硫化物系介在物の形状制御による低合金構造用鋼の機械的性質の改善とフラクトグラフィ

抄録

Structural low alloy steels, S45C and SNCM 439, have been studied to determine the effect of the shape of sulfide inclusions on improvement in the mechanical properties of quenched and highly tempered structural low alloy steels. The shape of sulfide inclusions was changed by varying the ratio of the hot-rolling reduction. Tensile and Charpy U-notch (CUN) impact tests were performed on the specimens with different testing orientations. Morphology of the sulfide inclusions was examined by an electron probe microanalizer and optical microscopy. Fractography was carried out by scanning electron microscopy. Modifing the shape of MnS inclusions from the stringer to the ellipse by decreasing hot-rolling reduction from 98 to 80% significantly increased room temperature fracture ductility in the transverse testing orientation independent of the steels. This also significantly improved the room temperature CUN impact energy in transverse testing orientation of the steels independent of notch orientation. However, the mechanical isotropy achieved by modifying the inclusions declined as test temperature decreased. Fractography revealed that the mechanical properties were improved when fracture occurred by ductile mode with MnS-void nucleation and growth, followed by localized shear between the inclusion-nucleated voids. The beneficial effect on the mechanical isotropy is discussed in terms of the model involving the formation of voids at inclusion sites and their growth and eventual coalescene by local shear during plastic deformation.