| Hot stamping technology which has the characteristics of high precision and good quality is widely used to produce ultra-high-strength stamping parts. And it can effectively avoid defects such as severe wrinkle and cracking which occurred in the conventional cold stamping process because of its unique design. However, study about the cooling system of hot stamping mainly focuses on design and optimization of cooling water channel of hot stamping die, but the researches about development and application of high heat conductivity mold material are little published in the journal. In this paper, based on H13, DIEVAR and HDLM the three different thermal conductivity mold material, using the metallo-thermo-mechanical multi-physical field coupling analysis method, the hot stamping process of B pillar is reproduced by means of DEFORM ? software. The dynamic evolution rule of various physical fields of sheet metal during the hot stamping process is analyzed, and the results are used to compare with that of the actual hot stamping to verify whether the simulation is reliable. Finally, the influence of different mold material, mold initial temperature and cooling speed on cooling behavior, organization evolution and the distribution of the tensile strength are discussed, which can provide a theoretical guidance for optimizing the hot stamping procedure and choosing mold material. The major conclusions can be summarized as follows:(1) Through measuring its hardness and toughness under various heat treatment process, this new type hot stamping die steel HDLM shows its optimal quenching and tempering process is 1080℃+640℃×2h+640℃×2h. At this moment, hardness is 53 HRC and impact energy is 170 J.(2) In terms of H13, DIEVAR and HDLM mould materials, due to the small difference among density and specific heat capacity of three mould materials, the main factor affecting thermal conductivity is the thermal diffusion coefficient of mold materials. Under the service temperature(200~400℃) of hot stamping process, thermal conductivity of HDLM steel is more than 40 W/(m·K), which is much higher than H13 steel DIEVAR die steel, and it is about 1.5 times than that of H13 steel.(3) At the forming process, the B pillar has small changes in temperature, and the temperature of the entire component is among 780~805℃ and with a continuous uniform distribution. At the quenching process, the martensitic transformation rate of bottom at the formed component is the fastest, the martensitic transformation rate of lateral wall is moderate, while that of rounded corner is slow. After quenching for 12 s, organization of B pillar almost completely change, and hardness of B pillar is greater than 48 HRC. At the same time, tensile strength of B pillar is more than 1460 Mpa, which is verified by experiment.(4) The effect of thermal conductivity of die steel on the cooling behavior of B pillar is significant. With the improvment of thermal conductivity of die steel, after forming phase, temperature and stress of the sheet respectively continues to decrease and increase, especially at rounded corners of the bottom. When the mold material is H13 steel, the martensitic transformation of B pillar starts to take place after quenching 3.8s, and it at least takes 13 s to make the temperature of B pillar below Ms. When the mold material is DIEVAR steel, the martensite starting and ending time has a relatively gentle reduction. While the mold material is HDLM steel, the martensitic transformation of B pillae begins to occur at 2.5s, only holding about 8.5 s can keep each part of B pillar fully translate into martensite, significantly shorten cooling time Comparing with the H13 steel, maintaining efficiency is increased by nearly 34.6%, effectively accelerate the hot stamping production rhythm and improve the production efficiency.(5) The improvement of mold temperature plays a vital role in forming parts.With increasing the mould initial temperature, after forming phase, the temperature of sheet has a relatively gentle rise, particularly in the lowest temperature of sheet.At the same time, the stress of B pillar presents a sharp dorp and with a continuous uniform distribution.When the initial temperature of the mould exceeds 100℃, the temperature drop rate of B pillar is very slow, which seriously restricts martensitic transformation, and tensile strength of B pillar is much lower than the agreed standard. The above reasons leads to prolonging the holding time and significantly reduce the efficiency of production, so we need to keep the initial temperature of the mold at lower levels in the practical production process.(6) The cooling speed has a gentle influence on on hot stamping parts. When the cooling speed of mold is more than 1 m/s, with increasing the colding speed, its influence on the temperature and stress field distribution of B pillar and mold is almost negligible. |
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