EP1509627A1

EP1509627A1 - PROCESS FOR PRODUCING COLD-ROLLED STEEL STRIP HAVING A Si-CONTENT OF AT LEAST 3.2 WT.-% USED FOR ELECTROMAGNETIC PURPOSES

Info

Publication number: EP1509627A1
Application number: EP03722586A
Authority: EP
Inventors: Yvan Houbaert; Carl-Dieter Wuppermann; Olaf Fischer; Jürgen Schneider
Original assignee: ThyssenKrupp Stahl AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2002-05-07
Filing date: 2003-05-02
Publication date: 2005-03-02
Anticipated expiration: 2023-05-02
Also published as: ATE303455T1; DE50301115D1; WO2003095683A1; AU2003229765A1; ES2248742T3; JP2005530033A; US20060086429A1; DE10220282C1; EP1509627B1

Abstract

The present invention relates to a cold-rolled steel strip or sheet in thicknesses of <=0.70 mm for electromagnetic applications, consisting of a steel containing (in % by weight) C: <0.01%, Si: 3.2-7%, Al: <2%, Mn: <=1%, the remainder being iron and usual impurities, which after smelting has been cast to form a base material, such as a slab, a thin slab, or a thin strip, which has then been heated through to temperature TR>1000° C. and has been final hot-rolled at a hot-rolling final temperature TF>800° C. to form a hot strip, which has then been cooled, starting from a temperature TC of the hot strip amounting to at least 750° C. but less than 850° C., at a cooling speed DeltaT/Deltat of at least 400° C./min. to a temperature of less than 300° C., subjected after cooling to a surface treatment such as mechanical descaling and/or pickling, after the surface treatment has been cold-rolled at a temperature amounting to maximum 500° C., and has finally been final-annealed.

Description

Cold-rolled steel strip with Si contents of at least 3.2% by weight for electromagnetic applications

The invention relates to a cold-rolled steel strip or sheet in thicknesses of 0,7 0.70 mm for electromagnetic applications with Si contents of at least 3.2% by weight and Al contents of less than 2% by weight, and a method for its manufacture. Cold strips or sheets produced on the basis of high-silicon FeSi steels are usually used as non-grain-oriented electrical sheets.

The term “non-grain-oriented electrical sheet” here means products falling under DIN EN 10106 (“final annealed electrical sheet”) and DIN EN 10165 (“non-final annealed electrical sheet”). In addition, more anisotropic grades are included as long as they are not considered grain-oriented electrical sheets. In this respect the terms "steel strip for electromagnetic purposes" and "steel sheet for electromagnetic purposes" as well as "electrical steel strip" and "electrical sheet metal" are used synonymously in the following.

FeSi steels with a maximum Si content of 3.5% by weight are usually used for the production of non-grain-oriented electrical sheets. FeSi steel alloys with such limited Si contents permit problem-free production in the usual way. In particular, by a limitation of the Si content to contents of <3.0% by weight ensures that, in a conventional procedure, the sheet obtained is free of cracks after cold rolling.

In the course of conventional production, after the steel alloy has melted, the melt is cast into a slab or thin slab. This primary material is then rolled into a hot strip in direct use without reheating or after cooling and reheating in a descaling, a pre-rolling and a hot rolling process, which is generally carried out in a multi-stand hot rolling mill. The hot strip is then subjected to a surface treatment which is usually carried out as pickling, which can be combined with annealing. If necessary, hot strip annealing is also carried out before the hot strip is cold rolled into cold strip. Finally, the strip is finally annealed or subjected to annealing with subsequent post-forming.

Even with Si contents of more than 3% by weight, the first difficulties with cold rolling in the form of high rolling forces and an increasing susceptibility to cracking are evident. During the cold rolling of hot strips produced from FeSi alloys with FeSi contents with more than 3.5% by weight, cracks regularly occur, which preclude the production of a high-quality electrical sheet product with thicknesses <0.75 mm via the conventional production route.

The difficulties in production are offset by the fact that the increase in the Si content leads to an increase in the electrical resistance and thus to a lowering of the magnetic losses in the application. For a number of applications, especially for audio, video, Small and very small machines used in data processing and medical technology, as well as for drives and for magnetic cores in electromagnetic applications that work at higher frequencies, are therefore made of FeSi alloys with Si contents in the range from 3.5% by weight to 7.0% .-% generated electrical sheets of particular interest. These highly silicon-containing materials have a high saturation magnetization compared to the other soft magnetic materials, such as amorphous Fe, FeNi, FeCo base alloys, nanocrystalline soft magnetic materials or soft magnetic ferrites. This higher saturation magnetization is combined with higher electrical resistance values and therefore lower magnetic losses compared to conventional electrotechnical steels, which enables use at higher frequencies.

FeSi materials with an Si content of approximately

6.5% by weight is available on the market. These products are manufactured using a chemical process

Deposition of a highly silicified FeSi layer on a conventional electrical steel and a subsequent one

Diffusion annealing.

In this way, the difficulties encountered in conventional production of sheets with high silicon contents can be avoided. However, additional work steps are required for this, which complicate the production and make it more expensive.

In the scientific literature there are numerous works in which the forming behavior of FeSi alloys with Si contents of more than 3.2% by weight is investigated and the possibilities of manufacturing such a steel on the usual metallurgical route have been considered. For example, G. Schlatte, W. Pietsch in the Zeitschrift für Metallkunde, volume 66 (1975), issue 11, page 661 ff., And W. Pepperhoff, W. Pietsch in Archiv Eisenhüttenwesen 47 (1976), No. 11, page 685 ff., mentions that a steel with up to approx. 6% by weight silicon can still be formed at around 400 ° C to 300 ° C (critical temperature: 300 ° C). Below a critical temperature dependent on the Si content, brittle behavior occurs and, as a result, a cold brittleness which does not permit cold working. Forming above the critical temperature, on the other hand, is possible for FeSi alloys with more than 4% by weight silicon, provided that the alloy processed in each case is cooled from temperatures below 700 ° C to a temperature below 400 ° C. The limitation of the deformability to a temperature range above the critical temperature, as found in the above-mentioned specialist articles, also severely limits the possibilities of manufacturing highly silicated electrical steel products using the conventional production route.

G. Rassmann, P. Klemm has found in Neue Hütte, issue 7, 8th year, 1963, page 403 ff. That for alloys with 5 and 6% by weight Si, cold rolling at 220 ° C or 350 ° C with a total forming up to about 40% and further rolling at room temperature can be realized. Comparable information can be found in US Pat. No. 3,099,176. With this type of two-stage cold rolling at different temperatures, however, the history of the material up to the cold rolling is not reflected. In practice, however, it turns out that, as the above-mentioned work by G. Schlatte and W. Pietsch confirms, such cold rolling cannot in reality be easily implemented for any hot strip that is manufactured in any way, so that hot strip production has a considerable influence on the Processability of a hot strip having the highest silicon content into cold strip.

In addition to the prior art mentioned above, it is known from EP 0 229 846 B1 to set the overall degree of deformation achieved during hot rolling as a function of the grain size before final rolling (finish hot rolling). However, this process path has the disadvantage that the grain size before final rolling depends on the conditions of reheating and roughing as well as on the respective chemical composition. As a result, the grain sizes present in the pre-rolled steel pre-product before entering the finished hot rolling mill cannot be clearly specified. In addition, the measurement of grain size in a production process that is continuously running in practice cannot be carried out with a technically and cost-justifiable effort.

EP 0 377 734 B1 describes a process for FeSi alloys in which, after the slab has been reheated, it is shaped at temperatures of not less than 600 ° C. and then used directly for further hot rolling or for reheating to temperatures of not less than 400 ° C followed by hot rolling. Cold rolling is then carried out to the final thickness. These process parameters are not specific for higher silicided alloys. Practice shows that when using the process steps known from EP 0 377 734 B1 for highly silicified FeSi alloys of the type processed according to the invention, no satisfactory work results can be achieved.

According to EP 0 467 265 A2, a highly siliconized FeSi steel can be cold rolled by cold rolling at sheet temperatures in the range from 120 ° C to 350 ° C. However, it is not specified how the hot strip is produced, which can be processed in this way. When this known method is used in practice, the problem arises that, as the above-mentioned specialist articles and the applicant's own investigations have shown, the processing of highly silicated electrical steel is not independent of the parameters observed during hot strip processing. Practical tests have shown that, in the conventional production of hot strip with a Si content of more than 3.5% by weight and subsequent cold rolling under the conditions specified in EP 0 467 265 A2, cracking regularly occurred in the first cold rolling pass.

Based on the prior art explained above, the object of the invention was to provide a practically producible, cold-rolled steel sheet or strip suitable for electromagnetic applications with thicknesses of at most 0.70 mm and an Si content of 3.5% by weight and to create more and to name a process with which such a product can be manufactured inexpensively.

With regard to the product, this task is solved by a cold-rolled steel strip or sheet in thicknesses of <0.70 mm for electromagnetic applications, which produces is made of a steel that contains (in% by weight) C: <0.01%, Si: 3.2 - 7%, Al: <2%, Mn: <1%, balance iron and usual impurities, and after melting to form a starting material, such as a slab, a thin slab or a thin strip, which was then heated to a temperature T _R > 1000 ° C and hot-rolled at a hot rolling end temperature T _F of> 800 ° C to a hot strip is then cooled from a temperature T _{c of} the hot strip of at least 750 ° C but less than 850 ° C to a temperature of less than 300 ° C at a cooling rate of at least 400 ° C / min, after cooling subjected to a surface treatment, such as mechanical descaling and / or pickling, after the surface treatment has been cold-rolled at a temperature T _{CR of} not more than 500 ° C. and finally annealed.

With regard to the method, the solution to the above-mentioned object according to the invention is that the following steps are carried out when producing a cold-rolled steel strip or sheet for electromagnetic applications:

Melting of a (in% by weight) C: <0.01%,

Si: 3.2 - 7%, AI: <2%, Mn: <1%, balance iron and steel containing common impurities,

Casting the steel into a starting material, such as a slab, a thin slab or a thin strip,

- warming the primary material to a temperature T _R > 1000 ° C, - finish hot rolling of the pre-heated material at a hot rolling end temperature T _F of> 800 ° C to a hot strip,

- Cooling of the hot strip after the finish hot rolling, starting from a temperature T _{c of} the hot strip of at least 750 ° C but less than 850 ° C with a cooling rate ΔT / Δt of at least 400 ° C / min to a temperature of less than 300 ° C .

- surface treatment of the cooled hot strip,

- Cold rolling the surface-treated hot strip at a temperature T _CR and not exceeding 500 ° C

- Final annealing of the cold-rolled steel strip or sheet obtained.

The invention is based on the knowledge that, based on a conventionally composed, highest content of silicon from -3.2% by weight to 7% by weight and Al contents of up to 2% by weight containing steel alloy while maintaining who uses conventional cold strip production steps to produce a high quality, especially crack-free cold strip, if

- the reheating temperature,

- the hot rolling end temperature,

- The targeted rapid cooling of the hot strip starting from a temperature lying in a certain temperature range after the end of the finish rolling and - The temperature of the strip during cold rolling

are coordinated with one another in the manner prescribed by the invention.

Surprisingly, it has been shown that excessive brittleness of the processed material can only be avoided by adhering to the combination of the relevant parameters according to the invention and that the hot strip has sufficient ductility for proper cold rolling, which is necessary for the production of crack-free electrical sheet with the desired final thickness of at most 0 , 70 mm, preferably at most 0.35 mm is required.

Each of the parameters concerned has an equivalent meaning. Thus it has been found that where the time set for the start of the cooling temperature range has been exceeded going beyond a tolerance range or below in such ^cases', no crack-free product could receive.

In cases where the hot rolling end temperature is more than 800 ° C but less than 850 ° C, the hot strip can be cooled immediately after the hot rolling. Otherwise, it must be waited for the rapid cooling to begin until the temperature of the hot strip has dropped to the range specified by the invention, within which the rapid cooling is to begin.

Of course, the hot strip cooled according to the invention can be coiled into a coil at a suitable point in time of the production process before it is fed to the cold strip for further processing. Of course, it is also possible to limit the production route according to the invention to panels. With regard to the transition from hot strip production to the production of the cold strip, the speed at which the hot strip is cooled rapidly after hot rolling is of particular importance. If the hot strip is further processed into cold strip within a period within which cold embrittlement does not yet occur even in the range of the lower limit of the cooling rate to be observed, then a crack-free cold-rolled steel product can be produced even at relatively low cooling rates. However, if a longer period of time, such as many days or weeks, elapses between hot strip production and cold rolling, a crack-free steel strip or sheet produced in accordance with the invention can still be safely produced for electromagnetic purposes by the fact that the

Cooling rate ΔT / Δt is at least 2000 ° C / min. With such a high cooling rate, the embrittlement effects to be expected with a longer storage period of the hot strip and a slower cooling can be avoided.

The primary material is preferably reheated at temperatures in the range from 1000 ° C. to 1190 ° C. in order to reliably avoid the formation of feyalite.

Particularly good electromagnetic properties of the cold-rolled electrical sheet obtained are obtained if the optionally pre-rolled pre-material is finish-rolled in a maximum of seven passes with a total forming of more than 90% to a final hot strip thickness of not more than 1.5 mm. It serves the same purpose if the degree of deformation during cold rolling is greater than 60% but less than 82%. is.

A further essential feature of the invention consists in that during cold rolling the upper limit of the temperature of the processed strip, which is specified by the invention, is maintained within the scope of the tolerance that is unavoidable due to production. Basically, it is therefore favorable if the hot strip is at room temperature at the start of cold rolling. The heat development during cold rolling which is unavoidable as a result of the input of deformation energy should preferably be managed in such a way that temperatures of <200 ° C are not exceeded. If cold rolling should nevertheless be carried out at elevated temperatures, taking into account the research results explained at the outset, these should be in the range of 200 ° C and 500 ° C. The time provided for preheating the hot strip before hot rolling should be limited to less than 20 minutes in order to avoid structural changes which would otherwise occur. These cause embrittlement.

The invention is suitable for the production of electrical sheets located in the lower region of the steels containing the highest silicon, 4.0-5.0% by weight Si, for the production of more than 5.0 wt.% Located in the middle region of the steels containing the highest silicon. % Of electrical sheets containing Si and for the production of 6.0 - 6.8% by weight of electrical sheets located in the upper region of the high-silicon steels. In this case, in particular in the alloys having the higher Si contents, the Al content the range of unavoidable impurities.

The invention is explained in more detail below on the basis of exemplary embodiments.

To demonstrate the effect of the invention, a HiSi steel and a LoSi steel were melted and cast into slabs. The alloys of the HiSi and LoSi steels are shown in Table 1.

Figures in% by weight Table 1

The slabs have been reheated to a reheating temperature T _R , pre-rolled and finally hot-rolled in a hot rolling mill comprising seven rolling stands at a hot rolling end temperature T _F to a hot strip with a thickness WB _D.

After leaving the hot rolling mill, the hot strip is at a temperature of at least 400 ° C / min

Cooling rate ΔT / Δt was cooled as soon as its temperature T _{c was} in the range from 750 ° C to 850 ° C. The hot strip cooled to room temperature in this way is then subjected to a mechanical pretreatment of its surfaces and then pickled. In order to demonstrate the influence of heating of the hot strip before cold rolling, some of the hot strips produced in the manner described above have been heated to a temperature T _CR within a time t _CR .

Total forming degrees Δ _K w have been achieved in cold rolling itself.

The process parameters observed in the course of the production are entered in table 2 for six cold strips E1 to E6 produced according to the invention.

*) RT = room temperature table 2

These examples show that despite the high silicon content of both HiSi and LoSi steel alloys, crack-free electrical sheets can be produced as long as the reheating temperature, the hot rolling end temperature, the temperature at which cooling begins, the cooling rate and the Temperature during hot rolling remain within the scope specified by the invention.

In order to further verify this, three cold strips VI to V3 and a cold strip V4 were produced from the HiSi alloy using the process steps used in the production of the samples E1 to E6 according to the invention, but with process parameters outside the specifications of the invention , The parameters in question are entered in Table 3 for the cold strips VI to V4 produced according to the invention and not for comparison.

) RT = room temperature

**) Cold rolling stopped due to Cracking in the first stitch

Table 3

It was shown that a deviation in just one process parameter means that crack-free cold strip can no longer be produced. For example, in comparative sample VI the excessively high temperature T _c , from which the rapid cooling took place, also leads to of the sample E1 according to the invention essentially corresponding parameters for crack formation lying within the invention. The temperature T _c which was too low for the comparison sample V2 and the cooling rate ΔT / Δt which was too low had the same effect in the comparison samples V3, V4.

Claims

P A T E N TAN S P RÜ C H E

Cold rolled steel strip or sheet in thicknesses of <0.70 mm for electromagnetic applications

- from a steel which (in% by weight) C: <0.01%,

AI: <2%,

Mn: <1%,

Balance iron and usual impurities, contains and

- after melting to form a starting material, such as a slab, a thin slab or a thin strip, has been cast,

- Which is then warmed to a temperature T _R > 1000 ° C and

- at a final hot rolling temperature T _F of> 800 ° C, hot rolling has been finished,

- subsequently cooling from a temperature T _{c of} the hot strip of at least 750 ° C but less than 850 ° C with a cooling rate ΔT / Δt of at least 400 ° C / min to a temperature of less than 300 ° C,

- after cooling, subjected to a surface treatment, such as mechanical descaling and / or pickling,

- cold rolled after surface treatment at a temperature T _CR not exceeding 500 ° C and - has finally been annealed.

2. Steel strip or sheet according to claim 1, d a d u r c h g e k e n n z e i c h n e t, that the hot strip had at room temperature at the start of cold rolling.

3. Steel strip or sheet according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the temperature reached during cold rolling was <200 ° C.

4. Steel strip or sheet according to claim 1, characterized in that the hot strip has been heated to a temperature of 200 ° C to 500 ° C within a period of less than 20 minutes before hot rolling and has been cold rolled at temperatures in this temperature range ,

5. Steel strip or sheet according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that the final annealing was carried out in a decarburizing atmosphere.

6. Steel strip or sheet according to one of claims 1 to 4, characterized in that the final annealing is carried out in a non-decarburizing atmosphere.

7. Steel strip or sheet according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that its thickness is at most 0.35 mm.

8. Steel strip or sheet according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that the steel contains 4.0-5.0% by weight of Si.

9. Steel strip or sheet according to one of claims 1 to 7, d a d u r c h g e k e n n z e i c h n e t, that the steel contains> 5.0 - 6.8 wt .-% Si.

10. Steel strip according to claim 9, d a d u r c h g e k e n n z e i c h n e t that the steel contains 6.0 - 6.8 wt .-% Si.

11. A method of manufacturing a cold rolled steel strip or sheet for electromagnetic applications, which comprises the following steps:

Melting of a (in% by weight) C: <0.01%,

Si: 3.2 - / 'S,

AI: <2%,

Mn: <1%,

Remainder of iron and steel containing common contaminants,

Casting the steel into a starting material, such as a slab, a thin slab or a thin strip, - warming the primary material to a temperature T _R > 1000 ° C,

- finish hot rolling of the pre-heated material at a final hot rolling temperature T _F of> 800 ° C to a hot strip,

- Cooling of the hot strip after the finish hot rolling, starting from a temperature T _{c of} the hot strip of at least 750 ° C. but less than 850 ° C. with a minimum

400 ° C / min cooling rate ΔT / Δt to a temperature less than 300 ° C,

- surface treatment of the cooled hot strip,

- Final annealing of the cold-rolled steel strip or sheet obtained.

12. The method of claim 11, d a d u r c h g e k e n n z e i c h n e t, that the cooling rate is ΔT / Δt> 2000 ° C / min.

13. The method according to any one of claims 11 or 12, d a d u r c h g e k e n n z e i c h n e t, that re-heating of the primary material takes place at temperatures in the range of 1000 ° C to 1190 ° C.

14. The method according to any one of claims 11 to 13, characterized in that the primary material is finish-rolled in a maximum of seven passes with a total deformation of more than 90% to a thickness of the hot strip of at most 1.5 mm.

15. The method according to any one of claims 11 to 14, d a d u r c h g e k e n n z e i c h n e t, that the degree of deformation during cold rolling is greater than 60% but less than 82%.

16. The method according to any one of claims 11 to 15, d a d u r c h g e k e n n z e i c h n e t, that the cold-rolled steel strip or sheet obtained has a thickness of at most 0.35 mm.

17. The method according to any one of claims 11 to 16, d a d u r c h g e k e n n z e i c h n e t, that the hot strip has room temperature at the start of cold rolling.

18. The method according to any one of claims 11 to 17, d a d u r c h g e k e n n z e i c h n e t, that the cold rolling is carried out at temperatures which are <200 ° C.

19. The method according to any one of claims 11 to 16, characterized in that the hot strip is heated to a temperature of 200 ° C to 500 ° C within a period of less than 20 minutes before the hot rolling and is cold rolled at temperatures in this temperature range.

20. The method according to any one of claims 11 to 19, d a d u r c h g e k e n n e e c h n e t, that the final annealing takes place in a decarburizing atmosphere.

21. The method according to any one of claims 11 to 20, that the final annealing is carried out in a non-decarburizing atmosphere.

22. The method according to one of claims 11 to 21, d a d u r c h g e k e n n z e i c h n e t, that the steel contains 4.0-5.0% by weight of Si.

23. The method according to claim 11 to 21, d a d u r c h g e k e n n z e i c h n e t that the steel contains> 5.0 - 6.8 wt .-% Si.

24. The method according to claim 23, wherein the steel contains 6.0-6.8% by weight of Si.

25. The method according to any one of claims 11 to 24, d a d u r c h g e k e n n z e i c h n e t, that the content of AI is limited to the area of the unavoidable impurities.