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Tensile tests were conducted in order to study the effects of nitrogen on the mechanical properties of the 0.2C–1.5Mn–1.5Si–0.04Al–(0.003–0.015)N steels often annealing at 800–830°C, followed by the austempering at 400–450°C. The results show that both tensile strength and elongation increase and the balance of tensile strength×elongation are improved upon nitrogen addition to the TRIP steel. It was found that the density of AlN precipitates increases with addition of nitrogen. The volume fraction of retained austenite increases due to AlN precipitation, because the precipitates retard the transformation of austenite during cooling and austempering. As a larger volume fraction of austenite remained after annealing has been transformed to martensite during deformation, elongation as well as strength has increased in the nitrogen added steel. It is also observed that the average grain size of ferrite and bainite decreases because of AlN precipitation that hinders grain growth. The refinement of ferrite and bainite by AlN precipitates also contributes to the increase in the strength of nitrogen added steels. more...

Tensile and dilatation tests were conducted in order to study the effects of Mn, Si and Nb on the transformation behavior in the 0.14C-(2.1∼2.4)Mn-(0.5∼1.0)Si-(0.00∼0.02)Nb TRIP steels. All the specimens for dilation tests were annealed under the condition to form the same austenite fraction, followed by cooling to the austempering temperature. The volume fraction of bainite transformed during austempering, ΔV B , and the martensite start temperature, M s , were measured. ΔV B was dependent on the austempering temperature and a minimum in ΔV B was obtained at 450°C for all the examined steels. The M s temperature was linearly decreased as ΔV B was increased. The decreasing rate of M s to ΔV B was not dependent on the steel composition, but on the austempering temperature, exhibiting the maximum value when the austempering temperature was 450°C. It was also observed that volume fraction of retained austenite was linearly increased with the decrease of M s for the steels austempered at various conditions. The highest elongation was obtained when the rate of M s to ΔV B was the maximum. M s was increased when the amount of Mn was increased from 2.1 % to 2.4 %. It was because Mn retarded the ferrite transformation rate, so that the soluble C diffused from the ferrite to the austenite was decreased. The addition of Si decreased M s because it activated the diffusion of C to austenite during the cooling to the austempering temperature. The addition of Nb was observed to give little influence on ΔV B and the M s . more...

The effect of sheet thickness on shear deformation and texture for the ferrite rolling has been studied. The shear deformation was estimated from the distorted shape of inserted wire before rolling and FEM simulation. The deformed shape of inserted wires and FEM results showed that the shear strain decreased with increasing initial sheet thickness. The measured and simulated textures also showed that the Goss component changed to Dillamore component as the thickness increased. From rolling simulation of 20 % reduction, the minimum shear deformation was found to exist for the given roll bite geometry and friction coefficient. more...

Thermodynamic and kinetic modelling was performed to describe the decomposition of austenite and the precipitation of alloy carbides in Nb microalloyed steels. Phase analysis of the Fe-C-Mn and the Fe-C-Nb systems was carried out considering local equilibrium and paraequilibrium, their nucleation behavior (matrix/grain boundary) and the shear energy during transformation. From the analysis, equilibrium transformation temperatures of each phase, the Gibbs energy changes for nucleation and compositions of elements at the phase boundary were derived. The transformation kinetics were formulated using classical nucleation and growth theories. Both local equilibrium and para-equilibrium conditions were considered to evaluate the partitioning of Mn during growth. The calculated TTT, CCT diagrams and the volume fraction of each phase were compared with the experimental results, and the most probable behaviors of γ/α transformation and NbC precipitation in Nb steels were discussed. more...

Effects of the service conditions on the roll surface deterioration in the front finishing stands of hot strip mill were analysed by observing the roll surface after each roll change and by calculating the roll surface temperature and the heat penetration depth. The roll surface temperature was varied with the rolling conditions and the steel grades rolled. The heat penetration depth was greatest at the F1 stand, and was gradually decreased in the latter stands. The roll surface deterioration was greatest at the F2 stand, which was thought to be due to the severe thermal fatigue of the F2 rolls. A parameter representing the thermal fatigue damage was developed by considering the effects fo roll surface temperature and heat penetration depth on the degradation of the roll surface. It was found that the parameter had a good correlation with the degree of the observed roll surface breakdown. more...