Ultra-high-strength steel sheet with excellent hole expansion and yield ratio and its manufacturing method

摘要

One aspect of the present invention is as follows: C: 0.05 to 0.2%, Si: 2.0% or less, Mn: 4.1 to 9.0%, P: 0.05% or less (0% by weight). %), S: 0.02% or less (excluding 0%), Al: 0.5% or less (excluding 0%), N: 0.02% or less (excluding 0%), balance Fe and Contains other unavoidable impurities, Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), V: 0.2% or less (excluding 0%), and Mo: 0.5% (excluding 0%) containing at least one selected from the following, satisfying the following relational expression 1, and the microstructure is volume fraction, retained austenite is 10 to 30%, and annealing is performed. Super-excellent in hole expandability and yield ratio, containing 50% or more of martensite and 20% or less of other phases including alpha martensite and epsilon martensite. On-strength steel sheet. Relational expression 1: C / 12 + Ti / 48 + Nb / 93 + V / 51 + Mo / 96 ≧ 0.015 (In the above relational expression 1, each element symbol is a value in which the content of each element is represented by% by weight.)