| 摘要 |
A forging steel excellent in hydrogen crack resistance is provided to effectively reduce hydrogen crack sensitivity of such forging steel by noticing that high clean steel in which light weight and fatigue strength required in a large steel forging such as a crank shaft are further improved highly efficiently damages hydrogen crack resistance, and a crank shaft manufactured by forging the forging steel is provided. A forging steel excellent in hydrogen crack resistance contains, by weight percent, 0.2 to 0.6% of C, 0.1 to 0.4% of Si, 0.7 to 1.5% of Mn, 1.0% or less of Ni, 1.2 to 3.5% of Cr, 0.1 to 0.6% of Mo, 0.01 to 0.05% of Al, and 0.005% or more of at least one of Ti, Zr, Hf and Nb, wherein an average value(an average circularity) of circularities of inclusions that are contained in steel and have the maximum subtense length of 1 mum or more is 0.5 or more, the number of inclusions having the maximum subtense length of 20 mum or more is less than 40 per 100 mm^2, and the number of inclusions having an average circularity of 0.25 or more and the maximum subtense length of 1 to 10 mum or more is 100 or more per 100 mm^2. |
