WO2019039824A1 - Cold-rolled steel sheet for enameling and manufacturing method therefor - Google Patents

Abstract

The present invention relates to a cold-rolled steel sheet for enameling and a manufacturing method therefor, the cold-rolled steel sheet comprising (in wt%), C (more than 0% and not more than 0.0015%), Mn (0.2 to 0.4%), Al (more than 0% and not more than 0.001%), P (more than 0% and not more than 0.02%), S (more than 0% and not more than 0.02%), Cr (0.05 to 0.1%), Nb (0.03 to 0.05%), N (more than 0% and not more than 0.002%), O (0.02 to 0.04%), and the balance Fe and other inevitable impurities, wherein the cold-rolled steel sheet comprises a Mn-Nb-Cr composite oxide and can improve moldability and fish-scaling resistance.

Description

Cold rolled steel sheet for enamel and manufacturing method thereof

The present invention relates to a cold rolled steel sheet for enamel and a method of manufacturing the same, and more particularly, to a cold rolled steel sheet for enamel which can improve formability and fish scale resistance and a method for manufacturing the same.

The enamel steel sheet is a kind of surface treatment product which is improved in corrosion resistance, weather resistance and heat resistance by applying vitreous glaze on a cold-rolled steel sheet as a base steel sheet and firing at a high temperature. Such an enamel steel sheet is mainly used for building exterior, home appliance, .

The conventional steel sheet for enamel has been known to prevent fish scale defects, which are known to be the most fatal defects in enamel products, through the decarburization annealing process or the like, or to improve the formability. However, this method has resulted in an increase in the cost of the product.

In recent years, in order to reduce the manufacturing cost, Ti is added and a steel sheet for enamel is manufactured using a continuous annealing process. However, the steel for enhancing the enamel has a high recrystallization temperature and must be annealed at a high temperature, resulting in low productivity and high manufacturing cost.

Particularly, Ti-based enamel steel has a limited number of castings due to the occurrence of TiN and intermetallic compounds during the casting process due to Ti addition, resulting in clogging of the nozzles, resulting in increased production cost and production load. Further, when a large amount of Ti is added, there is a problem that Ti adversely affects adhesion between the enamel steel sheet and the ceramic glaze layer.

Also, as a method for securing the fish scale property, a high acid content steel in which oxides are artificially produced in the steel by increasing the content of oxygen in the steel and the hydrogen absorption capacity is secured by using the oxide has been proposed. However, the high acid content of the high-oxygen steel has a high oxygen content, so there is a risk that the refractory loss is extremely high and there is a risk of operation such as the leakage of molten steel, the performance is low, and the manufacturing cost of the molten steel is high.

(Prior art document 1) KR2013-0073424 A

(Prior art document 2) KR2016-0041967A

The present invention provides a cold rolled steel sheet for an enamel capable of suppressing surface defects and improving moldability and a method for producing the same.

The present invention provides a cold rolled steel sheet for enamel which does not cause crack defects during processing by enhancing hydrogen absorption capability and a method for producing the same.

The cold rolled steel sheet for enamel according to the present invention contains, by weight%, C: more than 0 wt% to 0.0015 wt%, Mn: 0.2 to 0.4 wt%, Al: more than 0 wt% to 0.001 wt% 0.02 to 0.04% by weight, O: not more than 0.02% by weight, S: more than 0% by weight and not more than 0.02% by weight, Cr: 0.05 to 0.1% And the balance of Fe and other unavoidable impurities, and may include a Mn-Nb-Cr composite oxide.

The Mn-Nb-Cr composite oxide may have a size of 3 m or more and 100 / mm < 2 > or more.

The ratio (K1 / K2) of the Mn and Cr fraction (K1) to the Nb fraction (K2) in the Mn-Nb-Cr composite oxide may be 0.1 to 1.0.

The method for manufacturing a cold rolled steel sheet for enamel according to the present invention comprises the steps of: preparing a molten steel containing a Mn-Nb-Cr composite oxide; Casting a cast steel using the molten steel; A step of rolling the cast steel to produce a steel sheet; And annealing the steel sheet.

The process for producing the molten steel includes a process of deoxidizing molten steel for which the refining of the converter has been completed; Cr and Nb are added to the deoxidized molten steel to form a Mn-Nb-Cr composite oxide. The process for forming the Mn-Nb-Cr composite oxide comprises sequentially forming the Mn, Cr, .

And wire-feeding MnO when Mn is added.

C: not more than 0.0015 wt%, Mn: 0.2 to 0.4 wt%, Al: not less than 0 wt% and not more than 0.001 wt%, P: not more than 0 wt% and not more than 0.02 wt% % O, 0.02 to 0.04% by weight of O, 0 to 0.02% by weight of S, 0.05 to 0.1% by weight of Cr, 0.03 to 0.05% Molten steel containing the remaining Fe and other unavoidable impurities can be produced.

And a step of heat-treating the cast steel at a temperature of 1200 to 1300 ° C for 1 hour before the step of manufacturing the steel sheet.

The process for producing the steel sheet may include the steps of hot rolling the cast steel and finishing the hot rolling at a temperature of 800 to 900 ° C; Rolling the hot-rolled steel sheet at a temperature of 550 to 700 ° C; And cold rolling the hot-rolled steel sheet at a reduction ratio of 75 to 80%.

The annealing may be performed at 800 to 900 ° C. for 20 seconds or more.

According to the present invention, by adding niobium (Nb) to solidify solid carbon (C) and solid nitrogen (N) in steel in the form of NbC and NbN, the workability of the cold rolled steel sheet for enamel can be improved. In addition, carbon and nitrogen in the steel are removed as much as possible to minimize dislocation of potential movement in the base material, minimization of high-priced niobium input can be ensured, economical efficiency can be secured, and moldability can be improved.

In addition, it is possible to prevent the occurrence of fish scale defects due to hydrogen elution by forming a large amount of fine holes capable of adsorbing hydrogen in the base material to improve the hydrogen occlusion ability. At this time, the Mn-Nb-Cr composite oxide is formed in the base material to improve the fracture property, thereby forming a large amount of micropores.

1 is a graph showing the relationship between the amount of Nb and the steel formability.

FIG. 2 is a graph showing the relationship between the amount of Nb and the fish scale. FIG.

3 is a scanning electron micrograph of a Mn-Nb-Cr composite oxide according to an embodiment of the present invention.

FIG. 4 is an optical microscope photograph showing a state where the Mn-Nb-Cr composite oxide contained in the cold rolled steel sheet for enamel according to the embodiment of the present invention is broken.

5 is a flow chart showing a method of forming a Mn-Nb-Cr composite oxide in a method of manufacturing a cold rolled steel sheet for enamel according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know. In the description, the same components are denoted by the same reference numerals, and the drawings are partially exaggerated in size to accurately describe the embodiments of the present invention, and the same reference numerals denote the same elements in the drawings.

In order to ensure the formability of the enamel steel sheet, an extremely low carbon steel cold rolled steel sheet having a very low carbon (C) in the steel is used as a material. In addition, in order to secure the workability of the enamel steel sheet, the addition of a large amount of Ti in order to solidify the carbon (C) and nitrogen (N) remaining in the steelmaking process as the carbon / nitride of TiC and TiN in the base material, It forms the main part of the enamel steel sheet to be used. However, when the TiN mixed in the molten steel is exposed to the surface of the enamel steel sheet, bubble-like blister defects are caused on the surface of the enamel steel sheet. Further, Ti added in a large amount inhibits adhesion between the enamel steel sheet and the ceramic glaze layer. In order to improve the problem caused by Ti, the enamel steel system utilizing an oxide not containing Ti is poor in workability as it contains a large amount of oxygen in the material.

In addition, fish scale defects, which are one of the most fatal defects of the enamel steel sheet, are caused by the fact that the hydrogen dissolved in the steel during the manufacturing process of the enamel steel sheet is discharged to the surface of the steel due to the difference in solubility of the internal hydrogen ≪ / RTI > Therefore, in order to prevent fish scale defects, it is necessary to form a large amount of space in the steel sheet for adsorbing hydrogen dissolved in the steel. In general, enamel steel using existing precipitates utilizes FeP, TiS, TiN, BN, Cementite, etc. as a hydrogen occlusion source. However, such a precipitation-type hydrogen occlusion source temporarily immobilizes hydrogen in the atomic state of H ⁺ around the grain boundaries between the precipitate and the base material in which the atomic arrangement of the precipitate and the base material is not uniform. Therefore, Which causes a variation in the hydrogen occlusion capability and causes a quality variation.

Accordingly, the present invention solves the problem that the solid solution C and the solid solution N in the steel sheet are dissolved in the form of NbC and NbN through addition of Nb using Nb and the Mn-Nb-Cr composite oxide is formed in the steel sheet, This excellent cold rolled steel sheet for enamel can be provided.

A cold rolled steel sheet for enamel according to an embodiment of the present invention includes C: not less than 0% by weight, not more than 0.0015% by weight, Mn: 0.2 to 0.4% by weight, Al: not less than 0% 0.05 to 0.1% by weight of Cr, 0.03 to 0.05% by weight of Nb, 0 to 0.002% by weight of N, 0.002 to 0% by weight of O, 0.02 to 50% by weight of O, 0.04% by weight, the balance Fe and other unavoidable impurities, and may include a Mn-Nb-Cr composite oxide.

Hereinafter, the reason for adding each alloy component constituting the steel composition, which is one of the main features of the present invention, and the appropriate content range thereof will be described. Here, the content of each component means weight%.

FIG. 1 is a graph showing the relationship between the Nb input amount and the formability of the steel material, and FIG. 2 is a graph showing the relationship between the Nb input amount and the fish scale of the steel material.

C: more than 0 wt% to less than 0.0015 wt%

When C is added in an amount exceeding 0.0015% by weight, the amount of carbon employed in the steel is large, which hinders development of aggregate structure during annealing and lowers the formability. When aging occurs and processing is performed after a long period after production, surface defects Stretcher strain defect) is likely to occur, the upper limit value of C is limited to 0.0015 wt%. If C is more than 0.0015 wt%, the addition amount of Nb, which is an expensive element, must be increased, so that the economic efficiency is low and the upward of C is made 0.0015 wt%.

Mn: 0.2 to 0.4 wt%

Mn can be added to secure the strength of the steel sheet and to prevent hot shortness by precipitating solid sulfur in manganese sulfide in the steel. Therefore, when the content of manganese is less than 0.2% by weight, the possibility of occurrence of the heat-induced brittleness is high. Therefore, the lower limit is set to 0.2% by weight, and if the content of manganese exceeds 0.4% by weight, the moldability is greatly lowered, 0.4% by weight. Mn is an element that forms an initial Mn oxide (MnO) at the time of forming the Mn-Nb-Cr composite oxide. When Mn is less than 0.2% by weight, Mn which contributes to Mn oxide formation is insufficient and Mn oxide is not formed smoothly. To 0.2% by weight.

Nb: 0.03 to 0.05 wt%

Nb is the most important element in the present invention, and a large amount of NbC and NbN precipitates are precipitated to secure moldability and added for the purpose of Mn-Nb-Cr composite oxide formation. At this time, Nb may be included at a minimum of 0.03 wt% so that the difference between Mn-Cr and Nb in the Mn-Nb-Cr composite oxide is reduced.

When Nb is less than 0.03% by weight, the fish scale resistance can be ensured, but the formability may be deteriorated. When Nb is more than 0.05% by weight, workability is improved by adding Nb, And the fish scale property can be reduced.

In order to derive the optimum range of Nb as described above, the results of investigation of the formability and the fish scale according to the amount of Nb added are shown in FIG. 1 and FIG.

Referring to FIG. 1, it can be seen that the formability of the steel improves as the Nb content in the steel increases. However, since Nb is an expensive element, it is important from an industrial point of view to secure moldability by injecting Nb at a minimum in order to secure economical efficiency.

Referring to FIG. 2, it can be seen that as the amount of Nb added increases, the intrinsic fish scale becomes dull. This is because the amount of Nb having an affinity with oxygen is increased, oxygen of the steel is reduced, and the complex oxide formation amount is reduced, so that the microvoids are reduced and the fish scale property is weakened. Therefore, the addition amount of Nb is limited to 0.03 to 0.05% by weight, which is a range in which moldability and fish scale resistance can be secured at the same time.

Cr: 0.05 to 0.1 wt%

Cr is added next to Nb in the present invention as an important element for the purpose of improving the fish scale property. It is an essential element that binds with Mn-Nb oxide to form a large amount of Mn-Nb-Cr composite oxide. In order to secure the fish scale property, Cr should be added in an amount of 0.05 wt% or more, and when Cr is more than 0.1 wt%, the formability may be poor.

P: more than 0% by weight and not more than 0.02%

P is an element inhibiting the physical properties of steel. If it exceeds 0.02% by weight, the formability is significantly lowered, so the upper limit value is set to 0.02% by weight. However, since it is almost impossible to completely remove P from the steel, the lower limit is specified to be more than 0% by weight.

S: more than 0 wt% to less than 0.02 wt%

S is generally known as an element which hinders the physical properties of steel. When it exceeds 0.02% by weight, the ductility is greatly lowered and the upper limit value is limited to 0.02% by weight since it is likely to cause brittleness due to sulfur. Since the sulfide formed by S is formed by adhering to the composite oxide, it is preferable to reduce the content of S as much as possible because it inhibits the formation of micro-voids formed by crushing oxide after rolling and fills the micropores formed . However, since it is almost impossible to completely remove S in the steel, the lower limit is specified to be more than 0% by weight.

Al: more than 0% by weight and not more than 0.001% by weight

Since Al is used as a strong deoxidizing agent for removing oxygen in the molten steel, the addition amount is limited to 0.001 wt% or less. When the Al content exceeds 0.001% by weight, the Al-Nb composite oxide is formed, which is not Mn-Nb-Cr composite oxide, and the amount of micro-voids is remarkably reduced, . However, it is almost impossible to completely remove Al in the steel, so that the lower limit is specified as 0% by weight or more.

N: more than 0 wt% to less than 0.002 wt%

As the amount of N added increases, the formability becomes poor and the amount of Nb, which is an expensive alloy element, must be increased, so the upper limit value is limited to 0.002 wt%. However, since it is almost impossible to completely remove N in the steel, the lower limit is specified to be more than 0% by weight.

O: 0.02 to 0.04 wt%

O, the fish scale property was better as the amount was larger, but the upper limit value was limited to 0.04% by weight because the moldability was deteriorated when the content exceeded 0.04% by weight. When the content of O is less than 0.02% by weight, O for forming the Mn-Nb-Cr composite oxide is insufficient and the fish scale resistance is weakened to make the lower limit 0.02% by weight.

The composition of the enamel steel sheet according to the present invention having the same is, hot and cold rolling, when Mn-Nb-Cr complex oxides are crushed as fine public (micro-void) to a quantity formed state of hydrogen than the atomic state of H ⁺ molecules H ₂ gas to permanently store hydrogen, thereby making it possible to prevent fish scale defects.

In addition, since Mn-Nb-Cr composite oxide stable at high temperature is used as a hydrogen storage source, it is hardly influenced by the conditions of hot and cold rolling control, and quality deviation can be reduced.

In the case of the present invention, in order to increase the crushability of the composite oxide during hot rolling and cold rolling, a multi-element complex oxide can be formed by utilizing Mn, Nb and Cr in the steel. Generally, in the case of an oxide having a multi-component composition, it is excellent in the ability to be fractured under the same pressing force as compared with a single component oxide. This is because, when a multi-component system is formed, the composition is unevenly formed in the oxide due to the difference in affinity with oxygen. That is, the multi-component oxide is formed by the reduction reaction of the oxide at a high temperature of about 1600 ° C in a very short time, and thus the time for homogenizing the entire oxide is not sufficient. Therefore, oxides of heterogeneous composition formed in a multi-component state have different hardnesses depending on the composition of the oxides, so that they can be easily broken in hot and cold rolling compared to oxides uniformly formed in a single component system. A large amount of three-dimensional microvoids can be formed.

In the case of the Mn-Nb-Cr composite oxide of 3 탆 or more used in the present invention, the difference of the constituents in the composite inclusion causes fracture of the ferrite and Mn-Nb-Cr composite oxide having different strengths during hot rolling and cold rolling, ) Is formed in large quantity and is utilized as a hydrogen occlusion source.

FIG. 3 is a scanning electron micrograph of a Mn-Nb-Cr composite oxide according to an embodiment of the present invention, and FIG. 4 is a cross- In the same manner as in Example 1. FIG.

FIG. 3 shows the Mn-Nb-Cr composite oxide formed in the casting process, and the Mn-Nb-Cr composite oxide is formed in a lump shape. The Mn-Nb-Cr composite oxide is crushed in the course of rolling the slab and is formed into a flat plate as shown in FIG. The Mn-Nb-Cr composite oxide is broken and forms micropores around the micropores, and the micropores thus formed can be utilized as a space for storing hydrogen in the steel sheet.

On the other hand, the cold rolled steel sheet for enamel according to the embodiment of the present invention is characterized in that a nonmetallic inclusion having a particle size of 1 탆 or more, which is detected when an area of 500 mm 2 is observed with a scanning electron microscope, is determined by energy dispersive spectrometry (EDS) As a result, the Mn-Nb-Cr composite oxide inclusion was found to be a composite oxide containing Mn, Nb and Cr at the same time, and Mn and Cr and Nb fractions of inclusions having a particle diameter of 3 탆 or more, Can be calculated. Here, the ratio of Mn and Cr to Nb (K1 / K2) of the Mn-Nb-Cr composite oxide having a size of 3 탆 or more, Mn and Cr fraction is K1 and the fraction of Nb is K2, Mn And a value indicating the distribution of Cr and Nb, and the value may be 0.1 to 1.0. When K1 / K2 is less than 0.1, the size of the composite oxide is relatively small and the composition of Nb in the composite oxide is mostly occupied, so that the fracture between the interface of the composite oxide and the iron oxide during rolling does not occur, Fish scale property can not be secured. On the other hand, when K1 / K2 is more than 1.0, the content of Nb in the composite oxide is decreased, and the fracture ability is decreased, so that the micropores can not be sufficiently secured.

Further, the number of Mn-Nb-Cr composite oxides can be 100 or more per 1 mm < 2 > When the number of the Mn-Nb-Cr composite oxides is less than 100 per 1 mm 2, micro-pores can not be sufficiently secured, and the fish scale resistance is deteriorated.

Hereinafter, a method for manufacturing a rolled steel sheet for enamel according to an embodiment of the present invention will be described.

5 is a flowchart showing a method of forming a Mn-Nb-Cr composite oxide in a method of manufacturing a cold rolled steel sheet for enamel according to an embodiment of the present invention.

A method for manufacturing a rolled steel sheet for enamel according to an embodiment of the present invention comprises the steps of: preparing molten steel containing a Mn-Nb-Cr composite oxide; casting a cast steel; . ≪ / RTI >

The process of producing molten steel includes a process of deoxidizing molten steel that has been refined through decarburization using a vacuum degassing facility and a process of forming a Mn-Nb-Cr composite oxide by injecting Mn, Cr, and Nb into the deoxidized molten steel . ≪ / RTI >

Referring to FIG. 5, in the method of forming the Mn-Nb-Cr composite oxide, Mn is first added to the deoxidized molten steel to react with oxygen remaining in the molten steel to form MnO (S10). At this time, MnO can be wire-fed to form a large amount of MnO and be uniformly dispersed in molten steel. Thereafter, Cr is added to reduce MnO to form Mn-Cr-O (S20). Then, Nb is added to form a Mn-Nb-Cr multi-component complex oxide (S30). The reason for sequentially injecting Mn, Cr and Nb in this manner is to add a low oxygen-affinity component to form a large amount of Mn-Nb-Cr composite oxide.

Since the multi-component Mn-Nb-Cr composite oxide thus formed is heavier than the single-component oxide to be applied, it can not be separated by floating even if it is formed in the molten steel stage, so that it can be remained in molten steel in comparison with general oxide- Further, the present invention is characterized in that Al, which has a very high affinity with oxygen, is not added so that a large amount of the Mn-Nb-Cr composite oxide can be formed. In addition, since the amount of Ti can be omitted, it is possible to solve the problem caused by a large amount of Ti added to the known Ti based enamel steel.

C: not more than 0.0015 wt%, Mn: 0.2 to 0.4 wt%, Al: more than 0 wt% to not more than 0.001 wt%, P: not more than 0 wt% to not more than 0.02 wt% S: not more than 0 wt%, not more than 0.02 wt%, Cr: not more than 0.05 wt%, not more than 0.1 wt%, Nb: not more than 0.03 wt% And other unavoidable impurities.

Once molten steel is produced, the molten steel may be transferred to a continuous casting facility to cast the cast. The cast steel cast using the molten steel has the same composition as the molten steel.

When the cast steel is cast, it is charged into a heating furnace and then heat-treated at a temperature of 1200 to 1300 ° C for 1 hour.

When the heat treatment of the cast steel is completed, the steel sheet can be manufactured by rolling the cast steel.

The steel sheet can be manufactured by hot rolling the cast steel heated in the heating furnace. At this time, the finishing temperature of the hot rolling may be about 800 to 900 占 폚. When the finishing rolling temperature is lower than 800 ° C, the rolling resistance becomes too large during rolling and productivity deteriorates. On the other hand, when the finish rolling temperature exceeds 900 ° C, the oxide layer of the thermal expansion material is excessively grown and the yield is lowered. Therefore, it is preferable to perform finish rolling at 800 to 900 占 폚.

The steel sheet produced by hot rolling the cast steel is rolled up using a winder, and the coiling temperature may be about 550 to 700 ° C. At this time, if the coiling temperature is lower than 550 캜, the grain size formed in the steel sheet becomes small, and the formability may be deteriorated. On the other hand, when the temperature exceeds 700 ° C, an excessively hot-rolled oxide layer occurs. Therefore, the coiling temperature is preferably limited to 550 to 700 占 폚.

Thereafter, the hot-rolled steel sheet is subjected to pickling treatment to remove the oxide film formed on the surface thereof, and cold rolling is performed at a cold rolling reduction rate in the range of 75 to 80%. At this time, when the cold rolling reduction is lower than the suggested range, the development of the recrystallized aggregate structure is low and the formability is lowered, and the Mn-Nb-Cr composite oxide is not properly crushed and the micropores in the steel sheet are reduced. On the other hand, when the cold reduction ratio is higher than the range suggested, the ductility is lowered, and the micropores formed by crushing the Mn-Nb-Cr composite oxide are squeezed to reduce the absolute amount of the micropores. .

After the steel sheet is manufactured through hot rolling and cold rolling, the steel sheet is charged into the annealing furnace and continuous annealing is performed at a temperature of about 800 to 900 DEG C for 20 seconds or more.

The continuous annealing process is for imparting ductility and formability to the cold-rolled steel sheet. When the temperature is set at less than 800 ° C, recrystallization is not completed and it is difficult to secure ductility and formability. On the other hand, Requires too much heating equipment in the field, it is hard to heat up realistically, and the durability of the roll due to excessive high temperature is deteriorated. Therefore, it is preferable that the annealing is performed in the range of 800 to 900 DEG C in the continuous annealing. Further, even if the annealing time is too short, recrystallization is not completed, so that it is difficult to secure ductility and moldability

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the present invention by way of illustration and not to limit the scope of the present invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

A casting having the composition shown in Table 1 below was carried out by performing the converter-secondary refining-performance process. At this time, in the secondary refining process, deoxidation through decarburization was performed using a vacuum degassing facility, and then Mn, Cr and Nb were sequentially charged to form a Mn-Nb-Cr composite oxide. Thereafter, the cast steel was maintained in a heating furnace at 1250 DEG C for 1 hour and then subjected to hot rolling at a finishing hot rolling temperature of 900 DEG C, a coiling temperature of 650 DEG C, and a final thickness of 3.2 mm. The hot rolled steel sheet produced by hot rolling the cast steel was subjected to pickling treatment to remove the oxide film on the surface, and then cold rolling was performed. At this time, the cold reduction ratio was set to 78%, and a cold-rolled steel sheet having a thickness of 0.8 mm was produced.

	C (wt%)	Mn (wt%)	Al (wt%)	P (wt%)	S (wt%)	Si (wt%)	Nb (wt%)	Cr (wt%)	N (ppm)	O (ppm)	Oxide composition
Example 1	0.0013	0.35	<0.0005>	0.0101	0.015	<0.005>	0.031	0.07	17	373	Mn-Nb-Cr
Example 2	0.0012	0.26	<0.0005>	0.0156	0.013	<0.005>	0.039	0.08	16	320	Mn-Nb-Cr
Example 3	0.0010	0.23	<0.0005>	0.0121	0.012	<0.005>	0.042	0.10	15	281	Mn-Nb-Cr
Example 4	0.0009	0.26	<0.0005>	0.0184	0.015	<0.005>	0.047	0.06	17	345	Mn-Nb-Cr
Example 5	0.0010	0.27	<0.0005>	0.0156	0.017	<0.005>	0.034	0.09	18	292	Mn-Nb-Cr
Example 6	0.0012	0.25	<0.0005>	0.0135	0.013	<0.005>	0.036	0.06	16	266	Mn-Nb-Cr
Example 7	0.0013	0.31	&Lt; 0.05 &	0.0185	0.012	<0.005>	0.043	0.08	17	28	Al-Nb
Example 8	0.0011	0.29	<0.001>	0.0082	0.018	<0.005>	0.013	0.07	16	282	Mn-Nb-Cr
Example 9	0.0008	0.25	<0.001>	0.0123	0.015	<0.005>	0.082	0.06	19	167	Nb-Mn-Cr
Example 10	0.0042	0.35	<0.001>	0.0161	0.016	<0.005>	0.032	0.09	18	321	Mn-Nb-Cr

Then, cold rolled steel sheets were processed to investigate the characteristics of enamel and mechanical properties, and enamel treated specimens and tensile specimens were prepared and then subjected to continuous annealing.

The enamel treated specimens were cut into cold-rolled steel sheets with a size of 70 mm x 150 mm, and then subjected to continuous annealing at 830 ° C. After completion of the annealing, the substrate was thoroughly degreased, and then the lower oil glaze was applied and dried at 200 ° C for 10 minutes to completely remove water. The dried specimens were held at 830 ° C for 7 minutes, baked, and then cooled to room temperature. The specimens treated with Hae Yu enamel were again applied with an oily glaze, and then dried at 200 ° C for 10 minutes to completely remove moisture. The dried specimens were held at 800 ° C for 7 minutes, baked, and then air-cooled. At this time, the atmospheric conditions of the firing furnace were set at a dew point temperature of 30 占 폚 to apply harsh conditions in which fish scale defects were most likely to occur. After the enamel treatment was completed, the specimen was maintained at 200 ° C for 20 hours to accelerate the fish scale.

Thereafter, the number of fish scale defects formed on the specimen was visually observed, and the formability was evaluated in five steps of very good, excellent, normal, poor and very poor after measuring the R bar value.

When an area of 500 mm 2 was observed using an electron microscope, the non-metallic inclusions having a particle diameter of 1 탆 or more were analyzed by EDS to determine the inclusion complex oxide containing Mn, Nb and Cr at the same time, The distribution of the Mn-Nb-Cr composite oxide was analyzed by using inclusions having a particle diameter of 3 탆 or more when the diameter of the inclusions was converted into a circle. The number of Mn-Nb-Cr composite oxides was measured by an electron microscope using a point counting method with an image of 5000 to 40 fields, and then converted into 1 mm 2 by using an image analyzer. Respectively.

	Mn-Nb-Cr oxide distribution (K1 / K2)	Mn-Nb-Cr composite oxide (number / mm2)	Formability	Number of fish scale occurrences	compatibility
Example 1	0.56	280	Very good	0	○
Example 2	0.62	295	Very good	0	○
Example 3	0.26	320	Very good	0	○
Example 4	0.43	315	Very good	0	○
Example 5	0.83	268	Great	0	○
Example 6	0.39	324	Great	0	○
Example 7	0	0	Great	93 or more	×
Example 8	9.19	158	Very bad	23 or more	×
Example 9	0.11	198	Very good	46 or more	×
Example 10	0.23	270	Bad	0	×

Referring to the above Table 2, Examples 1 to 6 were able to secure the fish scale property because the number and size of the Mn-Nb-Cr composite oxide were within the range limited by the present invention, Chengdu was also very good.

On the other hand, in Example 7, Al ₂ O ₃ inclusions were formed due to a high Al content, and a part of Al-Nb, which is a microcrystalline inclusion containing Nb, was formed as shown in Table 1, And the fish scale was more than 93 in number. In Example 8, the number of Mn-Nb-Cr composite oxides falls within the range limited by the present invention. However, since the content of Nb is low and the K1 / K2 value indicating the distribution of Mn-Nb- Which is higher than the range. The formability was also very poor, and the fish ciches also occurred in 23 or more.

In the case of Example 9, although the distribution and the number of the Mn-Nb-Cr composite oxide fall within the range limited by the present invention, the Nb content is relatively high, so that the Mn-Nb- And the number of fish scales was more than 46 because there were few micro - vacancies capable of absorbing hydrogen. In Example 10, although the content of Nb was 0.032% by weight, the content of C which deteriorates the formability was as high as 0.0042% by weight and the formability was poor.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be construed as limited to the embodiments set forth herein. Those skilled in the art will appreciate that various modifications and equivalent embodiments may be possible. Accordingly, the technical scope of the present invention should be defined by the following claims.

The cold rolled steel sheet for enamel and the method of manufacturing the same according to the embodiment of the present invention are characterized in that niobium (Nb) is added to solidify solid carbon (C) and solid nitrogen (N) in steel in the form of NbC and NbN, A high-quality cold rolled steel sheet for an enamel improved in scaleability can be produced.

Claims (10)

1. Al: more than 0% by weight and not more than 0.001% by weight, P: not less than 0% by weight and not more than 0.02% by weight, S: not more than 0% by weight, Nb: more than 0% by weight and not more than 0.002% by weight, O: 0.02 to 0.04% by weight, and the balance of Fe and other unavoidable impurities are contained in an amount of not more than 0.02% by weight, Cr: 0.05-0.1% and,

   Mn-Nb-Cr composite oxide.
2. The method according to claim 1,

   The Mn-Nb-Cr composite oxide,

   A cold-rolled steel sheet for enamel having a size of not less than 3 μm and not smaller than 100 pieces / mm 2.
3. The method according to claim 1 or 2,

   Wherein a ratio (K1 / K2) of a fraction (K1) of Mn and Cr to a fraction (K2) of Nb in the Mn-Nb-Cr composite oxide is 0.1 to 1.0.
4. A process for producing a molten steel including a Mn-Nb-Cr composite oxide;

   Casting a cast steel using the molten steel;

   A step of rolling the cast steel to produce a steel sheet; And

   Annealing the steel sheet;

   Wherein the method comprises the steps of:
5. The method of claim 4,

   The process for producing the molten steel includes:

   A process of deoxidizing the molten steel after completion of the refining of the converter;

   And adding Mn, Cr and Nb to the deoxidized molten steel to form a Mn-Nb-Cr composite oxide,

   The process for forming the Mn-Nb-Cr composite oxide includes sequentially charging the Mn, Cr, and Nb.
6. The method of claim 5,

   And wire-feeding MnO when the Mn is added.
7. The method according to claim 5 or 6,

   In the process of manufacturing the molten steel,

   Al: more than 0% by weight and not more than 0.001% by weight, P: not less than 0% by weight and not more than 0.02% by weight, S: not more than 0% by weight, , Nb: more than 0% by weight and not more than 0.002% by weight, O: 0.02 to 0.04% by weight, and the balance of Fe and other unavoidable impurities are contained in an amount of not more than 0.02% by weight, Cr: 0.05-0.1% Wherein the molten steel is produced by a method comprising the steps of:
8. The method of claim 7,

   Prior to the process of manufacturing the steel sheet,

   And heat treating the cast steel at a temperature of 1200 to 1300 캜 for 1 hour.
9. The method of claim 8,

   The process of manufacturing the steel sheet comprises:

   Subjecting the cast steel to hot rolling and finishing the hot rolling at a temperature of 800 to 900 占 폚;

   Rolling the hot-rolled steel sheet at a temperature of 550 to 700 ° C; And

   Rolling the hot-rolled steel sheet at a reduction ratio of 75 to 80%;

   Wherein the method comprises the steps of:
10. The method of claim 9,

    The annealing process may include:

    At 800 to 900 占 폚 for at least 20 seconds.

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