Fractional Cooling Strategy of the Hot-Stamping Process and Its Influence on Formability and Mechanical Properties of Ultra-High-Strength Steel Parts

doi:10.1007/s40195-018-0832-3

Acta Metallurgica Sinica (English Letters) ›› 2019, Vol. 32 ›› Issue (3): 343-351.DOI: 10.1007/s40195-018-0832-3

所属专题： 2019年钢铁材料专辑

收稿日期:2018-07-05 修回日期:2018-08-15 出版日期:2019-03-10 发布日期:2019-02-22
作者简介:

Huan Liu is a Lecturer, Master’s Supervisor, College of Mechanics and Materials, Hohai University. He earned his Ph.D. from Southeast University in 2014 and then became a Lecturer in Hohai University. He was selected into the “Shuangchuang Program of Jiangsu Province” and “Dayu Scholars Program of Hohai University” in 2017. So far, he has published more than 30 scientific papers (indexed by SCI) and held 2 authorized Chinese patents. His papers were cited more than 200 times. His research interests mainly include design of high-strength and high ductility magnesium alloys, heat-resistant magnesium alloys, fabrication of fine-grained and ultra-fine-grained metallic materials, and biomedical materials.

Xian-Hua Chen is a Professor of Chongqing University and received his Doctor’s degree from Institute of Metal Research, Chinese Academy of Sciences in 2008. He is Director of Institute of Functional Mg Alloys in National Engineering Research Centre for Magnesium Alloys, Director of International Joint Laboratory for Light Alloys (Ministry of Education), Editorial Board of Acta Metallurgica Sinica (English Letters) (SCI). His research work is focused on new high-performance structural and functional magnesium alloys, and purification technology of magnesium alloys. He also worked in Materials Technology Laboratory of CANMET in Canada as visiting scientist during 2012-2013. He has 22 patents, 1 book and more than 60 SCI papers, including 2 science papers. His papers were cited more than 2700 times. He was awarded the Provincial and Ministerial S&T Prize in 2013, 2014 and 2017. He was the Chairman of “The 2nd China Youth Scholars Conference on Mg Alloys.”

Fractional Cooling Strategy of the Hot-Stamping Process and Its Influence on Formability and Mechanical Properties of Ultra-High-Strength Steel Parts

Xian-Hong Han¹, Cheng-Long Wang¹, Si-Si Chen^1,², Jun Chen¹()

1 Department of Plasticity Technology, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China
2 SAIC General Motors Co., Ltd, Shanghai 201206, China

Received:2018-07-05 Revised:2018-08-15 Online:2019-03-10 Published:2019-02-22

摘要/Abstract

Abstract:

The effects of forming temperature on the formability and product properties of hot-stamping boron steel B1500HS were investigated. Based on the fractional cooling strategy, boron steel sheets were heated to achieve full austenitization before they were removed from the furnace and cooled to the forming temperature using different cooling methods. Subsequently, they were simultaneously press-formed and quenched inside the tool until the martensitic transformation was finished. A series of thermal tensile tests were conducted to study the effects of forming temperatures on the stamping performance indices, including elongation, yield ratio, and hardening exponent. Then, the mechanical properties and microstructures of the hot-stamped products were characterized. Finally, an irregular part was formed using different fractional cooling strategies, while its formability and springback phenomena were discussed. The results show that using a fast-cooling method to reach 650 °C as the forming temperature optimizes the formability of the tested B1500HS boron steel. The best mechanical properties and smallest springback values were achieved using this optimal strategy.

Key words: Hot-stamping, Fractional cooling, Forming temperature, Formability

. [J]. Acta Metallurgica Sinica (English Letters), 2019, 32(3): 343-351.

Xian-Hong Han, Cheng-Long Wang, Si-Si Chen, Jun Chen. Fractional Cooling Strategy of the Hot-Stamping Process and Its Influence on Formability and Mechanical Properties of Ultra-High-Strength Steel Parts[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(3): 343-351.

图/表 14

Table 1 Chemical compositions (wt%) of B1500HS

C	Si	Mn	Al	B	S	P	Cr
0.23	0.24	1.3	0.046	0.0026	0.002	0.022	0.174

Fig. 1 Specimen for thermal tensile tests (unit: mm)

Fig. 2 True stress-strain curves at different forming temperatures

Table 2 Results of the stamping performance indicators against the forming temperature

Forming temperature (°C)	Yield strength (MPa)	Tensile strength (MPa)	Yield ratio	Elongation	Hardening exponent, n
600	159	324	0.491	0.269	0.25
650	143	294	0.486	0.344	0.21
700	131	259	0.506	0.309	0.21
750	122	230	0.530	0.305	0.19
800	106	191	0.555	0.292	0.18
850	94	163	0.577	0.271	0.15

Fig. 3 Stamping performance indicators at different forming temperatures: a yield strength, b yield ratio, c elongation, d hardening exponent n

Fig. 4 Thermal simulation scheme for the fractional cooling process

Fig. 5 Tensile specimen geometry for the thermal simulation experiments (unit: mm)

Table 3 Mechanical properties of B1500HS post different heat treatments

Forming temperature (°C)	σ_s (MPa)	σ_b (MPa)	δ (%)	σ_bδ (GPa%)
920	882	1558	10.5	16.3
800	1090	1547	10.7	16.6
750	1093	1538	10.8	16.6
700	1045	1547	11.9	18.4
650	1087	1530	12.8	19.6
600	1067	1563	12.7	19.8
550	1067	1549	12.8	19.8

Fig. 6 Microstructures of B1500HS at different forming temperatures. aQT = 920 °C, bQT = 650 °C

Fig. 7 Geometry of the unequal-height and unequal-width U-shaped Part (unit: mm)

Fig. 8 Hot-stamping experiment scheme with fractional cooling strategy

Fig. 9 Hot-stamping parts by integrating the fractional cooling process: with air cooling aQT = 550 °C, bQT = 600 °C, cQT = 650 °C, dQT = 700 °C, eQT = 750 °C, and with spray cooling fQT = 550 °C, gQT = 600 °C, hQT = 650 °C, iQT = 700 °C, jQT = 750 °C

Fig. 10 Section sketch of the springback analysis

Fig. 11 Springback results of different forming temperatures and cross sections. a Section 1, b Section 2, c Section 3, d Section 4, e Section 5, f Section 6, g Section 7, h Springback distribution (QT = 650 °C)

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Fractional Cooling Strategy of the Hot-Stamping Process and Its Influence on Formability and Mechanical Properties of Ultra-High-Strength Steel Parts

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