KR20190077178A - High strength cold rolled steel sheet having excellent surface property and manufacturing method for the same - Google Patents

Abstract

According to an embodiment of the present invention, a high-strength cold rolling steel sheet comprises: 0.002-0.3 wt% of C; 0.1-2.0 wt% of Si; 0.005-1.5 wt% of sol.AI; 0.3-8.0 wt% of Mn; 0.1 wt% or less of P; 0.04 wt% or less of S; 0.02 wt% or less of N; and the remainder consisting of Fe and inevitable impurities. Therefore, a Fe-Ni alloying surface formed by Ni supplied from the outside can be formed within 0.1-5 μm in the longitudinal direction from a surface of the high-strength cold rolling steel sheet.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-strength cold-rolled steel sheet having excellent surface quality and a method of manufacturing the same,

The present invention relates to a high-strength cold-rolled steel sheet having excellent surface quality and a method of manufacturing the same, and more particularly to a high-strength cold-rolled steel sheet having improved surface characteristics and effectively improving surface quality and a method of manufacturing the same.

In order to secure safety and light weight, the application range of the high strength steel sheet among the materials used for manufacturing automobiles is gradually increasing. In order to satisfy high strength characteristics, precipitation strengthened steel and solid solution strengthened steel have been developed. Recently, TRIP (Transformation Induced Plasticity Steel) has been developed which satisfies both high strength and processability. In order to secure the strength and workability of these high-strength steels, various alloying elements are added in comparison with general steels, and a large amount of highly oxidizable elements such as Mn, Si, Al and Cr are added in large amounts.

Since the hot rolling process is performed at a high temperature and a high oxygen partial pressure atmosphere, the highly oxidative elements diffuse into the grain boundaries and the mouth of the hot-rolled steel sheet to easily form oxides, which is called a hot internal oxide. When pickling and cold rolling the hot-rolled steel sheet with the hot-rolled inner oxide formed thereon, cracking of the inner oxide by intergranular etching of the inner oxide by pickling and cold rolling may occur, and the adhesion of the inner oxide remaining on the cold- Can greatly weaken. When the cold-rolled steel sheet having residual internal oxides embrittle by pickling and cold rolling is subjected to the annealing heat treatment, the residual internal oxides are detached from the cold-rolled steel sheet to adhere to or accumulate in the rolls, and the subsequent internal oxides adhered to or accumulated in the rolls A phenomenon of striking on the surface of the steel sheet, that is, a dent defect, may be caused.

During the annealing, elements having high oxidizability such as Si and Mn diffuse to the surface side of the cold-rolled steel sheet in order to combine with a small amount of oxygen included in the annealing atmosphere and form a surface oxide on the surface of the cold-rolled steel sheet. These surface oxides are also continuously adhered to the rolls in the plate of the cold-rolled steel sheet, thereby causing dent defects on the surface of the subsequent cold-rolled steel plate.

In order to improve the dent defect, a method of completely removing the internal oxide by pickling can be used, but the pickling time is excessively required, and the productivity may be lowered. Further, in order to completely remove the inner oxide, a portion corresponding to a thickness of about 50 μm from the surface of the hot-rolled steel sheet must be removed. As a result, the rate of water loss may remarkably decrease, and the acid tax control due to over-acidation may be difficult.

Korean Patent Publication No. 10-2002-0052857 (published on Apr. 4, 2002)

According to one aspect of the present invention, there can be provided a high-strength cold-rolled steel sheet capable of effectively suppressing occurrence of dent defects by improving surface layer characteristics and a method of manufacturing the same.

The object of the present invention is not limited to the above description. Those of ordinary skill in the art will have no difficulty understanding the further subject of the present invention from the general context of this specification.

A high strength cold rolled steel sheet excellent in surface quality according to an embodiment of the present invention comprises 0.002 to 0.3% of C, 0.1 to 2.0% of Si, 0.005 to 1.5% of sol.Al, 0.3 to 8.0% of Mn, , P: not more than 0.1%, S: not more than 0.04%, N: not more than 0.02%, and the balance of Fe and unavoidable impurities, and the Fe-Ni alloying layer can be formed in a depth direction of 0.1 to 5 μm.

The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet contains 0.1 to 0.7% of Cr, 0.1% or less of Mo, 0.1% or less of Ti, 0.1% or less of Nb, 0.005% or less of B and 0.01 to 0.05% As shown in FIG.

The Fe-Ni alloying layer may be formed of Ni supplied from the outside.

A method for producing a high strength cold rolled steel sheet having excellent surface quality according to an embodiment of the present invention comprises 0.002 to 0.3% of C, 0.1 to 2.0% of Si, 0.005 to 1.5% of sol.Al, 0.3 to 0.3% of sol.Al, To 8.0%, P: not more than 0.1%, S: not more than 0.04%, N: not more than 0.02%, and Fe and unavoidable impurities; Recrystallization annealing the Ni-coated cold-rolled steel sheet to form an Fe-Ni alloyed layer on the surface of the cold-rolled steel sheet; It is possible to cool the recrystallized annealed cold rolled steel sheet.

The Ni coating layer can be formed by attaching 50 to 500 mg of Ni per unit area (m ² ) to the surface of the cold-rolled steel sheet.

The Ni coating layer may be formed by electroplating.

The recrystallization annealing can maintain the cold-rolled steel sheet at a temperature of 600 to 960 캜 for 5 to 120 seconds.

The recrystallization annealing can heat-treat the cold-rolled steel sheet in a 3 to 70% H ₂ -N ₂ atmosphere gas controlled at a dew-point temperature of -60 to 10 ° C.

The recrystallized annealed cold rolled steel sheet can be cooled to a temperature range of 200 to 400 ° C at a cooling rate of 5 to 100 ° C / s.

The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet contains 0.1 to 0.7% of Cr, 0.1% or less of Mo, 0.1% or less of Ti, 0.1% or less of Nb, 0.005% or less of B and 0.01 to 0.05% As shown in FIG.

The high-strength cold-rolled steel sheet having excellent surface quality according to an embodiment of the present invention and the method of manufacturing the same can effectively prevent the internal oxide and the surface oxide from being detached by forming the Fe-Ni alloyed layer, It is possible to effectively prevent dent defects generated when the surface oxide adheres to the rolls.

Fig. 1 is an electron microscope image of a section of a residual internal oxide having weak adhesion after recrystallization annealing before recrystallization annealing of inventive steel 1; Fig.
Fig. 2 is an image of the surface oxides after recrystallization annealing of Invention Steel 1 and Invention Steel 2 by an electron microscope.
FIGS. 3 and 4 are graphs showing depth profiles of Mn and Si contents from the surface of Inventive Steel 1. FIG.
5 and 6 are graphs showing depth profiles of Mn and Si contents from the surface of Invention Steel 2, respectively.

The present invention relates to a high-strength cold-rolled steel sheet excellent in surface quality and a method of manufacturing the same, and the preferred embodiments of the present invention will be described below. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to explain the present invention to a person having ordinary skill in the art to which the present invention belongs.

Hereinafter, the composition content of the present invention will be described in more detail. Unless otherwise stated, the percentages of the present invention are based on weight.

A high strength cold rolled steel sheet excellent in surface quality according to an embodiment of the present invention comprises 0.002 to 0.3% of C, 0.1 to 2.0% of Si, 0.005 to 1.5% of sol.Al, 0.3 to 8.0% of Mn, , P: not more than 0.1%, S: not more than 0.04%, N: not more than 0.02%, and the balance of Fe and unavoidable impurities.

C: 0.002 to 0.3%

C is an important element added for securing strength and stabilizing retained austenite. Therefore, the present invention can limit the lower limit of the C content to 0.002%. However, when the C content is excessively added, there arises a problem that the weldability is weakened, so that the present invention can limit the upper limit of the C content to 0.3%.

Si: 0.1 to 2.0%

Si is an element that suppresses precipitation of carbide in ferrite and contributes to stabilization of retained austenite as an element that promotes diffusion of carbon in ferrite into austenite. Therefore, the present invention can limit the lower limit of the Si content to 0.1% in order to achieve this effect. However, if the content of Si is excessive, the hot and cold rolling properties are very high, and the formation of Si oxide on the surface of the steel sheet can inhibit the molten metalization. In the present invention, the upper limit of the Si content is limited to 2.0% can do.

sol.Al: 0.005 to 1.5%

Al is an element contributing to stabilization of retained austenite through inhibition of formation of carbide in ferrite. Therefore, the present invention can limit the lower limit of the Al content to 0.005% in order to achieve this effect. However, when the content of Al is excessively added, there is a problem that it is difficult to produce a sound slab through reaction with the mold flux during casting, and a surface oxide is formed to inhibit the phosphate treatment property. The upper limit can be limited to 1.5%.

Mn: 0.3 to 8.0%

Mn is an element contributing to the formation and stabilization of retained austenite, and is an element effective for securing strength and ductility. Therefore, the present invention can limit the lower limit of the Mn content to 0.3% in order to achieve this effect. On the other hand, when the Mn content is excessively added, the physical properties may be deteriorated by segregation caused in the continuous casting and hot rolling process, so that the upper limit of the Mn content can be limited to 8.0%.

P: not more than 0.1%

P is a solid solution strengthening element, however, when the content is excessively added, there is a possibility that the weldability is lowered and the risk of brittleness of steel is increased. Therefore, the present invention can limit the upper limit of the P content to 0.1%.

S: not more than 0.04%

S is an impurity element in the steel and is an element which hinders ductility and weldability of a steel sheet. Therefore, the present invention can limit the upper limit of the S content to 0.04%.

N: 0.02% or less

N is an element effective for stabilizing austenite, but if it is excessive, there is a great risk of brittleness and excessive AlN precipitation by binding with Al can deteriorate the performance quality. Therefore, the present invention can limit the upper limit of the N content to 0.02%.

A high strength cold rolled steel sheet having excellent surface quality according to an embodiment of the present invention includes 0.1 to 0.7% of Cr, 0.1% or less of Mo, 0.1% or less of Ti, 0.1% or less of Nb, 0.005% or less, and Sb: 0.01 to 0.05%.

Cr: 0.1-0.7%

Cr is an element for increasing hardenability and is an element effective for suppressing the formation of ferrite. Therefore, the present invention can limit the lower limit of the Cr content to 0.1% to achieve this effect. On the other hand, when the Cr content is excessively added, there arises a problem of an increase in cost due to an excessive amount of the feed, so that the present invention can limit the upper limit of the Cr content to 0.7%.

Mo: 0.1% or less

Mo, like Cr, is an element contributing to the improvement of strength, so that Mo can be selectively added for improving the strength. However, when the Mo content is excessively added, there arises a problem of an increase in cost due to an overload, so that the present invention can limit the upper limit of the Mo content to 0.1%.

Ti: 0.1% or less

Ti is a nitride-forming element and has an effect of reducing the concentration of N in the steel, so that a small amount of Ti can be added as needed. However, when the Ti content is excessively added, the carbon concentration and the strength of the martensite are reduced due to the precipitation of additional carbides in addition to the removal of the solid solution N. Therefore, the present invention can limit the upper limit of the Ti content to 0.1%.

Nb: not more than 0.1%

Nb is an element that segregates in the form of carbide in the austenite grain boundaries and increases the strength by suppressing the coarsening of the austenite grains during the annealing heat treatment. Therefore, Nb can be selectively added for strength improvement. However, when the Nb content is excessively added, a problem of increased cost due to an excessive amount of input occurs, so that the present invention can limit the upper limit of the Nb content to 0.1%.

B: not more than 0.005%

B is an element contributing to the strength improvement, so it can be added selectively for strength improvement. However, when the content of B is excessive, it is concentrated on the surface during the annealing process to greatly deteriorate the plating property, so that the upper limit of the B content can be limited to 0.005% in the present invention.

Sb: 0.01 to 0.05%

Since Sb is an element that inhibits oxidation of hot-rolled internal oxide, the present invention can limit the lower limit of the Sb content to 0.01% in order to achieve this effect. However, if the content of Sb is excessively added, the brittleness of the steel sheet may increase and the elongation may decrease. The present invention can limit the upper limit of the Sb content to 0.05%.

The present invention may include Fe and unavoidable impurities in addition to the above-mentioned steel composition. Unavoidable impurities can be intentionally incorporated in a conventional steel manufacturing process, and can not be entirely excluded, and the meaning of ordinary steel manufacturing industry can be understood easily. Further, the present invention does not exclude the addition of other compositions other than the above-mentioned steel composition in the whole.

In the high-strength cold-rolled steel sheet having excellent surface quality according to an embodiment of the present invention, the Fe-Ni alloying layer may be formed within 0.1 to 5 占 퐉 in the depth direction from the surface of the cold-rolled steel sheet. Further, according to an embodiment of the present invention, the Fe-Ni alloying layer may be formed of Ni supplied from the outside.

In addition, according to an embodiment of the present invention, a Ni coating layer may be formed by coating Ni on the surface of the cold-rolled steel sheet to form the Ni-Fe alloyed layer, followed by recrystallization annealing. Therefore, on the surface portion of the cold-rolled steel sheet, an Fe-Ni alloyed layer formed by reacting Ni of the Ni coating layer and Fe of the cold-rolled steel sheet may be formed. The Fe-Ni alloying layer can be formed within 0.1 to 5 占 퐉 in the depth direction from the surface of the cold-rolled steel sheet, and the surface quality of the cold-rolled steel sheet can be effectively secured by forming the Fe-Ni alloyed layer.

In the case of the cold-rolled steel sheet in which the Fe-Ni alloyed layer is formed as in the present invention, fine cracks existing in the residual internal oxide in the annealing process of the recrystallization in which the Fe-Ni alloyed layer is formed can be bonded by diffusion bonding. Therefore, the brittleness formed in the residual internal oxide during pickling and cold rolling can be alleviated, thereby effectively preventing the residual internal oxide from dropping out during the annealing process. In addition, in the case of the cold-rolled steel sheet having the Fe-Ni alloyed layer formed thereon, it is effectively prevented that the highly oxidizable components such as Si and Mn are diffused and concentrated to the surface side of the cold-rolled steel sheet during the recrystallization annealing process. The surface oxide formation of the high-strength cold-rolled steel sheet containing the high component in a predetermined amount or more can be effectively suppressed.

The thickness of the Fe-Ni alloyed layer may be 0.1 占 퐉 or more in order to diffuse and bond the residual internal oxide by formation of the Fe-Ni alloyed layer and to suppress surface diffusion of highly oxidizable components such as Si and Mn. Therefore, since the thickness of the Fe-Ni alloyed layer of the present invention is 0.1 占 퐉 or more, the effect of preventing the residual internal oxides and surface oxides from being formed by the formation of the Fe-Ni alloy layer can be sufficiently expected. In addition, when the Fe-Ni alloyed layer is formed to an excessive thickness, the phosphate treatment may be deteriorated due to the excessive Ni content of the surface layer, and the cost may increase due to excessive Ni loading. The Ni alloying layer may have a thickness of 5 탆 or less.

A method of manufacturing a high strength cold rolled steel sheet having excellent surface quality according to an embodiment of the present invention includes: forming a Ni coating layer on a surface of a cold rolled steel sheet having the above composition; Recrystallization annealing the Ni-coated cold-rolled steel sheet to form an Fe-Ni alloyed layer on the surface of the cold-rolled steel sheet; It is possible to cool the recrystallized annealed cold rolled steel sheet.

Hereinafter, a method for producing a high-strength cold-rolled steel sheet excellent in surface quality of the present invention will be described in more detail.

First, a method of manufacturing a hot-rolled steel sheet used for manufacturing a cold-rolled steel sheet of the present invention will be described. A steel slab having a composition corresponding to the composition of the cold-rolled steel sheet described above may be reheated, and a hot-rolled steel sheet may be wound to produce a hot-rolled steel sheet. The composition of the slab corresponds to the composition of the cold-rolled steel sheet described above, and the explanation of the reason for restricting the compositional content of the slab is superseded by the explanation of the reason for limiting the compositional content of the cold-rolled steel sheet described above.

The slab may be reheated to a temperature range of 1100 to 1300 ° C. If the reheating temperature is lower than 1100 ° C, there is a problem that the hot rolling load sharply increases. If the reheating temperature exceeds 1300 ° C, the reheating cost increases and the surface scale amount increases.

The finish rolling temperature during hot rolling is preferably Ar3 or higher. Below Ar3, ferrite and austenite bimetallic or ferrite reverse rolling is performed to produce a composite grain structure, which may cause malfunction due to fluctuations in the hot rolling load.

After the hot rolling, the hot-rolled steel sheet is rolled in a temperature range of 700 ° C or less. If the coiling temperature exceeds 700 캜, an oxide film on the surface of the steel sheet may be excessively generated to cause defects, and excessive formation of the hot internal oxide can not be effectively prevented.

The wound hot rolled steel sheet may be provided as a cold rolled steel sheet through pickling and cold rolling.

It is important to optimize the process conditions to remove only the surface oxides of the hot-rolled steel sheet and to prevent the internal oxides from being damaged, in order to prevent productivity and embrittlement of residual internal oxidation. To achieve this, optimal pickling speed is needed. If the cold rolling reduction after pickling exceeds 70% of the thermal expansion coefficient, a large amount of cracks are generated in the remaining internal oxides, so that the internal oxides may be removed during the annealing process. It is preferable to be carried out at a reduction rate.

Formation of Ni coating layer

The Ni coating layer can be formed by attaching Ni to the surface of the cold-rolled steel sheet. The Ni coating layer may be formed by electroplating, but the method of forming the Ni coating layer of the present invention is not limited thereto. The Ni coating layer can be formed by attaching 50 to 500 mg of Ni per unit area (m ² ) to the surface of the cold-rolled steel sheet. When the adhesion amount of Ni is less than 50 mg / mm ² , the Ni content is insufficient and the Fe-Ni alloying layer can not be formed to a depth of 0.1 μm or more from the surface of the cold-rolled steel sheet, And the effect of suppressing the deterioration of surface oxides can not be sufficiently expected. When the amount of Ni deposited exceeds 500 mg / mm ² , there is a high possibility that the Fe-Ni alloying layer is formed to a depth exceeding 5 μm from the surface of the cold-rolled steel sheet, and the excessive Ni content of the surface layer And the cost increase due to the excessive amount of input may occur.

Recrystallization annealing

The recrystallization annealing may be performed by maintaining the cold-rolled steel sheet on which the Ni coating layer is formed at a dew point temperature of -60 to 10 ° C in a 3 to 70% H ₂ -N ₂ atmosphere gas over a temperature range of 600 to 960 ° C for 5 to 120 seconds.

The dew point temperature of less than -60 ° C is difficult to keep at the production site, and the dew point temperature of more than 10 ° C can be oxidized to Fe of the steel sheet. The dew point temperature of the recrystallization annealing of the present invention may be -60 to 10 ° C .

When the hydrogen content of the atmosphere gas is less than 3%, it is impossible to reduce the trace amount of Fe oxide existing on the surface of the steel sheet. On the other hand, when the hydrogen content of the atmosphere gas exceeds 70%, the reduction effect of the Fe oxide on the surface of the steel sheet is excellent, but it is not economically preferable. Therefore, the hydrogen content of the atmosphere gas in the recrystallization annealing of the present invention can be 3 to 70%.

When the annealing temperature for recrystallization is 600 占 폚 or higher, recrystallization may be performed to secure the target microstructure for each steel type, and a sufficient temperature for forming the Fe-Ni alloying layer can be provided. However, when the recrystallization annealing temperature exceeds 950 deg. C, the lifetime of the annealing furnace may be reduced. As a result, the recrystallization annealing temperature of the present invention can be limited to 600 to 950 deg.

During recrystallization annealing, Ni of the Ni coating layer diffuses and moves toward the center portion side of the cold-rolled steel sheet, and thus an Fe-Ni alloyed layer can be formed. In the process of forming the Fe-Ni alloyed layer, fine cracks existing in the residual internal oxide can be diffusion-bonded by the diffusion movement of atoms for a high-temperature reducing atmosphere. Therefore, the adhesion of the residual internal oxide, which has been brittle by the pickling and cold rolling process, is restored, and dent defects caused by the internal oxide being removed and adhering to the roll can be effectively improved.

In addition, the Fe-Ni alloy layer formed during the recrystallization annealing can effectively prevent the phenomenon that the highly oxidizable components such as Si and Mn diffuse and move to the surface side of the cold-rolled steel sheet to be concentrated, Can be effectively suppressed. Therefore, it is possible to effectively improve the dent defect caused by the removal of the surface oxide and adhesion to the roll.

After completion of the recrystallization annealing, the cold rolled steel sheet can be cooled to a temperature range of 200 to 400 ° C at an average cooling rate of 5 to 100 ° C / s. It is possible to separate the sections by the primary cooling and the secondary cooling, and the cooling speed of the secondary cooling may be faster than that of the primary cooling. The average cooling rate may be 5 ° C / s or more in order to prevent transformation of austenite into pearlite in a ferrite and austenite bi-phase region or austenite single phase region by recrystallization annealing. On the other hand, when the cooling rate exceeds 100 ° C / s, the steel sheet may be deformed in the width direction due to the rapid cooling of the steel sheet. However, the cooling rate and the cooling arrival temperature can be suitably applied according to the microstructure and physical properties of the desired cold rolled steel sheet.

Hereinafter, the present invention will be described in more detail by way of examples.

(Example)

The steel having the composition shown in Table 1 below was melted to prepare a slab. The prepared slabs were maintained at 1200 ° C for 1 hour, finishing rolled at 900 ° C, cooled to 650 ° C to prepare hot rolled steel sheets, and cooled for 1 hour in a warming furnace. The depths of the hot internal oxides of the respective hot-rolled steel sheets were measured and are shown together in Table 1. Invention steel 1 containing relatively large amounts of Si and Mn forms a hot-rolled inner oxide depth of about 20 탆 while Inventive Steel 2 containing a relatively small amount of Si and Mn shows that there is no residual inner oxide .

Steel grade C Mn Si P S Sol.Al Ti Nb N Sb Hot-rolled inner oxide depth
(탆) Inventive Steel 1 0.096 2.75 1.08 0.011 0.0011 0.028 0.003 0.002 0.005 - 20 Invention river 2 0.004 0.61 0.29 0.088 0.0091 0.044 0.008 0.002 0.005 0.02 0

Each of the hot-rolled steel sheets was pickled at 50 ° C and 15 vol% HCl solution for 30 seconds to remove the scale of the steel sheets and cold-rolled at a reduction ratio of 50%. After cold rolling, Ni coating layer formation and recrystallization annealing were carried out under the conditions shown in Table 2. The internal oxides and surface oxides of the individual cold rolled steel sheets after the recrystallization annealing were observed, and the results were as follows.

division Steel grade Ni coating layer formation Annealing process Fe-Ni
Alloying layer thickness
(탆) Ni deposition amount
(mg / m ² ) Annealing furnace
dew point
Temperature
(° C) Annealing temperature
(° C) Retention time
(sec) Cooling temperature
(° C) Average
Cooling rate
(° C) Comparative Example 1 Inventive Steel 1 0 -50 800 65 320 22 0 Inventory 1 Inventive Steel 1 100 -50 800 65 320 22 0.3 Inventory 2 Inventive Steel 1 300 -50 800 65 320 22 0.5 Inventory 3 Inventive Steel 1 500 -50 800 65 320 22 0.8 Comparative Example 2 Invention river 2 0 -50 800 65 320 22 0 Honorable 4 Invention river 2 100 -50 800 65 320 22 0.5 Inventory 5 Invention river 2 300 -50 800 65 320 22 1.5 Inventory 6 Invention river 2 500 -50 800 65 320 22 2.0

Fig. 1 is an electron microscope image of a section of a residual internal oxide having weak adhesion after recrystallization annealing before recrystallization annealing of inventive steel 1; Fig.

As shown in FIG. 1, it can be seen that the shapes of residual internal oxides in the surface layer portions exhibit different behaviors depending on the amount of Ni adhered thereto. In the case of Comparative Example 1 in which the adhesion amount of Ni was 0 mg / mm ² , it was confirmed that the diffusion bonding was not sufficiently performed because the internal oxide was formed in a relatively wide range. On the other hand, in the case of Inventive Example 2 and Inventive Example 3 in which the adhesion amounts of Ni were 300 mg / mm ² and 500 mg / mm ² , diffusion bonding was promoted and most of the internal oxide cracks were bonded, As shown in Fig.

Fig. 2 is an image of the surface oxides after recrystallization annealing of Invention Steel 1 and Invention Steel 2 by an electron microscope.

In the case of Invention Steel 1, it can be seen that the grain boundaries are apparent as the amount of Ni deposited increases, which means that the amount of surface oxides decreases as the amount of Ni deposited increases. That is, in Inventive Examples 1 to 3, it can be confirmed that the amount of the surface oxide is significantly smaller than that of Comparative Example 1. Further, in the case of inventive steel 2, it can be seen that the circular surface oxide decreases remarkably as the deposition amount of Ni increases. That is, in the case of Inventive Examples 4 to 6, it can be confirmed that the surface oxide is significantly smaller than Comparative Example 2.

FIGS. 3 and 4 are graphs showing depth profiles of Mn and Si contents from the surface of inventive steel 1, and FIGS. 5 and 6 are graphs showing depth profiles of Mn and Si contents from the surface of inventive steel 2 .

3 and 4, it can be seen that as the amount of Ni deposited increases, Mn content and Si content in the surface layer of invention steel 1 are lowered. That is, the surface layer Mn and Si content of the inventive examples 1 to 3 were lower than the surface layer Mn and Si content of the comparative example 1, and the Mn and Si content of the surface layer were sequentially lowered from Inventive Example 1 to Inventive Example 3 .

5 and 6, it can be seen that Mn content and Si content in the surface layer of Inventive Steel 2 are lowered as Ni deposition amount increases. That is, Mn and Si content in the surface layer portions of Inventive Examples 3 to 6 are lower than those of the surface layer portion of Comparative Example 2, and Mn and Si content in the surface layer are sequentially lowered from Inventive Example 4 to Inventive Example 6 .

Therefore, the high-strength cold-rolled steel sheet according to one embodiment of the present invention and the method of manufacturing the same can effectively prevent the internal oxides and the surface oxides from being separated by forming the Fe-Ni alloyed layer, It is possible to effectively prevent the dent defect attached to the roll.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, Therefore, the technical idea and scope of the claims set forth below are not limited to the embodiments.

Claims (10)

0.002 to 0.3% of C, 0.1 to 2.0% of Si, 0.005 to 1.5% of sol.Al, 0.3 to 8.0% of Mn, 0.1% or less of P, 0.04% or less of S, 0.02% or less of N, Or less, and Fe-Ni alloying layer is formed within 0.1 to 5 占 퐉 in the depth direction from the surface and contains the remaining Fe and unavoidable impurities. The method according to claim 1,
The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet contains 0.1 to 0.7% of Cr, 0.1% or less of Mo, 0.1% or less of Ti, 0.1% or less of Nb, 0.005% or less of B and 0.01 to 0.05% Wherein the steel sheet has excellent surface quality. The method according to claim 1,
The Fe-Ni alloying layer is formed by Ni supplied from the outside, and has excellent surface quality. 0.002 to 0.3% of C, 0.1 to 2.0% of Si, 0.005 to 1.5% of sol.Al, 0.3 to 8.0% of Mn, 0.1% or less of P, 0.04% or less of S, 0.02% or less of N, A Ni coating layer is formed on the surface of the cold-rolled steel sheet containing the remaining Fe and unavoidable impurities;
Recrystallization annealing the Ni-coated cold-rolled steel sheet to form an Fe-Ni alloyed layer on the surface of the cold-rolled steel sheet;
Wherein the recrystallized annealed cold rolled steel sheet is cooled and the surface quality is improved. 5. The method of claim 4,
And the Ni coating layer is formed by attaching 50 to 500 mg of Ni per unit area (m ² ) to the surface of the cold-rolled steel sheet, to thereby obtain a high-strength cold-rolled steel sheet having excellent surface quality. 5. The method of claim 4,
Wherein the Ni coating layer is formed by electroplating and has excellent surface quality. 5. The method of claim 4,
Wherein the recrystallization annealing is performed by maintaining the cold-rolled steel sheet at a temperature of 600 to 960 캜 for 5 to 120 seconds. 8. The method of claim 7,
Wherein the recrystallization annealing is a heat treatment of the cold-rolled steel sheet in a 3 to 70% H ₂ -N ₂ atmosphere gas controlled at a dew point temperature of -60 to 10 ° C, thereby producing a high-strength cold-rolled steel sheet having excellent surface quality. 5. The method of claim 4,
And cooling the recrystallized and annealed cold rolled steel sheet to a temperature range of 200 to 400 ° C at a cooling rate of 5 to 100 ° C / s to obtain a high strength cold rolled steel sheet having excellent surface quality. 5. The method of claim 4,
The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet contains 0.1 to 0.7% of Cr, 0.1% or less of Mo, 0.1% or less of Ti, 0.1% or less of Nb, 0.005% or less of B and 0.01 to 0.05% Further comprising a step of forming a high-strength cold-rolled steel sheet having a high surface quality.

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