Isothermal transformation and precipitation behaviors of titanium microalloyed steels
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ORIGINAL PAPER
Isothermal transformation and precipitation behaviors of titanium microalloyed steels Xiang‑dong Huo1 · Kang He2 · Ji‑nian Xia3 · Lie‑jun Li4 · Song‑jun Chen4 Received: 15 December 2019 / Revised: 3 April 2020 / Accepted: 7 April 2020 © China Iron and Steel Research Institute Group 2020
Abstract The microstructure transformation and precipitation behavior of nano-carbides in Ti microalloyed steel during isothermal process were studied by a compression test on a Gleeble 3800 thermomechanical simulator and analyzed by optical microscopy, transmission electron microscopy and other methods. The results show that γ → α phase transformation and TiC precipitation take place in Ti microalloyed steel during the isothermal process, and time–temperature–transformation curve and precipitation–time–temperature (PTT) curve are all of “C”-type. During the isothermal process, the interphase precipitation of TiC mostly occurs at the period of the phase transformation, and the random precipitation of TiC mostly occurs on the ferrite after the phase transformation. The increment in yield strength at the initial stage of isothermal transformation mainly comes from phase transformation strengthening. With the increase in isothermal time, the precipitation hardening effect becomes more important for nucleation and growth of titanium carbides and eventually reaches the maximum value at the precipitation finished point of the PTT curve. Keywords Ti microalloyed steel · Isothermal process · Transformation · Precipitation hardening · Nanometer carbide
1 Introduction Titanium microalloyed steel with high strength is widely used in machinery, vehicles, construction and other industries [1–3]. Titanium nitrides and carbides in steel play an important role, such as inhibiting austenite grain growth, hindering austenite recrystallization and preventing dislocation movement [4–8]. Therefore, high-strength steel can be produced through grain refinement and precipitation hardening by using titanium microalloying technology [9–11]. In 2004, JFE corporation [12] developed a titanium microalloyed steel with a yield strength of 700 MPa. It was * Xiang‑dong Huo [email protected] 1
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China
2
Technology Quality Center, Shandong Iron and Steel Group Laiwu Co., Ltd., Laiwu 271104, Shandong, China
3
Technology and Quality Center, Shandong Iron and Steel Group Rizhao Co., Ltd., Rizhao 276826, Shandong, China
4
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China
found that precipitation hardening effect produced by nanosized (Ti, Mo) C on the ferrite matrix was about 300 MPa, which aroused a wide interest of metallurgist scholars and material researchers. In-depth researches [13–16] were carried out on the relationship between microstructure, property and precipitation behavior in titanium microalloyed steel, and it was gradually realized that the isothermal holding or
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