Microstructure And Properties Regulation And Mechanical Behavior Of High Strength High-manganese TRIP Steel

With the improvement of people’s living standards,the energy saving,emission reduction and safety issues of automobiles have been paid more and more attention.In order to meet the dual requirements of low emission and safety,the research and development and application of high-strength and high-plasticity steel for automobiles has become a research hotspot for researchers.Among the existing third-generation advanced high-strength steels,quenching and partitioning(Q&P)steels require a relatively complex Q&P process and precise temperature control,while medium-manganese TRIP steels often need an annealing process of up to several hours to obtain the target microstructure.These complex or time-consuming processes pose challenges to industrial applications.Therefore,it is urgent to explore the composition design,structure regulation principle and control technology in order to obtain high-strength and tough automotive steel with a simpler process.Starting from the design of the stacking fault energy and the alloy system,this topic aimed at the development of high-strength high-Mn TRIP steels with the activation of austenite→ε martensite→α’ martensite phase transformation during the deformation process.Meanwhile,the effects of B addition in high-Mn TRIP steels were explored.With the use of the strain-induced martensite reversion annealing process,fine-grained austenite+α’ martensite microstructure was obtained,cold-rolled high-strength high-Mn TRIP steels were developed,and the control of the microstructure and properties were realized with a simpler process.Microstructure evolution and mechanical behavior during the fabrication and deformation process were investigated by electron microscopes,X-ray diffraction and neutron diffraction.The main findings are as follows:The deformation behavior of the hot-rolled high-strength high-Mn TRIP steel was clarified.The rationality of the design of composition and stacking fault energy was verified.The heat treatment process of hot-rolled high manganese TRIP was also studied.During the annealing process of the hot-rolled test steel,recrystallization behavior was observed from 700℃.When the annealing temperature increased above 800℃,the proportion of recrystallized austenite grains increased,the recrystallized grains grew larger,and ε martensite and α’martensite were formed during the subsequent cooling process.For hot-rolled OB steel,the best mechanical properties were obtained when annealed at 700℃.with a yield strength of 347 MPa,a tensile strength of 1207 MPa,a total elongation of 45.3%,and a product of strength and elongation of 54.7 GPa·%.By adjusting the parameters of the reversion annealing process,the microstructure of high-strength and high-manganese TRIP steel can be regulated,and the coordination of different yield strength,tensile strength and total elongation can be obtained.The increase of annealing temperature or annealing time leads to the decrease of dislocation density,the increase of the proportion of austenite phase and the possible growth of recrystallized austenite,which in turn affects the mechanical properties.When annealed at a high temperature for a short time,a high proportion of austenite could be obtained by the high annealing temperature,and the recrystallization and growth of austenite were limited by the short holding time.Therefore,excellent mechanical properties could be reached.When annealed at 650℃ for 5 min,OB steel with the cold-rolled reduction of 25%reached the high tensile strength of 1411 MPa,with a total elongation of 41.5%and the product of strength and elongation of 58.6 GPa·%.Based on the distribution characteristics of Mn segregation in high-strength and high-manganese TRIP steel,the regulation of bimodal microstructure is realized,which effectively improves the mechanical properties of high-manganese TRIP steel.In the lower temperature reversion annealed samples,the micron-scale austenite grains retained in the Mn-rich region and the fine-grained austenite+a’martensite grains in the Mn-lean region constituted the bimodal structure.The finegrained structure brought an increase in strength,while the coarse-grained structure ensured a considerable total elongation.In addition,the difference in austenite stability caused by Mn segregation made the TRIP effect run through the entire deformation process and therefore,the TRIP effect was efficiently utilized.The addition of B in the high-strength high-Mn TRIP steel introduced the hard M2B phase and refined the austenite grains,which improved the yield strength and tensile strength of the test steel.In hot-rolled high-strength and high-manganese TRIP steel,the addition of B can increase the yield strength by over 120 MPa and stabilize the tensile strength above 1200 MPa with less reduction in elongation.However,B addition damages plasticity more than that in hot-rolled test steel.Meanwhile,its reinforcing effects are replaceable.Grain refinement can be achieved by adjusting the parameters of the reversion annealing process,while the strengthening of hard M2B can be replaced by α’ martensite.The strength of high-strength high-Mn TRIP steel after reversion annealing can be furtherly improved by increasing the cold rolling reduction rate or adjusting the composition to reduce the stacking fault energy system.The increase in the cold rolling reduction rate brings about an increase in the dislocation density and the refinement of the structure in the precursor.Under the appropriate heat treatment process,the strength and plasticity can be improved at the same time.Appropriately reducing the stacking fault energy system can also refine the precursor structure and improve the strength.Compared with increasing the rolling reduction,lowering the stacking fault energy system could obtain higher tensile strength of 179 MPa,but the elongation was relatively decreased to 30.5%.Based on neutron diffraction analysis,the redistribution of stress and strain in the deformation process of high-strength high-manganese TRIP steel and the relationship between the serration in the stress-strain curve and the discontinuous martensitic transformation are revealed.Multiple stages of stress-strain redistribution in the deformation process of hot-rolled high-Mn TRIP steel and reversion annealed high-Mn TRIP steel were observed.During the deformation of hot-rolled high-strength high-Mn TRIP steel,the stress increment before the formation of α’ martensite was mainly borne by austenite,and after the formation of α’ martensite,austenite and α’ martensite shared the stress increment.During the deformation process of the reversion annealed high-strength high-Mn TRIP steel,the successive yielding of austenite and α’ martensite occurs first.After the activation of austenite→ε martensite transformation,the increase in stress mainly came from orientations unfavorable for phase transformation,such as(200)oriented austenite.After the activation of ε martensite→α’ martensite transformation,α’ martensite began to bear more stress increments,and in the last stage of deformation,the stress increments were mainly from α’ martensite.In the high-Mn TRIP steels,ε martensite can bear few stresses during deformation.In addition,by analyzing phase fraction evolution during tensile,the serration in the stress-strain behavior of the high-strength high-Mn TRIP steel was proved to be strongly related to the discontinuous martensitic transformation.