Thermo-mechanical Controlling Process Study On Submicron Refinement Of Low Carbon Steel Grain

Steel is still the dominant structure material in the21century, and it does not exist a material can fully instead of iron and steel. It requires a major increase the obdurability of steel in the modern steel industry for the economic construction and social development, and the high value-added product development is more and more important. Because fine-grain strengthening means to be able to ensure that materials can enhance strength and not reduce ductility, and it is concerned in a new generation of iron and steel material in the development process of research workers.In this paper, the low carbon steel (of low carbon-steel+microalloying steel) as the object is studied about different heating process and original organization and rolling the cooling process, and study the method to achieve the ultimate generation organizations are also grain and sub-micron process, and analysis the high temperature deformation behavior and tissue in the fast heating and short supply heat preservation of the low carbon steel, this paper discusses the different technological factors on super fine grain austenite thermal deformation behavior and alpha?gamma phase change influence; In addition, trying to roll and product sub-micron steel in the laboratory, and the research has achieved the following results:(1) The effects of different heating speed, holding time and rolling temperature on the microstructure of low carbon steel and micro-alloy steel were studied. The results show that the grain size of the low carbon steel is about4μm, when the heating rate is50℃/s, holding temperature is1050℃, holding time is Is, and the deformation is800℃; the grain size of sub-micron microstructure of micro-Ti-TRIP is about0.4μm, when the heating rate is100℃/s, holding temperature is900℃, holing time is Is, and the deformation is650～700℃.(2) The rolling process of fast heating-short-term heat preservation-large deformation near Ar3temperature to refine the final grain size was studied. The results show that the microstructure can be affected by many factors, including original organization before heating, heating speed, heating temperature, holding time, deformation temperature and cooling process, and it is a coupling process for rolling process.(3) The effect of processing parameters on grain size in rapid heating rolling process conditions by thermal simulation test about influencing was studied. The experimental results show that the original organization, heating speed, holding time and heating temperature and deformation temperature and cooling process control is essential, and ferritic+pearlite with400℃temperature rolling among them is most favourable; Ti steel small degree with V steel about microstructure on the same process, TRIP steels performance better than the Nb-V-Ti steel, alloy steels performance better than carbon steel. Heating the faster the speed, the shorter the holding time, heating temperature is lower, and the ultimately grain size is extenuation, and the control of cooling process can phase strenthing.(4) The stress and strain curve through the quick heat to get sub-micron crystal low carbon steel and the continuous cooling transformation curves and the continuous heating transformation curves were studied. In the continuous heating phase change in interval:steel with the increase of the heating rate, phase transformation temperature, Ac1and Ac3difference in temperature between the heating up with the speed of shrinking; Ti element help to accelerate the ferrite austenitic phase transition occurred than V elements; holding some time after warm rolling delay ferritic-austenitic phase change, accelerate the occurrence the complete of ferrite and austenitic phase change. In the continuous cooling phase change of heat insulation time interval:extended delays austenite ferrite phase change happene and complete; Ti element help to accelerate austenite ferrite phase change happen than V elements, and V element is good for austenitic-ferritic phase transition complete; holding some time after warm rolling delay austenite ferrite phase change happened and complete.(5) Produce some high strength steel in laboratory, using the technology of rapid heating in temperature rolling process, and the microstrcture grain size of samples with0.6～0.8μm for Nb-V-Ti alloy steel and0.6μm for Ti microalloyingsteel were obtained. Both tensile strength can be about1000MPa even above, Nb-V-Ti steel yield ratio is higher, the TRIP steel quenching after rolling have high strength, the elongation of sample of air cooling after rolling is better, comprehensive analysis that the best rolling process is heat to930℃and heat rolling at750℃, the TRIP steel which is air cold after rolling can get better mechanics than sample quenching after rolling, consideration the elongation rate of sample, we can improve material strong plastic product by obtain a lot of bainite and austenite through bainite isothermalchange.