REGULATION OF MULTI-PHASE MICROSTRUCTURE AND MECHANICAL PROPERTIES IN A 700 MPa GRADE LOW CARBON LOW ALLOY STEEL WITH GOOD DUCTILITY

doi:10.11900/0412.1961.2014.00576

Acta Metall Sin

2015, Vol. 51

Issue (4): 407-416 DOI: 10.11900/0412.1961.2014.00576

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REGULATION OF MULTI-PHASE MICROSTRUCTURE AND MECHANICAL PROPERTIES IN A 700 MPa GRADE LOW CARBON LOW ALLOY STEEL WITH GOOD DUCTILITY

ZHOU Wenhao(

), XIE Zhenjia, GUO Hui, SHANG Chengjia

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083

Cite this article:

ZHOU Wenhao, XIE Zhenjia, GUO Hui, SHANG Chengjia. REGULATION OF MULTI-PHASE MICROSTRUCTURE AND MECHANICAL PROPERTIES IN A 700 MPa GRADE LOW CARBON LOW ALLOY STEEL WITH GOOD DUCTILITY. Acta Metall Sin, 2015, 51(4): 407-416.

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Abstract

Low carbon and low alloy steels require good combination of strength and ductility to ensure safety and stability of structures. Heat treatment in intercritical area can not only produce multi-phase microstructure, but also lead to the redistribution of alloying elements in different phases. Multi-step intercritical heat treatment is favorable to obtain retained austenite that is stabilized by repeated enrichment of alloying elements in reversed austenite and nanometer-sized precipitate that are primarily formed during tempering. Excellent mechanical properties are contributed by transformation-induced-plasticity effect of retained austenite and precipitation hardening effect of nanometer-size precipitates. In this work, the microstructural evolution and relative mechanical properties were investigated in a low carbon low alloy steel processed by a three-step heat treatment, namely, intercritical annealing, intercritical tempering and tempering. The microstructure was a typical dual-phase microstructure consisting of intercritical ferrite and bainite/martensite after intercritical annealing, and primarily comprised of intercritical ferrite, tempered bainite/martensite and retained austenite after intercritical tempering. Retained austenite with volume fraction of 29% distributed at the ferrite/bainite (martensite) boundaries and betweent bainitic/martensitic laths. Retained austenite was stabilized by enrichment of C, Mn, Ni and Cu in reversed austenite during the reversion transformation process. NbC precipitates with average size of 10 nm was formed in ferrite matrix and bainite/martensite, while Cu-containing particles in size range of 10~30 nm precipitated in ferrite and retained austenite during intercritical tempering and tempering process. The morphology of NbC precipitates was spherical, elliptical and irregular, and copper precipitates were spherical. With the combination of transformation-induced-plasticity (TRIP) effect of retained austenite and precipitation hardening, the steel possessed outstanding mechanical properties: yield strength > 700 MPa, tensile strength > 900 MPa, uniform elongation > 20%, and total elongation > 30%.

Key words: high performance intercritical heat treatment multi-phase microstructure retained austenite nanometer-sized precipitate

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	Wenhao ZHOU
	Zhenjia XIE
	Hui GUO
	Chengjia SHANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2014.00576 OR https://www.ams.org.cn/EN/Y2015/V51/I4/407

Fig.1 Schematic of heat treatment of the experimental steel (A_c1 and A_c3 are transformation start and finish temperatures from bcc to fcc during the reheating process, respectively. A_c1' and A_c1" are transformation start temperatures from bcc to fcc during the reheating process after the first- and second-step heat treatment, respectively)

Fig.2 Determination of critical point of experimental steel in different heat treatment process by dilatometric method

Fig.3 OM images of experimental steels processed after different heat treatments with air cooling

Fig.4 SEM images of experimental steels after different heat treatments with air cooling

Table 1 Mechanical properties of experimental steels after different heat treatment steps

Fig.5 EBSD images of retained austenite in sample A (a), sample B (b) and sample C (c), and corresponding XRD spectra (d)

Fig.6 Bright (a) and dark (b) field TEM images of sample B

Fig.7 TEM images (a, c, e) and statistical size analysis (b, d, f) of carbon replica extraction indicating the distribution of Nb-containing precipitates in hot-rolled sample (a, b), sample A (c, d) and sample B (e, f)

Table 2 Distribution of alloying elements in different phases in experimental steel annealed at 780 ℃ and tempered at 660 ℃ for 30 min at equilibrium state

Fig.8 TEM image (a) and statistical size analysis (b) of copper-containing precipitates in sample B, TEM images (c~e) and EDS (f) of Cu-containing precipitates in sample C

Table 3 Diffusion distances of austenite stabilizers elements at different temperatures (2D space)

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