THERMOMECHANICAL SIMULATION KINETICS TO DEVELOP HIGH STRENGTH IF AND MICROALLOYED STEELS

Abstract

The use of low carbon (C) microalloyed steels has been expanded in many strategic applications such as ship building, line pipe, building construction, bridges, storage tanks, pressure vessels etc. On the other hand, interstitial free (IF) steels are mainly used in automobile industry due to their superior formability. Recently, for potential weight reduction, the IF steels are also being intended to use for various structural parts viz. longitudinal beams, cross members, B-pillars etc. Throughout the world, continual strategy is to reduce the cost of raw materials and production without compromising its quality and to enhance the performance of the existing components. Thus, new scientific approach should be explored to optimize the processing parameters to achieve high strength low C microalloyed and IF steels through microstructural modification. Therefore, aim of the present study is to develop new methodology to improve mechanical properties of IF and microalloyed steels through design innovative thermomechanical control processing (TMCP) and investigate various mechanical properties, especially, tensile strength, fracture toughness and fatigue strength of both the steels. In the present study, first the hot/warm working behaviour of a Nb+Ti stabilized low C microalloyed and a Nb+Ti stabilized IF steels have been studied through physical simulation using a thermomechanical simulator (Gleeble 3800). The hot deformation simulation study has been conducted in the temperature range of 650-1100°C after austenitization at 1200°C for 2 min. The strain rates used in the study was in the range of 0.001-10s-1 for a total true strain of 0.7. The safe processing domains are determined using dynamic and modified dynamic materials models (MDMM) and α-parameter (related to workability map). Among the different models, the safe workable regions predicted by MDMM along with Poletti instability criteria are found to be the best suited regions to produce defect free structure. The constitutive equations have been developed to predict the flow stress of the steels precisely during isothermal hot compression tests and established the relationship between the total strain, strain rate, temperature and flow stress during the hot working. The predicted flow stress behaviour has been verified with the experimental results of the hot compression test data. Secondly, influence of cooling rates (low to ultrafast) on diffusion controlled and displacive transformation of the Nb+Ti stabilized IF and microalloyed steels has been thoroughly investigated with an objective to develop ferritic-bainitic microstructures with non-equiaxed morphology. The continuous cooling transformation behaviour has been studied in a thermomechanical simulator (Gleeble 3800) using cooling rates in the range of 1-150°C/s. On the basis of the dilatometry analysis of the samples at each cooling rate, continuous cooling transformation (CCT) diagrams have been constructed for both the steels and correlated with microstructural features at each cooling rate in different critical zones. Transformation mechanisms iv and kinetics of austenitic phase transformation to different phase morphologies at various cooling rates have been analysed in detail to correlate the microstructural evolution and mechanical properties. At very high cooling rates (120-150ºC/s), the austenite of the IF steel transforms into granular bainite in the massive ferrite laths. This typical multi-phase microstructure has exhibited an attractive combination of hardness (143HV) and impact toughness (157J), which is extremely vital for the dent resistance automobile applications. In case of the microlloyed steel, a mixture of lath bainite and lath martensitic structure is found to form at high cooling rate of ~80°C/s. This complex structure is highly attractive for ship-building applications due to its high hardness (325HV) and high impact toughness (174J). On the basis of the simulated data (i.e. safe workable zones, Ar3, Ar1 and Tnr), single and multi-phase control hot rolling and critical phase control multiaxial forging (MAF) have been conducted with the aims to obtain ultrahigh strength microalloyed/IF steels with significant amount of formability, fracture toughness and fatigue strength. Single phase control rolling was carried out in 3 selected critical phase regimes i.e. γ-recrystallization region at ~1050°C, γ-nonrecrystallization region at ~750°C and (α+γ) region at ~700°C. The best combination of yield strength (YS) and ductility of the quenched microalloyed steel specimen (YS=811MPa, %El=19) rolled in (α+γ) region is attributed to the dynamic recrystallization of ferrite grains to develop dual size ferritic grains (~2μm and ~30μm) along with martensite, high misorientation angle of matrix, precipitation of nanosize NbC (~10nm). While, a good combination of YS, ductility and impact toughness (YS=520MPa, %El=27, toughness=232J) of the forced air cooled samples of the same steel deformed at 1050°C is endorsed to the high fraction of acicular ferrite (76%), formation of degenerate pearlite and precipitation of nanosize TiC (~10nm). Similar kind of rolling schedule has been conducted for the IF steel specimens. In this case, γ-recrystallization region control rolling has been carried out at same temperature (i.e. ~1050°C). But other 2 critical regions have been selected at ~850°C (i.e. γ→α transition region) and 650°C (i.e. pure α-region) as per the critical temperatures. A bimodal equiaxed ferrite structure (fine ferrite of ~5μm size embedded with relatively larger size ferritic grains of 32μm) is obtained through the strain induced phase transformation of unstable γ during rolling at 850°C followed by air cooling to room temperature. In case of the rolling at α-region, improvement of the YS (>3-fold) is attributed to the formation of ultrafine ferrite grains of 1–3μm through dynamic recrystallization, strain-induced precipitation of nanosize NbC/TiC and micro-shear bands formation. Very short-annealing of the α-region rolled sample (~100s at 850°C) followed by forced air cooling is found to improve the formability without much affecting its YS. The avoidance of FeTiP phase formation (which deteriorates to form {111} recrystallization texture) and nucleation of ferrite grains within the v deformation bands (studied through EBSD) by the short-annealing treatment are accountable for regaining the formability. Finally, 3-steps multi-phase deformation schedules (schedule-I and II) have been designed in the safe workable regions to produce defect free ultrafine grained microstructure. In case of the innovative 3-steps multi-phase control rolling, the homogeneous ultrafine ferrite (UFF) grain structure (grain sizes in the range of 0.69-0.78μm) is obtained for the microalloyed steel samples as per schedule-I and II; whereas, the IF steel samples showed the maximum refinement of ferritic grains in the range of 0.83-0.88μm under similar processing conditions as per the schedule-I and II. Deformation induced transformed ferrite enhanced the subsequent dynamic recrystallization (DRX) (confirmed by EBSD) leading to the development of such UFF structure. It should be noted that, in single pass, extremely high deformation strain has not been imposed in the newly design TMCP routes (i.e. schedule-I and II). The best combination of the YS and ductility is achieved for the microalloyed (923MPa, 13.5% elongation) and IF steels (623MPa, 19% elongation) samples as per schedule-I. The improvement of the mechanical properties is ascribed to the development homogeneously distributed submicron size ferritic+martensitic structure in the microalloyed steel and submicron size ferritic grains in the IF steel. Thus, the present study could bring new excitement to the industrial applications. Moreover, single phase control multiaxial forging (MAF) was performed up to the maximum cumulative strain of 18 (0.4×3×15 cycles) for the microalloyed steel and 21.6 (for 18 cycles) for the IF steel samples, respectively, in 3 phase regimes (γ-region, γ→α transition region and pure α-region) separately. More than 4-fold increase in the YS (1027MPa) of the microalloyed steel specimen of after MAF (for 15 cycles at 650°C) as compared to that of the starting sample (251MPa) is ascribed to the development of submicron size (~280nm) ferrite grains and formation of nanosize (35nm) fragmented cementite (precipitated through repeated heating cycles) within the submicron size matrix grains. Similarly, the 18 cycles multiaxially forged (MAFed) IF steel sample (at pure ferritic region at 650°C) shows the highest value of YS (855MPa) with a total elongation of 11% as compared to the other two regions. The formation of submicron size ferrite grains in both the microalloyed (~280nm) and IF (320nm) steel samples by MAF is attributed to the mechanisms of formation of micro-shear bands in multiple directions, repeated change in the accumulated strain paths to promote dislocation activity, increase in the misorientation angle of grain boundaries and finally strain induced ferritic transformation with extensive recovery (confirmed by EBSD and TEM study). The theoretical YS has been estimated through analysis of different strengthening mechanisms and found to substantiate highly with the experimentally obtained results. vi Furthermore, fracture toughness analysis was carried out through determining apparent fracture toughness (KQ), equivalent energy fracture toughness (Kee) and J-integral values using 3-point bend test for some selected rolled and forged specimens. The best combination of the YS (1027MPa), ductility (8.3%) and fracture toughness (90MPa√m as per Kee) is achieved for the 15 cycles MAFed microalloyed steel samples. The complex microstructure of the sample is found to play the vital role to yield such high YS and fracture toughness with significant amount of ductility. In case of the IF steel, the 18 cycles MAFed sample shows the best combination of the YS (881MPa), ductility (11.2%) and fracture toughness (97MPa√m as per Kee). This is attributed to the formation of submicron size ferrite grains (~320nm) and high density of dislocation substructures with uniformly distributed nanosize precipitates. Finally, high cycle fatigue behaviour has been investigated for the selected rolled and forged samples which showed better combination of YS, ductility and fracture toughness. The (α+γ) region control MAFed (15 cycles) microalloyed steel specimen has revealed the highest fatigue strength survived for more than 10,53,972 cycles at a stress amplitude of 355MPa; whereas, the α-region control MAFed (18 cycles) samples could run for more than 9,13,659 cycles at a stress amplitude of 275MPa. The fatigue crack propagation is possibly blocked effectively by the fragmented nanosize Fe3C particles (35nm) and NbC/TiC (10nm) precipitates uniformly distributed within the submicron size ferritic microstructure in the MAFed microalloyed steel samples. And, the submicron size ferritic structure (320nm) along with uniformly distributed TiC/NbC phases is accountable for the high fatigue properties of the IF steel samples. Thus, the fatigue and fracture toughness behavior of both the ultrafine/submicron size grained microalloyed and IF steel specimens have been found to correlate well with the static mechanical properties (YS and ductility) of the corresponding sample. Therefore, the selected innovative deformation schedules could be effectively useful to produce high strength microalloyed and IF steels having good combination of ductility, fracture toughness and fatigue strength.