Provided is a high-strength steel sheet including: 0.12% to less than 0.17% of carbon (C), 0.3% to 0.8% of silicon (Si), 2.5% to 3.0% of manganese (Mn), 0.4% to 1.1% of chromium (Cr), 0.01% to 0.3% of aluminum (Al), 0.01% to 0.03% of niobium (Nb), 0.01% to 0.03% of titanium (Ti), 0.001% to 0.003% of boron (B), 0.04% or less of phosphorus (P), 0.01% or less of sulfur (S): 0.01% or less of nitrogen (N), and a balance of iron (Fe) and inevitable impurities. The contents of C, Si, and Al satisfy: [C]+[Si]+[Al])/5≤0.35 wt. A microstructure includes more than 1% to 4% or less of retained austenite, more than 10% to 20% or less of fresh martensite, 5% or less (excluding 0%) of ferrite, more than 50% to 70% or less of tempered martensite, and a balance of bainite.

A metal-strip rapid cooling apparatus includes a cooling fluid ejection device including one set of nozzles or a plurality of sets of nozzles arranged in a horizontal direction, and configured to eject a cooling fluid onto the metal strip from both sides of the metal strip; cooling fluid removing rolls configured to remove a remaining fluid from the metal strip onto which the cooling fluid has been ejected; and movable masking plates on both sides of a metal strip pass line along which the metal strip passes, the movable masking plates each disposed between the metal strip pass line and the nozzles, and configured to move in the horizontal direction to adjust a cooling start position and control a distance from the cooling start position to the cooling fluid removing rolls, the cooling start position positioned such that the metal strip starts to be cooled with the cooling fluid.

Provided is a method of manufacturing a high strength steel sheet having excellent workability, including: reheating a steel slab; hot-rolling the reheated steel slab at an Ar3 transformation point or higher; coiling in a temperature range of 400 to 700° C.; primary cooling at a cooling rate of 0.1° C./s or less to room temperature; producing a cold rolled steel sheet at a cold reduction ratio of 40 to 70%; continuously annealing the cold rolled steel sheet in a temperature range of Ac1+30° C. to Ac3−20° C.; secondary cooling at a cooling rate of 10° C./s or less to 630 to 670° C.; third cooling to 400 to 500° C. at a cooling rate of 5° C./s or more in a hydrogen; maintaining for 70 seconds or more; hot-dip galvanizing; and performing a final cooling to Ms or less at a cooling rate of 1° C./s or more.

To obtain a high-strength hot-dip galvanized steel sheet having excellent surface appearance even in the case where a steel strip containing Mn at a predetermined ratio or more to Si is subjected to hot-dip galvanizing treatment, a method of producing a hot-dip galvanized steel sheet using a continuous hot-dip galvanizing apparatus comprises: subjecting a steel strip to annealing, by conveying it in an annealing furnace; and subjecting the steel strip discharged from a cooling zone to hot-dip galvanizing using a hot-dip galvanizing line, to obtain a hot-dip galvanized steel sheet. The steel strip has a chemical composition containing, in mass %, Mn: 1.7% or more and 3.5% or less and Si: 0.2% or more and 1.05% or less and satisfying [Si]/[Mn]≤0.30. The chemical composition, a dew point of an atmosphere in the soaking zone, and a delivery temperature of the heating zone satisfy Formula (1).

A high-ductility, high-strength steel sheet having excellent close-contact bendability and a method for producing the same. The steel sheet has a specified chemical composition and a microstructure comprising, by area percentage, 50% or more of a ferrite phase, 5% to 30% of a pearlite phase, and 15% or less in total of bainite, martensite, and retained austenite, in which the area percentage of ferrite grains each containing three or more cementite grains having an aspect ratio of 1.5 or less is 30% or less, and the number of inclusions having a particle size of 10 μm or more present in a portion extending from a surface to a ¼ thickness position is 2.0 particles/mm.sup.2 or less.

A high-strength steel sheet with a tensile strength of 1,180 MPa or more has a predetermined chemical composition and a steel microstructure in which the area fraction of ferrite is 5% or less, the area fraction of martensite is 2% to 10%, the area fraction of bainite is 5% to 37%, the area fraction of tempered martensite is 42% to 65%, the volume fraction of retained austenite is 3% to 15%, the average grain size of ferrite and bainite is 3 μm or less, in a region extending 50 μm from a surface of the steel sheet in a through-thickness direction, and the average grain size of prior austenite grains is 10 μm or less, the average grain size of the prior austenite grains in the through-thickness direction is 0.9 or less of the average grain size thereof in a rolling direction.

One embodiment of the present invention discloses a member for automobile structure including a base steel sheet and a plating layer covering at least one surface of the base steel sheet, wherein the member for automobile structure has a tensile strength of 1350 MPa or greater and a yield strength of 900 MPa or greater, wherein the base steel sheet includes a martensite phase having an area fraction of 80% or greater, an iron-based carbide located inside the martensite phase and having an area fraction of less than 5% based on the martensite phase, and precipitates distributed inside the base steel sheet, wherein a mismatch dislocation exists at the interface between iron and the precipitates of the base steel sheet, and a difference between lattice constants of the iron and the precipitates is less than 25% of the lattice constants of the iron.

Provided are a high-yield-ratio high-strength galvanized steel sheet and a method for manufacturing thereof. The high-yield-ratio high-strength galvanized steel sheet has a steel sheet having a specified chemical composition and a metallographic structure including, in terms of area ratio, in terms of area ratio, 15% or less of ferrite, 20% or more and 50% or less of martensite, and bainite and tempered martensite in a total amount of 30% or more, and a galvanized layer formed on the steel sheet having a coating weight of 20 g/m.sup.2 to 120 g/m.sup.2 per side, in which a yield strength ratio is 65% or more, a tensile strength is 950 MPa or more, and Mn oxides are contained in the galvanized layer in an amount of 0.015 g/m.sup.2 to 0.050 g/m.sup.2.

A high-strength galvanized steel sheet of the present invention includes a steel sheet having a chemical composition containing a predetermined component element, and a steel structure in which an average grain size of inclusions containing at least one of Al, Si, Mg, and Ca and existing in an area extending from a surface to a position of ⅓ of a sheet thickness is 50 μm or less, and an average nearest distance between ones of the inclusions is 20 μm or more; and a galvanized layer provided on a surface of the steel sheet and having a coating weight per one surface of 20 g/m.sup.2 or more and 120 g/m.sup.2 or less, in which an amount of diffusible hydrogen contained in the steel is less than 0.25 mass ppm, and a tensile strength is 1100 MPa or more.

A zinc-plated steel sheet for hot stamping according to an aspect of the present invention includes a steel substrate and a plated layer provided on a surface of the steel substrate, in which the steel substrate contains, in % by mass, C: 0.10 to 0.5%, Si: 0.7 to 2.5%, Mn: 1.0 to 3%, and Al: 0.01 to 0.5%, with the balance being iron and inevitable impurities, and the steel substrate has, in the inside thereof, an internal oxide layer consists of an oxide containing at least one of Si and Mn having a thickness of 1 μm or more, and a decarburized layer having a thickness of 20 μm or less from an interface with the plated layer toward an internal direction of the steel substrate.