STEEL SHEET HAVING COMPOSITE STRUCTURE, SUPERIOR BENDABILITY AND LOW YIELD RATIO AND ITS MANUFACTURE

摘要

PURPOSE:To obtain the titled steel sheet by providing a structure of a single ferrite phase in which a phase formed by transformation at a low temp. is not practically present to the prescribed surface layers of a steel sheet having a low yield ratio and a composite structure. CONSTITUTION:A hot rolled steel sheet is heated to a temp. in the range in which ferrite and austenite phases coexist with each other, and it is cooled to obtain a hot rolled steel sheet having a low yield ratio and a composite metallic structure contg. a phase formed by transformation at a low temp. and dispersed in the matrix of a ferrite phase. In this method, the surfaces of the hot rolled steel sheet are decarburized before said heating and cooling to form decarburized surface layers of 20-100mum thickness. A structure of a single ferrite phase in which a phase formed by transformation at a low temp. is not practically present can be provided to the surface layers of 20-100mum thickness, so the bendability of the steel sheet can be considerably improved.