Jfe Steel Corporation

Kawasaki Steel Corporation

Saiji Matsuoka

Hiroshi Matsuda

Takeshi Fujita

Yoshimasa Funakawa

Tatsuya Nakagaito

Shinjiro Kaneko

Yoshiyasu Kawasaki

Yoshitsugu Suzuki

Yasushi Tanaka

Osamu Furukimi

Susumu Satoh

Kei Sakata

Takashi Obara

Masahiko Morita

Tetsuo Shimizu

Kozo Tsunoyama

Hideo Abe

Takashi Kawano

Takaaki Toyooka

Yuji Hashimoto

Takatoshi Okabe

Toshio Irie

Shusaku Takagi

Akira Yorifuji

Motoaki Itadani

Takaaki Hira

Nobuki Tanaka

Masanori Nishimori

Taro Kanayama

Yoshihiko Ono

Nobuyuki Nakamura

Yasunobu Nagataki

Kohei Hasegawa

Yasuhide Ishiguro

Yuki Toji

Hideyuki Kimura

Yusuke Fushiwaki

Toru Hoshi

Yoshikazu Kawabata

Toshiyuki Kato

Toshio Ichida

Yoshiharu Sugimoto

Masatoshi Aratani

Ikuo Yarita

Takayuki Futatsuka

Eiko Yasuhara

Shunichi Tsugawa

Shigeru Kobayashi

Toshio Fujimura

Norimasa Hirata

Arata Ueda

Takemi Yamamoto

Yoshio Yamazaki

Takashi Sekita

Koji Maitake

Toshiyuki Katoh

Tetsuya Kiyasu

Yoshiro Tsuchiya

Nobuhiko Uesugi

Shunji Ilzuka

Hidetaka Kawabe

Taro Yahiro

The present invention provides a method for manufacturing a high-strength galvannealed steel sheet. The method includes hot-rolling a slab to produce a hot-rolled sheet having a microstructure in which a total area ratio of bainite and martensite is 80% or more, cold-rolling the hot-rolled sheet to produce a cold-rolled steel sheet, continuously annealing the cold-rolled steel sheet by heating to 750&#xb0; C. to 900&#xb0; C. at an average heating rate of 8&#xb0; C./s or more from 500&#xb0; C. to an A1 transformation point, holding the steel sheet for 10 seconds or more, and then cooling the steel sheet to a temperature region of 300&#xb0; C. to 530&#xb0; C. at an average cooling rate of 3&#xb0; C./s or more from 750&#xb0; C. to 530&#xb0; C., galvanizing the steel sheet, and further coating-alloying the steel sheet in a temperature region of 540&#xb0; C. to 600&#xb0; C. for 5 to 60 seconds.

High-strength galvannealed steel sheet having excellent formability and fatigue resistance and method for manufacturing the same

A formable galvanized steel sheet includes, in terms of % by mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to 3.5% of Mn, 0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5% of Al, and 0.01 to 0.2% in total of at least one element selected from Ti, Nb and V, the remainder being Fe and unavoidable impurities, having a microstructure composed of, in terms of area fraction, 20 to 87% of ferrite, 3 to 10% in total of martensite and residual austenite, and 10 to 60% of tempered martensite, and a second phase composed of the martensite, residual austenite, and tempered martensite having an average crystal grain diameter of 3 &#x3bc;m or less, and having a ratio of absorption energy AE to tensile strength TS (AE/TS) not less than 0.063.

High tensile strength galvanized steel sheets excellent in formability and methods of manufacturing the same

The high-strength galvanized steel sheet having excellent formability has a component composition containing, on the basis of mass percent, 0.05 to 0.2% C, 0.5 to 2.5% Si, 1.5 to 3.0% Mn, 0.001 to 0.05% P, 0.0001 to 0.01% S, 0.001 to 0.1% Al, and 0.0005 to 0.01% N, the balance being Fe and incidental impurities; and the steel sheet has a microstructure including a ferritic phase and a martensitic phase including a tempered-martensitic phase, the ferritic phase having an area fraction of 30% or more relative to an entirety of the microstructure, the martensitic phase having an area fraction of 30 to 50% relative to the entirety of the microstructure, and the tempered-martensitic phase having an area fraction of 70% or more relative to an entirety of the martensitic phase.

High-strength galvanized steel sheet having excellent formability and method for manufacturing the same

A method of manufacturing a high-strength galvanized steel sheet includes hot-rolling a slab to form a steel sheet; during continuous annealing, heating the steel sheet to a temperature of 750&#xb0; C. to 900&#xb0; C. at an average heating rate of at least 10&#xb0; C./s at a temperature of 500&#xb0; C. to an Atransformation point; holding that temperature for at least 10 seconds; cooling the steel sheet from 750&#xb0; C. to a temperature of (Ms point&#x2014;100&#xb0; C.) to (Ms point&#x2014;200&#xb0; C.) at an average cooling rate of at least 10&#xb0; C./s; reheating the steel sheet to a temperature of 350&#xb0; C. to 600&#xb0; C.; holding that temperature for 10 to 600 seconds; and galvanizing the steel sheet.

Method for manufacturing high strength galvanized steel sheet with excellent formability

A high-strength galvanized steel sheet has excellent mechanical properties such as a TS of 1200 MPa or more, an El of 13% or more, and a hole expansion ratio of 50% or more and a method for manufacturing the same. A high-strength galvanized steel sheet excellent in formability contains 0.05% to 0.5% C, 0.01% to 2.5% Si, 0.5% to 3.5% Mn, 0.003% to 0.100% P, 0.02% or less S, and 0.010% to 0.5% Al on a mass basis, the remainder being Fe and unavoidable impurities, and has a microstructure which contains 0% to 10% ferrite, 0% to 10% martensite, and 60% to 95% tempered martensite on an area basis as determined by structure observation and which further contains 5% to 20% retained austenite as determined by X-ray diffractometry.

High-strength galvanized steel sheet with excellent formability and method for manufacturing the same

The present invention provides a high-strength galvanized steel sheet having excellent ductility, stretch flangeability, and fatigue resistance, and a method for manufacturing the same. A high-strength galvannealed steel sheet having excellent formability and fatigue resistance is characterized in that the steel sheet is composed of steel having a composition containing, by % by mass, C: 0.05% to 0.3%, Si: 0.5% to 2.5%, Mn: 1.0% to 3.5%, P: 0.003% to 0.100%, S: 0.02% or less, Al: 0.010% to 0.1%, and the balance including iron and unavoidable impurities, and the steel sheet has a microstructure containing 50% or more of ferrite, 5% to 35% of martensite, and 2% to 15% of pearlite in terms of an area ratio, the martensite having an average gain size of 3 &#x3bc;m or less and an average distance of 5 &#x3bc;m or less between adjacent martensite grains.

A steel sheet contains, in terms of percent by mass, C: 0.01 to 0.2%, Si: 2.0% or less, and Mn: 3.0% or less and has a martensite phase as dominant phase and ferrite with a grain size of 20 &#x3bc;m or less as a second phase. The ferrite is contained in area ratio of 1% to 30% and the amount of solute carbon being 0.01 percent by mass of more. The steel sheet can provide a hot-rolled steel sheet suitable for automobile steel sheet, i.e., has excellent press workability and excellent strain aging property whereby the tensile strength significantly increases by heat treatment at about the same temperature as typical baking process after the press-working. Moreover, hardening of the ferrite phase improves the fatigue strength after the strain aging.

Hot-rolled steel sheet, method for making the same, and worked body of hot-rolled steel sheet

A high strength galvanized steel sheet having a TS of 780 MPa or more and exhibiting excellent stretch frangeability and bendability and a method for manufacturing the same are provided. The component composition contains C: 0.05% to 0.15%, Si: 0.8% to 2.5%, Mn: 1.5% to 3.0%, P: 0.001% to 0.05%, S: 0.0001% to 0.01%, Al: 0.001% to 0.1%, N: 0.0005% to 0.01%, Cr: 0.1% to 1.0%, Ti: 0.0005% to 0.1%, B: 0.0003% to 0.003%, and the remainder composed of iron and incidental impurities, on a percent by mass basis. The microstructure includes 30% or more of ferrite phase and 30% or more, and 70% or less of martensite phase on an areal fraction basis, wherein regarding the above-described martensite phase, the proportion of a tempered martensite phase is 20% or more relative to the whole martensite phase and the proportion of a martensite phase having a grain diameter of 1 &#x3bc;m or less is 10% or less relative to the whole martensite phase.

High strength galvanized steel sheet excellent in formability

A high strength galvanized steel sheet with excellent workability, having a component composition containing C: 0.04% or more, and 0.15% or less, Si: 0.7% or more, and 2.3% or less, Mn: 0.8% or more, and 2.2% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, and the remainder composed of iron and incidental impurities on a percent by mass basis, and a microstructure including 70% or more of ferrite phase, 2% or more, and 10% or less of bainite phase, and 0% or more, and 12% or less of pearlite phase on an area fraction basis and 1% or more, and 8% or less of retained austenite phase on a volume fraction basis, wherein an average crystal grain diameter of ferrite is 18 &#x3bc;m or less and an average crystal grain diameter of retained austenite is 2 &#x3bc;m or less.

High strength galvanized steel sheet with excellent workability and method for manufacturing the same

A high strength galvanized steel sheet has a TS of 590 MPa or more and excellent processability. The component composition contains, by mass %, C: 0.05% to 0.3%, Si: 0.7% to 2.7%, Mn: 0.5% to 2.8%, P: 0.1% or lower, S: 0.01% or lower, Al: 0.1% or lower, and N: 0.008% or lower, and the balance: Fe or inevitable impurities. The microstructure contains, in terms of area ratio, ferrite phases: 30% to 90%, bainite phases: 3% to 30%, and martensite phases: 5% to 40%, in which, among the martensite phases, martensite phases having an aspect ratio of 3 or more are present in a proportion of 30% or more.

A high-strength galvanized steel sheet has a TS of at least 590 MPa and excellent ductility and stretch flangeability and a method for manufacturing the high-strength galvanized steel sheet. The galvanized steel sheet contains, on the basis of mass percent, C: 0.05% to 0.3%, Si: 0.01% to 2.5%, Mn: 0.5% to 3.5%, P: 0.003% to 0.100% or less, S: 0.02% or less, and Al: 0.010% to 1.5%. The total of Si and Al is 0.5% to 2.5%. The remainder are iron and incidental impurities, contain 20% or more of ferrite phase, 10% or less of martensite phase, and 10% to 60% of tempered martensite, on the basis of area percent, and 3% to 10% of retained austenite phase on the basis of volume fraction. The retained austenite has an average grain size of 2.0 &#x3bc;m or less.

High strength galvanized steel sheet with excellent formability

A high-strength galvanized steel sheet having excellent formability contains as a chemical component of steel on a mass percent basis: 0.05% to 0.3% of C; more than 0.60% to 2.0% of Si; 0.50% to 3.50% of Mn; 0.003% to 0.100% of P; 0.010% or less of S; 0.010% to 0.06% of Al; 0.007% or less of N; and the balance including Fe and inevitable impurities, and in the microstructure of the steel sheet, the standard deviation of nano-hardness is 1.50 GPa or less.

Zinc-plated high-tension steel sheet excellent in press formability

A high-strength galvanized steel sheet is provided which has a tensile strength of a level of about 590 MPa and superior formability, including a coating with good appearance. The galvanized steel sheet includes a base steel and a galvanized coating layer formed over the surface of the base steel. The galvanized steel sheet has a chemical composition containing 0.005% to 0.12% of C, 0.7% to 2.7% of Si, 0.5% to 2.8% of Mn, 0.1% or less of P, 0.07% or less of S, 1.0% or less of Al, 0.008% or less of N, and the balance being Fe and inevitable impurities on a mass basis, and a microstructure constituted of at least 90% of ferrite and 2% to 10% of martensite on an area basis. The ferrite has a Vickers hardness of 120 or more on average, and an inclusion is precipitated from the grain boundary with a length of 50% or less relative to the entire length of the grain boundary in the surface layer of the base steel with a depth of 3 &#x3bc;m from the interface between the coating layer and the base steel.

Galvanized high strength steel sheet

A method for manufacturing a high carbon hot-rolled steel sheet. The method including the steps of hot-rolling, primary cooling, secondary cooling, coiling, acid-washing and annealing. The primary cooling step is to cool the hot-rolled steel sheet down to a cooling termination temperature of 450&#xb0; C. to 600&#xb0; C. at a cooling rate of higher than 120&#xb0; C./sec. The secondary cooling step is to apply a secondary cooling to hold the primarily cooled hot-rolled steel sheet at a temperature of 450&#xb0; C. to 650&#xb0; C. until coiling.

Method for manufacturing a high carbon hot-rolled steel sheet

The present invention provides a steel sheet having a chemical composition comprising 0.15% or less C, 2.0% or less Si, 3.0% or less Mn, P, S, A1 and N in adjusted amounts, from 0.5 to 3.0% Cu, or one or more of Cr, Mo and W in a total amount of 2.0% or less, and having a composite structure comprising ferrite and martensite having an area ratio of 2% or more. The steel sheet is in the form of a high-strength hot-rolled steel sheet, a high-strength cold-rolled steel sheet, or a hot-dip galvanized steel sheet. There is thus available a steel sheet excellent in press-formability and in strain age hardening property as represented by a &#x394;TS of 80 MPa or more.

Hot-dip galvanized hot-rolled and cold-rolled steel sheets excellent in strain age hardening property

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Tokyo, Japan

Saiji Matsuoka