超微細粒組織を活用した低炭素鋼の強靭化

概要

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A low-carbon steel bar with an ultrafine elongated grain (UFEG) structure and an ultrafine exiaxed grain (UFG) structure was fabricated by multipass caliber rolling at 500°C and subsequent annealing at 500, 550, 600 and 650°C. The Charpy impact and static tensile tests were conductued at ambient temperature. The microstructures in the as-rolled bar and the annealed bars until 600°C consisted of an UFEG structure with a strong α-fiber texture. The both strength and impact energy increased in the as-rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain size of 0.73 μm. In the bars with the UFEG structure, many microcracks during an impact tests were observed near the main crack with fine dimples, and they were classified into two types: one normal to the striking direction (SD) and one with an angle of 45° to the SD. The occurrence of such brunching cracks corresponded to the spatial distribution of {100} cleavage planes and boundaries of the elongated grains. This tendency disappeared as annealing temperatures increased. The as-rolled bar exhibited the best strength-toughness balance in the annealed bars, conventional carbon steels and some UFG steels. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation.
一般社団法人日本機械学会の論文