EP0230320B1 - Tôle d'acier avec un revêtement organique durcissable à chaud et son procédé de fabrication - Google Patents

Tôle d'acier avec un revêtement organique durcissable à chaud et son procédé de fabrication Download PDF

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Publication number
EP0230320B1
EP0230320B1 EP87100847A EP87100847A EP0230320B1 EP 0230320 B1 EP0230320 B1 EP 0230320B1 EP 87100847 A EP87100847 A EP 87100847A EP 87100847 A EP87100847 A EP 87100847A EP 0230320 B1 EP0230320 B1 EP 0230320B1
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Prior art keywords
chromate
weight
bake hardenability
layer
resin
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EP0230320A1 (fr
Inventor
Shigeru C/O Technical Research Division Kobayashi
Shunichi C/O Technical Research Division Tsugawa
Taizo C/O Technical Research Division Mouri
Toshio C/O Technical Research Division Ichida
Takashi C/O Technical Research Division Obara
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JFE Steel Corp
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Kawasaki Steel Corp
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Priority claimed from JP61013229A external-priority patent/JPS62170340A/ja
Priority claimed from JP61290035A external-priority patent/JPS63143265A/ja
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    • 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
    • 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
    • C23C22/78—Pretreatment of the material to be coated
    • 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
    • C23C22/05—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 using aqueous solutions
    • C23C22/06—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 using aqueous solutions using aqueous acidic solutions with pH less than 6
    • C23C22/24—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 using aqueous solutions using aqueous acidic solutions with pH less than 6 containing hexavalent chromium compounds
    • C23C22/26—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 using aqueous solutions using aqueous acidic solutions with pH less than 6 containing hexavalent chromium compounds containing also organic compounds
    • C23C22/28—Macromolecular compounds
    • 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

Definitions

  • This invention relates to corrosion resistant, organic coated steel strips having improved bake hardenability and drawability and finding application in automobiles.
  • strip steel is pressed formed to a desired shape and then coated with a protective coating typically by electrophoretic painting followed by baking at elevated temperatures.
  • bake hardenability designates that the strip steel hardens during the baking of such a coating.
  • the bake hardenability of strip steel is evaluated in terms of an increase of yield strength by baking a 2% pre-stressed, steel strip at 170°C for 20 minutes and measuring the yield strength.
  • One conventional commercially available steel meeting such considerations is a class of zinc and zinc alloy plated steel strips having bake hardenability.
  • An organic coating is applied to a zinc or zinc alloy plated steel strip.
  • the organic coating on the steel must be baked at a temperature of higher than 150°C in orderto convert it into a hardened one.
  • the organic coated steel strip has been hardened prior to press forming and is thus not amenable to drawing.
  • EP-A-0 149 461 discloses a steel strip coated with a zinc alloy, a chromate layer, and an organic layer which contains silica, wherein a relatively large amount of 10 to 30 parts by weight of melamine resin is used as a curing agent of carboxylated polyethylene resin.
  • the curing agent may behave as a plasticizer when it is left after baking to make adverse effects of the properties of the coating.
  • From DE-A-3 151 115 also a basic organic coated steel strip comprising a steel substrate, a zinc alloy coating, a chromate coating, and an organic coating containing silica is known.
  • this product however, the bake hardenability, the use of extra low carbon steel as a substrate and the baking of the organic coating at a temperature beyond 150°C are not mentioned.
  • JP-A-58-224 175 relates to the manufacture of a surface-treated steel plate having a superior adhesive strength to paint after degreasing comprising the steps of treating a steel plate plated with Zn, a Zn alloy or AI with a chromating solution containing chromic acid, a water-soluble acrylic resin and a mineral acid, washing the treated plate in water, and subjecting the plate to a sealing treatment with an organic resin solution.
  • a surface-treatment of a steel plate having a bake hardenability of 3.5 to 6.1 kgf/mm 2 starting with an extra low carbon steel substrate, however, and subjecting the substrate to a chromate treatment after depositing a zinc or a zinc base alloy layer and baking an organic coating at a temperature of up to 150°C are not mentioned.
  • FR-A-1 576 677 discloses a process for the chromate treatment on Zn, Al and Fe metal surfaces and a treatment solution therefor.
  • a combination of an extra low carbon steel substrate, a layer of zinc base alloy, a chromate layer and an organic coating are not cited.
  • An object of the present invention is to provide a novel and improved organic coated steel strip which exhibits bake hardenability and good workability even after baking of the organic coating.
  • Another object of the present invention is to provide a method for making such an organic coated steel strip.
  • a further object of the present invention is to provide a method for making an organic coated steel strip having improved bake hardenability and capable of maintaining a high proportion of chromium fixed while preventing chromium from being dissolved out during alkaline degreasing and/or chemical conversion as used in an automotive coating process.
  • an organic coated steel strip having improved bake hardenability, of 3.5 to 6.1 kgf/mm 2 comprising
  • an organic coated steel strip having improved bake hardenability of 3.5 to 6.1 kgf/mm 2 comprising the steps of:
  • the chromate treatment is conducted using an aqueous chromate solution containing a chromate compound, a reducing agent, and at least one member selected from acid residues, resins and silica.
  • the present invention provides a highly corrosion resistant, organic coated steel strip capable of maintaining improved bake hardenability after press forming, which is prepared by preparing a drawing extra low carbon steel substrate having bake hardenability, depositing a layer of a zinc base alloy on one surface of the substrate, subjecting said substrate to a chromate treatment to form a chromate layer on the zinc base alloy layer, and applying an organic coating on the chromate layer and baking the coating at a temperature of up to 150°C.
  • automotive organic coated or painted steel strips must be baked at a temperature in excess of 150°C in order to convert the organic coating into hardened one.
  • the reason is that in case of Zincrometal @ with which cold rolled steel is coated, for example, baking must be effected at a sufficiently high temperature to cause the powder zinc-containing chromate to react with the steel substrate.
  • the use of a high-boiling solvent to dissolve a high molecular weight resin also requires a baking treatment at a temperature of higher than 150°C.
  • steel strips having a thin film organic coating other than the zinc rich paint are baked at a high temperature because a binding agent capable of crosslinking at a high temperature, for example, melamine resin must be added in the state of the art.
  • the present invention provides an organic coated steel strip having improved bake hardenability, comprising an extra low carbon steel substrate having bake hardenability, a layer of a zinc base alloy deposited on one surface of said substrate in a weight of 10 to 40 g/m 2 , a chromate layer formed on said zinc base alloy layer in a weight of at least 10 mg/m 2 calculated as metallic chromium, and an organic coating, optionally containing silica, attached to said chromate layer by baking at a temperature of up to 150°C.
  • the temperature at which the organic coating is baked is limited to 150°C or lower in order to maintain the bake hardenability. The reason will become clear from the description of a series of experiments.
  • the starting steel was an extra low carbon steel consisting of, in percentage by weight, 0.003% C, 0.01% Si, 0.16% Mn, 0.04% Al, 0.070% P, 0.026% Nb and balance essentially Fe.
  • the strip steel was cold rolled at a draft of 80% to a thickness of 0.7 mm, heat treated by soaking at 850°C for 30 seconds and cooled to 650°C at a rate of 45°C/sec. in a continuous annealing furnace, and then skin pass rolled at a draft of 1.0%.
  • the resulting extra low carbon steel strip having bake hardenability was determined for mechanical properties, exhibiting a yield strength (YS) of 20 kgf/mm 2 , a tensile strength (TS) of 35 kgf/mm 2 , an elongation (EI) of 45%, and a bake hardenability (BH) of 5 kgf/mm 2 .
  • YS yield strength
  • TS tensile strength
  • EI elongation
  • BH bake hardenability
  • the preferred cold rolled steel strips having bake hardenability with which the present invention starts are bake hardenable, cold rolled steel strips comprising, in percentage by weight, 0.001 to 0.008% of C, up to 0.5% of Si, 0.05 to 1.2% of Mn, up to 0.1 % of P, 0.01 to 0.08% of Al, the aluminum being at least 8 times the percent N, from 3 times the percent C to 8 times the percent C plus 0.02% of Nb, up to 0.05% of Ti, and balance essentially Fe, the strips being continuously annealed to have a BH value of 3 to 6 kgf/mm 2 .
  • the extra low carbon steel materials undergo little hardening during mild baking at temperatures of 150°C or lower so that they maintain their own bake hardenability.
  • Carbon, C ranges from 0.001 to 0.008 wt%. Contents of less than 0.001 wt% lead to the loss of the solid solution carbon contributing to bake hardening. Steels having more than 0.008 wt% of carbon exhibit too high yield strength and low ductility and r value (Lankford value).
  • Silicon, Si preferably ranges up to 0.5 wt%. In excess of 0.5 wt%, an oxide film will form to detract from chemical conversion amenability.
  • Mn preferably ranges from 0.05 to 1.2 wt%. Red shortness diminishes at less than 0.05 wt% whereas r value is reduced in excess of 1.2 wt%.
  • Phosphorus, P preferably ranges up to 0.1 wt%. Steel becomes brittle with P contents in excess of 0.1 wt%.
  • AI preferably ranges from 0.01 to 0.08 wt% and at least 8 times the percent N. At least 0.01 wt% of AI is necessary to fix nitrogen. AI contents of more than 0.08 wt% undesirably generate many inclusions. The function of AI to fix nitrogen provides the additional requirement that its content be at least 8 times the percent N.
  • Niobium, Nb preferably ranges from 3 times the percent C to 8 times the percent C plus 0.02%. Below the lower limit of 3 times the percent C, a too larger amount of solid solution carbon is left, inhibiting the formation of an aggregate structure contributing to drawability during cold rolling recrystallization. Ductility is impaired in excess of 8 times the percent C plus 0.02%.
  • Titanium, Ti preferably ranges up to 0.05 wt% because the bake hardenability of steel is lost in excess of this. limit.
  • the steel strips are preferably controlled to a BH value of 3 to 6 kgf/mm 2.
  • a value of less than 3 kgf/mm 2 is a substantial loss of bake hardenability.
  • Steels having BH values in excess of 6 kgf/mm z undergo severe deterioration upon aging and stretcher strain during working.
  • the extra low carbon steel strips are plated with zinc based alloys by any known deposition techniques, typical electrodeposition.
  • the zinc base alloy platings include Zn-Ni alloy platings preferably having a nickel content of 5 to 13 wt%; Zn-Fe alloy plating preferably having an iron content of 8 to 25 wt%; Zn-Co-A1 2 0 3 -Cr 2 0 3 alloy platings preferably having a cobalt content of 1 to 5 wt%; Zn-Al alloy platings preferably having an aluminum content of 1 to 15 wt%; Zn-Ni/Fe-P double-layered alloy platings preferably having a phosphorus content of 0.0003 to 5% by weight based on the weight of FeP; Zn-Fe/Fe-P double-layered alloy platings preferably having a phosphorus content of 0.0003 to 5% by weight based on the weight of Fe-P.
  • plating weight should range from 10 to 40 gram per square meter (g/m 2 ). Corrosion resistance is insufficient with less than 10 g/m 2 whereas plating weights in excess of 40 g/m 2 provide no additional benefit in corrosion resistance improvement and are thus uneconomical.
  • the contents of the respective elements are preferably limited to certain ranges.
  • the Zn-Ni alloys preferably have a nickel content of 5 to 13 wt%. Less than 5 wt% of Ni provides insufficient corrosion resistance whereas a plating containing more than 13 wt% of Ni is too hard..
  • the Zn-Fe alloys preferably have an iron content of 8 to 25 wt%. Less than 8 wt% of Fe provides insufficient corrosion resistance whereas red rust will often generate in excess of 25 wt% of Fe.
  • the Zn-Co-A1 2 0 3 -Cr 2 0 3 alloys preferably have a cobalt content of 1 to 5 wt%. Less than 1 wt% of Co provides insufficient corrosion resistance whereas more than 5 wt% of Co is uneconomical.
  • the Zn-Al alloys preferably have an aluminum content of 1 to 15 wt%. Less than 1 wt% of AI provides insufficient corrosion resistance whereas sacrificial corrosion prevention is lost in excess of 15 wt% of Al.
  • the Fe-P alloys for the double-layered Zn-Ni/Fe-P and Zn-Fe/Fe-P platings preferably have a phosphorus content of 0.0003 to 5% by weight based on the weight of Fe-P.
  • Platings having less than 0.0003 wt% of P are less susceptible to chemical conversion. More than 5 wt% of P is uneconomical because of reduced current efficiency during plating process.
  • the zinc base alloy platings are subjected to a chromate treatment in order to improve their adherence to subsequently applied organic coatings and hence, the corrosion resistance of the overall structure.
  • the chromate treatment is carried out to produce a chromate film of at least 10 mg/m 2 of metallic chromium. Either coating or electrolytic chromate treatment is advantageous in controlling the amount of chromate film to such a level. Chromate films of less than 10 mg/m 2 of metallic chromium have insufficient corrosion resistance and poor adherence to the subsequently applied organic coatings.
  • organic coating compositions used in the practice of the present invention contain as a main ingredient, a resin. selected from the following three groups:
  • resin (1) or (2) listed above there are contemplated some methods for assisting in fully hardening the resin by a heat treatment at a low temperature of up to 150°C, for example, the use of low temperature curing agents, for example, metal salt catalysts such as cobalt naphthenate, optionally in combination with amine curing agents such as diethylene triamine. With these curing agents added, the curing process can proceed at room temperature.
  • An organic coating may be more readily formed by using a method adequate for the particular type of organic resin used. Examples of the curing agents which exert their function upon heating include urea resins, melamine resins, benzoguanamine resins, block isocyanate resins, and phenol resins.
  • UV- or electron radiation-curable resins (3) are acrylic resin coatings utilizing a benzoin ether as a photopolymerization initiator and epoxy resin coatings utilizing an aromatic diazonium salt as a photopolymerization initiator.
  • Exemplary of the resins having an electron radiation-curable functional group there may be given acrylic and epoxy resins utilizing epoxy-acid addition reaction and polyurethane resins utilizing isocyanate-hydrogen group addition reaction.
  • These resins (3) can advantageously maintain the bake hardenability of the steel substrate substantially unchanged because they can be baked at temperatures of several ten degree in Centigrade.
  • the resinous coating composition of any of resins (1) to (3) is applied to the chromate layer to a thickness of 0.5 to 3 pm.
  • Organic coatings of less than 0.5 pm thick provide insufficient corrosion resistance whereas more than 3 pm thickness adversely affects weldability.
  • the resinous coating composition may further contain up to 60% by weight of silica sol for the purpose of improving corrosion resistance. Coating compositions containing more than 60% by weight of silica sol are too viscous and tend to gel.
  • the composite coated steel strips mentioned above are improved rust-preventive steel strips.
  • a chromate treatment is often employed as a preliminary treatment for the organic coating in order to enhance corrosion resistance.
  • a usual automotive part coating process proceeds according to the scheme of blank (organic coated steel) assembly alkaline degreasing chemical conversion electrophoretic deposition intermediate coating top coating. Since the temperature at which the chromate and resin films are baked is controlled relatively low to maintain bake hardenability according to the present invention, there is the likelihood that when a conventional chromate solution is used in the preliminary treatment, chromium be dissolved out during the alkaline degreasing and chemical conversion, imposing a problem to spent liquid disposal.
  • the present invention according to the other aspect provides a method for making an organic coated steel strip having improved bake hardenability, comprising the steps of:
  • the organic coated steel strips produced by the method of the present invention experience controlled dissolving out of chromium during alkaline degreasing and/or chemical conversion in the automotive coating process without a loss of corrosion resistance.
  • the chromate treatment will be described in more detail. We have made an experiment to examine the proportion of chromium fixed during alkaline degreasing and/or chemical conversion in the automotive coating process.
  • the starting steel strip is an extra low carbon steel consisting of, in percentage by weight, 0.003% C, 0.01% Si, 0.16% Mn, 0.04% Al, 0.070% P, 0.026% Nb, and balance essentially Fe.
  • a zinc base alloy, typically Zn-Ni alloy was plated on the strip in a plating weight of 10 to 40 g/m 2 , a chromate solution having a ratio of hexavalent to trivalent chromium (Cr 6+ /Cr 3+ ) of from 80/20 to 20/80 was applied and baked to the zinc base plating, and then a resin in water or solvent, typically epoxy resin was applied and baked to the chromate film. It is to be noted that the ratio of Crs+/Cr3+ was measured by the redox titration technique. The maximum temperature to which the strip was heated was from room temperature to 150°C for both the chromate and resin film baking steps.
  • the maximum baking temperature is limited to the range between room temperature and 150°C.
  • chromate solutions having added thereto methanol as a reducing agent and phosphoric acid as an acid residue was applied and baked to plated steel strips at temperatures of from room temperature to 150°C and then a resin was applied and baked to the chromate film at a temperature of from room temperature to 150°C.
  • the chromium fixing proportion that is, percent chromium fixed of the strips was plotted in FIG. 1 as a function of the Cr 6+ /Cr 3+ ratio of the chromate solution and the maximum baking temperature.
  • the range of Cr6+/Cr3+ ratio within which the percent chromium fixed is 80% or higher is depicted hatched as a favourable region in FIG. 1.
  • the reducing agent used is methanol and the additive used is phosphoric acid, similar results are obtained when other reducing agents are used and/or other additives such as acids other than phosphoric acid, resins and silica are used.
  • Examples of the reducing agents added to the chromate solution include methanol, aqueous hydrogen peroxide, ethylene glycol, succinic acid, succinimide, but are not limited thereto.
  • the reducing agent is added in an amount sufficient to provide a RAH of 0.2 to 1.9 per gram molecule of Cr0 3 .
  • RAH is the gram atoms of hydrogen contained in the reducing agent. With RAH of less than 0.2, the percent chromium fixed is reduced to an unacceptable level even when the additive as defined below is additionally used.
  • the chromate solution becomes gel if RAH is above 1.9.
  • the additive which is used in the chromate solution in combination with the reducing agent are selected from acid residues, resins, and silica. They are described in more detail.
  • Preferred acid residues are provided by such acids as phosphoric acid and boric acid. They are added in an amount to give a A -I /CrO 3 ratio of from 0.05 to 0.3 by weight wherein A- x represents an acid residue. Ratios of less than 0.05 will result in a percent chromium fixed of less than 80% whereas ratios of more than 0.3 will result in poor corrosion resistance.
  • Preferred examples of the resins added to the chromate solution include acrylic resins having an acid value of at least 250 and acrylic resins having acrylic acid monomer and/or methacrylic acid monomer added to stabilize them.
  • Silica added to the chromate solution is preferably colloidal silica. Silica is added in an amount to give a S!0 2 /CrO 3 ratio of from 0.3 to 3.0 by weight. Ratios of less than 0.3 will result in a percent chromium fixed of less than 80% whereas ratios of more than 3.0 will deteriorate the adherence of the chromate film to the underlying substrate.
  • the steel can maintain a significantly high percent chromium fixed at the end of alkaline degreasing and chemical conversion in the automotive coating process.
  • an experiment was made using an immersion type alkaline degreasing solution commonly used in the automotive coating process.
  • FIG. 2 graphically shows the percent chromium fixed as a function of the amount of methanol added as the reducing agent. It is seen that only the addition of methanol, that is, reducing agent mostly results in a percent chromium fixed of less than 80%. However, the RAH/Cr0 3 ratio range from 0.2 to 1.9 gives a ratio of Cr 6+ /Cr 3+ in the range of from 80/20 to 20/80. Then, the percent chromium fixed can be 80% or higher as seen from FIG. 1 by adding at least one additive selected from (1) acid residues, (2) resins, and (3) silica to the chromate solution while keeping the maximum baking temperature within the range of from room temperature to 150°C.
  • the resin used is an acidic acrylic resin.
  • the amount of resin added is expressed as a weight ratio of resin solids/Cr0 3 . It is seen that a resin solids/Cr0 3 ratio in the range between 0.1 and 20.0 is effective in improving the percent chromium fixed. Such a ratio of more than 20.0 adversely affects the adherence of chromate film to the underlying substrate, and hence, the workability and weldability of the product.
  • the silica used is ultrafine particulate silica anhydride.
  • the amount of silica added is expressed as a weight ratio of SiO 2 /CrO 3 . It is seen that a SiO 2 /CrO 3 ratio of at least 0.3 is effective in achieving a percent chromium fixed of 80% or higher. Such a ratio of more than 3.0 adversely affects the adherence of chromate film to the underlying substrate, and hence, the weldability of the product.
  • the reducing agent such as methanol, aqueous hydrogen peroxide and ethylene glycol is added to the chromate solution.
  • the reducing agent reduces chromic acid to lower the Cr 6+ /Cr 3+ ratio.
  • the percent chromium fixed is then increased because the proportion of hexavalent chromium which is more liable to dissolve away is decreased.
  • the additives, (1) acid residue, (2) resin, and (3) silica added to the chromate solution have the following functions.
  • Two sets of samples were prepared by applying a chromate solution having phosphoric acid residue added and an acid residue-free chromate solution followed by baking. Analysis of the samples from the acid residue-containing chromate solution indicates peaks probably attributable to the hydrate or hydroxide of trivalent chromium in addition to the peaks of trivalent and hexavalent chromium. No substantial difference is observed in the proportion of hexavalent chromium between the acid residue- introduced and free solutions. When the chromate films are evaluated as applied, the percent chromium fixed does not depend on the introduction of acid residue.
  • the Crs+/Cr3+ ratio of the solution remains unchanged, but a bond is created between hexavelent chromium and the resin at the end of baking to prevent the hexavalent chromium from dissolving out.
  • silica-containing chromate films indicates more OH groups than in silica-free chromate films.
  • the distribution of silicon in the chromate film is also determined to find that Si is concentrated in a surface layer.
  • silica forms a rigid film at the surface layer of the chromate film in which the chromate itself is present as the hydrate and hydroxide of trivalent chromium.
  • a resin is subsequently applied and baked to such a chromate film, the resin is firmly attached to the OH group of the chromate film so that the resulting resin coated steel strip is characterized by the controlled dissolving-out of chromium.
  • composition of cold rolled steel strips having bake hardenability used in a series of runs is shown in Table 1 together with their mechanical properties.
  • test specimens were prepared by depositing a zinc base alloy plating on the blank, subjecting the plating to a chromate treatment, and then applying an organic coating followed by baking.
  • the treating procedure is shown in Table 2.
  • test specimens were determined for corrosion resistance, weldability, workability, and bake hardenability, with the results shown in Table 3.
  • a salt spray test was carried out by crosshatching the organic coating on each specimen, and spraying a 5% NaCI solution at 35°C to the coating. The time was observed until red rust generated.
  • a cycle corrosion test was carried out by subjecting each specimen to corrosion cycles each consisting of spraying of 5% NaCl at 35°C for 4 hours, drying at 60°C for 2 hours, and allowing to stand in wet conditions at 50°C, RH 95% for 2 hours. The number of cycles was counted until red rust generated.
  • Each specimen in a disk form having a diameter of 90 mm was subjected to a cupping test by drawing to a cup shape having a diameter of 50 mm and a depth of 25 mm (Blank Hold Force 1 Ton).
  • An adhesive tape was applied to and removed from the worked area to determine the removal of coating as expressed in mg/circumference.
  • Each specimen was pre-stressed 2% and then subjected to a baking treatment at 170°C for 20 minutes.
  • the yield strength (YS) of the specimen was measured to determine an increase of YS in kgf/mm .
  • the cold rolled steel strips having bake hardenability used were the same as used in Example 1. Their compositions are shown in Table 1 together with their mechanical properties.
  • test specimens were prepared by depositing a zinc base alloy plating on the blank, subjecting the plating to a chromate treatment, and then applying an organic coating followed by baking.
  • the treating procedure is shown in Table 4.
  • test specimens were determined for corrosion resistance, weldability, workability, bake hardenability, and percent chromium fixed, with the results shown in Table 5.
  • Percent chromium fixed was determined by degreasing a specimen with a commonly used immersing alkaline degreasing solution. Using fluorescent X-ray analysis, the number of chromium counts was determined before and after the alkaline degreasing.

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  • Chemical Treatment Of Metals (AREA)

Claims (12)

1. Feuillard d'acier à revêtement organique ayant une capacité de durcissement à la cuisson améliorée, de 3,5 à 6,1 kgf/mm2, comprenant:
un substrat en acier à très faible teneur en carbone contenant de 0,001 à 0,008 pour cent en poids de carbone, et durcissable par cuisson,
une couche d'un alliage à base de zinc, déposée sur une surface du substrat, selon un poids de 10 à 40 g/m2,
une couche de chromate formée sur ladite couche d'alliage à base de zinc, selon un poids d'au moins 10 mg/m2, d'après calcul sur la base du chrome métallique, et
un revêtement organique d'une épaisseur de 0,5 à 3 pm, lié à ladite couche de chromate, par cuisson à une température allant jusqu'à 150°C.
2. Feuillard d'acier à revêtement organique selon la revendication 1, dans lequel le revêtement organique contient de la silice.
3. Procédé de fabrication d'un feuillard d'acier à revêtement organique, ayant une capacité de durcissement par cuisson améliorée, de 3,5 à 6,1 kgf/mm2, comprenant les étapes consistant:
à déposer une couche d'un alliage à base de zinc sur une surface d'un substrat en acier à très faible teneur en carbone, contenant de 0,001 à 0,008 pour cent en poids de carbone, et ayant une capacité de durcissement par cuisson, selon un poids de 10 à 40 g/m2,
à soumettre ce substrat à un traitement au chromate pour former une couche de chromate sur la couche d'alliage à base de zinc, selon un poids d'au moins 10 mg/m2, d'après calcul sur la base du chrome métallique, et
à appliquer un revêtement organique selon une épaisseur de 0,5 à 3 µm, sur la couche de chromate, et à cuire le revêtement à une température allant jusqu'à 150°C.
4. Procédé selon la revendication 3, dans lequel ledit traitement au chromate, met en oeuvre une solution aqueuse de chromate contenant un chromate, un agent réducteur, et au moins un composé choisi parmi des composés dérivés d'acide, des résines et la silice.
5. Procédé selon la revendication 4, dans lequel l'agent réducteur est choisi parmi le méthanol, le peroxyde d'hydrogène, l'éthylène glycol, l'acide succinique et le succinimide.
6. Procédé selon la revendication 5, dans lequel l'agent réducteur incorporé, est ajouté selon une quantité suffisante pour obtenir de 0,2 à 1,9 moles d'hydrogène par mole de Crûs.
7. Procédé selon la revendication 4, dans lequel le composé dérivé d'acide comprend un composé dérivé de l'acide phosphorique ou borique.
8. Procédé selon la revendication 7, dans lequel le composé dérivé d'acide est ajouté selon une quantité appropriée pour obtenir un rapport du composé dérivé d'acide à Cr03, de 0,05 à 0,3 en poids.
9. Procédé selon la revendiation 4, dans lequel la résine comprend une résine acrylique.
10. Procédé selon la revendication 9, dans lequel la résine est ajoutée selon une quantité appropriée pour obtenir un rapport de la résine à Cr03 de 0,1 à 20 en poids.
11. Procédé selon la revendication 4, dans lequel la silice comprend de la silice colloïdale.
12. Procédé selon la revendication 11, dans lequel la silice est ajoutée selon une quantité appropriée pour obtenir un rapport SiO2/CrO3, de 0,3 à 3,0 en poids.
EP87100847A 1986-01-24 1987-01-22 Tôle d'acier avec un revêtement organique durcissable à chaud et son procédé de fabrication Expired - Lifetime EP0230320B1 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
JP13229/86 1986-01-24
JP61013229A JPS62170340A (ja) 1986-01-24 1986-01-24 焼付硬化性に優れた有機被覆鋼板
JP61290035A JPS63143265A (ja) 1986-12-05 1986-12-05 焼付硬化性に優れた有機被覆鋼板の製造方法
JP290035/86 1986-12-05

Publications (2)

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EP0230320A1 EP0230320A1 (fr) 1987-07-29
EP0230320B1 true EP0230320B1 (fr) 1991-01-30

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EP87100847A Expired - Lifetime EP0230320B1 (fr) 1986-01-24 1987-01-22 Tôle d'acier avec un revêtement organique durcissable à chaud et son procédé de fabrication

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EP (1) EP0230320B1 (fr)
KR (1) KR900006812B1 (fr)
AU (1) AU583444B2 (fr)
CA (1) CA1259532A (fr)
DE (1) DE3767727D1 (fr)
ES (1) ES2020202B3 (fr)

Families Citing this family (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6480522A (en) * 1987-09-24 1989-03-27 Sumitomo Metal Ind Organic composite coated sheet of superior corrosion resistance
FR2638429B1 (fr) * 1988-10-19 1991-01-25 Toyo Kohan Co Ltd Tole d'acier revetue d'un film de resine polyester et son procede de production
JPH0688370B2 (ja) * 1989-03-31 1994-11-09 川崎製鉄株式会社 加工後耐食性に優れた有機被覆鋼板
US5456953A (en) * 1993-02-26 1995-10-10 Armco Steel Company, L.P. Method for coating bake hardenable steel with a water based chromium bearing organic resin
KR100216635B1 (ko) * 1996-01-19 1999-09-01 손욱 칼라 브라운관용 프레임의 제조방법
ITTO20030120A1 (it) * 2003-02-18 2004-08-19 Roberto Lanata Prodotto laminato e relativo procedimento di produzione.
IT1405319B1 (it) * 2010-12-27 2014-01-03 Fontana R D S R L Procedimento di rivestimento di pezzi metallici filettati

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3535168A (en) * 1967-10-13 1970-10-20 Hooker Chemical Corp Metal treating process
JPS55152055A (en) * 1979-05-16 1980-11-27 Toa Gosei Chem Ind Resin coated zinc plated steel plate and adhering vessel using said resin coated zinc plated steel plate as blank
DE3151115A1 (de) * 1980-12-24 1982-09-02 Nippon Kokan K.K., Tokyo "mit einer ueberzugszusammensetzung versehene stahlbleche mit guter korrosionsbestaendigkeit, anstreichbarkeit und korrosionsbestaendigkeit nach dem aufbringen des ueberzugs"
JPS6033192B2 (ja) * 1980-12-24 1985-08-01 日本鋼管株式会社 耐食性、塗料密着性、塗装耐食性のすぐれた複合被覆鋼板
JPS60149786A (ja) * 1984-01-17 1985-08-07 Kawasaki Steel Corp 耐食性に優れた亜鉛系合金電気めつき鋼板の表面処理方法

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AU6785387A (en) 1987-07-30
ES2020202B3 (es) 1991-08-01
KR900006812B1 (ko) 1990-09-21
DE3767727D1 (de) 1991-03-07
AU583444B2 (en) 1989-04-27
EP0230320A1 (fr) 1987-07-29
CA1259532A (fr) 1989-09-19
KR880007795A (ko) 1988-08-29

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