EP2547800B1 - Method for producing workpieces from lightweight steel having material properties that can be adjusted over the wall thickness - Google Patents

Method for producing workpieces from lightweight steel having material properties that can be adjusted over the wall thickness Download PDF

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Publication number
EP2547800B1
EP2547800B1 EP11715650.5A EP11715650A EP2547800B1 EP 2547800 B1 EP2547800 B1 EP 2547800B1 EP 11715650 A EP11715650 A EP 11715650A EP 2547800 B1 EP2547800 B1 EP 2547800B1
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Prior art keywords
forming
annealing
workpieces
hot
atmosphere
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German (de)
French (fr)
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EP2547800A1 (en
Inventor
Daniela John
Manuel Otto
Rune SCHMIDT-JÜRGENSEN
Thomas Evertz
Zacharias Georgeou
Bianca Springub
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Salzgitter Flachstahl GmbH
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Salzgitter Flachstahl GmbH
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    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0257—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/42—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for armour plate
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

  • the invention relates to a method for producing workpieces made of lightweight steel with material properties that can be adjusted via the wall thickness, according to the preamble of claim 1.
  • the rest of the iron including standard steel components.
  • Cr, Cu, Ti, Zr, V and Nb can be added as required.
  • This known lightweight steel has a partially stabilized ⁇ -mixed crystal structure with defined stacking fault energy with a z. T. multiple TRIP effect, which transforms the stress or strain-induced transformation of a face-centered ⁇ -mixed crystal (austenite) in an ⁇ -martensite (hexagonal closest packing), which then transforms in further deformation in a body-centered ⁇ -martensite and retained austenite.
  • TRIP Transformation Induced Plasticity
  • TWIP winning Induced Plasticity
  • a method for producing a composite strip of steel for example, from DE 101 24 594 A1 known. Thereafter, a ferritic core strip directly cast by the two-roll method is plated with an austenitic or high-alloy ferritic cold-rolled strip. Tubes with different material properties over the wall thickness are among others from the EP 0 944 443 B1 known. Here another tube is inserted into a tube and connected here, with different materials are used for outer and inner tube.
  • Another method for producing a composite material is known from DE 39 04 776 C2 in which by means of diffusion welding several layers of steel are joined together and these layers are alloyed by means of metalloids in a gas atmosphere in the form that adjusts a different concentration profile of the metalloids over the cross section of the flat product.
  • This method is complex and also has disadvantages in weight compared to only consisting of lightweight steel workpieces.
  • the object of the invention is to provide a method for producing workpieces made of austenitic lightweight steel, with which different material properties can be set variably in a simple and cost-effective manner while maintaining the weight advantage of the lightweight steel on the band or wall thickness.
  • the component or precursor is subjected to a decarburizing annealing under an oxidizing atmosphere in such a way that a ferritic or metastable austenite structure is formed in the near-surface regions, whose layer thickness via variation of the annealing parameters (temperature, holding time) and annealing atmosphere (gas composition, Partial horr) is adjustable and to produce a Property gradient of a subsequent accelerated cooling and / or cold working is subjected.
  • the core of the invention is to set a ferritic or ferritic-austenitic material for steel materials, which are permanently austenitic due to their alloy concept permanently high enough carbon contents, by targeted decarburization starting from the workpiece surface, with the all structural states of ferritic steels corresponding heating and cooling conditions can be produced.
  • ferritic or ferritic-austenitic material for steel materials, which are permanently austenitic due to their alloy concept permanently high enough carbon contents, by targeted decarburization starting from the workpiece surface, with the all structural states of ferritic steels corresponding heating and cooling conditions can be produced.
  • These include the structural components ferrite, bainite and in particular martensite and carbide in various morphologies.
  • steels whose forming, due to their chemical composition (stacking fault energy) preferably takes place via the formation of twins (TWIP), can be converted locally on the surface to a deformation-induced transformation of austenite into martensite (TRIP) after a targeted edge decarburization.
  • TWIP twins
  • deformation-induced martensite with correspondingly high hardness can then be produced, for example, during the cold forming of a sheet in the decarburized regions.
  • an initially unstable austenite is present which has the TRIP effect after forming.
  • the core is a stable austenite with the starting carbon content, which has twins after the forming and a high ductility at a lower hardness.
  • edge oxidation It is also known in carboniferous ferritic steel grades that during hardening or quenching a so-called edge oxidation can occur, which is responsible both for scaling of the surface and for decarburization in near-surface regions.
  • the decarburization is undesirable because the material loses hardness in these areas. Therefore, the maximum depth of decarburization is limited in standards and customer specifications (eg tempered steels or ball bearings).
  • the present invention leaves this prior art and goes the opposite way, in which the decarburization of austenitic lightweight steel combined with accelerated cooling and / or cold working is used specifically for increasing the hardness, can be adjusted with the different material properties in the sheet thickness direction.
  • the degree of decarburization and its depth can be adjusted starting from the workpiece surface.
  • the oxidizing Glühatmosphotre can z. B. be air or it can be selectively added oxygen or oxygen-containing gases, over the height of the gas partial pressure of the degree of decarburization can also be varied.
  • decarburization it is likewise possible to bring about decarburization by deliberately guiding the reheating conditions before hot rolling and / or between the hot rolling passes (temperature, holding times) under an oxidizing annealing atmosphere.
  • the degree of decarburization and its depth can then be precisely adjusted starting from the workpiece surface. For example, decarburization becomes more intense during a longer rolling time or lying time in the furnace and higher rolling temperature and acts deeper into the workpiece.
  • the degree of decarburization can be varied by the edge decarburized layer can be reduced again by balancing processes.
  • a gradient of the decarburization can be adjusted in a targeted manner over the thickness of the workpiece with corresponding properties after the subsequent targeted cooling and / or cold forming.
  • the height of the cooling rate and the degree of deformation influences the formation of martensite and thus the degree of hardening.
  • Such a material is particularly suitable for applications in which a high surface hardness combined with a high toughness is required, such as for bullet-proof components, because the material has a high Martensit with a very high energy absorption in the event of fire.
  • FIG. 1a FIG. 1b
  • Figure 1c Figure 1d C 0.7 0.7 0.7 0.7 al 2.5 2.5 2.5 2.5 2.5 Si 2.5 0.2 0.3 0.3 Mn 15 15 15 15 15
  • Micrographs of workpieces treated according to the invention for martensite formation and corresponding hardness measurements are shown in two micrographs ( FIG. 1a, 1b ).
  • the materials differ here in the Si content.
  • the micrographs show a layer of martensite of different thickness in the near-surface regions and the associated marked increase in hardness compared to the austenite microstructure in the matrix.
  • the steel shows according to FIG. 1a a significantly higher increase in hardness than the steel according to FIG. 1b ,
  • the necessary for the decarburization oxidizing annealing of the samples FIG. 1a and 1 b was performed under ambient atmosphere (air) at an annealing temperature of 1150 ° C and an annealing time of 1h.
  • the samples were not quenched after the annealing treatment, but only subjected to cold working to detect the TRIP effect (formation of strain-induced martensite).
  • FIGS. 1c and 1d show that, depending on the degree of decarburization, marginal areas with local twinning can also be set. Likewise, depending on the degree of decarburization, a variation of the carbide formation over the sheet thickness can be set.
  • workpieces made of lightweight steel must satisfy comparatively high demands with regard to processability, for example by cold forming, welding and / or corrosion protection (eg coatings containing zinc).
  • the process according to the invention is basically suitable for all austenitic alloys at room temperature, but especially for high-alloyed lightweight steels.
  • the method according to the invention advantageously offers the possibility of taking into account the specific requirements of the material properties of the finished component, in that they can be adjusted in a targeted manner over the strip thickness.

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

Die Erfindung betrifft ein Verfahren zur Herstellung von Werkstücken aus Leichtbaustahl mit über die Wanddicke einstellbaren Werkstoffeigenschaften gemäß dem Oberbegriff des Anspruches 1.The invention relates to a method for producing workpieces made of lightweight steel with material properties that can be adjusted via the wall thickness, according to the preamble of claim 1.

Im Folgenden werden unter Werkstücken Bauteile oder Vorprodukte für Bauteile, wie z. B. Bänder, Bleche oder Rohre, verstanden, die beispielsweise in Bereichen des Maschinen-, Anlagen-, Stahl- und Schiffbaus sowie insbesondere des Kraftfahrzeugbaus Anwendung finden.Below are under workpieces components or precursors for components such. As bands, sheets or tubes, understood that are used for example in areas of engineering, plant, steel and shipbuilding and in particular of motor vehicle application.

Gerade der stark umkämpfte Automobilmarkt zwingt die Hersteller ständig nach Lösungen zur Senkung des Flottenverbrauchs unter Beibehaltung eines höchstmöglichen Komforts und Insassenschutzes zu suchen. Dabei spielt einerseits die Gewichtsersparnis aller Fahrzeugkomponenten eine entscheidende Rolle, andererseits aber auch ein die passive Sicherheit der Passagiere förderndes Verhalten der einzelnen Bauteile bei hohen statischen und dynamischen Beanspruchungen im Betrieb und im Crashfall.Especially the highly competitive automotive market forces manufacturers to constantly look for solutions to reduce fleet consumption while maintaining the highest possible comfort and occupant protection. On the one hand, the weight saving of all vehicle components plays a decisive role, on the other hand, but also the passive safety of the passengers promoting behavior of the individual components with high static and dynamic stresses during operation and in the event of a crash.

In den letzten Jahren hat es große Entwicklungsfortschritte auf dem Gebiet der sogenannten Leichtbaustähle gegeben, die sich durch ein geringes spezifisches Gewicht bei gleichzeitig hohen Festigkeiten und Zähigkeiten auszeichnen (z. B. EP 0 489 727 B1 , EP 0 573 641 B1 , DE 199 00199 A1 ) sowie eine hohe Duktilität aufweisen und damit für den Fahrzeugbau von großem Interesse sind.In recent years, there has been great development progress in the field of so-called lightweight steels, which are characterized by a low specific weight with high strength and toughness at the same time (eg. EP 0 489 727 B1 . EP 0 573 641 B1 . DE 199 00199 A1 ) and have a high ductility and thus are of great interest to the vehicle industry.

Bei diesen im Ausgangszustand austenitischen Stählen wird durch den hohen Anteil von Legierungsbestandteilen mit einem spezifischen Gewicht weit unterhalb des spezifischen Gewichts von Eisen (Mn, Si, Al) eine für die Automobilindustrie vorteilhafte Gewichtsreduzierung unter Beibehaltung der bisherigen Konstruktionsbauweise erreicht.In these austenitic steels in the initial state, the high proportion of alloying constituents having a specific gravity well below the specific gravity of iron (Mn, Si, Al) achieves a weight reduction advantageous to the automobile industry while maintaining the previous design construction.

Aus der DE 10 2004 061 284 A1 ist z. B. ein Leichtbaustahl bekannt mit einer Legierungszusammensetzung (in Gew.%): C 0,04 bis < 1,0 Al 0,05 bis < 4,0 Si 0,05 bis ≤ 6,0 Mn 9,0 bis < 18,0 From the DE 10 2004 061 284 A1 is z. As a lightweight steel known with an alloy composition (in wt.%): C 0.04 to <1.0 al 0.05 to <4.0 Si 0.05 to ≤ 6.0 Mn 9.0 to <18,0

Rest Eisen einschließlich üblicher Stahlbegleitelemente. Optional können je nach Anforderung Cr, Cu, Ti, Zr, V und Nb zugegeben werden.The rest of the iron, including standard steel components. Optionally, Cr, Cu, Ti, Zr, V and Nb can be added as required.

Dieser bekannte Leichtbaustahl weist ein teilstabilisiertes γ-Mischkristall-Gefüge mit definierter Stapelfehlerenergie mit einem z. T. multiplen TRIP-Effekt auf, der die spannungs- oder dehnungsinduzierte Umwandlung eines flächenzentrierten γ-Mischkristalls (Austenit) in einen ε-Martensit (hexagonal dichteste Kugelpackung), der dann bei weiterer Verformung in einen raumzentrierten α-Martensit und Restaustenit transformiert.This known lightweight steel has a partially stabilized γ-mixed crystal structure with defined stacking fault energy with a z. T. multiple TRIP effect, which transforms the stress or strain-induced transformation of a face-centered γ-mixed crystal (austenite) in an ε-martensite (hexagonal closest packing), which then transforms in further deformation in a body-centered α-martensite and retained austenite.

Der hohe Umformgrad wird durch TRIP- (Transformation Induced Plasticity) und TWIP- (Twinning Induced Plasticity) Eigenschaften des Stahles erreicht.The high degree of deformation is achieved by TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) properties of the steel.

Zahlreiche Versuche haben zur Erkenntnis geführt, dass im komplexen Zusammenspiel zwischen Al, Si, Mn dem Kohlenstoffgehalt eine überragende Bedeutung zukommt. Er erhöht zum Einen die Stapelfehlerenergie und erweitert zum Anderen den metastabilen Austenitbereich. Dadurch können die verformungsinduzierte Martensitbildung und die damit verbundene Verfestigung und auch die Duktilität beeinflusst werden.Numerous experiments have led to the realization that in the complex interaction between Al, Si, Mn, the carbon content is of paramount importance. On the one hand, it increases the stacking fault energy and, on the other hand, expands the metastable austenite area. As a result, the deformation-induced martensite formation and the associated solidification and also the ductility can be influenced.

Mit diesen Leichtbaustählen können die Kundenanforderungen schon weitestgehend erfüllt werden, es besteht jedoch auch weiterhin der Wunsch nach beanspruchungsoptimierten Werkstücken aus Leichtbaustahl, die entsprechend den zu erwartenden Beanspruchungen im Betrieb in Wand- oder Blechdickenrichtung unterschiedliche Werkstoffeigenschaften
hinsichtlich Festigkeit, Zähigkeit, Verschleißfestigkeit, usw. aufweisen. Als Beispiel seien hier beschusssichere Fahrzeugteile genannt, bei denen das Bauteil eine oberflächenharte Schicht zur Abwehr von Projektilen und für ein hohes Energieaufnahmevermögen im Beschussfall eine hohe Zähigkeit der darunter liegenden Schicht aufweisen muss.With these lightweight steels customer requirements can be met as far as possible, but there is still the desire for stress-optimized workpieces made of lightweight steel, according to the expected stresses in operation in wall or sheet thickness different material properties
in terms of strength, toughness, wear resistance, etc. As an example, bullet-proof vehicle parts may be mentioned, in which the component must have a surface-hard layer for repelling projectiles and, for a high energy absorption capacity, a high toughness of the underlying layer in the event of a fire.

Ein Verfahren zum Herstellen eines Verbundbandes aus Stahl ist beispielsweise aus der DE 101 24 594 A1 bekannt. Hiernach wird ein nach dem Zweirollenverfahren direkt gegossenes ferritisches Kernband mit einem austenitischen oder hochlegierten ferritischen Kaltband plattiert. Rohre mit über die Wanddicke unterschiedlichen Werkstoffeigenschaften sind u. a. aus der EP 0 944 443 B1 bekannt. Hier wird in ein Rohr ein weiteres Rohr eingeschoben und hiermit verbunden, wobei unterschiedliche Werkstoffe für Außen- und Innenrohr verwendet werden.A method for producing a composite strip of steel, for example, from DE 101 24 594 A1 known. Thereafter, a ferritic core strip directly cast by the two-roll method is plated with an austenitic or high-alloy ferritic cold-rolled strip. Tubes with different material properties over the wall thickness are among others from the EP 0 944 443 B1 known. Here another tube is inserted into a tube and connected here, with different materials are used for outer and inner tube.

Nachteil dieser bekannten Verfahren ist der durch die Plattierung bedingte scharfe Sprung der Eigenschaften des Verbundwerkstoffes, der die den jeweiligen Anforderungen entsprechende optimale Anpassung der Eigenschaften über die Wand- bzw, Banddicke erschwert sowie die hohen Kosten für die Herstellung der Plattierung. Außerdem geht der Gewichtsvorteil der Leichtbaustähle durch die Plattierung mit konventionellen Stählen weitgehend verloren.Disadvantage of this known method is caused by the plating sharp jump in the properties of the composite material, which complies with the respective requirements corresponding optimal adjustment of the properties on the wall or strip thickness and the high cost of producing the plating. In addition, the weight advantage of lightweight steels is largely lost by plating with conventional steels.

Ein weiteres Verfahren zur Herstellung eines Verbundwerkstoffes ist aus der DE 39 04 776 C2 bekannt, bei dem mittels Diffusionsschweißen mehrere Schichten aus Stahl miteinander verbunden werden und diese Schichten mittels Metalloiden in Gasatmosphäre auflegiert werden in der Form, dass sich über den Querschnitt des Flachproduktes ein unterschiedliches Konzentrationsprofil der Metalloide einstellt.Another method for producing a composite material is known from DE 39 04 776 C2 in which by means of diffusion welding several layers of steel are joined together and these layers are alloyed by means of metalloids in a gas atmosphere in the form that adjusts a different concentration profile of the metalloids over the cross section of the flat product.

Hierdurch lassen sich über den Querschnitt des Verbundbandes unterschiedliche Werkstoffeigenschaften hinsichtlich Festigkeit und Zähigkeit einstellen.As a result, different material properties with regard to strength and toughness can be set over the cross section of the composite strip.

Auch dieses Verfahren ist aufwändig und weist ebenfalls Gewichtsnachteile gegenüber nur aus Leichtbaustahl bestehenden Werkstücken auf.This method is complex and also has disadvantages in weight compared to only consisting of lightweight steel workpieces.

Aus der DE 10 2007 039 013 B3 ist es schließlich bekannt, Warmband aus Vergütungsstahl einer entkohlenden Glühbehandlung zu unterziehen, um eine hohe Kernhärte einerseits und eine gute Verformbarkeit andererseits miteinander zu kombinieren.From the DE 10 2007 039 013 B3 Finally, it is known to subject hot strip made of tempered steel of a decarburizing annealing to combine a high core hardness on the one hand and good ductility on the other.

Aufgabe der Erfindung ist es, ein Verfahren zur Herstellung von Werkstücken aus austenitischem Leichtbaustahl anzugeben, mit dem auf einfache und kostengünstige Weise unter Beibehaltung des Gewichtsvorteils des Leichtbaustahls über die Band- bzw. Wanddicke unterschiedliche Werkstoffeigenschaften variabel eingestellt werden können.The object of the invention is to provide a method for producing workpieces made of austenitic lightweight steel, with which different material properties can be set variably in a simple and cost-effective manner while maintaining the weight advantage of the lightweight steel on the band or wall thickness.

Diese Aufgabe wird ausgehend vom Oberbegriff in Verbindung mit den kennzeichnenden Merkmalen des Anspruchs 1 gelöst. Vorteilhafte Weiterbildungen sind Gegenstand von Unteransprüchen.This object is achieved on the basis of the preamble in conjunction with the characterizing features of claim 1. Advantageous developments are the subject of dependent claims.

Nach der Lehre der Erfindung wird das Bauteil oder Vorprodukt einer entkohlenden Glühbehandlung unter oxidierender Atmosphäre unterzogen in der Weise, dass sich in den oberflächennahen Bereichen ein ferritisches oder metastabiles Austenitgefüge bildet, dessen Schichtdicke über Variation der Glühparameter (Temperatur, Haltezeit) und Glühatmosphäre (Gaszusammensetzung, Partialdruck) einstellbar ist und zur Erzeugung eines Eigenschaftsgradienten einer nachfolgenden beschleunigten Abkühlung und/oder einer Kaltumformung unterzogen wird.According to the teachings of the invention, the component or precursor is subjected to a decarburizing annealing under an oxidizing atmosphere in such a way that a ferritic or metastable austenite structure is formed in the near-surface regions, whose layer thickness via variation of the annealing parameters (temperature, holding time) and annealing atmosphere (gas composition, Partialdruck) is adjustable and to produce a Property gradient of a subsequent accelerated cooling and / or cold working is subjected.

Der Kern der Erfindung besteht darin, bei Stahlwerkstoffen, die aufgrund ihres Legierungskonzeptes permanent austenitisch sind und dabei ausreichend hohe Kohlenstoffgehalte aufweisen, durch eine gezielte Entkohlung ausgehend von der Werkstückoberfläche lokal einen ferritischen bzw. ferritisch-austenitischen Werkstoff einzustellen, mit dem alle Gefügezustände ferritischer Stähle durch entsprechende Erwärmungs- und Abkühlbedingungen herstellbar sind. Dazu gehören die Gefügebestandteile Ferrit, Bainit und insbesondere Martensit sowie Karbid in verschiedenen Morphologien.The core of the invention is to set a ferritic or ferritic-austenitic material for steel materials, which are permanently austenitic due to their alloy concept permanently high enough carbon contents, by targeted decarburization starting from the workpiece surface, with the all structural states of ferritic steels corresponding heating and cooling conditions can be produced. These include the structural components ferrite, bainite and in particular martensite and carbide in various morphologies.

Darüber hinaus können Stähle, deren Umformung aufgrund ihrer chemischen Zusammensetzung (Stapelfehlerenergie) bevorzugt über die Bildung von Zwillingen erfolgt (TWIP), nach einer gezielten Randentkohlung lokal an der Oberfläche zu einer verformungsinduzierten Umwandlung von Austenit in Martensit (TRIP) überführt werden.In addition, steels whose forming, due to their chemical composition (stacking fault energy) preferably takes place via the formation of twins (TWIP), can be converted locally on the surface to a deformation-induced transformation of austenite into martensite (TRIP) after a targeted edge decarburization.

Hierbei kann dann beispielsweise bei der Kaltumformung eines Bleches in den entkohlten Bereichen verformungsinduzierter Martensit mit entsprechend hoher Härte erzeugt werden. Dabei liegt in der gezielt entkohlten Randschicht ein zunächst instabiler Austenit vor, der nach der Umformung den TRIP-Effekt aufweist.In this case, deformation-induced martensite with correspondingly high hardness can then be produced, for example, during the cold forming of a sheet in the decarburized regions. In the deliberately decarburized boundary layer, an initially unstable austenite is present which has the TRIP effect after forming.

Bei Versuchen wurde durch das entkohlende Glühen in allen Proben eine Randentkohlung eingestellt, wie durch GDOES-Messungen gezeigt werden konnte. Die metallographischen Auswertungen zeigten bei allen Proben eine durch gezieltes Abkühlen und/oder Kaltumformung erzeugte Martensitbildung im Bereich der Werkstückoberfläche mit gleichzeitigem Härteanstieg im Randbereich des Werkstücks.In experiments, decarburization annealing in all samples set a surface decarburization, as evidenced by GDOES measurements. The metallographic evaluations showed for all samples a martensite formation in the area of the workpiece surface caused by deliberate cooling and / or cold forming with simultaneous increase in hardness in the edge region of the workpiece.

Dementsprechend konnte mit gezielter Randentkohlung durch eine Glühung an oxidierender Atmosphäre ein Gradientenwerkstoff hergestellt werden.Accordingly, it was possible to produce a gradient material with targeted edge decarburization by annealing in an oxidizing atmosphere.

Im Randbereich weist der so wärmebehandelte Stahl durch den reduzierten Kohlenstoffgehalt einen metastabilen Austenit auf, der bei der anschließenden Kaltumformung und/ oder bereits durch Abschrecken Martensit bildet und damit eine entsprechend hohe Härte aufweist. Im Kern liegt ein stabiler Austenit mit dem Ausgangskohlenstoffgehalt vor, der nach der Umformung Zwillinge und eine hohe Duktilität bei einer geringeren Härte aufweist.In the edge region of the so-heat-treated steel due to the reduced carbon content on a metastable austenite, which forms in the subsequent cold working and / or already by quenching martensite and thus has a correspondingly high hardness. The core is a stable austenite with the starting carbon content, which has twins after the forming and a high ductility at a lower hardness.

Eine der Wärmebehandlung anschließende Kaltumformung führte durch den auftretenden TRIP-Effekt zu einer Martensitbildung, verbunden mit einem erheblichen Anstieg der Härte.Cold forming following the heat treatment resulted in the formation of martensite due to the TRIP effect, which was associated with a considerable increase in hardness.

Bekannt ist, dass kohlenstoffhaltige ferritische Stahlsorten für das Härten oder Vergüten eingesetzt werden, um unterschiedliche Werkstoffeigenschaften von Werkstückoberfläche und Kern zu erzielen. Austenitische Stahlsorten können dagegen werkstoffbedingt nicht gehärtet werden.It is known that carboniferous ferritic steel grades are used for hardening or tempering in order to achieve different material properties of workpiece surface and core. Austenitic steel grades, however, can not be hardened due to the material.

Bekannt ist bei kohlenstoffhaltigen ferritischen Stahlsorten ebenfalls, dass beim Härten oder Vergüten eine sogenannte Randoxidation auftreten kann, die sowohl für eine Verzunderung der Oberfläche wie auch für eine Entkohlung in oberflächennahen Bereichen verantwortlich ist.It is also known in carboniferous ferritic steel grades that during hardening or quenching a so-called edge oxidation can occur, which is responsible both for scaling of the surface and for decarburization in near-surface regions.

Üblicherweise ist die Entkohlung unerwünscht, da der Werkstoff in diesen Bereichen an Härte verliert. Deshalb wird die maximale Tiefe der Entkohlung in Normen und Kundenspezifikationen beschränkt (z. B. Vergütungsstähle oder Kugellager).Usually, the decarburization is undesirable because the material loses hardness in these areas. Therefore, the maximum depth of decarburization is limited in standards and customer specifications (eg tempered steels or ball bearings).

Die vorliegende Erfindung verlässt diesen Stand der Technik und geht den entgegen gesetzten Weg, in dem die Entkohlung des austenitischen Leichtbaustahls kombiniert mit beschleunigter Abkühlung und/oder einer Kaltumformung gezielt zur Härtesteigerung eingesetzt wird, mit der unterschiedliche Werkstoffeigenschaften in Blechdickenrichtung eingestellt werden können.The present invention leaves this prior art and goes the opposite way, in which the decarburization of austenitic lightweight steel combined with accelerated cooling and / or cold working is used specifically for increasing the hardness, can be adjusted with the different material properties in the sheet thickness direction.

Gegenüber bekannten Verbundwerkstoffen aus ferritischen Stahlsorten lassen sich blechdickenabhängige Werkstoffeigenschaften auf einfache und kostengünstige Weise unter Beibehaltung der Gewichtsvorteile und der sonstigen vorteilhaften Eigenschaften des Leichtbaustahls realisieren. Mit Hilfe des erfindungsgemäßen Verfahrens ist es jetzt möglich, hochlegierte austenitische Leichtbaustähle für sogenannte Gradientenwerkstoffe einzusetzen. Die Entkohlung, d. h. die Ausbildung eines Gradientenwerkstoffs kann sowohl am Warmband wie auch am Kaltband vorgenommen werden, wobei die so behandelten Bänder mit einem metallischen Überzug versehen sein können. Als metallische Überzüge kommen z. B. auf Zn, wie auch auf Mg oder Al basierende Überzüge mit unterschiedlichen Legierungsanteilen in Frage.Compared with known composites of ferritic steel grades, sheet-thickness-dependent material properties can be realized in a simple and cost-effective manner while maintaining the weight advantages and other advantageous properties of the lightweight steel. With the aid of the method according to the invention it is now possible to use high-alloy austenitic lightweight structural steels for so-called gradient materials. Decarburization, d. H. the formation of a gradient material can be carried out both on the hot strip and on the cold strip, wherein the strips thus treated can be provided with a metallic coating. As metallic coatings z. B. on Zn, as well as on Mg or Al-based coatings with different proportions of alloy in question.

Mit einem solchen erfindungsgemäß hergestellten Gradientenwerkstoff wird der Anwendungsbereich bekannter Leichtbaustähle gerade im Automobilbereich deutlich vergrößert, wobei jeweils beanspruchungsoptimierte Werkstücke kombiniert mit den Vorteilen der Leichtbaustähle zum Einsatz gelangen.With such a gradient material produced according to the invention, the field of application of known lightweight structural steels is markedly increased, especially in the automotive sector, with stress-optimized workpieces combined with the advantages of lightweight structural steels being used in each case.

Darüber hinaus ist der durch die unterschiedlichen Gefüge einstellbare Gradient der Festigkeit für die Auslegung von Konstruktionen z. B. im Baubereich von Bedeutung.In addition, the adjustable by the different microstructure gradient of strength for the design of constructions z. B. in the construction sector of importance.

Durch eine gezielte Führung der Glühparameter (Temperatur, Haltezeit) sowie der oxidierenden Glühatmosphäre (Gaszusammensetzung, Partialdruck) bei der Wärmebehandlung kann der Grad der Entkohlung und dessen Tiefe ausgehend von der Werkstückoberfläche eingestellt werden.By a targeted management of the annealing parameters (temperature, holding time) and the oxidizing annealing atmosphere (gas composition, partial pressure) in the heat treatment, the degree of decarburization and its depth can be adjusted starting from the workpiece surface.

Beispielsweise wird bei längerer Glühzeit und höherer Glühtemperatur die Entkohlung intensiver und wirkt tiefer in das Werkstück ein. Die oxidierende Glühatmosphäre kann z. B. Luft sein oder es können gezielt Sauerstoff oder sauerstoffhaltige Gase zugesetzt werden, wobei über die Höhe des Gaspartialdrucks der Grad der Entkohlung ebenfalls variiert werden kann.For example, with longer annealing time and higher annealing temperature, the decarburization becomes more intense and acts deeper into the workpiece. The oxidizing Glühatmosphäre can z. B. be air or it can be selectively added oxygen or oxygen-containing gases, over the height of the gas partial pressure of the degree of decarburization can also be varied.

Ebenso ist es möglich, durch eine gezielte Führung der Wiedererwärmbedingungen vor dem Warmwalzen und/oder zwischen den Warmwalzstichen (Temperatur, Haltezeiten) unter oxidierender Glühatmosphäre eine Entkohlung herbeizuführen. In Kombination mit einer reduzierenden oder inerten Glühbehandlung kann der Grad der Entkohlung und deren Tiefe ausgehend von der Werkstückoberfläche anschließend genau eingestellt werden. Z. B. wird bei längerer Walzzeit bzw. Liegezeit im Ofen und höherer Walztemperatur die Entkohlung intensiver und wirkt tiefer in das Werkstück ein.It is likewise possible to bring about decarburization by deliberately guiding the reheating conditions before hot rolling and / or between the hot rolling passes (temperature, holding times) under an oxidizing annealing atmosphere. In combination with a reducing or inert annealing treatment, the degree of decarburization and its depth can then be precisely adjusted starting from the workpiece surface. For example, decarburization becomes more intense during a longer rolling time or lying time in the furnace and higher rolling temperature and acts deeper into the workpiece.

Durch die anschließende reduzierende oder inerte Glühbehandlung kann der Grad der Entkohlung variiert werden, indem die randentkohlte Schicht durch Ausgleichsprozesse wieder verringert werden kann. Hierdurch lässt sich über die Dicke des Werkstücks gezielt ein Gradient der Entkohlung einstellen mit entsprechenden Eigenschaften nach der anschließenden gezielten Abkühlung und/oder Kaltumformung.By the subsequent reducing or inert annealing treatment, the degree of decarburization can be varied by the edge decarburized layer can be reduced again by balancing processes. As a result, a gradient of the decarburization can be adjusted in a targeted manner over the thickness of the workpiece with corresponding properties after the subsequent targeted cooling and / or cold forming.

Die Höhe der Abkühlgeschwindigkeit und des Umformgrades beeinflusst dabei die Martensitbildung und damit den Grad der Härtung.The height of the cooling rate and the degree of deformation influences the formation of martensite and thus the degree of hardening.

Ein solcher Werkstoff ist besonders für Anwendungen geeignet, bei denen eine große Oberflächenhärte kombiniert mit einer hohen Zähigkeit gefordert wird, wie beispielsweise für beschusssichere Bauteile, da der Werkstoff eine hohe Randhärte (Martensit) mit einer sehr hohen Energieabsorption im Beschussfall aufweist.Such a material is particularly suitable for applications in which a high surface hardness combined with a high toughness is required, such as for bullet-proof components, because the material has a high Martensit with a very high energy absorption in the event of fire.

Folgende Legierungszusammensetzungen wurden bei den Betriebsversuchen verwendet in Gew.%: Figur 1a Figur 1b Figur 1c Figur 1d C 0,7 0,7 0,7 0,7 Al 2,5 2,5 2,5 2,5 Si 2,5 0,2 0,3 0,3 Mn 15 15 15 15 The following alloy compositions were used in the operating tests in% by weight: FIG. 1a FIG. 1b Figure 1c Figure 1d C 0.7 0.7 0.7 0.7 al 2.5 2.5 2.5 2.5 Si 2.5 0.2 0.3 0.3 Mn 15 15 15 15

Rest Eisen einschließlich üblicher Stahlbegleitelemente.The rest of the iron, including standard steel components.

Gefügeaufnahmen von erfindungsgemäß behandelten Werkstücken zur Martensitbildung und entsprechenden Härtemessungen werden in zwei Gefügebildern ( Figur 1a, 1b ) gezeigt. Die Werkstoffe unterscheiden sich hier im Si-Gehalt. Die Gefügebilder zeigen in den oberflächennahen Bereichen eine Schicht aus Martensit unterschiedlicher Dicke und den damit verbundenen deutlichen Härteanstieg gegenüber dem Austenitgefüge in der Matrix. Hier zeigt der Stahl gemäß Figur 1a einen deutlich höheren Härteanstieg als der Stahl gemäß Figur 1b . Micrographs of workpieces treated according to the invention for martensite formation and corresponding hardness measurements are shown in two micrographs ( FIG. 1a, 1b ). The materials differ here in the Si content. The micrographs show a layer of martensite of different thickness in the near-surface regions and the associated marked increase in hardness compared to the austenite microstructure in the matrix. Here the steel shows according to FIG. 1a a significantly higher increase in hardness than the steel according to FIG. 1b ,

Die zur Entkohlung notwendige oxidierende Glühbehandlung der Proben aus Figur 1a und 1 b wurde unter Umgebungsatmosphäre (Luft) durchgeführt bei einer Glühtemperatur von 1150°C und einer Glühdauer von 1h. Im vorliegenden Fall wurden die Proben nach der Glühbehandlung nicht abgeschreckt, sondern nur einer Kaltumformung zum Nachweis des TRIP-Effektes (Bildung von verformungsinduziertem Martensit) unterzogen.The necessary for the decarburization oxidizing annealing of the samples FIG. 1a and 1 b was performed under ambient atmosphere (air) at an annealing temperature of 1150 ° C and an annealing time of 1h. In the present case, the samples were not quenched after the annealing treatment, but only subjected to cold working to detect the TRIP effect (formation of strain-induced martensite).

Die Figuren 1c und 1d zeigen, dass abhängig vom Grad der Entkohlung ebenso Randbereiche mit lokaler Zwillingsbildung eingestellt werden können. Ebenso kann in Abhängigkeit vom Grad der Entkohlung eine Variation der Karbidbildung über die Blechdicke eingestellt werden.The Figures 1c and 1d show that, depending on the degree of decarburization, marginal areas with local twinning can also be set. Likewise, depending on the degree of decarburization, a variation of the carbide formation over the sheet thickness can be set.

Die zur Entkohlung notwendige oxidierende Glühbehandlung der Proben in Figur 1c und 1d fand während des Warmwalzens statt. Im Anschluss an das folgende Kaltwalzen wurde eine reduzierende Glühbehandlung mit unterschiedlichen Temperaturen ( Figur 1c : 750°C-Randschicht 30µm mit Zwillingen, Figur 1 d: 700°C - Randschicht 60µm mit Zwillingen) durchgeführt.The necessary for the decarburization oxidizing annealing of the samples in Figure 1c and 1d took place during hot rolling. Following the subsequent cold rolling, a reducing annealing treatment with different temperatures ( Figure 1c : 750 ° C edge layer 30μm with twins, FIG. 1 d: 700 ° C - edge layer 60μm with twins).

Werkstücke aus Leichtbaustahl müssen zudem vergleichsweise hohen Anforderungen hinsichtlich der Verarbeitbarkeit beispielsweise durch Kaltumformen, Schweißen und/oder Korrosionsschutz (z. B. Zink enthaltende Überzüge) genügen.In addition, workpieces made of lightweight steel must satisfy comparatively high demands with regard to processability, for example by cold forming, welding and / or corrosion protection (eg coatings containing zinc).

Beim Schweißen verzinkter austenitischer Leichtbaustähle kann es allerdings zu Problemen mit der sogenannten Flüssigmetallversprödung kommen. Hierbei kommt es in Folge der Erwärmung beim Schweißen im Grundmaterial zu einer Infiltration der Korngrenzen durch verflüssigtes Zinkmaterial des Überzugs. Dadurch verliert das Grundmaterial im Umfeld der Schweißzone seine Festigkeit und Duktilität, so dass die Schweißverbindung bzw. das an die Schweißverbindung angrenzende Grundmaterial die Anforderungen an die mechanischen Eigenschaften nicht mehr erfüllt, wodurch die Gefahr des vorzeitigen Versagens der Schweißverbindung steigt.When welding galvanized austenitic lightweight steels, however, it can lead to problems with the so-called liquid metal embrittlement. As a result of the heating during welding in the base material, the grain boundaries are infiltrated by liquefied zinc material of the coating. As a result, the base material loses its strength and ductility in the vicinity of the weld zone, so that the weld joint or the base material adjacent to the weld no longer meets the requirements of the mechanical properties, whereby the risk of premature failure of the weld increases.

Bei Versuchen hat sich herausgestellt, dass sich beim Schweißen hochmanganhaltiger Stähle durch Ausbildung eines martensitischen bzw. martensitisch-austenitischen Mischgefüges in den entkohlten oberflächennahen Bereichen der Korngrenzenangriff durch das schmelzflüssige Zinkmaterial wirksam verhindern lässt. Die oberflächenharte entkohlte Randschicht eignet sich dabei hervorragend als Zwischenschicht, um die Flüssigmetallversprödung bei verzinkten Leichtbaustählen wirksam zu vermeiden.Experiments have shown that the formation of a martensitic or martensitic-austenitic mixed structure in the decarburized near-surface regions can effectively prevent the grain boundary attack by the molten zinc material when welding high-manganese steels. The surface-hardened decarburized surface layer is ideally suited as an intermediate layer to effectively prevent liquid metal embrittlement in galvanized lightweight steels.

Die der Erfindung zugrunde liegende Idee ist nicht nur für Flachprodukte, wie Warm- und Kaltband, sondern auch für Profile und Rohre sowie daraus hergestellte Bauteile anwendbar. Für die Umformung kommen alle bekannten Verfahren der Kalt-, Warm- und Halbwarmformung in Betracht, wie Biegen, Tiefziehen, Stauchen, Aufweiten etc. aber beispielsweise auch das bekannte Innenhochdruckumformen oder Pressformhärten. Demzufolge kann die Herstellung erfindungsgemäßer Gradientenwerkstoffe beispielhaft über folgende Prozessrouten erfolgen:

  • Kalt- oder Warmumformung eines Werkstücks, wie z. B. eines Blechzuschnitts, zu einem Bauteil mit anschließendem oxidierenden Glühen des Bauteils und anschließender gezielter Abkühlung zum Härten der Oberfläche durch Umwandlung der entkohlten Bereiche zu Martensit
  • Innenhochdruckumformen eines Rohres bei angehobener Temperatur, die eine Entkohlung der Oberfläche zulässt, mit abschließender rascher Abkühlung (Härten)
  • Innenhochdruckumformen eines Rohres bei Raumtemperatur mit abschließender oxidierender Glühung des bereits umgeformten Bauteils und abschließender rascher Abkühlung (Härten)
  • Pressformhärten eines Werkstücks mit einer oxidierenden Glühung vor dem Umformen; Umformen bei erhöhter Temperatur im austenitischen Gefügezustand und anschließender rascher Abkühlung zur martensitischen Umwandlung der oberflächennahen, entkohlten Bereiche
  • oxidierendes Glühen zur Einstellung einer entkohlten Schicht, z. B. eines Bleches, mit anschließender gezielter Abkühlung (ohne Härtung) mit anschließender Kaltumformung
  • oxidierendes Glühen zur Einstellung einer entkohlten Schicht, z. B. eines Bleches mit anschließender gezielter Abkühlung (ohne Härtung) mit anschließendem Kaltwalzen zur gezielten Einstellung der Härteschichtdicke über die Bildung von Verformungsmartensit
  • oxidierendes Glühen z. B. eines Bleches mit anschließender gezielter Abkühlung (Härtung) und direkter Anwendung ohne weitere umformtechnische Beanspruchung
  • oxidierendes Glühen im Rahmen des Warmwalzprozesses zur Einstellung einer entkohlten Schicht mit anschließendem Kaltwalzen
  • oxidierendes Glühen im Rahmen des Warmwalzprozesses zur Einstellung einer entkohlten Schicht mit anschließendem Kaltwalzen und Glühen unter oxidierender Atmosphäre zur weiteren Entkohlung
  • oxidierendes Glühen im Rahmen des Warmwalzprozesses zur Einstellung einer entkohlten Schicht mit anschließendem Kaltwalzen und Glühen unter reduzierender oder inerter Atmosphäre zur Verringerung bzw. Einstellung der Entkohlung durch Ausgleichsprozesse.
The idea underlying the invention is applicable not only for flat products, such as hot and cold strip, but also for profiles and tubes and components made therefrom. For the transformation, all known methods of cold, warm and semi-warming into consideration, such as bending, deep drawing, upsetting, expansion, etc. but also, for example, the known hydroforming or compression molding. Accordingly, the production of gradient materials according to the invention can take place, for example, via the following process routes:
  • Cold or hot forming of a workpiece, such. B. a sheet metal blank, to a component with subsequent oxidizing annealing of the component and subsequent targeted cooling to harden the surface by converting the decarburized areas to martensite
  • Hydroforming of a pipe at elevated temperature, which allows decarburization of the surface, with final rapid cooling (hardening)
  • Hydroforming of a tube at room temperature with final oxidizing annealing of the already formed component and finally rapid cooling (hardening)
  • Press forming hardening of a workpiece with an oxidizing anneal prior to forming; Forming at elevated temperature in the austenitic microstructure and subsequent rapid cooling to the martensitic transformation of the near-surface, decarburized areas
  • oxidizing annealing to adjust a decarburized layer, e.g. As a sheet, followed by selective cooling (without hardening) followed by cold forming
  • oxidizing annealing to adjust a decarburized layer, e.g. B. a sheet with subsequent targeted cooling (without curing) followed by cold rolling for targeted adjustment of the hardness layer thickness on the formation of deformation martensite
  • oxidizing annealing z. As a sheet with subsequent targeted cooling (hardening) and direct application without further deformation stress
  • oxidizing annealing as part of the hot rolling process to set a decarburized layer followed by cold rolling
  • oxidizing annealing as part of the hot rolling process for setting a decarburized layer followed by cold rolling and annealing under an oxidizing atmosphere for further decarburization
  • oxidizing annealing in the course of the hot rolling process for setting a decarburized layer followed by cold rolling and annealing under a reducing or inert atmosphere to reduce or adjust decarburization by equalizing processes.

Das erfindungsgemäße Verfahren ist grundsätzlich für alle bei Raumtemperatur austenitischen Legierungen geeignet, insbesondere aber für hochlegierte Leichtbaustähle.The process according to the invention is basically suitable for all austenitic alloys at room temperature, but especially for high-alloyed lightweight steels.

Das erfindungsgemäße Verfahren bietet erstmals in vorteilhafter Weise die Möglichkeit, den spezifischen Anforderungen an die Werkstoffeigenschaften des fertigen Bauteils Rechnung zu tragen, indem diese über die Banddicke gezielt eingestellt werden können.For the first time, the method according to the invention advantageously offers the possibility of taking into account the specific requirements of the material properties of the finished component, in that they can be adjusted in a targeted manner over the strip thickness.

Zusammenfassend ergeben sich aus der Erfindung folgende Vorteile:

  • Einstellung benötigter Werkstoffeigenschaften über die Wanddicke durch einfaches entkohlendes Glühen mit anschließendem Härten oder mechanischer Umformung.
  • Gezielt beeinflusst werden können:
    • Verschleiß/Abrieb/Tribologie
    • Zunderbeständigkeit
    • Korrosionsschutz
    • Beschichtbarkeit
    • Beklebbarkeit
    • elektrische Eigenschaften
    • Schweißbarkeit (z. B. Widerstandspunktschweißbarkeit)
    • thermische Eigenschaften (Bimetall)
    • optische Eigenschaften (Aussehen)
    • Dämpfung
  • Realisierung von Kombinationen unterschiedlicher Oberflächen- und Materialkerneigenschaften.
In summary, the invention provides the following advantages:
  • Adjustment of required material properties over the wall thickness by simple decarburizing annealing with subsequent hardening or mechanical deformation.
  • Can be targeted:
    • Wear / abrasion / tribology
    • scaling resistance
    • corrosion protection
    • coatability
    • Beklebbarkeit
    • Electrical Properties
    • Weldability (eg resistance spot weldability)
    • thermal properties (bimetal)
    • optical properties (appearance)
    • damping
  • Realization of combinations of different surface and material core properties.

Claims (17)

  1. A method for producing workpieces from an austenitic lightweight steel having material properties that are adjustable across the wall thickness or strip thickness, having an alloy composition (in wt.%) consisting of: C 0.2 to ≤ 1.0 Al 0.05 to <15.0 Si 0.05 to ≤ 6.0 Mn 9.0 to <30.0
    remainder iron including common steel-accompanying elements with the optional addition of Cr, Cu, B, Ti, Zr, V and Nb with Cr ≤ 6.5; Cu ≤ 4.0; Ti+Zr ≤ 0.7; Nb+V ≤ 0.5, B ≤ 0.1,
    characterised in that
    the workpiece is subjected to a decarburising annealing treatment under an oxidising atmosphere in such a way that a ferritic or meta-stable austenitic structure is formed in the regions close to the surface, the layer thickness and properties of which can be adjusted via variation of the annealing parameters (temperature, holding time) and the annealing atmosphere (gas composition, partial pressure), and is subjected to a subsequent accelerated cooling and/or a cold forming to create a property gradient.
  2. The method according to claim 1,
    characterised in that
    forming takes place before, during or after the annealing treatment.
  3. The method according to claim 2,
    characterised in that
    the forming is a hot or cold forming.
  4. The method according to claim 3,
    characterised in that
    the forming is a hot or cold rolling.
  5. The method according to claim 4,
    characterised in that
    the depth of the decarburisation and the degree of the decarburisation created in the workpiece in the course of the reheating conditions before hot rolling and/or between the hot-rolling passes under an oxidising annealing atmosphere are adjusted in a targeted manner by a subsequent annealing process under a reducing or inert atmosphere.
  6. The method according to claim 4,
    characterised in that
    the depth and degree of decarburisation are adjusted in a targeted manner by reheating the workpiece between individual hot-rolling passes.
  7. The method according to claim 3,
    characterised in that
    the forming is an internal high-pressure forming.
  8. The method according to claim 3,
    characterised in that
    the forming is a deep drawing.
  9. The method according to claim 3,
    characterised in that
    the forming is a pressing.
  10. The method according to claim 3,
    characterised in that
    the forming is a press hardening.
  11. The method according to claim 2,
    characterised in that
    in the event that the forming is carried out after the annealing treatment, the accelerated cooling is carried out during the forming.
  12. The method according to one of claims 1 - 11,
    characterised in that
    the accelerated cooling is a quenching.
  13. The method according to one of claims 1 - 12,
    characterised in that
    the oxidising annealing atmosphere is ambient air.
  14. The method according to claim 13,
    characterised in that
    oxygen or oxygen-containing gases are added to the ambient air.
  15. Workpieces composed of austenitic lightweight steel having material properties that are adjustable across the strip thickness or wall thickness, having an alloy composition (in wt.%) consisting of: C 0.2 to ≤ 1.0 Al 0.05 to <15.0 Si 0.05 to ≤ 6.0 Mn 9.0 to <30.0
    remainder iron including common steel-accompanying elements with the optional addition of Cr, Cu, B, Ti, Zr, V and Nb with Cr ≤ 6.5; Cu ≤ 4.0; Ti+Zr ≤ 0.7; Nb+V ≤ 0.5, B ≤ 0.1,
    characterised in that the workpieces comprise layers that are decarburised compared with the matrix over the cross-section of the strip thickness or wall thickness.
  16. Annealed workpieces according to claim 15,
    characterised in that
    the workpieces have a metallic coating.
  17. Annealed workpieces according to claim 15,
    characterised in that
    the workpieces have a hardened structure in the decarburised skin layer.
EP11715650.5A 2010-03-16 2011-02-10 Method for producing workpieces from lightweight steel having material properties that can be adjusted over the wall thickness Active EP2547800B1 (en)

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DE102010034161.4A DE102010034161B4 (en) 2010-03-16 2010-08-10 Method for producing workpieces made of lightweight steel with material properties that can be adjusted via the wall thickness
PCT/DE2011/000128 WO2011113404A1 (en) 2010-03-16 2011-02-10 Method for producing workpieces from lightweight steel having material properties that can be adjusted over the wall thickness

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KR101707019B1 (en) 2017-02-15
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KR20130006461A (en) 2013-01-16
WO2011113404A1 (en) 2011-09-22
EP2547800A1 (en) 2013-01-23
US20130048150A1 (en) 2013-02-28
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DE102010034161A1 (en) 2011-09-22

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