Controlled rolling of low alloy steels

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Understanding Controlled Rolling Technology

The hot-rolling process has gradually evolved into a much more precisely controlled operation, increasingly applied to low alloy steels with compositions carefully chosen to provide optimum mechanical properties when hot deformation is complete. This sophisticated process, where various rolling stages are temperature-controlled, reduction amounts in each pass are predetermined, and finishing temperatures are precisely defined, is called controlled rolling. This technology has become critically important for obtaining reliable mechanical properties in steels for pipelines, bridges, and numerous other engineering applications.

In contrast, more highly alloyed steels can be subjected to heavy deformations under metastable austenitic conditions prior to transformation to martensite. This process, known as ausforming, enables the achievement of very high strength levels combined with excellent toughness and ductility.

Historical Development and Challenges

Before World War II, strength in hot-rolled low alloy steels was achieved by adding carbon up to 0.4% and manganese up to 1.5%, yielding stress levels of 350-400 MPa. However, these steels were essentially ferrite-pearlite aggregates that did not possess adequate toughness for many modern applications. The toughness, measured by the ductile/brittle transition, decreases dramatically with carbon content as the volume of pearlite in the steel increases. Furthermore, with welding becoming the primary fabrication technique, high carbon contents led to serious cracking problems that could only be eliminated by using lower carbon steels.

The significant advantage of producing fine ferrite grain sizes in these steels became apparent, leading to the gradual introduction of controlled rolling in the austenitic condition to achieve this objective.

Microalloying Elements and Grain Refinement

Fine ferrite grain sizes in finished steel were greatly enhanced by adding small concentrations (< 0.1 wt %) of grain refining elements such as niobium, titanium, vanadium, and aluminum. By adding these elements to steels containing 0.03-0.08% C and up to 1.5% Mn, it became possible to produce fine-grained material with yield strengths between 450 and 550 MPa and ductile/brittle transition temperatures as low as -70°C. These steels are now referred to as high strength low alloy steels (HSLA) or microalloyed steels. This progress from the relatively low strength of ordinary mild steel (220-250 MPa) over twenty years represents a major metallurgical development whose importance in engineering applications cannot be overstated.

Mechanisms of Grain Refinement

The primary grain refinement mechanism in controlled rolling is the recrystallization of austenite during hot deformation, known as dynamic recrystallization. This process is clearly influenced by temperature and the degree of deformation occurring during each pass through the rolls. However, in austenite without second phase particles, the high temperatures involved in hot rolling lead to marked grain growth, limiting grain refinement during subsequent working.

Controlling grain size at high austenitizing temperatures requires the finest possible grain boundary precipitate that will not dissolve completely in austenite, even at the highest working temperatures (1200-1300°C). The most effective grain refining elements are strong carbide and nitride formers, such as niobium, titanium, and vanadium, plus aluminum which forms only nitrides. Since both carbon and nitrogen are present in control-rolled steels, and nitrides are more stable than carbides, the most effective grain refining compounds are likely the respective carbo-nitrides, except for aluminum nitride. Through the combined use of controlled rolling and fine dispersions of carbo-nitrides in low alloy steels, ferrite grain sizes between 5 and 10 μm have been achieved in commercial practice.

Precipitation Strengthening Mechanisms

Solubility data indicates that in microalloyed steel, carbides and carbo-nitrides of Nb, Ti, and V precipitate progressively during controlled rolling as temperature decreases. While the primary effect of these fine dispersions is grain size control, dispersion strengthening also occurs. The strengthening from this mechanism depends on both particle size and interparticle spacing, which is determined by the precipitate volume fraction. These parameters depend primarily on the type of compound precipitating, which is determined by the steel's microalloying content. However, the maximum solution temperature reached and the detailed controlled rolling schedule are also important variables.

Current understanding reveals that precipitation occurs not only in austenite but continues during transformation to ferrite. The precipitation of niobium, titanium, and vanadium carbides occurs progressively as interphase boundaries move through the steel. Since this precipitation normally occurs on an extremely fine scale between 850 and 650°C, it likely represents the major contribution to dispersion strengthening. Given the higher solubility of vanadium carbide in austenite, this effect is most pronounced with vanadium, followed by titanium and niobium in decreasing order of effectiveness. If cooling through the transformation is rapid, leading to supersaturated acicular ferrite formation, carbides tend to precipitate within grains, usually on dislocations that are numerous in this ferrite type.

Optimizing Steel Compositions

When developing optimum compositions for microalloyed steels, the maximum volume fraction of precipitate that can be dissolved in austenite at high temperatures is achieved using stoichiometric compositions.

Modern control-rolled microalloyed steels employ at least three strengthening mechanisms contributing to final strength. The relative contribution from each is determined by steel composition and, equally important, the thermomechanical treatment details. First, solid solution strengthening increments come from manganese, silicon, and uncombined nitrogen. Second, grain size contribution to yield stress represents a substantial component whose magnitude is very sensitive to detailed thermomechanical history. Finally, dispersion strengthening provides a typical increment. The total result yields strengths between approximately 350 and 500 MPa.

Processing Parameters and Their Effects

The finishing temperature for rolling significantly affects grain size and therefore strength levels achieved for particular steels. Rolling through the transformation into completely ferritic conditions has become common practice to obtain fine subgrain structures in ferrite, providing additional strength contributions. Alternatively, rolling finishes above the γ/α transformation, and the transformation nature is altered by increasing cooling rates. Slow cooling rates obtained by coiling at particular temperatures produce lower strengths than rapid rates imposed by water spray cooling following rolling.

Dual Phase Steel Development

Dual phase steels, referred to as dual phase low alloy (DPLA) steels, exhibit continuous yielding without sharp yield points and relatively low yield stress (300-350 MPa) combined with rapid work hardening rates and high elongations (≈ 30%) providing excellent formability. Due to work hardening, yield stress in final formed products equals that of HSLA steels (500-700 MPa). The simplest steels in this category contain 0.08-0.2% C and 0.5-1.5% Mn, though vanadium-microalloyed steels are also suitable, while small additions of Cr (0.5%) and Mo (0.2-0.4%) are frequently used.

The simplest method for achieving duplex structure is intercritical annealing, where steel is heated to the (α + γ) region between Ac1 and Ac3 and held, typically at 790°C for several minutes, allowing small austenite regions to form in ferrite. Since transforming these regions to martensite is essential, recooling must be rapid or austenite must have high hardenability. This is achieved by adding 0.2-0.4% molybdenum to steel already containing 1.5% manganese. The required structure can then be obtained by air-cooling after annealing.

To eliminate extra heat treatment steps, dual phase steels have been developed that can achieve required structure during cooling after controlled rolling. These steels typically have 0.5% Cr and 0.4% Mo additions. After completing hot rolling around 870°C, steel forms approximately 80% ferrite on the water-cooled run-out table. The material then cools in the metastable region (510-620°C) below the pearlite/ferrite transformation, and during subsequent cooling, austenite regions transform to martensite.

Commercial Applications and Advantages

Microalloyed steels produced by controlled rolling represent an attractive proposition for many engineering applications due to their relatively low cost, moderate strength, excellent toughness and fatigue strength, and ready weldability. They have largely eliminated quenched and tempered steels in many applications.

These steels are most frequently available as control-rolled sheet, then cooled over temperature ranges between 750 and 550°C. Cooling temperature significantly influences final transformation temperature, affecting microstructure. Lower temperatures under identical conditions achieve higher strength.

The normal yield strength range for these steels varies from approximately 350 to 550 MPa. Strength is controlled by detailed thermomechanical treatment, varying manganese content from 0.5 to 1.5 wt%, and using microalloying additions ranging from 0.03 to above 0.1 wt%. Niobium is used alone or with vanadium, while titanium can be combined with other carbide-forming elements. Although interactions between these elements are complex, niobium generally precipitates more readily in austenite than vanadium as carbide or carbo-nitride, making it relatively more effective as a grain refiner. Vanadium carbide's greater solubility in austenite highlights this element's superior dispersion strengthening potential, shared to a lesser degree with titanium. Titanium also interacts with sulfur and can beneficially affect sulfide inclusion shapes. Since the total effect of these elements used together is not a simple sum of individual influences, the detailed metallurgy of these steels becomes extremely complex.

Pipeline Applications

One of the most extensive applications is in pipelines for natural gas and oil conveyance, where improved weldability due to overall lower alloying content (lower hardenability) and particularly lower carbon levels provides significant advantages.