Research On Formation And Controlling Factors Of Microstructure Of Dual-phase Steel Under High Cooling Rate

Dual-phase steel is the most widely used advanced high-strength steel in the automotive industry.The dual-phase steel currently used for body manufacturing is generally produced by cold rolling annealing process,with long manufacturing process,high energy consumption and high alloy cost.In this paper,low-cost dual-phase steel is taken as the research object,and the composition system of process simplification and alloy reduction is designed.The transformation law of ferrite-martensite biphasic structure under high cooling rate is studied.The influence of phase formation on the microstructure of dual-phase steel in hot rolling and cold rolling process was compared.The mechanical behavior of dual-phase steel under different conditions was analyzed,which provided a technical reference and theoretical basis for the industrial production of low-cost dual-phase steel.In order to simplify the process and reduce the alloy,two component systems are designed:C-Mn-Nb-Cr is designed by the carbon element below the peritectic zone of the Fe-Fe₃C phase diagram to meet the simple slab continuous casting and rolling process.Production requirements,through the regulation of strontium elements,fine-grained strengthening to compensate for the strength loss caused by carbon reduction;C-Mn-Si-Cr system through carbon,manganese and martensite volume fraction to achieve different intensity levels.Through the thermal simulation experiment,the two-phase structure control cooling process window is obtained:the ferrite transformation temperature of C-Mn-Si-Cr system is 560⁷30?,the martensite transformation temperature is?400?,and the martensite transformation cooling rate is?50?/s;the ferrite transformation temperature of C-Mn-Nb-Cr system is 650⁷30?,the martensite transformation temperature is?400?,and the martensite transformation cooling rate is?50?/s.The transformation rule and strengthening mechanism of duplex structure at high cooling rate were described.As the finishing temperature increases,the fine-grain strengthening effect is weakened,and the dislocation strengthening effect is increased.When the finishing rolling temperature exceeds 830?,the dislocation strengthening increase is larger than the fine-grain strengthening reduction.With the increase of isothermal temperature,the amount of ferrite transformation decreases,and the volume fraction of martensite increases after rapid cooling.With the extension of isothermal time,the amount of ferrite transformation increases and the grain size increases.The effect of niobium on the microstructure and properties of experimental steel was described.The addition of niobium reduces the solid solution content of carbon in austenite and the stability of supercooled austenite,promotes ferrite transformation,and decreases the volume fraction of martensite after rapid cooling;the precipitation of NbC consumes carbon in the supercooled austenite,causing the concentration of martensite carbon to decrease,resulting in a decrease in the strength of the duplex steel;Nb can refine austenite grains,but with the increase of Nb content,the refining effect is weakened.NbC precipitates in the rolling of austenite non recrystallized zone,which increases the lattice distortion energy of sub grain boundary and other parts,promotes the diffusion transformation,and increases the ferrite grain size in the isothermal process.The difference of microstructure and properties between hot rolling process and cold rolling process was analyzed.Nb and Ti precipitates in the hot-rolled dual-phase steel pinned grain boundaries,refine grains and promote nucleation,and the ferrite size is relatively small;the cold-rolled dual-phase steel martensite island is re-sintered by cold rolling and pearlite After austenitizing,it is obtained by rapid cooling,and the martensite size is relatively small.The hot-hardened dual-phase steel has better coordinating deformation ability between the soft and hard phases,stronger crack propagation resistance,and the extended flange performance can be twice that of the cold-rolled dual-phase steel.The mechanical behavior of hot rolled dual phase steel under different stress conditions was studied.The pre Strain Tensile test and low cycle fatigue test were carried out.The pre-strain increases the ferrite dislocation density and deposits at the ferrite/martensitic interface.When the total pre-strain is the same,the ferrite dislocation density of the secondary pre-strained specimen is significantly lower than that of the primary pre-strain specimen.Under low-cycle fatigue conditions,hot-rolled dual-phase steel has good plastic strain energy and excellent resistance to cyclic deformation.