JP2020007586A - HOT-DIP Al-Zn BASED PLATED STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

To provide a hot-dip Al-Zn based plated steel sheet capable of improving chemical convertibility, and a method for manufacturing the same.SOLUTION: The hot-dip Al-Si based plated steel sheet comprises: an alloy layer formed on the surface of a steel sheet and including at least Al, Si and Fe; and a plating layer formed on the surface of the alloy layer and including at least Al, Zn, Si and Cu. The Al content of the plating layer is 40% or more and 90% or less in atom%; the Si content of the plating layer is 0.2% or more and 3% or less in atom%. The Cu content of the plating layer is desirably 0.05% or more and 3% or less in atom%, and the plating layer desirably includes one or more kinds of elements selected from Mg, Ca, Ti and Mn of 10% or less in the total amount of each atom%.SELECTED DRAWING: None

Description

  The present invention relates to a hot-dip Al-Zn plated steel sheet and a method for producing the same.

Due to its excellent corrosion resistance, demand for hot-dip Al-Zn-based plated steel sheets having excellent corrosion resistance is growing mainly in the field of construction materials such as roofs and walls exposed outdoors for a long time. In the automobile field, as a part of measures against global warming, it is required to reduce CO ₂ emissions by improving fuel efficiency by reducing the weight of a vehicle body. For this reason, it is now strongly desired to reduce the weight by using a high-strength steel sheet and to reduce the gauge by improving the corrosion resistance of the steel sheet.

Japanese Patent No. 3599716 Japanese Patent No. 3843057 Japanese Patent No. 4090051 Japanese Patent No. 5404126

  However, when using a hot-dip Al-Zn-based plated steel sheet in the automobile field, particularly in a vehicle body such as an outer panel, there has been a problem that the chemical conversion property is poor.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-dip Al-Zn-based plated steel sheet capable of improving chemical conversion treatment and a method for manufacturing the same.

  The hot-dip Al—Zn-based plated steel sheet according to the present invention includes an alloy layer containing at least Al, Si, and Fe formed on the surface of the steel sheet, and at least Al, Zn, and Fe formed on the surface of the alloy layer. A plating layer containing Si and Cu, wherein the Al content of the plating layer is in the range of 40% to 90% in atomic% concentration, and the Si content of the plating layer is in atomic% concentration. In the range of 0.2% or more and 3% or less.

  The hot-dip Al-Zn-based plated steel sheet according to the present invention is characterized in that, in the above invention, the Cu content of the plating layer is in the range of 0.05% to 3% in atomic% concentration.

  The hot-dip Al—Zn-based plated steel sheet according to the present invention, in the above invention, wherein the plating layer contains at least one element selected from Mg, Ca, Ti, and Mn in a total of 10% or less in atomic% concentration. Is contained within the range.

The hot-dip Al—Zn-based plated steel sheet according to the present invention according to the above invention, wherein the diffraction intensity ΔI of the Al ₂ Cu phase [222] with respect to the diffraction intensity {I (Al)} of the Al phase [111] obtained by X-ray diffraction. (Al ₂ Cu)} The main peak intensity ratio Y = I (Al ₂ Cu) / I (Al) is 0.02 or less.

  The method for producing a hot-dip Al-Zn plated steel sheet according to the present invention is a method for producing a hot-dip Al-Zn plated steel sheet according to the present invention, and each of them has an atomic percent concentration of Al: 40% or more and 90% or less; : 0.2% or more and 3% or less, Fe: 2% or less, Cu: 0.05% or more and 3% or less, and 10% or less in total of one or more elements selected from Mg, Ca, Ti, and Mn , The balance consisting of Zn and unavoidable impurities, and immersing the steel sheet in a molten metal bath having a temperature in the range of 580 ° C to 620 ° C, and then the average cooling rate from 570 ° C to 500 ° C. At a rate of 10 ° C./s or more to perform rapid cooling.

  ADVANTAGE OF THE INVENTION According to this invention, the hot-dip Al-Zn-type plated steel sheet which can improve chemical conversion treatment property and its manufacturing method can be provided.

  Hereinafter, a hot-dip Al-Zn-based plated steel sheet and a method for manufacturing the same according to the present invention will be described. In the following description, the unit of the content of each element indicating the composition of the plating layer and the plating bath is “atomic% concentration (at%)”, and hereinafter, simply indicated by “%” unless otherwise specified. .

[Configuration of hot-dip Al-Zn plated steel sheet]
First, the configuration of the hot-dip Al-Zn-based plated steel sheet according to the present invention will be described.

  The hot-dip Al-Zn-based coated steel sheet according to the present invention includes a steel sheet, an alloy layer containing at least Al, Si, and Fe formed on the surface of the steel sheet, and at least Al, Zn formed on the surface of the alloy layer. And a plating layer containing Zn, Si, and Cu. The plating layer has a two-phase structure including a dendrite portion mainly composed of primary Al and a Zn-rich interdendrite portion.

  The Al content of the plating layer is in the range of 40% or more and 90% or less. When the Al content is less than 40%, the volume ratio of the Al-rich phase and the Zn-rich phase in the plating layer is reversed, and the corrosion resistance is extremely reduced. On the other hand, when the Al content is more than 90%, the Zn-rich phase disappears, and the plating layer becomes an Al single phase in which Zn is dissolved, and also in this case, the corrosion resistance is significantly reduced.

  The Si content of the plating layer is in the range of 0.2% or more and 3% or less. If the Si content is less than 0.2%, the growth of the Fe-Al alloy layer cannot be suppressed. On the other hand, when the Si content is more than 3%, single-phase Si may precipitate in the plating layer and become a starting point of local corrosion.

The Cu content of the plating layer is in the range of 0.05% or more and 3% or less. By containing Cu in the plating layer, a potential gradient is generated on the plating surface during the chemical conversion treatment, and the pH is locally increased to promote the formation of chemical conversion crystals. If the Cu content is less than 0.05%, a sufficient effect of improving the chemical conversion treatment property cannot be obtained. On the other hand, when the Cu content is more than 3%, the precipitation amount of the Al ₂ Cu phase sharply increases, and the corrosion resistance decreases.

  The plating layer may contain one or more elements selected from Mg, Ca, Ti, and Mn in a range of 10% or less in total. By containing these elements, the structure of the plating layer becomes finer, and it is possible to achieve a high degree of compatibility between improvement in chemical conversion treatment and more excellent corrosion resistance.

Ratio of main peak intensity of diffraction intensity {I (Al ₂ Cu)} of Al ₂ Cu phase [222] to diffraction intensity {I (Al)} of Al phase [111] obtained by X-ray diffraction Y = I (Al) ₂ Cu) / I (Al) is 0.02 or less. When the ratio Y is more than 0.02, it indicates that the Al ₂ Cu phase is excessively precipitated on the surface layer of the plating layer, and a film having poor corrosion resistance even with an alloy composition having excellent chemical conversion property. Could be. Although the correlation between the ratio Y and the production parameter is not clear, it tends to increase due to an increase in the amount of Cu in the bath and a decrease in the cooling rate after plating.

[Production method of hot-dip Al-Zn plated steel sheet]
The method for producing a hot-dip Al-Zn-based coated steel sheet according to the present invention is characterized in that, when manufacturing the hot-dip Al-Zn-based coated steel sheet according to the present invention, Al: 40% or more and 90% or less, Si: 0.2% or more and 3% or less. Hereinafter, Fe: 2% or less, Cu: 0.05% or more and 3% or less, one or more elements selected from Mg, Ca, Ti, and Mn are contained in a range of 10% or less in total, and the balance is After immersing the steel sheet in a molten metal bath composed of Zn and unavoidable impurities and having a temperature in the range of 580 ° C or more and 620 ° C or less, rapid cooling at an average cooling rate of 570 ° C to 500 ° C of 10 ° C / s or more is performed. Performing the following.

  Next, examples of the present invention will be described.

(Samples 1 to 40)
For all the hot-dip Al-Zn-based coated steel sheets to be used as samples, a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method is used as a base steel sheet. At 620 ° C., a line speed of 60 mpm, and an immersion time of 2 seconds, the composition of the plating bath was changed to various conditions, and a hot-dip Al—Zn-based plated steel sheet of each sample was manufactured. The composition of the plating bath was confirmed by collecting about 2 g of the plating bath used for producing the sample and performing chemical analysis. The composition of the plating bath for each sample is shown in Table 1 below. Table 1 below shows the cooling rate from 570 ° C. to 500 ° C. of cooling by nitrogen gas after immersion in the plating bath. Furthermore, as for the composition of the interface alloy layer and the plating layer, three arbitrary cross sections were cut out from the molten Al-Zn-based plated steel sheet of each sample by shearing, embedded in carbon resin, and observed by SEM-EDX. The average value of the semi-quantitative analysis values measured by EDX at any five points of the layer and the plating layer was used. Table 1 shows the compositions of the interface alloy layer and the plating layer of each sample.

  The ratio Y of the hot-dip Al-Zn-based plated steel sheet of each sample, the chemical conversion property, the bare corrosion resistance, and the corrosion resistance after coating were evaluated by the following methods.

1. Ratio Y
The ratio of the diffraction intensity {I (Al)} of the Al phase [111] to the main peak intensity of the diffraction intensity {I (Al ₂ Cu)} of the Al ₂ Cu phase [222] Y = I (Al ₂ Cu) / I ( Al) was determined by measuring a diffraction pattern with an X-ray diffractometer using Cu-Kα rays.

2. Chemical conversion treatment Using a commercially available chemical conversion chemical (PALBOND SX-35 manufactured by Nippon Parkerizing Co., Ltd.), the steel plate was subjected to chemical conversion under the conditions of bath temperature: 35 ° C, free fluorine concentration: 200 mass ppm, and treatment time: 120 seconds. After performing, the surface of the plated steel sheet was observed with a scanning electron microscope at 1000 magnifications for 10 fields of view. Regarding the morphology of the crystals, excellent (◎) means that a uniform chemical conversion crystal having an area ratio of 98% or more is generated in all 10 visual fields, and good (○) means that one visual field having an area ratio of less than 98% is recognized. The case where two visual fields having an area ratio of less than 98% were recognized was evaluated as acceptable (△), and the case where three or more visual fields having an area ratio of less than 98% were observed was evaluated as unacceptable (x). Here, the gap refers to a place where a uniform chemical conversion crystal is not generated, and the area ratio of the gap is calculated by binarizing the reflected electron image. Regarding the amount of attached crystals, three samples after the chemical conversion treatment test were immersed in a 20 g / L aqueous solution of ammonium bichromate for 15 minutes, and calculated from the difference in weight before and after as a value per test piece area. At all three points, the adhesion amount of the crystals was 2.0 g / m ² or more excellent (に お い て), 1.8 g / m ² or more and less than 2.0 g / m ² good (○), 1.5 g / G / m ² or more and less than 1.8 g / m ² were evaluated as acceptable (△), and those with less than 1.5 g / m ² even at one point were evaluated as unacceptable (x).

3. Bare corrosion resistance A salt spray test based on JIS Z2371-2000 was performed on the hot-dip Al-Zn-based plated steel sheet of each sample. The time until red rust of each sample was measured, and the bare corrosion resistance was evaluated according to the following criteria. The evaluation results are shown in Table 1 below.

：: Red rust occurrence time ≧ 600 hours ○: 300 hours ≦ Red rust occurrence time <600 hours Δ: 150 hours ≦ Red rust occurrence time <300 hours ×: Red rust occurrence time <150 hours

4. Corrosion resistance after painting About the hot-dip Al-Zn-based plated steel sheet of each sample, after shearing to the size of 70 mm x 80 mm, and performing zinc phosphate treatment as a chemical conversion treatment in the same manner as coating treatment for automobile outer panels, electrodeposition coating gave. Here, the zinc phosphate treatment and the electrodeposition coating were performed under the following conditions.

Zinc phosphate treatment: Using a commercially available chemical conversion treatment agent (PALBOND SX-35 manufactured by Nippon Parkerizing Co., Ltd.), forming a steel sheet under the conditions of bath temperature: 35 ° C., free fluorine concentration: 200 mass ppm, and treatment time: 120 seconds. Processing was performed.

Electrodeposition coating: Electrodeposition coating was performed using Kansai Paint's electrodeposition coating: GT-100 so that the film thickness became 15 μm.

  After the chemical conversion treatment and the electrodeposition coating, the edge of the evaluation surface 7.5 mm and the non-evaluation surface (back surface) are sealed with a tape, and the depth of reaching the base steel of the plated steel sheet with a cutter knife at the center of the evaluation surface. A cross-cut flaw having a length of 60 mm and a central angle of 60 ° was used as a sample for evaluation of corrosion resistance after painting.

  Using the sample for evaluation, a corrosion acceleration test was performed in a cycle specified in SAE J2334. After the corrosion promotion test was started from the wet state and was performed up to 60 cycles, the coating swelling width of the portion where the coating swelling from the scratch was the largest (maximum coating swelling width: one side with the scratch at the center) The maximum coating blister width was measured, and the corrosion resistance after coating was evaluated according to the following criteria. The evaluation results are shown in Table 1 below.

◎: maximum coating blister width ≦ 2.5 mm
：: 2.5 mm <maximum coating blister width ≦ 4.0 mm
Δ: 4.0 mm <maximum swollen film width ≦ 5.0 mm
×: Maximum coating swollen width> 5.0 mm

As shown in Table 1 below, in the comparative example, the chemical conversion treatment property was insufficient, or the Al ₂ Cu phase was excessive, and all had insufficient corrosion resistance after painting. On the other hand, the examples of the present invention were excellent in both chemical conversion property and corrosion resistance after painting. Thus, according to the present invention, it was confirmed that the post-paint corrosion resistance can be improved by improving the chemical conversion property of the hot-dip Al-Zn-based plated steel sheet.

The Cu content of the plating layer is preferably in the range of 0.05% or more and 3% or less. By containing Cu in the plating layer, a potential gradient is generated on the plating surface during the chemical conversion treatment, and the pH is locally increased to promote the formation of chemical conversion crystals. If the Cu content is less than 0.05%, a sufficient effect of improving the chemical conversion treatment property cannot be obtained. On the other hand, when the Cu content is more than 3%, the precipitation amount of the Al ₂ Cu phase sharply increases, and the corrosion resistance decreases.

Ratio of main peak intensity of diffraction intensity {I (Al ₂ Cu)} of Al ₂ Cu phase [222] to diffraction intensity {I (Al)} of Al phase [111] obtained by X-ray diffraction Y = I (Al) ₂ Cu) / I (Al) is preferably 0.02 or less. When the ratio Y is more than 0.02, it indicates that the Al ₂ Cu phase is excessively precipitated on the surface layer of the plating layer, and a film having poor corrosion resistance even with an alloy composition having excellent chemical conversion property. Could be. Although the correlation between the ratio Y and the production parameter is not clear, it tends to increase due to an increase in the amount of Cu in the bath and a decrease in the cooling rate after plating.

[Production method of hot-dip Al-Zn plated steel sheet]
The method for producing a hot-dip Al-Zn-based coated steel sheet according to the present invention is characterized in that, when manufacturing the hot-dip Al-Zn-based coated steel sheet according to the present invention, Al: 40% or more and 90% or less, Si: 0.2% or more and 3% or less. Hereinafter, Fe: 2% or less, Cu: 0.05% or more and 3% or less, one or more elements selected from Mg, Ca, Ti, and Mn are contained in a range of 10% or less in total, and the balance is After immersing the steel sheet in a molten metal bath consisting of Zn and unavoidable impurities and having a temperature in the range of 580 ° C or more and 620 ° C or less, the steel sheet is taken out, and the average cooling rate from 570 ° C to 500 ° C is set at 10 ° C / s. The above includes the step of performing rapid cooling.

(Samples 1 to 40)
For all hot-dip Al-Zn coated steel sheets to be used as samples, a cold-rolled steel sheet with a thickness of 0.8 mm manufactured by a conventional method is used as a base steel sheet. The infiltration temperature was 620 ° C., the line speed was 60 mpm, and the immersion time was 2 seconds. The composition of the plating bath was changed to various conditions, and hot-dip Al—Zn-based plated steel sheets of each sample were manufactured. The composition of the plating bath was confirmed by collecting about 2 g of the plating bath used for producing the sample and performing chemical analysis. The composition of the plating bath for each sample is shown in Table 1 below. Table 1 below shows the cooling rate from 570 ° C. to 500 ° C. of cooling by nitrogen gas after immersion in the plating bath. Furthermore, as for the composition of the interface alloy layer and the plating layer, three arbitrary sections were cut out from the molten Al-Zn-based plated steel sheet of each sample by shearing, embedded in a carbon resin, and observed by SEM-EDX. The average value of the semi-quantitative analysis values measured by EDX at any five points of the layer and the plating layer was used. Table 1 shows the compositions of the interface alloy layer and the plating layer of each sample.

Claims (5)

  An alloy layer containing at least Al, Si, and Fe formed on the surface of the steel sheet; and a plating layer containing at least Al, Zn, Si, and Cu formed on the surface of the alloy layer. The Al content of the plating layer is in the range of 40% to 90% in atomic% concentration, and the Si content of the plating layer is in the range of 0.2% to 3% in atomic% concentration. A hot-dip Al-Zn plated steel sheet.   The hot-dip Al-Zn-based coated steel sheet according to claim 1, wherein the Cu content of the plating layer is in a range of 0.05% or more and 3% or less in atomic% concentration.   3. The plating layer according to claim 1, wherein the plating layer contains at least one element selected from Mg, Ca, Ti, and Mn in a range of 10% or less in total of respective atomic% concentrations. 4. Hot-dip Al-Zn plated steel sheet. Ratio of main peak intensity of diffraction intensity {I (Al ₂ Cu)} of Al ₂ Cu phase [222] to diffraction intensity {I (Al)} of Al phase [111] obtained by X-ray diffraction Y = I (Al) ₂ Cu) / of the preceding claims, characterized in that I (Al) is 0.02 or less, the molten Al-Zn-based plated steel sheet according to any one.   The method for producing a hot-dip Al-Zn-based plated steel sheet according to any one of claims 1 to 4, wherein the atomic% concentration is Al: 40% to 90%, Si: 0.2%. 3% or less, Fe: 2% or less, Cu: 0.05% to 3%, and at least one element selected from Mg, Ca, Ti, and Mn in a range of 10% or less in total. After immersing the steel sheet in a molten metal bath whose balance is composed of Zn and unavoidable impurities and whose temperature is in the range of 580 ° C to 620 ° C, the average cooling rate from 570 ° C to 500 ° C is 10 ° C / s. A method for producing a hot-dip Al-Zn-based plated steel sheet, comprising the step of performing rapid cooling as described above.