Steel sheet for enameling, and manufacturing method therefor

Abstract

A steel sheet for enameling, according to an embodiment of the present invention, comprises, in weight%: 0.020 to 0.150% of Cu; 0.020 to 0.150% of Cr; 0.015 to 0.100% of Ni; 0.015 to 0.100% of Mo; 0.050 to 0.120% of Ti; and the balance of Fe and unavoidable impurities.

Description

Steel plate for enamel and method of manufacturing the same

[0001] One embodiment of the present invention relates to a steel plate for enamel and a method for manufacturing the same. More specifically, one embodiment of the present invention relates to a steel plate for enamel with improved enamel adhesion by adding Cu, Cr, Ni, Mo, and Ti to the steel components, and a method for manufacturing the same.

[0002] Enameled steel is a type of surface-treated product in which a vitreous glaze is applied to a base steel sheet, such as hot-rolled or cold-rolled steel, and then fired at high temperatures to enhance corrosion resistance, weather resistance, and heat resistance. Such enamel steel is used for architectural exteriors, home appliances, tableware, and as a material for various industries.

[0003] Initially, rimmed steel containing a relatively large amount of internal impurities was used for enamel steel sheets; however, with the recent active use of continuous casting methods to improve productivity, most materials are now being produced via continuous casting. Furthermore, regarding steel manufacturing, fishscale defects—one of the most critical flaws in enamel steel sheets—are a representative enamel defect that occurs when hydrogen dissolved in the steel during the manufacturing process becomes supersaturated within the steel during cooling after firing, and is subsequently released to the surface, causing the enamel layer to slough off in the shape of fish scales. Since the occurrence of such fishscale defects significantly reduces the value of enamel products, such as by causing concentrated rust formation in the affected areas, it is necessary to suppress their occurrence.

[0004] Furthermore, as enamel steel is primarily used as a material for structural components, there is a pursuit to enhance competitiveness by increasing the material's strength to achieve lightweight components. To this end, it is necessary to secure the yield strength following the firing heat treatment required for drying the glaze during the enamel process.

[0005] In one embodiment of the present invention, a steel plate for enamel and a method for manufacturing the same are provided. More specifically, in one embodiment of the present invention, by adding Cu, Cr, Ni, Mo, and Ti to the steel components, a steel plate for enamel with improved enamel adhesion and a method for manufacturing the same are provided.

[0006] A steel sheet for enamel according to one embodiment of the present invention comprises, in weight%, Cu: 0.020 to 0.150%, Cr: 0.020 to 0.150%, Ni: 0.015 to 0.100%, Mo: 0.015 to 0.100%, Ti: 0.050 to 0.120%, and the remainder being Fe and unavoidable impurities.

[0007] A steel plate for enamel according to one embodiment of the present invention may further include one or more of C: 0.0005 to 0.0100 wt%, Mn: 0.01 to 0.5 wt%, Al: 0.005 to 0.050 wt%, and S: 0.04 to 0.06 wt%.

[0008] A steel plate for enamel according to one embodiment of the present invention may contain Cu: 0.24 to 0.55 wt%, Cr: 0.020 to 0.200 wt%, Ni: 0.13 to 0.33 wt%, and Mo: 0.020 to 0.160 wt% in a concentration layer up to 50 μm from the surface of the steel plate.

[0009] The enrichment layer can satisfy Equation 1 below.

[0010] [Equation 1]

[0011] 1.30 ≤ 2× [Cu] + 2× [Ni] + 1.5 × [Mo] + 1.2× [Cr] + 0.3 ≤ 1.90

[0012] (In the above Equation 1, [Cu], [Ni], [Mo], and [Cr] represent the content (weight%) of Cu, Ni, Mo, and Cr in the enrichment layer, respectively.)

[0013] A steel plate for enamel according to one embodiment of the present invention may have a yield strength of 190 MPa or less after enamel sintering heat treatment.

[0014] A steel plate for enamel according to one embodiment of the present invention may have an enamel adhesion of 90% or more.

[0015] In one embodiment of the present invention, a steel plate for enamel may not have fish scale on the steel surface after enamel treatment.

[0016] A method for manufacturing a steel sheet for enamel according to one embodiment of the present invention comprises the steps of: hot rolling a slab containing, in weight percent, Cu: 0.020 to 0.150%, Cr: 0.020 to 0.150%, Ni: 0.015 to 0.100%, Mo: 0.015 to 0.100%, Ti: 0.050 to 0.120%, and the remainder being Fe and unavoidable impurities to produce a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

[0017] In the step of manufacturing hot-rolled steel sheets, the finishing hot rolling temperature may be 880 to 930℃.

[0018] In the step of manufacturing hot-rolled steel sheets, the coiling temperature may be 500 to 640℃.

[0019] In the step of manufacturing cold-rolled steel sheets, the reduction rate may be 75 to 90%.

[0020] The annealing step can be performed at a temperature of 770 to 820°C.

[0021] After the annealing step, the process may further include a step of enamel firing at a temperature of 750 to 900°C.

[0022] A steel plate for enamel according to one embodiment of the present invention has excellent fish scale resistance and enamel adhesion.

[0023] In addition, the steel plate for enamel according to one embodiment of the present invention can maintain high enamel adhesion by optimizing the surface characteristics of the steel plate.

[0024] 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 used solely to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section without departing from the scope of the invention.

[0025]

[0026] In this specification, when a part is described as "comprising" a certain component, it means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0027] In this specification, technical terms used are intended merely to refer to specific embodiments and are not intended to limit the invention. Singular forms used herein include plural forms unless phrases clearly indicate otherwise. The meaning of "comprising" as used in this specification specifies a particular characteristic, area, integer, step, action, element, and / or component, and does not exclude the presence or addition of other characteristics, areas, integers, steps, actions, elements, and / or components.

[0028] In this specification, the term “combination thereof” included in a Markush-type expression means one or more mixtures or combinations selected from a group consisting of components described in the Markush-type expression, and means including one or more selected from the group consisting of said components.

[0029] In this specification, when a part is referred to as being "on" or "on" another part, it may be immediately on or on the other part, or other parts may be involved between them. In contrast, when a part is referred to as being "immediately on" another part, no other parts are interposed between them.

[0030] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or highly formal sense unless otherwise defined.

[0031] Also, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

[0032] In one embodiment of the present invention, the meaning of including additional elements is that the remainder of iron (Fe) is replaced by an amount of the additional element.

[0033] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0034]

[0035] A steel sheet for enamel according to one embodiment of the present invention comprises, in weight%, Cu: 0.020 to 0.150%, Cr: 0.020 to 0.150%, Ni: 0.015 to 0.100%, Mo: 0.015 to 0.100%, Ti: 0.050 to 0.100%, and the remainder being Fe and unavoidable impurities.

[0036] First, I will explain the reason for limiting the composition of the steel plate.

[0037]

[0038] Cu: 0.020 to 0.150 wt%

[0039] Copper (Cu) is an element added to improve solid solution strengthening and enamel adhesion, and it has the effect of changing the surface roughness of the steel sheet, particularly during the pretreatment for pickling. If too little Cu is added, Fe dissolves too easily during the pickling process, resulting in a smooth surface after pickling and failing to form a sufficient wedge effect. On the other hand, if too much Cu is added, Cu itself strengthens acid corrosion resistance, which lowers the pickling speed during the acid treatment stage, making it difficult to obtain appropriate surface roughness characteristics of the steel sheet and potentially reducing enamel adhesion. Therefore, it contains 0.020 to 0.150 weight% of Cu. More specifically, it may contain 0.030 to 0.100 weight%.

[0040]

[0041] Cr: 0.020 to 0.150 wt%

[0042] Chromium (Cr) is an element that exists in the form of surface oxides or carbides within the steel. In this case, Cr is an element that is present in greater abundance within the steel sheet than within the glaze, and it plays a role in enhancing adhesion between the steel sheet and the enamel layer as it oxidizes at the interface between the glaze and the steel sheet. If Cr is added in small amounts, the amount of oxide is small, so it may not sufficiently perform the role of improving adhesion. If Cr is included in excessive amounts, a large amount of Cr-based oxides formed at the interface may hinder the formation of oxides of Ni, Cu, Mo, etc., which can perform a similar role; therefore, the upper limit is restricted to 0.150 weight%. More specifically, it may be included in an amount of 0.030 to 0.100 weight%.

[0043]

[0044] Ni: 0.015 to 0.100 wt%

[0045] Nickel (Ni) is an element traditionally included in glazes to enhance the adhesion between the steel plate and the enamel layer. However, due to recent concerns regarding its harmfulness to the human body, regulations have been established on the Ni content in the glaze layer. Consequently, Ni-Flash treatment, which creates a thin Ni layer on the surface of the steel, is performed as a representative pretreatment. In one embodiment of the present invention, the aim is to improve enamel adhesion by increasing the Ni content within the steel plate without artificially performing Ni-Flashing treatment. If the Ni content is lower than the lower limit, Ni-based oxides are not sufficiently present on the surface of the steel to perform the function of improving adhesion; conversely, if too much nickel is included, line defects caused by oxides may occur frequently during the production process. Therefore, Ni is included in an amount of 0.015 to 0.100 weight%. More specifically, it may be included in an amount of 0.020 to 0.080 weight%.

[0046]

[0047] Mo: 0.015 to 0.100 wt%

[0048] Molybdenum (Mo), like Cu, Cr, and Ni, is an element that forms surface oxides and plays a role in enhancing the adhesion between the steel sheet and the glassy glaze through covalent bonding. If too much Mo is included, the formation of unnecessary carbides and the refinement of grain size due to an increase in the recrystallization region can increase strength and degrade formability. Therefore, Mo is included in an amount of 0.015 to 0.100 weight%. More specifically, it may be included in an amount of 0.020 to 0.080 weight%.

[0049]

[0050] Ti: 0.050 to 0.120 wt%

[0051] Titanium (Ti) is a key element that improves hydrogen absorption and fish scale resistance in enamel steel, which primarily utilizes TiS-based precipitates. During the hot rolling process, Ti forms coarse TiS precipitates together with the large amount of sulfur (S) contained in the steel. During cold rolling, these precipitates interact with the matrix to form fine micro-voids between the Fe matrix and the TiS precipitates. These micro-voids absorb hydrogen generated by the dissociation of water vapor during the enamel firing process and trapped within the steel. Even when cooled to room temperature, hydrogen remains trapped on the surface to form hydrogen gas, which increases pressure and suppresses fish scale defects—a representative defect in enamel where the enamel layer is flaked off. In this case, if the Ti content is too low, sufficient precipitates are not formed during hot rolling, resulting in inferior F / S resistance. Conversely, if the Ti content is too high, complex oxides of Al and Ti can clog the continuous casting nozzle during the continuous casting process, leading to poor continuous casting performance and defects such as black lines. Accordingly, it contains 0.050 to 0.120 weight% of Ti. More specifically, it may contain 0.060 to 0.100 weight%.

[0052]

[0053] A steel plate for enamel according to one embodiment of the present invention may further include one or more of C: 0.0005 to 0.0100 wt%, Mn: 0.01 to 0.50 wt%, Al: 0.005 to 0.050 wt%, and S: 0.04 to 0.06 wt%.

[0054]

[0055] C: 0.0005 to 0.0100 wt%

[0056] Carbon (C) is a representative solid solution strengthening and carbide-forming element, but if added in excessive amounts, the amount of solid solution carbon in the steel increases, which hinders the development of texture after cold rolling and annealing, resulting in poor workability. Additionally, it can cause bubble defects due to bubbling in the enamel layer after enamel treatment. However, since C itself is an element that is difficult to remove during steelmaking and helps form a texture favorable for some formability during the cold rolling and annealing process, the C content may be included in an amount of 0.0005 to 0.0100 weight%. More specifically, it may be included in an amount of 0.0010 to 0.0070 weight%. Even more specifically, it may be included in an amount of 0.0020 to 0.0050 weight%.

[0057]

[0058] Mn: 0.01 to 0.50 wt%

[0059] Manganese (Mn) is a representative solid solution strengthening element that precipitates sulfur dissolved in steel in the form of manganese sulfide (MnS), thereby preventing hot shortness and promoting the precipitation of carbides. If too little Mn is added, there is a risk of hot shortness because the remaining sulfur, which cannot be precipitated in the form of TiS, is not sufficiently precipitated. However, if the Mn content is too high, it causes solid solution strengthening, which increases yield strength and can be a cause of impeding formability. Accordingly, Mn is included in an amount of 0.01 to 0.50 weight%. More specifically, Mn may be included in an amount of 0.10 to 0.30 weight%.

[0060]

[0061] Al: 0.005 to 0.050 wt%

[0062] Aluminum (Al) is used as a powerful deoxidizer to remove oxygen from molten steel during the steelmaking stage and is an element that removes dissolved nitrogen in a fixed form. It is also an element that is contained in large quantities in glazes. Although Al forms oxides on the surface of steel sheets to improve some enamel adhesion, if formed in large quantities on the surface, it actually weakens adhesion. Therefore, Al may be contained in an amount of 0.005 to 0.050 weight%. More specifically, Al may be contained in an amount of 0.0010 to 0.040 weight%.

[0063]

[0064] S: 0.04 to 0.06 wt%

[0065] Sulfur (S) is an element that combines with manganese to cause red-hot brittleness. However, in the present invention, S is an element that is essential to be added because it improves hydrogen absorption capacity by combining with Ti to form TiS precipitates. At this time, addition is necessary to form micro-voids to have hydrogen absorption capacity, but if too much is added, TiS is formed and the remaining S may combine with Mn to cause red-hot brittleness. Therefore, 0.04 to 0.06 weight% of S may be additionally included. More specifically, 0.042 to 0.047 weight% may be additionally included.

[0066] In addition to the aforementioned components, the present invention comprises Fe and unavoidable impurities. The addition of effective components other than those mentioned above is not excluded. If additional components are included, they may be added to replace the remainder of Fe.

[0067]

[0068] A steel plate for enamel according to one embodiment of the present invention may contain Cu: 0.24 to 0.55 wt%, Cr: 0.020 to 0.200 wt%, Ni: 0.13 to 0.33 wt%, and Mo: 0.020 to 0.160 wt% in a concentration layer up to 50 μm from the surface of the steel plate.

[0069] In one embodiment of the present invention, enamel adhesion can be improved by enriching Cu, Cr, Ni, and Mo in the enrichment layer. If Cu, Cr, Ni, and Mo are not sufficiently enriched, it is difficult to sufficiently improve enamel adhesion. If Cu, Cr, Ni, and Mo are enriched too much, problems may arise when Cu, Cr, Ni, and Mo are added in excess in the aforementioned steel grades. More specifically, the steel plate for enamel may contain Cu: 0.24 to 0.50 wt%, Cr: 0.024 to 0.180 wt%, Ni: 0.14 to 0.30 wt%, and Mo: 0.025 to 0.150 wt% in the enrichment layer up to 50 μm from the surface of the steel plate.

[0070]

[0071] The elemental content within the enrichment layer is measured using GDS (Glow Discharge Spectrometry). GDS is a device that measures elemental concentration in the depth direction from the surface of the steel plate, allowing for the determination of elemental distribution in the depth direction. GDS was utilized because it can perform analysis up to a depth of approximately 100 µm over a wide area and simultaneously measure most elements used in steel. In this invention, to analyze the adhesion between the steel plate and the enamel glaze, elemental measurements were performed in the depth direction after partially polishing the enamel layer on the steel plate; the measurements continued until the steel plate layer was exposed. The steel plate used for this analysis was an enamel-treated steel plate. Initially, information regarding the glaze layer, which is mainly composed of oxides, is obtained, but the trend changes to a transition zone where Fe increases, and then to a steel plate section composed mainly of Fe. The transition zone can be defined as a diffusion layer where elements within the plate and elements of the glaze layer mix; in particular, it can be said that the elements mainly detected at the interface between the steel plate and the glaze influence the adhesion.

[0072] Except for the aforementioned elements, they may be included in the same manner as the steel composition. Furthermore, the aforementioned steel composition refers to the average content relative to the total thickness of the steel sheet excluding the enrichment layer.

[0073] The enrichment layer can satisfy Equation 1 below.

[0074] [Equation 1]

[0075] 1.30 ≤ 2× [Cu] + 2× [Ni] + 1.5 × [Mo] + 1.2× [Cr] + 0.3 ≤ 1.90

[0076] (In the above Equation 1, [Cu], [Ni], [Mo], and [Cr] represent the content (weight%) of Cu, Ni, Mo, and Cr in the enrichment layer, respectively.)

[0077] Equation 1 represents the sum of elements that can improve enamel adhesion at the steel plate interface. These elements mainly exist in the form of covalent bonds between the steel plate and the enamel layer to enhance adhesion; however, if the value of Equation 1 is small, the enrichment of elements such as Cu is not sufficiently achieved, making it difficult to sufficiently improve enamel adhesion. If the value of Equation 1 is too large, oxides may be selectively formed only on the surface of the steel plate, which may actually cause a problem of reduced adhesion. More specifically, the value of Equation 1 may be between 1.35 and 1.70.

[0078] As described above, the cold-rolled steel sheet for enamel according to the present invention can have excellent formability even after enamel plastic treatment and excellent enamel characteristics. Specifically, the steel sheet for enamel according to one embodiment of the present invention may have a yield strength of 190 MPa or less after enamel plastic heat treatment. Regarding the yield strength after plastic heat treatment of the enamel steel sheet, when the material is utilized after enamel treatment, some bending occurs due to the high treatment temperature, and the material is straightened with a constant force to correct this. However, if the yield strength is too high, a phenomenon occurs where deformation suddenly acts in the opposite direction during straightening, making straightening difficult. Therefore, it is advantageous to have a low yield strength in terms of straightening after plastic deformation.

[0079] Since the yield strength measured by the tensile test method is a characteristic that can vary somewhat depending on the test conditions, in one embodiment of the present invention, the yield strength can be measured by applying a crosshead speed of 10 mm per minute, which represents the tensile speed per unit time.

[0080] The yield strength after enamel firing heat treatment obtained through this may be 190 MPa or less, and more specifically, may be 160 to 190 MPa.

[0081] A steel plate for enamel according to one embodiment of the present invention may have an enamel adhesion of 90% or more. By satisfying these physical properties, it can be applied as an enamel material even when using relatively inexpensive glazes. If the enamel adhesion is too low, the glaze layer may detach during distribution or handling after enamel treatment, reducing the marketability of the material as an enamel product. Consequently, enamel manufacturers may apply expensive glazes containing large amounts of components such as NiO to ensure stability, which can act as a factor in increasing costs. Therefore, it is ultimately necessary to ensure enamel adhesion even when using low-cost glazes. Furthermore, since a low enamel adhesion increases the rate of fish scale formation caused by hydrogen in the steel, it is desirable to ensure the highest possible enamel adhesion. More specifically, the enamel adhesion may be 90 to 97%. Enamel adhesion refers to a numerical value that quantifies the degree of detachment of the enamel glaze layer by applying a certain load to the enamel layer with a steel ball and evaluating the degree of electrical conductivity in that area, as defined in the American Society for Testing Materials standard ASTM C313-78.

[0082] A steel plate for enamel according to one embodiment of the present invention may not produce fish scale on the steel surface after enamel treatment. Fish scale is determined by visually measuring the surface condition after undergoing an enamel F / S accelerated test, in which enamel glaze is applied to the surface of the steel plate, heat-treated at 780 to 850°C for 10 to 15 minutes to bond the steel plate and the enamel layer, and then the material is placed in a 200°C chamber for 24 hours to create an environment where hydrogen can sufficiently escape. At this time, if no fish scale is present on the front or back surface of the test specimen, the material is considered to be resistant to fish scale.

[0083]

[0084] A method for manufacturing a steel sheet for enamel according to one embodiment of the present invention comprises the steps of: hot rolling a slab containing, in weight percent, Cu: 0.020 to 0.150%, Cr: 0.020 to 0.150%, Ni: 0.015 to 0.100%, Mo: 0.015 to 0.100%, Ti: 0.050 to 0.120%, and the remainder being Fe and unavoidable impurities to produce a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and annealing the cold-rolled steel sheet.

[0085] First, a slab satisfying the aforementioned composition is prepared. Molten steel with a composition adjusted to the aforementioned composition during the steelmaking stage can be manufactured into a slab through continuous casting. The alloy composition of the slab is substantially the same as the aforementioned enamel steel sheet. Since the alloy composition has been described above, a redundant explanation is omitted.

[0086] Before hot rolling the slab, the manufactured slab can be heated. By heating, the subsequent hot rolling process can be performed smoothly, and the slab can be homogenized. More specifically, heating may mean reheating.

[0087] At this time, the slab heating temperature may be 1150 to 1280°C. If the slab heating temperature is too low, the rolling load in the subsequent hot rolling process may increase rapidly, which can worsen workability. On the other hand, if the slab heating temperature is too high, not only will energy costs increase, but the amount of surface scale may increase, leading to material loss. More specifically, it may be 1180 to 1260°C.

[0088] After that, the heated slab is hot-rolled to produce a hot-rolled steel sheet.

[0089] At this time, the finishing hot rolling temperature may be 880 to 930°C. If the finishing hot rolling temperature is too low, the mixing of grains proceeds rapidly as rolling is finished in a low-temperature region, which may lead to a decrease in rollability and workability. On the other hand, if the finishing hot rolling temperature is too high, the peelability of surface scale decreases, and as uniform hot rolling is not achieved across the thickness, a decrease in impact toughness due to grain growth may occur. More specifically, the finishing hot rolling temperature may be 890 to 920°C. More specifically, the finishing hot rolling temperature may be 900 to 915°C.

[0090] Afterward, the hot-rolled steel sheet manufactured after hot rolling undergoes a coiling process. More specifically, it may be a hot-rolled coiling process.

[0091] At this time, the coiling temperature may be 500 to 640°C. The hot-rolled steel sheet may be cooled on a run-out table (ROT) before coiling. If the hot-rolled coiling temperature is too low, temperature non-uniformity in the width direction occurs during the cooling and holding process, which leads to variations in the formation of low-temperature precipitates, causing material variation and adversely affecting enamel properties. On the other hand, if the coiling temperature is too high, corrosion resistance decreases as carbide aggregation progresses, and problems may arise where cold-rollability is reduced due to the promotion of P intergranular segregation, as well as poor workability caused by microstructure coarsening in the final product. More specifically, the coiling temperature may be 510 to 630°C. More specifically, the coiling temperature may be 560 to 620°C.

[0092] The coiled hot-rolled steel sheet may additionally include a step of pickling the steel sheet before cold rolling.

[0093] Afterwards, the coiled hot-rolled steel sheet is manufactured into a cold-rolled steel sheet through cold rolling.

[0094] At this time, the cold reduction ratio can be 75 to 90%. If the cold reduction ratio is too low, the driving force for recrystallization in the subsequent heat treatment process is not secured, resulting in localized unrecrystallized grains remaining. Although strength increases, there is a problem in that workability is significantly reduced. Furthermore, as the ability to crush carbides formed during the hot rolling stage decreases, the number of sites capable of absorbing hydrogen is reduced, making it difficult to secure fish scale resistance. Moreover, considering the thickness of the final product, the thickness of the hot-rolled plate must be reduced, which can worsen rolling workability. On the other hand, if the cold reduction ratio is too high, the material hardens, worsening workability, and the load on the rolling mill increases, which can negatively affect operability. More specifically, the cold reduction ratio can be 77 to 88%. Even more specifically, the cold reduction ratio can be 80 to 85%.

[0095] After that, the cold-rolled steel sheet can be annealed to produce an annealed steel sheet.

[0096] In one embodiment of the present invention, the annealing temperature may be cracked at a temperature of 770 to 820°C. The cracking treatment may be within a range of 20 to 50 seconds depending on the production speed of the steel sheet. At this time, if the cracking temperature is too low, the deformation formed by cold rolling may not be sufficiently removed, and workability may be significantly reduced. On the other hand, if the cracking treatment temperature is too high, phenomena such as sticking of the steel sheets or severe deformation of the center of the steel sheet (heat buckle) may occur. More specifically, the cracking temperature may be 780 to 810°C. Meanwhile, if the cracking time is too short, workability may deteriorate. Conversely, if the cracking time is too long, defects such as orange peel caused by grain growth may occur. More specifically, the cracking time may be 25 to 40 seconds.

[0097] After the cold-rolled sheet annealing step, the process may further include a step of enamel firing the steel sheet at a temperature of 750 to 900°C. Through the enamel firing process, a hard enamel layer is applied to the surface of the steel sheet via a high-temperature heating and cooling step to room temperature, thereby obtaining various properties suitable for the intended use of the enamel product, such as chemical resistance and heat resistance. However, if the firing temperature is too low, there is a problem in that the adhesion properties of the enamel layer cannot be secured. On the other hand, if the firing temperature is too high, it may act as a factor in increasing costs due to the increased energy consumption and may also cause various defects, such as the formation of bubbles during enamel firing. Therefore, the enamel firing temperature may be applied at 780 to 890°C.

[0098]

[0099] The present invention will be explained in more detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and explain the invention in more detail, and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0100]

[0101] Examples

[0102] A slab was manufactured through a converter, secondary refining, and continuous casting process using an alloy composition containing the composition shown in Table 1 below in weight percent, with the remainder being iron (Fe) and unavoidable impurities. This slab was held in a furnace maintained at 1230°C for 3 hours and then subjected to hot rolling. At this time, the finishing hot rolling temperature was controlled to 910°C and the coiling temperature to 600°C, and the final thickness was 4.0 mm. The hot-rolled specimens were subjected to pickling to remove the oxide film from the surface, followed by cold rolling with a reduction rate of 82.5%. The specimens that had completed cold rolling were processed into enamel-treated specimens to investigate enamel properties and specimens to measure mechanical properties, and then subjected to continuous annealing heat treatment. The continuous annealing heat treatment was performed by taking 88 seconds to an annealing temperature of 810°C, holding at 810°C for 28 seconds, cooling to 650°C for 28 seconds, cooling to 450°C for 13 seconds, and then performing overaging heat treatment at 400°C for 180 seconds. Afterward, annealing was performed by cooling to room temperature for approximately 36 seconds.

[0103] The elemental content of the enrichment layer from the surface of the annealed steel sheet to 50 μm was analyzed by GDS, and the results are shown in Table 2 below.

[0104] The presence or absence of fish scale, enamel adhesion, and yield strength of the material obtained through the above process are shown in Table 2 below.

[0105]

[0106] The yield strength (MPa) was obtained by heat-treating the cold-rolled steel sheet for 15 minutes in a kiln maintained at a temperature of 820°C to simulate the firing process for drying the enamel glaze, and then preparing a tensile test specimen and conducting a tensile test at a crosshead speed of 10 mm / min.

[0107] The enamel-treated specimens were cut into appropriate sizes for each application to meet the test objectives. After the heat treatment was completed, the enamel-treated specimens were completely degreased, and a standard glaze (Check frit), which is relatively susceptible to fish scale defects, was applied and maintained at 300°C for 10 minutes to remove moisture. After drying, the specimens were fired at 800°C for 15 minutes to highlight the differences in enamel properties, such as enamel adhesion, and then cooled to room temperature. During this process, the atmosphere conditions of the firing furnace were set to a dew point of 20°C, a relatively harsh condition where fish scale defects are likely to occur.

[0108] The specimens after enamel treatment were subjected to an accelerated fish scale test by maintaining them in an oven at 200°C for 24 hours. After the accelerated fish scale treatment, the presence or absence of fish scale defects was visually observed, and "X" was marked if no fish scale defects occurred, and "O" was marked if they occurred.

[0109] Enamel adhesion, which evaluates the adhesion between the steel plate and the glaze, was expressed as an index of the degree of detachment of the enamel glaze layer by applying a certain load to the enamel layer with a steel ball and evaluating the degree of electrical conductivity in that area, as defined in the American Society for Testing Materials standard, ASTM C313-78. In this invention, the enamel adhesion evaluation result was set with a goal of securing adhesion of 95% or more in terms of ensuring application stability in relatively low-cost glazes.

[0110] Classification CmnS AlCuCrNiMoTi Remarks 10.00 20.20.0 450.0 30.0 500.0 500.0 500.0 500.0 90 Invention Example 20.00 50.20.0 450.0 30.0 500.0 500.0 500.0 500.0 90 Invention Example 30.00 20.30.0 450.0 30.0 500.0 500.0 500.0 500.0 90 Invention Example 40.00 20.20.0 400.0 30.0 500.0 500.0 500.0 500.0 90 Invention Example 50.00 20.20.0 450.0 10.0 500.0 500.0 500.0 500.0 90 Honorary 60.0020.20.0450.050.0500.0500.0500.0500.090 Invention Example 70.0020.20.0450.030.0200.0500.0500.0500.090 Invention Example 80.0020.20.0450.030.1500.0500. 0500.0500.090Invention Example 90.0020.20.0450.030.0500.0200.0500.0500.090Invention Example 100.0020.20.0450.030.0500.1500.0500.0500.090Invention Example 110.0020.20.045 0.030.0500.0500.0200.0500.090Invention Example 120.0020.20.0450.030.0500.0500.1000.0500.090Invention Example 130.0020.20.0450.030.0500.0500.0500.0200.090Bal Honorary 140.0020.20.0450.030.0500.0500.0500.1000.090 Invention Example 150.0020.20.0450.030.0500.0500.0500.0500.060 Invention Example 160.0020.20.0450.030.0500.05 00.0500.0500.120 Invention Example 170.0020.20.0450.030.0150.0200.0200.0200.0200.090 Comparative Example 180.0020.20.0450.030.1800.0200.0200.0200.090 Comparative Example 190.0020.20 .0450.030.0500.0150.0200.0200.090Comparative Example 200.0020.20.0450.030.0500.1800.0200.0200.090Comparative Example 210.0020.20.0450.030.0500.0200.0100.0200.090Comparative Example 220.0020.20.0450.030.0500.0200.1200.0200.090Comparative Example 230.0020.20.0450.030.0500.0200.0200.0100.090Comparative Example 240.0020.20.0450.030.0500.0200.0200.1200.090Comparative Example 250.0020.20.0450.030.0500.0200.0200.0200.030Comparative Example 260.0020.20.0450.030.0500.0200.0200.0200.130Comparative Example.

[0111] Classification Content in Thickening Layer (Weight%) Enamel Properties and Material Remarks CuCrNiMo Type 1 Presence / Absence of Fish Scale Enamel Adhesion (%) YP (MPa)10.3020.0600.1990.0741.485X95165Invention Example 20.3030.0590.2020.0711.487X92169Invention Example 30.2920.0610.2040.0791.484X93168Invention Example 40.3050.0590.1980.0741.488X92165Invention Example 50.2960.0600.2010.0781.483X93163Invention Example 60.3050.0570.1950.0771.484X91166Invention Example 70.2400.0590.2020. 0731.364X90163 Invention Example 80.5030.0610.2000.0721.887X96168 Invention Example 90.2970.0240.2040.0731.440X92164 Invention Example 100.3050.1830.1940.0741.629X90168 Invention Example 110.3000.0590.1430.0731.366X91164 Invention Example 120.2890.0570.3160.0731.688X96165 Invention Example 130.3060.0590.1950.0301.418X92165 Invention Example 14 0.2960.0550.2030.1561.598X96188Invention Example 150.2990.0590.2030.0741.486X90165Invention Example 160.3030.0600.1990.0731.486X95168Invention Example 170.2330.0240.1370.0301.114X84163Comparative Example 180.5670.0240.1330.0301.774X92168Comparative Example 190.2980.0180.1420.0301.247X88164Comparative Example 200.3060.2080.1430. 0291.491X85175Comparative Example 210.3000.0240.1210.0291.214X86163Comparative Example 220.3000.0240.3360.0331.650X89166Comparative Example 230.3000.0230.1410.0151.232X86160Comparative Example 240.2990.0230.1380.1831.476X92205Comparative Example 250.3040.0230.1390.0291.257085163Comparative Example 260.2970.0230.1440.0291.253X88165Comparative Example

[0112] As can be seen in Tables 1 and 2 above, the invention example satisfying the steel composition can be confirmed to have a properly formed enriched layer, no fish scale, excellent adhesion, and excellent yield strength.

[0113] On the other hand, if the steel composition is not properly controlled, the enrichment layer is not properly formed, resulting in some fish scale formation, inferior adhesion, and inferior yield strength.

[0114]

[0115] The present invention is not limited to the above embodiments and can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without changing the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1. A steel sheet for enamel comprising, in weight%, Cu: 0.020 to 0.150%, Cr: 0.020 to 0.150%, Ni: 0.015 to 0.100%, Mo: 0.015 to 0.100%, Ti: 0.050 to 0.120% and the remainder being Fe and unavoidable impurities.

2. In Paragraph 1, A steel sheet for enamel further comprising one or more of C: 0.0005 to 0.0100 wt%, Mn: 0.01 to 0.5 wt%, Al: 0.005 to 0.050 wt%, and S: 0.04 to 0.06 wt%.

3. In Paragraph 1, A steel sheet for enamel comprising Cu: 0.24 to 0.55 wt%, Cr: 0.020 to 0.200 wt%, Ni: 0.13 to 0.33 wt%, and Mo: 0.020 to 0.160 wt% in a concentration layer up to 50 μm from the surface of the steel sheet.

4. In Paragraph 3, The above-mentioned enrichment layer is a steel plate for enamel satisfying Formula 1 below. [Equation 1] 1.30 ≤ 2× [Cu] + 2× [Ni] + 1.5 × [Mo] + 1.2× [Cr] + 0.3 ≤ 1.90 (In Formula 1 above, [Cu], [Ni], [Mo], and [Cr] represent the content (weight%) of Cu, Ni, Mo, and Cr in the steel grade.) 5. In Paragraph 1, Steel plate for enamel with a yield strength of 190 MPa or less after enamel plastic heat treatment.

6. In Paragraph 1, Steel plate for enamel with an enamel adhesion of 90% or more.

7. In Paragraph 1, Steel plate for enamel that does not produce fish scale on the steel surface after enamel treatment.

8. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab comprising, in weight%, Cu: 0.020 to 0.150%, Cr: 0.020 to 0.150%, Ni: 0.015 to 0.100%, Mo: 0.015 to 0.100%, Ti: 0.050 to 0.120%, and the remainder being Fe and unavoidable impurities; A step of manufacturing a cold-rolled steel sheet by cold-rolling the above hot-rolled steel sheet; and A method for manufacturing a steel sheet for enamel, comprising the step of annealing the above cold-rolled steel sheet.

9. In Paragraph 8, A method for manufacturing an enamel steel sheet, wherein in the step of manufacturing the hot-rolled steel sheet, the finishing hot rolling temperature is 880 to 930℃.

10. In Paragraph 8, A method for manufacturing an enamel steel sheet in which the coiling temperature in the step of manufacturing the hot-rolled steel sheet is 500 to 640℃.

11. In Paragraph 8, A method for manufacturing an enamel steel sheet in which the reduction rate in the step of manufacturing the above cold-rolled steel sheet is 75 to 90%.

12. In Paragraph 8, The above annealing step is a method for manufacturing steel sheets for enamel, which involves crack treatment at a temperature of 770 to 820°C.

13. In Paragraph 8, A method for manufacturing an enamel steel sheet, comprising, after the step of annealing the cold-rolled steel sheet, an additional step of enamel firing the steel sheet at a temperature of 750 to 900°C.