Nippon Steel Sumitomo Metal Corporation

Fuminori Tasaki

Koji Noguchi

Tetsuya Yamada

Daisuke Maeda

Tatsuo Yokoi

Osamu Kawano

Tooru Akashi

Junji Haji

Hitoshi Nikaido

Takeo Itoh

Shingo Kuriyama

A hot-rolled steel sheet includes, as a chemical composition, by mass %, C: 0.030% to 0.10%, Mn: 0.5% to 2.5%, and Si+Al: 0.100% to 2.5%, in which the steel sheet has a microstructure including, by area fraction, ferrite: 80% or more, martensite: 3% to 15.0%, and pearlite: less than 3.0%, in which a number density of martensite having an equivalent circle diameter of 3 &#x3bc;m or more at a position which is at a depth of &#xbc; of the steel sheet thickness from the surface of the steel sheet, is 5.0 pieces/10000 &#x3bc;mor less, and the following Expression (1) is satisfied.&#x2265;1.00&#x2003;&#x2003;Expression (1) Here, R is an average martensite interval (&#x3bc;m) defined by the following Expression (2), and Dis a martensite average diameter (&#x3bc;m).={12.5&#xd7;(&#x3c0;/6)&#x2212;(&#x2154;)&#x2003;&#x2003;Expression (2) Here, Vis a martensite area fraction (%) and Dis the martensite average diameter (&#x3bc;m).

Hot-rolled steel sheet and method for producing same

The method for cooling a hot-rolled steel sheet of the invention includes a target ratio-setting process in which a top and bottom heat transfer coefficient ratio X1, at which a temperature standard deviation Y becomes a minimum value Ymin, is set as a target ratio Xt based on correlation data between a top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet and the temperature standard deviation Y of the hot-rolled steel sheet; and a cooling control process in which at least one of an amount of heat dissipated from a top surface by cooling and an amount of heat dissipated from a bottom surface by cooling of the hot-rolled steel sheet in the cooling section is controlled so that the top and bottom heat transfer coefficient ratio X of the hot-rolled steel sheet in the cooling section matches the target ratio Xt.

Method for cooling hot-rolled steel sheet

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Oita, Japan

Fuminori Tasaki