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Research Papers

Investigation on Microstructure and Martensitic Transformation Mechanism for the Warm-Stamped Third-Generation Automotive Medium-Mn Steel

[+] Author and Article Information
Xiaodong Li, Shuo Han, Daxin Ren, Ping Hu

School of Automotive Engineering,
State Key Lab of Structural Analysis
for Industrial Equipment,
Dalian University of Technology,
No. 2, Linggong Road,
Dalian 116024, China

Ying Chang

School of Automotive Engineering,
State Key Lab of Structural Analysis
for Industrial Equipment,
Dalian University of Technology,
No. 2, Linggong Road,
Dalian 116024, China
e-mail: yingc@dlut.edu.cn

Cunyu Wang, Han Dong

Central Iron & Steel
Research Institute (CISRI),
Beijing 100081, China

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received November 24, 2016; final manuscript received April 11, 2017; published online June 30, 2017. Assoc. Editor: Vadim V. Silberschmidt.

J. Eng. Mater. Technol 139(4), 041009 (Jun 30, 2017) (9 pages) Paper No: MATS-16-1344; doi: 10.1115/1.4037017 History: Received November 24, 2016; Revised April 11, 2017

With the development of the automotive industry, the application of the high-strength steel (HSS) becomes an effective way to improve the lightweight and safety. In this paper, the third-generation automotive medium-Mn steel (TAMM steel) is studied. The warm-stamped TAMM steel holds the complete and fine-grained martensitic microstructure without decarbonization layer, which contributes to high and well-balanced mechanical properties. Furthermore, the martensitic transformation mechanism of the TAMM steel is investigated by the dilatation tests. The results indicate that the effects of the loading method on the Ms temperature under different loads are different. The Ms temperature is hardly influenced under the tensile loads and low compressive load. However, it is slightly decreased under the high compressive load. Moreover, the effects of the strain and strain rate on the Ms temperature are insignificant and can be neglected. As a result, this research proves that the martensitic transformation of the TAMM steel is rarely influenced by the process parameters, such as stamping temperature, loading method, load, strain, and strain rate. The actual stamping process can be designed and controlled accurately referring to the continuous cooling transformation (CCT) curves to realize the required properties and improve the formability of the automotive part.

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Figures

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Fig. 1

Sample size of the room-temperature uniaxial tensile test (a) and schematic of sampling position (b)

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Fig. 2

Microstructures of three tested steels before stamping: (a) the uncoated 22MnB5 steel, (b) the Al–Si coated 22MnB5 steel, and (c) the TAMM steel

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Fig. 3

XRD diagram of TAMM steel before stamping

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Fig. 4

Tensile stress–strain curves before stamping

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Fig. 5

Microstructures of experimental steels after stamping: (a), (c), and (e) surface of uncoated 22MnB5 steel, coated 22MnB5 steel, and TAMM steel; (b), (d), and (f) core of uncoated 22MnB5 steel, coated 22MnB5 steel, and TAMM steel

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Fig. 6

Tensile stress–strain curves after stamping

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Fig. 7

Stress–strain curves of the isothermal compressive test of the TAMM steel

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Fig. 8

Samples of compressive dilatation tests under different compressive loads at 500 °C

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Fig. 9

Compressive load design of the warm-stamped TAMM steel

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Fig. 10

Radial dilatation curves of the TAMM steel under different stamping temperatures and compressive loads: (a) 500 °C, (b) 600 °C, and (c) 700 °C

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Fig. 11

Variations in the Ms temperature of the TAMM under different stamping temperatures and compressive loads

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Fig. 12

Samples of tensile dilatation tests under different loads at 500 °C

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Fig. 13

Radial dilatation curves of the TAMM steel under different tensile loads at 500 °C

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Fig. 14

Variations in the Ms temperature of the TAMM steel under different tensile loads at 500 °C

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Fig. 15

Microstructures of TAMM steels under tensile/compressive loads: (a) compressive and (b) tensile

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Fig. 16

Radial dilatation curves of the TAMM steel under different strains and strain rates

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Fig. 17

Variations in the Ms temperature of the TAMM steel under different strains and strain rates at 500 °C

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