초고강도 냉간압조용 중탄소 Cr-Mo 강 선재의 구상화 어닐링 거동에 대한 연구

초록

Medium-carbon Cr-Mo alloy steels for cold heading applications exhibit superior mechanical strength and heat resistance compared to conventional carbon steels, making them widely utilized in the automotive, machinery, and aerospace industries for high-strength fastening and precision-formed components. However, such high-strength steels often suffer from die wear and cracking during cold forming processes. To address these issues, optimization of spheroidizing annealing conditions through appropriate microstructural control via heat treatment is essential. In this study, spheroidizing annealing conditions were optimized for two steel grades: the widely used CHQ-grade SCM435 and a newly developed 1600 MPa-grade ultra-high-strength Cr-Mo alloy steel. A series of spheroidizing heat treatment conditions were applied to both steels, and their spheroidization behavior and mechanical characteristics were comparatively analyzed based on initial microstructural differences. Intercritical Annealing (IA) and Subcritical Annealing (SA) cycles were designed based on Ac1 and Ac3 transformation temperatures determined via DSC analysis. The results revealed that both steels exhibited optimal spheroidization and hardness reduction when subjected to IA conditions with a second heating temperature approximately 20?40?°C below Ac1. The lowest Vickers hardness was observed at 113 HV for SCM435 and 171 HV for the developed alloy, corresponding to an average decrease of 55% and 54.5% from their initial hardness, respectively. Microstructural observations showed that SCM435, which primarily consists of pearlite in a ferrite matrix, underwent effective spheroidization under both IA and SA conditions. In contrast, the bainitic structure of the developed alloy required prior austenitization under IA conditions to enable successful spheroidization.