Investigation On Conduction Heating Technology And Equipment Application For Ultra-high Strength Steel

With the increasingly serious of energy and environment crisis, energy saving and emission reducing are the direction of the future development of the automotive industry. The effective way to realize energy saving and emission reducing is to reduce the weight of the automobiles which does not lose the structural strength of automobiles. Hot stamping parts of ultra-high strength steel meet the requirements of lightweight and structural strength of the automobile.Hot stamping process began with the austenization and carbon diffusion homogenization of ultra-high strength steel. The process was completed by radiation heating (roller hearth furnaces or multilayer box furnaces) in the traditional hot stamping. The blank temperature was uniform by radiation heating between furnaces and blanks in the roller hearth furnaces. However, the roller hearth furnaces had the features of cost-intensive, low heating rates and energy efficiency, larger occupied space and little flexibility, affecting the hot stamping production efficiency. The conduction heating had the advantages of rapid heating, high energy efficiency and good flexibility, which solved the deficiencies of radiation heating.Rapid heating can refine the austenite grain and shorten the austenitizing time of ultra-high strength steel. And it can also improve the balance of strength and ductility of hot stamping parts. In this dissertation, the austenite mechanism, the heating specification of ultra-high strength steel, the variation of the microstructure and 3D optical microscopy of Al-Si coating of ultra-high strength steel by rapid conduction heating, and the key technologies of the rapid conduction heating test line of the full-size automobile safety parts, and so on, were systematically studied. The specific investigations were as follows:(1) A rapid heating of austenite growth kinetics model was established according to the principle of transformation kinetics and the thermal expansion curves obtaining from experiments. Considering the effects of heating rates on austenizing starting temperature and final temperature as well as austenite nucleation and growth, the influence regularity of heating rates, the maximum heating temperature on the austenizing process of ultra-high strength steel was explored based on the cellular automata model of the austenizing growth of ultra-high strength steel. The variation regularity of carbon concentration and austenite grain during the austenizing process was analyzed. The calculation results were aslo demonstrated by the tests. Finally the heating specification of rapid heating was developed.(2) The influence regularity of the heating rates, the heating temperature and the dwell times of rapid heating on the microstructure and 3D optical microscopy of Al-Si coating was studied according to the principle of Fe-Al-Si reaction-diffusion kinetics. And a multi-step heating process which was suitable for the Al-Si coating was proposed based on the adjustable heating rates of conduction heating. It provided the theoretical basis on the application of rapid conduction heating to the Al-Si coating of ultra-high strength steel.(3) A conduction heating experiment device was designed and developed based on the mechanical engineering, electrical engineering, control engineering and heat transfer theory. The device realized the closed-loop servo control of the processes of heating, forming and quenching of the blanks on the die. Based on solving the key technologies of temperature control, thermal expansion and temperature uniformity, the conduction heating test line of the full-size automobile safety parts was developed combining with the conduction heating experiment device and the hot stamping line. The test line realized the linkage control of the processes of rapid heating, transporting and forming.(4) According to the electrothermics theory, the influence regularity of electrical heating characteristics and temperature uniformity of the rectangular blanks by different heating control strategies were explored in the conduction heating device. The influence regularity of electrical heating characteristics and temperature distribution of different shapes of the blanks and the special blanks with holes, slots and grooves were also studied. Local conduction heating of the blanks was explored based on the selection characteristics of heating areas of conduction heating. The blanks containing non-heating region, heating region and non-heating region were achieved by local conduction heating. Finally, the mechanical properties of the hot stamping parts realize tailored distribution. The results showed that the balance of strength and ductility of the high strength region of the hot stamping parts by conduction heating was better than that of radiation heating.(5) The heating and the formability of the blanks of the front bumper parts in the conduction heating test line and the hot stamping test line were investigated, separately. The results showed that the uniform temperature of the blanks was achieved by the unequal radiation heat transfer technology. The demesion precision of the front bumper parts was ranged from +0.59mm to -0.84mm. The Vickers hardness of the uniform temperature regions exceeded 480HV. The tensile strength, elongation and the balance of strength and ductility of the uniform temperature regions exceeded 1490MPa,6.8% and 11.1GPa%, respectively.