Effect of Quenching Rate on Hardness and Microstructure of Hot-Stamped Steel

Abstract

The effect of cooling rate on hardness and microstructure of the hot-stamped boron steel with 0.2 mass% carbon was investigated. After sheet specimen with a thickness of 1.6 mm or 1.2 mm was heated up to 900°C for 4 min, it was press formed and simultaneously quench hardened with dies, or water-quenched. Simulated hot-stamping test was also carried out at various cooling rates. The Vickers hardness of quenched specimens was measured. The microstructure on the cross-section of quenched specimens was observed with optical microscope and transmission electron microscope. The microstrucure of hot-stamped specimen was composed of auto-tempered-martensite and was softer than water-quenched specimen which consisted of lath-martensite. Tempered martensite was distinguished from bainite by observation of cementite precipitation morphology. Cooling rate below the M_s point affects hardness significantly, even if cooling rate is higher than the upper critical cooling rate. Decrease in hardness caused by auto-tempering was formulated with the tempering parameter in which was taken account of integration of tempering effect.