Control of High Strength Low Alloy (HSLA) Steel Properties

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Most HSLA steels are furnished in the as-hot-rolled condition with ferritic-pearlitic microstructure. The exceptions are the controlled-rolled steels with an acicular ferrite microstructure and the dual-phase steels with martensite dispersed in a matrix of polygonal ferrite. These two types of HSLA steels use the formation of eutectoid structures for strengthening, while the ferritic-pearlitic HSLA steels generally require strengthening of the ferrite.

Most HSLA steels are furnished in the as-hot-rolled condition with ferritic-pearlitic microstructure. The exceptions are the controlled-rolled steels with an acicular ferrite microstructure and the dual-phase steels with martensite dispersed in a matrix of polygonal ferrite. These two types of HSLA steels use the formation of eutectoid structures for strengthening, while the ferritic-pearlitic HSLA steels generally require strengthening of the ferrite.

Pearlite is generally an undesirable strengthening agent in structural steels because it reduces impact toughness and requires higher carbon contents. Moreover, yield strength is largely unaffected by a higher pearlite content.

Strengthening Mechanisms in Ferrite

The ferrite in HSLA steels is typically strengthened by grain refinement, precipitation hardening, and, to a lesser extent, solid-solution strengthening. Grain refinement is the most desirable strengthening mechanism because it improves not only strength but also toughness.

Grain refinement is influenced by the complex effects of alloy design and processing methods. For example, the various methods of grain refinement used in the three different stages of hot rolling (that is, reheating, hot rolling, and cooling) include:

  • The addition of titanium or aluminum to retard austenite grain growth when the steel is reheated for hot deformation or subsequent heat treatment
  • The controlled rolling of microalloyed steels to condition the austenite so that it transforms into fine-grain ferrite
  • The use of alloy additions and/or faster cooling rates to lower the austenite-to-ferrite transformation temperature.

The use of higher cooling rates for grain refinement may require consideration of its effect on precipitation strengthening and the possibility of undesirable transformation products.

Precipitation strengthening occurs from the formation of finely dispersed carbonitrides developed during heating and cooling. Because precipitation strengthening is generally associated with a reduction in toughness, grain refinement is often used in conjunction with precipitation strengthening to improve toughness.

Precipitation strengthening is influenced by the type of carbonitride, its grain size, and, of course, the number of carbonitrides precipitated. The formation of MC is the most effective metal carbide in the precipitation strengthening of microalloyed niobium, vanadium, and/or titanium steels. The number of fine MC particles formed during heating and cooling depends on the solubility of the carbides in austenite and on cooling rates.

Steelmaking

Precise steelmaking operations are also essential in controlling the properties and chemistry of HSLA steels. Optimum property levels depend on such factors as the control of significant alloying elements and the reduction of impurities and nonmetallic inclusions.

Developments in secondary steelmaking such as desulphurization, vacuum degassing, and argon shrouding have enabled better control of steel chemistry and the effective use of microalloyed elements. Compositional limits for HSLA steel grades described in ASTM specifications the use of vacuum degassing equipment allows the production of interstitial-free (IF) steels. The IF steels exhibit excellent formability, high elongation, and good deep draw/ability.

Compositions and Alloying Elements

Chemical compositions for the HSLA steels are specified by ASTM standards. The principal function of alloying elements in these ferrite-pearlite HSLA steels, other than corrosion resistance, is strengthening of the ferrite by grain refinement, precipitation strengthening, and solid-solution strengthening. Solid-solution strengthening is closely related to alloy contents, while grain refinement and precipitation strengthening depend on the complex effects of alloy design and thermo-mechanical treatment.

Alloying elements are also selected to influence transformation temperatures so that the transformation of austenite to ferrite and pearlite occurs at a lower temperature during air cooling. This lowering of the transformation temperature produces a finer-grain transformation product, which is a major source of strengthening. At the low carbon levels typical of HSLA steels, elements such as silicon, copper, nickel, and phosphorus are particularly effective for producing fine pearlite. Element such as, manganese and chromium, which are present in both the cementite and ferrite, also strengthen the ferrite by solid-solution strengthening in proportion to the amount, dissolved in the ferrite.

In the presence of alloying elements, the practical maximum carbon content at which HSLA steels can be used in the as-cooled condition is approximately 0.20%. Higher levels of carbon tend to form martensite or bainite in the microstructure of as-rolled steels, although some of the higher-strength low-alloy steels have carbon contents that approach 0.30%.

The required strength is developed by the combined effect of:

  • Fine grain size developed during controlled hot roiling and enhanced by microalloyed elements (especially niobium)
  • Precipitation strengthening caused by the presence of vanadium, niobium, and titanium in the composition.

Nitrogen additions to high-strength steels containing vanadium are limited to 0.005% and have become commercially important because such additions enhance precipitation hardening. The precipitation of vanadium nitride in vanadium-nitrogen steels also improves grain refinement because it has a lower solubility in austenite than vanadium carbide.

Manganese is the principal strengthening element in plain carbon high-strength structural steels. It functions mainly as a mild solid-solution strengthener in ferrite, but it also provides a marked decrease in the austenite-to-ferrite transformation temperature. In addition, manganese can enhance the precipitation strengthening of vanadium steels and. to a lesser extent, niobium steels.

One of the most important applications of silicon is its use as a deoxidizer in molten steel. Silicon has a strengthening effect in low-alloy structural steels. In larger amounts, it increases resistance to scaling at elevated temperatures. Silicon has a significant effect on yield strength enhancement by solid-solution strengthening and is widely used in HSLA steels for riveted or bolted structures.

Copper in levels in excess of 0.50% also increases the strength of both low- and medium-carbon steels by virtue of ferrite strengthening, which is accompanied by only slight decreases in ductility. Copper can be retained in solid solution even at the slow rate of cooling obtained when large sections are normalized, but it is precipitated out when the steel is reheated to about 510 to 605°C (950 to 1125°F). At about 1% copper, the yield strength is increased by about 70 to 140 MPa regardless of the effects of other alloying elements. Copper in amounts up to 0.75% is considered to have only minor adverse effects on notch toughness or weldability. Copper precipitation hardening gives the steel the ability to be formed extensively and then precipitation hardened as a complex shape or welded assembly.

The atmospheric-corrosion resistance of steel is increased appreciably by the addition of phosphorus, and when small amounts of copper are present in the steel, the effect of the phosphorus is greatly enhanced. When both phosphorus and copper are present, there is a greater beneficial effect on corrosion resistance than the sum of the effects of the individual elements.

Chromium is often, added with copper to obtain improved atmospheric-corrosion resistance.

Nickel is often added to copper-bearing steels to minimize hot shortness.

Molybdenum in hot-rolled HSLA steels is used primarily to improve hardenability when transformation products other than ferrite-pearlite are desired. Molybdenum (0.15 to 0.30%) in microalloyed steels also increases the solubility of niobium in austenite, thereby enhancing the precipitation of NbC(N) in the ferrite. This increases the precipitation-strengthening effect of NbC(N).

Aluminum is widely used as a deoxidizer and was the first element used to control austenite grain growth during reheating. During controlled rolling, niobium and titanium are more effective grain refiners than aluminum.

Vanadium strengthens HSLA steels by both precipitation hardening the ferrite and refining the ferrite grain size. The precipitation of vanadium carbonitride in ferrite can develop a significant increase in strength that depends not only on the rolling process used, but also on the base composition. Carbon contents above 0.13 to 0.15% and manganese content of 1% or more enhances the precipitation hardening, particularly when the nitrogen content is at least 0.01%.

Titanium is unique among common alloying elements in that it provides both precipitation strengthening and sulfide shape control. Small amounts of titanium (<0.025%) are also useful in limiting austenite grain growth. However, it is useful only in fully killed (aluminum deoxidized) steels because of its strong deoxidizing effects, the versatility of titanium is limited because variations in oxygen, nitrogen, and sulfur affect the contribution of titanium as carbide strengthened.

Zirconium can also be added to killed high-strength low-alloy steels to improve inclusion characteristics, particularly in the case of sulfide inclusions, for which changes in inclusion shape improve ductility in transverse bending.

Boron has no effect on the strength of normal hot-rolled steel but can considerably improve hardenability when transformation products such as acicular ferrite are desired in low-carbon hot-rolled plate.

Treatment with calcium is preferred for sulfide inclusion shape control.

Controlled Rolling

The hot-rolling process has gradually become a much more closely controlled operation, and controlled rolling is now being increasingly applied to microalloyed steels with compositions carefully chosen to provide optimum mechanical properties at room temperature.

Controlled rolling is a procedure whereby the various stages of rolling are temperature controlled, with the amount of reduction in each pass predetermined and the finishing temperature precisely defined. This processing is widely used to obtain reliable mechanical properties in steels for pipelines, bridges, offshore platforms, and many other engineering applications. The use of controlled rolling has resulted in improved combinations of strength and toughness and further reductions in the carbon content of microalloyed HSLA steels.

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