Nippon Steel Corporation

Nippon Steel & Sumitomo Metal Corporation

Nec Corporation

Osamu Kawano

Yuzo Takahashi

Junji Haji

Junichi Wakita

Manabu Takahashi

Tatsuo Yokoi

Daisuke Maeda

Kazuyoshi Esaka

Hiroshi Abe

Tetsuya Yamada

Riki Okamoto

Akihiro Uenishi

Yukihisa Kuriyama

Yasuharu Sakuma

Hidesato Mabuchi

Kenichi Yamamoto

Osamu Yoshida

Hideaki Yamamura

Yoshihiro Suwa

Kazuya Ootsuka

Shinichi Araki

Fuminori Tasaki

Takehiro Hoshino

Kohsuke Kume

Satoshi Horioka

Yasuhiro Miyatani

Norio Ikenaga

The issue of the present invention is to provide a hot-rolled steel sheet with excellent press formability and method for producing the steel sheet, wherein the steel sheet has not only hole expandability but also stretch flanging workability by not assessing hole expandability for stretch flanging as conventional but an actual phenomena of side-bend elongation. To solve the issue, it is confirmed that the steel sheet are excellent in hole expandability and stretch flanging workability, wherein the steel sheet with a certain content of C, Si and Mn is characterized in that, in a metallic structure of said steel sheet, the area fraction of ferrite is 70% or more, the area fraction of bainite is 30% or less, the area fraction of either one or both of martensite and retained austenite is 2% or less, and with regard to respective average intervals (L, Land L), average diameters (D, Dand D) and number densities of a cementite, an inclusion and either one or both of martensite and retained austenite (n, nand n), a void formation/connection index L defined by formula 1 is 11.5 or more: 

Hot-rolled steel sheet with excellent press formability and production method thereof

Tooling used in a punching process which uses a die and projecting punch to form a worked material of steel sheet into a predetermined outer shape, in which tooling for punching, a radius of curvature Rd of a die cutting edge is from 0.03 mm to 0.2 mm and an angle (&#x3b1;) formed by a tangent drawn from a punch cutting edge to a projection shoulder and a direction perpendicular to a direction of movement of the punch is from 12&#xb0; to 72&#xb0;, and a punching method and shaping method using the same, are provided. Further, when making the die cutting edge (shoulder R part) a two-stage R of radii of curvature Rand Rand making a slant of a line passing through the centers of Rand R&#x3b2;, by making 0.03 mm&#x2264;R&#x2264;0.2 mm, 1&#x2264;R/R&#x2264;5, 0&#x2264;R(1&#x2212;sin &#x3b2;1)&#x2264;0.3t, and 30&#xb0;&#x2264;&#x3b2;&#x2264;90&#xb0;, punching hole expandability can be improved.

Tooling for punching steel sheet and punching method

A hot-rolled steel sheet includes, as a chemical composition, by mass %, C: 0.030% to 0.10%, Mn: 0.5% to 2.5%, and Si+Al: 0.100% to 2.5%, in which the steel sheet has a microstructure including, by area fraction, ferrite: 80% or more, martensite: 3% to 15.0%, and pearlite: less than 3.0%, in which a number density of martensite having an equivalent circle diameter of 3 &#x3bc;m or more at a position which is at a depth of &#xbc; of the steel sheet thickness from the surface of the steel sheet, is 5.0 pieces/10000 &#x3bc;mor less, and the following Expression (1) is satisfied.&#x2265;1.00&#x2003;&#x2003;Expression (1) Here, R is an average martensite interval (&#x3bc;m) defined by the following Expression (2), and Dis a martensite average diameter (&#x3bc;m).={12.5&#xd7;(&#x3c0;/6)&#x2212;(&#x2154;)&#x2003;&#x2003;Expression (2) Here, Vis a martensite area fraction (%) and Dis the martensite average diameter (&#x3bc;m).

Hot-rolled steel sheet and method for producing same

A hot rolled steel sheet includes, as a chemical composition, at least one selected from Ti, REM, and Ca, and includes, as a metallographic structure, a ferrite as a primary phase, at least one of a martensite and a residual austenite as a secondary phase, and plural inclusions, wherein a total length in the rolling direction of both inclusion-cluster whose length in the rolling direction is 30 &#x3bc;m or more and independent-inclusion whose length in the rolling direction is 30 &#x3bc;m or more is 0 mm to 0.25 mm per 1 mm.

Hot rolled steel sheet and method of producing same

The present invention provides a high-strength steel sheet including: C: 0.03 to 0.25 mass %, Si: 0.1 to 2.0 mass %, Mn: 0.5 to 3.0 mass %, P: not more than 0.05 mass %, T.O: not more than 0.0050 mass %, S: 0.0001 to 0.01 mass %, N: 0.0005 to 0.01 mass %, acid-soluble Al: more than 0.01 mass %, Ca: 0.0005 to 0.0050 mass %, and a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %, with a balance including iron and inevitable impurities, in which the steel sheet contains a chemical component on a basis of mass that satisfies 0.7&#x3c;100&#xd7;([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]&#x2266;70 and 0.2&#x2266;([Ce]+[La]+[Nd]+[Pr])/[S]&#x2266;10, the steel sheet contains compound inclusion including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at least one element of O and S, and a second inclusion phase having a component different from that of the first inclusion phase and containing at least one element of Mn, Si, and Al, the compound inclusion forms a spherical compound inclusion having an equivalent circle diameter in the range of 0.5 &#x3bc;m to 5 &#x3bc;m, and a ratio of the number of the spherical compound inclusion relative to the number of all inclusions having the equivalent circle diameter in the range of 0.5 &#x3bc;m to 5 &#x3bc;m is 30% or more.

High-strength steel sheet exhibiting excellent stretch-flange formability and bending workability, and method of producing molten steel for the high-strength steel sheet

A hot-rolled steel sheet according to the present invention is a steel sheet containing a predetermined components, and satisfying O&#x3c;S/Ca&#x3c;0.8, N&#x2212;14/48&#xd7;Ti&#x2267;'0' (zero) %. It is a high-strength hot-rolled steel sheet for a spiral pipe excellent in low-temperature toughness in which a pro-eutectoid ferrite fraction is 3% or more and 20% or less, and the other is a low-temperature transformation phase in a microstructure at a depth of a half thickness of a sheet thickness from a steel sheet surface, a number average crystal grain size of a whole of the microstructure is 2.5 &#x3bc;m or less, an area average grain size is 9 &#x3bc;m or less, a standard deviation of the area average grain size is 2.3 &#x3bc;m or less, and a reflected X-ray intensity ratio {211}/{111} in a direction and in a {111} direction relative to a plane in parallel to the steel sheet surface at the depth of the half thickness of the sheet thickness from the steel sheet surface is 1.1 or more. The steel sheet has both high-toughness and strength of API5L-X80 standard or more.

Hot-rolled steel sheet and manufacturing method thereof

On a cross section with a sheet width direction of a high-strength hot-rolled steel sheet set as a normal line, with regard to an inclusion having a major diameter of 3.0 &#x3bc;m or more, a maximum of a major diameter/minor diameter ratio expressed by (a major diameter of the inclusion)/(a minor diameter of the inclusion) is 8.0 or less, and a sum total of a rolling direction length per 1 mmcross section of a predetermined inclusion group composed of plural inclusions each having a major diameter of 3.0 &#x3bc;m or more and a predetermined extended inclusion having a length in a rolling direction of 30 &#x3bc;m or more is 0.25 mm or less. The plural inclusions composing the predetermined inclusion group congregate in both the rolling direction and a direction perpendicular to the rolling direction 50 &#x3bc;m or less apart from each other. The predetermined extended inclusion is spaced over 50 &#x3bc;m apart from all the inclusions each having a major diameter of 3.0 &#x3bc;m or more in at least either the rolling direction or the direction perpendicular to the rolling direction.

High-strength hot-rolled steel sheet and method of manufacturing the same

The present invention provides a steel sheet excellent in both a balance between strength and elongation and a balance between strength and hole expandability, in other words, a multi-phase steel sheet having an excellent balance between strength and hole expandability. The present invention is a multi-phase steel sheet excellent in hole expandability characterized in that:

Multi-phase steel sheet excellent in hole expandability and method of producing the same

An exemplary hot rolled steel sheet can included, in terms of percent by mass, C of 0.01 to 0.2%; Si of 0.01 to 2%; Mn of 0.1 to 2%; P of &#x2266;0.1%; S of &#x2266;0.03%; Al of 0.001 to 0.1%; N of &#x2266; 0.01%; and as a remainder, Fe and unavoidable impurities. For example, a microstructure can be substantially a homogeneous continuous-cooled microstructure, and an average grain size of the microstructure may be more than 8 &#x3bc;m and 30 &#x3bc;m or less. An exemplary method for manufacturing a hot rolled steel sheet can include subjecting a slab having the above composition to a rough rolling so as to obtain a rough rolled bar, subjecting the rough rolled bar to a finish rolling so as to obtain a rolled steel under conditions in which a finishing temperature is (Ar3 transformation point +50&#xb0; C.) or more; and starting cooling the rolled steel after 0.5 seconds or more pass from the end of the finish rolling at a temperature of the Ar3 transformation point or more. At least in the temperature range from the Ar3 transformation point can be cooled to 500&#xb0; C. at a cooling rate of 80&#xb0;C./sec or more, a further cooling can be effectuated until the temperature is 500&#xb0; C. or less to obtain a hot rolled steel sheet and coiling the hot rolled steel sheet.

Hot rolled steel sheet

An exemplary hot rolled steel sheet for processing can include, in terms of percent by mass, C of 0.01 to 0.2%; Si of 0.01 to 0.3%; Mn of 0.1 to 1.5%; P of &#x2266;0.1%; S of &#x2266;0.03%; Al of 0.001 to 0.1%; N of &#x2266;0.006%; and as a remainder, Fe and unavoidable impurities. The microstructure may include a main phase in the form of polygonal ferrite and a hard second phase, a volume fraction of the hard second phase is 3 to 20%, a hardness ratio (hardness of the hard second phase/hardness of the polygonal ferrite) is 1.5 to 6, and a grain size ratio (e.g., grain size of the polygonal ferrite/grain size of the hard second phase) is 1.5 or more.

Hot rolled steel sheet for processing and method for manufacturing the same

The object of the present invention is to provide high-strength steel sheets exhibiting high impact energy absorption properties, as steel sheets to be used for shaping and working into such parts to front side members which absorb impact energy upon collision, as well as a method for their production. The high-strength steel sheets of the invention which exhibit high impact energy absorption properties are press formable high-strength steel sheets with high flow stress during dynamic deformation characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength as when deformed in a strain rate range of 5&times;10 &tilde;5&times;10 (l/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength &sgr;d when deformed at a strain rate of 5&times;10 &tilde;5&times;10 (l/sec) after the pre-deformation, i.e. &sgr;d&minus;&sgr;s, is at least 60 MPa, and the work hardening coefficient between 5% and 10% of a strain is at least 0.130.

High-strength steel sheet highly resistant to dynamic deformation and excellent in workability and process for the production thereof

The object of the present invention is to provide high-strength steel sheets exhibiting high impact energy absorption properties, as steel materials, to be used for shaping and working into such parts as front side members of automobiles which absorb impact energy upon collision, as well as a method for their production. The high-strength steel sheets of the invention which exhibit high impact energy absorption properties are high-strength steel sheets with high flow stress during dynamic deformation characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the average value &sgr;dyn (MPa) of the flow stress in the range of 3&tilde;10% of equivalent strain when deformed in a strain rate range of 5&times;10,&tilde;5&times;10,(1/sec) after pre-deformation of greater than 0% and less than or equal to 10% of equivalent strain, satisfies the inequality: &sgr;dyn&gE;0.766&times;TS&plus;250 as expressed in terms of the maximum stress TS (MPa) in the static tensile test as measured in a strain rate range of 5&times;10,&tilde;5&times;10,(1/s) without deformation, and the work hardening coefficient between 1% and 5% of a strain is at least 0.080.

High-strength steel plate having high dynamic deformation resistance and method of manufacturing the same

High yield ratio-type, hot rolled high strength steel sheet excellent in

A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productivity and without requiring special alloy elements.

Process for producing a hot rolled steel sheet with high strength and

A hot rolled steel sheet with a high strength and a distinguished formability, and a process for producing the same are disclosed. The steel sheet comprises 0.15 to 0.4% by weight of C, 0.5 to 2.0% by weight of Si, and 0.5 to 2.0% by weight of Mn, the balance being iron and inevitable impurities, and has a microstructure composed of ferrite, bainite and retained austenite phases with the ferrite phase being in a ratio (V.sub.PF /d.sub.PF) of polygonal ferrite volume fraction V.sub.PF (%) to polygonal ferrite average grain size d.sub.PF (.mu.m) of 7 or more and the retained austenite phase being contained in an amount of 5% by volume or more on the basis of the total phases. The steel sheet can be produced with a high productiviity and without requiring special alloy elements.

Growing community of inventors

Tokyo, Japan

Osamu Kawano