JPS5833293B2 - Kokuromu Kounoseizouhouhou - Google Patents

Description

Detailed Description of the Invention The present invention utilizes a vacuum induction melting furnace (hereinafter referred to as VIP).
Cr) 25-40% (wt%, same below) carbon (C) 0
．． The present invention relates to a method for producing high chromium steel with a chromium content of 0.050% or less.

In general, in high chromium steel, as the amount of Cr is increased, corrosion resistance and oxidation resistance improve, but on the other hand, workability and toughness deteriorate.

It is known that this deterioration of workability and toughness is closely related to the amount of C contained in steel, and even if the amount of Cr is increased, if the amount of C is reduced to below a predetermined amount, the deterioration can be significantly suppressed. ing.

However, since Cr has a stronger affinity for C than Fe and reduces the activity of C, it becomes difficult to reduce C as the amount of Cr increases.

On the other hand, the amount of C required to obtain a certain level of workability and toughness is C
It tends to decrease as the r content increases, and it is considered extremely difficult to manufacture ultra-low carbon, high-chromium ferrite steel with excellent workability and toughness on an industrial scale.

For example, in a high chromium 7-elite steel containing 25% or more of Cr, in order to obtain sufficient toughness and sufficient workability including weldability at room temperature, the C content must be 50 ppl+ or less.

An example of a method for relatively stably obtaining such ultra-low carbon, high chromium steel is a VIP melting method.

In this case, the following two methods can be considered for reducing C using a balance sieve.

The first method is to lower the CO partial pressure of the atmosphere, and more specifically, methods are generally adopted such as increasing the degree of vacuum or covering the molten metal surface with an inert gas such as Ar gas to dilute the CO gas. .

The second method is to increase the oxygen concentration in molten steel.For this purpose, injection of oxidizing gas such as oxygen or CO2 or F
A method of adding oxides such as e2O3 and Cr2O3 is adopted.

By adopting this method, the dissolution amount can be reduced to 100k.
25% when refining with VIP of laboratory cheek model below g
It is easy to stably reduce the C content to below 50 ppmQ in the above high chromium steel, but even if the above method is adopted, when refining is carried out in an industrial-scale VIP with a melting amount of 1000 kg or more, in reality, electrolytic Unless you use high-purity raw materials such as chromium or electrolytic iron, you can stably achieve 50pp of C! It is extremely difficult to make it less than Il.

As is well known, C in the molten metal (molten steel) becomes CO gas from the molten metal surface according to the following reaction formula, and is discharged to the outside of the system.

C+0−+CO However, a part of this CO gas evaporates from the molten metal surface and reacts with the scattered atomic manganese (Mn) and Cr, which have a relatively high vapor pressure and are active, according to the following formula, and becomes C. It is reduced and contained in the deposits that adhere to the inner wall of the vacuum container, the crucible wall, and the crucible lid.

Mn + Co −) MnO+ C(1)2Cr+3
CO-+Cr2O3+3C(2) Among the vapor deposits containing C, the dust-like vapor deposits that adhere to the inner wall of the vacuum chamber grow and constantly peel off from the inner wall of the vacuum chamber, and some of them are deposited on the molten metal. Because it falls and dissolves into the molten metal, even if the molten metal is held under vacuum for a long enough time, the C concentration calculated by equilibrium theory will not be reached, and the decarburization rate will be much higher than the equilibrium C concentration. There is a marked decline and a tendency towards saturation.

Furthermore, some of the deposits that adhered to the inner wall of the crucible are washed away and dissolved into the molten metal during tapping, and a considerable amount of the dust that adhered to the inner wall of the vacuum vessel is removed due to the sudden increase in radiant heat during tapping. Because it flakes off, falls, and melts into the molten metal, it is extremely difficult to stably reduce C to 50 ppI11 or less in the ingot after tapping, even if the C content has been sufficiently reduced in the molten metal before tapping.

In small laboratory-scale reactors, the ratio of the size of the vacuum vessel to the size of the crucible is generally large, whereas in large industrial-scale reactors, the ratio is relatively small, and some of the above-mentioned phenomena, especially in the vacuum vessel, are The degree of C contamination caused by the adhering vapor deposits peeling off and falling is large, and its influence cannot be ignored.

In particular, C contamination due to such a phenomenon becomes noticeable when the furnace is operated continuously without cleaning and removing deposits, and therefore it becomes extremely difficult to increase the operating rate of the furnace.

The present invention aims to reduce C to 0.0050% or less in producing high chromium steel containing 25 to 40% Cr, and for this purpose, the steel is refined under specific conditions.

These will be explained in detail below.

First, raw materials are melted in a VIP under vacuum or an inert gas atmosphere to obtain 7-erochromium molten steel containing 25 to 40% Cr.

As a raw material, a 7-engine rochrome alloy containing 25 to 40% Cr may be used, and the composition may be adjusted using a normal low carbon ferrochrome alloy with a higher chromium content and a steel material such as industrial pure iron or iron scrap. Cr may be within the above range.

At this time, the amount of C at the time of melting of the molten metal, that is, the amount of burn-through C (cII
(represented by L) and the amount of Mn need to be suppressed to below a certain value.

C is emitted from molten steel as CO gas during vacuum refining, but as mentioned above, this CO gas is simultaneously evaporated from the surface of molten steel with high vapor pressure and strong affinity for oxygen.
It reacts with n and Cr, is reduced, and is contained in the form of free C in the vapor deposit mainly composed of these volatile elements.

Since this peels off or melts into the ingot as it grows, the C content tends to increase in the ingot.

In order to prevent an increase in the C concentration reached during vacuum refining and an increase in C during tapping due to such phenomena, and to maintain a stable C content of 0.0050% or less in the ingot, it is necessary to The amount of C, that is, the amount of burn-through C (CIIL) must be limited to 20 times or less of the target value, that is, 0.1% or less, and Mn must be limited to 0.2% or less.

Mn evaporates significantly more easily than Cr, and has a stronger affinity for oxygen than Mn.
The proportion of C produced by the reduction reaction of CO gas by n is relatively large, and even in high chromium steel, when Mn is reduced to 0.
By limiting C to 2% or less and 0.1% or less, it is possible to stably reduce C in the ingot to 0.0050% or less.

Regarding the refining temperature, if it is below 1600°C, the decarburization rate will be extremely slow and it will take a considerable amount of time to lower the target C to below 0.0050, which is not economical.

On the other hand, the higher the temperature, the faster the decarburization rate becomes, but the life of the refractory decreases significantly at temperatures above 1700°C.

In addition, since the activation energy for evaporation of Mn, Cr, etc. is larger than that for decarburization, the temperature dependence is large; at high temperatures, Cr, M, etc.
The evaporation of n increases, the amount of deposits generated and the total amount of C contained in the deposits increase, making it impossible to sufficiently suppress C contamination during tapping.

Considering this point, the molten metal temperature during vacuum refining was set at 1600℃.
-1700°C.

Next, the degree of vacuum during refining will be explained.

In vacuum refining, when the degree of vacuum is I Torr or higher, the diffusion of CO gas in the gas phase is suppressed and becomes rate-limiting, so the decarburization rate becomes slow.

On the other hand, below 1o-3 Torr, the increase in decarburization rate is saturated and Mn.

Since evaporation of Cr etc. tends to be further accelerated, Mn
A suitable degree of vacuum is 10'' Torr or more to prevent evaporation of Cr as much as possible and to obtain a sufficient decarburization rate.

Next, regarding the refining time required for decarburization, C
The decarburization rate in vacuum induction melting of high chromium steel containing Cr: 0.1% or less and Cr: 25 to 40% is given by the following equation with sufficient approximation.

Here, d C/d is the decarburization rate (%/5e
c) Kc Mass transfer coefficient for decarburization (crIL/5eC) L average molten metal method (n) CC concentration (%) Cs C saturation value (%) at which the decarburization rate becomes essentially zero, and as already mentioned Within the refining conditions of molten metal composition, degree of vacuum, and temperature of the present invention, 0.0015%<Cs≦0.0040% (5)
0.01cIrL/Sec≦Kc≦0.050crrL
/Sec (6).

When Cm is the burn-through C% obtained by integrating equation (3), the time t (minutes) required to lower C to the target Co% is given by the following equation.

2.303 Lcm t = −X−1o g−(7) 60 KcC.

(However, Co>Cs, 0.01≦Kc≦0.05)Co
In the present invention, it is set to 0.0050% or less, and 0,0
It cannot be lower than 0.015%, so 0.0015%<C
o≦0.0050% (8) Select according to the target C concentration.

Kc is the lower temperature within the scope of the present invention,
In addition, when the degree of vacuum is poor, the value is close to 0.01 cIrL/sec, and in the opposite case, the value is close to 0.05 cm/sec.

0.01-0.05 depending on the combination of temperature and degree of vacuum
Select as appropriate within the range of cIrt/sec.

If the time is less than the time given by equation (7), the C concentration of the molten steel will not decrease sufficiently, so naturally it will be impossible to obtain a steel ingot with the target C concentration.

Furthermore, even if the refining time is extended beyond that point, the decarburization rate will decrease (especially in the case of Cof-Cs), while the amount of evaporation of Cr, Mn, especially Cr, per unit time will remain constant with almost no change. Therefore, the layer of adhering dust in the furnace becomes thick and easily peels off and falls, making C contamination more likely to occur during tapping, which is not suitable for the purpose of the present invention.

Next, Claim 2 will be explained.

As already mentioned, a part of the deposited material mainly composed of volatile elements such as Mn and Cr, including C, also adheres to the inner wall of the crucible.

In addition, unrefined droplets with a relatively high C concentration are also scattered on the inner wall of the crucible due to the boiling water phenomenon caused by the burn-through process of the charged raw materials or the degassing reaction of CO, gas, H2 gas, etc. during the early stage of vacuum refining. adhere to.

Among these deposited metals, a part of the deposited metal on the tapping port side is washed by the molten steel during tapping and melts into the molten steel, so the C concentration tends to increase.

It is preferable to eliminate this influence when producing ultra-low carbon steel like the present invention.

In order to prevent C contamination during tapping due to such causes as much as possible, during vacuum refining, tilt the tapping port side of the crucible as far forward as possible without molten steel flowing out, and keep it in that state. It is sufficient to hold the furnace until the deposited metal is sufficiently melted, then return the furnace to an upright position and continue refining.

When the C concentration in the burn-through is as low as 0.03% or less and the Mn concentration in the burn-off is as low as 0.1% or less, it is sufficient to tilt the furnace forward once during vacuum refining.

If the C concentration and/or Mn concentration is relatively high and the above conditions are not satisfied, it is desirable to perform the forward tilting operation of the furnace two or more times as appropriate.The final forward tilting operation is performed when the C concentration of the molten steel is 0.02. It is appropriate to take the time to ensure that the amount is within the range of ~0.01%.

The high chromium steel of the present invention is not limited to ferritic steel as long as the Cr content is within the range of 25 to 40%, and may be an austenitic steel containing Ni or a two-phase steel having both an austenite phase and a ferrite phase.

In order to produce these final high chromium steel products, after reducing the C content to the desired level according to the present invention, by a conventional method,
For example, an AI-based or Ca-based deoxidizing agent is added to molten steel to deoxidize it, and then the steel is tapped.

Also, an appropriate amount of M is added to improve the properties of high chromium steel.

Or a small amount of Nb, Ta, V, Ti, Zr, Cu,
Of course, it is also possible to add alloying elements such as AI, and these are also included in the present invention.

Example 1 Using a 40KW vacuum melting furnace, low carbon ferrochrome,
Using electrolytic iron and electrolytic manganese as raw materials, the melting amount is 20 kg,
The compositions of Cr and Mn were mixed to be 30% and 0.04%, respectively, and dissolved in Ar gas at 1 atm.

Subsequently, by adding an appropriate amount of high carbon ferrochrome (approximately 8% C) to this molten steel, the C concentration of the blend was reduced to 0.03%.
It was varied from 0.2% to 0.2%.

Thereafter, vacuum refining was performed for 1 hour under the conditions of vacuum degree: 5×10 −3 to 1 O −2 Torr, molten steel temperature: 1630 to 1670° C., and steel was tapped in an Ar gas atmosphere of about 100 Torr.

After the steel ingot and crucible were sufficiently cooled, the inside of the furnace was opened to the atmosphere, the dust adhering to the vacuum container was collected, and the dust and steel ingot were analyzed for Cr, Mn, and C.

In addition, when dissolving in Ar gas at 1 atm, decarburization and Mn evaporation reactions can be almost ignored within the scope of this experiment. It was confirmed in advance that the concentrations were approximately equal.

Table 1 shows the steel ingot composition, dust composition, dust amount, amount of C in the dust (absolute amount), and percentage of the amount of C in the dust relative to the dissolved amount for each blending C%.

In addition, the amount of dust is calculated based on the amount of evaporation of Mn during vacuum refining.
% and the Mn% of the steel ingot after vacuum refining, and this value was divided by the Mn% in the dust.

From Table 1, as the blend C% and therefore the burn-through C% increase,
The C concentration in the dust and the absolute amount of C contained in the dust also increase, and when trying to obtain ultra-low carbon steel with a C content of 0.0050% or less, C It is known that pollution cannot be ignored.

Example 2 The blending composition was fixed as Cr: 30%, C: 0.06%,
The composition of Mn was varied between 0.04% and 0.5%, and the other aspects of the melting were as in Example 1.

Table 2 shows the steel ingot composition, dust composition, dust amount, amount of C in dust (absolute amount), and percentage of dissolved amount of Ci in dust for each Mn% blend.

From Table 2, it can be seen that, similar to the tendency observed when the blended C% in Table 1 is increased, as the blended Mn%, and therefore the burn-through Mn%, increases, the C concentration in the dust and the absolute amount of C contained in the dust increase. It is known that C contamination due to falling dust during the latter half of vacuum refining and during steel tapping cannot be ignored.

Example 3 In a vacuum melting furnace with a 900J melting capacity of 2000 kg, 30Cr-
2Mo, 25Cr-2ONi+ and 26Cr-6Ni-
Three types of 2Mo (each number indicates weight%).

), conditions A within the scope of the present invention shown in Table 3
For each steel type and condition, melting was performed three times in a row under conditions B and outside the range.

In either case, the degree of vacuum during vacuum refining is 8X10-'~4
X 10” Torr and molten steel temperature 1620~1
The temperature was maintained within the range of 670°C.

During the refining process, appropriate sampling was carried out, and a quick analysis of C was carried out. After confirming that the C content had been sufficiently reduced to 0.0050% or less, 0.2% aluminum shot was added to deoxidize, and the Steeled.

The mass transfer coefficient Kc of the decarburization rate calculated from the time required for decarburization refining is as follows for each alloy: 30 Cr −2Mo Kc = 0.023 to 0
．． 029 (1%/'3ec25Cr-2ONi
Kc=0.027~0.034KiAec26Cr-6N
i-2MoKc=0.020-0.028q-, and in any alloy, it is in the range of 0.01≦Kc≦0.05.

In any case in this experiment, the furnace was not tilted forward during vacuum refining.

FIG. 1 shows the C analysis values of the steel ingot under conditions A and B, respectively.

From Fig. 1, in the case of condition A, which is within the scope of the present invention, cleaning and removal of the same amount of dust adhering inside the furnace is omitted, and even if continuous melting is performed, the steel ingot stably contains 0.0050% C. Condition B, which is outside the scope of the present invention, can be reduced to
Especially in the case of continuous melting, even though the C content of the molten steel before tapping is 0.0050% or less, it becomes stable in the steel ingot due to C contamination during tapping. It is known that it is difficult to reduce C to 0.0050% or less.

Example 4 In order to investigate the effect of the forward tilting operation of the furnace during vacuum refining on the C concentration of the steel ingot, 30Cr-2Mo
For steel under condition A in Table 3, melting experiments were conducted three times in a row using a forward tilting operation of the furnace.

The number of times of forward tilting was two in each case, and since the vacuum refining was carried out for about 3 hours, it was carried out 1 hour and 2 hours after the start of the vacuum refining.

The other conditions were as in Example 3.

Figure 2 shows the C concentration in Example 3 when the forward tilting operation of the furnace is adopted.
A comparison with the C concentration in the case where L7 was not adopted is shown.

From this figure, in the former case, C is about 0.0005 compared to the latter.
% to about 0.0010%, and it is known that tilting the furnace forward during vacuum refining is highly effective in preventing C contamination during tapping and lowering the C concentration in the steel ingot.

[Brief explanation of drawings]

Figure 1 shows the relationship between the number of melts and 0%, and Figure 2 shows the relationship between the number of melts and 0% with and without forward tilting operation.
It is a figure showing the relationship of %.

Claims (1)

[Claims] 1. In a method for producing high chromium steel containing 25 to 40% chromium, carbon of 0.1% or less, A molten metal containing 0.2% or less manganese and 25-40% chromium is obtained, and then the temperature is 1600-1700°C and the degree of vacuum is 10.
~ I Torr, the amount of carbon in the molten metal Cm
(%), hold the time t (minutes) of the following equation related to the target carbon content co (%) of high chromium steel and the average molten metal depth L (α), 2.303 L Cm t = 60 XKclogC. (However, 0.01≦Kc≦0.05.0゜0015<C
o≦0.0050) A method characterized by reducing the carbon content to 0.0050% or less by decarburizing and refining. 2. In the method according to claim 1, t (minutes)
By tilting the tapping side of the crucible of the melting furnace forward at least once during the holding period, it is possible to eliminate the relatively high carbon concentration on the tapping side that has adhered to the inner surface of the crucible due to scattering or evaporation during decarburization. A method characterized by the addition of a step of melting the metal and then refining it.