EP2684975B1 - Cold rolled steel flat product and method for its production - Google Patents

Cold rolled steel flat product and method for its production Download PDF

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Publication number
EP2684975B1
EP2684975B1 EP12175756.1A EP12175756A EP2684975B1 EP 2684975 B1 EP2684975 B1 EP 2684975B1 EP 12175756 A EP12175756 A EP 12175756A EP 2684975 B1 EP2684975 B1 EP 2684975B1
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EP
European Patent Office
Prior art keywords
flat steel
steel product
temperature
cold
cooling
Prior art date
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EP12175756.1A
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German (de)
French (fr)
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EP2684975A1 (en
Inventor
Roland Sebald
Dorothea Mattissen
Sigrun Voss
Stefan Follner
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 Europe AG
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Priority to PL12175756T priority Critical patent/PL2684975T3/en
Application filed by ThyssenKrupp Steel Europe AG filed Critical ThyssenKrupp Steel Europe AG
Priority to ES12175756.1T priority patent/ES2614465T3/en
Priority to EP12175756.1A priority patent/EP2684975B1/en
Priority to US14/377,398 priority patent/US10344344B2/en
Priority to PCT/EP2013/064551 priority patent/WO2014009404A1/en
Priority to CN201380036484.9A priority patent/CN104471096B/en
Priority to BR112014021543-0A priority patent/BR112014021543B1/en
Priority to JP2015520969A priority patent/JP6236078B2/en
Priority to KR1020147021943A priority patent/KR102128563B1/en
Publication of EP2684975A1 publication Critical patent/EP2684975A1/en
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Publication of EP2684975B1 publication Critical patent/EP2684975B1/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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C21D8/0284—Application of a separating or insulating coating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—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
    • 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
    • C21D2211/009—Pearlite

Definitions

  • the invention relates to a cold rolled flat steel product and a process for its production.
  • flat steel products this refers to steel strips and sheets or blanks derived therefrom.
  • a dual-phase steel is known, which in addition to a strength of at least 950 MPa and good ductility also has a surface texture that allows using a simple manufacturing process, the flat product produced from this steel in the uncoated or provided with a corrosion protective coating state to a deforming a complex shaped component, such as a part of an automobile body.
  • This is achieved according to this prior art by the fact that the known dual-phase steel consists of 20-70% of martensite, up to 8% of retained austenite and the remainder of ferrite and / or bainite.
  • the known steel in wt .-%): C: 0.10 - 0.20%, Si: 0.10 - 0.60%, Mn: 1.50 - 2.50%, Cr: 0, 20 - 0.80%, Ti: 0.02-0.08%, B: ⁇ 0.0020%, Mo: ⁇ 0.25%, Al: ⁇ 0.10%, P: ⁇ 0.2%, S: ⁇ 0.01%, N: ⁇ 0.012% and the remainder iron and unavoidable impurities.
  • a flat steel product made from such a steel can be used as a hot strip or cold strip.
  • Si is used in the known steel to increase the strength by hardening the ferrite or bainite.
  • a minimum content of Si of 0.10 wt .-% is provided.
  • the Si content is limited to 0.6 wt%, with lower upper limits for the Si content be particularly preferred to minimize the risk of grain boundary oxidation.
  • the cutting gap is 8% to 14% of the sheet thickness.
  • the board is clamped in the test tool so that the degree of cutting of the hole is on the bottom.
  • the hold-down force is a maximum of 400 kN.
  • Below the tool then becomes a round 100 mm diameter punch against the sample and the board is arched until the hole edge fails.
  • the maximum hole diameter d M reached when a first crack of the hole edge occurs is detected, and the hole expansion ratio ⁇ M is determined as the ratio d 0 / d M expressed in "%".
  • the object of the invention to provide a manufacturable with simple means the flat steel product that has an optimum ductility despite high strength values, which is characterized by a high elongation and good hole expansion ratio ⁇ M.
  • a method should be given which allows the production of such a flat steel product in a simple manner.
  • the solution according to the invention of the abovementioned object is that during the production of a cold-rolled flat steel product according to the invention, the working steps specified in claim 4 are run through.
  • the structure of the flat steel product according to the invention is characterized in that it has 2-15% by volume, in particular at least 5% by volume, better still more than 8% by volume retained austenite.
  • the microstructure of a steel according to the invention is free in the technical sense of bainite and perlite.
  • the presence of effective bainite or pearlite content in the microstructure of a flat steel product according to the invention would impair its elongation at break and, consequently, its deformability, in particular the desired good hole widening properties. Due to the inventively specified contents of retained austenite, however, the required elongation at break of at least 15% is achieved, which has a flat steel product according to the invention.
  • a cold rolled flat steel product according to the invention has distinct differences.
  • Complex phase steels generally have a higher yield ratio, compared to the flat steel product according to the invention, with a lower "quality" calculated as the product of tensile strength Rm and elongation at break A80. This is due to the relatively high yield strength and the case attributed lower elongation of the known steels
  • the deformation behavior of the flat steel product according to the invention is similar to that of a dual-phase steel. However, a big difference can be found in the structures. While a flat steel product according to the invention has a residual austenite content of up to 15% Dual-phase steels no or only very low retained austenite contents.
  • TRIP steels in contrast to the flat steel product according to the invention, have significantly higher elongations at break. This results in grades (Rm * A80) of 20,000 MPa *% and more.
  • TRIP steels must be alloyed with increased contents of carbon, silicon and / or aluminum in order to achieve the so-called TRIP effect by means of sufficient stabilization of the retained austenite and secondly to achieve the corresponding strength.
  • such an alloy concept leads to a weldability that is significantly worse than that of a flat steel product according to the invention, in which high strength and, on the other hand, good weldability are attained by setting the contents of the alloying elements in an optimized manner, in particular with regard to the Si content.
  • the hole expansion ratio ⁇ M determined according to Marciniak is at least 6% for a flat steel product according to the invention, with hole expansion ratios ⁇ M of 7% and more regularly being achieved.
  • a flat steel product according to the invention has a high elongation at break of at least 15% and, associated therewith, a quality (Rm * A80) which is at least 14000 MPa *% at a minimum tensile strength Rm of 880 MPa.
  • the tensile strengths Rm Steel flat products according to the invention in the range of 880-1150 MPa.
  • the yield strength of a flat steel product according to the invention is at least 550 MPa, yielding yield strengths of 580 MPa and more on a regular basis. Typically, the yield strengths of flat steel products according to the invention are in the range of 580 - 720 MPa. For a flat steel product according to the invention, the yield ratio (ReL / Rm) is accordingly also 0.55-0.75.
  • the elongation at break A80 of a flat steel product according to the invention is at least 15%, with regular elongation at break A80 of up to 25% being achieved.
  • Carbon is present in a flat steel product of the invention at levels of 0.12-0.19% by weight to provide strength enhancement by interstitial solid solution formation and precipitation hardening to form cementite (Fe 3 C).
  • the minimum level of 0.12 wt% is necessary to achieve the desired strength.
  • the maximum content of 0.19% by weight should not be exceeded in order to meet in practice the weldability of Flat steel products of the type according to the invention requirements to meet.
  • Manganese is present in a flat steel product of the invention at levels of 1.5-2.5% by weight.
  • the addition of manganese increases yield strength and tensile strength.
  • a tensile strength Rm of at least 880 MPa and a yield strength ReL of at least 550 MPa, in particular at least 580 MPa possible.
  • More than 2.5% by weight of Mn should not be present in a steel according to the invention, since at higher Mn contents the risk of formation of manganese segregations increases, which may adversely affect the material behavior.
  • the content of silicon which is present in a steel flat product according to the invention in contents of> 0.60-1.0% by weight, is of particular importance with regard to the formation of the microstructure.
  • Si content By making the Si content more than 0.60% by weight, the formation of pearlite is suppressed, enabling carbonization of the austenite with carbon and concomitantly increasing the retained austenite stability.
  • the retained austenite transforms into martensite during forming, which provides additional strengthening.
  • Silicon also forms mixed crystals with iron, which increase the strength in the steel.
  • the positive effects of the presence of silicon in a flat steel product according to the invention can be used particularly reliably if the Si content is at least 0.65% by weight, in particular at least 0.7% by weight.
  • the Si content is limited to at most 1.0% by weight, such scale formation being limited in particular when the Si content is limited to not more than 0.95% by weight.
  • the steel constituting the flat steel product according to the invention is aluminum-killed. Accordingly, flat steel products according to the invention regularly contain more than 0.01% by weight and up to 0.1% by weight of aluminum.
  • Chromium is present in a flat steel product of the invention at levels of 0.2-0.6 wt%. Chromium strengthens the steel flat product according to the invention. In addition, in the course of the production of a flat steel product according to the invention, hot processing of the steel is delayed by the presence of Cr, the formation of bainite. A content of 0.2% by weight is required to achieve the necessary strength. The content is limited to 0.6 wt .-%, since tests have shown that too high a chromium content has an unfavorable effect on the elongation and concomitantly on the quality (Rm * A80) of the flat steel product according to the invention.
  • Titanium is added to a flat steel product according to the invention as a micro-alloying element in amounts of 0.05-0.15% by weight. Due to the presence of Ti, the steel has the finest precipitates of Ti (C, N) contributing to increase in strength and grain refining.
  • the grain size of the structure according to ASTM is less than or equal to 15, ie less than or equal to 1.9 ⁇ m.
  • a Ti content of at least 0.05 wt .-% is required, with the positive effect of Ti is particularly safe when the Ti content of the steel at least 0.07 wt .-%, in particular at least 0.09 wt .-%, is. From a content of 0.15 wt .-%, no further improvements of the effect of Ti occur.
  • a flat steel product according to the invention is suitable for applications in which higher degrees of deformation in combination with high strength values are necessary.
  • Typical examples of these applications are crash-relevant components such as side members and permanently loaded chassis parts during operation.
  • the steel flat product is cooled at a cooling rate of 8-100 K / s to an intermediate temperature of 450-550 ° C.
  • the cooling rate of at least 8 K / s is needed here in order to avoid the formation of perlite and bainite and nevertheless to generate sufficient amount of ferrite.
  • In the temperature range of 450 ° C to 550 ° C also finds the first enrichment of austenite with carbon instead.
  • the cooling in the first stage of the two-stage cooling can be carried out with any suitable, ensuring a sufficient cooling rate medium.
  • available cooling devices are used in practice. So the cooling can be done in moving air.
  • the cooling in the second stage of the two-stage cooling can be carried out according to a practical embodiment of the invention in that the flat steel product is cooled by contact with the cooled rollers.
  • the steel flat product can be cooled in the second stage of the two-stage cooling by a moving air flow.
  • the overaging treatment can be done, for example, by the steel flat product undergoing an environmentally shielded space in the overaging treatment.
  • the temperature of the flat steel product is set to 100-400 ° C. Based on the temperature with which the Steel flat product enters the overaging treatment, this adjustment of the temperature can be carried out as heating, cooling or holding.
  • the coating of the flat steel product with the metallic protective layer can be carried out particularly effectively electrolytically.
  • the figure shows a diagram in which the typical for an inventive annealing spans the temperature profile are shown over time.
  • the slabs were then thoroughly heated at an austenitizing temperature of 1100-1300 ° C., so that the slabs had a completely austenitic microstructure on entry into the subsequently passed hot rolling mill.
  • the slabs are then hot-rolled at the hot-rolling end temperatures WET given in Table 1b to hot-rolled strip with a thickness DKW of 1.8-4.6 mm, then air, to the respective values also given in Table 1b Reel temperature HT cooled and reeled at the rewinding temperature HT respectively achieved. Pickling was then optionally performed to remove any scale present on the hot strip prior to cold rolling to allow for optimum surface finish during subsequent cold rolling.
  • samples of the cold-rolled steel flat products thus obtained have been subjected to various heat treatments A - J in which they are heated in each case to an annealing temperature GT, then held at the annealing temperature GT over an annealing time tG, then in a first cooling stage with a first cooling rate r1 a first target temperature ZT1 and immediately thereafter in a second cooling stage with a second cooling rate r2 have been brought to a second target temperature ZT2.
  • the respectively obtained samples of cold rolled steel flat products are over an aging period of 250-710 s for a period of time tUeA at an over-aging temperature TUeA at 400-100 ° C at the end of the treatment, in an overburdened space of an overaging treatment been subjected.
  • the parameters set during heat treatment A - J respectively GT, tG, r1, ZT1, r2, ZT2 and tUeA are listed in Table 2.
  • the steel flat product samples produced from the non-composite steel melts 6 and 7 do not reach the lower limits of 880 MPa or 550 MPa, in particular 580 MPa, given their tensile strength Rm or their yield strength ReL, even if they have a lower limit Subjected to heat treatment, which is carried out in accordance with the invention.
  • the flat product samples assembled and heat-treated according to the invention regularly exceed these limits.
  • Table 1a stolen C Mn Si al Cr Ti P N S 1 0.17 1.9 0.72 0.04 0.37 0.114 0,012 0.0048 0.001 2 0.13 2.3 0.65 0.06 0.23 0.07 0,007 0.009 0,007 3 0.16 1.7 0.75 0.03 0.57 0.108 0,013 0,007 0,006 4 0.18 2.1 0.94 0.02 0.34 0.143 0.009 0,007 0.009 5 0, 14 1.5 0.83 0.08 0.48 0, 135 0,018 0,006 0,002 6 0.15 1.8 0.53 0.05 0.43 0.15 0,014 0,003 0,003 7 0.14 2.4 0.73 0.06 0.05 0.09 0.009 0,004 0.005 Content in wt .-%, balance iron and unavoidable impurities stole WET [° C] HT [° C] dW [mm] KWG [%] dKB [mm] D ° [%] According to the invention?

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

Description

Die Erfindung betrifft ein kaltgewalztes Stahlflachprodukt und ein Verfahren zu dessen Herstellung.The invention relates to a cold rolled flat steel product and a process for its production.

Wenn hier von "Stahlflachprodukten" die Rede ist, sind damit Stahlbänder und -bleche oder daraus gewonnene Zuschnitte gemeint.When referring to "flat steel products", this refers to steel strips and sheets or blanks derived therefrom.

Die Entwicklung gewichtsreduzierter Fahrzeuge, die die modernen Anforderungen an einen minimierten Kraftstoffverbrauch bei gleichzeitig optimaler Fahrgastsicherheit, hohem Komfort und Belastbarkeit erfüllen, ist in den letzten Jahren von der Automobilindustrie vorangetrieben worden.The development of weight-reduced vehicles, which meet the modern requirements for minimized fuel consumption while at the same time offering optimum passenger safety, comfort and endurance, has been driven by the automotive industry in recent years.

Aufgrund ihrer mechanischen Eigenschaften, insbesondere ihrer hohen Festigkeit und guten Verformbarkeit, sowie ihrer beherrschten Herstellung und Verarbeitung eignen sich insbesondere Stahlflachprodukte grundsätzlich für den Automobil-Karosseriebau optimal. Jedoch müssen für die gewünschte Gewichtseinsparung die Blechdicken der im Automobil eingesetzten Stahlflachprodukte reduziert werden. Zu diesem Zweck sind Stähle mit höheren Festigkeiten entwickelt worden, die gleichzeitig eine gute Umformbarkeit besitzen und somit für eine Leichtbauweise im Automobilbau besonders geeignet sind. Hierzu zählen moderne Mehrphasenstähle, wie Komplexphasen-Stähle, Dualphasenstähle und TRIP-Stähle.Due to their mechanical properties, in particular their high strength and good deformability, as well as their mastered production and processing in particular flat steel products are basically ideal for the automotive body shop. However, for the desired weight reduction, the sheet thicknesses of the flat steel products used in automobiles must be reduced. For this purpose steels with higher are Strengths have been developed, which at the same time have good formability and are thus particularly suitable for lightweight construction in the automotive industry. These include advanced multiphase steels such as complex-phase steels, dual-phase steels and TRIP steels.

Aus der EP 2 028 282 A1 ist ein Dualphasenstahl bekannt, der neben einer Festigkeit von mindestens 950 MPa und einer guten Verformbarkeit auch eine Oberflächenbeschaffenheit aufweist, die es unter Anwendung eines einfachen Herstellverfahrens erlaubt, das aus diesem Stahl erzeugte Flachprodukt im unbeschichteten oder mit einem vor Korrosion schützenden Überzug versehenen Zustand zu einem komplex geformten Bauteil, wie einem Teil einer Automobilkarosserie, zu verformen. Dies wird gemäß diesem Stand der Technik dadurch erreicht, dass der bekannte Dualphasenstahl zu 20 - 70 % aus Martensit, bis zu 8 % aus Restaustenit und als Rest aus Ferrit und / oder Bainit besteht. Dabei weist der bekannte Stahl (in Gew.-%): C : 0,10 - 0,20 %, Si: 0,10 - 0,60 %, Mn: 1,50 - 2,50 %, Cr: 0,20 - 0,80 %, Ti: 0,02 - 0,08 %, B: < 0,0020 %, Mo: < 0,25 %, Al: < 0,10%, P: < 0,2 %, S: < 0,01 %, N: < 0,012 % und als Rest Eisen und unvermeidbare Verunreinigungen auf. Ein aus einem solchen Stahl erzeugtes Stahlflachprodukt ist als Warmband oder Kaltband verwendbar. Si dient bei dem bekannten Stahl zur Steigerung der Festigkeit durch Härtung des Ferrits bzw. Bainits. Um diesen Effekt nutzen zu können, ist ein Mindestgehalt an Si von 0,10 Gew.-% vorgesehen. Gleichzeitig ist der Si-Gehalt jedoch auf 0,6 Gew.-% beschränkt, wobei niedrigere Obergrenzen für den Si-Gehalt als besonders bevorzugt herausgestellt werden, um die Gefahr von Korngrenzoxidation zu minimieren.From the EP 2 028 282 A1 is a dual-phase steel is known, which in addition to a strength of at least 950 MPa and good ductility also has a surface texture that allows using a simple manufacturing process, the flat product produced from this steel in the uncoated or provided with a corrosion protective coating state to a deforming a complex shaped component, such as a part of an automobile body. This is achieved according to this prior art by the fact that the known dual-phase steel consists of 20-70% of martensite, up to 8% of retained austenite and the remainder of ferrite and / or bainite. In this case, the known steel (in wt .-%): C: 0.10 - 0.20%, Si: 0.10 - 0.60%, Mn: 1.50 - 2.50%, Cr: 0, 20 - 0.80%, Ti: 0.02-0.08%, B: <0.0020%, Mo: <0.25%, Al: <0.10%, P: <0.2%, S: <0.01%, N: <0.012% and the remainder iron and unavoidable impurities. A flat steel product made from such a steel can be used as a hot strip or cold strip. Si is used in the known steel to increase the strength by hardening the ferrite or bainite. To be able to use this effect, a minimum content of Si of 0.10 wt .-% is provided. At the same time, however, the Si content is limited to 0.6 wt%, with lower upper limits for the Si content be particularly preferred to minimize the risk of grain boundary oxidation.

Neben der Eignung zur großvolumigen bzw. großflächigen Verformung zu einem Bauteil spielt insbesondere bei Stahlflachprodukten, die für den Karosseriebau verwendet werden sollen, auch das Verhalten bei lokal eng begrenzter Verformung eine wichtige Rolle. Solche Verformungen treten auf, wenn in ein Stahlflachprodukt bzw. eine daraus gebildete Platine oder ein aus einer solchen Platine geformtes Bauteil Öffnungen, Flansche, Durchstellungen, Ausstülpungen oder desgleichen eingeformt werden.In addition to the suitability for large-volume or large-scale deformation to a component, especially in the case of flat steel products, which are to be used for body construction, the behavior with locally narrow deformation plays an important role. Such deformations occur when in a flat steel product or a printed circuit board formed therefrom or a component formed from such a board openings, flanges, punctures, protuberances or the like are formed.

Als Maß für das Verhalten eines Flachmaterials bei einer solchen Verformung ist in Woestmann, S., Köhler, T., Schott, M., "Forming High-Strength Steels," SAE Technical Paper 2009-01-0802, 2009, doi:10.4271/2009-01-0802 das nach Marciniak so genannte Lochaufweitungsverhältnis λM vorgeschlagen worden, mittels dessen sich die Kantenrissempfindlichkeit eines Werkstoffs bei Verformungen der voranstehend erwähnten Art bewerten lässt. Die Untersuchung nach Marciniak sieht dabei vor, dass in eine rechteckige Platine, die in quer zur Walzrichtung 220 mm und in Walzrichtung 200 mm lang ist, mittig mit einem Stempel eine Lochung mit einem Durchmesser von 20 mm (d0) eingebracht. Der Schneidspalt beträgt dabei 8 % bis 14 % der Blechdicke. Für die Prüfung wird die Platine derart in das Prüfwerkzeug eingespannt, dass der Schneidgrad des Loches sich auf der Unterseite befindet. Die Niederhaltekraft beträgt maximal 400 kN. Unterhalb des Werkzeugs wird dann ein runder Stempel mit einem Durchmesser von 100 mm gegen die Probe verfahren und die Platine bis zum Versagen der Lochkante aufgewölbt. Der beim Auftreten eines ersten Risses der Lochkante erreichte maximale Lochdurchmesser dM wird erfasst und das Lochaufweitungsverhältnis λM als Verhältnis d0/dM, angegeben in "%", bestimmt.As a measure of the behavior of a flat material in such a deformation is in Woestmann, S., Kohler, T., Schott, M., "Forming High-Strength Steels," SAE Technical Paper 2009-01-0802, 2009, doi: 10.4271 / 2009-01-0802 the so-called hole widening ratio λ M according to Marciniak has been proposed, by means of which the edge crack sensitivity of a material can be evaluated in the case of deformations of the type mentioned above. The investigation according to Marciniak provides that in a rectangular board, which is 200 mm long in the transverse direction to the rolling direction and 200 mm long, centered with a punch a hole with a diameter of 20 mm (d 0 ) introduced. The cutting gap is 8% to 14% of the sheet thickness. For the test, the board is clamped in the test tool so that the degree of cutting of the hole is on the bottom. The hold-down force is a maximum of 400 kN. Below the tool then becomes a round 100 mm diameter punch against the sample and the board is arched until the hole edge fails. The maximum hole diameter d M reached when a first crack of the hole edge occurs is detected, and the hole expansion ratio λ M is determined as the ratio d 0 / d M expressed in "%".

Vor dem Hintergrund des voranstehend erläuterten Standes der Technik bestand die Aufgabe der Erfindung darin, ein mit einfachen Mitteln herstellbares Stahlflachprodukt anzugeben, das trotz hoher Festigkeitswerte eine optimale Verformbarkeit besitzt, die durch eine hohe Bruchdehnung und ein gutes Lochaufweitungsverhältnis λM gekennzeichnet ist. Darüber hinaus sollte ein Verfahren angegeben werden, das auf einfache Weise die Herstellung eines solchen Stahlflachprodukts ermöglicht.Against the background of the above-explained prior art, the object of the invention to provide a manufacturable with simple means the flat steel product that has an optimum ductility despite high strength values, which is characterized by a high elongation and good hole expansion ratio λ M. In addition, a method should be given which allows the production of such a flat steel product in a simple manner.

In Bezug auf das Stahlflachprodukt ist diese Aufgabe erfindungsgemäß dadurch gelöst worden, dass ein solches Stahlflachprodukt die in Anspruch 1 angegebenen Merkmale aufweist.With regard to the flat steel product, this object has been achieved according to the invention in that such a flat steel product has the features specified in claim 1.

In Bezug auf das Verfahren besteht die erfindungsgemäße Lösung der oben genannten Aufgabe darin, dass bei der Herstellung eines erfindungsgemäßen kaltgewalzten Stahlflachprodukts die in Anspruch 4 angegebenen Arbeitsschritte durchlaufen werden.With regard to the method, the solution according to the invention of the abovementioned object is that during the production of a cold-rolled flat steel product according to the invention, the working steps specified in claim 4 are run through.

Ein erfindungsgemäßes Stahlflachprodukt wird demnach aus einem Stahl erzeugt, der aus (in Gew.-%)

  • C: 0,12 - 0,19 %,
  • Mn: 1,5 - 2,5 %,
  • Si: >0, 60 - 1,0 %,
  • Al : ≤ 0,1 %,
  • Cr: 0,2 - 0,6 %,
  • Ti: 0,05 - 0,15 %,
und als Rest aus Eisen sowie herstellungsbedingt unvermeidbaren Verunreinigungen besteht. Dabei zählen zu den betreffenden unvermeidbaren Verunreinigungen (in Gew.-%) bis zu 0,1 % Mo, bis zu 0,03 % Nb, bis zu 0,03 % V, bis zu 0,0008 % B, bis zu 0,01 % S, bis zu 0,1 % P, bis zu 0,01 % N.A flat steel product according to the invention is accordingly produced from a steel which consists of (in% by weight)
  • C: 0.12-0.19%,
  • Mn: 1.5-2.5%,
  • Si:> 0.60-1.0%,
  • Al: ≦ 0.1%,
  • Cr: 0.2-0.6%,
  • Ti: 0.05-0.15%,
and the remainder is iron and production-related unavoidable impurities. Up to 0.1% Mo, up to 0.03% Nb, up to 0.03% V, up to 0.0008% B, up to 0, are included in the relevant unavoidable impurities (in% by weight). 01% S, up to 0.1% P, up to 0.01% N.

Gleichzeitig weist ein erfindungsgemäßes Stahlflachprodukt im kaltgewalzten Zustand

  • ein perlit- und bainitfreies Gefüge mit 4 - 20 Vol.-%, insbesondere mindestens 6 Vol.-% Martensit, 2 - 15 Vol.-% Restaustenit, Rest Ferrit,
  • eine Bruchdehnung A80 von mindestens 15 %,
  • eine Zugfestigkeit Rm von mindestens 880 MPa,
  • eine Streckgrenze ReL von mindestens 550 MPa und
  • ein Lochaufweitungsverhältnis λM von mehr als 6 %
auf.At the same time, a flat steel product according to the invention has a cold-rolled state
  • a perlite- and bainite-free structure with 4 - 20% by volume, in particular at least 6% by volume martensite, 2 - 15% by volume retained austenite, remainder ferrite,
  • an elongation at break A80 of at least 15%,
  • a tensile strength Rm of at least 880 MPa,
  • a yield strength ReL of at least 550 MPa and
  • a hole expansion ratio λ M of more than 6%
on.

Das Gefüge des erfindungsgemäßen Stahlflachprodukts zeichnet sich dadurch aus, dass es 2 - 15 Vol.-%, insbesondere mindestens 5 Vol.-%, besser noch mehr als 8 Vol.-% Restaustenit aufweist. Gleichzeitig ist das Gefüge eines erfindungsgemäßen Stahls im technischen Sinne frei von Bainit und Perlit. D. h., im kaltgewalzten Zustand sind im Gefüge eines erfindungsgemäßen Stahlflachprodukts allenfalls Spuren von Bainit oder Perlit vorhanden, die keinen Einfluss auf die technischen Eigenschaften des erfindungsgemäßen Stahlflachprodukts haben. Die Anwesenheit wirksamer Bainit- oder Perlit-Anteile im Gefüge eines erfindungsgemäßen Stahlflachprodukts würde dessen Bruchdehnung und damit einhergehend dessen Verformbarkeit, insbesondere die angestrebt guten Lochaufweitungseigenschaften, verschlechtern. Durch die erfindungsgemäß vorgegebenen Gehalte an Restaustenit wird jedoch die geforderte Bruchdehnung von mindestens 15 % erreicht, die ein erfindungsgemäßes Stahlflachprodukt besitzt.The structure of the flat steel product according to the invention is characterized in that it has 2-15% by volume, in particular at least 5% by volume, better still more than 8% by volume retained austenite. At the same time, the microstructure of a steel according to the invention is free in the technical sense of bainite and perlite. In other words, in the cold-rolled state, at most traces of bainite or perlite are present in the structure of a flat steel product according to the invention which have no influence on the technical properties of the flat steel product according to the invention. The presence of effective bainite or pearlite content in the microstructure of a flat steel product according to the invention would impair its elongation at break and, consequently, its deformability, in particular the desired good hole widening properties. Due to the inventively specified contents of retained austenite, however, the required elongation at break of at least 15% is achieved, which has a flat steel product according to the invention.

Gegenüber konventionellen modernen Mehrphasenstählen weist ein erfindungsgemäßes kaltgewalztes Stahlflachprodukt deutliche Unterschiede auf. Komplexphasenstähle weisen im Vergleich zum erfindungsgemäßen Stahlflachprodukt in der Regel ein höheres Streckgrenzenverhältnis bei einer als Produkt von Zugfestigkeit Rm und Bruchdehnung A80 berechneten geringeren "Güte" auf. Dies ist auf die verhältnismäßig hohe Streckgrenze und die dabei
geringere Dehnung der bekannten Stähle zurückzuführenCompared to conventional modern multiphase steels, a cold rolled flat steel product according to the invention has distinct differences. Complex phase steels generally have a higher yield ratio, compared to the flat steel product according to the invention, with a lower "quality" calculated as the product of tensile strength Rm and elongation at break A80. This is due to the relatively high yield strength and the case
attributed lower elongation of the known steels

Das Verformungsverhalten des erfindungsgemäßen Stahlflachprodukts ähnelt dem eines Dualphasen-Stahls. Ein großer Unterschied ist dabei jedoch in den Gefügen zu finden. Während ein erfindungsgemäßes Stahlflachprodukt einen Restaustenitanteil von bis zu 15 % aufweist, haben Dualphasen-Stähle keinen oder nur sehr geringe Restaustenitgehalte.The deformation behavior of the flat steel product according to the invention is similar to that of a dual-phase steel. However, a big difference can be found in the structures. While a flat steel product according to the invention has a residual austenite content of up to 15% Dual-phase steels no or only very low retained austenite contents.

TRIP-Stähle weisen im Gegensatz zum erfindungsgemäßen Stahlflachprodukt wesentlich höhere Bruchdehnungen auf. Hieraus ergeben sich in der Regel Güten (Rm*A80) von 20000 MPa*% und mehr. Jedoch müssen TRIP-Stähle mit erhöhten Gehalten an Kohlenstoff, Silizium und/oder Aluminium legiert sein, um zum einen durch eine hinreichende Stabilisierung des Restaustenits den sogenannten TRIP-Effekt zu erzielen und zum anderen die entsprechende Festigkeit zu erreichen. Ein solches Legierungskonzept führt jedoch zu einer Schweißeignung, die deutlich schlechter ist als die eines erfindungsgemäßen Stahlflachprodukts, bei dem durch eine insbesondere im Hinblick auf den Si-Gehalt optimierte Einstellung der Gehalte an den Legierungselementen einerseits hohe Festigkeiten und andererseits eine gute Schweißeignung erzielt werden.TRIP steels, in contrast to the flat steel product according to the invention, have significantly higher elongations at break. This results in grades (Rm * A80) of 20,000 MPa *% and more. However, TRIP steels must be alloyed with increased contents of carbon, silicon and / or aluminum in order to achieve the so-called TRIP effect by means of sufficient stabilization of the retained austenite and secondly to achieve the corresponding strength. However, such an alloy concept leads to a weldability that is significantly worse than that of a flat steel product according to the invention, in which high strength and, on the other hand, good weldability are attained by setting the contents of the alloying elements in an optimized manner, in particular with regard to the Si content.

Das nach Marciniak bestimmte Lochaufweitungsverhältnis λM beträgt bei einem erfindungsgemäßen Stahlflachprodukt mindestens 6 %, wobei regelmäßig Lochaufweitungsverhältnisse λM von 7 % und mehr erreicht werden.The hole expansion ratio λ M determined according to Marciniak is at least 6% for a flat steel product according to the invention, with hole expansion ratios λ M of 7% and more regularly being achieved.

Ein erfindungsgemäßes Stahlflachprodukt besitzt bei einer Mindestzugfestigkeit Rm von 880 MPa eine hohe Bruchdehnung von mindestens 15 % und damit einhergehend eine Güte (Rm*A80), die regelmäßig mindestens 14000 MPa*% beträgt. Typischerweise liegen die Zugfestigkeiten Rm erfindungsgemäßer Stahlflachprodukte im Bereich von 880 - 1150 MPa.A flat steel product according to the invention has a high elongation at break of at least 15% and, associated therewith, a quality (Rm * A80) which is at least 14000 MPa *% at a minimum tensile strength Rm of 880 MPa. Typically, the tensile strengths Rm Steel flat products according to the invention in the range of 880-1150 MPa.

Die Streckgrenze eines erfindungsgemäßen Stahlflachprodukts beträgt mindestens 550 MPa, wobei regelmäßig Streckgrenzen von 580 MPa und mehr erreicht werden. Typischerweise liegen die Streckgrenzen erfindungsgemäßer Stahlflachprodukte im Bereich von 580 - 720 MPa. Für ein erfindungsgemäßes Stahlflachprodukt beträgt das Streckgrenzenverhältnis (ReL/Rm) demnach ebenso regelmäßig 0,55 - 0,75.The yield strength of a flat steel product according to the invention is at least 550 MPa, yielding yield strengths of 580 MPa and more on a regular basis. Typically, the yield strengths of flat steel products according to the invention are in the range of 580 - 720 MPa. For a flat steel product according to the invention, the yield ratio (ReL / Rm) is accordingly also 0.55-0.75.

Die Bruchdehnung A80 eines erfindungsgemäßen Stahlflachprodukts beträgt mindestens 15 %, wobei regelmäßig Bruchdehnungen A80 von bis zu 25 % erreicht werden.The elongation at break A80 of a flat steel product according to the invention is at least 15%, with regular elongation at break A80 of up to 25% being achieved.

Aus der Dauerschwingfestigkeitsbestimmung nach DIN EN 50100 ergibt sich für erfindungsgemäße Stahlflachprodukte ein k-Wert, der regelmäßig höher als 4 ist.From the determination of fatigue strength according to DIN EN 50100, a k-value, which is regularly higher than 4, results for flat steel products according to the invention.

Kohlenstoff ist in einem erfindungsgemäßen Stahlflachprodukt in Gehalten von 0,12 - 0,19 Gew.-% vorhanden, um eine Festigkeitssteigerung durch interstitielle Mischkristallbildung und Ausscheidungshärtung unter Bildung von Zementit (Fe3C) zu bewirken. Der minimale Gehalt von 0,12 Gew.-% ist notwendig, um die erwünschte Festigkeit zu erreichen. Der maximale Gehalt von 0,19 Gew.-% sollte nicht überschritten werden, um den in der Praxis an die Schweißeignung von Stahlflachprodukten der erfindungsgemäßen Art gestellten Anforderungen zu genügen.Carbon is present in a flat steel product of the invention at levels of 0.12-0.19% by weight to provide strength enhancement by interstitial solid solution formation and precipitation hardening to form cementite (Fe 3 C). The minimum level of 0.12 wt% is necessary to achieve the desired strength. The maximum content of 0.19% by weight should not be exceeded in order to meet in practice the weldability of Flat steel products of the type according to the invention requirements to meet.

Mangan ist in einem erfindungsgemäßen Stahlflachprodukt in Gehalten von 1,5 - 2,5 Gew.-% vorhanden. Durch die Zugabe von Mangan werden Streckgrenze und Zugfestigkeit erhöht. So werden in einem erfindungsgemäßen Stahlflachprodukt durch die Anwesenheit von mindestens 1,5 Gew.-% Mangan eine Zugfestigkeit Rm von mindestens 880 MPa und eine Streckgrenze ReL von mindestens 550 MPa, insbesondere mindestens 580 MPa, ermöglicht. Mehr als 2,5 Gew.-% Mn sollte in einem erfindungsgemäßen Stahl nicht vorhanden sein, da bei höheren Mn-Gehalten die Gefahr der Entstehung von Manganseigerungen ansteigt, die sich ungünstig auf das Werkstoffverhalten auswirken können.Manganese is present in a flat steel product of the invention at levels of 1.5-2.5% by weight. The addition of manganese increases yield strength and tensile strength. Thus, in a flat steel product according to the invention by the presence of at least 1.5 wt .-% manganese a tensile strength Rm of at least 880 MPa and a yield strength ReL of at least 550 MPa, in particular at least 580 MPa, possible. More than 2.5% by weight of Mn should not be present in a steel according to the invention, since at higher Mn contents the risk of formation of manganese segregations increases, which may adversely affect the material behavior.

Dem Gehalt an Silizium, das in einem erfindungsgemäßen Stahlflachprodukt in Gehalten von >0,60 - 1,0 Gew.-% vorhanden ist, kommt eine besondere Bedeutung im Hinblick auf die Ausbildung des Gefüges zu. Indem der Si-Gehalt mehr als 0,60 Gew.-% beträgt, wird die Perlitbildung unterdrückt, was eine Anreicherung des Austenits mit Kohlenstoff ermöglicht und damit einhergehend die Restaustenitstabilität erhöht. Der Restaustenit wandelt bei der Umformung in Martensit um, wodurch eine zusätzliche Verfestigung erreicht wird. Silizium bildet zudem mit Eisen Mischkristalle, durch die die Festigkeit im Stahl gesteigert wird. Besonders sicher lassen sich die positiven Einflüsse der Anwesenheit von Silizium in einem erfindungsgemäßen Stahlflachprodukt dann nutzen, wenn der Si-Gehalt mindestens 0,65 Gew.-%, insbesondere mindestens 0,7 Gew.-%, beträgt. Um beim Warmwalzen ungünstige Zunderausbildung zu vermeiden, ist gleichzeitig der Si-Gehalt auf höchstens 1,0 Gew.-% beschränkt, wobei eine solche Zunderbildung insbesondere dann begrenzt wird, wenn der Si-Gehalt auf höchstens 0,95 Gew.-% begrenzt ist.The content of silicon, which is present in a steel flat product according to the invention in contents of> 0.60-1.0% by weight, is of particular importance with regard to the formation of the microstructure. By making the Si content more than 0.60% by weight, the formation of pearlite is suppressed, enabling carbonization of the austenite with carbon and concomitantly increasing the retained austenite stability. The retained austenite transforms into martensite during forming, which provides additional strengthening. Silicon also forms mixed crystals with iron, which increase the strength in the steel. The positive effects of the presence of silicon in a flat steel product according to the invention can be used particularly reliably if the Si content is at least 0.65% by weight, in particular at least 0.7% by weight. To hot rolling At the same time, the Si content is limited to at most 1.0% by weight, such scale formation being limited in particular when the Si content is limited to not more than 0.95% by weight.

Der Stahl, aus dem das erfindungsgemäße Stahlflachprodukt besteht, ist aluminiumberuhigt. Demzufolge enthalten erfindungsgemäße Stahlflachprodukte regelmäßig mehr als 0,01 Gew.-% und bis zu 0,1 Gew.-% Aluminium.The steel constituting the flat steel product according to the invention is aluminum-killed. Accordingly, flat steel products according to the invention regularly contain more than 0.01% by weight and up to 0.1% by weight of aluminum.

Chrom ist in einem erfindungsgemäßen Stahlflachprodukt in Gehalten von 0,2 - 0,6 Gew.-% vorhanden. Chrom wirkt im erfindungsgemäßen Stahlflachprodukt festigkeitssteigernd. Hinzukommt, dass bei der im Zuge der Herstellung eines erfindungsgemäßen Stahlflachprodukts erfolgenden Warmverarbeitung des Stahls durch die Anwesenheit von Cr die Bildung von Bainit verzögert wird. Ein Gehalt von 0,2 Gew.-% ist erforderlich, um die notwendige Festigkeit zu erreichen. Der Gehalt wird auf 0,6 Gew.-% begrenzt, da Versuche gezeigt haben, dass ein zu hoher Chromgehalt sich ungünstig auf die Dehnung und damit einhergehend auf die Güte (Rm*A80) des erfindungsgemäßen Stahlflachprodukts auswirkt.Chromium is present in a flat steel product of the invention at levels of 0.2-0.6 wt%. Chromium strengthens the steel flat product according to the invention. In addition, in the course of the production of a flat steel product according to the invention, hot processing of the steel is delayed by the presence of Cr, the formation of bainite. A content of 0.2% by weight is required to achieve the necessary strength. The content is limited to 0.6 wt .-%, since tests have shown that too high a chromium content has an unfavorable effect on the elongation and concomitantly on the quality (Rm * A80) of the flat steel product according to the invention.

Titan wird einem erfindungsgemäßen Stahlflachprodukt als Mikrolegierungselement in Gehalten von 0,05 - 0,15 Gew.-% zugegeben. Aufgrund der Anwesenheit von Ti weist der Stahl feinste Ausscheidungen von Ti(C,N) auf, die Festigkeitssteigerung und Kornfeinung beitragen. Die Korngröße des Gefüges ist nach ASTM kleiner oder gleich 15, d. h. kleiner oder gleich 1,9 µm. Um die gewünschten Ausscheidungen zu bilden, ist ein Ti-Gehalt von mindestens 0,05 Gew.-% erforderlich, wobei sich die positive Wirkung von Ti dann besonders sicher einstellt, wenn der Ti-Gehalt des Stahls mindestens 0,07 Gew.-%, insbesondere mindestens 0,09 Gew.-%, beträgt. Ab einem Gehalt von 0,15 Gew.-% treten keine weiteren Verbesserungen der Wirkung von Ti ein.Titanium is added to a flat steel product according to the invention as a micro-alloying element in amounts of 0.05-0.15% by weight. Due to the presence of Ti, the steel has the finest precipitates of Ti (C, N) contributing to increase in strength and grain refining. The grain size of the structure according to ASTM is less than or equal to 15, ie less than or equal to 1.9 μm. To the desired To form precipitates, a Ti content of at least 0.05 wt .-% is required, with the positive effect of Ti is particularly safe when the Ti content of the steel at least 0.07 wt .-%, in particular at least 0.09 wt .-%, is. From a content of 0.15 wt .-%, no further improvements of the effect of Ti occur.

Aufgrund seiner Eigenschaften eignet sich ein erfindungsgemäßes Stahlflachprodukt für Anwendungen, bei denen höhere Umformgrade in Kombination mit hohen Festigkeitswerten notwendig sind. Typische Beispiele für diese Verwendungen sind crashrelevante Bauteile wie Längsträger und auch dauerhaft im Betrieb belastete Fahrwerksteile.Due to its properties, a flat steel product according to the invention is suitable for applications in which higher degrees of deformation in combination with high strength values are necessary. Typical examples of these applications are crash-relevant components such as side members and permanently loaded chassis parts during operation.

Das erfindungsgemäße Verfahren zur Herstellung eines erfindungsgemäßen kaltgewalzten Stahlflachprodukts umfasst folgende Arbeitsschritte:

  • Eine Stahlschmelze, die (in Gew.-%) C: 0,12 - 0,19 %, Mn: 1,5 - 2,5 %, Si: >0,60 - 1,0 %, Al: ≤ 0,1 %, Cr: 0,2 - 0,6 %, Ti: 0,05 - 0,15 % und als Rest aus Eisen sowie herstellungsbedingt unvermeidbaren Verunreinigungen besteht, wird zu einem Vorprodukt vergossen, bei dem es sich um eine Bramme oder Dünnbramme handelt.
  • Das Vorprodukt wird bei einer 1100 - 1300 °C betragenden Austenitisierungstemperatur durcherwärmt, wobei diese Durcherwärmung ein von einer niedrigeren Temperatur ausgehendes Erwärmen umfassen kann oder als Halten der jeweiligen Bramme oder Dünnbramme unter Ausnutzung der nach ihrer Erzeugung in ihnen vorhandenen Wärme ausgeführt werden kann. Die Durchwärmung wird dabei unter Berücksichtigung der Geometrie des Vorprodukts und der Leistungsfähigkeit der zur Verfügung stehenden Erwärmungseinrichtung so durchgeführt, dass das Gefüge des Vorprodukts am Ende dieser Erwärmung vollständig austenitisch ist.
  • Das so bei der Austenitisierungstemperatur durcherwärmte Vorprodukt wird dann zu einem Warmband warmgewalzt, dessen Dicke typischerweise 1,8 - 4,7 mm beträgt. Die Temperaturführung in der mehrere, in der Regel fünf bis sieben Walzgerüste umfassenden Warmwalzstaffel wird so gewählt, dass in den ersten beiden Gerüsten der Warmwalzstaffel keine Rekristallisation stattfindet. Zu diesem Zweck sieht die Erfindung eine Warmwalzendtemperatur von 850 - 960 °C vor.
  • Das aus dem letzten Gerüst der Warmwalzstaffel austretende Warmband wird anschließend mit Luft, Wasser oder Luft und Wasser in Kombination auf eine 500 - 650 °C betragende Haspeltemperatur abgekühlt und bei dieser Temperatur gehaspelt. Bei einer Haspeltemperatur unterhalb von 500 °C wäre der Formänderungswiderstand im nachfolgenden Kaltwalzprozess zu hoch. Bei einer oberhalb von 650 °C liegenden Haspeltemperatur besteht die Gefahr, dass es zu in Bezug auf die Verformbarkeit schädlicher Korngrenzenoxidation kommt.
  • Zur Verbesserung seiner Oberflächenbeschaffenheit kann das Warmband optional gebeizt werden, wenn sich hierzu aufgrund von Qualitätsanforderungen die Notwendigkeit ergibt.
  • Das erhaltene Warmband wird nun zu einem kaltgewalzten Stahlflachprodukt kaltgewalzt, das typischerweise 0,6 - 2,5 mm dick ist. Dabei beträgt der beim Kaltwalzen erreichte Kaltwalzgrad mindestens 30 %, damit eine Rekristallisation überhaupt möglich ist. Um die Walzkräfte nicht zu hoch ansteigen zu lassen, sollte der Kaltwalzgrad 75 % nicht überschreiten.
  • Das kaltgewalzte Stahlflachprodukt wird dann einer Durchlaufglühung unterzogen. Dabei wird das Stahlflachprodukt zunächst auf eine 750 - 900 °C betragende Glühtemperatur erwärmt und bei dieser Glühtemperatur für mindestens 80 s, insbesondere für 80 - 300 s, gehalten. Die minimale Glühtemperatur von 750 °C und eine Haltedauer von mindestens 80 s sind notwendig, damit eine hinreichende Austenitisierung erreicht wird. Bei Glühtemperaturen von mehr als 900 °C würde die Bildung von Austenit zu stark gefördert. Dies würde zu einer Verschiebung der Gefügeanteile im Endprodukt führen, durch die die geforderte Festigkeit von 880 MPa nicht mehr gewährleistet wäre.
  • Nach dem Glühen wird das Stahlflachprodukt zweistufig abgekühlt.
The method according to the invention for producing a cold rolled flat steel product according to the invention comprises the following steps:
  • A molten steel containing (in% by weight) C: 0.12-0.19%, Mn: 1.5-2.5%, Si:> 0.60-1.0%, Al: ≤0, 1%, Cr: 0.2-0.6%, Ti: 0.05-0.15% and the balance of iron and inevitable impurities due to production, is poured into a precursor, which is a slab or thin slab is.
  • The precursor is heated at an austenitizing temperature of 1100-1300 ° C, which heating may include heating from a lower temperature, or as holding the respective slab or thin slab Utilization of the heat existing in them after their generation can be carried out. The heating is carried out taking into account the geometry of the precursor and the performance of the available heating device so that the structure of the precursor is completely austenitic at the end of this heating.
  • The pre-product thus heated at the austenitizing temperature is then hot rolled to a hot strip whose thickness is typically 1.8-4.7 mm. The temperature control in the several, usually five to seven rolling mills comprising hot rolling stand is chosen so that no recrystallization takes place in the first two stands of the hot rolling stand. For this purpose, the invention provides a hot rolling end temperature of 850-960 ° C.
  • The hot strip emerging from the last stand of the hot roll stand is subsequently cooled with air, water or air and water in combination to a coiler temperature of 500-650 ° C. and reeled at this temperature. At a reel temperature below 500 ° C, the deformation resistance in the subsequent cold rolling process would be too high. At a reel temperature higher than 650 ° C, there is a risk that it comes to deformability of harmful grain boundary oxidation.
  • To improve its surface finish, the hot strip may optionally be pickled if required by quality requirements.
  • The resulting hot strip is then cold rolled to a cold rolled flat steel product, which is typically 0.6-2.5 mm thick. The cold rolling degree achieved during cold rolling is at least 30%, so that recrystallization is possible at all. In order not to let the rolling forces rise too high, the cold rolling degree should not exceed 75%.
  • The cold-rolled steel flat product is then subjected to continuous annealing. The flat steel product is first heated to a 750 - 900 ° C amount annealing temperature and held at this annealing temperature for at least 80 s, in particular for 80 - 300 s. The minimum annealing temperature of 750 ° C and a holding time of at least 80 s are necessary in order to achieve sufficient austenitization. At annealing temperatures of more than 900 ° C, the formation of austenite would be promoted too much. This would lead to a shift in the proportion of microstructures in the end product, which would no longer guarantee the required strength of 880 MPa.
  • After annealing, the flat steel product is cooled in two stages.

In der ersten Stufe der Abkühlung wird das Stahlflachprodukt dabei mit einer Abkühlgeschwindigkeit von 8 - 100 K/s auf eine 450 - 550 °C betragende Zwischentemperatur abgekühlt. Die Abkühlgeschwindigkeit von mindestens 8 K/s wird hier benötigt, um die Bildung von Perlit und Bainit zu vermeiden und trotzdem hinreichend viel Ferrit entstehen zu lassen. In dem Temperaturbereich von 450 °C bis 550 °C findet zudem die erste Anreichung des Austenits mit Kohlenstoff statt.In the first stage of cooling, the steel flat product is cooled at a cooling rate of 8-100 K / s to an intermediate temperature of 450-550 ° C. The cooling rate of at least 8 K / s is needed here in order to avoid the formation of perlite and bainite and nevertheless to generate sufficient amount of ferrite. In the temperature range of 450 ° C to 550 ° C also finds the first enrichment of austenite with carbon instead.

In der zweiten Stufe der Abkühlung wird das Stahlflachprodukt dann von der Zwischentemperatur mit einer Abkühlgeschwindigkeit von mindestens 2 K/s auf 350 - 450 °C abgekühlt. Hierdurch wird ein Teil des maximal 20 % betragenden Martensitgehalts erreicht, durch den die 880 MPa betragende Mindestzugfestigkeit Rm eines erfindungsgemäßen Stahlflachprodukts gewährleistet wird.

  • Nachdem die Endtemperatur des zweistufigen Abkühlens erreicht ist, wird das Stahlflachprodukt überaltert. Die Endtemperatur nach einer Überalterungsdauer von 210 - 710 s beträgt 100 - 400 °C. Durch Diffusionsprozesse im Band beim Durchlaufen dieser Überalterungsbehandlung wird der Restaustenit ganz oder teilweise stabilisiert, um die Dehnfähigkeit des Stahlflachprodukts für nachfolgend an dem Stahlflachprodukt vorgenommene Umformungen zu erhöhen. Durch die Umwandlung von stabilisiertem Restaustenit in Martensit bei Umformprozessen wird zudem die Zugfestigkeit erhöht.
  • Im letzten Schritt der am kaltgewalzten Stahlflachprodukt vorgenommenen Wärmebehandlung wird das Stahlflachprodukt auf Raumtemperatur abgekühlt. Dabei kann aus dem nicht stabilisierten Restaustenit weiterer Martensit entstehen, der die Festigkeit des Stahlflachprodukts weiter erhöhen kann.
  • Anschließend wird das Band mit einem Dressiergrad von 0,2 % bis 2,0 % nachgewalzt. Ein Dressiergrad von 0,2 % wird benötigt, um die Planheit und die Oberflächenqualität einzustellen. Dressiergrade von 2 % sollen nicht überschritten werden, da sonst die Bruchdehnung zu stark absinkt.
  • Optional kann das Stahlflachprodukt abschließend mit einer metallischen Schutzschicht versehen werden, durch die beispielsweise ein für den jeweiligen Verwendungszweck ausreichender Korrosionsschutz gewährleistet wird.
In the second stage of the cooling, the flat steel product is then cooled from the intermediate temperature to 350-450 ° C at a cooling rate of at least 2 K / s. As a result, a portion of the maximum martensite content amounting to 20% is achieved by which the minimum tensile strength Rm of 880 MPa of a flat steel product according to the invention is ensured.
  • After the final temperature of the two-stage cooling is reached, the flat steel product is overaged. The final temperature after an aging period of 210 - 710 s is 100 - 400 ° C. Through diffusion processes in the strip when passing through this overaging treatment, the retained austenite is wholly or partly stabilized in order to increase the extensibility of the flat steel product for subsequent transformations made on the flat steel product. The conversion of stabilized retained austenite into martensite during forming processes also increases the tensile strength.
  • In the last step of the heat treatment performed on the cold-rolled steel flat product, the flat steel product is cooled to room temperature. In this case, from the unstabilized retained austenite further martensite can arise, which can further increase the strength of the flat steel product.
  • Subsequently, the strip is re-rolled with a degree of tempering of 0.2% to 2.0%. A degree of tempering of 0.2% is required to maintain the flatness and the Adjust surface quality. Dressing degrees of 2% should not be exceeded, otherwise the elongation at break drops too much.
  • Optionally, the flat steel product can finally be provided with a metallic protective layer which, for example, ensures sufficient corrosion protection for the respective intended use.

Die Abkühlung in der ersten Stufe der zweistufigen Abkühlung kann mit jedem geeigneten, eine ausreichende Abkühlgeschwindigkeit gewährleistenden Medium vorgenommen werden. Hierzu werden in der Praxis zur Verfügung stehende Kühleinrichtungen verwendet. So kann die Abkühlung in bewegter Luft erfolgen. Jedoch ist es auch denkbar, die Abkühlung mit Hilfe von Wasser vorzunehmen, das auf das Stahlflachprodukt gesprüht wird.The cooling in the first stage of the two-stage cooling can be carried out with any suitable, ensuring a sufficient cooling rate medium. For this purpose, available cooling devices are used in practice. So the cooling can be done in moving air. However, it is also conceivable to carry out the cooling with the aid of water, which is sprayed onto the flat steel product.

Die Abkühlung in der zweiten Stufe der zweistufigen Abkühlung kann gemäß einer praxisgerechten Ausgestaltung der Erfindung dadurch erfolgen, dass das Stahlflachprodukt durch Kontakt mit den gekühlten Rollen abgekühlt wird. Alternativ oder ergänzend kann das Stahlflachprodukt in der zweiten Stufe der zweistufigen Abkühlung durch einen bewegten Luftstrom gekühlt werden.The cooling in the second stage of the two-stage cooling can be carried out according to a practical embodiment of the invention in that the flat steel product is cooled by contact with the cooled rollers. Alternatively or additionally, the steel flat product can be cooled in the second stage of the two-stage cooling by a moving air flow.

Die Überalterungsbehandlung kann beispielsweise dadurch erfolgen, dass das Stahlflachprodukt bei der Überalterungsbehandlung einen gegenüber der Umgebung abgeschirmten Raum durchläuft. Dabei wird die Temperatur des Stahlflachprodukts auf 100 - 400 °C eingestellt. Ausgehend von der Temperatur, mit der das Stahlflachprodukt in die Überalterungsbehandlung eintritt, kann diese Einstellung der Temperatur als Erwärmen, Abkühlen oder Halten ausgeführt werden.The overaging treatment can be done, for example, by the steel flat product undergoing an environmentally shielded space in the overaging treatment. The temperature of the flat steel product is set to 100-400 ° C. Based on the temperature with which the Steel flat product enters the overaging treatment, this adjustment of the temperature can be carried out as heating, cooling or holding.

Die Beschichtung des Stahlflachprodukts mit der metallischen Schutzschicht kann besonders effektiv elektrolytisch erfolgen.The coating of the flat steel product with the metallic protective layer can be carried out particularly effectively electrolytically.

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

Die Figur zeigt ein Diagramm, in dem die für eine erfindungsgemäße Glühung typischen Spannen des Temperaturverlaufs über die Zeit dargestellt sind.The figure shows a diagram in which the typical for an inventive annealing spans the temperature profile are shown over time.

Sieben Stahlschmelzen 1 - 7, deren Zusammensetzungen in Tabelle 1a angegeben sind, sind zu Brammen vergossen worden, wobei die Stahlschmelzen 1 - 5 erfindungsgemäß und die Schmelzen 6 und 7 wegen ihres außerhalb der erfindungsgemäßen Vorgaben liegenden Si- bzw. Cr-Gehalts nicht erfindungsgemäß sind.Seven steel melts 1-7, the compositions of which are given in Table 1a, have been cast into slabs, with the molten steel 1 - 5 according to the invention and the melts 6 and 7 not being according to the invention because of their Si or Cr contents which are outside the specifications of the invention ,

Die Brammen sind anschließend bei einer 1100 - 1300 °C betragenden Austenitisierungstemperatur durcherwärmt worden, so dass die Brammen beim Eintritt in die nachfolgend durchlaufene Warmwalzstaffel ein vollständig austenitisches Gefüge besaßen.The slabs were then thoroughly heated at an austenitizing temperature of 1100-1300 ° C., so that the slabs had a completely austenitic microstructure on entry into the subsequently passed hot rolling mill.

Die Brammen sind dann bei den in Tabelle 1b angegebenen Warmwalzendtemperaturen WET zu Warmband mit einer Dicke dKW von 1,8 - 4,6 mm warmgewalzt, anschließend an Luft auf die jeweilige, ebenfalls in Tabelle 1b angegebene Haspeltemperatur HT abgekühlt und bei der jeweils erreichten Haspeltemperatur HT gehaspelt worden. Anschließend erfolgte optional ein Beizen, um auf dem Warmband vorhandenen Zunder vor dem Kaltwalzen zu entfernen und so optimale Oberflächenbeschaffenheiten beim nachfolgenden Kaltwalzen zu ermöglichen.The slabs are then hot-rolled at the hot-rolling end temperatures WET given in Table 1b to hot-rolled strip with a thickness DKW of 1.8-4.6 mm, then air, to the respective values also given in Table 1b Reel temperature HT cooled and reeled at the rewinding temperature HT respectively achieved. Pickling was then optionally performed to remove any scale present on the hot strip prior to cold rolling to allow for optimum surface finish during subsequent cold rolling.

Das daraufhin durchgeführte Kaltwalzen des jeweiligen Warmbands zu einem kaltgewalzten Stahlflachprodukt mit einer Dicke dKW erfolgte jeweils mit den auch in Tabelle 1b angegebenen Kaltwalzgraden KWG.The subsequent cold rolling of the respective hot strip to form a cold-rolled steel flat product with a thickness of DKW was carried out in each case with the cold rolling degrees KWG given in Table 1b.

Anschließend sind Proben der so erhaltenen kaltgewalzten Stahlflachprodukte verschiedenen Wärmebehandlungen A - J unterzogen worden, bei denen sie im Durchlauf jeweils auf eine Glühtemperatur GT erwärmt, dann über eine Glühdauer tG bei der Glühtemperatur GT gehalten, anschließend in einer ersten Abkühlstufe mit einer ersten Abkühlrate r1 auf eine erste Zieltemperatur ZT1 und unmittelbar daran anschließend in einer zweiten Abkühlstufe mit einer zweiten Abkühlrate r2 auf eine zweite Zieltemperatur ZT2 gebracht worden sind.Subsequently, samples of the cold-rolled steel flat products thus obtained have been subjected to various heat treatments A - J in which they are heated in each case to an annealing temperature GT, then held at the annealing temperature GT over an annealing time tG, then in a first cooling stage with a first cooling rate r1 a first target temperature ZT1 and immediately thereafter in a second cooling stage with a second cooling rate r2 have been brought to a second target temperature ZT2.

Nach der zweiten Stufe der Abkühlung sind die jeweils erhaltenen Proben der kaltgewalzten Stahlflachprodukte über eine Überalterungsdauer über eine 250 - 710 s betragende Dauer tUeA bei einer am Ende der Behandlung 400 - 100 °C betragenden Überalterungstemperatur TUeA in einem, gegenüber der Umgebung abgeschotteten Raum einer Überalterungsbehandlung unterzogen worden. Die bei den Wärmebehandlungen A - J jeweils eingestellten Parameter GT, tG, r1, ZT1, r2, ZT2 und tUeA sind in Tabelle 2 verzeichnet.After the second stage of cooling, the respectively obtained samples of cold rolled steel flat products are over an aging period of 250-710 s for a period of time tUeA at an over-aging temperature TUeA at 400-100 ° C at the end of the treatment, in an overburdened space of an overaging treatment been subjected. The parameters set during heat treatment A - J respectively GT, tG, r1, ZT1, r2, ZT2 and tUeA are listed in Table 2.

Nach einer Abkühlung auf die Raumtemperatur RT sind die Stahlflachproduktproben mit einem Nachwalzgrad von D°, wie in Tabelle 1b angegeben, nachgewalzt worden.After cooling to room temperature RT, the steel flat product samples were rethreaded with a degree of redrawing of D ° as indicated in Table 1b.

Die Eigenschaften der so erhaltenen Stahlflachproduktproben sind in Tabelle 3 zusammengefasst.The properties of the steel flat product samples thus obtained are summarized in Table 3.

Es zeigt sich, dass die aus den nicht erfindungsgemäß zusammengesetzten Stahlschmelzen 6 und 7 erzeugten Stahlflachproduktproben hinsichtlich ihrer Zugfestigkeit Rm bzw. ihrer Streckgrenze ReL die erfindungsgemäß vorgegebenen Untergrenzen von 880 MPa bzw. 550 MPa, insbesondere 580 MPa, auch dann nicht erreichen, wenn sie einer Wärmbehandlung unterzogen werden, die nach Maßgabe der Erfindung durchgeführt wird. Demgegenüber übertreffen die erfindungsgemäß zusammengesetzt und wärmebehandelten Stahlflachproduktproben regelmäßig diese Grenzwerte. Tabelle 1a Stahl C Mn Si Al Cr Ti P N S 1 0,17 1,9 0,72 0,04 0,37 0,114 0,012 0,0048 0,001 2 0,13 2,3 0,65 0,06 0,23 0,07 0,007 0,009 0,007 3 0,16 1,7 0,75 0,03 0,57 0,108 0,013 0,007 0,006 4 0,18 2,1 0,94 0,02 0,34 0,143 0,009 0,007 0,009 5 0, 14 1,5 0,83 0,08 0,48 0, 135 0,018 0,006 0,002 6 0,15 1,8 0,53 0,05 0,43 0,15 0,014 0,003 0,003 7 0,14 2,4 0,73 0,06 0,05 0,09 0,009 0,004 0,005 Gehaltsangaben in Gew.-%, Rest Eisen und unvermeidbare Verunreinigungen Tabelle 1b Stahl WET [°C] HT [°C] dW [mm] KWG [%] dKB [mm] D° [%] Erfindungsgemäß? 1 900 530 2,0 50 1,0 0,51 JA 2 910 560 2,0 45 1,1 0,69 JA 3 870 510 2,0 55 0,9 0,30 JA 4 950 635 4,6 50 2,4 1,50 JA 5 935 545 1,8 60 0,7 0,25 JA 6 910 530 2,4 50 1,2 0,96 NEIN 7 910 530 3,4 50 1,7 0,78 NEIN Tabelle 2 Wärmebehandlung GT [°C] tG, [s] r1 [K/s] ZT1 [°C] r2, [K/s] ZT2 [°C] TUeA [°C] tUeA [s] A 750 150 9, 3 500 4,3 400 260 440 B 810 150 11,6 500 4,3 400 280 440 C 840 150 13 500 4,3 400 290 440 D 900 150 15 500 4,3 400 270 440 E 810 200 8,4 500 3,1 400 150 600 F 810 90 18,5 500 6,9 400 350 250 G 810 150 13,4 450 4,3 350 300 440 H 810 150 9,7 550 4,3 450 320 500 I 810 150 14,8 500 4,3 350 350 440 J 810 150 17,9 500 4,3 350 400 440 Tabelle 3 Stahl Wärmebehandlung ReL [MPa] Rm [MPa] A80 [%] Gefügeanteile [%] Rm*A [MPa*%] ReL/Rm Steigung Wöhler-Kurve Lochaufweitung λ [%] Erfindungsgemäß? Ferrit Restaustenit Martensit 1 A 670 1123 15,1 75 5 20 16957 0,60 7,5 11,9 JA 1 B 660 1010 15,8 76 5 19 15958 0,65 8,9 12,5 JA 1 C 642 945 16,8 80 7 13 15876 0,68 12,4 13,8 JA 2 E 581 891 18,9 77 10 13 16840 0,65 11,5 7,4 JA 2 F 591 887 16, 8 80 9 11 14902 0,67 13,1 7,8 JA 2 G 602 901 19,2 78 12 10 17299 0,67 5,9 7,9 JA 3 B 651 922 16,2 80 11 9 14936 0,71 10,6 12,8 JA 3 C 629 892 17,1 79 13 8 15253 0,71 7,1 13,9 JA 3 D 591 882 19,4 81 12 7 17111 0,67 12,8 14 JA 4 F 632 1078 15,1 77 8 15 16278 0,59 13,2 12,3 JA 4 G 664 1023 17,2 79 9 12 17596 0,65 12,6 12,5 JA H 667 1031 17,3 76 10 14 17836 0,65 4, 6 12,2 JA 5 D 603 897 20,1 79 13 8 18030 0,67 8,3 13,4 JA 5 E 663 1030 16,2 77 10 13 16686 0,64 13,0 12,1 JA 5 I 657 1021 17,3 77 5 18 17663 0,64 9,1 12,3 JA 5 J 641 986 19,8 76 14 9 19523 0,65 7,6 15,4 JA 6 C 560 840 21,2 84 8 8 17808 0,67 11,5 10,3 NEIN 6 D 491 812 22 80 10 10 17864 0,60 11,9 12,2 NEIN 7 B 531 876 18,3 81 9 10 16031 0,61 5,8 5,9 NEIN 7 C 512 823 19,1 85 9 6 15719 0,62 9,9 5,7 NEIN It can be seen that the steel flat product samples produced from the non-composite steel melts 6 and 7 do not reach the lower limits of 880 MPa or 550 MPa, in particular 580 MPa, given their tensile strength Rm or their yield strength ReL, even if they have a lower limit Subjected to heat treatment, which is carried out in accordance with the invention. On the other hand, the flat product samples assembled and heat-treated according to the invention regularly exceed these limits. Table 1a stole C Mn Si al Cr Ti P N S 1 0.17 1.9 0.72 0.04 0.37 0.114 0,012 0.0048 0.001 2 0.13 2.3 0.65 0.06 0.23 0.07 0,007 0.009 0,007 3 0.16 1.7 0.75 0.03 0.57 0.108 0,013 0,007 0,006 4 0.18 2.1 0.94 0.02 0.34 0.143 0.009 0,007 0.009 5 0, 14 1.5 0.83 0.08 0.48 0, 135 0,018 0,006 0,002 6 0.15 1.8 0.53 0.05 0.43 0.15 0,014 0,003 0,003 7 0.14 2.4 0.73 0.06 0.05 0.09 0.009 0,004 0.005 Content in wt .-%, balance iron and unavoidable impurities stole WET [° C] HT [° C] dW [mm] KWG [%] dKB [mm] D ° [%] According to the invention? 1 900 530 2.0 50 1.0 0.51 YES 2 910 560 2.0 45 1.1 0.69 YES 3 870 510 2.0 55 0.9 0.30 YES 4 950 635 4.6 50 2.4 1.50 YES 5 935 545 1.8 60 0.7 0.25 YES 6 910 530 2.4 50 1.2 0.96 NO 7 910 530 3.4 50 1.7 0.78 NO heat treatment GT [° C] tG, [s] r1 [K / s] ZT1 [° C] r2, [K / s] ZT2 [° C] TUeA [° C] tUeA [s] A 750 150 9, 3 500 4.3 400 260 440 B 810 150 11.6 500 4.3 400 280 440 C 840 150 13 500 4.3 400 290 440 D 900 150 15 500 4.3 400 270 440 e 810 200 8.4 500 3.1 400 150 600 F 810 90 18.5 500 6.9 400 350 250 G 810 150 13.4 450 4.3 350 300 440 H 810 150 9.7 550 4.3 450 320 500 I 810 150 14.8 500 4.3 350 350 440 J 810 150 17.9 500 4.3 350 400 440 stole heat treatment ReL [MPa] Rm [MPa] A80 [%] Structure shares [%] Rm * A [MPa *%] Re L / Rm Slope Wöhler curve Hole widening λ [%] According to the invention? ferrite austenite martensite 1 A 670 1123 15.1 75 5 20 16957 0.60 7.5 11.9 YES 1 B 660 1010 15.8 76 5 19 15958 0.65 8.9 12.5 YES 1 C 642 945 16.8 80 7 13 15876 0.68 12.4 13.8 YES 2 e 581 891 18.9 77 10 13 16840 0.65 11.5 7.4 YES 2 F 591 887 16, 8 80 9 11 14902 0.67 13.1 7.8 YES 2 G 602 901 19.2 78 12 10 17299 0.67 5.9 7.9 YES 3 B 651 922 16.2 80 11 9 14936 0.71 10.6 12.8 YES 3 C 629 892 17.1 79 13 8th 15253 0.71 7.1 13.9 YES 3 D 591 882 19.4 81 12 7 17111 0.67 12.8 14 YES 4 F 632 1078 15.1 77 8th 15 16278 0.59 13.2 12.3 YES 4 G 664 1023 17.2 79 9 12 17596 0.65 12.6 12.5 YES H 667 1031 17.3 76 10 14 17836 0.65 4, 6 12.2 YES 5 D 603 897 20.1 79 13 8th 18030 0.67 8.3 13.4 YES 5 e 663 1030 16.2 77 10 13 16686 0.64 13.0 12.1 YES 5 I 657 1021 17.3 77 5 18 17663 0.64 9.1 12.3 YES 5 J 641 986 19.8 76 14 9 19523 0.65 7.6 15.4 YES 6 C 560 840 21.2 84 8th 8th 17808 0.67 11.5 10.3 NO 6 D 491 812 22 80 10 10 17864 0.60 11.9 12.2 NO 7 B 531 876 18.3 81 9 10 16031 0.61 5.8 5.9 NO 7 C 512 823 19.1 85 9 6 15719 0.62 9.9 5.7 NO

Claims (9)

  1. Cold-rolled flat steel product, which
    - is produced from a steel that is composed of (in % by weight)
    C: 0.12 - 0.19%,
    Mn: 1.5 - 2.5%,
    Si: > 0.60 - 1.0%
    Al: ≤ 0.1%,
    Cr: 0.2 - 0.6%,
    Ti: 0.05 - 0.15%
    with the remainder being iron and unavoidable impurities caused by the production process,
    and which comprises
    - a perlite- and bainite-free structure having 4 - 20% by vol. martensite, 2 - 15% by vol. residual austenite, remainder ferrite,
    - an elongation at break A80 of at least 15%,
    - a tensile strength Rm of at least 880 MPa,
    - a yield strength ReL of at least 550 MPa and
    - a hole expansion ratio λM of more than 6%.
  2. Cold-rolled flat steel product according to claim 1, characterised in that its Si content is at least 0.65% by weight.
  3. Cold-rolled flat steel product according to any one of the preceding claims, characterised in that its Ti content is at least 0.07% by weight.
  4. Method for producing a cold-rolled flat steel product created according to any one of claims 1 to 3, comprising the following steps:
    - casting a steel melt which is composed of (in % by weight)
    C: 0.12 - 0.19%,
    Mn: 1.5 - 2.5%,
    Si: > 0.60 - 1.0
    Al: ≤ 0.1%,
    Cr: 0.2 - 0.6%,
    Ti: 0.05 - 0.15%
    with the remainder being iron and unavoidable impurities caused by the production process,
    to form a primary product which is a slab or thin slab,
    - heating through the primary product to an austenitization temperature of 1,100 - 1,300 °C,
    - hot rolling the heated-through primary product to form a hot strip, wherein the hot-rolling end temperature is 850 - 960 °C,
    - cooling the hot strip to a coiling temperature of 500 - 650 °C
    - coiling the hot strip cooled to the coiling temperature,
    - optional pickling of the hot strip,
    - cold-rolling the hot strip to form a cold-rolled flat steel product, wherein the level of cold rolling achieved during cold rolling is at least 30%,
    - continuous annealing of the cold-rolled flat steel product, wherein during the course of continuous annealing the flat steel product
    - is heated to an annealing temperature of 750 - 900 °C and is held at this annealing temperature for 80 - 300 s and
    - following the annealing process is cooled in two stages, wherein the flat steel product
    - is cooled in the first cooling stage at a cooling rate of 8 - 100 K/s to an intermediate temperature of 450 - 550 °C and
    - is cooled in the second cooling stage from the intermediate temperature at a cooling rate of 2 - 100 K/s to 350 - 450 °C,
    - overageing the flat steel product for an overageing period of 210 - 710 s, wherein at the end of overageing the temperature is 100 - 400 °C,
    - cooling the flat steel product to room temperature,
    - temper rolling the flat steel product with a level of temper rolling of 0.2 - 2%,
    - optional coating of the flat steel product with a metallic protective layer.
  5. Method according to claim 4, characterised in that cooling in the first stage of the two-stage cooling process occurs in moving air.
  6. Method according to either of claims 4 to 5, characterised in that the flat steel product is cooled, at least in the second stage of the two-stage cooling, by contact with the cooled rollers.
  7. Method according to any one of claims 4 to 6, characterised in that the flat steel product is cooled in the second stage of the two-stage cooling process by a moving flow of air.
  8. Method according to claims 4 to 7, characterised in that during the overageing treatment the flat steel product passes through a space screened from the environment and in which the temperature of the flat steel product, starting from a maximum inlet temperature of 450 °C, is 100 - 400 °C at the end.
  9. Method according to any one of claims 4 to 8, characterised in that the coating with the metallic protective layer is made electrolytically.
EP12175756.1A 2012-07-10 2012-07-10 Cold rolled steel flat product and method for its production Not-in-force EP2684975B1 (en)

Priority Applications (9)

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ES12175756.1T ES2614465T3 (en) 2012-07-10 2012-07-10 Flat product of cold rolled steel and manufacturing process
EP12175756.1A EP2684975B1 (en) 2012-07-10 2012-07-10 Cold rolled steel flat product and method for its production
PL12175756T PL2684975T3 (en) 2012-07-10 2012-07-10 Cold rolled steel flat product and method for its production
PCT/EP2013/064551 WO2014009404A1 (en) 2012-07-10 2013-07-10 Cold-rolled flat steel product and method for the production thereof
US14/377,398 US10344344B2 (en) 2012-07-10 2013-07-10 Cold-rolled flat steel product and method for its production
CN201380036484.9A CN104471096B (en) 2012-07-10 2013-07-10 Flat cold-rolled bar product and its manufacture method
BR112014021543-0A BR112014021543B1 (en) 2012-07-10 2013-07-10 COLD LAMINATED FLAT STEEL PRODUCT AND PROCESS FOR ITS PRODUCTION
JP2015520969A JP6236078B2 (en) 2012-07-10 2013-07-10 Cold rolled steel sheet product and method for producing the same
KR1020147021943A KR102128563B1 (en) 2012-07-10 2013-07-10 Cold-rolled flat steel product and method for the production thereof

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
EP12175756.1A EP2684975B1 (en) 2012-07-10 2012-07-10 Cold rolled steel flat product and method for its production

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EP2684975A1 EP2684975A1 (en) 2014-01-15
EP2684975B1 true EP2684975B1 (en) 2016-11-09

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JP (1) JP6236078B2 (en)
KR (1) KR102128563B1 (en)
CN (1) CN104471096B (en)
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ES (1) ES2614465T3 (en)
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WO2015185956A1 (en) * 2014-06-06 2015-12-10 ArcelorMittal Investigación y Desarrollo, S.L. High strength multiphase galvanized steel sheet, production method and use
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KR102128563B1 (en) 2020-07-08
EP2684975A1 (en) 2014-01-15
CN104471096B (en) 2017-08-15
KR20150031407A (en) 2015-03-24
PL2684975T3 (en) 2017-08-31
WO2014009404A1 (en) 2014-01-16
BR112014021543B1 (en) 2020-03-17
JP2015528058A (en) 2015-09-24
ES2614465T3 (en) 2017-05-31
JP6236078B2 (en) 2017-11-22
US20150000797A1 (en) 2015-01-01
US10344344B2 (en) 2019-07-09

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