| 摘要 |
PURPOSE:To prevent internal grain coarsening at the time of carburizing treatment after warm forging, by adding Ni in combination with specific amounts of Al and N to a steel with a specific composition. CONSTITUTION:This case-hardending steel for warm forging has a composition consisting of, by weight, 0.10-0.30% C, <=0.50% Si, 0.3-2.0% Mn, 0.10-0.40% Ni, 0.60-2.0% Cr, 0.02-0.05% Al, 0.0010-0.0060% N, and the balance Fe and containing, if necessary, 0.10-0.30% Mo and also has a structure in which internal grains after carburizing treatment is formed into graded fine grains of grain size No. 6-10. In this steel, Ni is necessary in order to form initial austenite grains into graded fine grains of the above grain size number, neither coarse grains nor fine grains, at the time of carburizing heating by the combined addition with prescribed amounts of Al and N. For this purpose, >=0.10% is required as additive quantity and the above effect increases with the increase of the additive quantity, but, even if the additive quantity exceeds 0.40%, the above effect is saturated and steel manufacturing costs are needlessly increased.
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