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
  • 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
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • 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 with improved further processing properties.
• HSLA steels High Strength Low Alloy
• HSLA steels are characterized in particular by a combination of high strength and formability with relatively low alloy contents. They achieve their high strength through the addition of micro-alloying elements such as titanium, niobium or vanadium in connection with a controlled rolling and cooling process. Due to their low content of alloying elements, they also have excellent weldability and can be produced at low cost.
• metal sheets are usually first rolled and tempered in an additional production step.
• the sheet metal is regularly first reheated and quenched (hardened) and then tempered.
• high-strength strip plates can be produced from such a directly hardened hot strip in a conventional cut-to-length line.
• the strip sheets produced in this way can either be delivered to the end customer in an exclusively hardened condition and processed further.
• the depaneling process is often followed by an optional tempering of the sheets. This enables fine adjustment of the mechanical-technological properties. Furthermore, the scattering of the mechanical properties over the strip length and strip width can be reduced.
• the object of the present invention is to provide a high-strength flat steel product that eliminates these disadvantages.
• a further object is to provide an efficient method for producing this flat steel product.
• 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. Flat steel products according to the invention do not have a pronounced yield point, so that the yield point R e for these is to be understood as the yield point R p02 .
• the yield point R e is basically to be understood as parallel to the rolling direction.
• the yield point of the flat steel product 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 and wear-resistant applications.
• the microstructure of the hot-rolled flat steel product is preferably adjusted in such a way that its yield point Re is at least 890 MPa, particularly preferably at least 960 MPa.
• a flat steel product is understood here as a strip sheet, ie a sheet cut from a hot strip, the length and width of which are each significantly greater than its thickness. Therefore, when a flat steel product or a "sheet metal product” is mentioned below, this means a rolled product in the form of a sheet metal strip, from which, for example, vehicle, crane or infrastructure components as well as supporting structures or shields can be used in the upper or lower section Underground mining blanks or blanks are divided.
• “Shaped sheet metal parts” or “sheet metal components” are made from such a flat steel product or sheet metal product, the terms “shaped sheet metal part” and “sheet metal component” being used synonymously here.
• Infrastructure construction is understood to mean the manufacture of buildings, bridges, ships and aircraft.
• Vehicle construction here refers in particular to the construction of commercial vehicles, buses and trailers.
• Crane construction refers here in particular to the construction of mobile cranes, especially to the construction of crane booms.
• Components subject to wear and tear, such as e.g. B. troughs of dump trucks can be made of flat steel products according to the invention.
• alloy contents refers to the weight or mass, unless otherwise expressly stated. If element contents are given in formulas, this also means the corresponding alloy content in % by mass. Element contents in formulas are indicated by the symbol “%” followed by the element symbol, i.e. %C denotes the element content of carbon in % by mass. Information on the content of microstructure components refers to the surface (area %, FI%) observed in the metallographic section, unless otherwise stated, with the exception of the volume fraction of retained austenite determined by X-ray in % by volume (% by volume).
• 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.
• the hot-rolled flat steel product has a tensile strength R m that is greater than 730 MPa and less than 1700 MPa.
• the tensile strength is also determined in a tensile test according to DIN EN ISO 6892.
• Tensile strength is also generally understood to be parallel to the rolling direction.
• a tensile strength of at least 930 MPa, particularly preferably at least 980 MPa is preferably set. Since too high a tensile strength is associated with too little toughness, the tensile strength is preferably limited to a maximum of 1550 MPa.
• the tempering treatment of the hot-rolled and directly hardened hot strip as a coil in the bell-type annealer serves not only to specifically adjust the yield point and tensile strength level, but also to homogenize the mechanical properties across strip length, strip width and strip thickness.
• This process step reduces the spread of the yield point and the tensile strength over the strip length and strip width to a maximum of 100 MPa each.
• the yield strength R e and tensile strength R m over the length and width consequently vary by no more than 100 MPa (difference between the maximum value and the minimum value).
• the hot-rolled flat steel product has a ratio of yield point Re to tensile strength Re / Rm that is at least 0.84 and at most 0.95.
• This has the advantage that a flat steel product according to the invention can be used for bending and edging without any special pretreatment. This can be used, for example, to produce highly rigid structural components by roll profiling.
• yield point ratios R e /R m of at least 0.88 are preferably set.
• the yield strength ratio R e /R m is preferably limited to a maximum of 0.94.
• the restricted yield strength ratio of 0.84 to 0.94 offers an ideal combination of high safety through sufficient strong strain hardening in case of overloading and limited deflection during bending and folding by avoiding over hardening.
• the elongation at break A according to DIN EN ISO 6892, proportional sample, of the flat steel product according to the invention is between 5% and 25% parallel to the rolling direction.
• the minimum elongation at break is preferably 7% in order to ensure good formability.
• the hot-rolled flat steel product has a Brinell hardness (hereafter HB) for which the following applies: 47 + 0 , 2 ⁇ R e ⁇ hb ⁇ 0 , 4 ⁇ R e
• the Brinell hardness (HB) is determined as Brinell hardness HBW 5/750 according to DIN EN ISO 6506-1 1 mm below the surface as the mean value from at least 3 individual measurements of the flat steel product according to the invention.
• the hot-rolled flat steel product has a notched impact strength which is at least 25J/cm2 at a test temperature of -60°C and or which is at least 35J/cm2 at a test temperature of -40°C and or which is at least 50J/cm2 at a test temperature of -20°C.
• the specified minimum notched impact strength is determined as a Charpy V notch according to DIN EN ISO 148-1 parallel to the direction of rolling.
• the yield point is less than 890 MPa.
• the steel has a microstructure which comprises more than 50% by area of bainite, the remainder being martensite, ferrite, pearlite and retained austenite, with the proportion of ferrite and pearlite in particular being at most 10% by area, preferably at most 5% by area. is, ie the sum of the proportions of ferrite and pearlite is at most 10% by area, preferably at most 5% by area.
• the proportion of retained austenite is preferably at most 5% by volume.
• the structural components specified in this application are basically to be understood as being 1/3 of the sheet thickness.
• deviating structural components can occur due to segregation, and it can also occur on the sheet surface (e.g in an edge area of a maximum of 50 ⁇ m, in particular a maximum of 40 ⁇ m), deviations can occur due to edge decarburization.
• the yield point is at least 890MPa.
• the steel has a structure which comprises more than 50 FI% martensite, the remainder being bainite, ferrite, pearlite and residual austenite, with the proportion of ferrite and pearlite being at most 10 FI%, preferably at most 5 FI%. is, ie the sum of the proportions of ferrite and pearlite is at most 10% by area, preferably at most 5% by area.
• the proportion of retained austenite is preferably at most 5% by volume.
• the martensite has fine needles with a maximum needle width of 200 nm. Due to the tempering, the predominantly martensitic structure has a large number of finely distributed tempered carbides, whose size is preferably at most 200 nm.
• the density X Z of the tempering carbides is at least 10 11 m -2 , preferably at least 10 12 m -2 . This ensures good notched impact strength.
• the density of the tempering carbides is at most 10 14 m -2 , preferably at most 10 13 m -2 , in order to avoid excessive softening of the martensite.
• the flat steel product has, in particular, grain elongation of the former austenite grains in the rolling direction, with the ratio V ⁇ of austenite grain length in the rolling direction to austenite grain height in the normal direction of the sheet being between 1.3 and 5.0.
• the austenite grain length and the austenite grain height are determined on the former austenite grains by means of EBSD investigations.
• the minimum folding radius of the flat steel product for folding by 90° is five times the sheet thickness d W when the bending line is parallel and perpendicular to the rolling direction.
• the minimum bending radius is preferably four times the sheet thickness d W , also when the bending line is parallel and perpendicular to the rolling direction.
• the hot-rolled flat steel product according to the invention has good tempering resistance and is distinguished by high notched impact strength in the heat-affected zone of weld seams. In addition, it is excellently suited for edging and has good wear properties due to the high surface hardness.
• the combination of high strength and excellent formability combined with high notched impact strength achieved in a hot-rolled flat steel product according to the invention makes the flat steel product according to the invention particularly suitable for use in welded constructions for crane booms in telescopic crane construction, in vehicle construction, in infrastructure construction and for equipment used in surface or underground mining, such as supporting structures of shields and the like.
• a significant weight saving can also be achieved through the use of flat steel products according to the invention in the construction of commercial vehicles, such as semi-trailers, also known as “trailers”, for articulated lorries or trailers for trucks, in the manufacture of chassis parts and in the manufacture of vehicle wheels.
• a hot-rolled flat steel product according to the invention can be provided for further processing in the unpickled, pickled or blasted state.
• it can be coated with a metallic protective layer, with the zinc-based protective layers known from the prior art being particularly suitable for this purpose.
• Such Zn-based coatings can be applied in a practical manner to a flat steel product according to the invention, in particular by electrolytic galvanizing.
• Carbon (C) is primarily added to increase tensile strength and yield point. It exerts its effect on these two mechanical parameters through different mechanisms. On the one hand, up to a certain proportion of carbon can be present interstitially in dissolved form both in the body-centered cubic and in the face-centered cubic iron lattice and in this way cause an increase in strength. However, this mechanism is of secondary importance for the invention claimed here.
• the most important task of the carbon here is to enable a martensitic and/or bainitic structural transformation, which results in a significant increase in strength. The martensitic microstructural transformation is ensured by the greatly differing solubility of the carbon in the fcc and bcc lattice in conjunction with an increased cooling rate.
• the austenite-stabilizing effect of carbon ensures that the cooling rate required for martensite formation is reduced. In addition, however, it also causes a reduction in the martensite start temperature, so that lower temperatures have to be set for martensite formation. In order to bring about a defined increase in strength through martensite formation, a minimum carbon content is always required. Therefore, the minimum carbon content is set at 0.03% here. For safe compliance To ensure the required minimum values for yield point Re and tensile strength Rm even with unavoidable fluctuations in the process parameters during the manufacturing process, the minimum carbon content is preferably set at 0.04%. In order to achieve sufficiently high yield points and tensile strengths for the desired lightweight design potential, the minimum C content is particularly preferably set at 0.06%.
• C content is limited to a maximum of 0.3%.
• the C content is preferably set at a maximum of 0.25%. In order to ensure good toughness, a maximum C content of 0.20% is particularly preferably set.
• Manganese (Mn) fulfills three essential tasks. On the one hand, manganese forms a substitution mixed crystal with iron, which causes an increase in strength. Furthermore, manganese has an austenite-stabilizing effect and thus enables a martensitic transformation even at lower cooling rates. In addition, manganese has a high affinity for sulfur and binds it to form MnS. In this way, the formation of brittle phases such as FeS can be avoided. In order to ensure solid solution strengthening through manganese, a minimum manganese content of 0.1% is added. Furthermore, in order to promote the martensitic transformation, a manganese content of at least 0.5% is preferably adjusted.
• the manganese content is limited to a maximum of 2.5%, in order to avoid coarse plate martensite, the manganese content is preferably limited to a maximum of 1.7%. In order to additionally avoid segregation over the thickness of the material, the manganese content is particularly preferably limited to a maximum of 1.5%.
• the alloying element boron and of microalloying elements such as aluminum, niobium, titanium or vanadium—either individually or in combination with one another—is advantageous for adjusting the material properties of the present invention.
• the alloying element boron and of microalloying elements such as aluminum, niobium, titanium or vanadium—either individually or in combination with one another—is advantageous for adjusting the material properties of the present invention.
• the boron content is specified to be at least 0.0005%.
• a reliable improvement in the hardenability through free boron should also be guaranteed if the nitrogen is not completely bound. Therefore, the boron content is preferably set to be at least 0.0010%, more preferably at least 0.0015%. Since the strength-increasing effect initially increases with increasing boron content and falls again above a maximum, the boron content of the steel according to the invention is limited to a maximum of 0.007%. From a determine However, with the boron content, no significant improvement in hardenability is achieved, but toughness at the grain boundaries is reduced. Therefore, the boron content is preferably set to not more than 0.005%. In order to achieve the optimum properties in the flat steel product according to the invention, the boron content is limited to a maximum of 0.0035% in a particularly preferred variant.
• appropriate alloying measures for nitrogen binding e.g. B. by the targeted alloying of the elements Al, Ti, Nb and / or V, individually or in combination. Taking into account the practical conditions in economic steel production, nitrogen contents of up to 0.01% can be accepted as unavoidable contamination.
• N contents of at most 0.008%, particularly preferably at most 0.006%, are preferably set. Contents of less than 0.002% can technically only be avoided with great effort, which is why a content of at least 0.002% is tolerated for economic reasons.
• aluminum In steel production, aluminum usually fulfills the task of deoxidizing or "calming down" the molten steel.
• the high affinity of aluminum for oxygen is used here. By binding the oxygen to form Al 2 O 3 , the rising of oxygen bubbles ("boiling" of the melt) during steel production is avoided.
• An Al content of at least 0.01% is generally added to cool steel melts. In order to reduce austenite grain growth via fine AlN particles that are present in sufficient numbers, an Al content of at least 0.02% is preferably added. Excessive aluminum alloying leads to the formation of coarse Al 2 O 3 particles.
• the Al content is limited to a maximum of 0.3% in order to avoid problems when pouring the molten steel due to clogging of the dip tube with Al 2 O 3 particles.
• the maximum Al content is preferably limited to 0.2%.
• Ti titanium
• a titanium content of at least 0.002% can optionally be added.
• a Ti content of at least 0.005% is preferably alloyed in order to reliably avoid austenite grain growth through fine TiN particles that are present in sufficient numbers.
• a Ti content of at least 0.008% is preferably set. Since the formation temperature of titanium nitrides is significantly higher than that of boron nitrides, a titanium content of at least 0.015% can preferably be set in the case of a boron alloy for the purpose of binding nitrogen.
• a maximum Ti content of 0.2% can be added to the flat steel product according to the invention.
• the titanium content is preferably limited to a maximum of 0.04%, in particular to a maximum of 0.1%.
• the titanium content is particularly preferably limited to a maximum of 0.025%.
• the ratio %Ti/%N is limited to a maximum of 3.42.
• niobium Another optional alloying element is niobium (Nb).
• Nb niobium
• a niobium content of at least 0.002% is required for this.
• at least 0.005% Nb is preferably alloyed with the flat steel product according to the invention.
• the niobium content is preferably restricted to a maximum of 0.1%.
• the effects described are particularly effective with a preferred maximum niobium content of 0.05%.
• the binding of nitrogen is ensured by alloying in a combination of aluminum (Al) and niobium (Nb), optionally in combination with titanium.
• Al and Nb are also strong nitride and carbide or carbonitride formers.
• a combination of Al and Nb leaves a higher content of free nitrogen in the material than when only titanium (Ti) is alloyed with it, but this content is sufficiently low to avoid the formation of boron nitrides and thus to keep boron in solution.
• niobium content of at least 0.02% is required with a simultaneous addition of at least 0.09% Al. If the focus is only on binding nitrogen, the niobium content in this particularly preferred variant can be limited to a maximum of 0.03% and the aluminum content to a maximum of 0.15%.
• Chromium (Cr) can be included as an optional alloying element to increase strength and improve hardenability at levels of 0.05% to 2.5%.
• a minimum Cr content of 0.1% is preferably set. Cr also suppresses the formation of ferrite and pearlite during the cooling process, allowing full martensite and/or bainite formation even at slower cooling rates. This hardenability-increasing effect is particularly effective with a Cr content of at least 0.2%. Therefore, in the case of an optional alloying of Cr, a content of at least 0.2% is particularly preferably alloyed.
• the Cr content is preferably limited to a maximum of 1.2%.
• the Cr content is preferably set to 0.8% or less. To avoid coarse precipitation containing Cr, the Cr content is particularly preferably limited to a maximum of 0.5%.
• molybdenum (Mo) can be contained in the steel flat product according to the invention in amounts of 0.01% to 1% to increase strength and to improve hardenability.
• a minimum Mo content of 0.1% is preferably set.
• Mo effectively suppresses the formation of ferrite and pearlite during the cooling process and thus enables complete martensite and/or bainite formation even at lower cooling rates (hardenability increase).
• This hardenability-increasing effect is particularly effective with Mo contents of at least 0.20%. Therefore, in the case of an optional Mo alloy, a Mo content of at least 0.20% is particularly preferably added.
• the Mo content is preferably limited to a maximum of 0.5%, particularly preferably to a maximum of 0.3%.
• the sum of the proportions by weight of chromium (Cr) and molybdenum (Mo) is at most 1.2% by weight, preferably at most 0.85% by weight, particularly preferably at most 0.65% by weight.
• one element or more elements from the group “Si, Ni, Cu, V, Ca, Mg, REM, Be, Sb” can be present in the steel flat product according to the stipulations explained below.
• the flat steel product according to the invention can optionally have silicon (Si) in contents of 0.05% to 1.5%.
• Si forms a substitution mixed crystal with the iron lattice and thus causes a significant increase in strength.
• an Si content of at least 0.05% is alloyed.
• Si suppresses cementite formation from the austenite, leaving more carbon dissolved in the austenite, which in turn promotes the martensitic transformation.
• an Si content of at least 0.07% is preferably set.
• Si reduces the risk of undesired subsequent cementite formation in the martensite and thereby increases the resistance to an undesired reduction in strength in the heat-affected zone during welding and tempering.
• contents of at least 0.10% are particularly preferred.
• the Si content is limited to a maximum of 1.5%.
• high levels of silicon can lead to the formation of red scale. Rotzunder has an insulating effect on the surface of the material and can therefore significantly reduce the effect of the cooling water used to cool it down. This in turn has negative effects on the martensitic transformation.
• the content of Si if present at all in effective amounts, is preferably limited to a maximum of 0.6%.
• negative effects of the optional presence of Si can be avoided in a particularly reliable manner in that the optional Si content of the flat steel product according to the invention is particularly preferably limited to at most 0.35%.
• nickel (Ni) can be provided in the flat steel product according to the invention in contents of 0.05% to 10%.
• a minimum Ni content of 0.05% is required.
• a minimum Ni content of 0.15% is preferably added.
• Ni can also be alloyed to improve toughness. If this is desired, a minimum Ni content of 0.3% Ni is particularly preferred. A toughness-enhancing effect is effectively achieved up to a Ni content of 10%. Therefore, the Ni content is limited to a maximum of 10%.
• Ni is preferably limited to a maximum of 5%. Since the addition of Ni results in an increase in the carbon equivalent CEV and as a result weldability is adversely affected, the Ni content is preferably limited to a maximum of 1%, particularly preferably limited to a maximum of 0.5%, to ensure weldability.
• the flat steel product according to the invention can optionally contain copper (Cu) in a content of 0.01% to 1%. If the hardenability-increasing effect of Cu is to be exploited, a Cu content of at least 0.01% is added. A minimum Cu content of 0.03% is preferably set to ensure through-hardenability even with greater sheet thicknesses. In addition, Cu can improve weather resistance. In order to utilize this effect, a minimum Cu content of 0.1% is particularly preferred. In order to avoid a negative influence on the castability of the steel melt, the Cu content is limited to a maximum of 1%. In order to reduce a negative impact on toughness due to the formation of coarse Cu carbides, the Cu content is preferably limited to a maximum of 0.5%. In order to reliably avoid coarse Cu carbides, the Cu content is particularly preferably restricted to a maximum of 0.3%.
• Cu copper
• vanadium (V) can be added in contents of 0.002% to 0.2%.
• a minimum of 0.002% V must be added to increase yield and tensile strength through precipitation hardening due to the formation of vadium carbides and vanadium carbonitrides. If grain refinement is to be achieved at the same time, a minimum V content of 0.005% is preferably set.
• the V content is preferably limited to a maximum of 0.07%.
• the preferred V content is set at a maximum of 0.05%, particularly preferably at a maximum of 0.03%.
• vanadium carbides are formed as a result of tempering at temperatures above 180°C.
• the carbon bound in vanadium carbides is no longer available for sufficient strengthening of the martensite.
• the V content is limited to a maximum of 0.008%, which is particularly preferred compared to the V contents mentioned above.
• a flat steel product according to the invention can optionally contain 0.0005% to 0.005% Ca.
• contents of at least 0.001% are preferably added; for reasons of resource efficiency, the Ca content is preferably limited to a maximum of 0.004%.
• Ca is used as an optional alloying element
• a calcium to sulfur ratio (Ca/S, proportions in weight percent) of 0.5-2.5 should be set in a preferred embodiment.
• the Ca/S ratio should be at most 2.0.
• a flat steel product according to the invention can optionally contain 0.0005% to 0.005% Mg.
• metals from the group of rare earths REM—rare-earth metal
• cerium, lanthanum and neodymium is optionally possible.
• a content of at least 0.001% is optionally possible.
• the binding effect of the rare earths on sulfur, phosphorus and oxygen can reduce the segregation of these elements at grain boundaries, which makes it possible to increase toughness. At levels above 0.05% there is a risk of the formation of toughness-reducing precipitates. Therefore, the rare earth content is limited to a maximum of 0.05%.
• Beryllium (Be) can be used as an optional alloying element in levels from 0.001% up to 0.1% to increase wear resistance through the formation of high strength carbides and/or oxides.
• a Be content of at least 0.002% is preferably set. Since the toughness is greatly reduced if the content is too high, which is undesirable in the present case, the content is preferably limited to a maximum of 0.05%. Due to its toxicity, it is particularly preferred not to use Be as an optional alloying element.
• Antimony (Sb) can be added as an optional alloying element in contents of 0.001 to 0.3% in order to reduce the susceptibility to grain boundary oxidation and, when using higher contents, also to increase the corrosion resistance in acidic media by segregating at grain boundaries and there the tendency for hydrogen generation and thus for hydrogen-induced cracking is reduced or completely prevented.
• Sb content of at least 0.002%, particularly preferably at least 0.005%, is alloyed.
• the maximum content is preferably limited to 0.1%.
• the content is particularly preferably set at a maximum of 0.05%.
• Phosphorus (P) can be added as an optional alloying element to increase strength in amounts of 0.003% to 0.08%.
• the maximum P content is preferably set at 0.05%.
• the P content in the flat steel product according to the invention is preferably limited to at most 0.02%.
• Cobalt (Co) has a negative impact on hardenability and toughness. For technical reasons, however, traces of cobalt usually always remain in steel. Since the negative effects of cobalt generally only occur above 0.2%, its content is limited to a maximum of 0.2%.
• Tungsten (W) forms a Laves phase with molybdenum above certain levels. This can have a negative effect on the notched impact strength. For technical reasons, however, the tungsten content cannot usually be reduced to any desired extent, but in order to avoid negative influences it may not exceed 0.2% in accordance with the invention.
• Arsenic (As) and tin (Sn) can accumulate at grain boundaries at temperatures around 500 °C, causing embrittlement. In order to prevent these negative effects, the content of As and Sn in the steel flat product according to the invention should be limited to a maximum of 0.05%, preferably a maximum of 0.03%, in the usual way.
• sulfur (S) forms sulfides with iron or manganese (FeS or MnS). These have a negative impact on deformability and toughness. Therefore, the sulfur content is limited to at most 0.01%, preferably at most 0.008%, and more preferably at most 0.006%.
• the content in the flat steel product according to the invention is limited to a maximum of 0.001%, preferably to a maximum of 0.0005%, particularly preferably to a maximum of 0.0003%.
• Oxygen (O) combines in particular with aluminum to form oxides (Al 2 O 3 ). These reduce both toughness and fatigue strength. Therefore, the oxygen content is limited to a maximum of 0.03%, preferably a maximum of 0.02%, particularly preferably a maximum of 0.01%.
• Lead (Pb) is an unwanted by-element, so its content is limited to a maximum of 0.02%.
• All of the above-mentioned optional alloying elements can be present in small amounts as process-related unavoidable impurities, but are then not effective for the purposes of the present invention.
• the carbon equivalent CEV is at most 0.7, in particular at most 0.6.
• the specified maximum values for the carbon equivalent CEV result in better weldability.
• %C, %Mn, %Cu, %Ni, %Cr, %Mo and %V designate the respective content of this alloying element in percent by weight.
• the carbon equivalent CEV is limited to a maximum of 0.5 in a preferred variant.
• the carbon equivalent CET is at most 0.7, in particular at most 0.5, preferably at most 0.35.
• %C, %Mn, %Mo, %Cr, %Cu and %Ni indicate the respective content of this alloying element in percent by weight.
• the specified maximum values for the carbon equivalent CET result in better weldability.
• the flat steel product has cementite particles, the proportion of cementite particles with a diameter of 20 nm to 250 nm being greater than 98%.
• the mean value of the diameter of the cementite particles is in particular between 50 nm and 150 nm.
• the expression of the cementite particles is determined in a microstructure area characteristic of the material at approximately 1/3 of the sheet metal thickness.
• the majority (i.e. more than 50%) of Cr and Mo is built into the cementites and is not dissolved in the matrix.
• a hot-rolled flat steel product which has a high yield point R e without discontinuity in the stress-strain diagram, a high tensile strength R m and a high elongation at break A in combination with good bending ability.
• this preferred hot-rolled steel flat product excels characterized by a low yield strength ratio R e /R m which is advantageous for further processing.
• a flat steel product according to the invention is outstandingly suitable for stamping and mechanical cutting.
• Thermal cutting processes such as laser or plasma cutting can also be used without any problems when processing flat steel products according to the invention.
• 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 process engineering production of the flat steel product according to the invention takes place via hot rolling with direct hardening in the cooling section of the hot strip mill and subsequent tempering of the hot strip as a coil in a bell annealer, optionally under H2 or N2 protective atmosphere or a protective atmosphere consisting of a mixture of H2 and N2.
• a molten steel is produced with the above analysis, in order to set or shape specific properties of the hot-rolled flat steel product to be produced according to the invention.
• This melt is then cast in a conventional manner to form a preliminary product with a thickness dV .
• This preliminary product is typically a slab. Casting into thin slabs, cast strips or blocks is also possible.
• the pre-product produced in this way has a thickness d V between 2.5 mm and 350 mm.
• the preliminary product is heated to the austenitization temperature T WE , also referred to as the reheating temperature, with the analysis according to the invention.
• the reheating temperature of the steels of the present invention should be between 1100°C and 1350°C. However, the reheating temperature is preferably at least 1220° C. in order to reduce hardening in the subsequent rolling process due to the reheating temperature being too low and preferably at most 1320° C. in order to avoid melting of the slab surface and excessive austenite coarsening and to enable economical production. A homogeneous initial structure is also established in this temperature range. In addition, precipitates from the specifically alloyed micro-alloying elements 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 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.
• the pass reduction above T NR in the individual passes is at least 6% in each case in order to introduce sufficient deformation so that grain refinement is achieved by recrystallization.
• the pass reduction above T NR in the individual passes should be at least 8%. This ensures sufficient notched impact strength in the flat steel product according to the invention even at low tempering temperatures and with low alloy contents of Cr, Mo and Ni and thus low CEV and CET compared to classic analyzes for water tempering. During hot rolling above this temperature T NR , the austenite in the structure of the steel flat product recrystallizes completely.
• This minimum number n W 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 T NR .
• 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 flat steel product emerges from the last rolling pass.
• the cooling rate for this first cooling to a cooling stop temperature is 20-400°C/s, preferably at least 40°C/s, particularly preferably at least 60°C/s.
• 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 cooling stop temperature T KS is preferably at least 250° C. lower than the hot rolling end temperature T E , with cooling stop temperatures T KS of at most 550° C., in particular 500° C., being practical, provided they are not above T E ⁇ 250° C.
• microstructure of the flat steel product according to the invention and, associated therewith, its yield point R e and its other mechanical-technological properties explained above are adjusted via the selection of the cooling stop temperature T KS .
• the flat steel product has a microstructure that comprises more than 50% by area of bainite, the remainder being martensite, ferrite and retained austenite.
• the proportion of bainite in the structure can be determined by setting a cooling stop temperature T KS .
• T KS cooling stop temperature
• a bainite proportion of 100% by area, ie a completely bainitic structure can be achieved, for example, by choosing a cooling stop temperature T KS that is about 120 °C below the bainite start temperature BS (T KS ⁇ B S - 120 °C).
• the other microstructure components are martensite, up to 10% by area ferrite, preferably up to 5% by area ferrite and up to 5% by volume residual austenite, with the proportions of ferrite, martensite and residual austenite at correspondingly high proportions of the other Structural component can also be "0".
• T KS cooling stop temperatures
• the flat steel product has a microstructure that comprises more than 50 FI% martensite, the remainder being bainite, ferrite and retained austenite.
• the proportion of martensite can be adjusted by selecting the cooling stop temperature T KS .
• a cooling stop temperature T KS is required which is about 380 °C below the martensite start temperature MS ( T KS ⁇ MS - 380 °C).
• the other microstructure components are bainite, up to 10% by area ferrite, preferably up to 5% by area ferrite and up to 5% by area retained austenite, with the proportions of ferrite, bainite and retained austenite at correspondingly high proportions of the other Structural component can also be "0".
• the hot-rolled flat steel product which has been cooled to the cooling stop temperature, is coiled into a coil.
• the coil obtained is cooled a second time to room temperature.
• Room temperature is understood to mean a temperature in the range of 20°C - 50°C.
• the second cooling takes place slowly with a cooling rate which is at most 0.1 K/s, preferably at most 0.05 K/s. This avoids strong internal stresses after cooling in the coil.
• the coil obtained is tempered as a coil to set the mechanical-technological properties.
• This tempering process preferably takes place in a hood annealer.
• Pure hydrogen is preferably used as the protective gas because of the excellent heat transfer and the resulting improved temperature control during the tempering process.
• nitrogen or a mixture of hydrogen and nitrogen can also be used as protective gas.
• the annealing temperature T G during tempering must be selected depending on the analysis used and the mechanical properties to be set depending on the holding time.
• the annealing temperature T G is at least 170°C, preferably at least 200°C and at most 600°C, preferably at most 450°C.
• the steel flat product according to the invention must not be over-tempered in order to set the special microstructure with a large number of extremely fine cementite precipitations.
• the Hollomon-Jaffe parameter is preferably limited to a maximum of 20, in particular a maximum of 15, preferably a maximum of 13.3, particularly preferably a maximum of 12.1.
• Tempering of the flat steel product coiled into the coil is preferably followed by a third cooling to room temperature, the cooling rate being at least 10 K/h, preferably at least 20 K/h.
• the cooling rate can be up to 500 K/h.
• a coil manufactured in this way is then typically processed on a cut-to-length line to form flat strips.
• the special feature of this invention lies in the method described with tempering of the coil before further processing into strip sheets on a cut-to-length line.
• the yield point and tensile strength level can be adjusted and the scattering of the mechanical properties over the length of the strip can be reduced by tempering.
• the tempering step results in a pronounced yield point R eH instead of the yield point R p0.2 in the non-tempered state.
• This pronounced yield point is disadvantageous for bending and edging processes, as already explained.
• the yield point ratio R e /R m increases to values > 0.95 as a result of tempering and is therefore at an unfavorably high level for processing by the end customer and for the necessary production reliability and component safety.
• the processing of the coil into sheet metal takes place only after tempering.
• the coil is unwound and straightened and then divided into sheets.
• a plastic deformation takes place, whereby the pronounced yield point R eH , which was formed during the tempering process, is eliminated again.
• the yield strength ratio R e /R m is reduced to ⁇ 0.95, which ensures problem-free processing by the end customer.
• the slabs or blocks cast from melts A - D and F - J have each been reheated to an austenitizing temperature T WE , with which they entered a conventional reversing stand and then a conventional rolling mill to form a hot-rolling final temperature T ET to a To be hot-rolled steel strip with a thickness d W between 4 and 8 mm.
• T WE austenitizing temperature
• a strip having a thickness of 3 mm was cast from melt E, in order to then be hot-rolled to a thickness of 1.5 mm.
• Tests with different thicknesses d V (and d W ) resulted in similar properties and are therefore not shown in detail here.
• the flat steel products were first rolled over a minimum number n W of rolling passes at a temperature that was above the temperature T NR .
• the number n W was determined in the manner explained above from the thickness d V of the slabs and the final thickness d W of the respective hot-rolled flat steel product in the tests.
• the respective steel flat product was hot-rolled in at least one further rolling pass at a temperature below the temperature T NR .
• the hot-rolled steel strips obtained by hot-rolling were accelerated at a cooling rate ⁇ Q to a cooling stop temperature T KS .
• T KS After reaching the respective cooling stop temperature T KS , the steel strips were slowly cooled to room temperature at a cooling rate ⁇ Q '.
• Table 4 shows the steel (A - J) from which the steel flat product processed in the respective test was made, the width B C and the outer diameter D C and inner diameter d C of the coil produced, the selected annealing temperature T G in °C, the required minimum annealing time t G,min and the actual annealing time t G , the annealing temperature T G ' in K, the third cooling rate ⁇ QHG after annealing in the hood annealer and the Hollomon-Jaffe parameter H P .
• M is the structural proportion of tempered martensite
• B is the structural proportion of bainite
• F + P is the structural proportion of ferrite and pearlite
• RA is the structural proportion of retained austenite
• D Z the mean value of the diameter of the individual cementite particles and the proportion A Z in area % of cementite particles with a diameter between 20 nm and 250 nm based on the total proportion of cementite particles.
• the yield strength R e the tensile strength R m , the elongation A 5
• DIN EN ISO 148-1 the notched impact strength AV -20°C at a test temperature of -20 °C, A V -40°C at a test temperature of -40 °C and A V -40°C at a test temperature of -60 °C.
• the hardness Brinell HBW 5/750 was determined according to DIN EN ISO 6506-1. The results of these tests are summarized in Table 6. No pronounced yield point was found in any of the examples according to the invention, so that the yield point R p02 for Re is given in Table 6.

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