EP1398390B1 - Steel with a very fine ferritic and martensitic microstructure having a high tensile strength - Google Patents

Steel with a very fine ferritic and martensitic microstructure having a high tensile strength Download PDF

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EP1398390B1
EP1398390B1 EP02020294A EP02020294A EP1398390B1 EP 1398390 B1 EP1398390 B1 EP 1398390B1 EP 02020294 A EP02020294 A EP 02020294A EP 02020294 A EP02020294 A EP 02020294A EP 1398390 B1 EP1398390 B1 EP 1398390B1
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Prior art keywords
hot
rolled strip
cooling
process according
temperature
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EP02020294A
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German (de)
French (fr)
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EP1398390A1 (en
Inventor
Andreas Nuss
Thomas Dr.-Ing. Heller
Bernhard Dr.-Ing. Engl
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel AG
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Priority to ES02020294T priority Critical patent/ES2256378T3/en
Priority to DE50205631T priority patent/DE50205631D1/en
Priority to EP02020294A priority patent/EP1398390B1/en
Priority to AT02020294T priority patent/ATE316157T1/en
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—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
    • 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
    • C21D2201/00—Treatment for obtaining particular effects
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the invention relates to a method for producing a high-strength hot strip which has an ultrafine grain structure.
  • a method for producing hot strips of this type is known from EP 1 001 041 Al.
  • the slabs After re-heating to not more than 1150 ° C, the slabs are hot rolled in several steps to hot strip.
  • the hot rolling is carried out at temperatures at which dynamic recrystallization of the steel occurs.
  • low reduction levels are set, which should be in the last rolling step 13% to 30% and in all the last step preceding rolling steps 4% to 20%.
  • the desired fine-grained austenitic structure of the hot strip to be adjusted, in which the grain diameter of the austenite in the range of less than 4 .mu.m, in particular less than 2 microns.
  • the hot strip obtained is rapidly heated to 350 to 600 ° C temperature cooled to cause the formation of fine-grained ferrite.
  • the cooling rate is not less than 30 ° C / s.
  • the temperature at which the strip is subsequently coiled is, according to the known method, adapted to the most favorable temperature for the respective steel.
  • the steel obtained by the known method is characterized by a good deformability and at the same time comparably high strength.
  • tensile strengths are achieved which regularly exceed 500 MPa and can be up to 763 MPa.
  • the yield point is in the range of at least 420 MPa to 645 MPa.
  • the yield ratio of the known steel is regularly in the range of at least 0.75.
  • steel sheets having a very fine microstructure are produced in the process known from EP-A-0 945 522.
  • C and other optional components up to 1.0 wt% Cr and up to 3.0 wt% Mn.
  • the steel strips produced from such a steel composition using a reeling temperature not specified may, inter alia, have a bainitic, a bainitic / martensitic, a martensitic, a pearlitic or a pearlitic / martensitic structure.
  • the hot strips produced in this way can have ferrite grain sizes of less than 3 ⁇ m with further scattering.
  • a hot strip (W) which in particular continuously cast finished in the form of reheated or used directly from the casting heat slabs, thin slabs or cast strip and then continuously cooled.
  • the cooling takes place in at least two consecutive cooling phases (t CK , t LK ) accelerated cooling to a final temperature.
  • the first cooling phase (t CK ) of accelerated cooling sets in at the latest three seconds after the final rolling pass of finish rolling and the hot strip (W) is cooled during the first cooling phase (t CK ) accelerated cooling at a cooling rate of at least 150 ° C / s.
  • this steel When a TRIP steel containing 0.2% by mass of C, 1.8% by mass of Al and 1.6% by mass of Mn, the remainder of iron and unavoidable impurities is processed in accordance with this process, this steel has a fine microstructure with ferrite grains, whose diameter is less than 3 microns.
  • EP-A-0 969 112 describes a process for producing a dual-phase steel in which a steel containing, in addition to iron and unavoidable impurities, as Compulsory constituents (in% by weight) 0.02-0.25% C and in addition Mn and / or Cr in respective amounts of 0.15-3.5%, Si, Al and / or P in total contents of 0, 02 - 4.0% and other optional constituents, hot rolled at a hot rolling end temperature of 780 - 880 ° C and then cooled at a cooling rate of more than 5 ° C / s to reel temperatures below 350 ° C.
  • Compulsory constituents in% by weight
  • Mn and / or Cr in respective amounts of 0.15-3.5%
  • Si, Al and / or P in total contents of 0, 02 - 4.0% and other optional constituents
  • EP-A-0 969 112 show that high Si contents, high Mn contents and, if present, high Cr contents have been processed there in each case.
  • the hot strips produced from these steels have a structure with ferrite grains whose diameter is regularly above 4 ⁇ m.
  • EP-A-0 969 112 A method comparable to the prior art according to EP-A-0 969 112 is known from EP-A-0 952 235.
  • a steel is processed, which in addition to other alloying elements (in wt .-%) 0.03 - 0.3% C, 0.5 - 3.5% Mn and / or 0.5 -3.5 Cr and optionally 0.5 to 3.0% Si.
  • the concrete examples given in EP-A-0 952 235 either have Si contents of more than 0.9% or contain no chromium. Insofar as the examples in question have a martensite content, this is below 5%.
  • the diameter of the ferrite grains is in each case above 5 ⁇ m.
  • a method is also known from JP 61-034116, with which hot strips can be produced with high strength.
  • a hot strip is hot rolled at a hot rolling end temperature of 700 - 850 ° C, then at a cooling rate of at least 5 ° C / s cooled and finally reeled at a less than 500 ° C to 200 ° C amounting reel temperature.
  • the tapes produced in this way achieve a minimum tensile strength of 700 MPa. Their deformability is low. Thus, they have yield strength ratios which are each above 0.75.
  • the object of the invention was therefore to provide a method with which can reliably produce hot strips, in which the combination of deformability and strength is further optimized.
  • a hot strip is obtained, the structure of which is formed substantially completely martensitic / ferritic and free of pearlite.
  • the hot strip has a microstructure whose ferrite grains are ultrafine. This is achieved by the interaction of the inventively given composition of the processed steel, the comparatively high hot rolling end temperature of 750 ° C to 900 ° C and the following on the hot rolling cooling to a low coiler temperature, the upper limit inventively set to 250 ° C and from this upper limit can reach to room temperature.
  • an average ferrite diameter is set for this purpose, which is as small as possible taking into account the respective required properties. It is in hot strips according to the invention, whose tensile strength is in the range of 700 MPa to 900 MPa, on average preferably at 2.5 microns. In contrast, in the case of higher-strength hot strips according to the invention, the average ferrite grain size is preferably set to less than 2 ⁇ m, in particular less than 1 ⁇ m, in order to ensure optimum deformability even at the high strengths.
  • niobium-containing hot strips produced by the process according to the invention show a significantly finer ferrite grain than hot strips produced in the same way and containing no niobium.
  • niobium in the steel composition used according to the invention has a favorable effect on the strength of the resulting hot strip. According to the invention it is therefore provided that in the steel used at least 0.03% by mass of niobium are present. The positive influences of niobium are particularly noticeable then noticeable when higher carbon steels are processed.
  • the strength of inventively produced hot strip is also decisively influenced by the height of its martensite content.
  • tensile strengths in the range from 700 MPa to 1000 MPa can be obtained.
  • hot strip according to the invention which has a tensile strength of up to 800 MPa, typically has a 60% to 90% ferritic microstructure.
  • the ferrite content is already limited to about 40% to 60%. If strengths of more than 900 MPa are required, the ferrite content is adjusted to 10% to about 45% in accordance with the invention.
  • the martensite content the desired strength can be produced in a targeted manner.
  • the good deformability on the one hand and the high strength on the other hand lead to a particularly good ratio of deformability and tensile strength of the hot strips produced according to the invention.
  • the yield ratio formed in the ratio of the yield strength to the tensile strength of hot strips produced according to the invention is regularly below 0.7, ie significantly below the value which can be ascertained in the hot strips produced by the known method.
  • Characteristic of the optimized combination of properties of inventive hot strips is also that their product of tensile strength (Rm) and elongation at break (AS) measured in the longitudinal direction of the resulting hot strip regularly at least 15,000 MPa *%.
  • hot strips produced according to the invention are particularly suitable for the production of thin-walled components with complex shapes, which nevertheless are capable of absorbing large forces.
  • sheets obtained from hot strips according to the invention can be used particularly well for the production of components and structural elements of vehicle bodies, which are able to withstand high loads at low wall thicknesses, correspondingly low weight and complex design.
  • the expression of ultrafine-grained microstructure inventively produced hot strips can be directly influenced.
  • cooling has a direct influence on the martensite content and, consequently, on the strength of the hot strips.
  • FIGS. 1 and 2 show micrographs of four hot strips produced according to different variants of the method according to the invention.
  • Diagrams 1 and 2 graphically show the results of the experiments described below.
  • the melts S1, S2 were high carbon steels. With the exception of the niobium contents, the alloys of both steels S1, S2 are essentially identical. However, steel S1 does not contain niobium, while steel S2 has niobium contents. On the basis of steels S1 and S2, it was thus possible to demonstrate which positive effects the presence of niobium has in a high-carbon steel alloy processed according to the invention.
  • the slabs were hot rolled in a rolling scale each to 3.5 mm thick hot strips. After leaving the hot rolling stand, the hot strips obtained have been cooled in two stages with interposed cooling pause controlled to the coiler temperature, before they have been coiled into coils. The cooling was carried out in the first section with a first cooling rate K1 up to a cooling stop temperature Ts, was then interrupted for a break with the duration P, before it has been continued in the second cooling section with a second cooling rate K2 to the coiling temperature HT.
  • the Abfihlungsstopptemperatur Ts was placed in the temperature range at which it comes to ferrite formation. In this way, in each case a specific ferrite content was deliberately set in the resulting hot strip.
  • test series V2 and V3 have been carried out.
  • the hot rolling end temperature was lower in the test series V2 than in the test series V1 and in the test series V3 lower than in the test series V2.
  • the hot rolling was finished at the same hot rolling end temperature as in the test series V2.
  • the hot rolling was not followed by a multi-stage cooling, but the cooling to reel temperature was carried out continuously with a single high cooling rate K1 in one go.
  • Fig. 1 the micrograph of a hot strip is shown, which consists of both a high carbon content and a content of niobium-containing melt S2 in the Series V4 with an uninterrupted cooling after hot rolling has been generated.
  • the average size of the ferrite grains determined by a conventional image analysis is 1 ⁇ m in this example.
  • FIG. 2 shows the micrograph of a hot strip produced from the low carbon content and a minimized silicon content melt S4 likewise produced in the test series V4.
  • the average size of the ferrite grains in this example is 1.6 ⁇ m.
  • the invention provides according to an advantageous embodiment, a range of cooling rate, ranging from 10 K / s to 1000 K / s.
  • the invention provides that with cooling rates of at least 50 K / s is working. Compliance with this minimum cooling rate ensures that the respectively desired fineness is always achieved.
  • test series V1 to V4 are in Diag. 1 again shown graphically.
  • FIG. 2 shows the tensile strength of the hot strips produced in accordance with the invention from the melts S1-S4 as a function of the ferrite content. It can be clearly seen that higher tensile strengths are achieved with decreasing ferrite contents in favor of increasing martensite contents. At the same time goes from Diag. 2 shows the potential of each alloy.
  • hot strips can be produced that are in the high tensile strength range of 900 MPa to 1100 MPa.
  • the low-carbon steel compositions S3 and S4 can be used to produce hot strips with tensile strengths of at least 700 MPa.
  • the alloying elements carbon, niobium and silicon are the most important control variables in this order, with which the material properties of the microstructures produced according to the invention can be determined via an ultrafine martensitic / ferritic microstructure with respect to the ferrite grains Allow hot strips to be adjusted.

Abstract

The following composition (wt. %) is cast into optionally flat ingots: C 0.05-2, Si up to 0.9, P up to 0.06, Mn 0.6-1.2, Al up to 0.05%, Cr 0.02-0.6, Nb up to 0.08%, Ti up to 0.08%, V up to 0.08%, Mo up to 0.4%, Cu up to 1%, Ni up to 1%, remainder Fe and inevitable impurities. The ingot is heated to hot-rolling temperature of 750-950 degrees C. The hot sheet is cooled to normal temperatures, up to 250 degrees C, at the rate of at least 10 K/s, for winding into rolls. An Independent claim is included for corresponding sheet.

Description

Die Erfindung betrifft ein Verfahren zum Erzeugen eines hochfesten Warmbands, das eine ultrafeine Kornstruktur besitzt.The invention relates to a method for producing a high-strength hot strip which has an ultrafine grain structure.

Ein Verfahren zum Herstellen von Warmbändern dieser Art ist aus der EP 1 001 041 Al bekannt. Gemäß dem bekannten Verfahren wird ein (in Masse-%) 0,01 - 0,3 % C, bis zu 2,0 % Si, bis zu 3,0 % Mn, nicht mehr als 0,05 % P, 0,03 - 0,3 % Ti und als Rest Eisen sowie unvermeidbare Verunreinigungen enthaltender Stahl zu Brammen vergossen. Nach der Wiedererwärmung auf nicht mehr als 1150 °C werden die Brammen in mehreren Schritten zu Warmband warmgewalzt. Das Warmwalzen wird bei Temperaturen durchgeführt, bei denen es zu einer dynamischen Rekristallisation des Stahls kommt. Gleichzeitig werden geringe Reduktionsgrade eingestellt, die im letzten Walzschritt 13 % bis 30 % und in allen dem letzten Schritt vorangehenden Walzschritten 4 % bis 20 % betragen sollen. Auf diese Weise soll die gewünscht feinkörnige austenitische Struktur des Warmbandes eingestellt werden können, bei der die Korndurchmesser des Austenits im Bereich von weniger als 4 µm, insbesondere weniger als 2 µm liegen. In möglichst unmittelbarem Anschluss an das Warmwalzen wird das erhaltene Warmband schnell auf eine 350 bis 600 °C betragende Temperatur abgekühlt, um die Bildung von feinkörnigem Ferrit zu bewirken. Die Abkühlrate beträgt dabei nicht weniger als 30 °C/s. Die Temperatur, mit der das Band anschließend gehaspelt wird, ist gemäß dem bekannten Verfahren an die für den jeweiligen Stahl günstigste Temperatur anzupassen.A method for producing hot strips of this type is known from EP 1 001 041 Al. According to the known method, a (in mass%) 0.01-0.3% C, up to 2.0% Si, up to 3.0% Mn, not more than 0.05% P, 0.03 - 0.3% Ti and the remainder iron and unavoidable impurities containing steel cast into slabs. After re-heating to not more than 1150 ° C, the slabs are hot rolled in several steps to hot strip. The hot rolling is carried out at temperatures at which dynamic recrystallization of the steel occurs. At the same time low reduction levels are set, which should be in the last rolling step 13% to 30% and in all the last step preceding rolling steps 4% to 20%. In this way, the desired fine-grained austenitic structure of the hot strip to be adjusted, in which the grain diameter of the austenite in the range of less than 4 .mu.m, in particular less than 2 microns. As close as possible to the hot rolling, the hot strip obtained is rapidly heated to 350 to 600 ° C temperature cooled to cause the formation of fine-grained ferrite. The cooling rate is not less than 30 ° C / s. The temperature at which the strip is subsequently coiled is, according to the known method, adapted to the most favorable temperature for the respective steel.

Der nach dem bekannten Verfahren erhaltene Stahl zeichnet sich durch eine gute Verformbarkeit bei gleichzeitig vergleichbar hoher Festigkeit aus. So werden bei den in der EP 1 001 041 A1 angegebenen Beispielen Zugfestigkeiten erreicht, die regelmäßig über 500 MPa liegen und bis zu 763 MPa betragen können. Gleichzeitig liegt die Fließgrenze im Bereich von mindestens 420 MPa bis 645 MPa. Das Streckgrenzenverhältnis ("yield ratio") des bekannten Stahls liegt regelmäßig im Bereich von mindestens 0,75.The steel obtained by the known method is characterized by a good deformability and at the same time comparably high strength. Thus, in the examples given in EP 1 001 041 A1, tensile strengths are achieved which regularly exceed 500 MPa and can be up to 763 MPa. At the same time, the yield point is in the range of at least 420 MPa to 645 MPa. The yield ratio of the known steel is regularly in the range of at least 0.75.

Ein anderes Verfahren zur Herstellung von Warmband mit einem sehr feinen Gefüge mit Ferritkorngrößen von weniger als 4 µm durch Einsatz der "dynamischen Rekristallisation" ist aus der US-A-4,466,842 bekannt. Der zu diesem Zweck verwendete Stahl weist als Pflichtbestandteile neben Eisen 0,02 - 0,3 % C und 0,1 - 2,0 % Mn auf. Zusätzlich kann er nicht näher bezeichnete Gehalte an Nb, Ta, Mo und W umfassen. Um basierend auf einem solchen Stahl FerritKorndurchmesser von weniger als 2,5 µm zu erhalten, ist eine sehr niedrige Endwalztemperatur von weniger als 760 °C und eine mittlere Haspeltemperatur von ca. 400 °C erforderlich.Another method for producing hot strip with a very fine structure with ferrite grain sizes of less than 4 microns by using the "dynamic recrystallization" is known from US-A-4,466,842. The steel used for this purpose has as compulsory components in addition to iron 0.02 - 0.3% C and 0.1 - 2.0% Mn. In addition, it may include unspecified levels of Nb, Ta, Mo, and W. In order to obtain ferrite grain diameters of less than 2.5 μm based on such steel, a very low finish rolling temperature of less than 760 ° C and a mean reeling temperature of about 400 ° C are required.

Ebenso unter Ausnutzung der Möglichkeiten der dynamischen Rekristallisation werden beim aus der EP-A-0 945 522 bekannten Verfahren Stahlbleche mit sehr feinem Gefüge hergestellt. Der dabei verarbeitete Stahl kann neben Eisen und 0,01 - 0,3 Gew.-% C und anderen Wahlkomponenten bis zu 1,0 Gew.-% Cr und bis zu 3,0 Gew.-% Mn enthalten. Die aus einer solchen Stahlzusammensetzung unter Anwendung einer nicht näher angegebenen Haspeltemperatur hergestellten Stahlbänder können unter anderem ein bainitisches, ein bainitisch/martensitisches, ein martensitisches, ein perlitisches oder ein perlitisch/martensitisches Gefüge aufweisen. Die derart erzeugten Warmbänder können dabei bei weiter Streuung Ferritkorngrößen von weniger als 3 µm aufweisen.Likewise, taking advantage of the possibilities of dynamic recrystallization, steel sheets having a very fine microstructure are produced in the process known from EP-A-0 945 522. In addition to iron and 0.01 - 0.3 wt .-% C and other optional components up to 1.0 wt% Cr and up to 3.0 wt% Mn. The steel strips produced from such a steel composition using a reeling temperature not specified may, inter alia, have a bainitic, a bainitic / martensitic, a martensitic, a pearlitic or a pearlitic / martensitic structure. The hot strips produced in this way can have ferrite grain sizes of less than 3 μm with further scattering.

Ein weiteres Verfahren zur Herstellung von Warmbändern, die ein hohes Umformvermögen und eine erhöhte Festigkeit aufweisen, ist aus der DE 199 11 287 Cl bekannt. Gemäß diesem Verfahren wird ein Warmband (W), welches insbesondere aus Strangguß in Form von wiedererwärmten oder direkt aus der Gießhitze eingesetzten Brammen, Dünnbrammen oder aus gegossenem Band kontinuierlich fertiggewalzt und anschließend kontinuierlich abgekühlt. Das Abkühlen erfolgt dabei in mindestens zwei aufeinander folgenden Kühlphasen (tCK,tLK) beschleunigter Kühlung auf eine Endtemperatur. Die erste Kühlphase (tCK) beschleunigter Kühlung setzt spätestens drei Sekunden nach dem letzten Walzstich des Fertigwalzens ein und das Warmband (W) wird während der ersten Kühlphase (tCK) beschleunigter Kühlung mit einer Abkühlgeschwindigkeit von mindestens 150 °C/s gekühlt. Wird gemäß diesem Verfahren ein TRIP-Stahl mit 0,2 Masse-% C, 1,8 Masse-% Al und 1,6 Masse-% Mn, Rest Eisen und unvermeidbaren verunreinigungen verarbeitet, so besitzt dieser Stahl ein feines Gefüge mit Ferritkörnern, deren Durchmesser weniger als 3 µm beträgt.Another method for the production of hot strips, which have a high formability and increased strength, is known from DE 199 11 287 C1. According to this method, a hot strip (W), which in particular continuously cast finished in the form of reheated or used directly from the casting heat slabs, thin slabs or cast strip and then continuously cooled. The cooling takes place in at least two consecutive cooling phases (t CK , t LK ) accelerated cooling to a final temperature. The first cooling phase (t CK ) of accelerated cooling sets in at the latest three seconds after the final rolling pass of finish rolling and the hot strip (W) is cooled during the first cooling phase (t CK ) accelerated cooling at a cooling rate of at least 150 ° C / s. When a TRIP steel containing 0.2% by mass of C, 1.8% by mass of Al and 1.6% by mass of Mn, the remainder of iron and unavoidable impurities is processed in accordance with this process, this steel has a fine microstructure with ferrite grains, whose diameter is less than 3 microns.

In der EP-A-0 969 112 ist ein Verfahren zur Herstellung eines Dualphasen-Stahls beschrieben, bei dem ein Stahl, der neben Eisen und unvermeidbaren Verunreinigungen als Pflichtbestandteile (in Gew.-%) 0,02 - 0,25 % C und zusätzlich Mn und/oder Cr in Gehalten von jeweils 0,15 - 3,5 %, Si, Al und / oder P in Gehalten von insgesamt 0,02 - 4,0 % sowie weitere wahlbestandteile enthalten kann, bei einer Warmwalzendtemperatur von 780 - 880 °C warmgewalzt und dann mit einer Abkühlgeschwindigkeit von mehr als 5 °C/s auf Haspeltemperaturen abgekühlt, die unterhalb von 350 °C liegen. Die in der EP-A-0 969 112 zu diesem Verfahren angegebenen Ausführungsbeispiele zeigen jedoch, dass dort jeweils hohe Si-Gehalte, hohe Mn-Gehalte und, soweit vorhanden, hohe Cr-Gehalte verarbeitet worden sind. Die aus diesen Stählen erzeugten Warmbänder weisen ein Gefüge mit Ferritkörnern auf, deren Durchmesser regelmäßig oberhalb von 4 µm liegt.EP-A-0 969 112 describes a process for producing a dual-phase steel in which a steel containing, in addition to iron and unavoidable impurities, as Compulsory constituents (in% by weight) 0.02-0.25% C and in addition Mn and / or Cr in respective amounts of 0.15-3.5%, Si, Al and / or P in total contents of 0, 02 - 4.0% and other optional constituents, hot rolled at a hot rolling end temperature of 780 - 880 ° C and then cooled at a cooling rate of more than 5 ° C / s to reel temperatures below 350 ° C. However, the exemplary embodiments specified in EP-A-0 969 112 for this process show that high Si contents, high Mn contents and, if present, high Cr contents have been processed there in each case. The hot strips produced from these steels have a structure with ferrite grains whose diameter is regularly above 4 μm.

Ein mit dem Stand der Technik gemäß der EP-A-0 969 112 vergleichbares Verfahren ist aus der EP-A-0 952 235 bekannt. Bei diesem bekannten Verfahren wird ein Stahl verarbeitet, der neben anderen Legierungselementen (in Gew.-%) 0,03 - 0,3 % C, 0,5 - 3,5 % Mn und / oder 0,5 -3,5 Cr sowie wahlweise 0,5 - 3,0 % Si aufweisen kann. Die in der EP-A-0 952 235 konkret angegebenen Ausführungsbeispiele weisen entweder Si-Gehalte von mehr als 0,9 % auf oder enthalten kein Chrom. Soweit die betreffenden Beispiele einen Martensitgehalt aufweisen, liegt dieser jeweils unterhalb von 5 %. Gleichzeitig liegt der Durchmesser der Ferritkörner jeweils oberhalb von 5 µm.A method comparable to the prior art according to EP-A-0 969 112 is known from EP-A-0 952 235. In this known method, a steel is processed, which in addition to other alloying elements (in wt .-%) 0.03 - 0.3% C, 0.5 - 3.5% Mn and / or 0.5 -3.5 Cr and optionally 0.5 to 3.0% Si. The concrete examples given in EP-A-0 952 235 either have Si contents of more than 0.9% or contain no chromium. Insofar as the examples in question have a martensite content, this is below 5%. At the same time, the diameter of the ferrite grains is in each case above 5 μm.

Schließlich ist auch aus der JP 61-034116 ein Verfahren bekannt, mit dem sich Warmbänder mit hoher Festigkeit herstellen lassen. Dazu wird ein Warmband bei einer Warmwalzendtemperatur von 700 - 850 °C warmgewalzt, dann bei einer Abkühlgeschwindigkeit von mindestens 5 °C/s abgekühlt und schließlich bei einer weniger als 500 °C bis 200 °C betragenden Haspeltemperatur gehaspelt. Die so erzeugten Bänder erreichen eine Mindestzugfestigkeit von 700 MPa. Ihre Verformbarkeit ist dabei jedoch gering. So weisen sie Streckgrenzenverhältnisse auf, die jeweils oberhalb von 0,75 liegen.Finally, a method is also known from JP 61-034116, with which hot strips can be produced with high strength. For this purpose, a hot strip is hot rolled at a hot rolling end temperature of 700 - 850 ° C, then at a cooling rate of at least 5 ° C / s cooled and finally reeled at a less than 500 ° C to 200 ° C amounting reel temperature. The tapes produced in this way achieve a minimum tensile strength of 700 MPa. Their deformability is low. Thus, they have yield strength ratios which are each above 0.75.

Trotz der verbesserten Eigenschaften der nach den voranstehend erläuterten bekannten Verfahren erzeugten Bänder werden immer höhere Anforderungen von den Verarbeitern von warmbändern der in Rede stehenden Art an die Verformbarkeit einerseits und die Zugfestigkeit andererseits gestellt.Despite the improved properties of the tapes produced according to the above-described known methods, ever greater demands are placed on the formability on the one hand and the tensile strength on the other hand by the processors of hot tapes of the type in question.

Die Aufgabe der Erfindung bestand daher darin, ein Verfahren anzugeben, mit dem sich zuverlässig Warmbänder erzeugen lassen, bei welchen die Kombination aus Verformbarkeit und Festigkeit weiter optimiert ist.The object of the invention was therefore to provide a method with which can reliably produce hot strips, in which the combination of deformability and strength is further optimized.

Diese Aufgabe wird durch ein Verfahren zum Herstellen eines Warmbands mit einer Zugfestigkeit von mindestens 700 MPa, mit Streckgrenzverhältnissen, die für die Längsrichtung höchstens 0,65 und für die Querrichtung höchstens 0,66 betragen, und mit einer ultrafeinen ferritisch/martensitischen und perlitfreien Kornstruktur, die einen Anteil an Martensit von mindestens 15 % aufweist und bei der der durchschnittliche Durchmesser der Ferritkörner weniger als 2,5 µm beträgt, gelöst, welches folgende Schritte umfasst:

  • a) Vergießen einer (in Masse-%) C: 0,05 - 0,2 %,
    Si: < 0,9 %, P: < 0,06 %, Mn: 0,6 - 1,2 %,
    Al: < 0,05 %, Cr: 0,02 - 0,6 %, Nb: ≤ 0,08 %, Ti: ≤
    0,08 %, V: ≤ 0,08 %, Mo: ≤ 0,4 %, Cu: ≤ 1 %, Ni: ≤ 1 %, Rest Eisen und unvermeidbare Verunreinigungen enthaltenden Stahlschmelze zu einem Vormaterial, wie Brammen oder Dünnbrammen,
  • b) Warmwalzen des Vormaterials zu einem Warmband bei einer 750 °C bis 950 °C betragenden Warmwalzendtemperatur,
  • c) Kühlen des erhaltenen Warmbands auf eine Raumtemperatur bis 250 °C betragenden Haspeltemperatur mit einer mindestens 10 K/s betragenden Abkühlgeschwindigkeit,
  • d) Haspeln des abgekühlten Warmbands.
This object is achieved by a process for producing a hot strip with a tensile strength of at least 700 MPa, with yield ratios which are at most 0.65 for the longitudinal direction and at most 0.66 for the transverse direction, and with an ultrafine ferrite / martensite and pearlite-free grain structure, which has a martensite content of at least 15% and in which the average diameter of the ferrite grains is less than 2.5 μm, comprising the following steps:
  • a) casting one (in mass%) C: 0.05-0.2%,
    Si: <0.9%, P: <0.06%, Mn: 0.6 - 1.2%,
    Al: <0.05%, Cr: 0.02-0.6%, Nb: ≤ 0.08%, Ti: ≤
    0.08%, V: ≦ 0.08%, Mo: ≦ 0.4%, Cu: ≦ 1%, Ni: ≦ 1 %, Remainder containing iron and unavoidable impurities molten steel to a starting material, such as slabs or thin slabs,
  • (b) hot rolling the primary material into a hot strip at a hot rolling end temperature of 750 ° C to 950 ° C;
  • c) cooling the resulting hot strip to a coil temperature of up to 250 ° C. at room temperature with a cooling rate of at least 10 K / s,
  • d) reeling the cooled hot strip.

Mit der Erfindung wird ein warmband erhalten, dessen Gefüge im Wesentlichen vollständig martensitisch/ferritisch und frei von Perlit ausgebildet ist. Das Warmband weist dabei ein Gefüge auf, dessen Ferritkörner ultrafein sind. Dies wird durch das Zusammenwirken der erfindungsgemäß vorgegebenen Zusammensetzung des verarbeiteten Stahls, der vergleichsweise hohen Warmwalzendtemperatur von 750 °C bis 900 °C und der auf das Warmwalzen folgenden Abkühlung auf eine niedrige Haspeltemperatur erreicht, deren Obergrenze erfindungsgemäß auf 250 °C festgelegt und von dieser Obergrenze bis zur Raumtemperatur reichen kann.With the invention, a hot strip is obtained, the structure of which is formed substantially completely martensitic / ferritic and free of pearlite. The hot strip has a microstructure whose ferrite grains are ultrafine. This is achieved by the interaction of the inventively given composition of the processed steel, the comparatively high hot rolling end temperature of 750 ° C to 900 ° C and the following on the hot rolling cooling to a low coiler temperature, the upper limit inventively set to 250 ° C and from this upper limit can reach to room temperature.

Aufwändige Walzverfahren, wie sie beim Stand der Technik erforderlich sind, um die erwünschte Feinkörnigkeit des Gefüges zu erhalten, sind beim erfindungsgemäßen Verfahren nicht mehr erforderlich. Vielmehr wirken die einzelnen Parameter der Zusammensetzung des Stahls und des Verfahrens so zusammen, dass ein Warmband mit extrem feinem Gefüge erhalten wird. So lässt sich das erfindungsgemäße Verfahren sowohl über die eine konventionelle Brammenerzeugung umfassende Warmbandstraßenroute als auch unter Einbeziehung einer Gießwalzanlage durchführen, in der als Vormaterial Dünnbrammen erzeugt werden die anschließend "in line" in einem kontinuierlichen Prozess zu Warmband verarbeitet werden.Elaborate rolling processes, as required in the prior art in order to obtain the desired fineness of the structure, are no longer required in the process according to the invention. Rather, the individual parameters of the composition of the steel and of the process cooperate in such a way that a hot strip with an extremely fine structure is obtained. Thus, the method according to the invention can be using both a conventional slab production hot strip mill route and involving a casting mill, in which as a starting material thin slabs are produced which are then processed "in line" in a continuous process to hot strip.

Das extrem feine Gefüge erfindungsgemäß erzeugter Warmbänder führt zu deren besonders guten Dehnungskennwerten. Erfindungsgemäß wird dazu ein durchschnittlicher Ferritkorndurchmesser eingestellt, der unter Berücksichtigung der jeweils geforderten Eigenschaften möglichst gering ist. Er liegt bei erfindungsgemäßen Warmbändern, deren Zugfestigkeit im Bereich von 700 MPa bis 900 MPa liegt, im Mittel bevorzugt bei 2,5 µm. Bei höherfesten erfindungsgemäßen Warmbändern wird die durchschnittliche Ferritkorngröße dagegen bevorzugt auf unter 2 µm, insbesondere unter 1 µm, eingestellt, um auch bei den hohen Festigkeiten eine optimale Verformbarkeit zu gewährleisten.The extremely fine structure according to the invention produced hot strips leads to their particularly good elongation characteristics. According to the invention, an average ferrite diameter is set for this purpose, which is as small as possible taking into account the respective required properties. It is in hot strips according to the invention, whose tensile strength is in the range of 700 MPa to 900 MPa, on average preferably at 2.5 microns. In contrast, in the case of higher-strength hot strips according to the invention, the average ferrite grain size is preferably set to less than 2 μm, in particular less than 1 μm, in order to ensure optimum deformability even at the high strengths.

Wesentlichen Einfluss auf die Ausbildung derart ultrafeiner Ferritkörner im Gefüge erfindungsgemäß erzeugter Warmbänder hat die Anwesenheit von Niob. So zeigen nach dem erfindungsgemäßen Verfahren hergestellte, Niob enthaltende Warmbänder ein deutlich feineres Ferritkorn als in gleicher Weise erzeugte Warmbänder, die kein Niob enthalten.Substantial influence on the formation of such ultrafine ferrite grains in the microstructure of hot strips produced according to the invention has the presence of niobium. Thus, niobium-containing hot strips produced by the process according to the invention show a significantly finer ferrite grain than hot strips produced in the same way and containing no niobium.

Gleichzeitig wirkt sich die Anwesenheit von Niob in der erfindungsgemäß verwendeten Stahlzusammensetzung günstig auf die Festigkeit des erhaltenen Warmbands aus. Erfindungsgemäß ist daher vorgesehen, dass im verwendete Stahl mindestens 0,03 Masse-% Niob vorhanden sind. Die positiven Einflüsse von Niob machen sich dabei insbesondere dann bemerkbar, wenn höher kohlenstoffhaltige Stähle verarbeitet werden.At the same time, the presence of niobium in the steel composition used according to the invention has a favorable effect on the strength of the resulting hot strip. According to the invention it is therefore provided that in the steel used at least 0.03% by mass of niobium are present. The positive influences of niobium are particularly noticeable then noticeable when higher carbon steels are processed.

Entscheidend beeinflusst wird die Festigkeit erfindungsgemäß erzeugten Warmbands auch durch die Höhe seines Martensitgehalts. Durch eine Erhöhung der Martensitanteile bei gleichzeitiger Verminderung der Ferritanteile lassen sich Zugfestigkeiten im Bereich von 700 MPa bis 1000 MPa erhalten. So weist erfindungsgemäßes Warmband, das eine Zugfestigkeit von bis zu 800 MPa besitzt, typischerweise ein zu 60 % bis 90 % ferritisches Gefüge auf. Bei Zugfestigkeiten, die im Bereich von 800 - 900 MPa liegen, ist der Ferritgehalt dagegen schon auf ca. 40 % bis 60 % beschränkt. Werden Festigkeiten von mehr als 900 MPa benötigt, so wird dazu der Ferritgehalt erfindungsgemäß auf 10 % bis ca. 45 % eingestellt. Über den Martensitgehalt kann so die gewünschte Festigkeit gezielt hergestellt werden.The strength of inventively produced hot strip is also decisively influenced by the height of its martensite content. By increasing the martensite proportions while at the same time reducing the ferrite fractions, tensile strengths in the range from 700 MPa to 1000 MPa can be obtained. Thus, hot strip according to the invention, which has a tensile strength of up to 800 MPa, typically has a 60% to 90% ferritic microstructure. With tensile strengths which are in the range of 800-900 MPa, however, the ferrite content is already limited to about 40% to 60%. If strengths of more than 900 MPa are required, the ferrite content is adjusted to 10% to about 45% in accordance with the invention. By means of the martensite content, the desired strength can be produced in a targeted manner.

Die gute Verformbarkeit einerseits und die hohen Festigkeiten andererseits führen zu einem besonders guten Verhältnis von Verformbarkeit und Zugfestigkeit der erfindungsgemäß erzeugten Warmbänder. Dementsprechend liegt das als Verhältnis von Dehngrenze zur Zugfestigkeit gebildete Streckgrenzverhältnis bei erfindungsgemäß erzeugten Warmbändern regelmäßig unter 0,7, also deutlich unter dem Wert, der sich bei den nach dem bekannten Verfahren erzeugten Warmbändern feststellen lässt. Kennzeichnend für die optimierte Eigenschaftskombination erfindungsgemäßer Warmbänder ist ebenfalls, dass ihr Produkt aus Zugfestigkeit (Rm) und Bruchdehnung (AS) in Längsrichtung des erhaltenen Warmbands gemessen regelmäßig mindestens 15.000 MPa*% beträgt.The good deformability on the one hand and the high strength on the other hand lead to a particularly good ratio of deformability and tensile strength of the hot strips produced according to the invention. Accordingly, the yield ratio formed in the ratio of the yield strength to the tensile strength of hot strips produced according to the invention is regularly below 0.7, ie significantly below the value which can be ascertained in the hot strips produced by the known method. Characteristic of the optimized combination of properties of inventive hot strips is also that their product of tensile strength (Rm) and elongation at break (AS) measured in the longitudinal direction of the resulting hot strip regularly at least 15,000 MPa *%.

Aufgrund dieser besonderen Eigenschaftskombination eignen sich erfindungsgemäß erzeugte Warmbänder in besonderer Weise zur Herstellung von dünnwandigen Bauelementen mit komplexer Formgebung, die dennoch in der Lage sind, große Kräfte aufzunehmen. So lassen sich aus erfindungsgemäßen Warmbändern gewonnene Bleche besonders gut zur Herstellung von Bauteilen und Strukturelementen von Fahrzeugkarosserien einsetzen, die bei geringen Wandstärken, dementsprechend geringem Gewicht und aufwändiger Gestaltung in der Lage sind, hohe Belastungen aufzunehmen.Due to this special combination of properties, hot strips produced according to the invention are particularly suitable for the production of thin-walled components with complex shapes, which nevertheless are capable of absorbing large forces. Thus, sheets obtained from hot strips according to the invention can be used particularly well for the production of components and structural elements of vehicle bodies, which are able to withstand high loads at low wall thicknesses, correspondingly low weight and complex design.

Über die Art und Weise, in der das Warmband nach dem Verlassen der Fertigwarmwalzstaffel abgekühlt wird, lässt sich die Ausprägung des ultrafeinkörnigen Gefüges erfindungsgemäß erzeugter Warmbänder direkt beeinflussen. Gleichzeitig hat die Abkühlung unmittelbaren Einfluss auf den Martensitanteil und damit einhergehend auf die Festigkeit der Warmbänder.About the way in which the hot strip is cooled after leaving the Fertigwarmwalzstaffel, the expression of ultrafine-grained microstructure inventively produced hot strips can be directly influenced. At the same time, cooling has a direct influence on the martensite content and, consequently, on the strength of the hot strips.

Höchste Festigkeiten bei gleichzeitig feinster Kornausbildung werden erreicht, wenn die Abkühlung des Warmbands auf Haspeltemperatur in einem Schritt unterbrechungsfrei erfolgt. Soll dagegen ein Warmband erzeugt werden, das im Rahmen der Erfindung ein höheres Streckgrenzverhältnis besitzt, kann dies dadurch erreicht werden, dass das Warmband beim Abkühlen auf Haspeltemperatur zunächst mit einer 10 bis 200 K/s betragenden Abkühlgeschwindigkeit auf eine 600 - 700 °C betragende Zwischentemperatur abgekühlt wird, dass daraufhin die Abkühlung für 2 bis 6 Sekunden unterbrochen wird und dass das Warmband anschließend mit einer 10 bis 200 K/s betragenden Abkühlgeschwindigkeit auf die Haspeltemperatur gekühlt wird. Die Wahl eines geeigneten Verfahrens zur Abkühlung des nach dem Warmwalzen erhaltenen Warmbands auf die niedrige Haspeltemperatur ermöglicht so auf einfache Weise die optimale Anpassung der jeweiligen Eigenschaften des erfindungsgemäß erzeugten Warmbands an die vom Verwender gestellten Anforderungen.Highest strengths and at the same time the finest grain formation are achieved, if the cooling of the hot strip to coiling temperature takes place without interruption in one step. If, on the other hand, a hot strip is to be produced which has a higher yield ratio in the context of the invention, this can be achieved by first cooling the hot strip to a coiler temperature of from 10 to 200 K / s to a coil temperature of 600 to 700 ° C. during cooling to reel temperature is cooled, that then the cooling is interrupted for 2 to 6 seconds and that the hot strip is then cooled with a 10 to 200 K / s cooling rate to the coiler temperature. The choice of a suitable method for cooling the obtained after hot rolling Warmbands on the low reel temperature thus enables in a simple manner the optimal adaptation of the respective properties of the hot strip according to the invention to the requirements imposed by the user.

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

Die Figuren 1 und 2 zeigen Schliffbilder von vier nach unterschiedlichen Varianten des erfindungsgemäßen Verfahrens erzeugten Warmbändern.FIGS. 1 and 2 show micrographs of four hot strips produced according to different variants of the method according to the invention.

In den Diagrammen 1 und 2 sind die Ergebnisse der nachfolgend erläuterten Versuche graphisch dargestellt.Diagrams 1 and 2 graphically show the results of the experiments described below.

Es wurden zahlreiche Laborwalzversuche durchgeführt, bei denen vier Stahlschmelzen S1, S2, S3, S4 verwendet wurden, deren Zusammensetzungen in Tabelle 1 angegeben sind. Tabelle 1 C Mn P S Si Al N Cr Nb S1 0,188 1,05 0,004 0,004 0,82 0,030 0,0031 0,42 - S2 0,181 1,02 0,004 0,003 0,81 0,032 0,0022 0,42 0,047 S3 0,077 1,02 0,005 0,003 0,81 0,030 0,0020 0,41 0,045 S4 0,073 1,02 0,032 0,003 0,11 0,025 0,0022 0,41 0,057 Rest Eisen und unvermeidbare Verunreinigungen, Angaben in Masse-%, Numerous laboratory rolling trials were carried out using four steel melts S1, S2, S3, S4 whose compositions are given in Table 1. Table 1 C Mn P S Si al N Cr Nb S1 0.188 1.05 0,004 0,004 0.82 0,030 0.0031 0.42 - S2 0,181 1.02 0,004 0,003 0.81 0.032 0.0022 0.42 0.047 S3 0.077 1.02 0.005 0,003 0.81 0,030 0.0020 0.41 0,045 S4 0.073 1.02 0.032 0,003 0.11 0,025 0.0022 0.41 0.057 Remainder iron and unavoidable impurities, data in% by mass,

Bei den Schmelzen S1, S2 handelte es sich um Stähle mit hohem Kohlenstoffgehalt. Mit Ausnahme der Niob-Gehalte sind die Legierungen beider Stähle S1, S2 im Wesentlichen identisch. Der Stahl S1 enthält jedoch kein Niob, während der Stahl S2 Gehalte an Niob aufweist. Anhand der Stähle S1 und S2 konnte somit nachgewiesen werden, welche positiven Wirkungen die Anwesenheit von Niob in einer erfindungsgemäß verarbeiteten, höherkohlenstoffhaltigen Stahllegierung hat.The melts S1, S2 were high carbon steels. With the exception of the niobium contents, the alloys of both steels S1, S2 are essentially identical. However, steel S1 does not contain niobium, while steel S2 has niobium contents. On the basis of steels S1 and S2, it was thus possible to demonstrate which positive effects the presence of niobium has in a high-carbon steel alloy processed according to the invention.

In vergleichbarer Weise ist die Wirkung von Silizium anhand der Stähle S3 und S4 nachgewiesen worden. Bis auf den Si-Gehalt sind deren Zusammensetzungen im Wesentlichen gleich. Beim Stahl S3 ist jedoch der Si-Gehalt auf ein Minimum reduziert, während der Stahl S4 nennenswerte Gehalte an Silizium aufweist.In a similar way, the effect of silicon has been demonstrated using steels S3 and S4. Except for the Si content, their compositions are substantially the same. However, in the case of steel S3, the Si content is reduced to a minimum, while steel S4 has appreciable silicon contents.

Aus den Stählen S1 - S4 gegossene Brammen sind in vier unterschiedlichen Versuchsreihen V1 - V4 jeweils zu Warmband verarbeitet worden.Slabs cast from steels S1 - S4 have been processed to hot strip in four different test series V1 - V4.

In der Versuchsreihe V1 sind die Brammen in einer Walzstaffel jeweils zu 3,5 mm dicken Warmbändern warmgewalzt worden. Nach dem Verlassen der Warmwalzstaffel sind die erhaltenen Warmbänder in zwei Stufen mit zwischengeschalteter Kühlpause gesteuert auf die Haspeltemperatur abgekühlt worden, bevor sie zu Coils gehaspelt worden sind. Die Kühlung erfolgte im ersten Abschnitt mit einer ersten Abkühlgeschwindigkeit K1 bis zu einer Abkühlungsstopptemperatur Ts, wurde dann für eine Pause mit der Dauer P unterbrochen, bevor sie im zweiten Kühlabschnitt mit einer zweiten Abkühlgeschwindigkeit K2 bis auf die Haspeltemperatur HT fortgesetzt worden ist.In the test series V1, the slabs were hot rolled in a rolling scale each to 3.5 mm thick hot strips. After leaving the hot rolling stand, the hot strips obtained have been cooled in two stages with interposed cooling pause controlled to the coiler temperature, before they have been coiled into coils. The cooling was carried out in the first section with a first cooling rate K1 up to a cooling stop temperature Ts, was then interrupted for a break with the duration P, before it has been continued in the second cooling section with a second cooling rate K2 to the coiling temperature HT.

Die Abktihlungsstopptemperatur Ts wurde in den Temperaturbereich gelegt, bei dem es zur Ferritbildung kommt. Auf diese Weise wurde jeweils gezielt ein bestimmter Ferritgehalt in dem erhaltenen Warmband eingestellt.The Abfihlungsstopptemperatur Ts was placed in the temperature range at which it comes to ferrite formation. In this way, in each case a specific ferrite content was deliberately set in the resulting hot strip.

In gleicher Weise sind die Versuchsreihen V2 und V3 durchgeführt worden. Allerdings lag die Warmwalzendtemperatur bei der versuchsreihe V2 niedriger als bei der versuchsreihe V1 und bei der Versuchsreihe V3 niedriger als bei der Versuchsreihe V2.In the same way, the test series V2 and V3 have been carried out. However, the hot rolling end temperature was lower in the test series V2 than in the test series V1 and in the test series V3 lower than in the test series V2.

Bei der Versuchsreihe V4 ist das Warmwalzen bei der gleichen Warmwalzendtemperatur beendet worden wie in der Versuchsreihe V2. Allerdings schloss sich an das Warmwalzen keine mehrstufige Abkühlung an, sondern die Abkühlung auf Haspeltemperatur erfolgte mit einer einzigen hohen Abkühlgeschwindigkeit K1 kontinuierlich in einem Zuge.In the test series V4, the hot rolling was finished at the same hot rolling end temperature as in the test series V2. However, the hot rolling was not followed by a multi-stage cooling, but the cooling to reel temperature was carried out continuously with a single high cooling rate K1 in one go.

Die bei den Versuchsreihen V1 - V4 jeweils eingehaltenen Betriebsparameter "Warmwalzendtemperatur ET", "Abkühlgeschwindigkeit K1", "Abkühlungsstopptemperatur Ts", "Pausendauer P", "Abkühlgeschwindigkeit K2" und "Haspeltemperatur HT" und "Warmbanddicke d" sind in Tabelle 2 zusammengestellt. Tabelle 2 ET Kl Ts P K2 HT mm °C K/s °C s K/s °C mm V1 860 80 660 3 80 RT 3,5 V2 820 80 660 3 80 RT 3,5 v3 780 80 660 3 80 RT 3,5 V4 820 200 Abkühlung in einem Zug RT 3,5 The operating parameters "Hot rolling end temperature ET", "Cooling speed K1", "Cooling stop temperature Ts", "Pause duration P", "Cooling speed K2" and "Coiler temperature HT" and "Hot strip thickness d" respectively observed in the test series V1-V4 are summarized in Table 2. Table 2 ET kl ts P K2 HT mm ° C K / s ° C s K / s ° C mm V1 860 80 660 3 80 RT 3.5 V2 820 80 660 3 80 RT 3.5 v3 780 80 660 3 80 RT 3.5 V4 820 200 Cool down in one go RT 3.5

Die in "Längsrichtung L" und "Querrichtung Q" ermittelten mechanischen Eigenschaften "Dehngrenze Rp0,2", Zugfestigkeit Rn", " Streckgrenzverhältnis Rp0,2/Rm", "Gleichmaßdehnung Ag", "Dehnung AS", "Produkt aus Zugfestigkeit Rm und Dehnung A5 Rm*A5" sowie die jeweiligen Anteile des Gefüges an "Ferrit FA" "Bainit BA", "Martensit MA", "Restaustenit RA" und der jeweils ermittelte durchschnittliche "Ferritkorndurchmesser FØ" der in den Versuchsreihen V1 - V4 erhaltenen Warmbänder sind in den Tabellen 3a bis 3d angegeben.The mechanical properties determined in "longitudinal direction L" and "transverse direction Q""yield strength R p0.2 ", tensile strength R n "," yield ratio R p0.2 / R m "," uniform elongation A g "," elongation A S ", "Product of tensile strength R m and elongation A 5 R m * A 5 " and the respective fractions of the microstructure on "ferrite FA""bainiteBA","martensiteMA","residual austenite RA" and the respectively determined average "ferrite grain diameter FØ" The hot strips obtained in test series V1 - V4 are given in Tables 3a to 3d.

In Fig. 1 ist das Schliffbild eines Warmbands dargestellt, das aus der sowohl einen hohen Kohlenstoffgehalt als auch einen Gehalt an Niob aufweisenden Schmelze S2 in der Versuchsreihe V4 mit einer unterbrechungsfreien Abkühlung nach dem Warmwalzen erzeugt worden ist. Die durch eine konventionelle Bildanalyse ermittelte durchschnittliche Größe der Ferritkörner beträgt bei diesem Beispiel 1 µm.In Fig. 1, the micrograph of a hot strip is shown, which consists of both a high carbon content and a content of niobium-containing melt S2 in the Series V4 with an uninterrupted cooling after hot rolling has been generated. The average size of the ferrite grains determined by a conventional image analysis is 1 μm in this example.

Fig. 2 zeigt das Schliffbild eines aus der einen niedrigen Kohlenstoffgehalt und einen minimierten Siliziumgehalt aufweisenden Schmelze S4 ebenfalls in der Versuchsreihe V4 erzeugten Warmbands. Die durchschnittliche Größe der Ferritkörner beträgt bei diesem Beispiel 1,6 µm.FIG. 2 shows the micrograph of a hot strip produced from the low carbon content and a minimized silicon content melt S4 likewise produced in the test series V4. The average size of the ferrite grains in this example is 1.6 μm.

Bei einem Warmband, das aus der einen hohen Kohlenstoffgehalt jedoch keinen Niobanteil besitzenden Schmelze S1 in der Versuchsreihe V2 (Abkühlung mit Kühlpause) erzeugt worden ist, betrug die durchschnittliche Größe der Ferritkörner 2,6 µm.In the case of a hot strip which was produced from the melt S1 having a high carbon content but no niobium content in the experimental series V2 (cooling with cooling pause), the average size of the ferrite grains was 2.6 μm.

Bei einem Warmbands, das aus der einen niedrigen Kohlenstoffgehalt bei einem erhöhten Siliziumgehalt aufweisenden Schmelze S3 in der Versuchsreihe V1 hergestellt worden ist, konnte eine durchschnittliche Größe der Ferritkörner von 2 µm ermittelt werden.For a hot strip made from the low carbon content of the high-silica melt S3 in the experimental series V1, an average size of the ferrite grains of 2 μm could be determined.

Es zeigte sich, dass die feinsten Gefüge und größten Festigkeiten bei den Warmbändern vorlagen, die in der Versuchsreihe V4 ausgehend von der Warmwalzendtemperatur in einem Zuge mit hoher Abkühlgeschwindigkeit auf die Haspeltemperatur HT abgekühlt worden sind. Eine weitere Verfeinerung des Gefüges stellt sich ein, wenn noch höhere Abkühlgeschwindigkeiten eingestellt werden. Dementsprechend sieht die Erfindung gemäß einer vorteilhaften Ausgestaltung einen Bereich der Abkühlgeschwindigkeit vor, der von 10 K/s bis 1000 K/s reicht. Insbesondere sieht die Erfindung vor, dass mit Abkühlgeschwindigkeiten von mindestens 50 K/s gearbeitet wird. Die Einhaltung dieser Mindestkühlrate stellt sicher, dass die jeweils gewünschte Feinkörnigkeit immer erreicht wird.It was found that the finest microstructures and greatest strengths were in the hot strips, which have been cooled in the test series V4 from the hot rolling end temperature in a course with high cooling rate to the coiling temperature HT. Further refinement of the microstructure occurs when even higher cooling rates are set. Accordingly, the invention provides according to an advantageous embodiment, a range of cooling rate, ranging from 10 K / s to 1000 K / s. In particular, the invention provides that with cooling rates of at least 50 K / s is working. Compliance with this minimum cooling rate ensures that the respectively desired fineness is always achieved.

Gleichzeitig geht aus den Versuchsergebnissen der günstige Einfluss der Gehalte an Silizium und Niob auf die Feinkörnigkeit des Gefüges hervor. Dies wird insbesondere durch die Fig. 1 und 2 belegt.At the same time, the favorable results of the contents of silicon and niobium on the fine grain of the microstructure emerge from the test results. This is especially proven by FIGS. 1 and 2.

Die Ergebnisse der Versuchsreihen V1 bis V4 sind im Diag. 1 noch einmal graphisch dargestellt.The results of the test series V1 to V4 are in Diag. 1 again shown graphically.

Im Diag. 2 ist die Zugfestigkeit der in erfindungsgemäßer Weise aus den Schmelzen S1 - S4 erzeugten Warmbänder in Abhängigkeit vom Ferritgehalt dargestellt. Es ist klar zu erkennen, dass mit zu Gunsten von steigenden Martensitgehalten sinkenden Ferritgehalten höhere Zugfestigkeiten erreicht werden. Gleichzeitig geht aus Diag. 2 das Potential der einzelnen Legierung hervor. So lassen sich aus den hochkohlenstoffhaltigen Schmelzen S1 und S2 sicher Warmbänder erzeugen, die im hohen Zugfestigkeitsbereich von 900 MPa bis 1100 MPa liegen. Mit den niedrigen Kohlenstoffgehalte aufweisenden Stahlzusammensetzungen S3 und S4 dagegen lassen sich Warmbänder mit Zugfestigkeiten von mindestens 700 MPa herstellen. Damit ist klar, dass neben den Verfahrensparametern "Kühlgeschwindigkeit" und "Kühlpause" die Legierungselemente Kohlenstoff, Niob und Silizium in dieser Reihenfolge die wichtigsten Steuergrößen sind, mit denen sich über ein hinsichtlich der Ferritkörner ultrafeinen martensitisch/ferritischen Gefüge die Werkstoffeigenschaften der gemäß der Erfindung erzeugten Warmbänder einstellen lassen.In the diag. FIG. 2 shows the tensile strength of the hot strips produced in accordance with the invention from the melts S1-S4 as a function of the ferrite content. It can be clearly seen that higher tensile strengths are achieved with decreasing ferrite contents in favor of increasing martensite contents. At the same time goes from Diag. 2 shows the potential of each alloy. For example, from the high-carbon melts S1 and S2, hot strips can be produced that are in the high tensile strength range of 900 MPa to 1100 MPa. In contrast, the low-carbon steel compositions S3 and S4 can be used to produce hot strips with tensile strengths of at least 700 MPa. It is thus clear that in addition to the process parameters "cooling rate" and "cooling break", the alloying elements carbon, niobium and silicon are the most important control variables in this order, with which the material properties of the microstructures produced according to the invention can be determined via an ultrafine martensitic / ferritic microstructure with respect to the ferrite grains Allow hot strips to be adjusted.

Mit den Versuchen konnte ebenso nachgewiesen werden, dass sich mit dem erfindungsgemäßen verfahren sicher Warmbänder erzeugen lassen, deren Zugfestigkeit regelmäßig mehr als 700 MPa betrug. Gleichzeitig lag die Dehngrenze regelmäßig unterhalb von 500 MPa, so dass ein für die den erfindungsgemäß erzeugten Warmbändern zugedachten Anwendungen besonders günstiger Wert der Streckgrenzverhältnis von 0,65 bei keinem der untersuchten Warmbänder überschritten wurde.

Figure imgb0001
Figure imgb0002
 The experiments also proved that it is possible with the process according to the invention to reliably produce hot strips whose tensile strength regularly exceeded 700 MPa. At the same time, the yield strength was regularly below 500 MPa, so that a particularly favorable value of the yield ratio of 0.65 was not exceeded for any of the applications intended for the hot strips according to the invention in any of the hot strips investigated.
Figure imgb0001
Figure imgb0002

Claims (11)

  1. Process for producing high strength hot-rolled strip with a tensile strength of at least 700 MPa, having yield strength ratios which in the longitudinal direction amount to a maximum of 0.65 and in the transverse direction to a maximum of 0.66, and with an ultrafine ferrite/martensite and pearlite-free grain structure, which has a martensite content of at least 15% and wherein the average diameter of the ferrite grains is less than 2.5µm, comprising the following stages:
    a) Casting a steel melt containing (% by weight) C: 0.05 - 0.2% Si: <0.9% P: <0.06% Mn: 0.6 - 1.2% Al: <0.05 % Cr: 0.02 - 0.6% Nb: ≤0.08% Ti: ≤0.08% V: ≤0.08% Mo: ≤0.4% Cu: ≤1% Ni: ≤1%
    the balance being iron and incidental impurities into a starting material, such as slabs or thin slabs,
    b) Hot-rolling the starting material to a hot-rolled strip at a hot-rolling end temperature of 750°C - 950°C,
    c) Cooling the resultant hot-rolled strip to a coiling temperature of ambient temperature up to 250°C at a cooling rate of at least 10K per second,
    d) Coiling the cooled hot-rolled strip.
  2. Process according to Claim 1, characterized in that the hot-rolled strip is cooled to coiling temperature in one step.
  3. Process according to Claim 1, characterized in that when cooling to coiling temperature, the hot-rolled strip is first cooled at a cooling rate of 10 - 200K per second to an intermediate temperature of 600 - 700°C, and that there after cooling is interrupted for 2 - 6 seconds and subsequently the hot-rolled strip is cooled at a cooling rate of 10 - 200K per second to the coiling temperature.
  4. Process according to Claim 1, characterized in that the steel melt contains at least 0.03% by weight of Nb.
  5. Process according to any one_of the preceding claims, characterized in that the average diameter of the ferrite grains is less than 2µm.
  6. Process according to any one of the preceding claims, characterized in that the average diameter of the ferrite grains is less than 0.1µm.
  7. Process according to any one of the preceding claims, characterized in that the cooling rate is 10K per second up to 1,000K per second.
  8. Process according to any one of the preceding claims, characterized in that the cooling rate is at least 50K per second.
  9. Hot-rolled strip produced by the process according to any one of Claims 1 to 6, characterized in that the product of tensile strength (Rm) and elongation at rupture (A5) is at least 15,000 MPa*%.
  10. Hot-rolled strip according to Claim 9, characterized in that its tensile strength (Rm) is more than 900 MPa.
  11. Hot-rolled strip according to either of Claims 9 or 10, characterized in that its tensile strength is up to 1,100 MPa.
EP02020294A 2002-09-11 2002-09-11 Steel with a very fine ferritic and martensitic microstructure having a high tensile strength Revoked EP1398390B1 (en)

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ES02020294T ES2256378T3 (en) 2002-09-11 2002-09-11 HIGHLY RESISTANT FERRITIC / MARTENSITIC STEEL WITH VERY FINE STRUCTURE.
DE50205631T DE50205631D1 (en) 2002-09-11 2002-09-11 Ferritic / martensitic steel with high strength and very fine structure
EP02020294A EP1398390B1 (en) 2002-09-11 2002-09-11 Steel with a very fine ferritic and martensitic microstructure having a high tensile strength
AT02020294T ATE316157T1 (en) 2002-09-11 2002-09-11 FERRITIC/MARTENSITIC STEEL WITH HIGH STRENGTH AND VERY FINE STRUCTURE

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ES2325964T3 (en) * 2006-10-30 2009-09-25 Thyssenkrupp Steel Ag PROCEDURE FOR MANUFACTURING STEEL FLAT PRODUCTS FROM A MULTIPHASIC STEEL ALLOYED WITH SILICON.
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KR102596515B1 (en) 2014-12-19 2023-11-01 누코 코포레이션 Hot rolled light-gauge martensitic steel sheet and method for making the same

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