Carbon and Low Alloy steel Plates
Carbon and Low Alloy steel Plates
Steel plate is a flat-rolled steel product which is more than 200 mm wide and more than 6.0 mm thick or more than 1,220 mm wide and 5 mm thick. The steel plate rolling mills normally have a working-roll width ranging from 2,000 mm to 5,600 mm. Hence the width of the steel plates rolled in a plate mill normally ranges from 1,500 mm to 5,000 mm ( popular range is 2,000 mm to 3,000 mm). The thickness of the steel plates rolled in a plate mill ranges from 5 mm to 200 mm. Some plate mills, however, have the capability to roll steel plates of thickness higher than 200 mm.
Steel plate is normally used in the hot-finished condition, but the final rolling temperature can be controlled to improve both the strength and the toughness. Heat treatment is also used to improve the mechanical properties of some of the steel plates. Steel plate is mainly used in the construction of buildings, bridges, ships, railway wagons, storage tanks, pressure vessels, pipes, large machines and furnaces, and other heavy structures, for which good formability, weldability, and machinability are needed.
The loss of the desirable characteristics of good formability, weldability, and machinability with the increase in the carbon (C) content normally limits the grades of steel to the low-C and medium-C constructional grades, with the low-C grades dominating. Many alloy steels are also used for the production of the steel plates. In the final structure, however, alloy steel plate is sometimes heat treated to achieve mechanical properties superior to those typical of the hot-finished plate.
Steel plates are produced either from continuously cast slabs or individually cast ingots. Preparing these steel slabs or ingots for subsequent forming into plates can involve requirements regarding deoxidation practices, austenite grain size, and/or secondary melting practices.
Deoxidation practices – During the steelmaking process, segregation of C can occur when C reacts with the dissolved oxygen (O2) in the liquid steel (this reaction is favoured thermodynamically at lower temperatures). Hence, the practice of controlling dissolved O2 in the liquid steel before and during casting is an important aspect in improving the internal soundness and chemical homogeneity of cast steel. Deoxidation is also important in lowering the impact transition temperatures. Deoxidation can be achieved by vacuum processing or by adding deoxidizing elements such as aluminum (Al) or silicon (Si).
Steels are classified by their level of deoxidation as killed steel, semikilled steel, capped steel, and rimmed steel. The steel used for plates is normally either killed or semikilled. In case of ingot casting, semikilled steel is normally used since it is more economical than killed steel. Continuously cast steels are normally fully killed to assure internal soundness.
Killed steel is fully deoxidized, and from the viewpoint of minimum chemical segregation and uniform mechanical properties, killed steel represents the best quality available. Hence, killed steel is normally required when homogeneous structure and internal soundness of the plate are needed or when improved low temperature impact properties are the requirements. Killed steel can be produced either fine or coarse grained without adversely affecting soundness, surface, or cleanliness. Normally, heavy-gauge plates (thicker than 40 mm) are produced from killed steel to provide improved internal homogeneity.
Semikilled steel is deoxidized to a lesser extent that killed steel and hence does not have the same degree of chemical uniformity or freedom from surface defects as killed steel. This type of steel is used primarily on lighter gauge steel plate, for which high reductions from ingot to plate thicknesses minimize the structural and chemical variations found in the as-cast ingot.
Austenitic grain size is an important aspect during the production of several qualities of steel plates. Steel plate specifications for structural and pressure vessel applications normally require a steelmaking process which produces a fine austenitic grain size. When a fine austenitic grain size is specified, grain-refining elements are added during steelmaking. Al is effective in retarding austenitic grain growth, resulting in improved toughness for heat-treated (normalized or quenched and tempered) steels. Steels used in high-temperature service normally contain Al only in very small amounts since Al can affect strain-aging characteristics and graphitization. However, the addition of Al can be necessary for some grades of high-temperature steels (as well as most low-temperature steels) requiring good toughness. Other grain-refining elements, such as niobium (Nb), vanadium (V), and titanium (Ti), are used in high-strength low-alloy (HSLA) steels for grain refinement during rolling.
Melting practices – The steel for plate products can be produced by any of the primary steelmaking processes namely (i) basic oxygen steelmaking, or (ii) electric steelmaking. In addition, the steel can be further refined by secondary steelmaking processes such as vacuum degassing or various ladle treatments for deoxidation or desulphurization.
Vacuum degassing is used to remove dissolved O2 and H2 (hydrogen) from steel, thus reducing the number and size of indigenous nonmetallic inclusions. It also reduces the likelihood of internal fissures or flakes caused when the H2 content is in steel higher than the desired levels. Desulphurization is done by combining steel refining with the addition of ladle desulphurizing agents (e.g., calcium or rare earth additions) immediately before casting or teeming. Final sulphur (S) content in the steel used for steel plate production can be reduced to less than 0.005 %. Lower S content improves plate through-thickness properties and impact properties.
Liquid steel is then cast either in continuous casting machines into steel slabs or in ingot moulds. Steel ingots for some special grades of steel plates, are subjected to electro-slag remelting (ESR) process. In case of cast ingots, the ingots are first rolled in a slabbing mill where they are reduced in thickness to make a slab. The flowsheet of the production process for making steel slabs for the production of steel plate is shown in Fig 1.
Fig 1 Flowsheet of the production process for making steel slabs for steel plate
Plate production process – The slab is then inspected, and the surface is conditioned by grinding or scarfing to remove surface imperfections, and then heated in the reheating furnace prior to rolling to final plate thickness. The heated slabs are then rolled to final plate thickness. The plate can then be roller leveled and cooled.
Micro-alloyed HSLA steels can be controlled rolled for grain refinement. In this case, the reheating temperature is lower than usual, and the rolling practices are designed to impart heavy reductions at relatively low temperatures. This form of thermo-mechanically controlled processing (TMCP) is used for grain refinement, which results in plates with improved toughness and strength compared to conventional plate 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.
After cooling, steel plates are cut to size by shearing or thermal cutting. Following this operation, testing to confirm mechanical properties is customarily performed, and then the material is shipped to the customers. However, certain plate products need further processing such as heat treatment. Flowsheet of rolling and processing of steel plate in a plate mill is shown in Fig 2.
Fig 2 Flowsheet of rolling and processing of steel plate in a plate mill
Surface defects in steel plate
Certain characteristic surface defects which can weaken the steel plate can appear on hot finished steel. Chemical segregation which can alter properties across the section can also be present. Some of these imperfections are described below.
Seams – Seams are the most common imperfections found in hot-finished steel plate. These longitudinal cracks on the surface are caused by blowholes and cracks in the original ingot/slabs which have been rolled closed, but not welded. For many applications of the steel plates, seams are of minor consequence. However, seams are harmful for applications involving heat treating or upsetting or in certain parts subjected to fatigue loading.
Decarburization – Decarburization is a surface condition common to all hot-finished steel. It is produced during the heating and rolling operations when atmospheric O2 reacts with the heated surface thus removing C. This produces a soft, low-strength surface, which is often unsatisfactory for applications involving wear or fatigue.
Segregation – Alloying elements always segregate during the solidification of steel. Elements which are prone to segregation are C, Si, S, P (phosphorus), and Mn (manganese). The effect of segregation on the mechanical properties and fabricability of the steel plate is insignificant for most of its applications. However, segregation can produce difficulties in subsequent operations such as forming, welding, punching, and machining.
Although steel plates are mostly used in the hot-finished condition, the heat treatments as described below are applied to steel plates which are required to meet special requirements.
Normalizing – It consists of heating the steel above its critical temperature and cooling in air. This refines the grain size and provides improved uniformity of structure and properties of the hot-finished plate. When there is requirement of toughness in steel plates specified for certain thicknesses in some grades of normalized plate, accelerated cooling is to be used in place of cooling in still air from the normalizing temperature. Such cooling is accomplished by fans to provide air circulation during cooling or by a water spray or dip. Accelerated cooling is used most often in steel plates with heavy thicknesses to achieve properties comparable to those developed by normalizing of the plate in case of lighter thicknesses.
Quenching – It consists of heating the steel plate to a suitable austenitizing temperature, holding at that temperature, and then quenching in a suitable medium which depends on chemical composition and cross-sectional dimensions. As-quenched steel plates are hard, high in strength, and brittle. They are normally always tempered before being used in service.
Tempering – It consists of reheating the steel to a predetermined temperature below the critical range, then cooling under controlled conditions. This treatment is normally carried out after normalizing or quenching in order to get the desired mechanical properties. Those include a balance of strength and toughness to meet the specification requirements.
Stress relieving – It consists of heating the steel to a subcritical temperature to release stresses induced by such operations as flattening or other cold working, shearing, or gas cutting. Stress relieving is not intended to significantly modify the micro-structure or to achieve desired mechanical properties.
Types of steel plates
Steel plates are classified as per the composition, mechanical properties, and steel quality. The most common three general categories of steel plates are C steel plate, HSLA steel plate, and low alloy steel plate. These three categories of steel plates are produced in several steel grades qualities as shown in Tab 1.
|Tab 1 Steel grade qualities for steel plate production|
|Sl. No.||Carbon steel||HSLA steel||Low alloy steel|
|2||Structural quality||Structural quality||Structural quality|
|3||Drawing quality||Drawing quality||Drawing quality|
|4||Cold drawing quality||Cold drawing quality||Cold drawing quality|
|5||Cold pressing quality|
|6||Cold flanging quality||Cold flanging quality||Cold flanging quality|
|7||Forging quality||Forging quality|
|8||Pressure vessel quality||Pressure vessel quality||Pressure vessel quality|
|9||Ship building quality||Ship building quality||Ship building quality|
|10||Armour quality||Armour quality|
Carbon steels plate – It is normally produced in all qualities except armour and aircraft quality and is produced in many grades. Normally, C steel contains upto around 2 % of C and only residual quantities of other elements except those added for deoxidation, with Si content normally limited to 0.60 % and Mn content to around 1.65 %. The chemical composition requirements of standard C steel plate are normally covered in various standards. These steels are suitable for some structural applications. In addition, the C steel plates are also classified according to more specific requirements.
HSLA steels plate – HSLA steels offer higher mechanical properties and, in certain grades of these steels, higher resistance to atmospheric corrosion than conventional C structural steels. The HSLA steels are normally produced with emphasis on mechanical property requirements rather than the chemical composition limits. They are not considered alloy steels, even though there are deliberate additions of certain alloying elements at micro levels.
There are two groups of compositions in this category. The first is V and/or Nb steels, with a Mn content normally not exceeding 1.35 % maximum and with the addition of 0.2 % minimum Cu (copper) when specified. The second is high-strength intermediate Mn steels, with a Mn content in the range of 1.10 to 1.65 % and with the addition of 0.2 % minimum Cu when specified. Other elements normally added to HSLA steels to yield the desired properties include Si, Al, Cr (chromium), Ni (nickel), Mo (molybdenum), Ti (titanium), Zr (zirconium), B (boron), and N2 (nitrogen). HSLA steels have several qualities (Tab 1). The chemical compositions of HSLA steel plate are normally covered in various standards.
Low alloy steel plate – Steel is considered to be low-alloy steel when either of the two conditions is met. In the first condition, the maximum of the range given for the content of alloying elements exceeds one or more of the following limits, Mn – 1.65 %, Si – 0.60 %, and Cu – 0.60 %. As per the second condition, any definite range or specific minimum quantity of any of the following elements is specified or required within the limits of the recognized field of constructional alloy steels namely Al, B, Cr upto 3.99 %, Nb, Mo, Ni, Ti, V, Zr, Co (cobalt), W (tungsten), or any other alloying element added to obtain the desired alloying effect.
Alloying elements are added to hot-finished plates for several reasons, including improved corrosion resistance and/or improved mechanical properties at low or elevated temperatures. Alloying elements are also used to improve the hardenability of quenched and tempered plate.
Low-alloy steels normally need additional care throughout their production process. They are more sensitive to thermal and mechanical operations, the control of which is complicated by the varying effects of different chemical compositions. For getting the most satisfactory results, the steel user normally consult with steel producers regarding the working, machining, heat treating, or other operations to be employed in fabricating the steel, mechanical operations to be employed in the fabricating the steel, mechanical properties to be achieved, and the conditions of service for which the finished articles are intended.
The chemical composition requirements of low alloy steel plate are as per various standards. The low alloy steels can be suitable for some structural applications. The residual alloying elements can have effect on the mechanical properties of hot-finished steel plate. The alloying elements also have their effects on the hardenability and mechanical properties of quenched and tempered steels.
Quality of steel plate
Steel quality, as the term applies to steel plate, is indicative of several conditions, such as the degree of internal soundness, relative uniformity of mechanical properties and chemical composition, and relative freedom from harmful surface defects. The various types of steel plate quality are given in Tab 1.
The main quality descriptions used to describe steel plate are regular quality, structural quality, ship building quality, and pressure vessel quality. Special qualities include cold drawing quality, cold pressing quality, cold flanging quality, and forging quality C steel plate, along with drawing quality, armour quality, and aircraft quality alloy steel plate.
Regular quality steel plate – It is mainly the ordinary quality of C steel, which is applicable to plates with a maximum C content of 0.33 %. Plates of this quality are not expected to have the same degree of chemical uniformity, internal soundness, or freedom from surface defects which is associated with structural quality, ship building quality, armour quality or pressure vessel quality plate. Regular quality normally has standard composition ranges and is not usually produced to mechanical property requirements. Regular quality is analogous to merchant quality in case of steel bars since there are normally no restrictions on deoxidation, grain size, check analysis, or other metallurgical factors. Also, this quality steel plate can be satisfactorily used for applications similar to those of merchant quality steel bars, such as those involving mild cold bending, mild hot forming, punching, and welding for non critical parts of the machinery.
Structural quality steel plate – It is intended for general structural applications such as bridges, buildings, transportation equipment, and machined parts. The various specifications for structural quality steel plate are given in various standards. The majority of the structural steel plates is to meet the requirements concerning to both the chemical composition limits and the mechanical properties. However, some types of the structural steel plates are produced from the standard steels. These standard steels are as per the chemical compositions for the steel designations as specified in the standards. However, there are certain factors which affect the mechanical properties of hot finished C steel.
Ship building quality steel plate – Several qualities of steels are used for shipbuilding. These are (i) different grades of mild steels, (ii) HSLA steels, (iii) TMCP steels, (iv) normalized rolled steels, (v) high strength steels (HSS), (vi) new anti-corrosion steel plates for crude oil tankers which contribute to higher performance in ships through improved corrosion resistance, (vii) clad steel plates for chemical tankers, and (viii) stainless steels. The steels are to meet various shipbuilding requirements, such as reduction in welding man-hours, shortening of welding lines, elimination of cutting steps, stabilization of fabricated part quality and reduction in control costs.
Pressure vessel steel plate – Steel plates intended for fabrication into pressure vessels are to conform to specifications different from those of similar plate intended for structural applications. The major differences between the two groups of specifications are that pressure vessel plates are to meet requirements for notch toughness and have more stringent limits for allowable surface and edge defects. The specifications requirements for pressure vessel steel plates required to be met are given in various standards. All of the steel plate specifications are furnished according to both chemical composition limits and mechanical properties. Mechanical tests of pressure vessel steel plate involve a minimum of one tensile test for each as-rolled plate or a minimum of two tensile tests for quenched and tempered plates. The mechanical property requirements are given in the various standards.
Armour quality steel plate – Armour steel is basically a HSLA or low alloy structural steel which has been treated to have property of very high resistance to penetration. This property to the steel is normally imparted by the heat treatment usually by the thermo mechanical treatment. It is well known that the resistance to penetration of steel can be improved by increasing its texture intensity which can be obtained by thermo-mechanical treatment. The mass effectiveness of the armour increases with the hardness of the material. However, very hard armour tends to be brittle and to shatter when hit. The main alloying elements of the armour steel are Ni, Cr, and Mo. The P and S contents of this steel are to be very low (preferably less than 0.015 % of each element). Also there is requirement to have very low value of the dissolved gases like N2, O2, and H2 in this steel. Further, the steel is to be very clean steel with very low level of inclusions. It is also to be free from segregation.
Aircraft quality plates – These are used for important or highly stressed parts of aircraft, missiles, and other applications involving stringent requirements. Plates of this quality require exacting steelmaking, conditioning, and process controls in order to meet the internal cleanliness requirements. The primary requirements of this quality are a high degree of internal soundness, good uniformity of chemical composition, good degree of cleanliness, and a fine austenitic grain size. Aircraft quality plates can be produced in the hot-rolled or thermally treated condition.
Forging quality plates – These plates are intended for forging, quenching and tempering, or similar purposes or when uniformity of composition and freedom from harmful defects are important. Steel plates of this quality are produced from killed steel and are ordinarily furnished with the P content limited to 0.035 % maximum and the S content limited to 0.040 % maximum by heat analysis. Steel suitable for forging quality plate are given in the various standards. Plates of this quality can be produced to chemical ranges and limits and mechanical properties. When mechanical properties are specified, two tension tests from each heat are taken from the same locations at tests for structural quality.
Of the various mechanical properties normally determined for steel plate, yield strength is an important design criterion in structural applications. Tensile strength is also an important design consideration in many design codes, but is useful primarily as an indication of fatigue properties. Yield strength is a design criterion in most design codes when the ratio of yield strength to tensile strength is less than 0.5. Ductility, as measured by tensile elongation and reduction in area, is seldom in itself a valuable design criterion, but is sometimes used as an indication of toughness and suitability for certain applications.
The mechanical properties of steel plate in the hot finished condition are influenced by several variables, of which chemical composition is the most influential. Other factors include deoxidation practice, finishing temperature, plate thickness, and the presence of residual elements such as Ni, Cr, and Mo. For steels used in the hot finished condition (such as plate), C content is the single most important factor in determining mechanical properties.
Static tensile properties – Static tensile properties of the various grades, types, and classes of steel plates are covered in various standards. It is to be noted that some of these values vary with plate thickness and/or width. An example of the variation of tensile strength and elongation with thickness is shown in Fig 3, which presents the minimum expected values for 0.2 % C steel plate from 13 mm to 125 mm thick. Plate less than 13 mm thick shows even slightly higher tensile strength and lower elongation because of the increased amount of hot working during rolling and the faster cooling rates after rolling.
Fig 3 effect of thickness on the tensile properties of 0.2 C steel plates
When steel is produced to a mechanical property requirement, it is common practice to vary the C and Mn contents to compensate for size influence. The use of higher-than-average C and Mn contents to maintain yield strength as plate thickness increases is well known. The common mechanical properties of hot-finished steel plates are reliably related to each other, and this relation is relatively free from influence of composition for most of these properties. Residual alloying elements normally have a minor strengthening effect on hot-finished steel plates. Hardness is a relatively simple test to perform and is closely related to the tensile strength.
Fatigue strength – The high cycle (higher than 1 million) fatigue properties of hot finished steel plate, often called the fatigue limit, are more or less directly related to tensile strength and are greatly affected by the surface condition. The fatigue limit of machined samples is around 40 % of the tensile strength, depending on the surface finish. In contrast, unmachined hot rolled steel plates, when loaded so that fatigue stresses are concentrated at the surface, have a considerably lower fatigue limit because of decarburization, surface roughness, and other surface defects. For this reason, the location of maximum fatigue stresses is to be carefully considered. For structural members designed in hot finished steel plate, the surface is to be machined off from critically stressed areas or an allowance is to be made for the weakness of the hot-finished surface.
The presence of inclusions in hot finished steel plate can also have an adverse effect on the fatigue limit. Large inclusions are considered harmful under the dynamic stresses of impact or fatigue, and the effect is greater in the harder steels.
Low temperature impact energy -When notch toughness is an important consideration for steel plate, satisfactory service performance can be ensured by proper selection of the steel which behaves in a tough manner at its lower operating temperature. The Charpy V-notch tests and crack-starter drop-weight tests provide a fairly reliable indication of the tendency toward brittle fracture in service. The transition temperatures of hot-finished steel plates are controlled principally by their chemical composition and ferrite grain size. C is normally of primary importance because of its effect is raising the transition temperature, lowering the maximum energy values, and widening the temperature range between completely tough and completely brittle behaviour. Mn (upto around 1.5 %) improves low temperature properties.
Also, the transition temperature is affected by the deoxidation practice used. The transition temperature decreases and the energy absorption before fracture at normal temperatures increases in the order of rimmed, capped, semikilled, and killed steels. In addition, killed steels contain larger amounts of Si or Al than semikilled steels, and these elements improve low temperature toughness and ductility. Because of variations in finishing temperatures and cooling rates, plate thickness influences the grain size and hence the transition temperature.
Elevated temperature properties – The steel plate used in pressure vessel applications is frequently subjected to long term elevated temperatures. Of the C and low-alloy steels used for pressure vessel plate, the behaviour of 2.25 Cr-1 Mo steel at elevated temperatures has been studied more thoroughly than any other steel and has become the reference for comparing the elevated-temperature properties of low-alloy steels.
Directional properties – An important characteristic of steel plate, known as directionality or fibering, is to be considered. During the rolling operations, many inclusions, which are in a plastic condition at rolling temperatures, are elongated in the direction of rolling. At the same time, localized chemical segregates which have formed during solidification of the steel are also elongated. These effects reduce the ductility and impact properties transverse to the rolling direction, but have little or no effect on the strength.
Formability – The cold formability of steel plate is directly related to the yield strength and ductility of the material. The lower the yield strength, the smaller is the load needed to produce permanent deformation. High ductility allows large deformation without fracture. Hence, the lower C grades are most easily formed. Operations such as shearing and blanking are usually limited by the lack of the available facilities as the plate thickness increases. This also applies to bending operations. Of course, an adequate bend radius is to be used to avoid fracture. Because of directionality effects, the direction of bend is also important. When the axis of a bend is parallel to the direction of rolling, small bend radii are usually difficult to form because of the danger of cracking.
Machinability – Machining operations are normally performed with little difficulty on most plate steels having C upto about 0.50 %. Higher C steels can be annealed for softening. Steels with low C and Mn content having large quantities of free ferrite in the micro-structure can be too soft and sticky for good machining. Increasing the C content improves the machinability. Machining characteristics can be improved by factors which break up the chip as it is removed. This is normally done by the introduction of large numbers of inclusions such as manganese sulphides (MnS) or complex oxy-sulphides. These ‘free-machining’ steels are somewhat more costly, but are cost-effective when extensive machining is needed.
Weldability – It is a relative term which describes the ease with which sound welds possessing good mechanical properties can be produced in a material. The chief weldability factors are composition, heat input, and rate of cooling. These factors produce various effects, such as grain growth, phase changes, expansion, and contraction, which in turn determine weldability. Heat input and cooling rate are characteristics of the specific process and technique used and the thickness of the steel part being welded. Hence, weldability ratings are to state the conditions under which the rating is determined and the properties and soundness achieved.
For C steels, the C and Mn contents are the primary elements of the composition factor which determine the effect of the steel of given heating and cooling conditions. The great tonnage of steel used for welded applications consists of low C steel with C content upto 0.30 %. Normally, steels having a C content less than 0.15 % are readily weldable by any method. Steel with a C range of 0.15 % to 0.30 % can usually be welded satisfactorily without preheating, post heating, or special electrodes. For rather thick sections ( higher than 25 mm), however, special precautions such as 40 deg C minimum preheat, 40 deg C minimum temperature between weld passes, and a 540 deg C to 675 deg C stress relief is necessary.
High C and high Mn grades can frequently be welded satisfactorily if preheating, special welding techniques, and post heating and peening are used. In the absence of such precautions to control the rate of cooling and to eliminate high stress gradients, cracks can occur in the weld and base metal. In addition, base metal properties such as strength, ductility, and toughness are greatly reduced.
All the statements about the effect of C and Mn on weldability are to be qualified in terms of section size because of its relationship to heat input and cooling rate. In welding thicker sections, such as steel plate, the relatively cold base metal serves to greatly accelerate the cooling rate after welding with the result that plate thickness is a very important consideration. Figure 4 shows the effect of plate thickness and carbon equivalent (CE) on weldability as expressed in terms of a notch bend test.
Fig 4 Ratio (welded to unwelded) of bend angle for normalized steel plate