Process of making electrical steels

Abstract: Batch annealed, semi-processed and fully processed motor lamination steels are made by a process which combines an ultra low carbon composition (less than 0.01%) with the steps of pickle band annealing and light temper rolling (less than 1.0% reduction).

Description: Background of the Invention The present invention relates generally to the production of electrical steels, and more specifically to cold rolled, batch annealed and temper rolled motor lamination steels having good mechanical and magnetic properties, including low core loss and high permeability. Desired electrical properties of steels used for making motor laminations are low core loss and high permeability. Those steels which are stressed relief annealed after punching should have the mechanical properties which minimize distortion, warpage and delamination during the annealing of the lamination stacks. Continuously annealed, silicon steels are conventionally used for motors, transformers, generators and similar electrical products. Continuously annealed silicon steels can be processed by techniques well known in the art to obtain low core loss and high permeability. Since the steels are substantially free of strain, they can be used in the as-punched condition (commonly referred to as fully processed) or can be finally annealed by the electrical apparatus manufacturer after punching of the laminations (commonly referred to as semi-processed) to produce the desired magnetic properties with little danger of delamination, warpage, or distortion. A disadvantage of this practice is that the electrical steel sheet manufacturer is required to have a continuous annealing facility. In order to avoid a continuous annealing operation, practices have been developed to produce cold rolled motor lamination steel by normal cold rolled sheet processing including batch annealing followed by temper rolling. In order to obtain the desired magnetic properties of high permeability and low core loss, it has been considered necessary to temper roll the steel with a heavy reduction in thickness on the order of 7%. Electrical steels processed by batch annealing and heavy temper rolling followed by a final stress relief anneal after the punching operations develop acceptable core loss and permeability through a complete recrystallization process. Unfortunately, the heavy temper rolling necessary for development of magnetic properties often results in delamination, warpage and distortion of the intermediate product when it is annealed to the degree that it could be unsuitable for service. Summary of the Invention The present invention seeks to provide a batch annealed and temper rolled motor lamination steel having magnetic and mechanical properties similar to silicon electrical steels produced by continuous annealing without temper rolling. The present invention seeks to provide a batch annealed and temper rolled motor lamination steel which can be given a final stress relief anneal to achieve low core loss and high permeability without delamination, warpage or distortion of the intermediate product produced by the electrical product manufacturer. The invention also seeks to provide a batch annealed and temper rolled motor lamination steel which displays acceptable core loss and permeability without a final stress relief anneal operation. The present invention applies to the production of batch annealed and temper rolled motor lamination steels which are semi-processed, i.e. steels which are given a final stress relief anneal after punching, and fully processed steels, i.e. steels which are used in the as-punched condition without a final stress relief anneal. In both instances, the process of the invention is characterized by a composition having an ultra low carbon content less than 0.01%, preferably less than 0.005%, a pickle band anneal, and light temper rolling with a reduction in thickness of less than 1.0%, and, preferably, less than 0.5%. A preferred embodiment of the process provided by the invention for making both semi-processed and fully processed electrical steel comprises the steps of: hot rolling a slab into a strip having a composition consisting essentially of (% by weight): C up to 0.01 Si 0.20 - 1.35 Al 0.10 - 0.45 Mn 0.10 - 1.0 S up to 0.015 N up to 0.006 Sb up to 0.07 Sn up to 0.12 balance iron and unavoidable impurities followed by coiling, pickling, annealing, cold rolling and batch annealing the strip, and then temper rolling the strip with a reduction in thickness of less than 1.0%. In the case of semi-processed steel which is given a final stress relief anneal after punching, the steel can be hot rolled with a finishing temperature in either the austenite or ferrite region. Hot rolling with a finishing temperature in the austenite region results in optimum permeability after the stress relief anneal. Hot rolling with a finishing temperature in the ferrite region results in optimum core loss with lower permeability after the final stress relief anneal. In the case of fully processed steels which are not given a final stress relief anneal, optimum core loss and permeability are achieved when the steels are hot rolled with a finishing temperature in the austenite region. In the case of both semi-processed and fully processed steels, the combination of ultra low carbon content, pickle band annealing, and light temper rolling results in low core loss and high permeability. If the punched steel product is given a final stress relief anneal, the light temper roll of less than 1.0% and more particularly less than 0.5%, minimizes the residual stress that is thought to be responsible for the occurrence of delamination, warpage and distortion. Other objects and a fuller understanding of the invention will be had from the following description of preferred embodiments and the accompanying drawings. Brief Description of the Drawings FIG. 1 is a graph showing core loss (W/lb/mil) after stress relief annealing versus % temper elongation for four semi-processed steels, two of which are produced in accordance with the present invention. FIG. 2 is a graph showing permeability after stress relief annealing (Gauss/Oersted at an induction of 1.5 Tesla) versus % temper elongation for four semi-processed steels, two of which are made according to the present invention. Description of Preferred Embodiments As generally described above, the process of the present invention involves an ultra low carbon steel, i.e. a steel having a carbon content less than 0.01%, and, preferably, no greater than 0.005% by weight, which is pickle band annealed prior to cold rolling, batch annealed after cold rolling, and temper rolled with a light reduction in thickness, i.e. no greater than 1.0%, and, preferably, no greater than 0.5%. Steels processed in this manner are useful in semi-processed applications in which the intermediate products made by the electrical manufacturer are given a stress relief anneal and in fully processed applications in which the temper rolled steel sold by the steel sheet producer is used in the manufacture of as-punched intermediate products which are not given a final stress relief anneal. It has been found that in both instances the combination of ultra low carbon content, pickle band annealing and light temper rolling results in good magnetic and mechanical properties. The steel composition consists generally of up to 0.01% C, 0.20-1.35% Si, 0.10-0.45% Al, 0.10-1.0% Mn, up to 0.015% S, up to 0.006% N, up to 0.07% Sb, and up to 0.12% Sn. More specific compositions include less than 0.005% C, 0.25-1.0% Si, 0.20-0.35% Al, and less than 0.004% N. Suitable amounts of Sb are from 0.01-0.07% by weight, and, more preferably, from 0.03-0.05%. Less preferably, Sn may be used in a typical range of from 0.02-0.12%. In carrying out the process of the invention, a steel slab of the indicated composition is hot rolled into a strip, coiled, pickled and pickle band annealed. The strip is preferably coiled at a temperature no greater than 1200°F (648.9°C), and preferably, no greater than 1050°F (565.5°C). The lower coiling temperatures result in less subsurface oxidation in the hot band. Also, in the case of steels which are hot rolled with a finishing temperature in the ferrite region, coiling temperatures less than 1200°F (648.9°C) are preferred in order to retain the cold worked ferrite grain structure. The pickle band anneal is carried out at a temperature that usually ranges from about 1350°-1600°F (732.2°C - 871.1°C), and more specifically from 1400°-1550°F (760°C - 843.3°C). Following the pickle band anneal, the strip is cold rolled and batch annealed. The cold rolling reduction typically ranges from 70-80%. The batch anneal operation is carried out in a conventional manner at a coil temperature ranging from 1100°-1350°F (593.3°C - 732.2°C). In accordance with the invention, the batch annealed strip is temper rolled with a light reduction in thickness no greater than 1.0%, and, more preferably no greater than 0.5%. In the case of fully processed steels, the light temper roll is critical to obtaining low core loss and good permeability. In the case of semi-processed steels, the light temper roll is critical to avoiding delamination, warpage and distortion when the intermediate product is stress relief annealed. The following Table 1 sets forth the magnetic properties of semi-processed steels which were given a stress relief anneal. The stress relief anneal was carried out in a conventional manner by soaking for 90 minutes at 1450°F (787.8°C) in an HNX atmosphere having a dew point of from 50°-55°F (10°C - 13°C). The steels reported in Table 1 had a nominal composition of 0.35% Si, 0.25% A1, 0.55% Mn, 0.007% S, 0.004% N, 0.04% S, 0.03% Sb, and C in the amount indicated in the table. Examples %C Processing Magnetic Properties Core Loss (w/lb/mil) Permeability (G/Oe) Thickness (inch) A 0.005 Hot Rolling - 1720°F Finishing and 1420°F Coiling, Pickle, Pickle Band Anneal, Cold Roll, Batch Anneal, Temper Roll 0.5% 0.127 4035 0.0233 B 0.005 Hot Rolling - 1530°F Finishing and 1000°F Coiling, Pickle, Pickle Band Anneal, Cold Roll, Batch Anneal, Temper Roll 0.5% 0.116 2829 0.0214 C 0.02 Hot Rolling - 1720°F Finishing and 1420°F Coiling, Pickle, Cold Roll, Batch Anneal, Temper Roll 7% 0.123 2732 0.0220 1 lb = 0.45 kg ; 1 inch = 25.4 mm ; 1 mil = 25.4 µm 1000°F = 537.8°C ; 1420°F = 771°C ; 1530°F = 832.2°C ; 1720°F = 937.8°C The steels of Examples A and B were made according to the invention with a carbon content of 0.005% and a light temper reduction of 0.5%. Example A was hot rolled with a finishing temperature in the austenite region (1720°F = 937.8°C), while Example B was hot rolled with a finishing temperature in the ferrite region (1530°F = 832.2°C). It will be seen that rolling in the ferrite region improved the core loss while sacrificing some permeability. Example C is a 0.02% C steel which was given a heavy temper reduction of 7.0%. A comparison of the properties of Examples A and C shows the improvement in permeability which is achieved with the lower carbon level and lighter temper reduction. Figures 1 and 2 show the improved magnetic properties of semi-processed steels which are given a pickle band anneal in accordance with the invention compared to the properties of steels processed without a pickle band anneal. The steels had the same nominal composition as the steels reported in Table 1 and were give the same stress relief anneal. As shown in Figure 1, the two 0.005% C steels which were hot rolled with a finishing temperature in the austenite and ferrite regions and given a pickle band anneal exhibited the lowest core losses. The worst core loss occurred with a 0.02% carbon steel which was not given a pickle band anneal; a lower carbon content of 0.005% demonstrated better core loss. Referring to Figure 2, it will be seen that the two 0.005% carbon steels which were given a pickle band anneal exhibited the best permeability, while the two steels which were not given a pickle band anneal displayed lower permeabilities. The worst permeability was exhibited by a steel having a carbon content 0.02%. The following Table 2 sets forth the magnetic properties of fully processed steels, i.e. steels which were not given a final stress relief anneal. The steels reported in Table 2 had the same nominal composition as the steels reported in Table 1. Examples %C Processing Magnetic Properties Core Loss (w/lb/mil) Permeability (G/Oe) Thickness (inch) D 0.02 Hot Rolling - 1720°F Finishing and 1420°F Coiling, Pickle, Pickle Band Anneal, Cold Roll, Batch Anneal, Temper Roll 0.5% 0.193 941 0.0280 E 0.005 Hot Rolling - 1720°F Finishing and 1420°F Coiling, Pickle, Pickle Band Anneal, Tandem Roll, Batch Anneal, Temper Roll 0.5% 0.171 1244 0.0229 F 0.005 Plot Rolling - 1530°F Finishing and 1000°F Coiling, Pickle, Pickle Band Anneal, Cold Roll, Batch Anneal, Temper Roll 0.5% 0.213 951 0.0217 G 0.005 Hot Rolling - 1530°F Finishing and 1000°F Coiling, Pickle, Pickle Band Anneal, Cold Roll, Batch Anneal, Temper Roll 7% 0.248 634 0.0215 H 0.02 Hot Rolling - 1720°F Finishing and 1420°F Coiling, Pickle, Cold Roll, Batch Anneal, Temper Roll 7% 0.289 694 0.0253 The steel of Example D was made with a carbon content of 0.02%, while the steel of Example E was made in accordance with the invention from an ultra low carbon steel having a carbon content of 0.005%. Both steels were identically processed, including a pickle band anneal and a light temper reduction of 0.5%. It will be seen that lowering the carbon from 0.02% to 0.005% improved the as-punched/sheared magnetic properties. The steel of Example F was an ultra low carbon steel which was hot rolled to a finishing temperature in the ferrite region and given a light temper reduction of 0.5%. It will be seen that the magnetic properties of Example E which was a steel finished in the austenite region were superior to those of steel of Example F finished in the ferrite region. Thus, for fully processed applications, the preferred process of the invention involves finishing in the austenite region. The steel of Example G is an ultra low carbon content steel similar to Example F except that the steel of Example G was given a heavy temper reduction of 7.0%. It will be seen from a comparison of the magnetic properties of Examples F and G that the lowest core loss and highest permeability are achieved with a light temper reduction. Example H is a 0.02% carbon steel which was not given a pickle band anneal and was finished with a heavy temper reduction of 7.0%. A comparison of Examples D and H shows the improvement in as-punched/sheared magnetic properties achieved with light temper rolling and pickle band annealing versus heavy temper rolling and no pickle band annealing. Many modifications and variations of the invention will be apparent to those skilled in the art from the forgoing detailed description. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically disclosed.

Claims: In a method of making electrical steel strip characterized by low core loss and high permeability comprising the steps of: hot rolling a slab into a strip having a composition consisting of (% by weight): C up to 0.01 Si 0.20 - 1.35 Al 0.10 - 0.45 Mn 0.10 - 1.0 S up to 0.015 N up to 0.006 Sb up to 0.07 Sn up to 0.12 balance iron and unavoidable impurities followed by coiling, pickling, annealing, cold rolling, batch annealing, and temper rolling the strip with a reduction in thickness of less than 1.0%. The method of Claim 1 wherein said step of temper rolling is carried out with a reduction in thickness no greater than 0.5%. The method of Claim 1 or Claim 2 including the step of stress relief annealing the strip after temper rolling. The method of Claim 1 or Claim 2 in which the slab is hot rolled with a finishing temperature in the austenite region. The method of Claim 3 in which the slab is hot rolled with a finishing temperature in the austenite region. The method of Claim 3 in which the slab is hot rolled with a finishing temperature in the ferrite region. In a method of making electrical steel strip characterized by low core loss and high permeability comprising the steps of: producing a slab having a composition consisting of (% by weight): C up to 0.01 Si 0.20 - 1.35 Al 0.10 - 0.45 Mn 0.10 - 1.0 S up to 0.015 N up to 0.006 Sb up to 0.07 Sn up to 0.12 balance iron and unavoidable impurities hot rolling the slab into a strip with a finishing temperature in the ferrite region; coiling the strip at a temperature less than 1200°F (649°C) to retain the cold worked ferritic grain structure; pickling and pickle band annealing the strip at a temperature in the range of from 1350° - 1600°F (732° - 871°C); cold rolling the strip; batch annealing the strip at a temperature in the range of from 1100°-1350°F (593°-732°C), temper rolling the strip with a reduction in thickness no greater than 0.5%; and stress relief annealing the strip. The method of Claim 7 wherein the slab composition has a carbon content no greater than 0.005%. In a method of making electrical steel strip characterized by low core loss and high permeability comprising the steps of: producing a slab having a composition consisting of (% by weight): C up to 0.01 Si 0.20 - 1.35 Al 0.10 - 0.45 Mn 0.10 - 1.0 S up to 0.015 N up to 0.006 Sb up to 0.07 Sn up to 0.12 balance iron and unavoidable impurities hot rolling the strip with a finishing temperature in the austenite region, followed by coiling, pickling, annealing and cold rolling the strip, batch annealing the strip at a temperature in the range of from 1100°-1350°F (593°-732°C), and temper rolling the strip with a reduction in thickness no greater than 0.5%. In the method of Claim 9 wherein the slab composition has a carbon content no greater than 0.005. The method of Claim 9 or Claim 10 including the step of stress relief annealing the strip after temper rolling.

Inventors: BEATTY GERALD F, BUTLER JOHN F, LARSON ANN M R, LAUER BARRY A, BEATTY, GERALD F., BUTLER, JOHN F., LARSON, ANN M.R., LAUER, BARRY A.
Applicants: LTV STEEL CO INC, LTV STEEL COMPANY, INC.
Application year: 1995
Main classification: C21D8
IPC codes: C21D8