Study On Hot Stamping Process Intergrated With Fractional Cooling And Q&P Treatment

Despite the strength of conventional hot stamping parts is high,the plasticity is very low.Hot stamping process intergrated with Q&P(quenching & partitioning)treatment can improve the plasticity and obtain better mechanical properties.Based on HS+Q&P process,fractional cooling strategy is introduced to intergrate with hot stamping process and Q&P treatment in this paper.The specimen is firstly quenched to a suitable forming temperature by spray cooling method,which is followed by stamping and quenching steps conducted in the die.It is proved that new process ensures both high strength and plasticity which is as same as HS+Q&P process.Meanwhile,it lowers the cooling requirement of die and speeds up the production process.The main contents of this research are shown as follows:1.According to the characteristic of car body structures,an unequal-high and unequal-wide U-shaped part is designed and applied in B1500 HS steel conventional hot stamping.The effect of process parameters on mechanical properties and formability,especially springback of stamping parts were emphatically analyzed through metallographic observation,tensile test,hardness test,and laser scanning.2.Hot stamping process intergrated with fractional cooling is conduct and studied.Based on Gleeble3500 thermal simulation testing platform,the optimum forming temperature is determined by high temperature thermal tensile tests and thermal simulation experiments.Then fractional cooling technology auxiliary quenching device is designed for the improved hot stamping process intergrated with fractional cooling.To verify the operability and stamping performance under different forming temperatures,new hot stamping experiments of U-cap parts and unequal-high and unequal-wide U-shaped parts are conducted.The microstructure,mechanical properties and formability are discussed in detail.3.Based on HS + Q&P process,research on new hot stamping process which intergrated with fractional cooling technology and Q&P treatment is carried out by thermal simulation experiments.The effect of process parameters,partitioning time,partitioning temperature and cooling rate on the comprehensive mechanical properties are investigated.The result shows that compared with the conventional hot stamping process,the new process can maintain high strength and improve the elongation evidently.The product of strength and plasticity of B1500 HS after new process with appropriate process parameters is 33.1% higher than that after conventional hot stamping process.4.The mechanism of hot stamping process intergrated with fractional cooling and Q&P treatment is studied through qualitative analysis by OM,SEM observation and quantitative analysis of residual austenite content by XRD.The result shows that the microstructure of the thermal simulation specimen after new process is martensite-austenite dual phase structure.Soft residual austenite is obtained through the Q&P treatment which improves the plasticity.Fine and homogeneous martensite hard phase is obtained by fractional cooling technology which generates high strength and enhances the comprehensive mechanical properties.