KR20230094867A - Steel sheet for enamel and method of manufacturing the same - Google Patents

Abstract

In the steel sheet for porcelain enamel according to an embodiment of the present invention, in weight percent, C: 0.001 to 0.08%, Mn: 0.05 to 0.40%, Si: 0.001 to 0.03%, Al: 0.005 to 0.020%, P: 0.001 to 0.020 %, S: 0.001 to 0.020%, Cu: 0.03 to 0.08%, N: 0.006 to 0.015%, O: 0.003% or less (excluding 0%) and the balance including Fe and unavoidable impurities.
A steel sheet for porcelain enamel according to an embodiment of the present invention includes an oxide layer having a concentration gradient in which oxygen concentration decreases from the surface to the inside, and the thickness of the oxide layer is 0.006 to 0.030 μm.

Description

Steel sheet for enamel and its manufacturing method {STEEL SHEET FOR ENAMEL AND METHOD OF MANUFACTURING THE SAME}

One embodiment of the present invention relates to a steel sheet for enamel and a manufacturing method thereof. More specifically, one embodiment of the present invention relates to a high-strength enamel steel sheet having excellent yield strength, no bubble defects, excellent fishscale resistance and enamel adhesion, and a manufacturing method thereof.

Enameled steel sheet is a kind of surface treatment product that improves corrosion resistance, weather resistance, heat resistance, etc. by applying a glassy glaze on a base steel sheet such as a hot-rolled steel sheet or cold-rolled steel sheet and then firing it at a high temperature. These enamel steel sheets are used as materials for building exteriors, home appliances, tableware, and various industrial uses.

Although rimmed steel has been used as a steel sheet for enamel, most of the materials are being continuously cast as the continuous casting method is actively used in terms of recent productivity improvement. In addition, fishscale defects, which are one of the most fatal defects of enameled steel sheets in the manufacture of steel products, exist as supersaturated in the steel during the process of cooling after firing hydrogen dissolved in the steel during the manufacturing process of enamel products, and then spread to the surface of the steel. It is a representative enamel defect that occurs when the enamel layer is removed in the form of meat scales as it is released. When such a fishscale defect occurs, it is necessary to suppress the occurrence because rust is intensively generated in the defect area and the value of the enamel product is greatly reduced. In order to prevent fishscale defects, it is necessary to form a large amount of sites that can hold hydrogen dissolved in steel inside the steel. Accordingly, in order to prevent fish scale defects that deteriorate enamel or improve aging, open coil annealing (OCA), a type of phase annealing, is sometimes applied, but in this case, long-term heat treatment reduces productivity and increases manufacturing costs. There was a problem of high quality and large deviation in quality. In addition, in the open coil annealing method, it is difficult to control the amount of decarburization and the amount of decarburization is too large, and if the amount of carbon in the steel is too small, the grain boundaries of the steel sheet are softened, and cracks such as brittle fracture occur during product molding. In order to overcome the problems of poor productivity and increased manufacturing cost due to such long-term annealing, the recently developed steel sheet for enamel actively utilizes the continuous annealing process. etc. are used. However, even in this case, various quality problems, such as the addition of a large amount of carbonitride forming elements or the high rate of surface defects due to non-deoxidized compounds, and the increase in recrystallization temperature and the decrease in sheetability, as well as factors that decrease productivity and increase cost It's working.

In other words, since a large amount of titanium is added to the enamel steel sheet using titanium (Ti)-based precipitates to suppress the hydrogen reaction that causes fish scale, the nozzle is clogged by titanium nitride (TiN) and inclusions in the continuous casting stage of the steelmaking process. This frequently occurs and becomes a direct factor in reducing workability and increasing production load. In addition, TiN mixed in molten steel exists on the upper part of the steel sheet, causing a typical bubble defect, blister defect, and a large amount of added titanium also becomes a factor that inhibits the adhesion between the steel sheet and the glaze layer.

In addition, in the case of enamel steel, as it is mostly used as a material for structural members, the strength of the material is raised to strengthen competitiveness through weight reduction of the used member. To this end, it is necessary to secure the yield strength after firing heat treatment for drying the glaze in the enamel process.

In one embodiment of the present invention, it is intended to provide a steel sheet for enamel and a manufacturing method thereof. More specifically, in one embodiment of the present invention, it is intended to provide a high-strength enamel steel sheet having excellent yield strength, no bubble defects, excellent fishscale resistance and enamel adhesion, and a manufacturing method thereof.

In the steel sheet for porcelain enamel according to an embodiment of the present invention, in weight percent, C: 0.001 to 0.08%, Mn: 0.05 to 0.40%, Si: 0.001 to 0.03%, Al: 0.005 to 0.020%, P: 0.001 to 0.020 %, S: 0.001 to 0.020%, Cu: 0.03 to 0.08%, N: 0.006 to 0.015%, O: 0.003% or less (excluding 0%) and the balance including Fe and unavoidable impurities.

A steel sheet for porcelain enamel according to an embodiment of the present invention includes an oxide layer having a concentration gradient in which oxygen concentration decreases from the surface to the inside, and the thickness of the oxide layer is 0.006 to 0.030 μm.

The steel sheet for vitreous enameling according to an embodiment of the present invention may satisfy Equation 1 below.

[Equation 1]

0.02 ≤ ([Cu] × [Si])/[P] ≤ 0.19

(In Equation 1, [Cu], [Si], and [P] represent the contents (wt%) of Cu, Si, and P, respectively.)

The steel sheet for porcelain enamel according to an embodiment of the present invention may satisfy Equation 2 below.

[Equation 2]

0.5 ≤ [Al]/[N] ≤ 2.5

([Al] and [N] in Equation 2 indicate the content (wt%) of Al and N, respectively.)

The steel sheet for porcelain enamel according to an embodiment of the present invention contains Ti: 0.003 wt% or less, Co: 0.005 wt% or less, Nb: 0.003 wt% or less, Ni: 0.01 wt% or less, V: 0.003 wt% or less, and Mo: 0.01 wt% or less. One or more of the weight % or less may be further included.

The steel sheet for vitreous enameling according to an embodiment of the present invention may satisfy Equation 3 below.

[Equation 3]

0.75 ≤ C _1/2t - C _1/8t ≤ 2.45

(In Equation 3, C _1/2t represents the area fraction (%) of cementite at 1/2 of the total thickness of the steel sheet in the thickness direction of the steel sheet, and C _{1/8t represents} 1/8 of the total thickness of the steel sheet in the thickness direction of the steel sheet. It represents the area fraction (%) of cementite in the site.)

The steel sheet for vitreous enameling according to an embodiment of the present invention may satisfy Equation 4 below.

[Equation 4]

0.07 ≤ MV _1/8t - MV _Av ≤ 0.14

(In Equation 4, MV _1/8t is the micropore area fraction (%) at 1/8 of the total thickness of the steel sheet in the steel sheet thickness direction, and MV _Av means the average micropore area fraction (%).)

The steel sheet for vitreous enameling according to an embodiment of the present invention may satisfy Equation 5 below.

[Equation 5]

0.45 ≤ (R _a × 50 × S _e )/P _c ≤ 0.99

(In Equation 5, P _c is the number of surface irregularities per unit centimeter (cm), R _a is the average surface roughness value (μm), and S _e is the refining reduction (%).)

The steel sheet for porcelain enamel according to an embodiment of the present invention may have a yield strength of 220 MPa or more after enamel firing heat treatment.

The steel sheet for enamel enamel according to an embodiment of the present invention may have enamel adhesion of 95% or more.

The steel sheet for porcelain enamel according to an embodiment of the present invention may have a hydrogen permeability ratio of 600 sec/mm ² or more.

In the manufacturing method of the steel sheet for porcelain enamel according to an embodiment of the present invention, C: 0.01 to 0.1%, Mn: 0.05 to 0.40%, Si: 0.001 to 0.03%, Al: 0.005 to 0.020%, P: 0.001 to 0.020%, S: 0.001 to 0.020%, Cu: 0.03 to 0.08%, N: 0.006 to 0.015%, O: 0.003% or less (excluding 0%), and the balance Fe and unavoidable impurities. Manufacturing a hot-rolled steel sheet by rolling; Manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; and annealing the cold-rolled steel sheet.

The annealing step is annealing in a wet atmosphere having an oxidation capacity index (PH ₂ O/PH ₂ ) of 0.51 to 0.65.

In the step of manufacturing the hot-rolled steel sheet, the finish hot rolling temperature may be 850 to 910°C.

In the step of manufacturing the hot-rolled steel sheet, the coiling temperature may be 580 to 720 °C.

In the step of producing a cold-rolled steel sheet, the reduction ratio may be 60 to 90%.

The annealing step may be annealed at a temperature of 720 to 850 ° C for 20 to 180 seconds.

After the step of annealing the cold-rolled steel sheet, a step of temper rolling the annealed cold-rolled steel sheet may be further included.

After the step of annealing the cold-rolled steel sheet, a step of enamel firing the annealed cold-rolled steel sheet at a temperature of 780 to 850° C. may be further included.

The steel sheet for porcelain enamel according to an embodiment of the present invention has excellent fishscale resistance and enamel adhesion.

In addition, the steel sheet for vitreous enameling according to an embodiment of the present invention has remarkably improved enamel characteristics even through high-speed heat treatment, and can maintain a high level of strength after enamel firing heat treatment.

The steel sheet for porcelain enamel according to an embodiment of the present invention can prevent fishscale defects caused by hydrogen.

The steel sheet for porcelain enamel according to an embodiment of the present invention can prevent surface bubble defects as well as enameling properties.

The steel sheet for porcelain enamel according to an embodiment of the present invention can maintain high adhesion by optimizing the surface roughness characteristics.

In the steel sheet for vitreous enameling according to an embodiment of the present invention, residual nitrogen in the surface layer of the steel sheet suppresses crystal grain growth during enamel firing, so that stable material properties can be secured even after high-temperature firing.

1 is a schematic view of a cross section of a steel sheet for porcelain vitreous according to an embodiment of the present invention.
2 is a GDS (Glow Discharge Spectroscopy) analysis result for each depth of the steel sheet for enamel according to Inventive Example 3.

In this specification, terms such as first, second, and third are used to describe various parts, components, regions, layers, and/or sections, but are not limited thereto. 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.

In this specification, when a certain component is said to "include", it means that it may further include other components, not excluding other components unless otherwise stated.

In this specification, the technical terms used are only for referring to specific embodiments and are 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.

In this specification, the term "combination of these" included in the expression of the Markush form means a mixture or combination of one or more selected from the group consisting of the components described in the expression of the Markush form, It means including one or more selected from the group consisting of.

In this specification, 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 indicated, % 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.

In the steel sheet for porcelain enamel according to an embodiment of the present invention, in weight percent, C: 0.01 to 0.1%, Mn: 0.05 to 0.40%, Si: 0.001 to 0.03%, Al: 0.005 to 0.020%, P: 0.001 to 0.020 %, S: 0.001 to 0.020%, Cu: 0.03 to 0.08%, N: 0.006 to 0.015%, O: 0.003% or less (excluding 0%) and the balance including Fe and unavoidable impurities.

First, the reason for limiting the components of the steel sheet will be explained. The content of the elements in the final steel sheet may have a concentration gradient in the thickness direction, and the element content, which will be described later, represents an average of the content in the entire steel sheet 100 including the oxide layer 20 .

C: 0.001 to 0.08% by weight

If too much carbon (C) is added, the amount of dissolved carbon in the steel increases and the strength increases, but it hinders the development of texture during annealing, resulting in poor formability and can cause bubble defects due to bubbling in the enamel layer. . On the other hand, if C is too small, the structure grows and the target yield strength cannot be secured after firing, and the fraction of precipitates acting as a hydrogen storage source is also lowered, resulting in a vulnerability to fishscale defects.

In the steelmaking step, that is, carbon in the slab may contain 0.01 to 0.1% by weight. More specifically, carbon in the slab may include 0.02 to 0.08% by weight. Since decarburization occurs in an atmosphere with a high oxidation capability index during annealing, 0.001 to 0.08% by weight of carbon may be included in the finally manufactured steel sheet for porcelain enamel. More specifically, it may include 0.01 to 0.05% by weight. More specifically, it may contain 0.015 to 0.045% by weight.

Mn: 0.05 to 0.40% by weight

Manganese (Mn) is a representative solid solution strengthening element, and sulfur dissolved in steel is precipitated in the form of manganese sulfide (MnS) to prevent hot shortness and promote the precipitation of carbides. If too little Mn is added, it is difficult to obtain the above-mentioned effect sufficiently. On the other hand, if the content of Mn is too large, formability is deteriorated and the Ar ₃ transformation temperature is lowered, which may cause transformation and deformation during enamel firing. Therefore, 0.05 to 0.40% by weight of Mn may be included. More specifically, 0.07 to 0.038 wt% of Mn may be included.

Si: 0.001 to 0.03% by weight

Silicon (Si) is an element that promotes solid solution strengthening and formation of carbides that act as a hydrogen storage source. If too little Si is added, it is difficult to obtain the above-mentioned effect sufficiently. On the other hand, if too much Si is added, a high-concentration oxide film may be formed on the surface of the steel sheet, resulting in deterioration of enamel adhesion. Accordingly, 0.001 to 0.030% by weight of Si may be included. More specifically, it may contain 0.002 to 0.027% by weight.

Al: 0.005 to 0.020% by weight

Aluminum (Al) is used as a strong deoxidizer to remove oxygen from molten steel in the steelmaking stage, and is an element that fixes solid nitrogen. If too little Al is added, it is difficult to obtain the above-mentioned effect sufficiently. Conversely, if too much Al is added, aluminum oxide remains in the steel or on the surface of the steel, causing problems such as blisters in the enameling process. Dropping problems may occur. Therefore, 00.005 to 0.020 wt% of Al may be included. More specifically, it may contain 0.003 to 0.018% by weight.

P: 0.001 to 0.020% by weight

Phosphorus (P) is a representative material reinforcing element. If too little P is added, it is difficult to obtain the above-mentioned effect sufficiently. On the other hand, if too much P is added, a P segregation layer is formed inside the steel sheet to deteriorate formability, and also to deteriorate pickling properties of the steel, which may adversely affect enamel adhesion. Therefore, P may be included in the range of 0.001 to 0.020% by weight. More specifically, it may contain 0.003 to 0.018% by weight.

S: 0.001 to 0.020% by weight

Sulfur (S) is an element that causes red brittleness by combining with manganese. If too little S is added, a problem of deteriorating weldability may occur. If too much S is added, ductility is greatly reduced to deteriorate processability, and manganese sulfide is excessively precipitated, which may adversely affect the fishscale property of the product. Therefore, S may be included in an amount of 0.001 to 0.020% by weight. More specifically, it may contain 0.002 to 0.018% by weight.

Cu: 0.03 to 0.08% by weight

Copper (Cu) is an element added to improve solid solution strengthening and enamel adhesion. If too little Cu is added, the above effect cannot be obtained adequately. If too much Cu is added, the pickling rate is lowered in the acid treatment step, which is a pre-enamel process, so that proper roughness characteristics of the steel sheet surface cannot be obtained, and thus adhesion may be deteriorated. Therefore, Cu may be included in an amount of 0.03 to 0.08% by weight. More specifically, it may contain 0.032 to 0.078% by weight.

N: 0.006 to 0.015% by weight

Nitrogen (N) is a typical hardening element and is added to obtain a target yield strength after enamel firing. If too little N is included, the yield strength after firing the enamel may deteriorate. If too much N is included, moldability deteriorates and surface defects such as bubble defects may occur in the enamel treatment process. Therefore, 0.006 to 0.015 wt% of N may be included. More specifically, it may include 0.0065 to 0.0140% by weight.

O: 0.003% by weight or less

Oxygen (O) forms oxides, and such oxides not only cause dissolution loss of refractories in the steelmaking process, but also act as a factor inducing surface defects due to oxides on the surface during steel sheet manufacturing. Therefore, O may contain 0.003% by weight or less.

Regarding a manufacturing process to be described later, the oxide layer 20 may be formed in an annealing process. However, since the thickness of the oxide layer 20 is very thin compared to the entire steel sheet 100, there is substantially no change in the amount of oxygen in the entire steel sheet 100. The oxide layer 20 contains 5% by weight or more of oxygen. More specifically, 10 to 50% by weight of O may be included in the oxide layer 20 . The oxygen content in the oxide layer 20 means the average content in the oxide layer 20 .

The steel sheet for vitreous enameling according to an embodiment of the present invention may satisfy Equation 1 below.

[Equation 1]

0.02 ≤ ([Cu] × [Si])/[P] ≤ 0.19

(In Equation 1, [Cu], [Si], and [P] represent the contents (wt%) of Cu, Si, and P, respectively.)

If the value of Equation 1 is too low, the enamel adhesion may deteriorate. On the other hand, if the value of Equation 1 is too large, the inflow of gas to the surface increases, resulting in frequent enamel surface defects such as bubble defects, which deteriorates the reliability of the product. It can act as a triggering factor. Accordingly, the value of Equation 1 may be adjusted to 0.02 to 0.19 in order to secure enamel adhesion and suppress surface bubble defects. More specifically, the value of Equation 1 may be adjusted to 0.023 to 0.188.

The steel sheet for porcelain enamel according to an embodiment of the present invention may satisfy Equation 2 below.

[Equation 2]

0.5 ≤ [Al]/[N] ≤ 2.5

([Al] and [N] in Equation 2 indicate the content (wt%) of Al and N, respectively.)

In the case of nitrogen in steel, it reacts with aluminum to form nitride, and surplus nitrogen plays a role in securing strength by suppressing tissue growth at high temperatures and at the same time serving as a hydrogen storage source. It is necessary to consider the reactivity with nitrogen. Accordingly, the relationship between Al and N may satisfy Equation 2. If the value of Equation 2 is too low, it may cause workability deterioration as the amount of solid solution elements remaining in the steel increases. On the other hand, if the value of Equation 2 is too high, the rolling and annealing passability may deteriorate. More specifically, the value of Equation 2 may be 0.51 to 2.49.

In addition to the above components, the present invention includes Fe and unavoidable impurities. It does not preclude the addition of effective ingredients other than the above ingredients. Examples of unavoidable impurities include Ti, Co, Nb, Ni, V, and Mo. In one embodiment of the present invention, Ti, Co, Nb, Ni, V, Mo, etc. are not intentionally added, Ti: 0.003 wt% or less, Co: 0.005 wt% or less, Nb: 0.003 wt% or less, Ni: 0.01 At least one of wt% or less, V: 0.003 wt% or less, and Mo: 0.01 wt% or less may be further included.

1 shows a schematic view of a cross section of a steel sheet for vitreous enameling according to an embodiment of the present invention. As shown in FIG. 1, it includes an oxide layer 20 having a concentration gradient in which the oxygen concentration decreases from the surface of the steel sheet to the inside. The oxide layer 20 is distinguished from the steel substrate 10 containing less than 5 wt% of oxygen (O) in that it contains 5 wt% or more of oxygen (O). Specifically, when analyzing the oxygen concentration in the cross section of the steel sheet from the surface to the inside, the oxide layer 20 and the substrate 10 are divided based on the point containing 5% by weight of oxygen. When there are a plurality of points containing 5% by weight of oxygen, the innermost point is identified as the starting point.

The oxide layer 20 may include 90% by weight or more of Fe oxide.

Enamel products are products in which an organic glaze is applied on a steel plate, so it is very important to secure adhesion between the steel plate and the glaze. In general, the main component of the glaze is made of silicon-oxide (SiO ₂ ), and expensive glazes containing a large amount of NiO among the glaze components are often applied to prevent deterioration of adhesion with the steel plate.

In one embodiment of the present invention, a method for improving enamel adhesion by controlling the thickness of the oxide layer on the surface of the steel sheet was confirmed through repeated experiments. By managing the thickness of the oxide layer mainly composed of FeO-based within a certain range, covalent bonding with silicon (Si) atoms in the glaze layer was promoted to improve enamel adhesion. If the thickness of the oxide layer is too thin, it is difficult to secure enamel adhesion because the bonding force between the glaze layer and the steel sheet is low. Therefore, the thickness of the oxide layer 20 on the surface of the steel sheet was limited to 0.006 to 0.030 μm. More specifically, the oxide layer 20 may have a thickness of 0.007 to 0.028 μm. The thickness of the oxide layer 20 may be different throughout the steel sheet 100, and in one embodiment of the present invention, the thickness of the oxide layer 20 means the average thickness of the entire steel sheet 100.

Next, the steel sheet micropores and carbides of the present invention will be described.

The carbide used in the inventive steel is used as a hydrogen storage source that not only crushes itself in the cold rolling process due to the difference in ductility from the base material, or forms micropores by subsequent decarburization heat treatment, but also fixes hydrogen in the steel itself. . Therefore, the area fraction of such carbides affects enamel not only by itself but also by the mutual relationship with additive elements.

The steel sheet for porcelain enamel according to an embodiment of the present invention actively utilizes not only carbides such as Fe ₃ C (cementite) but also micropores due to decarburization as hydrogen occlusion positions by adjusting the steel components, and at the same time, It is possible to provide a high-strength cold-rolled steel sheet for enamel with excellent adhesion and fishscale resistance without surface defects and products thereof by controlling components and processes that affect adhesion, surface defects, etc. among components.

Cementite uniformly dispersed and precipitated during hot rolling is crushed during cold rolling and also acts as a decarburization reaction source through atmosphere control in the annealing process to form micropores, which are hydrogen storage sources, which effectively fix hydrogen in steel to cause fishscale defects. was able to suppress By controlling the fraction of carbides and micropores in the thickness direction by the continuous annealing decarburization operation and also controlling the behavior of oxides in the surface layer of the steel sheet, it has a great effect on enamel adhesion and suppression of cell defects.

Specifically, the steel sheet for porcelain enamel according to an embodiment of the present invention may satisfy Equation 3 below.

[Equation 3]

0.75 ≤ C _1/2t - C _1/8t ≤ 2.45

(In Equation 3, C _1/2t represents the area fraction (%) of cementite at 1/2 of the total thickness of the steel sheet in the thickness direction of the steel sheet, and C _{1/8t represents} 1/8 of the total thickness of the steel sheet in the thickness direction of the steel sheet. It represents the area fraction (%) of cementite in the site.)

Carbon present in the metal alloy combines with metal atoms to form carbides. One of the carbides formed at a relatively low temperature by combining iron with carbon is cementite. In general, cementite is formed between 250 and 700 ° C. in carbon steel, and at a higher temperature than this, it is coarsened into spherical particles. Cementite generated in the hot rolling step is crushed in the cold rolling process and also decomposed in the decarburization process to act as a source for occluding hydrogen. However, when these cementites are concentrated on the surface of the steel, they become a source that promotes the gasification reaction of carbon in the enamel firing process and become a factor that causes bubble defects. Therefore, it is necessary to strictly manage the carbide fraction in the thickness direction in order to suppress fishscale and bubble defects in enamel products. That is, if the difference in cementite area fraction in the thickness direction of the steel sheet is too small (C _1/2t - C _1/8t ), the decarburization reaction does not proceed smoothly, and the carbide area fraction of the surface layer increases, causing bubble defects after enamel firing. can act as a factor. On the other hand, if the difference in cementite area fraction in the steel sheet thickness direction (C _1/2t - C _1/8t ) is too large, the supply of sites capable of occluding hydrogen in the steel is insufficient, making it difficult to suppress the occurrence of fishscale defects. there is. Therefore, the difference in cementite area fraction in the thickness direction of the steel sheet in the thickness direction (C _1/2t - C _1/8t ) may be 0.75 to 2.45%, and more specifically, 0.78 to 2.43%. The reference plane for the cementite area fraction may be a cross section of the steel sheet in an arbitrary direction. Specifically, with respect to a surface parallel to the rolling surface (ND surface), it can be measured using a scanning electron microscope (SEM).

The steel sheet for vitreous enameling according to an embodiment of the present invention may satisfy Equation 4 below.

[Equation 4]

0.07 ≤ MV _1/8t - MV _Av ≤ 0.14

(In Equation 4, MV _1/8t is the micropore area fraction (%) at 1/8 of the total thickness of the steel sheet in the steel sheet thickness direction, and MV _Av means the average micropore area fraction (%).)

The cementite precipitated during hot rolling is crushed during cold rolling and decarburization heat treatment, so that fine pores are formed around them. The formed micropores act as hydrogen occlusion sources to suppress the occurrence of fishscale defects. For the micropores in the steel sheet, 10 pictures are taken at 1000x magnification using a scanning electron microscope (SEM) on a surface parallel to the rolling surface (ND surface), and the area fraction of the micropores in these areas is calculated using an image analyzer. can be measured using The average micropore area fraction can be measured by averaging the micropore area fractions measured at least three unbalanced sites. Specifically, it can be obtained by averaging the micropore area fractions measured at 1/8t, 1/4t, and 1/2t. In one embodiment of the present invention, it was confirmed that there is a region where fishscale and bubble defects can be simultaneously suppressed by controlling the area fraction distribution of these micropores for each region. In order to secure such an effect, it is necessary to manage the micropore area fraction difference (MV _1/8t - MV _Av ) to 0.07 to 0.14%. If the difference in micropore area fraction (MV _1/8t - MV _Av ) is too small, it is advantageous in terms of fish scale resistance, but problems such as deterioration of workability and frequent surface defects such as bubble defects may occur. On the other hand, if the difference in micropore area fraction (MV _1/8t - MV _Av ) is too large, the number of sites acting as hydrogen storage sources capable of fixing hydrogen in the steel is small, resulting in a problem in that the rate of fishscale defects in the product increases. . Therefore, the micropore micropore area fraction difference (MV _1/8t - MV _Av ) may be 0.07 to 0.14%, and more specifically, 0.073 to 0.137%.

The steel sheet for vitreous enameling according to an embodiment of the present invention may satisfy Equation 5 below.

[Equation 5]

0.45 ≤ (R _a × 50 × S _e )/P _c ≤ 0.99

(In Equation 5, P _c is the number of surface irregularities per unit centimeter (cm), R _a is the average surface roughness value (μm), and S _e is the refining reduction (%).)

If the value of Equation 5 is too small, the wedge effect of the surface of the steel sheet may be reduced, resulting in poor adhesion with the glaze. On the other hand, if the value of Equation 5 is too large, it may be difficult to secure the target material and enamel characteristics because the crystal grains of the steel sheet grow after the enamel firing process.

As described above, the high-strength enamel enamel steel sheet having excellent adhesion according to the present invention may have excellent enamel adhesion while having excellent strength characteristics.

In detail, the high-strength enamel steel sheet having excellent adhesion according to one embodiment of the present invention 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 influences dent resistance and shape freezing of members, and is usually measured by a tensile test method. In the case of enamel products, the yield strength at the processing inlet side of products produced and supplied by steelmakers is also important, but due to the nature of the product, it undergoes firing heat treatment at high temperatures for drying after enamel glaze treatment. At this time, the heat treatment may vary depending on the type of glaze used, but may be performed for 15 minutes at a temperature of 780 to 850 ° C. As described above, the characteristics of the enamel product were limited to 220 MPa or more because the yield strength after heat treatment in the enamel treatment process is a major factor in examining the stability of the product. Since the yield strength measured by the conventional tensile test method is a property that can change somewhat depending on the test conditions, in this evaluation, the cross head speed representing the tensile speed per unit time was applied at 10 mm per minute. The obtained yield strength after enamel firing heat treatment may be 220 MPa or more, and more specifically, 225 MPa or more. At this time, the upper limit of the yield strength is not particularly limited, but may be, for example, 350 MPa.

Enamel adhesion of the steel sheet for enamel according to an embodiment of the present invention may be 95% or more. By satisfying these physical properties, it can be applied as a material for enamel even when using a relatively inexpensive glaze. If the enamel adhesion is too low, the glaze layer falls off during distribution or handling after enamel treatment, and the marketability as an enamel material deteriorates. As a result, enamel houses apply expensive glazes with a large amount of ingredients such as NiO in consideration of stability, which increases cost. Since it acts as a factor of rise, efforts are being made to come up with a way to secure enamel adhesion even with low-cost glaze. Conventionally, if the enamel adhesion is 90% or more, it is classified as the best enamel product, but in one embodiment of the present invention, a method of securing enamel adhesion of 95% or more is proposed. In addition, if the enamel adhesion is low, the fishscale generation rate by hydrogen in the steel also increases, so it is desirable to secure as high an adhesion as possible. More specifically, the enamel adhesion may be 96% or more. Adhesion to enamel refers to the numerical value expressed by indexing the degree of delamination of the enamel glaze layer by evaluating the degree of current in this area after applying a certain load to the enamel layer with a steel ball as defined in the American Society for Testing and Materials (ASTM) C313-78. . The upper limit of the enamel adhesion is not particularly limited, but may be, for example, 100%.

The steel sheet for porcelain enamel according to an embodiment of the present invention may have a hydrogen permeability ratio of 600 sec/mm ² or more. The hydrogen permeability ratio is listed in the European standard (EN10209) as a representative index for evaluating fishscale resistance indicating resistance to fishscale defects, which are fatal defects when applied to enameled steel manufactured using cold-rolled steel sheet according to an embodiment of the present invention. The ability to fix hydrogen in the steel sheet was evaluated by the method described above. A value expressed by generating hydrogen in one direction of the steel plate and measuring the time (t _s , unit: second) for hydrogen to penetrate to the other side of the steel plate, dividing it by the square of the material thickness (t, unit: mm), It is expressed as t _s /t ² (unit: second/mm ² ). If the hydrogen permeability ratio is too low, when the fishscale defect resistance is evaluated by accelerated heat treatment at 200℃ for 24 hours after enameling, the defect rate is over 50%, which causes problems in using it as a stable enamel product. In order to secure, it is necessary to manage the hydrogen permeation ratio at 600 sec / mm ² or more. Also, more specifically, the hydrogen permeation ratio 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, 1700 sec/mm ² .

In the manufacturing method of the steel sheet for porcelain enamel according to an embodiment of the present invention, C: 0.01 to 0.1%, Mn: 0.05 to 0.40%, Si: 0.001 to 0.03%, Al: 0.005 to 0.020%, P: 0.001 to 0.020%, S: 0.001 to 0.020%, Cu: 0.03 to 0.08%, N: 0.006 to 0.015%, O: 0.003% or less (excluding 0%), and the balance Fe and unavoidable impurities. Manufacturing a hot-rolled steel sheet by rolling; Manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; Annealing the cold-rolled steel sheet; and temper-rolling the annealed cold-rolled steel sheet.

First, a slab satisfying the above composition is prepared. In the steelmaking step, the molten steel whose components are adjusted to the above-described composition may be manufactured into slabs through continuous casting. The alloy composition of the slab is substantially the same as that of the above-mentioned steel sheet for enamel. Since the alloy components have been described above, overlapping descriptions are omitted.

Before hot rolling the slab, the manufactured slab may be heated. By heating, the subsequent hot rolling process can be smoothly performed and the slab can be homogenized. More specifically, heating may mean reheating.

At this time, the slab heating temperature may be 1150 to 1280 ° C. If the slab heating temperature is too low, the rolling load rapidly increases in the subsequent hot rolling process, which may deteriorate workability. On the other hand, if the slab heating temperature is too high, it not only increases the energy cost, but also increases the amount of surface scale, which can lead to material loss. More specifically, it may be 1180 to 1260 ° C.

Thereafter, the heated slab is hot-rolled to manufacture a hot-rolled steel sheet.

At this time, the finish rolling temperature of the hot rolling may be 850 to 910 ℃. If the finish hot rolling temperature is too low, as the rolling is finished in a low temperature region, the grain granulation rapidly progresses, resulting in a decrease in rollability and workability. On the other hand, if the finish hot rolling temperature is too high, the peelability of the surface scale is lowered and the hot rolling is not uniform throughout the thickness, so that impact toughness may be reduced due to grain growth. More specifically, the finish hot rolling temperature may be 860 to 900 ° C.

Thereafter, the hot-rolled steel sheet manufactured after the hot rolling is subjected to a winding process. More specifically, it may be a hot-rolled coiling process.

At this time, the winding temperature may be 580 to 720 ℃. The hot-rolled steel sheet may be cooled on a run-out-table (ROT) before winding. If the hot-rolled coiling temperature is too low, temperature unevenness in the width direction occurs in the cooling and maintaining process, resulting in material deviation as low-temperature precipitate production changes, as well as adversely affecting enamel. On the other hand, if the coiling temperature is too high, the corrosion resistance deteriorates as the bulking of the tin oxide progresses, and the grain boundary segregation of P is promoted, resulting in a decrease in cold rolling properties and a problem of deteriorating workability due to coarsening of the structure in the final product. occurred. More specifically, the winding temperature may be 590 to 710 °C.

The coiled hot-rolled steel sheet may further include a step of pickling the steel sheet before cold rolling.

Then, the coiled hot-rolled steel sheet is manufactured into a cold-rolled steel sheet through cold rolling.

At this time, the cold reduction ratio may be 60 to 90%. If the cold rolling reduction is too low, since recrystallization driving force in the subsequent heat treatment process is not secured, unrecrystallized grains remain locally, increasing strength but significantly reducing workability. In addition, as the carbide crushing ability formed in the hot rolling step decreases, the number of sites capable of occluding hydrogen decreases, making it difficult to secure fishscale resistance, and considering the thickness of the final product, the thickness of the hot rolled sheet must be lowered, which deteriorates the rolling workability. there was. On the other hand, if the cold rolling reduction ratio is too high, there is a problem in that the material is hardened and workability is deteriorated, and the load of the rolling mill is increased to deteriorate workability. More specifically, the cold rolling reduction may be 63 to 88%.

Thereafter, the cold-rolled steel sheet may be annealed to produce an annealed steel sheet. In this case, the annealing treatment may refer to a continuous annealing treatment. Since cold-rolled materials have high strength due to high strain applied in cold rolling, but extremely poor workability, workability is secured by performing continuous annealing in a subsequent process. In one example of the present invention, the annealing temperature may be 720 to 850 ° C, and the appropriate holding time may be 20 to 180 seconds.

At this time, the annealing temperature may be 720 to 850 ℃. If the annealing temperature is too low, the strain formed by cold rolling is not sufficiently removed, and thus the workability is remarkably deteriorated, and the decarburization rate by atmospheric heat treatment is too low, so that the desired characteristics of the cold-rolled steel sheet for enamel cannot be secured. On the other hand, if the heat treatment temperature is too high, softening due to a decrease in high-temperature strength may reduce the annealing permeability due to plate breakage, and the opposite effect may occur in that the decarburization reaction is suppressed due to an increase in the thickness of the surface oxide layer. Therefore, the annealing temperature may be 720 to 850 ℃. More specifically, the annealing temperature may be 730 to 840 °C.

In addition, in one example of the present invention, the oxidation capacity (PH ₂ O/PH ₂ ) of the annealing atmosphere conditions may be 0.51 to 0.65. If the oxidation ability is too low, decarburization takes a long time and decarburization during continuous annealing decarburization deteriorates, making it difficult to secure enamel characteristics. On the other hand, if the oxidation ability is too high, there is a problem in that the rate of occurrence of surface defects increases due to the surface coating layer formed by peroxidation. Therefore, the oxidizing ability of atmospheric gas was limited to 0.51 to 0.65. More specifically, the oxidation ability may be 0.52 to 0.64.

In addition, the crack holding time in the annealing process may be 20 to 180 seconds. Even when the soaking time at the holding temperature is too short, non-recrystallized grains remain, greatly deteriorating formability, and decarburization in the thickness direction is not performed smoothly, which may act as a factor in deteriorating enamel. On the other hand, if the holding time is too long, abnormal crystal grain growth may occur due to decarburization reaction, which may cause problems such as deterioration in processability and deterioration of fishscale resistance due to material non-uniformity. Therefore, the holding time at the soaking temperature may be 20 to 180 seconds. More specifically, it may be 25 to 160 seconds.

The amount of carbon decarburized from the steel sheet through the above-described annealing process (carbon amount before annealing - carbon amount after annealing) may be 0.01 to 0.05% by weight.

Next, a step of temper rolling the heat-treated steel sheet after the step of annealing the cold-rolled steel sheet may be further included. Through temper rolling, it is possible to control the shape of the material and obtain the desired surface roughness. However, if the temper rolling reduction is too high, the material is hardened by work hardening, and during subsequent firing heat treatment, yield strength decreases rapidly due to tissue growth, resulting in poor dent resistance. Therefore, temper rolling can be applied with a reduction ratio of 2.5% or less. More specifically, the rolling reduction of temper rolling may be 0.3 to 2.2%.

Furthermore, an enamel firing step may be further included to dry the enamel-treated glaze after the step of annealing the cold-rolled steel sheet. Through the enamel firing process, by applying an enamel layer on the surface of the steel sheet through heating to a high temperature and cooling to room temperature, various properties suitable for the purpose of the enamel product, such as chemical resistance and heat resistance, can be obtained. However, if the firing temperature is too low, the enamel layer There is a problem that adhesion characteristics of cannot be secured, and on the other hand, if the firing temperature is too high, it acts as a cost increase factor due to the increase in energy sources used, so the firing temperature can be applied at 780 to 850 ° C. More specifically, the firing temperature may be 790 to 840 °C.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

Example

In weight percent, a slab was prepared through the converter-secondary refining-casting process with the composition shown in Table 1 below and the alloy components including the remainder iron (Fe) and unavoidable impurities. After holding this slab in a 1200 ° C heating furnace for 2 hours, hot rolling was performed. At this time, the final thickness of the hot-rolled steel sheet was 4.0 mm. The hot-rolled specimen was subjected to cold rolling at each reduction ratio after removing the oxide film on the surface through pickling treatment. The cold-rolled specimens were processed into enameled specimens to investigate enamel properties and specimens for mechanical property analysis, and heat treatment was performed. Finishing hot rolling temperature (FDT, Finishing Delivery Temperature), coiling temperature (CT, Coiling Temperature), cold rolling reduction rate, annealing temperature (AT, Annealing Temperature), holding time, oxidation ability, and enamel firing temperature (Firing Temperature) are shown in Table 2 below. arranged in

Table 3 shows the operability, enamel, material and texture characteristics of the material obtained through the above process for each manufacturing condition.

In the case of sheet permeability, “O” indicates operability of 90% or more compared to the productivity of ordinary materials in the continuous, hot-rolling, and cold-rolling processes, and “X” indicates productivity of 90% or less or defect occurrence rate of 10% or more.

The area fraction (%) of carbide was determined as the area fraction of carbide with respect to the entire field of view using an image analyzer after obtaining images of 20 fields of view at 500x magnification using an optical microscope.

Yield strength (MPa) is the result obtained by conducting a tensile test at a crosshead speed of 10 mm/min after preparing a tensile test piece after firing heat treatment for 15 minutes at each temperature to simulate the enamel glaze drying process for the steel plate.

Enamel treated specimens were cut into appropriate sizes for each purpose to meet the purpose of the test, and after the heat treatment was completely degreased, a 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. has been removed. After drying, the specimen was fired at a relatively low temperature of 800 °C for 15 minutes to highlight the difference in enamel characteristics such as adhesion, and then cooled to room temperature. Easy harsh conditions were chosen.

The specimens after the enamel treatment were subjected to a fishscale acceleration test in which they were maintained in an oven at 200 ° C. for 24 hours. After the fishscale acceleration treatment, the presence or absence of fishscale defects was observed with the naked eye, and the occurrence of fishscale defects was marked with “O” and “X” for occurrence.

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.

Bubble defects were visually observed for specimens kept in an oven at 200 ° C. for 24 hours after enamel treatment, and were judged in three stages: “O” excellent, “Δ” normal, and “X” poor, respectively.

The hydrogen permeation ratio is one of the indices for evaluating the resistance to fishscale, a fatal defect in 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. The output time (ts, unit: second) is measured and expressed as the square of the material thickness (t, unit: mm), expressed as ts/t ² (unit: second/mm ² ).

division C Mn Si Al P S N Cu O Ti ([Cu] × [Si])/[P] [Al]/
[N] invention steel 1 0.034 0.18 0.012 0.017 0.012 0.011 0.0081 0.039 0.0018 - 0.039 2.099 invention steel 2 0.052 0.32 0.016 0.009 0.008 0.005 0.0094 0.042 0.0011 - 0.084 0.957 invention steel 3 0.046 0.26 0.009 0.014 0.017 0.017 0.0075 0.064 0.0008 - 0.034 1.867 Invention Steel 4 0.078 0.29 0.018 0.008 0.011 0.009 0.0105 0.051 0.0015 0.001 0.083 0.762 invention steel 5 0.066 0.13 0.022 0.016 0.009 0.012 0.0122 0.072 0.0017 - 0.176 1.311 comparative steel 1 0.002 0.21 0.014 0.049 0.011 0.052 0.0034 0.045 0.0021 0.087 0.057 14.412 comparative steel 2 0.015 0.35 0.011 0.008 0.066 0.015 0.0095 0.019 0.0011 - 0.003 0.842 comparative lecture 3 0.002 0.79 0.007 0.003 0.018 0.010 0.0041 0.037 0.0354 - 0.014 0.732 comparative lecture 4 0.068 0.32 0.321 0.118 0.009 0.014 0.0082 0.311 0.0015 - 11.092 14.39 comparative steel 5 0.128 0.28 0.018 0.007 0.045 0.009 0.0074 0.002 0.0021 - 0.001 0.946

division steel grade
No. finish
Hot rolling temperature (℃) winding
temperature
(℃) cold
reduction rate
(%) Annealing
temperature
(℃) maintain
hour
(candle) tempering
Reduction rate (%) (Ra×50×Se)
/Pc oxidation ability
(P _H2O
/P _H2 ) enamel
Firing temperature (℃) oxide layer
thickness
(μm) after annealing
amount of carbon
(%) Invention example 1 invention steel 1 890 680 75 760 120 1.8 0.8591 0.53 820 0.021 0.015 Invention Example 2 invention steel 1 890 680 80 780 140 1.4 0.7457 0.53 820 0.020 0.015 Invention example 3 invention steel 1 890 680 85 820 60 0.8 0.4927 0.53 820 0.016 0.016 Invention example 4 invention steel 2 870 640 65 740 100 2.0 0.9032 0.57 800 0.011 0.026 Invention Example 5 invention steel 2 870 640 80 820 80 0.6 0.5250 0.57 800 0.018 0.019 Example 6 invention steel 3 890 660 75 780 120 0.8 0.6025 0.60 830 0.025 0.024 Example 7 Invention Steel 4 890 600 75 820 40 0.5 0.4907 0.58 830 0.021 0.036 Invention Example 8 invention steel 5 900 660 80 800 90 1.9 0.9265 0.52 830 0.018 0.038 Inventive Example 9 invention steel 5 900 680 80 830 100 0.8 0.4870 0.52 830 0.014 0.028 Comparative Example 1 invention steel 1 700 680 80 780 100 0.2 0.3231 0.35 820 0.002 0.026 Comparative Example 2 invention steel 2 870 640 45 640 15 2.8 1.1421 0.29 750 0.004 0.051 Comparative Example 3 invention steel 3 890 460 75 920 100 0.8 0.3257 0.67 880 0.041 0.009 Comparative Example 4 Invention Steel 4 890 640 95 800 240 1.4 0.3500 0.29 820 0.004 0.064 Comparative Example 5 comparative steel 1 960 640 75 830 40 0.5 0.1579 0.52 820 0.003 0.001 Comparative Example 6 comparative steel 2 900 600 75 880 100 1.2 0.4315 0.53 820 0.005 0.007 Comparative Example 7 comparative lecture 3 910 680 85 830 80 1.6 2.6462 0.7 820 0.001 0.001 Comparative Example 8 comparative lecture 4 880 680 75 820 100 0.8 0.3699 0.55 820 0.001 0.058 Comparative Example 9 comparative steel 5 880 680 75 820 80 1.2 0.2727 0.55 820 0.003 0.074

division permeability C _1/2t - C _1/8t MV _1/8t - MV _Av yield strength
(MPa) bubble defects
Occurrence or not fish scale
Occurrence or not Adhesion to enamel
(%) hydrogen permeation ratio
(second/㎟) Invention example 1 O 0.936 0.081 253 O O 99.4 946 Invention example 2 O 1.035 0.094 248 O O 99.1 908 Invention example 3 O 1.273 0.096 241 O O 100 1021 Invention Example 4 O 1.246 0.103 277 O O 99.2 754 Invention example 5 O 1.728 0.121 292 O O 99 902 Example 6 O 1.422 0.109 238 O O 100 1124 Example 7 O 2.119 0.135 312 O O 99.2 869 Invention Example 8 O 1.918 0.132 274 O O 99.3 725 Inventive Example 9 O 2.083 0.115 304 O O 99 647 Comparative Example 1 X 0.349 0.041 193 X X 82.5 514 Comparative Example 2 X 0.323 0.064 168 X X 65.9 428 Comparative Example 3 X 2.938 0.180 205 △ X 89.4 552 Comparative Example 4 X 0.709 0.042 246 X X 85.1 482 Comparative Example 5 O 0.004 0.002 142 O X 92.4 567 Comparative Example 6 X 0.051 0.003 167 X X 75.3 427 Comparative Example 7 O 0.003 0.031 164 O X 92.4 683 Comparative Example 8 O 0.526 0.052 206 X X 84.1 428 Comparative Example 9 X 0.559 0.047 247 X X 61.2 511

As can be seen from Tables 1 to 3, Inventive Examples 1 to 9 satisfying all of the component composition, manufacturing conditions, surface characteristics, oxide layer thickness, etc. not only have good sheet permeability, but also have carbide and micropore volume fractions and The related indices satisfied the limited range of the present invention, and enamel defects such as fish scale and bubble defects did not occur even under severe treatment conditions, and hydrogen permeation ratio of 600 sec/mm ² or more, enamel adhesion index of 95% or more, enamel firing After heat treatment, it was confirmed that the yield strength of 220 MPa or more was satisfied.

On the other hand, in the case of Comparative Examples 5 to 9, which do not satisfy the steel composition suggested in the present invention, the thickness of the surface oxide layer, the volume fraction of cementite and micropores in each thickness direction, the hydrogen permeability ratio, the enamel adhesion, etc. presented in the present invention are satisfied. Not only was it not possible, but in most cases, fish scale or bubble defects occurred even in visual observation after enamel treatment, resulting in applicability problems.

In addition, in Comparative Examples 1 to 4, which satisfied the steel composition suggested in the present invention, but did not properly control the oxidation ability in the annealing process, the thickness of the surface oxide layer was outside the range suggested in the present invention, and in this case, the enamel adhesion was 95%. It was confirmed that enamel defects such as bubble defects or fish scale occurred after enamel treatment, and sheetability was poor, and in some cases, the yield strength after enamel firing heat treatment was less than 220 MPa. characteristics could not be obtained.

Meanwhile, FIG. 2 shows the result of analyzing the component distribution in the thickness direction of the cold-rolled steel sheet for vitreous enameling according to Inventive Example 3 by GDS. It was confirmed that the innermost point where the oxygen content was 5% by weight was 0.016 μm, and the oxide layer 20 having a thickness of 0.016 μm was present on the surface.

The present invention is not limited to the above embodiments, but can be manufactured in a variety of different forms, and those skilled in the art to which the present invention pertains may take other specific forms without changing the technical spirit or essential features of the present invention. It will be understood that it can be implemented with Therefore, the embodiments described above should be understood as illustrative in all respects and not limiting.

100: steel sheet for enamel, 10: steel sheet substrate,
20: oxide layer

Claims (17)

In weight percent, C: 0.001 to 0.08%, Mn: 0.05 to 0.40%, Si: 0.001 to 0.03%, Al: 0.005 to 0.020%, P: 0.001 to 0.020%, S: 0.001 to 0.020%, Cu: 0.03 to 0.03% 0.08%, N: 0.006 to 0.015%, O: 0.003% or less (excluding 0%) and the balance including Fe and unavoidable impurities,
A steel sheet for porcelain enameling, comprising an oxide layer having a concentration gradient in which the oxygen concentration decreases from the surface to the inside, wherein the oxide layer has a thickness of 0.006 to 0.030 μm. According to claim 1,
A steel sheet for enamel that satisfies the following formula 1.
[Equation 1]
0.02 ≤ ([Cu] × [Si])/[P] ≤ 0.19
(In Equation 1, [Cu], [Si], and [P] represent the contents (wt%) of Cu, Si, and P, respectively.) According to claim 1,
A steel sheet for enamel that satisfies Expression 2 below.
[Equation 2]
0.5 ≤ [Al]/[N] ≤ 2.5
([Al] and [N] in Equation 2 indicate the content (wt%) of Al and N, respectively.) According to claim 1,
Ti: 0.003 wt% or less, Co: 0.005 wt% or less, Nb: 0.003 wt% or less, Ni: 0.01 wt% or less, V: 0.003 wt% or less, and Mo: 0.01 wt% or less steel plate. According to claim 1,
A steel sheet for enamel that satisfies Expression 3 below.
[Equation 3]
0.75 ≤ C _1/2t - C _1/8t ≤ 2.45
(In Equation 3, C _1/2t represents the area fraction (%) of cementite at 1/2 of the total thickness of the steel sheet in the thickness direction of the steel sheet, and C _{1/8t represents} 1/8 of the total thickness of the steel sheet in the thickness direction of the steel sheet. It represents the area fraction (%) of cementite in the site.) According to claim 1,
A steel sheet for enamel that satisfies Expression 4 below.
[Equation 4]
0.07 ≤ MV _1/8t - MV _Av ≤ 0.14
(In Equation 4, MV _1/8t is the micropore area fraction (%) at 1/8 of the total thickness of the steel sheet in the steel sheet thickness direction, and MV _Av means the average micropore area fraction (%).) According to claim 1,
A steel sheet for enamel that satisfies Expression 5 below.
[Equation 5]
0.45 ≤ (R _a × 50 × S _e )/P _c ≤ 0.99
(In Equation 5, P _c is the number of surface irregularities per unit centimeter (cm), R _a is the average surface roughness value (μm), and S _e is the refining reduction (%).) According to claim 1,
Steel sheet for enamel with a yield strength of 220 MPa or more after enamel firing heat treatment. According to claim 1,
Steel sheet for enamel with over 95% enamel adhesion. According to claim 1,
Steel sheet for enamel with a hydrogen permeability of 600 sec/mm ² or more. In weight percent, C: 0.01 to 0.1%, Mn: 0.05 to 0.40%, Si: 0.001 to 0.03%, Al: 0.005 to 0.020%, P: 0.001 to 0.020%, S: 0.001 to 0.020%, Cu: 0.03 to 0.03% Preparing a hot-rolled steel sheet by hot rolling a slab containing 0.08%, N: 0.006 to 0.015%, O: 0.003% or less (excluding 0%), and the balance Fe and unavoidable impurities;
manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; and
Including the step of annealing the cold-rolled steel sheet,
The step of annealing is a method of manufacturing a steel sheet for porcelain enamel by annealing in a wet atmosphere having an oxidation ability index (PH ₂ O/PH ₂ ) of 0.51 to 0.65. According to claim 11,
In the step of manufacturing the hot-rolled steel sheet,
A method for producing a steel sheet for porcelain enamel wherein the finish hot rolling temperature is 850 to 910 ° C. According to claim 11,
In the step of manufacturing the hot-rolled steel sheet,
A method for producing a steel sheet for porcelain enamel, wherein the coiling temperature is 580 to 720 ° C. According to claim 11,
A method of manufacturing a steel sheet for enamel wherein the reduction ratio is 60 to 90% in the step of manufacturing the cold-rolled steel sheet. According to claim 11,
The annealing step is annealing at a temperature of 720 to 850 ° C. for 20 to 180 seconds. According to claim 11,
After annealing the cold-rolled steel sheet, the method of manufacturing a steel sheet for enamel further comprising the step of temper rolling the annealed cold-rolled steel sheet. According to claim 11,
After annealing the cold-rolled steel sheet, the method of manufacturing a steel sheet for vitreous enameling further comprising the step of enamel firing the annealed cold-rolled steel sheet at a temperature of 780 to 850 ° C.

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