Patent ID: 6319338
Filing Date: 2001-11-20
Classification: C21D

Abstract:
A high strength steel sheet with high flow stress during dynamic deformation, characterized in that the steel sheet contains, in terms of wt %, C at from 0.03% to 0.3%, either or both Si and Al at a total of from 0.5% to 3.0% with the remainder Fe as a primary component, and the microstructure of the steel sheet in the final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominating phase, and the third phase including retained austenite at a volume fraction between 3% and 50%, wherein the average value of .sigma.dyn (MPa) of the flow stress in the range of 3-10% of equivalent strain when deformed in a strain rate range of 5.times.10.sup.2 -5.times.10.sup.3 (1/sec) after pre-deformation of greater than 0% and less than or equal to 10% of equivalent strain, satisfies the inequality: .sigma.dyn.gtoreq.0.766.times.TS+250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5.times.10.sup.-4 -5.times.10.sup.-3 (1/sec) without deformation, the value (M) determined by the solid solution (C) in the retained austenite and the average Mn equivalents of the steel material {Mneq=Mn+(Ni+Cr+Cu+Mo)/2}, defined by the equation M=678-428.times.(C)-33 Mneq is at least 70 and not greater than 250, the difference between the retained austenite volume fraction without pre-deformation and the retained austenite volume fraction after applying a pre-deformation of 5% of equivalent strain is at least 30% of the retained austenite volume fraction without pre-deformation, and the work hardening coefficient between 1% and 5% of a strain is at least 0.080 and of a strain yield strength is at least 40.