Effect of Alloying Elements in High Strength low alloy steel (HSLA)

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. Reduced Mn content in steels decreases the centerline microstructural banding . Also, Mn content of greater than 0.3% in pipeline steels causes hydrogen induced blister cracking on being subjected to sour service environment.

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.

Figure 1: Schematic of the different roles of Nb during thermomechanical processing

Niobium(Nb) in the HSLA steel has significant role in the thermomechanical process as shown in the figure 1. The precipitation behavior of Nb is mentioned later.

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 onlyslight 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.

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.

Titanium is unique among common alloying elements in that it provides both precipitation strengthening and sulfide shape control. Small amounts of titanium 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.