Method for making non-oriented magnetic steel sheet

Abstract: This invention is directed to a method of producing a non-oriented magnetic steel sheet involving a series of processes including performing hot rolling process on a steel slab containing no more than about 0.01 wt% C, no more than about 4.0 wt% Si, no more than about 1.5 wt% Mn, no more than about 1.5 wt% Al, no more than about 0.2 wt% P, and no more than about 0.01 wt% S, performing thereto at least one cold rolling process including an optional intermediate annealing process, and then performing a finishing annealing process. The hot rolling process further includes a step which reduces thermal irregularity formed during slab heating; this step involves maintaining a sheet bar, obtained by rough-rolling of the steel slab, at a temperature ranging from about 850° to 1,150°C. The hot rolling process also includes a step which promotes the growth of fine precipitated particles by applying strain to the sheet bar. Magnetic steel sheet thusly obtained possess uniform magnetic properties and thickness in the coil.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for making a non-oriented magnetic steel sheet having uniform magnetic characteristics and sheet shape in the coil product. Description of the Related Art Non-oriented magnetic steel sheets have been used in motors, dynamo-electric generators, and cores of transformers. Low core loss and high magnetic flux density are important magnetic properties required of non-oriented magnetic steel sheets, as these properties enhance the energy characteristics of the above-described devices. A demand for less irregularity in motor characteristics has coincided with the recent development of motors which are highly controllable through integrated circuits. Thus, non-oriented magnetic steel sheets which possess uniform magnetic characteristics and sheet shape, especially sheet thickness in the coil product of a non-oriented magnetic steel sheet, are in great demand as motor core materials. As a prior art method of producing uniform sheet thickness in the coil product, Japanese Patent Publication No. 57-60408 discloses a method which involves maintaining the finishing temperature of the hot rolling process within the α-phase temperature range. Furthermore, Japanese Patent Laid-Open No. 5-140649 discloses a steel containing extremely low quantities of N and S as a method of producing uniform sheet thickness in the coil product. From Fr-A-2 643 387 there is known a method of making non-oriented magnetic steel strips by hot direct rolling. In said conventional method the amounts of AIN and MnS which precipitate on the way of the direct rolling are decreased to such a level that they do not affect the magnetic properties by regulating the aluminum and sulphur amounts. Furthermore, unavoidable precipitating nitrides are precipitated coarsely as BN. For preventing non-uniform recrystallization after coiling, the upper limit of the coiling temperature is lower than 650°C. However, these prior art techniques cannot produce the uniformity presently demanded, thus there remains a great need for marked improvement. SUMMARY OF THE INVENTION It is an object of this invention to provide a method of producing a non-oriented magnetic steel sheet having uniform magnetic properties and uniform thickness in the coil product. The above object is achieved by the subject matter of claim 1. Preferred embodiments are defined in the subclaims. Hence, the present invention teaches a method for producing a non-oriented magnetic steel sheet which includes hot rolling a steel slab containing no more than about 0.01 wt% C, no more than about 4.0 wt% Si, no more than about 1.5 wt% Mn, no more than about 1.5 wt% Al, no more than about 0.2 wt% P, and no more than about 0.01 wt% S, performing at least one cold-rolling process including an optional intermediate annealing process, and then performing the finishing annealing process. The hot-rolling process further includes the steps of: coiling a sheet bar, obtained by rough-rolling the steel slab, into a coil having an inside diameter of at least about 100 mm and an outside diameter of no more than about 3,600 mm at a temperature ranging from about 850 to 1,150°C; uncoiling the coil; and performing a finishing hot rolling. According to the invention, the coiling of the sheet bar is preferably performed at a temperature T (°C) satisfying the following equation (1): 900.31 - 2.0183T + 1.4139×10 -3 T 2 - 3.0648×10 -7 T 3 - 326.7[Cwt%] + 11.8[Siwt%] -12.2[Mnwt%] +39.7[Pwt%] + 22.8[Alwt%] > 0 Furthermore, a light rolling step involving about a 3 to 15% rolling reduction is preferably performed after the finishing annealing process in order to improve the magnetic properties. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph illustrating the effect of sheet bar coiling on core loss; Fig. 2A and 2B are diagrams illustrating the effect of coil shape on magnetic properties; Fig. 3A and 3B are graphs showing the correlation between the α-phase stability index, G, and magnetic properties; and Fig. 4 is a graph showing the correlation between the α-phase stability index, G, and the α-phase fraction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of the experiments which led to the discovery of the present invention will be explained in detail below. Two steel slabs obtained by a continuous casting process and containing 0.003 wt% C, 0.4 wt% Si, 0.2 wt% Mn, 0.25 wt% Al, 0.05 wt% P, 0.005 wt% S, and the balance substantially Fe were heated to 1,150°C and roughly rolled so as to form sheet bars 30 mm thick. One of the sheet bars was immediately processed into a hot-rolled sheet by a finishing hot rolling. Another sheet bar was wound at 970°C into a coil having an inside diameter of 500 mm and an outside diameter of 1,400 mm, unwound and finish hot-rolled to form another hot-rolled sheet. The final temperature during the finish hot rolling of each sample was 840°C. Each hot-rolled sheet was cold-rolled to a thickness of 0.5 mm, and continuously annealed at 770°C for 30 seconds, then the thickness and magnetic properties in the longitudinal direction of each coil were measured. The evaluations of the magnetic properties and coil thickness were carried out at 30 m intervals on each coil product length, and the final results were determined by arithmetic average (X) and standard deviation σ as defined by the following equations (2) and (3): ( X ) = Σ X 1 n σ = Σ{ X 1 - ( X )} 2 n where X i represents a core loss W 15/50 measurement or a thickness measurement, and n represents the number points on the coil from which the measurements were taken (n = 133 in the experiments). In Fig. 1, blackened circles represent the results obtained from the conventionally-produced coil, i.e., the coil produced without winding (coiling) the sheet bar. Fig. 1 reveals that the core loss of the conventionally-produced coil significantly fluctuates at different positions on the coil. It was discovered that the positions on the coil which exhibited poor core loss corresponded to the positions between skids which were heated to a high temperature during the slab heating (a skid is a member supporting the slab in the slab heating furnace, and is usually cooled by water). Because non-homogeneous precipitated particles which worsen core loss values (i.e. increase core loss) are readily formed at higher slab heating temperatures, more non-homogeneous precipitated particles will be produced between skids (i.e., high temperature slab sections) during slab heating than at skid contact sections (i.e., low temperature slab sections) during the slab heating. Therefore, core loss values between skids are worse (higher) than core loss values at each skid contact section. The empty circles in Fig. 1 represent the results obtained from the coil produced with sheet bar coiling. Fig. 1 shows that there is less core loss fluctuation in the coil produced with sheet bar coiling as compared with the coil produced conventionally, i.e., without sheet bar coiling. The results of the magnetic property and thickness evaluations are shown in Table 1. The process of winding the sheet bar after rough-rolling minimized standard deviations of the magnetic properties and thickness. Further, excellent average magnetic properties were achieved as compared with the conventional process in which the sheet bar was rolled immediately after the rough-rolling. The thickness fluctuations in the coil produced by the conventional process (without sheet bar coiling) is due to the variable resistance to deformation across the hot-rolled sheet during finishing rolling. This variable resistance results from the temperature difference during slab heating between the skid section and the intermediate section between skids. Magnetic Induction B 50 (T) Core Loss W 15/50 (W/kg) Sheet Thickness (mm) Number of Measuring Points (X) σ (X) σ (X) σ n Without Sheet Bar Coiling 1.751 0.004 5.706 0.122 0.50 0.003 133 With Sheet Bar Coiling 1.762 0.001 5.315 0.031 0.50 0.001 133 Fig. 1 and Table 1 clearly demonstrate that magnetic properties are improved and that both magnetic properties and thickness become uniform in a coil by winding the sheet bar after rough-rolling. Possible mechanisms behind these improvements are as follows: (1) temperature fluctuation within the sheet bar during slab heating can be reduced by winding the sheet bar; and/or (2) strain caused by sheet bar coiling can promote the growth of fine precipitated particles. We conducted several investigations regarding the shape of the sheet bar. Fig. 2 shows the effects of the inside and outside diameter of the coil on magnetic properties. An outside diameter over about 3,600 mm causes an increased core loss average and a greater core loss standard deviation within a coil. Please refer to Fig. 2A and 2B, respectively. A larger outside diameter promotes non-uniform temperature and results in less strain being incorporated into the sheet bar during winding, thus precipitated particle growth may be hindered. Therefore, the outside diameter of the coil should not be over about 3,600 mm in order to promote uniform temperature and increase the strain from winding. On the other hand, an inside diameter of less than about 100 mm causes some surface defects in the form of cracks on the sheet bar. Consequently, the inside diameter of the coil should be about 100 mm or more. The results of our investigation into the effects of steel composition and sheet bar coiling temperature on the magnetic properties will be detailed below. Three steels, A, B and C, having the compositions shown in Table 2 were melted in a converter and vacuum degassing device, and slabs were prepared by a continuous casting process. The slabs were again heated, then rough-rolled to form sheet bars 40 mm thick. After coiling the sheet bars at various temperatures, a finishing hot rolling was performed on each sample. For the comparison, some sheet bars were hot-rolled without sheet bar coiling. The thickness of the each hot-rolled sheet after the finishing hot rolling was 2.0 mm. Then, the hot-rolled sheet was annealed at 900°C for 1 minute, cold-rolled to be 0.5 mm thick. Thereafter, continuous finishing annealing was performed at 800°C for 30 seconds, and an insulating coating treatment was performed to form the sheet product. The magnetic properties of test pieces cut from the plate product were evaluated through an Epstein test. Steel Composition (wt%) Sheet Bar Coiling Temperature (°C) C Si Mn P Al A 0.003 0.5 0.25 0.08 0.25 908 950 985 1020 1050 Without coiling B 0.003 0.25 0.25 0.08 0.5 910 985 1040 1050 1080 Without coiling C 0.003 0.4 0.45 0.08 0.25 900 920 980 1000 1080 Without coiling The results are plotted in Figs. 3A and 3B. Fig. 3A illustrates the correlation between α-phase stabilizing coefficient G (calculated from the sheet bar coiling temperature, see below) and average coil core loss, while Fig. 3B shows the correlation between the α-phase stabilizing coefficient G and the core loss standard deviation of a coil. The α-phase stabilizing coefficient G represents an index reflecting the stability of α-phase at a measured temperature. At a given temperature T (°C), G is expressed through the following equation (1): (1) G = 900.31 - 2.0183T + 1.4139×10 -3 T 2 - 3.0648×10 -7 T 3 - 326.7[C wt%] + 11.8[Si wt%] -12.2[Mn wt%] +39.7[P wt%] + 22.8[Al wt%] > 0 As shown Fig. 4 (discussed in detail below), G correlates well with α-phase fraction. Specifically, the α-phase fraction increases as G increases beyond 0, reflecting the stabilization of the α-phase. On the other hand, Fig. 3 shows the significant improvement in the average core loss, W 15/50 , and the core loss standard deviation σ on a coil after sheet bar coiling at a temperature satisfying G > 0 in equation (1). The reason for these improvements can be explained as follows. Fine precipitated particles which are formed during rough-rolling and improve core loss values can grow by means of the sheet bar coiling. With sheet bar coiling, the diffusion rate of the α-phase is about 10 times faster than that of the γ-phase, and the diffusion is a rate-determining stage in the growth of the fine precipitated particles. Thus, higher a α-phase fraction in a sheet bar coil promotes fine precipitated particle growth, increases the improvement of in core loss values, and reduces the standard deviation among core loss values within a coil. Accordingly, by controlling steel composition and coiling temperature so as to satisfy G > 0, a non-oriented magnetic steel having uniform core loss throughout the coil can be produced. The steel composition of the invention and a process illustrating the invention will now be explained in detail. C content should be not more than about 0.01 wt%. When the C content exceeds about 0.01 wt%, magnetic properties deteriorate due to C precipitation. The lower C content limit should be about 0.0001 wt% in view of economic feasibility. Si content should be not more than about 4.0 wt%. Although Si is a useful component for increasing specific resistance and decreasing core loss, an Si content over about 4.0 wt% causes poor formability during cold rolling. The lower limit is preferably set to about 0.05 wt% to ensure satisfactory specific resistance. Mn content should be not more than about 1.5 wt%. Although Mn is a useful component for increasing specific resistance and decreasing core loss, costs become prohibitively high when Mn content exceeds about 1.5 wt%. On the other hand, Mn can fix S as MnS, S being otherwise harmful to magnetic properties. Therefore, the lower limit of Mn is preferably set to about 0.1 wt% to ensure satisfactory magnetic properties. Al content should be not more than about 1.5 wt%. Although Al is a useful component for increasing specific resistance and decreasing core loss, an Al content over about 1.5 wt% causes poor formability during cold rolling. P content should be not more than about 0.2 wt%. Although P can be added to improve blanking ability, a P content over about 0.2 wt% causes poor formability during cold rolling. The lower P content limit should be about 0.0001 wt% in view of economic feasibility. S content should be not more than about 0.01 wt%. Because S forms MnS finely precipitated particles which hinder transfer of the magnetic domain walls and the growth of fine precipitated particles from the application of strain to the sheet bar, S content should be as small as possible. Any known additives, such as Sb, Sn, Bi, Ge, B, Ca, and rare earth metals, can be added to the steel to improve magnetic properties. The content of each additive is suitably not more than about 0.2 wt% in view of economic feasibility. A sheet bar is formed from a slab having the above composition by directly rough-rolling the slab or after re-heating the slab. The sheet bar is wound into a coil having an inside diameter not less than about 100 mm and outside diameter not more than about 3,600 mm. The winding is conducted within a temperature range of about 850 to 1,150°C. When the sheet bar temperature exceeds about 1,150°C, fine precipitated particle content increases during finishing hot rolling such that decreased uniformity in core loss within a coil and between coils results. On the other hand, a sheet bar coiling temperature less than about 850°C is not effective due to prolonged time required to cancel non-homogeneous precipitated particles and textures. A coiled sheet bar having an inside diameter of less than about 100 mm tends to form cracks or defects on the surface due to the larger curvature. A coiled sheet bar having an outside diameter of over about 3,600 mm exhibits poor temperature uniformity and experiences less strain during the coiling process, thereby inhibiting uniformity in magnetic properties and thickness. By coiling the sheet bar under the above conditions, uniform core loss and thickness can be attained in a coiled, non-oriented magnetic steel sheet. In addition, by controlling the sheet bar coiling temperature so that the α-phase stability index G satisfies G > 0, the average core loss as well as core loss uniformity will further improve. Thus, the sheet bar is preferably wound at a temperature satisfying G>0. The sheet bar coiling temperature represents the sheet bar average temperature during coiling, and remains substantially unchanged during coiling and uncoiling in general. However, when the average sheet bar temperature decreases during an extended coiling time, at least one average temperature during coiling or uncoiling should satisfy G>0. The coiled sheet bar is then unwound and hot-rolled for finishing to make hot-rolled sheet. Any self-annealing or hot-rolled sheet annealing may be incorporated as the need arises. The hot-rolled sheet annealing may be accomplished by either batch annealing (box annealing) or continuous annealing. Thereafter, a sheet having a predetermined thickness, for example 0.5 mm, is obtained by one or more cold rolling steps, and may include optional intermediate annealing steps. Subsequently, finishing annealing is performed to form the final product. Any insulating coating process may be performed after the finishing annealing. A continuous annealing may be preferably used for the finishing annealing in view of productivity and economics. Furthermore, a light-rolling process involving a rolling reduction of about 3 to 15% may be performed after the finishing annealing or the insulating coating process. A rolling reduction of less than about 3% or over about 15% diminishes the light-rolling effect of improving core loss values through the growth of coarse grains during the straightening annealing treatment. The invention will now be described through illustrative examples. The examples are not intended to limit the scope of the appended claims. EXAMPLE 1 After adjusting the steel composition in a converter and vacuum degassing device, slabs were prepared by continuous casting. When the slab temperature fell to 300°C, the slabs were reheated in a reheating furnace. Then, sheet bars 30 mm thick were obtained by rough-rolling the reheated slabs. After coiling the sheet bars, hot-rolled sheets were prepared from the sheet bar coil by finishing hot rolling. Some of the hot-rolled sheets were annealed. The hot-rolled sheets were then cold-rolled to a thickness of 0.5 mm, and continuous annealing was performed at 850°C for 30 seconds. The magnetic properties in the longitudinal direction and thickness of the coil products were measured. The length of the coil product was 4,000 m, and a measurement of the magnetic properties was carried out every 30 m on the coils. Table 3 shows the results of the magnetic property evaluations and thickness measurements, in addition to slab composition and the conditions under which hot rolling and sheet bar coiling were conducted. Table 3 reveals that examples where sheet bar coiling was performed after rough-rolling have superior (smaller) standard deviations of the magnetic properties and thickness, and superior (larger) average magnetic property values compared to those comparative examples conventionally produced in that finishing hot rolling was carried out immediately after rough-rolling. Among the Examples of the Invention, sample Nos. 1, 2, 8, 9, 13 and 14 satisfying G>0 exhibit excellent properties. Nos. 3 and 16, having a coiled sheet bar outside diameter over about 3,600 mm, failed to produce adequate sheet bar coiling effects. Nos. 4 and 12, having coiled sheet bar inside diameters under about 100. mm, formed many surface defects on the produced sheet. Furthermore, in No. 6, where the sheet bar coiling temperature was less than about 850°C, large deviations in the magnetic properties remained. Similarly, in No. 17, treated at a sheet bar coiling temperature over about 1,150°C, the averages and deviations of the magnetic properties are inferior to No. 13, which had a sheet bar coiling temperature less than about 1,150°C. EXAMPLE 2 After adjusting the steel composition in a converter and vacuum degassing device, slabs were prepared by continuous casting. When the slab temperature fell to 850°C, the slabs were reheated in a reheating furnace. Then, sheet bars 30 mm thick were obtained by rough-rolling the reheated slabs. After coiling the sheet bars, hot-rolled sheets were prepared from the sheet bar coil by finishing hot rolling. Some of the hot-rolled sheets were annealed. The hot-rolled sheets were then cold-rolled, and continuous annealing was performed at 770°C for 30 seconds, and thereafter a 5% light rolling was performed to obtain products 0.5 mm thick. Magnetic properties in the longitudinal direction and thickness of the coil products were measured. Table 4 shows the results of the magnetic property evaluations and thickness measurements, in addition to slab compositions and the conditions under which hot rolling and sheet bar coiling were conducted. Table 4 reveals that examples where sheet bar coiling was performed after rough-rolling have superior (smaller) standard deviations of the magnetic properties and thickness, and superior (larger) average magnetic property values compared to those comparative examples conventionally produced in that hot rolling finishing was carried out immediately after rough-rolling. Among the Examples of the Inventions, sample Nos. 18, 19, 25 and 30 satisfying G>0 exhibited excellent properties. Nos. 20 and 33, having a coiled sheet bar outside diameter over about 3,600 mm, failed to produce adequate sheet bar effects. Nos. 21 and 29, having coiled sheet bar diameters under about 100 mm, formed many surface defects on the produced sheet. Furthermore, in No. 23, where the sheet bar coiling temperature was less than about 850°C, large deviations in the magnetic properties remained. Similarly, in No. 34, treated at a sheet bar coiling temperature over about 1,150°C, the averages and deviations of the magnetic properties are inferior to No. 30, which had a sheet bar coiling temperature less than about 1,150°C. Although this invention has been described in connection with specific forms thereof, it will be appreciated that a wide variety of equivalents may be substituted for the specific elements described herein without departing from the scope of this invention defined in the appended claims.

Claims: A method of producing a non-oriented magnetic steel sheet, including the steps of: producing a steel slab having a steel slab composition; hot rolling said steel slab to form a sheet bar thereby producing a hot-rolled steel sheet; cold rolling said hot-rolled steel sheet at least once to form a cold-rolled steel sheet; and finish annealing said cold-rolled steel sheet to form said non-oriented magnetic steel sheet; wherein the chemical composition of said steel slab is controlled to comprise: not more than 0.01 wt.%C, not more than 4.0 wt.% Si, not more than 1.5 wt.% Mn, not more than 1.5 wt.% Al, not more than 0.2 wt.% P, and not more than 0.01 wt.% S; optionally not more than 0.2 wt.% each of Sb, Sn, Bi, Ge, B, Ca and rare earth metals, the balance being Fe apart from inevitable impurities; said hot rolling including rough-rolling said steel slab to form a sheet bar, coiling said sheet bar into a coil while said sheet bar is at a temperature ranging from 850° to 1,150°C, said coil having an inside diameter of not less than 100 mm and an outside diameter of not greater than 3,600 mm, uncoiling said coil to form an uncoiled sheet bar, and finish hot rolling said uncoiled sheet bar to hot rolled steel sheet. A method for producing a non-oriented magnetic steel sheet according to claim 1, wherein said coiling of said sheet bar is performed at a temperature T (°C) which satisfies the following relationship: 900.31 - 2.0183T + 1.4139 × 10 -3 T 2 - 3.0648 × 10 -7 T 3 - 326.7 [C wt%] + 11.8 [Si wt%] - 12.2 [Mn wt%] + 39.7 [P wt%] + 22.8 [Al wt%] > 0. A method for producing a non-oriented magnetic steel sheet according to claim 1 or 2, wherein a light rolling involving a rolling reduction of about 3 to 15% is performed after said finish annealing.

Inventors: OGINO ATSUSHI, SATO KEIJI, TAKASHIMA MINORU, OGINO, ATSUSHI, SATO, KEIJI, TAKASHIMA, MINORU, Ogino, Atsushi, c/o Chiba Works, Sato, Keiji, c/o Iron &amp; Steel Research Lab., Takashima, Minoru, c/o Iron &amp; Steel Research Lab.
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1995
Main classification: C21D8
IPC codes: B21B3, C22C38, C21D9, C21D8, B21C47