Experimental Research On Thermal-mechanical-phase Transformation Of Vehicle High Strength Steel Hot Forming Process

Combining hot forging with cold stamping technology, high strength steel hot forming technique has received widespread attention and application in auto parts manufacturing. It can satisfy requirements of light weight and high strength in using hot forming production, it is safe, inexpensive and reliable, and it can achieve the purpose of energy conservation. Non-isothermal phase transformation occurs in hot forming process, and sheet metal turns into high strength stamping part in this high temperature and rapid forming process. In the whole process, defects such as wrinkling and rupture occur easily, as a result, high strength and toughness of products and good formability in hot forming process are restricted to each other. Factors like thermal-mechanical-phase transformation and elastic-plastic deformation are important to explore optimization parameters of hot forming process.Formability and mechanical properties of hot forming high strength steel22MnB5was researched in this article. Firstly, rapid cooling process was added to optimize hot forming technology, it could reduce temperature of completely austenitized sheet to the optimal forming temperature before quenching, and the formability and mechanical properties of hot forming parts was analyzed comparatively with different cooling methods based on deep drawing box test. Secondly, the condition of martensite phase transformation was analyzed under different stress loading. Thirdly, hot forming B-pillar simulation of stress distribution was done by DYNAFORM, stress factors caused difference in formability and mechanical properties were analyzed by martensite phase transformation theory.The results demonstrate that:1) Defects such as wrinkling and rupture can be effectively decreased by using rapid cooling technology in hot forming process. Formability of sheet metal can be improved with optimized technology and optimal start forming temperature, and mechanical properties of high strength steel hot forming parts are guaranteed.2) Martensite phase transformation is affected by different loading ways, Tensile stress can promote martensite phase transformation, and martensite start temperature Ms improves obviously when tensile stress reaches a certain level. Martensite phase transformation is restrained with the increase of compression stress (deformation quantity is enough). With the loss of start forming temperature, the effect of compression stress to start forming temperature goes weakened. 3) Different distribution of stress has great influence on the mechanical properties and formability of forming parts. According to the contrastive analysis between optimized process and traditional process, this article found that punch rupture defect is easy to occur with traditional technology. For one thing, sheet metal do not reach the optimal hardening exponent when forming in high temperature, and metal flow is uneven, and materials in large deformation area do not get compensation timely; for another, due to the complexity of structure parts, degree of influence in different factors like stress is discrepant, which induce the difference of martensite phase transformation in the time and degree, and defects occurred.4) Due to the distribution of stress are diverse in different area, and it results in uneven transformation, martensite content and hardness is higher in high tensile stress area and low compression stress area because of higher martensite start temperature Ms, and martensite content and hardness is lower in low tensile stress area and high compression stress area because of lower martensite start temperature Ms.Through the implementation of the work in this paper, traditional hot forming technology has been improved, hot forming parameters have been optimized, the hot forming parts with optimizing technology on the premise of completely martensite phase transformation have realized the anticipated goal of microstructure refinement and macro mechanical properties improvement. This article provide a theoretical guidance and practical direction to mass production of automotive components with hot forming technology.