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AssigneesKabushiki Kaisha Kobe Seiko Sho, Shinshu Tlo Co., Ltd.
InventorsShushi Ikeda, Koichi Sugimoto, Yoichi Mukai, Hiroshi Akamizu
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The present invention provide a TRIP-type composite structure steel plate of the TPF steel type in which elongation and stretch flange formability at room temperature are improved by controlling the morphology of the second-phase structure. In a composite structure sheet steel comprising 0.02 to 0.12% C, 0.5 to 2.0% Si+Al and 1.0 to 2.0% Mn, with the remainder being Fe and unavoidable impurities, and comprising 80% or more polygonal ferrite (steel structure space factor) and 1 to 7% retained austenite, with the remainder being bainite and/or martensite, wherein the elongation and stretch flange formability of the composite sheet steel are improved by reducing the number of bulky, massive second phases with an aspect ratio of 1:3 or less and a mean grain size of 0.5 μm or more in the second phase of this composite structure, which comprises retained austenite and martensite.
A composite structure sheet steel with excellent elongation and stretch flange formability comprising 0.02 to 0. 12% C, 0.5 to 2.0% Si+Al and 1.0 to 2.0% Mn by mass percentage, with the remainder being Fe and unavoidable impurities, and comprising 80% or more polygonal ferrite (steel structure space factor) and 1 to 7% retained austenite (volume fraction measured by the saturation magnetization method), with the remainder being bainite and/or martensite, wherein a second-phase structure in the composite structure comprises martensite and retained austenite, and within the second-phase structure not more than 15 massive second-phase structures with an aspect ratio of 1:3 or less and a mean grain size of 0.5 μm or more are contained per 750 μm2 as observed under a scanning electron microscope at 4000×.
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