CS216653B2 - Method of decarbonization of the high-alloyed steels in vacuum - Google Patents

Abstract

In a process for removing carbon from a highly-alloyed steel, such as a chrome steel, the steel is treated by blowing oxygen into or on to the molten steel in a vacuum vessel to remove carbon from the melt. The temperature of the gases extracted from the vessel is monitored to detect an abrupt drop in the gas temperature, at which point the oxygen treatment is arrested and the temperature of the melt is immediately measured. The composition of the melt is then corrected on the basis of the known equilibrium concentration of carbon and chrome, or other constituent, which occurs at that measured melt temperature. The composition of the melt may be corrected either by adding further alloying substances thereto, or by further oxygen treatment, as the situation requires.

Description

The invention relates to a process for decarburizing high-alloy steels under vacuum, by blowing oxygen into or onto a steel melt.

It is known to decarburize high-alloy chromium steels under vacuum by blowing oxygen into or onto the steel melt to prevent loss of chromium. A disadvantage of this method is the difficulty of adjusting a certain final value of the carbon content in vacuum. So far, estimates have been made. Due to the highly fluctuating refining conditions, which are subject to numerous environmental influences, there is a danger that, for example, fluctuations in the amount of oxygen supplied do not achieve or exceed the desired carbon content. In this case, the desired composition can only be achieved after several sampling in vacuo, waiting for analysis results and subsequent additional oxygen blowing or carburizing.

These drawbacks are eliminated by a method of decarburizing high-alloy steels in vacuum, in particular chromium steels, by blowing oxygen into or onto the steel melt according to the invention, characterized in that the high-alloyed high carbon steel is decarburized under vacuum until the flue gas temperature drops suddenly. at least 20 ° C, after which the melt temperature is measured and the bath composition is corrected in accordance with the equilibrium carbon and chromium contents resulting from the measured melt temperature.

According to the invention, it is advantageous if the melt composition is corrected by the addition of alloying agents and / or by additional blowing of oxygen. Before correcting the composition of the melt, an element exothermic with the oxygen is first introduced into it.

With the method according to the invention, a certain final carbon content can be set with certainty without having to take samples during processing.

The process according to the invention makes it possible to achieve low carbon contents with certainty without time-consuming analyzes and significant chromium losses. Therefore, this method is suitable for use in the processing of large batches.

The method according to the invention is further described in detail by way of examples. The chromium-alloy steel is melted in a known manner in the furnace and the chromium, nickel, phosphorus and sulfur contents are adjusted to approximately the desired value. The carbon content of the melt lies well above the predetermined value in the furnace refining to prevent chromium losses and is, for example, about 0.2 to 1.0 why. The melt is tapped without slag or with a small amount of slag and is degassed, for example, in a degassing pan after argon, while stirring with argon. the pressure is reduced to approximately 1.33 kPa. Thereafter, oxygen is blown into the vacuum vessel, optionally into the melt or via the blower tube. In accordance with the oxygen flow rate, the temperature in the vacuum vessel rises.

Because oxygen combines with the carbon of the melt exothermically according to the equation

O ₂ + 2 C = 2 CO, a significant amount of heat is released and the temperature of the gases exhausted from the vacuum vessel is also considerably increased.

The temperature increase is used to monitor decarburization by a measurement technique. For example, the pumps are sized so that even if some CO is present, the pressure in the working vessel will remain in the range between 1.33 to 4 kPa. Experiments have shown that, while maintaining this pressure range in the working vessel, decarburization can be carried out up to a value that corresponds to a CO equilibrium of about 6.66 kPa at reasonably economical times. The carbon content of the melt at a CO equilibrium of 6.66 kPa depends on the amount of oxygen in the melt available for the reaction. SO, and which again depend on the melt temperature and its chromium content. It is apparent from the attached diagram that at a given partial pressure of CO, without the chromium being hit by grinding, the amount of oxygen is higher the higher the melt temperature. Accordingly, the achievable final carbon content is the lower the higher the melt temperature.

Order. the final carbon content has been reached,. . oxygen should be blown and suctioned off until the CO reaction is interrupted due to the equilibrium reached. If it continues to breathe, undesired chromium disintegration is caused. Achieving equilibrium can be seen by the sudden drop in the temperature of the exhaust gases due to the exothermic interruption. evolution of CO from the reaction of oxygen with carbon.

By measuring the temperature in the melt, at the time of the temperature drop, the achieved carbon content is checked. If this value is lower than the specified value, it may be. immediately add carbon to the melt under vacuum. Conversely, if the melt content achieved is higher than required,. then added to the melt under vacuum. an element which has high oxygen compatibility and reacts strongly exothermically, for example silicon. The amount of silicon to be added is selected such that the temperature required to adjust the specified amount of carbon is achieved by combustion with oxygen supplied. In the case of a short-term after-blowing of oxygen, the temperature rises again until the specified carbon content is reached, again as a result of a sudden drop in the temperature of the off-gases.

Described method. can naturally. to be carried out at a temperature corresponding to the specified carbon content. and heat loss,. . which would occur after. adequate degassing. without oxygen supply and from the reaction heat of the CO reaction, the required temperature at the beginning of the vacuum treatment is calculated. This melt temperature is adjusted at tapping or, if the melt temperature is too low, the required amount of silicon is added before or during the vacuum treatment to achieve the desired bath temperature and thus determined carbon content with the oxygen bubbles causing decarburization. The necessary tapping temperature for the melt can be determined based on known data according to the following formula:

To - T _E + ΔΤ _α + ΔΤ _Η T _r denoting:

This is the tapping temperature;

Te the final temperature required for decarburization according to the diagram in the drawing;

Pokles _α decrease (tapping eplothy;

Poklesη a drop in the temperature of the melt in the vacuum pan from the end of tapping to the end of blowing can be done with sufficient accuracy for 10 to 15 minutes. Thus, ΔΤη consists of traffic-dependent quantities that are easy to detect;

Tr is the heat of reaction resulting from the reaction between oxygen and carbon, as well as with other elements combining with oxygen. The following equation can be used to determine the Tr value:

Tr · = 300. (O / O Si) + 140. (O / o O + + 350 (% AI) + 130 (·% Cr) + + 90 (% Mi)

Accordingly, at a tapping temperature too low, silicon or aluminum in vacuum can be added to avoid too long a treatment time in the furnace before blowing oxygen, which increases the Tr value to the extent that To. is too low.

The method of the invention is. further explained on two examples.

Example 1

Melt of 50 tons with composition · v ° / o

0.5% carbon

0.35%. . silicon

0.5% manganese

18.0% chromium

10.0% nickel

0.015%. open

0.03 O / o phosphorus was tapped at 1630 ° C. The pan was placed in a vacuum vessel. and it was evacuated.

Consistent with the oxygen content of the melt, a slight CO reaction occurred and, as a result, the carbon content decreased by about. 0.02%. When the pressure reached 1.33 kPa, it was above. bath indicated blowing. pipe and blowing current

216В53 oxygen mass flow 1800 kg.tr ¹ The instantaneous CO reaction caused an increase in the flue gas temperature, measured at the beginning of the intake manifold, from about 300 ° C to about 500 ° C. After about 12 minutes the flue gas temperature suddenly dropped. The oxygen supply was immediately disconnected. A bath temperature of 1770 ° C was determined by measurement, which corresponds (from the diagram in the drawing) to a carbon content of 0.01%.

The melt composition was then corrected by addition

500 kg of ferromangan (80% Μη, 1% C), 150 kg of ferrochromium (70% Cr, 1.5% C) and 480 kg of ferrosilicon (75% Si).

This resulted in a composition melt

0,025 % carbon 0.7 % silicon 1.3 ° / o manganese 18.0 % chromium 10.0 % nickel 0.03 % sulfur 0.03 % phosphorus

in accordance with the desired carbon content of 0.02 to 0.03%.

was tapped at 1600 ° C and placed in a vacuum vessel. After reaching a pressure of 1.33 kPa in a vacuum vessel, it was purged with an oxygen stream of 1000 kg.tr- ¹ . After 10 minutes the flue gas temperature dropped. The steel temperature was 1670 ° C. Thus, according to the diagram in the drawing, a carbon content of about 0.022% was achieved. However, the prescribed carbon content of the finished melt was at most 0.03% carbon and at least 17.5% chromium. Accordingly, at least 1% of chromium had to be added. The addition of ferrochromium containing 1.5% carbon required an increase in the carbon content of 0.021%. 750 kg of ferrochromium (70% Cr) as well as 130 kg of ferrosilicon (75% Si) were added. As a result, the melt temperature decreased to 1630 ° C and the carbon content increased to 0.05 why. It was still blown briefly with an oxygen flow of 100 kg.h ^-1 . At the same time, the temperature rose by about 25 ° C and after barely 2 minutes there was a sudden noticeable drop in temperature. When the temperature in the melt was again measured, a temperature of 1650 ° C corresponding to 0.025% was found; carbon. Then 500 kg of ferrosilicon (75% Si) was added under vacuum and mixed. The final analysis revealed the composition

Example 2

Melt weighing 50 tons with composition

0.3% carbon

0.5% silicon

1.0% manganese

17.0 ° / o chromium

10.0% nickel

0.01% sulfur

0.025% phosphorus

0.025% carbon

0.7% silicon

0.9% manganese

17.8% chromium

10.0% nickel

0.01% sulfur

0.025% phosphorus corresponding to the requirements.

Claims (3)

Method for decarburizing high-alloy steels in vacuum, in particular chromium steels, by blowing oxygen into or onto a steel melt, characterized in that the high carbon alloyed steel is decarburized under vacuum until the flue gas temperature drops by at least 20 ° C, then the melt temperature is measured and the bath composition is corrected in accordance with the equilibrium carbon and chromium contents resulting from the measured melt temperature. 2. Method according to claim 1, characterized in that the composition of the bath is corrected by the addition of alloying agents and / or by additional blowing of oxygen. 3. A method according to claim 2, characterized in that, prior to correcting the bath composition, an exothermic oxygen-combining element is first introduced into the melt.