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 Ms 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.