WO2023113535A1 - Cold rolled steel sheet for enamel, having excellent anti-fishscale properties and method for manufacturing same - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet and a method for manufacturing same, the cold rolled steel sheet comprising, in wt%, 0.0003 to 0.003% of C, 0.25 to 0.55% of Mn, 0.001 to 0.03% of Si, 0.0005 to 0.0015% of Al, 0.01 to 0.03% of P, 0.001 to 0.010% of S, 0.03 to 0.08% of Cu, 0.008 to 0.015% of N, 0.1 to 0.3% of Mo, 0.025 to 0.055% of O, and a balance of Fe and inevitable impurities, and comprising an oxide layer, wherein the thickness of the oxide layer, which is formed from the surface of the cold rolled steel sheet toward the inside thereof, may be 0.006 to 0.030 μm.

Description

Cold-rolled steel sheet for enamel with excellent fishscale resistance and manufacturing method thereof

The present invention relates to a steel sheet, and more particularly, to a cold-rolled steel sheet for vitreous enameling with excellent fishscale resistance and a manufacturing method thereof.

Enameled steel sheet is a surface-treated product in which corrosion resistance, weather resistance, heat resistance, and chemical resistance are improved by applying a glassy glaze on a base steel sheet such as a hot-rolled steel sheet or a cold-rolled steel sheet and then firing at a high temperature. The enamel steel sheet is used as a material for building exteriors, home appliances, tableware, and various industrial uses.

Although rimmed steel was mainly used for the enamel steel sheet in the early stage, recently, as the continuous casting method is used to improve productivity, most of the materials are continuously cast by the continuous casting method. Fishscale defects, one of the problems of the enamel steel sheet, are supersaturated in the steel during the process of cooling after firing hydrogen dissolved in the steel during the manufacturing process of the enamel product, and then released to the surface of the steel over time. It is a defect that causes the enamel layer to fall off in the shape of meat vinyl.

When the fishscale defect occurs, there is a problem in that the value of the enamel product is greatly reduced because rust is intensively generated in the defect area. Accordingly, in the manufacture of the enamel steel sheet, in order to prevent the fishscale defect, various methods related to forming a large amount of sites capable of holding hydrogen dissolved in the steel inside the steel are utilized.

Specifically, a method of securing workability and fishscale resistance by applying an open coil annealing (OCA) method, which is a type of phase annealing, has been proposed. However, since the heat treatment process takes a relatively long time, the above method has problems in that productivity is lowered, manufacturing cost is increased, and quality deviation is greatly generated in the coil. In addition, since it is not easy to control the decarburization reaction in the heat treatment process, decarburization proceeds excessively, and the crystal grain boundaries of the steel sheet are softened after the enamel firing heat treatment, resulting in a decrease in shape freezing.

In this way, in order to overcome problems such as productivity inferiority and manufacturing cost increase due to long-time annealing, the recently developed enamel steel sheet utilizes a continuous annealing process. Enamel steel by the continuous annealing method using the continuous annealing process typically uses, for example, precipitates such as titanium (Ti) or inclusions secured by the non-deoxidation method as a hydrogen storage source based on ultra-low carbon steel. However, even in this case, as a large amount of carbonitride-forming elements are required, they act as a factor in cost increase and sheetability decrease, and the problem of surface defects caused by the generated precipitates or non-deoxidized inclusions is a fundamental problem to be solved. am.

As a large amount of titanium is added to suppress the reaction of hydrogen, which is the cause of fish scale, in the enamel steel sheet using titanium-based precipitates, exposure clogging due to titanium nitride (TiN) and inclusions occurs in the continuous casting stage of the steelmaking process. This causes a decrease in workability and a direct problem in production load. In addition, the titanium nitride mixed in the molten steel exists on the upper part of the steel plate and causes a blister defect, which is a typical bubble defect. In addition, a large amount of added titanium forms a titanium-based oxide layer, which inhibits the adhesion between the steel plate and the glaze layer. cause problems

In addition, in the case of enamel steel applied to enamel steel sheets, as most of the materials for structural members are used, the strength of the materials is raised to strengthen competitiveness through weight reduction of the members used. There is a demand for material properties after firing heat treatment in a high temperature range. In order to meet the above requirements, it is required to secure a yield strength of 220 MPa or more after firing the enameled steel.

A technical problem to be solved by the present invention is to provide a high-strength cold-rolled steel sheet for vitreous enameling, which has a yield strength of 220 MPa or more after enameling, no bubble defects, and excellent enamel adhesion and fishscale resistance.

Another technical problem to be solved by the present invention is to provide a method for manufacturing a cold-rolled steel sheet having the above advantages.

In the cold-rolled steel sheet according to an embodiment of the present invention, 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 including Fe and unavoidable impurities, including an oxide layer And, the thickness of the oxide layer formed in an inward direction from the surface of the cold-rolled steel sheet may be 0.006 to 0.030 μm.

In one embodiment, the cold-rolled steel sheet may satisfy Equation 1 below.

<Equation 1>

0.014 ≤ ([Cu] × [Si) / [P] ≤ 0.080

(In Equation 1, [Cu], [Si], and [P] mean the weight % content of Cu, Si, and P, respectively)

In one embodiment, the cold-rolled steel sheet may satisfy Equation 2 below.

<Equation 2>

0.0065 ≤ ([Al] × [Mo]) / ([C] + [N]) ≤ 0.0310

(In Equation 2, [Al], [Mo], [C] and [N] mean the weight % content of Al, Mo, C, and N, respectively)

In one embodiment, the cold-rolled steel sheet may satisfy Equation 3 below.

<Equation 3>

0.50 ≤ (R _{max ×} 20S _e ) / P _c ≤ 1.05

(In Equation 3 above, P _c is the number of surface irregularities per unit centimeter (cm), R _max is the maximum point roughness value (μm), and S _e is the temper reduction rate (%))

In one embodiment, the cold-rolled steel sheet may have a yield strength of 220 MPa or more after enamel firing heat treatment. In one embodiment, the cold-rolled steel sheet may have enamel adhesion of 95% or more. In one embodiment, the hydrogen permeability ratio of the cold-rolled steel sheet may be 600 sec/mm ² or more.

According to another embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet contains, 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 Fe and unavoidable impurities Reheating the slab to a temperature range of 1,150 to 1,280 ° C., hot rolling the heated slab to a finish hot rolling temperature range of 890 to 950 ° C., winding the hot-rolled hot-rolled steel sheet to a temperature range of 580 to 720 ° C. Step, cold-rolling the rolled hot-rolled steel sheet at a cold rolling reduction rate of 60 to 90%, annealing the cold-rolled cold-rolled steel sheet at an annealing temperature of 720 to 850 ° C. for 10 to 70 seconds, and a reduction rate of 2.5% or less It may include preparing an annealed sheet by temper rolling, and subjecting the annealed sheet to enamel firing heat treatment at a temperature range of 780 to 850 °C.

In one embodiment, a method of manufacturing a cold-rolled steel sheet may satisfy Equation 1 below.

<Equation 1>

0.014 ≤ ([Cu] × [Si) / [P] ≤ 0.080

(In Equation 1, [Cu], [Si], and [P] mean the weight % content of Cu, Si, and P, respectively)

In one embodiment, the method for manufacturing a cold-rolled steel sheet may satisfy Equation 2 below.

<Equation 2>

0.0065 ≤ ([Al] × [Mo]) / ([C] + [N]) ≤ 0.0310

(In Equation 2, [Al], [Mo], [C] and [N] mean the weight % content of Al, Mo, C, and N, respectively)

In one embodiment, a method for manufacturing a cold-rolled steel sheet may satisfy Equation 3 below.

<Equation 3>

0.50 ≤ (R _{max ×} 20S _e ) / P _c ≤ 1.05

(In Equation 3 above, P _c is the number of surface irregularities per unit centimeter (cm), R _max is the maximum point roughness value (μm), and S _e is the temper reduction rate (%))

The cold-rolled steel sheet according to an embodiment of the present invention provides a cold-rolled steel sheet excellent in enamel adhesion and fishscale resistance by controlling the steel composition, and is used in various applications such as home appliances, chemical appliances, kitchen appliances, sanitary appliances, and interior and exterior materials for buildings. Can be used for absence.

A method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention may provide a method for manufacturing a cold-rolled steel sheet having the above advantages.

1 is a schematic cross-sectional view of a cold-rolled steel sheet according to an embodiment.

Terms such as first, second and third are used to describe, but are not limited to, various parts, components, regions, layers and/or sections. These terms are only used to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Accordingly, a first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.

The terminology used herein is only for referring to specific embodiments and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. The meaning of "comprising" as used herein specifies particular characteristics, regions, integers, steps, operations, elements and/or components, and the presence or absence of other characteristics, regions, integers, steps, operations, elements and/or components. Additions are not excluded.

When a part is referred to as being “on” or “on” another part, it may be directly on or on the other part or may be followed by another part therebetween. In contrast, when a part is said to be “directly on” another part, there is no intervening part between them.

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having meanings consistent with related technical literature and currently disclosed content, and are not interpreted in ideal or very formal meanings unless defined.

In addition, unless otherwise specified, % means weight%, and 1ppm is 0.0001 weight%.

In one embodiment of the present invention, the meaning of further including an additional element means replacing and including iron (Fe) as much as the additional amount of the additional element.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments described herein.

1 is a schematic cross-sectional view of a cold-rolled steel sheet 100 according to an embodiment.

Referring to FIG. 1, a cold-rolled steel sheet 100 includes a steel sheet base material 10 and an oxide layer 20. It is formed in an inward direction from both surfaces of the steel sheet base material 10 and the cold-rolled steel sheet 100, and includes an oxide layer 20 that is distinguished from the steel sheet base material 10. Specifically, in the cold-rolled steel sheet 100 according to an embodiment of the present invention, in 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 Fe and unavoidable impurities. .

In the following, the reason for limiting the alloy components will be explained. Hereinafter, wt% may be expressed as %.

Carbon (C): 0.0003 to 0.03%

Carbon (C) is an element that affects steel sheet properties such as solid solution strengthening, aging, and cell defects. The carbon content may be 0.0003 to 0.0030%. Specifically, the carbon content may range from 0.0005 to 0.0028%.

When the carbon content is outside the upper limit of the range, the strength is increased, but the development of the texture during annealing is hindered, and the formability is lowered, and bubble defects are caused by bubbling of the enamel layer in the enamel firing process. There is a problem with When the carbon content is outside the lower limit of the range, the amount of precipitate acting as a site for occluding hydrogen in the steel decreases, and crystal grain growth occurs due to grain boundary cleaning in the firing step, resulting in a problem of deteriorating workability and enamel characteristics. there is.

Manganese (Mn): 0.25 to 0.55%

Manganese (Mn) is a typical solid solution strengthening element, which precipitates sulfur dissolved in steel in the form of manganese sulfide (MnS) to prevent hot shortness and promotes the precipitation of carbides. The manganese content may be 0.25 to 0.55%. Specifically, the manganese content may range from 0.27 to 0.53%.

When the content of manganese exceeds the upper limit of the range, problems in formability due to center segregation and lowering of the Ar3 transformation temperature of the steel cause transformation to occur during enamel firing, resulting in deformation of molded articles. When the content of manganese is outside the lower limit of the range, there is a problem in that the effect of preventing red-hot brittleness and encouraging precipitation of carbides is not expressed.

Silicon (Si): 0.001 to 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 range from 0.002 to 0.028%.

When the silicon content exceeds the upper limit of the above range, an oxide film is formed on the surface of the steel sheet, resulting in reduced enamel adhesion. When the content of the silicon is out of the lower limit of the range, there is a problem in that the effect of increasing the strength and forming the precipitate acting as a hydrogen storage source is not expressed.

Aluminum (Al): 0.0005 to 0.0015%

Aluminum (Al) is used as a strong deoxidizer to remove oxygen from molten steel in the steelmaking stage, and is a representative element that fixes solid nitrogen. The aluminum content may be 0.0005 to 0.0015%. Specifically, the aluminum content may range from 0.0006 to 0.0014%.

When the aluminum content is outside the upper limit of the range, the fraction of inclusions acting as a hydrogen storage source is reduced to significantly increase the occurrence of fishscale, and the amount of dissolved nitrogen is lowered to secure a target material after firing. there is a problem. When the content of the aluminum is out of the lower limit of the range, it is used as the deoxidizer, and there is a problem in that the effect of fixing solid nitrogen is not expressed.

Phosphorus (P): 0.001 to 0.010%

Phosphorus (P) is a solid solution strengthening element and an element that controls surface pickling. The phosphorus content may be 0.01 to 0.03%. Specifically, the phosphorus content may range from 0.011 to 0.028%.

When the phosphorus content is outside the upper limit of the above range, a segregation layer is formed inside the steel sheet to deteriorate moldability, and adhesion is deteriorated by promoting sulfuric acid reactivity in the enamel pretreatment process. When the phosphorus content is out of the lower limit of the range, it is difficult to secure target material and surface characteristics.

Sulfur (S): 0.001 to 0.010%

Sulfur (S) is an element that causes red brittleness by combining with manganese (Mn). The sulfur content may be 0.001 to 0.010%. Specifically, the sulfur content may range from 0.002 to 0.009%.

When the sulfur content is outside the upper limit of the range, ductility is greatly reduced to deteriorate processability, and manganese sulfide is excessively precipitated, which adversely affects the fishscale property of the product. When the sulfur content is out of the lower limit of the range, there is a problem of deteriorating weldability.

Copper (Cu): 0.03 to 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 range from 0.032 to 0.078%.

When the copper content is outside the upper limit of the range, the pickling speed is lowered in the acid treatment step, which is a pre-enamel process, so that the roughness characteristics suitable for the steel sheet cannot be obtained, resulting in a decrease in adhesion. When the content of copper is out of the lower limit of the range, there is a problem in that the effect such as the solid solution strengthening and the adhesion improvement is not expressed.

Nitrogen (N): 0.008 to 0.015%

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

Molybdenum (Mo): 0.1 to 0.3%

Molybdenum (Mo) is an element that secures stable strength and provides a hydrogen storage source by forming various precipitates and oxides. The molybdenum content may be 0.1 to 0.3%. Specifically, the molybdenum content may range from 0.11 to 0.29%.

When the content of molybdenum is out of the upper limit of the above range, there is a problem that not only the annealing sheetability is lowered, but also the occurrence rate of surface defects is increased. When the content of molybdenum is out of the lower limit of the range, there is a problem in that effects such as securing the strength and providing the Sous occlusion source are not expressed.

Oxygen (O): 0.025 to 0.055%

Oxygen (O) is an essential element in forming inclusions that act as a hydrogen storage source for enamel steel. The oxygen content may be 0.025 to 0.055%. Specifically, the oxygen content may range from 0.0255 to 0.0540%.

When the oxygen content is outside the upper limit of the range, there is a problem in that dissolution loss of the refractory material occurs severely in the steelmaking step and the occurrence rate of surface defects of the steel sheet increases. When the oxygen content is out of the lower limit of the range, there is a problem in that the effect of forming the inclusions is not expressed.

It contains iron (Fe) as the remainder. In addition, it may contain unavoidable impurities. The unavoidable impurities refer to impurities that are unavoidably mixed in the manufacturing process of steelmaking and grain-oriented electrical steel sheets. Since unavoidable impurities are widely known, detailed descriptions are omitted. In one embodiment of the present invention, the addition of elements other than the above-described alloy components is not excluded, and may be variously included within a range that does not impair the technical spirit of the present invention. When additional elements are included, they are included in place of Fe, which is the remainder.

In one embodiment, the cold-rolled steel sheet 100 may include titanium (Ti), niobium (Nb), chromium (Cr), and vanadium (V). The cold-rolled steel sheet for vitreous enameling with excellent fishscale resistance may optionally further contain at least one of Ti: 0.005% or less, Nb: 0.005% or less, Cr: 0.05% or less, and V: 0.003% or less.

As described above, the cold-rolled steel sheet 100 of the present invention not only does not arbitrarily add an element such as titanium (Ti), which has higher oxidizing properties than iron (Fe), but also controls the surface oxide layer, thereby improving the enamel adhesion between the steel sheet and the glaze. The same characteristics can be improved.

The oxide layer 20 is formed in an inward direction from both surfaces of the cold-rolled steel sheet 100, which is the base material 10, and can be classified based on a point containing 5% of oxygen. Specifically, the thickness of the oxide layer was divided based on the point containing 5% of oxygen by analyzing the oxygen concentration from the surface to the inside of the cross section of the steel sheet. More specifically, the thickness of the oxide layer was measured using GDS (Glow Discharge Spectroscopy) starting from the point containing 5% of oxygen.

In one embodiment, the thickness of the oxide layer 20 may range from 0.006 to 0.030 μm. Specifically, the thickness of the oxide layer 20 may be 0.007 to 0.028 μm. When the thickness of the oxide layer 20 is out of the upper limit of the above range, there is a problem in that the surface properties of the steel sheet are deteriorated. If the thickness of the oxide layer 20 is outside the lower limit of the above range, the bonding force between the glaze layer and the steel sheet decreases, making it difficult to secure normal adhesion, resulting in a decrease in fishscale resistance.

As such, an enamel product is a product in which an organic glaze is applied on a steel plate, and it is very important to secure adhesion between the steel plate and the glaze. Specifically, the main component of the glaze is composed of a silicon-oxide (SiO ₂ ) system, and there is a problem of applying an expensive glaze such as NiO among the glaze components in order to prevent a decrease in adhesion to the steel plate. The high-strength cold-rolled steel sheet for enamel having excellent enamel adhesion and fishscale resistance according to an embodiment of the present invention can improve enamel adhesion by controlling the thickness of the oxide layer on the surface of the steel sheet. Enamel adhesion can be improved by promoting covalent bonding with silicon (Si) atoms in the glaze layer by managing the thickness of the oxide layer composed of 90 wt% or more of iron oxide (FeO-based) within a certain range.

In one embodiment, the cold-rolled steel sheet 100 satisfies Equation 1 below.

<Equation 1>

0.014 ≤ ([Cu] × [Si) / [P] ≤ 0.080

(In Equation 1, [Cu], [Si], and [P] mean the weight % content of Cu, Si, and P, respectively)

Equation 1 is a correlation between phosphorus (P) and copper (Cu) and silicon (Si). Equation 1 may range from 0.014 to 0.080. Specifically, Equation 1 may range from 0.0142 to 0.0798. By satisfying the above range, the cold-rolled steel sheet 100 can suppress enamel adhesion and surface bubble defects.

When the value of Equation 1 exceeds the upper limit of the range, gas inflow into the surface portion of the steel sheet increases, resulting in surface defects such as bubble defects, thereby reducing product reliability. When the value of Equation 1 is outside the lower limit of the range, there is a problem in that enamel properties such as enamel adhesion are deteriorated as the surface is not modified in the sulfuric acid pretreatment process.

In one embodiment, the cold-rolled steel sheet 100 satisfies Equation 2 below.

<Equation 2>

0.0065 ≤ ([Al] × [Mo]) / ([C] + [N]) ≤ 0.0310

(In Equation 2, [Al], [Mo], [C] and [N] mean the weight % content of Al, Mo, C, and N, respectively)

Equation 2 is a correlation between aluminum (Al) and molybdenum (Mo) for carbon (C) and nitrogen (N). In the case of carbon (C) and nitrogen (N) in steel, it reacts with aluminum (Al) and molybdenum (Mo) to form carbonitrides, and surplus solid-solution elements suppress tissue growth even when heat treatment at high temperatures such as enamel firing is applied. By doing so, shape freezing is excellently controlled, and these precipitates play a role as a hydrogen storage source.

Accordingly, it is necessary to consider the reactivity of not only each alloying element but also complex solid-solution elements. Equation 2 above may be 0.0065 to 0.0310. Specifically, Equation 2 above may range from 0.0067 to 0.0305.

When the value of Equation 2 is outside the upper limit of the range, workability is good, but there is a problem in that the rolling and annealing sheetability is lowered and the manufacturing cost increases due to the increase in the amount of expensive alloy elements. When the value of Equation 2 is outside the lower limit of the range, there is a problem in that fishscale resistance is lowered as precipitation is not promoted, and workability is lowered as the amount of interstitial solid solution elements increases.

In one embodiment, the cold-rolled steel sheet 100 satisfies Equation 3 below.

<Equation 3>

0.50 ≤ (R _{max ×} 20S _e ) / P _c ≤ 1.05

(In Equation 3 above, P _c is the number of surface irregularities per unit centimeter (cm), R _max is the maximum point roughness value (μm), and S _e is the temper reduction rate (%))

Equation 3 above may be 0.50 to 1.05. Specifically, Equation 3 above may range from 0.505 to 1.00. When the value of Equation 3 is outside the upper limit of the range, crystal grains of the steel sheet grow after the enamel firing process, resulting in a problem in securing target material and enamel characteristics. When the value of Equation 3 is out of the lower limit of the range, the wedge effect of the surface of the steel sheet is reduced, resulting in a decrease in adhesion to the glaze.

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 materials used for structural members is a physical property that determines dent resistance and shape freezing of members. In the case of enamel products, not only the yield strength at the processing inlet side but also the yield strength after a long-term heat treatment at high temperature for drying after enamel glaze treatment acts as a major factor in examining the stability of the product. When the yield strength after the enamel firing heat treatment is 220 MPa or more, there is an advantage in that the stability of the product is excellent in the heat treatment step for drying after the glaze treatment.

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

In one embodiment, the cold-rolled steel sheet 100 may have a hydrogen permeability ratio of 600 sec/mm ² or more. Specifically, the hydrogen permeation rate may be 610 sec/mm ² or more. The upper limit of the hydrogen permeation ratio is not particularly limited, but may be, for example, 1,700 sec/mm ² . The hydrogen permeability ratio is a representative index for evaluating fishscale resistance indicating resistance to fishscale defects, which are fatal defects when enameled steel is applied, and means the ability to fix hydrogen into the cold-rolled steel sheet.

Specifically, the hydrogen permeation ratio is a value expressed by dividing the time taken by generating hydrogen in one direction of the steel sheet and penetrating the hydrogen in the other direction opposite to one direction of the steel sheet, and dividing it by the square of the thickness of the material. When the hydrogen permeation ratio is excessively low, when the resistance of fishscale defects is evaluated by accelerated heat treatment at 200 ° C. for 24 hours after enamel treatment, the defect rate is more than 50%, so there is a problem in using it as a stable enamel product.

According to another embodiment of the present invention, a method for manufacturing a cold-rolled steel sheet 100 includes, 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%, with the balance Fe and unavoidable impurities Reheating the steel slab, hot-rolling the heated slab, winding the hot-rolled hot-rolled steel sheet, cold-rolling the coiled hot-rolled steel sheet, annealing and temper-rolling the cold-rolled cold-rolled steel sheet for annealing A step of manufacturing a plate, and a step of subjecting the annealed plate to enamel firing heat treatment. A 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, and thus, duplicate descriptions will be omitted.

Reheating the steel slab is a step for smoothly performing a subsequent hot rolling process and homogenizing the steel slab. The heating may mean reheating.

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. When the upper limit of the temperature range is exceeded, the amount of surface scale increases at the slab heating temperature, resulting in increased material loss and increased energy costs due to the increase in heat source. When the temperature is outside the lower limit of the temperature range, the rolling load rapidly increases in the hot rolling process, resulting in deterioration in hot workability.

In one embodiment, the step of reheating the steel slab may be performed at a finish hot rolling temperature in the range of 890 to 950 °C. Specifically, the finish hot rolling temperature may be carried out in a temperature range of 900 to 945 ℃.

When the finish hot rolling temperature is out of the upper limit of the range, there is a problem in that the peelability of the surface scale decreases and the incidence of surface defects increases and the impact toughness decreases as uniform hot rolling property is not secured. When the finish hot rolling temperature is out of the lower limit of the range, there is a problem in that the grain aggregation rapidly deteriorates the rollability and workability as the hot rolling is finished in the low temperature region.

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

If the temperature is out of the upper limit of the temperature range, corrosion resistance is lowered during winding, and grain boundary segregation of phosphorus (P) is promoted, thereby reducing cold rolling property and adversely affecting the workability of the final product. When the temperature is out of the lower limit of the temperature range, there is a problem in that the material variation due to the difference in precipitation behavior increases and the enamel property deteriorates as the temperature unevenness in the width direction increases in the cooling and soaking process.

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

In one embodiment, the step of cold rolling the coiled hot-rolled steel sheet may be performed at a cold rolling reduction ratio of 60 to 90%. Specifically, the cold rolling reduction may be performed in the range of 63 to 88%.

When the cold rolling reduction ratio is out of the upper limit of the range, there is a problem in that the material is hardened and workability is lowered, as well as the cold rolling mill load is increased and workability is lowered. When the cold reduction ratio is out of the lower limit of the range, there is a problem in that workability is deteriorated because unreacted crystal grains remain locally as recrystallization driving force in the subsequent heat treatment process is not secured.

In one embodiment, the step of preparing an annealed sheet by annealing and temper rolling the cold-rolled cold-rolled steel sheet may be performed at an annealing temperature range of 720 to 850 ° C. during the annealing, for 10 to 70 seconds in the annealing temperature range. can be performed

When outside the upper limit of the annealing temperature range, there is a problem in that annealing passability is lowered due to softening due to a decrease in strength at high temperature. When the temperature is outside the lower limit of the annealing temperature range, the strain formed by cold rolling is not sufficiently removed, so that workability is significantly deteriorated and enamel characteristics cannot be secured.

When the annealing holding time is outside the upper limit of the range, the workability is good, but the crystal grain non-uniformity increases, resulting in deterioration of enamel characteristics. When the annealing holding time is out of the lower limit of the range, there is a problem in that formability deteriorates as recrystallization is not completed and unrecrystallized grains remain.

In one embodiment, the steps of preparing an annealed sheet by annealing and temper rolling the cold-rolled cold-rolled steel sheet may be performed at a reduction ratio of 2.5% or less during the temper rolling. Through the temper rolling, it is possible to control the shape of the material and obtain a desired surface roughness. Specifically, the temper rolling may be performed in the range of a reduction ratio of 0.3 to 2.2%.

When the range of the reduction ratio is out of the upper limit, there is a problem that the material is hardened by work hardening and workability is lowered. In addition, when the tempering reduction is high during firing heat treatment in the enamel process, strain-induced crystal grain growth occurs and the yield strength of the enamel product is significantly lowered, thereby reducing dent resistance.

In one embodiment, the step of enamel baking heat treatment of the annealed sheet may be a step of enamel firing heat treatment of the annealed sheet at 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.

When the temperature range is out of the upper limit, the surface defect rate increases due to the increase in the thickness of the oxide layer, and as the energy consumption increases, there is a problem that acts as a factor in increasing manufacturing cost. When the temperature range is outside the lower limit, the wettability of the glaze is lowered, and thus, adhesion to the enamel cannot be secured.

Hereinafter, a high-strength cold-rolled steel sheet for porcelain enamel with excellent fishscale resistance and a manufacturing method thereof according to the present invention will be described in detail through Examples and Comparative Examples. The following examples are references for explaining the present invention in detail, but the present invention is not limited thereto, and may be implemented in various forms.

In a method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention, carbon (C): 0.11 to 0.13%, silicon (Si): 0.05% or less, manganese (Mn): 0.1 to 0.6%, aluminum (Al): 0.02 to 0.06% phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, nitrogen (N): 0.006% or less, heating a steel slab containing the balance of Fe and other unavoidable impurities, the heated slab hot-rolling, cooling the hot-rolled hot-rolled steel sheet, winding the cooled hot-rolled steel sheet, cold-rolling the coiled hot-rolled steel sheet, annealing the cold-rolled cold-rolled steel sheet, and annealing the Secondary rolling of the cold-rolled steel sheet is included. A 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, and thus, duplicate descriptions will be omitted.

First, the steel slab is heated. Heating the steel slab may be performed at 1,150 °C or higher. In the step of heating the steel slab, a temperature of 1,150° C. or higher is required because the precipitate formed in the steel must be re-dissolved.

Next, in the step of hot-rolling the heated slab, a hot-rolled steel sheet may be obtained by hot-rolling the heated slab. In one embodiment, hot finish rolling may be performed on the steel slab at a temperature of Ar3 or higher. The temperature of Ar3 or higher may be 890 °C or higher. As the hot finish rolling is performed at a temperature of Ar3 or higher, rolling may be performed in the austenite single phase region. When rolling is performed in a range outside the above temperature range, there is a problem of poor rolling stability due to non-uniform material.

The step of cooling the hot-rolled hot-rolled steel sheet is cooled so as to be maintained for a time range of 150 to 1,200 seconds. If it is out of the lower limit of the time range, there is a problem in that a large amount of pearlite phase is formed and carbide with a large size is generated in the final material. When the size of the carbide is large, there is a problem in that the carbide becomes a starting point of a crack and reduces a hole expansion rate.

When the upper limit of the time range is exceeded, the generation 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 surface oxide layer is formed to be thick, there is a problem in that the pickling time before cold rolling is increased and the possibility of causing surface defects is greatly increased. Specifically, the step of cooling the hot-rolled hot-rolled steel sheet may be cooled to 710 °C to be maintained at a temperature between 710 and 860 °C for 150 to 1,200 seconds.

In the winding of the cooled hot-rolled steel sheet, the cooled hot-rolled steel sheet may be wound at 560 to 700 °C. The step of winding the cooled hot-rolled steel sheet in order to secure a grain size suitable for strength and workability may be controlled within the winding temperature range. When the coiling temperature is excessively low, there is a problem in that crystal grains become excessively fine, and in case the coiling temperature is excessively high, there is a problem in that crystal grains become excessively coarse.

In the cold rolling of the rolled hot-rolled steel sheet, a cold-rolled steel sheet may be manufactured by cold rolling at a reduction ratio of 80 to 95%. The thickness of the conventional hot-rolled steel sheet is 2 to 4 mm, and a reduction ratio of 80% or more is required to reduce the thickness of 0.4 mm.

When the reduction ratio is out of the upper limit, the deformation resistance due to rolling is excessively increased, making rolling difficult. If the reduction ratio is out of the lower limit value, there is a problem that the target thickness of the cold-rolled steel sheet cannot be reached.

In one embodiment, prior to the step of cold rolling the coiled hot-rolled steel sheet, the step of pickling may be further included. The pickling step may remove scale generated during hot rolling.

The step of annealing the cold-rolled cold-rolled steel sheet is annealed at a temperature of 620 to 760 ° C. to prepare an annealed steel sheet. Through annealing of the cold-rolled steel sheet, it is possible to remove internal stress formed during cold rolling and secure workability. In order to remove the internal stress and secure the processability, a process of annealing at a sufficiently high temperature is required to allow recrystallization to occur.

If the temperature is out of the upper limit value, a plate breakage defect due to a decrease in high-temperature strength may be caused. When the temperature is out of the lower limit, there is a problem in that recrystallization of the cold-rolled steel sheet having the component system of the present invention does not occur.

In the secondary rolling of the annealed cold-rolled steel sheet, a final steel sheet may be manufactured by secondary rolling the annealed cold-rolled steel sheet at a reduction ratio of 6 to 18%. If the reduction ratio is out of the upper limit, there is a problem in that the desired level of processability cannot be secured due to the decrease in elongation. If the reduction ratio is out of the lower limit value, there is a problem that it is not sufficient to obtain the target strength.

Hereinafter, a high-strength cold-rolled steel sheet for vitreous enameling with excellent fishscale resistance according to the present invention and a manufacturing method thereof will be described in detail through Examples and Comparative Examples. The following examples are references for explaining the present invention in detail, but the present invention is not limited thereto, and may be implemented in various forms.

<Steel slabs according to Inventive Steels 1 to 5 and Comparative Steels 1 to 5>

Table 1 below shows the composition of main components for Inventive Steels 1 to 5 and Comparative Steels 1 to 5. A slab was prepared through a converter, secondary refining, and casting process with the alloy components according to the composition of Table 1 below. In addition, the values of Equations 1 and 2 are shown according to the composition of the main components. Equation 1 represents ([Cu] × [Si])/[P], and Equation 2 represents ([Al] × [Mo])/([C] + [N]). [Cu], [Si], [P], [Al], [Mo], [C], and [N] in the above formulas 1 and 2 mean the respective weight%.

division	C	Mn	Si	Al	P	S	N	Cu	O	Mo	(Cu*Si)/P weight ratio	(Al*Mo)/(C+N) weight ratio
invention steel 1	0.0015	0.48	0.008	0.0012	0.019	0.008	0.0091	0.038	0.028	0.26	0.0160	0.0294
invention steel 2	0.0026	0.39	0.014	0.0009	0.024	0.005	0.0134	0.052	0.031	0.14	0.0303	0.0079
invention steel 3	0.0010	0.51	0.011	0.0013	0.017	0.007	0.0105	0.071	0.048	0.18	0.0459	0.0203
Invention Steel 4	0.0022	0.28	0.024	0.0010	0.015	0.003	0.0122	0.046	0.045	0.27	0.0736	0.0188
invention steel 5	0.0009	0.35	0.006	0.0007	0.026	0.004	0.0141	0.065	0.037	0.20	0.0150	0.0093
comparative steel 1	0.0052	0.34	0.012	0.0009	0.003	0.005	0.0124	0.025	0.002	0.12	0.1000	0.0061
comparative steel 2	0.0018	0.12	0.010	0.0325	0.026	0.003	0.0035	0.409	0.027	0.22	0.1573	1.3491
comparative lecture 3	0.0021	0.68	0.007	0.0003	0.038	0.007	0.0104	0.043	0.075	0.01	0.0079	0.0002
comparative lecture 4	0.0027	0.42	0.215	0.0009	0.025	0.004	0.0172	0.051	0.030	0.92	0.4386	0.0416
comparative steel 5	0.0672	0.29	0.008	0.0012	0.048	0.039	0.0097	0.053	0.045	0.28	0.0088	0.0044

Looking at Table 1, it can be confirmed that the comparative steels are not included in the composition range of the steel of the present invention compared to the inventive steels, and accordingly, it is confirmed that they are not included in the values of Equations 1 and 2 above. can

<Examples 1 to 9 and Comparative Examples 1 to 9>

In Table 2 below, cold-rolled steel sheets were manufactured according to the manufacturing conditions disclosed in Table 2 below for the slab. Specifically, after maintaining the slab in a heating furnace for 2 hours, hot rolling was performed, and at this time, the thickness of the hot-rolled steel sheet was adjusted to 4.0 mm. The hot-rolled hot-rolled steel sheet was subjected to cold rolling at each reduction ratio after pickling and removing the oxide film on the surface.

division	steel grade	slab reheat temperature (℃)	finish hot rolling temperature (℃)	winding temperature (℃)	cold reduction rate (%)	Annealing temperature (℃)	maintain hour (candle)	tempering reduction rate (%)	(R max *20S e )/P c value	enamel firing temperature (℃)
Example 1	invention steel 1	1220	910	680	70	750	60	0.8	0.5291	820
Example 2	invention steel 1	1220	910	680	75	780	40	1.3	0.7257	820
Example 3	invention steel 1	1220	910	680	80	830	15	1.9	0.9927	820
Example 4	invention steel 2	1240	940	640	65	780	50	0.5	0.6032	800
Example 5	invention steel 2	1240	940	640	85	820	20	1.5	0.8850	800
Example 6	invention steel 3	1260	920	660	75	750	35	0.8	0.5821	830
Example 7	Invention Steel 4	1240	920	620	75	820	20	1.2	0.6903	830
Example 8	invention steel 5	1250	920	620	80	800	40	1.9	0.9656	830
Example 9	invention steel 5	1250	920	620	70	830	40	2.0	0.9770	830
Comparative Example 1	invention steel 1	1080	750	680	80	880	90	0.8	0.4231	820
Comparative Example 2	invention steel 2	1220	940	780	50	820	30	2.9	1.2022	750
Comparative Example 3	invention steel 3	1250	920	480	75	680	5	0.8	0.3572	890
Comparative Example 4	Invention Steel 4	1250	970	620	95	820	20	0.1	0.1504	820
Comparative Example 5	comparative steel 1	1240	910	640	75	820	40	0.5	0.2015	820
Comparative Example 6	comparative steel 2	1240	910	640	75	820	30	1.2	0.4153	820
Comparative Example 7	comparative lecture 3	1240	910	640	80	800	60	1.6	1.3246	820
Comparative Example 8	comparative lecture 4	1240	910	640	80	800	20	0.8	0.3913	820
Comparative Example 9	comparative steel 5	1240	910	640	80	800	40	1.2	0.4234	820

Looking at Table 2, Examples 1 to 9 use the slabs of inventive steels 1 to 5 included in the composition range of the present invention, the slab reheating temperature, finish hot rolling temperature, winding temperature, cold reduction rate, annealing of the present invention It was carried out within the range of temperature, holding time, temper rolling rate, and enamel firing temperature. On the other hand, Comparative Examples 1 to 4 are controlled so that at least one of the manufacturing conditions in Table 2 does not correspond to the conditions of the present invention using the slabs of Inventive Steels 1 to 4 included in the composition range of the present invention. In Comparative Examples 5 to 9, comparative steels 1 to 5 of Table 1 were controlled so that the manufacturing conditions of Table 2 were included within the scope of the present invention.

<Measurement results of cold-rolled steel sheet properties>

Table 3 below shows the thickness of the oxide layer, sheet permeability, yield strength, cell defect occurrence or not, The presence or absence of fish scale, enamel adhesion, and hydrogen permeability are shown.

The thickness of the oxide layer is determined by analyzing the oxygen concentration from the surface of the steel sheet to the inside using GDS (Glow Discharge Spectroscopy), and by dividing the oxide layer and the base material based on the point containing 5% oxygen, to the point containing 5% oxygen. The thickness was measured, measured three times, and the average value was displayed. In the case of the sheet permeability, when the operability is 90% or more compared to the productivity of ordinary materials in the casting, hot rolling, and cold rolling processes, it is good (indicated by “○”), and the productivity is less than 90% or the defect rate is 10% or more is marked as defective (marked with “×”).

In order to simulate the enamel glaze drying process for the steel sheet, tensile test specimens were prepared (standard ASTM 13B) for specimens subjected to firing and heat treatment for 15 minutes at each temperature in a firing furnace, and tensile tests were performed at a crosshead speed of 10 mm/min. It was measured by conducting a test.

Enamel treatment specimens were cut into appropriate sizes for each purpose to meet the purpose of the test, and after heat treatment was completely degreased, standard glaze (Check Frit), which was relatively vulnerable to fish scale defects, was applied and maintained at 300 ° C for 10 minutes. to remove moisture. After drying, the specimen was fired at each firing temperature for 15 minutes to highlight the difference in enamel characteristics such as adhesion, and then cooled to room temperature. Severe conditions that were easy to achieve were selected.

The specimens after the enamel treatment were subjected to a fishscale acceleration test in which they were kept in an oven at 200 °C for 24 hours. After the fishscale acceleration process, the presence or absence of fishscale defects is observed with the naked eye, and when no fishscale defects occur, it is marked as good (marked with “○”), and when the fishscale defects occur, it is marked as bad (“○”). ×”).

Enamel adhesion, which evaluates the adhesion between the steel plate and the glaze, is evaluated by applying a certain load to the enamel layer with a steel ball as defined in ASTM C313-78, the American Society for Testing and Materials, and then evaluating the degree of conduction in this area. The degree was expressed as an index. In the present invention, the enamel adhesion evaluation results set the goal of securing adhesion of 95% or more in terms of securing application stability in relatively inexpensive glazes.

The bubble defects are judged as excellent (marked as “○”) and poor (marked as “x”), respectively, by visually observing the enamel surface of the specimen kept in an oven at 200 ° C. for 24 hours after enamel treatment. did The hydrogen permeation ratio is one of the indices for evaluating resistance to fishscale, a fatal defect of enamel. According to the test method indicated in the European standard (EN10209-2013), hydrogen is generated in one direction of the steel sheet and hydrogen permeates to the other side. Time (ts, unit: second) is measured, and this is a value expressed as the square of the material thickness (t, unit: mm), and is expressed as ts/t2 (unit: second/mm ² ).

division	oxide layer Thickness (㎛)	permeability	yield strength (MPa)	bubble defect Occurrence or not	fish scale Occurrence or not	Adhesion to enamel (%)	hydrogen permeation ratio (second/㎟)
Invention example 1	0.024	O	247	O	O	98.9	1008
Invention example 2	0.019	O	254	O	O	100.0	1126
Invention example 3	0.012	O	263	O	O	99.2	989
Invention example 4	0.010	O	256	O	O	99.5	883
Invention Example 5	0.017	O	277	O	O	99.4	1014
Example 6	0.026	O	309	O	O	99.3	942
Example 7	0.022	O	284	O	O	100.0	1203
Invention Example 8	0.015	O	251	O	O	98.8	869
Inventive Example 9	0.009	O	271	O	O	99.4	972
Comparative Example 1	0.036	X	202	O	X	75.5	548
Comparative Example 2	0.003	X	104	X	X	80.9	508
Comparative Example 3	0.002	X	148	X	X	82.4	498
Comparative Example 4	0.004	X	211	X	X	80.1	412
Comparative Example 5	0.003	O	158	X	X	87.4	525
Comparative Example 6	0.004	X	166	O	X	70.3	392
Comparative Example 7	0.001	O	169	X	X	82.4	438
Comparative Example 8	0.002	X	194	X	X	84.7	306
Comparative Example 9	0.003	O	241	X	X	58.4	284

Referring to Table 3, Inventive Examples 1 to 9 satisfying various characteristics such as component composition, manufacturing conditions, surface characteristics, and oxidation layer thickness of the present invention not only have good sheet-permeability, but also have relevant properties such as the thickness of the oxide layer. The indices met the scope of the present invention. In addition, inventive examples 1 to 9 do not generate enamel defects such as fish scale and bubble defects, and have a hydrogen permeability ratio of 600 sec/mm2 or more, an enamel adhesion index of 95% or more, and a yield strength of 220 MPa after firing and heat treatment of the enamel. By satisfying the above, it is possible to secure the characteristics targeted by the present invention. On the other hand, in the case of Comparative Examples 5 to 9, which do not satisfy the composition of the present invention, the thickness of the surface oxide layer, the hydrogen permeation ratio, and enamel adhesion were not satisfied, and in most cases, fish scale or air bubble defects occurred even in visual observation after enamel treatment, resulting in a problem in applicability to the target use. In addition, the composition of the present invention was satisfied, but , In Comparative Examples 1 to 4, in which manufacturing conditions in various annealing processes during hot rolling did not satisfy the management range of the present invention, the thickness of the surface oxide layer was outside the range suggested in the present invention, and the enamel adhesion was less than 95%. Or, it was confirmed that enamel defects such as bubble defects or fish scales occurred after enamel treatment, and the sheetability was poor, and the yield strength after enamel firing heat treatment was less than 220 MPa. As described above, the cold-rolled steel sheet for enamel having excellent enamel adhesion and fishscale resistance according to an embodiment of the present invention satisfies the above alloy composition and alloy range, thereby reducing the thickness of the oxide layer formed in the inward direction to an appropriate level. Through this, it is possible to provide a high-strength cold-rolled steel sheet for enamel with excellent fishscale resistance.

In addition, the cold-rolled steel sheet has significantly improved enamel characteristics even through high-speed continuous annealing operation, can maintain a high level of strength after enamel firing heat treatment, and optimizes surface roughness characteristics in the heat treatment and temper rolling step in a continuous annealing furnace to increase adhesion. In addition, stable material properties can be secured even after high-temperature firing by suppressing crystal grain growth during enamel firing, such as residual nitrogen in the surface layer of the steel sheet.

The present invention is not limited to the above embodiments and / or embodiments, but can be manufactured in various different forms, and those skilled in the art to which the present invention belongs may change the technical spirit or essential features of the present invention. It will be appreciated that it may be embodied in other specific forms without Therefore, it should be understood that implementations and/or examples described above are illustrative in all respects and not restrictive.

Claims (11)

1. In weight percent, 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 0.03% 0.08%, N: 0.008 to 0.015%, Mo: 0.1 to 0.3%, O: 0.025 to 0.055%, and the balance Fe and unavoidable impurities,

   contains an oxide layer;

   A cold-rolled steel sheet having a thickness of 0.006 to 0.030 μm of the oxide layer formed in an inward direction from the surface of the cold-rolled steel sheet.
2. According to claim 1,

   A cold-rolled steel sheet that satisfies Equation 1 below.

   <Equation 1>

   0.014 ≤ ([Cu] × [Si) / [P] ≤ 0.080

   (In Equation 1, [Cu], [Si], and [P] mean the weight % content of Cu, Si, and P, respectively)
3. According to claim 1,

   A cold-rolled steel sheet that satisfies Equation 2 below.

   <Equation 2>

   0.0065 ≤ ([Al] × [Mo]) / ([C] + [N]) ≤ 0.0310

   (In Equation 2, [Al], [Mo], [C] and [N] mean the weight % content of Al, Mo, C, and N, respectively)
4. According to claim 1,

   A cold-rolled steel sheet that satisfies Equation 3 below.

   <Equation 3>

   0.50 ≤ (R _{max ×} 20S _e ) / P _c ≤ 1.05

   (In Equation 3 above, P _c is the number of surface irregularities per unit centimeter (cm), R _max is the maximum point roughness value (μm), and S _e is the temper reduction rate (%))
5. According to claim 1,

   Cold-rolled steel sheet with a yield strength of 220 MPa or more after enamel firing heat treatment.
6. According to claim 1,

   Cold-rolled steel sheet with enamel adhesion of 95% or more.
7. According to claim 1,

   Cold-rolled steel sheet with a hydrogen permeability of 600 sec/mm ² or more.
8. In weight percent, 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 0.03% Reheating a steel slab containing 0.08%, N: 0.008 to 0.015%, Mo: 0.1 to 0.3%, O: 0.025 to 0.055% and balance Fe and unavoidable impurities to a temperature range of 1,150 to 1,280 °C;

   Hot rolling the heated slab at a finishing hot rolling temperature in the range of 890 to 950 ° C;

   Winding the hot-rolled hot-rolled steel sheet in a temperature range of 580 to 720 °C;

   cold-rolling the coiled hot-rolled steel sheet at a cold-rolling reduction ratio of 60 to 90%;

   Annealing the cold-rolled cold-rolled steel sheet at an annealing temperature of 720 to 850 ° C. for 10 to 70 seconds, and temper rolling at a reduction ratio of 2.5% or less to prepare an annealed sheet; and

   Method for producing a cold-rolled steel sheet comprising the step of subjecting the annealed sheet to enamel firing heat treatment in a temperature range of 780 to 850 ° C.
9. According to claim 8,

   A method for manufacturing a cold-rolled steel sheet that satisfies Equation 1 below.

   <Equation 1>

   0.014 ≤ ([Cu] × [Si) / [P] ≤ 0.080

   (In Equation 1, [Cu], [Si], and [P] mean the weight % content of Cu, Si, and P, respectively)
10. According to claim 8,

    A method for manufacturing a cold-rolled steel sheet that satisfies Equation 2 below.

    <Equation 2>

    0.0065 ≤ ([Al] × [Mo]) / ([C] + [N]) ≤ 0.0310

    (In Equation 2, [Al], [Mo], [C] and [N] mean the weight % content of Al, Mo, C, and N, respectively)
11. According to claim 8,

    A method for manufacturing a cold-rolled steel sheet that satisfies Equation 3 below.

    <Equation 3>

    0.50 ≤ (R _{max ×} 20S _e ) / P _c ≤ 1.05

    (In Equation 3 above, P _c is the number of surface irregularities per unit centimeter (cm), R _max is the maximum point roughness value (μm), and S _e is the temper reduction rate (%))

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