Patent ID: 9617626
Date: 2017-04-11
CPC Classifications: C21C,C22C

Claim:
1. A high-strength steel sheet comprising: C: 0.03 to 0.25 mass %, Si: 0.1 to 2.0 mass %, Mn: 0.5 to 3.0 mass %, P: not more than 0.05 mass %, T.O: not more than 0.0050 mass %, S: 0.0001 to 0.01 mass %, N: 0.0005 to 0.01 mass %, acid-soluble Al: more than 0.01 mass %, Ca: 0.0005 to 0.0050 mass %, a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %, and optionally one or more selected from the group consisting of: acid-soluble Ti: 0.008 to 0.20 mass %, Nb: 0.01 to 0.10 mass %, V: 0.01 to 0.10 mass %, Cu: 0.1 to 2 mass %, Ni: 0.05 to 1 mass %, Cr: 0.01 to 1 mass %, Mo: 0.01 to 0.4 mass %, B: 0.0003 to 0.005 mass %, and Zr: 0.001 to 0.01 mass % with a balance including iron and inevitable impurities, wherein: the steel sheet contains a chemical component on a basis of mass that satisfies 0.7<100×([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]≦70, and 0.2≦([Ce]+[La]+[Nd]+[Pr])/[S]≦10, where [Ce] is an amount of Ce contained, [La] is an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble Al] is an amount of acid-soluble Al contained, and [S] is an amount of S contained; the steel sheet has a compound inclusion including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at least one element of O and S, and a second inclusion phase having a component different from that of the first inclusion phase and containing at least one element of Mn, Si, Ti and Al; the compound inclusion forms a spherical compound inclusion having an equivalent circle diameter in a range of 0.5 μm to 5 μm; and a ratio of a number of the spherical compound inclusions relative to a number of all inclusions having the equivalent circle diameter in the range of 0.5 μm to 5 μm is 50% or more.