DEVELOPMENT OF HOT FORMING TECHNOLOGY FOR ULTRA HIGH STRENGTH STEEL AND ITS MECHANICAL PROBLEM

Hot forming is theadvanced technology that integrates traditional heat treatment andcold stamping. The ultra high strength component with the tensilestrength 1 600 MPa can be manufactured by hot forming. Theprinciple of hot forming, one-step method and multi-step methodtechnology, the key manufacturing technology in hot formingprocess are introduced. The evolutions of microstructure andmechanical property of the material are analyzed before hotforming, in hot forming and after hot forming. The material modelof austenitic phase at high temperature with dependence ontemperature and strain rate and its application method in hotforming are presented. The formability of the material in hotforming is studied. The influence of hardness ability value onformability is investigated and the optimized temperature range ispresented. The anisotropy of the rolling direction is analyzed andthe experimental results indicate the anisotropy of crystallattice disappear in hot forming. The constitutive relations ofthe material are descripted in hot forming, especially thethermal-mechanical-transformation coupled relationship. Themulti-phase mixed relationship is introduced to analyze theeffective thermo-mechanical parameters and mechanical propertiesof multi phases during the hot forming. By defining the concepts,the phase-transformation volume stress and phase-transformationplastic stress are expressed to explain the mechanism ofthermal-mechanical-transformation coupled relations. Then thenumerical simulation of hot forming is introduced, especially thedynamic explicit algorithm, the static explicit algorithm and thenumerical simulation of the temperature field based on theindependently developed commercial CAE software for sheet metalforming, named KMAS (King-Mesh Analysis System). The phasetransformation latent heat is introduced into the analysis oftemperature field during the hot forming process. Based on theanalysis of the crash problem for car body, the functional designmethods of hot-forming parts used in the vehicle body design arepresented. Finally prospective developments of hot forming aresuggested.