JPS582575B2 - Method for refining molten metal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a refining method for deoxidizing and/or desulfurizing molten metal.

It is well known to add refining flux to molten metal, such as molten metal, to effect deoxidation and desulfurization.

Various compositions of such fluxes are known, including CaO-Al2O3 as described in the present invention.
-CaF2-based fluxes are also known.

Further, as for the form of flux, powder mixed type and molten type are known, but each type has advantages and disadvantages.

For example, in methods such as injecting powder mixed type Franks into hot metal in a pig iron mixing car together with N2 gas, or methods in which injecting Ar gas into molten steel in a trap, etc., the refining effect is not so great because the powder turns into molten slag slowly. not high.

For this reason, the amount of CaO in the flux is lowered or more SiO2 or CaF2 is added to promote slag formation, but since CaO is essentially used in powder form, during mixing and storage, etc. CaO has a disadvantage in that it absorbs moisture and tends to increase the amount of hydrogen and oxygen in the steel.

Further, problems such as a decrease in the refining effect due to an increase in the amount of SiO2, and melting damage of the refractory wall of the ladle due to an increase in the amount of CaF2 occur.

On the other hand, molten flux has low hygroscopicity due to a lower ratio of CaO single phase, and since it has already been turned into slag, the refining effect itself is improved. The disadvantage is that the manufacturing cost increases.

The present invention has been made with attention to these circumstances, and the present invention is based on the method of processing melting etc. using a sintered flux.
This succeeded in establishing a processing method for a completely new concept.

The chemical component in the blacks is so-called CaO-Al
It is a 2O3-CaF2 system, and more specifically, C
A sintering mold configured such that the ratio represented by aO/Al2O3 falls within the range of 1 to 4, SiO2 is suppressed to 15% or less, and CaF2 satisfies the following conditions: 20≧CaF2 (%)≧0 The gist is that flux is added to molten metal for treatment.

The basic composition of the sintered flux is CaO-A.
The reason for selecting the 12O3-CaF2 system is as follows.

(1) CaO is a basic component that exhibits a strong desulfurization effect.

(2) For example, in Si-killed steel, (Si)+2
The reaction [O]→SiO2 progresses and SiO2 is produced.

However, CaO and Al2O3 have a strong affinity with Sin2 and exhibit a deoxidizing effect on Si-killed steel.

(3) CaO and A12O3 are stable even at high temperatures and do not decompose due to reaction with molten steel.

(4) If the proportion of CaO increases, the melting point as a flux increases, and there is a fear that the refining effect will decrease. Therefore, when the CaO content rate is high, the melting point is lowered by using CaF2.

Next, the reason for determining the blending ratio of each of these components as described above will be described.

When molten steel (5% Cr-0.5% Si steel) was deoxidized using sintered fluxes obtained by varying the ratio represented by CaO/A12O3 and the blended amount of SiO2, the results were as follows. The results shown in Figure 1 were obtained.

As is clear from the figure, regardless of CaO/Al2O3, the deoxidizing effect decreases when SiO2 exceeds 15%, and the upper limit of SiO2 was set at 15%.

Using the same flux as above, 5%Cr-0.5%Si
When steel was desulfurized, the results shown in Figure 2 were obtained.

The marks ◯, △, and ● in the figure have the same meanings as in Figure 1.

As seen in Figure 2, when CaO/Al2O3 is high, the desulfurization rate decreases rapidly after SiO2 exceeds 15%, and when CaO/A1203 is low, Si02
There is a nearly linear inverse proportional relationship between the content and the desulfurization rate, and in order to maintain a high desulfurization rate, it is desirable to reduce Si02.

In the end, even when the desulfurization rate is considered, the upper limit of SiO2 is 15%, and we came to the same conclusion as the above discussion regarding the deoxidation rate.

After various considerations based on these facts, we found that there is no particular restriction on the CaO/Al2O3 ratio when deoxidizing is the main purpose, but too much CaO will increase the melting point of the flux, so it is recommended to use less CaO. , i.e. CaO/Al2
It is recommended to reduce 03.

On the other hand, when desulfurization is the main purpose, CaO/Al2O
The larger the value of 3, the better the results.

However, when CaO/Al2O3 exceeds 4, CaO becomes excessive and the melting point rises rapidly, making it necessary to add a large amount of a melting point depressant such as CaF2.

However, if the amount of CaF2 added exceeds 20%, the erosion of the refractory wall becomes severe, so it is necessary to set the CaO/Al2O3 ratio such that the amount of CaF2 is less than 20%.As a result of various studies, this upper limit was set at 4. It was determined that

Regarding the lower limit, the CaO/Al2O3 ratio should be set at 1, since a decrease in CaO causes a decrease in deoxidation and desulfurization rates.
We have set the condition that the value shall not be lower than .

Next, we further investigated the content of CaF2 and found that the minimum necessary amount of CaF2 to suppress the rise in melting point caused by fluctuations in CaO/Al2O3 is given by the following formula.

Figure 3 is a graph that comprehensively shows the findings obtained above.
The shaded area in the figure indicates the condition range that satisfies all of the deoxidation rate, desulfurization rate, integrity of the firewall, etc.

There are no particular restrictions on other components in the flux according to the present invention, but lower oxides such as FeO and MnO are reduced by reaction with the molten steel and act to increase the oxygen concentration in the molten steel.

Therefore, it is recommended that consideration be given to the above-mentioned lower oxides so that their total sum is 1% or less.

Furthermore, there are no restrictions on the method for producing the sintered flux (sintering method) of the present invention.

When performing refining using these fluxes, the method of adding flux to the molten metal can be freely selected from known methods and methods that will be developed in the future. (1) N2 or Ar
A method of refining by injecting it into molten steel together with a carrier gas such as.

(2) When tapping steel from a converter, electric arc furnace, induction melting furnace, etc., a method of placing flux at the bottom of a ladle in advance and refining using stirring by the tapping flow.

(3) A method in which the flux is poured into a ladle after being tapped and refined under an atmospheric atmosphere, an inert atmosphere, or a reduced pressure atmosphere by adding gas injection stirring or stirring using electromagnetic force.

(4) Examples include a method of refining by adding flux to molten steel in an atmospheric induction melting furnace or a vacuum induction melting furnace.

Since the present invention is constructed as described above, the flux can be produced with a lower amount of heat than a molten flux, and the refining efficiency is extremely high compared to a powder-mixed flux.

Specifically, as will be clarified in the Examples below, the deoxidation rate and desulfurization rate are high, and the reaction rates of each are also fast.

Also, the number of large nonmetallic inclusions present in the molten steel was reduced, and the number of defects caused by the inclusions was reduced.

It is generally known that refining using flux increases the amount of hydrogen in molten steel, but since the sintered flux of the present invention has a reduced amount of adsorbed water, it is possible to keep the increase in hydrogen to a low level. I can do it.

Example 1 A sintered flux having the composition shown in Table 1 was manufactured.

When the mineral phase of Naomoto Blacks was identified using X-rays, it was found that 11
CaO・7Al2O3・CaF2, 3CaO・Al2O
3, β-2CaO・SiO2, SiO2・2Al2O3
, CaO, CaF2, etc.

On the other hand, low carbon Si-At killed steel (C: 0.10 to 0.15%) produced using a 60-ton converter, and medium carbon Si
-Al killed steel (C: ~0.45%) was placed in a ladle, and the above flux was blown into the ladle using Ar as a carrier gas.

As a comparative example, a conventional powder temperature table type flux (the composition of which is shown in Table 2) was similarly blown into the sample.

In each case, the amount of flux added was 300 kg per 86 tons of molten steel.

The results are shown in Table 3 and Figures 4-8.

The above table shows the oxygen concentration (%) in the sample sample taken from each part of the 110 medium billet, and the superiority of the deoxidation rate according to the present invention is clear.

FIG. 4 is a graph showing a comparison of ferrous steel medium (S) before and after flux injection, and it can be seen that the desulfurization rate of the present invention is extremely high compared to the case where mixed type fusix is used.

Figure 5 shows processing time after flux injection and molten steel [O]
The graph shown in FIG. 6 showing the change in amount over time is also a graph showing the change over time in [S], and the ○ and X marks in the figure have the same meanings as in FIG. 4.

As is clear from both graphs, the deoxidation rate and desulfurization rate are improved by using sintered Fusix.

Figures 7 and 8 are distribution diagrams of B-based inclusions observed in the billet in 110, Figure 7 is when a sintered type flux is used, Figure 8 is when a mixed type flux is used, As is clear from comparing these, the present invention has fewer large inclusions.

In addition, when we investigated the amount of (H) in molten steel, we found that (
The average increase in H) is 1.2p for mixed flux.
pm, and in the case of sintered flux it was 0.5 ppm.

Example 2 Sintered fluxes and mixed fluxes having the compositions shown in Table 4 were manufactured.

When medium-carbon Si-Al killed steel for mechanical structures was deoxidized in the same manner as described above, the results shown in Table 5 were obtained.

As shown in Table 5, the deoxidizing efficiency of the present invention was extremely good.

[Brief explanation of drawings]

Figures 1 and 2 are graphs showing the influence of the amount of SiO2, Figure 3 is a graph showing the relationship between CaO/Si02 and CaF2, and Figure 4 is a graph showing the relationship between CaO/Si02 and CaF2.
Figures 1 to 8 are graphs showing the difference between sintered flux and powder mixed flux.

Claims (1)

[Claims] 1. The ratio expressed by CaO/Al2Os satisfies the range of 1 to 4, and the SiO2 content is suppressed to 15% or less, and the two conditions expressed as 20≧CaF2 (%)≧0 are satisfied. 1. A method for refining molten metal, which comprises adding a sintered flux having a defined CaF2 content to molten metal.