Cold rolled steel sheet and hot dip zinc-coated cold rolled steel sheet having excellent bake hardenability, non-aging properties and formability, and process for producing same

A cold rolled steel sheet excellent in paint bake hardenability, non-aging properties and formability, which is composed of, in terms of percentage by weight, from 0.0005 to 0.0070% of C, from 0.001 to 0.8% of Si, from 0.8 to 4.0% of Mn, from 0.005 to 0.15% of P, from 0.0010 to 0.015% of S, from 0.005 to 0.1% of Al, from 0.0003 to 0.0060% of N, less than 0.0005% of B, said B satisfying the condition B/N <1.5, one or two elements selected from the group consisting of Ti in an amount from 0.003 to 0.1% and Nb in an amount from 0.003 to 0.1%, and the balance Fe and unavoidable impurities, and which has a mixed structure comprising phases transformed at low temperature in an amount greater than 5% based on the total volume and ferrite.

A hot dip zinc-coated cold rolled steel sheet excellent in paint bake hardenability, non-aging properties and formability, which consists essentially of, in terms of percentage by weight, from 0.0005 to 0.0070% of C, from 0.001 to 0.8% of Si, from 0.3 to 4.0% of Mn, from 0.003 to 0.15% of P, from 0.0005 to 0.015% of S, from 0.005 to 0.20% of Al, from 0.0003 to 0.0060% of N less than 0.0030% by weight of B, said B satisfying the condition B/N<1.5 and the balance Fe and unavoidable impurities, and which has a mixed structure comprising a product transformed at low temperature in an amount greater than 5% based on total volume and ferrite.

A hot dip zinc-coated cold rolled steel sheet excellent in paint bake hardenability, non-aging properties and formability, which is composed of, in terms of percentage by weight, from 0.0005 to 0.0070% of C, from 0.001 to 0.8% of Si, from 0.8 to 4.0% of Mn, from 0.005 to 0.15% of P, from 0.0010 to 0.015% of S, from 0.005 to 0.1% of Al, from 0.0003 to 0.0060% of N, less than 0.0005% of B, said B satisfying the condition B/N<1.5, one or two elements selected from the group consisting of Ti in an amount from 0.003 to 0.1% and Nb in an amount from 0.003 to 0.1%, and the balance Fe and unavoidable impurities, and which has a mixed structure comprising phases transformed at low temperature in an amount greater than 5% based on the total volume and ferrite.

A process for producing a cold rolled steel sheet excellent in paint bake hardenability, non-aging properties and formability, which comprises heating a slab consisting essentially of, in terms of percentage by weight, from 0.0005 to 0.0070% of C, from 0.001 to 0.8% of Si, from 0.3 to 4.0% of Mn, from 0.003 to 0.15% of P, from 0.0005 to 0.015% of S, from 0.005 to 0.20% of Al, from 0.0003 to 0.0060% of N less than 0.0030% by weight of B, said B satisfying the condition B/N<1.5 and the balance Fe and unavoidable impurities, hot rolling the slab and finishing hot rolling at a temperature of at least (Ar3-100).degree.C. to give a hot rolled steel strip, coiling the hot rolled steel strip in a temperature range from 800.degree. C. to room temperature, cold rolling the hot rolled steel strip after coiling at a reduction of at least 60% to give a cold rolled steel strip, and annealing the cold rolled steel strip by inserting the steel strip in an annealing furnace whose soaking temperature is adjusted in a temperature range from at least the .alpha..fwdarw..gamma. transformation temperature to up to the Ac3 transformation temperature and forming a mixed structure comprising phases transformed at a low temperature in an amount greater than 5% based on total volume and ferrite.

A process for producing a hot dip zinc-coated cold rolled steel sheet excellent in bake hardenability, non-aging properties and formability, which comprises heating a slab consisting essentially of, in terms of percentage by weight, from 0.0005 to 0.0070% of C, from 0.001 to 0.8% of Si, from 0.3 to 4.0% of Mn, from 0.003 to 0.15% of P, from 0.0005 to 0.015% of S, from 0.005 to 0.20% of Al, from 0.0003 to 0.0060% of N, less than 0.0030% by weight of B, said B satisfying the condition B/N<1.5 and the balance Fe and unavoidable impurities, hot rolling the slab and finishing hot rolling at a temperature of at least (Ar3-100).degree.C. to give a hot rolled steel strip, coiling the hot rolled steel strip in a temperature range from 800.degree. C. to room temperature, cold rolling the hot rolled steel strip after coiling at a draft of at least 60% to give a cold rolled steel strip, annealing the cold rolled steel strip by inserting the steel strip in an annealing furnace whose soaking temperature is adjusted in a temperature range from at least the .alpha..fwdarw..gamma. transformation temperature to up to the Ac3 transformation temperature, forming a mixed structure comprising phases transformed at a low temperature in an amount greater than 5% based on total volume and ferrite, and coating the annealed strip with zinc by immersing the strip in a hot dip zinc coating bath.