Showing papers by "Nobuhiro Fujita published in 2013"

High-strength cold-rolled steel sheet and process for manufacturing same

Abstract: This high-strength cold-rolled steel sheet has: a composition which contains, in mass%, 0.075 to 0.300% of C, 0.30 to 2.50% of Si, 1.30 to 3.50% of Mn, 0.001 to 0.050% of P, 0.0001 to 0.0100% of S, 0.001 to 1.500% of Al and 0.0001 to 0.0100% of N; a surface microstructure which comprises, in volume fraction, 3 to 10% of retained austenite and at most 90% of ferrite; and an internal microstructure which contains 3 to 20% of retained austenite in volume fraction. The internal microstructure is such observed at a depth of t/4 from the surface [wherein t is the thickness of the steel sheet]. Further, the Hvs/Hvb ratio exceeds 0.75 and is up to 0.90, Hvs being the hardness in the surface layer of the steel sheet and Hvb is the hardness at a depth of t/4 from the surface. The maximum tensile strength of the high-strength cold-rolled steel sheet is 700MPa or higher.

High-strength hot-rolled steel sheet for line pipe excellent on-site weldability and production method therefor

Abstract: PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet for a spiral pipe which can be used even in an area where severe buckling resistance and low-temperature toughness are required and in addition possesses strength equal to or greater than API 5L-X65 specification.SOLUTION: A high-strength hot-rolled steel sheet for a spiral pipe excellent in buckling resistance is a steel sheet containing specified components, satisfying N-14/48×Ti≥0%, and the balance comprising Fe and unavoidable impurity elements, and is characterized in that: the mean value of X-ray random intensity ratios of {100} to {223} orientation group in the sheet thickness center is 4 or less; the X-ray random intensity ratio of the crystal orientation of {332} is 4.8 or less; the pro-eutectoid ferrite fraction is 3% to 20%; the rest are low-temperature transformation products; the number-average crystal grain diameter of the entire micro-structure is 5 μm or less; the area-average grain diameter is 12 μm or less; and the standard deviation of the area-average particle diameters is 2 μm or less.

Galvanized steel sheet and manufacturing method thereof

Abstract: A galvanized steel sheet includes a steel sheet, and a coating layer on the surface of the steel sheet. The steel sheet has a microstructure including, one or two of martensite and bainite of a volume percentage of 20% to 99% in total, with a remainder composed of ferrite, residual austenite of a volume percentage of less than 8%, and perlite of a volume percentage of 10% or less, and has a tensile strength of 980MPa or more. The coating layer includes oxides including one or two or more chemical elements selected from Si, Mn, and Al, and includes Fe of a mass percentage of 15% or less, and the projected area ratio of the oxides is 10% to 90 % in a cross-sectional view including the steel sheet and the coating layer in a sheet thickness direction.

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

Abstract: A high-strength cold-rolled steel sheet includes, as a component composition, by mass %: C: 0.075% to 0.300%; Si: 0.30% to 2.50%; Mn: 1.30% to 3.50%; P: 0.001% to 0.050%; S: 0.0001% to 0.0100%; Al: 0.001% to 1.500%; and N: 0.0001% to 0.0100%, in which a surface microstructure contains residual austenite of 3% to 10% and ferrite of 90% or less by volume fraction, an inner microstructure at a depth of t/4 from the surface assuming that a sheet thickness is t contains residual austenite of 3% to 20% by volume fraction, a ratio Hvs/Hvb between a surface hardness Hvs of the steel sheet surface and a hardness Hvb at a depth of ¼ of the thickness is more than 0.75 to 0.90, and a maximum tensile strength is 700 MPa or more.

Present Situation and Future Trend of Ultra-High Strength Steel Sheets for Auto-Body

Abstract: Application of ultra-high strength steel sheets is one of the most important methods to satisfy weight reduction and crash safety of a vehicle. Recently, there is a trend to apply ultra-high strength steel sheets widely to auto-body parts. In this report, present situation of ultra-high strength steel sheets, especially with tensile strength more than 980MPa and 1,470MPa by hot-stamping, have been introduced.

High strength hot rolled steel sheet for line pipe excellent in on-site weldability and method for producing the same

Abstract: PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet for a line pipe, which has both high toughness and strength sustainable when used in a region where the ground is moved by freezing and melting like discontinuous tundra, in viewpoint of on-site weldability, such as transportation efficiency, remaining heat, and being free of post weld heat treatment, etc.SOLUTION: The high strength hot rolled steel sheet for a line pipe excellent in on-site weldability is a steel sheet containing predetermined components and satisfying N-14/48×Ti≥0%, wherein a pro-eutectoid ferrite fraction is less than 3% in microstructure at the center of sheet thickness, others are low-temperature transformation phases, in the entire microstructure, a number-average crystal grain size is 2.5 μm or less and an area-average grain size is 9 μm or less, and standard deviation of the area-average grain size is 2.3 μm or less.

Model for the Prediction of Microstructures and Mechanical Properties of Cold-rolled High Strength Steels

Abstract: Microstructure development of advanced high strength steels is quiet complicated and, thereby, it takes much time to develop advanced high strength steels. This paper presents a complete model which can simulate microstructure development of high strength steels during a heat treatment and the mechanical properties after the treatment. The model consisting of ferrite, bainite, martensite and predicting stressstrain curve of the steels is successively built based on metallurgical phenomena.

High-strength cold rolled steel sheet and manufacturing method thereof

Abstract: This high strength cold rolled steel sheet include a predetermined chemical composition wherein a microstructure at a 1/4 positon in a thickness direction from a surface includes, by volume%, a tempered martensite of 70.0% or more, a residual austenite of more than 3.0% and less than 10.0%, a total of a ferrite and a bainite of 25.0% or less, and a martensite of 5.0% or less, a microstructure at 25 [mu]m in the thickness direction from the surface, includes by volume%, a total of the ferrite and the bainite of 70% or more, and a total of the martensite and the tempered martensite of 30% or less, and at 25 [mu]m in the thickness direction from the surface, an average grain diameter of the martensite and the tempered martensite is 5.0 [mu]m or less, a tensile strength is 1310 MPa or more, an uniform elongation is 5.0% or more, a R/t, which is a ratio of limiting bend radius R and sheet thickness t, is 5.0 or less.

Experimental Approach for Investigation of Micro Void Behavior on Dual Phase Steel during Plastic Deformation

Abstract: In this research, micro void behavior of DPS, which has different phase volume fraction, is investigated experimentally. Prepared specimens have different amount of alloy element and heat treatment condition. Although both these specimens have almost the same strength and total elongation, these showed quite different local elongations, hole expanding ratio and bending ductility. In order to observe micro void behavior specimens, that have different strain levels, are prepared. These sectional areas are observed by laser microscope after cutting and polishing. The change ratio of micro void volume fraction of DPS, which showed high local ductility, is lower than that of the other DPS. Concerning low ductility DPS, small size void (less than 3µm diameter) increases rapidly not only at grain boundary but in the ferrite grain, especially at initial stage of deformation. On the other hand, nucleation of micro void of high ductility DPS is inhibited.

Zinc coated steel sheet and manufacturing method thereof

Abstract: A zinc coated steel sheet includes a steel sheet, and a coating layer on the surface of the steel sheet The steel sheet has a steel chemical composition including, by mass%, C: 005% to 040%, Si: 05% to 30%, and Mn: 15% to 30%, a microstructure including tempered martensite of a volume percentage of 30% or more, and austenite of a volume percentage of 8% or more, ferrite, and bainite, and a tensile strength of 980 MPa or more The coating layer includes oxides including one or more chemical elements selected from Si, Mn, and Al, and the projected area ratio of the oxides is 10% or more in a cross-sectional view including the steel sheet and the coating layer in a sheet thickness direction