JP3903580B2 - Method of melting high cleanliness steel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for melting high cleanliness steel, and more specifically, decarburized molten steel is removed at the end of so-called secondary refining using a vacuum smelting apparatus having a bottom blowing gas stirring function such as VOD. In deoxidation with an acid agent, the present invention relates to a technique for reducing the amount of slag entrained in molten steel and melting steel with less non-metallic inclusions.
[0002]
[Prior art]
Oxygen in molten steel is any form of so-called free oxygen dissolved in molten steel, deoxidation product inclusions produced by reaction with a deoxidizing agent, and slag inclusions produced by slag entrainment produced by refining Exists. The oxygen in the molten steel adversely affects the quality of the product steel produced by casting and rolling the molten steel. Usually, secondary refining is performed after converter refining so that it is as low as possible. Yes. One of the apparatuses used for the secondary refining is a vacuum refining apparatus 7 of a VOD (Vacuum Oxygen Decarburization) system as shown in FIG. It surrounds the ladle 2 containing the molten steel 1 with a sealed container 3 so that the inside of the container 3 can be depressurized. Further, an oxygen gas 5 is blown into the molten steel 1 through the lance 4 in the container 3 and a decarburizing function is blown, and an inert gas 6 is blown from the bottom of the ladle 2. And a function of stirring the molten steel 1.
[0003]
Conventionally, as a method for reducing oxygen in molten steel using such a VOD refining apparatus, the basicity (CaO / SiO ₂ ) of the slag formed on the molten steel surface is increased, and in the molten steel determined by the reaction between the slag and the metal. In addition to a method of reducing so-called free oxygen by utilizing the low equilibrium oxygen concentration, a method of levitating and separating the deoxidized product by enhancing the bottom blowing gas stirring has been adopted.
[0004]
However, in such a vacuum refining process, there is a molten slag 8 on the surface of the molten steel 1. Therefore, if the molten steel 1 is excessively agitated with the bottom blowing gas 6, the slag 8 is always caught in the molten steel 1. . At that time, when the particles of the slag 8 involved are large, they float up again in the molten steel 1 and are easily removed. However, particles having a size of several tens of μm or less are difficult to float and cannot be removed in many cases, so that the oxygen concentration in the molten steel 1 is increased and causes defects in the product steel. Nevertheless, since stirring with the bottom blowing gas 6 is indispensable for promoting the reduction of the slag 8 by the slag-metal reaction and reducing free oxygen, how to improve the entrainment of the slag 8 into the molten steel 1 in the refining under reduced pressure Was a problem.
[0005]
[Problems to be solved by the invention]
In view of this situation, the present invention provides a high-cleanness steel melting method that can reduce deoxidation products in molten steel and non-metallic inclusions caused by slag entrainment using a vacuum refining apparatus such as VOD. It is intended to provide.
[0006]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the inventor repeatedly researched how to produce a steel having a low free oxygen concentration of molten steel and a small amount of deoxidation products and slag by using a VOD vacuum refining apparatus. It was. Further, the present invention was completed by focusing on the molten steel stirring method at the end of the secondary refining, that is, at the time of deoxidation reduction treatment with metal Al, Fe-Si or the like.
[0007]
That is, the present invention is the vacuum spinning smelting molten steel decarburized in a device having a bottom-blown gas stirring function, in refining process of the molten steel for deoxidation treatment by introducing the deoxidizer subsequently, or introduction of the acid acceptor From 5 minutes to 30 minutes , the molten steel is stirred under the condition that the stirring power density (ε) defined by the following formula (1) is 300 watts / ton-steel or more and 500 watts / tonne-steel or less. Is a melting method of high cleanliness steel characterized in that stirring is performed for 5 minutes to 30 minutes under conditions where the stirring power density is 10 watts / ton-steel to 250 watts / ton-steel. .
[0008]
ε = 0.0285 · Q · T / W · log (1 + Z / 148 × 760 / P) (1)
ε: Stirring power density (Watt / ton-steel)
Q: Bottom blowing gas flow rate (normal liters / minute)
T: Steel bath temperature (K °)
W: Steel bath weight (tons)
Z: Bath depth (cm)
P: Degree of vacuum (torr)
Moreover, this invention is a melting method of the high cleanliness steel characterized by using the said vacuum smelting apparatus as VOD.
[0009]
In the present invention, a deoxidation product is formed, and the mixture is vigorously stirred while being sufficiently absorbed by the slag, and the deoxidation product is sufficiently floated, and after the slag has become uniform, the mixture is weakly stirred. Entrainment of deoxidation products and slag can be suppressed. As a result, a molten steel cleaner than before can be obtained.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
First, the molten steel 1 roughly decarburized in a steelmaking furnace (not shown) such as a converter is put into a ladle 2, and the ladle 2 is put into a vacuum refining device 7 having a stirring function by a bottom blowing gas 6. Transition.
Here, the VOD shown in FIG. 3 is adopted as the reduced pressure refining apparatus 7.
[0011]
In this VOD, as the first stage of vacuum refining, the inside of the sealed vessel 3 is depressurized, and oxygen gas 5 is blown onto the molten steel through the lance 4 to decarburize to the target carbon concentration. Subsequent to this oxygen blown decarburization, the top blown oxygen is stopped, and so-called “C—O decarburization” with dissolved oxygen in the molten steel may be performed. Then, after the decarburization is completed, as a second stage, a deoxidizer such as metal Al, Si or Fe—Si is introduced into the molten steel 1 to perform a deoxidation treatment. The present invention stirs the molten steel 1 during the deoxidation treatment by a method different from the conventional method, reduces the oxygen concentration of the obtained molten steel 1 from the conventional method, and suppresses the mixing of nonmetallic inclusions into the molten steel. It is.
[0012]
Specifically, the slag 8 is restrained from being entrained by vigorously stirring for at least 5 minutes from the introduction of the deoxidizer and then weakly stirring for at least 5 minutes. One example is shown in FIG. 1 as the relationship between time and stirring power density. In FIG. 1, the stirring power density is changed in a staircase shape only once, but the present invention is not limited to this example. That is, the change may be performed a plurality of times, or the power density may be continuously reduced.
[0013]
The reason why the stirring power density is 300 watts / ton-steel or more for at least 5 minutes after the deoxidizer is charged is as follows. Immediately after adding the deoxidizer, it is important to reduce the slag itself sufficiently and uniformly by the deoxidizer, in addition to the floating separation of nonmetallic inclusions. If the reduction of slag is insufficient at this stage, even if the inclusions are floated and separated, lower oxides such as chromium oxide, iron oxide and manganese oxide in the slag dissociate and free oxygen is introduced into the molten steel. It will continue to be supplied, and after the refining is finished, the molten steel is re-oxidized, which causes new non-metal oxides to be generated. This reaction all depends on the strength of stirring of the molten steel, and the effect becomes remarkable especially under strong stirring conditions of 300 watt / ton-steel or more. The reason why this strong stirring treatment is performed for at least 5 minutes is to allow the deoxidizer and the slag to sufficiently react and to make the slag uniform. The upper limit of the stirring power density is not particularly set. However, if the stirring is too intense, the molten steel is ejected from the molten steel container, or the metal is attached to the lid of the container and the operation is hindered. It is desirable to keep it at about watt / ton-steel. The upper limit of the treatment time is not particularly defined, but if it is too long, the refractory of the inner refractory of the molten steel container due to slag is generated, so it is preferable to keep it for about 30 minutes, more preferably about 20 minutes. .
[0014]
On the other hand, after the above treatment, the stirring treatment at a stirring power density of 250 watt / ton-steel or less is performed for at least 5 minutes for the following reason. At this time, reduction and homogenization of slag is no longer important, and non-metallic inclusions in the molten steel and slag entrained in the molten steel at the time of the above-mentioned strong stirring are measured to clean the molten steel. It is important. And it must avoid as much as possible that the slag on a steel bath surface is newly wound in molten steel. If the stirring power density exceeds 250 watts / ton-steel, the slag entrainment speed is higher than the slag and inclusion floating speed, so the stirring power density at this time is limited to 250 watts / ton-steel. And The lower limit of the stirring power density is not particularly provided, but it is necessary that the floating separation of slag and non-metallic inclusions by stirring is sufficiently performed within a predetermined time, so that the stirring power density is performed at 10 watt / ton-steel or more. Is good.
[0015]
This relatively weak stirring is performed for at least 5 minutes, because a minimum of 5 minutes of treatment is required to sufficiently float and separate inclusions and entrained slag. Although this treatment time is not particularly limited, as in the above-described treatment, if the treatment time is too long, the refractory of the lining refractory of the molten steel container is caused by slag, so about 30 minutes, more preferably 20 It is desirable to keep it to about a minute. Note that the stirring power density of the formula (1) is well known as representing gas stirring energy in a vacuum, and therefore the description thereof is omitted.
[0016]
Next, the inventor confirmed the implementation effect of the present invention with SUS304 stainless steel. Since the steel type does not carry out Ca treatment, the Ca component originally does not exist in the molten steel, but the fact that CaO is contained in the slag was used. That is, the amount of slag involved in the molten steel is set to Total. The comparison was made by the Ca concentration.
[0017]
The result is shown in FIG. The conventional method shown in FIG. 2 is a case where constant agitation is performed at a constant 300 watts / ton · steel during the deoxidation / reduction treatment period. -This is the case where the value is below steel. FIG. 2 shows that the T.V. in the molten steel after completion of the reduction / deoxidation treatment is higher than that in the case of constant treatment with the strong stirring of the conventional method. The Ca concentration decreases. Under the refining conditions of this steel type, only the CaO in the slag can be the total Ca source in the molten steel. Therefore, from this result, it is clear that the entrainment of the slag 8 into the molten steel 1 is reduced.
[0018]
【Example】
Example 1
160 tons of molten steel that has undergone rough decarburization in the converter is put into the ladle 2 and then oxygen blown in the VOD vacuum refining equipment. C: 0.055 wt%, Cr: 18.2 wt% 1 was obtained. Thereafter, 950 kg of Fe—Si alloy is added to the molten steel 1, the degree of vacuum in the apparatus is reduced to 3 torr, and Ar gas is blown as the bottom blowing gas 6 at 500 normal liters / minute, so-called deoxidation / reduction. Processing has started. Therefore, the initial stirring power density is 420 watts / ton-steel.
[0019]
After this state was continued for 15 minutes, the degree of vacuum in the apparatus was lowered to 40 torr, the Ar gas was changed to 100 N liters / minute, and the stirring power density was 47 watts / ton · steel for 7 minutes. It was.
As a result, C: 0.052 wt%, Cr: 18.3 wt%, Si: 0.35 wt%, O: 45 ppm, and T.I. Ca: 1 ppm molten stainless steel with less slag entrainment could be obtained. Further, the slab obtained from this molten steel was cut and the cross section was examined by microscopic observation, but the area ratio of the nonmetallic inclusions present was very small.
[0020]
(Example 2)
In the same manner as in Example 1, 160 tons of molten steel that had been coarsely decarburized in a converter was tapped into the ladle 2, and then oxygen blown in a VOD vacuum refining apparatus. C: 0.06 wt%, Cr: 18.1% by weight of molten steel was obtained. Thereafter, 1000 kg of Fe—Si alloy is added to the molten steel 1, the degree of vacuum in the apparatus is reduced to 3 torr, Ar gas of the bottom blowing gas 6 is blown at 500 normal liters / minute, and the stirring power density is increased. A deoxidation / reduction treatment of 420 watt / ton · steel was started.
[0021]
After maintaining this state for 15 minutes, with the vacuum inside the apparatus kept at 3 torr, the Ar gas was changed to 60 normal liters / minute, and the stirring power density was 50 watts / ton · steel for 8 minutes. went. As a result, C: 0.056 wt%, Cr: 18.2 wt%, Si: 0.38 wt%, O: 47 ppm, and T.I. Ca: 1 ppm molten stainless steel with less slag entrainment could be obtained. Further, the slab obtained from the molten steel was cut and the cross section was examined by microscopic observation, but the area ratio of the nonmetallic inclusions was almost the same as that of the example and was very small.
[0022]
(Conventional example)
In the same manner as in Example 1, 160 tons of molten steel that had been coarsely decarburized in a converter was tapped into the ladle 2, and then oxygen blown in a VOD vacuum refining apparatus. C: 0.06 wt%, Cr: 18.1% by weight of molten steel was obtained. Thereafter, 1000 kg of Fe—Si alloy is added to the molten steel 1, the degree of vacuum in the apparatus is reduced to 3 torr, Ar gas of the bottom blowing gas 6 is blown at 500 normal liters / minute, and the stirring power density is increased. A deoxidation / reduction treatment of 420 watt / ton · steel was started. Then, after this state was continued for 15 minutes, steel was produced. As a result, the molten steel components were C: 0.056% by weight, Cr: 18.2% by weight, Si: 0.38% by weight, O: 50 ppm, and T.I. Ca: 7 ppm, and it was revealed from the total Ca concentration that inclusions due to slag were involved. Moreover, when the slab obtained by this molten steel was cut | disconnected and the cross section was investigated by microscopic observation, the area ratio of a nonmetallic inclusion was 5 times of Example 1, 2, and the cleanliness was inferior. .
[0023]
【The invention's effect】
As described above, in the present invention, a deoxidation product is formed, and while being sufficiently absorbed by the slag, strong stirring is performed, and after the slag is easily caught in the molten steel, weak stirring is performed. It becomes possible to suppress the deoxidation product and slag from entraining. As a result, non-metallic inclusions are smaller than before, that is, clean molten steel can be obtained stably.
[Brief description of the drawings]
FIG. 1 is a diagram comparing the stirring power density patterns during reduction / deoxidation treatment between the conventional method and the method of the present invention, where (a) is the method of the present invention and (b) is the method of the conventional method.
[Fig. 2] Stirring power density of molten steel during reduction / deoxidation treatment and Total. It is a relationship figure with Ca density | concentration.
FIG. 3 is a view showing a VOD vacuum refining apparatus.
[Explanation of symbols]
1 Molten steel 2 Ladle 3 Sealed container 4 Lance 5 Oxygen gas 6 Inert gas (bottom blowing gas)
7 Vacuum refining equipment (secondary refining equipment)
8 Slag

Claims (2)

A vacuum rectification smelting molten steel decarburized in a device having a bottom-blown gas stirring function, in refining process of the molten steel for deoxidation treatment by introducing the deoxidizer subsequently,
The 30 minutes or less or more charged, 4, and 5 minutes of deoxidizer, and the molten steel below (1) stirring power density (epsilon) defined by the equation of 300 W / ton -steel 500 Watts / ton -steel or less And after that, stirring is performed for 5 minutes to 30 minutes under conditions where the stirring power density is 10 watts / ton-steel to 250 watts / ton-steel. Steel melting method.
ε = 0.0285 · Q · T / W · log (1 + Z / 148 × 760 / P) (1)
ε: Stirring power density (Watt / ton-steel)
Q: Bottom blowing gas flow rate (normal liters / minute)
T: Steel bath temperature (K °)
W: Steel bath weight (tons)
Z: Bath depth (cm)
P: Degree of vacuum (torr)   The method for melting high cleanliness steel according to claim 1, wherein the vacuum refining apparatus is VOD.

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