Influence of annealing temperatures on microstructure evolution and mechanical properties in a low-carbon steel
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ORIGINAL PAPER
Influence of annealing temperatures on microstructure evolution and mechanical properties in a low‑carbon steel You‑hui Jiang1 · Shun Yao2,3 · Wei Liu1 · Yun Han4 · Su‑peng Liu1 · Geng Tian1 · Ai‑min Zhao1 Received: 9 July 2019 / Revised: 9 December 2019 / Accepted: 16 December 2019 © China Iron and Steel Research Institute Group 2020
Abstract Quenching and partitioning (Q&P) heat treatments with different annealing temperatures and fixed initial quenching temperatures were applied to cold-rolled low-carbon steel with the initial microstructure of ferrite and pearlite, aiming to gain the same amount of austenite (preset value) before the partitioning stage. The chemical compositions of the material have been specially designed, containing 1.6 wt.% silicon and 0.8 wt.% aluminum to avoid the precipitation of carbides. The microstructure evolution of the investigated steel was characterized using a dilatometer, an optical microscope, a scanning electron microscope (SEM), an X-ray diffractometer, an electron backscattered diffraction and transmission electron microscope. Consequently, the microstructure of all samples looks quite similar. At the same time, according to SEM micrographs and dilatometer data, there are competitive reactions in Q&P process, such as the precipitation of carbides, the transformation of bainite and the formation of secondary martensite. Thus, the measured austenite is less than the preset values. Mechanical properties of the material were detected by uniaxial tensile tests. The results indicate that the ultimate tensile strength of the four groups of samples is similar, but the total elongation has a significant downward tendency with the increase in annealing temperatures. After annealing at 840 °C, the steel possesses great ultimate tensile strength of about 1200 MPa and optimum total elongation of about 20.37% with favorable products of strength and elongation of about 24.35 GPa%. Keywords Quenching and partitioning · Intercritical annealing · Microstructure evolution · Mechanical propriety · Retained austenite
1 Introduction In recent decades, various compositions and heat treatment processes have been used to produce advanced high-strength steel (AHSS) aiming to satisfy the sustainable green manufacturing of the automotive industry [1, 2]. For example, dual-phase steels, transformation-induced plasticity (TRIP) steels, and twinning-induced plasticity steels have relatively * Ai‑min Zhao [email protected] 1
Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China
2
Institute of Engineering and Technology, University of Science and Technology Beijing, Beijing 100083, China
3
China Shougang International Trade Engineering Co., Ltd., Beijing 100082, China
4
Shougang Research Institute of Technology, Beijing 100043, China
mature component systems and production processes [3]. Quenching and partitioning (Q&P) steels proposed by Speer et al. [4, 5] in 2003 are one of the candidates for the th
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