KR20150073795A - Oriented electrical steel sheet and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a grain-oriented electrical steel sheet, and a manufacturing method thereof. The method to manufacture a grain-oriented electrical steel sheet comprises: a step of providing a slab composed of 2.0-4.5 wt% of Si, 0.005-0.040 wt% of acid soluble Al, 0.20 wt% or less of Mn (excluding 0 wt%), 0.01 wt% or less of N (excluding 0 wt%), 0.01 wt% or less of S (excluding 0 wt%), 0.005-0.05 wt% of P, 0.03-0.08 wt% of C, 0.02-0.08 wt% of Sn, 0.01-0.1 wt% of Mo, and the remainder consisting of Fe and other inevitable impurities; a step of reheating the slab; a step of cold-rolling the slab after hot-rolling the slab; a step of annealing the cold-rolled steel sheet for decarburization and nitriding; and a step of finally annealing the steel sheet by spreading an annealing separator. According to the present invention, a silicon content in the steel sheet is increased and the steel sheet is heated at relatively high temperatures by optimally controlling the component system of Si, B, and others as a grain boundary segregation element among steel alloy elements; thereby manufacturing the grain-oriented electrical steel sheet having low iron loss, and excellent magnetism without warm rolling.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a directional electric steel sheet,

The present invention relates to a directional electrical steel sheet and a method of manufacturing the same, and more particularly, to a directional electrical steel sheet having improved magnetic properties and iron loss by controlling a component system and a method of manufacturing the same.

The oriented electrical steel sheet has a so-called Goss texture in which the orientation of all the grains on the steel sheet face is {110} plane and the crystal orientation in the rolling direction is parallel to the <001> axis, This is a very good soft magnetic material. In general, magnetic properties can be expressed by magnetic flux density and iron loss, and high magnetic flux density can be obtained by precisely aligning the orientation of the crystal grains in the {110} < 001 > orientation.

In general, to improve the magnetic properties of electric steel sheets, alloying element which can obtain a suppression effect similar to that of precipitates, unlike the technique of suppressing the growth of crystal grains by precipitates, is added so that secondary recrystallization annealing In the first recrystallization annealing process, the fraction of the goss texture in the primary recrystallization texture is increased to increase the secondary recrystallization microstructure fraction of the goss texture after the secondary recrystallization high-temperature annealing Technology, and a technique of uniformly distributing the size of the primary recrystallized grains so that the texture does not grow at all, which is caused by the nonuniformity of the primary recrystallized microstructure and does not contribute to the improvement of the magnetic properties at all.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems, and it is an object of the present invention to provide a steel sheet which is capable of optimally controlling components such as Si and B, A directional electric steel sheet having a low iron loss and excellent magnetic properties, and a method for producing the same.

The grain-oriented electrical steel sheet according to the present invention comprises 2.0 to 6.5% of Si, 0.040% or less of an acid soluble Al (not including 0%), 0.20% or less of Mn (not including 0% : Not more than 0.01% (not including 0%), S: not more than 0.01% (not including 0%), P: 0.005 to 0.05%, C: 0.04 to 0.12% Providing a slab comprising impregnating Fe and other inevitably incorporated impurities; Reheating the slab; Hot-rolling the slab to produce a hot-rolled steel sheet; Cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; Subjecting the cold-rolled steel sheet to decarburization annealing and steep annealing; And applying an annealing separator to perform final annealing; .

B in the slab may be 0.01 to 0.03%.

In the step of reheating the slab, the total amount of nitrogen reused in the slab may be 20 to 50 ppm.

In the step of reheating the slab, the reheating temperature may be a temperature range in which N and S to be dissolved are incompletely dissolved.

The method may further include a step of annealing the hot-rolled steel sheet by hot-

The average size of the precipitates in the steel sheet after annealing the hot-rolled sheet may be 200 to 3000 Å.

The step of producing the cold-rolled steel sheet can be rolled at a rate of 87% or more by one-time cold rolling.

The decarburization annealing and the steep annealing can be performed at 800 to 950 ° C.

The final annealing may be performed in a mixed atmosphere of nitrogen and hydrogen gas at a temperature rising period before the secondary recrystallization, and in a hydrogen atmosphere after completion of secondary recrystallization.

The grain-oriented electrical steel sheet according to the present invention comprises 2.0 to 6.5% of Si, 0.040% or less of an acid soluble Al (not including 0%), 0.20% or less of Mn (not including 0%), % Of P, 0.004 to 0.10% of C, 0.04 to 0.12% of C, 0.001 to 0.05 wt% of B, And the balance Fe and other inevitably incorporated impurities, and boron (B) or boron nitride is segregated in grain boundaries.

The B may be 0.01 to 0.03%.

In the grain-oriented electrical steel sheet according to the present invention, many goss particles with precise orientation are formed, and even after the first recrystallization heat treatment, a large number of grains remain, thereby increasing the number of goss particles causing secondary recrystallization. Therefore, after the secondary recrystallization, the crystal grain size decreases and the eddy current loss becomes small, so that the iron loss of the final product decreases, and the magnetic flux density also increases due to a large number of goss particles of the correct orientation.

Further, the grain-oriented electrical steel sheet according to the present invention does not cause plate breakage during rolling, so that the rolling property is increased and the workability is improved.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described in detail below. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims.

The grain-oriented electrical steel sheet according to an embodiment of the present invention contains 2.0 to 6.5% of Si, 0.040% or less of acid soluble Al (not including 0%), 0.20% or less of Mn (not including 0% ), N: not more than 0.01% (not including 0%), S: not more than 0.01% (not including 0%), P: 0.005 to 0.05%, C: 0.04 to 0.12%, B: %, Balance Fe and other inevitably incorporated impurities, and boron (B) or boron nitride is segregated in the grain boundaries.

The B may be 0.01 to 0.03%.

The reason for limiting the components of the present invention will be described.

[Si: 2.0 to 6.5 wt%]

Si is a basic composition of an electric steel sheet and plays a role of lowering the core loss by increasing the resistivity of the material.

When the Si content is less than 2.0 wt%, the resistivity is decreased, and the eddy current loss is increased to deteriorate the iron loss characteristic. In addition, during high temperature annealing, phase transformation between ferrite and austenite occurs, and the secondary recrystallization becomes unstable as well as the structure is severely damaged.

When the Si content exceeds 6.5 wt%, the magnetostrictive property and the magnetic permeability are remarkably poor.

[Acid soluble Al: 0.04 wt% or less]

In addition to AlN precipitated at the time of hot rolling and annealing of hot-rolled steel sheet, Al also has nitrogen ions introduced by ammonia gas in the annealing step after cold rolling combined with Al, Si and Mn existing in solid state in steel, Si, &lt; / RTI &gt; Mn) N and AlN type nitride to form a strong grain growth inhibitor.

When the acid soluble Al exceeds 0.040 wt%, a coarse nitride is formed and the crystal grain growth inhibiting ability is lowered.

[Mn: 0.20 wt% or less]

Mn has an effect of reducing the total iron loss by decreasing the eddy current loss by increasing the resistivity and reacting with the nitrogen introduced by the nitriding treatment together with Si to form precipitates of N (Al, Si, Mn) Which causes secondary recrystallization.

When Mn is more than 0.20 wt%, a large amount of (Fe, Mn) and Mn oxide are formed on the surface of the steel sheet in addition to Fe ₂ SiO ₄ , thereby hindering formation of a base coat formed during high temperature annealing, Since the phase transformation between ferrite and austenite is induced, the aggregate structure is seriously damaged and magnetic properties are greatly deteriorated. Therefore, the content of Mn is 0.20 wt% or less.

[N: 0.01 wt% or less]

N is preferably added in an amount of 0.01 wt% or less as an element that reacts with Al to form AlN.

If N exceeds 0.01 wt% in the steelmaking process, surface defects due to nitrogen diffusion are caused in the process after hot rolling, excessive nitrides are formed in the slab state, and the rolling property is lowered, so that the subsequent steps become complicated, Lt; / RTI &gt;

Further, in the annealing step after the cold rolling, N necessary for further forming nitrides such as N and AlN during the nitriding treatment using ammonia gas (Al, Si, Mn) can be reused.

[C: 0.04 to 0.12 wt%]

C is an element that causes phase transformation between ferrite and austenite and is an essential element for improving the rolling property of an electric steel sheet having a high brittleness and poor rolling property, but a carbide formed due to a magnetic aging effect when remaining in the final product It is an element that deteriorates magnetic properties.

If the content of C is less than 0.04% by weight in the range of the Si content according to the present invention, the phase transformation between ferrite and austenite does not work properly, resulting in nonuniformity of slab and hot rolled microstructure.

If it exceeds 0.12 wt%, not only a sufficient decarburization effect can not be obtained in the decarburization annealing process but also the secondary recrystallization texture is damaged due to the phase transformation phenomenon and the magnetic properties are deteriorated due to magnetic aging .

[S: 0.010 wt% or less]

S is an important element that reacts with Mn to form MnS.

When S is contained in an amount of 0.01 wt% or more, precipitates of MnS are formed in the slab to inhibit grain growth and it is difficult to control the microstructure in the subsequent process due to segregation at the center of the slab during casting. Further, since MnS is not used as a grain growth inhibitor in the present invention, S is preferably not added. However, it is preferably 0.01 wt% or less in consideration of the amount that is inevitably incorporated in the steelmaking process.

[B: 0.001 to 0.05 wt%]

B is kept in a solid state in the steel, and is used as a grain growth inhibitor by bonding with nitrogen introduced into the strip in the step of decarburization and nitriding. In addition, due to the increase of silicon content in steel, segregated grain boundaries are weakened to reduce the energy of grain boundaries, and grain size of hot rolled steel sheet and hot rolled and annealed steel sheet is miniaturized to improve ductility to alleviate brittleness due to increase of silicon content.

If the content of B is less than 0.001 wt%, not only the amount of reducing the grain boundary energy is insufficient, but also the amount of the inhibitor is insufficient to obtain a stable secondary recrystallized structure. When the content of B exceeds 0.05 wt%, the size of the inhibitor becomes large, A secondary recrystallized structure is obtained.

Therefore, the content of B is preferably 0.001 wt% or more and 0.05 wt% or less, more preferably 0.01 wt% or more and 0.03 wt% or less.

[P: 0.005 to 0.05 wt%]

P is segregated in the grain boundaries, and can interfere with the movement of the grain boundaries and at the same time can play an auxiliary role of suppressing grain growth and has an effect of improving {110} < 001 > When the content of P is less than 0.005 wt%, the effect of addition is not exhibited. When the content of P is more than 0.05 wt%, the brittleness is increased and the rolling property is greatly deteriorated.

Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described.

(Not including 0%), Mn: not more than 0.20% (not including 0%), N: not more than 0.01% (0%), , S: not more than 0.01% (not including 0%), P: 0.005 to 0.05%, C: 0.04 to 0.12%, B: 0.001 to 0.05 wt%, the remainder being Fe and other inevitably incorporated Thereby providing a slab containing impurities.

The slab is reheated. In the step of reheating the slab, the total amount of nitrogen reused in the slab may be 20 to 50 ppm.

The amount of reused N should be considered in consideration of the content of Al contained in the liquefied natural gas, because the nitrides used as the grain growth inhibitors are (Al, Si, Mn) N and AlN. That is, the reused N is influenced by the size and amount of the additional AlN formed in the decarburization annealing process. If the AlN size is the same, if the amount is too large, the grain growth restraining force increases, Secondary recrystallized microstructure can not be obtained. On the contrary, if the amount is too small, the crystal growth driving force of the primary recrystallized microstructure is increased, so that an appropriate secondary recrystallized microstructure can not be obtained similarly to the above-mentioned phenomenon.

In addition, it is preferable that the slab is reheated in a temperature range in which N and S which are dissolved in the step of reheating the slab are incompletely dissolved.

If N and S are completely dissolved, a large amount of nitride or sulfide is formed after annealing of the hot-rolled steel sheet, which makes it impossible to carry out a subsequent cold rolling step at a subsequent step, which requires an additional process. And the size of the primary recrystallized grains becomes considerably fine, so that proper secondary recrystallization can not be developed. For incomplete solution casting, it is preferable to reheat the slab to a temperature of 1250 DEG C or less.

The reheated slab is hot-rolled to produce a hot-rolled steel sheet.

Further, the hot-rolled steel sheet may further include a step of annealing the hot-rolled steel sheet.

In the hot-rolled hot-rolled sheet, there is a deformed structure stretched in the rolling direction due to the stress, and AlN or MnS precipitates during hot rolling. Therefore, in order to obtain uniform recrystallized microstructure and fine precipitate distribution of AlN before cold rolling, the hot rolled sheet is heated again to the temperature below the slab heating temperature to recrystallize the deformed structure, and sufficient austenite phase is secured, Annealing of the hot-rolled sheet is carried out to promote the employment of the grain growth inhibitor.

The annealing temperature of the hot-rolled sheet is preferably heated to 900 to 1200 ° C in order to maximize the austenite fraction, and then subjected to a crack heat treatment and then cooled. The average size of the precipitate in the strip after annealing annealing the hot-rolled sheet after applying the above-described heat treatment pattern may be in the range of 200 to 3000 ANGSTROM.

After the hot-rolled sheet annealing, cold-rolling is performed in a thickness of 0.10 mm to 0.50 mm, and a one-time cold rolling is performed without rolling annealing of the deformed structure in the middle, desirable.

With one strong cold rolling, orientations with low degrees of integration in the {110} < 001 > orientation are rotated in the direction of the deformation, and only the gothese grains best aligned with the {110} < 001 > orientation are present in the cold rolled plate. Therefore, in the two or more rolling methods, orientations having a low degree of integration are also present in the cold-rolled steel sheet, and secondary recrystallization is similarly performed at the time of the final high-temperature annealing to obtain low magnetic flux density and iron loss. Therefore, it is most preferable that the cold rolling is performed at a cold rolling rate of at least 87% by one-time cold rolling.

The cold-rolled cold-rolled steel sheet is subjected to decarburization and immersion treatment using ammonia gas. In the precipitation of N, AlN, (B, Si, Mn) N, (Al, B) N, BN and the like as inhibitors (Al, Si, Mn) by introducing nitrogen ions into the steel sheet using ammonia gas, And a method in which ammonia gas is used after completion of recrystallization or ammonia gas is used simultaneously so that nitrification can be performed simultaneously with decarburization, but the present invention is not limited thereto.

In the decarburization treatment and the nitriding treatment, the annealing temperature of the steel sheet is preferably in the range of 800 to 950 ° C. If the annealing temperature of the steel sheet is as low as 800 ° C or less, it takes a long time to decarburize and the SiO ₂ oxide layer is densely formed on the surface of the steel sheet to cause base coating defects. When heated to 950 ° C or more, And the crystal growth driving force is lowered so that stable secondary recrystallization is not formed.

Annealing time can be processed within 5 minutes considering productivity.

The tantalum and the soaked steel sheet are subjected to secondary recrystallization after the annealing separator is applied and then subjected to secondary recrystallization so that the {110} plane of the steel sheet is parallel to the rolling plane and the <001> direction is parallel to the rolling direction of {110} 001> forming a texture to produce a directional electrical steel sheet having excellent magnetic properties.

The final annealing is performed for formation of {110} < 001 > texture by secondary recrystallization, formation of a vitreous film by reaction of MgO with an oxide layer formed upon decarburization, and insulation removal and removal of impurities that impair magnetic properties. As a final high-temperature annealing method, the nitride as a grain growth inhibitor is protected by maintaining a mixed gas of nitrogen and hydrogen at a temperature rising period before the secondary recrystallization, so that the secondary recrystallization is well developed. After completion of the secondary recrystallization, For a long time to remove impurities.

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

[Example 1]

And the content of B was added by adding 3.2 wt% of Si, 0.055 wt% of C, 0.099 wt% of Mn, 0.0045 wt% of S, 0.0043 wt% of N, 0.028 wt% of an acid soluble Al, 0.028 wt% 1, and the remaining components were vacuum-melted in a directional electric steel sheet containing the remainder Fe and other inevitably contained impurities, and then the ingot was produced. Then heated to a temperature of 1200 캜, and hot-rolled to a thickness of 2.3 mm. The hot rolled sheet was heated to a temperature of 1050 占 폚, maintained at 950 占 폚 for 180 seconds, and quenched in water. The hot-rolled sheet was annealed at a temperature of 870 ° C for 180 seconds in a humidified atmosphere of hydrogen, nitrogen and ammonia, so that the nitrogen content was 200 ppm. Co-decarburized annealing annealing.

This steel sheet was coated with MgO as an annealing separator and finally annealed in a coiled state. In the final annealing, a mixed atmosphere of 25% nitrogen and 75% hydrogen was set up to 1200 ° C., and after reaching 1200 ° C., it was maintained in a 100% hydrogen atmosphere for 10 hours or more. Table 1 shows the measured values of magnetic properties for each condition.

B (wt%) _Iron loss (W _17/50 , W / kg) Flux density (B ₁₀ , Tesla) division - 0.888 1.891 Comparison 1 0.0003 0.890 1.892 Comparative material 2 0.0005 0.875 1.891 Comparative material 3 0.0006 0.872 1.935 Comparison 4 0.0052 0.811 1.925 Inventory 1 0.017 0.798 1.939 Inventory 2 0.024 0.798 1.938 Inventory 3 0.026 0.793 1.927 Invention 4 0.031 0.781 1.938 Invention Article 5 0.036 0.826 1.921 Inventions 6 0.041 0.822 1.931 Invention 7 0.049 0.831 1.883 Invention 8 0.056 0.954 1.885 Comparative material 5 0.068 0.975 1.883 Comparative material 6 0.075 0.945 1.889 Comparison 7

As can be seen in Table 1, the inventive material having an added amount of B in the range of 0.001 to 0.05 wt% has excellent magnetic characteristics as compared with the comparative material, and the added amount of B is 0.009 to 0.03 wt% It can be seen that there is a remarkable magnetic property improvement in the invention material.

[Example 2]

, The content of C was 0.053 wt%, the content of Mn was 0.097 wt%, the content of S was 0.0043 wt%, the content of N was 0.0047 wt%, the content of acid soluble Al was 0.028 wt%, the content of P was 0.027 wt% The remaining components were vacuum-melted in a directional electric steel sheet containing the remainder Fe and other inevitably contained impurities. The ingot was then heated to a temperature of 1200 캜 and hot-rolled to a thickness of 2.3 mm. The hot rolled sheet was heated to a temperature of 1050 占 폚, maintained at 950 占 폚 for 180 seconds, and quenched in water. The hot-rolled sheet was annealed at a temperature of 870 ° C for 180 seconds in a humidified atmosphere of hydrogen, nitrogen and ammonia so that the nitrogen content was 200 ppm. Co-decarburized annealing annealing.

This steel sheet was coated with MgO as an annealing separator and finally annealed in a coiled state. In the final annealing, a mixed atmosphere of 25% nitrogen and 75% hydrogen was set up to 1200 ° C., and after reaching 1200 ° C., it was maintained in a 100% hydrogen atmosphere for 10 hours or more. Table 2 shows the measured values of the magnetic properties for each condition.

Si (wt%) B (wt%) Iron loss
(W17 / 50, W / kg) Cold rolling property
O: No breakage
X: At least one fracture division 3.09 N / A 0.889 O COMPARISON 8 3.05 0.0005 0.881 O Comparative material 9 3.05 0.0006 0.887 O Comparative material 10 3.05 0.0008 0.885 O Comparative material 11 3.02 0.006 0.842 O Invention 9 3.04 0.018 0.824 O Inventions 10 3.06 0.036 0.839 O Invention invention 11 3.07 0.047 0.843 O Invention 12 3.12 0.051 0.879 O Comparative material 12 3.03 0.069 0.911 O Comparative material 13 3.41 N / A 0.888 O Comparative material 14 3.49 0.0002 0.884 O Comparative material 15 3.48 0.0004 0.881 O Comparative material 16 3.51 0.0008 0.881 O Comparative material 17 3.49 0.0039 0.822 O Invention invention 13 3.42 0.024 0.783 O Invention Article 14 3.45 0.035 0.801 O Invention material 15 3.42 0.049 0.813 O Invention material 16 3.42 0.057 0.874 O Comparative material 18 3.46 0.062 0.881 O Comparative material 19 3.87 N / A 0.889 X Comparative material 20 3.87 0.0002 0.882 X Comparative material 21 3.86 0.0004 0.882 X Comparative material 22 3.85 0.0005 0.882 X Comparative material 23 3.87 0.003 0.781 O Inventions 17 3.88 0.022 0.756 O Inventions 18 3.86 0.039 0.784 O Inventions 19 3.83 0.05 0.789 O Inventions 20 3.83 0.055 0.878 O Comparative material 24 3.82 0.069 0.889 X Comparative material 25 4.43 N / A 0.881 X Comparative material 26 4.46 0.0001 0.884 X Comparative material 27 4.42 0.0003 0.884 X Comparative material 28 4.46 0.0006 0.921 X Comparative material 29 4.43 0.007 0.766 O Invention Article 21 4.49 0.016 0.744 O Invention Article 22 4.46 0.037 0.762 O Invention Article 23 4.31 0.045 0.766 O Invention 24 4.45 0.051 0.876 O Comparative material 30 4.33 0.067 0.889 X Comparative material 31

As can be seen from the above Table 2, the inventive asphalt having an addition amount of B in the range of 0.001 to 0.05 wt% shows excellent rolling and magnetic properties as compared with the comparative material, and shows a remarkable difference as the silicon content increases . In addition, it can be seen that among the inventive materials, the magnetic properties are relatively superior to those of the inventive material having an addition amount of B of 0.009 to 0.03 wt%.

While the present invention has been described in connection with certain exemplary embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (15)

(Not including 0%), Mn: not more than 0.20% (not including 0%), N: not more than 0.01% (0%), S: not more than 0.01% (not including 0%), P: 0.005 to 0.05%, C: 0.04 to 0.12%, B: 0.001 to 0.05 wt%, the balance Fe and other inevitably incorporated impurities Providing a slab comprising the slab;
Reheating the slab;
Hot-rolling the slab to produce a hot-rolled steel sheet;
Cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;
Subjecting the cold-rolled steel sheet to decarburization annealing and steep annealing; And
Applying an annealing separator to perform final annealing;
Wherein the method comprises the steps of: The method according to claim 1,
And B in the slab is 0.009 to 0.03%. 3. The method according to claim 1 or 2,
Wherein the total amount of nitrogen recycled to the slab is 20 to 50 ppm in the step of reheating the slab. The method of claim 3,
Wherein the reheating temperature in the step of reheating the slab is a temperature range in which N and S to be dissolved are incompletely dissolved. 5. The method of claim 4,
Wherein the temperature range for incompletely fusing is 1250 DEG C or less. The method of claim 3,
Further comprising the step of annealing the hot rolled steel sheet by hot rolling,
Wherein the average size of the precipitates in the steel sheet after annealing the hot-rolled sheet is 200 to 3000 angstroms. The method according to claim 6,
Wherein the step of producing the cold-rolled steel sheet comprises a step of rolling the steel sheet at a ratio of not less than 87% 8. The method of claim 7,
Wherein the decarburization annealing and the steep annealing are performed at 800 to 950 占 폚. 9. The method of claim 8,
Wherein the final annealing is performed in a mixed atmosphere of nitrogen and hydrogen gas in a temperature rising period before the secondary recrystallization and in a hydrogen atmosphere after completion of secondary recrystallization. 10. The method of claim 9,
Wherein the step of performing the decarburization annealing and the steep annealing of the cold-rolled steel sheet is performed simultaneously with the decarburization annealing and the steep annealing. (Not including 0%), Mn: not more than 0.20% (not including 0%), N: not more than 0.01% (0%), , S: not more than 0.01% (not including 0%), P: 0.005 to 0.05%, C: 0.04 to 0.12%, B: 0.001 to 0.05 wt%, the remainder being Fe and other inevitably incorporated Wherein the boron (B) or boron nitride is segregated at grain boundaries. 12. The method of claim 11,
And B is 0.009 to 0.03%. (Not including 0%), Mn: not more than 0.20% (not including 0%), N: not more than 0.01% (0%), S: not more than 0.01% (not including 0%), P: 0.005 to 0.05%, C: 0.04 to 0.12%, B: 0.001 to 0.05 wt%, the balance Fe and other inevitably incorporated impurities Wherein the boron (B) or boron nitride having a content of recycled N in the range of 20 to 50 ppm after the slab heating is segregated in the grain boundaries. 14. The method of claim 13,
And B is 0.009 to 0.03%. 15. The method according to any one of claims 13 to 14,
Wherein the heated slab is subjected to hot rolling and then hot rolled sheet annealing, and the average size of the precipitates in the steel sheet after annealing the hot rolled steel sheet is 200 to 3000 Å.