EP0910676B1 - Process for producing a grain-orientated electrical steel sheet - Google Patents

Abstract

PCT No. PCT/EP97/03510 Sec. 371 Date Oct. 26, 1998 Sec. 102(e) Date Oct. 26, 1998 PCT Filed Jul. 3, 1997 PCT Pub. No. WO98/02591 PCT Pub. Date Jan. 22, 1998A process for producing a grain-oriented magnetic steel sheet in which a slab, made from a steel containing (in mass %) more than 0.005 to 0.10% C, 2.5 to 4.5% Si, 0.03 to 0.15% Mn, more than 0.01 to 0.05% S, 0.01 to 0.035% Al, 0.0045 to 0.012% N, 0.02 to 0.3% Cu, the remainder being Fe, including unavoidable impurities, is heated through and hot rolled to a final thickness between 1.5 and 7.0 mm. The hot strip is annealed and immediately cooled and cold rolled in one or several cold-rolling steps to the final thickness of the cold strip. The cold strip is subjected to a recrystallizing annealing process in a humid atmosphere containing hydrogen and nitrogen, with synchronous decarburization. A non-stick layer, essentially containing MgO, is applied to the surface of the decarburized cold strip which is then subjected to final annealing. The cold strip is then rolled into coils.

Description

The invention relates to a method for producing grain-oriented electrical sheet from which a slab is made a steel with (in mass%) more than 0.005 to 0.10% C, 2.5 to 4.5% Si, 0.03 to 0.15% Mn, more than 0.01 to 0.05% S, 0.01 to 0.035% Al, 0.0045 to 0.012% N, 0.02 up to 0.3% Cu, balance Fe including unavoidable Contamination at a temperature lower than that Solubility temperature for manganese sulfides, anyway below 1320 ° C, but above the solubility temperature for copper sulfides, is heated through, then with an initial temperature of at least 960 ° C and with a final temperature in the range of 880 to 1000 ° C up to the final hot strip thickness in the range from 1.5 to 7.0 mm is hot rolled, the hot strip then 100 to 600 s long at a temperature in the range of 880 to 1150 ° C annealed, then at a cooling rate greater than 15 K / s cooled and in one or more Cold rolling steps cold rolled to the final strip thickness is what the cold strip of a recrystallizing Annealing in moist, hydrogen and nitrogen containing atmosphere with simultaneous decarburization is subjected and after the double-sided application a release agent essentially containing MgO high temperature annealed and after applying one Insulating coating is finally annealed.

Such a method is described in DE 43 11 151 C1 disclosed. The slab preheating temperature drops to below the solubility temperature of MnS, in each Fall below 1320 ° C is due to the application of copper sulfide as an essential grain growth inhibitor possible. Its solubility temperature is so low that also lowered by preheating this Temperature and subsequent hot rolling in connection with the annealing of the hot rolled strip sufficient formation of this inhibitor phase is possible. MnS plays because of its much higher Solubility temperature as inhibitor and AlN, its solubility and excretion properties between those of Mn and Cu sulfide has only an insignificant part of the inhibition.

The aim of lowering the temperature before hot rolling is Avoiding liquid slag on the slabs, which Wear on the glow devices is reduced and that Production output increased.

EP-B-0 219 611 describes a method which a lowering of the slab preheating temperature in advantageously allows. In doing so (Al, Si) N particles used as grain growth inhibitors, which via a nitriding process into the finished strip thickness cold-rolled and decarburized strip can be introduced. As The measure to carry out this nitriding process is the Annealing atmosphere in the high-temperature annealing selected so that this has a nitriding ability, or it becomes nitriding Additives for glow repair, or combinations of cited both.

EP-B-0 321 695 describes a similar process described. As grain growth inhibitors only (Al, Si) N particles are used. It will additional information on the chemical composition made and another way one Nitriding treatment in connection with the Decarburization annealing demonstrated. Furthermore, the hint given that the slab preheating temperatures are preferred should be below 1200 ° C.

EP-B-0 339 474 also describes a process, but a nitriding treatment in the form of continuous annealing in the temperature range from 500 to 900 ° C. is carried out in detail in the presence of a sufficient amount of NH ₃ in the annealing gas. Furthermore, it is described in detail how the annealing nitriding treatment can be connected directly after the decarburization annealing. The goal here is the formation of (Al, Si) N particles as an effective grain growth inhibitor. It is particularly emphasized that with such a nitriding treatment, at least 100 ppm, but preferably more than 180 ppm, of nitrogen must be introduced. The slab preheating temperature should be below 1200 ° C.

EP-B-0 390 140 represents the special meaning of the Grain size distribution of the decarburized cold strip in particular and gives different methods for their determination on. The slab preheating temperature is always one Temperature specified below 1280 ° C. However Always recommended that the slabs be below 1200 ° C preheat all of the exemplary embodiments listed indicate 1150 ° C as preheating temperature.

In contrast, that known from DE 43 11 151 C1 Procedure the main advantage that Preheating temperatures are not as low as those above 1150 to 1200 ° C mentioned to have to choose. Im often applied mixed-rolling operation of a modern Hot rolling mill are often slab preheating temperatures from 1250 to 1300 ° C because of this Temperature range from hot rolling and energy technology View is particularly favorable. On the other hand, the Use of copper sulfide as an inhibitor decisive advantage, not by an additional one Technology to perform a nitriding treatment and to have to master, but can Grain growth inhibitor already at the beginning of the Generate production route directly. Further processing the hot strip to the finished product is made in this way considerably simplified.

The hot rolled strip is subjected to annealing in order to to excrete the copper sulfide particles, which the Form inhibitor phase. Then there is a Cold rolling to the finished strip thickness. Alternatively, you can the hot-rolled strip is first a first Cold rolling step are then subjected to the inhibitor-eliminating annealing and the last cold rolling to carry out the finished strip thickness. With this band eventually becomes a continuous one Decarburization annealing treatment carried out in a moist Nitrogen and hydrogen containing annealing atmosphere. At the beginning of this annealing treatment, the structure recrystallized and decarburized the tape. Then will an essentially containing MgO Anti-stick coating on the surface of decarburized Cold tape applied and the tape wound up Coils.

The decarburized cold strip coils thus produced are then subjected to a high temperature hood annealing in order to Formation of the Goss Texture on the Process of Initiate secondary recrystallization. Usually the coils with a heating rate of about 10 to 30 K / h slowly heated up in a glowing atmosphere There is hydrogen and nitrogen. At around 400 ° C Strip temperature, the dew point of the hot gas rises sharply because then the crystal water of the essentially MgO containing adhesive protection coating is released. At secondary recrystallization takes place at around 950 to 1020 ° C from. This is already the formation of the cast texture completed, but will continue except for one Temperature of at least 1150 ° C, preferably heated to at least 1180 ° C and at this temperature held for at least 2 to 20 h. This is necessary to the band of the inhibitor particles that are no longer required to clean, because otherwise they remain in the material and hinder the magnetization process in the finished product would. For an optimal cleaning process, after Completion of secondary recrystallization, usually from the beginning of the holding phase the hydrogen content in the Annealing atmosphere greatly increased, e.g. to 100%.

In the heating phase of the high-temperature annealing, a mixture of hydrogen and nitrogen is generally used as the annealing gas, a mixture of 75% hydrogen and 25% nitrogen being particularly common. With this gas composition, a certain nitrogen nitriding of the tape is effected, because with this stoichiometric composition there are enough NH ₃ molecules that are necessary for nitrogen nitriding. This increases the known inhibition based on AlN.

When using the one disclosed in DE 43 11 151 C1 Process in which the inhibition is not on AlN particles, but based on copper sulfide occur occasionally when using this type of annealing Scattering in the course of texture formation (secondary recrystallization) during high temperature annealing. This scatter has a direct effect on the magnetic Evaluate unfavorably. The object of the invention is now in this, during the high-glow these scatters to significantly reduce and thereby the expiry of Secondary recrystallization to stabilize the brought magnetic values to a very good level become.

To achieve this object, it is proposed according to the invention in the generic method that the cold strip for high-temperature annealing in an atmosphere containing less than 25% by volume of H ₂ , the rest nitrogen and / or noble gas, such as argon, is heated at least until the holding temperature is reached. After reaching the holding temperature, the H ₂ content can be steadily increased to 100%.

To evaluate the course of secondary recrystallization and To be able to compare, a number became identical decarburized cold strip samples from a laboratory simulation of subjected to operational high temperature hood annealing. When certain predetermined temperatures are reached during the heating, individual samples were given to this Stack removed. In these samples there were partial states of the Frozen material in this phase of annealing. The range between 900 was used as the temperature interval and 1045 ° C because there is secondary recrystallization expires. On all samples the Coercivity determined and against the Extraction temperature plotted in Fig. 1. The Coercivity is inversely proportional to the average grain size of the structure.

Then you can start secondary recrystallization as a sudden drop in the coercive force detect at a certain sampling temperature. This steep drop as an indicator of the beginning of the Secondary recrystallization is visible in Fig. 1. This Type of examination is called a "recrystallization test" referred to (see M. Hastenrath et al., Anales de Fisika B, Vol. 86 (1990), pp. 229-231). At the same time were on the contents of these recrystallization test samples Determines nitrogen and sulfur. This research showed that decarburized cold strip, which according to DE 43 11 151 was embroidered to a high degree when it with the usual annealing, which is 75% hydrogen and Contains 25% nitrogen in the heating phase, annealed becomes. At the same time, however, the sulfur content in the The course of this glow from strongly. But this means a weakening of inhibition due to the effect of Copper sulfides is based. This desulfurization takes place also in an inhomogeneous manner, from which the observed Variations in the magnetic values can be explained. Becomes but changed the high glow in the manner according to the invention and the hydrogen content during heating up limited to a maximum of 25 vol .-%, only one very much occurs weaker desulfurization. The sulfur content increases only noticeably decrease at higher temperatures when the Secondary recrystallization has already ended. This The situation is explained below using the examples demonstrated.

The use of low levels of hydrogen during the However, heating up phase also significantly increases that Oxidation potential of the annealing atmosphere, which is reflected in Individual cases unfavorable to the later training of the insulating phosphate layer and their adhesion can. This problem only occurs at the beginning of the Heating phase noticeable when the dew point of the hot gas by releasing water vapor from the Anti-adhesive coating increases significantly. A Change in the inhibitor phase occurs through desulfurization but not yet in at these low temperatures Appearance, but occurs only at higher temperatures on. To adversely affect the To avoid surface texture, the Gas composition changed during the heating phase become. So it is cheap to have a high-temperature glow Annealing atmosphere to start with a high Has hydrogen content, and under these conditions heat up to a temperature of 450 to 750 ° C. Then the glow atmosphere should change and on low hydrogen content, e.g. 5 to 10 vol% set and the heating until reaching the Hold level to be continued. From the beginning of the holding phase the hydrogen content is then increased in the usual way 100% increased.

From the examples, the effect of the inventive Measure clearly. Hot strips from melting with the in Chemical compositions listed in Table 1 according to the method described in DE 43 11 151 C1 decarburized cold rolled strip processed. This decarburized Cold strip was divided and three in operational trials subjected to different heat treatment:

"Reference" variant: The first high-temperature annealing referred to as "reference" corresponded to the prior art and contained an atmosphere of 75% by volume H ₂ + 25% by volume N ₂ in the heating phase. The temperature was raised from 15 K / h to a holding temperature of 1200 ° C., held at this temperature for 20 hours and then slowly cooled. An atmosphere of 100% H _{2 was} switched over from the start of the holding time.

"New" variant: The second high-temperature annealing, referred to as "new", represented the measure according to the invention and, in contrast to "Reference", contained an atmosphere of 10% by volume H ₂ + 90% by volume N ₂ in the heating phase.

Variant "inert": The third high-temperature annealing, referred to as "inert", also represented the measure according to the invention, however, in contrast to "new", the inert gas argon was used instead of N ₂ in the heating phase.

The ones compiled in Table 2 were used achieved magnetic properties. These values are in 2a and 2b graphically. Opposite the "Reference" high-temperature annealing (prior art) show the "New" and "inert" heat treatment variants according to the invention much more uniform magnetic values, represented by the polarization from which the stabilizing effect can be seen. These values are also at a high level. The comparison of the two variants according to the invention "new" and "inert" shows that nitrogen is the main component of the annealing gas is most suitable. The use of an inert gas like argon is not useful for cost reasons. The The "inert" variant also shows an improvement and stabilization of the magnetic properties what proves that nitrogen is the main component of Annealing atmosphere is not decisive for this, but the low hydrogen content.

Samples of decarburized cold strip recrystallization tests the above described type performed. It was also three variants formed with the corresponding ones Gas atmospheres in the heating phase as in the above described experiments.

Fig. 1 shows the steep drops of the Coercive field strength that in all three cases one Secondary recrystallization has taken place. The Individual recrystallization test samples were chemical analyzed for their nitrogen and sulfur content.

Fig. 3 shows the development of the nitrogen content and Fig. 4 shows the development of the sulfur content in Temperature interval from 900 ° C to 1045 ° C during the Warm-up phase of the high glow. For both representations were mean values of the measured values of all bands in the table 1 listed melts A to E. The bands were rolled to a finished strip thickness of 0.30 mm.

The development of the nitrogen content during the The heating-up phase in FIG. 3 shows the "reference" variant the expected high rise already at temperatures below 1020 ° C. In contrast, the increase is the "new" variant of the invention significantly weaker pronounced and is only at high temperatures dominant then when the secondary recrystallization has already been completed. In the case of also The variant "inert" according to the invention does not occur at all Increase in nitrogen content because of the glow gas does not contain nitrogen. Noticeable denitrification but occurs only at high temperatures above Secondary recrystallization. The effects of the two Hochglühvarianten invention on development the nitrogen content in the course of the annealing contrary. The effects on the magnetic Properties, however, is roughly the same. So can influencing the nitrogen content of material, the method disclosed in DE 43 11 151 C1 is not the cause of that improvement essential to the invention.

However, if one looks at the development of the Sulfur content during heating and compares the three variants considered here can be the mechanism of action of the method according to the invention easy to recognize: While in the "reference" variant the Sulfur content quite quickly in the course of heating, falls before the start of secondary recrystallization, is this drop in the variants according to the invention "New" and "inert" are much less pronounced. A The sulfur content is reduced with only one corresponding degradation of those acting as inhibitors Explain copper sulfides. In the case of the "reference" high-temperature variant this waste takes place quite well quickly, reducing the inhibitory effect early subsides and thereby the texture selection process at the beginning the secondary recrystallization certain scatter is subjected. By using an inventive The effect of the inhibitor phase becomes a high-glow variant extended in time, which is accordingly cheap the selection process in secondary recrystallization affects.

The development of the sulfur content differs between the inventive and the non-inventive annealing variants in a noteworthy manner only from strip temperatures above 900 ° C. Thus, the advantageous effect of the variant according to the invention also arises if the low-hydrogen incandescent atmosphere is only used at a later point in time during heating. If, for example, the use of very low-hydrogen glow atmospheres in the heating phase (e.g. 5% by volume hydrogen) should cause problems with the surface properties of the strip due to its very high oxidation potential, the method according to the invention can be modified as follows: The annealing starts with a hydrogen-rich one Annealing atmosphere. After a strip temperature of at least 450 ° C and at most 750 ° C has been reached, the composition of the annealing gas is changed and the annealing continued in a low-hydrogen atmosphere. In principle, it would be possible to change the annealing atmosphere only at 900 ° C, but it would be difficult to adequately achieve the strip temperature in a hood annealing device that is used for such annealing processes because of the high thermal capacity of the coiled material used and the resulting temperature gradients to specify exactly. Once the holding temperature of at least 1150 ° C. has been reached, the gas atmosphere is changed again and the hydrogen content is greatly increased, preferably to 100%. The effect of this modification of the method according to the invention is identical to that of the method according to the invention described above. Chemical composition of the 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

Claims (3)

1. A process for the production of grain-oriented electrical quality steel sheet, wherein a slab of steel containing (in % by weight)

   more than 0.005 to 0.10 % C,

   2.5 to 4.5 % Si,

   0.03 to 0.15 % Mn,

   more than 0.01 to 0.05 % S,

   0.01 to 0.035 % Al,

   0.0045 to 0.012 % N,

   0.02 to 0.3 % Cu,

   residue Fe, incl. unavoidable impurities

   is thoroughly heated at a temperature which is lower than the solubility temperature of manganese sulphide, at any rate below 1320 °C, but above the solubility temperature of copper sulphides, whereafter hot rolling is performed to a hot strip final thickness in the range of 1.5 to 7.0 mm from a starting temperature of at least 960 °C and with an end temperature in the range of 880 to 1000 °C, the hot strip then being annealed for 100 to 600 sec at a temperature in the range of 880 to 1150 °C and then cooled at a cooling rate higher than 15 K/sec and cold rolled to cold strip the final thickness in one or more cold rolling steps, whereafter the cold strip is subjected to a recrystallising annealing in a moist atmosphere containing hydrogen and nitrogen, accompanied by decarburisation, and after a separating agent containing mainly MgO has been applied to both sides, the cold strip is high temperature annealed, final annealing being performed after the application of an insulating coating,
   characterised in that for high temperature annealing the cold strip is heated in an atmosphere containing less than 25% by volume H₂, residue nitrogen and/or rare gas such as argon, until a holding temperature has been reached of 1150 to 1200 °C, preferably 1180 °C.
2. A process according to claim 1,
   characterised in that when the holding temperature has been reached, the H₂ component of the annealing gas atmosphere is steadily increased to 100%.
3. A process according to claims 1 and 2,
   characterised in that until a temperature in the range of 450 to 750 °C has been reached, the annealing gas atmosphere contains more than 50 % by volume H₂, the H₂ component being reduced to below 25 % by volume when said temperature has been exceeded, the H₂ component being increased to 100% when the holding temperature has been reached.

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