1、F-40 Numerical simulation of hot stamping of quenchable boron steel Zhongwen Xing1,Jun Bao1*,and Yuying Yang2 1School of Mechanical and Electrical Engineering,Harbin Institute of Technology,Harbin,China 2 School of Material Science and Engineering,Harbin Institute of Technology,Harbin,China ,*,and A
2、bstract:Quenchable boron steel is a new type of ultra high strength steel used for automotive parts to reduce the weight of automobiles and maintain the safety requirements.On the basis of experimental data of mechanics and thermal physical properties,a material model under hot stamping condition of
3、 quenchable steel was set up,and the numerical simulation to the whole hot stamping process of hot forming，quenching and spring-back of bending parts was made with ABAQUS software.The results show that the springback of hot stamping parts increases when blank holder force(BHF)decreases;and it increa
4、ses when the clearance between punch and dies increases and when the die radius increases.The simulation results are basically in agreement with experimental results.Keywords:quenchable boron steel,thermo-mechanical coupling,hot stamping,numerical simulation 1.Introduction Along with the development
5、 of automobile industry,more and more advanced high-strength and ultra high-strength steel is used to produce automotive parts.Such may reduce the weight of automobiles and then may improve the crash safety and low down the gas emissions.However,in the forming process of high-strength and ultra high
6、-strength steel sheet,there are such problems as forming force is big,springback is serious,fracture is possible and complex parts is hard to forming,etc.Today,a novel technology that use quenchable steel sheet to carry out hot stamping is considered as a solution to overcome above mentioned difficu
7、lties1,2.And it is researched by many automobile manufacturers and researchers in the world.Before stamping,the tensile strength of such steel sheet is about 600MPa,and the microstructure is ferritic and pearlite.The steel sheet is heated in the furnace and the temperature is kept to get full austen
8、itic,then is formed through hot stamping in die in which a cooling system is equipped to ensure that steel sheet may get martensite through quenching in die after forming.The tension strength of steel sheet after hot stamping may be improved to 250 of initial value,i.e.over 1500MPa.Plastic deformati
9、on,strong temperature drop and phase transformation of quenchable steel sheet arise in hot stamping process,i.e.it is a complete thermo-mechanical coupling process.Numerical simulation is the powerful tool for an analysis to thermo-mechanical coupling process.On the basis of acquired test data of me
10、chanics and thermal physical properties,a material model of quenchable steel was set up on the hot stamping conditions and a numerical simulation to the full process of hot forming，quenching and spring-back of bending parts was made with ABAQUS software to analyse and evaluate the processing technol
11、ogy of hot stamping.2.Finite element simulation 2.1 Material model In order to get the martensite on condition that quenchable steel sheet is formed in austenitic phase and is cooled rapidly.It is mainly to consider the affection to the relationship between stress and strain by temperature that Nort
12、onHoff law was adopted as the constitutive equation3,4.)/exp(TKmn=（1）Wherein,K is the material consistency,n the strain hardening,m the strain rate and the temperature parameter.These parameters were estimated by hot tensile tests on GLEEBLE 3800 mechanical system.Because bainite transformation is e
13、asy to arise when temperature is at between 650350,therefore,air blowing must be done during hot tension tests to restrain bainite transformations.The temperature change and phase transformation during hot stamping process leads to the change of thermal capacity，heat transfer coefficient and thermal
14、 expand coefficient of material.And currently,the cooling speed of instrument that may measure the thermal capacity,heat transfer coefficient and thermal expand coefficient can not reach the requirement of restraining the happening of phase transformations,except for the martensitic transformation.S
15、o the following methods were adopted to define such 3 coefficients.i.e.the parameter of material at any temperature is the linear combination of all phase volumes and the parameter of this phase at such temperature.For example,heat transfer coefficient of such material thermal at any temperature in
16、respect of austenitic and martensitic heat transfer coefficient and their own volume at such temperature.And it may be shown as the following equation:(T)=VAA(T)+VMM(T)（2）Wherein,the martensite fraction VM may be expressed by formula Koistinen-Marburger.The austenitic fraction VA may be calculated by equation(4).That is VM=1-exp(k(M s-T)（3）VA1-VM （4）The austenitic test sample is gained by heating on testing instrument,while martensitic test sample is got by pre-quenching.Respectively,test the he
