WO2025154575A1 - Method for cold rolling grain-oriented electromagnetic steel sheet, method for manufacturing same, and cold rolling equipment train - Google Patents

Abstract

Provided is a technique that makes it possible, when performing cold rolling using a single-stand rolling mill, to stably and productively manufacture a grain-oriented electromagnetic steel sheet having exceptional magnetic characteristics. The present invention provides a method for cold rolling a grain-oriented electromagnetic steel sheet, a method for manufacturing a grain-oriented electromagnetic steel sheet using the aforementioned cold rolling method, and a cold rolling equipment train used for the aforementioned cold rolling. In said method for cold rolling a grain-oriented electromagnetic steel sheet, the cold rolling is divided into primary cold rolling and secondary cold rolling when a cold-rolled sheet of a final sheet thickness is obtained by cold rolling in which the total rolling reduction rate is 80% or greater. In the primary cold rolling, said steel sheet is rolled to an intermediate sheet thickness using a tandem rolling mill, and in the secondary cold rolling, said steel sheet is rolled from the intermediate sheet thickness to the final sheet thickness by 1- to 3-pass rolling using a single-stand rolling mill having a work roll diameter such that the ratio thereof with respect to the work roll diameter of the final stand of the tandem rolling mill used for the primary cold rolling is 0.30-0.95, whereby the optimal value of the sheet-width-direction magnetic flux density B₈ of the end-product sheet is 1.920 T or greater.

Description

Cold rolling method and manufacturing method for grain-oriented electrical steel sheet, and cold rolling equipment train

The present invention relates to a method for cold rolling grain-oriented electrical steel sheets, a method for producing grain-oriented electrical steel sheets with excellent magnetic properties using the cold rolling method, and a cold rolling equipment line suitable for the cold rolling method. In this specification, "x to y" representing a range of values means x or more and y or less, and includes the boundary value.

Grain-oriented electrical steel sheet is a steel sheet with excellent magnetic properties that has a crystalline structure in which the <001> orientation, the easy axis of magnetization of iron, is highly concentrated in the rolling direction of the steel sheet, known as the Goss orientation. Such grain-oriented electrical steel sheet is generally manufactured using steel material that contains approximately 4.5 mass% or less of Si. Grain-oriented electrical steel sheet often also contains components that form inhibitors for secondary recrystallization, such as MnS, MnSe, and AlN.

On the other hand, Patent Document 1 proposes a technology for inducing secondary recrystallization without the inclusion of inhibitor-forming components, the so-called inhibitorless method. This inhibitorless method has the advantage of being able to produce grain-oriented electrical steel sheet at low cost, as it does not require high-temperature slab heating, which is necessary when using inhibitors. However, this technology is a technology that uses highly purified steel material to induce secondary recrystallization through texture control, that is, texture control (hereinafter also referred to as "texture inhibition"). Therefore, delicate control is required to create the texture.

In the manufacture of grain-oriented electrical steel sheets, the cold rolling conditions are important from the viewpoint of properly controlling the texture. In particular, in the manufacture of grain-oriented electrical steel sheets using steel materials that do not contain inhibitor-forming components, the quality of the texture is extremely important, as it has a significant effect on the quality of the magnetic properties.

For example, Patent Document 2 proposes a method of heat treating (aging) steel sheets at low temperatures during cold rolling as a technique for forming a good texture. Patent Document 3 proposes a technique in which the cooling rate during hot-rolled sheet annealing or annealing before finish cold rolling (final cold rolling) is set to 30°C/s or more, and interpass aging for 2 minutes or more is performed at least twice during finish cold rolling at a sheet temperature of 150-300°C. Patent Document 4 proposes a technique that uses the dynamic aging effect to immediately fix dislocations introduced during rolling with C (carbon) and N (nitrogen) by performing warm rolling, in which the steel sheet temperature is increased during rolling.

These techniques involve diffusing the solute elements C (carbon) and N (nitrogen) at low temperatures to fix dislocations introduced during rolling. In this way, the movement of dislocations is hindered, and shear deformation occurs during subsequent cold rolling, improving the rolling texture. The application of this technique can reduce the (111) fiber structure known as gamma fiber in the primary recrystallization texture after cold rolling. This has the effect of increasing the frequency of the {110}<001>, or Goss orientation, which is advantageous for magnetic properties.

The rolling mills used for the above cold rolling are generally single-stand reverse rolling mills (see, for example, Patent Document 5) and tandem rolling mills consisting of multiple stands (see, for example, Patent Document 6). However, from the viewpoint of applying the above-mentioned technology, it is considered more advantageous to use a reverse rolling mill that can hold the rolled steel sheet in a coil at high temperature for a long time and perform the so-called "aging treatment".

JP 2000-129356 A Japanese Unexamined Patent Publication No. 50-016610 Japanese Patent Application Publication No. 08-253816 Japanese Patent Application Publication No. 01-215925 Special Publication No. 54-013846 Special Publication No. 54-029182

As mentioned above, the conventional techniques disclosed in Patent Documents 2 to 4 all utilize the effect of aging treatment, which promotes the diffusion of C and N in the steel by maintaining the steel sheet temperature at a high temperature during or after rolling, and fixes dislocations introduced during rolling. Therefore, when using a tandem rolling mill, various studies are necessarily conducted on the premise of a very short aging treatment between stands. Also, when using a reverse rolling mill, various studies are conducted on the premise of a relatively long aging treatment after coiling. However, such aging treatment has the problem that it is extremely restricted by the equipment. For example, if one tries to secure an aging treatment time of 1 hour, rolling will have to be interrupted for 1 hour in the middle of reverse rolling, which significantly impairs productivity.

The present invention was made in consideration of the above problems with the conventional technology, and its purpose is to provide a technology that makes it possible to stably and productively manufacture grain-oriented electrical steel sheets with excellent magnetic properties when cold rolling is performed using a single-stand rolling mill. Specifically, the present invention proposes a cold rolling method for grain-oriented electrical steel sheets and a manufacturing method for grain-oriented electrical steel sheets using this cold rolling method, and provides a cold rolling equipment train suitable for these methods.

In order to solve the above problems, the inventors conducted extensive research focusing on the conditions for cold rolling steel sheets after hot rolling. As a result, they discovered that by dividing the cold rolling into a front and rear stage, using a tandem rolling mill for the front stage and a single-stand rolling mill for the rear stage, and controlling the reduction ratio and work roll diameter ratio in the front and rear cold rolling stages within appropriate ranges, grain-oriented electrical steel sheets with excellent magnetic properties can be produced productively and stably, which led to the development of the present invention.

The present invention based on the above findings provides: (A) a method for cold rolling grain-oriented electrical steel sheet to produce a cold-rolled sheet of a final sheet thickness by cold rolling with a total reduction of 80% or more, the cold rolling being divided into a front-stage cold rolling and a rear-stage cold rolling, and optionally intermediate annealing being performed between the front-stage cold rolling and the rear-stage cold rolling, the steel sheet having undergone the front-stage cold rolling being wound into a coil at a temperature of 80°C or more, and after a lapse of at least 30 minutes, the rear-stage cold rolling is performed, and when the intermediate annealing is performed, the annealing temperature is set to 700°C or more. the first stage cold rolling is performed using a tandem rolling mill to roll to an intermediate plate thickness with a reduction amount that is a ratio of the total reduction amount in the cold rolling to a thickness in the range of 0.50 to 0.95, and the second stage cold rolling is performed using a single-stand rolling mill to roll from the intermediate plate thickness to the final plate thickness in one to three passes, and the ratio of the work roll diameter of the single-stand rolling mill to the work roll diameter of the final stand of the tandem rolling mill is set to a range of 0.30 to 0.95, thereby making the best value of magnetic flux density _B8 in the plate width direction of the product plate 1.920 T or more.

In the cold rolling method for grain-oriented electrical steel sheet according to the present invention, (i) in the latter stage of cold rolling, it is more preferable to perform the cold rolling using one single-stand rolling mill with different work roll diameters, or to perform the cold rolling using multiple single-stand rolling mills with different work roll diameters.

The present invention also contains C: 0.01-0.10 mass%, Si: 2.0-4.5 mass%, Mn: 0.01-0.5 mass%, and further contains, as inhibitor-forming components, at least one group of components selected from group A; sol. Al: 0.010-0.05 mass% and N: 0.004-0.012 mass%, group B; at least one selected from S and Se: 0.01-0.05 mass% in total, and group C; sol. Al: 0.010-0.05 mass%, N: 0.004-0.012 mass%, and at least one selected from S and Se: 0.01-0.05 mass% in total,
Optionally, at least one component selected from Ni: 0.005 to 1.50 mass%, Cr: 0.005 to 0.3 mass%, Cu: 0.005 to 0.3 mass%, Sn: 0.005 to 0.3 mass%, Sb: 0.005 to 0.3 mass%, Mo: 0.005 to 0.3 mass%, Te: 0.005 to 0.3 mass%, Bi: 0.005 to 0.3 mass%, and P: 0.005 to 0.3 mass%, with the balance being F. a steel slab having a component composition consisting of Zn, Fe, and unavoidable impurities is hot-rolled to form a hot-rolled sheet, the hot-rolled sheet is cold-rolled with a total reduction of 80% or more to form a cold-rolled sheet of a final thickness, the cold-rolled sheet is subjected to primary recrystallization annealing that also serves as decarburization annealing, an annealing separator is applied to the surface of the steel sheet, and then secondary recrystallization is performed, followed by finish annealing for purification, wherein the cold rolling is performed using any of the above-mentioned cold rolling methods, thereby making the best value of magnetic flux density _B8 in the sheet width direction 1.920 T or more.

The present invention also provides a steel sheet containing C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, Mn: 0.01 to 0.5 mass%, and further containing sol. Al: less than 0.010 mass%, S: 0.0050 mass% or less, Se: 0.0050 mass% or less, and N: 0.0050 mass% or less, and optionally Ni: 0.005 to 1.50 mass%, Cr: 0.005 to 0.3 mass%, Cu: 0.005 to 0.3 mass%, Sn: 0.005 to 0.3 mass%, Sb: 0.005 to 0.3 mass%, Mo: 0.005 to 0.3 mass%, Te: 0.005 to 0.3 mass%, Bi: 0.005 to 0.3 mass%, and P: 0.005 to 0. The present invention proposes a method for producing grain-oriented electrical steel sheet, comprising the steps of hot rolling a steel slab having a composition comprising at least one component selected from among 0.15 to 0.3 mass% of Fe and the balance consisting of Fe and unavoidable impurities, cold rolling the hot rolled sheet with a total rolling reduction of 80% or more to produce a cold rolled sheet of a final thickness, subjecting the cold rolled sheet to primary recrystallization annealing also serving as decarburization annealing, applying an annealing separator to the surface of the steel sheet, and then subjecting the cold rolled sheet to secondary recrystallization and finish annealing for purification, wherein the cold rolling is performed using any of the above cold rolling methods to set the best magnetic flux density _B8 in the sheet width direction to 1.920 T or more.

The present invention also relates to a cold rolling equipment train consisting of a front-stage cold rolling mill and a rear-stage cold rolling mill used in the cold rolling method described in (a) above, characterized in that the front-stage cold rolling mill is a tandem rolling mill, the rear-stage cold rolling mill is a single-stand rolling mill, and the ratio of the work roll diameter of the rear-stage cold rolling mill to the work roll diameter of the final stand of the tandem rolling mill is within the range of 0.30 to 0.95.

The cold rolling equipment train of the present invention is a cold rolling equipment train consisting of a front-stage cold rolling mill and a rear-stage cold rolling mill used in the cold rolling method described in (i) above, in which the front-stage cold rolling mill is a tandem rolling mill and the rear-stage cold rolling mill is a single-stand rolling mill, and a more preferred solution is that the rear-stage cold rolling mill is one single-stand rolling mill with a changeable work roll diameter, or multiple single-stand rolling mills with different work roll diameters.

Moreover, a more preferable solution for the cold rolling equipment train according to the present invention is that it has at least one of the following configurations: (1) the tandem rolling mill has a function of controlling the coil winding temperature by adjusting at least one of the temperature and flow rate of the coolant sprayed onto at least one of the steel sheet surface and the work roll surface at the exit side of the final stand, and (2) a heat retention device is provided between the tandem rolling mill and the subsequent rolling mill, for maintaining the temperature of the steel sheet after cold rolling in the previous stage at 50°C or higher.

The present invention makes it possible to stably manufacture grain-oriented electrical steel sheets with excellent magnetic properties.

1 is a graph showing the influence of a work roll diameter ratio (WR diameter ratio) between a front-stage cold rolling mill and a rear-stage cold rolling mill on magnetic flux density and iron loss. 1 is a graph showing the influence of a work roll diameter ratio (WR diameter ratio) between a front-stage cold rolling mill and a rear-stage cold rolling mill on magnetic flux density, iron loss, and their variations in the sheet width direction.

First, the inventors conducted the following experiment to manufacture grain-oriented electrical steel sheets using steel materials that do not contain inhibitor-forming components, for which texture control is particularly important, and investigated the conditions necessary to improve magnetic properties.

(Experiment 1)
A steel material (slab) containing 0.050 mass% C, 3.3 mass% Si, and 0.04 mass% Mn, less than 0.0050 mass% S, Se, and O, less than 0.0025 mass% N, and the balance consisting of Fe and unavoidable impurities was melted. This steel slab not containing inhibitor-forming components was heated to a temperature of 1100 ° C. and hot rolled to obtain a hot rolled sheet having a thickness of 3.0 mm. The hot rolled sheet was then subjected to hot rolled sheet annealing at 1000 ° C. x 70 s to obtain a hot rolled annealed sheet. Next, the above hot-rolled annealed sheet was subjected to a first stage of cold rolling in which the steel sheet temperature between the third and subsequent stands was 120° C. or higher using a 5-stand tandem rolling mill with a work roll diameter of 250 mm, and rolled to the intermediate thickness shown in Table 1, and then wound into a coil. At this time, the sheet temperature (winding temperature) when wound into a coil was measured. Next, the steel sheet with the intermediate thickness was transported to a single-stand rolling mill with a work roll diameter of 200 mm, and subjected to 1 to 4 passes of cold rolling shown in Table 1 to obtain a cold-rolled sheet with a final thickness of 0.22 mm. At this time, the time (elapsed time) from the end of the first stage of cold rolling to the start of the second stage of cold rolling was variously changed. Thereafter, the cold-rolled sheet with the final thickness was subjected to primary recrystallization annealing, which also served as decarburization annealing, and an annealing separator was applied to the surface of the steel sheet, and then secondary recrystallization was performed, followed by finish annealing for purification treatment to obtain a product sheet.

A test piece for magnetic measurement was taken from the longitudinal center position of the product sheet coil obtained in this way. The magnetic flux density _B8 (magnetic flux density when a magnetic field of 800 A/m is applied) of the test piece was measured by the Epstein test defined in JIS C2550, and the results are also shown in Table 1. In addition, in Table 1, the ratio of the reduction amount of the cold rolling in the previous stage to the total reduction amount in the cold rolling (previous stage reduction amount/total reduction amount: hereinafter also simply referred to as "reduction amount ratio") is also shown. Here, the above reduction amount refers to the "sheet thickness reduction amount (mm) due to rolling".

The results in Table 1 above show that the magnetic properties are relatively good when the reduction ratio is in the range of 0.50 to 0.95. In addition, among the conditions under which these relatively good magnetic properties can be obtained, it can be seen that the magnetic properties are even better when the coil winding temperature after the first stage of cold rolling is 80°C or higher, and the elapsed time between the first and second stages of cold rolling is 30 minutes or more.

(Experiment 2)
A steel material (slab) containing 0.03 mass% C, 3.2 mass% Si, and 0.06 mass% Mn, less than 0.0050 mass% S, Se, and O, less than 0.0025 mass% N, and the balance consisting of Fe and unavoidable impurities was melted. This steel slab not containing inhibitor-forming components was heated to a temperature of 1100 ° C. and hot rolled to obtain a hot rolled sheet having a thickness of 3.0 mm and a width of 1000 mm. The hot rolled sheet was then subjected to hot rolled sheet annealing at 1000 ° C. x 70 s to obtain a hot rolled annealed sheet. Next, the hot-rolled annealed sheet was subjected to a first stage of cold rolling in which the steel sheet temperature between the third and subsequent stands was 120°C or higher using a 5-stand tandem rolling mill with a work roll diameter of 250 mm, to roll the sheet to an intermediate thickness of 0.6 mm. The sheet was then wound into a coil at a temperature of 100°C. Next, after 40 minutes had elapsed since the end of the first stage of cold rolling, the sheet was subjected to a second stage of cold rolling in which the final thickness of the sheet was 0.22 mm in two passes using various single-stand rolling mills with different work roll diameters. The work roll diameters used in the second stage of rolling were 75 mm, 80 mm, 140 mm, 150 mm, 160 mm, 235 mm, 250 mm, 265 mm, 330 mm, 335 mm, 340 mm, 510 mm, 520 mm, and 530 mm, which were 14 types. Thereafter, the cold-rolled sheet having the above-mentioned final sheet thickness was subjected to primary recrystallization annealing which also served as decarburization annealing, and an annealing separator was applied to the surface of the steel sheet. Thereafter, secondary recrystallization was caused, and then finish annealing for purification was performed to obtain the product sheet.

A sample material with a total width of 1000 mm was taken from the longitudinal center position of the product sheet coil obtained in this way. From the above sample material, 10 test pieces with a width of 100 mm and a length of 300 mm were taken with the rolling direction as the length direction. The test pieces were measured for magnetic flux density B ₈ (magnetic flux density when a magnetic field of 800 A/m is applied) and iron loss W _17/50 (iron loss per kg of steel sheet when magnetized to a magnetic flux density of 1.7 T in an AC magnetic field with an excitation frequency of 50 Hz) using a single sheet magnetic measuring device to obtain the best value in the sheet width direction. Here, the reason for obtaining the best value in the sheet width direction is to determine the best magnetic properties that can be obtained.

FIG. 1 shows the relationship between the ratio of the work roll diameters used in the front and rear cold rolling (rear WR diameter/front WR diameter), hereinafter also simply referred to as the "WR diameter ratio") and the best magnetic properties in the sheet width direction (magnetic flux density _B8 and iron loss W17 _/50 ) for the above measurement results. It can be seen from this figure that the iron loss is improved drastically when the WR diameter ratio is 0.95 or less. In addition, the magnetic flux density is also improved as the WR diameter ratio becomes smaller. Specifically, it was found that the best magnetic flux density _B8 in the sheet width direction can be stably set to 1.920 T or more by setting the ratio (WR diameter ratio) of the work roll diameter of the rear cold rolling to the work roll diameter of the front cold rolling (250 mm) to 0.95 or less.

The technology in this embodiment was developed by further improving on the above experimental results.

Next, we will explain the composition of the steel material (slab) used to manufacture the grain-oriented electrical steel sheet according to this embodiment. Hereinafter, the unit of content "%" for the composition of the components represents "mass%" unless otherwise specified.

C: 0.01-0.10%
C is an element effective in improving the primary recrystallization texture, and is contained at 0.01% or more from the viewpoint of obtaining the above effect. However, if the C content exceeds 0.10%, it leads to deterioration of the primary recrystallization texture. In addition, excessive C content makes it difficult to reduce the C content to, for example, 0.003% or less, which does not cause magnetic aging, by decarburization annealing. Therefore, the C content is set to the range of 0.01 to 0.10%. Preferably, the C content is 0.02% or more, and preferably, the C content is 0.08% or less.

Si: 2.0-4.5%
Silicon is an element effective in increasing the resistivity of steel and improving iron loss. From the viewpoint of obtaining the above effect, silicon is contained at 2.0% or more. However, if the silicon content exceeds 4.5%, the cold rolling property is significantly deteriorated, so the upper limit is set to 4.5%. Preferably, the silicon content is 2.5% or more, and more preferably, the silicon content is 4.0% or less.

Mn: 0.01~0.5%
Mn has the effect of improving workability in hot rolling, and is also a useful element from the viewpoint of controlling the formation of an oxide film during primary recrystallization annealing, so it is contained in an amount of 0.01% or more. However, if the Mn content exceeds 0.5%, the primary recrystallization texture deteriorates, leading to deterioration of magnetic properties, so the upper limit is set to 0.5%. The Mn content is preferably 0.03% or more, and more preferably 0.2% or less.

The components other than C, Si, and Mn mentioned above differ depending on whether a precipitation-type inhibitor, such as AlN, MnS, MnSe, etc., is used to induce secondary recrystallization or whether the above inhibitors are not used.

First, the use of a precipitation-type inhibitor to induce secondary recrystallization will be described.
When AlN is used as an inhibitor, the Al content is preferably in the range of 0.010 to 0.05% in terms of sol. Al (acid-soluble Al), and the N content is preferably in the range of 0.004 to 0.012%. If the sol. Al content is less than 0.010%, the amount of precipitated AlN is too small, resulting in insufficient inhibitor effect, secondary recrystallization failure, and a decrease in magnetic flux density. On the other hand, if the sol. Al content exceeds 0.05%, the secondary recrystallization tends to become unstable. In addition, if the N content is less than 0.004%, AlN does not precipitate properly during intermediate processes such as hot rolling and hot-rolled sheet annealing, making it difficult to control the grain size during hot-rolled sheet annealing, intermediate annealing, and even primary recrystallization annealing. On the other hand, if the N content exceeds 0.012%, it causes frequent occurrence of surface defects called blisters. Therefore, it is preferable that Al and N are in the above range. From the viewpoint of stabilizing the inhibitor during secondary recrystallization, the sol. Al content is more preferably 0.010% or more, and more preferably 0.035% or less. The N content is more preferably 0.008% or more, and more preferably 0.012% or less.

The N content of steel plate can be increased by applying a nitriding treatment in a process prior to the final annealing, so that at the material stage, for example in a steel slab, the N content may be 0.008% or less as long as it is 0.004% or more. When nitriding, the N content may be increased to about 0.05% in order to appropriately control the primary recrystallized grain size and obtain the inhibitor strength required for secondary recrystallization.

On the other hand, when a sulfide such as MnS or a selenide such as MnSe is used as an inhibitor, it is preferable to contain at least one selected from S and Se in a total range of 0.01 to 0.05%. If the total content of S and Se is less than 0.01%, the absolute amount of inhibitor will be insufficient. On the other hand, if it exceeds 0.05%, it will be difficult to purify by finish annealing. S and Se may be added alone, and in that case, the content is preferably in the range of 0.01 to 0.05% each. More preferably, the total content of S and Se is 0.015% or more, and more preferably, the total content of S and Se is 0.040% or less.

When using inhibitors to induce secondary recrystallization, the above-mentioned AlN-based inhibitors may be used in combination with MnS-based inhibitors or MnSe-based inhibitors.

On the other hand, as disclosed in Patent Document 1, when developing secondary recrystallized grains without using a precipitation-type inhibitor, it is preferable to reduce the inhibitor-forming components Al, N, S, and Se as much as possible. Specifically, it is preferable to reduce the Al content to less than 0.010% in sol. Al, the S content to 0.0050% or less, the Se content to 0.0050% or less, and the N content to 0.0050% or less. If the respective elements are contained outside the above values, it becomes difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition. In addition, it is preferable to keep N within the above range in order to prevent the formation of Si nitrides during the purification treatment of the final annealing.

In addition, if a precipitation-type inhibitor is not used, it is preferable to reduce the nitride-forming elements Ti, Nb, B, Ta and V to 0.0050% or less each in order to prevent interference with the texture inhibition effect and suppress deterioration of iron loss.

The steel material used to manufacture the grain-oriented electrical steel sheet according to this embodiment basically consists of Fe and unavoidable impurities, apart from the above-mentioned components. However, from the viewpoint of improving the hot-rolled sheet structure and enhancing the magnetic properties, Ni may be further contained in the range of 0.005 to 1.50% in addition to the above-mentioned components. Furthermore, for the purpose of improving the magnetic properties, at least one element selected from the grain boundary segregation type elements Cr, Cu, Sn, Sb, Mo, Te, Bi and P may be contained in the range of 0.005 to 0.3% each.

Next, a method for producing the grain-oriented electrical steel sheet according to this embodiment will be described.
First, the steel material (slab) that is the raw material for grain-oriented electrical steel sheet can be produced by melting steel satisfying the above-mentioned component composition in a commonly known refining process, and then by a conventional continuous casting method or an ingot making-slabbing rolling method.

Then, the steel material (slab) is heated to an appropriate temperature and then subjected to hot rolling. If the steel slab contains precipitation-type inhibitor-forming components, the heating temperature of the steel slab is preferably in the range of 1350 to 1450°C in order to completely dissolve Al, Se, S, etc. On the other hand, if the steel slab does not contain precipitation-type inhibitor-forming components, the heating temperature of the steel slab is preferably 1250°C or lower. This is because if the heating temperature of the steel slab is too high, the inhibitor-forming components that have dissolved during heating will precipitate unevenly and finely during hot rolling. This will locally inhibit grain boundary migration, resulting in an uneven grain size distribution and inhibiting the development of secondary recrystallized grains.

The conditions for the hot rolling that follows the heating of the steel slab may be in accordance with the usual methods used in the manufacture of grain-oriented electrical steel sheets. In addition, the steel sheet after the hot rolling, i.e., the hot-rolled sheet, may be subjected to hot-rolled sheet annealing in order to improve the magnetic properties, or it may not be necessary to perform hot-rolled sheet annealing in order to prioritize manufacturing costs.

Then, the hot-rolled sheet after the hot rolling or after the hot-rolled sheet annealing is subjected to cold rolling. This cold rolling is the most important step in this embodiment, and must be performed while satisfying the following conditions. First, the cold rolling in this embodiment must be performed in two separate steps, the front and rear, and the total reduction ratio of the front and rear cold rolling steps must be 80% or more. If the total reduction ratio is less than 80%, the dislocation density introduced by rolling cannot be sufficiently increased to promote the fixation of dislocations by strain aging and improve the texture. A preferred total reduction ratio is 84% or more, and a preferred total reduction ratio is 96% or less. Here, the "total reduction ratio" refers to the total thickness reduction amount by cold rolling, that is, the percentage of the value obtained by dividing the total reduction amount by the thickness before cold rolling.

Furthermore, intermediate annealing between the first and second cold rolling stages can be performed as desired. If intermediate annealing is performed, the annealing temperature must be set at 700°C or lower. If annealing above 700°C is performed during cold rolling, the rolled structure will recover and recrystallize, and the dislocations introduced by cold rolling will disappear, degrading the magnetic properties of the grain-oriented electrical steel sheet. For this reason, the annealing temperature for intermediate annealing is limited.

In addition, when switching from the first cold rolling to the second cold rolling, it is necessary to control the ratio of the reduction amount of the first cold rolling to the total reduction amount in cold rolling, that is, the reduction amount ratio, within the range of 0.50 to 0.95. Here, the total reduction amount in cold rolling is the difference between the hot rolled sheet thickness and the final cold rolled sheet thickness, and the reduction amount of the first cold rolling refers to the sheet thickness reduction amount in the first cold rolling. By setting the reduction amount ratio of the first cold rolling within the above range, sufficient dislocation density can be accumulated in the steel sheet. If the reduction amount ratio is less than 0.50, dislocation density is not sufficiently accumulated, while if it exceeds 0.95, the texture improvement effect by the second cold rolling, which is a feature of this embodiment, cannot be sufficiently obtained. Note that the preferable reduction amount ratio is 0.60 or more, and the preferable reduction amount ratio is 0.80 or less.

It is also important that the first stage of cold rolling is performed using a tandem rolling mill consisting of multiple rolling mills to roll to an intermediate plate thickness, and the second stage of cold rolling is performed using a rolling mill consisting of a single stand to roll from the intermediate plate thickness to the final plate thickness in one to three passes. By limiting the number of passes in the second stage of cold rolling to three or less, the reduction rate per pass in a state in which dislocations are locked can be increased, further promoting the formation of deformation bands.

Furthermore, the ratio of the work roll diameter of the rolling mill used in the latter cold rolling to the work roll diameter of the final stand of the tandem rolling mill used in the former cold rolling (WR diameter ratio) must be within the range of 0.30 to 0.95. By using smaller diameter rolls in the latter cold rolling compared to the former cold rolling, the frictional force on the steel sheet surface during rolling is increased, and large shear deformation is applied to the steel sheet surface layer. Specifically, the latter cold rolling is strongly pressed down with small diameter rolls, and shear stress is applied to the structure accumulated in {111}<112> in the former cold rolling. This causes crystal rotation to form a {110}<001> structure. As a result, iron loss can be improved by the formation of sharp {110}<001> and the refinement of the crystal grain size after secondary recrystallization due to the increase in {110}<001>. Therefore, in this embodiment, the work rolls used in the latter cold rolling are small diameter rolls with a WR diameter ratio of 0.95 or less, which is the ratio to the work roll diameter of the final stand of the former tandem rolling mill. However, if the WR diameter ratio is less than 0.30, the deformation resistance in the latter cold rolling increases, which may impair the sheet passing properties and rolling properties, so the lower limit of the WR diameter ratio is set to 0.30. The preferred WR diameter ratio is 0.35 or more, and the preferred WR diameter ratio is 0.90 or less.

If the subsequent cold rolling is performed in two or three passes, as long as the work roll diameter is within the range that satisfies the above conditions, each pass may be performed using work rolls of different diameters in one rolling mill, or each pass may be performed using multiple rolling mills having work rolls of different diameters.

Furthermore, after the first stage of cold rolling is completed, the steel sheet must be wound into a coil at a temperature of 80°C or higher, and the second stage of cold rolling must be performed after at least 30 minutes have passed. By satisfying the above conditions, the fixation of dislocations due to strain aging is promoted, and the effect of improving the texture can be further enhanced. The preferred coiling temperature is 100°C or higher, and the elapsed time is 60 minutes or more.

From the viewpoint of making the steel sheet temperature 80° C. or higher after the completion of the front-stage cold rolling, the tandem rolling mill used in the front-stage cold rolling according to this embodiment preferably has the following function: The function is to control the coil winding temperature by adjusting at least one of the temperature and flow rate of the coolant sprayed onto at least one of the steel sheet surface and the work roll surface at the delivery side of the final stand of the tandem rolling mill.
Generally, cold rolling is often performed while supplying rolling oil at 70° C. or less to the rolls, and therefore, in order to make the steel sheet temperature at the time of coil winding 80° C. or more, it is preferable to carry out control such as a function of wiping the oil so that the rolling oil does not come into contact with the steel sheet after cold rolling, or blowing off the rolling oil with high-pressure gas, air, etc. Considering manufacturability, it is more efficient to carry out the front and rear rolling stages continuously, but it is also an important factor to deliberately set a time of 30 min or more.

Moreover, from the viewpoint of promoting aging, the rolling mill used in the cold rolling of the grain-oriented electrical steel sheet according to this embodiment preferably has a heat retention device disposed between the front-stage tandem rolling mill and the rear-stage rolling mill for maintaining the temperature of the steel sheet after the front-stage cold rolling at 50°C or higher.
The temperature of the steel sheet at the start of cold rolling is not particularly limited, and so-called warm rolling is also included. However, if the temperature of the steel sheet at the start of cold rolling is too high, a problem of rolling oil burning occurs, so that the temperature is preferably 280° C. or lower.

The cold-rolled sheet having the final thickness is then subjected to primary recrystallization annealing, which also serves as decarburization annealing. The purpose of this primary recrystallization annealing is to recrystallize the cold-rolled sheet having a rolled texture, adjust it to an optimal primary recrystallization texture for secondary recrystallization, and reduce the carbon contained in the steel to a value at which magnetic aging does not occur. In addition, it is intended to form an oxide film necessary for forsterite film formation on the steel sheet surface. For this reason, the primary recrystallization annealing is preferably performed at a temperature of 750 to 900°C in an oxidizing wet hydrogen-nitrogen or wet hydrogen-argon atmosphere. In addition, the heating rate of the primary recrystallization annealing is preferably rapid heating of 50°C/s or more in order to improve the texture.

After the primary recrystallization annealing, the steel sheet is coated with an annealing separator. It is preferable that the main component of this annealing separator is magnesia (MgO), which forms a forsterite film on the steel sheet surface after the final annealing. It is preferable to add an appropriate amount of auxiliary agents such as Ti oxides and Sr compounds to this annealing separator to make the formation of the forsterite film uniform and improve peeling resistance.

The steel sheet coated with the annealing separator is wound on a coil and undergoes secondary recrystallization, and then is subjected to finish annealing at a temperature of 1100°C or higher to form a forsterite film and remove impurities such as inhibitor-forming components from the steel sheet (purification treatment). The atmosphere for this finish annealing is preferably _H2 atmosphere during the purification treatment, and _N2 , Ar, _H2 , or a mixture of these gases. By this purification treatment, the inhibitor-forming components Al, N, S, and Se contained in the steel sheet are reduced to the impurity level, specifically, Al: 0.008 mass% or less, N: 0.003 mass% or less, S: 0.003 mass% or less, and Se: 0.005 mass% or less. In the above-mentioned finish annealing, in order to more effectively carry out the secondary recrystallization, the steel may be isothermally held at a temperature close to the secondary recrystallization temperature for a long period of time, or may be slowly heated in the above-mentioned temperature range.

Then, the steel sheet after the above-mentioned finish annealing is preferably subjected to flattening annealing in order to correct the shape. Furthermore, in the above-mentioned flattening annealing, an insulating film may be applied to the surface of the steel sheet and baked to form it. The type of this insulating film is not particularly specified. For example, from the viewpoint of reducing iron loss, the technology disclosed in JP-A-50-79442 and JP-A-48-39338 can be applied. This technology forms a tension-imparting insulating film by applying a coating liquid containing phosphate, chromate, and colloidal silica to the surface of the steel sheet and baking it at a temperature of about 800°C. Furthermore, from the viewpoint of further reducing iron loss, a magnetic domain refining process may be performed by forming grooves on the surface of the steel sheet in any process after cold rolling, or by irradiating the surface of the steel sheet after the finish annealing with an electron beam or laser beam to introduce thermal strain.

The grain-oriented electrical steel sheet according to this embodiment manufactured as described above has excellent magnetic properties. Specifically, the best magnetic flux density _B8 in the sheet width direction is 1.920 T or more.

Example 1
A steel slab containing 0.040 mass% C, 3.3 mass% Si, and 0.05 mass% Mn, the other components shown in Table 2, and the balance consisting of Fe and unavoidable impurities, was melted. The steel slab was heated to a temperature of 1200 ° C. and then hot rolled to obtain a hot-rolled sheet having a thickness of 2.5 mm. The hot-rolled sheet was then subjected to hot-rolled sheet annealing at 1000 ° C. x 60 s. Thereafter, the steel sheet after the hot-rolled sheet annealing was rolled to the intermediate sheet thickness shown in Table 2 by cold rolling using a 4-stand tandem rolling mill with a work roll diameter of 400 mm in the final stand, with the steel sheet temperature between the stands from the third pass onwards being 150 ° C. or higher, and the cold rolling before winding into a coil at a temperature of 110 ° C. was performed. Some of the coils after the above-mentioned first stage cold rolling were then subjected to intermediate annealing at 1000° C. for 30 seconds.
Next, the cold-rolled sheet having the intermediate thickness was rolled into a cold-rolled sheet having a final thickness of 0.27 mm by the number of passes shown in Table 2 using a single stand rolling mill having a work roll diameter of 300 mm after 30 minutes or more had elapsed since the end of the previous stage of cold rolling.
Next, the cold-rolled sheet having the final thickness was rapidly heated at a heating rate of 80°C/s between 400°C and 700°C, and then subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 40s in a wet hydrogen atmosphere. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet after the primary recrystallization annealing and dried, and then secondary recrystallization was performed, followed by finish annealing for purification treatment.
Next, a coating liquid for an insulating coating containing phosphate-chromate-colloidal silica in a weight ratio of 3:1:2 was applied to the steel sheet surface after the above-mentioned finish annealing, and the steel sheet was subjected to flattening annealing at 850°C for 30 s, which served both to bake the insulating coating and to correct the shape, to obtain a product sheet.

Test pieces 30 mm wide x 280 mm long, with the rolling direction as the length direction, were cut out from multiple locations in the longitudinal direction of the product sheet coil thus obtained, with a total weight of 500 g or more. The magnetic flux density _B8 of the test pieces was measured by the Epstein test specified in JIS C2550, and the best value was evaluated.

The results of the above measurements are shown in Table 2. It can be seen from Table 2 that all of the steel sheets satisfying the conditions of this embodiment have excellent magnetic properties, with the best magnetic flux density _B8 value being 1.920 T or more.

Example 2
A steel slab containing 0.05 mass% C, 3.4 mass% Si, 0.08 mass% Mn, 0.015 mass% Al, 0.006 mass% N, 0.003 mass% S, and 0.012 mass% Se, with the balance being Fe and unavoidable impurities, was melted and produced. The steel slab was heated to a temperature of 1400°C and hot-rolled to form a hot-rolled sheet having a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1080°C for 60s, and then cold-rolled to an intermediate sheet thickness of 0.7 mm using a four-stand tandem rolling mill with a work roll diameter of 400 mm in the final stand, with the steel sheet temperature between stands from the third pass onwards being 150°C or higher. A preliminary cold rolling step was then performed at a temperature of 100°C before winding into a coil.
Next, after 40 min or more had elapsed since the previous stage of cold rolling, the cold-rolled sheet having the intermediate thickness was rolled in two passes using different single-stand rolling mills having six different work roll diameters of 80 mm, 160 mm, 250 mm, 330 mm, 400 mm, and 550 mm to form a cold-rolled sheet having a final thickness of 0.25 mm.
Next, the cold-rolled sheet having the above final thickness was subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 120s in a wet hydrogen atmosphere. After that, an annealing separator containing MgO as a main agent and 5 mass% _TiO2 was applied to the surface of the steel sheet after the primary recrystallization annealing, and then the steel sheet was subjected to secondary recrystallization and final annealing in which the steel sheet was held at 1200°C for 10 hours for purification.
Next, a coating liquid for an insulating coating containing phosphate-chromate-colloidal silica in a weight ratio of 3:1:2 was applied to the steel sheet surface after the finish annealing, and the steel sheet was subjected to flattening annealing at 850°C for 30 s, which served both to bake the insulating coating and to correct the shape, to obtain a product sheet.

A sample material was taken from the longitudinal center of the product plate coil thus obtained, and 10 test pieces, each 30 mm wide x 280 mm long, with the rolling direction as the longitudinal direction, were taken from the sample material in the plate width direction. The magnetic flux density _B8 (magnetic flux density when a magnetic field of 800 A/m was applied) and iron loss W17 _/50 (iron loss per kg of steel plate when magnetized to a magnetic flux density of 1.7 T in an AC magnetic field with an excitation frequency of 50 Hz) were measured using a single plate magnetic measuring device, and the average value and standard deviation σ were calculated.

Figure 2 shows the effect of the work roll diameter ratio (WR diameter ratio) of the front and rear cold rolling mills on the average magnetic flux density and iron loss values and their variation (3σ) across the sheet width for the above measurement results. It can be seen from this figure that all steel sheets whose cold rolling WR diameter ratios satisfy the conditions of the present invention have good magnetic flux density and iron loss properties and relatively small variations.

Example 3
In Example 1, the steel sheet (intermediate plate thickness 0.6 mm) produced from Steel 1 after the first stage of cold rolling was rolled in a single stand rolling mill with a work roll diameter of 330 mm and a single stand rolling mill with a work roll diameter of 250 mm, one pass each (total of two passes), in the second stage of cold rolling. Thereafter, a product coil of grain-oriented electrical steel sheet was produced under the same conditions as in Example 1. A sample material was taken from the center of the length direction of the product plate coil thus obtained, and 10 test pieces with a width of 30 mm and a length of 280 mm, with the rolling direction as the length direction, were taken from the sample material in the plate width direction, and the magnetic flux density _B8 was measured using a single plate magnetic measuring device. As a result, all magnetic flux densities _B8 in the plate width direction were within the range of 1.922±0.002 T. From these results, it was confirmed that, as long as the ratio of the diameter of the WR used in each pass of the latter cold rolling to the diameter of the WR in the preceding stage satisfies the conditions of this embodiment, good magnetic properties can be obtained even if the latter cold rolling is performed using multiple single-stand rolling mills with different work roll diameters.

The technology of this invention can also be applied to other steel products that use carbon in the steel to control the texture.

Claims (7)

A method for cold rolling grain-oriented electrical steel sheet, comprising cold rolling at a total reduction rate of 80% or more to produce a cold-rolled sheet having a final thickness, comprising the steps of:
The cold rolling is divided into a front-stage cold rolling and a rear-stage cold rolling,
Optionally, an intermediate annealing is performed between the first cold rolling and the second cold rolling.
The steel sheet after the first stage of cold rolling is wound into a coil at a temperature of 80° C. or higher, and after at least 30 minutes, the second stage of cold rolling is performed;
When the intermediate annealing is performed, the annealing temperature is set to 700° C. or less,
In the first stage cold rolling, a tandem rolling mill is used to roll the steel sheet to an intermediate thickness with a reduction ratio relative to the total reduction in the cold rolling being in the range of 0.50 to 0.95;
In the latter stage cold rolling, a single stand rolling mill is used to roll the sheet from the intermediate thickness to the final thickness in one to three passes,
A method for cold rolling a grain-oriented electrical steel sheet, characterized in that a ratio of a work roll diameter of the single stand rolling mill to a work roll diameter of a final stand of the tandem rolling mill is set in a range of 0.30 to 0.95, thereby making the best value of magnetic flux density _B8 in the sheet width direction of the product sheet 1.920 T or more. The method for cold rolling grain-oriented electrical steel sheet according to claim 1, characterized in that the latter stage of cold rolling is performed using one single-stand rolling mill with different work roll diameters, or using multiple single-stand rolling mills with different work roll diameters. Contains C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, and Mn: 0.01 to 0.5 mass%,
Further, as inhibitor-forming components, the composition contains at least one component selected from Group A: sol. Al: 0.010-0.05 mass% and N: 0.004-0.012 mass%, Group B: at least one selected from S and Se: 0.01-0.05 mass% in total, and Group C: sol. Al: 0.010-0.05 mass%, N: 0.004-0.012 mass%, and at least one selected from S and Se: 0.01-0.05 mass% in total,
Optionally, at least one component selected from Ni: 0.005 to 1.50 mass%, Cr: 0.005 to 0.3 mass%, Cu: 0.005 to 0.3 mass%, Sn: 0.005 to 0.3 mass%, Sb: 0.005 to 0.3 mass%, Mo: 0.005 to 0.3 mass%, Te: 0.005 to 0.3 mass%, Bi: 0.005 to 0.3 mass%, and P: 0.005 to 0.3 mass% is contained;
A method for producing a grain-oriented electrical steel sheet, comprising the steps of hot rolling a steel slab having a composition with the balance being Fe and unavoidable impurities to obtain a hot-rolled sheet, cold rolling the hot-rolled sheet with a total rolling reduction of 80% or more to obtain a cold-rolled sheet having a final sheet thickness, subjecting the cold-rolled sheet to primary recrystallization annealing also serving as decarburization annealing, applying an annealing separator to a surface of the steel sheet, and then subjecting the cold-rolled sheet to secondary recrystallization and then finish annealing for purification,
3. A method for producing a grain-oriented electrical steel sheet, comprising: applying the cold rolling method according to claim 1 or 2 to the cold rolling to set the best magnetic flux density _B8 in the sheet width direction to 1.920 T or more. Contains C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, and Mn: 0.01 to 0.5 mass%,
Further, it contains sol. Al: less than 0.010 mass%, S: 0.0050 mass% or less, Se: 0.0050 mass% or less, and N: 0.0050 mass% or less,
Optionally, at least one component selected from Ni: 0.005 to 1.50 mass%, Cr: 0.005 to 0.3 mass%, Cu: 0.005 to 0.3 mass%, Sn: 0.005 to 0.3 mass%, Sb: 0.005 to 0.3 mass%, Mo: 0.005 to 0.3 mass%, Te: 0.005 to 0.3 mass%, Bi: 0.005 to 0.3 mass%, and P: 0.005 to 0.3 mass% is contained;
A method for producing a grain-oriented electrical steel sheet, comprising the steps of hot rolling a steel slab having a composition with the balance being Fe and unavoidable impurities to obtain a hot-rolled sheet, cold rolling the hot-rolled sheet with a total rolling reduction of 80% or more to obtain a cold-rolled sheet having a final sheet thickness, subjecting the cold-rolled sheet to primary recrystallization annealing also serving as decarburization annealing, applying an annealing separator to a surface of the steel sheet, and then subjecting the cold-rolled sheet to secondary recrystallization and finish annealing for purification,
3. A method for producing a grain-oriented electrical steel sheet, comprising: applying the cold rolling method according to claim 1 or 2 to the cold rolling to set the best magnetic flux density _B8 in the sheet width direction to 1.920 T or more. A cold rolling equipment train comprising a front-stage cold rolling mill and a rear-stage cold rolling mill used in the cold rolling method according to claim 1,
The front-stage cold rolling mill is a tandem rolling mill, and the rear-stage cold rolling mill is a single-stand rolling mill;
a ratio of a work roll diameter of the rear cold rolling mill to a work roll diameter of a final stand of the tandem rolling mill is within a range of 0.30 to 0.95. A cold rolling equipment train including a front-stage cold rolling mill and a rear-stage cold rolling mill used in the cold rolling method according to claim 2,
The front-stage cold rolling mill is a tandem rolling mill, and the rear-stage cold rolling mill is a single-stand rolling mill;
6. The cold rolling facility train according to claim 5, wherein the downstream cold rolling mill comprises one single-stand rolling mill capable of changing the work roll diameter, or a plurality of single-stand rolling mills having different work roll diameters. (1) The tandem rolling mill has a function of controlling the coil winding temperature by adjusting at least one of the temperature and flow rate of the coolant sprayed onto at least one of the steel sheet surface and the work roll surface at the exit side of the final stand, and (2) a heat retention device is provided between the tandem rolling mill and a rolling mill in a subsequent stage, for maintaining the temperature of the steel sheet after cold rolling in the previous stage at 50°C or higher.
7. The cold rolling facility train according to claim 5, characterized in that it has at least one of the following configurations: