Steel sheet for enameling, and manufacturing method therefor

Abstract

A steel sheet for enameling, according to one embodiment of the present invention, comprises, by wt%, 0.01-0.05% of C, 0.10-0.80% of Mn, 0.001-0.03% of Si, 0.01-0.08% of Al, 0.001-0.02% of P, 0.001-0.02% of S, 0.005% or less (excluding 0%) of N, 0.003% or less (excluding 0%) of O, and the balance of Fe and inevitable impurities, and satisfies relation 1. [Relation 1] CVF[50 μm] / CVF[center]≤0.5 (In relation 1, CVF[50 μm] represents the area fraction (%) of cementite at a position 50 μm inside the steel sheet in the thickness direction from the surface of the steel sheet, and CVF[center] represents the area fraction (%) of cementite at 1 / 2 the thickness of the steel sheet.)

Description

Enameled steel plate and its manufacturing method

[0001] One embodiment of the present invention relates to an enamel steel sheet and a method for manufacturing the same. More specifically, one embodiment of the present invention relates to an enamel steel sheet with excellent fishscale resistance and a method for manufacturing the same.

[0002] Enameled steel sheet is used in a variety of industrial applications, including kitchenware, home appliances, boilers, tanks, agricultural silos, and building panels. For applications such as household sinks and ovens, where formability and surface appearance are crucial, ultra-low-carbon steel is utilized. For applications such as boilers, where mechanical strength and thermal stability are crucial, high-strength enameled steel sheet is required.

[0003] Enameled steel is formed by a glassy coating on the surface that bonds strongly with cold-rolled steel, imparting hardness, scratch resistance, acid corrosion resistance, high-temperature resistance, a high-gloss appearance, and color. Firing the enamel, which forms a glassy coating on the surface, creates an alloy layer at the rough interface between the steel sheets, bonding them strongly.

[0004] In addition to excellent adhesion between the glass and the steel, enameled steel must also possess excellent hydrogen absorption capacity. Fishscale defects, which occur when hydrogen is picked up during the enamel firing process, cause surface defects that peel off the enamel layer in a fish-scale pattern. This defect can lead to concentrated rust formation in the surrounding area, significantly reducing the value of the enamel product. Therefore, prevention is crucial. The hydrogen in fishscale defects originates from water molecules contained in the enamel glaze. Water reacts with the steel surface to form hydrogen ions, which then dissolve into the steel during the firing process. The hydrogen solubility in austenite is greater than that in ferrite, and the extremely low hydrogen solubility in ferrite at low temperatures contributes to the formation of fishscale defects. Hydrogen's high diffusivity allows it to diffuse to the glass / steel interface, forming H2 bubbles even at low temperatures, such as room temperature. These bubbles burst, forming scale-like defects.

[0005] In order to prevent fishscale defects, it is necessary to form a large number of sites inside the steel that can capture the hydrogen dissolved in the steel. To prevent fishscale defects that reduce enamel properties and to secure workability, open coil annealing (OCA), a type of batch annealing furnace (BAF) method, is applied. However, in this case, the heat treatment process takes several days, which reduces productivity due to long-term heat treatment, increases manufacturing costs, and causes large quality deviations within the coil.

[0006] However, the use of the latest continuous annealing process has overcome these productivity disadvantages and increased manufacturing costs. Enameled steels produced by the continuous annealing process are based on ultra-low carbon steel and utilize precipitates, such as titanium (Ti), or inclusions produced by the non-deoxidation process as hydrogen absorbers. However, even in this case, various problems arise, such as increased costs and reduced malleability due to the addition of large amounts of carbonitride-forming elements, and an increased incidence of surface defects due to precipitates and non-deoxidation products.

[0007] In particular, when titanium-based precipitates are used as hydrogen absorption sources, problems such as nozzle clogging caused by titanium nitride and inclusions arise. Furthermore, TiN mixed into the molten steel can cause blister defects, a typical bubble defect, if present on the upper surface of the steel sheet. Furthermore, the large amount of titanium added can form a titanium-based oxide layer, hindering the adhesion between the steel sheet and the glaze layer.

[0008] On the other hand, a representative method called high-oxygen enamel steel sheet uses a method of improving fishscale resistance by increasing the dissolved oxygen content inside the steel sheet and absorbing hydrogen by utilizing inclusions such as steel oxides.

[0009] However, high-oxygen enamel steel sheets also have a fundamental problem of high oxygen content, which causes extreme refractory dissolution, significantly reducing the casting productivity in the steelmaking process, and causing numerous surface defects.

[0010] One embodiment of the present invention provides an enamel steel sheet and a method for manufacturing the same. More specifically, one embodiment of the present invention provides an enamel steel sheet with excellent fishscale resistance and a method for manufacturing the same.

[0011]

[0012] An enamel steel sheet according to one embodiment of the present invention contains, in wt%, C: 0.01 to 0.05%, Mn: 0.10 to 0.80%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, P: 0.001 to 0.02%, S: 0.001 to 0.02%, N: 0.005% or less (excluding 0%), and O: 0.003% or less (excluding 0%), and the remainder includes Fe and unavoidable impurities, and satisfies the following formula 1.

[0013] [Formula 1]

[0014] CVF[50㎛] / CVF[center]≤0.5

[0015] (In Equation 1, CVF[50㎛] means the area fraction (%) of cementite at a position 50㎛ inside the steel plate in the thickness direction from the steel plate surface, and CVF[center] means the area fraction (%) of cementite at a position 1 / 2 of the steel plate thickness.)

[0016] An enamel steel sheet according to one embodiment of the present invention can satisfy the following equation 2.

[0017] [Formula 2]

[0018] Hv[50㎛] / Hv[center]≤0.75

[0019] (In Equation 2, Hv[50㎛] means the hardness at a position 50㎛ inside the steel plate in the thickness direction from the steel plate surface, and Hv[center] means the hardness at a position 1 / 2 of the steel plate thickness.)

[0020] The enamel steel sheet according to one embodiment of the present invention may further include at least one of Cu: 0.1 wt% or less, 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.

[0021] According to one embodiment of the present invention, the enamel steel sheet has a hydrogen permeability of 650 sec / mm.2 It could be strange.

[0022] A method for manufacturing an enamel steel sheet according to one embodiment of the present invention includes the steps of: hot-rolling a slab containing, in wt%, C: 0.01 to 0.05%, Mn: 0.10 to 0.80%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, P: 0.001 to 0.02%, S: 0.001 to 0.02%, N: 0.005% or less (excluding 0%), and O: 0.003% or less (excluding 0%) to manufacture a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

[0023] The annealing step includes a heating step and a soaking step of heating the steel sheet to a soaking temperature of 100°C or higher, the heating step being performed at a dew point temperature of 10 to 23°C, and the soaking step being performed at 0 to 13°C.

[0024] In the step of manufacturing hot-rolled steel sheets, the finishing hot rolling temperature can be 800 to 1000°C.

[0025] In the step of manufacturing hot rolled steel sheet, the coiling temperature can be 450 to 700°C.

[0026] In the manufacturing stage of cold rolled steel sheet, the reduction ratio can be 60 to 90%.

[0027] The cracking temperature can be between 700 and 850°C.

[0028] The heating step and the soaking step can be performed in an atmosphere of 3 to 20 vol% hydrogen and the remainder nitrogen.

[0029] The difference in the dew point temperature of the atmosphere during the heating stage and the soaking stage can be 3 to 15°C.

[0030] After the annealing step, a step of temper rolling the steel sheet at a reduction ratio of 0.5 to 3% may be further included.

[0031] The enamel steel sheet having excellent fish scale resistance according to one embodiment of the present invention can be used as a variety of industrial materials such as kitchenware, home appliances, boilers, tanks, agricultural silos, and building panels.

[0032] In this specification, 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 used only 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.

[0033] In this specification, when a part is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless specifically stated otherwise.

[0034] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include plural forms as well, unless the context clearly dictates otherwise. The term "comprising," as used herein, specifies a particular feature, region, integer, step, operation, element, and / or component, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0035] In this specification, the term "combination of these" included in the expression in the Makushi format means one or more mixtures or combinations selected from the group consisting of the components described in the expression in the Makushi format, and means including one or more selected from the group consisting of the components.

[0036] In this specification, when a part is referred to as being "on" or "over" another part, it may be directly on or over the other part, or there may be other parts intervening therebetween. Conversely, when a part is referred to as being "directly on" another part, there are no other parts intervening therebetween.

[0037] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0038] Also, unless otherwise stated, % means weight %, and 1 ppm is 0.0001 weight %.

[0039] In one embodiment of the present invention, the inclusion of additional elements means including the remaining iron (Fe) in an amount equivalent to the additional amount of the additional elements.

[0040] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0041]

[0042] An enamel steel sheet according to one embodiment of the present invention contains, in wt%, C: 0.01 to 0.05%, Mn: 0.10 to 0.80%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, P: 0.001 to 0.02%, S: 0.001 to 0.02%, N: 0.005% or less (excluding 0%), and O: 0.003% or less (excluding 0%), and the remainder includes Fe and unavoidable impurities.

[0043] First, we explain the reason for limiting the composition of the steel plate.

[0044]

[0045] C: 0.010 to 0.050 wt%

[0046] Carbon (C), if added in too much, increases the amount of dissolved carbon in the steel, which increases the strength, but it hinders the development of the texture during annealing, which reduces the formability and can cause bubble defects due to bubbling of the enamel layer. On the other hand, if C is too little, the texture grows, making it difficult to secure the target yield strength after firing, and the fraction of precipitates that act as a hydrogen absorption source is also low, which can cause the problem of susceptibility to fishscale defects. More specifically, C can be included in an amount of 0.013 to 0.045 wt%.

[0047]

[0048] Mn: 0.100 to 0.800 wt%

[0049] Manganese (Mn) is a representative solid solution strengthening element. It prevents hot shortness and promotes the precipitation of carbides by precipitating sulfur dissolved in steel in the form of manganese sulfide (MnS). If too little Mn is added, it is difficult to sufficiently obtain the aforementioned effect. On the other hand, if the Mn content is too high, the thickness center segregation zone develops significantly during casting of the steel slab in the casting process, which deteriorates the formability and lowers the Ar3 transformation temperature, which may cause a problem of deformation due to transformation during enamel firing. More specifically, Mn may be included in an amount of 0.150 to 0.770 wt%.

[0050]

[0051] Si: 0.001 to 0.030 wt%

[0052] Silicon (Si) is an element that promotes the formation of carbides, which act as hydrogen storage. If too little Si is added, it is difficult to fully achieve the aforementioned effects. Conversely, if too much Si is added, an oxide film may form on the steel sheet surface, reducing enamel adhesion. More specifically, the Si content may be 0.002 to 0.025 wt%.

[0053]

[0054] Al: 0.010 to 0.0800 wt%

[0055] Aluminum (Al) is an element added to molten steel for deoxidation. If too little Al is added, it is difficult to produce killed steel in a stable state. Conversely, if too much Al is added, while it is beneficial for increasing strength through grain refinement, it can also cause excessive inclusion formation during steelmaking, leading to nozzle clogging, as well as causing bubble defects such as blistering during the enameling process. More specifically, it can be included in an amount of 0.011 to 0.060 wt%.

[0056]

[0057] P: 0.001 to 0.020 wt%

[0058] Phosphorus (P) is the substitutional element with the best strengthening effect, and is the most effective element for securing yield strength without significantly reducing ductility. If too little P is added, it is difficult to fully achieve the aforementioned effect. On the other hand, if too much P is added, the possibility of brittle fracture greatly increases, which can cause slab fracture during hot rolling and impair the pickling property of the steel, which can also have a negative effect on enamel adhesion. More specifically, it can be included in an amount of 0.005 to 0.018 wt%.

[0059]

[0060] S: 0.001 to 0.020 wt%

[0061] Sulfur (S) is an impurity element that is inevitably contained in steel. It combines with manganese and other elements to form non-metallic inclusions, thereby significantly reducing the toughness of the steel. If too little S is added, the weldability may deteriorate. If too much S is added, not only will ductility decrease significantly, which will worsen workability, but manganese sulfide may also be excessively precipitated, which may have a negative effect on the fishscale resistance of the product. More specifically, it may be included in an amount of 0.003 to 0.015 wt%.

[0062]

[0063] N: 0 0.0050 wt% or less

[0064] Nitrogen (N) contributes to the strength of steel, but is difficult to control. In the case of high-solution nitrogen, it interacts with dislocations, causing numerous aging defects, poor formability, and problems such as the generation of bubble defects in the enamel treatment process. More specifically, it can be included in an amount of 0.0010 to 0.0040 wt%.

[0065]

[0066] O: 0.0030 wt% or less

[0067] Oxygen (O) is an essential element for forming oxides, and such oxides not only cause refractory material loss during the steelmaking process, but also act as a factor in causing surface defects caused by oxides on the surface during the production of steel sheets. Therefore, O may be included in an amount of 0.0030 wt% or less. More specifically, it may be included in an amount of 0.0003 to 0.0025 wt%.

[0068]

[0069] In addition to the above components, the present invention includes Fe and inevitable impurities. The addition of effective components other than the above components is not excluded. As inevitable impurities, Cu, Ti, Co, Nb, Ni, V, Mo, etc. can be mentioned. In one embodiment of the present invention, Cu, Ti, Co, Nb, Ni, V, Mo, etc. are not intentionally added, and one or more of Cu: 0.1 wt% or less, 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 may be further included.

[0070]

[0071] An enamel steel sheet according to one embodiment of the present invention satisfies the following equation 1.

[0072] [Formula 1]

[0073] CVF[50㎛] / CVF[center]≤0.50

[0074] (In Equation 1, CVF[≤50㎛] means the area fraction (%) of cementite at a location 50㎛ inside the steel plate in the thickness direction from the steel plate surface, and CVF[center] means the area fraction (%) of cementite at a location 1 / 2 of the steel plate thickness.)

[0075] If the ratio of the cementite area fraction within a depth of 50㎛ from the surface of the enamel steel plate to the cementite area fraction in the center of the steel plate is too high, it means that a large amount of cementite has been formed on the surface of the steel plate, which may lead to a problem of reduced hydrogen absorption capacity due to insufficient decarburization of the surface tissue. More specifically, the value of the left side of Equation 1 may be 0.10 to 0.45.

[0076] The cementite fraction can be calculated by grinding the steel sheet to a depth of 50 ㎛ from the surface of the steel sheet and to the center (1 / 2 thickness position), obtaining an image with an optical microscope of a plane parallel to the rolled surface (ND plane) of the steel sheet, and then using an image analyzer to obtain the cementite fraction for the entire field of view. In one embodiment of the present invention, in addition to cementite, the remaining microstructure may be composed of ferrite.

[0077] An enamel steel sheet according to one embodiment of the present invention can satisfy the following equation 2.

[0078] [Formula 2]

[0079] Hv[50㎛] / Hv[center]≤0.75

[0080] (In Equation 2, Hv[50㎛] means the hardness at a position 50㎛ inside the steel plate in the thickness direction from the steel plate surface, and Hv[center] means the hardness at a position 1 / 2 of the steel plate thickness.)

[0081] In one embodiment of the present invention, if the hardness at the surface is higher than that at the center, the problem of reduced hydrogen absorption capacity may occur, resulting in poor fish scale resistance. More specifically, the value of the left side of Equation 2 may be 0.40 to 0.73.

[0082]

[0083] According to one embodiment of the present invention, the enamel steel sheet has a hydrogen permeability of 650 sec / mm. 2It can be ideal. Hydrogen permeability is a representative index for evaluating the fishscale resistance, which indicates the resistance to fishscale defects, which are fatal defects when applying enamel steel manufactured using a cold-rolled steel sheet according to an embodiment of the present invention, and evaluates the ability to fix hydrogen in the steel sheet using a method listed in the European standard (EN10209-2013). It is the time (t) for hydrogen to be generated in one direction of the steel sheet and for hydrogen to permeate out to the opposite side of the steel sheet. s , unit: seconds) and divided by the square of the material thickness (t, unit: mm), and expressed as a value, t s / t 2 (Unit: sec / mm 2 ) is expressed as. If the hydrogen permeability ratio is too low, when evaluating the resistance to fish scale defects by performing accelerated heat treatment at 200℃ for 24 hours after enamel treatment, the defect rate is more than 50%, which causes problems in using it as a stable enamel product. Therefore, in order to secure a steel plate with excellent fish scale resistance, the hydrogen permeability ratio should be 650 sec / mm. 2 It is necessary to manage it as above. In addition, more specifically, the hydrogen permeability ratio is 690 sec / mm 2 It can be ideal. The upper limit of hydrogen permeability is not specifically limited, but for example, 1700 sec / mm 2 It could be.

[0084]

[0085] A method for manufacturing an enamel steel sheet according to one embodiment of the present invention includes the steps of: hot-rolling a slab containing, in wt%, C: 0.01 to 0.05%, Mn: 0.10 to 0.80%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, P: 0.001 to 0.02%, S: 0.001 to 0.02%, N: 0.005% or less (excluding 0%), and O: 0.003% or less (excluding 0%) to manufacture a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

[0086] First, a slab satisfying the aforementioned composition is prepared. Molten steel, whose composition has been adjusted to the aforementioned composition during the steelmaking process, can be manufactured into a slab through continuous casting. The alloy composition of the slab is substantially identical to that of the enamel-making steel sheet described above. Since the alloy composition has been previously described, a detailed explanation will be omitted.

[0087] Before hot rolling a slab, the manufactured slab can be heated. Heating facilitates the subsequent hot rolling process and homogenizes the slab. More specifically, heating can refer to reheating.

[0088] At this time, the slab heating temperature may be 1100 to 1300℃. If the slab heating temperature is too low, the rolling load in the subsequent hot rolling process will increase rapidly, and in particular, inclusions within the slab will not be sufficiently re-dissolved, which may cause surface defects after hot rolling. On the other hand, if the slab heating temperature is too high, the strength may be reduced as the final structure coarsens through abnormal grain growth of austenite. More specifically, it may be 1150 to 1250℃.

[0089] After that, the heated slab is hot rolled to produce hot rolled steel sheets.

[0090] At this time, the finishing rolling temperature of the hot rolling may be 800 to 1000℃. If the finishing hot rolling temperature is too low, there is a concern that the rolling load may increase significantly, elongated grains may develop due to delayed recrystallization, and the cold rolling property may be poor. On the other hand, if the finishing hot rolling temperature is too high, the structure of the steel plate may become coarse, making the steel material brittle, the scale may become thick, and the surface quality may be significantly deteriorated due to high-temperature rolling property scale defects. More specifically, the finishing hot rolling temperature may be 830 to 900℃.

[0091] Afterwards, the hot-rolled steel sheet manufactured after hot rolling goes through a coiling process. More specifically, this may be a hot-rolling coiling process.

[0092] At this time, the coiling temperature can be 450 to 700°C. If the hot-rolling coiling temperature is too low, the grains may become coarser, which can lead to poor cold-rollability. On the other hand, if the coiling temperature is too high, the precipitation of dissolved carbon and nitrogen may not be sufficient and may remain, resulting in poor workability and poor cold-rollability. More specifically, the coiling temperature can be 590 to 710°C.

[0093] The coiled hot rolled steel sheet may additionally include a step of pickling the steel sheet before cold rolling.

[0094] After that, the coiled hot-rolled steel plate is manufactured into cold-rolled steel plate through cold rolling.

[0095] At this time, the cold reduction ratio can be 50 to 90%. If the cold reduction ratio is too low, the amount of annealing recrystallization nuclei generated will be small, which may cause grains to grow too large during annealing, resulting in reduced strength and workability. On the other hand, if the cold reduction ratio is too high, the amount of nuclei generated will be too large, resulting in fine annealing recrystallization grains, which may result in reduced ductility. More specifically, the cold reduction ratio can be 75 to 85%.

[0096] After that, the cold rolled steel plate can be manufactured into annealed steel plate by annealing, and the annealing treatment here may mean continuous annealing treatment.

[0097] In one embodiment of the present invention, the annealing step includes a heating step of heating the steel sheet to a temperature of 100°C or higher and a soaking step. It is also possible to heat the steel sheet to a temperature higher than the soaking temperature in the heating step and then cool it to cause the soaking.

[0098] The heating step has a dew point temperature of 10 to 23°C. If the dew point temperature is too low during the heating step, local decarburization does not occur sufficiently, resulting in the formation of a large amount of cementite on the surface, and the desired properties of the cold-rolled steel sheet for enamel cannot be secured. If the dew point temperature is too high, the decarburization reaction causes grain growth, resulting in a deterioration of surface properties, and the formation of oxides may cause problems such as a decrease in workability due to material unevenness and a deterioration of fish-scale properties. More specifically, the heating step may have a dew point temperature of 13 to 20°C.

[0099] The cracking stage is a dew point temperature of 0 to 13°C. If the dew point temperature is too low during the cracking stage, local decarburization will not occur sufficiently, resulting in the formation of a large amount of cementite on the surface, and the desired properties of the cold-rolled steel sheet for enamel cannot be secured. If the dew point temperature is too high, the formation of oxides due to surface oxidation may cause problems such as deterioration of surface properties and deterioration of fish scale resistance. More specifically, the cracking stage may have a dew point temperature of 3 to 10°C.

[0100] The difference in the dew point temperature of the atmosphere during the heating and soaking stages can be 3 to 15°C. If the difference is too small or too large, decarburization can be insufficient or excessive, which can be problematic. The difference in dew point temperature of the atmosphere during the heating and soaking stages can be 5 to 10°C.

[0101] The soaking temperature may be between 700 and 850°C. If the soaking temperature is too low, the decarburization reaction rate is slow, and the low carbon solubility of ferrite may lead to the formation of a large amount of cementite on the surface, resulting in incomplete recrystallization and poor formability. If the soaking temperature is too high, shape defects due to sagging may occur during the annealing process. More specifically, the soaking temperature may be between 700 and 850°C.

[0102] The heating and soaking steps can be performed in an atmosphere containing 3 to 20% by volume of hydrogen and the remainder in a nitrogen atmosphere. If the hydrogen content is too low, the oxygen partial pressure at the surface can increase, leading to excessive decarburization and the formation of an excessively oxidizing atmosphere, which can be problematic. If the hydrogen content is too high, the hydrogen partial pressure at the surface can increase, making it difficult for the decarburization reaction to occur through oxygen. More specifically, the hydrogen content can be 5 to 15% by volume.

[0103] Annealing is preferably performed using a continuous annealing method, and the soaking time is preferably maintained to allow recrystallization to be completed, and can be performed in the range of 10 seconds to 30 minutes.

[0104]

[0105] After the annealing step, a step of temper rolling the steel sheet at a reduction ratio of 0.5 to 3% may be further included.

[0106] Temper rolling can control the shape of a material. If the reduction ratio is too low, shape correction may not be sufficient. If the reduction ratio is too high, excessive deformation may occur, reducing ductility. More specifically, the reduction ratio for temper rolling can be between 1.0 and 2.5%.

[0107] Furthermore, after the step of annealing the cold-rolled steel sheet, a step of enamel firing may be further included to dry the enameled glaze. Through the enamel firing process, by heating to a high temperature and cooling to room temperature, an enamel layer is applied to the surface of the steel sheet, thereby obtaining various properties suitable for the purpose of the enamel product, such as chemical resistance and heat resistance. However, if the firing temperature is too low, there is a problem that the adhesion properties of the enamel layer cannot be secured, and on the other hand, if the firing temperature is too high, it acts as a factor in increasing the cost due to an increase in the energy source used. Therefore, the firing temperature can be applied at 780 to 850°C. More specifically, the firing temperature may be 790 to 840°C.

[0108]

[0109] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0110]

[0111] Example

[0112] Slabs were manufactured with the composition shown in Table 1 below and the alloy components including the remainder iron (Fe) and unavoidable impurities in wt%. The slabs were maintained at 1200°C for 2 hours and then hot rolled. At this time, the final thickness of the hot-rolled steel sheet was 4.0 mm. The hot-rolled specimens were pickled to remove the oxide film on the surface and then cold rolled at each reduction ratio. Thereafter, they were annealed for 3 minutes under the conditions summarized in Table 2 below. The atmosphere contained 10% by volume of hydrogen and the remainder nitrogen.

[0113] The cementite fraction was obtained by obtaining images using an optical microscope at a depth of 50 ㎛ from the surface of the steel plate and at the center, and then using an image analyzer to obtain the cementite fraction for the entire field of view.

[0114] Hardness was measured using the Vickers hardness tester at a depth of 50 μm from the surface of the steel plate and at the center. A load of 100 g was applied, and five points were measured at 1 mm intervals to obtain the average value.

[0115] Hydrogen permeability is one of the indices that evaluates the resistance to fish scale, a fatal defect of enamel. It is a value expressed by dividing the time (ts, unit second) for hydrogen to be generated from one side of the steel plate and for hydrogen to permeate out from the opposite side by the experimental method specified in the European standard (EN10209-2013) by the square of the material thickness (d, unit mm), and expressed as ts / d.2 (Unit: sec / mm 2 ) is expressed as .

[0116] The tissue characteristics of the material obtained through the above process are shown in Table 3 below.

[0117] Steel type CMnSiAlPSNOInventive steel 10.0270.5650.0180.0250.0180.0080.00190.0016Inventive steel 20.0180.7510.0230.0540.0120.0060.00380.0012Inventive steel 30.0440.2470.0020.0110.0100.0050.00110.0005Inventive steel 40.0130.7660.0230.0370.0130.0080.002 70.0005 Invention steel 50.03 90.18 20.00 90.02 40.00 60.00 60.00 250.00 20 Comparison steel 10.00 20.19 20.00 30.01 60.01 00.00 70.00 190.00 20 Comparison steel 20.00 50.85 30.10 20.04 30.04 10.00 70.00 250.00 13 Comparison steel 30.07 70.89 90.25 10.02 80.01 80.00 30.00 350.00 15

[0118] ClassificationSteel gradeHot rolledCold rolledCold rolled sheetAnnealing Finishing temperature(℃)Coiling temperature(℃)Cold rolling reductionCracking temperature(℃)Heating dew point temperature(℃)Cracking dew point temperature(℃)Invention example 1Invention steel 189065083750155Invention example 2Invention steel 189065083800155Invention example 3Invention steel 1890650838002010Invention example 4Invention steel 288066080810155Invention example 5Invention steel 288066080830155Invention example 6Invention steel 384 0600808002010Invention Example 7Invention Class 3840600808002015Invention Example 8Invention Class 4890680807502010Invention Example 9Invention Class 4890680807802010Invention Example 10Invention Class 4890680808002010Invention Example 11Invention Class 4890680808202 010 Invention Example 12 Invention Steel 589066083800155 Invention Example 13 Invention Steel 589066083830155 Comparative Example 1 Invention Steel 1890650838802010 Comparative Example 2 Invention Steel 2880660807605-5 Comparative Example 3 Invention Steel 2880660807602510 Comparative Example 4 Invention Steel 384 0600808002015Comparative Example 5Inventive Steel 38406008080010-5Comparative Example 8Inventive Steel 3840600806502010Comparative Example 9Comparative Steel 189066083800155Comparative Example 10Comparative Steel 289066083800155Comparative Example 11Comparative Steel 386060083750155

[0119] Classification 1 Formula 2 Hydrogen permeability Invention example 10.34 0.73693 Invention example 20.27 0.67778 Invention example 30.16 0.59864 Invention example 40.42 0.62797 Invention example 50.39 0.63726 Invention example 60.21 0.49 1021 Invention example 70.14 0.43 1077 Invention example 80.43 0.69708 Invention example 90.410.66695 Invention example 100.310.51726 Invention example 110.330 .53738Invention Example 120.350.55990Invention Example 130.280.521056Comparative Example 10.870.85657Comparative Example 20.630.73678Comparative Example 30.800.91585Comparative Example 40.760.80541Comparative Example 50.750.79504Comparative Example 80.730.84607Comparative Example 900.95324Comparative Example 1000.94305Comparative Example 110.430.69534

[0120] As shown in Tables 1 to 3, in the case of an invention example that appropriately satisfies the alloy composition and manufacturing conditions, it can be confirmed that the cementite ratio on the surface and center of the steel plate is appropriately formed, and an excellent hydrogen permeability ratio can be secured.

[0121] On the other hand, Comparative Examples 1 to 6, which had satisfactory alloy composition but did not satisfy the manufacturing conditions, showed poor hydrogen permeability because the cementite ratio on the surface and center of the steel plate was not formed appropriately.

[0122] Comparative Examples 7 to 9 did not satisfy the alloy composition. In this case, it can be confirmed that the hydrogen permeability ratio is poor because the cementite ratio at the surface and center is not formed appropriately.

[0123]

[0124] The present invention is not limited to the above-described embodiments, but can be manufactured in a variety of different forms. Those skilled in the art will appreciate that the present invention can be implemented in other specific forms without altering the technical spirit or essential characteristics of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

Containing, in wt%, C: 0.01 to 0.05%, Mn: 0.10 to 0.80%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, P: 0.001 to 0.02%, S: 0.001 to 0.02%, N: 0.005% or less (excluding 0%) and O: 0.003% or less (excluding 0%), the remainder including Fe and unavoidable impurities, An enamel steel plate satisfying the following equation 1. [Formula 1] CVF[50㎛] / CVF[center]≤0.5 (In Equation 1, CVF[≤50㎛] means the area fraction (%) of cementite at a location 50㎛ inside the steel plate in the thickness direction from the steel plate surface, and CVF[center] means the area fraction (%) of cementite at a location 1 / 2 of the steel plate thickness.) In the first paragraph, An enamel steel plate satisfying the following equation 2. [Formula 2] Hv[50㎛] / Hv[center]≤0.75 (In Equation 2, Hv[50㎛] represents the hardness at a location 50㎛ inside the steel plate in the thickness direction from the steel plate surface, and Hv[center] represents the hardness at a location half the thickness of the steel plate.) In the first paragraph, An enamel steel sheet further comprising at least one of Cu: 0.1 wt% or less, 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. In the first paragraph, Hydrogen permeability is 650 sec / mm 2 Ideal enamel steel plate. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, in wt%, C: 0.01 to 0.05%, Mn: 0.10 to 0.80%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, P: 0.001 to 0.02%, S: 0.001 to 0.02%, N: 0.005% or less (excluding 0%) and O: 0.003% or less (excluding 0%), with the remainder being Fe and unavoidable impurities; A step of manufacturing a cold rolled steel sheet by cold rolling the hot rolled steel sheet; and A step of annealing the cold rolled steel sheet is included; The above annealing step includes a heating step of heating the steel plate to a cracking temperature of 100°C and a cracking step, The above heating step is performed at a dew point temperature of 10 to 23°C, A method for manufacturing an enamel steel plate, wherein the above cracking step is performed at 0 to 13°C. In paragraph 5, In the step of manufacturing the above hot rolled steel plate, A method for manufacturing an enamel steel sheet having a finishing hot rolling temperature of 800 to 1000°C. In paragraph 5, In the step of manufacturing the above hot rolled steel plate, A method for manufacturing an enamel steel sheet having a coiling temperature of 450 to 700°C. In paragraph 5, A method for manufacturing an enamel steel sheet having a reduction ratio of 60 to 90% in the step of manufacturing the above cold rolled steel sheet. In paragraph 5, A method for manufacturing an enamel steel plate having the above cracking temperature of 700 to 850°C. In paragraph 5, A method for manufacturing an enamel steel sheet, wherein the heating step and the soaking step are performed in an atmosphere of 3 to 20 volume% of hydrogen and the remainder of nitrogen. In paragraph 5, A method for manufacturing an enamel steel sheet, wherein the difference in dew point temperature of the atmosphere in the heating step and the soaking step is 3 to 15°C. In paragraph 5, A method for manufacturing an enamel steel sheet, further comprising, after the annealing step, a step of temper rolling the steel sheet at a reduction ratio of 0.5 to 3%.