STEEL SHEET WITH EXCELLENT AGING RESISTANCE PROPERTY AND METHOD FOR PRODUCING THE SAME

摘要

A steel sheet and a method for producing the same are disclosed. The steel sheet has a composition containing 0.015% to 0.05% C, less than 0.10% Si, 0.1% to 2.0% Mn, 0.20% or less P, 0.1% or less S, 0.01% to 0.10% Al, 0.005% or less N, and 0.06% to 0.5% Ti in percent by mass, C and Ti satisfying the inequality Ti*/C≧4, where Ti* (mass percent)=Ti-3.4N and Ti, C, and N represent the content (mass percent) of each element. The steel sheet has a microstructure which contains a ferrite phase as a base, in which the average grain diameter of the ferrite phase is 7 μm or more, and in which the ratio of the rolling-direction average grain diameter to thickness-wise average grain diameter of the ferrite phase is 1.1 or more.

主权项

1. A steel sheet with an excellent aging resistance property, having a composition containing 0.015% to 0.05% C, less than 0.10% Si, 0.1% to 2.0% Mn, 0.20% or less P, 0.1% or less S, 0.01% to 0.10% Al, 0.005% or less N, and 0.06% to 0.5% Ti in percent by mass, the remainder comprising Fe and inevitable impurities, C and Ti satisfying the following inequality (1); the steel sheet having a microstructure which contains a ferrite phase as a base, in which the average grain diameter of the ferrite phase is 7 μm or more, and in which the ratio dL/dt of the rolling-direction average grain diameter dL to thickness-wise average grain diameter dt of the ferrite phase is 1.1 or more; the steel sheet having a rolling-direction AI (aging index) value of 10 MPa or less, the rolling-direction AI value being defined as a value which is obtained in such a way that after a tensile specimen is taken such that a rolling direction coincides with a tensile direction, a pre-strain of 7.5% is applied to the tensile specimen, and the tensile specimen is aged at 100° C. for 30 minutes, the 7.5% pre-strained stress is subtracted from the yield stress:
Ti*/C≧4  (1) where Ti*=Ti-3.4N and Ti, C, and N represent the content (mass percent) of each element.