US3521695A - Method of producing a steel ingot - Google Patents

Description

y 2,8, 1970 B A. DIENER ETAL -3,521,95

' METHOD OF PRODUCING A STEEL INGOT Filed April 26, 1967 BUBBLE FORMATION- m/C/-\RBON ROD UNKILLED OR PARTIALLY KILLED CORE FIG. I

KILLED SOLIDIFIED SKIN LAYER INGOT MOLD V//////// //AI" PLATE CARRIER GAS WITH CARBON-CONTAINING MATERIAL 4 FIG .2 L YARBON MATER|AL ill ONTAINER OF MATERIAL iECO OSABLE IN AOLT STEEL United States Patent Int. Cl. B22d 27/18 US. Cl. 16455 7 Claims ABSTRACT OF THE DISCLOSURE A method of producing a steel ingot free of subcutaneous blowholes and voids, characterised in that after the formation of a skin layer which has solidified in the killed condition, there is added to the still liquid core of an ingot of killed steel an amount of carbon to produce semikilled or unkilled solidification.

SPECIFICATION This invention relates to manufacture of steel ingots.

It is known to pour steel ingots or slabs in the killed, unkilled, or semikilled condition, unkilled or semikilled solidification being generally preferred because of the reduction of shrinkage voids or cavities (pipes). The elimination of shrinkage voids reduces top waste considerably and thus leads to a not inconsiderable increase in production as compared with steel solidified in the killed state.

In the case of unkilled solidification, however, ingot segregation becomes an inconvenience because the alloying elements of the steel are distributed irregularly over the cross section of the ingot. Ingots solidified in the semikilled state do not have this disadvantage; in their case the distribution of the iron content, added elements and added alloying elements is homogeneous over the cross section of the ingot while there is relatively little shrinkage voids and little top waste. On the other hand, ingots solidified in the semikilled condition have the disadvantage that a part of the carbon monoxide that forms during solidification remains in the solidified skin layer of the ingot in the form of small blisters. Such subcutaneous blowholes constitute a very disagreeable defect which has a very detrimental action particularly in the further processing of the ingots. The formation of subcutaneous blowholes is frequently so serious that a considerable proportion of the skin layer must be removed by scarfing or similar measures.

It is known that the carbon monoxide produced during solidification leads to unkilled or semikilled solidification, depending on the amount formed. To what extent unkilled or semikilled solidification takes place in individual cases depends on the extent to which the steel deviates from its carbon-oxygen equilibrium at a given temperature. In order to achieve semikilled solidification, the carbon and oxygen contents are adjusted either by predeoxidation or by vacuum treatment so that they are appreciably beneath their respective equilibrium values, while the latter is nevertheless exceeded by carbon and oxygen segregation in the region of the solidification front. In the case of killed steels the free oxygen content of the steel is generally reduced, by adding elements having an afiinity for oxygen, such as aluminium, silicon, and titanium, to such an extent that carbon-oxygen equilibrium is not achieved during solidification.

It has now been found that a steel will solidify in the killed condition if its carbon and oxygen content is reduced to low values, for example, by vacuum treatment, and if the steel has a manganese content determined by ICC said values. Expressed differently, this means that the steel will solidify in the killed state with a given oxygen and manganese content provided its carbon content is below a determined value. The following conditions have been found for dependence of the carbon content in respect of killed solidification:

For 0 =20 10 c is -0.0035+0.06

For o =30 10 c) is 0.0055+0.06

For o =40 10 c is 0.007S+0.06

The interdependence between the oxygen, carbon, and manganese contents can be explained more fully by the following example.

EXAMPLE A steel containing 0.030% of oxygen, 0.42% of manganese, less than 0.002% of aluminium, and less than 0.01% of silicon requires a critical carbon content of 0.020% by application of the above conditions.

The steel was cast into two ten-ton slabs, one of which solidified in the completely killed condition with a carbon content of 0.017%, and which consequently had shrinkage voids. The slab solidified in the killed condition, without the addition of deoxidation agents, had a clean surface so that scarfing or chipping, such is frequently indispensable with aluminium killed slabs, was not necessary.

The second slab which had a carbon content of 0.022% solidified without forming shrinkage voids, although it had numerous subcutaneous blowholes. From this it is clear that the killed or semikilled solidification behaviour with a determined manganese content is only slightly dependent on the oxygen content but highly dependent on the carbon content of the steel.

Starting on this basis, the problem underlying the invention was to produce a steel ingot free of both subcutaneous blowholes and shrinkage voids.

According to this invention, solidification of the ingot is conducted in such a manner that it solidifies in the semikilled condition in the core and in the killed condition in the skin Zone.

More specifically, the invention consists in producing an ingot having a semikilled core and a killed skin zone by adding, after pouring, an amount of carbon, which is suificient for semikilled or unkilled solidification, to the core, which is still in the liquid condition, of an ingot of a vacuum killed steel, after the formation of a skin layer solidified in the killed condition. In this way, while the respective disadvantages are eliminated, the advantages of a steel solidified in the killed condition are combined with those of a steel solidified in the unkilled or semikilled condition.

This means that the ingot produced by the method of the invention is free of both subcutaneous blowholes and voids.

The timing of the addition of carbon to the liquid core of the ingot depends on the thickness of the skin layer which is necessary for the purpose of fault-free further processing. Since the thickness of the solidified skin layer increases continuously from the head to the foot of the ingot, the moment at which the carbon is added is advantageously determined on the basis of the thickness of the skin layer at the head of the ingot. The dependence'of the solidification on a time factor is obtained in the case of chill casting approximately from the following formula:

where d is the thickness of the solidified skin layer in 3 centimetres and t the time in minutes after commencement of solidification. Thus the timing of the addition of carbon after commencement of solidification can be determined in accordance with the equation:

(I being in this case the thickness of the skin layer free from subcutaneous blow holes which is required for faultfree further processing. If, accordingly a layer of, for example 2.5 cm., which is free from subcutaneous blow holes is required, the carbon must be added one minute after the commencement of solidification.

The quantity to be added in individual cases is determined from the difference between the carbon content of the melt which, in accordance with the above conditions, is required for semikilled or unkilled solidification, and the actual carbon content. For a slab solidified in the killed condition, which is described in the above example, an additional quantity of 0.003% of carbon would ac cordingly be necessary. It is, however, advisable in every case to add to the core of the ingot a quantity of carbon which slightly exceeds the amount necessary. Practice has shown that carbon additions of below 0.01% are generally sufficient, this corresponding to the addition of 1 kg. of carbon in the case of a ten-ton slab. Experiments have shown that the slight increase of the carbon content of the steel resulting from the addition of carbon in accordance with the invention does not result in any substantial deterioration of its technological properties. Since the carbon is added to the liquid core of the ingot for the purpose of preventing the formation of shrinkage voids after solidification of the outer zone which is free from subcutaneous blow holes, it is sufiicient for the carbon content to be increased solely in the region of the top of the ingot.

Our inventive method will be best understood from the following description of specific modes thereof when read in connection with the accompanying drawings, wherein:

FIGS. 1 to 4 are sectional views of an ingot mold showing four diiferent modes of performing the method of the invention for producing a steel ingot free of subcutaneous blow holes and voids.

According to the invention, as shown in FIG. 1, carbon is added to the steel by immersing carbon rods or rods of a carbon-containing material in the liquid core of the ingot. However, as shown in FIG. 2, containers holding carbon or carbon-containing material and adapted to dissolve or decompose in steel may also be introduced into the core of the ingot. Another possible method of introducing carbon or material containing carbon into the core of the ingot is shown in FIGS. 3 and 4 and comprises the use of a carrier gas or of carbon-containing gases which are decomposed in the presence of steel, such as for example, methane.

What is claimed is:

11. A method of producing a steel ingot free of subcutaneous blow holes and voids, which comprises the steps of first forming a steel ingot with a skin layer in killed condition by solidification of a killed liquid steel, and then adding to the still liquid core of the ingot an amount of carbon to produce solidification ranging from unkilled to partially killed solidification.

2. The method according to claim 1, in which the liquid steel is subjected to vacuum to adjust the carbon and oxygen content thereof to control the steel to killed condition prior to pouring the liquid steel into a mold.

3. The method according to claim 1 in which carbon is introduced by immersing a rod comprising carbon in the liquid core of the ingot.

4. The method according to claim 1 in which a carbon rod is immersed in the liquid core of the ingot.

5. The method according to claim 1 in which the carbon is introduced by immersing a container formed of material decomposable in steel into the liquid core of the ingot, the container containing carbon material.

6. The method according to claim 1 in which carbon is introduced or blown into the liquid core of the ingot by blowing a carrier gas having carbon-containing material in suspension therein.

7. The method according to claim 1 in which carbon is introduced by injecting carbon-containing gases which decompose in steel into the liquid core of the ingot.

References Cited UNITED STATES PATENTS 585,036 6/1897 Hunt 7548 1,294,209 2/1919 Walker. 2,518,738 8/1950 Woods. 2,853,376 9/1958 Blanc et al. 7548 2,837,800 6/1958 Hachiya et a1. 16456 2,952,534 9/1960 Quinn et al. 75-57 X 3,188,198 6/1965 Moore 7557 3,208,117 9/1965 Goedecke et al. 164--56 2,389,576 11/1945 Kinnear l6496 3,410,681 11/1968 Orban 16456 X 3,414,041 12/1968 Cutton 16456 3,436,209 4/1969 Lojas 16455 X 3,274,681 9/1966 Lohman 164-95 X FOREIGN PATENTS 599,323 10/1959 Italy. 325,003 9/ 1920 Germany.

J. SPENCER OVERHOLSER, Primary Examiner V. K. RISING, Assistant Examiner US. Cl. X.R.

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