Kobe Steel, Ltd.

Kabushiki Kaisha Kobe Seiko Sho

Toyota Jidosha Kabushiki Kaisha

Tetsuo Soshiroda

Motoo Satou

Yutaka Suzuki

Reiichi Suzuki

Tatsuya Asai

Yoichi Mukai

Michiharu Nakaya

Hiroki Nakajima

Kei Yamazaki

Masaaki Miura

Tetsuji Hoshika

Yoshinobu Omiya

Kazuya Kimijima

Tohru Shiogama

Yoshiki Tanaka

Yoshirou Tomioka

Tomohiro Kase

Kouichi Makii

Shinichirou Nakamura

Disclosed is a steel sheet, containing: Si: 0.20-2% (the term '%' herein means 'mass %', the same is true hereinbelow), Mn: 1-2.5%, a total mass of Si and Mn being 1.5% or more, and O: 0.002% or less (exclusive of 0%), C: 0.02-0.25%, P: 0.1% or less (exclusive of 0%), S: 0.05% or less (exclusive of 0%), Al&#x2014;0.02-0.2%, and N: 0.0015-0.015%. The steel sheet of the invention can be advantageously used for forming wide beads even in high-speed arc welding of 100 cm/min or higher.

Steel sheet having excellent weldability

Disclosed herein is a high-strength hot-rolled steel sheet which is characterized by high strength (in terms of tensile strength at 900 MPa level) and excellent combined formability expressed by balance between strength and ductility [tensile strength (TS)&#xd7;total elongation (El)] and balance between strength and stretch flangeability [tensile strength (TS)&#xd7;bore expanding ratio (&#x3bb;)]. The hot-rolled steel sheet contains C: no less than 0.02% and no more than 0.15%, Si: no less than 0.2% and no more than 2.0%, Mn: no less than 0.5% and no more than 2.5%, Al: no less than 0.02% and no more than 0.15%, Cu: no less than 1.0% and no more than 3.0%, Ni: no less than 0.5% and no more than 3.0%, and Ti: no less than 0.03% and no more than 0.5%. (% means mass %) It also has a metallographic structure in longitudinal cross section such that the sum of bainitic ferrite and granular bainitic ferrite accounts for no less than 85% by area.

High-strength hot-rolled steel sheet with excellent combined formability

A high-strength hot-rolled steel sheet containing C: 0.05 to 0.15%, Si: no more than 1.50% (excluding 0%), Mn: 0.5 to 2.5%, P: no more than 0.035% (excluding 0%), S: no more than 0.01% (including 0%), Al: 0.02 to 0.15%, and Ti: 0.05 to 0.2%, which is characterized in that its metallographic structure is composed of 60 to 95 vol % of bainite and solid solution-hardened or precipitation-hardened ferrite (or ferrite and martensite) and its fracture appearance transition temperature (vTrs) is no higher than 0&#xb0; C. as obtained by impact tests (% in terms of % by weight).

High strength hot rolled steel sheet excellent in bore expanding workability and method for production thereof

The present invention provides a high-strength steel material capable of suppressing formation of blowholes during welding even if the strength thereof is improved by nitriding, and a method of manufacturing such a high-strength steel material by using a low rolling force. A high-strength steel of the present invention contains 0.05% (percent by mass unless otherwise specified in describing chemical composition) or below C, 1% or below Si, 1.5% or below Mn, 0.05% or below P, 0.05% or below S, 0.05% or below Al, 0.02 to 0.3% Ti, and 0.020% or below N. The high-strength steel material has metallographic structure of a single phase of ferrite and contains Ti nitride grains having a maximum size of 20 nm or below and coherently precipitated in a density of 250 grains/&#x3bc;mor above. In the high-strength steel material, the ratio of the number of Ti nitride grains having a maximum size of 6 nm or below to that of Ti nitride grains having a maximum size of 20 nm or below is 80% or above.

High-strength steel excellent in weldability and process for production thereof

A high-strength hot-rolled steel sheet containing C: 0.05 to 0.15%, Si: no more than 1.50% (excluding 0%), Mn: 0.5 to 2.5%, P: no more than 0.035% (excluding 0%), S: no more than 0.01% (including 0%), Al: 0.02 to 0.15%, and Ti: 0.05 to 0.2%, which is characterized in that its metallographic structure is composed of 60 to 95 vol % of bainite and solid solution-hardened or precipitation-hardened ferrite (or ferrite and martensite) and its fracture appearance transition temperature (vTrs) is no higher than 0&#xb0; C. as obtained by impact tests. (% in terms of % by weight).

Disclosed is a hot rolled steel sheet in which it includes a steel having a chemical composition of 0.03 to 0.10% (the percent hereinafter representing % by mass) of C, 0.2 to 2.0% of Si, 0.5 to 2.5% of Mn, 0.02 to 0.10% of Al, 0.2 to 1.5% of Cr, 0.1 to 0.5% of Mo, and the residue of iron and inevitable contaminants, and in this steel sheet, at least 80% by area in longitudinal cross section has a martensitic structure. As a consequence, a high strength hot rolled steel sheet having a tensile strength of the level as high as 980 MPa or higher simultaneously with an excellent forming workability, and in particular, excellent stretch flangeability is provided at a relatively low cost.

High strength hot rolled steel sheet having excellent stretch flangeability and its production method

Disclosed is a high-strength hot-rolled steel sheet containing C in a range of 0.03 to 0.15 mass %, Mn in a range of 0.5 to 2 mass %, and Al in a range of 0.01 to 0.1 mass %, respectively, while controlling S to not more than 0.02 mass % (0% included), wherein the metallic structure thereof has a polygonal ferrite as the main phase, and contains martensite as a second phase, further containing P in a range of 0.030 to 0.08 mass %, and Cr in a range of 0.3 to 1.00 mass %, respectively, while controlling Si to not more than 0.1 mass % (0% included). Thus, a high-strength hot-rolled steel sheet superior in workability, and fatigue property, and excellent in surface quality is provided at a relatively low cost.

High strength hot rolled steel sheet superior in workability, fatigue property, and surface quality

There is provided a steel sheet which can simultaneously achieves the following objects: high strength is obtained by quenching with reliability; and excellent ductility is ensured, and further which is excellent in corrosion resistance, plating properties, and spot weldability. The steel sheet is so configured as to satisfy the following composition requirements: on a mass basis, C: 0.11 to 0.22%, Mn: 0.1 to less than 0.5%, Cr and/or Mo: a total amount of 0.1 to 0.5%, and B: 0.0005 to 0.005%, where C: the content of C (% by mass), Cr: the content of Cr (% by mass), and Mo: the content of Mo (% by mass), wherein T&gE;0.19 where T&equals;C&plus;(Cr&plus;Mo)/5.

Steel sheet excellent in ductility and strength stability after heat treatment

Hot rolled high strength steel sheet with excellent formability

Disclosed are alloying elements and microstructures suited for realizing a marked increase in strength of low-carbon or ultra-low-carbon steel plate using a high-density energy source such as a laser. Steel blanks satisfying both high formability and high strength requirements are provided which show sufficient press formability and yet can be markedly increased in strength by laser treatment or which have been markedly increased in strength by laser treatment in areas not to be subjected to severe forming.

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Kakogawa, Japan

Tetsuo Soshiroda