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Acta Metall Sin  2018, Vol. 54 Issue (8): 1105-1112    DOI: 10.11900/0412.1961.2017.00487
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Effect of Si and Mn Contents on the Microstructure and Mechanical Properties of Ultra-High Strength Press Hardening Steel
Kuanhui HU1,2(), Xinping MAO2, Guifeng ZHOU1,2, Jing LIU1, Zhifen WANG2
1 College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
2 Wuhan Branch of Baosteel Central Research Institute (R&D Center of Wuhan Iron & Steel Co., Ltd.),Wuhan 430080, China;
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Abstract  

It is very important to find out the mechanism of composition in steel. Many efforts have been put on the study of the effect of Si and Mn elements on the microstructure and mechanical properties of middle Mn steel, transformation induced plasticity (TRIP) steel and quenching and partitioning (Q&P) steel. But fewer studies were focused on the mechanism of Si and Mn contents in a press hardening steel. In this work, the microstructures after hot rolled and the fine martensite structure after hot stamping in ultra-high strength press hardening steel (PHS) with different Si and Mn contents were studied by OM, SEM, EBSD and TEM. The results showed that there are a great influence of Si and Mn contents on the microstructure and mechanical properties of PHS after hot rolled. The yield strength of the PHS increases from 552 MPa to 751 MPa, the ultimate tensile strength (UTS) increases from 757 MPa to 1124 MPa, and the microstructures are different with the Mn content rose from 0.57% to 1.21% and the other components remained the same. The UTS of the steels goes up as the Si content goes up from 0.25% to 0.38%, and the yield strength and the elongation show a fluctuation trend. After simulating hot stamping process at 950 ℃ and holding 5 min, the microstructure of the steels with different compositions is martensite, but it is different in the fine martensite structure and the average size of sub-grain; after hot stamping process, the comprehensive mechanical properties of the steel B with 0.30%C, 0.34%Si and 1.21%Mn are the most outstanding, the yield strength is 1161 MPa, the UTS is 1758 MPa, and the elongation is 6.5%; after hot stamping process, the microstructure of the steel B is fine lath martensite, and there is a large amount of dislocation in the martensite lath, and precipitates a small number of carbide. The mechanical properties of the ultra-high strength press hardening steels designed in this work is not obvious correlation before and after hot stamping process, and it is just a slight difference in martensite fine structure which is beneficial to controlling the performance stability of the mass industrial production.

Key words:  press hardening steel      martensite      undercooling austenite      ultra-high strength     
Received:  20 November 2017     
ZTFLH:  TG142  

Cite this article: 

Kuanhui HU, Xinping MAO, Guifeng ZHOU, Jing LIU, Zhifen WANG. Effect of Si and Mn Contents on the Microstructure and Mechanical Properties of Ultra-High Strength Press Hardening Steel. Acta Metall Sin, 2018, 54(8): 1105-1112.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00487     OR     https://www.ams.org.cn/EN/Y2018/V54/I8/1105

Steel C Si Mn P S Als Cr+Mo+Nb Fe
A 0.29 0.33 0.57 0.005 0.003 0.027 <0.80 Bal.
B 0.30 0.34 1.21 0.005 0.004 0.030 <0.80 Bal.
C 0.30 0.25 1.20 0.005 0.003 0.027 <0.80 Bal.
D 0.30 0.38 1.20 0.005 0.003 0.031 <0.80 Bal.
Table 1  Chemical compositions of the steels (mass fraction / %)
Fig.1  OM images of the steels A (a), B (b), C (c) and D (d) after finishing rolled at 870 ℃ (B—bainite, F—ferrite, P—pearlite, M—martensite)
Steel Before After
Rp0.2 / MPa Rm / MPa A50 mm / % Rp0.2 / MPa Rm / MPa A50 mm / %
A 552 757 20.4 1102 1686 7.2
B 751 1124 9.4 1161 1758 6.5
C 783 924 11.7 1073 1644 6.0
D 770 1196 12.6 1108 1705 6.5
Table 2  Mechanical properties of the steels before and after heat treatment
Fig.2  OM images of the steels A (a), B (b), C (c) and D (d) after heat treatment (950 ℃ for 5 min and oil quenching)
Fig.3  SEM images of the steels A (a), B (b), C (c) and D (d) after heat treatment
Fig.4  EBSD images of the steels A (a), B (b), C (c) and D (d) after heat treatment
Fig.5  TEM images of the steels A (a), B (b), C (c) and D (d) after heat treatment
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