JPS6252006B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for reducing the inclusion content of steel and for refining the structure of steel.

Impurities that cause inclusions in steel have the following components. i.e., oxides, sulfides, phosphides, silicates, aluminates, nitrides, arsenides, etc., or complexes of the above compounds, possibly complexes thereof.

The inclusion itself can be extrinsic or endogenous. It is well known that the growth of endogenous inclusions is initiated by changes in the supply or solubility of certain inclusion-removing alloys.

At the inclusion removal temperature, primary inclusions can be relatively easily removed from the steel bath under the action of the inclusion removal alloy. If appropriate inclusion removal alloys and methods are applied, removal will be nearly complete.

All alloys that produce insoluble inclusions with a lower specific gravity and lower melting point than that of steel are suitable for that purpose. The applied method should promote the flotation of inclusions present in the metal bath.

In the casting process following inclusion removal, the molten metal cools and secondary inclusions appear due to changes in the equilibrium constant. Removal of these secondary inclusions is more complicated than that of primary inclusions, and removal of all of them is practically impossible.

Between the liquidus line and the solidus line (i.e. liquid phase + solid phase 2
- in the phase region), removal of tertiary inclusions extending along the grain boundaries is not possible due to segregation of the inclusions. Furthermore, during various transformations, 4 particles strongly segregate on average locations (bubbles, grain boundaries, dislocations).
Removal of secondary inclusions is not possible due to reduced solubility.

Most inclusions in steel are oxide inclusions, which are the most harmful. Their removal or reduction is therefore extremely important. We therefore deal first and foremost with these inclusions, but at the same time it must be emphasized that the method can be applied equally to the removal of other inclusions.

The amount of oxide inclusions in steel at room temperature depends on the oxygen activity level, which can be affected by deoxidation.

Deoxidation is an extremely complex and intricate metallurgical process that depends on many factors (the deoxidizing ability of the deoxidizing element;
quantitative composition, melting point, solubility limits and rates, etc.
Furthermore, it is influenced by the temperature and degree of oxidation of the bath, the amount of other additives, the physical and chemical properties of the deoxidation products, which play an important role (growth and removal). Among these factors, the deoxidizing ability of the deoxidizing agent is extremely important from the viewpoint of the deoxidizing effect.

Although deoxidation is a somewhat complex metallurgical process, deoxidation is still carried out by simply introducing a deoxidizing agent onto the surface of the steel bath. Recently, blowing lances and inert gas streams have only been applied to introduce deoxidizers into molten metals.

In special cases, deoxidation is carried out in vacuum to avoid melting of the deoxidizing material and oxygen in the air.

Hungarian Patent No. 172104 deals with the removal of primary endogenous inclusions that segregate under the action of inclusion-removal alloys. The compositions of inclusion-removal alloys and various methods of removing inclusions from baths have been published.

This inclusion removal alloy, which is most suitable for removing inclusions from steel, contains 2-20% titanium, zirconium, niobium, hafnium, cerium, boron and residual iron, as well as 40-50% silicon, 15%
~30% aluminum, 10-25% calcium,
Contains 1.5-15% manganese.

The above solutions, however, are only suitable for removing primary inclusions and are not applicable to reducing the amount of secondary inclusions or refining the steel structure, respectively.

The object of the present invention is a method for reducing the content of secondary inclusions in steel and refining the steel structure.

According to the invention, inclusions are removed from the steel by an inclusion removal alloy containing calcium and/or magnesium under pressures equal to or higher than ambient pressure. A vacuum is then created to evaporate the calcium and/or magnesium from the steel bath.

It is advantageous to remove inclusions under high pressure, preferably 2 to 6 atmospheres. During boiling off, the vacuum level used is generally between 10 ^-3 and 10 Torr.

The apparatus for carrying out the invention consists of a closed chamber and a tundish and lance with a steel bath injection device. The chamber is equipped with a vacuum system. Preferably, the pressure source is associated with the injection means.

The essence of the present invention is to demonstrate that the deoxidizing ability of calcium - and especially of magnesium - is highly pressure dependent, and to reduce the inclusion content of steel as well as the microstructural refinement of steel. The essence of the method and apparatus we have invented is used in

We reached the above conclusion by undertaking deoxidation experiments using the alloy shown in the above-mentioned Hungarian patent. During the course of these experiments, deoxidation was accomplished by: (a) charging the deoxidizing material into the steel bath; (b) bubbling the deoxidizing agent through a lance with an inert gas; and ( c) Using a vacuum.

Experiments have shown that the best results are achieved using a lance and an inert gas. This was surprising since, in terms of the state of the art, the best results would have been expected from deoxidation in vacuum.

Later, deoxidation was carried out by applying a lance as well as an inert gas and following this step by creating a vacuum. Surprisingly good results were obtained with this method.
Its oxygen and sulfur contents were lower than ever, as was its hydrogen content. Although the deoxidizer actually contained significant amounts of magnesium and calcium, the inclusions contained almost no magnesium oxide and calcium oxide. It was also surprising that most inclusions were found inside the crystal lines rather than on the grain boundaries. The inclusions were small and the steel structure was surprisingly fine.

Further tests led to the conclusion that the best results were achieved by deoxidizing under pressure using alloys containing magnesium and calcium, followed by processing the steel in a vacuum. Ta.

Further details of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings.

To understand the invention, the effect of pressure changes on calcium and magnesium deoxidation is shown in FIG.

In the diagram of FIG. 1, the amount of thermodynamic standard free energy change is plotted against temperature. The thermodynamic standard free energy change is calculated from the following equation: △G゜=△H−T△S=−RTlnK _DFigure 1 shows that the deoxidizing capacity of calcium and magnesium is enhanced by increasing pressure. Clearly show what can be done. However, a decrease in pressure or the creation of a vacuum leads to a decrease in deoxidizing capacity.

If deoxidation occurs at 1600℃ and a pressure of 1 atm, point 1 is the deoxidation ability of calcium, point 2 is
indicates that of magnesium. If deoxidation is carried out at a pressure higher than 1 atm, the deoxidizing power of calcium increases at 1.6 atm to a value corresponding to point 1', and that of magnesium increases at 3.9 atm.
A value corresponding to point 2' is reached. This is also indicated by ΔG° in negative numbers.

Figure 1 also shows that at 1600°C, increasing the pressure above 1.6 atm by applying calcium and above 3.9 atm by applying magnesium has no effect and is meaningless.

However, if the deoxidation temperature is increased,
Pressure should also be increased accordingly.
It is clear that the increase in pressure at 1600°C is more effective when applying magnesium (3 times higher pressure produces a 3 times larger change in the value of ΔG°) than when applying calcium. .

If the deoxidation is carried out in vacuum, for example under about 0.001 atmospheres, the deoxidation capacity of calcium is reduced to a value corresponding to point 1'' and that of magnesium to point 2''. This phenomenon also makes the numbers more positive△
It is shown in G゜. Vacuum affects the value of ΔG° in the same way for both calcium and magnesium.

The essence of the invention is that the steel is deoxidized under pressure using alloys containing calcium and/or magnesium. After the deoxidation step is completed, the calcium and/or magnesium is almost completely evaporated from the steel by vacuum treatment.

The deoxidizing properties of calcium and magnesium improve with increased pressure and worsen with vacuum.
This means that at the deoxidizing temperature of steel under pressure,
It is possible to dissolve more calcium and magnesium, but the result is that the calcium and magnesium are evaporated in vacuum because their boiling points change with changes in pressure. By increasing the pressure their boiling points are raised, but in vacuum, as shown by the displacement of the breaking point (which is also the boiling point at a given pressure value) in FIG. The boiling point decreases.

Among the most important deoxidizing elements is calcium (1487
°C) and magnesium (1102 °C) have boiling points lower than the deoxidation temperature of steel (1600 °C).
For the implementation of the method described above, calcium and/or
Or an alloy containing magnesium is required.

The inclusion content of steel treated by this method is lower than that of steel treated by any of the previously known inclusion removal methods. None of the previous methods include means for applying pressure, and in this way the oxygen value corresponding to the value of point 1, corresponding point 2, can only be reached according to FIG. Point 1',
The low value indicated by the corresponding point 2' can only be reached by using the method according to the invention.

However, this is only one of the advantages of the method. Other advantages are illustrated in FIG.

Point 1'', point 2'' respectively represent the oxygen value in equilibrium with the remaining calcium and/or magnesium after deoxidation and evaporation of calcium and/or magnesium; ′, significantly higher than the oxygen values shown at point 2′, respectively.

Although the value of the equilibrium constant changes during cooling, secondary inclusions are such that for one of the deoxidizing elements remaining in the steel, the oxygen value changes due to the change in the value of the equilibrium constant. No segregation occurs until the lowest value shown in the process is reached. This point is easily second
Its location can be found on the diagram. If the curve representing the deoxidation characteristics of the deoxidizing element as a function of temperature intersects the straight line representing the lowest oxygen value, then
The breakpoint indicates the temperature at which the above-mentioned phenomenon occurs. These breakpoints are 3 ^× and 4 ^× . Point 3 ^× corresponds to a deoxidizing alloy containing silicon, aluminum, and magnesium; point 4 ^× corresponds to a deoxidizing alloy containing silicon, aluminum, and magnesium;
Aluminum, calcium, magnesium and rare earth metals (e.g. cerium = 48-56%, neodymium = 15-20%, praseodymium = 4-7%, lanthanum = 20-25%, other rare earth metals and impurities <1%) ) represents an alloy containing This allows for supercooling of the steel and segregation of solid secondary, tertiary and quaternary inclusions. The composition of these inclusions is
It is significantly different from that of primary inclusions. They contain very little calcium and/or magnesium or have no calcium and/or magnesium content at all. These segregations exist to a greater or lesser extent and play the role of crystal nuclei, which form an extremely fine steel structure. Unless a vacuum treatment is applied after deoxidation, which means that no evaporation of calcium and/or magnesium takes place, a liquid with a high calcium- and/or magnesium content and with a composition almost the same as that of the primary inclusions. Secondary inclusions will immediately begin to segregate during the cooling process due to the change in the equilibrium constant. As a result, the structure of the steel does not refine due to supercooling and lack of crystal nuclei. Inclusions will segregate along grain boundaries and affect the mechanical properties of the steel in a most undesirable manner.

The invention will be illustrated by the following examples.

Example 1 A deep drawable mild steel is made from a molten metal consisting of 0.1-0.2% carbon, 0.4-0.6% manganese, 0.05-0.1% silicon, 0.04-0.1% aluminum, up to 0.15% phosphorus, and up to 0.15% sulfur. Ta.

Removal of inclusions (deoxidation, desulfurization, dehydrogenation) is 1600
The experiments were carried out at 4 atm. The inclusion removal alloy contained 45% silicon, 25% aluminum, 4% magnesium and iron. The inclusion-removed alloy was added to the steel bath through a blow lance using argon. After removing inclusions, a vacuum of 10 ^-2 Torr was applied. In this method, 70 ppm of oxygen in the steel,
0.01% sulfur remained. After removing inclusions from similar alloys, the typical oxygen content is 100~
200ppm, sulfur content 0.012-0.015%.
The structure of the steel was surprisingly fine (average grain size: 0.015 as measured by Hungarian standard No. 2657). The grain size of similar steels is generally 0.028-0.03mm. The impact energy of steel treated using the method according to the invention is 16 mkp/mm ² at 20°C, -40°C
It became 6mkp/ ^mm2 . In the case of steel processed by conventional methods, the same values generally amount to values of 12-14 and 3-5 mkp/mm ² , respectively.

Example 2 Inclusions were removed from deep drawable mild steel according to Example 1. Inclusion removal alloy is 1620
℃ was added to the steel bath under standard atmospheric pressure.
The composition of the inclusion removal alloy was as follows: 50% silicon, 20% aluminum, 20% calcium.
Magnesium 1.5%, balance iron.

After removing inclusions, a vacuum of 10 ^-3 Torr was applied. After that treatment, the steel contains 50 ppm oxygen and 0.09% sulfur.
Contained. The average grain size was 0.018 mm.
The impact energy value is 16mkp/at 20℃.
mm ² , the value was 6 mkp/mm ² at −40°C.

Example 3 As shown in Example 2, inclusions were removed from the alloy at 1640°C and 4 atmospheres. The composition of the inclusion removal alloy was as follows: 40% silicon, 20% aluminum, 15% calcium, magnesium.
1.5%, balance iron. Blowing was carried out using argon using a Lance apparatus. The vacuum value after inclusion removal was 10 ^-1 Torr. The parameters of the steel obtained by means of this method were: oxygen content: 10 ppm, sulfur content: 0.008%, average grain size: 0.008 mm, impact energy. 20℃:
19mkp/ ^mm2 , -40℃: 8mkp/ ^mm2 .

The above-mentioned examples clearly show that the secondary inclusion content of the steel treated by the method according to the invention is reduced to a significant limit, the composition of the steel is refined and the mechanical properties are also improved. There is.

FIG. 3 shows the equipment applied for the treatment.

The apparatus consists of a chamber 1 in which a container 2 containing the alloy to be treated is placed. The chamber 1 can be sealed by a cover 3. The injection device 4 is connected to the cover 3. The inclusion removal alloy is placed inside the injector 4. The injection device 4 is equipped with a lance 6 that reaches the molten metal through a packing box 7 placed on the cover 3 of the chamber 1.

Chamber 1 is connected to a vacuum device 9.

The pressure device 5 is connected to the injection device 4 . The pressure device 5 serves, on the one hand, to blow in the inclusion-removal alloy and, on the other hand, to generate the necessary pressure to be able to remove the inclusions under pressure.

In the embodiment according to FIG. 3, the pressure device 5 may contain an inert gas, preferably argon.

The entire device is operated from an administration desk 10.

The device is operated as follows: - first stage, vessel 2 filled with pre-oxidized steel;
is placed in chamber 1, which is opened using a crane.

- second stage, the treatment chamber 1 is closed by a cover, which is equipped with an injector 4;

- third stage, the blowing with the aid of the pressure device 5 is started through the injection device 4; At the same time, the lance 6 of the injector 4 is submerged sufficiently deeply into the steel melt, and the chamber 1 is thus sealed by a packing box 7 placed over the blast lance 6.

- Fourth stage, the injector 4 is started and the alloy containing calcium and/or magnesium is injected into the steel.

The pressure in chamber 1 is increased by safety valve 8 to a preset value. At this point the injector 4 is stopped.

- Fifth stage, the vacuum device 9 is started and the pressure in the chamber 1 is gradually reduced. Thereafter, calcium and/or magnesium is evaporated from the steel.

- Sixth stage, the vacuum pump is stopped. The lance 6 of the injector 4 is lifted out of the steel bath and the gas flow is stopped.

- in the seventh step, the cover 3 is removed from the chamber 1;

- In the eighth stage, the container filled with treated steel is lifted out of the open chamber 1 using a crane and transported for casting.

The management desk 10 not only manages the entire process but also gives instructions on the operation of each device. All the above steps can be carried out in 10-20 minutes.

It is clear that inclusions are removed from steel in the most economical way by applying the method according to the invention, and that the simple equipment for carrying out the invention ensures the implementation of the method at low cost. It will be clear from the examples. The inclusion content of the steel produced by using this method is lower than that of ordinary steel, its structure is extremely fine, and its mechanical properties are also poor, even if the inclusions are removed by conventional methods. are better than those made of steel.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the deoxidizing effect of calcium and magnesium. Figure 2 shows the effect of vacuum treatment after deoxidation. FIG. 3 shows an apparatus applied to realize the method according to the invention. 1: Processing chamber, 2: Container, 3: Cover, 4: Injection device, 5: Pressure device, 6: Lance, 7: Packing box, 8: Safety valve, 9: Vacuum device, 10: Management desk.

Claims (1)

[Claims] 1. In a method for reducing the inclusion content of steel and refining its structure; (a) preparing molten steel; (b) an inclusion-removal alloy containing calcium and/or magnesium; is passed through the molten steel held under a pressure of at least 1 atmosphere; (c) the molten steel is then placed under vacuum to evaporate the calcium and/or magnesium from the molten steel; A method for reducing the inclusion content of steel and refining its structure. 2. A method according to claim 1, characterized in that the inclusion removal is carried out under a pressure of 2 to 6 atmospheres. 3. A method according to any one of claims 1 to 2, characterized in that the inclusion-removed alloy is injected into the metal bath by means of an inert gas through a blowing lance. 4. The method according to claim 3, characterized in that argon is used as the inert gas. 5. A method according to any one of claims 1 to 4, characterized in that a vacuum of between 5 10 ^-3 and 10 Torr is applied.