US3512957A

US3512957A - Steelmaking processes

Info

Publication number: US3512957A
Application number: US376420A
Authority: US
Inventors: Karl Brotzmann; Eric R Morgan
Original assignee: Dortmund Hoerder Huettenunion AG
Current assignee: Dortmund Hoerder Huettenunion AG
Priority date: 1963-08-10
Filing date: 1964-06-19
Publication date: 1970-05-19
Anticipated expiration: 1987-05-19
Also published as: DE1608595B1; LU46537A1; BE650825A; GB1011178A

Description

4 Sheets-Sheet l May 19, 1970 Filed June 19, 1964 May 19, 1970 l K, BROTZMANN ET AL 3,512,957'

STEELMAKING PROCESSES Fled June 19, 1964 4 Sheets-Sheet :3

May 19, 1970 K. BROTZMANN ET AL 3,512,957

STEELMAKING PROCESSES Filed June 19. 1964 4 Sheets-Sheet 5 Fig. 4

m /O/ Y STEELMAKINC- PROCESSES 4 Sheets-Sheet 4 Filed June 19, 1964 Fig. 5

M a 0 ORM. g 0u W90 Q MMU UDHQ mw. AWV JMQVQ n m nu Q 9 Q uM. s a ad vana o d a 0 0 0 United States Patent() Inf. c1. Calc 7/6, 7/10, 33/00 U.s. Cl. 75-49 7 claims To make deep-drawing steels resistant to aging, the steel is de-oxidised with aluminum leaving at least 0.025% of Al dissolved in the steel. Some of the aluminum oxide forming as a de-oxidation product is deposited as large oxide particles near the surface of the ingot on casting, so that the entire ingot must be flame-descaled. Apart from the cost of this descaling, production is greatly reduced ias a result. Also, small oxide particles are still left beneath the ingot surface and as a result the Al-killed steel cannot be used for purposes requiring best surface qualities. On theoth'er hand, the cold working properties of the steel are so superior to those of non-ageing-resistant steel that a considerable amount of deep-drawing sheets 'are produced in this way.

There has been no lack of attempts so to alter the steel structure as to obviate the considerable diiiiculties occurlring during its manufacture. For example, an attempt has been made to render an unkilled steel resistant to ageing by the addition of boron. Since, however, boron has a certain atiinity to oxygen, the boron greatly reduces the boiling of the unkilled steel so that in this case too the resultant steel is impure. A procedure that has therefore been adopted is to hold up the introduction of the boron until Va fewV minutes after pouring into the ingot, i.e., until a clean outer z'one has already formed. Apart from the difficulty of distributing the boron in the ingot, this steel has been unsatisfactory for lack of purity of the ingot. Finally, an attempt was made to use partially killed steels. Since these steels have a low oxygen content, it was thought that oxidation of the boron could be prevented.

Apart from insufficient purity, such attempts also resulted in considerable surface flaws, due mainly to the peripheral blow holes which are inevitable in a semi-killed steel and are situated just beneath the surface.

The object of the invention is to produce a deep-drawing steel which is resistant to ageing and has properties of Al-killed steel, under conditions which avoid the disadvantages of Al-killed steel and afford a very simple and economic production process.

To this end, according to the invention, the steel is tapped in an unkilled or partiallyA killed state, its free oxygen is largely removed, for example down to a residual contact between 0.005% and 0.010% by weight, by degasification of batches in a vacuum chamber, and the oxygen remaining in the steel is further reduced Iby the addition of aluminum in an amount between 0.001% and 0.005% by weight, after which between 0.05 kg. and 0.10 kg. of boron per metric ton of steel is added to the steel (which is an addition of 0.005 to 0.10% by weight, of boron) and the steel thus treated is poured rapidly into an ingot mould at a rate of climb in the mould of 80 cm. per minute, preferably 120 cm. per minute.

The process is best carried out using steel with a carbon content of about 0.08%, and this steel is tapped into a ladle in an unkilled state from an open-hearth furnace and is then vacuum-treated in batches. During this degasitcation process, individual batches weighing about 10% of the ladle content are periodically drawn into the degasitication chamber from the steel in the ladle through a pipe leading to the bottom of the degasilication chamber and are returned through the same pipe to the ICC ladle after degasification. The degasication of each batch takes about l15-20 seconds.

An example of a process in accordance with the invention will now be described with reference to the accompzlnylilng drawings which show the apparatus used and in w 1c FIG. l is a diagrammatic section through the apparatus at one stage of the process;

FIG. 2 is a similar section at a later stage;

FIGS. 3 and 4 are graphs showing the vacuum in the vacuum chamber during the process; and

FIG. 5 is a reproduction of an etched section of an ingot made by the process.

The apparatus comprises a degasilication chamber 1 having a pipe 2 leading to its base, a ladle 3 filled with the melt for degasiiication, a connection 4 from the chamber to a vacuum pump and a tank 5 for introduction of treatment or alloying substances into the degasication chamber 1. In the position shown in FIG. l, the pipe 2 dips into the charge in the ladle 3. In this position a batch of steel S is sucked into the tank where it is degasified. On raising the vacuum chamber to the position shown in FIG. 2 the degasified steel ows back through the pipe 2 into the ladle 3. This cycle of batch degasication is continued until the steel oxygen content has dropped to the required amount.

FIG. 3 shows the variation of the vacuum with time in the degasification chamber under these conditions. The abscissa of the diagram denotes time in minutes, and the ordinate vacuum pressure in torr (=mm. Hg). The vacuum, equal to about 50 torr at the beginning of the degasication operation, has dropped to about 5 torr after about 30 batch treatments. With a specifically dimensioned degasilication chamber the pressure rise for a flow of equal batches into the tank is a measure of the free oxygen content of the steel. The relationship between the pressure rise of the batch entering the degasication vessel and the free oxygen content of the steel is determined from samples taken from the ladle during prior treatments.

The result is shown in FIG. 4, which refers to the pressure rise per metric ton of steel taken in. In this diagram the abscissa denotes pressure change per quantity in torr/t, and the ordinate denotes free oxygen content in $5000 by weight. From FIG. 4 it will be apparent that a pressure rise of 0.2 torr per metric ton of steel corresponds to a steel free oxygen content of 0.009% by weight. If it is required to degasify to this value the steel of a 100 metric ton ladle, l0 metric tons of steel being drawn in each batch, the degasication is continued until the pressure rise on entry of steelinto the tank is equal to 2 torr. The pressure curve depends also on geometric conditions and the capacity of the suction apparatus used for the degasiiication plant. The conditions shown in FIG. 4 must therefore be determined empirically for each plant and each steel grade.

When the free oxygen content of the steel in the degasitication chamber has dropped to the required value, preferably to between 0.005 and 0.010% by weight, aluminium is introduced via the vessel 5 into the batch in the degasication tank until the free quantity of oxygen in the charge in the ladle after the discharge of the batch has dropped to 0.001-0.005% by weight, preferably 0.003% by weight. The quantity of aluminium required for the purpose can be calculated correctly because there is no incalculable burning away when the aluminium is added via the discharge vessel. In the case described, the required final oxygen content in the molten steel was obtained by the addition of grammes of aluminium per metric ton of Isteel. Boron was then added-again via the degasiticabatches of steel and return them to the ladle after the aluminium and boron have been added.

A melt prepared in this way was poured into moulds of a capacity of 15 metric tons each at a speed such that the moulds were Wull in about 11/2 minutes. The ingot head was quenched with water.

FIG. 5 shows the nature of a steel treated in this way. It shows the structure of an ingot cut open longitudinally. It will be seen that the steel has no peripheral blow holes otherwise found in half-killed steels. The core structure of the metal is loosened to a certain extent but there is no unsatisfactory pipe formation.

The ageing-resistant deep-drawing steel prepared by the method according to the invention has no oxide inclusion in and beneath the surface of the ingots, such as are found in aluminium-killed steel. The steel can therefore be rolled very satisfactorily and rough slabbing gives a surface quality such that no ame descaling is required. Another advantage of the steel is the high and uniform yield due to its favorable solidication structure. For example, an average yield of 93.5% was obtained for a charge, and varied only between 93 and 94% in the individual ingots.

I claim:

1. A process for the manufacture of deep-drawing steel which is resistant to ageing comprising the steps of tapping said steel in an at least partially unkilled state, degasifying said tapped steel in batches in a vacuum chamber to largely remove the free oxygen therefrom, adding aluminium to said steel to reduce its oxygen content further to between 0.001% and 0.005% by weight, adding between 0.05 kg.V and 0.10 kg. of boron per metric ton of steel to said Isteel and pouring said steel rapidly into an ingot mould at a rate of climb in said mould of at least 80 cm. per minute.

2. A process as claimed in claim 1, in which said degasifyng step is carried out until said free oxygen is reduced to a value of between 0.005% and 0.01% by Weight of said steel.

3. A process as claimed in claim 1, in which said pouring step takes place at a rate of climb in said mould of 120 cm. per minute.

4. The method of producing non-aging steels comprising the steps of subjecting a molten steel bath to a vacuum sufficient to cause removal of oxygen from the bath, maintaining the bath under vacuum until the oxygen level in 4 the bath is reduced to about 0.005% to about 0.01%, adding aluminium to the steel in an amount -sucient to react with the remaining oxygen to reduce the oxygen level below about 0.003% and adding 0.005% to 0.010% boron to the molten bath following the aluminum addition.

5. The method of producing non-aging steels comprising the steps of subjecting a molten steel bath to a vacuum sucient to cause removal of oxygen from the bath, maintaining the bath under vacuum until an oxygen level below about 0.01% is attained in the steel, adding an amount of aluminium to the steel suicient to reduce the oxygen level below about 0.003% and adding about 0.005 to 0.010% boron to the molten bath following the aluminium addition.

6. The method of producing non-aging -steels comprising the steps of subjecting a molten steel bath to a vacuum sufficient to cause removal of oxygen from the bath, maintaining the bath under vacuum until the oxygen level is reduced to about 0.005% to about 0.010%, adding an amount of aluminium sufficient to react with the remaining oxygen to reduce the oxygen level below about 0.003%, and thereafter adding about 0.005 to 0.010% boron to the molten bath.

7. The process of preparing a non-aging steel comprising the steps of subjecting a molten bath of steel containing about 0.08% carbon to a vacuum suicient to cause removal of oxygen from the bath, maintaining the bath under a vacuum until the oxygen level in the bath is reduced to about 0.005% to about 0.01%, adding aluminium to the steel to react with the remaining oxygen to reduce the level of oxygen below about 0.003%, adding about 0.005% to 0.10% of boron to the molten bath and casting the resulting molten bath.

References Cited UNITED STATES PATENTS 2,768,892 10/ 1956 Shoenberger 164-57 X 3,183,078 5/1965 Ohtake et al. 75-49 3,208,844 9/ 1965 Kato et al. 75-49 HENRY W. TARRING II, Primary Examiner U.S. Cl. X.R.

Claims

4. THE METHOD OF PRODUCING NON-AGING STEELS COMPRISING THE STEPS OF SUBJECTING A MOLTEN STEEL BATH TO A VACUUM SUFFICIENT TO CAUSE REMOVAL OF OXYGEN FROM THE BATH, MAINTAINING THE BATH UNDER VACUUM UNTIL THE OXYGEN LEVEL IN THE BATH IS REDUCED TO ABOUT 0.005% TO ABOUT 0.01%, ADDING ALUMINUM TO THE STEEL IN AN AMOUNT SUFFICIENT TO REACT WITH THE REMAINING OXYGEN TO REDUCE THE OXYGEN LEVEL BELOW ABOUT 0.003% AND ADDING 0.005% TO 0.010% BORON TO THE MOLTEN BATH FOLLOWING THE ALUMINUM ADDITION.