Abstract
For automotive structural parts, hot stamping is being increasingly used because of the need for both higher fuel efficiency and crashworthiness. The yield strength of hot-stamped steel sheet is lower than that of water-quenched steel sheet. The microstructure of hot-stamped low-carbon steel sheet comprises lath martensite and retained austenite. Due to the slow cooling rate below the martensite start temperature, the martensite formed by hot stamping is auto-tempered. To clarify the factor dominating the yield strength of the hot-stamped steel, the authors herein investigated the effect of microstructures on the yield strength with heat-treated specimens at various cooling rates and heating temperatures. Consequently, it was clarified that the yield strength of the auto-tempered low-carbon steel depends on grain, dislocation, solute carbon, carbide, and retained austenite. As far as the present experiment is concerned, the retained austenite is the most effective factor on the yield strength.
Highlights
Hot stamping is a promising process toward obtaining ultra-high strength steel sheet components with complex geometry
Hot stamping for automotive structural parts is being increasingly used because of the need for both higher fuel efficiency and crashworthiness
The sizes of packet and block are linearly related to the prior austenite grain size [1,2,3,4]
Summary
Hot stamping is a promising process toward obtaining ultra-high strength steel sheet components with complex geometry. The microstructure of hot-stamped steel sheet comprises lath martensite and retained austenite. The packet size becomes the effective grain size of low-carbon martensitic steel. The low-carbon martensitic steel formed by hot stamping is auto-tempered due to slow cooling rate below the Ms temperature. The effect of auto-tempering on the hardness of hot-stamped boron-bearing steel has been studied by the experiments of various cooling rates [18, 19]. The cooling rate below Ms temperature significantly affected the hardness and carbide morphology, a cooling rate was greater than the upper critical cooling rate The authors investigated possible microstructural factors of the yield strength, and estimated each contribution to clarify the factor dominating the yield strength of auto-tempered lath martensitic steel
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