Effect of quenching rate on hardness and microstructure of hot-stamped steel

• Published in: Journal of alloys and compounds Vol. 577; pp. S549 - S554
• Main Authors: Nishibata, Toshinobu, Kojima, Nobusato
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.11.2013
• Subjects: Auto-tempering; Bainite; Boron steel; Boron steels; Carbon; Cementite; Cooling rate; Die pressing; Hardness; Hot-stamping; Martensite; Microstructure; Quench-hardening; Quenching (cooling); Boron steel; Bainite; Auto-tempering; Hot-stamping; Martensite; Quench-hardening
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2011.12.154

► The hot-stamped specimens had an auto-tempered martensite microstructure. ► Below the Ms point, reducing the cooling rate significantly reduces the hardness in the hot-stamped-specimens. ► The major factor that causes the reduction in hardness in hot-stamping is auto-tempering. The effect of the cooling rate on the hardness and microstructure of the hot-stamped boron steel containing 0.2mass% carbon was investigated. Sheets with thicknesses of 1.6 and 1.2mm were heated to 900°C for 4min. They were then press formed and simultaneously quench hardened with dies or water quenched. Simulated hot-stamping tests were also performed at various cooling rates. The Vickers hardnesses of the quenched specimens were measured and their cross-sections were observed by optical microscopy and transmission electron microscopy. The hot-stamped specimens had an auto-tempered martensite microstructure and they were softer than the water-quenched specimens, which consisted of lath-martensite. Tempered martensite was distinguished from bainite by observing cementite precipitation. Below the Ms temperature, reducing the cooling rate significantly reduces the hardness, even when the cooling rate is higher than the upper critical cooling rate.