A galvanized steel sheet with high crack resistance performance and its production method

Abstract

The present invention provides a zinc-aluminum-magnesium coated steel sheet with high crack resistance and a production method thereof. The composition of the zinc-aluminum-magnesium coating is as follows: Al: 1% - 5%, Mg: 1% - 2%, Si: 0.05% - 0.15%, and the balance is Zn and inevitable impurity elements; by adjusting the Al / Mg content ratio, the proportion of the binary eutectic phase on the coating surface is < 14%; compared with the prior art, the present invention adjusts the Al / Mg content ratio in the coating to obtain a binary alloy phase with a suitable proportion, reducing the crack incidence rate; by adjusting the post-plating cooling process, the size of the zinc-rich phase is refined to obtain a favorable preferred orientation to inhibit the continuous expansion after crack initiation, so that it has better bending performance.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical technology, specifically to the field of steel-based hot-dip plating, and more specifically relates to a zinc-aluminum-magnesium coated steel sheet with high crack resistance and its production method. Background Art

[0002] In order to improve the corrosion resistance of traditional hot-dip galvanized steel sheets, save zinc resources, reduce product costs, and meet the growing demands of the automotive and construction industries for high corrosion resistance and high formability of steel sheets, a new generation of zinc-aluminum-magnesium alloy coated steel sheets has been developed by adding aluminum and magnesium elements to the hot-dip pure zinc coating. Compared with traditional pure zinc coatings, zinc-aluminum-magnesium coatings have better planar and notch corrosion resistance, wear resistance, and painting performance, etc. Over the years, researchers' studies on zinc-aluminum-magnesium coated steel sheets have mainly focused on the corrosion resistance and corrosion mechanism of the coatings, and it has been clarified that the stable corrosion products formed by magnesium and aluminum elements in the zinc-aluminum-magnesium coatings are the main reasons for the significantly improved corrosion resistance compared with pure zinc coatings; in contrast, the deformability of zinc-aluminum-magnesium coatings is relatively poor. For example, a large number of cracks will occur in the coatings during tensile or bending deformation, reducing the corrosion resistance of zinc-aluminum-magnesium coated steel sheets and affecting their applications in the fields of automotive and household appliance steels.

[0003] Therefore, while ensuring the corrosion resistance of zinc-aluminum-magnesium coated steel sheets, improving the crack resistance of zinc-aluminum-magnesium coatings during processing is an urgent problem to be solved for zinc-aluminum-magnesium coated steel sheets at present.

[0004] A patent with the publication number CN 114369782 A, published on April 19, 2022, discloses a hot-dip galvanized coated steel sheet without microcracks and its preparation method. The coated steel sheet includes a steel substrate and a coating; the coating includes N zinc-aluminum-magnesium coatings and N nickel-rich coatings; the zinc-aluminum-magnesium coatings and the nickel-rich coatings are arranged alternately, the innermost layer of the coating is a zinc-aluminum-magnesium coating, and the outermost layer of the coating is a nickel-rich coating; the zinc-aluminum-magnesium coating contains a zinc-rich phase, an aluminum-rich phase, and magnesium-silicon particles; the preparation method includes: obtaining a plating solution containing the chemical composition of the zinc-aluminum-magnesium coating; heating the plating solution to obtain a preheated plating solution; obtaining the steel substrate; heating the steel substrate and then immersing it in the preheated plating solution to obtain the steel substrate with a zinc-magnesium-aluminum coating; cooling the steel substrate with a zinc-magnesium-aluminum coating and then nickel infiltration to obtain the coated steel sheet; by the zinc-rich phase, aluminum-rich phase, and magnesium-silicon particles in the zinc-magnesium-aluminum coating and the cooperation with the nickel-rich coating, the generation of cracks is avoided. However, it contains Ni in the coating, resulting in a relatively high cost. Summary of the Invention

[0005] The object of the present invention is to provide a zinc-aluminum-magnesium coated steel sheet with high crack resistance and its production method. By adjusting the Al / Mg content ratio in the plating solution, a binary eutectic phase with a suitable proportion is obtained to reduce the crack incidence. By adjusting the post-plating cooling process, a texture with a favorable orientation is obtained in the zinc-rich phase to inhibit the continuous propagation after crack initiation, so that it has better formability while meeting the corrosion resistance. Moreover, the coating of the present invention does not contain Ni and has low cost.

[0006] The specific technical solution of the present invention is as follows:

[0007] A zinc-aluminum-magnesium coated steel sheet with high crack resistance, comprising a substrate and a zinc-aluminum-magnesium coating;

[0008] The composition of the zinc-aluminum-magnesium coating is Al: 1% - 5%, Mg: 1% - 2%, Si: 0.05% - 0.15%, and the balance is Zn and inevitable impurity elements;

[0009] The zinc-aluminum-magnesium coating mainly comprises a zinc-rich phase, a binary eutectic phase Zn + MgZn2 and a ternary eutectic phase Zn + Al + MgZn2;

[0010] The total content of the inevitable impurity elements is less than 1%;

[0011] The zinc-rich phase is the first-precipitated phase, grows in a dendritic shape, and accounts for a relatively large proportion;

[0012] The binary eutectic phase is precipitated after the zinc-rich phase and has a strip-shaped morphology with a large spacing;

[0013] The ternary eutectic phase is the last solidified phase of the coating, has a short rod-shaped or lamellar morphology and a small spacing, and is distributed around the zinc-rich phase together with the binary eutectic phase.

[0014] The morphologies of the zinc-rich phase, the binary eutectic phase Zn + MgZn2 and the ternary eutectic phase Zn + Al + MgZn2 are as Figure 1 、 Figure 2 shown.

[0015] The present invention adjusts the Al / Mg content ratio to 1.25 - 1.5, so that the proportion of the binary eutectic phase on the coating surface < 14%; specifically as follows:

[0016] When 1% ≤ Mg content ≤ 1.2%, control 1.42 < Al / Mg mass ratio ≤ 1.50. At this time, 13.56% ≤ mass proportion of the binary eutectic phase ≤ 13.83%;

[0017] When 1.2% < Mg content ≤ 1.4%, control 1.36 < Al / Mg mass ratio ≤ 1.42. At this time, 13.28% ≤ mass proportion of the binary eutectic phase < 13.56%;

[0018] When 1.4% < Mg content ≤ 1.6%, control 1.31 < Al / Mg mass ratio ≤ 1.36. At this time, 13.03% ≤ mass ratio of binary eutectic phase < 13.28%;

[0019] When 1.6% < Mg content ≤ 1.8%, control 1.28 < Al / Mg mass ratio ≤ 1.31. At this time, 12.76% ≤ mass ratio of binary eutectic phase < 13.03%;

[0020] When 1.8% < Mg content ≤ 2.0%, control 1.25 ≤ Al / Mg mass ratio ≤ 1.28. At this time, 12.52% ≤ mass ratio of binary eutectic phase < 12.76%.

[0021] The substrate thickness ranges from 0.3 mm to 2.0 mm, and the single-sided coating weight of the zinc-aluminum-magnesium coating ≤ 150 g / m 2 。

[0022] A production method of a zinc-aluminum-magnesium coated steel sheet with high crack resistance provided by the present invention includes: controlling the plating solution temperature at 425 - 440 °C, the strip outlet air knife temperature is 1 - 5 °C lower than the plating solution temperature, the air knife gas source is nitrogen at room temperature, and the nozzle type of the post-galvanizing aluminum-magnesium coating cooling device is "groove" type.

[0023] The production method further includes: controlling the temperature of the strip before entering the cooling fan at 350 ± 2 °C, and adopting a primary cooling rate after entering the cooling fan, with the cooling rate control range of 10 / s < cooling rate ≤ 20 °C / s; when 280 °C ≤ strip temperature < 350 °C, adopt a secondary cooling rate for cooling, with the cooling rate control range of 1 °C / s < cooling rate ≤ 3 °C / s.

[0024] Compared with the prior art, the present invention adjusts the Al / Mg content ratio in the coating to obtain a suitable proportion of binary eutectic phase, reducing the crack incidence rate; by adjusting the post-galvanizing cooling process, the zinc-rich phase obtains a favorable preferred orientation to inhibit the continuous expansion after crack initiation, enabling it to have better formability while meeting the corrosion resistance, and having high bending performance. Description of the Drawings

[0025] Figure 1 is the surface microstructure morphology of the zinc-aluminum-magnesium coating;

[0026] Figure 2 is the cross-sectional microstructure morphology of the zinc-aluminum-magnesium coating;

[0027] Figure 3 Initiation and propagation of cracks after bending of the zinc-aluminum-magnesium coating;

[0028] Figure 4 is the schematic diagram of the zinc-aluminum-magnesium coated steel sheet and the deformed force;

[0029] Figure 5 are the typical crystal planes and crystal directions of the HCP crystal;

[0030] Figure 6 is the schematic diagram of coating deformation and cracking;

[0031] Figure 7 is the surface morphology of the coating in Example 2 at 500×;

[0032] Figure 8 is the surface morphology of the coating in Comparative Example 3 at 500×;

[0033] Figure 9 is the surface morphology of the coating in Comparative Example 4 at 500×;

[0034] Figure 10 is the crack morphology of the coating in Example 2 at 23×;

[0035] Figure 11 is the crack morphology of the coating in Comparative Example 3 at 23×;

[0036] Figure 12 is the crack morphology of the coating in Comparative Example 4 at 23×;

[0037] Figure 13 is the surface morphology of the coating in Example 5 at 500×;

[0038] Figure 14 is the surface morphology of the coating in Comparative Example 9 at 500×

[0039] Figure 15 is the surface morphology of the coating in Comparative Example 10 at 500×;

[0040] Figure 16 is the crack morphology of the coating in Example 5 at 23×;

[0041] Figure 17 is the crack morphology of the coating in Comparative Example 9 at 23×;

[0042] Figure 18 is the crack morphology of the coating in Comparative Example 10 at 23×. Detailed implementation manners

[0043] To achieve the above object, the present invention provides a zinc-aluminum-magnesium coated steel sheet with high crack resistance performance, which includes a substrate and a coating; the composition of the zinc-aluminum-magnesium coating is Al: 1% - 5%, Mg: 1% - 2%, Si: 0.05% - 0.15%, and the balance is Zn and inevitable impurity elements. [[ID= 60]]

[0044] Reasons for the present invention to reduce the proportion of binary eutectic phase in the coating: Compared with the pure zinc coating, the zinc-aluminum-magnesium coating consists of three phases. During the bending forming process, the poor deformation consistency makes its bending performance significantly reduced. The inventor found that during the tensile deformation test of the coating, the phase that cracks preferentially in the coating is the binary eutectic phase and it spreads in the primary galvanized phase. The binary eutectic phase is composed of two phases, Zn and MgZn2. Since the crystal structures of both Zn and MgZn2 are hexagonal close-packed (HCP), the inventor's research found that the strong elastic anisotropy of the hexagonal close-packed crystal may be the main reason for its cracking during the deformation process. Next, the Young's moduli of single crystals of Zn, Mg, and MgZn2 are calculated to specifically illustrate the characteristics of their elastic anisotropy.

[0045] Let the Young's modulus of the hexagonal close-packed single crystal be E, and S 11 、S 33 、S 13 、S 44 be the compliance coefficients acting on different crystal planes, and θ be the angle between a certain crystal plane (h k i l) of the calculated cubic metal and the (0001) plane. Then, the relationship between E and the compliance coefficient is as follows:

[0046] 1 / E = S 11 (sin 4 θ) + S 33 (cos 4 θ) + (2S 13 + S 44 )(cos 2 θ)(sin 2 θ) (1)

[0047] The crystal plane angle is calculated by the following formula:

[0048] cosθ = 0.75×(a / c) 2 ×l×{0.75(a / c) 2 ×(h 2 + k 2 + hk + 0.75(a / c) 2 ×l 2 )}^(-1 / 2) (2)

[0049] The anisotropy coefficient is calculated by the following formula:

[0050] f E = S 11 / S 33 (3)

[0051] Where a / c is the ratio of lattice constants, and the lattice parameters of Zn, Mg, and MgZn2 are shown in Table 1; the compliance coefficients of single-crystalline close-packed hexagonal metals Zn, Mg, and MgZn2 are shown in Table 2. Using formula (2), the calculated angles between the (0001), (10-10), (10-11), (10-12) planes and the (0001) plane in the close-packed hexagonal single crystal are shown in Table 3.

[0052] Table 1 Lattice parameters of single crystals of Zn, Mg, and MgZn2 metals

[0053]

[0054]

[0055] Table 2 Compliance coefficients (TPa) of single crystals of Zn, Mg, and MgZn2 metals

[0056] Single crystal <![CDATA[S 11 > <![CDATA[S 33 > <![CDATA[S 13 > <![CDATA[S 44 > Zn 8.07 27.55 -7.02 25.25 <![CDATA[MgZn2]]> 9.90 8.45 -1.56 41.27 Mg 22.00 19.70 -4.96 60.98

[0057] Table 3 Angles between the crystal planes (0001), (10-10), (10-11), (10-12) and the (0001) crystal plane

[0058] Single crystal (0001) (10-10) (10-11) (10-12) Zn 0° 90° 64.99° 46.98° <![CDATA[MgZn2]]> 0° 90° 62.17° 43.45° Mg 0° 90° 61.92° 43.14°

[0059] Substituting the data in Tables 1, 2, and 3 into formula (1), the Young's moduli and anisotropy coefficients of single crystals of Zn, Mg, and MgZn2 with a close-packed hexagonal structure on different crystal planes are shown in Table 4.

[0060] Table 4 Young's moduli (GPa) of single crystals of Zn, Mg, and MgZn2 metals on the (0001), (10-10), (10-11), (10-12) crystal planes

[0061] Single crystal <![CDATA[E (0001) > <![CDATA[E (10-10) > <![CDATA[E (10-11) > <![CDATA[E (10-12) > <![CDATA[E (11-22) > <![CDATA[f E > Zn 36.30 123.92 125.49 90.39 122.01 0.29 <![CDATA[MgZn2]]> 116.55 85.91 76.02 75.65 74.97 1.36 Mg 50.76 45.45 43.28 43.28 43.04 1.12

[0062] From the calculation results in Table 4, it can be seen that the basal plane of the Mg single crystal has relatively low Young's moduli with similar values for both the prism plane and the pyramid plane, and the anisotropy coefficient is close to the ideal value of 1, indicating that the Mg single crystal is approximately isotropic; while for the Zn single crystal, the Young's modulus of the basal plane (0001) is significantly smaller than that on the (10-10) and (10-11) crystal planes, that is, the ability to resist uniaxial tensile deformation in the normal direction is significantly lower than that in other directions. In addition, its anisotropy coefficient is 0.29, which is significantly smaller than the ideal value of 1, indicating that the Zn single crystal has significant anisotropy; for the MgZn2 single crystal, the difference and value of the Young's moduli between the basal plane and the prism plane and the pyramid plane are smaller than those of the Zn single crystal, and its elastic anisotropy is between that of the Mg and Zn single crystals.

[0063] It is found in the research of the present invention that during the bending deformation test of the zinc-aluminum-magnesium coated steel sheet, cracks on the coating surface mainly occur in the binary eutectic phase of the coating, and a small part occurs in the Zn-rich phase and the ternary eutectic phase. At the same time, there is an obvious behavior in the coating that cracks initiate from the eutectic phase and extend to the adjacent Zn-rich phase. The C-axis

[0001] direction of the Zn-rich phase where cracks occur is mostly consistent with the loading direction. Since the Young's modulus of the (0001) plane of the Zn-rich phase is the lowest, that is, the ability to resist deformation is weaker than other crystal planes, cleavage fracture is likely to occur when there is a basal texture with <0001>∥RD (rolling direction). It shows that the elastic anisotropy of the Zn phase and the hard and brittle MgZn2 phase in the binary eutectic causes crack sources to generate after the coating deforms, and then extends in the adjacent Zn-rich phase with an unfavorable orientation ( Figures 3 - 6 ), therefore, for the deformation cracking phenomenon of the zinc-aluminum-magnesium coating, this patent provides a strategy to improve the crack resistance of the zinc-aluminum-magnesium coating during the deformation process.

[0064] The reason for setting the magnesium content in the coating of the present invention to 1%-2%: The improvement of the crack resistance of the coating is based on the premise that the coating has excellent corrosion resistance. After adding a certain amount of Mg element, when the coating contacts with media in the air such as water and carbon dioxide, in the initial stage of corrosion, Al element remains in the coating in an insoluble state, while Mg and Zn elements preferentially dissolve anodically to release a certain amount of Zn 2+ and Mg 2+ , and the oxygen reduction reaction occurs at the cathode to generate a certain amount of hydroxide ions. At this time, the local pH of the coating increases, and basic zinc salts (such as hydrozincite and chlorohydrozincite in a chloride ion environment) are formed and cover the coating surface. At this time, the magnesium ions existing in the coating can inhibit the decomposition of hydrozincite and chlorohydrozincite to form porous zinc oxide and zinc carbonate, so that the basic zinc salts formed in the initial stage of corrosion can exist stably, inhibiting the further corrosion of the coating; in the middle and late stages of corrosion, as the pH value continues to increase, the Al element in the coating dissolves in the alkaline environment to form Al(OH)4 -, thereby generating LDH (layered double hydroxide). LDH has a denser surface structure and a layered structure that is difficult for ions to penetrate, significantly improving the corrosion resistance of the zinc-aluminum-magnesium coating. If the Mg content in the coating is less than 1%, Mg cannot achieve the useful effects described above in the coating. When the Mg content in the coating is greater than 2%, the oxidation on the coating surface is difficult to control, and the binary eutectic phase in the coating will increase significantly, deteriorating the bending performance of the coating; when the Mg content is between 1% and 2%, the proportion of the binary eutectic phase in the coating can be effectively regulated through composition and production processes, so that the proportion of the binary alloy phase in the coating is controlled at a low level, reducing the formation probability of early cracks that cause stress relaxation during the deformation of the coating, reducing stress concentration, and enabling the coating to withstand a certain amount of plastic deformation without cracking and transfer stress to the surrounding zinc-rich phase, increasing the probability of activating the plastic deformation mechanism of cone surface slip or twinning in the zinc-rich phase, thereby improving the overall deformation ability of the coating while ensuring corrosion resistance.

[0065] The reason for setting the Al content in the coating higher than that of Mg: When the aluminum content in the coating is higher than that of magnesium, the binary eutectic reaction during the solidification of the coating will be postponed, and the remaining liquid phase will increase before the ternary eutectic reaction. After the coating solidifies, a microstructure with an increased proportion of the ternary phase, little change in the proportion of the zinc-rich phase, and a reduced proportion of the binary alloy phase is obtained, which is more conducive to improving the bending performance of the coating while ensuring the corrosion resistance of the coating; in addition, the higher Al element in the coating is supersaturated in the liquid phase and precipitates in the form of nanoscale particles in the zinc-rich phase at the end of solidification. The electrochemical coupling reaction with the zinc-rich phase at the initial stage of corrosion is more conducive to the preferential dissolution of zinc to form protective corrosion products.

[0066] The reason for strengthening the post-plating cooling to control the preferred orientation of the zinc-rich phase in the coating: When a high-component <0001>∥ND basal texture is generated in the coating, in addition to being able to obtain higher deformation resistance under external force and not being prone to cleavage fracture as described above, since the strengths of both the binary eutectic phase and the ternary eutectic phase in the coating are higher than that of the zinc-rich phase, when the coating is deformed by external force, the zinc-plated phase will bear most of the strain of the coating, indicating that the first-precipitated zinc-plated phase has higher microplasticity. It is found that when the zinc-plated phase has a high-component <0001>∥ND basal texture, a higher density of dislocations can be generated inside the grains of the zinc-plated phase when deformed by external force. At the same time, the interaction between the precipitated nanoscale Al particles and the dislocations is more intense, promoting slip or twinning inside the grains of the zinc-plated phase, making the zinc-rich phase prone to plastic deformation rather than cleavage-type brittle fracture.

[0067] After passing through the air knife, the strip steel is cooled to 350°C ± 2°C at a primary cooling rate. The control range of the cooling rate is: 10°C / s < cooling rate ≤ 20°C / s. When the strip steel temperature is between 280°C ≤ strip steel temperature < 350°C, it is cooled at a secondary cooling rate, and the control range of the cooling rate is 1°C / s < cooling rate ≤ 3°C / s. During the initial solidification process after the strip steel leaves the pot, if a relatively large cooling rate is adopted, a large temperature gradient is formed from the inside to the outside of the coating, which can generate a large growth driving force in the direction perpendicular to the coating during the initial solidification precipitation of the zinc-rich phase, facilitating the prior precipitation of the zinc-rich phase to form a <0001> ∥ ND basal texture, that is, the (0001) plane of the zinc-rich phase is parallel to the rolling plane of the steel plate. During the bending or stretching deformation process of the coating, the (0001) crystal plane of the zinc-rich phase forms an acute angle or is parallel to the external force direction, hindering crack propagation, thereby obtaining better crack resistance. In addition, the large supercooling degree formed by the relatively large cooling rate can promote the nucleation of the primary zinc phase, refine the solidification structure of the coating, and also inhibit the propagation behavior after crack generation, further enhancing the crack resistance of the coating during deformation.

[0068] Reasons for selecting the "groove" type for the nozzle of the cooling device: During the process of the strip steel leaving the zinc pot and entering the fan cooling, selecting the "groove" type air outlet nozzle can make the cooling of the coating surface more uniform, effectively inhibit the abnormal structure caused by uneven local cooling, ensure that the prior-precipitated zinc-rich phase can fully nucleate and grow, and refine the dendrite size of the zinc-rich phase at high cooling rates to generate a stronger <0001> ∥ RD texture.

[0069] By adjusting the Al / Mg content ratio in the plating solution, the content of the binary eutectic phase in the coating is reduced, the initiation of cracks is inhibited, and correspondingly, the content of the ternary eutectic is increased. By increasing the cooling rate after plating, fine zinc-rich phase dendrites with preferred orientation are obtained, thereby obtaining a zinc-aluminum-magnesium coating with excellent crack resistance.

[0070] Next, a zinc-aluminum-magnesium coated steel plate with high crack resistance and its production method according to the present invention will be described in detail with reference to examples, comparative examples, and experimental data.

[0071] A zinc-aluminum-magnesium coated steel plate with high crack resistance includes a substrate and a coating; a cold-rolled plate with a thickness of 1.8 mm is used as the substrate, the material is of CQ grade, and the single-sided coating weight is 130 g / m 2 ; The composition of the zinc-aluminum-magnesium coating is: Al: 1% - 5%, Mg: 1% - 2%, Si: 0.05% - 0.15%, and the rest are Zn and inevitable impurity elements.

[0072] The production methods of the zinc-aluminum-magnesium coated steel plates with high crack resistance in each example and comparative example include:

[0073] The plating solution temperature is 430 °C, the Si content in the plating solution is controlled at 0.1%, the temperature of the strip exiting the air knife is 426 °C, the air source of the air knife is nitrogen at room temperature, and the nozzle type of the cooling device is "groove" type; the temperature of the strip before entering the cooling fan is 350 °C, and the first-stage cooling rate is adopted after entering the cooling fan. The control range of the cooling rate is: 10 / s < cooling rate ≤ 20 °C / s; when 280 °C ≤ strip temperature < 350 °C, the second-stage cooling rate is adopted for cooling, and the control range of the cooling rate is 1 °C / s ≤ cooling rate ≤ 3 °C / s. The coating compositions and production process parameters of specific examples and comparative examples are shown in Table 5. The remainder not shown in Table 5 is Zn and inevitable impurity elements; the total content of inevitable impurity elements is less than 1%.

[0074] Table 5 Coating Compositions and Cooling Process Parameters of Each Example and Comparative Example

[0075]

[0076]

[0077] The anti-crack performance and corrosion resistance of the zinc-aluminum-magnesium coated steel sheets prepared according to the coating compositions and process parameters in the above examples and comparative examples were evaluated. The evaluation methods are as follows: 1) The 0T bending experiment was carried out on the coated steel sheets in the examples and comparative examples according to the regulations of the bending test in "GB / T 13448-2019 Test Methods for Color Coated Steel Sheets and Strips"; 2) If it is not clear enough to observe visually, a 10-fold magnifying glass can be used to observe whether there is coating peeling; 3) Use 3M transparent tape to stick tightly on the surface of the coating after bending, and then quickly lift the tape to observe whether there is metal coating powder adhered to the tape; 4) Observe and measure the crack width under an electron microscope, and the evaluation results are shown in Table 6; 5) Carry out the neutral salt spray test according to ASTM B117. The sample size is 75 mm × 150 mm, the surface is not passivated, and 3M tape is used to seal the edges of the sample. Place the sample in the neutral salt spray test chamber and monitor the initial red rust time of the sample.

[0078] Table 6 T-bending Performance Test Results of Coatings in Each Example and Comparative Example

[0079]

[0080]

[0081] The underlined data in the above table are the data that do not meet the requirements of the present invention.

[0082] As can be seen from the evaluation results in Table 6, although the Al / Mg content ratio in the coatings in Comparative Examples 2, 4, 6, 8, and 10 meets the requirements, the zinc-aluminum-magnesium coatings obtained at a relatively low cooling rate have powder shedding phenomenon and a relatively large average crack width. After the 0T bending test, there is no coating peeling and powder shedding on the galvanized aluminum-magnesium coating steel plates prepared in Examples 1-5 of the present invention, and the average crack width of the coating is lower. The surface microstructure of the coatings and the crack morphology after 0T bending in Examples 2 and 5 and the corresponding Comparative Examples 3, 4, 9, and 10 are listed. Compared with Comparative Examples 3, 4, 9, and 10, the binary eutectic phase on the coating surface in Examples 2 and 5 is significantly reduced, the ternary eutectic phase is significantly increased, the size of the galvanized phase is significantly refined, the average crack width of the coating is low and relatively uniform. Therefore, the zinc-aluminum-magnesium coating in the examples of the present invention has better crack resistance.

[0083] The above specific embodiments and examples illustrate the present invention, and the advantages and various effects of the present invention will be presented more clearly therefrom. Those skilled in the art should understand that these specific embodiments and examples are used to illustrate the present invention, rather than limiting the present invention.

Claims

1. A zinc-aluminum-magnesium coated steel sheet with high crack resistance performance, characterized in that, The zinc-aluminum-magnesium coated steel sheet with high crack resistance performance comprises a substrate and a zinc-aluminum-magnesium coating; The composition of the zinc-aluminum-magnesium coating is Al: 1% - 5%, Mg: 1% - 2%, Si: 0.05% - 0.15%, and the balance is Zn and inevitable impurity elements; the zinc-aluminum-magnesium coating mainly comprises a zinc-rich phase, a binary eutectic phase Zn + MgZn2, and a ternary eutectic phase Zn + Al + MgZn2; The Al / Mg content ratio is 1.25 - 1.50, and the proportion of the binary eutectic phase on the coating surface is < 14%; When 1% ≤ Mg content ≤ 1.2%, control 1.42 < Al / Mg mass ratio ≤ 1.

50. At this time, 13.56% ≤ the proportion of the binary eutectic phase ≤ 13.83%; When 1.2% < Mg content ≤ 1.4%, control 1.36 < Al / Mg mass ratio ≤ 1.

42. At this time, 13.28% ≤ the proportion of the binary eutectic phase < 13.56%; When 1.4% < Mg content ≤ 1.6%, control 1.31 < Al / Mg mass ratio ≤ 1.

36. At this time, 13.03% ≤ the proportion of the binary eutectic phase < 13.28%; When 1.6% < Mg content ≤ 1.8%, control 1.28 < Al / Mg mass ratio ≤ 1.

31. At this time, 12.76% ≤ the proportion of the binary eutectic phase < 13.03%; When 1.8% < Mg content ≤ 2.0%, control 1.25 ≤ Al / Mg mass ratio ≤ 1.

28. At this time, 12.52% ≤ the proportion of the binary eutectic phase < 12.76%; The production method of the zinc-aluminum-magnesium coated steel sheet with high crack resistance performance includes cooling to 350°C ± 2°C at a first-stage cooling rate after hot dip coating. The control range of the cooling rate is: 10°C / s < cooling rate ≤ 20°C / s. When the strip temperature is between 280°C ≤ strip temperature < 350°C, cooling is carried out at a second-stage cooling rate, and the control range of the cooling rate is 1°C / s < cooling rate ≤ 3°C / s.

2. The zinc-aluminum-magnesium coated steel sheet with high crack resistance according to claim 1, characterized in that, The thickness range of the substrate is 0.3mm - 2.0mm.

3. The zinc-aluminum-magnesium coated steel sheet with high crack resistance according to claim 1 or 2, characterized in that, The weight of the zinc-aluminum-magnesium coating on one side ≤ 150 g / m 2 .

4. A production method of a zinc-aluminum-magnesium coated steel sheet with high crack resistance according to any one of claims 1-3, characterized in that, The production method includes: controlling the bath temperature to be 425 - 440°C, cooling to 350°C ± 2°C at a first-stage cooling rate after hot dip coating. The control range of the cooling rate is: 10°C / s < cooling rate ≤ 20°C / s. When the strip temperature is between 280°C ≤ strip temperature < 350°C, cooling is carried out at a second-stage cooling rate, and the control range of the cooling rate is 1°C / s < cooling rate ≤ 3°C / s.

5. The production method according to claim 4, characterized in that, The production method includes: the temperature of the strip leaving the air knife is 1 - 5°C lower than the bath temperature.

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