Influence of Two-Step Bainite Transformation on Toughness in Medium-Carbon Micro/Nano-Structured Steel

doi:10.11900/0412.1961.2019.00065

Acta Metall Sin

2019, Vol. 55

Issue (12): 1503-1511 DOI: 10.11900/0412.1961.2019.00065

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Influence of Two-Step Bainite Transformation on Toughness in Medium-Carbon Micro/Nano-Structured Steel

WAN Xiangliang¹,HU Feng^1,^2,³(

),CHENG Lin^2,³,HUANG Gang^2,³,ZHANG Guohong^2,³,WU Kaiming^1,^2,³

1. The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
2. Hubei Collaborative Innovation Center for Advanced Steels, Wuhan University of Science and Technology, Wuhan 430081, China
3. International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan 430081, China

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WAN Xiangliang, HU Feng, CHENG Lin, HUANG Gang, ZHANG Guohong, WU Kaiming. Influence of Two-Step Bainite Transformation on Toughness in Medium-Carbon Micro/Nano-Structured Steel. Acta Metall Sin, 2019, 55(12): 1503-1511.

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Abstract

Micro/nano-structured bainitic steel provides a unique combination of ultra-high strength and high ductility due to their structure consisting of micro/nano-scale bainitic-ferrite and retained austenite, but the toughness is a little bit low. The retained austenite plays a leading role for the toughness, and it can significantly increase the toughness of micro/nano-structured bainitic steel by refining the size of blocky retained austenite and improving the content of film retained austenite. Simultaneously, the structure of retained austenite affects the stability of retained austenite, and even can change the micro-deformation and determine the toughness. This work has been refined retained austenite of medium-carbon bainitic steel by using two-step bainitic transformation to study phase transformation of retained austenite through heat treatment. The effect of retained austenite on the impact toughness in medium-carbon micro/nano-structured steels was analyzed by one (300 ℃ for 6 h) and two-step (300 ℃ for 2 h, then 250 ℃ for 24 h) bainitic transformation processes. The microstructure, phase fraction, misorientation, crystallographic grain size and impact energy of different heat treatment steels were observed, detected and analyzed. The results showed that the impact property of two-step bainitic transformation was significantly higher than that of one-step bainitic transformation in medium-carbon steel, which the impact energy in -40 ℃ increased from 31 J to 42 J. The main reason is the new bainitic ferrite was formed in two-step bainitic transformation, the untransformed retained austenite was divided and refined by new bainitic-ferrite, reducing the formation of massive martensite during water quenching after isothermal bainite process. It significantly improve the toughness of the steel because the fracture energy was increased, owing to making crack bifurcation and even preventing the propagation of cracks in the impact process. Through the above-mentioned studies, this research not only precisely refines the retained austenite structure, reveals the effect of retained austenite stability on deformation mechanism and resolves toughness mechanism, but also provides the theoretical guidance for the production of micro/nano-structured bainitic steels in combination with good toughness.

Key words: medium-carbon micro/nano-structured steel heat treatment process two-step bainitic transformation retained austenite impact toughness

Received: 11 March 2019

ZTFLH:

TG113.1

Fund: National Natural Science Foundation of China(No.51601134)

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	WAN Xiangliang
	HU Feng
	CHENG Lin
	HUANG Gang
	ZHANG Guohong
	WU Kaiming

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https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00065 OR https://www.ams.org.cn/EN/Y2019/V55/I12/1503

Table 1 Mechanical properties of the investigated steel

Fig.1 OM images of medium-carbon steel for one-step (a) and two-step (b) bainitic transformations

Fig.2 SEM images of medium-carbon steel for one-step (a, c, e) and two-step (b, d, f) bainitic transformations (RA/M—mixture of retained austenite and martensite, RA—retained austenite, BF—bainitic ferrite；area of dashed lines in Figs.2c and d show block RA/M)

Table 2 Volume fractions of phases for the investigated steel (%)

Fig.3 EBSD orientation maps of medium-carbon steel for one-step (a, b) and two-step (c, d) bainitic transformations(a, c) bcc structure (b, d) fcc structure

Fig.4 Grain boundary angular distributions of medium-carbon steel for one-step (a) and two-step (b) bainitic transformations

Fig.5 Effective grain size distributions of medium-carbon steel for one-step (a) and two-step (b) bainitic transformations

Fig.6 Calculated TTT curve (a) and bainitic transformation curve (b) (A→F indicate austenite transformed to ferrite, A→P indicate austenite transformed to pearlite, A→B indicate austenite transformed to bainite, A→M indicate austenite transformed to martensite, T₀—equilibrium transformation temperature,

T 0'

—equilibrium transformation temperature in strain energy, A_e3—para-equilibrium transformation temperature,

A e 3'

—para-equilibrium transformation temperature in strain energy)

Fig.7 Schematics of medium-carbon steel for one-step (a) and two-step (b) bainitic transformations (A_c3—temperature of ferrite completely transformed into austenite during heating, B_s—starting transformation temperature of bainite, M_s—starting transformation temperature of martensite, AT—austenitizing temperature, BT—bainitic transformation temperature, WQ—water quenching, γ₀—austenite microstructure in austenitizing process, γ₁—austenite microstructure in one-step bainitic transformation process, γ₂—austenite microstructure in two-step bainitic transformation process,B₁—bainite microstructure in one-step bainitic transformation process, B₂—bainite microstructure in two-step bainitic transformation process, M₂—martensite microstructure in water quenching process for one-step bainitic transformation, M₃—martensite microstructure in water quenching process for two-step bainitic transformation, C_i—carbon content of alloying element i, C_γ₀—carbon content of austenite microstructure in austenitizing process, C_γ₁—carbon content of austenite microstructure in one-step bainitic transformation process, C_γ₂—carbon content of austenite microstructure in two-step bainitic transformation process)

Fig.8 TEM images of medium-carbon steel for two-step bainitic transformations (Area of dashed lines show block RA/M)(a) new micro/nano-size bainitic-ferrite(b) nano-size martensite

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