JPH01246316A - Method for decarbonizing molten base steel for stainless steel under reduced pressure - Google Patents

Abstract

PURPOSE:To effectively improve decarbonizing efficiency in low carbon range without any oxidized loss of Cr by blowing low melting point slag composition powder into molten base steel for Cr-contained stainless steel by using oxygen-contained gas under reduced pressure. CONSTITUTION:In RH degassing apparatus, at the time of decarbonization treating the molten base steel for the stainless steel under reduced pressure atmosphere, the low melting point slag composition is blown into the molten steel 3 from tuyere 2 arranged at below the molten steel 3 surface in the reduced pressure vessel 1 by using O2-contained gas as carrier gas. By this method, in case the molten steel 3 is the low carbon stainless steel, Cr in the molten steel 3 is oxidized with O2 gas to form solid-state Cr2O3 just above tuyere 2. This Cr2O3 grain is absorbed and melted into the slag drip formed, by which the low melting point slag powder introduced in the molten steel 3 at the same time is melted, and shifted in the molten steel 3, to expose to the reduced pressure atmosphere. Then, as reaction velocity of O in liquid Cr2O3 with C in the molten steel 3 is extremely quick, the reduction of Cr2O3 with C is perfectly progressed during short time, which the slag drip stays in the molten steel 3.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for decarburizing stainless steel mother molten steel under reduced pressure, and particularly to an advantageous improvement in decarburization efficiency even in a low order range without oxidation loss of chromium. This is what we are trying to achieve.

(Prior Art) As the usage conditions for steel materials become more severe, the demand for stainless steel with excellent corrosion resistance and high-temperature strength increases, and the development of a highly efficient and inexpensive stainless steel melting process is desired. This is because, for example, in super ferrite stainless steel, which has extremely excellent corrosion resistance, and Ti-containing stainless steel, which has excellent heat resistance, the C and N contents are 110% each.
Although it is required to reduce it to 0 ppm or less or 50 ppm or less, since it is a Cr-containing steel, the activity of C and N decreases. This is because the processing time is significantly extended, making it difficult to match with continuous casting, and productivity is greatly impaired. In addition, in the case of stainless steel decarburization, decarbonization
The loss of expensive alloying elements due to the oxidation of Cr by the oxygen source supplied for the purpose becomes a problem, especially in the extremely low C range.

VOD instead of highly productive RH degassing process
In some cases, a process that degasses down to extremely low C and N is adopted, but in order to increase the efficiency of the VOD process, a large amount of blown Ar gas is required, and the free board inside the ladle is used. As a result, the amount of molten steel that can be processed per unit device decreases, which does not lead to improved productivity.

Against this background, with the aim of improving the decarburization rate of stainless steel in vacuum degassing equipment, especially in RH, Japanese Patent Publication No. 12810/1987 proposes a method of spraying 02 gas onto the surface of molten steel in the RH decompression chamber. However, the special public service in 1982
Publication No. 49968 proposes a technique for improving the decarburization oxygen efficiency while suppressing Cr oxidation by injecting 02 gas or Ar-〇□ mixed gas from the surface of the molten steel bath in the RH pressure chamber. has been done.

However, both of the above two techniques are effective when the C concentration before vacuum decarburization is extremely high, 0.4 to 0.6%; ．．
When the odor was lowered to 0.005% or less, the same effect could not be obtained even if the odor remained in the low odor region. In other words, when the C content is large, C+0→CO(g
) The decarburization reaction in molten steel is low in oxygen content and becomes an oxygen supply rate-limiting reaction. C
Since the rate-determining process changes due to the diffusion of

In addition, especially in the case of Cr-containing steel, since the supplied 0□ is consumed for both Cr oxidation and C oxidation, the CrzO
C oxidation by =, i.e. CrzOz + 3 C → 2 Cr + 3 Co
Unless there is some way to make the reaction (1) proceed sufficiently, disadvantages such as a prolongation of treatment time due to a decrease in oxygen decarburization efficiency and oxidation loss of Cr cannot be avoided.

CrzO:+ alone is solid in the molten steel processing temperature range of about 1600 to 1750°C, and the reaction in equation (1) does not proceed sufficiently unless the amount of molten steel is extremely high, so the temperature is 0.05%. In the vacuum degassing treatment described below, the problem has always been that the decarburization oxygen efficiency is extremely low at 70% or less.

In the above-mentioned Japanese Patent Publication No. 56-49968, in order to prevent a decrease in decarburization oxygen efficiency and an increase in Cr loss, it is necessary to reduce the oxygen delivery rate and change the 0□/inert gas ratio ( Although it is stated that increasing the amount of inert gas is effective, this statement also indicates that decarburization and oxygen efficiency decreases in the low carbon range. In the method disclosed in the publication, the decarburization rate in the low coal region inevitably decreases as the 02 supply rate decreases.

(Problem B to be solved by the invention) As mentioned above, in the decarburization treatment of stainless steel mother molten steel according to the conventional method, there is a problem in that the low carbon region cannot be treated efficiently without oxidation loss of Cr. I had left it behind.

The purpose of the present invention is to advantageously solve the above-mentioned problems, and to propose a new method for decarburizing stainless steel mother molten steel, which can be efficiently decarburized even in the low carbon range and is therefore highly productive.

(Means for Solving the Problems) This invention shows that the decrease in the decarburization rate of stainless steel in the low carbon region is caused by the reduction of Crz (h) by C (oxidation of C, that is, the decarburization reaction ) is assumed to be caused by the phenomenon of deviation into the ladle slag without being sufficiently affected.
It was developed based on the technical idea of changing the generated Cr, O, into a form that is more susceptible to reduction by Q.

In other words, the reduction reaction due to the amount of solid Crt (h particles) in molten steel is extremely slow and rate-limited by the diffusion of oxygen atoms from the inside of the solid particle to the particle surface.
, 03 in liquid form, the oxygen supply rate from Crz(h becomes extremely high.

When we tried to evaluate the reaction rate, we found that k CrzOz (solid) / k CrzO, (l
iquid) Yu (D o ROL 1 d / Do
L 1 Q u 1 d ) I / Zoc ((1
0-”cIl+z/s)/(10-”cm”/s))
""=10-""Here, Do can be expected to increase the reaction rate by 1oS/Z times the diffusion rate constant of oxygen atoms.

In other words, this invention provides a method for decarburizing chromium-containing molten stainless steel in a reduced-pressure atmosphere. This is a method for decarburizing stainless steel mother molten steel under reduced pressure, which comprises blowing slag composition powder into molten steel using an oxygen-containing gas as a carrier gas.

This invention will be specifically explained below.

A specific method for converting Crz (1+), which is originally solid in the decarburization temperature range, into a liquid is to use a slag composition with a low melting point composition along with gas containing 0 This is done by introducing the .

FIG. 1 shows, in cross section, an RH degassing device suitable for use in practicing the present invention.

In the figure, numeral 1 is a vacuum chamber, 2 is a slat provided at the bottom of the side wall of the vacuum chamber 1, 3 is molten steel, and 4 is a powder storage hopper.

Now, in the above-mentioned RH degassing device,
When slag composition powder with a low melting point is injected into molten steel 3 using 2 gas as a carrier gas, when the molten steel 3 is a low-order stainless steel, Cr in the molten steel is oxidized by the 0□ gas,
Solid C1 (h is formed just above the tuyere, but Cr2O
At the same time, the particles are absorbed and dissolved in the slag droplets formed by melting the low-melting point slag powder introduced into the molten steel, so that they move in the molten steel as Cr403-containing sludder droplets and are exposed to reduced pressure. It will be.

By the way, as mentioned above, the reaction rate between 1 Ω- from liquid Cr20i and neutral molten steel is extremely fast, so that the reduction of Cr2O3 completely progresses during the short time that the slag droplets stay in the molten steel. That's what I do.

Therefore, according to the above technology, almost 100% decarburization oxygen efficiency can be maintained without reducing the introduction rate of 0□ gas, making it possible to significantly increase the decarburization rate at low carbon dioxide levels. Become.

Regarding the slag composition introduced with O2-containing gas,
It is more easily reducible than Cr2O5, such as FeO.
Furthermore, it is also undesirable to contain a large amount of fluoride such as CaFt from the viewpoint of erosion of the refractory. Considering that the vacuum decarburization treatment temperature is 1,600 to 1,750 °C based on the preferential decarburization conditions determined thermodynamically, it is assumed that the melting point is about 1,600 °C or less and is more stable than Crz (h). Preferably, such a slag composition is Ca0-A l 203. Ca
O-5iOt, Ca0-AlzO+-5iOt systems, etc. are suitable.

Note that it is also possible to employ a slag system containing CaF as long as the content is not too large.

In addition, the supply rate of the slag composition powder relative to the 0□ gas supply rate is 0.5% of the slag composition powder immediately above the tuyere. The (1:r
,0. It is desirable that the concentration is at most 40%, preferably 20% or less.

If expressed numerically, it is as follows.

According to the inventors' research on this point, the slag powder supply rate does not necessarily have to satisfy the above formula;
It turned out to be somewhat small (but not problematic).
□It is not necessary for all of the Cr2O3 formed by the gas to be incorporated into the slag droplets; it is sufficient to have a sufficient effect even if some of it becomes CrJ3-containing sludder droplets and contributes to decarburization. ing.

Therefore, the conditional expression for the supply rate of the slag composition powder can be defined as follows.

Here, 0.2〈η≦1.0 (Example) Using the RH degassing equipment shown in Figure 1, RH decarburization treatment of 18% Cr stainless steel mother molten steel in 100 tons of molten steel was carried out. went. The molten steel before decarburization treatment was decarburized and refined using a top-bottom blowing converter to reduce the C concentration to 0.04%.

Now, to the above molten steel, oxygen gas was supplied at a rate of 30 ONm'/h from the inner tube with a double tube provided at the bottom of the vacuum chamber, and at the same time oxygen gas was supplied to the molten steel at a rate of 30 ONm'/h.
:1 composition slag composition powder (particle size: 150 μm or less)
was sent. Note that static gas for cooling the tuyere was blown from the outer tube of the tuyere at a rate of 2 ON m'/h.

Changes in C, Mn, and Cr content before and after treatment are
The decarburization time and Cr and Mn losses during treatment are also shown.

For comparison, the investigation results obtained when 0□ gas was simply blown according to the conventional method are also shown.

As is clear from the table, in the comparative example without slag powder injection, the amount of blown oxygen lost as CrzO is extremely large, and the amount of oxygen contributing to decarburization is small, so the required decarburization is The time is extremely long. This phenomenon is more noticeable when processing is aimed at low-intensity ranges, and as shown in Table 1, when attempting to lower the post-processing C from o, oos to 0.006% by just 0.002%, This indicates that most of the oxygen supplied during that time was wasted in oxidizing Cr.

In addition, in the comparative example shown in Table 1, an increase in the molten steel temperature due to heat generation due to excessive Cr oxidation was observed at an average rate of 2 to 3°C/sin, and this tendency was particularly noticeable in the low temperature range at the end of the process, making it difficult to operate. This was also a problem.

On the other hand, in the slag powder injection method according to the present invention, the loss of Cr as CrzO3 is extremely small (indicating that the supplied oxygen gas contributes to decarburization after forming Cr2O3). ing.

After melting 26% Cr and 0.1% C super ferritic stainless steel in a top-bottom blowing converter, the steel was tapped into a 100 mm ladle, and the steel was melted using the equipment shown in Figure 1. and subjected to RH decarburization treatment.

In this example, the Cr concentration was higher than in Example 1, and since preferential decarburization was thought to be more difficult, the 0□ blowing rate from the double-pipe feather inner tube was set to 20ONm3/h.

0 again! 40% Ca as slag powder injected with gas
O-40%5int-IQ%/1x(h-io%ca
The one made by Fz&ll was used.

Table 2 shows the component fluctuations before and after the treatment, along with the decarburization time and Cr loss during the treatment.

As is clear from the same table, according to the present invention, C5, which could previously only be produced by a long period of strong stirring VOO treatment,
0.003% super ferritic steel could be produced by R1 (processing). Also, the processing time was short and it was possible to fully transfer to continuous casting, making a large contribution to improving productivity.

As is clear from the above two examples, slag powder with a low melting point composition is blown in together with O1-containing gas under reduced pressure, and Crz-containing
The effect of promoting decarburization through the reaction between the Oz slag melt and C in the molten steel was overall excellent. In addition, although the example shows the case of pure 0□ gas as the Ot-containing gas,
Using a device like the one shown in Figure 2, the transportation of slag powder is carried out at A
A method may also be used in which the inert gas such as r is used and the gas is combined with the 02 gas. Furthermore, in the embodiments, explanation has been given only to a reflux type vacuum degassing device (R)l), but the present invention is not limited to only RH (vacuum degassing devices that do not involve reflux of molten steel, such as DH, VAD, VOD). The method can be similarly applied to processes such as the above, and the decarburization oxygen efficiency can be improved.

(Effects of the Invention) Thus, according to the present invention, high-speed processing for reducing the carbon content of stainless steel, which could not be achieved without a large loss of expensive Cr, can be effectively achieved while reducing Cr loss. Ultra-low carbon super ferritic steel with a Cr content of 20-30%, which could previously only be produced by vOD treatment with poor processing capacity or processing efficiency, can now be melted by short-time processing using RH, which has high productivity. Therefore, it not only improves the matching for continuous casting, but also greatly contributes to improving yield and productivity.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an RH degassing device suitable for use in carrying out the present invention, and FIG. 2 is a schematic diagram of another preferred device. Fig. 1 1-Masu 2--H'mouth 3-;Condition 4--Is it one tooth? A water ti-

Claims (1)

[Scope of Claims] 1. When decarburizing stainless steel mother molten steel containing chromium in a reduced pressure atmosphere, from a tuyere provided below the bath surface of the molten steel in a reduced pressure vessel or a lance immersed below the bath surface, A method for decarburizing stainless steel base molten steel under reduced pressure, characterized by injecting low melting point slag composition powder into molten steel using an oxygen-containing gas as a carrier gas.