Manufacturing method of grain-oriented electrical steel sheet

Abstract

Provided is a production method for obtaining a grain-oriented electrical steel sheet having excellent magnetic properties and in which an underlayer film generation state is satisfactory. In said method, an annealing-separation agent containing an oxide of Ti in a predetermined amount relative to MgO is applied to the surface of a steel sheet that has been subjected to cold rolling and decarburization-annealing, and then the annealing-separation agent is dried, after which the steel sheet is placed in the form of a coil in a final annealing furnace to subject the steel sheet to final annealing. In the process of increasing the temperature for the final annealing, the atmosphere at a time point at which the highest temperature of the coil reaches 1100°C is set to an atmosphere having 100 vol% of H2, and said atmosphere is changed to a H2 atmosphere containing 5 vol% or more of N2 during a period spanning from the time point at which the highest temperature of the coil reaches 1100°C until the temperature difference between the highest temperature and the lowest temperature of the coil is within 75°C and before a lapse of 15 hours.

Description

[Technical field]

[0001] The present invention relates to a method for producing a grain-oriented electrical steel sheet having excellent magnetic properties. [Background technology]

[0002] Grain-oriented electrical steel sheets have extremely excellent magnetic properties in the rolling direction, and are mainly used as materials for the iron cores inside transformers, etc. In recent years, there has been a demand for grain-oriented electrical steel sheets with further improved magnetic properties in order to improve the energy usage efficiency of such transformers, etc.

[0003] One of the measures to improve the magnetic properties is to promote secondary recrystallization in grain-oriented electrical steel sheets. During the finish annealing to promote secondary recrystallization, it is known that an annealing separator containing Ti is applied to the surface of the steel sheet in order to improve adhesion between the steel sheets and promote the formation of a forsterite undercoat. It is also known that the Ti contained in the annealing separator can penetrate into the steel sheet and deteriorate the formation of the undercoat and the magnetic properties.

[0004] As a measure to solve such problems, Patent Document 1 discloses a technique in which the atmosphere for purification annealing (finish annealing) during the production of grain-oriented electrical steel sheet is a mixed atmosphere of H2 and N2 in the temperature range of 1150 to 1250°C, thereby preventing Ti from penetrating or remaining in the steel. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-195072 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, in the finish annealing that also serves as purification annealing, it is common to coil the steel sheet and place it in an annealing furnace to perform annealing for a long period of time. In such finish annealing, when purification annealing is performed according to the description in the above-mentioned Patent Document 1, the penetration and retention of Ti in the steel was prevented in the center and inner wound parts of the coil during the annealing as shown in Figure 1, but the Ti content in the steel was high in the outer wound part, and the Ti content in the steel as a whole varied.

[0007] The present invention has been made in consideration of the above circumstances, and has an object to provide a method for manufacturing a grain-oriented electrical steel sheet that provides a grain-oriented electrical steel sheet in which the formation of a base coating is favorable and which has excellent magnetic properties. [Means for solving the problem]

[0008] As a result of extensive investigations to solve the above-mentioned problems, the inventors have discovered that by controlling the temperature of a coil made of a steel sheet and the atmospheric gas in the finish annealing step which induces secondary recrystallization in a grain-oriented electrical steel sheet, it is possible to effectively reduce the amount of Ti contained in an annealing separator applied to the surface of the steel sheet which penetrates into the steel sheet and the amount of Ti remaining in the steel, and to prevent variation therein; and ultimately to obtain a grain-oriented electrical steel sheet in which the formation of a base coating is favorable and which has excellent magnetic properties, thereby completing the present invention.

[0009] The gist of the present invention is as follows. 1. A method for producing a grain-oriented electrical steel sheet by cold rolling a steel sheet to a final thickness, subjecting the steel sheet to decarburization annealing, applying an annealing separator containing 1.0 to 20 parts by mass of Ti oxide per 100 parts by mass of MgO to the surface of the steel sheet and drying the steel sheet, and then placing the steel sheet in a coiled state in a finish annealing furnace and subjecting it to finish annealing, A method for producing a grain-oriented electrical steel sheet, wherein, during the temperature rise process of the final annealing, the atmosphere is a 100 vol% H2 atmosphere when the maximum temperature of the coil reaches 1100°C, and the 100 vol% H2 atmosphere is changed to an H2 atmosphere containing 5 vol% or more N2 from the time when the maximum temperature of the coil reaches 1100°C until the temperature difference between the maximum and minimum temperatures of the coil is within 75°C and 15 hours have elapsed. Effect of the Invention

[0010] According to the present invention, by setting an upper limit on the time for changing the atmosphere for final annealing from an H atmosphere to an atmosphere containing N during the temperature rise process of the final annealing and by changing the atmosphere as described above after reducing the temperature difference in the coil, it is possible to effectively suppress the retention and variation of Ti in the steel of the entire coil, and to obtain a grain-oriented electrical steel sheet with excellent magnetic properties and good formation of a base coating. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing Ti analysis values ​​in steel sheets of outer, middle and inner coils of conventional coiled steel sheets. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention will be described in detail below. The method for producing a grain-oriented electrical steel sheet according to the present invention is characterized in particular by the conditions for finish annealing. Note that hereinafter, when simply referring to a steel sheet, this means a grain-oriented electrical steel sheet.

[0013] In the manufacturing method of the present invention, a steel sheet that has been cold-rolled to a final thickness is subjected to decarburization annealing, and then an annealing separator containing 1.0 to 20 parts by mass of Ti oxide per 100 parts by mass of MgO is applied thereto, dried, and then finish annealing is performed. Here, the final sheet thickness is a sheet thickness generally used for grain-oriented electrical steel sheets, and specifically, is preferably in the range of 0.35 mm or less.

[0014] In the cold rolling in the present invention, the cold rolling conditions usually used in the production of grain-oriented electrical steel sheets can be used without any particular limitations.

[0015] In addition, in the above-mentioned decarburization annealing in the present invention, the conditions for decarburization annealing that are usually used in the production of grain-oriented electrical steel sheets can be used without any particular limitations.

[0016] In the manufacturing method of the present invention, an annealing separator containing 1.0 to 20 parts by mass of Ti oxide per 100 parts by mass of MgO is used. The Ti oxide is not particularly limited, but TiO2 is preferred. If the content of Ti oxide in the annealing separator is less than 1.0 part by mass per 100 parts by mass of MgO, the formation of the coating during final annealing becomes insufficient. On the other hand, if the content of Ti oxide in the annealing separator is more than 20 parts by mass per 100 parts by mass of MgO, the penetration of Ti into the steel increases, resulting in magnetic deterioration. From the same viewpoint, the content of Ti oxide in the annealing separator is preferably 2.0 parts by mass or more, and more preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, per 100 parts by mass of MgO.

[0017] In the present invention, the method and conditions for applying the annealing separator that are usually used in the production of grain-oriented electrical steel sheets can be used for applying the annealing separator without any particular limitations. The annealing separator can be dried using any method or condition that is commonly used in the manufacture of grain-oriented electrical steel sheets, without any particular limitations.

[0018] In the manufacturing method of the present invention, after the annealing separator is applied to the surface of the steel sheet and dried, the steel sheet is placed in a coiled state in a finish annealing furnace and finish annealed. During the temperature rise process of the finish annealing, it is necessary to make the atmosphere at least at the time when the maximum temperature of the coil reaches 1100°C an atmosphere of H2: 100 vol%. If this condition is not satisfied, nitrogen will penetrate into the steel sheet and deteriorate the magnetic properties. Note that, in the present invention, the coil may be allowed to reach its maximum temperature of 1100°C and continued to be heated while maintaining the atmosphere of 100 vol% H2 before the coil reaches its maximum temperature of 1100°C. In the present invention, the maximum temperature of the coil is defined as the temperature measured by the thermocouple located at the outer winding side of the sample coil, which is placed at two locations on the bottom of the coil, one on the outer winding side and the other at the center of the inner winding, while the minimum temperature of the coil is defined as the temperature measured by the thermocouple located at the center of the inner winding.

[0019] Furthermore, in the manufacturing method of the present invention, during the temperature rise process of the final annealing, from the time when the maximum temperature of the coil reaches 1100°C, until the temperature difference between the maximum and minimum temperatures of the coil is within 75°C and 15 hours have elapsed, the H2:100 vol% atmosphere is changed to an H2 atmosphere containing 5 vol% or more of N2. In the present invention, the timing for changing to an H2 atmosphere containing N2 is calculated from the point when the maximum coil temperature reaches 1100° C. The reason for calculating the timing from the point when the maximum coil temperature reaches 1100° C. is that if the temperature is lower than that, the Ti oxides (especially titanium oxide) in the annealing separator do not decompose and are not fixed in the coating as TiN.

[0020] In addition, the time from when the maximum temperature of the coil reaches 1100°C to when the atmosphere of H2:100 vol% is changed to the H2 atmosphere containing N2 is within 15 hours. If the time exceeds 15 hours, Ti will penetrate into the steel sheet and deteriorate the magnetic property. From the same viewpoint, the time from when the maximum temperature of the coil reaches 1100°C to when the atmosphere of H2:100 vol% is changed to the H2 atmosphere containing N2 is preferably within 12 hours, more preferably within 10 hours. On the other hand, the lower limit of the time is typically when the temperature difference between the maximum temperature and the minimum temperature of the coil is within 75°C. In addition, the lower limit of the time is specifically about 8 hours. If the time is less than 8 hours, the reaction between titanium and nitrogen may be insufficient. In addition, the coiled steel sheet used in the present invention may have extremely large dimensions. In such cases, the difference between the maximum and minimum coil temperatures is usually not within 75°C when the maximum coil temperature reaches 1100°C, and it takes a certain amount of time for this difference to fall within 75°C.

[0021] In the present invention, it is essential that the temperature difference between the maximum and minimum coil temperatures is within 75° C. at the timing when the H2:100 vol% atmosphere is changed to an H2 atmosphere containing N2 from the time when the maximum coil temperature reaches 1100° C. In other words, it is essential that the temperature difference between the maximum and minimum coil temperatures is within 75° C. at the latest by the time 15 hours have elapsed since the maximum coil temperature reached 1100° C. If the atmosphere is changed when the difference between the maximum and minimum temperatures exceeds 75°C, the formation of the base coating on the inner coil will deteriorate. On the other hand, the smaller the difference between the maximum and minimum temperatures of the coil when the atmosphere is changed, the better, and it can be as small as 0°C. In industrial practice, it is about 50°C. In order to control the temperature difference to the above-mentioned level, it is effective to apply a method of suppressing heat input from the outside, particularly heat input to the outer winding side, at an appropriate timing.

[0022] Furthermore, the H2 atmosphere containing N2 after the change is an atmosphere containing 5 vol% or more of N2. If the amount of N2 is less than 5 vol%, the reaction with titanium becomes insufficient. On the other hand, the upper limit of the amount of N2 is not particularly limited, but is about 25 vol% or less in consideration of productivity. From the same viewpoint, the content of N2 in the H2 atmosphere after the change is preferably 8 vol% or more, and more preferably 20 vol% or less, and more preferably 15 vol% or less.

[0023] In the present invention, the outer turn of the coil is defined as the range from the outermost circumference of the coil to 20% of the coil radius. In addition, in the present invention, the inner turn of the coil is defined as the range from the innermost circumference of the coil to 20% of the coil radius. In addition, in the present invention, the middle turn of the coil is defined as the range excluding the outer turn and the inner turn of the coil.

[0024] In the present invention, the outer winding portion, the inner winding portion, and the intermediate winding portion of the coil are defined as portions having the above-mentioned respective ranges.

[0025] The conditions for the final annealing other than those described above are not particularly limited, and may be determined according to conventional methods. As for the manufacturing conditions for the steel sheet not described above, any of them may be made according to conventional methods. EXAMPLES

[0026] The steel sheet was cold-rolled to a thickness of 0.23 mm and then subjected to decarburization annealing. An annealing separator containing 5 parts by mass of TiO2 per 100 parts by mass of MgO was then applied to the surface of the steel sheet and dried. The steel sheet was then placed in a coil state in a finish annealing furnace and subjected to finish annealing to produce a grain-oriented electrical steel sheet. Here, in the final annealing, thermocouples were placed at two locations on the bottom of the coil, at the outer winding and the center of the inner winding. In the final annealing, the atmosphere was changed to 100 vol% H2 when the maximum temperature of the coil (the temperature of the outer winding) reached 1100°C, and the atmosphere was changed from 100 vol% H2 to an H2 atmosphere containing 10 vol% N2 at the timing shown in Table 1. After that, when the maximum temperature of the coil reached 1200°C, the atmosphere was changed to 100 vol% H2, and the temperature was lowered after holding for 10 hours. The Ti content in the obtained grain-oriented electrical steel sheet (coil) was measured using energy dispersive X-ray analysis (EDX) according to a calibration curve prepared in advance. Table 1 shows the Ti content (remaining Ti) in the outer winding part of the coil.

[0027] In addition, the iron loss in the obtained grain-oriented electrical steel sheet (coil) was measured as follows: When magnetized to 1.7 T in an AC magnetic field with an excitation frequency of 50 Hz, the iron loss W per 1 kg of steel sheet was 17 / 50 Table 1 shows the iron loss W 17 / 50 Shows.

[0028] The grain-oriented electrical steel sheets (coils) thus obtained were visually observed for the state of formation of the undercoat film. Table 1 shows the observation results.

[0029] [Table 1]

[0030] As can be seen from Table 1, by setting an upper limit on the time required to change from an H2 atmosphere to an atmosphere containing N2 and by reducing the temperature difference inside the coil during this atmosphere change, it is possible to suppress the amount of Ti remaining in the steel of the entire coil (especially the amount of Ti remaining in the outer windings of the coil) and its variation, thereby obtaining a grain-oriented electrical steel sheet with good formation of a base coating and excellent magnetic properties.

Claims

[Claim 1] A steel sheet having a final thickness obtained by cold rolling is subjected to decarburization annealing, and then an annealing separator containing 1.0 to 20 parts by mass of Ti oxide per 100 parts by mass of MgO is applied to the surface of the steel sheet and dried. The steel sheet is then placed in a coil state in a finish annealing furnace and finish annealed to obtain a grain-oriented electrical steel sheet, During the temperature rise process of the final annealing, the atmosphere at the time when the maximum temperature of the coil reached 1100°C was H 2 : In an atmosphere of 100 vol%, from the time when the maximum temperature of the coil reaches 1100 ° C, the temperature difference between the maximum temperature and the minimum temperature of the coil is within 75 ° C and 8 hours or more have passed, and within 15 hours, 2 : 100 vol% atmosphere, 5 vol% or more N 2 Contains H 2 A manufacturing method for grain-oriented electrical steel sheet, changing the atmosphere.

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