Abstract
TWIP-cored three-layer steel sheets were newly fabricated by hot rolling of TWIP steel sheet surrounded by low-carbon (LC) or interstitial-free (IF) steel sheets. TWIP/LC or TWIP/IF interfaces were well bonded without pores or voids, while a few pearlites were thinly formed along the interfaces. The strengths and elongation of the TWIP-cored sheets increased as the volume fraction of TWIP-cored region increased, and were also well matched with the ones calculated by a rule of mixtures based on volume fraction or force fraction. According to digital image correlation and electron back-scatter diffraction analyses, very high strain hardening effect in the initial deformation stage and active twin formation in the interfacial region beneficially affected the overall homogeneous deformation in the TWIP-cored sheets without any yield point phenomenon occurring in the LC sheet and serrations occurring in the TWIP sheet, respectively. These TWIP-cored sheets can cover a wide range of yield strength, tensile strength, and ductility levels, e.g., 320~498 MPa, 545~878 MPa, and 48~54%, respectively, by controlling the volume fraction of TWIP-cored region, and thus present new applications to multi-functional automotive steel sheets requiring excellent properties.
Highlights
Deformable steels such as TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) steels have been actively developed in worldwide automotive industries to decrease CO2 emissions and increase fuel efficiency[1,2,3,4]
Even when the deformation proceeds further, pearlites existed in the interfacial region are well maintained without forming microvoids or microcracks (Fig. 3(c,d)), and the formation of localized bands is restrained by keeping positive strain rate sensitivity (SRS) values. These results indicate that the high hardening in the TWIP-cored region and the active twin formation in the TWIP/LC interfacial region beneficially affects the homogeneous deformation in the TWIP-cored sheets without any yield point phenomenon and serrations, respectively
TWIP-cored three-layer steel sheets containing thin low-carbon (LC) and interstitial-free (IF) steel surface layers were fabricated by hot-roll-bonding, and their tensile properties and flow behavior were investigated in relation with interfacial microstructural evolutions
Summary
Deformable steels such as TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) steels have been actively developed in worldwide automotive industries to decrease CO2 emissions and increase fuel efficiency[1,2,3,4]. In order to solve aforementioned shortcomings of TWIP steels, the control of C and Mn contents, addition of Al or Si, and optimization of fabrication processes have been applied[13,14,15,16] As another way, a multi-layered hybrid sheet fabrication, which is called a TWIP-cored sheet fabrication, is newly suggested in the present study. In the present study, six TWIP-cored three-layer steel sheets containing thin LC or IF steel surface layers were fabricated by direct solid-state hot-roll-bonding using commercial hot-rolling stands Tensile properties of these sheets as well as mono-layer TWIP, LC, and IF steel sheets were evaluated in order to obtain the basic data on applicability to vehicle body structures. Mechanisms of improved tensile properties were investigated in relation with effects of TWIP/LC or TWIP/IF interfaces on overall tensile properties by digital image correlation (DIC) and electron back-scatter diffraction (EBSD) analyses
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