The quantitative analysis of substructure in the martensite/bainite mixed structure,which is obtained from low-carbon NiCrMoV steels under different cooling conditions,was made by means of optical microscope (OM),scanning electron microscope (SEM),electron backscatter diffraction (EBSI)),and transmission electron microscope (TEM),in order to research the effect on toughness.The test results indicate that the toughness of the steel is enhanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0.38 μm.The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension.Furthermore,the effect of the block width on impact energy is much larger than that of the packet.Therefore,the block can be used as microstructural substructure to affect the toughness in low-carbon martensite steels,suggesting that the block size is "the effective grain size" for controlling toughness.
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