JPS6357714A - Manufacture of extra low p steel by refining - Google Patents

Abstract

PURPOSE:To safely and surely manufacture an extra low P steel in large quantities by discharging a molten steel satisfying prescribed conditions into a ladle whose slag line is made of a basic refractory and by carrying out slag removal and dephosphorization in the ladle. CONSTITUTION:A ladle whose slag line is made of a basic refractory so as to prevent Al2O3 from entering slag by leaching is prepd. as a ladle for dephosphorization. A molten steel contg. <=0.15wt% C is discharged into the ladle from a steel making furnace in an undeoxidized state and slag is removed by >=80%. A CaO-base flux is then added to the ladle to restrict the amount of Al2O3 to <=5% and the molten steel is dephosphorized. Slag produced by the dephosphorization is removed by >=80%.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is applicable to steel in which the P content ([P] amount) is approximately 10 ppm.
The present invention relates to a method for stably producing ultra-low P steel on an actual operational scale.

<Prior art and its problems> In recent years, the demand for steel materials with as low a P content as possible has been increasing, but conventionally, it has been difficult to melt steel with a reduced amount of [P] to the extent that it is called ultra-low Pli. The following manufacturing processes are adopted, including the steel tapping process and the molten steel film P process.
In the process, a ladle made of a refractory material mainly composed of Alto has generally been used as a reaction vessel. As a dephosphorizing agent, “CaOCaF z FeOFez
O3-based fluxes were commonly used.

However, the ultra-low P steel that can be stably produced by such conventional methods has a [P] content of less than 50 ppm at most, and with this method, there is almost no progress in removing P beyond that level. [P] amount: It is extremely difficult to stably melt steel with an amount of about IQ ppm on an actual operational scale.

<Means for solving the problem> Therefore, in order to meet the demand for ultra-low Pl, which has recently been increasing rapidly, the present inventors developed [Plffi 10pp]
Aiming to develop a method that can stably produce steel with a reduced Pw of around
When we conducted research to find out the reason why 4 was not stably obtained, we obtained the following findings (al~(gl). In other words, (al) A/ which has been commonly used as an agent
203 is a major factor that inhibits the dephosphorization reaction during dephosphorization.

Figure 1 shows 45Ca O-15Ca, which has the best P removal rate.
F t40FezOz flux with 8β20. P distribution between flux and metal ((P)/[
This study investigated changes in Pl). Furthermore, the fact that the above flux is excellent in removing r is that CaFz=15%
The flux composition and P between slag and metal regarding the flux when CaO and Fe2O3 are replaced with
It was found from the results of the distribution investigation (Figure 2) that the preferred range is CaO: 30 to 50% by weight, CaFz
: 15% by weight, remainder: The optimum value was selected from the composition of FezO3.

As is clear from this Figure 1, A i2 z O,
When 20% by weight of J is mixed, the P distribution decreases to about 1/3, which shows that Al2O2 in the slag must be suppressed as much as possible in order to sufficiently remove P in the molten steel dephosphorization process.

(b) There are three possible sources of A7!203 in slag: blast furnace slag, dephosphorization flux, and refractories in refining vessels. Since the slag is removed at the time of tapping the converter, the amount of intrusion is negligible, and the amount of intrusion from the P-free flux is as seen in JP-A-58-221218 when NazO3iOz-based flux is used. Although it may be necessary to set some restrictions, in the case of CaO-based fluxes, the amount is negligible, so AA! 03 The source and amount of intrusion is also troublesome because of the refractories in the smelting container, and AβZOX enters the slag.
In order to sufficiently prevent the contamination of the refining vessel, it is essential to change the material of the refractory material of the refining vessel.

That is, 81203 is CaO-CaF Z-FeO-Fe
It has a high solubility in zOi-based melts, and therefore, when injection of a dephosphorizing agent or gas bubbling is performed to promote the reaction between slag and metal, the Al2O2 in the refractory of the conventional refining vessel is is inevitably leached into the slag.

tc+ In addition, the amount of C in the steel before dephosphorization treatment, that is, the amount of [C], also affects the dephosphorization of molten steel.
In order to stably produce ultra-low Pl around pm [C]
It is necessary to keep the quantity low.

Figure 3 is a graph showing the relationship between the amount of [C] in molten steel before dephosphorization treatment and the dephosphorization rate.
At a high temperature of 1650℃, if the amount of [C] is high, [C + (Fed) = [Fel → -C]
O↑.

2 [P] ” 5 (Fed) ”' (PzOs
) + 5 [Decarburization represented by Fel and dephosphorization compete with each other, and the dephosphorization rate decreases as shown in FIG.

(d) Furthermore, when Iwasaki is deoxidized (when tapping steel from a steelmaking furnace), the re-P phenomenon appears markedly at the time of tapping, so [P
In order to stably produce ultra-low Pl after the l content reaches 10 ppmi, it is necessary to perform a dephosphorization treatment on undeoxidized molten steel.

Figure 4 shows the results of investigating the relationship between the [0] amount of molten steel tapped from a steelmaking furnace and the return P rate during tapping.
Normally, the amount of [0] is about 70.03 to 0.10% by weight.If the molten steel from the steelmaking furnace is deoxidized to reduce the amount of [0] to less than 0.03% by weight, the amount of regenerated P during tapping will be significant. It is clear that it will rise.

(e) [This was revealed by investigating the mass balance of P when melting ultra-low Pl with a P content of around 10 ppm, but the origin of most of the P that enters the steel is unknown. [In order to lower Pl ffl to the above level, it is necessary to perform thorough sludge removal treatment in a P removal treatment container (ladle) in addition to the above-mentioned measures.

(f) At this time, only one processing container (ladle) is used as a means for removing slag, and a slag dragger is used to remove phosphorus.
It is advantageous to adopt a method that removes most of the slag before and after dephosphorization.

Figure 5 shows the results of investigating the input balance of P depending on the slag removal method after P removal from the converter. )
When P is used, the amount of P mixed in from the adhering slag of the previous charge is doubled and introduced into the system, resulting in a large input of P of unknown origin at 67% and 49%, respectively, which is an obstacle to lowering the P content. On the other hand, if only a 250-ton ladle is used and most of the slag (more than 80%) is removed before and after P removal with a slag dragger, the P It can be seen that the unknown content in Imp 7) has decreased to 19%.

Figure 6 shows the sludge removal method after the converter departs from port and the entire ultra-low P steel melting process adopted in this study.

(g) In addition, in order to produce ultra-low PM with a [P] amount of around 10 ppm, it is necessary to reduce the [P] level in molten steel in advance from the viewpoint of material balance between slag and metal. Preferably, the [P] level of the molten steel supplied to the final deP step is preferably about 40 ppm or less.

FIG. 7 is a graph showing the relationship among P distribution, flux amount, input [P], and arrival [P]. In addition, the input [P] is 40 ppm and 20 ppm, and the P distribution is + (P) / [ when Al2O3 is mixed.
P] = 40) and the case where It z Ox in the slag is less than 5% ((P)/[P] = 100), respectively. Here, in order to stably achieve the attainment [P] < 10pp+n, it is necessary to aim for [P] = about 5 to 6 ppPa, but in the above case, (P) / [
If P] = 100, the reached [P] will be 9 ppm even if the flux amount is 40 Kg/T, so the input [P] is sufficiently low, that is, [P] < 10 ppm.
It is easy to understand that in order to obtain a steel of m, it is desirable that the [P] level of the molten steel supplied to the final deP step be 40 ppm or less.

This invention was made based on the above findings, and includes the following steps: From a steelmaking furnace, molten steel with a C content of less than 0.15% by weight is transferred to a ladle having at least a slag line made of a basic refractory. After tapping the steel in an undeoxidized state, 80% by weight or more of the slag is removed, and then quicklime-based flux is added to suppress J/l, o in the slag to 5.0% by weight or less, and P is removed. After that, by carrying out the operation of removing 80% by weight or more of the generated dephosphorized slag in the same ladle, it is possible to stably produce ultra-low P steel with a [P] level of around 10 ppm. It is characterized by the following points.

In addition, in the method of this invention, the C content in the molten steel to be treated is set to be less than 0.15% by weight because if the C content in the molten steel is 0.15% by weight or more, the dephosphorization rate will decrease. This is because the desired ultra-low P steel cannot be stably produced due to the low P steel, and the reason why undeoxidized molten steel is used is to suppress the re-P during tapping and produce ultra-low P steel as mentioned above. This is to ensure stable melting of.

In addition, the reason why the ladle for dephosphorization treatment is specified to have at least the slag line made of basic refractory material is to prevent Al2O3 from leaching into the slag from the refractory material of the ladle and lowering the dephosphorization rate. In order to prevent this, the slag line of the ladle is
g0-C, MgO・CrzO,.

This is because basic refractories mainly composed of CaO or the like have little effect on dephosphorization even if they elute. It also prevents as much as possible P mixing from the adhering slag of the previous charge to achieve the desired ultra-low p.
In order to achieve stable production of m, the number of ladle used was limited to one.

The reason why the amount of slag removed from the molten steel tapped from the steelmaking furnace is set to 80% by weight or more is because if the amount of slag removed is less than this, the input PJJ to the system will increase, resulting in the desired ultra-low P steel. This is because it cannot be done.

Now, if we calculate the P2O5 (i.e. (P2O3)) ratio in the slag and the amount of slag, we will get the result as shown in Table 1.As shown in Table 1, the slag before P removal If 80% by weight is removed at 50Kg/T (20% by weight remaining), 10Kg/T remains, and the input P to the system is 1 even considering only the slag (pzos).
．． 44 and 22 ppm for 0 and 0.5% respectively
As a result, ultra-low [P]m melting approaches the limit. Therefore, it is clear that the desired ultra-low PE cannot be obtained with a slag removal rate lower than this. Furthermore, if you leave reality, it will be 100%
It goes without saying that sludge removal of % is preferable.

The reason why A7!203 in the slag in Table 1 is particularly suppressed to 5.0% by weight or less is that if the value exceeds 5.0% by weight, the dephosphorization rate decreases and the desired ultra-low [P'Im] is still achieved. This is because it cannot be achieved stably.

As shown in Figure 1 above, the decrease in (P)/[Pi decreases linearly with the increase in the Aβ203 content in the slag, so ideally Al2O3 should be zero. is desirable. However, even if (AI!zOx) is set to 5.0% by weight or less, the P content in steel is reduced to 10p.
Because we can stably produce ultra-low [■]] steel around pm,
A7!203i in the slag was determined to be 5.0% by weight or less.

The reason why the dephosphorization slag generated in the dephosphorization process is removed at a ratio of 80% by weight or more is that in steel refining, reduction refining is performed at the end to add alloying elements and deoxidize. This is because if the ratio is less than 80% by weight, the amount of [P] increases, which becomes a major hindrance to achieving the desired ultra-low [p] steel.

Figure 8 shows the slag removal rate of P-free slag and [P] after reduction refining.
This is a graph showing the relationship with ffl, where the pre-depletion [P] is 20 ppm, the de-P4u [P] is 5 ppm, and (P)
/ [P] is 50 or 100 and the slag amount is 60Kg/
The data for T is also shown in Figure 8, when the slag removal rate = 80% by weight, (P)/[10.8 ppm for Pco50, (P)/'9 for P1 = 100.
．． It turns out that it is 3 ppm, which is just under 10 ppm. Therefore, in this case as well, it is necessary to have a slag removal rate of 80% by weight or more.

Next, the present invention will be explained by examples and in comparison with comparative examples.

〈Example〉 After depurating the hot metal to P = 0.015 to 0.025% by weight, quicklime was charged to a 250 ton converter at 35 to 50 h/T and [P] = 0. .06~0.0
Dephosphorized to 9% by weight and continued [C] = 0.03 to 0
．． 07% by weight, [0] = 0.04 to 0.08% by weight, was tapped into a ladle. In addition, CaO: 6Kg during tapping
/T, Scale: 4Kg/T and CaFz: 4
Steel tapping was performed at Kg/T.

Next, the composition is 45CaO15CaFz 40Fez○
The molten steel contained in a ladle was subjected to deP treatment in an LF furnace using a flux of 40 kg/T of No. 3, and further reductive deoxidation was performed.

The dephosphorization conditions at this time were as shown in Table 2.

In addition, in the LF furnace, the static gas is 0.005N r&/m1n-
The molten steel was subjected to pRn using T to promote dephosphorization.

Through the above treatment process, [P] of molten steel after being tapped from a converter
] The trends were investigated and the results are shown in Figure 9.

The results shown in FIG. 9 also show that in Inventive Example 1, the exposed steel film P
Later, [Pl = 49ppm was changed to LF deP
Since we were able to control (Al1203) to 0°7% by mass, it can be seen that we were able to achieve [Pl = 12 ppm after LF dephosphorization and [Pl = 14 ppm in the product].

In addition, in Example 2 of the present invention, [Pl = 20 ppm was obtained in the LF furnace strip in the final deP step, so in the LF furnace, [Pl = 5 ppm], and [P] = 61]I)
Each achieved Il+.

On the other hand, in Comparative Example 1 in which the refractory contained 82% Al1203, Comparative Example 2 in which the slag removal rate before dephosphorization was 60%, and Comparative Example 3 in which two ladles were used, the output was higher than in Example 1. Despite the low Pl level of the steel film, the Pl amount after dephosphorizing the molten steel in the LF furnace was more than 0.002% by weight, making it impossible to melt ultra-low P steel. As explained, according to this invention, the P content in the steel is extremely low, around 10 ppm! It has become possible to stably and reliably mass-produce l, and it has brought extremely useful effects industrially, such as being able to fully meet the growing demand for ultra-low P steel from various quarters.

[Brief explanation of drawings]

Figure 1 shows the amount of P mixed into the dephosphorized slag.
Figure 2 is a graph showing the relationship between the CaO content in the P removal flux and P distribution; C] A graph showing the relationship between I and the P removal rate. Figure 4 is a graph showing the relationship between the amount of [0] in molten steel and the amount of returned P at the time of steel tapping in a steelmaking furnace. Figure 5 is a graph showing the relationship between I and the P removal rate. A graph showing the input balance of P by method. Figure 6 is a schematic process diagram explaining the melting process and slag removal method for super-type P steel. Figure 7 is a diagram showing the P distribution, flux amount, and reached [P ] Figure 8 is a graph showing the relationship between slag removal rate and [P] amount after reduction refining; Figure 9 is a graph showing the relationship between slag removal rate and [P] amount after reduction refining; It is a graph showing the transition of [P].

Claims (2)

[Claims] (1) From a steelmaking furnace, molten steel with a C content of less than 0.15% by weight is tapped into a ladle in which at least the slag line is made of a basic refractory in an undeoxidized state.
After removing slag of more than 5% by weight and then adding quicklime-based flux to suppress Al_2O_3 in the slag to 5.0% by weight or less, dephosphorization is performed, and then 8% of the generated dephosphorized slag is
A method for producing ultra-low P steel, characterized in that an operation for removing 0% by weight or more of slag is carried out in the same ladle. (2) The method for producing ultra-low P steel according to claim 1, wherein the P level in the molten steel supplied to the final deP step is 40 ppm or less.