High Strength Low Alloy (HSLA) Steel

HSLA steels were originally developed in the 1960s for large-diameter oil and gas pipelines. The line pipe used in these projects required higher strength and toughness than mild carbon steel, and good weldability provided by a low carbon equivalent.

The oil & gas sector is still market where the most important applications for HSLA steels are found, but the automotive and the offshore & onshore structural engineering sectors now consume significant quantities of these alloys.

HSLA steels are available today with traditional ferritic-pearlitic, bainitic, martensitic and multiphase microstructures, each available in hot- or cold-rolled steels. The yield strength of contemporary HSLA steels ranges from 260 MPa to over 1000 MPa.

Typically, Mo is used in HSLA steel when the yield strength must be above 550 MPa, or a particular microstructure is demanded. Mo is particularly beneficial for producing bainite (and more specifically for its acicular ferrite variant) and the multiphase microstructures that appear in dual phase, complex phase or TRIP steels.

Mo in line pipe HSLA steel

Two technological developments made possible the successful launch of HSLA steel in large-diameter pipelines: the thermomechanical rolling process, and the use of niobium (Nb), vanadium (V) and titanium (Ti) as microalloying components. The combination of these developments permitted the manufacture of higher strength steels without additional expensive heat treatments. These early HSLA line pipe steels typically relied on reduced pearlite-ferrite microstructures to make line pipe grades up to X60 and X65. However, higher-strength line pipe required different approaches, including new processing routes and new steel chemical compositions. Extensive research in the 1970s and early 1980s successfully developed higher strengths than X70 using various steel compositions, many employing a Mo-Nb combination. With the introduction of new process technology like accelerated cooling, it became possible to develop even higher strengths with much leaner Mo-free alloy designs.

Mainstream HSLA line pipe steel typically contains 0.05 to 0.09% carbon, up to 2% manganese and small additions (usually max. 0.1%) of niobium, vanadium and titanium in various combinations. The preferred production route for this material is thermomechanical rolling to maximize grain refinement, thereby improving mechanical properties. Grain refinement is the only strengthening mechanism that improves both strength and toughness simultaneously.

Nevertheless, because many rolling mills either cannot apply the required cooling rates after finish rolling or do not even have the required accelerated cooling equipment, the only practical available solution is to use selected alloy additions like Mo to obtain the desired steel properties (See Tables 1 and 2). Furthermore, with X70 becoming the workhorse of modern pipeline projects and the increasing popularity of spiral pipe, the demand for cost-effective heavy-gauge plate and hot-rolled coil produced by a Steckel mill or conventional hot strip mill (HSM) has grown significantly in the last several years. Consequently, many of these mills use Mo alloys, having reintroduced and adapted the metallurgical developments made during the 1970’s for today’s increasing pipeline demands.

Table 1. Mo-containing X70-80 line pipe steel

There is a strong trend towards increased operating pressure for future long-distance gas pipelines, which will require steels with X80 properties and higher. Steel producers are making good progress in meeting this challenge, especially for heavy-gage hot strip. Here molybdenum is seeing a comeback, with additions between 0.1 and 0.3% that not only help to produce a very fine-grained structure but also substantially enhance the precipitation-hardening effect achieved with microalloying elements. Moreover, Mo alloying helps to promote a continuous yielding curve and avoid the so-called Bauschinger effect, which is important when strain-based design codes are specified.

Table 2. Typical steel chemistries for X70 and X80 Mo-bearing line pipe steels.

Mo in structural steel

Structural plate applications are extremely diverse, but the trend for all is higher strength at heavy or extra-heavy gage. This combination can bring even the most powerful cooling device to its limits, and hence Mo alloying becomes relevant. Today, grades with up to 700 MPa yield strength are being produced by thermomechanical rolling to heavy gage, replacing the more traditional and costly quench-and-temper route. Depending on the strength and toughness required, different cooling strategies such as accelerated cooling (ACC), heavy accelerated cooling (HACC) or direct quenching with self-tempering (DQST) have to be applied. In such sophisticated steels, Mo is combined with other alloying elements like Cr and Ni, and microalloying combinations of Nb, Ti and optionally B, in order to produce bainitic or acicular ferritic microstructures having extremely fine grain size. The higher-strength steel allows structural components to be manufactured from thinner plate, saving material and reducing transport, hoisting and welding costs.

Conventional structural HSLA steel has good strength at ambient temperature, but softens severely when exposed to elevated temperature. For this reason, these alloys are not specified for temperatures much higher than ambient temperature. This can be a problem when a building’s steel structure is accidentally subjected to heat produced by a fire. If the steel softens, the structure will collapse under its own weight. Therefore, fire-resistant steel must resist thermally activated deformation (creep) at elevated temperatures ranging from about 400-700°C for a period of up to several hours. In Japan, a minimum of 2/3 of the specified room temperature (RT) yield strength must be retained at 600°C for steel to be considered fire resistant. HSLA steels containing additions of Nb, Mo, V, and/or Ti exhibit superior strength at elevated temperatures when compared to plain carbon steels. Of these, MoNb alloyed steels with Mo contents up to 0.6%were found to be the best performers. Mo strengthens the steel both by solid solution hardening of ferrite and by secondary precipitation of Mo₂C particles. Nb provides grain refinement and forms NbC precipitates that give extra strength. Above that, Mo inhibits the coarsening of NbC precipitates at elevated temperature by segregating to the NbC-matrix interface.

Mo in automotive hot & cold rolled strip

No other industrial sector has pursued weight reduction as aggressively as the automotive industry. This has led to unprecedented steel innovation to produce alloys with high strength and good cold formability. Today, the body of a passenger car contains up to 80% high- strength steel, the majority of which is traditional (ferritic, ferritic-pearlitic or bainitic) HSLA steel and an increasing share is multiphase steel. The tensile strength of the established grades ranges up to 1500 MPa, with recent grades achieving 2000 MPa. Especially in those automotive steels where the yield strength exceeds 700 MPa, Mo alloying has its place. Just as in line pipe HSLA steel, Mo promotes the formation of bainitic microstructures that have higher strength than ferrite-pearlite microstructures. These bainitic steels are particularly interesting for structural reinforcement parts, wheels, chassis parts and truck frames. The synergetic interaction of Mo with microalloying elements like Nb and Ti has also led to the development of ultrahigh-strength ferritic steels. The strength in these steels is gained by massive precipitation hardening. The effect of Mo in these steels is manifold:

• Mo delays precipitation of microalloying elements during thermomechanical rolling
• Mo retards recrystallization during hot rolling by solute drag on the grain boundaries
• Mo retards transformation from austenite to ferrite, leading to finer grain size
• Mo prevents coarsening (Ostwald ripening) of fine NbC or TiC particles precipitated in ferrite (Figure 4)

Multiphase steel such as DP (dual phase), TRIP (TRansformation Induced Plasticity) and CP (Complex Phase) steel can be produced either directly from the rolling heat or by employing an additional heat treatment, usually after cold rolling. The latter is the route usually used to supply the automotive industry. The primary effect of Mo alloying is to modify the phase fields in the CCT diagram that define transformation cooling-rate processing windows, thereby minimizing property variations in the final strip product.

Regarding as-rolled dual-phase steel, the cooling pattern on the runout table must allow formation of sufficient proeutectoid ferrite matrix without nucleation of pearlite before final quenching transforms the remaining carbon-enriched austenite into martensite. This is usually realized in a two-step cooling process. Mo has a pronounced effect on the pearlite nose, delaying the onset of pearlite formation very effectively. Its retarding effect on the pro-eutectoid ferrite reaction is much smaller, thereby significantly increasing the window of allowable cooling rates and making a more robust production process.

In cold rolled strip, the amount of ferrite in the multiphase microstructure is adjusted by intercritical annealing between the Ar1 and Ar3 temperature. The newly formed austenite fraction enriches in carbon during this treatment, and then transforms into martensite under a sufficiently high cooling rate in a continuous annealing line (CAL) or a continuous galvanizing line (CGL). Mo alloying reduces the critical cooling rate required to produce a fully martensitic transformation. Hence, CGLs not specifically designed to produce DP steel can still be used, allowing the steelmaker more flexibility in terms of production planning and scheduling.

TRIP steel is not quenched below the martensite start temperature immediately after intercritical annealing, but rather to an intermediate temperature to form carbide-free bainite. After a holding period at this temperature, the carbide-free bainite transforms into retained austenite and bainitic ferrite. Mo makes this bainite transformation extremely sluggish. For longer holding times, a TRIP-aided DP steel can be obtained. Reducing the holding time in the bainitic region results in DP steel, the result of a preferential transformation into martensite. Mo additions can help to achieve higher martensite and lower retained austenite contents after processing. This increases tensile strength remarkably without deteriorating weldability too much in terms of carbon equivalent.