Disclosed is, as a high-strength steel plate of API X80 grade or higher with a thickness of 38 mm or more, a thick steel plate for structural pipes or tubes that exhibits high strength in the rolling direction and excellent Charpy properties at its mid-thickness part without addition of large amounts of alloying elements. The thick steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure at its mid-thickness part that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50%, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80% or more with respect to the whole area of the ferrite; a tensile strength of 620 MPa or more; and a Charpy absorption energy vE.sub.−20+ C. at −20° C. at the mid-thickness part of 100 J or more.

Disclosed is, as a high-strength steel plate of API X80 grade or higher with a thickness of 38 mm or more, a thick steel plate for structural pipes or tubes that exhibits high strength in the rolling direction and excellent Charpy properties at its mid-thickness part without addition of large amounts of alloying elements. The thick steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure at its mid-thickness part that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50%, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80% or more with respect to the whole area of the ferrite; a tensile strength of 620 MPa or more; and a Charpy absorption energy vE.sub.−20+ C. at −20° C. at the mid-thickness part of 100 J or more.

A method of manufacturing steel strip including the steps of: casting molten steel into slabs; reheating the slabs at 1150° C. or more for 1 hour or more; hot rolling the steel into a strip, preferably with an average F1 slab entry temperature above 1000° C.; coiling the hot rolled steel strip; batch annealing the steel strip: at an intercritical temperature (i.e. between Ac1 and Ac3), preferably below 700° C.; in non-oxidising and non-nitrogenated atmosphere; total annealing time at least 5 hours, preferably at least 10 hours to get Mn enrichment in austenite such that Mn content is at least 1.25 times bulk Mn content of the steel and C enrichment such that C content is at least 1.2 times bulk C content of the steel; cooling the steel after batch annealing in air, forced air or water quench.

A method of manufacturing steel strip including the steps of: casting molten steel into slabs; reheating the slabs at 1150° C. or more for 1 hour or more; hot rolling the steel into a strip, preferably with an average F1 slab entry temperature above 1000° C.; coiling the hot rolled steel strip; batch annealing the steel strip: at an intercritical temperature (i.e. between Ac1 and Ac3), preferably below 700° C.; in non-oxidising and non-nitrogenated atmosphere; total annealing time at least 5 hours, preferably at least 10 hours to get Mn enrichment in austenite such that Mn content is at least 1.25 times bulk Mn content of the steel and C enrichment such that C content is at least 1.2 times bulk C content of the steel; cooling the steel after batch annealing in air, forced air or water quench.

A high-strength structural steel material having excellent fatigue crack propagation inhibitory characteristics according to an aspect of the present invention contains, by weight, 0.02-0.12% of C, 1.7-2.5% of Mn, 0.01-0.8% of Si, 0.005-0.5% of Al, and the balance Fe and unavoidable impurities, wherein a microstructure of the structural steel sheet material is divided into a surface layer portion outside and a central portion inside along a thickness direction; the surface layer portion comprises tempered bainite as a matrix structure, fresh martensite as a second structure, and austenite as a residual structure; and the central portion comprises lath bainite.

Provided is a high-strength steel sheet having high impact resistance. The steel sheet includes: by weight %, carbon (C): 0.05% to 0.14%, silicon (Si): 0.01% to 1.0%, manganese (Mn): 1.5% to 2.5%, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.005% to 1.0%, phosphorus (P): 0.001% to 0.05%, sulfur (S): 0.001% to 0.01%, nitrogen (N): 0.001% to 0.01%, niobium (Nb): 0.005% to 0.06%, titanium (Ti): 0.005% to 0.11%, and the balance of iron (Fe) and inevitable impurities. The steel sheet has a microstructure comprising ferrite and bainite in a total area fraction of 90% or more. The steel sheet has a value of 0.05 to 1.0 as a shear texture ({110}<112>, {112}<111>) area ratio of a center region (ranging deeper than 1/10t to ½t in a thickness direction, t refers to thickness (mm)) and a surface region (ranging from a surface to 1/10t in the thickness direction).

A high-strength cold-rolled steel sheet and a hot-dipped galvanized steel sheet with low deviation of material properties and excellent formability, and a method for manufacturing same are provided. The present invention relates to a low-yield-ratio high-strength cold-rolled steel sheet with low deviation of directional material and excellent formability, comprising 0.05-0.15 wt % of C, 0.2-1.5 wt % of Si, 2.2-3.0 wt % of Mn, 0.001-0.10 wt % of P, 0.010 wt % or less of S, 0.01-0.10 wt % of sol.Al, 0.010 wt % or less of N, and the balance of Fe and impurities, satisfying Si/(Mn+Si)≦0.5, wherein the microstructure of the steel sheet includes 40% or more of ferrite, 10% or less of bainite, 3% or less of residual austenite, and a balance of martensite, and the area fraction of a Mn band present in the martensite phase is 5% or less.

Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 30% or more of a ferrite phase, 40% to 65% of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 40% to 55% and the total area ratio of a bainite phase having a grain diameter of more than 5 μm and/or a martensite phase having a grain diameter of more than 5 μm is 20% or less, and a tensile strength is 980 MPa or more.

Provided are a high-strength steel sheet and a method for manufacturing the steel sheet. The high-strength steel sheet has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 30% or more of a ferrite phase, 40% to 65% of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 μm from the surface in the thickness direction, the area ratio of a ferrite phase is 40% to 55% and the total area ratio of a bainite phase having a grain diameter of more than 5 μm and/or a martensite phase having a grain diameter of more than 5 μm is 20% or less, and a tensile strength is 980 MPa or more.

A high-strength galvanized steel sheet that includes a chemical composition containing, by mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and Fe and inevitable impurities. The steel sheet having a microstructure containing, by an area percentage basis, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), the other phases containing a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%).