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US20150218684A1 - Cold-Rolled Flat Steel Product and Method for the Production Thereof - Google Patents

Cold-Rolled Flat Steel Product and Method for the Production Thereof Download PDF

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Publication number
US20150218684A1
US20150218684A1 US14/417,659 US201314417659A US2015218684A1 US 20150218684 A1 US20150218684 A1 US 20150218684A1 US 201314417659 A US201314417659 A US 201314417659A US 2015218684 A1 US2015218684 A1 US 2015218684A1
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weight
flat steel
cold
steel product
content
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Brigitte Hammer
Thomas Heller
Frank Hisker
Rudolf Kawalla
Grzegorz Korpala
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel Europe AG
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    • 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
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • 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
    • 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
    • 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
    • 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
    • C22C38/02—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
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • C22C38/42—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
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the invention relates to a cold-rolled flat steel product having a tensile strength Rm of at least 1400 MPa and an elongation A80 of at least 5%. Products of this type are distinguished by a very high strength in combination with good elongation properties, and are suitable as such in particular for the production of components for motor vehicle bodies.
  • the invention similarly relates to a method for producing a flat steel product according to the invention.
  • flat steel product is to be understood here as meaning steel sheets or steel strips produced by a rolling process and also sheet bars and the like separated therefrom.
  • EP 1 466 024 B1 (DE 603 15 129 T 2 ) discloses a method for producing a flat steel product which is intended to have tensile strengths of considerably more than 1000 MPa.
  • a steel melt comprising (in % by weight) 0.0005-1% C, 0.5-10% Cu, up to 2% Mn, up to 5% Si, up to 0.5% Ti, up to 0.5% Nb, up to 5% Ni, up to 2% Al and as remainder iron and impurities which are unavoidable for production-related reasons is produced.
  • the melt is cast to form a strip, the thickness of which is at most 10 mm and which is cooled rapidly to a temperature of at most 1000° C.
  • the cast strip is hot-rolled with a conventional reduction rate.
  • the hot-rolling is ended at an end temperature at which all of the copper is still in a solid solution in the ferrite and/or austenite matrix.
  • the strip is subjected to a step of rapid cooling, in order to keep the copper in a supersaturated solid solution in the ferrite and/or austenite solution.
  • a cold strip can be rolled from the hot strip thus obtained with a degree of cold-rolling amounting to 40-80%.
  • This cold strip is then subjected to recrystallization annealing, during which it is brought as rapidly as possible to an annealing temperature lying in the region of 840° C. and held at said temperature, in order to bring the greatest possible proportion of the copper present in the steel into solution.
  • This is followed by rapid cooling to a temperature amounting to 400-700° C., at which Cu precipitations form once again.
  • precipitation hardening is intended to achieve the desired strength level of the steel.
  • the copper content is intended to increase the corrosion and embrittlement resistance of the steel through the formation of a protective oxide layer.
  • a further method for producing a cold strip of extreme strength is known from U.S. Pat. No. 7,591,977 B2.
  • a hot strip comprising (in % by weight) 0.1-0.25% C, 1.0-2.0% Si and 1.5-3.0% Mn is rolled with a degree of cold-rolling of 30-70% to form a cold strip, which is then subjected to a heat treatment completed in a continuous pass.
  • the cold strip is heated, in a first annealing step, to a first annealing temperature lying above the Ar3 temperature thereof, in order to bring carbides present in the cold strip into solution.
  • the cold strip thus provided achieves tensile strengths of up to 1180 MPa combined with an elongation of at least 9% and can be coated, if required, with a metallic layer affording protection against corrosion.
  • the object mentioned above is achieved according to the invention in that at least the working steps indicated in claim 12 are performed to produce a cold-rolled flat steel product according to the invention.
  • the cold-rolled flat steel product according to the invention is distinguished by the fact that it comprises, in addition to iron and unavoidable impurities (in % by weight):
  • the microstructure of the flat steel product according to the invention consists of bainite to an extent of at least 20% by volume, of residual austenite to an extent of 10-35% by volume and of martensite as remainder, it being self-evident that technically unavoidable traces of other microstructure constituents may be present in the microstructure of the flat steel product.
  • a cold-rolled flat steel product according to the invention provided in this way regularly achieves tensile strengths Rm of at least 1400 MPa and an elongation A80 of at least 5%.
  • the C content of the residual austenite is typically more than 1.0% by weight.
  • the method according to the invention for producing a flat steel product provided or being composed according to the invention comprises the following working steps:
  • a steel strip according to the invention has a three-phase microstructure, the dominant constituent of which is bainite and which moreover consists of residual austenite and also of martensite as remainder. It is optimal here that the bainite proportion is at least 50% by volume, in particular at least 60% by volume, and that the residual austenite proportion lies in the range of 10-25% by volume, with the remainder of the microstructure being made up here too in each case by martensite. The optimum martensite proportion is at least 10% by volume.
  • a microstructure having such a composition brings about the best combination of Rm*A80 with the required tensile strength.
  • the residual austenite is present in a cold strip according to the invention predominantly in film form with small, globular islands of block residual austenite having a grain size of ⁇ 5 ⁇ m, such that the residual austenite has a high stability in the initial state and, associated therewith, a low tendency towards undesirable transformation into martensite.
  • martensite is formed from this residual austenite (TRIP effect), and this increases the elongation at break.
  • Cold strip produced according to the invention regularly achieves tensile strengths Rm of more than 1400 MPa, with elongations A80 which similarly regularly lie above 5%. Accordingly, the quality Rm*A80 of flat steel products according to the invention is regularly above 7000 MPa*%, with qualities Rm*A80 of at least 13 500 MPa*% typically being achieved. A cold strip according to the invention as such has an optimum combination of extreme strength and sufficient deformability.
  • the martensite start temperature i.e. the temperature from which martensite forms in steel processed according to the invention, can be calculated on the basis of the procedure explained in the article entitled “Thermodynamic Extrapolation and Martensite-Start-Temperature of Substitutionally Alloyed Steels” by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180.
  • the C content of the flat steel product according to the invention can be set to at least 0.25% by weight, in particular at least 0.27% by weight or at least 0.28% by weight, it being possible for the effects achieved by the comparatively high carbon content to be utilized particularly reliably when the C content lies in the range of >0.25-0.5% by weight, in particular 0.27-0.4% by weight or 0.28-0.4% by weight.
  • the strength-increasing action of copper can also be utilized in a cold-rolled flat steel product according to the invention.
  • a minimum Cu content of 0.15% by weight, in particular at least 0.2% by weight Cu can be present in the flat steel product according to the invention.
  • Cu makes a particularly effective contribution to the strength if it is present in the flat steel product according to the invention in contents of at least 0.55% by weight, it being possible for negative effects of the presence of Cu to be limited by virtue of the fact that the Cu content is limited to at most 1.5% by weight.
  • the optional addition of Cr can also lower the martensite start temperature and suppress the tendency of the bainite to transform into pearlite or cementite. Moreover, in contents up to the upper limit of at most 2% by weight as predefined according to the invention, Cr promotes the ferritic transformation, with optional effects of the presence of Cr in the cold-rolled flat steel product according to the invention arising when the Cr content is limited to 1.5% by weight. The positive influence of Cr can be utilized particularly effectively if at least 0.3% by weight Cr is present in the flat steel product according to the invention.
  • Ti, V or Nb which is likewise optional, can support the formation of a finer-grained microstructure and promote the bainitic transformation.
  • these microalloying elements contribute to an increase in the hardness through the formation of precipitations.
  • the positive effects of Ti, V and Nb can be utilized in a particularly effective manner in the cold-rolled flat steel product according to the invention when the content of each of these elements lies in the range of 0.002-0.15% by weight, in particular does not exceed 0.1% by weight.
  • Si is present in a flat steel product according to the invention in contents of 0.4-2.5% by weight and brings about a considerable solid solution solidification.
  • the Si content can be set to at least 1.0% by weight.
  • Al can partly replace the Si content.
  • Al like Si, has a deoxidizing action during the steel production.
  • a minimum Al content of 0.01% by weight can be provided.
  • Higher contents of Al prove to be expedient, for example, when the addition of Al is intended to set the hardness or tensile strength of the steel to a relatively low value in favour of improved deformability.
  • a further function of Si and Al consists in suppressing the carbide formation in the bainite and therefore stabilizing the residual austenite by dissolved C down to low temperatures.
  • the positive influences of the simultaneous presence of Al and Si can thereby be utilized particularly effectively when the Si and Al contents within the limits predefined according to the invention satisfy the following condition: % Si+0.8% Al>1.2% by weight (where % Si: respective Si content in % by weight, % Al: respective Al content in % by weight).
  • the formation of the microstructure predefined according to the invention can be ensured in particular by virtue of the fact that the Mn, Cr, Ni, Cu and C contents of the steel processed according to the invention and accordingly the Mn, Cr, Ni, Cu and C contents of the flat steel product according to the invention satisfy the following condition
  • % Mn denotes the respective Mn content in % by weight
  • % Cr denotes the respective Cr content in % by weight
  • % Ni denotes the respective Ni content in % by weight
  • % Cu denotes the respective Cu content in % by weight
  • % C denotes the respective C content in % by weight.
  • the primary or preliminary product cast from a steel having a composition according to the invention is firstly brought to a temperature or held at a temperature which is sufficient to end the hot-rolling carried out proceeding from this temperature at a hot-rolling end temperature lying in the range of 830-1000° C.
  • the hot strip cools down on the roller table adjoining the rolling stand in question.
  • the hot strip passes into a coiling device, in which it is wound to form a coil.
  • the coiling temperature has to be at least 560° C., so that a relatively soft hot strip microstructure consisting of ferrite and pearlite is formed.
  • a temperature profile which is optimal for this purpose arises if the hot-rolling end temperature lies in the range of 850-950° C., in particular in the range of 880-950° C. To this end, it is typically the case that the preliminary product is heated to a temperature lying in the range of 1100-1300° C. or is held at this temperature before the hot-rolling.
  • the microstructure of the hot strip thus obtained consists primarily of ferrite and pearlite. The risk of grain boundary oxidation arising can be minimized by virtue of the fact that the coiling temperature is limited to at most 750° C.
  • the hot strip After the coiling, the hot strip is cold-rolled, it going without saying that the hot strip can be conventionally descaled by chemical or mechanical means before the cold-rolling.
  • the cold-rolling is effected with a degree of cold-rolling of at least 30%, in particular at least 45%, in order to accelerate the recrystallization and transformation during the subsequent annealing. It is generally the case that a better surface quality is also obtained by observing a correspondingly high degree of cold-rolling. Degrees of cold-rolling of at least 50% have proved to be particularly favourable for this purpose.
  • the cold strip obtained according to the invention completes an annealing cycle in a continuous pass, during which it is heated in a first annealing phase to a temperature of at least 800° C., preferably at least 830° C.
  • This first annealing phase lasts at least for such a period of time that the cold strip is completely austenitized. 50-150 s are typically required for this.
  • the product is quenched, the cooling rate being at least 8° C./s, in particular 10° C./s.
  • the target temperature for this quenching is a holding temperature which is at most 470° C. and is higher than the martensite start temperature MS, from which martensite forms in the microstructure of the cold strip.
  • the range of 300-420° C., in particular 330-420° C. can be used as an indication of the range in which the holding temperature is to lie.
  • the cold strip is held in the holding temperature range in the second annealing phase, to be precise until at least 20% by volume of the microstructure of the cold strip has transformed into bainite.
  • the hold can be carried out here as an isothermal hold at the holding temperature reached during the cooling or as a slow decrease in temperature within the holding temperature range.
  • the flat steel product produced according to the invention can be coated in a conventional manner with a metallic protective layer. This can be effected by hot-dip coating, for example. If annealing is required before the application of the metallic coating, the heat treatment provided according to the invention can be carried out in the course of this annealing.
  • the steel melts of corresponding composition were cast in a conventional manner to form a strand, from which slabs were separated.
  • the slabs were then heated in a similarly conventional manner to a reheating temperature.
  • the heated slabs were hot-rolled in a similarly conventional group of hot-rolling stands to form hot strips having a thickness of 2 mm.
  • the hot-rolling end temperature was in the range of 830-900° C. in each case.
  • the hot strips were cooled proceeding from this temperature to a coiling temperature lying above 560° C. and then coiled to form coils.
  • the hot strips thus obtained were descaled after the coiling and cold-rolled to form cold strip with degrees of cold-rolling of 50% after the descaling.
  • a relatively large number of specimens of these cold strips were then subjected to a heat treatment, in which they were heated in a first annealing step at a heating rate of at least 1.9° C./s to a first annealing temperature in the range of 830-850° C.
  • the cold strips were held at this temperature for a period of time of 120 s, until they had been completely heated through.
  • the holding temperatures T 2 in a first batch of tests were 300° C., 310° C., 330° C., 340° C., 375° C., 390° C. and 410° C.
  • the cold strip specimens were held at the respective holding temperature T 2 for an annealing duration t 2 .
  • the tensile strengths Rm achieved are plotted against the respective annealing temperature T 2 . It can be seen that the cold strip specimens produced from the steel S 5 each achieve the required minimum tensile strength of 1400 MPa only under certain annealing conditions, whereas the tensile strengths of the cold strip specimens produced from the other steels were always reliably above the minimum limit of 1400 MPa.
  • the comparatively low carbon content of the steel S 5 lying at the lower limit of the content range predefined according to the invention, has been identified as the reason for this.
  • the tensile strengths of the cold strip specimens produced from the steel S 4 are plotted against the annealing duration t 2 of the second annealing stage. It can be seen that the cold strip specimens held at a holding temperature of 310° C., 330° C. and 350° C., i.e. in the holding temperature range of 310-350° C., achieved the required tensile strength Rm of 1400 MPa, irrespective of the respective annealing duration t 2 .
  • the tensile strengths of the cold strip specimens produced from the steel S 5 are similarly plotted against the annealing duration t 2 of the second annealing stage. It can be seen here that the cold strip specimens held at a holding temperature of 350° C. and 390° C., i.e. in the holding temperature range of 350-390° C., achieve the required tensile strength Rm of 1400 MPa if the annealing duration t 2 is shorter than 145 s.
  • the elongation A80 of the cold strip specimens produced from the steel S 4 is plotted against the annealing duration t 2 of the second annealing stage.
  • the elongation A80 of the cold strip specimens produced from the steel S 5 is plotted against the annealing duration t 2 of the second annealing stage.
  • the cold strip specimens achieve the required elongation A80 of at least 5% irrespective of the respective holding temperature T 2 thereof and irrespective of the respective annealing duration t 2 . Accordingly, if a short annealing duration and suitably low holding temperatures T 2 are observed, it is also possible for a cold-rolled flat steel product according to the invention in which a high tensile strength Rm is combined with a sufficient elongation A80 to be produced from the steel S 5 , in spite of the comparatively low C content thereof.
  • FIG. 6 shows, in a section, a magnified view of a cross section of a cold strip according to the invention.
  • residual austenite blocks RA-b are marked and a point at which film-like residual austenite RA-f is present in a lamellar stratification is emphasized by being circled.

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  • Organic Chemistry (AREA)
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  • Crystallography & Structural Chemistry (AREA)
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US14/417,659 2012-07-27 2013-07-26 Cold-Rolled Flat Steel Product and Method for the Production Thereof Abandoned US20150218684A1 (en)

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EP12178332.8A EP2690184B1 (fr) 2012-07-27 2012-07-27 Cold rolled steel flat product and method for its production
EP12178332.8 2012-07-27
PCT/EP2013/065838 WO2014016421A1 (fr) 2012-07-27 2013-07-26 Produit plat en acier laminé à froid et son procédé de fabrication

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US20190003007A1 (en) * 2015-12-21 2019-01-03 Arcelormittal Method for Producing a High Strength Steel Sheet Having Improved Strength and Formability, and Obtained High Strength Steel Sheet

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EP2690184A1 (fr) 2014-01-29
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EP2690184B1 (fr) 2020-09-02
JP6202579B2 (ja) 2017-09-27
WO2014016421A1 (fr) 2014-01-30

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