Analysis On The Heat Transfer Coefficient Of The Third Generation Automotive Medium-Mn Steel In Warm/Hot Forming

The technology of high-strength steel hot forming can meet automotive lightweight and safety requirements, so it gets the favor of many manufacturers. The third generation of automotive high-strength steel keeps high strength and has superior ductility. It is expected to replace traditional automotive high-strength steel.Hot stamping is complex heat exchange process, which contains air-to-blank heat exchange and blank-die heat exchange. Changes of blank temperature affect its martensitic transformation process, thereby affecting its mechanical properties. Accurate heat transfer coefficient is critical for hot forming simulation.The adaptive round platform model and the mathematical model for solving heat transfer coefficient is used to research air, mold interface heat transfer coefficient in the whole hot stamping and stamping pressure, coating, mold materials and other factors on the heat transfer coefficient is explored. The specific contents are as follows:(1) When the Biot is less than 1, centralized parameter method is adopted; when Biot is larger than 0.1, the Beck method is used. Heat transfer coefficient throughout the whole hot forming process is obtained by the combination of both these two methods.(2) Heat transfer coefficients is explored under the pressure of IMPa,5MPa, 10MPa, 15MPa,20MPa coefficient at the variation with temperature. The greater the stamping pressure is, the greater the contact area. Heat transfer coefficients of the two interfaces is also large, and there is a relationship between the exponent, IHTC= 1459.8*P ? 0.3651 (W/m2.K).(3) Interfacial heat transfer coefficient is studied, the thicker oxide coating, smaller heat transfer coefficient. They approximately satisfy a certain function and expressed as:when x<0.2,1HTC=-8289.7x+2528.9, and when x>0.2, IHTC<500. x is the coating thickness, and its unit is mm.(4) When the sample is heated at high temperatures, it will produce oxide skin. The thicker the oxide coating, the heat transfer coefficient will be smaller. The researchers found that if the heating temperature is 850 ?, holding time of 9min, the loss of oxidation was only 2.8% and the Medium-Mn steel has good antioxidant capacity.The study found, under the same stamping pressure, the higher the heating temperature, the greater the weight loss oxidation, the smaller heat transfer coefficient will be.(5) Different die materials have different specific heat capacity, thermal conductivity. Comprehensive comparison of tw o kinds of die materials interface heat transfer coefficient under the pressure of 15MPa, the study found that 45# steel is about 1.3 times of H13 steel, and reach thermal equilibrium time of 45# steel is more quickly than H13.