EP0910676A1

EP0910676A1 - Process for producing a grain-orientated electrical steel sheet

Info

Publication number: EP0910676A1
Application number: EP97930498A
Authority: EP
Inventors: Manfred Espenhahn; Andreas Böttcher; Klaus Günther
Original assignee: Thyssen Stahl AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 1996-07-12
Filing date: 1997-07-03
Publication date: 1999-04-28
Anticipated expiration: 2017-07-03
Also published as: SK283881B6; SK1899A3; AU3442897A; CZ6899A3; DE19628136C1; CN1078256C; US6153019A; PL331166A1; RU2190025C2; TW425429B; CZ288875B6; WO1998002591A1; CN1219977A; DE59702901D1; ATE198629T1; AU710053B2; IN191758B; JP2000514506A; EP0910676B1; JP4369536B2

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

Process for the production of grain-oriented electrical sheet

The invention relates to a method for producing grain-oriented electrical sheet, in which a slab made of 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 to 0.3% Cu, balance Fe including unavoidable impurities a temperature which is lower than the solubility temperature for manganese sulfides, in any case below 1320 ° C, but above the solubility temperature for copper sulfides, is subsequently heated, with an initial temperature of at least 960 ° C and a final temperature in the range from 880 to 1000 ° C is hot-rolled to a final hot strip thickness in the range from 1.5 to 7.0 mm, the hot strip is then annealed for 100 to 600 s at a temperature in the range from 880 to 1150 ° C., then at a cooling rate of greater than 15 K. / s cooled and cold rolled in one or more cold rolling steps to the final cold strip thickness is, whereupon the cold strip is subjected to a recrystallizing annealing in a humid hydrogen and nitrogen-containing atmosphere with simultaneous decarburization and after high-temperature annealing on both sides of an essentially MgO-containing release agent is annealed and finally annealed after applying an insulating coating. Such a method is disclosed in DE 43 11 151 Cl. The slab preheating temperature can be reduced to below the solubility temperature of MnS, but in any case below 1320 ° C., by using copper sulfide as an essential grain growth inhibitor. Its solubility temperature is so low that preheating at this reduced temperature and the subsequent hot rolling in conjunction with the annealing of the hot-rolled strip also enable this inhibitor phase to be adequately formed. MnS plays no role as an inhibitor because of its much higher solubility temperature and A1N, whose solubility and excretion properties lie between those of Mn and Cu sulfide, has only an insignificant part in the inhibition.

The aim of lowering the temperature before hot rolling is to avoid liquid slag on the slabs, which reduces the wear on the annealing devices and increases the material production output.

EP-B-0 219 611 describes a method which also advantageously enables the slab preheating temperature to be reduced. (AI, Si) N particles are used as grain growth inhibitors, which are introduced into the strip cold-rolled and decarburized to a finished strip thickness using a nitriding process. As a measure to carry out this nitriding process, the annealing atmosphere during the high-temperature annealing is selected so that it has a nitriding ability, or nitriding additives for annealing, or combinations of the two, are mentioned. A similar process is described in EP-B-0 321 695. Only (AI, Si) N particles are used as grain growth inhibitors. Additional information on the chemical composition is given and another option for nitriding treatment in connection with the

Decarburization annealing demonstrated. Furthermore, it is indicated that the slab preheating temperatures should preferably 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 also the formation of (Al, Si) 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 pp, nitrogen must be introduced. The slab preheating temperature should be below 1200 ° C.

EP-B-0 390 140 emphasizes the particular importance of the grain size distribution of the decarburized cold strip and specifies various methods for its determination. In any case, a temperature of less than 1280 ° C is specified as the slab preheating temperature. However, the recommendation is always given to preheat the slabs below 1200 ° C, all of the exemplary embodiments mentioned indicate 1150 ° C as the preheating temperature.

In contrast, the method known from DE 43 11 151 Cl has the essential advantage that the preheating temperatures are not as low as those above 1150 to 1200 ° C mentioned to have to choose. In the often used mixed-rolling operation of a modern hot rolling mill, slab preheating temperatures of 1250 to 1300 ° C are often set, because this temperature range is particularly favorable from the point of view of hot rolling and energy technology. On the other hand, the use of copper sulfide as an inhibitor has the decisive advantage of not having to carry out and control a nitriding treatment using additional technology, but can generate the grain growth inhibitor directly at the beginning of the production process. The further processing of the hot strip to the finished product is considerably simplified in this way.

The hot rolled strip is annealed to remove the copper sulfide particles that are to form the inhibitor phase. This is followed by cold rolling to the finished strip thickness. As an alternative to this, the hot-rolled strip can first be subjected to a first cold rolling step in order to then carry out the annealing process which inhibits the inhibitor and the last cold rolling to the finished strip thickness. With this tape, it becomes a continuous one

Decarburization annealing treatment carried out in a humidifying atmosphere containing nitrogen and hydrogen. At the beginning of this annealing treatment, the structure is recrystallized and the strip decarburized. Then one containing essentially MgO

Anti-adhesive coating applied to the surface of the decarburized cold strip and the strip wound up into coils.

The decarburized cold-rolled coils thus produced are then subjected to high-temperature hood annealing in order to form the cast texture via the process of Initiate secondary recrystallization. Usually, the coils are slowly heated up at a heating rate of about 10 to 30 K / h in an annealing atmosphere consisting of hydrogen and nitrogen. At a strip temperature of around 400 ° C, the dew point of the annealing gas rises sharply because then the crystal water of the anti-adhesive coating essentially containing MgO is released. At about 950 to 1020 ° C, the secondary recrystallization takes place. Although the formation of the cast texture has already been completed, heating is continued to a temperature of at least 1150 ° C., preferably at least 1180 ° C., and the mixture is kept at this temperature for at least 2 to 20 hours. This is necessary in order to clean the belt from the inhibitor particles that are no longer required, because these would otherwise remain in the material and would impede the remagnetization process in the finished product. For an optimal cleaning process, after the secondary recrystallization has ended, usually from the beginning of the holding phase, the hydrogen content in the annealing atmosphere is greatly increased, for example 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 brought about 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 method disclosed in DE 43 11 151 Cl, in which the inhibition is based not on AlN particles but on copper sulfide, this type of high-temperature annealing occasionally occurs Scatters during the course of texture formation (secondary recrystallization) during high-temperature annealing. This scatter has a direct adverse effect on the magnetic values. The object of the invention is now to significantly reduce these scatterings during the high-temperature annealing and thereby to stabilize the course of the secondary recrystallization, as a result of which the magnetic values are brought to a very good level.

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%.

In order to be able to evaluate and compare the process of secondary recrystallization, a number of identically decarburized cold-strip samples were subjected to a laboratory simulation of the operational high-temperature hood annealing. When certain, predetermined temperatures were reached during heating, individual samples were taken from this stack. In these samples, partial states of the material were frozen in this phase of the annealing. The temperature range between 900 and 1045 ° C was chosen because the secondary recrystallization takes place there. The coercive field strength was determined on all samples and plotted against the removal temperature in FIG. 1. The coercive field strength is inversely proportional to the average grain size of the structure. Thereafter, the beginning of secondary recrystallization can be recognized as a sudden drop in the coercive field strength at a certain sampling temperature. This steep drop as an indicator of the start of secondary recrystallization is visible in FIG. 1. This type of examination is referred to as a "recrystallization test" (cf. M. Hastenrath et al., Anales de Fisika B, Vol. 86 (1990), pp. 229-231). At the same time, the nitrogen and sulfur contents were determined on these recrystallization test samples. These investigations showed that decarburized cold strip, which was produced in accordance with DE 43 11 151, is embroidered to a high degree when it is annealed with the usual high-temperature annealing which contains 75% hydrogen and 25% nitrogen in the heating phase. At the same time, however, the sulfur content drops sharply in the course of this high-temperature annealing. However, this means a weakening of the inhibition, which is based on the action of copper sulfides. This desulfurization also takes place in an inhomogeneous manner, which explains the observed scatter in the magnetic values. However, if the high-temperature annealing is changed in the manner according to the invention and the hydrogen portion is limited to a maximum of 25% by volume during the heating, only a much weaker desulfurization occurs. The sulfur content only drops noticeably at higher temperatures when the secondary recrystallization has already ended. This is demonstrated below using the examples.

However, the use of low amounts of hydrogen during the heating phase also significantly increases the oxidation potential of the annealing atmosphere, which in individual cases can have an unfavorable effect on the subsequent formation of the insulating phosphate layer and its adhesion. However, this problem only becomes noticeable at the beginning of the heating phase, when the dew point of the annealing gas increases significantly due to the release of water vapor from the adhesive protective coating. A change in the inhibitor phase due to desulfurization does not yet appear at these low temperatures, but only occurs at higher temperatures. In order to avoid an unfavorable influence on the surface properties, the gas composition should be changed during the heating phase. It is therefore favorable to start a high-temperature annealing with an annealing atmosphere which has a high hydrogen content and to heat up to a temperature of 450 to 750 ° C under these conditions. The annealing atmosphere should then be changed and a low hydrogen content, for example 5 to 10% by volume, set and the heating continued until the holding level has been reached. From the beginning of the holding phase, the hydrogen content is increased to 100% in the usual way.

The effect of the inventive measure becomes clear from the examples. Hot strips from melts with the chemical compositions listed in Table 1 were further processed to decarburized cold strip according to the process described in DE 43 11 151 Cl. This decarburized cold strip was split up and subjected to three different annealing tests:

"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. From the beginning of the cold period, an atmosphere of 100% H _{2 was used} . "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 magnetic properties shown in Table 2 were achieved. These values are shown graphically in FIGS. 2a and 2b. Compared to the "reference" high-temperature annealing (prior art), the high-temperature annealing variants according to the invention "new" and "inert" show significantly 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 "new" and "inert" according to the invention shows that nitrogen is the most suitable as the main constituent of the glow gas. The use of an inert gas such as argon does not make sense for cost reasons. However, the "inert" variant also shows an improvement and stabilization of the magnetic properties, which proves that the nitrogen as the main component of the annealing atmosphere is not decisive for this, but the low hydrogen content.

Before the annealing was carried out, samples of decarburized cold strip recrystallization tests of the type described above were carried out. Three variants were also formed with the corresponding gas atmospheres in the heating phase as in the experiments described above. 1 shows on the basis of the steep drops in the coercive field strength that a secondary recrystallization has taken place in all three cases. The individual recrystallization test samples were chemically 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 the temperature interval from 900 ° C. to 1045 ° C. during the heating phase of the high-temperature annealing. For both representations, mean values of the measured values of all bands of the melts A to E listed in Table 1 were formed. The strips were rolled to a finished strip thickness of 0.30 mm.

The development of the nitrogen content during the heating-up phase in FIG. 3 shows the expected high increase in the “reference” variant even at temperatures below 1020 ° C. In contrast, the increase in the "new" variant according to the invention is markedly weaker and becomes dominant only at high temperatures, then when the secondary recrystallization has already been completed. In the case of the "inert" variant according to the invention, there is no increase in the nitrogen content at all because the annealing gas contains no nitrogen. Noticeable denitrification only occurs at high temperatures above secondary recrystallization. The effects of the two high-glow variants according to the invention on the development of the nitrogen content in the course of the annealing are therefore opposite. However, the effects on the magnetic properties are approximately the same. Thus, influencing the nitrogen content in material which is produced by the process disclosed in DE 43 11 151 Cl cannot be the cause of the 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, the mechanism of action of the method according to the invention can be easily recognized: while in the "reference" variant the sulfur content is quite rapid during the heating process, even before the start of secondary recrystallization, this drop is much less pronounced in the “new” and “inert” variants according to the invention. A reduction in the sulfur content can only be explained by a corresponding breakdown of the copper sulfides acting as inhibitors. In the case of the "reference" high-glow variant, this drop takes place very quickly, as a result of which the inhibitory effect wears off at an early stage and the texture selection process at the beginning of the secondary recrystallization is subjected to certain scattering. By using a high-glow variant according to the invention, the effect of the inhibitor phase is prolonged, which has a correspondingly favorable effect on the selection process in the secondary recrystallization.

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 vol.% 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 in the following way: The annealing begins with a hydrogen-rich 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.

Table 1: Chemical composition of the test material in mass%

Table 2: Magnetic properties of the strips shown in the examples after different annealing

Claims

claims

l. Process for the production of grain-oriented electrical sheet, in which a slab made of 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% AI,

0.0045 to 0.012% N,

0.02 to 0.3% Cu,

Rest Fe including unavoidable

Contamination at a temperature lower than that

Solubility temperature for manganese sulfide, in any case below 1320 ° C, but above the solubility temperature for copper sulfide, is heated through, then with an initial temperature of at least 960 ° C and with a final temperature in the range from 880 to 1000 ° C to the final hot strip thickness Is hot rolled in the range of 1.5 to 7.0 mm, the hot strip is then annealed for 100 to 600 s at a temperature in the range of 880 to 1150 ° C, then cooled at a cooling rate of greater than 15 K / s and in one or several cold rolling steps are cold-rolled to the final cal-band thickness, whereupon the cold-strip is subjected to recrystallizing annealing in a humid hydrogen and nitrogen-containing atmosphere with simultaneous decarburization and after high-temperature annealing on both sides of an essentially MgO-containing release agent and finally annealing after the application of an insulating coating, characterized in 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, at least until the holding temperature is reached at at least 1150 ... 1200 ° C, preferably 1180 ° C, is heated.

2. The method according to claim 1, characterized in that after reaching the holding temperature, the H ₂ portion of the annealing gas atmosphere is continuously increased to up to 100%.

3. The method according to claim 1 and 2, characterized in that the annealing gas atmosphere contains up to a temperature in the range of 450 to 750 ° C more than 50 vol .-% H ₂ , that after exceeding this temperature, the H ₂ portion is reduced below 25% by volume and, after reaching the holding temperature, the H ₂ content is increased to up to 100%.