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A technology in the direction of steel plate and plate thickness, applied in the direction of furnace types, furnaces, heat treatment furnaces, etc., can solve the problems of reduced elongation and toughness, and achieve the effects of high strength, excellent impact energy absorption capacity, and excellent uniform elongation characteristics
Active Publication Date: 2021-08-17
NIPPON STEEL CORP
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However, generally, the higher the strength of the steel plate, the lower the elongation that affects the formability and the toughness that affects the impact characteristics.
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[0242] 1. Manufacture of steel plates for evaluation
[0243] Steels having the chemical compositions shown in Table 1 were melted in a converter and continuously cast to obtain 245 mm thick slabs.
[0244] [Table 1]
[0245] Table 1
[0246]
[0247] The obtained steel material (slab) was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of about 2.4 mm. The obtained hot-rolled steel sheet was subjected to heat treatment, pickling, and cold-rolling at the cold-rolling rate shown in Table 2 at a temperature and holding time to obtain the austenite phase fraction shown in Table 2 to obtain a steel sheet with a thickness of 1.4 mm. Cold rolled steel. The heat treatment of hot-rolled and hot-rolled steel sheets is carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
[0248] [Table 2]
[0249] Table 2
[0250]
[0251] The obtained cold-rolled steel sheets were subjected to primary annealing and sec...
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Abstract
A steel sheet having the following chemical composition in mass%: C: greater than 0.10% and less than 0.55%, Si: at least 0.001% and less than 3.50%, Mn: greater than 4.00% and less than 9.00%, sil.Al: at least 0.001% and less than 3.00%, P: not more than 0.100%, S: not more than 0.010%, N: less than 0.050%, O: less than 0.020%, Cr: at least 0% and less than 2.00%, Mo: 0 to 2.00%, W: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, B: 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Zr: 0 to 0.010%, REM: 0 to 0.010%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, Bi: 0 to 0.050%, and balance: Fe and impurities. In the L-cross section, at a position one-fourth of the thickness from the surface, the steel sheet has a metallographic composition, as an area ratio, of tempered martensite: 25 to 90%, ferrite: not more than 5%, retained austenite: 10 to 50%, and bainite: not more than 5%, wherein the area ratio of the total area of retained austenite crystal grains satisfying an area of at least 1 [mu]m2 and a grain circularity of at least 0.1, to the total area of the retained austenite, is less than 50%. The steel sheet satisfies CMn[gamma] / CMn[alpha]>=1.2.
Description
technical field [0001] The present invention relates to steel plates. Background technique [0002] In recent years, for the purpose of improving the fuel efficiency of automobiles and improving the crash safety, lightweight assembly of vehicle bodies based on the application of high-strength steel plates has prevailed. However, generally, the higher the strength of the steel sheet, the lower the elongation that affects the formability and the toughness that affects the crash properties. Therefore, in the development of high-strength steel sheets, it is an important subject to increase the strength without lowering the elongation and toughness. [0003] In order to improve the elongation, so-called TRIP steels utilizing the transformation-induced plasticity of retained austenite (residual γ) have hitherto been proposed (for example, Patent Document 1). [0004] Retained austenite is obtained by enriching and stabilizing C in austenite. For example, by adding carbide preci...
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