Zinc-aluminum-magnesium plated steel sheets and methods for manufacturing them.

Abstract

Invention details;

Description

Zinc-aluminum-magnesium coated steel plate and manufacturing method thereof Technical Field

[0001] The present invention relates to a steel material and a manufacturing method thereof, and in particular to a coated steel plate and a manufacturing method thereof. Background Art

[0002] At present, in order to produce steel plate products with longer service life, the market and users have increasingly higher requirements for the corrosion resistance of steel. Zinc-aluminum-magnesium coated steel plates have been recognized by the market for their excellent corrosion resistance.

[0003] In recent years, zinc-aluminum-magnesium (Zn-Aluminum-Magnesium) coated steel products have emerged on the market, boasting improved corrosion resistance. However, these Zn-Aluminum-Magnesium (Zn-Aluminum-Magnesium) coated steel products are susceptible to "blackening" during production, storage, transportation, and use, affecting both their appearance and performance.

[0004] Currently, there are differences in the technical approaches used to produce zinc-aluminum-magnesium coated steel sheets domestically and internationally. Overseas, the post-treatment coating applied to zinc-aluminum-magnesium coated steel sheets primarily utilizes a two-step process, first passivating the coating surface and then applying the desired coating. The resulting zinc-aluminum-magnesium coated steel sheets exhibit excellent blackening resistance. In contrast, domestic production of zinc-aluminum-magnesium coated steel sheets utilizes a one-step process, applying the desired coating to the coating surface in a single step. This one-step coating process is highly efficient, reducing equipment investment and costs, offering excellent economic benefits and promising application prospects.

[0005] However, the development of this post-treatment membrane and the control of the coating process have encountered some problems in practice. The zinc-aluminum-magnesium coated steel plates produced by the above-mentioned one-time coating process are prone to blackening, especially under conditions of humidity, heat, stacking, rain erosion, etc.

[0006] Summary of the Invention

[0007] In order to solve the above-mentioned problems existing in the prior art, the present invention provides a zinc-aluminum-magnesium coated steel plate, which has excellent blackening resistance, and its blackening resistance under hot and humid conditions is more significant, which can overcome the problem of easy blackening of existing zinc-aluminum-magnesium coated steel plate products, thereby ensuring that the steel plate has a good appearance during long-term use and meets the user's usage needs.

[0008] A first aspect of the present invention provides a zinc-aluminum-magnesium coated steel plate, comprising a steel substrate and a coating on the steel substrate, wherein the surface of the coating is further coated with a fingerprint-resistant film; the mass percentage of each component in the coating is: Al: 45%-65%, Mg: 0.2%-5.0%, Si: 0.1%-3.0%, Zr: 0.001%-0.15%, Ca: 0.001%-0.15%, and the remainder is Zn and other inevitable impurities.

[0009] It is known that the quality of the coating on the surface of the steel substrate directly affects the blackening resistance of the coated steel sheet. Therefore, in the present invention, the inventors have optimized the chemical composition of the coating.

[0010] In the coating of the zinc-aluminum-magnesium coated steel sheet of the present invention, the design principles of the chemical elements are as follows:

[0011] Al: Al is highly oxidizable, and its oxide film is very dense. The resulting aluminum-rich oxide film provides excellent protection for the coating, thereby imparting superior corrosion resistance. Therefore, to maximize the beneficial effects of Al, in the present invention, the mass percentage of Al in the coating is controlled between 45% and 65%.

[0012] Mg: Mg corrodes simultaneously with Zn to form corrosion products. These Mg corrosion products are denser and offer greater protection than those of pure Zn, improving the corrosion resistance of the coating. Therefore, to ensure the corrosion resistance of the coating, the present invention controls the Mg content in the coating to between 0.2% and 5.0% by weight.

[0013] Si: Adding an appropriate amount of Si to the coating of the zinc-aluminum-magnesium coated steel sheet described herein can mitigate the reaction between the substrate and the Al in the molten plating bath during the coating formation process, thereby ensuring substrate flatness and coating uniformity. Therefore, in the present invention, the mass percentage of Si in the coating is controlled between 0.1% and 3.0%.

[0014] Zr: An appropriate amount of Zr is added to the coating of the zinc-aluminum-magnesium coated steel sheet described herein. During the coating formation process, when the molten coating alloy cools, Zr reacts with Al to form Al3Zr particles. These particles participate in the peritectic reaction, refining and increasing the aluminum-rich phase on the steel sheet surface, thereby increasing the Al / Zn ratio of the coating surface and improving the corrosion resistance of the coating. Therefore, to maximize the beneficial effects of Zr, the mass percentage of Zr in the coating is controlled between 0.001% and 0.15%.

[0015] Ca: The role of Ca is to inhibit the formation of oxides on the surface of the molten plating bath, reduce slag production, and thus mitigate surface quality defects in the coated steel sheet caused by plating bath oxides and slag. Therefore, to maximize the beneficial effects of Ca, in the present invention, the mass percentage of Ca in the coating is controlled between 0.001% and 0.15%.

[0016] Preferably, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the mass percentage of each component in the coating satisfies at least one of the following:

[0017] Al: 50%-60%,

[0018] Mg: 1.0%-3.0%,

[0019] Si: 1.0%-2.0%,

[0020] Zr: 0.01%-0.1%,

[0021] Ca: 0.01%-0.1%.

[0022] Preferably, in the zinc-aluminum-magnesium coated steel plate described in the present invention, the thickness of the coating is 8-38 μm.

[0023] In the above technical solution of the present invention, the present invention controls the thickness of the coating to 8-38 μm. When the coating thickness is less than 8 μm, the coating has poor corrosion resistance, poor protection for the substrate, and reduced uniformity of the coating covering the substrate. Producing a coated steel sheet with excellent surface quality places high demands on the substrate and coating thickness control equipment, leading to production difficulties. When the coating thickness exceeds 38 μm, the coating thickness control equipment is very demanding, and it is difficult to ensure coating uniformity, making it difficult to produce zinc-aluminum-magnesium coated steel sheets with good surface quality.

[0024] Preferably, in the zinc-aluminum-magnesium coated steel sheet described herein, the anti-fingerprint film has a thickness of 0.5-2.5 μm. The composition of the anti-fingerprint film is not particularly limited in the present invention and can be a water-soluble resin commonly used in the prior art, such as acrylic resin or polyurethane resin. The coating method of the anti-fingerprint film is also not particularly limited and can be formed by spraying, roller coating, electrophoresis, or other methods. The anti-fingerprint film can enhance the corrosion resistance of the coated steel sheet and improve its appearance.

[0025] In the above-mentioned technical solution of the present invention, the thickness of the fingerprint-resistant film applied to the coating surface is controlled between 0.5 and 2.5 μm. When the thickness of the fingerprint-resistant film is less than 0.5 μm, the coverage of the coated steel sheet by the fingerprint-resistant film is reduced, resulting in localized coating leaks, which affects the corrosion resistance, blackening resistance, and formability of the coated steel sheet. When the thickness of the fingerprint-resistant film exceeds 2.5 μm, the requirements for film thickness control equipment will be very high, the uniformity of the fingerprint-resistant film will be difficult to ensure, the requirements for the subsequent heating and curing process will also be very high, and the conductivity and other properties of the fingerprint-resistant film will also be degraded.

[0026] More preferably, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the thickness of the anti-fingerprint film is 1.0-2.0 μm.

[0027] Preferably, in the zinc-aluminum-magnesium coated steel plate described in the present invention, the surface color difference ΔE of the zinc-aluminum-magnesium coated steel plate before and after the laminate is treated under wet heat conditions for 24 hours is less than 3, wherein ΔE is the difference between the surface brightness value of the zinc-aluminum-magnesium coated steel plate laminate before the wet heat treatment and the surface brightness value after the wet heat treatment for 24 hours.

[0028] In the technical solution described in the present invention, the laminated heat treatment involves stacking the steel plates together and subjecting them to a relatively high temperature (50°C) and relatively high humidity (95%). Under these conditions, tiny gaps exist between the stacked steel plates. Under the combined effects of water vapor and oxygen, the steel plates can undergo electrochemical corrosion. If the steel plates have poor blackening resistance, the surface of the steel plates will corrode, causing partial or complete blackening of the steel plates, resulting in blackening.

[0029] The inventors discovered that if a steel plate's resistance to blackening is poor, it will also blacken during storage or use in the presence of moisture, rain, or condensation. However, this is a long, slow process, and it takes a long time to detect the blackening problem. Furthermore, the inventors discovered that the conditions of wet and hot lamination are relatively harsh, with high temperatures, high humidity, and moisture, as well as the tiny gaps created by the laminations, which favor electrochemical corrosion and exacerbate corrosion on the steel plate's surface. If a steel plate's resistance to blackening is poor, blackening can be detected within 24 hours of lamination under wet and hot conditions, allowing the risk of blackening to be monitored.

[0030] In addition, the inventors also found that when the steel plate undergoes blackening, the brightness value of the steel plate surface will decrease. In the present invention, the color difference ΔE is the difference between the surface brightness value of the steel plate before wet heat treatment and the surface brightness value 24 hours after wet heat treatment. ΔE ≥ 3 indicates that the blackening phenomenon is visible to the naked eye, especially when the steel plate undergoes local blackening. The phenomenon is more obvious.

[0031] The zinc-aluminum-magnesium coated steel plate designed in the present invention has excellent blackening resistance under wet and hot conditions. The surface color difference ΔE of the zinc-aluminum-magnesium coated steel plate before and after being treated under wet and hot conditions for 24 hours is less than 3, that is, no blackening phenomenon visible to the naked eye occurs.

[0032] Correspondingly, another object of the present invention is to provide a method for manufacturing the above-mentioned zinc-aluminum-magnesium coated steel plate, which has a simple process and can effectively prepare the above-mentioned zinc-aluminum-magnesium coated steel plate.

[0033] The present invention relates to a method for manufacturing a zinc-aluminum-magnesium coated steel sheet, comprising the following steps performed in sequence:

[0034] (1) Pre-treating the steel substrate;

[0035] (2) The pretreated steel substrate enters the plating bath and is hot-dip plated in the plating bath to obtain a coated steel plate;

[0036] (3) The coated steel plate leaves the plating solution and is cooled in stages as follows: the first stage is air cooling to cool the steel substrate to below 300°C, with a cooling rate of 10-60°C / s; the second stage is roller cooling or roller cooling + air cooling to cool the steel plate to below 100°C, with a cooling rate of 60-100°C / s; the third stage is water cooling in a quenching tank to room temperature, with the plate temperature of the steel plate entering the quenching tank being below 100°C; the steel plate may be squeezed and dried afterwards;

[0037] (4) coating the anti-fingerprint film on the surface of the coated steel plate and heating and baking it at 100-160° C. to cure it;

[0038] (5) Cooling the coated steel plate to room temperature;

[0039] (6) Rolling the coated steel sheet into a finished coil.

[0040] In the above manufacturing method, the pretreatment in step (1) may include: cleaning the steel substrate to remove dirt and grease on the surface of the steel substrate, and then performing reduction annealing on the cleaned and degreased steel substrate to remove oxides on the surface of the steel substrate.

[0041] In the above step (2), the steel substrate is sent to a plating bath for hot dip plating, wherein the chemical composition of the plating bath is substantially the same as that of the steel substrate coating. By controlling the chemical elements of the plating bath, the chemical elements of the steel substrate coating can be controlled.

[0042] In the above step (3), after the coated steel plate leaves the plating solution, it can first pass through an air knife to blow nitrogen to control the coating thickness of the steel plate within the production requirements, while the coating is cooled to a certain extent. Afterwards, the coated steel plate needs to be cooled in sections, wherein:

[0043] The first stage is air cooling through a wind box to cool the steel plate to below 300°C at a cooling rate of 10-60°C / s;

[0044] The second stage uses roller cooling or roller cooling + air cooling, where the cooling rate of roller cooling is 60-100℃ / s. The second stage cools the steel plate to below 100℃, so that the plate temperature is lower than 100℃ when it enters the quenching tank.

[0045] After the steel plate cools to below 100°C, it undergoes the third stage of cooling, i.e. entering the quenching tank for water cooling.

[0046] When the steel plate enters the quenching tank at a temperature above 100°C, the surface coating will react with water, causing corrosion, affecting the subsequent application of the fingerprint-resistant film. Furthermore, the finished steel plate is more likely to turn black under hot and humid conditions. Therefore, the plate temperature must be kept below 100°C when entering the quenching tank.

[0047] Preferably, the steel plate is squeezed and dried after the staged cooling.

[0048] In addition, in step (4) of the manufacturing method described in the present invention, a fingerprint-resistant film can be coated on the surface of the coated steel plate by roller coating, and the thickness of the fingerprint-resistant film can be specifically controlled according to production requirements; after the fingerprint-resistant film is coated on the surface of the steel plate, it can be heated and baked at 100-160°C by electromagnetic induction heating to solidify the fingerprint-resistant film.

[0049] In the present invention, the heating and baking temperature needs to be controlled between 100-160°C. When the heating and baking temperature is lower than 100°C, the fingerprint-resistant film may not be fully baked and cured, the internal cross-linking and curing of the fingerprint-resistant film may not be thorough, the water-proof performance and other properties are deteriorated, and external water vapor can more easily pass through the fingerprint-resistant film and come into contact with the coating under the film to react, causing the coating surface to be corroded, resulting in the blackening of the coated steel plate. The blackening problem is more likely to occur under conditions such as water ingress or condensation in the steel coil, water ingress in the corrugated plate stack, and damp heat in the laminated sheets. When the heating and baking temperature is higher than 160°C, the fingerprint-resistant film may be overbaked, resulting in the deterioration of the fingerprint-resistant film performance. Moreover, too high a heating and baking temperature places high demands on the cooling section equipment after heating and baking, which will also lead to high energy consumption and increase production costs.

[0050] Preferably, in the manufacturing method of the present invention, in step (2), the temperature of the plating solution is 570-610° C. As an embodiment, in step (3), the second stage is roller cooling + air cooling, wherein the cooling rate of roller cooling is 60-100° C. / s, and the cooling rate of air cooling is 20-30° C. / s.

[0051] When the bath temperature is higher than 610°C, the reaction between the Fe in the steel substrate and the Al in the bath will intensify after the steel substrate enters the bath, which will lead to the thickening of the alloy layer of the main components Al and Fe formed at the interface between the coating and the steel substrate, resulting in the deterioration of the processability of the coated steel sheet. Moreover, when the bath temperature is too high, the dissolution of Fe in the steel substrate in the bath will intensify, and the evaporation of Zn in the furnace nose will also intensify, resulting in an increase in bottom slag and surface slag in the bath, and an increase in zinc ash in the furnace nose, resulting in a decrease in the surface quality of the coated steel sheet, an increase in zinc consumption, and an increase in production costs. In addition, when the bath temperature is too high, the corrosion of the cooling roller by the bath will also be intensified, reducing the life of the roller system equipment and increasing production risks.

[0052] When the plating bath temperature is below 570°C, the fluidity of the bath decreases, making it difficult to control the thickness of the coating, and thus making it difficult to ensure the uniformity and surface quality of the coating. Furthermore, when the bath temperature is too low, after the steel substrate enters the bath, the Fe in the substrate reacts with the Al in the bath to form an incomplete or thin alloy layer at the coating-substrate interface, thereby affecting the adhesion of the coating and the formability of the coated steel sheet. Therefore, considering the influence of the bath temperature, in the manufacturing method of the present invention, it is preferred to control the bath temperature between 570°C and 610°C.

[0053] Preferably, in the manufacturing method of the present invention, in step (4), the heating and baking is carried out at 110-140°C.

[0054] Preferably, in the manufacturing method of the present invention, in step (4), heating and baking are performed by electromagnetic induction heating.

[0055] Preferably, in the manufacturing method of the present invention, in step (5), cooling is performed by air cooling, wherein the cooling rate of the coated steel plate is 5-20°C / s.

[0056] During the cooling process of step (5), the cooling rate of the steel plate is preferably controlled at 5-20°C / s. When the cooling rate is higher than 20°C / s, the requirements for the cooling equipment are very high, and production is difficult to achieve. In addition, when the air cooling rate is too high, the fan flow is very large, the strip shakes significantly, and the accurate measurement of the thickness of the fingerprint-resistant film cannot be achieved, which is not conducive to the stable control of the film thickness. When the cooling rate is lower than 5°C / s, the fingerprint-resistant film cannot be fully cooled, which is not conducive to the aging of the film layer and leads to the degradation of the performance of the fingerprint-resistant film.

[0057] Compared with the prior art, the zinc-aluminum-magnesium coated steel sheet and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0058] The present invention provides a novel zinc-aluminum-magnesium-coated steel sheet with excellent blackening resistance. The steel substrate of the zinc-aluminum-magnesium-coated steel sheet is coated with a coating of optimized composition, and the coating is also coated with a fingerprint-resistant film. After lamination under wet-heat conditions for 24 hours, the surface color difference ΔE is less than 3, demonstrating excellent blackening resistance. This steel sheet overcomes the blackening problem of existing zinc-aluminum-magnesium-coated steel sheet products. The zinc-aluminum-magnesium-coated steel sheet of the present invention maintains a good appearance during long-term use, further enhancing the user experience and promoting the healthy development of zinc-aluminum-magnesium-coated steel sheet products.

[0059] In addition to the above advantages, the present invention also optimizes the design of the manufacturing method of zinc-aluminum-magnesium coated steel plates. The process is simple and can effectively improve the blackening resistance of the steel plates by controlling the post-processing coating process, heating and baking process, and cooling process, thereby ensuring that the prepared zinc-aluminum-magnesium coated steel plates have excellent blackening resistance. DETAILED DESCRIPTION

[0060] The zinc-aluminum-magnesium coated steel plate and the manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0061] Examples A1-A9 and Comparative Examples B1-B4

[0062] The zinc-aluminum-magnesium coated steel sheets of Examples A1-A9 were prepared by the following steps (1)-(6):

[0063] (1) Pretreatment of the steel substrate: A 0.6 mm thick cold-rolled hard steel plate was used as the substrate. After cleaning and degreasing, it was annealed in a N2-5 volume % H2 atmosphere at 770°C for 2 min.

[0064] (2) The pretreated steel substrate enters the plating bath and is hot-dip plated in the plating bath: the steel substrate is immersed in the plating bath for hot-dip plating, and the temperature of the plating bath is controlled to be 570-610° C. After immersion plating for 3 seconds, a coated steel sheet is obtained; wherein, the chemical composition of the plating bath used in each embodiment and comparative example is detailed in Table 1.

[0065] (3) The coated steel plate leaves the plating solution and passes through an air knife. The thickness of the coating is controlled by controlling the air knife blowing intensity, and then it is cooled in sections: the first section uses a bellows to cool the steel plate to below 300°C, with a cooling rate of 10-60°C / s; the second section uses roller cooling or roller cooling plus air cooling to cool the steel plate to below 100°C, with a cooling rate of 60-100°C / s for roller cooling and 20-30°C / s for air cooling; the third section uses water cooling in a quenching tank. The plate temperature when entering the quenching tank is below 100°C, and the final cooling temperature is room temperature. It should be noted that Comparative Examples B1-B3 only use air cooling and water cooling. The process of Comparative Example B4 meets the requirements of the present invention.

[0066] (4) The coated steel plate is coated with a fingerprint-resistant film by a roller coater, and the thickness of the fingerprint-resistant film is controlled by controlling the rotation speed and roller spacing of the roller coater; after the fingerprint-resistant film is coated on the surface of the coated steel plate, the steel plate passes through an induction heater, and the fingerprint-resistant film is heated and baked at 100-160°C using electromagnetic induction heating.

[0067] (5) The steel plate is cooled to room temperature by air cooling with a fan, and the cooling rate of the steel plate is controlled at 5-20℃ / s.

[0068] (6) When the steel plate cools to room temperature or close to room temperature, it is curled into a finished steel coil.

[0069] The specific parameters of the above process are listed in Table 2.

[0070] It should be noted that the present invention does not impose any particular restrictions on the steel substrate used. In practical applications, those skilled in the art can select steel substrates based on their needs, such as commonly used DC51 and S350 steel plates. The specific type of steel substrate has no direct correlation with the excellent blackening resistance of the coating formed on its surface.

[0071] Table 1 lists the plating solutions used for the zinc-aluminum-magnesium coated steel sheets of Examples A1-A9 and the comparative steels of Comparative Examples B1-B4, as well as the percentage by weight of the chemical elements in the resulting coatings. In the present invention, the coating is formed by cooling the plating solution, and its composition is substantially consistent with that of the plating solution.

[0072] Table 1. (wt.%, balance is Zn and other inevitable impurities)

[0073] Table 2 lists the specific process parameters used in the above process steps (1) to (6) for the zinc-aluminum-magnesium coated steel sheets of Examples A1 to A9 and the comparative steels of Comparative Examples B1 to B4.

[0074] Table 2. Note: In Table 2, “Air cooling rate after roller cooling in the second stage of segmented cooling” is not “0”, which means that roller cooling and air cooling are used in the second stage of cooling. “Air cooling rate after roller cooling in the second stage of segmented cooling” is “0”, which means that only roller cooling is used in the second stage of cooling.

[0075] The finished steel coils of Examples A1-A9 and Comparative Examples B1-B4 were sampled separately, and the samples of the steel coils of each Example and Comparative Example were simultaneously subjected to a moisture and heat resistance test under the same conditions. The steel plate laminates were treated under moisture and heat conditions for 24 hours. The lightness values ​​and color difference ΔE of the coated steel plate surfaces of each Example and Comparative Example before and after the test are listed in Table 3.

[0076] The specific procedure for the wet heat resistance test is as follows: Before the test, the lightness value of the steel plates to be tested is measured. The steel plates are then stacked together and clamped with a preload of 30 Newton meters (N·m). The secured steel plates are then placed in a wet heat chamber at 50°C and 95% humidity for 24 hours. The stacked steel plates are then removed and the lightness value of the steel plates' surfaces is measured again. This test effectively detects changes in the lightness value of the steel plates in Examples A1-A9 and Comparative Examples B1-B4 before and after the 24-hour wet heat treatment.

[0077] In addition, after the above test, the surface of the steel plate of each embodiment and comparative example was further visually observed to confirm whether the surface of the steel plate was blackened, that is, whether "blackening" occurred: if "blackening" occurred, it was recorded as "yes"; if "blackening" did not occur, it was recorded as "no", indicating that the steel plate still has good blackening resistance under hot and humid conditions. The relevant results are listed in Table 3 below.

[0078] Table 3 lists the results of the moisture and heat resistance tests of the finished steel coils of Examples A1-A9 and Comparative Examples B1-B4.

[0079] Table 3. Note: In Table 3, ΔE is the difference between the surface brightness value of the steel plate laminate before wet heat treatment and the surface brightness value of the laminate after wet heat treatment for 24 hours.

[0080] As shown in Table 3, under identical conditions, after the wet heat resistance test, the zinc-aluminum-magnesium-coated steel sheets of the present invention exhibited higher lightness values ​​than the control steel sheets, with surface lightness values ​​ranging from 68.49 to 72.08. Furthermore, the surface color differences ΔE of the zinc-aluminum-magnesium-coated steel sheets of Examples A1-A9 before and after the wet heat treatment for 24 hours ranged from 0.16 to 2.35, all less than 3, and no surface blackening was observed.

[0081] Comparative Examples B1-B4, however, showed a surface color difference ΔE greater than 3 before and after 24 hours of wet heat treatment. Furthermore, after the wet heat resistance test, the surfaces of the comparative steel plates in Comparative Examples B1-B4 turned black, exhibiting a blackening problem. Comparative Examples B1-B4 did not use the plating solution of the present invention to form the steel plate coatings, resulting in poor wet heat resistance and blackening resistance.

[0082] In summary, the zinc-aluminum-magnesium coated steel sheet produced using the technical solution of the present invention exhibits excellent blackening resistance, particularly under hot and humid conditions. The zinc-aluminum-magnesium coated steel sheet of the present invention has a wide range of applications, effectively meeting current market and user needs, and has promising promotional prospects and application value.

[0083] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0084] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.