Die Erfindung betrifft ein Verfahren zum Erzeugen eines hochfesten Warmbands mit einer Zugfestigkeit von mindestens 700 MPa und einer ultrafeinen ferritisch/martensitischen und perlitfreien Kornstruktur, bei der der durchschnittliche Durchmesser der Ferritkörner weniger als 2,5 µm beträgt. Dies wird erreicht durch folgende Schritte:

• a) Vergießen einer (in Masse-%) C: 0,05 - 0,2 %, Si: < 0,9 %, P: < 0,06 %, Mn: 0,6 - 1,2 %, Al: < 0,05 %, Cr: 0,02 - 0,6 %, Nb: ≤ 0,08 %, Ti: ≤ 0,08 %, V: ≤ 0,08 %, Mo: ≤ 0,4 %, Cu: ≤ 1 %, Ni: ≤ 1 %, Rest Eisen und unvermeidbare Verunreinigungen enthaltenden Stahlschmelze zu einem Vormaterial, wie Brammen oder Dünnbrammen,
• b) Warmwalzen des Vormaterials zu einem Warmband bei einer 750 °C bis 950 °C betragenden Warmwalzendtemperatur,
• c) Kühlen des erhaltenen Warmbands auf eine Raumtemperatur bis 250 °C betragenden Haspeltemperatur mit einer mindestens 10 K/s betragenden Abkühlgeschwindigkeit,
• d) Haspeln des abgekühlten Warmbands.

Die Veröffentlichung soll ohne Figur erfolgen.

A high strength and high ductility low carbon steel having a tensile strength of 800MPa or more, an uniform elongation of 5% or more, and an elongation to failure of 20% or more which is produced by a method comprising subjecting an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount being 0.01% or less and effective for accelerating martensitic transformation to processing and heat treatment to prepare a product having coarser size of austenite crystal grains and then to water-quenching, to provide a steel product having a martensite phase in an amount of 90% or more, and subjecting the steel product to a low strain processing, specifically a cold rolling at a total rolling reduction in thickness of 20% or more and less than 80%, and to a low temperature annealing at 500°C to 600°C, and a method for producing said high strength and high ductility low carbon steel.

A method of producing a ultra fine grain steel made of ferrite having a mean grain size of not larger that 3 µm as the base phase, after ingoting raw materials, by austenitizing the ingot by heating it to a temperature of at least an Ac 3 point, then, applying compression working of a reduction ratio of at least 50% at a temperature of from an Ae 3 point or lower to an Ar 3 point - 150°C, or at a temperature of at least 550°C, and thereafter, cooling, wherein the strain rate as compression working is in the range of from 0.001 to 10/second.

A steel product having a structure composed mainly of ferrite or ferrite plus pearlite or ferrite plus cementite. A steel pipe produced from this steel product by rolling at a ferrite recrystallization temperature such that the reduction of area is greater than 20%. This steel pipe is characterized by grain size not greater than 3 µm, preferably not greater than 1 µm, elongation greater than 20%, tensile strength (TS : MPa) and elongation (El : %) whose product is greater than 10000, and percent ductile fracture greater than 95%, preferably 100%, measured by Charpy impact test on an actual pipe at -100°C. The structure is characterized by C : 0.005-0.03%, Si : 0.01-3.0%, Mn : 0.01-2.0%, and Al : 0.001-0.10% on a weight basis, and is composed of ferrite or ferrite and a secondary phase, with ferrite grains being not greater than 3 µm and the secondary phase having an areal ratio not more than 30%. The steel pipe is produced from a steel pipe stock having the above-mentioned composition by heating at a temperature of (Ac₁ + 50°C) to 400°C and subsequently performing reducing on it at a rolling temperature of (Ac₁ + 50°C) to 400°C such that the cumulative reduction of diameter is greater than 20%. The reducing is preferably performed such that at least one of rolling passes reduces the diameter by more than 6% per pass. The steel pipe will have high ductility and high strength and will be superior in toughness and stress corrosion cracking resistance, if the content of C, Si, Mn, and other alloying elements is limited low and reducing is performed at the temperature specified above. The resulting steel pipe has good fatigue resistance and is suitable for use as line pipe.

A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (µm), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac₃ transformation point, and applying reducing at an average rolling temperature of θm (°C) and a total reduction ration Tred (%) within s temperature range of from 400 to Ac₃ transformation point, with di, θm and Tred being in a relation satisfying a prescribed equation.