On the Strengthening Effects Affecting Tensile and Low Cycle Fatigue Properties of Low-Alloyed Seismic/Fire-Resistant St
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On the Strengthening Effects Affecting Tensile and Low Cycle Fatigue Properties of Low‑Alloyed Seismic/Fire‑Resistant Structural Steels Jung‑Ho Sim1 · Tae‑Yeong Kim1 · Jun‑Yeon Kim1 · Chi‑Won Kim1 · Jun‑Ho Chung2 · Joonoh Moon3 · Chang‑Hoon Lee3 · Hyun‑Uk Hong1 Received: 19 July 2020 / Accepted: 25 August 2020 © The Korean Institute of Metals and Materials 2020
Abstract In the present study, low carbon ferritic and bainitic steels with different contents of Mo, Ti, and Nb were designed for both seismic and fire-resistant applications. The microstructure of steels containing 0.3 wt% Mo–0.02 wt% Nb (‘A’ hereinafter) was mainly composed of bainite. By contrast, the microstructure of steels with 0.2 wt% Mo–0.13 wt% Ti (‘B’ hereinafter) consisted of ferrite with a high density of nano-sized (Ti,Mo)-rich MX precipitates. The results showed that the bainitic microstructure (‘A’ steel) was quite favorable to high-temperature strength and thermal stability. The yield strength of ‘A’ steel at both room and 600 °C temperatures increased consistently with increasing thermal exposure time (600 °C/200–1000 h), since the precipitation of NbC particles occurred while maintaining bainitic ferrite platelets with a high density of dislocations during exposure. In the ‘B’ steel, the formation of nano-sized (Ti,Mo)-rich MX particles was effective to impede dislocation movement, leading to excellent plasticity (lower yield ratio) at room temperature. However, their contribution to precipitation hardening was not so much at 600 °C, as compared to the bainitic strengthening. During low cycle fatigue tests at room temperature, the main different feature between the two steels is that the ‘A’ steel showed cyclic softening while cyclic hardening was evident in the ‘B’ steel. The bainitic microstructure showed a better fatigue life due to increased ductility manifested by cyclic softening, by which dislocation cell was developed. Keywords Bainitic steel · Seismic and fire-resistance · Microstructure · Strengthening · Fatigue
1 Introduction Recently, there has been a tendency for the buildings to be constructed in the form of high-rise and multi-purposes. In addition, to secure the safety design of structures from natural disasters such as earthquakes and fire, the building standards have become more stringent [1–5]. Accordingly, many attentions have been paid to the development of highperformance low carbon steels, which are to be highly resistant to sudden damage. Earthquakes cause serious damage, * Hyun‑Uk Hong [email protected] 1
Department of Materials Science and Engineering, Changwon National University, Changwon 51140, Republic of Korea
2
Incheon R&D Team, Hyundai Steel Company, Incheon 22525, Republic of Korea
3
Ferrous Alloy Department, Korea Institute of Materials Science, Changwon 51508, Republic of Korea
and in many cases, earthquake events are followed by fire. This post-earthquake fire may cause more damage than the earthquake itself [6]. Low yield ratio (the ratio of yield strength to tensile strength) o
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