Preparation process of steel plate with black appearance and product

Abstract

The invention belongs to the technical field of metal surface treatment, discloses a preparation process of a steel plate with black appearance and a product, and aims to realize accurate control of the black appearance, according to the method, a black oxidation film with an anoxic structure is generated in situ by introducing a multi-phase coating with obvious oxidation activity difference and utilizing the ordered oxidation process of each phase. Specifically, the surface of the steel plate is plated with a zinc-aluminum-magnesium plating layer, and two-stage water vapor oxidation treatment is conducted, specifically, in the first stage, an Al-rich phase is preferentially oxidized to form a base film, and in the second stage, an MgZn2 phase is oxidized to form a black oxidation film. The brightness value of the obtained steel plate is less than or equal to 35, the thermal radiation heat dissipation performance is excellent, the film layer is crack-free, and the steel plate is suitable for long-term use. By limiting the reaction endpoint at the MgZn2 phase oxidation completion stage, the defects caused by Zn-rich phase oxidation are avoided, the problems that a traditional method is difficult in appearance control and poor in environmental protection property are solved, and the method is suitable for the field of high-end equipment shells.

Description

Technical Field

[0001] This invention belongs to the field of metal material surface treatment technology, and relates to a preparation process and product of a black steel plate. Background Technology

[0002] Black steel sheets are widely in demand in the high-end equipment casing sector due to their strong visual appeal and technological feel. Traditional manufacturing methods, such as coating, suffer from VOC emissions and difficulty in precisely controlling appearance uniformity. Single-phase coating oxidation, lacking clear stages in the reaction process, is prone to localized over- or under-oxidation, leading to appearance defects. Research has found that zinc-aluminum-magnesium coatings naturally contain Al-rich, MgZn2-rich, and Zn-rich phases, with significant differences in oxidation activity. By controlling reaction conditions, each phase can be oxidized sequentially, providing an ideal medium for precise control of the black appearance.

[0003] In existing technologies, the reaction sequence of zinc-aluminum-magnesium (ZA-Mg) coatings (Al-rich phase → MgZn2 phase → Zn-rich phase) and the process boundary of "terminating the reaction before the oxidation of the Zn-rich phase" are not clearly defined. Often, excessively long reaction times lead to the third stage, resulting in film defects; or insufficient reaction time results in incomplete oxidation of the MgZn2 phase and insufficient blackening (high brightness value). Therefore, precisely controlling the termination time of the second stage (after the completion of MgZn2 phase oxidation) is crucial to solving these problems. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to solve the above problems and provide a preparation process and product of black steel sheet, which achieves orderly and controllable oxidation process by introducing a multiphase system with obvious differences in oxidation activity.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A manufacturing process for a black-look steel sheet includes the following steps: 1) Substrate selection: Steel plate is selected as the substrate, and the surface of the steel plate is coated with zinc-aluminum-magnesium coating. The zinc-aluminum-magnesium coating contains Al-rich phase, MgZn2 phase and Zn-rich phase, and the oxidation activity of each phase is significantly different. The difference in oxidation activity of each phase is verified by thermodynamic calculation. The absolute value of Gibbs free energy change of Al-rich phase oxidation is the largest, followed by MgZn2 phase, and Zn-rich phase is the smallest. 2) Loading process: Using mechanical methods, the steel plate with zinc-aluminum-magnesium coating is placed in a closed space with opening and closing function; 3) Reducing atmosphere treatment: A reducing atmosphere is created in the enclosed space by circulating gas replacement through inert gas purging and reducing gas introduction; 4) heating treatment under protective atmosphere: using the pressure difference between inert gas and reducing atmosphere in the closed space, the inert gas is introduced to replace the reducing atmosphere; under the condition of continuous introduction of inert protective gas, the steel plate is heated; 5) stage oxidation treatment: high-temperature water vapor is introduced into the closed space, and the relative humidity is controlled, and the reaction is carried out in two stages: First stage: the Al-rich phase in the coating is preferentially oxidized to form an aluminum oxide film; Second stage: continue to react for a period of time, and the total reaction time is adjusted according to the reaction temperature, so that the MgZn2 phase in the coating is completely oxidized to generate ZnO particles, and the oxidation of the Zn-rich phase is avoided; the judgment standard at the end of this stage is that the characteristic peak of the Zn-rich phase is not obviously weakened, the characteristic peak of MgZn2 is basically disappeared, and oxygen vacancies are detected by XPS; 6) cooling treatment under protective atmosphere: after the reaction is completed, the inert gas is continuously introduced to replace the water vapor in the closed space by using the pressure difference between the inert gas and the gas in the closed space; 7) take-out treatment: when the temperature difference between the steel plate and the ambient atmosphere is ≤5℃, the closed space is opened by mechanical method, and the zinc-aluminum-magnesium steel plate with black oxide film on the surface is taken out.

[0006] Further, in the zinc-aluminum-magnesium coating, the content of Mg element is 1.00-10.00wt.%, the content of Al element is 3.00-18.00wt.%, and Zn element is the balance.

[0007] Further, in step 3), the number of gas replacement is ≥3 times, the purity of reducing gas is ≥99.99%, the treatment temperature of reducing atmosphere is 80-120℃, and the treatment time is 30-60min.

[0008] Further, in step 4), the purity of inert gas is ≥99.999%, the residual amount of reducing gas after replacement is ≤0.1vol.%, the average heating temperature of steel plate is controlled to 150℃±5℃, and the temperature difference between the highest point and the lowest point of steel plate is ≤10℃.

[0009] Further, in step 5), the temperature of the first stage is controlled at 150-200℃, the reaction time is 0.5-2h, and the temperature of the second stage is maintained at 110-160℃.

[0010] Further, in step 5), high-temperature water vapor is introduced into the closed space, and the relative humidity is controlled to be ≥95%.

[0011] Further, in step 6), the temperature of the introduced inert gas is between 25-40℃, and the content of water vapor after replacement is ≤5vol.%.

[0012] Furthermore, the sealed space in step 2) and the sealed space in step 5) are the same equipment or compatible independent equipment to ensure that there is no external pollution during the transfer of the steel plate; the temperature range of the sealed space is 25~300℃, the pressure range is -0.1~0.3MPa, and the sealing degree of the space gaps meets the requirement that the pressure drop within 30 minutes after gas replacement does not exceed 5%.

[0013] Furthermore, the inert gas is nitrogen or argon, and the reducing gas is hydrogen or carbon monoxide.

[0014] A black steel plate product is prepared using the preparation process described above. The product has a surface brightness value of ≤35, good heat absorption and release properties, and the oxide film is free of cracks.

[0015] Thermodynamic calculations show that the oxidation of zinc-aluminum-magnesium (ZAMg) coatings exhibits significant stages. The ΔG of the Al-rich phase oxidation reaction (calculated to be approximately -210 kcal / mol) is significantly lower than that of the MgZn2 phase oxidation reaction (calculated to be approximately -110 kcal / mol) and the Zn-rich phase oxidation reaction (calculated to be approximately -20 kcal / mol), indicating that it possesses the highest driving force and this oxidation reaction occurs preferentially. The ΔG of the MgZn2 phase oxidation reaction lies between that of the Al-rich and Zn-rich phase oxidation reactions, still exhibiting a relatively strong driving force. The MgZn2 phase oxidizes with water vapor to form MgO and ZnO phases. The Zn-rich phase oxidation reaction has the lowest driving force and the lowest reactivity, making it the least likely to occur. Therefore, from a thermodynamic perspective, the reaction sequence in the ZAMg coating microstructure should be: Al-rich phase (first stage), MgZn2 phase in the eutectic structure or bulk MgZn2 phase (second stage), and Zn-rich phase (third stage).

[0016] Based on scanning electron microscopy observations and elemental analysis, the first stage of the reaction involves the preferential oxidation of the Al-rich phase to form an aluminum oxide film, which then gradually diffuses into the surrounding area. The second stage results in the formation of an oxygen-deficient oxide with numerous oxygen vacancies, exhibiting a black appearance and thus providing excellent heat dissipation performance while inheriting the good processing properties of the coated steel sheet. When the oxidation reaction of the zinc-aluminum-magnesium coating becomes excessive, a third stage of Zn-rich phase oxidation occurs, which easily leads to a crack network and severely affects corrosion resistance. Therefore, overcoming the film crack defects caused by excessive reaction leading to the third stage in existing processes, while simultaneously avoiding poor blackening performance due to insufficient reaction, and precisely controlling the reaction at the completion stage of MgZn2 phase oxidation, achieving efficient and controllable preparation of a high-quality blackened film through two-stage control, is key to balancing performance and reliability.

[0017] This invention achieves efficient and defect-free preparation of black oxide films by adding reducing atmosphere pretreatment, protective atmosphere heating and cooling steps, and precise staged oxidation treatment. The core innovations include: Precise atmosphere control: Reducing atmosphere pretreatment removes trace oxidation impurities from the coating surface, while protective atmosphere heating and cooling prevent the introduction of external impurities during oxidation, thereby improving film purity and adhesion. Temperature uniformity control: Limit the average temperature and temperature difference of the steel plate during the heating stage to avoid uneven oxidation caused by local temperature deviations; Stage oxidation optimization: Based on the "reaction stage characteristics" of zinc-aluminum-magnesium coating oxidation, the oxidation temperature range and reaction endpoint determination criteria are clearly defined to ensure that the reaction terminates at the completion stage of MgZn2 phase oxidation and avoid crack defects caused by Zn-rich phase oxidation.

[0018] The beneficial effects of this invention are as follows: 1) Achieve proactive and precise control of the oxidation process: Based on the oxidation sequence of Al-rich phase → MgZn2 phase → Zn-rich phase, the reaction rhythm of each phase is actively regulated through a two-stage process to ensure uniform black appearance.

[0019] 2) Proactively avoid film defects and improve product reliability: By combining XRD and XPS analysis, the reaction is proactively terminated at the completion stage of MgZn2 phase oxidation. The process parameters obtained in this way can prevent cracks caused by Zn-rich phase oxidation from the source, significantly improve film integrity, and reduce the risk of corrosion in later use.

[0020] 3) Proactively shorten the reaction cycle and improve industrial efficiency: Through precise stage control and endpoint judgment, the ineffective reaction time is proactively reduced, energy consumption is reduced, and excessive oxidation of the membrane layer is avoided, which significantly improves the economy and efficiency of industrial production.

[0021] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 The results are thermodynamic calculations for each active phase in the zinc-aluminum-magnesium coating.

[0023] Figure 2 This is a scanning electron microscope image of Comparative Example 1 (first stage).

[0024] Figure 3 This is a scanning electron microscope image of Example 1 (second stage), in which the bright-colored granular material is the precipitated zinc oxide.

[0025] Figure 4 This is a scanning electron microscope image of Comparative Example 2 (Third Stage).

[0026] Figure 5 The XRD patterns are those of Comparative Example 1, Comparative Example 2, and Example 1.

[0027] Figure 6 This is the XPS spectrum of Example 1.

[0028] Figure 7 For the comparison of heat absorption and release performance between Comparative Example 1 and Example 1.

[0029] Figure 8 The elemental content of each region in the scanning electron microscope image of Comparative Example 1 is shown.

[0030] Figure 9 The element content of each point in the scanning electron microscope image of Example 1 is shown.

[0031] Figure 10 The elemental content of each point in the scanning electron microscope image of Comparative Example 2 is shown.

[0032] Figure 11 The XPS spectrum fitting results are for Example 1. Detailed Implementation

[0033] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0034] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0035] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0036] In the following embodiments, the steel plate pretreatment uses the following steps: 1) Select a 1mm thick cold-rolled steel sheet, coated with a zinc-aluminum-magnesium (Zn-6Al-3Mg) layer, and cut it into 150mm×100mm samples; the thermodynamic calculation results of each active phase in the zinc-aluminum-magnesium coating are as follows: Figure 1 As shown.

[0037] 2) Ultrasonic cleaning with anhydrous ethanol for 5 minutes to remove surface oil and impurities; 3) Dry in 80℃ hot air and set aside for later use.

[0038] The test methods are as follows: 1) Brightness value: The brightness value of the sample was tested using a Sanenshi TS7700 high-precision spectrophotometer to evaluate the degree of blackening. The lower the surface brightness value, the stronger the blackening of the sample.

[0039] 2) Heat Absorption and Release Performance: The heat absorption and release capabilities of the zinc-aluminum-magnesium alloy plate before and after blackening were compared and tested using a heat absorption and release test apparatus. Samples under the same working conditions were used on all four sides and the top of the test chamber. Quartz wool and a heating plate were placed on the bottom to heat the air inside the chamber. The gaps along the edges of the cuboid were sealed with black tape. A small hole was made on the top to facilitate the insertion of a temperature sensor for measuring the air temperature inside the chamber. The temperature inside the chamber was monitored and recorded in real time, and the differences in the final temperature inside the chamber under different working conditions were compared.

[0040] 3) Microscopic observation: The microstructure of the sample surface was characterized using a field emission scanning electron microscope (FESEM). The accelerating voltage was set to 10 kV, the beam spot size to 10 μm, and the working distance to 10 mm to balance image resolution with electron beam damage to the sample. Before testing, the sample was cut into 2 mm × 2 mm slices and fixed to the sample stage using conductive adhesive. All tests were conducted at an ambient temperature of (25 ± 2) ℃, relative humidity of (45 ± 5)%, and a vacuum degree close to 1 × 10⁻⁶. -4 The test was conducted in an mbar environment to reduce the interference of environmental factors on the test results.

[0041] 4) Elemental Analysis: The elemental composition of the sample surface was characterized using energy-dispersive spectroscopy. Before testing, the sample was cut into thin slices approximately 2mm × 2mm in size and fixed to the sample stage using conductive adhesive. During testing, the accelerating voltage was set to 20kV, the working distance was controlled at 10mm, and the acquisition time for scanning analysis was 50s. All tests were conducted under a vacuum of approximately 1×10⁻⁶. - 4 The X-ray signal was conducted in an environment of mbar to reduce the absorption and interference of air on the X-ray signal.

[0042] 5) Phase Analysis: X-ray diffraction was used to analyze the phase composition of the samples. A Cu Kα radiation source (λ=0.15406nm) was used as the incident light source, with the tube voltage set to 30kV and the tube current to 10mA. During the test, the scanning range was set to 10°~90° and the scanning rate was 0.02° / s. All tests were performed under ambient temperature of (25±1)℃ and relative humidity <40% to minimize the influence of environmental factors on the test results.

[0043] 6) Surface Chemical State: The surface chemical state of the samples was analyzed using X-ray photoelectron spectroscopy. Monochromatic Al Kα rays (hν = 1486.6 eV) were used as the excitation source. When acquiring the O 1s high-resolution spectrum, the scan range was set to 524–538 eV with an energy step of 0.1 eV. Peak fitting was performed on the O 1s spectrum peaks, and the binding energy was calibrated using surface contaminant carbon (C 1s, 284.8 eV) as a reference. Characteristic peaks were identified through Gaussian-Lorentzian mixing function fitting, and the peak area ratio of each component was calculated to analyze the evolution of the surface chemical state.

[0044] Example 1 Loading process: Place the pretreated sample in a sealed space that meets the requirements; Reducing atmosphere treatment: "Nitrogen purging - hydrogen (purity 99.995%) introduction" cycle replacement 3 times, temperature 100℃, treatment for 45 min; Heating under a protective atmosphere: Nitrogen gas (purity 99.999%) is introduced to replace hydrogen gas (residual amount 0.08 vol.%), and the mixture is heated to 150℃±3℃, with the temperature difference controlled within 8℃; Staged oxidation: High-temperature water vapor is introduced, with a relative humidity of 98%; the first stage is a reaction at 180℃ for 1 hour, and the second stage is a reaction at 140℃ for 9 hours. Cooling under protective atmosphere: Introduce nitrogen gas at 30°C; Remove the sample: When the temperature difference between the sample and the ambient temperature is 4°C, open the sealed space and remove the sample.

[0045] Test results: SEM ( Figure 3The brighter spherical particles precipitated on the surface are zinc oxide. Figure 9 Midpoint 1), the gray continuous phase is mainly magnesium oxide ( Figure 9 Midpoint 2), no cracks on the surface; XRD ( Figure 5 The "Example 1" shows that the characteristic peaks of MgZn2 have basically disappeared, while the peak intensity of ZnO is moderate. XPS ( Figure 6 The data shows that the O 1s peak corresponds to oxygen vacancies (accounting for 38.98%). Figure 11 ); Testing revealed that the sample had a brightness value of 31.67, and heat absorption and release tests showed a 6°C temperature difference in heat dissipation compared to the galvanized aluminum-magnesium plate it was treated with. Figure 7 This indicates that it has good heat absorption and release properties.

[0046] Comparative Example 1 Only the substrate selection, pretreatment, staged oxidation (same as the oxidation parameters in Example 1), and natural cooling steps are performed; there is no reducing atmosphere treatment, protective atmosphere heating and cooling.

[0047] Test results: SEM ( Figure 2 The surface shows a discontinuous dark film layer, mainly composed of aluminum oxide formed by Al-rich phase oxidation. Figure 8 In regions 2, 4, and 6, Al oxide film diffusion occurred, but no MgZn2 phase oxidation characteristics were observed. XRD ( Figure 5 The "Comparative Example 1" shows that the characteristic peaks of MgZn2 (PDF#34-0457) and Zn (PDF#01-1238) are clear, and there is no obvious ZnO peak.

[0048] Comparative Example 2 The first stage of the phased oxidation is carried out at 220°C (exceeding 150-200°C), and the remaining steps are the same as in Example 1.

[0049] Test results: SEM ( Figure 4 The surface is covered with a network of cracks, and the bright zinc-rich phase shows obvious cracks. Figure 10 (Midpoint 1, Point 2), this is a brittle film layer caused by excessive oxidation of the Zn-rich phase; XRD ( Figure 5 The "Comparative Example 2" shows that the characteristic peak of MgZn2 disappears, the characteristic peak of Zn is significantly weakened, and the ZnO peak (PDF#06-0664) is strong and sharp.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A manufacturing process for a black-looking steel sheet, characterized in that, Includes the following steps: 1) Substrate selection: Steel plate is selected as the substrate, and the surface of the steel plate is coated with zinc-aluminum-magnesium coating. The zinc-aluminum-magnesium coating contains Al-rich phase, MgZn2 phase and Zn-rich phase, and the oxidation activity of each phase is different. The absolute value of Gibbs free energy change of Al-rich phase is the largest, followed by MgZn2 phase, and Zn-rich phase is the smallest. 2) Loading process: The steel plate with zinc-aluminum-magnesium coating is placed in a sealed space with opening and closing function; 3) Reducing atmosphere treatment: A reducing atmosphere is created in the enclosed space by circulating gas replacement through inert gas purging and reducing gas introduction; 4) Heating treatment under a protective atmosphere: The pressure difference between the inert gas and the reducing atmosphere in the closed space is used to introduce the inert gas to replace the reducing atmosphere; The steel plate is heated under continuous inert protective gas supply. 5) Staged oxidation treatment: High-temperature water vapor is introduced into a sealed space, and the relative humidity is controlled to carry out the reaction in two stages: First stage: preferentially oxidize the Al-rich phase in the coating to form an aluminum oxide film; Second stage: Continue the reaction for a period of time. The total reaction time is adjusted according to the reaction temperature to ensure that the MgZn2 phase in the coating is completely oxidized to form ZnO particles, while avoiding the oxidation of the Zn-rich phase. The criteria for the end of this stage are that the characteristic peaks of the Zn-rich phase in XRD detection have not been significantly weakened, the characteristic peaks of MgZn2 have basically disappeared, and oxygen vacancies are detected by XPS. 6) Cooling treatment under a protective atmosphere: After the reaction is completed, the inert gas is continuously introduced to replace the water vapor in the sealed space by utilizing the pressure difference between the inert gas and the gas in the sealed space. 7) Removal process: When the temperature difference between the steel plate and the ambient air temperature is ≤5℃, open the sealed space and remove the zinc-aluminum-magnesium steel plate with a black oxide film on the surface.

2. The manufacturing process of the black-look steel sheet according to claim 1, characterized in that, In the zinc-aluminum-magnesium coating, the Mg content is 1.00~10.00 wt.%, the Al content is 3.00~18.00 wt.%, and the Zn content is the balance.

3. The manufacturing process of the black-look steel sheet according to claim 1, characterized in that, In step 3), the number of gas replacements is ≥3 times, the purity of the reducing gas is ≥99.99%, the reducing atmosphere treatment temperature is 80~120℃, and the treatment time is 30~60min.

4. The manufacturing process of the black-looking steel sheet according to claim 1, characterized in that, In step 4), the purity of the inert gas is ≥99.999%, and the residual amount of reducing gas after replacement is ≤0.1 vol.%; the average heating temperature of the steel plate is controlled to 150℃±5℃, and the temperature difference between the highest and lowest points of the steel plate is ≤10℃.

5. The manufacturing process of the black-looking steel sheet according to claim 1, characterized in that, In step 5), the temperature in the first stage is controlled at 150~200℃ and the reaction time is 0.5~2h. In the second stage, the temperature is maintained between 110~160℃.

6. The manufacturing process of the black-looking steel sheet according to claim 1, characterized in that, In step 5), high-temperature water vapor is introduced into the sealed space to control the relative humidity to ≥95%.

7. The manufacturing process of the black-look steel sheet according to claim 1, characterized in that, In step 6), the temperature of the inert gas introduced is between 25 and 40°C, and the water vapor content after displacement is ≤5 vol.%.

8. The manufacturing process of the black-looking steel sheet according to claim 1, characterized in that, The sealed space in step 2) and the sealed space in step 5) are the same equipment or compatible independent equipment to ensure that there is no external pollution during the transfer of steel plates; the temperature range of the sealed space is 25~300℃, the pressure range is -0.1~0.3MPa, and the sealing degree of the space gaps meets the requirement that the pressure drop within 30 minutes after gas replacement does not exceed 5%.

9. The manufacturing process of the black-looking steel sheet according to claim 1, characterized in that, The inert gas is nitrogen or argon, and the reducing gas is hydrogen or carbon monoxide.

10. A black-looking steel plate product, characterized in that, The product is prepared using the preparation process described in any one of claims 1 to 9, and the surface brightness value of the product is ≤35, and the oxide film is free of cracks.

Images