KR20190077965A - Manufacturing method of oriented electrical steel sheet - Google Patents

Abstract

The present invention provides a method for manufacturing an oriented electric steel sheet. The method for manufacturing an oriented electric steel sheet includes: a step of heating a slab including 2.0-6.0 wt% of Si; 0.02-0.08 wt% of C; 0.01 wt% or less (excluding 0 wt%) of N; and residues including Fe and inevitable impurities; a step of manufacturing a hot rolled sheet by hot rolling the slab; a step of annealing the hot rolled sheet; a step of cooling the hot rolled sheet annealed; a step of manufacturing a cold rolled sheet by cold rolling the cooled cold rolled sheet; a step of first recrystallizing and annealing the cold rolled sheet; and a step of secondly recrystallizing and annealing the cold rolled sheet firstly recrystallized and annealed. The step of hot rolling and annealing the hot rolled sheet includes a crack step of onetime or more. The cooling step cools the following equation 1 to satisfy the following equation 1. [equation 1]T_s<= 1045×n^0.14+20 (unit: °C)(In equation 1, T_s means crack temperature when the crack step is executed onetime or more. When executing the crack step multiple times, the T_s means the last crack temperature. n means a work hardening index of the hot rolled sheet annealed.)

Description

TECHNICAL FIELD [0001] The present invention relates to a method for manufacturing a directional electric steel sheet,

To a method for producing a directional electrical steel sheet. Specifically, the present invention relates to a method for producing a grain-oriented electrical steel sheet excellent in cold rolling and magnetic properties.

The oriented electrical steel sheet is a soft magnetic material having excellent magnetic properties in one direction or rolling direction because the texture of the gusset is {110} < 001 > in the rolling direction. Complicated processes such as component control in steelmaking, regeneration of slabs in hot rolling and hot-rolling process control, hot-rolled sheet annealing, cold rolling, primary recrystallization annealing, and secondary recrystallization annealing are required. It must be precisely and strictly controlled.

On the other hand, the quality and quantity of the Goss texture is attributed to the aggregate structure of the hot-rolled sheet, and the process control which enables the secondary recrystallization without damaging the Goss texture as much as possible through the hot-rolled sheet annealing, cold rolling and primary recrystallization annealing The factor is very important for commercial purposes. The origins of Goss assemblage are roughly divided into two groups, hot rolled sheet texture and cold rolled aggregate, as known by many scholars.

In the hot rolled sheet assembly texture, the annealing process of hot rolled sheet is important in the process of optimizing the hot rolled sheet aggregate structure in the post-process hot rolled sheet annealing process, and the latter cold rolled aggregate structure is already subjected to hot rolling annealing The hot-rolled sheet annealing process becomes very important in both cases.

The annealing of the hot-rolled sheet can be roughly classified into three stages. The first is a heating step for heating the hot-rolled sheet to reuse the coarse precipitates and impurities, controlling the highly deformed hot-rolled sheet microstructure relatively homogeneously, A cracking step for finely controlling the precipitation of reused residues in the heating step and stabilizing the microstructure in the heating step and a third step for cooling the precipitate controlled in the cracking step and the cooling step for keeping the microstructure stably to room temperature .

In the prior art, only a technique of annealing a hot-rolled steel sheet for improving the magnetic properties of a grain-oriented electrical steel sheet is presented, and no concrete technique for achieving both cold rolling productivity and magnetic property improvement has been made yet.

One embodiment of the present invention is to calculate the cold start temperature (T _s ) at which the cold rolling can be performed using the work hardening index (n) of the hot-rolled sheet and to control it to lower the fracture occurrence rate of the steel sheet during cold rolling A method for producing a directional electrical steel sheet is provided.

A method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention includes: 2.0 to 6.0% of Si, 0.02 to 0.08% of C, 0.01% or less of N (excluding 0%), the balance Fe and unavoidable impurities Heating the slab including the slab; Hot rolling the slab to produce a hot rolled sheet; Annealing the hot-rolled sheet by hot-rolling; Cooling the hot rolled sheet annealed; Cold-rolling the cooled hot-rolled sheet to produce a cold-rolled sheet; Subjecting the cold-rolled sheet to primary recrystallization annealing; And a second recrystallization annealing step of subjecting the cold-rolled sheet obtained by the first recrystallization annealing to a second recrystallization annealing step, wherein the step of annealing the hot rolled sheet comprises one or more cracking steps, Lt; / RTI &gt;

[Formula 1]

T _s ? 1045 占 n ^0.14 + 20 (unit: 占 폚)

(In the formula (1), T _s means the crack temperature at that time when the cracking step is performed once, and means the latest cracking temperature when it is performed a plurality of times. N is the temperature at which the hot- Means hardening index.)

In the cooling step, cooling may be performed to satisfy the following formula (2).

[Formula 2]

T _s ≤ 1621 × n + 500 (unit: ° C)

(In the expression (2), T _s means the crack temperature at that time when the cracking step is performed once and means the latest cracking temperature when it is performed a plurality of times. Means hardening index.)

In the cooling step, the work hardening index (n) of the hot-rolled sheet annealed in the hot-rolled sheet may be 0.005 to 0.228.

The step of annealing the hot-rolled sheet may include a first cracking step and a second cracking step, and the second cracking step may be performed at a temperature of 800 to 1200 ° C.

Wherein the slab is made of a material selected from the group consisting of 0.005 to 0.04% of Al, 0.01 to 0.2% of Mn, 0.01% or less of S (excluding 0%), 0.005 to 0.045% of P, 0.03 to 0.08% of Sn, % And Cr: 0.01 to 0.2%.

In the step of heating the slab, it may be heated to 1250 占 폚 or lower.

In the primary recrystallization annealing step, primary recrystallization annealing can be performed at 800 to 950 占 폚.

In the secondary recrystallization annealing step, secondary recrystallization can be completed at a temperature of not less than the primary recrystallization annealing temperature and not more than 1210 캜.

According to the method for producing a grain-oriented electrical steel sheet according to an embodiment of the present invention, the cooling start temperature (T _s ) at which cold rolling can be performed is calculated using the work hardening index (n) of the hot- It is possible to minimize the defective rate.

As a result, the quality of the grain-oriented electrical steel sheet can be improved and the productivity can be increased.

Fig. 1 is a graph showing the correlation of the cooling start temperature with the work hardening index (n).
FIG. 2 is a graph showing the temperature change with time in the annealing step of the hot-rolled sheet and the cooling step according to an embodiment of the present invention.

The terms first, second and third, etc. are used to describe various portions, components, regions, layers and / or sections, but are not limited thereto. These terms are only used to distinguish any moiety, element, region, layer or section from another moiety, moiety, region, layer or section. Thus, a first portion, component, region, layer or section described below may be referred to as a second portion, component, region, layer or section without departing from the scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not expressly express the opposite meaning thereto. Means that a particular feature, region, integer, step, operation, element and / or component is specified and that the presence or absence of other features, regions, integers, steps, operations, elements, and / It does not exclude addition.

When referring to a portion as being "on" or "on" another portion, it may be directly on or over another portion, or may involve another portion therebetween. In contrast, when referring to a part being "directly above" another part, no other part is interposed therebetween.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

Unless otherwise stated,% means% by weight, and 1 ppm is 0.0001% by weight.

In an embodiment of the present invention, the term further includes an additional element, which means that an additional amount of the additional element is substituted for the remaining iron (Fe).

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

Directional electric steel sheet manufacturing method

A method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention includes: 2.0 to 6.0% of Si, 0.02 to 0.08% of C, 0.01% or less of N (excluding 0%), the balance Fe and unavoidable impurities A step of hot rolling the slab to produce a hot rolled sheet, a step of annealing the hot rolled sheet to anneal the hot rolled sheet, a step of cooling the hot rolled sheet annealed, a step of cold rolling the cold rolled sheet, A step of producing a plate, a step of annealing the cold-rolled sheet by primary recrystallization, and a step of annealing the cold-rolled sheet subjected to the primary recrystallization annealing and secondary recrystallization annealing, wherein the step of annealing the hot rolled sheet comprises one or more cracking steps And cooled in a cooling step so as to satisfy the following formula (1).

[Formula 1]

T _s ? 1045 占 n ^0.14 + 20 (unit: 占 폚)

(In the formula (1), T _s means the crack temperature at that time when the cracking step is performed once, and means the latest cracking temperature when it is performed a plurality of times. N is the temperature at which the hot- Means hardening index.)

In the method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention, cooling may be performed so as to satisfy Equation (2).

[Formula 2]

T _s ≤ 1621 × n + 500 (unit: ° C)

(In the expression (2), T _s means the crack temperature at that time when the cracking step is performed once and means the latest cracking temperature when it is performed a plurality of times. Means hardening index.)

In the method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention, the slab may contain 0.005 to 0.04% of Al, 0.01 to 0.2% of Mn, 0.01% or less of S (excluding 0%), P : 0.005 to 0.045%, Sn: 0.03 to 0.08%, Sb: 0.01 to 0.05%, and Cr: 0.01 to 0.2%.

First, the reasons for limiting the slab component will be described below.

Si: 2.0 to 6.0%

Silicon (Si) is a basic composition of an electric steel sheet, and it plays a role of lowering the core loss by increasing the resistivity of the material. When Si is added too little, the resistivity decreases, the eddy current loss increases, the iron loss characteristic decreases, and during the decarburization annealing, the phase transformation between the ferrite and the austenite becomes active and the primary recrystallization texture is seriously damaged. In addition, during high-temperature annealing, phase transformation between ferrite and austenite occurs, and secondary recrystallization becomes unstable, and {110} < 001 >

On the other hand, when too much Si is added, when decarbonitization annealing, SiO ₂ and Fe ₂ SiO ₄ The oxide layer is excessively and densely formed, and the decarburization behavior can be delayed. As a result, the phase transformation between the ferrite and the austenite continuously occurs during the decarburization annealing, so that the primary recrystallization texture can be severely damaged. The nitriding behavior is delayed due to the delayed decarburization behavior caused by the formation of the dense oxide layer, so that nitrides such as N and AlN can not be sufficiently formed (Al, Si, Mn), so that sufficient crystal grain restraining force necessary for secondary recrystallization You can not.

In addition, the mechanical properties of the electrical steel sheet increase in brittleness and the toughness decreases, so that the occurrence rate of plate fracture during the rolling process is intensified, and the plate weldability is lowered, thereby failing to ensure easy workability. As a result, if the Si content is not controlled within the above-mentioned predetermined range, the formation of the secondary recrystallization becomes unstable, and the magnetic properties may be seriously damaged and the workability may be deteriorated.

C: 0.02 to 0.08%

Carbon (C) is an element which contributes to grain refinement by causing phase transformation between ferrite and austenite and contributes to improvement of elongation, and is an essential element for improving the rolling property of an electric steel sheet having poor brittleness.

However, in the case of remaining in the final product, the content can be controlled to an appropriate level because the carbide formed due to the magnetic aging effect is precipitated in the product plate to deteriorate the magnetic properties.

The content of C added in the slab is added in the range of 0.02 to 0.08%. When C is contained in the slab in an amount of less than 0.02% in the range of the Si content described above, the phase transformation between the ferrite and the austenite does not sufficiently occur, which causes unevenness of the slab and hot rolled microstructure, have.

On the other hand, after the heat treatment of annealing the hot-rolled sheet, the residual carbon in the steel sheet activates the fixing of the dislocations during the cold rolling, thereby increasing the shear deformation band and increasing the generation site of the Goss nucleus. Therefore, if the C content is more than 0.08% in the slab in the range of the Si content described above, a separate process or equipment is added to the slab. The decarburization annealing process can not obtain sufficient decarburization, and the secondary recrystallization texture is severely damaged due to the phase transformation caused by the decarburization annealing process. When the final product is applied to electric power equipment, the magnetic properties It may cause deterioration phenomenon.

N: not more than 0.01%

Nitrogen (N) is an important element that reacts with Al to form AlN, and the content of N added to the slab is added to 0.01% or less. If it is contained in an amount exceeding 0.01%, surface defects called blisters due to diffusion of nitrogen are caused in the process after hot rolling, and since too much nitride is formed in the slab state, rolling becomes difficult and the subsequent steps become complicated, May be caused.

On the other hand, in the annealing step after cold rolling, N is further required to form nitrides such as (Al, Si, Mn) N, AlN, (Si, Mn) N, Reinforce.

Al: 0.005 to 0.04%

In addition to AlN precipitated at the time of hot rolling and hot-rolled annealing, aluminum (Al) is combined with Al, Si, and Mn in which nitrogen ions introduced by ammonia gas exist in a solid state in steel during annealing after cold rolling (Al, Si, Mn) N and AlN type nitride to form a nitride, thereby acting as a strong grain growth inhibitor.

When the Al content is less than 0.005%, sufficient effect as an inhibitor can not be expected because the number and volume of the nitride are formed at a considerably low level. When the Al content exceeds 0.04%, coarse nitride is formed, Can fall.

Mn: 0.01 to 0.2 wt%

Manganese (Mn) is an element that decreases the total iron loss by decreasing the eddy current loss by increasing the resistivity same as Si. (Al, Si, Mn) precipitates by reacting with S in a lukewarm state to form Mn-based sulfides and reacting with nitrogen introduced by the nitriding treatment together with Si to form precipitates of N, thereby suppressing the growth of primary recrystallized grains 2 It is an important element for causing tea recrystallization. When the Mn content is less than 0.01%, sufficient effect as an inhibitor can not be expected because the number and volume of the precipitates are low, and when the Mn content exceeds 0.2%, Fe ₂ SiO ₄ In addition, (Fe, Mn) and Mn oxides are formed in large amounts, which hinders formation of a base coating formed during high-temperature annealing, which may deteriorate surface quality. Since the phase transformation between ferrite and austenite is caused in the high temperature annealing process, the aggregate structure is seriously damaged and the magnetic properties may be greatly deteriorated.

S: not more than 0.01%

When the content of sulfur (S) exceeds 0.01%, 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. Therefore, when MnS is not used as a crystal grain growth inhibitor, it may not be added more than the amount of S inevitably enters.

P: 0.005 to 0.045%

Phosphorus (P) segregates in the grain boundaries and interferes with grain boundary movement, and at the same time can play an auxiliary role of suppressing crystal grain growth and has an effect of improving {110} < 001 >

When the P content is less than 0.005%, the effect of addition is insignificant, and when the P content exceeds 0.045%, the brittleness is increased and the rolling property may be significantly deteriorated.

Sn: 0.03 to 0.08%

Tin (Sn) is known as a grain growth inhibitor because it is an element that interferes with the movement of grain boundaries as a grain boundary segregation element, like P. In the predetermined Si content range of the present invention, when annealing at a high temperature, since grain growth growth inhibiting ability for smooth secondary recrystallization behavior is insufficient, Sn which interferes with the movement of the grain boundaries by segregation to grain boundaries is necessarily required.

When the Sn content is less than 0.03%, the effect of improving the magnetic properties is insignificant. On the other hand, when the Sn content exceeds 0.08%, unless the heating rate is controlled or maintained for a predetermined time in the first recrystallization annealing section, the crystal grain restraining force is too strong to obtain a stable secondary recrystallization.

Sb: 0.01 to 0.05%

Antimony (Sb) segregates in grain boundaries like P and has the effect of suppressing the growth of crystal grains and has the effect of stabilizing secondary recrystallization. However, since the melting point is low, it is easy to diffuse to the surface during the primary recrystallization annealing, and there is an effect of preventing the decarburization or the steepness due to the formation of the oxide layer and the nitriding. Therefore, the addition of Sb above a certain level has an upper limit of addition because it inhibits decarburization and inhibits the formation of an oxide layer which forms the base coating.

When the Sb content is less than 0.01%, the grain growth inhibiting effect is insignificant. On the other hand, when the Sb content exceeds 0.05%, the crystal grain growth inhibiting effect and diffusion to the surface become severe, and thus stable secondary recrystallization can not be obtained and the surface quality can be deteriorated.

Cr: 0.01 to 0.2%

Cr promotes the formation of a {110} < 001 > texture during cold rolling by promoting the formation of a hard phase in the annealed sheet of hot rolled steel sheet, accelerating decarburization of C during decarburization annealing, It is possible to reduce the austenite phase holding time so as to prevent the phenomenon. It is possible to solve the disadvantage that formation of an oxide layer is inhibited by Sn and Sb among alloy elements used as a crystal grain growth supplementation inhibitor by promoting the formation of an oxide layer on the surface to be formed during the decarburization annealing process.

When the Cr content was less than 0.01%, the above-mentioned effect was less than when the Cr content was less than 0.01%. If the Cr content exceeds 0.2%, the formation of the oxide layer may be rather disadvantageous during the decarburization annealing process, and the decarburization and the soaking may be interrupted.

Co: 0.005 to 0.1%

Cobalt (Co) is an alloying element that is effective in increasing magnetic flux density to increase the magnetic flux density, and at the same time, it increases the specific resistance and reduces iron loss.

When the Co content is less than 0.005%, the effect of improving the magnetic flux density is insignificant, and sufficient iron loss reduction effect can not be expected. On the other hand, when the Co content is more than 0.1%, the production cost is increased due to the high price, and the amount of the austenite phase transformation is increased, which may have an unfair influence on the microstructure, precipitate and texture.

First, the slab is heated. When the slab is reheated, it can be heated to 1250 ° C or less. As a result, precipitates of Al-based nitride or Mn-based sulfide can be incompletely dissolved or completely dissolved depending on the chemical equivalence relationship between Al and N, M and S to be solved. Next, when the heating of the slab is completed, normal hot rolling is carried out so that the thickness of the hot-rolled sheet becomes 1.0 to 3.5 mm.

Thereafter, hot-rolled sheet annealing is performed. The step of annealing the hot-rolled sheet includes one or more cracking steps. Specifically, as shown in Fig. 2, the step of annealing the hot-rolled sheet may include a primary cracking step and a secondary cracking step, as shown in Fig. 2, after the heating step of heating the hot-rolled sheet. The primary cracking step may be performed at a temperature (T _e1 ) of 900 to 1300 ° C and the secondary cracking step may be performed at a temperature (T _e2 ) of 800 to 1200 ° C.

After the hot-rolled sheet is heated, it may undergo a primary cracking process for a predetermined time and then a secondary cracking process. If the cracking temperature is less than 800 ° C in the secondary cracking step, the amount of the precipitate in the hot-rolled steel sheet may be increased and the size may become finer. If the cracking temperature exceeds 1200 ° C, The size of the primary recrystallized grains becomes very uneven after the decarburization annealing, and the secondary recrystallization occurs unstably during the high temperature annealing, so that the magnetic properties may be lowered.

Next, in the cooling step, the hot-rolled sheet is cooled from the temperature not higher than the cooling start temperature (T _s ) calculated through the above-mentioned formula (2). Specifically, it can be cooled to room temperature of 20 to 30 占 폚.

The cooling start temperature (T _s ) means the cracking temperature at which the cracking step is carried out once, and means the latest cracking temperature when it is carried out a plurality of times. Specifically, the hot-rolled sheet there is the annealing upon performing a heating step and a step in one or more cracks, if done only one time of cracking step, and that the soaking temperature is the start temperature (T _s) cooling time, two times or more cracking step The lowest cracking temperature is the cooling start temperature T _s .

As shown in FIG. 2, if the step of cooling the hot-rolled sheet is performed at 20 ° C to 30 ° C, which is the room temperature (T ₀ ), even if the multi-stage cooling is performed in such a manner that the cooling rate changes at T _v , The cracking temperature T _e2 in the secondary cracking step is the cooling start temperature.

The work hardening index (n) means that the annealed hot-rolled steel sheet was subjected to a tensile test at a rate of 10 mm / min at room temperature after the steel sheet was processed with a tensile test piece of JIS-13B standard, and the elongation was measured between 5 and 10% do.

The temperature calculated by substituting the measured work hardening index (n) into 1045 x n ^0.14 +20, which is the right side of Equation 1, becomes a value equal to or higher than the cooling start temperature (T _s ) Only a slight level of cracking occurred. This can be confirmed from FIG.

Further, in the cooling step, the work hardening index (n) is substituted into 1621 x n + 500, which is the right side of the equation (2), becomes a value equal to or higher than the cooling start temperature (T _s ) The steel sheet showed good cold rolling resistance without any cracks. This can be confirmed from FIG.

That is, by controlling the relationship between the right term value of Equation 1 or the right term value of Equation 2 and the cooling start temperature T _s calculated through the work hardening index n of the hot-rolled sheet to satisfy Equation 1 or Equation 2, It is possible to minimize the defect rate during cold rolling. As a result, the quality of the grain-oriented electrical steel sheet can be improved and the productivity can be increased. In cold rolling as a subsequent step, since the rolling property is excellent and the goss texture is improved, a grain-oriented electrical steel sheet excellent in magnetic properties can be produced.

The work hardening index (n) of the hot-rolled sheet annealed in the hot-rolled sheet may be 0.005 to 0.228. (N) when the temperature calculated by 1621 x n + 500 and the temperature calculated by 1045 x n ^0.14 +20 are the same. When the work hardening index (n) is less than 0.005 or exceeds 0.228, the temperature value calculated by 1621 x n + 500 is higher than the temperature calculated by 1045 x n ^0.14 +20.

Next, after cooling the hot rolled sheet, cold rolling twice or more times including cold rolling to intermediate annealing is performed so that the thickness of the cold rolled sheet is 0.1 to 0.5 mm.

The cold-rolled steel sheet is subjected to decarburization, recrystallization of the deformed structure, and nitriding treatment using ammonia gas. In the precipitation of N, AlN, etc. which are inhibitors (Al, Si, Mn) by introducing nitrogen ions into the steel sheet by using ammonia gas, nitriding treatment is performed using ammonia gas after completion of decarburization and recrystallization, There is no problem in exerting the effect of the present invention in any of the methods using ammonia gas at the same time. In the decarburization treatment, recrystallization and nitridation treatment, the annealing temperature of the steel sheet can be heat-treated in the range of 800 to 950 ° C.

When the annealing temperature of the steel sheet is less than 800 ° C., decarburization takes a long time. If the annealing temperature exceeds 950 ° C., the recrystallized grains grow to a great extent and the crystal growth driving force drops, so that stable secondary recrystallization is not formed. The annealing time is not a big problem for exhibiting the effect of the present invention, but can be adjusted within 5 minutes in consideration of productivity.

The decarburized nitrided annealed steel sheet was subjected to annealing after MgO-based annealing was performed on the steel sheet by reducing and removing some or all of the oxide layers present in the outer oxide layer formed on the surface of the steel sheet in a reducing atmosphere Apply the agent. Thereafter, a {110} < 001 > aggregate structure in which the {110} planes of the steel sheet are parallel to the rolled surface and the < 001 > direction is parallel to the rolling direction can be formed by performing final annealing for a long time to cause secondary recrystallization.

Thereafter, in the secondary recrystallization annealing step, the secondary recrystallization can be completed at a temperature not lower than the primary recrystallization annealing temperature and not higher than 1210 캜. The purpose of the secondary recrystallization annealing is to provide insulating property by the formation of {110} < 001 > aggregate structure by secondary recrystallization, formation of a vitreous film by reaction of the oxide layer and MgO formed at decarburization, and removal of impurities that impair magnetic properties. Secondary recrystallization annealing is carried out by maintaining a mixed gas of nitrogen and hydrogen at a temperature rising period before the secondary recrystallization to protect the nitride as the grain growth inhibitor so that the secondary recrystallization can be well developed. After the secondary recrystallization is completed, % Hydrogen atmosphere for a long time to remove impurities.

Hereinafter, specific examples of the present invention will be described. However, the following examples are only a concrete example of the present invention, and the present invention is not limited to the following examples.

Example

0.05 wt% of C, 0.005 wt% of N, 0.025 wt% of Sol-Al, 0.01 wt% of Mn, 0.005 wt% of S, 0.025 wt% of P, 0.07 wt% of Sn, 0.02 wt% of Sb, 0.03 wt% of Cr, 0.03 wt% of Co, and Si were changed as shown in Table 1 below, and the remaining components were vacuum-melted in a directional electric steel sheet containing the remainder Fe and other inevitably contained impurities, And then heated to a temperature of 1150 캜 and hot-rolled to a thickness of 2.3 mm. Thereafter, the substrate was heated to 1080 캜, subjected to a primary crack heat treatment for a predetermined time, subjected to a secondary heat treatment for a predetermined time at the temperature shown in Table 1 below, and the cooling start temperature The cooling was performed as shown in Table 1.

The cold rolled steel sheet was cold rolled once to a thickness of 0.23 mm, and then subjected to simultaneous decarburization annealing so as to have a nitrogen content of 170 ppm at a temperature of 860 DEG C in a mixed hydrogen gas atmosphere of nitrogen and ammonia at a temperature of 860 DEG C Respectively.

This steel sheet was coated with MgO as an annealing separator and then subjected to secondary recrystallization annealing. The steel sheet was heated to 1200 DEG C in a mixed gas atmosphere of 25% nitrogen and 75% hydrogen, and after reaching 1200 DEG C, Hour, and then it was cooled.

Table 1 shows the results of the measurement of the plate fracture occurrence, the work hardening index and the magnetic properties after the annealing of the secondary recrystallization in the cold rolling according to the change of the cooling start temperature (T _s ).

Si (% by weight) Equation 1 Right value (℃) Equation 2 Right value (℃) Cooling start temperature (℃) n Fracture occurrence _Iron loss (W _17/50 ) division 2.11 851 815 760 0.194 X 0.876 Example 1 2.1 843 795 860 0.182 O 0.906 Comparative Example 1 2.12 858 836 550 0.207 X 0.872 Example 2 2.33 847 805 760 0.188 X 0.866 Example 3 2.33 841 790 860 0.179 O 0.897 Comparative Example 2 2.34 855 826 550 0.201 X 0.86 Example 4 2.7 844 797 760 0.183 X 0.855 Example 5 2.75 836 777 860 0.171 O 0.888 Comparative Example 3 2.72 851 815 550 0.194 X 0.845 Example 6 3.0 841 790 760 0.179 X 0.846 Example 7 3.01 828 758 860 0.159 O 0.882 Comparative Example 4 3.01 848 806 550 0.189 X 0.836 Example 8 3.1 841 790 760 0.179 X 0.832 Example 9 3.09 830 763 860 0.162 O 0.868 Comparative Example 5 3.11 843 795 550 0.182 X 0.824 Example 10 3.32 830 763 860 0.162 O 0.855 Comparative Example 6 3.34 845 798 550 0.184 X 0.811 Example 11 3.51 826 754 860 0.157 O 0.847 Comparative Example 7 3.55 844 797 550 0.183 X 0.799 Example 12 3.69 829 759 860 0.16 O 0.836 Comparative Example 8 3.71 841 790 550 0.179 X 0.792 Example 13 3.92 820 740 860 0.148 O 0.826 Comparative Example 9 3.94 835 774 550 0.169 X 0.78 Example 14 4.2 829 759 760 0.16 △ 0.787 Example 15 4.27 814 729 860 0.141 O 0.812 Comparative Example 10 4.19 833 771 550 0.167 X 0.775 Example 16 4.5 819 738 760 0.147 △ 0.773 Example 17 4.48 805 711 860 0.13 O 0.8 Comparative Example 11 4.54 826 753 550 0.156 X 0.767 Example 18 2.0 863 849 760 0.215 X 0.888 Example 19 2.01 856 829 960 0.203 O 0.915 Comparative Example 12 2.01 865 857 550 0.22 X 0.885 Example 20

In Table 1, the value of the right side of Equation 1 (° C) means the work hardening index (n) of the hot-rolled sheet annealed and the upper limit of the T _s temperature calculated through Equation 1. The value (° C) in Equation 2 means the work hardening index (n) of the hot-rolled sheet annealed in the hot-rolled sheet and the upper limit of the T _s temperature calculated by Equation (2).

The cooling start temperature (캜) means the actual temperature at which the actual cooling starts. Since the primary cracking step and the secondary cracking step have been performed, the cracking temperature in the secondary cracking step is the cooling starting temperature.

n means that the hot-rolled sheet annealed as a work hardening index was subjected to a tensile test at a rate of 10 mm / min and a stretching rate of 5 to 10% after a tensile test piece of JIS-13B standard was processed .

When no crack was observed on the surface of the steel sheet, it was indicated by X. When fine cracks were observed, it was indicated by DELTA, and when the steel sheet was broken, it was indicated by O.

The iron loss (W _17/50) is the average loss (W / kg) in the rolling direction to the rolling direction when the vertical magnetic flux density of the organic 1.7Tesla is at 50Hz frequency.

As shown in Table 1, in Examples 1 to 14, Example 16 and Examples 18 to 20, since the cooling start temperature (占 폚) is lower than the rightward value (占 폚) of Formula 1 and the rightward value (占 폚) No cracks were observed on the steel sheet. In Examples 15 and 17, a fine level of cracks was observed on the steel sheet after the cold rolling since the cooling start temperature (占 폚) was lower than the value of the right side of Equation 1 (占 폚) and higher than the value of right side of Equation 2 (占 폚).

On the other hand, in Comparative Examples 1 to 12, since the cooling start temperature (占 폚) was higher than the right side value (占 폚) of Equation 1, the steel sheet was broken after cold rolling.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and their equivalents. It will be understood that the invention may be embodied in other specific forms without departing from the spirit or scope of the invention. It is therefore to be understood that the embodiments and / or the examples described above are illustrative in all aspects and not restrictive.

Claims (8)

Heating a slab comprising Si: 2.0 to 6.0%, C: 0.02 to 0.08%, N: 0.01% or less (excluding 0%), the balance Fe and unavoidable impurities, in weight percent;
Hot rolling the slab to produce a hot rolled sheet;
Annealing the hot-rolled sheet by hot-rolling;
Cooling the hot rolled sheet annealed;
Cold-rolling the cooled hot-rolled sheet to produce a cold-rolled sheet;
Subjecting the cold-rolled sheet to primary recrystallization annealing; And
And secondary recrystallization annealing the cold-rolled sheet subjected to the primary recrystallization annealing,
Wherein the step of annealing the hot-rolled sheet comprises one or more cracking steps,
And cooling in the cooling step so as to satisfy the following formula (1).
[Formula 1]
T _s ? 1045 占 n ^0.14 + 20 (unit: 占 폚)
(In the formula (1), T _s means the crack temperature at that time when the cracking step is performed once, and means the latest cracking temperature when it is performed a plurality of times. N is the temperature at which the hot- Means hardening index.) The method according to claim 1,
In the cooling step,
(2). &Lt; / RTI &gt;
[Formula 2]
T _s ≤ 1621 × n + 500 (unit: ° C)
(In the expression (2), T _s means the crack temperature at that time when the cracking step is performed once and means the latest cracking temperature when it is performed a plurality of times. Means hardening index.) The method according to claim 1,
In the cooling step,
Wherein the working hardening index (n) of the hot-rolled sheet annealed is 0.005 to 0.228. The method according to claim 1,
Wherein the step of annealing the hot-rolled sheet includes a first cracking step and a second cracking step,
Wherein the secondary cracking step is performed at a temperature of 800 to 1200 占 폚. The method according to claim 1,
The slabs
P: 0.005 to 0.045%, Sn: 0.03 to 0.08%, Sb: 0.01 to 0.05%, and Cr: 0.005 to 0.04% 0.01 to 0.2%. &Lt; / RTI &gt; The method according to claim 1,
In the step of heating the slab,
And heating it to 1250 DEG C or less. The method according to claim 1,
In the primary recrystallization annealing step,
Wherein the first recrystallization annealing is performed at 800 to 950 占 폚. The method according to claim 1,
In the secondary recrystallization annealing step,
Wherein the secondary recrystallization is completed at a temperature of not less than the primary recrystallization annealing temperature and not more than 1210 占 폚.

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