Steels are most frequently classified according to their carbon and / or alloying elements content. The various classifications have become known under assorted designations, such as plain carbon steel, carbon-manganese steel, medium carbon steel, low alloy steel, and high strength low alloy (micro-alloyed) steel. Recently, a new classification of steel has introduced the steel processing technique as a categorization factor. These steels, known by various designations, are most frequently described as thermo -mechanical-control process (TMCP) steels. The boundaries between all the above classes are frequently diffused, they frequently overlap, and they are sometimes arbitrary.
TMCP steels are a relatively recent addition to the family of steels. These steels are normally produced with a combination of controlled rolling followed by accelerated cooling or in-line direct quenching. This processing allows the steel producer to develop a combination of high strength and high toughness while maintaining good weldability.
TMCP is a technology for steel production. It is a steel property control technology which has been developed to meet the requirements of higher strength and toughness. Steel grades produced utilizing the TMCP technology are called TMCP steels. TMCP is used to produce high performance steels which have excellent properties, such as high strength, excellent toughness, and excellent weldability. These steel properties are achieved by controlling the microstructure of steel through maximizing of grain refinement. TMCP steels are normally low carbon and low alloy steels. These steels are sometimes micro-alloyed. TMCP steels have been initially developed for ship building.
The special characteristics of TMCP technology include (i) low temperature heating of the steel, (ii) a reduction under high pressure in temperature range below re-austenitization temperature, (iii) when necessary, austenitic and ferrite duplex area rolled, and (iv) accelerated cooling after rolling.
TMCP is normally used to achieve excellent properties for steel products such as high strength, excellent toughness along with excellent weldability through maximizing of grain refinement. These steels have almost same formability and weldability compared with mild steels. The superior mechanical properties introduced to the steel through this processing route are virtually equivalent to those achieved by heat treating conventionally rolled or forged steel and hence TMCP is used as a substitute for heat treatments which need additional material handling and furnace facilities.
TMCP has several advantages which help to overcome issues related to the addition of major alloying elements and conventional heat treatments. TMCP steels with added micro-alloying elements have been developed to manage the conflicting requirements of strength, toughness, and weldability through grain refinement. TMCP effectively enables a reduction of the preheating temperature, hence lowering the cost of steel fabrication.
The development of the TMCP steels has a long history. The first commercial cooling method was probably the water quenching of reheated steel plates above their transformation temperature of around 900 deg C, which is called reheat quenching. Plate quenching and roller quenching processes date back to 1935 and 1966, respectively. These heat treatments, which consist of water quenching from elevated temperatures and subsequent heat treatment (tempering) at around 600 deg C, produce stronger and tougher plates. The reheat quenching process, however, does not consider the cooling rate and simply refers to a method of quenching undeformed austenite as rapidly as possible.
The first industrial-scale production process with online quenching on hot rolled steel plates was introduced in 1956 in Japan. Since then, there has been huge commercial production of steel plates with tensile strength ranging between 600 MPa and 800 MPa. This online quenching procedure for the hot rolled steel plates involves their direct quenching into water in a tank. It is the prototype of present-day TMCP in that it can result in enhanced strength, toughness, and weldability.
From the early 1960s, a considerable amount of studies have been conducted into the effects of rolling temperature on mechanical properties. From the results of such studies, a rolling procedure at a controlled temperature has been developed to refine the grain size. Controlled rolling has made a major contribution to the reduction in the addition of the alloying elements and the improvement of the weldability.
When TMCP is used in combination with controlled rolling, the accelerated cooling allows the austenite (gamma) to ferrite (alpha) transformation to be controlled. TMCP is now widely used to manufacture stronger and tougher steels with excellent weldability. The present approach of using water-cooled TMCP, which gives accelerated cooling after low temperature rolling and direct quenching, has been greatly improved in the 1980s. The continuous online control (CLCTM) process can cover a wide range of cooling parameters (i.e. heat transfer coefficient) and austenite conditions (i.e. unworked, recrystallized or non-recrystallized) and yields a variety of steel properties. Several concepts are employed in the cooling equipment. In one of the most sophisticated present configurations, the plate is flattened by a leveler before water cooling. This procedure is effective across a wide range of cooling rates for achieving uniform cooling throughout the plate plane and for realizing homogeneous mechanical properties. Fig 1 shows schematic of TMCP facility in plate rolling mill.
Fig 1 Schematic of TMCP facility in plate rolling mill
Although controlled rolling afforded relatively low productivity, it could be performed at a comparatively low temperature to achieve enhanced levels of strength and toughness. As the controlled rolling process is based on grain refinement, it can improve yield strength and toughness, but cannot improve tensile strength without the addition of the alloying elements. Accordingly, the improvement of weldability has been limited when applying this process. Since the late 1970s, effort has been made to combine TMCP (which improves both strength and toughness) with micro-alloying (which improves weldability).
TMCP technology is used for grain refinement, which results in steels with improved toughness and strength compared to conventional rolling. In some plate mills, controlled rolling is followed by accelerated cooling or direct quenching instead of air cooling. Attractive combinations of strength and toughness can be achieved by TMCP. TMCP involves both controlled rolling and controlled (accelerated) cooling to produce steels with a very fine grain size (ASTM 10 to 12). The main aim of TMCP is to increase strength and fracture toughness and improve weldability by reducing the carbon equivalent and controlling the chemical composition. In TMCP steels, minimum yield strengths is maximized between 290 MPa and 415 MPa by reducing the ferrite grain size and increasing the volume fraction of the second phase. Accelerated cooling is used to achieve these effects.
The replacement of conventional rolling plus post-rolling heat treatments by integrated controlled forming and cooling strategies implies important reductions in energy consumption, increases in productivity, and more compact facilities in the steel plant. The metallurgical challenges which this integration implies are relevant and impressive developments have been achieved since the development of the technology during the early 1980s. Several of the present practical integrated engineering achievements of TMCP were first demonstrated in Japan. The development of new steel grades and processing technologies devoted to thermo-mechanically control processed products is increasing and their implementation is being expanded to higher value added products and applications. A variation in cooling rate can be expected between surface and mid-thickness regions of thick steel sections.
Stronger and tougher steels have traditionally been produced by conventional alloying or heat treatments. Although alloying improves these properties, it also has some disadvantages, such as increased production cost and lower weldability. The steel thickness and toughness which can be achieved by alloying are hence limited. The application of this technology has initially being made in the production of steel plates. In recent years, TMCP has been effectively used in the hot rolling of bars, rods, and shapes also. Fig 2 shows the effect of cooling rate on strength and toughness of TMCP steels.
Fig 2 Effect of cooling rate on strength and toughness of TMCP steels
TMCP is a technological process where intended changes in structure and properties of steels result from the combined effect of plastic and heat treatment. The technology combines controlled rolling (thermo mechanical rolling) and controlled cooling (accelerated cooling) for the production of steel. The combination of hot working technologies with a thermal path, under controlled conditions provides opportunities to achieve required mechanical properties at lower costs. TMCP allows control of the micro-structure, phase transformation, and rolling of steels.
The achievement of mechanical properties and process stability during the TMCP depends on the chemical composition, process parameter control, and optimization, as well as post-forming cooling strategy, and thermal treatments. In addition to the metallurgical peculiarities and relationships between chemical composition, process, and final properties, the impact of advanced characterization techniques and innovative modelling strategies provides new tools to achieve further deployment of TMCP technology.
Control over micro-structure needs a good understanding of the effect of rolling mill variables on the resulting micro-structure. Variables such as preheating time and temperature, rolling deformation, deformation rate, inter-stand cooling and post-rolling controlled cooling affect the grain size distribution, recrystallization, and phase transformation kinetics which ultimately determine the final micro-structure and mechanical properties in the rolled product. Microstructural changes occurring at different stages in the rolling process affect the final micro-structure and properties of the rolled product. A typical thermo-mechanical cycle and microstructural evolution during the rolling process is shown Fig 3.
Fig 3 Time – temperature profile and microstructural evaluation in TMCP rolling
TMCP involves careful optimization of micro-alloying (e.g. niobium addition) with only small amounts added in each stage of the steel production process. It is also worth noting that minimizing the use of alloying elements in TMCP is one of the most advantageous features in terms of the weldability of the steels. The most fundamental properties needed for structural steels are strength and toughness. The crystalline grain size plays the most important role in controlling such properties of steels in terms of yield strength and toughness, which is represented by the ductile-brittle transition temperature.
A huge amount of work on the improvement of steel properties has been carried out. Yield strength can be expressed by the Hall–Petch relationship YS = K1 + Ky(d)-1/2, where YS is the yield strength, K1 and Ky are constants (independent of grain size) and d is the grain size. The ductile-brittle impact transition temperature for low-carbon ferrite-pearlite steels can also be expressed as a function of the grain size: Impact transition temperature (deg C) = −19 + 44[Si] +700[Nf]-1/2 + 2.2(pearlite) – 11.5(d)−1/2, where [Si], [Nf] and pearlite are the respective weight percentages of silicon, free nitrogen, and pearlite (a lamellar composite of ferrite and cementite) in the micro-structure.
Metallurgical aspects of TMCP
TMCP steels are recently developed steels which show considerably improved strength and toughness properties and weldability. The new technology of using accelerated cooling makes it possible to achieve the same strength level as conventional controlled rolled steels at lower carbon content (less than 0.06 %). Instead of strengthening by increasing the carbon content, TMCP steels derive their strength and toughness from the very fine ferrite and second-phase micro-structure (finely dispersed pearlite or bainite) which occur during the accelerated cooling of the production process.
The aim of TMCP is to achieve a fine and uniform acicular ferrite micro-structure instead of a ferrite / pearlite banded structure of conventional steels. The thermo-mechanical rolling process is characterized by deformation in the non-recrystallization region of austenite which can be carried upto an 80 % reduction in thickness so as to achieve the desired grain refinement.
In the production of the TMCP steels, the control of micro-structure starts at the reheating stage of the cast steel products itself when the grain size of the austenite is carefully controlled. In hot rolling stage, since the controlled rolling is carried out in the non recrystallization region, fine and worked austenite grains are formed. These fine austenite grains are transformed into fine acicular ferrite or upper bainite in the following stage of accelerated cooling. The final micro-structure of TMCP steels is very fine and uniform. Fig 4 shows comparison of the micro-structures of conventional and TMCP steels.
Fig 4 Comparison of the micro-structures of conventional and TMCP steels
The concept of micro-structural control by TMCP is schematically presented in Fig 5. Whereas controlled rolling technology enhances toughness mainly by refinement of the ferrite microstructure, TMCP steels achieves high strength by utilizing the transformation to ferrite and bainite in addition to enhanced toughness. Accelerated cooling affords improved productivity compared with inter-critical (austenite + ferrite region) rolling and minimizes any decrease in absorbed energy through the separation phenomena.
Fig 5 Concept of micro-structure control by TMCP
TMCP consists of two stages in series namely (i) controlled rolling, and (ii) a subsequent accelerated cooling process. During the rolling stage, the austenite grains are elongated into a pancake shape, which introduces crystallographic discontinuities such as ledges and deformation bands. These ledges and deformation bands remain until accelerated cooling starts when the rolling temperature is sufficiently low (below around 800 deg C). Recrystallization takes place when the temperature is sufficiently high (above around 900 deg C) and most of the ledges and deformation bands induced by deformation disappear. The retaining deformation ledges and bands can act as potential heterogeneous nucleation sites for the austenite to ferrite transformations and contribute to grain refinement. It is also worth noting that the heterogeneous deformation of austenite increases the grain surface area and the length of grain edges grain corners per unit volume.
The other feature of TMCP is its cooling process. During the accelerated cooling the growth of the transformed products is effectively suppressed and grain refinement is achieved by transformations where the aforementioned nucleation sites are introduced. The decrease in the transformation temperature caused by accelerated cooling induces strong changes in the Intra-granular structure. The transformation driving forces also contribute to grain size refinement through low temperature rolling followed by water quenching. The tensile strength can be widely controlled (from 500 MPa to over 800 MPa). It is also important to note that steelmaking and cast steel (slab, bloom, or billet) reheating processes have to be carefully controlled to achieve steels with high strength and toughness.
The micro-alloying elements control the micro-structure. Trace amounts of elements such as niobium (Nb) and titanium (Ti) in concentrations of the order of 0.01 % allow the micro-structure to be refined from the cast steel reheating to controlled-rolling and accelerated-cooling processes and enhance the strength of the final products. The effects of niobium, as an example of a micro-alloying element, are schematically shown in Fig 6. The size of the niobium precipitates formed during each process is roughly 300 nm (nano meters) at the cast product reheating temperature before rolling (1,000 deg C or higher), 50 nm during controlled rolling (around 800 deg C), and 10 nm at the transformation temperature (around 600 deg C) during cooling. In short, the size of newly formed precipitates decreases with decreasing in temperature as the process progresses. This is useful for microstructural control since the precipitates formed in the earlier processes are too large and hence useless for the subsequent processes.
Fig 6 Niobium precipitations at each stage of TMCP and their effects on the refinement of ferrite grains and precipitation hardening
It is hence necessary to maintain niobium in a solid solution so that it can be precipitated in sufficient quantities in the subsequent processes. The normal solubility product can be expressed by the equation −log10[Nb][C + (12/14)N] = −6770/T + 2.26, where the concentrations are expressed as percents and T is the absolute temperature. Niobium precipitates during the cast product reheating and prevents austenite grain growth through the pinning effect. During the subsequent rolling process, at below the recrystallization temperature (around 900 deg C), the driving force generated by the strain energy introduced by such rolling facilitates the precipitation of fine niobium carbides and / or nitrides. These fine precipitates prevent austenite grain recrystallization and hence coarsening. The effect of niobium on the recrystallization grain growth ratio G (micrometers per second) can be expressed as G = p(Fv/T)D(to the power q)exp[r(Nb)sol + s(Nb)pre – k/T]. Here p, q, r, s and k are constants, Fv is the strain energy of the crystals (Joules), D is the austenite grain size diameter before deformation (micrometers), and (Nb)sol and (Nb)pre are the solute and precipitated niobium concentrations (%) respectively.
During thermo-mechanical rolling, the strain-induced precipitation of micro-alloying elements such as niobium plays an important role in controlling the micro-structure. Some studies state that niobium delays the onset of austenite recrystallization. There is a lattice mismatch for niobium and vanadium precipitates in austenite, and hence the precipitates are located at crystalline defects in austenite (and also in ferrite). However, one of the studies using a three-dimensional atom probe has shown that the onset of recrystallization can be retarded by solute interstitial atoms and small substitutional- interstitial atomic clusters rather than by fine niobium carbo-nitrides.
Because of the non-recrystallized nature of austenite, there is a plentiful supply of heterogeneous ferrite nucleation sites (ledges and deformation bands) for the subsequent cooling process. Recent in situ observations by neutron diffraction have also demonstrated that niobium addition and austenite deformation increase the ferrite transformation temperature. Niobium also induces other effects: during the austenite to ferrite transformation upon cooling, it precipitates in the ferrite matrix and enhances its strength through the precipitation strengthening mechanism. Precipitates such as NbCN, VCN, TiC, and TiN show a NaCl crystal structure and do not fit well in the ferrite lattice. This incoherency between the ferrite and precipitates results in increased strength. TMCP parameters affect the microstructure and properties of niobium-titanium micro-alloyed steel. As an example, the yield strength and tensile strength increase with a decrease in the finishing-cooling or non-recrystallization rolling temperature.
The most notable effect of TMCP is that steel with the same strength as conventional steels can be produced with a lower carbon equivalent (i.e. with lower alloy addition) through microstructural control. It has been demonstrated that the value of the carbon equivalent needed for TMCP steel to attain the same strength is 0.04 % to 0.08 % lower than that needed for normalized steel. The carbon equivalent needed to produce tensile strength 490 MPa steel through the normalizing process is 0.39 % to 0.44 %, while it is 0.33 % to 0.36 % for TMCP steel. As a result, weldability (i.e. the preheating temperature needed to prevent cold cracking at the heat affected zone) has been considerably improved in TMCP steels. TMCP hence increases productivity in the fabrication of steel structures while enhancing safety and reliability.
TMCP steels production
When TMCP is chosen as the process route, the input cast steel is heated to a temperature which is normally used for hot rolling operations (around 1,200 deg C). The roughing operation during rolling in the rolling mill is carried out in a normal way, but the finish rolling is carried out at a lower temperature (around 750 deg C to 800 deg C) than the temperatures used in a normal rolling process. Plastic deformation at this lower temperature promotes fine grain sizes and retards precipitation. The final hot working can continue down to temperatures below the critical temperature of transformation from austenite to ferrite. This needs heavy rolling equipment capable of deforming the steel at low hot working temperatures. The optimum precipitate size and dispersion is achieved when the finish rolling temperature is around 775 deg C.
The cooling which follows brings the steel to the transformation temperature range, and the austenite to ferrite transformation results in fine ferrite grains and fine dispersed precipitates. For some TMCP steels, this last stage of cooling, during which transformation is completed, is accelerated by water cooling, to give a finer grain size. Accelerated cooling (AcC) can sometimes result in bainite formation as well as, or instead of, ferrite formation. There are several methods for TMCP, some of which are shown against conventional processes in Fig 7.
Fig 7 Comparison of conventional and TMCP routes of rolling
As shown in Fig 7, TMCP route broadly fall into three main categories. In the first category steel is controlled rolled down from the normalizing temperature, yet fully austenitic (above the Ar3 temperature) followed by a rapid cooling of around 10 deg C per second. The aim of this process is to refine the grain size by controlled rolling and to increase the strength by suppressing the formation of ferrite and pearlite in favour of a strong tough bainite. In the second category steel is controlled rolled both above the Ar3 and below that temperature, in the austenite ferrite mixed region. In addition to austenite grain refinement, the recrystallized grains are flattened and nucleation of fine ferrite is encouraged by the deformation. At a temperature above the AR1, the controlled rolling is interrupted, and followed by rapid cooling to room temperature or an intermediate temperature. In the third category, controlled rolling is performed as part of a preliminary processing, followed by cooling and reheating to just above the Ar3 temperature and then rapid cooling to well below the Ar1 temperature. The purpose of this technique is to develop the finest equiaxed austenite grain size before the controlled cooling begins.
By appropriate choice of deformation temperature and strain rate, the strength of steel can be increased. The strength of TMCP steel is higher than for normalized steel of the same composition. Hence, TMCP steel has a leaner composition (lower content of alloying elements) than conventional normalized steel of the same strength. Due to the fine and uniform acicular ferrite, TMCP steels have a higher strength and superior toughness. Fig 8 shows the relationship between mechanical properties, carbon equivalent and grain size.
Fig 8 Relationship between mechanical properties, carbon equivalent and grain size
Weldability of TMCP steels
As regards weldability of the TMCP steels due to its extra low equivalent carbon, TMCP steel virtually eliminates the need for preheating. This fact considerably improves welding efficiency, which has been the greatest problem in welding high strength steels. The weldability of TMCP steels is good since the content of the alloying elements of these steels can be kept very low, with carbon contents normally below 0.06 %. Yield strength levels as high as 700 MPa and above are possible with these steels. Normally, these steels can be welded without preheat. However, at the high strength levels, preheat can be needed in order to prevent cracking in the weld metal.
Hardenability is another important factor of weldability. Hardness level of TMCP steels is lower than conventional steels. TMCP steel is least susceptible to cold cracking or hydrogen cracking. Under adequate welding conditions, specific non low hydrogen type gravity electrodes can be used. The result is higher welding efficiency and weld joints with smooth beads which deliver remarkably improved performance.
TMCP steels considerably improve their resistance to hydrogen-induced cold cracking due to reduction in carbon content and carbon equivalent,. Hence, the concern with preheat, inter-pass temperature control, and PWHT (post weld heat treatment) during welding fabrication is not critical. However, HAZ (heat affected zone) softening, especially at high heat inputs, is a concern, since the favourable micro-structure of the TMCP steel is reverted during the slow cooling in the HAZ at high heat input levels.
HAZ fracture toughness properties are normally better in TMCP steels than normalized steels and adequate fracture toughness can frequently be obtained upto higher heat inputs. Indeed, some ‘high heat input resistant’ grades of steel are made by a TMCP route. There is some degree of softening in the heat affected zones of TMCP steels after welding. Reduction in joint strength, however, is unusual, in those which have not had accelerated cooling as part of the production process. TMCP steels which have been produced using an accelerated cooling method to achieve the desired properties are not normally resistant to high heat inputs, and a degradation of properties can occur on welding. This is because the cooling rate in the weld region can be slower than that of production. In these steels, it is important that the cooling rate is high, so that the grain size of the weld and HAZ can be maintained to give the desired properties. This is achieved by the use of moderately low heat input levels.
There are several reasons for the present interest in LBZs (local brittle zone). The need to reduce costs results in optimized structures having less redundancy and a large number of highly stressed joints. To reduce welding costs, narrow groove preparation is used which can result in an HAZ that is normal to the loading direction. Also, unlike normalized steel in which the HAZ yield strength is higher than that of the base steel, TMCP steel sometimes has an HAZ yield strength which is lower than that of both the weld metal and the base steel. Softening of the HAZ can also be expected if thermal cutting is used during fabrication. This behaviour can limit the application of thermal cutting to TMCP steels.
Application of TMCP steels
The application range of TMCP steels is within minimum yield strength of 350 MPa to 500 MPa. Since the TMCP steels afford good weldability, they are highly valued in industries. The typical applications for such steels are in shipbuilding including icebreakers, offshore structures, tanks and vessels especially for low temperature environments, pipelines, building structures, commercial vehicle, cranes and other general steel construction activities.
The popularity of TMCP reflects the advantages of TMCP steel such as enhanced strength and toughness coupled with excellent weldability. Another key issue which explains the success of TMCP steels is that alloy design, impurity control during the steelmaking process, segregation reduction, hydrogen removal, cast steel reheating, and the rolling and cooling processes are considered in both the upstream and downstream processes.
Advantages of TMCP steels
TMCP steels have higher strength and better toughness because of the fine and uniform acicular ferrite. These steels have higher arrestability than normalized steels. As regards to formability, TMCP steels have the better strain ageing property and line heating property when compared with conventional steels.
Normally, TMCP steels are good at resisting a reduction in toughness as a result of welding, since such steels normally have low carbon content. Some TMCP steels are excellent at resisting the toughness decrease, even at high heat inputs, as they have been processed to contain particles which control grain growth. These steels also have sufficiently high carbon content, or carbon equivalent levels to ensure that joint strength is not impaired at high heat inputs. Such steels are designated ‘high heat input resistant steels’.