EP3061838B1 - Blank bainite long product and method for producing the same - Google Patents

Blank bainite long product and method for producing the same Download PDF

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Publication number
EP3061838B1
EP3061838B1 EP16157945.3A EP16157945A EP3061838B1 EP 3061838 B1 EP3061838 B1 EP 3061838B1 EP 16157945 A EP16157945 A EP 16157945A EP 3061838 B1 EP3061838 B1 EP 3061838B1
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Prior art keywords
bainitic
blank
long product
steel
bis
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German (de)
French (fr)
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EP3061838A1 (en
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Mirkka Ingrid LEMBKE
Guido Olschewski
Heiko Haupt-Peter
Hans Roelofs
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Swiss Steel AG
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Swiss Steel AG
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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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • 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
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—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
    • 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
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the invention relates to a bare bainitic long product according to the preamble of claim 1 and uses thereof and a process for its preparation.
  • bainitic materials with a tensile strength of about 1 '300 MPa, for example, could increase the performance of common rails or injectors and thus of the entire internal combustion engine.
  • the WO 2009/090155 A1 describes such an application for a bainitic-ferritic material without TRIP effect.
  • EP 2103704 A1 describes such a product.
  • By the manufacturing process described therein can be dispensed with the addition of aluminum and titanium. This avoids the formation of abrasive oxide inclusions.
  • a high sulfur content of 0.04 to 0.25% additionally improves the machinability.
  • the sulfur forms with manganese sulfide inclusions, which on the one hand facilitate the chip breaking and on the other hand form a protective layer on the tool. Tool life and productivity are increased.
  • the manganese sulfides are also material weaknesses, which can cause cracks under dynamic load. This reduces the life expectancy of dynamically loaded components. Therefore, this steel can not be used for highly stressed safety components.
  • EP 1426453 A1 describes a forging of a bainitic steel, which should be suitable for machining.
  • the good machinability in this case also only by alloying with sulfur, tellurium, selenium, bismuth or lead (see there claims 6 and 7). Both the environmental compatibility and the suitability for highly stressed components is thereby limited.
  • WO 2011/124851 refers to components made by forging or machining long steel. It is therefore not drawn components, which are known to have a relatively low yield ratio.
  • the steels described therein have a low silicon content of at most 0.1% by weight.
  • EP 2103704 A1 describes a bainitic long product with a sulfur content of at least 0.04 wt .-%, wherein due to the manufacturing process, a ratio of yield strength to tensile strength of at most 0.82 is achieved.
  • WO 2011/124851 A2 describes a bainitic component with a yield strength to tensile strength ratio of 0.80 or less.
  • a bright bainitic long product with a bainitic microstructure that can be machined well in machining is currently not yet available for use in dynamically stressed components.
  • the object of the invention is to provide a bright bainitic long product, with which in particular the above disadvantages are avoided. Further objects of the invention relate to uses of the long product according to the invention. Yet another object of the invention is to provide a process for producing a bare bainitic long product.
  • the tensile test according to ISO 6892-1 is a standard procedure for characterizing the mechanical properties of a material. The associated terms are defined in the standard ISO 6892-1.
  • the yield strength Rp0.2 indicates which stress is present at a plastic strain of 0.2%. This parameter is used when there is no sharp transition between elastic and plastic material behavior.
  • the tensile strength Rm is the maximum stress in the measured stress-strain diagram. The tensile strength is reached the moment the sample starts to constrict (local taper of the sample cross-section).
  • the elongation at break A5 is the extension of the sample at the time of fracture, wherein the index 5 refers to the ratio of the initial measuring length L0 to the initial diameter d0.
  • the steel composition according to the invention can be machined significantly better at a constant predetermined tensile strength if the yield ratio Rp0.2 / Rm is above 0.87.
  • the significantly improved machinability manifests itself in significantly longer tool life.
  • the alloying components are selected such that, at conventional cooling rates from 0.1 to 8.0 K / s, a bainitic-martensitic microstructure always results with a tensile strength level of 900 to 1,200 MPa, without costly alloying elements and / or or special equipment for accelerated cooling from the rolling heat must be used.
  • the product In order to be able to adjust the properties according to the invention, the product must be drawn in the temperature range between room temperature and 600 ° C. and then tempered in a temperature range of 250 and 600 ° C.
  • the yield ratio Rp0.2 / Rm decreases, and according to the invention it is to be maintained above 0.87.
  • the yield ratio Rp0.2 / Rm usually falls below this value.
  • the yield ratio Rp0.2 / Rm is in the range of 0.87 to 0.94.
  • the upper limit of the carbon to 0.26% ensures that the tensile strength does not rise above 1'400 MPa. Higher strength values deteriorate the workability in the downstream drawing process or cutting process. Higher carbon contents also promote the formation of carbides, which adversely affects ductility.
  • Silicon suppresses the formation of hard and in machining abrasive Fe 3 C precipitates (cementite).
  • the chosen silicon concentration allows a one-hour annealing treatment at 400 ° C without coarse cementite precipitates can form (based on the description of the carbide-free bainite in WO 96/22396 ). Since silicon is an efficient solid solution in bainite, its content must be limited to at most 1.2 wt .-%, so as not to exceed the maximum desired tensile strength of 1 '400 MPa.
  • the lower limit in manganese to 1.20% by weight and chromium to 0.70% by weight ensures that a bainitic structure can be produced from the forming heat when the air cools down.
  • the manganese content must be limited to 1.70 wt .-%.
  • Chromium constricts the bainite area, which favors the formation of martensite. For this reason, the chromium content must be limited to 1.60 wt .-%.
  • molybdenum can be alloyed to suppress possible temper embrittlement.
  • the precipitation of iron carbides at the primary grain boundaries and a related loss of toughness can thus be prevented.
  • the molybdenum content is to be chosen as low as necessary and is therefore at most 0.3 wt .-%, in particular about 0.15 to 0.28 wt .-%.
  • Nickel improves notched impact strength and therefore has a negative impact on machinability. Therefore, the nickel content is limited to 0.30 wt%, more preferably about 0.05 to 0.1 wt%.
  • Sulfur is a steel pest. It forms manganese sulfide precipitates and weakens the microstructure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the sulfur content was limited to 0.03 wt .-%. Preferably, the sulfur content is 0.010 to 0.020 wt .-% and in particular about 0.015 wt .-%.
  • the addition of aluminum is not mandatory for the production of the product according to the invention and is therefore at most 0.01% by weight, in particular 0.005 to 0.009% by weight.
  • Phosphorus is a steel pest. It goes to the Austenitkorngrenzen and weakens the structure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the phosphorus content was limited to 0.03 wt .-% and is in particular 0.01 to 0.02 wt .-%.
  • Copper is a steel pest. At high copper contents, red brittleness occurs in hot forming. For this reason, the copper content is limited to 0.25 wt .-% and is in particular 0.1 to 0.2 wt .-%.
  • Hard oxide or nitride inclusions have a negative effect on the machinability above a certain size.
  • titanium titanium oxides and coarse titanium carbonitrides
  • aluminum hard Al 2 O 3 compounds
  • at Niobium and vanadium must also be assumed to have a negative effect.
  • the micro-alloying elements titanium, niobium and vanadium are therefore limited to 0.01 wt .-%.
  • Boron is usually alloyed with bainitic steels to suppress the formation of ferrite.
  • the nitrogen present in the steel must be bound with titanium. Since the addition of titanium is to be avoided, this alloy concept can not be used here. Boron will therefore be present only in traces to 0.001 wt .-%.
  • oxide inclusions should be set with an Al 2 O 3 content of ⁇ 50 wt .-%.
  • the metallurgical treatment according to claim 4 is carried out so that soft, glassy silicate inclusions arise with the following relative proportions by weight: 20 to 50% CaO, 25 to 65% SiO 2 and less than 30% Al 2 O 3 . The tool life of the tools used in machining is then significantly extended.
  • a heat treatment of up to 2 hours at 300 to 600 ° C. according to claim 7 may be expedient. Tensions introduced by strain hardening are thereby reduced.
  • the rods were shot peened to remove the iron oxide skin. Thereafter, the rods were either drawn at room temperature or in the temperature range between 350 and 450 ° C by a multi-stage die to the final dimension of ⁇ 40 mm and directed in a two-roll straightening machine. The final conductive heating at 350 resp. 500 ° C was used to reduce deformation-induced surface tensions.
  • the machinability of the manufactured variants was tested in a standard test at the ISF in Dortmund.
  • the test specimens used were double-turned and one-sided center bored rods with a length of 190 mm.
  • the experiments ran under Bechem Avantin 3309 emulsion with a concentration of about 6-8%.
  • the turning tool (CNMG120404-PF4215) was selected to allow the standard steel 42CrMo4 + QT (at Rm ⁇ 1'000 MPa) to have an industrially common machining time of 18 minutes without violating a wear mark width criterion of V Bmax ⁇ 300 ⁇ m.
  • the cutting speed was 200 m / min
  • Table 1 shows the steel compositions used for this comparison.
  • the composition of the existing oxide inclusions was analyzed by scanning electron microscopy using EDX.
  • the structural components of steel 1 were determined quantitatively in the scanning electron microscope (for bainite and ferrite) and in the X-ray diffractometer (for retained austenite). Within the measuring accuracy the differences of the three variants are small. The results are shown in Table 2 for the various heat treatments. For steel 2 there is a 100% compensation structure (tempered martensite). The steels 3 and 4, similar to steel 1, have a mixed structure of bainite, martensite and retained austenite.

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  • Engineering & Computer Science (AREA)
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  • Organic Chemistry (AREA)
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Description

Technisches GebietTechnical area

Die Erfindung betrifft ein blankes bainitisches Langprodukt gemäss dem Oberbegriff des Anspruchs 1 sowie Verwendungen davon und ein Verfahren zu dessen Herstellung.The invention relates to a bare bainitic long product according to the preamble of claim 1 and uses thereof and a process for its preparation.

Stand der TechnikState of the art

Tendenzen zum Downsizing oder Forderungen nach höherer Leistung bei geringerer Umweltbelastung in der Automobilindustrie führen zum Einsatz neuer Werkstoffe mit höherer Festigkeit.
Kontinuierlich aus der Umformhitze abgekühlte bainitische Stähle bieten dabei unter statischer Last ähnliche Festigkeits-und Zähigkeitseigenschaften wie wärmebehandelte Stähle mit Vergütungsgefüge. Während sich Vergütungsstähle unter dynamischer Last jedoch entfestigen, findet man bei den bainitischen Stählen eine Verfestigung. Aufgrund des TRIP-Effekts (verformungsinduzierte Umwandlung von Austenit in Martensit) verhalten sich bainitische Stähle auch vorteilhafter bei Überlasten (Spannungsspitzen). Dies macht sich in einer höheren Zeitfestigkeit bemerkbar.
Bei gleicher Ausgangsfestigkeit sind die Ermüdungseigenschaften (Dauerfestigkeit, Zeitfestigkeit, Kerbempfindlichkeit, Lebensdauer) der bainitischen Stähle den Vergütungsstählen deshalb deutlich überlegen.Downsizing tendencies or demands for higher performance with lower environmental impact in the automotive industry lead to the use of new materials with higher strength.
Bainitic steels cooled continuously from the forming heat offer similar strength and toughness properties under static load as heat-treated steels with a hardened structure. However, while tempered steels soften under dynamic load, the bainitic steels are solidified. Due to the TRIP effect (deformation-induced transformation of austenite into martensite), bainitic steels also behave more favorably with overloads (voltage peaks). This manifests itself in a higher time stability.
With the same initial strength, the fatigue properties (fatigue strength, fatigue strength, notch sensitivity, service life) of bainitic steels are therefore clearly superior to tempered steels.

Durch den Einsatz von bainitischen Werkstoffen mit einer Zugfestigkeit von ca. 1 '300 MPa liesse sich beispielsweise die Leistungsfähigkeit von Common-Rails oder Injektoren und damit des gesamten Verbrennungsmotors steigern. Die WO 2009/090155 A1 beschreibt eine solche Anwendung für einen bainitischferritischen Werkstoff ohne TRIP-Effekt.The use of bainitic materials with a tensile strength of about 1 '300 MPa, for example, could increase the performance of common rails or injectors and thus of the entire internal combustion engine. The WO 2009/090155 A1 describes such an application for a bainitic-ferritic material without TRIP effect.

Trotz der Vorteile haben sich diese innovativen Stähle bisher nur vereinzelt durchsetzen können. Die hohe Festigkeit bringt neue Herausforderungen für die Teilefertigung mit sich. In spanenden Prozessen erhöhen sich die Schnittkräfte und die Temperaturen am Werkzeug. Dies führt zu einer kürzeren Werkzeugstandzeit bzw. zu einer geringeren Produktivität. Durch Optimierung von Werkzeuggeometrie und Zerspanungsparametern können Produktivitätsverluste häufig teilweise wieder kompensiert werden (siehe z.B. Biermann, D.; Hartmann, H. Herausforderungen bei der Zerspanung höherfester Stähle. In: Stahl- und Gusszerspanung 2013 - Innovative Bearbeitungsverfahren für die wirtschaftliche Fertigung, 12.11.-13.11. 2013, Kassel, ISBN 978-3-942980-33-3, S. 129-138 .) Die inverse Korrelation zwischen Zerspanbarkeit und Festigkeit bleibt dennoch bestehen.Despite the advantages, these innovative steels have so far only been able to assert themselves in isolated cases. The high strength brings with it new challenges for parts production. In cutting processes, the cutting forces and the temperatures on the tool increase. This leads to a shorter tool life or to a lower productivity. By optimizing tool geometry and machining parameters, losses in productivity can often be partially compensated (see, eg Biermann, D .; Hartmann, H. Challenges in the machining of high-strength steels. In: Steel and Casting Machining 2013 - Innovative Machining Processes for Economic Production, 12.11.-13.11. 2013, Kassel, ISBN 978-3-942980-33-3, pp. 129-138 The inverse correlation between machinability and strength still remains.

Es gibt deshalb einen Bedarf an zerspanungsverbesserte hochfeste bainitische Stähle. Dies gilt insbesondere für Blankstahlprodukte, die in gezogener Ausführung für die spanende Bauteilfertigung eingesetzt werden.There is therefore a need for machined high strength bainitic steels. This applies in particular to bright steel products, which are used in a drawn version for cutting component production.

EP 2103704 A1 beschreibt ein solches Produkt. Durch das dort beschriebene Herstellverfahren kann auf die Zugabe von Aluminium und Titan verzichtet werden. Dadurch wird die Bildung von abrasiv wirkenden Oxideinschlüssen vermieden. Ein hoher Schwefelgehalt von 0.04 bis 0.25% verbessert die Zerspanbarkeit zusätzlich. Der Schwefel bildet mit Mangan Sulfideinschlüsse, die einerseits den Spanbruch erleichtern und andererseits eine schützende Schicht auf dem Werkzeug bilden. Die Werkzeugstandzeit und die Produktivität werden dadurch erhöht. Gleichzeitig sind die Mangansulfide aber auch Materialschwachstellen, an denen unter dynamischer Last Risse entstehen können. Dies reduziert die Lebenserwartung von dynamisch belasteten Bauteilen. Für hoch beanspruchte Sicherheitsbauteile kann dieser Stahl deshalb nicht eingesetzt werden. EP 2103704 A1 describes such a product. By the manufacturing process described therein can be dispensed with the addition of aluminum and titanium. This avoids the formation of abrasive oxide inclusions. A high sulfur content of 0.04 to 0.25% additionally improves the machinability. The sulfur forms with manganese sulfide inclusions, which on the one hand facilitate the chip breaking and on the other hand form a protective layer on the tool. Tool life and productivity are increased. At the same time, however, the manganese sulfides are also material weaknesses, which can cause cracks under dynamic load. This reduces the life expectancy of dynamically loaded components. Therefore, this steel can not be used for highly stressed safety components.

EP 1426453 A1 beschreibt ein Schmiedebauteil aus einem bainitischen Stahl, der für die spanende Bearbeitung geeignet sein soll. Die gute Zerspanbarkeit wird in diesem Fall ebenfalls erst durch Legieren mit Schwefel, Tellur, Selen, Wismut oder Blei erreicht (siehe dort Ansprüche 6 und 7). Sowohl die Umweltverträglichkeit als auch die Eignung für hochbeanspruchte Bauteile wird dadurch eingeschränkt. EP 1426453 A1 describes a forging of a bainitic steel, which should be suitable for machining. The good machinability in this case also only by alloying with sulfur, tellurium, selenium, bismuth or lead (see there claims 6 and 7). Both the environmental compatibility and the suitability for highly stressed components is thereby limited.

WO 2011/124851 betrifft Bauteile, die durch Schmieden oder spanabhebende Bearbeitung von Langstahl hergestellt werden. Es handelt sich also um nicht gezogene Bauteile, die bekanntermassen ein vergleichsweise niedriges Streckgrenzenverhältnis aufweisen. Die darin beschriebenen Stähle weisen einen geringen Siliziumgehalt von höchstens 0.1 Gew.-% auf. WO 2011/124851 refers to components made by forging or machining long steel. It is therefore not drawn components, which are known to have a relatively low yield ratio. The steels described therein have a low silicon content of at most 0.1% by weight.

EP 2103704 A1 beschreibt ein bainitisches Langprodukt mit einem Schwefelanteil von mindestens 0.04 Gew.-%, wobei aufgrund des Herstellungsverfahrens ein Verhältnis von Dehngrenze zu Zugfestigkeit von höchstens 0.82 erreicht wird. EP 2103704 A1 describes a bainitic long product with a sulfur content of at least 0.04 wt .-%, wherein due to the manufacturing process, a ratio of yield strength to tensile strength of at most 0.82 is achieved.

WO 2011/124851 A2 beschreibt ein bainitisches Bauteil mit einem Verhältnis von Dehngrenze zu Zugfestigkeit von höchstens 0.80. WO 2011/124851 A2 describes a bainitic component with a yield strength to tensile strength ratio of 0.80 or less.

Ein in der Zerspanung gut bearbeitbares blankes bainitisches Langprodukt in einem Abmessungsbereich von 5.0 bis 70 mm mit bainitischer Gefügestruktur steht heute für die Anwendung in dynamisch stark beanspruchten Bauteilen noch nicht zur Verfügung.A bright bainitic long product with a bainitic microstructure that can be machined well in machining is currently not yet available for use in dynamically stressed components.

Darstellung der ErfindungPresentation of the invention

Aufgabe der Erfindung ist es, ein blankes bainitisches Langprodukt bereitzustellen, mit dem insbesondere die obigen Nachteile vermieden werden. Weitere Aufgaben der Erfindung betreffen Verwendungen des erfindungsgemässen Langprodukts. Noch eine weitere Aufgabe der Erfindung besteht darin, ein Verfahren zur Herstellung eines blanken bainitischen Langprodukts anzugeben.The object of the invention is to provide a bright bainitic long product, with which in particular the above disadvantages are avoided. Further objects of the invention relate to uses of the long product according to the invention. Yet another object of the invention is to provide a process for producing a bare bainitic long product.

Die Bezeichnung "blankes Langprodukt" bezieht sich auf ein gezogenes, kalt umgeformtes Langprodukt nach DIN EN 10'277-1.The term "bright long product" refers to a drawn, cold-formed long product according to DIN EN 10'277-1.

Gelöst werden diese Aufgaben durch das im Anspruch 1 definierte blanke bainitische Langprodukt, die in den Ansprüchen 5 und 6 definierten Verwendungen sowie das im Anspruch 8 definierte Herstellverfahren.These objects are achieved by the defined in claim 1 bare bainitic long product, the uses defined in claims 5 and 6 and the method defined in claim 8.

Die nachfolgenden Gehaltsangaben in Prozent (%) bzw. in Teilen pro Million ("parts per million, ppm") beziehen sich - sofern nicht ausdrücklich anders angegeben - auf Gewichtsanteile.
Das erfindungsgemässe blanke bainitische Langprodukt weist einen Gewichtsanteil von

  • 0.16 bis 0.26 % Kohlenstoff,
  • 0.60 bis 1.20% Silizium,
  • 1.20 bis 1.70% Mangan,
  • 0.70 bis 1.60% Chrom,
  • bis zu 0.20% Nickel,
  • bis zu 0.30% Molybdän,
  • bis zu 0.03% Schwefel,
  • bis zu 0.01 % Aluminium,
  • bis zu 0.03% Phosphor,
  • bis zu 0.25% Kupfer,
  • bis zu 0.001 % Bor,
  • bis zu 0.01 % Titan,
  • bis zu 0.01 % Vanadium
  • bis zu 0.01 % Niob
  • 50 ppm bis 0.015% Stickstoff und
  • bis zu 0.01 % in oxidischen Einschlüssen gebundener Sauerstoff der Rest Eisen sowie stahlübliche Verunreinigungen,
wobei folgende Gefügebestandteile vorliegen
  • 60 bis 80% Bainit,
  • 5 bis 30% Martensit,
  • 0.5 bis 8 % Ferrit und
  • 3 bis 15% Restaustenit
und wobei der Zugversuch nach ISO 6892-1 ergibt:
  • eine Dehngrenze Rp0.2 = 950 bis 1'400 MPa
  • eine Zugfestigkeit Rm = 1'150 bis 1 '400 MPa und
  • eine Bruchdehnung A5 = 9.0 bis 17.0%
  • wobei 0.87 < Rp0.2/Rm < 0.99.
Das erfindungsgemässe Herstellungsverfahren umfasst folgende Schritte:
  • Herstellen einer Stahllegierung mit einem Gewichtsanteil von
    • 0.16 bis 0.26 % Kohlenstoff,
    • 0.60 bis 1.20% Silizium,
    • 1.20 bis 1.70% Mangan,
    • 0.70 bis 1.60% Chrom,
    • bis zu 0.20% Nickel,
    • bis zu 0.30% Molybdän,
    • bis zu 0.03% Schwefel,
    • bis zu 0.01 % Aluminium,
    • bis zu 0.03% Phosphor,
    • bis zu 0.25% Kupfer,
    • bis zu 0.001% Bor,
    • bis zu 0.01% Titan,
    • bis zu 0.01 % Vanadium
    • bis zu 0.01% Niob
    • 50 ppm bis 0.015% Stickstoff und
    • bis zu 0.01 % in oxidischen Einschlüssen gebundener Sauerstoff,
    • der Rest Eisen sowie stahlübliche Verunreinigungen,
    und Vergiessen zu einer Form vordefinierten Formates ("Stahlherstellung");
  • Wiedererwärmen des Einheitsformats, Warmwalzen zu Draht oder Stab und Abkühlen auf eine Temperatur unter 400°C mit einer Abkühlrate von 0.1 bis 8.0 K/s ("Walzen");
  • chemische oder mechanische Entfernung der äusseren Eisenoxidhaut ("Entzundern") ;
  • Ziehen des Walzstahls durch ein Werkzeug mit vordefiniertem Innenprofil ("Ziehen") bei einer Temperatur von 25 bis 600°C, wobei es sich vorzugsweise um ein Rund- oder Sechskantprofil handelt;
  • mechanisches Richten zur Erzeugung einer geeigneten Geradheit ("Richten");
  • Entspannen bei einer Temperatur von 250 bis 600°C ("Entspannen").
The following percentages by weight (%) or parts per million (ppm) refer to parts by weight unless expressly stated otherwise.
The inventive bare bainitic long product has a weight fraction of
  • 0.16 to 0.26% carbon,
  • 0.60 to 1.20% silicon,
  • 1.20 to 1.70% manganese,
  • 0.70 to 1.60% chromium,
  • up to 0.20% nickel,
  • up to 0.30% molybdenum,
  • up to 0.03% sulfur,
  • up to 0.01% aluminum,
  • up to 0.03% phosphorus,
  • up to 0.25% copper,
  • up to 0.001% boron,
  • up to 0.01% titanium,
  • up to 0.01% vanadium
  • up to 0.01% niobium
  • 50 ppm to 0.015% nitrogen and
  • up to 0.01% oxygen bound in oxidic inclusions the remainder iron as well as common impurities,
wherein the following structural constituents are present
  • 60 to 80% bainite,
  • 5 to 30% martensite,
  • 0.5 to 8% ferrite and
  • 3 to 15% retained austenite
and wherein the tensile test gives ISO 6892-1:
  • a yield strength Rp0.2 = 950 to 1400 MPa
  • a tensile strength Rm = 1'150 to 1'400 MPa and
  • an elongation at break A5 = 9.0 to 17.0%
  • where 0.87 <Rp0.2 / Rm <0.99.
The production method according to the invention comprises the following steps:
  • Producing a steel alloy with a weight fraction of
    • 0.16 to 0.26% carbon,
    • 0.60 to 1.20% silicon,
    • 1.20 to 1.70% manganese,
    • 0.70 to 1.60% chromium,
    • up to 0.20% nickel,
    • up to 0.30% molybdenum,
    • up to 0.03% sulfur,
    • up to 0.01% aluminum,
    • up to 0.03% phosphorus,
    • up to 0.25% copper,
    • up to 0.001% boron,
    • up to 0.01% titanium,
    • up to 0.01% vanadium
    • up to 0.01% niobium
    • 50 ppm to 0.015% nitrogen and
    • up to 0.01% oxygen bound in oxidic inclusions,
    • the remainder iron and common impurities,
    and casting into a form of predefined format ("steelmaking");
  • Reheating the unitary format, hot rolling into wire or rod and cooling to a temperature below 400 ° C with a cooling rate of 0.1 to 8.0 K / s ("rolling");
  • chemical or mechanical removal of the outer iron oxide skin ("descaling") ;
  • Pulling the rolled steel through a tool with a predefined inner profile ("pulling") at a temperature of 25 to 600 ° C, which is preferably a round or hexagonal profile;
  • mechanical straightening to produce a suitable straightness ("straightening");
  • Relax at a temperature of 250 to 600 ° C ("relax").

Bevorzugte Ausführungsformen sind in den abhängigen Ansprüchen definiert.Preferred embodiments are defined in the dependent claims.

Der Zugversuch nach ISO 6892-1 ist ein Standardverfahren zur Charakterisierung der mechanischen Eigenschaften eines Werkstoffs. Die zugehörigen Begriffe sind in der Norm ISO 6892-1 definiert. Die Dehngrenze Rp0.2 gibt an, welche Spannung bei einer plastischen Dehnung von 0.2% vorliegt. Diese Messgrösse wird dann verwendet, wenn kein scharfer Übergang zwischen elastischem und plastischem Werkstoffverhalten vorliegt. Die Zugfestigkeit Rm ist die maximale Spannung im gemessenen Spannungs-Dehnungsdiagramm. Die Zugfestigkeit wird in dem Moment erreicht, an dem die Probe anfängt einzuschnüren (lokale Verjüngung des Probenquerschnitts). Die Bruchdehnung A5 ist die Verlängerung der Probe beim Zeitpunkt des Bruchs, wobei sich der Index 5 auf das Verhältnis der Anfangsmesslänge L0 zum Anfangsdurchmesser d0 bezieht.The tensile test according to ISO 6892-1 is a standard procedure for characterizing the mechanical properties of a material. The associated terms are defined in the standard ISO 6892-1. The yield strength Rp0.2 indicates which stress is present at a plastic strain of 0.2%. This parameter is used when there is no sharp transition between elastic and plastic material behavior. The tensile strength Rm is the maximum stress in the measured stress-strain diagram. The tensile strength is reached the moment the sample starts to constrict (local taper of the sample cross-section). The elongation at break A5 is the extension of the sample at the time of fracture, wherein the index 5 refers to the ratio of the initial measuring length L0 to the initial diameter d0.

Während warmumgeformte bainitische Produkte (Schmiedebauteil, Stabstahl usw.) eine geringe Dehngrenze Rp0.2 und damit auch ein niedriges Rp0.2/Rm-Verhältnis aufweisen (in den Beispielen aus EP 1425453 liegt das Rp0.2/Rm-Verhältnis zwischen 0.6 und 0.7), können durch die Kombination von Ziehen und anschliessender Entspannungsbehandlung nahezu beliebige Rp0.2/Rm-Verhältnisse zwischen 0.6 und 1.0 eingestellt werden.While hot-formed bainitic products (forging, bar steel, etc.) have a low yield strength Rp0.2 and thus a low Rp0.2 / Rm ratio (in the examples of EP 1425453 if the Rp0.2 / Rm ratio is between 0.6 and 0.7), almost any Rp0.2 / Rm ratios between 0.6 and 1.0 can be set by the combination of pulling and subsequent relaxation treatment.

Es wurde überraschend gefunden, dass sich die erfindungsgemässe Stahlzusammensetzung bei konstant vorgegebener Zugfestigkeit deutlich besser zerspanbar ist, wenn das Streckgrenzenverhältnis Rp0.2/Rm über 0.87 liegt. Die deutlich verbesserte Zerspanbarkeit äussert sich in markant längere Werkzeugstandzeiten.It has surprisingly been found that the steel composition according to the invention can be machined significantly better at a constant predetermined tensile strength if the yield ratio Rp0.2 / Rm is above 0.87. The significantly improved machinability manifests itself in significantly longer tool life.

Bei dem erfindungsgemäss hergestellten blanken bainitischen Langprodukt sind die Legierungskomponenten so gewählt, dass bei üblichen Abkühlraten aus der Walzhitze von 0.1 bis 8.0 K/s immer ein bainitisch-martensitisches Gefüge mit Zugfestigkeitsniveau von 900 bis 1'200 MPa resultiert, ohne dass kostspielige Legierungselemente und/oder spezielle Einrichtungen zur beschleunigten Abkühlung aus der Walzhitze verwendet werden müssen. Damit die erfindungsgemässe Eigenschaften eingestellt werden können, muss das Produkt im Temperaturbereich zwischen Raumtemperatur und 600 °C gezogen und anschliessend in einem Temperaturbereich von 250 und 600°C angelassen werden.In the bare bainitic long product produced according to the invention, the alloying components are selected such that, at conventional cooling rates from 0.1 to 8.0 K / s, a bainitic-martensitic microstructure always results with a tensile strength level of 900 to 1,200 MPa, without costly alloying elements and / or or special equipment for accelerated cooling from the rolling heat must be used. In order to be able to adjust the properties according to the invention, the product must be drawn in the temperature range between room temperature and 600 ° C. and then tempered in a temperature range of 250 and 600 ° C.

Im Ziehprozess werden bei Abzügen über 10% (=Querschnittsreduktion) Streckgrenzenverhältnisse Rp0.2/Rm von 0.90 bis 1.00, insbesondere von 0.95 bis 1.00, eingestellt. Beim anschliessenden Anlassen sinkt das Streckgrenzenverhältnis Rp0.2/Rm, wobei es erfindungsgemäss auf über 0.87 zu halten ist. Bei Anlasstemperaturen über 600°C fällt das Streckgrenzenverhältnis Rp0.2/Rm in der Regel unter diesem Wert.In the drawing process yield ratios Rp0.2 / Rm from 0.90 to 1.00, in particular from 0.95 to 1.00, are set for prints over 10% (= reduction in cross section). During the subsequent tempering, the yield ratio Rp0.2 / Rm decreases, and according to the invention it is to be maintained above 0.87. At tempering temperatures above 600 ° C, the yield ratio Rp0.2 / Rm usually falls below this value.

Gemäss einer vorteilhaften Ausführungsform (Anspruch 2) liegt das Streckgrenzenverhältnis Rp0.2/Rm im Bereich von 0.87 bis 0.94.According to an advantageous embodiment (claim 2), the yield ratio Rp0.2 / Rm is in the range of 0.87 to 0.94.

Durch die untere Begrenzung des Kohlenstoffgehalts auf 0.16% wird in Kombination mit Mangan und Chrom sichergestellt, dass nur noch geringe Ferritanteile von 0.1 bis höchstens 8%, typischerweise 3 bis 5% im Gefüge vorliegen. Zu hohe Ferritanteile beeinträchtigen sowohl das Festigkeitsniveau wie auch die Kerbschlagzähigkeit des Produkts.Due to the lower limit of the carbon content to 0.16%, it is ensured in combination with manganese and chromium that only small amounts of ferrite from 0.1 to at most 8%, typically 3 to 5%, are present in the microstructure. Too high ferrite levels affect both the strength level and the impact strength of the product.

Durch die obere Begrenzung des Kohlenstoffs auf 0.26% wird gewährleistet, dass die Zugfestigkeit nicht über 1 '400 MPa ansteigt. Höhere Festigkeitswerte verschlechtern die Bearbeitbarkeit im nachgelagerten Ziehprozess oder Zerspanungsprozess. Höhere Kohlenstoffgehalte fördern ausserdem die Bildung von Karbiden, was die Duktilität nachteilig beeinflusst.The upper limit of the carbon to 0.26% ensures that the tensile strength does not rise above 1'400 MPa. Higher strength values deteriorate the workability in the downstream drawing process or cutting process. Higher carbon contents also promote the formation of carbides, which adversely affects ductility.

Silizium unterdrückt die Bildung von harten und in der Zerspanung abrasiven Fe3C-Ausscheidungen (Zementit). Die gewählte Siliziumkonzentration erlaubt eine einstündige Anlassbehandlung bei 400°C, ohne dass sich grobe Zementitausscheidungen bilden können (in Anlehnung an die Beschreibung des karbidfreien Bainits in WO 96/22396 ). Da Silizium ein effizienter Mischkristallverfestiger im Bainit ist, muss sein Gehalt auf höchstens 1.2 Gew.-% begrenzt werden, um die maximal gewünschte Zugfestigkeit von 1 '400 MPa nicht zu überschreiten.Silicon suppresses the formation of hard and in machining abrasive Fe 3 C precipitates (cementite). The chosen silicon concentration allows a one-hour annealing treatment at 400 ° C without coarse cementite precipitates can form (based on the description of the carbide-free bainite in WO 96/22396 ). Since silicon is an efficient solid solution in bainite, its content must be limited to at most 1.2 wt .-%, so as not to exceed the maximum desired tensile strength of 1 '400 MPa.

Durch die untere Begrenzung in Mangan auf 1.20 Gew.-% und Chrom auf 0.70 Gew.-% wird sichergestellt, dass bei Luftabkühlung ein bainitisches Gefüge aus der Umformhitze entstehen kann.The lower limit in manganese to 1.20% by weight and chromium to 0.70% by weight ensures that a bainitic structure can be produced from the forming heat when the air cools down.

Bei einem zu hohen Mangangehalt werden die Manganseigerungen ausgeprägt und das Gefüge wird sehr inhomogen. Aus diesem Grund muss der Mangangehalt auf 1.70 Gew.-% begrenzt werden.If the manganese content is too high, the manganese segregations are pronounced and the microstructure becomes very inhomogeneous. For this reason, the manganese content must be limited to 1.70 wt .-%.

Chrom schnürt das Bainitgebiet ein, was die Bildung von Martensit begünstigt. Aus diesem Grund muss der Chromgehalt auf 1.60 Gew.-% begrenzt werden.Chromium constricts the bainite area, which favors the formation of martensite. For this reason, the chromium content must be limited to 1.60 wt .-%.

Falls der Stahl im Temperaturbereich zwischen 250 und 600°C angelassen werden soll, kann Molybdän legiert werden, um eine mögliche Anlassversprödung zu unterdrücken. Die Ausscheidung von Eisenkarbiden an den Primärkorngrenzen und einen damit verbundenen Zähigkeitsverlust kann so verhindert werden. Aus Kostengründen ist der Molybdängehalt so niedrig wie notwendig zu wählen und beträgt demnach höchstens 0.3 Gew.-%, insbesondere ungefähr 0.15 bis 0.28 Gew.-%.If the steel is to be tempered in the temperature range between 250 and 600 ° C, molybdenum can be alloyed to suppress possible temper embrittlement. The precipitation of iron carbides at the primary grain boundaries and a related loss of toughness can thus be prevented. For cost reasons, the molybdenum content is to be chosen as low as necessary and is therefore at most 0.3 wt .-%, in particular about 0.15 to 0.28 wt .-%.

Nickel verbessert die Kerbschlagzähigkeit und wirkt sich deshalb negativ auf die Zerspanbarkeit aus. Deshalb wird der Nickelgehalt auf 0.30 Gew.-% begrenzt und beträgt insbesondere ungefähr 0.05 bis 0.1 Gew.-%.Nickel improves notched impact strength and therefore has a negative impact on machinability. Therefore, the nickel content is limited to 0.30 wt%, more preferably about 0.05 to 0.1 wt%.

Schwefel ist ein Stahlschädling. Es bildet Mangansulfidausscheidungen und schwächt das Gefüge. Dies wirkt sich bei dynamisch belasteten Bauteilen negativ auf die Ermüdungsfestigkeit aus. Aus diesem Grund wurde der Schwefelgehalt auf 0.03 Gew.-% begrenzt. Vorzugsweise beträgt der Schwefelgehalt 0.010 bis 0.020 Gew.-% und insbesondere ungefähr 0.015 Gew.-%.Sulfur is a steel pest. It forms manganese sulfide precipitates and weakens the microstructure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the sulfur content was limited to 0.03 wt .-%. Preferably, the sulfur content is 0.010 to 0.020 wt .-% and in particular about 0.015 wt .-%.

Die Zugabe von Aluminium ist für die erfindungsgemässe Herstellung des Produkts nicht zwingend und beträgt deshalb höchstens 0.01 Gew.-%, insbesondere 0.005 bis 0.009 Gew.-%.The addition of aluminum is not mandatory for the production of the product according to the invention and is therefore at most 0.01% by weight, in particular 0.005 to 0.009% by weight.

Phosphor ist ein Stahlschädling. Es geht an die Austenitkorngrenzen und schwächt das Gefüge. Dies wirkt sich bei dynamisch belasteten Bauteilen negativ auf die Ermüdungsfestigkeit aus. Aus diesem Grund wurde der Phosphorgehalt auf 0.03 Gew.-% begrenzt und beträgt insbesondere 0.01 bis 0.02 Gew.-%.Phosphorus is a steel pest. It goes to the Austenitkorngrenzen and weakens the structure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the phosphorus content was limited to 0.03 wt .-% and is in particular 0.01 to 0.02 wt .-%.

Kupfer ist ein Stahlschädling. Bei hohen Kupfergehalten kommt es in der Warmumformung zu Rotbrüchigkeit. Aus diesem Grund ist der Kupfergehalt auf 0.25 Gew.-% begrenzt und beträgt insbesondere 0.1 bis 0.2 Gew.-%.Copper is a steel pest. At high copper contents, red brittleness occurs in hot forming. For this reason, the copper content is limited to 0.25 wt .-% and is in particular 0.1 to 0.2 wt .-%.

Es ist zu bemerken, dass die Elemente P, S, Mo, Ni, Al und Cu nicht zulegiert werden und deshalb auch in beliebig geringen Mengenanteilen vorhanden sein können.It should be noted that the elements P, S, Mo, Ni, Al and Cu are not alloyed and therefore may be present in any small proportions.

Harte Oxid- oder Nitrideinschlüsse wirken sich ab einer gewissen Grösse negativ auf die Zerspanbarkeit aus. Insbesondere Titan (Titanoxide und grobe Titankarbonitride) und Aluminium (harte Al2O3-Verbindungen) verschlechtern sowohl die Zerspanbarkeit wie die Lebensdauer von dynamisch belasteten Bauteilen. Bei Niob und Vanadium muss ebenfalls von einer negativen Wirkung ausgegangen werden. Die Mikrolegierungselemente Titan, Niob und Vanadium werden deshalb auf 0.01 Gew.-% begrenzt.Hard oxide or nitride inclusions have a negative effect on the machinability above a certain size. In particular titanium (titanium oxides and coarse titanium carbonitrides) and aluminum (hard Al 2 O 3 compounds) deteriorate both the machinability and the life of dynamically loaded components. at Niobium and vanadium must also be assumed to have a negative effect. The micro-alloying elements titanium, niobium and vanadium are therefore limited to 0.01 wt .-%.

Bor wird bei den bainitischen Stählen in der Regel legiert, um die Bildung von Ferrit zu unterdrücken. Damit das Bor in diesem Sinne wirksam ist, muss der im Stahl vorhandene Stickstoff mit Titan abgebunden werden. Da die Zugabe von Titan zu vermeiden ist, kann dieses Legierungskonzept hier nicht angewendet werden. Bor wird deshalb nur in Spuren bis 0.001 Gew.-% vorhanden sein.Boron is usually alloyed with bainitic steels to suppress the formation of ferrite. For the boron to be effective in this sense, the nitrogen present in the steel must be bound with titanium. Since the addition of titanium is to be avoided, this alloy concept can not be used here. Boron will therefore be present only in traces to 0.001 wt .-%.

Dem erfindungsgemäss hergestellten blanken bainitischen Langprodukt wird kein Aluminium zugegeben. In Kombination mit einem hohen Siliziumgehalt und einer geringen Kalziumzugabe am Ende der metallurgischen Behandlung sollen gemäss Anspruch 3 Oxideinschlüsse mit einem Al2O3-Gehalt von < 50 Gew.-% eingestellt werden. Vorzugsweise wird die metallurgische Behandlung gemäss Anspruch 4 so vorgenommen, dass weiche, glasartige Silikateinschlüsse mit folgenden relativen Gewichtsanteilen entstehen: 20 bis 50% CaO, 25 bis 65% SiO2 und weniger als 30% Al2O3. Die Werkzeugstandzeit der in der Zerspanung eingesetzten Werkzeuge wird dann deutlich verlängert.No aluminum is added to the bare bainitic long product produced according to the invention. In combination with a high silicon content and a low calcium addition at the end of the metallurgical treatment according to claim 3 oxide inclusions should be set with an Al 2 O 3 content of <50 wt .-%. Preferably, the metallurgical treatment according to claim 4 is carried out so that soft, glassy silicate inclusions arise with the following relative proportions by weight: 20 to 50% CaO, 25 to 65% SiO 2 and less than 30% Al 2 O 3 . The tool life of the tools used in machining is then significantly extended.

Stickstoff geht an die Austenitkorngrenzen und schwächt das Gefüge. Dies wirkt sich bei dynamisch belasteten Bauteilen negativ auf die Ermüdungsfestigkeit aus Aus diesem Grund wurde der Stickstoffgehalt auf höchstens 0.0150 Gew.-% begrenzt. Ohne erheblichen Aufwand wird eine Untergrenze von 50 ppm nicht unterschritten.Nitrogen reaches the austenite grain boundaries and weakens the microstructure. This has a negative effect on the fatigue strength of dynamically loaded components. For this reason, the nitrogen content was limited to a maximum of 0.0150% by weight. Without significant effort, a lower limit of 50 ppm is not exceeded.

Vor der weiteren Bearbeitung des blanken bainitischen Langprodukts kann eine Wärmebehandlung bis 2 Stunden bei 300 bis 600°C gemäss Anspruch 7 sinnvoll sein. Durch die Kaltverfestigung eingebrachte Spannungen werden dadurch abgebaut.Before further processing of the bare bainitic long product, a heat treatment of up to 2 hours at 300 to 600 ° C. according to claim 7 may be expedient. Tensions introduced by strain hardening are thereby reduced.

Wege zur Ausführung der ErfindungWays to carry out the invention

Ein Ausführungsbeispiel der Erfindung wird nachfolgend unter Bezugnahme auf die Zeichnung näher beschrieben, dabei zeigt:

Fig. 1
die Ergebnisse für die Werkzeugstandzeit bei zwei Werten der Werkstofffestigkeit Rm in Abhängigkeit des Streckgrenzenverhältnisses Rp0.2/Rm.
Im Rahmen eines Ausführungsbeispiels wurde eine Stahlschmelze "Stahl 1" vergossen und anschliessend zu Stabstahl gewalzt. Die Herstellung der Stahlschmelze erfolgte nach dem Elektrostahl-Verfahren mit einer sekundärmetallurgischen Behandlung an einem Pfannenstand und anschliessendem Vergiessen zu 150x150 mm2-Knüppeln in einer kontinuierlichen Stranggussanlage. Die Knüppel wurden danach in einem Hubbalkenofen auf 1'150 bis 1'200°C wieder erwärmt und anschliessend zu Stabstahl in Durchmesser ∅43 mm gewalzt. Die Abkühlung der Stäbe nach der Walzung erfolgte an Luft (Kühlrate ca. 1.0 K/s).An embodiment of the invention will be described in more detail below with reference to the drawing, in which:
Fig. 1
the results for the tool life at two values of the material strength Rm as a function of the yield ratio Rp0.2 / Rm.
In one embodiment, a steel melt "steel 1" was cast and then rolled into bar steel. The molten steel was produced by the electrical steel process with a secondary metallurgical treatment on a ladle and subsequent casting to 150 × 150 mm 2 sticks in a continuous casting plant. The billets were then reheated in a walking beam oven to 1'150 to 1'200 ° C and then rolled to bar steel in diameter ∅43 mm. The cooling of the rods after rolling was carried out in air (cooling rate about 1.0 K / s).

In der Blankstahlfertigung wurden die Stäbe zur Entfernung der Eisenoxidhaut kugelgestrahlt. Danach wurden die Stäbe entweder bei Raumtemperatur oder im Temperaturbereich zwischen 350 und 450°C durch einen mehrstufigen Ziehstein auf das Endmass von ∅40 mm gezogen und in einer Zweiwalzen-Richtmaschine gerichtet. Die abschliessende konduktive Erwärmung bei 350 resp. 500°C diente zur Abbau von verformungsinduzierte Oberflächenspannungen.In the bright steel production, the rods were shot peened to remove the iron oxide skin. Thereafter, the rods were either drawn at room temperature or in the temperature range between 350 and 450 ° C by a multi-stage die to the final dimension of ∅40 mm and directed in a two-roll straightening machine. The final conductive heating at 350 resp. 500 ° C was used to reduce deformation-induced surface tensions.

Die Zerspanbarkeit der gefertigten Varianten wurde in einem Standardtest am ISF in Dortmund geprüft. Als Versuchsproben kamen beidseitig plangedrehte und einseitig zentriergebohrte Stäbe mit einer Länge von 190 mm zum Einsatz. Die Versuche liefen unter Emulsion vom Typ Bechem Avantin 3309 mit einer Konzentration von ca. 6-8%. Das Drehwerkzeug (CNMG120404-PF4215) wurde so ausgewählt, dass mit dem Standardstahl 42CrMo4+QT (bei Rm ≈ 1'000 MPa) eine industriell übliche Bearbeitungszeit von 18 min ermöglicht werden, ohne das ein Verschleissmarkenbreitekriterium von VBmax ≤ 300 µm verletzt wird. Bei einem Vorschub f von 0.3 mm/U und einer Schnitttiefe ap von 1 mm betrug die Schnittgeschwindigkeit 200 m/minThe machinability of the manufactured variants was tested in a standard test at the ISF in Dortmund. The test specimens used were double-turned and one-sided center bored rods with a length of 190 mm. The experiments ran under Bechem Avantin 3309 emulsion with a concentration of about 6-8%. The turning tool (CNMG120404-PF4215) was selected to allow the standard steel 42CrMo4 + QT (at Rm ≈ 1'000 MPa) to have an industrially common machining time of 18 minutes without violating a wear mark width criterion of V Bmax ≤ 300 μm. At a Feed f of 0.3 mm / rev and a cutting depth a p of 1 mm, the cutting speed was 200 m / min

Da sich die Zerspanbarkeit mit steigender Werkstofffestigkeit Rm abnimmt (die Werkzeugstandzeit verkürzt sich), macht ein vergleichender Test nur bei konstanter Werkstofffestigkeit Sinn. Bei vorgegebener Stahlzusammensetzung lassen sich durch die Variation der Produktions- und Wärmebehandlungsparameter jedoch nicht beliebige Kombinationen von Zugfestigkeit Rm und Streckgrenzenverhältnis Rp0.2/Rm einstellen. Aus diesem Grund wurden die hergestellten Varianten mit dem Stahl 2 und mit aus der Literatur bekannten Referenzstählen Stahl 3 und Stahl 4 verglichen, die ebenfalls am ISF in Dortmund mit demselben Testverfahren charakterisiert worden waren.As the machinability decreases with increasing material strength Rm (the tool life is shortened), a comparative test only makes sense with constant material strength. However, given a given steel composition, it is not possible to adjust any combination of tensile strength Rm and yield ratio Rp0.2 / Rm by varying the production and heat treatment parameters. For this reason, the manufactured variants were compared with the steel 2 and with reference steels known from the literature steel 3 and steel 4, which had also been characterized at the ISF in Dortmund by the same test method.

Tabelle 1 zeigt die Stahlzusammensetzungen, die für diesen Vergleich verwendet wurden. Die Zusammensetzung der vorhandenen oxidischen Einschlüssen wurde im Rasterelektronenmikroskop mit EDX analysiert. Die mittlere Zusammensetzung (bei 2'222 gemessene Einschlüssen) im normierten Dreistoffsystem CaO-Al2O3-SiO2 war CaO = 46.3%, Al2O3 = 27.9%, SiO2 = 25.8%.Table 1 shows the steel compositions used for this comparison. The composition of the existing oxide inclusions was analyzed by scanning electron microscopy using EDX. The average composition (inclusions measured at 2222) in the normalized ternary system CaO-Al 2 O 3 -SiO 2 was CaO = 46.3%, Al 2 O 3 = 27.9%, SiO 2 = 25.8%.

Die Gefügebestandteilen von Stahl 1 wurden quantitativ im Rasterelektronenmikroskop (für Bainit und Ferrit) und im Röntgendiffraktometer (für Restaustenit) bestimmt. Innerhalb der Messgenauigkeit sind die Unterschiede der drei Varianten gering. Die Ergebnisse sind in der Tabelle 2 für die verschiedenen Wärmebehandlungen dargestellt. Bei Stahl 2 liegt ein 100%-iges Vergütungsgefüge (angelassener Martensit) vor. Die Stähle 3 und 4 weisen ähnlich wie Stahl 1 ein Mischgefüge aus Bainit, Martensit und Restaustenit auf. Tabelle 1 Stahl 1* Stahl2 Stahl3 Stahl4 Gew.-% Gew.-% Gew.-% Gew.-% C 0.17 0.51 0.22 0.17 Si 1.20 0.26 0.49 1.46 Mn 1.49 0.67 1.72 1.52 P 0.015 0.016 0.019 0.013 S 0.015 0.007 0.013 0.017 Cr 1.21 1.06 1.58 1.32 Mo 0.28 0.20 0.30 0.06 Ni 0.07 0.18 0.19 0.16 Al 0.005 0.029 0.005 0.016 B - - - 0.0021 Cu 0.11 0.22 0.015 0.13 Nb - - 0.095 0.029 Ti - - 0.032 0.029 N 0.0099 0.0070 0.0258 0.0113 *erfindungsgemäss Tabelle 2 Bainit Martensit Restaustenit Ferrit % % % % Gezogen bei RT** Entspannt bei 350°C 71±3 23±5 3±3 3±1 Gezogen bei 400°C Entspannt bei 350°C 72±3 18±5 7±3 3±1 Gezogen bei 400°C Entspannt bei 500°C 76±3 11±5 9±3 4±1 **RT=Raumtemperatur The structural components of steel 1 were determined quantitatively in the scanning electron microscope (for bainite and ferrite) and in the X-ray diffractometer (for retained austenite). Within the measuring accuracy the differences of the three variants are small. The results are shown in Table 2 for the various heat treatments. For steel 2 there is a 100% compensation structure (tempered martensite). The steels 3 and 4, similar to steel 1, have a mixed structure of bainite, martensite and retained austenite. <u> Table 1 </ u> Steel 1 * Steel2 Stahl3 steel4 Wt .-% Wt .-% Wt .-% Wt .-% C 12:17 12:51 12:22 12:17 Si 1.20 12:26 12:49 1:46 Mn 1:49 0.67 1.72 1:52 P 0015 0016 0019 0013 S 0015 0007 0013 0017 Cr 1.21 1:06 1:58 1:32 Mo 12:28 12:20 12:30 12:06 Ni 12:07 12:18 12:19 12:16 al 0005 0029 0005 0016 B - - - 0.0021 Cu 12:11 12:22 0015 12:13 Nb - - 0095 0029 Ti - - 0032 0029 N 0.0099 0.0070 0.0258 0.0113 * According to the invention bainit martensite austenite ferrite % % % % Pulled at RT ** Relaxed at 350 ° C 71 ± 3 23 ± 5 3 ± 3 3 ± 1 Pulled at 400 ° C Relaxed at 350 ° C 72 ± 3 18 ± 5 7 ± 3 3 ± 1 Pulled at 400 ° C Relaxed at 500 ° C 76 ± 3 11 ± 5 9 ± 3 4 ± 1 ** RT = room temperature

Tabelle 3 zeigt die Ergebnisse für die Werkzeugstandzeit bei einer ausgewählten Werkstofffestigkeit von Rm = 1'230 ± 30 MPa. Die Korrelation zwischen Streckgrenzenverhältnis Rp0.2/Rm und Werkzeugstandzeit wurde überraschend festgestellt. Durch Erhöhung des Rp0.2/Rm-Verhältnisses konnte die Werkzeugstandzeit mehr als verdoppelt werden. Tabelle 3 A5 Rm Rp0.2/Rm Standzeit (VB = 300 µm) % MPa MPa Min Stahl 1* Gezogen bei RT Entspannt bei 350°C 11.6 1234 0.94* 12 Stahl 1* Gezogen bei 400°C Entspannt bei 350°C 10.5 1231 0.99* 20 Stahl 2 14.1 1202 0.85 8 Stahl 3 14.4 1253 0.7 6.5 *erfindungsgemäss Table 3 shows the tool life results for a selected material strength of Rm = 1,230 ± 30 MPa. The correlation between yield ratio Rp0.2 / Rm and tool life was surprisingly found. Increasing the Rp0.2 / Rm ratio more than doubled tool life. <u> Table 3 </ u> A5 rm Rp0.2 / Rm Service life (V B = 300 μm) % MPa MPa min Steel 1 * Drawn at RT Relaxed at 350 ° C 11.6 1234 0.94 * 12 Steel 1 * Pulled at 400 ° C Relaxed at 350 ° C 10.5 1231 0.99 * 20 Steel 2 14.1 1202 0.85 8th Steel 3 14.4 1253 0.7 6.5 * According to the invention

Für eine Überprüfung dieses Zusammenhangs bei Rm = 1'165 MPa standen nur zwei Stahlvarianten zur Verfügung (Tabelle 4). Da sich der Verschleiss aufgrund der tieferen Festigkeit langsamer einstellt, wurde das Kriterium für die Verschleissmarkenbreite auf 200 µm reduziert. Die Gegenüberstellung zeigt, dass bereits bei einem Rp0.2/Rm-Verhältnis von 0.88 deutlich besserer Werkzeugstandzeiten erreicht werden. Tabelle 4 A5 Rm Rp0.2/Rm Standzeit (VB = 200 µm) % MPa MPa Min Stahl 1* Gezogen bei 400°C Entspannt bei 500°C 16.2 1163 0.88* 18 Stahl 4 12.5 1167 0.67 9 *erfindungsgemäss For a review of this relationship at Rm = 1'165 MPa, only two steel variants were available (Table 4). Since the wear is slower due to the lower strength, the criterion for the wear mark width was reduced to 200 μm. The comparison shows that even with a Rp0.2 / Rm ratio of 0.88 significantly better tool life is achieved. <u> Table 4 </ u> A5 rm Rp0.2 / Rm Service life (V B = 200 μm) % MPa MPa min Steel 1 * Pulled at 400 ° C Relaxed at 500 ° C 16.2 1163 0.88 * 18 Steel 4 12.5 1167 0.67 9 * According to the invention

Die Ergebnisse aus den Tabellen 2 und 3 sind in der Figur 1 zusammengefasst.The results from Tables 2 and 3 are in FIG. 1 summarized.

Claims (8)

  1. A blank bainitic long product with weight fractions of
    0.16 to 0.26 % carbon,
    0.60 to 1.20% silicon,
    1.20 to 1.70% manganese,
    0.70 to 1.60% chromium,
    up to 0.20% nickel,
    up to 0.30% molybdenum,
    up to 0.03% sulfur,
    up to 0.01 % aluminum,
    up to 0.03% phosphorus,
    up to 0.25% copper,
    up to 0.001 % boron,
    up to 0.01 % titanium,
    up to 0.01 % vanadium
    up to 0.01 % niobium
    50 ppm to 0.015% nitrogen and
    up to 0.01 % oxygen bound in oxidic inclusions,
    the residue being iron and impurities usually contained in steel,
    wherein the following structural constituents are present:
    60 to 80% bainite,
    5 to 30% martensite,
    0.5 to 8% ferrite and
    3 to 15% residual austenite
    and wherein the drawing test according to ISO 6892-1 results in:
    a yield strength Rp0.2 = 950 to 1'400 MPa
    an ultimate tensile strength Rm = 1'150 to 1'400 MPa and
    a breaking elongation A5 = 9.0 to 17.0%,
    wherein 0.87 < Rp0.2/Rm < 0.99.
  2. The blank bainitic long product according to claim 1, wherein Rp0.2/Rm is 0.87 to 0.94.
  3. The blank bainitic long product according to claim 1 or 2, characterized in that it contains oxidic inclusions with less than 50 wt.-% Al2O3.
  4. The blank bainitic long product according to claim 3, wherein the oxidic inclusions have the following relative weight fractions: 20 to 50% CaO, 25 to 65% SiO2 and less than 30% Al2O3.
  5. Use of a blank bainitic long product according to one of claims 1 to 4 for a chip removing machining process.
  6. The use of a blank bainitic long product according to one of claims 1 to 4 for the production of dynamically stressed automotive components, particularly in the field of diesel injection.
  7. The use according to claim 5 or 6, wherein the blank bainitic long product is subjected to a heat treatment at 300 to 600°C for 2 hours prior to the chip removing machining process and/or or to a component production.
  8. A method for the production of a blank bainitic long product according to one of claims 1 to 4, comprising the following steps:
    - producing a steel alloy with weight fractions of
    0.16 to 0.26 % carbon
    0.60 to 1.20% silicon,
    1.20 to 1.70% manganese,
    0.70 to 1.60% chromium,
    up to 0.20% nickel,
    up to 0.30% molybdenum,
    up to 0.03% sulfur,
    up to 0.01 % aluminum,
    up to 0.03% phosphorus,
    up to 0.25% copper,
    up to 0.001% boron,
    up to 0.01% titanium,
    up to 0.01 % vanadium
    up to 0.01% niobium
    50 ppm to 0.015% nitrogen and
    up to 0.01% oxygen bound in oxidic inclusions,
    the residue being iron and impurities usually contained in steel, and casting to a mold of predefined format;
    - reheating of the unit format, hot-rolling to wire or bar and cooling to a temperature below 400°C with a cooling rate of 0.1 to 8.0 K/s;
    - chemical or mechanical removal of the outer iron oxide skin;
    - drawing of the rolled steel by means of a tool with predefined internal profile at a temperature of 25 to 600°C;
    - mechanical straightening;
    - relaxing at a temperature of 250 to 600°C.
EP16157945.3A 2015-02-27 2016-02-29 Blank bainite long product and method for producing the same Active EP3061838B1 (en)

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GB2297094B (en) 1995-01-20 1998-09-23 British Steel Plc Improvements in and relating to Carbide-Free Bainitic Steels
ITFI20010136A1 (en) 2001-07-18 2003-01-18 Eos S R L FIBERS, YARN AND FLAME-RESISTANT FABRICS, THEIR PREPARATION AND USE FOR THE PRODUCTION OF FLAME-RESISTANT, ANTI-HEAT AND INSULATION PRODUCTS
FR2847910B1 (en) 2002-12-03 2006-06-02 Ascometal Sa METHOD FOR MANUFACTURING A FORGED STEEL PIECE AND PART THUS OBTAINED
JP4716359B2 (en) * 2005-03-30 2011-07-06 株式会社神戸製鋼所 High strength cold-rolled steel sheet excellent in uniform elongation and method for producing the same
DE102008004371A1 (en) 2008-01-15 2009-07-16 Robert Bosch Gmbh Component, in particular a motor vehicle component, made of a dual-phase steel
ES2391312T3 (en) 2008-03-10 2012-11-23 Swiss Steel Ag Longitudinal hot rolled product and manufacturing process
FR2958660B1 (en) * 2010-04-07 2013-07-19 Ascometal Sa STEEL FOR MECHANICAL PIECES WITH HIGH CHARACTERISTICS AND METHOD FOR MANUFACTURING THE SAME.
WO2012048841A1 (en) * 2010-10-12 2012-04-19 Tata Steel Ijmuiden B.V. Method of hot forming a steel blank and the hot formed part
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