Enamel steel sheet and method for manufacturing the same

Abstract

A steel sheet for enamel according to an embodiment of the present application, the steel sheet comprising, in weight %, C: 0.0005 to 0.0030 %, Mn: 0.15 to 0.55 %, Si: 0.001 to 0.03 %, Al: 0.0001 to 0.002 %, P: 0.001 to 0.02 %, S: 0.001 to 0.03 %, Cu: 0.02 to 0.06 %, N: 0.005 to 0.012 %, Cr: 0.05 to 0.20 %, O: 0.03 to 0.06 %, with the balance comprising Fe and unavoidable impurities, comprising an oxide layer in a direction from the surface to the inside, the oxide layer having a thickness of 0.006 to 0.030 µm.

Description

Technical Field

[0001] One embodiment of the present invention relates to a steel sheet for enamel enamel and a method for manufacturing the same. More specifically, one embodiment of the present invention relates to a continuously annealed steel sheet for enamel enamel assembling with excellent enamel adhesion and anti-scaling properties, and excellent yield strength after enamel treatment, and a method for manufacturing the same. Background Technology

[0002] Enameled steel sheet is a surface-treated product made by coating a base steel sheet (e.g., hot-rolled or cold-rolled steel sheet) with a glass enamel coating and then sintering it at high temperatures, thereby improving its corrosion resistance, weather resistance, and heat resistance. This type of enamel-lined steel sheet is used in building exteriors, home appliances, tableware, and various industrial materials.

[0003] For a long time, rimmed steel has been used for enamel steel sheets, but recently continuous casting has been actively adopted to improve productivity, so most materials are now being continuously cast. Furthermore, one of the most fatal defects in steel manufacturing for enamel steel sheets is fishscale, a typical enamel defect caused by hydrogen dissolved in the steel being released from a supersaturated state to the steel surface during cooling after sintering, resulting in the enamel layer peeling off in a fish-scale pattern. If such fishscale defects occur, rust will concentrate at the defective sites, significantly reducing the value of the enamel product; therefore, it is necessary to suppress the formation of fishscale defects. To prevent fishscale defects, numerous sites need to be formed inside the steel to accommodate hydrogen dissolved in the steel. Therefore, to prevent fishscale defects that reduce enamel properties or improve aging performance, open coil annealing (OCA), one of the batch annealing methods, is also used. However, in this case, productivity decreases, manufacturing costs increase, and there are significant quality deviations due to the long heat treatment time. Furthermore, the loose-coil annealing method is problematic because it's difficult to control the amount of decarburization. Excessive decarburization, coupled with insufficient carbon in the steel, softens the grain boundaries, leading to cracking and brittle fracture during product forming. To overcome the reduced productivity and increased manufacturing costs caused by prolonged annealing, recently developed enamel steel sheets actively utilize continuous annealing processes. In this process, precipitates such as titanium or inclusions in undeoxidized steel are primarily used as hydrogen adsorption sources. However, even under these conditions, the surface defect rate is high due to the addition of numerous carbonitride-forming elements or undeoxidized compounds. Moreover, the increased recrystallization temperature leads to various quality issues, such as decreased sheet permeability, contributing to both reduced productivity and increased costs.

[0004] In other words, because titanium (Ti)-based enamel steel plates incorporate large amounts of titanium to suppress hydrogen reactions that cause scaling, sprue blockage due to titanium nitrides (TiN) and inclusions frequently occurs during the continuous casting process in steelmaking, directly contributing to decreased operability and reduced production load. Furthermore, the TiN mixed in with the molten steel exists in the upper part of the steel plate, not only causing blister defects (a typical bubble defect), but the large amount of added titanium also impairs the adhesion between the steel plate and the enamel layer.

[0005] Furthermore, in the case of enamel steel, since it is primarily used as a material for structural components, its competitiveness is enhanced by increasing its strength through lightweighting. Therefore, in the enamel process, it is necessary to ensure the yield strength after sintering heat treatment for drying the enamel. Summary of the Invention

[0006] Technical issues

[0007] One embodiment of the present invention aims to provide a steel sheet for enamel enamel and a method for manufacturing the same. More specifically, one embodiment of the present invention aims to provide a continuously annealed steel sheet for enamel enamel processing that does not produce bubble defects after enamel treatment and has excellent enamel adhesion and anti-scaling properties, as well as a method for manufacturing the same.

[0008] Technical solution

[0009] According to an embodiment of the present invention, the enamel steel sheet comprises, by weight %, C: 0.0005 to 0.0030%, Mn: 0.15 to 0.55%, Si: 0.001 to 0.03%, Al: 0.0001 to 0.002%, P: 0.001 to 0.02%, S: 0.001 to 0.03%, Cu: 0.02 to 0.06%, N: 0.005 to 0.012%, Cr: 0.05 to 0.20%, O: 0.03 to 0.06%, with the balance comprising Fe and unavoidable impurities.

[0010] According to an embodiment of the present invention, the enamel steel sheet includes an oxide layer from the surface inward, the oxide layer having a thickness of 0.006 to 0.030 μm.

[0011] According to an embodiment of the present invention, the enamel steel plate satisfies the following formula 1.

[0012] [Formula 1]

[0013] 3.05≤[Cu] / [P]≤5.10

[0014] (In Formula 1, [Cu] and [P] represent the content (by weight%) of Cu and P elements, respectively.

[0015] According to an embodiment of the present invention, the enamel steel plate satisfies the following formula 2.

[0016] [Equation 2]

[0017] 0.032≤([Cr] / 52+[Mn] / 32)×([N] / 14) / ([C] / 12)≤0.091

[0018] (In Equation 2, [Cr], [Mn], [N], and [C] represent the content (by weight%) of the elements Cr, Mn, N, and C, respectively.)

[0019] It also contains one or more of the following: Ti: less than 0.001 wt%, Nb: less than 0.001 wt%, Ni: less than 0.02 wt%, V: less than 0.001 wt%, and Mo: less than 0.02 wt%.

[0020] According to an embodiment of the present invention, the enamel steel plate satisfies the following formula 3.

[0021] [Formula 3]

[0022] 0.45≤(Ra×50×Se) / Pc≤0.99

[0023] (In Equation 3, Pc represents the number of surface bumps per unit centimeter (cm), Ra represents the average roughness value (μm), and Se represents the quenching and tempering reduction rate (%).)

[0024] According to an embodiment of the present invention, the yield strength of the enamel-lined steel plate after enamel heat treatment is 220 MPa or higher.

[0025] According to an embodiment of the present invention, the enamel-coated steel sheet has an enamel adhesion of 95% or more.

[0026] According to an embodiment of the present invention, the enamel-coated steel sheet has a hydrogen permeability of 600 seconds / mm. 2 above.

[0027] A method for manufacturing an enamel steel sheet according to an embodiment of the present invention comprises: hot rolling a slab to manufacture a hot-rolled steel sheet, wherein the slab comprises, by weight %, C: 0.0005 to 0.0030%, Mn: 0.15 to 0.55%, Si: 0.001 to 0.03%, Al: 0.0001 to 0.002%, P: 0.001 to 0.02%, S: 0.001 to 0.03%, Cu: 0.02 to 0.06%, N: 0.005 to 0.012%, Cr: 0.05 to 0.20%, O: 0.03 to 0.06%, with the balance comprising Fe and unavoidable impurities; cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; annealing the cold-rolled steel sheet; and temper rolling the annealed cold-rolled steel sheet.

[0028] The annealing process is carried out at 760°C to 840°C for 10 to 90 seconds.

[0029] In the process of manufacturing hot-rolled steel sheets, the final hot rolling temperature is between 910°C and 970°C.

[0030] In the process of manufacturing hot-rolled steel sheets, the coiling temperature is between 580°C and 720°C.

[0031] In the process of manufacturing cold-rolled steel sheets, cold rolling is performed at a reduction rate of 60 to 90%.

[0032] The quenching and tempering rolling process involves rolling with a reduction rate of 0.4% to 2.0%.

[0033] After the tempering and rolling step, the process also includes an enamel sintering step at a temperature of 780°C to 850°C.

[0034] Invention Effects

[0035] According to an embodiment of the present invention, the enamel-coated steel sheet has excellent enamel adhesion and anti-scaling properties.

[0036] Furthermore, according to an embodiment of the present invention, the enamel steel sheet maintains high adhesion by optimizing the surface roughness characteristics during the heat treatment and tempering rolling steps in the continuous annealing furnace after cold rolling.

[0037] In addition, according to an embodiment of the present invention, the enamel steel sheet prevents fishscale defects caused by hydrogen by forming Mn-Cr-based precipitates at high temperature and using them as a hydrogen adsorption source.

[0038] In addition, according to an embodiment of the present invention, the enamel steel plate inhibits grain growth during the enamel sintering process by the residual nitrogen in the surface layer of the steel plate, thereby ensuring stable material properties after high-temperature sintering. Attached Figure Description

[0039] Figure 1 This is a schematic cross-sectional view of an enamel steel plate according to an embodiment of the present invention.

[0040] Figure 2 It is based on the GDS (Glow Discharge Spectroscopy) analysis results of different depths of the enamel steel plate of Invention Example 3. Detailed Implementation

[0041] In this specification, the terms "first," "second," "third," etc., are used to describe various parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, and / or segment from another part, component, region, layer, and / or segment. Therefore, without departing from the scope of the invention, the first part, component, region, layer, and / or segment described below can also be described as a second part, component, region, layer, and / or segment.

[0042] In this specification, when a part is described as "containing" a certain component, it means, unless specifically stated to the contrary, that it may also contain other components, and does not exclude other components.

[0043] The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, the singular forms used herein are intended to include the plural forms as well. The word "comprising" as used in this specification can specifically refer to a particular feature, field, integer, step, action, element, and / or component, but does not exclude the presence or addition of other features, fields, integers, steps, actions, elements, components, and / or groups.

[0044] In this specification, "the combination of them" in the Markush form means a mixture or combination of one or more of the constituent elements described in the Markush form, implying that it includes one or more of the constituent elements described above.

[0045] In this specification, if a part is described as being on top of another part, then other parts may exist directly on top of or in between the other part. When a part is described as being directly on top of another part, there are no other parts in between.

[0046] Although not otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries should be interpreted as having the same meaning as disclosed in relevant technical literature and herein, and should not be interpreted in an idealized or overly formal sense.

[0047] Additionally, unless otherwise specified, % indicates weight, and 1 ppm is 0.0001 wt%.

[0048] In one embodiment of the present invention, the inclusion of an additional element means that a portion of the remaining iron (Fe) is replaced by an additional element, the amount of which is equivalent to the amount of the additional element added.

[0049] Embodiments of the present invention will be described in detail below to enable those skilled in the art to implement the invention. However, the invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0050] According to an embodiment of the present invention, the enamel steel sheet comprises, by weight %, C: 0.0005 to 0.0030%, Mn: 0.15 to 0.55%, Si: 0.001 to 0.03%, Al: 0.0001 to 0.002%, P: 0.001 to 0.02%, S: 0.001 to 0.03%, Cu: 0.02 to 0.06%, N: 0.005 to 0.012%, Cr: 0.05 to 0.20%, O: 0.03 to 0.06%, with the balance comprising Fe and unavoidable impurities.

[0051] First, explain the reasons for restricting the composition of the steel plate.

[0052] The elemental content in the final steel plate can have a concentration gradient in the thickness direction, and the elemental content, which will be described later, represents the average content in the entire steel plate 100 including the oxide layer 20.

[0053] C: 0.0005 to 0.0030% by weight

[0054] When excessive carbon (C) is added, the increased amount of dissolved carbon in the steel hinders the development of texture during cold rolling and annealing, resulting in poor formability and blister defects caused by blistering in the enamel layer. On the other hand, if too little C is added, the target yield strength cannot be guaranteed after the microstructure grows and sinters. The fraction of carbides that serve as hydrogen adsorption sites in the steel becomes lower, making it prone to scaling defects. More specifically, it can contain 0.00010 to 0.0028% by weight of carbon.

[0055] Mn: 0.15 to 0.55% by weight

[0056] Manganese (Mn), as a typical solid solution strengthening element, causes dissolved sulfur in steel to precipitate as manganese sulfide (MnS), thereby preventing hot shortness and promoting carbide precipitation. If the amount of Mn added is too small, the aforementioned effects are difficult to achieve. On the other hand, if the Mn content is too large, it leads to poor formability and lowers the Ar3 transformation temperature, potentially causing deformation during enamel sintering. Therefore, Mn can be contained in the form of 0.15 to 0.55% by weight. More specifically, Mn can be contained in the form of 0.20 to 0.55% by weight.

[0057] Si: 0.001 to 0.03% by weight

[0058] Silicon (Si) is an element that promotes the formation of carbides that act as a hydrogen adsorption source. If too little Si is added, the aforementioned effects are difficult to achieve. On the other hand, if too much Si is added, the formation of an oxide film on the steel plate surface may reduce enamel adhesion. Therefore, Si can be contained in quantities of 0.001 to 0.030% by weight. More specifically, it can be contained in quantities of 0.005 to 0.025% by weight.

[0059] Al: 0.0001 to 0.002% by weight

[0060] Aluminum (Al) acts as a strong deoxidizer in the steelmaking process to remove oxygen from molten steel and is also an element that fixes dissolved nitrogen. Since precipitates and inclusions in the steel are needed as hydrogen adsorption sources, potential deoxidation can be suppressed. Therefore, Al can be contained in quantities of 0.03 to 0.12% by weight. More specifically, it can be contained in quantities of 0.035 to 0.115% by weight. The upper limit of Al can be limited to 0.0020% by weight. It is preferable to contain as little Al as possible, thus the lower limit can be limited to 0.0001% by weight. More specifically, it can be contained in quantities of 0.0005 to 0.0015% by weight of Al.

[0061] P: 0.001 to 0.020% by weight

[0062] Phosphorus (P) is a typical material strengthening element. If too little P is added, the aforementioned effects are difficult to achieve. On the other hand, if too much P is added, it forms a segregation layer inside the steel sheet, which not only reduces formability but also worsens the pickling properties of the steel and may adversely affect enamel adhesion. Therefore, P can be contained in amounts from 0.001 to 0.020% by weight. More specifically, it can be contained in amounts from 0.005 to 0.015% by weight.

[0063] S: 0.001 to 0.030% by weight

[0064] Sulfur (S) is an element that combines with manganese to cause red-hot brittleness. If too little S is added, it can lead to deterioration of weldability. If too much S is added, it will significantly reduce ductility, resulting in poor workability, and may also adversely affect the scaling properties due to excessive manganese sulfide precipitation. Therefore, S can be contained in quantities of 0.001 to 0.030% by weight. More specifically, it can be contained in quantities of 0.005% to 0.025% by weight.

[0065] Cu: 0.020 to 0.060% by weight

[0066] Cu is an element added for solid solution strengthening and improving enamel adhesion. If too little Cu is added, the aforementioned effects cannot be fully achieved. If too much Cu is added, the pickling rate decreases in the acid treatment step, which is a pre-enamel treatment step, resulting in inadequate steel surface roughness characteristics and thus reduced adhesion. Therefore, it can contain 0.020 to 0.060% by weight of Cu. More specifically, it can contain 0.025 to 0.055% by weight.

[0067] N: 0.005 to 0.012% by weight

[0068] Nitrogen (N) is a typical hardening element added to achieve the target yield strength after enamel sintering. If the N content is too low, the yield strength of the enamel after sintering may be poor. However, if the amount added is increased, aging defects occur frequently, and formability deteriorates, potentially leading to bubble defects during the enamel finishing process. Therefore, 0.0050 to 0.0120% by weight of N may be included. More specifically, 0.0075 to 0.0110% by weight of N may be included.

[0069] Cr: 0.05 to 0.20% by weight

[0070] Chromium (Cr) is an effective element for improving strength and resistance to blistering in steel by forming precipitates and inclusions. If the amount of Cr added is too small, these effects cannot be fully achieved. If the Cr content is too high, it may concentrate on the surface, reducing enamel adhesion and increasing costs due to the addition of expensive ferroalloys. Therefore, the Cr content can be from 0.050 to 0.200% by weight. More specifically, it can contain 0.075 to 0.190% by weight.

[0071] O: 0.03 to 0.06% by weight

[0072] Oxygen (O) is an essential element for the formation of oxides, which serve as effective hydrogen adsorption sources to improve resistance to flaking. Insufficient O content will not provide adequate benefits. Excessive O content can lead to refractory material loss during steel plate manufacturing and may increase surface defects such as blacklines. Therefore, an O content of 0.0300 to 0.0600% by weight is recommended.

[0073] Regarding the manufacturing process described later, an oxide layer 20 can be formed during the final annealing process. However, the thickness of the oxide layer 20 is very thin relative to the overall steel plate 100, so the oxygen content in the overall steel plate 100 does not change substantially. The oxide layer 20 contains more than 5% by weight of oxygen. More specifically, the oxide layer 20 may contain 10 to 50% by weight of O. The oxygen content in the oxide layer 20 refers to the average content in the oxide layer 20.

[0074] According to an embodiment of the present invention, the enamel steel plate satisfies the following formula 1.

[0075] [Formula 1]

[0076] 3.05≤[Cu] / [P]≤5.10

[0077] In Formula 1, [Cu] and [P] represent the content (by weight%) of Cu and P elements, respectively.

[0078] If the value of Equation 1 is too low, the wedge effect may be reduced due to the inability to ensure appropriate surface properties during the pretreatment step, leading to poor enamel adhesion. Conversely, if the value of Equation 1 is too high, the surface roughness characteristics disappear, the enamel glaze flows downward, and the gas flowing into the surface increases, resulting in frequent enamel surface defects such as bubble defects, which may become a factor reducing product reliability.

[0079] Therefore, in order to ensure enamel adhesion and suppress surface bubble defects, the value of Formula 1 can be limited to 3.05 to 5.10. More specifically, the value of Formula 1 can be 3.20 to 5.00.

[0080] According to an embodiment of the present invention, the enamel steel plate satisfies the following formula 2.

[0081] 0.032≤([Cr] / 52+[Mn] / 32)×([N] / 14) / ([C] / 12)≤0.091

[0082] In Equation 2, [Cr], [Mn], [N] and [C] represent the content (by weight%) of the elements Cr, Mn, N and C, respectively.

[0083] Chromium and manganese in steel can react with carbon, nitrogen, sulfur, etc., to form carbonitrides or act as a composite precipitation source for these precipitates, thus improving processability and serving as a hydrogen adsorption source. Therefore, it is necessary to comprehensively consider the reactivity with carbon and nitrogen, as well as that of each element. This limits the value of Equation 2. If the value of Equation 2 is too low, the processability may deteriorate as the amount of residual solid solution elements in the steel increases. If the value of Equation 2 is too high, it will not only deteriorate the rolling and annealing properties but may also increase manufacturing costs. Therefore, the value of Equation 2 can be between 0.0320 and 0.0910. More specifically, it can be between 0.033 and 0.089.

[0084] In addition to the components described above, this invention contains Fe and unavoidable impurities, and does not exclude the addition of effective components other than those described above. Unavoidable impurities include, for example, Ti, Nb, Ni, V, and Mo. In one embodiment of this invention, Ti, Nb, Ni, V, and Mo are not intentionally added. It may contain one or more of the following: Ti: less than 0.001 wt%, Nb: less than 0.001 wt%, Ni: less than 0.02 wt%, V: less than 0.001 wt%, and Mo: less than 0.02 wt%.

[0085] Figure 1 The diagram shows a cross-sectional schematic of a steel plate for enamel application according to an embodiment of the present invention. Figure 1 As shown, an oxide layer 20 extends from the surface of the steel plate inwards. The oxide layer 20 contains more than 5% by weight of oxygen (O), which distinguishes it from the steel plate substrate 10, which has an oxygen (O) content of less than 5% by weight. Specifically, when analyzing the oxygen concentration from the surface inwards for a cross-section of the steel plate, the oxide layer 20 and the substrate 10 are distinguished based on the location where the oxygen content is 5% by weight. If there are multiple locations with an oxygen content of 5% by weight, the innermost location is used as the distinguishing point.

[0086] The oxide layer 20 may contain more than 90% by weight of Fe oxide.

[0087] Enameled products are made by coating steel plates with organic enamel, so ensuring good adhesion between the steel plate and the enamel is crucial. Typically, the main component of the enamel is silicon dioxide (SiO2). To prevent reduced adhesion to the steel plate, expensive enamels containing large amounts of NiO or similar compounds are often used.

[0088] In one embodiment of the present invention, repeated experiments confirmed that a method to improve enamel adhesion can be achieved by controlling the thickness of the oxide layer on the surface of the steel plate. By controlling the thickness of the oxide layer, which is mainly composed of FeO, within a certain range, covalent bonding with silicon (Si) atoms in the enamel layer is promoted, thereby improving enamel adhesion. For this purpose, the oxide layer thickness needs to be controlled between 0.006 and 0.030 μm. If the oxide layer thickness is too thin, the bonding force between the enamel layer and the steel plate is reduced, making it difficult to ensure enamel adhesion. On the other hand, if the oxide layer thickness is too thick, although it is beneficial to adhesion, there is a problem of deterioration in the surface properties of the steel plate. Therefore, the thickness of the oxide layer 20 on the surface of the steel plate is limited to 0.006 to 0.030 μm. More specifically, the thickness of the oxide layer 20 can be between 0.007 and 0.028 μm. The thickness of the oxide layer 20 may vary on the entire steel plate 100; in one embodiment of the present invention, the thickness of the oxide layer 20 refers to the average thickness relative to the entire steel plate 100.

[0089] According to an embodiment of the present invention, the enamel steel plate satisfies the following formula 3.

[0090] [Formula 3]

[0091] 0.45≤(Ra×50×Se) / Pc≤0.99

[0092] (In Equation 3, Pc represents the number of surface bumps per unit centimeter (cm), Ra represents the average roughness value (μm), and Se represents the quenching and tempering reduction rate (%).)

[0093] If the value of Equation 3 is too small, the wedge effect on the steel plate surface may be reduced, resulting in poorer adhesion to the enamel. On the other hand, if the value of Equation 3 is too large, it will be difficult to obtain the target material and enamel properties due to grain growth in the steel plate after the enamel sintering process. More specifically, the value of Equation 3 can be between 0.4600 and 0.9500.

[0094] As described above, the high-strength enamel steel sheet with excellent adhesion according to the present invention has excellent strength characteristics and excellent enamel adhesion.

[0095] Specifically, according to one embodiment of the present invention, a high-strength enamel steel sheet with excellent adhesion can have a yield strength of over 220 MPa after enamel firing heat treatment. The yield strength of materials used in structural components is a physical property affecting the component's resistance to denting and shape freezing, and is typically measured by tensile testing. For enamel products, the yield strength at the processing entry point is also important for products manufactured and supplied by steel manufacturers. However, due to the nature of the product, a high-temperature firing heat treatment is required after enamel glaze treatment for drying. The heat treatment can vary depending on the type of glaze used, but can be carried out at a temperature of 780 to 850°C for 15 minutes. As mentioned above, the properties of enamel products are limited to over 220 MPa because the yield strength after heat treatment during enamel processing is a major factor in verifying product stability. Since the yield strength measured by conventional tensile testing methods is subject to slight variations depending on test conditions, in this evaluation, the crosshead speed, representing the tensile speed per unit time, is 10 mm / min. The yield strength obtained after heat treatment of enamel firing can be above 220 MPa, more specifically, above 225 MPa. There is no particular upper limit to the yield strength, but it can be up to 350 MPa.

[0096] According to one embodiment of the present invention, the enamel adhesion of the steel sheet can be 95% or higher. By satisfying this property, it can be used as an enamel material even using relatively inexpensive enamels. If the enamel adhesion is excessively reduced, the enamel layer will peel off during distribution or handling after enamel processing, reducing its marketability as an enamel material. Therefore, enamel manufacturers, considering stability, use expensive enamels with large amounts of components such as NiO, thus increasing costs. Therefore, efforts are being made to develop a scheme that ensures enamel adhesion even when using inexpensive enamels. Generally, an enamel adhesion of 90% or higher is considered optimal for enamel products, but one embodiment of the present invention proposes a scheme that ensures an enamel adhesion of 95% or higher. Furthermore, if the enamel adhesion decreases, the rate of hydrogen-induced scaling in the steel will also increase; therefore, it is preferable to ensure the highest possible adhesion. In the present invention, excellent enamel adhesion of 95% or higher is also ensured in terms of adhesion characteristics and scaling control. More specifically, the enamel adhesion can be 96% or higher. Enamel adhesion refers to the electrical conductivity of an area after a steel ball has been applied to the enamel layer under a certain load, as defined in ASTM C313-78, and is expressed as an index of the degree of enamel enamel layer peeling off. There is no specific upper limit to enamel adhesion, but it can be 100%.

[0097] According to one embodiment of the present invention, the hydrogen permeability of the enamel-coated steel sheet can be 600 seconds / mm. 2The above. Hydrogen permeability is a typical index for evaluating resistance to blistering, which represents the resistance of enamel steel made from cold-rolled steel sheet according to an embodiment of the present invention to the fatal defect of blistering. The ability of the steel sheet to fix hydrogen is evaluated by a method included in European Standard (EN10209). This is measured by the time (t) it takes for hydrogen to permeate from one direction of the steel sheet to the opposite direction after hydrogen is generated in that direction. s The value is expressed as t (the unit is seconds) divided by the square of the material thickness (t, unit is mm). s / t 2 (Unit: seconds / mm) 2 If the hydrogen permeability is too low, the defect rate exceeds 50% when evaluating the resistance to blistering defects after enamel treatment and accelerated heat treatment at 200℃ for 24 hours, posing a problem for use as a stable enamel product. Therefore, to ensure steel plates with excellent resistance to blistering, the hydrogen permeability needs to be controlled at 600 seconds / mm². 2 That's all. Furthermore, more specifically, the hydrogen permeability can be 610 seconds / mm. 2 That's all. At this point, there's no specific upper limit to the hydrogen permeability, but it can be as low as 1700 seconds / mm. 2 .

[0098] A method for manufacturing an enamel steel sheet according to an embodiment of the present invention comprises: hot rolling a slab to manufacture a hot-rolled steel sheet, wherein the slab comprises, by weight percent, C: 0.0005 to 0.0030%, Mn: 0.15 to 0.55%, Si: 0.001 to 0.03%, Al: 0.0001 to 0.002%, P: 0.001 to 0.02%, S: 0.001 to 0.03%, Cu: 0.02 to 0.06%, N: 0.005 to 0.012%, Cr: 0.05 to 0.20%, O: 0.03 to 0.06%, with the balance being Fe and unavoidable impurities; cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; annealing the cold-rolled steel sheet; and temper rolling the annealed cold-rolled steel sheet.

[0099] First, prepare a slab that meets the aforementioned composition requirements. For molten steel whose composition has been adjusted to the aforementioned composition during the steelmaking process, a slab can be produced by continuous casting. The alloy composition of the slab is essentially the same as that of the aforementioned enamel steel plate. The alloy composition has already been described previously, so a repetition is omitted.

[0100] The manufactured slab is then heated. Heating facilitates subsequent hot rolling processes and homogenizes the slab. More specifically, heating can be reheating.

[0101] At this point, the slab heating temperature can be between 1150 and 1280°C. If the slab heating temperature is too low, the rolling load in the subsequent hot rolling process will increase sharply, which may lead to poor operability. On the other hand, if the slab heating temperature is too high, not only will energy costs increase, but the amount of surface oxide scale will also increase, which may lead to material loss. More specifically, it can be between 1180 and 1260°C.

[0102] Then, the heated slab is hot-rolled to produce hot-rolled steel sheets.

[0103] At this point, the final hot rolling temperature can be between 910 and 970°C. If the final hot rolling temperature is too low, grain mixing will occur rapidly due to ending rolling in the low-temperature region, potentially leading to reduced rollability and processability. On the other hand, if the final hot rolling temperature is too high, the peelability of the surface oxide scale will decrease, and the hot rolling will not be uniform across the entire thickness, which may result in grain growth and reduced impact toughness. More specifically, the final hot rolling temperature can be between 920 and 960°C.

[0104] Then, the hot-rolled steel sheet produced after hot rolling will undergo a coiling process. More specifically, it can be a hot-rolled coiling process.

[0105] At this point, the coiling temperature can be between 580 and 720°C. The hot-rolled steel sheet can be cooled on the run-out table (ROT) before coiling. If the hot-rolling coiling temperature is too low, uneven temperature distribution in the width direction occurs during the cooling and holding process, leading to changes in the formation of low-temperature precipitates. This not only causes material deviations but also negatively impacts enamel properties. On the other hand, if the coiling temperature is too high, the formation of carbides reduces corrosion resistance and promotes grain boundary segregation of phosphorus, resulting in reduced cold rollability and coarser microstructure in the product, leading to poorer processability. More specifically, the coiling temperature can be between 590 and 710°C.

[0106] Before cold rolling the coiled hot-rolled steel sheet, a pickling step may be included.

[0107] Then, the hot-rolled steel sheet is cold-rolled into cold-rolled steel sheet.

[0108] At this point, the cold rolling reduction rate can be 60% to 90%. If the cold rolling reduction rate is too low, the recrystallization driving force in the heat treatment process is not ensured, resulting in localized residual unrecrystallized grains, leading to increased strength but significantly reduced processability. Furthermore, due to the reduced breakage ability of carbides formed during the hot rolling step, the number of sites capable of adsorbing hydrogen decreases, making it difficult to ensure resistance to scaling. Moreover, considering the final product thickness, the thickness of the hot-rolled plate needs to be reduced, thus worsening rolling operability. On the other hand, if the cold rolling reduction rate is too high, material hardening not only worsens processability but also increases the load on the rolling mill, resulting in decreased operability. More specifically, the cold rolling reduction rate can be 63% to 88%.

[0109] Enameled steel sheets are then manufactured by continuously annealing the cold-rolled steel sheets. Cold-rolled materials, due to the high deformation during cold rolling, have high strength but extremely poor workability; therefore, heat treatment is performed in the subsequent processing environment to ensure workability and decarburization reaction. In one embodiment of the invention, the annealing temperature is 760°C to 840°C, and the appropriate holding time can be 10 to 90 seconds.

[0110] At this point, the annealing temperature is between 760℃ and 840℃. If the annealing temperature is too low, the deformation caused by cold rolling will not be fully removed, resulting in a significant reduction in processability. If the heat treatment temperature is too high, softening due to reduced strength at high temperatures will lead to sheet cracking, reducing the overall sheet performance. Therefore, the annealing temperature can be between 760℃ and 840℃. More specifically, the annealing temperature can be between 770℃ and 830℃.

[0111] Furthermore, the soaking time in continuous annealing processes can be between 10 and 90 seconds. If the soaking time at the holding temperature is too short, unrecrystallized grains will remain, significantly reducing formability. On the other hand, if the holding time is too long, grain growth will occur, leading to material softening and making it difficult to obtain the target material for subsequent sintering heat treatment. Therefore, the holding time at the annealing temperature can be between 10 and 90 seconds. More specifically, it can be between 15 and 80 seconds.

[0112] In addition, after the annealing step of the cold-rolled steel sheet, a temper rolling step can be included. Rolling allows control of the material's shape and the achievement of the desired surface roughness. However, if the temper rolling reduction is too high, work hardening can lead to a sharp decrease in yield strength due to microstructure growth during subsequent sintering heat treatment, resulting in poor dent resistance. Therefore, the temper rolling reduction can be between 0.4% and 2.0%. More specifically, the temper rolling reduction can be between 0.5% and 1.8%.

[0113] In addition, after the annealed steel sheet is quenched and tempered, an enamel sintering step may be included to dry the enamel-treated glaze.

[0114] The enamel sintering process, which involves high-temperature heating followed by room-temperature cooling, coats a steel plate with an enamel layer, achieving various properties suitable for enamel products, such as chemical resistance and heat resistance. However, if the sintering temperature is too low, the adhesion of the enamel layer cannot be guaranteed. On the other hand, if the sintering temperature is too high, the increased energy consumption leads to higher costs. Therefore, the sintering temperature can be between 780°C and 850°C. More specifically, the sintering temperature can be between 790°C and 840°C.

[0115] The invention will be further described in detail below by way of examples. However, the following examples are merely for illustrating the invention in more detail and are not intended to limit the scope of the invention. This is because the scope of the invention depends on the content described in the claims and what can be reasonably inferred therefrom.

[0116] Example

[0117] Slabs were manufactured using an alloy composition comprising the components listed in Table 1 (wt%), with the balance being iron (Fe) and unavoidable impurities, via a converter-secondary refining-continuous casting process. These slabs were then hot-rolled after being held in a furnace at 1230°C for 3 hours. The final thickness of the hot-rolled steel sheet was 4.0 mm. After hot rolling, the oxide film on the surface of the samples was removed by pickling, followed by cold rolling at a reduction rate. The cold-rolled samples were then processed into enamel-treated samples for investigating enamel properties and samples for mechanical property analysis, and subsequently subjected to heat treatment. The final hot rolling temperature, coiling temperature, cold rolling reduction rate, annealing temperature, and holding time are shown in Table 2.

[0118] The operability, enamel properties, and microstructure of the materials, ensured by the process described above, under different manufacturing conditions are shown in Table 3 below.

[0119] For plate performance, in continuous casting, hot rolling and cold rolling processes, if the workability is above 90% compared to the productivity of general materials, it is indicated as "O"; if the productivity is below 90% or the defect rate is above 10%, it is indicated as "X".

[0120] Machinability is evaluated by whether tensile strain occurs during the deep drawing and elongation process of enamel steel. If no defects occur, it is marked as "O"; if defects such as cracks occur, it is marked as "×".

[0121] The yield strength (MPa) was obtained by performing a tensile test at a crosshead speed of 10 mm / min after preparing the tensile specimen. To simulate the enamel glaze drying process, the specimen was prepared by sintering at 820°C for 15 minutes in a sintering furnace.

[0122] For samples that have undergone enamel sintering and have been kept in an oven at 200℃ for 24 hours, the presence of bubble defects on the enamel surface is visually observed. The surface condition is evaluated into three stages: "O" for excellent, "△" for average, and "X" for poor.

[0123] For enamel-treated specimens, they were cut to appropriate sizes according to different applications to meet the test objectives. After complete degreasing, the heat-treated specimens were coated with a standard enamel (Check frit) less susceptible to blistering defects and held at 300°C for 10 minutes to remove moisture. The dried specimens were then sintered at a lower temperature of 800°C for 20 minutes to highlight differences in enamel properties such as adhesion, and then cooled to room temperature. The sintering furnace was then subjected to harsh conditions with a dew point temperature of 30°C, conditions prone to blistering defects.

[0124] The enamel-treated samples were subjected to an accelerated scaling test in an oven at 200°C for 24 hours. After the accelerated scaling treatment, the presence of scaling defects was visually observed. If no scaling defects were found, it was marked as "O"; if scaling defects were found, it was marked as "X".

[0125] For evaluating the adhesion between the steel plate and the enamel, enamel adhesion is assessed using the method defined in ASTM C313-78, which involves applying a load to the enamel layer with a steel ball and evaluating the electrical conductivity of the area. This indexes the degree of enamel enamel layer detachment, thus representing enamel adhesion. In this invention, from the viewpoint of ensuring stability in the use of cheaper enamels, the target for enamel adhesion is set to ensure an enamel adhesion rate of over 95%.

[0126] For bubble defects, the enamel surface of the samples after enamel treatment and maintenance in an oven at 200°C for 24 hours was visually observed and evaluated using a three-step approach: “O” (excellent), “△” (average), and “X” (poor).

[0127] Hydrogen permeability, as one of the indices for evaluating resistance to the fatal defect of enamel spalling, is measured by an experimental method recorded in European standard (EN10209-2013) that measures the time (t) it takes for hydrogen to permeate from one direction of a steel sheet to the opposite direction after hydrogen is generated in one direction. s The value is expressed as t (the unit is seconds) divided by the square of the material thickness (t, unit is mm). s / t 2 (Unit: seconds / mm) 2 ).

[0128] Table 1

[0129]

[0130] Table 2

[0131]

[0132]

[0133] Table 3

[0134]

[0135] As shown in Tables 1 to 3, Examples 1 to 9 of the present invention, which satisfy the requirements for component composition, manufacturing conditions, and oxide layer thickness, not only exhibit excellent plate passability, but also meet the component ratios and relevant indices within the defined scope of the present invention. Even under harsh processing conditions, no enamel defects such as blistering and bubble defects are produced. Furthermore, a hydrogen permeability of 600 seconds / mm is also achieved. 2 The above-mentioned properties, enamel adhesion of 95% or higher, and yield strength of 220 MPa or higher after enamel sintering heat treatment, ensure the target characteristics of the present invention.

[0136] Comparative Examples 5 to 9 do not meet the chemical composition and component ratio requirements proposed in this invention, nor do they meet the requirements for surface oxide layer thickness, board permeability, processability, hydrogen permeability, enamel adhesion, yield strength, etc. Moreover, scale or bubble defects were observed in the naked eye after enamel treatment, so there are applicability issues.

[0137] On the other hand, even if the chemical composition and composition ratio proposed in this invention are met, if the annealing temperature is too low (Comparative Example 1), the annealing time is too short (Comparative Example 2), the annealing temperature is too high (Comparative Example 3), or the annealing time is too long (Comparative Example 4), an overly thick or thin oxide layer is formed, or the enamel adhesion is less than 95%, or enamel defects such as bubble defects or blistering occur after enamel treatment, resulting in poor board permeability. In some cases, the yield strength after enamel sintering treatment is less than 220 MPa, etc., and the target characteristics cannot be guaranteed overall.

[0138] Figure 2 The image shows the GDS analysis results for different thicknesses of the enamel steel sheet according to Example 5 of the invention. The innermost location with an oxygen content of 5% by weight is 0.015 μm, confirming the presence of an oxide layer 20 with a thickness of 0.015 μm on the surface.

[0139] This invention can be implemented in various ways and is not limited to the embodiments described above. Those skilled in the art will understand that the invention can be implemented in other specific ways without altering its technical concept or essential features. Therefore, it should be understood that the above embodiments are exemplary in all respects and are not restrictive.

[0140] Explanation of reference numerals in the attached figures

[0141] 100: Steel plate for enamel 10: Steel plate substrate

[0142] 20: Oxide layer

Claims

1. A steel plate for enamel enamel, wherein, The steel plate, by weight percent, comprises C: 0.0005 to 0.0030%, Mn: 0.15 to 0.55%, Si: 0.001 to 0.03%, Al: 0.0001 to 0.002%, P: 0.001 to 0.02%, S: 0.001 to 0.03%, Cu: 0.02 to 0.06%, N: 0.005 to 0.012%, Cr: 0.05 to 0.20%, O: 0.03 to 0.06%, with the balance comprising Fe and unavoidable impurities. The steel plate contains an oxide layer in the direction from the surface to the interior, the oxide layer having a thickness of 0.006 to 0.030 μm. The oxide layer contains more than 5% by weight of oxygen, and Among them, the following equations 1 to 3 are satisfied. [Formula 1] 3.05 ≤ [Cu] / [P] ≤ 5.10 In Formula 1, [Cu] and [P] represent the weight percentage of Cu and P elements, respectively. [Equation 2] 0.032 ≤ ([Cr] / 52 + [Mn] / 32)×([N] / 14) / ([C] / 12) ≤ 0.091 In Formula 2, [Cr], [Mn], [N], and [C] represent the weight percentage of Cr, Mn, N, and C elements, respectively. [Formula 3] 0.45 ≤ (Ra×50×Se) / Pc ≤ 0.99 In Equation 3, Pc represents the number of surface bumps per unit centimeter, Ra represents the average roughness value in μm, and Se represents the tempering reduction rate in % (%).

2. The enamel steel plate according to claim 1, wherein, It also contains one or more of the following: Ti: less than 0.001 wt%, Nb: less than 0.001 wt%, Ni: less than 0.02 wt%, V: less than 0.001 wt%, and Mo: less than 0.02 wt%.

3. The enamel-coated steel plate according to claim 1, wherein, The yield strength after sintering and heat treatment of enamel is above 220 MPa.

4. The enamel-coated steel plate according to claim 1 has an enamel adhesion of 95% or higher.

5. The enamel-coated steel plate according to claim 1, wherein the hydrogen permeability is 600 seconds / mm². 2 above.

6. A method for manufacturing a steel plate for enamel enamel, comprising: The step of hot rolling a slab to produce a hot-rolled steel sheet, wherein the slab, by weight percent, comprises C: 0.0005 to 0.0030%, Mn: 0.15 to 0.55%, Si: 0.001 to 0.03%, Al: 0.0001 to 0.002%, P: 0.001 to 0.02%, S: 0.001 to 0.03%, Cu: 0.02 to 0.06%, N: 0.005 to 0.012%, Cr: 0.05 to 0.20%, O: 0.03 to 0.06%, with the balance comprising Fe and unavoidable impurities, and satisfies the following formulas 1 and 2; The step of cold rolling the hot-rolled steel sheet to manufacture cold-rolled steel sheet; The step of annealing the cold-rolled steel sheet; The steps for quenching and tempering annealed cold-rolled steel sheets, and Following the quenching and tempering rolling step, the quenched and tempered steel sheet undergoes an enamel sintering process at a temperature of 790°C to 840°C. The annealing is performed at 760°C to 840°C for 10 to 90 seconds. [Formula 1] 3.05 ≤ [Cu] / [P] ≤ 5.10 In Formula 1, [Cu] and [P] represent the weight percentage of Cu and P elements, respectively. [Equation 2] 0.032 ≤ ([Cr] / 52 + [Mn] / 32)×([N] / 14) / ([C] / 12) ≤ 0.091 In Formula 2, [Cr], [Mn], [N], and [C] represent the weight percentage of Cr, Mn, N, and C elements, respectively. The steel plate contains an oxide layer in the direction from the surface to the interior, the oxide layer having a thickness of 0.006 to 0.030 μm. The oxide layer contains more than 5% by weight of oxygen. The steel plate used for enamel enamel meets the following formula 3: [Formula 3] 0.45 ≤ (Ra×50×Se) / Pc ≤ 0.99 In Equation 3, Pc represents the number of surface bumps per unit centimeter, Ra represents the average roughness value in μm, and Se represents the tempering reduction rate in % (%).

7. The method for manufacturing the enamel steel plate according to claim 6, wherein, In the process of manufacturing hot-rolled steel sheets, the final hot rolling temperature is between 910°C and 970°C.

8. The method for manufacturing the enamel steel plate according to claim 6, wherein, In the process of manufacturing hot-rolled steel sheets, the coiling temperature is between 580°C and 720°C.

9. The method for manufacturing the enamel steel plate according to claim 6, wherein, In the process of manufacturing cold-rolled steel sheets, cold rolling is performed with a reduction rate of 60 to 90%.

10. The method for manufacturing the enamel steel plate according to claim 6, wherein, The quenching and tempering rolling is performed with a reduction rate of 0.4% to 2.0%.

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