JP6034002B2 - Cold rolling method for grain-oriented electrical steel sheet containing high Si - Google Patents

Description

  The present invention relates to improvement of cold rolling property of grain-oriented electrical steel sheets used mainly as iron cores of transformers.

  Electrical steel sheets are broadly classified into non-oriented electrical steel sheets and directional electrical steel sheets. The former is mainly used as an iron core material for rotating devices such as motors and generators, and the latter is mainly used as an iron core material for stationary devices such as transformers.

  As for the electromagnetic characteristics of electrical steel sheets, “low iron loss” is required to reduce energy loss of equipment, and “high magnetic flux density” is required to reduce equipment size. Based on the recent demand for energy saving, a further low iron loss electromagnetic steel sheet has been demanded.

  As described in Non-Patent Document 1, the iron loss of the electrical steel sheet is composed of eddy current loss and hysteresis loss. In order to reduce eddy current loss in grain-oriented electrical steel sheets, it is necessary to reduce the plate thickness and increase the specific resistance (as one of the specific methods, high alloying, particularly high Si). In order to reduce the hysteresis loss, it is effective to improve the Goss orientation integration and reduce impurities and internal strain.

  In general, steel materials are known to become brittle if the Si content is increased to form a high alloy, but factors affecting ductility (toughness) in industrial production of metal materials (especially steel materials) Can be classified as follows.

  (A) Environmental (external) factors: test temperature, shape (size), processing (strain) speed, etc.

  (B) Matrix factor (physical property value of pure metal phase, intragranular strength): Lattice defects including components, phases, and / or precipitates.

  (C) Structure factor (grain boundary strength) as a polycrystal: crystal grain size, grain boundary strength, texture, etc.

  In the manufacture of grain-oriented electrical steel sheets, factors related to the manufacturing conditions are fixed (regulated) to ensure the required magnetic properties, and the Si content is increased almost independently to improve the iron loss properties. To be done. Therefore, in order to improve the cold-rollability of the high Si material and reliably perform industrial production, it is not necessary to consider the influence of the above factors (A) to (C) alone, (A) to (C) It is necessary to find out the effect that can be obtained by these factors affecting each other.

  For example, in the production of grain-oriented electrical steel sheets, from the viewpoint of primary recrystallization texture, until the decarburization annealing step, about 0.025 to 0.09% C is contained and the cooling rate is ensured. The microstructure of the rolled steel strip after annealing is a mixture of pearlite, bainite, and ferrite phase, and the grain size is about 10 to 50 μm. Therefore, the effect of improving the ductility due to the refinement of the crystal grain size cannot be expected. .

  Furthermore, the magnetic properties greatly depend on the secondary recrystallized texture and grain size of the product. To optimize them, it is necessary to optimize the two major elements of secondary recrystallization, the inhibitor and the primary recrystallized texture. is important.

  In particular, the primary recrystallization texture depends on various factors such as the component composition, but recently, for example, a correlation between the heating rate of the primary recrystallization annealing and the cold rolling reduction rate has been found, and the cold rolling rate is reduced. An attempt has been made to increase the size (see Patent Document 1). Therefore, there is a tendency to increase the thickness of the hot-rolled steel strip, and it is a reality that the breaking frequency in the cold rolling rapidly increases as the Si content increases.

  Against such a background, in the field of grain-oriented electrical steel sheets, about 3% by mass of Si was previously added, but further, the demand for increasing the Si content for increasing the specific resistance increased. Now it is over 3.2%. If the Si content exceeds 3.2%, the steel sheet becomes extremely brittle and the cold rollability deteriorates.

  For this reason, in industrial production, it is a reality that rolling is carried out without considerable difficulty. Therefore, there is a need for technical development to solve this problem.

  By the way, in order to improve the cold rolling property / productivity of the grain-oriented electrical steel sheet containing 3.2% or more of Si, (a) improving the quality of the surface / end face of the steel sheet, and / or (b) The temperature of the steel plate immediately before rolling is increased.

  As for (a), there is a method of sufficiently maintaining the trimming blade to reduce edge cracks (ear cracks), and a method of performing trimming at a high temperature and / or before heat treatment of a hot-rolled steel strip. . In addition, there are methods for reducing surface defects by taking care of surface defects at the slab stage and by taking measures against flying defects during hot rolling.

  For (b), for example, a method of increasing the temperature by a method of immersing the coil in a hot bath (hot water tank) before cold rolling, or a relatively high temperature (preferably without cooling to room temperature after heat treatment) There is a method of immediately starting cold rolling while maintaining (over 50 ° C.).

  However, even if these methods are carried out perfectly, breakage and cracking often occur due to the unique properties of the steel sheet. In particular, when Si exceeds 3.2%, many fractures and cracks occur at the start of the second pass rolling.

  Furthermore, as disclosed in Patent Document 2, it is not preferable from the viewpoint of magnetic properties to keep the steel strip warm between the annealing and cold rolling of the hot-rolled steel strip. In the production of the contained grain-oriented electrical steel sheet, a measure for improving the cold rolling property is awaited.

Japanese Patent No. 3943837 Japanese Patent Publication No. 50-026493 US Pat. No. 2,599,340 Japanese Patent Publication No. 40-015644 Japanese Patent Publication No.51-013469 Japanese Examined Patent Publication No. 61-060896 Japanese Patent No. 3481491 Japanese Patent Publication No. 54-013846

Metal Vol. 72 (2002) No. 11

It is an object of the present invention to provide prescribed component compositions, heat treatment conditions, cold rolling in order to ensure the required magnetic properties in the production of grain-oriented electrical steel sheets containing Si of 3.2% or more and less than 3.6%. It is to improve the cold rolling property on the assumption of the previous thickness.

  The inventors of the present invention have intensively investigated the breaking state of the steel sheet in cold rolling. As a result, when the cold rolling rate of the first pass is large, when the steel plate is wound around the reel at the start of rolling of the second pass, a crack occurs in the steel plate, and the fracture starts from this crack. I found.

  This invention was made | formed based on the said knowledge, The summary is as follows.

C: 0.025% or more, 0.09% or less, Si : 3.2% or more , less than 3.6%, Mn: 0.08% or more, 0.103% or less, Al: 0.00%. 022% or more, 0.033% or less, N: 0.006% or more, 0.0087% or less, Seq = S + 0.406 × Se: 0.010% or less, Sn + Sb: 0.02% or more, 0.30% It contained the following, the steel slab and the balance being Fe and unavoidable impurities hot rolling, then subjected to heat treatment, followed by subjecting the death cases ring, then subjected to one or more cold rolling, then Reversible cold in the cold rolling method in the production of grain-oriented electrical steel sheet having a series of steps of applying decarburization / primary recrystallization annealing, annealing separator coating, secondary recrystallization annealing, flattening annealing The first pass of cold rolling is performed locally using a rolling mill. Cold rolling method of the grain-oriented electrical steel sheet but which is characterized in that in the cold rolling rate of 2.5% or more.

  According to the present invention, when a grain-oriented electrical steel sheet having an Si content exceeding 3.2% is rolled by a reversible cold rolling mill, the cold rolling rate of the steel sheet is optimized by optimizing the cold rolling rate in the first pass. Can be significantly improved.

It is a figure which shows notionally the relationship between steel strip thickness and a cold rolling rate, and the degree of embrittlement. The solid line shows the relationship between elongation and the cold rolling rate, and the broken line shows the relationship between the thickness of the steel strip and the surface tension. It is a figure which shows the relationship between a cold rolling rate and local elongation according to Si content. It is a figure which shows the relationship of local elongation and the winding crack fracture | rupture rate at the time of cold rolling (number of fractures per 1000 tons) according to Si content. A local elongation of 2.5% corresponds to a breaking rate of 3 times / 1000 tons. In the actual production, this breakage rate is one break for every 40 coils, and this value or less is considered good.

  As described above, the present inventors diligently investigated the breaking state of a steel sheet in cold rolling. As a result, when the cold rolling rate of the first pass is large, when the steel plate is wound around the reel at the start of rolling of the second pass, a crack occurs in the steel plate, and the fracture starts from this crack. I found. Details will be described below.

  (X) The present inventors diligently investigated the form of fracture / cracking that occurs in cold rolling. In terms of material dynamics, the cold rolling process is organically linked by three elements: (1) winding and unwinding on a cylindrical reel, (2) rolling with a roll, and (3) tensioning with a reel. The steel plate is reduced to a predetermined thickness.

  First, “bending” related to “(1) winding around a cylindrical reel” which is the subject of the present invention will be described.

In general, when a thick metal material is bent, a tension acts on the outer surface and a compressive force acts on the inner surface. Assuming that the plate thickness is t and the winding reel diameter is D, the strain amount ξ at the outermost layer of the steel strip is ξ = t / (D + t). This is approximately ξ≈t / D− (t / D) ² , but generally, the reel diameter is often D = 508 mm (20 inches), and the plate thickness is t = 1. Since it is 8 to 4.0 mm, t << D and ξ≈t / D may be satisfied.

  That is, the strain amount ξ at the outermost layer of the steel strip increases in proportion to the plate thickness t. If the size of this ξ is greater than or equal to the yield strain of the material, plastic deformation occurs, and if it is greater than or equal to elongation, the material breaks. In this phenomenon, on the inner surface, a similar force (strain) acts symmetrically as a compression.

  By the way, in the reversible cold rolling mill, one or more cylindrical reels are arranged on the left and right, respectively, and a reduction stand is arranged between the reels. Therefore, as described above, when the plate thickness is reduced or the reel diameter is increased, ξ is reduced and the possibility of breakage is reduced.

  (Y) Next, regarding the properties of the material itself, in a normal steel material, when the degree of cold work increases, work hardening and elongation (toughness / ductility) decrease. This phenomenon has a monotonously decreasing relationship between the degree of processing and the degree of hardening, and takes an exponential relationship in consideration of accumulated processing.

  Thus, when comprehensively considering the rollability of the material, the steel strip thickness and the workability are in a trade-off relationship. As a result of intensive studies by the inventors, it has been found that the cold rolling process greatly depends not only on the bending (steel strip thickness factor) but also on the mechanical test value of the steel material.

  Next, the knowledge found by the present inventors, that is, “fracture occurs frequently due to winding cracks at the start of the second pass when the rolling reduction of the first pass is large”. .

  The embrittlement due to bending influenced by the plate thickness is proportional to the plate thickness, while the embrittlement due to work hardening follows an exponential relationship with the degree of work (rolling rate). The present inventors have found that there is an optimum point for optimizing these two monotonic phenomena.

  FIG. 1 conceptually shows the relationship between the steel strip thickness and the cold rolling rate and the degree of embrittlement. The solid line shows the relationship between elongation and the cold rolling rate, and the broken line shows the relationship between the thickness of the steel strip and the surface tension. The elongation is small (embrittlement) when the cold rolling rate is high, and is large when the cold rolling rate is low.

  On the other hand, the tension on the steel plate surface is proportional to the thickness of the steel strip. By combining these two relationships, the rolling property of the second pass is defined. The present inventors have found that the composite function has a minimum value, and the condition for achieving the minimum point is a preferable condition.

  As a result of studying that there are many cracks at the start of rolling of the second pass, it was found that the plate thickness of the second pass needs to be somewhat thick. Therefore, the relationship between the cold rolling rate and the local elongation was investigated for hot-rolled annealed steel strips of grain-oriented electrical steel sheets having different Si contents. The result is shown in FIG. It can be seen from FIG. 2 that local growth is important as an indicator.

  Here, the content of Si will be described. Hereinafter, “%” relating to the component composition means “% by mass”.

Si content shall be 3.2% or more by mass. If it is less than 3.2%, even if the present invention is not applied, no major problem occurs in cold rolling properties. If the Si content exceeds 4.0%, a problem arises in the sheet-passability of the steel strip even in processes other than cold rolling, which is not suitable for industrial production. When the Si content is 3.6% or more and 4.0%, if the present invention is applied, the fracture rate is reduced, but since it is close to the limit in industrial production, the Si content is preferably 3.6%. Is less than .

  Since the present invention relates to the general cold rolling property of grain-oriented electrical steel sheets utilizing the secondary recrystallization phenomenon, the type of inhibitor forming element (grain growth inhibitor) is not specified. The amount of the inhibitor forming element may be determined by a known technique.

  For example, in order to improve the secondary recrystallization, Al, N, Mn, S, Se, Sb, Sn, Bi, B and the like are added in combination in an amount of about 0.003 to 0.040%. Furthermore, the inhibitor may be built in either a completely solid solution type or a sufficiently precipitated type.

  Moreover, the presence or absence of nitriding after decarburization and primary recrystallization does not matter. That is, for example, the manufacturing method is not limited to any one of Patent Documents 3 to 7. The present invention can be applied to general cold rolling of grain-oriented electrical steel sheets.

  When it contains components other than Si, it is preferable to set it as the following content range.

  When C is less than 0.025%, the primary recrystallization texture becomes unsuitable, and when it exceeds 0.09%, decarburization becomes difficult and is not suitable for industrial production.

  In the case of containing Al, acid-soluble Al combines with N to form AlN, and mainly functions as a primary / secondary inhibitor. Some acid-soluble AlNs are formed before nitriding, and some are formed during high-temperature annealing after nitriding. In order to ensure the amount of both AlN, Al is preferably 0.022 to 0.033%. If it exceeds 0.033%, secondary recrystallization failure tends to occur, and if it is less than 0.022%, the Goss orientation integration degree may be significantly deteriorated.

  When nitriding in the post-process is premised, the content of N varies depending on the method of incorporation of the inhibitor, but 0.006% or less is desirable for the complete solid solution type, and 0.006% or more is desirable for the sufficiently precipitated type.

  If Mn is less than 0.03%, cracks are likely to occur in the hot-rolled steel strip, yield decreases, and secondary recrystallization is not stable. In the case of the complete solid solution type, if Mn exceeds 0.09%, MnS and MnSe increase, and the degree of solid solution becomes uneven depending on the location, causing a problem in stable production in actual industrial production. On the other hand, in the case of a sufficiently precipitated type, secondary recrystallization is not stable unless Mn is 0.08% or more.

  S and Se combine with Mn to act as an inhibitor, and are also useful as AlN precipitation nuclei. In the case of a completely solid solution type, it is preferable that Seq = S + 0.406 × Se, and 0.010 to 0.027% is contained. In the case of a sufficiently precipitated type, Seq is preferably 0.010% or less. When Seq is out of the specified range, secondary recrystallization becomes poor.

  Sn and Sb are effective in improving the primary recrystallization texture. Sn and Sb are grain boundary segregation elements, and have the effect of stabilizing secondary recrystallization and reducing the secondary recrystallization grain size. When the total amount of Sn and Sb is less than 0.02%, the effect of addition is extremely small. On the other hand, when Sn and Sb exceed 0.30% in total, it becomes difficult to be oxidized during decarburization annealing, remarkably hinders decarburization, and glass film formation becomes insufficient.

  Annealing of the hot rolled steel strip before cold rolling is essential. This annealing is usually performed at a temperature between 950 and 1170 ° C. for about 30 to 240 seconds. This annealing is indispensable for obtaining predetermined magnetic characteristics, and is aimed at optimizing and homogenizing the structure and inhibitors of the steel strip. The annealing conditions of the hot-rolled steel strip vary depending on the type of inhibitor and the forming method. Specifically, the conditions are as follows.

  (1) In the case of a completely solid solution type and AlN as a main inhibitor, the following conditions are satisfied regardless of the presence or absence of nitridation in the subsequent step (see Patent Documents 4, 7, etc.). The soaking temperature is defined by the content of the inhibitor element.

  (1a) One-stage cycle: soaking at 1050 to 1170 ° C. for 30 to 240 seconds, and then cooled at a cooling rate of 15 ° C./second or more.

  (1b) Two-stage cycle: The first stage is soaked at 1050 to 1170 ° C. for 15 to 80 seconds, then held at 870 to 970 ° C. for 20 to 180 seconds, and then cooled at a cooling rate of 15 ° C./second or more. .

  (2) Completely solid solution type, when AlN is not the main inhibitor (see Patent Document 5, etc.), soaking at 950 to 1050 ° C. for 30 to 180 seconds, then cooling rate of 10 ° C./second or more Cool with.

  (3) In the case of a sufficiently precipitated nitriding type using AlN as a main inhibitor (see Patent Document 6, etc.), the above condition (1a) is applied.

  As described above, regardless of the type of inhibitor and the incorporation, the hot-rolled steel strip is annealed at a temperature of 950 ° C. or higher. Crystallizes and becomes large and brittle.

  The present invention improves this embrittlement by annealing a hot-rolled steel strip regardless of the type of inhibitor.

  That is, in the hot rolling of the grain-oriented electrical steel sheet, since the coiling is basically performed at a low coiling temperature, the structure of the hot-rolled steel strip is a metal structure (crystal structure) while leaving the rolled structure, It is a structure with a small crystal grain size. Therefore, cold rolling can be performed as it is without annealing the hot-rolled steel strip. In this case, even if the Si content exceeds 3.2%, there is no problem unless a particularly large surface defect or the like is generated, so it is not included in the present invention.

Incidentally, so-called, there is a method for producing a twice cold rolling method comprising oriented electrical steel sheet, which eliminates the annealing of the hot rolled steel strip, hot rolled, Desuke over-rings, cold rolling, intermediate annealing, It consists of processes of intermediate cold rolling, primary recrystallization / decarburization annealing, and finish annealing.

  In this method, in order to stabilize the magnetic properties, the hot-rolled steel strip may be annealed before the first cold rolling. In this case, the hot-rolled steel strip is included in the present invention. When annealing is not performed, it is not included in the present invention for the reasons described above.

  In the production of grain-oriented electrical steel sheets, so-called “aging by warm rolling (aging between passes)” in which steel strips are held at 150 ° C. or higher during cold rolling to ensure magnetic properties (Patent Document 8). , See) has been applied. For this reason, the cold rolling mill is limited to a reversible cold rolling mill. Basically, the high alloy material as described above cannot be industrially produced unless it is a reversible cold rolling mill.

  As the reversible cold rolling mill, a conventional 4hi, 6hi, 12hi, Sendzimir rolling mill, CVC (variable crown control type) rolling mill or the like can be used, and it does not depend on the type of rolling equipment. In this case, the number of stands is up to two, but preferably one.

  Even if the number of rolling mills is increased, there is no problem with cold rolling itself, but the aging effect between passes is reduced, and the off-gauge becomes longer and the yield is lowered, so there are disadvantages in the production of grain-oriented electrical steel sheets. large. Further, in the case of the single cold rolling method, usually, 5 passes or more are performed, and the temperature of the steel strip is increased by warm rolling in the latter half of the pass, so that deterioration of rollability due to brittleness does not occur.

  Next, local elongation will be described. “Total elongation” in the tensile test of steel sheets is divided into “elastic elongation” and “breaking elongation (permanent elongation)”. Furthermore, “breaking elongation (permanent elongation)” is almost equal to “uniform elongation” and “ It is divided into “local growth”. Among these, the index of ductility is “local elongation”, and the greater the local elongation, the better the ductility.

  As already described, when a metal material is subjected to plastic working, it is work-hardened to reduce the total elongation and local elongation. If the cold rolling rate at which the local elongation is 2.5% or more is applied to the cold rolling of the first pass, winding cracks in the rolling of the second pass are remarkably reduced. The cold rolling rate applied in the first pass is mainly different depending on the Si content, and it is necessary to decrease the cold rolling rate in the first pass as the Si content is higher.

  As shown in FIG. 2, since the local elongation decreases as the Si content increases, the cold rolling rate is reduced. That is, if the material is a high Si material, the cold rolling rate in the first pass is further reduced, and the “cold rolling rate at which the local elongation is 2.5% or more” applied in the first pass is determined.

<Experimental example>
An experimental example according to the present invention will be described.

  FIG. 3 shows the relationship between the local elongation and the winding crack breaking rate during cold rolling (the number of breaks per 1000 tons) for each Si content for a sufficiently precipitation-nitrided grain-oriented electrical steel sheet. A local elongation of 2.5% corresponds to a breaking rate of 3 times / 1000 tons. In the actual production, this breakage rate is one break for every 40 coils, and this value or less is considered good.

  A sufficiently precipitation-nitrided grain-oriented electrical steel sheet is manufactured as follows.

  The Si content is changed in the range of 3.0 to 3.7%, and other elements are C: 0.065 to 0.075%, Al: 0.026 to 0.0285%, N: 0.0075. ~ 0.0088%, Mn: 0.097 to 0.103%, S: 0.0058 to 0.0075%, Sn: 0.054 to 0.067%, molten steel consisting of the remainder Fe and inevitable impurities is melted It was made into a grain-oriented electrical steel sheet slab.

  The slab was reheated at a temperature of 1200 ° C. or less and hot-rolled to obtain a 2.6 mm hot-rolled steel strip. Thereafter, the hot-rolled steel strip was soaked at 1120 ° C. for 20 seconds, then held at 900 ° C. for 100 seconds, and then cooled by water spray to room temperature at a cooling rate of 30 ° C./second.

  After removing the scale of the surface of the hot rolled steel strip after annealing, it was rolled with a reversible cold rolling mill. During cold rolling, the cold rolling reduction ratio of the first pass is changed in the range of 15 to 45%, the local elongation is changed to 1.5 to 7.0%, and cold rolling is performed. A 0.220 mm grain-oriented electrical steel sheet was used. In this case, the steel strip was not preheated before cold rolling.

  From FIG. 3, in the material having an Si content of less than 3.6%, when the local elongation is 2.5% or more, the winding crack breaking rate during the second pass rolling per 1000 t is 3 times or less, It can be seen that good cold rollability is exhibited.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
As in the experimental example, an example relating to a sufficiently precipitation-nitrided grain-oriented electrical steel sheet will be described.

  A steel slab having the composition shown in Table 1 was reheated at a temperature of 1200 ° C. or less and hot-rolled to obtain a 2.6 mm hot-rolled steel strip. Thereafter, the hot-rolled steel strip was soaked at 1120 ° C. for 20 seconds, then maintained at 900 ° C. for 100 seconds, and then cooled by water spray to room temperature at a cooling rate of 30 ° C./second.

  After removing the scale of the surface of the hot rolled steel strip after annealing, it was rolled with a reversible cold rolling mill. During cold rolling, the cold rolling reduction ratio of the first pass was changed and the local elongation was changed to perform cold rolling to obtain a directional electrical steel sheet having a final product thickness of 0.220 mm. During cold rolling, the breaking frequency per 1000 tons generated in the second pass was evaluated. The results are shown in Table 2.

  Inventive examples Y1 to Y5 having a local elongation of 2.5% or more after one pass of cold rolling have a crack breaking frequency of 3 times or less per 1000 tons in the second pass of cold rolling. In Comparative Examples Z1 to Z4 in which the local elongation after the pass is less than 2.5%, the cracking / breaking frequency is more than 3 times per 1000 tons.

  As described above, according to the present invention, when a grain-oriented electrical steel sheet having an Si content exceeding 3.2% is rolled by a reversible cold rolling mill, the cold rolling rate of the first pass is optimized, and the steel sheet The cold rolling property of can be remarkably improved. As a result, in the production of grain-oriented electrical steel sheets, productivity in cold rolling can be improved, so that the present invention has high applicability in the electrical steel sheet manufacturing industry.

Claims (1)

C: 0.025% or more, 0.09% or less, Si : 3.2% or more, less than 3.6%, Mn: 0.08% or more, 0.103% or less, Al: 0.00%. 022% or more, 0.033% or less, N: 0.006% or more, 0.0087% or less, Seq = S + 0.406 × Se: 0.010% or less, Sn + Sb: 0.02% or more, 0.30% It contained the following, the steel slab and the balance being Fe and unavoidable impurities hot rolling, then subjected to heat treatment, followed by subjecting the death cases ring, then subjected to one or more cold rolling, then Reversible cold in the cold rolling method in the production of grain-oriented electrical steel sheet having a series of steps of applying decarburization / primary recrystallization annealing, annealing separator coating, secondary recrystallization annealing, flattening annealing The first pass of cold rolling is performed locally using a rolling mill. A cold rolling method for grain-oriented electrical steel sheets, wherein the cold rolling rate is 2.5% or more.

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