EP0947597A2

EP0947597A2 - Method of producing a grain-oriented electrical steel sheet excellent in magnetic characteristics

Info

Publication number: EP0947597A2
Application number: EP99105071A
Authority: EP
Inventors: Tomoji c/oNIPPON STEEL CORP. YAWATA WORKS Kumano; Norikazu c/oNIPPON STEEL CORP YAWATA WORK Fujii; Katsuro c/o NITTETSU PLANT DESIGNING CORP Kuroki; Koji c/oNIPPON STEEL CORP. YAWATA WORKS Yamasaki; Yoshifumi NIPPON STEEL CORP. YAWATA WORK Ohata; Hisashi c/o NIPPON STEEL CORP. YAWATA WORKS Mogi
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1998-03-30
Filing date: 1999-03-23
Publication date: 1999-10-06
Anticipated expiration: 2019-03-23
Also published as: EP0947597B1; DE69923102T2; KR19990078406A; EP0947597A3; EP0947597B2; DE69923102D1; DE69923102T3

Abstract

A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics is provided wherein differentiated use of primary and secondary inhibitors enables production without high temperature slab heating, few glass film defects and no lack of uniformity in secondary recrystallization.

A slab containing 0.025 ∼ 0.10% of C, 2.5 ∼ 4.0% of Si as well as acid-soluble Al, Mn, Cu, N, and Seq = (S + 0.406 Se) of 0.008 ∼ 0.05% is reheated to a temperature of more than 1050°C and lower than 1350°C and hot-rolled into a hot-rolled strip. The hot-rolled strip is optionally annealed, subjected to at least one cold rolling with intermediate annealing, whereafter the sheet is decarburization annealed to have primary recrystallization grains after completion of the decarburization annealing of an average diameter of not less than 7 µm and less than 18 µm, nitrided up to the start of secondary recrystallization, coated with an annealing separator composed mainly of MgO, and subjected to final finish annealing.

Description

This invention relates to a method of producing a grain-oriented electrical steel sheet excellent in magnetic characteristics for use primarily in the cores of electrical transformers and the like.
A generally-known method used to produce a grain-oriented electrical steel sheet is to heat the slab to a very high temperature between 1350°C and a maximum of 1450°C and to hold (soak) the slab at this temperature of for a period sufficient to ensure uniform heating throughout its entirety. This is for putting MnS, AlN and the like into solid solution so that they function as inhibitors. Since this method requires the slab to be heated to a very high temperature, however, it involves various problems in actual production. For example, (1) the slag produced by melting of the slab surface layer causes difficult problems from the point of heating furnace maintenance, (2) the desired hot-rolling temperature is difficult to secure, and (3) yield is reduced by large edge cracks occurring in hot-rolled strip.
Various technologies aimed at avoiding the require for such high temperature slab heating have been proposed. These are classified into two categories.
Those classification in the first category, such as taught by Japanese Unexamined Patent Publication Nos. Sho 59-56522 and Hei 5-112827 and Hei 9-118964 adopt a method that combines use of AlN as inhibitor, a slab heating temperature under 1280°C and nitriding up to the start of secondary recrystallization following decarburization annealing. As taught by JP-A-2-182866, for example, in order to ensure good secondary recrystallization in this method, it is extremely important to control the average diameter of primary recrystallization grains after decarburization annealing to within a prescribed range, usually to within the range of 18 ∼ 35 µm. Since the method used for this control relies mainly on adjustment of the decarburization annealing temperature, however, the composition of the oxide layer after decarburization annealing is inevitably varied on top of the unavoidable variation in industrial production. This in turn may hinder formation of a glass film (containing spinel whose main component is forsterite) formed by reacting MgO as the main component with SiO₂ on the surface of the steel sheet. To prevent this hindrance from occurring, rigorous regulation of the conditions for glass film formation, including strict control of the MgO component, is required. In addition, the nitriding conditions become inconstant because of the variation in the composition of the oxide layer formed during decarburization and the resulting variation in the amount of nitrides formed may cause unstable secondary recrystallization. In particular, the method according to JP-A-9-118964, which contains a large amount of Mn in order to improve the iron loss of the steel sheet by increasing its resistivity, is not suitable for industrial production because it is very liable to produce defects in the glass film.
As taught, for example, by JP-A-6-322443, the technologies falling in the second category use Cu_xS (x = 1.8 or 2) as inhibitor and set the slab heating temperature to one not higher than the solution temperature of MnS and not lower than the solution temperature of Cu_xS. This method are characterized by not requiring an additional process such as the nitriding in the first category of methods. However, there is nothing novel in using Cu_xS as an inhibitor for controlling secondary recrystallization and the method is not suitable for producing a high permeability grain-oriented electrical steel sheet having weak texture by applying a final cold rolling reduction ratio of greater than 80% (Iron and Steel, p. 2029, No. 15, Vol. '0, No. 1984). Specifically, as shown in Fig. 4, this technology, which does not conduct nitriding after decarburization annealing up to the start of secondary recrystallization during finish annealing and does not make effective use of AlN, cannot stably provide a product with a high magnetic flux density exceeding 1.89T on an industrial scale. Fig. 4 shows the relationship between the magnetic flux density (B₈:T) and the iron loss (W_17/50:W/kg) of a thickness of 0.30 mm product. In Fig. 4,
, and
are examples when the heating rate in decarburization annealing was below 100°C/sec and above 100°C/sec, respectively, ⋄ represents the TGO of JP-A-6-322443 and the broken line indicates the results of inventor tests and shows the secondary recrystallization to be poor. ○ indicates Comparative Examples of the present invention. In addition, although JP-A-6-322443 calls for precipitation of at least 60% of the total N content in the hot-rolled strip state as AlN, in the case of the hot-rolled strip having the composition shown in the Examples (Mn, S, Al, N) and obtained at a slab heating temperature on the level of 1270°C, uniform precipitation of AlN and MnS, which are precipitates whose solubility product is a quadratic function, is extremely difficult industrially. It is therefore impossible to obtain uniform magnetic characteristics through out the coil length.
An object of this invention is to enable stable and simple production of a grain-oriented electrical steel sheet excellent in magnetic characteristics by compensating for the drawbacks of the technologies in both of the foregoing categories.
The gist of the present invention is as follows:
A first aspect of the invention provides a method for producing a grain-oriented electrical steel containing 2.5 ∼ 4.0% of Si, wherein at least one member selected from among sulfides and selenides is mainly used as a first inhibitor and at least one nitride formed by nitriding up to the start of secondary recrystallization following decarburization annealing is mainly used as a second inhibitor.
A second aspect of the invention provides a method for producing a grain-oriented electrical steel sheet comprising; the steps of reheating to a temperature of more than 1050°C and lower than 1350°C a slab comprising, by weight%, 0.025 ∼ 0.10% of C, 2.5 ∼ 4.0% of Si, 0.010 ∼ 0.035% of acid-soluble Al, 0.0030 ∼ 0.010% of N, Seq = (S + 0.406 Se) of 0.008 ∼ 0.05%, 0.02 ∼ 0.20% of Mn and the balance being Fe and unavoidable impurities, hot-rolling the slab into a hot-rolled strip, optionally annealing the hot-rolled strip, subjecting one cold rolling or to two or more cold rollings the hot-rolled strip with intermediate annealing to form a final sheet thickness, decarburization annealing the cold-rolled sheet, coating the sheet with an annealing separator composed mainly of MgO and subjecting final finish annealing, wherein the method characterized further comprising the step of nitriding the steel sheet from after the decarburization annealing up to the start of secondary recrystallization.
A third aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to the first or second aspect, wherein primary recrystallization grains after completion of the decarburization annealing have an average grain diameter of not less than 7 µm and less than 18 µm. (The average grain diameter means that the average grain diameter corresponds to a circle at the cross-sectional area in the rolling direction.)
A fourth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to third aspects, wherein 0.01 ∼ 0.30% of Cu is further included as a component of the slab.
A fifth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to fourth aspects, wherein the step of nitriding the steel sheet after decarburization annealing is effected on a running strip in a mixed gas atmosphere containing hydrogen, nitrogen and ammonia to increase the amount of nitrogen in the steel sheet by 0.001 ∼ 0.020 wt%.
A sixth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to fifth aspects, wherein heating during the decarburization annealing is conducted at a heating rate of not less than 100°C/sec from start of temperature rise to 650 ∼ 950°C.
A seventh aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to sixth aspects, wherein the strip temperature is adjusted to temperatures within the following ranges during the hot rolling: 850 + 2500 × Seq + 400 × Mn ≦ FOT(°C) ≦ 1100 + 3000 × Seq + 800 × Mn ≦ 1350°C where FOT: starting temperature of finishing hot-rolling (°C), 800 + 2500 × Seq + 400 × Mn ≦ FT(°C) ≦ 1050 + 3000 × Seq + 800 × Mn ≦ 1350°C where FT: finishing temperature of finishing hot-rolling (°C).
An eighth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to seventh aspects, wherein the hot-rolled strip annealing conditions are set to a maximum temperature of 950 ∼ 1150°C and an annealing period of not less than 30 sec and not more than 600 sec.
A ninth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to eighth aspects, wherein 0.02 ∼ 0.30% of at least one of Sn, Sb and P is further included as a component of the slab.
A tenth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to ninth aspects, wherein 0.02 ∼ 0.30% of Cr is further included as a component of the slab.
An eleventh aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to tenth aspects, wherein 0.03 ∼ 0.30% of Ni is further included as a component of the slab.
A twelfth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to eleventh aspects, wherein 0.008 ∼ 0.3% of at least one of Mo and Cd is further included as a component of the slab.
A thirteenth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to twelfth aspects, wherein the cold rolling is conducted at a final cold rolling reduction ratio of 80 ∼ 92%.
A fourteenth aspect of the invention provides a method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of the first to thirteenth aspects, wherein the strip is held in the temperature range of 100 ∼ 300°C for at least 1 min during at least one final cold rolling pass of the cold rolling.
The invention is described in more detail in connections with the drawings, in which:
Fig. 1 is a diagram showing the relationship between glass film defect ratio and Seq,
Fig. 2 is a diagram showing the relationship between magnetic property and nitriding level in the case of sheet thickness of 0.23 mm,
Fig. 3 is a diagram showing the relationship between magnetic property and nitriding level in the case of sheet thickness of 0.27 mm, and
Fig. 4 is a diagram showing the relationship between the magnetic flux density (B₈:T) and the iron loss (W_17/50:W/kg) of a thickness of 0.30 mm product. The open stars and solid stars are examples when the heating rate in decarburization annealing was below and above 100°C/sec, respectively.
The invention will now be explained in detail.
The most salient feature of this invention is that, in a method for producing a grain-oriented electrical steel sheet permitting a lower slab heating temperature than heretofore owing to avoidance of MnS as the main inhibitor for secondary recrystallization, it causes MnS (or MnSe), Cu_xS (or CuSe) etc. to function as primary inhibitors for controlling primary recrystallization grain diameter, causes nitrides (AlN, Si₃N₄ and individual or composite precipitates of Mn etc.) formed by nitriding conducted after decarburization annealing up to the start of secondary recrystallization in finish annealing to function as secondary inhibitors for controlling secondary recrystallization, and enables production of the grain-oriented electrical steel sheet excellent in magnetic characteristics by causing the two types of inhibitors to function effectively. In other words an object of the present invention is to metallurgically separate the functioning stages of inhibitors playing major roles in production of the grain-oriented electrical steel sheet and to cause each to fulfill its own function using different substances.
In production of the grain-oriented electrical steel sheet, the temperature of the decarburization annealing during which primary recrystallization takes place is generally low, i.e., no higher than 930°C. At this stage, therefore, the strong inhibitor formed during high-temperature hot rolling in the conventional method is unnecessary. Since the present invention uses sulfides and/or selenides as the primary inhibitors, the temperature dependence of the primary recrystallization grains is very small and, therefore, the temperature of the primary recrystallization annealing (actually the decarburization annealing) need not be greatly modified. This ensures very high stability of the primary oxide layer composition and the amount of nitrides formed by the ensuing nitriding, remarkably reduces glass film defects, and also eliminates nonuniformity of secondary recrystallization to enable stable industrial production.
On the other hand, in order to produce a sharp Goss orientation capable of providing good magnetic characteristics, the secondary recrystallization requires an inhibitor made stable against high temperatures by addition of sulfides and/or selenides. In the present invention, AlN formed by the nitriding mainly provides this stabilizing effect.
The reasons for limiting the content ranges of the slab components will now be explained.
When the C content is under 0.025%, the primary recrystallization texture becomes inappropriate, and when it is over 0.10%, the steel sheet becomes inappropriate for industrial production owing to difficulty of decarburization.
When the Si content is under 2.5%, desired iron loss property cannot be obtained, and when it is over 4.0%, the steel sheet becomes inappropriate for industrial production owing to extreme difficulty of cold rolling.
When the Mn content is under 0.02%, yield decreases because the hot-rolled strip becomes susceptible to cracking. When the Mn content is over 0.20%, problems arise regarding production stability in actual industrial production because the amount of MnS and/or MnSe increases to the point of causing local differences in the degree of their solid solution state and their amount. S and Se combine with Mn and Cu to form mainly primary inhibitors. When Seq which is a sulfur equivalent value represented by (S+0.406Se) is under 0.008%, however, the primary inhibitor strength becomes too weak to control the primary recrystallization. In this case, the variation in primary recrystallization grain diameter with decarburization annealing temperature becomes so large as to require change of the decarburization annealing temperature depending on the composition. On the other hand, when Seq is over 0.050%, the inhibitor strength becomes too strong, so that poor secondary recrystallization occurs owing to insufficient primary recrystallization.
Al combines with N to form AlN, which functions mainly as a secondary inhibitor. Some AlN is formed before nitriding and some is formed during the high-temperature annealing after nitriding. An Al content of 0.0010 ∼ 0.035% is needed to ensure the required amounts of AlN both before and after nitriding. When the Al content is outside this range, the primary recrystallization grain diameter becomes difficult to control and the secondary recrystallization therefore does not proceed stably.
As pointed out in the foregoing, the present invention uses mainly sulfides and selenides to control the primary recrystallization grains. However, AlN contained in the slab is also necessary for primary recrystallization grain control, and control of the primary recrystallization grain diameter is difficult when the N content is below 0.0030%. On the other hand, the upper limit of the N content is defined as 0.010% because at higher contents defects, i.e., blisters, occur on the steel sheet surface. Owing to this limitation, the amount of N contained in the slab is not sufficient to control the secondary recrystallization. This is why the nitriding explained later is necessary.
When the slab is heated to 1050°C or higher and hot-rolled under the present invention conditions, Cu is finely precipitated together with S and Se and manifests a primary inhibitor effect. As the precipitates also act as precipitation nuclei that more evenly disperse the AlN, they also play a role as secondary inhibitor and by this effect improve the secondary recrystallization. When Cu is present at under 0.02%, these effects decrease to the point of making stable production difficult, while presence of Cu at over 0.3% provides little or no additional effect and causes surface defects called "copper spills (Scab)" during hot rolling.
Sn, Sb and P contribute to improvement of the primary recrystallization texture. Cr has a beneficial effect on formation of a fosterite film (primary film, glass film). When the contents of these elements are below the ranges set out above, the beneficial effects on formation of a fosterite film are slight. When they are above the stated ranges, it becomes difficult to form a stable forsterite film (primary film, glass film). As Ni is highly effective for obtaining uniform dispersion of precipitates as primary and secondary inhibitors, its addition further improves and stabilizes the magnetic characteristics. It has no effect when added to less than 0.02%, while addition to over 0.3% makes formation of a forsterite film difficult because it impedes oxygen enrichment after decarburization annealing.
Mo and Cd also contribute to inhibitor strengthening by forming sulfides and selenides. They have no effect at a content under 0.008%, while when present at over 0.3%, they cause formation of enlarged precipitates that do not function as inhibitors that stabilize the magnetic characteristics.
The reasons for limiting the production steps in the present invention will now be explained.
Although the average diameter of the primary recrystallization grains after completion of decarburization annealing is specified as 18 ∼ 35 µm in Japanese Patent Application 06-046161, for example, in the present invention the average grain diameter of the primary recrystallization grains must be not less than 7 µm and less than 18 µm. This is an extremely important point of the invention as regards achieving excellent magnetic characteristics (particularly iron loss property). One reason for this is that, from the viewpoint of grain growth, the volume fraction of Goss oriented grains that can grow as secondary recrystallization grains becomes greater at the primary recrystallization stage when the primary recrystallization grain diameter is smaller (Materials Science Forum Vol. 204 ∼ 206, Part 2: pp: 631). Another is that the number of Goss nuclei becomes greater owing to the small grain diameter. Since the absolute number of Goss nuclei is therefore several times greater in this invention than when the average diameter of the primary recrystallization grains is 18 ∼ 35 µm, the secondary recrystallization grain diameter is also relatively smaller and the iron loss property is proportionally better.
Since, moreover, the small average diameter of the primary recrystallization grains increases the driving force of the secondary recrystallization, the secondary recrystallization starts early in the temperature of heating stage (at a low-temperature point) of the final finish annealing. In light of the current practice of final finish-annealing sheet in the coiled state, this means that the temperature hysteresis at different points of the coil up to the maximum temperature becomes more uniform (that the temperature increase rate becomes constant). As nonuniformity among different coil locations is therefore markedly reduced, the magnetic characteristics are highly stable.
The present invention requires that the steel sheet be nitrided between the completion of the decarburization annealing and the start of secondary recrystallization. This can be achieved either by the method of mixing nitrides (CrN, MnN and the like) with the annealing separator used during high-temperature annealing or by the method of nitriding the decarburization-annealed sheet as a running strip in an atmosphere containing ammonia. While either method can be used, the latter exhibits better stability in industrial production. When the amount of nitridation is below 0.001%, the secondary recrystallization is unstable, and when it is over 0.020%, many defects exposing the matrix occur in the primary film. The preferred range is 0.005 ∼ 0.015%.
The slab heating temperature prior to hot rolling is an important factor in this invention. Ultra-high temperature slab heating to a temperature exceeding 1350°C encounters severe difficulties in industrial production. Below the lower limit of 1050°C, on the other hand, the hot rolling becomes practically difficult and, moreover, the generation of primary inhibitor, a key point of the present invention, falls to an insufficient level that causes the primary recrystallization grain diameter to vary greatly with the decarburization annealing temperature. From the viewpoint of ease of hot rolling and the shape (crown) of the hot-rolled strip, the preferred slab heating temperature range is 1200 ∼ 1300°C.
The hot rolling temperature is, moreover, prescribed as: 850 + 2500 × Seq + 400 × Mn ≦ FOT (starting temperature of finishing hot-rolling) ≦ 1100 + 3000 × Seq + 800 × Mn ≦ 1350°C 800 + 2500 × Seq + 400 × Mn ≦ FT (finishing temperature of finishing hot-rolling) ≦ 1050 + 3000 × Seq + 800 × Mn ≦ 1350°C.
Below these ranges, sulfides and selenides precipitate excessively and fail to function as primary inhibitors. Since the primary recrystallization grain diameter therefore becomes highly dependent on decarburization annealing temperature, control becomes difficult in industrial production. Hot rolling temperatures above these ranges are not suitable in industrial production because, in actual production, the amount and state of MnS (MnSe) about solid solution comes to vary between different locations in the material, thereby causing local variation in secondary recrystallization.
In the present invention method, a slab is first produced by the conventional continuous casting method to have an initial thickness in the range of 150 mm to 300 mm, preferably 200 mm to 250 mm. It is also possible instead to use a so-called thin slab with an initial thickness in the range between about 30 mm and 70 mm. These ranges are advantageous in that no roughing rolling down to an intermediate thickness is needed at the time of producing the hot-rolled strip. If a slab or strip is produced beforehand by strip casting, moreover, a grain-oriented electrical steel sheet can be produced by the invention using a slab or strip having an even thinner initial thickness.
The heating method adopted for the hot rolling in industrial production is not limited to ordinary gas heating but can instead be induction heating or direct electric heating. No problem is encountered when the shape needed for these special heating methods is obtained by effecting breakdown on the cast slab. When the heating temperature is high, i.e., over 1300°C, this breakdown can be used to improve the texture and lower the amount of C. These are known techniques in the art.
In the cold rolling, when the final cold rolling reduction ratio is under 80%, the {110}〈001〉 texture broadens to the point of making it impossible to obtain high magnetic flux density. On the other hand, a final cold rolling reduction ratio of over 92% reduces the {110}〈001〉 texture to such an extremely low level that the secondary recrystallization becomes unstable.
The annealing of the hot-rolled strip is conducted mainly for the purpose of eliminating the nonuniformity of the texture/inhibitor dispersion that occurs in the strip during hot rolling. The annealing can be effected on the hot-rolled strip or be effected prior to the final cold rolling. At least one continuous annealing are preferably conducted before the final cold rolling in order to even out the heat hysteresis that arises during hot rolling.
Although the final cold rolling can be conducted at normal temperature, holding the strip the temperature range of 100 ∼ 300°C for at least one minute during at least one final cold rolling pass improves the primary recrystallization texture and markedly enhances the magnetic characteristics.
Making the heating rate between room temperature and 650 ∼ 950°C in the decarburization annealing not less than 100°C/sec improves the primary recrystallization texture and enhances the magnetic characteristics. Various methods are available for securing the heating rate. These include resistance heating, induction heating, and direct energy transfer heating. It is known from JP-B-(examined published Japanese patent application) 6-51887, among others, that speeding up the heating rate increases the Goss orientation in the primary recrystallization texture and reduces the secondary recrystallization grain diameter. While JP-B-6-51887 specifices a heating rate of not less than 40°C/sec, in the present invention the heating rate is effective even at 100°C/sec and is preferably 150°C/sec or higher. The decarburization annealing temperature is specified as not lower than 650°C because the effect is low below this temperature owing to incomplete recrystallization, and is specified as not higher than 950°C because decarburization annealing temperatures in excess of 950°C are not used in the production of grain-oriented electrical steel sheet.

〈Example 1〉

Table 1 shows the composition of molten steels produced by an ordinary method, and Table 2 shows the production conditions and the resulting product characteristics. Continuous annealing was conducted at 1100°C for 150 seconds followed by cooling at 20°C/sec. Annealing was then conducted at 850°C for 90 ∼ 150 seconds in a mixed atmosphere of H₂ and N₂ having a dew point of 65°C. This decarburization annealing was conducted at different heating rates of 50°C/sec, 110°C/sec and 180°C/sec. Nitriding by the designated method before and after coating with an annealing separator composed mainly of MgO and secondary recrystallization annealing were then conducted. The secondary recrystallization annealing was carried out in an atmosphere of N₂ = 25%, H₂ = 75% at 10 ∼ 20°C/hr to up to 1200°C. Following this purification annealing was conducted at a temperature of 1200°C for 20 or more hours in an atmosphere of H₂ = 100%. Application of an ordinarily used insulating tension coating and flattening annealing were then conducted.

〈Example 2〉

Fig. 1 shows the primary film defect rate when production was carried out under the following conditions using a material having the compositions set out below. Slabs comprising 0.045 ∼ 0.065% of C, 3.0 ∼ 3.51% of Si, 0.05 ∼ 0.10% of Mn, 0.0060 ∼ 0.0087% of N, 0.08 ∼ 0.20% of Cu, 0.020 ∼ 0.030% of Al, and amounts of Se and S selected to make Seq = 0.006 ∼ 0.050% were produced. Each was heated to 1200 ∼ 1300°C, formed into a 2.3 mm-thick hot-rolled strip, subjected to hot-rolled strip annealing at 980°C for 120 sec, pickled, cold-rolled to a sheet thickness of 1.55 mm, annealed at 1100°C for 150 sec, and final cold-rolled to a sheet thickness of 0.23 mm. At this time, the sheet was held at 180 ∼ 220°C for not less than 2 min in at least two passes. Then, in order to obtain an average primary recrystallization grain diameter of 13 ∼ 15 µm, the sheet was annealed in an atmosphere of H₂ = 75%, N₂ = 25% and dew point of 62°C at, in case (1) of Seq < 0.008%, an annealing temperature of 820 ∼ 870°C for 90 seconds, and at, in case (2) of Seq ≧ 0.008%, an annealing temperature of 850°C for 90 sec, whereafter 0.008 ∼ 0.012% nitriding was effected on a running strip in an ammonia atmosphere, an annealing separator composed mainly of MgO was applied, and high-temperature annealing was conducted. The high-temperature annealing was conducted to 1200°C in H₂ = 75%, N₂ = 25% at a temperature increase rate of 15°C/hr. Purification annealing was then conducted at 1200°C for 25 hours in H₂ = 100%. This was followed by application of an ordinarily used insulating tensile coating and flattening annealing.
As shown in Fig. 1, when Seq < 0.008%, the decarburization annealing temperature has to be changed to obtain primary recrystallization grains of uniform diameter, the oxide layer is not constant, and the primary film defect rate varies and is poor in absolute value. In contrast, when Seq ≧ 0.008%, the decarburization annealing temperature can be constant, the oxide layer is substantially constant, and the primary film defect rate is good and stable.
In passing it is noted that secondary recrystallization was good when Seq ≦ 0.050%.

〈Example 3〉

Production was carried out under the following conditions using a material having the compositions set out below. Figs. 2 and 3 show how the magnetic characteristics differed depending on whether or not nitriding was conducted in the case of sheets of 0.23 mm and 0.27 mm thickness. Slabs comprising 0.045 ∼ 0.065% of C, 3.00 ∼ 3.51% of Si, 0.05 ∼ 0.10% of Mn, 0.0060 ∼ 0.0087% of N, 0.08 ∼ 0.20% of Cu, 0.020 ∼ 0.030% of Al, and amounts of Se and S selected to make Seq = 0.010 ∼ 0.025% were produced. Each was heated to 1200 ∼ 1300°C, formed into a 2.3 mm-thick hot-rolled strip. In the case (1) where the final cold-rolled thickness was to be 0.23 mm, the strip was subjected to hot-rolled strip annealing at 980°C for 120 seconds, pickled, cold-rolled to a sheet thickness of 1.55 mm, annealed at 1100°C for 150 seconds, and final cold-rolled to the sheet thickness of 0.23 mm. In this case, the sheet was held at 180 ∼ 220°C for not less than 2 minutes in at least two passes. It was then annealed in an atmosphere of H₂ = 75%, N₂ = 25% and dew point of 62°C at an annealing temperature of 850°C for 90 seconds. In the case (2) where the final cold-rolled thickness was to be 0.27 mm, the strip was subjected to hot-rolled strip annealing at 1120°C for 120 seconds, pickled, and final cold-rolled to the sheet thickness of 0.27 mm. In this case, the sheet was held at 180 ∼ 220°C for not less than 2 minutes in at least two passes. It was then annealed in an atmosphere of H₂ = 75%, N₂ = 25% and dew point of 62°C at an annealing temperature of 850°C for 120 seconds.
Both nitrided (0.005 ∼ 0.013% nitriding on a running strip in an ammonia atmosphere) and un-nitrided 0.23 mm sheets and 0.27 mm sheets were produced. Each was applied with an annealing separator composed mainly of MgO and high-temperature annealed. The high-temperature annealing conducted to 1200°C at a temperature increase rate of 15°C/hr in (1) H₂ = 75%, N₂ = 25%, (2) H₂ = 50%, N₂ = 50%, (3) H₂ = 25%, N₂ = 75% and (4) H₂ = 10%, N₂ = 90%. Purification annealing was then conducted at 1200°C for 25 hours in H₂ = 100%. This was followed by application of an ordinarily used insulating tensile coating and flattening annealing. The results are shown in Figs. 2 and 3. As can be seen in Figs. 2 and 3, irrespective of which of the atmospheres (1) ∼ (4) the nitriding was conducted in at the time of the high-temperature annealing temperature rise, both the 0.23 mm sheets and the 0.27 mm sheets exhibited excellent magnetic characteristics while the sheets that were not nitrided exhibited inferior magnetic characteristics despite secondary recrystallization.
Since application of the production method of this invention sharply reduces occurrence of primary film defects and eliminates nonuniformity of secondary recrystallization, stable industrial production becomes possible and excellent magnetic characteristics can be secured.

Claims

A method for producing a grain-oriented electrical steel containing 2.5 ∼ 4.0% of Si, wherein at least one member selected from among sulfides and selenides is used as a first inhibitor and at least one nitride formed by nitriding up to the start of secondary recrystallization following decarburization annealing is used as a second inhibitor.
A method for producing a grain-oriented electrical steel sheet comprising the steps of reheating to a temperature of more than 1050°C and lower than 1350°C a slab comprising, by weight%, 0.025 ∼ 0.10% of C, 2.5 ∼ 4.0% of Si, 0.010 ∼ 0.035% of acid-soluble Al, 0.0030 ∼ 0.010% of N, Seq = (S + 0.406 Se) of 0.008 ∼ 0.05%, 0.02 ∼ 0.20% of Mn , optionally at least one selected from 0.02 ∼ 0.30% of at least one of Sn, Sb and P, 0.01 ∼ 0.30% of Cu, 0.02 ∼ 0.30% of Cr, 0.03 ∼ 0.30% of Ni and 0.008 ∼ 0.3% of at least one of Mo and Cd and the balance being Fe and unavoidable impurities, hot-rolling the slab into a hot-rolled strip, optionally annealing the hot-rolled strip, subjecting one cold rolling or to two or more cold rollings the hot-rolled strip with intermediate annealing to form a final sheet thickness, decarburization annealing the cold-rolled sheet, coating the sheet with an annealing separator composed mainly of MgO and subjecting final finish annealing, wherein the method is characterized by further comprising the step of nitriding the steel sheet from after the decarburization annealing up to the start of secondary recrystallization.
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to claim 1 or 2, wherein primary recrystallization grains after completion of the decarburization annealing have an average grain diameter of not less than 7 µm and less than 18 µm.
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of claims 1 to 3, wherein the step of nitriding the steel sheet after decarburization annealing is effected on a running strip in a mixed gas atmosphere containing hydrogen, nitrogen and ammonia to increase the amount of nitrogen in the steel sheet by 0.001 ∼ 0.020 wt%.
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of claims 1 to 4, wherein heating during the decarburization annealing is conducted at a heating rate of not less than 100°C/sec from start of temperature rise to 650 ∼ 950°C.
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of claims 1 to 5, wherein the strip temperature is adjusted to temperatures within the following ranges during the hot rolling. 850 + 2500 × Seq + 400 × Mn ≦ FOT(°C) ≦ 1100 + 3000 × Seq + 800 × Mn ≦ 1350°C FOT:starting temperature of finishing hot-rolling (°C), 800 + 2500 × Seq + 400 × Mn ≦ FT(°C) ≦ 1050 + 3000 × Seq + 800 × Mn ≦ 1350°C where FT: finishing temperature of finishing hot-rolling (°C).
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of claims 1 to 6, wherein the hot-rolled strip annealing conditions are set to a maximum temperature of 950 ∼ 1150°C and an annealing period of not less than 30 seconds and not more than 600 seconds.
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of claims 1 to 7, wherein the cold rolling is conducted at a final cold rolling reduction ratio of 80 ∼ 92%.
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of claims 1 to 8, wherein the strip is held in the temperature range of 100 ∼ 300°C for at least 1 min during at least one final cold rolling pass of the cold rolling.
A method for producing a grain-oriented electrical steel sheet excellent in magnetic characteristics according to any of claims 1, wherein at least one selected from 0.02 ∼ 0.30% of at least one of Sn, Sb and P, 0.01 ∼ 0.30% of Cu, 0.02 ∼ 0.30% of Cr, 0.03 ∼ 0.30% of Ni and 0.008 ∼ 0.3% of at least one of Mo and Cd is further included as a component of the slab.
A grain-oriented electrical steel sheet obtainable by a method according to any one of claims 1 to 10.