Investigation Into Rapid Heating And TTP Process Of High Strength Steel

With the increasing requirements of automotive industry for reduced weight,energy saving and emission reducing,hot stamping technology has been increasingly applied.Heating blanks for austenitizing is the first step of hot stamping.Traditional radiation heating method was featured as low efficiency,while conductive heating technology has brilliant prospect for rapid heating rate and high heating efficiency.So rapid heating process and the influence on austenitizing and mechanical properties was investigated in this paper.Base on the results,TTP(Tailored Tempering Properties)process with conductive heating technology was also investigated.Firstly,rapid heating process and the influence on austenitizing and mechanical properties was investigated in this paper.Owing to rapid heating rate,nucleation rate of austenitizing increased,and holding time decreased,so the quantitative relationship between heating rate and the most appropriate holding time was built based on experimental results.And also relationship between heating rate and grain size was investigated.At last the quantitative relationship between heating rate and mechanical properties was built by means of Hall-Petch relationship.According to the demand of mechanical properties of parts,conductive heating parameters could be formulated with the application of this relationship,such as heating rate and holding time.In order to make conductive heating technology applied,conductive heating equipment was developed.Appropriate process parameters could be formulated through this model.And also the way to improve temperature uniformity of blanks was proposed for improvement of practical application value of conductive heating technology.Utilization of blanks was increased from 63.8%to 91.6%through this way.Secondly,based on results of the influence of rapid heating process on austenitizing and mechanical properties,TTP process with conductive heating technology was investigated by the way of local austenitizing.Analytical method about temperature distribution of conductive heating blanks was firstly investigated in this part.Temperature distribution in different heating condition could be obtained through this analytical method,so that the formulation of mechanical properties could be came ture.TTP process of rectangular blanks and trapezoidal blanks was secondly investigated in this part.For rectangular blanks,both methods of local heating and heat preservation at middle could make mechanical properties gradient distribution.From low temperature zone to high temperature zone,Vickers hardness increased from 180HV to 520HV,tensile strength increased from 435MPa to 1532MPa,and microstructure was transformed from pearlite/ferrite to martensite.Because fragile transitional zone with the method of heat preservation at middle was bigger,local heating method was more appropriate for rectangular blanks.For trapezoidal blanks,the method of local heating and heat preservation at single side were investigated.From low temperature zone to high temperature zone,Vickers hardness was increased from 190HV to 505HV.Because the percentage of high temperature zone with local heating method was too small,the method of heat preservation at single side was more appropriate for trapezoidal blanks.CSP hot rolled plate was one of blanks with short process.Owing to low cost and less investment,its replacement to cold rolled plate was development tendency in hot stamping industry.Because of crassi initial microstructure,mechanical properties of parts made from hot rolled plate was worse.Conductive heating technology could improve mechanical properties,so the application of conductive heating technology into CSP hot rolled plate was investigated at last.Results showed that tensile strength of hot rolled plate was 1393MPa when heating rate was 10~oC/s.This value can't meet the requirement of mechanical properties of parts,while tensile strength was 1587MPa when heating rate was 100~oC/s,this result indicated that hot rolled plate could be raw materials of hot stamping industry by means of conductive heating process.Comparing holding time of the two materials,the most appropriate holding time of hot rolled plate was longer than that of cold rolled plate when heating rate is low,while the condition was inverse when heating rate was high.To summary,the investigation of rapid heating process can not only offer theoretical direction for hot stamping industry,but also make TTP process with low cost and high efficiency come ture.And also rapid heating process guaranteed the application of hot rolled plate in hot stamping industry.