EP2009120A2 - Use of an extremely resistant steel alloy for producing steel pipes with high resistance and good plasticity - Google Patents

Use of an extremely resistant steel alloy for producing steel pipes with high resistance and good plasticity Download PDF

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Publication number
EP2009120A2
EP2009120A2 EP08011681A EP08011681A EP2009120A2 EP 2009120 A2 EP2009120 A2 EP 2009120A2 EP 08011681 A EP08011681 A EP 08011681A EP 08011681 A EP08011681 A EP 08011681A EP 2009120 A2 EP2009120 A2 EP 2009120A2
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EP
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Prior art keywords
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steel alloy
steel
manganese
silicon
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EP08011681A
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German (de)
French (fr)
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EP2009120B1 (en
EP2009120A3 (en
Inventor
Uwe Dr.-Ing. Diekmann
Andreas Dr.-Ing. Frehn
Alexander Dr.-Ing. Redenius
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Benteler Deustchland GmbH
Benteler Automobiltechnik GmbH
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Benteler Automobiltechnik GmbH
Benteler Stahl Rohr GmbH
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    • 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
    • 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
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the invention relates to the use of a steel alloy according to the features of patent claim 1.
  • the state of the art for steel pipes with increased strength can be described by micro-alloyed fine grain steels with ferritic-pearlitic structure, for example steel StE 460.
  • this steel achieves breaking strengths of 650 - 750 MPa and elongation at break of approximately 20 - 25%.
  • the product of strength and elongation at break is usually about 16,000 - 18,000 [MPa *%].
  • This combination of properties allows a good cold workability, eg by pulling, pressing, thread rolling.
  • the properties of the StE 460 achieved by variations of steel alloy 20MnV6.
  • the solid solution hardening by the alloying element manganese together with the precipitation of vanadium carbonitrides causes a comparatively high strength at a moderate cost.
  • the strength is generally adjusted by varying the carbon content in the range between 0.12 and 0.22%.
  • vanadium, titanium and niobium also play an important role as micro-alloying elements.
  • the micro-alloying elements are generally alloyed in small proportions of up to 0.2%, the amount and choice of the micro-alloying elements being dependent on thermoforming, eg hot-rolled strip production.
  • the structure of a classic StE 460 consists of a mixture of ferrite and pearlite and is generally formed by cooling in air after rolling or austenitizing.
  • An advantage of these steels is the property, by a so-called normalization, generally carried out in the form of austenitization and cooling in air, restore the initial structure and the initial properties even after a complex manufacturing history.
  • a further increase in the strength through additional alloying elements leads to increased costs and to a pronounced decrease in the elongation at break, so that the desired cold workability is not guaranteed.
  • additional heat treatment such as soft annealing before forming, this problem can be easily overcome.
  • this procedure is also associated with increased costs.
  • the described ferritic-pearlitic structure of the state of the art steel tubes has in addition to the only moderate ratio of strength and ductility additional disadvantages.
  • the microstructures ferrite-perlite are not evenly distributed but show a pronounced linearity, the first consequence of which is a pronounced anisotropy of the properties brings and leads in the cold forming to undesirable effects. For example, there are significant differences along and across the rolling direction.
  • Welded steel pipes are often produced by pressure welding.
  • the strip edges are heated by resistance heating (high-frequency or direct current) and then welded at high pressure with significant plastic deformation, without a molten phase is formed.
  • Such welding methods are therefore covered by the term solid state welding methods.
  • a great advantage of the welding process described is the extremely high welding speed, which is significantly higher than other methods, e.g. above that of the laser beam welding, and thus brings a superior cost-effectiveness.
  • pressure welding of ferritic-pearlitic steels however, the formation of the weld bead as a result of the necessary plastic deformation results in the effect that pearlite rows are deflected and reach the surface in the region of the weld zone.
  • brittle cementite lamellae of the pearlitic structure constituent form metallurgical notches, which in the worst case emerge perpendicular to the surface.
  • These fins can already be used during the following processing, e.g. Calibration of pipes for roundness, lead to cracks.
  • these notches mean that even with high static strength no high dynamic strength can be achieved. Consequently, pearlite-free structures are particularly suitable for producing high-strength press-welded steel tubes.
  • TRIP steels usually contain over 0.2% carbon, with the silicon content often exceeding 1.5%.
  • the microstructure of these steels has a ferritic-bainitic base matrix containing retained austenite constituents, which are converted to hard martensite during transformation of the steel.
  • the retained austenite is stabilized by alloying elements and a special heat treatment.
  • the advantage of the TRIP steel lies in the good forming properties at high strengths and high breaking strengths.
  • a TRIP steel has a high solidification capacity even with large changes in shape and a high energy absorption capacity, which is maintained even under dynamic load.
  • TRIP steels generally require a complex and technically difficult heat treatment to stabilize the desired amount of retained austenite to room temperature.
  • the TRIP heat treatment generally consists of accelerated cooling from the austenite region to prevent perlite formation and holding for a few minutes at temperatures just above the martensite start temperature. This heat treatment requires a complex process control and is difficult to implement reliably in conventional production facilities of plants for pipe production.
  • ferritic-bainitic steels (FB steels) which have strengths of 500-1,000 MPa and exhibit better properties than ferrite-pearlitic materials in terms of forming behavior Steels of equal strength.
  • FB steels ferritic-bainitic steels
  • the achievable plastic deformations at strengths above 700 MPa are still too low.
  • the production of ferritic-bainitic steels generally requires a so-called thermomechanical treatment, ie special rolling and cooling conditions. For this reason, conventional ferritic-bainitic steels are mainly available as hot-rolled strip.
  • TRIP steels and FB steels can not yet be normalized analogously to ferritic-pearlitic steels, since during normalization the necessary cooling conditions are not guaranteed.
  • the first three steels shown have a much higher carbon content and also differ in the other elements of Although the presented TRIP steel (number 3) achieves comparable mechanical properties, for the processing is, however, as already explained, an expensive to implement temperature-time curve during production required.
  • Material characteristics of Docol 1000 DP, TRIP steel RA-K 42/80 and FB-W 600 are only available in strip material. Therefore, the table also indicates the A80 instead of the A5 elongation for the DP / TRIP and FB steel.
  • the A80 elongation is used for strip material due to sample geometry, as opposed to strip tensile.
  • the invention is based on the object of demonstrating how steel pipes with high strength and good formability can be produced without costly heat treatment and without costly alloying concepts, wherein the elongation at break should at least equal the steel StE 460 and wherein the steel pipes have a breaking strength above 700 MPa should.
  • the solution of the problem of the invention is achieved by a new structure concept and its alloy implementation.
  • the new alloy concept is based on the avoidance of perlite and on the setting of a ferritic-bainitic structure with small amounts of lamellar retained austenite. As a result, favorable, low yield strength ratios are achieved for cold forming.
  • the product of breaking strength and elongation at break reaches very good values of more than 20,000 [MPa *%].
  • This microstructure is achieved by adapting the chemical composition to predefined cooling conditions of the steel tubes from the austenite region. The cooling conditions are described by a continuous cooling with cooling rates between 0.5 K / sec and 5 K / sec.
  • the alloy concept prevents the formation of perlite in this cooling zone.
  • ferrite or bainitic ferrite and a Residual phase or several residual phases which, depending on the cooling conditions, consist of lower bainite and martensite with lamellar retained austenite.
  • the steel is characterized by excellent formability in the cold state, as well as by a high breaking strength at high elongation at break, which is caused by the strong solidification due to the multi-phase character.
  • the pipes are intended to be cold formed in further processing.
  • the alloys show a basic ferritic structure with bainite, martensite and partially retained austenite, the grain sizes being 10-20 ⁇ m for the rolling conditions not optimized here. Occasionally it comes to the formation of fine and small pearlite nests, which are not arranged in a row. By improving the hot rolling conditions, the microstructures can be significantly improved and thus also the properties of the materials.
  • alloys 2 and 3 after improved hot rolling conditions, ie from the standard production of seamless tubes measuring 36 ⁇ 3.6 mm with final rolling temperature 860 ° C.
  • Alloy 2 was chosen by way of example because it has a high fracture toughness.
  • Alloy 3 was chosen as an example because it has a high strength.
  • alloys 2 and 3 after hot rolling of seamless tubes Rp0.2 [MPa] Rm [MPa] A5 [%] Fracture Z [%] True Breaking Voltage [MPa] Alloy 2 375 677 32 68 1310 Alloy 3 545 960 24 55 1610
  • Tubes made of such a steel have a pearlite-free multi-phase structure and open up a variety of applications and uses, some of which are exemplified below.
  • hot-rolled steel alloy pipes Due to the excellent relationship between strength and ductility, hot-rolled steel alloy pipes have particular advantages in subsequent cold forming processes, eg drawing, rotary kneading, spinning, thread rolling, extrusion, compression, autofretting, bending.
  • the steel alloy can be used to produce high-strength and cost-effective cold-drawn steel tubes, eg drill pipes, line pipes, diesel injection lines, cylinder tubes, tubes for airbag generators, and pipes for side impact beams for motor vehicles produce.
  • work hardening is used to achieve high strength.
  • Soft annealing before cold drawing is not required. Tempering is optionally possible after cold drawing, depending on the desired strength. Stresses in the range of well over 1,000 MPa up to 1,600 MPa are possible.
  • the alloy is distinguished by the fact that no pearlite line appears, so that the tubes react less sensitively to internal defects caused by pleats.
  • the tubes produced from the steel alloy are also particularly suitable for further processing by hydroforming.
  • the excellent deformation behavior of the steel alloy results in hydroforming advantages, since high component strengths can be achieved using the steel alloy.
  • the production of welded pipes from the alloy is also advantageously possible.
  • the alloy concept allows the production of hot strip and cold strip. Compared to conventional DP steels and TRIP steels, a comparatively simple temperature-time control is required.
  • the alloy can be normalized up to a plate thickness of 4 mm, ie Develops the target structure in case of air cooling.
  • the low carbon content results in only comparatively low hardness peaks in the welded seam of welded pipes. this applies especially in comparison to TRIP steels, which show a high degree of hardening with twice the carbon content. Due to the lack of pearlite brittleness, advantages arise in classical and very economical pressure welding.
  • the alloy concept also advantageously allows beam welding by means of laser beam or electron beam.
  • the advantage of the alloy concept is also the low carbon content and the normalization capability of the tubes.
  • the steel alloy is suitable for the production of tubes for chassis applications in the automotive industry. Due to the good breaking strength-Umform zucchinis ratio more complex components are conceivable, which could not be produced with the previous steel grades or only with great technical and therefore benefited insomniaßem effort. In addition, the low carbon content in combination with the other alloying elements ensures good weldability.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Manufacturing & Machinery (AREA)
  • Physics & Mathematics (AREA)
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  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

The steel alloy comprises 0.06-0.15 wt.% of carbon, 1-0 wt.% of silicon, 0.7-2.2 wt.% of manganese, 0.1-0.8 wt.% of chromium, 0.2 wt.% or less of molybdenum, 0.1 wt.% or less of aluminum, 0,2wt.% of vanadium, 0.02 wt.% or less of nitrogen, 0.06 wt.% of niobium, 0.2 wt.% or less of copper, and 0.2 wt.% or less of nickel. 0.001-0.004 wt.% of boron, 0.001-0.05 wt.% of titanium, 0.2 wt.% or less of tungsten, and 0.15 wt.% or less of tungsten are also provided. Iron and remainder have impurities, where the sum of silicon, manganese, chrome and copper is 3-4%.

Description

Die Erfindung betrifft die Verwendung einer Stahllegierung gemäß den Merkmalen des Patentanspruchs 1.The invention relates to the use of a steel alloy according to the features of patent claim 1.

Der Stand der Technik für Stahlrohre mit erhöhter Festigkeit kann durch mikrolegierte Feinkornstähle mit ferritisch-perlitischem Gefüge, beispielsweise den Stahl StE 460 beschrieben werden. Bei Streckgrenzen von 460 - 490 MPa erreicht dieser Stahl Bruchfestigkeiten von 650 - 750 MPa und Bruchdehnungen von ca. 20 - 25 %. Das Produkt aus Festigkeit und Bruchdehnung beträgt in der Regel ca. 16.000 - 18.000 [MPa*%]. Diese Eigenschaftskombination ermöglicht eine gute Kaltumformbarkeit, z.B. durch Ziehen, Drücken, Gewindewalzen. Klassisch werden die Eigenschaften des StE 460 durch Variationen der Stahllegierung 20MnV6 erreicht. Dabei bewirkt die Mischkristallverfestigung durch das Legierungselement Mangan zusammen mit der Ausscheidung von Vanadium-Carbonitriden eine vergleichsweise hohe Festigkeit bei moderaten Kosten. Bei den genannten höherfestern mikrolegierten Feinkornstählen wird die Festigkeit durch Variation des Kohlenstoffgehalts im allgemein im Bereich zwischen 0,12 und 0,22 % eingestellt. Neben Vanadium spielen auch Titan und Niob eine wichtige Rolle als Mikrolegierungselement. Die Mikrolegierungselemente werden allgemein in kleinen Anteilen von bis zu 0,2 % zulegiert, wobei Menge und Wahl der Mikrolegierungselemente von der Warmformung, z.B. Warmbandherstellung, abhängig sind.The state of the art for steel pipes with increased strength can be described by micro-alloyed fine grain steels with ferritic-pearlitic structure, for example steel StE 460. At yield strengths of 460 - 490 MPa, this steel achieves breaking strengths of 650 - 750 MPa and elongation at break of approximately 20 - 25%. The product of strength and elongation at break is usually about 16,000 - 18,000 [MPa *%]. This combination of properties allows a good cold workability, eg by pulling, pressing, thread rolling. Classically, the properties of the StE 460 achieved by variations of steel alloy 20MnV6. Here, the solid solution hardening by the alloying element manganese together with the precipitation of vanadium carbonitrides causes a comparatively high strength at a moderate cost. In the case of the higher-strength microalloyed fine-grain steels mentioned, the strength is generally adjusted by varying the carbon content in the range between 0.12 and 0.22%. Besides vanadium, titanium and niobium also play an important role as micro-alloying elements. The micro-alloying elements are generally alloyed in small proportions of up to 0.2%, the amount and choice of the micro-alloying elements being dependent on thermoforming, eg hot-rolled strip production.

Das Gefüge eines klassischen StE 460 besteht aus einer Mischung aus Ferrit und Perlit und entsteht allgemein durch Abkühlung an der Luft nach dem Walzen oder Austenitisieren. Ein Vorteil dieser Stähle ist die Eigenschaft, durch eine so genannte Normalisierung, allgemein in Form einer Austenitisierung und Abkühlung an Luft durchgeführt, das Ausgangsgefüge und die Ausgangseigenschaften auch nach einer komplexen Herstellgeschichte wieder herzustellen.The structure of a classic StE 460 consists of a mixture of ferrite and pearlite and is generally formed by cooling in air after rolling or austenitizing. An advantage of these steels is the property, by a so-called normalization, generally carried out in the form of austenitization and cooling in air, restore the initial structure and the initial properties even after a complex manufacturing history.

Eine weitere Steigerung der Festigkeit durch zusätzliche Legierungselemente, beispielsweise in Form einer Mischkristallhärtung, führt zu erhöhten Kosten und zu einer starken Abnahme der Bruchdehnung, so dass die gewünschte Kaltumformbarkeit nicht gewährleistet ist. Mit einer zusätzlichen Wärmebehandlung, wie z.B. dem Weichglühen vor der Umformung, kann dieses Problem in Grenzen umgangen werden. Diese Vorgehensweise ist jedoch ebenfalls mit erhöhten Kosten verbunden.A further increase in the strength through additional alloying elements, for example in the form of solid solution hardening, leads to increased costs and to a pronounced decrease in the elongation at break, so that the desired cold workability is not guaranteed. With an additional heat treatment, such as soft annealing before forming, this problem can be easily overcome. However, this procedure is also associated with increased costs.

Das beschriebene ferritisch-perlitische Gefüge der dem Stand der Technik entsprechenden Stahlrohre hat neben dem nur moderaten Verhältnis aus Festigkeit und Duktilität zusätzliche Nachteile. Die Gefügeanteile Ferrit-Perlit sind nicht gleichmäßig verteilt sondern zeigen eine ausgeprägte Zeiligkeit, die als erste Konsequenz eine ausgeprägte Anisotropie der Eigenschaften mit sich bringt und bei der Kaltumformung zu unerwünschten Effekten führt. Z.B. ergeben sich deutliche Unterschiede längs und quer zur Walzrichtung.The described ferritic-pearlitic structure of the state of the art steel tubes has in addition to the only moderate ratio of strength and ductility additional disadvantages. The microstructures ferrite-perlite are not evenly distributed but show a pronounced linearity, the first consequence of which is a pronounced anisotropy of the properties brings and leads in the cold forming to undesirable effects. For example, there are significant differences along and across the rolling direction.

Normalerweise liegen die Perlitzeilen parallel zur Oberfläche und stören die meisten Anwendungen nicht. Bei der Herstellung von Stahlrohren ergeben sich jedoch zum Teil Nachteile.Normally, the pearlite lines are parallel to the surface and do not disturb most applications. In the production of steel pipes, however, there are some disadvantages.

Geschweißte Stahlrohre werden oft durch ein Pressschweißen hergestellt. Dabei werden die Bandkanten durch Widerstandsbeheizung (Hochfrequenz-oder Gleichstrom) aufgeheizt und dann mit hohem Druck unter deutlicher plastischer Verformung verschweißt, ohne dass eine schmelzflüssige Phase entsteht. Derartige Schweißverfahren fallen deshalb unter den Begriff der Festkörperschweißverfahren. Ein großer Vorteil des beschriebenen Schweißverfahrens ist die extrem hohe Schweißgeschwindigkeit, die deutlich über der anderer Verfahren, z.B. über der des Laserstrahlschweißens, liegt und damit eine überlegene Wirtschaftlichkeit mit sich bringt. Beim Pressschweißen von ferritisch-perlitischen Stählen entsteht aber durch die Herausbildung der Schweißwulst infolge der notwendigen plastischen Verformung der Effekt, dass Perlit-Zeilen abgelenkt werden und im Bereich der Schweißzone an die Oberfläche gelangen. Dabei bilden spröde Zementitlamellen des perlitischen Gefügebestandteils metallurgische Kerben, die im schlimmsten Fall senkrecht an der Oberfläche austreten. Diese Lamellen können bereits während der folgenden Verarbeitung, z.B. Kalibrierung der Rohre auf Rundheit, zu Anrissen führen.Welded steel pipes are often produced by pressure welding. The strip edges are heated by resistance heating (high-frequency or direct current) and then welded at high pressure with significant plastic deformation, without a molten phase is formed. Such welding methods are therefore covered by the term solid state welding methods. A great advantage of the welding process described is the extremely high welding speed, which is significantly higher than other methods, e.g. above that of the laser beam welding, and thus brings a superior cost-effectiveness. In pressure welding of ferritic-pearlitic steels, however, the formation of the weld bead as a result of the necessary plastic deformation results in the effect that pearlite rows are deflected and reach the surface in the region of the weld zone. Here, brittle cementite lamellae of the pearlitic structure constituent form metallurgical notches, which in the worst case emerge perpendicular to the surface. These fins can already be used during the following processing, e.g. Calibration of pipes for roundness, lead to cracks.

Insbesondere bei dynamisch belasteten Bauteilen führen diese Kerben dazu, dass auch bei hoher statischer Festigkeit keine hohe dynamische Festigkeit erreicht werden kann. Demzufolge sind perlitfreie Gefüge besonders geeignet, hochfeste pressgeschweißte Stahlrohre herzustellen.Especially with dynamically loaded components, these notches mean that even with high static strength no high dynamic strength can be achieved. Consequently, pearlite-free structures are particularly suitable for producing high-strength press-welded steel tubes.

Ein ähnliches Problem durch umgelenkte Perlitzeilen kann bei der Herstellung nahtloser Stahlrohre entstehen. Hier kommt es oft während der Warmformung zur Bildung von so genannten Fältelungen. Diese Fältelungen verschweißen im Allgemeinen während des Fertigungsprozesses und stellen dann makroskopisch kleine Fehler dar. Es entstehen jedoch auch hier senkrecht zur Oberfläche austretende spröde Zementit-Lamellen, die ebenfalls die Dauerfestigkeit negativ beeinflussen. Demzufolge ist auch für die Herstellung nahtloser Rohre die Verwendung von perlitfreien Gefügen vorteilhaft, wenn lokale Fältelungen nicht ausgeschlossen werden können.A similar problem due to deflected pearlite lines can arise in the production of seamless steel tubes. This often leads to the formation of so-called pleats during thermoforming. These pleats generally weld during the manufacturing process and then set macroscopically small defects. However, brittle cementite lamellae emerge perpendicular to the surface and have a negative effect on fatigue strength. Consequently, the use of pearlite-free structures is also advantageous for the production of seamless tubes, if local crinkles can not be ruled out.

Es ist bekannt, dass durch die gezielte Einstellung von Restaustenit im Gefüge das Produkt aus Bruchdehnung und Bruchfestigkeit verbessert werden kann. Der so genannte TRIP-Effekt (TRansformation Induced Plasticity) ermöglicht vergleichsweise hohe Dehnungen bei hohen Festigkeiten. TRIP-Stähle enthalten üblicherweise über 0,2 % Kohlenstoff, wobei der Siliziumgehalt oft über 1,5 % liegt. Das Gefüge dieser Stähle weist eine ferritisch-bainitische Grundmatrix auf, die Restaustenit-Bestandteile enthält, welche bei der Umformung des Stahls in harten Martensit umgewandelt werden. Der Restaustenit wird durch Legierungselemente und eine spezielle Wärmebehandlung stabilisiert. Der Vorteil des TRIP-Stahls liegt in den guten Umformeigenschaften bei hohen Festigkeiten sowie hohen Bruchfestigkeiten. Ein TRIP-Stahl besitzt ein hohes Verfestigungsvermögen auch bei großer Formänderung und ein hohes Energieabsorptionsvermögen, das auch bei dynamischer Belastung erhalten bleibt. Allerdings ist bei TRIP-Stählen allgemein eine aufwändige und technisch schwierig zu realisierende Wärmebehandlung erforderlich, um die gewünschte Menge Restaustenit bis Raumtemperatur zu stabilisieren. Die TRIP-Wärmebehandlung besteht allgemein aus einer beschleunigten Abkühlung aus dem Austenitgebiet zur Verhinderung einer Perlitbildung und ein Halten von wenigen Minuten auf Temperaturen kurz oberhalb der Martensit-Starttemperatur. Diese Wärmebehandlung setzt eine aufwändige Prozessregelung voraus und ist in üblichen Produktionsanlagen von Werken zur Rohrherstellung schwierig prozesssicher umsetzbar.It is known that the product of elongation at break and breaking strength can be improved by the targeted adjustment of retained austenite in the microstructure. The so-called TRIP effect (TRANSformation Induced Plasticity) enables comparatively high strains at high strengths. TRIP steels usually contain over 0.2% carbon, with the silicon content often exceeding 1.5%. The microstructure of these steels has a ferritic-bainitic base matrix containing retained austenite constituents, which are converted to hard martensite during transformation of the steel. The retained austenite is stabilized by alloying elements and a special heat treatment. The advantage of the TRIP steel lies in the good forming properties at high strengths and high breaking strengths. A TRIP steel has a high solidification capacity even with large changes in shape and a high energy absorption capacity, which is maintained even under dynamic load. However, TRIP steels generally require a complex and technically difficult heat treatment to stabilize the desired amount of retained austenite to room temperature. The TRIP heat treatment generally consists of accelerated cooling from the austenite region to prevent perlite formation and holding for a few minutes at temperatures just above the martensite start temperature. This heat treatment requires a complex process control and is difficult to implement reliably in conventional production facilities of plants for pipe production.

Aus dem Bereich der Bandherstellung sind ferritisch-bainitische Stähle (FB-Stähle) bekannt, die Festigkeiten von 500 - 1000 MPa aufweisen und bezogen auf das Umformverhalten bessere Eigenschaften zeigen als ferritisch-perlitische Stähle gleicher Festigkeit. Allerdings sind die erreichbaren plastischen Verformungen bei Festigkeiten über 700 MPa noch zu gering. Auch die Herstellung ferritisch-bainitische Stähle erfordert allgemein eine so genannte thermomechanische Behandlung, d.h. besondere Walz- und Abkühlbedingungen. Aus diesem Grund werden übliche ferritisch-bainitische Stähle vorwiegend als Warmband angeboten.From the field of strip production, ferritic-bainitic steels (FB steels) are known which have strengths of 500-1,000 MPa and exhibit better properties than ferrite-pearlitic materials in terms of forming behavior Steels of equal strength. However, the achievable plastic deformations at strengths above 700 MPa are still too low. Also, the production of ferritic-bainitic steels generally requires a so-called thermomechanical treatment, ie special rolling and cooling conditions. For this reason, conventional ferritic-bainitic steels are mainly available as hot-rolled strip.

Aus den vorgenannten Gründen folgt auch, dass TRIP-Stähle und FB-Stähle bisher nicht analog zu ferritisch-perlitischen Stählen normalisiert werden können, da bei einer Normalisierung die notwendigen Abkühlbedingungen nicht gewährleistet werden.For the reasons mentioned above, it follows that TRIP steels and FB steels can not yet be normalized analogously to ferritic-pearlitic steels, since during normalization the necessary cooling conditions are not guaranteed.

Zum Vergleich sind die chemische Analyse und die zugehörigen mechanischen Kennwerte für einen Stahl StE 460 (Firma Benteler Stahl/Rohr GmbH), einen hochfesten Dulphasen (DP)-Stahl (Docol 1000 DP, Fa. SSAB Swedish Steel GmbH), einen TRIP-Stahl (RA-K 42/80, Fa. Thyssen Krupp GmbH), einen FB-Stahl (FB-W 600, Fa. Thyssen Krupp GmbH) mit aufgeführt. C Si Mn Cr Al V Nb N Ti B StE460 0,18 0,23 1,37 0,15 0,03 0,09 0,037 0,01 0,00 0,00 Docol 1000 DP 0,15 0,2 1,5 0,04 0,015 RA-K 42/80 0,22 1,5 2 0,5 0,7 FB-W 600 0,09 0,03 1,46 0,02 0,031 0,001 0,032 0,008 0,001 0,0001 Werkstoffkennwerte: Rp0,2 [MPa] Rm [MPa] A5 [%] StE 460 473 709 19,47 Docol 1000 DP 700 - (950) 1000 - 1200 5* RA-K 42/80 453 832 23,1 * FB-W 600 534 592 19,8* *) Dehnung A80 For comparison, the chemical analysis and the associated mechanical characteristics for a steel StE 460 (Benteler Stahl / Rohr GmbH), a high-strength Dulphasen (DP) steel (Docol 1000 DP, SSAB Swedish Steel GmbH), a TRIP steel (RA-K 42/80, Fa. Thyssen Krupp GmbH), a FB steel (FB-W 600, Fa. Thyssen Krupp GmbH) listed. C Si Mn Cr al V Nb N Ti B StE460 0.18 0.23 1.37 0.15 0.03 0.09 0.037 0.01 0.00 0.00 Docol 1000 DP 0.15 0.2 1.5 0.04 0,015 RA-K 42/80 0.22 1.5 2 0.5 0.7 FB-W 600 0.09 0.03 1.46 0.02 0.031 0.001 0.032 0,008 0.001 0.0001 Material characteristics: Rp0.2 [MPa] Rm [MPa] A5 [%] StE 460 473 709 19.47 Docol 1000 DP 700 - (950) 1000 - 1200 5 * RA-K 42/80 453 832 23.1 * FB-W 600 534 592 19.8 * *) Elongation A80

Die ersten drei dargestellten Stähle haben einen wesentlich höheren Kohlenstoffgehalt und unterscheiden sich auch in den anderen Elementen von der Zusammensetzung der im Folgenden vorgestellten und erfindungsgemäßen Legierungen 1 bis 4. Der vorgestellte TRIP-Stahl (Nummer 3) erreicht zwar vergleichbare mechanische Eigenschaften, für die Verarbeitung ist jedoch, wie bereits erläutert, ein aufwendig zu realisierender Temperatur-Zeit-Verlauf während der Produktion erforderlich. Werkstoffkennwerte vom Docol 1000 DP, vom TRIP-Stahl RA-K 42/80 sowie vom FB-W 600 liegen nur von Bandmaterial vor. Deshalb wird in der Tabelle auch die A80- anstatt der A5-Dehnung für den DP- / TRIP- und FB-Stahl angegeben Die A80-Dehnung wird bei Bandmaterial aufgrund der Probengeometrie im Unterschied zu Stabzugproben verwendet.The first three steels shown have a much higher carbon content and also differ in the other elements of Although the presented TRIP steel (number 3) achieves comparable mechanical properties, for the processing is, however, as already explained, an expensive to implement temperature-time curve during production required. Material characteristics of Docol 1000 DP, TRIP steel RA-K 42/80 and FB-W 600 are only available in strip material. Therefore, the table also indicates the A80 instead of the A5 elongation for the DP / TRIP and FB steel. The A80 elongation is used for strip material due to sample geometry, as opposed to strip tensile.

Der Erfindung liegt die Aufgabe zu Grunde, aufzuzeigen, wie Stahlrohre mit hoher Festigkeit und guter Umformbarkeit ohne aufwändige Wärmebehandlung und ohne kostenintensive Legierungskonzepte hergestellt werden können, wobei die Bruchdehnung mindestens der des Stahls StE 460 entsprechen soll und wobei die Stahlrohre eine Bruchfestigkeit über 700 MPa aufweisen sollen.The invention is based on the object of demonstrating how steel pipes with high strength and good formability can be produced without costly heat treatment and without costly alloying concepts, wherein the elongation at break should at least equal the steel StE 460 and wherein the steel pipes have a breaking strength above 700 MPa should.

Diese Aufgabe wird durch die Verwendung einer Stahllegierung mit den Merkmalen des Patentanspruchs 1 gelöst.This object is achieved by the use of a steel alloy having the features of patent claim 1.

Die erfindungsgemäße Lösung der Aufgabe wird durch ein neues Gefügekonzept und deren legierungstechnische Umsetzung erreicht. Das neue Legierungskonzept basiert auf der Vermeidung von Perlit und auf der Einstellung eines ferritisch-bainitischen Gefüges mit geringen Anteilen an lamellarem Restaustenit. Dadurch werden für die Kaltumformung günstige, niedrige Streckgrenzenverhältnisse erreicht. Das Produkt aus Bruchfestigkeit und Bruchdehnung erreicht hingegen sehr gute Werte von über 20.000 [MPa*%]. Dieses Gefüge wird durch Anpassung der chemischen Zusammensetzung an vordefinierte Abkühlbedingungen der Stahlrohre aus dem Austenitgebiet erreicht. Die Abkühlbedingungen werden beschrieben durch eine kontinuierliche Abkühlung mit Abkühlraten zwischen 0,5 K/sec und 5 K/sec. Das Legierungskonzept verhindert in diesem Abkühlbereich die Bildung von Perlit. Es entsteht vielmehr Ferrit bzw. bainitischer Ferrit und eine Restphase oder mehrere Restphasen, die abhängig von den Abkühlbedingungen aus unterem Bainit und Martensit mit lamellarem Restaustenit bestehen. Der Stahl zeichnet sich durch exzellente Umformbarkeit im kalten Zustand, sowie durch eine hohe Bruchfestigkeit bei hoher Bruchdehnung aus, die durch die starke Verfestigung infolge des mehrphasigen Charakters verursacht wird. Die Rohre sind dafür vorgesehen in der Weiterverarbeitung kalt umgeformt zu werden.The solution of the problem of the invention is achieved by a new structure concept and its alloy implementation. The new alloy concept is based on the avoidance of perlite and on the setting of a ferritic-bainitic structure with small amounts of lamellar retained austenite. As a result, favorable, low yield strength ratios are achieved for cold forming. The product of breaking strength and elongation at break, on the other hand, reaches very good values of more than 20,000 [MPa *%]. This microstructure is achieved by adapting the chemical composition to predefined cooling conditions of the steel tubes from the austenite region. The cooling conditions are described by a continuous cooling with cooling rates between 0.5 K / sec and 5 K / sec. The alloy concept prevents the formation of perlite in this cooling zone. Rather, ferrite or bainitic ferrite and a Residual phase or several residual phases which, depending on the cooling conditions, consist of lower bainite and martensite with lamellar retained austenite. The steel is characterized by excellent formability in the cold state, as well as by a high breaking strength at high elongation at break, which is caused by the strong solidification due to the multi-phase character. The pipes are intended to be cold formed in further processing.

Das Legierungskonzept beruht auf folgenden grundsätzlichen Überlegungen:

  • Absenkung des C-Gehalts auf 0,06 bis 0,15 % um nur ein geringes Potential zur Bildung von Zementit zu bilden und Härtespitzen bei einer schweißtechnischen Verarbeitung zu verringern.
  • Erhöhung des Si-Gehalts auf 1,0 % oder mehr, um eine Zementitbildung zu unterdrücken.
  • Einstellung eines Mischkristall-Gehaltes (MK-Gehalt) von mehr als 3 % um die kritische Abkühlungsgeschwindigkeit zur Ferrit-/Perlit-Bildung zu verringern und weniger als 4 % um eine hinreichende Duktilität zu erhalten. Dabei besteht der MK-Gehalt aus der Summe der Elemente, die mit Eisen im Austenit Substitutions-MK bilden. Vorteilhaft aus Kostengründen ist hierbei die Verwendung von Si, Mn, Cr und Cu. Kobalt, Nickel, Molybdän können ebenfalls einen entsprechenden Beitrag leisten, haben jedoch Nachteile bezüglich der Kosten. Durch Variation von Cr, Mn, Si innerhalb der beschriebenen Grenzen können zusätzlich die Umwandlungstemperaturen entsprechend der vorgesehenen Verarbeitungsprozesse eingestellt werden, sowie der gewünschte Grad an Mischkristallverfestigung eingestellt werden.
  • Sicherstellung von wenigstens 0,001 und höchstens 0,004 gelöstes freies Bor im Austenit, um im Wesentlichen die Ferritkeimbildung an den Austenitkorngrenzen zu unterdrücken. Hierfür ist das Abbinden des Stickstoffs im Stahl durch Elemente wie Titan, Zirkonium und/oder Hafnium notwendig. Die Menge der Zugabe des oder der Refraktärmetalls/-metalle ergibt sich aus der Stöchiometrie der entsprechenden Nitride. Aus Kostengründen ist dabei die Verwendung von Titan sinnvoll.
  • Sicherstellung eines Stickstoffgehalts, der eine vorteilhafte Keimbildung durch Refraktär-Nitride erlaubt. Der gewünschte Phasenanteil hängt von den angestrebten Warmformstufen ab. Dabei muss beachtet werden, dass Refraktärnitride als harte Phasenanteile auch metallurgische Kerben bilden, die eine Dauerfestigkeit bei höchstfesten Stählen negativ beeinträchtigen können. Stickstoffgehalte in einem Bereich von 0,005% und 0,015% sind vorteilhaft.
  • Gezielter Einsatz der Mikrolegierungselemente Niob und Vanadium für die Bildung von Keimen und die Beeinflussung der Rekristallisation beim Warmwalzen. Niob in Gehalten von 0,02 - 0,05 hat sich als vorteilhaft herausgestellt.
  • Absenkung des Gehalts teurer Legierungselemente (Mo, V, Ni, W) auf maximal 0,3 %, aus Kostengründen vorzugsweise maximal 0,15 %.
  • Verwendung eines geringen Aluminium-Gehalts (maximal 0,1 %), der sonst zur Bildung von harten Aluminiumoxiden führen würde, um die Dauerfestigkeit zu verbessern.
The alloy concept is based on the following fundamental considerations:
  • Reduction of the C content to 0.06 to 0.15% in order to form only a small potential for the formation of cementite and to reduce hardness peaks during a welding process.
  • Increasing the Si content to 1.0% or more to suppress cementite formation.
  • Adjustment of a mixed crystal content (MK content) of more than 3% to reduce the critical cooling rate for ferrite / pearlite formation and less than 4% to obtain a sufficient ductility. The MK content consists of the sum of the elements that form substitution MK with iron in austenite. Advantageous for reasons of cost here is the use of Si, Mn, Cr and Cu. Cobalt, nickel, molybdenum can also contribute, but have disadvantages in terms of cost. In addition, by varying Cr, Mn, Si within the described limits, the transformation temperatures can be adjusted according to the intended processing, and the desired degree of solid solution hardening can be set.
  • Ensuring at least 0.001 and at most 0.004 dissolved free boron in austenite to substantially suppress ferrite nucleation at the austenite grain boundaries. For this purpose, the setting of the nitrogen in the steel by elements such as titanium, zirconium and / or Hafnium necessary. The amount of addition of the refractory metal (s) results from the stoichiometry of the corresponding nitrides. For cost reasons, the use of titanium makes sense.
  • Ensuring a nitrogen content that allows for beneficial nucleation by refractory nitrides. The desired phase proportion depends on the desired thermoforming stages. It should be noted that refractory nitrides form as hard phase fractions also metallurgical notches, which can adversely affect fatigue strength in high-strength steels. Nitrogen contents in the range of 0.005% and 0.015% are advantageous.
  • Targeted use of the micro-alloying elements niobium and vanadium for the formation of nuclei and the influence of recrystallization during hot rolling. Niobium at levels of 0.02-0.05 has been found to be advantageous.
  • Reduction of the content of expensive alloying elements (Mo, V, Ni, W) to a maximum of 0.3%, for cost reasons, preferably a maximum of 0.15%.
  • Use of a low aluminum content (maximum 0.1%), which would otherwise lead to the formation of hard aluminas to improve fatigue strength.

In den nachfolgenden Tabellen sind beispielhaft chemische Zusammensetzungen von ausgewählten Versuchslegierungen, die entsprechend des oben angegebenen Werkstoffkonzepts erstellt wurden, zusammen mit ihren Werkstoffkennwerten angegeben. Das untersuchte Material wurde dabei ohne besondere Prozesssteuerung in einem Laborwalzwerk warmgewalzt und anschließend normalisiert, d.h. bei 950 °C austenitisiert und bei einer Blechdicke von 5 mm an Luft abgekühlt. Demzufolge sind die angegebenen Werte für nicht optimierte Verarbeitungsschritte ermittelt.In the following tables, by way of example, chemical compositions of selected experimental alloys that have been prepared in accordance with the material concept given above, together with their material characteristics, are indicated. The investigated material was hot rolled without special process control in a laboratory mill and then normalized, ie austenitized at 950 ° C and cooled at a sheet thickness of 5 mm in air. As a result, the specified values for non-optimized processing steps are determined.

Alle Angaben bei der chemischen Analyse sind in Massenprozent. Rp02 kennzeichnet die technische Elastizitätsgrenze, Rm die Bruchfestigkeit und A5 die Bruchdehnung. Versuchslegierungen C Si Mn Cr Al W Nb N Ti B Nr. 1 0,09 1,49 1,52 0,15 0,01 <0,01 0,04 0,01 0,05 0,002 Nr. 2 0,08 1,51 0,87 0,62 0,01 0,17 0,05 0,01 0,05 0,002 Nr. 3 0,08 1,51 2,02 0,16 0,01 <0,01 0,05 0,01 0,04 0,002 Nr. 4 0,08 1,97 1,52 0,14 0,01 <0,01 0,05 0,01 0,05 0,002 Mo, Cu, Ni jeweils unter 0,15%, V jeweils unter 0,03 % All data in the chemical analysis are in mass percent. Rp02 denotes the technical elastic limit, Rm the breaking strength and A5 the breaking elongation. experimental alloys C Si Mn Cr al W Nb N Ti B number 1 0.09 1.49 1.52 0.15 0.01 <0.01 0.04 0.01 0.05 0,002 No. 2 0.08 1.51 0.87 0.62 0.01 0.17 0.05 0.01 0.05 0,002 No. 3 0.08 1.51 2.02 0.16 0.01 <0.01 0.05 0.01 0.04 0,002 No. 4 0.08 1.97 1.52 0.14 0.01 <0.01 0.05 0.01 0.05 0,002 Mo, Cu, Ni each less than 0.15%, V each less than 0.03%

Die nachfolgende Tabelle gibt die Werkstoffkennwerte der vier Legierungen nach dem Normalisieren wieder. Rp0,2 [MPa] Rm [MPa] A5 [%] Legierung 1 343 710 30 Legierung 2 338 592 35 Legierung 3 419 831 25 Legierung 4 352 698 31 The following table shows the material characteristics of the four alloys after normalizing. Rp0.2 [MPa] Rm [MPa] A5 [%] Alloy 1 343 710 30 Alloy 2 338 592 35 Alloy 3 419 831 25 Alloy 4 352 698 31

Die Legierungen zeigen ein ferritisches Grundgefüge mit Bainit, Martensit und partiell Restaustenit, wobei die Korngrößen bei den hier nicht optimierten Walzbedingungen bei 10-20 µm liegen. Vereinzelt kommt es auch zur Ausbildung feiner und kleiner Perlit-Nester, die jedoch nicht zeilig angeordnet sind. Durch Verbesserung der Warmwalzbedingungen können die Gefüge deutlich verbessert werden und damit auch die Eigenschaften der Werkstoffe.The alloys show a basic ferritic structure with bainite, martensite and partially retained austenite, the grain sizes being 10-20 μm for the rolling conditions not optimized here. Occasionally it comes to the formation of fine and small pearlite nests, which are not arranged in a row. By improving the hot rolling conditions, the microstructures can be significantly improved and thus also the properties of the materials.

Die nachfolgende Tabelle zeigt beispielhaft Werkstoffkennwerte für die Legierung 2 und 3 nach verbesserten Warmwalzbedingungen, d.h. aus der Standard-Fertigung nahtloser Rohre der Abmessung 36 x 3,6 mm mit Endwalztemperatur 860 °C. Legierung 2 wurde beispielhaft gewählt, weil sie eine hohe Bruchzähigkeit aufweist. Legierung 3 wurde beispielhaft gewählt, weil sie eine hohe Festigkeit aufweist.The following table shows, by way of example, material characteristics for alloys 2 and 3 after improved hot rolling conditions, ie from the standard production of seamless tubes measuring 36 × 3.6 mm with final rolling temperature 860 ° C. Alloy 2 was chosen by way of example because it has a high fracture toughness. Alloy 3 was chosen as an example because it has a high strength.

Exemplarisch die Werkstoffkennwerte der Legierungen 2 und 3 nach dem Warmwalzen nahtloser Rohre: Rp0,2 [MPa] Rm [MPa] A5 [%] Brucheinschnürung Z[%] Wahre Bruchspannung [MPa] Legierung 2 375 677 32 68 1.310 Legierung 3 545 960 24 55 1.610 As an example, the material characteristics of alloys 2 and 3 after hot rolling of seamless tubes: Rp0.2 [MPa] Rm [MPa] A5 [%] Fracture Z [%] True Breaking Voltage [MPa] Alloy 2 375 677 32 68 1310 Alloy 3 545 960 24 55 1610

Durch Absenkung der Endwalztemperaturen konnte bei beiden Legierungen die Korngröße auf ca. 5 µm deutlich verringert werden und das Gefüge homogener entwickelt werden. Die Eigenschaften konnten deutlich verbessert werden. Bemerkenswert ist der Anstieg von Streckgrenze und Zugfestigkeit bei praktisch gleich bleibender Bruchdehnung und hoher Gleichmaßdehnung. Bemerkenswert ist weiterhin, dass eine wahre Bruchspannung von 1.200 - 1.600 MPa erreicht wird, was für Stähle mit weniger als 0,1 % Kohlenstoffgehalt als untypisch bezeichnet werden kann. Legierung 3 erreicht damit die Festigkeit und die Duktilität von TRIP-Stählen bei deutlich abgesenktem C-Gehalt und signifikant vereinfachter Temperatur-Zeit Führung.By lowering the final rolling temperatures, the grain size of both alloys could be significantly reduced to about 5 μm and the microstructure could be developed more homogeneously. The properties could be significantly improved. Noteworthy is the increase in yield strength and tensile strength with practically constant elongation at break and high uniform elongation. It is also noteworthy that a true fracture stress of 1,200 - 1,600 MPa is achieved, which can be considered as untypical for steels with less than 0.1% carbon content. Alloy 3 achieves the strength and ductility of TRIP steels with significantly reduced C content and significantly simplified temperature-time guidance.

Aus einem derartigen Stahl hergestellte Rohre besitzen ein perlitfreies mehrphasiges Gefüge und eröffnen eine Vielzahl von Anwendungsgebieten bzw. Verwendungsmöglichkeiten, von denen nachfolgend einige beispielhaft genannt werden.Tubes made of such a steel have a pearlite-free multi-phase structure and open up a variety of applications and uses, some of which are exemplified below.

Warmgewalzte Rohre aus der Stahllegierung haben bedingt durch das exzellente Verhältnis zwischen Festigkeit und Duktilität besondere Vorteile bei nachfolgenden vorwiegend kalt durchgeführten Verformungsprozessen, z.B. Ziehen, Rundkneten, Drückwalzen, Gewindewalzen, Fließpressen, Stauchen, Autofrettieren, Biegen. Grundsätzlich lassen sich mit der Stahllegierung höchstfeste und kostengünstige kaltgezogene Stahlrohre, z.B. Bohrrohre, Leitungsrohre, Dieseleinspritzleitungen, Zylinderrohre, Rohre für Airbaggeneratoren sowie Rohre für Seitenaufprallträger für Kraftfahrzeuge herstellen. Im Ziehprozess wird für das Erreichen hoher Festigkeiten eine Kaltverfestigung genutzt. Ein Weichglühen vor dem Kaltziehen ist nicht erforderlich. Ein Anlassen ist nach dem Kaltzug optional je nach gewünschter Festigkeitslage möglich. Festigkeiten in einer Größenordnung von deutlich über 1.000 MPa bis hin zu 1.600 MPa sind möglich.Due to the excellent relationship between strength and ductility, hot-rolled steel alloy pipes have particular advantages in subsequent cold forming processes, eg drawing, rotary kneading, spinning, thread rolling, extrusion, compression, autofretting, bending. In principle, the steel alloy can be used to produce high-strength and cost-effective cold-drawn steel tubes, eg drill pipes, line pipes, diesel injection lines, cylinder tubes, tubes for airbag generators, and pipes for side impact beams for motor vehicles produce. In the drawing process, work hardening is used to achieve high strength. Soft annealing before cold drawing is not required. Tempering is optionally possible after cold drawing, depending on the desired strength. Stresses in the range of well over 1,000 MPa up to 1,600 MPa are possible.

Zudem ist mit diesem Werkstoff die Herstellung von Rohren mit kaltgewalztem Gewinde möglich, wie es beispielsweise bei Gerüstrohren oder bei Ankerrohren für Felsanker erforderlich ist. Ausgehend von einer bereits hohen Ausgangsbruchfestigkeit werden hierbei die Streckgrenze und die Bruchfestigkeit durch Kaltverfestigung weiter angehoben. Durch nur vergleichsweise geringe Plastifizierung besteht nach dem Gewindewalzen eine Restdehnung des fertigen Bauteils von über 15 % bei einer Bruchfestigkeit von deutlich über 850 MPa.In addition, with this material, the production of pipes with cold-rolled thread is possible, as is required, for example, scaffold tubes or anchor tubes for rock anchors. Starting from an already high initial breaking strength, the yield strength and the breaking strength are further increased by work hardening. Due to only comparatively low plasticization, there is a residual elongation of the finished component of more than 15% after the thread rolling and a breaking strength of considerably more than 850 MPa.

Insbesondere bei mit hohem Innendruck belasteten Bauteilen zeichnet sich die Legierung dadurch aus, dass keine Perlit-Zeiligkeit auftritt, so dass die Rohre weniger empfindlich auf Innenfehler, die durch Fältelungen hervorgerufen werden, reagieren.Particularly in the case of components subjected to high internal pressure, the alloy is distinguished by the fact that no pearlite line appears, so that the tubes react less sensitively to internal defects caused by pleats.

Die aus der Stahllegierung hergestellten Rohre eignen sich demzufolge auch besonders für die Weiterbearbeitung durch Innenhochdruckumformung. Durch das ausgezeichnete Verformungsverhalten der Stahllegierung ergeben sich für die Innenhochdruckumformung Vorteile, da unter Verwendung der Stahllegierung hohe Bauteilfestigkeiten realisierbar sind.Consequently, the tubes produced from the steel alloy are also particularly suitable for further processing by hydroforming. The excellent deformation behavior of the steel alloy results in hydroforming advantages, since high component strengths can be achieved using the steel alloy.

Neben der Herstellung nahtloser Rohre ist auch die Herstellung geschweißter Rohre aus der Legierung vorteilhaft möglich. Das Legierungskonzept erlaubt die Herstellung von Warmband und Kaltband. Gegenüber üblichen DP-Stählen und TRIP-Stählen ist eine vergleichsweise einfache Temperatur-Zeitführung erforderlich. Darüber hinaus ist die Legierung bis hin zur Blechdicke von 4 mm Normalisierungsfähig, d.h. Entwickelt das Zielgefüge bei Luftabkühlung. Durch den niedrigen Kohlenstoffgehalt ergeben sich nur vergleichsweise niedrige Härtespitzen in der Schweißnaht von geschweißten Rohren. Dies gilt insbesondere im Vergleich zu TRIP-Stählen, die mit dem doppelten Kohlenstoffgehalt eine hohe Aufhärtung zeigen. Durch die fehlende Perlit-Zeiligkeit ergeben sich Vorteile beim klassischen und sehr wirtschaftlichen Pressschweißen. Das Legierungskonzept erlaubt ebenso vorteilhaft ein Strahlschweißen mittels Laserstrahl oder Elektronenstrahl. Der Vorteil des Legierungskonzepts liegt auch hier in dem niedrigen Kohlenstoffgehalt und in der Normalisierungsfähigkeit der Rohre.In addition to the production of seamless pipes, the production of welded pipes from the alloy is also advantageously possible. The alloy concept allows the production of hot strip and cold strip. Compared to conventional DP steels and TRIP steels, a comparatively simple temperature-time control is required. In addition, the alloy can be normalized up to a plate thickness of 4 mm, ie Develops the target structure in case of air cooling. The low carbon content results in only comparatively low hardness peaks in the welded seam of welded pipes. this applies especially in comparison to TRIP steels, which show a high degree of hardening with twice the carbon content. Due to the lack of pearlite brittleness, advantages arise in classical and very economical pressure welding. The alloy concept also advantageously allows beam welding by means of laser beam or electron beam. The advantage of the alloy concept is also the low carbon content and the normalization capability of the tubes.

Insbesondere eignet sich die Stahllegierung zur Herstellung von Rohren für Fahrwerksanwendungen im Automobilbau. Durch das gute Bruchfestigkeits-Umformbarkeits-Verhältnis sind komplexere Bauteile denkbar, die mit den bisherigen Stahlsorten gar nicht bzw. nur unter großem technischen und damit kostenmäßigem Aufwand produziert werden konnten. Zudem ist durch den niedrigen Kohlenstoffgehalt in Kombination mit den anderen Legierungselementen eine gute Schweißbarkeit gewährleistet.In particular, the steel alloy is suitable for the production of tubes for chassis applications in the automotive industry. Due to the good breaking strength-Umformbarkeits ratio more complex components are conceivable, which could not be produced with the previous steel grades or only with great technical and therefore kostenmäßem effort. In addition, the low carbon content in combination with the other alloying elements ensures good weldability.

Claims (6)

Verwendung einer Stahllegierung, die in Massenanteilen aus Kohlenstoff (C) 0,06 - 0,15 Silizium (Si) 1,0 oder mehr Mangan 0,7 - 2,2 Chrom (Cr) 0,1 - 0,8 Molybdän (Mo) 0,2 oder weniger Aluminium (Al) 0,1 oder weniger Vanadium (V) 0,2 oder weniger Stickstoff (N) 0,02 oder weniger Niob (Nb) 0,06 oder weniger Kupfer (Cu) 0,2 oder weniger Nickel (Ni) 0,2 oder weniger Bor (B) 0,001- 0,004 Titan (Ti) 0,001- 0,05 Wolfram (W) 0,2 oder weniger
und Eisen sowie erschmelzungsbedingter Verunreinigungen als Rest besteht, wobei die Summe von Silizium (Si) + Mangan (Mn) + Chrom (Cr) + Kupfer (Cu) in einem Bereich von 3 bis 4 % liegt, und wobei die Stahllegierung ein feines, weitgehend perlitfreies, mehrphasiges Gefüge bestehend aus Ferrit mit eingelagertem Bainit sowie Martensit mit Restaustenit aufweist, wobei das Produkt aus Bruchfestigkeit und Bruchdehnung 20.000 [MPa*%] übersteigt, wobei die Festigkeit Rm im unverformten Zustand mehr als 600 MPa beträgt zur Herstellung von Stahlrohren hoher Festigkeit mit guter Umformbarkeit.Use of a steel alloy, in mass proportions Carbon (C) 0.06 - 0.15 Silicon (Si) 1.0 or more manganese 0.7 - 2.2 Chrome (Cr) 0.1 - 0.8 Molybdenum (Mo) 0.2 or less Aluminum (Al) 0.1 or less Vanadium (V) 0.2 or less Nitrogen (N) 0.02 or less Niobium (Nb) 0.06 or less Copper (Cu) 0.2 or less Nickel (Ni) 0.2 or less Boron (B) 0.001-0.004 Titanium (Ti) 0.001-0.05 Tungsten (W) 0.2 or less
and iron and melt impurities as the remainder, wherein the sum of silicon (Si) + manganese (Mn) + chromium (Cr) + copper (Cu) is in a range of 3 to 4%, and wherein the steel alloy is a fine, broad pearlite-free, multi-phase structure consisting of embedded bainite ferrite and martensite with retained austenite, the product of breaking strength and breaking elongation exceeding 20,000 [MPa *%], the strength Rm in the undeformed state being more than 600 MPa for the production of high strength steel tubes with good formability. Verwendung nach Anspruch 1, dadurch gekennzeichnet, dass die Stahllegierung in Massenanteilen besteht aus Kohlenstoff (C) 0,06 - 0,15 Silizium (Si) 1,0 - 2,0 Mangan (Mn) 0,7 - 2,2 Chrom (Cr) 0,1 - 0,8 Molybdän (Mo) 0,15 oder weniger Aluminium (AI) 0,05 oder weniger Vanadium (V) 0,15 oder weniger Stickstoff (N) 0,02 oder weniger Niob (Nb) 0,06 oder weniger Kupfer (Cu) 0,2 oder weniger Nickel (Ni) 0,2 oder weniger Bor (B) 0,001- 0,004 Titan (Ti) 0,001- 0,05 Wolfram (W) 0,15 oder weniger
und Eisen sowie erschmelzungsbedingter Verunreinigungen als Rest, wobei die Summe von Silizium (Si) + Mangan (Mn) + Chrom (Cr) + Kupfer (Cu) in einem Bereich von 3 bis 3,8 % liegtUse according to claim 1, characterized in that the steel alloy consists in mass fractions Carbon (C) 0.06 - 0.15 Silicon (Si) 1.0 - 2.0 Manganese (Mn) 0.7 - 2.2 Chrome (Cr) 0.1 - 0.8 Molybdenum (Mo) 0.15 or less Aluminum (AI) 0.05 or less Vanadium (V) 0.15 or less Nitrogen (N) 0.02 or less Niobium (Nb) 0.06 or less Copper (Cu) 0.2 or less Nickel (Ni) 0.2 or less Boron (B) 0.001-0.004 Titanium (Ti) 0.001-0.05 Tungsten (W) 0.15 or less
and iron and melt-caused impurities as the remainder, wherein the sum of silicon (Si) + manganese (Mn) + chromium (Cr) + copper (Cu) is in a range of 3 to 3.8% Verwendung nach Anspruch 1, dadurch gekennzeichnet, dass die Stahllegierung in Massenanteilen besteht aus: Kohlenstoff (C) 0,06 - 0,10 Silizium (Si) 1,2 - 1,8 Mangan (Mn) 1,4 - 2,2 Chrom (Cr) 0,1 - 0,4 Molybdän (Mo) 0,15 oder weniger Aluminium (AI) 0,05 oder weniger Vanadium (V) 0,15 oder weniger Stickstoff (N) 0,02 oder weniger Niob (Nb) 0,02 - 0,06 Kupfer (Cu) 0,2 oder weniger Nickel (Ni) 0,2 oder weniger Bor (B) 0,001- 0,004 Titan (Ti) 0,001- 0,05 Wolfram (W) 0,15 oder weniger
und Eisen sowie erschmelzungsbedingter Verunreinigungen als Rest, wobei die Summe von Silizium (Si) + Mangan (Mn) + Chrom (Cr) + Kupfer (Cu) in einem Bereich von 3 bis 3,8 % liegtUse according to claim 1, characterized in that the steel alloy in mass proportions consists of: Carbon (C) 0.06 - 0.10 Silicon (Si) 1.2 - 1.8 Manganese (Mn) 1.4 - 2.2 Chrome (Cr) 0.1-0.4 Molybdenum (Mo) 0.15 or less Aluminum (AI) 0.05 or less Vanadium (V) 0.15 or less Nitrogen (N) 0.02 or less Niobium (Nb) 0.02-0.06 Copper (Cu) 0.2 or less Nickel (Ni) 0.2 or less Boron (B) 0.001-0.004 Titanium (Ti) 0.001-0.05 Tungsten (W) 0.15 or less
and iron and melt-caused impurities as the remainder, wherein the sum of silicon (Si) + manganese (Mn) + chromium (Cr) + copper (Cu) is in a range of 3 to 3.8% Verwendung einer Stahllegierung entsprechend Anspruch 1 oder 2 oder 3 zur Herstellung nahtloser warmgewalzter Rohre mit guter Kaltumformbarkeit.Use of a steel alloy according to claim 1 or 2 or 3 for the production of seamless hot-rolled tubes with good cold workability. Verwendung einer Stahllegierung entsprechend Anspruch 1 oder 2 oder 3 zur Herstellung geschweißter Stahlrohre durch Schmelzschweißen und Festkörperschweißen.Use of a steel alloy according to claim 1 or 2 or 3 for the production of welded steel tubes by fusion welding and solid-state welding. Verwendung einer Stahllegierung entsprechend Anspruch 4 oder 5 zur Herstellung kaltgezogener Stahlrohre hoher Festigkeit.Use of a steel alloy according to claim 4 or 5 for the production of cold-drawn steel tubes of high strength.
EP08011681.7A 2007-06-27 2008-06-27 Use of an extremely resistant steel alloy for producing steel pipes with high resistance and good plasticity Not-in-force EP2009120B1 (en)

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