Jfe Steel Corporation

Yuma Honda

Yoshihiko Ono

Shimpei Yoshioka

Nobuyuki Nakamura

Yoshimasa Funakawa

Shinjiro Kaneko

Kozo Harada

Takuya Hirashima

Akinori Nakamura

Hiroshi Matsuda

Koichi Taniguchi

Noriaki Kohsaka

Kenji Kawamura

Takeshi Murai

Tadachika Chiba

A steel sheet has a tensile strength of 1310 MPa or higher, a specified chemical composition, and a steel microstructure containing martensite at an area ratio of 70% or more, bainite at an area ratio of 30% or less, and ferrite and retained austenite at a total area ratio of 5% or less, in which, at a ¼ thickness position of the steel sheet, a number density of carbides having long axes of 0.5 μm or more is 60000 carbides/mmor less, in a ¼-to-¾ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 10 grains/mmor more and 30 grains/mmor less, and, in a surface-to-¼ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 27 grains/mmor less.

Steel sheet, member, and production methods therefor

A thin steel sheet has a specific chemical composition. The thin steel sheet has a microstructure in which ferrite is present in an area fraction of 5% or more and 60% or less, as-quenched martensite is present in an area fraction of 10% or less (including 0%), retained austenite is present in an area fraction of 5% or more and 20% or less, and upper bainite, lower bainite, and tempered martensite are present in a total area fraction of more than 15% and less than 85%; BCC iron that has a misorientation of 1° or less and surrounds retained austenite having an aspect ratio of 2.5 or higher is present in an area fraction of 5% or more and 70% or less; and a top 10% of retained austenite in terms of an equivalent circular diameter has an average aspect ratio of 2.5 or higher.

Thin steel sheet and method for manufacturing same

A high-strength steel sheet having a low yield ratio and a method for producing the same. The high-strength steel sheet has a specified chemical composition and a microstructure in which ferrite is present as a major phase, and martensite is present in an area fraction of 10% or greater and less than 50% relative to an entire area of the microstructure. The martensite has an average grain diameter of 3.0 μm or less, in an entirety of the martensite, a proportion of martensite having an aspect ratio of 3 or less is 60% or greater, and the martensite having an aspect ratio of 3 or less has a carbon concentration of 0.30% or greater and 0.90% or less in mass %.

High-strength steel sheet and method for producing the same

A steel sheet having a specified chemical composition and a tensile strength of 1,320 MPa or more, and methods for producing the steel sheet. The steel sheet has a specified microstructure including martensite and bainite, the total area fraction of the martensite and the bainite being 95% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite. The forumulae [% Ti]+[% Nb]>0.007 and [% Ti]×[% Nb]≤7.5×10are satisfied in the chemical composition.

Steel sheet, member, and methods for producing them

A steel sheet having a specified chemical composition and a tensile strength of 1,320 MPa or more, and methods for producing the steel sheet. The steel sheet has a specified microstructure including martensite and bainite, the total area fraction of the martensite and the bainite being 92% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite. The forumulae [% Ti]+[% Nb]>0.007 and [% Ti]×[% Nb]≤7.5×10are satisfied in the chemical composition.

A high-strength steel sheet of the present invention has a specific chemical composition. Furthermore, in the steel sheet, a degree of Mn segregation in a specific region is 1.5 or less; a maximum P concentration in a specific region is 0.08 mass % or less; in a specific region, at least one specific MnS particle group is present, the number of specific MnS particle groups is 2.0 or fewer per 1 mm, and the number of specific oxide-based inclusions is 8 or fewer per 1 mm; of all oxide-based inclusions, oxide-based inclusions having a specific composition are present in a number ratio of 80% or greater; the microstructure includes, in terms of a volume fraction, 30 to 95% martensite, 5 to 70% ferrite phase, less than 30% (and 0% or greater) bainite, and less than 2.0% (and 0% or greater) austenite phase; and a tensile strength is 980 MPa or greater.

High-strength steel sheet and method for producing same

The high-strength steel sheet according to the present invention includes a specific chemical composition, a steel structure in which a total area fraction of martensite and bainite in a position of ¼ of a sheet thickness is 92% or more and 100% or less, the balance in a case where the total area fraction is not 100% contains retained austenite, and an area fraction of ferrite in a region extending up to 10 μm in a sheet thickness direction from a surface is 10% or more and 40% or less, in which a tensile strength is 1320 MPa or more, and a Vickers hardness in a position of 15 μm in the sheet thickness direction from the surface satisfies a specified formula.

High-strength steel sheet and method for manufacturing same

A cold rolled steel sheet having a high strength, an aging resistance, a high yield ratio and a small anisotropy of tensile strength is obtained by hot rolling and cold rolling a steel material containing in percent by mass C: 0.06-0.14%, Si: less than 0.50%, Mn: 1.6-2.5%, Nb: not more than 0.080% (including 0%), Ti: not more than 0.080% (including 0%), provided that Nb and Ti are contained in an amount of 0.020-0.080% in total, subjecting a cold rolled steel sheet continuous annealing including steps of soaking-annealing at a temperature of 840-940&#xb0; C. for a holding time of 30-120 seconds, cooling from the soaking temperature to 600&#xb0; C. at a rate of not less than 5&#xb0; C./s, retaining in a temperature range of 600-500&#xb0; C. for 30-300 seconds and then conducting a secondary cooling to apply such a steel structure that martensite is finely dispersed into ferrite base.

Cold rolled steel sheet and method of manufacturing the same

A high-strength cold rolled steel sheet having mechanical characteristics having a tensile strength of not less than 780 MPa, a yield ratio of not more than 70%, and a small in-plane anisotropy of a tensile characteristicis obtained by hot rolling a steel slab comprising by mass % C: 0.07 to 0.12%, Si: not more than 0.7%, Mn: 2.2 to 2.8% and Ti and Nb: 0.02 to 0.08% in total, and cold rolling the sheet, followed by continuous annealing to form a steel texture comprised of ferrite having an area ratio of 40 to 80% with respect to the whole texture, and a second phase constituted by tempered martensite, fresh martensite and bainite, wherein the total area ratio of the bainite and the tempered martensite to the second phase is 50 to 80%, and the aspect ratio of the fresh martensite is in the range of 1.0 to 1.5.

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

A high-strength steel sheet having high yield ratio, excellent stretch flange formability, and resistance to secondary working embrittlement. The steel sheet has a composition containing C: 0.02% to less than 0.10%, Si: less than 0.3%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% to 0.10%, N: 0.010% or less, and Nb: 0.003% to less than 0.070% on a mass basis, the remainder being Fe and inevitable impurities. A steel microstructure of the steel sheet contains ferrite: 90% or more and a total of pearlite, martensite, retained austenite, and cementite: 0% to 10% on an area fraction basis, in which the average grain size of the ferrite is 15.0 &#x3bc;m or less, and in which the average aspect ratio of the ferrite is 1.2 or more; and a tensile strength of 500 MPa or less.

High-strength steel sheet and method for manufacturing the same

A high-strength steel sheet having a high yield ratio and excellent stretch flangeability, and a method for producing the high-strength steel sheet. The high-strength steel sheet has a composition containing, by mass: C: 0.02% or more and less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% or more and 0.10% or less, N: 0.010% or less, Nb: 0.005% or more and less than 0.070%, and the balance being Fe and inevitable impurities; and a microstructure including, by area: ferrite: 85% or more, pearlite: 0% to 15%, and a total of martensite, retained austenite and cementite: 0% to 3%. The average crystal grain diameter of the ferrite is 15.0 &#x3bc;m or less. The grain diameter of a Nb carbide is 5 to 50 nm. The amount of Nb carbide precipitate is 0.005% to 0.050% in terms of volume fraction.

A high-strength steel sheet having a high yield ratio and a reduced difference in strength between the center and edges of the steel sheet in the width direction. The high-strength steel sheet includes a composition containing, by mass, C: 0.02% or more and less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% or more and 0.10% or less, N: 0.010% or less, Nb: 0.005% or more and less than 0.070%, and the balance being Fe and inevitable impurities. Additionally, the steel sheet has a microstructure including, by area, ferrite: 90% or more, pearlite: 0% to 10%, and the total of martensite, retained austenite, and cementite: 0% to 3%. The average crystal grain diameter dof the ferrite at the center of the steel sheet in the width direction is 15.0 &#x3bc;m or less.

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Fukuyama, Japan

Yuma Honda