Effect of Ausforming on the Microstructures and Mechanical Properties of an Ultra-High Strength Bainitic Steel

doi:10.11900/0412.1961.2020.00310

Acta Metall Sin

2021, Vol. 57

Issue (6): 749-756 DOI: 10.11900/0412.1961.2020.00310

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Effect of Ausforming on the Microstructures and Mechanical Properties of an Ultra-High Strength Bainitic Steel

LIU Man¹^,², HU Haijiang¹^,²(

), TIAN Junyu¹^,², XU Guang¹^,²

^1.State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
^2.Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China

Cite this article:

LIU Man, HU Haijiang, TIAN Junyu, XU Guang. Effect of Ausforming on the Microstructures and Mechanical Properties of an Ultra-High Strength Bainitic Steel. Acta Metall Sin, 2021, 57(6): 749-756.

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Abstract

Ultra-high-strength bainitic steels with excellent combinations of strength and ductility may be the new generation of metallurgical interest. However, there still exist some production problems, such as long transformation times due to low-temperature processing and difficulty in tailoring the elongation. In this work, both ausforming and austempering were used to investigate the effects of deformation on the transformation and microstructure in a medium-carbon bainitic steel. The Gleeble 3500 simulator, SEM, TEM, XRD, and tensile tests were used to analyze the effects of ausforming on retained austenite, the strength and plasticity of bainitic steel. The results show that ausforming at 300^oC with a strain of 0.2 not only accelerates the kinetics of isothermal transformation, but also refines the bainitic microstructure and optimizes the retained austenite and its stability. The stability of the retained austenite is affected by the carbon content and dislocation density, and the carbon content can be increased by prolonging the duration of the isothermal stage. The volume fraction of retained austenite is increased by ausforming because of the enhanced dislocation density, which leads to ultra-high-strength bainitic steel with excellent properties of a tensile strength of 1733 MPa and ductility of 15.7%.

Key words: bainitic steel retained austenite ausforming microstructure mechanical property

Received: 17 August 2020

ZTFLH:

TG142.1

Fund: National Natural Science Foundation of China(51704217);Major Projects of Technology Innovation of Hubei Province(2017AAA116)

About author: HU Haijiang, associate professor, Tel: 13638676695, E-mail: huhaijiang@wust.edu.cn

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	Junyu TIAN
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https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00310 OR https://www.ams.org.cn/EN/Y2021/V57/I6/749

Fig.1 Schematic of thermal simulation experiment (M_s—martensite transformation start temperature)

Fig.2 SEM images of non-deformed (a) and deformed (b) samples at room temperature (M—martensite, BF—bainite ferrite, RA—retained austenite)

Fig.3 Low (a, b) and high (c, d) magnified TEM images of non-deformed (a, c) and deformed (b, d) samples

Fig.4 Changes of relative dilatation of non-deformed and deformed samples during isothermal holding

Fig.5 The bainitic transformation rate curves of non-deformed and deformed samples

Fig.6 XRD spectra of non-deformed and deformed samples

Table 1 Results of tensile tests of non-deformed and deformed samples

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