EP0266422B1

EP0266422B1 - Process for producing low core loss, thin, unidirectional silicon steel plate having excellent surface properties

Info

Publication number: EP0266422B1
Application number: EP86902022A
Authority: EP
Inventors: Yukio Kawasaki St. Corp. Techn. Res. Di. Inokuti; Yoh Kawasaki St. Corp. Techn. Res. Div. Ito
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1986-03-25
Filing date: 1986-03-25
Publication date: 1990-11-28
Anticipated expiration: 2006-03-25
Also published as: EP0266422A4; EP0266422A1; DE3675945D1; WO1987005945A1; EP0266422B2

Abstract

A process for consistently producing a low core loss, thin, unidirectional silicon steel plate. The process makes it possible to produce a low core loss, unidirectional silicon steel plate having a thickness of 0.1 to 0.25 mm for use in producing transformers with advantageously avoiding deterioration of surface properties by considering chemical ingredients of steel, optimizing rolling conditions, particularly cold-rolling conditions, and forming allomeric microzones on the surface of a steel plate. The steel plate does not undergo deterioration by stress-relieving annealing.

Description

In connection with the improvement of surface properties in low iron loss grain oriented silicon steel sheets, particularly thin sheets as well as the improvement of magnetic flux density by the control of secondary recrystallized grain, the technical content disclosed throughout the specification proposes results from research and development capable of producing the above silicon steel sheets in stable steps.
The grain oriented silicon steel sheets can be utilized as a core for transformer and other electrical machinery and equipment, and are required to have a high magnetic flux density (represented by 8₁₀ value) and a low iron loss (represented by W_17/50 value).
Up to the present, there have been many attempts to achieve the above requirement, and grain oriented silicon steel sheets having a low iron loss with a magnetic flux density, 8₁₀ value of not less than 1.89T and an iron loss, W_17/50 value of not more than 1.05 W/kg are manufactured today.
However, the production of grain oriented silicon steel sheets having a lower iron loss is particularly urgent in view of the energy crisis. In this connection, a system of granting a bonus on super-low iron loss silicon steel sheets (Loss evaluation system) is widely spread in European and America.
Recently, the following methods have been proposed as methods of producing grain oriented silicon steel sheets having a considerably reduced iron loss value.
That is, as disclosed in each of Japanese Patent Application Publication No. 57-2,252, Japanese Patent Application Publication No. 58-53,419, Japanese Patent Application Publication No. 58-5,968, Japanese Patent Application Publication No. 58-26,405, Japanese Patent Application Publication No. 58-26,406, Japanese Patent Application Publication No. 58-26,407 and Japanese Patent Application Publication No. 58-36,051, there is a method wherein an artificial grain boundary is introduced into the surface of the grain oriented silicon steel sheet by utilizing an AIN precipitation phase as an inhibitor for . inhibiting the growth of crystal grains in an unsuitable direction on finish annealing and irradiating a laser beam onto the steel sheet surface at an interval of several mm in a direction substantially perpendicular to the rolling direction to thereby reduce the iron loss through the artificial grain boundary.
In such methods of introducing artificial grain boundary, however, regions of high transformation density are locally formed, so that there is a problem that the resulting products are stably used only at a low temperature of below about 350°C.
In the production of the grain oriented silicon steel sheet utilizing the AIN precipitation phase as mentioned above, it is necessary to conduct the heating of slab before hot rolling at a temperature higher than that of ordinary steel for the dissociation and solution of MnS coexistent with AIN as an inhibitor, but when the slab heating is carried out at such a high temperature, hot tear is caused at the slab heating or hot rolling which facilitates the occurrence of surface defects in the product. More particularly the surface properties of the product are considerably degraded when the content of Si obstructing the hot workability exceeds 3.0%.
In this respect, as disclosed in Japanese Patent laid open No. 59-85,820, the inventors have noticed that when utilizing the AIN precipitation phase, a silicon steel material having a high Si content of Si: 3.1 - 4.5% is essentially a material suitable for obtaining a high magnetic flux density, low iron loss product, and have found that the surface properties can be made good even at the high Si content by enriching the surface layer of the steel material with Mo before the hot rolling, as a means for solving the degradation of surface properties. According to this means, the surface properties of the product are greatly improved as compared with the former case, but if it is particularly intended to thin the gauge of the product to 0.23 - 0.17 mm for obtanining low iron loss, there remains a large problem in that the improving effect on surface properties is small.
Aside from this, the utilization of AIN precipitation phase is naturally dependent on a strong one-stage cold rolling process, so that if it is intended to manufacture a thinned product, the secondary recrystallized grains become very unstable, and it is difficult to grow the secondary recrystallized grains highly aligned in Goss orientation.
Lately, Japanese Patent laid open No. 59-126,722 has disclosed that in order to stably manufacture thinned products by utilizing the AIN precipitation phase at high Si content, a two-stage cold rolling process largely different from the conventional strong one-stage cold rolling process is particularly applied to a hot rolled material containing small amounts of Cu and Sn in addition to AIN.
This is effective for stably reducing the iron loss of the thinned product, but has yet many problems in that it is difficult to obtain products having excellent surface properties because high-temperature heating of the slab is usually required under a state of increasing Si and in that the cost of the product becomes considerably higher because of the small amounts of Sn and Cu which are added for stabilizing secondary recrystallized grains.
As methods of reducing the iron loss of grain oriented silicon steel sheet, there are fundamentally considered the following methods;

(_D the increasing of the Si content in silicon steel;
@ the thinning of the product gauge;
the increasing of the purity of the steel sheet;
the growing of secondary recrystallized fine grains without lowering the degree of alignment of the secondary recrystallized grain in Goss orientation in the product.

At first, it has been attempted to increase the Si content to a value higher than the usual value of 3.0% as regards method (1), or to thin the product gauge from the usual values of 0.35, 0.30 mm to 0.23, 0.20 mm as regards method ②. In any case, however, there are caused problems in that the secondary recrystallized texture becomes non-uniform and the degree of alignment in Goss orientation is lowered.
In addition, when the Si content is increased from the usual value according to method CD, hot brittleness becomes evident, and hot tear is caused in the slab heating or hot rolling to considerably degrade the surface properties of the product as previously mentioned.
On the other hand, the development of the improvement of steel purity ③ or orientation @ is considered to be marginal at the present. For example, the Goss orientation of secondary recrystallized grains in the existing products is aligned within 3° - 4° on average with respect to the rolling direction, so that it is very difficult in metallurgy to make the crystal grain small under such a highly aligned state.
Considering the recent trend of the aforementioned conventional techniques under the background of the above situations, it is an object of the invention to provide a method of stably and advantageously producing grain oriented silicon steel thin sheets having very excellent surface properties, a considerably small iron loss and a high magnetic flux density on an industrial scale.
Accordingly the present invention provides a method of producing low iron loss grain oriented silicon thin steel sheets comprising subjecting a steel slab comprising 3.1 to 4.5 wt % Si, 0.003 to 0.1 wt % Mo, 0.005 to 0.06 wt % acid soluble Al, at least one of S and Se in a total amount of not more than 0.005 to 0.1 wt % and optionally 0.005 to 0.2 wt % Sb, the remainder being Fe, incidental impurities and, optionally, incidental elements to a hot rolling to form a steel sheet, and subjecting said steel sheet to

(i) a primary cold rolling at a reduction of 10 to 60%,
(ii) an intermediate annealing including a heating stage from 500°C to 900°C and a cooling stage from 900°C to 500°C wherein the heating and cooling rates respectively are not less than 5^OCs-',
(iii) a secondary cold rolling at a reduction of 75% to 90% to form a steel sheet having a final gauge of 0.1 to 0.25 mm,
(iv) a decarburization and primary recrystallization annealing in a wet hydrogen atmosphere and
(v) a high temperature finish annealing.

In a first embodiment of the invention, the optional incidental elements include one or more of: 0.02, to 2 wt % Mn, 0.030-0.080 wt % C, one or more of Sn, Cu and B in a total amount of not more than 0.5 wt %.
In a second embodiment of the invention, either before or after the decarburization and primary recrystallization annealing the steel sheet is subjected to a treatment which causes the formation at the high temperature finish annealing, of heterogeneous microareas on the steel sheet surface.
In a third embodiment of the invention heterogeneous microareas are formed on the surface of the steel sheet after the high temperature finish annealing.
The inventors have found that when a grain oriented silicon steel thin sheet is produced by utilizing the AIN precipitation phase at a high silicon content of 3.1 - 4.5 wt%, products having excellent surface properties are obtained by adding a small amount of Mo to a steel material. Also the production of grain oriented silicon steel sheets having a low iron loss is made possible at very stable steps by the adoption of a two-stage cold rolling process including an intermediate annealing with rapid heating and rapid cooling, and as a result the above invention and embodiments thereof have been accomplished.
For a better understanding of the invention reference will be made to the following figures, in which:

Fig. 1 is a graph showing the relation of the magnetic properties of the product to the reductions at the primary cold rolling and secondary cold rolling and the state of the surface properties;
Fig. 2 is a graph showing the relation of the heating rate and the cooling rate in the intermediate annealing to the magnetic properties of the product; and
Fig. 3 is a graph showing the relation of the magnetic properties of the product to the reductions at the primary cold rolling and the secondary cold rolling and the state of the surface properties.

Firstly, the invention will be described in detail with respect to experimental examples resulting in the success of the invention.
A steel ingot (test steel I) containing C: 0.048 wt%, Si: 3.40 wt%, Mo: 0.025 wt%, acid soluble AI: 0.026 wt% and S: 0.025 wt% and a steel ingot (comparative steel I) containing C: 0.053 wt%, Si: 3.42 wt%, acid soluble AI: 0.027 wt%, S: 0.024 wt%, Sn: 0.11 wt% and Cu: 0.09 Wt% were each heated at 1,420 °C for 4 hours to perform the dissociation and solution of the inhibitor, and thereafter hot rolled each to form a hot rolled steel sheet of 2.2 mm in thickness.
Then, the hot rolled steel sheet was subjected to a primary cold rolling at a reduction of not more than 70% and further to an intermediate annealing at 1,050°C for 3 minutes. In the intermediate annealing, the heating from 500°C to 900°C was carried out by a rapid heating treatment of 10°C/s, and the cooling from 900°C to 500°C was carried out by a rapid cooling treatment of 15°C/s.
Thereafter, the steel sheet was subjected to a secondary cold rolling at a reduction of 70% - 91 % to obtain a cold rolled steel sheet having a final gauge of 0.20 mm, which was then subjected to decarburization and primary recrystallization annealing at 850°C in a wet hydrogen atmosphere.
Then, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet, which was subjected to a secondary recrystallization annealing by raising temperature between 850°C - 1,100°C at 8°C/hr and further to a high-temperature finish annealing or a purification annealing in a dry hydrogen atmosphere at 1,200°C for 10 hours.
The magnetic properties of the resulting product and the ratio of surface defect produced (the ratio of the surface defect block existing on the steel sheet surface is represented by %) are shown in Fig. 1.
As seen from plots shown by the mark. in Fig. 1, the product made from the test steel I containing Mo is good in the magnetic properties when the reduction at primary cold rolling is 10 - 60% (particularly 20 - 40%), and the ratio of the surface defect produced in the product is noticed to be not more than 2% (not more than 0.5% when the reduction at primary cold rolling is within a range of 20 - 25%).
On the contrary, in the product made from the comparative steel I of conventional composition, the 8₁₀ value and W_17/50 value are somewhat poorer than those of the test steel I as magnetic properties as seen from plots shown by the mark 0 in the same figure, and particularly the ratio of the surface defect produced in the product is as extremely high as 6 - 18%.
Then, a steel ingot (test steel II) containing C: 0.049%, Si: 3.45%, Mo: 0.02%, acid soluble Al: 0.028% and S: 0.026% was heated at 1,410°C for 5 hours to perform the dissociation and solution of the inhibitor, and then hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness.
Thereafter, the hot rolled steel sheet was subjected to a primary cold rolling at a reduction of about 40% and further to an intermediate annealing at 1,050°C for 3 minutes. In the intermediate annealing, each of the heating rate from 500°C to 900°C and the cooling rate from 900°C to 500°C was varied within a range of 1°C - 100°C.
The steel sheet after the intermediate annealing was subjected to a secondary cold rolling at a reduction of about 83% to obtain a cold rolled steel sheet having a final gauge of 0.23 mm, which was then subjected to decarburization and primary recrystallization annealing at 850°C in a wet hydrogen atmosphere, an application of an annealing separator mainly composed of MgO onto the steel sheet surface, a secondary recrystallization annealing by raising temperature from 850°C to 1,100°C at 10°C/hr, and a purification annealing in a dry hydrogen atmosphere at 1,200°Cfor 10 hours. The magnetic properties of the resulting product are shown in Fig. 2.
As seen from Fig. 2, products having considerably improved magnetic properties can be obtained when the heating rate from 500°C to 900°C at the intermediate annealing and the cooling rate from 900°C to 500°C after the intermediate annealing are not less than 5°C/s, particularly not less than 10°C/s.
The reason for the improvement of properties by such rapid heating and rapid cooling treatment in the intermediate annealing is considered to be due to the fact that the secondary recrystallized texture with {110} < 001 > orientation is preferentially grown as the inventors have previously disclosed in Japanese Patent laid open No. 59-35,625 (previously mentioned). Moreover, the production method of the grain oriented silicon steel thin sheet through the utilization of the AIN precipitation phase by the two-stage cold rolling process in the aforementioned Japanese Patent laid open No. 59-126,722, applies only the AIN micro-precipitation treatment through quenching treatment after normalized annealing in the conventional strong one-stage cold rolling process to the cooling stage of the intermediate annealing after the primary cold rolling, while according to the present invention it is newly elucidated that excellent magnetic properties are obtained only by the combination of rapid cooling at the intermediate annealing with rapid heating at the heating stage of the intermediate annealing and particularly by the addition of Mo.
Developmental details of the invention will be described below.
A continuously cast slab (test steel A) containing C: 0.046 wt%, Si: 3.36 wt%, Mo: 0.026 wt%, Sb: 0.025 wt%, acid soluble Al: 0.024 wt% and Se: 0.020 wt% and a continuously cast slab (comparative steel B) containing C: 0.049%, Si: 3.45%, acid soluble AI: 0.025 wt%, Sb: 0.023 wt% and Se: 0.022 wt% were each heated at 1,360°C for 3 hours to perform the dissociation and solution of the inhibitor, and then hot rolled each to form a hot rolled steel sheet of 2.2 mm in thickness.
Thereafter, each hot rolled steel sheet was subjected to a normalized annealing at 1,050°C for 2 minutes and quenched.
Then, each steel sheet was subjected to a primary cold rolling at a reduction of about 40% and further to an intermediate annealing at 1,000°C for 2 minutes. In the intermediate annealing, the heating from 500°C to 900°C was carried out by a rapid heating treatment of 10°C/s, and the cooling from 900°C to 500°C was carried out by a rapid cooling treatment of 12°C/s.
Thereafter, the steel sheets were subjected to a secondary cold rolling at a reduction of 85% to obtain cold rolled steel sheets having a final gauge of 0.20 mm, which were subjected to decarburization and primary recrystallization annealing at 830°C in a wet hydrogen atmosphere.
After an annealing separator mainly composed of MgO was applied to the steel sheet surface, the steel sheets were subjected to a secondary recrystallization annealing by raising temperature from 850°C at a rate of 10°C/hr, a purification annealing in a dry hydrogen atmosphere at 1,200°C for 10 hours, a baking treatment with an insulation coating and a strain relief annealing at 800°C for 3 hours.
The magnetic properties of the resulting products and the ratio of the surface defect produced therein (a ratio of surface the defect block existing in the steel sheet surface is represented by %) are shown in Table 1.
As seen from the magnetic properties and surface properties of the products shown in Table 1, the magnetic properties of the product made from the test steel A containing Mo therein are good in that the 8₁₀ value is 1.94 T and the W_17/50 value is 0.82 W/kg, and it is noted that the ratio of the surface defect produced in the product is 1.8%.
On the contrary, the magnetic properties of the product made from the comparative steel B of the conventional composition are bad in that B₁₀ is 1.93 T and W_17/50 is 0.85 W/kg as compared with those of the test steel B containing Mo therein, and particularly the ratio of the surface defect produced in the product is as extremely high as 8%.
The developmental details of the second embodiment will be described below.
A steel ingot (test steel III) containing C: 0.053%, Si: 3.43%, Mo: 0.023%, acid soluble AI: 0.028% and S: 0.027% and a steel ingot (comparative steel II) containing C: 0.056%, Si: 3.46%, acid soluble AI: 0.026%, S: 0.026%, Sn: 0.1% and Cu: 0.1% was heated at 1,430°C for 3 hours to perform the dissociation and solution of the inhibitor, and then hot rolled each to form a hot rolled steel sheet of 2.2 mm in thickness.
Thereafter, the hot rolled steel sheets were subjected to a primary cold rolling at a reduction of not more than 70% and further to an intermediate annealing at 1,100°C for 3 minutes. In the intermediate annealing, the heating from 500°C to 900°C was carried out by a rapid heating treatment at a heating rate of 13°C/s, and the cooling from 900°C to 500°C after the intermediate annealing was carried out by a rapid cooling treatment at a cooling rate of 18°C/s.
The steel sheets were then subjected to a secondary cold rolling at a reduction of 70% - 91 % to obtain cold rolled steel sheets having a final gauge of 0.20 mm. In this case, a warm rolling at 250°C was carried out in the course of the cold rolling.
After the surface of the steel sheets was degreased at a temperature of 110°C, an aqueous diluted solution of MgS0₄ (0.01 molll at 80°C) was applied at an interval of 5 mm and a width of 0.5 mm in a direction perpendicular to the rolling direction by spraying. For reference, there was also provided a sample where steel sheet surface was only degreased (reference example).
Each of these samples was subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere, and after an annealing separator mainly composed of MgO was applied to the steel sheet surfaces, the samples were further subjected to a secondary recrystallization annealing by raising temperature from 850°C to 1,100°C at 10°C/hr and a purification annealing in a dry hydrogen atmosphere at 1,200°C for 10 hours.
The magnetic properties of the resulting products and the ratio of the surface defect produced therein (the ratio of the surface defect block existing in the steel sheet surface is represented by %) are shown in Fig. 3.
As seen from Fig. 3, the test steels III containing Mo therein (mark ■, D) have good magnetic properties when the reduction at the primary cold rolling is from 10 to 60% (particularly 20 - 40%), and it is noted that the ratio of surface the defect produced in the product is not more than 3% (particularly not more than 1.0% when the reduction at the primary cold rolling is within a range of 20 - 50%). On the contrary, the properties of the comparative steels II of the conventional composition (mark A, 0), 8₁₀ value and W_17/50 value are somewhat poorer than those of Mo containing steel, and the ratio of the surface defect produced in the product is as extremely high as 6 - 20%.
When aqueous diluted solution of MgS0₄ is applied to the surface of the finally cold rolled steel sheet by spraying at an interval of 5 mm and a width of 0.5 mm in a direction perpendicularto the rolling diretion, the magnetic properties are noteably good in that the W_17/50 value is 0.72 W/kg when the reduction at the primary cold rolling is 30 - 40% (reduction at secondary cold rolling, 87 - 85%) as shown in plots of the mark t of the test steel III, and the ratio of the surface defect produced in the product is as good as not more than 1%.
On the other hand, even in the application treatment for the comparative steel II containing no Mo, the W_17/50 value of iron loss is as good as 0.75 W/kg when the reduction at the primary cold rolling is 30 ~ 40% as shown in plots of the mark A, but the ratio of the surface defect produced in the product is as high as 6 - 7%.
Thus, these experimental examples show that the production of low iron loss grain oriented silicon steel thin sheet having excellent surface properties is achieved by combining the addition of a small amount of Mo to high silicon steel material, the adoption of a two-stage cold rolling process, and the application of a solution or suspension of chemicals exemplified by the aqueous diluted solution of MgS0₄ to the surface of the finally cold rolled steel sheet.
This point has previously been proposed by the inventors as a method of producing a low iron loss grain oriented silicon steel sheet by alternately forming decarburization promotion areas or decarburization delay areas on the steel sheet surface before the decarburization and primary recrystallization annealing, in a direction substantially perpendicular to the rolling direction thereby to non- homogeneously grow secondary recrystallized grains and introduce heterogeneous microareas as partially mentioned in Japanese Patent laid open No. 60-39,124, which method is used together with the two-stage cold rolling process including the intermediate annealing of rapid heating and rapid cooling prior to the application to the finally cold rolled steel sheet surface, whereby the stable growth of secondary recrystallized grains can particularly be achieved. Furthermore, it is effective to apply the method of alternately forming the decarburization promotion areas or decarburization delay areas on the steel sheet surface after the decarburization and primary recrystallization annealing, a part of which has already been disclosed in Japanese Patent laid open No. 60-89,521.
A steel ingot (test steel C) containing C: 0.048%, Si: 3.41 %, Mo: 0.024%, acid soluble AI: 0.025%, Sb: 0.025% and S: 0.026% and a steel ingot (test steel C) containing C: 0.052%, Si: 3.38%, acid soluble Al: 0.023% and S: 0.025% were each heated at 1,420°C for 3 hours to perform the dissociation and solution of inhibitor and hot rolled each to form a hot rolled steel sheet of 2.0 mm in thickness.
Thereafter, each hot rolled steel sheet was subjected to a two-stage cold rolling (reduction at primary cold rolling: 50%, reduction at secondary cold rolling: 80%) with an intermediate annealing at 980°C for 3 minutes to obtain cold rolled steel sheets having a final gauge of 0.20 mm.
In the intermediate annealing, the heating from 500°C to 900°C was carried out by a rapid heating treatment at a heating rate of 10°C/s, and the cooling from 900°C to 500°C after the intermediate annealing was carried out at a cooling rate of 13°C/s.
After the steel sheets were subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C, AI₂0₃ powder as a reaction inhibiting substance between annealing separator and Si0₂ in the subscale of the steel sheet was linearly adhered to the steel sheet surfaces under conditions that the adhered amount was 0.5 g/m², the adhesion width in a direction substantially perpendicular to the rolling direction of steel sheet was 2 mm and the repeated interval was 8 mm before the annealing seprator mainly composed of MgO was applied to the annealed steel sheet surfaces. After the application of the annealing separator mainly composed of MgO, the steel sheets were subjected to a secondary recrystallization annealing by raising the temperature from 850°C to 1,050°C at 10°C/hr, a purification treatment at 1,200°C for 8 hours, a baking treatment with an insulation coating and a strain relief annealing at 800°C for 3 hours.
For the comparison, a grain oriented silicon steel sheet was produced by a method of applying an annealing separator mainly composed of MgO, omitting the adhesion treatment of A1₂0₃ powder according to the usual manner, which was a comparative example.
Upon the examination of the coating state, a grey and homogeneous forsterite layer was formed over the front surface of the steel sheet in the comparative example, while in the areas coated with A1₂0₃ powder was formed a forsterite layer having a thickness thinner by 0.7 um.
The magnetic properties and surface properties of these products are shown in Table 2.
As seen from the magnetic properties and the surface properties of the products shown in Table 2, the magnetic properties of the product made from the test steel C containing Mo therein are good in that B₁₀ is 1.94 T and W_17/50 is 0.84 W/kg when the MgO annealing separator is uniformly applied to steel sheet according to the usual manner after the decarburization and primary recrystallization annealing, and the ratio of the surface defect produced in the product is 0.4%. Further, when the same test steel C after the decarburization and primary recrystallization annealing is locally coated with A1₂0₃ and further with MgO to form a non-uniform forsterite layer thereon, it is noted that 8₁₀ is 1.94 T, W_17/50 is 0.77 W/kg and the ratio of the surface defect produced in the product is 0.5%.
On the contrary, the magnetic properties of the product made from the comparative steel D of the conventional composition are 8₁₀ of 1.93 T and W_17/50 of 0.86 - 0.90 W/kg depending upon the handling conditions after the decarburization and primary recrystallization annealing and are poorer than those of the test steel C containing Mo therein, and the ratio of the surface defect produced in the product is as extremely high as 9 - 10%.
As this point has partially been disclosed in Japanese Patent laid open No. 60-92,479, it is useful as a method of producing a low iron loss grain oriented silicon steel plate by forming areas of different thickness in the forsterite layer constituting the surface layer of the grain oriented silicon steel sheet, to finely divide the width of magnetic domain.
The experimental details of the third embodiment will be described below.
A steel ingot (test steel E) containing C: 0.053%, Si: 3.43%, Mo: 0.026%, acid soluble AI: 0.029%, Se: 0.021% and Sb: 0.020% and a steel ingot (test steel F) containing C: 0.058%, Si: 3.49%, acid soluble AI: 0.026%, S: 0.026%, Cu: 0.1% and Sn: 0.05% were each heated at 1,420°C for 5 hours to perform the dissociation and solution of inhibitor and hot rolled each to form a hot rolled steel sheet of 2.0 mm in thickness.
Then, the hot rolled-steel sheets were subjected to a normalized annealing at 1,080°C for 2 minutes, quenched and subjected to two-stage cold rolling (reduction at primary cold rolling: 50%, reduction at secondary cold rolling: 80%) through an intermediate annealing at 950°C for 3 minutes to obtain a cold rolled steel sheets having a final gauge of 0.20 mm.
In the intermediate annealing, the heating from 500°C to 900°C was carried out by a rapid heating treatment at 11°C/s, and the cooling from 900°C to 500°C after the intermediate annealing was carried out at a cooling rate of 12°C/s..
After decarburization and primary recrystallization annealing was carried out in a wet hydrogen atmosphere at 850°C, the steel sheets were coated at their surface with an annealing separator mainly composed of MgO, and subjected to a secondary recrystallization annealing by raising the temperature from 850°C to 1,050°C at a heating rate of 12°C/hr and further to a purification annealing in a dry hydrogen atmosphere at 1,220°C for 5 hours.
Thereafter, parts of the steel sheets were irradiated with a YAG laser at an interval of 8 mm in a direction perpendicular to the rolling direction of steel sheets (laser irradiating conditions: pulse distance D = 0.4 mm, interval of irradiation row I = 6 mm, pulse frequency fa = 8 KHz, energy per spot of steel sheet E = 3.5 x 10-³J) to introduce a microstrain thereinto, which steel sheets were pickled with a solution of H₂S0₄ (60%) at 80°C and immersed into SbCI₃.
After the thus treated steel sheets were subjected to a baking treatment with an insulation coating composed mainly of phosphate and colloidal silica, they were subjected to recovery of laser irradiated position and a recrystallization treatment serving as a strain relief at 800°C for 3 hours to obtain a final product.
For the comparison, the steel sheet after the finish annealing was subjected to the baking treatment with the insulation coating and further to a strain relief annealing at 800°C for 3 hours.
The magnetic properties and surface properties of the resulting products are shown in Table 3.
As seen from the magnetic properties and surface properties of the product shown in Table 3, the magnetic properties of the product from the test steel E containing Mo therein are good in that B₁₀ is 1.94 T and W_17/50 is 0.84 W/kg when the insulation coating is formed according to the usual manner after the finish annealing, and the ratio of the surface defect produced in the product is 0.2%.
Further, when the sheet of the same test steel E after the finish annealing is subjected to laser irradiation, pickling, immersion in SbCl₃ solution, formation of an insulation coating and recovery and recrystallization annealing serving as a strain relief, the magnetic properties are very good in that 8₁₀ is 1.94 T and W_17/50 is 0.76 W/kg, and it is noted that the ratio of the surface defect produced in the product is 0.4%.
On the contrary, the magnetic properties of the product made from the comparative steel F of the conventional composition are 8₁₀ of 1.93 T and W_17/50 of 0.85 - 0.90 W/kg depending upon the handling conditions after the finish annealing and are poorer than those of the test steel E containing Mo therein, and the ratio of the surface defect produced in the product is as extremely high as 9 - 11%.
A part of the construction of the above method is a method wherein iron loss is reduced by irradiating with laser the surface of the grain oriented silicon steel sheet after the finish annealing in a direction substantially perpendicular to the rolling direction to introduce artificial grain boundary thereinto as disclosed in Japanese Patent Application Publication No. 57-2,252, Japanese Patent Application Publication No. 57-53,419, Japanese Patent Application Publication No. 58-5,968, Japanese Patent Application Publication No. 58-26,405, Japanese Patent Application Publication No. 58-26,406, Japanese Patent Application Publication No. 58-26,407 and Japanese Patent Application Publication No. 58-36,051. However, this method locally forms high transformation density areas, so that it has a drawback that the method is merely used only at low temperature. On the other hand, the low iron loss grain oriented silicon steel sheet can advantageously be produced by a method wherein microstrain is introduced through laser irradiation, and the base metal is completely exposed through pickling to react with Sb at a high temperature, and recovery and recrystallization of local areas is accelerated to form heterogeneous microareas onto the steel sheet surface. The latter method is an epock-making method that the degradation of iron loss is not caused even when being subjected to high-temperature heating treatment, which is different from the laser irradiated product sheet as mentioned above, and a part of the construction of this method is disclosed in Japanese Patent laid open No. 60-255,926.
As mentioned above, the invention makes possible the production of grain oriented silicon steel sheets having good iron loss and surface properties at stable steps by the addition of Mo to the steel material, the adoption of a two-stage cold rolling process, a restriction of heating and cooling rates at the intermediate annealing, and the further formation of heterogeneous microareas onto the steel sheet in the decarburization and primary recrystallization annealing or after the finish annealing, which is different from the aforementioned conventional techniques in the fundamental idea and is fairly superior in the effect obtained by the adoption of these steps as compared with the conventional techniques.
In each of the above inventions, Si is an element effective for increasing the electrical resistance of silicon steel sheet to reduce eddy current loss as previously mentioned, and is particularly required to be not less than 3.1 wt% for reducing the iron loss of the thinned product. However, when the Si amount exceeds 4.5 wt%, brittle fracture is prone to be caused in the cold rolling, so that the Si amount is limited to a range of 3.1 - 4.5 wt%. On the other hand, the Si amount in the conventional grain oriented silicon steel sheet utilizing AIN as an inhibitor is about 2.8 - 3.0 wt%, but if the Si amount is increased, the surface properties of product as in the comparative steels I, III of Figs. 1, 3 are considerably degraded. In the invention, the prevention of the occurrence of surface defects is made possible by adding 0.003 - 0.1 wt% of Mo to the steel material.
When the amount of Mo added to the steel material is less than 0.003 wt%, the force improving the magnetic properties and preventing the occurrence of surface defect is weak, while when it exceeds 0.1 %, the decarburization in the steel is delayed at the decarburization step, so that the amount should be limited to a range of 0.003 - 0.1 wt%.
AI forms a fine precipitate of AIN by bonding to N contained in steel and acts as a strong inhibitor. Particularly, in order to grow secondary recrystallized grains highly aligned in Goss orientation in the production of grain oriented silicon steel thin sheet, acid soluble AI is necessary to be within a range of 0.005 - 0.06 wt%.
When the amount of acid soluble AI is less than 0.005 wt%, the precipitated amount of AIN fine precipitates as an inhibitor is lacking and the growth of secondary recrystallized grains in {110} < 001 > orientation is insufficient, while when it exceeds 0.06 wt%, the growth of secondary recrystallized grains in {110} < 001 > orientation is also considerably degraded.
S and Se form dispersed precipitation phases of MnS or MnSe together with AIN to promote the inhibitor effect. If the amount of S or Se in total is less than 0.005 wt%, the inhibitor effect of MnS or MnSe is weak, while when the total amount exceeds 0.1 wt%, the hot and cold workabilities are considerably degraded, so that the amount of at least one of S, Se in total should be within a range of 0.005 - 0.1 wt%. Even in such a total amount range, if the S amount is less than 0.005 wt%, or if the Se amount is less than 0.003 wt%, the inhibitor effect is lacking, while if either of the amounts exceeds 0.05 wt%, the hot and cold workabilities are degraded, so that it is desirable that the S amount is within a range of 0.005 - 0.05 wt% and the Se amount is within a range of 0.003 - 0.05 wt%.
In the invention and embodiments thereof, it is particularly expected that Sb exerts control of the primary recrystallized grain growth. When the amount is less than 0.005 wt%, the effect is small, while when it exceeds 0.2 wt%, the magnetic flux density is lowered to reduce the magnetic properties, so that the amount should be within a range of 0.005 - 0.2 wt%.
The steel material adapted for the method of the invention should contain 3.1 - 4.5% of Si and small amounts of Mo, Al, S and Se and further Sb as mentioned above, but there is no obstacle to the presence of other well-known elements added to ordinary silicon steel.
For instance, it is preferable to contain about 0.02 - 2 wt% of Mn.
Further, Ci is required to produce y transformation in a part of the steel sheet during the annealing of the hot rolled steel sheet in connection with the fine precipitation of AIN. The C amount is suitably within a range of about 0.030 - 0.080 wt% when the Si amount is within a range of 3.1 - 4.5 wt% according to the invention.
Moreover, at least one of Sn, Cu and B added to ordinary silicon steel as a well-known inhibitor for primary recrystallized grain growth may be contained in a total amount of not more than 0.5 wt%, and also it is generally accepted to contain a slight amount of inevitable elements such as Cr, Ti, V, Zr, Nb, Ta, Co, Ni, P, As and so on.
The invention will be described with reference to a series of production steps below.
Firstly, LD converter, open hearth and other well-known steel making processes can be used as the means for melting the steel material used in the method according to the invention. It is a matter of course that the above means may be used together with vacuum treatment or vacuum dissolution.
As the means for the production of slabs, the usual ingot making-bloom rolling as well as continuous casting may preferably be used.
The thus obtained silicon steel slab is heated in the well-known method and then subjected to a hot rolling. The thickness before hot rolling obtained by the hot rolling is different by the reduction of the subsequent cold rolling step, but it is usually desirable to be about 1.5 - 3.0 mm.
According to the invention, the addition of a small amount of Mo to the steel material is an essential feature for obtaining silicon steel sheets having good surface properties. As disclosed in Japanese Patent laid open No. 59―85,820 by the inventors, a means for enriching Mo in the surface layer of the steel sheet by applying an Mo compound to the surface up to the completion of the hot rolling may naturally be used.
Then, the hot rolled steel sheet after the completion of the hot rolling is subjected to a primary cold rolling. According to circumstances, the steel sheet is subjected to a normalized annealing within a temperature range of 900 - 1,200°C and a quenching treatment for obtaining a finely uniformized dispersion of C into the hot rolled steel sheet before the primary cold rolling.
The reduction at primary cold rolling is somewhat different in accordance with the gauge of the product, but it is limited to 10 - 60% (desirably 20 - 50%) for obtaining the thinned product having good properties according to the invention as seen from Figs. 1 and 3.
The intermediate annealing is carried out at a temperature of 900 - 1,100°C for about 30 seconds - 30 minutes. In order to stably obtain good magnetic properties, it is desirable that the heating from 500°C to 900°C and the cooling from 900°C to 500°C after the intermediate annealing are carried out at a rate of not less than 5°C/s, preferably not less than 10°C/s. Such rapid heating and rapid cooling treatments may be performed by a well-known means such as a continuous furnace, a batch furnace or the like.
The secondary cold rolling is adapted at a reduction of 75 - 90% as seen from Figs. 1 and 3, whereby a cold rolled steel sheet having a final gauge of 01. - 0.25 mm is finished.
The invention is to produce high magnetic flux density electromagnetic steel thin sheets. The steel sheets having good properties are obtained by finishing the hot rolled steel sheet of about 1.5 - 3.0 mm in thickness at the reduction of each of the cold rolling and secondary cold rolling shown in Figs. 1 and 3 into a cold rolled steel thin sheet having a final gauge of 0.1 - 0.25 mm.
In this case, an ageing treatment at 50 - 600°C may be performed through a plurality passes as disclosed in Japanese Patent Application Publication No. 54-13,866.
The thus cold rolled thin sheet of 0.1 - 0.25 mm in gauge is subjected to a decarburization annealing serving as a primary recrystallization within a temperature range of about 750 - 870°C. The decarburization annealing may be usually performed in a wet hydrogen atmosphere having a dew point + about 30 - 65°C or in a mixed gas atmosphere of hydrogen and nitrogen for several minutes.
Then, the steel sheet after the decarburization annealing is coated with an annealing separator mainly composed of MgO and subjected to a finish annealing to grow secondary recrystallized grains in {110} < 001 > orientation. The concrete conditions for the finish annealing may be the same as in the well-known ones, but it is usually desirable that the secondary recrystallized grains are grown by raising the temperature up to 1,150 - 1,250°C at a heating rate of 3 - 50°C/hr and then a purification annealing is carried out in a dry hydrogen atmosphere for 5 - 20 hours.
Although the steel sheet of final product gauge after the final cold rolling is subjected to a surface degreasing treatment and further to decarburization and primary recrystallization annealing treatment, a treatment for forming heterogeneous microareas onto the steel sheet surface through subsequent high-temperature finish annealing is previously performed in the decarburization and primary recrystallization annealing, i.e. before or after this annealing and then the high-temperature finish annealing is performed as previously mentioned in the second embodiment, or the laser irradiation is performed as mentioned in the third embodiment, whereby low iron loss grain oriented silicon sheets can be produced.
As previously mentioned, the treatment for the formation of heterogeneous microareas can use the following methods:

_CD The decarburization promotion areas or decarburization delay areas are formed on the steel sheet surface by applying a coating agent in a direction substantially perpendicular to the rolling direction in the decarburiztion and primary recrystalilization annealing.
@ Microstrain is introduced into the steel sheet surface after the high-temperature finish annealing or an area acting as a different tension is formed thereon at local positions by laser, discharge working, scriber or ballpen-like microsphere.
(₃) An uneven temperature area is formed on the steel sheet surface at local positions by heat treatment.

In the method Q, the decarburization promotion area and decarburization delay area are alternately formed on the steel sheet surface at substantially an equal width at intervals of 1 - 50 mm as previously disclosed in Japanese Patent laid open No. 60-39,124. The narrower the width of these areas, the finer the primary recrystallized texture, and hence the secondary recrystallized grain becomes finer. Since the secondary recrystallized grain size of the product is usually within a range of 1.5 - 25 mm, when the primary recrystallized texture is varied on the steel sheet surface at a width corresponding to not more than 2 times of the secondary recrystallized grain size or a width of 3 - 50 mm, it is possible to obtain finer secondary recrystallized grains.
The effect of applying the coating agent to the steel sheet surface is sufficiently developed even at the one-side surface, but it is more enhanced when being applied to both-side surfaces of the steel sheet. As the application method to the steel sheet surface, it is considered that the application with a grooved or uneven rubber roll is optimal, but a spraying method after the covering of unnecessary areas with a making plate may be used.
Moreover, the coating solution for forming the decarburization promotion area and decarburization delay area on the steel sheet surface may be prepared according to the teaching published by the inventors (Y. Inokuti: Trans. ISIJ, Vol. 15 (1975), P. 324), which is quoted below by way of precaution. Decarburization promotion agent: MgCl₂·6H₂O, Mg(NO₃)₂·6H₂O, CaCl₂·2H₂O, Ca(NO₃)₂·4H₂O, SrCl₂·2H₂O, Sr(NO₃)₂·4H₂O, BaCl₂·2H₂O, Ba(NO₃)₂, KCI, KMnO₄, K₂P₂O₇, KBr, KClO₃, KBrO₃, KF, NaCI, Na10₄, NaOH, NaHP0₄, NaH₂PO₄·2H₂O, _NaF, NaHCO₃·Na₂O₅, Na₄P₂O₇·10H₂O, Nal·(NH₄)₂Cr₂O₇, Cu(NO₃)₂·3H₂O, Fe(NO₃)₃·9H₂O, Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, Pd(NO₃)₂, Zn(CH₃COO), Zn(NO₃)₂·6H₂O and so on. Decarburization delay agent: K₂S, Na₂S₂O₃·5H₂O, Na₂S·9H₂O, MgS0₄, SrS0₄, Al₂(SO₄)₃·18H₂O, S₂Cl₂, NaHS0₃, FeSO₄·7H₂O, KHS0₄, Na₂S₂O₈, K₂S₂O₇, Ti(SO₄)₂·3H₂O, CuSO₄·5H₂O, ZnSO₄·7H₂O, CrSO₄·7H₂O, (NH₄)₂S₂O₈, H₂SO₄, H₂SeO₃, SeOCl₂, Se₂Cl₂, Se0₂, H₂SeO₄, K₂Se, Na₂Se, Na₂Se0₃, K₂SeO₃, Na₂Se0₄, K₂SeO₄, H₂TeO₄·2H₂O, Na₂TeO₃, K₂TeO₃, K₂TeO₄·3H₂O, TeCl₄, Na₂Te0₄, Na₂AsO₂, H₃As0₄, AsCl₃, (NH₄)₃AsO₄, KH₂AsO₄, SbOCI, SbCl₃, SbBr₃, Sb₂(SO₄)₃, Sb₂0₃, BiCl₃, Bi(OH)₃, BiF₃, NaBi0₃, Bi₂(SO₄)₃, SnCl₂·2H₂O, Snl₂, PbCl₂, PbO(OH)₂, Pb(N0₃)₂ and so on.
Therefore, it is clear that the non-treated area is formed as a delay area in the treatment using only the former agent or as a promotion area in the treatment using only the latter agent.
The method of forming the microareas on the steel sheet surface after the decarburization and primary recrystallization annealing with a secondary recrystallization promoting or controlling agent may be performed according to the teaching of Japanese Patent laid open No. 60-89,521, which is quoted below by way of precaution.

(a) Secondary recrystallization promoting agents of S, Se, Te, As, Sb, Bi, Sn and Pb:
- S compound: K₂S, Na₂S₂O₃·5H₂O, Na₂S·9H₂O, MgS0₄, SrS0₄, Al₂(SO₄)₃·18H₂O, S₂Cl₂, NaHSO₃, FeSO₄·7H₂O, KHS0₄, Na₂S₂0₈, K₂S₂O₇, Ti(SO₄)₂·3H₂O, CuSO₄·5H₂O, ZnSO₄·7H₂O, CrSO₄·7H₂O, (NH₄)₂S₂O₈, H₂S0₄
- Se compound: H₂SeO₃, SeOCl₂, Se₂Cl₂, Se0₂, H₂SeO₄, K₂Se, Na₂Se, Na₂SeO₃, K₂Se₂O₃, Na₂SeO₄, K₂SeO₄
- Te compound: H₂TeO₄.2H₂O, Na₂Te0₃, K₂TeO₃, K₂TeO₄.3H₂O, TeCl₄, Na₂TeO₄
- As compound: Na₂AsO₂, H₃As0₄, AsCl₃, (NH₄)₃As0₄, KH₂As0₄
- Sb compound: SbOCI, SbCI₃, SbBr₃, Sb₂(SO₄)₃, Sb₂O₃
- Bi compound: BiCI₃, Bi(OH)₃, BiF₃, NaBi0₃, Bi₂(SO₄)₃
- Sn compound: SnCl₂·2H₂O, Snl₂
(b) Secondary recrystallization controlling agents of Ce, C, Na, K, Mg and Sr:
- Ce compound: Ce0₂, Ce(NO₃)₂·6H₂O, CeCl₃·7H₂O
- Ca compound: CaCl₂, Ca(NO₃)₃·6H₂O, CaHPO₄·2H₂O
- Na compound: NaOH, NaCI, Na₂HP0₄, Na₂Cr₂O₇·2H₂O, Na₄P₂O₇·10H₂0, NaHC0₃, NalO₄
- K compound: KN0₂, KCI, KMn0₄, KN0₃, KClO₃
- Mg compound: MgCl₂·6H₂O, Mg(NO₃)₂·6H₂O
- Sr compound: SrCl₂·2H₂O, Sr(NO₃)₂·4H₂O
- Ba compound: BaCl₂·2H₂O, Ba(NO₃)₂

In the method ②, the conditions for the introduction of microstrain through, for example, laser treatment are sufficient according to the teachings of the well-known articles (Japanese Patent laid open No. 60-96,720 and the like). By way of precaution, the preferred conditions are mentioned as follows:
As the laser, a YAG laser pulse generating multimode is optimal. The preferable irradiation conditions of laser treatment for steel sheet surface are
On the other hand, the conditions for the introduction of microstrain through discharge working treatment are sufficient according to the teachings of the well-known articles (Japanese Patent Application Publication No. 57-18,810 and the like). By way of precaution, the preferred conditions are mentioned as follows.
Moreover, the conditions for the introduction of microstrain at local positions through scriber (pushing) or ballpen-like microsphere are sufficient according to the teaching of the well-known article (Japanese Patent Application Publication No. 58-59,68). By way of precaution, the preferred conditions are mentioned as follows.
The method @, i.e. the formation of temperature differences on the steel sheet surface through heat treatment may be performed according to the teachings of well-known articles (Japanese Patent laid open No. 60-103,132 and the like). By way of precaution, the preferred conditions are mentioned as follows.

Difference between temperature of
high-temperature treated steel
sheet and usual annealing

The method for non-uniform heat treatment through these repeated annealing treatments (for example, Japanese Patent laid open No. 59-100,221, Japanese Patent laid open No. 59-100,222, Japanese Patent laid open No. 60-103,120 and the like) may be performed by any one of conventional well-known means such as local heating with a flash lamp, infrared ray lamp, high frequency induction heating, a pulse type heat treatment and so on.
In case of the method C_D among the above methods, the annealing separator mainly composed of MgO is applied to the treated steel sheet surface and then the high-temperature finish annealing is performed to grows the secondary recrystallized grains strongly aligned in {110} < 001 > orientation. The concrete conditions of the finish annealing may be the same as in the conventional well-known annealing method, but it is usually desirable that the temperature is raised up to 1,150 - 1,250⁰C at a heating rate of 3 - 50°C/hr to grow the secondary recrystallized grains and then a purification annealing is carried out in a dry hydrogen atmosphere for 5 - 20 hr.
Onto the forsterite layer at the steel sheet surface after the finish annealing is formed an insulation coating for guaranteeing sure insulation. In this case, as previously disclosed in the third embodiment, heterogeneous microareas are formed onto the finish annealed steel sheet surface to produce low iron loss grain oriented silicon steel sheets.
In this case, the introduction of artificial grain boundary through laser irradiation process disclosed in Japanese Patent Application Publication No. 57-2,252, Japanese Patent Application Publication No. 57-53,419, Japanese Patent Application Publication No. 58-5,968, Japanese Patent Application Publication No. 58-26,405, Japanese Patent Application Publication No. 58-26,406, Japanese Patent Application Publication No. 58-26,407, Japanese Patent Application Publication No. 58-36,051 has a drawback in that it is merely used stably at only low temperature, so that it is necessary to adopt a method of forming non-homogeneous areas onto the steel sheet surface without degrading the magnetic properties even after the high-temperature strain relief annealing.
As the formation of heterogeneous microareas without degradation of magnetic properties even after the high-temperature annealing, there may be used the following methods:

a. Areas having different thicknesses of forsterite layer are formed onto the steel sheet surface;
b. A coating having a different tension is formed on the forsterite layer;
c. After the forsterite layer is locally removed by using a layer or the like as mentioned above, the formed local areas are subjected to a recovery and recrystallization treatment serving as a strain relief annealing to form non-uniform areas.

The method may be performed according to a method previously disclosed in Japanese Patent laid open No. 60-92,479. By way of precaution, there are mentioned the following four methods:

a-i) Method of locally adhering a substance inhibiting reaction with the annealing separator to the steel sheet surface in an amount of not more than 1 g/m² prior to the application of annealing separator at the step for applying the annealing separator to the steel sheet surface after the primary recrystallization annealing.

In this method, oxides such as Si0₂, A1₂0₃, Zr0₂ and so on as well as metals such as Zn, AI, Sn, Ni, Fe and so on are mentioned as reaction inhibiting substances. When the amount of the reaction inhibiting substance adhered exceeds 1 g/m², the reaction inhibiting effect becomes excessive and the forsterite layer is not formed. Therefore, it is necessary to control the amount of forsterite layer thickness reduced by limiting the amount of the reaction inhibiting substance to not more than 1 g/m². Moreover, anyone of application, spraying, plating, printing, static painting and the like may be utilized as a means for adhering the reaction inhibiting substance to the steel sheet.

a-ii) Method of locally adhering a water repellant substance against an annealing separator slurry (suspension of water and annealing separator) to the steel sheet surface in an amount of not more than 0.1 g/m² prior to the application of annealing separator at the step for applying the annealing separator to the steel sheet surface after the primary recrystallization annealing.

As the water repellent substance, oil paint, varnish and the like are advantageously adaptable. This substance inhibits the contact between the steel sheet surface and the annealing separator to delay the reaction of forsterite formation and form the reduced area of forsterite thickness. However, when the amount of the substance adhered exceeds 0.1 ₉1m², the reaction delaying effect becomes excessive to form no forsterite layer, so that it is necessary to control the reduced amount of forsterite layer thickness by limiting the amount of the substance to not more than 0.1 g/m^2. Moreover, as a means for adhering the water repellent substance to the steel sheet, the application, spraying, printing, static painting and the like may be used as in the case of using the aforementioned reaction inhibiting substance.

a-iii) Method of locally adhering a substance as an oxidant for Si in steel to the steel sheet surface in an amount of not more than 2 g/m² prior to the application of annealing separator at the step for applying the annealing separator to the steel sheet surface after the primary recrystallization annealing.

This substance oxidizes Si in steel at high temperature in the subsequent finish annealing to increase the amount of Si0₂ grains in subscale of steel sheet surface, whereby the thickness of forsterite layer after the finish annealing is increased to locally form layer of increased thickness on the steel sheet surface. As the oxidizer, oxides such as FeO, Fe₂0₃, Ti0₂ and so on, reducible silicates such as Fe₂Si0₄ and so on, hydroxides such as Mg(OH)₂ and so on are advantageously adaptable. When the amount of the oxidizer adhered exceeds 2 g/m², the layer thickness becomes too thick to lose the adhesion force to the steel sheet and peel off the layer, and consequently the given object cannot be achieved.

a-iv) Method of forming the reduced-thickness-areas by removing the forsterite layer formed on the steel sheet surface after the secondary recrystallization so as not to apply plastic strain to the surface of base metal.

As such a method, there are chemical polishing and electrolytic polishing as well as removal with a rotating conical whetstone, removal with an iron needle under a light pressure, optical removal with a laser beam having a properly adjusted output, and the like. Particularly, when the laser beam is used as the optical removal means, it has an advantage that a plurality of different thickness areas can efficiently be formed at a single operation by taking a plurality of beams from a light source or irradiating the beam over the whole surface in the presence of a proper masking.
In method b, i.e. the method of forming different tension coatings on the forsterite layer, the thermal expansion coefficient of the insulation coating is not more than 8.5 x 10-6 1rC and the coefficient between different coatings is not less than 1.1 as disclosed in Japanese Patent laid open No. 60-103,182, which may be achieved by alternately applying and baking the conventionally known different coating solutions at an interval of 1 - 30 mm.
In method c as disclosed in Japanese Patent laid open No. 60-255,926 or Japanese Patent laid open No. 60―89,545, the steel sheet layer is peeled off from the steel sheet surface after the finish annealing by means of a laser or a means for application of stress such as scriber, and a part of the base metal is removed with an acid such as hydrochloric acid, nitric acid or the like, and then the treated steel sheet is immersed in an aqueous solution of an inorganic compound containing a semi-metal, a metal or the like to fill in the removed portion, which is thereafter subjected to recovery and recrystallization annealing serving as a strain relief annealing to form non-uniform areas.
Further, in order to guarantee a good insulating property, an insulation coating composed mainly of phosphate and colloidal silica is applied and baked to the above treated sheet. It is naturally required for use in transformers having a capacity as large as 1,000,000 KVA. The formation of such an insulation coating may be performed by using the conventionally well-known process as it is.
After the formation of such an insulation coating, the strain relief annealing is carried out at a temperature of not lower than 600°C. The method according to the invention has a characteristic that the degradation of magnetic properties is not caused even after such a high-temperature annealing.

Example 1

A continuously cast slab containing C: 0.059%, Si: 3.49%, MO: 0.024%, acid soluble Al: 0.034%, S: 0.029% was heated at 1,430°C for 3 hours and hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness. Thereafter, the steel sheet was subjected to a primary cold rolling at a reduction of about 50% and further to an intermediate annealing at 1,100°C for 3 minutes. In the intermediate annealing, a rapid heating treatment of 12°C/s was performed from 500°C to 900°, and a rapid cooling treatment of 15°C/s was performed from 900°C to 500°C after the intermediate annealing.
Thereafter, the steel sheet was subjected to a cold rolling at a reduction of about 80% to obtain a cold rolled steel sheet having a final gauge of 0.20 mm, which was then subjected to a primary recrystallization annealing serving as a decarburization in a wet hydrogen atmosphere at 830°C.
After a secondary recrystallization was carried out by raising temperature from 850°C to 1,100°C at 10°C/hr, a purification annealing was performed in a dry hydrogen atmosphere at 1200°C for 10 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were B_io: 1.93 T and W_17/50: 0.80 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 0.8%.

Example 2

A continuously cast slab containing C: 0.064%, Si: 3.39%, Mo: 0.19%, acid soluble Al: 0.029% Se: 0.020%, Sb: 0.022% was heated at 1,420°C for 4 hours and hot rolled to a thickness of 2.2 mm. Thereafter, the steel sheet was subjected to a primary cold rolling at a reduction of about 40% and further to an intermediate annealing at 1,100°C for 2 minutes. In the intermediate annealing, a rapid heating treatment of 12°C/s was performed from 500°C to 900°C, and a rapid cooling treatment of 18°C/s was performed from 900°C to 500°C after the intermediate annealing.
Thereafter, the steel sheet was subjected to a secondary cold rolling at a reduction of about 83% to obtain a cold rolled steel sheet having a final gauge of 0.23 mm, which steel sheet was then subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C.
After an annealing separator mainly composed of MgO was applied to the steel sheet surface, a secondary recrystallization was performed by raising the temperature from 850°C to 1,100°C at 10°C/hr, and then a purification annealing was performed in a dry hydrogen at 1,200°C for 15 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were B_io: 1.93 T and W_",_5o: 0.80 w/kg, and the surface properties were very good as the ratio of the surface defect block produced was 0.6%.

Example 3

A steel ingot containing C: 0.058%, Si: 3.59%, Mo: 0.035%, acid soluble Al: 0.033%, S: 0.023%, Cu: 0.15%, Sn: 0.11% was hot rolled to form a hot rolled steel sheet of 2.0 mm in thickness which was then subjected to a primary cold rolling (reduction: about 40%). Thereafter, the steel sheet was subjected to an intermediate annealing at 1,050°C for 5 minutes, wherein the heating from 500°C to 900°C was performed by a rapid heating treatment of 18°C/s and the cooling from 900°C to 500°C was performed by a rapid cooling treatment of 20°C/s.
Next, the steel sheet was subjected to a strong cold rolling at a reduction of about 89% to obtain a cold rolled steel sheet having a final gauge of 0.17mm, during which a warm rolling at 300°C was performed. Then, the steel sheet was subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C, a secondary recrystallization by raising the temperature from 850°C to 1,100°C at 15°C/hr, and a purification annealing in a dry hydrogen atmosphere at 1,200°C for 15 hours. In the resulting product, the magnetic properties were B₁₀: 1.93 T and W_17/50: 0.76 w/kg, and the surface properties were good as the ratio of the surface defect block produced was 0.9%.

Example 4

A continuously cast slab containing C: 0.064%, Si: 3.45%, Mo: 0.025%, acid soluble Al: 0.025%, S: 0.028% was heated at 1420°C for 4 hours and hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness. Then, the steel sheet was subjected to a primary cold rolling at a reduction of about 30% and furtherto an intermediate annealing at 1,080°C for 3 minutes. In the intermediate annealing, a rapid heating treatment of 13°C/s was performed from 500°C to 900°C, and a rapid cooling treatment of 18°C/s was performed from 900°C to 500°C.
Then, the steel sheet was subjected to a cold rolling at a reduction of about 85% to obtain a cold rolled steel sheet having a final gauge of 0.23 mm. After the steel sheet (surface temperature: 70°C) was degreased, an aqueous diluted solution of MgS0₄ (0.01 mol/I) at 85°C was applied by spraying with a jig of 0.5 mm in width at an interval of 5 mm in a direction substantially perpendicular to the rolling direction to alternately form applied areas and non-applied areas. The steel sheet was then subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C. After the application of an annealing separator mainly composed of MgO, the steel sheet was slowly heated from 850°C to 1,100°C at 10°C/hr and then subjected to a purification annealing in a hydrogen atmosphere at 1,200°C for 10 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were B₁₀: 1.93 T and W_17/50: 0.82 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 1.2%.

Example 5

A continuously cast slab containing C: 0.066%, Si: 3.5%, Mo: 0.035%, acid soluble Al: 0.030%, S: 0.026%, Sb: 0.026%, Sn: 0.1%, Cu: 0.1% was heated at 1,430°C for 4 hours and hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness. Then, the steel sheet was subjected to a primary cold rolling at a reduction of about 40% and further to an intermediate annealing at 1,050°C for 5 minutes. In the intermediate annealing, a rapid heating treatment of 15°C/s was performed from 500°C to 900°C, and a rapid cooling treatment of 20°C/s was performed from 900°C to 500°C after the intermediate annealing.
Next, the steel sheet was subjected to a cold rolling at a reduction of about 85% to obtain a cold rolled steel sheet having a final gauge of 0.20 mm, during which a warm rolling at 250°C was performed.
After the steel sheet surface was degreased and held at a surface temperature of about 100°C, a mixed solution of MgS0₄ (0.01 mol/I) and Mg(N0₃)₂ (0.01 mol/I) (90°C) was applied to the steel sheet surface with a rubber roll having an uneven surface to alternately form applied areas and non-applied areas. The steel sheet was then subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 850°C. After the application of an annealing separator mainly composed of MgO, the steel sheet was slowly heated from 850°C to 1,100°C at 8°C/hr and subjected to a purification annealing in a hydrogen atmosphere at 1,200°C for 10 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were B₁₀: 1.94 T and W_17/50: 0.73 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 1.2%.

Example 6

A continuously cast slab containing C: 0.058%, Si: 3.40%, Mo: 0.026%, Se: 0.021%, acid soluble Al: 0.030%, Sb: 0.025% was heated at 1,430°C for 3 hours and hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness. Then, the steel sheet was subjected to a primary cold rolling at a reduction of about 50% and further to an intermediate annealing at 1,100°C for 3 minutes. In the intermediate annealing, a rapid heating treatment of 12°C/s was performed from 500°C to 900°C, and a rapid cooling treatment of 15°C/s was performed from 900°C to 500°C after the intermediate annealing.
Thereafter, the steel sheet was subjected to a cold rolling at a reduction of about 80% to obtain a cold rolled steel sheet having a final gauge of 0.20 mm, which was then subjected to a primary recrystalization annealing serving as a decarburization in a wet hydrogen atmosphere at 830°C.
Prior to the application of an annealing separator mainly composed of MgO, A1₂0₃ powder as a reaction inhibiting substance against the annealing separator and Si0₂ in subscale of steel sheet was linearly adhered to the steel sheet surface under the conditions that the adhesion amount was 0.3 glm², the adhesion width in a direction substantially perpendicular to the rolling direction of steel sheet was 1.5 mm, and interval was: 8 mm, and thereafter the annealing separator mainly composed of MgO was applied thereto.
Thereafter, the steel sheet was subjected to a secondary recrystallization by raising the temperature from 850°C to 1,100°C at 10°C/hr and further to a purification annealing in a hydrogen atmosphere at 1,200°C for 10 hours. In the steel sheet surface after the finish annealing, the forsterite layer having a thickness thinner by 0.6 µm was formed on the area coated with A1₂0₃ powder.
After an insulation coated composed mainly of phosphate and colloidal silica was baked on the forsterite layer, strain relief annealing was performed at 800°C for 3 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were B₁₀: 1.94 T and W_17/50: 0.78 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 0.9%.

Example 7

A continuosuly cast slab containing C: 0.054%, Si: 3.36%, Mo: 0.024%, acid soluble AI: 0.025%, Se: 0.020% was heated at 1,420°C for 4 hours and hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness. Then, the steel sheet was subjected to a primary cold rolling at a reduction of about 40% and further to an intermediate annealing at 1,100°Cfor 2 minutes. In the intermediate annealing, a rapid heating treatment of 12°C/s was performed from 500°C to 900°C, and a rapid cooling treatment of 18°C/s was performed from 900°C to 500°C after the intermediate annealing.
Thereafter, the steel sheet was subjected to a secondary cold rolling at a reduction of about 83% to obtain a cold rolled steel sheet having a final gauge of 0.23 mm, which was then subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C.
Next, the steel sheet was irradiated linearly with a pulse laser (line width: 0.3 mm) at an interval of 8 mm in a direction perpendicular to the rolling direction, and thereafter a solution of SbC1₃ (0.01 mol/I, 90°C) was applied at the laser irradiated position.
After an annealing separator mainly composed of MgO was applied to the steel sheet surface, a secondary recrystalliazation was performed by raising the temperature from 850°C to 1,100°C at 10°C/hr, and then a purification annealing was performed in a dry hydrogen atmosphere at 1,200°C for 15 hours.
After the formation of an insulation coating composed mainly of phosphate and colloidal silica, the steel sheet was subjected to a strain relief annealing at 800°C for 2 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were 8₁₀: 1.94 T and W_17/50: 0.79 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 0.8%.

Example 8

A steel ingot containing C: 0.054%, Si: 3.49%, Mo: 0.025%, acid soluble AI: 0.30%, SO: 0.022%, Cu: 0.15%, Sn: 0.10% was hot rolled to form a hot rolled steel sheet of 2.0 mm in thickness, which was subjected to a primary cold rolling (reduction: about 40%). Then, the steel sheet was subjected to an intermediate annealing at 1,050°C for 5 minutes, wherein the heating from 500°C to 900°C was carried out by a rapid heating treatment of 18°C/s, the cooling from 900°C to 500°C after the intermediate annealing was carried out by a rapid cooling treatment of 20°C/s.
Thereafter, the steel sheet was subjected to a strong cold rolling at a reduction of about 89% to obtain a cold rolled steel sheet having a final gauge of 0.17 mm, during which a warm rolling at 300°C was performed. Then, the steel sheet was subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C, before which an electron beam was scanned at a width of 0.5 mm and an interval of 12 m in a direction perpendicular to the rolling direction form non-uniform heat areas.
After an annealing separator mainly composed of MgO was applied to the steel sheet surface, a secondary recrystallization was performed by raising the temperature from 850°C to 1,100°C at 15°C/hr, and a purification annealing was performed in a dry hydrogen atmosphere at 1,200°C for 15 hours.
After the baking of an annealing separator composed mainly of phosphate and colloidal silica, a strain relief annealing was performed at 800°C for 5 hours. In the resulting product, the magnetic properties were B_io: 1.94 T and W_17/50: 0.77 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 1.2%.

Example 9

A continuously cast slab containing C: 0.057%, Si: 3.35%, Mo: 0.025%, acid soluble Al: 0.020%, Se: 0.022%, Sb: 0.023% was heated at 1,420°C for 4 hours and hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness. Then, the steel sheet was subjected to a primary cold rolling at a reduction of about 30% and further to an intermediate annealing at 1,080°C for 3 minutes. In the intermediate annealing, a rapid heating treatment of 13°C/s was performed from 500°C to 900°C, and a rapid cooling treatment of 18°C/s was performed from 900°C to 500°C after the intermediate annealing.
Thereafter, the steel was subjected to a cold rolling at a reduction of about 85% to obtain a cold rolled steel sheet having a final gauge of 0.23 mm, which was then subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C. After the application of an annealing separator mainly composed of MgO, the steel sheet was slowly heated from 850°C to 1,100°C at 10°C/hr and subjected to a purification annealing in a hydrogen atmosphere at 1,200°C for 10 hours.
After microstrain was introduced by linearly (line width: 0.5 mm) irradiating with a pulse laser at an interval of 11 mm in a direction perpendicular to the rolling direction, the steel sheet was pickled and immersed in a solution of SbC1₃ (0.01 mol/I, 90°C).
After the formation of an insulation coating composed mainly of phosphate and colloidal silica, the steel sheet was subjected to recovery/recrystallization annealing serving as a strain relief annealing at 800°C for 5 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were B_io: 1.94 T and W_17/50: 0.78 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 1.1%.

Example 10

A continuously cast slab containing C: 0.056%, Si: 3.41%, Mo: 0.25%, acid soluble Al: 0.030%, Se: 0.020%, Sn: 0.1 %, Cu: 0.1% was heated at 1,430°C for 4 hours and hot rolled to form a hot rolled steel sheet of 2.2 mm in thickness. Then, the steel sheet was subjected to a primary cold rolling at a reduction of about 40% and further to an intermediate annealing at 1,050°C for 5 minutes. In the intermediate annealing, a rapid heating treatment of 15°C/s was performed from 500°C to 900°C, and a rapid cooling treatment of 20°C/s was performed from 900°C to 500°C after the intermediate annealing.
Thereafter, the steel sheet was subjected to a secondary cold rolling at a reduction of about 85% to obtain a cold rolled steel sheet of 0.20 mm in gauge, during which a warm rolling at 250°C was performed. Then, the steel sheet was subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 850°C, coated with an annealing separator mainly composed of MgO, slowly heated from 850°C to 1,100°C at 8°C/hr, and subjected to a purification annealing in a hydrogen atmosphere at 1,200°C for 10 hours.
After a scriber was applied to the steel sheet surface at a width of 0.5 mm and an interval of 8 mm in a direction perpendicular to the rolling direction, an insulation coating composed mainly of phosphate and colloidal silica was baked, and recovery recrystallization annealing serving as a strain relief annealing was performed at 800°C for 5 hours. The magnetic properties and surface properties of the resulting product were as follows.
The magnetic properties were B_io: 1.94 T and W_",_5o: 0.76 w/kg, and the surface properties were very good as the ratio of surface defect block produced was 1.1 %.
As seen from the above explanations, the invention has a remarkable effect that grain oriented silicon steel thin sheets having a low iron loss in that the 8₁₀ value is not less than 1.92 T and W_17/50 value is not more than 0.85 W/kg (0.23 mm thickness) and very excellent surface properties can be produced industrially and stably. Particularly, products having excellent iron loss properties and surface properties can be produced at stable steps by including Mo and AI into a steel material, subjecting a steel sheet to two-stage cold rolling process to obtain a final cold rolled steel sheet, and forming heterogeneous microareas onto the steel sheet surface in decarburization and primary recrystallization annealing or after finish annealing to grow a non-uniform and fine secondary recrystallized texture in Goss orientation.

Claims

1. A method of producing low iron loss grain oriented silicon thin steel sheets comprising subjecting a steel slab comprising 3.1 to 4.5 wt% Si, 0.003 to 0.1 wt% Mo, 0.005 to 0.06 wt% acid soluble Al, at least one of S and Se in a total amount of not more than 0.005 to 0.1 wt% and optionally 0.005 to 0.2 wt% Sb, the remainder being Fe, incidental impurities and, optionally, incidental elements to a hot rolling to form a steel sheet, and subjecting said steel sheet to

(i) a primary cold rolling at a reduction of 10 to 60%,

(ii) an intermediate annealing including a heating stage from 500°C to 900°C and a cooling stage from 900°C to 500°C wherein the heating and cooling rates respectively are not less than 5'Cs-¹,

(iii) a secondary cold rolling at a reduction of 75 to 90% to form a steel sheet having a final gauge of 0.1 to 0.25 mm,

(iv) a decarburization and primary recrystallization annealing in a wet hydrogen atmosphere and

(v) a high temperature finish annealing.

2. A method as claimed in claim 1 wherein the optional incidental elements include one or more of: 0.02 to 2 wt% Mn, 0.030-0.080 wt% C, and one or more of Sn, Cu and B in a total amount of not more than 0.5 wt%.

3. A method as claimed in claim 1 or 2 wherein either before or after the decarburization and primary recrystallization annealing the steel sheet is subjected to a treatment which causes the formation at the high temperature finish annealing, of heterogeneous microareas on the steel sheet surface.

4. A method as claimed in claim 1 or 2 wherein heterogeneous microareas are formed on the surface of the steel sheet after the high temperature finish annealing.