Kobe Steel, Ltd.

Voestalpine Stahl Gmbh

Toshio Murakami

Hideo Hata

Shushi Ikeda

Katsura Kajihara

Junya Naitou

Keisuke Okita

Elijah Kakiuchi

Tatsuya Asai

Kenji Saito

Muneaki Ikeda

Naoki Mizuta

Akira Ibano

Masaaki Miura

Tomokazu Masuda

Tadao Murata

Kosuke Shibata

Yukihiro Utsumi

Fumio Yuse

Hirokazu Natsumeda

Shinji Sato

Michiharu Nakaya

Masao Kinefuchi

Andreas Pichler

Thomas Kurz

Junichiro Kinugasa

Atsuhiro Shiraki

Takahiro Ozawa

Shinobu Nakayama

Toshiya Nakata

Rie Okada

Keita Nakayama

Disclosed is a steel sheet including predetermined elements in each predetermined amount, with the balance consisting of iron and inevitable impurities, wherein a total area ratio of bainite, bainitic ferrite, martensite, residual austenite, and a martensite-austenite mixed structure is 95% or more and 100% or less, a total area ratio of ferrite and pearlite is less than 5%, an area ratio of the martensite-austenite mixed structure is 5% or more and 30% or less, an average section length of the martensite-austenite mixed structure is 0.32 μm or less, and a proportion of an area of a region in ferrite, bainitic ferrite, and martensite where no cementite is present to the total area of ferrite, bainitic ferrite, and martensite is 3.0% or more and 5.0% or less.

Steel sheet

The present invention provides a high-strength steel sheet, which can be used in various applications including automobile parts and exhibits excellent collision safety and excellent moldability, and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet according to an aspect of the present invention satisfies a predetermined chemical composition and has a metallographic microstructure having ferrite fraction: 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area proportion and retained austenite fraction: 5% to 30% in terms of volume proportion, and in the high-strength steel sheet, the skewness of IQ as analyzed by the EBRD method is −1.2 to −0.3 when the skewness is expressed by a predetermined relational expression in a case where crystal grains having a bcc structure and a bct structure are regarded as an aggregation of regions having an area of 0.05 μm.

High-strength steel sheet and method for producing same

Disclosed is a high-strength sheet containing: C: 0.15% by mass to 0.35% by mass, a total of Si and Al: 0.5% by mass to 3.0% by mass, Al: 0.01% by mass or more, N: 0.01% by mass or less, Mn: 1.0% by mass to 4.0% by mass, P: 0.05% by mass or less, and S: 0.01% by mass or less, with the balance being Fe and inevitable impurities, wherein the steel structure satisfies that: a ferrite fraction is 5% or less, the total fraction of tempered martensite and tempered bainite is 60% or more, the amount of retained austenite is 10% or more, MA has an average size of 1.0 &#x3bc;m or less, retained austenite has an average size of 1.0 &#x3bc;m or less, retained austenite having a size of 1.5 &#x3bc;m or more accounts for 2% or more of the total amount of retained austenite, and the amount of solute nitrogen in a steel sheet is 0.002% by mass or less.

Disclosed is a high-strength sheet containing: C: 0.15% by mass to 0.35% by mass, a total of Si and Al: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass, P: 0.05% by mass or less (including 0% by mass), S: 0.01% by mass or less (including 0% by mass), and Ti: 0.01% by mass to 0.2% by mass, with the balance being Fe and inevitable impurities, wherein the steel structure satisfies that: a ferrite fraction is 5% or less, a total fraction of tempered martensite and tempered bainite is 60% or more, a retained austenite fraction is 10% or more, a fresh martensite fraction is 5% or less, retained austenite has an average grain size of 0.5 &#x3bc;m or less, retained austenite having a grain size of 1.0 &#x3bc;m or more accounts for 2% or more of the total amount of retained austenite, and a prior austenite grain size is 10 &#x3bc;m or less.

High-strength steel sheet and production method for same

A steel sheet for hot pressing includes in a chemical composition, in percent by mass, C of 0.1% to 0.4%, Si of greater than 0% to 2.0%, Mn of 0.5% to 3.0%, P of greater than 0% to 0.015%, S of greater than 0% to 0.01%, B of 0.0003% to 0.01%, N of greater than 0% to 0.05%, Al in a content of 2&#xd7;[N]% to 0.3% at a Si content of greater than 0.5% to 2.0%; or Al in a content of (0.20+2&#xd7;[N]&#x2212;0.40&#xd7;[Si]N)% to 0.3% at a Si content of 0% to 0.5%, where [N] and [Si] are contents of N and Si, respectively, in mass percent, with the remainder being iron and inevitable impurities, where contents of Ti, Zr, Hf, and Ta, of the inevitable impurities, are each controlled to 0.005% or lower. The steel sheet includes nitride-based inclusions with an equivalent circle diameter of 1 &#x3bc;m or more in a number density of 0.10 per square millimeter.

Steel sheet for hot pressing and hot pressed article using the same

A high-strength steel sheet has a chemical composition including 0.05% to 0.3% of C, 1% to 3% of Si, 0.5% to 3% of Mn, 0% to 0.1% of P, 0.001% to 0.1% of Al, and 0.002% to 0.03% of N, in mass percent; further includes iron and impurities; and has a structure including 50% to 90% of bainitic ferrite, 5% to 20% of retained austenite (&#x3b3;), a total of 10% to 50% of martensite and the retained austenite, and 0% to 40% of polygonal ferrite, in area percent based on the entire structure. The retained austenite has a carbon content (C&#x3b3;) of 0.5% to 1.2% by mass, an average equivalent circle diameter of 0.2 to 2 &#x3bc;m, and an average aspect ratio (maximum diameter/minimum diameter) of less than 3.0. The high-strength steel sheet excels both in elongation and deep drawability while having a strength of 980 MPa or more.

Highly formable high-strength steel sheet, warm working method, and warm-worked automobile part

Provided is an ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 2 area% or more of a region having a structure that includes 90% or more of martensite and 0.5% or more of residual austentite by area ratio relative to the entire structure, the local Mn concentration in said region being at least 1.1 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more.

Ultra-high-strength steel sheet having excellent delayed fracture resistance at cut end thereof

A high strength cold-rolled steel sheet has a component composition containing specific amounts of C, Si, Mn, P, S, N and Al, respectively with a remainder being iron and inevitable impurities. The steel sheet contains 95% or more of martensite in terms of area ratio, and contains 5% or less (inclusive of 0%) of residual austenite and ferrite in terms of a total area ratio. An average size of a carbide is 60 nm or less in terms of an equivalent circle diameter, and a number density of the carbide having the equivalent circle diameter of 25 nm or more is 5.0&#xd7;10pieces or less per mm. The steel sheet has a yield strength of 1,180 MPa or more and a tensile strength of 1,470 MPa or more.

High-strength cold-rolled steel sheet having excellent bendability

A spot welding method joins metal sheets by: clamping a stacked section of two or more stacked metal sheets with a pair of electrodes at a load of 100 N or more; applying pressure around the electrodes at a load, which is 5-1000% of the load due to the pair of electrodes, using a pressurizing member, which applies pressure on the stacked section in regions that are 20% or more of the total outer circumference of the electrode tips and for which the continuous regions of the entire outer circumference of the electrode tips that are not pressed are 30% or less (including 0%) of the entire outer circumference of the electrode tips; and passing welding current from the electrodes to the stacked section.

Spot welding method

The present disclosure relates to a high-strength hot-dip galvannealed steel sheet excellent in bake hardenability and bendability and having a component composition containing, in mass %, C: from 0.05 to 0.30%, Si: from 0.5 to 3.0%, Mn: from 0.2 to 3.0%, P: from 0 to 0.10%, S: from 0 to 0.010%, N: form 0 to 0.010%, and Al: from 0.001 to 0.10%, with the remainder being iron and unavoidable impurities. The steel sheet has a steel structure containing, in terms of area percentage, martensite: form 50 to 85% and ferrite: 0% or more and less than 5%, with the remainder being bainite. The steel sheet has a dislocation density of 5.0&#xd7;10mor more, a solute carbon amount of 0.08 mass % or more and a tensile strength of 1180 MPa or more.

High-strength hot-dip galvannealed steel sheet having excellent bake hardening property and bendability

In the present invention, a press-formed product is manufactured by heating a steel sheet for hot pressing use to a temperature of 900&#xb0; C. or above and 1,100&#xb0; C. or below, the steel sheet for hot pressing use having a predetermined chemical component composition, some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, having an average equivalent circle diameter of 6 nm or less, and the precipitated Ti amount and the total Ti amount in the steel fulfilling the relationship represented by formula (1) shown below, thereafter starting press-forming, and holding at the bottom dead point and cooling to a temperature lower than the martensite transformation starting temperature Ms while securing the average cooling rate of 20&#xb0; C./s or more within a tool.Precipitated Ti amount(mass %)&#x2212;3.4[N]&#x3c;0.5&#xd7;[total Ti amount (mass %)&#x2212;3.4[N]]&#x2003;&#x2003;(1)(In the formula (1), [N] represents the content (mass %) of N in the steel.).

Method for manufacturing press-formed product and press-formed product

This high-strength steel sheet contains, in mass %, 0.05 to 0.3% of C, 1 to 3% of Si, 0.5 to 3% of Mn, up to 0.1% (inclusive of 0%) of P, up to 0.01% (inclusive of 0%) of S, 0.001 to 0.1% of Al and 0.002 to 0.03% of N with the balance consisting of iron and unavoidable impurities, and has a microstructure which comprises, in area fraction relative to the microstructure, 40 to 85% of bainitic ferrite, 5 to 20% of retained austenite (&#x3b3;), 10 to 50% (in total) of martensite and &#x3b3;, and 5 to 40% of ferrite. The retained austenite (&#x3b3;) has a C concentration of 0.5 to 1.0 mass %, while the quantity of &#x3b3;present in the ferrite grains is 1% or more (in area fraction) relative to the microstructure.

High-strength steel sheet with excellent warm formability and process for manufacturing same

A hot-dip galvanized steel sheet contains specific amounts of C, Si, Mn, P, S, Ti, Al, and N with the remainder being iron and unavoidable impurities. Bainitic ferrite, martensite, retained &#x3b3;, and ferrite (&#x3b1;) are present each in a specific area ratio. The remainder &#x3b3; has a specific C concentration. Sub-grains in the recrystallized &#x3b1; and un-recrystallized &#x3b1; have a specific grain diameter. The surface area ratio of &#x3b1; and worked &#x3b1; having a grain diameter of 5 &#x3bc;m or more is 5% or less. The average particle diameter of TiC particles inside &#x3b1; grains is 10 nm or less.

High-strength hot-dip galvanized steel sheet having excellent yield strength and formability

A hot-press formed product can be achieved which has regions corresponding to a shock resistant portion and an energy absorption portion within a single formed product without applying a welding method and achieves the balance of high strength and elongation with a high level according to each region by means of having a first forming region exhibiting a metal structure containing martensite: 80-97 area % and retained austenite: 3-20 area % respectively, the remaining structure being 5 area % or less, and a second forming region exhibiting a metal structure containing annealed martensite or annealed bainite: 30-97 area %, martensite as quenched: 0-67 area %, and retained austenite: 3-20 area %.

Hot-press formed product and method for manufacturing same

In a steel sheet having a specific chemical composition and having a microstructure including ferrite that is a soft first phase by 20-50% in terms of the area ratio, the remainder being tempered martensite and/or tempered bainite that is a hard second phase, the microstructure of steel of a surface layer section of the steel sheet from the surface to the depth of 100 &#x3bc;m and a center section of t/4-3t/4 (t is the sheet thickness) is controlled.

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Kobe, Japan

Toshio Murakami