A high strength cold rolled steel sheet having excellent burring properties includes: by weight %, 0.13-0.25% of carbon (C), 1.0-2.0% of silicon (Si), 1.5-3.0% of manganese (Mn), 0.08-1.5% of aluminum (Al)+chrome (Cr)+molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur (S), 0.01% or less of nitrogen (N), and the balance of Fe and inevitable impurities; and, by area fraction, 3-25% of ferrite, 20-40% of martensite, 5-20% of residual austenite. The ferrite has an average grain size of 2 μm or less at the reference point of 4/t (wherein t refers to a steel sheet thickness), with the average ratio between lengths in the thickness direction and in the rolling direction being 1.5 or less.

A novel combination of heat treatment steps includes the steps of carburizing a component fabricated of a medium carbon alloy steel at an elevated temperature for between three and six hours, subjecting the component to an austempering bath and holding it there for between fifteen and two hundred forty minutes and finally cooling the component to room temperature to allow martensitic transformation. These steps may be followed with cryogenic treatment to reduce retained austenite if needed. The process produces components with low distortion, high surface hardness, from HRC 56 to 62, and high surface compressive residual stress.

A novel combination of heat treatment steps includes the steps of carburizing a component fabricated of a medium carbon alloy steel at an elevated temperature for between three and six hours, subjecting the component to an austempering bath and holding it there for between fifteen and two hundred forty minutes and finally cooling the component to room temperature to allow martensitic transformation. These steps may be followed with cryogenic treatment to reduce retained austenite if needed. The process produces components with low distortion, high surface hardness, from HRC 56 to 62, and high surface compressive residual stress.

A thick-walled high-strength steel plate with excellent low-temperature toughness (Charpy impact and CTOD properties of a weld bond) in a multilayer weld zone, and a method for manufacturing the steel plate.

A thick-walled high-strength steel plate with excellent low-temperature toughness (Charpy impact and CTOD properties of a weld bond) in a multilayer weld zone, and a method for manufacturing the steel plate.

The invention relates to a steel sheet having a tensile strength of 1180 MPa or more, which excels in workability and cold brittleness resistance. The high-strength steel sheet contains 0.10% to 0.30% of C, 1.40% to 3.0% of Si, 0.5% to 3.0% of Mn, 0.1% or less of P, 0.05% or less of S, 0.005% to 0.20% of Al, 0.01% or less of N, 0.01% or less of O, as well as Fe and inevitable impurities. The steel sheet has: (i) a ferrite volume fraction of 5% to 35% and a bainitic ferrite and/or tempered martensite volume fraction of 60% or more; (ii) a MA constituent volume fraction of 6% or less (excluding 0%); and (iii) a retained austenite volume fraction of 5% or more.

The invention relates to a steel sheet having a tensile strength of 1180 MPa or more, which excels in workability and cold brittleness resistance. The high-strength steel sheet contains 0.10% to 0.30% of C, 1.40% to 3.0% of Si, 0.5% to 3.0% of Mn, 0.1% or less of P, 0.05% or less of S, 0.005% to 0.20% of Al, 0.01% or less of N, 0.01% or less of O, as well as Fe and inevitable impurities. The steel sheet has: (i) a ferrite volume fraction of 5% to 35% and a bainitic ferrite and/or tempered martensite volume fraction of 60% or more; (ii) a MA constituent volume fraction of 6% or less (excluding 0%); and (iii) a retained austenite volume fraction of 5% or more.

Austempered steel for components requiring high strength and high ductility and/or fracture toughness, which has a silicon content of 3.1 weight-% to 4.4 weight-% and a carbon content of 0.4 weight-% to 0.6 weight-%. The microstructure of the austempered steel is ausferritic or superbainitic.

Austempered steel for components requiring high strength and high ductility and/or fracture toughness, which has a silicon content of 3.1 weight-% to 4.4 weight-% and a carbon content of 0.4 weight-% to 0.6 weight-%. The microstructure of the austempered steel is ausferritic or superbainitic.

A high-strength high-ductility steel sheet has a specific component composition and a steel microstructure containing, in terms of an area ratio relative to the entire microstructure, 8% or more of a retained austenite, with the remainder being at least one of bainite, martensite, tempered bainite, and tempered martensite. With respect to the carbon concentration in the retained austenite, an average carbon concentration is from 0.9 to 1.2 mass %, a standard deviation of the carbon concentration distribution is 0.35 mass % or more, and an area ratio of a region having the carbon concentration of 1.5 mass % or more, relative to the entire microstructure, is 1.0% or more.