Effect Of Alloying Elements And Technology On Microstructure And Properties Of 980MPa Grade Q&P Steel

With the continuous development of the automotive industry,in addition to the increasingly high requirements on the automobile safety performance,automobile lightening has gradually become the focus and consensus of the industry.Under the industrial background,the emergence and development of advanced high-strength steel represented by Quench-Distribution?Q&P?steel have become an effective method for realizing lightweighting of the vehicle body,which has attracted widespread attention from a large number of experts and scholars at home and abroad.The structure which has a good combination of strength and toughness,and broad application prospects shows that martensite is used as the matrix and is accompanied by the presence of retained austenite after treated by the Q&P process.Although there have been many researches on Q&P steel at home and abroad,the research on the mechanism of organization and control is still incomplete,and there are still some problems to be solved,such as the influence of alloying elements,the optimization of annealing process,and the stability of retained austenite and so on.The main content of this study is low-carbon Si-Mn 980MPa grade Q&P steel,based on which,four different alloy compositions of Q&P steels are designed by adding microalloy strenghtening element Nb and changing the contents of Si and Mn.The influence of steel composition and annealing process on the microstructure and the stability of retained austenite content were investigated through the experiments of continuous cooling transformation,hot rolling in the middle stage and annealing in cold rolling.The test methods used during the experiment include metallographic analysis?OM?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,X-ray diffraction?XRD?,and mechanical property testing?static stretching?.After the study of this topic,the main conclusions obtained are as follows:?1?The addition of Mn increases the austenitic phase area of the test steel,decreases the temperature of the Ac₁,Ac₃ points and the M_s,M_f points,and reduces the critical transition rate of the steel.The addition of Si increases the island-like martensite or the massive martensite of the tested steel.With the increase of Si and Mn contents,the area of ferrite and bainite reduces,the critical transformation cooling rate of martensite decreases,and the pearlite transformation is inhibited.When the Mn content increases to 2.8%,no pearlite transformation can be observed in the microstructure.The addition of Nb refines the microstructure of the tested steel,makes the distribution of the microstructure more uniform,and significantly improves the stability of the test steel.?2?Under the same Q&P process that annealing at 850°C,quenching at 250°C and dividing at 280°C,No.2 test steel with Nb element added and No.3 test steel with high Si content have good combination of strength and plastic,and the product of strength and plasticity can reach 17GPa%,of which the strength of No.2 test steel containing Nb reaches 980MPa level,and the strength of high Si test steel reaches1070MPa level.The experimental results show that the increase of Si content and the addition of Nb element are beneficial to improve the mechanical properties of Q&P steel.?3?In this experiment,for Nb-containing No.2 test steel,when quenched at230-250°C and assigned at 250-320°C,the tensile strength can reach 980MPa or more,and the maximum plastic product can reach 18.26GPa%.For the high Si steel No.3test steel,when annealed at 850°C,quenched at 230°C and assigned at 280°C,the product of strength and plasticity can reach the maximum 19.22GPa%.The addition of Nb can enhance the strength not only by grain refinement but also by precipitation,which can be showed as the behavior that the Nb carbide precipitated was also found on the Nb-containing test steel substrate by the TEM observation.?4?Based on the different alloy compositions of the four test steels,different Q&P processes were used to find that the elongation and plasticity of the test steel were mainly determined by the content and morphology of retained austenite at room temperature.The content of retained austenite at room temperature was affected by the combination of the annealing temperature,quenching temperature and partition temperature in the Q&P process.In terms of retained austenite morphology,the retained austenite distributed in fine islands of film contributes the most to the elongation,while the uniform and finely distributed ferrite in the structure improves the plasticity of the test steel.