US2745731A - Method for making low carbon 18-8 type heats - Google Patents

Description

May 15, 1956 w, ESE, JR, ETAL 2,745,731

METHOD FOR MAKING LOW CARBON 18-8 TYPE HEATS Filed Nov. 22, 1954 E Q S Q Q m MS R k EQC RQQQQQQGWQ 1708579 .1/V30U3d INVENTORS Gar/and W Reese Jn and GeorgeE. Schmidt Unite dtates METHOD FOR MAKING LOW CARBQN 18-8 TYPE HEATS Application November 22, 1954, Serial No. 470,303

5 Claims. (Cl. 75-12) This invention relates to improvements in the method of making steel and in particular to a method of making steel having an extra low carbon content.

Heretofore there have been many types of steels made, having different characteristics depending upon the ultimate use thereof. For example, certain stainless steels, such as Type 304, are not suited for use at a temperature in the range between 750 F. and 1500 F. because such steels are subject to sensitization, a condition wherein the carbon precipitates at the grain boundaries in the form of chromium carbide, thus depleting the adjacent areas of the grain of its requisite chromium content. Since chromium is the principal element used in stainless steel for developing resistance to corrosion, when the chromium is depleted in certain areas of these steels as a result of the precipitation of chromium carbide, sensitized steel such as Type 304 is subject to an accelerated form of corrosion known in the trade as intergranular corrosion However, sensitization in Type 304 stainless steel takes place only if the steel is exposed to a temperature above 750 F. and held at such temperature for a period of time. In order to overcome the sensitization of stainless steels such as Type 304, Types 321 and 347 stainless steels were developed, titanium and columbium, respectively, being employed as stabilizers to prevent the precipitation of chromium carbide at the grain boundaries when the steels are subjected to temperatures in the range between 750 F. and 1500 F.

While the stabilized stainless steels can be used in the sensitization range without the precipitation of chromium carbide, such steels are more costly than Type 304 stainless steel. It is also more difiicult to produce such stabilized steels since an extremely close control is required in the melting and rolling and difiiculties are encountered in the finishing operation to produce the required surface quality. High scrap losses are thus encountered in producing the stabilized stainless steels.

It has been found that sensitization in 18-8 type stainless steels can in most cases be overcome by lowering the carbon content to below 0.03% maximum. This low carbon content has been achieved by injecting gaseous oxygen into the molten metal to oxidize the carbon from the molten metal bath. However, in obtaining a sufficiently low carbon content, that is, below 0.03% maximum in the final analysis of the solidified metal, many difiiculties have been encountered. For example, very high temperatures over long periods of time have been employed in the refining with the result that the life of the refractories have been shortened. Further, in certain melting practices, it has been necessary to use exatom tremely large volumes of oxygen in the melt with the "ice result that oxidizable elements such as chromium and iron have been oxidized into the slag. It has also been difficult to prevent carbon contamination such as is encountered in ordinary steel mill practices. All of these difiiculties have placed a premium on the cost of pro-- ducing steel having an extra low carbon content.

An object of this invention is to provide a method for producing stainless steel having a carbon content below 0.03% maximum carbon.

Another object of this invention is to provide a method for producing extra low carbon stainless steel in which a preferred sequence of steps are employed to produce an extremely low carbon content of below 0.03% maximum carbon in a substantially shortened time period, without the use of extremely high temperatures, and without undue less of metal from the initially melted metal.

A more specific object of this invention is to provide a method for producing stainless steel having an extra low carbon content below 0.03% maximum carbon in which are initially melted charge of scrap metal devoid of substantial amounts of chromium and manganese is recarburized to produce a reboiling action, and simultaneously with the reboiling action, inject gaseous oxygen into the reboiling molten metal bath to thereby lower the carbon content to substantially below 0.03% maximum carbon in less than 8 minutes from the start of the oxygen blow and without attaining excessively high temperatures.

These and other objects of this invention will become apparent from the following description when taken in conjunction with the drawing, the single figure of which is a typical carbon-time curve, resulting from the practice of this invention.

In its broader aspects, this invention makes use of a preferred sequence of steps which are part of the overall melting procedure. The preferred sequence of steps comprises recarburizing an initially melted metal bath by the addition of a predetermined mass of pig iron, which pig iron produces a reboiling action within the molten metal bath. Simultaneously with a reboiling action, a predetermined amount of gaseous oxygen is injected into the molten metal bath to reduce the carbon content to substantially below 0.03% maximum within a time period of less than 8 minutes from the start of the oxygen injection.

In practicing this invention, it is preferred to follow a virgin metal melting procedure in producing extra low carbon steels. Such a virgin metal melting procedure comprises melting a charge of scrap metal which is substantially devoid of oxidizable elements, such as chromium, manganese, silicon and aluminum, but which has predetermined maximum amounts of the elements sulphur, phosphorus and copper. A predetermined quantity of scrap metal is charged into an electric arc furnace of the carbon or graphite electrode type together with slag forming material such as burnt lime where required. This charge is melted through the action of the carbon arcs to form a molten metal bath. The carbon content of the molten metal bath is then adjusted through the use of either ore additions or gaseous oxygen, or the combination of both. After thecarbon content is adjusted to the predetermined low level, the alloying elements needed in the final chemical analysis are added either in the form of virgin metals, for example, electrolytic manganese or in the form of ferro- 3 alloys for example, ferrochromium, ferrosilicon, ferromanganese and the like. The molten metal bath is then refined to obtain the desired chemical analysis and thereafter tapped into a ladle from which the molten metal is teemed into ingots.

In employing the virgin metal melting procedure in this invention, scrap known to the trade as low phos phorous carbon scrap (04% max P) is charged into the electric arc furnace together with predetermined quantities of non-oxidizable meltal such as nickel and molybdenum' 'a's may be required for compositional purposes together with slag. forming materials such as burnt lime if such slag forming material is required. A typical composition of' low phosphorous scrap material which is charged is less than 0.20% maximum carbon, about 0.10% to 0.50% manganese, less than 0.040% maximum phosphorus and sulphur, and the balance substantially iron. It will be appreciated, however, that some of the iron will be charged in its oxide form, for example FezOs, from the rust and mill scale encountered such scrap. It is to be noted that the low phosphorous" scrap material is devoid of substantial amounts of oxidizabl'e elements such as chromium and manganese which tend to block. the carbon removal. Since these oxidizable elements would also be lost during the subsequent injection of oxygen into the molten metal bath as will be described hereinafter, there is a twofold purpose for excluding them from the initially charged scrap.

The nickel and/or molybdenum is preferably charged in the oxide form, such as nickel oxide and molybdic oxide; Since each of the metals nickel and molybdenum is quite soluble in molten iron and since iron oxide is more stable than either nickel oxide or molybdic. oxide, the nickel oxide and molybdic oxide readily decompose in molten metal bath to release nickel and molybdenum which combine with the iron in the molten metal bath leavingv the oxygen, which has. also been released, free to combine with carbon in the melt to thereby lower the carbon content to a value slightly in excess of 0.03% carbon- It. is desired that approximately 105% of all of the iron to. be required in. the final analysis, minus that which will be later added in the form of ferrochromium, ferromanganese and ferrosilicon which is used to obtain the requisite. chromium, manganese and silicon, respectively, be. charged and initially melted together with approximately 95 of each of the non-oxidizable metals, nickeland molybdenum, and the slag forming ingredients burnt lime and/or fluorspar if required. it is desired to have approximately 105 of the requisite iron content, since. it, is preferred that the final additions of chromium, manganese and silicon. in the form of ferroalloys be so minimized as not to unnecmsarily chill the molten. metal bath by their addition prior to tapping the heat. Not more than 95% of the required amounts of the non-oxidizable metals, nickel and molybdenum, is. contained in the initial charge for it has been found that where large amounts of iron oxidized into the slag the. non-oxidizable metals, nickel and molybdenum, remaining in. the molten metal bath will be present in larger quantities: than: those required by the final anaonce of about 95% of the non-oxidizable metals in the 7 mum. carbon content of the initially charged. scrap material may be upto 0.20% maximum carbon in melting practice it is preferred to increase the initial carbon content of the charge up to about 0.50% by the addition of carbon to prevent high metal loss during oxidation. With such a charge, the carbon content of the molten metal bath is effectively reduced to a value slightly in excess of 0.03% by the available oxygen from the initial charge of scrap and the non-oxidizable metals. The carbon content of the initially melted metal bath is dependent upon the amount of carbon in the initial charge and the amount of available oxygen from the initial charge of scrap and from the nickel oxide and/ or molybdic oxide. During the melting of the charge described, the bath of molten metal attains a temperature of about 2900 F. While this. may seem, to be a relatively high superheat, the attainment of this temperature is necessary for the efiective removal or" carbon as described hereinafter.

Ordinarily, the molten metal bath is covered with a large volume of slag which has been obtained partially through the oxidation of some iron together with any of the oxidizable elements such as manganese and. silicon which form part of the initially charged scrap material. Upon completion of the initial melting of the charge approximately of the slag is removed in order to reduce the volume thereof. After deslagging,, the molten metal bath is then ready for. further processing.

In order to lower the carbon content of the initially melted metal bath to a value belowthe minimum value of 0.03% referred to hereinbefore, gaseous oxygen has. been previously injected into the melt. This practice, however, has not proven to be satisfactory for the reaction causes the iron of the melt to be preferentially oxidized into the slag. Moreover, an extremely large volume of oxygen is needed to reduce the carbon content to below 0.03%. This in turn raises the temperature of the molten metal bath, which in some cases may be above 3200 F., thereby causing attrition to the furnace lining and substantially shortening. the life of the refractories.

In accordance with this invention, pig iron containing approximately 4% carbon, about 6% silicon and the balance substantially iron with incidental impurities usually found in pig iron is then added to the molten metal bath in an amount ranging between 2% to 6% of the weight of the bath. Such additions of pig iron increases the carbon content of the molten metal bath from slightly in excess of 0.03% to a carbon content in the range between 0.07% to 0.20% and causes a violent reboiling action of the melt. Simultaneously with the reboiling action, gaseous oxygen is injected into the molten metal bath at a predetermined flow rate for a period of time of from 3' to 8 minutes to efiectively reduce the carbon content to below 0.030% and in most cases to below 0.020% carbon. In all cases, the simultaneous injection of gaseous oxygen during the rebelling of the melt caused by the pig iron additions, effectively lowers the carbon content to below 0.030% and in most instances. to below 0.020%- While the exact phenomenon is not completely understood, it is thought that the resulting extremely low carbon content just described is obtained, by the effect occasioned by providing the large carbon content of be.- tween 0.07% and 0.20% in the molten metal bathv so that when the required gaseous oxygen is introduced therein during, the reboiling action caused by the addition of pig iron, the intimate mixing of the oxygen and contact thereof with the large mass of carbon will result in. a more complete reaction of the carbon and oxygen to efiectively reduce the carbon content to less than 0.030%.

The oxygen injection is preferably made at a flow rate so as to introduce not less than 1200 cubic feet per hour per ton of metal bath, into the metal bath and preferably as. a, flow.- rate greater than 1500' cubic feet per hour per ton- 'of' molten metal bath simultaneously with the re boiling action of the molten metal bath for a, time of between 3 to 8 minutes. As a specificexample oi" the flow rate used in practicing this invention, in the making of a ton heat of steel, a flow rate of about 24,000 cubic feet per hour of gaseous oxygen into the metal bath has effectively reduced the carbon content of the molten metal bath to below 0.020% maximum in a time period between 3 to 8 minutes from the start of the oxygen injection into the reboiling molten metal bath. In making larger heats, for example, ton heats, a flow rate of 31,700 cubic feet per hour of gaseous oxygen into the metal bath during the reboiling action has effectively reduced the carbon content of the molten bath to less than 0.020% in the same time interval. It will be appreciated that the diameter of the lance used to deliver the gaseous oxygen into the molten metal bath will determine the pressure needed to deliver the requisite amount of oxygen required to reduce the carbon content to the desired level within the time period of from 3 to 8 minutes. It is also to be noted that in referring to a 10 ton or a 25 ton heat of steel, these references are directed to the approximate expected tap weight. In actual practice when the process of this invention is used, the mass of the molten metal which is actually treated in accordance with the teachings of this invention will be considerably less than the tap weight and usually in the order of 70 to 75% by weight of the expected tap weight. Thus in the making of a 25 ton heat as referred to hereinbefore, the flow of 31,700 cubic feet of oxygen per hour is based on only about 18% tons of metal bath, or at a rate of about 1,700 cubic feet per hour per ton. It should be noted further that if the basic variables are changed, the flow rate also changes somewhat. For example, the carbon-oxygen equilibrium will vary with the temperature of the molten metal bath, therefore, when the bath is at a high temperature at the start of the process of this invention and pig iron is added, the equilibrium conditions are destroyed. The addition of the pig iron adds an excess of carbon and silicon to the molten metal bath; therefore, the oxygen must be added as rapidly as possible to promote the carbon and silicon exothermic reactions thereby raising the bath temperature. The oxygen is continuously added to efiect a new carbon-oxygen equilibrium at a higher temperature to lower the carbon content and provide an excess of oxygen over that required to establish the new equilibrium. In all cases, however, the flow rate of oxygen must be not less than 1200 cubic feet per hour per ton of molten metal bath in order to effect the reduction in carbon to a value of less than 0.03% in a time of from 3 to 8 minutes.

Referring now to the curve of the drawing, there is graphically illustrated a typical carbon-time curve representative of the effective reduction in the carbon content as described hereinbefore. As illustrated, there are three portions to the curve, namely, recarburizing, reboiling and decarburizing. The recarburizing portion of the curve is dependent upon the temperature of the molten metal bath, it being noted that the carbon content of the melt is increased during the recarburizing to a value in the range of 0.07% to 0.20% as referred to hereinbelow. A carbon content of 0.11% for the recarburized melt has been used for purposes of illustration. It is also to be noted that at higher temperatures for the melt, that is, above 3000 F., the recarburization period will be shorter than at lower temperatures, because the pig iron is melted faster and reboiling occurs sooner. In practice the recarburizing period may be from 2 to about 5 minutes depending on how fast the operators charge the pig iron into the furnace. As soon as the pig iron is charged, the oxygen lance is introduced into the melt to inject oxygen therein. Since the gaseous oxygen is injected, preferably by means of the lance injection method, into the molten metal bath simultaneously with the reboiling action, the recarburization period is terminated with the reboiling action. It will be appreciated that in practicing this invention not all of the carbon available from the pig iron goes into solution in the metal bath since the oxygen content injected therein during the reboiling action is so great that part of the carbon is oxidized before it goes into solution. The reboiling action occasioned by the addition of cold pig iron to the molten metal bath is dependent upon the temperature of the molten bath, the amount of pig iron charged and the time needed to start the melting of the cold pig iron. Thus, if the temperature of the molten metal bath is above 300 F. and only 2% of the weight of the bath in the form of pig iron is added to the molten metal bath, the time period of the reboiling action is considerably shorter than if the temperature of the bath is below 300 F. and about 6% of the weight of the molten metal bath in the form of pig iron is added to the molten metal bath.

As shown in the drawing, the decarburizing portion of the curve extends from the time the gaseous oxygen is first injected into the molten metal bath until the required amount of oxygen is delivered to the molten metal bath. The points on the curve of the drawing represent the carbon contents of the molten metal bath as measured during the making of commercial heat of steel. Thus by inspection of the curve in the drawing, it is apparent that in accordance with this invention the carbon content of the steel is lowered to a value below 0.03% carbon within 3 to 8 minutes after the start of the oxygen blow.

After the carbon content of the molten metal bath is reduced to a value below 0.03% and preferably below 0.02% in accordance with the teachings of this invention, the molten metal bath may be further processed by reducing the slag volume followed by the addition of the metal components to give the required final analysis of the solidified metal. In further processing the molten metal while preventing carbon contamination, it is preferred to employ the process described and claimed in application Serial No. 195,489, now Patent No. 2,704,247 issued March 15, 1955 to W. G. Connor.

In order to more clearly illustrate the teachings of this invention with respect to the general melting procedure as followed in an electric arc furnace, the following stepby-step general melting procedure is outlined giving the preferred steps in making a heat of stainless steel having an extra low carbon content including the steps prior to and subsequent to the recarburizing and blowing with oxygen simultaneously with the reboiling action as described hereinbefore.

1. The original furnace charge must be the so-called virgin charge; that is, it must be substantially free of chromium and manganese but may contain nickel-iron alloy material and in some instances molybdenum. Chromium in particular is withheld as it will block the carbon removal at a later stage. This charge should contain about 95 of all of the non-oxidizable metal, nickel and molybdenum to be required and 105% of all of the iron to be required minus that which will be later added as ferrochromium, ferromanganese and ferrosilicon. This stipulation is necessary so that the so-called final additions will be minimized and will not unnecessarily chill the bath near the conclusion of the process.

2. Most of the nickel and molybdenum to of each) should be charged prior to melting the charge. This provides oxygen to react with carbon as well as a substantial saving over the cost of the ferro alloy or the pure metal. The charged iron should be of the low phosphorus, low sulphur common steel variety.

3. Power is applied to completely melt the charge and develop a temperature of about 2900 F. therein. The charged oxygen in the form of nickel oxide and molybdic oxide as well as other metallic oxides such as FezOs is effective to partially decarburize the heat during melt down to a value slightly in excess of 0.03% carbon.

4. Turn power oif and deslag about 80%, simply to reduce slag volume.

5. Add pig iron in an amount equivalent to 2% to 6% and preferably between 3% to 4% of the bath weight. At the start of the reboil action from about 2 to about 5 minutes after the addition of the pig iron, start blowing with gaseous oxygen. The oxygen blow shonldal-waysbe concurrent with'theturbulent reboil to obtain --the best mixing of gasand'metal and the mosteffective carbon removalin the shortest time. The'blow is maintained a perind of 3 to 8 minutes-at not less than 1200 cubic feet per hour ofoxygen per ton of metal bath toefiectively-lower the carboncontent tobelow 0.03% and'preferably to below .02%. The superheat of the melt facilitates the effective removal of the carbon during :the effervescent period. "The relatively short blowgives the'least furnace lining attrition.

"6. Deslag 75-to 85%, preferably 80%.

"7. Add ferrosilicon containingabout 75 silicon in -an amountofabout 40 to 50 pounds per'tonio'f expected tap weight. 'This siliconserves as a source for heat by .exothermic reaction'during a subsequent oxygen blow at the rate of 6.6 F. for-each 0.01% 'ofssilicon so oxidized.

8. Add substantially two-thirds of the'low carbon ferrochromium and immediately resume .theoxygen blow by blowing until allthe ferrochromium is :melted. Add the balance of the ferrochromium and resume oxygen blow until, by calculation from thexvolume of oxygen used, the silicon-contentis'lowered-intothe final specification range. The molten metal bath must not be overblown at this point, or else excessive chromium Will be oxidized into the slag.

9. Turnthepower on sufiicientlyto flux a new slag of burnt lime and aluminum'to partiallyreduce CrzOs. This slag 'should'be oxide rich, not a rich reducing slag receptive to carbon pickup.

10. Add into the furnace 1 /2 to 2 pounds per ton of pig aluminum.

ll. Add manganese as 0.07% maximum carbon ferromanganese-orielectrolyticmanganese to approach the lower=limit of the manganese range. Turn off the power. It is possible to add the aluminurn of step 10 and manganese of this stepin='the ladle during refurnacing or reladling.

'12. At this point, the molten metal contained within the ifurnace is stirred to prevent stratification andlast additionsofalloying components for chemistryshould cool the bath to altemperarure slightly above the optimum tapping'temperature of2770 to 2800 F.

13. 'Turn .t'he power on, adjusting the voltage toobtain ailongarc'to keep'the carbon electrodes remote from the bath. The current should be adjusted to contain only sutficient heat tomaintainslagfluidity.

* 1"4. *Sarnplefor final chemistry.

.1'5. .On'preliminary analysis report the necessary additionsfof chromium anduickel. If Type 316 stainless steel is being made, the necessary additions of 'ferromolybdenum shouldbe added to the molten metal bath. The precedingprocedure having been followed, a minimum of metal loss through oxi'dation will have been experienced. Thelosses may thus be calculated at 12 to 13%.

:16. Turn the power off and stir the'bath to mix well.

17. 'Deslag approximatelyQ'O to 95% and redress the slag with lime and aluminum.

18. Add manganese and silicon sufficient to bring said elcments within the specification range.

119. After mixing well, tap'at once when the additions have been dissolved. If rare earth elements are to be added as on Type 316, slag off 100% before tapping and make the rare :earth additions to the ladle.

While the foregoing outlines a preferred procedure, it will be appreciated that this invention is not limited thereto. Variations and alterations in the foregoin steps maybe made. as .Well me different:melting procedure may be l-USEd, for .example,.a procedure 'formaking extra low carbon 'SAE type steel.

As-an illustration of the results obtained through the use of the process-of this'invention, reference may be had to Table I in which the carbon contents of ditferent types o'fstainless steel produced in accordance with this invention' are recorded both after the decarburization period and as final contentsitogether with the size of theh'eats.

Table I Carbon Con Final .Carbon *Heat No. Amount Alloy tent (p c Content Type after Decar 'burization 1 perceu) 81299 '10 Ton Heat- 303 0. 016 0. 022 .do 304 0. 015 30; 028 316 0. 013 0. 027 316 0. 018 0. 023 316 0.020 0; 029 316 0. 013 0. 023 304 -0. 019 0. 023 316 0. 010 0. 020 304 0. 012 0. 028 304 -0. 019 0. 026 304 0. 013 0.020 316 0. 019 0. 026 304 0. 022 0. 028 316 0. 011 0. 024 304 0. 015 0. 025 316 0.019 0. 025

Approx 280 Tons Ave. 0016 Ave. 0.024

sumption while making possible an overall "savings in-the cost of repairs and maintenance as well as life of the refractories. 'Powerconsumption-is also reduced by'the use vof this ;-process for'the pig iron which is used for recarburization contains substantial amounts of silicon which when oxidized throughthe injection of oxygen into 7 the moltenmetal bathincreases the temperature which increasein temperature would have to be otherwise .provided through the carbon arcs. through oxidation into the slag by oxygen injection is substantially lowered, the loss being less than 13% of the weightof the molten metal bath. This is aadistinct improvement over-the experience in industry which reported losses of metallics greater than 15%. Further, furnace lining attrition is substantially-reduced through the shortened periodcf oxygenblow during the reboiling action thereby-providing longer life to the refractories.

.In addition to the-foregoing advantages, thepresentinventionmakes it possible to have a substantially :continuous operation of the,process,,for the results .ofstep 5 in the outline givenhereinbefore are so reliable that it 'isno longer necessary to wait for carbon analysis before continuingthe process. In all cases, it has been found Where carbon analysis has been taken immediately after step 5 of the outline that the carbon is less than 0.03% and in more than of the cases below 0.02%. The delay in Waiting for carbon analysis is thus completely eliminated resulting in a further savings in .time and costs in the making of a heat of low carbon stainless steel.

We claim:

1. -In the method of producing steel "having a carbon content below 0.03% maximum by means-of an electric arc furnace 'using a virgin metal melting procedure in which a predetermined mass ofscrap -metal, 'devoidof substantial'amounts of chromium and manganese, together with predetermined amounts ofmetal of the nonoxidizable' metals nickel and molybdenum is-initially arc melted to form amolten metal bath and the carbon'icon tent is-then adjusted to a predetermined levelafterwhich predetermined amounts of chromium, manganese silicon are added and the moltenmetal is refined'to afpre determinedchemical analysis prior to t-appingthe molten metal into a ladle, the improvement comprising, recarburizing the initially arc melted metal bathrby jadding pig iron to produce a .reb'oiling action, and injectinggas- The loss .of V-metallics eous oxygen simultaneously with the reboiling of the molten metal bath to lower the carbon content of the molten metal bath to below 0.020% by weight of the molten metal bath.

2. In the method of producing steel having a carbon content below 0.03% maximum by means of an electric arc furnace using a virgin metal melting procedure in which a predetermined mass of scrap metal, devoid of substantial amounts of chromium and manganese, together with predetermined amounts of metal of the nonoxidizable metals nickel and molybdenum is initially arc melted to form a molten metal bath and the carbon content is then adjusted to a predetermined level after which predetermined amounts of chromium, manganese, silicon and aluminum are added and the molten metal is refined to a predetermined chemical analysis prior to tapping the molten metal into a ladle, the improvement comprising, recarburizing the initially arc melted metals by adding pig iron in an amount of about 2% to about 6% of the weight of the molten metal oath to the molten metal bath to produce a reboiling action, and injecting gaseous oxygen simultaneously with the reboiling of the molten metal bath to lower the carbon content of the molten metal bath to below about 0.020% by weight of the molten metal bath.

3. In the method of producing steel having a carbon content below 0.03% maximum by means of an electric arc furnace using a virgin metal melting procedure in which a predetermined mass of scrap metal, devoid of substantial amounts of chromium and manganese, together with predetermined amounts of metal of the non-oxidizable metal nickel and molybdenum is initially arc melted to form a molten metal bath and the carbon content is then adjusted to a predetermined level, after which predetermined amounts of chromium, manganese, silicon and aluminum are added and the molten metal is refined to a predetermined chemical analysis prior to tapping the molten metal into a ladle, the improvement comprising, recarburizing the initially arc melted metal by adding pig iron in an amount of about 2% to about 6% of the weight of the molten metal bath to the molten metal bath to produce a reboiling action, and injecting gaseous oxygen simultaneously with the reboiling of the molten metal bath, the injection of the gaseous oxygen being continued for a time period ranging from about 3 to about 8 minutes to lower the carbon content of the molten metal to below about 0.020% by Weight of the molten metal bath.

4. In the method of producing steel having a carbon content below 003% maximum by means of an electric arc furnace using a virgin metal melting procedure in which a predetermined mass of scrap metal, devoid of substantial amounts of chromium and manganese, together with predetermined amounts of metal of the non-oxidizable metals nickel and molybdenum is initially arc melted to form a molten metal bath and the carbon content is then adjusted to a predetermined level after which predetermined amounts of chromium, manganese, silicon and aluminum are added and the molten metal is refined to a predetermined chemical analysis prior to tapping the molten metal into a ladle, the improvement comprising, recarburizing the initially arc melted metals by adding pig iron in an amount of about 2% to about 6% of the weight of the molten metal bath to the molten metal bath to produce a reboiling action, and injecting gaseous oxygen simultaneously with the reboiling of the molten metal bath, the injection of gaseous oxygen being continued for a time period ranging from about 3 to about 8 minutes at a flow rate of not less than 1200 cubic feet per hour per ton of molten metal of gaseous oxygen to lower the carbon content of the molten metal to below about 0.020% by Weight of the molten metal.

5. In the method of producing steel having a carbon content below 0.03% maximum by means of an electric arc furnace using a virgin metal melting procedure in which a predetermined mass of scrap metal, devoid of substantial amounts of chromium and manganese, together with predetermined amounts of metal of the nonoxidizable metals nickel and molybdenum is initially are melted to form a molten metal bath and the carbon content is then adjusted to a predetermined level after which predetermined amounts of chromium, manganese, silicon and aluminum are added and the molten metal is refined to a predetermined chemical analysis prior to tapping the molten metal into a ladle, the improvement comprising, recarburizing the initially arc melted metals by adding pig iron in an amount of about 4% of the weight of the molten metal bath to the molten metal bath to produce a reboiling action, and injecting gaseous oxygen simultaneously with the reboiling of the molten metal bath, the injection of the gaseous oxygen being continued for a time period of about three minutes at a flow rate of gaseous oxygen of 1700 cubic feet per hour per ton to lower the carbon content of the molten metal to below 0.020% by weight of the molten metal.

No references cited.

Claims (1)

1. IN THE METHOD OF PRODUCING STEEL HAVING A CARBON CONTENT BELOW 0.03% MAXIMUM BY MEANS OF AN ELECTRIC ARC FURNACE USING A VIRGIN METAL MELTING PROCEDURE IN WHICH A PREDETERMINED MASS OF SCRAP METAL, DEVOID OF SUBSTANTIAL AMOUNTS OF CHROMIUM AND MANGANESE, TOGETHER WITH PREDETERMINED AMOUNTS OF METAL OF THE NONOXIDIZABLE METALS NICKEL AND MOLYBDENUM IS INITIALLY ARC MELTED TO FORM A MOLTEN METAL BATH AND THE CARBON CONTENT IS THEN ADJUSTED TO A PREDETERMINED LEVEL AFTER WHICH PREDETERMINED AMOUNTS OF CHROMIUM, MANGANESE AND SILICON ARE ADDED AND THE MOLTEN METAL IS REFINED TO A PREDETERMINED CHEMICAL ANALYSIS PRIOR TO TAPPING THE MOLTEN METAL INTO A LADLE, THE IMPROVEMENT COMPRISING, RECARBURIZING THE INITIALLY ARC MELTED METAL BATH BY ADDING PIG IRON TO PRODUCE A REBOILING ACTION, AND INJECTING GASEOUS OXYGEN SIMULTANEOUSLY WITH THE REBOILING OF THE MOLTEN METAL BATH TO LOWER THE CARBON CONTENT OF THE MOLTEN METAL BATH TO BELOW 0.020% BY WEIGHT OF THE MOLTEN METAL BATH.

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