초고강도 베이나이트강의 미세조직과 기계적 성질에 미치는 탄소함량과 항온열처리 온도의 영향

• Published in: 대한금속·재료학회지, 57(6) Vol. 57; no. 6; pp. 335 - 342
• Main Authors: 황보승, Seung Hwangbo, 이홍범, Hong-bum Lee, 송영범, Young-beum Song, 함진희, Jinhee Ham, 김홍규, Hong-kyu Kim, 서동우, Dong-woo Suh
• Format: Journal Article
• Language: Korean
• Published: 대한금속재료학회 05.06.2019; 대한금속·재료학회
• Subjects: austenite; bainitic ferrite; heat treatment; high strength; impact toughness; 재료공학
• ISSN: 1738-8228; 2288-8241
• DOI: 10.3365/KJMM.2019.57.6.335

The effect of carbon content and isothermal heat treatment conditions on the microstructure evolution and mechanical properties of ultra-high strength bainitic steels was investigated. A reduction in carbon content from 0.8 wt% to 0.6 wt% in super-bainite steel with typical chemistry effectively improved not only the Charpy impact toughness but also the strength level. This suggests that reducing the carbon content is a very promising way to obtain better mechanical balance between strength and impact toughness. The higher Charpy impact toughness at a lower carbon content of 0.6 wt% is thought to result from a reduction in austenite fraction, and refinement of the austenite grain. The coarse austenite grains have a detrimental effect on impact toughness, by prematurely transforming to deformation-induced martensite during crack propagation. Mechanical properties were also affected by the isothermal treatment temperature. The lower isothermal temperature enhanced the formation of bainitic ferrite with a refined microstructure, which has a beneficial influence on strength, but reduces impact toughness. The lower impact toughness at lower isothermal temperature is attributed to the sluggish redistribution of carbon from the bainitic ferrite into the surrounding austenite. Higher solute carbon in the bainitic ferrite contributes to an increase of strength, but at the same time, encourages a propensity to cleavage fracture. (Received March 6, 2019; Accepted May 2, 2019)