WO2012167930A1 - Hot formable strip, sheet or blank, process for the production thereof, method for hot forming a product and hot formed product - Google Patents

Hot formable strip, sheet or blank, process for the production thereof, method for hot forming a product and hot formed product Download PDF

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Publication number
WO2012167930A1
WO2012167930A1 PCT/EP2012/002416 EP2012002416W WO2012167930A1 WO 2012167930 A1 WO2012167930 A1 WO 2012167930A1 EP 2012002416 W EP2012002416 W EP 2012002416W WO 2012167930 A1 WO2012167930 A1 WO 2012167930A1
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Prior art keywords
less
sheet
strip
hot
blank
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PCT/EP2012/002416
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French (fr)
Inventor
Robert Bleeker
Original Assignee
Tata Steel Ijmuiden B.V.
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Publication date
Application filed by Tata Steel Ijmuiden B.V. filed Critical Tata Steel Ijmuiden B.V.
Priority to EP12728979.1A priority Critical patent/EP2718027A1/en
Priority to CN201280027154.9A priority patent/CN103582531A/en
Publication of WO2012167930A1 publication Critical patent/WO2012167930A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
    • B05D7/14—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to metal, e.g. car bodies
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D37/00—Tools as parts of machines covered by this subclass
    • B21D37/16—Heating or cooling
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • 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
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • 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
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
    • C23C22/74—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process for obtaining burned-in conversion coatings
    • 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
    • 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
    • C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
    • C23C28/322—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer only coatings of metal elements only
    • C23C28/3225—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer only coatings of metal elements only with at least one zinc-based 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
    • 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
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D2350/00—Pretreatment of the substrate
    • B05D2350/60—Adding a layer before coating
    • B05D2350/65—Adding a layer before coating metal layer
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05D2701/00—Coatings being able to withstand changes in the shape of the substrate or to withstand welding
    • 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
    • C23C2222/00—Aspects relating to chemical surface treatment of metallic material by reaction of the surface with a reactive medium
    • C23C2222/20—Use of solutions containing silanes

Definitions

  • the invention relates to a hot formable strip, sheet or blank which is formable at a temperature of 600° C or above, comprising a substrate of hot formable steel coated with a corrosion protective coating.
  • the invention also relates to a process for producing such a strip, sheet or blank, a method for hot forming a product and a hot formed product made therefrom.
  • Such a coated strip, sheet or blank is known from EP 0971044A1 , relating to an
  • Uncoated boron steels are known to form Fe oxides during the heat treatment preceding the hot forming step in a die, as a consequence whereof loose and thick oxide layers are formed on the surface, which can pollute and damage the surface of the die. Moreover, such oxide layers interfere with the welding process of the formed product during the subsequent use of the formed product, and also contaminate subsequent painting processes. Therefore, the oxide layers have to be removed after the hot forming process of the uncoated steel products, which is inefficient and costly.
  • coated boron steels have been developed, and the boron steel substrate has been covered with a metallic coating such as an Al-Si coating or a Zn based coating.
  • a metallic coating such as an Al-Si coating or a Zn based coating.
  • oxide layer due to the high temperature needed for the hot forming process, which oxide layer is formed from the coating material.
  • a zinc-oxide layer makes that it is difficult to spot weld the hot formed products. Good spot weldability is needed for the automotive industry, where hot formed parts are used.
  • a steel strip, sheet or blank for hot forming at a temperature of Ac1 or above comprising a substrate of hot formable steel coated with a corrosion protective coating, wherein the coated steel substrate is covered by a siloxane layer.
  • siloxane layer is suitable to reduce the extent of oxidation during the hot forming of a steel strip, sheet and blank coated with a corrosion protective coating.
  • the siloxane layer also reduces zinc losses due to evaporation.
  • the siloxane coated material When hot formed, the siloxane coated material will be oxidised to leave a thin adhering silicon oxide layer, which has a low contact resistance and consequently good weldability. Because of the reduced oxidation of the corrosion protective coating, the siloxane layer also improves the effectiveness of the coating for corrosion protection of the steel.
  • the steel substrate is for hot forming at Ac1 -temperature or above. Such steel substrates can be used for hot forming and quenching of the hot formed product, which usually provides a tensile strength of 1500 MPa or more.
  • Siloxane layers are normally used as chromate replacing passivation treatment or as phosphate or chromate replacing conversion treatment to improve adhesion with adhesives or paints. They are formed after curing of a water-based silane film eventually containing alcohol as well, applied by roll coating, dip/squeezing or spray/squeezing and contain silicon atoms that are bound with oxygen atoms and/or hydroxyl groups as well as with the carbon atoms of hydrocarbon and/or hydrocarbyl groups. The carbon atoms of the hydrocarbon and/or hydrocarbyl groups can be bound with each other or with intermediate amine or carbonyl groups and/or sulphur atoms.
  • the siloxane layer originates from a bis- silane containing an amount of carbon such that C/Si ⁇ 1.
  • the siloxane layer thus adds optimal to the protection of the steel due to the little formation of carbon dioxide resulting in an almost porous-free, homogeneous silicon oxide layer, which has shown to provide best welding possibilities for the hot formed product.
  • the thickness of the siloxane layer is such that at most 200 mg Si/m 2 remains in the silicon oxide layer after hot forming, preferably between 20 and 100 mg Si/m 2 , and more preferably between 40 and 100 mg Si/m 2 .
  • the inventors have found that the thickness of the metal-phosphate layer should be at most 200 mg Si/m 2 since with higher thickness the silicon oxide layer will itself hamper the contact resistance of the spot welded material. Moreover, also the costs of the layer become too high. Therefore, thinner layers are preferred.
  • the corrosion protective layer is a zinc or zinc alloy layer. A coating of zinc or a zinc alloy gives a very good, active corrosion protection of the steel substrate.
  • the zinc or zinc alloy layer is a galvannealed layer containing Fe up to 70 weight % and preferably containing Fe from 8 to 12 weight %. It has been found that a siloxane layer as mentioned above on this type of coating will give good spot weldability. Using this galvannealed zinc or zinc alloy layer makes it easier to hot form the coated steel as well.
  • the zinc or zinc alloy layer is a galvannealed layer containing Fe up to 70 weight % and preferably containing Fe from 1 to 5 weight %.
  • the zinc alloy layer consists of 0.3 to 2.3 weight % magnesium and 0.6 to 2.3 weight % aluminium, optional less than 0.2 weight % of one or more additional elements, the remainder being zinc and unavoidable impurities.
  • An optional element that could be added in a small amount, less than 0.2 weight %, could be Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn, Bi and Sb are usually added to form spangles. This type of coating has an improved corrosion resistance as compared to usual zinc layers.
  • the corrosion protective layer has a thickness of 2 to 25 m. These thicknesses are suitable to produce hot formed parts for automotive purposes.
  • the hot formable steel substrate is a steel substrate, having a composition in weight percent:
  • the hot formable steel substrate is a steel substrate, having a composition in weight percent:
  • the amount of boron is usually between 0.001 and 0.005 weight %.
  • the hot formable steel substrate is a steel substrate, having a composition in weight percent:
  • the amount of boron is usually between 0.001 and 0.005 weight %.
  • the steel substrate has a thickness between 0.5 and 3.0 mm. Such thicknesses are used in the automotive industry.
  • the invention also relates to a process for producing a hot formable strip, sheet or blank as described above, wherein a substrate of hot formable steel coated with a corrosion protective coating is covered by a solution of silane, whereafter the strip, sheet or blank is heated to form the siloxane layer. In this way it is possible to provide the steel substrate with the siloxane layer as exemplified above.
  • the silane solution is applied on the coated steel strip, sheet or blank as a wet film having a thickness of 1 to 10 ml/m 2 by spraying or dipping and subsequently squeezing or by roll coating, preferably at the end of a coating line, and the wet film is dried and cured at a temperature of between 70 °C and 200 °C, preferably between 70 °C and 100 °C.
  • This process of applying the wet film is easy to perform and gives an even wet layer on the coated substrate, which is dried and cured in the usual manner, for instance in an oven.
  • the siloxane covered corrosion protection coated substrate embodies one or more of the features of the strip, sheet or blank as exemplified above.
  • the invention moreover relates to a method for hot forming a product using a blank as described above or using a blank formed from a strip or sheet as described above, wherein the blank or a pre-form formed out of the blank is heated to the Ac1- temperature or above, the blank or pre-form is hot formed in a hot forming die, and the hot formed product is quenched.
  • This is the usual hot forming process, which is now performed using a blank according to the invention or a blank produced from the strip or sheet according to the invention.
  • the product formed has a good weldability.
  • the invention furthermore relates to a hot formed product using the strip, sheet or blank as described above or produced according to the method above, wherein the coated hot formed substrate has a contact resistance below 1 mOhm.
  • the contact resistance was measured according to a method described in DVS2929:2007 with the exceptions that a 7.5kN force and 300mm electrode radius were used.
  • a contact resistance of below 1 mOhm stands for good weldability.
  • Fig. 1 shows the contact resistance of heated zinc coated steel for two different siloxane layer thickness as well as the expected siloxane layer thickness needed to achieve good weldability ( ⁇ 1 mOhm).
  • Fig. 2A shows a zinc coated substrate without post treatment after heating.
  • Fig.2B. shows the same substrate with a siloxane layer after heating.
  • silane namely bis-(trimethoxysilyl)ethane (BTSE) (which is commercially available) was chosen for the investigation.
  • This silane was applied as a water-based solution for two different concentrations namely 0.5 % weight % and 3 weight % by roll-coating, resulting in a wet film thickness of ca. 10 ml/m 2 to give a siloxane coating weight of respective 2 and 12 mg Si/m 2 after drying in an oven.
  • the substrates were 22MnB5 steel sheets coated with a zinc coating that had an additional heat treatment to have 12 % Fe in the coating.
  • Table 1 below shows the results the inventors obtained for different post-treatments after a heat cycle of 6 min in a chamber furnace heated to 880 °C . It will be clear that the applied siloxane layer increases the amount of zinc remaining in the sheets, contributing to a better corrosion protection, and reduces the amount of zinc-oxide, which reduces contact resistance. However, the difference between both concentrations is too small to find a significant effect of concentration. Nevertheless, it is clear that by using higher silane concentrations we will achieve the needed contact resistance lower than 1 mOhm that is required for hot formed parts in the automotive industry to provide good welding.
  • Figure 1 tells us that when we extrapolate the results (dotted line), we at least have to apply siloxane layers containing > 20 mg Si/m 2 and preferably > 40 mg Si/m 2 , taking deviation into account, to achieve a contact resistance lower than 1 mOhm.
  • Figure 2 shows the difference between a zinc coated substrate without post treatment (Fig. 2A) and with the siloxane layer (Fig. 2B). It is clear from the figures that the use of the siloxane layer minimizes the oxide layer formed in the zinc coating due to the heating of the silane treated zinc coated steel.

Abstract

The invention relates to a hot formable strip, sheet or blank which is formable at a temperature of 600° C or above, comprising a substrate of hot formable steel coated with a corrosion protective coating. According to the invention, the coated steel substrate is covered by a siloxane layer. The invention also relates to a process to produce such strip, sheet or blank, a method for hot forming a product using such a strip, sheet or blank and a hot formed product using such a strip, sheet or blank.

Description

HOT FORMABLE STRIP, SHEET OR BLANK, PROCESS FOR THE PRODUCTION THEREOF, METHOD FOR HOT FORMING A PRODUCT AND HOT FORMED
PRODUCT The invention relates to a hot formable strip, sheet or blank which is formable at a temperature of 600° C or above, comprising a substrate of hot formable steel coated with a corrosion protective coating. The invention also relates to a process for producing such a strip, sheet or blank, a method for hot forming a product and a hot formed product made therefrom.
Such a coated strip, sheet or blank is known from EP 0971044A1 , relating to an
Al-Si coated boron steel; the process of hot forming a zinc coated boron steel is known from for instance EP 1143029A1.
Uncoated boron steels are known to form Fe oxides during the heat treatment preceding the hot forming step in a die, as a consequence whereof loose and thick oxide layers are formed on the surface, which can pollute and damage the surface of the die. Moreover, such oxide layers interfere with the welding process of the formed product during the subsequent use of the formed product, and also contaminate subsequent painting processes. Therefore, the oxide layers have to be removed after the hot forming process of the uncoated steel products, which is inefficient and costly.
To overcome the above problems and to improve the corrosion performance of the steel, coated boron steels have been developed, and the boron steel substrate has been covered with a metallic coating such as an Al-Si coating or a Zn based coating. However, it has been found that during the hot forming process the coatings are covered by an oxide layer due to the high temperature needed for the hot forming process, which oxide layer is formed from the coating material. A zinc-oxide layer makes that it is difficult to spot weld the hot formed products. Good spot weldability is needed for the automotive industry, where hot formed parts are used.
It is an object of the invention to provide a coated strip, sheet or blank suitable for hot forming that has a good spot weldability after hot forming.
It is a further object of the invention to provide a coated strip, sheet or blank suitable for hot forming that is economical to produce.
It is also an object of the invention to provide a process for the production of a strip, sheet or blank that meets one or more of the objects hereinabove.
It is furthermore an object of the invention to provide a method for hot forming a product and a hot formed product that has a good spot weldability. According to the invention one or more of the above objects are reached with a steel strip, sheet or blank for hot forming at a temperature of Ac1 or above, comprising a substrate of hot formable steel coated with a corrosion protective coating, wherein the coated steel substrate is covered by a siloxane layer.
The inventors have found that such a siloxane layer is suitable to reduce the extent of oxidation during the hot forming of a steel strip, sheet and blank coated with a corrosion protective coating. The siloxane layer also reduces zinc losses due to evaporation. When hot formed, the siloxane coated material will be oxidised to leave a thin adhering silicon oxide layer, which has a low contact resistance and consequently good weldability. Because of the reduced oxidation of the corrosion protective coating, the siloxane layer also improves the effectiveness of the coating for corrosion protection of the steel. The steel substrate is for hot forming at Ac1 -temperature or above. Such steel substrates can be used for hot forming and quenching of the hot formed product, which usually provides a tensile strength of 1500 MPa or more.
Siloxane layers are normally used as chromate replacing passivation treatment or as phosphate or chromate replacing conversion treatment to improve adhesion with adhesives or paints. They are formed after curing of a water-based silane film eventually containing alcohol as well, applied by roll coating, dip/squeezing or spray/squeezing and contain silicon atoms that are bound with oxygen atoms and/or hydroxyl groups as well as with the carbon atoms of hydrocarbon and/or hydrocarbyl groups. The carbon atoms of the hydrocarbon and/or hydrocarbyl groups can be bound with each other or with intermediate amine or carbonyl groups and/or sulphur atoms.
According to a preferred embodiment the siloxane layer originates from a bis- silane containing an amount of carbon such that C/Si≤ 1. The siloxane layer thus adds optimal to the protection of the steel due to the little formation of carbon dioxide resulting in an almost porous-free, homogeneous silicon oxide layer, which has shown to provide best welding possibilities for the hot formed product.
According to a preferred embodiment, the thickness of the siloxane layer is such that at most 200 mg Si/m2 remains in the silicon oxide layer after hot forming, preferably between 20 and 100 mg Si/m2, and more preferably between 40 and 100 mg Si/m2. The inventors have found that the thickness of the metal-phosphate layer should be at most 200 mg Si/m2 since with higher thickness the silicon oxide layer will itself hamper the contact resistance of the spot welded material. Moreover, also the costs of the layer become too high. Therefore, thinner layers are preferred. According to a preferred embodiment the corrosion protective layer is a zinc or zinc alloy layer. A coating of zinc or a zinc alloy gives a very good, active corrosion protection of the steel substrate.
According to a preferred embodiment the zinc or zinc alloy layer is a galvannealed layer containing Fe up to 70 weight % and preferably containing Fe from 8 to 12 weight %. It has been found that a siloxane layer as mentioned above on this type of coating will give good spot weldability. Using this galvannealed zinc or zinc alloy layer makes it easier to hot form the coated steel as well.
According to an embodiment the zinc or zinc alloy layer is a galvannealed layer containing Fe up to 70 weight % and preferably containing Fe from 1 to 5 weight %.
It is possible that the zinc alloy layer consists of 0.3 to 2.3 weight % magnesium and 0.6 to 2.3 weight % aluminium, optional less than 0.2 weight % of one or more additional elements, the remainder being zinc and unavoidable impurities. An optional element that could be added in a small amount, less than 0.2 weight %, could be Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn, Bi and Sb are usually added to form spangles. This type of coating has an improved corrosion resistance as compared to usual zinc layers.
Preferably the corrosion protective layer has a thickness of 2 to 25 m. These thicknesses are suitable to produce hot formed parts for automotive purposes.
According to a preferred embodiment the hot formable steel substrate is a steel substrate, having a composition in weight percent:
C between 0.04 and 0.5 %
Mn between 0.5 and 3.5 %
Si less than 2.0 %
Cr less than 1.0 %
Ti less than 0.2 %
Al less than 2.0 %
Mo less than 0.5 %
P less than 0.1 %
N less than 0.015 %
S less than 0.05 %
B less than 0.015 %
the remainder being Fe and unavoidable impurities.
These steel types are suitable for hot forming, wherein the alloying elements can still be chosen within limits. Usually the amount of boron is between 0.001 and 0.005 weight %. Preferably, the hot formable steel substrate is a steel substrate, having a composition in weight percent:
C between 0.04 and 0.5 %
Mn between 0.5 and 3.5 %
Si less than 2.0 %
Cr less than 1.0 %
Ti less than 0.2 %
Al less than 2.0 %
Mo less than 0.5 %
P less than 0.1 %
Nb less than 0.1 %
N less than 0.015 %
S less than 0.05 %
B less than 0.015 %
the remainder being Fe and unavoidable impurities.
These steel types have better mechanical properties when Nb is added. Also in this embodiment the amount of boron is usually between 0.001 and 0.005 weight %.
Preferably, the hot formable steel substrate is a steel substrate, having a composition in weight percent:
C between 0.04 and 0.5 %
Mn between 0.5 and 3.5 %
Si less than 2.0 %
Cr less than 1.0 %
Ti less than 0.2 %
Al less than 2.0 %
Mo less than 0.5 %
P less than 0.1 %
N less than 0.015 %
S less than 0.05 %
B less than 0.015 %
W less than 0.3 %
the remainder being Fe and unavoidable impurities.
These steel types have better mechanical properties when W is added. Also in this embodiment the amount of boron is usually between 0.001 and 0.005 weight %.
Preferably the steel substrate has a thickness between 0.5 and 3.0 mm. Such thicknesses are used in the automotive industry. The invention also relates to a process for producing a hot formable strip, sheet or blank as described above, wherein a substrate of hot formable steel coated with a corrosion protective coating is covered by a solution of silane, whereafter the strip, sheet or blank is heated to form the siloxane layer. In this way it is possible to provide the steel substrate with the siloxane layer as exemplified above.
According to a preferred embodiment the silane solution is applied on the coated steel strip, sheet or blank as a wet film having a thickness of 1 to 10 ml/m2 by spraying or dipping and subsequently squeezing or by roll coating, preferably at the end of a coating line, and the wet film is dried and cured at a temperature of between 70 °C and 200 °C, preferably between 70 °C and 100 °C. This process of applying the wet film is easy to perform and gives an even wet layer on the coated substrate, which is dried and cured in the usual manner, for instance in an oven.
Preferably, the siloxane covered corrosion protection coated substrate embodies one or more of the features of the strip, sheet or blank as exemplified above.
The invention moreover relates to a method for hot forming a product using a blank as described above or using a blank formed from a strip or sheet as described above, wherein the blank or a pre-form formed out of the blank is heated to the Ac1- temperature or above, the blank or pre-form is hot formed in a hot forming die, and the hot formed product is quenched. This is the usual hot forming process, which is now performed using a blank according to the invention or a blank produced from the strip or sheet according to the invention. Using such a blank with a siloxane layer according to the invention, the product formed has a good weldability.
The invention furthermore relates to a hot formed product using the strip, sheet or blank as described above or produced according to the method above, wherein the coated hot formed substrate has a contact resistance below 1 mOhm. The contact resistance was measured according to a method described in DVS2929:2007 with the exceptions that a 7.5kN force and 300mm electrode radius were used. A contact resistance of below 1 mOhm stands for good weldability.
The invention will be elucidated referring to some background information and a number of experiments hereinafter, shown in the accompanying figures.
Fig. 1 shows the contact resistance of heated zinc coated steel for two different siloxane layer thickness as well as the expected siloxane layer thickness needed to achieve good weldability (< 1 mOhm).
Fig. 2A shows a zinc coated substrate without post treatment after heating. Fig.2B. shows the same substrate with a siloxane layer after heating.
Due to the high temperatures during hot forming, zinc coated material oxidises rapidly. Due to the nature of the formed oxide, the contact resistance of the material will increase. In general, a hot formed material with a contact resistance larger than 1 mOhm is regarded as unweldable. After a commonly used heat treatment of 6 minutes in a furnace of 900 °C , the contact resistance is usually larger than 5 mOhm. The amount of oxide can be reduced by a lower furnace temperature and/or shorter dwell times in the furnace but this will seriously affect the robustness of the hot forming process. Low furnace temperatures make the material susceptible to ferrite formation which will decrease the final material strength. A short dwell time minimizes the flexibility of the process, since sometimes the samples have to remain in the furnace for a longer time period due to technical issues at the pressing station behind the furnace.
It is the opinion of the inventors that the amount of oxide should be lowered (without lowering the furnace temperature or shortening the maximum furnace dwell time) in order to maintain a very robust hot forming process. Therefore they investigated several post-treatments that would minimize the total amount of oxidation at comparable heating cycles.
One type of silane namely bis-(trimethoxysilyl)ethane (BTSE) (which is commercially available) was chosen for the investigation. This silane was applied as a water-based solution for two different concentrations namely 0.5 % weight % and 3 weight % by roll-coating, resulting in a wet film thickness of ca. 10 ml/m2 to give a siloxane coating weight of respective 2 and 12 mg Si/m2 after drying in an oven. The substrates were 22MnB5 steel sheets coated with a zinc coating that had an additional heat treatment to have 12 % Fe in the coating.
Table 1 below shows the results the inventors obtained for different post-treatments after a heat cycle of 6 min in a chamber furnace heated to 880 °C . It will be clear that the applied siloxane layer increases the amount of zinc remaining in the sheets, contributing to a better corrosion protection, and reduces the amount of zinc-oxide, which reduces contact resistance. However, the difference between both concentrations is too small to find a significant effect of concentration. Nevertheless, it is clear that by using higher silane concentrations we will achieve the needed contact resistance lower than 1 mOhm that is required for hot formed parts in the automotive industry to provide good welding.
Figure 1 tells us that when we extrapolate the results (dotted line), we at least have to apply siloxane layers containing > 20 mg Si/m2 and preferably > 40 mg Si/m2, taking deviation into account, to achieve a contact resistance lower than 1 mOhm. Amount Amount of Zn in
Post treatment Contact resistance
of ZnO sheet
(mOhm) (g/m2) (g/m2) mean Standard deviation
oil 12 5.9 68 48
BTSE (2 mg Si/m2 8.3 2.1 52 77
BTSE (12 mg Si/m2) 5.2 2.8 44 67
Table 1
Figure 2 shows the difference between a zinc coated substrate without post treatment (Fig. 2A) and with the siloxane layer (Fig. 2B). It is clear from the figures that the use of the siloxane layer minimizes the oxide layer formed in the zinc coating due to the heating of the silane treated zinc coated steel.

Claims

Steel strip, sheet or blank for hot forming at a temperature of Ac1 or above, comprising a substrate of hot formable steel coated with a corrosion protective coating, characterised in that the coated steel substrate is covered by a siloxane layer.
Strip, sheet or blank according to claim 1 wherein the siloxane layer originates from a bis-silane containing an amount of carbon such that C/Si < 1.
Strip, sheet or blank according to any one of the preceding claims, wherein the thickness of the siloxane layer is such that at most 200 mg Si/m2 remains in the silicon layer after hot forming, preferably between 20 and 100 mg Si/m2 and more preferably between 40 and 100 mg Si/m2
Strip, sheet or blank according to any one of the preceding claims, wherein the corrosion protective layer is a zinc or zinc alloy layer.
Strip, sheet or blank according to claim 4, wherein the zinc or zinc alloy layer is a galvannealed layer containing Fe up to 70 weight % and preferably containing Fe from 8 to 12 weight % .
Strip, sheet or blank according to any one of the preceding claims, wherein the corrosion protective layer has a thickness of 2 to 25 μηη.
Strip, sheet or blank according to any one of the preceding claims, wherein the hot formable steel substrate is a steel substrate, having a composition in weight percent:
C between 0.04 and 0.5 %
Mn between 0.5 and 3.5 %
Si less than 2.0 %
Cr less than 1.0 %
Ti less than 0.2 %
Al less than 2.0 %
Mo less than 0.5 %
P less than 0.1 % N less than 0.015 %
S less than 0.05 %
B less than 0.015 %
the remainder being Fe and unavoidable impurities.
8. Strip, sheet or blank according to any one of the preceding claims, wherein the hot formable steel substrate is a steel substrate, having a composition in weight percent:
C between 0.04 and 0.5 %
Mn between 0.5 and 3.5 %
Si less than 2.0 %
Cr less than 1.0 %
Ti less than 0.2 %
Al less than 2.0 %
Mo less than 0.5 %
P less than 0.1 %
Nb less than 0.1 %
N less than 0.015 %
S less than 0.05 %
B less than 0.015 %
the remainder being Fe and unavoidable impurities.
9. Strip, sheet or blank according to any one of the preceding claims, wherein the hot formable steel substrate is a steel substrate, having a composition in weight percent:
C between 0.04 and 0.5 %
Mn between 0.5 and 3.5 %
Si less than 2.0 %
Cr less than 1.0 %
Ti less than 0.2 %
Al less than 2.0 %
Mo less than 0.5 %
P less than 0.1 %
N less than 0.015 %
S less than 0.05 %
B less than 0.015 % W less than 0.3 %
the remainder being Fe and unavoidable impurities.
10. Strip, sheet or blank according to any one of the preceding claims, wherein the steel substrate has a thickness between 0.5 and 3.0 mm.
11. Process for producing a hot formable strip, sheet or blank according to any one of the preceding claims, wherein a substrate of hot formable steel coated with a corrosion protective coating is covered by a solution of silane, whereafter the strip, sheet or blank is heated to form the siloxane layer.
12. Process according to claim 11 , wherein the silane solution is applied on the coated steel strip, sheet or blank as a wet film having a thickness of 1 to 10 ml/m2 by spraying or dipping and subsequently squeezing or by roll coating, preferably at the end of a coating line, and the wet film is dried and cured at a temperature of between 70 °C and 200 °C , preferably between 70 °C and 100 °C
13. Method for hot forming a product using a strip, sheet or blank according to any one of the claims 1 - 10, wherein the strip, sheet or blank, or a pre-form formed out of the blank is heated to the Ac1 -temperature or above, the blank or preform is hot formed in a hot forming die, and the hot formed product is quenched.
14. Hot formed product using the strip, sheet or blank according to any one of the claims 1 - 10 or produced according to claim 13, having a contact resistance below 1 mOhm.
PCT/EP2012/002416 2011-06-07 2012-06-06 Hot formable strip, sheet or blank, process for the production thereof, method for hot forming a product and hot formed product WO2012167930A1 (en)

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