JPS5931426B2 - Electroslag hot top method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electroslag hot top method that supplies molten slag onto the internal steel surface of a riser and maintains the slag composition optimally.

For guild copper steel ingots, a riser is generally attached to the head of the ingot to prevent defects caused by solidification shrinkage, and various methods have been devised to keep the riser warm. There is.

Molten slag is placed on the steel surface of the feeder, an electrode is immersed in the slag, and a current is passed between the electrode and the steel ingot or between multiple electrodes, and the molten steel inside the feeder is heated using the resistance heat generated by the slag. The method of heating is the electroslag hole method (ES).
(also called HT method or EST method).

This method is authentic! An actual example from J-VEW's Kabfuenberg factory has been reported.

(Special Public Interest Publication No. 47-39817 and Electric
Furnace Proceedings Vol 3
3°Dec, 1975 pl, 85/189) It is said that other Italian special steel factories, Chill II, are also implementing this method.

At the Kapfuenberg factory, this method is called the BEST method, and its outline is shown in FIG.

That is, 1 is a surface plate, 2 is a mold, 3 is a riser mold, 4 is a power supply, and 5
is an electrode, 6 is a slag, and 7 is a solidifying steel ingot.

In order to carry out the electroslag hot top method (hereinafter abbreviated as EST method), there must be a molten slag layer on the steel surface of the riser.

In order to create this molten slag, it is possible to use a method (cold start method) in which a powdered or solid slag-forming agent is poured onto the surface of the molten steel in a riser and melted by an arc to turn it into molten slag (cold start method). The period of melting is the original EST
This is not recognized as legal implementation time, and as a result, the EST treatment effect is diminished.

Therefore, it is desirable to transfer molten slag onto the inner steel surface of the feeder of the steel ingot after ingot making and heat the feeder and energize it (hot start method), but the slag is melted in a separate slag melting furnace. Preparation requires equipment and personnel for slag melting work.

The present invention has been made as a result of intensive research in view of the above points, and the gist thereof is to transfer the molten electric furnace reduction stage slag remaining in the ladle after the completion of ingot making work onto the molten steel in the feeder frame using appropriate means. After pouring, the slag composition is adjusted to a CaO/SiO2 weight ratio of 1.5 to 1.5 by adding no slag-forming agent or adding an appropriate amount of slag-forming agent.
5.0, (CaO + MgO) / (S io2 + AI2
The electroslag hot top method is characterized by maintaining a weight ratio of 1.0 to 3.0, CaF2<20% by weight, and (T, Fe+T, Mn)<2% by weight.

However, (T, Fe) The total amount of iron (Fe) contained in the varnish slag, that is, the total amount of Fe in metallic iron and iron oxide (including FeO1Fe304, Fe2O3) (T, Mn) Mn and Mn contained in the varnish slag
By applying the total amount of Mn in the compound and the present invention, special work and equipment for separately preparing molten slag are not required, and by adjusting the slag composition, desulfurization expected in the electroslag hot top method can be achieved. It is possible to improve the effectiveness and cleanliness of inclusions, and at the same time, it is possible to prevent an increase in the hydrogen content in the steel ingot.

The present invention will be explained in detail below.

Slag during electric furnace reduction is discharged into the ladle as molten steel is tapped.

As a result of studies conducted by the present inventors, it has become clear that this in-pot slag is suitable as EST slag.

The required properties of EST slag are (1) appropriate electrical resistance in the molten state, (2) appropriate melting point, (3) appropriate fluidity, (4) stability of chemical composition, (5) desulfurization ability, (6) )
(7) ability to absorb nonmetallic inclusions suspended in molten steel; and (8) low hydrogen solubility.

Some of the measures to address these demands may be mutually contradictory.

Adjustment of the composition of the EST slag, especially its basicity, is an important factor.

From experience with the electroslag remelting method (ESR method), increasing the content of Al2O3, MgO, etc. in the slag increases the electrical resistance of the molten slag, but the melting point of the slag becomes high (and fluidity is impaired).

Furthermore, if the CaO content in the Surak offshore is increased, the desulfurization effect will be greater (although this will increase the risk of increased absorption of hydrogen into the steel ingot).

Blending CaF2 has the effect of lowering the melting point of the slag and increasing its fluidity, but if it is added too much, the fluidity becomes too good.
During the EST treatment, it flows into the gap between the steel ingot and the mold, causing cracking of the steel ingot.

Furthermore, if the feeder frame is lined with bricks, the lining bricks will be severely damaged by erosion, making it difficult to continue the EST work.

On the other hand, CaF2 is more expensive than other slag-forming agents, and it is more economical to incorporate a smaller amount of CaF2.

Next, if we reconsider the required characteristics of EST slag,
In order to expect a desulfurization effect, the weight ratio of Cab/S i0 is
2>1.5, (CaO+Mg0)/(5i02+Al2
03)>1.0 is required.

On the other hand, the aO content in the ESTS slag should not be too high in order to prevent the slag from absorbing hydrogen and thus picking up the hydrogen content of the EST steel ingot.

In addition, the weight ratio of Cab/Si
02 < 5.0, (CaO+Mg0)/(SiO3+
A'203)<3.0.

In the case of such a slag composition, it is sufficient that CaF2 supplementarily increases the fluidity of the slag, and its content is 20%.
The following is fine.

EST slag with such a composition is compositionally stable, has desulfurization ability and deoxidizing ability, and can simultaneously capture and absorb nonmetallic inclusions suspended in molten steel.

Oxides of Fe, Mn, Cr, etc. are found in nonmetallic inclusions when molten steel is not sufficiently deoxidized, and when they come into contact with killed molten steel, Si, which is a deoxidizing alloying element,
Reacts with AI etc.

These compositions are not preferable in EST slag either.

Therefore, FeIMn oxide must be present in a small amount in the EST slag, and it is desirable that the analytical value in the form of T, Fe + T, Mn is 2% by weight (hereinafter all % by weight is simply referred to as %).

As a result of the above consideration 1.5 < Cab/S i 02
< 5.0.1, 0 < (CaO + MgO) /
(S 102 + AI 203 )<3.0, CaF
It has been found that there is an optimum range of EST slag of 2<20% (T, Fe + T, Mn) and 2%.

This slag composition is also in the composition range of the slag during reduction in the electric furnace and the slag in the ladle after tapping in the electric furnace.

That is, it has been found that it is possible to use the slag in the ladle that has been tapped in the electric furnace as LEST slag.

Using the slag in the pot as EST slag does not require special equipment such as a slag melting furnace for EST, and there is no need to separately consume a slag-forming agent, and there is no need to deploy personnel for preliminary slag melting. It is extremely economical to start the EST without having to worry about it.

The slag in the pot may be transferred to the self-melting steel surface of the riser.

It may be poured directly into the pot, or passed through a transfer gutter.

In the present invention, the slag in the pot is used as the EST slag after being transferred, and at that time, if necessary, a relatively small amount of CaO1 is added.
Adjusting the slag composition by adding other slag-forming agents does not pose a side pipe problem in terms of operation.

The main points of the present invention, such as the transfer of the slag in the ladle to the self-melting steel surface of the feeder and the limitations regarding the slag composition, also apply to the case where the feeder frame is a water-cooled mold (Kapfenberg factory). (chill factory).

In the EST method, molten slag is placed on the steel surface of the riser and kept at high temperature for a long time.

For this reason, the Kapfenberg factory mentioned above uses a water-cooled die riser frame.

When using a refractory-lined riser, it is necessary to consider the material of the refractory.

Chromite-containing refractories have the disadvantage that Cr is reduced and eluted into molten steel, and Al2O3-based refractories with high slag resistance are expensive.

As a result of a trial test, it was found that Magdrone is the most suitable material for lining the EST feeder.

Examples of the present invention are shown below.

Example 1 Molten steel composition 0.29%C10, 32%Si, 0.69%
Mn, 0.009%P, 0.006%S, Steel ingot weight 50 tons Electrode used 14″$graphite electrode 3 Booster Mag-Doro brick lining slag composition CaF29.1%, Ca038.7%, 5
i0219.8%, A120315.2%, MgO13
,4%, T, Fe<1%, 80.12% EST energizing time 22 hours EST power consumption unit 230 Kwh / Zulfur print of longitudinal section of TEST treated steel ingot (Figure 2) Steel ingot of EST treated steel ingot Cleanliness of inclusions on the shaft (Figure 3) From Figure 2, the feeder part and the head on the steel ingot shaft are white with sulfur print, proving that they were drained during the ESTO. .

Furthermore, as shown in FIG. 3, the cleanliness is significantly improved even in the high sedimentation area at the bottom of the steel ingot where many oxide inclusions gather, and the cleanliness of inclusions is high overall.

These confirm the effectiveness of the method of the present invention.

Although the use of the steel ingot produced by the hot top method of the present invention is not particularly specified, an example of its application to a steel ingot for a turbine rotor will be shown as a second embodiment.

Example 2 Molten steel composition 0.29%C10, 25%Si, 0.73%
Mn, 0.005%S, 09%Cr, 1.01%Mo.

0.22%■ Steel ingot weight 60.51-inch Electrodes used: 3 14"f graphite electrodes Riser Mag-Doro brick lining slag composition CaF212.0%, Ca038.5%5
i0217.7%, A120314. ．． 7%, Mg01
5.4%, T, FeO, 7%, T, Mn 0.0% EST treatment time 22 hours EST power consumption 226 Kwh/T Longitudinal section sulfur print of EST treated steel ingot (Fig. 4) EST treated steel ingot Comparison of the cleanliness of inclusions on the axis of the steel ingot and the segregation of C and S with the conventional steel ingot (Figure 5) Example 2
The EST of the present invention is applied to steel ingots for high-pressure steam turbine rotors.
It is an application of the law.

As seen in FIG. 4, there is no concentration of 5S, which is related to solidification, and no segregation or grain defects are observed in the steel ingot.

Furthermore, (2) Segregation was only slightly observed at the top of the steel ingot, and the internal properties of the steel ingot were significantly improved compared to conventional steel ingots.

Furthermore, as shown in FIG. 5, the inclusion cleanliness of the bottom part of the steel ingot and the upper part of the steel ingot main body is improved, and both the positive and negative segregation of C is better than that of the conventional steel ingot.

Note that the conventional steel ingot is produced by melting in an electric furnace and forming ingots in a vacuum tank.

The molten steel composition (element content weight %) of this steel ingot is as follows. This steel ingot has a unit weight of 60 TON, and has a molten steel composition similar to the EST steel ingot shown as an example, and a steel ingot unit weight of E.
The results of a cutting investigation similar to the ST steel ingot are shown for comparison with the EST steel ingot.

Furthermore, it can be seen from the sulfur print in Figure 4 that no concentration of S is observed in the steel ingot feeder section, which is an indication of the effect of the EST treatment.

The difference in the appearance of Δ segregation in the sulfur prints of the carbon steel in Figure 2 and the Cr-Mo-■ steel in Figure 4 is due to the difference in the composition system of the steel ingot,
It is well known that V steel ingots have less Δ segregation than carbon steel.

However, while component defects due to △ segregation have been found in turbine rotors made of conventional steel ingots such as CrMo'V, the rotor subjected to the EST treatment of the present invention has no product defects due to steel ingot defects. (It has been confirmed with a practical rotor that this is not allowed.)

As described above, the present invention is an industrially extremely ideal method for supplying high-grade clean steel ingots, and is particularly useful for manufacturing steam turbine rotors, rotor shafts for generators, etc., which have strict characteristics requirements.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the principle of the BEST method (B6hler Electr. Slag Toppig method). FIG. 2 is a diagram showing a sulfur print of a longitudinal section of an EST steel ingot. Figure 3 A2 is a diagram showing the inclusion cleanliness in the height direction on the axis of the steel ingot. FIG. 4 is a diagram showing a sulfur print of a steel ingot obtained according to the present invention. Figure 5 A2 is a comparison diagram of the quality and properties of the steel ingot according to the present invention and the steel ingot produced by the conventional method. 1...Surface plate, 2...Mold, total...
・Riser mold, 4...Power source, 5...Electrode, 6...Slag, 7...Steel ingot, A...
...Comparative steel ingot (conventional method), B...EST steel ingot (invention).

Claims (1)

[Scope of Claims] 1. After the molten electric furnace reduction stage slag remaining in the ladle after the completion of the ingot-making operation is transferred onto the molten steel in the riser frame using an appropriate means, no slag-forming agent is added or the slag is added as appropriate. The slag composition is adjusted to a Cab/Si02 weight ratio of 1.5 to 1.5 by adding an amount of sludge forming agent.
5.0, (CaO+Mg0)/(5i02+Al203
) 10-301CaF2<20% by weight by weight, and (
T, Fe + T, Mn) <An electroslag hot top method characterized by maintaining the content at 2% by weight or less. However, (T, Fe) The total amount of iron (Fe) contained in the varnish slag, that is, the total amount of Fe in metallic iron and iron oxide (including Fed, Fe3O4, and Fe2O3) (T, Mn) Mn and Mn
Total amount of Mn in the compound.