EP3305935B9 - High strength flat steel product and use of a high strength flat steel product - Google Patents

High strength flat steel product and use of a high strength flat steel product Download PDF

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Publication number
EP3305935B9
EP3305935B9 EP17191293.4A EP17191293A EP3305935B9 EP 3305935 B9 EP3305935 B9 EP 3305935B9 EP 17191293 A EP17191293 A EP 17191293A EP 3305935 B9 EP3305935 B9 EP 3305935B9
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EP
European Patent Office
Prior art keywords
flat steel
content
steel product
steel
hot
Prior art date
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Application number
EP17191293.4A
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German (de)
French (fr)
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EP3305935A1 (en
EP3305935B1 (en
Inventor
Alexander GAGANOV
Wolfgang Gervers
Andreas Kern
Gabriel Kolek
Elena Schaffnit
Hans-Joachim Tschersich
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
ThyssenKrupp AG
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ThyssenKrupp Steel Europe AG
ThyssenKrupp AG
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Application filed by ThyssenKrupp Steel Europe AG, ThyssenKrupp AG filed Critical ThyssenKrupp Steel Europe AG
Priority to SI201431325T priority Critical patent/SI3305935T1/en
Priority to EP17191293.4A priority patent/EP3305935B9/en
Priority to PL17191293T priority patent/PL3305935T3/en
Priority to DK17191293.4T priority patent/DK3305935T3/en
Priority to ES17191293T priority patent/ES2745046T3/en
Publication of EP3305935A1 publication Critical patent/EP3305935A1/en
Publication of EP3305935B1 publication Critical patent/EP3305935B1/en
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • 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
    • 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
    • C21D8/0247—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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

Definitions

  • the invention relates to a high-strength flat steel product with a yield strength of 700-850 MPa and with at least 70% by volume bainitic structure and to a use of such a high-strength steel flat product.
  • Flat steel products of the type in question are typically rolled products, such as steel strips or sheets, as well as blanks and blanks made therefrom.
  • High-strength flat steel products in particular in the field of commercial vehicle construction an increasing importance, as they reduce the dead weight of the vehicle and a Increase the payload.
  • a low weight not only contributes to the optimal use of the technical performance of the respective drive unit, but also supports resource efficiency, cost optimization and climate protection.
  • a significant reduction in the dead weight of steel sheet constructions can be achieved by increasing the mechanical properties, in particular the strength of each processed flat steel product.
  • modern toughened steel products intended for commercial vehicle construction are also expected to have good toughness properties, good brittle fracture resistance behavior and optimum suitability for cold forming and welding.
  • the steel slab After pouring and solidification of the melt in the known method, the steel slab is reheated to a temperature range whose lower limit is determined depending on the C and Nb contents of each potted steel and whose upper limit is 1170 ° C. Subsequently, the reheated slab is pre-rolled at a final temperature which is 1080-1150 ° C. After a pause of 30-150 seconds, during which the pre-rolled slab is maintained at 1000-1080 ° C, the pre-rolled slab is then hot-rolled to a hot-rolled strip. The degree of deformation of the last pass of the hot rolling should be 3 - 15%.
  • the hot rolling is completed at a hot rolling end temperature which is at least the Ar3 temperature of the processed steel and is at most 950 ° C.
  • the hot strip obtained is cooled at a cooling rate of more than 15 ° C / s to a coiling temperature of 450 - 550 ° C, where it is coiled into a coil.
  • the grain boundary density of the carbon present in solid solution should be 1 - 4.5 atoms / nm 2 and the size of the grains of cementite precipitated at the grain boundaries should not exceed 1 ⁇ m .
  • the flat steel products produced in this way and produced by the known method should have tensile strengths of more than 780 MPa and have yield strengths of up to 726 MPa at sufficiently high-dose alloy contents.
  • the hot strip produced in the known manner should have a combination of properties which is particularly suitable for use in automobile construction. Optimum surface finish is achieved by limiting the reheat temperature to which the slab is heated prior to hot rolling to the above-mentioned temperature range and thus avoiding excessive scale formation which would be incorporated into the hot strip surface during hot rolling.
  • the microstructure of the sheet consists of at least 40 area% bainite and the remainder ferrite or martensite, with the structure consisting of Ti (C, N) precipitates of sizes 10 nm or less in a range of at least 10 10 precipitates / mm3 available are.
  • the ratio Hvs / Hvc of a hardness Hvs measured at a distance of 10 ⁇ m from the surface of the sheet to the hardness Hvc in the center of the sheet thickness is 0.85 or more.
  • a steel sheet the (in wt .-%) of 0.06% C, 0.18% Si, 1.75% Mn, 0.082% P, 0.0044% S, 0.04% Al, 0 , 0035% N, 0.092% Ti, 0.075% Nb and 0.0015% B, balance iron and unavoidable impurities, one (in area%) to 70% bainite, 5% ferrite and 25% martensite existing structure and a yield strength of 960 MPa.
  • the object of the invention was to specify a high-strength steel sheet having mechanical properties optimized with regard to use in automobile construction and also an optimized surface finish.
  • the invention achieves this object by the flat steel product specified in claim 1.
  • a flat steel product according to the invention with a yield strength of 700-850 MPa and with at least 70% by volume bainitic structure can be produced by the following working steps: a) melting a molten steel, the (in wt .-%) from C: 0.05-0.08%, Si: 0.015 - 0.500%, Mn: 1.60 - 2.00%, P: up to 0.025%, S: up to 0.010%, al: 0.020-0.050%, N: up to 0.006%, Cr: up to 0.40%, Nb: 0.060-0.070%, B: 0.0005 - 0.0025%, Ti: 0.090 - 0.130%, and technically unavoidable impurities including up to 0.12% Cu, up to 0.100% Ni, up to 0.010% V, up to 0.004% Mo and up to 0.004% Sb, and as the remainder of iron consists; b) pouring the melt into a slab; c) reheating the slab to a reheating temperature of 1200 - 1300 °
  • This method is based on a steel alloy whose alloying constituents and alloy contents are coordinated with one another within narrow limits in such a way that maximized mechanical properties and optimized surface textures are achieved in the case of an operationally reliable procedure.
  • Cu, Ni, V, Mo and Sb occur as accompanying elements, which enter the steel processed according to the invention as a technically unavoidable impurity in the steelmaking process. Their contents are limited to amounts which are ineffective in relation to the properties of the steel processed according to the invention.
  • the Cu content is limited to max. 0.12 wt .-%, the Ni content to less than 0.1 wt%, the V content to at most 0.01 wt .-%, the Mo content to less than 0.004 wt .-%, and the Sb content is also limited to less than 0.004 wt%.
  • an austenitizing temperature which is 1200-1300 ° C.
  • the upper limit of the temperature range to which the slab is heated to austenitise should not be exceeded in order to avoid coarsening of the austenite grain and increased scale formation.
  • reheating temperature 1200 - 1300 ° C, it is not yet to increased formation of Rotzunder that would reduce the surface quality of the invention produced according to the flat steel product.
  • Rotzunder forms in the processing of composite slabs according to the invention exclusively during the hot rolling process (steps d), e) of the process), if after reheating too much primary scale on the slab surface is present.
  • the lower limit of the reheating temperature is set so that the desired homogenization of the structure is ensured with a uniform temperature distribution. From this temperature, a largely complete dissolution of the coarse Ti and Nb carbonitride precipitates present in the respective slab begins in the austenite.
  • fine Ti or Nb carbonitride precipitates can be newly formed which, as explained, make a significant contribution to increasing the strength properties. In this way, it is ensured that the flat steel products produced and assembled according to the invention regularly have a minimum yield strength of 700 MPa.
  • the rewarming temperature during austenitisation of the respective slab is at least 1200 ° C., in order to achieve the desired effect of the most complete possible dissolution of the TiC and NbC precipitates.
  • the austenitizing temperature is below 1200 ° C.
  • the amount of carbide precipitates of Ti and Nb dissolved in austenite is so small that the effects used according to the invention do not occur.
  • a rewarming temperature below 1200 ° C. would therefore result in the processing of flat steel products which are composed according to the alloy selection optimized according to the invention, that the required strength properties are not achieved.
  • the most complete possible dissolution of the TiC and NbC precipitates can be ensured with particular certainty if the reheating temperature is at least 1250 ° C.
  • a flat steel product meeting the highest quality requirements for its surface finish can be produced by completely removing the scale present on the slab before rough rolling. This can be done by the slab surface after the Furnace discharge and as soon as possible immediately before the rough rolling is completely descaled. For this purpose, the slab can go through a conventional scale scrubber.
  • the time t_1 the transfer of the slab from the workstation ("reheating (step c)") or the optional "post-reheating" removal of the primary scale (step c ') ") may be started of finish hot rolling (step e)) is required, limited to a maximum of 300 s.
  • the transport time between the descaling unit and the roughing stand should not exceed 30 s. With such a short transport time, no or at most a harmless thin oxide layer can thus form on the previously descaled slab.
  • step d the respectively processed slab is pre-rolled at a rough rolling temperature of 950-1250 ° C.
  • the total reduction in pre-rolling amounts to at least 50%.
  • the lower limit of the predetermined for the rough rolling temperature range and the minimum value of the total stitch decrease ⁇ hv are set so that the recrystallization processes in the respective pre-rolled slab can run completely. In this way, the formation of a fine-grained austenitic structure is ensured before the finish rolling, whereby optimized toughness and elongation at break properties of the steel flat product produced according to the invention are achieved.
  • the dwell and pause time t_2 between rough rolling and finish rolling is limited to 50 seconds to avoid undesirable austenite grain growth.
  • Pre-rolling is followed, in step e), by hot rolling of the pre-rolled slab into a hot-rolled flat steel product having a hot strip thickness of typically 3-15 mm.
  • Flat steel products with such thicknesses are also referred to in the jargon as "heavy plate”.
  • the final temperature of hot rolling is 800 - 880 ° C.
  • the comparatively low hot rolling end temperature enhances the effect of hot rolling.
  • the upper limit of the range of the hot rolling finish temperature is set so that no recrystallization of the austenite takes place during rolling in the hot rolling finishing line. This also contributes to the expression of a fine-grained structure.
  • the lower limit temperature is at least 800 ° C, so that no ferrite forms during rolling.
  • the cooling break after hot rolling is at most 10 seconds to prevent undesirable microstructural changes between hot rolling and controlled accelerated cooling.
  • the choice of reel temperature has a decisive influence on precipitation hardening.
  • the reel temperature range is selected so that it is below the bainite start temperature, on the one hand, and at the precipitation maximum for the formation of carbonitride precipitates, on the other hand.
  • the cooling conditions are chosen so that the hot rolled flat steel product immediately before the reel has a bainitic structure with a phase content of at least 70 vol .-%.
  • Another Bainit Guess then runs off in the reel.
  • the high cooling rate avoids the formation of unwanted phase components.
  • the cooling rate of the cooling after hot rolling can be limited to 150 K / s.
  • the yield strength of the manner explained above produced hot-rolled steel flat products is reliably 700 - 850 MPa. Their elongation at break is in each case at least 12%. Equally regularly, steel flat products according to the invention achieve tensile strengths of 750-950 MPa.
  • the notched impact work determined for products according to the invention is in the range from 50 to 110 J at -20 ° C. and in the range from 30 to 110 J at -40 ° C.
  • Steel flat products produced in the manner explained above have a fine-grained structure with an average grain size of at most 20 ⁇ m in order to achieve good elongation at break and toughness.
  • the reheated slabs have been transported in less than 30 s to a scale washer in which the primary scale adhering to them has been removed from the slabs.
  • the slabs emerging from the scale scrubber have then been transported to a roughing stand, where they have been pre-rolled with a roughing temperature TVW and an overall reduction in the total value ⁇ hv achieved via rough rolling.
  • the pre-rolled slabs were finished hot rolled in a finished hot rolling mill to hot strip with a thickness of BD and a width BB.
  • the hot rolling has been completed with a total decrease in the finished heat transfer scale ⁇ hf at a hot rolling end temperature TEW.
  • the finished hot-rolled flat steel product leaving the last stand has been cooled to a coiling temperature HT by intensive cooling with water at a cooling rate dT of 50-120 K / s , After cooling, the flat steel products already had at least 70% by volume bainitic structure.
  • the tensile tests for determining the yield strength ReH, the tensile strength Rm and the elongation at break A were carried out according to DIN EN ISO 6892-1 on longitudinal samples of the hot strips.
  • the notched bar impact tests to determine the impact energy Av at -20 ° C and -40 ° C and -60 ° C were carried out on longitudinal samples according to DIN EN ISO 148-1.
  • the structural investigations were carried out by light microscope and scanning electron microscope. For this, the samples were taken from a quarter of the bandwidth, prepared as a longitudinal section and with Nital (ie alcoholic nitric acid containing a proportion of 3% by volume of nitric acid) or Sodium disulfite etched. The determination of the structural components was carried out by means of area analysis in sample position 1/3 sheet thickness, as in H. Schumann and H. Oettel "Metallography" 14th edition, 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim described.
  • the mechanical properties and the structural constituents of the hot strips produced according to the invention are given in Table 3.
  • the band sheets produced according to the method of the present invention have high strength properties with good toughness properties and good elongation at break.
  • the yield strengths of the hot strips produced in the above manner are between 700 MPa and 790 MPa.
  • the elongation at break is at least 12% and the tensile strength 750-880 MPa.
  • the notch impact work at -20 ° C is in the range 60 to 100 J.
  • the notch impact work is 40 to 75 J and at -60 ° C, the impact energy is at 30 - 70 J.

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

Description

Die Erfindung betrifft ein hochfestes Stahlflachprodukt mit einer Streckgrenze von 700 - 850 MPa und mit einem zu mindestens 70 Vol.-% bainitischen Gefüge sowie eine Verwendung eines solchen hochfesten Stahlflachprodukts.The invention relates to a high-strength flat steel product with a yield strength of 700-850 MPa and with at least 70% by volume bainitic structure and to a use of such a high-strength steel flat product.

Bei Stahlflachprodukten der hier in Rede stehenden Art handelt es sich typischerweise um Walzprodukte, wie Stahlbänder oder Bleche sowie daraus hergestellte Zuschnitte und Platinen.Flat steel products of the type in question are typically rolled products, such as steel strips or sheets, as well as blanks and blanks made therefrom.

Alle Angaben zu Gehalten der in der vorliegenden Anmeldung angegebenen Stahlzusammensetzungen sind auf das Gewicht bezogen, sofern nicht ausdrücklich anders erwähnt. Alle nicht näher bestimmten, im Zusammenhang mit einer Stahllegierung stehenden "%-Angaben" sind daher als Angaben in "Gew.-%" zu verstehen.All information on contents of the steel compositions given in the present application are by weight, unless expressly stated otherwise. All unspecified "% figures" in connection with a steel alloy are therefore to be understood as statements in "% by weight".

Hochfeste Stahlflachprodukte haben insbesondere im Bereich des Nutzfahrzeugbaus eine wachsende Bedeutung, da sie eine Reduzierung des Eigengewichts des Fahrzeugs und eine Steigerung der Nutzlast ermöglichen. Ein geringes Gewicht trägt nicht nur zur optimalen Nutzung der technischen Leistungsfähigkeit des jeweiligen Antriebsaggregats bei, sondern unterstützt die Ressourceneffizienz, Kostenoptimierung und den Klimaschutz.High-strength flat steel products, in particular in the field of commercial vehicle construction an increasing importance, as they reduce the dead weight of the vehicle and a Increase the payload. A low weight not only contributes to the optimal use of the technical performance of the respective drive unit, but also supports resource efficiency, cost optimization and climate protection.

Eine entscheidende Reduzierung des Eigengewichts von Stahlblechkonstruktionen kann durch eine Steigerung der mechanischen Eigenschaften, insbesondere der Festigkeit des jeweils verarbeiteten Stahlflachprodukts erreicht werden. Neben einer hohen Festigkeit werden von modernen, für den Nutzfahrzeugbau vorgesehenen Stahlflachprodukten aber auch gute Zähigkeitseigenschaften, ein gutes Sprödbruchwiderstandsverhalten sowie eine optimale Eignung zum Kaltumformen und Schweißen erwartet.A significant reduction in the dead weight of steel sheet constructions can be achieved by increasing the mechanical properties, in particular the strength of each processed flat steel product. In addition to high strength, modern toughened steel products intended for commercial vehicle construction are also expected to have good toughness properties, good brittle fracture resistance behavior and optimum suitability for cold forming and welding.

Es ist bekannt, dass diese Eigenschaftskombination durch Wahl eines geeigneten Legierungskonzepts und ein spezielles Herstellverfahren erreicht werden kann. Bei konventionellen Verfahren zum Herstellen hochfester Grobbleche mit einer Mindeststreckgrenze von 700 MPa wird wie folgt vorgegangen. Zunächst werden die Brammen warmgewalzt und nach dem Walzen an Luft abgekühlt. Danach werden die Bleche wiedererwärmt, gehärtet und einer Anlassbehandlung unterzogen. Der Prozess enthält also mehrere Stufen, um die mechanischen Eigenschaften zu erreichen. Die Vielzahl der damit verbundenen Herstellschritte führt zu vergleichbar hohen Herstellkosten. Auch ist eine exakte Verfahrensführung erforderlich, um die gewünschten Zähigkeitseigenschaften und Oberflächenqualitäten zu erreichen.It is known that this combination of properties can be achieved by choosing a suitable alloying concept and a specific manufacturing method. In conventional processes for producing high-strength plates with a minimum yield strength of 700 MPa, the procedure is as follows. First, the slabs are hot rolled and cooled after rolling in air. Thereafter, the sheets are reheated, cured and subjected to a tempering treatment. The process thus contains several stages in order to achieve the mechanical properties. The large number of associated production steps leads to comparatively high production costs. Also, exact process control is required to achieve the desired toughness properties and surface qualities.

Aus der EP 2 130 938 A1 ist ein Verfahren zur Herstellung eines warmgewalzten Stahlflachprodukts bekannt, bei dem eine Schmelze zu Brammen vergossen wird, die neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,01 - 0,1 Gew.-% C, 0,01 - 0,1 Gew.-% Si, 0,1 - 3 Gew.-% Mn, nicht mehr als 0,1 Gew.-% P, nicht mehr als 0,03 Gew.-% S, 0,001 - 1 Gew.-% Al, nicht mehr als 0,01 Gew.-% N, 0,005 - 0,08 Gew.-% Nb und 0,001 bis 0,2 Gew.-% Ti enthält, wobei für den jeweiligen Nb-Gehalt %Nb und den jeweiligen C-Gehalt %C gilt: %Nb x %C ≤ 4.34 x 10-3.From the EP 2 130 938 A1 For example, there is known a method of producing a hot-rolled flat steel product in which a melt is cast into slabs which contain, in addition to iron and unavoidable impurities (in% by weight) 0.01-0.1% by weight C, 0.01-0 , 1 wt% Si, 0.1-3 wt% Mn, not more than 0.1 wt% P, not more than 0.03 wt% S, 0.001-1 wt% Al, not more than 0.01 wt .-% N, 0.005 - 0.08 wt .-% Nb and 0.001 to 0.2 wt .-% Ti, wherein for the respective Nb content% Nb and the respective C Content% C is:% Nb x% C ≤ 4.34 x 10 -3 .

Nach dem Abgießen und Erstarren der Schmelze wird bei dem bekannten Verfahren die Stahlbramme bis in einen Temperaturbereich wiedererwärmt, dessen Untergrenze in Abhängigkeit der C- und Nb-Gehalte des jeweils vergossenen Stahls bestimmt wird und dessen Obergrenze 1170 °C beträgt. Anschließend wird die wiedererwärmte Bramme bei einer Endtemperatur vorgewalzt, die 1080 - 1150 °C beträgt. Nach einer 30 - 150 Sekunden betragenden Pause, bei der die vorgewalzte Bramme bei 1000 - 1080°C gehalten wird, wird die vorgewalzte Bramme dann zu einem Warmband fertig warmgewalzt. Der Umformgrad des letzten Stichs des Warmwalzens soll 3 - 15 % betragen.After pouring and solidification of the melt in the known method, the steel slab is reheated to a temperature range whose lower limit is determined depending on the C and Nb contents of each potted steel and whose upper limit is 1170 ° C. Subsequently, the reheated slab is pre-rolled at a final temperature which is 1080-1150 ° C. After a pause of 30-150 seconds, during which the pre-rolled slab is maintained at 1000-1080 ° C, the pre-rolled slab is then hot-rolled to a hot-rolled strip. The degree of deformation of the last pass of the hot rolling should be 3 - 15%.

Gemäß dem bekannten Verfahren wird das Warmwalzen bei einer Warmwalzendtemperatur beendet, die mindestens der Ar3-Temperatur des verarbeiteten Stahls entspricht und höchstens 950 °C beträgt. Nach dem Ende des Warmwalzens wird das erhaltene Warmband mit einer Abkühlgeschwindigkeit von mehr als 15 °C/s auf eine Haspeltemperatur von 450 - 550 °C abgekühlt, bei der es zu einem Coil gehaspelt wird.According to the known method, the hot rolling is completed at a hot rolling end temperature which is at least the Ar3 temperature of the processed steel and is at most 950 ° C. After the end of the hot rolling, the hot strip obtained is cooled at a cooling rate of more than 15 ° C / s to a coiling temperature of 450 - 550 ° C, where it is coiled into a coil.

Im so erzeugten Warmband soll die Korngrenzdichte des in fester Lösung vorliegenden Kohlenstoffs 1 - 4,5 Atome/nm2 und die Größe der an den Korngrenzen ausgeschiedenen Zementitkörner nicht mehr als 1 µm betragen. Die in dieser Weise beschaffenen und nach dem bekannten Verfahren hergestellten Stahlflachprodukte sollen bei ausreichend hoch dosierten Legierungsgehalten Zugfestigkeiten von mehr als 780 MPa aufweisen und Streckgrenzen besitzen, die bis zu 726 MPa betragen. Auf diese Weise soll das in der bekannten Weise erzeugte Warmband eine für die Verwendung im Automobilbau besonders geeignete Eigenschaftskombination aufweisen. Eine optimale Oberflächenbeschaffenheit soll dabei dadurch erreicht werden, dass die Wiedererwärmungstemperatur, auf die die Bramme vor dem Warmwalzen erwärmt wird, auf den oben genannten Temperaturbereich beschränkt und so eine übermäßige Bildung von Zunder, der beim Warmwalzen in die Warmbandoberfläche eingearbeitet würde, vermieden wird.In the hot strip thus produced, the grain boundary density of the carbon present in solid solution should be 1 - 4.5 atoms / nm 2 and the size of the grains of cementite precipitated at the grain boundaries should not exceed 1 μm . The flat steel products produced in this way and produced by the known method should have tensile strengths of more than 780 MPa and have yield strengths of up to 726 MPa at sufficiently high-dose alloy contents. In this way, the hot strip produced in the known manner should have a combination of properties which is particularly suitable for use in automobile construction. Optimum surface finish is achieved by limiting the reheat temperature to which the slab is heated prior to hot rolling to the above-mentioned temperature range and thus avoiding excessive scale formation which would be incorporated into the hot strip surface during hot rolling.

Neben dem voranstehend erläuterten Stand der Technik ist aus der EP 2 436 797 A1 ein hochfestes Stahlblech bekannt, dass (in Masse-%) aus 0,03 - 0,10 % C; 0,01 - 1,5 % Si; 1,0 - 2,5 % Mn; bis zu 0,1 % P; bis zu 0,02 % S; 0,01 - 1,2 % Al; 0,06 - 0,15 % Ti und bis zu 0,01 % N, und als Rest Eisen und unvermeidbare Verunreinigungen besteht. Die Zugfestigkeit des Stahlblechs beträgt mindestens 590 MPa und das Verhältnis aus Zugfestigkeit und Streckgrenze mindestens 0,80. Das Gefüge des Blechs besteht zu mindestens 40 Flächen-% aus Bainit und als Rest aus Ferrit oder Martensit, wobei in dem Gefüge Ti (C, N) - Ausscheidungen mit Größen von 10 nm oder weniger in einem Bereich von mindestens 1010 Ausscheidungen / mm3 vorhanden sind. Das Verhältnis Hvs/Hvc einer in einem Abstand von 10 µm von der Oberfläche des Blechs gemessenen Härte Hvs zur Härte Hvc in der Mitte der Blechdicke beträgt 0,85 oder mehr. Bei einem in der EP 2 436 797 A1 beschriebenen Ausführungsbeispiel weist ein Stahlblech, das (in Gew.-%) aus 0,06 % C, 0,18 % Si, 1,75 % Mn, 0,082 % P, 0,0044 % S, 0,04 % Al, 0,0035 % N, 0,092 % Ti, 0,075 % Nb und 0,0015 % B, Rest Eisen und unvermeidbare Verunreinigungen besteht, ein (in Flächen-%) zu 70 % aus Bainit, zu 5 % aus Ferrit und zu 25 % aus Martensit bestehendes Gefüge sowie eine Streckgrenze von 960 MPa auf.In addition to the above-described prior art is from the EP 2 436 797 A1 a high-strength steel sheet known that (in mass%) from 0.03 to 0.10% C; 0.01-1.5% Si; 1.0-2.5% Mn; up to 0.1% P; up to 0.02% S; 0.01-1.2% Al; 0.06 - 0.15% Ti and up to 0.01% N, and the balance iron and unavoidable impurities. The tensile strength of the steel sheet is at least 590 MPa and the ratio of tensile strength to yield strength is at least 0.80. The microstructure of the sheet consists of at least 40 area% bainite and the remainder ferrite or martensite, with the structure consisting of Ti (C, N) precipitates of sizes 10 nm or less in a range of at least 10 10 precipitates / mm3 available are. The ratio Hvs / Hvc of a hardness Hvs measured at a distance of 10 μm from the surface of the sheet to the hardness Hvc in the center of the sheet thickness is 0.85 or more. At one in the EP 2 436 797 A1 described embodiment, a steel sheet, the (in wt .-%) of 0.06% C, 0.18% Si, 1.75% Mn, 0.082% P, 0.0044% S, 0.04% Al, 0 , 0035% N, 0.092% Ti, 0.075% Nb and 0.0015% B, balance iron and unavoidable impurities, one (in area%) to 70% bainite, 5% ferrite and 25% martensite existing structure and a yield strength of 960 MPa.

Vor dem Hintergrund des voranstehend erläuterten Standes der Technik bestand die Aufgabe der Erfindung darin, ein hochfestes Stahlblech mit im Hinblick auf die Verwendung im Automobilbau optimierten mechanischen Eigenschaften und einer ebenso optimierten Oberflächenbeschaffenheit anzugeben.Against the background of the prior art described above, the object of the invention was to specify a high-strength steel sheet having mechanical properties optimized with regard to use in automobile construction and also an optimized surface finish.

Die Erfindung löst diese Aufgabe durch das in Anspruch 1 angegebene Stahlflachprodukt.The invention achieves this object by the flat steel product specified in claim 1.

Vorteilhafte Ausgestaltungen der Erfindung sind in den abhängigen Ansprüchen angegeben und werden nachfolgend wie der allgemeine Erfindungsgedanke im Einzelnen erläutert.Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

Ein erfindungsgemäßes Stahlflachprodukt mit einer Streckgrenze von 700 - 850 MPa und mit einem zu mindestens 70 Vol.-% bainitischen Gefüge lässt sich durch folgende Arbeitsschritte erzeugen:
a) Erschmelzen einer Stahlschmelze, die (in Gew.-%) aus C: 0,05 - 0,08 %, Si: 0,015 - 0,500 %, Mn: 1,60 - 2,00 %, P: bis zu 0,025 %, S: bis zu 0,010 %, Al: 0,020 - 0,050 %, N: bis zu 0,006 %, Cr: bis zu 0,40 %, Nb: 0,060 - 0,070 %, B: 0,0005 - 0,0025 %, Ti: 0,090 - 0,130 %, sowie aus technisch unvermeidbaren Verunreinigungen, zu denen bis zu 0,12 % Cu, bis zu 0,100 % Ni, bis zu 0,010 % V, bis zu 0,004 % Mo und bis zu 0,004 % Sb gehören, und
als Rest aus Eisen
besteht;
b) Vergießen der Schmelze zu einer Bramme;
c) Wiedererwärmen der Bramme auf eine Wiedererwärmungstemperatur von 1200 - 1300 °C;
d) Vorwalzen der Bramme bei einer 950 - 1250 °C betragenden Vorwalztemperatur und einer über das Vorwalzen erzielten Gesamtstichabnahme von mindestens 50 %;
e) Fertigwarmwalzen der vorgewalzten Bramme, wobei das Fertigwarmwalzen bei einer Warmwalzendtemperatur von 800 - 880 °C beendet wird;
f) innerhalb von höchstens 10 s nach dem Fertigwarmwalzen einsetzendes intensives Kühlen des fertig warmgewalzten Stahlflachprodukts mit einer Abkühlgeschwindigkeit von mindestens 40 K/s auf eine 550 - 620 °C betragende Haspeltemperatur;
g) Haspeln des fertig warmgewalzten Stahlflachprodukts.A flat steel product according to the invention with a yield strength of 700-850 MPa and with at least 70% by volume bainitic structure can be produced by the following working steps:
a) melting a molten steel, the (in wt .-%) from C: 0.05-0.08%, Si: 0.015 - 0.500%, Mn: 1.60 - 2.00%, P: up to 0.025%, S: up to 0.010%, al: 0.020-0.050%, N: up to 0.006%, Cr: up to 0.40%, Nb: 0.060-0.070%, B: 0.0005 - 0.0025%, Ti: 0.090 - 0.130%, and technically unavoidable impurities including up to 0.12% Cu, up to 0.100% Ni, up to 0.010% V, up to 0.004% Mo and up to 0.004% Sb, and
as the remainder of iron
consists;
b) pouring the melt into a slab;
c) reheating the slab to a reheating temperature of 1200 - 1300 ° C;
d) pre-rolling the slab at a roughing temperature of 950 - 1250 ° C and a total reduction of at least 50% in the roughing;
e) finish hot rolling the pre-rolled slab, finishing the finish hot rolling at a hot rolling end temperature of 800 - 880 ° C;
f) intensive cooling of the finished hot-rolled flat steel product starting within a maximum of 10 s after finish hot rolling at a cooling rate of at least 40 K / s to 550 - 620 ° C reel temperature;
g) Coiling the finished hot-rolled flat steel product.

Diesem Verfahren liegt eine Stahllegierung zu Grunde, deren Legierungsbestandteile und Legierungsgehalte in engen Grenzen so aufeinander abgestimmt sind, dass bei einer betriebssicher durchzuführenden Verfahrensweise jeweils maximierte mechanische Eigenschaften und optimierte Oberflächenbeschaffenheiten erzielt werden.This method is based on a steel alloy whose alloying constituents and alloy contents are coordinated with one another within narrow limits in such a way that maximized mechanical properties and optimized surface textures are achieved in the case of an operationally reliable procedure.

Wie nachfolgend erläutert, sind Legierungsbestandteile und Legierungsgehalte der im Arbeitsschritt a) erschmolzenen Stahllegierung so ausgewählt, dass sich bei Einhaltung der hier vorgegebenen Arbeitsschritte zuverlässig ein warmgewalztes Stahlflachprodukt mit einer Eigenschaftskombination erzeugen lässt, die es für die Verwendung im Stahlleichtbau, insbesondere im Bereich des Nutzfahrzeugbaus, besonders geeignet macht:

C:
Der Kohlenstoffgehalt des erfindungsgemäß verarbeiteten Stahls beträgt 0,05 - 0,08 Gew.-%. Um die gewünschten Festigkeitseigenschaften zu erreichen, ist ein C-Gehalt von wenigstens 0,05 Gew.-% erforderlich. Falls jedoch der Kohlenstoffgehalt zu hoch ist, werden die Zähigkeitseigenschaften bzw. die Schweißbarkeit und die Umformbarkeit des erfindungsgemäß verarbeiteten Stahls beeinträchtigt. Aus diesem Grund ist der Kohlenstoffgehalt auf höchstens 0,08 Gew.-% begrenzt.
Si:
Silizium wird bei dem erfindungsgemäß verarbeiteten Stahl als Desoxidationsmittel sowie zum Verbessern der Festigkeitseigenschaften eingesetzt. Wenn jedoch der Siliziumgehalt zu hoch ist, werden die Zähigkeitseigenschaften, insbesondere die Zähigkeit in der Wärmeeinflusszone von Schweißverbindungen, stark beeinträchtigt. Aus diesem Grund soll der Siliziumgehalt des erfindungsgemäß verarbeiteten Stahls 0,50 Gew.-% nicht überschreiten. Zur sicheren Vermeidung von Störungen der Oberflächenqualität kann der Siliziumgehalt auf max. 0,25 Gew.-% beschränkt werden.
Mn:
Mangan wird zur Einstellung der gewünschten Festigkeitseigenschaften bei guten Zähigkeitseigenschaften dem erfindungsgemäß verwendeten Stahl in Gehalten von 1,6 - 2,0 Gew.-% zugegeben. Wenn der Mangangehalt weniger als 1,60 Gew.-% beträgt, werden die geforderten Festigkeitseigenschaften nicht mit der ausreichenden Sicherheit erreicht. Durch die Beschränkung des Mn-Gehalts auf max. 2,00 Gew.-% wird eine Verschlechterung der Schweißbarkeit, der Zähigkeitseigenschaften, der Umformbarkeit und des Seigerungsverhaltens vermieden.
P:
Das Begleitelement Phosphor verschlechtert die Kerbschlagarbeit und die Umformbarkeit. Der Phosphorgehalt soll daher die Obergrenze von 0,025 Gew.-% nicht überschreiten. Optimaler Weise ist der P-Gehalt auf weniger als 0,015 Gew.-% beschränkt.
S:
Schwefel verschlechtert die Kerbschlagarbeit und die Umformbarkeit eines erfindungsgemäß verarbeiteten Stahls infolge von MnS-Bildung. Aus diesem Grund darf der S-Gehalt eines erfindungsgemäß verarbeiteten Stahls höchstens 0,010 Gew.-% betragen. Ein derart niedriger Schwefelgehalt kann in an sich bekannter Weise z. B. durch eine CaSi-Behandlung erzielt werden. Um die negativen Einflüsse von Schwefel auf die Eigenschaften des erfindungsgemäß verarbeiteten Stahls sicher auszuschließen, kann der S-Gehalt auf max. 0,003 Gew.-% beschränkt sein.
Al:
Aluminium wird ebenfalls als Desoxidationsmittel verwendet und behindert infolge von AlN-Bildung die Vergröberung des Austenitkorns beim Austenitisieren. Liegt der Aluminiumgehalt unter 0,020 Gew.-%, laufen die Desoxidationsprozesse nicht vollständig ab. Übersteigt der Aluminiumgehalt jedoch die Obergrenze von 0,050 Gew.-%, so können sich Al2O3-Einschlüsse bilden. Diese wirken sich negativ auf den Reinheitsgrad und die Zähigkeitseigenschaften aus.
N:
Das Begleitelement Stickstoff bildet mit Aluminium AlN oder mit Titan TiN. Wenn jedoch der Stickstoffgehalt zu hoch ist, werden die Zähigkeitseigenschaften verschlechtert. Um dies zu verhindern, ist bei einem erfindungsgemäß verarbeiteten Stahl die Obergrenze für den Stickstoff-Gehalt auf 0,006 Gew.-% festgesetzt.
Cr:
Chrom kann einem erfindungsgemäß verarbeiteten Stahl optional zugegeben sein, um seine Festigkeitseigenschaften zu verbessern. Wenn der Chromgehalt zu hoch ist, werden allerdings die Schweißbarkeit und Zähigkeit in der Wärmeeinflusszone negativ beeinflusst. Daher ist bei einem erfindungsgemäß verarbeiteten Stahl die obere Grenze für den Chromgehalt auf 0,40 Gew.-% festgesetzt.
Nb:
Niob ist in einem erfindungsgemäß verarbeiteten Stahl enthalten, um die Festigkeitseigenschaften durch Kornfeinung der Austenitstruktur beim temperaturgesteuerten Walzen bzw. durch Ausscheidungshärtung beim Haspeln zu unterstützen. Hierzu sind im erfindungsgemäß verarbeiteten Stahl 0,060 - 0,070 Gew.-% Nb vorhanden. Liegt der Niobgehalt unterhalb dieses Bereichs, werden die Festigkeitseigenschaften nicht erreicht. Liegt der Nb-Gehalt über der Obergrenze dieses Bereichs, verschlechtert sich die Schweißbarkeit und die Zähigkeit in der Wärmeeinflusszone einer Schweißung.
B:
Der Borgehalt eines erfindungsgemäß verarbeiteten Stahls beträgt 0,0005 - 0,0025 Gew.-%. B wird zur Unterstützung der Festigkeitseigenschaften und zur Verbesserung der Härtbarkeit verwendet. Zu hohe Borgehalte verschlechtern jedoch die Zähigkeitseigenschaften.
Ti:
Titan trägt ebenfalls zur Verbesserung der Festigkeitseigenschaften durch Verhinderung des Kornwachstums beim Austenitisieren bzw. durch Ausscheidungshärtung beim Haspeln bei. Um dies zu gewährleisten, betragen die Ti-Gehalte eines erfindungsgemäß verarbeiteten Stahls 0,09 - 0,13 Gew.-%. Liegt der Titangehalt unter 0,09 Gew.-%, werden die erfindungsgemäß angestrebten Festigkeitswerte nicht erreicht. Wird die Obergrenze des vorgegebenen Ti-Gehaltsbereichs überschritten, verschlechtern sich die Schweißbarkeit und die Zähigkeit in der Wärmeeinflusszone einer Schweißung.
As explained below, alloy constituents and alloy contents of the steel alloy melted in step a) are selected such that a hot rolled flat steel product with a combination of properties can be reliably produced while maintaining the work steps specified here, which is suitable for use in lightweight steel construction, in particular in the field of commercial vehicle construction. particularly suitable:
C:
The carbon content of the steel processed according to the invention is 0.05-0.08% by weight. To achieve the desired strength properties, a C content of at least 0.05% by weight is required. However, if the carbon content is too high, the toughness properties and weldability and formability of the steel processed according to the present invention are impaired. For this reason, the carbon content is limited to at most 0.08 wt .-%.
Si:
Silicon is used in the inventively processed steel as a deoxidizer and to improve the strength properties. However, if the silicon content is too high, the toughness properties, especially the toughness in the heat affected zone of welded joints, are greatly impaired. For this reason, the silicon content of the steel processed according to the invention should not exceed 0.50% by weight. To safely avoid surface quality problems, the silicon content can be reduced to max. 0.25 wt .-% be limited.
Mn:
Manganese is added to adjust the desired strength properties with good toughness properties of the steel used in the invention in amounts of 1.6 to 2.0 wt .-%. If the manganese content is less than 1.60 wt%, the required strength properties are not achieved with sufficient certainty. By limiting the Mn content to max. 2.00 wt.%, Deterioration of weldability, toughness properties, formability and segregation behavior is avoided.
P:
The accompanying element phosphor deteriorates the impact work and the formability. The phosphorus content should therefore not exceed the upper limit of 0.025 wt .-%. Optimally, the P content is limited to less than 0.015 wt%.
S:
Sulfur degrades the impact work and the formability of a steel processed according to the invention due to MnS formation. For this reason, the S content of a steel processed according to the invention may not exceed 0.010% by weight. Such a low sulfur content can be in a conventional manner z. B. can be achieved by a CaSi treatment. In order to reliably exclude the negative effects of sulfur on the properties of the steel processed according to the invention, the S content can be reduced to max. Be limited to 0.003 wt .-%.
al:
Aluminum is also used as a deoxidizer and hinders austenitizing coarsening of austenite due to AlN formation. If the aluminum content is below 0.020% by weight, the deoxidation processes do not proceed completely. However, if the aluminum content exceeds the upper limit of 0.050% by weight, Al 2 O 3 inclusions may form. These have a negative effect on the degree of purity and the toughness properties.
N:
The companion nitrogen forms aluminum with AlN or with titanium TiN. However, if the nitrogen content is too high, the toughness properties are deteriorated. In order to prevent this, in a steel processed according to the invention, the upper limit for the nitrogen content is set at 0.006% by weight.
Cr:
Chromium may optionally be added to a steel processed according to the invention to improve its strength properties. If the Chromium content is too high, however, the weldability and toughness in the heat affected zone are adversely affected. Therefore, in an inventively processed steel, the upper limit of the chromium content is set to 0.40 wt%.
Nb:
Niobium is included in a steel processed according to the present invention to promote the strength properties by grain refining of the austenite structure in temperature controlled rolling or by precipitation hardening during coiling. For this purpose, 0.060-0.070% by weight Nb are present in the steel processed according to the invention. If the niobium content is below this range, the strength properties are not achieved. If the Nb content exceeds the upper limit of this range, the weldability and toughness in the heat affected zone of a weld are deteriorated.
B:
The boron content of a steel processed according to the invention is 0.0005-0.0025% by weight. B is used to support the strength properties and to improve the hardenability. Excessive boron contents, however, degrade the toughness properties.
Ti:
Titanium also contributes to the improvement in strength properties by preventing grain growth during austenitizing or precipitation hardening during coiling. In order to ensure this, the Ti contents of a steel processed according to the invention are from 0.09 to 0.13% by weight. If the titanium content is below 0.09% by weight, the strength values desired according to the invention are not achieved. If the upper limit of the given Ti content range is exceeded, the weldability and toughness in the heat affected zone of a weld deteriorate.

Cu, Ni, V, Mo und Sb treten als Begleitelemente auf, die als technisch unvermeidbare Verunreinigung im Prozess der Stahlerzeugung in den erfindungsgemäß verarbeiteten Stahl gelangen. Ihre Gehalte sind auf Mengen beschränkt, die in Bezug auf die erfindungsgemäß angestrebten Eigenschafen des erfindungsgemäß verarbeiteten Stahls unwirksam sind. Dazu ist der Cu-Gehalt auf max. 0,12 Gew.-%, der Ni-Gehalt auf weniger als 0,1 Gew-%, der V-Gehalt auf höchstens 0,01 Gew.-%, der Mo-Gehalt auf weniger als 0,004 Gew.-% und der Sb-Gehalt ebenfalls auf weniger als 0,004 Gew.-% beschränkt.Cu, Ni, V, Mo and Sb occur as accompanying elements, which enter the steel processed according to the invention as a technically unavoidable impurity in the steelmaking process. Their contents are limited to amounts which are ineffective in relation to the properties of the steel processed according to the invention. For this purpose, the Cu content is limited to max. 0.12 wt .-%, the Ni content to less than 0.1 wt%, the V content to at most 0.01 wt .-%, the Mo content to less than 0.004 wt .-%, and the Sb content is also limited to less than 0.004 wt%.

Um eine gute Schweißbarkeit zu erreichen, können der C-, der Mn-, der Cr-, der Mo-, der V-, der Cu- und der Ni-Gehalt des erfindungsgemäßen Stahls innerhalb der erfindungsgemäß vorgegebenen Grenzen so eingestellt werden, dass für das nach der Formel CE = % C + % Mn / 6 + % Cr + % Mo + % V / 5 + % Cu + % Ni / 15

Figure imgb0001

  • mit %C = jeweiliger C-Gehalt in Gew.-%,
  • %Mn = jeweiliger Mn-Gehalt in Gew.-%,
  • %Cr = jeweiliger Cr-Gehalt in Gew.-%,
  • %Mo = jeweiliger Mo-Gehalt in Gew.-%,
  • %V = jeweiliger V-Gehalt in Gew.-%,
  • %Cu = jeweiliger Cu-Gehalt in Gew.-%,
  • %Ni = jeweiliger Ni-Gehalt in Gew.-%,
berechnete Kohlenstoffäquivalent CE gilt: CE ≤ 0,5 Gew . - %
Figure imgb0002
 In order to achieve a good weldability, the C, Mn, Cr, Mo, V, Cu and Ni content of the steel according to the invention can be adjusted within the limits specified in the invention so that that according to the formula CE = % C + % Mn / 6 + % Cr + % Mo + % V / 5 + % Cu + % Ni / 15
Figure imgb0001
  • with% C = respective C content in% by weight,
  • % Mn = respective Mn content in% by weight,
  • % Cr = respective Cr content in% by weight,
  • % Mo = respective Mo content in wt.%,
  • % V = respective V content in wt%,
  • % Cu = respective Cu content in% by weight,
  • % Ni = respective Ni content in wt.%,
Calculated carbon equivalent CE: CE ≤ 0.5 weight , - %
Figure imgb0002

Nach dem Gießen der Bramme wird auf eine Austenitisierungstemperatur wiedererwärmt, die 1200 - 1300 °C beträgt. Der obere Grenzwert des Temperaturbereichs, auf den die Bramme zur Austenitisierung erwärmt wird, sollte nicht überschritten werden, um eine Vergröberung des Austenitkorns und eine vermehrte Zunderbildung zu vermeiden. Im hier vorgegebenen Bereich der Wiedererwärmungstemperatur von 1200 - 1300 °C kommt es noch nicht zur erhöhten Bildung von Rotzunder, der die Oberflächenqualität des erfindungsgemäß erzeugten Stahlflachprodukts mindern würde. Rotzunder bildet sich bei der Verarbeitung erfindungsgemäß zusammengesetzter Brammen ausschließlich beim Warmwalzvorgang (Arbeitsschritte d), e) des Verfahrens), wenn nach der Wiedererwärmung zu viel Primärzunder auf der Brammenoberfläche vorhanden ist.After the slab has been cast, it is reheated to an austenitizing temperature which is 1200-1300 ° C. The upper limit of the temperature range to which the slab is heated to austenitise should not be exceeded in order to avoid coarsening of the austenite grain and increased scale formation. In the here specified range of reheating temperature of 1200 - 1300 ° C, it is not yet to increased formation of Rotzunder that would reduce the surface quality of the invention produced according to the flat steel product. Rotzunder forms in the processing of composite slabs according to the invention exclusively during the hot rolling process (steps d), e) of the process), if after reheating too much primary scale on the slab surface is present.

Der untere Grenzwert der Wiedererwärmungstemperatur ist dagegen so festgesetzt, dass bei gleichmäßiger Temperaturverteilung die angestrebte Homogenisierung des Gefüges gewährleistet ist. Ab dieser Temperatur setzt eine weitestgehend vollständige Auflösung der in der jeweiligen Bramme vorhandenen groben Ti- und Nb-Karbonitridausscheidungen im Austenit ein. Beim abschließenden Haspeln des fertig warmgewalzten Stahlflachprodukts (Arbeitsschritt g) des Verfahrens) können sich dann feine Ti- oder Nb-Karbonitridausscheidungen neu bilden, die, wie erläutert, einen wesentlichen Beitrag zur Erhöhung der Festigkeitseigenschaften leisten. Auf diesem Wege ist gewährleistet, dass die erfindungsgemäß erzeugten und zusammengesetzten Stahlflachprodukte regelmäßig eine Mindeststreckgrenze von 700 MPa besitzen.The lower limit of the reheating temperature, however, is set so that the desired homogenization of the structure is ensured with a uniform temperature distribution. From this temperature, a largely complete dissolution of the coarse Ti and Nb carbonitride precipitates present in the respective slab begins in the austenite. During the final coiling of the finished hot-rolled flat steel product (step g) of the process) As a result, fine Ti or Nb carbonitride precipitates can be newly formed which, as explained, make a significant contribution to increasing the strength properties. In this way, it is ensured that the flat steel products produced and assembled according to the invention regularly have a minimum yield strength of 700 MPa.

Dabei beträgt die Wiedererwärmungstemperatur bei der Austenitisierung der jeweiligen Bramme mindestens 1200 °C, um den angestrebten Effekt der möglichst vollständigen Auflösung der TiC- und NbC-Ausscheidungen zu erreichen. Bei einer unter 1200 °C liegenden Austenitisierungstemperatur ist die Menge der im Austenit gelösten Karbidausscheidungen von Ti und Nb dagegen so gering, dass die erfindungsgemäß genutzten Effekte nicht eintreten. Eine unterhalb von 1200 °C liegende Wiedererwärmungstemperatur hätte daher bei der Verarbeitung von Stahlflachprodukten, die aus entsprechend der erfindungsgemäß optimierten Legierungsauswahl zusammengesetzt sind, zur Folge, dass die geforderten Festigkeitseigenschaften nicht erreicht werden. Besonders sicher lässt sich die möglichst vollständige Auflösung der TiC- und NbC-Ausscheidungen dann gewährleisten, wenn die Wiedererwärmungstemperatur mindestens 1250 °C beträgt.The rewarming temperature during austenitisation of the respective slab is at least 1200 ° C., in order to achieve the desired effect of the most complete possible dissolution of the TiC and NbC precipitates. On the other hand, when the austenitizing temperature is below 1200 ° C., the amount of carbide precipitates of Ti and Nb dissolved in austenite is so small that the effects used according to the invention do not occur. A rewarming temperature below 1200 ° C. would therefore result in the processing of flat steel products which are composed according to the alloy selection optimized according to the invention, that the required strength properties are not achieved. The most complete possible dissolution of the TiC and NbC precipitates can be ensured with particular certainty if the reheating temperature is at least 1250 ° C.

Ein Stahlflachprodukt, das höchste Qualitätsanforderungen an seine Oberflächenbeschaffenheit erfüllt, kann dadurch erzeugt werden, dass vor dem Vorwalzen der auf der Bramme vorhandene Zunder vollständig entfernt wird. Dies kann dadurch geschehen, dass die Brammenoberfläche nach dem Ofenaustrag und möglichst unmittelbar vor dem Vorwalzen vollständig entzundert wird. Hierzu kann die Bramme einen konventionellen Zunderwäscher durchlaufen.A flat steel product meeting the highest quality requirements for its surface finish can be produced by completely removing the scale present on the slab before rough rolling. This can be done by the slab surface after the Furnace discharge and as soon as possible immediately before the rough rolling is completely descaled. For this purpose, the slab can go through a conventional scale scrubber.

Zur Erzeugung eines Stahlflachprodukts mit optimierter Oberflächenbeschaffenheit kann die Zeit t_1, die der Transfer der Bramme von der Arbeitsstation ("Wiedererwärmung (Arbeitsschritt c)") oder der optional nach dem Wiedererwärmen durchlaufenen "Entfernung des Primärzunders (Arbeitsschritt c')") bis zum Beginn des Fertigwarmwalzens (Arbeitsschritt e)) benötigt, auf maximal 300 s beschränkt werden. Dies schließt optimaler Weise das Vorwalzen ein. In einer so kurzen Transferzeit wird nur eine so geringe Menge an Primärzunder neu gebildet, dass der sich daraus beim Warmwalzen bildende Rotzunder für die Qualität der Oberfläche des nach dem Warmwalzen erhaltenen Stahlflachprodukts unschädlich ist. Im Fall, dass eine Entzunderung vor dem Vorwalzen durchgeführt wird, sollte die Transportdauer zwischen dem Entzunderungsaggregat und zum Vorwalzgerüst maximal 30 s betragen. Bei einer so kurzen Transportdauer kann sich somit keine oder allenfalls eine unschädliche dünne Oxidschicht auf der zuvor entzunderten Bramme bilden.In order to produce a flat steel product with optimized surface finish, the time t_1, the transfer of the slab from the workstation ("reheating (step c)") or the optional "post-reheating" removal of the primary scale (step c ') ") may be started of finish hot rolling (step e)) is required, limited to a maximum of 300 s. This optimally includes pre-rolling. In such a short transfer time, only such a small amount of primary scale is newly formed that the red scale forming therefrom during hot rolling is harmless to the surface quality of the flat steel product obtained after hot rolling. In the case that descaling is carried out before roughing, the transport time between the descaling unit and the roughing stand should not exceed 30 s. With such a short transport time, no or at most a harmless thin oxide layer can thus form on the previously descaled slab.

Im Arbeitsschritt d) wird die jeweils verarbeitete Bramme bei einer Vorwalztemperatur von 950 - 1250 °C vorgewalzt. Die beim Vorwalzen erzielte Stichabnahme beträgt insgesamt mindestens 50 %. Als gesamte Stichabnahme Δhv ist dabei das aus der Differenz der Dicken der Bramme vor (Dicke dVv) und nach (Dicke dNv) dem Vorwalzen und der Dicke dVv der Bramme vor dem Vorwalzen gebildete Verhältnis bezeichnet Δ hv % = dVv − dNv / dVv × 100 % .

Figure imgb0003
In step d), the respectively processed slab is pre-rolled at a rough rolling temperature of 950-1250 ° C. The total reduction in pre-rolling amounts to at least 50%. The total loss Δhv is the ratio formed by the difference between the thicknesses of the slab before (thickness dVv) and after (thickness dNv) the pre-rolling and the thickness dVv of the slab before rough-rolling Δ hv % = dVv - DNV / dVv × 100 % ,
Figure imgb0003

Die untere Grenze des für die Vorwalztemperatur vorgegebenen Bereichs und der Mindestwert der Gesamtstichabnahme Δhv sind dabei so festgesetzt, dass die Rekristallisationsvorgänge in der jeweils vorgewalzten Bramme vollständig ablaufen können. Auf diese Weise ist die Entstehung eines feinkörnigen austenitischen Gefüges vor dem Fertigwalzen gewährleistet, wodurch optimierte Zähigkeits- und Bruchdehnungseigenschaften des erfindungsgemäß erzeugten Stahlflachprodukts erreicht werden.The lower limit of the predetermined for the rough rolling temperature range and the minimum value of the total stitch decrease Δhv are set so that the recrystallization processes in the respective pre-rolled slab can run completely. In this way, the formation of a fine-grained austenitic structure is ensured before the finish rolling, whereby optimized toughness and elongation at break properties of the steel flat product produced according to the invention are achieved.

Die Verweil- und Pausenzeit t_2 zwischen dem Vorwalzen und dem Fertigwalzen ist auf 50 s beschränkt, um ein unerwünschtes Austenitkornwachstum zu vermeiden.The dwell and pause time t_2 between rough rolling and finish rolling is limited to 50 seconds to avoid undesirable austenite grain growth.

Auf das Vorwalzen folgt im Arbeitsschritt e) das Warmwalzen der vorgewalzten Bramme zu einem warmgewalzten Stahlflachprodukt mit einer Warmbanddicke, die typischerweise 3 - 15 mm beträgt. Stahlflachprodukte mit solchen Dicken werden in der Fachsprache auch als "Grobblech" bezeichnet.Pre-rolling is followed, in step e), by hot rolling of the pre-rolled slab into a hot-rolled flat steel product having a hot strip thickness of typically 3-15 mm. Flat steel products with such thicknesses are also referred to in the jargon as "heavy plate".

Die Endtemperatur des Warmwalzens liegt dabei bei 800 - 880 °C. Durch Einhaltung dieses Warmwalz-Endtemperaturbereichs wird ein stark gestrecktes Austenitkorn im Gefüge des erhaltenen Warmbands erreicht. Durch die vergleichbar niedrige Warmwalz-Endtemperatur wird der Effekt des Warmwalzens verstärkt. Im Gefüge des erhaltenen Warmbands ist versetzungsreicher Austenit vorhanden. Dieser wandelt sich nach einer Intensivkühlung (Arbeitsschritt f)) zu einem versetzungsreichen, feinstrukturierten Bainit um, so dass die Streckgrenze angehoben wird. Die obere Grenze des Bereichs der Warmwalz-Endtemperatur ist so festgesetzt, dass keine Rekristallisation des Austenits beim Walzen in der Warmwalzfertigstraße stattfindet. Auch dies trägt zur Ausprägung eines feinkörnigen Gefüges bei. Die untere Grenztemperatur beträgt mindestens 800 °C, damit sich kein Ferrit beim Walzen bildet.The final temperature of hot rolling is 800 - 880 ° C. By maintaining this hot rolling final temperature range, a highly stretched austenite grain is achieved in the microstructure of the obtained hot strip. The comparatively low hot rolling end temperature enhances the effect of hot rolling. In the structure of the hot strip obtained is dislocation rich austenite. This transforms after an intensive cooling (step f)) to a dislocated, finely structured bainite, so that the yield strength is raised. The upper limit of the range of the hot rolling finish temperature is set so that no recrystallization of the austenite takes place during rolling in the hot rolling finishing line. This also contributes to the expression of a fine-grained structure. The lower limit temperature is at least 800 ° C, so that no ferrite forms during rolling.

Die beim Fertigwalzen erzielte Stichabnahme Δhf beträgt insgesamt mindestens 70 %, wobei hier die Stichabnahme Δhf nach der Formel Δhf = (dVf-dNf)/dVf x 100 % (mit dVf = Dicke des Walzguts beim Einlauf in die Fertigwarmwalzstaffel und dNf = Dicke des Walzguts am Auslauf der Fertigwarmwalzstaffel) berechnet wird. Durch die hohe Stichabnahme Δhf findet die Phasenumwandlung aus stark umgeformtem Austenit statt. Dies wirkt sich positiv auf die Feinkörnigkeit aus, so dass im Gefüge des erfindungsgemäß erzeugten Stahlflachprodukts geringe Korngrößen vorliegen.The total reduction achieved during finish rolling .DELTA.hf is at least 70%, here the drop decrease .DELTA.hf according to the formula .DELTA.hf = (dVf-dNf) / dVf x 100% (with dVf = thickness of the rolling stock at the inlet to the final hot-rolling stand and dNf = thickness of the rolling stock at the outlet of the Fertigwarmwalzstaffel) is calculated. Due to the high reduction Δhf, the phase transformation takes place from strongly formed austenite. This has a positive effect on the fine granularity, so that small particle sizes are present in the microstructure of the steel flat product produced according to the invention.

Nachdem das fertig warmgewalzte Stahlflachprodukt aus dem letzten Gerüst der Fertigwarmwalzstraße ausgetreten ist, setzt innerhalb von höchstens 10 s eine intensive Abkühlung ein, bei der das warmgewalzte Stahlflachprodukt mit einer Abkühlgeschwindigkeit dT von mindestens 40 K/s auf eine Haspeltemperatur von 550 - 620 °C abgekühlt wird.After the finished hot-rolled flat steel product has left the last stand of the finished hot rolling mill, intensive cooling takes place within a maximum of 10 seconds during which the hot-rolled flat steel product is cooled to a reeling temperature of 550-620 ° C. with a cooling rate dT of at least 40 K / s becomes.

Die Kühlpause nach dem Warmwalzen beträgt höchstens 10 s, um zu verhindern, dass es zwischen Warmwalzen und gesteuertem beschleunigten Abkühlen zu unerwünschten Gefügeveränderungen kommt.The cooling break after hot rolling is at most 10 seconds to prevent undesirable microstructural changes between hot rolling and controlled accelerated cooling.

Durch Einhaltung des vorgegebenen Bereichs der Haspeltemperatur werden die Voraussetzungen für die Bildung eines bainitischen Gefüges des erfindungsgemäß erzeugten Stahlflachprodukts geschaffen.By observing the predetermined range of the reel temperature, the conditions for the formation of a bainitic structure of the steel flat product produced according to the invention are created.

Gleichzeitig hat die Wahl der Haspeltemperatur entscheidenden Einfluss auf die Ausscheidungshärtung. Dazu ist der Haspeltemperaturbereich so gewählt, dass er einerseits unterhalb der Bainitstarttemperatur, anderseits im Ausscheidungsmaximum für die Bildung von Karbonitridausscheidungen liegt. Eine zu tiefe Haspeltemperatur würde jedoch dazu führen, dass das Ausscheidungspotenzial nicht mehr nutzbar wäre und somit die geforderte Mindeststreckgrenze nicht mehr erreicht würde. Die Abkühlbedingungen sind dabei so gewählt, dass das warmgewalzte Stahlflachprodukt unmittelbar vor dem Haspeln ein bainitisches Gefüge mit einem Phasenanteil von mindestens 70 Vol.-% aufweist. Eine weitere Bainitbildung läuft dann im Haspel ab. Im Hinblick auf die geforderte Eigenschaftskombination optimal erweist es sich dabei, wenn das Gefüge des so erzeugten warmgewalzten Stahlflachprodukts nach dem Haspeln im technischen Sinne vollständig aus Bainit besteht. Dies wird durch Einhaltung des vorgegebenen Bereichs der Haspeltemperatur erreicht.At the same time, the choice of reel temperature has a decisive influence on precipitation hardening. For this purpose, the reel temperature range is selected so that it is below the bainite start temperature, on the one hand, and at the precipitation maximum for the formation of carbonitride precipitates, on the other hand. However, a reel temperature which is too low would mean that the precipitation potential would no longer be usable and thus the required minimum yield strength would no longer be reached. The cooling conditions are chosen so that the hot rolled flat steel product immediately before the reel has a bainitic structure with a phase content of at least 70 vol .-%. Another Bainitbildung then runs off in the reel. With regard to the required combination of properties, it proves to be optimal if the structure of the hot-rolled flat steel product produced in this way consists entirely of bainite after reeling in the technical sense. This is achieved by maintaining the specified range of reel temperature.

Durch die hohe Abkühlgeschwindigkeit wird die Bildung von unerwünschten Phasenbestandteilen vermieden. Um dabei ein optimal planes Stahlflachprodukt zu erhalten, kann die Abkühlgeschwindigkeit der Abkühlung nach dem Warmwalzen auf 150 K/s beschränkt werden.The high cooling rate avoids the formation of unwanted phase components. In order to obtain an optimally flat flat steel product, the cooling rate of the cooling after hot rolling can be limited to 150 K / s.

Die Streckgrenze der in der voranstehend erläuterten Weise erzeugten warmgewalzten Stahlflachprodukte beträgt zuverlässig 700 - 850 MPa. Ihre Bruchdehnung liegt dabei jeweils bei mindestens 12 %. Genauso regelmäßig erreichen erfindungsgemäße Stahlflachprodukte Zugfestigkeiten von 750 - 950 MPa. Die für erfindungsgemäße Produkte ermittelte Kerbschlagarbeit liegt bei -20 °C im Bereich von 50 - 110 J und bei -40 °C im Bereich von 30 - 110 J.The yield strength of the manner explained above produced hot-rolled steel flat products is reliably 700 - 850 MPa. Their elongation at break is in each case at least 12%. Equally regularly, steel flat products according to the invention achieve tensile strengths of 750-950 MPa. The notched impact work determined for products according to the invention is in the range from 50 to 110 J at -20 ° C. and in the range from 30 to 110 J at -40 ° C.

In der voranstehend erläuterten Weise erzeugte Stahlflachprodukte weisen ein feinkörniges Gefüge mit einer mittleren Korngröße von höchstens 20 µm auf, um eine gute Bruchdehnung und Zähigkeit zu erreichen.Steel flat products produced in the manner explained above have a fine-grained structure with an average grain size of at most 20 μm in order to achieve good elongation at break and toughness.

Dabei liegen bei der hier erläuterten Verfahrensweise die voranstehend genannten Eigenschaften bei einem warmgewalzten Stahlflachprodukt im Walzzustand nach dem Haspeln vor. Eine weitere Wärmebehandlung zur Einstellung oder Ausprägung bestimmter für die zugedachte Verwendung als hochfestes Blech im Nutzfahrzeugbau wichtiger Eigenschaften ist nicht notwendig.In the process described here, the abovementioned properties are present in a hot-rolled flat steel product in the rolling state after reeling. A further heat treatment for the adjustment or expression of certain important properties for the intended use as a high-strength sheet in commercial vehicle construction is not necessary.

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

Stahlschmelzen A - E mit der in Tabelle 1 angegebenen Zusammensetzung sind erschmolzen und in bekannter Weise zu Brammen 1 - 26 vergossen worden.Steel melts A - E with the composition given in Table 1 have been melted and cast in a known manner into slabs 1 - 26.

Anschließend sind die aus den Stählen A - E bestehenden Brammen auf eine Wiedererwärmungstemperatur TW durcherwärmt worden.Subsequently, the slabs made of the steels A - E have been heated to a reheating temperature TW.

Aus dem Wiedererwärmungsofen sind die wiedererwärmten Brammen in weniger als 30 s zu einem Zunderwäscher transportiert worden, in dem der auf ihnen haftende Primärzunder von den Brammen entfernt worden ist.From the reheating furnace, the reheated slabs have been transported in less than 30 s to a scale washer in which the primary scale adhering to them has been removed from the slabs.

Die aus dem Zunderwäscher austretenden Brammen sind dann zu einem Vorwalzgerüst transportiert worden, wo sie mit einer Vorwalztemperatur TVW und einer über das Vorwalzen erzielten Gesamtstichabnahme Δhv vorgewalzt worden sind.The slabs emerging from the scale scrubber have then been transported to a roughing stand, where they have been pre-rolled with a roughing temperature TVW and an overall reduction in the total value Δhv achieved via rough rolling.

Anschließend sind die vorgewalzten Brammen in einer Fertigwarmwalzstaffel zu Warmbändern mit einer Dicke BD und einer Breite BB fertig warmgewalzt worden. Das Warmwalzen ist jeweils mit einer gesamten Stichabnahme in der Fertigwarmstaffel Δhf bei einer Warmwalzendtemperatur TEW beendet worden. Die Zeit, die zwischen dem Austritt aus dem Zunderwäscher und dem Beginn des Fertigwarmwalzens vergangen ist, betrug jeweils weniger als 300 s.Subsequently, the pre-rolled slabs were finished hot rolled in a finished hot rolling mill to hot strip with a thickness of BD and a width BB. The hot rolling has been completed with a total decrease in the finished heat transfer scale Δhf at a hot rolling end temperature TEW. The time that elapsed between the exit from the scale washer and the beginning of the finish hot rolling, each less than 300 s.

Das aus dem letzten Gerüst austretende fertig warmgewalzte Stahlflachprodukt ist nach einer Pause t_p von 1 - 7 s, in der es an Luft langsam abgekühlt ist, mittels Intensivkühlung mit Wasser mit einer Abkühlrate dT von 50 - 120 K/s auf eine Haspeltemperatur HT abgekühlt worden. Nach der Abkühlung wiesen die Stahlflachprodukte bereits ein zu mindestens 70 Vol.-% bainitisches Gefüge auf.After a break t_p of 1-7 s, during which it has slowly cooled in air, the finished hot-rolled flat steel product leaving the last stand has been cooled to a coiling temperature HT by intensive cooling with water at a cooling rate dT of 50-120 K / s , After cooling, the flat steel products already had at least 70% by volume bainitic structure.

Bei dieser Haspeltemperatur HT sind die erhaltenen Warmbänder jeweils zu einem Coil gehaspelt worden. Im Zuge der Abkühlung der Stahlflachprodukte im Coil kam es zur vollständigen Umwandlung des Gefüges in Bainit, so dass die erhaltenen Stahlflachprodukte ein im technischen Sinne zu 100 Vol.-% banitisches Gefüge besaßen.At this reel temperature HT, the hot strips obtained have each been coiled into a coil. In the course of the cooling of the flat steel products in the coil, the structure was completely transformed into bainite, so that the obtained in the technical sense to 100 vol .-% banitisches structure possessed.

In den Tabellen 2a,2b sind die bei der Verarbeitung der Brammen 1 - 26 jeweils eingestellten Verfahrensparameter Wiedererwärmungstemperatur TW, Vorwalztemperatur TVW, über das Vorwalzen erzielte Gesamtstichabnahme Δhv, Zeit t_1 zwischen dem nach dem Wiedererwärmen und vor dem Vorwalzen durchgeführten Entzundern und Beginn des Fertigwarmwalzens, Zeit t_2 Zeit zwischen Vorwalzen und Warmwalzen, über das Fertigwalzen insgesamt erzielte Stichabnahme Δhf, Endwalztemperatur TEW, Kühlpause t_p zwischen dem Ende des Warmwalzens und dem Beginn der forcierten Abkühlung, Abkühlgeschwindigkeit dT, Haspeltemperatur HT, Banddicke BD und Bandbreite BB angegeben.In Tables 2a, 2b, the process parameters reheating temperature TW, rough-rolling temperature TVW set in the processing of the slabs 1-26, total decay Δhv over the roughing, time t_1 between descaling after reheating and pre-rolling, and start of final hot rolling are shown. Time t_2 Time between rough rolling and hot rolling, total reduction achieved via finish rolling Δhf, final rolling temperature TEW, cooling pause t_p between the end of hot rolling and the beginning of forced cooling, cooling rate dT, reel temperature HT, belt thickness BD and belt width BB.

Die mechanischen Eigenschaften sowie das Gefüge der erhaltenen Warmbänder sind untersucht worden.The mechanical properties as well as the microstructure of the obtained hot strips have been investigated.

Die Zugversuche zur Ermittlung der Streckgrenze ReH, der Zugfestigkeit Rm und der Bruchdehnung A wurden nach DIN EN ISO 6892-1 an Längsproben der Warmbänder durchgeführt.The tensile tests for determining the yield strength ReH, the tensile strength Rm and the elongation at break A were carried out according to DIN EN ISO 6892-1 on longitudinal samples of the hot strips.

Die Kerbschlagbiegeversuche zur Ermittlung der Kerbschlagarbeit Av bei -20 °C bzw. -40 °C und -60 °C wurden an Längsproben nach DIN EN ISO 148-1 durchgeführt.The notched bar impact tests to determine the impact energy Av at -20 ° C and -40 ° C and -60 ° C were carried out on longitudinal samples according to DIN EN ISO 148-1.

Die Gefügeuntersuchungen erfolgten mittels Lichtmikroskop und Rasterelektronenmikroskop. Dafür wurden die Proben aus einem Viertel der Bandbreite entnommen, als Längsschliff präpariert und mit Nital (d. h. alkoholische Salpetersäure, die einen Salpetersäureanteil von 3 Vol.-% enthält) oder Natriumdisulfit geätzt. Die Bestimmung der Gefügebestandteile erfolgte mittels Flächenanalyse in Probenlage 1/3 Blechdicke, wie in H. Schumann und H. Oettel "Metallografie" 14. Auflage, 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim beschrieben.The structural investigations were carried out by light microscope and scanning electron microscope. For this, the samples were taken from a quarter of the bandwidth, prepared as a longitudinal section and with Nital (ie alcoholic nitric acid containing a proportion of 3% by volume of nitric acid) or Sodium disulfite etched. The determination of the structural components was carried out by means of area analysis in sample position 1/3 sheet thickness, as in H. Schumann and H. Oettel "Metallography" 14th edition, 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim described.

Die mechanischen Eigenschaften und die Gefügebestandteile der erfindungsgemäß erzeugten Warmbänder sind in Tabelle 3 angegeben. Die gemäß dem Verfahren der vorliegenden Erfindung hergestellten Bandbleche weisen hohe Festigkeitseigenschaften bei guten Zähigkeitseigenschaften sowie guter Bruchdehnung auf.The mechanical properties and the structural constituents of the hot strips produced according to the invention are given in Table 3. The band sheets produced according to the method of the present invention have high strength properties with good toughness properties and good elongation at break.

Die Streckgrenzen der in der voranstehend erläuterten Weise erzeugten Warmbänder liegen zwischen 700 MPa und 790 MPa. Die Bruchdehnung beträgt mindestens 12 % und die Zugfestigkeit 750 - 880 MPa. Die Kerbschlagarbeit bei -20 °C liegt im Bereich 60 bis 100 J. Bei -40 °C beträgt die Kerbschlagarbeit 40 bis 75 J und bei -60 °C liegt die Kerbschlagarbeit bei 30 - 70 J. Tabelle 1 Stahl C Si Mn P S Al N Cr Nb B Ti Cu Ni V Mo Sb A 0,060 0,42 1,77 0,012 0,0010 0,034 0,0046 0,04 0,062 0,0020 0,110 0,02 0,03 0,010 0,004 0,004 B 0,053 0,49 1,75 0,015 0,0014 0,034 0,0049 0,06 0,066 0,0020 0,091 0,02 0,03 0,005 0,004 0,004 C 0,061 0,22 1,79 0,014 0,0021 0,050 0,0047 0,04 0,063 0,0019 0,097 0,02 0,02 0,003 0,004 0,004 D 0,065 0,20 1,8 0,014 0,0021 0,040 0,0047 0,04 0,065 0,0005 0,110 0,02 0,02 0,003 0,004 0,004 E 0,070 0,03 1,89 0,011 0,0014 0,042 0,0051 0,04 0,060 0,0005 0,130 0,02 0,03 0,008 0,004 0,004 Angaben in Gew.-%, Rest Eisen und unvermeidbare Verunreinigungen Tabelle 2a Nr. Stahl TW Δhv TVW t_1 t_2 Δhf TEW t_p dT HT BD BB [°C] [%] [°C] [s] [s] [%] [°C] [s] [K/s] [°C] [mm] [mm] 1 A 1293 85 1070 220 40 90 905 1 100 600 4 1525 2 A 1296 80 1065 220 40 92 915 1 100 600 4 1525 3 A 1288 80 1045 225 40 92 895 2 100 605 4 1525 4 A 1287 85 1045 230 42 90 880 2 100 605 4 1530 5 A 1269 82 1055 230 40 91 890 2 100 600 4 1525 6 A 1300 82 1050 240 45 82 835 3 70 600 8 1545 7 A 1296 82 1050 245 41 82 810 4 70 600 8 1545 8 A 1305 76 1060 240 42 86 825 4 70 600 8 1755 9 A 1247 76 1040 260 44 83 800 6 50 580 10 1530 10 B 1291 80 1060 230 40 90 910 2 100 600 5 1630 11 B 1309 80 1110 240 44 90 870 2 100 610 5 1630 12 B 1288 85 1070 230 40 88 890 2 100 600 5 1540 13 B 1304 76 1055 240 40 90 860 2 90 600 6 1540 14 B 1285 85 1030 255 42 75 800 5 50 590 10 1550 15 B 1296 85 1100 210 40 93 850 2 120 600 3 1280 16 B 1298 82 1090 200 40 93 900 1 120 580 3 1275 Tabelle 2b Nr. Stahl TW Δhv TVW t_1 t_2 Δhf TEW t_p dT HT BD BB [°C] [%] [°C] [s] [s] [%] [°C] [s] [K/s] [°C] [mm] [mm] 17 B 1206 82 1067 205 40 93 870 1 120 610 3 1275 18 C 1289 85 1040 260 45 75 800 6 50 550 10 1550 19 C 1291 85 1090 235 42 85 880 2 90 605 6 1535 20 C 1214 82 1070 230 40 91 865 2 100 600 4 925 21 D 1290 85 1090 205 40 93 890 1 120 620 3 1280 22 D 1285 82 1080 200 40 93 900 1 120 575 3 1275 23 E 1290 76 1060 260 43 83 800 6 50 598 10 1550 24 E 1290 78 1090 235 40 89 860 3 90 615 6 1535 25 E 1290 80 1040 260 45 76 800 7 50 590 12 1530 26 E 1285 78 1045 260 45 73 822 7 50 570 15 1530 Tabelle 3 Nr. Stahl Lage am Coil Zugversuch, längs Kerbschlagbiegeversuch, längs Gefügebestandteile ReH Rm A Av-20°C Av-40°C Av-60°C [MPa] [MPa] [%] [J] [J] [J] Vol. % 1 A Anfang 770 852 19,0 n.b. n.b. n.b. 100 Bainit 2 A Anfang 762 837 17,0 n.b. n.b. n.b. 100 Bainit 3 A Anfang 749 819 18,0 n.b. n.b. n.b. 100 Bainit 4 A Anfang 754 818 21,0 n.b. n.b. n.b. 100 Bainit 5 A Anfang 737 809 24,0 n.b. n.b. n.b. 100 Bainit 6 A Anfang 736 834 20,3 70 44 31 100 Bainit 7 A Anfang 739 842 15,7 81 62 31 100 Bainit 8 A Anfang 716 817 17,2 62 40 31 100 Bainit 9 A Anfang 733 832 23,5 79 68 65 100 Bainit 10 B Anfang 750 852 16,0 n.b. n.b. n.b. 100 Bainit 11 B Anfang 752 841 22,0 n.b. n.b. n.b. 100 Bainit 12 B Anfang 736 829 20,0 n.b. n.b. n.b. 100 Bainit 13 B Anfang 734 860 17,0 99 48 33 100 Bainit 14 B Anfang 717 846 18,0 84 58 30 100 Bainit 15 B Anfang 782 864 23,0 n.b. n.b. n.b. 100 Bainit 16 B Anfang 779 857 24,0 n.b. n.b. n.b. 100 Bainit 17 B Anfang 720 819 23,0 n.b. n.b. n.b. 100 Bainit 18 C Anfang 705 813 19,1 97 73 30 100 Bainit 19 C Anfang 718 783 24,0 80 60 31 100 Bainit 20 C Anfang 710 790 24,0 n.b. n.b. n.b. 100 Bainit 21 D Anfang 720 850 22,0 n.b. n.b. n.b. 100 Bainit 22 D Anfang 760 823 22,0 n.b. n.b. n.b. 100 Bainit 23 E Anfang 712 820 20,0 97 73 30 100 Bainit 24 E Anfang 713 825 23,0 80 60 31 100 Bainit 25 E Anfang 733 809 21,0 72 53 42 100 Bainit 26 E Anfang 727 821 19,2 83 76 67 100 Bainit "n.b." = "nicht bestimmt" The yield strengths of the hot strips produced in the above manner are between 700 MPa and 790 MPa. The elongation at break is at least 12% and the tensile strength 750-880 MPa. The notch impact work at -20 ° C is in the range 60 to 100 J. At -40 ° C, the notch impact work is 40 to 75 J and at -60 ° C, the impact energy is at 30 - 70 J. Table 1 steel C Si Mn P S al N Cr Nb B Ti Cu Ni V Mo sb A 0,060 0.42 1.77 0,012 0.0010 0.034 0.0046 0.04 0.062 0.0020 0,110 0.02 0.03 0,010 0,004 0,004 B 0.053 0.49 1.75 0,015 0.0014 0.034 0.0049 0.06 0.066 0.0020 0.091 0.02 0.03 0.005 0,004 0,004 C 0,061 0.22 1.79 0,014 0.0021 0,050 0.0047 0.04 0.063 0.0019 0.097 0.02 0.02 0,003 0,004 0,004 D 0,065 0.20 1.8 0,014 0.0021 0,040 0.0047 0.04 0,065 0.0005 0,110 0.02 0.02 0,003 0,004 0,004 e 0,070 0.03 1.89 0.011 0.0014 0,042 0.0051 0.04 0,060 0.0005 0.130 0.02 0.03 0,008 0,004 0,004 Data in wt .-%, balance iron and unavoidable impurities No. steel TW Δhv TVW t_1 t_2 △ Hf TEW t_p dT HT BD BB [° C] [%] [° C] [s] [s] [%] [° C] [s] [K / s] [° C] [mm] [mm] 1 A 1293 85 1070 220 40 90 905 1 100 600 4 1525 2 A 1296 80 1065 220 40 92 915 1 100 600 4 1525 3 A 1288 80 1045 225 40 92 895 2 100 605 4 1525 4 A 1287 85 1045 230 42 90 880 2 100 605 4 1530 5 A 1269 82 1055 230 40 91 890 2 100 600 4 1525 6 A 1300 82 1050 240 45 82 835 3 70 600 8th 1545 7 A 1296 82 1050 245 41 82 810 4 70 600 8th 1545 8th A 1305 76 1060 240 42 86 825 4 70 600 8th 1755 9 A 1247 76 1040 260 44 83 800 6 50 580 10 1530 10 B 1291 80 1060 230 40 90 910 2 100 600 5 1630 11 B 1309 80 1110 240 44 90 870 2 100 610 5 1630 12 B 1288 85 1070 230 40 88 890 2 100 600 5 1540 13 B 1304 76 1055 240 40 90 860 2 90 600 6 1540 14 B 1285 85 1030 255 42 75 800 5 50 590 10 1550 15 B 1296 85 1100 210 40 93 850 2 120 600 3 1280 16 B 1298 82 1090 200 40 93 900 1 120 580 3 1275 No. steel TW Δhv TVW t_1 t_2 △ Hf TEW t_p dT HT BD BB [° C] [%] [° C] [s] [s] [%] [° C] [s] [K / s] [° C] [mm] [mm] 17 B 1206 82 1067 205 40 93 870 1 120 610 3 1275 18 C 1289 85 1040 260 45 75 800 6 50 550 10 1550 19 C 1291 85 1090 235 42 85 880 2 90 605 6 1535 20 C 1214 82 1070 230 40 91 865 2 100 600 4 925 21 D 1290 85 1090 205 40 93 890 1 120 620 3 1280 22 D 1285 82 1080 200 40 93 900 1 120 575 3 1275 23 e 1290 76 1060 260 43 83 800 6 50 598 10 1550 24 e 1290 78 1090 235 40 89 860 3 90 615 6 1535 25 e 1290 80 1040 260 45 76 800 7 50 590 12 1530 26 e 1285 78 1045 260 45 73 822 7 50 570 15 1530 No. steel Location on the coil Tensile test, longitudinal Charpy impact test, longitudinal Structural constituents Deer rm A Av-20 ° C Av-40 ° C Av-60 ° C [MPa] [MPa] [%] [J] [J] [J] Vol% 1 A Beginning 770 852 19.0 nb nb nb 100 bainite 2 A Beginning 762 837 17.0 nb nb nb 100 bainite 3 A Beginning 749 819 18.0 nb nb nb 100 bainite 4 A Beginning 754 818 21.0 nb nb nb 100 bainite 5 A Beginning 737 809 24.0 nb nb nb 100 bainite 6 A Beginning 736 834 20.3 70 44 31 100 bainite 7 A Beginning 739 842 15.7 81 62 31 100 bainite 8th A Beginning 716 817 17.2 62 40 31 100 bainite 9 A Beginning 733 832 23.5 79 68 65 100 bainite 10 B Beginning 750 852 16.0 nb nb nb 100 bainite 11 B Beginning 752 841 22.0 nb nb nb 100 bainite 12 B Beginning 736 829 20.0 nb nb nb 100 bainite 13 B Beginning 734 860 17.0 99 48 33 100 bainite 14 B Beginning 717 846 18.0 84 58 30 100 bainite 15 B Beginning 782 864 23.0 nb nb nb 100 bainite 16 B Beginning 779 857 24.0 nb nb nb 100 bainite 17 B Beginning 720 819 23.0 nb nb nb 100 bainite 18 C Beginning 705 813 19.1 97 73 30 100 bainite 19 C Beginning 718 783 24.0 80 60 31 100 bainite 20 C Beginning 710 790 24.0 nb nb nb 100 bainite 21 D Beginning 720 850 22.0 nb nb nb 100 bainite 22 D Beginning 760 823 22.0 nb nb nb 100 bainite 23 e Beginning 712 820 20.0 97 73 30 100 bainite 24 e Beginning 713 825 23.0 80 60 31 100 bainite 25 e Beginning 733 809 21.0 72 53 42 100 bainite 26 e Beginning 727 821 19.2 83 76 67 100 bainite "nb" = "not determined"

Claims (9)

  1. Hot rolled flat steel product with a yield strength of 700-850 MPa, an elongation at breaking of at least 12% and a minimum of 70 vol% bainitic microstructure, made from a steel alloy comprising (in wt%) C: 0.05 - 0.08 %, Si: 0.015 - 0.500 %, Mn: 1.60 - 2.00 %, P: up to 0.025 %, S: up to 0.010 %, Al: 0.020 - 0.050 %, N: up to 0.006 %, Cr: up to 0.40 %, Nb: 0.060 - 0.070 %, B: 0.0005 - 0.0025%, Ti: 0.090 - 0.130 %,
    as well as technically unavoidable
    impurities, including up to 0.12 % Cu, up to 0.100 % Ni, up to 0.010 % V, up to 0.004 % Mo and up to 0.004 % Sb, and
    iron as the remainder.
  2. Flat steel product according to claim 1,characterised in that the carbon equivalent CE of the steel alloy from which the flat steel product is made is CE < 0.5 wt % ,
    Figure imgb0006
     the carbon equivalent being calculated according to the formula CE = % C + % Mn / 6 + % Cr + % Mo + % V / 5 + % Cu + % Ni / 15
    Figure imgb0007
     with
    %C = respective C content in wt%,
    %Mn = respective Mn content in wt%,
    %Cr = respective Cr content in wt%,
    %Mo = respective Mo content in wt%,
    %V = respective V content in wt%
    %Cu = respective Cu content in wt%,
    %Ni = respective Ni content in wt%,
  3. Flat steel product according to any one of the preceding claims, characterised in that its thickness is 3 - 15 mm.
  4. Flat steel product according to any one of the preceding claims, characterised in that its tensile strength is 750-950 MPa.
  5. Flat steel product according to any one of the preceding claims, characterised in that its notch impact energy at -20°C is 50-110 J.
  6. Flat steel product according to any one of the preceding claims, characterised in that it has a microstructure which, except for technically unavoidable other structural components, is exclusively bainitic.
  7. Flat steel product according to any one of the preceding claims, characterised in that the mean grain diameter of its microstructure is at most 20 µm.
  8. Flat steel product according to any one of the preceding claims, characterised in that its S content is at most 0.003 wt%.
  9. Use of a flat steel product formed according to any one of the preceding claims in the construction of commercial vehicles.
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