JPS5890360A - Preventing method for reoxidation and nitrogen absortion of molten steel for cntinuous casting - Google Patents

Abstract

PURPOSE:To suppress the reoxidation and nitrogen absorption of molten steel particularly in a non-steady stage and to produce ingots having high purity and high cleanliness in high yields by adding metals or alloys contg. Mg or Ca to a tundish for continuous casting. CONSTITUTION:The following operations are carried out in a non-steady stage, for example, prior to charging of molten steel after exchanging of a tundish. More specifically, gaseous Ar is supplied into the tundish preheated with a burner in an ordinary way to substitute the atmosphere therein with said gas as far as possible. In this state, metals or alloys contg. Mg or Ca are charged into the tundish, then the molten steel is charged therein. As a result, the O2 and N2 in the atmosphere near the molten steel in the tundish are decreased by the respective reactions expressed by formulas, whereby the reoxidation and nitrogen absorption of the molten steel are prevented.

Description

[Detailed description of the invention] Non-inventive continuous casting process: suppressing re-oxidation and a theft of molten steel in the +411K unsteady part to produce high-purity, high-clean slabs at a high yield. Regarding the method for.

In recent years, advances in S-steel refining technology have made it possible to make the molten steel in the composter highly pure and clean before it is poured. On the other hand, even in continuous casting, advances have been made in technology to prevent contact between molten steel and the atmosphere, such as through the use of thorough mud seals and the use of long nozzles and immersion nozzles, so that nitrification during air oxidation of HA steel can be ignored under steady injection conditions. It has become possible to suppress the nitrogen content to about K, making it possible to produce slabs with low nitrogen content and few oxide inclusions. This is because in a steady state, the falling flow of molten steel can be separated from the outside air by a V-ring nozzle or an immersion nozzle, and the molten steel surface in the rust layer during tandying is covered with 7 lux. This is because the amount of oxygen and nitrogen supplied to the vicinity of the molten steel surface can be limited to a small amount.A small amount of oxide inclusions has the effect of preventing porosity in the steel material and improving mechanical properties. On the other hand, the low nitrogen content is effectively used to reduce aging and improve mechanical properties such as softening.

However, one issue that remains unresolved at this point is the unsteady region of continuous casting, that is, the initial stage of injection when the tandyer is replaced and the molten steel is replaced during the run, and the flow of molten steel during the initial run when the ladle is changed to the ladle warehouse. There is re-oxidation and nitrification. As a result, the molten steel in the tundice is contaminated, and the components and properties of the slab are correspondingly inferior to those in the steady portion. If there are strict requirements regarding the inclusion content or Fi-threatening content of the steel material, the presence of this unsteady portion will lead to a decrease in yield. In particular, recently, as the capacity of Tandijio has increased, the range of influence that reoxidation and nitrification have on steel slabs in the unsteady region is increasing.

The above problem is that in the unsteady continuous casting region, direct contact between the molten steel surface and the atmosphere cannot be avoided by coating the molten steel with the sludder coating as in the steady region, and the atmosphere in the cast iron is kept as low as possible.
False, N:% that can be lowered even if you try to replace it with gas
Since there is a limit to the amount of molten steel, a reaction between the molten steel and 01 and N3 in the atmosphere inevitably occurs. and fLK versus Nl s o in the atmosphere.

Although the conventional idea is that NhO is reduced by dilution with a gas such as Ar, it is also possible to consciously supply a substance that reacts with NhO, thereby preventing the reaction with molten steel. No suitable method has been found that is sufficiently effective in trenches and has no side effects. For example, supplying hydrocarbon gas can lower the OsN in the atmosphere and reduce reoxidation of molten steel. However, as a side effect, the molten steel absorbs hydrogen, which poses a problem. Maku, N.
, I am speechless about it. Adding carbon powder or the like helps to reduce the temperature to 0.000 g, but it poses the problem of carburization of molten steel and is powerless against nitrogen. Other substances that evaporate when in contact with molten steel, such as soda compounds and carbonates.

It is also possible to charge P-lye ice into the tundish, but both will stably prevent both re-oxidation and nitrogen absorption, and will have a negative impact on the molten steel composition! None of them had any negative impact on tundish refractories or the working environment.

In view of the above-mentioned circumstances, this was obtained as a result of an experimental study conducted from the viewpoint of preventing the reaction between molten steel and gas in the unsteady part of continuous casting), and its characteristics are as follows. This is a spinning method for reoxidation and nitrogen absorption of molten steel for continuous casting in which a metal or alloy containing Mg or Oa is added to the tundish for continuous casting.

Hereinafter, a detailed explanation will be given based on specific examples.

In the continuous casting method of the present invention, it is applied to a device for supplying molten steel to a mold, generally a tundish.

First, we will discuss the work procedure after replacing the tundish and before starting injection. The tundish is preheated with a burner as usual, and when the injection start time approaches, KAr gas is supplied inside the tundish to replace the atmosphere in the furnace as much as possible, and immediately before injection, the KMg inside the tundish is heated.
Alternatively, a metal or alloy containing hO plating metal is added. When molten steel is injected in that state, Mg (solid), +jos→MgO...(1) 3Mg
(Solid) + Ns-+Mg1N, -(2) Mg (solid) 1Mg (vapor)... (3) Mg (recruited air) + 2 false → M
gO...(4) 3Mg (steam) + N snow → Mgs
Ns...(5)' Each reaction occurs. In addition o
If l', 4 ago! Reactions of the same type as e & (1) to (5) occur. As a result, the atmosphere near the molten steel in the tandisier was reduced to zero.
．． , N, *reduced: Re-oxidation and nitrification of m steel are prevented.

This method stably prevents re-oxidation and nitrification.
must consider the following conditions:

(a) The sum of Mg+Oa is on BOXO.

In order to prevent re-oxidation and nitrification of molten steel, the above formula (1)
l (not only the 2-O reaction but also the above formula (4) * (5
) in the atmosphere in contact with the exposed molten steel surface.
1+N! % must be sufficiently reduced. Ninthly, Mx (or O
It is necessary that the evaporation rate of s) is above a limit value,
Furthermore, 6 cases had Mg+Oa≧30X (in terms of weight).

Pure ML or pure 04 can be used, but any alloy can be used.
Almost the same effect can be expected if the condition of Mg 10m≧30N-1 (weight x) is satisfied. However, if 9 alloys or composite agents are used, they may cause undesirable changes in the molten steel composition and are not common.
Targeting materials close to pure metals, M is particularly inexpensive in terms of availability.
g What to say about K.

(b) Adding an amount of Mg that is more than the minimum required amount depending on the average air residual rate in the tundish.

Figure 1 shows the average residual air rate (Ox4 +
It shows how the relationship between the amount of Mg added and the amount of Mg added affects the reoxidation and nitrogen absorption amount of molten steel. The amount of reoxidation is calculated from the decrease in deoxidizing components in molten steel and the increase in oxygen content.As the amount of 0Mg added increases, the amount of reoxidation in molten steel decreases monotonically, but on the other hand, the amount of nitrogen absorption decreases. It was observed that unless the amount of Mg added exceeds a certain value, the value decreases by 1, or in some cases it actually increases compared to when FiMg is not used. This is because, despite a decrease in the nitrogen partial pressure (P) of the surrounding atmosphere (Mg (solid or vapor) + 10 rich → Mg0), the amount of nitrogen adsorbed does not decrease, but rather increases (under the same injection conditions, the amount of nitrogen adsorbed increases). The amount of nitrogen is approximately proportional to ρ・kK).

From Figure 1, it can be seen that in order to stably prevent reoxidation, it is desirable that the horizontal axis in Figure 1 be 40 or more, and in order to stably prevent reoxidation and nitrogen absorption, it is desirable that it be 80 or more. . It was found that when 5M11 alloy or Mg alloy (K Oa) is used, almost the same results as in Fig. 1 can be obtained if the atomic weight is corrected for the Mg alloy amount (i.e., MK gold alloy). For alloys, as Mg weight,
If Os is used, the weight per work shall be 1.7 times the Mg weight)
.

@ Make the size distribution of Mg appropriate.

To prevent re-oxidation and nitrification of molten steel, the unsteady time limit Mg must effectively reduce the atmospheric O, N, *11! The ease with which the steel reacts with 01*Nl in the atmosphere is intense at the beginning of injection, and gradually shifts to a steady state. While the steady state continues, the atmosphere inside the tandy is kept at low false and low N.

Therefore, a ninth test was conducted by changing the size structure of the metal Mg added to the injection direct IIjK. That is.

According to the results of mixing 1113~7mm average diameter and 15~25mm in various ratios, as shown in Figure 2, 15~25mm has the most stable reoxidation when 30~7ON. - The effect of preventing nitrogen absorption was obtained. (In addition, the relative value 1 in the figure corresponds to the case using the conventional method) Furthermore, Mg
It is also possible to add additional input. in this way,
Immediately before (within 3 minutes) before starting injection when replacing the tundish, add a predetermined amount of metal or alloy containing MgTo or 01 to the inside of the K tundish (preferably within about 1111 degrees from the drop point of the injection stream). By adding it, re-oxidation and nitrogen absorption can be stably prevented.

It should be noted that hydrogen absorption in the molten steel at the initial stage of injection is mainly caused by water vapor generated from the refractories of the tendice. On the other hand, if 4Mg'', Os metal or metal alloy is added, the amount of hydrogen absorption can be suppressed to about >S degrees.

Next, we will discuss the case where only the electrical controller is changed while the tundish is filled with molten steel. In this case %, usually
After the pinning nozzle is turned back and injection is started, the long nozzle is immersed, so re-oxidation or nitrification of the molten steel occurs at the same time as the rolling of the slab.

In this case, addition of a metal or mineral alloy of MII% Oa is also effective. The method of addition is to the place where the injection stream falls.

There are methods such as adding a bag of 9Mg and 01 metals or alloys before the start of injection, and a method of blowing gas onto the place where the injection stream falls. In the former case, the slag is pushed away by the force of the evaporating gas, causing the KIF steel to fall, thereby suppressing the 01wN1 reaction and preventing the slag from being rolled up.

Figure 1IE3 shows the effect of M on reoxidation and absorption i1 of molten steel.
An example of the influence of gO particle size is shown. In this case Kn3~7■
A stable effect was obtained when Mg particles accounted for 70% or more.a If the Mg grains are too large, the evaporation rate is slow and sMK is rolled into the molten steel, causing spitustein 10 in the molten steel, which is undesirable. The same thing is true when spraying with mulch gas.

It should be noted that the possibility that the 9M and medium Oa used in this treatment will produce harmful oxides and electrified substances in the molten steel is extremely low, because the molten steel targeted in the single treatment is generally deoxidized steel. Therefore, the oxygen content in the molten steel is low (therefore, no deoxidation products are newly generated in the molten steel), and the purpose of Mg or Otori is to alloy it with the molten steel. , Adopts an addition method that reacts with Ns to form oxides and nitrides + Oxides and nitrides such as Mg are rolled into the molten steel. This is because there are circumstances such as decomposition at the molten steel temperature even if it is molten steel. Furthermore, since 5Ml1 and Oa reduce the slab and increase its viscosity, it also has the secondary effect of preventing re-oxidation of molten steel by slag. Further %MgT
The reaction product with the atmosphere of O or Oa does not cause undesirable effects such as lowering the melting point during the reaction with the mendish refractory, and therefore does not cause inclusions caused by the tundish refractory.

Example (1) 3501 low carbon M-killed steel (0:α
05X, At: 0.02X, N: 14ppm, 0: 1
2 ppm) was preheated to an internal temperature of 1200°C and 9% M
When injecting through a long nozzle into a nine-toned steel lined with HO (inner volume: 2 d, steady molten steel amount: 40 tons),
The average gas composition in the tundish is Murnia 8Xs (%:
4%, "!: 17%", then the metal or alloy containing Type 3 II C) Mg or 01 shown in Table 1 was introduced, and after 1.5 minutes the injection was carried out. started.

Table 1 Metal islands or alloys are 15earO from the falling position of the molten steel flow.
I threw it in.

O average for the first part of slab O 3QtK.

The amount of inclusions, the amount of oxygen, the amount of nitrogen, and the amount of hydrogen absorbed were as shown in Table 2. It was found that the addition of 0Mg or IdOa significantly suppressed reoxidation and nitrogen absorption of s11 hydrogen in the unsteady region. Recognize.

Table 2 (no) Regarding the same molten steel as in Example (1), one minute before starting the injection after changing the % smoothness in the same tundish, Mg (particle size α3~0.7■, 2kt) packed in a paper bag was used.
) was put into the place L where the injection stream falls (Case D).Separately, a 0. '3
~+17 ■Q:) Spraying Mg from 1 minute before the start of injection to 1 minute after the start of injection is 9 (case 1) @ in each case,
The average amount of inclusions, oxygen amount, pot background, and absorption amount of 5 tons of slabs after restarting injection are as shown in Table 3.Table 3 In any case, Ml addition is higher than in the conventional method. The re-oxidation and absorption-IIVG inhibitory effects have been confirmed.

As described above, by applying the present invention, it is possible to reliably prevent re-oxidation and nitrification of molten steel in the unsteady state of continuous casting, and efficiently produce high-purity, high-clean molten steel in a composter. , can be tied to highly clean cast slabs (becomes,
It has great industrial effects.

[Brief explanation of drawings]

FIG. 1 is a chart showing the influence of the relative relationship between the average air residual rate in the tundish and the amount of Ml added in the tundish on the amount of reoxidation and nitrogen absorption of molten steel at the start of injection when the tundish is changed. A chart showing the influence of the particle size structure of metallic Mg on the amount of reoxidation and the amount of nitrogen absorption of molten steel at the start of injection when the second rl JFi tundish is changed. The third wJ is a chart showing the influence of the particle size structure of metallic Mg on the amount of reoxidation and the amount of nitrogen absorption in molten steel at the start of injection when the pot is changed. Agent: Patent attorney Masamitsu Akizawa and 2 others (MiN entry@zero"lJ'Mv'(Nz+(h)%)-(
Quantity content atS〕0 20 40 60
110 100 gold AMl Nou1715-JOtnm
ey> ttyr proportion c weight t%)

Claims (1)

[Claims] (1) SMM or O in tandem for continuous casting
A method for preventing re-oxidation and nitrogen absorption in molten steel for continuous casting, characterized by adding a metal or alloy containing a.