EP0966547A1 - Method for producing a highly resistant, very ductile steel strip - Google Patents

Method for producing a highly resistant, very ductile steel strip

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Publication number
EP0966547A1
EP0966547A1 EP98916911A EP98916911A EP0966547A1 EP 0966547 A1 EP0966547 A1 EP 0966547A1 EP 98916911 A EP98916911 A EP 98916911A EP 98916911 A EP98916911 A EP 98916911A EP 0966547 A1 EP0966547 A1 EP 0966547A1
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EP
European Patent Office
Prior art keywords
steel
strip
hot
hot strip
strength
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Granted
Application number
EP98916911A
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German (de)
French (fr)
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EP0966547B1 (en
Inventor
Bernhard Engl
Günter STICH
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
ThyssenKrupp Steel Europe AG
Original Assignee
ThyssenKrupp Stahl AG
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Publication of EP0966547A1 publication Critical patent/EP0966547A1/en
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Classifications

    • 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
    • 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
    • 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 method for producing a steel strip with a high strength of at least 900 MPa and good formability.
  • the characteristic values relevant for the forming can be obtained from the tensile test with great meaning for practice.
  • the elongation at break and the n-value are important measures.
  • the n-value is characteristic of the deformability under a stretching stress. This is the predominant deformation mechanism in most sheet metal parts of a vehicle.
  • the n-value is in relatively good agreement with the yield point ratio, which is also a measure of the strength of a material that can be used in practice.
  • the highest possible values for the elongation at break (A) and the strengthening value (n value) are aimed for.
  • the object of the invention is to develop strip steels which have a high strength, paired with good formability and high component strength.
  • the cooling speed on the outfeed roller table is at least 30 ° C / s and the reel temperature is 300 to 600 ° C.
  • the particular economic importance of the method according to the invention consists in the possibility of production as a hot strip in thicknesses below 2.0 mm, e.g. 1.5 mm.
  • the manufacturing process thus does not necessarily require the complex manufacturing process of cold strip production with the additional steps of cold rolling and final annealing.
  • This material concept also includes the option of surface finishing applied at the factory. For example, an electrolytically deposited zinc layer can be applied. The enormous improvement in corrosion protection through a zinc coating can be assumed as a known fact. It is also known that high-strength steels tend to become brittle due to hydrogen absorption during the electrolytic galvanizing process. It could be shown that the steel strip according to the invention remains free from these dreaded galvanizing problems. The meaning of the alloying elements and the manufacturing parameters are described below.
  • Carbon is required for structural hardening and for the formation of fine precipitates.
  • the content should be limited to 0.1 to 0.2%.
  • Silicon increases the hardness of the mixed crystal, which requires at least 0.3%. For reasons of weldability and to avoid unfavorable scale formation, the content should be limited to 0.6%.
  • Manganese at a content of at least 1.5% delays the conversion and causes the formation of hard conversion products. In order to avoid impermissibly strong micro segregations, the content should be max. Limit 2.0%.
  • Phosphorus can be used to further increase solid-solution strengthening, but should not exceed 0.08% for reasons of weldability.
  • Chromium promotes the formation of a bainite-rich final structure in at least 0.3%. In order not to delay the conversion too much, its content should be limited to max. Be limited to 0.80%.
  • Titanium or zirconium can be used to form fine precipitates with a hardening effect.
  • the effect clearly drops at levels above 0.2%.
  • the maximum value is therefore set at 0.2%.
  • Niobium can also be used for precipitation hardening. At least 0.04% should preferably be added. For reasons of effectiveness, the content is limited to max. 0.08% set.
  • Boron improves hardenability at levels in the range of 0.0005 to 0.005%. According to the current state of knowledge, it is used for this purpose in martensitic steel. It has surprisingly been found that boron in the present case also causes a significant increase in strength in the bainitic basic structure with only a slight reduction in formability.
  • the final roll temperature should be in the range of homogeneous austenite and therefore not below 800 ° C, on the one hand to ensure sufficiently low resistance to deformation and on the other hand to keep deformation-induced precipitations low.
  • the cooling conditions should be selected so that a conversion to pearlite is avoided and the conversion takes place largely in the bainite stage. Shares of martensite can contribute to further solidification. In addition, solidification should be achieved by separating the finest particles. This requires a cooling of the final roll temperature with a cooling rate of at least 30 ° C / s. This cooling process must be ended at a temperature below 600 ° C by winding the strip on a reel and then cooling it in the coil.
  • Table 1 shows the chemical compositions of the strip steels 1 and 2 and steel 3, a comparative martensitic steel, produced according to the invention.
  • Table 2 shows the characteristic mechanical properties of the strip steels 1 and 2 produced according to the invention and of the comparative steel 3, which was tempered to the values given in Table 2 by a subsequent heat treatment.
  • Table 3 shows the influence of low reel temperature and a subsequent heat treatment on the properties of a strip steel produced according to the invention of the composition of steel 1 in Table 1.
  • Low reel temperatures of preferably 330 ° C can significantly increase the strength properties, see Example 4 in Table 3.
  • Another object of the invention is to achieve the advantageous effect of a subsequent heat treatment. It has surprisingly been found that the forming properties can be further increased by the thermal treatment of the steel strip produced according to the invention in the temperature range between 500 and 850 ° C. Examples 4, 5 and 6 in Table 3 show the effect of such a heat treatment on the steel 1 with the composition according to Table 1. This achieves a material state which offers advantages for components which overall still have high strengths, especially yield strengths with good ones Formability, must have.
  • This property profile is suitable for the production of cold-formed parts with a high energy absorption capacity (example 5a). By selecting higher annealing temperatures, high strengths can be achieved with extraordinarily low yield strength ratios or, in the same way, high hardening with good elongation values (Examples 5b, 6a to 6c).
  • Example 7 in Table 3 shows that the strip steel 1 produced according to the invention after cold rolling with a 50% degree of deformation and subsequent annealing likewise achieves high strengths with an even further improved yield strength ratio compared to the only hot-rolled strip steels 1 and 2.

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

Abstract

The invention relates to a method for producing a highly resistant (at least 900MPa), very ductile steel strip. The steel, containing (in mass per cent); 0.10 to 0.20 % C; 0.30 to 0.60 % Si; 1.50 to 2.00 % Mn; max. 0.08 % P; 0.30 to 0.80 % Cr; up to 0.40 % Mo; up to 0.20 % Ti and/or Zr; up to 0.08 % Nb; the remainder being Fe and unavoidable impurities, is melted, cast in slabs and then rolled out into a hot rolled strip. The roll end temperature is above 800oC, the cooling speed on the delivery roller table is at least 30oC/s and the reel temperature is 300 to 600oC.

Description

Verfahren zur Herstellung eines Bandstahles mit hoher Festigkeit und guter UmformbarkeitProcess for the production of a steel strip with high strength and good formability
Die Erfindung betrifft ein Verfahren zur Herstellung eines Bandstahles mit hoher Festigkeit von mind. 900 MPa und guter Umformbarkeit.The invention relates to a method for producing a steel strip with a high strength of at least 900 MPa and good formability.
Die Forderung nach Reduzierung des Treibstoffverbrauchs von Fahrzeugen macht die Anwendung von Leichtbaukonzepten erforderlich. Leichte Konstruktionen können durch eine Verringerung der Blechdicken erreicht werden. Zum Ausgleich des dadurch bedingten Verlustes an Festigkeit des Bauteils muß die Festigkeit des Werkstoffs erhöht werden. Eine Steigerung der Festigkeit bewirkt normalerweise eine Verminderung der Verformbarkeit. Im Fahrzeugbau eingesetzte Bleche müssen durch eine Umformung in die aus Design- und Funktionsgründen erforderliche Endform gebracht werden. Wenn die Steigerung der Festigkeit und die damit verbundene verschlechterte Umformbarkeit zu groß werden, kommt es zum Versagen beim Umformen durch lokale Einschnürung und Reißen. Aus diesem Grund ist eine Steigerung der Festigkeit begrenzt.The demand for reducing the fuel consumption of vehicles makes it necessary to use lightweight construction concepts. Light constructions can be achieved by reducing the sheet thickness. To compensate for the loss of strength of the component caused by this, the strength of the material must be increased. An increase in strength normally results in a reduction in deformability. Sheets used in vehicle construction must be formed into the final shape required for design and functional reasons. If the increase in strength and the associated deteriorated formability become too great, the forming will fail due to local constriction and tearing. For this reason, an increase in strength is limited.
Die Entwicklung von Stählen zielte stets auf eine Verbesserung des Verformbarkeit/Festigkeit-Verhältnisses ab.The development of steels has always aimed to improve the formability / strength ratio.
Im Festigkeitsbereich unter 500 MPa konnten bereits beachtliche Erfolge hinsichtlich einer Reduzierung der Blechdicke durch Einsatz von phosphorlegierten oder mikrolegierten Stählen erzielt werden. Noch bessere Ergebnisse wurden mit Bake-hardening-Stählen erzielt. Im Festigkeitsbereich zwischen 500 und 800 MPa lieferten die Entwicklungen der Dualphasen- und der TRIP- (Transfor- mation-i_nduced plasticity) Stähle recht gute Umformbarkeitswerte .In the strength range below 500 MPa, considerable success has already been achieved in reducing the sheet thickness by using phosphorus-alloyed or micro-alloyed steels. Even better Results were achieved with bake hardening steels. In the strength range between 500 and 800 MPa, the developments of dual-phase and TRIP (Transformation-i_nduced plasticity) steels gave quite good formability values.
Die für die Umformung relevaten Kennwerte können mit hoher Aussagekraft für die Praxis aus dem Zugversuch gewonnen werden. Besonders die Bruchdehnung und der n-Wert (Maß für das Verfestigungsvermögen) stellen wichtige Maßzahlen dar. Der n-Wert ist kennzeichnend für die Verformbarkeit unter einer Streckziehbeanspruchung. Diese ist bei den meisten Blechteilen eines Fahrzeugs der vorherrschende Verformungsmechanismus. Der n-Wert steht in verhältnismäßig guter Übereinstimmung mit dem Streckgrenzenverhältnis, das ebenfalls ein für die Praxis brauchbares Maß für das Verfestigungsvermögen eines Werkstoffs darstellt.The characteristic values relevant for the forming can be obtained from the tensile test with great meaning for practice. In particular, the elongation at break and the n-value (measure of the strengthening strength) are important measures. The n-value is characteristic of the deformability under a stretching stress. This is the predominant deformation mechanism in most sheet metal parts of a vehicle. The n-value is in relatively good agreement with the yield point ratio, which is also a measure of the strength of a material that can be used in practice.
Um den Vorteil einer Erhöhung der Festigkeit zur Reduzierung der Blechdicke möglichst weitgehend ausnutzen zu können, werden möglichst hohe Werte der Bruchdehnung (A) und des Verfestigungswertes (n-Wert) angestrebt.In order to be able to take advantage of an increase in strength to reduce the sheet thickness as much as possible, the highest possible values for the elongation at break (A) and the strengthening value (n value) are aimed for.
Stähle mit sehr hohen Festigkeiten über 800 MPa können sehr effizient zur Gewichtsoptimierung von crashrelevanten Teilen, wie Türaufprallträger, Stoßfängerträger, eingesetzt werden. Dazu muß die Blechdicke jedoch von z.B. über 2,0 mm auf Dicken unter 2,0 mm, etwa auf 1,5 mm, abgesenkt werden. Solch höchstfeste Stahlerzeugnisse konnten in der Vergangenheit nur als kaltgewalzte Bleche zur Verfügung gestellt werden. Vor allem im Bereich höchster Festigkeiten über 800 MPa reichen beim Einsatz herkömmlicher Werkstoffkonzepte zur Herstellung von Kaltband oder Warmband die Verformungseigenschaften nicht aus, um Bleche zu brauchbaren Teilen umzuformen. Die hohe Festigkeit wird dabei durch die Einstellung von martensitischen Gefügen erzielt. Die Streckgrenzen ist aber bei solchen Stählen ebenfalls sehr hoch. Die daraus resultierenden Werte für das Streckgrenzenverhältnis bzw. die Verfestigung sind entsprechend niedrig. Dies führt neben der geringen Umformbarkeit außerdem zu hohen Rückfederungswerten, so daß Preßteile nur schwierig oder gar nicht formgerecht herstellbar sind.Steels with very high strengths of over 800 MPa can be used very efficiently to optimize the weight of crash-relevant parts such as door impact beams and bumper beams. To do this, however, the sheet thickness must be reduced from, for example, over 2.0 mm to thicknesses below 2.0 mm, for example to 1.5 mm. In the past, such high-strength steel products could only be made available as cold-rolled sheets. Especially in the area of the highest strengths above 800 MPa, when using conventional material concepts for the production of cold and hot strip, the deformation properties are not sufficient to convert sheet metal into usable parts. The high strength is achieved by setting martensitic structures. The yield strength is also very high with such steels. The resulting values for the yield point ratio or the hardening are correspondingly low. In addition to the low formability, this also leads to high springback values, so that pressed parts can be produced only with difficulty or not at all in the correct shape.
Aufgabe der Erfindung ist nun Bandstähle zu entwickeln, die ein hohes Verfestigungsvermögen, gepaart mit guter Umformbarkeit und hoher Bauteilfestigkeit aufweisen.The object of the invention is to develop strip steels which have a high strength, paired with good formability and high component strength.
Zur Lösung dieser Aufgabe wird erfindungsgemäß ein Verfahren vorgeschlagen, bei dem ein Stahl, bestehend aus (in Masse-%)To achieve this object, a method is proposed according to the invention in which a steel consisting of (in mass%)
0, 10 bis 0,20 % C0.10 to 0.20% C
0,30 bis 0, 60 % Si0.30 to 0.60% Si
1,50 bis 2,00 % Mn max. 0,08 % P1.50 to 2.00% Mn max. 0.08% P
0,30 bis 0,80 % Cr bis 0,40 % Mo bis 0,20 % Ti und/oder Zr bis 0,08 % Nb0.30 to 0.80% Cr to 0.40% Mo to 0.20% Ti and / or Zr to 0.08% Nb
Rest Fe und unvermeidbare VerunreinigungenBalance Fe and unavoidable impurities
erschmolzen, zu Brammen abgegossen wird und anschließend zu Warmband ausgewalzt wird, wobei die Walzendtemperatur oberhalb 800 °C, die Abkühlgeschwindigkeit auf dem Auslaufrollgang mindestens 30 °C/s und die Haspeltemperatur 300 bis 600 °C betragen.is melted, poured into slabs and then rolled into hot strip, the final roll temperature above 800 ° C, the cooling speed on the outfeed roller table is at least 30 ° C / s and the reel temperature is 300 to 600 ° C.
Die gezielte Einstellung sehr feiner MikroStrukturen, bestehend aus weichen und harten Phasen nebeneinander, kombiniert mit einer Verteilung feinster Ausscheidungen, eröffnete die Möglichkeit attraktiver, bisher nicht bekannter Verarbeitungs- und Gebrauchseigenschaften. Eine Gefügehärtung durch Mehrphasigkeit in Verbindung mit Härtung durch Feinkorn und feine Teilchen verursachen dabei einen multiplen Verfestigungsvorgang.The targeted setting of very fine microstructures, consisting of soft and hard phases next to each other, combined with a distribution of the finest excretions, opened up the possibility of attractive, previously unknown processing and usage properties. Structural hardening by multiphase in combination with hardening by fine grain and fine particles cause a multiple hardening process.
Die besondere wirtschaftliche Bedeutung des erfindungsgemäßen Verfahrens besteht in der Herstellungsmöglichkeit als Warmband in Dicken unter 2,0 mm, z.B. 1,5 mm. Das Herstellungsverfahren erfordert somit nicht unbedingt den aufwendigen Fertigungsprozeß einer Kaltbanderzeugung mit den zusätzlichen Schritten einer Kaltwalzung und abschließenden Glühung.The particular economic importance of the method according to the invention consists in the possibility of production as a hot strip in thicknesses below 2.0 mm, e.g. 1.5 mm. The manufacturing process thus does not necessarily require the complex manufacturing process of cold strip production with the additional steps of cold rolling and final annealing.
Das vorliegende Werkstoffkonzept beinhaltet auch die Möglichkeit werkseitig aufgebrachter Oberflächenveredelung. So kann beispielsweise eine elektrolytisch abgeschiedene Zinkschicht aufgebracht werden. Die enorme Verbesserung des Korrosionsschutzes durch eine Zinkschicht kann als bekannte Tatsache vorausgesetzt werden. Weiterhin ist bekannt, daß höchstfeste Stähle zur Versprödung durch eine Wasserstoffaufnähme beim elektrolytischen Verzinkungsvorgang neigen. Es konnte gezeigt werden, daß der erfindungsgemäße Bandstahl frei von diesen gefürchteten Verzinkungsproblemen bleibt. Im folgenden werden die Bedeutung der Legierungselemente und der Fertigungsparameter beschrieben.This material concept also includes the option of surface finishing applied at the factory. For example, an electrolytically deposited zinc layer can be applied. The enormous improvement in corrosion protection through a zinc coating can be assumed as a known fact. It is also known that high-strength steels tend to become brittle due to hydrogen absorption during the electrolytic galvanizing process. It could be shown that the steel strip according to the invention remains free from these dreaded galvanizing problems. The meaning of the alloying elements and the manufacturing parameters are described below.
Kohlenstoff wird zur Gefügehärtung und zur Bildung von Feinstausscheidungen benötigt. Aus Gründen der Schweißbarkeit sollte der Gehalt auf 0,1 bis ,0,2 % begrenzt werden.Carbon is required for structural hardening and for the formation of fine precipitates. For reasons of weldability, the content should be limited to 0.1 to 0.2%.
Silizium erhöht die Härte des Mischkristalls, wozu mindestens 0,3 % erforderlich sind. Aus Gründen der Schweißbarkeit und zur Vermeidung ungünstiger Zunderausbildung ist der Gehalt auf 0,6 % zu begrenzen.Silicon increases the hardness of the mixed crystal, which requires at least 0.3%. For reasons of weldability and to avoid unfavorable scale formation, the content should be limited to 0.6%.
Mangan bei einem Gehalt von mindestens 1,5 % verzögert die Umwandlung und bewirkt die Bildung harter Umwandlungsprodukte. Zur Vermeidung unzulässig starker Mikroseigerungen ist der Gehalt auf max. 2,0 % zu begrenzen .Manganese at a content of at least 1.5% delays the conversion and causes the formation of hard conversion products. In order to avoid impermissibly strong micro segregations, the content should be max. Limit 2.0%.
Phosphor kann zur weiteren Steigerung der Mischkristallverfestigung eingesetzt werden, sollte aber aus Gründen der Schweißbarkeit einen Gehalt von 0,08 % nicht übersteigen.Phosphorus can be used to further increase solid-solution strengthening, but should not exceed 0.08% for reasons of weldability.
Chrom fördert bei mindestens 0,3 % die Bildung eines bainitreichen Endgefüges. Um die Umwandlung nicht zu stark zu verzögern, sollte sein Gehalt auf max. 0,80 % begrenzt werden.Chromium promotes the formation of a bainite-rich final structure in at least 0.3%. In order not to delay the conversion too much, its content should be limited to max. Be limited to 0.80%.
Titan oder Zirkonium lassen sich zur Bildung von Feinstausscheidungen mit aushärtender Wirkung benutzen. Die Wirkung läßt bei Gehalten über 0,2 % deutlich nach. Deshalb ist der Maximalwert auf 0,2 % festgesetzt. Niob läßt sich ebenfalls zur Ausscheidungshärtung einsetzen. Es sollten bevorzugt mindestens 0,04 % zulegiert werden. Aus Gründen der Wirksamkeit ist der Gehalt auf max. 0,08 % festgelegt.Titanium or zirconium can be used to form fine precipitates with a hardening effect. The effect clearly drops at levels above 0.2%. The maximum value is therefore set at 0.2%. Niobium can also be used for precipitation hardening. At least 0.04% should preferably be added. For reasons of effectiveness, the content is limited to max. 0.08% set.
Bor verbessert die Härtbarkeit bei Gehalten im Bereich von 0,0005 bis 0,005 %. Dazu wird es nach dem Kenntnisstand bei martensitisch umwandelnden Stählen eingesetzt. Es hat sich überraschenderweise herausgestellt, daß Bor auch im vorliegenden Fall im bainitischen Grundgefüge eine signifikante Steigerung der Festigkeit bei nur geringer Erniedrigung der Umformbarkeit hervorruft .Boron improves hardenability at levels in the range of 0.0005 to 0.005%. According to the current state of knowledge, it is used for this purpose in martensitic steel. It has surprisingly been found that boron in the present case also causes a significant increase in strength in the bainitic basic structure with only a slight reduction in formability.
Die Walzenendtemperatur sollte im Bereich des homogenen Austenits und damit nicht unter 800 °C liegen, um zum einen ausreichend niedrige Formänderungswiderstände zu gewährleisten und zum anderen verformungsinduzierte Ausscheidungen gering zu halten.The final roll temperature should be in the range of homogeneous austenite and therefore not below 800 ° C, on the one hand to ensure sufficiently low resistance to deformation and on the other hand to keep deformation-induced precipitations low.
Die Abkühlbedingungen sind so zu wählen, daß eine Umwandlung zu Perlit vermieden wird und die Umwandlung weitestgehend in der Bainitstufe erfolgt. Anteile von Martensit können zu weiterer Verfestigung beitragen. Des weiteren soll eine Verfestigung durch Ausscheidung von feinsten Teilchen erzielt werden. Dazu ist eine Abkühlung von Walzendtemperatur mit einer Abkühlgeschwindigkeit von mindestens 30 °C/s erforderlich. Dieser Abkühlvorgang ist bei einer Temperatur unter 600 °C zu beenden, indem das Band auf einen Haspel aufgewickelt wird und danach im Coil abkühlt.The cooling conditions should be selected so that a conversion to pearlite is avoided and the conversion takes place largely in the bainite stage. Shares of martensite can contribute to further solidification. In addition, solidification should be achieved by separating the finest particles. This requires a cooling of the final roll temperature with a cooling rate of at least 30 ° C / s. This cooling process must be ended at a temperature below 600 ° C by winding the strip on a reel and then cooling it in the coil.
Die Erfindung wird anhand der nachfolgenden Beispiele beschrieben. In Tabelle 1 sind die chemischen Zusammensetzungen der erfindungsgemäß hergestellten Bandstähle 1 und 2 und Stahl 3, einem martensitischen Vergleichsstahl, mitgeteilt .The invention is described on the basis of the following examples. Table 1 shows the chemical compositions of the strip steels 1 and 2 and steel 3, a comparative martensitic steel, produced according to the invention.
In der Tabelle 2 sind die kennzeichnenden mechanischen Eigenschaften der erfindungsgemäß erzeugten Bandstähle 1 und 2 und des Vergleichsstahls 3, der durch eine nachgeschaltete Wärmebehandlung auf die in Tabelle 2 angegebenen Werte angelassen wurde, aufgeführt.Table 2 shows the characteristic mechanical properties of the strip steels 1 and 2 produced according to the invention and of the comparative steel 3, which was tempered to the values given in Table 2 by a subsequent heat treatment.
Ein Eigenschaftsvergleich weist die großen Vorteile des erfindungsgemäß erzeugten Bandstahls klar auf. Er weist eine höhere Bruchdehnung und ein besseres Streckgrenzenverhältnis als Maß für die Verfestigung auf.A comparison of properties clearly shows the great advantages of the steel strip produced according to the invention. It has a higher elongation at break and a better yield strength ratio as a measure of the strengthening.
Tabelle 3 zeigt den Einfluß niedriger Haspeltemperatur und einer nachfolgenden Wärmebehandlung auf die Eigenschaften eines erfindungsgemäß erzeugten Bandstahls der Zusammensetzung des Stahls 1 in Tabelle 1 auf. Durch niedrige Haspeltemperaturen von vorzugsweise 330 °C können deutlich Steigerungen der Festigkeitseigenschaften erreicht werden, siehe Beispiel 4 in Tabelle 3.Table 3 shows the influence of low reel temperature and a subsequent heat treatment on the properties of a strip steel produced according to the invention of the composition of steel 1 in Table 1. Low reel temperatures of preferably 330 ° C can significantly increase the strength properties, see Example 4 in Table 3.
Ein weiterer Gegenstand der Erfindung besteht in der Erzielung der vorteilhaften Wirkung einer nachfolgenden Wärmebehandlung. Es hat sich überraschenderweise herausgestellt, daß durch die thermische Behandlung des erfindungsgemäß erzeugten Bandstahls im Temperaturbereich zwischen 500 und 850 °C die Umformeigenschaften noch weiter gesteigert werden können. Die Beispiele 4, 5 und 6 in Tabelle 3 zeigen die Wirkung einer solchen Wärmebehandlung an dem Stahl 1 mit der Zusammensetzung gemäß Tabelle 1. Dadurch wird ein Werkstoffzustand erreicht, der Vorteile für Bauteile bietet, die insgesamt noch hohe Festigkeiten, vor allem Streckgrenzen bei guter Umformbarkeit, aufweisen müssen. Dieses Eigenschaftsbild bietet sich für die Herstellung von kaltprofilierten Teilen mit einem hohen Energieaufnahmevermögen an (Beispiel 5a) . Durch Wahl höherer Glühtemperaturen können hohe Festigkeiten bei außerordentlich niedrigen Streckgrenzenverhältnissen bzw. gleichbedeutend hoher Verfestigung bei guten Dehnungswerten erreicht werden (Beispiele 5b, 6a bis 6c) .Another object of the invention is to achieve the advantageous effect of a subsequent heat treatment. It has surprisingly been found that the forming properties can be further increased by the thermal treatment of the steel strip produced according to the invention in the temperature range between 500 and 850 ° C. Examples 4, 5 and 6 in Table 3 show the effect of such a heat treatment on the steel 1 with the composition according to Table 1. This achieves a material state which offers advantages for components which overall still have high strengths, especially yield strengths with good ones Formability, must have. This property profile is suitable for the production of cold-formed parts with a high energy absorption capacity (example 5a). By selecting higher annealing temperatures, high strengths can be achieved with extraordinarily low yield strength ratios or, in the same way, high hardening with good elongation values (Examples 5b, 6a to 6c).
Viele warmgewalzte Erzeugnisse zeigen den Nachteil, daß sie ihre vorteilhaften Eigenschaften verlieren, wenn sie anschließend kaltgewalzt und rekristallisierend geglüht werden. Für den erfindungsgemäß erzeugten Bandstahl wurde jedoch gefunden, daß er auch nach anschließendem Kaltwalzen und Glühen ebenfalls vorteilhafte Eigenschaften aufweist. So zeigt Beispiel 7 in Tabelle 3, daß der erfindungsgemäß erzeugte Bandstahl 1 nach einer Kaltwalzung mit 50 % Verformungsgrad und anschließender Glühung ebenfalls hohe Festigkeiten bei noch weiter verbessertem Streckgrenzenverhältnis gegenüber den nur warmgewalzten Bandstählen 1 und 2 erreicht. Many hot-rolled products have the disadvantage that they lose their advantageous properties if they are subsequently cold-rolled and recrystallized annealed. However, it was found for the strip steel produced according to the invention that it also has advantageous properties even after subsequent cold rolling and annealing. Example 7 in Table 3 shows that the strip steel 1 produced according to the invention after cold rolling with a 50% degree of deformation and subsequent annealing likewise achieves high strengths with an even further improved yield strength ratio compared to the only hot-rolled strip steels 1 and 2.
Tabelle 1 (Masse-%)Table 1 (mass%)
martensitischer Vergleichsstahlmartensitic comparison steel
Tabelle 2Table 2
VergleichsstahlReference steel
Re - StreckgrenzeR e - yield strength
Rm - ZugfestigkeitR m - tensile strength
Ag - GleichmaßdehnungA g - uniform expansion
A5 - BruchdehnungA 5 - Elongation at break
A8o _ BruchdehnungA 8 o _ elongation at break
WET - WalzendtemperaturWET - final roll temperature
HT - Haspeltemperatur Tabelle 3HT - reel temperature Table 3
kaltgewalzt mit 50 % cold rolled with 50%

Claims

Patentansprüche claims
1. Verfahren zur Herstellung von Bandstahl mit hoher Festigkeit von mind. 900 MPa und guter Umformbarkeit, bestehend aus (in Masse-.)1. Process for the production of steel strip with a high strength of at least 900 MPa and good formability, consisting of (in mass.)
0, 10 bis 0,20 % C0.10 to 0.20% C
0,30 bis 0, 60 % Si0.30 to 0.60% Si
1,50 bis 2,00 % Mn max. 0,08 % P1.50 to 2.00% Mn max. 0.08% P
0,30 bis 0,80 % Cr bis 0,40 % Mo bis 0,20 % Ti und/oder Zr bis 0,08 % Nb0.30 to 0.80% Cr to 0.40% Mo to 0.20% Ti and / or Zr to 0.08% Nb
Rest Fe und unvermeidbare Verunreinigungen,Balance Fe and unavoidable impurities,
der erschmolzen, zu Brammen abgegossen wird und anschließend zu Warmband ausgewalzt wird, wobei die Walzendtemperatur oberhalb 800 °C, diewhich is melted, poured into slabs and then rolled into hot strip, with the final roll temperature above 800 ° C, the
Abkühlgeschwindigkeit auf dem Auslaufrollgang mindestens 30 °C/s und die Haspeltemperatur 300 bis 600 °C betragen.The cooling speed on the outfeed roller table is at least 30 ° C / s and the reel temperature is 300 to 600 ° C.
2. Verfahren nach Anspruch 1, d a d u r c h g e k e n n z e i c h n e t , d a ß das Warmband bei einer Temperatur von maximal 550 °C gehaspelt wird. 2. The method according to claim 1, characterized in that ß the hot strip is coiled at a maximum temperature of 550 ° C.
3. Verfahren nach Anspruch 1, d a d u r c h g e k e n n z e i c h n e t , d a ß das Warmband bei maximal 350 °C gehaspelt wird.3. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t, that the hot strip is coiled at a maximum of 350 ° C.
4. Verfahren nach einem der Ansprüche 1 bis 3, d a d u r c h g e k e n n z e i c h n e t , d a ß das Warmband nicht unter 330 °C gehaspelt wird.4. The method according to any one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t, that the hot strip is not coiled below 330 ° C.
5. Verfahren nach einem oder mehreren der Ansprüche 1 bis 4, d a d u r c h g e k e n n z e i c h n e t , daß das Warmband auf eine Enddicke von max. 2,0 mm gewalzt wird.5. The method according to one or more of claims 1 to 4, d a d u r c h g e k e n n z e i c h n e t that the hot strip to a final thickness of max. 2.0 mm is rolled.
6. Verfahren nach einem oder mehreren der Ansprüche 1 bis 5, d a d u r c h g e k e n n z e i c h n e t , daß das Warmband dressiert wird.6. The method according to one or more of claims 1 to 5, d a d u r c h g e k e n n z e i c h n e t that the hot strip is trained.
7. Verfahren nach einem oder mehreren der Ansprüche 1 bis 6, d a d u r c h g e k e n n z e i c h n e t , daß das Band gebeizt und metallisch beschichtet wird.7. The method according to one or more of claims 1 to 6, d a d u r c h g e k e n n z e i c h n e t that the strip is pickled and coated with metal.
8. Verfahren nach Anspruch 7, d a d u r c h g e k e n n z e i c h n e t , d a ß die metallische Beschichtung elektrolytisch aufgebracht wird.8. The method of claim 7, d a d u r c h g e k e n n z e i c h n e t, that the metallic coating is applied electrolytically.
9. Verfahren nach Anspruch 7, d a d u r c h g e k e n n z e i c h n e t , d a ß die metallische Beschichtung im Schmelztauchverfahren aufgebracht wird.9. The method of claim 7, d a d u r c h g e k e n n z e i c h n e t, that the metallic coating is applied by the hot-dip method.
10. Verfahren nach einem oder mehreren der Ansprüche 1 bis 6, d a d u r c h g e k e n n z e i c h n e t , daß das Warmband im Bereich von 500 bis 850 °C geglüht wird . 10. The method according to one or more of claims 1 to 6, characterized in that the hot strip is annealed in the range of 500 to 850 ° C.
11. Verfahren nach einem oder mehreren der Ansprüche 1 bis 6, d a d u r c h g e k e n n z e i c h n e t , daß nach dem Warmwalzen eine Kaltwalzung von mind. 30 % und eine Durchlaufglühung bei Temperaturen zwischen 700 und 900 °C durchgeführt wird.11. The method according to one or more of claims 1 to 6, d a d u r c h g e k e n n z e i c h n e t that after hot rolling a cold rolling of at least 30% and a continuous annealing is carried out at temperatures between 700 and 900 ° C.
12. Verfahren nach einem der Ansprüche 1 bis 11, d a d u r c h g e k e n n z e i c h n e t , d a ß dem Stahl max. 0,15 % Mo zulegiert wird.12. The method according to any one of claims 1 to 11, d a d u r c h g e k e n n z e i c h n e t, d a ß the steel max. 0.15% Mo is added.
13. Verfahren nach einem oder mehreren der Ansprüche 1 bis 12, d a d u r c h g e k e n n z e i c h n e t , daß dem Stahl mindestens 0,04 % Ti und/oder Zr zulegiert wird.13. The method according to one or more of claims 1 to 12, d a d u r c h g e k e n n z e i c h n e t that at least 0.04% Ti and / or Zr is alloyed to the steel.
14. Verfahren nach einem oder mehreren der Ansprüche 1 bis 13, d a d u r c h g e k e n n z e i c h n e t , daß dem Stahl 0,0005 bis 0,005 % B zulegiert wird.14. The method according to one or more of claims 1 to 13, d a d u r c h g e k e n n z e i c h n e t that 0.0005 to 0.005% B is alloyed to the steel.
15. Verfahren nach einem oder mehreren der Ansprüche 1 bis 14, d a d u r c h g e k e n n z e i c h n e t , daß dem Stahl mindestens 0,04 % Nb zulegiert wird. 15. The method according to one or more of claims 1 to 14, d a d u r c h g e k e n n z e i c h n e t that at least 0.04% Nb is alloyed to the steel.
EP98916911A 1997-03-13 1998-03-10 Method for producing a highly resistant, very ductile steel strip Expired - Lifetime EP0966547B1 (en)

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DE19710125A DE19710125A1 (en) 1997-03-13 1997-03-13 Process for the production of a steel strip with high strength and good formability
DE19710125 1997-03-13
PCT/EP1998/001376 WO1998040522A1 (en) 1997-03-13 1998-03-10 Method for producing a highly resistant, very ductile steel strip

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DE19710125A1 (en) 1998-09-17
ATE206472T1 (en) 2001-10-15
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CN1252105A (en) 2000-05-03
AR010130A1 (en) 2000-05-17
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