Estimation And Experimental Study Of Heat Transfer Coefficient In Hot Forming Of Ultra High Strength Steels

Automotive high strength steel hot stamping technology supply a comprehensive solution for material formability and part intensity. During hot stamping process, the red-hot steel plate has been in a condition of heat exchange with the outside, the temperature history of the plate has a direct impact on microstructure formation and therefore the end-use of the parts. Heat transfer characteristics of the material with the outside are characterized by heat transfer coefficients (HTC), and HTC is determined by the physical parameters of the material and external media and the contact condition. In the different stages of the forming process, the heat exchange modes are different, and the mathematical descriptions of the heat transfer coefficients and the determined factors are different.To obtain the HTC under different stages of the forming process (heating, transport, stamping, quenching), heat transfer modes in the different stages of the forming process was analyzed on the basis of the basic theory of heat transfer, and two perspectives was presented to obtain various stages HTC:one is solving the HTC theoretically by material basic parameters and heat transfer model, the other is estimate the HTC reversely through basic heat transfer experiments and inverse estimation modes. Hence the HTC under Air-cooling, gap conduction, contact conduction and the pipeline heat convection were investigated and analyzed.(1) The main heat exchange includes heat radiation and air heat convection during transport. The improved Newton’s law of cooling was used to estimate the coefficients of air cooling in this research, and the HTC mainly depend on the temperature and size of the material, coefficient of thermal radiation, and temperature and flow rate of air. The higher the material temperature and the greater the flow rate of air, the greater the heat transfer coefficients.(2) Gap conduction can be regarded as the comprehensive effect of the multilayer material conduction and heat radiation. The improved Newton’s law of cooling was used to estimate the coefficients of gap conduction in this research, and the HTC mainly depend on temperature of the material and the gap size. The higher the material temperature and the smaller the gap size, the greater the heat transfer coefficients.(3) Contact conduction can be regarded as the comprehensive effect of contact point conduction, mezzanine air conduction and heat radiation, wherein the conductance of the contact point is the biggest determinant of a heat transfer coefficient of the interface, mezzanine air conduction has small effect, and the radiation effect is almost negligible. In this paper, the improved Newton’s law of cooling and Beck sequence estimation method were both adopted to estimate the heat transfer coefficients. The HTC depend on two basic factors, one is the interface material properties, and the other is the real contact area. Material properties depend on temperature and phase organization, and the real contact area was affected by work pressure.(4) In this paper, the idea of steady-state inverse estimation was used to estimate the pipeline convective heat transfer coefficients. The HTC mainly depend on the size of and the wall roughness the pipe, the properties and velocity of the fluid medium. The larger the diameter and the faster the flow rate, the greater the heat transfer coefficients.Finally, the heat transfer coefficients obtained by the experiment were taken into the finite element software, and the simulation result was compared with the experimental data, and therefore the validity and reliability of the experimental and inverse methods were verified.