JP6435983B2 - Method for refining molten steel - Google Patents

Description

  The present invention is used in secondary refining of molten steel such as carbon steel, low alloy steel, and alloy steel, and a method for refining molten steel for performing treatments such as degassing, decarburization, temperature adjustment, component adjustment, and inclusion removal. About.

  Generally, molten steel decarburized in a converter is subjected to a steel refining process called secondary refining. Secondary refining includes a number of processing methods such as inert gas blowing refining, tank degassing refining, and arc heating refining, but the vacuum tank connected to the vacuum exhaust system and the riser pipe attached to the bottom of the vacuum tank and the descent The riser is provided with a recirculation gas blowing tuyere, and after the riser and downcomer are immersed in the molten steel in the ladle, the molten steel in the ladle is sucked into the vacuum chamber, 2. Description of the Related Art Molten steel refining treatment equipment (hereinafter referred to as RH) that circulates molten steel by blowing recirculation gas from the tuyere is widely used.

  In RH, the molten steel is circulated between the vacuum tank and the ladle so that the reaction such as degassing in the vacuum tank and the mixing in the ladle proceed simultaneously. For this reason, it is most important to smoothly circulate the molten steel in order to enhance various refining effects in RH.

  As an index indicating the circulation of the molten steel, an index which is an annular flow rate is widely used, and is indicated by a molten steel mass moving between the vacuum tank and the ladle per unit time.

  The recirculation flow rate depends on the flow rate of the recirculation gas blown into one dip tube (rising tube) and the cross-sectional area of the dip tube, but the upper limit value of the recirculation gas flow rate depends on the cross-sectional area of the dip tube. Changing the flow of molten steel in the vacuum chamber by increasing the area or the inner diameter of the dip tube is the mainstream of the technical idea for enhancing the RH refining capacity. A technique is disclosed in which a pipe is formed integrally and a partition is provided between the two to increase the effective cross-sectional area of both, thereby increasing the flow rate of the molten steel.

  In Patent Document 2, the shape of the vacuum tank is an ellipse, and two dip pipes are arranged in the long axis direction, thereby eliminating the stay of molten steel and slag circulating in the vacuum tank, and the decarburization reaction or dehydrogenation reaction. A technique for promoting the above is disclosed.

  In Patent Document 3, swirl flows to the molten metal in the vacuum chamber by shifting at least one of the positions of the upper end opening and the lower end opening of the rising tube and the positions of the upper end opening and the lower end opening of the descending tube in the circumferential direction of the vacuum chamber. And a technique for increasing the degassing rate and inclusion removal rate is disclosed.

  In Patent Document 4, the shape of the dip tube is a double tube structure, the molten steel is sucked up from the inner tube into the vacuum chamber, the molten steel is discharged from the outer tube into the ladle, and the circulation rate of the molten steel between the ladle and the vacuum chamber is increased. However, a technique for improving the degassing rate is disclosed.

Japanese Patent Laid-Open No. 2-200721 JP-A-4-272120 Japanese Patent Laid-Open No. 6-10027 JP-A-8-193215

  As described above, a number of techniques for increasing the ring flow rate have been disclosed in the prior art. However, in the prior art, a method for macroscopically grasping and analyzing the flow in the entire apparatus is generally used.

  For example, as a typical development method, there is a uniform mixing time measurement method. In this method, by measuring the mixing time, the influence of the apparatus shape and apparatus operating conditions on the entire mixing, that is, the entire flow in the RH, is quantified, and the optimum mixing time is measured. You can explore the conditions.

  By applying such a development technique, a number of excellent ring flow high level stabilization techniques have been developed. These technological developments greatly improve the various functions required by RH, such as degassing of hydrogen and nitrogen, decarburization to extremely low concentrations, final adjustment of alloy component concentration, temperature adjustment, and removal of non-metallic inclusions. Has been improved.

  On the other hand, in recent years, demands for non-metallic inclusions (hereinafter simply referred to as inclusions) in high-performance, high-function steel materials have become stricter, and further cleanliness is required. At the same time, as the demand for high-performance, high-function steel materials increases, it is also required to produce steel materials with high cleanliness more efficiently. However, there are some problems in the prior art to meet these requirements.

  The first problem is a decrease in productivity. Since the time required to remove inclusions necessary to achieve high cleaning becomes longer, the RH treatment time is greatly extended. For this reason, productivity will fall significantly.

  The second problem is the limit of cleanliness. Since removal of minute inclusions, particularly inclusions of 20 μm or less, is difficult in mass-produced steel, it has been considered that there is a limit to the cleanability obtained particularly in mass-produced steel.

  That is, although it has become possible to produce steel having higher cleanliness than before by the prior art, it has been difficult to satisfy the recent high requirement level for cleanliness.

  Furthermore, in order to improve cleanliness, methods such as adding processing by a refining device other than RH that is generally used, or remodeling the RH device itself, can be envisaged. However, these methods are costly and productive. A new problem arises in this aspect.

  This invention solves the problem in the said prior art, and provides the RH refining method of the molten steel which can have high cleanliness in a short time, without adding the big remodeling and refining process of a RH apparatus. It is said.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors, in addition to the downflow in the downcomer of the RH apparatus, by controlling to a specific condition in the molten steel refining treatment method using the RH apparatus. It has been found that the cleanliness of the molten steel is improved by the generation of a circulating flow, and the agglomeration and coalescence of inclusions progressing and being lifted and removed.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(A) a vacuum tank connected to an evacuation device; a riser pipe and a downfall pipe attached to the bottom of the vacuum tank; the riser pipe is provided with a bubbling tuyere of reflux gas; After the dipping pipe is immersed in the molten steel in the ladle, the molten steel in the ladle is sucked into the vacuum chamber, and then blown into the ascending pipe and the recirculation gas is blown from the tuyere to circulate the molten steel. In the refining treatment method,
The relation between the inner wall of the riser and the inner wall of the downcomer, the minimum horizontal distance D, the inner diameter d of the downcomer, and the bath depth H in the vacuum chamber satisfies the following formula (1). Method.
0.03 ≦ H / (D + d) ≦ 0.10 (1)
(B) The molten steel refining method according to (a), wherein a flux containing 30% by mass or more of CaO is added to the molten steel in the vacuum chamber or the molten steel in the ladle.

  According to the present invention, steel having high cleanliness can be produced in a short time without adding significant remodeling or refining treatment of the RH device.

Schematic diagram of the flow in a normal RH descending dip tube Schematic diagram of flow in controlled RH downside dip tube Diagram showing the relationship between A value and inclusion number index Diagram showing the relationship between A value and uniform mixing time index Figure comparing the number of inclusions with and without flux The figure which shows the relationship between CaO concentration in flux and the number of inclusions

  Hereinafter, the present invention will be described in detail.

  In adding studies for solving the above-mentioned problems, the present inventors pay attention not to the macroscopic flow in the entire RH device such as the ring flow rate but to the microscopic flow generated in each part of the device. And examined.

  Using the RH type water model experimental apparatus (water volume 150 L) and particles (diameter 0.5 mm or less) simulating inclusions, the flow state and inclusion movement of each part of the RH were closely observed, and the following was found.

  In the RH vacuum degassing apparatus, as shown in FIG. 1, molten steel 9 in the ladle is sucked up into the vacuum chamber 1 as an upward flow 6 from the ascending pipe 2, and from the RH descending side immersion pipe (hereinafter referred to as the descending pipe 3). A flow returning to the molten steel 9 in the ladle is formed as the downward flow 7. As shown in FIG. 1, the flow in the downcomer 3 is a downflow 7 that flows down from the vacuum chamber 1 to the molten steel 9 in the ladle, and the downcomer 3 is filled with the downflow 7. However, by controlling to a specific condition, a circulating flow is generated in the downcomer in addition to the downflow. This flow is shown in FIG. It was confirmed that inclusion agglomeration proceeded by the circulating flow 8 generated in the downcomer 3 shown by a broken line in FIG.

  Since the inclusions are taken into larger inclusions by this aggregation and coalescence, the floating becomes easy and the RH treatment time required for improving the cleanliness can be shortened. In addition, since fine inclusions that have a low flying speed and are difficult to float and separate are taken into larger inclusions, a high cleanliness with reduced fine inclusions that has been difficult in the past can be obtained.

Therefore, the circulation flow generation condition and inclusion removal effect were quantified by the water model experiment. From the experiment, it was concluded that the generation of the circulating flow and the change in the number of inclusions can be arranged by equation (1). In addition, H is the bath depth in a stationary state where no reflux is performed.
0.03 ≦ H / (D + d) ≦ 0.1 (1)
Where D: shortest horizontal distance between the inner wall of the riser and the inner wall of the downcomer, d: inner diameter of the downcomer, H: bath depth in the vacuum chamber

  Hereinafter, H / (D + d) is defined as an A value. FIG. 3 shows the relationship between the A value and the simulated inclusion particle number index in water 180 s after the start of RH reflux. The number of inclusions was indexed with the number at A value = 0.01 being 1. The circulation was stopped 180 s after the start of RH reflux, 100 cc of water was collected from the water in the ladle, and the number of particles separated by filtration was counted using a stereomicroscope. It can be seen from FIG. 3 that the number of inclusions is reduced when the A value is in the range of 0.03 to 0.1. From the result of visual observation of the experiment, when the A value was 0.1 or less, it was confirmed that the circulating flow 8 shown in FIG. 2 was generated. Aggregation and coalescence proceed, and as a result, the number of inclusions is reduced.

  On the other hand, when the A value is less than 0.03, the circulating flow 8 is generated, but the number of inclusions is increased. The reason for this will be described. FIG. 4 shows the relationship between the A value and the uniform mixing time. The uniform mixing time was indexed assuming that the uniform mixing time when the A value was 0.2 was 1. It can be seen from FIG. 4 that when the A value is less than 0.03, the uniform mixing time increases abruptly. That is, the flow of the entire RH, that is, the ring flow rate is reduced. From the result of the visual observation of the experiment, when the A value is less than 0.03, the circulating flow 8 in the downcomer 3 shown in FIG. 2 develops, and the original downward flow 7 from the vacuum chamber 1 to the molten steel 9 in the ladle is generated. It was confirmed that the ring flow rate was reduced. Therefore, when the A value is less than 0.03, inclusion coalescence and coalescence are promoted. On the other hand, the circulating flow of the entire RH is reduced, so that the removal of inclusions is difficult to proceed.

From the above, it has the vacuum chamber 1 connected to the vacuum exhaust device 11, the rising pipe 2 and the down pipe 3 attached to the bottom of the vacuum tank 1, and the rising pipe 2 has the bubbling tuyere 12 for circulating gas. The riser 2 and the downfall pipe 3 are immersed in the molten steel 9 in the ladle and the molten steel 9 in the ladle is sucked into the vacuum chamber 1, and then blown into the riser 2 and the reflux gas is blown from the tuyere 12. In the molten steel refining apparatus for circulating molten steel, the relationship between the shortest horizontal distance D between the inner wall of the riser 2 and the inner wall of the downcomer 3, the inner diameter d of the downcomer 3, and the bath depth H in the vacuum chamber is expressed by the following formula (1) The inclusions can be greatly reduced by the molten steel refining treatment method characterized by satisfying the above.
0.03 ≦ H / (D + d) ≦ 0.1 (1)
H can be obtained by the following method. When the atmospheric pressure is P ′ (Pa) and the pressure in the vacuum chamber is P (Pa), the drop h (m) between the molten steel surface 4 in the ladle and the molten steel surface 5 in the vacuum chamber is expressed by equation (2).
h = {(P′−P) + ρ _S · g · h _S } / (ρ _M · g) (2)
Here, ρ _M : molten steel density (kg / m ³ ), ρ _S : slag density (kg / m ³ ), h _S : slag thickness (m), g: gravitational acceleration (m / s ² ). Next, assuming that the vertical distance between the lower end surface of the dip tube and the bottom surface of the tank in the vacuum chamber is h ′ (m) and the immersion depth of the dip tube in molten steel is h _d (m), H (m) is (3) It is shown by the formula.
H = h− (h′−h _d ) (3)

  Next, the inventors focused on the circulating flow 8 in the downcomer 3 and studied further methods for removing inclusions. In the previous experiment, we investigated the conditions for promoting the floating removal of inclusions by utilizing the aggregation and coalescence of inclusions. Further, in order to promote the floating removal of inclusions, the size of inclusions after aggregation and coalescence should be increased, but there is a limit in the aggregation and coalescence of inclusions. Therefore, a method has been conceived in which large particles to be aggregated and coalesced with inclusions are positively added to the molten steel. In the same experiment as FIG. 3, flux particles having a particle diameter of 4 mm were continuously added by a method of spraying on the water surface in the vacuum chamber, and the number of simulated inclusion particles in water was measured. The number of inclusions was indexed with A value = 0.01 and the number without flux as 1. The circulation was stopped 180 s after the start of RH reflux, 100 cc of water was collected from the water in the ladle, and the number of particles separated by filtration was counted using a stereomicroscope.

  The results are shown in FIG. FIG. 5 also shows the results shown in FIG. 3 as experimental results without flux. From the figure, it is clear that the number of inclusions decreases as a result of the addition of flux, but this is caused by coalescence of flux particles with inclusion simulation particles.

  Furthermore, it can be seen that the inclusion removal effect by the flux is further enhanced in the A value range of 0.03 to 0.1 in which the circulating flow 8 is generated and uniform mixing can be achieved. This is because coalescence of the flux particles and inclusions is further promoted by the circulating flow.

The above water model experiment found that inclusion removal was promoted by flux addition. The flux composition capable of adsorbing inclusions when in contact with inclusions was investigated by molten steel experiments. 15 kg of molten steel was maintained at 1873 K, and a deoxidizer such as Al and Si was added in a predetermined amount in a range of 0.01 to 0.2% by mass concentration in the molten steel to generate inclusions. Thereafter, flux was added to the molten steel and held for 10 minutes, and then a sample was taken from the molten steel, and the number of inclusions of 5 μm or more was measured with an optical microscope. The flux used in the experiment is CaO, 0 to 50% CaO, and the remainder is Al ₂ O ₃ or MgO CaO—Al ₂ O ₃ based flux, CaO—MgO flux. The results are shown in FIG. In any flux, the number of inclusions at a CaO concentration = 0% was taken as 1 and indexed. FIG. 6 shows that the number of inclusions can be stably reduced by setting the CaO concentration to 30% or more regardless of the type of flux. Accordingly, the CaO concentration in the flux is preferably 30% by mass or more.

  From the above, in addition to satisfying the formula (1), further inclusion removal is possible by adding a flux containing 30 mass% or more of CaO to the molten steel in the vacuum chamber or the molten steel in the ladle.

  Embodiments for carrying out the present invention will be described below.

  In order to carry out the present invention with the existing RH, there is a method of adjusting both the inner diameter d of the downcomer pipe and the bath depth H in the vacuum chamber, or either d or H, but it is easiest to adjust H. It is.

  As a method of adjusting H, a method of changing the stack height of the tank bottom refractories in the vacuum chamber, and when there is a margin in the immersion tube length, H is adjusted by adjusting the vertical distance between the vacuum chamber and the ladle. And a method of adjusting the degree of vacuum.

  However, in the method of adjusting the vertical distance, since the immersion depth of the dip tube into the molten steel in the ladle becomes shallow, problems such as entrainment of ladle slag are likely to occur. Further, in the method of adjusting the degree of vacuum, when the adjustment in the tank pressure is increased, the effect is slightly reduced because the ring flow rate is reduced.

  Therefore, when adjusting H, a more stable effect can be obtained by adjusting the stacking height of the refractories in the vacuum chamber.

  In addition, since the present invention does not restrict other RH treatments such as alloy adjustment and degassing treatment, component adjustment by alloy addition and degassing treatment can be carried out under existing conditions. Further, in order to promote inclusion removal, H may be adjusted after component adjustment or degassing treatment.

  However, in the process of blowing oxygen gas to the surface of the molten steel through the upper blowing lance installed in the vacuum chamber, it is necessary to pay attention to the adjusted H and the dynamic pressure of the upper blowing oxygen gas jet on the molten steel surface in the vacuum chamber. When an oxygen gas jet having a high dynamic pressure is used, if H is excessively shallow, the refractory at the bottom of the tank may be damaged. Therefore, it is necessary to adjust H or the lance height.

  Moreover, when performing the process which sprays oxygen gas on the molten steel surface via the top blowing lance installed in the vacuum chamber, it is desirable to implement before this invention. This is because in the treatment of blowing oxygen gas, inclusions are generated by the reaction of the blown oxygen with molten steel, Al, Si, Mn, etc. in the molten steel, and therefore it is desirable to carry out before the present invention for removing inclusions.

  For the same reason, it is desirable that metals and alloys that form inclusions by deoxidation, such as rare earth elements such as Al, Si, Mn, Ti, and Ce, and Zr, are also completed before the present invention is implemented. That is, it is desirable that the processing in the case of carrying out the present invention is performed in the order of deoxidation element addition, oxygen spraying, and the present invention.

  The atmospheric pressure in the vacuum chamber when carrying out the present invention may be a pressure that can maintain the molten steel reflux, but is preferably 7 kPa or less, and more preferably 1 kPa or less. When the pressure exceeds 7 kPa and the pressure is high, the circulation time is slow and the processing time becomes long. On the other hand, by setting the pressure to 1 kPa or less, the inclusion supply speed to the circulation flow is increased, so that the effect is more stable.

  The treatment time that satisfies the conditions of the present invention is preferably from 3 minutes to 20 minutes, and more preferably from 6 minutes to 10 minutes. If it is less than 3 minutes, it is difficult for all the molten steel to come into contact with the circulating flow 8, and by setting it to 6 minutes or more, the molten steel can come into contact with the circulating flow 8 twice, so that the effect becomes more stable. If it exceeds 10 minutes, the effect is saturated, and if it is longer than 20 minutes, wear of the refractory is induced.

The flux used in the invention of claim 2 needs to be a substance having a specific gravity lighter than that of molten steel. Specifically, CaO, CaO—Al ₂ O ₃ , CaO—CaF ₂ , and CaO—MgO are widely used. The CaO-based flux is acceptable. In addition, the use of CaO—Al ₂ O ₃ or CaO—Al ₂ O ₃ —SiO ₂ type fluxes is promoted by the use of a low melting point flux with a high liquid phase rate in molten steel, which promotes agglomeration with inclusions. Is desirable. In addition, Si, Al, Ca, Mg, Ti, rare earth metals, etc. as auxiliary deoxidizing materials may be mixed with these oxide fluxes in the range of 5 to 20% by mass.

  The flux can be added to the molten steel by any method, such as by blowing it into the molten steel in the ladle or by spraying it on the molten steel surface in the vacuum chamber. desirable.

  The amount of flux added is desirably 3 to 5 kg per ton of molten steel. If it is less than 3 kg / ton, the removal efficiency of fine inclusions is slightly lowered, and if it exceeds 5 kg / ton, the effect is saturated.

  The flux addition rate is desirably 0.1 to 1.3 kg / min per ton of molten steel. If it is less than 0.1 kg / min · ton, the processing time becomes too long and the operation load increases. On the other hand, if the speed exceeds 1.3 kg / min · ton, the agglomeration and coalescence of the fluxes proceed excessively and become enlarged, so that it is difficult to be caught in the molten steel, and the effect is reduced.

  A 300 ton molten steel decarburized in the converter was taken out into the ladle. Al, Si, Mn, and CaO were added into the ladle at the time of steel removal to reduce oxygen in the molten steel and FeO in the slag in the ladle.

  After steeling out, the ladle was immediately transferred to RH and processing was started. The shortest horizontal distance D between the inner wall of the riser 2 and the inner wall of the downcomer 3 and the inner diameter d of the downcomer 3 were used in the RH vacuum degassing apparatus. After the treatment was started, an alloy was added to the molten steel while recirculating at a vacuum tank pressure of 9 kPa to adjust the molten steel components. Thereafter, the pressure in the vacuum chamber was adjusted to 5 kPa, and after adding Al to the molten steel, oxygen gas was sprayed onto the molten steel through an upper blowing lance installed in the vacuum chamber to adjust the molten steel temperature.

After completion of oxygen top blowing, the pressure inside the vacuum chamber was adjusted to 800 Pa, and a reflux treatment was performed. The reflux time was 10 minutes. At this time, the bath depth H in the vacuum chamber was changed by adjusting the distance between the vacuum chamber and the ladle, and the number of inclusions was evaluated. In some cases, a CaO—Al ₂ O ₃ powder flux (100 mesh under) with a CaO concentration in the flux of 0 to 90% was blown up for 5 minutes from the start of reflux through an upper blowing lance in the vacuum chamber. . The rate of flux addition was 1 kg per ton per ton of molten steel. Table 1 shows the A value calculated by the formula of A = H / (D + d).

The number of inclusions was evaluated by an optical microscope after collecting a sample from the molten steel after 10 minutes of reflux treatment. The observation visual field is 100 mm ² . The results are shown in Table 1. The number of inclusions was arranged with the number in test number 1 as 1. Furthermore, the size of the largest inclusion observed in the observation field was defined as the maximum inclusion size, and the maximum inclusion size in Test No. 1 was arranged as 1.

  The results are shown in Table 1. Numerical values that fall outside the scope of the present invention are underlined.

  Test numbers 1 to 10 are comparative examples not according to the present invention, test numbers 1 to 5 are no flux top blowing, and test numbers 6 to 10 are flux top blow. Test numbers 11 to 22 are invention examples, test numbers 11 to 15 are examples that satisfy the conditions of the invention (a) without blowing on the flux, and test numbers 16 and 17 are conditions that satisfy the conditions of the invention (a), but the flux Examples where the composition does not satisfy the conditions of the invention (b), test numbers 18 to 22 are examples of satisfying the conditions of the invention (b).

  When test numbers 1 to 5 and test numbers 6 to 10 are compared, both the number of inclusions and the maximum size are slightly reduced, but it is not sufficient. On the other hand, when the test numbers 1 to 5 and the test numbers 11 to 15 according to the present invention are compared, it can be seen that both the number of inclusions and the maximum size are significantly reduced. Furthermore, when the test numbers 11 to 17 and the test numbers 18 to 22 are compared, it can be seen that both the number of inclusions and the maximum size are further reduced when the flux satisfying the condition of the invention (b) is added. Since the reduction effect by this flux is larger than the reduction effect of test numbers 6 to 10 with respect to test numbers 1 to 5, it is important to perform flux addition that satisfies the conditions of invention (b) under the conditions of invention (a). It can be understood that.

DESCRIPTION OF SYMBOLS 1 Vacuum tank 2 Rising pipe 3 Downcomer pipe 4 Molten steel surface in a ladle 5 Molten steel surface in a vacuum tank 6 Upflow 7 Downflow 8 Circulating flow 9 Molten steel in a ladle 10 Slag 11 in a ladle Vacuum exhaust device 12 Blowing tuyere

Claims (2)

A vacuum chamber connected to an evacuation device, and a riser tube and a downcomer tube attached to the bottom of the vacuum chamber, and the riser tube is provided with a bubbling tuyere of reflux gas, the riser tube and the downcomer After the pipe is immersed in the molten steel in the ladle and the molten steel in the ladle is sucked into the vacuum chamber, the molten steel is smelted using a refining treatment device that circulates the molten steel by blowing it into the ascending pipe and blowing the circulating gas from the tuyere. In the method
The relation between the inner wall of the riser and the inner wall of the downcomer, the minimum horizontal distance D, the inner diameter d of the downcomer, and the bath depth H in the vacuum chamber satisfies the following formula (1). Method.
0.03 ≦ H / (D + d) ≦ 0.10 (1)   The method for refining molten steel according to claim 1, wherein a flux containing 30 mass% or more of CaO is added to the molten steel in the vacuum chamber or the molten steel in the ladle.