Abstract:
The magnetic properties of silicon-iron are improved by adding tin, and by both adding tin and lowering the sulfur content the weld brittleness is reduced in addition to improving the magnetic properties.

Description:
The present invention relates generally to the art of producing electrical steel and is more particularly concerned with a novel method of producing singly oriented silicon-iron sheet having both good weldability characteristics and excellent magnetic properties, and is also concerned with the resulting new product. 
     CROSS REFERENCE 
     This invention is related to the invention disclosed and claimed in U.S. patent application Ser. No. 837,504 filed of even date herewith and assigned to the assignee hereof and directed to the novel concept of limiting sulfur content of silicon-iron to not more than 0.018 percent and using copper as a partial substitute for sulfur as a grain growth inhibitor during the final anneal and thereby reducing or eliminating weld brittleness while retaining excellent magnetic properties in the resulting product. 
     BACKGROUND OF THE INVENTION 
     The sheet materials to which this invention is directed are usually referred to in the art as &#34;electrical&#34; silicon steels or, more properly, silicon-irons and are ordinarily composed principally of iron alloy with about 2.2 to 4.5 percent silicon and relatively minor amounts of various impurities and very small amounts of carbon. These products are of the &#34;cube-on-edge&#34; type, more than about 70 percent of their crystal structure being oriented in the (110) [001] texture, as described in Miller Indices terms. 
     Such grain-oriented silicon-iron sheet products are currently made commercially by the sequence of hot rolling, heat treating, cold rolling, heat treating, again cold rolling and then final heat treating to decarburize, desulfurize and recrystallize. Ingots are conventionally hot-worked into a strip or sheet-like configuration less than 0.150 inch in thickness, referred to as &#34;hot-rolled band.&#34; The hot-rolled band is then cold rolled with appropriate intermediate annealing treatment to the finished sheet or strip thickness usually involving at least a 50 percent reduction in thickness, and given a final or texture-producing annealing treatment. As an alternative practice, set forth, for example, in my U.S. Pat. No. 3,957,546, assigned to the assignee hereof, the hot-rolled band is cold rolled directly to final gauge thickness. 
     In these boron- and nitrogen-containing silicon-irons, strong restraint to normal grain growth and thus promotion of secondary recrystallization to a precise (110) [001] grain orientation is the result of controlling the ranges of these constituents. The sulfur effective for this purpose is that which is not combined with strong sulfide-forming elements such as manganese, a presently unavoidable impurity in iron and steel. Thus, the total sulfur is necessarily greater than that necessary to provide its grain growth inhibition effect. 
     It is also generally recognized in the art that the presence of high total sulfur and a small quantity of boron can lead to marked brittleness in welds made in the silicon-iron alloy. Because of this weld brittleness, it has not been generally possible to weld two hot rolled coils together for cold rolling as would be a desirable operating practice since reducing the sulfur content for that purpose would have the result of degrading the magnetic properties of the metal. Having that choice usually means foregoing the advantage of good weldability. 
     SUMMARY OF THE INVENTION 
     I have discovered that in certain silicon-iron heats containing boron and nitrogen the sulfur requirement for grain growth inhibition can be met to a greater or lesser degree through the use of tin. Further, I found that tin additions for that purpose do not increase weld brittleness and that the magnetic properties are superior to those of higher sulfur heats without tin. In other words, I have discovered how, through the use of tin, to produce heats having magnetic properties excelling those associated with high sulfur content and having the desirable weld characteristics associated with low sulfur content. 
     Specifically, I have found that the foregoing new results can be consistently achieved by adding up to 0.10 percent tin to alloys containing as little as 0.010 percent sulfur, the amount of tin required being greater the lower the sulfur content. 
     Another finding that I have made is that magnetic properties can be still further enhanced in silicon-iron to which tin has thus been added by applying the boron-containing coating to the cold rolled silicon-iron sheet prior to the final heat treatment. 
     The initial hot rolling temperature has likewise been found to have a noticeable effect on permeability in these tin-addition silicon-iron alloys. Thus, sheets of the foregoing composition hot rolled from 1200°-1300° C consistently have higher permeability than those hot rolled from 1100°-1150° C. 
     In view of these several discoveries of mine, those skilled in the art will understand that this invention has both method and product aspects. The product is a cold rolled sheet containing boron, nitrogen, sulfur and tin in controlled amounts enabling development of desired magnetic properties and weldability in the finished sheet material. The product by which the sheet material is produced is likewise novel, particularly in the relation between the sulfur and tin contents. 
     Briefly described, in its article aspect this invention takes the form of a cold rolled silicon-iron sheet product containing 2.2 to 4.5 percent silicon and from three to 35 parts per million boron, from 30 to 75 ppm nitrogen in the above stated ratio range of boron, from 0.02 to 0.05 percent manganese, 0.005 to 0.025 percent sulfur and tin in amounts ranging from 0.01 to 0.10 percent, with the highest tin content being associated with the lowest sulfur content. 
     Similarly described, the method of this invention comprises the steps of providing a silicon-iron melt for the foregoing composition, casting the melt and hot rolling the resulting billet to produce a sheet-like body, cold rolling the hot rolled body to provide a sheet of final gauge thickness, and subjecting the resulting cold rolled sheet to a heat treatment to decarburize it and develop (110) [001] secondary recrystallization in it. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows the effects of tin on the permeability and the 60 Hertz losses at 17 kB. 
     FIG. 2 plots the permeability of two alloys without a boron addition to the coating. 
     FIG. 3 plots the permeability of the two alloys with a boron addition to the coating. 
     FIG. 4 shows the magnetic properties of 10 heats after a final anneal; 5 of said heats having a tin addition. 
     FIG. 5 shows the magnetic properties of 10 heats after a final anneal, 5 of said heats having a tin addition. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In carrying out this invention, one may provide the cold-rolled sheet product described above by preparing a silicon-iron melt of the required chemistry, and then casting and hot rolling to intermediate thickness. Thus, the melt on pouring will contain from 2.2 to 4.5 percent silicon, from about three to 35 ppm boron and about 30 to 90 ppm nitrogen in the ratio range to boron of 1 to 15 parts to 1, manganese from 0.02 to 0.05 percent, and sulfur and tin in the ranges stated above, the remainder being iron and small amounts of incidental impurities. Following anneal, the hot band is cold rolled with or without intermediate anneal to final gauge thickness and then decarburized. 
     The resulting fine-grained, primary recrystallized, silicon-iron sheet product in whatever manner produced is provided with a magnesia coating for the final texture-developing anneal. Preferably, the coating step is accomplished electrolytically as described in U.S. Pat. No. 3,054,732, referenced above, a uniform coating of Mg(OH) 2  about 0.5 mil thick thereby being applied to the sheet. Boron may be incorporated in the resulting coating in the amount and for the purpose stated above by dipping the coated strips in aqueous boric acid solution or the like. 
     As the final step of the process of this invention, the thus-coated sheet is heated in hydrogen to cause secondary grain growth which begins at about 950° C. As the temperature is raised at about 50° C per hour to 1000° C, the recrystallization process is completed and heating may be carried on to up to 1175° C if desired to insure complete removal of residual carbon, sulfur and nitrogen. 
     The following illustrative, but not limiting, examples of my novel process as actually carried out with the new results indicated above will further inform those skilled in the art of the nature and special utility of this invention. 
     EXAMPLE I 
     Four laboratory heats were melted in an air induction furnace under an argon cover using electrolytic iron and 98 percent ferrosilicon, all containing 3.1 percent silicon, 0.025 percent manganese, 0.012 percent sulfur, 5-10 parts per million boron, 45-75 parts per million nitrogen, 0.10 percent copper and 0.035 percent chromium. Tin was added in different amounts to the separate heats to provide a range of tin content from 0.002-0.045 percent. Compositions of these heats, as analyzed, are set out in Table I: 
     
                       TABLE I______________________________________Heat    % Mn     % S     Mn/S   % Sn   ppm N______________________________________1       0.025    0.012   2.0    0.002  692       0.024    0.012   2.0    0.010  743       0.026    0.012   2.1    0.020  464       0.025    0.011   2.3    0.045  49______________________________________ 
    
     Slices 1.75 inch thick were cut from ingots cast from these melts and were hot rolled from 1250° C in six passes to a thickness of about 90 mils. Following pickling, the hot band samples were heat treated at 950° C, the time between 930° and 950° C being about 3 minutes. The hot bands were then cold rolled directly to 11 mils final gauge thickness. Then Epstein-size strips of the cold-rolled material were decarburized to less than 0.006 percent by heating for 2 minutes at 800° C in 20° C dew point hydrogen. With 0.10 percent tin, the carbon level after the decarburization heat treatment is approximately 0.010 percent. This leads to higher losses but does not affect permeability. Lower carbon levels and losses may be achieved through use of an annealing atmosphere of higher dew point. The decarburized strips were brushed with milk of magnesia to a weight gain of about 40 milligrams per strip and boron additions were made to some of the magnesia coated strips using a 0.5 percent boric acid solution which deposited sufficient boron on the coating that if it were all taken up by the silicon-iron, the boron content of the metal would be increased by 12 parts per million. The resulting coated strips, including both those brushed with the boric acid solution and those not so treated, were subjected to a final anneal consisting of heating at 40° C per hour from 800° C to 1175° C in dry hydrogen and holding at the latter temperature for 3 hours. 
     The effects of tin on the permeability and the 60 Hertz losses at 17kB are shown on the chart of FIG. 1 on which permeability at 10H is plotted against percent tin in the melt. Curve A represents the boron-containing coating specimens while Curve B represents those having coatings which were boron-free. The losses in milliwatts per pound are entered adjacent to the corresponding data points on each of the curves. As evident from the data depicted on the chart, the presence of as little as 0.010 percent tin, particularly with boron added to the coating, results in a substantial improvement in magnetic properties. With these alloys essentially the full benefit in this respect of the presence of tin is attained with 0.020 percent. 
     EXAMPLE II 
     In another experiment like that of Example I, two laboratory heats were melted in an air induction furnace under an argon cover using electrolytic iron and 98 percent ferrosilicon, both containing 3.1 percent silicon, 10 parts per million boron and 40-50 parts per million nitrogen and otherwise having the compositions stated in Table II. 
     
                       TABLE II______________________________________Heat      % Mn      % S      % C    % Sn______________________________________5         0.028     0.013    0.036  &lt;0.0026         0.026     0.013    0.035  0.02______________________________________ 
    
     Processing from the melt stage to finally annealed condition was as described in Example I except that hot rolling was carried out at five different  temperatures and the boron content of the coatings was greater, being equivalent to 15 parts per million on the basis of the substrate silicon-iron sheet or strip material. The permeability values for alloys 5 and 6 are plotted in FIG. 2 when final annealed without a boron addition to the coating, and in FIG. 3 with a boron addition to the coating. The losses in milli-watts per pound are entered adjacent to the corresponding data points on each of the curves representing Heats 5 and 6, as indicated. 
     The superiority of the heat-containing tin is evident from a comparison of the magnetic properties, and particularly the permeabilities, in FIGS. 2 and 3. Even without boron in the coating, the permeability is greater than 1900 or close to 1900 when hot rolled from 1200° C and 1250° C, and with boron in the coating the permeabilities exceed 1900 when rolled from all but the lowest temperature. 
     EXAMPLE III 
     In a third experiment like those of Examples I and II, seven heats each containing 3.1 percent silicon, 0.1 percent copper and 0.03 percent chromium were prepared to the compositions stated in Table III. 
     
                       TABLE III______________________________________Heat  % Mn     % S     % C   ppm B  ppm N  % Sn______________________________________7     0.028    0.013   0.036 7      43     &lt;0.0028     0.026    0.013   0.035 8      39     0.029     0.025    0.014   0.034 6      38     0.04710    0.025    0.009   0.035 4      38     &lt;0.00211    0.025    0.009   0.035 4      38     0.02312    0.027    0.010   0.035 5      35     0.04813    0.024    0.008   0.036 8      36     0.097______________________________________ 
    
     Processing through the final anneal was as set forth in Example I, except that five different hot rolling temperatures were used as set out in Example II. Also, boron was incorporated in some of the magnesia coatings as described in Example II and indicated in Tables IV and V, the boron content of the coating in each instance being equivalent to 12 parts per million on the basis of the substrate silicon-iron sheet or strip material. The magnetic properties of the silicon-iron strip material made and tested in the course of this experiment are set out in Table IV (heats containing 0.013 percent sulfur) and Table V (heats containing 0.009 percent sulfur). 
     
                                           TABLE IV__________________________________________________________________________MAGNETIC PROPERTIES AFTER THE FINAL ANNEAL OF HEATS WITH 0.013% SULFUR  Heat 7          Heat 8          Heat 9  MgO     MgO+B   MgO     MgO+B   MgO     MgO+BHot Rolling  mwpp    mwpp    mwpp    mwpp    mwpp    mwppTemp., ° C  17kB      μ10H          17kB              μ10H                  17kB                      μ10H                          17kB                              μ10H                                  17kB                                      μ10H                                          17kB                                              μ10H__________________________________________________________________________1100   1280      1451          1218              1518                  1244                      1530                          832 1808                                  929 1732                                          727 18781150   1273      1487          874 1739                  767 1848                          707 1903                                  788 1878                                          720 18931200    987      1680          701 1856                  714 1894                          699 1924                                  681 1920                                          665 19321250    847      1774          699 1862                  707 1912                          705 1912                                  702 1919                                          674 19281300   1071      1657          937 1696                  762 1859                          709 1907                                  734 1912                                          688 1920__________________________________________________________________________ 
    
     
                                           TABLE V__________________________________________________________________________MAGNETIC PROPERTIESAFTER THE FINAL ANNEAL OF HEATS WITH 0.009% SULFURHeat 10               Heat 11Hot   MgO     MgO+B   MgO     MgO+BRolling mwpp    mwpp    mwpp    mwppTemp., ° C 17kB     μ10H         17kB             μ10H                 17kB                     μ10H                         17kB                             μ10H__________________________________________________________________________1100  &gt;1300     1455         &gt;1300             1471                 &gt;1300                     1481                         1162                             16111150  &gt;1300     1465         1258             1493                 1170                     1623                         742 18441200  1285     1472         1256             1515                 743 1855                         682 18921250  1226     1530         1037             1648                 705 1888                         679 18981300  1292     1507         906 1750                 923 1737                         735 1860Heat 12               Heat 13Hot   Mgo     MgO+B   MgO     MgO+BRolling mwpp    mwpp    mwpp    mwppTemp., ° C 17kB     μ10H         17kB             μ10H                 17kB                     μ10H                         17kB                             μ10H__________________________________________________________________________1100  1359     1507         970 1701                 --  --  --  --1150  1148     1618         732 1861                 --  --  --  --1200   839     1775         683 1890                 804 1881                         776 19301250   789     1813         678 1906                 --  --  --  --1300  1199     1589         798 1812                 --  --  --  --__________________________________________________________________________ 
    
     EXAMPLE IV 
     In a fourth experiment like that of Examples I, II and III, 10 heats each containing 3.1 percent silicon, 0.10 percent copper, 0.03 percent chromium, 0.04 percent carbon, 0.035 percent manganese, 5-10 parts per million boron and 35-65 ppm nitrogen were prepared. To five heats 0.05 percent tin was added, whereas no tin was added to the other five heats. Compositions of these heats, as analyzed, and the welding behavior of material produced from them are set out in Table VI. 
     
                       TABLE VI______________________________________                         Parallel                                TransverseHeat  % Mn     % S     % Sn   Crack  Cracks/Meter______________________________________14    0.034    0.010   &lt;0.002 No      015    0.035    0.013   &lt;0.002 No     1616    0.037    0.016   &lt;0.002 No     6417    0.038    0.019   &lt;0.002 Yes    17318    0.034    0.022   &lt;0.002 Yes    19219    0.034    0.010   0.045  No      420    0.035    0.013   0.040  No     3721    0.032    0.015   0.046  No     6522    0.036    0.017   0.045  No     7523    0.035    0.019   0.049  --     --______________________________________ 
    
     Table VI indicates that as the sulfur content is increased, the frequency of cracks in the weld increases and with 0.019 percent sulfur or greater, a crack also develops in the weld parallel to its length. The tests yielding these results and leading to the conclusion that the occurrence of cracks in primarily dependent upon sulfur content were carried out through simulated welding which involved running a tungsten electrode (1/16-inch diameter) above (1/32 inch) the surface of a 60-mil thick cold rolled strip specimen clamped in a fixture. With a current of 50 amperes and electrode travel at a rate of eight inches per minute, a molten zone of 100 to 150 mils wide was obtained. After a pass with the electrode, the test specimens fell into three categories: 
     (1) those with a prominent crack running the length of the weld (&#34;parallel crack&#34; in Table I) and with other small cracks in the weld; 
     (2) those without a parallel crack but with occasional cracks in and adjacent to the weld oriented at an angle to the weld (&#34;transverse cracks&#34; in Table I); and 
     (3) those free from cracks, which was confirmed by using a dye penetrant in general use for crack detection purposes. 
     This test exaggerates the tendency for the material to develop cracks, it being anticipated that a material that develops only transverse cracks in the evaluation would be weldable with the proper techniques. 
     In FIGS. 4 and 5 are shown the magnetic properties of the 10 heats after the final anneal. No boron was added to the coating prior to the anneal. Adjacent to the data points are the losses at 17 kilogausses and 60 Hertz. The superior magnetic properties of the heats containing tin are evident. It is apparent from the welding behavior outlined in Table VI and the magnetic properties in FIGS. 4 and 5 that with an addition of tin high permeability and low losses can be achieved in heats sufficiently low in sulfur as not to exhibit a &#34;parallel crack&#34; in the welding evaluation.