HOT-ROLLED STEEL SHEET FOR NITRIDING AND COLD-ROLLED STEEL SHEET FOR NITRIDING WITH EXCELLENT FATIGUE STRENGTH AND MANUFACTURING METHOD THEREFOR, AS WELL AS AUTOMOBILE PARTS OF EXCELLENT FATIGUE STRENGTH USING SAME

摘要

A hot-rolled steel sheet for nitriding or cold-rolled steel sheet for nitriding of excellent fatigue strength, wherein the steel contains appropriate amounts of Cr, V, and B in particular, and the dislocation density within 50 mum from the surface in the sheet thickness direction is 2.0 to 10.0 times the dislocation density at the ¼ position in the sheet thickness direction. The manufacturing method therefor is characterized in that after acid pickling the hot-rolled steel sheet or cold-rolled steel sheet comprising said components, skin pass rolling is performed under the conditions of the rolling reduction being 0.5 - 5.0%, and the ratio F/T of the line load F (kg/mm), which is the rolling mill load divided by the steel sheet board width, to the load per unit area T (kg/mm2), which is applied on the longitudinal direction of the steel sheet, being 8000 or more. Said hot-rolled steel sheet or cold-rolled steel sheet is then molded to produce nitrided automobile parts.