EP1352982A2 - Stainless steel, method for manufacturing of stress cracking free workpieces and product made thereof - Google Patents

Stainless steel, method for manufacturing of stress cracking free workpieces and product made thereof Download PDF

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Publication number
EP1352982A2
EP1352982A2 EP03008317A EP03008317A EP1352982A2 EP 1352982 A2 EP1352982 A2 EP 1352982A2 EP 03008317 A EP03008317 A EP 03008317A EP 03008317 A EP03008317 A EP 03008317A EP 1352982 A2 EP1352982 A2 EP 1352982A2
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Prior art keywords
content
steel
cold
strip
ferrite
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German (de)
French (fr)
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EP1352982B1 (en
EP1352982A3 (en
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Gabriele Dr. Brückner
Hans-Joachim Dr. Rer. Nat. Krautschick
Wolfgang Dr. Rer. Nat. Schlump
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Outokumpu Nirosta GmbH
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ThyssenKrupp Nirosta GmbH
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C22C38/58—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
    • 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
    • C21D2211/005—Ferrite
    • 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
    • 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
    • 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
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling

Definitions

  • the invention relates to a stainless steel made of such steel by cold forming generated component and a method for producing stress-free molded parts.
  • Metastable or unstable austenitic stainless Steels are characterized by very good formability stretching stress.
  • the good Deformability of such stainless steels includes in this justifies that there is stretching deformation too deformation-induced martensite formation that Cause TRIP effect.
  • the object of the invention an inexpensive to produce stainless steel create, even with conventional cold forming insensitive to the development of stress cracks is, as well as a method for producing to specify stress-free, cold-formed components.
  • the starting point of the invention is the idea instead of usually for stainless steels of the generic type provided single-phase purely austenitic structure to go over a two-phase mixed structure, which consists of Austenite and ferrite is formed. It will Mixed structure by alloying Si and / or Mo and partly by lowering the Ni content or by Exchange of Ni by Cu discontinued. The austenite will stabilized so far that the formation of martensite at Deformation no longer leads to stress cracks.
  • the austenite is not stabilized, however Alloying expensive austenite formers, but by the Addition of ferrite formers such as Si, Mo, Al with formation of a two-phase structure. This structure leads to the fact that in the ⁇ phase the austenite formers are enriched and the Ferrite formers are depleted.
  • the ⁇ -ferrite content should not be higher than 40% to ensure good ductility of the steel guarantee.
  • This is achieved in one alloy according to the invention in that the contents Cr, Mo, Si, Al, Mn, Ni, Cu, C and N so on top of each other be voted that the factor t formed with them in Range is 1.3 to 1.8. In this way the required for high resistance to stress cracking Weighting of the austenite and ferrite components in the structure of the steel according to the invention unerringly achieved.
  • the M d30 temperature is the temperature at which 50% of the austenite to martensite conversion occurred after cold working of 30%. Above this temperature, on the other hand, there is a reduced conversion (see Material Science Steel, Volume 2, publisher: Association of German Ironworkers, 1985, Springer-Verlag Berlin Heidelberg New York Tokyo, Verlag Stahleisen mbH Düsseldorf, Chapter D 10.3.2). Accordingly, if the upper limit of the M d30 temperature prescribed by the invention is observed, it is ensured that the formation of martensite, which is unfavorable with regard to the development of stress cracks, is largely suppressed. This is set reliably by the two-phase structure of the steel sought according to the invention.
  • the two-phase structure leads from steel according to the invention to separate the Alloying elements.
  • Austenite formers such as Mn, Ni, Cu, C, N
  • ferrite formers such as Cr, Si and Mo
  • the storage the austenite in the ⁇ phase stabilizes the Austenite content beyond the basic analysis and reduced or suppresses the formation of martensite.
  • the figure shows a micrograph for a sample piece P. to represent the spatial distribution of the phases of a steel according to the invention, specifying the Sample coordinate system, in which the vertical aligned sheet metal normal BN perpendicular to that through the Cross direction Qr and the rolling direction Wr spanned Level stands.
  • the composition of the comparative steel V1 is below that Material number 1.4301 and that of the comparative steel V2 known under the material number 1.4376.
  • Table 1 the proportions of the ⁇ -ferrite in the structure and the respective strain-induced martensite portion recorded. Table 1 also shows whether it is at cold forming of steels E1 to E12 and V1 and V2 cracking occurred.
  • sample P shows the spatial distribution of austenitic and ferritic Structural components.
  • the cold strips obtained were used to form sheet blanks a diameter of 150 mm, from which then using a 75 mm diameter Stamp cups were deep drawn. It turned out that result from the steels E1 to E12 according to the invention Cold strips obtained can easily be molded into wells were cold formed while it was cold formed generated from the conventional comparative steels V1, V2 Cold strips on a large scale to create Tension cracks came.
  • Table 2 shows the M d30 temperatures calculated from the alloy contents for the steels E1, E2 and E3 according to the invention and for the comparative steels V1 and V2. It shows that stress cracks certainly do not occur if the M d30 temperature of the respective steel does not exceed -55 ° C.

Abstract

Stainless steel having a structure with at least 15 vol.% ferrite and a balance of austenite contains alloying additions of chromium and manganese, and optionally nickel, silicon, molybdenum, aluminum and copper. Stainless steel having a structure with at least 15 vol.% ferrite and a balance of austenite comprises (in wt.%): 0.02-0.08 carbon (C), 0.1-0.5 nitrogen (N), 16.0-20.0 chromium (Cr), 6.0-12.0 manganese (Mn), at most 9.05 nickel (Ni), at most 3.0 silicon (Si), at most 3.0 molybdenum (Mo), at most 2.0 aluminum (Al), at most 3.0 copper (Cu) and a balance of iron (Fe) and impurities (where 1.3 less than t less than 1.8 and t = (%Cr + 2%Mo + 1.5%Si + 3%Al -5)/(0.3%Mn + %Ni + 0.5%Cu + 15(%C + %N) +2)). Md30 temperature of the austenitic phase is not more than -55degrees C (where Md30 (degrees C) = 413 - 462(%C + %N) - 9.2%Si - 8.1%Mn - 13.7%Cr - 9.5(%Ni + %Cu) - 18.5%Mo). An Independent claim is also included for a process for the production of crack-free molded parts made from the above stainless steel. Preferred Features: The stainless steel structure comprises not more than 40 vol.% ferrite.

Description

Die Erfindung betrifft einen nichtrostenden Stahl, ein aus einem solchen Stahl bestehendes, durch Kaltumformen erzeugtes Bauteil sowie ein Verfahren zum Herstellen von spannungsrißfreien Formteilen.The invention relates to a stainless steel made of such steel by cold forming generated component and a method for producing stress-free molded parts.

Metastabile bzw. instabile austenitische nichtrostende Stähle zeichnen sich durch sehr gute Verformbarkeit bei streckziehender Beanspruchung aus. Die gute Verformbarkeit derartiger Edelstähle ist u.a. darin begründet, daß es bei der streckziehenden Verformung zu verformungsinduzierter Martensitbildung kommt, die den TRIP-Effekt hervorrufen.Metastable or unstable austenitic stainless Steels are characterized by very good formability stretching stress. The good Deformability of such stainless steels includes in this justifies that there is stretching deformation too deformation-induced martensite formation that Cause TRIP effect.

Es hat sich gezeigt, daß der Umfang der Martensitbildung bei bekannten Stählen der voranstehend genannten Art abhängig ist von der jeweiligen Verformungsbeanspruchung. So kommt es bei einer mehrachsigen Beanspruchung zu einer deutlich ausgeprägteren Martensitbildung als im reinen Zugversuch.It has been shown that the extent of martensite formation in known steels of the type mentioned above depends on the respective strain. So there is a multi-axis load significantly more pronounced martensite formation than in pure Tensile test.

Diese Eigenart bekannter austenitischer Edelstähle bringt bei der Herstellung von Bauteilen, bei der unter mehrachsiger Belastung besonders hohe Umformgrade erzielt werden, die Gefahr der Entstehung von Spannungsrissen mit sich. Diese entstehen in Folge einer durch die komplexe Verformungsbeanspruchung extensiven Martensitbildung. Die Martensitbildung zieht hohe, im Bauteil verbleibende Spannungen nach sich, die als Ursache für die Entstehung der Spannungsrisse angesehen werden. Die betreffenden Effekte sind in einem Artikel von M. Weingräber, A. Gräber, erschienen in Bleche Rohre Profile 32 (1985), S. 7782, erläutert worden.This peculiarity of well-known austenitic stainless steels brings in the manufacture of components, in the under Multi-axis loading achieves particularly high degrees of deformation with the risk of stress cracks yourself. These arise as a result of the complex Deformation stress extensive martensite formation. The The formation of martensite attracts high amounts remaining in the component Tensions after themselves that cause the emergence the stress cracks are considered. The concerned Effects are in an article by M. Weingräber, A. Graves, published in Bleche Rohrs Profile 32 (1985), p. 7782.

Es ist in der Praxis versucht worden, die Entstehung von Spannungsrissen zu vermeiden, indem die Verformung der aus diesen Stählen gewonnenen Bleche zu Bauteilen bei erhöhten Temperaturen durchgeführt wird. Auf diese Weise läßt sich zwar eine übermäßige Martensitbildung vermeiden. Die dazu erforderliche Erwärmung macht den Kaltverformungsprozeß jedoch aufwendig und erschwert die großtechnische Nutzbarkeit.In practice, the emergence of Avoid stress cracks by deforming the Sheets made from these steels contribute to components elevated temperatures is carried out. In this way excessive martensite formation may occur avoid. The necessary warming does that Cold forming process, however, complex and difficult industrial usability.

Ebenso ist versucht worden, die bekannten Stähle durch Zugaben an Mn, Ni und Cu gegenüber einer Martensitbildung zu stabilisieren. Die dazu erforderlichen Mengen an Legierungselementen erhöhen jedoch die Kosten der Stahlerzeugung, so daß auch dieser Weg zu einer Verschlechterung der Wirtschaftlichkeit und Vermarktbarkeit von bekannten Stählen der in Rede stehenden Art führt.Attempts have also been made to test the known steels Additions to Mn, Ni and Cu compared to martensite formation to stabilize. The quantities required for this However, alloying elements increase the cost of Steel production, so that this way too Deterioration of economy and Marketability of known steels in question standing type leads.

Ausgehend von dem voranstehend erläuterten Stand der Technik bestand die Aufgabe der Erfindung somit darin, einen kostengünstig herstellbaren nichtrostenden Stahl zu schaffen, der auch bei konventioneller Kaltumformung unempfindlich gegen die Entstehung von Spannungsrissen ist, sowie ein Verfahren zur Herstellung von spannungsrißfreien, kaltumgeformten Bauteilen anzugeben. Based on the status of the Technology was therefore the object of the invention an inexpensive to produce stainless steel create, even with conventional cold forming insensitive to the development of stress cracks is, as well as a method for producing to specify stress-free, cold-formed components.

In Bezug auf den Stahl wird diese Aufgabe durch einen nichtrostenden Stahl gelöst, dessen Gefüge mindestens 15 Vol.-% δ-Ferrit und als Rest Austenit aufweist, mit folgender Zusammensetzung (in Gew.-%): C 0,02 - 0,08 %, N 0,1 - 0,5 %, Cr 16,0 - 20,0 %, Mn 6,0 - 12,0 %, Ni ≤ 9,05 %, Si ≤ 3,0 %, Mo ≤ 3,0 %, Al ≤ 2,0 %, Cu ≤ 3,0 %, Rest Fe und unvermeidbare Verunreinigungen,
wobei für t = %Cr + 2%Mo + 1,5%Si + 3%Al - 5 0,3%Mn + %Ni + 0,5%Cu + 15(%C + %N) + 2 gilt 1,3 < t < 1,8 und für die Md30-Temperatur TMd30 der austenitischen Phase gilt TMd30 ≤ -5560 °C
mit TMd30[°C]= 413 - 462(%C+%N) - 9,2%Si - 8,1%Mn - 13,7%Cr - 9,5(%Ni+%Cu) - 18,5%Mo wobei mit %C der C-Gehalt, %N der N-Gehalt, %Si der Si-Gehalt, %Al der Al-Gehalt, %Mn der Mn-Gehalt, %Cr der Cr-Gehalt, %Ni der Ni-Gehalt, %Mo der Mo-Gehalt und %Cu der Cu-Gehalt der jeweiligen Stahlzusammensetzung bezeichnet sind. With regard to steel, this problem is solved by a stainless steel, the structure of which has at least 15 vol.% Δ-ferrite and the rest austenite, with the following composition (in wt.%): C 0.02 - 0.08%, N 0.1 - 0.5%, Cr 16.0 - 20.0%, Mn 6.0 - 12.0%, Ni ≤ 9.05%, Si ≤ 3.0%, Mo ≤ 3.0%, al ≤ 2.0%, Cu ≤ 3.0%, Balance Fe and unavoidable impurities,
being for t = % Cr + 2% Mo + 1.5% Si + 3% Al - 5 0.3% Mn +% Ni + 0.5% Cu + 15 (% C +% N) + 2 applies 1.3 <t <1.8 and for the M d30 temperature T Md30 of the austenitic phase, TM d30 ≤ -5560 ° C applies
With TM d30 [° C] = 413 - 462 (% C +% N) - 9.2% Si - 8.1% Mn - 13.7% Cr - 9.5 (% Ni +% Cu) - 18.5% Mo where with% C the C content,% N the N content,% Si the Si content,% Al the Al content,% Mn the Mn content,% Cr the Cr content,% Ni the Ni content ,% Mo the Mo content and% Cu the Cu content of the respective steel composition.

Überraschend hat sich gezeigt, daß ein solcher Stahl schon aufgrund seiner erfindungsgemäßen Zusammensetzung und seines sich infolge dessen einstellenden Gefüges in so hohem Maße unempfindlich gegen Spannungsrisse ist, daß sich aus ihm auch unter Einwirkung komplexer Verformungsbeanspruchungen spannungsrißfreie Bauteile durch Kaltumformung erzeugen lassen. Aufwendiger Wärmebehandlungen oder kostspieliger Erhöhung der Gehalte an wertvollen Legierungsbestandteilen bedarf es dazu nicht.Surprisingly, it has been shown that such a steel does due to its composition according to the invention and as a result of its structure in so is highly insensitive to stress cracks that become more complex even under the influence of it Deformation stresses, stress-free components can be produced by cold forming. consuming Heat treatments or costly increases in levels This requires valuable alloy components Not.

Ausgangspunkt der Erfindung ist der Gedanke, anstelle des üblicherweise für Edelstähle der gattungsgemäßen Art vorgesehenen einphasigen rein austenitischen Gefüges zu einem zweiphasigen Mischgefüge überzugehen, welches aus Austenit und Ferrit gebildet ist. Dabei wird das Mischgefüge durch Zulegieren von Si und/oder Mo und teilweise unter Absenkung des Ni-Gehaltes bzw. durch Austausch von Ni durch Cu eingestellt. Der Austenit wird dabei soweit stabilisiert, daß die Martensitbildung bei Verformung nicht mehr zu Spannungsrissen führt.The starting point of the invention is the idea instead of usually for stainless steels of the generic type provided single-phase purely austenitic structure to go over a two-phase mixed structure, which consists of Austenite and ferrite is formed. It will Mixed structure by alloying Si and / or Mo and partly by lowering the Ni content or by Exchange of Ni by Cu discontinued. The austenite will stabilized so far that the formation of martensite at Deformation no longer leads to stress cracks.

Die Austenitstabilisierung erfolgt jedoch nicht durch Zulegieren von teuren Austenitbildner, sondern durch die Zugabe von Ferritbildner, wie Si, Mo, Al unter Ausbildung eines zweiphasigen Gefüges. Dieses Gefüge führt dazu, daß in der γ-Phase die Austenitbildner angereichert und die Ferritbildner abgereichert werden.The austenite is not stabilized, however Alloying expensive austenite formers, but by the Addition of ferrite formers such as Si, Mo, Al with formation of a two-phase structure. This structure leads to the fact that in the γ phase the austenite formers are enriched and the Ferrite formers are depleted.

Um diesen Effekt sicher zu erreichen, muß ein Mindestanteil von 15 Vol-% δ-Ferrit vorhanden sein. To achieve this effect safely, a At least 15 vol% δ-ferrite must be present.

Gleichzeitig sollte der δ-Ferritanteil jedoch nicht höher als 40 % liegen, um eine gute Verformbarkeit des Stahls zu gewährleisten. Erreicht wird dies in einer erfindungsgemäßen Legierung dadurch, daß die Gehalte an Cr, Mo, Si, Al, Mn, Ni, Cu, C und N so aufeinander abgestimmt werden, daß der mit ihnen gebildete Faktor t im Bereich von 1,3 bis 1,8 liegt. Auf diese Weise wird die für eine hohe Spannungsrißunempfindlichkeit erforderliche Gewichtung der Austenit- und Ferritanteile am Gefüge des erfindungsgemäßen Stahls zielsicher erreicht.At the same time, however, the δ-ferrite content should not be higher than 40% to ensure good ductility of the steel guarantee. This is achieved in one alloy according to the invention in that the contents Cr, Mo, Si, Al, Mn, Ni, Cu, C and N so on top of each other be voted that the factor t formed with them in Range is 1.3 to 1.8. In this way the required for high resistance to stress cracking Weighting of the austenite and ferrite components in the structure of the steel according to the invention unerringly achieved.

Mit der Md30-Temperatur ist die Temperatur bezeichnet, bei der nach einer Kaltumformung von 30 % die Umwandlung von Austenit in Martensit zu 50 % abgelaufen ist. Oberhalb dieser Temperatur tritt dagegen eine verminderte Umwandlung auf (s. Werkstoffkunde Stahl, Band 2, Herausgeber: Verein Deutscher Eisenhüttenleute, 1985, Springer-Verlag Berlin Heidelberg New York Tokio, Verlag Stahleisen m.b.H. Düsseldorf, Kapitel D 10.3.2). Demzufolge ist bei Einhaltung der durch die Erfindung vorgeschriebenen Obergrenze der Md30-Temperatur sichergestellt, daß die im Hinblick auf die Entstehung von Spannungsrissen ungünstige Martensitbildung weitestgehend unterdrückt wird. Diese Weise wird durch das erfindungsgemäß angestrebte zweiphasige Gefüge des Stahls sicher eingestellt.The M d30 temperature is the temperature at which 50% of the austenite to martensite conversion occurred after cold working of 30%. Above this temperature, on the other hand, there is a reduced conversion (see Material Science Steel, Volume 2, publisher: Association of German Ironworkers, 1985, Springer-Verlag Berlin Heidelberg New York Tokyo, Verlag Stahleisen mbH Düsseldorf, Chapter D 10.3.2). Accordingly, if the upper limit of the M d30 temperature prescribed by the invention is observed, it is ensured that the formation of martensite, which is unfavorable with regard to the development of stress cracks, is largely suppressed. This is set reliably by the two-phase structure of the steel sought according to the invention.

In erfindungsgemäßem Stahl sind die einzelnen Legierungselemente unter Berücksichtigung der folgenden Überlegungen eingestellt worden:

  • C und N seigern besonders starkt. Sie sind sehr starke Austenitbildner, die beispielsweise bis zu 30 mal wirksamer sind als Ni. Darüber hinaus führen ihre Gehalte zu einer besonders starken Absenkung der Md30-Temperatur (Gewichtungsfaktor "-462"). Zudem ist ihre Löslichkeit im Ferrit sehr viel geringer als im Austenit, so daß sie eindeutig bevorzugt in den Austenit diffundieren und so zu seiner Stabilisierung beitragen. Daher ist erfindungsgemäß für N ein Mindestgehalt von 0,1 Gew.-% und für C ein Mindestgehalt von 0,02 Gew.-% vorgeschrieben. Die Obergrenze des für den N-Gehalt angegebenen Bereichs ergibt sich aus der begrenzten Löslichkeit von Stickstoff in erfindungsgemäßem Stahl. Bei Gehalten von mehr als 0,08 Gew.-% Kohlenstoff kann es zu einer im Hinblick auf die Kaltverformbarkeit des Stahls unerwünschten Chromkarbidbildung kommen.
  • Ni, Mn und Cu werden als Austenitbildner dem erfindungsgemäßen Stahl zugegeben, wobei Mn zu einer Erhöhung der N-Löslichkeit in der Schmelze beiträgt und Cu die Md30-Temperatur in einem ähnlichen Umfang herabsetzt wie Ni.
  • Al kann in erfindungsgemäßem Stahl vorhanden sein, um die Hitzebeständigkeit zu verbessern. Bei Gehalten von mehr als 2 Gew.-% neigen die Stähle jedoch zur Ausbildung versprödender Phasen.
  • Cr ist in erfindungsgemäßem Stahl in den angegebenen Grenzen in erster Linie zur Verbesserung der Korrosionsbeständigkeit enthalten. Gleichzeitig erhöht auch Cr die Stickstofflöslichkeit in der Schmelze. Die Untergrenze des Cr-Gehaltsbereiches muß eingehalten werden, um die Bildung von unerwünschtem martensitischen Gefüge zu verhindern.
  • Si und Mo sind als Ferritbildner in den angegebenen Grenzen im erfindungsgemäßen Stahl enthalten, wobei Mo zu einer Erhöhung der N-Löslichkeit in der Schmelze beiträgt und gleichzeitig die Korrosionsbeständigkeit verbessert.
    • In the steel according to the invention, the individual alloying elements have been set taking into account the following considerations:
  • C and N are particularly strong. They are very strong austenite formers that are, for example, up to 30 times more effective than Ni. In addition, their contents lead to a particularly strong reduction in the M d30 temperature (weighting factor "-462"). In addition, their solubility in ferrite is much lower than in austenite, so that they diffuse preferentially into the austenite and thus contribute to its stabilization. Therefore, according to the invention, a minimum content of 0.1% by weight is prescribed for N and a minimum content of 0.02% by weight for C. The upper limit of the range given for the N content results from the limited solubility of nitrogen in steel according to the invention. With a content of more than 0.08% by weight of carbon, chromium carbide formation which is undesirable with regard to the cold deformability of the steel can occur.
  • Ni, Mn and Cu are added to the steel according to the invention as austenite formers, Mn contributing to an increase in the N-solubility in the melt and Cu reducing the M d30 temperature to a similar extent as Ni.
  • Al can be present in steel according to the invention to improve heat resistance. At levels of more than 2% by weight, the steels tend to form brittle phases.
  • Cr is contained in the steel according to the invention within the stated limits primarily to improve the corrosion resistance. At the same time, Cr also increases nitrogen solubility in the melt. The lower limit of the Cr content range must be observed in order to prevent the formation of undesirable martensitic structures.
  • Si and Mo are contained as ferrite formers within the specified limits in the steel according to the invention, Mo contributing to an increase in the N-solubility in the melt and at the same time improving the corrosion resistance.

    • Durch die Zweiphasigkeit des Gefüges eines erfindungsgemäß beschaffenen Stahls können keine langreichenden Spannungen durch Martensitbildung aufgebaut werden. Schon aus diesem Grund weist erfindungsgemäßer Stahl eine gegenüber herkömmlichen Stählen deutlich verminderte Spannungsrißanfälligkeit auf.Due to the two-phase structure of the one according to the invention procured steel cannot withstand long-term stresses be built up by martensite formation. Already from this The reason is that steel according to the invention contrasts with one conventional steels significantly reduced Susceptibility to stress cracking.

    Des weiteren führt die Zweiphasigkeit des Gefüges von erfindungsgemäßem Stahl zu einer Entmischung der Legierungselemente. Austenitbildner, wie Mn, Ni, Cu, C, N, diffundieren in den Austenit, während sich Ferritbildner, wie Cr, Si und Mo, im Ferrit anreichern. Die Einlagerung der Austenitbildner in der γ-Phase stabilisiert den Austenitanteil über der Grundanalyse hinaus und vermindert bzw. unterdrückt die Martensitbildung.Furthermore, the two-phase structure leads from steel according to the invention to separate the Alloying elements. Austenite formers, such as Mn, Ni, Cu, C, N, diffuse into the austenite while ferrite formers, such as Cr, Si and Mo, accumulate in the ferrite. The storage the austenite in the γ phase stabilizes the Austenite content beyond the basic analysis and reduced or suppresses the formation of martensite.

    In Bezug auf das Verfahren zum Herstellen von spannungsrißfreien Formteilen wird die oben genannte Aufgabe dadurch gelöst, daß ein solches Verfahren folgende Arbeitsschritte umfaßt:

    • Erschmelzen eines gemäß Anspruch 1 beschaffenen Stahls,
    • Vergießen des Stahls zu einem Vormaterial,
    • Warmwalzen des Vormaterials zu einem Warmband bei Temperaturen von insbesondere 1050 °C bis 1180 °C,
    • Glühen des Warmbands bei insbesondere über 1100 °C liegenden Temperaturen,
    • Beizen des Warmbands,
    • Kaltwalzen des Warmbands zu einem Kaltband,
    • Wärmebehandeln des Kaltbands, wobei die Temperatur bei der Wärmebehandlung des Kaltbands vorzugsweise mindestens 1100 °C beträgt,
    • Konfektionieren des Kaltbandes zu Blechzuschnitten und
    • Kaltumformen der Blechzuschnitte zu den Formteilen, wobei sich diese Kaltumformung insbesondere als Tiefziehen durchführen läßt, bei dem mehrachsige Verformungsbelastungen auf das Werkstück wirken.
    With regard to the method for producing stress-free molded parts, the above-mentioned object is achieved in that such a method comprises the following steps:
    • Melting a steel obtained according to claim 1,
    • Casting the steel into a raw material,
    • Hot rolling the primary material to a hot strip at temperatures of in particular 1050 ° C. to 1180 ° C.,
    • Annealing the hot strip at temperatures above 1100 ° C in particular,
    • Hot strip pickling,
    • Cold rolling the hot strip into a cold strip,
    • Heat treatment of the cold strip, the temperature during the heat treatment of the cold strip preferably being at least 1100 ° C.,
    • Assembling the cold strip into sheet metal blanks and
    • Cold forming of the sheet metal blanks into the shaped parts, this cold forming can be carried out in particular as deep drawing, in which multi-axis deformation loads act on the workpiece.

    Praktische Versuche haben ergeben, daß sich ausgehend vom erfindungsgemäß beschaffenen Stahl bei Beachtung der erfindungsgemäßen Vorgehensweise spannungsrißfreie Formteile kaltformen lassen. Diese halten während der bei hohen Umformgraden durchgeführten Umformung auch solchen Belastungszuständen ohne weiteres stand, die bei konventionellen Stählen regelmäßig zur Entstehung von Spannungsrissen führen.Practical tests have shown that starting from Steel procured according to the invention while observing the Procedure according to the invention free of stress cracks Allow molded parts to be cold formed. These last during the high degrees of deformation performed even such Stress conditions stood without further ado conventional steels regularly to the formation of Cause stress cracks.

    Nachfolgend wird die Erfindung anhand von Ausführungsbeispielen näher erläutert.The invention is described below with reference to Embodiments explained in more detail.

    Die Figur zeigt für ein Probenstück P eine Gefügeaufnahme zur Darstellung der räumlichen Verteilung der Phasen eines erfindungsgemäßen Stahls unter Angabe des Probenkoordinatensystems, in welchem die vertikal ausgerichtete Blechnormale BN senkrecht auf der durch die Querrichtung Qr und die Walzrichtung Wr aufgespannten Ebene steht.The figure shows a micrograph for a sample piece P. to represent the spatial distribution of the phases of a steel according to the invention, specifying the Sample coordinate system, in which the vertical aligned sheet metal normal BN perpendicular to that through the Cross direction Qr and the rolling direction Wr spanned Level stands.

    Es wurden Chargen von erfindungsgemäß zusammengesetzten Stählen E1 bis E12 erschmolzen, deren Zusammensetzungen in Tabelle 1 angegeben sind. Zum Vergleich wurden Vergleichsstähle V1, V2 erzeugt, deren Zusammensetzungen ebenfalls in Tabelle 1 eingetragen sind.Batches of compositions composed according to the invention were made Steels E1 to E12 melted, their compositions are given in Table 1. For comparison Comparative steels V1, V2 generated their compositions are also entered in Table 1.

    Die Zusammensetzung des Vergleichsstahls V1 ist unter der Werkstoffnummer 1.4301 und die des Vergleichsstahls V2 unter der Werkstoffnummer 1.4376 bekannt. Zusätzlich sind in der Tabelle 1 die Anteile des δ-Ferrit am Gefüge und der jeweils verfomungsinduzierte Martensitanteil verzeichnet. Ebenso ist in Tabelle 1 angegeben, ob es bei der Kaltverformung der Stähle E1 bis E12 und V1 sowie V2 zu einer Rißbildung kam.The composition of the comparative steel V1 is below that Material number 1.4301 and that of the comparative steel V2 known under the material number 1.4376. In addition are in Table 1 the proportions of the δ-ferrite in the structure and the respective strain-induced martensite portion recorded. Table 1 also shows whether it is at cold forming of steels E1 to E12 and V1 and V2 cracking occurred.

    Die Stähle E1 bis E12 sowie V1 und V2 wurden zu Brammen vergossen, warmgewalzt und geglüht. Die erhaltenen Warmbänder wurden anschließend auf eine Enddicke von 1,5 mm kaltgewalzt und einer abschließenden Wärmbehandlung bei 1100 °C unterzogen.Steels E1 to E12 as well as V1 and V2 became slabs cast, hot rolled and annealed. The received Hot strips were then cut to a final thickness of 1.5 mm cold rolled and a final one Subjected to heat treatment at 1100 ° C.

    Das in der Figur dargestellte, aus einem wärmebehandelten, aus der erfindungsgemäßen Schmelze E6 erzeugten Kaltband gewonnene Probenstück P zeigt die räumliche Verteilung der austenitischen und ferritischen Gefügeanteile. In dem Probenstück erscheinen die Austenitanteile A hell, während die Ferritanteile F dunkler dargestellt sind.The one shown in the figure heat-treated, from the melt E6 according to the invention generated cold strip obtained sample P shows the spatial distribution of austenitic and ferritic Structural components. The appear in the sample Austenite fractions A bright, while the ferrite fractions F are shown darker.

    Die erhaltenen Kaltbändern wurden zu Blechronden mit einem Durchmesser von 150 mm konvektioniert, aus denen anschließend mittels eines im Durchmesser 75 mm messenden Stempels Näpfchen tiefgezogen wurden. Es zeigte sich, daß sich aus den erfindungsgemäßen Stählen E1 bis E12 gewonnene Kaltbänder problemlos zu Näpfchen-Formteilen kaltumformen ließen, während es bei der Kaltumformung der aus den konventionellen Vergleichsstählen V1,V2 erzeugten Kaltbänder im großen Umfang zur Entstehung von Spannungsrissen kam.The cold strips obtained were used to form sheet blanks a diameter of 150 mm, from which then using a 75 mm diameter Stamp cups were deep drawn. It turned out that result from the steels E1 to E12 according to the invention Cold strips obtained can easily be molded into wells were cold formed while it was cold formed generated from the conventional comparative steels V1, V2 Cold strips on a large scale to create Tension cracks came.

    In Tabelle 2 sind beispielhaft für die erfindungsgemäßen Stähle E1, E2 und E3 sowie für die Vergleichsstähle V1 und V2 die aus den Legierungsgehalten berechneten Md30-Temperaturen angegeben. Es zeigt sich, daß Spannungsrisse dann sicher nicht auftreten, wenn die Md30-Temperatur des jeweiligen Stahls -55 °C nicht überschreitet.Table 2 shows the M d30 temperatures calculated from the alloy contents for the steels E1, E2 and E3 according to the invention and for the comparative steels V1 and V2. It shows that stress cracks certainly do not occur if the M d30 temperature of the respective steel does not exceed -55 ° C.

    Zum Nachweis ihrer Korrosionsbeständigkeit wurden an den erfindungsgemäßen Stählen E1 bis E12 gemäß den Vorschriften der EN ISO 3651-1 IK-Prüfungen im geschweißten Zustand durchgeführt. Dabei erwiesen sich alle untersuchten Stähle E1 bis E12 sowohl im Grundwerkstoff als auch in der Schweißnaht und der Wärmeeinflußzone als IK-beständig.To prove their corrosion resistance, the Steels E1 to E12 according to the invention according to Regulations of EN ISO 3651-1 IK tests in welded condition. It turned out to be all investigated steels E1 to E12 both in Base material as well as in the weld seam and the Heat affected zone as IK resistant.

    Lochfraßpotentialmessungen, welche potentiostatisch in 0,5 % - NaCl bei 30 °C durchgeführt wurden, ergaben eine chemische Beständigkeit der Stähle E1 bis E12, die mindestens auf dem Niveau des konventionellen Stahls V1 liegen.

    Figure 00110001
    Pitting potential measurements, which were carried out potentiostatically in 0.5% NaCl at 30 ° C, showed a chemical resistance of steels E1 to E12, which is at least at the level of conventional steel V1.
    Figure 00110001

    Claims (8)

    Nichtrostender Stahl, dessen Gefüge mindestens 15 Vol.-% δ-Ferrit und als Rest Austenit aufweist, mit folgender Zusammensetzung (in Gew.-%): C 0,02 - 0,08 %, N 0,1 - 0,5 %, Cr 16,0 - 20,0 %, Mn 6,0 - 12,0 %, Ni ≤ 9,05 %, Si ≤ 3,0 %, Mo ≤ 3,0 %, Al ≤ 2,0 %, Cu ≤ 3,0 %,
    Rest Fe und unvermeidbare Verunreinigungen,
    wobei für t = %Cr + 2%Mo + 1,5%Si + 3%Al - 5 0,3%Mn + %Ni + 0,5%Cu + 15(%C + %N) + 2 gilt 1,3 < t < 1,8 und für die Md30-Temperatur der austenitischen Phase gilt Md30 ≤ -55 °C mit Md30[°C]= 413 - 462(%C+%N) - 9,2%Si - 8,1%Mn - 13,7%Cr - 9,5(%Ni+%Cu) - 18,5%Mo wobei mit %C der C-Gehalt, %N der N-Gehalt, %Si der Si-Gehalt, %Al der Al-Gehalt %Mn der Mn-Gehalt, %Cr der Cr-Gehalt, %Ni der Ni-Gehalt, %Mo der Mo-Gehalt und %Cu der Cu-Gehalt der jeweiligen Stahlzusammensetzung bezeichnet sind.    Stainless steel, the structure of which has at least 15 vol.% Δ-ferrite and the rest austenite, with the following composition (in wt.%): C 0.02 - 0.08%, N 0.1 - 0.5%, Cr 16.0 - 20.0%, Mn 6.0 - 12.0%, Ni ≤ 9.05%, Si ≤ 3.0%, Mo ≤ 3.0%, al ≤ 2.0%, Cu ≤ 3.0%,
    Balance Fe and unavoidable impurities,
    being for t = % Cr + 2% Mo + 1.5% Si + 3% Al - 5 0.3% Mn +% Ni + 0.5% Cu + 15 (% C +% N) + 2 applies 1.3 <t <1.8 and for the M d30 temperature of the austenitic phase, M d30 ≤ -55 ° C also applies M d30 [° C] = 413 - 462 (% C +% N) - 9.2% Si - 8.1% Mn - 13.7% Cr - 9.5 (% Ni +% Cu) - 18.5% Mo where with% C the C content,% N the N content,% Si the Si content,% Al the Al content% Mn the Mn content,% Cr the Cr content,% Ni the Ni content, % Mo the Mo content and% Cu the Cu content of the respective steel composition.         Stahl nach Anspruch 1, dadurch gekennzeichnet, daß sein Gefüge höchstens 40 Vol.-% δ-Ferrit aufweist.Steel according to claim 1, characterized in that its structure has at most 40 vol.% Δ ferrite. Verfahren zum Herstellen von spannungsrißfreien Formteilen umfassend folgende Arbeitsschritte: Erschmelzen eines gemäß Anspruch 1 oder 2 beschaffenen Stahls, Vergießen des Stahls zu einem Vormaterial, Warmwalzen des Vormaterials zu einem Warmband, Glühen des Warmbands, Beizen des Warmbands, Kaltwalzen des Warmbands zu einem Kaltband, Wärmebehandeln des Kaltbands, Konfektionieren des Kaltbandes zu Blechzuschnitten, Kaltumformen der Blechzuschnitte zu den Formteilen. Process for producing stress-free molded parts comprising the following steps: Melting a steel procured according to claim 1 or 2, Casting the steel into a raw material, Hot rolling of the primary material into a hot strip, Hot strip annealing, Hot strip pickling, Cold rolling the hot strip into a cold strip, Heat treating the cold strip, Assembling the cold strip into sheet metal blanks, Cold forming of the sheet metal blanks into the molded parts. Verfahren nach Anspruch 3, dadurch gekennzeichnet, daß das Warmwalzen bei 1050 °C bis 1180 °C betragenden Temperaturen erfolgt.A method according to claim 3, characterized in that the hot rolling is carried out at temperatures between 1050 ° C and 1180 ° C. Verfahren nach Anspruch 3 oder 4, dadurch gekennzeichnet, daß das Glühen des Warmbands bei Temperaturen oberhalb 1100 °C erfolgt. A method according to claim 3 or 4, characterized in that the hot strip is annealed at temperatures above 1100 ° C. Verfahren nach einem der Ansprüche 3 bis 5,
    dadurch gekennzeichnet, daß die Temperatur bei der Wärmebehandlung des Kaltbands mindestens 1100 °C beträgt.    Method according to one of claims 3 to 5,
    characterized in that the temperature during the heat treatment of the cold strip is at least 1100 ° C.         Verfahren nach einem der Ansprüche 3 bis 6 oder 4,
    dadurch gekennzeichnet, daß die Kaltumformung als Tiefziehen durchgeführt wird.    Method according to one of claims 3 to 6 or 4,
    characterized in that the cold forming is carried out as deep drawing.         Durch Kaltumformung hergestelltes Formteil bestehend aus einem gemäß Anspruch 1 oder 2 beschaffenen Stahl.Molded part made by cold forming from a steel procured according to claim 1 or 2.
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