JPS613815A - Manufacture of high chromium steel - Google Patents

Abstract

PURPOSE:To obtain a dead soft high Cr steel by refining a steel bath by blowing under prescribed conditions while keeping the concn. of carbon in the steel bath and the temp. of the bath at a prescribed value each so as to inhibit the oxidation of Cr in the bath. CONSTITUTION:The concn. of carbon in a steel bath in a refining furnace having a top and bottom blowing function is regulated to <=2%, and the temp. of the bath is regulated to 1,650-1,800 deg.C. The steel bath is then refined by blowing under conditions which satisfy <=30 BOC represented by the equation [where Qo2 is the total flow rate (Nm<3>/min) of gaseous oxygen fed from a lance and a nozzle, W is the amount (ton) of molten steel (%C) is the concn. (wt%) of carbon, and gamma is uniform mixing time (sec)]. The oxidation of Cr in the steel bath is inhibited.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing high chromium-containing alloy steel, particularly stainless steel.

<Prior Art> When decarburizing stainless steel, a major problem was that chromium, which has a strong oxidizing power, escapes expensive chromium as chromium oxide. In particular, chromium oxidation is large at the final stage of blowing when the carbon concentration has decreased, so it is necessary to reduce the chromium oxide by introducing an expensive ferrosilicon alloy after the carbon concentration has been reduced to a predetermined level. Ta.

In order to suppress chromium mistakes in this low carbon range, a method of reducing the partial pressure of carbon monoxide at the flame point is widely used9. The so-called AOD method in which gas is injected (Sawamura: Tetsu to Hagane, 63 (1977), P1953), or the VOD method in which oxygen is pumped through a top blowing lance under vacuum (Moriya et al. 2 Tetsu to Hagane 63 (1977), P2O70) )and so on.

However, both methods have disadvantages in that they require a long time because the high chromium alloy is manufactured using hot metal with a high carbon concentration, and the Ka and feeding speed are low.

For this purpose, normally, nine rough decarburization steps are first performed in a converter, and then final decarburization is performed in an AOD or VOD furnace. However, when blowing high chromium steel in a converter, the rim is easily oxidized, so it is not possible to blow down to a low carbon concentration using the normal method, so the AOD + VOD method is used. The load is getting bigger.

In recent years, with the development of the so-called top-bottom blowing combined blowing method, attempts have been made to suppress chromium oxidation in the converter, and a method has been adopted in which argon and oxygen are mixed through a bottom-blowing nozzle. This method is basically the same technology as the AOD method, and even with this method, escape of chromium is not sufficiently suppressed. (Yamada et al.: Tetsu to Hagane, 69 (1983),
P1886).

On the other hand, in ordinary steel blowing, a BOC value determined by the balance between oxygen supply rate and stirring power has been proposed (Kai et al., Nitetsu to Hagane, 68 (1982), P1946.), and B
It is known that the iron concentration in slag can be lowered by controlling the OC value, but unlike refining ordinary steel, high chromium steel has a property of containing high chromium, which is the property of chromium itself. Since there are many special factors such as oxidation properties and reactivity in high temperature ranges, the conditions for suppressing the escape of chromium by the top-bottom blowing combined blowing method have not been completely clarified until now.

<Problems to be Solved by the Invention> The present invention has been made to solve the above-mentioned problems, and uses a top-bottom blowing composite blowing method to maintain the temperature at 1650°C or higher and 1800°C or lower. However, it is a decarburization method for high chromium steel that makes it possible to suppress chromium oxidation to a low carbon concentration by blowing while controlling the BOC value defined by the following formula to 30 or less, Since it is possible to obtain high chromium steel with a low carbon concentration in a converter, the load on the final decarburization process, AOD and VOD, is significantly reduced, preventing chromium from escaping, and it is also possible to omit the process. It can be done.

However, QO, is the total flow rate of oxygen gas supplied from the lance and nozzle (N = 310), W is the molten steel (1 ()), [C] is the carbon concentration (weight) (4 cents), and τ is, Uniform Mixing Time (Seconds) Means for Solving the Problem> The method for producing high chromium steel according to the present invention will be described below.

In the present invention, oxidation loss of chromium is determined by the rate of oxidation of chromium by oxygen supplied from top-blown oxygen and the rate at which chromium oxide once generated is reduced again by being mixed into the stirred steel bath. This is based on the confirmation of the fact that the situation is determined by mutual agreement. According to this idea, even if the rate of oxidation of chromium by top-blown or storm-blown oxygen is increased, if the rate of reduction in the steel bath is increased, even if chromium oxide is once produced, it will eventually be generated according to the following equation: It is reduced and does not remain, and decarburization is relatively promoted.

Cr0n + nC → Cr + nc.

Thus, the rate of oxidation of chromium by the supplied oxygen is considered to be approximately proportional to the rate of oxygen supply. on the other hand,
The rate of reduction in a steel bath is thought to be governed by factors that determine the frequency with which chromium oxide once formed is sprinkled into the bath, and factors that determine the rate at which the chromium oxide that has been sprinkled is reduced.

Of these, the former can be represented by stirring power,
As a more quantitative value, the uniform mixing time (τ: seconds) determined by the following equation can be used.

In addition, in the latter case, the reduction reaction of chromium oxide with carbon in steel is thought to be rate-determining the diffusion of carbon in the liquid phase.
It is caused by the carbon concentration in the steel bath.

Here, H is bath depth (cR), ρeFi bath severity (k19
/ff13) ”V, B' ”V and T are bottom blowing gas, respectively.
The input energy (W) per unit volume of the bath due to the top blowing gas
att/--5ee);7. T is, QB is the total gas flow rate (Nm3/
minutes), Q, is the total gas flow rate (N
m7 min), TL is the steel bath temperature (K), vL is the bath volume (N
Insu, P2 is atmospheric pressure (103311/cn?), η
is the expansion angle (deg,) of the top blowing lance, M is the number of moles of the top blowing gas, n is the number of holes in the top blowing lance, d is the tip exit diameter of the top blowing lance hole (m), and X is the top blowing lance Distance between and bath surface (m)
represents.

Therefore, the oxidation of chromium should be suppressed by changing the three parameters mentioned above: the pickle, the uniform mixing time, the oxygen supply rate, and the carbon concentration. Although the BOC value proposed for ordinary steel includes these three parameters, the value usually applied is 1000 or more, and under these conditions it cannot be applied to high chromium steel at all. Furthermore, it has been found that chromium oxidation is greatly affected by temperature, and therefore it is not possible to effectively prevent chromium loss and decarburize even if refining is carried out using BOC alone.

Therefore, as a result of conducting numerous experiments and examining decarburization and chromium loss, we found that the steel bath temperature was set at 1,650℃ or higher, and 18
By blowing while keeping the temperature below 00℃ and controlling the BOC value to below 30, chromium loss during decarburization and refining can be reduced.
It has become clear that it is possible to suppress and control carbon levels down to the low carbon concentration range.

This method makes it possible to incorporate most of the final decarburization process, or all of the final decarburization process performed by the conventional AOD and VOD methods, into the top-bottom blowing combined blowing converter, greatly increasing the production of high-chromium steel. improved.

The BOC value is determined by three factors: uniform mixing time, oxygen supply rate, and carbon concentration.
However, in a normal top-bottom blowing converter, the uniform mixing time is mostly determined by the bottom-blowing gas flow and amount, so if the bottom-blowing gas flow rate is kept constant during blowing, , it can be assumed that the uniform mixing time remains unchanged. Therefore, in order to control the BOC value, the top blowing acid rate is reduced so that the BOC value becomes 30 or less with respect to the carbon concentration depending on the blowing progress determined empirically. In this case, in order to progress decarburization as quickly as possible, the BOC value must be 3.
It is best to continuously reduce the oxygen supply rate in accordance with decarburization so that the oxygen concentration becomes 0. In addition, if the bottom blowing gas flow rate is changed during blowing, for example, by increasing the bottom blowing gas flow rate in the low coal region, the oxygen delivery rate at which the BOC value reaches 30 will also increase, resulting in high-speed decarburization. This is recommended because it can also be done.

By the way, the present invention only applies when the steel bath temperature is 1650°C.
The above can be applied when the temperature is 1800℃ or less, and 16
At temperatures below 50°C, chromium oxidation is severe and no matter how low the BOC is, chromium loss cannot be reduced. In this sense, a region with a carbon concentration of 2 or more has a temperature of 1650°C.
This cannot be done beyond the above, and the object of the present invention is lost. Moreover, if the steel bath temperature exceeds 1800° C., the refractories will suffer severe melting and loss, which is unrealistic.

<Example and Effects> Next, an example of the method for manufacturing high chromium steel according to the present invention will be described. First, using a small melting furnace, argon was used as the bottom blowing gas, oxygen was used as the top blowing gas, and the flow rates were varied to investigate the relationship between the BOC value and chromium loss. As a result, as shown in Figure 1, the 'BOC value was reduced to 3.
It was found that if the chromium loss is suppressed to 0 or less, the chromium loss is 0.5% or less.

In response to this, we conducted an experiment using top-bottom blowing combined blowing. In this case, using oxygen for both the top-blown gas and the bottom-blown gas, the conditions are expected empirically such that the bottom-blown oxygen flow rate is 0.8 Nm3/T min and the uniform mixing time (τ) is about 18 seconds. Blowing was carried out by changing the amount of oxygen supplied from the top blowing lance in accordance with the decrease in carbon concentration. As a result, for example, as shown in Figure 2, the amount of oxygen supplied is slightly reduced near the end of refining, but the BOC is 3.
When using an oxygen feed/4' turn that exceeds 0, the chromium loss after refining (after the experiment) as shown in Table 1 despite the addition of a slag reducing agent such as Fe-8i at the final stage. is extremely large at approximately 5%.

Table 1 On the other hand, when an oxygen supply pattern in which the BOC is suppressed to 30 or less as shown in FIG. 3 is used, the chromium loss can be reduced to a very small 0.5% as shown in Table 2.

As described above, by using the method for producing high chromium steel according to the present invention, decarburization in the converter can be carried out while suppressing loss of chromium, making it possible to produce high chromium steel with extremely low carbon content.

In addition, the subsequent V
This is an excellent manufacturing method that can reduce the load on the smelting furnace, such as OD, and omit processes.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the BOC value and chromium loss obtained in a small melting furnace, Figure 2 is a diagram showing the conventional method of slightly reducing the amount of oxygen fed at the final stage of refining in a converter, and Figure 3 1 is a diagram showing an example of a refining method according to the present invention in which the BOC value is 30 or less.

Claims (1)

[Claims] A method for producing high chromium-containing steel using a refining furnace having a top-bottom blowing function, wherein the carbon concentration in the steel bath is 2% or less,
Moreover, the oxidation of chromium in the steel bath is suppressed by blowing while keeping the steel bath temperature within 1650 to 1800°C and controlling the BOC value defined by the following formula to 30 or less. A method of manufacturing high chromium steel. BOC=Qo_2/[(W/τ)×[%C]] However, Qo_2 is the total flow rate of oxygen gas supplied from the lance and nozzle (Nm^3/min) W is the amount of molten steel (tons) [% C] is carbon concentration (weight percent) τ is uniform mixing time (seconds)