Manufacturing method of grain-oriented electrical steel sheets

Abstract

A method for producing a grain-oriented electromagnetic steel sheet, the method preventing a break during cold rolling when a grain-oriented electromagnetic steel sheet is produced by subjecting a steel slab, which contains, in mass%, 0.03% to 0.08% of C, 2.0% to 5.0% of Si, 0.005% to 1.0% of Mn, less than 0.010% of Al, 0.006% or less of N and 0.0060% or less of O, while containing S and Se within the range where (S + 0.405 × Se) is 0.0015% to 0.0060%, to hot rolling, hot rolled sheet annealing, cold rolling, decarburization annealing that doubles as primary recrystallization annealing, and subsequent finish annealing, by performing at least two consecutive passes of rolling within the temperature range of 1050°C to 1150°C during the hot rolling, and setting the inter-pass time between the two passes to 60 s or less, the draft of each pass to 20% or more, and the strain rate to 15 s-1 or more; and a hot rolled sheet which is used for this production method.

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing grain-oriented electrical steel sheets that have excellent cold-rolling properties and magnetic properties, and to hot-rolled steel sheets used in the above manufacturing method. In this invention, the above hot-rolled steel sheets are also referred to as "hot-rolled sheets." [Background technology]

[0002] Grain-oriented electrical steel sheets are generally treated with precipitates called inhibitors in the final finish annealing process to maintain the Goss orientation ({110}). <001> It is manufactured by a method that preferentially recrystallizes the ) grains. As the above inhibitor, for example, AlN and MnS are proposed in Patent Document 1, and MnS and MnSe are proposed in Patent Document 2, both of which have been put into practical use industrially.

[0003] While methods using these inhibitors are useful for stably developing secondary recrystallized grains with Goss orientation, they require the slab to be heated to a high temperature of 1300°C or higher before hot rolling, as the inhibitor precipitates must be finely dispersed. Therefore, this method has problems such as increased scale loss and reduced yield, as well as increased thermal energy costs, equipment costs, and complicated equipment maintenance.

[0004] On the other hand, as a technology to solve the above problems, a manufacturing method that does not use inhibitors (inhibitor-less method) has also been proposed. For example, Patent Document 3 proposes a technology that uses a higher-purity steel material that does not contain inhibitor-forming components and induces secondary recrystallization by controlling the texture (structure). However, when manufacturing grain-oriented electrical steel sheets using a steel material that does not contain inhibitor-forming components, the grain boundary strength decreases due to its high purity, and fracture is more likely to occur during cold rolling, which has been one of the factors hindering industrial-scale production.

[0005] To address this problem, Patent Document 4 proposes a technique that controls the recrystallization process after hot rolling, thereby achieving both further stabilization of secondary recrystallization and suppression of fracture during cold rolling. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 40-015644 [Patent Document 2] Special Publication No. 51-013469 [Patent Document 3] Japanese Patent Publication No. 2000-129356 [Patent Document 4] Japanese Patent Publication No. 2003-226916 [Overview of the project] [Problems that the invention aims to solve]

[0007] The technology disclosed in Patent Document 4 above controls the thermal history during hot rolling and hot-rolled sheet annealing to specific conditions, thereby making the crystalline structure after hot-rolled sheet annealing more uniform, and thereby stabilizing secondary recrystallization and suppressing fracture during cold rolling. However, hot-rolled sheets for grain-oriented electrical steel sheets that do not contain inhibitor-forming components are inherently prone to fracture during cold rolling, so the above technology alone is not sufficient to completely prevent fracture during cold rolling.

[0008] This invention has been made in view of the above-mentioned problems of the prior art, and its purpose is to propose a method for stably manufacturing grain-oriented electrical steel sheets with excellent magnetic properties without fracture during cold rolling, even when using steel materials that do not contain inhibitor-forming components, and to provide a hot-rolled sheet to be used in the manufacturing method. [Means for solving the problem]

[0009] To solve the above problems, the inventors diligently investigated the reasons why hot-rolled sheets that do not contain inhibitor-forming components are prone to fracture during cold rolling. As a result, they found that the cause of fracture during cold rolling is not only the decrease in grain boundary strength due to high purity, but also the significant involvement of sulfides and selenides present in the center (segregated portion) of the steel sheet thickness. They also found that in order to prevent fracture during cold rolling caused by the above sulfides and selenides, it is necessary to optimize the hot-rolling pass schedule and hot-rolled sheet annealing conditions, and to further improve the uniformity of the crystal structure after hot-rolled sheet annealing. Based on these findings, they developed the present invention.

[0010] Based on the above findings, the present invention relates to a steel slab having a composition containing C: 0.03~0.08 mass%, Si: 2.0~5.0 mass%, Mn: 0.005~1.0 mass%, Al: less than 0.010 mass%, N: 0.006 mass% or less, and O: 0.0060 mass% or less, and containing S and Se in the range of (S + 0.405 × Se) 0.0015~0.0060 mass%, with the remainder being Fe and unavoidable impurities. This steel slab is rolled in at least two consecutive passes at a temperature range of 1050~1150°C, with an inter-pass time of 60 s or less, a reduction ratio of 20% or more in each pass, and a strain rate of 15 s -1 We propose a method for manufacturing grain-oriented electrical steel sheets, which involves hot rolling to obtain a hot-rolled sheet, hot-rolled sheet annealing, cold rolling once or two or more times with intermediate annealing in between to obtain a cold-rolled sheet, decarburization annealing which also serves as primary recrystallization annealing, and then finish annealing.

[0011] The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet, characterized in that the hot rolling is performed in a temperature range of 850 to 950°C for at least one pass, the hot-rolled sheet annealing after hot rolling is performed at a temperature of 850°C or higher, and the sum of the cooling time from 750°C to 650°C during the cooling process after hot rolling and the heating time from 600°C to 700°C during the heating process of the hot-rolled sheet annealing is 20 seconds or less.

[0012] Furthermore, the present invention's method for manufacturing the grain-oriented electrical steel sheet is characterized by holding the steel sheet in its original state for 1.0 s or more after at least one pass of rolling in the temperature range of 850 to 950°C.

[0013] Furthermore, the steel slab in the method for manufacturing the grain-oriented electrical steel sheet of the present invention is characterized in that, in addition to the above component composition, it further contains at least one component selected from groups A to C below. Note ·Group A; Ni: 1.50mass% or less Group B: At least one selected from Sn: 0.50 mass% or less, Sb: 0.50 mass% or less, Cu: 0.50 mass% or less, Mo: 0.50 mass% or less, Co: 0.0100 mass% or less, P: 0.50 mass% or less, Cr: 1.50 mass% or less, B: 0.0200 mass% or less, and Bi: 0.0200 mass% or less. • C group; at least one selected from Nb: 0.0200 mass% or less, Ti: 0.0200 mass% or less, Te: 0.0200 mass% or less, Ga: 0.0100 mass% or less, and Zn: 0.500 mass% or less.

[0014] Furthermore, the present invention has a component composition containing C: 0.03~0.08 mass%, Si: 2.0~5.0 mass%, Mn: 0.005~1.0 mass%, Al: less than 0.010 mass%, N: 0.006 mass% or less, and O: 0.0060 mass% or less, and containing S and Se in the range of (S + 0.405 × Se) 0.0015~0.0060 mass%, with the remainder being Fe and unavoidable impurities, and has a region of 50 mm in width of 1 / 5 of the plate thickness, including the center of the plate thickness cross-section along the rolling direction. 2 The number density of precipitates containing sulfides, selenides, and their compounds with a major axis of 4 μm or larger is 0.30 particles / mm³. 2 The following is a hot-rolled sheet for grain-oriented electrical steel.

[0015] The hot-rolled sheet of the present invention is characterized in that, in addition to the above component composition, it further contains at least one component selected from groups A to C below. Record ·Group A; Ni: 1.50 mass% or less ·Group B; at least one selected from Sn: 0.50 mass% or less, Sb: 0.50 mass% or less, Cu: 0.50 mass% or less, Mo: 0.50 mass% or less, Co: 0.0100 mass% or less, P: 0.50 mass% or less, Cr: 1.50 mass% or less, B: 0.0200 mass% or less, and Bi: 0.0200 mass% or less ·Group C; at least one selected from Nb: 0.0200 mass% or less, Ti: 0.0200 mass% or less, Te: 0.0200 mass% or less, Ga: 0.0100 mass% or less, and Zn: 0.500 mass% or less

Advantages of the Invention

[0016] According to the present invention, since the cold rolling property of a hot rolled sheet not containing an inhibitor forming component can be improved, it is possible to stably manufacture a grain-oriented electromagnetic steel sheet having good magnetic properties without causing breakage during cold rolling.

Brief Description of the Drawings

[0017] [Figure 1] It is a graph comparing the breakage occurrence rate in cold rolling when the technology of Patent Document 4 is applied with the breakage occurrence rate of the prior art. [Figure 2] It is a graph comparing the breakage occurrence rate in cold rolling when the technology of the present invention is applied with the breakage occurrence rates when the technologies of the prior art and Patent Document 4 are applied.

Modes for Carrying Out the Invention

[0018] First, the investigation that became the契机 for developing the present invention will be described. The inventors conducted a detailed investigation on coils that still broke during cold rolling even when the technology of Patent Document 4 was applied. As a result, in the portion where the steel sheet broke, a portion where the steel sheet was separated into two sheets with the front and back surfaces separated was observed.

[0019] Elemental mapping analysis of this separation surface using EPMA revealed precipitates consisting of multiple sulfides and selenides. Therefore, the relationship between the fracture rate during cold rolling and (S + 0.405 × Se), a parameter representing the sum of the number of S and Se atoms, was investigated. As shown in Figure 1, it was found that the fracture rate tended to increase when the S and Se content (S + 0.405 × Se) exceeded 0.0015 mass%, and this tendency became more pronounced above 0.0025 mass%. This trend was completely unknown to the public and is a novel discovery in this invention.

[0020] Based on these findings, the following fracture mechanism was hypothesized. Conventional methods for manufacturing grain-oriented electrical steel sheets without using inhibitors, such as the method described in Patent Document 4, use a highly purified steel material with reduced S and Se content to 0.0050 mass% or less, and induce secondary recrystallization by appropriately controlling the texture of the hot-rolled sheet. Therefore, it is generally believed that the lower the S and Se content, the more stably secondary recrystallization occurs, and in Patent Document 4, the S and Se content is reduced to approximately 0 to 0.0020 mass% each.

[0021] S (S) is generally removed during the steelmaking refining process by being incorporated into the slag through reduction treatment. However, since many elements, including C (Carbon), are removed through oxidation treatment, S, once removed, can return from the slag to the steel during oxidation treatment, a process known as "resulfurization." Therefore, stably reducing S requires the addition of large amounts of auxiliary materials, which contributes to increased manufacturing costs. On the other hand, unlike S, Se (Se) is often brought in from scrap used to reduce costs rather than from the raw materials themselves, but its removal presents similar difficulties to S. For this reason, steel materials containing S and Se within a range that does not cause problems with secondary recrystallization are used in the manufacture of grain-oriented electrical steel sheets.

[0022] In the manufacturing method of grain-oriented electrical steel sheets using inhibitors, it is necessary to completely dissolve the inhibitor-forming components contained in the steel material (slab) into the steel. Therefore, prior to hot rolling, the slab is heated to a high temperature of 1350-1450°C. However, when inhibitors are not used, complete dissolution of the inhibitor-forming components as described above is unnecessary, so the slab heating temperature is set to a relatively low temperature of around 1250°C or less, which does not adversely affect hot rolling.

[0023] Furthermore, S and Se are known to be elements that readily cause segregation. For example, when slabs are manufactured using a continuous casting method, they can segregate and become highly concentrated in the center of the slab's thickness where it solidifies, and can also form precipitates such as sulfides and selenides. The above-mentioned central segregation and precipitates are reduced by dissolution and diffusion through slab heating prior to hot rolling. However, if the S and Se content is high, or if the slab heating temperature is low, sulfides and selenides may remain as precipitates in the center of the slab's thickness.

[0024] In the areas where fracture occurred during cold rolling, a section resembling a two-layer delamination was observed in the center of the sheet thickness, and precipitates of sulfides and selenides were observed on the delamination surface. From this, it is thought that the sulfides and selenides remaining in the center of the sheet thickness acted as the starting point for delamination, leading to fracture.

[0025] Therefore, to prevent fracture during cold rolling, the slab should be heated before hot rolling so that the inhibitor-forming components are completely dissolved. However, the advantage of manufacturing grain-oriented electrical steel sheets using steel materials without inhibitors lies in reducing manufacturing costs by lowering the slab heating temperature, and this advantage cannot be enjoyed if the slab is heated to a high temperature. Furthermore, if high-temperature slab heating is to be performed, then the conventional method using inhibitors should be applied.

[0026] Therefore, the inventors investigated a method to neutralize sulfides and selenides present as precipitates near the center of the slab thickness due to central segregation, not by completely dissolving them through high-temperature heating of the slab, but by destroying the precipitates during hot rolling. As a result, they found that this can be achieved by performing at least two consecutive passes of high-pressure, high-strain rolling under appropriate conditions in a specific temperature range of hot rolling.

[0027] Furthermore, while it is generally known that the interface between sulfides and selenides and the base metal is a crack initiation point, we found that not all sulfide and selenide precipitates adversely affect cold rolling. Rather, large precipitates, specifically precipitates with a major axis of 4 μm or more that are stretched in the rolling direction by hot rolling, particularly adversely affect fracture. We believe this is because the deformability of the precipitates and the base metal differs, and when the steel stretches during rolling, shear force is generated at the large interface, causing fracture. Therefore, to prevent fracture during cold rolling, a 50 mm area with a width of 1 / 5 of the plate thickness, including the center of the plate thickness cross-section along the rolling direction, is necessary. 2 The number density of precipitates such as sulfides and selenides with a major axis of 4 μm or larger is 0.30 particles / mm³. 2 We also found that the following needs to be reduced. This invention is the result of further improvements to the above findings.

[0028] Next, the component composition of the steel material used in the manufacturing method of the grain-oriented electrical steel sheet of the present invention will be described. C:0.03~0.08mass% This invention is characterized by the fact that large sulfide and selenide precipitates present in the center of a slab are fractured to a size of less than 4 μm in the major axis, without fracture during cold rolling, by hot rolling at least two consecutive passes under appropriate conditions in a specific temperature range (1050~1150°C). Normally, hot rolling of Si-containing steel is performed in a temperature range where almost a single layer of ferrite is formed. However, this temperature range coincides with the temperature range where austenite is formed, albeit in small amounts, and for reasons described later, sulfide and selenide precipitates become easily fragmented and fractured. Therefore, in order to enable hot rolling in the above two-phase range, carbon content of 0.03 mass% or more is included. On the other hand, if carbon content exceeds 0.08 mass%, it becomes difficult to reduce it to 0.0050 mass% or less, where magnetic aging does not occur, even after decarburization annealing. Therefore, carbon content is set in the range of 0.03~0.08 mass%. Preferably, the range is 0.03 to 0.06 mass%.

[0029] Si: 2.0~5.0 mass% Si is an effective element for increasing the resistivity of steel and reducing iron loss, and to obtain this effect fully, it is necessary to include at least 2.0 mass%. On the other hand, if it exceeds 5.0 mass%, the steel becomes brittle, significantly degrading its cold-rolling properties and increasing the risk of fracture when passing through manufacturing equipment, so the upper limit should be 5.0 mass%. Preferably, it is in the range of 2.8 to 4.5 mass%.

[0030] Mn: 0.005~1.0mass% Mn is a useful element because it improves hot workability and also controls the oxide film formed during primary recrystallization; therefore, it should be included in a concentration of 0.005 mass% or more. On the other hand, if it exceeds 1.0 mass%, the texture of the primary recrystallization deteriorates, leading to a degradation of magnetic properties; therefore, the upper limit should be 1.0 mass%. Preferably, the concentration is in the range of 0.01 to 0.5 mass%.

[0031] Al: less than 0.010 mass%, N: 0.006 mass% or less This invention relates to a technology for manufacturing grain-oriented electrical steel sheets using steel materials that do not contain inhibitor-forming components. Therefore, Al is limited to less than 0.010 mass%, and N is limited to 0.006 mass% or less. If Al and N exceed the above values, it becomes difficult to obtain a good secondary recrystallized structure even with microstructure control. Furthermore, N must be 0.006 mass% or less from the viewpoint of preventing the formation of Si nitrides during the purification annealing of the finish annealing, which degrades the magnetic properties. Preferably, Al is 0.008 mass% or less, and N is 0.0050 mass% or less.

[0032] O:0.0060mass% or less Since oxygen forms oxides and degrades the magnetic properties of the final product plate, its content is limited to 0.0060 mass% or less. Preferably, it is 0.0030 mass% or less.

[0033] S+0.405×Se:0.0015~0.0060mass% S and Se are harmful elements in this invention because they form sulfides and selenides, causing fracture during cold rolling, and form inhibitors, degrading magnetic properties. Therefore, it is generally desirable to have lower levels of these elements. In particular, in order to stably induce secondary recrystallization without using inhibitors, S and Se need to be reduced to (S + 0.405 × Se) 0.0060 mass% or less. However, excessive reduction of S and Se does not adversely affect magnetic properties or fracture during cold rolling, but it leads to increased manufacturing costs. Therefore, the lower limit of (S + 0.405 × Se) is set to around 0.0015 mass%. Preferably, it is in the range of 0.0020 to 0.0050 mass%.

[0034] The steel material used in the manufacturing method of grain-oriented electrical steel sheets of the present invention consists of Fe and unavoidable impurities as the remainder of the above components. However, it may contain at least one component selected from the following components as needed.

[0035] Ni: 1.50mass% or less Ni is a useful element for improving the microstructure of hot-rolled steel sheets and enhancing their magnetic properties. To obtain this effect, it is preferable to add 0.005 mass% or more. On the other hand, excessive addition of Ni destabilizes secondary recrystallization and actually degrades the magnetic properties, so it is preferable to set the upper limit at 1.50 mass%. More preferably, it is in the range of 0.01 to 1.00 mass%.

[0036] At least one selected from Sn: 0.50 mass% or less, Sb: 0.50 mass% or less, Cu: 0.50 mass% or less, Mo: 0.50 mass% or less, Co: 0.0100 mass% or less, P: 0.50 mass% or less, Cr: 1.50 mass% or less, B: 0.0200 mass% or less, and Bi: 0.0200 mass% or less. Sn, Sb, Cu, Mo, Co, P, Cr, B, and Bi are grain boundary segregation elements and have the effect of improving magnetic properties. However, if the upper limit of the above range is exceeded, the development of secondary recrystallized grains will be inhibited. Therefore, when adding these elements, it is preferable to keep the amount below the upper limit. In order to reliably obtain the above addition effect, it is preferable to add Sn: 0.01 mass% or more, Sb: 0.005 mass% or more, Cu: 0.01 mass% or more, Mo: 0.01 mass% or more, Co: 0.0001 mass% or more, P: 0.0050 mass% or more, Cr: 0.01 mass% or more, B: 0.0005 mass% or more, and Bi: 0.0005 mass% or more.

[0037] At least one selected from Nb: 0.0200 mass% or less, Ti: 0.0200 mass% or less, Te: 0.0200 mass% or less, Ga: 0.0100 mass% or less, and Zn: 0.500 mass% or less. Nb, Ti, Te, Ga, and Zn are elements that form carbonitride precipitates. While not strictly necessary in manufacturing methods that do not use inhibitors, adding trace amounts can improve magnetic properties, provided they are within the range where solid solution is possible even at relatively low slab heating temperatures. However, adding more than the above upper limit will destabilize secondary recrystallization. Furthermore, even within the above range, secondary recrystallization may be destabilized depending on the slab heating temperature and manufacturing conditions, so caution is required when using them. To reliably obtain the above additive effect, it is preferable to add Nb: 0.0005 mass% or more, Ti: 0.0005 mass% or more, Te: 0.0005 mass% or more, Ga: 0.0001 mass% or more, and Zn: 0.0001 mass% or more.

[0038] Next, the method for manufacturing grain-oriented electrical steel sheets according to the present invention will be described. The steel material (slab) used in the manufacture of the grain-oriented electrical steel sheet of the present invention is manufactured by melting steel adjusted to a component composition suitable for the present invention as described above, using a known refining method such as a converter or electric furnace, and further vacuum degassing treatment if necessary, and then by a commonly known continuous casting method. Alternatively, thin cast slabs with a thickness of 100 mm or less may be manufactured using a direct casting method.

[0039] Next, the slab is heated to a predetermined temperature before being subjected to hot rolling, but it may also be subjected to hot rolling immediately after continuous casting without heating. Here, it is preferable that the slab heating temperature before hot rolling be 1250°C or lower. This is because, in the component system of the present invention which does not contain inhibitor-forming components, there is no need to solid-solve inhibitor-forming components. However, if the slab heating temperature is too low, the load of hot rolling will increase, which may impair manufacturing stability. Furthermore, as will be described later, in the present invention it is essential to carry out hot rolling at 1050°C or higher in at least two consecutive passes, so it is preferable that the heating temperature be 1100°C or higher.

[0040] The slabs heated to the relatively low temperatures described above are then subjected to hot rolling, consisting of rough rolling and finish rolling, to produce hot-rolled steel sheets. However, at such low slab heating temperatures, precipitates of sulfides and selenides that segregated and precipitated in the center of the slab's thickness during slab manufacturing, or precipitates of these compounds, may not completely dissolve and remain, potentially degrading the cold-rollability.

[0041] Therefore, in order to break down and reduce the size of large sulfide and selenide precipitates that are harmful to cold rolling, the present invention provides the following for hot rolling: at least two consecutive passes of rolling are performed in a temperature range of 1050 to 1150°C, the inter-pass time between the two passes is 60 s or less, the reduction ratio of each pass in the two passes is 20% or more, and the strain rate is 15 s -1 It is necessary to do the above.

[0042] As mentioned above, in steel containing 0.03 to 0.08 mass% carbon, the temperature range of 1050 to 1150°C is the temperature range in which austenite is formed in ferrite. Austenite has higher deformation resistance than ferrite and is difficult to deform even when reduced in temperature. Therefore, when austenite is present, the surrounding ferrite is forced to deform unevenly. Consequently, rolling in this temperature range contributes greatly to the fracture of precipitates such as sulfides and selenides that have precipitated in the ferrite. Furthermore, the above fracture effect increases with increasing strain, so in this invention, the reduction ratio during rolling in the above temperature range is set to 20% or more. Preferably, it is 25% or more.

[0043] However, simply increasing the reduction ratio in one pass is not sufficient. This is because even if dislocations are introduced by rolling under the above conditions in one pass, the high temperature in the above temperature range causes the microstructure to recover and the dislocation density to quickly decrease. Therefore, high-pressure rolling is performed for at least two consecutive passes in the above temperature range, with the inter-pass time between the two passes being 60s or less, and the strain rate of each pass being 15s -1By doing so, it becomes possible to break large sulfides and selenides precipitated in ferrite by increasing the dislocation density and promoting non-uniform deformation more. The strain rate of each preferred pass is 18 s -1 or less, and the inter-pass time is 50 s or less.

[0044] Here, the above strain rate is a value calculated using the following Ekelund's formula. TIFF0007859489000001.tif23166v R : roll peripheral speed (mm / s), R´: roll radius (mm), h1: plate thickness on the roll inlet side (mm), r: reduction ratio (%)

[0045] By adopting the above hot rolling conditions, it becomes possible to break large sulfide, selenide precipitates or their composite precipitates existing near the center of the plate thickness of the hot rolled steel sheet. As a result, the number density of precipitates with a major axis of 4 μm or more, which is harmful to cold rolling, existing in the center of the plate thickness cross section can be reduced to 0.30 pieces / mm 2 or less. A more preferable number density of precipitates is 0.1 piece / mm 2 or less.

[0046] Note that the number density of precipitates such as the above sulfides and selenides is obtained by collecting a test piece from the center of the plate width of the hot rolled sheet, and in the region 1 / 10 of the plate thickness from the center of the plate thickness cross section along the rolling direction of the test piece, that is, in the region of 1 / 5 of the plate width including the center of the plate thickness, for 50 mm 2 or more, observing with a scanning electron microscope SEM, and counting the number of precipitates in which sulfides, selenides and their compounds are combined and have a major axis of 4 μm or more.

[0047] Furthermore, in hot rolling, as disclosed in Patent Document 4, it is preferable to perform at least one pass of rolling in a temperature range of 850°C to 950°C in order to homogenize the grain size distribution of the primary recrystallized structure. The reason for having a structure with a uniform grain size distribution will be explained below. Crystal grains tend to grow more easily the larger they are. However, in the primary recrystallized structure, the grain boundary energy between Goss-oriented grains and surrounding crystal grains is high, while the grain boundary energy of crystal grains other than Goss-oriented grains is low. Therefore, by making the grain size uniform, the advantage of grain growth due to the energy difference is further emphasized, and Goss-oriented grains can be selectively grown. However, if the rolling temperature exceeds 950°C, processing strain is not sufficiently accumulated due to recovery, and good recrystallization does not occur. On the other hand, if the temperature is below 850°C, the temperature is too low, and recrystallization immediately after rolling hardly occurs.

[0048] Furthermore, from the viewpoint of further improving magnetic properties, it is preferable to hold the steel sheet without further processing for 1.0 s or more after passing it in the temperature range of 850°C to 950°C. The pass in which the sheet is held for 1.0 s or more after passing can be any hot rolling pass, as long as it can be rolled in the temperature range of 850 to 950°C and held without further processing for 1.0 s or more after passing. For example, the first pass of finish rolling, where the rolling speed is slow and the time until the next pass is long, or the final pass of finish rolling, where the time from rolling to the start of cooling can be freely set, are suitable.

[0049] The hot-rolled steel sheet described above is cooled and wound into a coil, after which it undergoes hot-rolled sheet annealing. In this process, in order to achieve complete recrystallization of the hot-rolled sheet, it is important to limit the sum of the cooling time from 750°C to 650°C during the cooling process after hot rolling and the heating time from 600°C to 700°C during the heating process to reach the maximum temperature (850°C or higher) of the hot-rolled sheet annealing to 20 seconds or less.

[0050] The reason for this is that in a composition system containing a large amount of inhibitor-forming components, the recovery after hot rolling, i.e., the dissipation of processing strain by holding at a high temperature, is at a negligible level. However, in steel with reduced inhibitor-forming components as in the present invention, if the hot-rolled sheet is left in the above temperature range for a long time, recovery occurs rapidly, causing the processing strain to dissipate, and it becomes impossible to secure enough strain necessary for recrystallization during hot-rolled sheet annealing. The reason why the temperature ranges for controlling the cooling and heating times are different as described above is that the temperature range in which processing strain is released differs slightly between the cooling process after hot rolling and the heating process during hot-rolled sheet annealing.

[0051] Furthermore, the maximum temperature reached during hot-rolled sheet annealing is preferably 850°C or higher, from the viewpoint of promoting recrystallization and highly developing the Goss structure in the product sheet. If the hot-rolled sheet annealing temperature is below 850°C, the band structure formed by hot rolling will remain, making it difficult to obtain a uniform primary recrystallized structure, and potentially inhibiting the development of secondary recrystallization.

[0052] On the other hand, it is preferable to set the upper limit of the hot-rolled sheet annealing temperature to around 1130°C. If the temperature exceeds 1130°C, inhibitor-forming components inevitably mixed into the steel will dissolve and re-precipitation unevenly during cooling, making it difficult to obtain a uniform primary recrystallized structure, and potentially inhibiting the development of secondary recrystallization. Furthermore, if the annealing temperature exceeds 1130°C, the grain size after annealing becomes too coarse, which is also disadvantageous in obtaining a uniform primary recrystallized structure.

[0053] The steel sheet, after annealing the hot-rolled sheet described above, is subjected to one cold-rolling or two or more cold-rolling processes with an intermediate annealing in between to obtain a cold-rolled sheet of the final thickness (product thickness). For the cold-rolling process to obtain the final thickness (final cold-rolling), it is desirable to employ warm rolling, where the steel sheet temperature is raised to 80-150°C during rolling, or to perform inter-pass aging, where the steel sheet temperature is raised to 100-300°C between passes, once or multiple times, from the viewpoint of developing the Goss structure.

[0054] The cold-rolled sheet, after reaching its final thickness, is then subjected to decarburization annealing, which also serves as primary recrystallization annealing. The purpose of this annealing is to primary recrystallize the cold-rolled sheet, which has a rolled structure, to create a primary recrystallized structure optimized for secondary recrystallization, and to reduce the carbon content in the steel to 0.0050 mass% or less, preferably 0.0030 mass% or less, so that magnetic aging does not occur, while also forming an oxide film layer (subscale) on the surface of the steel sheet necessary for forsterite film formation. For this reason, the atmosphere used in the decarburization annealing is preferably an oxidizing atmosphere containing H2, such as humid hydrogen nitrogen or humid hydrogen argon, and the soaking temperature is preferably in the range of 750 to 900°C. Furthermore, when heating to the soaking temperature, the texture of the primary recrystallized structure can be further improved by setting the heating rate between 550 and 680°C to 100°C / s. More preferably, it is 200°C / s or higher. In addition, after decarburization annealing, a silicating treatment may be performed to increase the Si content.

[0055] After the decarburization annealing described above, the steel sheet is then coated with an annealing release agent, followed by finish annealing to induce secondary recrystallization, and the Goss orientation ({110} <001> ) develops grains. As the above annealing separating agent, an annealing separating agent mainly composed of MgO may be used to form a forsterite film on the steel plate surface during finish annealing. In this case, by adding an appropriate amount of auxiliary agents such as Ti oxide or Sr compounds to the annealing separating agent, the formation of the forsterite film can be further promoted. In particular, the addition of the above auxiliary agents, which homogenize the formation of the forsterite film, is also advantageous in improving the peel resistance of the film. Alternatively, instead of an annealing separating agent mainly composed of MgO, an annealing separating agent mainly composed of Al2O3 or the like may be used to suppress film formation.

[0056] For the finish annealing described above, it is preferable to heat to a temperature of 800°C or higher in order to induce secondary recrystallization. The heating rate up to 800°C is not particularly limited as it does not significantly affect the magnetic properties. The atmosphere for the finish annealing may be N2, Ar, H2, or a mixture of these gases. To further promote secondary recrystallization, it is preferable to maintain isothermal heating for 20 to 80 hours at a temperature of 800 to 1000°C where secondary recrystallization occurs. However, similar effects can be obtained by slowly heating within the above temperature range at a heating rate of 10°C / hr or less, so isothermal heating is not always necessary. Furthermore, if trace amounts of impurities remain in the final product, they will precipitate and lead to deterioration of magnetic properties, so it is preferable to perform a purification treatment in a hydrogen atmosphere by heating to a temperature of 1100°C or higher after completing secondary recrystallization. This heating also forms a forsterite film.

[0057] After the above-mentioned finish annealing, the steel sheet may be subjected to planar annealing to shape the sheet after removing any unreacted annealing separating agent. Furthermore, an insulating coating may be applied and baked onto the surface of the steel sheet during, or before or after, this planar annealing process. The insulating coating can be any conventionally known coating. For example, a tension-imparting insulating coating is preferred, which is applied to the steel sheet using a coating solution containing phosphate-chromate-colloidal silica, as described in Japanese Patent Publication No. 48-39338 and Japanese Patent Publication No. 50-79442, and then baked at a temperature of approximately 800°C. [Examples]

[0058] Experiments were conducted to systematically manufacture grain-oriented electrical steel sheets using a steel slab as the raw material, which does not contain inhibitor-forming components and has a composition of C:0.03~0.05 mass%, Si:3.3~3.4 mass%, Mn:0.07 mass%, Al:0.005~0.008 mass%, N:0.006 mass% or less, O:0.0060 mass% or less, and S and Se (S + 0.405 × Se) at a concentration of 0.0060 mass% or less, with the remainder being Fe and unavoidable impurities. Specifically, a steel slab with the above composition was heated to a temperature of 1220°C, followed by hot rolling consisting of 4 passes of rough rolling and 30 consecutive passes of finish rolling to produce a hot-rolled sheet with a thickness of 2.2 mm. In this process, the second pass of rough rolling was performed at a temperature of 1100°C with a reduction ratio of 35% and a strain rate of 30 s. -1 The third pass was performed at a temperature of 1080°C with a reduction ratio of 35% and a strain rate of 30s. -1 The interval between the second and third passes was set to 50 seconds. The finish rolling was performed in the temperature range of 880 to 950°C, with the final rolling completion temperature being 880°C. The cooling time from 750°C to 650°C after hot rolling was set to 7 seconds, and the coil winding temperature was set to 570°C. Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing in a continuous annealing furnace to a maximum temperature of 1020°C. During this process, the heating time from 600°C to 700°C was controlled to 11 seconds.

[0059] Next, the hot-rolled steel sheets, after annealing, were cold-rolled to obtain cold-rolled sheets with a final thickness of 0.260 mm. During this process, the number of coils that fractured during cold rolling was counted. In cases where a coil fractured multiple times, or where a coil fractured in an unsteady section at the leading or trailing end, it was counted as one coil fracture.

[0060] Figure 2 is a graph showing the fracture rate in cold rolling organized by (S + 0.405 × Se), with the results shown alongside Figure 1. From this figure, it can be seen that applying the hot rolling method and hot-rolled sheet annealing conditions of the present invention reduces the fracture rate in cold rolling in the region where the S and Se content in the steel sheet is 0.0015 mass% or more in terms of (S + 0.405 × Se), and that the reduction effect is particularly significant when (S + 0.405 × Se) is 0.0025 mass% or more.

[0061] Next, the cold-rolled steel sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a heating rate of 250°C / s between 550°C and 680°C, a soaking temperature of 800°C, and a soaking time of 30 s, thereby reducing the carbon content in the steel sheet to 0.0050 mass% or less. Then, an annealing separation agent consisting of MgO: 95 mass% and TiO2: 5 mass% was applied to the surface of the steel sheet in the form of a water slurry, dried, and then subjected to finish annealing to induce secondary recrystallization. Subsequently, after removing the unreacted annealing separation agent from the steel sheet after the finish annealing, a coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3 was applied to the surface of the steel sheet and baked at 800°C to form a tension-imparting insulating film.

[0062] Test pieces were taken from the center of the coil in the length direction and the center of the plate width direction from the resulting product plate, and their magnetic properties were measured. As a result, it was confirmed that the product had magnetic properties that were equal to or better than those before the application of the present invention. [Examples]

[0063] A steel slab containing C:0.04 mass%, Si:3.3 mass%, and Mn:0.05 mass%, with other components as shown in Table 1, and the remainder consisting of Fe and unavoidable impurities, was heated to 1200°C. It was then hot-rolled to a thickness of 2.0 mm using a 5-pass rough rolling process followed by a series of 1-pass finish rollings, and wound into a coil at 500°C. The reduction ratio and strain rate were kept the same for the 4th and 5th rough rolling passes, while the first finish rolling pass was performed between 850°C and 950°C. The reduction ratio, strain rate, and inter-pass time for the 4th and 5th rough rolling passes, the inter-pass time between the 1st and 2nd finish rolling passes, the finish rolling completion temperature, and the cooling time between 750°C and 650°C after finish rolling were varied as shown in Table 2.

[0064] A test specimen was taken from the center of the length and width of the hot-rolled sheet obtained as described above. The microstructure of the sheet thickness cross-section parallel to the rolling direction was observed using SEM, and a 50 mm region with a width of 1 / 5 of the sheet thickness, including the center of the sheet thickness cross-section along the rolling direction, was identified. 2 The number density of precipitates of sulfides and selenides with a major axis of 4 μm or larger, as well as precipitates composed of these compounds, was measured.

[0065] Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing with a maximum temperature of 1000°C. During this process, the heating time from 600°C to 700°C was varied as shown in Table 2. Then, the steel sheet after the hot-rolled sheet annealing was subjected to primary cold rolling to 1.7 mm at 100°C using a reverse rolling mill, followed by intermediate annealing at 900°C for 1 min, and then secondary cold rolling again using the reverse rolling mill to obtain a cold-rolled sheet with a final thickness of 0.22 mm. At this time, the fracture rate for the primary and secondary cold rolling combined was counted in the same manner as in Example 1. In the secondary cold rolling process, inter-pass aging was performed at least once between passes, in which a coil wound at 200°C was held for 0.5 hours or more for aging treatment. Subsequently, a decarburization annealing process, which also served as primary recrystallization annealing, was performed with a heating rate of 300°C / s between 550°C and 680°C, a soaking temperature of 840°C, and a soaking time of 60 s, thereby reducing the carbon content in the steel to 0.0050 mass% or less. Next, an annealing separation agent consisting of MgO: 95 mass% and TiO2: 5 mass% was prepared as a water slurry and applied to the surface of the steel sheet. After drying, a finish annealing process was performed to induce secondary recrystallization. After removing the unreacted annealing separation agent from the steel sheet after the finish annealing, a coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3 was applied to the surface of the steel sheet and baked at 800°C to form a tension-imparting insulating coating.

[0066] Test pieces were taken from the center of the coil in both the length and width directions of the resulting product sheet, and their magnetic properties (magnetic flux density B8) were measured. The results are shown in Table 2. From these results, it can be seen that steel sheets manufactured under conditions conforming to the present invention not only have a reduced fracture rate during cold rolling, but also exhibit superior magnetic properties.

[0067] [Table 1]

[0068] [Table 2]

Claims

1. A steel slab having a composition containing C: 0.03 to 0.08 mass%, Si: 2.0 to 5.0 mass%, Mn: 0.005 to 1.0 mass%, Al: less than 0.010 mass%, N: 0.006 mass% or less, and O: 0.0060 mass% or less, and containing S and Se in the range of (S + 0.405 × Se) 0.0015 to 0.0060 mass%, with the remainder being Fe and unavoidable impurities, is rolled in at least two consecutive passes at a temperature range of 1050 to 1150°C, with an inter-pass time of 60 s or less, a reduction ratio of 20% or more in each pass, and a strain rate of 15 s －１ A method for manufacturing grain-oriented electrical steel sheets, comprising: hot rolling to obtain a hot-rolled sheet, hot-rolled sheet annealing, cold rolling once or two or more times with intermediate annealing in between to obtain a cold-rolled sheet, decarburization annealing that also serves as primary recrystallization annealing, and then finish annealing.

2. The above hot rolling is performed in a temperature range of 850 to 950°C for at least one pass. The hot-rolled sheet annealing described above is carried out at a temperature of 850°C or higher, A method for manufacturing grain-oriented electrical steel sheets according to claim 1, characterized in that the sum of the cooling time from 750°C to 650°C in the cooling process after hot rolling and the heating time from 600°C to 700°C in the heating process of hot-rolled sheet annealing is 20 seconds or less.

3. The method for manufacturing a grain-oriented electrical steel sheet according to claim 2, characterized in that, after rolling in at least one pass in the temperature range of 850 to 950°C, the steel sheet is held in that state for 1.0 s or more.

4. The method for manufacturing grain-oriented electrical steel sheets according to any one of claims 1 to 3, characterized in that the steel slab further contains, in addition to the above-mentioned component composition, at least one component selected from groups A to C below. Note ・Group A; Ni: 1.50 mass% or less Group B: At least one selected from Sn: 0.50 mass% or less, Sb: 0.50 mass% or less, Cu: 0.50 mass% or less, Mo: 0.50 mass% or less, Co: 0.0100 mass% or less, P: 0.50 mass% or less, Cr: 1.50 mass% or less, B: 0.0200 mass% or less, and Bi: 0.0200 mass% or less. Group C: At least one selected from Nb: 0.0200 mass% or less, Ti: 0.0200 mass% or less, Te: 0.0200 mass% or less, Ga: 0.0100 mass% or less, and Zn: 0.500 mass% or less.

Images