair the toughness.
Nb steels are produced by controlled rolling, recrystallization controlled rolling, accelerating cooling, and direct quenching. The recrystallization controlled rolling of Nb steel can be effective without Ti, while recrystallization rolling of V micro-alloyed steels needs Ti for grain refinement. Also, much less Nb is needed, and Nb-Ti steels can be recrystallization controlled rolled at higher temperatures.
Vanadium-niobium micro-alloyed steels – Steels which are micro-alloyed with both V and Nb provide higher yield strength in the conventionally hot rolled condition than that achievable with either element alone. As conventionally hot rolled, the V-Nb micro-alloyed steels derive almost all of their increased strength from precipitation strengthening and hence have high ductile-brittle transition temperatures. If the steel is controlled rolled, the addition of both V and Nb together is especially advantageous for increasing the yield strength and lowering ductile-to-brittle transition temperatures by grain refinement. Normally V-Nb micro-alloyed steels are made with relatively low C content (less than 0.1 % C). This reduces the quantity of pearlite and improves toughness, ductility, and weldability. These steels are frequently referred to as pearlite-reduced steels.
Niobium-molybdenum micro-alloyed steels – These steels can have either a FP microstructure or an acicular ferrite microstructure. In FP Nb micro-alloyed steels, the addition of Mo increases the yield strength and tensile strength by around 20 MPa and 30 MPa, respectively, per 0.1 % Mo, over a range of 0 % Mo to 0.27 % Mo.
The principal effect of Mo on the microstructure is to alter the morphology of the pearlite and to introduce upper bainite as a partial replacement for pearlite. However, since the individual strength values of pearlite and bainite are somewhat similar, it has been proposed that the strength increase is due to solid-solution strengthening and improved Nb(CN) precipitation strengthening caused by a Nb-Mo synergism. The interaction between Nb and Mo (or V) has been proposed as an explanation for the increase in precipitation strengthening by the addition of Mo. This effect has been attributed to the reduced precipitation in austenite from an increase in solubility arising from a decrease in C activity brought about by Mo. With lesser precipitation in austenite, more precipitates can form in the ferrite, resulting in improved strength. Also, Mo has been identified in the precipitates themselves. Its presence can increase their strengthening effectiveness by increasing coherency strains and / or by increasing the volume fraction of precipitation.
Vanadium-nitrogen micro-alloyed steels – V combines more strongly with N2 than Nb does and forms VN precipitates in V-N2 micro-alloyed steel. Additions of N2 to high-strength steels containing V have become commercially important since the additions enhance precipitation hardening. Precipitation hardening can be accompanied by a drop in notch toughness, but this can frequently be overcome by lowering the C content. The precipitation of vanadium nitride (VN) also acts as a grain refiner. Some producers use N2 additions to assist in the precipitation strengthening of controlled-cooled steel sheet and plate with thicknesses above 9.5 mm. However, delayed cracking is a major problem in these steels. The use of N2 is not advised for steels which are to be welded because of its detrimental effect on notch toughness in the heat-affected zone (HAZ).
Titanium micro-alloyed steels – Ti in low C steel forms into a number of compounds. These compounds provide grain refinement, precipitation strengthening, and sulphide shape control. However, since Ti is also a strong deoxidizer, it can be used only in fully killed (Al deoxidized) steels so that Ti is available for forming into compounds other than titanium oxide (TiO2). Commercially, steels precipitation strengthened with Ti are produced in thicknesses up to 9.5 mm in the minimum yield strength range from 345 MPa to 550 MPa, with controlled rolling needed to maximize strengthening and improve toughness.
Like V and / or Nb micro-alloyed steels, Ti micro-alloyed steels are strengthened by mechanisms which involve a combination of grain refinement and precipitation strengthening. The combination depends on the quantity of alloying element additions and processing methods. In reheated or continuously cast steels, small amounts of Ti (less than or equal to 0.025 %) are effective grain refiners since the austenite grain growth is retarded by Titanium nitride (TiN). Small quantities of Ti are also effective in recrystallization controlled rolling since TiN retards the grain growth of the recrystallized austenite. In conventional controlled rolling, however, Ti is a moderate grain refiner, causing less refinement than Nb but more than V.
In terms of precipitation strengthening, a sufficient quantity of Ti is needed to form titanium carbide (TiC). Small percentages of Ti (less than 0.025 %) form mainly TiN, which has an effect on austenite grain growth but little effect on precipitation strengthening since the precipitates formed in the liquid are too coarse. Increasing the Ti content leads to the formation first of Ti-containing manganese sulphide inclusions (Mn, Ti)S, and then of globular carbo-sulphides, Ti4C2S2 (which provide sulphide shape control). The formation of Ti4C2S2 is accompanied by and followed by TiC formation, which can be used for the precipitation strengthening of low C steels. For the determination of the quantity of Ti which is available for precipitation strengthening, the total Ti content is to be adjusted for the formation of the coarse, insoluble TiN and carbo-sulphides which do not participate in the precipitation strengthening.
Experimentally observed strength increases from TiC precipitation have ranged up to 440 MPa for very fine particles (less than 30 angstrom) and a relatively large fraction of precipitate. If sufficient quantities of Ti are used, TiC can provide more precipitation strengthening than either V or Nb. However, since higher levels of precipitation strengthening are normally associated with reduced toughness, grain refinement is necessary to improve toughness. Ti is a moderate grain refiner (compared to V and Nb hot rolled steels), and the high levels of precipitation strengthening of Ti micro-alloyed steels result in a severe reduction in toughness. Using only Ti as a strengthener in high strength hot rolled steel has also result in unacceptable variability in mechanical properties.
Titanium-niobium micro-alloyed steels – Although precipitation-strengthened Ti steels have limitations in terms of toughness and variability of mechanical properties, studies have shown that an addition of Ti to low C-Nb micro-alloyed steels results in an overall improvement in properties. Ti increases the efficiency of Nb since it combines with the N2 forming TiN, hence preventing the formation of niobium nitride (NbN). This allows for increased solubility of Nb in the austenite resulting in subsequent increased precipitation of Nb(C,N) particles in the ferrite. The addition of 0.04 % Ti to steel containing various quantities of Nb consistently produce a yield strength increase of around 105 MPa for a coiling temperature of 675 deg C. Hot rolled Ti-Nb micro-alloyed steel is effective in achieving yield strengths of 550 MPa in FP steels. An addition of either V or Mo can raise yield strengths to 690 MPa.
As-rolled pearlitic structural steels – These steels are a specific group of steels in which enhanced mechanical properties (and, in some cases, resistance to atmospheric corrosion) are achieved by the addition of moderate quantities of one or more alloying elements other than C. Some of these steels are C-Mn steels and differ from ordinary C steels only in having a higher Mn content. Other pearlitic structural steels contain small amounts of alloying elements, which are added to improve weldability, formability, toughness, and strength.
The as-rolled pearlitic structural steels are characterized by as-rolled yield strengths in the range of 290 MPa to 345 MPa. They are not intended for quenching and tempering and are not to be subjected to such treatment. For certain applications, they can be annealed, normalized, or stress relieved, or processed which can alter mechanical properties. When these steels are used in welded structures, care is to be taken in grade selection and in the specification of the welding process details. Certain grades can be welded without preheating or post-heating. The basic disadvantages of these steels are that the pearlitic microstructure raises the ductile-to-brittle transition temperature but does not improve yield strengths. In addition, the high C content (relative to other micro-alloyed steels) reduces weldability.
Acicular ferrite (low-C bainite) steels – Another approach to the development of micro-alloyed steels is to achieve a very fine, high strength acicular ferrite microstructure, instead of the normal polygonal ferrite microstructure, during the cooling transformation of ultra-low C (less than 08 %) steels with sufficient hardenability (by additions of Mn, Mo, and / or B). Nb can also be used for precipitation strengthening and grain refinement. The principal difference between the structure of acicular ferrite (which is also referred to as low C bainite) and that of polygonal ferrite is that the former is characterized by a high dislocation density and fine, highly elongated grains which are not shown n polygonal ferrite.
Acicular ferrite steels can be obtained by quenching or, preferably, by air cooling with suitable addition of alloying elements for hardenability. The principal advantage of this type of micro-alloyed steel is the unusual combination of high yield strengths (415 MPa to 690 MPa), high toughness, and good weldability.
The major application of acicular ferrite steel involves oil pipelines in arctic conditions. This application needs a combination of high strength, superior toughness, excellent resistance to HIC, and first-rate field weldability. In answer to these needs, tough acicular ferrite steel has been developed for line pipe through the optimization of C and Nb content, the addition of B, and / or the application of on-line accelerated cooling. In this pipe, optimum C content ranges from 0.01 % to 0.05 %. Below 0.01 % C, grain boundaries in the HAZ are embrittled, resulting in inter-granular, HIC, and loss of toughness in the HAZ. The addition of B and / or the application of on-line accelerated cooling ensure both high strength and superior toughness, along with desirable welding properties.
Dual-phase steels – These steels have a microstructure with 80 % to 90% polygonal ferrite and 10 % to 20 % martensite islands dispersed throughout the ferrite matrix. These steels have a low yield strength and continuous yielding behaviour. Hence, these steels form just like low strength steel, but they can also provide high strength in the finished component because of their rapid work-hardening rate. Typical as-shipped yield strength is 310 MPa to 345 MPa.
Dual-phase steels can be produced from low C in three ways namely (i) inter-critical austenitization of C-Mn steels followed by rapid cooling, (ii) hot rolling with ferrite formers such as Si and transformation-delaying elements such as Cr (chromium), Mn, and / or Mo, and (iii) continuous annealing of cold-rolled C-Mn steel followed by quenching and tempering.
Interstitial-free (IF) Steels – IF steels with very low interstitial contents show excellent formability with low yield strength, high elongation, and good deep drawability. With the addition of carbo-nitride-forming elements, the deep drawability and the non-aging properties are further improved. The effect of Nb, unlike that of other micro-alloying elements, is to improve the planar anisotropy, reducing earing. This is due to the finer grain size already in the hot rolled steel prior to cold rolling. Ti is also added to improve the effectiveness of Nb.
Inclusion shape controlled steels – An important development in micro-alloyed steels is the use of inclusion shape control. Sulphide inclusions, which are plastic at rolling temperatures and thus elongate and flatten during rolling, adversely affect ductility in the short transverse (through thickness) direction. The main objective of inclusion shape control is to produce sulphide inclusions with negligible plasticity at even the highest rolling temperatures.
The preferred method for sulphide shape control involves Ca-Si ladle additions. However, sulphide shape control is also performed with small additions of REEs, Zr, or Ti which change the shape of the sulphide inclusions from elongated stringers to small, dispersed, almost spherical globules. This change in the shape of sulphide inclusions substantially increases transverse impact energy and improves formability. Inclusion shape control has introduced with the advent of hot rolled steel sheet and light plate having yield strength of 550 MPa in the as-rolled condition. This technology has also been extended to include grades with lower yield strengths ranging from 310 MPa to 550 MPa. Inclusion shape control with REEs is seldom used since REEs produce relatively dirty steel. Sulphide inclusion shape control performs several important roles in micro-alloyed steels. It improves transverse impact energy, and it can minimize lamellar tearing in welded structures by improving through-thickness properties which are critical in constrained weldments.
Control of micro-alloyed steel properties
Majority of the micro-alloyed steels are furnished in the as hot rolled condition with FP microstructure. The exceptions are the controlled rolled steels with an acicular ferrite microstructure and the dual-phase steels with martensite dispersed in a matrix of polygonal ferrite. These two types of micro-alloyed steels use the formation of eutectoid structures for strengthening, while the FP micro-alloyed steels normally need strengthening of the ferrite. Pearlite is normally an undesirable strengthening agent in structural steels since it reduces impact toughness and needs higher C content. Moreover, yield strength is largely unaffected by a higher pearlite content.
Strengthening mechanisms of ferrite in micro-alloyed steels
The ferrite in micro-alloyed steels is typically strengthened by grain refinement, precipitation hardening, and, to a lesser extent, solid-solution strengthening. Grain refinement is the most desirable strengthening mechanism since it improves not only strength but also toughness.
Grain refinement – It is influenced by the complex effects of alloy design and processing methods. For example, the various methods of grain refinement used in the three different stages of hot rolling (that is, reheating, hot rolling, and cooling) include (i) the addition of Ti or Al to retard austenite grain growth when the steel is reheated for hot deformation or subsequent heat treatment, (ii) the controlled rolling of the micro-alloyed steels to condition the austenite so that it transforms into fine grain ferrite, and (iii) the use of alloy additions and / or faster cooling rates to lower the austenite-to-ferrite transformation temperature. The use of higher cooling rates for grain refinement can need consideration of its effect on precipitation strengthening and the possibility of undesirable transformation products.
Precipitation strengthening – Precipitation strengthening occurs from the formation of finely dispersed carbo-nitrides develop during heating and cooling. Since precipitation strengthening is normally associated with a reduction in toughness, grain refinement is frequently used in conjunction with precipitation strengthening to improve toughness.
Precipitation strengthening is influenced by the type of carbo-nitride, its grain size, and, of course, the number of carbo-nitride particles precipitated. The formation of metal carbide (MC) is the most effective in the precipitation strengthening of micro-alloyed Nb, V, and / or Ti micro-alloyed steels. The number of fine MC particles formed during heating and cooling depends on the solubility of the carbides in austenite and on cooling rates.
Steelmaking operations – Precise steelmaking operations are also essential in controlling the properties and chemistry of the micro-alloyed steels. Optimum property levels depend on such factors as the control of significant alloying elements and the reduction of impurities and non-metallic inclusions. Developments in secondary steelmaking such as desulphurization, vacuum degassing, and argon shrouding have enabled better control of steel chemistry and the effective use of micro-alloyed elements. In particular, the use of vacuum degassing process allows the production of IF steels. The IF steels show excellent formability, high elongation, and good deep drawability.
Composition and alloying elements
Chemical compositions for the micro-alloyed steels are specified in the standards. The principal function of alloying elements in the FP micro-alloyed steels, other than corrosion resistance, is strengthening of the ferrite by grain refinement, precipitation strengthening, and solid solution strengthening. Solid-solution strengthening is closely related to the content of the alloying elements, while grain refinement and precipitation strengthening depend on the complex effects of alloy design and thermo-mechanical treatment.
Alloying elements are also selected to influence transformation temperatures so that the transformation of austenite to ferrite and pearlite occurs at a lower temperature during air cooling. This lowering of the transformation temperature produces a finer-grain transformation product, which is a major source of strengthening. At the low C levels typical of micro-alloyed steels, elements such as Si, Cu, P, and Ni (nickel) are particularly effective for producing fine pearlite. Elements such as Mn and Cr, which are present in both the cementite and ferrite, also strengthen the ferrite by solid-solution strengthening in proportion to the amount dissolved in the ferrite. The role of various alloying elements in the micro-alloyed steels is given below.
Carbon – It markedly increases the amount of pearlite in the microstructure and is one of the more strong, as well as economical, strengthening elements. However, higher C contents reduce weldability and the impact toughness of steel. Increases in pearlite content are also ineffective in improving yield strength, which is frequently the main strength criterion in structural steel applications.
In the presence of alloying elements, the practical maximum C content at which micro-alloyed steels can be used in the as rolled condition is around 0.2 %. Higher levels of C tend to form martensite or bainite in the microstructure of as-rolled steels, although some of the higher strength micro-alloyed steels have C contents which approach 0.3 %. Several of the micro-alloyed steels have C contents of 0.06 % or even lower, yet are still able to develop yield strengths of 345 MPa to 620 MPa. C levels as low as 0.03% is utilized in some alloy designs. The required strength is developed by the combined effect of (i) fine grain size developed during controlled hot rolling and enhanced by micro-alloying elements (especially Nb), and (ii) precipitation strengthening caused by the presence of V, Nb, and Ti in the composition.
Nitrogen – N2 in quantities upto around 0.02 % has been used to achieve strengths typical of micro-alloyed steels and at reasonable cost. For C and C-Mn steels, such a practice is limited to light gauge steel products since the increase in strength is accompanied by a drop in notch toughness. In some applications, N2 contents are limited to 0.005 %. N2 additions to high strength steels containing V have become commercially important since such additions improve precipitation hardening. The precipitation of VN in V-N2 micro-alloyed steels also improves grain refinement since it has a lower solubility in austenite than VC.
Manganese – Mn is the principal strengthening element in plain C high strength structural steels when it is present in quantities over 1 %. It functions mainly as a mild solid-solution strengthener in ferrite, but it also provides a marked decrease in the austenite-to-ferrite transformation temperature. In addition, Mn can enhance the precipitation strengthening of V micro-alloyed steels and, to a lesser extent, Nb micro-alloyed steels.
Silicon – One of the most important applications of Si is its use as a deoxidizer in the liquid steel. It is normally present in fully deoxidized structural steels in quantities upto 0.35 %, which ensures the production of sound, dense cast steel. Si has a strengthening effect in micro-alloyed structural steels. In larger amounts, it increases resistance to scaling at high temperatures. Si has a considerable effect on yield strength improvement by solid-solution strengthening and is widely used in the micro-alloyed steels for riveted or bolted structures. It can be used upto 0.3 % in weldable steels. Higher quantities of Si produce deterioration in notch toughness and weldability.
Copper – Around 0.2 % Cu is used to provide resistance to atmospheric corrosion. Its effect on resistance to corrosion is improved when P is present in quantities higher than around 0.05 %. Cu in levels in excess of 0.5 % also increases the strength of both low C and medium C steels by virtue of ferrite strengthening, which is accompanied by only slight decreases in ductility. In quantities over around 0.6 %, Cu can precipitate as Cu, which precipitation strengthens the ferrite. Cu can be retained in solid solution even at the slow rate of cooling obtained when large sections are normalized, but it is precipitated out when the steel is reheated to around 510 deg C to 605 deg C. At around 1 % Cu, the yield strength is increased by around 70 MPa 140 MPa, regardless of the effects of other alloying elements.
Cu in quantities upto 0.75 % is considered to have only minor adverse effects on notch toughness or weldability. Cu precipitation hardening gives the steel the ability to be formed extensively (while still low in strength) and then precipitation hardened as a complex shape or welded assembly. This avoids the distortion or difficulty encountered by quenching compared with components made from conventionally heat treated alloy steel.
Steels containing more than 0.5 % Cu can show hot shortness, with the result that cracks and a rough surface can develop during hot working. Careful control of oxidation during heating and prevention of overheating can minimize these conditions. The addition of Ni in amounts equal to at least one-half of the Cu content is very beneficial to the surface quality of Cu bearing steels. Cu is also added to steels in quantities 0.25 % to 0.35 % for improving resistance to HIC in aqueous environments and H2S (hydrogen sulphide) environments.
Phosphorus – P is an effective solid-solution strengthener in ferrite. It also enhances corrosion resistance but causes a decrease in ductility. P at low levels (less than 0.05 %) can also cause embrittlement through segregation to the prior-austenite grain boundaries. The atmospheric corrosion resistance of steel is increased appreciably by the addition of P, and when small amounts of Cu are present in the steel, the effect of the P is greatly enhanced. When both P and Cu are present, there is a greater beneficial effect on corrosion resistance than the sum of the effects of the individual elements.
Chromium – Cr is frequently added with Cu to achieve improved atmospheric corrosion resistance. Upon exposure to the atmosphere, a steel composition with around 0.12 % P, 0.85 % Cr, and 0.40 % Cu develops a particularly adherent, dense oxide coating which is characteristic of weathering steels.
Nickel – Ni can be added in amounts upto around 1 % in several micro-alloyed steels and in quantities upto 5 % in high strength heat treated alloy steels. It moderately increases strength by solution hardening of the ferrite. In the micro-alloyed steels, it improves atmospheric-corrosion resistance and, when present in combination with Cu and / or P, increases the seawater corrosion resistance of steels. Ni is frequently added to Cu bearing steels to minimize hot shortness.
Molybdenum – Mo in hot rolled micro-alloyed steels is used primarily to improve hardenability when transformation products other than ferrite-pearlite are desired. For example, Mo is an essential ingredient for producing as rolled acicular ferrite steels. In addition, it effectively improves higher temperature properties. Mo (0.15 % to 0.3 %) in micro-alloyed steels also increases the solubility of Nb in austenite, thereby improving the precipitation of NbC(N) in the ferrite. This increases the precipitation-strengthening effect of Nb(C,N). Mo has also been shown to join the Nb(C,N) precipitates, which further raises the yield strength.
Niobium – Small additions of Nb (0.03 % to 0.05 %) increase yield strength by a combination of precipitation strengthening and grain refinement. Nb is a more effective grain refining element than V since NbC is more stable in austenite than VC at typical rolling temperatures. The lower solubility of NbC in austenite provides more stable precipitate particles, which pin the austenite grain boundaries and hence retard austenite grain growth.
Aluminum – Al is widely used as a deoxidizer and was the first element used to control austenite grain growth during reheating. During controlled rolling, Nb and Ti are more effective grain refiners than Al. Ti, Nb, Zr, and V are also effective grain growth inhibitors during reheating. However, for steels which are heat treated (quenched and tempered), these four elements can have adverse effects on hardenability since their carbides are quite stable and difficult to dissolve in austenite prior to quenching.
Vanadium – V strengthens micro-alloyed steels both by precipitation hardening the ferrite and refining the ferrite grain size. The precipitation of V(CN) in ferrite can develop a considerable increase in strength which depends not only on the rolling process used, but also on the base composition. C content above 0.13 % to 0.15 % and Mn content of 1 % or more improves the precipitation hardening, particularly when the N2 content is at least 0.01 %. Grain size refinement depends on thermal processing (hot rolling) variables, as well as the V content.
Titanium – Ti is unique among the normal alloying elements in that it provides both precipitation strengthening and sulphide shape control. Small quantities of Ti (less than 0.025 %) are also useful in limiting austenite grain growth. However, it is useful only in fully killed (Al deoxidized) steels because of its strong deoxidizing effects. The versatility of Ti is limited since variations in O2, N2, and S affect the contribution of Ti as a carbide strengthener.
Zirconium – Zr can also be added to killed high strength micro-alloyed steels to improve inclusion characteristics, particularly in the case of sulphide inclusions, for which changes in inclusion shape improve ductility in transverse bending.
Boron – B has no effect on the strength of normal hot rolled steel but can considerably improve hardenability when transformation products such as acicular ferrite are desired in low C hot-rolled steel plate. Its full effect on hardenability is achieved only in fully deoxidized (Al killed) steels.
Rare earth elements – REEs principally cerium (Ce), lanthanum (La), and praseodymium Pr), can be used to provide shape control of sulphide inclusions. Sulphide inclusions, which are plastic at rolling temperatures and thus elongate and flatten during rolling, adversely affect ductility in the short transverse (through-thickness) direction. The main role of REEs is to produce rare earth sulphide and oxy-sulphide inclusions, which have negligible plasticity at even the highest rolling temperatures. However, presently excessive quantities of Ce (higher than 0.02 %) and other REEs are seldom used since they produce relatively dirty steels. Treatment with Ca is preferred for sulphide inclusion shape control.
The hot rolling process has gradually become a much more closely controlled operation, and controlled rolling is now being increasingly applied to micro-alloyed steels with compositions carefully chosen to provide optimum mechanical properties at room temperature. Controlled rolling is a procedure whereby the various stages of rolling are temperature controlled, with the quantity of reduction in each pass predetermined and the finishing temperature precisely defined. This processing is widely used to achieve reliable mechanical properties in steels for pipelines, bridges, offshore platforms, and many other engineering applications. The use of controlled rolling has resulted in improved combinations of strength and toughness and further reductions in the C content of micro-alloyed steels. This reduction in C content improves not only toughness but also weldability.
The basic objective of controlled rolling is to refine and / or deform austenite grains during the rolling process so that fine ferrite grains are produced during cooling. Controlled rolling can be performed on C steels but is most beneficial in steels with V and / or Nb additions. During hot rolling, the undissolved carbo-nitrides of V and Nb pin austenite grain boundaries and thus retard austenite grain growth. However, in C steels, the temperatures involved in hot rolling lead to marked austenite grain growth, which basically limits the benefit of grain refinement by controlled rolling. Controlled rolling is performed on strip, plate, and bar mills but not on continuous hot strip mills. On a hot strip mill, the water cooling on the run out table ensures a fine grain size.
The three methods of controlled rolling are namely (i) conventional controlled rolling, (ii) recrystallization controlled rolling, and (iii) dynamic recrystallization controlled rolling. These three methods use different techniques for grain refinement, but they are all preceded by a roughing operation to refine grain size by repeated recrystallization. In the roughing stage, stable carbo-nitride precipitates are desirable since they pin the grain boundaries of the recrystallized austenite and hence prevent their growth. Nb is more effective than V in preventing austenite grain growth during rolling since Nb forms precipitates which are less soluble than VC in austenite. Roughing can achieve austenite grain sizes of the order of 20 micrometers. The austenite grains are then either deformed or further refined by controlled rolling during finishing operations.
Conventional controlled rolling – It is based on the deformation, or flattening (pan-caking), of austenite grains so that a large number of nucleation sites exist on the deformed austenite grain boundaries and on the deformation bands with the austenite grains. These nucleation sites allow the formation of very fine grain ferrite during transformation cooling. This process needs a total reduction of upto 80 % in a temperature range where the austenite deforms but does not recrystallize.
Nb is the most effective alloying element for grain refinement by conventional controlled rolling. During the rolling reductions at temperatures below 1,040 deg C, the Nb in solution suppresses recrystallization by solute drag or by strain-induced Nb(C,N) precipitation on the deformed austenite and slip planes. The strain-induced precipitates are too large to affect precipitation strengthening but are beneficial for two reasons. They allow additional suppression of recrystallization by preventing migration of austenite grain sub-boundaries, and they provide a large number of nuclei in the deformed austenite for the formation of fine ferrite particles during cooling. The strain-induced precipitates in the austenite detract from the precipitation-hardening potential of the ferrite by removing the available Nb from austenite solid solution. However, a useful measure of precipitation strengthening is possible in controlled-rolled Nb micro-alloyed steels.
The controlled rolling of Nb steels can lead to ferrite grain sizes in the range of 5 micrometers to 10 micrometers (ASTM grain size numbers 10 to 12). Since the precipitation of Nb(CN) in the austenite during hot rolling retards recrystallization and raises the temperature at which recrystallization of austenite ceases (the recrystallization stop temperature), a broader temperature range is possible for hot working the steel to produce highly deformed austenite. The optimum amount of Nb to suppress recrystallization between passes can be as little as 0.02 %. Ti, Zr, and V are not as effective as Nb in raising the recrystallization stop temperature. Ti and Zr nitrides formed during solidification and upon cooling of the steel do not readily dissolve upon reheating to hot rolling temperatures. Although these nitrides can prevent grain coarsening upon reheating, they are not effective in preventing recrystallization, since insufficient Ti or Zr remains in solution at the rolling temperature to precipitate on deformed austenite boundaries during hot rolling and hence suppress austenite recrystallization.
V, on the other hand, is so soluble that precipitation does not readily occur in the austenite at normal hot rolling temperatures. The concentrations of Nb, Ti, V, C, and N2, the degree of strain, the time between passes, the strain rate, and the temperature of deformation all influence recrystallization during the hot working.
Recrystallization controlled rolling – Although conventional controlled rolling can lead to very fine ferrite grain sizes, the low finishing temperature (750 deg C to 900 deg C) of this method leads to increased rolling loads for heavy plate and thick-walled seamless tube. For thicker sections, recrystallization controlled rolling is used to refine austenite grain size. This process can result in ferrite grain sizes of the order of 8 micrometers to 10 micrometers. Recrystallization controlled rolling involves the recrystallization of austenite at successively lower temperatures below roughing temperatures but still above 900 deg C. Recrystallization is not to be sluggish for this method to succeed, and hence V can be beneficial since VC is readily dissolved at rolling temperature and hence unavailable for suppressing recrystallization. However, V micro-alloyed steels need stable carbo-nitrides, such as TiN, to retard grain growth after recrystallization. Nb micro-alloyed steels, on the other hand, can undergo recrystallization controlled rolling at higher temperatures with Nb(C,N) precipitates eventually forming. This precipitation of Nb(C,N) restricts austenite grain growth and can preclude the need for a Ti addition.
Dynamic recrystallization controlled rolling – It is used when there is insufficient time for recrystallization between rolling passes. This process initiates recrystallization during deformation and needs appreciable reductions (e.g. 100 %) to achieve an austenite grain size of 10 micrometers. With low temperature finishing, dynamic recrystallization can result in ferrite grain as fine as 3 micrometers to 6 micrometers.
Inter-pass cooling in controlled rolling – One of the major drawbacks with conventional controlled rolling is excessive processing time, especially on single stand mills lacking facilities for removing steel plates off line during the hold period. Hence, some studies have examined the possibility of reducing processing time by the application of accelerated cooling both during the delay period and between individual roughing and finishing passes. In general, it appears that the process time can be shortened by 30 % to 40 %, depending on exactly when forced cooling is applied in the rolling schedule. The as-rolled microstructure is not affected very much by intermittent cooling between passes but prolonged exposure to water during the hold period causes the temperature of the surface layers of the steel plate to fall below Ar3. On resumption of rolling, this surface shell is reaustenitized by heat conduction from the interior of the plate and the surface austenite grain size is thereby considerably finer than that in the rest of the material, and this difference persists, of course, in the microstructure after rolling. Such non-uniformity of microstructure and properties in the through thickness direction is hardly desirable, but it is unlikely to be directly detrimental to any particular bulk property of the steel plate (with the possible exception of bendability). Plates processed with shorter rolling times by the application of accelerated cooling between passes and / or under holding appear to show equivalent strength and toughness properties to those controlled rolled in the conventional manner.
Normalizing – Normalizing, as with C steels, can be used to refine the FP grain size in the micro-alloyed steels. However, since micro-alloyed steels can involve some strengthening by precipitation hardening, normalizing can cause a reduction in precipitation strengthening when stable carbo-nitride precipitates coarsen at typical austenitizing temperatures for normalizing. For example, normalizing a conventionally hot rolled Nb micro-alloyed steel reduces the yield strength considerably so that much of the precipitation strengthening increment that is due to Nb in the as-rolled condition is lost.
At the normal austenitizing temperatures for normalizing, most of the Nb has not been taken into solution and is present as relatively coarse precipitates. Thus, Nb inhibits grain coarsening, but provides little or no precipitation strengthening when the steel is normalized. However, the grain refining accounts for the improved low temperature impact properties and most of the small increase in strength, compared to a Nb free steel.
Normalized Nb micro-alloyed steels hence provide a fairly good combination of yield strength and ductile-to-brittle impact transition temperatures, even in semi-killed steels (which contain free N2 detrimental to impact toughness). Nb micro-alloyed steels, along with Ti, and V steels, in the form of normalized, heavy gauge steel plate find important applications in offshore and general construction, shipbuilding, and pressure vessels. Normalized steels of this type are always fully Al killed.
Although normalized Nb micro-alloyed steels have a finer ferrite grain size than do normalized C-Mn steels, when strength is of paramount importance, controlled rolling of Nb micro-alloyed steels is preferred over normalizing since controlled rolling also provides a measure of precipitation strengthening. V, on the other hand, causes marked precipitation strengthening with limited grain refinement upon normalizing since practically all of the V is in solution at normal normalizing temperatures. The strengthening with V in the normalized steels is far greater than can be achieved in the normalized Nb micro-alloyed steels. In the normalized condition, V semi-killed structural steels also have impact properties superior to those of the semi-killed Nb micro-alloyed steels since V combines with part of the N2, thereby reducing the free N2 content.
Mechanical properties of hot rolled micro-alloyed steels
The three major types of micro-alloyed steel microstructures are dual-phase microstructures, acicular ferrite microstructures, and FP microstructures. Hot rolled steels with FP microstructures are the most common micro-alloyed steels. Commercially available micro-alloyed steels with FP microstructures have yield strengths ranging upto 700 MPa, which is almost a four-fold increase over the 200 MPa yield strength of conventional hot rolled plain C steel. The increased strengths are developed by variations in composition and processing, but compositions having similar strengths are produced in different ways by different producers. There are three principal micro-alloying additions namely Nb, V, and Ti. Other additions can be made, depending on processing capabilities (particularly the cooling facilities) and on property requirements for the finished steel. In order to achieve good transverse properties, Ca- Si ladle additions (or, less frequently, additions of REEs) are used in Nb or V containing steels to control the shape of sulphide inclusions. For micro-alloyed compositions containing Ti, ladle treatments for sulphide shape control are not needed. Ti itself has the desired effect on the shape of sulphide inclusions. In recent years, Ca treatment has replaced REEs in sulphide inclusion shape control.
Tensile properties – Tensile properties of hot rolled FP micro-alloyed steels are influenced by alloying elements and production methods. In V micro-alloyed steels, yield strengths are influenced by Mn and N2 contents, and cooling rates. The cooling rate, which determines the level of precipitation strengthening in the V micro-alloyed steels, depends on the temperature, the product thickness, and the cooling medium. For thin products, temperature is the primary factor influencing cooling rates. Yield strengths in Nb micro-alloyed steels are also affected by rolling procedures and cooling rates.
The notch toughness – The notch toughness of micro-alloyed structural steels, as evaluated by Charpy-impact or drop-weight tests, is normally superior to that of structural C steels. The transition temperatures of the former also are lower. In the presence of a stress raiser, brittle failure is less likely to occur at subnormal temperatures in steels with lower transition temperatures. The transition temperatures of micro-alloyed steels in the as-rolled or normalized conditions are controlled principally by chemical composition (particularly C) and ferrite grain size.
Notch toughness is reduced when the FP micro-alloyed steels are strengthened by precipitation hardening. The notch toughness of V micro-alloyed steels can be improved by normalizing or recrystallization controlled rolling. The notch toughness Nb micro-alloyed steels is normally improved by conventional controlled rolling, although the effects of recrystallization controlled rolling are under study.
Brittle fracture – Majority of the structural steels show a transition in fracture behaviour, that is, from ductile to brittle when the temperature is lowered to some critical temperature which is known as the nil-ductility transition temperature (NDTT). This temperature is defined as that temperature at which steel loses its ability to flow plastically in the presence of a sharp, crack-like discontinuity. At and below the NDTT, a brittle cleavage fracture initiates from this discontinuity when stresses approaching the yield strength are reached in the volume of material surrounding the discontinuity. Once initiated, brittle fracture can propagate easily through regions of the structure which are subjected only to low levels of applied stress. In some steels, the transition from ductile to brittle fracture can occur at relatively high temperatures if a mechanical or metallurgical notch is present. If no sharp notch or crack is present, temperatures as low as minus 75 deg C are necessary to produce brittle fracture.
For welded structures, since it is assumed that sharp notches are present; this makes brittle fracture at normal operating temperatures an important concern. Majority of structural members remain within elastic loadings, except for corners, cutouts, and similar locations where slight yielding can occur. For this type of service, brittle fracture is possible when certain conditions exist namely (i) the temperature is below the characteristic NDTT value for the steel being used, (ii) a crack-like notch is present, and (iii) load values are sufficiently large to raise the nominal stress level in the area of the notch to values close to or exceeding the yield strength.
At and below the NDTT, the effects of residual stress can be considered. Weldments can contain residual stresses as high as 80 % of the original yield strength. In these cases, an applied stress of only 20 % of the yield strength is sufficient to initiate brittle fracture. All three factors, high stress, low temperature, and crack like flaws, are to be present for brittle fracture to initiate. However, as flaw size increases, the stress needed for crack initiation decreases. With larger flaw sizes, the levels can be well below the yield strength, and residual stresses play an increasingly important role.
At temperatures slightly above or below the NDTT, localized plastic deformation is needed to initiate brittle fracture, and residual stresses are less harmful. At temperatures well below the NDTT, design is critical. The nominal stress (the sum of applied stress and residual stress) is not to exceed the yield strength at the locations of crack like flaws. In general, the lower the temperature below NDTT, the less severe the notch is to be to initiate brittle fracture. For very thin sections, stresses act in a manner which makes brittle fracture considerations less critical.
Each type of micro-alloyed steel has a range of NDTT values which depends on chemical composition, as-rolled ferritic grain size, and other variables; a range of 35 deg C is not uncommon. Grain coarsening can occur in different steel products because of rolling practice, which increases the NDTT to above the maximum values.
Directionality of properties – In micro-alloyed steels, the changes in mechanical properties resulting from the use of Nb and V, together with controlled rolling, result in improved yield strength, weldability, and toughness. The ferrite grain size is reduced, with an attendant increase in yield strength. Because of this increase, any reduction in toughness due to the precipitation strengthening can normally be tolerated. The remaining properties, however, are typical only of samples tested in the direction of rolling. In the transverse direction, toughness is reduced considerably, and formability is inadequate because of the characteristic shape of non-metallic inclusions which, during rolling, become elongated in the rolling direction.
For Al killed steels the low-shelf energy in the transverse direction is caused primarily by elongated sulphide inclusions. Reducing the S content to 0.01 % is not sufficient to eliminate directionality. To prevent the sulphides from becoming excessively elongated during hot rolling, it is necessary to alter their composition. This can be done by adding elements such as Zr, Ti, Ca, or REEs, which form sulphides having high melting points. Sulphides with high melting points are less plastic at hot rolling temperatures and cannot be deformed readily. REEs additions can effectively improve transverse toughness so that when the Ce-to-S ratio is between 1.5 and 2, the transverse upper-shelf energy approaches that of the longitudinal direction. Of the elements mentioned above, Ca and REEs are the most frequently used materials for improving transverse or through-thickness properties.
Fatigue characteristics of micro-alloyed steels – Several structural applications for the micro-alloyed steels involve cyclic loading. The fatigue behaviour of these steels hence becomes important. Some of the fatigue characteristics of the micro-alloyed steels are compared to similar characteristics of hot rolled low C steels are given below.
- Micro-alloyed steels have fatigue properties equivalent or superior to those of hot rolled low C steel. Micro-alloyed steels of the three major compositional approaches (V, Nb, and Ti) have similar properties.
- Although the notch sensitivity is somewhat higher, micro-alloyed steels have greater notch fatigue resistance than hot rolled low C steels.
- Large plastic pre-strains tend to deteriorate the fatigue life of both the micro-alloyed and hot rolled low C steels, particularly the response of the micro-alloyed steels to cyclic loading at high-strain amplitudes. Tensile pre-strains are more detrimental to fatigue resistance than are compressive pre-strains for both classes of material.
- Majority of the gains in monotonic strength achieved by work hardening are not retained under cyclic loading for either micro-alloyed or hot rolled low C steel.
Forming of micro-alloyed steels
Micro-alloyed steels are normally formed at room temperature using conventional equipment. Cold forming is not to be done at temperatures below 10 deg C. As a class, micro-alloyed steels are inherently less formable than low C steels because of their greater strength and lower ductility. This reduces their ability to distribute strain. The greater strength makes micro-alloyed steels necessary to use higher forming pressure and to allow for more spring-back compared to low C steels. However, micro-alloyed steels have good formability, and straight bends can be made to relatively tight bend radii, especially with the grades having lower strengths and greater ductility. Further, micro-alloyed steels can be stamped to relatively severe shapes such as automotive bumper facings, wheel spiders, and engine-mounting brackets.
With the advent of inclusion shape control, cold formability has been substantially improved. Any grade produced with inclusion shape control can be more severely formed than a grade of the same strength level produced without inclusion shape control. Inclusion shape control enables the steel to be formed to nearly the same extent in both the longitudinal and transverse directions and is responsible for the moderately good formability of the micro-alloyed steels such as the grades having 550 MPa yield strengths. As with any metal, the bendability of the micro-alloyed steels is inversely proportional to the strength and thickness.
Micro-alloyed steels can be hot formed. However, hot forming normally alters mechanical properties, and a particular issue which arises in many applications is that some of the more recent thermo-mechanical processing techniques (such as controlled rolling) used for steel plates in particular are not suitable where hot forming is used during fabrication. The property deterioration can also be expected for accelerated-cooled steels. This problem can be circumvented by the use of a rolling finishing temperature which coincides with the hot-forming temperature (900 deg C to 930 deg C). Subsequent hot forming hence simply repeats this operation, and deterioration in properties is then small or even absent provided that grain growth does not occur.
Most of the cases hot forming at temperatures is not to be below 650 deg C. In some cases, satisfactory results can be achieved with certain grades by forming at temperatures between 815 deg C and 900 deg C without appreciable hardening after cooling. However, hot forming at these temperatures can result in material with mechanical properties equivalent to those of annealed or normalized material.
Welding of micro-alloyed steels
Micro-alloyed steels are readily welded by any of the welding processes used for plain C structural steels, including shielded metal arc, submerged arc, flux-cored arc, gas metal arc, and electrical resistance methods of welding. Welding can normally be done without the need for preheat or post-heat. Because the C content of the micro-alloyed steels is low, these steels show hardening characteristics (in the HAZ) similar to those of plain C steels with similar low C content. Micro-alloyed steels do not show the severe hardening in the HAZ which characterizes some plain C steels (those with sufficient C contents to attain yield and tensile strengths comparable to those of micro-alloyed steels). Weldability normally decreases with increasing C content.
For shielded metal arc welding, both mild steel covered electrodes and low H2 electrodes can be used. Use of the latter is frequently advised in order to preclude the need for preheating except when welding thick, highly restrained sections. Submerged arc welding is frequently preferred because of the high production rates which can be attained. However, notch toughness of the weld metal in submerged arc welds can vary because of such factors as base metal composition, filler metal composition, type of welding flux, welding speed, current, voltage, and joint preparation. The choice of particular steel is frequently influenced greatly by its ability to be used in welded construction, with less attention being given to the influence of steel composition on weld metal composition. In a submerged arc weld, the base metal can sometimes account for as much as 50 % to 75 % of the total weld metal. Base metal composition, hence, can have an important effect on weld microstructure.
With constructional micro-alloyed plate steels, the use of low H2 electrodes and other precautions to minimize H2 pickup is advised. Various electrodes are available to match base metal strength and toughness. Preheating is normally needed for all thicknesses over 25 mm and for highly restrained joints. Specific preheating temperatures depend largely on the grade, thickness, and welding process used, normally range from 40 deg C to 200 deg C.
Electrodes containing V are to be avoided for any weldments which are to be subsequently stress relieved since reheating at temperatures below the critical temperature causes precipitation of VC and raises the transition temperature of the weld metal.
Application of micro-alloyed steels
Micro-alloyed steels gained their first use as structural shapes and plates in the early 1960s because of their ability to be welded with ease. By the early 1970s, they were also used in pipelines in both high-temperature and severe arctic conditions. Later in the 1970s, concurrent with the energy crisis, another dominant application involved the use of micro-alloyed steels to reduce the weight of parts and assemblies in automobiles and trucks. In the 1980s, bars, forgings, and castings have emerged as applications of particular interest. For example, weldable and non-weldable reinforcing bars are available, and high-strength forgings in the as-forged condition are being used to replace Q&T steels. Shapes such as elbows and fittings for pipelines are also being cast out of micro-alloyed steel.
Micro-alloyed steels typically have an edge over other materials when their unique properties allow (i) weight reductions, (ii) new or more efficient designs with improved performance, (iii) attractive reductions in fabrication / manufacturing costs, and (iv) reduced transportation costs from weight reductions. These benefits can come in combinations, depending on the application.
Micro-alloyed steels are used in a wide variety of applications, and properties can be tailored to specific applications by the combination of composition and structures achieved in processing at the mill. The micro-alloyed steels is normally usd in structural applications which includes bridges, buildings, off shore applications including oil and gas production platforms, oil and gas pipe lines and tubular components, electricity pylons, pressure vessels, penstocks, steel piling, ship building, railway tracks, trucks, trailers, cold formed products, crane and lifting equipment, heavy equipment including earthmoving and mining equipment, tanks, and reinforcing bars.