Cold-rolled steel sheet and method for producing the same

摘要

A cold-rolled steel sheet of the present invention which has a composition containing, in terms of % by mass, C: 0.05-0.30%, Si: 3.0% or less (including 0%), Mn: 0.1-5.0%, P: 0.1% or less (including 0%), S: 0.010% or less (including 0%), and Al: 0.001-0.10%, and remainder being mainly iron, and which has a structure comprising, in terms of area ratio, 10-80% ferrite, less than 5% (including 0%) of the sum of retained austenite and martensite, and a hard phase as the remainder. The steel sheet gives a KAM value frequency distribution curve in which the relationship between the proportion of frequency having a KAM value ≦0.4, XKAM≦0.4°, and the area ratio of ferrite, Vα satisfies XKAM≦0.4°/Vα≧0.8 and the proportion of frequency having a KAM value in the range of 0.6-0.8, XKAM=0.6-0.8° is 10-20%. In the hard phase adjoining the ferrite, cementite, grains having an equivalent circle diameter of 0.1 μm or larger exist so that three or less such cementite grains are dispersed per μm2 of the hard phase. The steel sheet has improved balance between elongation and stretch flangeability and has better formability.

主权项

1. A cold-rolled steel sheet comprising:
a component composition, comprising, in terms of % by mass: iron, C: 0.05-0.30%, Si: 3.0% or less (including 0%), Mn: 0.1-5.0%, P: 0.1% or less (including 0%), S: 0.010% or less (including 0%) and Al: 0.001-0.10%; and having a structure, comprising, in terms of area ratio: 10-80% of ferrite as a soft phase; less than 5% (including 0%) of a sum of retained austenite, martensite and a mixed structure of retained austenite and martensite; and a hard phase comprising at least one tempered substance selected from the group consisting of tempered martensite and tempered bainite; wherein, in a frequency distribution curve of a Kernel Average Misorientation value (KAM value), a relation between a proportion of frequency having a KAM value of 0.4° or less to a total frequency XKAM≦0.4° (unit: %) and an area ratio of ferrite Vα (unit: %) satisfies XKAM≦0.4°/Vα≧0.8; and a proportion of frequency having a KAM value of 0.6-0.8° to a total frequency XKAM=0.6-0.80° is 10-20%; and wherein a dispersion state of cementite grains having an equivalent circle diameter of 0.1 μm or more and existing at an interface between the ferrite and the hard phase is three grains or less per 1 μm2 of the hard phase.