High-strength hot-rolled steel sheet and method of manufacturing the same

摘要

On a cross section with a sheet width direction of a high-strength hot-rolled steel sheet set as a normal line, with regard to an inclusion having a major diameter of 3.0 μm or more, a maximum of a major diameter/minor diameter ratio expressed by (a major diameter of the inclusion)/(a minor diameter of the inclusion) is 8.0 or less, and a sum total of a rolling direction length per 1 mm2 cross section of a predetermined inclusion group composed of plural inclusions each having a major diameter of 3.0 μm or more and a predetermined extended inclusion having a length in a rolling direction of 30 μm or more is 0.25 mm or less. The plural inclusions composing the predetermined inclusion group congregate in both the rolling direction and a direction perpendicular to the rolling direction 50 μm or less apart from each other. The predetermined extended inclusion is spaced over 50 μm apart from all the inclusions each having a major diameter of 3.0 μm or more in at least either the rolling direction or the direction perpendicular to the rolling direction.

主权项

1. A high-strength hot-rolled steel sheet containing:
in mass %, C: 0.02% to 0.1%; Si: 0.001% to 3.0%; Mn: 0.5% to 3.0%; P: 0.1% or less; S: 0.01% or less; Al: 0.001% to 2.0%; N: 0.02% or less; Ti: 0.03% to 0.3%; and Nb: 0.001% to 0.06%, the steel sheet further containing at least one element selected from the group consisting of: Cu: 0.001 to 1.0%; Cr: 0.001 to 1.0%; Mo: 0.001 to 1.0%; Ni: 0.001 to 1.0%; and V: 0.01 to 0.2%, the balance being composed of Fe and inevitable impurities, a parameter Q expressed by Mathematical expression 1 below being 30.0 or more, a microstructure being made of a ferrite structure, a bainite structure, or a structure mixed with the ferrite structure and the bainite structure, an average grain size of grains included in the microstructure being 6 μm or less, an X-ray random intensity ratio of {211} plane on a rolled surface being 2.4 or less, and on a cross section with a sheet width direction set as a normal line,
with regard to inclusions having a major diameter of 3.0 μm or more, a maximum of a major diameter/minor diameter ratio expressed by (a major diameter of the inclusion)/(a minor diameter of the inclusion) being 8.0 or less,a sum total of a rolling direction length per 1 mm2 cross section of an inclusion group composed of plural inclusions each having a major diameter of 3. 0 μm or more and an extended inclusion having a length in a rolling direction of 30 μm or more being 0.25 mm or less,the plural inclusions composing the inclusion group congregating in both the rolling direction and a direction perpendicular to the rolling direction 50 μm or less apart from each other, andthe extended inclusion being spaced over 50 μm apart from all the inclusions each having a major diameter of 3.0 μm or more in at least either the rolling direction or the direction perpendicular to the rolling direction,Q=[Ti]48/[S]32(Mathematical⁢⁢expression⁢⁢1) wherein [Ti] indicates the Ti content (mass %) and [S] indicates the S content (mass %).