Nippon Steel Corporation

Kohei Nakada

Takeshi Toyoda

Takeshi Kawachi

Ryo Tabata

Kazuki Matsuda

Hitoshi Nikaido

Yui Yamashita

Hot-rolled steel sheet having a chemical composition including, in mass %, C: 0.02 to 0.20%, Si: 0.01 to 1.50%, Mn: 0.10 to 3.00%, P: 0.10% or less, S: 0.010% or less, Al: 0.005 to 0.100%, Ti: 0.02 to 0.20%, N: 0.001 to 0.010%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.00%, Mo: 0 to 0.40%, Nb: 0 to 0.060%, V: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0050%, O: 0.0100% or less, and the balance: Fe and impurities; in which: a steel micro-structure includes, in area %, ferrite: 60 to 80%, and a total of ferrite and bainite: 90% or more; an average of the crystal grain size of ferrite and bainite is 7.0 μm or less, a standard deviation of the crystal grain size is 2.0 μm or less; and a standard deviation of a diameter of Ti carbo-nitrides is 10 nm or less.

Hot-rolled steel sheet and weld joint, and methods for producing same

As a burring processed member able to suppress the formation of fatigue cracks in a burring part, one having a structure satisfying the following relations (1) to (3) is disclosed: 3≤Ra≤100 . . . (1), 3.0<h . . . (2), 24+r−[TS/(40+0.28×Ra)]<h . . . (3), where, 'Ra' is an arithmetic mean roughness (μm) of the burring end face, “h” is a height (mm) from the first surface to the burring end face, “r” is a radius of curvature (mm) of the curved wall part, and “TS” is the tensile strength (MPa) of the sheet-shaped part.

Burring processed member

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Tokyo, Japan

Kohei Nakada