Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon

Definitions

• the present invention relates to hot-dip galvanized steel materials.
• Hot-dip Zn plating is an inexpensive method of rust prevention for steel, and is used in a variety of fields where rust prevention is required, such as civil engineering, construction, and automotive.
• Patent Document 1 describes the production of plated steel sheets using a so-called continuous hot-dip plating method, in which a steel sheet is continuously immersed in a hot-dip plating bath. The plated steel sheet is then processed into the shape of the part.
• the continuous hot-dip plating method is used to form zinc plating and Zn alloy plating containing Al and Mg. With the continuous hot-dip plating method, the thickness of the plating layer is relatively thinner than with the post-plating method described below.
• Patent Document 2 discloses a method in which an object to be plated, which has been processed into a predetermined shape, is immersed in a plating bath.
• This method is also known as a batch-type hot-dip plating method or a post-plating method.
• the post-plating method the object to be plated is immersed in the plating bath for one minute or more, so the plating layer tends to be thicker.
• corrosion resistance tends to be inferior, and manufacturability is also significantly inferior.
• plated steel sheets are extremely susceptible to corrosion in environments where they remain wet for long periods of time. Therefore, increasing the thickness of the plating layer to improve water corrosion resistance is an important factor in extending the life of plated steel sheets. Therefore, if a Zn alloy plating layer containing a large amount of Al could be produced using the continuous hot-dip plating method to a thickness comparable to that achieved using post-plating methods, it would be possible to efficiently produce steel sheets that provide long-term corrosion protection.
• Coated steel sheets produced by the continuous hot-dip galvanizing method are used in the civil engineering and building materials fields and require long-term corrosion resistance.
• the thickness of the coating layer on coated steel sheets produced by this method is often around 20 to 30 ⁇ m. This is due to the process characteristics of the continuous hot-dip galvanizing method, which involve lifting the molten metal when the steel sheet is removed from the coating bath, gas wiping, and air-cooling solidification. Increasing the removal speed from the coating bath increases the coating layer thickness, but because this makes surface appearance control difficult, the upper limit of the coating layer thickness is strictly limited by wiping adjustments.
• the present invention was made in consideration of the above circumstances, and aims to provide hot-dip galvanized steel material that has excellent corrosion resistance, water-wet corrosion resistance, and workability.
• a hot-dip plated steel material having a base steel material and a coating layer disposed on a surface of the base steel material, wherein the average chemical composition of the coating layer is, in mass%, Al: more than 10% and less than 45%; Mg: 4.0 to 15.0%, Si: 0-2.00%, Cr: 0-3.00%, Mo: 0-3.00%, Sn: 0 to 0.7%, Bi: 0-0.3%, In: 0 to 0.3%, Ca: 0-0.60%, Y: 0 to 0.3%, La: 0 to 0.3%, Ce: 0-0.3%, Sr: 0-0.3%, Li: 0 to 0.3%, Ni: 0-1.0%, Cu: 0 to 1.0%, Ag: 0-0.25%, Sb: 0 to 0.25%, Pb: 0 to 0.25%, B: 0 to 0.50%, P: 0 to 0.50%, Ti: 0 to 0.25%, Co: 0-0.2
• the present invention provides hot-dip galvanized steel products with excellent corrosion resistance, water-wet corrosion resistance, and workability.
• FIG. 1 is a diagram for explaining formulas (1) and (2), and is a schematic diagram showing the results of X-ray diffraction measurement.
• the inventors have conducted extensive research into hot-dip galvanized steel material that has a coating layer containing Al, Mg, and Zn and is manufactured using a continuous hot-dip galvanizing method, and that has a high coating layer mass, excellent corrosion resistance and water-wet corrosion resistance, and excellent workability, as well as a method for manufacturing the same.
• Alloying elements such as Al and Mg have a lower specific gravity than Zn.
• the specific gravity of alloying elements is closely related to viscosity in the molten state. That is, molten metal containing high concentrations of Al, Mg, etc. has a lower viscosity. For this reason, in continuous hot-dip plating, when a steel sheet is passed through a plating bath at approximately 500°C at a constant line speed, the amount of hot-dip plating bath that adheres to the steel sheet and is lifted up with it is significantly less than in the case of pure Zn plating. Therefore, the development of wiping technology and other techniques is essential to consistently produce plating layers with thicknesses exceeding 20 ⁇ m.
• the molten metal that makes up the plating layer is light due to its low specific gravity, it is easily blown away during wiping blows. Furthermore, because it is prone to wrinkling, it is difficult to produce thick plating layers, even with improvements in wiping technology and other techniques.
• elements such as Si are sometimes added to plating baths to enhance their performance.
• added elements such as Si tend to bond with elements such as Fe and Mg, and can also form floating dross, which can easily settle in the plating bath.
• Floating dross is particularly likely to form in plating baths at around 500°C.
• the inventors therefore conducted extensive research to solve the above problems.
• adding Cr increases the specific gravity and viscosity of the coating bath, making it possible to increase the coating adhesion weight on the base steel during threading.
• Mo is an element that has a similar effect to Cr.
• the reaction between Al and Fe is suppressed, making it possible to significantly suppress the growth of the interfacial alloy layer.
• This makes it possible to produce hot-dip plated steel that has excellent corrosion resistance, water-wet corrosion resistance, and workability, and has a relatively large coating mass of the plating layer.
• % used to indicate the concentration of each element in the chemical composition means “mass %.”
• a numerical range expressed using “ ⁇ ” means a range that includes the numbers written before and after " ⁇ ” as the lower and upper limits. Furthermore, when the numbers written before and after " ⁇ ” are followed by "greater than” or “less than,” the numerical range does not include these numbers as the lower or upper limit.
• the base steel material to be plated is, for example, mainly a steel plate, but there is no particular limitation on its size.
• the steel plate may be any steel plate that can be applied to a normal hot-dip galvanizing process. Specifically, this applies to steel plates that can be applied in processes such as continuous hot-dip galvanizing lines (CGLs) where the steel plate is immersed in molten metal and solidified.
• the size of the steel plate may be, for example, 10 mm or less in thickness and 2000 mm or less in width, but the steel plate size is not limited to these.
• the thickness of the base steel plate is preferably 0.25 to 10.00 mm, more preferably 0.40 to 10.00 mm, 0.50 to 10.00 mm, or 0.5 to 6.00 mm.
• base steel There are no particular restrictions on the material of the base steel.
• Examples of applicable base steel include general steel, pre-plated steel thinly plated with various metals, Al-killed steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, some high-alloy steels (steels containing corrosion-resistant strengthening elements such as Ni and Cr), soft steel wire, hard steel wire, spring steel, steel cord, steel for bolts, and steel wire for bridge cables.
• JIS Applicable steels include various plated steels such as JIS G 3321:2022 (hereinafter, plating to use plated steel sheets as base steel is also referred to as "pre-plating", and plated steel sheets or plated steel sheets as base steel are also referred to as "pre-plated steel sheets or pre-plated steel sheets”); rolled steel for building structures specified in JIS G 3136:2022; various high-tensile steels specified in JIS G 3113:2018, JIS G 3134:2018, JIS G 31
• a Ni-plated layer, a Cr-plated layer, or a Mo-plated layer may be provided in advance as a pre-plating on the surface of the base steel material in a coating amount of 0.3 to 5.0 g/m 2.
• a base steel material provided with such a pre-plated layer it is possible to suppress the growth of an Al-Fe-based interfacial alloy layer, suppress the occurrence of powdering, etc., and also improve workability.
• the manufacturing process for the base steel material includes common processes such as pig iron and steel making using a blast furnace or electric furnace, hot rolling, pickling, cold rolling, and heat treatment, but the base steel material of this embodiment may have undergone any of these processes, and the processing conditions for each process are not limited.
• the Zn—Al—Mg alloy layer is made of a Zn—Al—Mg alloy, which means a ternary alloy containing Zn, Al, and Mg.
• the thickness of the Zn—Al—Mg alloy layer may be 4 to 80 ⁇ m, and if necessary, the lower limit may be 7 ⁇ m, 10 ⁇ m, 15 ⁇ m, or 20 ⁇ m, and the upper limit may be 70 ⁇ m, 60 ⁇ m, or 50 ⁇ m.
• the plating layer may have a single-layer structure of a Zn-Al-Mg alloy layer, or a laminated structure including a Zn-Al-Mg alloy layer and an Al-Fe interfacial alloy layer.
• the Zn-Al-Mg alloy layer it is preferable for the Zn-Al-Mg alloy layer to be the layer that forms the surface of the plating layer.
• the Al-Fe-based interface alloy layer only contributes to corrosion resistance, it influences the adhesion of the plating layer during processing of hot-dip galvanized steel and the workability (presence or absence of cracks).
• the Al-Fe-based interface alloy layer may affect powdering resistance, which indicates the degree of peeling of the plating layer during processing.
• a thinner Al-Fe-based interface alloy layer reduces the number of crack initiation points in the plating layer during processing, thereby improving powdering resistance. Therefore, for hot-dip galvanized steel that may be subjected to high processing during use, it is preferable that the Al-Fe-based interface alloy layer be as thin as possible.
• the Al-Fe-based interface alloy layer may partially contain small amounts of AlFe phase, Al 3 Fe phase, Al 5 Fe 2 phase, etc. Furthermore, since the plating bath also contains a certain concentration of Zn, the Al-Fe-based interface alloy layer may contain a small amount of Zn or Si, which tends to accumulate at the interface.
• Al More than 10% and Less than 45% Al is an element that mainly constitutes the plating layer.
• the Al concentration exceeds 10%, the melting point of the plating layer becomes higher than that of pure Zn.
• Cr which is necessary to increase the thickness of the plating layer in this embodiment, does not form a solid solution with Zn or Mg. Therefore, a certain amount of Al is required to contain Cr. Since the minimum concentration required for this purpose is more than 10%, the Al concentration is set to more than 10%.
• Al is an essential element for forming Cr-containing intermetallic compounds or Mo-containing intermetallic compounds.
• the Al concentration is preferably 18% or more, more preferably 25% or more.
• a Cr concentration of 0.03% or higher is preferred.
• the Cr concentration is more preferably 0.10% or higher, 0.30% or higher, 0.50% or higher, 0.70% or higher, or 1.00% or higher.
• Si 0-2.00% Si suppresses the Al-Fe reaction, thereby suppressing the formation of an Al-Fe-based interface alloy layer. Furthermore, Si is incorporated into a portion of the Al-Fe-based interface alloy layer to form an Al-Fe-Si compound. Without Si, the Al-Fe reaction becomes active, the thickness of the Al-Fe alloy layer increases, powdering occurs during processing, and corrosion resistance is significantly impaired. Since Si is not necessary, the lower limit of the Si concentration is 0%. However, if Si is contained in an amount of 0.01% or more, the growth rate of the thickness of the interface alloy layer slows. Therefore, the Si concentration is preferably 0.01% or more. The Si concentration is preferably 0.10% or more, or 0.20% or more.
• each element has an upper limit to its concentration, and if too much is added, Mg will be absorbed from the Cr-containing intermetallic compound, preventing the Cr-containing intermetallic compound from forming and resulting in a thinner plating thickness. Therefore, the Sn concentration should be 0.7% or less, and the Bi and In concentrations should be 0.3% or less. Furthermore, the total concentration ⁇ X of Sn, Bi, and In should be 0.7% or less. The total amount ⁇ X is preferably 0.5% or less, or 0.4% or less.
• the Ca concentration should be 0.60% or less.
• the Ca concentration is preferably 0.50% or less, or 0.40% or less.
• the elements in the element group Yb have a common effect. When the concentration of any one of these elements is 0.10% or more, the effect of improving water-wet corrosion resistance is obtained. Therefore, it is preferable that the concentration of any one of these elements is 0.10% or more. However, if the total concentration of these elements, ⁇ Yb, becomes excessive, various intermetallic compounds are formed, causing the viscosity of the coating bath to become extremely high.
• Element group Yc B 0 to 0.50%
• P 0 to 0.50%
• P ⁇ Yc 0 to 0.50%
• the elements in element group Yc have a common effect.
• concentration of any one of these elements is 0.05% or more
• the concentration of any one of these elements is 0.05% or more.
• the viscosity of the coating bath becomes extremely high. As a result, when the base steel is pulled out of the coating bath, the amount of molten metal adhering to the base steel decreases, the thickness of the coating layer becomes extremely thin, and the corrosion resistance deteriorates.
• the concentration of any one of these elements is 0.10% or more, or the total amount ⁇ Z of the concentrations of these elements is 0.10% or more.
• the concentration of each element in element group Z is set to 0.25% or less.
• the concentration of each element in element group Z is preferably 0.20% or less and 0.10% or less, respectively.
• the total amount ⁇ Z is set to 0.25% or less, preferably 0.20% or less, and more preferably 0.10% or less.
• the plating layer according to this embodiment is a Zn-Al-Mg alloy plating, and therefore contains a Zn phase, an Al phase, and an MgZn biphase .
• the plating layer according to this embodiment also contains one or both of a Cr-containing intermetallic compound and a Mo-containing intermetallic compound. Furthermore, the plating layer according to this embodiment may contain other intermetallic compounds.
• Mg 2 Sn, Mg 9 Sn 5 and Mg 3 In tend to combine with Mg to form Mg 2 Sn, Mg 9 Sn 5 and Mg 3 In, but these are formed independently of each other regardless of reaction with Cr and Mo.
• a glow discharge optical emission spectroscopy (GDS) device is preferably used to analyze the components in the depth direction within the plating layer.
• the inventors used a LECO Japan 850A glow discharge optical emission spectroscopy device, but the measurement device is not limited to this.
• the analysis conditions are argon pressure: 0.27 MPa, output power: 30 W, output voltage: 1000 V, and discharge area: within a circular area with a diameter of 4 mm. Measurements are performed from the surface of the plating layer in the depth direction to a position at least 1/4 of the plate thickness away from the surface of the base steel sheet.
• the analysis range of depth direction analysis using GDS extends from the plating surface to the Zn-Al-Mg alloy layer, Al-Fe alloy layer, and part of the base steel.
• the sputtering depth of the cross section is measured using, for example, a Surfcom 130A manufactured by Tokyo Seimitsu Co., Ltd.
• GDS analysis provides an elemental distribution profile of the plating layer in the depth direction.
• the elemental distribution profile shows the concentration distribution of each element in the depth direction, assuming the total amount of detected elements to be 100%.
• the evaluation range is defined as "the region from 0.1 x t thickness from the surface of the coating layer (hot-dip coated steel) to the thickness position where an Fe concentration of 40% of the base steel Fe concentration (Fe concentration at a position 1/4 of the thickness of the hot-dip coated steel from the surface of the hot-dip coated steel) is detected.”
• the region from the surface of the coating layer to 0.1 x t thickness from the surface of the coating layer is excluded from the evaluation range because it is an area where GDS analysis errors can be large.
• areas near the boundary between the coating layer and the base steel may be affected by an Al-Fe interfacial alloy layer.
• the coating layer thickness can be increased to 15 ⁇ m or more, preferably 20 ⁇ m or more, 25 ⁇ m or more, or more than 30 ⁇ m, whereas in the case of a coating layer without Cr and Mo, the maximum thickness was limited to around 30 ⁇ m. If a region where the total amount ⁇ A of Cr and Mo is 0.03% or more exists continuously in the thickness direction for 1.0 ⁇ m or more, the coating layer thickness can be increased to 15 ⁇ m or more, preferably 20 ⁇ m or more, 25 ⁇ m or more, or more than 30 ⁇ m.
• the base material for plating in an element distribution profile obtained by GDS analysis from the surface of the plating layer to a position at least 1/4 of the thickness of the hot-dip plated steel, if the thickness position at which an Fe concentration of 95% of the Fe concentration in the base steel (the Fe concentration at a position 1/4 of the thickness of the hot-dip plated steel from the surface of the hot-dip plated steel) is detected is defined as the interface between the plating layer and the base steel, it is preferable that the average value of the sum of the Cr and Mo concentrations ⁇ A exceeds 0.50% in the region from the interface to 1.0 ⁇ m toward the surface of the plating layer.
• the X-ray diffraction pattern of the plating layer surface satisfies the following formulas (1) and (2): (Imax(10.5° ⁇ 11.0°)/(I(10.5°)+0.2 ⁇ (
• Imax (k to m°) is the average chemical composition of the plating layer. Therefore, when the average chemical composition of the plating layer has a Ca concentration of 0.05% or more, a Cr concentration or a Mo concentration of 0.05% or more, and satisfies the relationship ⁇ Ya ⁇ Si+Cr+Mo, it is preferable that the X-ray diffraction pattern of the plating layer surface satisfies the following formulas (1) and (2): (Imax(10.5° ⁇ 11.0°)/(I(
• the Ca concentration is 0.05% or more
• the Cr concentration or the Mo concentration is 0.05% or more, and the relationship ⁇ Ya ⁇ Si+Cr+Mo is satisfied, it becomes possible to detect Al18Cr2Mg3 and the like by X-ray diffraction measurement.
• Cr - containing intermetallic compounds Al18Cr2Mg3 and Al20CaCr are intermetallic compounds with excellent corrosion resistance. When these metals are contained in the plating layer to an extent that they can be detected by X-ray diffraction, corrosion resistance in a water-wet environment is improved.
• X-ray diffraction measurements are performed using Cu-K ⁇ radiation at an X-ray output of 50 kV and 300 mA.
• Imax (10.5° - 11.0°) in equation (1) is the maximum value of X-ray diffraction intensity between diffraction angles of 10.5° and 11.0°.
• I(10.5°) is the X-ray diffraction intensity at a diffraction angle of 10.5°
• Imax (11.0°) is the X-ray diffraction intensity at a diffraction angle of 11.0°.
• Imax (20.2° - 20.5°) is the maximum value of the X-ray diffraction intensity between diffraction angles of 20.2° and 20.5°.
• I(20.2°) is the X-ray diffraction intensity at a diffraction angle of 20.2°, and I(20.5°) is the X-ray diffraction intensity at a diffraction angle of 20.5°.
• the numerators in formulas (1) and (2) are the intensities corresponding to the diffraction peaks of the Cr-containing compound or Mo-containing compound, and are the maximum diffraction intensities of the diffraction peaks, including background intensity. Because measurement errors in X-ray diffraction can cause the diffraction peaks to deviate from 10.6° or 20.4°, the maximum values between 10.5° and 11.0° and between 20.2° and 20.5° are obtained.
• the Al-Fe interfacial alloy layer may not be clearly visible due to the influence of the pre-plated layer (Cr, Mo, Ni).
• the thickness of the Al-Fe interfacial alloy layer is obtained by measuring the thickness in any three fields of view using an SEM and calculating the average value.
• the thickness of the Zn-Al-Mg alloy layer can be determined to the nearest 1 ⁇ m by calculating the average thickness of the plating layer obtained by observing three fields of view at 200x magnification, subtracting the average thickness of the Al-Fe interfacial alloy layer from the plating layer thickness, and rounding off the result.
• the hot-dip plated steel material according to this embodiment is preferably produced by a continuous hot-dip plating method. However, due to size restrictions of the base steel material, it can also be produced by a batch-type hot-dip plating method, if necessary.
• the base steel material with a sufficiently reduced surface is immersed in a reduced state in a coating bath.
• the temperature of the coating bath is 600°C or higher.
• the coating bath is stirred by bubbling N2 gas through the coating bath.
• the flow rate of the N2 gas bubbling is 0.05 m/sec or higher. If the coating bath is not stirred under these conditions, floating dross consisting of Cr compounds or Mo compounds will settle in the coating bath and become bottom dross, and these intermetallic compounds will not penetrate into the coating layer, making it impossible to obtain a coating layer with the desired composition.
• the immersion time of the base steel in the plating bath should be in the range of 1 to 5 seconds. Immersion times longer than 5 seconds are not recommended, as they will result in the formation of a thick interfacial alloy layer.
• the thickness of the plating layer is adjusted immediately by wiping. After wiping is complete, the material is cooled so that the time it takes to reach 560°C from the plating bath temperature is 3 to 5 seconds.
• a high-gas plating layer can be easily produced with precise control of its thickness. Therefore, wiping conditions do not need to be particularly limited. However, if a cooling method such as rapid cooling by submerging in water or rapid cooling by spraying wiping gas from a wiping nozzle at a high pressure exceeding 0.1 MPa is adopted, it is not desirable because the plating layer cannot be controlled favorably.
• Thickness of plating layer Over 40 to 50 ⁇ m Time required to reach 360°C from 560°C: Within 20 seconds
• electrolytic chromate treatment which forms a chromate film through electrolysis
• reactive chromate treatment which forms a film by using a reaction with the material and then washes away excess treatment solution
• paint-on chromate treatment which applies the treatment solution to the object to be coated and then dries it without rinsing with water to form a film. Any of these treatments may be used.
• the average value of the total amount ⁇ A was calculated in the region extending from the interface between the coating layer and the base steel material to a depth of 1.0 ⁇ m toward the surface of the coating layer.
• the average value of the total amount ⁇ A obtained is shown in the "Average value of ⁇ A" column in the table.
• the inner and outer tapes were attached to black cardboard to check for the presence of peeled plating powder in the processed area.
• the evaluation criteria were as follows: “G” was considered pass, and "B” was considered fail.
• Tables 4-1 to 4-8 show the results as "powdering.”
• Red rust formation cycles exceed 900 cycles
• S Red rust formation cycles exceed 750...
• A+++” Red rust formation cycles exceed 600...
• A++” Red rust formation cycles exceed 480...
• A+” Red rust formation cycles exceed 300
• the sample size for evaluating water-wet corrosion resistance was 40 mm x 120 mm x 1.6 mm for samples using hot-rolled steel sheet as the base sheet for plating, and 40 mm x 120 mm x 0.8 mm for samples using cold-rolled steel sheet as the base sheet for plating.
• a 0.001% by volume NaCl aqueous solution at room temperature and pH 5.0 ⁇ 0.1 was dripped onto the center of the sample in the width and length directions from a height of 50 mm at a rate of 1 ⁇ l/second. The period until red rust of 1 mm diameter or more appeared at the dripped area was evaluated.
• the evaluation criteria were as follows: "S,” “A+++,” “A++,” “A+,” and "A” were considered pass, and "B” was considered fail.
• the results are shown in Tables 4-1 to 4-8 as "water-wet corrosion resistance.”
• the examples in the table exhibited excellent corrosion resistance, water-wet corrosion resistance, and workability.

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