Tetsuo Yamamoto

Bio: Tetsuo Yamamoto is an academic researcher. The author has contributed to research in topics: Ultimate tensile strength & Ferrite (iron). The author has an hindex of 9, co-authored 18 publications receiving 564 citations.

Development of High Strength Hot-rolled Sheet Steel Consisting of Ferrite and Nanometer-sized Carbides

Abstract: A ferritic steel precipitation-strengthened by manometer-sized carbides was developed to obtain a high strength hot-rolled sheet steel having tensile strength of 780 MPa grade with excellent stretch flange formability. Manganese in a content of 1.5% and molybdenum in a content of 0.2 % were added to 0.04 % carbon Ti-bearing steel in order to lower austenite-ferrite transformation temperature for fine carbides and to retard generating of pearlite and large cementites, respectively. Tensile strength of hot-rolled sheet steel increased with titanium content and it was achieved to 800 MPa in a 0.09 % Ti steel. Microstructure of the 0.09 %Ti steel was ferrite without pearlite and large cementites. Fine carbides of 3 nm in diameter were observed in rows in the ferrite matrix of the 0.09 % Ti steel with transmission electron microscope. The characteristic arrangement of the nanometer-sized carbides indicates that the carbides were formed at austenite-ferrite interfaces during transformation. By energy dispersive X-ray spectroscopy, the carbides were found to contain molybdenum in the same atomic concentration as titanium. Crystal structure of the nanometer-sized carbides was determined to be NaCI-type by X-ray diffractometry. The calculated amount of precipitation-strengthening by the carbides was approximately 300 MPa. This is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. Based on the results obtained in the laboratory investigation, mill trial was carried out. The developed hot-rolled high strength sheet steel exhibited excellent stretch flange formability.

High formability and high tensile strength hot rolled steel sheet having excellent material uniformity, production method therefor, and working method therefor

Abstract: PROBLEM TO BE SOLVED: To provide a high formability, high tensile strength, hot rolled steel sheet having excellent material uniformity which has a reduced variation in material in a coil, particularly that of yield strength in the width direction and can be industrially put into practical use, to provide a production method therefor, and to provide a working method therefor. SOLUTION: The steel containing, by weight, ≤0.1% C and 0.02 to 0.2% Ti, and containing one ore more kinds of metals selected from 0.05 to 0.6% Mo and 0.01 to 1.5% W is cast. The steel is thereafter hot-rolled, is coiled round a coil, and is heat-treated. By the process, carbides containing Ti and one or more kinds of metals selected from Mo and W are dispersively precipitated in the ranges satisfying the following (1) into a structure mainly made up of ferrite, and the difference in yield stress between the central part and edge parts in the width direction of the steel sheet is ≤30 MPa: 0.1≤Ti/[48{(Mo/96)+(W/184)}]≤3.5 (1). COPYRIGHT: (C)2004,JPO

Ultrahigh tensile strength steel sheet having excellent workability, production method therefor, and working method therefor

Abstract: PROBLEM TO BE SOLVED: To provide an ultrahigh tensile strength steel sheet which has excellent elongation and strength-flanging properties as the indexes of workability, to provide a production method, and to provide a working method therefor. SOLUTION: The ultrahigh tensile strength steel sheet having excellent workability so as to satisfy tensile strength of >=950 MPa has a composition containing, by weight, 0.07 to 0.15% C, =0.25 are dispersedly precipitated.

High tensile hot rolled steel sheet having excellent elongation and stretch-flanging property, production method therefor and working method therefor

Abstract: PROBLEM TO BE SOLVED: To provide a high tensile hot rolled steel sheet which has an excellent elongation and excellent stretch-flanging properties, and is suitably used for a member having a complex cross-sectional shape on pressing such as a suspension of an automobile, and to provide a production method therefor, and a working method therefor. SOLUTION: The steel sheet has a composition containing, by weight, <=0.06% C, <=0.3% Si, 1 to 2% Mn, <=0.06% P, <=0.005% S, <=0.06% Al, <=0.006% N, 0.07 to 0.45% Mo, 0.03 to 0.10% Ti and <=0.08% Nb, and has a ferritic structure in which carbonitride of Ti or Ti and Nb and carbide of Mo are respectively independently and/or compositely precipitated.

High-strength hot rolled steel sheet

Abstract: The present invention relates to a high strength hot rolled steel sheet containing 0.15% or less C, 0.02 to 0.35% Ti, and 0.05 to 0.7% Mo by weight percentage and consisting essentially of a matrix of ferrite structure single phase and fine precipitates with a grain size of smaller than 10 nm dispersed in the matrix, for example, a high strength hot rolled steel sheet which consists essentially of 0.06% or less C, 0.5% or less Si, 0.5 to 2.0% Mn, 0.06% or less P, 0.005% or less S, 0.1% or less Al, 0.006% or less N, 0.02 to 0.10% Ti, 0.05 to 0.6% Mo by weight percentage, and the balance being Fe, wherein fine precipitates with a grain size of smaller than 10 nm are dispersed in a matrix of ferrite structure single phase at a number per unit volume of 5×104/μm3 or higher. This steel sheet, which has tensile strength of not lower than 550 MPa, high elongation and excellent stretch flangeability, is suitable for intricately shaped automotive chassis parts such as a suspension arm.

Precipitation hardening of high-strength low-alloy steels by nanometer-sized carbides

Abstract: The effects of Ti, Ti–Mo, and Ti–Nb microalloy additions on the precipitation strengthening in three experimental high-strength low-alloy steels have been investigated. The objective of this work was to study the carbide precipitation under the conditions of continuous cooling and interrupted cooling. It was found that titanium molybdenum complex carbide, (Ti, Mo)C, can strongly maintain nanometer-scaled sizes and has the largest contribution to the hardness as compared to titanium carbide, TiC, and titanium niobium complex carbide, (Ti, Nb)C. The result emphasizes that (Ti, Mo)C particles possess an excellent behavior of thermal stability.

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.