66results about How to "Excellent stretch flangeability" patented technology

The present invention provides a high-strength steel sheet having a tensile strength (TS) of at least 780 MPa, excellent ductility and stretch flangeability, and excellent material-quality stability, achieved by: having a specific component composition; the Mn volume divided by the B volume being not more than 2100; the steel structure having, in terms of area ratio, a total of ferrite and bainitic ferrite of 25-80%, and 3-20% martensite, and having, in terms of volume fraction, at least 10% retained austenite; the average crystalline particle diameter of the retained austenite being not more than 2[mu]m; the average Mn volume (mass%) in the retained austenite being at least 1.2 times the Mn volume (mass%) in the steel; and retained austenite aggregates, in which seven or more retained austenite crystal particles having the same orientation are grouped together, accounting for, in terms of area ratio, at least 60% of the total retained austenite.

The present invention relates to a high strength hot rolled steel sheet consisting of 0.04 to 0.15% C, 1.5% or less Si, 0.5 to 1.6% Mn, 0.04% or less P, 0.005% or less S, 0.04% or less Al, 0.03 to 0.15% Ti, 0.03 to 0.5% Mo, by mass, and balance of Fe and inevitable impurities, and having a microstructure consisting of ferrite containing precipitates, second phase of bainite and / or martensite, and other phase, wherein the percentage of the ferrite containing precipitates is 40 to 95%, and the percentage of the other phase is 5% or less. For example, the high strength hot rolled steel sheet having a thickness of 1.4 mm shows a tensile strength of 780 MPa or higher, an elongation of 22% or higher elongation, and a hole expansion ratio of 60% or higher, thus the steel sheet is suitable for reinforcing members automobile cabin and crash worthiness members of automobile.

Disclosed is a hot-dip galvanized steel sheet with high tensile strength that is provided with both strength and processability (elongation and stretch flangeability) and a method for producing the same. Specifically disclosed is a hot-dip galvanized steel sheet with high tensile strength that has tensile strength of 980 MPa or greater and superior processability. This hot-dip galvanized steel sheet has a composition containing 0.07 - 0.13% C, 0.3% or less Si, 0.5 - 2.0% Mn, 0.025% or less P, 0.005% or less S, 0.0060% or less N, 0.06% or less Al, 0.10 - 0.14% or less Ti, and 0.15 - 0.30% V (% by mass) such that S and N satisfy Ti >= 0.10 + (N / 14*48 + S / 32*48) and 0.8 <= (Ti / 48 + V / 51) / (C / 12) <= 1.2 (wherein C, Ti, V, S and N are the content for each element in % by mass). The composition further contains 0.04 - 0.1% V in solid solution and 0.05% or less Ti in solid solution, and the remainder is Fe and unavoidable impurities. The hot-dip galvanized steel sheet has a matrix that is 97% or more in areal proportion of the overall structure of the ferrite phase and a hot-dip galvanized coating or alloyed hot-dip galvanized coating on the surface of a hot-rolled steel sheet that has fine carbide that has an average particle diameter of less than 10 nm and contains Ti and V dispersed and precipitated, with a structure forming 0.007 or more in areal proportion of the overall structure of that fine carbide.

The present invention provides: a high-strength cold-rolled steel sheet which has excellent stretch flange properties, and which has a chemical composition comprising, in mass%, 0.050-0.090% of C:, 0.05% or less of Si, 1.5-2.0% of Mn, 0.030% or less of P, 0.0050% or less of S, 0.005-0.1% of Al, 0.01% or less of N, 0.005-0.050% of Ti, 0.020-0.080% of Nb and a remainder made up by Fe and unavoidable impurities, has a tissue composed of, in vol%, 50-77% of a ferrite phase, 20-50% of a bainite phase, 2-10% of a martensite phase and 1-5% of a residual austenite phase, and has such high strength that the tensile strength (TS) is 590 MPa or more, a strength-elongation balance (TSEl) of 16000 MPa% or more and a strength-(hole expanding ratio) balance (TS[lambda]) of 40000 MPa% or more; and a process for producing the high-strength cold-rolled steel sheet.

Disclosed is a steel sheet including 0.10 to 0.20 mass % of C, 0.8 to 2.5 mass % of Si, 1.5 to 2.5 mass % of Mn, and 0.01 to 0.10 mass % of Al, wherein P is limited to less than 0.1 mass %, and S is limited to less than 0.002 mass % and more than 0 mass %, and the structure includes bainitic ferrite and residual austenite such that area percentage of the bainitic ferrite in relation to the entire structure is at least 70%, area percentage of the residual austenite is 2 to 20%, and total area percentage of polygonal ferrite and quasi-polygonal ferrite is up to 15%, and proportion of the residual austenite having an average particle size of up to 5 μm in the residual austenite is at least 60%. The steel sheet is a TRIP steel sheet which has bainitic ferrite as its matrix, and it has a tensile strength (TS) of at least 980 MPa as well as an excellent stretch flangeability.

Provided are: a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, having good workability as evaluated by ductility and stretch-flangeability, and having excellent collision characteristics; and a method for producing said steel sheet. In this high-strength cold-rolled steel sheet, the metal structure at a position of 1 4 sheet thickness satisfies (1)-(4) below. (1) The area ratio of ferrite is 0-10%, inclusive, and the balance is a hard phase comprising at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite, said hard phase including quenched martensite and retained austenite. (2) The volume ratio V[gamma] of retained austenite is 5-30%, inclusive. (3) The area ratio VMA of an MA structure in which quenched martensite and retained austenite are combined is 3-25%, inclusive, and the average equivalent circle diameter of the MA structure is 2.0 [mu]m or less. (4) The ratio VMA / V[gamma] of the area ratio VMA of the MA structure to the volume ratio V[gamma] of the retained austenite is 0.50-1.50.

A hot-dip galvanized cold-rolled steel sheet has a tensile strength of 750 MPa or higher, a composition consisting, in mass percent, of C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and 3.0% or less, and optionally containing one or more types of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM, and Bi, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.010% or less, and a main phase as a low-temperature transformation product and a second phase as retained austenite. The retained austenite volume fraction is more than 4.0% and less than 25.0% of the whole structure, and has an average grain size of less than 0.80 □m. A number density of retained austenite grains having a grain size of 1.2 □m or more is 3.0□10−2 / □m2 or less.

A high-strength cold-rolled steel sheet excellent in ductility, work hardenability, and stretch flangeability, and having tensile strength of 780 MPa or more includes: a chemical composition containing, in mass percent, C: more than 0.020% to less than 0.30%, Si: more than 0.10% to 3.00% or less, Mn: more than 1.00% to 3.50% or less; and metallurgical structure whose main phase is a low-temperature transformation product, and whose secondary phase contains retained austenite. The retained austenite has a volume fraction relative to overall structure of more than 4.0% to less than 25.0% and an average grain size of less than 0.80 μm, and of the retained austenite, the number density of retained austenite grains whose grain size is 1.2 μm or more is 3.0×10−2 grains / μm2 or less.

Provided is a high-strength hot-rolled steel sheet that exhibits little inter-coil variability in mechanical properties and excellent stretch flangeability. By mass, said steel sheet contains more than 0.010% and no more than 0.06% carbon, up to 0.3% silicon, up to 0.8% manganese, up to 0.03% phosphorus, up to 0.02% sulfur, up to 0.1% aluminum, up to 0.01% nitrogen, and 0.05-0.10% titanium, with the remainder comprising iron and unavoidable impurities. The amounts of silicon and manganese are minimized, the amount of segregation and the like is reduced, and strength variation due to inter-coil position differences is decreased. The structure of the steel sheet is such that the area fraction of a ferrite phase is at least 95%, the mean diameter of ferrite crystal grains is at least 1 [mu]m, and TiC with a mean grain diameter of at most 7 nm is dispersed inside the ferrite crystal grains. This results in a high-strength hot-rolled steel sheet that maintains a yield strength of at least 530 MPa.

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.

This steel sheet has a specific chemical composition and is provided with a structure represented by, in terms of area ratio, 30-95% ferrite and 5-70% bainite. When a crystal grain is defined as a region which is surrounded by grain boundaries having a misorientation of 15D or higher and for which the equivalent circle diameter is 0.3 Mum or larger, the proportion of crystal grains having an intragranular misorientation of 5-14D relative to all of the crystal grains is 20-100% in terms of area ratio. The average aspect ratio of ellipses equivalent to the crystal grains is 5 or lower. The average distribution density of the total of Ti carbides and Nb carbides having a particle size of 20 nm or larger in the ferrite grain boundaries is 10 particles / Mum or lower.

This steel sheet has a specific chemical composition and is provided with a structure represented by, in terms of area ratio, 0 - 30% ferrite and 70 - 100% bainite. When a crystal grain is defined asa region which is surrounded by grain boundaries having a misorientation of 15 DEG or higher and for which the equivalent circle diameter is 0.3 mu m or larger, the proportion of crystal grains havingan intragranular misorientation of 5 - 14 DEG relative to all of the crystal grains is 20 - 100% in terms of area ratio. The grain boundary number density of a solid solution of C, or the total grainboundary number density of a solid solution of C and a solid solution of B is 1 particle / nm<2> to 4.5 particles / nm<2> inclusive. The average particle size of cementite precipitated in the grain boundaries is no larger than 2 mu m.