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摘要
During rolling and heat treatment, due to the cooling rate and transformation kinetics from surface to center of large thickness marine steel are greatly different, which makes it difficult to completely reach the critical cooling rate on the cross section of the thick plate, thus forming martensite and bainite multiphase structure. Effect of multi-scale LM/LB substructure on strength and toughness of test steel by cross section microstructure of ultra-heavy plate observation and statistical analysis. The test results show that the tensile strength and yield strength are slightly lowered from the surface to the center, but the low temperature impact properties of the thick plate at different positions of the section are significantly different. (the average value of impact energy of the surface, 1/4 thickness(1/4θ (θ represents thickness)) and center at -40 ℃ were 139, 204.7 and 172 J, respectively). Due to the higher density lattice defects of martensite in the EBSD band slope (BS) diagram shows the characteristics of lower image quality, in combination with the Gaussian multi-peak fitting technique to quantitatively determine the mixed structure of 1/4θ lath martensite (LM) and lath bainite (LB) (68.3%LM+ 31.7%LB), two-phase mixture makes 1/4θ to achieve a high large-angle grain boundary ratio and lowest Kernel average misorientation (KAM) distribution, Geometrically necessary dislocation (GND) densities, making the 1/4θ have better impact toughness. Martensitic non-diffusive transformation is greatly affected by nucleation rate, while bainite transformation is jointly affected by nucleation rate and atomic diffusion rate, the cross-section effect of thick plate changes the nucleation rate and atomic diffusion rate, further affecting the multi-scale LM/LB substructure size. The Hall-Petch equation is introduced to describe the influence of LM/LB sub-structures at different scales on yield strength and impact toughness. The correlation coefficient R, between sub-structure size and strength and toughness is obtained by linear fitting numerical analysis. The larger the R-value is, the greater the influence of sub-structure on strength and toughness of materials is. Therefore, it is determined that lath and block are effective control units to determine the yield strength and toughness of quenched and tempered marine steel, respectively. ? 2023 Chinese Society for Metals.
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