A Novel Design Of Martensitic Stainless Steels With Ultrahigh Strength And Toughness

The problems of environmental pollution,energy and security about automotive industry are worse and worse,the key to solve the problems is to develop novel advanced high strength automobile steels.Q&P steels contain the advantages of TRIP,DP and martensitic steels as a new generation of high strength steels,the structures which have high strength and plasticity are only consisted of martensite and retained austenite at ambient temperature,but the production process of Q&P steels are low-efficiency as the water quenching is difficult to control accurate temperature among 200? and 300? about strip steels.Quenching and tempering&partitioning(Q-T&P)process can be a good solution to this problem by designing the chemical composition to reduce the Ms under 200?,the quenching temperature is reduced to the temperature which water quenching technology can control easily.Martensitic stainless steels not only have outstanding corrosion resistance,but also have high strength and plasticity by quenching and tempering&partitioning process,the steels with thes excellent properties can be well applied to the high strength automobile steels.This paper has designed novel martensitic stainless steels with lower martensite transformation temperature according to the Q-T&P process in order to get enough retained austenite when quenched to room temperature.The influence of solution treatment temperature on the strength and plasticity through Q-T&P process of the new designed martensitic stainless steels is to be studied and try to find out the best heat treatment process to get ultrahigh strength and plasticity by observation of micro structure and test of mechanical properties.The research processes are as follows:(1)Design chemical composition of ix?vel martensitic stainless steels:the Cr13 martensitic stainless steels with different carbon content are chosen,appropriate contents of Si and Mn are added to reduce the martensite transformation temperature to 200?,except for these functions,Si element can inhibit the precipitation of cementite in the process of tenpering,Mn element increases the hardenability of steels and has an impact on the stability of retained austenite;Two kinds of martensitic stainless steels of 2Cr13-Sil.5Mn3 and 3Cr13-Si1.5Mn2 have been designed according to Ms theory formula and K-M equation(2)Development of properties:the influences of different solution and tempering treatment tenperatures on the strength and plasticity of the new designed martensitic stainless steels through Q-T&P process are to be studied to achieve super-high strength and plasticity,the optimum heating treatment process has to be ensured by these measurements;The steel of 2Crl3-Sil.5Mn3 has gotten yield strength of 1100MPa,tensile strength of 1600MPa and elongation of 15%after solution treating at 1100? for 5min then quenched to ambient temperature and tempering at 300? for lOmin as followed;The steel of 3Cr13-Sil.5Mn2 has gotten yield strength of 1400MPa,tensile strength of 1880MPa and elongation of 11%after solution treating at 1050? for 30min then quenched to ambient temperature and tempering at 250? for 60min as followed.(3)Research of mechanism:to understand the difference on the properties of material dealt with different Q-T&P process by synthetic analysis combined with the micro structure,phase and mechanical properties and realize the mechanism of super-high strength and plasticity of novel martensitic stainless steels.The chromium carbides at grain boundary which dissolved incompletely can inhibit grain grew up and reduce the volume fraction of twin martensite to improve the plasticity,but these carbides also lower the impact toughness of the material.