Carbon and Low-Alloy Steel Plate

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Carbon and Low-Alloy Steel Plate

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Carbon and Low-Alloy Steel Plate

Messaggioda Aldebaran » 10/05/2010, 14:05

Carbon and Low-Alloy Steel Plate
Revised by F.B. Fletcher, Lukens Steel Company
Introduction
STEEL PLATE is any flat-rolled steel product more than 200 mm (8 in.) wide and more than 6.0 mm (0.230 in.) thick or
more than 1220 mm (48 in.) wide and 4.6 mm (0.180 in.) thick. The majority of mills for rolling steel plate have a
working-roll width between 2030 and 5600 mm (80 and 220 in.). Therefore, the width of product normally available
ranges from 1520 to 5080 mm (60 to 200 in.). Most steel plate consumed in North America ranges in width from 2030 to
3050 mm (80 to 120 in.) and ranges in thickness from 5 to 200 mm ( 3
16
to 8 in.). Some plate mills, however, have the
capability to roll steel more than 640 mm (25 in.) thick.
Steel plate is usually used in the hot-finished condition, but the final rolling temperature can be controlled to improve
both strength and toughness. Heat treatment is also used to improve the mechanical properties of some plate.
Steel plate is mainly used in the construction of buildings, bridges, ships, railroad cars, storage tanks, pressure vessels,
pipe, large machines, and other heavy structures, for which good formability, weldability, and machinability are required.
The impairment of these desirable characteristics with increasing carbon content usually limits the steel to the low-carbon
and medium-carbon constructional grades, with the low-carbon grades predominating. Many alloy steels are also
produced as plate. In the final structure, however, alloy steel plate is sometimes heat treated to achieve mechanical
properties superior to those typical of the hot-finished product.
Steelmaking Practices
Steel plate is produced from continuously cast slabs or individually cast ingots or slabs. Preparing these steel slabs or
ingots for subsequent forming into plates may involve requirements regarding deoxidation practices, austenite grain size,
and/or secondary melting practices.
Deoxidation Practices. During the steelmaking process, segregation of carbon can occur when carbon reacts with the
dissolved oxygen in the molten steel (a reaction that is favored thermodynamically at lower temperatures). Therefore, the
practice of controlling dissolved oxygen in the molten metal before and during casting is an important factor 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 or silicon.
Steels are classified by their level of deoxidation: killed steel, semikilled steel, capped steel, and rimmed steel. The steel
used for plates is usually either killed or semikilled. Semikilled steel is commonly used for casting ingots because 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. Therefore, killed steel is generally specified when
homogeneous structure and internal soundness of the plate are required or when improved low-temperature impact
properties are desired. Killed steel can be produced either fine or coarse grained without adversely affecting soundness,
surface, or cleanliness. Generally, heavy-gage plate (thicker than 38 mm, or 1 1
2
in.) is produced from killed steel to
provide improved internal homogeneity.
Semikilled steel is deoxidized to a lesser extent that killed steel and therefore does not have the same degree of
chemical uniformity or freedom from surface imperfections as killed steel. This type of steel is used primarily on lightergage
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. Steel plate specifications for structural and pressure vessel applications may require a
steelmaking process that produces a fine austenitic grain size. When a fine austenitic grain size is specified, grain-refining
elements are added during steelmaking.
Aluminum 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 only very small quantities of
aluminum because aluminum may affect strain-aging characteristics and graphitization. However, the addition of
aluminum may be necessary for some high-temperature steels (as well as most low-temperature steels) requiring good
toughness. Other grain-refining elements, such as niobium, vanadium, and titanium, are used in high-strength low-alloy
(HSLA) steels for grain refinement during rolling (see the article "High-Strength Structural and High-Strength Low-Alloy
Steels" in this Volume).
Melting Practices. The steel for plate products can be produced by the following primary steelmaking processes: open
hearth, basic oxygen, or electric furnace. In addition, the steel can be further refined by secondary processes such as
vacuum degassing or various ladle treatments for deoxidation or desulfurization.
Vacuum degassing is used to remove dissolved oxygen and 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 hydrogen
content is higher than desired. A small cost premium is associated with the specification of vacuum degassing.
Desulfurization. By combining steel refining with the addition of ladle desulfurizing agents (for example, calcium or
rare earth additions) immediately before casting or teeming, final plate steel sulfur content can be reduced to less than
0.005%. Lower sulfur content improves plate through-thickness properties and impact properties, but adds to the cost of
the steel.
Platemaking Practices
As noted earlier, steel plates are produced from either continuous-cast slabs, pressure-cast slabs, or ingots. Steel ingots are
typically between 380 and 1140 mm (15 and 45 in.) thick. There ingots first pass through a slabbing mill where they are
reduced in thickness to make a slab. The slab is then inspected, and the surface is conditioned by grinding or scarfing to
remove surface imperfections, and then reheated in furnaces prior to rolling to final plate thickness. Continuous-cast slabs
and pressure-cast slabs are normally heated and rolled to final plate thickness in a single operation. The plate can then be
roller leveled and cooled.
Microalloyed HSLA steels can be controlled rolled for grain refinement (see the article "High-Strength Structural and
High-Strength Low-Alloy Steels" in thisVolume). 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 thermomechanically
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, 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 fabricator. Certain plate products,
however, require further processing such as heat treatment.
Plate Imperfections
Certain characteristic surface imperfections that can weaken the plate may appear on hot-finished steel; chemical
segregation that can alter properties across the section may also be present. Some of these imperfections are discussed
below.
Seams are the most common imperfections found in hot-finished steel. These longitudinal cracks on the surface are
caused by blowholes and cracks in the original ingot that have been rolled closed, but not welded. For many plate
applications, 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, a surface condition common to all hot-finished steel, is produced during the heating and rolling
operations when atmospheric oxygen reacts with the heated surface, removing carbon. This produces a soft, low-strength
surface, which is often unsatisfactory for applications involving wear or fatigue. For this reason, critical parts or at least
critical areas of parts are usually machined to remove this weakened surface.
Segregation. Alloying elements always segregation during the solidification of steel. Elements that are especially prone
to segregation are carbon, phosphorus, sulfur, silicon, and manganese. The effect of segregation on mechanical properties
and fabricability is insignificant for most plate steel applications. However, segregation may produce difficulties in
subsequent operations such as forming, welding, punching, and machining.
Heat Treatment
Although most steel plate is used in the hot-finished condition, the following heat treatments are applied to plate that must
meet special requirements.
Normalizing 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 toughness requirements are specified for certain thicknesses in some grades of normalized plate, accelerated
cooling must be used in lieu 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 plates
with heavy thicknesses to obtain properties comparable to those developed by normalizing material in the lighter
thicknesses.
Quenching consists of heating the steel to a suitable austenitizing temperature, holding at that temperature, and
quenching in a suitable medium that depends on chemical composition and cross-sectional dimensions. As-quenched
steels are hard, high in strength, and brittle. They are almost always tempered before being placed in service.
Tempering consists of reheating the steel to a predetermined temperature below the critical range, then cooling under
suitable conditions. This treatment is usually carried out after normalizing or quenching to obtain desired mechanical
properties. Those include a balance of strength and toughness to meet the designer's requirements.
Stress relieving 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
microstructure or to obtain desired mechanical properties.
Types of Steel Plate
Steel plate is classified according to composition, mechanical properties, and steel quality. The three general categories of
steel plate considered in this article are carbon steel plate, low-alloy plate, and high-strength low-alloy (HSLA) steel
plate. These three categories of steel plate are available in the steel plate quality levels given in Table 1. Further
discussion on these various quality levels is provided in the section "Steel Plate Quality" in this article.
General Categories
Carbon steel plate is available in all quality levels except aircraft quality (Table 1) and is available in many grades.
Generally, carbon steel contains carbon up to about 2% and only residual quantities of other elements except those added
for deoxidation, with silicon usually limited to 0.60% and manganese to about 1.65%. The chemical composition
requirements of standard carbon steel plate are listed in Table 2. These steels may be suitable for some structural
applications when furnished according to ASTM A 830 and A 6. In addition to the carbon steels listed in Table 2, other
carbon steel plates are also classified according to more specific requirements in various ASTM specifications (see the
section "Steel Plate Quality" in this article).
Table 2 Standard carbon steel plate compositions applicable for structural applications
When silicon is required, the following ranges and limits are commonly used for nonresulfurized carbon steel: 0.10% max, 0.07-
0.15%, 0.10-0.20%, 0.15-0.30%, 0.35% max, 0.20-0.40, or 0.30-0.60%.
Low-Alloy Steel Plate. Steel is considered to be low-alloy steel when either of the following conditions is met:
· The maximum of the range given for the content of alloying elements exceeds one or more of the
following limits: 1.65% Mn, 0.60% Si, and 0.60% Cu
· Any definite range or definite minimum quantity of any of the following elements is specified or
required within the limits of the recognized field of constructional alloy steels: aluminum, boron,
chromium up to 3.99%, cobalt, niobium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium,
or any other alloying element added to obtain the desired alloying effect
Alloying elements are added to hot-finished plates for various 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 generally require additional care throughout their manufacture. They are more sensitive to thermal and
mechanical operations, the control of which is complicated by the varying effects of different chemical compositions. To
secure the most satisfactory results, consumers 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
fabricating the steel; mechanical properties to be obtained; and the conditions of service for which the finished articles are
intended.
The chemical composition requirements of standard low-alloy steel plate are listed in Table 3. These low-alloy steels may
be suitable for some structural applications when furnished according to ASTM A 6 and A 829. The effect of residual
alloying elements on the mechanical properties of hot-finished steel plate is discussed in the section "Mechanical
Properties" in this article. The effect of alloying elements on the hardenability and mechanical properties of quenched and
tempered steels is discussed in the articles "Hardenable Carbon and Low-Alloy Steels" and "High-Strength Structural and
High-Strength Low-Alloy Steels" in this Volume.
High-strength low-alloy steels offer higher mechanical properties and, in certain of these steels, greater resistance
to atmospheric corrosion than conventional carbon structural steels. The HSLA steels are generally produced with
emphasis on mechanical property requirements rather than the chemical composition limits. They are not considered alloy
steels as described in the American Iron and Steel Institute (AISI) steel products manuals, even though utilization of any
intentionally added alloy content would technically qualify as such.
There are two groups of compositions in this category:
· Vanadium and/or niobium steels, with a manganese content generally not exceeding 1.35% maximum
and with the addition of 0.2% minimum copper when specified
· High-strength intermediate-manganese steels, with a manganese content in the range of 1.10 to 1.65%
and with the addition of 0.2% minimum copper when specified
Other elements commonly added to HSLA steels to yield the desired properties include silicon, chromium, nickel,
molybdenum, titanium, zirconium, boron, aluminum, and nitrogen. The chemical compositions of ASTM structural
quality and pressure vessel quality plates made of HSLA steel are listed in Table 4. More information on HSLA steels is
provided in the article "High-Strength Structural and High-Strength Low-Alloy Steels" in this Volume.
Steel Plate Quality
Steel quality, as the term applies to steel plate, is indicative of many conditions, such as the degree of internal soundness,
relative uniformity of mechanical properties and chemical composition, and relative freedom from injurious surface
imperfections. The various types of steel plate quality are indicated in Table 1.
The three main quality descriptors used to describe steel plate are regular quality, structural quality, and pressure vessel
quality. Special qualities include cold-drawing quality, cold-pressing quality, cold-flanging quality, and forging quality
carbon steel plate, along with drawing quality and aircraft quality alloy steel plate. Quality descriptors that have been used
in the past include flange quality and firebox quality carbon and alloy steel plate and marine quality carbon steel plate.
However, use of these descriptors has been discontinued in favor of pressure vessel quality.
Regular quality is the most common quality of carbon steel, which is applicable to plates with a maximum carbon
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 imperfections that is associated with structural quality or pressure vessel quality
plate. Regular quality is usually ordered to standard composition ranges and is not customarily produced to mechanical
property requirements. Regular quality is analogous to merchant quality for bars because there are normally no
restrictions on deoxidation, grain size, check analysis, or other metallurgical factors. Also, this quality plate can be
satisfactorily used for applications similar to those of merchant quality bars, such as those involving mild cold bending,
mild hot forming, punching, and welding for noncritical parts of machinery.
Structural quality steel plate is intended for general structural applications such as bridges, buildings, transportation
equipment, and machined parts. However, some structural steel plate (ASTM A 829 and A 830 in Table 5) is produced
from the standard steels listed in Tables 2 and 3. These steels can be furnished only according to the chemical
compositions specified by SAE/AISI steel designations. Factors affecting the mechanical properties of hot-finished
carbon steel are discussed in the section "Mechanical Properties" in this article.
Pressure Vessel Plate. Steel plate intended for fabrication into pressure vessels must 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 plate must meet requirements for notch toughness and has more stringent limits for
allowable surface and edge imperfections.
Table 8 lists the various ASTM specifications for pressure vessel steel plate. All of these steel plate specifications are
furnished according to both chemical composition limits and mechanical properties.
Aerospace Materials Specifications AMS-2301. 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 supplied in the hot-rolled or thermally treated condition.
Forging quality plates are intended for forging, quenching and tempering, or similar purposes or when uniformity of
composition and freedom from injurious imperfections are important (see ASTM A 827). Plates of this quality are
produced from killed steel and are ordinarily furnished with the phosphorus content limited to 0.035% maximum and the
sulfur content limited to 0.040% maximum by heat analysis.
Mechanical Properties
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 in the United
States, 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 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 nickel, chromium, and molybdenum. For steels used in the hotfinished
condition (such as plate), carbon content is the single most important factor in determining mechanical
properties.
The static tensile properties of the various grades, types, and classes of steel plate covered by ASTM specifications
are listed in Tables 7 and 10. It should 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. 1, which presents the minimum
expected values for 0.20% C steel plate from 13 to 125 mm ( 1
2
to 5 in.) thick. Plate under 13 mm ( 1
2
in.) thick would
show 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.
The distribution of the tensile properties obtained for a larger number of heats of A 285, A 515, and A 516 steel plate is
illustrated in , which also shows the distribution of the carbon and manganese content. The use of the carbon and
manganese contents to control mechanical properties is clearly shown ; higher carbon and manganese contents
accompany higher yield strengths.
Fatigue Strength. The high-cycle (>1 million) fatigue properties of hot-finished steel, 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 specimens is about 40% of the tensile strength, depending on the surface finish. In contrast, unmachined hotrolled
steel, when loaded so that fatigue stresses are concentrated at the surface, will have a considerably lower fatigue
limit because of decarburization, surface roughness, and other surface imperfections. For this reason, the location of
maximum fatigue stresses should be carefully considered; for structural members designed in hot-finished steel, the
surface should be machined off from critically stressed areas or an allowance made for the weakness of the hot-finished
surface.
The presence of inclusions in hot-finished steel may 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, satisfactory service
performance can be ensured by proper selection of the steel that will behave 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 steels are controlled principally by
their chemical composition and ferrite grain size. For the steels considered in this article, carbon is 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 behavior.
Manganese (up to about 1.5%) improves low-temperature properties. Also, as mentioned previously, 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 silicon or aluminum than semikilled steels, and these elements improve lowtemperature
toughness and ductility. Because of variations in finishing temperatures and cooling rates, plate thickness
influences the grain size and therefore the transition temperature. Extensive data on the impact properties of hot-finished
steel are given in the article "Notch Toughness of Steels" in this Volume.
-Temperature Properties. The steel plate used in pressure vessel applications is often subjected to longterm
elevated temperatures. Of the carbon and low-alloy steels used for pressure vessel plate, the behavior of 2.25Cr-1Mo
steel (ASTM A 387, Class 22, in Table 9) 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. Further information
on the elevated-temperature properties of 2.25Cr-1Mo steel can be found in the article "Elevated-Temperature Properties
of Ferritic Steels" in this Volume.
Directional Properties. An important characteristics of steel plate, known as directionality or fibering, must 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 that 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 strength.
Fabrication Considerations
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 the load required to produce permanent deformation; high ductility allows large
deformation without fracture. Therefore, the lower-carbon 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 must be used to avoid fracture.
Because of fibering 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 usually performed with little difficulty on most plate steels up to about 0.50%
C. Higher-carbon steels can be annealed for softening. Steels with low carbon and manganese content, such as 1015, with
large quantities of free ferrite in the microstructure may be too soft and gummy for good machining. Increasing the
carbon content (to a steel such as 1025) improves the machinability.
Machining characteristics can be improved by factors that break up the chip as it is removed. This is usually
accomplished by the introduction of large numbers of inclusions such as manganese sulfides or complex oxysulfides.
These "free-machining" steels are somewhat more expensive, but are cost-effective when extensive machining is
involved.
Weldability is a relative term that 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 metal part being welded. Therefore, weldability ratings should state the conditions under which the rating was
determined and the properties and soundness obtained.
For carbon steels, the carbon and manganese contents are the primary elements of the composition factor that determine
the effect of the steel of given heating and cooling conditions. The great tonnage of steel used for welded applications
consists of low-carbon steel, up to 0.30% C.
Generally, steels with a carbon content less than 0.15% are readily weldable by any method. Steel with a carbon range of
0.15 to 0.30% can usually be welded satisfactorily without preheating, postheating, or special electrodes. For rather thick
sections (>25 mm, or 1 in.), however, special precautions such as 40 °C (100 °F) minimum preheat, 40 °C (100 °F)
minimum temperature between weld passes, and a 540 to 675 °C (1000 to 1250 °F) stress relief may be necessary.
Higher-carbon and higher-manganese grades can often be welded satisfactorily if preheating, special welding techniques,
and postheating and peening are used. In the absence of such precautions to control the rate of cooling and to eliminate
high stress gradients, cracks may occur in the weld and base metal. In addition, base metal properties such as strength,
ductility, and toughness may be greatly reduced.
All comments about the effect of carbon and manganese on weldability must be qualified in terms of section size because
of its relationship to heat input and cooling rate. In welding thicker sections, such as 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.
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