A hot-rolled steel sheet has a predetermined chemical composition in which a microstructure includes 99% or more of martensite by volume fraction and a remainder in microstructure including residual austenite and ferrite, in a cross section parallel to a rolling direction, an average aspect ratio of prior austenite grains is less than 3.0, a proportion of sulfides having an aspect ratio of more than 3.0 among sulfides having an area of 1.0 μm.sup.2 or more is 1.0% or, less, in a thickness middle portion, and a pole density of {211} <011> orientation is 3.0 or less, and a tensile strength TS is 980 MPa or higher.

Disclosed in the present invention is complex-phase steel having high hole expansibility. The complex-phase steel has a microstructure of ferrite and bainite. The complex-phase steel having high hole expansibility comprises the following chemical elements in percentage by mass: C: 0.06-0.09%, Si: 0.05-0.5%, Al: 0.02-0.1%, Mn: 1.5-1.8%, Cr: 0.3-0.6%, Nb≤0.03%, Ti: 0.05-0.12%, and the balance of Fe and inevitable impurities. In addition, also disclosed in the present invention is a manufacturing method for the foregoing complex-phase steel having high hole expansibility. The method comprises the following steps: (1) smelting and casting; (2) heating; (3) hot-rolling; (4) phosphorous removal; (5) laminar cooling: a relaxation time period is controlled to be 0-8 s, and a laminar cooling rate is 40-70° C./s; (6) coiling; (7) leveling; and (8) pickling. The complex-phase steel having high hole expansibility can simultaneously satisfy the requirements for hole expansibility and good plasticity.

Disclosed in the present invention is complex-phase steel having high hole expansibility. The complex-phase steel has a microstructure of ferrite and bainite. The complex-phase steel having high hole expansibility comprises the following chemical elements in percentage by mass: C: 0.06-0.09%, Si: 0.05-0.5%, Al: 0.02-0.1%, Mn: 1.5-1.8%, Cr: 0.3-0.6%, Nb≤0.03%, Ti: 0.05-0.12%, and the balance of Fe and inevitable impurities. In addition, also disclosed in the present invention is a manufacturing method for the foregoing complex-phase steel having high hole expansibility. The method comprises the following steps: (1) smelting and casting; (2) heating; (3) hot-rolling; (4) phosphorous removal; (5) laminar cooling: a relaxation time period is controlled to be 0-8 s, and a laminar cooling rate is 40-70° C./s; (6) coiling; (7) leveling; and (8) pickling. The complex-phase steel having high hole expansibility can simultaneously satisfy the requirements for hole expansibility and good plasticity.

Provided is a high strength steel sheet that has a tensile strength of 1180 MPa or more and a uniform elongation of 6 % or more. The high strength steel sheet has a chemical composition that contains predetermined components with a MSC of 3.0 mass% to 4.2 mass%. The high strength steel sheet has a microstructure including upper bainite in an area fraction of 70 % or more as a main phase, fresh martensite and retained austenite in a total area fraction of 7 % to 30 %, with the retained austenite having an area fraction of 2 % or more. The high strength steel sheet has a mechanical property with a uniform elongation of 6 % or more and a tensile strength of 1180 MPa or more.

Provided is a high strength steel sheet that has a tensile strength of 1180 MPa or more and a uniform elongation of 6 % or more. The high strength steel sheet has a chemical composition that contains predetermined components with a MSC of 3.0 mass% to 4.2 mass%. The high strength steel sheet has a microstructure including upper bainite in an area fraction of 70 % or more as a main phase, fresh martensite and retained austenite in a total area fraction of 7 % to 30 %, with the retained austenite having an area fraction of 2 % or more. The high strength steel sheet has a mechanical property with a uniform elongation of 6 % or more and a tensile strength of 1180 MPa or more.

A steel sheet having a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, high LME resistance, and good weld fatigue properties. The steel sheet has a specific chemical composition and a specific steel microstructure. Crystal grains containing an oxide of Si and/or Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in the range of 3 to 10 μm, the lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy a specified formula.

A steel sheet having a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, high LME resistance, and good weld fatigue properties. The steel sheet has a specific chemical composition and a specific steel microstructure. Crystal grains containing an oxide of Si and/or Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in the range of 3 to 10 μm, the lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy a specified formula.

A steel sheet for hot stamping includes an amount of 0.17 wt % to 0.25 wt % of carbon (C), an amount of 0.3 wt % to 1.0 wt % of silicon, an amount of 0.6 wt % to 1.0 wt % of manganese (Mn), an amount of 0.02 wt % or less of phosphorus (P), an amount of 0.01 wt % or less of sulfur (S), an amount of 0.1 wt % to 1.0 wt % of aluminum (Al), an amount of 0.001 wt % to 0.005 wt % of boron (B), an amount of 0.01 wt % to 0.1 wt % of titanium (Ti), an amount of 0.02 wt % to 0.06 wt % of niobium (Nb), an amount of 0.3 wt % to 1.0 wt % of a sum of at least one of chromium (Cr), nickel (Ni), and molybdenum (Mo), and the balance of iron (Fe) and other unavoidable impurities; and also a microstructure including ferrite and pearlite.

After low-temperature finish rolling has been performed on a steel material having a certain chemical composition, cooling is performed at an average cooling rate of 10° C./s or higher to a temperature of 500° C., rapid cooling is further performed in a temperature range from a Ms temperature to a temperature of (Ms temperature - 200° C.), coiling is thereafter performed in a low temperature range of 250° C. or lower, and the coiled steel sheet is uncoiled and further subjected to rolling with a certain amount or more of rolling load per unit width and the like. Consequently, it is possible to obtain a high-strength hot-rolled steel sheet having a microstructure including, in terms of area fraction, 95% or more of a martensite phase at a position located at ¼ of the thickness of the steel sheet, in which an average aspect ratio of prior austenite grains is 3.0 or more.

After low-temperature finish rolling has been performed on a steel material having a certain chemical composition, cooling is performed at an average cooling rate of 10° C./s or higher to a temperature of 500° C., rapid cooling is further performed in a temperature range from a Ms temperature to a temperature of (Ms temperature - 200° C.), coiling is thereafter performed in a low temperature range of 250° C. or lower, and the coiled steel sheet is uncoiled and further subjected to rolling with a certain amount or more of rolling load per unit width and the like. Consequently, it is possible to obtain a high-strength hot-rolled steel sheet having a microstructure including, in terms of area fraction, 95% or more of a martensite phase at a position located at ¼ of the thickness of the steel sheet, in which an average aspect ratio of prior austenite grains is 3.0 or more.