Kenji Oi

Bio: Kenji Oi is an academic researcher from Fujitsu. The author has contributed to research in topics: Welding & Ultimate tensile strength. The author has an hindex of 13, co-authored 92 publications receiving 555 citations. Previous affiliations of Kenji Oi include Kawasaki Steel Corporation.

Method for producing high tensile steel sheet

Abstract: A method for producing a high tensile steel sheet, which comprises casting a steel having a chemical composition, in mass %, that C: 0.02 to 0.18%, Si: 0.05 to 0.5 %, Mn: 0.5 to 2.0 %, Al: 0.005 to 0.1 %, N: 0.0005 to 0.008 %, P: 0.03 % or less, S: 0.03 % or less, P: 0.03 % or less, and the balance: Fe and inevitable impurities, subjecting the resultant steel to hot rolling so as for the sheet to have a prescribed sheet thickness, without cooling it to its Ar3 transformation point or lower or after re-heating it to its Ac3 transformation point or higher, subsequently subjecting the rolled sheet to the direct quenching or accelerated cooling from its Ar3 transformation point or higher, to cool it to 400˚C or lower, and then tempering the cooled sheet to such a degree that a portion of the sheet being central in the thickness direction has a highest temperature of 520˚C or higher, with an average rate of the temperature rise in the above portion of the sheet of 1˚C/s or higher for the range from 460˚C to a prescribed tempering temperature being its Ac1 transformation point or lower, by the use of a heating apparatus provided being directly connected to the same production line as that for a roller and an apparatus for direct quenching or accelerated cooling. The above method allows the production of a high tensile strength sheet which is superior to a conventional sheet in the balance between tensile strength and toughness before PWHT and after PWHT and has a tensile strength of 570 MPa (N/mm2) or more, by adopting, in the tempering treatment for a quenched and tempered steel sheet, a specific value as the rate of temperature rise in a portion of the steel sheet being central in the thickness direction.

High tensile steel products excellent in the resistance to delayed fracture and process for production of the same

Abstract: The invention provides high tensile steel products which have tensile strength of 600MPa or above and are excellent in the resistance to delayed fracture and thus suitable for construction and industrial machinery use and a process for the production of the same. A steel which contains C, Si, Mn, Al, N, P, S and, if necessary, one or more elements selected from among Mo, Nb, V, Ti, Cu, Ni, Cr, W, B, Ca, REM and Mg with the balance consisting of Fe and unavoidable impurities and in which the average of aspect ratios of prior austenite grains is 3 or above over the whole plate thickness; a steel as described above in which the cementite covering ratio in the lath boundary is 50% or below; a steel as described above in which the safety index of delayed fracture resistance is 75% or above; and a process for the production of high tensile steel products excellent in the resistance to delayed fracture which comprises casting a steel as described above, either inhibiting the cast steel from being cooled to a temperature of the Ar3 transformation temperature or below or reheating the cast steel to a temperature of the Ac3 transformation temperature or above, hot-rolling the resulting cast steel at a reduction of 30% or above in the non-recrystallization region, cooling the hot-rolled steel from a temperature of the Ar3 transformation temperature or above to a temperature of 350°C or below at a cooling rate of 1°C/s or above, and then tempering the resulting rolled steel at a temperature of the Ac1 transformation temperature or below.

High-tensile steel sheet and production method thereof

Abstract: PROBLEM TO BE SOLVED: To provide a production method of a high-tensile steel sheet excellent in strength/toughness balance after PWHT (post-weld heat treatment) compared with conventional materials, by regulating the temperature-rising rate at the center of the sheet thickness during an annealing treatment of a quenched/annealed material. SOLUTION: In the production method of the high-tension steel sheet, steel comprising, by mass, 0.02-0.18% C, 0.05-0.5% Si, 0.5-2.0% Mn, 0.005-0.1% Al, 0.0005-0.008% N, ≤0.03% P, ≤0.03% S and the balance being Fe and unavoidable impurities is cast and, without being cooled to a temperature equal to or below the Ar 3 transformation point or after being re-heated to a temperature equal to or above the Ac 3 transformation point, the steel is subjected to hot-rolling to a prescribed thickness. Then, the steel is either subjected to direct quenching from a temperature equal to or above the Ar 3 transformation point or is cooled to 400°C or below through accelerated cooling. The steel is subsequently annealed so that the center of the sheet in the thickness direction reaches a maximum temperature of ≥520°C. Here, annealing is performed by using a heating apparatus which is directly connected to a production line comprising a roller and a direct quenching apparatus or an accelerated cooling device and raising the temperature at an average rate of ≥1°C/s at the center of the sheet thickness from 460°C to the prescribed annealing temperature not higher than the Ac 1 transformation point. COPYRIGHT: (C)2006,JPO&NCIPI

High toughness thick steel plate excellent in toughness in weld heat-affected zone, and its production

Abstract: PROBLEM TO BE SOLVED: To refine ferrite grains after rolling and to improve toughness in weld heat-affected zone as well as strength by specifying the composition of a (low C)Mn steel containing Ti, Nb, V, etc., and also specifying the carbon equivalent of the steel and the size and number of contained TiN, respectively. SOLUTION: The steel has a composition consisting of, by weight ratio, 0.01-0.18% C, 0.02-0.60% Si, 0.60-2.00% Mn, ≤0.030% P, ≤0.015% S, 0.0015-0.100% Al, 0.005-0.30% Ti, 0.0050-0.0200% N, and the balance Fe with inevitable impurities. Further, the carbon equivalent Ceq, determined by equation, is 0.36-0.45%, and TiN of ≤0.05 μm circle-equivalent diameter and TiN of 0.03-0.27 μm circle-equivalent diameter are contained by ≤1×10 3 pieces/mm 2 and <1×(10' to 10 5 ) pieces/mm 2 , respectively. If necessary, one or ≥2 elements among specific amounts of Cu, Ni, Mo, Nb, V, B, etc., can further be added to the above composition. Moreover, this steel is formed into thick steel plate by controlled rolling. COPYRIGHT: (C)1999,JPO

Welding method of thick steel plate

Abstract: PROBLEM TO BE SOLVED: To provide a welding method of a thick steel plate which is suitable for pipe making welding of large diameter steel pipes such as UOE steel pipe with a thickness of ≥30 mm and spiral steel pipe. SOLUTION: In welding a steel material with a thickness of ≥30 mm or more from both sides, an electrode for gas shielded arc welding is arranged, as multi-electrode welding as necessary, in the front in the welding direction of the first electrode in multi-electrode submerged arc welding for hybrid welding. In the hybrid welding, the multi-electrode submerged arc welding is performed with a heat input to satisfy an expression (1) while, desirably, the gas shielded arc welding is performed with a heat input to satisfy an expression (2). If the gas shielded arc welding is multi-electrode welding, the wire diameter to be applied for the first electrode is ≥1.4 mm, with a current density set at ≥500 A/mm 2 . Provided that 0.18t-3≤Q S ≤0.35t-5.5 (1), where t is the thickness (mm) of the steel material and Q S is a welding heat input (KJ/mm) of multi-electrode submerged arc welding, and that Q G ≤0.17t-1.5 (2), where t is the thickness (mm) of the steel material and Q G is a welding heat input (KJ/mm) of gas shielded arc welding. COPYRIGHT: (C)2011,JPO&INPIT

High strength steel sheet and method for manufacturing the same

Abstract: Disclosed is a high-strength steel sheet having excellent processability and a tensile strength of 980 MPa or more. The high-strength steel sheet has the following composition (in mass%): C: 0.1 to 0.3% inclusive, Si: 2.0% or less, Mn: 0.5 to 3.0% inclusive, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less, with the remainder being Fe and unavoidable impurities. The steel structure of the high-strength steel sheet comprises 50% or more of martensite, 50% or less (including 0%) of ferrite, 10% or less (including 0%) of bainite, and 10% or less (including 0%) of residual austenite. In the high-strength steel sheet, the half width in the degree distribution of the nano-hardness obtained by the measurement of the hardness distribution of the martensite is 2.0 GPa or more, and the steel sheet has a tensile strength of 980 MPa or more.

High-strength steel sheet and process for production thereof

Abstract: Disclosed are an ultra-high-strength steel sheet having both a tensile strength of as high as 1400MPa or above and excellent formability and an advantageous process for manufacturing the same. A high-strength steel sheet having both a composition which contains by mass C: 0.12 to 0.50%, Si: 2.0% or less, Mn: 1.0 to 5.0%, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less with the balance being Fe and unavoidable impurities, and a structure which comprises, in terms of area fraction, autotempered martensite: 80% or more, ferrite: less than 5%, bainite: 10% or less, and retained austenite: 5% or less and in which the average number of precipitated iron carbide particles of 5nm to 0.5[mu]m in the autotempered martensite is 5OE04 or above per mm2.

Advances in friction stir spot welding

Abstract: Friction stir spot welding (FSSW) is a variation of linear Friction Stir Welding (FSW), which was invented to compete with resistance spot welding (RSW) and riveting of lightweight alloys in the au...

High strength hot rolled steel sheet and method for producing same

Abstract: Provided are a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent in terms of bending workability and a method for manufacturing the steel sheet. A high-strength hot-rolled steel sheet having a tensile strength TS of 980 MPa or more is manufactured by controlling the steel sheet to have a chemical composition containing C: more than 0.1% and 0.2% or less, Si: 0.5% or more and 3.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.05% or less, S: 0.004% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.15% or less, V: more than 0.10% and 0.30% or less, and the balance being Fe and inevitable impurities. Surface regions have a microstructure including mainly a ferrite phase, and an inner region has a microstructure including mainly a bainite phase. The proportions of the surface regions in the thickness direction of the steel sheet are 1.0% or more and 5.0% or less of the whole thickness respectively from the upper and lower surfaces of the steel sheet.

Titanium microalloyed steels

Abstract: Compared with niobium and vanadium, titanium has been regarded as a relative minor element in microalloyed (MA) steels. More recently, titanium compounds in MA steels have been recognised as having...