High-strength cold-rolled steel sheet for enamel having excellent adhesion and method for manufacturing the same

Abstract

A high-strength cold-rolled steel sheet for enamel according to an embodiment of the present application, a method of manufacturing the same, the steel sheet comprising, in weight %, C: 0.01 to 0.1 %, Mn: 0.05 to 0.4 %, Si: 0.001 to 0.03 %, Al: 0.03 to 0.12 %, P: 0.02 to 0.04 %, S: 0.001 to 0.02 %, Cu: 0.03 to 0.08 %, Mo: 0.13 to 0.30 %, N: 0.004 % or less, O: 0.003 % or less, the balance comprising Fe and inevitable impurities, the high-strength cold-rolled steel sheet having an oxide layer thickness of 0.006 to 0.030 µm formed in a direction from the surface inward, satisfying Formula 1 and Formula 2. No bubble defects are generated after the enamel treatment, the resistance to spalling is excellent, and the enamel adhesion is excellent.

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 processing that does not produce bubble defects after enamel treatment and has excellent enamel adhesion and anti-scaling properties, 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 machinability, open coil annealing (OCA), one of the batch annealing furnace 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] On the other hand, even with high-oxygen enamel steel plates that increase the dissolved oxygen content inside the steel plate and utilize inclusions such as oxides in the steel to adsorb hydrogen to ensure resistance to flaking, the high oxygen content already results in extremely severe melting loss of the refractory material. This not only greatly reduces the continuous casting productivity in the steelmaking process but also leads to frequent surface defects.

[0006] Furthermore, since enamel steel is primarily used as a material for structural components, increasing the material's strength reduces the weight of the components used, thereby enhancing competitiveness. Therefore, in enamel engineering, the yield strength after the high-temperature heat treatment required for enamel drying must be maintained above 220 MPa.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Korean Patent Registration No. 10-1353643 (January 14, 2014) Summary of the Invention

[0010] Technical issues

[0011] One embodiment of the present invention aims to provide a high-strength cold-rolled steel sheet for enamel with excellent enamel adhesion 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 processing with a yield strength of 220 MPa or higher after enamel treatment, free from bubble defects, and exhibiting excellent enamel adhesion and anti-scaling properties, and a method for manufacturing the same.

[0012] The purpose of this invention is not limited to the above-mentioned purposes, and other purposes not mentioned will be clearly understood by those skilled in the art from the following description.

[0013] Technical solution

[0014] To achieve the aforementioned objective, an embodiment of the present invention provides a high-strength cold-rolled steel sheet for enamel, comprising, by weight %, C: 0.01 to 0.1%, Mn: 0.05 to 0.4%, Si: 0.001 to 0.03%, Al: 0.03 to 0.12%, P: 0.02 to 0.04%, S: 0.001 to 0.02%, Cu: 0.03 to 0.08%, Mo: 0.13 to 0.30%, N: less than 0.004%, O: less than 0.003%, with the balance including Fe and unavoidable impurities. The high-strength cold-rolled steel sheet for enamel has an oxide layer thickness of 0.006 to 0.030 μm formed from the surface to the interior, satisfying Formulas 1 and 2 below.

[0015] [Formula 1]

[0016] 0.95≤Cu / P≤2.95

[0017] [Equation 2]

[0018] 0.20≤(Mo / 96) / (C / 12+N / 14)≤0.90

[0019] (In Equations 1 and 2, Cu, P, Mo, C and N each represent the content (by weight%) of the element.)

[0020] According to one embodiment, a high-strength cold-rolled steel sheet for enamel has an oxide layer extending from the surface inwards, the oxide layer having a thickness of 0.006 to 0.030 μm.

[0021] According to one embodiment, the high-strength cold-rolled steel sheet for enamel coating satisfies the following formula 3.

[0022] [Formula 3]

[0023] 0.75≤C v ≤2.45

[0024] (In Equation 3, C) v The difference in cementite volume fraction (C) 1 / 2t □C 1 / 8t ), C 1 / 2t and C 1 / 8t Each represents the volume fraction of cementite in the center and 1 / 8 portion of the steel plate along its thickness direction.

[0025] According to one embodiment, the high-strength cold-rolled steel sheet for enamel coating satisfies the following formula 4.

[0026] [Formula 4]

[0027] 0.07≤MV v ≤0.14

[0028] (in the formula (4)MV) vMicrocavity volume fraction difference (MV) 1 / 8t □MV Av ),MV 1 / 8t The microvoid volume fraction (%) and MV of the 1 / 8 section along the thickness direction of the steel plate. Av (mean: average microcavity volume fraction (%))

[0029] According to one embodiment, the yield strength of the high-strength enamel cold-rolled steel sheet after enamel sintering heat treatment is above 220 MPa.

[0030] According to one embodiment, the enamel adhesion of the high-strength cold-rolled steel sheet for enamel is 95% or higher.

[0031] According to one embodiment, the hydrogen permeability of the high-strength cold-rolled steel sheet for enamel is 600 seconds / mm² or higher.

[0032] A method for manufacturing a high-strength cold-rolled steel sheet for enamel according to another embodiment of the present invention includes: hot rolling a slab to manufacture a hot-rolled steel sheet, wherein the steel sheet comprises, by weight %, C: 0.01 to 0.1%, Mn: 0.05 to 0.4%, Si: 0.001 to 0.03%, Al: 0.03 to 0.12%, P: 0.02 to 0.04%, S: 0.001 to 0.02%, Cu: 0.03 to 0.08%, Mo: 0.13 to 0.30%, N: less than 0.004%, O: less than 0.003%, and the balance including Fe and unavoidable impurities, satisfying Formulas 1 and 2 below.

[0033] The steps of cold rolling the hot-rolled steel sheet to manufacture cold-rolled steel sheet; and

[0034] The step of annealing the cold-rolled steel sheet

[0035] [Formula 1]

[0036] 0.95≤Cu / P≤2.95

[0037] [Equation 2]

[0038] 0.20≤(Mo / 96) / (C / 12+N / 14)≤0.90

[0039] (In Equations 1 and 2, Cu, P, Mo, C and N each represent the content (by weight%) of the element.)

[0040] The annealing step can be performed at temperatures between 720°C and 850°C, and with an oxidation capacity index (P... H2O / P H2 Perform for 20 to 180 seconds in a humid environment with a pH of 0.51 to 0.65.

[0041] Invention Effects

[0042] According to an embodiment of the present invention, a high-strength cold-rolled steel sheet for enamel meets the above-mentioned chemical composition and content range, and simultaneously satisfies formulas (1) and (2), thereby controlling the thickness of the oxide layer formed from the surface to the interior to an appropriate thickness. This shell provides a high-strength cold-rolled steel sheet for enamel with excellent anti-scaling and enamel adhesion.

[0043] The high-strength cold-rolled steel sheet for enamel according to the present invention improves productivity and machinability by using low-carbon steel with excellent surface properties in the range of 0.01 to 0.1% C during the steelmaking stage.

[0044] Furthermore, by optimizing the furnace atmosphere and controlling the oxide layer thickness and the volume fraction of carbides in the steel during heat treatment in a continuous annealing furnace after cold rolling, the enamel properties obtained through high-speed heat treatment are significantly improved, and a high level of strength after enamel sintering heat treatment can be maintained.

[0045] According to an embodiment of the present invention, an enamel-enameled steel sheet exhibiting excellent resistance to blistering and enamel adhesion utilizes cementite, which acts as a low-temperature precipitate, to promote a decarburization reaction during a continuous annealing process by controlling the environment. The cementite is uniformly dispersed during hot rolling, and the microcavities formed by cold rolling and the decarburization reaction act as hydrogen adsorption sources, thereby preventing hydrogen-induced blistering defects. On the other hand, residual carbon on the surface layer of the steel sheet due to the gasification reaction during enamel sintering can also contribute to bubble defects in enamel products. Therefore, the present invention controls the distribution of carbides and microcavities in the thickness direction of the cold-rolled steel sheet, thereby not only improving enamel properties but also preventing surface bubble defects. Attached Figure Description

[0046] Figure 1 This is a cross-sectional schematic diagram of a high-strength cold-rolled steel sheet for enamel coating according to an embodiment of the present invention.

[0047] 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

[0048] The following will describe in detail the high-strength cold-rolled steel sheet for enamel and its manufacturing method according to the present invention. The accompanying drawings are provided to fully convey the spirit of the invention to those skilled in the art. Therefore, the invention is not limited to the drawings provided below and can be embodied in other forms; the drawings may be exaggerated to illustrate the spirit of the invention. Unless otherwise defined, all terms, including technical or scientific terms used in this specification, have the same meaning as commonly understood by those skilled in the art. In the following description and drawings, descriptions of well-known functions and structures that may unnecessarily obscure the essence of the invention will be omitted.

[0049] In this specification, when a part is described as "containing" a certain element, it means, unless specifically stated to the contrary, that it may also contain other elements, and does not exclude other elements.

[0050] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel contains, by weight %, C: 0.01 to 0.1%, Mn: 0.05 to 0.4%, Si: 0.001 to 0.03%, Al: 0.03 to 0.12%, P: 0.02 to 0.04%, S: 0.001 to 0.02%, Cu: 0.03 to 0.08%, Mo: 0.13 to 0.30%, N: less than 0.004%, O: less than 0.003%, with the balance including Fe and unavoidable impurities. The oxide layer formed from the surface to the interior of the high-strength cold-rolled steel sheet has a thickness of 0.006 to 0.030 μm, satisfying Formulas 1 and 2 below.

[0051] [Formula 1]

[0052] 0.95≤Cu / P≤2.95

[0053] [Equation 2]

[0054] 0.20≤(Mo / 96) / (C / 12+N / 14)≤0.90

[0055] (In Equations 1 and 2, Cu, P, Mo, C and N each represent the content (by weight%) of the element.)

[0056] According to an embodiment of the present invention, a high-strength cold-rolled steel sheet for enamel meets the above-mentioned chemical composition and content range, and simultaneously satisfies formulas (1) and (2), thereby controlling the thickness of the oxide layer formed from the surface to the interior to an appropriate thickness. This shell provides a high-strength cold-rolled steel sheet for enamel with excellent anti-scaling and enamel adhesion.

[0057] The high-strength cold-rolled steel sheet for enamel according to the present invention improves productivity and machinability by using low-carbon steel with excellent surface properties in the range of 0.01 to 0.1% C during the steelmaking stage.

[0058] Furthermore, by optimizing the furnace atmosphere and controlling the oxide layer thickness and the volume fraction of carbides in the steel during heat treatment in a continuous annealing furnace after cold rolling, the enamel properties obtained through high-speed heat treatment are significantly improved, and a high level of strength after enamel sintering heat treatment can be maintained.

[0059] According to an embodiment of the present invention, an enamel-enameled steel sheet exhibiting excellent resistance to blistering and enamel adhesion utilizes cementite, a low-temperature precipitate, to promote decarburization reaction during a continuous annealing process by controlling the environment. The cementite is uniformly dispersed during hot rolling, and the microcavities formed by cold rolling and the decarburization reaction act as hydrogen adsorption sources, thereby preventing hydrogen-induced blistering defects.

[0060] On the other hand, due to the gasification reaction during enamel sintering, residual carbon in the surface layer of the steel plate can also become a factor causing bubble defects in enamel products. Therefore, in this invention, the distribution of carbides and microcavities is controlled in the thickness direction of the cold-rolled steel plate, thereby not only improving the enamel properties, but also preventing the generation of surface bubble defects.

[0061] The reasons for limiting the content of chemical components are described below. Unless otherwise stated, all units below are by weight %.

[0062] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel has a carbon (C) content of 0.01 to 0.1%, preferably 0.02 to 0.08%.

[0063] When carbon (C) is added in excess (more than 0.1%), the strength of the steel increases due to the increased amount of dissolved carbon. However, this also hinders the development of texture during annealing, leading to poor formability and blister defects caused by blistering in the enamel layer. On the other hand, if the C content is too low (below 0.01%), the fraction of carbides that serve as hydrogen adsorption sites in the steel decreases, making it prone to scaling defects.

[0064] Regarding the manufacturing process described later, during the final annealing process, due to decarburization occurring in an environment with a high oxidation index, the carbon content in the slab and the final steel plate will differ. Since decarburization proceeds to approximately 0.01 to 0.05% by weight, the carbon content in the final steel plate can be 0.01 to 0.05% by weight. The carbon content in the final steel plate can have a concentration gradient along the thickness direction, and the aforementioned carbon content represents the average carbon content throughout the steel plate 100 with oxide layer 20. More specifically, the carbon content in the final steel plate can be 0.015 to 0.045% by weight.

[0065] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel has a manganese (Mn) content of 0.05 to 0.40%, preferably 0.07 to 0.38%.

[0066] 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 exceeds 0.40%, it leads to poor formability and lowers the Ar3 transformation temperature, potentially causing deformation during enamel sintering.

[0067] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel has a silicon (Si) content of 0.001 to 0.03%, preferably 0.002 to 0.0278%.

[0068] Silicon (Si) is an element that promotes the formation of carbides that act as a hydrogen adsorption source. If the amount of Si added is too little, below 0.001%, it is difficult to achieve the aforementioned effect. On the other hand, if the amount of Si added is too much, exceeding 0.03%, the formation of an oxide film on the steel plate surface may reduce the adhesion of the enamel.

[0069] According to an embodiment of the present invention, the aluminum (Al) content of the high-strength cold-rolled steel sheet for enamel can be from 0.03 to 0.12%, preferably from 0.035 to 0.115%.

[0070] Aluminum (Al) acts as a strong deoxidizer in steelmaking, removing oxygen from molten steel and fixing dissolved nitrogen, thereby improving aging performance. If the amount of Al added is too low (below 0.03%), the aforementioned effects are difficult to achieve. On the other hand, if the amount of Al added is too high (exceeding 0.12%), alumina will be present in the steel or on its surface, potentially causing blistering defects in the enamel finishing process.

[0071] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel has a phosphorus (P) content of 0.02 to 0.04%, preferably 0.022 to 0.038%.

[0072] Phosphorus (P) is a typical material strengthening element. If the amount of P added is too low, below 0.02%, it is difficult to achieve the aforementioned effects. On the other hand, if the amount of P added is too high, exceeding 0.04%, a segregation layer will form inside the steel plate, which will not only reduce formability but also worsen the pickling properties of the steel and may also have an adverse effect on the adhesion of enamel.

[0073] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel has a sulfur (S) content of 0.001 to 0.020%, preferably 0.002 to 0.018%.

[0074] Sulfur (S) is an element that combines with manganese and causes red-hot brittleness. If the amount of S added is too little, below 0.001%, it may lead to problems with weldability. If the amount of S added is too much, exceeding 0.020%, the ductility will be greatly reduced, resulting in poor workability. In addition, the excessive precipitation of manganese sulfide may also have an adverse effect on the scaling performance.

[0075] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel has a copper (Cu) content of 0.03 to 0.08%, preferably 0.032 to 0.078%.

[0076] Copper (Cu) is an additive element used for solid solution strengthening and improving the adhesion of enamel. To achieve its effect, it needs to be added at a level of 0.03% or more. However, adding too much will reduce the pickling speed in the acid treatment step of the enamel pretreatment process, making it difficult to obtain a suitable surface roughness of the steel plate. It is preferable to limit its upper limit to 0.08%.

[0077] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel has a molybdenum (Mo) content of 0.13 to 0.30%, preferably 0.15 to 0.28%.

[0078] Molybdenum (Mo) combines with carbon in steel to form carbides or exists in a solid solution state. It not only strengthens the material, but the precipitates also act as sites for hydrogen adsorption in the steel, effectively improving its anti-scaling properties. To achieve these effects, it is necessary to add at least 0.13%. Adding too much increases the recrystallization temperature, reduces annealing workability, and increases costs due to the addition of ferroalloys. Preferably, the upper limit is limited to 0.30%.

[0079] According to an embodiment of the present invention, the nitrogen (N) content of the high-strength cold-rolled steel sheet for enamel can be less than 0.004%, preferably from 0.0005 to 0.0037%.

[0080] Nitrogen (N) is a typical hardening element, but if the amount added increases, aging defects occur frequently, and formability deteriorates, potentially leading to bubble defects in the enamel finishing process. Therefore, the upper limit for N is limited to 0.004% by weight.

[0081] According to an embodiment of the present invention, the nitrogen (N) content of the high-strength cold-rolled steel sheet for enamel can be less than 0.003%, preferably from 0.0001 to 0.0019%.

[0082] Oxygen (O) is an essential element for the formation of oxides. These oxides not only cause refractory loss during the steelmaking process but also contribute to surface defects caused by surface oxides during steel plate manufacturing. Therefore, the amount of O added to the slab can be less than 0.003% by weight.

[0083] Regarding the manufacturing process described later, during the final annealing process, due to decarburization in an environment with a high oxidation index, some oxygen will penetrate, thereby forming an oxide layer 20. 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.

[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 and Nb. In one embodiment of this invention, Ti and Nb are not intentionally added. The high-strength cold-rolled steel sheet for enamel may selectively contain one or more of Ti (less than 0.005%) and Nb (less than 0.003%).

[0085] On the other hand, in the steel of the present invention, not only are elements such as titanium (Ti), which have a higher oxidizing power than iron (Fe), not added, the enamel adhesion performance between the steel plate and the enamel can be improved by controlling the surface oxide layer.

[0086] 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.

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

[0088] In one embodiment of the present invention, repeated experiments confirmed that a scheme 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] Therefore, it is necessary not only to adjust the chemical composition and content, but also to satisfy Equations 1 and 2.

[0090] If neither of the two formulas is satisfied, the thickness of the oxide layer may not be within the range of 0.006 to 0.030 μm.

[0091] [Formula 1]

[0092] 0.95≤Cu / P≤2.95

[0093] [Equation 2]

[0094] 0.20≤(Mo / 96) / (C / 12+N / 14)≤0.90

[0095] (In Equations 1 and 2, Cu, P, Mo, C and N each represent the content (by weight%) of the element.)

[0096] To ensure the adhesion performance and surface properties of enamel products, it is preferable to satisfy Formula 1. When the value of Formula 1 is less than 0.95, there is a problem of reduced enamel adhesion.

[0097] On the other hand, when the value of Equation 1 exceeds a certain level, the amount of gas flowing into the surface increases, and surface defects such as bubble defects occur frequently, which becomes the main reason for reducing product reliability.

[0098] To ensure enamel adhesion and suppress surface bubble defects, the value of Formula 1 is preferably limited to 0.95 to 2.95, more preferably to 0.98 to 2.90.

[0099] In addition, molybdenum in steel reacts with carbon, nitrogen, etc. to form carbon nitride compounds, thereby improving processability and acting as a hydrogen adsorption source. Therefore, it is necessary not only to consider the reactivity of each element but also to comprehensively consider its reactivity with carbon and nitrogen. In this case, it is preferable to satisfy Equation 2.

[0100] If the value of Equation 2 is too small, below 0.20, the increased amount of solid solution elements remaining in the steel will lead to a deterioration in workability. Conversely, if the value of Equation 2 is too large, exceeding 0.90, it may not only cause a decrease in the passability of rolling and annealing, but may also become a factor in increasing manufacturing costs. The value of Equation 2 is more preferably between 0.22 and 0.88.

[0101] Next, the rationale for limiting the volume fraction of carbides in the steel plate microvoid and hot rolling process of the present invention will be described.

[0102] The carbides used in the steel of this invention have different ductility from the base material. During the cold rolling process, the carbides themselves break down or form microvoids through subsequent decarburization heat treatment. Moreover, the carbides themselves also act as a hydrogen adsorption source to fix hydrogen in the steel. Therefore, such carbide fraction not only affects the enamel properties on its own, but also its relationship with the added elements.

[0103] The high-strength cold-rolled steel sheet for enamel with excellent adhesion proposed in this invention is achieved by adjusting the steel composition as a hydrogen adsorption site, mainly utilizing carbides such as Fe3C (cementite), and actively utilizing micro-voids generated by decarburization. At the same time, the components and processes in the steel composition that affect enamel adhesion and surface defects are controlled to provide enamel steel sheets and products with no surface defects and excellent enamel adhesion and anti-scaling properties.

[0104] The uniformly dispersed and precipitated cementite during hot rolling breaks down during cold rolling. Furthermore, the annealing process, by controlling the environment as a decarburization reaction source, forms microcavities that act as hydrogen adsorption sources. This effectively fixes hydrogen in the steel, thus suppressing scaling defects. By controlling the carbide and microcavity fractions in the thickness direction through continuous annealing and decarburization operations, and by controlling the oxide behavior of the steel plate surface layer, it also has a significant effect on enamel adhesion and suppressing bubble defects.

[0105] On the other hand, unlike high-temperature precipitates / inclusions that form during high-temperature solidification, one embodiment of the present invention utilizes carbides that are stable at low temperatures. This prevents operational degradation such as refractory melt-off or continuous casting nozzle blockage, which are problematic in existing enamel steels, and also prevents surface defects such as black lines. The carbide fraction is not only closely related to the total carbon content in the steel, but also has a significant impact from operating conditions.

[0106] According to an embodiment of the present invention, a high-strength cold-rolled steel sheet for enamel coating satisfies the following formula 3.

[0107] [Formula 3]

[0108] 0.75≤C v ≤2.45

[0109] (In Equation 3, C) v The difference in cementite volume fraction (C) 1 / 2t □C 1 / 8t ), C 1 / 2t and C 1 / 8t Each represents the volume fraction of cementite in the center and 1 / 8 portion of the steel plate along its thickness direction.

[0110] Carbon in metal alloys combines with metal atoms to form carbides, and one type of carbide formed by the combination of iron and carbon at relatively low temperatures is cementite. In ordinary carbon steel, cementite forms at temperatures between 250 and 700°C, and at even higher temperatures, it coarsens into spherical granules. The cementite generated during the hot rolling process breaks down during cold rolling and decomposes during decarburization, becoming a source of adsorbed hydrogen. However, if this cementite is concentrated on the surface of the steel, it becomes a source that promotes carbon vaporization during enamel sintering and can also contribute to bubble defects. Therefore, to suppress blistering and bubble defects in enamel products, it is necessary to strictly control the carbide volume fraction along the thickness direction. That is, if the difference in cementite volume fraction along the thickness direction of the cold-rolled steel sheet is C... v If the carbon content is too low (less than 0.75), the decarburization reaction will not proceed smoothly, thus increasing the carbide fraction in the surface layer and becoming a factor causing bubble defects after enamel sintering. On the other hand, if the carbon content is too low (less than 0.75), the decarburization reaction will not proceed smoothly, resulting in an increase in the carbide fraction in the surface layer, which in turn becomes a factor causing bubble defects after enamel sintering. v When the value exceeds 2.45, the supply of hydrogen adsorption sites in the steel is insufficient, making it difficult to suppress the formation of scaling defects. Therefore, the difference in cementite volume fraction C along the thickness direction... v It can be from 0.75% to 2.45%. More preferably, C v It can range from 0.78% to 2.42%.

[0111] According to an embodiment of the present invention, a high-strength cold-rolled steel sheet for enamel coating satisfies the following formula 4.

[0112] [Formula 4]

[0113] 0.07≤MV v ≤0.14

[0114] (in the formula (4)MV) v Microcavity volume fraction difference (MV) 1 / 8t □MV Av ),MV 1 / 8t The microvoid volume fraction (%) and MV of the 1 / 8 section along the thickness direction of the steel plate.Av (mean: average microcavity volume fraction (%))

[0115] Cementite precipitated during hot rolling breaks down during cold rolling and decarburization heat treatment, forming microcavities around it. These microcavities act as hydrogen adsorption sources, thus suppressing the formation of blistering defects. For microcavities in cold-rolled steel sheets, ten images were taken at 1000x magnification using a scanning electron microscope on the surface parallel to the rolling face (ND face), and the area fraction of microcavities in these areas was measured using an image analyzer. In one embodiment of the invention, by controlling the area fraction distribution of these microcavities in different locations, regions capable of simultaneously suppressing blistering and bubble defects were identified. To ensure this effect, the difference in microcavity volume fraction MV needs to be measured. v Controlled within 0.07% to 0.14%. If the difference in microvoid volume fraction MV... v When the value is too small (less than 0.07), although it is beneficial for preventing scaling, it also leads to problems such as deterioration in workability and frequent surface defects such as bubbles. On the other hand, if the MV... v When the value exceeds 0.14, the number of sites in the steel that can fix hydrogen and serve as hydrogen adsorption sources decreases, potentially leading to a higher rate of blistering defects in the product. Therefore, the difference in microvoid volume fraction MV... v Limited to 0.07% to 0.14%. More preferably, MV v It can range from 0.073% to 0.137%.

[0116] As mentioned above, the high-strength cold-rolled steel sheet for enamel according to the present invention not only has excellent strength characteristics, but also excellent enamel adhesion.

[0117] According to one embodiment, the yield strength of the high-strength enamel cold-rolled steel sheet after enamel sintering heat treatment is above 220 MPa.

[0118] The yield strength of materials used in structural components is a physical property that affects the component's resistance to indentation and its shape freeze-thaw resistance. It is usually measured and analyzed by tensile testing.

[0119] For enamel products, although the yield strength at the processing inlet side produced and supplied by the steel mill is also important, due to the characteristics of the product, it requires a relatively long period of high-temperature heat treatment for drying after enamel glaze treatment. The heat treatment varies depending on the type of glaze used, but in the method of this invention, the evaluation was performed after heat treatment at 830°C for 15 minutes.

[0120] Thus, the characteristics of enamel products make the yield strength after heat treatment during the enamel processing a primary factor in evaluating product stability. On the other hand, the yield strength measured by the tensile testing method can vary depending on the tensile testing method; therefore, in this evaluation, the crosshead speed, representing the tensile testing speed per unit time, is set to 10 mm / min. The yield strength after enamel firing heat treatment measured by this method can be 220 MPa or higher, more preferably 225 MPa or higher. At this point, there is no particular upper limit on the yield strength; for example, it can be 350 MPa.

[0121] According to an embodiment of the present invention, the enamel adhesion of a high-strength cold-rolled steel sheet for enamel 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. While there is no specific upper limit to enamel adhesion, it can be, for example, 100%.

[0122] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet for enamel coating has a hydrogen permeability of 600 seconds / mm. 2 above.

[0123] 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 using a method included in European Standard (EN10209). Hydrogen permeability is measured as 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 / t2 (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. While there's no specific upper limit on hydrogen permeability, it could be, for example, 2500 seconds / mm. 2 .

[0124] Next, the manufacturing method of the aforementioned high-strength cold-rolled steel sheet for enamel will be explained.

[0125] Specifically, according to another embodiment of the present invention, a method for manufacturing a high-strength cold-rolled steel sheet for enamel includes: hot rolling a slab to manufacture a hot-rolled steel sheet, wherein the steel sheet comprises, by weight percent, C: 0.01 to 0.1%, Mn: 0.05 to 0.4%, Si: 0.001 to 0.03%, Al: 0.03 to 0.12%, P: 0.02 to 0.04%, S: 0.001 to 0.02%, Cu: 0.03 to 0.08%, Mo: 0.13 to 0.30%, N: less than 0.004%, O: less than 0.003%, and the balance comprising Fe and unavoidable impurities, satisfying the following formulas 1 and 2.

[0126] The steps of cold rolling the hot-rolled steel sheet to manufacture cold-rolled steel sheet; and

[0127] The step of annealing the cold-rolled steel sheet

[0128] [Formula 1]

[0129] 0.95≤Cu / P≤2.95

[0130] [Equation 2]

[0131] 0.20≤(Mo / 96) / (C / 12+N / 14)≤0.90

[0132] (In Equations 1 and 2, Cu, P, Mo, C and N each represent the content (by weight%) of the element.)

[0133] First, prepare slabs that meet the aforementioned composition, Formula 1, and Formula 2. For molten steel whose composition has been adjusted to the aforementioned composition during the steelmaking process, slabs can be produced by continuous casting. As mentioned earlier, the C and O contents will change somewhat during the annealing process of cold-rolled steel sheets, while other alloy compositions are essentially the same as those for enamel steel sheets. The alloy chemical compositions, Formula 1, and Formula 2, have already been described, so a repetition is omitted.

[0134] The manufactured slab is then heated. Heating facilitates subsequent hot rolling processes and homogenizes the slab. More specifically, heating can be reheating.

[0135] At this point, the slab heating temperature can be between 1150 and 1280°C. More specifically, it can be between 1180 and 1260°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.

[0136] Then, the heated slab is hot-rolled to produce hot-rolled steel sheets.

[0137] At this point, the final hot rolling temperature can be between 850 and 910°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 860 and 900°C.

[0138] 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.

[0139] 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.

[0140] Before cold rolling the coiled hot-rolled steel sheet, a pickling step may be included.

[0141] Then, the hot-rolled steel sheet is cold-rolled into cold-rolled steel sheet.

[0142] 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%.

[0143] Then, enamel steel sheets are 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 machinability. Therefore, heat treatment is performed in subsequent processes to ensure machinability and decarburization reaction.

[0144] In one embodiment of the invention, during the heat treatment of cold-rolled steel sheets, the oxidation capacity (P) is controlled. H2O / P H2 To optimize carbon atom diffusion rates and promote outward diffusion of carbon atoms from the material, decarburization is improved. Therefore, based on the optimized management standards for the decarburization annealing process, the decarburization temperature is set within the range of 720 to 850°C, and the oxidation capacity (P...) is... H2O / P H2 Heat treatment is performed in a humid environment with a temperature of 0.51 to 0.65, with an appropriate holding time of 20 to 180 seconds.

[0145] At this point, the heat treatment temperature can be between 720 and 850°C. If the decarburization annealing temperature is too low, the deformation formed by cold rolling will not be sufficiently removed, resulting in a significant reduction in workability. Furthermore, the decarburization rate due to environmental heat treatment will be too low, thus failing to ensure the intended characteristics of the cold-rolled steel sheet for enamel. On the other hand, if the heat treatment temperature is too high, softening due to reduced high-temperature strength will lead to sheet cracking, resulting in reduced annealing permeability. Moreover, the increased thickness of the surface oxide layer will inhibit the decarburization reaction. Therefore, the heat treatment temperature is limited to between 720 and 850°C. More preferably, the annealing temperature can be between 730 and 840°C.

[0146] At this point, the oxidation capacity (P) of the heat treatment environment conditions. H2O / P H2The oxidation capacity can be between 0.51 and 0.65. If the oxidation capacity is too low, decarburization will take a long time, and the decarburization performance will deteriorate during continuous annealing, making it difficult to ensure the enamel properties. On the other hand, if the oxidation capacity is too high, there is a problem of a high rate of surface defects caused by the surface film formed by over-oxidation. Therefore, the oxidation capacity of the ambient gas is limited to 0.51 to 0.65. More specifically, the oxidation capacity can be between 0.52 and 0.64.

[0147] Furthermore, the soaking time in the continuous annealing process can be from 20 to 180 seconds. If the soaking time at the holding temperature is too short, unrecrystallized grains will remain, significantly reducing formability and hindering the decarburization reaction in the thickness direction, thus contributing to poor enamel properties. On the other hand, if the holding time is too long, abnormal grain growth will occur due to the decarburization reaction, leading to reduced processability and deterioration of the scaling performance caused by material inhomogeneity. Therefore, the soaking time at the soaking temperature can be from 20 to 180 seconds. More preferably, it can be from 25 to 160 seconds.

[0148] In addition to the annealing step of cold-rolled steel sheet, a temper rolling step can also be included. Temper rolling allows control of the material's shape and the attainment of the desired surface roughness. However, if the temper reduction rate is too high, work hardening can occur, leading to material hardening and decreased machinability. Therefore, temper rolling can employ a reduction rate of less than 3%. Specifically, the reduction rate in temper rolling can be between 0.3% and 2.5%.

[0149] Furthermore, after the quenching and tempering of the cold-rolled steel sheet, a high-temperature heat treatment step can be included to dry the enamel coating. Through a firing process, high-temperature heating and cooling are performed to coat the steel sheet surface with an enamel layer, thereby obtaining various properties of enamel products, such as chemical resistance and heat resistance. If the temperature is too low, the adhesion of the enamel layer will not be guaranteed; if the temperature is too high, it will increase costs due to the increased energy consumption. Therefore, the firing temperature can be between 780 and 850°C. More preferably, the firing temperature can be between 790 and 840°C.

[0150] The high-strength cold-rolled steel sheet for enamel and its manufacturing method according to the present invention will be further described in detail below by way of examples. However, the following examples are only for illustrating the invention in more detail by way of example 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. Although not otherwise defined, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0151] Additionally, unless otherwise specified, % indicates weight, and 1 ppm is 0.0001 wt%.

[0152] [Invention Examples 1 to 9, Comparative Examples 1 to 9]

[0153] 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. The slabs were then hot-rolled after being held in a furnace at 1200°C for 1 hour. 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 finishing delivery temperature (FDT), coiling temperature (CT), cold rolling reduction rate, annealing temperature (AT), holding time, oxidation capacity, and enamel firing temperature are shown in Table 2.

[0154] 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.

[0155] For plate permeability, in continuous casting, hot rolling and cold rolling processes, if the permeability is above 90% compared to the permeability of general materials, it is indicated as "0"; if the permeability is below 90% or the defect rate is above 10%, it is indicated as "X".

[0156] For the carbide fraction, after ensuring images of 20 fields of view at 500x magnification using an optical microscope, the carbide fraction relative to the entire field of view area was determined using an image analyzer.

[0157] For yield strength (MPa), in order to simulate the drying process of enamel glaze on steel plates, tensile specimens were prepared after being heat-treated at various temperatures in a firing furnace for 15 minutes, and tensile tests were conducted at a crosshead speed of 10 mm / min to measure the yield strength.

[0158] 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 15 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 20°C, conditions prone to blistering defects.

[0159] 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".

[0160] 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%.

[0161] 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).

[0162] 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 ).

[0163] Table 1

[0164]

[0165]

[0166] Table 2

[0167]

[0168] Table 3

[0169]

[0170] 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 good plate permeability, but also meet the limitations of the present invention in terms of carbide content, micro-void fraction, and related indices. Even under harsh processing conditions, no enamel defects such as blistering and bubble defects are generated, and the hydrogen permeability of 600 seconds / mm is also met. 2 The above-mentioned properties, including enamel adhesion of 95% or higher and yield strength of 220 MPa or higher after enamel firing heat treatment, ensure the target characteristics of the present invention.

[0171] Comparative Examples 5 to 9, which do not meet the alloy composition requirements proposed in this invention, mostly fail to meet the requirements of the present invention, such as surface oxide layer thickness, cementite in different thickness directions, microvoid volume fraction, hydrogen permeability, and enamel adhesion. Moreover, scale or bubble defects were observed in the naked eye after enamel treatment, thus raising applicability issues.

[0172] On the other hand, although the chemical composition proposed in this invention is satisfied, if the following conditions are not met: the final rolling temperature of hot rolling (Comparative Example 1), the hot rolling coiling temperature (Comparative Examples 2 and 3), the cold rolling reduction rate during cold rolling (Comparative Examples 2 and 4), the annealing temperature during annealing (Comparative Examples 1 and 3), the holding time (Comparative Examples 2 and 4), the oxidation capacity (Comparative Examples 3 and 4), and the firing temperature during enamel firing (Comparative Examples 2 and 4), the surface oxide layer thickness exceeds the range proposed in this invention, the enamel adhesion is less than 95%, enamel defects such as bubble defects or scale bursts occur after enamel treatment, the board continuity is poor, and the yield strength after enamel firing heat treatment is less than 220 MPa, etc., which generally cannot ensure the target characteristics.

[0173] Figure 2 The image shows the GDS analysis results for different thicknesses of the enamel steel sheet according to Example 4 of the invention. The innermost location with an oxygen content of 5% by weight is 0.016 μm, confirming the presence of an oxide layer 20 with a thickness of 0.016 μm on the surface.

[0174] 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.

[0175] Explanation of reference numerals in the attached figures

[0176] 100: Steel plate for enamel 10: Steel plate substrate

[0177] 20: Oxide layer

Claims

1. A high-strength cold-rolled steel sheet for enamel coating, wherein, The steel plate, by weight percent, comprises C: 0.01 to 0.1%, Mn: 0.05 to 0.4%, Si: 0.001 to 0.03%, Al: 0.03 to 0.12%, P: 0.02 to 0.04%, S: 0.001 to 0.02%, Cu: 0.03 to 0.08%, Mo: 0.13 to 0.30%, N: less than 0.004%, O: less than 0.003%, with the balance including Fe and unavoidable impurities. The high-strength cold-rolled steel sheet for enamel coating has an oxide layer thickness of 0.006 to 0.030 μm from the surface inwards. Satisfy equations 1 to 4 below, [Formula 1] 0.95≤Cu / P≤2.95 [Equation 2] 0.20≤(Mo / 96) / (C / 12+N / 14)≤0.90 In Equations 1 and 2, Cu, P, Mo, C, and N each represent the weight percentage of their respective elements. [Formula 3] 0.75≤C v ≤2.45 In Equation 3, C v The difference in cementite volume fraction C 1 / 2t -C 1 / 8t C 1 / 2t and C 1 / 8t Each represents the volume fraction of cementite in the center and 1 / 8 portion of the steel plate along its thickness direction. [Formula 4] 0.07≤MV v ≤0.14 In Equation 4, MV v The difference in microcavity volume fraction MV 1 / 8t -MV Av MV 1 / 8t The microvoid volume fraction (%) is located at 1 / 8 of the thickness of the steel plate. Av The average microcavity volume fraction is %.

2. The high-strength cold-rolled steel sheet for enamel as described in claim 1, wherein, The high-strength enamel-coated cold-rolled steel sheet has a yield strength of over 220 MPa after enamel sintering heat treatment.

3. The high-strength cold-rolled steel sheet for enamel as described in claim 1, wherein, The enamel adhesion of the high-strength enamel-coated cold-rolled steel sheet is above 95%.

4. The high-strength cold-rolled steel sheet for enamel as described in claim 1, wherein, The hydrogen permeability of the high-strength cold-rolled steel sheet for enamel coating is 600 seconds / mm. 2 above.

5. A method for manufacturing a high-strength cold-rolled steel sheet for enamel enamel, comprising: The step of hot rolling a slab to produce a hot-rolled steel sheet, wherein the steel sheet comprises, by weight percent, C: 0.01 to 0.1%, Mn: 0.05 to 0.4%, Si: 0.001 to 0.03%, Al: 0.03 to 0.12%, P: 0.02 to 0.04%, S: 0.001 to 0.02%, Cu: 0.03 to 0.08%, Mo: 0.13 to 0.30%, N: less than 0.004%, O: less than 0.003%, with the balance including Fe and unavoidable impurities, and satisfying Formulas 1 and 2 below. The steps of cold rolling the hot-rolled steel sheet to manufacture cold-rolled steel sheet; and The step of annealing the cold-rolled steel sheet in, The annealing step is performed at a temperature of 720°C to 850°C and an oxidation capacity index P. H2O / P H2 Perform for 20 to 180 seconds in a humid environment with a pH of 0.51 to 0.

65. [Formula 1] 0.95≤Cu / P≤2.95 [Equation 2] 0.20≤(Mo / 96) / (C / 12+N / 14)≤0.90 In Equations 1 and 2, Cu, P, Mo, C, and N each represent the weight percentage of their respective elements.

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