| With the continuous improvement of people's living standards and the increasing awareness of environmental protection,the serious shortage of nickel resources and environmental problems in stainless steel hinder the development of industry.Therefore,many stainless steel factories have begun to accelerate the production of stainless steel with low or no nickel.Ferritic stainless steel is an iron-based alloy with chromium content more than 10.5%,which is nickel saving stainless steel.Compared with austenitic stainless steel,ferritic stainless steel has higher thermal conductivity,lower expansion coefficient and excellent stress corrosion resistance.In addition,ferritic stainless steel is magnetic,low cost and stable price.However,the mechanical properties of ferritic stainless steel are poor,such as tensile strength and strong plasticity,which are far lower than that of austenitic stainless steel..In this study,the 430 ferritic stainless steel by intrducing the quenching-partitioning process(Q&P),and the effects of partitioning temperature and time and deformation temperature on the microstructure and mechanical properties of ferritic stainless steel by Q&P,especially the austenite and its mechanical stability are studied.The main findings are as follows:(1)The microstructure evolution of ferritic stainless steel during different process are studied.It is shown that the microstructure after hot rolling consists of martensite and ferrite;after annealing it is composed of ferrite and precipitate M23C6;after cold rolling,the microstructure is characterized by elongated grains,with a uniform distribution of precipitates has been observed;after Q&P treatment,the multiphase microstructure composed of polygonal ferrite,martensite and some amount of retained austenite is obtained,and the retained austenite is mainly in a thin film form.(2)The effect of partitioning temperature on the microstructure and mechanical properties of ferritic stainless steel by Q&P is studied.As the partitioning temperature rises,the content of retained austenite and the carbon content,grain size and in-grain defects such as stacking fault of retained austenite gradually increases,the average local misorientation and hardness of the surrounding phase of retained austenite(martensite)decreases,and the mechanical stability of retained austenite increases.It shows that the mechanical stability is mainly affected by the carbon content,and the grain size and surroundings have little effect on it.Therefore,the tensile strength gradually decrease,the elongation gradually increase and the change of work hardening exponent during tensile deformation gradually decrease with the increase of partitioning temperature.(3)The effect of partitioning time on the microstructure and mechanical properties of ferritic steel by Q&P is studied.As the increase of the partition time,the content of retained austenite first increases and then decreases slightly,the carbon content gradually decreases but the change is not large,the grain size increases first and then decreases slightly,and the hardness of the surrounding phase of retained austenite(martensite)decreases.At the same time,the mechanical stability of retained austenite continues to increase.At different partitioning times,the surrounding phase of retained austenite is a key factor affecting its stability.It is precisely because of the changes in the above-mentioned microstructure and mechanical stability of retained austenite that as the partitioning time increases,the tensile strength gradually decreases,the elongation gradually increases and the change of work hardening exponent during tensile deformation gradually decrease with the increase of partitioning temperature.(4)The effect of deformation temperature on the microstructure and mechanical properties of ferritic stainless steel by Q&P is studied.As the deformation temperature increases,the mechanical stability of retained austenite increases firstly and then decreases.When the deformation temperature is 300?,the mechanical stability of retained austenite is the highest,and TRIP effect is not easy to occur.At the deformation temperature below 300?,increasing deformation temperature is beneficial to increasing the stacking fault energy of retained austenite and weakening stress concentration,thereby decreasing the tendency for the occurence of strain-induced martensite transformation and eventually increasing the mechanical stability of the retained austenite.At the deformation temperature above 300?,increasing the deformation temperature is beneficial to promoting the precipitation of carbides in martensite,the decomposition of retained austenite and thereby decreasing the stability.At deformation temperature above 200?,PLC effect occurs during tensile deformation.With the increase of the deformation temperature,the uniform elongation and the elongation after fracture decrease firstly and then increase and the tensile strength decreases firstly and then increases to about 795MPa.Wihch can be explained by the combination of TRIP effect and PLC effect... |
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