EP2453027A1 - Thermoformed product and method for producing same - Google Patents

Thermoformed product and method for producing same Download PDF

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Publication number
EP2453027A1
EP2453027A1 EP11188717A EP11188717A EP2453027A1 EP 2453027 A1 EP2453027 A1 EP 2453027A1 EP 11188717 A EP11188717 A EP 11188717A EP 11188717 A EP11188717 A EP 11188717A EP 2453027 A1 EP2453027 A1 EP 2453027A1
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EP
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Prior art keywords
hot
steel product
manganese
chromium
nickel
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German (de)
French (fr)
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EP2453027B1 (en
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Hans Roelofs
Giovanni Mastrogiacomo
Ulrich Hugo Urlau
Francisca Garcia Caballero
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Swiss Steel AG
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Swiss Steel AG
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/004Heat treatment of ferrous alloys containing Cr and Ni
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/005Heat treatment of ferrous alloys containing Mn
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/08Ferrous alloys, e.g. steel alloys containing nickel
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/001Austenite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/002Bainite
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/008Martensite

Definitions

  • the invention relates to a method for producing a steel product according to the preamble of claim 1 and to a hot-formed product producible therewith.
  • the tempering treatment aims at a tempered martensitic steel structure and allows the setting of a high level of toughness with still good strength.
  • a broadly used tempered steel is the material 42CrMo4. At a tensile strength of about 1000 MPa, this steel still achieves a Charpy impact work (ISO-V at room temperature) of 200 J.
  • Annealing steels with a martensite and bainite mixed structure can also have good property combinations.
  • the WO 2007/017161 describes such a steel for thick-walled seamless tubes (with up to 30 mm wall thickness). After quenching (cooling rate> 30 K / s) from the forming heat, a dominant martensitic microstructure with up to 40% bainite is formed. Irrespective of the original austenite grain size (primary structure), the martensitic structure has a good notched impact strength as soon as the martensite grain size is ⁇ 3 ⁇ m.
  • EP 0 845 544 describes such a microalloyed bainitic steel with C ⁇ 0.12%, which has a tensile strength of more than 1000 MPa at room temperature.
  • the steel is austenitized again after rolling and then quenched at a cooling rate> 17 K / s. This cooling rate is still significantly higher than that of the air-cooled long products in conventional rolling mills.
  • EP 0 775 756 describes another bainitic-martensitic steel for the production of forgings.
  • the tensile strength should be> 1 000 MPa and the Charpy notched impact strength ISO-U is> 50 J / cm 2 (or impact strength ISO-U> 25 J).
  • the described steel composition necessarily requires an accelerated cooling from the forming heat, so that these values can be achieved.
  • the exemplary embodiments show that the cooling rate should be> 14 K / s.
  • This technical teaching can not be applied in conventional forging and hot rolling processes.
  • the implementation is limited to small components in which even in the core still large cooling rates can be achieved.
  • JP2007 284774 describes a similar bainitic steel for the production of wire rod, which is characterized by good fatigue properties and good cold workability.
  • an accelerated cooling is required from the forming heat.
  • the cooling rates of the embodiments are between 15 and 50 K / s. The achievable impact strength is not apparent from the description.
  • GB 2 297 094 describes a carbide-free bainitic steel that can be made from the forming heat cooled in air.
  • the steel is designed for the production of rails and is characterized by a good wear resistance and a good fatigue behavior.
  • the notched impact strength of the Material was not the focus of this development.
  • the Charpy impact work ISO-V at room temperature is only at 20 to 40 years.
  • the in CN 1 477 226 steel described after air cooling may contain the following mixed structure: granular bainite, lower bainite, martensite, retained austenite. It achieves a tensile strength of 850 to over 1,400 MPa. However, to give good toughness, the steel must be heat treated (annealed) after hot working. The carbon present then migrates into the present austenite films, and at a tensile strength of about 900 MPa, a Charpy impact toughness ISO-U greater than 110 J / cm 2 (or> 55 J) can be achieved.
  • the object of the invention is to provide an improved hot-worked steel product and a method for its production, with which in particular the above disadvantages are avoided.
  • the cooling from the forming heat is particularly preferably carried out in such a way that the temperature range between 600 and 400 ° C. is passed through at a cooling rate of 0.1 to 4.0 K / s.
  • the mass number Bs used in the above condition corresponds to a known empirical approach for the bainite start temperature in Kelvin [ W. Steven, and AJ Haynes, JISI 183, pp. 349-359 (1956 )].
  • the alloying components are chosen so that at normal cooling rates from 0.1 to 8.0 K / s rolling heat always a bainitic-martensitic microstructure results with tensile strength of 900 to 1400 MPa, without costly alloying elements and / or special Facilities for accelerated cooling from the rolling heat must be used.
  • the Charpy ISO-V impact test at room temperature is> 100 years.
  • the lower limit of the carbon content to 0.03 wt .-% is ensured in combination with manganese, chromium and molybdenum that there are no ferrite in the structure. Ferrite levels affect both the strength level and the impact strength of the product.
  • the upper limit of the carbon to 0.20 wt .-% ensures that the tensile strength does not rise above 1400 MPa. Higher strength values degrade machinability in the downstream drawing or machining process. Higher carbon contents also promote the formation of carbides, which adversely affects ductility.
  • the lower limit of 2.00% by weight in manganese ensures that a bainite start temperature below 800 K can be achieved without expensive alloying additions. These deep bainite start temperature ensures a fine steel structure, which consists predominantly of lower bainite.
  • a manganese content of at least 2.55% by weight is used. In particular, it is proven to use a manganese content of at least 2.80 wt .-%, for example, about 3.0 wt .-%.
  • Molybdenum suppresses the grain boundary segregation of embrittling elements such as phosphorus.
  • An addition of at least 0.15 wt .-% molybdenum thus improves the tempering resistance of the steel. If no downstream heat treatment takes place, the addition of molybdenum is not mandatory.
  • the hot-worked steel product has a molybdenum content of 0.15 to 0.50% by weight.
  • Chromium may be alloyed in place of manganese to adjust the bainite start temperature.
  • the use of chromium is more expensive than the use of manganese. Since manganese segregates strongly, it may nevertheless make sense for certain applications to replace part of the manganese with chromium. Since chromium increases the risk for the formation of chromium-rich nitrides and carbides, which can lead to a deterioration of the toughness, the chromium content is limited to 2.0% by weight.
  • the addition of silicon is not necessary to achieve the desired properties.
  • a metered addition of silicon suppresses carbide formation.
  • a preferred embodiment of the product according to the invention therefore contains 0.40 to 0.80% by weight of silicon.
  • Nickel improves Charpy impact strength at low temperatures. In general, the properties are sufficient without addition of nickel. For cost reasons, the nickel content is limited to 1.0 wt .-%.
  • Phosphorus is a steel pest. It goes to the Austenitkorngrenzen and weakens the structure. For this reason, the phosphorus content was limited to 0.035 wt .-%.
  • ferrite formation should be avoided as far as possible. This can be ensured by a sufficiently rapid cooling of the hot-formed product. If the cooling rate is insufficient, addition of boron may additionally be provided. Boron goes to the austenite grain boundaries and suppresses ferrite formation. In this case, a boron content of 10 to 50 ppm is sufficient.
  • the chemical composition of the steel should be chosen such that, after cooling in air, a structure is created which is predominantly composed of lower bainite. This is preferably achieved by setting the bainite start temperature Bs low enough. For this reason, the Bs temperature should not be more than 800 K. The low transformation temperature ensures a very fine microstructure, which is decisive for achieving the high notched impact strength.
  • a sufficiently fine microstructure is achieved if the mean grain size of the dominant bainitic secondary microstructure is less than 5 ⁇ m.
  • the grain size is defined by the linear distance between grain boundaries.
  • the crystallographic orientation at the grain boundary should change by more than 15 °.
  • a too low selected Bs temperature slows the kinetics of bainite formation. It produces significantly less bainite and the structure becomes dominant martensitic.
  • the Bs temperature should therefore be above 700 K.
  • the bainite start temperature should preferably be between 750 and 800 K.
  • Austenite is not completely transformed into bainite during structural transformation. In order to dominate the properties of the lower bainite, however, should be at least 60% of the structure of lower bainite.
  • Austenite which does not convert to bainite during hot-dip cooling, is either stabilized to a sufficient carbon content or converts to martensite at lower temperatures. At an average carbon content in the steel of 0.05% by weight, it is expected that there will be no retained austenite in the structure and the resulting martensite may be up to 40%.
  • the manganese content of the steel is more than 2.0% by weight, a microscopically uneven manganese distribution in the industrially produced product is to be expected (segregation zones). For this reason, the transformation behavior of austenite during cooling from hot working may vary locally. Thus, isolated grains of ferrite, granular bainite or upper bainite can not be completely excluded. As long as their ingredients are small, they will not affect the good properties of the product. Therefore, up to 10% granular or upper bainite and up to 2% ferrite are permissible for the product produced according to the invention.
  • the final rolling temperature at the last hot working was 1,040 to 1,060 ° C.
  • an accelerated air cooling with a cooling rate of approx. 5.5 K / s was set between 900 and 600 ° C.
  • the rods were cooled in still air.
  • the room temperature Charpy impact values determined at nine melts are in Fig. 1 as a function of the bainite start temperature Bs (determined according to Steven & Hayns). It has been discovered that the melts with Bs ⁇ 800 K always have a good notched impact strength.
  • Carbon, manganese, molybdenum were used in the trial melts to adjust the Bs temperature. No chromium and nickel were alloyed. The measured chromium and nickel contents (as accompanying elements or impurities in the steel) were between 0.05 and 0.09 wt .-%.
  • the three steels according to the invention (“steels 7 to 9”) are compared with the six non-novel steels ("steels 1 to 6") in Tables 1 and 2.
  • the properties and the microstructure of the non-inventive steel 2 are similar to steel 1.
  • the bainite start temperature is slightly lower and the microstructure is accordingly somewhat finer ( Fig. 3 ).
  • the structure consists predominantly of a carbide-free granular bainite.
  • the quantitative microstructure analysis (by means of X-ray diffraction for austenite and quantitative SEM analysis for ferrite, bainite and M / A phase fractions) revealed the following structural composition: 80% bainite (dominant granular), 17% martensite and 3% retained austenite.
  • the microstructure changes from granular bainite to pale lower bainite.
  • LOM micrograph
  • the much finer microstructure can be seen ( Fig. 5 ).
  • the strength increases markedly compared to steels 1 and 2, but the impact value remains low.
  • Reason for the unsatisfactory toughness are coarse grains of granular, upper bainite, which are embedded in a matrix of fine lower bainite.
  • the structure consists of 87% bainite, 10% martensite and 3% retained austenite.
  • the microstructure is not as fine as it appears in the LOM or in the scanning electron microscope.
  • EBSD Electro Back Scattering Diffraction
  • Fig. 7 shows the structure of the inventive steel 7 compared to the steel 6.
  • the structure has become even finer in steel 7.
  • the quantitative analysis gives the following structure composition: 70 to 72% lower bainite and 28 to 30% self-tempered martensite. Rough structural components such as ferrite or granular upper bainite are missing.
  • the mean grain size determined by EBSD is correspondingly small. It is 4.51 ⁇ m ( ⁇ 1.09 ⁇ m) and thus only half the size of steel 6.

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  • Engineering & Computer Science (AREA)
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  • Metallurgy (AREA)
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Abstract

The process, comprises subjecting a hot form at 900-1300[deg] C, and cooling in air, where an average austenite grain size after a final hot form is smaller than 50 mu m and the cooling step occurs from the hot form at resting or moving air so that the temperature of 400-600[deg] C with a cooling rate of 0.1-4.0 K/s is passed through. The steel product comprises: carbon (0.03-0.20%); manganese (2-4.%); chromium (0.05-2%); nickel (0.05-1%); phosphorus (0.035%); molybdenum (0.5%); nitrogen (0.02%); aluminum (0.04%); boron (0.005%); titanium (0.10%); silicon (0.8%); and residual iron. The process, comprises subjecting a hot form at 900-1300[deg] C, and cooling in air, where an average austenite grain size after a final hot form is smaller than 50 mu m and the cooling step occurs from the hot form at resting or moving air so that the temperature of 400-600[deg] C with a cooling rate of 0.1-4.0 K/s is passed through. The steel product comprises: carbon (0.03-0.20%); manganese (2-4.%); chromium (0.05-2%); nickel (0.05-1%); phosphorus (0.035%); molybdenum (0.5%); nitrogen (0.02%); aluminum (0.04%); boron (0.005%); titanium (0.10%); silicon (0.8%); and residual iron and/or steel impurities. A weight percentage of carbon, manganese (2.55%), chromium, nickel and molybdenum satisfies the equation as given in the specification. The steel product is formed with the structural components having lower bainite (60-95%), granular or upper bainite (10%), martensite (40%), austenite (20%), and ferrite (2%). An independent claim is included for a hot-pressed steel product.

Description

Technisches GebietTechnical area

Die Erfindung betrifft ein Verfahren zur Herstellung eines Stahlprodukts gemäss dem Oberbegriff des Anspruchs 1 sowie ein damit herstellbares warmumgeformtes Produkt.The invention relates to a method for producing a steel product according to the preamble of claim 1 and to a hot-formed product producible therewith.

Stand der TechnikState of the art

Für massive Bauteile, die dynamisch oder schlagartig beansprucht werden, sind hohe Anforderungen an die Duktilität des eingesetzten Stahls gestellt. Aus diesem Grund werden die Endeigenschaften des Werkstoffs in der Regel über eine Wärmebehandlung eingestellt. Die Vergütungsbehandlung zielt auf ein angelassenes martensitisches Stahlgefüge und erlaubt die Einstellung eines hohen Zähigkeitsniveaus bei gleichzeitig noch guter Festigkeit. Ein breit eingesetzter Vergütungsstahl ist der Werkstoff 42CrMo4. Dieser Stahl erreicht bei einer Zugfestigkeit von ca. 1'000 MPa noch eine Charpy-Kerbschlagarbeit (ISO-V bei Raumtemperatur) von 200 J.For massive components that are stressed dynamically or suddenly, high demands are placed on the ductility of the steel used. For this reason, the final properties of the material are usually adjusted via a heat treatment. The tempering treatment aims at a tempered martensitic steel structure and allows the setting of a high level of toughness with still good strength. A broadly used tempered steel is the material 42CrMo4. At a tensile strength of about 1000 MPa, this steel still achieves a Charpy impact work (ISO-V at room temperature) of 200 J.

Vergütungsstähle mit einem Mischgefüge aus Martensit und Bainit können ebenfalls gute Eigenschaftskombinationen aufweisen. Die WO 2007/017161 beschreibt einen solchen Stahl für dickwandige Nahtlos-Rohre (mit bis zu 30 mm Wandstärke). Nach dem Abschrecken (Abkühlrate > 30 K/s) aus der Umformhitze entsteht ein dominant martensitisches Gefüge mit bis zu 40 % Bainit. Das martensitische Gefüge weist unabhängig von der ursprünglichen Austenitkorngrösse (Primärgefüge) eine gute Kerbschlagzähigkeit auf, sobald die Martensit-Korngrösse <3 µm beträgt.Annealing steels with a martensite and bainite mixed structure can also have good property combinations. The WO 2007/017161 describes such a steel for thick-walled seamless tubes (with up to 30 mm wall thickness). After quenching (cooling rate> 30 K / s) from the forming heat, a dominant martensitic microstructure with up to 40% bainite is formed. Irrespective of the original austenite grain size (primary structure), the martensitic structure has a good notched impact strength as soon as the martensite grain size is <3 μm.

Eine weitere Absenkung der Abkühlrate (aus der Umformwärme) führt zu dominant bainitischen Gefügen. Entsprechende Patentveröffentlichungen beziehen sich insbesondere auf Schmiedeteile und Schienen. Das resultierende Bainit-Gefüge ist in der Regel sehr kerbempfindlich, so dass die Charpy-Kerbschlagzähigkeit typischerweise an Proben mit U-Kerbe ermittelt wird.A further reduction of the cooling rate (from the forming heat) leads to dominant bainitic structures. Corresponding patent publications relate in particular to forgings and rails. The resulting bainite microstructure is usually very notch sensitive, so the Charpy notched impact strength typically determined on samples with U notch.

In EP 0 845 544 wird ein solcher mikrolegierter bainitischer Stahl mit C ≤ 0.12 % beschrieben, der bei Raumtemperatur eine Zugfestigkeit von über 1'000 MPa aufweist. Um die gewünschten Eigenschaften zu erreichen, wird der Stahl nach der Walzung wieder austenitisiert und anschliessend mit einer Abkühlrate > 17 K/s abgeschreckt. Diese Abkühlrate liegt immer noch deutlich über denjenigen der an Luft abgekühlten Langprodukte in konventionellen Walzwerken.In EP 0 845 544 describes such a microalloyed bainitic steel with C ≤ 0.12%, which has a tensile strength of more than 1000 MPa at room temperature. In order to achieve the desired properties, the steel is austenitized again after rolling and then quenched at a cooling rate> 17 K / s. This cooling rate is still significantly higher than that of the air-cooled long products in conventional rolling mills.

EP 0 775 756 beschreibt einen weiteren bainitisch-martensitischen Stahl für die Herstellung von Schmiedeteilen. Nach dem Schmieden soll die Zugfestigkeit > 1'000 MPa betragen und die Charpy-Kerbschlagzähigkeit ISO-U ist > 50 J/cm2 (bzw. Kerbschlagarbeit ISO-U > 25 J). Die beschriebene Stahlzusammensetzung erfordert jedoch zwingend eine beschleunigte Abkühlung aus der Umformhitze, damit diese Werte erreichbar sind. Die Ausführungsbeispiele zeigen, dass die Abkühlrate > 14 K/s sein soll. Diese technische Lehre lässt sich in konventionellen Schmiede- und Warmwalzprozessen nicht anwenden. Ausserdem beschränkt sich die Umsetzung auf kleine Bauteile, bei denen auch im Kern noch grosse Kühlraten erreicht werden können. EP 0 775 756 describes another bainitic-martensitic steel for the production of forgings. After forging, the tensile strength should be> 1 000 MPa and the Charpy notched impact strength ISO-U is> 50 J / cm 2 (or impact strength ISO-U> 25 J). However, the described steel composition necessarily requires an accelerated cooling from the forming heat, so that these values can be achieved. The exemplary embodiments show that the cooling rate should be> 14 K / s. This technical teaching can not be applied in conventional forging and hot rolling processes. In addition, the implementation is limited to small components in which even in the core still large cooling rates can be achieved.

JP2007 284774 beschreibt einen ähnlichen bainitischen Stahl zur Herstellung von Walzdraht, der sich durch gute Ermüdungseigenschaften und durch eine gute Kaltumformbarkeit auszeichnet. Zur Erreichung des erfindungsgemässen Gefüges wird wiederum eine beschleunigte Abkühlung aus der Umformhitze verlangt. Die Abkühlraten der Ausführungsbeispiele liegen zwischen 15 und 50 K/s. Die erreichbare Kerbschlagzähigkeit geht aus der Beschreibung nicht hervor. JP2007 284774 describes a similar bainitic steel for the production of wire rod, which is characterized by good fatigue properties and good cold workability. To achieve the inventive structure again an accelerated cooling is required from the forming heat. The cooling rates of the embodiments are between 15 and 50 K / s. The achievable impact strength is not apparent from the description.

GB 2 297 094 beschreibt einen karbid-freien bainitischen Stahl, der aus der Umformhitze an Luft abgekühlt hergestellt werden kann. Der Stahl ist für die Herstellung von Schienen konzipiert und zeichnet sich durch eine gute Verschleissbeständigkeit und ein gutes Ermüdungsverhalten aus. Die Kerbschlagzähigkeit des Werkstoffs stand bei dieser Entwicklung nicht im Vordergrund. Die Charpy-Kerbschlagarbeit ISO-V bei Raumtemperatur liegt nur bei 20 bis 40 J. GB 2 297 094 describes a carbide-free bainitic steel that can be made from the forming heat cooled in air. The steel is designed for the production of rails and is characterized by a good wear resistance and a good fatigue behavior. The notched impact strength of the Material was not the focus of this development. The Charpy impact work ISO-V at room temperature is only at 20 to 40 years.

Der in CN 1 477 226 beschriebene Stahl kann nach der Luftabkühlung folgende Mischgefüge enthalten: granularer Bainit, unterer Bainit, Martensit, Restaustenit. Er erreicht eine Zugfestigkeit von 850 bis über 1'400 MPa. Damit eine gute Zähigkeit resultiert, muss der Stahl nach der Warmumformung jedoch nochmals wärmebehandelt (angelassen) werden. Der vorhandene Kohlenstoff wandert dann in die vorliegenden Austenitfilme, und bei einer Zugfestigkeit von ca. 900 MPa lässt sich eine Charpy-Kerbschlagzähigkeit ISO-U von über 110 J/cm2 (oder > 55 J) erreichen.The in CN 1 477 226 steel described after air cooling may contain the following mixed structure: granular bainite, lower bainite, martensite, retained austenite. It achieves a tensile strength of 850 to over 1,400 MPa. However, to give good toughness, the steel must be heat treated (annealed) after hot working. The carbon present then migrates into the present austenite films, and at a tensile strength of about 900 MPa, a Charpy impact toughness ISO-U greater than 110 J / cm 2 (or> 55 J) can be achieved.

Obwohl die Eigenschaften der bainitisch-martensitischen Stähle bisher vielversprechend sind, liegt bislang keine Beschreibung für die Herstellung massiver Bauteile (Durchmesser bzw. Wandstärke > 10 mm) vor, mit der

  • nach Luftabkühlung aus der Umformhitze bei Raumtemperatur Charpy-Kerbschlagzähigkeiten an V-gekerbten Proben von > 100 J erreichbar sind
  • für ein breites Spektrum an Bauteil-Abmessungen bzw. Walzabmessungen konstante Eigenschaften erreicht werden können.
Although the properties of bainitic-martensitic steels have been promising so far, so far there is no description for the production of solid components (diameter or wall thickness> 10 mm), with the
  • After air cooling from the forming heat at room temperature, Charpy impact strengths at V-notched specimens> 100J can be achieved
  • For a wide range of component dimensions or rolling dimensions constant properties can be achieved.

Darstellung der ErfindungPresentation of the invention

Aufgabe der Erfindung ist es, ein verbessertes warmumgeformtes Stahlprodukt sowie ein Verfahren zu dessen Herstellung bereitzustellen, mit dem insbesondere die obigen Nachteile vermieden werden.The object of the invention is to provide an improved hot-worked steel product and a method for its production, with which in particular the above disadvantages are avoided.

Gelöst werden diese Aufgaben durch das im Anspruch 1 definierte Verfahren sowie durch das im Anspruch 5 definierte warmumgeformte Stahlprodukt.These objects are achieved by the method defined in claim 1 and by the defined in claim 5 hot-formed steel product.

Die nachfolgenden Gehaltsangaben in Prozent (%) bzw. in Teilen pro Million ("parts per million, ppm") beziehen sich - sofern nicht ausdrücklich anders angegeben - auf Gewichtsanteile.The following percentages by weight (%) or parts per million (ppm) refer to parts by weight unless expressly stated otherwise.

Beim erfindungsgemässen Verfahren zur Herstellung eines Stahlprodukts wird ein Stahl mit einem Gewichtsanteil von:

  • 0.03 bis 0.20 % Kohlenstoff (C),
  • 2.00 % bis 4.00 % Mangan (Mn),
  • 0.05 bis 2.00 % Chrom (Cr),
  • 0.05 bis 1.00% Nickel (Ni),
  • bis zu 0.035% Phosphor (P),
  • bis zu 0.5% Molybdän (Mo),
  • bis zu 0.02% Stickstoff (N),
  • bis zu 0.04% Aluminium (Al),
  • bis zu 0.005% Bor (B),
  • bis zu 0.10% Titan (Ti),
  • bis zu 0.8% Silizium (Si),
der Rest Eisen sowie stahlübliche Beimengungen,
einer Warmumformung bei 900 bis 1300°C unterzogen und danach an Luft abkühlt, wobei die mittlere Austenitkorngrösse nach dem letzten Warmumformungschritt kleiner ist als 50 µm und wobei die Abkühlung aus der Umformhitze an ruhender oder bewegter Luft so geschieht, dass der Temperaturbereich zwischen 800 und 500°C mit einer Kühlrate von 0.1 bis 8.0 K/s durchlaufen wird.In the process according to the invention for producing a steel product, a steel with a weight fraction of:
  • 0.03 to 0.20% carbon (C),
  • 2.00% to 4.00% manganese (Mn),
  • 0.05 to 2.00% chromium (Cr),
  • 0.05 to 1.00% nickel (Ni),
  • up to 0.035% phosphorus (P),
  • up to 0.5% molybdenum (Mo),
  • up to 0.02% nitrogen (N),
  • up to 0.04% aluminum (Al),
  • up to 0.005% boron (B),
  • up to 0.10% titanium (Ti),
  • up to 0.8% silicon (Si),
the remainder iron as well as steel admixtures,
subjected to hot working at 900 to 1300 ° C and then cooled in air, wherein the average Austenitkorngrösse after the last hot forming step is less than 50 microns and wherein the cooling from the forming heat to stationary or moving air is done so that the temperature range between 800 and 500 ° C is passed through with a cooling rate of 0.1 to 8.0 K / s.

Besonders bevorzugt wird die Abkühlung aus der Umformhitze so durchgeführt, dass der Temperaturbereich zwischen 600 und 400°C mit einer Kühlrate von 0.1 bis 4.0 K/s durchlaufen wird.The cooling from the forming heat is particularly preferably carried out in such a way that the temperature range between 600 and 400 ° C. is passed through at a cooling rate of 0.1 to 4.0 K / s.

Die prozentualen Gewichtsanteile x(i) von Kohlenstoff, Mangan, Chrom, Nickel und Molybdän erfüllen dabei die folgende Bedingung: 700 < Bs = 1 ʹ 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo < 800

Figure imgb0001
The percentages by weight x (i) of carbon, manganese, chromium, nickel and molybdenum meet the following condition: 700 < bs = 1 ' 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Not a word < 800
Figure imgb0001

Hierdurch wird erreicht, dass nach Luftabkühlung aus einem Temperaturbereich von 900 bis 1'300°C folgende Gefügebestandteile vorliegen:

  • 60 bis 95% unterer Bainit,
  • bis zu 10% granularer oder oberer Bainit,
  • bis zu 40% Martensit,
  • bis zu 20% Restaustenit, und
  • bis zu 2% Ferrit.
This ensures that after air cooling from a temperature range of 900 to 1'300 ° C the following structural components are present:
  • 60 to 95% lower bainite,
  • up to 10% granular or upper bainite,
  • up to 40% martensite,
  • up to 20% retained austenite, and
  • up to 2% ferrite.

Die in obiger Bedingung verwendete Masszahl Bs entspricht einem an sich bekannten Erfahrungsansatz für die Bainit-Starttemperatur in Kelvin [ W. Steven, and A.J. Haynes, JISI 183, pp. 349-359 (1956 )].The mass number Bs used in the above condition corresponds to a known empirical approach for the bainite start temperature in Kelvin [ W. Steven, and AJ Haynes, JISI 183, pp. 349-359 (1956 )].

Bei dem erfindungsgemäss hergestellten Produkt sind die Legierungskomponenten so gewählt, dass bei üblichen Abkühlraten aus der Walzhitze von 0.1 bis 8.0 K/s immer ein bainitisch-martensitisches Gefüge mit Zugfestigkeitsniveau von 900 bis 1'400 MPa resultiert, ohne dass kostspielige Legierungselemente und/oder spezielle Einrichtungen zur beschleunigten Abkühlung aus der Walzhitze verwendet werden müssen. Die Charpy-ISO-V Kerbschlagarbeit bei Raumtemperatur ist > 100 J.In the product according to the invention, the alloying components are chosen so that at normal cooling rates from 0.1 to 8.0 K / s rolling heat always a bainitic-martensitic microstructure results with tensile strength of 900 to 1400 MPa, without costly alloying elements and / or special Facilities for accelerated cooling from the rolling heat must be used. The Charpy ISO-V impact test at room temperature is> 100 years.

Durch die untere Begrenzung des Kohlenstoffgehalts auf 0.03 Gew.-% wird in Kombination mit Mangan, Chrom und Molybdän sichergestellt, dass keine Ferritanteile im Gefüge vorliegen. Ferritanteile beeinträchtigen sowohl das Festigkeitsniveau wie auch die Kerbschlagzähigkeit des Produkts.The lower limit of the carbon content to 0.03 wt .-% is ensured in combination with manganese, chromium and molybdenum that there are no ferrite in the structure. Ferrite levels affect both the strength level and the impact strength of the product.

Durch die obere Begrenzung des Kohlenstoffs auf 0.20 Gew.-% wird gewährleistet, dass die Zugfestigkeit nicht über 1'400 MPa ansteigt. Höhere Festigkeitswerte verschlechtern die Bearbeitbarkeit im nachgelagerten Ziehprozess oder Zerspanungsprozess. Höhere Kohlenstoffgehalte fördern ausserdem die Bildung von Karbiden, was die Duktilität nachteilig beeinflusst.The upper limit of the carbon to 0.20 wt .-% ensures that the tensile strength does not rise above 1400 MPa. Higher strength values degrade machinability in the downstream drawing or machining process. Higher carbon contents also promote the formation of carbides, which adversely affects ductility.

Die untere Begrenzung in Mangan auf 2.00 Gew.-% stellt sicher, dass eine Bainitstarttemperatur unter 800 K ohne teure Legierungszusätze erreichbar ist. Diese tiefe Bainitstarttemperatur gewährleistet ein feines Stahlgefüge, welches dominant aus unterem Bainit besteht. Gemäss einer bevorzugten Ausführungsform wird ein Mangangehalt von mindestens 2.55 Gew.-% verwendet. Insbesondere bewährt es sich, einen Mangangehalt von mindestens 2.80 Gew.-%, beispielsweise ungefähr 3.0 Gew.-% zu verwenden.The lower limit of 2.00% by weight in manganese ensures that a bainite start temperature below 800 K can be achieved without expensive alloying additions. These deep bainite start temperature ensures a fine steel structure, which consists predominantly of lower bainite. According to a preferred embodiment, a manganese content of at least 2.55% by weight is used. In particular, it is proven to use a manganese content of at least 2.80 wt .-%, for example, about 3.0 wt .-%.

Eine weitere Zugabe von Mangan senkt die Bainit-Starttemperatur nochmals ab. Das Bainitgebiet engt sich ein und der resultierenden Bainitanteil nimmt zugunsten von Restaustenit und Martensit ab. Bei Überschreitung der oberen Begrenzung in Mangan liegt das gewünschte bainitische Gefüge nicht mehr vor. Das Gefüge wird dann dominant martensitisch sein.Another addition of manganese reduces the bainite start temperature again. The bainite area narrows and the resulting bainite decreases in favor of retained austenite and martensite. If the upper limit in manganese is exceeded, the desired bainitic structure is no longer present. The microstructure will then be dominant martensitic.

Molybdän unterdrückt die Korngrenzensegregation von versprödenden Elementen wie Phosphor. Eine Zugabe von mindestens 0.15 Gew.-% Molybdän verbessert somit die Anlassbeständigkeit des Stahls. Falls keine nachgelagerte Wärmebehandlung stattfindet, ist die Zugabe von Molybdän nicht zwingend erforderlich.Molybdenum suppresses the grain boundary segregation of embrittling elements such as phosphorus. An addition of at least 0.15 wt .-% molybdenum thus improves the tempering resistance of the steel. If no downstream heat treatment takes place, the addition of molybdenum is not mandatory.

Ein Molybdängehalt über 0.5 Gew.-% fördert die Bildung von kohlenstoffreichen Martensitinseln. Diese führen zu einer markanten Verschlechterung der Zähigkeit des Stahls. Aus diesem Grund soll der Molybdängehalt maximal 0.5 Gew.-% betragen. Vorteilhafterweise weist das warmumgeformte Stahlprodukt einen Molybdängehalt von 0.15 bis 0.50 Gew.-% auf.A molybdenum content above 0.5% by weight promotes the formation of carbon-rich martensite islands. These lead to a marked deterioration of the toughness of the steel. For this reason, the molybdenum content should be at most 0.5% by weight. Advantageously, the hot-worked steel product has a molybdenum content of 0.15 to 0.50% by weight.

Chrom kann an Stelle von Mangan zulegiert werden, um die Bainitstarttemperatur einzustellen. Die Verwendung von Chrom ist jedoch kostspieliger als die Verwendung von Mangan. Da Mangan stark seigert, kann es für gewisse Anwendungen dennoch sinnvoll sein, einen Teil des Mangans durch Chrom zu ersetzen. Da Chrom das Risiko für die Bildung von chromreichen Nitriden und Karbiden erhöht, was zu einer Verschlechterung der Zähigkeit führen kann, wird der Chromgehalt auf 2.0 Gew.% begrenzt.Chromium may be alloyed in place of manganese to adjust the bainite start temperature. However, the use of chromium is more expensive than the use of manganese. Since manganese segregates strongly, it may nevertheless make sense for certain applications to replace part of the manganese with chromium. Since chromium increases the risk for the formation of chromium-rich nitrides and carbides, which can lead to a deterioration of the toughness, the chromium content is limited to 2.0% by weight.

Die Zugabe von Silizium ist nicht notwendig, um die gewünschten Eigenschaften zu erreichen. Eine dosierte Zugabe von Silizium unterdrückt die Karbidbildung. Eine bevorzugte Ausführung des erfindungsgemäss hergestellten Produkts enthält deshalb 0.40 bis 0.80 Gew.-% Silizium.The addition of silicon is not necessary to achieve the desired properties. A metered addition of silicon suppresses carbide formation. A preferred embodiment of the product according to the invention therefore contains 0.40 to 0.80% by weight of silicon.

Nickel verbessert die Charpy-Kerbschlagzähigkeit bei tiefen Temperaturen. In der Regel reichen die Eigenschaften auch ohne Nickelzugabe aus. Aus Kostengründen wird der Nickelgehalt auf 1.0 Gew.-% begrenzt.Nickel improves Charpy impact strength at low temperatures. In general, the properties are sufficient without addition of nickel. For cost reasons, the nickel content is limited to 1.0 wt .-%.

Die Zugabe von Aluminium ist für die erfindungsgemässe Herstellung des Produkts nicht zwingend. Falls eine spätere Wärmebehandlung des Produkts, z.B. eine Einsatzhärtung zur Einstellung einer verschleissfesten Oberfläche, notwendig ist, dann kann die Austenitkornstabilität über eine Aluminiumzugabe gewährleistet werden. In diesem Fall sind Aluminiumgehalte von 0.02 bis 0.04 Gew.% üblich.The addition of aluminum is not mandatory for the production of the product according to the invention. If a subsequent heat treatment of the product, e.g. case hardening is necessary to set a wear-resistant surface, then austenite grain stability can be ensured by adding aluminum. In this case, aluminum contents of 0.02 to 0.04 wt.% Are common.

Phosphor ist ein Stahlschädling. Es geht an die Austenitkorngrenzen und schwächt das Gefüge. Aus diesem Grund wurde der Phosphorgehalt auf 0.035 Gew.-% begrenzt.Phosphorus is a steel pest. It goes to the Austenitkorngrenzen and weakens the structure. For this reason, the phosphorus content was limited to 0.035 wt .-%.

Da sich schon geringfügige Ferritanteile negativ auf die Kerbschlagzähigkeit auswirken können, soll die Ferritbildung möglichst vermieden werden. Durch eine ausreichend schnelle Abkühlung des warmumgeformten Produkts kann dies gewährleistet werden. Falls die Abkühlrate nicht ausreicht, kann zusätzlich eine Zugabe von Bor vorgesehen werden. Bor geht an die Austenitkorngrenzen und unterdrückt die Ferritbildung. In diesem Fall ist einen Borgehalt 10 bis 50 ppm ausreichend.Since even small amounts of ferrite can have a negative impact on the notched impact strength, ferrite formation should be avoided as far as possible. This can be ensured by a sufficiently rapid cooling of the hot-formed product. If the cooling rate is insufficient, addition of boron may additionally be provided. Boron goes to the austenite grain boundaries and suppresses ferrite formation. In this case, a boron content of 10 to 50 ppm is sufficient.

Eine Zugabe von 0.03 bis 0.10 Gew.-% Titan stellt sicher, dass der im flüssigen Stahl gelösten Stickstoff von bis zu 0.02 Gew.-% während der Erstarrung des Stahls in Form von Titankarbonitriden ausgeschieden wird. Dies ist die Voraussetzung dafür, dass elementares Bor an die Austenitkorngrenzen gelangen kann und nicht in Form von Bornitriden vorliegt. Falls kein Bor zulegiert wird, muss keine Titanzugabe vorgesehen werden.An addition of 0.03 to 0.10% by weight of titanium ensures that the nitrogen dissolved in the liquid steel of up to 0.02% by weight is precipitated during the solidification of the steel in the form of titanium carbonitrides. This is the prerequisite for elemental boron to reach the austenite grain boundaries and not in the form of boron nitrides. If no boron is alloyed, no titanium addition must be provided.

Die chemische Zusammensetzung des Stahls ist so zu wählen, dass nach der Abkühlung an Luft ein Gefüge entsteht, welches dominant aus unterem Bainit besteht. Dies wird bevorzugt dadurch erreicht, dass die Bainit-Starttemperatur Bs niedrig genug eingestellt wird. Aus diesem Grund soll die Bs-Temperatur nicht mehr als 800 K betragen. Die tiefe Umwandlungstemperatur gewährleistet eine sehr feine Gefügestruktur, was für die Erreichung der hohen Kerbschlagzähigkeit entscheidet ist.The chemical composition of the steel should be chosen such that, after cooling in air, a structure is created which is predominantly composed of lower bainite. This is preferably achieved by setting the bainite start temperature Bs low enough. For this reason, the Bs temperature should not be more than 800 K. The low transformation temperature ensures a very fine microstructure, which is decisive for achieving the high notched impact strength.

Eine ausreichend feine Gefügestruktur ist erreicht, wenn die mittlere Korngrösse des dominant bainitischen Sekundärgefüges kleiner ist als 5 µm. Die Korngrösse ist dabei über den linearen Abstand zwischen Korngrenzen definiert. Die kristallografische Orientierung an der Korngrenze soll sich um mehr als 15° ändern.A sufficiently fine microstructure is achieved if the mean grain size of the dominant bainitic secondary microstructure is less than 5 μm. The grain size is defined by the linear distance between grain boundaries. The crystallographic orientation at the grain boundary should change by more than 15 °.

Eine zu tief gewählte Bs-Temperatur verlangsamt die Kinetik der Bainitbildung. Es entsteht deutlich weniger Bainit und das Gefüge wird dominant martensitisch. Die Bs-Temperatur soll deshalb über 700 K liegen. Damit sich möglichst viel Bainit einstellt, ist die Bainitstarttemperatur bevorzugt zwischen 750 und 800 K zu wählen.A too low selected Bs temperature slows the kinetics of bainite formation. It produces significantly less bainite and the structure becomes dominant martensitic. The Bs temperature should therefore be above 700 K. To set as much bainite as possible, the bainite start temperature should preferably be between 750 and 800 K.

Der Austenit wird während der Gefügeumwandlung nicht vollständig in Bainit umgewandelt. Damit die Eigenschaften des unteren Bainits dominieren, soll jedoch mindestens 60% des Gefüges aus unterem Bainit bestehen.Austenite is not completely transformed into bainite during structural transformation. In order to dominate the properties of the lower bainite, however, should be at least 60% of the structure of lower bainite.

Der Austenit, der sich während der Abkühlung aus der Warmumformung nicht in Bainit umwandelt, ist entweder über einen ausreichenden Kohlenstoffgehalt stabilisiert oder er wandelt sich bei tieferen Temperaturen in Martensit um. Bei einem mittleren Kohlenstoffgehalt im Stahl von 0.05 Gew.-% wird im Gefüge voraussichtlich kein Restaustenit vorliegen und der resultierende Martensitanteil kann bis zu 40% betragen.Austenite, which does not convert to bainite during hot-dip cooling, is either stabilized to a sufficient carbon content or converts to martensite at lower temperatures. At an average carbon content in the steel of 0.05% by weight, it is expected that there will be no retained austenite in the structure and the resulting martensite may be up to 40%.

Bei einem Kohlenstoffgehalt von 0.2 Gew.-% wird ein Teil des Austenits während der Abkühlung aus der Warmumformung stabilisiert. Es können noch 20% Restaustenit im Endprodukt vorliegen.At a carbon content of 0.2% by weight, a part of the austenite is stabilized during the cooling from the hot working. There may still be 20% retained austenite in the final product.

Da der Mangangehalt des Stahls über 2.0 Gew.-% beträgt, ist mit einer mikroskopisch ungleichmässigen Manganverteilung im grosstechnisch hergestellten Produkt zu rechnen (Seigerungszonen). Aus diesem Grund kann das Umwandlungsverhalten des Austenits während der Abkühlung aus der Warmumformung lokal variieren. So können vereinzelte Körner aus Ferrit, granularem Bainit oder oberem Bainit nicht gänzlich ausgeschlossen werden. Solange deren Bestandteile gering sind, werden sie die guten Eigenschaften des Produkts nicht beeinträchtigen. Deshalb sind bis zu 10 % granularer oder oberer Bainit und bis zu 2 % Ferrit für das erfindungsgemäss hergestellte Produkt zulässig.Since the manganese content of the steel is more than 2.0% by weight, a microscopically uneven manganese distribution in the industrially produced product is to be expected (segregation zones). For this reason, the transformation behavior of austenite during cooling from hot working may vary locally. Thus, isolated grains of ferrite, granular bainite or upper bainite can not be completely excluded. As long as their ingredients are small, they will not affect the good properties of the product. Therefore, up to 10% granular or upper bainite and up to 2% ferrite are permissible for the product produced according to the invention.

Kurze Beschreibung der ZeichnungenBrief description of the drawings

Ausführungsbeispiele der Erfindung werden nachfolgend anhand der Zeichnungen näher beschrieben, dabei zeigen:

Fig. 1
die Abhängigkeit der ISO-V-Kerbschlagarbeit von der Bainit-Starttemperatur;
Fig. 2
ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 1;
Fig. 3
ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 2;
Fig. 4
ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 6;
Fig. 5
ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 7;
Fig. 6
Gefügebilder bei 3000X Vergrösserung im hochauflösenden Rasterelektronenmikroskop: oben Stahl 6 und unten Stahl 7, und
Fig. 7
das Ergebnis einer EBSD-Untersuchung am Beispiel von Stahl 7; die Sekundär-Korngrössen wurden entlang den Linien d1, d2 und d3 ausgewertet; die mittlere Korngrösse beträgt 4.51 µm (± 1.09 µm).
Exemplary embodiments of the invention will be described in greater detail below with reference to the drawings, in which:
Fig. 1
the dependence of the ISO-V impact energy on the bainite start temperature;
Fig. 2
a microstructure (etchant: LePera) for a 18 mm steel bar 1;
Fig. 3
a microstructure (etchant: LePera) for a 18 mm steel bar steel 2;
Fig. 4
a microstructure (etchant: LePera) for a steel 18 mm steel bar 6;
Fig. 5
a microstructure (etchant: LePera) for a steel 18 mm steel bar 7;
Fig. 6
Micrographs at 3000X Magnification in the high-resolution scanning electron microscope: steel 6 at the top and steel 7 at the bottom and steel 7 at the bottom
Fig. 7
the result of an EBSD study using the example of Stahl 7; the secondary grain sizes were evaluated along lines d1, d2 and d3; the average particle size is 4.51 μm (± 1.09 μm).

Wege zur Ausführung der ErfindungWays to carry out the invention

Die entscheidenden Zusammenhänge wurden bei Versuchen an Laborschmelzen entdeckt. Es wurden Stahlblöcke von Ø 100 x 400 mm gefertigt und zu Ø 50 mm Rundknüppel ausgeschmiedet. Zur Simulation einer Walzung in einem konventionellen Warmwalzwerk wurden die Knüppel in 8 Stichen auf Ø 18 mm ausgewalzt:The crucial connections were discovered during laboratory melt tests. Steel blocks of Ø 100 x 400 mm were produced and forged to a Ø 50 mm round billet. To simulate a rolling process in a conventional hot rolling mill, the billets were rolled in 8 passes to Ø 18 mm:

Die Endwalztemperatur bei der letzten Warmumformung war 1'040 bis 1'060°C. Zur Simulation der Abkühlung bei dünnen Drähten wurde zwischen 900 und 600°C eine beschleunigte Luftabkühlung mit einer Kühlrate von ca. 5.5 K/s eingestellt. Während der Gefügeumwandlung unter 600°C wurden die Stäbe an ruhender Luft abgekühlt.The final rolling temperature at the last hot working was 1,040 to 1,060 ° C. To simulate the cooling of thin wires, an accelerated air cooling with a cooling rate of approx. 5.5 K / s was set between 900 and 600 ° C. During structural transformation below 600 ° C, the rods were cooled in still air.

Die an neun Schmelzen ermittelten Charpy-Kerbschlagwerte bei Raumtemperatur sind in Fig. 1 in Abhängigkeit von der Bainit-Starttemperatur Bs (ermittelt nach Steven & Hayns) dargestellt. Es wurde entdeckt, dass die Schmelzen mit Bs < 800 K immer eine gute Kerbschlagzähigkeit aufweisen.The room temperature Charpy impact values determined at nine melts are in Fig. 1 as a function of the bainite start temperature Bs (determined according to Steven & Hayns). It has been discovered that the melts with Bs <800 K always have a good notched impact strength.

In den Versuchsschmelzen wurden Kohlenstoff, Mangan, Molybdän verwendet, um die Bs-Temperatur einzustellen. Es wurde kein Chrom und Nickel legiert. Die gemessenen Chrom- und Nickelgehalte (als Begleitelemente bzw. Verunreinigungen im Stahl) lagen zwischen 0.05 und 0.09 Gew.-%.Carbon, manganese, molybdenum were used in the trial melts to adjust the Bs temperature. No chromium and nickel were alloyed. The measured chromium and nickel contents (as accompanying elements or impurities in the steel) were between 0.05 and 0.09 wt .-%.

Die drei erfindungsgemässen Stähle ("Stähle 7 bis 9") sind den sechs nicht-erfindungsgemässen Stählen ("Stähle 1 bis 6") in den Tabellen 1 und 2 gegenübergestellt. Der wesentliche Unterschied zwischen den erfindungsgemässen und den nicht-erfindungsgemässen Stählen besteht in der Feinheit und in der Morphologie der Mikrostruktur. Diese wird über die chemische Zusammensetzung des Stahls, gemäss Bs (in K) = 1103 - 270C - 90Mn - 70Cr - 37Ni - 83Mo < 800 K eingestellt. Ausgewählte Gefügebilder sind in Fig. 2 bis 5 dargestellt.The three steels according to the invention ("steels 7 to 9") are compared with the six non-novel steels ("steels 1 to 6") in Tables 1 and 2. The essential difference between the novel and non-inventive steels is the fineness and the morphology of the microstructure. This is set by the chemical composition of the steel, according to Bs (in K) = 1103-270C-90Mn-70Cr-37Ni-83Mo <800K. Selected micrographs are in Fig. 2 to 5 shown.

Stahl 1 besteht dominant aus einem grobkörnigen granularen Bainit (Fig. 2). Vereinzelt wurden auch Ferritkörner gefunden. Die Zugfestigkeit Rm ist entsprechend niedrig. Die ISO-V-Kerbschlagarbeit bei Raumtemperatur fällt unter 100 J.Steel 1 is dominantly a coarse granular bainite ( Fig. 2 ). Occasionally ferrite grains were also found. The tensile strength Rm is correspondingly low. The ISO-V impact work at room temperature falls below 100 years.

Die Eigenschaften und die Mikrostruktur des nicht-erfindungsgemässen Stahls 2 sind ähnlich zu Stahl 1. Die Bainit-Starttemperatur ist etwas niedriger und das Gefüge ist entsprechend etwas feiner (Fig. 3). Das Gefüge besteht dominant aus einem karbid-freien granularen Bainit. Die quantitative Gefügeanalyse (mittels Röntgendiffraktion für Austenit und quantitative REM-Analyse für Ferrit, Bainit und M/A-Phasenanteile) ergab folgende Gefügezusammensetzung: 80 % Bainit (dominant granular), 17 % Martensit und 3 % Restaustenit.The properties and the microstructure of the non-inventive steel 2 are similar to steel 1. The bainite start temperature is slightly lower and the microstructure is accordingly somewhat finer ( Fig. 3 ). The structure consists predominantly of a carbide-free granular bainite. The quantitative microstructure analysis (by means of X-ray diffraction for austenite and quantitative SEM analysis for ferrite, bainite and M / A phase fractions) revealed the following structural composition: 80% bainite (dominant granular), 17% martensite and 3% retained austenite.

Bei weiterer Absenkung der Bainit-Starttemperatur ändert die Gefügestruktur von granularem Bainit zu lattenförmigem unterem Bainit. Bereits im Schliffbild (LOM) des nicht-erfindungsgemässen Stahls 6 ist die deutlich feinere Gefügestruktur zu erkennen (Fig. 5). Die Festigkeit steigt im Vergleich zu den Stählen 1 und 2 markant an, aber die Kerbschlagzähigkeit bleibt niedrig. Grund für die unbefriedigende Zähigkeit sind grobe Körner aus granularem, oberem Bainit, die in einer Matrix aus feinem unteren Bainit eingelagert sind. Das Gefüge besteht zu 87 % aus Bainit, 10 % aus Martensit und 3 % Restaustenit.Upon further lowering of the bainite start temperature, the microstructure changes from granular bainite to pale lower bainite. Already in the micrograph (LOM) of the non-inventive steel 6, the much finer microstructure can be seen ( Fig. 5 ). The strength increases markedly compared to steels 1 and 2, but the impact value remains low. Reason for the unsatisfactory toughness are coarse grains of granular, upper bainite, which are embedded in a matrix of fine lower bainite. The structure consists of 87% bainite, 10% martensite and 3% retained austenite.

Aufgrund der vorliegenden Körner aus granularem Bainit ist die Gefügestruktur nicht so fein, wie sie im LOM oder auch im Rasterelektronenmikroskop erscheint.Due to the grains of granular bainite present, the microstructure is not as fine as it appears in the LOM or in the scanning electron microscope.

Um die Feinheit der Struktur sichtbar zu machen wurden EBSD-Untersuchungen ("Electron Back Scattering Diffraction") durchgeführt. Mit dieser Methode werden die kristallografische Orientierungen in der Mikrostruktur gemessen. Eine Korngrenze liegt vor, wenn sich die kristallografische Orientierung um mehr als 15° ändert. Die mittlere lineare Ausdehnung der Körner kann so bestimmt werden. Für Stahl 6 ist die mittlere Korngrösse 10.1 µm (± 0.93 µm).To visualize the fineness of the structure, EBSD (Electron Back Scattering Diffraction) studies were performed. This method measures the crystallographic orientations in the microstructure. A grain boundary exists when the crystallographic orientation changes by more than 15 °. The average linear extent of the grains can thus be determined. For steel 6, the mean grain size is 10.1 μm (± 0.93 μm).

Fig. 7 zeigt das Gefüge des erfindungsgemässen Stahls 7 im Vergleich zum Stahl 6. Das Gefüge ist bei Stahl 7 nochmals feiner geworden. Die quantitative Analyse gibt folgende Gefügezusammensetzung: 70 bis 72 % unterer Bainit und 28 bis 30 % selbstangelassener Martensit. Grobe Gefügebestandteile wie Ferrit oder granularer oberer Bainit fehlen. Die mittels EBSD bestimmte mittlere Korngrösse ist entsprechend klein. Sie liegt bei 4.51 µm (± 1.09 µm) und ist somit nur halb so gross wie bei Stahl 6. Fig. 7 shows the structure of the inventive steel 7 compared to the steel 6. The structure has become even finer in steel 7. The quantitative analysis gives the following structure composition: 70 to 72% lower bainite and 28 to 30% self-tempered martensite. Rough structural components such as ferrite or granular upper bainite are missing. The mean grain size determined by EBSD is correspondingly small. It is 4.51 μm (± 1.09 μm) and thus only half the size of steel 6.

Die erfindungsgemässen Stähle repräsentieren eine Stahlzusammensetzung mit hoher Festigkeit (ca. 1'000 MPa) und Zähigkeit (ISO-V bei Raumtemperatur ist 150 bis 200 J):

  • Sie können mit konventioneller Walz- und Schmiedetechnologie hergestellt werden. Eine Feinung des Primärgefüges durch Absenkung der Temperatur im letzten Umformschritt unterhalb der Rekristallisations-Stopptemperatur ist nicht zwingend gefordert. Im Ausführungsbeispiel liegt eine Austenitkorngrösse von ca. 30 µm vor.
  • Die Eigenschaften werden bei tiefen Abkühlraten erreicht (hier an ruhender Luft), so dass massive Bauteile daraus gefertigt werden können (im Beispiel 18 mm Walzstahl). Die Abkühlrate entspricht der Herstellung von Stabstahl im Durchmesserbereich von 40 bis 50 mm.
  • Die Eigenschaften werden direkt aus der Warmumformung erreicht. Eine nachgelagerte Wärmebehandlung (z.B. ein Anlassen) ist nicht zwingend notwendig.
  • Der Einsatz von teuren Legierungsmitteln (wie Mikrolegierungsmittel oder Nickel und Molybdän) ist nicht zwingend notwendig.
Figure imgb0002
Figure imgb0003
The steels according to the invention represent a steel composition with high strength (about 1000 MPa) and toughness (ISO-V at room temperature is 150 to 200 J):
  • They can be manufactured using conventional rolling and forging technology. A refining of the primary structure by lowering the temperature in the last forming step below the recrystallization stop temperature is not mandatory. In the exemplary embodiment, an austenite grain size of approximately 30 μm is present.
  • The properties are achieved at low cooling rates (here in still air), so that massive components can be made from them (in the example, 18 mm rolled steel). The cooling rate corresponds to the production of steel bars in the diameter range from 40 to 50 mm.
  • The properties are achieved directly from hot forming. A subsequent heat treatment (eg tempering) is not absolutely necessary.
  • The use of expensive alloying agents (such as microalloying agent or nickel and molybdenum) is not mandatory.
Figure imgb0002
Figure imgb0003

Claims (15)

Verfahren zur Herstellung eines Stahlprodukts, wobei man einen Stahl mit einem Gewichtsanteil von 0.03 bis 0.20 % Kohlenstoff (C), 2.00 % bis 4.00 % Mangan (Mn), 0.05 bis 2.00 % Chrom (Cr), 0.05 bis 1.00% Nickel (Ni), bis zu 0.035% Phosphor (P), bis zu 0.5% Molybdän (Mo), bis zu 0.02% Stickstoff (N), bis zu 0.04% Aluminium (Al), bis zu 0.005% Bor (B), bis zu 0.10% Titan (Ti), bis zu 0.8% Silizium (Si), der Rest Eisen sowie stahlübliche Verunreinigungen, einer Warmumformung bei 900 bis 1300°C unterzieht, und danach an Luft abkühlt, wobei die mittlere Austenitkorngrösse nach dem letzten Warmumformungschritt kleiner ist als 50 µm und wobei die Abkühlung aus der Umformhitze an ruhender oder bewegter Luft so geschieht, dass der Temperaturbereich zwischen 800 und 500°C mit einer Kühlrate von 0.1 bis 8.0 K/s durchlaufen wird, dadurch gekennzeichnet, dass die prozentualen Gewichtsanteile x(i) von Kohlenstoff, Mangan, Chrom, Nickel und Molybdän die folgende Bedingung erfüllen: 700 < Bs = 1 ʹ 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo < 800
Figure imgb0004
wobei ein Stahlprodukt mit folgenden Gefügebestandteilen gebildet wird: 60 bis 95% unterer Bainit, bis zu 10% granularer oder oberer Bainit, bis zu 40% Martensit, bis zu 20% Restaustenit, und bis zu 2% Ferrit.
A process for producing a steel product, wherein a steel having a weight fraction of 0.03 to 0.20% carbon (C), 2.00% to 4.00% manganese (Mn), 0.05 to 2.00% chromium (Cr), 0.05 to 1.00% nickel (Ni), up to 0.035% phosphorus (P), up to 0.5% molybdenum (Mo), up to 0.02% nitrogen (N), up to 0.04% aluminum (Al), up to 0.005% boron (B), up to 0.10% titanium (Ti), up to 0.8% silicon (Si), the remainder iron and common impurities, subjected to hot working at 900 to 1300 ° C, and then cooled in air, wherein the average austenite grain size after the last hot forming step is less than 50 microns and wherein the cooling from the forming heat to static or moving air is such that the temperature range between 800 and 500 ° C is passed through with a cooling rate of 0.1 to 8.0 K / s, characterized in that the percentage by weight x (i) of carbon, manganese, chromium, nickel and molybdenum satisfy the following condition: 700 < bs = 1 ' 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Not a word < 800
Figure imgb0004
wherein a steel product is formed with the following structural constituents: 60 to 95% lower bainite, up to 10% granular or upper bainite, up to 40% martensite, up to 20% retained austenite, and up to 2% ferrite.
Verfahren nach Anspruch 1, wobei der prozentuale Gewichtsanteil von Mangan mindestens 2.55% beträgt.The method of claim 1, wherein the weight percentage of manganese is at least 2.55%. Verfahren nach Anspruch 1 oder 2, wobei die Abkühlung aus der Umformhitze an ruhender oder bewegter Luft so geschieht, dass der Temperaturbereich zwischen 600 und 400°C mit einer Kühlrate von 0.1 bis 4.0 K/s durchlaufen wird.The method of claim 1 or 2, wherein the cooling from the forming heat to stationary or moving air is done so that the temperature range between 600 and 400 ° C is passed through with a cooling rate of 0.1 to 4.0 K / s. Verfahren nach einem der Ansprüche 1 bis 3, wobei die prozentualen Gewichtsanteile x von Kohlenstoff, Mangan, Chrom, Nickel und Molybdän die Bedingung erfüllen: 750 < Bs = 1 ʹ 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo < 800.
Figure imgb0005
Method according to one of claims 1 to 3, wherein the percentages by weight x of carbon, manganese, chromium, nickel and molybdenum satisfy the condition: 750 < bs = 1 ' 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Not a word < 800th
Figure imgb0005
Warmumgeformtes Stahlprodukt, herstellbar mit einem Verfahren nach einem der vorangehenden Ansprüche, mit einem Gewichtsanteil von 0.03 bis 0.20 % Kohlenstoff (C), 2.00 % bis 4.00 % Mangan (Mn), 0.05 bis 2.00 % Chrom (Cr), 0.05 bis 1.00% Nickel (Ni), bis zu 0.035% Phosphor (P), bis zu 0.5% Molybdän (Mo), bis zu 0.02% Stickstoff (N), bis zu 0.04% Aluminium (Al), bis zu 0.005% Bor (B), bis zu 0.10% Titan (Ti), bis zu 0.8% Silizium (Si), der Rest Eisen sowie stahlübliche Verunreinigungen, wobei die prozentualen Gewichtsanteile x von Kohlenstoff, Mangan, Chrom, Nickel und Molybdän die folgende Bedingung erfüllen: 700 < Bs = 1 ʹ 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo < 800
Figure imgb0006
wobei das Stahlprodukt folgende Gefügebestandteile aufweist: 60 bis 95% unterer Bainit, bis zu 10% granularer oder oberer Bainit, bis zu 40% Martensit, bis zu 20% Restaustenit, und bis zu 2% Ferrit.
Hot-formed steel product producible by a process according to any one of the preceding claims, having a weight fraction of 0.03 to 0.20% carbon (C), 2.00% to 4.00% manganese (Mn), 0.05 to 2.00% chromium (Cr), 0.05 to 1.00% nickel (Ni), up to 0.035% phosphorus (P), up to 0.5% molybdenum (Mo), up to 0.02% nitrogen (N), up to 0.04% aluminum (Al), up to 0.005% boron (B), up to 0.10% titanium (Ti), up to 0.8% silicon (Si), the remainder iron and common impurities, wherein the percentages by weight x of carbon, manganese, chromium, nickel and molybdenum satisfy the following condition: 700 < bs = 1 ' 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Not a word < 800
Figure imgb0006
the steel product having the following structural constituents: 60 to 95% lower bainite, up to 10% granular or upper bainite, up to 40% martensite, up to 20% retained austenite, and up to 2% ferrite.
Warmumgeformtes Stahlprodukt nach Anspruch 5, wobei die mittlere Korngrösse des dominant bainitischen Sekundärgefüges kleiner ist als 5 µm.Hot-formed steel product according to claim 5, wherein the mean grain size of the dominant bainitic secondary structure is smaller than 5 μm. Warmumgeformtes Stahlprodukt nach Anspruch 5 oder 6, wobei die prozentualen Gewichtsanteile x von Kohlenstoff, Mangan, Chrom, Nickel und Molybdän die Bedingung erfüllen: 750 < Bs = 1 ʹ 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Mo < 800.
Figure imgb0007
A hot-formed steel product according to claim 5 or 6, wherein the weight percentages x of carbon, manganese, chromium, nickel and molybdenum satisfy the condition: 750 < bs = 1 ' 103 - 270 x C - 90 x Mn - 70 x Cr - 37 x Ni - 83 x Not a word < 800th
Figure imgb0007
Warmumgeformtes Stahlprodukt nach einem der Ansprüche 5 bis 7, mit folgenden Gefügebestandteilen: 60 bis 95% unterer Bainit, bis zu 10% granularer oder oberer Bainit, bis zu 30% Martensit, bis zu 5% Restaustenit, und bis zu 2% Ferrit. Hot-formed steel product according to one of claims 5 to 7, having the following structural constituents: 60 to 95% lower bainite, up to 10% granular or upper bainite, up to 30% martensite, up to 5% retained austenite, and up to 2% ferrite. Warmumgeformtes Stahlprodukt nach einem der Ansprüche 5 bis 8, wobei der Siliziumgehalt 0.40 bis 0.80 Gew.-% beträgt.A hot-worked steel product according to any one of claims 5 to 8, wherein the silicon content is 0.40 to 0.80 wt%. Warmumgeformtes Stahlprodukt nach einem der Ansprüche 5 bis 9, wobei der Borgehalt 10 bis 50 ppm beträgt.A hot-worked steel product according to any one of claims 5 to 9, wherein the boron content is 10 to 50 ppm. Warmumgeformtes Stahlprodukt nach einem der Ansprüche 5 bis 10, wobei der Titangehalt 0.03 bis 0.10 Gew.-% beträgt.A hot-worked steel product according to any one of claims 5 to 10, wherein the titanium content is 0.03 to 0.10% by weight. Warmumgeformtes Stahlprodukt nach einem der Ansprüche 5 bis 11, mit einer Mindest-Wandstärke bzw. Mindest-Durchmesser von 10 mm.Hot-formed steel product according to one of claims 5 to 11, with a minimum wall thickness or minimum diameter of 10 mm. Warmumgeformtes Stahlprodukt nach einem der Ansprüche 5 bis 12, mit einer Charpy-Kerbschlagzähigkeit ISO-V bei Raumtemperatur von über 100 J.Hot-formed steel product according to one of claims 5 to 12, having a Charpy notched impact strength ISO-V at room temperature of more than 100 years. Warmumgeformtes Stahlprodukt nach einem der Ansprüche 5 bis 13, mit einer Zugfestigkeit Rm bei Raumtemperatur von 800 bis 1400 MPa.Hot-formed steel product according to one of claims 5 to 13, having a tensile strength Rm at room temperature of 800 to 1400 MPa. Verwendung eines warmumgeformten Produkts nach einem der Ansprüche 5 bis 14 für die Verarbeitung in einer Presse zur Herstellung von Kaltfliesspress- oder Kaltstauchteilen.Use of a hot formed product according to any one of claims 5 to 14 for processing in a press for the production of cold extrusion or cold upsetting parts.
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EP0775756A1 (en) 1995-11-27 1997-05-28 ASCOMETAL (Société anonyme) Steel for the production of forgings with bainitic structure, and production process of forged parts
EP0845544A1 (en) 1996-11-26 1998-06-03 Ascometal Steel product made from bainitic steel and process for making the steel product
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EP0845544A1 (en) 1996-11-26 1998-06-03 Ascometal Steel product made from bainitic steel and process for making the steel product
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