Jfe Steel Corporation

Lingling Yang

Tatsuya Nakagaito

Noriaki Kohsaka

Shunsuke Yamamoto

Yuki Takeda

Shotaro Terashima

Yoshiyasu Kawasaki

Katsuya Hoshino

Yoshimasa Funakawa

Takeshi Yokota

Yuki Toji

Hidekazu Minami

Yusuke Wada

Tomomi Kanazawa

Gosuke Ikeda

Nakagaito Tatsuya

A steel sheet having a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, high LME resistance, and good weld fatigue properties. The steel sheet has a specific chemical composition and a specific steel microstructure. Crystal grains containing an oxide of Si and/or Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in the range of 3 to 10 μm, the lowest Si concentration Land the lowest Mn concentration Lin the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration Tand a Mn concentration Tat a quarter thickness position of the steel sheet satisfy a specified formula.

Steel sheet, member, and method for producing them

A high strength galvanized steel sheet is disclosed having a tensile strength of 1,320 MPa or more. A chemical composition of a steel sheet contains, by mass, C: 0.150 to 0.450%, Si: 0.80 to 3.00%, Mn: 2.00 to 4.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 0.100%, O: not more than 0.0100%, and N: not more than 0.0100%, with a balance of Fe and inevitable impurities. An amount of diffusible hydrogen in steel is not more than 0.60 mass ppm. A total of tempered martensite and bainite is 60 to 95%, retained austenite is 5 to 30%, and retained austenite having an aspect ratio of 5.5 or higher with respect to an entire retained austenite is not more than 50%, and fresh martensite having a diameter of 2.0 μm or less is not more than 20%.

High-strength galvanized steel sheet and member, and method for manufacturing same

A high-strength steel sheet that has a predetermined component composition, that has a steel microstructure in which, in a thickness cross-section in a rolling direction, an area percentage of ferrite ranges from 5% to 30%, a total area percentage of tempered martensite and bainite ranges from 40% to 90%, pearlite constitutes 0% to 5%, a total area percentage of fresh martensite and retained γ ranges from 5% to 30%, a ratio of a total area percentage of the fresh martensite and the retained γ to a total area percentage of the tempered martensite, bainite, and pearlite is 0.5 or less, and a ratio of the fresh martensite and the retained γ adjacent to the ferrite with respect to the fresh martensite and the retained γ is 30% or more in total area percentage, and that has a yield strength of 550 MPa or more.

High-strength steel sheet and method for producing the same

A steel sheet with a tensile strength (TS) of 1180 MPa or more, a member, and a method for producing them. In a region of the steel sheet within 4.9 μm in the thickness direction, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more. The lowest Si concentration Land the lowest Mn concentration Lin the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration Tand a Mn concentration Tat a quarter thickness position of the steel sheet satisfy the following formula (1):≤()/4  (1).

A steel sheet according to the present invention has a specific chemical composition and a steel microstructure including, in terms of area fraction, ferrite: 40% or more and 70% or less, a total of bainite and tempered martensite: 5% or more and 30% or less, retained austenite: 4% or more and 18% or less, fresh martensite: 8% or more and 35% or less, and the remainder: 5% or less. Cementite particles are present in the retained austenite, a ratio of an area fraction of the cementite particles in the retained austenite to an area fraction of the retained austenite is 5% or more and 25% or less, and the steel sheet has a tensile strength of 780 MPa or more and less than 980 MPa.

Steel sheet, member, and methods for manufacturing the same

A high-strength steel sheet that has a predetermined component composition, that has a steel microstructure in which an area percentage of ferrite ranges from 5% to 50% in a thickness cross-section in a rolling direction, a total area percentage of fresh martensite and retained γ ranges from 2% to 30%, each of the fresh martensite and the retained γ has an average grain size of 5 μm or less, and a ratio of the fresh martensite and the retained γ adjacent only to ferrite with respect to the fresh martensite and the retained γ from a surface to 200 μm in the thickness direction is 30% or less in total area percentage, and that has a yield strength of 550 MPa or more.

A steel sheet having a tensile strength (TS) of 1180 MPa or more, high LME resistance, and good weld fatigue properties. The steel sheet has a specific chemical composition and a specific steel microstructure. Crystal grains containing an oxide of Si and/or Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in the range of 3 to 10 μm, the lowest Si concentration Land the lowest Mn concentration Lin the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration Tand a Mn concentration Tat a quarter thickness position of the steel sheet satisfy a specified formula.

A cold rolled steel sheet of the present invention has a specific chemical composition and a steel microstructure in which the average aspect ratio of ferrite crystal grains is 2.0 or less; an rvalue that is the average value of values obtained by measuring the r-value represented by a first predetermined formula in three places apart with spacings of 200 mm in the sheet width direction is 1.20 or more, and |&#x394;r| that is the average value of values obtained by measuring, in three places apart with spacings of 200 mm in the sheet width direction, |&#x394;r| that is the absolute value of &#x394;r represented by a second predetermined formula is 0.40 or less and the difference between the largest value and the smallest value of |&#x394;r| among the three places is 0.15 or less.

Cold rolled steel sheet and method for manufacturing the same

Provided are a high-strength steel sheet having a yield strength of 550 MPa or more and excellent fatigue-strength of a weld and a method for manufacturing the steel sheet. A high-strength steel sheet has a specified chemical composition, a steel microstructure observed in a cross section in a thickness direction parallel to a rolling direction including 40% to 75% of a martensite phase in terms of volume fraction, in which a total volume fraction of martensite grains whose average grain diameter ratios with respect to adjacent ferrite grains are &#xbc; or more and 1 or less is 60% or more with respect to an entire martensite phase, and a yield strength (YP) of 550 MPa or more.

High-strength steel sheet and method for manufacturing the same

Provided are a high-strength steel sheet having a yield strength of 550 MPa or higher and having a small amount of springback and width-direction uniformity in material properties as well as a manufacturing method therefor. The high-strength steel sheet has a yield strength (YP) of 550 MPa or higher and has a specific component composition and a microstructure containing a ferrite phase, 40 to 70% of a martensite phase in area ratio, and 5 to 30% of a bainite phase in area ratio, where: an average grain size of the martensite phase is 2 to 8 &#x3bc;m and an average grain size of the ferrite phase is 11 &#x3bc;m or less on a cross-section in the thickness direction and in a direction orthogonal to a rolling direction; and the average grain size of the ferrite phase is 3.0 times or less the average grain size of martensite.

High-strength steel sheet and manufacturing method therefor

Provided are a high-strength coated steel sheet and a method for manufacturing the same. The high-strength coated steel sheet has a base steel sheet and a coating layer formed on a surface of the base steel sheet. The base steel sheet has a specified chemical composition and a microstructure, including a martensite phase and a ferrite phase. A volume fraction of the martensite phase is 50% to 80%. A volume fraction of tempered martensite with respect to the whole martensite phase is 50% or more and 85% or less. An average grain diameter of the ferrite phase is 13 &#x3bc;m or less. A volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more. Yield strength (YP) of the high-strength coated steel sheet is 550 MPa or more.

High-strength coated steel sheet and method for manufacturing the same

Provided are a high-strength steel sheet having high strength of a yield strength of 550 MPa or more and a method for manufacturing the same. The high-strength steel sheet has a specified chemical composition and a microstructure, where observed in a cross section in a thickness direction perpendicular to a rolling direction, including a martensite phase having a volume fraction of 50% to 80%, and a ferrite phase having an average grain diameter of 13 &#x3bc;m or less, wherein a volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more, and wherein an average length in a longitudinal direction (in a width direction of the steel sheet) of the ferrite grains is 20 &#x3bc;m or less, and a yield strength (YP) of 550 MPa or more.

Provided are a steel sheet with excellent weldability, and a production method therefor. The steel sheet is characterized by having a specific composition and a metallographic structure containing, in terms of an area ratio, ferrite of 25% or more and 65% or less, martensite having iron-based carbides precipitated in the grains of 35% or more and 75% or less, and the balance structure other than the ferrite and the martensite of 20% or less (including 0%) in total, the average grain diameters of the ferrite and the martensite being respectively 5 &#x3bc;m or lower, the total of concentration of Si and Mn at interface between the ferrite and the martensite being, in terms of an atomic concentration, 5% or more, and having a tensile strength of 900 MPa or higher.

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Tokyo, Japan

Lingling Yang