LU80693A1 - PROCESS FOR THE MANUFACTURE OF SEMI-CALM STEEL - Google Patents

Description

»Λ A 548

PATENT

Applicant: ARBED S.A.

Avenue de la Liberté LUXEMBOURG

Process for manufacturing semi-calm steel, "

The present invention relates to a method of manufacturing a steel showing the favorable properties of one semi-quenched steel.

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It is known that the heads of calmed steel ingots exhibit a more or less pronounced shrinkage which leads to the heads falling. The extent of this shrinkage and of the resulting fall can only be reduced by costly means / such as weighting and / or reheating.

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Semi-calm steels, on the other hand, release during their solidification in the ingot mold quantities of gas which vary according to the degree of calming and the chemical composition of the steel. These gases compensate for the space which would be due to shrinkage at the head of the ingot, thanks to the formation of a multitude of hollows or blows.

In fact, the ferrostatic pressure which prevails inside the ingot in the process of solidification pushes the gas bubbles towards the upper half of it. On the other hand, depending on the degree of calming and the extent of the release of gases, the blasts formed are found more or less near the surface of the ingot where they can lead to defects on the finished products during the subsequent processing of the ingots.

If a correct semi-calm steel therefore normally presents a thick skin, of excellent quality, it may be that the appearance of blisters which are harmful to the skin of the ingot itself.

Attempts have therefore been made to adjust the quantities of gas released during solidification by adding sedatives to the steel in determined quantities. However, it has proved impossible to direct the evolution of the gases in a completely reproducible manner, given that the factors which govern solidification are numerous and difficult to control.

The object of the invention is therefore to propose a method for manufacturing a steel showing the favorable properties of semi-quenched steel, in particular with regard to the composition of the cast ingots, while avoiding the disadvantages normally inherent in l semi-calm steel.

This object is achieved by the process according to the invention which provides for pouring a completely quenched steel into the ingot mold and for treating said steel by acting inside the ingot mold during or after casting a gas capable of creating inside the ingot body a multitude of hollows. This gas is preferably a neutral gas or a reducing gas blown into the mass of the metal not yet solidified.

The idea which is at the basis of the process according to the invention consists in taking advantage first of all of the fact that the completely quenched steel solidifies following the casting without the formation of gas and consequently without there being a gas evolution which would take place in an uncontrollable manner or difficult to control and then to obtain a semi-calmed shade by controlled gas insufflation. This insufflation of gas into quenched steel cannot alter the chemical composition of the steel, since the gas in question does not cause oxidation, so that the risk of a deviation in the grade of the steel that we proposed to manufacture, is discarded. Furthermore, the blown gas creates inside the body of the ingot during solidification a multitude of mini-recesses whose volume is sufficient to compensate for the shrinkage which would have formed by contraction at the head of an ingot cast from a steel that has calmed down without adding gas.

The overall result is an ingot with excellent quality skin, as well as a uniform interior situation with a multitude of mini-hollows which are distributed uniformly throughout the body of the ingot.

The process according to the invention therefore makes it possible to obtain a steel which combines the favorable properties of semi-calm steel with those of totally quenched steel, while minimizing the falling of the ingot heads.

According to a first technique, provision is made for insufflation from above inside the ingot mold, using a submerged lance.

“'-4.- The advantage of this insufflation technique lies in the fact that it is possible to change the immersion depth of said lance during insufflation, which can result in an optimization of the degree of distribution. gas blown through the body of the ingot.

A second technique provides for blowing the gas from below inside the mold, for example through. a porous stone or a tube housed in the base of the mold.

> The advantage of this technique lies in the fact that it is practical and inexpensive. In addition there are no tools to operate as is the case for insufflation from above.

According to the invention, the blown gases are gases which show better solubility in liquid steel at high temperature than at low temperature.

The gases which are taken into account to be blown in during the execution of the process according to the invention are preferably neutral gases, such as nitrogen or else reducing gases, such as hydrogen or even compounds of both, such as NO3 or N2H ^. Similarly, it is not excluded to cause the effect described in whole or in part by gases generated by the thermal decomposition of blown solid products or by metal vapors.

Hydrogen has the property of being less soluble in steel at low temperature than at high temperature and leading to the formation of bubbles or non-oxidized hollows during the solidification of the steel. This gas is therefore particularly suitable for serving during the execution of the process according to the invention, at least during the casting of steel grades which are not sensitive to the formation of flakes.

The gas blowing according to the invention is therefore an addition to the steel of an ingredient which, at lower temperatures, creates a multitude of hollows throughout the ingot. This addition is an entirely different maneuver than stirring, in which gases are injected for the sole purpose of causing agitation or eddies inside the liquid steel.

According to the invention, it is advantageous to close the head of the mold at the end of casting using iron filings or a thin sheet cover.

This makes it possible to reinforce the retention effect of gases soluble in liquid steel at high temperatures.

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Claims (9)

1. A method of manufacturing a steel showing the favorable properties of 11 semi-calm steel, in particular as regards the composition of cast ingots, characterized in that a completely calmed steel is poured into the ingot mold and that the 'is acted on said steel during or after casting a gas capable of creating inside the body of the ingot a multitude of hollows. 2. Method according to claim 1, characterized in that the gas is blown from above into the mold using a lance. 3. Method according to claim 1, characterized in that the gas is blown from below inside the mold, in particular through a porous stone. i 4. Method according to claims 1-3, characterized in that the gases blown inside the mold are gases which show in the liquid steel better solubility at high temperature than at low temperature. 5. Method according to claims 1-4, characterized in that the blown gas is a neutral gas resp. a reducing gas. 6. Method according to claims 1-5, characterized in that the blown gases are hydrogen, nitrogen, ammonia or hydrazine. ’- 7 - 7. Method according to claim 1, characterized in that at least part of the treatment gas is generated inside the metal for thermal decomposition of a blown solid product. 8. Method according to claim 1, characterized in that all or part of the treatment is carried out by metal vapors. 9. Method according to claims 1-8, characterized in that the head of the mold is closed at the end of casting with t> using iron or aluminum filings or using a cover in thin sheet metal. / t * "