Patent ID: 9200342
Filing Date: 2015-12-01
Classification: C21D,C22C

Abstract:
1. A hot-rolled steel sheet, comprising by mass %: C=0.02% to 0.06%; Si=0.05% to 0.5%; Mn=1% to 2%; Nb=0.05% to 0.12%; Ti=0.005% to 0.02%; P≦0.03%; S≦0.005%; V≦0.003%; Al=0.005% to 0.1%; N=0.0015% to 0.006%; Ca=0.0005% to 0.003%; V 0.15% (0 (zero) % is not included); Mo≦0.3% (0 (zero) % is not included) N−14/48×Ti≧0 (zero) %; and the remaining portion made up of Fe and inevitable impurity elements, wherein a polygonal ferrite fraction is 3% or more and 20% or less, and the others are a low-temperature transformation phase and pearlite of 1% or less in a microstructure at a depth of a half thickness of a sheet thickness from a steel sheet surface, a number average crystal grain size of a whole of the microstructure is 1 μm or more and 2.5 μm or less, an area average grain size is 3 μm or more and 9 μm or less, a standard deviation of the area average grain size is 0.8 μm or more and 2.3 μm or less, and a reflected X-ray intensity ratio {211}/{111} in a {211} direction and in a {111} direction relative to a plane in parallel to the steel sheet surface at the depth of the half thickness of the sheet thickness from the steel sheet surface is 1.1 or more, and wherein the steel sheet has a strength of the API-X80 standard and a ductile fracture rate (SA) at −20° C. of 96% or more, and wherein the sheet thickness is 16 mm or more.