METHOD OF PRODUCING DIRECTIONAL SILICON STEEL SHEET HAVING EXCELLENT MAGNETIC CHARACTERISTICS AND CONTINUOUS INTERMEDIATE ANNEALING EQUIPMENT

Abstract: This invention effectively smoothens the steel sheet surface after final cold rolling by removing oxidation scale produced on the steel sheet surface in the production steps of grain oriented silicon steel sheets, particularly after intermediate annealing and at a stage before the final cold rolling, or further forming grooves onto the steel sheet surface along the rolling direction, and hence can utilize high speed tandem rolling for the final cold rolling, whereby the production of grain oriented silicon steel sheets having excellent magnetic properties is realized in a high productivity.

Description: This invention relates to a method of producing grain oriented silicon steel sheets having improved magnetic properties and a continuous intermediate annealing equipment therefor, and more particularly it is concerned with advantageously enhancing the iron loss properties by improving the surface state of the steel sheets before the final cold rolling step. Grain oriented silicon steel sheets are mainly used as cores for transformers and other electrical machinery, and are required to have excellent magnetic properties, particularly magnetization property and iron loss property. The magnetic properties of grain oriented silicon steel sheets are strongly affected by not only the sheet quality but also the surface properties. For example, the smaller the surface roughness, the better the magnetic properties as disclosed in Japanese Patent laid open No. 59-38326. Therefore, a rolling treatment rendering the surface roughness of the steel sheet into a center-line average roughness Ra of not more than 0.4 µm, which is referred to as so-called bright finishing, is adopted at the cold rolling stage. As the surface roughness or specific surface area increases, the surface enriching amount of MnS or MnSe acting as an agent inhibiting normal growth of the crystal gain (inhibitor) increases thus reducing the inhibitor effect inside the steel sheet during the secondary recrystallization annealing step. Consequently, the growth of recrystallized gains is insufficient. Further, when the surface roughness of the finally cold rolled steel sheet becomes rough, not only is the unevenness of the surface of the product sheet large, but also the insulating film formed on the sheet surface is thick and uneven so that, when the product sheet is magnetized, the movement of the magnetic domains is obstructed. Furthermore, when the steel sheet contains 2.5 ∼ 4.0 wt% (hereinafter shown by % simply) of Si as in grain oriented silicon steel sheets, it is very brittle and is more likely to break as compared with ordinary steel. Also the deformation resistance is very high, so that the cold rolling has generally to be carried out at a low speed of not more than about 700 mpm using a reverse mill such as a Sendzimir mill having a small roll diameter (roll diameter: about 80 mm). Therefore, the rolling efficiency is low and the productivity is poor. The surface roughening caused by oxidation scale will now be described. The hot rolled sheet used as a base sheet for silicon steel sheets is subjected to two or more-times cold rolling with an intermediate annealing to obtain a sheet having a thickness appropriate to the final product. During the intermediate annealing, oxidation scale is produced at a thickness of about 0.2 ∼ 3 µm on the surface of the steel sheet. This oxidation scale consists mainly of silicon dioxide (Si0₂) and is very hard. It acts as an abrasive on the rolling roll and causes the roll surface to become worn. The worn roll causes the surface of the steel sheet to become rough. The applicants have previously proposed a method wherein the silicon steel sheet having a scale layer adhered to its surface after the intermediate annealing, is rolled in a cold tandem rolling machine line while descaling the sheet by means of a descaling device particularly arranged between a first stand and a second stand in Japanese Patent laid open No. 63-119925 in order to reduce the wear of the rolling roll. In the above method, however, there still remain the following problems: 1) The surface of the rolling roll in the first stand is roughened by the scale which shortens the life of the roll, so that the roll has to be frequently exchanged. 2) The broken scale adheres to the surface of the roll and is transferred to the surface of the steel sheet to roughen the surface. 3) When the reduction ratio of the first stand is not less than about 30%, the steel sheet surface after the rolling is roughened by the scale pushed into the steel sheet. 4) The descaling device has to be large because it needs to be synchronised with the speed of the high speed tandem mill. Surface roughening caused by the rolling lubricant will now be described. Fig. 2 of the accompanying drawings is a side view diagrammatically showing a section of a steel sheet being rolled by a rolling roll. For simplification of explanation, it is assumed that the surfaces of rolling roll 2 and steel sheet 1 are smooth before the rolling. During the rolling, a rolling oil is normally used for mitigating the rolling load, but this figure illustrates the case where no rolling oil is used. In this figure, the contact between the rolling roll 2 and the steel sheet 1 starts from a point A. At this point A, the steel sheet 1 begins to undergo plastic deformation. The steel sheet 1 and the rolling roll 2 metallically contact each other because there is no rolling oil. Therefore, the rolling load considerably increases, and consequently rolling may be impossible. On the contrary, Fig. 3 of the accompanying drawings shows diagrammatically a section of the steel sheet 1 being rolled by the rolling roll 2 in the case where rolling oil is used. When the viscosity of the rolling oil is large and particularly when the diameter of the rolling roll or the rolling speed in the tandem mill is large, the pressure of the rolling oil 3 produced in the wedge-shaped region of the bite of the rolling roll 2 reaches the yield stress of the steel sheet 1 at a point B on the way to the point A (the contact point between the rolling roll 2 and the steel sheet shown in Fig. 2). Therefore, the steel sheet 1 is subjected to plastic deformation, but this is a free deformation in the rolling oil 3 so that unevenness is caused in the sheet. Furthermore, the rolling oil 3 enters the rolled region, and the deformation increases to increase the unevenness. When the unevenness becomes larger than the thickness of the oil film, the oil film is broken resulting in contact between the roll and the steel sheet at point C. The convex portions of the steel sheet 1 contacted by the rolling roll 2 are flattened by the rolling roll 2, but the concave portions are not flattened because the concave portions contain rolling oil 3. Hence the concave portions remain as they are thus making the surface of the steel sheet rough. An example of this uneven state is shown in Fig. 4 of the accompanying drawings. This shows a so-called three-dimensional profile obtained by measuring the height direction (Z) of the unevenness while moving the probe of a surface roughness meter in the lengthwise direction (X) on the surface of the steel sheet and repeating the measurement while moving the probe in the widthwise direction (Y) by a given amount. The concave portions of the steel sheet can be made smaller by reducing the viscosity of the rolling oil but the smoothness obtained by bright finishing cannot be achieved. It is an object of the present invention to advantageously solve the aforementioned problems and to provide a method of advantageously producing grain oriented silicon steel sheets which can be subjected to high speed tandem rolling without causing degradation of the surface properties to attain an improvement in productivity and a reduction in cost and also to provide a continuous intermediate annealing equipment suitable for direct use in the above method. The inventors have made various studies in order to solve the above problems and found that even when the cold rolling is carried out at a high speed in a tandem mill, if the steel sheet is subjected to a treatment to improve the surface state of the sheet (i.e. a descaling and groove forming treatment after the intermediate annealing followed by the final cold rolling) the surface property of the steel sheet after the rolling can be raised to that of bright sheet obtained by bright finishing. Accordingly, one aspect of the present invention provides a method of producing a grain oriented silicon steel sheet having improved magnetic properties by subjecting a hot rolled sheet of silicon steel containing from 0.02 to 0.1% of carbon, from 2.5 to 4.0% of silicon and an inhibitor for normal crystal grain growth to two or more cold rollings with an intermediate annealing therebetween to achieve a final sheet thickness and then subjecting the sheet to decarburization annealing and finish annealing wherein the final cold rolling is effected by tandem rolling and a descaling step is carried out characterised in that the descaling step is carried out after said intermediate annealing and before the final cold rolling and in that grooves are formed along the rolling direction of the sheet after said intermediate annealing and before the final cold rolling. According to another aspect of the present invention there is provided a continuous intermediate annealing equipment for the production of grain oriented silicon steel sheets which comprises a continuous annealing furnace for effecting intermediate annealing during cold rolling of a silicon steel sheet and a sweeping device for forming grooves in the surface of the sheet along the rolling direction, the device being located at the delivery side of the continuous annealing furnace. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 shows a three-dimensional profile of a cold rolled silicon steel sheet subjected to a final cold tandem rolling after a surface treatment according to the invention; Figs. 2 and 3 are side views schematically showing a steel sheet being rolled by a rolling roll; Fig. 4 shows a three-dimensional profile of a cold rolled silicon steel sheet after being cold rolled according to a conventional method; Fig. 5 is a schematic view illustrating the flow of rolling oil when a steel sheet provided at its surface with fine grooves in accordance with the invention is subjected to rolling; and Fig. 6 is a schematic view of a preferred embodiment of continuous intermediate annealing equipment according to the invention. First, there will be described the reason why the chemical composition of the starting steel material used according to the invention is limited to the above range. C: 0.02 ∼ 0.1% C is an element useful not only for effectively contributing to the uniformization of hot rolled and cold rolled textures but also for enhancing the alignment of the Goss orientation component in the recrystallized texture during repeated cold rolling and annealing to obtain the final sheet thickness. When the amount is less than 0.02%, the addition effect is poor, while when it exceeds 0.1%, the temperature needed to dissolve the inhibitor, such as S, Se or the like, during the slab heating has to be increased. Thus there is a reduction of the inhibiting force of the inhibitor due to poor solution and also the decarburization during the decarburization annealing becomes difficult. Therefore, the carbon amount is limited to a range of from 0.02 to 0.1%. Si: 2.5 ∼ 4.0% Si effectively enhances the electric resistance to reduce the iron loss. When the amount is less than 2.5%, a sufficient reduction of iron loss cannot be expected and also a part or whole of the steel sheet undergoes γ transformation during the high temperature annealing resulting in disorder of crystal orientation. When it exceeds 4.0%, the cold workability is considerably degraded. Therefore, the amount is limited to a range of from 2.5 to 4.0%. As inhibitor use may be made of the so-called MnS system composed of Mn, S, Se, Sb and the like or the Aℓ system. For example, when using the MnS system, the following composition is preferable. Mn: 0.03 ∼ 0.15%, one or two of S, Se and Sb: 0.008 ∼ 0.080% Any of Mn, S, Se and Sb are useful as inhibitor forming elements. However, when these elements are outside the above range, sufficient inhibiting of normal grain growth is not usually achieved, so that each of these elements is preferred to be added in an amount according to the above range. Further, Mo may be added in an amount of about 0.005 ∼ 0.02% for preventing slab breakage during the hot rolling, if necessary. Molten steel adjusted to the above preferable composition is rendered into a slab by means of an ingot making-blooming process or a continuous casting process and is then subjected to a hot rolling. Then, the hot rolled sheet is subjected to 2 or more cold rolling steps with intermediate annealing to obtain a cold rolled sheet of final thickness. In accordance with the invention, smoothening of the steel sheet surface is attained by sweeping the surface of the steel sheet to form grooves in the rolling direction after the intermediate annealing and before the final cold rolling step and thereafter carrying out the final cold rolling. That is, the steel sheet is subjected to a sweeping treatment such as grinding, polishing or the like to remove oxidation scale produced on the surface of the steel sheet during the intermediate annealing and form shallow grooves having a depth of about 1 ∼ 50 µm in the rolling direction of the steel sheet. Thereafter, the steel sheet is subjected to final cold rolling whereby a smooth surface equal to that of bright sheet is obtained on the steel sheet as shown in Fig. 1. The step of subjecting the steel sheet to a sweeping treatment such as grinding, polishing or the like in accordance with the invention is believed to have the following effects:- 1) the oxidation scale is effectively removed from the steel sheet surface so that the concave portions resulting from the scale are eliminated. 2) strain is introduced into the crystal grains beneath the surface so that unevenness due to the plastic deformation during the rolling is reduced. 3) the rolling oil escapes from the resulting fine grooves so that the pressure of the rolling oil reduces. The fine grooves do not have a bad influence upon the surface after rolling to final thickness. The term "sweeping the surface" as used herein means that the steel sheet surface is ground or polished in the rolling direction by means of a sweeping device in the form of, for example, a grinding or polishing tool such as a polishing belt using a polishing paper, a cylindrical polishing sleeve, a polishing non-woven fabric, a brush containing abrasive grains therein, an elastic grinding roll, or a brush formed of metal wires. The method used may be selected by taking equipment cost, equipment size, running cost, treating quantity and the like into consideration. The above sweeping treatment may be carried out by arranging the sweeping device at the entrance side of the rolling machine. However, it is more advantageous to locate the device at the delivery side of the intermediate annealing furnace for continuously treating the steel sheet. The reason for this is that, when the sweeping device is arranged at the entrance side of the rolling machine, it needs to be synchronised with the high rolling speed, so that not only does the device have to be made large but also control is difficult. On the other hand, when the device is arranged at the delivery side of the intermediate annealing furnace, the sheet passing speed is fairly low so the device can be made smaller and control is easy. In Fig. 6 there is schematically shown a preferred embodiment of the continuous intermediate annealing equipment according to the invention. Numerals 10a and 10b are entrance side and delivery side loopers; 11a, 11b and 11c are bridle rolls; and 12 is a continuous intermediate annealing furnace which is comprised of a heating zone 12-a, a soaking zone 12-b and a cooling zone 12-c. Numeral 13 denotes a device for sweeping the steel sheet surface. The steel sheet surface after the intermediate annealing is swept by the steel sheet surface sweeping device which is arranged at the delivery side of the continuous annealing furnace 12. When the swept steel sheet is subjected to a final cold rolling, it is more advantageous that the roughness of the rolling roll in at least the final pass is not more than 0.30 µm Ra and the viscosity at 50°C of the rolling oil is from 2 to 15 cSt in order to obtain a surface after rolling which has a roughness of not more than 0.4 µm Ra. That is, during oil lubrication rolling, the rolling oil is usually supplied to the sheet or to the roll as an emulsion obtained by emulsifying and suspending oil particles in water so that the emulsion is spread over the sheet surface and drawn into the wedge-like portion defined by the sheet and the roll at the entrance side of the roll bite through a hydrodynamic effect (the so-called wedge effect) so as to enter into the roll bite and form concave portions on the steel sheet. If the roughness of the rolling roll exceeds 0.30 µm Ra, there is a risk that the roughness of the sheet surface will become larger than 0.4 µm due to the unevenness resulting from the transcription of the roughness of the rolling roll and the concave portions resulted from the rolling oil. If the viscosity of the rolling oil at 50°C exceeds 15 cSt, the roughness of the sheet surface is apt to become larger than 0.4 µm when high speed rolling is carried out in a tandem rolling machine having a rolling roll diameter of about 600 mm. The following Examples illustrate the invention. Example 1 A hot rolled sheet of silicon steel containing C: 0.045%, Si: 3.35%, Mn: 0.065%, Se: 0.017% and Sb: 0.027% and having a thickness of 2.5 mm was subjected to normalizing annealing at 1000°C for 30 seconds, pickled, cold rolled to 0.64 mm, and subjected to an intermediate annealing at 980°C for 90 seconds to prepare two samples A and C. Thereafter, sample A was ground at its surface in a direction parallel to the rolling direction with a polishing belt of grain size #100. Sample C was not subject to grinding. Each of these samples was finished to a final sheet thickness of 0.23 mm in a 3-stand tandem mill provided with a rolling roll having a roll diameter of 350 mm and a roll surface roughness of 0.1 µm Ra at a final stand rolling speed of 1000 mpm and using a rolling oil having a viscosity of 8 cSt/50°C and a concentration of 3%. After the surface average roughness (Ra) of the portion rolled at a rolling speed of 1000 mpm was measured, each sample was subjected to decarburization annealing, coated with an annealing separator, and then subjected to a finish annealing at 860°C for 60 hours and at 1200°C for 5 hours. The iron loss (W 17/50 ) and magnetic flux density (B₁₀) of the thus obtained grain oriented silicon steel sheets were measured to obtain the results shown in Table 1. Classification Sample Average surface roughness Ra (µm) W 17/50 (W/kg) B₁₀ (T) Invention Example A 0.20 0.83 1.923 Comparative Example C 0.55 0.90 1.900 As can be seen from Table 1, sample A obtained according to the invention had excellent surface properties and magnetic properties as compared with comparative Example C. Example 2 A hot rolled sheet of silicon steel containing C: 0.038%, Si: 3.05%, Mn: 0.070%, Se: 0.020% and having a thickness of 2.7 mm was pickled, cold rolled to 0.74 mm, and subjected to an intermediate annealing at 970°C for 40 seconds to prepare two samples D and F. Thereafter, as described in Example 1, the sample D was polished at its surface with a brush containing abrasive grains of grain size #240 in a direction parallel to the rolling direction. Further, the intermediately annealed sample F was a comparative example and was not subjected to polishing. Each sample was finished to a final sheet thickness of 0.27 mm in a 3-stand tandem mill as in Example 1 at a final stand rolling speed of 1700 mpm and using a rolling oil having a viscosity of 15 cSt/50°C and a concentration of 3%. After the surface average roughness (Ra) of the portion rolled at a rolling speed of 1700 mpm was measured, each sample was subjected to decarburization annealing, coated with an annealing separator, and then subjected to a finish annealing at 860°C for 60 hours and at 1200°C for 5 hours. The iron loss (W 17/50 ) and magnetic flux density (B₁₀) of the thus obtained grain oriented silicon steel sheets were measured to obtain the results shown in Table 2. Classification Sample Average surface roughness Ra (µm) W 17/50 (W/kg) B₁₀ (T) Invention Example D 0.25 1.16 1.883 Comparative Example F 0.60 1.21 1.862 As can be seen from Table 2, sample D according to the invention had excellent surface properties and magnetic properties as compared with the comparative Example. Example 3 A hot rolled sheet of silicon steel containing C: 0.050%, Si: 3.10%, S: 0.027% and acid soluble Aℓ: 0.030% was subjected to normalizing annealing at 1170°C for 90 seconds, cold rolled to a sheet thickness of 0.3 mm, and then subjected to an intermediate annealing at 980°C for 60 seconds to prepare two samples G and I. Thereafter, as described in Example 2, sample G was polished with a brush containing abrasive grains of grain size #240 in a direction parallel to the rolling direction. The intermediately annealed sample I was a comparative example and was not subjected to polishing. Each sample was finished to a final sheet thickness of 0.27 mm in the same 3-stand tandem mill as in Example 1 at a final stand rolling speed of 1700 mpm and using a rolling oil having a viscosity of 15 cSt/50°C and a concentration of 3%. After the surface average roughness (Ra) of the portion rolled at the rolling speed of 1700 mpm was measured, each sample was subjected to decarburization annealing, coated with an annealing separator, and then subjected to a finish annealing at 860°C for 60 hours and at 1200°C for 5 hours. The iron loss (W 17/50 ) and magnetic flux density (B₁₀) of the thus obtained grain oriented silicon steel sheets were measured to obtain the results shown in Table 3. Classification Sample Average surface roughness Ra (µm) W 17/50 (W/kg) B₁₀ (T) Invention Example G 0.24 0.97 1.942 Comparative Example I 0.60 1.05 1.920 As can be seen from Table 3, sample G according to the invention had excellent surface properties and magnetic properties as compared with the comparative Example I. Example 4 A hot rolled sheet of silicon steel containing C: 0.045%, Si: 3.35%, Mn: 0.065%, Se: 0.017% and Sb: 0.027% and having a thickness of 2.5 mm was subjected to normalizing annealing at 1000°C for 30 seconds, pickled, cold rolled to 0.64 mm, and then subjected to an intermediate annealing at 900°C for 90 seconds to prepare three samples J, K and O. Thereafter, in sample J the scale was broken by a tension leveller and swept out by an elastic grinding roll of grain size #240. Sample K was chemically descaled by pickling in hydrochloric acid and then subjected to a sweeping with a similar elastic grinding roll. Sample O was left untreated after the intermediate annealing. Then, each of the samples J, K and O was finished to a final sheet thickness of 0.23 mm in a final stand rolling mill having a roll diameter of 600 mm, and a roll roughness of 0.1 µm Ra at a final stand rolling speed of 1000 mpm and a reduction ratio of 20% using a rolling oil having a viscosity of 2 cSt/50°C and a concentration of 3%. After the surface average roughness (Ra) of the portion rolled at the rolling speed of 1000 mpm was measured, each sample was subjected to decarburization annealing, coated with an annealing separator, and then subjected to a finish annealing at 860°C for 60 hours and at 1200°C for 5 hours. The iron loss (W 17/50 ) and magnetic flux density (B₁₀) of the thus obtained grain oriented silicon steel sheets were measured to obtain the results shown in Table 4. Classification Sample Average surface roughness Ra (µm) W 17/50 (W/kg) B₁₀ (T) Invention Example J 0.15 0.82 1.925 K 0.15 0.82 1.925 Comparative Example O 0.55 0.90 1.900 According to the invention, even when the grain oriented silicon steel sheets are rolled at a high speed in a tandem mill having a large roll diameter, a good surface state having a surface average roughness of not more than 0.4 µm can be maintained, and hence grain oriented silicon steel sheets having excellent magnetic properties can be obtained with a high productivity.

Claims: A method of producing a grain oriented silicon steel sheet having improved magnetic properties by subjecting a hot rolled sheet of silicon steel containing from 0.02 to 0.1% of carbon, from 2.5 to 4.0% of silicon and an inhibitor for normal crystal grain growth to two or more cold rollings with an intermediate annealing therebetween to achieve a final sheet thickness and then subjecting the sheet to decarburization annealing and finish annealing wherein the final cold rolling is effected by tandem rolling and a descaling step is carried out characterised in that the descaling step is carried out after said intermediate annealing and before the final cold rolling and in that grooves are formed along the rolling direction of the sheet after said intermediate annealing and before the final cold rolling. A method according to claim 1, wherein descaling and groove forming are carried out by sweeping the sheet surface. A method according to claim 2, wherein the descaling and groove forming are carried out by means of a grinding or polishing tool. A method according to claim 1, wherein said sheet is first descaled and the sheet is subsequently subjected to sweeping to form the grooves. A method according to any one of claims 1 to 4, wherein at least the final pass in the final cold rolling is carried out using a rolling roll having a surface roughness (Ra) of not more than 0.30 µm and a rolling oil having a viscosity at 50°C of 2 to 15 cSt. A continuous intermediate annealing equipment for the production of grain oriented silicon steel sheets which comprises a continuous annealing furnace for effecting intermediate annealing during cold rolling of a silicon steel sheet and a sweeping device for forming grooves in the surface of the sheet along the rolling direction, the device being located at the delivery side of the continuous annealing furnace.

Inventors: KITAMURA K KAWASAKI STEEL CORP, NAITO T KAWASAKI STEEL CORP MI, SUGANUMA N KAWASAKI STEEL CORP, KITAMURA, K. KAWASAKI STEEL CORP. TECH. RESEARCH, NAITO, T. KAWASAKI STEEL CORP. MIZUSHIMA WORKS, SUGANUMA, N. KAWASAKI STEEL CORP. MIZUSHIMA WORKS
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: C21D8
IPC codes: C21D8