Effect of Austenitizing Temperature on the Microstructure and Mechanical Properties of a 2 GPa Ultra-High Strength Steel

doi:10.11900/0412.1961.2024.00005

Acta Metall Sin

2025, Vol. 61

Issue (9): 1353-1363 DOI: 10.11900/0412.1961.2024.00005

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Effect of Austenitizing Temperature on the Microstructure and Mechanical Properties of a 2 GPa Ultra-High Strength Steel

ZHANG Tianyu¹, ZHANG Peng², XIAO Na³, WANG Xiaohai², LIU Guoqiang², YANG Zhigang¹, ZHANG Chi¹(

)

1 School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2 National Key Laboratory of Special Vehicle Design and Manufacturing Integration Technology, Norinco Group Inner Mongolia First Machinery Group Co. Ltd., Baotou 014030, China
3 Beijing Automotive Technology Center Co. Ltd., Beijing 101300, China

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ZHANG Tianyu, ZHANG Peng, XIAO Na, WANG Xiaohai, LIU Guoqiang, YANG Zhigang, ZHANG Chi. Effect of Austenitizing Temperature on the Microstructure and Mechanical Properties of a 2 GPa Ultra-High Strength Steel. Acta Metall Sin, 2025, 61(9): 1353-1363.

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Abstract

Recently, 2 GPa grade ultra-high strength steel has emerged as a potential candidate material for torsional axles. Nevertheless, studies focusing on the correlation between the microstructure and mechanical properties are scarce. To address this, the current study investigates the impact of austenitizing temperature on the microstructure, tensile mechanical properties, and static torsional properties of 2 GPa grade ultra-high strength steel used in torsion axles. Various techniques including SEM, EBSD, AES, TEM, uniaxial tension, and static torsion are employed. During the austenitizing process, the lamellar cementite in the initial microstructure (composed of pearlite and a minimal amount of ferrite) first spheroidized and then dissolved. This spheroidization process is primarily dominated by discontinuity-assisted spheroidization, with minimal contributions from termination-migration-assisted spheroidization. With increasing austenitizing temperature, the cementite gradually transforms from (Fe, Cr, V)₃C to Fe₃C, eventually completely dissolving (at the austenitizing temperature of 950 ^oC). Moreover, the precipitation temperature range of vanadium carbide is consistent with the temperature range of undissolved cementite. Additionally, austenitizing at 850 ^oC and tempering at 220 ^oC results in better tensile properties, including a yield strength of 1580 MPa, tensile strength of 2062 MPa, uniform elongation of 8.4%, and total elongation of approximately 12.7%. These improvements are attributed to the precipitation strengthening enabled by cementite and vanadium carbide and the fine grain strengthening provided by fine martensite block sizes. The static torsion test results show that when the austenitizing temperature is 800 and 850 ^oC, the sample shows the best shear moduli and torsional yield strength due to the increased cementite and vanadium carbide contents, along with fine martensite block sizes. With increasing austenitizing temperature, the shear plastic deformation zone expands, and the predominant fracture mechanism changes from shear fracture to shear ductile fracture, resulting in higher torsional strengths.

Key words: austenitizing temperature cementite tensile mechanical property torsional property

Received: 11 January 2024

ZTFLH:

TG142

Fund: Inner Mongolia Autonomous Region Science and Technology Major Special Project(2020ZD0027);Open Research Fund from National Key Laboratory of Special Vehicle Design and Manufacturing Integration Technology(GZ-2022KF005)

Corresponding Authors: ZHANG Chi, professor, Tel: (010)62782361, E-mail: chizhang@mail.tsinghua.edu.cn

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	ZHANG Tianyu
	ZHANG Peng
	XIAO Na
	WANG Xiaohai
	LIU Guoqiang
	YANG Zhigang
	ZHANG Chi

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https://www.ams.org.cn/EN/10.11900/0412.1961.2024.00005 OR https://www.ams.org.cn/EN/Y2025/V61/I9/1353

Fig.1 Schematic of the heat treatment process (A_c3—austenite transformation finish temperature, AC—air cooling, OQ—oil quenching)

Fig.2 SEM images of the hot-rolling sample (a, b), AES selected area (c) and the element profiles (d)

Fig.3 SEM images of the sample subjected to different austenitizing temperatures of 800 ^oC (a), 850 ^oC (b), 900 ^oC (c), 950 ^oC (d), and 1000 ^oC (e) and tempering at 220 ^oC (The corresponding samples are named Q800T, Q850T, Q900T, Q950T, and Q1000T), and change curves of cementite diameter and volume fraction with austenitizing temperature (f) (Arrows in Figs.3a-c represent the cementites)

Fig.4 Element profiles of precipitate in the Q800T (a), Q850T (b), and Q900T (c, d) samples
(a-c) element distributions of cementite (d) element distributions of VC

Fig.5 TEM images and corresponding STEM-EDS mappings of the Q800T (a), Q850T (b), Q900T (c), and Q950T (d) samples

Fig.6 EBSD orientation imaging mappings of the Q800T (a), Q850T (b), Q900T (c), Q950T (d), and Q1000T (e) samples and statistical diagram of martensitic block sizes (f)

Fig.7 Grain boundary distributions in the Q800T (a), Q850T (b), Q900T (c), Q950T (d), and Q1000T (e) samples (blue line: 2°-15°, red line: 15°-45°, yellow line: > 45°) and statistical diagram of high angle grain boundary ratios (f)

Fig.8 Tensile mechanical properties of the samples with different heat treatments
(a) strength (b) elongation

Fig.9 Torsional shear stress-strain curves of the samples with different heat treatments (Fig.9b is the enlarged view of Fig.9a)

Table 1 Torsion properties of the samples with different heat treatments

Fig.10 Driving force of VC varies with the austenitizing temperature calculated by Thermo-Calc

Fig.11 SEM images of torsional fracture surfaces of the Q800T (a-c), Q850T (d-f), and Q1000T (g-i) samples
(a, d, g) overviews of the fracture surfaces (b, c, e, f, h, i) crack propagation regimes

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