Effect of Austenitizing Temperature on Microstructures and Mechanical Properties of Si-Mn Hot-Rolled Plate After Quenching and Partitioning Treatment
CHENG Yuanyao1,2, ZHAO Gang1,2, XU Deming1,2(), MAO Xinping3, LI Guangqiang1,2
1 Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China 2 State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 3 Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
CHENG Yuanyao, ZHAO Gang, XU Deming, MAO Xinping, LI Guangqiang. Effect of Austenitizing Temperature on Microstructures and Mechanical Properties of Si-Mn Hot-Rolled Plate After Quenching and Partitioning Treatment. Acta Metall Sin, 2023, 59(3): 413-423.
The production of quenching and partitioning (Q&P) steel using hot-rolled steel instead of cold-rolled steel can significantly reduce the manufacturing process time and cost. However, the initial microstructures of hot-rolled and cold-rolled steels are different, which affect the microstructures and mechanical properties of Q&P steel. Because most studies used Q&P steel prepared from cold-rolled steel, the microstructures and mechanical properties of Q&P steel prepared from hot-rolled steel are unclear. This study examines the microstructures and mechanical properties of Q&P Si-Mn steel prepared from hot-rolled steel as a function of the austenitizing temperature. The results showed that the ferrite in the Q&P Si-Mn steel produced from the hot-rolled steel had lath-type and blocky-type morphologies. The observed ferrite morphology could influence the morphology of the adjacent retained austenite. The lath-type and blocky-type ferrite surrounding the retained austenite was mainly observed as the thin lath and blocky types, respectively. The ferrite and retained austenite contents decreased with increasing austenitizing temperature. In addition, the corresponding yield and tensile strengths increased gradually with a concomitant decrease in elongation and the product of strength and elongation. When the austenitizing temperature was 810oC, the product of strength and elongation of the Q&P Si-Mn steels produced from hot-rolled steel reached 28.36 GPa·%, which was approximately 36% higher than that of Q&P980 produced industrially from cold-rolled steel. The higher product of strength and elongation of Q&P Si-Mn steel produced from hot-rolled steel may be related to the different morphologies of ferrite, which might control the morphology and stability of the adjacent retained austenite. These experimental results could provide a theoretical basis for preparing Q&P steel from hot-rolled steel instead of cold-rolled steel.
Fig.1 Schematic of quenching and partitioning (Q&P) Si-Mn steel heat treatments (Ac1—start temperature of pearlite transformed to austenite, Ms—initial temperature of martensite transformation, Mf—finish temperature of martensite transformation)
Fig.2 OM (a) and SEM (b) images of the Si-Mn hot-rolled plate (B, F, M, BF, and LF refer to bainite, ferrite, martensite, blocky ferrite, and lath-like ferrite, respectively)
Fig.3 SEM images of the Q&P Si-Mn steels under austenitizing temperatures of 810oC (a), 840oC (b), 870oC (c), 910oC (d), and 950oC (e) (RA, M1, and M2/RA refer to retained austenite, primary martensite, and secondary martensite/retained austenite island, respectively)
Fig.4 XRD spectra (a) and volume fraction and carbon content of retained austenite (b) of Q&P Si-Mn steels under different austenitizing temperatures
Fig.5 EBSD images of the Q&P Si-Mn steels under austenitizing temperatures of 810oC (a), 870oC (b), and 950oC (c) (The green, gray, and black structures represent RA, F or M1, and M2, respectively)
Fig.6 TEM images of the Q&P Si-Mn steels under austenitizing temperatures of 810oC (a, b), 870oC (c, d), and 950oC (e, f) (Insets in Figs.6b and d show the selected area electron diffraction patterns of blocky RA and film-like RA, respectively)
Fig.7 Engineering stress-strain curves of Q&P Si-Mn steels under different austenitizing temperatures
Fig.8 Yield strength (a), tensile strength (b), elongation (c), and product of strength and elongation (d) of Q&P Si-Mn steels under different austenitizing temperatures
Fig.9 True stress-true strain (a) and strain hardening rate (b) curves of the Q&P Si-Mn steels under austenitizing temperatures of 810, 870, and 950oC
TA / oC
Stage I
Stage II
Stage III
810
ε < 0.015
0.015 < ε < 0.166
0.166 < ε < 0.207
870
ε < 0.018
0.018 < ε < 0.112
0.112 < ε < 0.135
950
ε < 0.020
0.020 < ε < 0.088
0.088 < ε < 0.109
Table 1 Range of the true strain of every stage in the strain hardening rate curve of the Q&P Si-Mn steels under austenitizing temperatures of 810, 870, and 950oC
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