GB1594826A

GB1594826A - Electrical steels

Info

Publication number: GB1594826A
Application number: GB48596/77A
Authority: GB
Original assignee: British Steel Corp
Current assignee: British Steel Corp
Priority date: 1977-11-22
Filing date: 1977-11-22
Publication date: 1981-08-05
Also published as: FR2409313B2; FR2409313A2; BE872142R; DD140568A6; IT1160946B; IT7869670A0; AU538791B2; SE7811978L; CA1119919A; AU4183578A; SE427473B; DE2850249A1; PL211104A1; YU271778A; JPS5493622A

Description

(54) IMPROVEMENTS IN ELECTRICAL STEELS (71) We, BRITISH STEEL CORPORA TION, a Corporation incorporated and existing under the Iron and Steel Act 1967 whose principal office is at 33 Grosvenor Place, London, S.W.1 do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the production of grain oriented silicon steels intended for electromagnetic applications. More specifically the invention is concerned with such steels having a "cube on edge" texture or in terms of Miller indices a (110) [001] texture.

Such steels are used primarily as the core material for transformers and other electric machines and it is desirable that they should be characterised by a high magnetic permeability together with a low core loss at a given flux density.

Such magnetic properties may be optimised by adopting a process in which individual grains within a polycrystalline sheet of the steel become aligned with the crystalline directions yielding the best magnetic properties lying in the plane of the sheet and parallel to the rolling direction.

One conventional process for producing grain oriented steel sheets is based on the recognition that a fine dispersed precipitate of manganese sulphide is responsible for producing a final well oriented grain structure. The present invention is based on the appreciation that this dispersion of manganese sulphide may be supplemented by a precipitate of vanadium nitride and that the magnetic properties of strip derived from a starting material containing these two precipitates may be up-graded by suitable selection of process parameters.

According to one aspect of the present invention a method of producing silicon steel strip intended for electromagnetic ap plications comprises hot rolling to sheet a slab having by weight, 2% to 4% of silicon, 0.05% to 0.1% of manganese, 0.02% to 0.035% of sulphur, 0.02% to 0.06% of carbon, 0.002% to 0.01% of nitrogen, and 0.01% to 0.1% of vanadium together with incidental impurities, and cold rolling the hot band with a final reduction of at least 70% before decarburising at a temperature within the range 900 to 10500C.

Preferably the cold reduction is in a single stage. In this case the hot band must be annealed suitably at a temperature within the range 900 to 11000C and cooled at a reflected rate. Alternatively, two stage cold reduction with an intermediate anneal may be employed. In this case the intermediate anneal should be the same as the hot band anneal, followed by the same selected cooling rate, and the hot band anneal may be omitted if desired. The second cold reduction must be at least 70%.

Suitably the slab is reheated and hot rolled to a gauge in the range 1.90 - 3.00 mm. and preferably 2.30 - 2.85 mm. which permits a higher cold reduction when rolling to the commercially desirable final gauges within the range .28 - .35mm. The hot band is then side-trimmed and continuously annealed at a temperature in the range 900 to 11000C for between 2 and 5 minutes. A preferable range is 950 to 10750C.

It has been found that selection of the cooling rate after this anneal is important for the vanadium bearing steel in obtaining the best final magnetic properties. A cooling rate of 5 - 16 C/sec down to a temperature in the range 750 to 9000C gives optimum results; followed by more rapid cooling down to substantially room temperature, such as natural air cooling, forced air or gas jet cooling, or water spray cooling. This represents a two stage cooling cycle and may be associated with the need to precipitate vanadium nitride in a specific morphology after hot band annealing, but to prevent further particle coarsening which may occur during a prolonged cool to room temperature.

After hot band annealing the steel is pickled and cold-rolled to final gauge, with or without low-temperature ageing during the rolling, in such a way that the final cold reduction is between 70 and 95%. After cold-rolling the steel is decarburised on a continuous annealing line in an atmosphere of wet hydrogen or a wet nitrogen/hydrogen mixture and at a temperature in the range 900-1050"C and preferably 950-10500C for about 4 minutes. This decarburisation temperature range is distinctly higher than the 800-850"C range conventionally used for grain-oriented steel production and has been found to overcome the difficulties in obtaining complete secondary recrystallisation in this material when using conventional and relatively lower temperature decarburising. After decarburisation the strip is magnesia coated and box-annealed at a temperature above 1100 C for some 24 hours.

Examples of the invention will now be described: Example 1 A steel melt derived from an open hearth refining process was innoculated in the ladle with vanadium to given an overall composition of 0.022% carbon, 0.026% sulphur, 0.0056% nitrogen, 0.092% manganese, 2.86% silicon and 0.062% vanadium the balance being iron and incidental impurities. Slabs made from ingots of this composition were reheated at 1400"C and rolled to hot-band of 2.85mm thickness at a finish temperature of 960 C. The hot-band which now contained dispersed precipitates of manganese sulphide and vanadium nitride as grain-growth inhibitors, was annealed at 1050"C for 5 minutes, cooled to 850"C at an average rate of 10.5 C/sec and oil-quenched to room temperature. The material was cold-rolled to a final gauge of .35mm and during rolling an ageing anneal of 200"C for 5 minutes was applied to the material at 6 equispaced gauges during the reduction.

After cold reduction the strip was decarburised at 1050"C for 4 minutes in hydrogen with a dew-point of + 60"C prior to coating and box-annealing in conventional manner.

A steel produced by this method gave a core loss of 1.30 W/Kg at 1.7T and 50 Hz and a permeability of 1.92T at H=1.0 kA/m.

Example 2 Hot-band stock produced as above was annealed at 10000C for 5 minutes, cooled to 850"C at 7.7"C/sec and oil-quenched. After cold-rolling to .35mm with a similar ageing sequence, and decarburising at 1050"C for 5 minutes, the strip was coated and boxannealed in the conventional manner. This steel had a core loss of 1.34 W/kg at 1.7T and 50Hz and a permeability of 1.89T at H= 1.0 kA/m.

Example 3 Hot-band stock produced as above was annealed at 1050"C for 5 minutes, cooled to 750"C at a rate of 11.0 C/sec and cold-rolled to .35 mm using the ageing sequence described above. After decarburising at 1050"C for 5 minutes, the strip was coated and box-annealed in the conventional manner. This steel had a core loss of 1.39 W/kg at 1.7T and 50 Hz and a permeability of 1.89T at H= 1.0 kA/m.

Example 4 Hot-band stock produced according to Example 1 was annealed at 10000C for 2 minutes, cooled to 900"C in air at 150cos then quenched into oil at about room temperature. The annealed hot-band was then cold-rolled with an ageing treatment to .35mm decarburised at 10000C for 4 minutes and box-annealed at 12000C for 24 hours.

Steel made in this way had a core loss at 1.7T and 50Hz of 1.32 W/kg and a permeability of 1.89T at H= 1.0 kA/m.

Example 5 Hot-band was processed as in Example 4, except that after initial annealing the hotband was cooled at 60C/s to 900"C then quenched into oil at about room temperature. The steel made in this way had a core loss at 1.7T and 50Hz of 1.34 W/kg and a permeability of 1.89T at H= 1.0 kA/m..

Example 6 Hot-band stock made according to Example 1 was annealed at 1050"C for 2 minutes and cooled to 9000C at 8"C/s before quenching into oil at substantially room temperature. The annealed hot-band was then coldrolled with ageing to .35mm, decarburised at 1050"C for 4 minutes and box-annealed at 1200"C for 24 hours. Steel made by this route had a core loss of 1.30 W/kg at 1.7T and 50 Hz, and a permeability at H= 1.0 kA/m of 1.93T.

Example 7 Hot-band made as in Example 1 was initial annealed at 1025"C for 2 minutes, and cooled to 900"C in air at 160C/s and oilquenched to room temperature. The annealed hot-band was then cold-rolled with ageing, decarburised at 1050"C for 4 minutes and box-annealed at 1200"C for 24 hours. Steel made in this way had a core loss of 1.37 W/kg at 1.7T and 50 Hz and a permeability at H= 1.0 kA/m of 1.88T.

Example 8 The same hot-band stock was processed as in Example 7 except that the decarburisa tion was carried out at 1000"C for 4 minutes.

This steel had a core loss of 1.34 W/kg at 1.7T and 50Hz and a permeability at H= 1.0 kA/m of 1.89T.

Example 9 A sample of steel from the stock as in Example 1 was annealed at 9500C for 5 minutes, cooled to 850"C at 5 C/sec and forced air cooled to room temperature. The sample was cold-rolled without ageing, de carburised at 1050"C for 4 minutes and box-annealed at 1200"C for 24 hours. Steel processed on this route had a core loss at 1.7T and 50Hz of 1.36 W/kg, and a permea bility of 1.91T.

WHAT WE CLAIM IS: 1. A method for producing silicon bear ing steel strip intended for electro-magnetic applications comprising hot rolling to sheet a slab having by weight, 2% to 4% of silicon, 0.05% to 0.1% of manganese, 0.02% to 0.035% of sulphur, 0.02% to 0.06% of carbon, 0.002% to 0.01% of nitrogen, and 0.01% to 0.1% of vanadium, together with incidental impurities, and cold rolling the hot band with a final reduction of at least 70% before decarburising at a temperature within the range 900 to 1050"C.

2. A method as claimed in Claim 1 wherein cold reduced strip is decarburise annealed at a temperature within the range 900 to 10500C.

3. A method as claimed in Claim 2 wherein the decarburise anneal is at a temperature within the range 950 to 1050"C.

4. A method as claimed in Claim 3 or Claim 4 wherein the decarburise anneal is a continuous anneal.

5. A method as claimed in any preced ing Claim wherein the cold reduction is in a single stage and the hot band is annealed and cooled at a selected rate before cold reduction.

6. A method as claimed in Claim 5 wherein the hot band anneal is at a tempera ture within the range 900 to 11000C.

7. A method as claimed in Claim 6 wherein the hot band anneal is at a tempera ture within the range 950 to 1075"C.

8. A method as claimed in Claim 5, 6 or 7 wherein the selective cooling is at a rate of 5 - 16"C per second to a temperature within the range 750 to 9000C.

9. A method as claimed in any one of Claims 5 to 8 wherein the cooling is followed by rapid cooling to substantially room temperature.

10. A method as claimed in Claim 9 wherein the rapid cooling is by natural or forced air, gas jet or water spray.

11. A method as claimed in any one of Claims 1 to 4 wherein cold reduction is in two stages with an intermediate anneal followed by cooling at a selected rate.

12. A method as claimed in Claim 11 wherein the intermediate anneal is within the temperature range 900 to 11000C.

13. A method as claimed in Claim 12 wherein the intermediate anneal is at a temperature within the range 950 to 10750C.

14. A method as claimed in Claim 11, 12 or 13 wherein the selective cooling is at a rate of 5 - 16"C per second to a temperature within the range 750 to 9000C.

15. A method as claimed in any one of Claims 11 to 14 wherein the selective cooling is followed by rapid cooling to substantially room temperature.

16. A method as claimed in Claim 15 wherein the rapid cooling is by natural or forced air, gas jet or water spray.

17. A method as claimed in any one of Claims 11 to 16 wherein the hot band is annealed before cold reduction.

18. A method as claimed in Claim 17 wherein the hot band is annealed at a temperature within the range 900 to 1100 C.

19. A method as claimed in Claim 18 wherein the hot band anneal is at a temperature within the range 950 to 1075"C.

20. A method as claimed in any preceding Claim wherein the slab is re-heated and hot rolled to a gauge within the range 1.90 to 3.00 mm.

21. A method as claimed in Claim 20 wherein the slab is hot rolled to a gauge within the range 2.30 to 2.85 mm.

22. A method as claimed in any preceding Claim wherein the annealed hot band cooled at a rate of 5 to 16"C per second to a temperature within the range 750 to 900"C.

23. A method for producing silicon bearing steel strip for electro magnetic applications substantially as herein described with reference to the examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Example 7 Hot-band made as in Example 1 was initial annealed at 1025"C for 2 minutes, and cooled to 900"C in air at 160C/s and oilquenched to room temperature. The annealed hot-band was then cold-rolled with ageing, decarburised at 1050"C for 4 minutes and box-annealed at 1200"C for 24 hours. Steel made in this way had a core loss of 1.37 W/kg at 1.7T and 50 Hz and a permeability at H= 1.0 kA/m of 1.88T. Example 8 The same hot-band stock was processed as in Example 7 except that the decarburisa tion was carried out at 1000"C for 4 minutes. This steel had a core loss of 1.34 W/kg at 1.7T and 50Hz and a permeability at H= 1.0 kA/m of 1.89T. Example 9 A sample of steel from the stock as in Example 1 was annealed at 9500C for 5 minutes, cooled to 850"C at 5 C/sec and forced air cooled to room temperature. The sample was cold-rolled without ageing, de carburised at 1050"C for 4 minutes and box-annealed at 1200"C for 24 hours. Steel processed on this route had a core loss at 1.7T and 50Hz of 1.36 W/kg, and a permea bility of 1.91T. WHAT WE CLAIM IS:

1. A method for producing silicon bear ing steel strip intended for electro-magnetic applications comprising hot rolling to sheet a slab having by weight, 2% to 4% of silicon, 0.05% to 0.1% of manganese, 0.02% to 0.035% of sulphur, 0.02% to 0.06% of carbon, 0.002% to 0.01% of nitrogen, and 0.01% to 0.1% of vanadium, together with incidental impurities, and cold rolling the hot band with a final reduction of at least 70% before decarburising at a temperature within the range 900 to 1050"C.

24. Silicon bearing steel strip for electro

magnetic applications when produced by the method of any preceding Claim.