EP2489748B1 - Hot-rolled steel surface product produced from a complex phase steel and method for the manufacture - Google Patents

Hot-rolled steel surface product produced from a complex phase steel and method for the manufacture Download PDF

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Publication number
EP2489748B1
EP2489748B1 EP11154973.9A EP11154973A EP2489748B1 EP 2489748 B1 EP2489748 B1 EP 2489748B1 EP 11154973 A EP11154973 A EP 11154973A EP 2489748 B1 EP2489748 B1 EP 2489748B1
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EP
European Patent Office
Prior art keywords
steel
weight
content
complex phase
hot
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EP11154973.9A
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German (de)
French (fr)
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EP2489748A9 (en
EP2489748A1 (en
Inventor
Brigitte Dr. Hammer
Jörg Dipl.-Ing. Mertens
Günther Dipl.-Ing Stich
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel Europe AG
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Priority to EP11154973.9A priority Critical patent/EP2489748B1/en
Application filed by ThyssenKrupp Steel Europe AG filed Critical ThyssenKrupp Steel Europe AG
Priority to KR1020137024831A priority patent/KR20140005293A/en
Priority to CN201180067938.XA priority patent/CN103380217B/en
Priority to PCT/EP2011/074251 priority patent/WO2012110165A1/en
Priority to CA2825240A priority patent/CA2825240A1/en
Priority to JP2013553818A priority patent/JP5864619B2/en
Priority to US13/985,420 priority patent/US20140041767A1/en
Publication of EP2489748A1 publication Critical patent/EP2489748A1/en
Publication of EP2489748A9 publication Critical patent/EP2489748A9/en
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0268—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the invention relates to a hot rolled flat steel product made from a complex phase steel and to a process for producing such a product.
  • a flat steel cold-rolled from a dual-phase steel which, in addition to a tensile strength of at least 950 MPa and good ductility, also has a surface finish which allows, using a simple manufacturing process, the flat product produced from this steel in uncoated or anti-corrosion coating provided state to deform a complex-shaped component, such as a part of an automobile body.
  • the steel according to the invention consists of 20-70% of martensite, up to 8% of retained austenite and the remainder of ferrite and / or bainite and (in% by weight): C: 0.10-0, 20%, Si: 0.10-0.60%, Mn: 1.50-2.50%, Cr: 0.20-0.80%, Ti: 0.02-0.08%, B: ⁇ 0.0020%, Mo: ⁇ 0.25%, Al: ⁇ 0.10%, P: ⁇ 0.2%, S: ⁇ 0.01%, N: ⁇ 0.012%, and the remainder contains iron and unavoidable impurities ,
  • the in practice of such steel produced flat steel products achieve tensile strengths of up to 1050 MPa.
  • a cooling of the hot strip with a cooling speed on the outlet roller table of at least 30 ° C / s so that the conversion of the steel is largely carried out in the bainite and a conversion of the steel is avoided to perlite.
  • Parts of martensite in the structure of the hot strip can further increase the tensile strengths.
  • the comparatively rapid cooling contributes to the precipitation of very fine particles, which further increases the strength.
  • the cooling process is to be stopped at a temperature below 600 ° C by winding the tape on a reel and then cooling in the coil on.
  • the hot strip thus obtained regularly reaches tensile strengths of up to 1150 MPa.
  • the steel sheet should be (in mass%) 0.05 - 0.3% C, 0.01 - 3.0% Si, 0.5 - 3.0% Mn, 0.01 - 0.1% Al and as Containing residual Fe and unavoidable impurities and have a structure consisting mainly of tempered martensite and annealed bainite.
  • the object of the invention was to provide a flat steel product in which further increased tensile strengths with good elongation properties and thus accompanied by good deformation properties are combined. Likewise, a method for producing such a flat steel product should be specified.
  • the complex phase steel used for the production of a hot rolled flat steel product according to the invention contains, in addition to iron and unavoidable impurities (in% by weight) C: 0.13-0.2%, Mn: 1.8-2.5%, Si: 0.70 - 1.3%, Al: up to 0.1%, P: up to 0.1%, S: up to 0.01%, Cr: 0.25 - 0.70%, optionally Mo, where the sum the Cr and Mo contents are 0.25-0.7%, Ti: 0.08-0.2%, and B: 0.0005-0.005%.
  • a steel flat product hot-rolled from the steel according to the invention has high strength combined with good elongation.
  • the microstructure of a flat steel product according to the invention is due to its narrow limits selected alloy characterized in that its structure consists of at most 10% by volume of retained austenite, 10 to 60% by volume of martensite, at most 30% by volume of ferrite and the remainder of bainite, the proportion being at least 10% Vol .-% should be.
  • Perlite is present in a steel flat product according to the invention at most in ineffective traces, the perlite content is reduced as possible to a minimum.
  • flat steel products according to the invention thus achieve a tensile strength Rm which is more than 1100 MPa, in particular regularly reaches at least 1150 MPa and more, and a yield strength Re of likewise regularly at least 720 MPa.
  • Rm tensile strength
  • Re yield strength
  • Carbon is added in the complex phase steel used according to the invention for the surface hardening and for the formation of ultrafine precipitates.
  • C in the inventively predetermined contents of 0.13-0.2% by weight, a sufficiently high martensite and bainite content for the desired hardness is formed in the microstructure.
  • carbon hinders the formation of the desired high bainitic structure content.
  • higher C contents have a negative effect on the weldability, which is for the application of the material according to the invention, for example in the field of automotive engineering is of particular importance.
  • the advantageous effect of carbon in a steel used to produce a flat steel product according to the invention can be used particularly reliably if the C content is 0.15-0.18% by weight, in particular not more than 0.17% by weight.
  • Manganese at a level of at least 1.8% by weight, retards the conversion and causes the formation of hard, strength increasing conversion products. Thus, the presence of Mn supports the formation of martensite. To avoid unduly high microsegregations, the content is according to the invention to max. 2.5 wt .-% limited, with the beneficial effects of Mn then occur particularly safe when the Mn content of the steel according to the invention is limited to 2.05-2.2 wt .-%.
  • Si also serves to increase the strength by, on the one hand, promoting the solid-solution hardening of the ferrite or bainite and, on the other hand, stabilizing the retained austenite.
  • the retained austenite content contributes to increasing elongation and strength (TRIP effect).
  • steel according to the invention has 0.70-1.3% by weight of Si, in particular at least 0.75% by weight of Si.
  • the strength and elongation-increasing effect occurs in particular when the Si content of a steel according to the invention is at least 0.75% by weight, in particular at least 0.85% by weight.
  • the steel constituting the flat steel product of the invention is Al-killed.
  • Aluminum is used in the melting of a steel according to the invention for deoxidizing and for setting nitrogen which may be present in the steel.
  • Al may be added in amounts of less than 0.1% by weight to the steel according to the invention, the desired effect of Al occurring particularly safely if its contents in the range of 0.01-0.06 wt. -%, in particular 0.020 - 0.050 wt .-%, are.
  • Phosphorus can be used to further increase the solid solution hardening, but for reasons of weldability should not exceed a content of 0.1 wt .-% because of the otherwise increasing risk of the formation of segregations.
  • Chromium inhibits ferrite and pearlite formation at levels of at least 0.25% by weight. Accordingly, it promotes the formation of a hardened structure and thus the strength of the steel used for the flat steel product according to the invention. In order not to delay the conversion too much, its content should be reduced to max. 0.7 wt .-% are limited.
  • the Cr content of a steel according to the invention By limiting the Cr content of a steel according to the invention to 0.7% by weight, the risk of grain boundary oxidation is reduced and the good elongation properties of the steel according to the invention are ensured. Also, adhering to this upper limit, a surface of the steel flat product produced from the steel is achieved, which can be well provided with a metallic coating.
  • the optionally present levels of molybdenum, like Cr, contribute to increasing the strength of a steel according to the invention by promoting the formation of ultrafine precipitates and martensite in the structure of the steel.
  • the presence of Mo does not adversely affect the coatability of the flat product with a metallic coating and its ductility. Practical experiments have shown that the positive effects of Mo up to contents of 0.25% by weight, in particular 0.22% by weight, can be used particularly effectively, even from a cost point of view. For example, contents of 0.05% by weight Mo have a positive effect on the properties of the steel according to the invention.
  • the sum of the Cr and Mo contents in a steel used according to the invention is limited to 0.25-0.7% by weight.
  • titanium in contents of at least 0.08 to at most 0.2 wt .-%, in particular 0.09 to 0.15 wt .-%, can be in inventive steel, the formation of ultrafine precipitates in the form of TiC or Ti ( C, N) with hardening effect and cause grain refining.
  • Another positive effect of Ti is the setting of possibly present nitrogen, so that the formation of boron nitrides in the steel according to the invention is prevented.
  • the presence of Ti thus also ensures, in the case of an addition of boron to increase the strength, that the boron can fully develop its effect in the dissolved state.
  • the positive effect of Ti in a steel according to the invention can be used with particular reliability if its Ti content is 0.11-0.13% by weight.
  • Boron improves hardenability in steel used in the present invention when B is present at levels of 0.0005-0.005 weight percent.
  • the favorable effects of B on the alloy according to the invention are particularly reliable when the B content of the steel according to the invention is set at 0.001-0.002% by weight.
  • procured flat steel products are characterized by a particularly high granularity, a high yield strength and increased strength.
  • the proportions of martensite, bainite and ultrafine precipitates in its structure contribute to the high strength.
  • the residual austenite and ferrite portions of the microstructure ensure its good elongation properties.
  • the hot strips can be provided with a metallic protective coating before or after their transformation into a component. This can be done by hot-dip galvanizing or electrolytic coating.
  • a molten steel with a below the alloy of according to the invention used steel cast to a precursor, which is typically a strand which is cut into slabs or thin slabs.
  • the precursor is heated to a temperature of 1150-1350 ° C to ensure a fully austenitic structure of the steel for the subsequent hot rolling and to bring the precipitates into solution.
  • the precursor is then hot rolled to a hot strip, the final temperature of hot rolling being 800-950 ° C.
  • the rolling end temperature should be in the range of the homogeneous austenite and thus not below 800 ° C in order to keep deformation-induced precipitations low and to enable the expression of the desired structure composition.
  • the hot strip obtained is cooled at a cooling rate which is at least 30 ° C / s, to the respectively selected coiler temperature.
  • the cooling conditions are to be chosen so that a conversion to perlite is avoided and the conversion is largely carried out so that the high bainite levels and the inventively given proportions of martensite and retained austenite are obtained.
  • the cooling process is terminated when the inventively predetermined range of reel temperature of 400 - 570 ° C. is reached, in which the Bainitcut of the steel according to the invention is reached.
  • the correspondingly cooled hot strip is then wound into a coil and cooled further in the coil. This leads to further transformations in bainite and martensite and to the formation of excretion.
  • steel according to the invention is particularly suitable for the production of highly loaded profiles in practical use as well as for crash and strength-relevant components for vehicle bodies.
  • the blocks were heated to 1270 ° C and hot rolled from this temperature to hot strip with a thickness of 2.5 mm.
  • the hot rolling end temperature was 900 ° C.
  • the hot strip obtained after hot rolling at a cooling rate of 80 ° C / s and at a temperature of 490 ° C in the oven was slowly cooled to simulate cooling in the coil.
  • the obtained hot strip had a tensile strength Rm of 1192 MPa and an elongation A80 of 10.5% transversely to the rolling direction.
  • the microstructure obtained consists of 35-40 vol.% Martensite, about 5 vol.% Ferrite, 6 vol.% Retained austenite and the remainder bainite.
  • the hot strips produced in the manner described above after hot rolling were first cooled to a temperature of 75 ° C and then slowly further in the oven to room temperature, to simulate cooling in the coil here.
  • the hot strips thus obtained had a tensile strength Rm of 1550 MPa and a comparatively low elongation A80 of 5.9%. They were predominantly martensitic.
  • the above-described hot strips after hot rolling were first cooled to a temperature corresponding to the "coiler temperature” of 600 ° C and then again slowly cooled to room temperature to simulate the cooling in the coil.
  • the hot strips thus obtained had a tensile strength Rm of 955 MPa and an elongation A80 of 15.5%.
  • the microstructure consisted of ferrite with a perlite content of 25-30% by volume.
  • the blocks were heated to 1270 ° C and hot rolled from this temperature to hot strip with a thickness of 2.5 mm.
  • the hot rolling end temperature was 900 ° C.
  • the hot strip obtained after hot rolling at a cooling rate of 80 ° C / s has been cooled to a "reel temperature" of 550 ° C, from which in turn the Coilabkühlung has been simulated in the manner already described above.
  • the resulting hot strip had a tensile strength Rm of 1180 MPa and an elongation A80 of 11%.
  • Their structure had a martensite content of 35-40% by volume, a residual austenite content of 7.5% by volume, a ferrite content of 10% by volume and the remainder bainite
  • a steel with the alloy according to the invention indicated in Table 3 has been melted and cast into a strand.
  • the slabs separated from the strand are then reheated to a temperature of about 1260 ° C, then hot rolled with a hot rolling temperature WET to hot strips with a thickness D and finally cooled at a cooling rate V T on a coiling temperature HT, in which they to a Coil have been reeled.
  • the respectively set parameters and the mechanical properties of the resulting hot strips are given in Table 4.
  • the hot strip obtained in the operating trial 3c had a significantly lower tensile strength than the hot strips obtained in the temperature range according to the invention, due to the high coiling temperature due to a high ferrite content (and perlite).

Description

Die Erfindung betrifft ein aus einem Komplexphasenstahl hergestelltes warmgewalztes Stahlflachprodukt und ein Verfahren zur Herstellung eines solchen Produktes.The invention relates to a hot rolled flat steel product made from a complex phase steel and to a process for producing such a product.

Aus der EP 2 028 282 A1 ist ein aus einem Dualphasenstahl kaltgewalztes Stahlflachprodukt bekannt, der neben einer Zugfestigkeit von mindestens 950 MPa und einer guten Verformbarkeit auch eine Oberflächenbeschaffenheit aufweist, die es unter Anwendung eines einfachen Herstellverfahrens erlaubt, das aus diesem Stahl erzeugte Flachprodukt im unbeschichteten oder mit einem vor Korrosion schützenden Überzug versehenen Zustand zu einem komplex geformten Bauteil, wie einem Teil einer Automobilkarosserie, zu verformen. Dies wird dadurch erreicht, dass der erfindungsgemäße Stahl zu 20 - 70 % aus Martensit, bis zu 8 % aus Restaustenit und als Rest aus Ferrit und / oder Bainit besteht und (in Gew.-%): C: 0,10 - 0,20 %, Si: 0,10 - 0,60 %, Mn: 1,50 - 2,50 %, Cr: 0,20 - 0,80 %, Ti: 0,02 - 0,08 %, B: < 0,0020 %, Mo: < 0,25 %, Al: < 0,10 %, P: ≤ 0,2 %, S: ≤ 0,01 %, N: ≤ 0,012 % sowie als Rest Eisen und unvermeidbare Verunreinigungen enthält. Die in der Praxis aus solchem Stahl hergestellten Stahlflachprodukte erreichen Zugfestigkeiten von bis zu 1050 MPa.From the EP 2 028 282 A1 For example, a flat steel cold-rolled from a dual-phase steel is known, which, in addition to a tensile strength of at least 950 MPa and good ductility, also has a surface finish which allows, using a simple manufacturing process, the flat product produced from this steel in uncoated or anti-corrosion coating provided state to deform a complex-shaped component, such as a part of an automobile body. This is achieved by the fact that the steel according to the invention consists of 20-70% of martensite, up to 8% of retained austenite and the remainder of ferrite and / or bainite and (in% by weight): C: 0.10-0, 20%, Si: 0.10-0.60%, Mn: 1.50-2.50%, Cr: 0.20-0.80%, Ti: 0.02-0.08%, B: < 0.0020%, Mo: <0.25%, Al: <0.10%, P: ≤ 0.2%, S: ≤ 0.01%, N: ≤ 0.012%, and the remainder contains iron and unavoidable impurities , The in practice of such steel produced flat steel products achieve tensile strengths of up to 1050 MPa.

Eine andere Möglichkeit der Herstellung eines hochfesten Stahls ist in der EP 0 966 547 B1 beschrieben. Gemäß dem dort erläuterten Verfahren wird ein Stahl der (in Gew.-%) 0,10 - 0,20 % C, 0,30 - 0,60 % Si, 1,50 - 2,00 % Mn, max. 0,08 % P, 0,30 - 0,80 % Cr, bis 0,40 % Mo, bis 0,20 % Ti und/oder Zr, bis 0,08 % Nb, Rest Fe und unvermeidbare Verunreinigungen enthält, erschmolzen, zu Brammen abgegossen und anschließend zu Warmband ausgewalzt. Die Walzendtemperatur liegt dabei oberhalb 800 °C. Anschließend erfolgt eine Abkühlung des Warmbands mit einer Abkühlgeschwindigkeit auf dem Auslaufrollgang von mindestens 30 °C/s, so dass die Umwandlung des Stahls weitestgehend in der Bainitstufe erfolgt und eine Umwandlung des Stahls zu Perlit vermieden wird. Anteile von Martensit im Gefüge des Warmbands können die Zugfestigkeiten weiter steigern. Des Weiteren trägt die vergleichbar schnelle Abkühlung zur Ausscheidung von feinsten Teilchen bei, durch die die Festigkeit weiter gesteigert wird. Der Abkühlvorgang ist bei einer Temperatur unter 600 °C zu beenden, indem das Band auf einen Haspel aufgewickelt wird und danach im Coil weiter abkühlt. Das so erhaltene Warmband erreicht regelmäßig Zugfestigkeiten von bis zu 1150 MPa.Another way of producing a high strength steel is in the EP 0 966 547 B1 described. According to the process described there, a steel of (in wt.%) 0.10-0.20% C, 0.30-0.60% Si, 1.50-2.00% Mn, max. 0.08% P, 0.30-0.80% Cr, up to 0.40% Mo, up to 0.20% Ti and / or Zr, up to 0.08% Nb, remainder containing Fe and unavoidable impurities, melted, poured into slabs and then rolled out to hot strip. The rolling end temperature is above 800 ° C. Subsequently, a cooling of the hot strip with a cooling speed on the outlet roller table of at least 30 ° C / s, so that the conversion of the steel is largely carried out in the bainite and a conversion of the steel is avoided to perlite. Parts of martensite in the structure of the hot strip can further increase the tensile strengths. Furthermore, the comparatively rapid cooling contributes to the precipitation of very fine particles, which further increases the strength. The cooling process is to be stopped at a temperature below 600 ° C by winding the tape on a reel and then cooling in the coil on. The hot strip thus obtained regularly reaches tensile strengths of up to 1150 MPa.

Des Weiteren ist in Pyshmintsev I. Y. et al. "Microstructure and properties of hot-rolled high strength multiphase steels for automotive application", STEEL RESEARCH, Düsseldorf, DE, Bd. 73, Nr. 9, 1. September 2002, S. 392 - 402, ISSN: 0177-4832 , über Ergebnisse einer Untersuchung der Wirkung des Zulegierens von Kombinationen der Elemente Mo, Cr und B auf das Umwandlungsverhalten von Bainit und die Mikrostruktur warmgewalzter hochfester Stähle für Feinbleche berichtet worden, die Gehalte an Ti und Nb, 0,05 und 0,15 Gew.-% C sowie 1,5 Gew.-% Mn aufweisen. Die betreffenden Untersuchungen bezogen sich auch auf die Wechselwirkung zwischen den Gefügen, die sich in den betrachteten Stählen während des Haspelvorgangs bei verschiedenen Temperaturen bilden, und den Festigkeitseigenschaften der untersuchten Stähle.Furthermore, in Pyshmintsev IY et al. "Microstructure and properties of hot-rolled high strength multiphase steels for automotive applications", STEEL RESEARCH, Dusseldorf, DE, Vol. 73, No. 9, September 1, 2002, pp. 392-402, ISSN: 0177-4832 reported results of an investigation of the effect of adding combinations of the elements Mo, Cr and B on the conversion behavior of bainite and the microstructure of hot rolled high-tensile steels for thin plates, the contents of Ti and Nb, 0.05 and 0.15 wt. -% C and 1.5 wt .-% Mn have. The investigations concerned also the interaction between the microstructures formed in the considered steels during the coiling process at different temperatures and the strength properties of the steels investigated.

Schließlich ist aus der US 2009/0277547 A1 ein hochfestes Stahlblech bekannt, das ein optimales Dehnungs- und Verformungsverhalten besitzen soll. Das Stahlblech soll (in Masse-%) 0,05 - 0,3 % C, 0,01 - 3,0 % Si, 0,5 - 3,0 % Mn, 0,01 - 0,1 % Al und als Rest Fe und unvermeidbare Verunreinigungen enthalten und ein Gefüge aufweisen, das überwiegend aus angelassenem Martensit und geglühtem Bainit besteht.Finally, out of the US 2009/0277547 A1 a high-strength steel sheet is known, which should have an optimal expansion and deformation behavior. The steel sheet should be (in mass%) 0.05 - 0.3% C, 0.01 - 3.0% Si, 0.5 - 3.0% Mn, 0.01 - 0.1% Al and as Containing residual Fe and unavoidable impurities and have a structure consisting mainly of tempered martensite and annealed bainite.

Vor dem Hintergrund des voranstehend erläuterten Standes der Technik bestand die Aufgabe der Erfindung darin, ein Stahlflachprodukt zu schaffen, bei dem weiter gesteigerte Zugfestigkeiten mit guten Dehnungseigenschaften und damit einhergehend guten Verformungseigenschaften kombiniert sind. Ebenso sollte ein Verfahren zum Herstellen eines solchen Stahlflachproduktes angegeben werden.Against the background of the prior art described above, the object of the invention was to provide a flat steel product in which further increased tensile strengths with good elongation properties and thus accompanied by good deformation properties are combined. Likewise, a method for producing such a flat steel product should be specified.

In Bezug auf den Stahl ist diese Aufgabe durch einen Komplexphasenstahl mit der in Anspruch 1 genannten Zusammensetzung und Gefügestruktur gelöst worden.With regard to steel, this object has been achieved by a complex phase steel having the composition and microstructure specified in claim 1.

Das die voranstehend angegebene Aufgabe erfindungsgemäß lösende Verfahren ist durch die in Anspruch 14 angegebenen Maßnahmen gekennzeichnet.The above-mentioned object according to the invention-solving method is characterized by the measures specified in claim 14.

Vorteilhafte Ausgestaltungen der Erfindung sind in den abhängigen Ansprüchen angegeben und werden nachfolgend wie der allgemeine Erfindungsgedanke im Einzelnen erläutert.Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

Der für die Herstellung eines erfindungsgemäß warmgewalzten Stahlflachprodukts verwendete Komplexphasenstahl enthält neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) C: 0,13 - 0,2 %, Mn: 1,8 - 2,5 %, Si: 0,70 - 1,3 %, Al: bis zu 0,1 %, P: bis zu 0,1 %, S: bis zu 0,01 %, Cr: 0,25 - 0,70 %, optional Mo, wobei die Summe der Cr- und Mo-Gehalte 0,25 - 0,7 % beträgt, Ti: 0,08 - 0,2 % und B: 0,0005 - 0,005 %.The complex phase steel used for the production of a hot rolled flat steel product according to the invention contains, in addition to iron and unavoidable impurities (in% by weight) C: 0.13-0.2%, Mn: 1.8-2.5%, Si: 0.70 - 1.3%, Al: up to 0.1%, P: up to 0.1%, S: up to 0.01%, Cr: 0.25 - 0.70%, optionally Mo, where the sum the Cr and Mo contents are 0.25-0.7%, Ti: 0.08-0.2%, and B: 0.0005-0.005%.

Dank seines Komplexphasengefüges besitzt ein aus dem erfindungsgemäßen Stahl warmgewalztes Stahlflachprodukt eine hohe Festigkeit bei gleichzeitig guter Dehnung. Dabei ist das Gefüge eines erfindungsgemäßen Stahlflachprodukts in Folge seiner in engen grenzen ausgewählten Legierung dadurch gekennzeichnet, dass sein Gefüge höchstens zu 10 Vol.-% aus Restaustenit, zu 10 - 60 Vol.-% aus Martensit, zu höchstens 30 Vol.-% aus Ferrit und als Rest aus Bainit besteht, wobei der Anteil mindestens 10 Vol.-% betragen soll. Perlit ist in einem erfindungsgemäßen Stahlflachprodukt allenfalls in unwirksamen Spuren vorhanden, wobei der Perlit-Anteil möglichst auf ein Minimum reduziert ist.Thanks to its complex phase structure, a steel flat product hot-rolled from the steel according to the invention has high strength combined with good elongation. In this case, the microstructure of a flat steel product according to the invention is due to its narrow limits selected alloy characterized in that its structure consists of at most 10% by volume of retained austenite, 10 to 60% by volume of martensite, at most 30% by volume of ferrite and the remainder of bainite, the proportion being at least 10% Vol .-% should be. Perlite is present in a steel flat product according to the invention at most in ineffective traces, the perlite content is reduced as possible to a minimum.

Erfindungsgemäße Stahlflachprodukte erzielen so im warmgewalzten Zustand eine Zugfestigkeit Rm, die mehr als 1100 MPa beträgt, insbesondere regelmäßig mindestens 1150 MPa und mehr erreicht, und eine Streckgrenze Re von ebenso regelmäßig mindestens 720 MPa. Für seine Bruchdehnung A80 können gleichzeitig Werte von mehr als 7 %, insbesondere mehr als 8 %, garantiert werden. Diese hohe Festigkeit gepaart mit den vergleichbar guten Dehnungseigenschaften sind durch die erfindungsgemäße Einstellung des Komplexphasengefüges erzielt worden.In the hot-rolled state, flat steel products according to the invention thus achieve a tensile strength Rm which is more than 1100 MPa, in particular regularly reaches at least 1150 MPa and more, and a yield strength Re of likewise regularly at least 720 MPa. For its elongation at break A80 values of more than 7%, in particular more than 8%, can be guaranteed at the same time. This high strength paired with the comparable good elongation properties have been achieved by the adjustment of the complex phase structure according to the invention.

Kohlenstoff wird im erfindungsgemäß verwendeten Komplexphasenstahl zur Gefügehärtung und zur Bildung von Feinstausscheidungen beigefügt. So bildet sich durch die Anwesenheit von C in den erfindungsgemäß vorgegebenen Gehalten von 0,13 - 0,2 Gew.-% ein für die gewünschte Härte ausreichend hoher Martensit- und Bainitanteil im Gefüge. Bei Gehalten von mehr als 0,20 Gew.-% behindert Kohlenstoff die Entstehung des gewünscht hohen bainitischen Gefügeanteils. Auch wirken sich höhere C-Gehalte negativ auf die Schweißeignung aus, was für die Anwendung des erfindungsgemäßen Materials beispielsweise im Bereich des Automobilbaus von besonderer Bedeutung ist. Besonders sicher kann die vorteilhafte Wirkung von Kohlenstoff in einem zur Herstellung eines erfindungsgemäßen Stahlflachprodukts verwendeten Stahls dann genutzt werden, wenn der C-Gehalt 0,15 - 0,18 Gew.-%, insbesondere höchstens 0,17 Gew.-%, beträgt.Carbon is added in the complex phase steel used according to the invention for the surface hardening and for the formation of ultrafine precipitates. Thus, due to the presence of C in the inventively predetermined contents of 0.13-0.2% by weight, a sufficiently high martensite and bainite content for the desired hardness is formed in the microstructure. At levels greater than 0.20% by weight, carbon hinders the formation of the desired high bainitic structure content. Also, higher C contents have a negative effect on the weldability, which is for the application of the material according to the invention, for example in the field of automotive engineering is of particular importance. The advantageous effect of carbon in a steel used to produce a flat steel product according to the invention can be used particularly reliably if the C content is 0.15-0.18% by weight, in particular not more than 0.17% by weight.

Mangan verzögert bei einem Gehalt von mindestens 1,8 Gew.-% die Umwandlung und bewirkt die Bildung harter, festigkeitssteigernder Umwandlungsprodukte. So wird durch die Anwesenheit von Mn die Entstehung von Martensit unterstützt. Zur Vermeidung unzulässig starker Mikroseigerungen ist der Gehalt erfindungsgemäß auf max. 2,5 Gew.-% begrenzt, wobei die vorteilhaften Einflüsse von Mn dann besonders sicher eintreten, wenn der Mn-Gehalt des erfindungsgemäßen Stahls auf 2,05 - 2,2 Gew.-% beschränkt ist.Manganese, at a level of at least 1.8% by weight, retards the conversion and causes the formation of hard, strength increasing conversion products. Thus, the presence of Mn supports the formation of martensite. To avoid unduly high microsegregations, the content is according to the invention to max. 2.5 wt .-% limited, with the beneficial effects of Mn then occur particularly safe when the Mn content of the steel according to the invention is limited to 2.05-2.2 wt .-%.

Si dient in einem erfindungsgemäß verwendeten Stahl ebenfalls zur Steigerung der Festigkeit, indem es einerseits die Mischkristallhärtung des Ferrits bzw. Bainits unterstützt und andererseits den Restaustenit stabilisiert. Der Restaustenitanteil trägt dabei zur Erhöhung von Dehnung und Festigkeit bei (TRIP-Effekt). Um die angestrebt hohen mechanischen Kennwerte zu erreichen, weist erfindungsgemäßer Stahl 0,70 - 1,3 Gew.-% Si, insbesondere mindestens 0,75 Gew.-% Si auf. Die festigkeits- und dehnungssteigernde Wirkung tritt dabei insbesondere dann ein, wenn der Si-Gehalt eines erfindungsgemäßen Stahls mindestens 0,75 Gew.-%, insbesondere mindestens 0,85 Gew.-% beträgt. Im Hinblick darauf, dass ein aus einem erfindungsgemäßen Stahl erzeugtes Flachprodukt eine für die weitere Verarbeitung und erforderlichenfalls aufgetragene Beschichtungen optimale Oberflächenbeschaffenheit besitzen soll, ist gleichzeitig die Obergrenze des Si-Gehaltes auf 1,3 Gew.-% festgelegt worden. Auch ist bei Einhaltung dieser Obergrenzen die Gefahr von Korngrenzenoxidation minimiert. Dabei lässt sich ein ungünstiger Einfluss von Si auf die Eigenschaften des erfindungsgemäß verwendeten Stahls dadurch mit noch größerer Sicherheit vermeiden, dass der Si-Gehalt des erfindungsgemäßen Stahls auf 1,1 Gew.-%, insbesondere 0,95 Gew.-%, beschränkt ist.In a steel used according to the invention, Si also serves to increase the strength by, on the one hand, promoting the solid-solution hardening of the ferrite or bainite and, on the other hand, stabilizing the retained austenite. The retained austenite content contributes to increasing elongation and strength (TRIP effect). In order to achieve the desired high mechanical characteristics, steel according to the invention has 0.70-1.3% by weight of Si, in particular at least 0.75% by weight of Si. The strength and elongation-increasing effect occurs in particular when the Si content of a steel according to the invention is at least 0.75% by weight, in particular at least 0.85% by weight. In terms of The fact that a flat product produced from a steel according to the invention should have an optimum surface finish for further processing and, if necessary, applied coatings has at the same time set the upper limit of the Si content to 1.3% by weight. Even if these upper limits are adhered to, the risk of grain boundary oxidation is minimized. In this case, an unfavorable influence of Si on the properties of the steel used according to the invention can thereby be prevented with even greater certainty that the Si content of the steel according to the invention is limited to 1.1% by weight, in particular 0.95% by weight ,

Der Stahl, aus dem das erfindungsgemäße Stahlflachprodukt besteht, ist Al-beruhigt. Aluminium wird bei der Erschmelzung eines erfindungsgemäßen Stahls zur Desoxidation und zum Abbinden von gegebenenfalls in dem Stahl enthaltenem Stickstoff genutzt. Zu diesem Zweck kann dem erfindungsgemäßen Stahl erforderlichenfalls Al in Gehalten von weniger als 0,1 Gew.-% zugegeben werden, wobei die gewünschte Wirkung von Al dann besonders sicher eintritt, wenn dessen Gehalte im Bereich von 0,01 - 0,06 Gew.-%, insbesondere 0,020 - 0,050 Gew.-%, liegen.The steel constituting the flat steel product of the invention is Al-killed. Aluminum is used in the melting of a steel according to the invention for deoxidizing and for setting nitrogen which may be present in the steel. For this purpose, if necessary, Al may be added in amounts of less than 0.1% by weight to the steel according to the invention, the desired effect of Al occurring particularly safely if its contents in the range of 0.01-0.06 wt. -%, in particular 0.020 - 0.050 wt .-%, are.

Phosphor kann zur weiteren Steigerung der Mischkristallverfestigung eingesetzt werden, sollte aber aus Gründen der Schweißbarkeit wegen der sonst steigenden Gefahr der Bildung von Seigerungen einen Gehalt von 0,1 Gew.-% nicht übersteigen.Phosphorus can be used to further increase the solid solution hardening, but for reasons of weldability should not exceed a content of 0.1 wt .-% because of the otherwise increasing risk of the formation of segregations.

Bei unterhalb der erfindungsgemäß vorgegebenen Obergrenze liegenden Gehalten an Schwefel wird die Bildung von MnS bzw. (Mn,Fe)S in dem erfindungsgemäß verwendeten Stahl unterdrückt, so dass eine gute Dehnbarkeit des erfindungsgemäßen Stahlflachprodukts gewährleistet ist. Dies ist insbesondere dann der Fall, wenn der S-Gehalt unter 0,003 Gew.-% liegt.At below the inventively given upper limit levels of sulfur, the formation of MnS or (Mn, Fe) S is suppressed in the steel used in the invention, so that a good extensibility of the flat steel product according to the invention is ensured. This is especially the case when the S-content is less than 0.003 wt .-%.

Chrom behindert bei Gehalten von mindestens 0,25 Gew.-% die Ferrit- und Perlitbildung. Es fördert dementsprechend die Bildung eines Härtungsgefüges und damit die Festigkeit des für das erfindungsgemäße Stahlflachprodukt verwendeten Stahls. Um die Umwandlung nicht zu stark zu verzögern, sollte sein Gehalt auf max. 0,7 Gew.-% begrenzt werden. Indem der Cr-Gehalt eines erfindungsgemäßen Stahls auf 0,7 Gew.-% beschränkt ist, ist die Gefahr der Entstehung von Korngrenzenoxidation vermindert und die guten Dehnungseigenschaften des erfindungsgemäßen Stahls gesichert. Auch wird bei Einhaltung dieser Obergrenze eine Oberfläche des aus dem Stahl erzeugten Stahlflachproduktes erreicht, die gut mit einer metallischen Beschichtung versehen werden kann.Chromium inhibits ferrite and pearlite formation at levels of at least 0.25% by weight. Accordingly, it promotes the formation of a hardened structure and thus the strength of the steel used for the flat steel product according to the invention. In order not to delay the conversion too much, its content should be reduced to max. 0.7 wt .-% are limited. By limiting the Cr content of a steel according to the invention to 0.7% by weight, the risk of grain boundary oxidation is reduced and the good elongation properties of the steel according to the invention are ensured. Also, adhering to this upper limit, a surface of the steel flat product produced from the steel is achieved, which can be well provided with a metallic coating.

Die optional vorhandenen Gehalte an Molybdän tragen wie Cr zur Erhöhung der Festigkeit eines erfindungsgemäßen Stahls bei, indem es die Bildung von Feinstausscheidungen und von Martensit im Gefüge des Stahls unterstützt. Dabei wirkt sich die Anwesenheit von Mo nicht negativ auf die Beschichtbarkeit des Flachproduktes mit einer metallischen Beschichtung und seiner Dehnbarkeit aus. Praktische Versuche haben gezeigt, dass sich die positiven Einflüsse von Mo bis zu Gehalten von 0,25 Gew.-%, insbesondere 0,22 Gew.-%, auch unter Kostengesichtspunkten besonders effektiv nutzen lassen. So wirken sich bereits Gehalte von 0,05 Gew.-% Mo positiv auf die Eigenschaften des erfindungsgemäßen Stahls aus.The optionally present levels of molybdenum, like Cr, contribute to increasing the strength of a steel according to the invention by promoting the formation of ultrafine precipitates and martensite in the structure of the steel. The presence of Mo does not adversely affect the coatability of the flat product with a metallic coating and its ductility. Practical experiments have shown that the positive effects of Mo up to contents of 0.25% by weight, in particular 0.22% by weight, can be used particularly effectively, even from a cost point of view. For example, contents of 0.05% by weight Mo have a positive effect on the properties of the steel according to the invention.

Um jedoch zu verhindern, dass die Bruchdehnung durch einen zu hohen Martensitanteil negativ beeinflusst wird, ist die Summe der Cr- und Mo-Gehalte in einem erfindungsgemäß verwendeten Stahl auf 0,25 - 0,7 Gew.-% beschränkt.However, in order to prevent the elongation at break from being adversely affected by an excessively high martensite content, the sum of the Cr and Mo contents in a steel used according to the invention is limited to 0.25-0.7% by weight.

Mit Titan in Gehalten von mindestens 0,08 bis höchstens 0,2 Gew.-%, insbesondere 0,09 - 0,15 Gew.-%, lässt sich in erfindungsgemäßem Stahl die Bildung von Feinstausscheidungen in Form von von TiC bzw. Ti(C,N) mit aushärtender Wirkung unterstützen und eine Kornfeinung bewirken. Eine weitere positive Wirkung von Ti besteht in der Abbindung eventuell vorhandenen Stickstoffs, so dass die Bildung von Bornitriden im erfindungsgemäßen Stahl verhindert wird. Durch die Anwesenheit von Ti wird somit im Fall einer Zugabe von Bor zur Festigkeitssteigerung auch sichergestellt, dass das Bor seine Wirkung im gelösten Zustand voll entfalten kann. Besonders sicher kann die positive Wirkung von Ti in einem erfindungsgemäßen Stahl genutzt werden, wenn sein Ti-Gehalt 0,11 - 0,13 Gew.-% beträgt.With titanium in contents of at least 0.08 to at most 0.2 wt .-%, in particular 0.09 to 0.15 wt .-%, can be in inventive steel, the formation of ultrafine precipitates in the form of TiC or Ti ( C, N) with hardening effect and cause grain refining. Another positive effect of Ti is the setting of possibly present nitrogen, so that the formation of boron nitrides in the steel according to the invention is prevented. The presence of Ti thus also ensures, in the case of an addition of boron to increase the strength, that the boron can fully develop its effect in the dissolved state. The positive effect of Ti in a steel according to the invention can be used with particular reliability if its Ti content is 0.11-0.13% by weight.

Bor verbessert bei erfindungsgemäß verwendetem Stahl die Härtbarkeit, wenn B in Gehalten von 0,0005 - 0,005 Gew.-% vorhanden ist. Im Austenit seigert Bor an die Korngrenzen und behindert die Ferrit- und Perlitbildung. Dabei ruft Bor eine signifikante Steigerung der Festigkeit bei nur geringer Erniedrigung der Umformbarkeit hervor. Die günstigen Einflüsse von B auf die erfindungsgemäße Legierung stellen sich dann besonders sicher ein, wenn der B-Gehalt des erfindungsgemäßen Stahls auf 0,001 - 0,002 Gew.-% festgelegt ist.Boron improves hardenability in steel used in the present invention when B is present at levels of 0.0005-0.005 weight percent. In austenite, boron seizes the grain boundaries and hinders ferrite and pearlite formation. Boron causes a significant increase in strength with little reduction in formability. The favorable effects of B on the alloy according to the invention are particularly reliable when the B content of the steel according to the invention is set at 0.001-0.002% by weight.

In erfindungsgemäßer Weise beschaffene Stahlflachprodukte zeichnen sich durch eine besonders hohe Feinkörnigkeit, eine hohe Streckgrenze und eine erhöhte Festigkeit aus. Die in seinem Gefüge enthaltenen Anteile an Martensit, Bainit und Feinstausscheidungen tragen zur hohen Festigkeit bei. Die Restaustenit- und Ferritanteile des Gefüges stellen seine guten Dehnungseigenschaften sicher.In accordance with the invention procured flat steel products are characterized by a particularly high granularity, a high yield strength and increased strength. The proportions of martensite, bainite and ultrafine precipitates in its structure contribute to the high strength. The residual austenite and ferrite portions of the microstructure ensure its good elongation properties.

Sollen erfindungsgemäß erzeugte Stahlflachprodukte besonders gegen Korrosion geschützt werden, so können die Warmbänder vor oder nach ihrer Umformung zu einem Bauteil mit einem metallischen Schutzüberzug versehen werden. Dies kann durch Feuerverzinken, oder elektrolytisches Beschichten erfolgen.If steel flat products produced according to the invention are to be particularly protected against corrosion, the hot strips can be provided with a metallic protective coating before or after their transformation into a component. This can be done by hot-dip galvanizing or electrolytic coating.

Bei der erfindungsgemäßen Erzeugung eines erfindungsgemäßen warmgewalzten Stahlflachprodukts mit einer Zugfestigkeit von mehr als 1100 MPa und dem voranstehend erläuterten Gefüge wird zunächst eine Stahlschmelze mit einer unter die Legierung des erfindungsgemäß verwendeten Stahls fallende Zusammensetzung zu einem Vorprodukt vergossen, bei dem es sich typischerweise um einen Strang, der zu Brammen oder Dünnbrammen geschnitten wird, handelt.In the inventive production of a hot rolled flat steel product according to the invention with a tensile strength of more than 1100 MPa and the structure explained above, first a molten steel with a below the alloy of according to the invention used steel cast to a precursor, which is typically a strand which is cut into slabs or thin slabs.

Anschließend wird das Vorprodukt auf eine 1150 - 1350 °C betragende Temperatur erwärmt, um für das anschließend erfolgende Warmwalzen ein vollständig austenitisches Gefüge des Stahls zu gewährleisten und die Ausscheidungen in Lösung zu bringen.Subsequently, the precursor is heated to a temperature of 1150-1350 ° C to ensure a fully austenitic structure of the steel for the subsequent hot rolling and to bring the precipitates into solution.

Ausgehend von dieser Erwärmungstemperatur wird das Vorprodukt dann zu einem Warmband warmgewalzt, wobei die Endtemperatur des Warmwalzens 800 - 950 °C beträgt. Die Walzendtemperatur sollte im Bereich des homogenen Austenits und damit nicht unter 800 °C liegen, um verformungsinduzierte Ausscheidungen gering zu halten und die Ausprägung der angestrebten Gefügezusammensetzung zu ermöglichen.Starting from this heating temperature, the precursor is then hot rolled to a hot strip, the final temperature of hot rolling being 800-950 ° C. The rolling end temperature should be in the range of the homogeneous austenite and thus not below 800 ° C in order to keep deformation-induced precipitations low and to enable the expression of the desired structure composition.

Nach dem Warmwalzen wird das erhaltene Warmband mit einer Abkühlgeschwindigkeit, die mindestens 30 °C/s beträgt, auf die jeweils gewählte Haspeltemperatur abgekühlt. Die Abkühlbedingungen sind so zu wählen, dass eine Umwandlung zu Perlit vermieden wird und die Umwandlung weitestgehend so erfolgt, dass die hohen Bainit-Anteile sowie die erfindungsgemäß vorgegebenen Anteile an Martensit und Restaustenit erhalten werden.After hot rolling, the hot strip obtained is cooled at a cooling rate which is at least 30 ° C / s, to the respectively selected coiler temperature. The cooling conditions are to be chosen so that a conversion to perlite is avoided and the conversion is largely carried out so that the high bainite levels and the inventively given proportions of martensite and retained austenite are obtained.

Der Abkühlvorgang wird beendet, wenn der erfindungsgemäß vorgegebene Bereich der Haspeltemperatur von 400 - 570 °C erreicht ist, in der die Bainitstufe des erfindungsgemäßen Stahls erreicht ist. Das entsprechend abgekühlte Warmband wird dann zu einem Coil gewickelt und kühlt im Coil weiter ab. Dabei kommt es zu weiteren Umwandlungen in Bainit und Martensit und zur Ausscheidungsbildung.The cooling process is terminated when the inventively predetermined range of reel temperature of 400 - 570 ° C. is reached, in which the Bainitstufe of the steel according to the invention is reached. The correspondingly cooled hot strip is then wound into a coil and cooled further in the coil. This leads to further transformations in bainite and martensite and to the formation of excretion.

Aufgrund seiner besonderen Kombination aus hoher Festigkeit und guten Dehnungseigenschaften eignet sich erfindungsgemäßer Stahl insbesondere zur Herstellung von im praktischen Einsatz hoch belasteten Profilen sowie für crash- und festigkeitsrelevante Bauteile für Fahrzeugkarosserien.Due to its special combination of high strength and good elongation properties, steel according to the invention is particularly suitable for the production of highly loaded profiles in practical use as well as for crash and strength-relevant components for vehicle bodies.

AUSFÜHRUNGSBEISPIELEEMBODIMENTS Versuch 1Trial 1

Unter Laborbedingungen ist ein Stahl mit der in Tabelle 1 angegebenen Zusammensetzung erschmolzen und zu Blöcken vergossen worden.Under laboratory conditions, a steel having the composition given in Table 1 has been melted and cast into blocks.

Anschließend sind die Blöcke auf 1270 °C erwärmt und ausgehend von dieser Temperatur zu Warmband mit einer Dicke von 2,5 mm warmgewalzt worden. Die Warmwalzendtemperatur betrug 900 °C.Subsequently, the blocks were heated to 1270 ° C and hot rolled from this temperature to hot strip with a thickness of 2.5 mm. The hot rolling end temperature was 900 ° C.

Das erhaltene Warmband ist nach dem Warmwalzen mit einer Abkühlgeschwindigkeit von 80 °C/s und bei einer Temperatur von 490 °C im Ofen langsam abgekühlt worden, um das Abkühlen im Coil zu simulieren.The hot strip obtained after hot rolling at a cooling rate of 80 ° C / s and at a temperature of 490 ° C in the oven was slowly cooled to simulate cooling in the coil.

Das erhaltene Warmband wies quer zur Walzrichtung eine Zugfestigkeit Rm von 1192 MPa und eine Dehnung A80 von 10,5 % auf. Das erhaltene Gefüge besteht aus 35 - 40 Vol.-% Martensit, etwa 5 Vol.-% Ferrit, 6 Vol.-% Restaustenit und als Rest aus Bainit.The obtained hot strip had a tensile strength Rm of 1192 MPa and an elongation A80 of 10.5% transversely to the rolling direction. The microstructure obtained consists of 35-40 vol.% Martensite, about 5 vol.% Ferrite, 6 vol.% Retained austenite and the remainder bainite.

Für einen ersten Vergleich sind die in der voranstehend erläuterten Weise erzeugten Warmbänder nach dem Warmwalzen zunächst auf eine Temperatur von 75 °C und anschließend im Ofen langsam weiter auf Raumtemperatur abgekühlt worden, um auch hier das Abkühlen im Coil zu simulieren. Die so erhaltenen Warmbänder wiesen eine Zugfestigkeit Rm von 1550 MPa und eine vergleichbar niedrige Dehnung A80 von 5,9 % auf. Sie waren überwiegend martensitisch.For a first comparison, the hot strips produced in the manner described above after hot rolling were first cooled to a temperature of 75 ° C and then slowly further in the oven to room temperature, to simulate cooling in the coil here. The hot strips thus obtained had a tensile strength Rm of 1550 MPa and a comparatively low elongation A80 of 5.9%. They were predominantly martensitic.

Für einen zweiten Vergleich sind die voranstehend erläuterten Warmbänder nach dem Warmwalzen zunächst auf eine der "Haspeltemperatur" entsprechende Temperatur von 600 °C abgekühlt und daraufhin wiederum langsam auf Raumtemperatur gekühlt worden, um die Abkühlung im Coil zu simulieren. Die so erhaltenen Warmbänder wiesen eine Zugfestigkeit Rm von 955 MPa und eine Dehnung A80 von 15,5 % auf. Das Gefüge bestand aus Ferrit mit einem Perlit-Anteil von 25 - 30 Vol.-%.For a second comparison, the above-described hot strips after hot rolling were first cooled to a temperature corresponding to the "coiler temperature" of 600 ° C and then again slowly cooled to room temperature to simulate the cooling in the coil. The hot strips thus obtained had a tensile strength Rm of 955 MPa and an elongation A80 of 15.5%. The microstructure consisted of ferrite with a perlite content of 25-30% by volume.

Versuch 2Trial 2

Ebenfalls unter Laborbedingungen ist ein Stahl mit der in Tabelle 2 angegebenen Zusammensetzung erschmolzen und zu Blöcken vergossen worden. Im Unterschied zu dem beim ersten Versuch untersuchten Stahl enthielt dieser Stahl zusätzlich 0,25 Gew.-% Mo.Also under laboratory conditions, a steel having the composition shown in Table 2 was melted and cast into blocks. Unlike the case of steel, this steel additionally contained 0.25 wt.% Mo.

Anschließend sind die Blöcke auf 1270 °C erwärmt und ausgehend von dieser Temperatur zu Warmband mit einer Dicke von 2,5 mm warmgewalzt worden. Die Warmwalzendtemperatur betrug 900 °C.Subsequently, the blocks were heated to 1270 ° C and hot rolled from this temperature to hot strip with a thickness of 2.5 mm. The hot rolling end temperature was 900 ° C.

Das erhaltene Warmband ist nach dem Warmwalzen mit einer Abkühlgeschwindigkeit von 80 °C/s auf eine "Haspeltemperatur" von 550 °C abgekühlt worden, ab der wiederum die Coilabkühlung in der voranstehend schon beschriebenen Weise simuliert worden ist.The hot strip obtained after hot rolling at a cooling rate of 80 ° C / s has been cooled to a "reel temperature" of 550 ° C, from which in turn the Coilabkühlung has been simulated in the manner already described above.

Das erhaltene Warmband wies eine Zugfestigkeit Rm von 1180 MPa und eine Dehnung A80 von 11 %. Ihr Gefüge wies einen Martensitanteil von 35 - 40 Vol.-%, einen Restaustenit-Gehalt von 7,5 Vol.-%, einen Ferrit-Gehalt von 10 Vol.-% und als Rest Bainit aufThe resulting hot strip had a tensile strength Rm of 1180 MPa and an elongation A80 of 11%. Their structure had a martensite content of 35-40% by volume, a residual austenite content of 7.5% by volume, a ferrite content of 10% by volume and the remainder bainite

Versuch 3Trial 3

Für einen Betriebsversuch 3a - 3c ist ein Stahl mit der in Tabelle 3 angegebenen erfindungsgemäßen Legierung erschmolzen und zum Strang vergossen worden. Die von dem Strang abgeteilten Brammen sind anschließend auf eine Temperatur von ca. 1260 °C wiedererwärmt, daraufhin mit einer Warmwalzendtemperatur WET zu Warmbändern mit einer Dicke D warmgewalzt und schließlich mit einer Abkühlrate VT auf einer Haspeltemperatur HT abgekühlt worden, bei der sie zu einem Coil gehaspelt worden sind. Die jeweils eingestellten Parameter und die mechanischen Eigenschaften der erhaltenen Warmbänder (quer zur Walzrichtung bestimmt) sind in Tabelle 4 angegeben.For an operational test 3a-3c, a steel with the alloy according to the invention indicated in Table 3 has been melted and cast into a strand. The slabs separated from the strand are then reheated to a temperature of about 1260 ° C, then hot rolled with a hot rolling temperature WET to hot strips with a thickness D and finally cooled at a cooling rate V T on a coiling temperature HT, in which they to a Coil have been reeled. The respectively set parameters and the mechanical properties of the resulting hot strips (determined transversely to the rolling direction) are given in Table 4.

Es zeigte sich, dass das beim Betriebsversuch 3c erhaltene Warmband aufgrund der zu hohen Haspeltemperatur durch einen hohen Ferritanteil (und Perlit) eine deutlich niedrigere Zugfestigkeit aufwies als die in den Versuchen 3a und 3b erhaltenen, im erfindungsgemäßen Temperaturbereich gehaspelten Warmbänder.It was found that the hot strip obtained in the operating trial 3c had a significantly lower tensile strength than the hot strips obtained in the temperature range according to the invention, due to the high coiling temperature due to a high ferrite content (and perlite).

Versuch 4Trial 4

Für einen zum Vergleich durchgeführten weiteren Betriebsversuch V ist ein Stahl mit der in Tabelle 5 angegebenen, aufgrund ihres deutlich zu geringen Si-Gehaltes und ihrer ebenfalls zu geringen Gehalte an Mn, Cr und Ti nicht erfindungsgemäßen Legierung erschmolzen und zu einem Strang vergossen worden, von dem Brammen abgeteilt worden sind. Die Brammen sind anschließend auf eine Temperatur von 1250 °C wiedererwärmt, daraufhin mit einer Warmwalzendtemperatur WET zu einem Warmband mit einer Dicke D warmgewalzt und schließlich mit einer Abkühlrate VT auf einer Haspeltemperatur HT abgekühlt worden, bei der sie zu einem Coil gehaspelt worden sind. Die jeweils eingestellten Parameter und die mechanischen Eigenschaften des erhaltenen Warmbands sind in Tabelle 6 angegeben.For a comparison carried out for further operational test V, a steel with the specified in Table 5, due to their significantly too low Si content and their too low levels of Mn, Cr and Ti not inventive alloy melted and cast into a strand of the slabs have been divided. The slabs were then reheated to a temperature of 1250 ° C, then hot rolled to a hot strip having a thickness D at a hot rolling end temperature WET, and finally cooled at a cooling rate V T at a coiling temperature HT at which they were coiled into a coil. The parameters set and the mechanical properties of the resulting hot strip are given in Table 6.

Es zeigte sich, dass das beim Vergleichsversuch V erhaltene Warmband zwar eine hohe Zugfestigkeit aufwies, dass jedoch seine Dehnungseigenschaften ungenügend waren. Tabelle 1 C Si Mn P S Al Cr Mo N Ti B 0,15 0,89 2,06 0,013 0,004 0,041 0,36 - 0,0047 0,12 0,0011 Rest Eisen und unvermeidbare Verunreinigungen, Angaben in Gew.-% Tabelle 2 C Si Mn P S Al Cr Mo N Ti B 0,155 0,87 1,94 0,013 0,006 0,029 0,36 0,25 0,0050 0,11 0,0009 Rest Eisen und unvermeidbare Verunreinigungen, Angaben in Gew.-% Tabelle 3 C Si Mn Al Cr Mo N Ti B 0,16 0,86 2,05 0,033 0,33 - 0,004 0,12 0,002 Rest Eisen und unvermeidbare Verunreinigungen, Angaben in Gew.-% Tabelle 4 Versuch WET [°C] D [mm] VT [°C/s] HT [°C] Re [MPa] Rm [MPa] A80 [%] Erfindungsgemäß? 3a 900 3 50 470 794 1194 11,0 JA 3b 900 1,6 90 500 825 1203 9,6 JA 3c 910 1,6 85 590 693 789 14,7 NEIN Tabelle 5 C Si Mn Al Cr Mo N Ti B 0,12 0,08 1,40 0,029 0,23 - 0,004 0,04 0,001 Rest Eisen und unvermeidbare Verunreinigungen, Angaben in Gew.-% Tabelle 6 Versuch WET [°C] D [mm] VT [°C/s] HT [°C] Re [MPa] Rm [MPa] A80 [%] Erfindungsgemäß? V 860 1,6 125 50 1012 1284 6,3 NEIN It was found that although the hot rolled strip obtained in Comparative Experiment V had high tensile strength, its elongation properties were insufficient. Table 1 C Si Mn P S al Cr Not a word N Ti B 0.15 0.89 2.06 0,013 0,004 0,041 0.36 - 0.0047 0.12 0.0011 Balance iron and unavoidable impurities, data in% by weight C Si Mn P S al Cr Not a word N Ti B 0,155 0.87 1.94 0,013 0,006 0,029 0.36 0.25 0.0050 0.11 0.0009 Balance iron and unavoidable impurities, data in% by weight C Si Mn al Cr Not a word N Ti B 0.16 0.86 2.05 0.033 0.33 - 0,004 0.12 0,002 Balance iron and unavoidable impurities, data in% by weight attempt WET [° C] D [mm] V T [° C / s] HT [° C] Re [MPa] Rm [MPa] A80 [%] According to the invention? 3a 900 3 50 470 794 1194 11.0 YES 3b 900 1.6 90 500 825 1203 9.6 YES 3c 910 1.6 85 590 693 789 14.7 NO C Si Mn al Cr Not a word N Ti B 0.12 0.08 1.40 0,029 0.23 - 0,004 0.04 0.001 Balance iron and unavoidable impurities, data in% by weight attempt WET [° C] D [mm] V T [° C / s] HT [° C] Re [MPa] Rm [MPa] A80 [%] According to the invention? V 860 1.6 125 50 1012 1284 6.3 NO

Claims (14)

  1. Hot-rolled flat steel product which has a high tensile strength of at least 1100 MPa and good elasticity and which is produced from a complex phase steel, which contains (in % by weight)
    C: 0.13 - 0.2 %,
    Mn: 1.8 - 2.5 %,
    Si: 0.70 - 1.3 %,
    Al: 0.01 - 0.1 %,
    P: up to 0.1 %,
    S: up to 0.01 %,
    Cr: 0.25 - 0.70 %,
    optionally Mo, the total of the Cr and Mo contents being 0.25 - 0.7 %,
    Ti: 0.08 - 0.2 %
    B: 0.0005 - 0.005 %,
    and as the remainder iron and inevitable impurities,
    and which has a structure which consists of at the most 10 % by volume of residual austenite, 10 - 60 % by volume of martensite, at the most 30% by volume of ferrite and at least 10% by volume of bainite.
  2. Flat steel product according to claim 1, characterised in that the C content of the complex phase steel is from 0.15 to 0.18 % by weight.
  3. Flat steel product according to either of the preceding claims, characterised in that the C content of the complex phase steel is at the most 0.17 % by weight.
  4. Flat steel product according to any one of the preceding claims, characterised in that the Mn content of the complex phase steel is 2.05 - 2.2 % by weight.
  5. Flat steel product according to any one of the preceding claims, characterised in that the Si content of the complex phase steel is at least 0.75 % by weight.
  6. Flat steel product according to any one of the preceding claims, characterised in that the Si content of the complex phase steel is at the most 1.1 % by weight.
  7. Flat steel product according to claim 5 or claim 6, characterised in that the Si content of the complex phase steel is at least 0.85 % by weight.
  8. Flat steel product according to any one of the preceding claims, characterised in that the Si content of the complex phase steel is at the most 0.95 % by weight.
  9. Flat steel product according to any one of the preceding claims, characterised in that the Al content of the complex phase steel is from 0.02 to 0.05 % by weight.
  10. Flat steel product according to any one of the preceding claims, characterised in that the Cr content of the complex phase steel is from 0.30 to 0.40 % by weight.
  11. Flat steel product according to any one of the preceding claims, characterised in that the Ti content of the complex phase steel is from 0.09 to 0.15 % by weight.
  12. Flat steel product according to claim 9 or claim 10, characterised in that the Ti content of the complex phase steel is from 0.11 to 0.13 % by weight.
  13. Flat steel product according to any one of the preceding claims, characterised in that the B content of the complex phase steel is 0.001 - 0.002 % by weight.
  14. Method for producing a hot-rolled flat steel product, comprising the following operating steps:
    - casting a complex phase steel produced according to any one of claims 1 to 13 to form a preliminary product,
    - heating the preliminary product to a temperature of 1150 - 1350°C,
    - hot rolling the preliminary product to form a hot-rolled strip, the end temperature of the hot rolling being 800 - 950°C,
    - cooling the hot-rolled strip obtained at a cooling speed which is at least 30°C/s,
    - coiling the hot-rolled strip obtained at a coiling temperature of 400 - 570 °C.
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EP11154973.9A EP2489748B1 (en) 2011-02-18 2011-02-18 Hot-rolled steel surface product produced from a complex phase steel and method for the manufacture
CN201180067938.XA CN103380217B (en) 2011-02-18 2011-12-29 The hot-rolled steel sheet product be made up of Multiphase Steel and manufacture method thereof
PCT/EP2011/074251 WO2012110165A1 (en) 2011-02-18 2011-12-29 Hot rolled flat steel product produced from a complex phase steel, and method for producing same
CA2825240A CA2825240A1 (en) 2011-02-18 2011-12-29 Hot-rolled flat steel product produced from a complex phase steel and method for the production thereof
KR1020137024831A KR20140005293A (en) 2011-02-18 2011-12-29 Hot rolled flat steel product produced from a complex phase steel, and method for producing same
JP2013553818A JP5864619B2 (en) 2011-02-18 2011-12-29 Hot rolled flat steel product manufactured from composite phase steel and method for manufacturing the same
US13/985,420 US20140041767A1 (en) 2011-02-18 2011-12-29 Hot-Rolled Flat Steel Product Produced from a Complex Phase Steel and Method for the Production Thereof

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