US3885958A - Method of producing chromium containing alloys - Google Patents

Abstract

A chromium-containing alloy, having very low contents of nitrogen and carbon, is produced by refining with a small but effective addition of aluminum, or an aluminum alloy, preferably an aluminum-lithium alloy.

Description

United States Patent Grunbaum et al.

[451 May 27, 1975 METHOD OF PRODUCING CIIROMIUM CONTAINING ALLOYS Inventors: Gunnar Grunbaum; John Erik Wallen; John Kjell Gustafsson; Gustaf Widmark, all of Sandviken,

Sweden Assignee: Sandvik Aktiebolag, Sandviken,

Sweden Filed: Dec. 21, 1973 Appl. No.: 427,094

Foreign Application Priority Data Dec. 29, l972 Sweden 17l46/72 US. Cl 75/58; 75/1305 Int. Cl C2lc 7/04; C22c 39/!4 Field of Search 75/58, 130.5

Primary Examiner-C. Lovell Assistant ExaminerPeter D. Rosenberg Attorney, Agent, or FirmPierce, Scheffier & Parker [57] ABSTRACT A chromium-containing alloy, having very low contents of nitrogen and carbon, is produced by refining with a small but effective addition of aluminum, or an aluminum alloy, preferably an aluminum-lithium alloy.

6 Claims, 2 Drawing Figures METHOD OF PRODUCING CHROMIUM CONTAINING ALLOYS The present invention relates to a method of producing chromium-containing alloys with low contents of nitrogen and carbon.

In ferritic chromium steels normal contents of carbon or nitrogen cause material problems such as lowered impact strength, a high (ductile-to-brittle) transition temperature, and disposition towards intergranular corrosion. These phenomena are ascribed to chromium-containing nitrideand carbide precipitations which take place as a result of the too low solubility of carbon and nitrogen in ferrite.

Prior investigations have uncovered the fact that such chromium-containing precipitations can be avoided by alloying the steel with elements which bind carbon and nitrogen more strongly than chromium does, for example with titanium or niobium. It has been found, however, that this treatment gives a material with inferior impact strength, probably because of the formed titanium or niobium carbonitrides. Furthermore, welding of such material is difficult as this easily causes precipitation of chromium-containing particles which in its turn means lowered corrosion properties. Finally, it can be mentioned that titanium and niobium carbonitrides and oxide inclusions collect into streaks which cause surface defects in the material, particularly after grinding or polishing.

By drastically decreasing the contents of carbon and nitrogen far below the normal level, a ferrtic chromium steel is obtained which has very good corrosion properties and a low transition temperature. It should be mentioned that a normal carbonand nitrogen content is in this case:

150 I500 ppm 200 500 ppm where l ppm =0.000l% by weight.

In chromium steels with said improved properties the total contents of carbon and nitrogen have to be reduced to 100 ppm or lower. Such steels have been introduced lately. They have the following compositions:

Cr Ti.Nb C+N l8 28 of O 3 may be present very low Also for austenitic stainless chromiumnickel steels ex tremely low levels of carbon and nitrogen are of interest. The corrosion properties are improved and the ductility is increased, which among other things means a better workability. It is, however, associated with great difficulties to produce steels with total carbon and nitrogen contents of about or below 100 ppm in the presence of a high chromium content. It may for example be mentioned that the solubility of nitrogen in iron, free from chromium, is 500 ppm, while iron with 70 Cr can dissolve up to 5,000 ppm nitrogen.

Therefore, special methods have been tried in order to successfully meet this situation. Such a method is electron beam refining of a chromium steel, in a high vacuum. upon a series of watercooled hearths. This way is complicated; it has high operating costs and dcmands great investment. In a vacuum induction furnace a very low carbon content, l0-2U ppm, can be reached by a lengthy vacuum treatment. The content of nitrogen remains, however, usually at levels above lUU ppm. By combining vacuum induction and vacuum arc remelting (VAR) or double VAR a sum of carbon and nitrogen below ppm has been obtained. This is also a very expensive method, however.

In a conventional arc furnace lower contents than about 250 ppm carbon and 250 ppm nitrogen cannot be practically produced in the presence of chromium. If a process step including vacuum decarburizing with oxygen gas is followed after the procedure in the arc furnace, a low carbon content of about 50-80 ppm can be reached, but the nitrogen content remains at l50200 ppm. Similar conditions apply to the AOD- process (argon-oxygen-decarburizing), in which the chromium-containing charge is decarburizcd by means of a mixture of oxygen and argon. Remelting of chro mium and iron with low contents of carbon and nitrogen in an inert atmosphere in, for example, a HF-(high frequency induction) furnace is impeded because available chromium raw materials such as ferrochromium have too high contents of nitrogen and carbon.

From the above-mentioned technological findings it is obvious that the removal of nitrogen is a greater problem than that of carbon refining. The carbon is removed by oxidation to carbon monoxide, while nitrogen is removed by the reaction: N (dissolved) V: N (gas). This reaction is very slow, kinetically considered.

In an earlier known process the nitrogen problem is solved in the following way. Chromium pig iron produced in a cupola furnace is decarburized in an LD- converter to about 0.3 C, after which the melt is vacuum-oxidized to the above-mentioned low content of carbon. A low nitrogen content of less than 100 ppm is reached thereby that the carbonand silicon-rich chromium pig iron having a relatively low nitrogen content is refined from nitrogen by the very vigorous evolution of gas in the LD-converter during a decrease of the carbon content from 5 to 0.3 percent. The method pre-supposes, however, the opportunity of using special equipment. In the known process the low contents of carbon and nitrogen have been obtained at a Cr content of about 18 percent, thus a rather low content of chromium.

By the present invention a low nitrogen content is obtained by using strong nitrideformers to separate the nitrogen from a melt. This nitrogen-impoverished melt is then refined with respect to carbon and finished to steel in, for example. an AOD-converter.

In the following, the invention will be discussed with reference to the appended drawing, in which FIG. 1 is a diagram showing equilibria of Al and N in aluminum nitride formation; and

FIG. 2 is a diagram showing influence of lithium in cooperation with aluminum in achieving a low content of nitrogen.

Studies of the free energy in forming nitrides from pure components show that titanium and zirconium are stronger nitride formers than is, for example, aluminum. Attempts to decrease the nitrogen content to very low values by means of Ti and Zr fail in practice. however. The interaction between nitrogen and added nitride former thus affects in such a way that Ti and Zr, but not Al, increase the solubility of nitrogen. In the di agram of FIG. I, there is shown the equilibrium be- Curve C "7: Cr 2 Temp. C

A 0 I535 B 0 25 I500 C 2 25 I300 D 2 25 I500 The diagram in FIG. 1 is thus specifically constructed for the purpose of explaining the invented method, and it shows that a low content of nitrogen in the presence of a high content of chromium ought to be obtained by an addition aluminum to a melt containing carbon at a low temperature, i.e., a temperature near the melting temperature. As an example, according to the equilibrium 5 A] at 1,300 C. corresponds to 2 ppm nitrogen in a steel melt with 2 C and Cr. The importance of chromium is shown in the curves A and B. It may be mentioned that in case of titanium, the dissolved N- content first decreases with increasing Ti-content, but already at a Ti-level of about 1 with the equilibrium content of N in the melt again increases.

The reason for non-observance of the possibility of nitride-precipitation with aluminum may besides an insufficient theoretical foundation be that a relatively high carbon content and a low temperature are essential. (Compare curves B and C in the diagram, FIG. 1. The nitrogen content according to the equilibrium is 50 times lower on the curve C compared to B).

In the following text. specific examples will be given of nitrogen refining according to the invention.

EXAMPLE 1 In a 50 kilogram open HF-furnace iron, FeCr and FeMo were melted to the following analysis:

N 298 ppm Fe remainder. besides normal impurities.

After a temperature of 1.350C. had been adjusted, Almetal corresponding to 25% Al in the melt was introduced. After homogenization of the melt, test samples were taken which. upon analysis. showed a nitrogen content of 93 ppm.

EXAMPLE 2 The same charging was used as in the foregoing example. Starting analysis:

Continued N 309 ppm Si 0.30 9; Mo 1.03 9;

Al-metal corresponding to 5.0 percent was added at 1.350" C. After mixing. a sample taken from the furnace showed 57 ppm nitrogen.

EXAMPLE 3 The same material was used as in examples I and 2. Starting analysis:

C 157 7( Cr 25.8 if N 289 ppm Si 0.34 it Mo 1.00 9;

Al-metal corresponding to 9 percent was added at 1.350 C. The nitrogen content was 40 ppm in samples taken from the furnace. From the examples [-3 it is obvious that the nitrogen is reduced with increased additions of Al. See also the diagram in FIG. 2.

EXAMPLE 4 The same test as in the examples l, 2 and 3 was repeated but in a 200 kilogram open HF furnace and at a carbon content of about 3 percent. Starting analysis:

C 2.70 7r Cr 25.2 /r N about 294 ppm Si 0.5l 71 Mo 0.95 71 Al-metal was added corresponding to 9.0 7r Al at l,300 C. In samples taken from the furnace after homogenizing the melt, the nitrogen content was 45 ppm. Thus, a higher content of carbon had not caused any lower nitrogen content than in the compared case. example 3.

EXAMPLE 5 The influence of the chromium content was examined. A charge was produced in a 50 kilogram open HF furnace. Starting analysis:

At the temperature of 1.350 C., Al-metal was added corresponding to 5.0 7( Al. A sample taken from the furnace after mixing showed 30 ppm nitrogen. The example illustrates, in a comparision with example 2, that the refining is facilitated at a lower chromium content.

The examples show that a strong decrease in the nitrogen level is obtained by the help of aluminum. The contents are, however. far above the equilibrium values according to FIG. 1. This might depend upon incomplete separation of the nitride particles from the melt. For this reason the following test was made:

EXAMPLE 6 Four charges of the same kind as in examples 1-4 were prepared in a 50 kilogram open HF furnace. The starting analysis of the melts averaged:

C ll Pi Cr 25.7 i N 320 ppm Si 0.4 F; Mo 1.05 01' Fe remainder At the temperature of 1 ,350C. in the four tests quantities of an aluminum-lithium alloy consisting essentially of 5 Li and 95 "/1 Al was introduced into the melt to a content corresponding to 1.0, 2.0, 2.5 and 8.0 Li. After mixing the melt, nitrogen test samples were taken. The analysis showed 80, 45, and 20 ppm nitrogen (in the above order) after the increasing addition of Al and Li. compared to pure aluminum, aluminum-lithium gave a lower content of nitrogen, see the diagram FIGv 2. This diagram shows the N content as a function of added content of Al respectively LiAl. The upper curve sums up the results in the examples [-3, while the lower curve gives the result of example 6.

EXAMPLE 7 Pure lithium was added in two charges to control the effect in the foregoing example. The amounts were 0.05 and 0.20 Li, respectively. The content of nitrogen before the addition was 285 ppm. After addition of Li the content was 250 ppm nitrogen. No important effect was thus obtained by an addition of Li as such.

EXAMPLE 8 The influence of changing from an iron-base to a nickel-base alloy was examined. A charge was produced in a 50 kg HF furnace. Starting analysis:

N 287 ppm Si (H8 "/1 Ni remainder besides normal impurities Al-metal corresponding to 5 percent was added at l,350 C. A sample taken from the furnace showed 43 ppm nitrogen.

EXAMPLE 9 In an open 50 kg HF furnace a chromium-nickel steel was melted with the following analysis:

Ni l0.2 5%

N 327 ppm Si 0.48 if Fe remainder besides normal impurities At the temperature l350 C. Al-metal corresponding to 572 was added. A sample taken after homogenization of the melt showed 46 ppm nitrogen.

EXAMPLE l0 In a 5000 kg arc furnace a melt was produced. Starting analysis:

C 2.68 Ct Si 0.54 Ct Mo 1 IO "/5 Fe remainder besides normal impurities The melt was tapped into a ladle where Al-mctal corresponding to 6percent was added A sample taken after addition of aluminum showed 35 ppm nitrogen and 5.1% Al. In order to remove aluminum and carbon the melt was transferred to an ADD-converter where aluminum and carbon were oxidized. The analysis in the converter after this period was:

C 35 ppm Cr 22.1 7r

N 47 ppm Al 50 ppm Example 8 shows the result when using a nickel base alloy. The example 9 relates to a Cr-Ni steel. The example l0 shows the final removal of C and Al which should be done in a larger charge and not in a small 50 kg furnace. (Examples 2, 3 and 4 have iron as remainder.)

The given examples all relates to chromium steel. As nickel has an increasing effect upon the activity of the nitrogen, it is self-evident that the invention is applicable also to chromium steels. It is also self-evident that other Al-alloys than pure Al and LiA] can be used, for example CaAl and MgA].

in the practical performance of the invention the raw materials may for example be melted in a normal arc furnace so that a carbon content of 1-2 percent is obtained. Such a charge may be based upon cheap raw materials and also contain a high content of circulating internal scrap. The latter material has great economical importance. The melt is then taken for example to an ADD-converter, vacuum furnace or the like, in which aluminum or aluminum-lithium is added to the melt. After that, the normal process of removing carbon is carried out. An intermediate slag separation preferably is made, because aluminum gives rise to great amounts to slag during the oxidation. When A] is oxidized by oxygen gas, a great amount of heat is evolved. The metal bath may then be cooled by substituting chromium ore for the oxygen gas or by adding suitable scrap. By adding chromium ore the reducing properties of aluminum are used for supplying chromium in a cheap way and with good heat economy.

The nitrogen refining addition of Al or LiAl may also be performed in a ladle. The metal must, however, be protected from nitrogen pick up from the air when the content of the ladle is taken to the unit for oxidation of carbon. If the ladle has a high freeboard, meaning that the level of the melt is far below the upper edge of the ladle, vacuum oxidation can be done directly in the la die.

It is also possible to granulate or in other ways disintegrate the nitrogen-refined chromium alloy. Decarburizing can then be performed in solid state in known ways, for example annealing in vacuum. The obtained carbonand nitrogen-impoverished chromium alloy may then be melted together with likewise carbonand nitrogen-impoverished iron under an argon atmosphere. for example. in an HF furnace.

From the results it is obvious that at least 1 percent, and preferably at the least 3 percent. Al in the form of aluminum metal must be added to the melt in order to make an essential reduction of the nitrogen content possible (compare the diagram FIG. 2).

As has been mentioned earlier. the carbon content should be relatively high in the melt bath when the addition of Al is effected. Carbon has a favorable inlluence, as it in itself decreases the solubility of nitrogen as well as lowering the melting point of the melt bath. A suitable content of carbon has shown to be at least 1 at C.

The invented method is particularly applicable to melts containing between and 7: Cr. At actual contents of Cr and C the melt should have a temperature between l 300-l ,500 C.

The above-described method and the specific examples shown have generally discussed the conditions for removal of nitrogen and carbon from a chromiumcontaining base melt of iron. It is. naturally, within the scope of the invention to effect corresponding operations with chromium-containing base melts of other metals for which the prerequisite conditions are similar.

We claim:

1. Method of producing a chromium-containing alloy having very low contents of nitrogen and carbon not amounting, in toto, to more than 0.01 percent by 8 weight. which comprises melting a charge containing at least 1 percent by weight of carbon and consisting essentially of a chromium-containing initial material together with a base metal and desired alloying elements;

adding to the melt a nitridable member of the group consisting of aluminum and aluminum alloys. in an amount corresponding to at least 3 percent by weight of said melt. thereby converting nitrogen to aluminum nitride in the form of a constitutent of the resulting slag; and

thereafter refining the molten charge by oxidizing carbon and residual aluminum and removing slag containing aluminum nitride and aluminum oxide.

2. Method according to claim 1. wherein the melt has a content of Cr between 15 and 30 percent.

3. Method according to claim 1, in which the melt is heated to a temperature between l,300l ,500 C. before Al is added.

4. Method according to claim 1, wherein the added aluminum is in the form of an alloy consisting essen tially of Al and a member of the group consisting of Li, Ba, Ca and Mg.

5. Method according to claim 4, in which carbon and aluminum remaining in the melt after the nitrogen refining operation are removed from the melt by oxidation.

6. Method according to claim 5, in which the oxidation of carbon and aluminum is effected with the aid of added chromium ore or scrap.

Claims (6)

1. METHOD OF PRODUCING A CHRONIUM-CONTAINING ALLOY HAVING VERY LOW CONTENTS OF NITROGEN AND CARBON NOT AMOUNTING IN TO TOO MORE THAN 0.01 PERCENT BY WEIGHT, WHICH COMPRISES MELTING A CHARGE CONTAINING AT LEAST 1 PERCENT BY WEIGHT OF CARBON AND CONSISTING ESSENTIALLY OF A CHRONIUM-CONTAINING INITIAL MATERIAL TOGETHER WITH A BASE METAL AND DESIRED ALLOYING ELEMENTS; ADDING TO THE MELT A NITRIDABLE MEMBER OF THE GROUP CONSISTING OF ALUMINIUM AND ALUMINIUM ALLOYS, IN AN AMOUNT CORRESPONDING TO AT LEAST 3 PERCENT BY WEIGHT OF SAID MELT THEREBY CONVERTING NITROGEN TO ALUMINIUM NITRIDE IN THE FORM OF A CONSTITUENT OF THE RESULTING SLAG; AND THEREAFTER REFINING THE MOLTEN CHARGE BY OXIDIZING CARBON AND RESIDUAL ALLIMINUM AND REMOVING SLAG CONTAINING ALUMINUM NITRIDE AND ALUMINUM OXIDE.

2. Method according to claim 1, wherein the melt has a content of Cr between 15 and 30 percent.

3. Method according to claim 1, in which the melt is heated to a temperature between 1,3001,500* C., before Al is added.

4. Method according to claim 1, wherein the added aluminum is in the form of an alloy consisting essentially of Al and a member of the group consisting of Li, Ba, Ca and Mg.

5. Method according to claim 4, in which carbon and aluminum remaining in the melt after the nitrogen refining operation are removed from the melt by oxidation.

6. Method according to claim 5, in which the oxidation of carbon and aluminum is effected with the aid of added chromium ore or scrap.

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