KR19980044924A - Manufacturing method of directional electric steel sheet by low temperature slab heating method - Google Patents

Abstract

The present invention relates to a method for producing a grain-oriented electrical steel sheet having excellent magnetic flux density characteristics by adding a specific component controlled to be able to heat a slab at a low temperature and then stabilizing the secondary recrystallization and improving the directionality, For the formation of recrystallization breakfast, T ₂ = 600 + 9 × 10 ⁴ / A + 1 (° C) for primary nucleation of the secondary recrystallized structure with excellent direction and cracking at 400 ~ 650 ℃ for 15 ~ 25 hours. And a secondary slit for 15 to 25 hours at a satisfactory temperature to have a B ₁₀ value of 1.85 Tesla or more and to prevent magnetic deterioration of the inner coil portion.

Description

Manufacturing method of directional electric steel sheet by low temperature slab heating method

The present invention relates to a method of manufacturing a directional electric steel sheet used as an iron core material such as a transformer, a generator and other electronic equipment, and more particularly, to a method for manufacturing a directional electric steel sheet by adding a specific component controlled to heat a low temperature slab, To produce a directional electric steel sheet having excellent magnetic flux density characteristics.

Generally, the grain oriented electrical steel sheet has a texture in which the orientation of the grain is aligned in the (110) [001] direction, and the product has excellent magnetic properties in the cold rolling direction.

The magnetic properties of the grain-oriented electrical steel sheet are mainly represented by the magnetic flux density and the iron loss. The magnetic flux density is a magnetic flux density B ₁₀ induced in the iron core by a magnetic field of 1000 A / m and an iron loss is 1.7 Tesla The energy loss, W _17/50 , which is wasted as inferiority in the iron core when the magnetic flux density is obtained, is evaluated.

If a material with high magnetic flux density is used, it is possible to manufacture a small-sized and small-sized electric device. The smaller the iron loss, the greater the electric energy loss can be.

The (100) [001] texture is obtained by using a secondary recrystallization phenomenon. The secondary recrystallization is a crystal grains of a specific azimuth among fine grains formed by ordinary primary recrystallization, which is called a so-called Goss azimuth 110) [001] (usually referred to as a nucleus of secondary recrystallization) is abnormally grown throughout the specimen. When such a secondary recrystallization occurs completely and its directionality is excellent, Is known to be obtained.

In order to obtain a good magnetic flux density, it is necessary to stabilize the secondary recrystallization and simultaneously improve the directionality of the secondary recrystallization. For this purpose, the size of the primary recrystallized grains is uniform and the orientation of the primary recrystallized grains Tissue) is well encapsulated in the nucleus of the secondary recrystallization, and the secondary recrystallization in the growth process of the secondary recrystallization adheres to the ideal orientation, that is, it is advantageous to develop the secondary recrystallization with excellent orientation It is said to be. To achieve this goal, proper alloy design and appropriate process control accordingly are required.

Korean Patent Application No. 93-23751 discloses a steel slab which is subjected to hot rolling, pickling, primary cold rolling, intermediate decarburization annealing, secondary cold rolling, secondary annealing (non-recrystallization conditions) As a precondition for improving the directionality of the secondary recrystallization, the secondary annealing is performed at a temperature at which the primary recrystallization is not formed, so that the primary recrystallization occurs at the high temperature annealing , The temperature at which the secondary recrystallization starts is increased by increasing the primary recrystallization grain size, and thus a superior magnetic flux density is obtained. This is because the second recrystallization temperature is high and the probability of secondary recrystallization is increased only by the first recrystallization bins having a very small deviation between the rolling direction and (110).

However, the above-mentioned known technology has a problem that the magnetic property of the inner portion is deteriorated when the product is produced in the coil state, and the magnetic deviation in the longitudinal direction is caused as a whole. This magnetic deviation is presumed to be caused by a temperature difference between the inner coil portion and the outer coil portion or the central portion during the final high-temperature annealing in the above-described manufacturing process. Therefore, in the conventional high temperature annealing conditions, And the second recrystallization process is different.

Korean Patent Application No. 94-34279 discloses that the secondary recrystallization starting temperature varies depending on the amount of acid soluble Al as a method for solving the above problems, and it is known that T5? 950 + 5 占 (? 10 ³ ) + 5 (° C), where T _{5 is the} titration specific secondary cracking temperature, and α is the acid-soluble Al content (wt%).

However, this method is based on the accurate measurement of the acid soluble Al content, and it is difficult to apply it in consideration of the fact that the analysis error of the acid soluble Al content is about 5 to 8% in the range of 0.010 to 0.020%.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and it is an object of the present invention to provide a method of manufacturing a slab which can heat a slab at a temperature of 1250 to 1320 캜, The magnetic flux density of B ₁₀ > = 1.85 Tesla can be stably secured while preventing magnetic deterioration of the inner coil portion by optimizing the magnetic flux density.

A feature of the present invention having such an object is to induce nucleation of a secondary recrystallized structure having excellent directionality and finally to improve the directionality of the completed secondary recrystallization, , It is noted that the primary recrystallization texture changes before the start of the secondary recrystallization, and the primary and secondary cracking temperatures are changed during the high-temperature annealing together with the secondary annealing temperature. Thereafter, And the directionality (which can be replaced with the magnetic flux density value) was investigated. As a result, with the primary cracking treatment at 400 to 600 ° C. for 15 to 25 hours during the high temperature annealing, T ₂ = 600 + 9 × (T ₂ : titration secondary cracking temperature (° C), A: secondary annealing temperature (° C)) for 10 to 25 hours at a temperature satisfying 10 ⁴ / A + 10 .

The present invention relates to a steel sheet comprising, by weight%, C 0.025-0.043%, Si 2.95-3.30, Mn 0.12-0.43% S 0.007%, acid soluble Al 0.008-0.018%, N 0.0085-0.0110% % Of Cu, 0.4 to 0.55% of Cu, 0.03 to 0.06% of Cr, and the remainder of Fe, to obtain a silicon steel slab having a thickness of 150 to 350 mm;

Heating the silicon steel slab at 1250 to 1320 캜 for 3 to 6 hours and then hot-rolling the steel slab to a thickness of 1.8 to 2.5 mm;

The hot-dip coil is subjected to pre-annealing and pickling at 600 to 950 ° C for 30 seconds to 10 minutes and then at 830 to 930 ° C for 30 seconds to 10 minutes in a mixed gas atmosphere of wet nitrogen and hydrogen having a dew point of 30 to 70 ° C Followed by cold rolling twice, including intermediate annealing, to a final cold rolled coil having a thickness of 0.23 to 0.35 mm;

The hot-rolled coil is subjected to secondary annealing at a temperature of 450 to 600 ° C for 30 seconds to 10 minutes to form a recovered structure. Subsequently, in the secondary annealing coil, TiO ₂ is contained in an amount of 1 to 5% After application of the re-annealing separator;

Heating the annealing separator coating coil to 1170 to 1230 캜 in a mixed gas atmosphere of hydrogen or hydrogen and nitrogen at an elevated temperature of 10 to 50 캜 / hr to form a secondary recrystallized structure;

Forming a glass film at the above-mentioned temperature, 1170 to 1230 ° C, and heat-annealing at a high temperature for 10 to 30 hours to remove impurities; and heating the slab at a high temperature In order to form primary recrystallization during the annealing temperature, primary cracking is carried out at a temperature of 400 to 650 ° C for 15 to 25 hours and T ₂ = 600 + 9 × 10 ⁴ / A + 10 (℃) [where, T _2: fair secondary soaking temperature (℃), a: a secondary annealing temperature (℃)] characterized in that the temperature 15-25 hours secondary cracking in satisfying, B ₁₀ value And a low temperature slab heating method capable of preventing magnetic deterioration of the inner coil portion.

Hereinafter, the present invention will be described in more detail.

The composition and conditions of the silicon steel slab used in the present invention should satisfy the following conditions.

If C is less than 0.025%, the crystal grains grow coarsely during the heating of the slab, and the development of secondary recrystallization becomes unstable when the final high-temperature annealing is performed. If 0.043% is exceeded, it takes a long time for intermediate decarburization annealing.

When Si is less than 2.95%, excellent iron loss property is not obtained, and when it exceeds 3.30%, cold rolling property deteriorates, which is not preferable.

Mn is an element which forms austenite in the slab and facilitates the solidification of A1N. If it is added in an amount of less than 0.12%, the formation amount of austenite becomes too small. If it exceeds 0.43%, roll force is excessively increased The plate shape is not uniform because it becomes uneven.

If S is excessively added, the S segregation at the central portion of the slab is increased and homogenized. Since the slab must be heated to a temperature higher than the range of the present invention, it is preferable that the slab is contained at 0.007% or less.

Acid soluble Al and N are elements necessary for the formation of A1N precipitates. When the acid soluble Al is less than 0.008%, the directionality of the secondary recrystallization deteriorates and the magnetic flux density lowers. When the content of Al exceeds 0.008%, the secondary recrystallization becomes unstable.

On the other hand, if N is less than 0.0085%, the amount of A1N becomes insufficient, and when N is more than 0.011%, it is not preferable because Bilster type defects easily occur in the product.

In the present invention, P is limited to 0.015% or less, which is a controllable amount without causing an increase in cost in steelmaking because the steel sheet may cause plate breakage during cold rolling when the content of Mn is larger than usual.

Cu is an element that stabilizes the secondary recrystallization by contributing to solidification and fine precipitation of A1N as an austenite forming element. If it is added in an amount of less than 0.4%, the effect is insignificant and secondary recrystallization occurs unstably to degrade the magnetic properties. On the other hand, when it exceeds 0.55%, the decarburization degrades and the intermediate decarburization annealing time is lengthened I do not.

The Cr is an element that allows precipitates such as A1N to be uniformly distributed in the steel sheet after hot rolling. However, when the addition amount of Cr is less than 0.03%, the effect becomes insignificant. When the addition amount of Cr exceeds 0.06%, the effect is not shown to be larger. Therefore, it is added to the amount of 0.06% or less to reduce the cost increase due to the addition of the expensive alloy It is good.

The steel component of the present invention is as described above, and the others are composed of Fe and inevitable trace impurities. The above-mentioned silicon steel material can be used as a material of the present invention even when it is produced by any conventional melting method, roughing method, and playing method.

Next, the heating conditions of the silicon steel slab composed of the above-described steel components are preferably 1250 to 1320 DEG C for 3 to 6 hours. At the heating temperature and time under the range of the present invention, solidification of the precipitate such as A1N becomes insufficient and excellent magnetic properties can not be obtained. In addition, at the heating temperature and time exceeding the range of the present invention, So that the present invention is excluded from the scope of the present invention.

If the thickness of the slab is too thin, the productivity of hot rolling is deteriorated. If the thickness of the slab is too thick, the slab heating time should be long. Therefore, it is preferable to control the thickness to 150 to 350 mm.

The hot rolled coil is usually made of a hot rolled coil having a thickness of 1.8 to 2.5 mm in consideration of a subsequent optimum cold rolling reduction by ordinary hot rolling.

Preliminary annealing is preferably carried out at a temperature of 600 to 950 ° C for 30 seconds to 10 minutes to prevent pickling and to prevent coarsening of A1N.

It is not preferable since it is difficult to form a film which is easy to pick up at a heating temperature and time which is less than the range of the present invention, and the time required for the pickling process is increased. Secondary recrystallization occurs at the heating temperature and time exceeding the range of the present invention, It is not good because it becomes unstable.

The pre-annealing and picked-up coils are adjusted to a final thickness by cold rolling two times with intermediate decarburization annealing interposed therebetween. If the intermediate decarburization annealing temperature is less than 830 占 폚 or the annealing time is less than 30 seconds, it is difficult to lower the residual carbon content to below the allowable limit. If the temperature exceeds 903 占 폚 or the annealing time exceeds 10 minutes, The crystal grains are coarsened and the secondary recrystallization becomes unstable.

If the dew point of the atmospheric gas is less than 30 ° C during the intermediate decarburization annealing, the decarburization becomes insufficient, and if it exceeds 70 ° C, a dense surface scale is formed to deteriorate the cold rolling property.

When the thickness of the final cold-rolled coil is less than 0.23 mm, the secondary recrystallization is not well developed, and when it exceeds 0.35 mm, excellent iron loss characteristics can not be obtained.

If the temperature of the secondary annealing is less than 450 ° C or the time is less than 30 seconds, the surface flatness of the cold rolled coil is lowered, which is not preferable because the coil sticking phenomenon occurs after the finish annealing at a high temperature. After the secondary annealing, a fine primary recrystallization is generated and the excellent magnetic flux density can not be obtained, and the annealing time exceeding 10 minutes is not economical.

In the case of high-temperature annealing, the heating rate should be controlled within the range of 10-50% / hr in order to form a proper primary recrystallization texture and to completely induce secondary recrystallization. As the atmospheric gas for the high-temperature annealing, it is preferable to use dry hydrogen or a mixed gas of hydrogen and nitrogen to form a glass film and remove residual impurities such as N, S and the like.

When the high-temperature cracking is performed at a temperature lower than 1170 占 폚 or when it is cracked for less than 10 hours, it is difficult to form a satisfactory grating film and to remove a smooth impurity. When cracking occurs at a temperature exceeding 1230 占 폚 or over 30 hours, Are excluded from the scope of the present invention.

The reason for limiting the numerical values for the two-time cracking treatment during the elevation of the high temperature annealing, which is the core of the present invention, is as follows.

When the primary cracking temperature is less than 400 ° C or the primary cracking time is only 15 hours, the primary recrystallization structure is not formed in the primary cracking process and remains as a recovery structure. As a result, The recrystallized structure is formed very coarse and the secondary recrystallization driving force (grain boundary energy) is reduced, which results in unstable secondary recrystallization, which is not preferable.

If the temperature exceeds 650 ° C, the primary recrystallization size becomes too small and the secondary recrystallization occurs at a low temperature. At this time, the inner coil portion having a lower temperature than the coil outer portion or the central portion is unfavorable because the secondary recrystallization becomes unstable .

Also, the primary cracks exceeding 25 hours do not cause a decrease in magnetic flux density or magnetic deterioration of the inner coil portion, but are uneconomical.

Secondary cracks are very important for the nucleation of the secondary recrystallized structure having excellent directionality. The inventors of the present invention have found that when the secondary cracking temperature is appropriately adjusted according to the secondary annealing temperature, that is, T ₂ = 600 + The second annealing is carried out at a temperature that satisfies the following formula: 9 x 10 ⁴ / A + 10 (° C) where T _{2 is the} appropriate secondary cracking temperature (° C) and A is the secondary annealing temperature It was confirmed that an excellent magnetic flux density was obtained by improving the directionality.

When the secondary cracking time is less than 15 hours, the directionality improvement effect of the secondary recrystallization does not appear, so that the excellent magnetic flux density can not be obtained. Secondary cracks exceeding 25 hours do not show the improvement of the magnetic flux density .

The coil surface on which the inorganic glass coating is formed by the high-temperature annealing may be subjected to a tension-imparting coating after the high-temperature annealing for the purpose of improving the insulating property and improving the iron loss by micro-

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

[Example 1]

0.04% C, 0.05% Si, 0.005% Mn, 0.25% Mn, 0.005% S, 0.005% And the remainder Fe was prepared.

The slab was heated at 1300 占 폚 for 5 hours and then hot-rolled to obtain a hot-rolled coil having a thickness of 2.0 mm. Subsequently, preliminary annealing and pickling for 2 minutes at 930 ° C, primary cold-rolling were performed to adjust the thickness to 0.6 mm, and then intermediate annealing was performed at 870 ° C for 3 minutes in a 25% H ₂ + 75% N ₂ atmosphere having a dew point of 50 ° C , Followed by final cold rolling to a thickness of 0.285 mm;

Secondary annealing was performed by varying the annealing temperature and time as shown in Table 1 below. At this time, a dry 5% H ₂ + 95% N ₂ mixed gas was used as the atmosphere gas, and then an annealing separator composed of 1.5% TiO 2 and MgO by weight% was applied to the surface of the steel sheet and then subjected to finish annealing.

As shown in Table 1, the finish annealing was performed by varying the temperature and time at the time of secondary cracking for primary cracking to form a primary recrystallized structure and secondary recrystallization nucleation with excellent directionality.

Thereafter, the temperature was raised to 1200 ° C at a heating rate of 25 ° C / hr to cause secondary recrystallization, followed by a heat treatment cycle of cooling for 10 hours at the above temperature for cooling to remove impurities. As the atmospheric gas, 25% N ₂ + 75% H ₂ was used, and pure gas was used in the 1200 ° C cracking period.

As described above, the primary recrystallization microstructure, the secondary recrystallization development rate, and the magnetic properties after the self-aging heat treatment were applied to the coils which changed the secondary annealing temperature and time and changed the primary and secondary crack time- The properties are shown in Table 1 below. The primary recrystallized microstructure was observed by optical microscopy at 3% of the cross-sectional microstructure of the specimen taken just before the initiation of the second-stage cracking during the temperature increase of the final high-temperature annealing. The secondary recrystallization development rate was , And the microstructure exposed by etching with a 20% hydrochloric acid solution heated to about 80 ° C by the coil portion was observed. The magnetic flux density was measured by a single-plate magnetometer for the coil outer portion where no magnetic deterioration occurred B ₁₀ (magnetic flux density induced at 1000 A / m excitation force), and the magnetic deterioration of the inner coil portion was judged by observing the iron loss value change in the coil length direction using a continuous iron loss meter.

[Table 1]

A primary cracking of the high-temperature annealing temperature was raised 400 ~ 650 ℃, 15 ~ 25 sigan performed, along the secondary cracking in the second annealing temperature _{T 2 = 600 + 9 × 10} 4 / A + 10 (℃) [ where, T ₂ (About 30 to about 80%) of the second order cracks (Inventive Material 1 to Inventive Material 8) at a temperature that satisfies the following conditions: the optimum secondary cracking temperature (占 폚), A: the secondary annealing temperature (占 폚) (B ₁₀ < / = 1.85 Tesla), and the magnetic deterioration phenomenon of the inner coil portion did not occur. The secondary recrystallization of the secondary recrystallization was excellent.

On the other hand, in the case of 380 占 폚 (Comparative material 1) where the primary cracking temperature is lower than the range of the present invention and 10 hours in which the primary cracking time is less than the range of the present invention (Comparative material 6), the primary recrystallization grain size becomes too large, The secondary recrystallization was unstable. As a result, excellent magnetic flux density was not obtained. In the case of 670 캜 at which the primary crack temperature exceeded the range of the present invention (Comparative materials 2 and 3), the primary recrystallization grain size was too small, The secondary recrystallization was fully developed at the outer portion and the center portion of the coil, but the secondary recrystallization occurred unstably at the outer portion of the coil, resulting in magnetic deterioration.

Even when the primary cracking condition is within the scope of the present invention, when the secondary cracking temperature deviates from the desired value (comparative materials 4 and 5) and the secondary cracking time is too short (comparative material 8), the secondary recrystallization nuclei Was not generated, although the magnetic deterioration phenomenon of the inner coil portion did not appear, but the magnetic flux density was inferior.

The present invention has been accomplished by adding a specific component controlled to enable low-temperature slab heating and then stabilizing the secondary recrystallization and improving the directionality, thereby obtaining a directional electric steel sheet having excellent magnetic flux density characteristics.

Claims (1)

0.005 to 0.0110%, P: 0.015%, Cu: 0.005 to 0.03%, C: 0.025 to 0.043%, Si: 2.95 to 3.30, Mn: 0.12 to 0.43% : 0.4 to 0.55%, Cr: 0.03 to 0.06%, and the remainder Fe, to prepare a silicon steel slab having a thickness of 150 to 350 mm; The hot-dip coil is subjected to pre-annealing and pickling at 600 to 950 ° C for 30 seconds to 10 minutes and then at 830 to 930 ° C for 30 seconds to 10 minutes in a mixed gas atmosphere of wet nitrogen and hydrogen having a dew point of 30 to 70 ° C Followed by cold rolling twice, including intermediate annealing, to a final cold rolled coil having a thickness of 0.23 to 0.35 mm; The secondary cold coercive rolling coils are subjected to secondary annealing at a temperature of 450 to 600 ° C for 30 seconds to 10 minutes to form a recovered structure. Subsequently, in the secondary annealing coil, TiO ₂ is added in an amount of 1 to 5% After applying an annealing separator composed of MgO; Heating the annealing separator coating coil to 1170 to 1230 캜 in a mixed gas atmosphere of hydrogen or hydrogen and nitrogen at an elevated temperature of 10 to 50 캜 / hr to form a secondary recrystallized structure; Subsequently, at a temperature of 1170 to 1230 ° C, a high temperature annealing is performed to form a glassy coating film and finish with a thermal cycle for 10 to 30 hours to remove impurities. In order to form primary recrystallization buffers during the high-temperature annealing, primary cracking is performed at a temperature of 400 to 650 ° C for 15 to 25 hours and T ₂ = 600 + 9 × 10 ⁴ / A + 10 (占 폚) for 15 to 25 hours at a temperature that satisfies A + 10 (占 폚).