Jfe Steel Corporation

Katsumi Nakajima

Yoshimasa Funakawa

Nobuyuki Nakamura

Hayato Saito

Takayuki Murata

Noriaki Moriyasu

Tetsuya Mega

Takeshi Yokota

Katsumi Yamada

Koichi Nakagawa

Yuji Tanaka

Yasunobu Nagataki

Chikara Kami

Kazuhiro Seto

Takayuki Futatsuka

Satoshi Kinoshiro

Kazuhiko Yamazaki

Kazutaka Okimoto

Takahiko Ogura

Tamako Ariga

Reiko Mizuno

A high strength hot rolled steel sheet with good formability, which has good magnetic properties, excellent punchability, and a tensile strength of 780 MPa or more. The high strength hot rolled steel sheet having a chemical composition containing C: 0.070% to 0.140%, Si: 0.10% to 1.00%, Mn: 1.00% to 1.80%, P: 0.050% or less, S: 0.0050% or less, N: 0.0080% or less, Al: 0.010% to 0.100%, Ti: 0.050% to 0.150%, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, and 95% or more of bainite microstructure on a volume fraction basis, wherein carbides constituting 80% or more of total precipitation carbides are dispersed at grain boundaries of bainitic ferrite constituting the bainite microstructure and 80% or more of total precipitation carbides have particle grain sizes of 20 to 300 nm.

High strength hot rolled steel sheet having tensile strength of 780 MPa or more

A steel slab having a composition containing C: more than 0.07% and 0.2% or less, Si: 2.0% or less, Mn: 1.0% to 3.0%, Al: 0.1% or less, Ti: 0.05% to 0.3%, and V: 0.05% to 0.3% on a mass percent basis is heated to 1100&#xb0; C. or more and is subjected to rough rolling and finish rolling. In the finish rolling, the total rolling reduction of two final passes is 30% or more, and the finish rolling temperature ranges from (Atransformation temperature) to (Atransformation temperature+120&#xb0; C.). Cooling is started within 2 seconds after the finish rolling. Coiling is performed at an average cooling rate of 40&#xb0; C./s or more at a coiling temperature in the range of 300&#xb0; C. to 500&#xb0; C. The resulting high-strength hot-rolled steel sheet has a microstructure in which a bainite phase constitutes more than 90% by volume, the average lath interval of bainite is 0.45 &#x3bc;m or less, and the ratio of Fe-based carbide precipitated in bainite lath to all Fe-based carbide is 10% or more, has a high tensile strength of 900 MPa or more, and has significantly improved punchability in mass production.

High-strength hot-rolled steel sheet and method for producing the same

A hot-rolled steel sheet for a generator rim contains a structure containing a ferrite phase with an areal ratio of 95% or more in which precipitates containing Ti and V whose average grain diameter is less than 10 nm are precipitated in crystal grains of the ferrite phase. The ferrite phase has an average crystal grain diameter within the range of 2 &#x3bc;m or more and less than 10 &#x3bc;m. The hot-rolled steel sheet for a generator rim has strength with a yield strength YS in a rolling direction of 700 MPa or more and electromagnetic properties with a magnetic flux density Bof 1.5 T or more and a magnetic flux density Bof 1.6 T or more.

Hot-rolled steel sheet for generator rim and method for manufacturing the same

A high-strength hot-rolled steel sheet including a chemical composition containing, in percent by mass, 0.05% to 0.12% of C, 0.05% to 1.0% of Si, 0.5% to 1.8% of Mn, 0.04% or less of P, 0.0030% or less of S, 0.005% to 0.07% of Al, 0.006% or less of N, 0.05% to 0.15% of Ti, and the balance being Fe and incidental impurities, in which, in a region in the range of &#x215b; to &#x215c; of the sheet thickness, the content of Ti*, which is Ti existing as precipitates, is 0.3&#xd7;[Ti] to 0.6&#xd7;[Ti], where [Ti] is the Ti content, and the steel sheet has a microstructure in which the area fraction of the bainite phase in the entire structure is more than 95%.

High-strength hot-rolled steel sheet and method for producing same

A high strength hot-rolled steel sheet has a tensile strength of not less than 780 MPa and exhibits excellent stretch flangeability and excellent fatigue resistance. A steel has a composition containing C at 0.05 to 0.15%, Si at 0.2 to 1.2%, Mn at 1.0 to 2.0%, P at not more than 0.04%, S at not more than 0.005%, Ti at 0.05 to 0.15%, Al at 0.005 to 0.10% and N at not more than 0.007%.

Method for manufacturing high strength hot-rolled steel sheet having excellent stretch flangeability and fatigue resistance

A high-strength steel sheet includes a composition containing, in mass percent, 0.08% to 0.20% of carbon, 0.2% to 1.0% of silicon, 0.5% to 2.5% of manganese, 0.04% or less of phosphorus, 0.005% or less of sulfur, 0.05% or less of aluminum, 0.07% to 0.20% of titanium, and 0.20% to 0.80% of vanadium, the balance being iron and incidental impurities.

High strength steel sheet and method for manufacturing the same

A steel sheet including C at 0.05 to 0.15%, Si at 0.2 to 1.2%, Mn at 1.0 to 2.0%, P at not more than 0.04%, S at not more than 0.0030%, Al at 0.005 to 0.10%, N at not more than 0.005% and Ti at 0.03 to 0.13%, the balance being Fe and inevitable impurities, includes surface regions having an area fraction of bainite of less than 80% and an area fraction of a ferrite phase with a grain diameter of 2 to 15 &#x3bc;m of not less than 10%, the surface regions extending from both surfaces of the steel sheet each to a depth of 1.5 to 3.0% relative to a total sheet thickness, as well as an inner region other than the surface regions having an area fraction of a bainite phase of more than 95%, and has a tensile strength of not less than 780 MPa.

High strength hot rolled steel sheet having excellent bendability and method for manufacturing the same

Described is a high tensile strength hot rolled steel sheet having high strength and formability, and a manufacturing method. It has tensile strength &#x2267;980 MPa and excellent formability, and specifically identified ranges by mass % of C, Si, Mn, P, S, N, Al, Ti, V, Solute V, and Solute Ti; (ii) microstructure with fine carbides dispersion precipitated therein, the fine carbides containing Ti and V and having the average particle diameter &#x3c;10 nm, as well as volume ratio with respect to the entire microstructure &#x2267;0.007; and matrix as ferrite phase having area ratio with respect to the entire microstructure &#x2267;97%. C, Ti, V, S and N satisfy (1) Ti&#x2267;0.08 +(N/14&#xd7;48+S/32&#xd7;48) and (2) 0.8&#x2266;(Ti/48+V/51)/(C/12)&#x2266;1.2, where 'C', 'Ti', &#x201c;V&#x201d;, &#x201c;S&#x201d; and &#x201c;N&#x201d; represent contents (mass %) of corresponding elements, respectively.

High tensile strength hot rolled steel sheet having excellent formability and method for manufacturing the same

The invention provides a high strength cold rolled steel sheet comprising ferrite phases and second phases, in which the mean grain size of the ferrite phases is 20 &#x3bc;m or less, the volume fraction of the second phase is 0.1% or more to less than 10%, the absolute value |&#x394;r| of in-plane anisotropy of r value is less than 0.15, and the thickness is 0.4 mm or more. The high strength cold rolled steel sheet of the present invention has a tensile strength of 370 to 590 MPa, and has excellent stretchability, dent resistance, surface precision, secondary working embrittlement, anti-aging, and surface appearance, therefore it is suitable for outer panels of automobile.

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Tokyo, Japan

Katsumi Nakajima