Nippon Steel Corporation

Tetsuya Hirashima

Takeshi Toyoda

Riki Okamoto

Natsuko Sugiura

Provided is a hot rolled steel sheet comprising a predetermined composition wherein the hot rolled steel sheet comprises ferrite with an average orientation difference in the same grain of 0.5 to 5.0&#xb0; in 30 to 70 vol %, the ferrite and martensite in a total of 90 vol % or more, and a balance microstructure of 10 vol % or less, has an average grain size of the ferrite of 0.5 to 5.0 &#x3bc;m, and has an average grain size of the martensite and the balance microstructure of 1.0 to 10 &#x3bc;m. Provided is a method for producing a hot rolled steel sheet comprising rolling where two or more consecutive passes of rolling including a final pass are performed under conditions of a rolling temperature: A point or more and less than Aepoint, a strain rate: 1.0 to 50/sec, and a time between passes: within 10 seconds and where a total strain amount of all passes satisfying the conditions is 1.4 to 4.0, cooling by a 20&#xb0; C./sec or more average cooling rate, and coiling the steel sheet at room temperature or more and less than 300&#xb0; C.

Hot rolled steel sheet and method for producing same

In a hot-rolled steel sheet having a predetermined chemical composition and having a metallographic structure including 90 vol % or greater of martensite and 0 vol % to 10 vol % of a residual structure, the residual structure includes one or both of bainite and ferrite, the average prior austenite grain size in an L-section parallel to a rolling direction and an average prior austenite grain size in a C-section parallel to a direction orthogonal to the rolling direction are 1.0 &#x3bc;m to 10.0 &#x3bc;m the aspect ratio associated with the prior austenite grain size is 1.8 or less, the average grain size of the residual structure in the L-section and the average grain size of the residual structure in the C-section are 5.0 &#x3bc;m or less, and the aspect ratio associated with the average grain size of the residual structure is 2.0 or less.

Hot-rolled steel sheet and production method therefor

Provided is a hot rolled steel sheet comprising a predetermined composition wherein the hot rolled steel sheet comprises first ferrite with an average orientation difference in the same grain of 0.5 to 5.0&#xb0; in 30 to 70 vol %, at least one type of structures among bainite and second ferrite with an average orientation difference of 0 to less than 0.5&#xb0; and the first ferrite in a total of 95 vol % or more, a balance microstructure of 5 vol % or less, has an average grain size of the first ferrite of 0.5 to 5.0 &#x3bc;m, and has an average grain size of the other structures of 1.0 to 10 &#x3bc;m. Provided is a method for producing a hot rolled steel sheet comprising rolling where two or more consecutive passes of rolling including a final pass are performed under conditions of a rolling temperature: A point or more and less than Aepoint etc., and where a total strain amount of all passes satisfying the conditions is 1.4 to 4.0, cooling by a 20 to 50&#xb0; C./sec average cooling rate, and coiling the steel sheet at 300&#xb0; C. to 600&#xb0; C.

The hot-rolled steel sheet includes, in % by mass, 0.10% or more and 0.50% or less of C; 0.10% or more and 3.0% or less of Si; 0.5% or more and 3.0% or less of Mn; 0.10% or less of P; 0.010% or less of S; 1.00% or less of Al; 0.010% or less of N; 0% or more and 0.20% or less of Ti; 0% or more and 0.100% or less of Nb; 0% or more and 0.0060% or less of Ca; 0% or more and 0.50% or less of Mo; and 0% or more and 1.00% or less of Cr; with the balance comprising Fe and impurities, and an average grain size of prior austenite in a structure is 0.1 &#x3bc;m or larger and 3.0 &#x3bc;m or smaller, and a sheet crown quantity corresponding to a thickness difference between a width center portion and a portion away, by 10 mm, from a width edge portion in the widthwise direction toward the width center portion is 80 &#x3bc;m or smaller.

Hot-rolled steel sheet

A hot rolled steel sheet according to an aspect of the present invention has a predetermined chemical composition, and a metallographic structure in which martensite is the major structure, wherein an average length of grains in an L direction parallel to a rolling direction is 0.2 &#x3bc;m or more and 5.0 &#x3bc;m or less, an average length of grains in a C direction parallel to a transverse direction is 0.1 &#x3bc;m or more and 5.0 &#x3bc;m or less, the ratio of the average length in the C direction to the average length in the L direction is 0.2 or more and 5.0 or less, and the tensile strength is 1180 MPa or more.

Hot rolled steel sheet and method for manufacturing same

A hot rolled steel sheet is provided, which is excellent in collision characteristics, excellent in anisotropy of toughness, and high in strength. The hot rolled steel sheet is characterized by containing, by mass %, C: 0.10% to 0.50%, Si: 0.10% to 3.00%, Mn: 0.5% to 3.0%, P: 0.100% or less, S: 0.010% or less, Al: 1.00% or less, N: 0.010% or less and a balance of Fe and impurities, wherein a metal structure at position of &#xbc; thickness from surface in L-cross-section of the steel sheet comprises prior austenite grains of average value of aspect ratios of 2.0 or less, average grain size of 0.1 &#x3bc;m to 3.0 &#x3bc;m, and coefficient of variation of a standard deviation of grain size distribution/average grain size of 0.40 or more, and a texture with X-ray diffraction intensity ratio of {001}&#x3c;110&#x3e;orientation for random samples of 2.0 or more, and the steel sheet has tensile strength of 1180 MPa or more.

Provided is a hot rolled steel sheet comprising a predetermined composition wherein the hot rolled steel sheet comprises a dual structure of, by area fraction, a structural fraction of a martensite phase of 10 to 40% and a structural fraction of a ferrite phase of 60% or more, has an average grain size of ferrite grains of 5.0 &#x3bc;m or less, and has a coverage rate of martensite grains by ferrite grains of more than 60%. Also provided is a method for producing a hot rolled steel sheet comprising rolling a steel sheet wherein the respective rolling loads of the final three rolling stands are 80% or more of an immediately previous rolling stand and an average value of these rolling temperatures is 800 to 950&#xb0; C., and forcibly cooling, then coiling the steel sheet wherein the forcibly cooling includes cooling started within 1.5 seconds after the rolling ends and cooling the steel sheet by a 30&#xb0; C./second or more average cooling rate down to 600 to 750&#xb0; C., natural cooling for 3 seconds or more and 10 seconds or less, and cooling by a 30&#xb0; C./second or more average cooling rate down to 200&#xb0; C. or less.

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Tokyo, Japan

Tetsuya Hirashima