US3874954A

US3874954A - Method of preparing iron silicon alloys with high silicon content for cold working requiring ductility

Info

Publication number: US3874954A
Application number: US141377A
Authority: US
Inventors: Erwin Wildschutz
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1970-05-11
Filing date: 1971-05-07
Publication date: 1975-04-01
Anticipated expiration: 1992-04-01
Also published as: DE2024525B1; GB1356342A; BE766995A; CA948529A; FR2091679A5

Abstract

Method of preparing iron silicon steel alloys having silicon content in excess of 4.5% by weight, the remainder being iron and impurities, for cold working; an ingot of said alloy is hot worked, e.g. hot rolled at a temperature in excess of 650* centigrade to obtain an intermediate product which, in turn, is worked (e.g. rolled) during cooling from 650* C down to at least 350* C, the resulting stock is cold worked while maintaining temperature below 350* C.

Description

D United States Patent 11 1 [111 3,874,954

Wildschutz Apr. 1, 1975 [54] METHOD OF PREPARING IRON SILICON 1,912,129 /1933 Detwiler 148/111 ALLOYS WITH HIGH SILICON CONTENT 1,992,039 2/1935 Reinhardt 148/1 11 FOR COLD WORKING REQUIRING 5:??33233 Z1133; $135.25;111:1:111111113111113113.1539?I? DUCTILITY 2,209,686 7/1940 75 lnventor'. Erwin Wildschutz, Krefeld, 2235532 3/1941 Germany 2,765,246 /1956 3,144,363 8/1964 Aspden et a1 148/111 1 3] Assignee: Mannesmann Aktiengesellschaft, 3,147,157 9/1964 Grenoble 148/111 Dusseldorf Germany Albert et a1 11 3,522,114 7/1970 Knuppel et a1. 148/111 1 fl y 1971 3.671.337 6/1972 Kumai et a1. 148/111 21 Appl. 190.; 141,377

Primary ExaminerWalter R. Satterfield At ,A F' -RlfH.S d Foreign Application Priority Data wme'v gem or lrm a legemun M"ll.l970 2 11 Germany 20 4525 [57] S CT 1 1 Cl N 4 /123 L, 148/ Method of preparing iron silicon steel alloys having 148/112 silicon content in excess of 4.5% by weight, the re- [51] Int. Cl. H011 l/04 mainder being iron and impurities, for cold working; 1 Field Of Search 8/ 31-55; an ingot of said alloy is hot worked, e.g. hot rolled at 75/123 L a temperature in excess of 650 Centigrade to obtain an intermediate product which, in turn, is worked [56] References Cited (e.g. rolled) during cooling from 650 C down to at UNITED STATES PATENTS least 350 C, the resulting stock is cold worked while 1.739.126 12/1929 Currv 148/111 maintaining temperature below 1.852.836 4/1932 Corson 148/111 1.898.061 2 1933 Otto 148/111 18 Clams N0 Drawmgs METHOD OF PREPARING IRON SILICON ALLOYS WITH HIGH SILICON CONTENT FOR COLD WORKING REQUIRING DUCTILITY The present invention relates to a method for working iron silicon alloys and particularly for preparing an intermediate product having ductility sufficient for subsequent cold working. The alloy may have a silicon content above 4.571 by weight. preferably 4.5 to 7.5% (by weight) silicon. the remainder being iron plus inevitable impurities. Moreover. it is presumed that a casting is hot worked into a semi-finished product hereinafter referred to as intermediate product, eg plate. stock or the like.

Iron silicon alloys with low content of impurities. also called silicon steel. have particularly interesting magnetic properties. Employment of thin sheets made of such silicon steel in electric machines and transformers is quite desirable; due to the development of texture sheets with preferred direction of magnetization, these sheets are mostly made through cold rolling. It has to be observed that silicon steel is harder and more brittle at room temperature if the silicon content is increased. Therefor. cold rolling of silicon steel is limited to a silicon content of not more than 3.6% by weight silicon.

Silicon steel. having a higher Si-content. is usually hot rolled and is used as quasi isotropic sheets in electrical machines. AC magnetization losses at the usual commercial power frequency exhibit a minimum if the sheets are about 0.35 to 0.50 mm thick. Such sheets result from particularly hot rolling of plates. by using folded over sheet bars or plates are doubled or stacked. However. yield deteriorates and maintaining tolerances becomes more difficult with increasing silicon content. Most importantly. the cold brittleness of higher silicon content products renders subsequent cold working of hot rolled sheets more difficult. For example. stamping or punching of parts will almost inevitably lead to development of cracks and burrs along cutting edges. For these reasons. a silicon content of about 4.571 by weight is deemed just about the upper limit for such steel: higher contents render the steel practically unworkable.

It must be observed now that a silicon content in excess of 4.59? improves the magnetic properties of the steel significantly. Particularly. such steel has very low magnetostriction and. most importantly. the electric (ohmic) resistance increases. and the anisotropic energy decreases. so that eddy current and hysteresis losses drop. Nevertheless. the mechanical properties deteriorate with increasing silicon content. Little is gained. if such material has excellent magnetic properties but cannot be worked. Such steel has practically no ductility as evidenced by tensile tests and by measurement of resulting changes in cross section. by bending tests. by testing the impact energy in notch impact bending tests. etc.

It is an object of the present invention to suggest preparatory process for iron-silicon alloys. e.g. silicon steel with more than 4.5% by weight silicon. going the route through suitably prepared intermediate products. so that final working steps. requiring some ductility. can be carried out at room temperature. as is necessary. for example. to obtain particular parts for electrical equipment and machinery.

In accordance with the preferred embodiment of the invention. it is suggested to provide a first intermediate product by hot working a casting ingot or the like at temperature above 650 centigrade. For example. a cast ingot is hot rolled at these temperatures to obtain a slab. sheet bar plate etc. That first stage intermediate product is subsequently. continuously worked into a second intermediate product during cooling from 650 C. e.g.. to about 350 C and/or below. so that the product retains significant ductility. Subsequent cold working of that second intermediate product is carried out while maintaining the temperature below 350 C. The workingwhile-cooling of the first intermediate product may succeed immediately the hot working production of the first intermediate product. or may be delayed. In case of immediately following working. the starting temperature is at 650 C and working proceeds as the slab cools. In case the working-while-cooling is delayed. the first intermediate product should not be al' lowed to cool below 650 C. However. there are cases where the first intermediate product must be allowed to cool below 650 C. down to room temperature or where such cooling is inevitable. Now. this first intermediate product must be re-heated at least up to 650 C and annealed at that temperature. preferably for at least one hour; working of the first intermediate product into the second intermediate product is carried out during a cooling phase, while the temperature drops from 650 to 250 C. The annealing should preferably last more than 3. up to 7 hours.

The process as improved in accordance with the invention begins with a silicon steel ingot which has been cast in vacuum or in a protective atmosphere so that gaseous impurities are eliminated as much as possible. It should be noted that silicon steel when not sufficiently degassed, may develop bubbles under the surface during rolling. Thus. the nitrogen content should be below 0.004%.

The hot ingot may then be hot rolled. stripped. descaled and hot rolled again at a final rolling temperature of about 750 to 650 to arrive at the first intermediate products. from which to proceed to the workingduring-cooling step. This operation is carried out in a range of temperature from 650 to 350 centigrade. Working-during-cooling should proceed at a high forming speed. i.e.. the rate of deformation should be high with large reduction in cross section for each step. in a continuous sequence of single steps or with only short passes between steps. particularly during cooling through the critical range of 500 to 425 C.

Preferably. the first intermediate product is a slab that is rolled during cooling; there should be as many rolling passes as possible while the temperature drops. The principle purpose of this working-during-cooling is to disturb continuously (i.c. as frequently as possible) internal ordering processes that tend to influence the incomplete ordered super lattice and tends to build up lattice structure. which is responsible for brittleness. This disturbing is continued until the temperature has dropped far enough that there is no longer tendency to form lattice structure by decreased speed of diffusion. sufficient to establish significant brittleness and low ductility.

The rolling-while-cooling is preferably carried out with rolls of pronounced camber. as such rolls furnish forces sufficiently high to roll steel of high silicon content. The size reduction per pass is to be selected that the material. in fact. flows uniformly. as otherwise lobes and other defects are formed similar as known from sheet bar rolling, inevitably resulting in the formation of cracks. Therefor, lubrication is preferred at lower temperature, using steel hardening oil or brightannealing oil. After a reduction by about 70%, the duetility is significantly increased.

Steel sheets made in that manner, as second intermediate products, and having thickness, e.g. below 1.5 mm, may now be worked at temperatures as low as room temperature. Depending upon the final size additional cold rolling may be in order and is possible at this stage. Clearly, for obtaining particularly reduced thickness of a final product, the respective thickness of first and second intermediate product should be adapted accordingly. Particularly, as long as the product is still warm after rolling during-cooling, such supplemental cold rolling should follow immediately to take advantage of the elevated temperature. Therefor, the hot rolling may reduce the plate below 5 mm gauge. The rolling-during-cooling will thereafter reduce the plate thickness below 1.5 mm, even below 1 mm and subsequent cold rolling may reduce the sheet thickness further.

It is important that the resulting ductility at room temperature suffices for subsequent working, such as cutting, stamping or punching, which working steps can be carried out now without formation of cracks or burrs. The second intermediate product, though sufficiently ductile, is still somewhat harder than the usual silicon steel, so that the tools should be sharpened or resharpened frequently. Such possible inconvenience is readily acceptable as long as the yield from the stock is as high as, in fact. it is. It is apparent that final annealing, necessary for reasons of obtaining particular magnetic properties, should be carried out on the final product, not just on the sheets used as stock. The final annealing renders the material brittle again, but as additional forming or working is not needed thereafter, there will be no problem. The stacking or other assembly of the parts made is not impeded by the resulting brittleness.

The invention is not only of immediate advantage to the final working, but also for making ofthe steel itself. Neither the annelaing period nor the working duration is critical, so that strips can be made under tension. This is of advantage for obtaining significant reduction in cross section and rather planar strips.

Silicon steel resists change in shape to an increasing degree with increasing silicon content, particularly with dropping temperature. For example, the deformation resistance of 671 Si-steel doubles for a temperature drop from 1100 C to [000 C and quadruples for a drop to 850; the resistance is eight fold increased when the temperature has dropped to 700 C. On the other hand. the deformation resistance remains about the same in the range from 550 C to 150 C. The deformation resistance has values about twice the values fora 3/( Si-steel. As a consequence, a hot rolling stand and drawing equipment must be very strong.

The first intermediate product is worked into the second intermediate product, e.g., by means of rolling. The rolling forces available are, thus. high, so is the working temperature, at least in the beginning. Therefor, one can use a multi-roll mill of the Sendzimir variety with solid. hard metal rolls and coiling furnace similar to a hot rolling mill ofthe Steckel variety. Such a reversing mill can be used also for finish rolling the plate or sheet stock to final gauge, even when using rolls of smaller diameter.

It can, thus, be'seen that.the-=invention relates to working of normally brittle silicon steel with a silicon content in excess of4.57r by weight silicon, particularly from 4.571 to 7.5% siliconpHowever, this upper limit and the, presumably minimum content of additives and/or impurities in the steel, is not to be regarded as critical limit for practicing the invention. Rather, these types of steels are those that are of particular economic interest as to magnetic properties, and which prior to the invention were found impossible to work with.

EXAMPLE A teemed one-ton (metric) slab ingot, having dimensions of about 190 X 650 X 1 mm ofb /r silicon steel was moved to a rolling mill and stripped. The ingothad temperature of about 650 C to 700 C and was placed in a soaking pit and heated by and with this furnace. After drawing at 1100 C the slab was descaled and rolled in a four-high plate rolling mill. That kind of mill usually permit reduction to sheet gauge about 5 to 8 mm thickness, at final temperatures of about 750 C to 650 C. 1n view of the limited capabilities of such mill as to further rolling. the sheets were divided at these plate gauges, cooled to room temperature and mechan-' ically descaled.

The cooled 5 mm gauge sheets were then rc-heated up to 650 C and that temperature was maintained for an hour. At that point, the first intermediate product as defined was ready for the working process that ensures sufficient ductility. This slab or plate was then rolled while cooling in a four-high mill, in a continuing sequence of passes, without intermediate heating but under utilization of natural, regular cooling. there were about 15 to 20 passes and the sheet gauge was reduced to below 1.5 mm. This working-during-cooling ensured sufficient ductility.

1n a modified example, plates thicker than 5 mm (first intermediate product) were re-heated up to 850 to 900 C and hot rolled in a two-high mill to reduce thickness to about 3.5 to 1.75 mm. There were several intermediate heating steps at 870 C, the number of heating steps depending on the gauge ofthe plate as hot rolled at first. Immediately thereafter, the plates were transferred to the four-high, cold rolling mill and reduced in size (below 1 mm) in plural passes as the rolled sheet cooled.

The first pass of rolling while-cooling in the four-high mill results in gauge reduction by about 15 to 20%; during this first pass the temperature drops from 650 C to about 510 C. The gauge reduction at the subsequent passes were chosen so-that the entire intermediate rolling was carried out at constant rolling force per pass. Under consideration of the strong roll chamber that force was about 850 kiloponds per millimeter sheet width (1 kilopond 2.205 lbf.) The resulting sheets were rolled still further as the temperature dropped to room temperature, downto sheet gauge of 0.5 to 0.3 mm, in an experimentalcase even down to 0.03 mm. The resulting second intermediate product (sheet stock) were tested by breaking off edges, but there were no edge fissures. Also, the sheets could be cut at room temperature without development of cracks, and they could be stamped and punched withoutdevelopment of cracks or burrs. Strips-o 0.3 mm thickness has bending values of up to 10 times. bending back and force at room temperature. The specific ohmic resistance was 80 micro ohms centimeter. The sheets were quasi isotropic. Thus, any residual difference in magnetization in longitudinal and transverse directions, similar to such difference of usual hot rolled sheets, could be eliminated entirely in these silicon steel sheets by annealing in a magnetic field. Loses of magnetization reversal had V values of 0.6 watts per kilogram of annealed sheet stock at 0.5 mm sheet gauge.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

l claim:

1. Method of working iron silicon steel alloys having silicon content between 4.5 /1 to 7.5 by weight, the remainder being iron and impurities, providing the steps of:

first. hot working a casting of said alloy at a tempera ture in excess of (150 centrigrade to obtain a first intermediate product;

second, working and forming the first intermediate product into a second intermediate product during cooling from (150 C down to at least 350 C prior to final working; and

third, making parts from the unannealed second intermediate product which has not been reheated above 350 C and while avoiding increasing its temperature above 350 C, the making of parts in accordance with the third step excluding rolling and being ofthe variety which is impeded by brittleness because of formation of edge cracks and burrs, and being one of the following cutting, punching, stamping, the brittleness having been maintained sufficiently low by the second step and by maintaining the temperature below 350 C, so that cracks and burrs are avoided during the cutting, punching or stamping for making such parts.

2. Method as in claim I, wherein the working of the first intermediate product follows immediately its production by the first step.

3. Method as in claim 1. wherein the first intermediate product is maintained at 650C until worked during cooling pursuant to the second step.

4. Method as in claim I, wherein the first intermediate product cools or is cooled down to about room temperature prior to the second step; a fourth step preceding the second step including reheating the first intermediate product and annealing same at about 650C, the second step including working as the previously so annealed first product cools from 650 to 250 C. final annealing of the resulting second product being carried out after cutting, punching or stamping.

5. Method as in claim 4. the annealing to last at least for 1 hour.

6. Method as in claim 4. the annealing to last from 3 to 7 hours.

7. Method as in claim 1, the second step including working at a high rate of deformation as the first intermediate product cools from 500 to 425C.

8. Method as in claim I the first step including hot rolling a cast slab into a plate of several mm gauge. the second step including rolling the hot rolled plate during cooling in plural passes. reducing the thickness further.

9. Method as in claim I. including reheating the first intermediate product to a temperature above (150C and hot rolling same prior to rolling-during-cooling below (C.

10. A method for preparing silicon steel sheet stock with a silicon content in-between 4.5 /1 and 7.5 /r by weight for making parts by stamping, punching or cutting from such sheet stock, such making requiring ductility and low brittleness and resulting in the formation of cracks and burrs along the edges when the sheet stock used is too brittle, comprising the steps of:

hot rolling a cast ingot into a plate or sheet bar at a temperature in excess of 650 C;

rolling the plate or sheet bar into sheet stock in plural passes during cooling of the plate or sheet bar from a temperature above 650 C, and at such rate and strength that the forming lattice structure is continuously disturbed and a uniform flow is maintained during each pass at least until the temperature has dropped below 425 C to reduce the brittleness of the resulting sheet stock; and maintaining the resulting low brittleness by keeping the temperature of the sheet stock below 350 C and avoiding re-heating above that temperature, at least until after stamping, punching or cutting parts from the sheet stock as succeeding any and all rolling of said sheet stock. ll. Method as in claim 10, wherein the hot rolling step is succeeded by rolling without cooling, the plate or sheet bar being subsequently reheated and annealed for at least about 1 hour at a temperature not lower than 650C, the rolled-during-cooling step succeeding immediately the annealing.

12. Method as in claim 10, the hot rolling carried out in several steps, the first step reducing the casting slab to a plate of thickness in excess of 5 mm, the subsequent hot rolling reducing the plate to below 3.5 mm, the rolling during cooling reducing the plate gauge below 1 mm.

13. Method as in claim 10, the hot rolling carried out to provide plate stock below 5 mm gauge, the rollingduring-cooling reducing the plate stock to sheet stock below 1.5 mm gauge.

14. A method as in claim 10, wherein the sheet stock as rolled while the temperature drops is cold rolled sub sequently, prior to the stamping, punching or cutting parts.

15. A method as in claim 10, wherein the rolling is continued as the rolled plate or sheet bar cools to a temperature below 350 C.

16. A method as in claim 15, wherein the rolling is continued as rolled plate or sheet bar cools to a temperature below 250 C.

17. Method as in claim 8, and including cold rolling following the second step, but prior to said third step. 18. Method of making parts from silicon steel sheet stock with a silicon content in-betwcen 4.5 and 7.5 "/1 by weight, comprising the steps of hot rolling an ingot into a plate or sheet bar at a temperature in excess of 650 C;

rolling the plate or sheet bar in plural passes during cooling from a temperature in excess of 650 C. so that internal lattice structure in the steel bar tending to form during cooling is disturbed by each of said plural passes, such disturbing to be continued at least while the bar cools during such rolling from above 650 C to about 350 C, so as to obtain sheet stock of high ductility; and

making parts from such sheet stock through cutting,

punching or stamping. the sheet stock having been prevented from reheating to a temperature above 350 C at any time following said last mentioned rolling step prior to and until the parts have been

Claims

1. METHOD OF WORKING IRON SILICON STEEL ALLOYS HAVING SILICON CONTENT BETWEEN 4.5% TO 7.5% BY WEIGHT, THE REMAINDER BEING IRON AND IMPURITIES, PROVIDING THE STEPS OF: FIRST, HOT WORKING A CASTING OF SAID ALLOY AT A TEMPERATURE IN EXCESS OF 650* CENTRIGRADE TO OBTAIN A FIRST INTERMEDIATE PRODUCT; SECOND, WORKING AND FORMING THE FIRST INTERMEDIATE PRODUCT INTO A SECOND INTERMEDIATE PRODUCT DURING COOLING FROM 650*C DOWN TO AT LEAST 350*C PRIOR TO FINAL WORKING, AND THIRD, MAKING PARTS FROM THE UNANNEALED SECOND INTERMEDIATE PRODUCT WHICH HAS NOT BEEN REHEATED ABOVE 350*C AND WHILE AVOIDING INCREASING ITS TEMPERATURE ABOVE 350* C, THE MAKING OF PARTS IN ACCORDANCE WITH THE THIRD STEP EXCLUDING ROLLING AND BEING OF THE VARIETY WHICH IS IMPEDED BY BRITTLENESS BECAUSE OF FORMATION OF EDGE CRACKS AND BURRS, AND BEING ONE OF THE FOLLOWING CUTTING, PUNCHING, STAMPING, THE BRITTLENESS HAVING BEEN MAINTAINED SUFFICIENTLY LOW BY THE SECOND STEP AND BY MAINTAINING THE TEMPERATURE BELOW 350*C, SO THAT CRACKS AND BURRS ARE AVOIDED DURING THE CUTTING, PUNCHING OR STAMPING FOR MAKING SUCH PARTS.

2. Method as in claim 1, wherein the working of the first intermediate product follows immediately its production by the first step.

3. Method as in claim 1, wherein the first intermediate product is maintained at 650*C until worked during cooling pursuant to the second step.

4. Method as in claim 1, wherein the first intermediate product cools or is cooled down to about room temperature prior to the second step; a fourth step preceding the second step including reheating the first intermediate product and annealing same at about 650* C, the second step including working as the previously so annealed first product cools from 650* to 250* C, final annealing of the resulting second product being carried out after cutting, punching or stamping.

5. Method as in claim 4, the annealing to last at least for 1 hour.

6. Method as in claim 4, the annealing to last from 3 to 7 hours.

7. Method as in claim 1, the second step including working at a high rate of deformation as the first intermediate product cools from 500* to 425*C.

8. Method as in claim 1 the first step including hot rolling a cast slab into a plate of several mm gauge, the second step including rolling the hot rolled plate during cooling in plural passes, reducing the thickness further.

9. Method as in claim 1, including reheating the first intermediate product to a temperature above 650*C and hot rolling same prior to rolling-during-cooling below 650*C.

10. A method for preparing silicon steel sheet stock with a silicon content in-between 4.5 % and 7.5 % by weight for making parts by stamping, punching or cutting from such sheet stock, such making requiring ductility and low brittleness and resulting in the formation of cracks and burrs along the edges when the sheet stock used is too brittle, comprising the steps of: hot rolling a cast ingot into a plate or sheet bar at a temperature in excess of 650* C; rolling the plate or sheet bar into sheet stock in plural passes during cooling of The plate or sheet bar from a temperature above 650* C, and at such rate and strength that the forming lattice structure is continuously disturbed and a uniform flow is maintained during each pass at least until the temperature has dropped below 425* C to reduce the brittleness of the resulting sheet stock; and maintaining the resulting low brittleness by keeping the temperature of the sheet stock below 350* C and avoiding re-heating above that temperature, at least until after stamping, punching or cutting parts from the sheet stock as succeeding any and all rolling of said sheet stock.

11. Method as in claim 10, wherein the hot rolling step is succeeded by rolling without cooling, the plate or sheet bar being subsequently reheated and annealed for at least about 1 hour at a temperature not lower than 650*C, the rolled-during-cooling step succeeding immediately the annealing.

13. Method as in claim 10, the hot rolling carried out to provide plate stock below 5 mm gauge, the rolling-during-cooling reducing the plate stock to sheet stock below 1.5 mm gauge.

14. A method as in claim 10, wherein the sheet stock as rolled while the temperature drops is cold rolled subsequently, prior to the stamping, punching or cutting parts.

15. A method as in claim 10, wherein the rolling is continued as the rolled plate or sheet bar cools to a temperature below 350* C.

16. A method as in claim 15, wherein the rolling is continued as rolled plate or sheet bar cools to a temperature below 250* C.

17. Method as in claim 8, and including cold rolling following the second step, but prior to said third step.

18. Method of making parts from silicon steel sheet stock with a silicon content in-between 4.5 % and 7.5 % by weight, comprising the steps of hot rolling an ingot into a plate or sheet bar at a temperature in excess of 650* C; rolling the plate or sheet bar in plural passes during cooling from a temperature in excess of 650* C, so that internal lattice structure in the steel bar tending to form during cooling is disturbed by each of said plural passes, such disturbing to be continued at least while the bar cools during such rolling from above 650* C to about 350* C, so as to obtain sheet stock of high ductility; and making parts from such sheet stock through cutting, punching or stamping, the sheet stock having been prevented from reheating to a temperature above 350* C at any time following said last mentioned rolling step prior to and until the parts have been made.