Study on Microstructure and Toughness of Simulated Coarse Grain Heated Zone in Normalized V-N-Ti and Nb-V-Ti Marine Steel

doi:10.11901/1005.3093.2020.206

Chinese Journal of Materials Research

2020, Vol. 34

Issue (12): 939-948 DOI: 10.11901/1005.3093.2020.206

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Study on Microstructure and Toughness of Simulated Coarse Grain Heated Zone in Normalized V-N-Ti and Nb-V-Ti Marine Steel

SHI Zhongran¹^,²(

), ZHAO Qingkai³, LIU Denghui³, WANG Tianqi¹^,², CHAI Xiyang¹^,², LUO Xiaobing¹^,², CHAI Feng¹^,²

^1.Department of Structural Steels, Central Iron and Steel Research Institute, Beijing 100081, China
^2.National New Material Production and Application Demonstration Platform (Advanced Marine Engineering and High-tech Ship Materials), Luoyang 471000, China
^3.Cnooc China Limited Tianjing Branch, Tianjin 300452, China

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SHI Zhongran, ZHAO Qingkai, LIU Denghui, WANG Tianqi, CHAI Xiyang, LUO Xiaobing, CHAI Feng. Study on Microstructure and Toughness of Simulated Coarse Grain Heated Zone in Normalized V-N-Ti and Nb-V-Ti Marine Steel. Chinese Journal of Materials Research, 2020, 34(12): 939-948.

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Abstract

The microstructure and toughness of the simulated coarse grain heat affected zone (CGHAZ) of the normalized microalloying offshore steels V-N-Ti and Nb-V-Ti were investigated by means of a welding thermal simulator. The results show that the perculiar microstructure led to better CGHAZ toughness for the V-N-Ti steel. For V-N-Ti steel, the high N content increases the precipitation temperature of Ti-rich (Ti, V)(C, N) particles and the ferrite nucleation ability, so that refines the original austenite grain and decreases the effective grain size with high boundary tolerance angle 15^o, hence, the fine polygonal ferrite could efficiently deflect or even arrest the propagation of cleavage microcracks, so it had good low temperature CGAHZ toughness. While there exist chain-like M-A on original austenite grain boundaries, coarse original austenite grains and larger amount of fine grains with high boundary tolerance angle 15^o that may be responsible to the lower simulated CGHAZ toughness of Nb-V-Ti steel.

Key words: V-N-Ti steel normalized steel simulated CGHAZ polygonal ferrite

Received: 30 May 2020

ZTFLH:

TG406

Fund: Shandong Provincial Natural Science Foundation of China(ZR201911170022)

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	Zhongran SHI
	Qingkai ZHAO
	Denghui LIU
	Tianqi WANG
	Xiyang CHAI
	Xiaobing LUO
	Feng CHAI

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https://www.cjmr.org/EN/10.11901/1005.3093.2020.206 OR https://www.cjmr.org/EN/Y2020/V34/I12/939

Table1 Chemical composition of the tested steel (mass fraction, %)

Fig.1 Schematic diagram of heat affected zone of multilayer multipass welding

Fig.2 Sampling diagram of welding joint

Table 2 Welding parameters of double submerged-arc welding

Fig.3 Impact of simulated CGHAZ at different temperature. (a) simulated CGHAZ; (b) welded joint

Fig.4 Simulated CGHAZ microstructures of experimental steels. (a, b) V-N-Ti steel；(c, d) Nb-V-Ti steel

Fig.5 TEM micrographs of M-A in simulated CGHAZ. (a, b) V-N-Ti steel; (c, d) Nb-V-Ti steel

Fig.6 Prior austenite grain size of simulated CGHAZ in experiment steels. (a) V-N-Ti steel, (b) Nb-V-Ti steel

Fig.7 EBSD orientation images with grain boundary misorientation distribution at cooling times of t_8/5 30 s. (a, c) V-N-Ti steel; (b, d) Nb-V-Ti steel

Fig.8 TEM carbon replica analysis micrographs of particles in simulated CGHAZ. (a, c) V-N-Ti steel; (c, d) Nb-V-Ti steel

Fig.9 SEM micrographs showing impact fracture surface morphologies. (a) V-N-Ti steel, (b) Nb-V-Ti steel

Fig.10 SEM analysis of cross-sectional area beneath fracture surface for the impact specimen of experimental steels fractured at -60℃. (a) V-N-Ti steel; (b, c) Nb-V-Ti steel

Fig.11 Mass fraction of second phase precipitates as a function of temperature for the experimental steels. (a) V-N-Ti steel; (b) Nb-V-Ti steel

Fig.12 The effect grain size with tolerance 15^o was distinguished by EBSD. (a) V-N-Ti steel; (b) Nb-V-Ti steel

Table 3 Results of M-A quantitative statistics

Fig.13 SEM micrograph and precipitates morphology for simulated CGHAZ in V-N-Ti steel (a) SEM micrograph and (b) precipitate morphology and chemical composition

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