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CARBON AND LOW-ALLOY STEEL SHEET AND STRIP

10/05/2010, 13:24

Introduction
CARBON AND LOW-ALLOY STEEL SHEET AND STRIP have much in common with, but also some significant
differences from, their flat-rolled plate counterparts. Categorizing these products on the basis of dimensions alone is
impossible because of their interchangeability and the overlapping of sizes. Mechanical processing must also be taken
into consideration when classifying these hot mill products. Generally, sheet and strip are produced as coils that are rolled
in only one direction while using water to accelerate cooling. Plate products, on the other hand, are made from slabs that
can be cross rolled (rolled parallel and perpendicular to the slab length) and that are air cooled. Plate is available in widths
up to 5080 mm (200 in.). Strip refers to hot-rolled coils less than 305 mm (12 in.) wide or cold-rolled coils less than 608
mm (23 15
16
in.) wide. The maximum sheet width currently available from most manufacturers is 1830 mm (72 in.),
although a few manufacturers have the capability of producing 1930 mm (76 in.) wide sheet in a 2130 mm (84 in.) mill
facility.
Note
* David Hudok, Weirton Steel Corporation; J.K. Mahaney, Jr., S.A. Kish, and A.P. Cantwell, LTV Steel
Company; Elgin Van Meter, Empire-Detroit Steel Division
Plain Carbon Steel
Plain carbon steel sheet and strip are used primarily in consumer goods. These applications require materials that are
serviceable under a wide variety of conditions and that are especially adaptable to low-cost techniques of mass production
into articles having good appearance. Therefore, these products must incorporate, in various degrees and combinations,
ease of fabrication, adequate strength, excellent finishing characteristics to provide attractive appearance after fabrication,
and compatibility with other materials and with various coatings and processes.
The steels used for these products are supplied over a wide range of chemical compositions (see Tables 1 and 2);
however, the vast majority are unalloyed, low-carbon steels selected for stamping applications, such as automobile bodies
and appliances. Thus, this section of the article will focus on low-carbon steel applications. For these major applications,
typical compositions are 0.03 to 0.10% C, 0.15 to 0.50% Mn, 0.035% P (max), and 0.04% S (max).
Generally, rimmed (or capped) ingot cast steel has been used because of its lower price. More recently, these steels have
been replaced by killed steels produced by the continuous casting process. This process is inherently suited to the
production of killed steels. Where strain aging is to be avoided and/or when exceptional formability is required, steel
killed with aluminum, regardless of the method of casting or manufacture, is preferred. Further details regarding
steelmaking and deoxidation practice are given in the articles "Steel Processing Technology" and "Classification and
Designation of Carbon and Low-Alloy Steels" in this Volume.
Quality Descriptors for Carbon Steels
The descriptors of quality used for hot-rolled plain carbon steel sheet and strip and cold-rolled plain carbon steel sheet
include structural quality, commercial quality, drawing quality, and drawing quality, special killed (Table 4(a)). Some of
the as-rolled material made to these qualities is subject to surface disturbances known as coil breaks, fluting, and stretcher
strains; however, fluting and stretcher strains will not be produced during subsequent forming if the material is temper
rolled and/or roller leveled immediately prior to forming. It should be noted that any beneficial effects of roller leveling
deteriorate rapidly in nonkilled steel. In addition to the requirements listed below for the various qualities of plain carbon
steel sheet and strip, special soundness can also be specified.
Commercial quality (CQ) plain carbon steel sheet and strip are suitable for moderate forming; material of this quality
has sufficient ductility to be bent flat on itself in any direction in a standard room-temperature bend test. Commercial
quality material is not subject to any other mechanical test requirements, and it is not expected to have exceptionally
uniform chemical composition or mechanical properties. However, the hardness of cold-rolled CQ sheet is ordinarily less
than 60 HRB at the time of shipment.
Drawing Quality. When greater ductility or more uniform properties than those afforded by commercial quality are
required, drawing quality (DQ) is specified. Drawing quality material is suitable for the production of deep-drawn parts
and other parts requiring severe deformation. When the deformation is particularly severe or resistance to stretcher strains
is required, drawing quality, special killed (DQSK) is specified. When either type of drawing quality material is specified,
the supplier usually guarantees that the material is capable of being formed into a specified part within an established
breakage allowance. The identification of the part is included in the purchase order. Ordinarily, DQ or DQSK material is
not subject to any other mechanical requirements, nor is it normally ordered to a specific chemical composition.
Special killed steel is usually an aluminum-killed steel, but other deoxidizers are sometimes used to obtain the desired
characteristics. In addition to severe drawing applications, it is specified for applications requiring freedom from
significant variations in mechanical properties or freedom from fluting and stretcher strains in temper-rolled material
without subsequent roller leveling prior to forming. Special killed steels also have inherent characteristics that increase
their formability.
Structural quality (SQ), formerly called physical quality (PQ), is applicable when specified strength and elongation
values are required in addition to bend tests (Table 6). Minimum values of tensile strength ranging up to 690 MPa (100
ksi) in hot-rolled sheet and strip and up to 1035 MPa (150 ksi) in cold-rolled sheet are available. Cold-rolled strip, which
does not have a quality descriptor, is available in five tempers that conform to specified Rockwell hardness ranges and
bend test requirements (Table 5). It should be noted that steels with yield strengths exceeding 275 MPa (40 ksi) or tensile
strengths greater than 345 MPa (50 ksi) are referred to as high-strength structural or high-strength low-alloy steels. These
materials are described elsewhere in this Section of the Volume (see the articles "Classification and Designation of
Carbon and Low-Alloy Steels" and "High-Strength Structural and High-Strength Low-Alloy Steels").
Mechanical Properties of Carbon Steels
The commonly measured tensile properties of plain carbon steel sheet and strip are not readily related to their
performance in fabrication; the relationship between formability and values of the strain-hardening exponent, n, and the
plastic strain ratio, r (determined in tensile testing), is discussed in the article "Sheet Formability of Steels" in this
Volume. The mechanical properties of commercial quality, drawing quality, and drawing quality, special killed sheet and
strip are not ordinarily used in specifications unless special strength properties are required in the fabricated product.
It should be noted that the ranges are broader and the sheet harder for the hot-rolled than for the cold-rolled materials and
that cold-rolled drawing quality, special killed sheet is produced to a narrower range of mechanical properties than coldrolled
drawing quality sheet, which is a rimmed steel grade. There is a great deal of overlapping in properties between
commercial quality and drawing quality sheet.
Mill Heat Treatment of Cold-Rolled Products
Unless a hard temper is desired, cold-rolled carbon steel sheet and strip are always softened to improve formability. This
is usually accomplished at the mill by a recrystallization heat treatment such as annealing or normalizing.
Annealing. Low-temperature recrystallization annealing, or process annealing, can be used to soften cold-rolled lowcarbon
steel. When done as a batch process, this type of annealing is known as box annealing. It is carried out by placing
coils on a bottom plate and then enclosing them with a cover within which a protective gas atmosphere is maintained. A
bell-type heating furnace is then placed over the atmosphere container. After heating to approximately 595 to 760 °C
(1100 to 1400 °F), the charge is allowed to soak until the temperature is uniform throughout. The heating furnace is then
removed, and the charge is allowed to cool in the protective atmosphere before being uncovered.
Cold-rolled steel can be batch annealed in coil form under a protective atmosphere. Some producers use a 100% hydrogen
atmosphere in an effort to shorten annealing cycles.
Instead of box annealing, coils can also be treated by continuous annealing. With this process, which is usually intended
to provide a fully recrystallized grain structure, coils are unwound and passed through an annealing furnace. The uncoiled
steel strip passes through several different thermal zones of the furnace that serve to heat, soak, and cool the steel before it
exits the furnace and is recoiled. This anneal cycle is very rapid and can be measured in seconds or minutes (as opposed
to hours or days with a box anneal cycle). Generally, the rapid anneal cycle of a continuous anneal process results in
material properties that are less ductile than those resulting from a box anneal cycle. However, continuous annealing
results in more uniformity of properties throughout the length of a coil.
Open-coil annealing is used when uniform heating and/or gas contact across the entire width of the coil is required (for
example, to obtain decarburization over the entire surface during production of material for porcelain enameling). In this
process, the coils are loosely wound, permitting gas to flow freely between the coil convolutions. Annealing temperatures
may be higher than those used in conventional box annealing.
Normalizing consists of heating the sheet or strip to a temperature above the Ac3 point (~925 °C, or 1700 °F, for a steel
that contains less than 0.15% C) in a continuous furnace containing an oxidizing atmosphere, then cooling to room
temperature at a controlled rate (usually in still air). This treatment recrystallizes and refines the grain structure by phase
transformation. Low-metalloid steel (enameling iron) for porcelain enameling is normalized rather than annealed because
this steel will not readily recrystallize at box-annealing temperatures.
Surface Characteristics
The surface texture of low-carbon cold-rolled steel sheet and strip can be varied between rather wide limits. For
chromium plating and similar finishes, a smooth, bright sheet or strip surface is necessary, but for porcelain enameling
and many drawing operations, a rougher surface texture (matte finish) is preferred. In porcelain enameling, roughness
tends to improve the adherence and uniformity of the coating; in certain drawing operations where heavy pressures are
developed, the rougher type of surface is believed to retain more lubricant, thus aiding formation of the sheet by reducing
friction and die galling.
Minor surface imperfections and slight strains are less noticeable on a dull surface than on a bright one. However, the
surfaces of parts to be painted should not be so rough that the paint will not cover them adequately. A very smooth, bright
surface can be obtained on sheet or strip by utilizing ground and polished rolling-mill rolls, and a dull (matte) surface can
be obtained by either grit blasting or etching the rolls. For the purpose of evaluating surface roughness, an appropriate
instrument is employed that measures the average height of surface asperities (peaks) in microinches and the number of
peaks per inch that exceed a given height.
Cold-rolled sheet or strip can also be purchased with coined patterns that form a geometric design or that simulate such
textures as leather grain. Such products are available in commercial quality, drawing quality, and drawing quality, special
killed material. The texture is rolled into the steel surface after the sheet or strip has been annealed and thus has an effect
on properties similar to that of a heavy temper-rolling pass. This effect, plus the notch effect of the pattern itself,
somewhat reduces the formability of the sheet or strip.
Stretcher Strains. When loaded in tension, practically all hot-rolled or as-annealed cold-rolled plain carbon steels,
whether rimmed, capped, or killed, exhibit a sharp upper yield point, a drop in load to the lower yield point, and
subsequent plastic deformation at a nearly constant load (known as yield point elongation). The plastic deformation that
occurs within this yield point elongation is accompanied by the formation of visible bands of deformation on the product
surfaces. These bands are called stretcher strains or Lüders lines, and they can be aesthetically undesirable.
The tendency for stretcher strains to occur can be prevented through elimination of yield point elongation. In rimmed or
capped steels, this is accomplished by subjecting the steel to small amounts of plastic deformation, usually by temper
rolling, tension leveling, and/or roller leveling. Because overstraining the steel by these practices can increase strength
and generally decrease ductility, it is usually desirable to strain the steel only by the amount required to eliminate yield
point elongation. When properly processed, a killed steel, such as DQSK, provides a product with no yield point
elongation.
Strain Aging. In rimmed or capped (but not killed) carbon steels, deformation (such as by temper rolling) following by
aging for several days or more at or slightly above room temperature will result in a return of the upper yield point and
yield point elongation, increases in yield and tensile strengths, and a decrease in ductility. This treatment, called strain
aging, may be desirable if the increase in strength can be used to advantage. However, strain aging often causes problems
due to reduced formability and stretchability and the return of both yield point elongation and a propensity for stretcher
strains. Further temper rolling may eliminate yield point elongation, but it will not restore stretchability. In applications
where the appearance of stretcher strains is objectionable, killed steels, which are resistant to aging, are preferable to
rimmed and capped steels. For ingot casting, however, rimmed and capped steels are generally superior in inherent
surface quality, are lower in cost, and are preferred over killed steel as long as the occurrence of stretcher strains is not a
problem.
Strain aging is related to the presence of nitrogen in solid solution in the steel and is affected by time and temperature,
with longer times and higher temperatures producing greater aging. The strain-aging rate is also dependent on the amount
of deformation that has occurred and is increased when the deformation occurs at higher temperatures or lower strain
rates. Another important variable that affects strain aging is the amount of nitrogen in solution. Killed carbon steels have
very little susceptibility to strain aging because their nitrogen content is essentially chemically combined with aluminum.
Rimmed and capped steels, however, tend to strain age because they contain greater amounts of nitrogen in solid solution
(typically 6 to 30 ppm).
Control of Flatness
Plain carbon steel sheet is ordinarily sold to two standards of flatness:
· Commercial flatness, which is used where flatness is important but not critical
· The stretcher-level standard of flatness, which is required when little or no forming is to be done and the
product is required to be flat and free from waves or oil can, or when flatness is necessary to ensure
smooth automatic feeding of forming equipment.
The permissible variations for the flatness of hot- and cold-rolled sheet have been established by the Technical Committee
of the American Iron and Steel Institute and are given in the AISI Steel Products Manual. Commercial flatness can
usually be produced by roller leveling or by temper rolling and roller leveling, but where very flat sheet is required,
producers may have to resort to stretcher leveling, tension leveling, or other leveling processes.
In temper rolling, the steel is cold reduced, usually by 1
2
to 2%, which is also effective for removing yield point
elongation and preventing stretcher strains.
In roller leveling, a staggered series of small-diameter rolls alternately flexes the steel back and forth. The rolls are
adjusted so that the greatest deformation occurs at the entrance end of the rolls and less flexing occurs at the exit end.
Stretcher strains can also be eliminated by roller leveling, as long as the deformation is great enough to remove yield
point elongation. Dead-soft annealed sheet cannot be made suitable for production of exposed parts by roller leveling
because the rolls kink the sheet severely, producing leveler breaks. The deformed areas or kinks will not deform further
upon stretching and will appear as braised welts after forming.
Stretcher Leveling. Leveling by stretching cut lengths of the temper-rolled sheet lengthwise between jaws (stretcher
leveling) is a more positive means of producing flatness. Elongation (stretching) during stretcher leveling may vary from
about 1 to 3%, which exceeds the elastic limit of the steel and therefore results in some permanent elongation. The sheet
must be of a killed or a capped steel having nearly uniform properties so that it will spring back uniformly across its full
width and remain flat. It may be necessary to use killed steel having nearly uniform properties so that, after stretching,
strain markings do not develop.
Tension Leveling. Another flattening process that is used for steel sheet is tension leveling, which combines the effects
of stretcher and roller leveling. The sheet is pulled to a stress near its yield point while it is simultaneously flexed over
small rolls; the combined tension and bending produce yielding at the flex points.
Modified Low-Carbon Steel Sheet and Strip
In addition to the low-carbon steel sheet and strip products already discussed in this article, there are numerous additional
products available that are designed to satisfy specific customer requirements. These products are often made with lowcarbon
steels having chemical compositions slightly modified from those discussed earlier.
To be considered a plain low-carbon grade, a steel should contain no more than 0.25% C, 1.65% Mn, 0.60% S, and 0.60%
Cu, but it may also contain small amounts of other elements, such as nitrogen, phosphorus, and boron, that are effective in
imparting special characteristics when present singly or in combination. The modified low-carbon steel grades discussed
below are designed to provide sheet and strip products having increased strength, formability, and/or corrosion resistance.
Carbon-Manganese Steels. Manganese is a solid-solution strengthening element in ferrite and is also effective in
increasing hardenability. Manganese in amounts ranging from 1.0 to 1.5% is added to low-carbon steel (0.15 to 0.25% C)
to provide enhanced strength (yield strength of about 275 MPa, or 40 ksi) with good ductility in hot-rolled and cold-rolled
sheet and strip. Components fabricated from these higher-manganese steels can be heat treated by quenching and
tempering to provide enhanced strength with good toughness (see the article "High-Strength Structural and High-Strength
Low-Alloy Steels" in this Volume).
Carbon-Silicon Steels. Silicon, like manganese, is an effective ferrite-strengthening element and is sometimes added
in amounts of about 0.5%, often in combination with 1.0 to 1.5% Mn, to provide increased strength in low-carbon hotrolled
and cold-rolled steel sheet and strip.
Nitrogenized and Rephosphorized Steels. Nitrogen is a strong interstitial strengthener, and phosphorus is an
effective solid-solution strengthener in ferrite. Either about 0.010 to 0.015% N or 0.07 to 0.12% P is added to low-carbon
steel to provide hot-rolled and cold-rolled sheet and strip with yield strength in the range of 275 to 345 MPa (40 to 50 ksi)
for low-cost structural components for buildings and automotive uses. Formed parts produced from nitrogenized steel can
be further strengthened to yield strengths in the range of 415 to 485 MPa (60 to 70 ksi) as the result of strain aging that
occurs at paint-curing temperatures.
Boron Steels. Boron is a strong carbide-and nitride-forming element and increases strength in quenched and tempered
low-carbon steels through the formation of martensite and the precipitation strengthening of ferrite. Boron-containing
killed carbon steels are available as low-cost replacements for the high-carbon and low-alloy steels used for sheet and
strip. The low-carbon boron steels have better cold-forming characteristics and can be heat treated to equivalent hardness
and greater toughness for a wide variety of applications, such as tools, machine components, and fasteners.
Copper Steels. Copper in amounts up to 0.5% is not only a mild solid-solution strengthener in ferrite, but it also
provides enhanced atmospheric corrosion resistance together with improved paint retention in applications involving full
exposure to the weather. Therefore, copper-bearing (0.20% Cu, minimum) steel is often specified by customers for use in
sheet and strip for structures subject to atmospheric corrosion. Essentially all low-carbon steel sheet and strip products
can be supplied in copper-bearing grades, if so specified. Copper-bearing steels, which are also referred to as weathering
steels, are also described in the article "High-Strength Structural and High-Strength Low-Alloy Steels" in this Volume.
References cited in this section
1. Steel--Plate, Sheet, Strip, Wire, Vol 01.03. Annual Book of ASTM Standards, American Society for Testing
and Materials
2. Materials, Vol 1, SAE Handbook, Society of Automotive Engineers, 1989
Low-Alloy Steel**
Low-alloy steel sheet and strip are used primarily for those special applications that require the mechanical properties
normally obtained by heat treatment. A sizeable selection of the standard low-alloy steels are available as sheet and strip,
either hot-rolled or cold rolled. The most commonly available alloys are listed in Table 7, along with their chemical
compositions. In addition to standard low-alloy steels, high-strength low-alloy (HSLA) and dual-phase steels are available
as sheet or strip for applications requiring tensile strengths in the range of 290 to 760 MPa (42 to 110 ksi), and ultrahighstrength
steels or maraging steels for applications requiring tensile strengths above 1380 MPa (200 ksi). These steels are
discussed in the articles "High-Strength Structural and High-Strength Low-Alloy Steels," "Dual-Phase Steels,"
"Ultrahigh-Strength Steels" and "Maraging Steels" in this Volume.
Production of Sheet and Strip
As described earlier in this article, steel sheet and strip are flat-rolled products that can be rolled to finished thickness on
either a hot mill or a cold mill. Hot-rolled steel sheet and strip are normally produced by passing heated slabs through a
continuous mill consisting of a series of roll stands, where the thickness is progressively reduced to the desired final
dimension.
Cold-rolled low-alloy steel sheet and strip are normally produced from pickled and annealed hot-rolled bands of
intermediate thickness by cold reduction to desired thickness in a single-stand mill or tandem mill. Intermediate anneals
may be required to facilitate cold reduction or to obtain the mechanical properties desired in the finished product. Cold
rolling can produce thinner gages than can be obtained by hot rolling.
Low-alloy steel sheet and strip are produced in thicknesses similar to those typical of HSLA steel sheet and strip (Table
8). In general, tolerances similar to those given in the general requirements for hot-rolled and cold-rolled low-alloy and
HSLA steel sheet and strip, ASTM A 505, apply to all low-alloy and HSLA steel sheet and strip. Available thicknesses
and tolerances may vary among producers, due mainly to the interrelation between steel quality and rolling practice, as
influenced by the equipment available for rolling the product.
Quality Descriptors
As it is used for steel mill products, the term quality relates to the general suitability of the mill product to make a given
class of parts. For low-alloy steel sheet and strip, the various quality descriptors imply certain inherent characteristics,
such as the degree of internal soundness and the relative freedom from harmful surface imperfections.
The quality descriptors used for alloy steel sheet and plate include regular quality, drawing quality, and aircraft quality,
which are covered by ASTM specifications. The general requirements for these qualities include bearing quality and
aircraft structural quality. Aircraft quality requirements are also defined in Aerospace Material Specifications (AMS).
Regular Quality. Low-alloy steel sheet and strip of regular quality are intended principally for general or miscellaneous
applications where moderate drawing and/or bending is required. A smooth finish free of minor surface imperfections is
not a primary requirement. Sheet and strip of this quality do not have the uniformity, the high degree of internal
soundness, or the freedom from surface imperfections that are associated with other quality descriptors for low-alloy sheet
and strip.
Regular quality low-alloy steel sheet and strip are covered by ASTM A 506. One or more of the following characteristics
may be specified by the purchaser: chemical composition, grain size, or mechanical properties (determined by tensile and
bend tests.)
Drawing quality describes low-alloy steel sheet and strip for applications involving severe cold working such as deepdrawn
or severely formed parts. Drawing quality low-alloy sheet and strip are rolled from steel produced by closely
controlled steelmaking practices. The semifinished and finished mill products are subject to testing and inspection
designed to ensure internal soundness, relative uniformity of chemical composition, and freedom from injurious surface
imperfections. Spheroidize annealing is generally specified so the mechanical properties and microstructure are suitable
for deep drawing or severe forming. Drawing quality low-alloy steel sheet and strip are covered by ASTM A 507.
No standard test can fully evaluate resistance to breakage during deep drawing because successful drawing is affected by
die clearances, die design, speed of drawing, lubricants, ironing, grade of steel, and any alteration of hardness, ductility,
or surface condition that may develop during drawing. Thus, it cannot be assumed that merely specifying drawing quality
steel will ensure a capability for drawing or forming a specific part under a given set of manufacturing conditions.
Manufacturing trials may be necessary before purchase orders can be written for production material.
Bearing quality describes low-alloy steel sheet and strip intended for antifriction bearing parts. The steels are generally
AISI-SAE alloy carburizing grades or AISI-SAE high-carbon chromium grades. These steels are produced using
steelmaking and conditioning practices that are intended to optimize internal soundness and to provide a known size,
shape, and distribution of non-metallic inclusions. Standards of acceptance for microstructural quality are commonly
reviewed and agreed upon between producer and purchaser for each order. Alternatively, internal soundness and
microcleanliness can be determined by using immersion ultrasonic testing techniques to agreed-upon acceptance
standards. More detailed information on low-alloy bearing steels can be found in the article "Bearing Steels" in this
Volume.
Aircraft quality describes low-alloy steel sheet and strip for important or highly stressed parts of aircraft, missiles, and
similar applications involving stringent performance requirements, especially in terms of internal cleanliness. The special
mill practices required for producing aircraft quality sheet and strip include careful selection of the raw materials charged
into the melting furnace, exceptionally close control of the steelmaking process, cropping and discarding more of the
ingot than is normal during primary reduction, selection of specific heats or portions of heats for fulfillment of a given
customer order, and using exceptionally close control over process variables during reheating and rolling. Aircraft quality
low-alloy steel sheet and strip generally have an austenitic grain size predominantly ASTM No. 5 or finer, with grains as
coarse as ASTM No. 3 permissible. Grain size tests are normally made on rerolling slabs or billets.
Aircraft quality low-alloy steel sheet and strip are covered by Aerospace Material Specifications (AMS 6454A, for
example). Material of this quality is ordinarily certified that it has been produced as aircraft quality.
Aircraft structural quality low-alloy steel sheet and strip meet all the requirements of aircraft quality mill products
described above. In addition, they meet specified requirements for mechanical properties, which may include tensile
strength, yield strength, elongation, bend test results, or results of other similar tests. Many specimens from each heat
must be tested to ensure compliance with the required mechanical properties.
Mill Heat Treatment
Hot-rolled regular quality low-alloy steel sheet and strip are normally available from the producer either as-rolled or heat
treated. Standard mill heat-treated conditions are annealed, normalized, or normalized and tempered. Cold-rolled regular
quality product is normally available only in the annealed condition.
Hot-rolled and cold-rolled drawing quality alloy steel sheet and strip are normally furnished by the producer in the
spheroidize-annealed condition. They can be purchased in the as-rolled condition if they are to be spheroidize annealed by
the user.
Aircraft quality products are normally furnished in a heat-treated condition. Hot-rolled products may be annealed,
spheroidize annealed, normalized, or normalized and tempered by the producer. Cold-rolled products are normally
furnished only in the annealed or spheroidize-annealed condition.
Annealing is done by heating the steel to a temperature near or below the lower critical temperature and holding at that
temperature for a sufficient period, followed by slow cooling in the furnace. This process softens the sheet or strip for
further processing, but not to the same degree as spheroidize annealing.
Spheroidize annealing involves prolonged heating at a temperature near or slightly below the lower critical
temperature, followed by slow cooling. The objective of this process is to change the form of the carbides in the
microstructure to a globular (spheroidal) shape, which produces the greatest degree of softening.
Normalizing consists of heating the sheet or strip to a temperature 55 to 70 °C (100 to 125 °F) above Ac3 and then
cooling to room temperature at a controlled rate (usually in still air). This treatment recrystallizes and refines the grains by
phase transformation and can be used to obtain the desired mechanical properties.
Tempering consists of reheating steel to a predetermined temperature below the lower critical temperature, holding for a
specified length of time, and then cooling under suitable conditions. When it is carried out as part of a mill heat treatment,
tempering is done after normalizing to obtain the desired mechanical properties by modifying the as-normalized
microstructure.
Quenching and tempering (or hardening) is normally reserved for the user to apply as one of the final steps in the
fabricating process.
Mechanical Properties
In most instances, the mechanical properties of low-alloy steel furnished by the producer are of little consequence because
they will be altered by heat treatment during fabrication. For low-alloy steel sheet and strip to be used in the mill
condition, mechanical properties will vary, depending on both chemical composition and mill processing. Table 9 lists
typical tensile properties for chromium-molybdenum low-alloy steel sheet and strip used for pressure vessels. Usually,
low-alloy steel sheet and strip are custom produced to fulfill specific customer orders. Where necessary, any mechanical
property requirements can be made part of the purchase order.
Because the chief benefits of low-alloy steel sheet and strip accrue to the user only after the finished part is heat treated,
the mechanical properties of heat-treated low-alloy steels are the ones of greatest importance. These properties can be
determined from hardenability curves (see the article "Hardenability Curves" in this Volume) and heat-treating guides
such as those found in the articles "Hardenable Carbon and Low-Alloy Steels" and "Hardenability of Carbon and Low-
Alloy Steels" in this Volume. In general, only those properties typical of through-hardened steel of the specific grade
under consideration need to be considered. Except for the most shallow hardening grades used at thicknesses at or near
the upper limit for sheet and strip, parts made of low-alloy steel sheet or strip will through harden when quenched. Many
grades will through harden when quenched in a slow medium such as oil and may even through harden when air cooled.
The possibility of oil quenching or air cooling should always be considered for hardening thin parts, especially when
warping or distortion during hardening need to be minimized.
Parts made of low-alloy steel sheet and strip are sometimes carburized or carbonitrided to improve the mechanical
properties or wear resistance of the surface layer. In some cases, parts that are difficult to form when made of a mediumcarbon
low-alloy steel can be formed from low-carbon low-alloy steel and then carburized to a uniform but higher carbon
content.
Reference cited in this section
1. Steel--Plate, Sheet, Strip, Wire, Vol 01.03. Annual Book of ASTM Standards, American Society for Testing
and Materials
Note cited in this section
** The term low-alloy steel rather than the more general term alloy steel is being used in this article as well as
other articles in this Section of the Handbook. See the article "Classification and Designations of Carbon
and Low-Alloy Steels" for definitions of various steel types.
Direct Casting Methods
Because of the large investment needed to build conventional steelmaking casting and rolling facilities, the focus over the
last ten years has been on reducing production costs and simplifying the overall steelmaking process. For the most part,
cost savings have been achieved by the progression of casting technology from ingot to continuous casting, which
eliminates soaking and breakdown hot rolling of large ingots. The following table compares the continuous cast share (in
percent) for the United States, the European Economic Community (EEC), Japan, and the total world:
Conventional continuous casting of steels requires the casting of a 150 to 250 mm (6 to 10 in.) thick by 800 to 2200 mm
(31 to 86 in.) wide slab that is subsequently rolled down to a thickness of 1.5 to 25 mm (0.05 to 1.0 in.) utilizing a hot
strip mill having both four-stand roughing and six- or seven-stand finishing mills (Fig. 3). This process requires a high
degree of reduction and the equivalent input of energy.
Direct casting processes are alternatives to conventional slab casting processes. Direct casting processes for steel flat
products could be defined as any casting process that produces a casting as close as possible to the final product
dimensions of the next processing step. By this definition, direct casting could also be termed near-net shape casting
because the final cast dimensions would approach the final product dimensions (Ref 3).
Presently, there are three direct casting alternatives. Listed in increasing order according to how close they come to
producing near-net shape dimensions, these processes are (Ref 3):
· Thin slab casting
· Thin strip casting
· Spray casting
The flowcharts summarize the key operations involved in these three alternative direct casting processes and
compare them with those of a continuous casting process in an integrated steel production facility.
Thin Slab Casting. Of the three direct casting processes listed above, only the thin slab casting process is being used
commercially. In thin slab casting, a slab 40 to 60 mm (1.5 to 2.5 in.) is produced. Hot rolling is not completely
eliminated in this process, but the amount of reduction necessary to produce strip is greatly reduced. However, the need
for a heating furnace and a roughing mill is eliminated . In addition, thin slab casting yields a finer grain structure
and a better finish than that obtained with conventional continuous casting technology.
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