WO1999053106A1 - Method for producing grain-oriented anisotropic electrotechnical steel sheets - Google Patents

Abstract

The invention relates to a method for producing grain-oriented anisotropic electrotechnical steel sheet, containing 0.008 to 0.025 % aluminium, 2.8 to 3.5 % silicon, 0.1 to 0.7 % copper, 0.005 to 0.012 nitrogen, less than 0.06 % carbon, 0.1 to 0.3 % manganese and less than 0.02 % sulphur. The method comprises the following steps: melting the raw material and producing slabs; heating said slabs to 1230 to 1300 DEG C; hot rolling; two-stage cold rolling; tempering; preferably, applying a magnesium oxide-based coating; and high-temperature annealing. The temperature T for heating the slabs prior to hot rolling is adjusted in dependence on the Al-content of the steel according to the following ratio: T( DEG C) = 1230 + ((Al%) - 0.008) * 5300 + 20, Al % being the proportion in wt. % of aluminium in the melt.

Description

 1

Process for the production of grain-oriented anisotropic, electrotechnical steel sheets

The present invention relates to the field of iron metallurgy or black metallurgy, and in particular to a process for the production of grain-oriented anisotropic electrotechnical steel sheets, and in particular for the production of grain-oriented electrotechnical steel sheets with a small thickness of typically 0.2 to 0.4 mm, as used in the Manufacture of magnetic lines used by transformers ..

According to the application or operating conditions in a transformer, a finished electrotechnical steel sheet should have a high magnetic flux density or induction and low power losses during the remagnetization. Furthermore, they should have permanent surface insulation, which is usually created by applying a ceramic layer to the surface.

To achieve the required quality of the magnetic properties, it is necessary to provide a perfect cubic rib texture in the steel. This texture is developed in a slow heating process in the high-temperature annealing step, whereby the normal grain growth is slowed down by the so-called inhibitor phase. Disperse particles of aluminum nitride, manganese sulfide, manganese selenide, boron nitride and the like can take on the role of an inhibitor phase.

The technological processes for steel production are different for different types of inhibitor phases. High temperature slab heating at 1400 ° C (a slab or plate typically has a thickness of 200 mm) before the hot rolling step (rolling to 2.5 mm) is the special characteristic for steel with a sulfide inhibitor. This warming has 2

The aim is to dissolve the MnS in Mn and S and their subsequent elimination during rapid cooling in the course of hot rolling.

The said process is extremely non-technological, requires special equipment, is labor-intensive and is accompanied by metal losses due to oxidation.

A number of patents, e.g. US-A-3, 287, 183 recommends the simultaneous use of MnS and AlN (so-called sulfide-nitride variant) as the inhibitor phase. In this known method, hot rolling of the slabs with a specified content of C, Si, acid-soluble Al (e.g. AlN) and S is carried out. The hot-rolled sheet thus obtained is cold-rolled to reduce the thickness in the range between 5 and 40% and then annealed in the temperature range 950 to 1200 ° C. in order to produce AlN precipitates. Then there is a second cold rolling to reduce thickness in the range between 81 and 95%. Finally, the usual decarburization and high-temperature annealing steps are carried out. This known method is referred to below as the prototype method.

It is now known that copper is an effective inhibitor that enhances the {554} <225> and {110} <001> orientations in the texture of primary recrystallization. Therefore, an additional inhibition by a copper-containing phase is used in some cases. In each of the above cases, the slabs are heated to a high temperature, typically 1400 ° C, before hot rolling.

Experiments carried out for steels with a nitride inhibitor (increased content of AI and low content of S) showed that the influence of copper on the texture and the magnetic properties was not shown for steels which according to 3

conventional technology have been treated by decarburization annealing in the final thickness.

The US-A-3, 873, 388 with the title "Process for the production of electrotechnical sheets with high magnetic permeability" recommends the addition of copper in the order of 0.24 to 0.75% for the steel with a sulfide inhibitor.

In this regard, steels with a nitride inhibitor are different because they do not require high temperature heating before the hot rolling step. However, these steels with only one inhibitor (for example AlN) show instability in the secondary recrystallization, which results in significant differences in the magnetic properties between different furnace cycles and in a lower yield of steel with high quality of the magnetic properties. In other words, the magnetic properties are unstable. Furthermore, the ceramic coating is not uniform in length and width.

It has been found that shortening the inhibitor phase in these steels does not allow the generation of a sufficient volume of {lll} <112> texture which is necessary for the successful growth of the edge grains in the process step of the subsequent secondary recrystallization.

For low carbon steel, an improvement in texture was observed in either slow warming or warming in the area of primary recrystallization, in which context the best development of the preferred {111} <112> or {554} <225> orientations of precipitate particles with sizes of 20 to 30 mm and with a density of 3 to 10 cm ^"1 corresponds. 4

In the prototype process from which the present invention is based, heating is reduced during primary recrystallization before hot rolling. Furthermore, the electrotechnical steel with additives of Se or S can be heated for 0.5 to 10 minutes at temperatures of 660 to 650 ° C before the decarburization annealing.

However, an experiment on steel with AlN as an inhibitor using this method alone showed no significant improvement in the magnetic properties, but also no impairment of the magnetic properties.

The object of the present invention is to provide a method for producing electrotechnical steel sheets with aluminum nitride inhibitor, which offers improved magnetic properties and a more stable secondary recrystallization of the steel.

This object is achieved by the method specified in claim 1.

The {lll} <112> orientation in the steel was strengthened with an aluminum nitride inhibitor (without copper or with an addition of 0.4 to 0.7% copper) before the secondary recrystallization. To generate a sufficient amount of phase-forming impurities from Al and N in the solid solution, the temperature T of the slab heating is set depending on the Al content according to the following equation (1):

T (° C) = 1230 + ((Al%) - 0.008) * 5300 + 20 (1)

where Al% denotes the percentage by weight of aluminum in the melt. 5

The method according to the invention creates steel sheets with excellent magnetic properties and stable secondary recrystallization in steel with nitride inhibition without copper or with an addition of 0.4 to 0.7% copper.

Preferred further developments are specified in the subclaims.

The invention is based on the knowledge that the degree of dissolution of aluminum nitride (AlN) is mainly determined by the concentration of the aluminum, by the heating temperature and by the heating time of the slabs. The relationship between the austenite and ferrite structures also plays a role in the process of allowing the slabs to protrude. All of these parameters must be selected so that the nitride dissolution takes place at temperatures at which oxidation and melting of the slab surfaces does not take place.

However, since a temperature of more than 1300 ° C results in a melting of the surface of the metal and since the liquid phase that forms thereby deteriorates the working conditions of the annealing furnace, the concentration of aluminum must be chosen so that the dissolution of nitrides at lower temperatures is guaranteed. This is achieved according to the invention in that the slab heating temperature is set as a function of the aluminum content.

According to a preferred development, the steel sheet is wound up at temperatures of 520 to 570 ° C. after hot rolling. As experiments have shown, the addition of nitrides can not only prevent the growth of the grains before secondary recrystallization, but can also control the entire process of primary recrystallization by strengthening the orientations in the matrix that are favorable for texture formation. This requires a portion of the nitrogen 6 stay in the solid solution. This is achieved in that the temperature after hot rolling is reduced to 540 to 570 ° C. when the strips are wound up.

According to a further preferred development, slow heating for high-temperature annealing is carried out at a heating rate of 5 to 15 ° C./hour in the temperature range from 400 to 700 ° C. Disperse nitrogen precipitates, which are responsible for the formation of the {lll} <112> texture, appear in the final thickness of the cold-rolled material at the start of primary recrystallization (polygonization) when slow heating (5 - 15 ° C / Hour) in the temperature range of 400 - 700 ° C during high-temperature annealing or during a special heat treatment (aging).

According to a further preferred development, decarburization annealing is carried out in the step of vacuum treatment of the non-deoxidized liquid steel or in the step of heat treatment of the steel sheet with the initial, intermediate or final thickness. Decarburization can advantageously alternatively by vacuum treatment of liquid steel or by heat treatment of rolled strips in the initial (typically 2.5 mm after hot rolling), intermediate (typically 0.7 mm after first cold rolling) or final thickness (typically after second cold rolling) 0.3 mm).

According to a further preferred development, 0.4 to 0.7% copper is added to the steel. This creates an effective combination of aluminum and copper nitrides, which are separated from the supersaturated solid solution in the same temperature range. Copper plays a very different role here than in the above-mentioned US Pat. No. 3,873,388. The particularly preferred features of the present invention are therefore:

1. the specified temperature of the slab heating before the hot rolling step depending on the Al content of the steel;

2. the low winding temperature (520 - 570 ° C);

3. the slow heating (5 - 15 ° C / hour) of the cold-rolled steel of the final thickness in the temperature range of 400 - 700 ° C in the high-temperature annealing phase or in a special heat treatment to separate disperse AlN particles before primary recrystallization;

4. decarburization of the steel in the liquid state or in the form of rolled strips with the initial, intermediate or final thickness; and

5. the addition of 0.3 to 0.7% copper to the steel.

The invention is particularly applicable to steels with the following composition: 2.8 to 3.5% Si, 0.030 to 0.045% C, 0.10 to 0.30% Mn, 0.003 to 0.020% S, 0.008 - 0.025% Al, 0 , 4 - 0.7% Cu.

If the aluminum concentration falls below 0.008%, sufficient structural stabilization by the aluminum nitride is not possible, since a large part of the nitrogen is bound in silicon nitride. When the aluminum concentration rises above 0.025%, the nitrides dissolve in the high temperature range (above 1320 ° C.), the slab surfaces melting and a useful feature of the process according to the invention being lost. The temperature of the slab heating should therefore be 1230 to 1300 ° C, preferably 1250 to 1270 ° C. This prevents the surface from melting and reduces the energy consumption for heating to a minimum.

Exemplary embodiments of the present invention are explained in more detail below with reference to the accompanying drawings.

Show it:

1 shows the distribution of the magnetic flux density W800 (T) for 0.3 mm thick steel, which was produced by the method according to the invention (curve 1) and that according to the usual method with sulfide inhibition (curve 2); and

FIG. 2 shows the distribution of the specific losses PI 7/50 (W / kg) for the example according to FIG. 1.

Two furnace runs in converters with a capacity of 350 tons of the chemical compositions listed below were carried out to test the proposed method:

Steel- Si C Mn S AI P Cu melt

1 3.03 0.032 0.24 0.003 0.012 0.011 0.54

92 2.90 0.034 0.20 0.004 0.025 0.012 0.50

9

Slabs cast in one piece were heated in furnaces with walking beams to a temperature T, which was determined according to equation (1) above using the Al content.

They were left to stand at the above temperature for 1.5 to 2 hours and then hot-rolled into strips with a thickness of 2.5 mm and wound up at a winding temperature in the range from 520 to 570 ° C.

After pickling the hot rolled strips 2.5 mm thick, they were cold rolled to 0.65 mm thick, annealed in a humid nitrogen-hydrogen mixture environment, and then cold rolled to a final thickness of 0.3 mm.

A magnesium suspension was then applied to the strips with the final thickness. Then high temperature annealing took place at 1150 ° C. Disperse precipitation particles made of AlN, which effects the secondary recrystallization, were produced in the process step of slowly heating (5-25 ° C./hour) the cold-rolled strip with the final thickness in the high-temperature annealing phase.

Various decarburization processes were tested: in the liquid state, on rolled strips with the initial and the intermediate thickness.

The parameters of the treatment and the magnetic properties obtained are shown in the table below.

To make a comparison, the steels were treated in accordance with the underlying prototype method, and the parameters of the method proposed according to the invention were also slightly varied. 10

As can be seen from the table and FIGS. 1 and 2, there are mediocre magnetic properties when the prototype process is followed (experiments 1 to 3). The same applies to some modifications to the machining parameters (Experiments 7, 8)

Only steel sheets in which the essential parameters were set according to the invention (experiments 4 to 6) show a stable high quality of the magnetic properties. A deviation from any parameter set according to the invention worsens the steel quality. The positive influence of the copper alloy is also clearly evident from the comparison of the properties of melts 1 and 2 in the context of experiments 4 to 6.

The implementation of the invention under industrial conditions thus confirmed its great effectiveness.

Table 1

Experiment furnace process Slab winding heat treatment Magneti e e ment, heating with the primary properties S

Melting temperature temperature recrystallization no. Sation B800 PI.7 / 50 (T) (W / kg)

1 1 prototype 1260-1280 560-580 20 ° C / min from 1.82 1.30

2 1260-1280 560-580 400 to 600 ° C

2 1 prototype 1300-1320 600-620 20 ° C / min of 1.81 1.32

2 1300-1320 600-620 400 to 600 ° C

3 1 prototype 1240-1260 630-650 20 ° C / min of 1.82 1.33

2 1240-1260 630-650 400 to 600 ° C

4 1 Invention 1230-1250 520-540 5 ° C / min of 1.88 1.14

2 1280-1300 540-560 400 to 700 ° C 1.85 1.22

5 1 Invention 1230-1250 520-540 15 ° C / min of 1.87 1.15

2 1280-1300 540-560 400 to 700 ° C 1.85 1.21

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Table 1 (continued)

6 1 Invention 1230-1250 520-540 10 ° C / min of 1.87 1.13

2 1280-1300 540-560 400 to 700 ° C 1.85 1.20 O

7 1 dev. Para1230-1250 520-540 10 ° C / min from 1.85 1.26 OöJϊ o

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2 meters 1280-1300 540-560 400 to 700 ° C 1.83 1.29

8 1 dev. Para1280-1300 620-640 15 ° C / min from 1.84 1.28 2 meters 400 to 700 ° C

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Claims

13 1. Process for the production of grain-oriented anisotropic electrotechnical steel sheets with 0.008 to 0.025% aluminum, and in particular with 2.8 to 3.5% silicon, 0.1 to 0.7% copper, 0.005 to 0.012% nitrogen, less than 0 , 06% carbon, 0.1 to 0.3% manganese and less than 0.02% sulfur with the following steps: a) melting the raw material and making slabs; b) heating the slabs to 1230 to 1300 ° C; c) hot rolling; d) two-stage cold rolling; e) tempering; f) preferably applying a coating based on magnesium oxide; and g) high temperature annealing; because of the fact that The setting of the slab heating temperature T before hot rolling as a function of the Al content of the steel is carried out according to the following relationship: T (° C) = 1230 + ((Al%) - 0.008) * 5300 + 20 where Al% denotes the percentage by weight of aluminum in the melt. 14 2. The method according to claim 1, characterized in that after hot rolling, a winding of the steel sheet is carried out at temperatures of 520 to 570 ° C. 3. The method according to claim 1 or 2, characterized in that a slow heating for high temperature annealing is carried out at a heating rate of 5 to 15 ° C / hour in the temperature range of 400 to 700 ° C. 4. The method according to claim 1, 2 or 3, characterized in that a decarburization annealing is carried out in the step of heat treatment of the non-deoxidized liquid steel or in a step of heat treatment of the steel sheet with the initial, intermediate or final thickness. 5. The method according to any one of claims 1 to 3, characterized in that 0.4 to 0.7% copper is added to the steel.