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 mechanicalstrength and heat resistance compared to conventional carbon steels, making them widely utilized in theautomotive, machinery, and aerospace industries for high-strength fastening and precision-formedcomponents. However, such high-strength steels often suffer from die wear and cracking during cold formingprocesses. To address these issues, optimization of spheroidizing annealing conditions through appropriatemicrostructural control via heat treatment is essential. In this study, spheroidizing annealing conditions wereoptimized for two steel grades: the widely used CHQ-grade SCM435 and a newly developed 1600 MPa-gradeultra-high-strength Cr-Mo alloy steel. A series of spheroidizing heat treatment conditions were applied to bothsteels, and their spheroidization behavior and mechanical characteristics were comparatively analyzed basedon initial microstructural differences. Intercritical Annealing (IA) and Subcritical Annealing (SA) cycles weredesigned based on Ac1 and Ac3 transformation temperatures determined via DSC analysis. The resultsrevealed that both steels exhibited optimal spheroidization and hardness reduction when subjected to IAconditions with a second heating temperature approximately 20–40 °C below Ac1. The lowest Vickers hardnesswas observed at 113 HV for SCM435 and 171 HV for the developed alloy, corresponding to an averagedecrease of 55% and 54.5% from their initial hardness, respectively. Microstructural observations showed thatSCM435, which primarily consists of pearlite in a ferrite matrix, underwent effective spheroidization underboth IA and SA conditions. In contrast, the bainitic structure of the developed alloy required prioraustenitization under IA conditions to enable successful spheroidization. A quantitative evaluation using themorphological shape factor (F) confirmed there was a clear inverse relationship between hardness and Fvalues, indicating that this factor can serve as a reliable metric for assessing cold formability and theeffectiveness of spheroidizing heat treatment.

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