Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/561—Continuous furnaces for strip or wire with a controlled atmosphere or vacuum
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
  • C23C2/29—Cooling or quenching
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips

Definitions

• the present application relates to a method for producing a high-strength flat steel product provided with a metallic coating and a coated flat steel product.
• High-strength steels are characterized by a high proportion of alloying elements that increase the strength of the material, such as silicon, manganese and chromium.
• a surface-refining layer is often required to prevent material corrosion.
• a surface-finishing layer can be applied, for example, electrolytically or by means of hot-dip coating, which is also referred to as hot-dip coating.
• Zinc-based coatings, which are applied by hot-dip coating are of particular technical importance for corrosion protection.
• the boundary layer between the anti-corrosion layer and the steel substrate or the base material is understood to be the layer that begins with the layer between the anti-corrosion layer and the base material, in which the zinc and iron content have the same value in % by weight, down to a depth of 300 nm into the base material.
• An accumulation of one or more of the elements silicon, manganese and chromium in the boundary layer has a negative effect on the service properties of the coated flat steel product. For example, the adhesion of the anti-corrosion layer on the base material deteriorates. However, the formability of the coated flat steel product is also limited.
• EP2540854B1 is known an ultra-high-strength cold-rolled steel sheet containing in % by mass 0.15-0.30% C, 0.01-1.8% Si, 1.5-3.0% Mn, not more than 0.05% P, not more than 0.005% S, 0.005-0.05% Al and not more than 0.005% N, optionally further one or more elements of 0.001-0.10% Ti, 0.001-0.10% Nb, 0, 01-0.50% V, 0.0001-0.005% B, 0.01-0.50% Cu, 0.01-0.50% Ni, 0.01-0.50% Mo and 0.01- 0.50% Cr and having a soft surface portion containing at least 90% tempered martensite.
• the steel sheet has a tensile strength of not less than 1270 MPa.
• the steel sheet is decarburized in an atmosphere having a high dew point of 30°C at 700-800°C for 15-60 minutes. Decarburizing annealing in an atmosphere with a high dew point for a relatively long period of time results in a decarburized, ductile surface layer, which is then subjected to a coating treatment.
• This oxide layer which consists only of iron, manganese and silicon, is that due to the lack of chromium and a thickness of the oxide layer of up to 5 ⁇ m, the adhesion of metallic coatings can be impaired. In addition, a deterioration in local formability is to be expected between the soft, decarburized and easily formable ferrite layer and the harder and brittle oxide layer.
• the object of the invention was to provide a method for producing a high-strength steel flat product coated by means of a hot-dip coating system, which has good adhesion of the metallic coating on the Guaranteed steel substrate and good formability of the coated flat steel product.
• a high-strength, coated steel flat product should be specified, which has good adhesion of the metallic coating on the steel substrate and good forming properties.
• the object was achieved in that at least the method steps specified in claim 1 are completed in the production of a high-strength, coated steel flat product.
• the invention is based on the finding that the distribution of the main alloying elements silicon, manganese and chromium in the boundary layer has a significant influence on the adhesion of the anti-corrosion coating. This applies in particular to zinc-based anti-corrosion coatings.
• Silicon, manganese and chromium are strong oxide formers. In theory, silicon has a higher affinity for oxygen than manganese, manganese has a higher affinity for oxygen than chromium, and chromium has a higher affinity for oxygen than iron. It would therefore be expected that, depending on the respective proportion of the element under consideration, silicon oxides form first in the boundary layer before manganese oxides and before chromium oxides. This applies under the assumption of equilibrium states that can only be reached theoretically and under ideal conditions, according to which all phases are present as pure phases and the formation of mixed phases is excluded, while the reaction kinetics and diffusion processes are not taken into account.
• the distribution of silicon, manganese and chromium in the boundary layer can vary greatly and that the distribution can be influenced by the production parameters, such as the set temperatures and gas atmosphere.
• step a) a hot-rolled flat steel product produced by means of conventional casting and hot-rolling processes is made available.
• the hot-rolled flat steel product provided in step a) is uncoated, ie it has no metallic anti-corrosion coating.
• the uncoated flat steel product forms the steel substrate or the base material for the metallic anti-corrosion coating that is applied in step i).
• the uncoated flat steel product comprises a steel, in particular it consists of a steel, of the composition explained in more detail below.
• the carbon content of the steel of a flat steel product according to the invention is 0.1-0.5% by weight.
• Carbon (C) influences the formation and stabilization of austenite. During quenching, which is carried out to form martensite, and during the subsequent annealing treatment, any retained austenite is replaced by C stabilized.
• the C content has a strong influence on the strength of the martensite formed during the cooling in step f) with a cooling rate ThetaQ, as well as on the strength of the martensite formed during the last cooling step in step k) with a cooling rate ThetaB2 is formed.
• the C content should be at least 0.1% by weight in order to ensure the austenite-stabilizing and strength-increasing effect.
• the carbon content is at least 0.12% by weight in order to be able to use the austenite-stabilizing and strength-increasing effect of carbon particularly effectively.
• the martensite start temperature is shifted to lower and lower temperatures, so that if the C content is too high, no martensite or only a small proportion of martensite can be formed.
• the carbon content of the steel of a steel flat product according to the invention is limited to at most 0.5% by weight, preferably at most 0.4% by weight.
• the steel of a flat steel product according to the invention contains either manganese or silicon or both manganese and silicon.
• the Mn content is limited to a maximum of 3 0.0% by weight, preferably at most 2.7% by weight. Excessively high manganese contents also cause excessive manganese enrichment in the boundary layer between the anti-corrosion coating and the steel substrate and thus lead to poor adhesion. For this reason too, the Mn content is limited to at most 3.0% by weight, preferably at most 2.7% by weight.
• the silicon content is 0.7-2.5% by weight, preferably at least 0.9% by weight.
• Silicon (Si) contributes to the suppression of cementite formation. During the formation of cementite, carbon is bound in the form of carbides. By suppressing the formation of cementite, free carbon is available, which contributes to the stabilization of the retained austenite and thus to an improvement in elongation. This effect can also be partially achieved by alloying aluminum. If the Si content is too high, silicon can accumulate in the boundary layer between the anti-corrosion coating and the base material, which leads to poor adhesion of the anti-corrosion coating.
• the Si content is limited to at most 2.5% by weight, in particular to less than 2.5% by weight. In a preferred embodiment, the Si content is limited to a maximum of 1.5% by weight in order to further reduce the risk of red scale formation, which can occur during hot strip production.
• the chromium content of the steel of a flat steel product according to the invention is 0.05-1% by weight.
• Chromium (Cr) helps increase strength and is an effective inhibitor of pearlite.
• the accumulation of Cr in the boundary layer between the anti-corrosion coating and the base material leads to improved adhesion.
• the Cr content is at least 0.05% by weight, preferably at least 0.1% by weight.
• Cr increases the risk of severe grain boundary oxidation, which adversely affects weldability and surface quality.
• the Cr content is limited to at most 1.0% by weight. In a preferred embodiment, the Cr content is limited to a maximum of 0.6% by weight for cost reasons, which also contributes to further minimizing the risk of grain boundary oxidation.
• Aluminum (Al) can optionally be contained in the steel of a flat steel product according to the invention at 0.01-1.5% by weight.
• Al can be used for deoxidation and for binding any nitrogen present.
• Al can also be used to suppress cementite.
• the addition of Al increases the austenitization temperature of the steel. If higher annealing temperatures can be set, Al can be alloyed with up to 1.5% by weight. Since aluminum increases the annealing temperature required for complete austenitization and complete austenitization is only possible with difficulty at Al contents above 1.5% by weight, the Al content of the steel is one flat steel product according to the invention to a maximum of 1.5% by weight, preferably a maximum of 1.0% by weight. In a preferred embodiment, the Al content is limited to at most 0.1% by weight, in particular to 0.01-0.1% by weight, in order to limit the austenitization temperature.
• Phosphorus (P), sulfur (S) and nitrogen (N) have a negative effect on the mechanical and technological properties of flat steel products according to the invention, which is why their presence in flat steel products according to the invention should be avoided if possible.
• Phosphorus (P) has an unfavorable effect on weldability, which is why the P content should be at most 0.02% by weight, preferably less than 0.02% by weight.
• sulfur (S) leads to the formation of MnS or to the formation of (Mn,Fe)S, which has a negative effect on elongation. Therefore, the S content is limited to values of at most 0.005% by weight, preferably less than 0.005% by weight.
• N Nitrogen (N), both in interstitially dissolved form and as nitride, for example in combination with titanium, niobium or vanadium, leads to embrittlement of the steel, which can have a negative effect on formability, which is why the N content is limited to a maximum of 0.008 % by weight, preferably to less than 0.008% by weight.
• Steels of flat steel products according to the invention can optionally contain molybdenum (Mo) in amounts of 0.05-0.5% by weight.
• Mo molybdenum
• Mo promotes the suppression of pearlite formation, and for this purpose it can be contained in the steel in an amount of at least 0.05% by weight.
• the Mo content is limited to at most 0.5% by weight, in particular less than 0.5% by weight.
• Steels of flat steel products according to the invention can optionally contain boron (B) in amounts of 0.0004-0.001% by weight. Boron segregates onto the phase boundaries and blocks their movement. This supports the formation of a fine-grain structure, which improves the mechanical properties of the steel flat product. In order to bring about an improvement in the mechanical properties, boron can be added in amounts of at least 0.0004% by weight. When alloying boron, sufficient Ti or Nb should preferably be available to bind N, which prevents the formation of harmful boron nitrides.
• steels of flat steel products according to the invention can contain one or more micro-alloying elements in total contents of 0.001 to 0.3% by weight.
• micro-alloy elements are understood to mean the elements titanium (Ti), niobium (Nb) and vanadium (V). Titanium or niobium or a combination of both are preferably used.
• the micro-alloying elements can form carbides with carbon, which in the form of very finely divided precipitates contribute to greater strength. With a total content of microalloying elements of at least 0.001% by weight, preferably at least 0.005% by weight, precipitates can form which lead to the freezing of grain and phase boundaries during austenitizing.
• the total concentration of the microalloying elements should be at most 0.3% by weight, preferably at most 0.2% by weight.
• the hot-rolled flat steel product is first pickled in a conventional manner and then subjected to cold rolling.
• the cold rolling reduces the thickness of the flat steel product by at least 37%, in particular by more than 37%.
• the reduction in thickness refers to the difference between the initial thickness of the flat steel product before the first cold rolling pass and the final thickness of the flat steel product after the last cold rolling pass.
• Cold rolling with a thickness reduction of at least 37% causes mechanical homogenization of the material and leads to a particularly fine-grained structure with an average grain size of less than 30 ⁇ m in the cold-rolled state.
• the very fine-grained structure created by cold rolling provides many nucleation sites for the formation of austenite grains for the following austenitizing annealing, which consequently also leads to very fine-grained austenite.
• the grain-refining effect can be intensified if a thickness reduction of preferably at least 42% is set during cold rolling.
• Mechanical homogenization of the material facilitates the setting of the targeted ratio of Si, Mn and Cr in the boundary layer between the anti-corrosion coating and the steel substrate, which takes place in the further course of work.
• step c) the cold-rolled flat steel product is heated to an annealing temperature THZ above the Ar temperature of the steel, which can also be referred to as the holding zone temperature, in order to enable a complete transformation of the microstructure into austenite.
• the soak zone temperature THZ can be limited to a maximum of 950°C to save on operating costs.
• the heating to THZ takes place in two stages.
• the flat steel product is first heated at a heating rate Theta_H1 of 5 - 50 K/s until a turning temperature TW of 200 - 400 °C is reached. Above the turning temperature T_W, the heating takes place at a heating rate Theta_H2 of 2 - 10 K/s until the holding zone temperature THZ is reached.
• the first heating rate Theta_H1 is not equal to the second heating rate Theta_H2.
• Theta_H2 is less than Theta_H1.
• the flat steel product is heated in a continuous furnace.
• the flat steel product is heated in a furnace equipped with ceramic radiant tubes, which is particularly advantageous for reaching strip temperatures above 900°C.
• the indirect heating avoids undesirably strong oxidation of the steel surface combined with the formation of an oxide layer, since the oxygen components required for combustion do not come into contact with the material.
• a gas mixture in a closed burner and the heat transfer in this case is by radiation.
• a furnace is also known as a Radiant Tube Furnace or RTF.
• step d) the flat steel product is held at the holding zone temperature THZ for a holding time tHZ of 5-15 s.
• the holding time tHZ should not exceed 15 seconds in order to avoid the formation of a coarse austenite grain and irregular austenite grain growth and thus negative effects on the formability of the steel flat product.
• the holding time should last at least 5 s in order to achieve complete transformation into austenite and a homogeneous C distribution in the austenite.
• the atmosphere in which the flat steel product is kept contains 3-7% by volume of hydrogen.
• the rest of the atmosphere is made up of nitrogen moistened with water vapor and unavoidable impurities, with a nitrogen content of 93 - 97% by volume being aimed at and the sum of all components being 100% by volume.
• the information on the furnace atmosphere composition relates to atmosphere compositions resulting in a total of 100% by volume.
• the atmosphere consists in particular of 3-7% by volume of hydrogen and the remainder of nitrogen moistened with water vapor and avoidable impurities.
• the amount of water vapor in the atmosphere is controlled by the dew point.
• the dew point is reduced to values of -22°C to 0°C, preferably to values of at most -5°C, in particular to values of -22°C to -5°C, and particularly preferably to values of at least -20°C and/or at most -15 °C, in particular adjusted to values from -20 °C to -15 °C.
• the concentration profile of the elements Si, Mn and Cr in the boundary layer can be controlled by the dew point and concentration profiles of the elements Si, Mn and Cr in the boundary layer can be obtained.
• the proportion of water vapor is described using the dew point.
• the dew point corresponds to the temperature at which the water condenses in a volume of gas. If the dew point values are low, the proportion of water in the gas mixture is low. As the dew point rises, the proportion of water in the gas mixture increases.
• the moistened gas mixture in the furnace atmosphere in combination with the facilitated diffusion during annealing, initially leads to an enrichment of the elements Mn, Si and Cr, which have a higher oxygen affinity than iron, on the surface of the base material. Due to the small size difference between manganese and iron, Mn diffuses faster in the iron lattice than Cr or Si. Chromium exhibits slightly slower diffusion than Mn, while silicon diffuses significantly more slowly.
• the proportion of water vapor in the furnace atmosphere in particular during holding in step d), is more than 0.070% by volume, particularly preferably at least 0.080% by volume.
• the proportion of water vapor in the furnace atmosphere is at most 1.0% by volume, preferably at most 0.8% by volume.
• One finding of the present invention is that high Si and Mn contents in the boundary layer impair coatability, whereas Cr has no negative influence, but even a positive influence on the adhesion of the anti-corrosion coating if the abovementioned ratio is maintained. Maintaining the ratio of the oxide-forming elements Si, Mn and Cr in the boundary layer leads not only to excellent adhesion of the anti-corrosion coating but also to good formability of the coated flat steel product.
• the flat steel product is heated in step c) and/or held in step d) in a radiant tube furnace.
• the combustion gases containing oxygen do not come into contact with the flat steel product, since the gas mixture to be burned is in is burned in a closed burner and heat transfer is by radiation.
• decarburization of the surface and severe oxidation of the surface of the uncoated steel flat product and the formation of a covering oxide layer can be reduced and preferably avoided.
• step e the flat steel product is cooled to a temperature TLK.
• the cooling starts after the end of the holding in step d). In particular, cooling starts immediately after holding, and thus at the latest after the maximum holding time of 15 s has expired.
• the temperature TLK is no more than 150 °C below the A3 temperature of the steel of the steel flat product in order to avoid the formation of ferrite.
• the duration for cooling from THZ to TLK is at least 50 s and at most 300 s.
• the cooling carried out in step e) can also be referred to as controlled and slow cooling.
• the flat steel product is further cooled from temperature TLK to a cooling stop temperature TAB.
• the TLK is cooled down to TAB at a cooling rate ThetaQ, which is at least 30 K/s. Cooling can also be referred to as rapid cooling.
• ThetaQ cooling rate is at least 30 K/s to avoid the formation of ferrite and the formation of bainite. Cooling can preferably be carried out at up to 120 K/s, which can be achieved, for example, by using modern gas jet cooling.
• the cooling stop temperature TAB lies between the martensite start temperature TMS, ie the temperature at which a martensitic transformation begins, and a temperature which is up to 175°C lower than TMS. The following applies: (TMS-175°C) ⁇ TAB ⁇ TMS.
• step h) the steel flat product is heated at a maximum heating rate ThetaB1 of 80 K/s to a treatment temperature TB of 450 - 500 °C in order to enrich residual austenite with carbon from the supersaturated martensite.
• the formation of carbides and the decomposition of residual austenite are avoided by observing a total treatment time of 10 - 1000 s for this work step.
• the treatment temperature TB is matched to the subsequent hot-dip coating treatment.
• TB is also a suitable temperature for immersion in a zinc-based molten bath.
• the heating takes place at a heating rate of at most 80 K/s, in particular less than 80 K/s, in order to ensure sufficient redistribution of the carbon to guarantee.
• the heating can be implemented, for example, by using radiant tubes or by using a booster.
• step i) the flat steel product is subjected to a coating treatment, in particular a hot dip coating.
• the steel flat product runs through a coating bath with a zinc-based molten bath composition.
• the temperature of the molten bath is preferably 450-500.degree.
• a suitable molten bath composition can contain, for example, up to 2% by weight Al, up to 2% by weight Mg, the remainder zinc and unavoidable impurities, in particular from up to 2% by weight Al, up to 2% by weight Mg, The rest consists of zinc and unavoidable impurities.
• a suitable molten bath composition can contain, for example, up to 1% by weight Al, the remainder being zinc and unavoidable impurities contain, in particular consist of up to 1 wt .-% Al, remainder zinc and unavoidable impurities.
• a molten bath composition can contain 1-2% by weight Al, 1-2% by weight Mg, the remainder zinc and unavoidable impurities, in particular 1-2% by weight Al, 1-2% by weight % Mg, remainder zinc and unavoidable impurities.
• the flat steel product can be subjected to a galvannealing treatment in an optional step j). To do this, it is tempered for a duration tGA of 10 s - 60 s at a temperature TGA of 500 - 565 °C.
• step k the coated flat steel product is cooled to room temperature at a cooling rate ThetaB2 of at least 5 K/s, preferably more than 5 K/s.
• the martensite formed in the course of the method according to the invention by the second quenching in step k) is referred to as untempered martensite.
• the martensite created by the first quenching after austenitizing, which is subjected to heating in step h), is also referred to as tempered martensite.
• the atmosphere compositions through which the flat steel product passes in the further work steps, in particular in work steps e) to k), can be adapted to the furnace atmosphere of the holding process of work step d).
• an atmosphere is preferably set that contains 3-7% by volume of hydrogen and the balance with steam, preferably with at least 0.070% by volume, particularly preferably with at least 0.080% by volume, more preferably with at most 1.0% by volume, particularly preferably containing at most 0.8% by volume, water vapor, humidified nitrogen and unavoidable impurities.
• the method according to the invention for producing a high-strength flat steel product provided with a metallic anti-corrosion coating comprises no further work steps and thus exclusively the work steps mentioned under a) - k).
• a product according to the invention comprises a steel substrate which comprises a steel, preferably consisting of a steel, which consists of (in % by weight): 0.1-0.5% C, at least one element selected from the group consisting of Mn and Si , where the Mn content is 1.0 - 3.0% and the Si content is 0.7 - 2.5%, 0.05 - 1% Cr, up to 0.020% P, up to 0.005% S, up to 0.008% N, and optionally from one or more of the following elements 0.01 - 1.5% Al, 0.05 - 0.5% Mo, 0.0004 - 0.001% B and optionally from a total of 0.001 - 0, 3% V, Ti and Nb, and the balance consists of iron and unavoidable impurities.
• a steel substrate which comprises a steel, preferably consisting of a steel, which consists of (in % by weight): 0.1-0.5% C, at least one element selected from the group consisting of Mn and Si , where the Mn content is 1.0
• the steel substrate has a microstructure containing 5-20% by volume retained austenite, less than 5% by area bainite, less than 10% by area ferrite and at least 80% by area martensite, of which at least 75% by area is tempered martensite and less than 25% by area is untempered martensite.
• the structure of the product according to the invention consists of 5-20 vol. % of which at least 75% by area is tempered martensite and less than 25% by area is untempered martensite.
• a high proportion of martensite is used to achieve the desired strength.
• the ductility can be influenced by the proportion of tempered martensite. All of the martensite present in the structure is composed of tempered and untempered martensite, with the possibility that there is no untempered martensite.
• microstructural proportions for retained austenite is based on vol. % and for other microstructural components such as martensite, ferrite and bainite, on area %.
• the structure is particularly fine-grained and preferably has an average grain size of less than 30 ⁇ m. Due to the fineness of the microstructure, it is advisable to carry out the microstructure investigations using a scanning electron microscope (SEM) with a magnification of at least 5000x. An examination using X-ray diffraction (XRD) according to ASTM E975 is recommended as a suitable method for the quantitative determination of retained austenite.
• SEM scanning electron microscope
• XRD X-ray diffraction
• the product according to the invention also comprises a metallic protective coating, preferably a Zn-based anti-corrosion coating.
• a suitable anti-corrosion coating contains up to 2% by weight Al, up to 2% by weight Mg, the remainder zinc and unavoidable impurities, in particular the anti-corrosion coating consists of up to 2% by weight Al and up to 2% by weight Mg , remainder Zn and unavoidable impurities.
• the anti-corrosion coating has 1-2% by weight Al, 1-2% by weight Mg, the remainder zinc and unavoidable impurities, in particular it consists of 1-2% by weight Al, 1-2% by weight % Mg, remainder zinc and unavoidable impurities.
• the anti-corrosion coating has up to 1% by weight Al, remainder zinc and unavoidable impurities, in particular it consists of up to 1% by weight Al, remainder zinc and unavoidable impurities.
• the coated steel flat product according to the invention has a ratio of the sum of Si and Mn to Cr of at least 1.7 and at most 15 in the boundary layer between the anti-corrosion coating and the steel substrate according to the following relationship: 1.7 ⁇ [(Si + Mn) / Cr]_GS ⁇ 15 with Si: Si wt% content in the boundary layer, Mn: Mn wt% content in the boundary layer, Cr: Cr wt% content in the boundary layer.
• [(Si + Mn) / Cr]_GS is smaller than [(Si + Mn) / Cr]_GW ensures that the steel flat product has good adhesion of the metallic coating on the steel substrate and good forming properties. This effect can be achieved particularly reliably if [(Si+Mn)/Cr]_GS is preferably less than 0.9*[(Si+Mn)/Cr]_GW, particularly preferably less than 0.6*[(Si+Mn ) / Cr]_GW is.
• the coated flat steel products preferably have a tensile strength Rm of at least 600 MPa, a yield point Rp02 of at least 400 MPa and an elongation A80 of at least 7%, in particular more than 7%.
• Tensile strengths of 950 to 1500 MPa are typically achieved. Yield strength values are typically at least 700 MPa. The yield point is below the achieved tensile strength. The yield point is typically below 950 MPa.
• the coated flat steel products have excellent adhesion of the anti-corrosion coating, preferably level 1 adhesion determined by the ball impact test in accordance with SEP 1931, on the steel substrate, and very good formability.
• Hole expansion for example, can be used as a measure of formability. The hole expansion is typically at least 25%.
• the product of tensile strength and hole expansion can also be used as a measure of formability. In a preferred embodiment, the product of tensile strength and hole expansion is at least 20,000 MPa*%, preferably at least 25,000 MPa*%
• the tensile strength, yield point and elongation were determined according to DIN EN ISO 6892, sample form 2, the adhesion was determined using a ball impact test KST according to SEP 1931 and the hole expansion was determined according to ISO 16630.
• the element distribution in the boundary layer and in the areas adjacent to the boundary layer can be carried out using the glow discharge spectroscopy method (Glow Discharge Optical Emission Spectroscopy, GDOES for short).
• GDOES glow discharge spectroscopy
• a GDOES measuring device from Leco can be used for this. With GDOES it is possible to carry out the quantitative determination of elements in layer structures along the layer thickness.
• the beginning of the boundary layer can be determined using GDOES by using the intersection of the curves of the Zn content and the Fe content as the starting point of the boundary layer, which extends 300 nm from this intersection into the base material.
• the flat steel product according to the invention is produced by the method according to the invention explained above.
• melts AG were produced with the compositions given in Table 1, from which 11 hot strips with a thickness of 1.8 to 2.5 mm were produced in a conventional manner.
• the melts C, E, F and G correspond to the specifications for the steel composition according to the invention, whereas the melts A and B have too low Si contents and the melt D has too low a Si content and too high an Al content.
• the hot strips were pickled in a conventional manner and further processed with the production parameters given in Table 2.
• the hot strips were each rolled into cold strips with the degree of cold rolling "KWG” given in Table 2, the cold strips were each heated to a turning temperature "TW” at a first, faster heating rate “ThetaH1” and then at a second, slower heating rate “ThetaH2” brought to the holding zone temperature "THZ", at which they were held for the duration "tHZ" of 5 to 15s in an atmosphere with a dew point "TP".
• the cold strips were first slowly cooled to an intermediate temperature "TLK” within a period of time “tLK” of 50 to 300 s, then quickly quenched from the intermediate temperature “TLK” at a cooling rate "ThetaQ” to a cooling stop temperature "TAB”, at which they were held for a duration "tQ” of 10 to 60s.
• the flat steel products were then heated to a treatment temperature "TB” at a heating rate "ThetaB1" of at most 80K/s. The steel flat products were not kept at the treatment temperature.
• the flat steel products were then subjected to hot dip coating, carried out in an otherwise conventional manner, in a melt bath with the following composition: up to 2% by weight Al, up to 2% by weight Mg, the remainder zinc and unavoidable impurities.
• the steel flat products of melts A - F were finally quenched to room temperature at a cooling rate "ThetaB2" of at least 5 K/s.
• the flat steel products of melt G were first tempered at a temperature TGA for a duration tGA and only after tempering were they quenched to room temperature at a cooling rate of at least 5 K/s.
• the structural investigations were carried out on cross-sections at 1/3t layer, i.e. on sections which were taken from a third of the sheet thickness of the steel substrate.
• the sections were prepared for scanning electron microscopy (SEM) examination and treated with a 3% Nital etch. Due to the fineness of the microstructure, the microstructure was characterized by means of REM observation at a magnification of 5000x.
• the quantitative determination of the retained austenite was carried out by means of X-ray diffraction (XRD) according to ASTM E975.
• XRD X-ray diffraction
• the element content of the base material was determined using the combustion analysis ICP-OES (inductively coupled plasma optical emission spectrometry) in a 1/3t layer.
• the mechanical properties yield strength "Rp02”, tensile strength “Rm” and elongation "A80" were tested according to DIN EN ISO 6892:2009, specimen form 2, on longitudinal specimens which were taken from the middle of the steel flat products.
• the adhesion of the zinc-based anti-corrosion coating was determined as KST according to SEP 1931 and the hole expansion was determined according to ISO16630.
• samples C4, C5, E8 and F10 produced according to the invention have very low values for the ratio [(Si+Mn)/Cr]_GS of at most 15. At the same time, these samples show excellent adhesion of the anti-corrosion coating of less than 1.5 and very good hole expansion of over 25%. In comparison, samples of steels of the same strength class, but with a value higher than 15 for [(Si+Mn)/Cr]_GS, show poorer formability and poorer coating adhesion.
• Sample E9 shows that it is possible to achieve adequate values for the product of tensile strength and hole expansion (tensile strength*hole expansion) if the nitrogen in the gas mixture is not sufficiently moistened with water vapor and the dew point is therefore too low of the anti-corrosion coating is affected.
• the increasing difference between the yield point and the tensile strength in the annealed material means that the product of tensile strength*hole expansion no longer achieves adequate values.

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• 2018
  • 2018-09-26 CN CN201880098183.1A patent/CN112789358B/zh active Active
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