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AssigneeNippon Steel Corporation
InventorsNaoki Yoshinaga, Kohsaku Ushioda, Osamu Akisue, Yoshikazu Matsumura, Kunio Nishimura
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The present invention provides a cold-rolled steel sheet and a hot-dip galvanized steel sheet having good bake hardenability, non-aging properties at room temperature and good formability and a process for producing the same. An extra low carbon steel or an extra low carbon steel containing at least one element selected from Ti and Nb is annealed at a temperature of not lower than the AC.sub.3 transformation point to bring the structure after annealing to a structure of low-temperature transformation products. This makes it possible to provide a steel sheet that has a combination of high paint-bake hardenability and non-aging properties at room temperature and is excellent also in formability in respect of average r value (deep drawability) and elongation (punch stretchability). In particular, with respect to paint-bake hardenability, a BH property on a high level up to about 10 kgf/mm.sup.2 can be imparted according to need, and it is possible to provide a cold-rolled steel sheet and a hot-dip galvanized cold-rolled steel sheet that also have a non-aging property at room temperature.
A cold-rolled steel sheet having excellent bake hardenability and non-aging properties at room temperature, consisting essentially of, in terms of % by weight, 0.0005 to 0.0070% of C, 0.001 to 0.8% of Si, 0.3 to 4.0% of Mn, 0.002 to 0.15% of P, 0.0005 to 0.015% of S, 0.005 to 0.1% of Al and 0.0003 to 0.0060% of N with the balance consisting of Fe and unavoidable impurities and composed of a structure of only low-temperature transformation products consisting of at least one member selected from the group consisting of massive ferrite, bainite, Widmanstatten ferrite, martensite, and acicular ferrite, said cold-rolled steel sheet having a bake hardenability property, BH, of not less than 5 kgf/mm.sup.2.
A cold-rolled steel sheet having excellent bake hardenability and moldability, consisting essentially of, in terms of % by weight, 0.0005 to 0.0070% of C, 0.001 to 0.8% of Si, 0.01 to 4.0% of Mn, 0.002 to 0.15% of P, 0.0005 to 0.015% of S, 0.005 to 0.1% of Al and 0.0003 to 0.0060% of N and further consisting essentially of at least one additional element selected from the group consisting of 0.003 to 0.1% to Ti and 0.003 to 0.1% of Nb with the balance consisting of Fe and unavoidable impurities and composed of a structure of only low-temperature transformation products, said cold-rolled steel sheet having a bake hardenability property, BH, of not less than 5 kgf/mm.sup.2.
A hot-dip galvanized cold-rolled steel sheet having excellent bake hardenability and non-aging properties at room temperature, consisting essentially of, in terms of % by weight, 0.0005 to 0.0070% C, 0.001 to 0.8% of Si, 0.3 to 4.0% of Mn, 0.002 to 0.15% of P, 0.0005 to 0.015% of S, 0.005 to 0.1% of Al and 0.0003 to 0.0060% of N with the balance consisting of Fe and unavoidable impurities and composed of a structure of only low-temperature transformation products, said hot-dip galvanized cold-rolled steel sheet having a bake hardenability property, BH, of not less than 5 kgf/mm.sup.2.
A hot-dip galvanized cold-rolled steel sheet having excellent bake hardenability and moldability, consisting essentially of, in terms of % by weight, 0.0005 to 0.0070% of C, 0.001 to 0.8% of Si, 0.01 to 4.0% of Mn, 0.002 to 0.15% of P, 0.0005 to 0.015% of S, 0.005 to 0.1% of Al and 0.0003 to 0.0060% of N and further consisting essentially of at least one additional element selected from the group consisting of 0.003 to 0.1% of Ti and 0.003 to 0.1% of Nb with the balance consisting of Fe and unavoidable impurities and composed of a structure of only low-temperature transformation products, said hot-dip galvanized cold-railed steel sheet having a bake hardenability property, BH, of not less than 5 kgf/mm.sup.2.
A process for producing a cold-rolled steel sheet having excellent bake hardenability and non-aging properties at room temperature, comprising the steps of:
A process for producing a cold-rolled steel sheet having excellent bake hardenability and non-aging properties at room temperature, comprising the steps of:
A process for producing a hot-dip galvanized cold-rolled steel sheet having excellent bake hardenability and non-aging properties at room temperature, comprising the steps of:
A process for producing a hot-dip galvanized cold-rolled steel sheet having excellent bake hardenability and non-aging properties at room temperature, comprising the steps of:
Dependencies are inferred from the claim text.