EP1918403B1 - Process for manufacturing steel flat products from a steel forming martensitic structure - Google Patents

Process for manufacturing steel flat products from a steel forming martensitic structure Download PDF

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Publication number
EP1918403B1
EP1918403B1 EP06123137A EP06123137A EP1918403B1 EP 1918403 B1 EP1918403 B1 EP 1918403B1 EP 06123137 A EP06123137 A EP 06123137A EP 06123137 A EP06123137 A EP 06123137A EP 1918403 B1 EP1918403 B1 EP 1918403B1
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EP
European Patent Office
Prior art keywords
hot
strip
rolled strip
cold
rolled
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Not-in-force
Application number
EP06123137A
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German (de)
French (fr)
Other versions
EP1918403A1 (en
Inventor
Brigitte Dr.-Ing. Hammer
Dr.-Ing. Heller Thomas
Johann Wilhelm Dr. Schmitz
Jochen Dr. Wans
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
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ThyssenKrupp Steel AG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to PL06123137T priority Critical patent/PL1918403T3/en
Application filed by ThyssenKrupp Steel AG filed Critical ThyssenKrupp Steel AG
Priority to DE502006003831T priority patent/DE502006003831D1/en
Priority to EP06123137A priority patent/EP1918403B1/en
Priority to AT06123137T priority patent/ATE432373T1/en
Priority to ES06123137T priority patent/ES2325961T3/en
Priority to US12/447,623 priority patent/US20100096047A1/en
Priority to CN2007800394302A priority patent/CN101528967B/en
Priority to PCT/EP2007/061389 priority patent/WO2008052918A1/en
Priority to KR1020097007484A priority patent/KR101458577B1/en
Priority to JP2009533821A priority patent/JP5350252B2/en
Publication of EP1918403A1 publication Critical patent/EP1918403A1/en
Application granted granted Critical
Publication of EP1918403B1 publication Critical patent/EP1918403B1/en
Not-in-force legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/041—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing involving a particular fabrication or treatment of ingot or slab
    • C21D8/0415—Rapid solidification; Thin strip casting
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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 flat steel products, such as strips or sheet metal blanks, from high-strength martensitic steels.
  • Such MS steels belong to the group of multiphase steels. These are usually steels whose properties are determined by the type, amount and arrangement of the phases of the structure. The structure therefore has at least two phases (eg ferrite, martensite, bainite). This gives them a strength / formability combination that is superior to conventional steels.
  • multiphase steels are of particular interest for the automotive industry because of their high strength, they allow the use of lower material thicknesses and concomitantly a reduction in vehicle weight and, secondly, the safety of the vehicle body in the event of a collision (crash behavior) .
  • multiphase steels with at least constant strength of the overall body allow a reduction in the sheet thickness of a component produced from such multiphase steels compared to a body produced from conventional steels.
  • multiphase steels are melted in the converter steelworks and cast on a continuous casting plant into slabs or thin slabs, which are then hot rolled into hot strip and coiled.
  • the mechanical properties of the hot strip can be varied.
  • the hot strips can be cold rolled to cold strip to provide thinner sheet thicknesses ( EP 0 910 675 B1 . EP 0 966 547 B1 . EP 1 169 486 B1 . EP 1 319 725 B1 . EP 1 398 390 A1 ).
  • a problem in the production of flat products made of high-strength multiphase steels with tensile strengths of more than 800 MPa is that high rolling forces must be applied when rolling such steels. This requirement has the consequence that high-strength hot strips made of steels of the type in question can usually only be made available in width and thickness with the currently customarily available production facilities, which do not meet the demands made today in the field of automobile construction more fully. Above all, tapes of small thicknesses with sufficient widths can be poorly represented on conventional systems. It also turns out in conventional practice in practice difficult to produce multiphase steels cold strips with strengths of more than 800 MPa.
  • the cast strip is then hot rolled in-line in one or more passes of between 25% and 70% strain to a hot strip.
  • the final temperature of hot rolling is above the Ar 3 temperature.
  • the hot strip obtained is then cooled in two stages. In the first stage of this cooling, a cooling rate of 5 - 100 ° C / s is maintained until a temperature between 400 - 550 ° C is reached. At this temperature, the hot-rolled strip is left to rest for a period of time required to allow bainitic transformation of the steel with a residual austenite content greater than 5%. The formation of perlite should be avoided.
  • the conversion process is stopped by the beginning of the second stage of cooling, in which the hot strip is brought to a temperature below 400 ° C, then at a lying below 350 ° C reel to a Wind coil.
  • Such steels have relatively high strengths with good ductility. However, the strength is not sufficient for many applications, especially in the field of automotive engineering.
  • the object of the invention was therefore to provide a method with which extremely high-strength steel flat products can be produced over a wide range of geometric dimensions with reduced manufacturing outlay.
  • this object has been achieved by a method according to claim 1 for the production of steel flat products, according to the invention in a martensitic structure forming steel, the 0.15 - 0.19% C, 0.80 - 1.20% Mn, up to 0.030% P, up to 0.004% S, 0.60 - 1.00% Si, up to 0.05% Al, up to 0.0060% N, 0.30 - 0 , 60% Cr, 0.040-0.070% Nb and the remainder contains iron and unavoidable impurities, is cast into a cast strip of 1 to 4 mm thickness, in which the cast strip is in a continuous operation with a degree of deformation greater than 20 % in-line is hot rolled to a hot strip of 0.5 - 3.2 mm thickness at a hot rolling end temperature in the range 900 - 1050 ° C, and the hot strip is coiled at a reel temperature of 350 ° C or less, then that a hot strip is obtained whose tensile strength
  • the invention uses the possibility of strip casting to process a particularly high-strength martensitic steel to a hot strip.
  • the cast band with it itself already has a small thickness, in the course of hot rolling of this band, only relatively small degrees of deformation must be maintained in order to produce flat products with low thicknesses, as required in particular in the automotive industry.
  • the method according to the invention it is easily possible to produce hot strips with the method according to the invention, which have a maximum property distribution of at most 1.5 mm and from which, for example, elements for the support structure of an automobile can be produced.
  • the invention makes it possible to manufacture high-strength hot strips consisting of a martensitic steel of the specified composition processed according to the invention, whose width is more than 1,200 mm, in particular more than 1,600 mm.
  • the use according to the invention of the strip casting method in the processing of high-strength steels of the type assembled according to the invention offers, in addition to the above-mentioned advantages, due to its process-specific properties and manipulated variables (eg hot rolling end temperature, cooling, reel temperature). the possibility of safely shedding critical steel compositions of the type processed according to the invention also with regard to their solidification behavior.
  • the very rapid solidification of the cast strip which is characteristic of strip casting, leads to a significantly reduced risk of the formation of center segregations compared with conventional production, with the result that the hot strip produced according to the invention has a particularly uniform distribution of properties and microstructure over its cross section and its length.
  • a further particular advantage of the procedure according to the invention is that the hot strip produced according to the invention has high strengths of at least 1400 MPa, without having to observe a special cooling cycle of the hot strip between the end of the hot rolling and the coiling, as described, for example, in US Pat EP 1 072 689 B1 is prescribed by the need for a cooling phase.
  • it merely has to be ensured that the hot rolling ends in a relatively narrow temperature window and that the reeling is also carried out in a precisely defined temperature range. In between there is a one-stage cooling down.
  • a further advantage of the procedure according to the invention is that an extension of the range of mechanical properties of the strip produced according to the invention based on only one steel analysis can be achieved by varying the cooling and rolling conditions.
  • Hot strips produced according to the invention are particularly suitable for further processing into cold rolled strip. Accordingly, a practice-oriented embodiment of the invention provides that the hot strip is cold rolled to a cold strip having a thickness of 0.5-1.4 mm, in particular 0.7 mm to 1.3 mm, as is required for the construction of automobile bodies.
  • the cold strip can be annealed at an annealing temperature of 750-850 ° C.
  • tensile strengths of at least 800 MPa can be reliably ensured.
  • the breaking elongation A 50 of the cold strip is just as safe at least 10%.
  • the cold strip is provided in a conventional manner with a metallic coating, which may be, for example, a galvanizing.
  • the tapes cast from steels A and B were hot rolled immediately after tape casting in-line at a hot rolling end temperature WET to a hot strip whose thickness was 1.25 mm. Subsequently, each hot strip obtained has been cooled directly in a cooling step to a coiler temperature HT and coiled. After coiling, the hot strips produced from steels A and B each had a tensile strength R m and an elongation at break A 80 , which, like the hot rolling end temperature WET and reel temperature HT, respectively, maintained in their preparation, are given in Table 2. Table 2 stole WET [° C] HT [° C] R m [MPa] A 80 [%] A 900 290 1414 5.2 B 980 290 1405 6.0
  • the hot strip produced from the steel A was cold rolled after the coiling and pickling to a 0.7 mm thick cold strip and annealed at a temperature of 780 ° C in the flow to recrystallize the strip.
  • the tensile strength R m of the cold-rolled strip obtained in this way was 654 MPa.

Abstract

The method for the production of flat steel products useful in automotive industry, comprises casting a steel into a cast strip having a thickness of 1-4 mm, hot-rolling the cast strip in-line into a hot-rolled strip having a thickness of higher than 1.5 mm in a continuous process at 900-1050[deg] C, and coiling the hot-rolled strip at 550-620[deg] C to obtain a hot-rolled strip, which has a minimum tensile strength R m of 1400 MPa and a minimum breaking elongation A 8 0 of 5%. The hot-rolled strip is cold-rolled with a thickness of 0.5-1.4 mm at 750-850[deg] C to obtain a cold-rolled strip. The method for the production of flat steel products useful in automotive industry, comprises casting a steel into a cast strip having a thickness of 1-4 mm, hot-rolling the cast strip in-line into a hot-rolled strip having a thickness of higher than 1.5 mm in a continuous process at 900-1050[deg] C, and coiling the hot-rolled strip at 550-620[deg] C to obtain a hot-rolled strip, which has a minimum tensile strength R m of 1400 MPa and a minimum breaking elongation A 8 0 of 5%. The steel forms a complex phase structure. The shaping degree is greater than 20%. The width of the hot-rolled strip is more than 1.600 mm. The hot-rolled strip is cold-rolled with a thickness of 0.5-1.4 mm at 750-850[deg] C to obtain a cold-rolled strip, which has a minimum tensile strength of 600 MPa and a minimum breaking elongation A 8 0 of 15%. The cold- or hot-rolled strip is provided with a metallic coating, which is galvanizing.

Description

Die Erfindung betrifft ein Verfahren zur Herstellung von Stahl-Flachprodukten, wie Bändern oder Blechzuschnitten, aus hochfesten martensitischen Stählen. Derartige MS-Stähle gehören zur Gruppe der Mehrphasenstähle. Bei diesen handelt es sich üblicherweise um Stähle, deren Eigenschaften durch Art, Menge und Anordnung der Phasen des Gefüges bestimmt werden. Im Gefüge liegen daher mindestens zwei Phasen vor (z. B. Ferrit, Martensit, Bainit). Dadurch haben sie eine gegenüber konventionellen Stählen überlegene Festigkeits- / Umformbarkeitskombination.The invention relates to a method for producing flat steel products, such as strips or sheet metal blanks, from high-strength martensitic steels. Such MS steels belong to the group of multiphase steels. These are usually steels whose properties are determined by the type, amount and arrangement of the phases of the structure. The structure therefore has at least two phases (eg ferrite, martensite, bainite). This gives them a strength / formability combination that is superior to conventional steels.

Aufgrund dieser Besonderheiten sind Mehrphasenstähle insbesondere für den Automobilbau von großem Interesse, da sie aufgrund ihrer hohen Festigkeit zum einen die Verwendung geringerer Materialstärken und damit einhergehend eine Reduzierung des Fahrzeuggewichts erlauben und zum anderen die Sicherheit der Fahrzeugkarosserie im Fall eines Zusammenstoßes (Crash-Verhalten) verbessern. So ermöglichen Mehrphasenstähle bei mindestens gleich bleibender Festigkeit der Gesamtkarosse eine Reduzierung der Blechdicke eines aus solchen Mehrphasenstählen hergestellten Bauteils gegenüber einer aus konventionellen Stählen hergestellten Karosserie.Because of these peculiarities, multiphase steels are of particular interest for the automotive industry because of their high strength, they allow the use of lower material thicknesses and concomitantly a reduction in vehicle weight and, secondly, the safety of the vehicle body in the event of a collision (crash behavior) , Thus, multiphase steels with at least constant strength of the overall body allow a reduction in the sheet thickness of a component produced from such multiphase steels compared to a body produced from conventional steels.

Üblicherweise werden Mehrphasenstähle im Konverterstahlwerk erschmolzen und auf einer Stranggießanlage zu Brammen oder Dünnbrammen vergossen, die dann zu Warmband warmgewalzt und gehaspelt werden. Durch eine gezielt gesteuerte Abkühlung des Warmbands nach dem Warmwalzen mit dem Ziel einer Einstellung bestimmter Gefügeanteile können dabei die mechanischen Eigenschaften des Warmbandes variiert werden. Die Warmbänder können darüber hinaus zu Kaltband kaltgewalzt werden, um auch dünnere Blechdicken zur Verfügung zu stellen ( EP 0 910 675 B1 , EP 0 966 547 B1 , EP 1 169 486 B1 , EP 1 319 725 B1 , EP 1 398 390 A1 ).Usually, multiphase steels are melted in the converter steelworks and cast on a continuous casting plant into slabs or thin slabs, which are then hot rolled into hot strip and coiled. By a controlled controlled cooling of the hot strip after hot rolling with the aim of adjusting certain microstructures, the mechanical properties of the hot strip can be varied. In addition, the hot strips can be cold rolled to cold strip to provide thinner sheet thicknesses ( EP 0 910 675 B1 . EP 0 966 547 B1 . EP 1 169 486 B1 . EP 1 319 725 B1 . EP 1 398 390 A1 ).

Ein Problem bei der Fertigung von Flachprodukten aus hochfesten Mehrphasenstählen mit Zugfestigkeiten von mehr als 800 MPa besteht darin, dass beim Walzen derartiger Stähle hohe Walzkräfte aufgebracht werden müssen. Diese Anforderung hat zur Folge, dass in der Regel mit den derzeit üblicherweise zur Verfügung stehenden Fertigungsanlagen hochfeste Warmbänder aus Stählen der in Rede stehenden Art vielfach nur in einer Breite und Dicke zur Verfügung gestellt werden können, die den heute im Bereich des Automobilbaus gestellten Anforderungen nicht mehr in vollem Umfang gerecht werden. Vor allem Bänder geringer Dicken bei ausreichenden Breiten lassen sich auf konventionellen Anlagen schlecht darstellen. Auch erweist es sich bei konventioneller Vorgehensweise in der Praxis als schwierig, aus Mehrphasenstählen Kaltbänder mit Festigkeiten von mehr als 800 MPa herzustellen.A problem in the production of flat products made of high-strength multiphase steels with tensile strengths of more than 800 MPa is that high rolling forces must be applied when rolling such steels. This requirement has the consequence that high-strength hot strips made of steels of the type in question can usually only be made available in width and thickness with the currently customarily available production facilities, which do not meet the demands made today in the field of automobile construction more fully. Above all, tapes of small thicknesses with sufficient widths can be poorly represented on conventional systems. It also turns out in conventional practice in practice difficult to produce multiphase steels cold strips with strengths of more than 800 MPa.

Ein alternativer Weg der Herstellung von Stahlbändern aus einem Mehrphasenstahl ist in der EP 1 072 689 B1 ( DE 600 09 611 T2 ) vorgeschlagen worden. Gemäß diesem bekannten Verfahren wird zur Herstellung von dünnen Stahlbändern zunächst eine Stahlschmelze, die (in Gew.-%) 0,05 und 0,25 % C, in Summe 0,5 - 3 % Mn, Cu und Ni, in Summe 0,1 - 4 % Si und Al, in Summe bis zu 0,1 % P, Sn, As und Sb, in Summe weniger als 0,3 % Ti, Nb, V, Zr und REM sowie jeweils weniger als 1% Cr, Mo und V, Rest Eisen und unvermeidbare Verunreinigungen enthält, zu einem gegossenen Band mit einer Dicke von 0,5 - 10 mm, insbesondere 1 - 5 mm, vergossen. Das gegossene Band wird anschließend in-Line in ein oder mehreren Stichen mit einem zwischen 25 % und 70 % liegenden Umformgrad zu einem Warmband warmgewalzt. Die Endtemperatur des Warmwalzens liegt dabei oberhalb der Ar3-Temperatur. Nach dem Ende des Warmwalzens wird das erhaltene Warmband dann zweistufig abgekühlt. In der ersten Stufe dieser Abkühlung wird eine Abkühlgeschwindigkeit von 5 - 100 °C/s eingehalten, bis eine zwischen 400 - 550 °C liegende Temperatur erreicht ist. Bei dieser Temperatur lässt man dann das Warmband für eine Pausenzeit verweilen, die benötigt wird, um eine bainitische Umwandlung des Stahls mit einem Restaustenitanteil von mehr als 5 % zu ermöglichen. Die Bildung von Perlit soll dabei vermieden werden. Nach einer für die Einstellung der geforderten Gefügestruktur ausreichenden Pausenzeit wird der Umwandlungsvorgang durch den Beginn der zweiten Stufe der Abkühlung abgebrochen, bei der das Warmband auf eine Temperatur unter 400 °C gebracht wird, um es anschließend bei einer unter 350 °C liegenden Haspeltemperatur zu einem Coil zu wickeln.An alternative way of producing steel strip from a multiphase steel is in the EP 1 072 689 B1 ( DE 600 09 611 T2 ) has been proposed. According to this known method is used for the production of thin Steel strips are first a molten steel, the (in wt .-%) 0.05 and 0.25% C, in total 0.5 - 3% Mn, Cu and Ni, in total 0.1 - 4% Si and Al, in Sum up to 0.1% P, Sn, As and Sb, in total less than 0.3% Ti, Nb, V, Zr and REM and in each case less than 1% Cr, Mo and V, balance iron and unavoidable impurities cast into a cast strip having a thickness of 0.5 - 10 mm, in particular 1 - 5 mm. The cast strip is then hot rolled in-line in one or more passes of between 25% and 70% strain to a hot strip. The final temperature of hot rolling is above the Ar 3 temperature. After the end of the hot rolling, the hot strip obtained is then cooled in two stages. In the first stage of this cooling, a cooling rate of 5 - 100 ° C / s is maintained until a temperature between 400 - 550 ° C is reached. At this temperature, the hot-rolled strip is left to rest for a period of time required to allow bainitic transformation of the steel with a residual austenite content greater than 5%. The formation of perlite should be avoided. After a sufficient for the adjustment of the required microstructure pause time, the conversion process is stopped by the beginning of the second stage of cooling, in which the hot strip is brought to a temperature below 400 ° C, then at a lying below 350 ° C reel to a Wind coil.

Mit der in der EP 1 072 689 B1 beschriebenen Vorgehensweise soll eine einfache Herstellung von Warmband mit bainitischen Gefügeanteilen aus einem Mehrphasenstahl möglich sein, das TRIP-Eigenschaften ("TRIP" = "Transformation Induced Plasticity") aufweist.With the in the EP 1 072 689 B1 described procedure is a simple production of hot strip with bainitic microstructural parts of a multi-phase steel is possible, the TRIP properties ("TRIP =" T ransformation I nduced P lasticity ") has.

Derartige Stähle weisen relativ hohe Festigkeiten bei guter Verformbarkeit auf. Allerdings reichen die Festigkeiten für viele Anwendungsfälle insbesondere im Bereich des Automobilbaus nicht aus.Such steels have relatively high strengths with good ductility. However, the strength is not sufficient for many applications, especially in the field of automotive engineering.

Die Aufgabe der Erfindung bestand daher darin, ein Verfahren zur Verfügung zu stellen, mit dem sich extrem hochfeste Stahl-Flachprodukte über eine große Bandbreite von geometrischen Abmessungen bei vermindertem Herstellaufwand erzeugen lassen.The object of the invention was therefore to provide a method with which extremely high-strength steel flat products can be produced over a wide range of geometric dimensions with reduced manufacturing outlay.

Ausgehend von dem voranstehend erläuterten Stand der Technik ist diese Aufgabe durch ein Verfahren gemäß Anspruch 1 zum Herstellen von Stahl-Flachprodukten gelöst worden, bei dem erfindungsgemäß ein ein martensitisches Gefüge bildender Stahl, der 0,15 - 0,19 % C, 0,80 - 1,20 % Mn, bis zu 0,030% P, bis zu 0,004% S, 0,60 - 1,00% Si, bis zu 0,05 % Al, bis zu 0,0060 % N, 0,30 - 0,60 % Cr, 0,040 - 0,070 % Nb und als Rest Eisen und unvermeidbare Verunreinigungen enthält, zu einem gegossenen Band mit einer Dicke von 1 - 4 mm vergossen wird, bei dem das gegossene Band in einem kontinuierlichen Arbeitsablauf mit einem Umformgrad von mehr als 20 % in-Line bei einer im Bereich von 900 - 1050 °C liegenden Warmwalzendtemperatur zu einem Warmband mit einer Dicke von 0,5 - 3,2 mm warmgewalzt wird und bei dem das Warmband bei einer höchstens 350 °C betragenden Haspeltemperatur gehaspelt wird, so dass ein Warmband erhalten wird, dessen Zugfestigkeit Rm mindestens 1400 MPa bei einer Bruchdehnung A80 von mindestens 5 % beträgt.Based on the above-described prior art, this object has been achieved by a method according to claim 1 for the production of steel flat products, according to the invention in a martensitic structure forming steel, the 0.15 - 0.19% C, 0.80 - 1.20% Mn, up to 0.030% P, up to 0.004% S, 0.60 - 1.00% Si, up to 0.05% Al, up to 0.0060% N, 0.30 - 0 , 60% Cr, 0.040-0.070% Nb and the remainder contains iron and unavoidable impurities, is cast into a cast strip of 1 to 4 mm thickness, in which the cast strip is in a continuous operation with a degree of deformation greater than 20 % in-line is hot rolled to a hot strip of 0.5 - 3.2 mm thickness at a hot rolling end temperature in the range 900 - 1050 ° C, and the hot strip is coiled at a reel temperature of 350 ° C or less, then that a hot strip is obtained whose tensile strength R m at least 1400 MPa at a Bruchdeh A 80 of at least 5%.

Die Erfindung nutzt die Möglichkeit des Bandgießens dazu, einen besonders hochfesten martensitischen Stahl zu einem Warmband zu verarbeiten. Da das gegossene Band dabei selbst schon eine geringe Dicke besitzt, müssen im Zuge des Warmwalzens dieses Bandes nur relativ geringe Umformgrade eingehalten werden, um Flachprodukte mit geringen Dicken zu erzeugen, wie sie insbesondere im Bereich der Automobilindustrie benötigt werden. So ist es durch Vorgabe einer entsprechenden Ausgangsdicke des gegossenen Bandes problemlos möglich, mit dem erfindungsgemäßen Verfahren Warmbänder herzustellen, die bei einer optimalen Eigenschaftsverteilung eine Dicke von höchstens 1,5 mm aufweisen und aus denen sich beispielsweise Elemente für die Tragstruktur eines Automobils fertigen lassen.The invention uses the possibility of strip casting to process a particularly high-strength martensitic steel to a hot strip. As the cast band with it itself already has a small thickness, in the course of hot rolling of this band, only relatively small degrees of deformation must be maintained in order to produce flat products with low thicknesses, as required in particular in the automotive industry. Thus, by specifying a corresponding output thickness of the cast strip, it is easily possible to produce hot strips with the method according to the invention, which have a maximum property distribution of at most 1.5 mm and from which, for example, elements for the support structure of an automobile can be produced.

Aufgrund der geringen Umformgrade während des Warmwalzens sind die dazu erforderlichen Walzkräfte verglichen mit den beim Warmwalzen von Brammen oder Dünnbrammen bei der konventionellen Vorgehensweise erforderlichen Kräften gering, so dass mit dem erfindungsgemäßen Verfahren problemlos Warmbänder von großer Breite erzeugt werden können, die deutlich über der Breite von in konventioneller Weise erzeugbaren Warmbändern derselben Festigkeits- und Dickenklasse liegen. So erlaubt es die Erfindung, hochfeste, aus einem martensitischen Stahl der angegebenen erfindungsgemäß verarbeiteten Zusammensetzung bestehende Warmbänder sicher zu fertigen, deren Breite mehr als 1.200 mm, insbesondere mehr als 1.600 mm beträgt.Due to the low degrees of deformation during hot rolling, the required rolling forces are low compared to the forces required in the hot rolling of slabs or thin slabs in the conventional approach, so that can be easily produced by the inventive method hot strips of large width, well above the width of be produced in a conventional manner hot strips of the same strength and thickness class. Thus, the invention makes it possible to manufacture high-strength hot strips consisting of a martensitic steel of the specified composition processed according to the invention, whose width is more than 1,200 mm, in particular more than 1,600 mm.

Die erfindungsgemäße Anwendung des Bandgießverfahrens bei der Verarbeitung von hochfesten Stählen der erfindungsgemäß zusammengesetzten Art bietet neben den voranstehend genannten Vorteilen auf Grund seiner verfahrensspezifischen Eigenschaften und Stellgrößen (z. B. Warmwalzendtemperatur, Abkühlung, Haspeltemperatur) die Möglichkeit, auch hinsichtlich ihres Erstarrungsverhaltens kritische Stahlzusammensetzungen der erfindungsgemäß verarbeiteten Art sicher zu vergießen. So führt die für das Bandgießen charakteristische sehr rasche Erstarrung des gegossenen Bandes zu gegenüber einer konventionellen Fertigung deutlich verminderten Gefahr der Entstehung von Mittenseigerungen mit der Folge, dass das erfindungsgemäß erzeugte Warmband über seinen Querschnitt und seine Länge eine besonders gleichmäßige Eigenschaftsverteilung und Gefügestruktur aufweist.The use according to the invention of the strip casting method in the processing of high-strength steels of the type assembled according to the invention offers, in addition to the above-mentioned advantages, due to its process-specific properties and manipulated variables (eg hot rolling end temperature, cooling, reel temperature). the possibility of safely shedding critical steel compositions of the type processed according to the invention also with regard to their solidification behavior. Thus, the very rapid solidification of the cast strip, which is characteristic of strip casting, leads to a significantly reduced risk of the formation of center segregations compared with conventional production, with the result that the hot strip produced according to the invention has a particularly uniform distribution of properties and microstructure over its cross section and its length.

Ein weiterer besonderer Vorteil der erfindungsgemäßen Vorgehensweise besteht darin, dass erfindungsgemäß erzeugtes Warmband hohe Festigkeiten von mindestens 1400 MPa aufweist, ohne dass dazu ein besonderer Abkühlzyklus des Warmbands zwischen dem Ende des Warmwalzens und des Haspelns eingehalten werden muss, wie dies beispielsweise in der EP 1 072 689 B1 durch die Notwendigkeit einer Kühlphase vorgeschrieben ist. Bei der Durchführung des erfindungsgemäßen Verfahrens muss lediglich sichergestellt sein, dass das Warmwalzen in einem relativ eng umgrenzten Temperaturfenster beendet und auch das Haspeln in einem exakt definierten Temperaturbereich durchgeführt wird. Dazwischen findet eine einstufige Abkühlung statt.A further particular advantage of the procedure according to the invention is that the hot strip produced according to the invention has high strengths of at least 1400 MPa, without having to observe a special cooling cycle of the hot strip between the end of the hot rolling and the coiling, as described, for example, in US Pat EP 1 072 689 B1 is prescribed by the need for a cooling phase. When carrying out the method according to the invention, it merely has to be ensured that the hot rolling ends in a relatively narrow temperature window and that the reeling is also carried out in a precisely defined temperature range. In between there is a one-stage cooling down.

Ein weiterer Vorteil der erfindungsgemäßen Vorgehensweise besteht darin, dass eine Erweiterung der Spannbreite der mechanischen Eigenschaften des erfindungsgemäß erzeugten Bandes basierend auf nur einer Stahlanalyse durch eine Variation der Abkühl- und Walzbedingungen erreicht werden kann.A further advantage of the procedure according to the invention is that an extension of the range of mechanical properties of the strip produced according to the invention based on only one steel analysis can be achieved by varying the cooling and rolling conditions.

Erfindungsgemäß erzeugte Warmbänder eignen sich besonders zur Weiterverarbeitung zu kaltgewalztem Band. Dementsprechend sieht eine praxisgerechte Ausgestaltung der Erfindung vor, dass das Warmband zu einem Kaltband mit einer Dicke von 0,5 - 1,4 mm, insbesondere 0,7 mm bis 1,3 mm kaltgewalzt wird, wie es zum Bau von Automobilkarosserien benötigt wird. Um während des Kaltwalzens entstehende Verfestigungen zu beseitigen, kann das Kaltband bei einer Glühtemperatur von 750 - 850 °C geglüht werden. Für auf diese Weise aus dem erfindungsgemäß hergestellten Warmband erzeugtes Kaltband können Zugfestigkeiten von mindestens 800 MPa sicher gewährleistet werden. Die Bruchdehnung A50 des Kaltbands beträgt dabei ebenso sicher mindestens 10 %.Hot strips produced according to the invention are particularly suitable for further processing into cold rolled strip. Accordingly, a practice-oriented embodiment of the invention provides that the hot strip is cold rolled to a cold strip having a thickness of 0.5-1.4 mm, in particular 0.7 mm to 1.3 mm, as is required for the construction of automobile bodies. In order to eliminate solidification during cold rolling, the cold strip can be annealed at an annealing temperature of 750-850 ° C. For cold rolled strip produced in this way from the hot strip produced according to the invention, tensile strengths of at least 800 MPa can be reliably ensured. The breaking elongation A 50 of the cold strip is just as safe at least 10%.

Gemäß einer weiteren vorteilhaften Ausgestaltung der Erfindung wird das Kaltband in an sich bekannter Weise mit einer metallischen Beschichtung versehen, bei der es sich beispielsweise um eine Verzinkung handeln kann.According to a further advantageous embodiment of the invention, the cold strip is provided in a conventional manner with a metallic coating, which may be, for example, a galvanizing.

Nachfolgend wird die Erfindung anhand von Ausführungsbeispielen näher erläutert.The invention will be explained in more detail by means of exemplary embodiments.

In zum Nachweis der Wirkung der Erfindung durchgeführten Versuchen sind zwei erfindungsgemäß zusammengesetzte Stähle A und B mit der in Tabelle 1 angegebenen Zusammensetzung erschmolzen und in einer konventionellen Zweiwalzengieß-Maschine zu gegossenem Band vergossen worden, das 1,6 mm dick war. Tabelle 1 (Angaben in Gew.-%) C Mn P S Si Al N Cr Nb A 0,157 1,25 0,006 0,005 0,72 0,021 0,0049 0,46 0,060 B 0,157 1,08 0,005 0,004 0,77 0,022 0,0060 0,48 0,064 In tests carried out to demonstrate the effect of the invention, two steels A and B of the invention having the composition shown in Table 1 were melted and cast in a conventional two-roll casting machine into cast strip 1.6 mm thick. Table 1 (in% by weight) C Mn P S Si al N Cr Nb A 0,157 1.25 0,006 0.005 0.72 0,021 0.0049 0.46 0,060 B 0,157 1.08 0.005 0,004 0.77 0,022 0.0060 0.48 0.064

Die aus den Stählen A und B gegossenen Bänder sind im unmittelbaren Anschluss an das Bandgießen in-Line bei einer Warmwalzendtemperatur WET zu einem Warmband warmgewalzt worden, dessen Dicke 1,25 mm betrug. Anschließend ist das jeweils erhaltene Warmband direkt in einem Kühlschritt auf eine Haspeltemperatur HT abgekühlt und gehaspelt worden. Nach dem Haspeln wiesen die aus den Stählen A und B erzeugten Warmbänder jeweils eine Zugfestigkeit Rm und eine Bruchdehnung A80 auf, die wie die bei ihrer Herstellung jeweils eingehaltene Warmwalzendtemperatur WET und Haspeltemperatur HT in Tabelle 2 angegeben sind. Tabelle 2 Stahl WET [°C] HT [°C] Rm [MPa] A80 [%] A 900 290 1414 5,2 B 980 290 1405 6,0 The tapes cast from steels A and B were hot rolled immediately after tape casting in-line at a hot rolling end temperature WET to a hot strip whose thickness was 1.25 mm. Subsequently, each hot strip obtained has been cooled directly in a cooling step to a coiler temperature HT and coiled. After coiling, the hot strips produced from steels A and B each had a tensile strength R m and an elongation at break A 80 , which, like the hot rolling end temperature WET and reel temperature HT, respectively, maintained in their preparation, are given in Table 2. Table 2 stole WET [° C] HT [° C] R m [MPa] A 80 [%] A 900 290 1414 5.2 B 980 290 1405 6.0

Das aus dem Stahl A erzeugte Warmband ist nach dem Haspeln und Beizen zu einem 0,7 mm dicken Kaltband kaltgewalzt und bei einer Temperatur von 780 °C im Durchlauf geglüht worden, um das Band zu rekristallisieren.The hot strip produced from the steel A was cold rolled after the coiling and pickling to a 0.7 mm thick cold strip and annealed at a temperature of 780 ° C in the flow to recrystallize the strip.

Bei einer Bruchdehnung A50 von 21,8 % betrug die Zugfestigkeit Rm des so erhaltenen Kaltbandes 654 MPa.At an elongation at break A 50 of 21.8%, the tensile strength R m of the cold-rolled strip obtained in this way was 654 MPa.

Claims (9)

  1. Method for manufacturing flat steel products,
    - wherein a steel that forms a martensitic microstructure with the following composition (in wt. %) C: 0.15 - 0.19 % Mn: 0.80 - 1.20 % P: ≤ 0.030 % S: ≤ 0.004 % Si: 0.60 - 1.00 % Al: ≤ 0.05 % N: ≤ 0.0060 % Cr: 0.30 - 0.60 % Nb: 0.040 - 0.070 %
    remainder iron and unavoidable impurities is cast into a cast strip having a thickness of 1 - 4 mm,
    - wherein the cast strip is hot-rolled in-line into a hot-rolled strip having a thickness ranging from 0.5 to 3.2 mm in a continuous process at a final hot-rolling temperature ranging from 900 to 1050 °C, the deformation degree being greater than 20 %, cooled in one step and
    - wherein the hot-rolled strip is coiled at a coiling temperature of at most 350 °C,
    - so as to obtain a hot-rolled strip, which has a minimum tensile strength Rm of 1400 MPa at a minimum breaking elongation A80 of 5 %.
  2. Method according to Claim 1, characterized in that the width of the hot-rolled strip is greater than 1,200 mm, in particular greater than 1,600 mm.
  3. Method according to any one of the preceding claims, characterized in that the thickness of the hot-rolled strip is 1.5 mm at most.
  4. Method according to any one of the preceding claims, characterized in that the hot-rolled strip is cold-rolled into cold-rolled strip having a thickness of 0.5 - 1.4 mm.
  5. Method according to Claim 4, characterized in that the cold-rolled strip is annealed at an annealing temperature of 750 - 850 °C.
  6. Method according to Claim 4 or 5, characterized in that the minimum tensile strength of the cold-rolled strip is 600 MPa.
  7. Method according to any one of Claims 4 to 6, characterized in that the cold-rolled strip has a minimum breaking elongation A50 of 15 %.
  8. Method according to any one of the preceding claims, characterized in that the hot-rolled strip or cold-rolled strip is provided with a metallic coating.
  9. Method according to Claim 8, characterized in that the metallic coating is a zinc coating.
EP06123137A 2006-10-30 2006-10-30 Process for manufacturing steel flat products from a steel forming martensitic structure Not-in-force EP1918403B1 (en)

Priority Applications (10)

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DE502006003831T DE502006003831D1 (en) 2006-10-30 2006-10-30 Process for producing steel flat products from a steel forming a martensitic structure
EP06123137A EP1918403B1 (en) 2006-10-30 2006-10-30 Process for manufacturing steel flat products from a steel forming martensitic structure
AT06123137T ATE432373T1 (en) 2006-10-30 2006-10-30 METHOD FOR PRODUCING FLAT STEEL PRODUCTS FROM A STEEL FORMING A MARTENSITIC STRUCTURE
ES06123137T ES2325961T3 (en) 2006-10-30 2006-10-30 PROCEDURE FOR MANUFACTURING STEEL FLAT PRODUCTS FROM A STEEL FORMING A MARTENSITICAL STRUCTURE.
PL06123137T PL1918403T3 (en) 2006-10-30 2006-10-30 Process for manufacturing steel flat products from a steel forming martensitic structure
CN2007800394302A CN101528967B (en) 2006-10-30 2007-10-24 Method for manufacturing flat steel products from a steel forming a martensitic structure
US12/447,623 US20100096047A1 (en) 2006-10-30 2007-10-24 Method for manufacturing flat steel products from a steel forming a martensitic microstructure
PCT/EP2007/061389 WO2008052918A1 (en) 2006-10-30 2007-10-24 Method for manufacturing flat steel products from a steel forming a martensitic structure
KR1020097007484A KR101458577B1 (en) 2006-10-30 2007-10-24 Method for manufacturing flat steel products from a steel forming a martensitic structure
JP2009533821A JP5350252B2 (en) 2006-10-30 2007-10-24 Process for producing flat steel products from steel forming a martensitic microstructure

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EP1918403B1 true EP1918403B1 (en) 2009-05-27

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ATE432373T1 (en) 2009-06-15
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EP1918403A1 (en) 2008-05-07
US20100096047A1 (en) 2010-04-22
KR101458577B1 (en) 2014-11-07
ES2325961T3 (en) 2009-09-25
KR20090090301A (en) 2009-08-25
PL1918403T3 (en) 2009-10-30
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CN101528967B (en) 2011-06-15
JP2010508434A (en) 2010-03-18

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