WO2014166630A1 - Product formed by hot forming of metallic coated steel sheet, method to form the product, and steel strip - Google Patents

Product formed by hot forming of metallic coated steel sheet, method to form the product, and steel strip Download PDF

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Publication number
WO2014166630A1
WO2014166630A1 PCT/EP2014/000951 EP2014000951W WO2014166630A1 WO 2014166630 A1 WO2014166630 A1 WO 2014166630A1 EP 2014000951 W EP2014000951 W EP 2014000951W WO 2014166630 A1 WO2014166630 A1 WO 2014166630A1
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WIPO (PCT)
Prior art keywords
hot
product
steel
zone
optionally
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PCT/EP2014/000951
Other languages
French (fr)
Inventor
Shangping Chen
Guido Cornelis Hensen
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Tata Steel Ijmuiden Bv
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Publication date
Application filed by Tata Steel Ijmuiden Bv filed Critical Tata Steel Ijmuiden Bv
Priority to ES14716760T priority Critical patent/ES2891582T3/en
Priority to EP14716760.5A priority patent/EP2984198B1/en
Publication of WO2014166630A1 publication Critical patent/WO2014166630A1/en

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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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • 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
    • C23C2/261—After-treatment in a gas atmosphere, e.g. inert or reducing atmosphere
    • 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
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
    • C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
    • C23C28/321—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer with at least one metal alloy layer
    • 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
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
    • C23C28/34—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates
    • C23C28/345—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates with at least one oxide layer

Definitions

  • the invention relates to a product formed by hot forming of a metallic coated steel sheet, wherein the formed product has a substrate layer of hot formed steel, a metal affected zone and a diffusion coating layer on the substrate layer, and optionally a metal oxide layer on the diffusion coating layer.
  • the invention also relates to a method for producing such a product, and to a steel strip for use in the method.
  • Products formed by hot forming are well known in the art. Such products have the advantage that, starting from a blank having a low strength, products with high mechanical properties (such as a high tensile strength) can be produced, which products do not show spring-back. However, during production the steel oxidizes. For this reason, in recent years metallic coated blanks are used to produce hot formed products. As a coating aluminium or an aluminium alloy, or zinc or a zinc alloy can be used.
  • the steel substrate usually is a so-called boron steel.
  • a product formed by hot forming of a metallic coated steel sheet, wherein the formed product has a substrate of hot formed steel, a metal affected zone and a diffusion coating layer on the substrate, and optionally a metal oxide layer on the diffusion coating layer, wherein a low carbon zone is present between the substrate layer and the metal affected zone.
  • the metal affected zone is the zone directly under the diffusion layer, where the metal of the coating has penetrated into the substrate steel in a small amount under industrial production conditions.
  • the inventors have realized that the small amount of the coating element such as Zn will segregate at the original austenite grain boundaries in the metal affected zone, which is the main cause for the formation of the microcracks during forming, and therefore, the depth of the metal affected zone should be minimized.
  • the inventors have found that the presence of a low carbon zone in the substrate steel before coating can reduce the depth of the metal affected zone between the substrate and the diffusion coating layer during production process. In the one hand, the inventors assume that the low carbon layer will help to minimize liquid zinc penetration in the substrate and/or solid zinc diffusion into the substrate.
  • the inventors assume that the low carbon zone forms a ductile layer between the substrate and the metal affected zone during heating for hot press forming, which will dissipate tensions during the hot forming and/or cold forming (after hot forming), resulting in an improved formability. Therefore, the tendency to form micro-cracks during hot forming is reduced. The bendability of the coated sheets is also increased.
  • the metal of the metallic coated steel sheet is zinc or a zinc alloy, or aluminium or an aluminium alloy. These are nowadays the coatings that provide the best corrosion protection of the hot formed products.
  • the low carbon zone has a carbon content of at most 0.01 weight% C. With a carbon content of at most 0.01 weight%, the low carbon zone has the required ductility.
  • the metal affected zone has a thickness less than 10 ⁇ , preferably less than 5 pm.
  • the metal affected zone should be as small as possible.
  • the thickness of the coating layer and the hot press forming temperature vary.
  • the low carbon layer has a thickness of up to 30 ⁇ , preferably between 5 and 30 ⁇ , more preferably between 5 and 20 pm, most preferably between 5 and 10 pm.
  • the thickness of the low carbon layer should be not so large to influence the mechanical properties of the hot formed product.
  • the metal of the metallic coated steel sheet is galvannealed zinc, or a zinc alloy containing (in weight %) 0.1- 6 Al, 0 - 6 Mg and optionally at most 0.2 weight% of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi each, the remainder being zinc and unavoidable impurities, and preferably 0.1 - 3 Al, 0 - 3 Mg and at most 0.2 weight% of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi each, the remainder being zinc and unavoidable impurities, or wherein the metal of the metallic coated steel sheet is a zinc-nickel alloy containing (in weight%) 0.2 - 7 Ni, the remainder being zinc and unavoidable impurities, or wherein the metal of the metallic coated steel sheet is an aluminium alloy containing (in weight%) 6 - 12 Al and/or 1 - 5 Fe, the remainder being aluminium alloy
  • These coatings can provide a good corrosion protection during and after the hot forming process.
  • 1.0 - 2.5 Al and 1.0 - 2.5 Mg is present, more preferably 1.5 - 1.8 Al and 1.5 - .8 Mg.
  • the substrate layer of hot formed steel has been made from a steel having the following composition in weight %:
  • Mn 0.5 - 3.0, preferably 1.0 - 2.5
  • Si 0.1 - 0.5, preferably 0.1 - 0.4
  • Ti up to 0.2, preferably up to 0.1
  • Al up to 0.2, preferably up to 0.1
  • Nb up to 2, preferably up to 0.1
  • V up to 2, preferably up to 0.1
  • Steel having this composition is suitable for hot forming.
  • a method for producing a metallic coated hot formed product comprising the following steps:
  • a decarburized zone is formed at both sides of the steel strip before the steel strip is coated with a metallic coating, also at both sides of the steel strip.
  • the depth of the decarburized zone has to be such that after the hot forming of the product, a low carbon zone is still present between the steel substrate and the metal affected zone.
  • the hot forming is performed by either heating the blank, hot pressing the blank into a formed product, and quenching the formed product, or cold pressing the blank into a preformed product, heating the preformed product, hot pressing the preformed product into a formed product, and quenching the formed product.
  • These two hot forming methods are usually called the direct method and the indirect method. After quenching the hot formed product must be trimmed, and optionally part or all of the metal oxides are removed.
  • the decarburized zone has been provided to a depth of 20 to 50 pm, preferably a depth of 35 to 45 pm, more preferably a depth of 30 to 40 pm.
  • the decarburization zone in the hot formed product will be reduced, also depending on the thickness of the metallic coating and the heating before the hot forming step.
  • the thickness of the metal affected zone is reduced; and a low carbon layer is eventually formed between the substrate and the metal affected zone during coating and the following production processes.
  • the thickness of the coating layer and the hot press forming temperature the thickness of the diffusion coating layer and the thickness of the metal affected zone vary. Therefore, the thickness of the decarburized zone in the steel sheet should be controlled such that it is larger than the sum of the, thickness of the diffusion coating layer and the thickness of the metal affected zone to ensure the formation of the low carbon layer.
  • the decarburized zone is provided in an annealing line by applying a dew point higher than - 20 °C in an (N2 + H2) atmosphere. In this way, during the continuous annealing a decarburized zone is formed on both sides of the steel strip.
  • annealing is performed at 740 to 860 °C for 30 - 240 seconds in a N2 + 2-5% H2 atmosphere with a dew point in the range from -15 °C to 5 °C. It has been found that in this way a proper decarburized zone is formed having the required depth.
  • the decarburized zone is provided by cooling a hot rolled strip of hot formable steel from a finishing rolling temperature between the Ac1 and the Ac3 temperature to the coiling temperature between 450 °C and 750 °C. During the cooling of the strip on the run-out table the decarburized zone is formed.
  • the metallic coating on the steel strip is provided as a zinc or zinc alloy, or aluminium or aluminium alloy coating, more preferably by using hot-dip coating. These coatings are the most used for hot forming.
  • the strip of hot formable steel used has the following composition in weight %:
  • Mn 0.5 - 3.0, preferably 1.0 - 2.5
  • Si 0.1 - 0.5, preferably 0.1 - 0.4
  • Ti up to 0.2, preferably up to 0.1
  • Al up to 0.2, preferably up to 0.1
  • Nb up to 2, preferably up to 0.1
  • V up to 2, preferably up to 0.1
  • a hot formable steel strip for use in the method according to the second aspect of the invention having a composition in weight % of:
  • Mn 0.5 - 3.0, preferably 1.0 - 2.5
  • Si 0.1 - 0.5, preferably 0.1 - 0.4
  • Ti up to 0.2, preferably up to 0.1
  • Al up to 0.2, preferably up to 0.1
  • Nb up to 2, preferably up to 0.1
  • V up to 2
  • W up to 3
  • the strip at both sides has a decarburized zone to a depth of 20 to 50 ⁇ , preferably a depth of 30 to 40 ⁇ .
  • This steel strip at both sided provided with a decarburized zone, is suitable for use in the method for producing a hot formed product according to the invention, which has a better bendability and a better resistance against micro- cracks .
  • FIG. 1 gives a schematic description of the process steps according to the invention.
  • Figure 2 shows the bending angle of the coated steel according to the invention in comparison with the prior art.
  • FIG. 1 shows the sequence of steps in the process from start to finished product schematically.
  • Step A shows the substrate 1 of the steel strip that is the starting point of the process.
  • Step B shows that in the top layer of the substrate a decarburization zone 2 has been formed.
  • the decarburization zone is part of the steel strip, for clarity the decarburization is described as a layer on top of the substrate.
  • a decarburization zone is formed.
  • the decarburization zone can for instance be formed during continuous annealing, as described hereunder.
  • step C a coating layer 3 has been applied on the substrate with the decarburization layer.
  • the coating layer can for instance be applied using a hot dip coating process.
  • Step D shows the endpoint of the process, after the hot forming of a blank with the structure as seen in step C.
  • an oxide layer 6 of the coating material is formed.
  • a diffused layer 5 is formed, where the steel from the decarburization zone has diffused with the coating material.
  • a metal affected zone 4 is formed, where the decarburized zone has been partially consumed by the diffusion coating layer.
  • Between the substrate and the metal affected zone is shown what remains of the decarburization zone in the form of a low carbon zone 2.
  • the thickness of the oxide layer, the diffused layer and the metal affected zone will depend on a number of variables.
  • the type of metallic coating is such a variable, and also the thickness of this coating layer, but probably also the heating time of the blank before the hot press forming, and the heating temperature of the blank.
  • a decarburization zone with a depth of 30 to 40 pm can be used (as shown in step B of Figure 1), so as to obtain a low carbon zone in the range of 5 to 10 pm in the hot formed product (as shown in step D of Figure 1).
  • the required decarburization depth on the substrate of the steel strip can be found for a different coating, another coating thickness, and, if needed, another furnace temperature and heating time.
  • a cold rolled strip of 22MnB5 steel having a thickness of 1.5 mm is continuously annealed at a temperature of approximately 800° C during about 120 seconds in a N2 + 2% H2 atmosphere with a dew point of -5° C. In this way, a decarburization zone with a depth of approximately 35 pm is obtained.
  • the steel strip is then cooled to about 460° C at a rate of about 10° C/sec, held for about 2 seconds at 460° C, and then hot-dipped in a Zinc bath of 460 °C.
  • the zinc bath contains 0.19 wt% aluminium and 0.011 wt% iron.
  • the coated steel strip is wiped with nitrogen to a thickness of 130 g/m2. The coated strip is then annealed.
  • Figure 2 shows a three-point bending test to measure the bendability.
  • the bending angle is shown on the vertical axis, indicated with A.
  • tests with prior art zinc coated steel sheet are shown as column PA (prior art), and tests with the zinc coated steel sheet according to the present invention are shown as the column PI (present invention).
  • column PA prior art zinc coated steel sheet
  • column PI present invention
  • two rollers with a diameter of 30 mm are disposed at a distance of twice the sheet thickness plus 0.5 mm.
  • the hardened sheet is placed thereon and then subjected to stress with a bending punch having a radius of 0.4 mm at the same distance, respectively, from the rollers.
  • the time, the distance between the contact of the bending punch with the sample and the original position thereof, and the force are measured and recorded.
  • the angle is calculated from the distance.
  • the bending angle at maximum force without cracking on the surface of the specimen is applied as test criterion. It can be seen that for a steel sheet (1.5 mm) of the 22MnB5 type with zinc coated GA130 according to the prior art (PA) a bending angle of about 50° can be reached, whereas with comparable zinc coated steel produced according to the invention (PI), a bending angle of about 75° can be reached. This is a mayor improvement.

Abstract

The invention relates to a product formed by hot forming of a metallic coated steel sheet, wherein the formed product has a substrate layer of hot formed steel, a metal affected zone and a diffusion coating layer on the substrate layer, and optionally a metal oxide layer on the diffusion coating layer. According to the invention a low carbon zone is present between the substrate layer and the metal affected zone. The invention also relates to a method for producing a metallic coated hot formed product, and to a steel strip to be used in the method.

Description

PRODUCT FORMED BY HOT FORMING OF METALLIC COATED STEEL SHEET, METHOD TO FORM THE PRODUCT, AND STEEL STRIP
The invention relates to a product formed by hot forming of a metallic coated steel sheet, wherein the formed product has a substrate layer of hot formed steel, a metal affected zone and a diffusion coating layer on the substrate layer, and optionally a metal oxide layer on the diffusion coating layer. The invention also relates to a method for producing such a product, and to a steel strip for use in the method.
Products formed by hot forming are well known in the art. Such products have the advantage that, starting from a blank having a low strength, products with high mechanical properties (such as a high tensile strength) can be produced, which products do not show spring-back. However, during production the steel oxidizes. For this reason, in recent years metallic coated blanks are used to produce hot formed products. As a coating aluminium or an aluminium alloy, or zinc or a zinc alloy can be used. The steel substrate usually is a so-called boron steel.
It has been found that the use of such coatings has the disadvantage that the formability of the steel sheet is hampered, and more specific, the bendability of the coated steel sheets after hot forming is reduced; and that especially for zinc (alloy) coated steel sheets micro-cracks are formed during the hot and/or cold forming process. Bendability is the property of being easily bent without breaking, measured by bending angle at maximum force in a three-point bending test. Higher bendability can have a positive influence on the crash behaviour. Micro-cracks are usually present in the coating, but due to high stresses in the product micro-cracks can also be introduced in the surface of the substrate steel. These micro-cracks occur in particular in areas that are formed with a high degree of deformation.
It is an object of the present invention to provide a metallic coated hot formed product having an improved formability.
It is another object of the present invention to provide a zinc coated hot formed product having less and/or smaller micro-cracks. It is also an object of the invention to provide a method for producing a metallic coated hot product, having improved bendability and/or less/smaller micro-cracks.
Furthermore, it is an object of the invention to provide a hot formable steel strip that can be used for forming such products.
According to a first aspect of the invention a product is provided formed by hot forming of a metallic coated steel sheet, wherein the formed product has a substrate of hot formed steel, a metal affected zone and a diffusion coating layer on the substrate, and optionally a metal oxide layer on the diffusion coating layer, wherein a low carbon zone is present between the substrate layer and the metal affected zone.
The metal affected zone is the zone directly under the diffusion layer, where the metal of the coating has penetrated into the substrate steel in a small amount under industrial production conditions. The inventors have realized that the small amount of the coating element such as Zn will segregate at the original austenite grain boundaries in the metal affected zone, which is the main cause for the formation of the microcracks during forming, and therefore, the depth of the metal affected zone should be minimized. The inventors have found that the presence of a low carbon zone in the substrate steel before coating can reduce the depth of the metal affected zone between the substrate and the diffusion coating layer during production process. In the one hand, the inventors assume that the low carbon layer will help to minimize liquid zinc penetration in the substrate and/or solid zinc diffusion into the substrate. On the other hand, the inventors assume that the low carbon zone forms a ductile layer between the substrate and the metal affected zone during heating for hot press forming, which will dissipate tensions during the hot forming and/or cold forming (after hot forming), resulting in an improved formability. Therefore, the tendency to form micro-cracks during hot forming is reduced. The bendability of the coated sheets is also increased.
Preferably the metal of the metallic coated steel sheet is zinc or a zinc alloy, or aluminium or an aluminium alloy. These are nowadays the coatings that provide the best corrosion protection of the hot formed products.
According to a preferred embodiment the low carbon zone has a carbon content of at most 0.01 weight% C. With a carbon content of at most 0.01 weight%, the low carbon zone has the required ductility.
According to a preferred embodiment the metal affected zone has a thickness less than 10 μιη, preferably less than 5 pm. The metal affected zone should be as small as possible. By providing a proper decarburized zone in the steel sheet before coating, the thickness of the metal affected zone is reduced; and a low carbon layer is eventually formed between the substrate and the metal affected zone during coating and the following production processes.
Depending on the coating types, the thickness of the coating layer and the hot press forming temperature, the thickness of the diffusion coating layer and the thickness of the metal affected zone vary.
Preferably, the low carbon layer has a thickness of up to 30 μιη, preferably between 5 and 30 μηι, more preferably between 5 and 20 pm, most preferably between 5 and 10 pm. The thickness of the low carbon layer should be not so large to influence the mechanical properties of the hot formed product.
Preferably, the metal of the metallic coated steel sheet is galvannealed zinc, or a zinc alloy containing (in weight %) 0.1- 6 Al, 0 - 6 Mg and optionally at most 0.2 weight% of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi each, the remainder being zinc and unavoidable impurities, and preferably 0.1 - 3 Al, 0 - 3 Mg and at most 0.2 weight% of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi each, the remainder being zinc and unavoidable impurities, or wherein the metal of the metallic coated steel sheet is a zinc-nickel alloy containing (in weight%) 0.2 - 7 Ni, the remainder being zinc and unavoidable impurities, or wherein the metal of the metallic coated steel sheet is an aluminium alloy containing (in weight%) 6 - 12 Al and/or 1 - 5 Fe, the remainder being aluminium and unavoidable impurities. These coatings can provide a good corrosion protection during and after the hot forming process. Usually at most 0.1 weight% of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi each is present in the zinc alloy, or even at most 0.02 weight%. Preferably in the zinc alloy 1.0 - 2.5 Al and 1.0 - 2.5 Mg is present, more preferably 1.5 - 1.8 Al and 1.5 - .8 Mg.
According to a preferred embodiment the substrate layer of hot formed steel has been made from a steel having the following composition in weight %:
C: 0.10 - 0.5, preferably 0.15 - 0.4
Mn: 0.5 - 3.0, preferably 1.0 - 2.5
Si: 0.1 - 0.5, preferably 0.1 - 0.4
Cr: up to 1.0, preferably up to 0.8
Ti: up to 0.2, preferably up to 0.1
Al: up to 0.2, preferably up to 0.1
P: up to 0.1 , preferably up to 0.08
S: up to 0.05, preferably up to 0.04
B: 0.0005 - 0.08, preferably 0.0005 - 0.04
optionally Nb up to 2, preferably up to 0.1
optionally V up to 2, preferably up to 0.1
optionally W up to 3, preferably up to 0.1
the remainder being iron and unavoidable impurities.
Steel having this composition is suitable for hot forming.
According to a second aspect of the invention a method for producing a metallic coated hot formed product is provided, the method comprising the following steps:
- providing a strip of hot formable steel having a decarburized zone with a controlled depth at both sides of the strip of steel;
- providing a metallic coating on the decarburized steel strip;
- cutting a blank from the coated strip of metal;
- hot forming the coated blank or preformed part into a metallic coated hot formed product.
According to the invention a decarburized zone is formed at both sides of the steel strip before the steel strip is coated with a metallic coating, also at both sides of the steel strip. The depth of the decarburized zone has to be such that after the hot forming of the product, a low carbon zone is still present between the steel substrate and the metal affected zone.
Preferably the hot forming is performed by either heating the blank, hot pressing the blank into a formed product, and quenching the formed product, or cold pressing the blank into a preformed product, heating the preformed product, hot pressing the preformed product into a formed product, and quenching the formed product. These two hot forming methods are usually called the direct method and the indirect method. After quenching the hot formed product must be trimmed, and optionally part or all of the metal oxides are removed.
According to a preferred method the decarburized zone has been provided to a depth of 20 to 50 pm, preferably a depth of 35 to 45 pm, more preferably a depth of 30 to 40 pm. During the subsequent method steps the decarburization zone in the hot formed product will be reduced, also depending on the thickness of the metallic coating and the heating before the hot forming step. By providing a proper decarburized zone in the steel sheet before coating, the thickness of the metal affected zone is reduced; and a low carbon layer is eventually formed between the substrate and the metal affected zone during coating and the following production processes. Depending on the coating types, the thickness of the coating layer and the hot press forming temperature, the thickness of the diffusion coating layer and the thickness of the metal affected zone vary. Therefore, the thickness of the decarburized zone in the steel sheet should be controlled such that it is larger than the sum of the, thickness of the diffusion coating layer and the thickness of the metal affected zone to ensure the formation of the low carbon layer.
According to a first preferred embodiment of the method the decarburized zone is provided in an annealing line by applying a dew point higher than - 20 °C in an (N2 + H2) atmosphere. In this way, during the continuous annealing a decarburized zone is formed on both sides of the steel strip.
Preferably, annealing is performed at 740 to 860 °C for 30 - 240 seconds in a N2 + 2-5% H2 atmosphere with a dew point in the range from -15 °C to 5 °C. It has been found that in this way a proper decarburized zone is formed having the required depth.
According to a second preferred embodiment of the method the decarburized zone is provided by cooling a hot rolled strip of hot formable steel from a finishing rolling temperature between the Ac1 and the Ac3 temperature to the coiling temperature between 450 °C and 750 °C. During the cooling of the strip on the run-out table the decarburized zone is formed. Preferably, the metallic coating on the steel strip is provided as a zinc or zinc alloy, or aluminium or aluminium alloy coating, more preferably by using hot-dip coating. These coatings are the most used for hot forming.
According to a preferred embodiment the strip of hot formable steel used has the following composition in weight %:
C: 0.10 - 0.5, preferably 0.15 - 0.4
Mn: 0.5 - 3.0, preferably 1.0 - 2.5
Si: 0.1 - 0.5, preferably 0.1 - 0.4
Cr: up to 1.0, preferably up to 0.8
Ti: up to 0.2, preferably up to 0.1
Al: up to 0.2, preferably up to 0.1
P: up to 0.1 , preferably up to 0.08
S: up to 0.05, preferably up to 0.04
B: 0.0005 - 0.08, preferably 0.0005 - 0.04
optionally Nb up to 2, preferably up to 0.1
optionally V up to 2, preferably up to 0.1
optionally W up to 3, preferably up to 0.1
the remainder being iron and unavoidable impurities.
Steel with such a composition is suitable for hot forming.
According to a third aspect of the invention a hot formable steel strip for use in the method according to the second aspect of the invention is provided, the steel strip having a composition in weight % of:
C: 0.10 - 0.5, preferably 0.15 - 0.4
Mn: 0.5 - 3.0, preferably 1.0 - 2.5
Si: 0.1 - 0.5, preferably 0.1 - 0.4
Cr: up to 1.0, preferably up to 0.8
Ti: up to 0.2, preferably up to 0.1
Al: up to 0.2, preferably up to 0.1
P: up to 0.1 , preferably up to 0.08
S: up to 0.05, preferably up to 0.04
B: 0=0005 = 0.08, preferably 0.0005 - 0.04
optionally Nb up to 2, preferably up to 0.1
optionally V up to 2, preferably up to 0.1 optionally W up to 3, preferably up to 0.1
the remainder being iron and unavoidable impurities,
wherein the strip at both sides has a decarburized zone to a depth of 20 to 50 μηι, preferably a depth of 30 to 40 μηι.
This steel strip, at both sided provided with a decarburized zone, is suitable for use in the method for producing a hot formed product according to the invention, which has a better bendability and a better resistance against micro- cracks .
The invention will be elucidated using the accompanying figures, also providing an example.
Figure 1 gives a schematic description of the process steps according to the invention.
Figure 2 shows the bending angle of the coated steel according to the invention in comparison with the prior art.
Figure 1 shows the sequence of steps in the process from start to finished product schematically. Step A shows the substrate 1 of the steel strip that is the starting point of the process. Step B shows that in the top layer of the substrate a decarburization zone 2 has been formed. Though the decarburization zone is part of the steel strip, for clarity the decarburization is described as a layer on top of the substrate. Though not shown, usually on both sides of the substrate a decarburization zone is formed. The decarburization zone can for instance be formed during continuous annealing, as described hereunder. In step C a coating layer 3 has been applied on the substrate with the decarburization layer. The coating layer can for instance be applied using a hot dip coating process. Step D shows the endpoint of the process, after the hot forming of a blank with the structure as seen in step C. On top, an oxide layer 6 of the coating material is formed. Under the oxide layer, a diffused layer 5 is formed, where the steel from the decarburization zone has diffused with the coating material. Directly below the diffused layer a metal affected zone 4 is formed, where the decarburized zone has been partially consumed by the diffusion coating layer. Between the substrate and the metal affected zone is shown what remains of the decarburization zone in the form of a low carbon zone 2. These layers are usually already formed during the heating of the blank before the hot press forming in a hot forming press.
It will be clear that the thickness of the oxide layer, the diffused layer and the metal affected zone will depend on a number of variables. Clearly the type of metallic coating is such a variable, and also the thickness of this coating layer, but probably also the heating time of the blank before the hot press forming, and the heating temperature of the blank.
However, the inventors have found that when a GA 130 coating is used and the blank is heated in a preheated furnace of 900° C during approximately 5 minutes, a decarburization zone with a depth of 30 to 40 pm can be used (as shown in step B of Figure 1), so as to obtain a low carbon zone in the range of 5 to 10 pm in the hot formed product (as shown in step D of Figure 1). With some routine experiments the required decarburization depth on the substrate of the steel strip can be found for a different coating, another coating thickness, and, if needed, another furnace temperature and heating time.
An example for the production of a decarburization zone is as follows. A cold rolled strip of 22MnB5 steel having a thickness of 1.5 mm is continuously annealed at a temperature of approximately 800° C during about 120 seconds in a N2 + 2% H2 atmosphere with a dew point of -5° C. In this way, a decarburization zone with a depth of approximately 35 pm is obtained.
The steel strip is then cooled to about 460° C at a rate of about 10° C/sec, held for about 2 seconds at 460° C, and then hot-dipped in a Zinc bath of 460 °C. The zinc bath contains 0.19 wt% aluminium and 0.011 wt% iron. After the hot dip coating, the coated steel strip is wiped with nitrogen to a thickness of 130 g/m2. The coated strip is then annealed.
Figure 2 shows a three-point bending test to measure the bendability. The bending angle is shown on the vertical axis, indicated with A. In the figure tests with prior art zinc coated steel sheet are shown as column PA (prior art), and tests with the zinc coated steel sheet according to the present invention are shown as the column PI (present invention). In this three-point bending test, two rollers with a diameter of 30 mm are disposed at a distance of twice the sheet thickness plus 0.5 mm. The hardened sheet is placed thereon and then subjected to stress with a bending punch having a radius of 0.4 mm at the same distance, respectively, from the rollers. The time, the distance between the contact of the bending punch with the sample and the original position thereof, and the force are measured and recorded. The angle is calculated from the distance. The bending angle at maximum force without cracking on the surface of the specimen is applied as test criterion. It can be seen that for a steel sheet (1.5 mm) of the 22MnB5 type with zinc coated GA130 according to the prior art (PA) a bending angle of about 50° can be reached, whereas with comparable zinc coated steel produced according to the invention (PI), a bending angle of about 75° can be reached. This is a mayor improvement.

Claims

1. Product formed by hot forming of a metallic coated steel sheet, wherein the formed product has a substrate layer of hot formed steel, a metal affected zone and a diffusion coating layer on the substrate layer, and optionally a metal oxide layer on the diffusion coating layer, characterised in that a low carbon zone is present between the substrate layer and the metal affected zone. 2. Product according to claim 1 , wherein the metal of the metallic coated steel sheet is zinc or a zinc alloy, or aluminium or an aluminium alloy.
3. Product according to claim 1 or 2, wherein the low carbon zone has a carbon content of at most 0.01 weight% C.
4. Product according to any one of the preceding claims, wherein the metal affected zone has a thickness of less than 10 pm, preferably of less than 5 pm. 5. Product according to any one of the preceding claims, wherein the low carbon layer has a thickness of up to 30 pm, preferably between 5 and 30 pm, more preferably between 5 and 20 pm, most preferably between 5 and 10 pm. 6. Product according to any one of the preceding claims, wherein the metal of the metallic coated steel sheet is galvannealed zinc, or a zinc alloy containing (in weight%) 0.1 - 6 Al, 0 - 6 Mg and optionally at most 0.2 weight% of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and/or Bi each, the remainder being zinc and unavoidable impurities, and preferably 0.1 - 3 Al, 0 - 3 Mg and at most 0.2 weight% of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce,
Cr, Ni, Zr and/or Bi each, the remainder being zinc and unavoidable impurities, or wherein the metal of the metallic coated steel sheet is a zinc-nickel alloy containing (in weight%) 0.2 - 7 Ni, the remainder being zinc and unavoidable impurities, or wherein the metal of the metallic coated steel sheet is an aluminium alloy containing (in weight%) 6 - 12 Al and/or 1 - 5 Fe, the remainder being aluminium and unavoidable impurities.
Product according to any one of the preceding claims, wherein the substrate layer of hot formed steel has been made from a steel having the following composition in weight %:
C: 0.10 - 0.5, preferably 0.15 - 0.4
Mn: 0.5 - 3.0, preferably 1.0 - 2.5
Si: 0.1 - 0.5, preferably 0.1 - 0.4
Cr: up to 1.0, preferably up to 0.8
Ti: up to 0.2, preferably up to 0.1
Al: up to 0.2, preferably up to 0.1
P: up to 0.1 , preferably up to 0.08
S: up to 0.05, preferably up to 0.04
B: 0.0005 - 0.08, preferably 0.0005 - 0.04
optionally Nb up to 2, preferably up to 0.1
optionally V up to 2, preferably up to 0.1
optionally W up to 3, preferably up to 0.1
the remainder being iron and unavoidable impurities.
Method for producing a metallic coated hot formed product, the method comprising the following steps:
providing a strip of hot formable steel having a decarburized zone with a controlled depth at both sides of the strip of steel;
providing a metallic coating on the decarburized steel strip;
cutting a blank from the coated strip of metal;
hot forming the coated blank or preformed part into a metallic coated hot formed product.
Method according to claim 8, wherein the hot forming is performed by either heating the blank, hot pressing the blank into a formed product, and quenching the formed product, or cold pressing the blank into a preformed product, heating the preformed product, hot pressing the preformed product into a formed product, and quenching the formed product.
10. Method according to claim 8 or 9, wherein the decarburized zone has been provided to a depth of 20 to 50 pm, preferably a depth of 35 to 45 pm, more preferably a depth of 30 to 40 pm.
11. Method according to any one of claims 8 - 10, wherein the decarburized zone is provided in an annealing line by applying a dew point higher than - 20 °C in an (N2 + H2) atmosphere.
Method according to claim 11 , wherein annealing is performed at 740 to 860 °C for 30 -240 seconds in a N2 + 2-5% H2 atmosphere with a dew point in the range from -15 °C to 5 °C.
Method according to any one of claims 8 - 10, wherein the decarburized zone is provided by cooling a hot rolled strip of hot formable steel from a finishing rolling temperature between the Ac1 and the Ac3 temperature to the coiling temperature between 450 °C and 750 °C.
Method according to any one of claims 8 - 13, wherein the metallic coating on the steel strip is provided as a zinc or zinc alloy, or aluminium or aluminium alloy coating, preferably by using hot-dip coating.
Method according to any one of claims 8 - 14, wherein the strip of hot formable steel used has the following composition in weight %:
C: 0.10 - 0.5, preferably 0.15 - 0.4
Mn: 0.5 - 3.0, preferably 1 ,0 - 2.5
Si: 0.1 - 0.5, preferably 0.1 - 0.4
Cr: up to 1.0, preferably up to 0.8 Ti: up to 0.2, preferably up to 0.1
Al: up to 0.2, preferably up to 0.1
P: up to 0.1 , preferably up to 0.08
S: up to 0.05, preferably up to 0.04
B: 0.0005 - 0.08, preferably 0.0005 - 0.04
optionally Nb up to 2, preferably up to 0.1
optionally V up to 2, preferably up to 0.1
optionally W up to 3, preferably up to 0.1
the remainder being iron and unavoidable impurities.
16. Hot formable steel strip for use in the method according to any one of claims 8 - 15, the steel strip having a composition in weight % of:
C: 0.10 - 0.5, preferably 0.15 - 0.4
Mn: 0.5 - 3.0, preferably 1.0 - 2.5
Si: 0.1 - 0.5, preferably 0.1 - 0.4
Cr: up to 1.0, preferably up to 0.8
Ti: up to 0.2, preferably up to 0.1
Al: up to 0.2, preferably up to 0.1
P: up to 0.1 , preferably up to 0.08
S: up to 0.05, preferably up to 0.04
B: 0.0005 - 0.08, preferably 0.0005 - 0.04
optionally Nb up to 2, preferably up to 0.1
optionally V up to 2, preferably up to 0.1
optionally W up to 3, preferably up to 0.1
the remainder being iron and unavoidable impurities,
wherein the strip at both sides has a decarburized zone provided to a depth of 20 to 50 μιη, preferably a depth of 30 to 40 pm.
PCT/EP2014/000951 2013-04-10 2014-04-09 Product formed by hot forming of metallic coated steel sheet, method to form the product, and steel strip WO2014166630A1 (en)

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