ULTRA HIGH STRENGTH STEEL SHEET WITH AN EXCELLENT HEAT TREATMENT PROPERTY FOR HOT PRESS FORMING, QUENCHED MEMBER, AND MANUFACTURING METHOD FOR SAME

摘要

The present invention relates to an ultra high strength steel sheet with an excellent heat treatment property, wherein the steel sheet contains, by weight %, C: 0.2 to 0.5%, Si: 0.01 to 1.5%, Mn: 0.5 to 2.0%, P: 0.1% or less (but not 0%), S: 0.03% or less (but not 0%), soluble Al: 0.1% or less (but not 0%), N: 0.01 to 0.1%, and Cr: 0.1 to 2.0%, and a remainder consisting of Fe and other impurities, and has a tensile strength measured before hot press forming of 800 MPa or less. The invention also relates to a manufacturing method for same. Further, the present invention relates to a quenched member formed by hot press forming and quick freezing the steel sheet to a tensile strength of 1800 MPa or higher, and relates to a manufacturing method for same. The present invention provides a steel sheet the tensile strength of which when measured before hot press forming is 800MPa or less and the tensile strength of which when measured after hot press forming and quick freezing is 1800 MPa or higher. Moreover, the steel sheet of the present invention has a yield strength increased to 100 MPa or higher after coating.