THERMOMECHANICAL CONTROLLED PROCESSING OF (Nb+V)-MICROALLOYED STEEL: MICROSTRUCTURE AND MECHANICAL PROPERTIES

Abstract

Low-carbon microalloyed (MA) steels have been expanded in many strategic applications, i.e., ship-building, pipeline, automotive, construction, bridges, etc. Currently, industries are trying to reduce the cost of raw materials and production without compromising its quality. Thus, a new scientific approach should be explored to optimize the processing parameters to achieve highstrength steels through hot-rolling, coiling, and microstructural modification techniques. Therefore, the present study aims to optimize the processing parameters and develop a new methodology to improve the mechanical properties of low-carbon Nb+V stabilized MA steel. In this work, strain-rate dependent workability and flow instability have been investigated. Uniaxial compression tests have been conducted in the temperature domain of 700-1100 °C at several strain-rates (0.01 to 10 s-1). The results show that a good sample processing window arises at medium strain-rates (0.1 to 1 s-1), whereas flow instability occurs at extreme (10 s-1 and 0.01 s-1) strain-rate. The reasons for flow instability are the formation of micro-cracks or void nucleation at 0.01 s-1, whereas flow localization and shear banding at 10 s-1. In both cases (0.01 and 10 s-1), the serration arises at periodic intervals with negative strain-rate sensitivity. At a higher strain-rate (10 s-1), the flow instability dominates till 1100 °C because of relieving the deformation-induced stored energy mainly by dynamic recovery (DRV). At lower deformation temperatures (i.e., 700-800 °C), fine ferrite grains nucleate around shear bands by the diffusional transformation of austenite. The dynamic recrystallization (DRX) is either absent or incomplete in agreement with the experimental determination of Tnr (non-recrystallization temperature) and texture analysis. The overall flow instability characteristics substantiate well with the dynamic materials model (DMM). Furthermore, the constitutive equations have been developed using activation energy (Q) and various material constants to anticipate the impact of strain-rate and deformation temperature on flow stress in α+γ and γ phases separately. The Q has been estimated to be 367.3 kJ/mol and 411.5 kJ/mol for α+γ and γ phases, respectively, with a respective stress exponent (n) value of 14.2 and 5.9. The model’s flow stress is correlated with the experimental value for both α+γ and γ phases regions with a worthy fitting value (R) of 0.988 and 0.969, respectively. A detailed analysis of Q, n, and σ for DRV and DRX using the constitutive models suggests that the hot deformation processes have been primarily governed by dislocation climb and glide mechanisms. Further, a novel thermomechanical controlled processing (TMCP) has been designed to reduce the gauge thickness of automotive body sheets strategically. The sample processing results in an ultimate tensile strength of 1059 MPa with 21 % ductility. The critical factor enabling superior strength-ductility is grain refinement by DRX and deformation-induced ferrite transformation (DIFT). The DRX contributes to the uniform distribution of refined ferrite grains with sizes within 2 μm at the intercritical temperature domain (800-700 °C). In contrast, the DIFT occurs earlier, just above the upper intercritical temperature, but contributes less to the grain refinement. The microstructural transformation through DRX has been discussed in detail, along with a thermodynamic analysis of the driving force for DIFT. The role of hot-rolling followed by coiling temperatures on microstructural formation, precipitation kinetics, and its mechanical properties have been further investigated. The sample has been subjected to 50 % thickness reduction by hot-rolling at 1100 °C and then coiled for 1 h at 650-500 °C. The microstructure reveals a duplex/triplex phase mixture depending on the coiling temperature, affecting its mechanical properties. For the coiling temperature of 650-550 °C, the pro-eutectoid ferrite+pearlite phase mixture is evolved, resulting in an adequate ductility of ~28 % with a tensile strength of ~650 MPa. The strength-ductility combination is not altered much at 500 °C (i.e., near the bainitic transformation temperature domain), though a trace quantity (~5 %) of Widmanstatten ferrite (WF) or degenerate pearlite (DP) is nucleated along with the pro-eutectoid ferrite+pearlite microstructure. The lower volume fraction of niobium and vanadium-rich carbides (~0.5 %) with fcc crystal structure, which are primarily incoherent with the ferrite matrix phase, offer a substantial amount of precipitation strengthening (32-40 MPa) as compared to the grain refinement strengthening (126-156 MPa) and dislocation strengthening (115-150 MPa). Furthermore, different martensitic structures have been developed by hot-rolling followed by quenching and tempering at various temperatures. Hot-rolling+quenching (HRQ) results in an ultimate tensile strength of 1404 MPa along with 14.4 % ductility, the highest strength-ductility combination attained till now in this steel grade. The superior strength is attributed to the lath martensite supersaturated with carbon atoms (by diffusionless transformation), dislocation (due to shape accommodation), and spherical NbC precipitates. However, the strength is mainly governed by dislocation (653 MPa) and grain-size (277 MPa) strengthening. During tempering, the variation in lath morphology is observed due to carbon segregation into favorable defect sites (i.e., grain/lath boundaries and dislocations) to nucleate cementite and by dislocation annihilation. The cementite’s shape changes from vermicular to spherical (to reduce interfacial energy) by spheroidization and element enriching at higher tempering temperatures (500-600 °C). At 300-400 °C tempering temperatures, ductility decreases (13.2 %) along with tensile strength (1135 MPa), which is attributed to temper embrittlement (due to de-cohesion at cementite/matrix interfaces). However, at 500-600 °C, the strength decreases drastically (1003 MPa), but the ductility and impact-toughness increase to 18-18.6 % and 41±1 J due to the fracture resistance provided by spherical carbides. The crystallography analysis depicts that the V7-V12 and V19-V24 variants are found to be thermally stable; however, the frequency decreases with increasing tempering temperatures. Furthermore, variants of low-interfacial energy pairs, such as V1/V2 and V1/V3, do not substantially reduce during tempering, contributing to the strengthductility combination during tempering processes. In addition to the static properties, high-cycle fatigue (HCF) behavior (i.e., dynamic properties) has been investigated for the HRQ and T500 samples compared to the AR material. For the HCF test, samples were loaded axially under the stress-controlled mode with a stress ratio (R) of -1 at a sinusoidal-type loading frequency of 20 Hz. The HCF testing of all the specimens was conducted at room temperature (25±5 °C), and the tests were continued till 2×106 cycles or the final fracture, whichever occurred first. The stress level at which the loaded sample survived 2×106 cycles was considered run-out, and stress amplitude is defined as the endurance limit for the applied stress ratio (R). The results indicate that the endurance limit is the highest for the HRQ sample, which can be correlated with the YS value. The fractographs can be divided into three different zones: (i) crack initiation region, (ii) crack propagation region, and (iii) fast fracture region. The particle-induced cracking (due to MnS inclusions) in the crack initiation region is observed for all the HCF-tested samples. The formation of striation, secondary cracks, and tear ridges have been observed in the crack propagation region. The striation width is the lowest (0.6 μm) for the HRQ sample, signifying the better crack resistance/hindrance properties. Furthermore, the dimples formation occurs in the fast fracture region. The effect of unlubricated dry-sliding wear behavior of the as-received (AR), HRQ, and tempered at 500 °C (T500) samples has been examined for disc brake application. Effects of normal load on specific wear-rate (SWR), worn-surface, and sub-surface characteristics have been discussed, enlightening relevant deformation mechanisms. The SWR is the lowest (2.91×10-5 mm3m-1N-1) for the HRQ sample, primarily due to its better hardness, strength, and toughness combination. SEM investigation of worn surfaces confirmed that abrasion (i.e., micro-cutting and ploughing) and oxidative wear mechanisms are the dominant phenomena in all the samples. The sub-surface analysis indicates the severely deformed region and bending of martensite is only 1-2 μm and 7 μm for the HRQ sample, whereas it (bending) increases to 3-5 μm and 15 μm, respectively, in the T500 specimen.