X-ray diffraction analysis is one of powerful tools on the dislocation analysis and this method can be applied reasonably for many metals with isotropic crystal structure such as bcc and fcc. In this study, modified Williamson-Hall analysis was applied for martensitic steels containing 0.006 -0.26 mass% carbon and proved that the value of dislocation density increases with increasing the carbon content. However, martensitic steels containing solute carbon have bct structure characterized by different lattice constants on a-axis and c-axis. With increasing solute carbon, a-axis shrinks but c-axis is elongated. This leads to the peak séparation in an X-ray diffraction peak and causes an increase of the full-width at half-maximum (FWHM) in the diffraction peak. This suggests that the value of dislocation density is over estimated due to the effect of peak separation in as quenched martensitic steels with solute carbon. It was found that the increment of apparent dislocation density Δρ' is expressed by the following equation as a function of the amount of solute carbon (mass%C), independent of the values of true dislocation density and the screw component of dislocation. Δρ[m −2 ] =1.68×10 17 (mass%C) 2 As a result, it is concluded that the true dislocation density is constant at 4.5×10 15 m 2 in martensitic steels which have solute carbon more than 0.14 mass% at least.

In order to fabricate new type of low carbon dual phase steel with excellent local deformability, cold-deformed lath martensite was intercritically annealed at ferrite and austenite two phase region at very high heating rate of 500 K/s by high-frequency induction heating in a low carbon chromium-molybdenum steel (0.15%C-1.15%Cr-0.15%Mo). No recrystallization of lath martensite to ferrite took place upon heating because of the fast heating and the pinning effect of very ne M 3 C particles. Consequently, austenite grains densely nucleated in the non-recrystallized lath martensite matrix during intercritical annealing, resulting in the formation of very ne dual phase structure consisting of tempered lath martensite matrix and nely dispersed fresh martensite grains. The very ne structured dual phase steel exhibited higher local deformability while maintaining good strength-elongation balance compared with the dual phase steel with recrystallized-ferrite matrix due to the high density of distribution of ne fresh martensite grains and low hardness ratio of fresh martensite to matrix.

The formation mechanism of a carbonized layer was investigated under low-pressure and low-temperature process conditions. The initial carbonized layer under those conditions was formed epitaxially using the silicon atoms sublimated from substrate and the carbon atoms of the gas source. This result is suggested from the consideration of pit formation mechanism at the Si/SiC interface. After the initial layer was formed, the carbonized layer grew by the diffusion process of the carbon atoms through the crystal defects in the initially formed layer. This is suggested from that the thickness of the carbonized layer increases linearly with the square root of the process time. The growth rate seemed to be determined by the concentration of carbon atoms taken into the SiC.

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