DE69316114T2 - Thick grain-oriented electrical steel sheets with excellent magnetic properties - Google Patents

Description

The invention relates to a thick grain-oriented electrical steel sheet which has excellent magnetic properties and is suitable for use as a material for the core of a transformer or the like.

Presentation of the state of the art

Since grain oriented electrical steel sheet is mainly used as a core material for transformers and other electrical equipment, it is required to have excellent magnetic properties, primarily excellent magnetization and core loss properties. Generally, the magnetization property is expressed as the B8 value of the flux density at a magnetic field of 800 A/m, and the core loss property is expressed as the W17/50 value of the core loss at a frequency of 50 Hz and magnetization to 1.7 Tesla.

The core loss property is mainly determined by the flux density. In general, the higher the flux density, the better the core loss property. However, an increase in the flux density causes a simultaneous increase in the size of the secondary recrystallization grains and thus deteriorates the core loss property to their respective extent. On the other hand, by controlling the magnetic domain, the core loss property can be improved regardless of the diameter of the secondary recrystallization grains.

Grain oriented electrical steel sheet is manufactured by utilizing the phenomenon of secondary recrystallization in the finish annealing step to develop a Goss texture in which the axes {110} of the grains are aligned to the sheet surface while their axes <001> to the rolling direction In order to obtain good magnetic properties, the easily magnetizable axis <001> must have a high degree of alignment with the rolling direction.

JP-B-40-15644 and JP-B-51-13469 teach typical processes for producing grain-oriented electrical steel sheet with high flux density. JP-B-40-15644 describes a process using MnS and AlN as main inhibitors, and JP-B-51-13469 describes a process using MnS, MnSe, Sb and the like as main inhibitors. Therefore, appropriate size, morphology and distribution control of the precipitates acting as inhibitors is an indispensable requirement in the currently available technology.

On the other hand, as a result of transformer manufacturers' efforts to increase the energy efficiency and reduce the cost of their products, the laminated core sector has seen an increasing demand for thick grain-oriented electrical steel sheet that can reduce the number of laminations. In addition, the large rotating machinery sector has long been interested in using grain-oriented electrical steel sheet. Here, too, there is a particularly high demand for thick grain-oriented electrical steel sheet that can reduce the number of laminations.

Since a higher sheet thickness generally leads to poorer core loss properties, there has been a strong demand for the development of a thick grain-oriented electrical steel sheet with excellent magnetic properties.

JP-A-3-72027 discloses a method for producing an oriented electrical steel sheet with high magnetic flux density and good core loss properties. The electrical steel sheet is produced from a steel melt containing ≤ 0.12% C and 2.5 to 4.5% Si. JP-A-3-72027 discloses steel sheet thicknesses in the range of 0.10 to 0.285 mm. In addition, the sheet has a magnetic induction of ≥ 1.88 T and a magnetic loss (W15/50) of ≤ 0.42 W/kg.

Summary of the invention

The object of the invention is to provide an improved thick grain-oriented electrical steel sheet that has good magnetic properties. This object is solved by the features of the patent claims.

Fig. 1 is a diagram illustrating how the core loss property of a product sheet is affected by its carbon content and flux density.

Fig. 2 is a diagram showing how the core loss property of a product sheet is affected by the shape factor of the grain boundary and the degree of deviation of crystal orientation in the grains.

Fig. 3 is a diagram showing how the core loss property of a product sheet is affected by the sheet thickness in inventive products and comparative products.

Fig. 4 shows a typical grain pattern of a thick grain-oriented electrical steel sheet according to the invention.

The invention provides a grain-oriented electrical steel sheet having excellent magnetic properties, the electrical steel sheet containing 2.5 to 4.5 wt% Si, having a thickness of 0.36 to 1.00 mm, a C content of at most 0.0050 wt%, a magnetic flux density B₈ of at least 1.83 T and an SF (average value) of less than 0.80, where SF is an index representing the boundary configuration characteristics of the individual sheet grains having the same area as a circle having a diameter of 5 mm and

SF = (grain area x 4π) / (grain boundary length)²

is defined as, where the SF (mean value) is the mean value of the individual SF values, its grains with a diameter of more than 5 mm have a crystal orientation deviation of 0.2 to 4 degrees relative to the crystal orientation at the grain center, and as a product sheet with a thickness of t (mm) it has a core loss w17/50 (W/kg) of at most 3.3 x t + 0.35.

The grain oriented electrical steel sheet of the invention is produced by successively carrying out the following steps The following steps may be performed: casting molten steel obtained by a conventional steelmaking process as a strand or by the ingot making process; when the ingot making process is used, slab rolling the ingot into a slab; hot rolling the slab into a hot-rolled sheet; annealing the hot-rolled sheet as required; cold rolling the sheet in one pass or cold rolling in two or more passes with intermediate annealing; decarburization annealing of the cold-rolled sheet; and final finish annealing of the decarburized sheet.

The inventors conducted a broad investigation into the conditions required to realize good magnetic properties in the process of producing thick grain oriented electrical steel sheet. This enabled them to determine the requirements that the product must meet.

In the following, their findings are explained using experimental results.

Fig. 1 shows the effect of C content and flux density of the product on the core loss property of the product.

In this experiment, a silicon steel slab containing by weight 3.21 to 3.30% Si, 0.025 to 0.085% C, 0.025 to 0.030% acid-soluble Al, 0.0075 to 0.0086% N, 0.070 to 0.161% Mn, 0.005 to 0.029% S, and the balance Fe and unavoidable impurities was heated at 1150 to 1380 ºC for 1 h, the slab was hot-rolled into a 2.8 mm thick hot-rolled sheet, a portion of the hot-rolled sheet was annealed at 900 to 1100 ºC while another portion thereof was not annealed, and the sheets were rolled to a thickness of about 83%. of 0.48 mm cold rolled.

The cold-rolled sheets thus obtained were subjected to decarburization annealing (atmosphere: 25% N₂ and 75% H₂; dew point: 65 ºC) in the temperature range of 810 to 860 ºC for 250 s. Then, a section of each sheet was subjected to nitriding treatment to increase N by 0.0102 to 0.0195% using NH3 gas during 30 s supplementary annealing at 750 ºC, while another portion of each sheet was not subjected to any nitriding treatment. The sheets were coated with an annealing separator consisting mainly of MgO, the coated sheets were wound into (5 t) coils of 200 to 1500 mm inner diameter, and the coils were finally finish annealed by heating to 1200 ºC at a temperature rise rate of 15 ºC/h in an annealing atmosphere containing 10 to 100% N2 (balance H2) and holding at 1200 ºC in an H2 annealing atmosphere for 20 h.

A tensile coating was applied to the coils, then they were cut to size for a single sheet testing machine, straightened and stress-relieved at 850 ºC for 4 hours, after which the magnetic properties were measured. The finished product thickness was 0.50 mm.

As shown in Fig. 1, products with good core loss characteristics, i.e., W17/50 of 2.00 W/kg or less, were obtained only under the conditions of carbon content of 0.0050% or less and flux density B8 of 1.83 T or more. However, even when these conditions were met, there were cases where W17/50 exceeded 2.00 W/kg. The reason for this was carefully investigated.

The results of the study are explained below.

Fig. 2 relates to those of the products of the test of Fig. 1 which had a carbon content of 0.0050% or less and a flux density B₈ of 1.83 T or more, and shows how the core loss property of these products is influenced by the shape factor (SF) of the grain boundary of grains having the same area as a circle with a diameter of more than 5 mm and the degree of deviation (Δθ) of the crystal orientation in grains with a diameter of more than 5 mm.

The shape factor (SF) of the grain boundary was determined as SF = (grain area x 4π) / (grain boundary length)²

and used to quantify the grain boundary configuration. The value of SF is 1 for a circular grain and decreases with increasing irregularity (unevenness) of the grain boundary configuration.

The degree of deviation (Δθ) of crystal orientation in grains with a diameter of more than 5 mm represents the difference in the orientation in the grain relative to that at the grain centroid. When secondary recrystallization occurs in the wound state as in the invention and the strip ring is subsequently stretched to form the product, the deviation (Δθ) of crystal orientation in the grains generally tends to increase with increasing distance from the centroid in the rolling direction.

The SF value was measured by image analysis, while the Δθ value was measured using an electron channeling pattern (ECP).

Each point in Fig. 2 corresponds to a sample sized for the Single Sheet Tester (SST) prepared under the experimental conditions of Fig. 1. SF is expressed as the mean (SF (mean)) for 101 to 151 grains with diameters over 5 mm, and Δθ is expressed as the mean (Δθ (mean)) of the maximum orientation deviations (orientation difference between the point at the center of gravity and the point furthest from the center of gravity in the rolling direction) from 81 to 113 grains.

As shown in Fig. 2, all products that satisfied the conditions of SF (mean) < 0.80 and Δθ (mean) (degrees) = 0.2 to 4 showed good magnetic properties with W17/50 ≤ 2.00 W/kg.

In furtherance of their investigation, the inventors manufactured products of 0.36 to 1.00 mm in thickness using the same slabs as starting materials as in the explanation of Fig. 1 and under the same processing conditions as in the explanation of Fig. 1, except that the thickness of the hot-rolled sheets was 2.3 to 5.0 mm.

The experimental results for these products are shown in Fig. 3.

As is clear from Fig. 3, the products satisfying all the conditions of the invention, i.e., the conditions of C ≤ 0.0050%, B 8 ≥ 1.83 T, SF (average) < 0.80 and Δθ (average) (degree) = 0.2 to 4, showed an excellent core loss characteristic with a W17/50 value of at most 3.3 x t + 0.35 (where W17/50 is the core loss characteristic in W/kg and t is the product thickness in mm).

Although the mechanism by which the invention produces its effect has not yet been established with absolute certainty, the inventors have reached the preliminary conclusions described below.

While the core loss characteristic improves with increasing flux density, the improvement is generally reduced in magnitude to the extent that the increase in magnetic flux density simultaneously causes the large grain diameter. However, when the sheet thickness is large as in the invention, the probability that the product grains become excessively large tends to be small. This means that with a thick product as in the invention, the correlation between magnetic flux density and core loss becomes more pronounced.

On the other hand, residual C in the product forms carbides, which prevent movement of the magnetic domain walls during magnetization and thereby deteriorate the core loss property. In a thick product such as in the invention, the probability of insufficient decarburization in the decarburization annealing step is high, so limiting the C content in the product is particularly important.

The basic principle of the invention is to achieve a determined combination of grain boundary configuration of the product and crystal orientation deviation. The tendency for magnetic spike domains to form in the vicinity of the grain boundaries becomes even more pronounced when there is a crystal orientation deviation in the grains.

In addition, if the irregularity of the grain boundary configuration becomes high (SF becomes low), as in the invention, the resulting increase in the grain boundary area increases the Frequency of occurrence of magnetic spike domains. The increased number of magnetic spike domains produced by the invention causes magnetic domain refinement when the sheet is given the tensile stress by the glass film and the coating, thereby improving the core loss property.

Since it is difficult to realize a magnetic domain refinement effect only by a simple means (e.g., increasing the tensile stress imparted to the sheet) in a thick sheet as in the invention, it becomes necessary to achieve a good core loss property by combining control of grain boundary configuration and control of intra-grain crystal orientation deviation as in the invention.

The reason for the limitations with regard to the features constituting the invention is explained below.

Although there are no particular restrictions on the composition of the slab used in the invention, in order to stabilize the magnetic flux density of the product and to facilitate decarburization to the desired value, the C content of the slab is preferably in the range of 0.025 to 0.075 wt%.

In order to achieve an improved core loss property, the product sheet of the present invention preferably contains 2.5 to 4.5% Si. As inhibitor-forming elements, Al, N, Mn, S, Se, Sb, B, Cu, Nb, Cr, Sn, Ti, Bi, etc. may be added. While there is no particular limitation on the temperature at which the slab is heated, it is preferable to use a heating temperature of 1300°C or less due to energy costs and the like. In the subsequent step, the heated slab is hot-rolled into a hot-rolled sheet. If necessary, the hot-rolled sheet is annealed and then cold-rolled in one pass or in two or more passes with intermediate annealing to reduce it to the finished sheet thickness.

The reduction in the final cold rolling is not specifically limited, but a reduction of at least 80% is preferred in view of increasing the magnetic flux density (B₈ value) of the product. A reduction of at least 80% in the final cold rolling ensures that the decarburized sheet has appropriate amounts of grains with sharp orientation {110} <100> and grains with coincident orientation (grains with orientation {111} <112> or similar) which are likely to be eroded by the grains with orientation {110} <100>. This can give a B₈ value of at least 1.83 T.

After the final cold rolling, the cold-rolled sheet is decarburization annealed at 700 to 1000 ºC. Since the product of the invention is thick (0.36 to 1.00 mm), the time required for decarburization to the required value tends to be long. To shorten the time required, it is appropriate to reduce the C content of the molten steel, increase the temperature during decarburization annealing and/or raise the dew point of the annealing atmosphere.

If the inhibitor strength is insufficient to cause secondary recrystallization in the decarburized sheet, a nitriding treatment using NH3 gas or another inhibitor-strengthening measure is preferably carried out.

After the sheet has been coated with an annealing separator consisting mainly of MgO, it is wound into a strip ring with an inner diameter of 10 to 100,000 mm and then subjected to a final annealing. If the inner diameter during the annealing is in this range, the presence of a crystal orientation deviation of 0.2 to 4 degrees relative to the grain center in the sheet grains over 5 mm in diameter can be guaranteed.

Thereafter, the final product is obtained by stress relieving the sheet and applying a tensile coating. To improve the core loss property of the product, it is preferably subjected to magnetic domain control using a laser beam or the like.

The final product sheet must have a Si content of 2.5 to 4.5% by weight. If the content is less than 2.5%, it is difficult to obtain a good core loss property, while above 4.5% content, a brittleness problem occurs in normal cold rolling.

The product of the invention is thick. In particular, it has a thickness of 0.36 to 1.00 mm. In some cases, a sheet with a thickness of less than 0.36 mm can achieve a good core loss property without satisfying the conditions of the invention. A sheet over 1.00 mm is undesirable because the time required for decarburization to the value required by the invention becomes so long that an unacceptable increase in production costs occurs.

The product must have a C content of 0.0050% or less and a flux density B8 of at least 1.83 T. According to Fig. 1, the reason for this is that these are the ranges required for good core loss characteristics. A C content of 0.0030% or less is preferred.

The shape factors SF representing the boundary configuration characteristics of the sheet grains with the same area as a circle with a diameter greater than 5 mm shall have a mean value (a mean value for the sheet referred to as "SF (mean)") of at least 0.80.

The degree of deviation (Δθ) of crystal orientation in grains with a diameter of over 5 mm must be in the range of = 0.2 to 4 degrees. According to Fig. 2, the reason for this is that this is the range required for a good core loss property.

The invention is not limited to a specific method for controlling the SF value, and a selection can be made by controlling the grain diameter of the primary recrystallization before the occurrence of the secondary recrystallization, using a grain boundary separating element such as Sn, and adjusting the inhibitor strength during the secondary recrystallization.

The invention is not limited to a specific method for controlling the Δθ value, and a strip ring having a diameter suitable for the grain diameter of the product can be finally finished annealed, or the heat history can be between solidification and slab heating can be used to control the slab grain size. As for the effect of this Δθ value, the presence of the prescribed crystal orientation deviation in even a single grain leads to an improvement in the core loss property.

When the above product requirements are met, a thick grain oriented electrical steel sheet is obtained with a good core loss property W17/50 of not more than 3.3 x t + 0.35 (where W17/50 is the core loss property in W/kg and t is the product thickness in mm).

By utilizing the effect combination of C content control in the product sheet, magnetic flux density control, grain boundary configuration control and intra-grain crystal orientation deviation control according to the invention, a thick grain-oriented electrical steel sheet having excellent magnetic properties can be obtained. Therefore, the invention can be expected to make a very important contribution industrially.

Examples example 1

A slab containing (in wt%) 0.053% C, 3.26% Si, 0.15% Mn, 0.006% S, 0.029% acid soluble Al, 0.0076% N, and the balance Fe and unavoidable impurities was heated to 1150 ºC and then hot rolled to a hot rolled sheet of 2.8 mm.

The hot-rolled sheet was annealed by holding at 1120ºC and then at 900ºC, and the annealed sheet was cold-rolled to a thickness of 0.38 mm with a reduction of about 86%. A section (1) of the sheet was decarburization annealed at 800ºC for 150 s, a second section (2) at 830ºC for 150 s, and a third section (3) at 860ºC for 200 s (atmosphere: 25% N₂ and 75% H₂; dew point: 65ºC). Then, the annealed sheets were subjected to nitriding treatment by annealing at 750ºC for 30 s in an annealing atmosphere containing NH₃ gas.

After nitriding, the N content of the sheets was 0.0195 to 0.0211 wt.%. The sheets were then coated with an annealing separator consisting mainly of MgO, wound into 5-ton coils with an inner diameter of 600 mm and then subjected to a final finish annealing process in which they were heated to 1200 ºC at 15 ºC/h and held at 1200 ºC for 20 h.

In this final finish annealing, an atmosphere of (25% N₂ + 75% H₂) was used during the temperature increase phase, and an atmosphere of 100% H₂ was used during the holding phase at 1200 ºC. Thereafter, the coils were coated with a tensile coating, cut into single sheet tester (SST) sized specimens, straightened, stress relieved at 850 ºC, and tested for magnetic properties. The thickness of the final product sheet was 0.40 mm. Table 1 shows the property values of the sheets treated under each condition. Fig. 4 shows the grain pattern of the thick grain oriented electrical steel sheet of the present invention. In this illustration, B indicates the grain center of gravity. The crystal orientation difference between B and A was 3 degrees and that between B and C was 2.4 degrees. Table 1

Note: SF and Δθ are the mean values specified in the text.

Example 2

A first slab (1) containing 0.045% C, 3.01% Si, 0.14% Mn, 0.008% S, 0.035% acid-soluble Al, 0.0061% N, 0.05% Sn and the balance Fe and unavoidable impurities and a second slab (2) of the same composition with the exception that that the Sn content was below 0.01%, were heated to 1150 ºC and hot rolled to a thickness of 2.3 mm.

Without annealing, the hot-rolled sheets were cold-rolled to a thickness of 0.48 mm with a reduction of about 79%. The cold-rolled sheets were annealed at 830 ºC for 300 s (atmosphere: 25% N₂ and 75% H₂; dew point: 62 ºC) and then treated under the same conditions as in Example 1. The thickness of the final product sheets was 0.50 mm. Table 2 shows the property values of the sheets treated under the respective conditions. Table 2

Note: SF and Δθ are the mean values specified in the text.

Example 3

A first slab (1) containing 0.078% C, 3.21% Si, 0.12% Mn, 0.009% S, 0.034% acid-soluble Al, 0.0060% N, and the balance Fe and unavoidable impurities, a second slab (2) of the same composition except that the C content was 0.053%, and a third slab (3) of the same composition except that the C content was 0.039% were heated at 1200 ºC and hot rolled to a thickness of 3.0 mm.

Without annealing, the hot-rolled sheets were cold-rolled to a thickness of 0.58 mm with a reduction of about 81%. The cold-rolled sheets were annealed at 830 ºC for 450 s (atmosphere: 25% N₂ and 75% H₂; dew point: 62 ºC) and were then treated under the same conditions as in Example 1. The thickness of the final product sheets was 0.60 mm.

Table 3 shows the property values of the sheets treated under the respective conditions. Table 3

Note: SF and Δθ are the mean values specified in the text.

Claims (1)

1. Thick grain oriented electrical steel sheet having excellent magnetic properties, the electrical steel sheet containing 2.5 to 4.5 wt% Si, measuring 0.36 to 1.00 mm in thickness, having a C content of not more than 0.0050 wt%, a magnetic flux density B₈ of at least 1.83 T and has an SF mean value of less than 0.80, where SF is an index representing the boundary configuration characteristics of the individual sheet grains with the same area as a circle with a diameter of over 5 mm and is defined as SF = (grain area x 4π) / (grain boundary length)² where the SF mean value is the mean of the individual SF values, its grains with a diameter of over 5 mm have a crystal orientation deviation of 0.2 to 4 degrees with respect to that at the grain center, and as a product sheet with a thickness of t (mm), it has a core loss W17/50 (W/kg) of not more than 3.3 x t + 0.35.