JP5375678B2 - Non-oriented electrical steel sheet with excellent iron loss and magnetic flux density - Google Patents

Description

  The present invention relates to a non-oriented electrical steel sheet that is used as an iron core material for motors and transformers and has excellent iron loss and magnetic flux density.

  Non-oriented electrical steel sheets are widely used as iron core materials for various motors, transformers, ballasts and the like for heavy electrical equipment and home appliances. Commercially, the grades are classified by iron loss, and they are used according to the design characteristics of the motor and transformer. In recent years, there has been a demand for further reduction of iron loss from the viewpoint of energy saving and further increase in magnetic flux density from the viewpoint of miniaturization of electrical equipment. There is a strong demand.

  Against this background, many technologies aimed at improving iron loss and magnetic flux density have been disclosed so far, including optimization of components, addition of special elements, provision of hot-rolled sheet annealing, higher temperature of finish annealing, etc. Has been put to practical use. One of the factors that these technologies are trying to control is the form of precipitates and is considered to be an important factor because it strongly affects the material properties.

  In general, when fine precipitates are present in a steel sheet, grain growth during annealing is inhibited and iron loss is deteriorated. In particular, fine sulfides are known to inhibit grain growth and significantly deteriorate iron loss. To improve grain growth of steel sheets, the amount of sulfides can be reduced or coarsened, or made harmless. There is a need to. Although the reduction of S is direct in reducing the amount of sulfide, desulfurization, denitrification intensification, and degassing treatment for a long time are required in the secondary scouring process, and an increase in steelmaking cost is inevitable.

  In addition, it is difficult to manufacture steel sheets with excellent characteristics and low cost due to the increased cost of technology such as hot rolling annealing, which is an additional manufacturing process, or higher final finishing annealing temperature. there were.

As another method, a technique for lowering the hot rolling heating temperature has been proposed. For example, Patent Literature 1 discloses a technique for preventing fine precipitation of MnS during hot rolling by heating a rough bar during hot rolling to 950 to 1150 ° C.
However, simply lowering the hot rolling heating temperature in this way does not improve the characteristics sufficiently, and in order to obtain good characteristics, it is necessary to limit the type and state of the sulfide within a specific range. In addition, lowering the hot rolling heating temperature also lowers the hot rolling temperature range, so the rolling load becomes larger, and recrystallization and grain growth after hot rolling become insufficient, conversely deteriorating magnetic properties. In some cases.

Japanese Patent Laid-Open No. 11-61257

  The present invention has been made in view of such a situation, and a non-oriented electrical steel sheet that is extremely excellent in both iron loss and magnetic flux density by optimizing basic components and improving the manufacturing process without giving a new process. It is something that can be obtained.

  The present inventors have studied the optimum production conditions (especially hot rolling conditions) in order to find a production method of a non-oriented electrical steel sheet that can control the sulfide form and obtain good magnetic properties, and particularly among sulfides. It has been found that magnetic properties are good when the amount, size and density of the sulfide containing Cu are within a specific range, and for that purpose, especially hot rolling conditions (temperature, time, history, cooling rate, etc.) The present invention has been completed by clarifying that control of the above is important.

That is, the state of sulfide (especially Cu-containing sulfide) in the final product is limited to a specific range, and hot rolling conditions are effectively utilized as a manufacturing method therefor, that is, Cu: 0.2% or less Is held for 30 minutes or more in a temperature range of 900 ° C. to 1100 ° C., and then hot rolling is started without holding for 30 or more at a temperature of 1150 ° C. or more, and finish hot rolling The non-oriented electrical steel sheet obtained by controlling the cooling rate immediately after hot rolling in the middle and finish is characteristic.
The gist of the present invention is as follows.

(1) By mass%, C: 0.0050% or less, Si: 0.05 to 3.5%, Mn: 3.0% or less, Al: 3.0% or less, S: 0.008% or less, P: 0.15% or less, N: 0.0050% or less, Cu: 0.2% or less, the balance being Fe and inevitable impurities, and (S which is Cu sulfide) / (S in steel ) ≦ 0.2 and (Cu sulfide S) / (Mn sulfide S) ≦ 0.2, a steel further having a diameter of 0.03 to 0.20 μm in the steel plate. The number density of sulfides contained is 0.5 piece / μm ³ or less, and the sulfide containing Cu having a diameter of 0.03 μm or more and 1.0 μm or less in the steel sheet has an average diameter of 0.05 μm or more and a diameter of 0.2 μm. A non-oriented electrical steel sheet characterized in that the ratio of the number of particles having a size of 05 μm or less is 50% or less.

  As described above, according to the present invention, it is possible to obtain a non-oriented electrical steel sheet that is inexpensive and extremely excellent in both iron loss value and magnetic flux density.

The details of the present invention will be described below together with the reasons for limitation. All the contents are mass%.
C has an upper limit of 0.0050% because the magnetic properties are significantly deteriorated by magnetic aging. Further, from the viewpoint of reducing iron loss, C is preferably 0.0020% or less.

  Si is set to 0.05 to 3.5% in view of the balance between magnetic properties and plate passing properties. If it is less than 0.05%, good magnetic properties cannot be obtained, and if it exceeds 3.5%, the plate-passability in the production process is significantly deteriorated due to embrittlement.

  Mn is an important element in the present invention because it reacts with S to form a sulfide. Usually, when Mn is little in the middle, fine MnS may precipitate during hot rolling, and iron loss and magnetic flux density may be remarkably deteriorated. However, in the present invention, since the adverse effect can be avoided by controlling the hot rolling heating condition within a specific range, the lower limit of Mn is not particularly provided. On the other hand, when the amount of Mn is increased, the amount of MnS that is re-dissolved and re-precipitated in the hot rolling step is reduced, so that adverse effects due to MnS are reduced. This effect is saturated when it exceeds 1.0%, but the presence of the solid solution Mn itself suppresses the generation of the {111} orientation, which is disadvantageous for the magnetic flux density, and improves the magnetic flux density. There is no. In view of cost, the upper limit of Mn is set to 3.0%.

  From the viewpoint of promoting the formation of AlN, increasing the size of AlN, and improving the crystal grain growth, there is no need to provide a lower limit for Al. Even if the amount of Al exceeds 1.0%, the effect of promoting the precipitate form is saturated, but since solid solution Al reduces the iron loss by increasing the electrical resistance, it is advantageous in terms of characteristics to contain a large amount It is. However, since molten steel containing high Al deteriorates the operability during casting, the upper limit is made 3.0%.

  S is directly related to the amount of sulfide. If the content of S is large, even if the hot rolling heating conditions are controlled within the range of the present invention, the amount of precipitation increases and the grain growth is inhibited, so the upper limit is made 0.008%. In order to further improve the magnetic properties of the steel sheet, it is preferable to set it to 0.005% or less.

P has the effect of increasing the hardness of the steel sheet and improving the punchability, so the necessary addition amount is determined by the desired punching hardness. However, if the content is excessive, the magnetic flux density deteriorates, so the upper limit is made 0.15%.
If the content of N is large, the amount of nitride increases and the crystal grain growth is inhibited, so the upper limit is made 0.0050%. In order to further improve the magnetic properties of the steel sheet, it is preferable to set it to 0.0030% or less.

  Cu forms sulfides like Mn. However, since the influence of magnetic properties via sulfides is greater than that of Mn sulfides, Cu is a particularly important element in the present invention. Even with a small content of Cu, fine sulfides may be formed during hot rolling, particularly in the processes after finish rolling, and iron loss and magnetic flux density may be significantly degraded. For this reason, although it is preferable that it is as few as possible, it is difficult to set it to 0 because it is inevitably contained in the steel plate from raw materials and scraps mixed in the manufacturing process. On the other hand, if contained excessively, the magnetic flux density is reduced, so the upper limit is made 0.2%.

Next, the state of sulfide, which is an important limiting factor of the present invention, will be described.
In the present invention, control of a sulfide containing Cu is particularly important. In general, the types and forms of sulfides include not only the amount of sulfide-forming elements such as S, Mn, Ti, Mg, Ca and Cu, and production conditions such as hot rolling, but also O, C, oxides, and carbon in the case of complex precipitation. It also varies depending on the content of nitride-forming elements.
In the present invention, (S which is Cu sulfide) / (S in steel) ≦ 0.2 and (S which is Cu sulfide) / (S which is Mn sulfide) ≦ 0.2. .
This is because it is important to reduce the amount of Cu sulfide having a particularly large adverse effect on magnetic properties among sulfides. In particular, the ratio of sulfides other than Cu, such as MnS, which is mainly used in the steel of the present invention, is reduced. It is important to. In order to obtain a sufficient effect, the ratio is preferably 0.1 or less, and more preferably 0.05 or less.

  Here, (S which is Cu sulfide) is a value obtained by quantifying the amount of Cu in the residue obtained by electrolytic extraction of the steel sheet, and converting it to the mass of S as Cu / S = 2/1 in atomic ratio. In addition, (M which is Mn sulfide) is a value obtained by quantifying the amount of Mn in the residue obtained by electrolytic extraction of the steel sheet, and converting it to the mass of S as Mn / S = 1/1 by atomic ratio. .

  In order to obtain the effects of the present invention, the amount of sulfide that can be known by chemical analysis, as well as the control of size, density, and the like that can be determined by direct observation using a microscope or the like, are important. In particular, even when the analytical value cannot be detected by the chemical analysis as described above and the analytical value is 0, a fine and trace amount of sulfide may be observed in direct observation. Controlling sulfides can be important in the present invention. It should be noted that not only sulfides but also composite precipitates with oxides or carbides are considered. When composite precipitates are formed, it is difficult to specify the type of one precipitate and the size of each compound, but when one precipitate is clearly divided into a sulfide part and the other Shall be judged as one sulfide.

  In the present invention, for the sulfide, an extracted replica obtained by the SPEED method is observed with an electron microscope with EDX. The composition is determined by analysis by EDX, and when the non-metallic element mainly observed is S, the sulfide is determined. In addition, because of its small size, the characteristic spectrum of S is not clear, but Mn, Cu, etc. are detected, and no clear spectrum of O, etc. is observed, and morphology comparison with other precipitates that can be identified as sulfides From the present invention, a precipitate that can be almost determined as a sulfide is also considered as a sulfide. Those that are very small in size and do not show a clear characteristic spectrum in the EDX spectrum are excluded from the sulfides to be considered in the present invention. Since this minimum size is approximately 0.03 μm, 0.03 μm is set as the lower limit in the present invention.

The diameter and number of sulfides are measured in a visual field where there is no bias. In the present invention, the magnification is set so that the number of sulfides of a target diameter is about 500 in one field of view, and 10 fields of view are randomly selected. Dividing by the visual field area and the electrolytic thickness by the SPEED method, the average diameter was obtained by dividing the sum of the individual sulfide diameters by the number.
Here, it goes without saying that it is necessary to measure all the target sulfides in the field of view. The number of sulfides and the diameter can also be obtained by using image analysis or the like.
Moreover, although what extended the shape may be seen, about the thing where a shape is not isotropic, let the average of a major axis and a minor axis be the diameter of the precipitate.

  The number density of the precipitates is consumed while the steel sheet is electrolyzed as Fe divalent ions (Fe2 +), and all the precipitates remaining as residues during electrolysis are on the replica. Calculated as supplemented to. In the ordinary replica production by the present inventors, electrolysis is performed with an electric quantity of 50 C (Coulomb) / cm 2 on the surface area of the sample. Become.

  About the sulfide measured as described above, the number density of the sulfide containing Cu having a diameter of 0.03 to 0.20 μm is 0.5 piece / μm 3 or less and containing Cu having a diameter of 0.03 μm or more and 1.0 μm or less. With respect to the sulfide to be produced, good magnetic properties can be obtained by setting the ratio of the number of those having an average diameter of 0.05 μm or more and a diameter of 0.05 μm or less to 50% or less. If the number density is a specific numerical value or more, especially when the proportion of fine particles increases, the grain growth property is remarkably inhibited, and good characteristics cannot be obtained.

Next, the manufacturing method of the non-oriented electrical steel sheet according to the present invention as described above will be described.
The non-oriented electrical steel sheet of the present invention can be obtained by hot rolling, pickling, cold rolling, and recrystallization annealing of a steel slab comprising the above-described components. Furthermore, heat treatment may be performed after hot rolling.

  Among these, the hot rolling conditions particularly important for obtaining the electrical steel sheet of the present invention will be described. In particular, the temperature history before hot rolling and the cooling rate during finish hot rolling and immediately after finish rolling are important requirements, and by controlling this within a predetermined range, the non-oriented electrical steel sheet of the present invention can be surely obtained. Can be obtained.

  That is, the effect of the invention becomes remarkable by maintaining for 30 minutes or more in a temperature range of 900 to 1100 ° C. before hot rolling. If this condition is not satisfied, the above-mentioned sulfide distribution is outside the optimum range defined in the present invention, and the characteristics deteriorate.

The details of this mechanism are not clear, but heating at high temperatures basically increases the amount of sulfide dissolved during heating, increasing the amount of fine sulfides as it precipitates during the subsequent temperature drop in the hot rolling process. It is thought to make it.
On the other hand, when the heating temperature is lowered, the finishing temperature of hot rolling is lowered and the subsequent winding temperature is also lowered, so that it is impossible to expect sulfide growth during winding. In addition, when the finishing temperature is lowered, recrystallization and grain growth are less likely to occur, and the processed structure may remain on the hot-rolled sheet, which may impair the final characteristics.

For this purpose, it is effective to start rolling by holding at high temperature for a short time after holding at low temperature. In this case, particularly, the surface layer becomes high temperature and the temperature drop during the rolling of the entire plate is suppressed, whereby the recrystallization and growth of the hot rolled structure is promoted and the properties in the final product are improved.
This short-time high-temperature heating does not provide the effect of promoting recrystallization and grain growth when the temperature is 1150 ° C. or lower, and when the heating time exceeds 30 minutes, it becomes difficult to control the sulfide form.

  In order to preferably control the sulfide form, it is preferable that the cooling conditions during and after the hot finish rolling are also in a specific range. The average cooling rate during finishing hot rolling is 50 ° C./second or less, and the average cooling rate for 3 seconds after finishing hot rolling is 20 ° C./second or less. The temperature range in which the control of the cooling rate is important is generally a temperature range lower than the heating temperature before hot rolling because the temperature of the steel material drops due to contact with cooling water or a roll during hot rolling.

  The mechanism by which the cooling rate in this temperature range greatly affects the characteristics is not clear, but Cu and Mn also have a balance with the content, and Cu sulfide is generated in a lower temperature range, just around the hot rolling finish temperature. It is thought that it is formed in the temperature range, and the cooling rate in this temperature range has a strong influence on the control and amount of Cu sulfide. In detail, it is considered that a mechanism that takes into account the processing-induced precipitation due to rolling works.

  The form of the sulfide can be controlled to some extent by the heat treatment after finish hot rolling. In this case, the precipitate form can be controlled more preferably when the coiling temperature is 700 ° C. or higher. Further, the same effect can be obtained by subjecting the hot-rolled plate to heat treatment at 700 ° C. or more and 1200 ° C. or less for 5 seconds to 10 minutes.

The manufacturing process after pickling does not need to be special at all, and the effects of the present invention can be obtained in the same manner as in a normal method for manufacturing a non-oriented electrical steel sheet.
In addition, the non-oriented electrical steel sheet obtained by finishing annealing by the manufacturing method demonstrated above retains the outstanding iron loss value and magnetic flux density even if it performs strain relief annealing after that.

  In addition, the effect of the present invention is not only the so-called full-process non-oriented electrical steel sheet used as a motor while suppressing the introduction of strain after annealing. The present invention can also be applied to a so-called semi-processed non-oriented electrical steel sheet in which characteristics are improved by using induced grain growth.

  Furthermore, Sn, W, for the purpose of further improving the magnetic characteristics, additional functions as members such as strength, corrosion resistance and fatigue characteristics, as well as production in the manufacturing process such as casting, annealing, and scrap use Addition or inevitable mixing of trace elements such as Mo, Sb, Cr, Ni, Co does not impair the effects of the present invention.

  Steels having the components shown in Table 1 were melted and formed into slabs by continuous casting, and hot-rolled under the hot rolling conditions shown in Table 2 to obtain hot-rolled sheets having a thickness of 2 mm. After pickling the hot-rolled sheet, it was cold-rolled to 0.50 mm at a rolling reduction of 75%, and then subjected to continuous annealing under the conditions shown in Table 2 to obtain a product. Tables 3 and 4 show the state of precipitates and magnetic properties of the obtained steel sheets.

In addition, since the characteristics of the electrical steel sheet greatly vary depending on the amounts of Si, Mn, and Al contained, the effect of the present invention was evaluated by three groups having almost the same content, that is, test Nos. 1, 2, 3 (Steel A, B), Test No. 4, 5, 6 (steel C, D), test no. Performed in 7, 8, 9 (Steel E, F).
Further, the magnetic characteristics were measured by measuring the characteristics of 0 °, 45 °, and 90 ° from the rolling direction of the coil with a sample having a size of 55 mm × 55 mm, and evaluated by the in-plane average of the steel sheet obtained by the following formula. All measurements were performed as cut out.
(X0 + 2 × X45 + X90) / 4
X0, X45, X90: Characteristics of 0 °, 45 °, 90 ° from the coil rolling direction

  From these results, it can be seen that the steel sheet within the scope of the present invention is extremely excellent in iron loss value and magnetic flux density. On the other hand, when the precipitation state is outside the range of the present invention, the characteristics are poor.

Claims (1)

In mass%, C: 0.0050% or less, Si: 0.05 to 3.5%, Mn: 3.0% or less, Al: 3.0% or less, S: 0.008% or less, P: 0 .15% or less, N: 0.0050% or less, Cu: 0.2% or less, the balance being Fe and inevitable impurities, and (Cu sulfide S) / (S in steel) ≦ 0 .2 and (Sulfur that is Cu sulfide) / (S that is Mn sulfide) ≦ 0.2, and further containing Cu having a diameter of 0.03 to 0.20 μm in the steel sheet For the sulfide containing Cu having a number density of 0.5 pieces / μm ³ or less and Cu in the steel sheet having a diameter of 0.03 μm or more and 1.0 μm or less, the average diameter is 0.05 μm or more and the diameter is 0.05 μm or less. A non-oriented electrical steel sheet characterized in that the ratio of the number of a certain thing is 50% or less.