Methods for melting low-carbon steel

Abstract

To provide a method for melting a low carbon steel for starting OB at an appropriate timing for shortening a decarburization treatment time.SOLUTION: In the method for melting low carbon steel, oxygen gas is blown on molten steel in a de-compressible refining apparatus when decarburizing molten steel using the refining apparatus, and the blowing of oxygen gas is started at a predetermined elapsed time from the start of decompression determined according to the free oxygen concentration in molten steel before decarburization and the oxygen gas flow rate per ton of molten steel to be blown. A predetermined elapsed time TOB (seconds) from the start of depressurization satisfies equation (1). k×QO2×Oini+C1≤TOB≤k×QO2×Oini+C2...(1) where Oini is in the range of 350≤Oini≤700, k is an adjustment factor, C1 and C2 are adjustment intercept, QO2 is a flow rate of oxygen gas blown (Nm3 / (ton h)), and Oini is free oxygen concentration in molten steel before decarburization (ppm).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for melting low-carbon steel, and more particularly to a method for decarburizing molten steel in a reflux-type vacuum degassing apparatus, such as an RH vacuum degassing apparatus. [Background technology]

[0002] When melting low-carbon steel (for example, carbon concentration: 0.0050% or less) used for automotive body panels, a reflux-type vacuum decarburization process is performed using a reflux-type vacuum decarburization apparatus, such as an RH vacuum degassing apparatus (hereinafter referred to as "RH"), to decarburize the molten steel to a predetermined carbon concentration by exposing it to a reduced-pressure atmosphere and discharging the carbon dissolved in the molten steel as CO gas. Industrially, when performing vacuum decarburization, it is required to meet the specified temperature, composition, and cleanliness in a short time, and shortening the decarburization time is an issue that directly impacts manufacturing costs.

[0003] In this case, the factors that greatly affect the decarburization time are the pressure inside the vacuum chamber and the free oxygen concentration in the molten steel. According to Non-Patent Document 1, the amount of decarburization per unit time, i.e., the decarburization rate, can be increased by increasing the CO partial pressure, which is proportional to the concentration product of C and free oxygen in the molten steel, as well as by lowering the atmospheric pressure. For example, Patent Document 1 discloses a method for controlling the exhaust volume per ton of molten steel processed to 70 kg / Hr·ton or more when the pressure inside the vacuum chamber is 300 torr, 40 kg / Hr·ton or more at 200 torr, 25 kg / Hr·ton or more at 100 torr, 12 kg / Hr·ton or more at 50 torr, and 10 kg / Hr·ton or more at 10 torr.

[0004] On the other hand, in order to increase the CO partial pressure, rapid decarburization is performed by vacuum decarburizing molten steel produced in a converter or electric furnace without deoxidizing it, while the free oxygen concentration in the molten steel is still high. For example, when decarburizing from 0.0350 mass% to 0.0050 mass% carbon concentration after tapping from a converter, considering the atomic weight, a free oxygen concentration of at least 0.040 mass% or more, corresponding to the carbon concentration of 0.030 mass% to be decarburized, is required. However, during vacuum decarburization, carbon in the molten steel reacts with free oxygen and is discharged as CO gas, and both carbon and free oxygen decrease simultaneously. As the process progresses, the CO partial pressure decreases, and the decarburization rate slows down. Therefore, in order to suppress the decrease in the decarburization rate as decarburization progresses, it is necessary to maintain a free oxygen concentration above a predetermined level during decarburization. For example, Patent Document 2 discloses a method for controlling the dissolved oxygen in steel to 0.040% or more from the start of the process until it reaches 0.0030% when performing decarburization under reduced pressure.

[0005] As a means of increasing the free oxygen concentration in molten steel, when performing vacuum decarburization, an operation called oxygen blowing (OB) is often performed, in which oxygen gas is supplied to the molten steel from a lance installed in the vacuum chamber. When OB is performed in RH, the atmosphere in the vacuum chamber is composed of Ar gas for refluxing the molten steel, the supplied oxygen gas, and CO gas generated by decarburization. When reducing the pressure inside the vacuum chamber in situations where a large amount of highly reactive CO gas is generated, a depressurization device called a steam ejector is often used. However, as the amount of gas generated in the vacuum chamber increases, the load on the depressurization device also increases, and the depressurization rate decreases. Therefore, it can be seen that performing OB has both the effect of improving the decarburization rate by increasing the partial pressure of CO in the molten steel and the negative effect of reducing the rate of pressure drop inside the vacuum chamber. Generally, the effect of improving the decarburization rate is greater than the effect of reducing the rate of pressure drop inside the vacuum chamber, so OB is often performed in the early stages of decarburization to increase the decarburization rate. This is because, when the pressure inside the vacuum chamber has dropped completely, the acid deposition efficiency is low when the same amount of oxygen is blown from above, and the utilization efficiency of the blown oxygen is higher when the over-gassing (OB) is performed when the pressure inside the vacuum chamber has not dropped completely at the beginning of the process. Furthermore, if the timing of OB is delayed when the free oxygen concentration is low at the beginning of the process, the decarburization rate may decrease due to insufficient free oxygen concentration. For example, Patent Document 3 discloses a method that includes a process to maintain the maximum decarburization rate by supplying oxygen-containing gas to the molten iron inside the vacuum chamber while performing degassing treatment in the range of a set vacuum degree of 50 torr or higher inside the vacuum chamber. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2000-178635 [Patent Document 2] Japanese Patent Application Laid-Open No. 63-190113 [Patent Document 3] Japanese Patent Application Publication No. 4-176812 [Patent Document 4] Japanese Patent Publication No. 2021-152191 [Non-patent literature]

[0007] [Non-Patent Document 1] Yoshihiko Higuchi et al., "Effects of [C], [O] and Vacuum Decarburization on RH Vacuum Decarburization," Iron and Steel, 84 (1998), 709. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Since over-obstruction (OB) increases the free oxygen concentration in molten steel, if the sole purpose is to improve the decarburization rate, it is sufficient to maintain a high free oxygen concentration by performing OB until the desired carbon concentration is reached. However, after decarburization, aluminum (Al) is often added to reduce free oxygen, and the steel is melted down as Al-killed steel. Therefore, if OB is performed excessively, the amount of Al required to reduce the free oxygen to below the desired value increases, and the amount of Al2O3 generated also increases. As a result, some of the generated Al2O3 remains in the molten steel as nonmetallic inclusions, worsening the cleanliness of the molten steel. Generally, the free oxygen concentration is adjusted within a range that does not decrease the decarburization rate, and the amount of OB is determined according to the carbon concentration before RH treatment and the molten steel temperature.

[0009] However, while prior art indicates the lower limit of free oxygen and describes the supply of oxygen gas during the degassing process, it is unclear at what timing to start OB (Oxygen Overflow) to reach the desired carbon concentration as quickly as possible, and therefore has not yet achieved the shortest possible time required for decarburization.

[0010] Therefore, the present invention aims to provide a method for melting low-carbon steel that starts OB at an appropriate timing to shorten the decarburization treatment time. [Means for solving the problem]

[0011] When performing OB, a part of the oxygen gas sprayed from the lance is supplied into the molten steel, and the rest is discharged into the atmosphere. The oxygen supply rate to the molten steel during OB, that is, the rate of increase in the free oxygen concentration per unit molten steel and per unit time, is affected by the height of the lance for oxygen supply installed above the molten steel, the oxygen flow rate, and the atmospheric pressure. However, if the same oxygen gas supply conditions, that is, the same lance height, oxygen flow rate, and atmospheric pressure, the lower the oxygen concentration in the molten steel, the greater the concentration difference between the oxygen concentration on the molten steel surface, so the ratio of the oxygen gas sprayed from the lance that is supplied to the molten steel (oxygen absorption efficiency) increases, and the oxygen supply rate to the molten steel increases. During vacuum decarburization, as time passes, the oxygen in the molten steel reacts with C and is discharged out of the system as CO gas. Therefore, when spraying the same amount of oxygen onto the molten steel under the same oxygen gas supply conditions, when performing OB after decarburization has progressed to a certain extent, the amount of oxygen supplied to the molten steel is larger, and the oxygen concentration in the molten steel itself after decarburization to the same C concentration becomes higher.

[0012] In addition to this, the exhaust behavior in the vacuum chamber changes according to the supply of oxygen gas into the vacuum chamber during OB and the decarburization rate, that is, the CO generation rate. That is, when performing vacuum decarburization by RH, even when performing OB with the same amount of oxygen, the time required to decarburize to a predetermined C concentration differs depending on the OB start timing.

[0013] As a result of the inventors' investigation of the free oxygen concentration before vacuum decarburization, the amount of oxygen supplied by OB, and the OB start timing in actual operation, it was found that when the free oxygen concentration in the molten steel before vacuum decarburization is high, starting OB after a certain degree of decarburization progresses rather than starting OB immediately after the start of treatment results in a shorter time to reach a predetermined C concentration. On the other hand, it was found that when the free oxygen concentration before vacuum decarburization is low, the decarburization time becomes longer as the OB start is delayed. As a result of intensive study based on this, it was found that the decarburization rate can be improved by adjusting the OB start timing according to the free oxygen concentration before vacuum decarburization and the oxygen gas flow rate per ton of molten steel to be sprayed.

[0014] Specific means for the problem of at what timing to start OB to minimize the decarburization time are as follows. (A) A method for melting low-carbon steel in which oxygen gas is blown onto molten steel in a refining apparatus capable of reducing pressure during decarburization treatment of the molten steel, characterized in that the blowing of oxygen gas is started at a predetermined elapsed time from the start of pressure reduction according to the free oxygen concentration in the molten steel before decarburization treatment and the oxygen gas flow rate per ton of molten steel to be blown. (B) The method for melting low-carbon steel according to (A), characterized in that the predetermined elapsed time from the start of pressure reduction satisfies the formula (1). k×Q O2 ×O ini +C1≦T OB ≦k×Q O2 ×O ini +C2···(1) However, 350≦O ini ≦700 range k: adjustment coefficient, C1, C2: adjustment intercept Q O2 : Oxygen gas flow rate per ton of molten steel to be blown (Nm 3 / (ton·h)) O ini : Free oxygen concentration in molten steel before decarburization treatment (ppm) T OB : Predetermined elapsed time from the start of pressure reduction (seconds)

Effect of the Invention

[0015] By applying the present invention, it is possible to determine the OB start timing at which the decarburization time is the shortest according to the free oxygen concentration before vacuum decarburization. Therefore, many costs such as steam, refractories, and alloys for heating can be reduced by shortening the treatment time.

Brief Description of the Drawings

[0016] [Figure 1] Change behavior of CO partial pressure and tank internal pressure during RH treatment

Mode for Carrying Out the Invention

[0017] [Definitions of terms in this invention] Low-carbon steel refers to steel with a carbon (C) concentration of 0.0050% or less. To produce low-carbon steel, molten steel produced in a steelmaking furnace (primary refining furnace) such as a converter or electric furnace is received into a ladle and decarburized using a refining device capable of reduced pressure, such as an RH, to adjust it to the desired carbon concentration.

[0018] Decarburization refers to the process of removing carbon (C) as CO gas from molten steel by treating it under reduced pressure using a refining apparatus equipped with a vacuum chamber capable of reduced pressure, such as an RH chamber. The higher the concentration of carbon and oxygen (O) in the molten steel, the faster the decarburization rate during reduced pressure treatment. Therefore, the steel is often treated in a state of so-called undeoxidized steel, without adding deoxidizing agents such as aluminum when tapping it from the steelmaking furnace.

[0019] Free oxygen concentration refers to the concentration of oxygen dissolved in molten steel.

[0020] Oxygen gas blowing (OB) refers to the operation of blowing oxygen gas from above into the molten steel in a vacuum chamber via a lance installed inside the vacuum chamber during decarburization treatment in equipment such as RH, with the aim of increasing the free oxygen concentration in the molten steel. Oxygen gas blowing can be performed even during the decarburization treatment. The free oxygen concentration in the molten steel can be measured even during the decarburization treatment using an oxygen sensor based on the principle of an oxygen concentration cell.

[0021] The acid formation efficiency during over-burden (OB) refers to the ratio of the amount of oxygen actually supplied to the molten steel to the amount of oxygen gas supplied. The acid formation efficiency is affected by the free oxygen concentration in the molten steel during OB; the lower the free oxygen concentration, the better the acid formation efficiency. During the reduced pressure treatment, the decarburization reaction progresses and the amount of free oxygen decreases, so if the same amount of oxygen is blown in, the acid formation efficiency of OB will be better as it gets closer to the end of the decarburization stage.

[0022] [Molten steel composition according to the present invention] This invention assumes that steel molten in a steelmaking furnace is poured into a ladle in an undeoxidized state and then subjected to reduced pressure treatment in an apparatus equipped with a vacuum chamber capable of reducing pressure, such as an RH chamber. The carbon concentration in the molten steel before treatment is approximately 0.02% (200 ppm) to 0.07% (700 ppm), and the free oxygen concentration is approximately 0.03% (300 ppm) to 0.08% (800 ppm). In addition to carbon and oxygen, manganese and silicon, which have weak deoxidizing power, may also be present. Furthermore, unavoidable impurities such as phosphorus, sulfur, nitrogen, and other elements may be present.

[0023] [Decarburization treatment method in the present invention] This invention relates to a refining apparatus equipped with a vacuum chamber capable of reducing pressure, such as an RH chamber, which is used to reduce the carbon concentration in the molten steel in the ladle under reduced pressure in the vacuum chamber, thereby reducing the carbon concentration in the molten steel to 0.005% (50 ppm) or less, and is intended for the production of low-carbon steel. The following describes a decarburization treatment method using an RH chamber.

[0024] Molten steel before vacuum treatment can be melted in steelmaking furnaces such as converters and electric furnaces. The molten steel melted in the steelmaking furnace is tapped into a ladle and transported to the RH (Reduced Heat Treatment) facility in the ladle. While alloys can be added during tapping, it is preferable to tap the molten steel in an undeoxidized state without adding deoxidizing agents such as aluminum in order to increase the decarburization rate. Furthermore, by analyzing a sample taken from the molten steel after tapping, the carbon concentration before RH treatment can be determined. At the same time, the molten steel temperature and free oxygen concentration can be determined.

[0025] The immersion tube is immersed in the molten steel in the ladle, which has been transported to the RH chamber. The vacuum chamber is depressurized, and an inert gas is flowed through one end of the immersion tube, thereby initiating reflux and circulating the molten steel between the vacuum chamber and the ladle. At this time, the partial pressure of CO gas in the molten steel, P, is determined by multiplying the concentration product of C and free oxygen (O) in the molten steel by the equilibrium constant. COIn contrast, when the pressure P inside the vacuum chamber decreases, the carbon that can no longer dissolve in the molten steel reacts with oxygen to form CO gas, which is discharged to the gas phase and evacuated from the top of the vacuum chamber, allowing the decarburization reaction to proceed. Since the decarburization reaction occurs through the reaction C + O = CO, 16 g of oxygen reacts for every 1 mole of carbon (12 g). Therefore, as the decarburization reaction progresses, the free oxygen concentration in the molten steel also decreases. In this invention, to increase the reduced free oxygen concentration, OB is performed during the decarburization process. After the carbon concentration is reduced to below a predetermined level, aluminum is added to the molten steel to further reduce the free oxygen and stop the decarburization reaction. Generally, the decarburization time is about 10 to 20 minutes.

[0026] [Decompression Procedure] Generally, in industrial RH (Reduced Heat) systems, the pressure inside the vacuum chamber is reduced using a vacuum pump, steam ejector, and booster pump. Each piece of equipment has a set applicable pressure range, and the pressure reducing device used is switched according to the pressure inside the vacuum chamber. At this time, the pressure reducing device is switched automatically or manually after confirming that the pressure inside the vacuum chamber has dropped to the target pressure. However, if the exhaust is slow, it is necessary to wait for the pressure inside the chamber to drop to the target value. If the amount of gas generated inside the vacuum chamber is large, the rate of pressure reduction will decrease. Depending on the timing of the out-of-bust (OB), the exhaust may not keep up with the amount of gas generated, which can lead to a significant delay in switching the pressure reducing device and a longer decarburization process time. Therefore, optimizing the OB timing is important.

[0027] [Amount of oxygen blown in by OB] If the free oxygen concentration in molten steel decreases during decarburization, the decarburization reaction rate also decreases. Therefore, if the free oxygen concentration decreases during the decarburization reaction, the free oxygen concentration can be increased by blowing oxygen gas from the top of the vacuum chamber through a lance, i.e., by OB (overflow).

[0028] First, the free oxygen concentration emitted as CO is calculated based on the carbon concentration that will be decarburized by the end of the decarburization process. A free oxygen concentration of around 400 ppm at the end of decarburization is preferable because it allows the decarburization rate to be maintained until the end of decarburization. The amount of oxygen (Nm³) to be blown in by the OB is determined considering the acid deposition efficiency of the OB, so that the sum of the free oxygen concentration at the start of decarburization and the oxygen concentration supplied by the OB matches the sum of the free oxygen concentration consumed during decarburization and the free oxygen concentration (around 400 ppm) required to maintain the decarburization rate. 3 ) can be decided.

[0029] [The concept of optimizing out-of-bounds timing] Figure 1 shows the changes over time in the partial pressure of CO in the molten steel in the ladle and the pressure inside the tank during reduced pressure treatment with RH. As described in Non-Patent Literature 1, the vacuum decarburization rate is determined by the difference between the partial pressure of CO, which is obtained by multiplying the concentration product of C and O in the molten steel by the equilibrium constant, and the ambient pressure (pressure inside the tank). In Figure 1, the horizontal axis represents the treatment time, the solid line represents the partial pressure of CO obtained from the concentration product of C and O in the molten steel in the ladle, and the dashed line represents the pressure inside the tank. Decarburization begins when the pressure inside the tank becomes lower than the partial pressure of CO. The fact that the partial pressure of CO does not decrease immediately after the start of treatment, when the pressure inside the tank is lower than the partial pressure of CO, indicates that the pressure inside the tank has not decreased sufficiently and the circulation flow rate is still low, so decarburization hardly progresses for the molten steel in the ladle as a whole. The slope of both the partial pressure of CO and the pressure inside the tank decreases as the pressure decreases. Points where the slope of the solid line (partial pressure of CO) changes particularly represent the start of the decarburization phenomenon, the start and end of OB, and deoxidation due to Al addition. The points where the slope of the dashed line (tank pressure) changes significantly represent changes in the amount of gas generated due to the start of decarburization and the start and end of over-burning (OB), as well as the switching of the depressurization device. As decarburization progresses and the concentrations of carbon and free oxygen decrease, the partial pressure of CO decreases. Also, as the tank pressure decreases, the vacuum pump, steam ejector, booster pump, and depressurization device are switched. When OB occurs, the free oxygen concentration in the molten steel increases, or the rate of decrease in free oxygen concentration slows down, causing the partial pressure of CO to shift upward or its rate of decrease to slow down. In addition, the behavior of the decrease in tank pressure changes depending on the oxygen gas blown into the vacuum tank and the CO gas generated by decarburization. Since the decarburization rate is determined by the difference between the partial pressure of CO and the tank pressure, different OB start timings affect the decarburization behavior. Specifically, if the OB timing is too early, the exhaust in the initial stages of decarburization is hindered, delaying the decrease in tank pressure and potentially lowering the decarburization rate. If the OB timing is too late, the effect of OB is not obtained until OB occurs, and the decrease in the CO concentration product is not suppressed, resulting in a decrease in the total decarburization rate. Therefore, there is an optimal over-the-counter (OB) timing depending on the conditions of the molten steel.

[0030] When melting low-carbon steel, the undeoxidized steel melted in the steelmaking furnace is treated with RH (Respiratory Heat Treatment), so the carbon (C) and free oxygen (BO) concentrations in the molten steel before RH treatment cannot be controlled and are not constant. However, the carbon concentration at the end of decarburization in RH is generally constant depending on the standard components, and the amount of OB (Oxidized Beam) is adjusted so that the free oxygen concentration is approximately 400 ppm at the end of decarburization. Therefore, working backward from the timing of the end of decarburization, even if the carbon and free oxygen concentrations before treatment are different, if the standard components are the same, the behavior of the carbon and free oxygen concentrations from the time OB ends until the end of decarburization will be generally the same.

[0031] In this case, assuming the initial free oxygen concentration is the same, a lower initial carbon concentration results in less oxygen being needed for decarburization, thus reducing the amount of oxygen-based oxygen (OB). Conversely, a higher initial carbon concentration results in more oxygen being needed for decarburization, thus increasing the amount of OB.

[0032] As mentioned above, if the OB start timing is too early, initial decarburization proceeds quickly, but the exhaust load increases, delaying the timing of switching the depressurization device and thus extending the total decarburization time. On the other hand, if the OB start timing is too late, the exhaust load in the initial stages of treatment is small, and there is no delay in switching the depressurization device, but the time during which the effect of increasing the decarburization rate by increasing the CO partial pressure is received is shortened, thus extending the total decarburization time. Therefore, it is clear that there is an optimal OB start timing depending on the operating conditions. When we examined the optimal OB start timing for each operating condition, we found that if the oxygen gas flow rate at OB is the same, the OB start timing is determined by the free oxygen concentration before treatment, regardless of the C concentration. This is because the optimal OB start timing is roughly determined by the balance between the timing of increasing the amount of CO gas produced and the OB oxygen flow rate, which affect the exhaust load in the initial stages of treatment, and the timing of increasing free oxygen to increase the decarburization rate, and the influence of changes in the amount of CO gas produced due to differences in initial C concentration and the magnitude of the increase in the decarburization rate is small.

[0033] When the oxygen gas flow rate injected during over-boiling (OB) is high, the amount of acid deposited per unit time also increases, thus shortening the OB time itself. Therefore, under conditions of high oxygen gas flow rate, if the carbon and free oxygen concentrations at the end of OB are roughly the same, the OB start timing will be delayed as the OB time will be shortened. Not all of the oxygen gas injected during OB reacts with the molten steel; some of the injected oxygen gas is discharged into the atmosphere inside the tank without reacting with the molten steel (without acid deposition).

[0034] Based on the above, when performing decarburization treatment under the condition of blowing oxygen gas, the optimal OB start timing T will minimize the treatment time. OB (seconds) is calculated using the adjustment coefficient k and the oxygen gas flow rate Q. O2 (Nm 3 / (ton·h)), untreated free oxygen O ini It is given by equation (1), which consists of (ppm) and adjustment sections C1 and C2. k×Q O2 ×O ini +C1≦T OB ≤k×Q O2 ×O ini +C2···(1)

[0035] Considering the carbon concentration and free oxygen concentration at the end of decarburization, if the carbon concentration and free oxygen concentration at the end of OB are adjusted to the same value, the oxygen gas flow rate Q O2 Free oxygen concentration before treatment: ini If the value is large, the start of OB can be delayed. For this reason, in equation (1), the oxygen gas flow rate Q O2 Free oxygen before treatment O ini If there is a positive correlation with the OB start timing, that is, if both values ​​are large, then the OB start timing T OB This will result in a slower calculation.

[0036] In equation (1), the adjustment coefficient k is a parameter that includes the acid deposition efficiency. k can be determined by extracting the region of the OB start timing that resulted in the shortest treatment time under the same pre-treatment free oxygen conditions, based on the results of multiple decarburization treatments performed with varying oxygen gas flow rate, pre-treatment free oxygen, and OB start timing, and then extracting the slope from the relationship between the two.

[0037] Furthermore, the adjustment intercepts C1 and C2 at the end of the right-hand and left-hand sides of equation (1) are values ​​linked to the post-treatment free oxygen concentration and can be determined as the intercepts of the relationship when calculating k as described above.

[0038] The present invention applies to a pre-treatment free oxygen concentration of 350 ppm or more and 700 ppm or less. When the pre-treatment free oxygen concentration is less than 350 ppm, the pre-treatment carbon concentration is often also high, and the over-boiling (OB) and decarburization processes take longer than usual. In such cases, there is no need to apply the present invention; OB can be started immediately after the start of processing. Furthermore, the amount of oxygen gas blown during OB should be 0.55 Nm³ per ton of molten steel. 3 Even under conditions where the amount is greater than that, the operational variability is large and the process often differs from normal decarburization treatment, so this condition is outside the scope of application of the present invention. On the other hand, when the free oxygen concentration before treatment exceeds 700 ppm, depending on the C concentration before treatment, there is often no need for OB, so this condition is also outside the scope of application of the present invention.

[0039] Furthermore, if the carbon concentration after decarburization is higher than 0.0050%, the effect of variations in carbon concentration due to delayed mixing in the ladle during decarburization in the high-carbon region outweighs the effect of reducing processing time by optimizing the timing of OB start, and a clear reduction in processing time cannot be obtained. Therefore, the favorable effects of the present invention can be achieved when the carbon concentration after decarburization is 0.0050% or less.

[0040] Based on the above considerations, the present invention diligently searched for the optimal OB (Oxygen Blowing) start timing that results in the most efficient decarburization behavior using actual production equipment. As a result, it was found that OB is started after a predetermined elapsed time from the start of depressurization, which is determined by the free oxygen concentration in the molten steel before decarburization and the oxygen gas flow rate being blown. Furthermore, the present invention was completed by identifying this OB start timing and clarifying its range. In addition, the upper limit of the oxygen blowing amount to which the present invention can be applied and the carbon concentration after decarburization were clarified. [Examples]

[0041] First, with the aim of determining the adjustment coefficient k, adjustment intercepts C1 and C2 in equation (1) of the present invention, a primary refining treatment was performed in a converter, and 450 tons of molten steel that had not been deoxidized was received in a ladle and decarburized in RH. The molten steel temperature was in the range of 1600°C to 1620°C. Under conditions where the pre-treatment free oxygen was 400 ppm and the pre-treatment carbon was 350 ppm, and under conditions where the pre-treatment free oxygen was 600 ppm and the initial carbon was 300 ppm, the OB start timing was varied from 60 seconds after reflux to 360 seconds after reflux, and the time until decarburization was completed ([C]=25 ppm) was compared. The steel type melted was a steel type with an upper limit of carbon concentration of 30 ppm, and the carbon concentration in the molten steel at the end of decarburization was estimated using the method described in Patent Document 4, and Al was added when the carbon concentration (calculated value) decreased to 20 ppm to complete decarburization. Furthermore, the amount of oxygen (Nm³) sprayed by the OB is adjusted according to the pre-treatment free oxygen concentration, taking into account the acid formation efficiency, so that the free oxygen concentration in the molten steel is 400 ppm before the addition of Al. 3 The settings were adjusted as shown in Table 1. The oxygen gas flow rate during over-exposure was 2500 Nm³. 3 The value is / h. To compare the exact decarburization time, the time at which the C concentration reached 25 ppm was calculated from the C analysis value after melting using the method described in Non-Patent Literature 1. The results are shown in Table 1. Even with the same pre-treatment free oxygen, pre-treatment C, and OB oxygen amount, the time at which the C concentration reached 25 ppm differed depending on the OB start timing, indicating that an optimal OB start timing exists. Similar preliminary tests were conducted with oxygen gas flow rates of 2000 and 3000 Nm³ at OB. 3 The study was repeated under the condition of / h, and based on these results, the adjustment coefficient k in equation (1) was determined to be 0.108. Furthermore, from these results, the intercepts were determined to be C1 = -150 and C2 = -120.

[0042] [Table 1]

[0043] Primary refining was performed in a converter, and 450 tons of molten steel, which had not been deoxidized, was received in a ladle and decarburized in a RH (Restoration Heat) chamber. Before the RH treatment, the carbon concentration ranged from 0.02% (200 ppm) to 0.06% (600 ppm), the free oxygen concentration ranged from 0.035% (350 ppm) to 0.070% (700 ppm), and the molten steel temperature ranged from 1600°C to 1620°C. A two-legged immersion tube connected to a vacuum chamber was immersed in the molten steel in the ladle that had been transported to the RH chamber. The vacuum chamber was depressurized, and Ar gas was introduced from one side of the immersion tube to reflux the molten steel.

[0044] The conditions under which the OB start timing was left to the operator between 60 and 360 seconds after the start of processing (=comparative example), and T calculated using equation (1) OB Table 2 shows the results of comparing the time it takes for the C concentration to reach 25 ppm under the conditions set within the range indicated (=inventive example). Except for the OB start timing, the processing conditions were the same for the comparative example and the inventive example. In the comparative example, the average time it took for the C concentration to reach 25 ppm was 15.6 minutes, while in the inventive example it was 15.2 minutes, indicating an average reduction in processing time of 0.4 minutes. From these results, it is clear that low-carbon steel can be efficiently manufactured by vacuum processing using the method of the present invention.

[0045] [Table 2]

Claims

[Claim 1] A method for producing low-carbon steel, comprising blowing oxygen gas into molten steel in a refining apparatus that can reduce pressure when decarburizing molten steel, wherein the blowing of oxygen gas is started at a predetermined time elapsed from the start of pressure reduction, which is determined according to the free oxygen concentration in the molten steel before decarburization and the oxygen gas flow rate per ton of molten steel to be blown. A method for melting low-carbon steel, characterized in that the predetermined elapsed time from the start of reduced pressure satisfies equation (1). k×Q O2 ×O ini +C 1 ≦T OB ≦k×Q O2 ×O ini +C 2 ... (1) However, within the range of 350 ≤ O ini ≤ 700 k: Adjustment coefficient, C1, C2: Adjustment intercept Q O2: Oxygen gas flow rate per ton of molten steel sprayed (Nm³ / (ton·h)) O ini: Free oxygen concentration (ppm) in molten steel before decarburization treatment. TOB: Predetermined elapsed time (seconds) from the start of decompression.

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