Characterization of Phase Transformations During Graded Thermo-Mechanical Processing of Press-Hardening Sheet Steel 22Mn
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NTRODUCTION
DUE to their advantageous specific properties such as a high strength combined with a sufficient residual deformability and a high-energy absorption in the event of a crash, press-hardened components find increasing use in the automotive industry.[1] The martensitic microstructure of a press-hardened part, which reaches tensile strengths of up to 1500 MPa, develops during the simultaneous forming and quenching in cooled dies.[2] While a cooling rate above the critical rate of 25 K/s is required for the formation of a fully martensitic microstructure according to the continuous cooling transformation (CCT) diagram of 22MnB5, softer phases such as ferrite, perlite, and bainite are formed at lower cooling rates.[3] The formation of these phases is influenced by hot deformation, whereby a shift of the ferritic and bainitic area to shorter times has been observed at a hot deformation degree of 20 or 40 pct.[4–6] A specific adjustment of a mixed microstructure can produce a material with both high strength and elongation, for which phase compositions of martensite and bainite or martensite and ferrite are favorable.[7]
A. REITZ, O. GRYDIN, and M. SCHAPER are with the Department of Material Science, Paderborn University, Warburger Straße 100, 33098 Paderborn, Germany. Contact e-mail: [email protected] Manuscript submitted June 4, 2020. Article published online September 18, 2020 5628—VOLUME 51A, NOVEMBER 2020
The process route of direct press hardening generally consists of austenitizing, hot forming, and quenching the parts.[5] A diffusional transformation of austenite into ferrite or bainite is preferred in non-isothermally deformed boron steels according to Shi et al., which also resulted in a lower martensite start temperature Ms.[8] Zhou et al. found that austenitization at a temperature of 800 °C or 850 °C leads to a microstructure consisting of undissolved pearlite, fragmentary ferrite, and martensite.[9] Press-hardened parts made of 22MnB5 reach hardness values of 450 HV, which is 50 HV lower compared to water quenched 22MnB5 parts due to a self-tempering of martensite.[10] The crash performance of body in white components produced by press hardening is related to the microstructure of the parts.[7] Furthermore, this microstructure is dependent on the deformation temperature and strain, the cooling rate, the initial grain size, the chemical composition, etc.[6] In order to achieve an optimum modification of the mechanical properties of multiphase steels, knowledge of the phase compositions and the morphology of the individual phases is essential.[11] In addition, tailored hot formed parts are produced by press hardening with areas of lower strength but higher ductility to improve the crashworthiness of the part.[1,2] To reach such tailored properties in one component, different heating or cooling paths are applied within particular blank regions.[2] The data on the change in phase transformation kinetics as a function of thermal and strain conditions are of primary importance for the correct design of tailored
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