A high strength pressed member has excellent ductility and stretch flangeability and tensile strength of 780-1400 MPa, with a predetermined steel composition and steel microstructure relative to the entire microstructure of steel sheet, where area ratio of martensite 5-70%, area ratio of retained austenite 5-40%, area ratio of bainitic ferrite in upper bainite 5% or more, and total thereof is 40% or more, 25% or more of martensite is tempered martensite, polygonal ferrite area ratio is above 10% and below 50% to the entire microstructure of steel sheet, and average grain size is 8 μm or less, average diameter of a group of polygonal ferrite grains is 15 μm or less, the group of polygonal ferrite grains represented by a group of ferrite grains of adjacent polygonal ferrite grains, and average carbon content in retained austenite is 0.70 mass % or more and tensile strength is 780 MPa or more.

High-strength steel sheet and method for manufacturing same

A high-yield-ratio, high-strength cold-rolled steel sheet has a composition containing, in terms of % by mass, C: 0.13% to 0.25%, Si: 1.2% to 2.2%, Mn: 2.0% to 3.2%, P: 0.08% or less, S: 0.005% or less, Al: 0.01% to 0.08%, N: 0.008% or less, Ti: 0.055% to 0.130%, and the balance being Fe and unavoidable impurities. The steel sheet has a microstructure that contains 2% to 15% of ferrite having an average crystal grain diameter of 2 um or less in terms of volume fraction, 5 to 20% of retained austenite having an average crystal grain diameter of 0.3 to 2.0 μm in terms of volume fraction, 10% or less (including 0%) of martensite having an average grain diameter of 2 μm or less in terms of volume fraction, and the balance being bainite and tempered martensite, and the bainite and the tempered martensite having an average crystal grain diameter of 5 μm or less.

High-yield-ratio high-strength cold rolled steel sheet and production method therefor

A method of producing an L-shaped product (10) which includes an L-shaped flat top wall (12), an inside wall (14) extending and connected along an inside edge (12c) of the top wall, and an outside wall (16) extending and connected along an outside edge (12d) of the top wall, each of the inside and outside walls being terminated by flange (18, 20) substantially parallel to the top wall, is disclosed. The method comprises the steps of: providing sheet metal material; providing a drawing die assembly; and stamping the sheet metal material by the drawing die assembly to produce an intermediate product. The method further comprises the steps of: providing a bending die assembly; and stamping the intermediate product by the bending die assembly to produce an L-shaped product completed.

Method for manufacturing l-shaped product

The present invention produces a curved article (10) having a hat-shaped cross section by means of press molding without generating cracks or wrinkles. When producing by means of press molding a curved article (10) having a roughly hat-shaped cross section comprising a top plate (11), vertical walls (12a, 12b) extending in parallel to each other along the ridge line (14a, 14b) of the top plate, and outward-facing flanges (13a, 13b) joined to the tips of the vertical walls, a starting material metal plate (601) is sandwiched and bound by a die (502) and a blank holder (505) and press molded while the starting material metal plate (601) is sandwiched and bound by a pad (503) and a punch (504).

Method for producing curved article and skeleton structure member for automobile body shell

Effect of shearing clearance and angle on stretch-flange formability evaluated by saddle-type forming test

PROBLEM TO BE SOLVED: To suppress twist in a negative direction and also to suppress occurrence of fold lines and crease marks generated in a vertical wall part outside a curvature, when bend-forming is applied on a channel member with a curved channel section using a two-stage bend-forming method. SOLUTION: The forming method employs the two-stage bend-forming method for applying a first forming to produce a preliminary formed body equipped with a curved channel section and then applies a second forming on the preliminary formed body. In the preliminary formed body, an angle θpa, which is determined by a preliminary horizontal wall part 11a and a preliminary vertical wall part 12a, is set to be larger than an angle θp, which is determined by a horizontal wall part and a vertical wall part of the channel member. Also, in relation to the θpa, a bending angle βc of a preliminary first bending point 14ao outside the curvature of the preliminary curved channel section 1ca is set to be smaller than a bending angle βs of a preliminary first bending point 14ao outside the curvature of the preliminary straight channel section 1sa. (βs-βc) is desirable to set at 10° or larger. COPYRIGHT: (C)2010,JPO&INPIT

Bend-forming method of channel member

PROBLEM TO BE SOLVED: To provide a high-rigidity steel plate improved in both elongation and local deformability and excellent in processability, and to provide its manufacturing method.SOLUTION: The steel plate contains C, Si, Mn, Al, P and S, and its remaining part is composed of iron and inevitable impurities, and a metal organization of the steel plate contains bainite, polygonal ferrite, residual austenite and annealed martensite. (1) When viewing the metal organization by using a scanning electronic microscope, (1a) the bainite is constituted of a composite organization of high-temperature region-created bainite and low-temperature region-created bainite, an area ratio a of the high-temperature region-created bainite is 10 to 80% with respect to the whole of the metal organization, and a total area ratio b of the low-temperature region-created bainite and the annealed martensite is 10 to 80% with respect to the whole of the metal organization, and (1b): an area ratio c of the polygonal ferrite is 10 to 50% with respect to the whole of the metal organization. (2) A volume ratio of the residual austenite which is measured by a saturation magnetization method is not smaller than 5% with respect to the whole of the metal organization.

High-rigidity steel plate excellent in processability and its manufacturing method

A press forming method forming an extended part by extension in a later forming period after deep drawing, the deep drawing process carried out with working at 100-250° C., and the extension forming process carried out colder at less than 50° C., whereby the extended part is formed by extension in a cup shaped low parts formed by deep drawing. Thus, press forming products containing formed elements can be deep drawing formed and extension formed at a high forming rate of 10 mm/sec or greater, which can assure high productivity. By making a steel plate temperature 100-350° C. during press forming and by making the forming rate in the later forming period where extension forming is carried out slower than the forming rate in an earlier forming period where extension forming is not carried out, cracking of the extended part can be prevented and press forming limitations can be improved.

Press forming method for steel plate

PROBLEM TO BE SOLVED: To suppress the generation of bent streaks and wrinkle marks which are generated at the vertical wall portions and flange portions on the outside of a curvature, suffering shrinkage deformation when bending a channel member including the curved channel portion by a method for forming by a two-stage bending. SOLUTION: In the method for forming a curved channel member, when the channel member including the curved channel portion 1c is formed, firstly a temporally formed body is subjected to a first molding and, next, the temporally formed body is subjected to the second molding. In the temporary formed body, one or several beads 21 which are bulged outward from the flat surface of the temporary vertical wall 12oa are formed along the lower part on the side of a temporary flange 13oa from the upper part on the side of a temporary lateral wall portion 11a of the temporary vertical wall portion 12oa, on the temporary vertical wall 12oa on the side suffering shrinkage deformation in the temporary curved channel portion corresponding to the curved channel portion. The beads 21 are formed so that the upper part on the side of the temporary lateral wall 11a is higher than the lower part on the side of the temporary flange 13oa, in the height from the flat portion. COPYRIGHT: (C)2011,JPO&INPIT

Method of forming curved channel member

Provided are: a high-strength steel sheet which is improved in both elongation and local formability and thus exhibits excellent workability; and a manufacturing method thereof. The high-strength steel sheet contains C, Si, Mn, Al, P and S with the remainder including iron and unavoidable impurities, and has a metal structure which includes polygonal ferrite, bainite, tempered martensite, and retained austenite. In the metal structure, (1) the bainite has a composite microstructure including both a high-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of 1 µm or more and a low-temperature-formed bainite having an average distance between adjacent regions of retained austenite and/or carbide of less than 1 µm each identified upon observation with a scanning electron microscope; and (2) the retained austenite is present in a volume percentage of 5% or more of the entire metal structure as determined by a saturation magnetization measurement.

High-strength steel sheet with excellent workability and manufacturing process therefor

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