Effect Of Heat Treatment Process On Microstructure And Properties Of 700L High Strength Automotive Large Beam Steel

The low grade automobile girder steel produced by iron and steel enterprises are gradually unable to meet the demands of the market with the continuous advancement of automobile lightweight process and the continuous improvement of the quality requirements of automobile girder steel plate by middle and lower reaches of automobile enterprises.It is imperative to research,develop and apply the high strength automobile girder steel plates.In recent years,the composition,production technology and microstructure of high strength beam steels have been studied and reported in many literatures.The strength is improved in high titanium microalloyed automobile beam steel with high strength by adding microalloyed element Ti,which has obvious cost advantage.However,there are few reports on the basic thermodynamic data and the effect of heat treatment process on high strength and high titanium automobile beam steel,which are not systematic and comprehensive.This paper takes high strength and high titanium automobile beam steel 700L as the research object to investigate the thermodynamic behavior of second phase precipitation and kinetic behavior of grain growing in high titanium steel.Optical microscope,scanning electron microscope,transmission electron microscope,impact test and hardness test were used to study the microstructure and mechanical properties of the steel,based on the influence of quenching temperature and tempering temperature on the microstructure and mechanical properties of the steel,so as to explore its strengthening and toughening mechanism and obtain the optimal heat treatment process parameters and provide some theoretical reference for heat treatment process control of high strength automobile beam steel.Thermo-Calc software was used to establish the thermodynamics database of precipitation behavior of second phase particle in high strength and high titanium automotive beam steel.The second phases precipitated in order of precipitation temperature including Ti₄C₂S₂,FCC?A1#3,FCC?A1#2,MNS,Cementite,M₂P?C₂₂,_M5C2,M₂₃C₆ and M₇C₃ in the process of equilibrium cooling.Ti₄C₂S₂ phase precipitates in the solid-liquid region,which inhibites the precipitation of Mn S.Moreover,Mn S and Ti₄C₂S₂ phases are not affected by element content,and only the precipitation/decomposition temperature has a slight change.FCC?A1#3 phase is mainly composed of Ti,Nb,C and N elements,which mainly precipitates in the form of Ti(C,N)since the end of solidification at about 1498?.FCC?A1#2 and FCC?A1#3 are isomorphic and heterogeneous relations,which mainly precipitates in the form of(Ti,Nb)C at about 1266?.The precipitatation amount of FCC?A1#2 and the contents of Ti and Nb elements in the phase are much larger than that of FCC?A1#3 phase.The precipitation temperatures and total precipitates of FCC?A1#3 and FCC?A1#2increased with the increase of Ti content.(2)The experimental results show that the average austenite grain size of the experimental steel increases exponentially with the increase of quenching temperature,and the growth rate slows down with the increase of soaking time,and the effect of quenching temperature on grain size is much greater than soaking time;When the quenching temperature reaches or exceeds 1100?,the rapid growth trend of grain is obvious;the average grain size is larger than the experimental grain size of ferrite after holding at 1000?for more than 60min;Therefore,the quenching temperature should be controlled below 1100?and the soaking time should not exceed 60min to avoid abnormal grain growth during the heat treatment.(3)Beck,Hillert and Arrhenius equations were used to fit the austenite grain growth process of experimental steels,among which the Arrhenius equations(D(28)26003t^0.367exp(7)-114657/RT(8))also considered the effects of quenching temperature and soaking time.The calculated value of grain size is in good agreement with the experimental value,which has better fitting effect and prediction accuracy;The Ostwald ripening model was used to analyze the diffusion of Nb element in the steel,which is the controlling element of the Ostwald ripening process of FCC?A1#2 phase,based on the thermodynamic data of the precipitation of the second phase particles;The relationship between the volume fraction of FCC?A1#2 phase and the mean particle radius with temperature can be used to reflect and reasonably explain the changing rule of austenite grain size of experimental steel.(4)The experimental steel was treated at different quenching and tempering temperatures,and the microstructure of the experimental steel shown obviously different characteristics.With the increase of temperature,the GB structure decreases,the number of lamellar ferrite gradually increases,the width of the plate increases,the austenite grain boundary gradually becomes clear,and the number of white precipitates on the ferrite matrix and boundary increases.With the increase of tempering temperature,the massive ferrite grew up,the decomposition phenomenon of M/A structure and residual austenite was obvious.A small number of equiaxis ferrite appeared,the amount of tempering precipitates increased.And the particles appeared coarsening and growing phenomenon when the tempering temperature exceeded 600?.The microhardness of the experimental steel increases significantly due to the effect of"diffusion hardening",with the increase of quenching temperature.But the microhardness only decreases slightly with the increase of tempering temperature,which is attributed to the secondary hardening of carbide counteracting the microstructure softening caused by the increase of tempering temperature.The low temperature impact energy fluctuates up and down under different quenching and tempering conditions,which is mainly related to the regularity of the second phase particles and the matrix.The experimental steel manifested the best low temperature impact toughness at 600?,the main reason is that there are a large number of bainitic ferrite with high density dislocation and the second phase nanoparticles with appropriate size and uniform distribution in the steel matrix.The comprehensive strengthening effect brought by the two is the intrinsic reason for the high toughness performance.