Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D8/0226—Hot rolling
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  • 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
  • C21D8/0242—Flattening; Dressing; Flexing
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  • 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
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  • 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
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/24—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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/28—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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
• • 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 high-strength flat steel product with a minimum yield point of 680 MPa, which has a specifically adjusted ratio of yield point and tensile strength, as a result of which the flat steel product has excellent formability.
• the high strength of flat steel products enables the construction of components that withstand high mechanical loads and at the same time have a low component weight. This property allows, among other things, applications in the field of commercial vehicle construction and mobile crane construction.
• a high strength of the flat steel product is also helpful in applications in which a flat steel product is subject to abrasive wear, since this is accompanied by a high level of hardness, which counteracts the wear. Such applications are, for example, tipper bodies or materials handling equipment where abrasive wear occurs.
• the object of the present invention is to provide a high-strength flat steel product which has improved forming properties.
• a further object is to provide an efficient method for producing this flat steel product.
• the aim of the present invention is to use a conventional straightening machine to influence the yield point and thus set the ratio of yield point and tensile strength according to the requirements.
• the yield point R e of a flat steel product is understood to mean the upper yield point R eH if the flat steel product has a pronounced yield point. Otherwise (that is, for flat steel products without a pronounced yield point), the yield point of the flat steel product is understood to mean the yield point R p02 within the meaning of this application.
• a defined ratio of yield point and tensile strength is important for subsequent forming, since this ratio allows a statement to be made about the hardening of the material.
• An excessively pronounced hardening which is associated with small ratios of yield point and tensile strength, leads to a rapidly increasing force in a forming process, so that there is a minimum ratio of yield point and tensile strength of 0.6 in the flat steel product according to the invention.
• More favorable behavior due to strain hardening with an upper limit results in particular with a minimum ratio of yield point and tensile strength of 0.7, preferably with a minimum ratio of yield point and tensile strength of 0.8, particularly preferably with a minimum ratio of yield point and tensile strength of 0.85.
• the maximum ratio of yield point and tensile strength is therefore below 0.97, in particular below 0.95, preferably below 0.93, particularly preferably below 0.9.
• the yield strength of the flat steel product Re according to the invention which is determined according to DIN EN ISO 6892, is at least 680 MPa in order to ensure sufficient strength for structural applications and applications subject to wear.
• the minimum yield strength is 890 MPa to enable efficient designs.
• the flat steel product according to the invention is preferably characterized by a ratio of the yield point to the modulus of elasticity which is at most 0.01. The smaller this ratio, the lower the elastic elongation that has to be applied before the yield point is reached.
• the achievable plastic part of the bend depends not only on the system dimensions and the roll adjustment but also on the ratio of the yield point to the modulus of elasticity.
• a maximum ratio of yield point and modulus of elasticity of, in particular, 0.0085 is therefore favorable.
• a maximum ratio of yield point to modulus of elasticity of 0.007 is preferred, and a maximum ratio of yield point to modulus of elasticity of 0.0055 is particularly preferred.
• the modulus of elasticity describes the linear-elastic behavior of solid bodies and the proportional relationship between stress and strain during deformation.
• the modulus of elasticity increases with the resistance that a material offers to its elastic deformation. It is measured here in accordance with DIN EN ISO 6892-1.
• the selected alloy contents of carbon and manganese specifically set a modulus of elasticity of less than 215 GPa, preferably less than or equal to 210 GPa and particularly preferably less than or equal to 206 GPa, with the modulus of elasticity greater than 190, preferably greater than or equal to 195 and particularly preferably greater than or equal to is 198GPa.
• the values are preferably measured along the rolling direction.
• the flat steel product according to the invention is characterized by excellent formability, which is the case for typical applications in bending operations. This excellent formability is characterized by the lowest possible ratio of the minimum bending radius to the thickness dw of the steel flat product.
• test strips of the material to be tested are bent with any orientation to the rolling direction, but one that is constant in a series of tests, with a steadily decreasing bending radius.
• the convex bending side is subjected to an optical check, possibly supported by magnifying optics. If no cracks are visible, the test is passed. If cracks are found, the previously used bend radius at which no cracking was found shall be taken as the minimum possible bending radius.
• a minimum crack length of 10 ⁇ m is defined as the limit value for defining a crack.
• the ratio of the minimum bending radius to the thickness of the flat steel product is at most 4, in particular at most 2.5, preferably at most 2.1.
• the ratio of minimum bending radius and thickness is at most 4.5, in particular at most 3.0, preferably at most 2.5.
• the structure consists in particular of bainite, martensite and retained austenite.
• the term “bainite” specifically includes bainitic ferrite and dislocation-rich ferrite.
• the term “martensite” also includes tempered martensite. The proportions of the structural components mentioned below always refer to an evaluation based on the area.
• the microstructure preferably comprises at least 50% bainite, at most 10% by volume, preferably at most 5% by volume, of martensite.
• the microstructure can comprise 100% by volume of bainite.
• the steel has a microstructure which comprises more than 50% by volume martensite, at most 10% by volume, preferably at most 5% by volume ferrite, the remainder being bainite.
• the microstructure can comprise 100% by volume of martensite.
• the hot-rolled flat steel product has a thickness dw of 1.5 mm to 25 mm, in particular up to 20 mm.
• the thickness is preferably at least 2.0 mm, in particular at least 3.0 mm, in order to enable sufficiently rigid constructions.
• the maximum thickness is preferably 15 mm since a weight reduction is possible in this way.
• Carbon (C) is present in the steel substrate primarily to increase tensile strength and yield strength. With C contents of at least 0.03% by weight, the effect in the flat steel product according to the invention can be used efficiently.
• the interstitial solubility of carbon in both the face-centered cubic and the body-centered cubic lattice structure enables such an increase in strength.
• the solubility varies within the different lattice structures, the presence of C can also lead to a martensitic phase transformation. In this case, the carbon in the body-centered structure is forcibly dissolved by a sufficiently high cooling rate and thus leads to a tetragonal distortion of the cubic system.
• This martensite transformation results in a significant increase in strength, which occurs particularly reliably in the case of process-typical variations, preferably in the flat steel product according to the invention with C contents from 0.06% by weight.
• carbides can form between C and other alloying elements, which also contribute to increased strength. These carbides are either harder than the surrounding matrix or distort the matrix to such an extent that its hardness increases. This increase in hardness has a particularly positive effect on the wear resistance of the flat steel product according to the invention.
• the carbon content should preferably not fall below 0.07% by weight. At the same time, the C content has a lowering effect on the martensite start temperature.
• an upper limit for the C content of at most 0.65% by weight is recommended.
• the suitability for welding is also influenced by the C content. Particularly good suitability for welding can be ensured, preferably with a maximum C content of 0.4% by weight.
• the C content should preferably be limited to a maximum of 0.2% by weight.
• Mn manganese
• Mn occupies regular lattice positions in the steel substrate.
• the substitution atoms distort the cubic lattice due to their atomic radius, which differs from that of the iron atoms, and thus increase the strength.
• Mn should be present in the flat steel product according to the invention in amounts of at least 0.1% by weight.
• Mn is used as a deoxidizing agent due to its high oxygen affinity.
• a preferably set minimum content of 0.5% by weight has a calming effect on the melt of the flat steel product according to the invention.
• Mn also has a high affinity for sulfur production-related mostly in the form of unavoidable impurities in the flat steel product according to the invention.
• Mn By preferably adding Mn contents of at least 0.7% by weight, this affinity can bind the sulfur (to MnS) and thus avoid the formation of brittle phases (e.g. FeS). Mn tends to form segregations over the material thickness, which worsen the mechanical-technological properties of the flat steel product according to the invention. Such segregation can be curbed by a limit value of the Mn content of at most 3.0% by weight in order to homogeneously ensure the corresponding property profile of the flat steel product according to the invention. Furthermore, with higher Mn contents, the suitability for welding and the forming behavior of flat steel products according to the invention can be adversely affected.
• the negative effects on the joinability can preferably be largely suppressed by limiting the Mn content to a maximum of 2.5% by weight.
• the flat steel product according to the invention With a higher Mn content, the flat steel product according to the invention becomes more sensitive to overheating and tends to be brittle when tempered.
• a maximum Mn content of 2.0% by weight is preferably added.
• additional elements can optionally be added in order to fulfill the desired mechanical-technological properties to a particular extent. All optional elements may be present in the form of impurities below the minimum levels specified herein without significantly affecting the properties of the product.
• silicon (Si) forms a substitution mixed crystal in the flat steel product according to the invention, which leads to an increase in strength.
• the addition of at least 0.05% by weight achieves a level of strength as described in the profile of the flat steel product according to the invention.
• Si also has the ability to bind oxygen present as an undesirable impurity in the flat steel product according to the invention and thereby calm the melt.
• Si in the steel flat product according to the invention increases the resistance to an undesirable reduction in strength within the heat-affected zone during welding and tempering.
• Al can optionally be used as an alloying element.
• Al is usually used to calm the melt. By binding the oxygen to Al2O3, the rise of oxygen bubbles is avoided.
• an Al content of at least 0.01% by weight is necessary in order to utilize this effect.
• Al is also used for grain refinement. Al also binds the optional alloying element nitrogen (N) and aluminum nitrides are formed. These improve nucleation and impede grain growth due to the resulting high nucleus density, as a result of which more small grains are formed and the toughness of the flat steel product according to the invention is increased.
• An Al content of at least 0.02% by weight is preferably required for grain refinement. Since boron (B) can optionally be used in the flat steel product according to the invention to achieve high hardness, good bonding of the nitrogen contained is important. With the optional presence of niobium (Nb), Al can reduce the formation energy of niobium nitrides and carbonitrides, whereby the atomic boron free can improve the properties of the steel flat product according to the invention. Furthermore, a sufficient Al content lowers the density. In order to be protected against possible process-related fluctuations in the N content that can only be avoided with great effort and to set a lower density, an Al content of at least 0.070% by weight should preferably be selected.
• the Al content is particularly preferably set to at least 0.085% by weight. Due to the high affinity for oxygen, the resulting Al2O3 particles coarsen at high Al contents. In order to prevent the precipitation of coarse particles, which have a negative impact on the mechanical-technological properties, an Al content of a maximum of 1.5% by weight should not be exceeded.
• the Al content contained affects the castability. In order to ensure good castability, an Al content of at most 0.4% by weight should preferably be set.
• a flat steel product according to the invention with an Al content of preferably at most 0.15% by weight leads to optimal utilization of the alloyed aluminum if there are no requirements for density reduction.
• B boron
• B contents of at least 0.0001% by weight can be added.
• B must be present atomically in the steel substrate for its hardenability-increasing effect.
• additional elements must be added which bind the nitrogen that may be present as an optional alloying element or undesired impurity to such an extent that the formation of boron nitride is prevented.
• Preferred versions are the combination of the B alloy either with aluminum (Al) in combination with niobium (Nb) or with titanium (Ti), which due to their affinity for nitrogen preferably function as nitride and carbide or carbonitride formers.
• a minimum B content of 0.0005% by weight should preferably be set.
• B preferentially segregates at austenite grain or phase boundaries, which suppresses ferritic nucleation and shifts the ferritic-pearlitic phase transformation to longer cooling times.
• Niobium (Nb) can optionally be added to the melt to bind the nitrogen (N) possibly contained as an alloying element or as an unavoidable impurity in the flat steel product according to the invention. Due to the high temperature resistance of the niobium nitrides, carbides and carbonitrides, they impede grain growth before, during and after the rolling process. The resulting finer microstructure has improved toughness properties. In order to obtain a sufficient effect, an Nb content of at least 0.002% by weight is required. The formation of such nitrocarbides, nitronitrides and carbonitrides requires a relatively high formation energy, which has to be introduced in the form of high temperatures.
• the formation energy can be reduced and the formation of boron nitrides can be prevented more efficiently.
• a preferably adjusted Nb content of at least 0.005% by weight ensures a precipitation-hardening effect of the resulting particles.
• carbides and carbonitrides should preferably not fall below an Nb content of at least 0.010% by weight. In order to ensure complete binding of the nitrogen despite process-typical variations, it is particularly preferred to add at least 0.020% by weight.
• Nb contents that exceed 0.2% by weight no further improvement in the mechanical-technological properties can be seen, which is why this value is provided as the maximum limit.
• Nb contents of at most 0.1% by weight should preferably be set.
• a high Nb content increases the recrystallization temperature TNR markedly. This results in a more elongated former austenite texture with constant manufacturing processes. This can also be reflected in anisotropic material behavior.
• the desired toughness properties of the flat steel product according to the invention in the transverse direction can be ensured particularly reliably by an Nb content of preferably at most 0.08% by weight.
• the isotropy of the strength properties can particularly preferably be achieved by maximum Nb contents of 0.035% by weight.
• Chromium (Cr) can contribute to increasing strength in certain concentrations.
• a Cr content of at least 0.05% by weight is necessary.
• the through-hardenability of Cr-containing alloy concepts which is particularly advantageous for larger thicknesses, stems from the active suppression of the formation of ferrite and pearlite by the Cr content. This enables a complete martensitic or bainitic transformation even at lower cooling rates.
• a positive additional effect of Cr as an alloying element is the toughness-increasing character.
• a Cr content of at least 0.1% by weight should preferably be introduced.
• the corrosion resistance of Cr makes the alloying element interesting for a wide variety of applications.
• the scale resistance of the flat steel product according to the invention can be improved precisely in combination with other optional elements such as silicon (Si) or aluminum (Al).
• a Cr content of at least 0.2% by weight is preferably selected for this purpose. From the point of view of joining technology, Cr is a hindrance, since the weldability decreases noticeably with increasing content. In order to ensure the joinability of the flat steel product according to the invention, the Cr content should be limited to a maximum of 2.5% by weight. In addition to the increase in strength through the mechanism of solid solution strengthening, Cr also forms carbides, which increase the yield point and improve toughness at the same time.
• a maximum Cr content of 1.5% by weight should preferably be maintained.
• a maximum content of 0.8% by weight, particularly preferably maximum 0.5% by weight should be set in the flat steel product according to the invention, since the efficiency of the Cr addition decreases with increasing contents .
• Cr contents of up to 0.04% by weight are in the range of unavoidable impurities.
• the optional alloying element molybdenum (Mo) has similar properties to chromium (Cr), which is why both are preferably used in combination.
• Mo chromium
• an Mo content of at least 0.01% by weight should be set.
• Mo reduces the tendency of the steel flat product according to the invention to become brittle by tempering and improves the high-temperature strength.
• an Mo content of at least 0.05% by weight is preferably present.
• the through-hardenability of a flat steel product according to the invention can be improved by adding specific Mo contents.
• a content of at least 0.20% by weight should preferably be chosen in order to optimally utilize the properties of the Mo.
• An increase in the Mo content above 1.0% by weight is avoided for economic reasons, since this does not entail any mechanical-technological benefit and increases the costs unnecessarily.
• the increasing strength due to the addition of Mo correlates with a decreasing formability, as a result of which forming manufacturing processes are significantly influenced when processing a flat steel product according to the invention.
• the Mo content should preferably be limited to a maximum of 0.5% by weight in order not to jeopardize the hot formability.
• An increased Mo content also has an increasing effect on the breakpoint A1, which is why Mo contents of at most 0.30 wt avoid.
• Mo contents of up to 0.005% by weight are in the range of unavoidable impurities.
• Ti titanium
• N nitrogen
• TiN titanium nitrides
• the precipitation strengthening of the titanium nitrides formed is noticeable at Ti contents of at least 0.008% by weight, which is why this value is preferably used as the lower limit.
• this value In addition to the affinity of Ti for nitrogen, it can also form bonds with the carbon contained in the flat steel product according to the invention.
• the titanium carbides or carbonitrides produced in this way have a precipitation-hardening character in the flat steel product according to the invention.
• it is particularly preferable to have a Ti content of at least 0.015 Wt .-% are introduced.
• the titanium carbides, nitrides and carbonitrides have a grain-refining effect during thermomechanical rolling and correspondingly improve the toughness of the flat steel product according to the invention.
• the addition of titanium also markedly increases the recrystallization stop temperature TNR.
• the maximum Ti content is limited to 0.3% by weight.
• the toughness properties, which are of great importance for a flat steel product according to the invention, can drop noticeably as a result of the formation of coarse TiN if the Ti content is too high and N is present at the same time. Therefore, the Ti content should preferably be limited to a maximum of 0.2% by weight.
• Titanium carbides, nitrides and carbonitrides with a small diameter are advantageous in terms of mechanical and technological properties. Coarse precipitations have negative effects on the fatigue strength properties of the steel flat product according to the invention under cyclic loading.
• the formation of coarse carbides can preferably be limited or even completely avoided by a maximum Ti content of 0.1% by weight.
• Vanadium (V) can optionally be used as an alloying element in the flat steel product according to the invention.
• V contents of at least 0.002% by weight are advantageous in order to ensure the level of strength of the flat steel product according to the invention.
• a V content of at least 0.005% by weight is preferably set.
• V can contribute to grain refinement, preferably at levels of at least 0.008% by weight.
• the strength and the toughness of the flat steel product according to the invention are also increased in this way.
• V can also have a transformation-retarding effect on the alloy.
• V-contents that exceed 0.15% by weight are not advisable, since the small further improvement in properties through higher contents does not justify the significant increase in costs associated with this.
• the V content is preferably limited to a maximum of 0.07% by weight.
• V increases the recrystallization temperature TNR of the steel flat product noticeably, which leads to a stretched austenite grain and anisotropic material behavior.
• the effect of V is rather weak compared to niobium (Nb). Nevertheless, V contents of at most 0.03% by weight are preferably used in order to reliably contain this effect.
• a maximum V content of 0.01% by weight is particularly preferably alloyed.
• the V content can also be set to higher contents if, in particular, the tendency to crack can be reliably avoided through appropriate process control during welding can.
• the main focus of this variant is the formation of temper carbides in the flat steel product according to the invention. Their strength-increasing effect is noticeable with V contents of at least 0.02% by weight. V contents of at least 0.08% by weight are preferably selected for this purpose. For reasons of cost, the V content is limited to a maximum of 0.5% by weight. In order to prevent the coarsening of the tempering carbides, a maximum content of 0.3% by weight is preferably used. For optimum utilization of the mechanisms of action, V contents of 0.1% by weight are preferably used.
• Ni nickel
• the critical cooling rate is reduced by Ni contents of at least 0.05% by weight, which can result in improved hardening and hardening.
• Ni contents of at least 0.15% by weight are preferably maintained in order to achieve the level of hardness that is desired for the flat steel product according to the invention over the entire material thickness and with little sensitivity to technically caused fluctuations in the process parameters.
• the deformability and toughness of the flat steel product according to the invention are also improved by the addition of Ni.
• Ni contents of at least 0.3% by weight are preferably set for this purpose.
• Ni contents above 10% by weight are not recommended, since a further increase does not have an additional positive effect on the mechanical-technological properties.
• increased concentrations of Ni promote the tempering embrittlement of the flat steel product according to the invention.
• the effect can preferably be curbed by Ni contents of at most 5% by weight. Since the weldability is adversely affected by the addition of Ni, the Ni content is preferably limited to a maximum of 1% by weight to ensure weldability. Efficient use of Ni in the flat steel product according to the invention is given with particularly preferred contents of at most 0.5% by weight. Even without the described optional addition of nickel, a certain Ni content can occur as an unavoidable impurity. In that case, the Ni content is at most 0.05% by weight, preferably at most 0.04% by weight.
• Cu copper
• Cu contents of at least 0.005% by weight can be used to improve the hardenability of the flat steel product according to the invention.
• the tempering resistance of the flat steel product according to the invention is improved by the addition.
• Cu contents of at least 0.03% by weight are preferably set.
• P phosphorus
• Cu improves the corrosion resistance of the steel flat product according to the invention against atmospheric corrosion.
• a Cu content of at least 0.10% by weight is preferably used.
• the risk of red cracking during production is minimized by a maximum Cu content of 1.0% by weight.
• Cu influences the weldability of the flat steel product according to the invention.
• Cu contents are therefore preferred of a maximum of 0.5 wt.
• a maximum Cu content of 0.3% by weight is preferably selected.
• the contents of Cu and Ni are adjusted in such a way that the sum of the corresponding two alloy contents in % by weight satisfies the limits given above for Ni.
• Ca is an optional alloying element for the steel flat product of the present invention.
• Ca is used as a desulfurizing agent.
• Ca contents of at least 0.0001% by weight are recommended in order to efficiently bind together with Mn the sulphur, which can be present as an unavoidable impurity in the flat steel product according to the invention due to the production process.
• Ca causes non-metallic inclusions to be rounded in shape, which can improve fatigue strength and toughness properties. This mechanism is particularly noticeable with Ca contents of at least 0.0003% by weight, which is why this is preferably chosen as the minimum content.
• Ca also changes the plasticity of sulfides such as MnS.
• Ca dissolves in the MnS and forms a mixed sulphide, which leads to an increase in hardness.
• Ca reduces the strain of MnS and suppresses the formation of extended sulfides.
• the disadvantages of non-metallic inclusions are curbed particularly reliably with Ca contents of at least 0.0005% by weight, which is why this value is specified as the preferred upper limit. Due to resource efficiency, Ca contents exceeding 0.008% by weight are avoided.
• an increased Ca content can impair the mechanical-technological properties of the flat steel product according to the invention. In order to rule this out, a maximum Ca content of 0.0065% by weight must be maintained, with the optimum utilization of the Ca being given at a maximum of 0.005% by weight.
• rare earth elements such as e.g. As cerium, lanthanum, neodymium, praseodymium and yttrium possible.
• the addition can result in particular in an increase in strength.
• Contents of at least 0.001% by weight improve the mechanical-technological properties of the flat steel product according to the invention.
• the binding effect of the rare earths on sulfur, phosphorus and oxygen can reduce segregation at grain boundaries, which increases toughness.
• Rare earth contents above 0.05% by weight are not recommended for cost reasons. This upper limit also prevents the formation of additional precipitates, which in turn can reduce the toughness properties.
• N is also an optional alloying element.
• N can have a strength-increasing effect on the flat steel product according to the invention.
• N contents of at least 0.002% by weight must be observed.
• Many optional alloying elements used in the steel flat product according to the invention have a high affinity for N, which leads to the formation of various nitrides. With the appropriate simultaneous use of nitrogen and at least one of the optional alloying elements listed in connection with nitrogen, these can also increase the strength of the flat steel product according to the invention.
• N contents of at least 0.003% by weight are selected, preferably at least 0.004% by weight being added.
• nitrides such as As titanium nitrides
• Some nitrides are very coarse and angular and therefore have a rather negative effect on the strength of the flat steel product according to the invention.
• boron nitride is undesirable because the mechanism of action of B is prevented by the setting of the alloying element. Therefore, when adding boron and nitrogen, sufficient titanium or aluminum in combination with niobium must be present to ensure the efficient binding of the N.
• a maximum N content of 0.01% by weight is set. N contents of at most 0.008% by weight are preferably maintained in order to ensure process-reliable production. N contents of at most 0.006% by weight are preferably added in order to be able to set completely, in particular if boron is optionally added.
• Phosphorus (P) can likewise optionally be added to the flat steel product according to the invention.
• P can have a strength-increasing effect.
• the negative influence of P on the toughness properties predominates, which severely limits the resistance to crack propagation. Therefore, a maximum P content of 0.15% by weight is not exceeded.
• the P content should preferably be limited to a maximum of 0.05% by weight and preferably a maximum of 0.02% by weight.
• Tin (Sn) is also an optional alloying element.
• Sn can lead to improved corrosion resistance.
• a Sn content of at least 0.001% by weight is required for this.
• Sn At temperatures around 500 °C, Sn accumulates along grain boundaries, which inhibits hydrogen recombination there, which is the reason for the improved resistance to acidic media.
• these local Sn segregations can lead to embrittlement of the flat steel product according to the invention to lead.
• a maximum Sn content of 0.04% by weight is not exceeded, but preferably a maximum of 0.03% by weight and more preferably a maximum of 0.02% by weight for optimum mechanical-technological properties of the material according to the invention Steel flat product selected.
• arsenic can also have an advantageous effect on the mechanical-technological properties of the flat steel product according to the invention.
• the As deposits can predestine the grain boundaries for brittle fracture. Therefore, an As content of at most 0.02% by weight is maintained.
• the toughening effect of As is controlled by preferably adding a maximum As content of 0.015% by weight.
• the As content is preferably limited to a maximum of 0.01% by weight.
• Oxygen (0) can also be used as an optional alloying element.
• Some alloying elements which are used to produce a flat steel product according to the invention have a high affinity for oxygen.
• Aluminum preferably bonds with the oxygen contained to form stable oxides. With 0 contents of at least 0.001% by weight, non-metallic inclusions are formed, which impede the movement of dislocations and thus contribute to an increase in strength. Therefore, if oxygen is deliberately added and is not only present as an impurity, this value is set as the lower limit. Higher 0 contents lead to coarser oxides, which can reduce the toughness and fatigue strength of the flat steel product according to the invention. In order to effectively limit the formation of large oxides, a maximum O content of 0.03% by weight is specified.
• an oxide coating which can occur with higher 0 contents, can have a negative effect on the castability and rollability of the flat steel product according to the invention, which is why an upper limit of 0.02% by weight is preferably observed.
• the castability of the flat steel product according to the invention is preferably stabilized to the extent that the formation of alumina is restricted.
• Co Co
• Co contents of at least 0.01% by weight are to be added to the steel product according to the invention.
• Co contents of preferably at least 0.05% by weight and preferably at least 0.1% by weight the strength-increasing character of Co is particularly pronounced.
• Co can reduce hardenability.
• the negative effect of Co becomes noticeable to, which is why this value is not exceeded.
• Co contents of at most 0.7% by weight and preferably at most 0.5% by weight are preferably maintained.
• W tungsten
• W contents of at least 0.005% by weight are added.
• the addition ensures that the recrystallized austenite grains do not become too coarse in the fully austenitized state and thus noticeably reduce the strength. This is preferably ensured by W contents of at least 0.01% by weight.
• W tends to form carbides, which in turn can impede dislocation movement and increase strength.
• preference is given to using W contents of at least 0.015% by weight.
• W can also be used in combination with other elements for micro-alloying.
• Laves phases can form, especially in combination with molybdenum (Mo), which would impair the notched impact strength of the flat steel product according to the invention.
• Mo molybdenum
• W contents of a maximum of 0.2% by weight are maintained.
• W contents of at most 0.15% by weight and preferably at most 0.1% by weight are selected.
• zirconium (Zr) can optionally be added within the same limits.
• the contents of W and Zr are adjusted in such a way that the sum of the corresponding two alloy contents in % by weight satisfies the limits given above for W.
• Be beryllium
• Be contents of at least 0.001% by weight, high-strength carbides and oxides can form.
• Coarse non-metallic inclusions can have a counterproductive effect on the mechanical-technological properties of the flat steel product according to the invention, which is why Be contents of 0.1% by weight are not exceeded.
• Be has a particularly efficient effect at contents of preferably at most 0.05% by weight and preferably at most 0.02% by weight.
• the use of Be should be dispensed with due to its toxicity through substitution with other optional alloying elements.
• Antimony (Sb) can be added to the steel flat product according to the invention as an optional alloying element. With contents of at least 0.001% by weight, Sb can form segregations at grain boundaries, which reduces hydrogen recombination at these. Thus, Sb can improve the corrosion resistance of the flat steel product according to the invention in acidic media. In addition, Sb can suppress the anodic reaction during the corrosion process, which is advantageously used preferably at Sb contents of at least 0.002% by weight and preferably at least 0.005% by weight. From an economic point of view, Sb contents of a maximum of 0.3% by weight make sense. In addition to the improved corrosion resistance, however, the Sb segregations along the grain boundaries also have an embrittling effect on the flat steel product according to the invention. In order to curb this, an Sb content of at most 0.1% by weight and preferably at most 0.05% by weight is provided.
• the manganese sulfide formed has a positive effect on the machinability of the flat steel product according to the invention. This positive effect is present with S contents of at least 0.0002% by weight. However, for a noticeable effect, preferably at least 0.0005% by weight, and more preferably at least 0.0008% by weight, is added.
• an S content of at most 0.02% by weight is preferably present. Exceeding this leads to increased sulfide formation, which has a negative effect on the toughness, ductility and deformability of the flat steel product according to the invention. Furthermore, S tends to form segregations, which represent preferred failure points in the material. In order to limit this effect, the maximum S content of 0.01% by weight is preferably not exceeded. S contents of at most 0.005% by weight are preferably set in order to limit the negative effect of the sulfur.
• Lead (Pb) is an optional alloying element which can have a positive effect on the machinability of the flat steel product according to the invention.
• the addition of at least 0.0001% by weight results in short chips and clean cut surfaces. Due to the toxicological classification of lead, its content is limited to a maximum of 0.02% by weight.
• the remainder consists of iron and elements whose presence is unavoidable for production reasons.
• the contents of such impurities are to be kept as low as possible within the limits that can be achieved economically and with justifiable technical effort.
• H hydrogen
• the introduction of hydrogen (H) is almost unavoidable due to its small atomic size. It is known that H has an embrittling effect on materials.
• H attaches itself to lattice defects, locally reducing the yield stress there and thus facilitating material failure.
• the H content in a flat steel product according to the invention is restricted to a maximum of 10 ppm. However, the H content preferably does not exceed 5 ppm and is preferably at most 3 ppm.
• Said molten steel can preferably also contain one or more optional elements which have been explained in detail in relation to the flat steel product. Likewise, the content of C and Mn can be within the preferred ranges discussed.
• the melt produced in this way is conventionally cast into a pre-product of thickness dv.
• An alternative casting to thin slabs, cast strips or blocks is also possible.
• the pre-product is completely heated to the austenitization temperature T WE , with the heating being able to consist of heating the pre-product to this temperature or the pre-product being kept at the respective temperature after casting.
• the austenitizing temperature T WE is at least 1100° C. and at most 1350° C., with an austenitizing temperature of at least 1220° C. being preferred with a view to avoiding excessive hardening in the subsequent hot rolling process. Melting of the surface of the preliminary product and excessive grain coarsening can be safely avoided if the austenitizing temperature is limited to a maximum of 1320 °C. In the temperature range between 1220 °C and 1320 °C, an optimally homogeneous initial structure is also set and previously existing precipitations are reliably dissolved.
• the preliminary product is hot-rolled to form the hot-rolled flat steel product at a final hot-rolling temperature T E of at least 770° C., in particular at least Ar3+20K.
• T E final hot-rolling temperature
• the temperature of the rolled flat steel product decreases continuously with each pass down to the final rolling temperature T E , at which the hot-rolled flat steel product leaves the last pass.
• the final rolling temperature must be at least 770 °C. If the final rolling temperature T E is at least 20° C. above the Ar3 temperature of the flat steel product according to the invention, the formation of ferrite is avoided in a particularly reliable manner.
• the element symbols are usually used as abbreviations for the element contents in % by weight. For C, therefore, the carbon content in % by weight must be entered in the formula.
• the at least two hot rolling passes above the recrystallization temperature have the advantage that a fine, multiple recrystallized austenite structure results, since above this temperature the austenite recrystallizes completely in the structure of the steel flat product.
• the approximate calculation of the recrystallization temperature is carried out according to the Effect of Chemical Compostion on Critical Temperatures of Microalloyed Steels", Boratto et al., THERMEC '88, Proceedings, Iron and Steel Institute of Japan, Tokyo, 1988, pp. 383-390 specified method.
• This minimum number nw of rolling passes above T NR has the advantage that recrystallization results in an optimally fine-grained structure.
• the hot rolling of the preliminary product includes at least one hot rolling pass, which is carried out at a temperature below the recrystallization temperature TNR.
• the final hot rolling temperature T E is therefore lower than the recrystallization temperature T NR .
• the last rolling pass or passes will be performed at a temperature below the recrystallization temperature. This suppresses the recrystallization of the austenite during the last rolling pass (or the last rolling passes in the case of several rolling passes below the recrystallization temperature).
• the degree of deformation ⁇ is preferably at least 0.25 over all hot rolling passes that are carried out at a temperature below the recrystallization temperature T NR .
• the degree of deformation is defined as the absolute value of the natural logarithm of the ratio of these two thicknesses.
• the obtained hot-rolled flat steel product is cooled immediately after the hot-rolling. Due to the design of hot rolling mills known from the prior art and the associated cooling devices, the term “immediately” describes cooling that begins a maximum of 8 s after the steel flat product emerges from the last rolling pass. Water, which is applied to the flat steel product in a conventional manner in a conventional cooling section, is particularly suitable as a coolant.
• the hot-rolled flat steel product has a yield strength of at least 890 MPa.
• the hot-rolled flat steel product obtained is cooled at a cooling rate ⁇ Q of at least 40 K/s to a cooling stop temperature of T KS of at most T E -250 K.
• the cooling rate ⁇ Q is preferably at least 60K/s.
• the cooling stop temperature T KS is preferably at most 550° C., in particular at most 500° C., provided that it is then not above T E ⁇ 250° C.
• the hot-rolled flat steel product has a yield point that is less than 890 MPa.
• the hot-rolled flat steel product obtained is cooled at a cooling rate ⁇ Q of at most 40 K/s to a cooling stop temperature of T KS between 500°C and 700°C.
• the cooling rate ⁇ Q is preferably at most 35 K/s, particularly preferably at most 30 K/s.
• the cooling stop temperature T KS is preferably between 550°C and 650°C, particularly preferably between 570°C and 630°C.
• the hot-rolled flat steel product obtained is cooled to room temperature at a cooling rate ⁇ Q of at most 0.1 K/s, in particular at most 0.05 K/s.
• the strength of the flat steel product can be set very precisely via the respective cooling stop temperature in combination with the subsequent slow cooling down to room temperature by self-tempering of the microstructure or the targeted formation of precipitations.
• the flat steel product according to the invention is optionally heat treated.
• the heat treatment is preferably carried out by self-tempering due to the residual heat still present in the material after the accelerated cooling.
• a heat treatment by annealing is particularly preferred, since in this case the desired mechanical properties can be set precisely.
• the heat treatment by self-tempering takes place directly from the rolling heat due to the increased temperature remaining in the core during cooling with spray water cooling on the surfaces of the steel flat product.
• the residual heat from the inner area leads to a self-starting effect.
• the heat treatment by annealing takes place after the steel flat product has been cooled to room temperature and then reheated in an annealing device known in the prior art, such as a batch annealing device or a continuous annealing device.
• a key figure is defined in the form of the product of the annealing temperature in K and the annealing time at this temperature in h.
• the defined annealing parameter is expediently above 4000 K*h and below 18000 K*h. Annealing parameters above 4300 K*h and below 9000 K*h have proven to be advantageous with regard to the set microstructure and productivity.
• the first bend introduced in the straightening process takes place in particular against the pre-curvature of the strip or in the direction of the pre-curvature of the strip. preferably, the first bend is against the pre-curvature of the strip, since in this way a higher level of plasticization can be set in the straightening process, and the required flatness and the desired internal stress distribution can thus be set in a simple manner.
• the straightening process takes place in at least one straightening mill, in particular straightening takes place in an at least two-stage process, comprising a pre-straightening process and a fine-straightening process.
• This division into two separate straightening steps makes it possible to consciously carry out the first straightening step to be designed in such a way that a residual curvature remains against which the first bend can take place in the subsequent fine straightening process.
• a straightening step is quite common in the manufacture of flat steel products.
• the aim is to achieve sufficient flatness for all subsequent processes.
• the inventors have recognized that this customary process step can be used to improve the forming properties. It was found that the straightening step with deliberately introduced plasticizing has a decisive influence on the ratio of yield point and tensile strength according to the invention and on the ratio of yield point and modulus of elasticity of the flat steel product. A connection between the ratio of the minimum bending radius r and the thickness dW of the steel flat product could also be determined.
• the plastification is generally a measure of the plastic deformation of the material to be straightened in straightening mills and indicates the percentage of the strip cross-section that is plastically deformed during bending in a bending triangle of a straightening mill.
• R e is the yield point of the material to be straightened in MPa
• E is the modulus of elasticity of the material to be straightened in MPa
• d W is the thickness of the material to be straightened in mm
• c is the curvature in mm -1 caused by bending in a bending triangle.
• the curvature of the material to be straightened depends directly on the position of the rollers in relation to each other in a bending triangle. A low position of the roller, which dips between the two opposite rollers, results in more bending and therefore in a larger value for the curvature c.
• the definition of plasticization still allows negative values for plasticization.
• a straightening mill comprises a plurality of rollers which are arranged alternately on different sides of the flat steel product.
• the rolls have a roll diameter.
• the crests of the rollers are at a distance from each other in the rolling direction, resulting in bending triangles from three consecutive rollers in the rolling direction.
• the introduced plasticization results from the relative Roll position of the rolls of the straightening mill in the respective bending triangle to each other.
• the focus is on plasticizing in the first bending triangle of the straightening mill.
• the relative role position is commonly referred to as the engagement.
• the specification of the position relates to a reference position, in which the crests of all straightening rollers lie on a horizontal line. This position of the rollers is referred to as 0 pitch.
• negative throws are associated with a roll apex "dipping" between any two other roll crests and correspondingly more bending is applied, whereas positive throws result in a clearance between the roll apexes and therefore less bending is applied.
• the plasticization is therefore at least 75%. In the case of a pronounced yield point, it is advantageous to eliminate this before further forming. Accordingly, in particular, a plasticization of at least 82% is to be set. In order to introduce sufficient plasticization to eliminate flatness errors at the same time, the plasticization is preferably at least 85%. The introduction of excessive plasticization leads to an excessive reduction in the yield point ratio, so that a maximum plasticization of 95% cannot be exceeded. Furthermore, in order not to excessively reduce the remaining deformability of the material, the maximum plasticization is in particular 94%, preferably 90%.
• a flat steel product according to the invention is thus available, which is characterized by high strength and, at the same time, good formability.
• Table 2 initially shows the chemical compositions of the respective steel in % by weight.
• the steel also consists of iron and unavoidable impurities.
• the steel melts composed in this way are cast into a preliminary product in the form of a slab. Now follows a special rolling process, the parameters of which are given in Table 3. First, the pre-product is completely heated to an austenitization temperature T WE .
• the preliminary product is then hot-rolled to form the hot-rolled flat steel product at a hot-rolling finish temperature T E .
• the hot-rolling finish temperature T E is more than 770 °C and more than Ar3+20K.
• the temperature of the rolled flat steel product decreases continuously with each pass down to the final rolling temperature T E , at which the hot-rolled flat steel product leaves the last pass.
• Table 3 also gives the recrystallization temperature as it is according to the publication " Effect of Chemical Compostion on Critical Temperatures of Microalloyed Steels", Boratto et al., THERMEC '88, Proceedings, Iron and Steel Institute of Japan, Tokyo, 1988, pp. 383-390 calculated.
• the austenitization temperature T WE at which the hot rolling step starts, is above the respective recrystallization temperature TNR.
• the first rolling passes inevitably take place at a temperature above the recrystallization temperature TNR.
• at least two rolling passes take place above the recrystallization temperature TNR.
• the specific number of hot rolling passes nw above the recrystallization temperature T NR is also given in Table 3.
• At least one hot rolling pass was carried out below the recrystallization temperature T NR .
• the number of hot rolling passes below the recrystallization temperature is denoted by n W without Rx and is given in Table 3.
• the degree of deformation ⁇ without RX is at least 0.05 over all hot rolling passes that are carried out at a temperature below the recrystallization temperature TNR.
• the hot-rolled flat steel product obtained was cooled at a cooling rate ⁇ Q of at least 40 K/s to a cooling stop temperature of T KS of at most T E -250K.
• This cooling parameter ensures that the hot-rolled steel flat product is sufficiently quenched, resulting in a yield strength of at least 890 MPa, as shown in Table 5.
• the hot-rolled flat steel product obtained was cooled at a cooling rate ⁇ Q of at most 40 K/s to a cooling stop temperature of T KS of at most T E -250K.
• Example 1 was heat-treated by self-tempering due to the residual heat still present in the material after the accelerated cooling.
• Example 2 has undergone a heat treatment by annealing.
• the annealing treatment takes place within the framework of specified annealing parameters, which are calculated from the annealing temperature in °C and the annealing time in hours.
• the annealing parameters of the exemplary embodiments are expediently above 4000 K*h and below 18000 K*h.
• the mechanical-technological characteristics and the microstructure composition of the hot-rolled flat steel products obtained in this way are given in Table 5 below.
• the exemplary embodiments 1, 2, 3, 7 and 8 almost exclusively have martensite or tempered martensite as structural components, which is associated with yield strengths of greater than 890 MPa.
• the exemplary embodiments 4, 5 and 6 have ferrite and bainite as structural components, which is associated with yield strengths of greater than 890 MPa.
• the tensile strength R m and yield point R e according to ISO 6892 are listed. In all design variants, the ratio of yield point and tensile strength is at least 0.6 and at most 0.97.
• Table 5 also shows that the ratio of the yield point Re to the modulus of elasticity E of the flat steel product is a maximum of 0.01.
• the ratio of the minimum bending radius to the thickness of the steel flat product r/dw is a maximum of 4 for yield points of less than 1100 MPa and is a maximum of 4.5 for yield points of more than 1100 MPa.
• Table 5 Mechanical-technological parameters and microstructure ⁇ /b> # R e R m E R e /R m R e /E r/d W structure in MPa in MPa GPa 1 1089 1172 210 0.93 0.005 2.1 > 99% martensite 2 973 1049 199 0.93 0.005 2 > 99% tempered martensite 3 1268 1505 195 0.84 0.007 4.2 > 99% martensite 4 714 767 196 0.93 0.004 1.1 dislocation-rich ferrite 5 721 773 206 0.93 0.004 1 > 85% bainite, remainder dislocation-rich ferrite 6 707 779 202 0.91 0.004 1.1 dislocation-rich ferrite 7 1292 1523 199 0.85 0.00

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