Abstract: A high-strength galvanized steel sheet with high yield ratio having excellent ductility and stretch flange formability, the steel sheet having a chemical composition containing, by mass %, C: 0.04% or more and 0.13% or less, Si: 0.9% or more and 2.3% or less, Mn: 0.8% or more and 2.4% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% or more and 0.1% or less, N: 0.008% or less, and the balance being Fe and inevitable impurities and a microstructure including, in terms of area ratio, 94% or more of a ferrite phase and 2% or less of a martensite ferrite phase, wherein mean grain size of ferrite is 10 ?m or less, Vickers hardness of ferrite is 140 or more, mean grain size of carbide particles existing at grain boundaries of ferrite is 0.5 ?m or less, and aspect ratio of carbide particles existing at the grain boundaries of ferrite is 2.0 or less.

Abstract: A high-strength galvanized steel sheet having excellent formability and crashworthiness, including a component composition containing 0.03% to 0.13% C, 1.0% to 2.0% Si, 2.4% to 3.5% Mn, 0.001% to 0.05% P, 0.0001% to 0.01% S, 0.001% to 0.1% Al, 0.0005% to 0.01% N, and 0.0003% to 0.01% B on a mass basis, the remainder being Fe and unavoidable impurities, and a microstructure containing a tempered martensitic phase and a bainitic phase such that the sum of an area fraction of the tempered martensitic phase and an area fraction of the bainitic phase is 30% or more (the area fraction of the martensitic phase is 30% or more in the absence of the bainitic phase), wherein a distance of closest approach of the tempered martensitic phase is 10 ?m or less and the contents of C, Mn, and B satisfy (1): (% Mn)+1000×(% B)?35 ×(% C) ??(1).

Abstract: A high-strength cold rolled steel sheet has excellent formability and crashworthiness and includes, on a mass % basis, C: 0.05 to 0.3%, Si: 0.3 to 2.5%. Mn: 0.5 to 3.5%, P: 0.003 to 0.100%, 5: 0.02% or less, Al: 0.010 to 0.5%, the balance being iron and unavoidable impurities, the high-strength cold rolled steel sheet having a microstructure including 20% or more of ferrite on an area fraction basis, 10 to 60% of tempered martensite on an area fraction basis, 0 to 10% of martensite on an area fraction basis, and 3 to 15% of retained austenite on a volume fraction basis.

Abstract: 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 ?m or less.

Abstract: A high-strength galvanized steel sheet contains C: 0.04% or more and 0.10% or less, Si: 0.7% or more and 2.3% or less, Mn: 0.8% or more and 2.0% or less, P: 0.03% or less, S: 0.003% or less, Al: 0.1% or less, and N: 0.008% or less on a mass percent basis, and the remainder of iron and incidental impurities. The C content [C%] (% by mass) and the Si content [Si%] (% by mass) satisfy [C%]×[Si%]?0.20. A ferrite phase constitutes 75% or more, a bainitic ferrite phase constitutes 1% or more, a pearlite phase constitutes 1% or more and 10% or less, and a martensite phase constitutes less than 5% on an area ratio basis. The area ratio of the martensite phase/(the area ratio of the bainitic ferrite phase + the area ratio of the pearlite phase) is 0.6 or less.

Abstract: A steel sheet has the chemical composition containing, by mass %, C: 0.04 to 0.13%, Si: 0.9 to 2.3%, Mn: 0.8 to 1.8%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.008% or less, the remainder being Fe and the inevitable impurities and a microstructure including, in terms of area ratio, a ferrite phase of 80% or more, a bainitic ferrite phase of 1.0% or more, a pearlite phase of 1.0 to 10.0%, and a martensite phase of 1.0% or more and less than 5.0%, wherein the mean grain size of ferrite is 14 ?m or less, the mean grain size of martensite is 4 ?m or less, the mean free path of martensite is 3 ?m or more, the Vickers hardness of ferrite is 140 or more, and the relationship area ratio of martensite/(area ratio of bainitic ferrite+area ratio of pearlite) 0.6 is satisfied.

Abstract: 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 ?m or less and an average distance of 5 ?m or less between adjacent martensite grains.

Abstract: 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 ?m or less and an average crystal grain diameter of retained austenite is 2 ?m or less.

Abstract: 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.

Abstract: 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 ?m from the interface between the coating layer and the base steel.

Abstract: A high-strength cold-rolled steel sheet and high-strength galvanized steel sheet has a TS of 1180 MPa or more and excellent formability including stretch flangeability and bendability. The high-strength cold-rolled steel sheet contains 0.05% to 0.3% C, 0.5% to 2.5% Si, 1.5% to 3.5% Mn, 0.001% to 0.05% P, 0.0001% to 0.01% S, 0.001% to 0.1% Al, 0.0005% to 0.01% N, and 1.5% or less Cr (including 0%) on a mass basis, the remainder being Fe and unavoidable impurities.

Abstract: 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 ?m or less.

Abstract: 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.

Abstract: A high-strength hot-rolled steel sheet has a tensile strength (TS) of 540 to 780 MPa, only small variations in strength, and excellent uniformity in strength using a general-purpose Ti-containing steel sheet, which is inexpensive. The high-strength hot-rolled steel sheet includes, on a mass percent basis, 0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, and the balance being Fe and incidental impurities. The microstructure have a volume fraction of polygonal ferrite of 70% or more, and the amount of Ti present in a precipitate having a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula (1): Ti*=[Ti]?48/14×[N] (1) where [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent by mass), respectively, of the steel sheet.

Abstract: 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 ?m or less is 10% or less relative to the whole martensite phase.

Abstract: A high-strength hot-rolled steel sheet has a tensile strength (TS) of 540 to 780 MPa, only small variations in strength, and excellent uniformity in strength using a general-purpose Ti-containing steel sheet, which is inexpensive. The high-strength hot-rolled steel sheet includes, on a mass percent basis, 0.05%-0.12% C, 0.5% or less Si, 0.8%-1.8% Mn, 0.030% or less P, 0.01% or less S, 0.005%-0.1% Al, 0.01% or less N, 0.030%-0.080% Ti, and the balance being Fe and incidental impurities. The microstructure have a bainitic ferrite fraction of 70% or more, and the amount of Ti present in a precipitate having a size of less than 20 nm is 50% or more of the value of Ti* calculated using formula (1): Ti*=[Ti]?48/14×[N] (1) where [Ti] and [N] represent a Ti content (percent by mass) and a N content (percent by mass), respectively, of the steel sheet.

Abstract: A high tensile strength galvanized steel sheet with excellent formability and anti-crush properties contains, 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, 0.007% or less of N, in addition, 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, the steel sheet 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 ?m or less.

Abstract: 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 ?m from the interface between the coating layer and the base steel.

Abstract: The present invention presents a high tensile strength cold rolled steel sheet having excellent formability, impact resistance and strain age hardening characteristics, and the production thereof. As a specific means, a slab having a composition which contains, by mass %, 0.15% or less of C, 0.02% or less of Al, and 0.0050 to 0.0250% of N at N/Al of 0.3 or higher, and has N in a solid solution state at 0.0010% or more, is first hot rolled at the finish rolling delivery-side temperature of 800° C. or above, and is subsequently coiled at the coiling temperature of 750° C. or below to prepare a hot rolled plate. Then, after cold rolling, the hot rolled plate is continuously cooled at a temperature from the recrystallization temperature to 900° C. at a holding time of 10 to 120 seconds, and is cooled by primary cooling in which the hot rolled plate is cooled to 500° C. or below at a cooling rate of 10 to 300° C.

Abstract: The present invention presents a high tensile strength cold rolled steel sheet having excellent formability, impact resistance and strain age hardening characteristics, and the production thereof. As a specific means, a slab having a composition which contains, by mass %, 0.15% or less of C, 0.02% or less of Al, and 0.0050 to 0.0250% of N at N/Al of 0.3 or higher, and has N in a solid solution state at 0.0010% or more, is first hot rolled at the finish rolling delivery-side temperature of 800° C. or above, and is subsequently coiled at the coiling temperature of 750° C. or below to prepare a hot rolled plate. Then, after cold rolling, the hot rolled plate is continuously cooled at a temperature from the recrystallization temperature to 900° C. at a holding time of 10 to 120 seconds, and is cooled by primary cooling in which the hot rolled plate is cooled to 500° C. or below at a cooling rate of 10 to 300° C.