US3869283A

US3869283A - Alloying steels

Info

Publication number: US3869283A
Application number: US341451A
Authority: US
Inventors: Emrys Davies; John Roy Rippon
Original assignee: British Steel Corp
Current assignee: United Engineering Steels Ltd
Priority date: 1969-10-15
Filing date: 1973-03-15
Publication date: 1975-03-04
Anticipated expiration: 1992-03-04

Abstract

A method of adding an alloying addition, such as lead, to a ferrous metal, by bubbling inert gas, such as argon, through the ferrous metal whilst the metal is contained in a ladle and adding the alloying addition to the turbulent zone created by the passage of the inert gas. The temperature of the ferrous metal in the ladle is maintained above its customary teeming temperature during the adding of the alloying addition. When all the alloying addition has been added the temperature of the metal is allowed to cool to its customary teeming temperature whilst the inert gas is continued to be passed. The ferrous metal is then teemed from the ladle into moulds.

Description

United States Patent [191 Davies et al.

[ Mar. 4, 1975 ALLOYING STEELS [73] Assignee: British Steel Corporation, London,

England [22] Filed: Mar. 15, 1973 [21] Appl. No.: 341,451

Related US. Application Data [63] Continuation of Ser. No. 80,553, Oct. 14, 1970,

abandoned.

[30] Foreign Application Priority Data Oct. 15, 1969 Great Britain 50609/69 I {52] U.S. Cl. 75/129, 75/130.5 {51] Int. Cl. C22C 33/00 [58] Field of Search 75/129, 130, 130.5, 49,

[56] References Cited UNITED STATES PATENTS 2,915,380 12/1959 Hilty ..75/60 3,321,300 5/1967 Worner 75/49 3,545,960 12/1970 McClellan 75/59 3,547,622 12/1970 Hutchinson 75/49 3,574,603 4/1971 Rassenfoss 75/61 3,671,224 6/1972 North 75/129 3,672,869 6/1972 Niehaus 75/46 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Bacon & Thomas [57] ABSTRACT A method of adding an alloying addition, such as lead, to a ferrous metal, by bubbling inert gas, such as argon, through the ferrous metal whilst the metal is contained in a ladle and adding the alloying addition to the turbulent Zone created by the passage of the inert gas. The temperature of the ferrous metal in the ladle is maintained above its customary teemingtemperature during the adding of the alloying addition. When all the alloying addition has been added the temperature of the metal is allowed to cool to its customary teeming temperature whilst the inert gas is continued to be passed. The ferrous metal is then teemed from the ladle into moulds.

16 Claims, 3 Drawing Figures PATENTEU 41975 3. 869,283

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PATENTEDHAR 419-75 snmaur PATENTEDMR 1975 SHEET 3 (IF 3 FiQZL.

ALLOYING STEELS This application is a continuation of Ser. No. 80,553, filed Oct. 14, 1970, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the addition to iron and alloys of iron, hereinafter all referred to as ferrous metals," of an alloying addition, and particularly an alloying addition for the purpose of improving the machinability of the ferrous metals.

2. Description of Prior Art It is well known to add a machinability-improving alloying addition to ferrous metals, but the alloying addition has in general low or negligible solid solubility in ferrous metals so that it is difficult to obtain a good recovery of the alloying addition in the ferrous metal while ensuring that the part of the alloying addition which is retained in the ferrous metal is present in the form of uniformly distributed fine particles.

The presence of large globules of the alloying addition or of a heavy segregation thereof which are commonly found in the bottom of the ingot means that a larger part of the bottom of the ingot must be discarded than the usual discard at the bottom of a rolled ingot.

Furthermore, if the alloying addition is intended to improve the machinability of the ferrous metal, it is important that the alloying addition be dispersed as uniformly as possible throughout the ferrous metal ingot so as to improve the machinability substantially uniformly throughout the ingot as well as avoiding the Iocalised segregation which would be liable to impair the mechanical properties of the ferrous metal.

Also. the fumes generated when the alloying addition is added to ferrous metals may be toxic, in which case the fumes may be efficiently removed throughout the casting process.

The alloying addition may, for example, include one, some or all of the elements lead, selenium and tellurium. in the form of pure metals or in the form of alloys, or as mineral compounds.

SUMMARY OF THE INVENTION An object of the invention is the provision of an improved method of adding the alloying addition to ferrous metals, whereby a high recovery of the alloying addition is obtained in the ingot and an ingot is produced in which the alloying addition is present in the form of a uniformly distributed and finely divided mi cro-dispersion, with a reduction in the segregation of the alloying addition as compared with most commonly used methods so that a reduced proportion of the ingot need be discarded.

A further object of the invention is the provision of an improved method as described above wherein any toxic fumes generated by the alloying addition may be efficiently and easily removed.

According to the present invention we provide a method of adding alloying addition to a ferrous metal, the method comprising the steps of; passing a gas through the ferrous metal, the ferrous metal being heated to a temperature above its customary teeming temperature, the passage ofthe gas being such that turbulence is created in the ferrous metal and a turbulent zone at the surface thereof; simultaneously adding said alloying addition to the ferrous metal at or adjacent to the turbulent zone whilst removing any toxic fumes generated; continuing to pass the gas through the ferrous metal until the temperature of the metal reaches its customary teeming temperature; and'teemin'g the metal into moulds whilst removing any toxic fumes generated by the alloying addition.

The ferrous metal may be tapped from a furnace into a tapping ladle and the gas may be passed through the ferrous metal whilst the metal is in the tapping ladle.

Preferably, the alloying addition is added slowly and in a finely divided form.

Preferably also, the gas is an inert gas such as argon which will not affect the composition of the metal.

The term ferrous metal as used herein includes alloy and other steels and the method of this invention is particularly applicable to ferrous metals of which the carbon content does not exceed 2.0 percent.

Up to 0.50 percent lead may be added to such ferrous metals.

Lead has negligible solid solubility in steel, but is slightly soluble in liquid steel; the solubility of lead increases as the temperature of the steel rises.

Steel is normally tapped from the furnace and cast at temperatures as near its solidifcation temperature as possible since the higher the teeming temperature the more prone is the ingot to defects. It has been found if lead is added to the steel during casting, or in the mould, the temperature of the steel is too low for adequate solubility of the lead, and the ingot tends to contain large globules of lead and to have heavy segregation of lead towards the bottom of the ingot.

Further, the fumes generated when lead is added to steel, are highly toxic and must be efficiently removed throughout the casting process.

Similar problems are encountered with other alloying additions.

Methods hitherto used for adding a machinability improving alloying addition, for example lead, to ferrous metals, for example, plain carbon steel, have obtained recovery of 15 percent to 64 percent lead in the ingot and required approximately 25 percent of the ingot to be discarded because segregation of lead particles and normal metallurgical waste.

The use ofthe method of this invention for the addition of lead to a steel can result in a recovery of about percent lead and requires only the normal ingot discard to be taken to remove pipe and surface defects. Similar advantageous results are obtained with other alloying elements.

BRIEF DESCRIPTION OF THE DRAWINGS One embodiment of the invention will now be described with reference to the accompanying drawings wherein:

FIG. I is a diagrammatic side elevation, partly in section, of a furnace and ladle for use in carrying out the method of the present invention,

FIG. 2 is a diagrammatic cross-sectional view. to an enlarged scale, of the ladle of FIG. 1 showing it in operative position with a fume hood,

FIG. 3 is a diagrammatic cross-sectional view of the apparatus shown in FIG. 2 but with the cross-section taken so as to show the argon inlet and pressure gun for adding lead,

FIG. 4 is a view, to an enlarged scale, of the part of FIG. 3 enclosed in the circle marked 4,

the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring firstly to FIG. 1 of the drawings, there is shown a conventional electric arc furnace, indicated at 10, pivotally mounted in conventional manner on a base 11. In FIG. 1, the furnace is shown in its tilted position for discharging its contents into a 60-ton tapping ladle 12 which is suspended, in conventional manner, from an overhead travelling crane, part of the hook of which is shown at 13. The tapping ladle 12 is of conventional construction and is provided with a pouring orifice 14 which can be closed by a ceramic plug 15 controlled by a mechanical linkage 16.

Referring now to FIGS. 2 and 3 of the drawings, the tapping ladle 12 can be transported by the overhead crane so that it is positioned in a pit 17 formed in the floor 18 of the melting shop.

A steel fume hood 19 lined with a refractory 20 is mounted on an arm 21 for pivotal movement about a fixed axis 22 so that the fume hood 19 can be moved into position to cover the tapping ladle 12 when the ladle is positioned within the pit 17.

The fume hood 19 is provided with ducting 23 which communicates with fixed ducting 24, when the hood 19 is in its operative position, which extends to a conventional fume extractor plant illustrated diagrammatically at 25.

As best shown in FIGS. 3 and 5, means are provided in the base of the tapping ladle 12 for introducing an inert gas. This means comprises a porous brick 26 of generally frusto-conical configuration retained in a correspondingly shaped refractory retaining formation 27. A flexible armoured hose 28 extends from an argon supply to the undersurface of the porous brick 26 and suitable connecting means are provided between the hose and the brick so that the hose can be connected to the brick when the ladle is positioned in the pit 17.

An aperture 29 is formed in the fume hood 19 to enable a conventional pressure gun 30 to be introduced into the ladle. The pressure gun is of conventional construction and includes a pressurised hopper 31 and a nozzle 32 in the form of an open-ended funnel with a solid central cone. Air under pressure is applied to the hopper 31 so as to force lead shot from the hopper to the nozzle 32, which is arranged to disperse the stream of lead particles into a spray.

Referring now particularly to FIG. 6 of the drawings, the tapping ladle 12, after the introduction of lead, can be raised by the overhead crane out of the pit 17, and transported until its discharge orifice 14 is positioned over the ingate of a set of eight ingot moulds 33 arranged for bottom casting in conventional manner.

Movable ducting 34 is provided to cover the eight ingot moulds and the ingate. The ducting 34 extends to fixed ducting 35 which leads to a conventional fume extractor plant.

By way of example, the addition of lead to a low alloy steel using the above apparatus will now be described. In this example the following procedure was carried out.

The steel of the present example made in the electric arc furnace 10 was a low alloy steel having a furnace analysis as set out below:

Iron and usual impurities balance The steel was tapped from the furnace 10 by tilting the furnace to the position shown in FIG. 1 so that the steel issued in the stream 39 and entered the 60-ton tapping ladle 12 at a temperature of 1 680C, that is approximately C above its customary tapping temperature.

When the tapping ladle 12 was filled the furnace 10 was returned to its upright position and the ladle 12 was lifted and transported by the overhead crane until it was positioned in the pit 17, as shown in FIGS. 2 and 3. The flexible armoured hose 28 was then connected to the porous brick 26, the fume hood 19 was pivoted about its axis 22 into operative position over the ladle 12, as shown in FIGS. 2 and 3, and the pressure gun 30 was introduced through the aperture 29 in the fume hood 19.

Argon gas was then bubbled through the steel at a pressure of approximately to p.s.i. at a flow rate of between 20 to 25 cubic feet per minute.

The introduction of the argon created turbulence in the steel and a turbulent zone 36 at the surface 37 of the steel.

Whilst the argon was being passed a spray of finely divided lead particles was introduced into the steel adjacent to the turbulent zones 36 by the pressure gun 30. The fume extractor plant 25 was operated to remove the toxic fumes generated by the lead on addition of the lead to the steel. A total weight of 330 lbs of lead was added as slowly as possible over a period of 15 to 20 minutes and the argon was passed continuously.

The high temperature of the steel ensured the solution of the lead addition and the turbulence created in the steel by the argon and the slow addition of the lead particles ensured the uniform distribution of the lead and prevented the segregation of the lead and the formation of lead globules.

When all the lead had been added passage of the argon was continued until the temperature of the steel had reached its customary teeming temperature of l,570C.

The fume hood 19 was pivoted about its axis 22 out of operative position with the ladle 12 and then the overhead crane was engaged with the ladle l2 and the ladle was lifted and transported to a further position where its discharge orifice 14 was positioned above the ingate of the ingot moulds 33, as shown in FIG. 6.

The linkage 16 was then operated to lift the ceramic plug 15 and so enable the steel in the ladle 12 to leave the discharge orifice 14 in a stream 38 and enter the ingate of the ingot moulds 33 in conventional manner. During this operation the movable ducting 34 was positioned over the ingot moulds and ingate, as shown in FIG. 6, and the fume extractor plant was operated to move any further toxic fumes given off during the casting operation.

The additional turbulence created by teeming also helped to ensure uniform distribution of the lead.

The ingots produced were tested to determine the lead distribution and content. The tests performed were as follows.

A photographic paper was soaked for 2 minutes in 5 percent sodium hydroxide solution and was placed on a coarsely ground cross-section of a billet product of the ingot. The paper was left for 3 minutes and then placed in 5 percent sodium sulphite solution, the prints were then washed and dried.

On examination, the lead distribution shows on the paper as a brown speckled pattern, the intensity of the pattern indicating the distribution.

In a second test a short piece of the billet was sawn and placed in a small furnace at a minimum temperature of 600C. Any lead particles on the surface of the billet melt and exude, forming droplets on the billet surface. The sample was then cooled and the droplets examined. The presence of droplets gives an indication of large particles present in the sample.

In a third test, a short piece of sawn billet was held in a shaping machine and cuttings were taken from the whole cross-section of the billet. The lead content of the cuttings was then determined chemically in conventional manner.

In a fourth test billets obtained from the ingot were tested ultrasonically. The ultrasonic beam passed through the cross-section of the billet and a signal was recorded on an oscilloscope by the returning sound beam reflected from the opposite base of the billet. Any interference with this beam by large lead particles given an altered signal on the oscilloscope.

We have found in commercial production that it is convenient to carry out the last mentioned test on all the billets produced according to the method of the present invention as a routine quality control test whilst the first three tests described above are conducted on random samples taken from different ingot positions as is considered appropriate during a production run.

In the case of the example described above, it was found that the ingots which were produced had an average lead content of 0.18 percent indicating that 70 percent of the lead had been recovered in the ingot. It was also found that substantially all of the lead was uniformly distributed as a finely divided micro-dispersion and it was found to be unnecessary to discard any part of an ingot due to gross segregation of lead particles, although the normal casting discard had still to be made.

In the example above described the ferrous metal was heated to the above mentioned temperature above its customary tapping temperature by superheating the ferrous metal in the furnace before tapping the furnace into the ladle. In this case, the argon case may be at or near room temperature.

Alternatively, the gas may be heated to a temperature such that the temperature of the superheated ferrous metal is maintained at a temperature above its customary tapping temperature, i.e., 1,610C whilst it is in the ladle.

Still further alternatively, it is proposed that the ferrous metal may be heated to a temperature above its customary tapping temperature by passing gas through the ferrous metal in the ladle, the gas having been heated to a temperature sufficiently high to cause the ferrous metal to reach this temperature.

In such cases where the temperature of the gas is above room temperature, after introduction of the lead, the temperature of the gas is reduced to or near room temperature to enable the temperature of the metal to reach its customary teeming temperature.

Although argon has been described herein as the gas passed into the steel to create the turbulence, other inert gases may be used if desired so long as the inert gas used does not affect the properties of the steel in an undesirable manner.

Although the addition of lead has been described hereinbefore in the example given other alloying additions such as selenium and tellurium, either alone or in combination may be added to a ferrous metal to improve its machineability by the method of the present invention and if desired other alloying additions may also be added to a ferrous metal using the method of the present invention.

Instead of using a pressure gun to add the alloying addition the alloying addition may be added by any other of the known means which are available in the art, for example, by means of gravity feed from a hopper.

In this specification all compositions are expressed in percentage by weight.

If desired the inert gas may be introduced into the ladle through more than one porous brick.

We claim:

1. A method of adding an alloying addition to a par ticular steel, the method comprising the steps of heating the steel in a furnace to a temperature substantially above that at which said particular steel is normally heated for tapping into a ladle for teeming into ingot molds, tapping the steel from the furnace into a ladle while it is at a temperature substantially above that at which it is customarily tapped from the furnace for teeming into ingot molds, passing gas through the steel in the ladle, the passage of gas being such that turbulence is created in the steel and a turbulent zone is created at the surface thereof, simultaneously adding said alloying addition to the steel in the region of said turbulent zone while said steel is at a temperature sufficient to ensure solution of said alloying addition, removing any toxic fumes generated by the alloying addition, continuing to pass gas through the steel so as to maintain turbulence therein and permitting the steel to cool until the temperature thereof reaches the customary teeming temperature for said particular steel, and teeming the steel into molds.

2. A method according to claim 1, wherein the alloying addition is added slowly and in finely divided form.

3. A method according to claim 1, wherein the gas in an inert gas which does not affect the composition of the metal.

4. A method according to claim 3, wherein the gas is argon.

5. A method according to claim 1, wherein the steel includes a carbon content not in excess of 2.0 percent.

6. A method according to claim 1, wherein the alloying addition is lead.

7. A method according to claim 1, wherein the alloying addition is lead and up to 0.5 percent lead is added.

8. A method according to claim 1, wherein the gas is introduced into the steel at substantially room temperature.

9. A method according to claim 1, wherein the gas is introduced into the steel at an elevated temperature so asto maintain the steel at a temperature above its customary tapping temperature sufficient to ensure solution of said alloying addition.

10. A method according to claim 1, wherein the gas is introduced into the steel at a temperature sufficiently high to raise the temperature of the steel to a temperature above its customary tapping temperature sufficient to ensure solution of said alloying addition.

11. A method according to claim 9, wherein the temperature of the gas is reduced to room temperature or to substantially room temperature after addition of the alloying addition to enable the temperature of the steel to cool to its customary teeming temperature.

12. A method according to claim 10, wherein the temperature of the gas is reduced to room temperature or to substantially room temperature after addition of the alloying addition to enable the temperature of the steel to cool to its customary teeming temperature.

13. A method according to claim 1, wherein the gas is introduced into the ladle at the bottom thereof.

14. A method according to claim 1, wherein said customary tapping temperature is l,610C.

15. A method according to claim 1, wherein the steel is heated to a temperature of about 1,680C.

16. A method of adding'lead to steel to assure a uniform distribution of lead particles finely divided as a micro-dispersion in the steel when solidified, including the steps of containing molten steel to which lead is to be added in a vessel at a temperature above its customary tapping temperature, adding enough lead to the steel to obtain a lead content substantially equal to the solubility limit under existing process conditions, retaining the steel within the vessel for a time after the lead is added and permitting it to cool until the steel is at its customary teeming temperature, removing steel containing lead essentially only in the dissolved state from the vessel and solidifying the steel removed from the vessel.

Claims

1. A METHOD OF ADDING AN ALLOYING ADDITION TO A PARTICULAR STEEL, THE METHOD COMPRISING THE STEPS OF HEATING THE STEEL IN A FURNACE TO A TEMPERATURE SUBSTANTIALLY ABOVE THAT AT WHICH SAID PARTICULAR STEEL IS NORMALLY HEATED FOR TAPPING INTO A LADLE FOR TEEMING INTO INGOT MOLDS, TAPPING THE STEEL FROM THE FURNACE INTO A LADEL WHILE IT IS A TEMPERATURE SUBSTANTIALLY ABOVE THAT AT WHICH IT IS CUMSTOMARILY TAPPED FROM THE FURNACE FOR TEEMING INTO INGOT MOLDS, PASSING GASS THROUGH THE STEEL IN THE LADEL, THE PASSAGE OF GAS BEING SUCH THAT TURBULENCE IS CREATED IN THE STEEL AND A TURBULENT ZONE IS CREATED AT THE SURFACE THEREOF, SIMULTANEOUSLY ADDING SAID ALLOYING ADDITION TO THE STEEL IN THE REGION OF SAID TURBULENT ZONE WHILE SAID STEEL IS AT A TEMPERATURE SUFFICIENT TO ENSURE SOLUTION OF SAID ALLOYING ADDITION, REMOVING ANY TOXIC FUMES GENERATED BY THE ALLOYING ADDITION, CONTINUING TO PASS GAS THROUGH THE STEEL SO AS TO MAINTAIN TURBULENCE THEREIN AND PERMITTING THE STEEL TO COOL UNTIL THE TEMPERATURE THEREOF REACHES THE CUSTOMARY TEEMING TEMPERATURE FOR SAID PARTICULAR STEEL, AND TEEMING THE STEEL INTO MOLDS.

4. A method according to claim 3, wherein the gas is argon.

6. A method according to claim 1, wherein the alloying addition is lead.

9. A method according to claim 1, wherein the gas is introduced into the steel at an elevated temperature so as to maintain the steel at a temperature above its customary tapping temperature sufficient to ensure solution of said alloying addition.

14. A method according to claim 1, wherein said customary tapping temperature is 1,610*C.

15. A method according to claim 1, wherein the steel is heated to a temperature of about 1,680*C.

16. A method of adding lead to steel to assure a uniform distribution of lead particles finely divided as a micro-dispersion in the steel when solidified, including the steps of containing molten steel to which lead is to be added in a vessel at a temperature above its customary tapping temperature, adding enough lead to the steel to obtain a lead content substantially equal to the solubility limit under existing process conditions, retaining the steel within the vessel for a time after the lead is added and permitting it to cool until the steel is at its customary teeming temperature, removing steel containing lead essentially only in the dissolved state from the vessel and solidifying the steel removed from the vessel.