Influence of Finish-Rolling Conditions on Microstructure and Mechanical Properties of Low-Alloy Mn-Ni-Cr-Mo Steel Grade

Abstract

A physical simulation of the thermomechanical processing of the Mn-Ni-Cr-Mo low-alloy steel was performed in the laboratory rolling mill Tandem in the Institute of Modelling and Control of Forming Processes at VŠB – Technical University of Ostrava. The task was to determine the influence of the finish rolling temperature on the structural and mechanical properties of the rolled products. After different modes of rolling and slow cooling in the furnace, the final structure of the tested samples was in all cases composed of ferrite, bainite and islands of martensite. The finish rolling temperature markedly influenced a part by volume of the individual phases as well as the structure homogeneity. The results of the tensile tests at room temperature indicated that the studied steel did not show any pronounced dependence of the yield stress on the finish rolling temperature in the investigated range of values (750 – 1000 °C). On the other hand, the closely corresponding dependences of the ultimate tensile stress and elongation exhibited a considerable and very complex course, which can be explained mainly by the martensite fraction originating during the last stage of the final air cooling from temperature 600 °C.

Archives of Metallurgy and Materials

“…Furthermore, the diagram constructed by calculation in the program QTSteel does not reflect the temperature drop at the beginning of the martensitic transformation with the decreasing cooling rate and the pink curve representing the area of martensite ends already at the cooling rate of 11.5°C• s -1 . Certain differences, mainly in the size of individual structural phases, between the CCT diagrams determined by dilatometry and those calculated mathematically are evident also in the works of the authors [16][17][18]. It is evident already from this comparison that the programs operating on the basis of universal computational models do not always provide a completely accurate information on the final structure of the heat treated steels.…”

“…The CCT diagrams are usually plotted using physical methods, and one of the most widely used methods is the dilatometric analysis [13][14][15]. Other possibilities for their plotting consist in the use of special computational computer programs, such as for example JMatPro software, EWI software or QTSteel software [16][17][18], the advantage of which lies in obtaining information on the structural condition of the investigated material within a few seconds after entering the chemical composition, conditions of austenitization and cooling rates.…”