Process Optimization And Experimental Analysis Of Strength And Toughness For Hot Stamping High Strength Steels

As an effective economical energy measure, the light weighting development direction has become one of the important research subjects in automobile industry. So far, new materials with lighter weight are the main approach to reduce the vehicle weight. And making use of the hot-stamping high strength steels (HSHSS) not only reduces the weight of automobiles, but also improves crash safety, therefore more and more the mentioned steels are used to produce automotive parts. Despite HSHSS’high strength and light weight, low ductility and toughness has become one of the major hurdles for the application of HSHSS in automobile body. However, optimizing process parameters scientifically and effectively are very crucial to improve strength and toughness.In order to research the effect of heating temperature, holding time, start deformation temperature, tempering temperature and tempering time on the strength and toughness of hot stamping boron steel SG1.6with the thickness of1.6mm, an L25(56) orthogonal experiment which was analyzed by means of comprehensive evaluation was designed based on strength, ductility and fracture toughness. Notable factors and the range of levels were obtained according to the results of orthogonal test, response surface methodology was carried out to get the optimal hot stamping and tempering techniques based on strength and toughness of the mentioned steel through analyzing test points continuously.(1) The results of orthogonal experiment indicate that the notable factors which affect the strength and toughness of HSHSS are heating temperature, start deformation temperature and tempering temperature, such as heating temperature mostly affects the strength, start deformation temperature and tempering temperature mostly affects the ductility and toughness. The high strength and excellent fracture toughness is obtained when the heating temperature is890～920℃, the holding time is1min, the start deformation temperature is650～700℃, the tempering temperature is100℃, and the tempering time is30min.(2) The optimization results of single objective indicate that the predicted maximum value of tensile strength is1577.34MPa, the predicted maximum value of elongation is10.363%, the predicted maximum value of tear strength is2116.93MPa, and the predicted maximum value of unit initiation energy is565.255N/mm. The optimization results of multi-objective indicate that the predicted values of tensile strength, elongation, tear strength and unit initiation energy are not less than1570.51MPa,10.059%,2067.30MPa,471.39N/mm. when the heating temperature is890～899.71℃. start deformation temperature is682.10～700℃and the tempering temperature is100～101.57℃.(3) The optimum parameters were used for B pillar hot stamping test, properties of specimen such as tear strength and unit initiation energy of the mentioned steel is improved by11.44%and21.55%respectively. Tensile Strength and hardness of the mentioned steel is decreased by2.34%and2.09%respectively. Toughness is increased substantially on the basis of a small decrease of strength.