고강도 강판의 홀 확장성에 관한 연구

Abstract

Hot rolled steel sheets, widely used in automobile parts such as wheel and structural component, are required to have higher press formability regarding stretch-flangeability as well as stretch-formability. Stretch-flangeability is evaluated as a hole expansion ratio (HER) measured by a hole expansion test that represents changes in the diameter of a material before failure during the extension of a sheared hole edge. However, the hole expansion ratio is not well predicted by standard approaches of material formability evaluation such as the forming limit diagram (FLD). Thus there are many controversial issues on parameters influencing on the hole expansion ratio such as the tensile properties and microstructural aspects of various steel grades. Multiphase AHSSs, such as DP and TRIP steels have higher strength and ductility compared with conventional high strength steels. Nevertheless, they exhibit poor stretch-flangeability due to the formation of voids at the interface between soft ferrite matrix and hard second phases in the initial stage of press-forming. Thus, they are limited to use of complicated shape parts. Although the lower hole expansion ratio in multiphase steels is due to the easy void formation, it is not sufficient to explain the fracture processes of steel sheets in all forming modes because some multiphase steels show high stretch-formability. In this study, commercial steels with various types and grades were used to investigate the metallurgical/mechanical factors influencing on the hole expansion ratio. It was found that a hole expansion ratio of materials is strongly affected by hole surface states depending on hole piercing methods. On the respect of microstructures, major factors influencing on the hole expansion properties are a void formation mechanism and a strain-hardening rate. Steels with the hard second phases showed that voids were formed by strain incompatibility between a ferrite matrix and hard second phase, which lead fast crack propagation along the interfaces. In particular, voids in DP steels were formed by interface decohesion of ferrite/martensite, which was elucidated by the crystallographic orientation relation as well as the strain incompatibility of two phases. Also, it was found that materials with a high strain-hardening rate showed lower hole expansion ratios. In order to understand the effect of strain hardening rate on the hole expansion ratio, FE simulations were carried out to analyze the development of stress and strain distributions in specimens during the deformation processes. 1GPa grade DP and martensitic steels with different yield strength were used in this simulation. Simulation results showed that in a high strain-hardening rate material excessive hardening occurred on the hole edge and shear stress increased on hole surface that leaded to the fast failure. Conclusively, we analyzed the void formation and crack propagation mechanisms during tensile and hole expansion tests and clarified the relationship between metallurgical/mechanical factors and a hole expansion ratio. Also, a new evaluation index was proposed to estimate the stretch-flangeability of materials. Based on these analysis, precipitation strengthened ferritic steels were fabricated to obtain a tensile strength of 980MPa with high stretch-flangeability by using the concept of a high yield strength (or low strain-hardening rate) and the absence of coarse hard second phase. And hole expansion properties of these steels were also evaluated by a new index.