Nippon Steel Corporation

Shinsuke Kai

Hiroshi Shuto

Akifumi Sakakibara

Mutsumi Sakakibara

Daisuke Maeda

Tatsuo Yokoi

Koutarou Hayashi

Jun Ando

Yoshihiro Serizawa

Teruki Hayashida

Kazuya Ootsuka

Hiroshi Kaido

Hiroshi Tanei

Shota Ishitsuka

Takaaki Tsutsumi

This hot-rolled steel sheet has a predetermined chemical composition, the number % of crystal grains of bainite that are in contact with both tempered martensite and residual austenite is 80% or more of all crystal grains of the bainite, a C concentration in the residual austenite is 0.80 mass % or more, an average crystal grain size of the residual austenite is 0.70 μm or less, and a standard deviation of Vickers hardness is 25 HV0.01 or less.

Hot-rolled steel sheet

This hot-rolled steel sheet has a predetermined chemical composition, and in a case where the thickness is denoted by t, the metallographic structure at a t/4 position from the surface contains one or both of tempered martensite and lower bainite at a volume percentage of 90% or more, the tensile strength is 980 MPa or more, and the average Ni concentration on the surface is 7.0% or more.

Hot-rolled steel sheet and method for manufacturing same

This hot-rolled steel sheet has a predetermined chemical composition. The metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite. The average grain size of the metallographic structure excluding the residual austenite is 7.0 μm or less. The C concentration in the residual austenite is 0.5 mass % or more. The number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10carbides/mmor more.

A hot-rolled steel sheet includes, by mass %, C: 0.030% or more and less than 0.075%, Si+Al: 0.08% to 0.40%, Mn: 0.5% to 2.0%, and Ti: 0.020% to 0.150%, and includes a microstructure having a ferrite and a martensite. The hot-rolled steel sheet includes a microstructure having, by area %, 90% to 98% of the ferrite, 2% to 10% of the martensite, 0% to 3% of a bainite, and 0% to 3% of a pearlite. In the martensite, the number proportion of martensite grains having a hardness of 10.0 GPa or more is 10% or less, and a ratio N1/N2 of the number N1 of martensite grains having a hardness of 8.0 GPa or more and less than 10.0 GPa to the number N2 of martensite grains having a hardness of less than 8.0 GPa is 0.8 to 1.2.

A plurality of water removing nozzles of a water removing apparatus comprise one or more of a single far water removing nozzle and a far water removing nozzle group. The single far water removing nozzle forms a far end water removal single area that does not include one end but includes the other end in the width direction of a steel sheet conveyance plane. The far water removing nozzle group forms the far end water removal single area and one or more inner water removal single areas that do not include either end in the width direction of the steel sheet conveyance plane, in such a manner that the far end water removal single area and the one or more inner water removal single areas are aligned in order from the one end side to the other end side while overlapping with each other in the width direction of the steel sheet conveyance plane and aligned in order from the upstream side to the downstream side without overlapping in the conveyance direction.

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Tokyo, Japan

Shinsuke Kai