US3764407A - Method for producing a mono directional silicon steel sheet - Google Patents

Abstract

A MONO-DIRECTIONAL SILICON STEEL SHEET HAVING EXCELLENT ELECTROMAGNETIC CHARACTERISTIC IS PRODUCED BY USING A SILICON STEEL CONTAINING 2-4% OF SI AND SMALL AMOUNT OF AL, TI, ZR OR TA OR AT LEAST TWO OF AL, TI, ZR, AND TA AND SUBJECTING THE STEEL SHEET TO NITRIDING TREATMENT AT 450*750*C. BEFORE A FINAL ANNEALING

Description

1973 HIROSHI HIRANO ETAL 3,754,407

METHOD FOR PRODUCING A MONO-DIRECTIONAL SILICON STEEL SHEET Filed Feb. 9, 1972 v 3 Sheets-Sheet 1 Oct. 9, 1973 HIROSHI HIRANO ETAL 3,754,407

METHOD FOR PRODUCING A MONODIRECTIONAL SILICON STEEL SHEET Filed Feb. 9, 1972 5 Sheets-Sheet 2 METHOD FOR PRODUCING A MONO-DIRECTIONAL SILICON STEEL SHEET Filed Feb. 9, 1972 HlROSHl HIRANO ETAL Oct. 9, 1973 3 Sheets-Sheet 5 N0. /28 N0. 53 N0. /36

No. /i3

U.S. Cl.,14tl-'--112 2 Claims ABSTRACT QF.THE...,DISCLSURE A mono-directional silicon steel sheet having excellent electromagnetic characteristic is produced by using a silicon steel containing 24% of Si and small amount of Al, Ti, Zr or Ta or at le'ast'two of.Al, Ti, Zr and Ta and subjecting the steel sheet to nitriding treatment at 450- 750 C. before. a final annealing.

This in a continuation-in-part of our application, Ser. No. 817,485 filed on Apr. 18, 1969, now abandoned.

This invention relates to amethod for producing a mono-directionalsilicon steel sheet having excellent electromagnetic characteristics. The term sheet herein used also includes a strip.

The mono-directional-silicon steel sheet used-for core materialof a transformer, etc. ispolycrystallin'e. When each crystal grain whch is a constructive element of the siliconsteel sheet is oriented in .the directionof (110) 100 a soft electromagnetic material can be produced having a low iron loss anda high magnetic flux density by passing magnetic flux in the direction of 100 Such electromagnetic material has been produced on an industrial scale. However, in the industrial production of said materiah the establishment of a technique for obtaining a stable secondary recrystallization structure has been a problem and many proposals have been made as a possible solution. Most of the conventional methods for producing a mono-directional silicon steel sheet, with some exceptions, have in common the utilization of a dispersed fine' precipitatetopreventnormal grain growth of primary recrystallization. However, the control of the precipitation state of the dispersed fine precipitate must be monitored through the,v entire production run beginning with the initial chemical composition of an ingot, heating of the ingot, hot rolling, water cooling after hot rolling, coiling temperature of steel strip and the subsequent cold rolling, annealing, etc. As a result, unusual and careful quality control mustbe used to produce a mono-directional silicon steel sheet wilth good repro ducibility. and in a high yield. 7

As is well known, for example, MnS is widely used to restrict normal grain growth and to obtain complete secondary recrystallization. However, in the event of MnS, the content of Mn and S must precisely be controlled to ensure solution of MnS during heating of the "UnitedStates Patent 0 ice iron including inevitable impurities, to which a small amount of one element selected from the group consisting of Al, Ti, Zr and Ta or two or more of elements selected from the group consisting of Al, Ti, Zr and Ta are added as a nitride producing element, to hot rolling, and cold rolling to the given thickness, decarburization, etc. in accordance with the conventional methods, to nitriding treatment for nitriding said nitride producing elements and to the final annealing treatment to orient the crystal direction to the direction of (110) after the final annealing.

According to this invention, elements, Al, Ti, Zr and Ta added to the silicon steel are nitrided by a nitriding treatment for attaining secondary recrystallization before the final annealing to produce nitrides of said elements and said nitrides act efiectively as dispersed precipitate for preventing the normal grain growth of primary recrystallization. These elements also ensure the production of a mono-directional silicon steel sheet having secondary recrystallization structure. Adding nitride producing elements to silicon steel and nitriding said elements with a suitable nitriding treatment immediately before the final annealing to produce the nitrides simplify the practice of our process since the extensive monitoring of the several stages is unnecessary to our process.

A characteristic of this invention is the addition of said nitride producing elements to the silicon steel used. When said element is added alone, Al should be added within the range of 0.01-0.07% by weight, Ti within 0.01 0.2% by weight, Zr within 0.01-0.3% by weight and Ta within ODS-0.5% by weight. When two or more of them are added, Ti, Zr, Ta and A1 are added within the ranges as mentioned above, but the total amount should not exceed 0.5%.

The reasons for the restrictions of the amounts of the nitride producing elements as mentioned above are as follows: (i) when the amounts are less than their lower limits, the amount of nitride produced is insuflicient to prevent the normal grain growth of the primary recrystallization grains and no growth of stable secondary recrystallization is observed and (ii) when the amounts are more than the upper limits, excess nitrides are precipitated to prevent the growth of the secondary recrystallization.

Si is added primarily for improving magnetic characteristic and is added in an amount of 24% in view of industrial production. Furthermore, the silicon steel should contain carbon in an amount of 0.01-0.10% for the following reasons. Since carbon has a bad effect on magnetic property, the amount should be as small as possible. However, when carbon content is less than 0.01 in the production of steel ingot, insufficient deoxidation of thesteel takes place. Since steel containing a large amount of oxygen deteriorates the magnetic property of the steel, the lower limit of carbon is specified as 0.01%. When the amount of carbon is more than 0.1%, decarburization becomes difficult and time consuming and further the texture of the sheet upon cold rolling and annealing is also affected. Therefore, the carbon content is restricted to 0.01-0.10%.

Another characteristic of this invention is that a silicon steel sheet containing said nitride producing elements is subjected to nitriding treatment immediately before final annealing. Said nitriding treatment can be attained merely by heating the silicon steel sheet in an atmosphere of pure nitrogen gas, hydrogen-nitrogen mixture gas capable of nitriding or ammonia decomposition gas from liquid ammonia. The heating temperature in said nitriding treatment must be between from 450 to 750 C. for industrial production. This is because the silicon steel sheet must be heated at a temperature as low as possible in order to avoid softening and coil set of the sheet due to high tem- 3 V 4, perature. As is known, thedsofteniilg aid coil setnare af- TABLE 1 fected b thickness and wi th of e s eet as we as mv triding tie mperature. Hence, since the nitriding treatment Temp Elementm 8011mm EDI-000% EN is carried out after the completion of the primary recrysm t E tas mp u 'Ii=0.0 7% 2Ti=0.03% tallization by clecarburization treatment in a continuous 5 N 0001 Q0014 furnace, the nitriding temperature within the range of 1,350- N gggg 8.3853 450 to 750 C. can be determined with due regard to T1 010304 010261 coil set due to coil softening, and by criteria known to l 100 N 3;; those skilled in the art, the most economical temperature Ti Q6391 0: 3 within said range can accordingly be optionally selected. 10 0-0309 385g; The heating temperature in the nitriding treatment, N i 1565' 010087 which depends upon, nitrogen partial pressure and the T1 8.8333 kind and amount of nitride producing elements contained in the steel, is about 2-20 hours within which time secondary recrystallization can be achieved.

Conditions other than those above mentioned which 10g [percent Zr] [percent w+ have not been specified as a characteristic of this mven- T tion may be suitably chosen and employed. For example, for rolling the silicon steel to a desired thickness, the steel is hot rolled, thereafter is subjected to pickling and an- Temll Elementm 2N=0'009% nealing, and then is subjected to cold rolling which int ment Elementascompound =0.03% eludes twice and single cold rolling methods. According N Nm solution .0002 045 to the single cold rolling method, a steel sheet of 1.5-2.5 N as Zr mm. in thickness is cold rolled with a reduction rate of 5 70-90% to a desired thickness and is subjected to nitriding {N treatment after decarburization step. According to the Zr double cold rolling method, a steel sheet of 2-3 mm. in N thickness is first subjected to a rolling with a reduction rate of 50-80%, then to decarburization treatment at 700-900 C., thereafter to the second cold rolling with a reduction rate of 40-60%, again to decarburization treatment, and then to nitriding treatment. 2 800 I v The nitride forming elements in this invention must be log [percent Ta] [percent N]=---+6.08 classified into two groups from the thermodynamic point T of view, that is, the groups of Ti and Zr, and A1 and Ta. TABLE? Titanium and zirconium form more stable compounds with Element in Solution EN=0'009% nitrogen that any other commonly used alloying elelements, that is, titanium and zirconium nitrides are much Temp (0 meat Element 85 P 2113:0270 more stable than aluminum and tantalum nitrides. N Nin solution; 0.007 The thermodynamic data concerning the equilibrium L350 figg between Ti and N or Zr and N in ferrite, particularly Ta g T--- 0-028 nsolutioii.-.. 0.0042 in 3 percent silicon steel are still lacking. 1,100- N {N as 'IaN. 0. 0048 In order to attempt a qualitative estimation of the be- Ta 8- g: haviors of these nitride forming elements in 3 percent N Soluflo 010002 silicon steel, the thermodynamic relationships in gamma 900 ga i n s gl iitiom 88323 or alpha iron shown below are taken and the quantities of Ta {Ta as TaN 011141 nitrogen in solution or as compound with each nitride gigging element are calculated under the given condi- I j The empirical formula and the results of calculation 1 [percent A1] [percent N]= 1.95 are as follows. T

TABLE 4 Element in solution EN=0.015% EN=0.009% Temperature C.) merit Element as compound 2Al=0.05% 2A1=0.03% EA1=0.05% EAl=0.03% 1,350 --{N n i H Al {Al in solution 1,100 N 8: 8888 8188?? A1 88888 N 01 0018 01 0045 900 0. 0132 0. 0105 0. 0245 0. 0097 0.0255 0. 020a ('Y)-Ti-N EFe(a)-Al-N log [percent Ti] [percent N -+4 72 lo 8300 p j V j I T g percent Al] [percent N] 1.6g

' TABLE I V Element in solution 2N=0.015% 2N=0.009%

Temperature C.) ment I Element as compound 2A1=0.05% ZA1=0.03% 2Al=0.05% 2Al=0.03%

N 0.0012 0. 0001 0. 0003 900 0. 0138 0. 0089 0. 0087 0. 0034 0. 0329 0. 0132 0.0266 0. 0171 0. 0168 N 0. 0005 0.00003 0.00007 800 N as A 0.0145 0.00 0.0089 Al A1 in solution 0.0020 0.0327 0.0128 l as l 0. 0280 0. 0173 0. 0172 N V 0. 0001 0. 00001 700 0. 0149 0. 0090 0. 0006 0. 0013 0. 0327 0. 0127 0.0287 0.0173 0.0173

According to-the-=results of the calculation above, the followingconclusionsare obtained.

( Ee-Ti-N'.

As is clearly shown in Table 1j,jmost of the nitrogen is fixed as TiN at 1350 C.,when 'N content is 0.009 percent and Ti content is 0.03 percent; 1

It seems that the siie of molten steel are generally quite large and they are distributed coarsely -i n solidified 7 steel. Such kinds of titanium nitrides, of course, will, to'some extent restrict grain boundary migration, but entire prevention of normal grain growth is impossible. v

'lFIGpl-is a photograph (X 1,500) showing an example of; a large titanium nitride particle which is seemed to be formed in the molten steel, and the effect of the tea striction of grain-boundary migration.

If the contents of Ti and N in steel are kept at a constant value and the size of titanium nitrides are quite a large as shown in EIG. 1.,--the totalnumberof-titanium nitrides become small. In the consequence, absolute pre-. vention of normaLgrain growth-- is- -impossible and complete secondary recrystallizationfcannot'be obtained. in addition, in the event of titanium nitrides, the control of the size and distribution ofsuch nitrides by heat treatment is virtually impossible. This is clearly shown in Tablel.

Accordingly in this invention, only titanium is added to. the original melt to the substantial exclusion of nitro gen, the latter if present being containedin the melt as an incidental impurity.

The material mentioned above. was-processed following .the process shownin examples.

. FIG. 2 isa photograph (X1,500) showing the finely dispersed titanium nitrides formed by nitriding treatment prior to final annealing for secondary recrystallization; It is clearly shown that in the case of a strong nitride forming element like titanium, nitriding at low temperature such as 750 is the most excellent means to get finely dispersed titanium nitrides. The size of titanium nitrides shown in FIG. 2 is about 70 A. in average and much smaller than the one shown in FIG. 1.

The is one ofthe characte'fisticsof this invention.

Zirconium is thermodynamically strong nitride form 1 ing element next totitanium. 1

Accordingly the behavior of ZrN is similar to the one of TiN. It is observed from Table 2r that a partof ZrN goes into solution at a'temper'ature'of from about 1100 C. to '1350 C. when the contents of Zr and um 0.03%- and 0.009%, respectively. It is. also observedfromiTable' 1 that only minor amount of .TiN, on the contrary. to

the case of ZrN, goes into solution.

This is amarked difference between both'nitride form ing elements.

According to -Table 2, whenthe content ofZr'is-"0.03%, about, half of thetotal nitrogen-goes into solution at. but whe the o t n o ZP-, .$..Q-Q3i L! .-Q LQ. the nitrogen remains still unsolved. It" is, therefore, im-

shown titanium nitrides formed in possible to control the size and distribution of zirconium 5 nitrides by successive treatments such as slab heating,

hot rolling and the control of coiling temperature.

The formation of zirconium nitrides by nitriding process prior to final annealing for secondary recrystallization is also a quite elfective method to obtain finely dispersed zironium nitrides.

to subsequent heat treatment. However, said control by the heat treatment process requires severe limitations of temperature through the rolling, decarburization and denitrification ste-ps.' Therefore, it is very diflicult to obtainproducts having stable properties in the industrial scale production.

As is shown in Tables 4 and 5, the behavior of the aluminum nitrides closely resembles to that of tantalum 0 nitrides. It is, therefore, possible to control their size and I distribution by successive treatments after slabbing.

Accordingly, both methods of adding of nitrogen can be used for the same purpose above mentioned. In this invention, however, the addition of nitrogen from annealing atmosphere is intended to restrict the nitrides formation to the minimum quantity required. The nitrides formed adversely affect the magnetic properties of the final product. The amount of nitrides, therefore, must -be-limited to minimal and they must be re- 5 moved after obtaining secondary recrystallization structure.

'The denitriding from the 3 percent silicon steel (containing) these nitride forming elements using hydrogen is possible and depends on the content of nitride forming elements and nitrogen, and the thermodynamic properties of the nitrides.

" As to Zr, Al and Ta, the nitrides of these elements are 'less stable than titanium nitride, and consequently the amount of the nitrides of these three elements can be so easily decreased by hydrogen reduction after annealing to achieve secondary recrystallization.

and 9. The analysis of the nitrogen in a 3 percent silicon steel is so difiicult that a modified Kjeldahl technique was applied, but some uncertainties in the accuracy still remain. As tantalum forms slightly less stable nitride than aluminum, it can easily be removed by denitriding in hydrogen.-

- It was found that titanium nitride is so stable that the denitriding of the material containing Ti is almost impossible.

Consequently the selection of the nitride forming elements must be made based on the grade of a final prodnet or an end use of the product, and in accordance with the working process. A 7

The results of the denitriding are shown in Tables 7= TABLE 6.-CHEMICAL COMPOSITION OF ondary recrystallization was carried out for 3 hrs. at THE SPECIMEN 1150 C v No of Specimen 163 FIG. 3 shows photographs indicating the remarkable C O eflects of nitriding for the secondary recrystallization of Si 5 the specimens, namely, shows macro-structures'after the Mn 012 annealing for secondary recrystallization of each speci- P O 002 men. The photographs of the upper row in FIG. 3 show the macro-structures of the specimens without nitridin s 0.001 g Zr 0 057 namely, those vacuum heated and the' photographs of the 7 I: 00024 lower row show macro-structures of the specimens sub- TABLE 7.--THE RESULTS OF THE DENITRIDING TREATMENTS i N soluble Ninsoluble 2N v Numbers of specimens (percent) (percent) (percent) Treatment 0. 0019 0.0005 0. 0024 As cold rolled. 0.0028 0. 0010 0. 0038 Nitrlding for 1 hr. at 700 C. in 18% H, plus N1 atmosphere. 0.0049 0.0018 0.0067 Nitriding for 3 hrs. at 700 C. in pure nitrogen. 0. 0038 0. 0010 0. 0048 Denitriding for 2 hrs. at 1,150 O. in pure hydrogen. 0. 0027 0. 0012 0.0039 Denitriding for 4 hrs. at 1,150 C. in pure hydrogen. 0. 0037 0.0007 0. 0044 Denitriding for 8 hrs. at 1,150 C. in pure hydrogen. 0.0065 0.0005 0.0070 Nitriding for 5 hrs. at 850 C. in pure nitrogen; 0. 0040 Trace 0. 0040 Denitriding for 2 hrs. at 1,150 C. in pure hydrogen. 0. 0034 0. 0021 0. 0055 Denitriding for 4 hrs. at 1,150 C. in pure hydrogen. 0. 0037 0. 0005 0. 0042 Denitriding for 8 hrs. at 1,150 O. in pure hydrogen.

TABLE 8.CHEMICAL COMPOSITION OF I THE SPECIMEN jected to nitriding for 5 hours at 750 C. in pure N; of Specimen atmosphere. (No. 1560.01-8% A1, No. 1-600.064% Ti, C 05 N0. 162-0.033% Zr, N0. 164O.l1% Zr and N0. 1 66 Si 3.03 0.10% Ta). 7 Mn 11 FIGS. 4 to 6 are photographs which show macrostrucp 0,011 ture of mono-directional silicon steel sheet obtained S 0.004 under various conditions according to this invention. 1 0.019 The following examples illustrate this invention. I ZN 0.0129

TABLE 9.THE RESULTS OF THE DENI'IRIDING TREATMENTS N soluble N insoluble ZN Numbers of specimens (percent) (percent) (percent) Treatment p 0.0112 0. 0017 0. 0129 Nitrlding for 10 hrs. at 750 C. in pure nitrogen. 0. 0029 0. 0020 0. 0049 Denitridlng for 4 hrs. at 1,000 C. in pure hydrogen. 0. 0012 0. 0032 0. 0044 Denitriding for 4 hrs. at 1,050 O; in pure hydrogen. 0.0015 0.0021 0. 0030 Denitriding for 4 hrs. at 1,100 C. in pure hydrogen. 0. 0010 0. 0020 0. 0030 Denitriding for 4 hrs. at 1,150 C. in pure hydrogen. 0. 0008 0. 0016 0.0024 Denitriding tor4 hrs. at 1,200 C. in pure hydrogen. 0. 0018 0. 0021 0. 0039 Denitridlng for 24 hrs. at 1,000 C. in pure hydrogen. 0. 0018 0.0020 0.0033 Denitrlding for 24 hrs. at 1,050 C. in pure hydrogen. 0. 0007 0. 0025 0.0032 Denitriding for 24 hrs. at 1,100 C. in pure hydrogen. 0. 0006 0. 0015 0. 0021 Denitriding for 24 hrs. at 1,150 O. in pure hydrogen. 0. 0006 0.0010 0. 0016 Denitrlding for 24 hrs. at 1,200 C. in pure hydrogen.

TABLE 10.-CHEMICAL COMPOSITION OF p v THE SPECIMEN Steels having the chemical compositions as shown in of specimen 160 the Table 12 below were prepared and ingots of 100 C 0.063 kg. were produced therefrom. Each ingot was heated Si 3,07 at 12 C. and then extended into a l of Mn 11 100 mm. in thicknessyx 1 mm. in width x 150 mm. in P 0.003 55 l ngth by forging. Th'en said slab was heat d to 1.250 c, s 0.004 and thereafter hot rolled to a thickness of 2.3 mm. Each A1 0.006 hot rolled steel was subjected to pickling, thereafter an: Ti 0.064 p I v TABLE 11.THE RESULTS OF THE NITRIDING OF THE SPECIMEN CONTAINING TITANIUM a N soluble N insoluble 2N Numbers of specimens (percent) (percent) (percent) Treatment 032: s-sss s-ae; stn. t 2%% c. H

1 1n or r. 0. 0042 0. 0030 0. 0063 Nitriding for a hrsi et 700 (fin are n it i gg ri p 0. 0044 0. 0019 0.0072 Nitriding for 5 hrs. at 850 C. in pure nitrogen.

In addition, Table 7, 9 and 11 show the increase of nitrogen content by the nitriding carried out under the nealing (heated at 800 C. fo r 20 minutes), the first cold various nitriding conditions. The remarkable increase of lling (to 0.70 mm. in thickness), decarburization annitrogen contents can be recognized in every case. nealing (at 780 C. for 20 minutes), the'sec ond cold The finely di p nitride pl'eciPltates formed rolling,(to 0.35 mm. in thickness), decarburization an: nitriding greatly contribute to the secondary recrystal lization of these specimens.

. t n atmos here how b low, an th In order to distinguish the effect of the fine precipitates men m? p as S n e ereafter' the final annealing. The annealing between the first cold tta ned wthout the reel itates, the gig in ti in i ng one of each elemefit of Ti, Zr, Ta 011mg and the Second rolling y f the and Al were heated for 5 hrs. in vacuum or nitrogen decarburizafiqn treatment is o always n s y mthe atmosphere a 750 C., and then the annealing for seccase of steel havinga low carbon m nealing (at 780 C. for 20 minutes), then nitriding treat;

TABLE 12.CHEMIOAL COMPOSITIONS (WT. PERCENT) OF TESTING MATERIALS Si Mn P s Cu Ni Cr Al Ti Ta Zr EXAMPLE 1 1. A method of producing a mono-directional silicon A steel sheet of 0.35 mm. in thickness after the second Ste cold rolling and decarburization treatment, was subjected to nitriding treatment at 500 C. for 10 hours in ammonia gas produced from liquid ammonia and then was cooled to room temperature. Thereafter it was heated in an argon atmosphere from room temperature to 900 C., kept for 2 hours at this temperature, heated to 1150 C., and thereafter was held in a hydrogen atmosphere at 1150 C. for 4 hours. The macro-structure of thus obtained silicon steel sheet is shown in FIG. 4.

EXAMPLE 2 Each steel sheet of 0.35 mm. in thickness, after the second cold rolling and decarburization treatment was subjected to nitriding treatment at 650 C. for 18 hours in a pure nitrogen gas atmosphere and then was heated to 900 C. in an argon atmosphere, kept 2 hours at this temperature, again heated to 1150 C., and thereafter was held at 1150 C. for 4 hours in a hydrogen atmosphere. The macro-structure of the thus obtained steel sheet is shown in FIG. 5.

EXAMPLE 3 Each steel sheet of 0.35 mm. in thickness, after the second cold rolling and decarburization treatment, was subjected to nitriding treatment at 750 C. for 4 hours in a pure nitrogen atmosphere and then was heated to 900 C. in an argon atmosphere, kept 2 hours at this temperature, again heated to 1150 C., and thereafter was held at 1150 C. for 4 hours. The macro-structure of thus obtained steel sheet is shown in FIG. 6.

The nitriding atmosphere employed in Examples 2 and 3 may be nitrogen atmosphere containing hydrogen (a mixture gas of less than of hydrogen and balance of nitrogen) which is capable of nitriding the treated steel sheet. Sufficient nitriding effect was seen with such mixture atmosphere but photographs of macro-structure ob tained using such atmosphere were not taken.

'Regarding Zr containing steel (type 128), a hot rolled sheet of 1.5 mm. in thickness (aside from those mentioned above) was also prepared and was rolled to a sheet of 0.35 mm. in thickness by only one pronounced cold rolling to provide the same result.

The percentages referred to herein are by weight.

What is claimed is:

e1 sheet, comprising the sequential steps of (i) selecting a hot and cold rolled silicon steel consisting, aside from incidental impurities, of (a) from 2 to 4% silicon, (b) from 0.01 to 0.10% carbon,

and (c) a nitride producing element selected from the group consisting of Al, Ti, Zr and Ta and which when incorporated alone is in the range of 0.01-

0.07% Al, 0.01-0.2% Ti, 0.01- 0.3% Zr and 0.05-

0.5% Ta and when incorporated as two or more elements are within the indicated ranges, the total being however not greater than 0.5

(ii) subjecting said hot and cold rolled steel to decarburization treatment,

(iii) nitriding in a nitriding atmosphere the clecarburized steel at a temperature of 450 C.-750 C. for

2 to 20 hours to form a finely dispersed nitride precipitate of said element in said steel; and (iv) subjecting the steel to denitriding treatment at an annealing temperature of between 1,000-1,200 C.

in a hydrogen atmosphere for a time sufiicient to remove said nitrides.

2. A method according to claim 1, wherein the nitriding atmosphere is selected from the group consisting of pure nitrogen gas, hydrogen-nitrogen mixture gas and ammonia gas from liquid ammonia.

References Cited UNITED STATES PATENTS 3,214,303 10/ 1965 Fiedler 148-1l1 3,147,158 9/1964 Fiedler 148-111 2,802,761 8/ 1957 Fast 148-113 3,184,346 5/1965 Fiedler 148-31.55 3,163,564 12/1964 Taguchi et a1. 148-111 3,287,183 11/1966 Taguchi et al. 148-111 3,671,337 6/1972 Kumai et a1 148-113 2,528,216 10/ 1950 Dunn et al 148-16.6 2,114,802 4/ 1938 Kinzel 148-16.6 2,067,896 1/ 1937 Babinet 148-16.6

L. DEWAYNE RUTLEDGE, Primary Examiner W. R. SATTERFIELD, Assistant Examiner U.S. Cl. X.R. 148-113, 111

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