JPS61159506A - Slag for dephosphorizing molten iron - Google Patents

Abstract

PURPOSE:To manufacture easily a steel material having a very low concn. of P at a low cost by blending CaO-CaF2-SiO2 slag having a specified composition with a proper amount of Na2O and by adding the resulting slag to molten iron to dephosphorize highly the molten iron at a low consumption unit of slag. CONSTITUTION:CaO-CaF2-SiO2 slag having a composition consisting of >=7wt% SiO2, 10-42wt% CaF2 and the balance CaO and satisfying equations CaO<=3.17.SiO2+1.7.CaF2 and CaO/SiO2>=2.1 is blended with <=3wt% Na2O to obtain dephosphorizing slag having high dephosphorizing power, reducing the consumption unit of slag and enabling reduction in the cost of dephosphorization. Molten iron is dephosphorized by adding the dephosphorizing slag, and a steel material having a very low concn. of P is easily obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a dephosphorizing slag that is added to molten iron such as hot metal or molten steel to dephosphorize the molten iron.

[Conventional technology] Dephosphorization of molten iron is performed by dephosphorizing molten iron while it is being transported from a blast furnace to a converter.
This is done by adding dephosphorization slag to molten steel that has been decarburized and refined in a converter, or to molten steel in a ladle that has been tapped from a converter.

In this case, in order to efficiently advance the dephosphorization reaction and obtain molten iron with a low phosphorus concentration, the selection of the dephosphorization slag is important.

The higher the dephosphorizing ability of slag, the lower the phosphorus concentration of molten iron can be reduced with a lower unit of slag addition. Conventional dephosphorization slag mainly contains CaO-3 as added in the converter.
In2-FeO-based slag, cao-CaF2-31o2-FeO-based slag and Na2O-SiO2-based slag are added to hot metal and molten steel.

The dephosphorization abilities of these slags are shown in Table 1 below.

In Table 1, slag A is used for hot metal processing, and N a 2 O
- S i OZ system or CaO-CaF2-3iO2-
Slag B is a FeO-based slag, which is added in a converter, and is a CaO-31o2-FeO-based slag, and slag C is used for molten steel processing, and is a CaO-CaF-5in.
It is a 2-FeO-based slag.

However, the phosphorus distribution is the ratio of the phosphorus concentration in the slag after dephosphorization (P) to the phosphorus concentration in the hot metal [P] (P)/CPI.

[Problems to be Solved by the Invention] As described above, in conventional slag, even when hot metal is added, which is advantageous for dephosphorization and allows for low-temperature treatment, the phosphorus content is at most l 000, and the phosphorus content is at most l 000, For this reason, the slag consumption rate to reduce the phosphorus concentration to the required value is high, and the dephosphorization cost is high, and there is a limit to the increase in the male loading of the slab due to operational reasons. . Therefore, even if it is attempted to obtain molten iron with an extremely low phosphorus concentration, there is a limit to the phosphorus concentration that can be lowered by dephosphorization treatment.

This invention was made in view of the above circumstances, and has a high dephosphorization ability, can reduce the slag consumption rate and the dephosphorization cost, and can easily produce steel materials with extremely low phosphorus concentration. The purpose of the present invention is to provide a molten iron dephosphorization slag that can be produced in the following manner.

[Means for Solving the Problems] The molten iron dephosphorization slag according to the present invention is a molten iron dephosphorization slag that is added to molten iron to dephosphorize the molten iron. It is characterized in that the 3iO-based slag contains Na2O in a proportion of 3% or less.

(S iO2)≧7 10≦(CaF2)≦42 (Cao)≦3, 1 7 fist (S
i O2)'+ 1.7 ・(Ca F 2 ) (Ca O) / (S i O2) ≧ 2.1 set, (
) indicates the concentration (weight 96) of the component, (Cao) + (C\F ) + (S i O2
)=100.

[Example] The present invention will be specifically described below with reference to the attached drawings. The dephosphorization slag according to this invention is quicklime (CaO
), calcium fluoride CCaF2) and silica (SiO
This slag is based on a ternary slag consisting of 7) and contains a small amount of sodium oxide (Na 2 O ).

The inventors of the present application have conducted various experimental studies in order to develop a slag with high dephosphorization ability, and as a result, the CaO-CaF-
In S t O2 type slag, Trical? It has been found that slag whose composition is adjusted so that sium silicate (3CaO.SiO2) and quicklime coexist has a high dephosphorizing ability. Furthermore, when such slag contains sodium oxide, the dephosphorization ability is further increased. Moreover, iron oxide (F e O) may be contained in order to increase the oxygen potential, if necessary.

The present invention has been made based on such knowledge, and the composition of added slag is controlled so that it falls within the composition range specified below at the dephosphorization treatment temperature of hot metal or molten steel. First, the slag used in the dephosphorization treatment of molten steel will be explained based on FIG. 1, which is a phase diagram of CaO-CaF-S i O2 ternary slag. Since the slag needs to be melted at the dephosphorization treatment temperature, the melting point of CaO-CaF2-3iO2') ternary slag is 200℃ higher than the treatment temperature and 200℃ lower than the treatment temperature. It is necessary that the composition be between the two. In the dephosphorization treatment of molten steel, the treatment temperature is usually 1575°C, so the above composition has a liquidus line of 1775°C and 1375°C (175°C, respectively) in Fig. 1.
This is a region sandwiched between 0°C and 1350°C (near the liquidus line).

A region where the liquidus line is lower than 1375°C is preferable because the slag has a composition that easily melts, but it has the disadvantage that the concentration of CaF2 increases, which increases the erosion of the refractory and lowers the dephosphorization ability. . Therefore, in this invention, the melting point is 1375°C or higher, that is, 200°C lower than the processing temperature.
Control the slag composition so that the temperature is above low. Furthermore, if the melting point is higher than the processing temperature, it is thought that the flux will not melt, but the dephosphorization slag according to the present invention also contains Na2O in addition to these three components. j2-5 Even in the liquidus region of 1775° C. in the phase diagram of iO2-based slag, the slag actually used is in a molten state. Therefore, in the present invention, the slag composition of the ternary slag is set to a region having a melting point of 200° C. higher than the processing temperature.

In this invention, since a slab in the coexistence region of tricalcium silicate and quicklime is used, 2CaO・S
The region where iO2 exists is outside the composition range of the dephosphorization slag according to the present invention.

In the region where CaO exists alone, the composition range is determined as follows. In other words, 200°C higher than the processing temperature
Point X, which is a region sandwiched between liquidus lines of high temperature and 200° C. low temperature, where these liquidus lines intersect with the boundary line between the CaO single region and the tricalcium silicate/quicklime coexistence region.

This is an area partitioned by a straight line connecting points X and Y, where the proportion of Ca F 2 is 10% higher, along these liquidus lines. In order to increase the dephosphorization ability, it is desirable to increase the activity of CaO, but increasing the concentration of CaO
The concentration of F2 also increases, which is undesirable, so Ca
The m composition range of the dephosphorization slag according to the present invention is defined as the range in which the F 2 concentration increases by 10%.

The composition range of the slag determined as described above is the area shown by diagonal lines in FIG. In other words.

The relative proportions of Cab, CaF and S*02 are determined so that they fall within the above composition range, and based on this slab, Na2O is contained in a proportion of 3% by weight or less.
The higher the Na2O, the higher the phosphorus distribution value, but the NaO
The concentration of Na2O is set at 3% because of the high cost of refractories, the high content of Na2O, the severe melting and loss of refractories, and the tendency to generate dust.
Set as below. If necessary, FeO may be contained in order to maintain the oxygen potential.

In FIG. 2, the composition range of dephosphorization slag to be added to hot metal is shown in the shaded area. The method for determining the 0 composition range is the same as that for slag to be added to molten steel. When dephosphorizing hot metal, the treatment temperature is usually as low as 1300°C, so the composition range is lower than when dephosphorizing molten steel.
The ratio of F2 is moving to the higher side.

As described above, the optimal slag composition range for dephosphorization differs depending on the processing temperature, and the composition range (excluding Na2O) that includes the cases of molten steel processing and hot metal processing is CaO-
The poor condition of the Ca F -S t O2 type slag is shown in the shaded area. This region is expressed by the following inequality.

(SiO7)≧7 1O≦(CaF2)≦42 (CaO)≦3, 17 ・(S i 02)+1.7
- (CaF2) (CaO) / (S t 02 ) ≧ 2.1 set,
() indicates the concentration (% by weight) of the component.

Also, (CaO)+ (CaF)+ (S 1o2
)=100.

Next, embodiments of the present invention will be specifically described. First, an example in which molten steel is subjected to dephosphorization treatment will be described.

After melting low carbon steel in a magnesia crucible in the atmosphere using a 5 kg high frequency furnace, a flux calculated to have the above composition was added at a ratio of 6 kg to the melt. Thereafter, the magnesia pipe was immersed in the molten steel, and argon gas was blown into the molten steel at a flow rate of 0.5 Nl/min to stir the molten steel and promote the dephosphorization reaction.

During the reaction period of about 20 minutes, samples were taken from the molten steel and slag and their compositions were analyzed. The results are shown in Tables 2 and 3.

Table 2 1113 As shown in Table 2 and Table 3, after adding blacks,
After 10 minutes, the phosphorus concentration in the molten steel became 1Oppm or less, the phosphorus content exceeded 1000, and the molten steel was dephosphorized with a high dephosphorization ability that could not be obtained with conventional dephosphorization slag.

Table 4 shows the phosphorus content of the slag according to the present invention controlled within the range shown in the shaded area of 1Vli (molten steel dephosphorization treatment), and the phosphorus content of the slab whose composition is outside the shaded area of Figure 1. The phosphorus content shown for comparison is the phosphorus content obtained 20 minutes after adding the dephosphorization slag. 1, ■, ■, and ■ indicate the compositions of the slags of Comparative Examples 1 and 2°3.4 in Table 4, respectively.

The composition of each slag is as shown in Table 5 below.

As is clear from this Table 4 and FIG.
2, 3, 4) even if they contain Na2O,
The slag of Comparative Example 1, which has an extremely lower phosphorus distribution than the slag according to the present invention, has a slightly higher phosphorus distribution than the slags of other comparative examples, but because of the high concentration of CaF2, the erosion of refractories is It's too intense to be practical.

On the other hand, FIG. 4 is a graph showing the phosphorus distribution and implantation Lp of the slag according to the present invention in comparison with the phosphorus distribution and implantation of the conventional dephosphorization slag. Since phosphorus distribution depends on temperature, the horizontal axis of the figure shows the temperature. In the figure, the broken line shows the CaO-CaF2 system, and the dashed line shows the Lp of the Na2O-SiO2 system slag, which is surrounded by the broken line. As is clear from FIG. 4, the slag according to the present invention has a phosphorus concentration several to several tens of times higher than that of conventional slag at all temperatures. It has distributed planting.

Next, an example in which hot metal is dephosphorized will be described. Table 6 shows the phosphorus distribution and implantation of the dephosphorization slag according to the present invention in comparison with the phosphorus distribution and implantation of the slag that deviates from the composition specified in the present invention, and Table 7 shows the composition of each slag.

Table 6 Table 7 The phosphorus content in Table 6 is the phosphorus content when 30 minutes have passed after addition of the flux, and the slag according to the present invention has an extremely high value of about 1000G. There is. In this manner, the slag according to the present invention provides an extremely high phosphorus content that cannot be obtained with conventional slags, regardless of the processing temperature.

[Effects of the Invention] As described above in detail, according to the present invention, since the dephosphorizing ability of slag is extremely high, the unit of addition of slag can be reduced, and the dephosphorization fust can be reduced. Further, according to the present invention, it is possible to easily produce a steel material with an extremely low phosphorus concentration that cannot be obtained with conventional dephosphorization slag.

[Brief explanation of the drawing]

1 to 3 are phase diagrams showing the composition range of the slag according to the present invention, and FIG. 4 is a graph diagram showing the effects of the present invention.

Claims (1)

[Claims] In the molten iron dephosphorization slag that is added to molten iron to dephosphorize the molten iron, CaO-CaF_ has a composition represented by the following inequality.
A molten iron dephosphorization slag characterized by containing 2-SiO_2-based slag and Na_2O in a proportion of 3% or less. (SiO_2)≧7 10≦(CaF_2)≦42 (CaO)≦3.17・(SiO_2) +1.7・(CaF_2) (CaO)/(SiO_2)≧2.1 Indicates the concentration (wt%), (CaO) + (CaF_2) + (SiO_2) = 100
and standardize it.