EP3913105A1 - Produit plat en acier et son procédé de fabrication - Google Patents

Produit plat en acier et son procédé de fabrication Download PDF

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Publication number
EP3913105A1
EP3913105A1 EP21173504.8A EP21173504A EP3913105A1 EP 3913105 A1 EP3913105 A1 EP 3913105A1 EP 21173504 A EP21173504 A EP 21173504A EP 3913105 A1 EP3913105 A1 EP 3913105A1
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EP
European Patent Office
Prior art keywords
scale
layer
flat steel
steel substrate
steel product
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP21173504.8A
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German (de)
English (en)
Inventor
Christian Mertin
Christian Mussmann
Dr. Frank Friedel
Jörg Ruck
Daniel Rütten
Jenny Schulte
Sebastian Weber
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ThyssenKrupp Steel Europe AG
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ThyssenKrupp Steel Europe AG
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Publication of EP3913105A1 publication Critical patent/EP3913105A1/fr
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    • 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
    • C21D1/84—Controlled slow cooling
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • 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
    • 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

Definitions

  • the present invention relates to a hot-rolled flat steel product, comprising a steel substrate and a layer of scale contacting the steel substrate.
  • the present invention also relates to a method for producing a flat steel product according to the invention.
  • the invention also relates to the use of a flat steel product according to the invention.
  • the flat steel products according to the invention are rolled products, such as steel strips, steel sheets or blanks and blanks obtained therefrom, the thickness of which is significantly less than their width and length.
  • Black hot strip is a hot-rolled flat steel product with an unpickled surface and a layer of scale.
  • the main phase components of the scale layer are usually magnetite (Fe3O4), wustite (FeO), hematite (Fe2O3) and iron (Fe).
  • the scale on the surface is removed to avoid these negative effects.
  • the standard way of removing scale is pickling, for example in a hydrochloric acid pickling solution, shot blasting or sandblasting. This means additional work steps.
  • the scale can no longer be used as a lubricant in forming processes.
  • WO 2018/186265 A1 discloses a 3.0-20 ⁇ m thick layer of scale made of Fe and Fe3O4 on hot-rolled flat steel products, which is characterized by its black color and its adhesion.
  • the average particle diameter in the scale layer is less than 3.0 ⁇ m and the Fe content increases from the surface of the scale layer in the direction of the steel substrate.
  • the final rolling temperature is limited to 800 to 950 ° C and cooling after the last pass starts after 2 s at the latest.
  • WO 2020/065372 A1 describes a flat steel product with a 5-40 ⁇ m thick layer of scale containing at least 50% magnetite and ferrite, less than 50% wustite and less than 10% hematite.
  • the adhesive strength of the scale layer is more than 80%.
  • the flat steel product is produced by preheating, hot rolling with a final rolling temperature of more than 800 ° C, cooling to a coiling temperature of less than 650 ° C with a cooling rate of 2-30 ° C / s and subsequent cooling with a cooling rate of less than 2 ° C / s.
  • the end JP 2004-043888 A a steel sheet with a scale layer of more than 4 ⁇ m and an Fe3O4 proportion of more than 50% is known, the Fe3O4 proportion being formed close to the surface.
  • the scale layer is characterized by an excellent blackening and does not contain any Fe.
  • the flat steel product is produced at a final rolling temperature of 800 ° C or higher and is cooled to less than 650 ° C at a cooling rate of greater than 50 ° C / s and is coiled at a temperature of greater than 600 ° C.
  • JP 2012-162778 A A hot-rolled flat steel product with a layer of scale is disclosed which has a thickness of less than 10 ⁇ m and a proportion of voids of 0.10-3.0%.
  • the scale layer consists of at least 50% Fe3O4 and 0-10% Fe2O3.
  • the flat steel product is produced at a final roller temperature between 700 ° C and 900 ° C and is coiled at 450 ° C to 650 ° C.
  • the task was to create a flat steel product which, compared to conventionally manufactured flat steel products, has a homogeneous and firmly adhering layer of scale in order to avoid various manufacturing and quality defects in further processing. Due to its high adhesive strength, the firmly adhering layer of scale should not flake off, or only to a small extent, and if possible also after forming, cutting and welding processes can be painted or galvanized directly. Flaked scale can, in the form of finely divided dust, impair optical measuring equipment or, in agglomerated form, cause impressions on formed components. Firmly adhering scale thus reduces the contamination of the system and leads to lower costs and better system performance due to longer cleaning intervals and less rework.
  • a flat steel product consisting of a steel substrate which has a carbon content of less than 0.8% by weight and a layer of scale contacting the steel substrate, which in direct contact at the interface between the steel substrate and the scale layer has a maximum proportion of residual desertite 30%, preferably a maximum of 20% and particularly preferably a maximum of 10%.
  • the interface and direct contact mean the line that separates the areas of scale and steel substrate from one another in the metallographic section through a discrete phase transition (microscopically recognizable through different gray values). Only those areas with a minimum thickness of 0.5 ⁇ m are taken into account when determining the proportion of residual desertite.
  • the proportion of residual desertite at the interface results in the metallographic section from the summed up length of the line sections that are covered with residual desertite and the length of the entire line that separates the scale from the substrate.
  • the length of the entire line is at least 100 ⁇ m.
  • the solution according to the invention of the above-mentioned object consists in completing the work steps specified in claim 9 in the production of a flat steel product according to the invention.
  • the inevitable impurity indicates an impurity which is inevitably contained or causes a marked increase in production cost in order to avoid its inclusion, such as an impurity contained in a raw material.
  • the customary limit values apply to unavoidable contamination in the steel substrate.
  • the adhesive strength serves as a characteristic value for the adhesion of the scale layer to the steel substrate. It is a measure of the amount of scale that is released from the steel substrate or the scale layer after mechanical stress and is determined with the help of a tensile or bending test and subsequent surface test under defined conditions.
  • a high adhesive strength is characterized by the fact that the scale layer on the steel substrate is largely retained during or after the mechanical load by tensile or bending tests and only small amounts of scale come off.
  • the amount of detached scale can preferably be determined by means of image analysis or reflection methods.
  • a high adhesive strength of the scale layer is characterized by values greater than 60%, preferably greater than 70%, particularly preferably greater than 80%.
  • a coating is applied to the flat steel product.
  • the flat steel product can be galvanized, for example by hot-dip galvanizing or electro-galvanizing, or painted.
  • the required steel substrate is characterized by a carbon content "C" of less than 0.8%, in particular less than 0.5%, preferably up to 0.3%, preferably less than 0.12%.
  • Carbon contents of up to 0.8% are required to produce a firmly adhering layer of scale on steel. Due to the recalescence, higher contents lead to excessive reheating during the phase transition, which means that there is a risk of the scale oxidizing to form hematite and magnetite and the breakdown into magnetite and iron is suppressed.
  • a carbon content of 0.002% is preferably beneficial for producing sufficient strength in the steel.
  • silicon "Si" diffuses to the interface between the steel substrate and the scale layer, where it increases the adhesion of the scale layer during hot rolling due to its oxides. As a result, the scale cannot be completely removed during rolling and the build-up of scale is uneven and increased.
  • very high silicon contents lead to the formation of fayalite particles and red scale, which has a negative impact on the roughness and appearance of the subsequent product. Therefore, the content should be reduced to at least 1.5% or less.
  • silicon promotes the formation of the scale layer according to the invention and therefore a lower limit in the range of unavoidable impurities of 0.01% is set.
  • Manganese "Mn” is preferably a strong austenite former and promotes grain refinement. If the stabilization of the austenite leads to a lowering of the transformation temperatures, the resulting stresses in the colder and already converted scale can be reduced more poorly, which results in a preliminary damage to the scale layer.
  • the grain refinement in the base material leads to a layer of scale with smaller areas of the same orientation, which inhibits the growth of cracks.
  • Aluminum “Al” is preferably used as a deoxidizer and, due to the formation of AlN, prevents the coarsening of the austenite grain during austenitizing. If the aluminum content is below 0.02%, the deoxidation processes do not take place completely. However, if the aluminum content exceeds the upper limit of 1%, Al2O3 inclusions and a layer of scale that is difficult to remove during the rolling process can form. The excessive growth of the layer leads to a reduced adhesive strength of the scale layer.
  • Phosphorus "P” is a surface-active element and accumulates at interfaces. Similar to the embrittlement at grain boundaries, it thereby worsens the adhesion between the scale layer and the steel substrate, which leads to poorer adhesion.
  • the maximum content of phosphorus must therefore be limited to at least 0.15%. No positive effect of phosphorus on the expression of the firmly adhering scale layer according to the invention was found. However, even low levels of phosphorus increase the yield point and tensile strength. In order to use the strength-increasing effect of phosphorus, contents ⁇ 0.005% are required.
  • Nitrogen “N” can be used as an optional alloying element in levels of up to 0.02% to form nitride and / or improve hardenability.
  • the content must therefore be limited to a maximum of 0.02%.
  • contents of ⁇ 0.0005% are required.
  • Niobium "Nb” is preferably used to support the strength properties through grain refinement of the austenite structure during temperature-controlled rolling or through precipitation hardening during cooling and can be up to 0.2%. If the niobium content is above 0.2%, niobium carbonitrides can no longer be completely dissolved even in melts with very low nitrogen and carbon contents, which in turn has a negative effect on the subsequent mechanical-technological properties. In order to utilize the positive effect of niobium on grain refinement and precipitation hardening, contents ⁇ 0.005% are required.
  • Titanium "Ti” is preferably used to support the strength properties by preventing grain growth during austenitizing or by precipitation hardening during coiling and is at most 0.22%. If the upper limit is exceeded, formability, weldability and toughness deteriorate due to the formation of coarse titanium precipitates. In order to use the positive effect of titanium on grain growth and precipitation hardening, contents ⁇ 0.005% are required.
  • Vanadium "V” can optionally be used to increase the strength due to the formation of carbonitrides and is limited to less than or equal to 0.2%. In order to obtain the strength-increasing effect through the formation of precipitates, contents ⁇ 0.005% are required.
  • Nickel “Ni” preferably improves the adhesion between the scale layer and the steel substrate, but increases the material costs and is therefore not alloyed or only in very small quantities. unless this is necessary to achieve the mechanical-technological properties, where it is mainly used to improve the toughness.
  • the nickel content is below 0.5%.
  • at least 0.01% Ni, preferably at least 0.05% Ni is added.
  • Copper "Cu” is an accompanying element and should be limited to a maximum of 0.5%. If the content is too high, it worsens the weldability and, due to its strong tendency to segregate in the steel, it can lead to a defective surface. Smaller amounts of copper can help increase strength in the form of the finest precipitates. This requires levels of ⁇ 0.05%.
  • Chromium "Cr” can optionally be added and is a strong solid solution strengthener and promotes grain refinement by retarding the transformation. In quenched and tempered steels, chromium is also of particular importance for through hardenability. However, in excessively large amounts, chromium leads to a stronger layer of scale in the rolling process, which is difficult to remove. This leads to an excessive build-up of scale, which in turn has a negative effect on the adhesive strength of the scale. Therefore the maximum chromium content is limited to 2%. Solid solution hardening and the positive effect of chromium on the conversion retardation occur from contents of 0.05%, which results in the lower limit for optional alloying.
  • Molybdenum "Mo” optionally promotes the bainitic transformation and leads to a refinement of the structure and precipitations. In martensitic steels, it also increases the tempering resistance to a particular degree. The positive properties described here occur from contents ⁇ 0.02%. However, due to the high costs, the content is limited to 1.5%.
  • Boron "B” can optionally be added up to 0.005% to support the strength properties or improve hardenability. From a boron content of 0.005%, the toughness properties deteriorate due to embrittlement at grain boundaries. In order to ensure the reliable effectiveness of boron, contents of ⁇ 0.0005% are necessary for technically common impurities in the melt.
  • Calcium "Ca” can optionally be used to mold the non-metallic inclusions. As a result, inter alia the toughness can be improved. Ca contents ⁇ 0.0005% are required to ensure improved toughness. If the Ca content is above 0.015%, this can have a negative effect on the degree of purity of the melt and damage the shell.
  • Cobalt "Co” can be used as an optional alloying element with levels of at least 0.05% to increase hardness. Contents below 0.05% show no noticeable effect, but can be tolerated. For cost reasons, the Co content is limited to a maximum of 1%.
  • Beryllium "Be” can be used as an optional alloying element in contents of up to 0.1% in order to increase the wear resistance through the formation of high-strength carbides and / or oxides. In addition, it leads to an excessive increase in cost. For a reliable setting of the effectiveness, contents of at least 0.002% are used. It is particularly preferred not to use beryllium because of its toxicity.
  • Antimony "Sb” can be added as an optional alloying element in contents of up to 0.3% in order to reduce the susceptibility to grain boundary oxidation and, if higher contents are used, also to increase the corrosion resistance in acidic media by segregating and inclining at grain boundaries for hydrogen generation and thus for hydrogen-induced crack formation reduced or completely prevented.
  • higher contents lead to an excessive increase in costs.
  • a content of at least 0.001% is required to achieve a reliable effect of the addition.
  • Tin “Sn” can be added as an optional alloying element to increase the corrosion resistance in acidic media and can be used for this purpose with an alloy content of up to 0.3%. A content of at least 0.001% is used to ensure at least a slight effectiveness. To avoid a deterioration in the toughness of the material, an upper limit of 0.3% or less is required.
  • Tungsten "W” and / or zirconium “Zr” can be added as optional alloying elements individually or in combination for grain refinement.
  • These optional alloying elements like Ti, can be used as micro-alloying elements in order to form strength-increasing carbides, nitrides and / or carbonitrides. To ensure their effectiveness, contents of at least 0.005% are required in each case. Both optional alloying elements are to be limited to a maximum of 0.2% each, since higher contents can have a detrimental effect on the material properties, in particular on the toughness properties of the flat steel product.
  • Rare earth metals such as cerium, lanthanum, neodymium, praseodymium, yttrium and others, which are individually or collectively abbreviated as SEM, can be added as optional alloying elements in order to bind S, P and / or O and the formation of oxides and / or sulfides as well To reduce or avoid phosphorus segregations at grain boundaries and thus to increase the toughness. In order to achieve a recognizable effect, a content of at least 0.0005% is added when using SEM. The SEM content is limited to a maximum of 0.05% in order not to form too many additional precipitates, which can negatively affect the toughness.
  • the flat steel product may contain, as unavoidable impurities, one or more of the elements from the group of oxygen, hydrogen and arsenic, which are not specifically alloyed as alloying elements.
  • Oxygen "O" is an undesirable, but for technical reasons usually unavoidable impurity in the base material.
  • the maximum content for O is given as up to 0.005%, in particular up to 0.002%.
  • Hydrogen "H” as the smallest atom in interstitial spaces in steel, can be very mobile and can lead to cracks in the flat steel product when it cools down after hot rolling, especially in high-strength steels.
  • the possible contamination with hydrogen is therefore reduced to a content of a maximum of 0.001%, in particular a maximum of 0.0004%, preferably a maximum of 0.0002%.
  • Arsenic "As” is an impurity that can be present in the hot-rolled flat steel product, the content being limited to a maximum of 0.02% in order to avoid negative influences.
  • the adhesion of the scale layer to the steel substrate is referred to as adhesive strength and has a significant influence on the solution of the defined task.
  • the proportion of firmly adhering scale must be on average at least 60%, preferably at least 70%, particularly preferably at least 80%.
  • the mean value is formed from representative individual measurements of the scale adhesion test of the top and bottom of the sheet.
  • a preferred feature of the invention is the limited variation in adhesive strength between the top and bottom of the tape.
  • the difference in the measured adhesive strength between the individual measurements on the upper and lower side of the belt may be a maximum of 30%, preferably a maximum of 25%, particularly preferably a maximum of 20%. This limitation of the difference in the measured adhesive strength values leads to a homogeneous adhesive strength on both belt surfaces and thus represents an important factor in the quality assurance of the scale properties.
  • the adhesive strength of the scale layer on the steel substrate is controlled by various factors. On the one hand through the adhesion of the scale to the steel substrate (adhesion), on the other hand through the cohesion within the scale (cohesion).
  • the adhesion of the scale layer to the steel substrate and its cohesion are in turn largely determined by the scale layer thickness, as well as by the scale layer composition and the morphology of the scale layer.
  • the adhesive strength of the scale layer on the steel substrate is increased if the scale layer has iron precipitates and the ratio of the length to the width of the iron precipitates is on average at least 2: 1, preferably at least 5: 1, particularly preferably at least 10: 1.
  • the elongated shape of the iron precipitates increases the adhesion and cohesion of the scale layer.
  • the adhesive force is increased because the iron precipitates formed at the phase boundary between the scale layer and the steel substrate combine with the base material, because of their elongated shape "migrate" into the scale layer and thus "cling” it to the base material.
  • the cohesive force is increased because the iron precipitates formed within the scale layer, due to their elongated shape, also reinforce the scale layer, similar to a composite material.
  • the thickness of the scale layer makes a decisive contribution to the adhesive strength.
  • phase composition of the scale after cooling to room temperature is a construct of magnetite (Fe3O4) and iron (Fe) and also contains, in particular, proportions of residual desertite (FeO) and hematite (Fe2O3).
  • the flat steel product according to the invention is characterized in that the proportion of magnetite (Fe3O4) and iron (Fe) in the scale layer is at least 60%, preferably at least 70%, particularly preferably at least 80%, with the proportion of iron in the total content in particular of magnetite and iron is at least 1%.
  • the proportion of residual desertite is a maximum of 30%, preferably a maximum of 20% and particularly preferably a maximum of 10% in order to achieve a sufficiently high adhesive strength, since residual desertite lowers the adhesion.
  • the high adhesive strength of the scale layer on the steel substrate is produced when the steel substrate is strongly interlocked with the scale layer. Toothing means, on the one hand, elongated iron precipitates at the phase boundary between the scale layer and the steel substrate, which are in direct contact with the base material and which, due to their elongated shape, "migrate” into the scale layer and thus “cling” it to the base material.
  • the "micro-roughness" of the steel substrate surface at the phase boundary between the scale layer and the steel substrate also forms a form of toothing. With micro-roughness is meant the roughness that is detected on a microscopic scale.
  • a tooth system that produces the high adhesive strength according to the invention is present when the areas of the scale layer and micro-roughness (5) or Alternate scale layer and iron precipitation (6) n times, so that n / L ⁇ 0.2 ⁇ m-1, in particular n / L ⁇ 0.5 ⁇ m-1, preferably n / L ⁇ 1 ⁇ m-1, where n is the number of Corresponds to intersections and the line (1) is at least 50 ⁇ m.
  • the "micro-roughness" or iron precipitations must each have a width (y) between 0.01 and 2 ⁇ m and must not be island-shaped.
  • the volume fraction of cracks and pores before the deformation in the scale layer may be a maximum of 20%, in particular 15%, preferably 10%, in order to ensure sufficiently high adhesion and cohesion in the scale layer and thus to increase the adhesive strength.
  • a quantitative determination of the volume fraction can be carried out on the light microscope image or in the scanning electron microscope with the help of the usual methods for determining area proportions, such as. B. point analysis, line intersection method or digital image analysis using the segmentation threshold method.
  • the image section viewed must be selected to be representative of the entire sample.
  • the flat steel product is characterized by a yield point greater than 165 MPa, in particular in the range between 165 and 900 MPa, preferably between 315 and 850 MPa, particularly preferably between 315 and 700 MPa.
  • the tensile strength of the flat steel product according to the invention is in the range from 250 to 1000 MPa, in particular between 250 and 900 MPa, preferably between 450 and 900 MPa and particularly preferably between 450 and 800 MPa.
  • the elongation at break of the flat steel product according to the invention is greater than or equal to 8%, in particular greater than or equal to 10%, regardless of the sample geometry. This is preferably in the range between 10 and 35%, particularly preferably between 12 and 30%.
  • the mechanical parameters are determined in accordance with DIN EN ISO 6892-1.
  • the scale layer breaks into microscopic, compact fragments during tensile deformation, which do not crumble and which are separated from one another by vertical cold cracks.
  • Vertical cold cracks here mean those cracks whose path is a maximum of twice as long as the shortest distance between the phase boundary between the scale layer and steel substrate and the scale surface.
  • the steel substrate preferably has a structure made of ferrite or pearlite or bainite or a combination of these phases, optionally contains the components martensite or retained austenite in contents of less than 5%, and in particular precipitates in the form of cementite, carbides, nitrides or carbonitrides.
  • an organic, inorganic or metallic layer or a combination of these can be applied to the flat steel product.
  • the steel melt according to the invention can contain impurities such as oxygen, hydrogen or arsenic.
  • a steel melt with the specified alloying elements must be melted and cast into a preliminary product, in particular a slab, a thin slab, a cast strip or a block with a thickness d between 2.5 and 600 mm.
  • the hot rolling of the heated preliminary product from step c) to a hot rolled flat steel product with the thickness d takes place in one or more rolling passes and leads to a scale layer thickness ZSD after the coil has cooled, the hot rolling end temperature TEW of the hot rolled flat steel product obtained when leaving the last hot rolling pass at least 770 ° C is to prevent the scale from breaking too much during rolling due to the increase in the hardness of the scale, and at most 950 ° C so that the scale layer does not exceed the optimum thickness according to formula (1).
  • the coiling temperature from step d) is in the range from 400.degree. C. to 700.degree. C., preferably in the range from 450.degree. C. to 680.degree. C., particularly preferably 450.degree. C. to 650.degree.
  • the coiling temperature determines the composition of the scale layer before slowed cooling and has a significant influence on the subsequent adhesive strength. Too high a coiling temperature above 700 ° C, preferably above 680 ° C and preferably above 650 ° C leads to undesired overoxidation of the iron oxide phase Wüstitzu magnetite and hematite, especially in the edge area on the outer surfaces of the coil.
  • the minimum reel temperature required is 400 ° C, preferably 450 ° C.
  • step e In order to develop the chemical and structural features of the scale layer according to the invention, specific cooling conditions from step e) must be set after reeling. In terms of process technology, a certain dwell time in a certain temperature range must be observed during cooling.
  • the cooling time from 500 ° C. to 350 ° C. is referred to as t5 / 3.5 and must be at least 3 hours, in particular at least 5 hours, preferably at least 8 hours, in order to form the chemical and structural features according to the invention.
  • intermediate heating and reheating is also possible in order to maintain the cooling time as long as the temperature does not fall below the corresponding specification.
  • Corresponding cooling times are necessary in order to allow the decomposition reaction of residual desert to magnetite and iron to proceed almost completely with a sufficiently high driving force and kinetics caused by the temperature.
  • the temperature values given here can be determined using a thermographic camera or thermocouple, for example.
  • the temperature can be determined by means of a numerical calculation, taking into account the decisive thermal effects (Heat conduction, radiation, convection, course of enthalpy including phase transformation) and boundary conditions take place.
  • the cooling from step e) takes place in open storage.
  • the "open" (A) storage can take place under air in a warm environment, where compliance with the residence times results, for example, from radiant heat from the warm environment.
  • "Open” storage means cooling variants in which the hot-rolled product according to the invention with the firmly adhering layer of scale, without insulation, has direct contact with the atmosphere in freely moving air.
  • An example is storage in which the hot-rolled product according to the invention with the firmly adhering layer of scale is surrounded by hot surfaces such as slabs or coils. These hot surfaces should preferably have a temperature of at least 400.degree.
  • the cooling from step e) takes place in closed storage, preferably under a protective hood, in which case, in addition to air, protective gas can also be used as the surrounding medium.
  • “Closed” storage means cooling variants in which the flat steel product according to the invention with the firmly adhering layer of scale is thermally insulated and there is no exchange of the surrounding medium with the atmosphere.
  • the cooling time can be extended here by thermal radiation from nearby hot surfaces and / or by insulating the protective hood itself and / or by one or more heating processes, for example by placing another heating hood over the protective hood and adding the heat via thermal conduction and convection is brought up to the flat steel product with the firmly adhering layer of scale. It is also possible here for the flat steel product to be reheated in order to achieve the necessary holding time.
  • the classic hood annealing can be mentioned as an example of an extension of the cooling time in a closed system.
  • the surrounding medium can be a protective gas.
  • the protective gas is hydrogen
  • the flat steel product cooled under H2 can be galvanized.
  • an iron layer forms on the surface of the scale layer, which preferably enables galvanization.
  • an organic, inorganic or metallic layer or a combination of these can be applied for the coating in step f).
  • This can preferably include galvanizing, aluminizing, painting or similar coating steps.
  • the flat steel product with the firmly adhering layer of scale according to the invention can be produced using different process routes.
  • the flat steel product can also be produced using a continuous casting plant or strip casting.
  • the essential properties of the firmly adhering scale layer according to the invention are produced by the cooling process after reeling.
  • the targeted control of the process parameters limits the thickness of the scale layer and prevents cracks parallel to the strip plane.
  • a controlled and slowed cooling process after reeling controls and accelerates the decomposition of the wustite, so that a high amount of magnetite and iron is present at room temperature and the proportion of hematite and residual wustite is kept low or avoided.
  • the thickness of the scale layer is primarily set by the process parameters during the finish rolling up to the coiler.
  • the thinnest possible layer of scale can be created by specifically setting the rolling speed, pass reduction, rolling end temperature and coiling temperature according to the process parameters described.
  • a thin layer of scale according to formula (1) is important, because the thinner the layer of scale, the lower the probability that there is a critical defect in the layer of scale, which leads to the flaking of scale within the layer of scale and thus reduces the adhesive strength.
  • the adhesive strength of the scale layer is essentially determined by the scale composition and morphology.
  • the decomposition mechanism of wüstite plays a decisive role in this. In addition to the composition of the initial waste, the decomposition mechanism depends largely on the temperature after reeling and the subsequent cooling rate. If the temperature of a hot-rolled flat steel product falls below 570 ° C, Wüstit is no longer stable. If the cooling is as slow as possible, wüstite breaks down into the stable phases magnetite and iron. The iron deposited in the scale layer increases their cohesion. The iron precipitated directly at the phase boundary between the steel substrate and the scale layer leads to good "interlocking" of the scale layer with the steel. It is known that a structure with a high proportion of these two phases is best suited to be able to develop a particularly high adhesive strength of the scale. Fractions of residual desertite or hematite should be avoided, as both phases damage the adhesive strength.
  • the flat steel product according to the invention is suitable for a large number of applications, depending on customer requirements. Possible areas of application for flat steel products, consisting of steel substrate and contacting scale layer, are primarily to be seen where unpickled hot strip is further processed through various cutting, welding and forming processes.
  • Typical applications of the flat steel product are components of construction and agricultural machines. It is also suitable for frame constructions, preferably in the truck and trailer sector, side members in the commercial vehicle sector or for components that are manufactured from sheet metal using laser cutting systems. Molds for fiber cement panels and concrete cladding in the industrial sector are also possible application examples.
  • the flat steel products according to the invention can be used both for applications in which the scale layer is removed by blasting and for those applications in which the scale layer remains on the finished component.
  • the clean, homogeneous and firmly adhering layer of scale according to the invention leads to an improvement in production and quality, especially during forming and cutting processes, since dense scale dusts are avoided, which would otherwise be indented in agglomerated form can lead or, in finely divided form, hinder optical measuring devices. This reduces rework costs and the cleaning intervals can be increased, which increases the performance of the system.
  • Another advantage of the clean, homogeneous and firmly adhering scale layer of the flat steel product is that it can be used as a forming aid.
  • flat steel products with a clean, homogeneous and firmly adhering layer of scale do not limit the thermal cutting and joining of the firmly adhering scale, on the contrary.
  • the electrical resistance is reduced by the additionally precipitated iron in the scale layer, which makes it suitable for joining with all resistance pressure welding processes, as well as for cutting with all electricity-based cutting processes, such as. B. the plasma jet cutting is improved.
  • the laser-based cutting and joining processes also show advantages. Compared to flat steel products with low and fluctuating scale adhesion or scale-free surfaces, the clean, homogeneous and firmly adhering scale layer z. B. increases process stability and improves seam cleanliness.
  • the conventional production route took place via the hot strip mill. This includes the steps of heating the slab to TA, pre-rolling and multiple descaling, finishing rolling in several stands with the final rolling temperature TEW, cooling with water or standing air and winding on the coiler to form a coil with the coiling temperature HT.
  • Tests on the casting and rolling plant were carried out on the production of thin slabs with direct reheating to TA after complete solidification, TA being generally lower than in a conventional hot strip mill. After descaling, the slabs are rolled in several passes to the final thickness d and then cooled with water and reeled.
  • Tests on the strip caster were rolled directly to thickness d after casting the strip with the final rolling temperature TEW and then cooled with water as in a conventional hot strip mill and then reeled.
  • the flat steel products were stored as free-standing coils in a hall, designated as storage method A, or as coils in a heat-insulated hood, designated as storage method B, for further cooling.
  • the surrounding medium was varied in the hood. Both air and protective gas were used.
  • Table 3 shows the maximum measured differences in the adhesive strength values between the upper and lower side of the strip of the flat steel product.
  • microstructural properties of the scale are listed for the individual tests. These include the maximum permitted scale layer thickness (ZSDmax), the measured scale layer thickness (ZSD), the volume proportion of magnetite and iron in the entire scale layer (Fe3O4 + Fe), the volume proportion of iron in magnetite and iron (Fe), the area proportion of the interface of Scale layer to steel substrate covered with residual desertite (FeOrest), the ratio of length to width of the iron precipitates in the scale (L A / B A ), the measure of the degree of interlocking that is present (n / L), the volume proportions of the cracks and pores in the scale layer (V pores + cracks ) and whether the material can be galvanized.
  • ZSDmax maximum permitted scale layer thickness
  • ZSD measured scale layer thickness
  • ZSD volume proportion of magnetite and iron in the entire scale layer
  • Fe2O4 + Fe the volume proportion of iron in magnetite and
  • Test 1 not according to the invention, with composition A did not comply with the required cooling time from 500 ° C. to 350 ° C. (t5 / 3.5). The measures to delay the cooling were not sufficient. This has resulted in incomplete disintegration of the desert, which is expressed by the high proportion of residual desert at the phase boundary between the steel substrate and the scale layer. As a result, the adhesive strength is too low. Tests 2 and 3 according to the invention show that it is possible to produce a firmly adhering layer of scale for composition A through adapted cooling conditions.
  • Experiment 22 which is not according to the invention, also shows the importance of maintaining the maximum permissible scale layer thickness ZSDmax, which was not achieved here due to an excessively high HT. As a result of the rolling process, as in test 6, this was exceeded here, which in turn led to inadequate adhesive strength.
  • the adhesion strength is determined in the scale adhesion test using image analysis and / or the reflection method.
  • a sample that has been machined on the edges is cut off Figure 1a or Figure 1b
  • An adhesive film (3) is applied to both sides and then stretched by 5%.
  • the scale flaked off by the stretching collects on the adhesive film (3) and can be evaluated by image analysis or reflection methods.
  • the value is given as an average.
  • the mean value is formed from the representative values of the scale adhesion test on the top and bottom of the sheet
  • the sample Before stretching, the sample must have been processed on the edges in order to remove the material influences caused by cutting or flame cutting and similar processing types during sampling.
  • FIG 1a it is shown how the adhesive film (3) is applied to a tensile specimen mold.
  • Figure 1b it is shown how the adhesive film (3) is applied to a bending specimen mold.
  • Commercially available transparent adhesive film such as the dc-fix film from Konrad Hornschuh AG, can be used as the adhesive film (3). Before sticking, however, make sure that the surface is cleaned and degreased. This can ensure that, for. B. Dust is completely removed from the air and the adhesive film (3) can be pressed on without bubbles.
  • the adhesive film is applied centrally to the sample, the width of the adhesive film (3) corresponding to the width (2) for the tensile specimen shape or the width (6) of the bending specimen shape.
  • Any protruding film should be cut off, for example with a cutter or scissors, in order to avoid the formation of bubbles due to the ingress of air when the protrusion is moved.
  • the length of the adhesive film (3) should be at least the area (4) shown for the tensile specimen shape or at least the area (7) shown for the bending specimen shape in order to be able to catch enough flaked scale.
  • the length of the adhesive film (3) on the bending specimen should be at least half the total length of the specimen (5).
  • the proportion of flaked scale on the adhesive film (3) can be evaluated either via image analysis or the reflection method and then easily converted to the proportion of firmly adhering scale and thus to the adhesive strength.
  • the adhesive film (3) is peeled off the sample after the stretching process and the detached scale particles on the adhesive film are evaluated using digital image analysis using the segmentation threshold method.
  • Figure 2 shows an example of a representative area of an adhesive film that has been evaluated by means of image analysis.
  • a representative area of the adhesive film selected by the tester is recorded by the program and the percentage of the area of detached scale is evaluated, which in Figure 2 is shown in black.
  • Adhesion Strength % 100 % - Share expired O ⁇ tinder
  • the adhesive strength can also be evaluated via the scale detachment using a reflection method.
  • the adhesive film (3) is also detached from the sample and the proportion of detached scale is determined by the amount of light reflected.
  • the measurement of the reflected light can e.g. B. with the device Ci60 Color Spectrophotometer from x-rite.
  • the scale particles sticking to the film cause light to be reflected.
  • the proportion of reflected light can be used as a measure of the detachment of scale. The smaller the light reflection, the higher the scale detachment.
  • the corresponding scale adhesion can be assigned to the degree of reflection by means of a series of guidelines drawn up beforehand. To create the series of guidelines, the scale adhesion is determined by means of image analysis and compared to the measured reflection values. From a graphic created in this way as in Figure 4 shown, a series of guidelines can be derived.
  • the samples for the scale adhesion test of industrially manufactured hot strip are preferably taken from representative areas of the coils. Material is considered to be representative if it is removed from the belt edge at least 30 mm from the belt edge and over the belt length at least 6 m after the start of the belt and 6 m before the end of the belt.
  • phase components and characteristics of the scale layer can be determined by means of light microscopy, scanning electron microscopy, X-ray diffractometry, electron back scattered diffraction (EBSD) and / or glow discharge optical emission spectroscopy (GD-OES).
  • EBSD electron back scattered diffraction
  • GD-OES glow discharge optical emission spectroscopy
  • the scale layer thickness and the phase composition within the scale layer are determined with a light microscope on the vertical metallographic section of the scaled hot strip directly after polishing.
  • the individual phases of the scale layer can be distinguished by their coloration in the bright field contrast.
  • the compact magnetite and the residual desertite show a light gray color.
  • the magnetite that emerged from the decay of the desertite is interspersed with white-colored iron deposits.
  • the hematite phase appears blue-gray.
  • the metallographic section must be etched in a 10% aqueous HCl solution for approx. 1-2 seconds. Thereafter, the residual desertite is clearly separated from the other phase components of the scale layer as a dark gray colored scale phase.
  • a quantitative determination of the phase composition can be carried out on the light microscopic image with the help of the usual methods, such as. B. point analysis, line intersection method or digital image analysis can be carried out using the segmentation threshold value method.
  • the phases of the scale layer can also be identified and quantified.
  • the identification is based on the different lattice types and lattice parameters of the individual scale phases wüstite, magnetite and hematite, which lead to distinctive and clearly assignable peak patterns in the X-ray diffraction diagram.
  • quantitative information on phase proportions can also be derived from the peak-height ratios.
  • the scanning electron microscope is used in particular to characterize the iron deposits precipitated in the magnetite. This concerns the amount of iron content and the more detailed description of the form of the iron precipitates. For example, to determine the iron precipitates for the toothing at the phase boundary between the steel substrate and the scale layer, the higher resolution of the scanning electron microscope image is also necessary.
  • the backscatter electron image in the material contrast is particularly suitable for distinguishing the precipitated iron from the surrounding oxide phase.
  • a fractured scale surface can also be used to characterize the iron precipitates that occur in the magnetite during the Wüstitzer fall.
  • Both light microscopy and scanning electron microscopy can be used to characterize cracks and pores within the scale layer. Particular attention should be paid to an artifact-free metallographic preparation of the samples.
  • the image section viewed must be selected to be representative of the entire sample.
  • the local characterization of residual desertite areas can, as described above, be carried out with the aid of an etching technique (10% HCl) in combination with light microscopy.
  • an etching technique (10% HCl)
  • EBSD electron back scattered diffraction
  • the residual desert can be clearly identified due to its characteristic lattice parameters and measured locally and quantitatively within previously selected measurement areas (mappings).
  • GD-OES Glow Discharge Optical Emission Spectroscopy
  • Protective gas 10 Table 3 attempt stolen ZSD in ⁇ m ZSDmax in ⁇ m Fe 3 O 4 + Fe in% Fine % L A / B A of iron FeO rest in% V pores + cracks in% n / L in ⁇ -1
  • Adhesive strength in% ⁇ adhesive strength OS / US in% Can be galvanized 1* A. 13th 15th 68 2 12th 38 18th 0.41 39 45 no 2 A. 11 13.4 96 4th 8th 3 2 0.76 91 11 Yes 3 A. 22nd 27 96 4th 7th 3 8th 1.26 89 14th Yes 4 * B. 12th 15.8 57 2 3 19th 5 0.86 41 28 no 5 B. 11 17.4 86 4th 11 3 6th 1.53 95 17th Yes 6 * B.

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CN114262239A (zh) * 2021-12-28 2022-04-01 江苏大学 一种炭/炭复合材料高温抗氧化涂层的制备方法
CN115976400A (zh) * 2022-10-09 2023-04-18 燕山大学 一种耐腐蚀钢及其制备方法和应用

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