WO2020111739A2 - Cold-rolled steel plate for enameling and method for manufacturing same - Google Patents

Abstract

The present invention relates to a cold-rolled steel plate for enameling and a method for manufacturing same. The cold-rolled steel plate for enameling according to an embodiment of the present invention comprises, based on wt%, 0.05 to 0.09% of C, 0.1 to 0.3% of Mn, 0.001 to 0.03% of Si, 0.01 to 0.08% of Al, 0.001 to 0.02% of S, 0.01 to 0.15% of Cu, 0.005% or less of N (excluding 0%), the balance of Fe, and inevitable impurities, wherein the area fraction of micro-voids is 0.3 to 0.8%, the relationship index of enameling defects (D) represented by [Relation Formula 1] below satisfies 0.45 to 4.50, the relationship index of adhesion (A) represented by [Relation Formulae 2 and 3] satisfies 0.007 to 0.185, and the relationship index of formability (F) satisfies 3500 to 7000. [Relation Formula 1] D = ([Mn] x [Cu] / [S]) x (D_void / [C]) [Relation Formula 2] A = ([Mn] x [Cu] x D_cementite}) / [Si] [Relation Formula 3] F = ([Al]/[N]) x (D_cementite/[C])

Description

Cold rolled steel sheet for enamel and its manufacturing method

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

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

In the conventional steel sheet for enamel, the enamel layer known as the most fatal defect in enamel products is eliminated in the form of meat scales to prevent fishscale defects that degrade enamel, or to improve the formability. As it was manufactured, there was a problem in that manufacturing costs were high and quality variations were frequent. In order to overcome the problems of productivity inferiority and manufacturing cost increase due to such annealing for a long time, a continuous annealing process is used for a steel plate for enamel recently developed, wherein titanium (Ti) or boron (B) is mainly used as a hydrogen storage source. In addition, these precipitates are utilized. However, even in this case, as a large amount of carbonitride-forming elements have to be added, the incidence of surface defects is high, and the recrystallization temperature rises, so high-temperature heat treatment is required, leading to a decrease in productivity and an increase in cost.

In particular, in the case of a titanium (Ti)-based enamel steel sheet, as a large amount of titanium is added to suppress the reaction of hydrogen, which is a cause of 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 lowering workability and production load. In addition, when TiN mixed in the molten steel is present on the upper portion of the steel sheet, not only causes a blister defect, which is a representative bubble defect, but also a large amount of added titanium is a factor that inhibits the adhesion between the steel sheet and the glaze layer.

On the other hand, even in the case of a high-oxygen enamel steel sheet that secures fish-scale resistance by absorbing hydrogen by using inclusions such as oxides in steel by increasing the dissolved oxygen content inside the steel sheet, the content of oxygen is high and the refractory melting is extremely severe. In addition to significantly lowering the performance of performance in the steelmaking process, there is a problem in that surface defects frequently occur.

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

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%), including residual Fe and unavoidable impurities, the micropore area fraction is 0.3 to 0.8%, and expressed by the following [Relational Formula 1] The enamel defect relationship index (D) to be satisfied satisfies 0.45 to 4.50.

[Relationship 1]

In the above relational expression 1, [Mn], [Cu], [S] and [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 a cold rolled steel sheet ).

The steel sheet may further include 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%).

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 seconds/mm ^{2 or} more.

On the other hand, the method of manufacturing a 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 residual Fe and unavoidable impurities; Heating the slab; Hot-rolling the heated slab to produce a hot-rolled steel sheet having a carbide volume fraction of 2.5 to 7.0%; Winding the hot rolled steel sheet; Cold-rolling the wound hot-rolled steel sheet at a reduction ratio of 60 to 90% to produce a cold-rolled steel sheet; And annealing heat treatment of the cold-rolled steel sheet; and the adhesion relationship index (A) represented by the following [Relational Formula 2] satisfies 0.007 to 0.185.

[Relationship 2]

In the relational expression 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.

In the method for manufacturing a cold rolled steel sheet, the workability relationship index (F) expressed by [Relational Formula 3] may satisfy 3500 to 7000.

[Relationship 3]

In the relational expression 3, [Al], [N] and [C] 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 slab may further include 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%).

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

In the step of winding the hot rolled steel sheet; In the, it may be to roll the hot rolled steel sheet at 640 ℃ to 750 ℃.

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

In the step of heat-treating the annealing of the cold-rolled steel sheet, the cold-rolled steel sheet may be maintained for at least 30 seconds.

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 thus can be used in household appliances, chemical equipment, kitchen equipment, sanitary equipment, and interior and exterior materials of buildings.

The cold-rolled steel sheet for enameling according to an embodiment of the present invention suppresses the chemical composition of the steel material within an appropriate range and controls the enamel defect, adhesion, and processability relational index, thereby providing high enamel adhesion and hydrogen permeation ratio of the produced cold-rolled steel sheet. Can be secured. Therefore, the fatal defects of fish scale and bubble defects of the enameled steel sheet can be suppressed, and the enamel properties are significantly improved.

The cold-rolled steel sheet for enameling 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 to prevent fish scale defects caused by hydrogen. In addition, compared to the precipitate system precipitated in the solidification step at high temperature, stable carbide at low temperature is used as a hydrogen storage source, which deteriorates the workability of operations such as melting of refractory material or clogging of the playing nozzle, which has been a problem in conventional enamel steel. And surface defects such as blacklines.

The cold-rolled steel sheet for enameling according to an embodiment of the present invention can also improve the adhesion between the steel sheet and the glaze as elements such as titanium (Ti) having a high oxidizing property compared to Fe are not added.

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

1 is an 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 portion, component, region, layer or section described below may be referred to as a second portion, component, region, layer or section without departing from the scope of the present invention.

In the present specification, when a part is to “include” a certain component, it means that the component may further include other components, not to exclude other components, unless otherwise stated.

In this specification, the terminology used is only for referring to a specific embodiment, and is not intended to limit the present invention. The singular forms used herein include plural forms unless the phrases clearly indicate the opposite. As used herein, the meaning of “comprising” embodies a particular property, region, integer, step, action, element, and/or component, and the presence or presence of other properties, regions, integers, steps, action, element, and/or component. It does not exclude addition.

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

In the present specification, when it is said that a part is "on" or "on" another part, it may be directly on or on another part, or another part may be involved therebetween. In contrast, if one part is referred to as being “just above” another part, no other part is interposed therebetween.

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which the present invention pertains. Commonly used dictionary-defined terms are further interpreted as having meanings consistent with related technical documents and currently disclosed contents, and are not interpreted as ideal or very formal meanings unless defined.

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

In one embodiment of the present invention, the meaning of further including an additional element means that the remaining amount of iron (Fe) is replaced by an additional amount of the additional element.

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

Cold rolled steel sheet according to an embodiment of the present invention in weight percent, 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%), residual Fe and unavoidable impurities, and a micropore area fraction of 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 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%), residual Fe and unavoidable impurities. In addition, P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%) and Ti: 0.001% or less (excluding 0%) may further include one or more. Thereafter, a step of heating the slab is performed, and the hot-rolled slab is hot-rolled to produce a hot-rolled steel sheet having a carbide volume fraction of 2.5 to 7.0%. The process after that 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 amount of solid carbon in steel increases, the strength increases, and annealing hinders the development of aggregates, thereby deteriorating formability and causing bubble defects due to bubbling of the enamel layer. When too little C is added, there may be a problem that fish fraction defects are vulnerable due to a low fraction of carbide serving as a site for storing hydrogen in the steel.

Manganese (Mn): 0.1 to 0.3%

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

Silicon (Si): 0.001 to 0.03%

Si is an element that promotes the formation of a carbide that acts as a hydrogen storage source, and in order to obtain such an effect, it is necessary to add 0.001% or more. On the other hand, if it is added too much, there may be a problem of forming an oxide film on the surface of the steel sheet to degrade 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 deoxidizer to remove oxygen from molten steel, and it is an element that improves aging by fixing solid nitrogen, and for this effect, addition of 0.01% or more is required. On the other hand, if too much is added, aluminum oxide may remain in the steel or on the surface of the steel, thereby causing a bubble defect in the enameling process. More specifically, it may be 0.02 to 0.044%.

Phosphorus (P): 0.002 to 0.02%

P is a typical material strengthening element, and at least 0.002% of addition is required to obtain this effect. On the other hand, if P is increased, a segregation layer is formed inside the steel sheet to deteriorate the formability as well as to deteriorate the pickling property of the steel, which may adversely affect enamel adhesion. More specifically, it may be 0.005 to 0.02%.

Sulfur (S): 0.001 to 0.02%

S is an element that causes red-brittle brittleness in combination with manganese, and in the case where too much is added, ductility deteriorates significantly, which can deteriorate processability and 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 concentrates on the surface of the steel sheet and improves the bonding strength between the enamel layer and the steel sheet, and for this, it is necessary to add 0.01% or more. However, if too much is added, the reaction between the enamel layer and the steel sheet is suppressed, thereby causing bubble defects and deteriorating processability. 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 if the amount added is increased, aging defects are frequently formed, poor moldability is generated, and there may be a problem of generating bubble defects in the enameling 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 an oxide, and such an oxide not only causes melting of the refractory material in the steelmaking step, but can also act as a factor that causes surface defects due to oxides on the surface of the steel sheet.

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

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

In addition to the above components, the present invention includes Fe and unavoidable impurities. The addition of effective ingredients other than the above ingredients is not excluded.

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

In the case of the carbide used in the present invention steel, the carbide itself is crushed during cold rolling due to a ductility difference with the base material, and the micropores formed thereby are used as a hydrogen storage source for fixing hydrogen in the river. This plays a large 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 enamel property not only alone but also by the interaction with the added elements.

Fishscale defects, one of the most fatal defects of enamel steel plates, are supersaturated in the steel during the cooling process after hydrogen is fired in the steel during the manufacturing process of the enamel product, and then released into the surface of the steel to release the enamel layer. It is an enamel defect caused by dropping out of the meat scales. When such a fish-scale defect occurs, it is necessary to suppress the occurrence of the enamel product, as it significantly reduces the value of the enamel product, such as intensive rust on the defect site. In order to prevent fishscale defects, it is necessary to form a large amount of sites inside the steel that can trap hydrogen dissolved in the steel. Therefore, the cold rolled steel sheet for enamel using precipitates and inclusions is proposed. Representative hydrogen storage sources widely 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 controls carbide components such as Fe ₃ C (cementite) as a storage position for hydrogen by controlling the steel component, and also affects enamel adhesion, surface defects, and formability among steel components. It is intended to provide a steel sheet excellent in enamel adhesion and fish scale resistance without surface defects by controlling the components and processes affecting it. In the case of cementite, which is a low-temperature precipitate, micro-voids formed by uniformly dispersing during hot rolling and 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 at a high temperature at a solidification stage, stable carbide at low temperature is used as a hydrogen storage source, which deteriorates the workability of operations such as melting of refractory material or clogging of a performance nozzle, which has been a problem in conventional enamel steel. And surface defects such as blacklines.

The fraction of carbide not only has a close relationship with the total amount of carbon in the steel, but is also greatly influenced by operating conditions. On the other hand, in the case of the steel of the present invention, the adhesion between the steel sheet and the glaze could be improved as the addition of elements such as titanium (Ti) having a high oxidation property compared to Fe is very small. In addition, the present invention example suppresses the generation of sulfate hydrate (H ₂ SO ₄ -H ₂ O) generated on the surface of the steel sheet in the enamel pre-treatment step by controlling the amount of copper (Cu) and the area fraction of the micropores, resulting in fishscale resistance and It can be seen that the occurrence of bubble defects is significantly improved.

Below is the volume fraction of carbide in the hot rolled steel sheet in the middle of manufacturing cold rolled steel sheet, the area fraction of micropores in the final cold rolled steel sheet, 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 volume fraction of carbide in a hot rolled steel sheet.

The carbide may be cementite (Fe ₃ C). The carbon present in the metal alloy bonds with a metal atom to form a carbide. Iron is combined with carbon to form a carbide, which is called cementite. Normally, in carbon steel, cementite is formed near 250 to 700°C, and at higher temperatures, it becomes coarse to spherical particles. Among steel materials, such as white cast iron, carbon is almost in the form of cementite, and because it has excellent abrasion resistance, it is used in areas of high wear, such as a ball mill.

When the volume fraction of carbides generated in the hot rolling step is too small, the fraction of carbides that can be provided as a source for storing hydrogen by crushing in the cold rolling process decreases, thereby reducing the micropore area fraction of the steel sheet via cold rolling. There may be a problem in that it is difficult to suppress fish scale and the like because there is a limit to fixing hydrogen. On the other hand, when the carbide fraction is too high, it is advantageous in terms of forming micro-pore as a hydrogen storage source, and is effective in improving fish-scale resistance. However, there may be a problem that processability and corrosion resistance are deteriorated by too many pores.

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

The micropore area fraction means the micropore area fraction in a 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 of hydrogen in the enameled steel sheet, thereby suppressing the occurrence of fish scale defects in which the enamel layer, which is an enamel defect caused by hydrogen, falls out into the shape of meat scales. The micropores in the cold-rolled steel sheet were measured using a scanning electron microscope, and after taking 10 pictures at a magnification of 1000, the area fraction of micropores in these areas was measured using an image analyzer. 1 shows an example of a microscopic public observation photograph of Inventive Example 4 photographed as described above. If the area fraction of the micropore is too low, there may be a problem that the fish scale defect rate of the enamel product increases due to the small number of sites that act as a hydrogen storage source that can fix hydrogen in the river. Although it is advantageous in terms of fish scale, there may be a problem in that workability and surface defects are frequently caused by an increase in microporosity. 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 , indicating resistance to fish scale and bubble defects in which the enamel layer falls out of the shape of meat scales /[C]) It was necessary for the value to satisfy the above range. Here, [Mn], [Cu], [S], and [C] are values obtained by dividing the weight percent of each element by 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 fish scale defects in enamel products increases because a site capable of sufficiently storing hydrogen that causes fish scale defects may not be obtained, whereas 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 that the workability and surface characteristics of the product are deteriorated. More specifically, it may be 0.5 to 4.2.

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

It was necessary for the values of the adhesiveness index (A), A=([Mn]x[Cu]xD _cementite} )/[Si] associated with enamel adhesion to satisfy the above range. Here, [Mn], [Cu], and [Si] are values obtained by dividing% by weight of each element by atomic weight, and D _cementite represents the volume fraction of carbide in the hot-rolled steel sheet. If the value of A is too small, as the concentration of the interfacial layer concentrated on the surface decreases, the amount of formation of aluminum oxide or the like increases, and there may be a problem of deteriorating the adhesion between the enamel glaze layer and the iron. If it is too large, the amount of gas (Gas) generated on the surface of the steel sheet during the calcination heat treatment increases, which may cause a bubble defect, and thus, the adhesion may be significantly reduced. More specifically, it may be 0.008 to 0.180.

Machinability relation 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 solid solution elements 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 each element weight% by atomic weight, and D _cementite is a volume fraction of carbide in a hot rolled steel sheet. When the workability relationship index, F is too small, as the amount of solid solution in the steel increases, aging defects such as bending or surface defects occur in the enamel processing step, thereby 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 that workability deteriorates.

Next, the pickling loss 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 an enamel material. If the pickling loss is too low, the roughness of the surface of the steel sheet is not so great that the adhesion between the glaze and the steel sheet deteriorates as the adsorption capacity of the glaze decreases, and problems such as dropping of the enamel layer may occur. On the other hand, when it is too high, a flattening operation is performed on the surface layer of the steel sheet, thereby deteriorating adhesiveness and increasing the occurrence of bubble defects. More specifically, the pickling loss may be 12 to 35gr/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, which indicates the resistance of fish-scale defects, which are fatal defects of enamel steel produced using cold-rolled steel sheets according to an embodiment of the present invention, and is a method registered in European Standard EN 10209. Experiment to evaluate the ability to fix hydrogen in the steel sheet. It is a value expressed by dividing by the square of the material thickness (t, unit: mm) by measuring the time (t _s , unit: second) at which hydrogen is generated in one direction of the steel sheet and hydrogen permeates to the other side of the steel sheet. It is represented by 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 fish scale defect rate is more than 50% when accelerated heat treatment at 200℃ for 24 hours after enamel treatment, and there is a problem in using it as a stable enamel product. For this, it is necessary to manage the hydrogen permeation ratio to 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 satisfying the above-described composition is prepared. The molten steel whose components are adjusted to the above-described composition in the steelmaking step may be manufactured as slabs through continuous casting.

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

At this time, the slab heating temperature may be 1150 to 1250°C. If the slab heating temperature is too low, the rolling load increases rapidly during subsequent hot rolling, which can deteriorate workability. On the other hand, if the slab heating temperature is too high, it not only increases energy cost, but also increases the amount of surface scale, which can lead to material loss.

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

At this time, the volume fraction of carbide of the hot rolled steel sheet may be 2.5 to 7.0%. The description of the carbide volume fraction is mentioned and is 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, the crystal grains rapidly progress and tend 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 falls and uniform hot rolling may not be performed over the entire thickness, and thus impact toughness due to grain growth may appear. More specifically, the finish hot rolling temperature may be 860 to 890°C.

Thereafter, the hot-rolled steel sheet produced after the hot rolling is finished undergoes a winding process. More specifically, it may be a hot rolling 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 temperature of the hot-rolled coil is too low, temperature fluctuations in the width direction occur in the cooling and maintaining process, and as the formation of low-temperature precipitates varies, it not only causes material deviation, but also adversely affects enamel properties. On the other hand, if the coiling temperature is too high, corrosion resistance is lowered as the bulking of the nitride proceeds, and a problem of deteriorating processability is caused by coarsening of the tissue in the final product. More specifically, the coiling temperature may be 650 to 710°C.

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

Thereafter, 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%. When the cold reduction rate is too small, strength is increased by locally remaining unrecrystallized grains due to low recrystallization driving force in a subsequent heat treatment process, but there is a problem in that workability is remarkably deteriorated. In addition, as the crushing capacity of the carbide formed in the hot rolling step decreases, the site capable of storing hydrogen decreases, so it is difficult to secure the fish scale resistance of the enamel product, and considering the thickness of the final product, the thickness of the hot rolled steel sheet must be lowered. There is also a problem that the rolling workability is deteriorated. On the other hand, if the cold rolling reduction ratio is too high, the material is hardened to deteriorate workability, and the load of the rolling mill is increased, thereby deteriorating the cold rolling workability. More specifically, the cold rolling reduction rate may be 65 to 88%.

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

At this time, the heat treatment temperature may be 700 to 850 ℃. If the continuous annealing temperature is too low, there is a problem in that workability is significantly deteriorated as deformation formed by cold rolling is not sufficiently removed. On the other hand, if the heat treatment temperature is too high, softening due to a decrease in high temperature strength causes plate fracture, which significantly reduces the mailing performance of the operation. More specifically, the annealing temperature may be 750 to 840°C.

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

In addition, after the step of annealing the cold rolled steel sheet may further include a step of temper rolling the heat-treated steel sheet. Although the shape of the material can be controlled and the desired surface roughness can be obtained through temper rolling, if the temper rolling reduction is too high, the material hardens and the workability deteriorates due to work hardening, so temper rolling can be applied at a rolling reduction of 3% or less. have. Specifically, the rolling reduction of temper rolling may be 0.3 to 2.5%.

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

[Example]

Experiment and evaluation

Table 1 shows the chemical composition of the steels. The balance contains Fe and unavoidable impurities. The slab via the converter-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 4.0 mm. The hot-rolled specimen was cold-rolled after removing the oxide film on the surface through pickling. After the cold rolling was completed, the specimen was processed into an enameled specimen to investigate the enamel property and a tensile specimen for mechanical property analysis, followed by continuous annealing heat treatment.

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

Table 3 shows enamel properties by manufacturing conditions of the material secured through the above process.

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

Carbide fractions of the present invention steel and comparative steel were obtained by obtaining an image of 20 fields of view at 500 times magnification with an optical microscope, and then obtaining this as the fraction of carbide for the entire field of view using an image analyzer.

The enameled specimen was cut to an appropriate size to meet the test purpose, and after the heat treatment was completed, the enameled specimen was completely degreased and then applied with a check frit relatively vulnerable to fish scale defects and maintained at 300°C for 10 minutes. To remove moisture. The dried specimen was subjected to a firing treatment at 830°C for 20 minutes, and then cooled to room temperature. At this time, the atmosphere condition of the firing furnace was a dew point temperature of 30°C, and harsh conditions that caused fish scale defects to occur were selected. The enamel-finished specimens were subjected to fish scale acceleration experiments maintained in an oven at 200° C. for 24 hours.

The presence or absence of occurrence of fishscale defects after the fishscale acceleration treatment was visually observed, and was indicated as "O" when no fishscale defects occurred and "X" when it occurred.

The enamel adhesion index, which evaluated the adhesion between the steel sheet and the glaze, was determined by applying a certain load to the enamel layer with a steel ball as defined in the American Society for Testing and Materials, ASTM C313-78, and then evaluating the degree of energization of the enamel layer. The degree of dropout was indicated by indexing. In the case of the adhesiveness index in the present invention, the goal was to secure at least 90% in terms of securing stability during use of the enamel layer.

Bubble defects were determined by three stages of "O" excellent, "Δ" normal, and "X" defective, respectively, by visually observing the surface of the enamel layer for a specimen kept in an oven at 200°C for 24 hours after the enamel treatment.

The hydrogen permeation ratio is an index for evaluating the resistance to fish scale, a fatal defect of enamel steel, and generates hydrogen in one direction of the steel sheet according to the method registered in the European standard, EN 10209 (2013). The time (t _s , unit seconds) that is transmitted, is measured and divided by the square of the material thickness (t, unit mm), expressed as t _s /t ² (unit seconds/mm ² ). In this case, as described above, it was necessary to secure at least 950 seconds/mm ² to obtain stable properties of the enamel steel.

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

From here,

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

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

It is defined as F = ([Al]/[N]) x (D _cementite /[C]),

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

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

[S] is the weight% of S divided by the atomic weight of S (32),

[C] is the weight% of C divided by the atomic weight of C (12),

[Si] is the weight% of Si divided by the atomic weight of Si (28),

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

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

In addition, D _void is an area fraction (%) of micropores in a cold rolled steel sheet,

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

Experiment result

As can be seen from Tables 1 to 3, Inventive Examples 1 to 9 satisfying both alloy composition, microstructure properties, and manufacturing conditions have good mailing properties, and carbides and micropore fractions and associated indices are used in the present invention. Satisfies the limited range of, and not only did fish scale and bubble defects occur under severe processing conditions, but also satisfied the enamel adhesion index of 90% or more, hydrogen permeation ratio of 950 seconds/mm ² or more, and pickling loss of 10 to 40gr/m ² Thus, it was possible to secure the properties targeted by the present invention.

On the other hand, although the chemical composition suggested in the present invention was satisfactory, it was found that Comparative Examples 1 to 4, which did not satisfy the range of manufacturing conditions, could not secure the target characteristics. That is, as shown in Table 3, there was a problem in that mailability was poor (Comparative Examples 1, 3 and 4), and 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 confirmed that a bubble defect or a fish scale defect occurred after the enamel treatment, so that the targeted properties could not be secured overall.

Comparative Examples 5 to 8 are the cases in which the manufacturing conditions suggested in the present invention were satisfied but the alloy composition was not satisfied. In Comparative Examples 5 to 8, most of the target carbides and micropore fractions of the present invention, hydrogen permeability ratio, enamel adhesion index, pickling loss, etc. were not satisfied, and fish scale and bubble defects also occurred in visual observation after enamel treatment.

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

Claims (11)

1. In weight percent, 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% Below (excluding 0%), the balance includes Fe and unavoidable impurities,

   The micropore area fraction is 0.3-0.8%,

   Cold-rolled steel sheet for enamel that satisfies the enamel defect relationship index (D) represented by the following [Relational Formula 1], which satisfies 0.45 to 4.50.

   [Relationship 1]

   

   (In the above equation 1, [Mn], [Cu], [S], [C] is the value by dividing the weight percent of each element by the atomic weight of each element, D _void is the area fraction of micro-pore in the cold rolled steel sheet ( %).)
2. According to claim 1,

   P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%) and Ti: 0.001% or less (excluding 0%), cold rolled steel sheet for enamels further comprising one or more of.
3. According to claim 1,

   The cold rolled steel sheet for enameling, wherein the pickling loss of the cold rolled steel sheet is 10 to 40 gr/m ² .
4. According to claim 1,

   The cold rolled steel sheet for enameling, wherein the cold rolled steel sheet has a hydrogen permeability ratio of 950 seconds/mm ² or more.
5. In weight percent, 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% Hereinafter (excluding 0%), preparing a slab containing the residual Fe and unavoidable impurities;

   Heating the slab;

   Hot-rolling the heated slab to produce a hot-rolled steel sheet having a carbide volume fraction of 2.5 to 7.0%;

   Winding the hot rolled steel sheet;

   Cold-rolling the wound hot-rolled steel sheet at a reduction ratio of 60 to 90% to produce a cold-rolled steel sheet; And

   Annealing the cold-rolled steel sheet;

   Including,

   A method of manufacturing a cold-rolled steel sheet for enamel that satisfies 0.007 to 0.185 of the adhesion relationship index (A) represented by the following [Relational Formula 2].

   [Relationship 2]

   

   (In the relational expression 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.)
6. The method of claim 5,

   Method of manufacturing a cold-rolled steel sheet for enamel satisfying a workability relationship index (F) expressed by the following [Relational Formula 3] meets 3500 to 7000.

   [Relationship 3]

   

   (In the relational expression 3, [Al], [N], and [C] are the values 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.)
7. The method of claim 5,

   The slab of P: 0.002 to 0.02%, O: 0.002% or less (excluding 0%) and Ti: 0.001% or less (excluding 0%) further comprising one or more of the cold rolled steel sheet for enamel Manufacturing method.
8. The method of claim 5,

   In the step of manufacturing the hot-rolled steel sheet; In,

   Method of manufacturing a cold-rolled steel sheet for enamel hot rolling the heated slab at a rolling temperature of 850 ℃ to 900 ℃.
9. The method of claim 5,

   In the step of winding the hot-rolled steel sheet; In,

   Method of manufacturing a cold-rolled steel sheet for enamel winding the hot-rolled steel sheet at 640 ℃ to 750 ℃.
10. The method of claim 5,

    In the annealing heat treatment of the cold-rolled steel sheet; In,

    A method of manufacturing a cold-rolled steel sheet for enameling, wherein the cold-rolled steel sheet is annealed at 700°C to 850°C.
11. The method of claim 5,

    In the annealing heat treatment of the cold rolled steel sheet; In,

    Method of manufacturing a cold-rolled steel sheet for enamel to hold the cold-rolled steel sheet for 30 seconds or more.

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