| Title |
Study on the Spheroidizing Annealing Behavior of Medium-Carbon Cr-Mo Steel for Ultra High Strength Cold Heading Quality Wire Rods |
| Authors |
강현우(HyeonWoo Kang) ; 장병록(ByoungLok Jang) |
| DOI |
https://doi.org/10.3365/KJMM.2025.63.10.803 |
| ISSN |
1738-8228(ISSN), 2288-8241(eISSN) |
| Keywords |
UNS A93003; Localised corrosion; Immersion; EIS |
| Abstract |
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. A quantitative evaluation using the
morphological shape factor (F) confirmed there was a clear inverse relationship between hardness and F
values, indicating that this factor can serve as a reliable metric for assessing cold formability and the
effectiveness of spheroidizing heat treatment. |