KR20000016310A - Process for manufacturing grain-oriented electrical steel sheet - Google Patents

Abstract

PURPOSE: A grain-oriented electrical steel sheet manufacturing process has a slab soaked at a temperature lower than the dissolution temperature of manganese sulfide and higher than that of copper sulfide. CONSTITUTION: The process comprises the steps of:preparing a slab from Fe steel containing 0.005 to 0.10 wt.% of C, 2.5 to 4.5 wt.% of Si, 0.01 to 0.05 wt.% of Si, 0.01 to 0.05 wt.% of S, 0.01 to 0.035 wt.% of AL, 0.045 to 0.012 wt.% of N, 0.02 to 0.3 wt.% of Cu and unavoidable impurities; heating the slab at a temperature below the dissolution temperature of manganese sulfide, lower than 1320°C and higher than the dissolution temperature of copper sulfide; hot-rolling the slab at the initial temperature of at least 960°C and the final temperature of 880 to 1000°C into a hot-rolled strip 1.5 to 7.0 mm in thickness; annealing the hot-rolled strip at 880 to 1150°C for 100 to 600 seconds, cooling the annealed strip at a rate of 15 K/s, and cold-rolling the strip into a cold-rolled strip; annealing the cold-rolled strip under the humid atmosphere containing hydrogen and nitrogen for synchronous decarbonization and recrystallization; applying a dispersing agent containing MgO to both sides of the strip; annealing the strip at high temperature; applying an insulating layer; and finally annealing the strip. The cold-rolled strip for the hot annealing step is heated under the atmosphere containing less than 25 volume % of hydrogen and nitrogen and/or noble gas for the rest, until a threshold temperature.

Description

Unidirectional Electronic Steel Sheet Manufacturing Process

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing a grain-oriented electrical steel sheet, in which the mass (in% by mass) is 0.005 to 0.10% or more of carbon (C), and 2.5 to 4.5% of silicon (Si). ), 0.03 to 0.15% manganese (Mn), 0.01 to 0.05% or more sulfur (S), 0.01 to 0.035% aluminum (Al), 0.0045 to 0.012% nitrogen (N) and 0.02 to 0.3% Slabs made from steel of iron (Cu) containing copper (Cu) and the remainder containing unavoidable impurities, at any rate, but at a temperature below the melting temperature of manganese sulfide, but at 1320 ° C. Heated to a temperature lower than and higher than the dissolution temperature of copper sulfide; It is then hot rolled to an initial temperature of at least 960 ° C. and a final temperature in the range of 880 to 1000 ° C., resulting in a hot strip 1.5 to 7.0 mm thick. The hot strip is then annealed for 100 to 600 seconds at a temperature in the range from 880 to 1150 ° C., followed by instant cooling at a cooling rate exceeding 15 K / s and one or several cold rolling steps Cold rolled through to the final thickness of the cold strip. The cold strip is then subjected to a recrystallising annealing process, in addition to synchronous decarburisation in a humid atmosphere containing hydrogen and nitrogen, and basically comprising a magnesium oxide (MgO) After the parting agent) is applied to both sides, it is annealed at high temperature, and the finishing layer is annealed after the insulation layer is applied.

This process is disclosed in DE 43 11 151 C1. It is possible to lower the preheat temperature of the slab to below the melting temperature of manganese sulfide (MnS), but at any rate but below 1320 ° C. by using copper sulfide as a significant grain growth inhibitor. Because of its low dissolution temperature, it is possible to properly form such an inhibitor phase even by preheating at this lowered temperature and subsequent hot rolling and annealing of the hot rolled strip. Manganese sulfide does not act as an inhibitor because of its high dissolution temperature, and aluminum nitride (AlN), whose solubility and elimination properties are intermediate between the dissolution and removal properties of manganese sulfide and copper sulfide, is merely insignificant. Participate in the inhibition.

The purpose of lowering the temperature prior to hot rolling is to prevent the formation of liquid slag deposits on the slab, thereby reducing wear of the annealing plant and increasing production yield.

EP-B-0219 611 discloses a process that also allows for a drop in slab preheating temperature in an advantageous manner. Here, (Al, Si) N-particles are used as grain growth inhibitors, which are cold rolled to the final thickness and injected into the decarburized strip through a nitriding process. As a method for performing this nitriding process, the annealing atmosphere during annealing coarse grains is selected to have nitriding capability, or other nitriding additives are used for annealing separation, or both A method combining these is disclosed.

EP-B-0 321 695 discloses a similar process. As grain growth inhibitors, only (Al, Si) N-particles are used exclusively. Other details of the chemical composition have been disclosed and show additional possibilities for nitriding in conjunction with the decarburization. It has also been shown that it is desirable to maintain the slab preheating temperature below 1200 ° C.

According to the process disclosed in EP-B-0 339 474, a continuous annealing type of nitriding treatment is carried out in the temperature range of 500 to 900 ° C in the presence of an appropriate amount of ammonia (NH ₃ ) in the annealing gas. It shows in detail how it is done. In addition, a detailed description has been made as to how the annealing nitriding treatment can immediately follow the decarburization annealing treatment. Here too the aim is to form (Al, Si) N-particles as effective grain growth inhibitors. It is particularly emphasized here that at least 100 ppm, preferably at least 180 ppm, of nitrogen must be charged for this nitriding treatment. The slab preheating temperature should be below 1200 ° C.

EP-B-0 390 140 highlights in particular the particular importance of the grain size distribution of decarburized cold strips and provides several methods for the determination. In each case, slab preheating temperatures below 1280 ° C. were stated. However, it is recommended to preheat the slab below 1200 ° C. All examples of the process show 1150 ° C. as the preheat temperature.

In the process according to DE 43 11 151 C1, an important advantage is disclosed that the preheating temperature does not need to be selected to be as low as 1150 to 1200 ° C. In the mixed rolling operation of modern hot rolling mills which are often used, the slab preheating temperature is often set from 1250 to 1300 ° C., which is seen in terms of power engineering and hot rolling techniques. It is because a range is especially preferable. In addition, the use of copper sulfide as an inhibitor is an important advantage, thus eliminating the need to carry out nitriding treatments according to a separate technique and directly generating grain growth inhibitors early in the production process. In this way the further process of treating the hot strip as a final product is greatly simplified.

The hot rolled strip is annealed to remove copper sulfide particles forming the phase of the inhibitor. Cold rolling is then carried out to the thickness of the final strip. As an alternative to this, the hot rolled strip can be passed through a first cold rolling step before annealing to remove the inhibitor and final cold rolling up to the thickness of the final strip. This strip is subsequently subjected to continuous decarburization annealing in a humid annealing atmosphere containing hydrogen and nitrogen. At the beginning of this annealing treatment, the microstructure is recrystallized and the strip is decarburized. Then, a non-stick layer comprising essentially magnesium oxide (MgO) is applied to the surface of the decarburized cold strip which is then rolled into a coil.

The decarburized cold strip coil produced in this manner is then hot annealed in a hooded furnace to initiate the formation of a Goss texture through a secondary recrystallization process. Usually the coil is slowly heated at a heating rate of approximately 10-30 K / h in an annealing atmosphere containing hydrogen and nitrogen. At a strip temperature of approximately 400 ° C., the dew point of the annealing gas rises rapidly because the crystal water of the non-stick layer (basically containing magnesium oxide) which has been applied is released at this stage. Secondary recrystallization occurs at approximately 950 to 1020 ° C. Thus, even if goth tissue formation has already been completed, the temperature nevertheless continues to increase to reach at least 1150 ° C., preferably at least 1180 ° C., which temperature is maintained for at least 2 to 20 hours. This is necessary to remove the inhibitor particles from the strip which are no longer used because otherwise these particles may remain in the material and interfere with the magnetic reversal process in the final product. In order to ensure an optimal cleaning process, when secondary recrystallization is completed, usually from the beginning of the holding phase, the hydrogen content in the annealing atmosphere is significant, for example up to 100%. Is increased to a degree.

During the heating step of the coarse grain annealing, a mixture of hydrogen and nitrogen is generally used as an annealing gas, inter alia a mixture of 75% hydrogen and 25% nitrogen. By this gas composition, the nitrogen content of the strip is increased by a certain amount, since this stoichiometric composition mixture contains a sufficient number of ammonia (NH ₃ ) molecules required for nitrogenisation. In this way, based on aluminum nitride (AlN), the inhibitory action is further increased.

During the process described in DE 43 11 151 C1, where the inhibitory action is based on copper sulfide rather than on aluminum nitride particles, when this type of coarse grain annealing proceeds, sometimes texture formation (secondary recrystallization) Dispersions during the process can occur during hot annealing. This dispersion has a direct and undesirable effect on magnetic values. It is therefore an object of the present invention to significantly reduce this dispersion during coarse grain annealing and in this way to stabilize the progress of secondary recrystallization so that the magnetic values are at very good levels.

In order to achieve this object, in the generic process according to the invention, the cold strip for hot annealing contains less than 25 vol% (vol.%) Of hydrogen (H ₂ ) and the rest is nitrogen. And / or in an atmosphere that is a noble gas such as argon, at least until it reaches a holding temperature. After reaching the holding temperature, the hydrogen content can be increased gradually to 100%.

In order to evaluate and compare the progress of secondary recrystallization, a number of identically decarburized cold strip specimens were placed in laboratory simulations under operating conditions of hot annealing in a hooded furnace. As soon as certain predetermined temperatures were reached while being heated, each specimen was taken out of this stack. In these specimens, the substates of the materials at the stage of coarse grain annealing were were frozen. The temperature range from 900 to 1045 ° C. was chosen as the temperature interval because secondary recrystallization occurs in this range. For all samples, coercive field strength was determined and shown graphically for sampling temperature in FIG. 1. The coercive force is inversely proportional to the average grain size of the microstructure. Thus, the onset of secondary recrystallization can be recognized through a sudden drop in the coercivity at any sampling temperature. This sudden drop, indicating the start of secondary recrystallization, can be seen in FIG. 1. This type of test is referred to as the "recrystallization test" (Analys de Risica B, M. Hastenlas et al., Vol. 86 (1990) pp. 229-231). At the same time, nitrogen and sulfur contents were determined from these recrystallization test specimens. According to these investigations, decarburized cold strips made according to DE 43 11 151 are also highly nitrided when annealed in a conventional coarse grain annealing process containing 75% hydrogen and 25% nitrogen in the heating step. . At the same time, however, the sulfur content is reduced to a significant extent during this coarse grain annealing. This implies a weakening of the inhibitory effect due to the influence of copper sulfide. The desulfurization action also takes place in an inhomogeneous manner, which explains the dispersion of magnetic values is observed. However, if the coarse grain annealing is varied according to the invention and the hydrogen content is limited to a maximum of 25% by volume during heating, only a significantly reduced degree of desulfurization occurs. The sulfur content is reduced to only perceptible at high temperatures, with secondary recrystallization already completed. This fact is illustrated below by some examples.

However, applying a low hydrogen content in the heating step also significantly increases the oxidation potential of the annealing atmosphere, which in each case may adversely affect the formation and subsequent attachment of the insulating phosphoric acid layer. However, this problem is only detected at the beginning of the heating phase when the dew point of the annealing gas rises markedly as a result of the release of water vapor from the non-fixed layer. However, at such low temperatures, it is not yet clear whether there is any change in the inhibitor phase as a result of the desulfurization action; This only occurs at high temperatures. To avoid undesirable effects on the surface condition, the gas composition in the heating step must be changed. Accordingly, it is preferable to start the coarse grain annealing at a predetermined annealing temperature in an annealing atmosphere having a high hydrogen content and to heat it to a temperature of 450 to 750 占 폚 in this state. The annealing atmosphere is then changed such that a low hydrogen content of, for example, 5 to 10% by volume must be set, and heating must be continued until the maintenance step is reached. From the beginning of the maintenance step, the hydrogen content is increased by 100% in the usual way.

The examples illustrate the effect of the means according to the invention. The hot strips from the molten charges of the chemical composition according to Table 1 became decarburized cold strips after an additional process according to the process disclosed in DE 43 11 151 C1. This decarburized cold strip was divided and subjected to three different coarse grain annealing during the test run.

"Reference" variant: The first variant, termed the "reference" variant, is according to the prior art and in the heating step contained an atmosphere of 75 vol% H ₂ + 25 vol% N ₂ . Heating proceeded from ambient temperature to a holding temperature of 1200 ° C. at a rate of 15 K / h; It was kept at this temperature for 20 hours and then slow cooling started. From the beginning of the holding period, the atmosphere was changed to 100% hydrogen (H ₂ ).

"New" variant: A second coarse grain annealing, named "new", provided a means according to the invention, which, in contrast to "reference", was 10 vol% H ₂ + 90 vol% in the heating step. The atmosphere of N ₂ was included.

"Inert" variant: also "inactive", the third coarse-grain annealing of the named but presented a means according to the invention, which in the "new" and, N ₂ instead of the heating step the inert gas is argon to be contrasted Was used.

In this way, the magnetic properties shown in Table 2 were obtained. These values are shown graphically in FIGS. 2A and 2B. Compared to "reference" coarse grain annealing (according to the prior art), the coarse grain annealing variant of "new" and "inert" according to the present invention is significantly more unified magnetic, represented by polarization. Enemy values are shown, thereby showing a stabilizing effect. In addition, these values are at a high level. Comparing the two variants "new" and "inert" according to the invention, it is shown that nitrogen is the most suitable as a major element of the annealing gas. For cost reasons, the use of inert gases such as argon is not suitable. However, the "inert" strain also shows improvement and stability of the magnetic properties, thus demonstrating that the main component of the annealing atmosphere is the decisiveness of the oxygen content, not nitrogen.

Prior to performing coarse grain annealing, samples of decarburization recrystallization tests of the kind described above were performed. Here too, three variations were made in each gas atmosphere of the heating step as in the above experiment.

Figure 1 shows a sharp drop in coercive force where secondary recrystallization occurred in all three cases. Each recrystallization test specimen was chemically analyzed to determine nitrogen and sulfur content.

3 and 4 show the development of nitrogen content and sulfur content at temperature intervals of 900 ° C. to 1045 ° C. in the heating step of the coarse grain annealing treatment, respectively. In the above two figures, the average measurements of all strips of the melting charges A to E listed in Table 1 were calculated. The strip was rolled to a final thickness of 0.30 mm.

In the case of the “reference” variant, the nitrogen content development during the heating step shown in FIG. 3 has already seen the expected high increase even at temperatures below 1020 ° C. In contrast, the increase in "new" strain according to the invention is much less and only significant at elevated temperatures after secondary recrystallization is completed. Also in the case of the "inert" variant according to the invention, no increase in nitrogen content occurs at all because the annealing gas does not contain nitrogen. However, a noticeable decrease in nitrogen content occurs only at high temperatures above secondary recrystallization. Therefore, the effects on the nitrogen content development during the annealing period of the two coarse grain modifications according to the present invention are different. However, the effect on the cognitive properties is about the same. Thus, the effect on the nitrogen content in the material produced according to the process disclosed in DE 43 11 151 C1 cannot be the reason for the improvements which are the subject matter of the present invention.

However, if one examines the development of the sulfur content during the heating period and compares the three variants examined, the effective mechanism of the process according to the invention is easily recognized: in the case of the "reference" variant the sulfur content is While descending considerably fast-even before the start of secondary recrystallization-such a drop is much less in the "new" and "inactive" variants according to the invention. The reduction in sulfur content can only be explained by the corresponding reduction in copper sulfide acting as an inhibitor. In the case of a "baseline" coarse grain annealing variant, this drop occurs quite rapidly, so that the inhibitory effect disappears quickly and thus the texture selection process in the early stages of secondary recrystallization is applied to some dispersions. Will be. By applying the modifications according to the invention, the influence of the inhibitor phase is magnified over time and thus has a favorable influence on the selection step during secondary recrystallization.

The development of the sulfur content only differs between the coarse grain annealing process according to the prior art and the coarse grain annealing process according to the present invention only at strip temperatures of 900 ° C. or higher. Thus, the beneficial effect of the modification according to the invention also takes place only if an anhydrous annealing atmosphere is applied only later in the heating step. For example, if the application of an annealing atmosphere with very little hydrogen (eg 5 vol% hydrogen) in the heating step causes problems with the surface condition of the strip due to the high oxidation potential, the process according to the invention It can be changed as follows: start the annealing in a hydrogen-rich annealing atmosphere, and after obtaining a strip temperature of at least 450 ° C and a maximum of 750 ° C, change the composition of the annealing gas to continue the annealing in an atmosphere of low hydrogen . In principle, once the temperature of 900 ° C. is reached, the above described annealing atmosphere can be varied, but the hood furnace used for this coarse grain annealing process is characterized by the high heat capacity of the loaded coiled material and Due to the temperature gradients that follow, it can be difficult to determine the strip temperature with adequate accuracy. Once the holding temperature of at least 1150 ° C. is reached, the gas atmosphere is once again changed to significantly change the hydrogen content, preferably up to 100%. In terms of effects only, the modification of this process according to the invention is identical to the process according to the invention as described above earlier.

Table 1) Chemical composition of test material in mass%

C Mn S Si Cu Al N Melt A: 0.061% 0.080% 0.023% 3.08% 0.068% 0.020% 0.0079% Melt B: 0.048% 0.089% 0.024% 3.20% 0.077% 0.022% 0.0086% Melt C: 0.058% 0.097% 0.022% 3.21% 0.070% 0.021% 0.0073% Melt D: 0.057% 0.081% 0.027% 3.12% 0.078% 0.022% 0.0074% Melt E: 0.085% 0.081% 0.023% 3.20% 0.071% 0.023% 0.0085%

Table 2) Magnetic properties of the strips described in the examples following different coarse grain annealing processes

Coarse grain annealing type "standard" "new" "Inactive" Melt J800 (T) P 1.7 (W / kg) J800 (T) P 1.7 (W / kg) J800 (T) P 1.7 (W / kg) A 1.91 1.11 1.94 0.91 1.93 1.00 B 1.94 1.03 1.93 0.95 1.92 1.04 C 1.92 1.06 1.94 0.91 1.93 1.01 D 1.89 1.15 1.93 0.95 1.93 0.99 E 1.91 1.09 1.94 0.92 1.93 1.03 medium 1.912 1.09 1.936 0.93 1.925 1.01

Claims (3)

As a unidirectional electromagnetic steel sheet manufacturing process, (by mass%) 0.005 to 0.10% or more of carbon (C), 2.5 to 4.5% of silicon (Si), Sulfur (S) of 0.01 to 0.05% or more, 0.01 to 0.035% of aluminum (Al), 0.045 to 0.012% nitrogen (N), 0.02 to 0.3% copper (Cu), The rest is heated at a rate below the melting temperature of manganese sulfide at a rate below the melting temperature of manganese sulfide but at a temperature below the melting temperature of copper sulfide, slab made from steel, iron (Fe) containing unavoidable impurities. ; Then hot rolled to an initial temperature of at least 960 ° C. and a final temperature of 880 to 1000 ° C. such that the final thickness of the hot strip is 1.5 to 7.0 mm; The hot strip is then annealed at temperatures ranging from 880 to 1150 ° C. for 100 to 600 seconds and then immediately cooled to a cooling rate exceeding 15 K / s, followed by one or more cold rolling steps to the final thickness of the cold strip. Cold rolled; The cold strip is then subjected to recrystallization annealing with simultaneous decarburization in a humid atmosphere containing hydrogen and nitrogen, and then annealing at high temperature after addition of a separator comprising magnesium oxide (MgO) on both sides, Next, in the step of manufacturing a unidirectional electrical steel sheet subjected to an annealing treatment after applying an insulating layer, The cold strip for hot annealing comprises a holding temperature of at least 1150 to 1200 ° C. and preferably 1180 ° C. in an atmosphere containing less than 25 volume% hydrogen and the remainder being a noble gas such as nitrogen and / or argon. The unidirectional electromagnetic steel sheet manufacturing process characterized in that it is heated to. The method of claim 1, After reaching the holding temperature, the hydrogen content of the annealing gas atmosphere is gradually increased to 100%, characterized in that the manufacturing process of the steel sheet. The method according to claim 1 or 2, The annealing gas atmosphere contains at least 50% by volume of hydrogen until the temperature reaches 450 to 750 ° C; When the temperature is exceeded, the hydrogen content is reduced below 25% by volume and after reaching the holding temperature, the hydrogen content is increased by 100%.