| 摘要 |
PROBLEM TO BE SOLVED: To obtain the steel which is composed of bainitic structure and in which the length-to-width ratio of old austenite grains in the region between the surface layer and a position at a specific depth from the surface layer is regulated to a specific value or above by regulating the temp. at the time of thread rolling of a bolt steel of specific composition and also regulating the cooling method after thread rolling. SOLUTION: At the time of applying thread rolling to a steel having a composition consisting of, by weight, 0.15-0.50% C, 0.05-2.0% Si, 0.2-2.0% Mn, 0.005-0.1% A1, and the balance Fe, thread rolling is performed at least at 700-900 deg.C and then held, without delay, at 200-500οC for 10-3000sec. by this method, the steel for machine structural use, which has bainitic structure and in which the length-to-width ratio (aspect ratio) of old austenite grains in the region between the surface layer of a screw and a position at a depth of >=300μm from surface layer is regulated to >=1.2 and also to >=1300MPa tensile strength and excellent delayed fracture resistance are provided, can be obtained. Moreover, prescribed amounts of Ti, B, Cr, Mo, Ni, Cu, V, Nb, Ta, and W can further be added to the above composition.
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