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EP2383353B2 - High-strength, manganese-containing steel, flat steel product made from such steel and methods for its production - Google Patents
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EP2383353B2 - High-strength, manganese-containing steel, flat steel product made from such steel and methods for its production - Google Patents

High-strength, manganese-containing steel, flat steel product made from such steel and methods for its production

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Publication number
EP2383353B2
EP2383353B2 EP11164339.1A EP11164339A EP2383353B2 EP 2383353 B2 EP2383353 B2 EP 2383353B2 EP 11164339 A EP11164339 A EP 11164339A EP 2383353 B2 EP2383353 B2 EP 2383353B2
Authority
EP
European Patent Office
Prior art keywords
content
steel
hot
flat steel
steel product
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Active
Application number
EP11164339.1A
Other languages
German (de)
French (fr)
Other versions
EP2383353A3 (en
EP2383353B1 (en
EP2383353A2 (en
Inventor
Dr.-Ing. Jens-Ulrik Becker
Dr.-Ing. Jian Bian
Dr. Brigitte Hammer
Dr. Thomas Heller
Christian Höckling
Dr.-Ing. Harald Hofmann
Dipl.-Ing. Matthias Schirmer
Oliver Bülters
Thomas Rieger
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
Original Assignee
ThyssenKrupp Steel Europe AG
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Classifications

    • 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
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/041—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing involving a particular fabrication or treatment of ingot or slab
    • C21D8/0415—Rapid solidification; Thin strip casting
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0421—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0447—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing
    • C21D8/0478—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for drawing, e.g. for deep-drawing involving a particular surface treatment
    • 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

Definitions

  • a method for producing hot-rolled strips from a formable, in particular cold-deep-drawable, lightweight structural steel, which is to possess high tensile strength and TRIP and/or TWIP properties, is derived from the WO 2005/061152 A1 known. According to this process, a steel melt is cast in a horizontal strip casting plant to a near-net-shape, flow-stabilized and bend-free pre-strip in the range between 6 and 15 mm and subsequently subjected to further processing.
  • the steel used contains, in addition to iron and unavoidable impurities (in wt.%), C: 0.04–1.0%, Al: 0.05– ⁇ 4.0%, Si: 0.05–6.0%, Mn: 9.0–30.0%, and optionally Cr: up to 6.5%, with Cr contents of 0.2–0.3% being preferred, Nb and V in total contents of up to 0.06%, and Ti and Zr in total contents of up to 0.7%.
  • Cr contents contents of 0.2–0.3% being preferred
  • Nb and V in total contents of up to 0.06%
  • Ti and Zr in total contents of up to 0.7%.
  • the effect of chromium is seen as stabilizing the ⁇ -martensite and improving corrosion resistance.
  • higher Cr contents are recommended at Mn contents of 9–18%, while lower Cr contents are considered sufficient at Mn contents above 18%.
  • WO 2005/061152 A1 it specifies how this ratio should be set in concrete terms.
  • EP 0 425 058 A1 A use of a killed-cast steel containing 0.15–0.25% C, 3.40–6.10% Mn, 0–1.0% Ni, 0–1.0% Cr, 0–1.0% Mo, 0–0.15% V, max. 0.03% P, max. 0.03% S, max. 0.6% Si, max. 0.05% Al, balance iron and usual impurities, as a material for the manufacture of tubes for reinforcing motor vehicle doors, provided that the following relationship for the sum of the alloying elements (in wt.%) is satisfied: Mn + Ni + Cr + Mo + 10 ⁇ V ⁇ 4 , 5 shareholderss ⁇ %
  • the object of the invention was to create a flat steel product with good strength and good formability from a steel that can be produced more cost-effectively than the known high-manganese steels and at the same time has high elongation at break values and thus significantly improved formability.
  • the invention proposes a material concept according to which a steel, in addition to iron and unavoidable impurities, consists of (in wt.%) C: 0.02 - 0.5% Mn: 7 - 12.0% Yes: 0.05 - 1.0% Al: up to 3.0% Cr: 1 - 4.0% Cu: up to 2.0% Ni: up to 2.0% N: up to 0.05% P: up to 0.05% S: up to 0.01% consists of and optionally contains one or more elements from the group "V, Nb, Ti", where the sum of the contents of these elements is at most 0.5%.
  • microstructure of a flat steel product produced from such a steel according to the invention typically consists of 30 - 100% hardened microstructure (martensite, tempered martensite or bainite), while the remainder of the microstructure is austenitic.
  • a steel according to the invention due to its medium-range manganese content, can be produced at significantly reduced alloying and manufacturing costs, both by continuous casting and by strip casting.
  • carbon determines, on the one hand, the strength of martensite and, on the other hand, the quantity and stability of the retained austenite. Excessively high carbon contents negatively affect the weldability and toughness of the steel, for example, through the formation of chromium carbides.
  • the carbon content of manganese steels of the type according to the invention is therefore below 0.5 wt.%, with optimal properties being achieved when the carbon content is limited to less than 0.2 wt.%, and in particular less than 0.1 wt.%.
  • the carbon content of a steel according to the invention is at least 0.02 wt.%, in particular at least 0.03 wt.%, for example at least 0.05 wt.%.
  • Manganese is an austenite former. It delays the transformation of ferrite, pearlite, and bainite, thus stabilizing austenite up to the martensite start temperature. Manganese promotes the formation of cubically or hexagonally distorted martensite ( ⁇ - or ⁇ -martensite). These manganese martensites are characterized by high strength and significantly higher toughness compared to carbon-induced, cubically distorted ⁇ -martensite. If the manganese content is too low, bainite forms upon cooling, resulting in lower strength and elongation at break. Conversely, if the manganese content is too high, there is a risk that the entire austenite will remain stable until room temperature.
  • the manganese content of 7–12% specified according to the invention allows for the creation of a martensite matrix with a retained austenite component in the microstructure. This effect is particularly likely to occur if the Mn content is at least 7 wt.%, whereby the positive effects of manganese in a steel according to the invention can be optimized by limiting the upper limit of the Mn content to 10 wt.%, in particular to less than 9 wt.%, for example to up to 8.5 wt.%.
  • Aluminum and silicon are strong ferrite formers. Both elements counteract the influence of the austenite formers carbon and manganese.
  • the essential function of the elements silicon (Si) and aluminum (Al) in a steel according to the invention is to suppress carbide precipitation in the martensite matrix and thus promote the stability of the retained austenite.
  • Si and Al lead to solid solution hardening and reduce the specific gravity of the steel.
  • the Si and Al content is too low, carbide precipitation may not be effectively suppressed.
  • processing becomes more difficult in both continuous casting and strip casting processes.
  • the invention provides for limiting the Si content to a maximum of 1 wt.%, whereby the positive effects of the presence of Si can be effectively utilized if the Si content of the steel according to the invention is at least 0.05 wt.%, in particular 0.1 wt.%.
  • the negative effects of Si can be particularly reliably excluded by limiting the Si content to 0.7 wt.%, in particular 0.5 wt.%.
  • the Al content can be set at a minimum of 0.01 wt. %, in particular 0.02 wt. %, while negative influences of Al can be particularly reliably excluded if the Al content of a steel according to the invention is limited to 2 wt. %, in particular 1 wt. %,.
  • the presence of copper, chromium, and nickel generally improves the resistance of a steel according to the invention to various corrosion mechanisms.
  • the positive effect of Cu and Ni can be utilized particularly reliably by adding these elements to the steel according to the invention in amounts totaling at least > 0 wt.%, and in particular 0.1 wt.%.
  • negative effects of the presence of Cu and/or Ni in steels according to the invention are avoided by limiting the Cu and Ni content to a maximum of 1 wt.% each, or by limiting the total Cu and Ni content to a maximum of 2 wt.%, and in particular 1 wt.%.
  • the presence of chromium (Cr) in a steel according to the invention specifically reduces the risk of stress corrosion cracking. Cr also contributes to increased strength. These positive effects can be observed from a Cr content of 0.1 wt.%, with the positive effect of Cr being particularly reliable when the Cr content, as in the steel according to the invention, is at least 1 wt.%.
  • the Cr content of a steel according to the invention is limited to a maximum of 4 wt.% because higher contents can lead to the formation of Cr carbides, which can negatively affect the ductility of the steel. Such negative effects can be particularly reliably prevented by limiting the Cr content to a maximum of 2 wt.%.
  • the presence of Cr in a steel according to the invention has an optimal effect when the Cr content is 1–2 wt.%.
  • Ti, Nb, and V which can be present in a total of up to 0.5 wt.% in a steel according to the invention, contribute to grain refinement and increased strength. Total contents of Ti, Nb, and V exceeding 0.5 wt.% do not enhance this effect.
  • the strength-enhancing effect of Ti, Nb, and V can be utilized particularly effectively and efficiently when the total content of these microalloying elements in a steel according to the invention is limited to 0.3 wt.%, particularly 0.2 wt.%.
  • the positive effect of the microalloying elements mentioned here is already achieved when the total content is at least 0.025 wt.%. In the case of Ti, its content is advantageously limited to a maximum of 0.15 wt.% to prevent coarse Ti precipitates.
  • the austenitic microstructure can be further stabilized by the addition of nitrogen in contents of up to 0.05 wt.%, particularly 0.03 wt.%. This effect occurs even when the nitrogen content of a steel according to the invention is at least 0.002 wt.%, in particular at least 0.0025 wt.%, with an optimal effect being achieved when the nitrogen content is limited to a maximum of 0.025 wt.%.
  • the phosphorus content of a steel according to the invention is limited to a maximum of 0.05 wt.%, preferably 0.03 wt.%, in order to reliably exclude negative influences of this element.
  • the sulfur content of a steel according to the invention is limited to a maximum of 0.01 wt.%, in particular 0.005 wt.%.
  • the alloy concept according to the invention is designed to enable the formation of hardened microstructures with or without retained austenite in hot-rolled strip.
  • the martensite start temperature M ⁇ sub>S ⁇ /sub> of a steel alloyed according to the invention is above room temperature
  • the martensite finish temperature M ⁇ sub> F ⁇ /sub> of a steel composed according to the invention is below room temperature.
  • the steel flat product according to the invention is an uncoated hot-rolled strip.
  • the possibilities for producing hot-rolled or cold-rolled strips made of manganese steel are summarized in the accompanying diagram. Specifically, they comprise the following processing steps:
  • the castability of Mn steels according to the invention is improved as a result of the reduction in Mn content.
  • One method for producing hot-rolled strip is conventional continuous casting.
  • a steel according to the invention proves particularly advantageous because it allows for a reduced hot-rolled strip thickness of less than 2.5 mm. This is due to the fact that its forming resistance is significantly reduced compared to conventional high-manganese steels as a result of the lower manganese content.
  • the higher austenite content is achieved by annealing the hot-rolled strip. This reduces the strength and significantly increases the elongation at break. After hot-rolled strip annealing, up to 70% austenite content is achieved, depending on the analytical approach; this is primarily responsible for the improved elongation at break. Since a martensite matrix is present in unannealed hot-rolled strip, it is difficult to process it directly into cold-rolled strip. Therefore, hot-rolled strip annealing can also serve the purpose of softening the hot-rolled strip for cold rolling. Both hood annealing and continuous annealing are suitable methods for hot-rolled strip.
  • Cold rolling of the annealed or unannealed hot-rolled strip further reduces the strip thickness and improves strip flatness.
  • Subsequent annealing eliminates work hardening for component manufacturing and results in an optimal microstructure with an increased austenite content.
  • Both hot-annealed and cold-annealed strip can be finished either electrolytically, by hot-dip galvanizing (following cold-annealing), or by other strip coatings. It is also possible to apply an organic coating to the resulting steel strip.
  • the desired microstructure of a steel according to the invention typically comprising 30-100% hardened microstructure (martensite, tempered martensite or bainite) and the remainder being austenite, can be achieved by hot forming and quenching the steel.
  • the resulting hot strip exhibited a tensile strength (Rm) of 1400 MPa and an elongation at break (A80) of 7%.
  • the retained austenite content of its microstructure was 14%.
  • a hot-rolled strip consisting of iron and unavoidable impurities, and containing (in wt%) 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 1.5% Cr, 0.18% Ni, 0.13% Cu, 0.02% N, and 0.079% V, was subjected to hood annealing at a temperature of 650°C for 40 hours.
  • the annealed hot-rolled strip exhibited a tensile strength Rm of 1030 MPa and an elongation at break A50 of 23%.
  • the austenite content of its microstructure was 30%.
  • a hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 0.6% Cr, 0.18% Ni, 0.13% Cu, 0.02% N, and 0.079% V was cold-rolled with a total deformation of 50% and subsequently annealed continuously at a temperature of 680 °C.
  • the tensile strength Rm of the resulting cold-rolled strip was 1120 MPa with an elongation at break A50 of 21%.
  • the austenite content of the microstructure was 30%.
  • the resulting hot strip exhibited a tensile strength (Rm) of 1345 MPa and an elongation at break (A80) of 5%.
  • the retained austenite content of its microstructure was 5.5%.
  • the hot-rolled strip obtained according to Example 5 was subjected to hot-roll annealing at 300 °C for a period of 10 minutes.
  • the annealed hot-rolled strip exhibited a tensile strength Rm of 1100 MPa and an elongation at break A80 of 8%.
  • the annealed hot-rolled strip exhibited a tensile strength Rm of 1300 MPa and an elongation at break A80 of 8%.
  • the cast strip exhibited a tensile strength Rm of 1380 MPa and an elongation at break A50 of 6%.
  • the proportion of retained austenite in the microstructure of the resulting cast strip was 2%.
  • the proportion of retained austenite in the microstructure of the strip after annealing was 35%.
  • a hot-rolled strip consisting of iron and unavoidable impurities, with (in wt%) 0.1% C, 7% Mn, 0.20% Si, 0.01% N, and 2.6% Cr, was annealed at 920 °C for three minutes, then transferred to a quenching tank within 7 seconds and quenched in water. Alternatively, quenching in oil would have yielded the same result. After quenching, its tensile strength Rm was 1450 MPa with an elongation at break A80 of 11%. The product RmxA80 was therefore approximately 16,000 MPa x%.
  • the microstructure of the hot-rolled strip obtained in this way consisted of cubically distorted ⁇ -martensite and small volume fractions of approximately 5% each of austenite and hexagonally distorted ⁇ -martensitanium.
  • Hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 1.5% Cr, 0.18% Ni, 0.13% Cu, 0.002% N, and 0.08% V was cold-rolled and subsequently hot-dip galvanized.
  • the galvanized cold-rolled strip exhibited a tensile strength Rm of 1300 MPa and an elongation at break A50 of 15%.
  • the retained austenite content of the resulting cast strip was 20%.
  • the oil-quenched steel exhibited a tensile strength Rm of 1390 MPa at an elongation at break A80 of 12%.
  • the product Rm*A was therefore 16680 MPa%.
  • the water-quenched steel exhibited a tensile strength Rm of 1350 MPa at an elongation at break A80 of 12%.
  • the product Rm*A for the water-quenched steel was therefore 16200 MPa%.
  • the microstructure of the steel consisted of cubically distorted ⁇ -martensite and small volume contents of tough austenite (approx. 4%) and hexagonally distorted ⁇ -martensite (approx. 6%).
  • the steel quenched in oil exhibited a tensile strength Rm of 1315 MPa and an elongation at break A80 of 12.1%.
  • the product Rm*A was therefore 15910 MPa%.
  • the steel quenched in water exhibited a tensile strength Rm of 1285 MPa and an elongation at break A80 of 12.3%.
  • the product Rm*A for the water-quenched steel was therefore 15810 MPa%.
  • the microstructure of the steel consisted of cubically distorted ⁇ -martensite and small volume contents of tough austenite (approx. 7%) and hexagonally distorted ⁇ -martensite (approx. 5%).
  • the steel quenched in oil exhibited a tensile strength Rm of 1350 MPa and an elongation at break A80 of 10.8%.
  • the product Rm*A was therefore 14580 MPa%.
  • the steel quenched in water exhibited a tensile strength Rm of 1350 MPa and an elongation at break A80 of 10.6%.
  • the product Rm*A was therefore 14310 MPa%.
  • the microstructure of the steel consisted of cubically distorted ⁇ -martensite and small volume contents of tough austenite (approx. 12%).
  • the inventive method achieves an improved combination of component strength and residual deformation capacity compared to the prior art for hot-formed high-strength materials, which is characterized by high values of the product of tensile strength and respective elongation at break.

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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

Für den modernen Fahrzeugbau werden in zunehmendem Maße höherfeste Stähle wie Dualphasen (DP)-Stähle, Complexphasen (CP)-Stähle, TRIP-Stähle oder Martensitstähle (MS)-Stähle eingesetzt.For modern vehicle construction, higher-strength steels such as dual-phase (DP) steels, complex-phase (CP) steels, TRIP steels or martensitic (MS) steels are increasingly being used.

Durch die hohe Festigkeit dieser Stähle erhöht sich die Fahrsicherheit. Zugleich können immer leichtere Autokarosserien gestaltet werden, die aufgrund ihres verminderten Gewichts und der damit einhergehenden Einsparung an benötigter Antriebsenergie besonders umweltfreundlich sind.The high strength of these steels increases driving safety. At the same time, increasingly lighter car bodies can be designed, which, due to their reduced weight and the associated savings in required drive energy, are particularly environmentally friendly.

Ein Problem bei der Entwicklung hochfester Stähle besteht darin, dass sich ihre Umformeigenschaften (Bruchdehnung) üblicherweise mit steigender Festigkeit immer mehr verschlechtert. Ein Beispiel für diesen Effekt ist ein hochfester Dualphasen-Stahl, der bei einer Festigkeit von 1000 MPa nur noch eine Bruchdehnung A80 von ca. 12 % erwarten lässt. Die vergleichbar geringe Bruchdehnung kann dazu führen, dass der Werkstoff bei der Bauteilumformung versagt.One problem in the development of high-strength steels is that their forming properties (elongation at break) typically deteriorate with increasing strength. An example of this effect is a high-strength dual-phase steel that, at a strength of 1000 MPa, can only be expected to have an A80 elongation at break of approximately 12%. This comparatively low elongation at break can lead to material failure during component forming.

Die Entwicklung von hochmanganhaltigen Stählen, d.h. Stählen mit Mn-Gehalten von mehr als 15 Gew.-%, zielte deshalb darauf ab, eine hohe Festigkeit mit hervorragender Umformbarkeit zu kombinieren. Bei einer Festigkeit von 1000 MPa bietet dieses Werkstoffkonzept eine Bruchdehnung A80 von 50 %. Jedoch sind diese Werkstoffkonzepte aufgrund des hohen Mangangehalts und den vergleichbar aufwändigen Erzeugungsprozessen sehr kostenintensiv.The development of high-manganese steels, i.e., steels with Mn contents exceeding 15 wt.%, therefore aimed to combine high strength with excellent formability. At a strength of 1000 MPa, this material concept offers an elongation at break (A80) of 50%. However, these material concepts are very expensive due to the high manganese content and the comparatively complex production processes.

Aus der WO 2007/000156 A1 sind Beispiele für hochfeste austenitisch-martensitische Leichtbaustähle bekannt, die mit Chrom, Silizium, Nickel, Mangan und Aluminium legiert sind und eine Zugfestigkeit von > 800 - 1200 MPa bei einer Bruchdehnung > 25 % aufweisen. Bei Mn-Gehalten von (in Gew.-%) > 2,5 und < 30 %, Cr-Gehalten von > 0,5 und < 18 %, einem Si-Gehalt von > 1 % und < 4 % und einem Al-Gehalt von > 0,05 und < 4 % sollen ein Chrom- und ein Nickel-Äquivalent in Abhängigkeit von den jeweiligen Gehalten an Cr, Mo, Si, W, Mn, N, Co, Cu und Al jeweils so eingestellt werden, dass für die beiden Äquivalente angegebene Grenzwertpaare eingehalten werden. Konkret weisen die Beispiele, die diesen Anforderungen gerecht werden, jeweils hohe Si-Gehalte in Kombination mit jeweils hohen Ni-Gehalten und variierten Cr-Gehalten auf.From the WO 2007/000156 A1 Examples of high-strength austenitic-martensitic lightweight steels alloyed with chromium, silicon, nickel, manganese, and aluminum are known, exhibiting a tensile strength of > 800–1200 MPa and an elongation at break of > 25%. For Mn contents (in wt.%) > 2.5% and < 30%, Cr contents > 0.5% and < 18%, Si contents > 1% and < 4%, and Al contents > 0.05% and < 4%, a chromium and a nickel equivalent should be adjusted, depending on the respective contents of Cr, Mo, Si, W, Mn, N, Co, Cu, and Al, such that specified limit pairs are met for both equivalents. Specifically, the examples that meet these requirements exhibit high Si contents in combination with high Ni contents and varying Cr contents.

Ein Verfahren zum Erzeugen von Warmbändern aus einem umformbaren, insbesondere gut kalt tiefziehfähigen Leichtbaustahl, der eine hohe Zugfestigkeit und TRIP- und/oder TWIP-Eigenschaften besitzen soll, ist aus der WO 2005/061152 A1 bekannt. Gemäß diesem Verfahren wird eine Stahlschmelze in einer horizontalen Bandgießanlage endabmessungsnah sowie strömungsberuhigt und biegefrei zu einem Vorband im Bereich zwischen 6 und 15 mm vergossen und anschließend einer Weiterbehandlung zugeführt.A method for producing hot-rolled strips from a formable, in particular cold-deep-drawable, lightweight structural steel, which is to possess high tensile strength and TRIP and/or TWIP properties, is derived from the WO 2005/061152 A1 known. According to this process, a steel melt is cast in a horizontal strip casting plant to a near-net-shape, flow-stabilized and bend-free pre-strip in the range between 6 and 15 mm and subsequently subjected to further processing.

Konkret wird dazu ein Horizontal-Bandgießverfahren eingesetzt. Der dazu verwendete Stahl enthält neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) C: 0,04 - 1,0 %, Al: 0,05 - < 4,0 %, Si: 0,05 - 6,0 %, Mn 9,0 - 30,0 % sowie optional Cr: bis 6,5 %, wobei Cr-Gehalte von 0,2 - 0,3 % als bevorzugt angegeben sind, Nb und V in Gehalten von in Summe bis zu 0,06 % und Ti und Zr in Gehalten von in Summe bis zu 0,7 % vorhanden sein können. Die Wirkung von Chrom wird dabei darin gesehen, dass es den ε-Martensit stabilisiert und die Korrosionsbeständigkeit verbessert. Zu diesem Zweck werden höhere Cr-Gehalte bei Mn-Gehalten von 9 - 18 % empfohlen, während bei Mn-Gehalten von über 18 % niedrigere Cr-Gehalte für ausreichend gehalten werden. An keiner Stelle der WO 2005/061152 A1 wird allerdings angegeben, wie dieses Verhältnis konkret eingestellt werden soll.Specifically, a horizontal strip casting process is used. The steel used contains, in addition to iron and unavoidable impurities (in wt.%), C: 0.04–1.0%, Al: 0.05–<4.0%, Si: 0.05–6.0%, Mn: 9.0–30.0%, and optionally Cr: up to 6.5%, with Cr contents of 0.2–0.3% being preferred, Nb and V in total contents of up to 0.06%, and Ti and Zr in total contents of up to 0.7%. The effect of chromium is seen as stabilizing the ε-martensite and improving corrosion resistance. For this purpose, higher Cr contents are recommended at Mn contents of 9–18%, while lower Cr contents are considered sufficient at Mn contents above 18%. At no point in the WO 2005/061152 A1 However, it specifies how this ratio should be set in concrete terms.

Eine weitere Möglichkeit höchstfeste Bauteile darzustellen, ist das Warmpresshärten konventioneller Warmumformstähle. Nach dem Press-Hardening - nach vorheriger Vollaustenitisierung - weisen diese Stähle ein martensitisches Gefüge auf, das allerdings ein relativ geringes Restverformungsvermögen besitzt.Another way to produce high-strength components is through the hot press hardening of conventional hot-forming steels. After press hardening – following prior full austenitization – these steels exhibit a martensitic microstructure, which, however, has a relatively low residual ductility.

Neben dem voranstehend erläuterten Stand der Technik ist aus der EP 0 425 058 A1 eine Verwendung eines beruhigt vergossenen Stahles, der 0,15 - 0,25 % C, 3,40 - 6,10 % Mn, 0 - 1,0 % Ni, 0 - 1,0 % Cr, 0 - 1,0 % Mo, 0 - 0,15 % V, max. 0,03 % P, max. - 0,03 % S, max. - 0,6 % Si, max. 0,05 % Al, Rest Eisen und übliche Verunreinigungen enthält, als Werkstoff zur Herstellung von Rohren zur Verstärkung von Kraftfahrzeugtüren mit der Maßgabe bekannt, dass die folgende Beziehung für die Summe der Legierungsanteile (in Gewichts-%) erfüllt ist: Mn + Ni + Cr + Mo + 10 × V ≧ 4 , 5 Gewichts − % In addition to the state of the art explained above, the following also applies: EP 0 425 058 A1 A use of a killed-cast steel containing 0.15–0.25% C, 3.40–6.10% Mn, 0–1.0% Ni, 0–1.0% Cr, 0–1.0% Mo, 0–0.15% V, max. 0.03% P, max. 0.03% S, max. 0.6% Si, max. 0.05% Al, balance iron and usual impurities, as a material for the manufacture of tubes for reinforcing motor vehicle doors, provided that the following relationship for the sum of the alloying elements (in wt.%) is satisfied: Mn + Ni + Cr + Mo + 10 × V ≧ 4 , 5 Gewichts − %

Vor dem Hintergrund des voranstehend erläuterten Standes der Technik bestand die Aufgabe der Erfindung darin, ein Stahlflachprodukt mit guter Festigkeit und guter Verformbarkeit aus einem Stahl zu schaffen, der sich kostengünstiger herstellen lässt als die bekannten hochmanganhaltigen Stähle und gleichzeitig hohe Bruchdehnungswerte und damit einhergehend eine deutlich verbesserte Umformbarkeit besitzt.Against the background of the prior art explained above, the object of the invention was to create a flat steel product with good strength and good formability from a steel that can be produced more cost-effectively than the known high-manganese steels and at the same time has high elongation at break values and thus significantly improved formability.

In Bezug auf den Stahl ist diese Aufgabe erfindungsgemäß durch den in Anspruch 1 angegebenen Stahl gelöst worden.With regard to the steel, this problem has been solved according to the invention by the steel 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 in detail below, as is the general concept of the invention.

Die Erfindung schlägt ein Werkstoffkonzept vor, gemäß dem ein Stahl, der neben Eisen und unvermeidbaren Verunreinigungen aus (in Gew.-%) C: 0,02 - 0,5 %, Mn: 7 - 12,0 %, Si: 0,05 - 1,0 %, Al: bis zu 3,0 %, Cr: 1 - 4,0 %, Cu: bis zu 2,0 %, Ni: bis zu 2,0 %, N: bis zu 0,05 %, P: bis zu 0,05 %, S: bis zu 0,01 % besteht und optional ein Element oder mehrere Elemente aus der Gruppe "V, Nb, Ti" enthält, wobei die Summe der Gehalte an diesen Elementen höchstens gleich 0,5 % ist.The invention proposes a material concept according to which a steel, in addition to iron and unavoidable impurities, consists of (in wt.%) C: 0.02 - 0.5% Mn: 7 - 12.0% Yes: 0.05 - 1.0% Al: up to 3.0% Cr: 1 - 4.0% Cu: up to 2.0% Ni: up to 2.0% N: up to 0.05% P: up to 0.05% S: up to 0.01% consists of and optionally contains one or more elements from the group "V, Nb, Ti", where the sum of the contents of these elements is at most 0.5%.

Das Gefüge eines aus einem solchen erfindungsgemäßen Stahl erzeugten Stahlflachprodukts besteht typischerweise zu 30 - 100 % aus Härtungsgefüge (Martensit, angelassener Martensit oder Bainit), während der Rest des Gefüges austenitisch ist.The microstructure of a flat steel product produced from such a steel according to the invention typically consists of 30 - 100% hardened microstructure (martensite, tempered martensite or bainite), while the remainder of the microstructure is austenitic.

Im Vergleich zu den bekannten hochmanganhaltigen Stählen lässt sich ein erfindungsgemäßer Stahl aufgrund seiner in einem mittleren Gehaltsbereich liegenden Mn-Gehalten zu deutlich verminderten Legierungs- und Erzeugungskosten sowohl bei der Erzeugung über Strangguss als auch bei der Erzeugung über ein Bandgussverfahren herstellen. Kohlenstoff bestimmt bei einem erfindungsgemäßen Stahl zum einen die Festigkeit von Martensit und zum anderen die Menge und die Stabilität des Restaustenits. Bei zu hohen Kohlenstoffgehalten wird die Schweißbarkeit und Zähigkeit des Stahls, z. B. durch Bildung von Cr-Karbiden, negativ beeinflusst. Idealerweise liegt daher der Kohlenstoffgehalt von Mn-Stählen der erfindungsgemäßen Art unter 0,5 Gew.-%, wobei sich optimale Eigenschaften ergeben, wenn der C-Gehalt auf weniger als 0,2 Gew.-%, insbesondere weniger als 0,1 Gew.-%, beschränkt ist. Bei zu geringem Kohlenstoffgehalt wird jedoch die Menge und Stabilität des verbleibenden Restaustenits beeinträchtigt. Deshalb beträgt der C-Gehalt eines erfindungsgemäßen Stahls mindestens 0,02 Gew.-%, insbesondere mindestens 0,03 Gew.-%, beispielsweise mindestens 0,05 Gew.-%.Compared to known high-manganese steels, a steel according to the invention, due to its medium-range manganese content, can be produced at significantly reduced alloying and manufacturing costs, both by continuous casting and by strip casting. In a steel according to the invention, carbon determines, on the one hand, the strength of martensite and, on the other hand, the quantity and stability of the retained austenite. Excessively high carbon contents negatively affect the weldability and toughness of the steel, for example, through the formation of chromium carbides. Ideally, the carbon content of manganese steels of the type according to the invention is therefore below 0.5 wt.%, with optimal properties being achieved when the carbon content is limited to less than 0.2 wt.%, and in particular less than 0.1 wt.%. However, excessively low carbon contents impair the quantity and stability of the remaining retained austenite. Therefore, the carbon content of a steel according to the invention is at least 0.02 wt.%, in particular at least 0.03 wt.%, for example at least 0.05 wt.%.

Mangan ist ein Austenitbildner. Es verzögert die Umwandlung von Ferrit, Perlit und Bainit und stabilisiert damit Austenit bis zur Martensitstarttemperatur. Mangan fördert dabei die Ausbildung von kubisch oder hexagonal verzerrtem Martensit (α- oder ε-Martensit). Diese Mangan-Martensite zeichnen sich durch hohe Festigkeiten und einer gegenüber C-induziertem, kubisch verzerrtem α-Martensit wesentlich höheren Zähigkeit aus. Bei zu geringem Mangangehalt entsteht bei der Abkühlung Bainit, was eine niedrigere Festigkeit und Bruchdehnung mit sich bringt. Bei zu hohem Mangangehalt besteht dagegen die Gefahr, dass der gesamte Austenit bis Raumtemperatur stabil bleibt. Der erfindungsgemäß vorgegebene Mangangehalt von 7 - 12 % ermöglicht dagegen die Einstellung einer Martensitmatrix mit einem Restaustenitanteil im Gefüge. Besonders sicher tritt dieser Effekt ein, wenn der Mn-Gehalt mindestens 7 Gew.-% beträgt, wobei eine Optimierung der positiven Einflüsse von Mangan in einem erfindungsgemäßen Stahl dadurch erzielt werden kann, dass die Obergrenze des Mn-Gehalts auf 10 Gew.-%, insbesondere auf weniger 9 Gew.-%, beispielsweise auf bis zu 8,5 Gew.-%, beschränkt wird.Manganese is an austenite former. It delays the transformation of ferrite, pearlite, and bainite, thus stabilizing austenite up to the martensite start temperature. Manganese promotes the formation of cubically or hexagonally distorted martensite (α- or ε-martensite). These manganese martensites are characterized by high strength and significantly higher toughness compared to carbon-induced, cubically distorted α-martensite. If the manganese content is too low, bainite forms upon cooling, resulting in lower strength and elongation at break. Conversely, if the manganese content is too high, there is a risk that the entire austenite will remain stable until room temperature. The manganese content of 7–12% specified according to the invention, however, allows for the creation of a martensite matrix with a retained austenite component in the microstructure. This effect is particularly likely to occur if the Mn content is at least 7 wt.%, whereby the positive effects of manganese in a steel according to the invention can be optimized by limiting the upper limit of the Mn content to 10 wt.%, in particular to less than 9 wt.%, for example to up to 8.5 wt.%.

Aluminium und Silizium sind starke Ferritbildner. Beide Elemente wirken dem Einfluss der Austenitbildner C und Mn entgegen. Die wesentliche Aufgabe der Elemente Si und Al besteht in einem erfindungsgemäßen Stahl darin, die Karbidausscheidung in der Martensitmatrix zu unterdrücken und damit die Stabilität des Restaustenits zu fördern. Gleichzeitig führen Si und Al zu einer Mischkristallhärtung und reduzieren das spezifische Gewicht des Stahls. Bei zu geringem Si- und Al-Gehalt kann die Karbidausscheidung jedoch möglicherweise nicht effektiv unterdrückt werden. Bei zu hohen Gehalten an Si und Al wird dagegen die Verarbeitung sowohl bei einer Erzeugung über ein Strangguss- als auch bei einer Erzeugung über ein Bandgussverfahren erschwert.Aluminum and silicon are strong ferrite formers. Both elements counteract the influence of the austenite formers carbon and manganese. The essential function of the elements silicon (Si) and aluminum (Al) in a steel according to the invention is to suppress carbide precipitation in the martensite matrix and thus promote the stability of the retained austenite. At the same time, Si and Al lead to solid solution hardening and reduce the specific gravity of the steel. However, if the Si and Al content is too low, carbide precipitation may not be effectively suppressed. Conversely, if the Si and Al content is too high, processing becomes more difficult in both continuous casting and strip casting processes.

Deshalb sieht die Erfindung vor, den Si-Gehalt auf max. 1 Gew.-% zu beschränken, wobei die positiven Effekte der Anwesenheit von Si dadurch effektiv genutzt werden können, wenn der Si-Gehalt des erfindungsgemäßen Stahls mindestens 0,05 Gew.-%, insbesondere 0,1 Gew.-%, beträgt. Die negativen Einflüsse von Si können dadurch besonders sicher ausgeschlossen werden, dass der Si-Gehalt auf 0,7 Gew.-%, insbesondere 0,5 Gew.-%, beschränkt wird.Therefore, the invention provides for limiting the Si content to a maximum of 1 wt.%, whereby the positive effects of the presence of Si can be effectively utilized if the Si content of the steel according to the invention is at least 0.05 wt.%, in particular 0.1 wt.%. The negative effects of Si can be particularly reliably excluded by limiting the Si content to 0.7 wt.%, in particular 0.5 wt.%.

Um die vorteilhafte Wirkung von Al sicher nutzen zu können, kann der Al-Gehalt auf mindestens 0,01 Gew.- %, insbesondere 0,02 Gew.-%, festgelegt werden, während negative Einflüsse von Al besonders sicher dann auszuschließen sind, wenn der Al-Gehalt eines erfindungsgemäßen Stahls auf 2 Gew.-%, insbesondere 1 Gew.-%, beschränkt wird.In order to be able to safely utilize the advantageous effect of Al, the Al content can be set at a minimum of 0.01 wt. %, in particular 0.02 wt. %, while negative influences of Al can be particularly reliably excluded if the Al content of a steel according to the invention is limited to 2 wt. %, in particular 1 wt. %,.

Durch die Anwesenheit von Kupfer, Chrom und Nickel wird grundsätzlich der Widerstand eines erfindungsgemäßen Stahls gegen verschiedene Korrosionsmechanismen verbessert. Die positive Wirkung von Cu und Ni lässt sich dabei dadurch besonders sicher nutzen, dass diese Elemente mit in Summe mindestens > 0 Gew.-%, insbesondere 0,1 Gew.-%, betragenden Gehalten dem erfindungsgemäßen Stahl zugegeben werden. Dagegen werden negative Auswirkungen der Anwesenheit von Cu und / oder Ni in erfindungsgemäßen Stählen dadurch vermieden, dass der Gehalt an Cu und Ni jeweils max. 1 Gew.-% beträgt bzw. der Gehalt an Cu und Ni in Summe auf maximal 2 Gew.-%, insbesondere 1 Gew.-%, beschränkt ist.The presence of copper, chromium, and nickel generally improves the resistance of a steel according to the invention to various corrosion mechanisms. The positive effect of Cu and Ni can be utilized particularly reliably by adding these elements to the steel according to the invention in amounts totaling at least > 0 wt.%, and in particular 0.1 wt.%. Conversely, negative effects of the presence of Cu and/or Ni in steels according to the invention are avoided by limiting the Cu and Ni content to a maximum of 1 wt.% each, or by limiting the total Cu and Ni content to a maximum of 2 wt.%, and in particular 1 wt.%.

Durch die Anwesenheit von Cr wird in einem erfindungsgemäßen Stahl die Gefahr der Entstehung von Spannungsrisskorrosion gezielt vermindert. Zudem trägt Cr zur Festigkeitssteigerung bei. Ab einem Gehalt von 0,1 Gew.-% Cr sind diese positiven Effekte zu beobachten, wobei die positive Wirkung von Cr dann besonders sicher eintritt, wenn der Cr-Gehalt, wie im erfindungsgemäßen Stahl, mindestens 1 Gew.-%, beträgt. Der Cr-Gehalt eines erfindungsgemäßen Stahls ist auf max. 4 Gew.-% beschränkt, weil bei höheren Gehalten Cr-Karbide entstehen können, die die Duktilität des Stahls negativ beeinflussen können. Solche negativen Effekte können dadurch besonders sicher ausgeschlossen werden, dass der Cr-Gehalt auf max. 2 Gew.-% beschränkt wird. Optimal wirkt sich die Anwesenheit von Cr in einem erfindungsgemäßen Stahl aus, wenn der Cr-Gehalt 1 - 2 Gew.-% beträgt.The presence of chromium (Cr) in a steel according to the invention specifically reduces the risk of stress corrosion cracking. Cr also contributes to increased strength. These positive effects can be observed from a Cr content of 0.1 wt.%, with the positive effect of Cr being particularly reliable when the Cr content, as in the steel according to the invention, is at least 1 wt.%. The Cr content of a steel according to the invention is limited to a maximum of 4 wt.% because higher contents can lead to the formation of Cr carbides, which can negatively affect the ductility of the steel. Such negative effects can be particularly reliably prevented by limiting the Cr content to a maximum of 2 wt.%. The presence of Cr in a steel according to the invention has an optimal effect when the Cr content is 1–2 wt.%.

Ti, Nb und V, die in Gehalten von in Summe bis zu 0,5 Gew.-% in einem erfindungsgemäßen Stahl vorhanden sein können, tragen zur Kornfeinung und Festigkeitssteigerung bei. In Summe oberhalb von 0,5 Gew.-% liegende Gehalte an Ti, Nb und V führen zu keiner Steigerung dieses Effekts. Besonders zielsicher und ressourcenschonend lässt sich die festigkeitssteigernde Wirkung von Ti, Nb und V dann nutzen, wenn die Summe der Gehalte an diesen Mikrolegierungselementen bei einem erfindungsgemäßen Stahl auf 0,3 Gew.-%, insbesondere 0,2 Gew.-%, beschränkt ist. Die positive Wirkung der hier genannten Mikrolegierungselemente stellt sich dabei bereits dann ein, wenn die Summe ihrer Gehalte mindestens 0,025 Gew.-% beträgt. Im Falle der Anwesenheit von Ti wird dessen Gehalt vorteilhafterweise auf max. 0,15 Gew.-% beschränkt, um grobe Ti-Ausscheidungen zu verhindern. Durch die Zugabe von Stickstoff in Gehalten von bis zu 0,05 Gew.-%, insbesondere 0,03 Gew.-%, kann das austenitische Gefüge zusätzlich stabilisiert werden. Dieser Effekt tritt bereits dann ein, wenn der N-Gehalt eines erfindungsgemäßen Stahls mindestens 0,002 Gew.-%, insbesondere mindestens 0,0025 Gew.-%, beträgt, wobei sich ein optimaler Einfluss ergibt, wenn der N-Gehalt auf max. 0,025 Gew.-% beschränkt ist.Ti, Nb, and V, which can be present in a total of up to 0.5 wt.% in a steel according to the invention, contribute to grain refinement and increased strength. Total contents of Ti, Nb, and V exceeding 0.5 wt.% do not enhance this effect. The strength-enhancing effect of Ti, Nb, and V can be utilized particularly effectively and efficiently when the total content of these microalloying elements in a steel according to the invention is limited to 0.3 wt.%, particularly 0.2 wt.%. The positive effect of the microalloying elements mentioned here is already achieved when the total content is at least 0.025 wt.%. In the case of Ti, its content is advantageously limited to a maximum of 0.15 wt.% to prevent coarse Ti precipitates. The austenitic microstructure can be further stabilized by the addition of nitrogen in contents of up to 0.05 wt.%, particularly 0.03 wt.%. This effect occurs even when the nitrogen content of a steel according to the invention is at least 0.002 wt.%, in particular at least 0.0025 wt.%, with an optimal effect being achieved when the nitrogen content is limited to a maximum of 0.025 wt.%.

Die P-Gehalte eines erfindungsgemäßen Stahls sind auf maximal 0,05 Gew.-%, bevorzugt 0,03 Gew.-%, beschränkt, um negative Einflüsse dieses Elements sicher auszuschließen.The phosphorus content of a steel according to the invention is limited to a maximum of 0.05 wt.%, preferably 0.03 wt.%, in order to reliably exclude negative influences of this element.

Aus demselben Grund ist der S-Gehalt eines erfindungsgemäßen Stahls auf max. 0,01 Gew.-%, insbesondere 0,005 Gew.-%, beschränkt.For the same reason, the sulfur content of a steel according to the invention is limited to a maximum of 0.01 wt.%, in particular 0.005 wt.%.

Grundsätzlich gilt, dass das erfindungsgemäße Legierungskonzept so abgestimmt ist, dass die Entstehung von Härtungsgefüge mit oder ohne Restaustenit im Warmband ermöglicht wird. Das heißt: Die Martensitstarttemperatur MS eines im Rahmen der Erfindung legierten Stahls liegt oberhalb und die Martensitfinishtemperatur MF eines erfindungsgemäß zusammengesetzten Stahls liegt unterhalb der Raumtemperatur.In principle, the alloy concept according to the invention is designed to enable the formation of hardened microstructures with or without retained austenite in hot-rolled strip. This means that the martensite start temperature M <sub>S</sub> of a steel alloyed according to the invention is above room temperature, and the martensite finish temperature M<sub>F</sub> of a steel composed according to the invention is below room temperature.

Das erfindungsgemäße Legierungskonzept ermöglicht die Einstellung eines Härtungsgefüges mit bis zu 70 % Austenit. Je nach Legierungslage können folgende Phasen auftreten:

  • Stabiler Austenit,
  • Metastabiler Austenit mit Fähigkeit zur spannungsinduzierten Martensitbildung (TRIP-Effekt),
  • C- und/oder Mn- verzerrter kubischer α-Martensit,
  • Hexagonal verzerrter ε-Martensit,
  • Bainit.
The alloy concept according to the invention enables the creation of a hardened microstructure with up to 70% austenite. Depending on the alloy layer, the following phases can occur:
  • Stable austenite,
  • Metastable austenite with the ability to form stress-induced martensite (TRIP effect),
  • C- and/or Mn- distorted cubic α-martensite,
  • Hexagonally distorted ε-martensite,
  • Bainit.

Ein Verfahren zur Herstellung eines Stahlflachprodukts, umfasst folgende Arbeitsschritte:

  • Erschmelzen einer erfindungsgemäß zusammengesetzten Stahlschmelze,
  • Erzeugen eines Ausgangsprodukts für ein anschließendes Warmwalzen, indem die Stahlschmelze zu einem Strang, von dem mindestens eine Bramme oder Dünnbramme als Ausgangsprodukt für das Warmwalzen abgeteilt wird, oder über Zwei-Rollen-Bandguss zu einem gegossenen Band vergossen wird, das als Ausgangsprodukt dem Warmwalzen zugeführt wird,
  • Wärmebehandeln des Ausgangsprodukts, um das Ausgangsprodukt auf eine Warmwalzstarttemperatur von 1150 - 1000 °C zu bringen,
  • Warmwalzen des Ausgangsprodukts zu einem Warmband mit einer Dicke von höchstens 2,5 mm, wobei das Warmwalzen bei einer 1050 - 800 °C betragenden Warmwalzendtemperatur beendet wird,
  • Haspeln des Warmbands zu einem Coil bei einer Haspeltemperatur ≤ 700 °C,
  • wobei sich an das Haspeln jeweils optional die folgenden Arbeitsschritte anschließen können:
    • Glühen des Warmbands bei einer 250 - 950 °C betragenden Warmbandglühtemperatur,
    • Kaltwalzen des geglühten Warmbands in einem Schritt oder in mehreren Schritten zu einem Kaltband mit einer Dicke von höchstens 60 % der Dicke des Warmbands,
    • Glühen des Kaltbands bei einer 450 - 950 °C betragenden Kaltbandglühtemperatur,
    • Beschichten der Oberfläche des Warmbands oder des Kaltbands mit einem metallischen Korrosionsschutzüberzug,
    • Beschichten der Oberfläche des Warmbands oder des Kaltbands mit einem organischen Überzug.
A process for manufacturing a flat steel product comprises the following steps:
  • Melting of a steel melt composed according to the invention,
  • Producing a starting product for subsequent hot rolling by casting the molten steel into a strand from which at least one slab or thin slab is separated as the starting product for hot rolling, or by casting a strip via two-roll strip casting, which is then fed to the hot rolling process as the starting product.
  • Heat treatment of the starting product to bring it to a hot rolling start temperature of 1150 - 1000 °C,
  • Hot rolling of the starting product into a hot strip with a thickness of no more than 2.5 mm, wherein the hot rolling is terminated at a hot rolling temperature of 1050 - 800 °C,
  • Winding the hot strip into a coil at a winding temperature ≤ 700 °C,
  • The following steps can optionally follow the reeling process:
    • Annealing of the hot strip at a hot strip annealing temperature of 250 - 950 °C,
    • Cold rolling of the annealed hot strip in one or more steps to a cold strip with a thickness of no more than 60% of the thickness of the hot strip,
    • Annealing of the cold-rolled strip at a cold-rolled strip annealing temperature of 450 - 950 °C,
    • Coating the surface of the hot-rolled or cold-rolled strip with a metallic corrosion protection coating,
    • Coating the surface of the hot-rolled or cold-rolled strip with an organic coating.

Bei dem erfindungsgemäßen Stahlflachprodukt handelt es sich um ein unbeschichtetes Warmband. Die Möglichkeiten der Erzeugung von Warm- oder Kaltbändern, die aus Mn-Stahl bestehen, sind in dem beigefügten Diagramm zusammengefasst. Im Einzelnen umfassen sie folgende Bearbeitungsschritte:The steel flat product according to the invention is an uncoated hot-rolled strip. The possibilities for producing hot-rolled or cold-rolled strips made of manganese steel are summarized in the accompanying diagram. Specifically, they comprise the following processing steps:

WarmbanderzeugungHot strip production

Gegenüber Hoch-Mn-Stählen ist die Vergießbarkeit erfindungsgemäßer Mn-Stähle in Folge der Absenkung des Mn-Gehaltes verbessert.Compared to high-Mn steels, the castability of Mn steels according to the invention is improved as a result of the reduction in Mn content.

Eine erste Möglichkeit der Warmbanderzeugung besteht im konventionellen Strangguss. Dabei erweist sich ein erfindungsgemäßer Stahl als besonders vorteilhaft, weil er eine geringere Warmbanddicke von weniger als < 2,5 mm erlaubt. Dies ist darin begründet, dass sein Umformwiderstand in Folge der Absenkung des Mn-Gehaltes gegenüber konventionellen hochmanganghaltigen Stählen deutlich reduziert ist.One method for producing hot-rolled strip is conventional continuous casting. In this process, a steel according to the invention proves particularly advantageous because it allows for a reduced hot-rolled strip thickness of less than 2.5 mm. This is due to the fact that its forming resistance is significantly reduced compared to conventional high-manganese steels as a result of the lower manganese content.

Es ist ebenfalls möglich, Mn-Stähle durch Bandgießen herzustellen. Beim Bandgießen sind Warmbanddicken von weniger als 2,0 mm realisierbar.It is also possible to produce manganese steels by strip casting. Hot-cast strip thicknesses of less than 2.0 mm are achievable.

WarmbandglühungHot strip annealing

Durch die Glühung des Warmbandes werden die höheren Austenitanteile eingestellt. Danach verringert sich die Festigkeit, und die Bruchdehnung nimmt deutlich zu. Nach der Warmbandglühung wird bis zu 70 % Austenit je nach Analysenkonzept eingestellt, der für die Verbesserung der Bruchdehnung hauptverantwortlich ist. Da eine Martensitmatrix im ungeglühten Warmband vorliegt, ist es schwierig, es direkt zu Kaltband zu prozessieren. Somit kann eine Warmbandglühung auch dem Zweck dienen, das Warmband für das Kaltwalzen zu entfestigen. Für die Warmbandglühung kommt sowohl eine Haubenglühung als auch eine Durchlaufglühung in Frage.The higher austenite content is achieved by annealing the hot-rolled strip. This reduces the strength and significantly increases the elongation at break. After hot-rolled strip annealing, up to 70% austenite content is achieved, depending on the analytical approach; this is primarily responsible for the improved elongation at break. Since a martensite matrix is present in unannealed hot-rolled strip, it is difficult to process it directly into cold-rolled strip. Therefore, hot-rolled strip annealing can also serve the purpose of softening the hot-rolled strip for cold rolling. Both hood annealing and continuous annealing are suitable methods for hot-rolled strip.

Kaltwalzen und GlühenCold rolling and annealing

Durch Kaltwalzen des geglühten oder des ungeglühten Warmbandes (dann mit optimierter Haspeltemperatur) wird die Banddicke weiter reduziert und die Bandplanheit verbessert. Die nachfolgende Glühung beseitigt die Kaltverfestigung für die Bauteilherstellung und führt zur optimalen Gefügeeinstellung mit erhöhtem Austenitanteil.Cold rolling of the annealed or unannealed hot-rolled strip (then with an optimized coiling temperature) further reduces the strip thickness and improves strip flatness. Subsequent annealing eliminates work hardening for component manufacturing and results in an optimal microstructure with an increased austenite content.

OberflächenveredelungSurface finishing

Sowohl das geglühte Warmband als auch das geglühte Kaltband können entweder elektrolytisch oder durch Feuerverzinkung (im Anschluss an die Kaltbandglühung) oder durch sonstige Bandbeschichtung veredelt werden. Es ist ebenfalls möglich, das jeweils erhaltene Stahlband mit einer organischen Beschichtung zu versehen.Both hot-annealed and cold-annealed strip can be finished either electrolytically, by hot-dip galvanizing (following cold-annealing), or by other strip coatings. It is also possible to apply an organic coating to the resulting steel strip.

WarmformungHot forming

Das angestrebte Gefüge eines erfindungsgemäßen Stahls mit typischerweise 30 - 100 % Härtungsgefüge (Martensit, angelassener Martensit oder Bainit) und als Rest Austenit kann dadurch erreicht werden, dass der Stahl warmgeformt und abgeschreckt wird.The desired microstructure of a steel according to the invention, typically comprising 30-100% hardened microstructure (martensite, tempered martensite or bainite) and the remainder being austenite, can be achieved by hot forming and quenching the steel.

Auf Grundlage der erfindungsgemäßen Stähle ist es demnach möglich, durch Warmumformung mit anschließender Härtung höchstfeste Bauteile zu erzeugen, deren Restverformungsvermögen aufgrund der Bildung harter, aber vergleichsweise zäher Phasen gegenüber konventionellen hochfesten Stählen signifikant verbessert ist.Based on the steels according to the invention, it is therefore possible to produce extremely strong components by hot forming followed by hardening, whose residual deformation capacity is significantly improved compared to conventional high-strength steels due to the formation of hard, but comparatively tough phases.

AusführungsbeispieleExamples of implementation Beispiel 1Example 1

Eine Stahlschmelze, die neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,1 % C, 10 % Mn, 0,4 % Si, 0,008 % N, 1,6 % Al und 2 % Cr enthielt, ist im Strangguss vergossen und bei einer Warmwalzendtemperatur ET von 900°C zu einem Warmband warmgewalzt worden, das anschließend bei einer Haspeltemperatur HT von 650 °C gehaspelt worden ist. Das so erhaltene Warmband wies eine Zugfestigkeit Rm von 1400 MPa und eine Bruchdehnung A80 von 7 % auf. Der Restaustenit-Anteil seines Gefüges betrug 14 %.A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 10% Mn, 0.4% Si, 0.008% N, 1.6% Al, and 2% Cr, was continuously cast and hot-rolled at a hot rolling temperature (ET) of 900°C to produce a hot strip, which was then coiled at a coiling temperature (HT) of 650°C. The resulting hot strip exhibited a tensile strength (Rm) of 1400 MPa and an elongation at break (A80) of 7%. The retained austenite content of its microstructure was 14%.

Beispiel 2Example 2

Eine Stahlschmelze, die neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,1 % C, 10 % Mn, 0,4 % Si, 0,008 % N, 1,6 % Al und 1,6 % Cr enthielt, ist in einer Bandgießmaschine zu einem gegossenen Band vergossen und bei einer Warmwalzendtemperatur ET von 900 °C zu einem Warmband warmgewalzt worden, welches anschließend bei einer Haspeltemperatur HT von 650 °C gehaspelt worden ist. Anschließend ist eine Haubenglühung durchgeführt worden. Das so erhaltene Band wies eine Zugfestigkeit Rm von 990 MPa und eine Bruchdehnung A50 von 27,5 % auf. Der Restaustenit des erhaltenen Warmbands betrug nach dem Glühen 60 %.A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 10% Mn, 0.4% Si, 0.008% N, 1.6% Al, and 1.6% Cr, was cast into a strip in a strip casting machine and hot-rolled at a hot rolling temperature (ET) of 900 °C. This strip was then coiled at a coiling temperature (HT) of 650 °C. A hood anneal was subsequently performed. The resulting strip exhibited a tensile strength (Rm) of 990 MPa and an elongation at break (A50) of 27.5%. The retained austenite of the hot-rolled strip after annealing was 60%.

Beispiel 3Example 3

Ein Warmband, das neben Eisen und unvermeidbaren Verunreinigungen aus (in Gew.-%) 0,1 % C, 7 % Mn, 0,13 % Si, 0,02 % Al, 1,5 % Cr, 0,18 % Ni, 0,13 % Cu, 0,02 % N und 0,079 % V bestand, ist einer Haubenglühung bei einer Glühtemperatur von 650°C über eine Glühzeit von 40 h unterzogen worden. Das geglühte Warmband wies eine Zugfestigkeit Rm von 1030 MPa und eine Bruchdehnung A50 von 23 % auf. Der Austenit-Anteil seines Gefüges betrug 30 %.A hot-rolled strip, consisting of iron and unavoidable impurities, and containing (in wt%) 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 1.5% Cr, 0.18% Ni, 0.13% Cu, 0.02% N, and 0.079% V, was subjected to hood annealing at a temperature of 650°C for 40 hours. The annealed hot-rolled strip exhibited a tensile strength Rm of 1030 MPa and an elongation at break A50 of 23%. The austenite content of its microstructure was 30%.

Beispiel 4 (nicht erfindungsgemäß)Example 4 (not according to the invention)

Ein Warmband, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,1 % C, 7 % Mn, 0,13 % Si, 0,02 % Al, 0,6 % Cr, 0,18 % Ni, 0,13 % Cu, 0,02 % N und 0,079 % V enthielt, ist mit einer Gesamtverformung von 50 % kaltgewalzt und anschließend bei einer 680 °C betragenden Glühtemperatur im Durchlauf geglüht worden. Die Zugfestigkeit Rm des erhaltenen Kaltbands betrug 1120 MPa bei einer Bruchdehnung A50 von 21 %. Der Austenit-Anteil des Gefüges betrug 30 %.A hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 0.6% Cr, 0.18% Ni, 0.13% Cu, 0.02% N, and 0.079% V, was cold-rolled with a total deformation of 50% and subsequently annealed continuously at a temperature of 680 °C. The tensile strength Rm of the resulting cold-rolled strip was 1120 MPa with an elongation at break A50 of 21%. The austenite content of the microstructure was 30%.

Beispiel 5 (nicht erfindungsgemäß)Example 5 (not according to the invention)

Eine Stahlschmelze, die neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,11 % C, 5 % Mn, 0,39 % Si, 0,008 N und 1,5 % Al sowie 0,6 % Cr enthielt, ist im Strangguss vergossen und bei einer Warmwalzendtemperatur ET von 900 °C zu einem Warmband warmgewalzt worden, das anschließend bei einer Haspeltemperatur HT von 650 °C gehaspelt worden ist. Das so erhaltene Warmband wies eine Zugfestigkeit Rm von 1345 MPa und eine Bruchdehnung A80 von 5 % auf. Der Restaustenit-Anteil seines Gefüges betrug 5,5 %.A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.11% C, 5% Mn, 0.39% Si, 0.008% N, 1.5% Al, and 0.6% Cr, was continuously cast and hot-rolled at a hot rolling temperature (ET) of 900 °C to produce a hot strip, which was then coiled at a coiling temperature (HT) of 650 °C. The resulting hot strip exhibited a tensile strength (Rm) of 1345 MPa and an elongation at break (A80) of 5%. The retained austenite content of its microstructure was 5.5%.

Das gemäß Beispiel 5 erhaltene Warmband ist über eine Glühzeit von 10 min. einer Warmbandglühung bei 300 °C unterzogen worden. Das geglühte Warmband wies eine Zugfestigkeit Rm von 1100 MPa bei einer Bruchdehnung A80 von 8 % auf.The hot-rolled strip obtained according to Example 5 was subjected to hot-roll annealing at 300 °C for a period of 10 minutes. The annealed hot-rolled strip exhibited a tensile strength Rm of 1100 MPa and an elongation at break A80 of 8%.

Beispiel 7Example 7

Ein entsprechend Beispiel 2 zusammengesetztes Warmband ist über eine Glühzeit von 10 min. einer Warmbandglühung bei 300 °C unterzogen worden. Das geglühte Warmband wies eine Zugfestigkeit Rm von 1300 MPa bei einer Bruchdehnung A80 von 8 % auf.A hot-rolled strip, as described in Example 2, was subjected to hot-roll annealing at 300 °C for 10 minutes. The annealed hot-rolled strip exhibited a tensile strength Rm of 1300 MPa and an elongation at break A80 of 8%.

Beispiel 8Example 8

Aus einer Stahlschmelze, die neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,12 % C, 7 % Mn, 0,11 % Si, 1,6 % Al, 0,3 % Ni, 0,1 % Cu, 0,007 % N und 0,01 % V und 0,5 % Cr enthielt, ist zu einem gegossenen Band vergossen worden. Das gegossene Band wies eine Zugfestigkeit Rm von 1380 MPa bei einer Bruchdehnung A50 von 6 % auf. Der Anteil des Restaustenits am Gefüge des erhaltenen gegossenen Bands betrug 2 %. Nach einer Haubenglühung betrug seine Zugfestigkeit Rm 1050 MPa und seine Bruchdehnung A50 22 %. Der Anteil des Restaustenits am Gefüge des Bands betrug nach dem Glühen 35 %.A steel melt containing, in addition to iron and unavoidable impurities (in wt.%), 0.12% C, 7% Mn, 0.11% Si, 1.6% Al, 0.3% Ni, 0.1% Cu, 0.007% N, 0.01% V, and 0.5% Cr, was cast into a strip. The cast strip exhibited a tensile strength Rm of 1380 MPa and an elongation at break A50 of 6%. The proportion of retained austenite in the microstructure of the resulting cast strip was 2%. After hood annealing, its tensile strength Rm was 1050 MPa and its elongation at break A50 was 22%. The proportion of retained austenite in the microstructure of the strip after annealing was 35%.

Beispiel 9Example 9 Beispiel 6 (nicht erfindungsgemäß)Example 6 (not according to the invention)

Ein Warmband, das neben Eisen und unvermeidbaren Verunreinigungen aus (in Gew.-%) 0,1 % C, 7 % Mn, 0,20 % Si, 0,01 % N und 2,6 % Cr bestand, ist über drei Minuten einer Glühung bei 920 °C unterzogen, anschließend innerhalb von 7 s in ein Abschreckbecken überführt und dort in Wasser abgeschreckt worden. Alternativ wäre auch mit demselben Ergebnis eine Abschreckung in Öl möglich gewesen. Nach dem Abschrecken betrug seine Zugfestigkeit Rm 1450 MPa bei einer Bruchdehnung A80 von 11 %. Das Produkt RmxA80 betrug demnach ca. 16.000 MPa x %. Das Gefüge des auf diese Weise erhaltenen Warmbands bestand aus kubisch verzerrtem α-Martensit und geringen Volumenanteilen von jeweils ca. 5 % an Austenit und hexagonal verzerrtem ε-Martensitanteilen.A hot-rolled strip, consisting of iron and unavoidable impurities, with (in wt%) 0.1% C, 7% Mn, 0.20% Si, 0.01% N, and 2.6% Cr, was annealed at 920 °C for three minutes, then transferred to a quenching tank within 7 seconds and quenched in water. Alternatively, quenching in oil would have yielded the same result. After quenching, its tensile strength Rm was 1450 MPa with an elongation at break A80 of 11%. The product RmxA80 was therefore approximately 16,000 MPa x%. The microstructure of the hot-rolled strip obtained in this way consisted of cubically distorted α-martensite and small volume fractions of approximately 5% each of austenite and hexagonally distorted ε-martensitanium.

Beispiel 10 (nicht erfindungsgemäß)Example 10 (not according to the invention)

Ein Warmband, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,1 % C, 7 % Mn, 0,13 % Si, 0,02 % Al, 1,5 % Cr, 0,18 % Ni, 0,13 % Cu, 0,002 % N und 0,08 % V enthielt, ist zu einem Kaltband kaltgewalzt und anschließend feuerverzinkt worden. Das verzinkte Kaltband wies eine Zugfestigkeit Rm von 1300 MPa bei einer Bruchdehnung A50 von 15 % auf. Der Anteil des Restaustenits am Gefüge des erhaltenen gegossenen Bands betrug 20 %.Hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.1% C, 7% Mn, 0.13% Si, 0.02% Al, 1.5% Cr, 0.18% Ni, 0.13% Cu, 0.002% N, and 0.08% V, was cold-rolled and subsequently hot-dip galvanized. The galvanized cold-rolled strip exhibited a tensile strength Rm of 1300 MPa and an elongation at break A50 of 15%. The retained austenite content of the resulting cast strip was 20%.

Beispiel 11 (nicht erfindungsgemäß)Example 11 (not according to the invention)

Ein Warmband, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,08 % C, 8 % Mn, 0,15 % Si, 0,02 % Al, 1 % Cr, 0,2 % Ni, 0,15 % Cu, 0,015 % N und 0,06 % V enthielt, ist zu einem Kaltband kaltgewalzt und anschließend einer Haubenglühung bei einer Glühtemperatur von 550 °C unterzogen worden. Nach der Haubenglühung betrug seine Zugfestigkeit Rm 1080 MPa und seine Bruchdehnung A50 25 %. Der Anteil des Restaustenits am Gefüge des gegossenen Bands lag nach dem Glühen bei 30 %.A hot-rolled strip containing, in addition to iron and unavoidable impurities (in wt.%), 0.08% C, 8% Mn, 0.15% Si, 0.02% Al, 1% Cr, 0.2% Ni, 0.15% Cu, 0.015% N, and 0.06% V, was cold-rolled and subsequently subjected to a hood annealing process at an annealing temperature of 550 °C. After hood annealing, its tensile strength Rm was 1080 MPa and its elongation at break A50 was 25%. The proportion of retained austenite in the microstructure of the cast strip after annealing was 30%.

Beispiel 12 (nicht erfindungsgemäß)Example 12 (not according to the invention)

Ein Stahlblech, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,05 % C, 0,06 % Si, 1,1 % Cr, 0,01 % N und 10 % Mn enthielt, ist innerhalb von drei Minuten auf 920 °C erwärmt worden. Anschließend ist das Blech innerhalb von 7 s in jeweils ein Abschreckbecken transferiert worden, in dem es in Öl- oder Wasser abgeschreckt worden ist. Der in Öl abgeschreckte Stahl wies eine Zugfestigkeit Rm von 1390 MPa bei einer Bruchdehnung A80 von 12 % auf. Das Produkt Rm*A betrug dementsprechend 16680 MPa%. Der in Wasser abgeschreckte Stahl wies eine Zugfestigkeit Rm von 1350 MPa bei einer Bruchdehnung A80 von 12 % auf. Das Produkt Rm*A betrug für den wasser-abgeschreckten Stahl dementsprechend 16200 MPa%. Nach der Öl- oder Wasserabschreckung bestand die Mikrostruktur des Stahls aus kubisch verzerrtem α-Martensit und geringen Volumengehalten aus zähem Austenit (ca. 4 %) sowie hexagonal verzerrten ε-Martensit (ca. 6 %).A steel sheet containing, in addition to iron and unavoidable impurities (in wt%), 0.05% C, 0.06% Si, 1.1% Cr, 0.01% N, and 10% Mn, was heated to 920 °C within three minutes. The sheet was then transferred within 7 seconds to quenching tanks, where it was quenched in either oil or water. The oil-quenched steel exhibited a tensile strength Rm of 1390 MPa at an elongation at break A80 of 12%. The product Rm*A was therefore 16680 MPa%. The water-quenched steel exhibited a tensile strength Rm of 1350 MPa at an elongation at break A80 of 12%. The product Rm*A for the water-quenched steel was therefore 16200 MPa%. After oil or water quenching, the microstructure of the steel consisted of cubically distorted α-martensite and small volume contents of tough austenite (approx. 4%) and hexagonally distorted ε-martensite (approx. 6%).

Beispiel 13 (nicht erfindungsgemäß)Example 13 (not according to the invention)

Ein Stahlblech, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,05 % C, 10 % Mn, 0,06 % Si, 0,009 % N, 1,1 % Cr und 1 % Ni enthielt, ist innerhalb von drei Minuten auf 920 °C erwärmt worden. Anschließend ist das Blech innerhalb von 7 s in jeweils ein Abschreckbecken transferiert worden, in dem es in Öl- oder Wasser abgeschreckt worden ist. Der in Öl abgeschreckte Stahl wies eine Zugfestigkeit Rm von 1315 MPa bei einer Bruchdehnung A80 von 12,1 % auf. Das Produkt Rm*A betrug dementsprechend 15910 MPa%. Der in Wasser abgeschreckte Stahl wies eine Zugfestigkeit Rm von 1285 MPa bei einer Bruchdehnung A80 von 12,3 %. Für den wasserabgeschreckten Stahl betrug das Produkt Rm*A demnach 15810 MPa%. Nach Öl- oder Wasserabschreckung bestand die Mikrostruktur des Stahls aus kubisch verzerrtem α-Martensit und geringen Volumengehalten aus zähem Austenit (ca. 7 %) sowie hexagonal verzerrten ε-Martensit (ca. 5 %).A steel sheet containing, in addition to iron and unavoidable impurities (in wt%), 0.05% C, 10% Mn, 0.06% Si, 0.009% N, 1.1% Cr, and 1% Ni, was heated to 920 °C within three minutes. The sheet was then transferred within 7 seconds to quenching tanks, where it was quenched in either oil or water. The steel quenched in oil exhibited a tensile strength Rm of 1315 MPa and an elongation at break A80 of 12.1%. The product Rm*A was therefore 15910 MPa%. The steel quenched in water exhibited a tensile strength Rm of 1285 MPa and an elongation at break A80 of 12.3%. The product Rm*A for the water-quenched steel was therefore 15810 MPa%. After oil or water quenching, the microstructure of the steel consisted of cubically distorted α-martensite and small volume contents of tough austenite (approx. 7%) and hexagonally distorted ε-martensite (approx. 5%).

Beispiel 14 (nicht erfindungsgemäß)Example 14 (not according to the invention)

Ein Stahlblech, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) 0,1 % C, 10 % Mn, 0,06 % Si, 0,009 % N, 1,1 % Cr und 1,5 % Al enthielt, ist innerhalb von drei Minuten auf 920 °C erwärmt worden. Anschließend ist das Blech innerhalb von 7 s in jeweils ein Abschreckbecken transferiert worden, in dem es in Öl- oder Wasser abgeschreckt worden ist. Der in Öl abgeschreckte Stahl wies eine Zugfestigkeit Rm von 1350 MPa bei einer Bruchdehnung A80 von 10,8 % auf. Das Produkt Rm*A betrug dementsprechend 14580 MPa%. Der in Wasser abgeschreckte Stahl wies eine Zugfestigkeit Rm von 1350 MPa bei einer Bruchdehnung A80 von 10,6 %. Für den wasserabgeschreckten Stahl betrug das Produkt Rm*A demnach 14310 MPa%. Nach der Öl- oder Wasserabschreckung bestand die Mikrostruktur des Stahls aus kubisch verzerrtem α-Martensit und geringen Volumengehalten aus zähem Austenit (ca. 12 %).A steel sheet containing, in addition to iron and unavoidable impurities (in wt%), 0.1% C, 10% Mn, 0.06% Si, 0.009% N, 1.1% Cr, and 1.5% Al, was heated to 920 °C within three minutes. The sheet was then transferred within 7 seconds to quenching tanks, where it was quenched in either oil or water. The steel quenched in oil exhibited a tensile strength Rm of 1350 MPa and an elongation at break A80 of 10.8%. The product Rm*A was therefore 14580 MPa%. The steel quenched in water exhibited a tensile strength Rm of 1350 MPa and an elongation at break A80 of 10.6%. For the water-quenched steel, the product Rm*A was therefore 14310 MPa%. After oil or water quenching, the microstructure of the steel consisted of cubically distorted α-martensite and small volume contents of tough austenite (approx. 12%).

Insgesamt wird durch die erfindungsgemäße Vorgehensweise eine gegenüber dem Stand der Technik für warmumgeformte höchstfeste Materialien verbesserte Kombination aus Bauteilfestigkeit und Restverformungsvermögen erzielt, welche durch hohe Werte des Produkts aus Zugfestigkeit und jeweiliger Bruchdehnung charakterisiert ist. Overall, the inventive method achieves an improved combination of component strength and residual deformation capacity compared to the prior art for hot-formed high-strength materials, which is characterized by high values of the product of tensile strength and respective elongation at break.

Claims (15)

  1. Flat steel product having a thickness of a maximum of 2.5 mm and an elongation at break A80 which is at least 4% and a tensile strength which is from Rm of 900 to 1500 MPa and which in addition to iron and inevitable impurities consists of (in % bei weight)
    C: 0.02 - 0.5%,
    Mn: 7 - 12.0%,
    Si: 0.05 - 1.0%,
    Al: up to 3.0%,
    Cr: 1 - 4.0%,
    Cu: up to 2.0%,
    Ni: up to 2.0%,
    N: up to 0.05%,
    P: up to 0.05%,
    S: up to 0.01 %,
    and
    optionally one or more elements from the group "V, Nb, Ti", wherein the sum of the contents of these elements is at a maximum equal to 0.5%,
    wherein the structure of the steel consists of 30 - 100% of hardening structure (martensite, tempered martensite or bainite), whilst the remainder of the structure is austenitic, wherein the flat steel product is an uncoated hot strip.
  2. Flat steel product according to claim 1, characterized in that its C content is at least 0.03 % by weight.
  3. Flat steel product according to any one of the preceding claims, characterized in that the Mn content thereof is a maximum of 10 % by weight.
  4. Flat steel product according to any one of the preceding claims, characterized in that the Mn content thereof is less than 9.5 % by weight.
  5. Flat steel product according to any one of the preceding claims, characterized in that the Si content thereof is a maximum of 0.5 % by weight.
  6. Flat steel product according to any one of the preceding claims, characterized in that the Al content thereof is a maximum of 2 % by weight.
  7. Flat steel product according to any one of the preceding claims, characterized in that the Cr content thereof is at least 0.5 % by weight.
  8. Flat steel product according to any one of the preceding claims, characterized in that the Cr content thereof is a maximum of 3 % by weight.
  9. Flat steel product according to any one of the preceding claims, characterized in that the Cr content thereof is a maximum of 2 % by weight.
  10. Flat steel product according to any one of the preceding claims, characterized in that the Cu content thereof is a maximum of 1 % by weight.
  11. Flat steel product according to any one of the preceding claims, characterized in that the Ni content thereof is a maximum of 1 % by weight.
  12. Flat steel product according to any one of the preceding claims, characterized in that the N content thereof is at least 0.0025 % by weight.
  13. Flat steel product according to any one of the preceding claims, characterized in that the N content thereof is a maximum of 0.03 % by weight.
  14. Flat steel product according to any one of the preceding claims, characterized in that the sum of the contents of the optionally present elements from the group "V, Nb, Ti" is at a maximum equal to 0.3 % by weight.
  15. Flat steel product according to any one of the preceding claims, characterized in that the optionally present content of Ti is at a maximum equal to 0.15 % by weight.
EP11164339.1A 2010-04-30 2011-04-29 High-strength, manganese-containing steel, flat steel product made from such steel and methods for its production Active EP2383353B2 (en)

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