A flat product of steel with yield strength Rp 0.2 of 660 to 820 MPa, BH2 value greater than 30 MPa, a hole expansion ratio greater than 30%, and a microstructure having a first main component at a proportion of at least 50%, including one or more individual components of ferrite, tempered bainite, and tempered martensite, each with less than 5% carbides, and a second main component at a proportion of 5% to 50%, including one or more individual components of martensite, residual austenite, bainite or perlite, with the steel having a following chemical composition (in weight %): C: 0.04 to 0.12; Si: 0.03 to 0.8; Mn: 1 to 2.5: P: max. 0.08; S: max. 0.01; N: max. 0.01; Al: up to 0.1; Ni+Mo; up to 0.5; Nb: up to 0.08; Ti: up to 0.2; Nb+Ti: min, 0.03; Cr: up to 0.6; the remainder being iron including unavoidable steel-associated elements.

A method of manufacturing a hot-stamped part includes: inserting a blank into a heating furnace including a plurality of sections with different temperature ranges; step heating the blank in multiple stages; and soaking the blank at a temperature of about Ac3 to about 1,000° C., wherein in the step of heating the blank, a temperature condition in the heating furnace satisfies the following equation: 0<(Tg−Ti)/Lt<0.025° C./mm, where Tg denotes a soaking temperature (° C.), Ti denotes an initial temperature (° C.) of the heating furnace, and Lt denotes a length (mm) of step heating sections.

A method is for producing a high strength coated steel sheet having a yield stress YS>800 MPa, a tensile strength TS>1180 MPa, and improved formability and ductility. The steel contains: 15%≤C≤0.25%, 1.2%≤Si≤1.8%, 2%≤Mn≤2.4%, 0.1%≤Cr≤0.25%, Al≤0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature higher than Ac3 and lower than 1000° C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 250° C. and 350° C., to obtain a structure consisting of at least 60% of martensite and a sufficient austenite content such that the final structure contains 3% to 15% of residual austenite and 85% to 97% of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 430° C. and 480° C. and maintained at this temperature for a partitioning time Pt between 10 s and 90 s, then hot dip coated and cooled to the room temperature.

A steel sheet has a predetermined chemical composition and a metal structure represented by, in area fraction, polygonal ferrite: 40% or less, martensite: 20% or less, bainitic ferrite: 50% to 95%, and retained austenite: 5% to 50%. In area fraction, 80% or more of the bainitic ferrite is composed of bainitic ferrite grains that have an aspect ratio of 0.1 to 1.0 and have a dislocation density of 8×10.sup.2 (cm/cm.sup.3) or less in a region surrounded by a grain boundary with a misorientation angle of 15° or more. In area fraction, 80% or more of the retained austenite is composed of retained austenite grains that have an aspect ratio of 0.1 to 1.0, have a major axis length of 1.0 μm to 28.0 μm, and have a minor axis length of 0.1 μm to 2.8 μm.

A high-strength steel sheet has a specific composition and a microstructure. In the microstructure, the area fraction of elongated ferrite phase grains having an aspect ratio of 3 or more is 1% or less, the average crystal grain size of martensite included in a region extending 50 μm from a surface of the steel sheet is 20 μm or less, the content of oxide particles having a minor axis length of 0.8 μm or less in the region extending 50 μm from the surface of the steel sheet is 1.0×10.sup.10 particles/m.sup.2 or more, and the content of coarse oxide particles having a minor axis length of more than 1 μm in the region extending 50 μm from the surface of the steel sheet is 1.0×10.sup.8 particles/m.sup.2 or less. The content of hydrogen trapped in the steel sheet is 0.05 ppm by mass or more.

The steel material according to the present disclosure has a chemical composition consisting of, in mass %, C: 0.20 to 0.35%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr 0.25 to 0.80%, Mo: 0.20 to 2.00%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.0020 to 0.0100% and O: 0.0100% or less, with the balance being Fe and impurities, and satisfying Formula (1). A number density of precipitates having an equivalent circular diameter of 400 nm or more is 0.150 particles/μm.sup.2 or less. The yield strength is within a range of 655 to 965 MPa. A dislocation density ρ is 7.0×10.sup.14 m.sup.−2 or less.
5×Cr—Mo-2×(V+Ti)≤3.00  (1)

This hot-rolled steel sheet has a predetermined chemical composition, in which in a case where the thickness is denoted by t, a metallographic structure at a t/4 position from the surface includes, by area fraction, 77.0% to 97.0% of bainite or tempered martensite, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite, in the metallographic structure, the average grain size excluding the residual austenite is 7.0 μm or less, the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10.sup.6 carbides/mm.sup.2 or more, a tensile strength is 980 MPa or more, and an average Ni concentration on the surface is 7.0% or more.

The present invention relates to the application of at least partially bainitic or interstitial martensitic heat treatments on steels, often tool steels or steels that can be used for tools. The first tranche of the heat treatment implying austenitization is applied so that the steel presents a low enough hardness to allow for advantageous shape modification, often trough machining. Thus a steel product is obtained which can be shaped with ease and whose hardness can be raised to a higher working hardness with a simple heat treatment at low temperature (below austenitization temperature).

A steel sheet includes a predetermined chemical composition and a metal structure represented by, in area fraction, ferrite: 50% to 95%, granular bainite: 5% to 48%, martensite: 2% to 30%, and upper bainite, lower bainite, tempered martensite, retained austenite, and pearlite: 5% or less in total.

A cold rolled and heat treated steel sheet having a composition with the following elements, expressed in percentage by weight: 0.10%≤Carbon≤0.5%,1%≤Manganese≤3.4%, 0.5%≤Silicon≤2.5%, 0.03%≤Aluminum≤1.5%, 0%≤Sulfur≤0.003% 0.002%≤Phosphorus≤0.02%, 0%≤Nitrogen≤0.01% and can contain one or more of the following optional elements 0.05%≤Chromium≤1%, 0.001%≤Molybdenum≤0.5%, 0.001%≤Niobium≤0.1%, 0.001%≤Titanium≤0.1%, 0.01%≤Copper≤2%, 0.01%≤Nickel≤3%, 0.0001%≤Calcium≤0.005%, 0%≤Vanadium≤0.1%, 0%≤Boron≤0.003%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.010%, 0%≤Zirconium≤0.010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet having in area fraction, 10 to 30% Residual Austenite, 10 to 40% Bainite, 5% to 50% Annealed Martensite, 1% to 20% Quenched Martensite and less than 30% Tempered Martensite, wherein the cumulated amounts of Bainite and Residual Austenite is more than or equal to 25%.