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Hot forming of boron steel has become increasingly popular due to the demand of both weight reduction of body parts and enhanced safety for passengers. The most commonly used boron steel grade for hot forming is 22MnB5, with an ultimate tensile strength of ~1,500MPa and quite low total elongation of ~6% in the martensitic state. Recent research mainly focused on improving the ductility of hot formed steels by either introducing a post-hot forming tempering treatment for 22MnB5 or developing an alternative class of hot formed steels such as a medium Mn steel. However, the dimensional variability of the cold-stamped TRIP parts is too high due to the high instantaneous plastic modulus at all strains; whereas hot forming provides a new opportunity for these types of steel grades. This present work will focus on the microstructure characteristics and mechanical behavior of the warm-rolled 0.17C-6.5Mn-1.1Al-0.22Mo-0.05Nb steel, and compared with the commercial hot formed 22MnB5 steel and the conventional multi-stage thermo-mechanical process including hot rolling, cold rolling and intercritical annealing. It can be concluded that the microstructure and mechanical characteristics strongly depended on the warm rolling temperature. The best combination of UTS and TE (~32.0GPa%) was achieved in the 660°C-warm rolled specimen, which is much higher than 12.3GPa% for the hot formed 22MnB5 steel, and is also comparable to 38.3GPa% for the annealed CR specimen.

Grain size and texture are important factors which affects the coating properties of hot-dip galvanized steel. In this study, different skin passing parameters are controlled to achieve the optimal control of grain size distribution and texture of zinc coating. Specimens were prepared from industry CGL and its coating grain size and texture were evaluated using metallography microscope, SEM and EBSD. The results show that uniform and fine grain size can be achieved by controlled skin-passing. Also, twinning of zinc which normally considered harmful for coating during stamping can be dramatically decreased. Furthermore, the EBSD analysis show that by controlled skin-passing, basal texture component ratio increased distinguishingly which is believed beneficial for stamping. Thus, the further potential benefits for curbing stamping defects of hot-dip galvanized steel are also intriguing.

Electron backscatter diffraction (EBSD) was used to study the microstructure and formation mechanism of tailor welded sheet of dissimilar steel. The microstructure of weld zone mainly was matensite, the microstructure of heat affected zone (HAZ) were matensite, bainite and ferrite. The analysis of EBSD showed that the microstructure of weld zone was coarser and it had obvious texture. The microstructure in HAZ consisted of equiaxed grains principally. And there was no obvious preferred orientation. A lot of grain boundary and dislocation improved hardness and reduced plasticity for weld and HAZ. The grains of two kind steels had no significant preferred orientation. A large number of high-angle boundaries made the base metal show good strength and plasticity.

The morphologies and crystal orientations of the shear bands in cold-rolled ultra-low carbon steel were characterized by using TEM and EBSD technique. One important finding is that some shear bands develop in α orientated grains in addition to γ and rotated Goss orientated grains. Shear bands originate at grain boundaries and spread to ingrains. The shear bands in α and γ grains have {111} <112> and (110) [001] orientation, respectively, which conform to the preferential orientation of the recrystallization grains. It is believed that at the margin and in the interior of the shear bands in α grains are ideal recrystallization nucleation sites because there are large orientation gradient, strain gradient and lattice curvature in these regions. The present investigation provides an experiment evidence for clarifying the nucleation and crystal orientation of recrystallization grains during annealing.

Shot peening is a mechanical surface modification technology, which can extend the fatigue life of materials by introducing work hardening, compressive stress, and/or some additional microstructural change in surface layer resulting in a restraint of crack initiation and propagation on the surface. In this study, SUS304, which has high formability and corrosion resistance, was shot peened and fatigued for the determination of their effect on the evolution of microstructures. The fatigue of the specimens were carried out at three different cycles, followed by second shot peening and finish fatigue of 10⁶ cycles. The microstructures of the specimens were investigated using OM, EBSD, SED and EDS. The resulting mechanical property such as microhardness and residual stress was also investigated. Deformed layer of ~100μm and mechanical twins were observed after fatigue and shot peening test. The top surface layer of shot peened specimen showed the highest twin density and microhardness. The increase of the fatigue before shot peening caused increase and deepening of the compressive residual stress. However, the finish fatigue of 10⁶ cycles decreased overall compressive residual stress.