STEEL SHEET PLATED WITH ZINC-ALUMINUM-MAGNESIUM-CALCIUM ALLOY BY MEANS OF HOT DIPPING AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed in the present invention is a steel sheet plated with a zinc-aluminum-magnesium-calcium alloy by means of hot dipping, which steel plate comprises a steel substrate and an alloy plating on a surface of the steel substrate. The chemical elements of the alloy plating include Zn and inevitable impurities, and the alloy plating further comprises the following chemical elements in percentages by mass: Al: 12-27%, Mg: 2-8%, Ca: 0.02-5%, and Si: 0.15-1.0%, wherein the mass percentage contents of Al, Mg and Ca in the alloy plating further satisfy the following relations: 4%(Mg+Ca)10%, and Al/(Mg+Ca)2.5. In addition, further disclosed in the present invention is a manufacturing method for the steel sheet plated a zinc-aluminum-magnesium-calcium alloy by means of hot dipping. The method comprises the steps of: (1) immersing a steel substrate annealed in a non-oxidizing atmosphere into a zinc-aluminum-magnesium-calcium alloy plating solution; and (2) after the plated strip steel is taken out from a plating bath pot, subjecting same to air-jet cooling by means of a cooling spray box at a cooling speed of 10 C./s until the temperature of the plated strip steel is lower than 100 C., and then placing the plated strip steel in a water quenching tank for cooling with water.

Claims

1. A steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating, comprising a steel substrate and an alloy coating on a surface of the steel substrate, wherein chemical elements of the alloy coating include Zn and unavoidable impurities, and the alloy coating further comprises the following chemical elements in mass percentages: Al: 12-27%, Mg: 2-8%, Ca: 0.02-5%, Si: 0.15-1.0%; wherein the mass percentages of Al, Mg and Ca in the alloy coating further satisfy the following relationships: 4%(Mg+Ca)10%, Al/(Mg+Ca)2.5.

2. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 1, wherein the mass percentages of the chemical elements in the alloy coating are: Al: 12-27%, Mg: 2-8%, Ca: 0.02-5%, Si: 0.15-1.0%, and a balance of Zn and unavoidable impurities; wherein the mass percentages of Al, Mg and Ca in the alloy coating further satisfy the following relationships: 4%(Mg+Ca)10%, Al/(Mg+Ca)2.5.

3. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 1, wherein the alloy coating further comprises either or both of Ti: 0.01-0.1% and B: 0-0.05%.

4. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 1, wherein the alloy coating has a microstructure comprising: an Al-rich phase, a MgZn.sub.2 phase, a Zn-rich phase and a Mg.sub.2Si phase, and a granular intermetallic compound enriched with elements Mg and Ca.

5. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 4, wherein the granular intermetallic compound includes at least one of the following: MgZn.sub.2, Mg.sub.2Si, Al.sub.2Ca, Al.sub.4Ca, Al.sub.2CaSi.sub.2, and Ca.sub.3Zn.

6. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 1, wherein the alloy coating has a hardness of 140-240 Hv.

7. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 1, wherein a single-sided coating amount of the alloy coating is 120-300 g/m.sup.2.

8. A method for manufacturing the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 1, comprising steps of: (1) immersing a steel substrate annealed in a non-oxidizing atmosphere in a plating liquid of a zinc-aluminum-magnesium-calcium alloy, wherein the plating liquid of the zinc-aluminum-magnesium-calcium alloy has temperature of 460-560 C.; (2) removing a coated strip steel from a plating bath; subjecting the coated strip steel to gas jet cooling using a cooling jet box, with a cooling rate controlled to be 10 C./s, until a temperature of the coated strip steel is lower than 100 C.; and then placing the coated strip steel in a water quenching tank for water cooling.

9. The method according to claim 8, wherein in step (1), when the steel substrate is immersed in the plating liquid of the zinc-aluminum-magnesium-calcium alloy, a relationship between a temperature of the steel substrate T.sub.Fe and a temperature of the plating liquid of the zinc-aluminum-magnesium-calcium alloy T.sub.Zn is controlled to satisfy: (T.sub.Zn5 C.)T.sub.Fe(T.sub.Zn+20 C.).

10. The method according to claim 8, wherein in step (2), before the coated strip steel is cooled to a temperature of 340 C., a cooling gas jetted from the cooling jet box is N.sub.2 with 3% by volume of O.sub.2 compressed at ambient temperature.

11. The method according to claim 8, wherein in step (2), the temperature of the coated strip steel is controlled to be 250 C. when it reaches a top roller.

12. The method according to claim 8, wherein a first-stage cooling jet box is set at a height of 2-4 m from a surface of the plating liquid of the zinc-aluminum-magnesium-calcium alloy in the plating bath.

13. The method according to claim 8, wherein a surface of the plating bath is covered with a sealing housing, and an atmosphere inside the sealing housing is: an inert gas with 3% by volume of O.sub.2.

14. The method according to claim 8, wherein the coated strip steel between the first-stage cooling jet box and the plating bath is protected using a sealing box.

15. The method according to claim 8, wherein before the coated strip steel is cooled to a temperature of 340 C., a sealing box is arranged for the strip steel between the cooling jet box and the plating bath, and inert atmosphere protection is employed.

16. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 2, wherein the alloy coating further comprises either or both of Ti: 0.01-0.1% and B: 0-0.05%.

17. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 2, wherein the alloy coating has a microstructure comprising: an Al-rich phase, a MgZn.sub.2 phase, a Zn-rich phase and a Mg.sub.2Si phase, and a granular intermetallic compound enriched with elements Mg and Ca.

18. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 17, wherein the granular intermetallic compound includes at least one of the following: MgZn.sub.2, Mg.sub.2Si, Al.sub.2Ca, Al.sub.4Ca, Al.sub.2CaSi.sub.2, and Ca.sub.3Zn.

19. The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to claim 2, wherein the alloy coating has a hardness of 140-240 Hv, and/or a single-sided coating amount of the alloy coating is 120-300 g/m.sup.2.

20. The method according to claim 8, wherein the mass percentages of the chemical elements in the alloy coating are: Al: 12-27%, Mg: 2-8%, Ca: 0.02-5%, Si: 0.15-1.0%, and a balance of Zn and unavoidable impurities; and the mass percentages of Al, Mg and Ca in the alloy coating further satisfy the following relationships: 4%(Mg+Ca)10%, Al/(Mg+Ca)2.5.

Description

DESCRIPTION OF THE DRAWINGS

[0069] FIG. 1 is an electron microscope photograph showing the surface of the alloy coating of the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating in Example 1.

[0070] FIG. 2 is an electron microscope photograph showing the cross-section of the alloy coating obtained by rapid cooling the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating in Example 3.

[0071] FIG. 3 is an electron microscope photograph showing the cross-section of the alloy coating obtained by slowly cooling the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating in Example 4.

[0072] FIG. 4 is a photograph schematically showing the cracks at the 0T bend in Example 3.

[0073] FIG. 5 is a photograph schematically showing the cracks at the 0T bend in Example 4.

DETAILED DESCRIPTION

[0074] The steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to the present disclosure and the manufacturing method thereof will be further explained and illustrated below with reference to the specific Examples and the drawings of the specification. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present disclosure.

Examples 1-10 and Comparative Examples 1-10

[0075] For the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating according to the present disclosure, the inventors have imposed no special restrictions on the composition or process for the steel substrate, and those skilled in the art can arbitrarily select a cold-rolled or hot-rolled substrate in light of the target use of the product.

[0076] The steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 and the comparative steel sheets in Comparative Examples 1-10 were all prepared by the following steps: [0077] (1) A low-carbon Al-killed steel cold-rolled sheet having a substrate thickness of 0.8 mm was used as a steel substrate. The steel substrate annealed in a non-oxidizing atmosphere was sent into a plating bath, and immersed in a plating liquid of a zinc-aluminum-magnesium-calcium alloy having one of the various compositions as shown in Table 1. The temperature of the plating liquid of the zinc-aluminum-magnesium-calcium alloy was controlled to be 460-560 C. When the steel substrate was immersed in the plating liquid of the zinc-aluminum-magnesium-calcium alloy, the relationship between the temperature of the steel substrate T.sub.Fe and the temperature of the plating liquid of the zinc-aluminum-magnesium-calcium alloy T.sub.Zn was controlled to satisfy: (T.sub.Zn-5 C.)T.sub.Fe(T.sub.Zn+20 C.). The steel substrate was lifted up after being immersed in the plating liquid for 3 seconds. A sealing housing was arranged on the surface of the plating bath to cover it, and the atmosphere inside the sealing housing was controlled to be: an inert gas with 3% by volume of O.sub.2. [0078] (2) After the coated strip steel was removed from the plating bath, the coated strip steel was firstly subjected to gas jet cooling using a cooling jet box. The height of the first-stage cooling jet box was controlled to be 2-4 m from the surface of the plating liquid of the zinc-aluminum-magnesium-calcium alloy in the plating bath. The coated strip steel between the first-stage cooling jet box and the plating bath was protected using a sealing box. The temperature of the coated strip steel when it reached the top roller was controlled to be 250 C., and the cooling rate was controlled to be 10 C./s, until the temperature of the coated strip steel was lower than 100 C. Then, the coated strip steel was placed in a water quenching tank for water cooling, so as to obtain a coated strip steel with a single-sided coating amount of 140 g/m.sup.2; [0079] wherein before the coated strip steel was cooled to a temperature of 340 C., the cooling gas was ambient-temperature compressed N.sub.2, and the volume content of O.sub.2 therein was controlled to be 3%. After the coated strip steel was cooled to a temperature of 340 C., the above special control was no longer performed, and the cooling gas jetted from the remaining cooling jet boxes was ambient-temperature compressed air.

[0080] In the present disclosure, for evaluating the influence of the coating composition on the corrosion resistance of the steel sheet, the inventors prepared several steel sheets hot-dipped with zinc-aluminum-magnesium-calcium alloy coatings, which had the same coating amount (i.e., a single-sided coating amount of 140 g/m.sup.2), but obtained by plating with plating liquids of different compositions.

[0081] It should be noted that in the present disclosure, the chemical compositions of the alloy coatings used for the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 and the related processes implemented all meet the design specification requirements of the present disclosure.

[0082] However, the comparative steel sheets in Comparative Examples 1-10 do not meet the requirements of the present disclosure in terms of the chemical composition design of the alloy coating and/or the design of the process parameters. Comparative Example 10 listed is the GI reference.

[0083] Table 1 lists the mass percentages of the various chemical elements in the zinc-aluminum-magnesium-calcium alloy plating liquids used for the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 and the comparative steel sheets in Comparative Examples 1-10.

[0084] Table 1: Mass percentages of the chemical elements in the zinc-aluminum-magnesium-calcium alloy plating liquids used in the Examples and Comparative Examples (the balance is Zn and unavoidable impurities)

TABLE-US-00001 TABLE 1 Mass percentages of the chemical elements in the zinc-aluminum-magnesium-calcium alloy plating liquids used in the Examples and Comparative Examples (the balance is Zn and unavoidable impurities) Al Si Mg Ca Ti B Mg + Ca Al/(Mg + Ca) No. (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) Ex. 1 12.8 0.2 2.5 2.5 0 0 5.00 2.56 Ex. 2 20.1 0.4 2.5 4.5 0.01 0 7.00 2.87 Ex. 3 20.1 0.6 4.3 2.3 0.01 0.01 6.60 3.05 Ex. 4 20.1 0.6 4.3 2.3 0.01 0.03 6.60 3.05 Ex. 5 20.1 0.6 4.3 2.3 0.03 0.03 6.60 3.05 Ex. 6 20.1 0.5 7.5 0.02 0.01 0 7.52 2.67 Ex. 7 20.1 0.5 7.5 0.02 0.05 0 7.52 2.67 Ex. 8 26.3 0.8 2.5 4.5 0.01 0.04 7.00 3.76 Ex. 9 26.3 0.8 4.8 4.5 0.07 0.04 9.30 2.83 Ex. 10 26.3 0.8 7.8 0.05 0.09 0 7.85 3.35 Comp. Ex. 1 6.3 0.2 2.5 2.5 0 0 5.00 1.26 Comp. Ex. 2 12.8 0.4 1.5 0.05 0.01 0.02 1.55 8.26 Comp. Ex. 3 20.1 0.6 0.0 4.5 0.01 0 4.50 4.47 Comp. Ex. 4 20.1 0.6 4.3 2.3 0.01 0.07 6.60 3.05 Comp. Ex. 5 20.1 0.6 7.5 0.02 0.15 0 7.52 2.67 Comp. Ex. 6 26.3 0.8 4.8 4.5 0.01 0 9.30 2.83 Comp. Ex. 7 26.3 0.8 7.8 2.8 0.01 0 10.60 2.48 Comp. Ex. 8 26.3 0.8 7.8 2.8 0.01 0 10.60 2.48 Comp. Ex. 9 38.5 1.0 2.5 1.0 0.01 0 3.50 11.00 Comp. Ex. 10 0.02 0 0 0 0 0 0 0

[0085] In the present disclosure, the steel substrate was immersed in a plating liquid of a zinc-aluminum-magnesium-calcium alloy for plating, and an alloy coating was formed on the surface of the steel substrate. Therefore, the chemical composition of a plating liquid of a zinc-aluminum-magnesium-calcium alloy in Table 1 above can be understood to be equivalent to the chemical composition of the alloy coating formed in the corresponding Example or Comparative Example.

TABLE-US-00002 TABLE 2 Specific process parameters used in the process steps for the Examples and Comparative Examples Zinc- Distance from Temper- aluminum- first-stage ature of magnesium- cooling coated calcium jet box to strip Steel alloy zinc-aluminum- steel substrate plating magnesium- arriving temper- liquid calcium alloy at Cooling ature temperature Atmosphere plating liquid top roller rate No. ( C.) ( C.) protection surface (m) ( C.) ( C./s) Ex. 1 485 490 Yes 2 170 30 Ex. 2 505 510 Yes 4 200 15 Ex. 3 505 510 Yes 4 180 30 Ex. 4 515 510 Yes 3 230 10 Ex. 5 515 510 No 3 235 10 Ex. 6 515 510 Yes 4 220 20 Ex. 7 520 510 Yes 3 220 20 Ex. 8 525 530 No 2 240 25 Ex. 9 525 530 Yes 2 240 25 Ex. 10 565 550 Yes 4 230 25 Comp. Ex. 1 450 430 Yes 4 170 25 Comp. Ex. 2 495 490 Yes 5 160 30 Comp. Ex. 3 510 510 Yes 3 190 15 Comp. Ex. 4 510 510 Yes 3 200 20 Comp. Ex. 5 505 510 Yes 4 180 20 Comp. Ex. 6 590 580 No 10 250 25 Comp. Ex. 7 525 530 No 10 220 25 Comp. Ex. 8 525 530 Yes 6 200 25 Comp. Ex. 9 560 550 No 6 220 30 Comp. Ex. 10 450 460 No 10 170 25 Table 2 lists the specific process parameters in the above process steps for the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 and the comparative steel sheets in Comparative Examples 1-10.

[0086] Note: In Table 2 above, atmosphere protection means that in step (2), before the coated strip steel was cooled to a temperature of 340 C., a sealing box was arranged between the cooling jet box and the plating bath, and the atmosphere inside the sealing box was controlled to be: an inert gas with 3% by volume of O.sub.2.

[0087] In order to verify the implementation effectiveness of this case and prove the superior effects of this case over the prior art, the alloy coatings of the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings obtained in Examples 1-10 and the alloy coatings of the comparative steel sheets obtained in Comparative Examples 1-10 were analyzed and tested. Table 3 lists the analysis and test results of the alloy coatings of the Examples and Comparative Examples.

[0088] The relevant analysis and test methods are as follows: [0089] (1) The coating surface quality was rated by observing with naked eyes the number of defects such as transverse fine lines, zinc ash, and zinc slag on the alloy coating surfaces in the Examples and Comparative Examples. The evaluation criteria are as follows: [0090] Evaluation : No obvious defects; [0091] Evaluation : number of defects 4/100 m; [0092] Evaluation : number of defects 5-10/100 m; [0093] Evaluation x: number of defects >10/100 m.

[0094] In summary, when the coating surface quality was evaluated as or , it's considered to be able to meet the surface quality control requirement of the present disclosure. [0095] (2) From the coated steel sheets obtained in Examples 1-10 and Comparative Examples 1-10, 120200 mm samples of the Examples and Comparative Examples were taken respectively, and 1800 0T bending was performed. A tape was applied and then peeled off. Dezincification was observed with naked eyes to evaluate the bonding strength between the coating and the substrate. The evaluation criteria are as follows: [0096] Evaluation : no peeling from the coating; [0097] Evaluation : 1-2 peeling points on the tape visible to naked eyes; [0098] Evaluation : 3-10 peeling points on the tape visible to naked eyes; [0099] Evaluation x: peeling points >10, or peeling off in pieces from the coating.

[0100] In summary, when the T-bend adhesion evaluation was or o, it's considered to be able to meet the coating bonding strength requirement of the present disclosure. [0101] (3) For the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 and the comparative steel sheets in Comparative Examples 1-10 prepared under different composition and process conditions, the coated steel sheet of each of the Examples and Comparative Examples was cut into a sample of 15070 mm in size.

[0102] According to the accelerated test standard for cyclic exposure of metals and alloys to salt mist, dry and wet conditions as specified in ISO 14993-2018, the corrosion resistance of the samples of the Examples and Comparative Examples at the flat surface, cut edge and T-bend was tested, and the number of cycles corresponding to the occurrence of 5% red rust on the coating surface, cut edge and T-bend under different conditions was recorded. The GI coating (pure zinc coating) having a single-sided coating amount of 140 g/m.sup.2 in Comparative Example 10 was used as a reference, and the multiple by which the corrosion resistance of the alloy coating of the coated steel sheet in each of Examples 1-10 and Comparative Examples 1-9 was increased relative to the GI coating (pure zinc coating) in Comparative Example 10 was calculated.

[0103] In addition, the alloy coating of the coated steel sheet in each of the Examples and Comparative Examples was further tested. The coating hardness was tested according to the method specified in GB/T 9790-2021 Metallic materialsVickers and Knoop microhardness tests of metallic and other inorganic coatings, with a load of 100 gf used to test the coating hardness H.sub.v100g of each of the Examples and Comparative Examples.

[0104] Table 3 lists the analysis and test results of the alloy coatings of the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 and the comparative steel sheets in Comparative Examples 1-10.

TABLE-US-00003 TABLE 3 Analysis and test results of alloy coatings in Examples and Comparative Examples Number of cycles Number of cycles Number of cycles for red rust at flat for red rust at cut for red rust at 0T surface edge bend Coating hardness Multiple Multiple Multiple Multiple relative to relative to relative to relative to Coating Comp. Ex. Comp. Ex. Comp. Ex. Comp. surface T bend 10 10 10 Ex. 10 No. quality adhesion Cycles reference Cycles reference Cycles reference Hv.sub.100g reference Ex. 1 210 10.5 200 13.3 180 12.0 145 2.2 Ex. 2 260 13.0 220 14.7 230 15.3 160 2.5 Ex. 3 280 14.0 230 15.3 220 14.7 200 3.1 Ex. 4 260 13.0 230 15.3 200 13.3 210 3.1 Ex. 5 260 13.0 230 15.3 190 12.7 200 3.1 Ex. 6 300 15.0 250 16.7 250 16.7 240 3.7 Ex. 7 300 15.0 260 17.3 245 16.3 235 3.6 Ex. 8 320 16.0 290 19.3 310 20.7 180 2.8 Ex. 9 320 16.0 300 20.0 300 20.0 200 3.1 Ex. 10 300 15.0 210 14.0 230 15.3 230 3.5 Comp. 130 6.5 130 8.7 70 4.7 120 1.8 Ex. 1 Comp. 120 6.0 100 6.7 80 5.3 130 2.0 Ex. 2 Comp. 220 11.0 200 13.3 200 13.3 105 1.6 Ex. 3 Comp. 180 9.0 150 10.0 130 8.7 180 2.8 Ex. 4 Comp. 250 12.5 200 13.3 220 14.7 220 3.4 Ex. 5 Comp. X X 310 15.5 280 18.7 300 20.0 190 2.9 Ex. 6 Comp. X X 280 14.0 200 13.3 120 8.0 240 3.7 Ex. 7 Comp. 320 16.0 250 16.7 200 13.3 240 3.7 Ex. 8 Comp. 280 14.0 80 5.3 160 10.7 130 2.0 Ex. 9 Comp. 20 1 15 1 15 1 65 1 Ex. 10

[0105] As it can be seen from Table 3, in the present disclosure, the alloy coatings of the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings obtained in Examples 1-10 all had excellent surface quality, and the coatings were well bonded to the substrates. The coating surface quality evaluation results and T-bend adhesion evaluation results were all or o, meeting the requirements of the present disclosure.

[0106] In addition, it's not difficult to see from the comparative tests of Examples 1-10 and the GI reference (Comparative Example 10) for corrosion resistance that the alloy coatings of the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 according to the present disclosure exhibited larger cycle numbers in the corrosion resistance test at the flat surface, cut edge and T-bend, indicating better corrosion resistance. The cycle number corresponding to the occurrence of 5% red rust on the flat surface was 10.5-16 times that of the GI reference; the cycle number corresponding to the occurrence of 5% red rust on the cut edge was 13.3-20 times that of the GI reference; and the cycle number corresponding to the occurrence of 5% red rust on the 0T bend was 12-20.7 times that of the GI reference.

[0107] In addition, the alloy coatings of the steel sheets hot-dipped with the zinc-aluminum-magnesium-calcium alloy coatings in Examples 1-10 also exhibited higher coating hardness which was 2.2-3.7 times that of the GI reference, indicating excellent scratch resistance.

[0108] In Comparative Example 1 designed in the present disclosure, the content of the Al element in the coating was rather low (6.3%), and the corrosion resistance of the coating at the flat surface was only 6.5 times that of the GI reference. In addition, its Al/(Mg+Ca) value was rather low, and the coating surface quality was poor. In contrast, in Example 1, the (Mg+Ca) content remained unchanged relative to Comparative Example 1, while the content of the Al element in the coating was increased to 12.8%. As a result, the corrosion resistance of the coating at the flat surface reached 10.5 times that of the GI reference, and the corrosion resistance at both the cut edge and 0T bend was improved, also indicating good surface quality.

[0109] In Example 2, the content of the Al element in the coating was increased to 20.1%, and the upper limit of the (Mg+Ca) content in the plating liquid was increased accordingly, which could not only improve the corrosion resistance of the coating, but also provide better coating surface quality. In addition, the hardness and scratch resistance of the coating were also increased with the increase of intermetallic compound forming elements such as Mg and Ca. However, in Comparative Example 9, the content of the Al element in the coating was further increased to 38.5% such that the content of the Al element was too high. Although the corrosion resistance of the finally resulting coating at the flat surface was still good, the corrosion resistance of the coating at the cut edge dropped sharply, with the cycle number for the red rust at the cut edge of the coating being only 5.3 times that of the GI reference.

[0110] As it can be seen from Comparative Example 2, although the Al content in the coating was 12.8%, the total amount of (Mg+Ca) was relatively low, only 1.55%, which also affected the corrosion resistance of the coating, especially the corrosion resistance at the cut edge and bend. The overall performance was even worse than the coating in Comparative Example 1. In Example 1, the total amount of (Mg+Ca) in the coating reached 5%, and the corrosion resistance of the coating was improved significantly. However, as it can be seen from Comparative Example 8, when the (Mg+Ca) content in the coating exceeded the range defined by the present disclosure, it would be difficult to obtain good coating surface quality. Under the premise of atmosphere protection, the corrosion resistance of the coating was acceptable, but the surface defects of the coating had increased. In Comparative Example 7, under the same preparation conditions but without N.sub.2 protection, the surface quality of the coating degraded seriously, and the corrosion resistance of the coating, especially the corrosion resistance at the T-bend processing area, was reduced seriously due to the large-scale cracking and peeling caused by defects. As it can be seen from a comparison between Example 5 and Example 4, when there was no atmosphere protection, the surface quality of the coating was somewhat decreased than that under inert atmosphere protection conditions, and the transverse fine lines on the coating surface increased somewhat, but there was no significant influence on the other performances.

[0111] As it can be seen from Comparative Example 3, when designing the coating, even with no addition of the Mg element, adding Ca alone also played a good role in improving the corrosion resistance, but the effect of adding the Ca element alone on improving the hardness of the coating was limited. The hardness of the coating was only 105 Hv, still far from the scratch resistance desired by the present disclosure. In contrast, in Example 3, while the total amount of (Mg+Ca) in the coating was not much different, further addition of the Mg element to the coating contributed more to the improvement of the hardness of the coating.

[0112] In Example 4, 0.03% B was further added on the basis of Example 3, and it had no significant influence on the surface quality or corrosion resistance of the coating. However, in Comparative Example 4, the B element was added in an excessive amount, which increased the defects on the coating surface, thereby affecting the corrosion resistance. The effect of the Ti element was the same. Compared with Example 7, in Comparative Example 5, Ti was added in an excessive amount, which caused a large number of defects on the coating surface, thereby reducing the corrosion resistance to a certain extent while affecting the aesthetic appearance. Therefore, although both of them had a certain effect of refining the structure, the amount added should be selected judiciously within the range specified in the present disclosure.

[0113] In addition, as it can be seen from Comparative Example 6, the temperature of the plating liquid of the zinc-aluminum-magnesium-calcium alloy in Comparative Example 6 didn't meet the design requirement of the present disclosure. When the temperature of the plating liquid of the zinc-aluminum-magnesium-calcium alloy was too high, the FeAl inhibiting layer was significantly thickened. Moreover, without post-plating atmosphere protection, the surface quality of the coating was seriously affected. Although these surface defects had little influence on the corrosion resistance of the flat surface of the coating, severe cracking and peeling of the coating were observed at the 0T bend, and the corrosion resistance of the deformed parts was deteriorated seriously.

[0114] FIG. 1 is an electron microscope photograph showing the surface of the alloy coating of the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating in Example 1.

[0115] In the present disclosure, the surface morphology of the alloy coating of the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating obtained in Example 1 is shown in FIG. 1. In this embodiment, the addition of the Ca element led to the formation of white star-shaped intermetallic compound particles of Al.sub.2Ca and Al.sub.4Ca on the surface of the alloy coating, and the Mg element also formed black Mg.sub.2Si particles.

[0116] FIG. 2 is an electron microscope photograph showing the cross-section of the alloy coating obtained by rapid cooling the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating in Example 3.

[0117] FIG. 3 is an electron microscope photograph showing the cross-section of the alloy coating obtained by slowly cooling the steel sheet hot-dipped with a zinc-aluminum-magnesium-calcium alloy coating in Example 4.

[0118] FIG. 4 is a photograph schematically showing the cracks at the 0T bend in Example 3.

[0119] FIG. 5 is a photograph schematically showing the cracks at the 0T bend in Example 4.

[0120] Comparing FIG. 2 and FIG. 3, it can be seen from the difference in the cross-sectional morphologies of the coatings of Example 3 and Example 4 (FIG. 2 and FIG. 3) that after the coated strip steel was removed from the plating bath, the cooling rate of the rapid cooling had a more significant influence on the structure morphology than the addition of the structure refining elements. The difference in microstructure morphology had no obvious influence on the corrosion resistance, but the cracks on the coating surface at the T-bend were relatively fine when the cooling rate was fast (as shown in FIGS. 4 and 5).

[0121] However, regardless of whether the slow cooling shown in Example 4 (at a cooling rate of 10 C./s) or the rapid cooling shown in Example 3 (at a cooling rate of 30 C./s) was used, the interior of the alloy coating obtained finally was composed of an Al-rich phase, a MgZn.sub.2 phase, a Zn-rich phase and a Mg.sub.2Si phase, with no obvious intermetallic compound particles other than Mg.sub.2Si observed on its cross section, and the Ca-containing granular intermetallic compound tended to be enriched in the surface layer of the coating.

[0122] It should be noted that the prior art in the protection scope of the present disclosure is not limited to the Examples set forth in the present application file. All prior art that does not contradict the solution of the present disclosure, including but not limited to prior patent documents, prior publications, prior public uses, etc., can be included in the protection scope of the present disclosure.

[0123] In addition, it should be noted that combinations of the various technical features in this case are not limited to the combinations described in the claims of this case or the combinations described in the specific Examples. All technical features recorded in this case can be combined freely or associated in any way unless a contradiction occurs.

[0124] It should also be noted that the Examples listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above Examples, and changes or modifications made thereto can be directly derived from the present disclosure or easily conceived of by those skilled in the art, all of which fall within the protection scope of the present disclosure.