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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
• • 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
  • C21D8/0242—Flattening; Dressing; Flexing
• • 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
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • 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
  • C22C38/26—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
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • 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
  • C21D2211/001—Austenite
• • 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 steel with good forming and surface properties and its manufacturing process, as well as a component made of the steel.
• the flat steel products described in the invention are typically rolled products, such as steel strips or sheets, as well as blanks and plates made therefrom.
• uncoated steel flat product is to be understood as meaning that there is no corrosion coating, for example through hot-dip coating or electrolytically produced coatings, on the surface of the flat steel product.
• data on the contents of the various microstructure components refer in each case to the area of a microsection of a sample of the respective product (specified in area percentage "area %"), unless expressly stated otherwise.
• the microstructure is determined on cross-sections subjected to etching with 3% Nital (alcoholic nitric acid).
• the microstructure is determined using a scanning electron microscope at 5000x magnification to determine the proportion of plate-like and other non-plate-like bainite, and at 20,000x to 50,000x magnification to determine the plate length, width, and plate spacing.
• the proportion of retained austenite is determined by X-ray diffraction (XRD) according to ASTM E975.
• All strip temperatures in the process can be determined, for example, using a commercially available pyrometer.
• High-strength steels with good forming properties are known from the state of the art for applications in, for example, automotive engineering.
• High-strength steels are characterized by a high proportion of alloying elements, which contribute to increased strength. At the same time, the steel must exhibit good elongation properties.
• From the EP 2 710 158 A1 is an ultra-high-strength, cold-rolled steel sheet which consists in mass% 0.10 - 0.50% C, 0.01 - 2.5% Si, 1.0 - 3.5% Mn, not more than 2.5% Al, not more than 0.020% P, not more than 0.005% S, and not more than 0.02% N, optionally further one or more elements from 0.01 - 0.50% Cr, 0.01 - 0.5% Mo, 0.01 - 0.1% V, 0.001 - 0.15% Ti, 0.02 - 0.05% Nb, where the sum of V, Ti, Nb ⁇ 0.2%, 0.0005 - 0.005% B, not more than 0.01% Ca and a structure of less than 5% ferrite, less than 5% bainite, 5-70% untempered martensite, 5-30% retained austenite, and 25-80% tempered martensite, with at least 99% of the iron carbides contained in the tempered martensite having a size of less than 500 nm.
• High-strength steels are often electrolytically coated or hot-dip coated, e.g. in the EP 2 258 886 B1
• the object of the present invention is to provide a high-strength flat steel product with good elongation properties and good surface properties, as well as a manufacturing process for it. Furthermore, it may be necessary for the flat steel product with the good surface properties to also have optimized forming properties.
• the manufacturing process includes at least the following steps: a. Providing a cold-rolled flat steel product comprising a steel consisting of the following elements: C: 0.10 - 0.5%; Mn: 1.0 - 3.0%; Si: 0.9-1.7%; P: ⁇ 0.020%; S: ⁇ 0.005%; N: ⁇ 0.008% and optionally one or more of the following elements AI: 0.01 - 1.5%; Cr: 0.05 - 1%; Mon: 0.05 - 0.2%; B: 0.0004 - 0.002%; Cu: 0.05 - 0.2%; 0.005% ⁇ Ti+Nb+V ⁇ 0.2%; and the remainder consists of iron and unavoidable elements. b. Heating the cold-rolled flat steel product to a furnace inlet temperature.
• the method for producing a high-strength, uncoated flat steel product comprises no further working steps and the method consists of steps a to j.
• no further temperature changes occur between two consecutive steps; this particularly preferably applies to all pairs of consecutive steps. This means, for example, that after step e) (cooling to T 5 ), T 6 is set and maintained directly.
• the provided cold-rolled flat steel product is manufactured in a conventional manner.
• This conventional method includes casting the steel into a slab, reheating the slabs, hot rolling, coiling the hot strip, pickling the hot strip, and cold rolling the hot strip.
• the following explanations regarding the composition apply to the inventive composition of the slab and the inventive flat steel product and their optional variations.
• Carbon "C” is present in the steel according to the invention in amounts of 0.10% to 0.5%. Carbon supports the formation and stabilization of austenite in the steel according to the invention. Stabilization occurs particularly during quenching and the subsequent annealing treatment. Furthermore, the addition of C imparts high strength to the steel, as the strength of the martensite formed during the process is increased. Therefore, the C content should be at least 0.10%, preferably 0.12%, particularly preferably 0.15%. On the other hand, the martensite initiation temperature shifts to increasingly lower temperatures with increasing C content, so that possibly no or only an insufficient proportion of low-temperature phases can be formed. For this reason, the C content in the steel according to the invention should be a maximum of 0.5%, preferably 0.4%, particularly preferably 0.5%.
• Silicon is required to achieve the special microstructure in this invention because it delays cementite formation.
• An excessively high cementite content would result in the carbon being bound in carbides, making it unavailable to stabilize the residual austenite during the process, and elongation would deteriorate. Therefore, silicon must be present in the steel according to the invention at a level of at least 0.9%, preferably at least 1.05%, particularly preferably at least 1.10%.
• an excessively high silicon content leads to poor surface quality, so the steel according to the invention should contain a maximum of 1.7%, particularly preferably a maximum of 1.5%.
• the steel according to the invention comprises C ⁇ 0.16% and Si ⁇ 1.2%, preferably C ⁇ 0.15% and Si ⁇ 1.2%, particularly preferably C ⁇ 0.15% and Si ⁇ 1.1%.
• it can particularly preferably have C ⁇ 0.12%, particularly preferably C ⁇ 0.15%, and preferably Si ⁇ 0.9%, particularly preferably Si ⁇ 1.05%.
• the steel according to the invention has C > 0.16% and Si > 1.2%, preferably C > 0.16% and Si ⁇ 1.25%, particularly preferably C ⁇ 0.18% and Si ⁇ 1.3%, especially preferably C ⁇ 0.20% and Si ⁇ 1.4%.
• it can particularly preferably have C ⁇ 0.5%, particularly preferably C ⁇ 0.5% and preferably Si ⁇ 1.7%, particularly preferably Si ⁇ 1.5%.
• the steel according to the invention contains manganese (Mn). At a content of 1.0% or more, Mn enables martensite formation by suppressing pearlite formation. A content of at least 1.2% has proven advantageous, and a content of at least 1.5% is particularly advantageous. However, an excessively high Mn content can lead to severe segregation, which is why the Mn content is limited to 3.0%. Furthermore, a high Mn content severely limits weldability and reduces corrosion resistance. Therefore, a maximum Mn content of 2.5% and, in particular, 2.3% has proven particularly advantageous.
• P phosphorus
• S can lead to the formation of Mn sulfides, which severely impair formability properties. Therefore, in the steel according to the invention, the content is limited to 0.005%, although a restriction to 0.004% and especially to 0.005% may be advantageous. Sulfur contamination cannot be completely avoided during steelmaking.
• Nitrogen "N” can lead to the formation of coarse nitrides at levels above 0.010%, resulting in impaired formability. To avoid these nitrides, a maximum content of 0.008% has proven particularly advantageous. Nitrogen contamination cannot be completely avoided during steelmaking.
• unavoidable impurities In addition to the impurities P, S, and N discussed above, other elements may also be present as impurities in steel. These additional elements are summarized under the term “unavoidable impurities.”
• the total content of these "unavoidable impurities” is preferably a maximum of 0.2%, preferably a maximum of 0.1%.
• the optional alloying elements "Al, Cr, Mo, B, Cu, Ti, Nb" described above, for which a lower limit is specified, may also occur in the steel substrate as unavoidable impurities in contents below the respective lower limit. In this case, they are also counted as "unavoidable impurities," whose total content is limited to a maximum of 0.2%, preferably a maximum of 0.1%.
• Aluminum can be added to the steel according to the invention for deoxidation and to bind any nitrogen that may be present.
• Aluminum can also be used to increase the residual austenite content.
• a higher residual austenite content results from the addition of aluminum by delaying the formation of cementite precipitates.
• an aluminum content of at least 0.01%, preferably 0.05% has proven advantageous in the flat steel product according to the invention.
• an excessively high aluminum content can lead to the formation of coarse aluminum nitrides, which have an embrittling effect and thus to poorer formability.
• higher aluminum contents can lead to poorer casting behavior, as aluminum compounds can lead to clogging. Therefore, the present invention provides for a limitation of the aluminum content to 1.5%, preferably 0.8%, particularly preferably 0.4%.
• Chromium (Cr) is an effective pearlite inhibitor and contributes to strength.
• a chromium content of at least 0.10% has proven particularly advantageous.
• chromium can lead to grain boundary oxidation through the formation of Cr oxides. Therefore, the chromium content is limited to 1.0%, preferably 0.9%.
• Molybdenum (Mo) also forms fine, strength-enhancing carbon nitrides in small amounts. Therefore, an addition of at least 0.05% has proven beneficial. However, the strength-enhancing effect of carbon nitrides is exhausted as soon as the molybdenum content becomes too high. Furthermore, high molybdenum contents can impair cold formability and weldability. A maximum content of 0.2%, preferably 0.10%, and particularly preferably 0.07%, has proven advantageous in this case.
• boron "B” leads to a fine-grained microstructure, as boron segregates at the phase boundaries and blocks their movement.
• at least 0.0004%, particularly preferably at least 0.0005% can be added to the steel according to the invention.
• the effect of B is saturated at a maximum content of 0.002%.
• the addition of copper (“Cu”) to the flat steel product according to the invention can form very fine, strength-enhancing Cu precipitates. Therefore, an addition of at least 0.05%, preferably 0.10%, can be advantageous in the present invention. However, the copper content should be limited to 0.2%, as otherwise, so-called red brittleness, i.e., cracks in the slab, can occur during the hot rolling process.
• microalloying elements (preferably Ti and/or Nb and/or V) can be added to the steel according to the invention.
• MLEs microalloying elements
• boron is not considered a microalloying element.
• the cold-rolled flat steel product is heated to a furnace inlet temperature.
• the furnace inlet temperature is the temperature at the center of the sheet when entering the furnace.
• the furnace inlet temperature is preferably at least 10 °C, more preferably 15 °C.
• the furnace inlet temperature should preferably not exceed 100 °C, more preferably 50 °C, and especially preferably 35 °C.
• the flat steel product according to the invention is heated from T 0 to a temperature T 1 in an atmosphere A 1.
• the temperature T 1 is at least 650 °C, preferably 670 °C.
• the temperature T 1 is a maximum of 750 °C, preferably 750 °C, since recrystallization processes begin above this temperature and the process conditions must be adjusted according to step d).
• the atmosphere A 1 set according to the invention is reducing. It preferably comprises at least 2% hydrogen "H 2 ", preferably 3% H 2 , particularly preferably 5% H 2 .
• H 2 hydrogen
• the H2 content should be limited to a maximum of 20%, preferably 10%.
• up to 0.5% oxygen " O2 ", particularly traces of O2 , and up to 0.5% water “ H2O " can be added to the atmosphere. Both proportions must be limited to minimize the selective oxidation of base alloying elements.
• the remainder of the preferred atmosphere is nitrogen " N2 ", preferably 80%, particularly preferably 85%, especially preferably 90%.
• the atmosphere consists of the described proportions of H2 , O2 , H2O , and N2 .
• the dew point of the T P1 is at least -60°C, preferably -55°C, particularly preferably -45°C. Furthermore, the dew point should not exceed -5 °C, preferably -10 °C, and especially preferably -15 °C, as otherwise selective oxidation of base alloying elements may occur. This would lead to undesirable surface defects on the final steel surface.
• the heating rate ⁇ 1 in step c) between 500 °C and T 1 in step c) is at least 2 °C/s, preferably 4 °C/s, and a maximum of 50 °C/s, preferably 10 °C/s.
• the heating rate of at least 2 °C/s can delay the selective oxidation of the base alloying elements until T 1 is reached and further minimize it.
• step d) the flat steel product is heated from a temperature T 1 to a temperature T 4 and soaked at a temperature T 4 in a reducing atmosphere A 4 .
• the flat steel product is heated and soaked for at least t 4 ⁇ 5 s, preferably 10 s, particularly preferably 15 s.
• the minimum value of t 4 results from the fact that residual oxides on the starting material surface are not sufficiently reduced back to metallic Fe at an exposure time of ⁇ 5 s compared to the reduction conditions according to the invention.
• the time should be limited to t4 ⁇ 300 s, preferably 180 s, as otherwise coarsening of the austenite grain will occur, which will negatively affect the mechanical properties.
• the oxide layer has a thickness of less than 100 nm; particularly preferably, the oxide layer formed in step c), in particular the FeO layer, is completely reduced to metallic iron.
• the preferably set atmosphere A 4 comprises at least 2% hydrogen "H 2 ", preferably 3% H 2 , particularly preferably 5% H 2 .
• a 1 A 4 .
• the set hydrogen content can ensure that the atmosphere is reducing, particularly with regard to iron.
• the H 2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons.
• up to 0.5% oxygen “O 2 ", in particular traces of O 2 , and up to 0.5% water “H 2 O” can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements.
• the remainder of the preferred atmosphere is added to nitrogen "N 2 ", preferably 80%, particularly preferably 85%, especially preferably 90%.
• the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O and N 2 .
• the dew point of the T P2 is at least -60 °C, preferably -40 °C, particularly preferably -35 °C. Furthermore, the dew point should not be greater than 0 °C, preferably -10 °C, particularly preferably -16 °C, since otherwise unwanted oxide formation may occur.
• the heating of the flat steel product in step d) can be carried out by keeping it at a constant temperature T 4. Under a constant Temperature fluctuations of a maximum of ⁇ 5 °C are to be understood, as a more precise setting is not possible due to process technology. This particular design is particularly suitable when a relatively low T4 temperature has been set for analytical purposes.
• the heating rate should not exceed 10 °C/s, preferably 8 °C/s, particularly preferably 4 °C/s.
• the heating rate should be at least 0.5 °C/s, preferably 1.0 °C/s, particularly preferably 2.5 °C/s.
• the preferably set atmosphere A 2 comprises at least 2% hydrogen "H 2 ", preferably 3% H 2 , particularly preferably 5% H 2 .
• the set hydrogen content ensures that the atmosphere is reducing, particularly with respect to iron. This prevents uncontrolled oxidation and allows a thin oxide layer, in particular less than 100 nm, to be achieved.
• the H 2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons.
• up to 0.5% oxygen "O 2 ", in particular traces of O 2 , and up to 0.5% water “H 2 O” can be added to the atmosphere. Both proportions must be limited to minimize the selective oxidation of base alloy elements.
• the remainder of the preferred atmosphere is nitrogen "N 2 ", preferably 80%, particularly preferably 85%, especially preferably 90%.
• the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 .
• the dew point of the T P2 of the atmosphere A 2 is at least -60 °C, preferably -30 °C. Furthermore, the dew point should not be greater than -5 °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements may occur.
• the temperature T 5 is at most (T MS + 40 °C), preferably (T MS + 20 °C), particularly preferably T MS , to ensure a sufficient martensite content or sufficient nucleation for bainite in the final structure.
• the temperature T 5 should be at least (T MS - 175 °C). In this step, the so-called primary martensite is formed.
• the temperature T 5 must be ⁇ 550 °C to avoid selective re-oxidation on the steel surface.
• the cooling rate ⁇ 5 should be at least 10 °C/s, particularly preferably 20 °C/s.
• the cooling rate ⁇ 5 should be limited to a maximum of 100 °C/s, preferably 50 °C/s, particularly preferably 30 °C/s.
• the minimum value of u 5 results from the fact that if the cooling rate is too low, an unwanted ferritic and/or bainitic transformation cannot be ruled out.
• the maximum value of u 5 is limited by the fact that there is an excessively high risk of unwanted (selective) re-oxidation of the steel surface.
• the preferably set atmosphere A5 in step e) comprises at least 2% hydrogen " H2 ", preferably 3% H2 , particularly preferably 5% H2 .
• the set hydrogen content ensures that the atmosphere is reducing, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface.
• the H2 content should preferably be limited to a maximum of 80%, preferably 50%, for economic reasons.
• up to 0.5% oxygen " O2 ", in particular traces of O2 , and up to 0.5% water “ H2O " can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements.
• the remainder of the preferred atmosphere is nitrogen " N2 ", preferably 80%, particularly preferably 85%, especially preferably 90%.
• the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 .
• the dew point of the T P5 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not be greater than 0 °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements may occur.
• a 4 is not equal to A 5 , as this prevents uncontrolled entrainment of hydrogen from atmosphere A 4 into atmosphere A 5 .
• This allows the hydrogen content in atmosphere A 5 to be precisely adjusted to the required amount to prevent selective oxidation, and there is no excess hydrogen present that could undesirably diffuse into the steel and lead to hydrogen embrittlement. This can be achieved by structural separation, particularly a lock system.
• the minimum value for T 6 is (T MS - 175 °C), preferably (T MS - 150 °C).
• the maximum value is T MS , preferably (T MS - 75 °C).
• T 6 T 5 .
• T P6 T P5 .
• the flat steel product should be held at T 6 for at least 1 s, preferably at least 4 s, particularly preferably at least 9 s, since this achieves a homogeneous temperature distribution in the material according to the invention, which ensures the formation of a particularly fine and uniform microstructure of primary martensite and residual austenite across the cross-section of the flat steel product.
• the holding time t 6 is limited to 60 s for economic reasons. In a particular embodiment, for flat steel product thicknesses ⁇ 1.0 mm, the holding time is 10 s - 60 s.
• the preferably set atmosphere A6 in step f) comprises at least 2% hydrogen " H2 ", preferably 3% H2 , particularly preferably 5% H2 .
• the set hydrogen content ensures that the atmosphere is reducing, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface.
• the H2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons.
• up to 0.5% oxygen " O2 ", in particular traces of O2 , and up to 0.5% water “ H2O " can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements.
• the remainder of the preferred atmosphere is added to nitrogen " N2 ", preferably 80%, particularly preferably 85%, especially preferably 90%.
• the atmosphere consists of the described proportions of H2 , O2 , H2O and N2 .
• the dew point of T P6 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not exceed (-5) °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements may occur.
• a 6 is not equal to A 5 , as this prevents uncontrolled entrainment of hydrogen from atmosphere A 4 into atmosphere A 5 .
• This allows the hydrogen content in atmosphere A 5 to be precisely adjusted to the required amount to prevent selective oxidation, and there is no excess hydrogen present that could undesirably diffuse into the steel and lead to hydrogen embrittlement. This can be achieved by structural separation, particularly a lock system.
• the temperature T 7 is at most 510 °C, particularly preferably 500 °C, since otherwise an undesirable decrease in the strength of the flat steel product occurs.
• T 7 can be ⁇ 400 °C, preferably 350 °C, since this allows the desired strength and elongation to be better achieved.
• the temperature T 7 should be greater than T MS , preferably greater than T MS +50 °C, in order to enrich the residual austenite in the base material structure with C from the supersaturated primary martensite or bainite.
• the heating rate ⁇ 7 should be at least 2 °C/s, particularly preferably 4 °C/s, as otherwise unwanted carbides may form, which bind the carbon and are not available for enrichment in the retained austenite.
• the cooling rate ⁇ 7 should be limited to a maximum of 100 °C/s, preferably 50 °C/s.
• the preferably adjusted atmosphere A7 in step g) comprises at least 2% hydrogen " H2 ", preferably 3% H2 , particularly preferably 5% H2 .
• the adjusted hydrogen content ensures that the atmosphere is reducing, particularly with regard to iron. This prevents uncontrolled oxidation and re-oxidation on the surface.
• the H2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons.
• up to 0.5% oxygen " O2 ", in particular traces of O2 , and up to 0.5% water “ H2O " can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloy elements.
• the remainder of the preferred atmosphere is nitrogen "N 2 ", preferably 80%, particularly preferably 85%, especially preferably 90%.
• the atmosphere consists of the described proportions of H 2 , O 2 , H 2 O, and N 2 .
• the dew point of the T P7 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not be greater than 0 °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements can occur.
• a 7 is equal to A 6 .
• the temperature T 8 is a maximum of 510 °C, particularly preferably 500 °C, since otherwise an undesirable decrease in the strength of the flat steel product would occur.
• T 8 can be ⁇ 400 °C, preferably 350 °C, since this allows the desired strength and elongation to be better achieved.
• t 8 can particularly preferably be set to > 400 s to provide sufficient energy to enrich the retained austenite with C from the supersaturated primary martensite and bainite.
• the temperature T 8 should be greater than T MS , preferably greater than T MS +50 °C, in order to enrich the residual austenite in the base material structure with C from the supersaturated primary martensite and bainite.
• T 7 T 8 .
• the flat steel product should be held at T 8 for at least 10 s, preferably at least 15 s, and particularly preferably at least 20 s, since otherwise there is insufficient diffusion time for C to accumulate in the residual austenite.
• the holding time t 8 is limited to 120 s, preferably 100 s, since otherwise an undesirably high carbide content would form in the basic structure.
• the preferred atmosphere A 8 in step h) comprises at least 2% hydrogen "H 2 ", preferably 3% H 2 , particularly preferably 5% H 2 . Due to the adjusted hydrogen content It can be ensured that the atmosphere is reducing, particularly with regard to iron. This avoids uncontrolled oxidation and re-oxidation on the surface can be avoided.
• the H2 content should preferably be limited to a maximum of 20%, preferably 10%, for economic reasons.
• up to 0.5% oxygen " O2 ", in particular traces of O2 , and up to 0.5% water “ H2O " can be added to the atmosphere. Both proportions must be limited in order to minimize the selective oxidation of base alloying elements.
• the remainder of the preferred atmosphere is added to nitrogen " N2 ", preferably 80%, particularly preferably 85%, especially preferably 90%.
• the atmosphere consists of the described proportions of H2 , O2 , H2O and N2 .
• the dew point of the T P8 is at least -60 °C, preferably -40 °C. Furthermore, the dew point should not be greater than (-5) °C, preferably -10 °C, particularly preferably -15 °C, since otherwise selective oxidation of base alloy elements can occur.
• a 8 A 7
• step i) the flat steel product is cooled at a cooling rate of ⁇ 10 > 5 °C/s to a temperature T 10 , where T 10 ⁇ 60 °C.
• T 10 ⁇ 60 °C.
• the minimum value of u 10 is determined for technical and economic reasons to avoid making the required cooling section unnecessarily long. If T 10 > 60 °C, preferably T 10 > 40 °C, this can lead to surface defects during the subsequent skin-passing process.
• the skin-passing can be carried out with at least two forming passes. Skin-passing serves to improve flatness, fine-tune the mechanical properties by ultimately increasing strength, and to imprint a defined fine surface structure into the surface via the skin-pass roll structure.
• the skin-passing according to the invention achieves the desired roughness and peak count of the surface.
• the minimum value of D is determined by the fact that with a skin pass degree of ⁇ 0.1%, preferably 0.2%, insufficient rolling force is applied to optimize the flatness and meet the inventive minimum requirements for R a and R pc .
• the maximum value of D is limited by the fact that the product properties cannot be further improved by a D degree > 0.8%, preferably 0.5%, but the technical effort increases disproportionately due to the necessary rolling force or multi-pass re-skin pass.
• skin pass is preferably carried out in-line, i.e. in a continuous process in the same plant together with the upstream annealing treatment.
• skin pass can also be carried out in a subsequent process on a stand-alone skin pass stand or in a combination of in-line and offline skin pass, particularly if more than one forming pass is necessary to achieve the inventive limits of D, R a and R pc .
• the applied surface texturing can be based on a deterministic or stochastic fine structure.
• a preferred embodiment is to apply stochastic surface texturing during skin-passing in order to optimize the friction behavior between the steel surface and the tool during forming into the component in the oiled or greased state.
• a stochastic surface structure offers the advantage that, at high compressive loads, the lubricant can flow out of the stress zone via microchannels that open up between the peaks and valleys of the surface texture. This allows a more even distribution of the lubricant over the entire surface where contact occurs between the tool and the flat steel product during the forming process. Furthermore, a stochastic basic structure ensures flow and adhesion properties for organic or metallic coatings, which can be additionally applied to the flat steel product according to the invention if required.
• Skin-passing can be performed either wet (under water or oil) or dry (without liquid media). Dry skin-passing offers the advantage that no liquid residues can be carried over, which later lead to surface defects in the form of corrosion. Nevertheless, if necessary, dry skin-passing can be supported by applying a small amount of a rapidly evaporating forming aid, especially if a high degree of skin-passing is to be achieved.
• the microhardness is determined according to DIN EN ISO 6507.
• interactions between the various metallic and oxide components of the steel surface and the furnace rolls can occur. This can lead to growths on the furnace rolls, which in turn can cause surface defects in the steel strip.
• the uncoated flat steel product can be provided with an additional coating or conversion layer, in a particular embodiment chromating or phosphating.
• the flat steel product can be coated with a metallic zinc-based corrosion protection layer.
• the coating can be applied using a PVD process or by electrolytic deposition.
• This coating is preferably applied by electrolytic deposition with a zinc layer thickness d of ⁇ 2 - ⁇ 10 ⁇ m.
• the minimum value of d is based on the fact that at a layer thickness of ⁇ 2 ⁇ m, the desired cathodic corrosion protection cannot be adequately ensured. At d > 10 ⁇ m, however, the forming and welding properties can be negatively affected.
• a roughness R a of both ⁇ 0.5 ⁇ m and > 1.8 ⁇ m should be avoided, as such values can lead to adverse friction behavior during subsequent forming into the component.
• R PC should not be ⁇ 40 cm -1 to ensure sufficiently good optical properties even after painting.
• the tensile strength R m is ⁇ 1000 MPa, preferably R m ⁇ 980 MPa.
• the steel according to the invention can particularly preferably have C ⁇ 0.12%, particularly preferably C ⁇ 0.15% and preferably Si ⁇ 0.9%, particularly preferably Si ⁇ 1.05%.
• the steel according to the invention can particularly preferably have C ⁇ 0.5%, particularly preferably C ⁇ 0.5% and preferably Si ⁇ 1.7%, particularly preferably Si ⁇ 1.5%.
• the flat steel product has a bending angle > 80% and a hole expansion > 25%.
• the flat steel product according to the invention has a structure consisting of ⁇ 80% bainite and/or martensite, of which at least 75% of the martensite is tempered, ⁇ 5% residual austenite and ⁇ 10% ferrite.
• the present microstructure consists of 80% bainite and/or martensite.
• the bainite is preferably bainitic ferrite. 75%, preferably 80%, particularly preferably 90% of the martensite is tempered during the process according to the invention.
• a maximum of 25%, particularly preferably 20%, particularly preferably 10% of the martensite in the microstructure according to the invention is untempered.
• the microstructure of a flat steel product according to the invention contains at least 5% residual austenite.
• Residual austenite has a positive effect on the formability and elongation of martensite-containing steels.
• Austenite stabilized down to room temperature can be elongated to a greater extent than other microstructure components by utilizing the TRIP effect, while simultaneously achieving higher work hardening. Due to the limitation of austenite-stabilizing alloying elements such as C and Mn for weldability reasons, a residual austenite content greater than 20% is not possible with the described manufacturing process.
• the flat steel product according to the invention has a microstructure containing a maximum of 10% ferrite, preferably 5%, particularly preferably 3%, to ensure the required high strength.
• the ferrite present is polygonal ferrite.
• the atmosphere A 4 and the dew point T P4 in step d) can be adjusted so that H 2 O/H 2 ⁇ 0.957, preferably H 2 O/H 2 ⁇ 0.90, particularly preferably H 2 O/H 2 ⁇ 0.80.
• This further reduces the thin oxide layer that forms in the atmosphere A 4.
• the surface layer i.e. the portion that is at a maximum distance of 10 ⁇ m from the surface, is chemically changed.
• carbon diffuses out of the material and the surface layer becomes depleted of carbon. As a result, no carbon is present in step h) to stabilize the residual austenite.
• the chemical change in the surface region can be supported by the targeted inflation of NH 3 , which can have a positive effect on the mechanical properties close to the surface and additionally inhibits the external selective oxidation of the base alloy elements. Therefore, in this preferred embodiment, the ratio of the residual austenite content in the RA Bulk material compared to the residual austenite content in the surface layer RA Surface is RA Surface / RA Bulk ⁇ 80%.
• the surface layer is defined as the area of the steel that is at a maximum distance of 10 ⁇ m from the surface. The surface area is therefore softer than the material, resulting in good forming properties. In this particular embodiment, a flat steel product with the inventive good surface and good formability can be achieved.
• a component for structural lightweight construction in automotive engineering can be formed from a flat steel product according to the invention.
• samples F112 and B102 underwent stochastic surface texturing during the skin-passing step.
• Sample A1 was subsequently coated with a conversion layer.
• a furnace roller was coated with a coating with a microhardness of 8001 HV0.3 and a roughness of 4 ⁇ m.
• Steel alloys B and DF have the steel composition according to the invention.
• Steel alloy B was tested under different manufacturing parameters (B102 - B106).
• Tests B102, BIOS and B106 were carried out under process conditions according to the invention and show good surface conditions with good mechanical properties.
• Test B104 did show good surface properties, but due to a T4 temperature ⁇ Ac3 -30 °C the carbon cannot be distributed homogeneously in the austenite structure, which results in an excessively low proportion of tempered martensite.
• example B105 on the other hand, the structure according to the invention is achieved, but the example has poor surface properties, i.e. no roughness according to the invention. These poor surface properties are due to the dew points Tp6 and Tp7 not according to the invention.
• Examples D107, D108, F112, and F113 were produced according to the inventive process and exhibited good surface properties, i.e., roughness. Furthermore, the residual austenite in the surface layer had a RA_Surface / RA_Bulk ratio of ⁇ 80%. In contrast, in Example D9, a good surface could not be achieved due to a non-inventive skin-passing degree. In Example D110, however, the non-inventive dew points T p6 and T p7 lead to the selective oxidation of base alloying elements and poor surface properties.
• Steel alloys A and E have a silicon content that is not in accordance with the invention; all further process steps are within the inventive range. Due to the low silicon content, a high proportion of bainite and carbides forms in the microstructure. This results in a low residual austenite content and a high proportion of tempered martensite. Therefore, examples A101, E110, and E111 are not in accordance with the invention.
• Steel alloy C has a carbon and silicon content that is not in accordance with the invention because too much fresh martensite is formed. Both examples C105 and C106 therefore have a non-inventive proportion of tempered martensite. C106 also has a non-inventive proportion of ferrite and retained austenite. Table 1 No.
• Bainite + Martensite Proportion of tempered martensite retained austenite ferrite Rp0.2 Rm A80 Roughness Ra Peak number Rpc A101 77 40% 3 20 585 883 17 0.65 55 B102 84 88% 12 4 880 1199 16 0.8 52 B103 88 91% 11 1 940 1181 14 0.7 54 B104 84 50% 8 8 719 1245 10 1.3 58 C105 94 56% 6 0 759 1091 10 0.8 50 C106 82 31% 3 15 673 1101 13 0.9 51 D107 88 80% 10 2 900 1055 16 1.2 57 D108 90 76% 9 1 853 1021 13 1.4 60 D109 81 81% 14 5 843 1098 17 2.1 71 E110 82 47% 3 15 679 1209 9 0.7 55 E111 93 54% 3 4 873 1219 6 1.1 56 F112 84 89% 16 0 1239 1482 17 1 54 F113 78 93% 17 5 1158 1489 22 1 53 B105 80 88% 15 5 8

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