CN113508186B - Molten Al-Zn-Mg-Si-Sr plated steel sheet and method for producing same - Google Patents
Molten Al-Zn-Mg-Si-Sr plated steel sheet and method for producing same Download PDFInfo
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- CN113508186B CN113508186B CN201980093402.1A CN201980093402A CN113508186B CN 113508186 B CN113508186 B CN 113508186B CN 201980093402 A CN201980093402 A CN 201980093402A CN 113508186 B CN113508186 B CN 113508186B
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 137
- 239000010959 steel Substances 0.000 title claims abstract description 137
- 238000004519 manufacturing process Methods 0.000 title description 28
- 238000007747 plating Methods 0.000 claims abstract description 194
- 229910045601 alloy Inorganic materials 0.000 claims abstract description 60
- 239000000956 alloy Substances 0.000 claims abstract description 60
- 229910001278 Sr alloy Inorganic materials 0.000 claims abstract description 28
- 239000000203 mixture Substances 0.000 claims abstract description 17
- 239000010410 layer Substances 0.000 claims description 230
- 210000001787 dendrite Anatomy 0.000 claims description 26
- 229910052710 silicon Inorganic materials 0.000 claims description 22
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- 239000011247 coating layer Substances 0.000 claims description 8
- 230000007797 corrosion Effects 0.000 abstract description 103
- 238000005260 corrosion Methods 0.000 abstract description 103
- 238000000034 method Methods 0.000 description 22
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- 238000000576 coating method Methods 0.000 description 21
- 239000011701 zinc Substances 0.000 description 17
- 230000000694 effects Effects 0.000 description 14
- 238000011282 treatment Methods 0.000 description 12
- 238000001816 cooling Methods 0.000 description 11
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- 238000002149 energy-dispersive X-ray emission spectroscopy Methods 0.000 description 7
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 7
- 238000012545 processing Methods 0.000 description 7
- 238000003786 synthesis reaction Methods 0.000 description 7
- 229910019018 Mg 2 Si Inorganic materials 0.000 description 6
- 229910018137 Al-Zn Inorganic materials 0.000 description 5
- 229910018573 Al—Zn Inorganic materials 0.000 description 5
- 238000005452 bending Methods 0.000 description 5
- 239000000155 melt Substances 0.000 description 5
- 229910052782 aluminium Inorganic materials 0.000 description 4
- 230000005540 biological transmission Effects 0.000 description 4
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- 238000012360 testing method Methods 0.000 description 3
- 230000037303 wrinkles Effects 0.000 description 3
- 229910052725 zinc Inorganic materials 0.000 description 3
- 229910018571 Al—Zn—Mg Inorganic materials 0.000 description 2
- 238000012935 Averaging Methods 0.000 description 2
- 229910019064 Mg-Si Inorganic materials 0.000 description 2
- 229910019406 Mg—Si Inorganic materials 0.000 description 2
- 229910000676 Si alloy Inorganic materials 0.000 description 2
- 230000001133 acceleration Effects 0.000 description 2
- 238000005275 alloying Methods 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- ZCDOYSPFYFSLEW-UHFFFAOYSA-N chromate(2-) Chemical compound [O-][Cr]([O-])(=O)=O ZCDOYSPFYFSLEW-UHFFFAOYSA-N 0.000 description 2
- 238000005253 cladding Methods 0.000 description 2
- 239000010960 cold rolled steel Substances 0.000 description 2
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- 230000002829 reductive effect Effects 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- 229910000838 Al alloy Inorganic materials 0.000 description 1
- 229910018125 Al-Si Inorganic materials 0.000 description 1
- 229910018520 Al—Si Inorganic materials 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- 229910007570 Zn-Al Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 230000004888 barrier function Effects 0.000 description 1
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- 239000010703 silicon Substances 0.000 description 1
- 239000002210 silicon-based material Substances 0.000 description 1
- HPALAKNZSZLMCH-UHFFFAOYSA-M sodium;chloride;hydrate Chemical compound O.[Na+].[Cl-] HPALAKNZSZLMCH-UHFFFAOYSA-M 0.000 description 1
- 238000005507 spraying Methods 0.000 description 1
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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
- C23C2/12—Aluminium or alloys based thereon
-
- 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
- C23C2/26—After-treatment
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C18/00—Alloys based on zinc
- C22C18/04—Alloys based on zinc with aluminium as the next major constituent
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C21/00—Alloys based on aluminium
- C22C21/10—Alloys based on aluminium with zinc as the next major constituent
-
- 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
-
- 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
- C23C2/26—After-treatment
- C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
- C23C2/29—Cooling or quenching
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Physics & Mathematics (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Thermal Sciences (AREA)
- Coating With Molten Metal (AREA)
Abstract
The purpose of the present invention is to provide a molten Al-Zn-Mg-Si-Sr plated steel sheet that has good surface appearance and that has excellent corrosion resistance in the processed portion. In order to achieve the above object, the present invention is characterized in that the plating layer has the following composition: the alloy layer comprises an interface alloy layer existing at the interface with a base steel sheet and a main layer existing on the alloy layer, wherein an Al-Si-Sr alloy having an average length diameter of 1 [ mu ] m or less is present between the main layer and the interface alloy layer.
Description
Technical Field
The present invention relates to a molten Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in a processed portion while having good surface appearance, and a method for producing the same.
Background
Since the molten al—zn-based plated steel sheet can achieve both the sacrificial corrosion resistance of Zn and the high corrosion resistance of Al, the molten zinc-plated steel sheet also exhibits high corrosion resistance. For example, patent document 1 discloses a molten al—zn-based plated steel sheet containing 25 to 75 mass% of Al in a plating layer. Further, since the molten al—zn-plated steel sheet has excellent corrosion resistance, there has been an increasing demand in recent years mainly in the field of building materials such as roofs and walls exposed outdoors for a long period of time, and in the field of civil engineering and construction such as guardrails, wiring pipes, soundproof walls.
The cladding layer of the molten Al-Zn-based plated steel sheet is composed of a main layer and an interface alloy layer existing at the interface between the base steel sheet and the main layer, the main layer mainly composed of a portion containing Zn and having dendrites of Al solidified (dendrite portion of α -Al phase) and a portion having residual dendrite gaps (inter-dendrite) mainly containing Zn, and has a structure in which a plurality of α -Al phases are laminated in the film thickness direction of the cladding layer. With such a characteristic film structure, since the corrosion path from the surface becomes complicated, it is difficult for corrosion to easily reach the base steel sheet, and the molten al—zn-based plated steel sheet can achieve excellent corrosion resistance as compared with the molten zinc-plated steel sheet having the same plating layer thickness.
In addition, a technique for further improving corrosion resistance by adding Mg to a molten al—zn-based plated layer is known. As a technique related to a molten Al-Zn-based plated steel sheet containing Mg (molten Al-Zn-Mg-Si plated steel sheet), for example, patent document 2 discloses an Al-Zn-Mg-Si plated steel sheet containing an Al-Zn-Si alloy containing Mg in a plating layer, the Al-Zn-Si alloy being an alloy containing 45 to 60 wt% of elemental aluminum, 37 to 46 wt% of elemental zinc, and 1.2 to 2.3 wt% of elemental silicon, the concentration of Mg being 1 to 5 wt%.
Among them, the molten al—zn-based plated steel sheet containing Mg disclosed in patent document 2 has the following problems: although having excellent corrosion resistance, a wrinkle-like defect (hereinafter referred to as "wrinkle-like defect") is easily generated by an oxide layer generated on the surface of the plating layer, and the appearance of the plating layer surface is impaired.
For this reason, for example, patent document 3 discloses a technique for improving the surface appearance of a molten al—zn-based plated steel sheet by incorporating Sr in the plating layer.
Patent document 4 discloses a technique for improving workability by incorporating Sr in a coating layer in a molten Al-Zn-Mg-based coated steel sheet.
Prior art literature
Patent literature
Patent document 1: japanese patent publication No. 46-7161
Patent document 2: japanese patent No. 5020228
Patent document 3: japanese patent No. 3983932
Patent document 4: japanese patent No. 6368730
Disclosure of Invention
(technical problem to be solved by the invention)
Since Sr is contained in the coating layer in the molten al—zn-based coated steel sheet of patent document 3 and patent document 4, the occurrence of wrinkles-like defects can be suppressed, and the surface appearance can be improved.
However, regarding the Sr-containing molten al—zn-based plated steel sheets of cited patent document 3 and cited patent document 4, there are the following problems: when a steel sheet is processed, corrosion starts to propagate from a crack generated, and as a result, the corrosion resistance of the processed portion is reduced.
In view of the above, an object of the present application is to provide a molten Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in a processed portion while having good surface appearance, and a method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in a processed portion while having good surface appearance.
(technical means for solving the technical problems)
As a result of the studies by the inventors of the present application to solve the above problems, it was found that when the plating layer is corroded, mg is formed in the plating layer 2 Since Si preferentially dissolves and Mg in the corrosion product formed on the surface of the plating layer is concentrated, high corrosion resistance is obtained, and therefore, even if a crack is generated in the plating layer, if the crack is of a certain width, the corrosion product fills the crack, and exposure of the base steel sheet can be suppressed, and therefore, corrosion resistance of the processed portion can be sufficiently maintained. Further, the results of further repeated studies have revealed that: since the al—si—sr alloy present between the interface alloy layer present at the interface with the base steel sheet and the main layer present on the alloy layer (hereinafter, also referred to as "plating main layer") is hard and has low ductility, and the surface shape of the interface alloy layer is made convex, cracks are likely to occur in the plating main layer, and when the width of the generated cracks is large, the corrosion resistance of the processed portion is adversely affected. It was further found that: in this al—si—sr alloy, since the size is kept as small as possible (specifically, the average long diameter is 1 μm or less), even when cracks are generated in the main layer of the plating layer during processing of the steel sheet, the width of the cracks can be prevented from becoming large, and therefore, the corrosion product acts to fill in the cracks generated, and excellent corrosion resistance of the processed portion can be achieved.
The present invention has been completed based on the above-described findings, and the gist thereof is as follows.
1. A molten Al-Zn-Mg-Si-Sr plated steel sheet, characterized in that,
the coating layer had the following composition: which contains 25 to 70 mass% of Al, 0.6 to 5 mass% of Si, 0.1 to 10 mass% of Mg and 0.001 to 1.0 mass% of Sr, and the balance of Zn and unavoidable impurities,
the plating layer is composed of an interface alloy layer existing at the interface with the base steel plate and a main layer existing above the alloy layer, and an Al-Si-Sr alloy with an average length diameter of 1 μm or less is arranged between the main layer and the interface alloy layer.
2. The molten Al-Zn-Mg-Si-Sr coated steel sheet as described in claim 1, wherein the interface alloy layer contains 0.001 mass% or more of Sr.
3. The molten Al-Zn-Mg-Si-Sr plated steel sheet as defined in any one of the above 1 or 2, characterized by comprising Mg as observed in a cross section in the thickness direction of the plated layer 2 The Si has a long diameter of 10 μm or less.
4. The molten Al-Zn-Mg-Si-Sr plated steel sheet as defined in any one of the above 1 to 3, characterized in that, regarding the Si phase observed in the cross section in the thickness direction of the plating layer, the area ratio of the Si phase is relative to the Mg phase observed in the cross section in the thickness direction of the plating layer 2 The total ratio of the area ratios of Si and Si phases is 30% or less.
5. The molten Al-Zn-Mg-Si-Sr plated steel sheet as set forth in any one of the above 1 to 4, wherein the main layer has dendrite portions of an α -Al phase, and the average inter-dendrite arm distance of the dendrite portions and the thickness of the plated layer satisfy the following formula (1):
t/d≥1.5……(1)
t: thickness of plating layer (μm), d: average dendrite inter-arm spacing (μm).
6. A method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet, characterized by using a plating bath having a bath temperature of 585 ℃ or less and having the following composition: which contains 25 to 70 mass% of Al, 0.6 to 5 mass% of Si, 0.1 to 10 mass% of Mg and 0.001 to 1.0 mass% of Sr, and the balance of Zn and unavoidable impurities,
when the steel sheet is subjected to the melt plating, the temperature of the steel sheet at the time of entering the plating bath (entering plate temperature) is set to a temperature obtained by adding 20 ℃ to the bath temperature of the plating bath (plating bath temperature +20℃).
7. The method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet as described in the above 6, wherein the entering temperature of the steel sheet is not higher than the bath temperature of the plating bath.
8. The method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet as described in the above 6 or 7, characterized by comprising performing molten plating on the steel sheet, and then cooling the steel sheet at an average cooling rate of 30 ℃/s or more until the sheet temperature is a temperature obtained by subtracting 150 ℃ from the bath temperature of the plating bath (plating bath temperature-150 ℃).
(effects of the invention)
According to the present invention, it is possible to provide a molten Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in a processed portion while having good surface appearance, and a method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in a processed portion while having good surface appearance.
Drawings
Fig. 1 is a view showing the state of an interface between a main layer and an interface alloy layer in a cross section of a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention in the thickness direction of the plated layer as observed by a scanning transmission electron microscope.
Fig. 2 (a) is a view showing the state of each element of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention by an energy dispersive X-ray spectrometry (SEM-EDX) of a scanning electron microscope, FIG. 2 (b) is a view of the molten Al-Zn-Mg-Si-Sr plated steel sheet as shown in FIG. 2 (a), for observing the presence of Mg in the primary layer 2 Si and a method of Si phase.
Fig. 3 is a diagram for explaining a method of measuring the distance between dendrite arms.
FIG. 4 is a diagram for explaining a flow of a composite cycle test (JASO-CCT) of Japanese automotive standards.
Detailed Description
(molten Al-Zn-Mg-Si-Sr plated steel sheet)
The molten Al-Zn-Mg-Si-Sr plated steel sheet, which is the object of the present invention, has a plating layer on the surface of the steel sheet, the plating layer being composed of an interface alloy layer present at the interface with the base steel sheet and a main layer present on top of the alloy layer. In addition, the plating layer has the following composition: contains 25 to 70 mass% of Al, 0.6 to 5 mass% of Si, 0.1 to 10 mass% of Mg and 0.001 to 1.0 mass% of Sr, and the balance is composed of Zn and unavoidable impurities.
The Al content in the plating layer is 40 to 70 mass% in terms of balance between corrosion resistance and operation. If the Al content of the main layer of the plating layer is 25 mass% or more, good corrosion resistance can be ensured. The main layer is mainly composed of a portion (dendrite portion of α -Al phase) supersaturated with Zn and solidified with Al dendrites and a portion (inter-dendrite portion) of remaining dendrite gaps, and can realize a structure in which the dendrite portions are laminated in the film thickness direction of the plating layer and are excellent in corrosion resistance. Further, the more dendrite portions of the α -Al phase are stacked, the more complicated the corrosion travel path becomes, the more difficult it is for corrosion to easily reach the base steel sheet, and thus the corrosion resistance is improved. From the same viewpoint, the Al content in the plating layer is preferably 40 mass% or more. On the other hand, if the Al content in the plating layer exceeds 70 mass%, the Zn content having a sacrificial corrosion-preventing effect on Fe becomes small, and the corrosion resistance is deteriorated. Therefore, the Al content in the plating layer is set to 70 mass% or less. Further, if the Al content in the plating layer is 65 mass% or less, the adhesion amount of plating becomes small, and even when the base steel sheet is easily exposed, the plating layer has a sacrificial corrosion preventing effect on Fe, and sufficient corrosion resistance can be obtained. Therefore, the Al content of the plating main layer is preferably 65 mass% or less.
The Si in the plating layer is added to the plating bath for the purpose of suppressing the growth of an interface alloy layer formed at the interface with the base steel sheet, and is inevitably contained in the main layer. In the case of the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present inventionIn this case, if the molten plating treatment is performed by containing Si in the plating bath, the base steel sheet is immersed in the plating bath, and at the same time, fe on the surface of the steel sheet undergoes an alloying reaction with Al or Si in the bath, thereby producing an alloy composed of fe—al and/or fe—al—si based compounds. The generation of the Fe-Al-Si based interface alloy layer can suppress the growth of the interface alloy layer. Further, when the Si content in the plating layer is 0.6 mass% or more, the growth of the interface alloy layer can be sufficiently suppressed. On the other hand, when the Si content of the plating layer exceeds 5%, the workability in the plating layer is lowered, and Si is likely to precipitate in the cathode region. The precipitation of the Si phase can be suppressed by increasing the content of Mg and making a relationship between the Si content and the Mg content as described later, but in this case, the production cost increases or the content of Mg is increased 2 The reduction in workability caused by the increase in the amount of Si makes it more difficult to control the composition of the plating bath. Therefore, the Si content in the plating layer is set to 5% or less. Further, if the aspect of more reliably suppressing the growth of the interface alloy layer or the precipitation of Si phase is considered, it is possible to cope with the alloy as Mg 2 In the case where Si is consumed, the Si content in the plating layer is preferably set to be more than 2.3% to 3.5%.
The plating layer contains 0.1 to 10 mass% of Mg. When the main layer of the plating layer is corroded, mg is contained in the corrosion product, and the stability of the corrosion product is improved, and the progress of corrosion is delayed, as a result, the effect of improving corrosion resistance can be obtained. More specifically, mg present in the main layer of the plating layer combines with Si as described above to produce Mg 2 Si. The Mg is 2 Si is dissolved in the initial stage when the plated steel sheet is corroded, and Mg is contained in the corrosion product. Mg contained in the corrosion product has an effect of densifying the corrosion product, and can improve stability of the corrosion product and barrier properties to external corrosion factors.
The reason why the Mg content of the plating layer is 0.1 mass% or more is that Si is contained in the plating layer in the above concentration range In this case, mg can be produced by setting the Mg concentration to 0.1 mass% or more 2 Si can obtain corrosion delay effect. From the same viewpoint, the Mg content of the plating layer is preferably 1 mass% or more, more preferably 3 mass% or more. On the other hand, the reason why the Mg content of the plating layer is 10 mass% or less is that, when the Mg content of the plating layer exceeds 10%, the effect of improving the corrosion resistance is saturated, and the manufacturing cost increases and the composition control of the plating bath becomes difficult. From the same viewpoint, the Mg content of the plating layer is preferably 6 mass% or less.
Further, by setting the Mg content in the plating layer to 1 mass% or more, corrosion resistance after coating can be improved. If the coating layer of the conventional molten Al-Zn-based coated steel sheet containing no Mg is brought into contact with the atmosphere, dense and stable Al is formed immediately around the alpha-Al phase 2 O 3 The solubility of the α -Al phase is extremely low compared with the solubility of the Zn-rich phase in the interdendritic phase due to the protective effect of the oxide film. As a result, when a coated steel sheet using a conventional al—zn-based coated steel sheet as a substrate is damaged in a coating film, selective corrosion of a Zn-rich phase is caused at a coating film/coating interface starting from a damaged portion, and the coated steel sheet proceeds deep into the whole of the coating film, causing large expansion of the coating film, so that the corrosion resistance after coating is poor. Therefore, from the viewpoint of obtaining excellent post-coating corrosion resistance, the Mg content in the plating layer is preferably 1 mass% or more, more preferably 3 mass% or more.
On the other hand, in the case of a coated steel sheet using a molten al—zn-based coated steel sheet containing Mg in the coating layer, mg precipitated in the dendrites 2 Si phase or Mg-Zn compound (MgZn) 2 、Mg 32 (Al,Zn) 49 Etc.) dissolve out at the initial stage of corrosion, and Mg is mixed into the corrosion product. Since the corrosion product containing Mg is very stable and thus the corrosion is suppressed in the initial stage, it is possible to suppress the selective corrosion from the Zn-rich phase, which is problematic when the conventional al—zn-based plated steel sheet is used for the coated steel sheet of the substrateThe resulting large film expansion. As a result, the molten al—zn-based plated steel sheet containing Mg in the plating layer exhibits excellent post-coating corrosion resistance. When the Mg content in the coating layer is less than 1 mass%, the amount of Mg eluted during corrosion is small, and there is a risk that the corrosion resistance after coating does not improve. In addition, when the Mg content in the plating layer exceeds 10 mass%, not only is the effect saturated, but also corrosion of Mg compounds occurs strongly, and the solubility of the entire plating layer excessively increases, and as a result, even if the corrosion product is stabilized, the dissolution rate increases, and therefore, there is a possibility that a large expansion width occurs, and the corrosion resistance after coating may deteriorate. Therefore, in order to stably obtain excellent post-coating corrosion resistance, it is preferable that the Mg content in the plating layer is 10 mass% or less.
The plating layer contains 0.001 to 1.0 mass% of Sr. By containing Sr in the coating layer, the surface appearance of the molten Al-Zn-Mg-Si-Sr coated steel sheet of the present invention can be improved while suppressing the occurrence of wrinkles.
The wrinkles are formed on the surface of the plating layer and are observed as white streaks on the surface of the plating layer. Such streak defects are easily generated in the case where a large amount of Mg is added to the plating layer. Therefore, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, sr is contained in the plating layer, whereby Sr is preferentially oxidized in the plating layer surface layer over Mg, and oxidation reaction of Mg is suppressed, whereby occurrence of the streak-like defect can be suppressed.
The Sr content in the plating layer is required to be 0.001 mass% or more. This is to obtain an effect of suppressing the occurrence of the streak-like defect described above. From the same viewpoint, the Sr content in the plating layer is preferably 0.005 mass% or more, more preferably 0.01 mass% or more, and particularly preferably 0.05 mass% or more. On the other hand, the Sr content in the plating layer is required to be 1.0 mass% or less. The reason is that if the Sr content becomes excessive, the effect of suppressing the streak-like defect is saturated, so that it is disadvantageous in terms of cost. From the same viewpoint, the Sr content in the plating layer is preferably 0.7 mass% or less, more preferably 0.5 mass% or less, and particularly preferably 0.3 mass% or less.
Further, the plating layer includes: unavoidable impurities contained in a base steel sheet component mixed into plating by a reaction between a plating bath and the base steel sheet or an ingot used in bath building in the plating bath during the plating treatment. As a base steel sheet component to be mixed into the plating, fe may be contained in an amount of about several%. Examples of the type of unavoidable impurities in the plating bath include Fe, mn, P, S, C, nb, ti, B as a base steel sheet component. Further, as impurities in the ingot, fe, pb, sb, cd, as, ga, V and the like can be mentioned. Further, as for Fe in the plating layer, it is impossible to distinguish and quantify Fe mixed from the base steel sheet from Fe present in the plating bath. The total content of the unavoidable impurities is not particularly limited, but from the viewpoint of maintaining corrosion resistance and uniform solubility of plating, the total content of the unavoidable impurities other than Fe is preferably 1 mass% or less.
In the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the plating layer may further contain at least one or more selected from Cr, ni, co, mn, ca, V, ti, B, mo, sn, zr, li, ag and the like known as a stabilizing element for corrosion products in Zn-Al plating, at a content of less than 1% of each element. If the content of each of these elements is less than 1%, the effects disclosed in the present invention are not hindered, and further improvement in corrosion resistance can be achieved by the corrosion product stabilizing effect.
The interface alloy layer is a layer existing at the interface with the base steel sheet in the plating layer, and is, as described above, a compound of Fe-Al-type and/or Fe-Al-Si-type that is inevitably produced by an alloying reaction between Fe on the steel sheet surface and Al or Si in the plating bath. Since the interface alloy layer is hard and brittle, if grown thicker, it becomes a starting point for crack generation during processing, and therefore, it is preferable to thin.
The interface alloy layer preferably contains 0.001 mass% or more of Sr. The reason is that, when 0.001 mass% or more of Sr is contained in the interface alloy layer, sr reduces the surface energy of the fe—al alloy generated at the interface in molten al—zn, and therefore, the irregularities of the interface shape on the plating main layer side of the interface alloy layer are smoothed, and the workability (crack resistance) at the time of bending processing can be further improved. Further, although red rust is generated by corrosion of the steel sheet after corrosion of the plating main layer, when Sr is contained in the interface alloy layer, the time until red rust is generated can be prolonged by improving the corrosion resistance of the interface alloy layer. From the same viewpoint, the Sr content in the interface alloy layer is preferably 0.005 mass% or more, more preferably 0.01 mass% or more.
The Sr concentration in the interface alloy layer is preferably 10 mass% or less. The reason is that if the Sr concentration in the interface alloy layer exceeds 10 mass%, the hardness of the interface alloy layer may become high and the workability may be lowered.
The Sr in the interface alloy layer can be quantitatively analyzed by STEM-EDX analysis described later.
In the molten Al-Zn-Mg-Si-Sr plated steel sheet according to the present invention, the plating layer is characterized in that an Al-Si-Sr alloy having an average long diameter of 1 μm or less is present in a local portion between the main layer and the interface alloy layer.
In the present invention, sr is contained in the plating layer, whereby an al—si—sr alloy is inevitably formed at the interface between the main layer and the interface alloy layer of the plating layer, but by controlling the size thereof, excellent surface appearance can be achieved, and corrosion resistance of the processed portion can be improved.
In the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, since the plating layer contains Mg 2 Si, while at the time of corrosion of the plating layer, the Mg 2 Si is preferentially dissolved, and Mg dissolved in corrosion products generated on the surface of the plating layer is concentrated, whereby excellent corrosion resistance can be found. But at the same time When Sr is contained in the plating layer for the purpose of suppressing wrinkles and defects, the following problems are involved: as described above, an al—si—sr alloy is formed at the interface between the main layer and the interface alloy layer (in a state of being sandwiched between the main layer and the interface alloy layer), and as a result, cracks are likely to occur in the plating layer at the time of processing the steel sheet, and the width of the generated cracks becomes large, thereby causing a decrease in corrosion resistance of the processed portion. Therefore, in the molten al—zn—mg—si—sr plated steel sheet of the present invention, since the al—si—sr alloy is controlled to be small in size (such that the average long diameter becomes 1 μm or less) at the time of formation, even when cracks occur in the plating layer, a sufficient amount of corrosion products can be filled in the cracks at the time of corrosion of the steel sheet, mg can be concentrated in the vicinity of the surface of the corrosion products, and as a result, the occurrence of wrinkles-like defects can be suppressed, and excellent corrosion resistance of the processed portion can be realized.
Here, fig. 1 is a photograph of a cross section of a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention observed by a Scanning Transmission Electron Microscope (STEM). As can be seen from fig. 1, it is known that an Al-Si-Sr alloy exists between the main layer of the plating layer and the interface alloy layer.
Table 1 below shows the results of analysis of the chemical compositions of the portions indicated by 1 to 4 in fig. 1. As is clear from table 1, it is known that the portions 1 to 3 in fig. 1 are all interface alloy layers mainly composed of Fe, al, si, and Zn, whereas the portion indicated by 4 in fig. 1 is basically composed of Al, si, and Sr, that is, is an al—si—sr alloy, and is an alloy different from the interface alloy layers.
In addition, the al—si—sr alloy does not need to be present at all interfaces between the main layer and the interface alloy layer, and is present at a part of interfaces between the main layer and the interface alloy layer, as shown in fig. 1. The al—si—sr alloy (in a state sandwiched between the main layer and the interface alloy layer) is present at the interface between the main layer and the interface alloy layer, and is not formed inside the main layer or the interface alloy layer.
TABLE 1
TABLE 1
The average long diameter of the Al-Si-Sr alloy that is present between the main layer and the interface alloy layer is required to be 1 μm or less. As described above, by suppressing the size of the al—si—sr alloy to 1 μm or less in average length diameter, even when cracks occur in the plating layer, a sufficient amount of corrosion products can be filled in the cracks when the steel sheet corrodes, and excellent corrosion resistance of the processed portion can be achieved. From the same viewpoint, the average long diameter of the Al-Si-Sr alloy is preferably 0.8 μm or less.
The long diameter of the al—si—sr alloy is the longest diameter among the particles of the al—si—sr alloy in the observation field.
The average length of the al—si—sr alloy can be calculated by using, for example, a Scanning Transmission Electron Microscope (STEM). As shown in fig. 1, the cross section of the plating layer in the thickness direction was observed, the long diameter of the al—si—sr alloy was measured for each particle in the observation field, and the average diameter was calculated. Regarding observation by TEM, the visual field of five arbitrarily selected portions can be observed, and the long diameters of all al—si—sr alloys can be measured, and the average of these long diameters can be taken as the long diameter of the al—si—sr alloy.
In addition, regarding Mg observed in a cross section in the thickness direction of the plating layer 2 Si is preferably 10 μm or less in both major axes, and more preferably 8 μm or less. Regarding Mg contained in the plating layer 2 Si contributes to the effect of improving corrosion resistance as described above, but causes hardening of the plating main layer and decreases workability, so that in a conventional molten Al-Zn-Mg-based plated steel sheet, sufficient workability and corrosion resistance of the processed portion cannot be obtained. Therefore, by reducing the Mg in the plating layer (in such a manner that the long diameter becomes 10 μm or less) 2 Si size, even when cracks are generated in the plating layer, the width or length of the cracks can be suppressed to be smallAs a result, when the steel sheet is corroded, a sufficient amount of the corrosion product can be filled in the cracks, and Mg can be concentrated in the vicinity of the surface of the corrosion product, thereby greatly improving the corrosion resistance of the processed portion.
In addition, so-called Mg observed in a cross section in the thickness direction of the plating layer 2 The long diameter of Si is one Mg 2 The longest diameter in Si. In addition, regarding the Mg 2 The long diameter of Si refers to Mg observed in a cross section in the thickness direction of the plating layer 2 The Si particles have an average long diameter of 10 μm or less. For example, when 10 arbitrary sites are observed in cross section, all Mg can be used 2 Whether the average length of Si particles is 10 μm or less is determined.
In addition, mg in the cross section in the thickness direction of the above-mentioned plating layer 2 The observation of Si can be performed by using a scanning electron microscope and by using an energy dispersive X-ray spectrometry (SEM-EDX), for example.
For example, as shown in fig. 2 (a), after the cross-sectional state of the plating layer in the thickness direction is obtained, mg and Si are mapped separately as shown in fig. 2 (b) (Mg is represented by red, and Si is represented by blue). Then, out of the mapped Mg and Si, those overlapping at the same position (the portion indicated by purple in (b) of fig. 2) can be set as Mg 2 Si. Mg can be calculated from the ratio of the sum of the areas of the purple portions to the area of the plating layer in the observed field of view 2 Area ratio of Si (B%).
Further, since Si is contained as a constituent component in the plating layer, as described above, an Si phase may be formed in the plating layer depending on the composition of Si and Mg in the plating layer. However, from the viewpoint of further improving corrosion resistance and workability, it is preferable to suppress the formation of the Si phase as much as possible.
In particular, it was found in the present invention that Mg, which improves corrosion resistance 2 The content ratio of Si phase, which becomes a cathode portion and deteriorates corrosion resistance when the Si and the plating layer are corroded, is important. That is, the essence of the present invention is that even Mg, which improves corrosion resistance 2 Si ofIf the amount of Si phase that deteriorates corrosion resistance is large, good corrosion resistance cannot be ensured, and therefore the ratio is controlled to be a certain value or less.
Accordingly, in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the relative amount of Mg observed in the cross section in the thickness direction of the plated layer, as measured by the method shown below 2 The area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer (area ratio of the Si phase/Mg 2 The total area ratio of Si and Si phase) is preferably 30% or less, more preferably 10% or less.
In addition, regarding the method of deriving the area ratio of the Si phase, for example, the method is similar to the above-mentioned Mg 2 Si can be similarly performed by using a scanning electron microscope and by using an energy dispersive X-ray spectrometry (SEM-EDX).
As described above, after the cross-sectional state of the plating layer in the thickness direction is obtained (fig. 2 (a)), mg and Si are mapped respectively (fig. 2 (b)). Then, among the mapped Mg and Si, the portion indicated by blue in fig. 2 (b) where Mg is not present at the position where Si is present can be regarded as Si phase. The area ratio (a%) of the Si phase can be calculated from the ratio of the sum of the areas of the blue portions to the area of the plating layer in the field of view being observed.
In addition, relative to Mg observed in a cross section in the thickness direction of the plating layer 2 The area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer (area ratio of the Si phase/Mg 2 Total area ratio of Si and Si phase: z%) can be calculated as (a%/(a% +b%) ×100%).
Here, the relative Mg observed in the cross section in the thickness direction of the plating layer described above 2 The area ratio of the Si phase, which is the total area of Si and Si phases, is obtained by averaging the area ratios of the Si phases observed in the cross section of 10 sites randomly selected in the plating layer.
The area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer (area ratio of the Si phase in the observation field: a%) is preferably 10% or less, and more preferably 3% or less.
Here, the area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer is obtained by averaging the area ratios of the Si phase observed in the cross section of 10 randomly selected portions of the plating layer.
Further, from the viewpoint of further improving the initial corrosion resistance of the plated steel sheet, the area ratio of the Si phase observed on the surface of the plated layer (the area ratio of the Si phase in the observation field) is preferably 1% or less, more preferably 0.5% or less. Here, the method of deriving the area ratio of the Si phase on the surface of the plating layer can be performed by using a scanning electron microscope and using an energy dispersive X-ray spectrometry (SEM-EDX) as in the case of observing the cross section. The area ratio can be obtained by a cross-sectional observation method, and the area ratio of the Si phase observed on the surface of 10 randomly selected portions of the plating layer can be averaged.
Further, from the viewpoint of further improving initial corrosion resistance, the same applies to Mg observed on the surface of the plating layer 2 The area ratio of Si phase observed in the cross section in the thickness direction of the plating layer (area of Si phase/Mg 2 The total area of Si and Si phase) is preferably 20% or less, more preferably 10% or less. The actual observation method and the area ratio determination method are based on the cross-sectional observation method described above.
Further, when the main layer of the plating layer or the interface alloy layer is observed by a scanning electron microscope, the observation is performed after polishing and/or etching the cross section or surface of the plating layer. There are several methods for polishing the cross section or surface and etching methods, and the method is not particularly limited as long as it is a method used when the cross section or surface of the plating layer is usually observed. In addition, observation and analysis conditions by a scanning electron microscope were used. For example, the acceleration voltage is 5kV to 20kV, and the magnification is about 500 to 5000 times in the secondary electron image or the reflected electron image.
As the observation conditions using the scanning transmission electron microscope (STEM-EDX), for example, if a cross-section sample of the FIB-machined plating layer is at an acceleration voltage of 200kV and a magnification of about 1000 to 50000, the plating layer can be clearly observed and analyzed.
In addition, the main layer of the plating layer has dendrite portions of an α -Al phase, and the average inter-dendrite arm distance of the dendrite portions and the thickness of the plating layer preferably satisfy the following formula (1).
t/d≥1.5……(1)
t: thickness of plating layer (μm), d: average dendrite arm spacing (μm)
By satisfying the above expression (1), the arm of the dendrite portion composed of the α -Al phase can be relatively reduced, and the inter-dendrite path that is preferentially corroded can be ensured to be long, thereby further improving corrosion resistance.
The inter-dendrite arm distance of the dendrite part refers to a center distance (dendrite arm spacing) between adjacent dendrite arms. In the present invention, for example, as shown in fig. 3, the surface of the polished and/or etched plating layer main layer is observed under magnification (for example, at 200 x) using a Scanning Electron Microscope (SEM) or the like, and the intervals of dendrite arms (secondary dendrite arms) having the second wide intervals are measured as described below in a randomly selected field of view. More than three secondary dendrite arms are selected (three among a-B are selected in fig. 3), and a distance (distance L in fig. 3) is measured along the direction in which the arms are arranged. Then, the measured distance was divided by the number of dendrite arms (L/3 in FIG. 3), and the inter-dendrite arm distance was calculated. The inter-dendrite arm distances are measured at three or more positions in one field of view, an average of the inter-dendrite arm distances obtained respectively is calculated, and the calculated average is used as the average inter-dendrite arm distance.
The film thickness of the plating layer is preferably 10 μm to 30 μm, more preferably 20 μm to 25 μm, from the viewpoint of achieving both workability and corrosion resistance at a high level. The reason is that, when the plating layer is 10 μm or more, sufficient corrosion resistance can be ensured, and when the plating layer is 30 μm or less, workability can be sufficiently ensured.
Further, the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention can also be produced into a surface-treated steel sheet further comprising a chemically synthesized coating film and/or a coating film on the surface thereof.
(method for producing molten Al-Zn-Mg-Si-Sr coated steel sheet)
Next, a method for producing the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention will be described.
The method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet according to the present invention is characterized by using a plating bath having the following composition and a bath temperature of 585 ℃ or lower, wherein when molten plating is performed on a steel sheet, the temperature of the steel sheet at the time of entry into the plating bath (entry plate temperature) is set to a temperature (plating bath temperature +20 ℃ or lower) obtained by adding 20 ℃ to the bath temperature of the plating bath, and the plating bath has the following composition: it contains 25 to 70 mass% of Al, 0.6 to 5 mass% of Si, 0.1 to 10 mass% of Mg and 0.001 to 1.0 mass% of Sr, and the remainder is composed of Zn and unavoidable impurities.
The molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the above-described production method has excellent surface appearance and also has excellent corrosion resistance in the processed portion.
In the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, there is no particular limitation, but from the viewpoint of production efficiency or quality stability, a continuous type molten plating apparatus is generally used.
The type of the base steel sheet used for the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention is not particularly limited. For example, a hot-rolled steel sheet or strip subjected to pickling and rust removal, or a cold-rolled steel sheet or strip obtained by cold-rolling these can be used.
The conditions of the pretreatment step and the annealing step are not particularly limited, and any method can be used.
In the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the plating bath has the following composition: it contains 25 to 70 mass% of Al, 0.6 to 5 mass% of Si, 0.1 to 10 mass% of Mg and 0.001 to 1.0 mass% of Sr, and the remainder is composed of Zn and unavoidable impurities.
Thus, a molten Al-Zn-Mg-Si-Sr plated steel sheet having a desired composition can be obtained. The type, content, and action of each element contained in the plating bath are described in the molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention.
The molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the production method of the present invention has substantially the same composition as the plating bath as a whole. Thus, the control of the composition of the main layer can be precisely performed by controlling the plating bath composition.
In the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, the Mg and Si contents in the plating bath preferably satisfy the following formula (2).
M Mg /(M Si -0.6)≥1.0……(2)
M Mg : content of Mg (mass%), M Si : si content (mass%)
By making the Mg and Si contents in the plating bath satisfy the above-described relational expression, the formed plating layer can suppress the generation of Si phases (for example, the area ratio of the Si phase observed in the cross section in the thickness direction of the plating layer becomes 10% or less, and the area ratio of the Si phase observed in the surface of the plating layer becomes 1% or less), and can achieve further improvement in workability and corrosion resistance.
From the same point of view, M Mg /(M Si -0.6) is more preferably 2.0 or more, still more preferably 3.0 or more.
In the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet, the bath temperature of the plating bath is 585 ℃ or less, preferably 580 ℃ or less. By controlling the bath temperature to 585 ℃ or lower, the aforementioned Al-Si-Sr alloy having an adverse effect on workability can be suppressed Coarse growth can reduce large Mg 2 Amount of Si. In addition, the effect of inhibiting the growth of the interface alloy layer is also achieved. In the case where the bath temperature of the plating bath exceeds 585 ℃, the size of the Al-Si-Sr alloy becomes large, or large Mg, even if rationalization is achieved with respect to the steel sheet temperature at the time of entering the plating bath 2 The amount of Si also increases, and since the interface alloy layer grows thicker, the desired workability and corrosion resistance of the processed portion cannot be obtained.
In the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet according to the present invention, the temperature of the steel sheet at the time of entering the plating bath (entering plate temperature) is set to a temperature obtained by adding 20 ℃ to the bath temperature of the plating bath (plating bath temperature +20℃). The reason why the entering plate temperature is controlled to be equal to or lower than a predetermined temperature is that if the entering plate temperature is high, the bath temperature in the vicinity of the steel sheet increases when the steel sheet enters the bath, and the same adverse effect as the high bath temperature occurs. The reason is that by controlling the entering plate temperature, the temperature of the Al-Si-Sr alloy or Mg in the plating layer existing between the main layer and the interface alloy layer 2 Si can be controlled to be large, and growth of the interface alloy layer can be suppressed.
From the same viewpoint, the entering plate temperature of the steel sheet is preferably a temperature obtained by adding 10 ℃ to the bath temperature of the plating bath (plating bath temperature +10℃ C.) or less, and more preferably a bath temperature of the plating bath or less.
Further, in the method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet of the present invention, it is preferable that the steel sheet is subjected to molten plating and then cooled at an average cooling rate of 30 ℃/s or more until the sheet temperature is a temperature obtained by subtracting 150 ℃ from the bath temperature of the plating bath (plating bath temperature-150 ℃). Concerning Mg formed in the above-mentioned plating layer 2 Si, an Al-Si-Sr alloy formed between the main layer and the interface alloy layer, is known to be easily produced in a temperature range from the bath temperature of the plating bath to a temperature obtained by subtracting 150 ℃ from the bath temperature of the plating bath (plating bath temperature-150 ℃) and M can be suppressed by increasing the cooling rate in the temperature range to 30 ℃/sec or more on averageg 2 Growth of Si particles, al-Si-Sr alloys, and can more reliably reduce large Mg 2 Si and Al-Si-Sr alloy. Further, by increasing the cooling rate of the steel sheet after the melt plating, the growth of the interface alloy layer can be suppressed, and as a result, excellent corrosion resistance of the processed portion can be achieved. From the same viewpoint, the cooling of the steel sheet after the melt plating is preferably performed at an average cooling rate of 35 ℃/sec or more, and more preferably at an average cooling rate of 40 ℃/sec or more.
The average cooling rate is obtained by obtaining the time until the steel sheet reaches a temperature obtained by subtracting 150 ℃ from the plating bath temperature and dividing 150 ℃ by the time.
In the production method of the present invention, conditions other than the bath temperature and the entering plate temperature at the time of the melt plating and the cooling conditions after the melt plating are not particularly limited, and a molten al—zn—mg—si—sr plated steel sheet can be produced according to a usual method.
The molten Al-Zn-Mg-Si-Sr plated steel sheet obtained by the production method of the present invention may further have a chemical synthesis treatment film formed on the surface thereof (chemical synthesis treatment step) or a coating film formed in a separate coating apparatus (coating film forming step).
The chemical synthesis treatment film can be formed by, for example, a chromate treatment or a chromate-free chemical synthesis treatment in which a chromate treatment liquid or a chromate-free chemical synthesis treatment liquid is applied and a drying treatment for bringing the temperature of the steel sheet to 80 to 300 ℃ is performed without washing. These chemical synthesis treatment films may be single-layered or multi-layered, and in the case of multi-layered, a plurality of chemical synthesis treatments may be sequentially performed.
Examples of the coating film include roll coater coating, curtain coating, and spray coating. After the organic resin-containing coating material is applied, the coating film can be formed by heat drying by a method such as hot air drying, infrared heating, or induction heating.
Examples
(samples 1 to 31)
A cold-rolled steel sheet having a sheet thickness of 0.5mm produced by a usual method was used as a base steel sheet, and the production of molten Al-Zn-based plated steel sheets of samples 1 to 31 was carried out in a continuous molten plating apparatus. The composition of the plating bath used in the production was substantially the same as that of the plating layer of each sample shown in table 2, and the cooling rates up to the bath temperature of the plating bath, the entering plate temperature of the steel sheet, and the temperature obtained by subtracting 150 ℃ from the bath temperature of the plating bath were shown in table 2.
Then, for each sample of the obtained molten al—zn-based plated steel sheet, a cross-sectional view was made at a random one site by an energy dispersive X-ray spectrometry (SEM-EDX) using a scanning electron microscope.
The conditions of the formed plating layer and the manufacturing conditions of the plating were measured or calculated for each sample, and are shown in table 2.
(evaluation)
The following evaluations were performed for each sample of the molten al—zn-based plated steel sheet obtained as described above. The evaluation results are shown in table 2.
(1) Surface appearance
For each sample of the molten Al-Zn-based plated steel sheet, the surface of the plated layer (both surfaces of each sample) was visually observed in an observation field of about 1000mm to 1600mm in terms of the width X length of the steel sheet of 1000 mm.
The observation results were evaluated according to the following criteria.
O: no wrinkle-like defects were observed at all on either the front or back surface
X: a wrinkle-like defect is observed in at least one of the front surface and the back surface
(2) Evaluation of corrosion resistance of bending portion (corrosion resistance of processing portion)
For each sample of the molten Al-Zn-Mg-Si-Sr plated steel sheet, three sheets of the same plate thickness were sandwiched on the inner side and subjected to 180 ° bending processing (3T bending), and then a japanese automobile standard composite cycle test (JASO-CCT) was performed on the outer side of the bending. The JASO-CCT is a test in which brine is sprayed, dried, and humidified under specific conditions as shown in FIG. 4.
The number of cycles until red rust was generated was measured for each sample processing unit, and evaluated according to the following criteria.
And (3) the following materials: the number of the red rust generation cycles is more than or equal to 400
O: the number of the red rust generation cycles is less than or equal to 300 and less than 400
X: the number of cycles of red rust generation is less than 300 cycles
As is clear from the results in table 2, each sample of the present invention example was excellent in balance between surface appearance and corrosion resistance of the processed portion, as compared with each sample of the comparative example.
(industrial applicability)
According to the present invention, it is possible to provide a molten Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in a processed portion while having good surface appearance, and a method for producing a molten Al-Zn-Mg-Si-Sr plated steel sheet having excellent corrosion resistance in a processed portion while having good surface appearance.
Claims (5)
1. A molten Al-Zn-Mg-Si-Sr plated steel sheet, characterized in that,
the coating layer had the following composition: which contains 25 to 70 mass% of Al, 0.6 to 5 mass% of Si, 0.1 to 10 mass% of Mg and 0.001 to 1.0 mass% of Sr, and the balance of Zn and unavoidable impurities,
the plating layer is composed of an interface alloy layer existing at the interface with the base steel plate and a main layer existing above the alloy layer, and an Al-Si-Sr alloy with an average length diameter of 1 μm or less is arranged between the main layer and the interface alloy layer.
2. The molten Al-Zn-Mg-Si-Sr plated steel sheet according to claim 1, wherein 0.001 mass% or more of Sr is contained in the interface alloy layer.
3. The molten Al-Zn-Mg-Si-Sr plated steel sheet according to claim 1 or 2, wherein Mg observed in a cross section in a thickness direction of the plated layer 2 The Si has a long diameter of 10 μm or less.
4. The molten Al-Zn-Mg-Si-Sr plated steel sheet according to claim 1 or 2, wherein, regarding the Si phase observed in the cross section in the thickness direction of the plating layer, the area ratio of the Si phase is relative to Mg observed in the cross section in the thickness direction of the plating layer 2 The total ratio of the area ratios of Si and Si phases is 30% or less.
5. The molten Al-Zn-Mg-Si-Sr plated steel sheet according to claim 1 or 2, wherein the main layer has a dendrite part of an α -Al phase whose average inter-dendrite arm distance and the thickness of the plated layer satisfy the following formula (1):
t/d≥1.5……(1)
t: thickness of plating layer (μm), d: average dendrite inter-arm spacing (μm).
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| CN112342482A (en) * | 2020-10-19 | 2021-02-09 | 首钢京唐钢铁联合有限责任公司 | Hot-dip aluminum-zinc steel plate and preparation method thereof |
| KR20230082045A (en) | 2020-10-30 | 2023-06-08 | 제이에프이 스틸 가부시키가이샤 | Hot-dip Al-Zn-Si-Mg-Sr coated steel sheet, surface treated steel sheet and painted steel sheet |
| AU2021369097B2 (en) | 2020-10-30 | 2024-08-22 | Jfe Galvanizing & Coating Co., Ltd. | HOT-DIP Al-Zn-Si-Mg COATED STEEL SHEET, SURFACE-TREATED STEEL SHEET, AND PRE-PAINTED STEEL SHEET |
| WO2022091850A1 (en) | 2020-10-30 | 2022-05-05 | Ｊｆｅスチール株式会社 | HOT DIPPED Al-Zn-Si-Mg COATED STEEL SHEET, SURFACE-TREATED STEEL SHEET, AND COATED STEEL SHEET |
| US12043900B2 (en) * | 2021-01-18 | 2024-07-23 | Nippon Steel Corporation | Plated steel material |
| CN113046672A (en) * | 2021-03-11 | 2021-06-29 | 江苏中远稀土新材料有限公司 | Rare earth multi-element alloy coating for surface of metal support and hanger and hot galvanizing process thereof |
| WO2023181427A1 (en) * | 2022-03-24 | 2023-09-28 | Ｊｆｅ鋼板株式会社 | Molten al-zn-plated steel sheet and method for producing same |
| JPWO2023181426A1 (en) * | 2022-03-24 | 2023-09-28 |
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- 2019-11-14 KR KR1020217031006A patent/KR20210133266A/en not_active IP Right Cessation
- 2019-11-14 SG SG11202109473SA patent/SG11202109473SA/en unknown
- 2019-11-14 WO PCT/JP2019/045521 patent/WO2020179148A1/en active Application Filing
- 2019-11-14 CN CN201980093402.1A patent/CN113508186B/en active Active
- 2019-12-06 TW TW108144630A patent/TWI737066B/en active
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| CN1369020A (en) * | 1999-08-09 | 2002-09-11 | 新日本制铁株式会社 | Zn-Al-Mg-Si alloy plated steel product having excellent corrosion resistance and method for preparing the same |
| CN1461740A (en) * | 2002-03-11 | 2003-12-17 | 东曹株式会社 | High durability quartz glass, its manufacturing method unit and device using it |
| CN104955975A (en) * | 2013-01-31 | 2015-09-30 | Jfe钢板株式会社 | HOT-DIP Al-Zn GALVANIZED STEEL PLATE AND METHOD FOR PRODUCING SAME |
| JP2016166414A (en) * | 2015-03-02 | 2016-09-15 | Ｊｆｅ鋼板株式会社 | MOLTEN Al-Zn-Mg-Si PLATED SHEET STEEL AND PRODUCTION METHOD THEREOF |
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Also Published As
| Publication number | Publication date |
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| CN113508186A (en) | 2021-10-15 |
| SG11202109473SA (en) | 2021-09-29 |
| TW202033790A (en) | 2020-09-16 |
| WO2020179148A1 (en) | 2020-09-10 |
| TWI737066B (en) | 2021-08-21 |
| KR20210133266A (en) | 2021-11-05 |
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