Study On Ferrite Transformation And Precipitation Behavior Based On Mo-Ti Micro Alloyed Steels

Japan's JFE company a full ferritic matrix of precipitation hardening of hot-rolled steel,while maintaining high strength while taking into account the high elongation and high formability,can be widely used in automobile beams,B-pillar.Based on the addition of Cu,Mo,Ti and V,the ferritic transformation and precipitation behavior of nano-scale composite precipitates were studied,and the theoretical thermodynamics and kinetics of the precipitates were analyzed.The following work:(1)The effects of continuous cooling and continuous cooling on the ferrite transformation were studied by continuous cooling experiment.The dynamic CCT curves of the experimental steels were obtained under different conditions.Rapid cooling can effectively shorten the start time of ferrite transformation and reduce the ferrite transformation initiation temperature,so as to effectively refine the ferrite grains.The decrease of deformation temperature can refine ferrite grains and improve the microhardness of ferrite grains.The increase in deformation,the same can refine the ferrite grain,improve the micro-hardness of ferrite.(2)Through the static isothermal quenching experiment,the kinetic curves of ferritic transformation and static TTT curves were obtained,and the nose point temperature of Mo-TiCu microalloyed steel was determined to be 600?.Mo-Ti-V micro-alloy steel nose point temperature of 580?.In the dynamic isothermal quenching experiment,the isothermal temperature,isothermal time on the ferrite transformation.(3)For the Mo-Ti-Cu microalloyed steel,there are two types of precipitates in the ferrite grains:?-Cu and(Ti,Mo)C,respectively,and ?-Cu is found at 600?.(Ti,Mo)C precipitates at 620?680?,and as the temperature rises,the precipitate of Cu precipitates decreases rapidly.The precipitation of(Ti,Mo)C at 600? isotherm is not found.And the interphase precipitation of precipitated ?-Cu and(Ti,Mo)C in a ferrite grain at the same time were not observed.The isothermal phase of the ferrite grains precipitated during the isothermal process from 2h to 8h at 620?,and the interphase precipitation of the carbides disappeared.Which was coarsened from 13.24nm to 38.48nm,but still less than the theoretical value.Cu and the matrix to maintain K-S relationship:[11-1]ferrite//[011]?-cu,(1-10)ferrite//(1-1-1)?-Cu,carbide and ferrite matrix(111)carbide//(011)ferrite,[01-1]carbide//[100]ferrite and(-1-11)carbide//(01-1)ferrite,[110]carbide//[100]ferrite The lattice constants of ferrite,?-Cu and carbides are 0.370 nm and 0.414 nm,respectively.Calculated by precipitation hardening theory found that at 620?,the carbide can provide the highest precipitation hardening value of 360.18MPa.(4)The solid solubility product formula of(Ti,Mo)C is calculated by the thermodynamic model of carbides.At high temperature,the precipitates are mainly rich in Ti.the precipitation of Mo is almost not precipitated at high temperature.And the precipitation amount of Mo gradually increases.(Mo,Ti)C in the range of 650-600?,the precipitation free energy decreases with the temperature decreasing,and reaches the peak at 600?.The critical temperature of precipitation of ?-Cu was 688.56?.The precipitation kinetics of(Mo,Ti)C was calculated and found to be the fastest precipitation temperature at 750?.