| Since the 21st century,with people’s pursuit of a better life,the safety,performance,and aesthetics of automobiles have gradually become the development direction of the automobile industry.With the increases in automobile production and ownership,China is facing increasingly serious energy consumption,environmental pollution,and traffic congestion problems,this led to the emergence of the concept of automobile lightweight,and the development of low-cost,high-performance advanced high strength steel(AHSS)is one of the important means of automobile lightweight.The hot stamping(HS)process has been the main technology for auto parts manufacturing since its appearance in the 1980s.Although hot stamping steel has high strength,its plasticity is very low.In order to solve the plastic defects of hot stamping steel,the quenching & partitioning process was combined with the hot stamping process,that is,the HS-Q&P process.This process can greatly improve the plasticity of hot stamping steel under the premise of a small amount of reduced strength,so as to obtain automobile steel with higher comprehensive mechanical properties.It is the future development direction of the third-generation advanced high-strength steel.Through literature reading and experimental exploration,with the help of simulation software such as JMat Pro and Thermo-Calc,a material suitable for hot stamping quenching &partitioning process was choicen.Through characterization methods such as OM,SEM,EBSD,universal electronic testing machine,hardness tester,etc.,the microstructure evolution and the change trend of mechanical properties of the tested steel under the different processes were analyzed,and it is found that the two-phase zone hot stamping-quenching & partitioning process can obtain a better structure due to the element partitioning process in the two-phase zone and the grain refinement process of the hot stamping process.Therefore,under the hot stampingquenching & partitioning process,the influence of the holding time of different two-phase regions on the volume fraction of the tissue,microstructure orientation,mechanics,hardness,and carbon content of retained austenite of the tested steel was further studied,in order to explore effectiveness of the diffusion behavior of the two elements in the heat treatment process,that is,the Mn element partitioning in the two-phase zone heat preservation process and the C element partitioning in the quenching & partitioning process,,the following conclusions are obtained:(1)Through the simulation of JMatPro,it is verified that the crystal grain of the tested steel can maintain a relatively small level at 800℃,which is beneficial to the comprehensive mechanical properties of the tested steel,and through the Mn partitioning process,it was verified that in this experiment,there are two kinds of element diffusion behaviors,namely,the Mn partitioning behavior existed in the heat preservation process of the two-phase zone,and with the increase of the holding time,the effect of Mn partitioning was enhanced,which laid the foundation for the choice of heat treatment parameters for the different processes.(2)The microstructure and mechanics of the tested steel under the different processes were characterized.It was found that in addition to the hot stamping process,lath martensite appeared in the quenching & partitioning process,the two-phase zone hot stamping-quenching& partitioning process and the completely austenite zone hot stamping-quenching & partitioning process.The width of the lath is 6-10μm,and the width of the martensite lath of the two-phase zone tested steel is smaller than that of the completely austenitic zone tested steel.Compared with the quenching & partitioning process,the two-phase zone hot stamping-quenching &partitioning process can improve the strength of the tested steel under the premise of ensuring the plasticity.Compared with the complete austenite zone hot stamping-quenching &partitioning process,the two-phase zone hot stamping-quenching & partitioning process can improve the plasticity of the tested steel under the premise of ensuring the strength.(3)Under the two-phase zone hot stamping-quenching & partitioning process,the microstructure of the tested steel is martensite,ferrite and retained austenite,and the martensite is lath-like and block-like.When the holding time of the two-phase zone is short,the martensite is not standard and the spacing between the laths is larger.As the holding time increases,the martensite grains become larger and the martensite laths become regular.As time further increases,part of the martensite becomes flattened,and block-like martensite is formed.The increase of ferrite presents a block-like shape or a lath shape that fits with lath martensite.(4)The SEM structure was processed and calculated by image processing software,and it was found that as the holding time increases,the content of martensite in the structure increases slightly,from 50% in the initial state to about 60%.In the EBSD characterization,it was found that the retained austenite is distributed in the grain boundary between martensite and ferrite in a point-line or chain-like shape.By analyzing the contrast maps of the tested steel when holding for 5 and 20 min respectively,the proportion of martensite and ferrite volume fractions was further verified,and the results were similar to those calculated by the SEM structure.(5)Under the two-phase zone hot stamping-quenching & partitioning process,with the increase of the holding time,the tensile strength of the tested steel shows a declining trend.When the holding time exceeds 20 min,the tensile strength drops significantly.Both the elongation and the product of strength and elongation of the tested steel showed a trend of first increasing and then decreasing.When the holding time is 20 min,the elongation is 13.72% and the product of strength and elongation is 18.93%.(6)As the holding time increases,the volume fraction of retained austenite in the tested steel is positively correlated with the carbon content in the retained austenite.When the holding time is 20 min,the volume fraction of retained austenite reaches the highest value,which is8.65%,and the carbon content in austenite is 1.16%.(7)When the holding time is 5min,the alloying element content of the ferrite in the tested steel is high,which is unfavorable to the coordination of the plasticity and deformation of the ferrite.With the increase of holding time,due to the diffusion behavior of the two elements,that is,partitioning of Mn element in the two-phase zone and C element in the quenching &distribution process,the degree of enrichment of alloying elements in the structure is changed.As a result,although the volume fraction of ferrite in the tested steel decreases,the elongation tends to increase due to the plasticizing effect of austenite and martensite laths.This shows that through the element-differentiated structure and the deformation coordination between different structures,the two-phase zone hot stamping-quenching & partitioning steel has obtained a better structure distribution and better comprehensive mechanical properties. |
