Patent Publication Number: US-7214277-B2

Title: Method for manufacturing non-oriented electrical steel sheet having high magnetic flux density

Description:
BACKGROUND OF THE INVENTION  
     The present application claims priority to Japanese Application 2003-106992, filed in Japan on Apr. 10, 2003 and which is herein incorporated by reference in its entirety. 
     FIELD OF THE INVENTION  
     The present invention relates to a method for manufacturing low iron loss non-oriented electrical steel sheet having extremely high magnetic flux density in the L and C directions. 
     DESCRIPTION OF THE RELATED ART  
     Non-oriented electrical steel sheets are used for large sized generators, motors, and small sized stationary electric devices such as stabilizers or devices for audio goods. 
     Cut-outs of steel sheets, such as shown in  FIG. 1 , have magnetic paths formed mainly in the rolling direction (hereinafter referred to as L direction) and in the perpendicular direction to the L direction (hereinafter referred to as the C direction). Recently a split core shown in  FIG. 1  or a stator core formed by arranging cut-out T-shaped steel sheets annularly has been used for manufacturing an electric motor. A low iron loss non-oriented electrical steel sheet having high magnetic flux density in L and C direction has been demanded for these products. 
     A quench solidification method is one of the manufacturing methods for making the non-oriented electrical steel sheet having high magnetic flux density. In quench solidification, molten steel is solidified on a moving cooling wall to form a steel cast strip, and the steel cast strip is cold-rolled to a predetermined thickness then annealed in a final step to become a non-oriented electrical steel sheet. In Unexamined Japanese Patent Application Publication No. 62-240714 (JP &#39;714), a method is disclosed where a steel cast strip having a mean grain size equal to or more than 50 μm is prepared and then the steel cast strip is rolled so as to establish a reduction rate of more than 50%. In Example 1 (JP &#39;714), it is reasonable to conclude that the starting steel material contains equiaxial crystals, since the starting steel cast strip is disclosed to have crystals having a mean grain size of 0.5 mm and the thickness of the strip is 1.4 mm. It is also disclosed that the texture suitable for the stated purpose is obtained by controlling the cold reduction rate. For example, a {100}&lt;001&gt; type texture suitable for a small stationary electric device is obtained with more than a 50% reduction rate and a {100}&lt;025&gt; type texture suitable for a rotational machine is obtained with more than a 70% cold reduction rate.  FIG. 2  in JP &#39;714 shows that there is a relationship between a cold reduction rate and a magnetic flux density in both L and C directions, i.e., the magnetic flux density decreases as the reduction rate exceeds 70%. 
     Large sized generators, small sized stationary devices and motors having a split core require a steel sheet having high magnetic flux density in both L and C directions to save energy and resources. However, a non-oriented electrical steel sheet having a very high magnetic flux density (especially in both L and C directions) can not be obtained by the method disclosed in JP &#39;714, because: (a) the molten steel is solidified at a cold reduction rate exceeding 70% on a moving cooling wall; and (b) the steel cast strip has crystals having mean grain size of more than 50 μm. As disclosed infra, under conditions (a) and (b), the magnetic flux density increases with increasing cold reduction rate until the reduction rate hits about 70% at which point the magnetic flux begins to decrease. 
     Generally it is known that cracks are likely to occur by rolling a steel cast strip at room temperature where the steel cast strip is obtained with a quench solidification method because the steel cast strip obtained with quench solidification method is very brittle. 
     SUMMARY OF THE INVENTION  
     An object of the invention is to provide a method for manufacturing a low iron loss non-oriented electrical steel sheet having extremely high magnetic flux density in L and C directions which can not be obtained by the method disclosed in JP &#39;714. 
     The object is accomplished by the following method. 
     A method for manufacturing non-oriented electrical steel sheet having high magnetic flux density comprising the steps of: preparing a molten steel containing, in mass %, 0.008% or less of C, 1.8% to 7% of (Si+2Al), 0.02 to 1.0% of Mn, 0.005% or less of S, 0.01% or less of N, and the balance Fe and unavoidable impurities; solidifying the molten steel on at least one moving cooling wall to form a steel cast strip; cold-rolling the steel cast strip to a predetermined thickness; and finally annealing the cold-rolled steel; wherein the {100} pole intensity, which is a ratio of the integrated inverse pole intensity of the {100} plane at the midplane of a steel cast strip [for a given sample of the cold-rolled steel] to the integrated inverse pole intensity of {100} plane for a “random” sample in which crystal grains have random orientations, is at least 4 and a cold reduction rate of the cold-rolling is between 70% and 85%. 
     In an embodiment of the invention, the superheating degree of the molten steel before the solidification is 70° C. or more. A superheating degree of molten steel is defined as a difference between the molten steel temperature at the casting and liquidus temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  shows applications of non-oriented electrical steel sheet and a blank layout for the application. 
         FIG. 2  is a graph showing a relationship between a {100} pole intensity and a magnetic flux density B 50 . 
         FIG. 3   a ) is a photo showing a solidified structure of a steel cast strip of which the {100} pole intensity is 1.3. 
         FIG. 3   b ) is a photo showing solidified structure of a steel cast strip of which the {100} pole intensity is 6.4. 
         FIG. 4  is a graph showing a relationship between a cold reduction rate and a magnetic flux density B 50 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     It was found that it is very effective to control the solidified structure, the texture of a steel cast strip and the cold reduction rate (applied to the strip within a certain narrow range) in the quench solidification method in order to manufacture the non-oriented electrical steel sheet having high magnetic flux density.  FIG. 2  shows an example of experimental results performed by the inventors. Molten steel containing, in mass %, 0.0011 to 0.0013% of C, 3.1% of Si, 1.1% of Al, 0.26% of Mn, 0.0022 to 0.0026% of S and 0.0013 to 0.0016% of N, was quench solidified by a twin roll method under various conditions to form a steel cast strip with 1.6 mm thickness. The steel cast strip was cold-rolled at room temperature with a 78% reduction rate to form a 0.35 mm thick steel sheet and the steel sheet was finally annealed at 1075° C. for 30 seconds.  FIG. 2  shows that there is a relationship between: a) a ratio of the integrated inverse pole intensity of {100} at the midplane of a steel cast strip to the integrated inverse pole intensity of {100} plane for a “random” sample in which crystal grains have random orientations (herein referred to as simply “{100} pole intensity”); and b) a magnetic flux density B 50  of the steel sheet in L, C and LC directions.  FIG. 2  indicates that magnetic flux density increases as {100} pole intensity exceeds  4 . In the samples of  FIG. 2 , different {100} pole intensities were prepared by changing the superheating degree of molten steel. 
       FIG. 3  is two photos of strips of the solidified structures. The structure shown in  FIG. 3B  has a {100} pole intensity of 6.4 and the structure shown in  FIG. 3A  has a {100} pole intensity of 1.3. In the photos, the vertical direction is the thickness direction of the steel cast strip and the horizontal direction is the casting direction. In  FIG. 3B , the sample having a {100} pole intensity of 6.4 has well-developed columnar crystals extending from the surface toward the center layer. On the contrary, in  FIG. 3A , the sample having a {100} pole intensity of 1.3 has a lot of spherical equiaxial crystals and almost no columnar crystals. In view of this, it was found that it is important to form a texture which is rich in {100}&lt;0vw&gt; by developing as much columnar crystals as possible. 
       FIG. 4  shows the relationship between the cold reduction rate and the magnetic flux density B 50  with respect to samples of strips obtained by cold-rolling a steel cast strip at room temperature, having a {100} pole intensity of 5.0 which was obtained in the experiment of  FIG. 2 , with various cold reduction rates, and annealing the strip in a final step at 1075° C. for 30 seconds.  FIG. 4  indicates that the highest magnetic flux density is obtained by cold-rolling a steel cast strip of 5.0 {100} pole intensity at 70–85% cold reduction rate. 
     It was found by the inventors that under the temperature condition of cold-rolling adopted for the samples in  FIG. 3  and  FIG. 4 , edge cracks form in some samples. Table 1, below, shows the relationship between temperature of cold-rolling, depth of edge cracks in a case where cracks are found, and magnetic flux density B50 with respect to samples of strips obtained by cold-rolling a steel cast strip with reduction rate of 78%, having a {100} pole intensity of 5.0 which was obtained in the experiment of  FIG. 2 , at various rolling temperatures, and annealing the strip in a final step at 1075° C. for 30 seconds. 
     As shown in Table 1, it is newly found that edge cracks are prevented and an increase of 0.01 T for the magnetic flux density B50 is achieved by cold-rolling of a steel cast strip at a temperature above 180° C. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Depth of 
                   
               
               
                   
                   
                   
                 edge cracks 
               
               
                   
                   
                   
                 in a case 
               
               
                   
                   
                 Temperature of 
                 which cracks 
               
               
                   
                   
                 Cold-Rolling 
                 are found 
               
               
                   
                 No. 
                 (° C.) 
                 (mm) 
                 B50 LC (T) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1 
                 20 
                 50 
                 1.732 
               
               
                   
                 2 
                 50 
                 45 
                 1.732 
               
               
                   
                 3 
                 100 
                 20 
                 1.737 
               
               
                   
                 4 
                 150 
                 10 
                 1.739 
               
               
                   
                 5 
                 180 
                 No cracks 
                 1.743 
               
               
                   
                 6 
                 250 
                 No cracks 
                 1.745 
               
               
                   
                 7 
                 350 
                 No cracks 
                 1.746 
               
               
                   
                 8 
                 370 
                 No cracks 
                 1.746 
               
               
                   
                   
               
            
           
         
       
     
     In an embodiment of the present invention, the annealing step is performed in a range of 750–1250° C. for 10–180 seconds. Preferably, the annealing step is performed in a range of 850–1200° C. for 20–180 seconds. Most preferably, the annealing step is performed in a range of 1000–1200° C. for 25–60 seconds. 
     As mentioned above, in the Unexamined Japanese Patent Application Publication No. 62-240714, a method is proposed where a steel cast strip having a mean grain size equal to or more than 50 μm pared and then the steel cast strip is rolled so as to establish a cold reduction rate of more than 50%. In this reference, however, it is reasonable to conclude that equiaxial crystals are used in the starting material. This conclusion is based on the observation that the data given in FIG. 2 of Example 1 of JP &#39;714 is of a strip having a mean grain size of crystals of 0.5 mm and a thickness of 1.4 mm. This steel sample has a reduction in the magnetic flux density as the cold reduction rate exceeds 70%. 
     In the present invention, it is newly found that the high magnetic flux density can be obtained by using a steel cast strip having columnar crystals and applying a cold reduction rate of 70–85%. While the sample having a {100} pole intensity of 1.3 is recognized to have equiaxial grains in the center layer of the strip as shown in  FIG. 3A , the B 50  in LC direction is 1.69 T at a cold reduction rate of 78%. As shown in  FIG. 2 , in the region of the present invention where the texture of a steel strip is rich in {100}&lt;0vw&gt; having developed columnar crystals using a 70–85% cold reduction rate, the B 50  in the LC direction is more than 1.72 T, which leads to increasing the magnetic flux density by 0.03 T or more. 
     It is also newly found that edge cracks are prevented and an increase of 0.01 T for the magnetic flux density is achieved by cold-rolling of a steel cast strip at a temperature above 180° C., as shown in Table 1, above. 
     In the steel sheet of the invention, in mass %, the C content is up to 0.008% so that a dual-phase of austenite and ferrite is not formed and a single phase is formed of ferrite which develops as much columnar crystals as possible. Preferably, the C content is 0.0002% to 0.008%. 
     If the (Si+2Al)% is 1.8% or more and the C % is 0.008% or less, a dual-phase of austenite and ferrite is not formed but a single phase of ferrite is formed, which encourages the columnar crystals to develop. When (Si+2Al) % exceeds 7%, cold-rollability deteriorates. So the upper limit of (Si+2Al) % is 7% and the lower limit is 1.8%. 
     Mn % is 0.02% to 1% to improve the brittleness. If the Mn content exceeds 1%, the magnetic flux density deteriorates. 
     S % is 0.005% or less to avoid formation of fine sulfides which have an adverse affect on iron loss. Preferably, the S content is 0.0002% to 0.005%. 
     N % is 0.01% or less to avoid formation of fine nitrides such as AlN which have an adverse affect on iron loss. Preferably, the N content is 0.0002% to 0.01%. 
     Molten steel is solidified on at least one moving cooling wall to form a steel cast strip. The single roll method and twin roll method can be used. 
     The {100} pole intensity should be 4 or more. High magnetic flux density is obtained when columnar crystals are developed in the steel cast strip and the {100} pole intensity is 4 or more as shown in  FIG. 2  and  FIG. 3 . 
     It is effective to adjust a superheating degree of molten steel in order to control the {100} pole intensity. A superheating degree of molten steel is defined as a difference between the molten steel temperature at the casting and the liquidus temperature. As shown in the example below, a superheating degree of 70° C. or more enable a {100} pole intensity of 4 or more. 
     The reduction rate of cold-rolling is applied at 70–85 %. As shown in  FIG. 4 , in the cases when the reduction rate is less than 70% or more than 85%, a high magnetic flux density can not be obtained. 
     Preferably, cold-rolling before annealing is performed at a temperature between 180 and 350° C. As shown in Table 1 above, in the cases when the cold-rolling is performed below 180° C., there is a possibility that edge cracks will form. In the cases when the cold-rolling is performed above 350° C., the increase in the magnetic flux density B50 is saturated. A strip can be cold-rolled at a temperature above 180° C. by rolling a quench solidified strip before the temperature of the strip comes down below 180° C. A strip can also be heated above 180° C. with using an external heating device such as an electric furnace and a gas oven. 
     EXAMPLE 1  
     Molten steel containing, in mass %, 0.0009% of C, 3.0% of Si, 0.20% of Mn, 1.2% of Sol. Al, 0.0007 to 0.0018% of S and 0.0018 to 0.0024% of N, was quench solidified by the twin roll method under various superheating degrees to form steel cast strips with various thicknesses. The liquidus temperature of the steel was 1490° C. Then the steel cast strips were pickled, cold-rolled to steel sheets of 0.35 mm thickness at room temperature, annealed at 1075° C. for 30 seconds and finally coated with an insulation coating. Table 2 below, shows the relationship between a cold reduction rate, magnetic properties and the {100} pole intensity. It was found that the combination of {100} pole intensity of 4 or more and cold reduction rate of 70 to 85% can provide high magnetic flux density. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Super- 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 {100} 
                 heating 
                 Steel cast 
                 Cold 
               
               
                   
                 pole 
                 degree 
                 strip thick- 
                 reduction 
                 W 15/50  LC 
                 B 50  L 
                 B 50  C 
                 B 50  LC 
               
               
                 No. 
                 intensity 
                 (° C.) 
                 ness (mm) 
                 rate (%) 
                 (W/kg) 
                 (T) 
                 (T) 
                 (T) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 2.3 
                 30 
                 1.59 
                 78 
                 2.07 
                 1.729 
                 1.669 
                 1.699 
                 Comp. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 2 
                 3.5 
                 55 
                 1.59 
                 78 
                 2.06 
                 1.734 
                 1.691 
                 1.713 
                 Comp. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 3 
                 4.1 
                 72 
                 1.59 
                 78 
                 2.03 
                 1.746 
                 1.705 
                 1.726 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 4 
                 5.5 
                 88 
                 1.59 
                 78 
                 2.01 
                 1.739 
                 1.720 
                 1.730 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 5 
                 6.4 
                 100 
                 1.59 
                 78 
                 1.98 
                 1.734 
                 1.733 
                 1.734 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 6 
                 5.5 
                 89 
                 0.88 
                 60 
                 2.05 
                 1.735 
                 1.697 
                 1.716 
                 Comp. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 7 
                 5.6 
                 90 
                 1.09 
                 68 
                 2.05 
                 1.738 
                 1.700 
                 1.719 
                 Comp. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 8 
                 5.3 
                 88 
                 1.25 
                 72 
                 2.03 
                 1.741 
                 1.707 
                 1.724 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 9 
                 5.4 
                 88 
                 1.75 
                 80 
                 1.99 
                 1.744 
                 1.718 
                 1.731 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 10 
                 5.2 
                 85 
                 2.19 
                 84 
                 2.02 
                 1.724 
                 1.720 
                 1.722 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 11 
                 5.3 
                 87 
                 2.50 
                 86 
                 2.07 
                 1.710 
                 1.699 
                 1.705 
                 Comp. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                   
               
            
           
         
       
     
     EXAMPLE 2  
     Table 3 below, shows the relationship between temperature of cold-rolling, a cold reduction rate, depth of edge cracks, the {100} pole intensity and magnetic properties with respect to samples of strips obtained by cold-rolling a steel cast strip to steel sheets of 0.35 mm thickness, which was obtained for preparing the sample No. 9 of Example 1 in Table 2, at various rolling temperatures, annealing the strip at 1075° C. for 30 seconds and applying an insulating membrane on the strip. According to a method of the present invention, a non-oriented electrical steel having high magnetic flux density without edge cracks can be manufactured by adopting conditions of a cold reduction rate of the cold-rolling between 70° C. and 85%, {100} pole intensity of at least 4 and a cold-rolling temperature between 180 and 350° C. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Super- 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 {100} 
                 heating 
                 Steel cast 
                 Cold 
                 Cold 
                 Depth of 
               
               
                   
                 pole 
                 degree 
                 strip thick- 
                 reduction 
                 reduction 
                 edge cracks 
                 W 15/50  LC 
                 B 50  L 
                 B 50  C 
                 B 50  LC 
                   
               
               
                 No. 
                 intensity 
                 (° C.) 
                 ness (mm) 
                 rate (%) 
                 temp. (° C.) 
                 (mm) 
                 (W/kg) 
                 (T) 
                 (T) 
                 (T) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 12 
                 5.4 
                 88 
                 1.75 
                 80 
                 20 
                 55 
                 1.99 
                 1.744 
                 1.718 
                 1.731 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 13 
                 5.4 
                 88 
                 1.75 
                 80 
                 150 
                 20 
                 1.99 
                 1.746 
                 1.720 
                 1.733 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 14 
                 5.4 
                 88 
                 1.75 
                 80 
                 180 
                 0 
                 1.98 
                 1.753 
                 1.726 
                 1.740 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 15 
                 5.4 
                 88 
                 1.75 
                 80 
                 210 
                 0 
                 1.96 
                 1.754 
                 1.729 
                 1.742 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                 16 
                 5.4 
                 88 
                 1.75 
                 80 
                 350 
                 0 
                 1.96 
                 1.754 
                 1.729 
                 1.741 
                 Inv. 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ex. 
               
               
                   
               
            
           
         
       
     
     According to a method of the present invention, a low iron loss non-oriented electrical steel sheet having extremely high magnetic flux density in the L and C directions can be manufactured, which is suitable for use as an iron core for a large size electric generator, a small size stationary electric device, a motor (including split core), etc.