JP4565264B2 - Method for rolling hot rolled steel strip for grain-oriented electrical steel and method for producing grain-oriented electrical steel sheet - Google Patents

Description

  The present invention relates to a rolling method of a hot rolled steel strip for directional electrical steel used in the manufacture of directional electrical steel sheets used for iron cores of transformers and other electrical equipment, and more particularly, a slab for directional electrical steel heated at high temperature. Of producing directional electrical steel sheet hot-rolled steel strip with high product yield by preventing the occurrence of ear cracks when hot-rolling steel, and directional electrical steel sheet using hot-rolled steel strip obtained thereby It relates to a method of manufacturing.

  The grain-oriented electrical steel sheet is used as a soft magnetic material mainly as an iron core material for a transformer or a rotating device, and is required to have a high magnetic flux density and a small iron loss and magnetostriction. With the recent deterioration of energy situation and the increase in power investment due to the aging of power transmission facilities, there is an increasing need to economically supply grain-oriented electrical steel sheets with excellent magnetic properties.

  In order to obtain a grain-oriented electrical steel sheet having excellent magnetic properties, it is necessary to obtain a secondary recrystallized structure in which crystal grains are highly accumulated in the so-called Goth orientation, that is, the {110} <001> orientation. A general method for producing such a grain-oriented electrical steel sheet is to perform hot rolling by heating a slab for grain-oriented electrical steel containing an appropriate inhibitor, and then subjecting it to hot rolling as necessary once or once. A cold-rolled sheet with the final product thickness is obtained by cold rolling at least twice with intermediate annealing, and after decarburization annealing is applied, an annealing separator mainly composed of MgO is applied to form a coil. It consists of a series of processes for winding and high-temperature finishing annealing.

  Each of these processes has a great impact on product quality. Of these, the slab heating and the subsequent hot rolling process have a secondary recrystallized structure that is highly accumulated in the Goth direction throughout the process. It means that the inhibitor to be obtained is in an appropriate precipitation dispersion state. Therefore, slab heating is performed in a high temperature range of 1250-1450 ° C for a long time to dissociate and dissolve the inhibitor component, and then rough rolling and finish rolling are performed under appropriate conditions to optimize the inhibitor precipitation state. Processing is performed.

  However, when such high-temperature and long-time slab heating is performed, abnormal growth of slab crystal grains is induced and coarse crystal grains are generated. Among the coarsened grains, those present at the slab side end are difficult to be reduced during the rough rolling stage, so that recrystallization does not proceed easily. Grain remained, which was the cause of cracks (hereinafter simply referred to as “ear cracks”) generated in the ears of the hot-rolled steel strip. Since this edge crack causes the steel strip to break in the subsequent cold rolling process, it is necessary to cut the edge prior to cold rolling, resulting in a decrease in product yield and an increase in cost. Such a phenomenon is particularly noticeable when a so-called continuous cast slab obtained by continuous casting is used as a material. This is because, in continuous cast slabs, the columnar crystal structure generated with rapid solidification tends to abnormally grow during slab heating, and coarse unrecrystallized grains have poor toughness, and cracks are likely to occur during hot finish rolling. Because.

  As a means for preventing such ear cracks, for example, Patent Document 1 discloses a method of promoting recrystallization of coarse grains by changing a rolling schedule during rough rolling. Patent Document 2 discloses a method for controlling the temperature difference between the start and end of finish rolling, and Patent Document 3 reduces the temperature difference in the longitudinal and width directions of the material to be rolled before finish rolling. A method is disclosed. Furthermore, Patent Literature 4 and Patent Literature 5 disclose a method of performing sheet bar width reduction.

Japanese Patent Publication No.57-4690 JP 55-62124 A JP 57-165102 A Japanese Patent Publication No. 64-3564 Japanese Patent Publication No. 3-68432

  However, the means described in Patent Document 1 has a problem that since the change of the rolling schedule depends only on the horizontal roll, a sufficient stress is not applied to the side surface of the material to be rolled and the effect is poor. On the other hand, the means disclosed in Patent Documents 2 and 3 both prevent ear cracks by reducing the temperature non-uniformity of the material to be rolled during finish rolling. It does not act directly on the edge (hereinafter simply referred to as “steel strip ear”) and is not a fundamental solution. In addition, the means disclosed in Patent Documents 4 and 5 are easily affected by fluctuations in the slab heating condition, and may not sufficiently prevent the ear cracks.

  In view of the problem that none of the above-disclosed conventional means is a means for fundamentally solving ear cracks, the present invention provides hot rolled steel for grain-oriented electrical steel for effectively preventing ear cracks. It aims at proposing the rolling method of a band, and the manufacturing method of a grain-oriented electrical steel sheet.

  The present inventor has investigated in detail the influence of the slab heating state before hot rolling on the occurrence of ear cracks, and that decarburization has progressed in the slab surface layer along with the slab heating, and decarburization of this slab surface layer. When the amount falls within a predetermined range, the effect of sheet bar width reduction disclosed in Patent Document 5 often appears, and further, during the series of steps, an appropriate amount of width reduction rolling is performed before rough rolling, thereby causing cracks. The inventors have found that it can be effectively prevented and have completed the present invention.

In the present invention, a slab for grain-oriented electrical steel containing C: 0.01-0.08% and Si: 2.5-4.1% by mass ratio is heated to 1000-1250 ° C. by a gas heating furnace. Further, after rolling slabs at 1250-1450 ° C. by an induction heating furnace, a rolling method for hot rolled steel strip for directional electrical steel that performs rough rolling and finish rolling, and the slab heating conditions for the starting material are as follows: adjusted to a ratio of C content in the center layer of the slab to the C content of up to the surface layer 5mm slab width direction sides to be subjected to rough rolling (Cs) (Cc) a (Cs / Cc) becomes 0.40 or more 0.86 or less The slab heating conditions for the starting material are adjusted , and after the heating in the gas heating furnace, the rolling reduction at a rolling reduction of 3% or more is performed at least once before the rough rolling.

  In the above invention, the width reduction rolling may be performed after heating by the gas heating furnace and before heating by the induction heating furnace, and after the width reduction rolling, before the heating by the induction heating furnace, a reduction ratio of 5 % Or more rolling.

  Moreover, the width reduction rolling can be performed after heating by an induction heating furnace. These width rolling can be performed either after heating by the gas heating furnace, before heating by the induction heating furnace, or after heating by the gas heating furnace and after heating by the induction heating furnace, but should be performed in both of them. You can also.

  A directional electrical steel sheet is manufactured by performing cold rolling, decarburization annealing and final finish annealing on the directional electrical steel strip obtained by the rolling method of the hot rolled steel strip for directional electrical steel described in the above method. can do.

  According to the present invention, it is possible to effectively prevent the edge cracks generated in the steel band edge portion during finish rolling of the hot rolled steel band for directional electromagnetic steel. This eliminates the need for edge cutting prior to cold rolling or reduces the amount of edge cutting, thereby improving product yield and thus reducing costs.

In the present invention, basically, the composition component of the starting material is 1000% by a gas heating furnace with respect to a slab for grain-oriented electrical steel containing C: 0.01 to 0.08% and Si: 2.5 to 4.1% in a trade volume ratio. In the rolling method for hot-rolled steel strips for grain-oriented electrical steel sheets in which rough rolling and finish rolling are performed after heating to ~ 1250 ° C and further slab heating at 1250 to 1450 ° C by an induction heating furnace, the following (1 ) To (3) to satisfy the purpose.
(1) Adjusting the slab heating conditions, the ratio (Cs / Cc) of the C content (Cc) of the center layer of the slab to the C content (Cs) of the surface layer up to 5 mm on the side surface in the slab width direction subjected to rough rolling Must be 0.40 or more and 0.86 or less.
(2) After the heating in the gas heating furnace, before the rough rolling, the width rolling at a rolling reduction of 3% or more is performed at least once.
(3) In the above (1), when the width reduction rolling is performed before the heating in the induction heating furnace, the rolling reduction of 5% or more is performed following the width reduction rolling, if necessary.
Hereinafter, the above conditions will be described sequentially.

(Starting material components)
The basic composition of the slab, which is the starting material of the present invention, contains C: 0.01 to 0.08%, Si: 2.5 to 4.1%, and Mn: 0.03 to 0.10% as necessary, one of S and Se Or 2 types (total amount): 0.005-0.1%, Sol.Al:0.01-0.05%, N: 0.004-0.012% is contained, and the remainder consists of Fe except an unavoidable impurity.

  C is effective for improving the crystal structure by utilizing the α-γ transformation during hot rolling, but if it is too much, decarburization becomes difficult, so 0.01 to 0.08% is set. If Si is too small, the electrical resistance of the steel sheet will decrease and eddy current loss will increase, so iron loss will deteriorate. If it is too much, cold rolling will be difficult and scrap will increase due to breakage, etc. To do. Mn is a component that forms an inhibitor, but if it is excessive, the particle diameter of the inhibitor becomes coarse and the grain growth inhibiting power decreases, so the content is made 0.08 to 0.10%. Se and S are components that form inhibitors together with Mn and Cu. However, if excessive, surface defects due to grain boundary cracking during hot rolling increase, and purification during finish annealing becomes difficult. Therefore, the total content is set to 0.005 to 0.1%. Al and N are components that form AlN as an inhibitor. If the Al content is too small, the magnetic flux density will decrease. If the Al content is too large, the secondary recrystallization will not be stable, so Al should be in the range of 0.01-0.05% as sol.Al (acid-soluble Al). If the magnetic flux density decreases and is too large, surface defects due to blistering during slab heating increase, so the range is 0.004 to 0.012%.

  In addition to the above basic components, Sb or Sn can be contained as a grain boundary segregation inhibitor for improving magnetic properties. However, if the content is too small, the effect of improving the magnetic properties is small. If the content is too large, the brittleness is lowered and the forsterite insulating film is adversely affected. Is preferred. In order to improve the properties of the forsterite green film, it is effective to contain 0.03 to 0.20% of Cu. Furthermore, in order to improve the crystal structure by utilizing the surface α-γ transformation during hot rolling, it is also effective to contain 0.05 to 0.8% Ni. Each of these elements can be used alone or in any combination to obtain the desired effect. The balance is Fe except for inevitable impurities, but it is also possible to contain various elements that can be arbitrarily added in the production of grain-oriented electrical steel sheets, such as Cr, Mo, P, etc. .

  The starting material having the above composition can be produced by preparing molten steel having a predetermined composition and making it into a slab by a known method, for example, a normal ingot-making method or a continuous casting method.

(Slab heating conditions)
The slab as the starting material is heated to 1000 to 1250 ° C. using a gas heating furnace such as a pusher type or a walking beam type. Then, it heats at high temperature using the induction heating furnace provided after the slab heating furnace. If the heating temperature of this high temperature heating is lower than 1250 ° C, the inhibitor component is not sufficiently dissolved, and a sufficient precipitated dispersed phase cannot be formed in the subsequent process. The yield is lowered and the life of the furnace is shortened, so the temperature is set to 1250 to 1450 ° C.

  In the present invention, in this slab heating stage (including both gas heating and induction heating), the C content (Cs) up to a surface layer of 5 mm (hereinafter simply referred to as “slab surface layer 5 mm”) on the side surface in the width direction of the slab subjected to rough rolling. The slab heating conditions are adjusted so that the ratio (Cs / Cc) of the C content (Cc) of the central layer of the slab (hereinafter simply referred to as “slab central layer”) to 0.40 or more and 0.86 or less.

  Fig. 1 shows a slab for grain-oriented electrical steel sheets made of Fe except for C: 0.058%, Si: 3.0%, Mn: 0.071%, S: 0.018%, N: 0.005%, and the remainder unavoidable impurities. Heating within the temperature range of 1100-1200 ° C, then heating in the temperature range of 1350-1450 ° C using an induction furnace, followed by rough rolling and finish rolling, C content up to 5mm slab surface layer It is a graph which shows the relationship between the ratio (Cs / Cc) of C content (Cc) of the slab center layer with respect to the amount (Cs) and the depth of the edge crack generated in the finish-rolled hot-rolled steel strip. As shown in FIG. 1, the ear crack depth can be reduced by setting the Cs / Cc ratio to 0.40 to 0.86. In addition, in FIG. 1, (circle) is what carried out width reduction rolling in the range of 3 to 5% before rough rolling, and (triangle | delta) is what did not perform width reduction rolling before rough rolling, The effect of reducing ear cracks is observed in all cases.

  In addition, the C content (Cs) up to 5 mm of the slab surface layer when obtaining the above results, the C content (Cc) of the slab center layer is finished with a slab having the same composition as that subjected to finish rolling. After cooling the slab heated under the same conditions as provided, drill in the center of the slab width direction and the center of the slab length direction in the direction perpendicular to the slab side (slab width direction) The C content (Cs) up to 5 mm from the slab surface is determined by the chips obtained from the slab surface to 5 mm, and the slab center by the 5 mm chips in the width direction when further drilling is performed to reach the center of the slab width direction. The C content (Cc) of the layer is determined. In addition, the ear crack depth is measured by rewinding the coiled hot-rolled steel strip obtained by finish rolling and individually measuring the depth of the ear crack that appears in the hot-rolled steel strip. The maximum value at is the ear crack depth.

  Causes of ear cracking when the ratio (Cs / Cc) of the C content (Cc) of the center layer of the slab to the C content (Cs) of the slab surface layer up to 5 mm is outside the above range When Cs / Cc is lower than 0.40, the surface layer decarburization is large, so the surface layer becomes α single phase and the structure is not improved by α-γ transformation, and the slab crystal grains are excessively coarsened. On the other hand, if it is higher than 0.86, the surface decarburized layer, that is, the α single-phase part is thin, so the void generated in the α + γ phase immediately below it is a sheet. It is presumed that this is because it easily reaches the surface layer of the bar and deep ear cracks occur regardless of the slab crystal grains.

  Therefore, in the present invention, when the slab is heated, the C content of the slab surface layer up to 5 mm, in other words, the decarburization amount and the decarburization depth of the surface layer are adjusted to the optimum range, thereby (Cs / Cc) within the above range. It is necessary to keep it inside. Such a (Cs / Cc) ratio can be achieved by adjusting the oxygen concentration in the gas heating furnace, preferably in the range of 0.1 to 5.0% in the gas heating furnace. In the present invention, the slab heating is performed by the induction heating furnace following the gas heating furnace. At that time, the decarburization amount and the decarburization depth of the slab surface layer are adjusted by adjusting the atmosphere in the gas heating furnace. The atmosphere is preferably non-oxidizing.

  Fig. 2 shows the same conditions as those obtained in Fig. 1, with heating in a gas heating furnace, heating in an induction heating furnace, rough rolling, edge rolling, and finish rolling, and (1) gas Immediately after heating in the heating furnace, (2) after the gas heating and further heating by the induction heating furnace, when the width reduction rolling is performed, respectively, the reduction ratio of the width reduction rolling and the finish-rolled hot rolled steel strip It is a graph showing the relationship with the generated ear crack search.

  From the above results, it can be seen that, in any case, the depth of the ear crack becomes small when the reduction amount of the width reduction rolling is 3% or more. In this case, the method for determining the C content (Cs) up to 5 mm of the surface layer of the slab and the C content (Cc) of the central layer of the slab is the same as that for obtaining the result of FIG. The Cc ratio was set to an average level of 0.60 to 0.70.

  As apparent from the above, in the present invention, in the slab heating stage, immediately after heating by the gas heating furnace or after heating by the gas heating furnace and after heating by the induction heating furnace, the width rolling rolling with a width rolling reduction rate of 3% or more is achieved. In this case, the ear crack depth is 2 mm or less. Such a result can be obtained when the width reduction rolling is performed both after heating in the gas heating furnace and in the induction heating furnace.

  In the view of the present inventor, the cause of the occurrence of ear cracks in the steel strip after finish rolling is that the columnar crystals of the continuous cast slab are huge (greater than 50 mm, for example) in the slab heating stage. Although it grows, the strain introduced by normal rough rolling cannot sufficiently recrystallize such huge grains, and it becomes a state where coarse stretched grains and fine recrystallized grains are mixed, and the sheet after rough rolling The coarse stretched grains that remain without recrystallizing on both sides of the bar discontinuously jump out, resulting in complex undulations, which causes uneven width expansion and local stress concentration during the finish rolling stage. It is estimated that cracks occur inside the ears under triaxial stress and eventually lead to ear cracks.

  The width reduction before the rough rolling of the present invention is to recrystallize the coarse grains in which the columnar crystals of the continuous cast slab have grown enormously in the slab heating stage, and thereby recrystallize. It cuts off the ear cracking mechanism. Therefore, in the present invention, the width reduction ratio before rough rolling requires 3% or more at any stage. There is no need to set the upper limit of the width reduction ratio. However, in order to suppress the occurrence of surface defects such as edge seams (defects in which the wrinkles generated at the end of the plate have turned around the surface near the end), the width reduction ratio is preferably 15% or less. . In addition, when the width reduction rolling is performed both immediately after the gas heating and further after the induction heating, the width reduction rolling of 3% or more is performed, thereby reducing the size of the coarse grains generated at the end of the slab. Can be done automatically.

  A known method can be applied to the width reduction method. For example, a vertical roll may be used, and the reduction may be performed by a sizing press. Any means can be used as long as coarse particles produced at the end of the slab can be obtained.

  In the present invention, when the width reduction rolling is performed immediately after heating in the gas heating furnace, sometimes the slab cross-section becomes a dogbone shape by the width reduction rolling, and it is difficult to charge the induction heating furnace. In such a case, it is effective to carry out rolling (horizontal rolling) that reduces the thickness at a rolling reduction of 5% or more following the width rolling.

  As described above, slabs that have been subjected to at least one width reduction rolling with a reduction ratio of 3% or more after the heating in the gas heating furnace are finished into a hot-rolled steel strip by rough rolling and finish rolling. It is done. That is, after a sheet bar of about 20 to 60 mm is obtained by rough rolling, it is finished to a hot rolled steel strip of about 1.6 to 3 mm by finish rolling and wound into a coil by a normal process. The hot-rolled steel strip finished in this way has an ear crack depth of 3 mm or less, and the shape of the ear part is extremely good without occurrence of a double plate due to dogbone.

  The means for rough rolling and finish rolling are not particularly limited. If necessary, an edge burner or heat insulation cover may be used to prevent the temperature of the side portion of the seat bar from decreasing, and the sheet bar may be subjected to width reduction rolling to correct the shape of the seat bar ear. Good.

  The hot-rolled steel strip thus obtained is finished into a grain-oriented electrical steel sheet by performing cold rolling, decarburization annealing and final finish annealing by a conventional method. The conditions for that may follow what can be generally adopted. Examples of such general means are as follows. That is, if necessary, hot-rolled sheet annealing that is preferably 800 ° C. or higher and 1100 ° C. or lower is performed, and then cold rolling is performed once or two or more times with intermediate annealing between them to obtain a cold-rolled sheet having a final finished thickness. To do. The resulting cold-rolled sheet is decarburized and annealed, and then an annealing separator is applied, and final finish annealing is performed at a high temperature of around 1200 ° C to develop a secondary recrystallized structure and form a forsterite film on the surface. Let After final finish annealing, shape correction is performed by flattening annealing as necessary, and an insulating coating that applies tension to the steel sheet surface is applied to improve iron loss. It should be noted that various means employed in the manufacture of so-called high magnetic flux density electrical steel sheets, such as mirror finishing, magnetic domain subdivision, and immersion method after decarburization annealing, are free, and the effects of the present invention It does not prevent.

  Contains C: 0.069%, Si: 3.24%, Mn: 0.068%, Se: 0.020%, Sol.Al: 0.027%, N: 0.009%, and Sb: 0.038%, with the balance being Fe and inevitable A molten steel having a composition composed of mechanical impurities is made into a 210 mm thick slab by continuous casting, heated at a temperature of 1200 ° C. by a gas heating furnace, and subsequently heated to 1380-1410 ° C. in a non-oxidizing atmosphere by an induction heating furnace, A rough rolling at a final rolling reduction ratio of 50% was performed to obtain a 35 mm thick sheet bar, and finish rolling was performed in a range of finish rolling start temperatures of 1040 to 1120 ° C. to hot rolling to a plate thickness of 2.6 mm.

  In the above series of steps, the slab center layer with respect to the C content (Cs) up to 5 mm of the slab surface layer to be subjected to rough rolling by changing the oxygen concentration in the gas furnace by controlling the nitrogen gas supply amount into the gas heating furnace The ratio of C content (Cc) (Cs / Cc) was varied. This control is based on the C content (Cs) of the slab surface layer up to 5 mm and the C content (Cc) of the slab center layer, and the ratio of these (Cs / Cc). ) Was made into a table and the oxygen concentration in the gas furnace based on the table was controlled. Further, the slabs adjusted to have Cs, Cc and Cs / Cc were subjected to width reduction rolling at various reduction ratios after heating in a gas heating furnace and before rough rolling in accordance with the present invention. These operational data are summarized in Table 1.

The hot-rolled sheet thus obtained is subjected to hot-rolled sheet annealing at 1000 ° C. for 30 minutes, and the final cold-rolled sheet having a thickness of 0.30 mm by two cold rollings sandwiching the intermediate annealing at 1150 ° C. for 30 seconds. It was. The obtained cold-rolled sheet was alkali degreased to clean the surface, and then decarburized and annealed at 840 ° C. for 120 s in a wet hydrogen atmosphere. Next, a MgO-based annealing separator containing 5% TiO ₂ by mass ratio was applied, and finish annealing was performed at 1200 ° C. for 10 hours in an H ₂ atmosphere. Thereafter, a coating mainly composed of magnesium phosphate and colloidal silica was applied. Table 1 shows the results of investigation on the magnetic flux density B _{8 at} a magnetic field of 800 A / m and the iron loss W _{17/50 at 1.7} T-50 Hz of the product thus obtained. Table 1 also shows operation data and product evaluation when the width reduction rolling performed as a comparison is not performed.

  As shown in Table 1, Nos. 1 to 7 satisfying the conditions of the present invention have small ear cracks, good shape, and excellent magnetic properties of the final product. In contrast, Nos. 11 to 14 where Cs / Cc is out of the range of 0.40 to 0.86 have a large ear crack depth. Moreover, even if No. 8 which did not perform width reduction and No. 9 to 10 whose width reduction amount is less than 3%, the ear crack depth is large.

Graph showing the relationship between the ratio of C content (Cc) at the center of the slab to the C content (Cs) up to 5 mm of the slab surface (Cs / Cc) and the depth of the ear cracks produced in the finished hot-rolled steel strip It is. It is a graph which shows the relationship between the slab width reduction and the ear crack depth which arose in the hot-rolled steel strip by finish rolling.

Claims (6)

The slab for grain-oriented electrical steel containing C: 0.01-0.08% and Si: 2.5-4.1% by mass ratio as the composition of the starting material is heated to 1000-1250 ° C in a gas heating furnace, and further induction heating A rolling method of a hot rolled steel strip for directional electrical steel that performs rough rolling and finish rolling after performing slab heating at 1250 to 1450 ° C. in a furnace,
The starting material so that the ratio (Cs / Cc) of the C content (Cc) of the central layer of the slab to the C content (Cs) of the surface layer in the width direction of the slab subjected to rough rolling up to 5 mm is (Cs / Cc) is 0.40 or more and 0.86 or less While adjusting the slab heating conditions for
A rolling method of a hot-rolled steel strip for grain-oriented electrical steel, characterized in that after the heating in the gas heating furnace, width rolling at a rolling reduction of 3% or more is performed at least once before rough rolling.   2. The method of rolling a hot rolled steel strip for grain-oriented electrical steel according to claim 1, wherein the width reduction rolling is performed after heating by the gas heating furnace and before heating by the induction heating furnace.   The rolling method for hot steel strips for grain-oriented electrical steel according to claim 2, wherein after the width reduction rolling, rolling is performed at a reduction rate of 5% or more before heating by an induction heating furnace.   2. The method of rolling hot-rolled steel strip for directional electromagnetic steel according to claim 1, wherein the width reduction rolling is performed after heating by an induction heating furnace.   The width reduction rolling is performed both after heating by the gas heating furnace and before heating by the induction heating furnace and after heating by the gas heating furnace and after heating by the induction heating furnace. Rolling method for hot rolled steel strip for directional electromagnetic steel. Performing cold rolling, decarburization annealing and final finish annealing on the directional electromagnetic steel strip obtained by the rolling method of the hot rolled steel strip for directional electrical steel according to any one of claims 1 to 5. A method for producing a grain-oriented electrical steel sheet characterized by the above.

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