US20070012382A1 - Steel product excellent in corrosion resistance and corrosion fatigue resistance, and surface treatment method therefor - Google Patents

Steel product excellent in corrosion resistance and corrosion fatigue resistance, and surface treatment method therefor Download PDF

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US20070012382A1
US20070012382A1 US11/484,988 US48498806A US2007012382A1 US 20070012382 A1 US20070012382 A1 US 20070012382A1 US 48498806 A US48498806 A US 48498806A US 2007012382 A1 US2007012382 A1 US 2007012382A1
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steel product
surface treatment
atoms
range
ratio
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Takeshi Suzuki
Yoshiki Ono
Kazuya Ikai
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NHK Spring Co Ltd
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NHK Spring Co Ltd
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Assigned to NHK SPRING CO. LTD. reassignment NHK SPRING CO. LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: IKAI, KAZUYA, ONO, YOSHIKI, SUZUKI, TAKESHI
Publication of US20070012382A1 publication Critical patent/US20070012382A1/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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • 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
    • C23C22/07—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 phosphates
    • C23C22/08—Orthophosphates
    • C23C22/20—Orthophosphates containing aluminium cations
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium

Definitions

  • the present invention relates to a steel product excellent in corrosion resistance and corrosion fatigue resistance, and a surface treatment method therefor.
  • compositions of steel products are adjusted or coating films are formed by a surface treatment or the like in steel products used under corrosive environments.
  • the purpose thereof is to prevent deterioration of static strength and fatigue strength characteristics due to corrosive thinning and occurrence of corrosion pits of the steel product from decreasing, and to prevent the appearance from worsening due to generation of rusts.
  • a coating film treatment with a zinc-containing phosphate system may be applied relatively easily.
  • corrosion resistance of the coating film is insufficient.
  • steel products plated with a Zn—Al—Si base molten alloy (trade name Galvanium Steel, manufactured by Nittetsu Steel Sheet Corporation) have been known as backside-treated steel products having both a sacrificial corrosion protective action of Zn and a self-repair action of Al.
  • this molten alloy plating requires a plating bath temperature of 400° C. or more. For this reason, the method cannot be employed when a decrease of the mechanical strength caused by heating the steel product during immersion in a molten alloy is of problem.
  • Patent document 1 discloses an organic-coated steel sheet excellent in corrosion resistance.
  • the organic-coated steel sheet is produced by forming a chemical conversion coating film on a zinc-plated steel sheet followed by forming an organic coating film containing aluminum phosphate.
  • the process of the steel sheet disclosed in patent document 1 is complex with high processing cost since the chemical conversion treatment should be applied before forming the organic coating film containing aluminum phosphate.
  • Patent document 2 discloses a low iron loss unidirectional electromagnetic steel sheet and a method of producing the same.
  • the electromagnetic steel sheet has a coating film comprising a first layer having a Young's modulus of 100 GPa or more and a difference of a linear coefficient of expansion of 2 ⁇ 10 ⁇ 6 or more from that of the steel sheet, and a second layer containing aluminum phosphate.
  • the steel sheet is baked at a temperature in the range of 400 to 1000° C. after applying a coating liquid and drying the coating film for forming the second layer.
  • it has been a problem in patent document 2 that the mechanical strength of the steel product decreases when the steel product is baked at a temperature in the range of 400 to 1000.
  • an object of the present invention is to provide a steel product excellent in corrosion resistance and corrosion fatigue resistance and a surface treatment method therefor, wherein problems of high production cost, complex production process, hydrogen embrittlement and reduced strength of materials can be solved as a surface treatment method capable of substituting conventional surface treatment methods such as electroplating, chemical conversion coating and molten alloy plating.
  • a steel product excellent in corrosion resistance and corrosion fatigue resistance comprising a coating film on the surface of the steel product, wherein a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 0.5% or more in an average composition of the coating film, and the number of Al atoms in the average composition of the coating film is higher than the number of Al atoms in an average composition of the steel product before surface treatment.
  • a steel product excellent in corrosion resistance and corrosion fatigue resistance comprising a coating film on the surface of the steel product, wherein a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 0.5% or more in an average composition in a region from the surface to a depth of 5 ⁇ m of the steel product, and the number of Al atoms in an average composition of the coating film is higher than the number of Al atoms in an average composition of the steel product before surface treatment.
  • a surface treatment method for a steel product comprising rinsing the steel product after allowing an aqueous solution at room temperature having an acidity ratio in the range of 3.3 to 5.7 and containing Al ions to contact the surface of the steel product; and drying the steel product.
  • a surface treatment method for a steel product comprising: drying the steel product after allowing an aqueous solution at room temperature having an acidity ratio in the range of 3.3 to 5.7 and containing Al ions to contact the surface of the steel product; and drying again the steel product after rinsing.
  • FIG. 1A is a flowchart illustrating a first example of surface treatment according to the invention.
  • FIG. 1B is a flowchart illustrating a second example of surface treatment according to the invention.
  • the present inventors have completed the following invention through intensive studies on a mechanism of corrosion of a steel product and a chemical conversion treatment method.
  • a steel product excellent in corrosion resistance and corrosion fatigue resistance has a coating film on the surface of the steel product, wherein a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 0.5% or more in an average composition of the coating film, and the number of Al atoms in the average composition of the coating film is higher than the number of Al atoms in an average composition of the steel product before surface treatment.
  • a steel product excellent in corrosion resistance and corrosion fatigue resistance has a coating film on the surface of the steel product, wherein a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 0.5% or more in an average composition in a region from the surface to a depth of 5 ⁇ m of the steel product, and the number of Al atoms in an average composition of the coating film is higher than the number of Al atoms in an average composition of the steel product before surface treatment.
  • the measurement of “a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms, and selectively added Si, Mn and Cr atoms” is based on elementary analysis, and the analysis is sufficient by measuring a region from the surface of the steel product to a depth of 5 ⁇ m from the surface. This region may be only the coating film or both the coating film and base material.
  • the phrase “selectively added Si, Mn and Cr” means that these atoms are not added at all or at least one atom of them is added.
  • the ratio of the number of Al atoms to the total number of atoms of Fe, C, Al, P and O, and selectively added Si, Mn and Cr in the invention 2 is preferably 0.5% or more, particularly preferably 3% or more.
  • the proportion of the number of Al atoms is less than 0.5%, both sacrificial corrosion protective action and passive state forming action to be described below are weak to fail in obtaining sufficient corrosion resistance and corrosion fatigue resistance.
  • a surface treatment method for a steel product according to a third embodiment of the invention comprises rinsing the steel product after allowing an aqueous solution at room temperature having an acidity ratio in the range of 3.3 to 5.7 and containing Al ions to contact the surface of the steel product followed by drying the steel product.
  • a surface treatment method for a steel product comprises: drying the steel product after allowing an aqueous solution at room temperature having an acidity ratio in the range of 3.3 to 5.7 and containing Al ions to contact the surface of the steel product; and drying again after rinsing.
  • the acidity ratio is determined to be in the range of 3.3 to 5.7 in the methods cited in the items 3 and 4 of the invention for the following reasons. That is, the coating film is suppressed from being formed due to too severe corrosion of the steel product when the acidity ratio is less than 3.3. On the other hand, the formation of the coating film takes a long period of time since the corroding action of the steel product is too weak when the acidity ratio exceeds 5.7.
  • the acidity ratio is more preferably in the range of 3.8 to 5.4.
  • aqueous solution containing Al ions it is preferable in the methods cited in the items 3 and 4 of the invention to allow the aqueous solution containing Al ions to contact the surface of the steel product for 30 seconds or more at a temperature in the range of 40 to 50° C.
  • the contact period is less than 30 seconds, the corroding action of the steel product is insufficient to fail in obtaining a coating film excellent in corrosion resistance and corrosion fatigue resistance.
  • aqueous solution containing Al ions it is also preferable in the methods cited in the items 3 and 4 of the invention to allow the aqueous solution containing Al ions to contact the surface of the steel product for 180 seconds or more at 30° C.
  • the contact period is less than 180 seconds, the corroding action of the steel product is insufficient to fail in obtaining a coating film excellent in corrosion resistance and corrosion fatigue resistance.
  • the drying temperature is prescribed to be 50° C. or less because, when the temperature exceeds 50° C., denseness of the coating film decreases due to too rapid evaporation of water to result in a decrease of the strength of the coating film.
  • the treatment work is easy with a low processing cost in the steel product subjected to surface treatment in the invention, and excellent corrosion resistance and corrosion fatigue resistance are manifested without causing hydrogen embrittlement and decreased strength of the material.
  • FIG. 1A shows an example of the surface treatment according to the invention, which is a first example of the surface treatment in which a drying step 2 as a post treatment is carried out after a rinsing step 1 .
  • FIG. 1B shows another example of the surface treatment according to the invention, which is a second example of the surface treatment in which the rinsing step 1 and a second drying step 2 b as post treatments are sequentially carried out after a first drying step 2 a.
  • the surface of the steel product is preferably washed with an aqueous cleaning solution containing an organic solvent or a surfactant in advance for degreasing as in the pre-treatments in FIGS. 1A and 1B , or subjected to descaling with an acid solution.
  • aqueous cleaning solution containing an organic solvent or a surfactant in advance for degreasing as in the pre-treatments in FIGS. 1A and 1B , or subjected to descaling with an acid solution.
  • Water to be used for rinsing in FIGS. 1A and 1B desirably contains Cl as small as possible.
  • the solution for use in the treatment in FIGS. 1A and 1B (referred to the treatment solution hereinafter) is obtained by adding aluminum phosphate (AlPO 4 ) in water, and further adding phosphoric acid (H 3 PO 4 ) for dissolving AlPO 4 .
  • the concentration by weight of AlPO 4 is favorably in the range of 1 to 10%.
  • the concentration of AlPO 4 exceeds 10%, the steel product may be eroded with the acid because the concentration of H 3 PO 4 added for dissolving AlPO 4 increases.
  • concentration of AlPO 4 is below 1%, processability decreases since AlPO 4 is frequently replenished.
  • industrial water, city water and distilled water may be used as water used for the solvent.
  • Cl is preferably removed as much as possible.
  • any methods such as immersion, air spray and brush coating may be used for allowing the treatment solution to contact the steel product. While the mechanism for forming the coating film has not been fully elucidated, it is conjectured as follows.
  • Aluminum primary phosphate (Al(H 2 PO 4 ) 3 ), H 3 PO 4 and AlPO 4 are in an equilibrium state represented by the following formula (1) in the treatment solution.
  • H 3 PO 4 interacts with Fe as shown in the following formula (2), and the concentration of H 3 PO 4 decreases in the solution near the surface of the steel product as shown in the formula (1). Accordingly, the equilibrium shown in the formula (1) shifts to the right side, and hardly soluble AlPO 4 seems to precipitate on the surface of the steel product to form a coating film.
  • the coating film is considered to be formed based on the steel product corroding action of H 3 PO 4 and deposition of hardly soluble AlPO 4 formed by decomposition of Al(H 2 PO 4 ) 3 . It may be also considered that dissolved Fe is contained in AlPO 4 formed as described above, and a film comprising Al, Fe, P and O is formed. Accordingly, the molar ratio between H 3 PO 4 and Al(H 2 PO 4 ) 3 is important in the treatment condition by the treatment solution of the invention, and specifically, control of the acidity ratio is important.
  • the term “acidity ratio” as used herein refers to a ratio of a point of acidity of total phosphoric acid (H 3 PO 4 and Al(H 2 PO 4 ) 3 ) to a point of acidity of free phosphoric acid (H 3 PO 4 ) in the treatment solution.
  • the “point” is as described below.
  • a treatment solution (10 cc) is neutralized with 0.1 N sodium hydroxide (NaOH) by adding 2 to 3 drops of methyl orange solution as an indicator at room temperature.
  • the point of acidity of free phosphoric acid is the volume of the aqueous NaOH solution represented by a cc unit when the color of the solution changes to orange.
  • the point of total acidity is the volume of the aqueous NaOH solution represented by the cc unit when the same solution is neutralized as described above by adding 2 to 3 drops of phenolphthalein solution and the color of the solution changes to pale pink.
  • the acidity ratio of the treatment solution can be controlled by adding a basic aqueous solution such as an aqueous NaOH solution.
  • a treatment solution ready for forming the coating film may be obtained for maintaining the equilibrium in formula (1) by increasing the acidity ratio, or by decreasing the amount of H 3 PO 4 .
  • the acidity ratio is desirably in the range of 3.3 to 5.7, particularly of 3.8 to 5.4.
  • the acidity ratio is less than 3.3 (too much H 3 PO 4 )
  • the coating film is suppressed from being formed due to too vigorous corrosion of the steel product.
  • the acidity ratio exceeds 5.7 (to little H 3 PO 4 )
  • a long period of time is necessary for forming the coating film since the corrosion action for the steel product is weak.
  • the required contact periods of the steel product with the treatment solution are 1000 seconds or more, 180 seconds or more and 30 seconds or more when the temperatures of the treatment solution are room temperature, 30° C. and in the range of 40 to 50° C., respectively.
  • the upper limit temperature of the treatment solution is desirably 50° C. or less since temperature control is difficult at a higher temperature while the cost for maintaining the temperature is high.
  • the steel product may be immediately rinsed (first example of surface treatment: A) after the above-mentioned treatment as shown in FIG. 1A , or may be rinsed after drying (second example of surface treatment: B) after the above-mentioned treatment as shown in FIG. 1B . Since drying is applied for removing adhered water, the steel product may be left at room temperature or in a heated atmosphere. Otherwise, a conventional drying furnace may be used.
  • the temperature of the first drying step in the second example of the surface treatment is desirably 50° C. or less. A temperature exceeding 50° C. is not preferable since water is so rapidly evaporated that denseness of the coating film decreases to reduce the strength of the coating film.
  • the steel product treated as described above is expected to have a sacrificial corrosion protective action by the Al component in the coating film formed on the surface and an action for forming a passive film by oxidation of the Al component.
  • the concentration of Al atoms in the coating film at the surface of the steel product is quantitatively analyzed by elementary analysis in the region from the surface to a depth of 5 ⁇ m using an EDX (Energy Dispersive X-ray spectroscopy) device, and is corrected by ZAF correction (Z: difference of emitted X-ray intensity depending on difference of sample compositions; A: absorption X-ray in sample; F: fluorescence excitation by emitted X-rays in sample).
  • the ratio of A (the ratio of the number of Al atoms) to B should be 0.5% or more, particularly desirably 3% or more.
  • the ratio of the number of Al atoms is less than 0.5%, both the above-mentioned sacrificial corrosion protective action and action for forming a passive film are so weak that sufficient corrosion resistance and corrosion fatigue resistance cannot be obtained.
  • a surface treatment with a low cost is possible since aluminum phosphate, phosphoric acid and distilled water are cheaply and readily available and the treatment is simple. Since the treatment solution is weakly acidic and the amount of hydrogen generated from the steel product is very small, the steel product is substantially free from hydrogen embrittlement. Further, all the treatments are performed at 50° C. or less, so that the strength of the materials is hardly reduced.
  • the surface of a steel product SAE9254 (Fe-0.56% C-1.42% Si-0.75% Mn-0.68% Cr) with a diameter of 4 mm and a length of 20 to 80 mm was treated under the conditions shown in Table 1.
  • the steel product was a material obtained by removing in advance solid and thick oxidized scales by blast treatment.
  • the pre-treatment procedure comprises washing the steel product with distilled water, degreasing with acetone, and then removing the scale again by immersing the steel product in dilute aqueous hydrochloric acid. Then, the steel product to be treated was immersed in the treatment solution under the conditions shown in Table 1, washed with distilled water, and allowed to spontaneously dry (the condition in which the column of drying at 50° C.
  • the ratio of the number of Al atoms on the surface of the steel product, corrosion resistance and corrosion fatigue resistance were evaluated by using the surface-treated product as a test material.
  • the ratio of the number of Al atoms in the region from the surface to a depth of 5 ⁇ m was quantitatively analyzed through elementary analysis by using the EXD device, and the measured value was corrected by ZAF.
  • the ratio of A to B was calculated and evaluated based on the result of the quantitative analysis, where A denotes the number of Al atoms and B denotes the total number of Fe, C, Al, P and O atoms, and of Si, Mn, Cr atoms that are optionally added.
  • the material with a ratio of 3% or more was represented by “3 or more”, the product with a ratio in the range of 0.5% or more and less than 3% was represented by “0.5 to 3”, and the product with a ratio of less than 0.5% was represented by “less than 0.5”.
  • the corrosion resistance was evaluated by leaving the test material in a constant temperature-constant humidity chamber (26° C., 95% RH) for 200 hours. The proportion of the rusted area to the total area after leaving for 200 hours was visually evaluated.
  • the products with a surface ratio of the rusted surface of less than 80%, in the range of 80% or more and less than 90%, and 90% or more were evaluated as “best (a level having quite excellent corrosion resistance)”, “good (a level having excellent corrosion resistance)” and “poor (a level having poor corrosion resistance)”, respectively, by using the proportion of the rusted area C in Comparative Example 1 as a standard.
  • the corrosion fatigue resistance was evaluated by repeating the steps of vibrating a test material left under salt water spray (35° C., 5% NaCl) for 30 minutes at a shear stress T of 733 ⁇ 441 MPa for 3,000 times (1.5 Hz, for about 33 minutes), and leaving the test material in a constant temperature-constant humidity chamber (26° C., 95% RH) until the test material was broken.
  • the corrosion fatigue resistance was evaluated as “best (a level quite excellent in corrosion fatigue resistance)” when the duration count was 120% or more, as “good (a level excellent in corrosion fatigue resistance)” when the duration count was in the range of 10% or more and less than 120%, and as “poor (a level poor in corrosion fatigue resistance” when the duration count was less than 110% on the basis of the duration count D in Comparative Example 1 as a standard.
  • the steel product in Comparative Example 1 is a non-treated product, and serves as a standard for evaluating corrosion resistance and corrosion fatigue resistance.
  • TABLE 1 Surface treatment condition Evaluation Surface Bath Immersion Temperature Evaluation Evaluation of corrosion treatment Acidity temperature period of drying of ratio of of corrosion fatigue example ratio (° C.) (seconds) step (° C.) Al atoms resistance resistance Comparative — — — — — Less than 0.5 Poor Poor example 1 Comparative B 2.0 Room 180 50 Less than 0.5 Poor Poor example 2 temperature Comparative B 2.5 Room 180 50 Less than 0.5 Poor Poor example 3 temperature
  • Example 4 B 5.0 Room 180 50 3 or more Best Best temperature Example 5 B 5.4 Room 180 50 3 or more Best Best temperature Example 6 B 5.7 Room 180 50 0.5 to 3 Good Good temperature
  • Steel products in Comparative Examples 2 and 3 and in Examples 1 to 6 are in accordance with the second examples of the surface treatment in FIG. 1B . That is, the products were dried in air by heating at 50° C. (first drying step) after immersing in the treatment solution at room temperature for 180 seconds, and then spontaneously dried again (second drying step) after rinsing, where the acidity ratio of the treatment solution was changed in the range of 2.0 to 5.7.
  • the ratio of the number of Al atoms is “0.5 to 3” or “3 or more” in the products in Examples 1 to 6 in which the acidity ratio is in the range of 3.3 to 5.7.
  • Corrosion resistance and corrosion fatigue resistance are “good (excellent level)” or “best (quite excellent level)” in the products in Examples 1 to 6 in which the acidity ratio is in the range of 3.3 to 5.7.
  • both the corrosion resistance and corrosion fatigue resistance are “best (quite excellent level)” in the products in Examples 2 to 5 in which the acidity ratio is in the range of 3.8 to 5.4.
  • both the corrosion resistance and corrosion fatigue resistance are “poor (poor level)” in the products in Comparable Examples 2 and 3 in which the activity ratio is 2.5 or less.
  • the steel products in Examples 7 to 10 are in accordance with the second example of the surface treatment in FIG. 1B . That is, the products were dried by heating at 50° C. in air after immersing in the treatment solution with an acidity ratio of 4.7 at room temperature, and then spontaneously dried after rinsing, where the immersion period was changed in the range form 5 to 1000 seconds.
  • the atomic ratio of the number of Al atoms is “0.5 to 3” or “3 or more” in all these examples. In particular, the ratio of the number of Al atoms is “3 or more” in the products in all Examples 8 to 10 in which the immersion period is 30 seconds or more.
  • Corrosion resistance and corrosion fatigue resistance are “good (excellent level)” or “best (quite excellent level)” in the products in all the examples.
  • corrosion resistance and corrosion fatigue resistance are “best (quite excellent level)” in the products in Examples 9 and 10 in which the immersion period is 360 seconds or more.
  • the steel products in Comparative Examples 4 to 7 and Example 11 are in accordance with the first example of the surface treatment in FIG. 1A . That is, the products were rinsed and spontaneously dried after immersing in the treatment solution with an acidity ratio of 4.7 at room temperature, where the immersion period was changed in the range of 5 to 1000 seconds.
  • the atomic ratio of the number of Al atoms is “3 or more” in the product in Example 11 with an immersion period of 1000 seconds. However, the ratio of the number of Al atoms is “less than 0.5” in the products in Comparative Examples 4 to 7 with an immersion period of 360 seconds or more.
  • Corrosion resistance and corrosion fatigue resistance are “best (quite excellent level)” in the product in Example 11 with an immersion period of 1000 seconds. However, corrosion resistance and corrosion fatigue resistance are “poor” (poor level) in the products in Comparative Examples 4 to 7 with an immersion period of 360 seconds or less.
  • the steel products in Comparative Examples 8 and 9 and Examples 12 to 14 are in accordance with the first example of the surface treatment in FIG. 1A . That is, the products were rinsed and spontaneously dried after immersing in the treatment solution with an acidity ratio of 4.7 at 30° C., where the immersion period was changed in the range form 5 to 1000 seconds.
  • the atomic ratio of the number of Al atoms is “0.5 to 3” or “3 or more” in the products in Examples 12 to 14 with an immersion period of 180 seconds or more.
  • the ratio of the number of Al atoms in the product in Example 14 with an immersion period of 1000 seconds is “3 or more”.
  • the ratio of the number of Al atoms is “less than 0.5” in the products in Comparative examples 8 and 9 with an immersion period of 30 seconds or less.
  • Corrosion resistance and corrosion fatigue resistance are “good (excellent level)” or “best (quite excellent level)” in the products in Examples 12 to 14 with an immersion period of 180 seconds or more.
  • corrosion resistance and corrosion fatigue resistance are “best (quite excellent level)” in the product in Example 14 with an immersion period of 1000 seconds.
  • Corrosion resistance and corrosion fatigue resistance are “poor (poor level)” in the products in Comparative Examples 8 and 9 with an immersion period of 30 seconds or less.
  • the steel products in Comparative Examples 10 and 11 and Examples 15 to 22 are in accordance with the first example of the surface treatment in FIG. 1A . That is the products were rinsed and spontaneously dried after immersing in the treatment solution with an acidity ratio of 4.7 at 40° C. or 50° C., where the immersion period was changed in the range of 5 to 1000 seconds.
  • the ratio of the number of Al atoms is “3 or more” in the products in Examples 15 to 22 with an immersion period of 30 seconds or more. However, the ratio of the number of Al atoms is “less than 0.5” in the products in Comparative Examples 10 and 11 with an immersion period of 5 seconds.
  • Corrosion resistance and corrosion fatigue resistance are “good (excellent level)” or “best (quite excellent level)” in the products in Examples 15 to 22 with an immersion period of 30 or more.
  • corrosion resistance and corrosion fatigue resistance are “best (quite excellent level)” in the products in Examples 16 to 18 and Examples 20 to 22 with an immersion period of 180 seconds.
  • corrosion resistance and corrosion fatigue resistance are “poor (poor level)” in the products in Comparative Examples 10 and 11 with an immersion period of 5 seconds.
  • the invention is not restricted to a steel product SAE9254 having the above-mentioned diameter and length, instead the invention is applicable to other steel products at a practical stage in the range not departing from the spirit of the invention. Specifically, the invention is applicable to all the steel products containing at least Fe and C with optionally added at least one of Si, Mn and Cr.
  • the surface treatment conditions for example, bath temperature, immersion period and the like
  • the surface treatment conditions are not restricted to those as set forth in the examples, but an appropriate combination is possible in the range not modifying the spirit of the invention.

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CN1924097B (zh) 2011-02-23
EP1743955B1 (de) 2012-05-02
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