KR102179214B1 - Cold-rolled steel sheet for enamel and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet for enamel and a manufacturing method thereof.
The cold-rolled steel sheet for enamel according to an embodiment of the present invention is in wt%, C: 0.05 to 0.09%, Mn: 0.1 to 0.3%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, S: 0.001 to 0.02 %, Cu: 0.01 to 0.15%, N: 0.005% or less (excluding 0%), including the balance Fe and inevitable impurities, the microporous area fraction is 0.3 to 0.8%, expressed by the following [Relationship 1] The enamel defect relationship index (D) satisfies 0.45 to 4.50, the adhesion relationship index (A) represented by the following relationships 2 and 3 is 0.007 to 0.185, and the workability relationship index (F) satisfies 3500 to 7000.
[Relationship 1] D = ([Mn] x [Cu] / [S]) x (D _void / [C])
[Relationship 2] A = ([Mn] x [Cu] x D _cementite} ) / [Si]
[Relationship 3] F = ([Al]/[N]) x (D _cementite /[C])

Description

Cold-rolled steel sheet for enamel and its manufacturing method {COLD-ROLLED STEEL SHEET FOR ENAMEL AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a cold-rolled steel sheet for enamel and a manufacturing method thereof. More specifically, it relates to an enamel cold-rolled steel sheet and a manufacturing method that can prevent the occurrence of bubble defects after the enamel treatment, and has excellent enamel adhesion and fish scale resistance.

Enamel steel plate is a kind of surface treatment product that improves corrosion resistance, weather resistance and heat resistance by applying a glassy glaze on a steel plate such as a hot rolled steel plate or a cold rolled steel plate, and then firing at a high temperature. These enameled steel sheets are used for exterior construction, home appliances, tableware, and various industrial materials.

Conventional enamel steel plates are decarburized annealing or phase annealing methods to prevent fishscale defects that reduce enamel properties by dropping the enamel layer, which is known as the most fatal defect in enamel products, to reduce enamel properties or to improve formability. As the manufacturing process increases, there is a problem in that the manufacturing cost is increased and quality deviations are frequent. In order to overcome the problem of productivity inferiority and manufacturing cost increase due to such annealing for a long time, a continuous annealing process is used for the recently developed steel sheet for enamel. In this case, titanium (Ti) or boron (B) is mainly used as a hydrogen storage source. By adding these precipitates are utilized. However, even in this case, since a large amount of carbonitride-forming elements must be added, the incidence of surface defects is high, and the recrystallization temperature is increased to perform high-temperature heat treatment, which causes a decrease in productivity and an increase in cost.

In particular, in the case of a steel sheet for titanium (Ti)-based enamel, as a large amount of titanium is added to suppress the reaction of hydrogen that causes fish scale, titanium nitride (TiN) and inclusions occur in the continuous casting step of the steelmaking process. Nozzle clogging occurs frequently, which is a direct factor in workability and production load. In addition, when TiN mixed in the molten steel is present on the top of the steel plate, it not only causes blister defects, which are typical bubble defects, but also the added titanium is a factor that hinders the adhesion between the steel plate and the glaze layer.

On the other hand, even in the case of a steel sheet for high oxygen-based enamel that secures fish scale resistance by using inclusions such as oxides in the steel by increasing the dissolved oxygen content inside the steel sheet, the refractory melting loss is extremely high because of the high oxygen content. There is a problem in that the performance productivity in the steelmaking process is greatly reduced, as well as surface defects are frequent.

The present invention is to provide a cold-rolled steel sheet for enamel and a method of manufacturing the same. More specifically, it is intended to provide a cold-rolled steel sheet for enamel and a manufacturing method that can prevent the occurrence of bubble defects after the enamel treatment, and has excellent enamel adhesion and fish scale resistance.

The cold-rolled steel sheet for enamel according to an embodiment of the present invention is, by weight, C: 0.05 to 0.09%, Mn: 0.1 to 0.3%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, S: 0.001 to 0.02%, Cu: 0.01 to 0.15%, N: 0.005% or less (excluding 0%), the balance contains Fe and inevitable impurities, the fine pore area fraction is 0.3 to 0.8%, expressed by the following [Relationship 1] The enamel defect relationship index (D) satisfies 0.45 to 4.50.

[Relationship 1]

D = ([Mn] x [Cu] / [S]) x (D _void / [C])

In the above relational equation 1, [Mn], [Cu], [S] and [C] are values obtained by dividing the weight% of each element by the atomic weight of each element, and D _void is the area fraction of micropores in the cold rolled steel sheet (% ).

The steel sheet may further include one or more of P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%), and Ti: 0.001% or less (excluding 0%).

The pickling loss of the cold rolled steel sheet may be 10 to 40 gr/m ² .

The hydrogen permeation ratio of the cold-rolled steel sheet may be 950 sec/mm ^{2 or} more.

On the other hand, the manufacturing method of the cold-rolled steel sheet for enamel according to an embodiment of the present invention, by weight, C: 0.05 to 0.09%, Mn: 0.1 to 0.3%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08% , S: 0.001 to 0.02%, Cu: 0.01 to 0.15%, N: 0.005% or less (excluding 0%), preparing a slab containing the balance Fe and inevitable impurities; Heating the slab; Hot rolling the heated slab to prepare a hot-rolled steel sheet having a carbide volume fraction of 2.5 to 7.0%; Winding a hot-rolled steel sheet; Manufacturing a cold-rolled steel sheet by cold rolling the wound hot-rolled steel sheet at a reduction ratio of 60 to 90%; And annealing and heat treatment of the cold-rolled steel sheet, and an adhesion relationship index (A) represented by the following [Relational Formula 2] satisfies 0.007 to 0.185.

[Relationship 2]

A = ([Mn] x [Cu] x D _cementite} ) / [Si]

In the above relational equation 2, [Mn], [Cu] and [Si] are values obtained by dividing the weight% of each element by the atomic weight of each element, and D _cementite represents the carbide volume fraction (%) of the hot rolled steel sheet.

In the manufacturing method of the cold-rolled steel sheet, the workability relation index (F) expressed by [Relational Expression 3] may satisfy 3500 to 7000.

[Relationship 3]

F = ([Al]/[N]) x (D _cementite /[C])

In the above relational formula 3, [Al], [N] and [C] are values obtained by dividing the weight% of each element by the atomic weight of each element, and D _cementite represents the carbide volume fraction (%) of the hot rolled steel sheet.

The slab may further include one or more of P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%), and Ti: 0.001% or less (excluding 0%).

In the step of manufacturing a hot-rolled steel sheet; In, the heated slab may be hot-rolled at a rolling temperature of 850°C to 900°C.

In the step of winding the hot-rolled steel sheet; in, it may be to wind the hot-rolled steel sheet at 640°C to 750°C.

In the step of annealing heat treatment of the cold-rolled steel sheet; in, the cold-rolled steel sheet may be annealing heat treatment at 700 to 850 °C.

In the step of annealing and heat treatment of the cold-rolled steel sheet, it may be to maintain the cold-rolled steel sheet for 30 seconds or more.

The cold-rolled steel sheet for enamel according to an embodiment of the present invention is excellent in fish scale resistance and enamel adhesion, and can be used in home appliances, chemical devices, kitchen appliances, sanitary appliances, and interior and exterior materials of buildings.

In the cold-rolled steel sheet for enamel according to an embodiment of the present invention, since the chemical composition of the steel material is suppressed within an appropriate range and the relationship index of enamel defects, adhesion and workability is controlled, the high enamel adhesion and hydrogen permeability of the produced cold-rolled steel sheet are improved. Can be secured. Accordingly, it is possible to suppress fishscale and bubble defects, which are fatal defects of the enameled steel sheet, and the enamel properties are remarkably improved.

The cold-rolled steel sheet for enamel according to an embodiment of the present invention utilizes cementite, which is a low-temperature precipitate. Cementite is uniformly dispersed during hot rolling, and micro-voids formed by crushing during cold rolling act as a storage source of hydrogen, thereby preventing fishscale defects caused by hydrogen. In addition, compared to the precipitate system precipitated in the solidification step at high temperatures, since the stable carbide at low temperatures is used as a hydrogen storage source, the workability of operations such as melting loss of refractories and clogging of the playing nozzle, which has been a problem in the existing enamel steel, deteriorates. And it is possible to prevent the occurrence of surface defects such as blackline.

In the cold-rolled steel sheet for enamel according to an embodiment of the present invention, the adhesion between the steel sheet and the glaze may be improved as an element such as titanium (Ti) having higher oxidation properties than Fe is not added.

The cold-rolled steel sheet for enamel according to an embodiment of the present invention can be made by continuous casting, and as it can be produced by continuous annealing heat treatment, it is possible to significantly lower the steelmaking cost and process cost, thereby lowering the mail order property and manufacturing cost, Productivity is high.

1 is a SEM photograph of Inventive Example 4, which is an embodiment of the present invention.

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 the present specification, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless otherwise stated.

In this specification, the terminology used is only for referring to specific embodiments, and is not intended to limit the invention. Singular forms as used herein also include plural forms unless the phrases clearly indicate the opposite. The meaning of “comprising” as used in the specification specifies a specific characteristic, region, integer, step, action, element and/or component, and the presence of another characteristic, region, integer, step, action, element and/or component, or It does not exclude additions.

In the present specification, the term "combination of these" included in the expression of the Makushi format refers to one or more mixtures or combinations selected from the group consisting of components described in the expression of the Makushi format, and the components It means to include one or more selected from the group consisting of.

In the present specification, when a part is referred to as being "on" or "on" another part, it may be directly on or on another part, or another part may be involved in between. In contrast, when a part is referred to as being “directly above” another part, no other part is intervened.

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

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

In an embodiment of the present invention, the meaning of further including an additional element means to include the remaining iron (Fe) by replacing the amount of the additional element.

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

The cold-rolled steel sheet according to an embodiment of the present invention is by weight %, C: 0.05 to 0.09%, Mn: 0.1 to 0.3%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, S: 0.001 to 0.02%, Cu: 0.01 to 0.15%, N: 0.005% or less (excluding 0%), the balance Fe and inevitable impurities are included, and the fine pore area fraction is 0.3 to 0.8%. In addition, the steel sheet, P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%) and Ti: 0.001% or less (excluding 0%) It may further include one or more of.

In addition, the slab prepared in the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention is in weight%, C: 0.05 to 0.09%, Mn: 0.1 to 0.3%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, S: 0.001 to 0.02%, Cu: 0.01 to 0.15%, N: 0.005% or less (excluding 0%), balance Fe and inevitable impurities are included. In addition, it may further include one or more of P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%), and Ti: 0.001% or less (excluding 0%). After that, the slab is heated, and the heated slab is hot-rolled to produce a hot-rolled steel sheet having a carbide volume fraction of 2.5 to 7.0%. The subsequent process will be described later.

First, the reason for limiting the components of the cold-rolled steel sheet will be described.

Carbon (C): 0.05 to 0.09%

If too much C is added, the strength increases due to the increase in the amount of solid solution carbon in the steel, impedes the development of the texture during annealing, deteriorating formability, and causing bubble defects due to bubbling of the enamel layer. If too little C is added, there may be a problem that the fraction of carbides acting as sites for storing hydrogen in the steel is low, making it vulnerable to fishscale defects.

Manganese (Mn): 0.1 to 0.3%

Mn is a representative solid solution strengthening element, and it is added to prevent hot shortness by depositing solid solution sulfur in steel as manganese sulfide (MnS), and at least 0.1% or more needs to be added to secure such an effect. On the other hand, if the content of manganese is too high, there may be a problem of deteriorating the moldability.

Silicon (Si): 0.001 to 0.03%

Si is an element that promotes the formation of carbides acting as a hydrogen storage source, and it needs to be added at least 0.001% to obtain such an effect. On the other hand, if too much is added, there may be a problem in that an oxide film is formed on the surface of the steel sheet, thereby reducing the enamel adhesion. More specifically, it may be 0.005 to 0.025%.

Aluminum (Al): 0.01 to 0.08%

Al is used as a strong deoxidizing agent to remove oxygen from molten steel, and as an element that improves aging by fixing solid solution nitrogen, it needs to be added at least 0.01% for such an effect. On the other hand, if too much is added, there may be a problem in that the aluminum oxide remains in the steel or on the surface of the steel, causing bubble defects in the enamel treatment process. More specifically, it may be 0.02 to 0.044%.

Phosphorus (P): 0.002 to 0.02%

P is a representative material reinforcing element, and it needs to be added at least 0.002% to obtain such an effect. On the other hand, when P increases, not only does a segregation layer form inside the steel plate, which deteriorates the formability, but also worsens the pickling property of the steel, which can adversely affect the enamel adhesion. More specifically, it may be 0.005 to 0.02%.

Sulfur (S): 0.001 to 0.02%

S is an element that combines with manganese to cause red heat embrittlement, and when too much is added, ductility is greatly deteriorated to deteriorate workability and may adversely affect fish scale properties due to excessive precipitation of manganese sulfide. More specifically, it may be 0.003 to 0.015%.

Copper (Cu): 0.01 to 0.15%

Cu is an element that is concentrated on the surface of the steel sheet to improve the bonding strength between the enamel layer and the steel sheet, and for this, it needs to be added at least 0.01%. However, if too much is added, there may be a problem in that the reaction between the enamel layer and the steel sheet is rather suppressed, causing bubble defects as well as deteriorating workability. More specifically, it may be 0.03 to 0.13%.

Nitrogen (N): 0.005% or less (excluding 0%)

N is a typical hardening element, but when the amount of addition increases, there may be a problem in that aging defects occur, moldability deteriorates, and bubble defects occur in the enamel treatment process. More specifically, it may be 0.004% or less.

Oxygen (O): 0.002% or less (excluding 0%)

O is an essential element in forming oxides, and such oxides not only cause melting and loss of refractories in the steel making step, but also may act as a factor causing surface defects caused by oxides on the surface of the steel sheet.

Titanium (Ti): 0.001% or less (excluding 0%)

Ti is known as an element added in order to improve processability and suppress the reaction of hydrogen, which causes fish scale. However, in the steel type according to the exemplary embodiment of the present invention, when a large amount of Ti is added, there is a problem of causing a bubble defect such as a blister. In addition, as described above, nozzle clogging due to titanium nitride (TiN) and inclusions frequently occurs in the continuous casting step of the steelmaking process, resulting in lower workability and a direct factor of production load. Therefore, when Ti is included, it may be limited to 0.001% or less.

In addition to the above components, the present invention includes Fe and unavoidable impurities. It does not exclude the addition of effective ingredients other than the above ingredients.

Next, the area fraction of fine pores of the cold-rolled steel sheet of the present invention and the volume fraction of carbides in the hot rolling step of the slab to be prepared will be described.

In the case of the carbide used in the steel of the present invention, the carbide itself is crushed during cold rolling due to the difference in ductility from the base material, and the fine pores formed thereby are used as a hydrogen storage source to fix hydrogen in the steel. This plays a big role in suppressing fishscale defects, so it is very important to manage the carbide fraction within a certain range. On the other hand, such a carbide fraction affects the enamel property not only by itself but also by the interrelationship with the additional elements.

Fishscale defect, which is one of the most fatal defects of enamel steel plate, is supersaturated in the steel during the process of cooling after firing of hydrogen dissolved in the steel during the manufacturing process of the enamel product, and then discharged to the surface of the steel, causing the enamel layer to be formed. It is an enamel defect caused by dropping it into a scale shape of meat. When such a fishscale defect occurs, it is necessary to suppress the occurrence of the enamel product because it greatly reduces the value of the enamel product, such as intensive occurrence of rust in the defective area. In order to prevent fishscale defects, it is necessary to form a large amount of sites in the river that can hold the hydrogen dissolved in the steel. Accordingly, the cold-rolled steel sheet for enamel using precipitates and inclusions is presented. Representative hydrogen storage sources known so far include non-metallic inclusions such as MnO and CrO, precipitates such as BN, TiN, and TiS, and micro-voids generated by rolling.

The steel sheet for enamel proposed in the present invention mainly utilizes carbides such as Fe ₃ C (cementite) as a hydrogen storage location by controlling the steel component, and at the same time affects enamel adhesion, surface defects and formability among the steel components. It is intended to provide a steel plate with excellent enamel adhesion and fish scale resistance without surface defects by controlling the components and processes that are affected. In the case of cementite, which is a low-temperature precipitate, micro-voids formed by being uniformly dispersed during hot rolling and crushed during cold rolling act as an occlusion source for hydrogen, preventing fishscale defects caused by hydrogen. In addition, compared to the precipitate system precipitated in the solidification stage at high temperatures, since the stable carbide at low temperatures is used as a hydrogen storage source, the workability of operations such as melting loss of refractory materials and clogging of the playing nozzle, which has been a problem in the existing enamel steel, is worse And it is possible to prevent the occurrence of surface defects such as blackline.

The fraction of carbide is not only closely related to the total carbon content in the steel, but is also greatly influenced by operating conditions. On the other hand, in the case of the steel according to the present invention, since the addition of elements such as titanium (Ti) having higher oxidation properties than Fe is very small, the adhesion between the steel plate and the glaze can be improved. In addition, the example of the present invention suppresses the generation of sulfuric acid hydrate (H ₂ SO ₄ -H ₂ O) generated on the surface of the steel sheet in the enamel pretreatment step by controlling the amount of copper (Cu) and the area fraction of micropores. It can be seen that the occurrence of bubble defects is remarkably improved.

The following is the volume fraction of carbides in the hot-rolled steel sheet in the middle of cold-rolled steel sheet manufacturing, the area fraction of micropores in the cold-rolled steel sheet as the final product, the enamel defect relationship index (D), the adhesion relationship index (A), and the workability relationship index (F). Will be explained.

Volume fraction of carbide (D _cementite ): 2.5 to 7.0%

The carbide volume fraction means the carbide volume fraction in the hot rolled steel sheet.

The carbide may be cementite (Fe ₃ C). Carbon present in a metal alloy combines with metal atoms to form carbides, and when iron combines with carbon to form carbides, it is called cementite. In general, in carbon steel, cementite is formed around 250 to 700°C, and at higher temperatures, it becomes coarse into spherical particles. Among the steel materials, materials such as white cast iron have almost carbon in the form of cementite, and have excellent abrasion resistance, so they are used in areas with severe wear such as ball mills.

When the volume fraction of carbides generated in the hot rolling step is too small, the fraction of carbides that can be crushed in the cold rolling process and provided as a source for storing hydrogen decreases, resulting in a decrease in the fine pore area fraction of the steel sheet through cold rolling. There may be a problem in that it is difficult to suppress such as fish scale because there is a limit in fixing hydrogen. On the other hand, when the carbide fraction is too high, it is advantageous in terms of formation of micropores as a hydrogen storage source, and is effective in improving fish scale resistance, but there may be a problem in that workability and corrosion resistance are lowered by too many pores.

Area fraction of micropores (D _void ): 0.30 to 0.80%

The area fraction of fine pores means the fraction of fine pores in the cold-rolled steel sheet.

In the case of cementite, which is a low-temperature precipitate, it is uniformly dispersed during hot rolling and crushed during cold rolling, thereby forming fine pores around the precipitate. The formed micropores act as a storage source for hydrogen in the enamel steel plate, thereby suppressing the occurrence of fishscale defects in which the enamel layer, which is an enamel defect caused by hydrogen, falls out into meat scales. The micropores in the cold-rolled steel sheet were taken using a scanning electron microscope at a magnification of 1000 times, and then the area fraction of the micropores occupied in these areas was measured using an image analyzer. 1 shows an example of a microscopic public observation photograph of Inventive Example 4 taken as described above. If the area fraction of micropores is too low, there may be a problem that the fishscale defect rate of the enamel product increases as there are few sites that act as hydrogen storage sources that can fix hydrogen in the river. Although it was advantageous in terms of fish scale, there may be a problem in that workability and surface defects occur frequently due to an increase in micropores. More specifically, it may be 0.30 to 0.75%.

Enamel defect relationship index, D=([Mn]x[Cu]/[S])x(D _void /[C]): 0.45 to 4.50

Enamel defect relationship index (D), D=([Mn]x[Cu]/[S])x(D _void , which represents the resistance to fishscale and bubble defects in which the enamel layer falls off into meat scales. It was necessary that the value of /[C]) satisfies the above range. Here, [Mn], [Cu], [S], and [C] are values obtained by dividing the weight% of each element by the atomic weight, and D _void is the area fraction of micropores in the cold rolled steel sheet. If the D value is too low, there may be a problem that the occurrence of fishscale defects of enameled products increases because it is not possible to secure a site capable of sufficiently storing hydrogen that causes fishscale defects. On the other hand, the D value is too high. In this case, although it was advantageous in terms of securing a hydrogen storage source, there may be a problem of deteriorating the processability and surface characteristics of the product. More specifically, it may be 0.5 to 4.2.

Adhesion relationship index, A=([Mn]x[Cu]xD _cementite} )/[Si]: 0.007 to 0.185

It was necessary for the adhesion relationship index (A), A=([Mn]x[Cu]xD _cementite} )/[Si] to satisfy the above range. Here, [Mn], [Cu], and [Si] are values obtained by dividing the weight% of each element by the atomic weight, and D _cementite represents the volume fraction of carbides in the hot-rolled steel sheet. If the A value is too small, the amount of aluminum oxide formed increases as the concentration of the interfacial layer concentrated on the surface decreases, resulting in a problem of lowering the adhesion between the enamel glaze layer and the base iron. If it is too large, the amount of gas generated on the surface of the steel sheet during plastic heat treatment increases, causing bubble defects, and thus there may be a problem in that adhesion is significantly reduced. More specifically, it may be 0.008 to 0.180.

Processability relationship index, F=([Al]/[N])x(D _cementite /[C]): 3500 to 7000

Since the workability of enamel steel also has a close relationship with the carbide fraction and the solid solution element in the steel, it may be necessary to manage the workability relationship index, F, within the above range. Here, [Al], [N], and [C] are values obtained by dividing the weight% of each element by the atomic weight, and D _cementite is the volume fraction of carbides in the hot rolled steel sheet. If the workability relational index, F, is too small, as the amount of solid solution in the steel increases, aging defects such as bends or surface defects occur in the enamel processing step, increasing the defect rate of the molded product. On the other hand, if the F value is too high As the fraction of fine precipitates increases, there may be a problem in that workability deteriorates.

Next, the pickling reduction and hydrogen permeation ratio of the cold-rolled steel sheet of the present invention will be described.

The cold-rolled steel sheet according to an embodiment of the present invention may have a pickling loss of 10 to 40 gr/m ² . By satisfying these properties, it can be applied as a material for enamel. If the pickling loss is too low, there is no roughness on the surface of the steel plate, and the adhesion between the glaze and the steel plate is deteriorated, resulting in a problem such as dropping of the enamel layer. On the other hand, when it is too high, the surface layer of the steel sheet is flattened, thereby deteriorating the adhesion and increasing the occurrence of bubble defects. More specifically, the pickling reduction may be 12 to 35 gr/m ² .

The cold-rolled steel sheet according to an embodiment of the present invention may have a hydrogen permeation ratio of 950 seconds/mm ^{2 or} more. The hydrogen permeation ratio is an index for evaluating the fish scale resistance indicating the resistance of fish scale defects, which is a fatal defect of enamel steel manufactured using the cold rolled steel sheet according to an embodiment of the present invention, and is a method registered in European standard EN 10209. Experiment and evaluate the ability to fix hydrogen in the steel plate. It is a value expressed by dividing this by the square of the material thickness (t, unit: mm) by measuring the time (t _s , unit: second) that generates hydrogen in one direction of the steel plate and the hydrogen permeates through the other side of the steel plate. It is expressed as t _s /t ² (unit: second/mm ² ). Specifically, the hydrogen permeation ratio may be 950 seconds/mm ^{2 or} more. If the hydrogen permeation ratio is too small, the fishscale defect rate is more than 50% when accelerated heat treatment at 200℃ for 24 hours after enamel treatment, so there is a problem in using it as a stable enamel product, so it is necessary to secure a steel plate with excellent fish scale resistance. For this, it is necessary to manage the hydrogen permeation ratio to be 950 seconds/mm ² or more. In addition, more specifically, the hydrogen permeation ratio may be 1000 seconds / mm ^{2 or} more.

Hereinafter, a method of manufacturing the cold-rolled steel sheet of the present invention will be described in detail.

First, a slab that satisfies the above composition is prepared. The molten steel whose components are adjusted to the above-described composition in the steel making step can be manufactured into slabs through continuous casting.

After that, the prepared slab is 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 1250 ℃. If the slab heating temperature is too low, the rolling load rapidly increases during subsequent hot rolling, resulting in poor workability. On the other hand, if the slab heating temperature is too high, not only the energy cost increases, but also the amount of surface scale increases, which can lead to material loss.

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

In this case, the volume fraction of carbides of the hot-rolled steel sheet may be 2.5 to 7.0%. Since the description of the carbide volume fraction has been mentioned, it will be omitted.

In addition, the finish rolling temperature of hot rolling may be 850 to 900°C. If the finish hot rolling temperature is too low, as the rolling is finished in the low-temperature region, mixing of crystal grains rapidly proceeds, which tends to cause 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 deteriorated, and as uniform hot rolling is not performed throughout the thickness, the impact toughness due to grain growth may decrease. More specifically, the finish hot rolling temperature may be 860 to 890°C.

Thereafter, the hot-rolled steel sheet produced after hot rolling is finished undergoes a winding process. More specifically, it may be a hot-rolled winding process.

At this time, the coiling temperature may be 640 to 750°C. The hot-rolled steel sheet can be cooled in a run-out-table (ROT) before winding. If the hot-rolled coiling temperature is too low, temperature non-uniformity in the width direction occurs in the cooling and maintaining process, causing material variation as well as the enamel properties as the low-temperature precipitates are different. On the other hand, if the coiling temperature is too high, not only the corrosion resistance decreases as the lumping of the tin oxide proceeds, but also the problem of deteriorating the workability due to the coarsening of the structure in the final product occurred. More specifically, the coiling temperature may be 650 to 710°C.

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

After that, the wound hot-rolled steel sheet is manufactured into a cold-rolled steel sheet through cold rolling.

At this time, the cold reduction rate may be 60 to 90%. If the cold reduction rate is too small, the strength is increased but the workability is remarkably deteriorated as the unrecrystallized grains remain locally as the recrystallization driving force in the subsequent heat treatment process is low. In addition, as the crushing ability of the carbide formed in the hot rolling step decreases, the number of sites capable of storing hydrogen decreases, making it difficult to secure the fish scale resistance of the enameled product, and considering the thickness of the final product, the thickness of the hot rolled steel sheet must be reduced. There is also a problem that makes rolling workability worse. On the other hand, if the cold rolling reduction rate is too high, the material is hardened and workability is deteriorated, and the load on the rolling mill increases, thereby reducing the workability of cold rolling. More specifically, the cold reduction ratio may be 65 to 88%.

After that, the cold-rolled steel sheet can be annealed and heat treated. More specifically, hot rolling annealing heat treatment may be performed. The cold rolled material has high strength due to the deformation applied in cold rolling, but is extremely inferior in workability. Therefore, the target strength and workability are secured by performing heat treatment in a subsequent process.

In this case, the heat treatment temperature may be 700 to 850°C. If the continuous annealing temperature is too low, there is a problem that the workability is significantly deteriorated as the deformation formed by cold rolling is not sufficiently removed. On the other hand, if the heat treatment temperature is too high, the plate breakage occurs due to softening due to a decrease in high-temperature strength, resulting in a problem that greatly deteriorates the plateability of the operation. More specifically, the annealing temperature may be 750 to 840 °C.

In addition, during the heat treatment, the heat treatment holding time may be 30 seconds or more. Even if the cracking time at the heat treatment holding temperature is too short during the heat treatment, since unrecrystallized grains remain and act as a factor that greatly deteriorates the formability, a holding time of 30 seconds or more may be required.

In addition, it may further include temper rolling the heat-treated steel sheet after the annealing of the cold-rolled steel sheet. Through temper rolling, the shape of the material can be controlled and the desired surface roughness can be obtained.However, if the temper reduction rate is too high, the material is hardened by work hardening and workability deteriorates, so temper rolling can be applied with a reduction rate of 3% or less. have. Specifically, the reduction ratio of temper rolling may be 0.3 to 2.5%.

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

[Example]

Experiment and evaluation

Table 1 shows the chemical composition of these steels. The balance contains Fe and unavoidable impurities. The slab through the conversion furnace-secondary refining-casting process was maintained in a heating furnace at 1200° C. for 1 hour and then hot-rolled. At this time, the final thickness of the hot-rolled steel sheet was 4.0mm. The hot-rolled specimen was subjected to cold rolling after removing the oxide film on the surface through pickling. The cold-rolled specimen was processed into an enamel-treated specimen to investigate the enamel properties and a tensile specimen for mechanical property analysis, and then subjected to continuous annealing heat treatment.

Table 2 shows the manufacturing conditions for each process of hot rolling, winding, cold rolling, and continuous annealing processes applied to each of the steels.

Table 3 shows the enamel properties for each manufacturing condition of the material secured through the above process.

In the case of mail order, "O" is indicated when the operability is 90% or more compared to the productivity of the normal material in the performance-hot rolling-cold rolling process, and "X" is indicated when the productivity is 90% or less or the defect occurrence rate is 10% or more.

The carbide fraction of the inventive steel and the comparative steel was obtained as the fraction of the carbide with respect to the total field of view using an image analyzer after obtaining an image of 20 fields of view at 500 times magnification with an optical microscope.

Enamel-treated specimens were cut to a suitable size to meet the purpose of the test, and after heat-treated enamel-treated specimens were completely degreased, a standard glaze relatively vulnerable to fishscale defects (check frit) was applied and maintained at 300°C for 10 minutes. To remove moisture. The dried specimen was sintered at 830°C for 20 minutes and then cooled to room temperature. At this time, the atmosphere condition of the sintering furnace was a dew point temperature of 30°C, and a severe condition where fish scale defects were likely to occur was selected. The specimens after the enamel treatment were subjected to a fishscale accelerated experiment that was maintained in an oven at 200°C for 24 hours.

After the fishscale acceleration treatment, the presence or absence of a fishscale defect was visually observed, and when a fishscale defect did not occur, "O" and when a fishscale defect occurred, "X" was indicated.

The enamel adhesion index, which evaluates the adhesion between the steel plate and the glaze, is determined by applying a certain load to the enamel layer with a steel ball as defined in the American Material Testing Association Standard, ASTM C313-78, and then evaluating the degree of energization of the enamel glaze layer. The degree of dropout was expressed as an index. In the case of the adhesion index in the present invention, the aim was set to secure 90% or more in terms of securing stability during use of the enamel layer.

Bubble defects were determined by visual observation of the surface of the enamel layer with respect to the specimen kept in an oven at 200° C. for 24 hours after the enamel treatment, and were evaluated in three stages: "O" excellent, "△" normal, and "X" defective.

The hydrogen permeation ratio is an index that evaluates the resistance to fish scale, which is a fatal defect of enamel steel, and according to the method registered in European standard EN 10209 (2013), hydrogen is generated from one direction of the steel plate and hydrogen is generated on the other side of the steel plate. The time to penetrate through (t _s , unit seconds) is measured, divided by the square of the material thickness (t, unit mm), and expressed as t _s /t ² (unit seconds/mm ² ). In the case of this value, it was necessary to secure 950 sec/mm ² or more in order to obtain stable properties of the enamel steel as described above.

Pickling reduction (unit gr/m ² ) is the test method shown in European Standard, EN 10209 (2013). After cutting and degreasing the steel sheet for enamel, it is immersed in a sulfuric acid solution maintained at 70g/l, 70℃ for 7 minutes. It was calculated from the weight loss. Even in the case of this value, it was necessary to satisfy the range of 10 to 40 gr/m ² .

Chemical composition of these steels (unit: wt%) division C Mn Si Al P S Cu N O River 1 0.055 0.17 0.018 0.036 0.009 0.005 0.042 0.0019 0.0012 River 2 0.078 0.23 0.011 0.044 0.015 0.011 0.069 0.0025 0.0007 River 3 0.086 0.28 0.009 0.025 0.006 0.008 0.124 0.0028 0.0015 River 4 0.067 0.21 0.021 0.039 0.011 0.011 0.031 0.0035 0.0018 River 5 0.081 0.14 0.006 0.041 0.01 0.004 0.082 0.0038 0.0011 River 6 0.003 0.25 0.312 0.001 0.042 0.01 0.321 0.0027 0.0251 River 7 0.062 0.06 0.011 0.003 0.009 0.013 0 0.0019 0.0015 River 8 0.12 1.21 0.005 0.049 0.038 0.005 0.013 0.0032 0.0418 River 9 0.042 0.25 0.017 0.096 0.011 0.039 0.029 0.0108 0.0011

Manufacturing conditions of these steels division Steel grade
No. Wrap-up
Hot rolled
Temperature
(℃) Winding
Temperature
(℃) Cold
Reduction rate
(%) Annealing
Temperature
(℃) maintain
time
(second) D _cementite
(%) A value F value D _void
(%) D value Invention Example 1 River 1 870 680 70 780 48 2.73 0.0086 5852 0.31 0.8780 Inventive Example 2 River 1 870 680 75 800 40 2.73 0.0086 5852 0.32 0.9064 Invention Example 3 River 2 870 720 80 820 40 3.21 0.0101 6881 0.33 0.9347 Invention Example 4 River 2 880 700 75 770 36 4.81 0.0552 6753 0.61 1.2309 Invention Example 5 River 2 880 720 85 830 42 4.51 0.0518 6332 0.63 1.2712 Invention Example 6 River 3 880 660 75 800 64 5.62 0.1725 3630 0.75 4.1290 Invention Example 7 River 4 880 700 80 820 42 5.36 0.0132 5547 0.59 0.5685 Invention Example 8 River 5 880 680 75 780 42 6.27 0.0954 5197 0.74 2.8603 Invention Example 9 River 5 880 720 75 830 42 6.62 0.1008 5487 0.76 2.9376 Comparative Example 1 River 1 760 680 75 620 42 1.62 0.0051 3473 0.15 0.4249 Comparative Example 2 River 1 870 680 48 820 20 2.73 0.0086 5852 0.12 0.3399 Comparative Example 3 River 2 880 560 93 830 40 1.95 0.0224 2738 0.11 0.2220 Comparative Example 4 River 3 880 780 75 880 40 7.81 0.2397 5045 0.96 5.2851 Comparative Example 5 River 6 880 680 75 830 42 0.01 0.0000 8 0.23 67.118 Comparative Example 6 River 7 880 680 75 810 42 1.67 0.0000 265 0.23 0.0000 Comparative Example 7 River 8 880 680 75 810 42 8.95 0.2240 7106 1.85 5.2910 Comparative Example 8 River 9 880 680 75 810 42 1.52 0.0052 2002 0.19 0.0917

From here,

D = ([Mn] x [Cu] / [S]) x (D _void / [C])

A = ([Mn] x [Cu] x D _cementite} ) / [Si]

F = ([Al]/[N]) x (D _cementite /[C])

[Mn] is the value obtained by dividing the weight% of Mn by the atomic weight of Mn (55),

[Cu] is a value obtained by dividing the weight% of Cu by the atomic weight of Cu (64),

[S] is the value obtained by dividing the weight% of S by the atomic weight of S (32),

[C] is the value obtained by dividing the weight% of C by the atomic weight of C (12),

[Si] is a value obtained by dividing the weight% of Si by the atomic weight of Si (28),

[Al] is a value obtained by dividing the weight% of Al by the atomic weight of Al (27),

[N] is a value obtained by dividing the weight% of N by the atomic weight of N (14).

In addition, D _void is the area fraction (%) of fine pores in the cold rolled steel sheet,

D _cementite is the carbide volume fraction (%) of the hot rolled steel sheet.

Characteristics of these steels by manufacturing conditions division Mail order Bubble defect
Occurrence or not Fish scale
Occurrence or not Enamel adhesion
Indices (%) Hydrogen permeation ratio
(Sec/mm ² ) Pickling reduction
(gr./m ² ) Invention Example 1 O O O 96.4 1221 31.2 Inventive Example 2 O O O 95.9 1098 27.5 Invention Example 3 O O O 98.3 1139 29,6 Invention Example 4 O O O 99.1 1358 18.5 Invention Example 5 O O O 97.7 1249 20.8 Invention Example 6 O O O 98.2 1435 29.6 Invention Example 7 O O O 97.6 1605 33.3 Invention Example 8 O O O 99.3 1488 14.6 Invention Example 9 O O O 98.7 1516 21.3 Comparative Example 1 X X X 87.2 865 38.4 Comparative Example 2 O △ X 78.5 781 37.9 Comparative Example 3 X X X 81.5 567 42.2 Comparative Example 4 X △ X 76.7 922 42.3 Comparative Example 5 O X X 80.3 427 7.9 Comparative Example 6 X X X 65.7 735 51.4 Comparative Example 7 O X X 50.3 889 54.3 Comparative Example 8 X X X 60.3 681 46.8

Experiment result

As can be seen from Tables 1 to 3, Inventive Examples 1 to 9 satisfying all of the alloy composition, microstructure characteristics, and manufacturing conditions have good mail-order properties, as well as carbide and microporous fractions and related indices. The limited range of was satisfied, and fish scale and bubble defects did not occur even under severe treatment conditions, and the enamel adhesion index was 90% or more, the hydrogen permeation ratio was 950 sec/mm ² or more, and the pickling reduction range was 10 to 40 gr/m ^2. Thus, the target characteristics of the present invention could be secured.

On the other hand, it can be seen that in Comparative Examples 1 to 4, in which the chemical composition suggested by the present invention was satisfied, but the manufacturing condition range was not satisfied, target characteristics could not be secured. That is, as shown in Table 3, there was a problem that the mail order was deteriorated (Comparative Examples 1, 3 and 4), the hydrogen permeation ratio was lower than the target (Comparative Examples 1 to 4), or the enamel adhesion index was less than 90% (Comparative In Examples 1 to 4), it was possible to confirm that a bubble defect or a fishscale defect occurred after the enamel treatment, so that the target characteristics as a whole could not be secured.

Comparative Examples 5 to 8 are cases in which the manufacturing conditions suggested in the present invention are satisfied, but the alloy composition is not satisfied. In Comparative Examples 5 to 8, not only did not satisfy the target carbide and microporous fraction, hydrogen permeability ratio, enamel adhesion index, pickling reduction, etc. of the present invention, but also fishscale and bubble defects occurred in visual observation after enamel treatment.

The present invention is not limited to the above embodiments, but may be manufactured in a variety of different forms, and those of ordinary skill in the art to which the present invention pertains may It can be understood that Therefore, it should be understood that the embodiments described above are illustrative in all respects and not limiting.

Claims (11)

In% by weight, C: 0.05 to 0.09%, Mn: 0.1 to 0.3%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, S: 0.001 to 0.011%, Cu: 0.01 to 0.15%, N: 0.004% Below (excluding 0%), including the balance Fe and inevitable impurities,
The fine pore area fraction is 0.3 to 0.8%,
The cold-rolled steel sheet for enamel that satisfies the enamel defect relationship index (D) represented by the following [Relational Formula 1] of 0.45 to 4.50.
[Relationship 1]
D = ([Mn] x [Cu] / [S]) x (D _void / [C])
(In the above relational equation 1, [Mn], [Cu], [S], [C] are values obtained by dividing the weight percent of each element by the atomic weight of each element, and D _void is the area fraction of micropores in the cold rolled steel sheet ( %).)
The method of claim 1,
P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%) and Ti: 0.001% or less (excluding 0%), a cold-rolled steel sheet for enamel further comprising at least one of.
The method of claim 1,
The cold-rolled steel sheet for enamel of 10 to 40 gr/m ² of the pickling loss of the cold-rolled steel sheet.
The method of claim 1,
The cold-rolled steel sheet for enamel having a hydrogen permeation ratio of 950 sec/mm ² or more of the cold-rolled steel sheet.
In% by weight, C: 0.05 to 0.09%, Mn: 0.1 to 0.3%, Si: 0.001 to 0.03%, Al: 0.01 to 0.08%, S: 0.001 to 0.011%, Cu: 0.01 to 0.15%, N: 0.004% Or less (excluding 0%), preparing a slab containing the balance Fe and unavoidable impurities;
Heating the slab;
Hot rolling the heated slab to manufacture a hot-rolled steel sheet having a carbide volume fraction of 2.5 to 7.0%;
Winding the hot-rolled steel sheet;
Producing a cold-rolled steel sheet by cold rolling the wound hot-rolled steel sheet at a reduction ratio of 60 to 90%; And
Annealing and heat treating the cold rolled steel sheet;
Including,
The adhesion relationship index (A) represented by the following [Relational Formula 2] satisfies 0.007 to 0.185,
Heating the slab; in the slab heating temperature is 1150 to 1250 ℃,
In the step of manufacturing the hot-rolled steel sheet; In, the heated slab is hot-rolled at a rolling temperature of 850 ℃ to 900 ℃
Method of manufacturing cold rolled steel sheet for enamel.
[Relationship 2]
A = ([Mn] x [Cu] x D _cementite} ) / [Si]
(In the above relational equation 2, [Mn], [Cu] and [Si] are values obtained by dividing the weight percent of each element by the atomic weight of each element, and D _cementite represents the carbide volume fraction (%) of the hot-rolled steel sheet.)
The method of claim 5,
A method of manufacturing a cold-rolled steel sheet for enamel that satisfies the workability relationship index (F) represented by the following [Relational Formula 3] of 3500 to 7000.
[Relationship 3]
F = ([Al]/[N]) x (D _cementite /[C])
(In the above relational equation 3, [Al], [N] and [C] are values obtained by dividing the weight% of each element by the atomic weight of each element, and D _cementite represents the carbide volume fraction (%) of the hot rolled steel sheet.)
The method of claim 5,
The slab further comprises at least one of P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%) and Ti: 0.001% or less (excluding 0%). Manufacturing method.
delete The method of claim 5,
In the step of winding the hot-rolled steel sheet;
A method of manufacturing a cold-rolled steel sheet for enamel winding the hot-rolled steel sheet at 640°C to 750°C.
The method of claim 5,
Annealing heat treatment of the cold rolled steel sheet; In,
A method of manufacturing a cold-rolled steel sheet for enamel by annealing and heat treatment at 700°C to 850°C.
The method of claim 5,
Annealing heat treatment of the cold rolled steel sheet; In,
A method of manufacturing a cold-rolled steel sheet for enamel for holding the cold-rolled steel sheet for 30 seconds or more.

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