Study On Hot Stamping Process And Microstructure And Properties Of Nb-V Alloyed High Strength Steel

With the rapid development of the automotive industry and the increasing requirements for environmental protection,the application of high-strength steel to achieve light weight has gradually become an important way for the automotive industry to save energy and reduce emissions,and hot stamping technology is an important means of processing and forming high-strength steel for automobiles.At present,commercial 1500 MPa grade 22 Mn B5 steel is widely used in the hot stamping manufacturing process,and its elongation is only about 5-6%,which restricts its promotion and application to key automotive components to a certain extent.The addition of micro-alloying elements such as Nb and V can further refine the grains of the hot-formed steel,thereby effectively improving the mechanical properties of the hot-formed steel.This article takes Nb and V micro-alloyed steel as the object,adopts experimental and theoretical calculations and other means to study and analyze the suitable process parameters and second phase particle precipitation in the hot stamping process of niobium-vanadium alloyed high-strength steel,and comparatively study Nb,V Effect of V alloy element on hydrogen embrittlement sensitivity of high strength steel.The conclusions of the study are as follows:(1)According to the hot stamping process of niobium-vanadium alloyed high-strength steel,a plate simulation quenching experiment was used to study and analyze the different holding temperature and holding time on the structure,pre-austenite grains and mechanical properties of the steel,and obtained suitable hot stamping heating condition.Experimental research on holding temperature shows that when the holding temperature is lower than 870 ?,the steel is not fully austenitized.After quenching,the steel structure is a mixture of martensite and ferrite.The tensile strength and yield strength of the quenched steel vary As the holding temperature increases,the elongation decreases.When the holding temperature is higher than870 ?,it can be fully austenitized and fully martensite after quenching.At this time,the factors affecting the mechanical properties of the quenched steel are mainly the original Austenitic grain size,tensile strength change range is not large,yield strength and elongation slightly decreased;with the increase of holding temperature,austenite grains gradually become larger,uneven grain coarseness occurs at 960 ?.The experimental results of different holding time show that: Niobium-vanadium steel can be fully austenitized for 3 minutes and above;the austenite grains gradually grow with the extension of the holding time,and rapid coarsening occurs after 7 minutes of insulation;After austenitizing,the tensile strength,yield strength and elongation of the quenched steel decrease with the increase of holding time.The tensile strength of niobium-vanadium high-strength steel has a maximum value of 1570.78 MPa when kept for 3 minutes.Comprehensive experimental results and actual operation requirements,the hot stamping heating condition of niobium-vanadium high-strength steel is determined as: holding temperature is 930 ?,holding time is 3min.(2)JMat Pro thermodynamic calculation and experimental analysis were used to comparatively study the precipitation behavior of the second phase particles in the niobium-vanadium alloy steel during hot forming and its effect on the structure and properties of the steel.The results show that: Nb and V elements are almost all present in the M(C,N)phase in the steel;Nb-containing particles are precipitated with Ti at the austenite grain boundary,and with V and Ti in the crystal;V-containing particles are mainly precipitated in the crystal in the form of V and Ti compound.Nb and V alloy elements can refine the structure and improve the mechanical properties and cold bending properties of steel.(3)Through the comparative experimental analysis of niobium vanadium high strength steel and 22 Mn B5 steel,the influence of Nb and V alloy elements on the hydrogen embrittlement sensitivity of high strength steel was studied.The results show that the hydrogen embrittlement index of the niobium-vanadium alloyed steel is slightly lower than that of 22 Mn B5 steel;the niobium-vanadium high-strength steel exhibits quasi-cleavage characteristics at the fracture location after hydrogen charging,while the22 Mn B5 steel is a mixed feature of quasi-cleavage and intergranular fracture;Nb,V alloy elements can refine the grains,and carbides in the crystals as hydrogen traps,effectively improving the resistance to hydrogen-induced cracking of niobium-vanadium high-strength steels.