JPS5921368B2 - Manufacturing method of high chromium steel by vacuum oxygen blowing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high chromium steel, and more specifically, in refining using a vacuum oxygen blowing method (hereinafter referred to as VOD method), the amount of oxidation of Cr, Mn, etc. throughout the entire process is determined. In order to suppress the amount of decarburization in preliminary refining, VO
The present invention relates to a method for manufacturing high chromium steel characterized by increasing the amount of decarburization in method D.

Generally, in order to suppress chromium oxidation in high-chromium molten metal (including hot metal and molten steel) and preferentially advance the decarburization reaction, oxygen is supplied into the molten metal while keeping the 00 partial pressure in the atmosphere low. This is necessary, and one known method is the VOD method using a pressure reduction means.

However, decarburization in the VOD method described above is particularly difficult in the carbon region (C
= 0.30 to 1.20%), the amount of oxygen supplied and the decarburization rate are almost proportional, so if the amount of oxygen supplied is increased in an attempt to increase the decarburization rate, the amount of exhaust gas will increase and the vacuum Due to the limited exhaust capacity of the system, the degree of vacuum deteriorates and induces chromium oxidation loss, and as the decarburization rate increases, the amount of CO gas generated from the molten metal increases, resulting in an increase in splash and Equipment damage accidents often occur due to boiling orders.

For this reason, in order to avoid the above-mentioned problems and maintain stable operation within a finite time cycle, the pre-refining step is carried out with oxygen blowing, and the [C] level before VOD treatment is reduced to 0.30.
A commonly used method is to reduce the load on the VOD process by adjusting the amount to about 0.50%, but as preliminary decarburization is performed under atmospheric pressure, oxidation of chromium in the molten metal is unavoidable, and the entire process is affected. The reality is that most of the yield loss, ie, recovery loss of Si, AI, Cr, and Mn for reduction, occurs in this preliminary decarburization step.

The present invention was made to solve the above-mentioned problems, and it reduces the amount of decarburization during preliminary decarburization as much as possible, and reduces the amount of VO
Chromium 10 reduces the amount of oxidation in the entire process by increasing the amount of decarburization in the D process, improving yield.
% or more, and will be described in detail below.

First, in carrying out the method of the present invention, in the preliminary decarburization step, as the [C] in the molten metal decreases, the deoxidizing power of C decreases relatively, and finally the metal components in the molten metal, such as Cr.
, Mn, Fe, etc., and the oxidation of the metal components in the molten metal progresses at an accelerated pace.

Therefore, high chromium steel is produced by blow-stopping (C) and blow-stopping the bell above this limit point to prevent oxidation of the metal components in the molten metal.

In addition, as the blow-off [C] level increases, the amount of oxygen blowing decreases, and the rise in molten metal temperature is also suppressed, so there is no risk of furnace roughness even without the use of composite material, and conventional operation is possible. It is possible to prevent the vicious cycle of temperature rise → furnace roughness → metal material input → melting remaining → yield drop, which was a drawback of the conventional method.

Further, due to the decrease in the basic unit of reducing Si due to the decrease in the amount of oxidation of the metal component in the molten metal, the amount of 5102 generated during the reduction period is suppressed, and the basicity of the slag during the reduction period increases.

Then, the reduction reaction is significantly promoted, and the recovery rate of Cr and Mn can be improved.

In addition, preliminary decarburization is omitted for those with a low burn-through [C] level.

Next, VOD was added to the high chromium pig iron obtained by the method described above.
However, when decarburizing in a vacuum, the active generation of CO gas causes increased splash and rapid boiling, which damages equipment and makes it impossible to continue operations. Sometimes.

However, in the method of the present invention, the carbon range is 0.30 to 1.2
At 0%, the amount of oxygen supplied is excessive (more than 0.5Nm''/Tm1n), the amount of generated gas is greater than the exhaust capacity of the vacuum system, and the degree of vacuum is deteriorated, creating a high-temperature peroxidation state by the middle stage of decarburization. .

After that, the blowing acid is stopped and the degree of vacuum is gradually restored, allowing natural decarburization to occur due to the oxygen source abundantly stored in the molten metal and CC) in the molten metal, until the target [C] level is reached. This method reduces metal oxides in the molten metal while decarburizing it.

Carbon range 0.30-1.2%, 02 oxygen delivery rate 0.5 Nm
3/ The reason why the vacuum natural decarburization was limited to 10 minutes or more was based on several tests, and if it was outside this range, the above conditions could not be obtained and favorable results could not be obtained. It was.

The Cr recovery rate of the high chromium steel manufactured by the method described above throughout the entire process is approximately 2% higher than that of the high chromium steel manufactured by the conventional method (preliminary decarburization to 0.4% C1). was gotten.

Next, the manufacturing method of the present invention will be explained in more detail based on experimental results on 5US304 steel.

First, high chromium pig iron was obtained by melting scrap, high carbon Fe-Cr, and high carbon Fe-Ni in a normal graphite electrode arc furnace.

In this case, some degree of metal oxidation in the molten metal was unavoidable due to the intrusion of the atmosphere from the furnace mouth, but the absolute amount was small and by adding a small amount of Si in the reduction stage of the post-process, it was possible to
0% recovery was possible.

When performing preliminary decarburization, it is better to set the molten metal temperature to a higher temperature to reduce Cr.
It is known that oxidation is low, but empirically, 1620
There is no problem in actual operation if it is above ~1630℃, and since preliminary decarburization is done by dipping and blowing an oxygen lance coated with a refractory in the molten metal, Mn , Cr, etc., proceed at an accelerated pace.

Here, the equilibrium relationship between [C] and Cr in the oxidation reaction of high chromium pig iron is determined by the relational expression of D, C, and Hilty.

That is, 1 og (%Cr)-Pco/C'10C':
1=-13800/T+8.76 In the above formula, Pc
o=1 atm, (%C'r'1-18.5%, T
= 1973°, no Cr oxidation occurs (C)
The level is [C''l: 0.32%, but if a decarburization reaction is occurring, it is necessary that the molten metal maintains a certain degree of peroxidation, and accordingly this equilibrium [C] The oxidation of (lcr) occurs even when the temperature exceeds this value.
>(Cr) - The relationship between temperature was investigated.

The results are shown in FIG.

As is clear from Figure 1, the oxygen level in the molten metal during decarburization remains slightly higher than the C-0 equilibrium line in stainless steel, and reaches the [C] level below point A, which intersects with the Cr-0 equilibrium line. It has been found that when the air is blown down, it becomes lower than the C-0 equilibrium line.

Anyway, below point A, the equilibrium of (C'l) is established with (Cr) rather than with [C], and if oxygen is further supplied, oxidation of Cr progresses. It shows.

Furthermore, the relationship between the [C] level in the molten metal and the amount of Cr oxidation at the time of blow-off is clear from Figure 2.
It can be seen that the increase in the amount of Cr oxidation is suppressed by setting the level to C-shaped 0.80% or higher.

Note that when burn-through [C] is 1.2% or less, it is desirable to omit preliminary decarburization because it increases the oxidation loss of Cr.

The high chromium pig iron obtained in this way cannot be decarburized in vacuum by refining using the VOD method, and in general, when high carbon hot metal is decarburized in vacuum, there are the problems mentioned above. In order to solve these problems, the present invention is carried out by the method described below.

That is, even if the amount of CO gas generated is the same, the lower the atmospheric pressure, the more intense the boiling of the molten metal becomes.

This is because at the same temperature, the volume expansion increases as the pressure decreases due to the physics that pressure x gas volume = constant, and as the pressure increases, the decarburization efficiency decreases, resulting in the amount of CO gas generated. suppressed.

Therefore, in a high carbon region, by setting the oxygen supply rate so as to exceed the evacuation capacity of the ejector in terms of CO gas, the generated gas deteriorates the degree of vacuum and the generation of CO gas is suppressed.

For example, a normal Ezekku designed to decarburize from 0.30 to 0.40% has an exhaust capacity of 1 to 1.2 k
g/Tm1n, and the oxygen supply amount corresponding to the exhaust capacity is approximately 0.4 to 0.5 Nm/Tm1n.
As shown in the figure, the above-mentioned objective can be achieved by maintaining the amount of oxygen supplied above this level.

However, in the method of the present invention, since the decarburization efficiency is artificially deteriorated in the high carbon region, Crft is naturally accompanied. It is necessary to carry out reduction using the residual [C] in the molten metal in the medium to low carbon range where there is no more carbon.

In this case, if the [C] level at which the above-mentioned strong blowing is completed is too low, decarburization will be completed before sufficient reduction is completed in the latter half, resulting in metal loss.

Moreover, if the above [C] level is too high, boiling will occur.

As stated above, this (C, as a rubel, C=
: ~0.30% is optimal.

Note that C deoxidation may be performed immediately after strong ejaculation, but since there is a risk of bumping due to a sudden decrease in the degree of vacuum, a transition period is provided and intermediate oxygen ejaculation is performed while the vacuum level is increased. It is desirable to move to C deoxidation by lowering the

If the C deoxidation is too short, the oxide reduction effect will be insufficient, so it is essential to carry out the deoxidation for 10 minutes or more while stirring with an inert gas.

FIG. 3 shows an example of an operation pattern in vacuum decarburization, where A is an operation pattern according to the conventional method, and FIG. 3A is an operation pattern according to the method of the present invention.

As is clear from Fig. 3, the method of the present invention maintains the amount of oxygen supplied in a high carbon range above the amount corresponding to the evacuation capacity of the ejector, worsens the degree of vacuum compared to the conventional method, and prevents the generation of CO gas. Because it is suppressed, intense boiling can be prevented.

. Figure 4 shows the oxidation status of Cr when the operation pattern shown in Figure 3 is used.As is clear from the figure, oxidation by the method of the present invention is suppressed to zero, and therefore Table 1. As shown in the total yield of all processes, C
It can be seen that this method has remarkable effects when compared with the conventional method in terms of r recovery rate, Si consumption rate for reduction, etc.

Table 2 shows the changes in the content of component elements for each process when using the method of the present invention in SUS 304 steel. is for medium [C] formulation.

In addition, the method of the present invention is 5 US 316.321.

It goes without saying that it is also applicable to austenitic steels such as 310, 400 series ferrites, martensitic steels, and other stainless steels.

[Brief explanation of the drawing]

Figure 1 is a relationship between CC)-(0]-(Cr') in the molten metal and temperature, Figure 2 is a relationship between [C] in the molten metal and the amount of Cr oxidation at the time of blow-off, and Figure 3 is in vacuum. This figure shows an example of an operation pattern in decarburization. Figure A shows an operation pattern according to the conventional method, Figure 4 shows an operation pattern according to the method of the present invention, and Figure 4 shows [C] the operation pattern when the conventional method and the method of the present invention are used. % and the amount of Cr conversion.

Claims (1)

[Claims] 1. If the molten metal composition [C] is less than 0.80%, preliminary decarburization is omitted; If the
C. If the molten metal composition [C] is 0.80 to 1.20%, the preliminary decarburization is omitted, or if the preliminary decarburization is performed, the blow-off [C] is 0.80%. With the above, the molten metal of A, C and C was heated to a carbon range of 0.30 to 1.20 by vacuum oxygen blowing method.
A method for producing high chromium steel, which comprises blowing the 02 supply amount at 0.5Nyl/Tmln or more in terms of %, and then performing vacuum natural decarburization for at least 10 minutes.