Cold-rolled steel sheet for enamel coating with excellent resistance to fish scale and method for manufacturing the same

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, the cold-rolled steel sheet containing, by weight%, 0.0003-0.003% C, 0.25-0.55% Mn, 0.001-0.03%, 0.0005-0.0015%, P: 0.01-0.03%, S: 0.001-0.010%, Cu: 0.03-0.08%, N: 0.008-0.015%, Mo: 0.1-0.3%, O: 0.025-0.055%, and the balance being Fe and unavoidable impurities, the cold-rolled steel sheet including an oxide layer, the oxide layer formed from the surface toward the inside of the cold-rolled steel sheet having a thickness of 0.006-0.030 μm.

Description

[Technical Field]

[0001] The present invention relates to steel sheets, and more particularly to cold-rolled steel sheets for enamel coating with excellent resistance to fish scale, and to a method for manufacturing the same. [Background technology]

[0002] Enameled steel sheets are surface-treated products in which a vitreous glaze is applied to a base steel sheet, such as a hot-rolled or cold-rolled steel sheet, and then fired at high temperatures to improve corrosion resistance, weather resistance, heat resistance, and chemical resistance. These enamel-coated steel sheets are used for building exteriors, home appliances, tableware, and various industrial applications.

[0003] Initially, rimmed steel was mainly used for the aforementioned enamel steel sheets, but recently, continuous casting has been adopted to improve productivity, and most of the materials are now continuously cast using this method. One of the problems with the aforementioned enamel steel sheets is the fishscale defect, which occurs when hydrogen dissolved in the steel during the manufacturing process of enamel products becomes supersaturated in the steel during the cooling process after firing, and is released to the surface of the steel over time, causing the enamel layer to peel off in a fishscale-like pattern.

[0004] When the aforementioned fish scale defects occur, rust develops intensively at the defective area, which significantly reduces the value of the enamel product. Therefore, in the manufacturing of enamel steel sheets, various methods are used to prevent fish scale defects by creating a large number of sites within the steel that can capture hydrogen dissolved in the steel.

[0005] Specifically, a method has been proposed to ensure workability and resistance to fish scale by applying the Open Coil Annealing (OCA) method, a type of phase annealing. However, this method has the problem that the heat treatment process takes a relatively long time, which reduces productivity and increases manufacturing costs, as well as causing significant quality deviations within the coil. Furthermore, because it is not easy to control the decarburization reaction in the heat treatment process, if decarburization is carried out excessively, the grain boundaries of the steel sheet soften after enamel firing heat treatment, leading to a decrease in shape-freezing properties.

[0006] Thus, in order to overcome problems such as decreased productivity and increased manufacturing costs due to prolonged annealing, recently developed enamel steel sheets utilize a continuous annealing process. Enamel steel produced by the continuous annealing method using the aforementioned continuous annealing process typically uses ultra-low carbon steel as the base, and as a hydrogen storage source, it uses precipitates such as titanium (Ti) or inclusions secured by the non-deoxidation method. However, even in this case, a large amount of carbonitride-forming elements is required, which leads to increased costs and decreased treadability, and the problem of surface defects caused by the generated precipitates or non-deoxidized inclusions is a problem that must be fundamentally solved.

[0007] Enameled steel sheets that utilize titanium precipitates require the addition of large amounts of titanium to suppress the hydrogen reaction that causes fish scale. This leads to nozzle clogging due to titanium nitride (TiN) and inclusions during the continuous casting stage of the steelmaking process, resulting in reduced workability and direct problems with production load. Furthermore, the titanium nitride mixed into the molten steel is present on the upper surface of the steel sheet, inducing blister defects, a typical type of bubble defect. In addition, the large amount of added titanium forms a titanium oxide layer, hindering adhesion between the steel sheet and the glaze layer.

[0008] Furthermore, in the case of enamel steel used in enamel steel sheets, the goal is to enhance the strength of the material and strengthen competitiveness by reducing the weight of the components used, largely by using the same material as structural members. For this reason, the material properties after the high-temperature firing heat treatment performed in the enamel process to dry the glaze are required. To meet these requirements, it is necessary to ensure that the yield strength of the enamel steel after firing is 220 MPa or higher. [Overview of the project] [Problems that the invention aims to solve]

[0009] The technical problem that this invention aims to solve is to provide a high-strength cold-rolled steel sheet for enamel coating that has a yield strength of 220 MPa or more after enamel treatment, is free from void defects, and exhibits excellent enamel adhesion and resistance to fish scale.

[0010] Another technical problem that the present invention aims to solve is to provide a method for manufacturing cold-rolled steel sheets having the aforementioned advantages. [Means for solving the problem]

[0011] A cold-rolled steel sheet according to one embodiment of the present invention contains, by weight percent, C: 0.0003~0.003%, Mn: 0.25~0.55%, Si: 0.001~0.03%, Al: 0.0005~0.0015%, P: 0.01~0.03%, S: 0.001~0.010%, Cu: 0.03~0.08%, N: 0.008~0.015%, Mo: 0.1~0.3%, O: 0.025~0.055%, and the remainder being Fe and unavoidable impurities, and includes an oxide layer, the thickness of which is 0.006~0.030 μm, formed in the direction from the surface of the cold-rolled steel sheet toward the interior.

[0012] In one embodiment, the cold-rolled steel sheet satisfies the following formula 1. <Expression 1> 0.014≦([Cu]×[Si) / [P]≦0.080 (In Formula 1 above, [Cu], [Si], and [P] represent the weight percentage content of Cu, Si, and P, respectively.)

[0013] In one embodiment, the cold-rolled steel sheet satisfies the following formula (2). [Formula (2)] 0.0065 ≦ ([Al] × [Mo]) / ([C] + [N]) ≦ 0.0310 (In the above formula (2), [Al], [Mo], [C], and [N] respectively represent the weight % contents of Al, Mo, C, and N)

[0014] In one embodiment, the cold-rolled steel sheet satisfies the following formula (3). [Formula (3)] 0.50 ≦ (R max × 20S e ) / P c ≦ 1.05 (In the above formula (3), P c represents the number of surface irregularities per centimeter (cm), R max represents the maximum peak roughness value (μm), S e represents the temper rolling reduction rate (%))

[0015] In one embodiment, the yield strength of the cold-rolled steel sheet after borax firing heat treatment may be 220 MPa or more. In one embodiment, the borax adhesion of the cold-rolled steel sheet may be 95% or more. In one embodiment, the hydrogen permeation ratio of the cold-rolled steel sheet may be 600 seconds / mm 2 or more.

[0016] According to another embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet comprises reheating a steel slab containing, by weight %, C: 0.0003 to 0.003%, Mn: 0.25 to 0.55%, Si: 0.001 to 0.03%, Al: 0.0005 to 0.0015%, P: 0.01 to 0.03%, S: 0.001 to 0.010%, Cu: 0.03 to 0.08%, N: 0.008 to 0.015%, Mo: 0.1 to 0.3%, O: 0.025 to 0.055% and the balance of Fe and inevitable impurities in a temperature range of 1,150 to 1,280°C; hot-rolling the heated slab in a range of finish hot-rolling temperature of 890 to 950°C; coiling the hot-rolled steel sheet in a temperature range of 580 to 720°C; cold-rolling the coiled hot-rolled steel sheet at a cold reduction rate of 60 to 90%; annealing the cold-rolled steel sheet at an annealing temperature of 720 to 850°C for 10 to 70 seconds, and performing temper rolling at a reduction rate of 2.5% or less to produce an annealed sheet; and performing borax firing heat treatment on the annealed sheet in a temperature range of 780 to 850°C.

[0017] In one embodiment, the method for manufacturing a cold-rolled steel sheet satisfies the following Formula 1. <Formula 1> 0.014 ≤ ([Cu] × [Si]) / [P] ≤ 0.080 (In the above Formula 1, [Cu], [Si], and [P] respectively represent the weight % contents of Cu, Si, and P)

[0018] In one embodiment, the method for manufacturing a cold-rolled steel sheet satisfies the following Formula 2. <Formula 2> 0.0065 ≤ ([Al] × [Mo]) / ([C] + [N]) ≤ 0.0310 (In the above Formula 2, [Al], [Mo], [C], and [N] respectively represent the weight % contents of Al, Mo, C, and N)

[0019] In one embodiment, the method for manufacturing a cold-rolled steel sheet satisfies the following Formula 3. <Formula 3> 0.50 ≤ (R max × 20S e ) / P c ≤ 1.05 (In formula 3 above, P c R is the number of surface irregularities per unit centimeter (cm). max This is the maximum point roughness value (μm), S e This represents the rate of reduction in tempering (%). [Effects of the Invention]

[0020] The cold-rolled steel sheet according to one embodiment of the present invention provides a cold-rolled steel sheet with excellent enamel adhesion and fish scale resistance by controlling the steel composition, and can be used for a variety of components such as home appliances, chemical equipment, kitchen equipment, sanitary equipment, and interior and exterior building materials.

[0021] A method for manufacturing cold-rolled steel sheets according to another embodiment of the present invention can provide a method for manufacturing cold-rolled steel sheets having the aforementioned advantages. [Brief explanation of the drawing]

[0022] [Figure 1] This is a schematic cross-sectional view of a cold-rolled steel sheet according to one embodiment. [Modes for carrying out the invention]

[0023] The terms first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited to these. These terms are used solely to distinguish one part, component, region, layer, or section from other parts, components, regions, layers, or sections. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section without departing from the scope of the invention.

[0024] The technical terms used herein are for the sole purpose of referring to specific embodiments and are not intended to limit the invention. The singular forms used herein also include plural forms unless the wording explicitly indicates otherwise. The meaning of “including” as used in this specification is to embody certain characteristics, domains, integers, stages, operations, elements, and / or components, and does not exclude the presence or addition of other characteristics, domains, integers, stages, operations, elements, and / or components.

[0025] When we say that one part is "on top of" another part, it means that it is directly on top of the other part, or that the other part is in between. In contrast, when we say that one part is "directly on top of" another part, there is no other part in between.

[0026] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as those generally understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have the meaning corresponding to the relevant technical literature and the present disclosures, and are not interpreted in an ideal or highly formal sense unless otherwise defined.

[0027] Also, unless otherwise specified, % means weight percent, and 1 ppm is 0.0001 weight percent.

[0028] In one embodiment of the present invention, the meaning of further including additional elements is that the additional amount of the additional elements replaces the remaining iron (Fe).

[0029] The embodiments of the present invention will be described below in detail so that they can be easily implemented by a person with ordinary skill in the art to which the present invention pertains. However, the present invention can be realized in various different forms and is not limited to the embodiments described herein.

[0030] Figure 1 is a schematic cross-sectional view of a cold-rolled steel sheet 100 according to one embodiment.

[0031] Referring to Figure 1, the cold-rolled steel sheet 100 comprises a steel sheet base material 10 and an oxide layer 20. The oxide layer 20 is formed inward from both the steel sheet base material 10 and the cold-rolled steel sheet 100 and is separated from the steel sheet base material 10. Specifically, the cold-rolled steel sheet 100 according to one embodiment of the present invention contains, by weight percent, C: 0.0003~0.003%, Mn: 0.25~0.55%, Si: 0.001~0.03%, Al: 0.0005~0.0015%, P: 0.01~0.03%, S: 0.001~0.010%, Cu: 0.03~0.08%, N: 0.008~0.015%, Mo: 0.1~0.3%, O: 0.025~0.055%, and the remainder being Fe and unavoidable impurities.

[0032] The following explains the reasons for limiting the alloy composition. Hereafter, wt% can be expressed as %.

[0033] Carbon (C): 0.0003~0.03% Carbon (C) is an element that affects steel sheet properties such as solid solution strengthening, aging, and bubble defects. The carbon content may be between 0.0003% and 0.0030%. Specifically, the carbon content may be in the range of 0.0005% to 0.0028%.

[0034] If the carbon content exceeds the upper limit of the range, the strength increases, but during annealing, it hinders the development of the texture, reducing formability and causing problems such as bubble defects due to bubbling in the enamel layer during the enamel firing process. If the carbon content exceeds the lower limit of the range, the amount of precipitates that act as sites for absorbing hydrogen in the steel decreases, and grain growth occurs due to the cleaning of grain boundaries during the firing stage, which reduces workability and enamel properties.

[0035] Manganese (Mn): 0.25~0.55% Manganese (Mn) is a representative solid solution strengthening element that prevents hot shortness by precipitating sulfur dissolved in steel in the form of manganese sulfide (MnS) and promotes the precipitation of carbides. The manganese content may be 0.25 to 0.55%. Specifically, the manganese content may be in the range of 0.27 to 0.53%.

[0036] If the manganese content exceeds the upper limit of the range, problems arise such as a decrease in formability due to the formation of central segregation and a decrease in the Ar3 transformation temperature of the steel, causing transformation to occur during enamel firing and deformation of the molded product. If the manganese content exceeds the lower limit of the range, problems arise such as the inability to prevent red-hot brittleness and the promotion of carbide precipitation.

[0037] Silicon (Si): 0.001~0.030% Silicon (Si) is an element that promotes the formation of precipitates that increase strength and act as a hydrogen storage source. The silicon content may be 0.001 to 0.030%. Specifically, the silicon content may be in the range of 0.002 to 0.028%.

[0038] If the silicon content exceeds the upper limit of the range, there is a problem in that an oxide film is formed on the surface of the steel plate, reducing the adhesion of the enamel. If the silicon content exceeds the lower limit of the range, there is a problem in that the effect of increasing strength and forming the precipitate that acts as a hydrogen storage source does not occur.

[0039] Aluminum (Al): 0.0005~0.0015% Aluminum (Al) is used as a powerful deoxidizing agent to remove oxygen from molten steel during the steelmaking process and is a representative element that fixes dissolved nitrogen. The aluminum content may be between 0.0005% and 0.0015%. Specifically, the aluminum content may be in the range of 0.0006% to 0.0014%.

[0040] If the aluminum content exceeds the upper limit of the range, the fraction of inclusions that act as hydrogen storage sources decreases, leading to a significant increase in fish scale formation. Furthermore, the amount of dissolved nitrogen decreases, resulting in the problem of not being able to secure the target material after firing. If the aluminum content exceeds the lower limit of the range, the aluminum is used as a deoxidizing agent and does not exhibit the effect of fixing dissolved nitrogen.

[0041] Phosphorus (P): 0.001~0.010% Phosphorus (P) is a solid solution strengthening element and an element that controls surface pickling properties. The phosphorus content may be 0.01 to 0.03%. Specifically, the phosphorus content may be in the range of 0.011 to 0.028%.

[0042] If the phosphorus content exceeds the upper limit of the range, it not only causes a segregation layer to form inside the steel sheet, reducing formability, but also promotes sulfuric acid reactivity in the enamel pretreatment process, reducing adhesion. If the phosphorus content exceeds the lower limit of the range, it becomes difficult to secure the target material and surface properties.

[0043] Sulfur (S): 0.001~0.010% Sulfur (S) is an element that combines with manganese (Mn) to induce red-hot brittleness. The sulfur content may be 0.001 to 0.010%. Specifically, the sulfur content may be in the range of 0.002 to 0.009%.

[0044] If the sulfur content exceeds the upper limit of the range, it not only significantly reduces ductility and thus workability, but also causes excessive manganese sulfide precipitation, negatively impacting the fish scale properties of the product. If the sulfur content exceeds the lower limit of the range, it causes a decrease in weldability.

[0045] Copper (Cu): 0.03~0.08% Copper (Cu) is an element added to improve solid solution strengthening and adhesion. The copper content may be 0.03 to 0.08%. Specifically, the copper content may be in the range of 0.032 to 0.078%.

[0046] If the copper content exceeds the upper limit of the range, the pickling rate decreases during the acid treatment step, which is a pre-treatment step for enamel coating, resulting in a problem where the steel sheet does not have the appropriate roughness characteristics and the adhesion is reduced. If the copper content exceeds the lower limit of the range, there is a problem where effects such as solid solution strengthening and improved adhesion do not occur.

[0047] Nitrogen (N): 0.008~0.015% Nitrogen (N), along with carbon (C), is a representative interstitial solid solution strengthening element, and is an element used to ensure the target strength level after the firing process. The nitrogen content may be 0.008 to 0.015%. Specifically, the nitrogen content may be 0.0083 to 0.0145%.

[0048] Molybdenum (Mo): 0.1-0.3% Molybdenum (Mo) is an element that provides stable strength by forming various precipitates and oxides and serves as a hydrogen storage source. The molybdenum content may be 0.1 to 0.3%. Specifically, the molybdenum content may be in the range of 0.11 to 0.29%.

[0049] If the molybdenum content exceeds the upper limit of the range, it not only reduces the annealing properties but also increases the rate of surface defects. If the molybdenum content exceeds the lower limit of the range, it does not provide the effects of ensuring strength and providing a hydrogen storage source.

[0050] Oxygen (O): 0.025~0.055% Oxygen (O) is an essential element for forming inclusions that act as hydrogen storage sources in enamel steel. The oxygen content may be between 0.025% and 0.055%. Specifically, the oxygen content may be in the range of 0.0255% to 0.0540%.

[0051] If the oxygen content exceeds the upper limit of the range, severe melting of the refractory material occurs during the steelmaking process, leading to a problem of a high rate of surface defects in the steel plate. If the oxygen content exceeds the lower limit of the range, the effect of forming the inclusions does not occur.

[0052] The remainder consists of iron (Fe). It may also contain unavoidable impurities. Unavoidable impurities refer to impurities that are inevitably mixed in during the steelmaking and grain-oriented electrical steel sheet manufacturing processes. Since unavoidable impurities are widely known, a detailed explanation will be omitted. In one embodiment of the present invention, the addition of elements other than the alloy components described above is not excluded, and a variety of elements are included within a range that does not hinder the technical concept of the present invention. If additional elements are included, they are included in place of the remainder Fe.

[0053] In one embodiment, the cold-rolled steel sheet 100 may contain titanium (Ti), niobium (Nb), chromium (Cr), and vanadium (V). The cold-rolled steel sheet for enamel coating, which has excellent resistance to fish scale, may further selectively contain one or more of the following: Ti: 0.005% or less, Nb: 0.005% or less, Cr: 0.05% or less, and V: 0.003% or less.

[0054] Thus, the cold-rolled steel sheet 100 of the present invention not only does not arbitrarily add elements such as titanium (Ti), which has a higher oxidation rate than iron (Fe), but also improves properties such as enamel adhesion between the steel sheet and the glaze by controlling the surface oxide layer.

[0055] The oxide layer 20 is formed inward from both surfaces of the cold-rolled steel sheet 100, which is the base material 10, and can be divided based on the point where oxygen content reaches 5%. Specifically, the thickness of the oxide layer was determined by analyzing the oxygen concentration inward from the surface of the steel sheet cross-section and dividing it based on the point where oxygen content reaches 5%. More specifically, the thickness of the oxide layer was measured using GDS (Glow Discharge Spectroscopy) starting from the point where oxygen content reached 5%.

[0056] In one embodiment, the thickness of the oxide layer 20 may be in the range of 0.006 to 0.030 μm. Specifically, the thickness of the oxide layer 20 may be 0.007 to 0.028 μm. If the thickness of the oxide layer 20 exceeds the upper limit of the above range, there is a problem that the surface properties of the steel sheet will deteriorate. If the thickness of the oxide layer 20 exceeds the lower limit of the above range, there is a problem that the bonding strength between the glaze layer and the steel sheet will decrease, making it difficult to ensure enamel adhesion and reducing fish scale resistance.

[0057] Thus, enamel products are products in which an organic glaze is applied to a steel plate, and ensuring adhesion between the steel plate and the glaze is extremely important. Specifically, the main component of the glaze is composed of silicon oxide (SiO2) and there is a problem in applying expensive glazes such as NiO to prevent a decrease in adhesion to the steel plate. A high-strength cold-rolled steel plate for enamel coating, which has excellent enamel adhesion and fish scale resistance according to one embodiment of the present invention, can improve enamel adhesion 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 composed of 90% by weight or more of iron oxide (FeO-based), to a certain range, covalent bonding with silicon (Si) atoms in the glaze layer can be promoted, thereby improving enamel adhesion.

[0058] In one embodiment, the cold-rolled steel sheet 100 satisfies the following formula 1. <Expression 1> 0.014≦([Cu]×[Si) / [P]≦0.080 (In Formula 1 above, [Cu], [Si], and [P] represent the weight percentage content of Cu, Si, and P, respectively.)

[0059] Formula 1 above represents the correlation between copper (Cu) and silicon (Si) with respect to phosphorus (P). Formula 1 may be in the range of 0.014 to 0.080. Specifically, Formula 1 may be in the range of 0.0142 to 0.0798. By satisfying the above range, the cold-rolled steel sheet 100 can suppress enamel adhesion and surface void defects.

[0060] If the value of Equation 1 exceeds the upper limit of the range, gas inflow into the steel plate surface increases, leading to surface defects such as air bubbles and reducing the reliability of the product. If the value of Equation 1 falls below the lower limit of the range, surface modification does not occur during the sulfuric acid pretreatment process, resulting in a decrease in enamel properties such as enamel adhesion.

[0061] In one embodiment, the cold-rolled steel sheet 100 satisfies the following formula 2. <Expression 2> 0.0065≦([Al]×[Mo]) / ([C]+[N])≦0.0310 (In Formula 2 above, [Al], [Mo], [C], and [N] represent the weight percentage content of Al, Mo, C, and N, respectively.)

[0062] Equation 2 above represents the correlation between carbon (C) and nitrogen (N) and aluminum (Al) and molybdenum (Mo). In the case of carbon (C) and nitrogen (N) in steel, they react with aluminum (Al) and molybdenum (Mo) to form carbonitrides. The excess solid solution elements not only suppress microstructure growth even when heat treatments at high temperatures, such as in the enamel firing process, are applied, resulting in excellent control over shape freeze-freezing properties, but these precipitates also act as hydrogen storage sources.

[0063] Therefore, it is necessary to consider the reactivity of not only each alloying element but also the combined solid solution elements. Equation 2 above may be in the range of 0.0065 to 0.0310. Specifically, Equation 2 above may be in the range of 0.0067 to 0.0305.

[0064] If the value of Equation 2 exceeds the upper limit of the range, the workability is good, but the rolling and annealing passability decreases, and there is a problem that it induces an increase in manufacturing costs due to the increased use of expensive alloying elements. If the value of Equation 2 falls below the lower limit of the range, not only does the resistance to fish scale decrease because precipitation is not promoted, but there is also a problem that the workability decreases due to an increase in the amount of interstitial solid solution elements.

[0065] In one embodiment, the cold-rolled steel sheet 100 satisfies the following formula 3. <Expression 3> 0.50≦(R max ×20S e ) / P c ≤1.05 (In formula 3 above, P c R is the number of surface irregularities per unit centimeter (cm). max This is the maximum point roughness value (μm), S e This represents the rate of reduction in tempering (%).

[0066] The value of Equation 3 above may be between 0.50 and 1.05. Specifically, the value of Equation 3 above may be in the range of 0.505 to 1.00. If the value of Equation 3 above exceeds the upper limit of the range, there is a problem in that the crystal grains of the steel plate grow after the enamel firing treatment, making it difficult to ensure the target material and enamel properties. If the value of Equation 3 above falls below the lower limit of the range, there is a problem in that the wedge effect on the surface of the steel plate decreases, reducing adhesion with the glaze.

[0067] In one embodiment, the cold-rolled steel sheet 100 may have a yield strength of 220 MPa or more after enamel firing heat treatment. The yield strength of a material used in structural members is a physical property that affects dent resistance and the ability of the member to freeze shape. In the case of enamel products, not only the yield strength at the start of processing, but also the yield strength after the stage of heat treatment at high temperature for a long time for drying after enamel glaze treatment acts as a major factor in considering the stability of the product. When the yield strength after enamel firing heat treatment is 220 MPa or more, there is an advantage that the product has excellent stability during the heat treatment stage for drying after glaze treatment.

[0068] In one embodiment, the cold-rolled steel sheet 100 may have an enamel adhesion of 95% or more. Specifically, the enamel adhesion may be 96% or more. Within this range, the cold-rolled steel sheet 100 can be used as an enamel material even when using inexpensive glazes. If the enamel adhesion is lower than the above range, there is a problem that the rate of fish scale formation due to hydrogen in the steel increases.

[0069] In one embodiment, the cold-rolled steel sheet 100 has a hydrogen permeation ratio of 600 seconds / mm². 2 The above may also be applicable. Specifically, the hydrogen permeation ratio may be 610 seconds / mm 2 The above may also be true. The upper limit of the hydrogen permeation ratio is not particularly limited, but for example, 1,700 seconds / mm 2 This may also be the case. The hydrogen permeation ratio is a representative index for evaluating the fish scale resistance, which indicates the resistance to fish scale defects, a fatal defect when enamel steel is applied, and represents the ability to fix hydrogen within the cold-rolled steel sheet.

[0070] Specifically, the hydrogen permeation ratio is a value obtained by measuring the time it takes for hydrogen to permeate out in the opposite direction to the direction in which hydrogen is generated in one direction of the steel plate, and then dividing this time by the square of the thickness of the material. If the hydrogen permeation ratio is excessively low, when evaluating the resistance to fish scale defects by accelerating heat treatment at 200°C for 24 hours after enamel treatment, a defect rate of 50% or more occurs, which poses a problem for the use of the product as a stable enamel product.

[0071] A method for manufacturing cold-rolled steel sheet 100 according to another embodiment of the present invention is as follows, by weight %, C: 0.0003~0.003%, Mn: 0.25~0.55%, Si: 0.001~0.03%, Al: 0.0005~0.0015%, P: 0.01~0.03%, S: 0.001~0.010%, Cu: 0.03~0.08%, N: 0.008~0.015%, Mo: 0.1~0.3%, The process includes the steps of: reheating a steel slab containing 0.025-0.055% O and the remainder Fe and unavoidable impurities; hot rolling the heated slab; winding the hot-rolled steel sheet; cold rolling the wound hot-rolled steel sheet; annealing and temper-rolling the cold-rolled steel sheet to produce an annealed sheet; and enamel-fired heat treatment of the annealed sheet. The detailed description of the steel slab is the same as that of the cold-rolled steel sheet described above, to the extent that it does not contradict the description, so redundant explanations are omitted.

[0072] The step of reheating the steel slab is a step to facilitate the subsequent hot rolling process and to homogenize the steel slab. This heating can refer to reheating.

[0073] In one embodiment, the step of reheating the steel slab may be a step of hot rolling in the range of 1,150 to 1,280°C. Specifically, the reheating temperature range may be 1,150 to 1,280°C. If the temperature exceeds the upper limit of the temperature range, the amount of surface scale increases at the slab heating temperature, leading to greater material loss, and the problem of increased energy costs due to the rise in the heat source arises. If the temperature falls below the lower limit of the temperature range, the rolling load increases rapidly during the hot rolling process, leading to a problem of reduced hot workability.

[0074] In one embodiment, the step of reheating the steel slab is performed at a finish hot rolling temperature in the range of 890 to 950°C. Specifically, the finish hot rolling temperature is performed in the range of 900 to 945°C.

[0075] If the finishing hot rolling temperature exceeds the upper limit of the range, the surface scale delamination rate decreases, the surface defect rate increases, and the impact toughness decreases because uniform hot rolling properties cannot be ensured. If the finishing hot rolling temperature falls below the lower limit of the range, the hot rolling is finished in a low-temperature region, which causes rapid grain mixing and leads to a decrease in rollability and workability.

[0076] In one embodiment, the step of winding the hot-rolled steel sheet is performed in a temperature range of 580 to 720°C. Specifically, the temperature range may be 590 to 700°C. The hot-rolled steel sheet can be cooled on a run-out table (ROT) before winding.

[0077] If the temperature exceeds the upper limit of the aforementioned temperature range, corrosion resistance during winding decreases, promoting grain boundary segregation of phosphorus (P) and reducing cold rolling properties, which also negatively affects the processability of the final product. If the temperature falls below the lower limit of the aforementioned temperature range, the non-uniformity of the temperature in the width direction increases during the cooling and soaking process, leading to a larger material deviation due to differences in precipitation behavior and a decrease in enamel properties.

[0078] In one embodiment, after the step of winding the hot-rolled steel sheet, the step of pickling the hot-rolled steel sheet may be further included. The pickling step can remove the scale generated during hot rolling.

[0079] In one embodiment, the step of cold-rolling the wound hot-rolled steel sheet is performed with a cold reduction ratio of 60 to 90%. Specifically, the cold reduction ratio is in the range of 63 to 88%.

[0080] If the cold reduction ratio exceeds the upper limit of the range, not only does the material harden and its workability decrease, but the load on the cold rolling mill increases, leading to a decrease in workability. If the cold reduction ratio falls below the lower limit of the range, the recrystallization driving force in the subsequent heat treatment process is not secured, resulting in the problem of unrecrystallized grains remaining locally and a decrease in workability.

[0081] In one embodiment, the step of producing an annealed sheet by annealing and temper-rolling a cold-rolled steel sheet is carried out at an annealing temperature range of 720 to 850°C for 10 to 70 seconds.

[0082] If the annealing temperature exceeds the upper limit of the aforementioned annealing temperature range, there is a problem of reduced annealing passability due to softening caused by a decrease in high-temperature strength. If the annealing temperature falls below the lower limit of the aforementioned annealing temperature range, the deformation formed by cold rolling is not sufficiently removed, resulting in a significant decrease in workability and a problem of not being able to ensure enamel properties.

[0083] If the annealing holding time exceeds the upper limit of the range, the workability is good, but the non-uniformity of the crystal grains increases, which degrades the enamel properties. If the annealing holding time falls below the lower limit of the range, recrystallization is not completed and unrecrystallized grains remain, resulting in a decrease in formability.

[0084] In one embodiment, the step of producing an annealed sheet by annealing and temper-rolling a cold-rolled steel sheet is performed with a reduction ratio of 2.5% or less during the temper-rolling. The shape of the material can be controlled by the temper-rolling, and a desired surface roughness can be obtained. Specifically, the temper-rolling is performed with a reduction ratio in the range of 0.3 to 2.2%.

[0085] If the reduction ratio exceeds the upper limit, work hardening occurs, leading to a decrease in workability. Furthermore, if the temper reduction ratio is high during the heat treatment in the enamel process, abnormal growth of strain-induced crystal grains occurs, significantly reducing the yield strength of the enamel product and decreasing its dent resistance.

[0086] In one embodiment, the step of enamel firing heat treatment of the annealed plate may be a step of enamel firing heat treatment of the annealed plate in a temperature range of 780 to 850°C. The enamel firing heat treatment may be a step for drying the enamel-treated glaze. Specifically, the temperature range may be 790 to 840°C.

[0087] If the temperature range exceeds the upper limit, the surface defect rate increases due to the increased thickness of the oxide layer, and the increased energy consumption contributes to higher manufacturing costs. If the temperature range falls below the lower limit, the wettability of the glaze decreases, making it impossible to ensure proper enamel adhesion.

[0088] The following describes in detail, through examples and comparative examples, a high-strength cold-rolled steel sheet for enamel coating with excellent fishscale resistance according to the present invention and a method for manufacturing the same. The following examples are for reference to illustrate the present invention in detail, and the present invention is not limited thereto and can be realized in various forms.

[0089] A method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention includes the steps of: heating a steel slab containing carbon (C): 0.11-0.13%, silicon (Si): 0.05% or less, manganese (Mn): 0.1-0.6%, aluminum (Al): 0.02-0.06%, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, nitrogen (N): 0.006% or less, with the remainder being Fe and other unavoidable impurities; hot-rolling the heated slab; cooling the hot-rolled steel sheet; winding the cooled hot-rolled steel sheet; cold-rolling the wound hot-rolled steel sheet; annealing the cold-rolled steel sheet; and secondary-rolling the annealed cold-rolled steel sheet. The detailed description of the steel slab is the same as that of the cold-rolled steel sheet described above, to the extent that it does not contradict the description of the cold-rolled steel sheet described above, so redundant descriptions are omitted.

[0090] First, the steel slab is heated. This heating step is performed at a temperature of 1,150°C or higher. This temperature is necessary because the precipitates formed in the steel must be redissolved during this heating step.

[0091] Next, in the step of hot rolling the heated slab, a hot-rolled steel sheet can be obtained by hot rolling the heated slab. In one embodiment, the steel slab is hot-finished at a temperature of Ar3 or higher. The temperature of Ar3 or higher may be 890°C or higher. By performing the hot-finish rolling at a temperature of Ar3 or higher, rolling is performed in the austenite single-phase region. If rolling is performed in a range exceeding the above temperature range, there is a problem that the rolling stability decreases due to the non-uniform material.

[0092] The cooling step for hot-rolled steel sheets is to be maintained within a time range of 150 to 1,200 seconds. If the cooling time falls below the lower limit of this range, a large amount of pearlite phase is formed, leading to the problem of large carbides being generated in the final material. When the carbides are large, they can become the starting point for cracks, reducing the pore expansion rate.

[0093] If the time exceeds the upper limit of the aforementioned time range, the formation of the pearlite phase is suppressed, but there is a problem in that a thick surface oxide layer is formed due to oxidation of the surface of the hot-rolled steel sheet. When the aforementioned surface oxide layer is formed thickly, not only is the pickling time before cold rolling lengthened, but the possibility of inducing surface defects also increases significantly. Specifically, the step of cooling the hot-rolled steel sheet can be cooled to 710°C so that it is held at a temperature between 710 and 860°C for 150 to 1,200 seconds.

[0094] The step of winding the cooled hot-rolled steel sheet allows the cooled hot-rolled steel sheet to be wound at 560-700°C. In order to ensure a crystal grain size appropriate for strength and workability, the winding step of the cooled hot-rolled steel sheet can be controlled within the winding temperature range. If the winding temperature is excessively low, there is a problem of the crystal grains becoming excessively fine, and if the winding temperature is excessively high, there is a problem of the crystal grains becoming excessively coarse.

[0095] In the cold-rolling stage of the wound hot-rolled steel sheet, a cold-rolled steel sheet can be produced by cold-rolling with a reduction ratio of 80-95%. The thickness of a typical hot-rolled steel sheet is 2-4 mm, and a reduction ratio of 80% or more is required to reduce the thickness by 0.4 mm.

[0096] If the reduction ratio exceeds the upper limit, the deformation resistance due to rolling increases excessively, making rolling difficult. If the reduction ratio falls below the lower limit, the target thickness of the cold-rolled steel sheet cannot be reached.

[0097] In one embodiment, the step of pickling the wound hot-rolled steel sheet may be further included before the step of cold-rolling it. The pickling step can remove the scale generated during hot-rolling.

[0098] The annealing step of cold-rolled steel sheets involves annealing them at a temperature of 620 to 760°C to produce annealed steel sheets. This annealing of the cold-rolled steel sheets removes the internal stresses formed during cold rolling and ensures workability. To remove these internal stresses and ensure workability, a process of annealing at a sufficiently high temperature is necessary to induce recrystallization.

[0099] If the temperature exceeds the upper limit, it may induce plate fracture defects due to a decrease in high-temperature strength. If the temperature falls below the lower limit, there is a problem in that recrystallization does not occur in the cold-rolled steel sheet having the component system of the present invention.

[0100] In the secondary rolling stage of the annealed cold-rolled steel sheet, the final steel sheet can be manufactured by secondary rolling the annealed cold-rolled steel sheet at a reduction ratio of 6 to 18%. If the reduction ratio exceeds the upper limit, there is a problem in that the desired level of workability cannot be secured due to a decrease in the elongation ratio. If the reduction ratio falls below the lower limit, there is a problem in that it is not sufficient to obtain the target strength. [Examples]

[0101] The following describes in detail, through examples and comparative examples, a high-strength cold-rolled steel sheet for enamel coating with excellent fishscale resistance according to the present invention and a method for manufacturing the same. The following examples are for reference to illustrate the present invention in detail, and the present invention is not limited thereto and can be realized in various forms.

[0102] <Steel slabs made from inventive steels 1-5 and comparative steels 1-5> Table 1 below shows the composition of the main components for invention steels 1-5 and comparative steels 1-5. Slabs were manufactured using the alloy compositions shown in Table 1, through a converter, secondary refining, and continuous casting process. The values ​​of Equations 1 and 2 are shown based on the composition of the main components. Equation 1 represents ([Cu]×[Si]) / [P], and Equation 2 represents ([Al]×[Mo]) / ([C]+[N]). In Equations 1 and 2, [Cu], [Si], [P], [Al], [Mo], [C], and [N] represent their respective weight percent.

[0103] [Table 1]

[0104] As can be seen from Table 1, the comparative steel is not included in the steel composition range of the present invention compared to the inventive steel, and thus it can be confirmed that it is not included in the values ​​of Formulas 1 and 2 above.

[0105] <Examples 1-9 and Comparative Examples 1-9> Table 2 below shows the results of manufacturing cold-rolled steel sheets from the aforementioned slabs using the manufacturing conditions disclosed in Table 2. Specifically, the slabs were held in a heating furnace for 2 hours, followed by hot rolling, at which time the thickness of the hot-rolled steel sheets was adjusted to 4.0 mm. After hot-rolling, the hot-rolled steel sheets were pickled to remove the oxide film from the surface, and then cold-rolled at the respective reduction ratios.

[0106] [Table 2]

[0107] As can be seen in Table 2, Examples 1 to 9 were carried out using slabs of Inventive Steel 1 to 5, which are included in the composition range of the present invention, and within the slab reheating temperature, finish hot rolling temperature, coiling temperature, cold reduction ratio, annealing temperature, holding time, temper rolling ratio, and enamel firing temperature range of the present invention. In contrast, Comparative Examples 1 to 4 were carried out using slabs of Inventive Steel 1 to 4, which are included in the composition range of the present invention, and controlled so that at least one of the manufacturing conditions in Table 2 does not correspond to the conditions of the present invention. Furthermore, Comparative Examples 5 to 9 were carried out using Comparative Steel 1 to 5 in Table 1, and controlled so that the manufacturing conditions in Table 2 fall within the range of the present invention.

[0108] <Measurement results of properties of cold-rolled steel sheet> Table 3 below shows the thickness of the oxide layer, sheet treadability, yield strength, presence or absence of bubble defects, presence or absence of fish scale formation, enamel adhesion, and hydrogen permeation ratio of Examples 1 to 9 and Comparative Examples 1 to 9, which were manufactured using the inventive steel and comparative steel described in Table 1 under the manufacturing conditions of Table 2.

[0109] The thickness of the oxide layer was determined by analyzing the oxygen concentration from the surface of the steel sheet inward using GDS (Glow Discharge Spectroscopy). The oxide layer and the base material were separated using the point containing 5% oxygen as a reference, and the thickness up to the point containing 5% oxygen was measured. This measurement was taken three times, and the average value was expressed. In the case of sheet pulverability, if the operability in the continuous casting, hot rolling, and cold rolling processes is 90% or more compared to the productivity of normal materials, it is indicated as good (indicated by "○"). If the productivity is less than 90% or the defect rate is 10% or more, it is indicated as poor (indicated by "×").

[0110] The yield strength was measured by preparing tensile test specimens (according to ASTM 13B standard) from test specimens that had been heat-treated in a firing furnace at various temperatures for 15 minutes to simulate the enamel glaze drying process on steel plates, and then conducting tensile tests at a crosshead speed of 10 mm / min.

[0111] The enamel-coated test specimens were cut to appropriate sizes according to their intended use, and after heat treatment, the enamel-coated specimens were completely degreased. A standard glaze (Check Frit), which is relatively susceptible to fish scale defects, was applied, and the specimens were held at 300°C for 10 minutes to remove moisture. After drying, the specimens were fired at their respective firing temperatures for 15 minutes to highlight the differences in enamel properties such as adhesion, and then cooled to room temperature. During this process, the firing furnace atmosphere was set to a harsh condition with a dew point of 20°C, which is prone to the occurrence of fish scale defects.

[0112] After the enamel coating was completed, the test specimens underwent a fish-scale acceleration experiment by being held in a 200°C oven for 24 hours. After the accelerated fish-scale treatment, the presence or absence of fish-scale defects was observed visually. If no fish-scale defects occurred, it was marked as good (indicated by "○"), and if fish-scale defects occurred, it was marked as poor (indicated by "×").

[0113] Enamel adhesion, which evaluates the adhesion between the steel plate and the glaze, is defined in the American Society for Testing and Materials standard, ASTM C313-78. This is done by applying a constant load to the enamel layer with a steel ball and then evaluating the degree of electrical conductivity at that point, thereby indexing the degree of detachment of the enamel glaze layer. In this invention, the evaluation results for enamel adhesion were set with the goal of achieving an adhesion rate of 95% or higher to ensure the application stability of relatively inexpensive glazes.

[0114] The aforementioned bubble defects were determined by visually observing the enamel surface of test specimens that had been held in a 200°C oven for 24 hours after enamel treatment, and were judged as excellent (indicated by "○") or poor (indicated by "×"). The hydrogen permeation ratio is one of the indices used to evaluate the resistance to fish scale, a fatal defect in enamel. It is calculated by generating hydrogen in one direction of the steel plate using the experimental method described in the European standard (EN10209-2013), measuring the time (ts, in seconds) it takes for the hydrogen to permeate to the opposite side, and expressing this value as the square of the material thickness (t, in mm), resulting in ts / t² (in seconds / mm). 2 It is represented as ).

[0115] [Table 3]

[0116] Referring to Table 3 above, Invention Examples 1 to 9, which satisfy all the diverse characteristics of the present invention, such as the component composition, manufacturing conditions, surface properties, and oxide layer thickness, not only have good sheet permeability but also satisfy the scope of the present invention in related indices such as oxide layer thickness. Furthermore, Invention Examples 1 to 9 not only do not produce enamel defects such as fish scale and bubble defects, but also have a hydrogen permeability ratio of 600 seconds / mm 2As described above, the enamel adhesion index of 95% or higher and the yield strength of 220 MPa or higher after enamel firing heat treatment are satisfied, ensuring the properties targeted by the present invention. In contrast, in the case of Comparative Examples 5 to 9, which do not satisfy the composition of the present invention, not only do most fail to satisfy the thickness of the surface oxide layer, hydrogen permeability ratio, and enamel adhesion presented in the present invention, but in most cases, fish scale and pore defects occur even when observed with the naked eye after enamel treatment, posing problems for applicability to the target applications. Furthermore, in Comparative Examples 1 to 4, which satisfied the composition of the present invention, but whose manufacturing conditions in the various annealing processes during hot rolling did not satisfy the control range of the present invention, the thickness of the surface oxide layer exceeded the range presented in the present invention, it was confirmed that the enamel adhesion was less than 95%, and enamel defects such as pore defects or fish scale occurred after enamel treatment, resulting in poor sheet treadability and a yield strength of less than 220 MPa after enamel firing heat treatment, thus failing to secure the target properties overall.

[0117] Thus, by satisfying the alloy composition and alloy range, the cold-rolled steel sheet for enamel coating, according to one embodiment of the present invention, which has excellent enamel adhesion and fish scale resistance, can control the thickness of the oxide layer formed in the interior direction to an appropriate level, thereby providing a high-strength cold-rolled steel sheet for enamel coating with excellent fish scale resistance.

[0118] Furthermore, the cold-rolled steel sheet exhibits significantly improved enamel properties through high-speed continuous annealing operation, maintains a high strength level after enamel firing heat treatment, and allows for optimization of surface roughness characteristics during the heat treatment and temper rolling stages in a continuous annealing furnace to enhance adhesion. By suppressing grain growth during enamel firing, such as residual nitrogen in the inner surface layer of the steel sheet, stable material properties can be ensured even after high-temperature firing.

[0119] The present invention is not limited to the embodiments and / or examples described above, and can be manufactured in a variety of different forms. A person with ordinary skill in the art to which the present invention belongs will understand that the invention can be carried out in other specific forms without altering the technical idea or essential features of the invention. Therefore, the embodiments and / or examples described above should be understood to be illustrative and non-limiting in all respects.

Claims

1. In weight percent, it consists of C: 0.0003-0.003%, Mn: 0.25-0.55%, Si: 0.001-0.03%, Al: 0.0005-0.0015%, P: 0.01-0.03%, S: 0.001-0.010%, Cu: 0.03-0.08%, N: 0.008-0.015%, Mo: 0.1-0.3%, O: 0.025-0.055%, and the remainder being Fe and unavoidable impurities. Contains an oxidized layer, The thickness of the oxide layer formed in the direction from the surface of the cold-rolled steel sheet is 0.006 to 0.030 μm. The following equation 1 is satisfied, A cold-rolled steel sheet that satisfies the following equation 2. <Formula 1> 0.014≦([Cu]×[Si) / [P]≦0.080 (In Formula 1 above, [Cu], [Si], and [P] represent the weight percentage content of Cu, Si, and P, respectively.) <Formula 2> 0.0065≦([Al]×[Mo]) / ([C]+[N])≦0.0310 (In formula 2 above, [Al], [Mo], [C], and [N] represent the weight percentage content of Al, Mo, C, and N, respectively.)

2. The cold-rolled steel sheet according to claim 1, wherein the yield strength after enamel firing heat treatment is 220 MPa or more.

3. The cold-rolled steel sheet according to claim 1, wherein the enamel adhesion is 95% or more.

4. Hydrogen permeation ratio of 600 seconds / mm ２ The cold-rolled steel sheet according to claim 1.

5. The process involves reheating a steel slab, composed of, by weight percent, C: 0.0003–0.003%, Mn: 0.25–0.55%, Si: 0.001–0.03%, Al: 0.0005–0.0015%, P: 0.01–0.03%, S: 0.001–0.010%, Cu: 0.03–0.08%, N: 0.008–0.015%, Mo: 0.1–0.3%, O: 0.025–0.055%, and the remainder Fe and unavoidable impurities, in a temperature range of 1,150–1,280°C. The heated slab is hot-rolled at a finish hot-rolling temperature in the range of 890 to 950°C. The process involves winding the hot-rolled steel sheet at a temperature range of 580 to 720°C, The process involves cold-rolling the wound hot-rolled steel sheet at a cold reduction ratio of 60-90%, The process involves annealing a cold-rolled steel sheet at an annealing temperature of 720 to 850°C for 10 to 70 seconds, and then temper-rolling it to a reduction ratio of 2.5% or less to produce an annealed sheet. The process includes the step of heat-treating the annealed plate by enamel firing at a temperature range of 780 to 850°C. A method for manufacturing a cold-rolled steel sheet, wherein the thickness of the oxide layer formed in the inward direction from the surface of the cold-rolled steel sheet is 0.006 to 0.030 μm.

6. A method for manufacturing a cold-rolled steel sheet according to claim 5, satisfying the following formula 1. <Formula 1> 0.014≦([Cu]×[Si) / [P]≦0.080 (In Formula 1 above, [Cu], [Si], and [P] represent the weight percentage content of Cu, Si, and P, respectively.)

7. A method for manufacturing a cold-rolled steel sheet according to claim 5, satisfying the following formula 2. <Formula 2> 0.0065≦([Al]×[Mo]) / ([C]+[N])≦0.0310 (In formula 2 above, [Al], [Mo], [C], and [N] represent the weight percentage content of Al, Mo, C, and N, respectively.)

8. A method for manufacturing a cold-rolled steel sheet according to claim 5, satisfying the following formula 3. <Formula 3> 0.50≦(R ｍａｘ ×20S ｅ ) / P ｃ ≦1.05 (In the above formula 3, P ｃ R is the number of surface irregularities per unit centimeter (cm). ｍａｘ This is the maximum point roughness value (μm), S ｅ This represents the rate of reduction in tempering (%).

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