The Effects of Process Temperatures on the Microstructure and Tensile Properties of Warm-Stamped Nb-Bearing Medium-Mn St
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THE hot stamping process has been widely employed to fabricate complex-shaped automotive components with high strength, such as pillars, roof rails, and bumpers,[1–3] due to the difficulty in making such components by cold stamping. In the hot stamping process, a blank of B-bearing 22MnB5 steel is austenitized at ~ 900 C to 950 C, stamped at ~ 700 C to 800 C, and then quenched in press dies with cooling channels. During die quenching, martensitic transformation occurs, resulting in high yield strength (YS, ~ 1000 MPa), ultimate tensile strength (UTS, ~ 1500 MPa), and moderate total elongation (TE, ~ 8 pct) at room temperature.[1–10] However, for example, the hot stamping process for automotive pillars using 22MnB5 steel has the following problems[11–16]; the high energy cost due to high austenitizing temperature (~ 900 C to JAE-HOON NAM and YOUNG-KOOK LEE are with the Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Republic of Korea. Contact e-mail: [email protected] JEONGHO HAN is with the Department of Materials Science and Engineering, Yonsei University and also with the Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea. Manuscript submitted June 18, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
950 C), the limitation of coating material, the low durability of press dies, and the low productivity due to the holding time of 5 to 10 seconds for die quenching. To resolve these problems, a warm stamping process using medium-Mn steel with high hardenability has been studied[17–20] because this strategy can lower austenitizing temperature and replace die quenching by air cooling. For example, Fe-5Mn-0.1C[17,18] and Fe-5Mn0.1C-0.23Si[19] (wt pct) steels were hot-rolled, annealed, cold-rolled, and intercritically annealed again to obtain a dual-phase microstructure of austenite and ferrite. For warm stamping, specimens were austenitized at 800 C for 4 to 5 minutes, stamped at ~ 500 C prior to austenite decomposition, and then die-quenched at the rates of 30 to 60 C/s to obtain a fully martensitic microstructure. Both warm-stamped medium-Mn steels revealed tensile properties comparable to hot-stamped 22MnB5 steel: YS (1220 MPa), UTS (1420 MPa), and TE (11 pct) for Fe-5Mn-0.1C (wt pct) steel, and YS (1050 MPa), UTS (1520 MPa), and TE (11.3 pct) for Fe-5Mn-0.1C-0.23Si (wt pct) steel. Pan et al.[20] performed warm stamping using hot-rolled Fe-5.6Mn-0.19C-1.2Al-0.05Nb-0.22Mo (wt pct) steel with a triple-phase microstructure: martensite, ferrite, and retained austenite. A set of hot-rolled specimens was reheated at 700 C for 5 minutes, and stamped at ~ 650 C. Another set of specimens was reheated at 760 C for 5 minutes, and stamped at ~ 710 C. Both reheating
temperatures (700 C and 760 C) are below the Ae3 temperature of the steel used and belong to the intercritical region. After warm stamping, all specimens were die-quenched at a rate of 20 C/s. The average tensile properties of warm-stamped specimens are as follow
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