JP2006328546A - Method for producing clean steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of easily producing a clean steel extremely low in oxygen content without impairing productivity by performing the whole process of a secondary refining under a pressure-reducing condition. <P>SOLUTION: In the producing method for clean steel, the secondary refining for molten steel is separated into a first process and a second process, and in the first process, the refining using slag under reduced pressure to the molten steel held in a ladle and the temperature-rising of the molten steel, are performed, and successively, in the second process, the treatment for separating and removing inclusions by circulating the molten steel between the ladle and a reduced pressure vessel. It is desirable to be ≤1.33×10<SP>4</SP>Pa vacuum degree in the atmosphere in contact with the molten steel, ≤1.25 L/min/t supplying quantity of inert gas from the bottom part of the ladle, ≤50 kW input power and ≥10 min the circulating time of the molten steel. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing clean steel with a low content of non-metallic inclusions, and more specifically, in the secondary refining of molten steel, the clean steel is stabilized using a vacuum arc degassing furnace and a reflux vacuum degassing apparatus. And the method of melting.

  In recent years, clean steel typified by bearing steel has been required to have higher cleanliness. To achieve this, the content of non-metallic inclusions that greatly affect fatigue strength and cold workability is reduced. There is a need to reduce the oxygen content, which is a guideline for the cleanliness of steel. Correspondingly, the production of clean steel has been conventionally performed by the vacuum arc degassing method (hereinafter also referred to as “VAD method”) or the ladle refining (LF) -circular vacuum degassing (RH) process method (hereinafter referred to as “VAD method”) , Also referred to as the “LF-RH method”). However, the VAD method and the LF-RH method have the following problems.

  In the production of clean steel by the VAD method, since the refining process is performed under reduced pressure, there is an advantage that it is not affected by the oxidation of molten steel due to air intrusion during the process, but the slag in the ladle during the degassing process Since it is difficult to control the flow, there are inclusions in the steel due to slag and inclusions due to refractory erosion due to slag flow. For these reasons, the oxygen content in steel that can be stably produced by the VAD method is about 8 ppm, which is not sufficient to meet the strict standards for today's clean steel.

  In order to solve this problem, Patent Document 1 has a refining furnace vessel that accommodates a molten steel ladle having an inert gas inlet at the bottom, and an electrode for arc heating, and heating or removing the molten steel under vacuum. A refining furnace that performs gas, and has a cylindrical refractory (snorkel) that is immersed in the molten steel during degassing to secure an exposed portion of the molten steel. A refining furnace is disclosed in which a ratio to the inner diameter of the pan is defined and the immersion depth of the cylindrical refractory into the molten steel is adjustable. If this furnace is used, it becomes difficult to stir the molten steel in the ladle by appropriately increasing the inner diameter of the snorkel immersed in the molten steel, and therefore it is possible to suppress the slag entrainment. However, on the other hand, since the refractory is newly immersed in the molten steel, the amount of refractory that is melted by arc heating increases, and the amount of inclusions attributed to the refractory may increase.

  On the other hand, in the production of clean steel by the LF-RH method, the refining by slag and the floating separation function of inclusions are shared by the LF and RH processes, so the amount of inclusions in steel due to slag and refractory refractories is Compared to the case of the VAD method, the level is low. However, since the LF treatment is performed in an air atmosphere, the entrainment of air cannot be completely prevented, which causes the oxygen content in the steel to increase.

  In response to this problem, Patent Document 2 discloses that in a steel manufacturing process in which molten steel is transferred to a ladle refining furnace and refined and then subjected to reflux vacuum degassing, refining in the ladle refining furnace is performed for 60 minutes or less. And a manufacturing method of high cleanliness steel in which degassing is performed for 25 minutes or more by setting the ring flow rate of molten steel by a reflux vacuum degassing apparatus to 8 times or more of the total molten steel. According to the method disclosed in this document, high purity steel is manufactured by preventing the increase in oxygen content in steel due to contamination from refractories or slag and preventing the formation of large inclusions. It is supposed to be possible.

  However, from the viewpoints of productivity and manufacturing cost, the circulation time that is industrially acceptable in the reflux type vacuum degassing apparatus is usually limited to about 15 to 20 minutes at the longest. Therefore, it is difficult to stably obtain a steel having a low oxygen content by applying this method to actual operation. For this reason, the oxygen content of steel that can be stably produced by the LF-RH process method is about 6 ppm. Is not enough.

JP-A-9-170015 (Claims and paragraph [0010]) JP 2001-342516 A (Claims and paragraphs [0007] to [0012])

  The present invention has been made in view of the problems of the above-described conventional methods, and by purifying molten steel under reduced pressure, it is easy to clean the oxygen content of 4.5 ppm or less without hindering productivity. It aims at providing the manufacturing method of the clean steel which can manufacture steel.

  In order to solve the above-mentioned problems, the present inventors effectively remove inclusions in steel derived from slag and refractory refractories without being oxidized by the atmosphere, with a reduction in productivity. The present invention has been completed by studying a method for producing clean steel that can easily melt clean steel having a low oxygen content, and obtaining the following findings (a) to (d).

  (A) Secondary refining of molten steel is separated into molten steel refining using slag (hereinafter also referred to as “slag refining”) and molten steel heating process, and separation and removal process of inclusions in molten steel by circulating molten steel. By performing both steps under reduced pressure, it is possible to produce a clean steel having an extremely low oxygen content by preventing the oxidation of molten steel by the atmosphere and efficiently removing inclusions in the steel. .

(B) In order to sufficiently suppress the oxidation of molten steel by the atmosphere in both steps (a) and to promote stirring of the molten steel, the degree of vacuum of the atmosphere is set to 1.33 × 10 ⁴ Pa or less (100 Torr or less). It is preferable. In addition, when performing slag refining of molten steel, an inert gas such as argon gas is supplied as a stirring gas from the bottom of the ladle, and the supply amount is in a range in which it is difficult to promote slag entrainment in the molten steel. It is preferable to use 1.25 liters (L) / min or less (hereinafter also referred to as “1.25 L / min / t or less”) per ton (t) of molten steel.

  (C) When the molten steel is heated, from the viewpoint of suppressing the deterioration of the cleanliness of the molten steel due to the melting loss of the ladle refractory, the input electric energy for the heating is 50 kW or less per 1 ton (t) of molten steel (hereinafter, And “50 kW / t or less”).

  (D) In the step of separating and removing inclusions in the molten steel by circulating the molten steel, it is preferable to set the circulating time of the molten steel to 10 minutes or more in order to sufficiently separate and remove the inclusions in the molten steel.

  This invention is completed based on said knowledge, The summary exists in the manufacturing method of the clean steel shown by following (1)-(5).

  (1) Secondary refining of molten steel is separated into a first step and a second step. In the first step, the molten steel accommodated in the ladle is subjected to refining using slag under reduced pressure and a heat treatment of the molten steel. In the subsequent second step, the inclusions are separated and removed by circulating the molten steel between the ladle and the decompression tank (hereinafter also referred to as “first invention”). ).

(2) In the secondary refining of the molten steel, the degree of vacuum of the atmosphere in contact with the molten steel is 1.33 × 10 ⁴ Pa or less. Hereinafter also referred to as “second invention”).

  (3) In performing refining using the slag, the inert gas is supplied from the bottom of the ladle at a flow rate of 1.25 liters (L) / min or less per 1 ton (t) of molten steel. (1) The manufacturing method of the clean steel as described in (2) (henceforth "the 3rd invention").

  (4) Any of the above (1) to (3), wherein the amount of electric power supplied for heating is 50 kW or less per 1 ton (t) of molten steel when the molten steel is heated. The method for producing clean steel according to (1) (hereinafter also referred to as “fourth invention”).

  (5) The method for producing clean steel according to any one of (1) to (4) above, wherein in the separation and removal of inclusions by recirculation of the molten steel, the recirculation time of the molten steel is 10 minutes or more. (Hereinafter also referred to as “fifth invention”).

  In the present invention, "secondary refining" is a steel produced from a converter or arc melting furnace in the steel making process, for the purpose of adjusting the amount of gas components in the molten steel, reducing the amount of inclusions, etc. It means refining that is performed in an atmosphere such as an inert gas atmosphere under reduced pressure.

“Under reduced pressure” means that the atmospheric pressure is 1.6 × 10 ⁴ Pa or less (120 Torr or less), and “clean steel” means the oxygen content, that is, dissolved oxygen in steel and inclusions in inclusions. It means steel having a total oxygen content of 4.5 ppm or less, which is the total amount of oxygen.

  The “inert gas” means an element belonging to Group 18 of the periodic table, for example, a gas such as argon, helium, or neon, and argon gas is generally used in a steel refining process.

  According to the method for producing clean steel of the present invention, the slag refining and heating process of molten steel and the separation and removal process of inclusions are separated, and the treatment in the series of refining processes is performed in a reduced-pressure atmosphere. Without receiving, the inclusions in the steel derived from the slag and the refractory refractory can be efficiently separated and removed. As a result, according to the method of the present invention, it is possible to stably and easily produce a clean steel having a very low oxygen content in the steel without a decrease in productivity due to an extension of the reflux time in the RH degassing process. can do.

  In the method of the present invention, as described above, secondary refining of molten steel is separated into a first step and a second step, and in the first step, refining using slag under reduced pressure is performed on the molten steel contained in the ladle. In addition, in the subsequent second step, the molten steel is subjected to a heat treatment, and in the subsequent second step, the inclusion is separated and removed by circulating the molten steel between the ladle and the vacuum tank. Hereinafter, the method of the present invention will be described in more detail.

  FIG. 1 is a system diagram schematically showing a method for producing clean steel according to the present invention. In FIG. 1, a portion surrounded by a broken line is a process range targeted by the present invention.

  1) The molten steel oxidatively refined by an arc melting furnace or converter is adjusted to a desired component composition and temperature, and then poured into a ladle.

  2) After removing the slag in the ladle with a slag stopper, slag dragger, etc., place the ladle in a vacuum tank such as a vacuum arc degassing furnace (VAD) and add slag to the top of the molten steel in the ladle. To do.

3) In the first step of secondary refining, an inert gas such as argon gas is supplied as a stirring gas from the bottom of the ladle, and the molten steel in the ladle is reduced and refined (treatment indicated by (a) in FIG. 1). . The degree of vacuum of the atmosphere in contact with the molten steel during the refining treatment needs to be a degree of vacuum that can sufficiently prevent the molten steel from being oxidized by the atmosphere and promote stirring of the molten steel, and is 1.33 × 10 ⁴ Pa. The following is preferable.

In order to obtain the expected effect, it is not necessary to specify the lower limit of the degree of vacuum.However, in actual operation, if the set vacuum level is too low, the time required to reach the set vacuum level becomes longer. As a result, the processing time is extended. Therefore, from the viewpoint of operational efficiency, the degree of vacuum is preferably 1.33 × 10 ³ Pa or more.

  4) In the first step of secondary refining of 3) above, in order to confirm the preferable flow rate range of the inert gas supplied as the stirring gas from the ladle bottom, the supply amount of the inert gas is adjusted to the oxygen content in the steel. A survey test was conducted on the effect. The test was performed by using molten steel having the component composition shown in Table 1 and changing the Ar gas supply amount under the conditions shown in Table 2.

  As shown in Table 2, in the above investigation test, in the VAD method used in the first step of secondary refining, the flow rate of Ar gas supplied from the bottom of the ladle into the molten steel was changed, and the oxygen content in the steel The effects on

  FIG. 2 is a graph showing the effect of the Ar gas supply amount in the first step on the oxygen content in steel after the treatment according to the method of the present invention. According to the results of FIG. 2, the oxygen content in the steel decreases as the Ar gas supply rate decreases, and in order to keep the oxygen content in the steel to 4.0 ppm or less, the Ar gas supply amount is 1.25 L / It turns out that it is preferable to set it as min / t or less. The reason for this is that slag entrainment in molten steel is promoted when the flow rate of the inert gas is too large.

  In addition, since the stirring power of molten steel will fall if there is too little flow volume of an inert gas, it is preferable that Ar gas supply amount shall be 0.63 L / min / t or more from a viewpoint of ensuring the effect of reductive refining.

  5) In parallel with the reduction refining in the first step of secondary refining in 3) above, the molten steel is subjected to a heat treatment (treatment shown by (b) in FIG. 1). The heat increase may be performed by a general heat increase method in which the molten steel is energized through an electrode inserted in the molten steel.

  In order to investigate the effect of the input power amount for the heat treatment of the molten steel on the oxygen content in the steel, the molten steel having the component composition shown in Table 1 was used in the same manner as described above. The test was conducted by changing the conditions of the VAD method as shown in Table 3.

  FIG. 3 is a diagram showing the effect of the input heat intensity for heating in the first step on the oxygen content in steel after the treatment according to the present invention method. From the results shown in FIG. 3, in order to increase the oxygen content in the steel as the heating power input unit increases, and to reduce the oxygen content in the steel to 4.0 ppm or less, the heating power input power source is increased. It can be seen that it is preferable to set the unit (heating input power amount per 1 ton of molten steel) to 50 kW / t or less. The reason is that if the heating power input unit for heating is higher than 50 kW / t, the amount of erosion of the ladle refractory increases and the cleanliness of the molten steel tends to decrease.

  6) Furthermore, the coarse adjustment of the component composition of the molten steel may be performed together with the reduction refining and the temperature increase of the molten steel in the second step of the secondary refining of the above 3) to 5) (treatment shown by (c) in FIG. ).

7) By recirculating the molten steel after reduction refining, heating of the molten steel and rough adjustment of the component composition using a recirculating vacuum degassing apparatus such as an RH vacuum degassing apparatus in the second step of secondary refining. The molten steel is degassed (treatment indicated by (d) in the figure). The final adjustment of the molten steel component composition can be performed together with the above treatment. In the second step of secondary refining, if the reflux time of the molten steel is short, there is a possibility that the inclusions are not sufficiently separated and removed, so the reflux time is preferably 10 minutes or more. Note that the reflux time is preferably 15 minutes or less from the viewpoint of ensuring productivity and suppressing manufacturing costs.
Moreover, it is preferable that the vacuum degree in a decompression tank shall be 1.33 * 10 < ⁴ > Pa or less similarly to the case of (a) in the 1st process of the said secondary refining.

  According to the above method, the slag refining function of molten steel and the separation and removal function of inclusions in the molten steel are separated, and a series of refining treatments are performed under reduced pressure, so the influence of molten steel oxidation by the atmosphere is completely eliminated Thus, inclusions in steel caused by slag and refractory refractories can be separated and removed efficiently. As a result, it is possible to stably and easily manufacture a clean steel having an oxygen content in steel of 4.5 ppm or less.

  In order to confirm the effect of the method for producing clean steel according to the present invention, the following steel production tests were conducted and the results were evaluated.

  Using molten steel having the component composition shown in Table 1, refining was performed according to the process shown in FIG. Table 4 shows the test conditions for the inventive examples and comparative examples.

  In the same table, test numbers 1 to 6 are tests for the present invention examples that satisfy the conditions specified in the present invention. The VAD method is used as the first step of secondary refining, and the RH vacuum is used as the second step. A degassing method was used. On the other hand, test number 7 is a test for a comparative example in which only the processing by the VAD method in the first step of the secondary refining is performed and the processing in the second step is not performed. This is a test for a comparative example using the LF method in which the treatment under atmospheric pressure is performed as one step and the LF-RH method using the RH method as the second step.

  FIG. 4 is a diagram showing a state of reduction of the oxygen content in steel in the test for the inventive example and the comparative example.

  In the test numbers 1 to 6 of the present invention examples that satisfy the conditions specified in the present invention, the oxygen content in the steel is 3 in comparison with the test numbers 7 and 8 of the comparative examples that do not satisfy the conditions specified in the present invention examples. It is reduced to about 5-4.5 ppm, and the cleanliness of steel is remarkably improved.

  Among the test numbers 1 to 6, the tests of test numbers 1 and 6 are the degree of vacuum specified by the second invention, the supply amount of inert gas specified by the third invention, the input power amount specified by the fourth invention and 5 This test satisfies all conditions of the reflux time of molten steel specified in the invention. In these tests No. 1 and No. 6, Test No. 2 that does not satisfy the above-mentioned vacuum condition, Test No. 3 that does not satisfy the inert gas supply amount condition, Test No. 4 that does not satisfy the input power amount condition, and The oxygen content in the steel is lower than in any case of test number 5 that does not satisfy the condition of the molten steel reflux time, and particularly good cleanliness is obtained.

  In contrast to the above-described examples of the present invention, in test numbers 7 and 8, which are tests for comparative examples, the oxygen content in the steel was as high as about 6 to 8 ppm, and the cleanliness of the steel was inferior.

  According to the method for producing clean steel of the present invention, the slag refining and heating process of molten steel and the separation and removal process of inclusions are separated, and the treatment in the series of refining processes is performed in a reduced-pressure atmosphere. Without being received at all, the inclusions in the steel derived from the slag and the refractory refractory can be separated and removed efficiently. As a result, according to the method of the present invention, it is possible to stably and easily produce a clean steel having a very low oxygen content in the steel without a decrease in productivity due to an extension of the reflux time in the RH degassing process. can do. Therefore, the method of the present invention can be widely applied in the refining technical field as a clean steel manufacturing method that can easily manufacture clean steel having an oxygen content of 4.5 ppm or less without impairing productivity.

It is a systematic diagram which shows the method of this invention typically. It is a figure which shows the effect of the Ar gas supply amount in the 1st process which has on the oxygen content in the steel after the process by this invention method. It is a figure which shows the effect of the input power basic unit for the heating in the 1st process which has on the oxygen content in steel after the process by this invention method. It is a figure which shows the reduction condition of the oxygen content in steel by this invention method and a comparative method.

Claims (5)

  The secondary refining of the molten steel is separated into the first step and the second step. In the first step, the molten steel accommodated in the ladle is subjected to refining using slag under reduced pressure and the heat treatment of the molten steel. A process for producing clean steel characterized in that in two steps, the inclusions are separated and removed by circulating the molten steel between the ladle and the vacuum tank. 2. The method for producing clean steel according to claim 1, wherein in performing the secondary refining of the molten steel, a degree of vacuum of an atmosphere in contact with the molten steel is 1.33 × 10 ⁴ Pa or less.   The refining using the slag is characterized in that an inert gas is supplied from the bottom of the ladle at a flow rate of 1.25 liters (L) / min or less per 1 ton (t) of molten steel. 2. The method for producing clean steel according to 2.   The clean steel according to any one of claims 1 to 3, wherein the amount of electric power supplied for heating is set to 50 kW or less per ton (t) of molten steel in performing the heating of the molten steel. Production method. The method for producing clean steel according to any one of claims 1 to 4, wherein in the separation and removal of inclusions by recirculation of the molten steel, the recirculation time of the molten steel is 10 minutes or more.

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