Jfe Steel Corporation

Noriaki Moriyasu

Kazuhiko Yamazaki

Yoshimasa Funakawa

Shunsuke Toyoda

Hayato Saito

Takayuki Murata

Katsumi Nakajima

Sumio Kaiho

Takeshi Yokota

Shinjiro Kaneko

Nobuyuki Nakamura

Kaneharu Okuda

Tetsuo Shimizu

Taro Kizu

Tetsuya Mega

Kazunori Tahara

Kana Sasaki

Tamako Ariga

Masahide Watabe

Reiko Mizuno

Kentaro Irisa

Shigeyuki Nabeshima

A high-strength hot rolled steel sheet and a method for manufacturing the high-strength hot rolled steel sheet are disclosed. The high-strength hot rolled steel sheet includes a specific chemical composition, and a steel structure in which a lower bainite phase and/or a tempered martensite phase at 90% or more in terms of a total area fraction is contained as a dominant phase, an average grain size of the dominant phase is 10.0 μm or less, and an amount of Fe in Fe-based precipitates is 0.70% or less in mass %, in which an arithmetic average roughness (Ra) of a surface is 2.50 μm or less, and a tensile strength TS is 1180 MPa or more.

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

There are provided a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and a production method therefor. The high-strength hot-rolled steel sheet has a predetermined component composition and a microstructure containing 75.0% or more by area and less than 97.0% by area of a primary phase composed of an upper bainite phase, the primary phase having an average grain size of 12.0 &#x3bc;m or less, and more than 3.0% by area and 25.0% or less by area of a secondary phase that is a structure composed of one or two of a lower bainite phase and/or a tempered martensite phase, and a martensite phase, in which the number density of grains of the secondary phase having an equivalent circular diameter of 0.5 &#x3bc;m or more is 150,000 grains/mmor less, and the steel sheet has an arithmetic mean surface roughness of 2.00 &#x3bc;m or less.

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

A high-strength hot-rolled steel sheet that has excellent punching workability and hole expandability, and a method for manufacturing the same. The hot-rolled steel sheet has a tensile strength of 980 MPa or more. The hot-rolled steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, N, Ti, Cr, and B, and has a microstructure including a bainite phase having an area ratio of 85% or more as a main phase, and a martensite phase or martensite-austenite constituent having an area ratio of 15% or less as a second phase, the balance being a ferrite phase. The second phase has an average grain diameter of 3.0 &#x3bc;m or less, prior-austenite grains have an average aspect ratio of 1.3 or more and 5.0 or less, and recrystallized prior-austenite grains have an area ratio of 15% or less relative to non-recrystallized prior-austenite grains.

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

There is provided a galvanized steel sheet more highly excellent in terms of punchability. The steel sheet has a specified chemical composition and a microstructure including a ferrite phase and a tempered bainite phase in a total amount of 95% or more in terms of area ratio, in which the average grain diameter of the microstructure is 5.0 &#x3bc;m or less, the amount of Fe precipitated is 0.10 mass % or more, the amount of Ti, Nb, and V precipitated in the form of precipitates having a grain diameter of less than 20 nm is 0.025 mass % or more in terms of precipitate C equivalent, and half or more of precipitates having a grain diameter of less than 20 nm are formed at random.

Galvanized steel sheet and method for manufacturing the 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 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

A high-strength hot-rolled steel sheet has a tensile strength (TS) of 540 to 780 MPa, only small variations in strength, and excellent uniformity in strength using a general-purpose Ti-containing steel sheet, which is inexpensive. The high-strength hot-rolled steel sheet includes, on a mass percent basis, 0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, and the balance being Fe and incidental impurities. The microstructure have a bainitic ferrite fraction of 70% or more, and the amount of Ti present in a precipitate having a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula (1): Ti*=[Ti]&#x2212;48/14&#xd7;[N] (1) where [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent by mass), respectively, of the steel sheet.

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

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Tokyo, Japan

Noriaki Moriyasu