Method for producing grain-oriented electrical steel sheet, and hot-rolled sheet for grain-oriented electrical steel sheet

Abstract

A method for producing a grain-oriented electrical steel sheet, and a hot-rolled sheet used in this production method, in which a steel slab containing, in mass%, 0.03-0.08% of C, 2.0-5.0% of Si, 0.005-1.0% of Mn, less than 0.010% of Al, 0.006% or less of N, and 0.0060% or less of O, and containing S and Se in the range of 0.0015-0.0060% in terms of (S + 0.405 Se) is hot-rolled, hot-rolled sheet annealing is performed, cold rolling is performed, decarburization annealing is performed, which is also used as primary recrystallization annealing, and then hot-rolled sheet annealing is performed, in order to manufacture a grain-oriented electrical steel sheet by performing hot rolling and final annealing, at least two consecutive passes of the hot rolling are performed in the temperature range of 1050-1150 DEG C, the inter-pass time of the two passes is 60 s or less, the reduction rate of each pass is 20% or more, and the strain rate is 15 s <-1 > or more, whereby breakage during cold rolling is prevented.

Description

Technical Field

[0001] The present invention relates to a method for producing a grain-oriented electrical steel sheet having excellent cold rolling properties and magnetic properties, and a hot-rolled steel sheet used in the above-mentioned production method. In the present invention, the above-mentioned hot-rolled steel sheet is also referred to as a "hot-rolled sheet". Background Art

[0002] Grain-oriented electrical steel sheets are generally subjected to Goss orientation ({110} <001> As the above-mentioned inhibitor, for example, AlN and MnS are proposed in Patent Document 1, and MnS and MnSe are proposed in Patent Document 2, all of which have been put into practical use in industry.

[0003] Although the method using these inhibitors is a useful method for stably developing secondary recrystallized grains having a Goss orientation, since the precipitates serving as the inhibitors must be finely dispersed, the steel slab must be heated to a high temperature of 1300° C. or higher before hot rolling. Therefore, this method has problems such as increased scale loss and reduced yield, increased thermal energy cost, increased equipment cost, and complicated equipment maintenance.

[0004] On the other hand, as a technology to solve the above-mentioned problems, a manufacturing method without using an inhibitor (inhibitor-free method) has also been proposed. For example, Patent Document 3 proposes a technology that uses a higher-purified steel material that does not contain inhibitor-forming components and expresses secondary recrystallization by grain (texture) control. However, when a grain-oriented electrical steel sheet is manufactured using a steel material that does not contain inhibitor-forming components, due to the high purity, there is a problem that the grain boundary strength is reduced and fracture is easily caused during cold rolling, which becomes a factor that hinders industrial-scale production.

[0005] To solve this problem, Patent Document 4 proposes a technique for achieving both further stabilization of secondary recrystallization and suppression of fracture during cold rolling by controlling the recrystallization process after hot rolling.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Publication No. 40-015644

[0009] Patent Document 2: Japanese Patent Publication No. 51-013469

[0010] Patent Document 3: Japanese Patent Application Publication No. 2000-129356

[0011] Patent Document 4: Japanese Patent Application Publication No. 2003-226916 Summary of the invention

[0012] Problems to be solved by the invention

[0013] The technology disclosed in the above-mentioned Patent Document 4 controls the thermal history during hot rolling and hot-rolled sheet annealing to specific conditions, thereby making the crystal structure after hot-rolled sheet annealing more uniform, 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 fracture during cold rolling cannot be completely prevented by the above-mentioned technology alone.

[0014] The present invention has been made in view of the above-mentioned problems existing in the prior art, and its purpose is to propose a method for stably manufacturing a oriented electromagnetic steel sheet with excellent magnetic properties without fracture during cold rolling even when a steel raw material containing no inhibitor-forming components is used, and to provide a hot-rolled sheet used in the manufacturing method.

[0015] Methods used to solve problems

[0016] In order to solve the above problems, the inventors have repeatedly conducted in-depth studies focusing on the reasons why hot-rolled sheets that do not contain inhibitor-forming components are prone to fracture during cold rolling. As a result, it was found that the reason for fracture during cold rolling is that, in addition to the reduction in grain boundary strength caused by high purification, it is also largely related to sulfides and selenides present in the center of the plate thickness (segregation part) of the steel plate. 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 the hot-rolled sheet annealing conditions, and further improve the uniformity of the crystal structure of the hot-rolled sheet after annealing, thereby developing the present invention.

[0017] The present invention based on the above findings proposes a method for manufacturing a grain-oriented electrical steel sheet, wherein the grain-oriented electrical steel sheet contains 0.03-0.08 mass % of C, 2.0-5.0 mass % of Si, 0.005-1.0 mass % of Mn, less than 0.010 mass % of Al, less than 0.006 mass % of N, and less than 0.0060 mass % of O, and contains S and Se in the range of 0.0015-0.0060 mass % (S+0.405×Se), and the balance is Fe and unavoidable impurities. The steel billet with the component composition is hot-rolled to form a hot-rolled sheet, the hot-rolled sheet is annealed, and then cold-rolled once or twice or more with intermediate annealing is performed to form a cold-rolled sheet, the cold-rolled sheet is subjected to decarburization annealing which also serves as a recrystallization annealing, and then final annealing, wherein the hot rolling comprises: performing at least two consecutive passes of rolling in a temperature range of 1050 to 1150° C., and setting the pass time between the two passes to be less than 60 s, setting the reduction rate of each pass to be more than 20%, and setting the strain rate to 15 s-1 above.

[0018] The hot rolling in the method for manufacturing the oriented electromagnetic steel sheet of the present invention is characterized in that at least one pass is carried out within a temperature range of 850 to 950°C, the hot rolled sheet annealing after the hot rolling is carried out at a temperature above 850°C, and the sum of the cooling time from 750°C to 650°C in the cooling process after the hot rolling and the heating time from 600°C to 700°C in the heating process of the hot rolled sheet annealing is set to less than 20s.

[0019] Furthermore, the method for producing the grain-oriented electrical steel sheet of the present invention is characterized in that after at least one pass of rolling in the temperature range of 850 to 950° C., the steel sheet is kept as it is for 1.0 second or more.

[0020] Furthermore, the steel slab in the method for producing a grain-oriented electrical steel sheet according to the present invention is characterized by containing, in addition to the above-mentioned component composition, at least one component selected from the following groups A to C.

[0021] Group A: Ni: 1.50 mass % or less;

[0022] 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;

[0023] Group C: at least one selected from the group consisting of 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.

[0024] In addition, the present invention is a hot-rolled sheet for grain-oriented electrical steel sheet, which has 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 contains S and Se in the range of 0.0015-0.0060 mass% as (S+0.405×Se), and the balance is Fe and inevitable impurities, and has a composition of 50 mm in an area of ​​1 / 5 of the plate thickness including the plate thickness center portion in a plate thickness cross section along the rolling direction. 2 The number density of precipitates composed of sulfides, selenides and their compounds with a long diameter of 4 μm or more is 0.30 pieces / mm2 the following.

[0025] The hot-rolled sheet of the present invention is characterized by containing at least one component selected from the following groups A to C in addition to the above-mentioned component composition.

[0026] Group A: Ni: 1.50 mass % or less;

[0027] 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;

[0028] Group C: at least one selected from the group consisting of 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.

[0029] Effects of the Invention

[0030] According to the present invention, the cold rolling property of a hot-rolled sheet containing no inhibitor-forming component can be improved, and thus a grain-oriented electrical steel sheet having excellent magnetic properties can be stably manufactured without causing breakage during cold rolling. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a graph comparing the fracture incidence rate during cold rolling when the technology of Patent Document 4 is applied with the fracture incidence rate of the conventional technology.

[0032] Figure 2 This is a graph comparing the fracture incidence rate during cold rolling when the technology of the present invention is applied and the fracture incidence rate when the conventional technology and the technology of Patent Document 4 are applied. DETAILED DESCRIPTION

[0033] First, the investigation that led to the development of the present invention will be described.

[0034] The present inventors conducted a detailed investigation of a coil that broke during cold rolling even when the technology of Patent Document 4 was applied. As a result, a portion where the steel plate was separated into two plates at the front and back sides was found in the broken portion of the steel plate.

[0035] The separation surface was subjected to elemental mapping analysis using EPMA, and the results showed that precipitates consisting of multiple sulfides and selenides were confirmed on the separation surface. Therefore, the relationship between the fracture incidence rate during cold rolling and (S+0.405×Se), which is a parameter representing the sum of the atomic numbers of S and Se, was further investigated. As a result, it was found that Figure 1 As shown, when the content of S and Se exceeds 0.0015 mass % as (S+0.405×Se), the fracture incidence tends to increase, and it is more significant when it is 0.0025 mass % or more. Such a tendency is completely unknown in the past and is a newly discovered matter in the present invention.

[0036] Based on these findings, the following fracture mechanism was presumed.

[0037] In the conventional method for manufacturing a grain-oriented electrical steel sheet without using an inhibitor, for example, the method described in Patent Document 4, a steel raw material is used in which the S and Se contents are reduced to 0.0050 mass % or less, respectively, and secondary recrystallization is performed by appropriately controlling the texture of the hot-rolled sheet. Therefore, it is believed that the lower the S and Se contents are, the more stable the secondary recrystallization is performed, and in Patent Document 4, the contents of S and Se are also reduced to about 0 mass % to about 0.0020 mass %.

[0038] S is usually removed by being introduced into the slag through a reduction treatment during the refining process of steelmaking, but a large number of elements represented by C are removed by an oxidation treatment, so sometimes the temporarily removed S returns from the slag to the steel during the oxidation treatment, which is called "resulfurization". Therefore, in order to stably reduce S, it is necessary to add a large amount of auxiliary raw materials, which becomes a reason for the increase in manufacturing costs. On the other hand, unlike S, Se is mostly not derived from raw materials, but is brought in from scrap used to reduce costs, but has the same difficulty as S in removing it. Therefore, in the manufacture of grain-oriented electrical steel sheets, steel raw materials containing S and Se are used within the range where secondary recrystallization does not cause problems.

[0039] In the method for manufacturing a grain-oriented electrical steel sheet using an inhibitor, it is necessary to completely dissolve the inhibitor-forming components contained in the steel material (steel slab) in the steel, so the steel slab is heated to a high temperature of 1350 to 1450° C. before hot rolling. However, when no inhibitor is used, complete solid solution of the inhibitor-forming components as described above is not required, so the steel slab heating temperature is a relatively low temperature of about 1250° C. or less that does not adversely affect hot rolling.

[0040] In addition, S and Se are also known as elements that are prone to segregation. For example, when a steel slab is manufactured by a continuous casting method, S and Se are segregated and highly concentrated in the thickness center of the steel slab where the steel slab is finally solidified, and further form precipitates such as sulfides and selenides. The above-mentioned center segregation and precipitates are melted and diffused by heating the steel slab before hot rolling, thereby reducing them. However, when the content of S and Se is high and the steel slab heating temperature is low, sulfides and selenides may remain as precipitates in the thickness center of the steel slab.

[0041] In the part where the steel sheet fractured during cold rolling, a part where the steel sheet was separated into two plates was found in the center of the plate thickness, and sulfide and selenide precipitates were observed on the separated surface. It is believed that the sulfide and selenide remaining in the center of the plate thickness became the starting point of separation and caused the fracture.

[0042] Therefore, in order to prevent fracture during cold rolling, the steel slab may be heated before hot rolling so that the inhibitor-forming components are completely dissolved. However, the advantage of the method for manufacturing a grain-oriented electromagnetic steel sheet using a steel raw material without using an inhibitor is that the manufacturing cost can be reduced by lowering the heating temperature of the steel slab, but this advantage cannot be enjoyed when the steel slab is heated at a high temperature. In addition, if the steel slab is heated at a high temperature, the conventional method using an inhibitor can be applied.

[0043] Therefore, the present inventors have studied a method of destroying and harmlessly treating precipitates during hot rolling, instead of a method of completely dissolving sulfides and selenides that exist as precipitates near the thickness center of the steel slab due to center segregation by high temperature heating of the steel slab. As a result, they have found that the above-mentioned object can be achieved by performing at least two consecutive passes of high pressure and high strain rate rolling under appropriate conditions within a specific temperature range of hot rolling.

[0044] In addition, it is generally known that the interface between sulfide, selenide and steel base becomes the starting point of cracks, but it was found that the precipitates of the above-mentioned sulfide and selenide have no adverse effect on cold rolling. Large precipitates, specifically precipitates with a long diameter of more than 4 μm extending along the rolling direction by hot rolling, have a particularly adverse effect on fracture. It is believed that this is because the deformation ability of the precipitates is different from that of the steel base. Therefore, when the steel is extended during rolling, shear force is generated at the large interface and damage occurs. It was also found that in order to prevent fracture during cold rolling, it is necessary to cover an area of ​​50 mm with a width of 1 / 5 of the plate thickness including the center portion of the plate thickness section along the rolling direction. 2 The number density of precipitates such as sulfides and selenides with a long diameter of more than 4 μm in the inner part is reduced to 0.30 pieces / mm 2 the following.

[0045] The present invention has been completed by further improving the above findings.

[0046] Next, the component composition that the steel material used in the method for producing the grain-oriented electrical steel sheet of the present invention should have will be described.

[0047] C: 0.03-0.08% by mass

[0048] The present invention is characterized in that, by performing at least two consecutive hot rollings under appropriate conditions within a specific temperature range (1050-1150°C), large sulfide and selenide precipitates present in the center of the steel billet are broken into a size with a long diameter of less than 4 μm, which does not break during cold rolling. Generally, hot rolling of Si-containing steel is carried out in a temperature range in which a ferrite single phase is roughly formed, but the above temperature range coincides with the temperature range in which austenite is formed although in small amounts, and for reasons described later, sulfide and selenide precipitates are easily divided and broken. Therefore, in order to enable hot rolling in the above two-phase region, 0.03 mass% or more of C is contained. On the other hand, when C exceeds 0.08 mass%, it is difficult to reduce it to less than 0.0050 mass% where magnetic aging does not occur even if decarburization annealing is performed. Therefore, C is set to a range of 0.03-0.08 mass%. Preferably, it is in a range of 0.03-0.06 mass%.

[0049] Si: 2.0-5.0 mass%

[0050] Si is an element effective in increasing the resistivity of steel and reducing iron loss. In order to fully obtain this effect, it is necessary to contain at least 2.0 mass%. On the other hand, when it exceeds 5.0 mass%, the steel becomes brittle, the cold rolling property deteriorates significantly, or the risk of fracture when the plate is passed through the manufacturing equipment increases, so the upper limit is set to 5.0 mass%. The range of 2.8 to 4.5 mass% is preferred.

[0051] Mn: 0.005-1.0 mass%

[0052] Mn has the effect of improving hot workability. In addition, it is also a useful element from the viewpoint of controlling the oxide film formed during primary recrystallization, so it is contained in an amount of 0.005 mass % or more. On the other hand, when it exceeds 1.0 mass %, the primary recrystallization texture deteriorates, resulting in deterioration of magnetic properties, so the upper limit is set to 1.0 mass %. It is preferably in the range of 0.01 to 0.5 mass %.

[0053] Al: less than 0.010 mass%, N: 0.006 mass% or less

[0054] The present invention is a technology for manufacturing a grain-oriented electrical steel sheet using a steel raw material that does not contain inhibitor-forming components. Therefore, Al is limited to less than 0.010 mass%, and N is limited to less than 0.006 mass%. When Al and N exceed the above values, it is difficult to obtain a good secondary recrystallization structure even if the structure is controlled. In addition, from the perspective of preventing the formation of Si nitrides during the purification annealing of the final annealing and deterioration of magnetic properties, N also needs to be set to less than 0.006 mass%. Preferably, Al: less than 0.008 mass%, N: less than 0.0050 mass%.

[0055] O: 0.0060 mass % or less

[0056] O forms oxides and deteriorates the magnetic properties of the final product sheet, so the content is limited to 0.0060 mass % or less, preferably 0.0030 mass % or less.

[0057] S+0.405×Se: 0.0015 to 0.0060 mass%

[0058] S and Se form sulfides and selenides, which cause fractures during cold rolling, or form inhibitors to deteriorate magnetic properties. They are harmful elements in the present invention, so the lower the better. In particular, in order to stably perform secondary recrystallization without using inhibitors, S and Se need to be reduced to less than 0.0060 mass% in terms of (S+0.405×Se). However, although excessive reduction of S and Se has no adverse effect on magnetic properties and fractures during cold rolling, it leads to an increase in manufacturing costs, so the lower limit of (S+0.405×Se) is set to about 0.0015 mass%. Preferably, it is in the range of 0.0020 to 0.0050 mass%.

[0059] In the steel material used in the method for producing a grain-oriented electrical steel sheet of the present invention, the balance other than the above components is basically Fe and inevitable impurities. However, at least one component selected from the following components may be contained as necessary.

[0060] Ni: 1.50 mass % or less

[0061] Ni is an element useful for improving the steel sheet structure of hot-rolled plates and improving magnetic properties. In order to obtain this effect, it is preferably added at 0.005% by mass or more. On the other hand, when Ni is added excessively, secondary recrystallization becomes unstable and magnetic properties deteriorate instead. Therefore, the upper limit is preferably set to 1.50% by mass. More preferably, it is in the range of 0.01 to 1.00% by mass.

[0062] 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

[0063] Sn, Sb, Cu, Mo, Co, P, Cr, B and Bi are grain boundary segregation type elements, which have the effect of improving magnetic properties. However, when the upper limit value of the above range is exceeded, the development of secondary recrystallized grains is hindered. Therefore, in the case of addition, it is preferably set to be below the above upper limit value. It should be noted that in order to reliably obtain the above-mentioned addition effect, it is preferred 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.

[0064] At least one selected from the group consisting of 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

[0065] Nb, Ti, Te, Ga and Zn are elements that form carbonitride precipitates. They are not necessarily required in the manufacturing method without using inhibitors, but as long as they are within the range where they can be dissolved even when the steel billet is heated at a relatively low temperature, adding them in very small amounts also has the effect of improving the magnetic properties. However, when the addition exceeds the above upper limit, the secondary recrystallization becomes unstable. In addition, even within the above range, depending on the heating temperature of the steel billet and the manufacturing conditions, the secondary recrystallization may sometimes be unstable, so care should be taken when using it. It should be noted that in order to reliably obtain the above-mentioned addition effects, it is preferred 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.

[0066] Next, a method for producing a grain-oriented electrical steel sheet according to the present invention will be described.

[0067] The steel raw material (steel billet) used to manufacture the grain-oriented electrical steel sheet of the present invention is manufactured as follows: the steel adjusted to the above-mentioned component composition suitable for the present invention is melted by a known refining method such as a converter, an electric furnace, etc. and further subjected to vacuum degassing treatment as needed, and then manufactured by a generally known continuous casting method. It should be noted that a thin cast sheet with a thickness of 100 mm or less can also be manufactured by a direct casting method.

[0068] Next, the steel billet is heated to a predetermined temperature and then subjected to hot rolling, but it may also be directly subjected to hot rolling without heating after continuous casting. Here, the heating temperature of the steel billet before the hot rolling is preferably set to 1250°C or less. This is because, for the component system of the present invention that does not contain inhibitor-forming components, it is not necessary to dissolve the inhibitor-forming components. However, when the heating temperature of the steel billet is too low, the load of hot rolling sometimes increases, hindering manufacturing stability. In addition, as described later, in the present invention, it is necessary to implement hot rolling at or above 1050°C in at least two consecutive passes, so the heating temperature is preferably set to 1100°C or more.

[0069] The steel slab heated to a relatively low temperature is then subjected to hot rolling consisting of rough rolling and finish rolling to produce a hot rolled steel sheet (hot rolled sheet). However, at the relatively low steel slab heating temperature as described above, there is a possibility that the sulfide and selenide precipitates that are segregated and precipitated in the thickness center of the steel slab during steel slab manufacturing, or the precipitates formed by the composite of their compounds, are not completely dissolved and remain, thereby deteriorating the cold rolling properties.

[0070] Therefore, in the present invention, in order to destroy the large sulfide and selenide precipitates that are harmful to cold rolling and make them smaller and harmless, in the hot rolling, it is necessary to perform at least two consecutive rolling passes in the temperature range of 1050-1150°C, and the time between the two passes is set to 60s or less, the reduction rate of each of the two passes is set to 20% or more, and the strain rate is set to 15s. -1 above.

[0071] As described above, for steel containing 0.03 to 0.08 mass % of C, the temperature range of 1050 to 1150°C is the temperature range in which austenite is formed in ferrite. Austenite has a higher deformation resistance than ferrite and is difficult to deform even when pressed. Therefore, if austenite exists, the ferrite around it has to be deformed unevenly. Therefore, rolling within this temperature range greatly contributes to the destruction of precipitates such as sulfides and selenides precipitated in ferrite. In addition, the more strain is applied, the greater the above-mentioned destructive effect is, so in the present invention, the reduction rate of rolling within the above-mentioned temperature range is set to 20% or more. Preferably, it is 25% or more.

[0072] However, sufficient effect cannot be obtained by simply increasing the reduction rate in one pass. The reason is that even if dislocations are introduced by rolling in one pass under the above conditions, the structure will recover and the dislocation density will immediately decrease due to the high temperature in the above temperature range. Therefore, by performing at least two consecutive passes of high pressure rolling in the above temperature range, setting the time between the two passes to less than 60 seconds, and setting the strain rate of each pass to 15 seconds, the steel structure can be improved. -1 The above can further increase the dislocation density and further promote the uneven deformation, thereby destroying the large sulfides and selenides precipitated in the ferrite. The strain rate of each pass is preferably 18s -1 The time between the above passes is less than 50s.

[0073] Here, the strain rate is a value calculated using the following Ekelund's equation.

[0074]

[0075] v R : Roller circumferential speed (mm / s), R´: Roller radius (mm), h 1 : Thickness at the roller entrance side (mm), r: Reduction rate (%)

[0076] By adopting the above hot rolling conditions, large sulfide, selenide precipitates or their composite precipitates existing near the center of the plate thickness of the hot rolled steel plate can be destroyed. As a result, the number density of precipitates with a major diameter of 4 μm or more, which are harmful to cold rolling and exist in the center of the plate thickness section, can be reduced to 0.30 pieces / mm. 2 The more preferred number density of precipitates is 0.1 pcs / mm 2 the following.

[0077] It should be noted that the number density of the above-mentioned precipitates such as sulfides and selenides is measured as follows: a test piece is cut from the central portion of the hot-rolled plate width, and a scanning electron microscope SEM is used to measure the area of ​​the plate thickness section along the rolling direction that is 1 / 10 of the plate thickness from the center of the plate thickness, that is, the area with a width of 1 / 5 of the plate thickness including the center of the plate thickness, for 50 mm. 2 The above observation was performed by counting the number of precipitates having a major diameter of 4 μm or more among precipitates composed of sulfide, selenide, and a composite of these compounds.

[0078] Furthermore, in hot rolling, as disclosed in Patent Document 4, in order to make the grain size distribution of the primary recrystallization structure uniform, it is preferred to perform at least one rolling in a temperature range of 850°C to 950°C. The reason for forming a structure with uniform grain size distribution is explained. It has the characteristic that the larger the crystal grain size, the easier the grain growth. However, the primary recrystallization structure is in a state where the grain boundary energy between the Gossian oriented grains and the surrounding grains is high and the grain boundary energy of the grains other than the Gossian oriented grains is low. Therefore, by forming a uniform grain size, the superiority of the grain growth caused by the energy difference can be made more significant, and the Gossian oriented grains can be selectively grown. However, when the above-mentioned rolling temperature exceeds 950°C, the processing strain cannot be fully accumulated due to recovery, so good recrystallization does not occur. On the other hand, when it is lower than 850°C, the temperature is too low and recrystallization just after rolling hardly occurs.

[0079] In addition, from the viewpoint of further improving the magnetic properties, after the above-mentioned pass in the temperature range of 850°C to 950°C, it is preferred that the steel plate is not subjected to processing and is kept in the original state for more than 1.0s. For the pass that is kept for more than 1.0s after the above-mentioned pass, any pass in hot rolling can be used as long as it can be rolled in the temperature range of 850 to 950°C and can be kept for more than 1.0s without being subjected to processing after the pass. For example, it is preferred that the first pass of finishing rolling with a slow rolling speed and a long time until the next pass, and the final pass of finishing rolling with the time from rolling to the start of cooling can be freely set.

[0080] After the hot rolled steel sheet is cooled and coiled into a coil, the hot rolled sheet is annealed. At this time, in order to make the hot rolled sheet completely recrystallized, it is important to limit 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 to reach the highest reaching temperature (above 850°C) of the hot rolled sheet annealing to less than 20s.

[0081] The reason is that, for a component system containing a large amount of inhibitor-forming components, the recovery caused by keeping the steel at a high temperature after hot rolling, i.e., the dissipation of the processing strain, is at a negligible level. However, for a steel in which the inhibitor-forming components are reduced as in the present invention, if the hot-rolled sheet is left in the above-mentioned temperature range for a long time, the recovery occurs rapidly and the processing strain dissipates, and the strain required for recrystallization in the hot-rolled sheet annealing cannot be fully ensured. It should be noted that the reason why the temperature ranges for managing the time during cooling and heating are different as described above is because the temperature ranges for releasing the processing strain are subtly different in the cooling process after hot rolling and the heating process of hot-rolled sheet annealing.

[0082] In addition, from the viewpoint of promoting recrystallization and highly developing the Gossian structure in the product sheet, the maximum reaching temperature of the hot rolled sheet annealing is preferably set to 850° C. or higher. When the hot rolled sheet annealing temperature is lower than 850° C., the banded structure formed during hot rolling may remain, making it difficult to obtain a uniform primary recrystallization structure, which hinders the development of secondary recrystallization.

[0083] On the other hand, the upper limit of the hot-rolled sheet annealing temperature is preferably set to about 1130°C. When the temperature exceeds 1130°C, the inhibitor-forming components inevitably mixed in the steel may be dissolved and unevenly re-precipitated during cooling, so it is still difficult to obtain a whole-grained primary recrystallization structure, which hinders the development of secondary recrystallization. In addition, when the annealing temperature exceeds 1130°C, the grain size after annealing becomes too coarse, which is disadvantageous for obtaining a whole-grained primary recrystallization structure.

[0084] The steel sheet after the hot-rolled sheet annealing is subjected to one cold rolling or two or more cold rollings with intermediate annealing to obtain a cold-rolled sheet of the final sheet thickness (product sheet thickness). It should be noted that, from the viewpoint of developing the Gaussian structure, the cold rolling (final cold rolling) to obtain the final sheet thickness is preferably performed by warm rolling in which the temperature of the steel sheet is raised to 80 to 150° C. during rolling, or by performing inter-pass aging in which the temperature of the steel sheet is raised to 100 to 300° C. between passes for aging treatment once or twice or more.

[0085] The cold-rolled sheet having the final sheet thickness is then subjected to a decarburization annealing which also serves as a primary recrystallization annealing. The purpose of this annealing is to make the cold-rolled sheet with the rolling structure recrystallize once, to make a primary recrystallization structure that is optimal for secondary recrystallization, and to reduce the C in the steel to a level below 0.0050 mass % and preferably below 0.0030 mass % where magnetic aging does not occur, and to form an oxide film layer (sub-scale) required for the formation of a forsterite film on the surface of the steel sheet. Therefore, it is preferred that the atmosphere in the above-mentioned decarburization annealing be a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere containing H 2 The annealing is carried out in an oxidizing atmosphere and the soaking temperature is set to the range of 750 to 900°C. In addition, when heating to the above soaking temperature, the texture of the primary recrystallization 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 more. In addition, after the decarburization annealing, siliconizing treatment to increase the amount of Si can be performed.

[0086] After the decarburization annealing, the steel sheet is coated with an annealing separator on the surface of the steel sheet, and then subjected to final annealing to achieve secondary recrystallization, thereby achieving a Goss orientation ({110} <001> ) grains are well developed. As the above-mentioned annealing separator, an annealing separator with MgO as the main component can be used to form a forsterite film on the surface of the steel sheet during the final annealing. At this time, by adding an appropriate amount of Ti oxide, Sr compound and other additives to the annealing separator, the formation of the forsterite film can be further promoted. In particular, the addition of the above-mentioned additives that make the formation of the forsterite film uniform also effectively plays a role in improving the peeling resistance of the film. In addition, an annealing separator with Al as the main component can also be used. 2 O 3 The annealing separator having MgO as the main component is replaced by an annealing separator having MgO as the main component to suppress the formation of the film.

[0087] In order to achieve secondary recrystallization, the above-mentioned final annealing is preferably heated to a temperature of 800°C or higher. The heating rate to 800°C has no significant effect on the magnetic properties and is therefore not particularly limited. The atmosphere for the final annealing may be N 2 , Ar, H 2 Or any of their mixed gases. It should be noted that in order to further promote secondary recrystallization, it is preferably kept isothermally for 20 to 80 hours near the temperature of 800 to 1000°C where secondary recrystallization occurs. However, the same effect can be obtained even if slowly heated at a heating rate of less than 10°C / hour in the above temperature range, so isothermal holding is not necessarily required. In addition, if trace impurities remain in the final product, they will precipitate and cause deterioration of the magnetic properties. Therefore, it is preferred to heat to a temperature of more than 1100°C after the completion of the secondary recrystallization and then perform purification treatment in a hydrogen atmosphere. In addition, by this heating, a forsterite coating is formed.

[0088] After the above-mentioned final annealing, the unreacted annealing separator is then removed and then a flattening annealing is performed to adjust the shape of the steel sheet. In addition, during the flattening annealing, or before or after the flattening annealing, an insulating coating may be applied to the surface of the steel sheet and sintered. The type of the above-mentioned insulating coating may be any conventionally known insulating coating. For example, it is preferred to apply a coating liquid containing phosphate-chromate-colloidal silica as described in Japanese Patent Publication No. 48-39338 and Japanese Patent Publication No. 50-79442 to the steel sheet and then sinter it at a temperature of about 800°C to obtain a tension-imparting insulating coating.

[0089] Example 1

[0090] The following experiment was conducted: a steel billet containing 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, S and Se (S+0.405×Se) of 0.0060 mass% or less, the balance of Fe and unavoidable impurities, and no inhibitor-forming components was used as the raw material, and a grain-oriented electrical steel sheet was manufactured according to the process. Specifically, after the steel billet having the above composition was heated to a temperature of 1220°C, hot rolling consisting of four passes of rough rolling and two or more consecutive passes of finish rolling was performed to produce a hot-rolled sheet with a thickness of 2.2 mm. At this time, the second pass of rough rolling was set to a temperature of 1100°C, a reduction rate of 35%, and a strain rate of 30s -1 The third pass was set at 1080°C with a reduction of 35% and a strain rate of 30s -1 , the time between the second and third passes is set to 50s. In addition, the finish rolling is carried out in the temperature range of 880-950°C, and the final rolling end temperature is set to 880°C. In addition, the cooling time from 750°C to 650°C after hot rolling is set to 7s, and the coiling temperature is set to 570°C. Next, the hot-rolled sheet is annealed in a continuous annealing furnace with the maximum reaching temperature set to 1020°C. At this time, the heating time from 600°C to 700°C in the heating process is controlled to 11s.

[0091] Next, the steel sheet after annealing the hot rolled sheet was cold rolled to obtain a cold rolled sheet with a final thickness of 0.260 mm. At this time, the number of coils that broke during the cold rolling was counted. It should be noted that in the case of multiple breaks in the same coil or in the case of breaks in the unstable parts at the front and rear ends of the coil, the breaks were also counted as one coil.

[0092] Figure 2 The above-mentioned fracture incidence rate during cold rolling is summarized as (S+0.405×Se) and the results are recorded in the above-mentioned Figure 1 . As can be seen from the figure, by applying the hot rolling method and hot rolled sheet annealing conditions of the present invention, the incidence of fracture during cold rolling in the region where the content of S and Se contained in the steel sheet is 0.0015 mass % or more as calculated by (S+0.405×Se) is reduced, and the reduction effect is particularly significant when (S+0.405×Se) is 0.0025 mass % or more.

[0093] Next, the cold-rolled steel sheet was subjected to a decarburization annealing which also served as a 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 30s, to reduce the carbon content of the steel sheet to 0.0050 mass % or less. Next, a decarburization annealing process was performed, which also served as a primary recrystallization annealing process, 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 30s. 2 An annealing separator composed of 5:5 mass % is made into a water slurry and applied to the surface of the steel sheet. After drying, a final annealing is performed to cause secondary recrystallization. Next, after removing the unreacted annealing separator from the steel sheet after the final annealing, a coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3 is applied to the surface of the steel sheet, and sintered at 800°C to form a tension-imparting insulation film.

[0094] The obtained product sheet was tested by cutting test pieces from the center of the coil in the longitudinal direction and the center of the sheet width direction, and measuring the magnetic properties. The results showed that the product had magnetic properties that were equal to or better than those before the application of the present invention.

[0095] Example 2

[0096] A steel slab having a composition containing 0.04 mass % of C, 3.3 mass % of Si and 0.05 mass % of Mn, and containing various components shown in Table 1 as other components, with the balance consisting of Fe and inevitable impurities, was heated to a temperature of 1200° C., and then subjected to hot rolling consisting of five passes of rough rolling and two or more consecutive passes of finish rolling to produce a hot rolled sheet having a sheet thickness of 2.0 mm, which was then coiled at 500° C. At this time, the reduction ratio and strain rate of the fourth and fifth passes of the rough rolling were made the same, the first pass of the finish rolling was performed at 850 to 950° C., and the reduction ratio and strain rate and the inter-pass time of the fourth and fifth passes of the rough rolling, the inter-pass time between the first and second passes of the finish rolling, the finish rolling end temperature, and the cooling time from 750° C. to 650° C. after the finish rolling were variously changed as shown in Table 2.

[0097] A test piece was cut from the center of the length direction and the center of the width direction of the hot rolled sheet obtained as described above, and the structure of the plate thickness section parallel to the rolling direction was observed by SEM. The area 50 mm wide including the center of the plate thickness of 1 / 5 of the plate thickness of the plate thickness section along the rolling direction was measured. 2 The number density of sulfide and selenide precipitates with a long diameter of 4 μm or more and the number density of precipitates composed of their compounds.

[0098] Next, the hot rolled sheet is subjected to hot rolled sheet annealing with a maximum reaching temperature of 1000°C. At this time, the heating time from 600°C to 700°C of the heating process is changed as shown in Table 2. Next, the steel sheet after the hot rolled sheet annealing is cold rolled once at a temperature of 100°C to 1.7 mm using a reversible rolling mill, and after an intermediate annealing of 900°C × 1 minute, a reversible rolling mill is used again for secondary cold rolling to form a cold rolled sheet with a final plate thickness of 0.22 mm. At this time, the combined fracture incidence of the primary cold rolling and the secondary cold rolling is counted in the same manner as in Example 1. It should be noted that in the secondary cold rolling, at least one inter-pass aging treatment is implemented between two or more passes in which the coiled material after being coiled at 200°C is kept for more than 0.5 hours. Then, a decarburization annealing was performed, which also served as a primary recrystallization annealing, 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 60s, to reduce the carbon content of the steel to 0.0050% by mass or less. 2 An annealing separator composed of 5:5 mass % is made into a water slurry and applied to the surface of the steel sheet. After drying, a final annealing is performed to cause secondary recrystallization. Next, after removing the unreacted annealing separator from the steel sheet after the final annealing, a coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3 is applied to the surface of the steel sheet, and sintered at 800°C to form a tension-imparting insulation film.

[0099] For the product sheet thus obtained, test pieces were cut from the center of the coil in the longitudinal direction and the center of the sheet width direction, and the magnetic properties (magnetic flux density B 8 ), and the results are also shown in Table 2. From the results, it can be seen that the steel sheet manufactured under the conditions suitable for the present invention not only has a low fracture incidence rate during cold rolling, but also has excellent magnetic properties.

[0100]

[0101]

Claims

1. A method for manufacturing a grain-oriented electrical steel sheet, wherein: A steel billet having a composition comprising 0.03 to 0.08 mass % of C, 2.0 to 5.0 mass % of Si, 0.005 to 1.0 mass % of Mn, less than 0.010 mass % of Al, 0.006 mass % or less of N, and 0.0060 mass % or less of O, wherein S and Se are contained in a range of 0.0015 to 0.0060 mass % as (S+0.405×Se), and the balance is Fe and inevitable impurities, is hot rolled to produce a steel billet having a composition comprising 0.03 to 0.08 mass % of C, 2.0 to 5.0 mass % of Si, 0.005 to 1.0 mass % of Mn, less than 0.010 mass % of Al, 0.006 mass % or less of N, and 0.0060 mass % or less of O, and wherein S and Se are contained in a range of 0.0015 to 0.0060 mass % as (S+0.405×Se), and the balance is Fe and inevitable impurities. The hot rolled sheet is formed into a hot rolled sheet, the hot rolled sheet is subjected to hot rolled sheet annealing, and then, cold rolling is performed once or twice or more with intermediate annealing to form a cold rolled sheet, the cold rolled sheet is subjected to decarburization annealing which also serves as a recrystallization annealing, and then, final annealing is performed, wherein the hot rolling comprises: performing at least two consecutive passes of rolling in a temperature range of 1050 to 1150° C., and setting the pass time between the two passes to be less than 60 s, setting the reduction rate of each pass to be more than 20%, and setting the strain rate to be 15 s -1 above.

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

3. The method for manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein: After at least one rolling step in the temperature range of 850 to 950° C., the steel plate is kept as is for more than 1.0 second.

4. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 3, characterized in that: The steel slab further contains at least one component selected from the following groups A to C in addition to the above component composition. 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 the group consisting of 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.

5. A hot-rolled sheet for grain-oriented electrical steel sheet, comprising: 0.03-0.08 mass % of C, 2.0-5.0 mass % of Si, 0.005-1.0 mass % of Mn, less than 0.010 mass % of Al, 0.006 mass % or less of N, and 0.0060 mass % or less of O, and containing S and Se in the range of 0.0015-0.0060 mass % as (S+0.405×Se), with the balance being Fe and unavoidable impurities, In the plate thickness section along the rolling direction, the area with a width of 1 / 5 of the plate thickness including the plate thickness center is 50 mm. 2 The number density of precipitates composed of sulfides, selenides and their compounds with a long diameter of 4 μm or more is 0.30 pieces / mm 2 the following.

6. The hot rolled plate for grain-oriented electrical steel sheet according to claim 5, characterized in that: In addition to the above-mentioned composition, it further contains at least one component selected from the following groups A to C, 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 the group consisting of 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