KR101417225B1 - High strength cold rolled steel sheet with excellent stretch flangeability and method of manufacturing the same - Google Patents

Abstract

In one aspect of the present invention, a high strength cold rolled steel sheet excellent in stretch flangeability comprises 0.05 to 0.08% of C, 0.01 to 0.10% of Si, 1.5 to 2.5% of Mn, 0.01 to 0.10% of Al, % Of Fe, 0.007% or less of S, 0.04 to 0.09% of Mo, 0.02 to 0.06% of Cr, 0.5 to 0.9% of Co, the balance of Fe and other unavoidable impurities and the microstructure is composed of 85 to 98% Ferrite) and 2 to 15% Martensite. In another aspect of the present invention, there is provided a method for producing a high strength cold rolled steel sheet excellent in stretch flangeability, which comprises 0.05 to 0.08% of C, 0.01 to 0.10% of Si, 1.5 to 2.5% of Mn, 0.10%, P: not more than 0.007%, S: not more than 0.007%, Mo: 0.04 to 0.09%, Cr: 0.02 to 0.06%, Co: 0.5 to 0.9%, the balance Fe and other unavoidable impurities , Hot-rolling the reheated slab at 870 to 910 ° C, winding the hot-rolled steel sheet at 550 to 590 ° C, cold-rolling the rolled steel sheet at a reduction ratio of 50 to 90% Annealing the steel sheet at 770 to 800 ° C for recrystallization annealing.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength cold rolled steel sheet having excellent stretch flangeability and a method of manufacturing the same.

The present invention relates to a cold-rolled steel sheet used as a material for automobiles, home appliances, and the like, and a manufacturing method thereof.

Recently, cold rolled steel sheets used in automobiles, household appliances, etc. are required to have excellent strength and elongation flangeability. High-strength steel plates having high tensile strength have been actively employed to lighten the vehicle body and secure passenger stability. These high-strength steel plates have been developed in close relation with various legal regulations surrounding the automobile industry such as the automobile safety regulation law, the fuel consumption regulation law, the exhaust gas regulation law, and the fuel economy regulation by the high price has been strengthened, As a result, research and development have been accelerated, and many kinds of high strength steel sheets have been developed.

Generally, precipitation strengthened steel or ferrite / pearlite steel of a ferrite base has been used to increase the strength, and there has been a problem that the ductility and stretch flangeability of such steel decrease with increasing strength. Therefore, to solve this problem, a technique of securing a stretch flange and ductility by forming a mixed structure composed of an equiaxed ferrite or acicular ferrite and bainite has been proposed.

Such a technique is disclosed in Patent Document 1. This technique suggests a method of improving stretch flangeability by suppressing segregation of P by performing low temperature winding while suppressing the amount of retained austenite as much as possible during winding, And Patent Document 2. This technology relates to a hot-rolled steel sheet having a strength of 690 MPa or more and having both an elongation and a stretch flangeability at the same time. The ferrite-bainite structure is mainly composed of ferrite having a ferrite ratio of 80% It has been proposed to control the crystal grains having a ratio (ds / dl) of the short diameter (ds) to the long diameter (dl) of the grains to 0.1 or more of 80% or more. However, The stretch flangeability is deteriorated.

As another technique, Patent Document 3 is mentioned. The main technique is to wind the steel sheet at a temperature of less than 400 DEG C in order to produce a high-strength hot-rolled steel sheet having excellent elongation flangeability and excellent molding processability, Lt; 0 > C, the heat transfer coefficient suddenly changes, and the temperature hit ratio is lowered during the winding operation, so that it is difficult to control the microstructure.

Patent Document 4 discloses another technique. In order to improve elongation flangeability, the fraction of bainite is controlled to 90% or more. However, in this case, the ductility is lowered, .

Therefore, there is currently limited research on steel sheets that have excellent stretch flangeability and excellent strength, and it is necessary to conduct research.

Japanese Patent Laid-Open No. 1996-269538 Korean Patent Publication No. 2003-55339 Japanese Laid-Open Patent Application No. 2008-001984 Japanese Patent Laid-Open No. 2008-069425

An aspect of the present invention is to provide a cold rolled steel sheet excellent in elongation flangeability and excellent in strength and a method of manufacturing the same.

In one aspect of the present invention, a high strength cold rolled steel sheet excellent in stretch flangeability comprises 0.05 to 0.08% of C, 0.01 to 0.10% of Si, 1.5 to 2.5% of Mn, 0.01 to 0.10% of Al, % Of Fe (excluding 0% by weight), 0.007% or less of S, 0.04 to 0.09% of Mo, 0.02 to 0.06% of Cr, 0.5 to 0.9% of Co, 0.5 to 0.9% of Co and the balance of Fe and other inevitable impurities. To 98% of ferrite and 2 to 15% of martensite.

A method of manufacturing a high strength cold rolled steel sheet excellent in stretch flangeability according to another aspect of the present invention comprises the steps of: C: 0.05 to 0.08%, Si: 0.01 to 0.10%, Mn: 1.5 to 2.5%, Al: 0.01 to 0.10% Of Cr, 0.02 to 0.06% of Cr, 0.5 to 0.9% of Co, the balance Fe and other unavoidable impurities, P: not more than 0.007% (excluding 0 wt%), S: not more than 0.007%, Mo: 0.04 to 0.09% Hot rolling the reheated slab at a temperature of 870 to 910 占 폚, winding the hot-rolled steel sheet at 550 to 590 占 폚, cold-rolling the rolled steel sheet at a reduction ratio of 50 to 90% And heat-treating the cold-rolled steel sheet by recrystallization annealing at 770 to 800 ° C.

According to the present invention, the microstructure of a steel sheet is controlled by a ferrite of 85 to 98% and martensite of 2 to 15% through the technique of the present invention, and a circular shape having a diameter of 2 탆 or less Can be provided at a density of 15 pieces / mm < 2 > or more. Such a steel sheet can have a tensile strength of 490 MPa or more and an elongation of 27% or more.

The inventors of the present invention have conducted intensive studies to derive a high strength cold rolled steel sheet excellent in stretch flangeability. As a result, the present inventors have found that by controlling the component system of the steel sheet and appropriately controlling the microstructure of the steel sheet with ferrite and martensite, It is possible to produce a cold rolled steel sheet which simultaneously secures excellent stretch flangeability and high strength by controlling the number density.

Hereinafter, a cold-rolled steel sheet as one aspect of the present invention will be described in detail.

Carbon (C): 0.05 to 0.08 wt%

C is an Austenite stabilizing element that minimizes pearlite structure and carbide inside the ferrite structure in the hot rolled steel sheet and refines the grain. The re-use of the composite precipitates is partially reused in the annealing process of the cold-rolled steel sheet to provide fine crystal grains having a size of about 10 to 30 μm and to restrict the volume ratio of martensite appearing at grain boundaries to 15% or less, And plays a role in developing good texture 111. When the content of C is less than 0.05% by weight, stable austenite can not be secured in the critical temperature region, and martensite is not produced in an appropriate fraction after cooling, so that it is difficult to ensure proper strength. On the other hand, if the content of C exceeds 0.08% by weight, ductility can not be ensured and weldability is deteriorated. Therefore, in the present invention, the content of C is preferably controlled to 0.05 to 0.08% by weight.

Silicon (Si): 0.01 to 0.10 wt%

The Si increases the strength by solid solution strengthening as a ferrite stabilizing element while suppressing precipitation of cementite during holding at a temperature of 350 to 600 캜 after the annealing heat treatment and the C is concentrated to austenite And contributes to formation of martensite and improvement of ductility upon cooling. However, when the content of Si is less than 0.01% by weight, the austenite stabilizing effect described above is lowered. On the other hand, when the content of Si exceeds 0.10% by weight, the surface properties are deteriorated and the Si oxide is concentrated to deteriorate both the weldability and the plating ability. Therefore, in the present invention, the Si content is preferably controlled to 0.01 to 0.10 wt%.

Manganese (Mn): 1.5 to 2.5 wt%

Since Mn is an element stabilizing austenite, it delays decomposition of austenite into pearlite during cooling to 300 to 580 DEG C after annealing. Therefore, while cooling to room temperature, manganese structure, which is a low temperature transformation phase, . In addition, it has an effect of enhancing the strength by strengthening of the solid solution, and is very effective in preventing hot cracking of the slab by forming MnS inclusions in combination with sulfur (S) in the steel. When the content of Mn is less than 1.5% by weight, it is difficult to delay decomposition of austenite into pearlite. On the other hand, when the content of Mn is more than 2.5% by weight, not only the slab cost is remarkably increased but also the weldability and the formability are deteriorated. Therefore, in the present invention, the content of Mn is preferably controlled to 1.5 to 2.5 wt%.

Aluminum (Al): 0.01 to 0.10 wt%

The Al is used as a deoxidizing agent and is an element that inhibits cementite precipitation like Si and stabilizes austenite by delaying the progress of transformation. Since unnecessary solid solution N in the steel is precipitated as AlN by adding Al to the austenite stabilization minimum effect limit of 0.01% by weight or more, the Al is segregated at the grain boundary in the high temperature region to make the carbide fine in the hot-rolled steel grain. However, when the content of Al exceeds 0.10%, clogging of the nozzle occurs during continuous casting, and the hot brittleness and ductility are remarkably lowered by Al oxide or the like during casting, and the surface defect is liable to occur. Therefore, in order to remove quality defects due to Al segregated at grain boundaries in the high temperature region. In the present invention, the content of Al is preferably controlled to 0.01 to 0.10 wt%.

Phosphorus (P): 0.007% by weight or less (excluding 0% by weight)

The P increases strength by solid solution strengthening, and when added together with Si inhibits cementite precipitation while maintaining the temperature at 300 to 580 ° C, and promotes carbon enrichment with austenite. However, when the content of P is more than 0.007% by weight, it is disadvantageous to the secondary process brittleness and it may lower the adhesion of the zinc plating and lower the alloying property. Therefore, in the present invention, the content of P is preferably controlled to 0.007% by weight or less.

.

Sulfur (S): 0.007% by weight or less

The S is inevitably contained as an impurity, which is combined with Fe to form FeS, thereby causing hot brittleness. Therefore, it is desirable to suppress the content to the maximum. In theory, it is advantageous to limit the content of S to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the S content is preferably limited to 0.007% by weight.

Molybdenum (Mo): 0.04 to 0.09 wt%

The molybdenum is preferably added in an amount of 0.04% by weight or more, which is the minimum value of the effect of re-dissolving in the annealing process to reuse the carbon bonded to Mo in the composite precipitate. However, when it exceeds 0.09% by weight, the stock capacity is decreased, so that formation of the low temperature transformation phase is difficult and the cost is remarkably increased. Therefore, in the present invention, the Mo content is preferably controlled to 0.04 to 0.09% by weight.

Cr (Cr): 0.02 to 0.06 wt%

The Cr improves the hardenability and is a very effective element for stably forming a low temperature transformation phase, which leads to refinement of carbide, delaying the rate of spheroidization, refining grain refinement, inhibiting growth of crystal grains, and ferrite strengthening element. It is also effective in suppressing the softening of the heat affected zone (HAZ) at the time of welding. However, when the content of Cr is less than 0.02% by weight, the bond with C becomes too small to be reused. On the other hand, when the content of Cr exceeds 0.06% by weight, the hardness increase of the HAZ (weld heat affected zone) becomes too large. Therefore, in the present invention, the content of Cr is preferably controlled to 0.02 to 0.06% by weight.

Cobalt (Co): 0.5 to 0.9 wt%

Like Co, Mn is an austenite-generating element and inhibits precipitation of cementite while maintaining the temperature at 300 to 580 캜, like Si and Al, and slows the progress of transformation. Further, when the proper amount of Co is added, it plays a role of simultaneously improving strength and moldability. Further, since Co is an element harder to oxidize than Fe, Co is suppressed from being oxidized such as Si and Al which is concentrated on the surface during the annealing for recrystallization annealing to deteriorate the plating adhesion, thereby improving the plating adhesion. When the content of Co is less than 0.5% by weight, the plating adhesion improving effect can not be obtained. On the other hand, when the content of Co exceeds 0.9 wt%, the strength is increased but the ductility is lowered. Therefore, in the present invention, the content of Co is preferably controlled to 0.5 to 0.9 wt%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

As the steel sheet satisfying the above-mentioned composition, it is preferable that the microstructure is controlled by 85 to 98% of ferrite and 2 to 15% of martensite. By controlling the microstructure as described above, both the elongation flange strength and the strength can be improved.

It is also preferable that the number of inclusions having a diameter of 2 탆 or less in the circular shape existing in the steel sheet is controlled to be 15 / mm 2 or more. The number density of inclusions having a circle-equivalent diameter of 2 占 퐉 or less is focused on observations by a scanning electron microscope (SEM) or the like, and the reason why the inclusions of 2 占 퐉 or less are dispersed at 15 / mm2 or more is that The oxygen potential is lowered and the synergistic effect of the refinement of the MnS inclusions is considered. Thereby, a mechanism for relieving the stress concentration occurring at the time of forming the stretch flange acts, and the effect of drastically improving the hole expandability is obtained.

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

The slab having the above-mentioned composition is obtained through steelmaking and then through molten steel or continuous casting. The slabs are manufactured through a hot rolling step, a winding step, a cold rolling step and an annealing step to produce steel plates having desired mechanical properties. The production conditions for each step will be described in detail below.

Hot rolling process

The slab formed as described above can be hot rolled. The hot rolling finishing temperature is 870 ~ 910 ℃ and the cooling is controlled to make the hot-rolled structure finer. At this time, if the hot rolling temperature is low, drawability deteriorates due to generation of coarse grain in the crystal structure by strain annealing, so that hot rolling is performed at an appropriate rolling temperature to obtain a fine hot rolled structure. After hot rolling, it is preferable to use a high-pressure descaling device or remove the scale of the surface by strong pickling.

Winding process

The hot-rolled steel sheet can be rolled at a temperature of 550 to 590 캜. The carbide is smoothly formed in the wound state to minimize the amount of dissolved carbon and to minimize the formation of nitrogen dissolved in the steel by precipitating AlN as much as possible. Such a coiling temperature is a temperature for obtaining a structure for obtaining optimum mechanical properties after cold rolling and recrystallization heat treatment. When the coiling temperature is lower than 550 캜, cold rolling is difficult due to bainite or martensite structure. On the other hand, when the coiling temperature exceeds 590 DEG C, the final microstructure is coarsened and it is difficult to produce a steel sheet having sufficient strength.

Cold rolling process

The wound hot rolled steel sheet may be pickled and then cold rolled. At this time. It is preferable to control the cold reduction rate to 50 to 90%. Cold rolling transforms the hot-rolled structure and its strain energy becomes energy for the recrystallization process. When the cold rolling reduction is less than 50%, the above-described deformation effect is small. On the other hand, when the reduction rate exceeds 90%, the cold rolling is difficult to realize in practice, and the complex precipitates in the hot-rolled steel sheet are decomposed during rolling to develop (100) texture in the initial stage of recrystallization, , Cracks may be formed at the edge of the steel sheet, and plate breakage may occur.

Annealing heat treatment

The cold-rolled steel sheet may be subjected to recrystallization annealing. At this time, annealing is preferably continuous annealing. The recrystallization annealing improves the drawability by developing (111) texture through recrystallization and grain growth, and dissolves the dissolved carbon by redissolving the fine complex precipitates. Here, the annealing temperature is preferably controlled to 770 to 800 占 폚, and it is preferable to perform recrystallization annealing for 10 to 200 seconds. The recrystallization annealing heat treatment must be performed between the Ac1 transformation point and the Ac3 transformation point in order to make a two-phase structure of ferrite and austenite. At a temperature lower than 770 DEG C, too much time is required for re-use of cementite, At the temperature, the austenite volume fraction becomes too large, resulting in a decrease in the carbon concentration of the austenite.

After the annealing, the steel sheet can be cooled to a temperature of 300 to 580 ° C at a rate of 10 to 30 ° C / sec. When the cooling rate is less than 10 DEG C / sec, most of the austenite is transformed into pearlite structure or forms bainite structure during cooling. On the other hand, when the cooling rate exceeds 30 DEG C / sec, the cooling termination temperature deviation in the width direction and the longitudinal direction becomes too large, making it impossible to produce a steel sheet of uniform quality. Due to the nature of the production line, the cooling can be quenched to a final temperature at a rate of 2 to 100 ° C / sec.

As described above, the steel sheet can be cooled to a temperature of 300 to 580 ° C and maintained at that temperature for 10 minutes or less to generate stable martensite even at room temperature after the final cooling. When the temperature is less than 300 ° C, a considerable amount of the structure transforms into martensite and the moldability is deteriorated. On the other hand, when the temperature exceeds 580 占 폚, there is a problem that the austenite is transformed into bainite.

Through such a process, the microstructure of the steel sheet can be controlled to 85 to 98% of ferrite and 2 to 15% of martensite. In addition, the steel sheet can control the number density of circular inclusions having a diameter of 2 탆 or less existing in the steel sheet to 15 / mm 2 or more.

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 invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

Steel slabs having the compositions shown in the following Table 1 were prepared and subjected to hot rolling, cold rolling and annealing heat treatments under the process conditions shown in Table 2 to produce cold rolled steel sheets.

The cold-rolled steel sheet produced by the above method was observed for microstructure and is shown in Table 1 below. The tensile strength, the elongation, and the tensile strength, elongation, and elongation values are shown in Table 2 below. In addition, in the following Table 2, when the tensile strength is 490 MPa, the elongation percentage (%) is at least 27%, and TS El is at least 13,230 MPa% △, and when any one of them is not satisfied, it is marked with "×".

The density of the inclusions was measured and shown in Table 2 below.

The hole expandability, which is an index for evaluating elongation flangeability, is measured when a circular hole is formed in a specimen and then expanded by using a conical punch. When the hole is enlarged by a punch until the crack at the edge of the hole penetrates at least one direction in the thickness direction Can be expressed as a ratio with respect to the size of the initial hole, and specifically, the hole expanding ratio can be derived by the following equation (1). The hole expanding ratios for the respective examples are calculated and the hole expanding ratios are shown in Table 2 below. The results are shown in Table 2 below when the hole expanding ratio is 100 or more, and when the hole expanding ratio is 100 or less.

(Equation 1)? = (Dh-Do) / Do * 100

(? is the hole expansion ratio (%), Do is the initial hole diameter (mm), Dh is the hole diameter after fracture (mm))

division C Mn P S Si Mo Al Cr Co Bera
It
(%) Marten
site
(%) Inventory 1 0.051 2.5 0.0067 0.0021 0.01 0.04 0.012 0.02 0.51 85 15 Inventory 2 0.076 1.5 0.0057 0.0034 0.02 0.09 0.029 0.04 0.67 87 13 Inventory 3 0.071 1.6 0.0049 0.0051 0.08 0.07 0.045 0.05 0.89 93 7 Honorable 4 0.065 1.7 0.0059 0.0029 0.06 0.05 0.027 0.03 0.66 91 9 Inventory 5 0.077 1.6 0.0061 0.0066 0.05 0.07 0.021 0.06 0.82 92 8 Inventory 6 0.071 1.7 0.0054 0.0069 0.04 0.08 0.032 0.05 0.77 87 13 Honorable 7 0.052 2.3 0.0045 0.0067 0.10 0.06 0.039 0.02 0.54 98 2 Honors 8 0.063 2.2 0.0034 0.0054 0.09 0.05 0.065 0.04 0.64 88 12 Proposition 9 0.069 1.9 0.0032 0.0070 0.08 0.07 0.072 0.06 0.84 86 14 Inventory 10 0.054 2.2 0.0039 0.0067 0.02 0.08 0.094 0.03 0.55 91 9 Comparative Example 1 0.051 1.7 0.0088 0.0059 0.02 0.05 0.033 - - 87 13 Comparative Example 2 0.063 1.9 0.0091 0.0067 0.04 0.07 0.035 - - 88 12 Comparative Example 3 0.071 2.3 0.0076 0.0061 0.05 0.09 0.028 - - 99 One Comparative Example 4 0.051 2.2 0.0061 0.0079 0.08 0.07 0.029 - - 86 14 Comparative Example 5 0.076 1.7 0.0098 0.0061 0.09 0.08 0.033 - - 91 9 Comparative Example 6 0.091 1.3 0.0112 0.0089 0.12 - 0.039 0.02 - 84 16 Comparative Example 7 0.097 1.4 0.0097 0.0091 0.13 - 0.029 0.04 - 82 18 Comparative Example 8 0.115 2.6 0.0102 0.0094 0.11 - 0.034 0.05 - 79 21 Comparative Example 9 0.123 2.8 0.0097 0.0072 0.15 - 0.023 0.07 - 81 19 Comparative Example 10 0.101 2.9 0.0087 0.0074 0.19 - 0.021 0.08 - 80 20

(Note that the unit of each element is% by weight)

division FDT
(° C) CT
(° C) SS
(° C) TS
(MPa) Hand
(%) TS × El
(MPa%) Density of inclusions having a circular diameter of 2 占 퐉 or less (pieces / mm2) hole
Expansion rate
(%) TS-El
evaluation hole
Scalability evaluation Inventory 1 871 577 771 491 30 14730 25 100 ○ ○ Inventory 2 883 587 799 597 27 16119 26 101 ○ ○ Inventory 3 909 555 787 610 27 16470 34 141 ○ ○ Honorable 4 901 567 777 498 29 14442 16 121 ○ ○ Inventory 5 897 589 774 533 28 14924 15 119 ○ ○ Inventory 6 899 573 794 576 28 16128 17 121 ○ ○ Honorable 7 905 551 767 594 27 16038 31 110 ○ ○ Honors 8 874 559 781 520 28 14560 35 136 ○ ○ Proposition 9 888 562 789 541 28 15148 18 123 ○ ○ Inventory 10 899 564 794 613 27 16551 19 121 ○ ○ Comparative Example 1 901 549 777 497 24 11928 4 111 △ ○ Comparative Example 2 897 545 774 515 23 11845 6 121 △ ○ Comparative Example 3 899 547 794 536 22 11792 8 90 △ × Comparative Example 4 905 539 777 576 22 12672 11 88 △ × Comparative Example 5 874 533 774 584 21 12264 14 121 △ ○ Comparative Example 6 888 593 794 487 25 12175 9 89 △ × Comparative Example 7 897 597 767 456 25 11400 8 87 × × Comparative Example 8 899 603 771 444 26 11544 6 87 × × Comparative Example 9 901 591 799 460 25 11500 7 89 × × Comparative Example 10 897 612 794 471 26 12246 One 97 × ×

(Where FDT is the hot finish temperature, CT is the coiling temperature, SS is the annealing temperature, TS is the tensile strength, and El is the elongation)

As shown in Tables 1 and 2, Inventive Examples 1 to 5 can provide a steel sheet excellent in tensile strength, elongation, and hole expandability as an example satisfying all the constituents and manufacturing conditions controlled by the present invention I could confirm.

On the other hand, in Comparative Examples 1 to 10, the elongation was lower than the range controlled by the present invention, and the value of TS * El was also lower than the range controlled by the present invention. In addition, it was confirmed that the density of the inclusions was also lower than the range controlled by the present invention.

Claims (6)

(% By weight), S: 0.007% or less, S: 0.05 to 0.08%, Si: 0.01 to 0.10%, Mn: 1.5 to 2.5% The ferritic stainless steel according to the present invention contains 0.04 to 0.09% of Mo, 0.02 to 0.06% of Cr, 0.5 to 0.9% of Co, the balance of Fe and other unavoidable impurities. The microstructure contains ferrite of 85 to 98% and martensite of 2 to 15% (TS) of not less than 490 MPa, an elongation (El) of not less than 27%, and a tensile strength (TS) of not less than TS / Of 13,230 MPa% or more and a hole expansion ratio of 100 or more.
delete delete delete (% By weight), S: 0.007% or less, S: 0.05 to 0.08%, Si: 0.01 to 0.10%, Mn: 1.5 to 2.5% Reheating the slab containing 0.04 to 0.09% of Mo, 0.02 to 0.06% of Cr, 0.5 to 0.9% of Co, the balance Fe and other unavoidable impurities;
Hot-rolling the reheated slab at 870 to 910 占 폚;
Winding the hot-rolled steel sheet at 550 to 590 캜;
Cold rolling the rolled steel sheet at a reduction ratio of 50 to 90%; And
And heat-treating the cold-rolled steel sheet by annealing at 770 to 800 ° C for recrystallization annealing.
The method of claim 5,
Wherein the annealing is performed for 10 to 200 seconds. ≪ RTI ID = 0.0 > 21. < / RTI >