Novel Steel Compositions and Processing Technologies

1. Introduction and Scope

Steel compositions and processing technologies have been researched for more than 100 years. This work is still being continued because the optimisation criteria are changing with global economy transitions. A higher strength with a simultaneously high ductility is not enough. Energy consumption and the overall cost become equality important parameters of the processing technology, in addition to its ability to assure property formation.

The mechanical properties of steel (as for any other metal alloy) are formed under the influence of five mechanisms: solid solution strengthening (depends on alloy composition), grain boundary strengthening (determined by the grain size), phase balance (number, volume fraction, and strength of each phase), precipitation strengthening (particle size and number density), and dislocation density. The resulting properties may vary in different directions depending on the microstructure banding, deformation texture, or chemical segregation.

A solid solution may be expensive because it requires the addition of alloying elements in significant quantities. Alloying elements vary in their effectiveness and price. The effectiveness of alloy additions can be enhanced with particle precipitation—particles are a stronger barrier to the dislocation motion than solute atoms and provide a higher strengthening effect for the same alloying element concentrations. However, particle precipitation may require a complex heat treatment that would increase energy consumption and cost. Therefore, precipitation during controlled cooling after hot deformation or solidification after casting is preferred. Phase balance variation can be achieved with complex alloying, and this is utilised in austenitic and duplex stainless steels. However, even in a plain carbon steel, the multi-phase (ferrite–bainite–martensite) microstructure can be easily obtained during controlled cooling, following the polymorphic transformation in Fe from the face centred cubic to the body cantered cubic structure. This approach would probably be the cheapest way to improve mechanical properties. A small grain size can be obtained through multiple recrystallisation cycles supported with high-temperature particle precipitation (which would pin grain boundaries and prevent grain growth)—this requires substantial deformation strains. The dislocation density can be increased, for example, via deformation at lower temperatures or accelerated cooling. Increased strains (multiple deformation cycles) and deformation at a lower temperature will increase energy consumption and cost.

2. Contributions

As mentioned in [Contribution 1], the type, quantity, size, morphology, and distribution of non-metallic inclusions have a significant influence on the corrosion resistance and mechanical properties of steel products. Therefore, inclusion control is of primary importance during liquid steel refining and hot rolling. Calcium additions to the H08A welding steel studied in [Contribution 1] transformed MnS, Al₂O₃, and MnS-Al₂O₃ inclusions into CaOAl₂O₃, MgO-CaO-Al₂O₃, and MgO-Al₂O₃-MnS complexes with a lower melting temperature. The spherical shape of these complexes, the lower number of coarse particles, and the more uniform spatial distribution of the particles will positively influence the microstructure and mechanical property development.

Precipitates formed in liquid or during solidification influence the grain structure formation in the solid state. This happens through grain boundary pinning, the prevention of grain boundary migration, and, therefore, the retardation of grain growth. Grain growth may happen during high-temperature reheating prior hot rolling or after recrystallisation during rolling if the rolling speeds are too slow. The authors of [Contribution 2] studied the effect of precipitates on grain coarsening during the pseudo-carburising of a 20CrMnTi gear steel. Carburising, and its associated solid solution and precipitation strengthening, is an economic way to improve the surface properties and reduce the cost of a component’s main body. However, grain growth adversely affects strength and toughness. In the studied 20CrMnTi steel, grain coarsening was related to the dissolution of small and the growth of large (Ti, Mo)(C, N) particles. This effect was more pronounced at a higher carburising temperature (980 and 970 °C were compared), and the grain coarsening rate was measured to be 2.34 mm/min at 980 °C and 0.79 mm/min at 970 °C. The optimisation of the Ti and Mo contents was proposed to reduce the dissolution rate of (Ti, Mo)(C, N) precipitates and grain growth.

The precipitates formed at lower deformation temperatures or during cooling provide precipitation strengthening. Interphase precipitation, when particles are densely dispersed at a similar distance from each other, is known to provide a strong strengthening effect. In [Contribution 3], a novel CrVNb micro-alloyed steel was proposed. The investigation of the thermomechanical processing technology in Gleeble suggested the optimum cooling parameters required to guarantee the maximum strengthening effect from interphase precipitation. Thus, simulated coiling at 650 °C and holding for 30 min resulted in the highest yield stress of 960 MPa, an ultimate tensile strength of 1100 MPa, and an elongation to failure of 25%. The finish rolling temperature was found to affect the ferrite grain size and the ratio of precipitates formed in austenite and ferrite. If a significant amount of solute is consumed for precipitation in austenite and during the subsequent growth of strain-induced precipitates, then a lower fraction of interphase and random precipitates forms in ferrite, resulting in a lower strength. An extended time to simulate coiling temperature resulted in the growth of interphase precipitates and the precipitation of random ones in ferrite, which are less effective in strengthening.

The development of the steel composition should be carried out simultaneously with the processing technology optimisation, as the best processing parameters vary with alloy composition. Thus, in [Contribution 4], three chemistry-processing combinations were studied: (1) 0.14C-1.77Mn-0.03Nb steel was heated at different rates for annealing after cold rolling to various strains—it was found that a maximum cold deformation of 75% and a moderate heating rate of 15 C/s provided the best combination of strength and ductility because of the fine grain size and high martensite volume fraction; (2) 0.14C-1.77Mn-0.03Nb steel with different hot rolled microstructures (ferrite/pearlite, ferrite/bainite, and fully martensitic) was cold rolled and annealed—both the ferrite–bainite and martensite–bainite microstructures for all cold strains resulted in annealed microstructures with necklace martensite morphology and finer ferrite grains compared to the ferrite–pearlite hot rolled material; and (3) 0.14C-1.3Mn-0.048Nb steel with an increased Nb and reduced Mn content exhibited finer hot rolled grains and reduced solid solution strengthening—after annealing with the standard parameters only the 45% cold-reduced material produced a finer ferrite grain size than the standard material, whereas the 60% and 75% cold-reduced samples required a higher heating rate to achieve finer ferrite grain sizes due to rapid recrystallisation and growth kinetics. In a two-phase microstructure, fine ferrite grain size and the random distribution of small second-phase islands provide the best combination of mechanical properties. Although mechanical properties were not presented in [Contribution 4], an increased Nb content would probably result in worse properties than an increased Mn concentration, due to a larger grain size range after annealing.

The hot-rolled or cold-rolled flat and section products frequently undergo further shaping during the manufacturing of welded structures, pipes, and automotive frames and bodies. Therefore, not only the rolled strength and toughness matter but also the work-hardening, i.e., the ability to further strengthen during cold forming without failure. In [Contribution 5], work-hardening was studied for an ultra-fine-grained steel containing 0.16C-0.21Si-0.45Mn, which was cold rolled or cryo-rolled and annealed in the temperature range of 400–600 °C. A relatively coarse ferrite phase and a larger number of fine intragranular cementite particles strongly contribute to work hardening. The intragranular cementite particles generate geometrically necessary dislocations around them, while the intergranular cementite particles result in a decreased dislocation accumulation in ferrite, impairing the strength of the grain boundaries. The cryo-rolling process substantially improved the tensile strength of the studied steel but concurrently deteriorated the work hardening behaviour.

In summary, it is worth noting that the cheapest way to improve properties would be a lean chemical composition, a carefully developed high-temperature deformation schedule to obtain a desired grain size at minimum strain, and controlled cooling to assure particle precipitation and the formation of secondary phases. Cooling rate, holding times, and temperatures will determine the particle size distributions and number density, as well as the volume fraction of secondary phases. Some of these modern approaches were studied in the papers published in this Special Issue.