JP4463701B2 - Decarburization method for molten stainless steel and method for producing ultra-low carbon stainless steel - Google Patents

Description

  The present invention relates to a decarburization method in refining molten stainless steel, and more particularly to an efficient decarburization method suitable for production of extremely low carbon steel, and a method of producing ultralow carbon stainless steel using the method.

  In the refining of molten stainless steel, finish decarburization processing is performed by AOD method, VOD method or the like. Among these, the VOD method performs decarburization processing by oxygen blowing under vacuum, and has an advantage that decarburization refining can be performed while lowering the CO partial pressure under vacuum and suppressing oxidation of Cr in molten steel. . In the steelmaking process of stainless steel using the VOD method, stainless steel melted in an electric furnace is roughly decarburized by oxygen blowing in a decarburization furnace such as a converter, and then steeled in a ladle for vacuum refining. After adjusting the components of the molten steel in the ladle in the VOD process, it is made into a slab by a continuous casting method or the like.

In the VOD process, when the ladle containing the molten steel after the completion of rough decarburization is set in a sealed container connected to the vacuum exhaust device, the exhaust operation is started, and when the predetermined vacuum degree is reached, the upper blow lance is used. Oxygen is blown into the molten steel, and decarburization is performed by vacuum oxygen blowing (vacuum oxygen blowing period). The degree of vacuum in the container gradually increases and finally reaches 10 Torr (1333 Pa) or less. Usually, oxygen blowing is finished after this stage. In a typical determination method at the end of oxygen blowing, the CO and CO ₂ concentrations in the exhaust gas are measured and the decarburization amount is calculated over time, and the calculated value is converted into the C content in the molten steel. When the C content is reached, the oxygen blowing is finished. Thereafter, deoxidation without vacuum oxygen blowing, that is, decarburization treatment is performed by stirring the molten steel with a bottom-blown Ar gas or the like in vacuum, and further decarburization proceeds (vacuum decarburization period).

In the vacuum oxygen blowing period, when the decarburization progresses and becomes a low carbon region, the decarburization rate decreases, and the CO and CO ₂ concentrations as exhaust gas components significantly decrease. For this reason, it is difficult to properly determine the end point of oxygen blowing. Moreover, the accuracy of the end point control decreases due to various disturbances. In the low carbon region, oxidation of Cr by oxygen blowing is inevitable. The produced Cr oxide is taken into the slag, causing a decrease in the fluidity of the slag. When the fluidity of slag decreases, the decarburization efficiency decreases in the subsequent vacuum decarburization period.

  Therefore, in the VOD process, it is important to accurately adjust to a predetermined C content while minimizing the Cr oxide concentration in the slag, but the vacuum oxygen blowing is completed to satisfy these requirements. It is not easy to control the time. This is especially true when ultra-low carbon steel is melted. If the predetermined C content is not achieved, re-vacuum oxygen blowing is required, resulting in a decrease in productivity. On the other hand, if excessive vacuum oxygen blowing is performed, the oxidation of Cr increases due to the excessive supply of oxygen, and the fluidity of the slag is deteriorated, thereby inhibiting decarburization during the vacuum decarburization period.

  As a decarburizing method for molten stainless steel, Patent Document 1 discloses that the dip tube is immersed in the molten steel in the ladle, the inside of the dip tube is depressurized, an inert gas is supplied from the bottom, and oxygen blowing is performed while stirring. Next, a method for performing degassing under reduced pressure is described. According to this method, decarburization with suppressed oxidation of Cr can be achieved by defining the degree of vacuum in the dip tube and the amount of C in the molten steel after oxygen blowing. However, in order to suppress the oxidation of Cr by this method, it is necessary to control oxygen blowing so that it does not become excessive. As a result, it is difficult to reduce C to an extremely low range by oxygen blowing, The load entrusted to decarburization after smelting tends to increase. For this reason, the case where the refining time increases and the target C amount is not achieved easily occurs.

  In Patent Document 2, when high-Cr steel is refined in a vacuum smelting furnace, various fluxes are added to reduce excess Cr oxide in the slag, thereby improving the slag fluidity and slag / metal contact stirring. Has been disclosed to promote the decarburization reaction in the vacuum decarburization period. However, sampling and analysis of slag is indispensable for optimizing such slag components with high accuracy, leading to an increase in operation time. If the analysis is not performed, the slag composition must be relied on the estimated value, and it is difficult to control the C content accurately and stably.

In Patent Document 3, the basicity (% CaO) / (% SiO ₂ ) of slag is adjusted to 1.5 to 3.5 in advance before oxygen blowing in the VOD process. By monitoring the total amount and the total amount of oxygen in the exhaust gas, estimating the amount of Cr oxide in the slag from the difference, and performing end point control to end oxygen blowing when the estimated amount of Cr oxide is less than 40%, A method for improving the decarburization efficiency in the vacuum decarburization period is disclosed. However, this method requires time and effort for adjusting the slag basicity. In addition, strict end point control is required to stop oxygen blowing when the amount of Cr oxide in the slag does not increase too much. If the end point time is shifted, the target decarburization amount may not be achieved, There is a problem that component fluctuations easily occur due to excessive consumption of deoxidizing elements such as Si.

Japanese Patent Laid-Open No. 11-50133 JP 57-41312 A JP-A-8-260030

  As described above, in order to efficiently decarburize molten stainless steel in the VOD process, it is important to reduce the C content as much as possible while suppressing the oxidation of Cr as much as possible during the vacuum oxygen blowing process. In the period, it is important to advance decarburization by stirring with inert gas in a slag state with good fluidity where the Cr oxide concentration is not too high. However, it is difficult to control the end point of oxygen blowing to achieve both the suppression of oxidation of Cr and sufficient decarburization. Eventually, the load tends to increase in the subsequent vacuum decarburization period or further final component adjustment. The vacuum decarburization efficiency can be improved to some extent by strengthening the stirring of the molten steel, but this requires a cost for increasing the amount of Ar used and facility enhancement. Further, if the stirring time is lengthened, decarburization proceeds along with this, but this is not a good idea because the productivity is lowered.

  In addition to this problem, there is a so-called “C pickup” problem. C pickup may contain high concentration of C in the bullion or slag lump adhering to the slag line of the ladle. After the vacuum decarburization by vacuum oxygen blowing and stirring, the component adjustment It is considered that the metal and slag lump are melted before and after the stage, and the C content in the molten steel is inadvertently increased. The problem of C pick-up can sometimes be a serious blow in the melting of ultra-low carbon steel. The conventional refining method has not solved the problem of C pickup.

  The present invention improves the decarburization efficiency in the vacuum decarburization period without performing special end point control of vacuum oxygen blowing, and is a simple and highly productive decarburization of stainless steel that can cope with C pickup. Is to provide a method.

  As a result of various studies, the inventors do not leave the adjustment of the slag components used in the vacuum decarburization period to the end point control of the vacuum oxygen blowing, but work efficiency by performing the reduction operation after the completion of the vacuum oxygen blowing, We found it very advantageous in terms of cost and productivity. Then, by measuring the oxygen activity in the molten steel after vacuum oxygen blowing, it is possible to estimate the amount of Cr oxide in the slag, and based on that, the amount of addition of the reducing agent (deoxidizing agent) can be accurately determined. I found out that I could decide. In the present invention, efficient vacuum decarburization is performed in such a state that the amount of Cr oxide in the slag is optimized.

  That is, the decarburization method of molten stainless steel provided in the present invention is a decarburization process of molten stainless steel in which decarburization is performed by stirring with inert gas after completion of oxygen blowing in a vacuum vessel. The oxygen activity in the molten steel was measured before or after the start, and the Cr oxide concentration in the slag was estimated from the correlation between the oxygen activity in the molten steel and the Cr oxide concentration in the slag that had been obtained in advance. The deoxidizer is added to the molten steel so that the Cr oxide concentration in the range of 10 to 30% by mass ”is performed at least once, so that the Cr oxide concentration in the slag is 10 For example, decarburization by stirring with an inert gas is allowed to proceed under a vacuum of 10 Torr (1333 Pa) or less in a state of ˜30 mass%.

  The vacuum container is a container whose inside is maintained in a vacuum state, and “vacuum” here means a reduced pressure state of approximately 120 Torr (about 16000 Pa) or less. As the deoxidizer, one or two of Al and Fe-Si (ferrosilicon) can be used. Fe-Si is an alloy containing approximately 60 to 80% by mass of Si, and is generally used as a Si raw material in steelmaking.

  Moreover, in this invention, the manufacturing method of the ultra-low carbon stainless steel (for example, ferritic stainless steel) which reduces C content to 0.010 mass% or less by such a decarburization method is provided.

  According to the present invention, when the molten stainless steel is decarburized in the process of “vacuum oxygen blowing → vacuum decarburization by stirring”, the deoxidation is performed after the vacuum oxygen blowing is completed after adjusting the components of the slag used in the vacuum decarburization period. This can be done by adding an agent. For this reason, it is not necessary to employ special end point control. There is no need to adjust slag basicity before oxygen blowing. Moreover, even if oxygen blowing is performed somewhat excessively, it can be sufficiently optimized by the subsequent slag component adjustment operation. Furthermore, the C pickup will also be improved to a level where there is no problem in commercial production. Therefore, the present invention enables the production of ultra-low carbon stainless steel by a quick and simple method.

When stainless steel molten steel is vacuum-oxygen blown, the oxidation of Cr is more likely to occur preferentially than the decarburization reaction as the carbon content decreases, and as a result, the concentration of Cr oxide in the slag increases. Although the Cr oxide concentration in the slag depends on the operating conditions, it tends to gradually increase when the decarburization reaction is stagnant. Therefore, after the decarburization reaction has stagnated, the decarburization reaction does not proceed efficiently even if oxygen blowing is continued further. Moreover, the increase in the Cr oxide concentration in the slag causes a decrease in the fluidity of the slag, and the decarburization efficiency in the vacuum decarburization period is remarkably lowered. In the present invention, as described later, Cr oxide increased in the slag is reduced by the addition of a deoxidizer, so that the end point control of the vacuum oxygen blowing may be performed by a conventional method. That is, the CO and CO ₂ concentrations in the exhaust gas are measured and the decarburization rate is monitored over time. For example, an extremely low carbon stainless steel having a Cr content of 11 to 30% by mass and a final target value of C of 0.010% by mass or less. In the case of melting steel, the oxygen blowing may be terminated when the decarburization rate reaches 10-25 ppm / min.

  In the vacuum decarburization period, decarburization proceeds by C in the molten steel mainly reacting with oxygen contained in the solid state in the slag (hereinafter referred to as “solid oxygen”). Solid oxygen exists mainly in the form of metal oxide, and Cr oxide is one of important oxygen sources in the decarburization reaction. That is, it is effective that the slag in the vacuum decarburization period contains Cr oxide to some extent. As a result of various studies, it has been found that when the Cr oxide concentration in the slag is less than 10% by mass, the decarburization reaction in the vacuum decarburization period is not sufficiently accelerated.

  On the other hand, if the Cr oxide concentration in the slag is too high, the fluidity of the slag decreases as described above, and the decarburization reaction efficiency decreases. When vacuum oxygen blowing is performed by a conventional general method, the Cr oxide concentration in the slag after the completion of vacuum oxygen blowing may be 40% by mass or more. According to the study by the inventors, it has been found that it is extremely effective to improve the decarburization efficiency when the Cr oxide concentration in the slag is 30% by mass or less in the vacuum decarburization period.

  Therefore, in the present invention, in the vacuum decarburization period, the molten steel is stirred in a state where the slag whose Cr oxide concentration is adjusted to 10 to 30% by mass is brought into contact with the molten steel, and the decarburization proceeds. In order to adjust the Cr oxide concentration in the slag, a deoxidizer such as Al or Fe-Si is added to the molten steel. In this case, the slag component adjustment operation can be performed as follows.

  First, before or after the start of decarburization by stirring with an inert gas, the oxygen activity in the molten steel is measured using an oxygen sensor. As a specific measurement time, the end point of vacuum oxygen blowing is suitable. In general, there is a correlation between the oxygen activity in the molten steel and the oxygen activity in the slag, but as a result of detailed studies by the inventors, there is a difference between the oxygen activity in the molten steel and the Cr oxide concentration in the slag. Was also found to be correlated. Therefore, if a calibration curve showing the relationship between the oxygen activity in the molten steel and the Cr oxide concentration in the slag is prepared in advance for the molten steel of the composition, the Cr oxide in the slag is determined from the measured oxygen activity in the molten steel. The concentration can be estimated quickly. The oxygen sensor can use a solid electrolyte such as zirconia.

  Next, the Cr oxide concentration in the slag is estimated from the measured value of the oxygen activity in the molten steel using the above calibration curve, and the difference between the estimated value and the target value (range of 10 to 30% by mass) is obtained. The amount of deoxidizer sufficient to reduce the corresponding amount of Cr oxide is calculated. Then, that amount of deoxidizer is added to the molten steel. The added deoxidizer dissolves in the steel, the dissolved deoxidizer component reacts with the Cr oxide in the slag, Cr is reduced, and the slag has a predetermined Cr oxide concentration. In this state where the optimized slag and molten steel are in contact with each other, stirring of an inert gas such as Ar is continued, and decarburization in an extremely low carbon region is efficiently advanced. The inert gas is blown at the bottom. A series of slag component adjustment operations as described above and subsequent decarburization by inert gas stirring may be performed a plurality of times.

A strong deoxidizer such as Al or Fe-Si is used as a deoxidizer added to adjust the Cr oxide concentration in the slag. When the deoxidizer is added, an additive such as CaO or CaF ₂ may be added as long as the Cr oxide concentration in the slag does not deviate from 10 to 30%. When this slag component adjustment operation is carried out, the fluidity of the slag is improved, and the bullion and slag lump attached to the ladle slag line due to the addition of the deoxidizer are also dissolved, and once the C pickup is However, since the Cr oxide concentration in the slag is optimized, the slag / metal reaction in the vacuum decarburization stage is activated, and C is contained in the molten steel without requiring special strong stirring or long-time stirring. The quantity reaches the target very low level. That is, in the present invention, the problem of the C pickup is solved by rather positively dissolving the deposits that cause the C pickup. The end point of the vacuum decarburization period varies depending on the target C concentration, but is determined by a conventional general method, for example, a decarburization rate calculated from analysis values of CO and CO ₂ concentration in exhaust gas. be able to.

About 80 tons of ferritic stainless steel with Cr content shown in Table 1 is melted in an electric furnace, followed by rough decarburization by oxygen blowing in a converter, and after confirming the composition, the steel is put into a ladle. did. The ladle in which the molten stainless steel was accommodated was set in a sealed container of the VOD device, and evacuation was started. About 5 minutes after the start of evacuation, the pressure in the sealed container reached 120 Torr (about 16000 Pa). At this point, oxygen blowing was started using the top blowing lance. At about the same time, Ar was blown from the porous nozzle at the bottom of the ladle, and gas stirring of the molten steel was started. In the vacuum oxygen blowing period, the change in the decarburization rate was monitored by a conventional method by measuring the CO and CO ₂ concentrations in the exhaust gas. In any charge, vacuum oxygen blowing was completed when the decarburization rate reached 10-25 ppm / min. The degree of vacuum at this time was 10 Torr (1333 Pa) or less. After that, vacuuming and Ar bottom stirring were continued.
The chemical analysis values of the final components of each charge shown in Table 1 are as shown in Table 2.

  About charge No. 1-6 (example of this invention), the oxygen activity in molten steel was measured using the zirconia oxygen sensor immediately after completion | finish of vacuum oxygen blowing. The results are shown in Table 1. Based on a calibration curve representing the correlation between “oxygen activity in molten steel and Cr oxide concentration in slag” obtained in advance by separate experiments for each steel type, Cr oxidation in slag is determined from the measured oxygen activity. The object concentration was estimated. The results are also shown in Table 1. The target values of the Cr oxide concentration in the slag during the vacuum decarburization period are set to the values shown in Table 1, respectively, and an amount of Cr oxide corresponding to the difference between the estimated value of the Cr oxide concentration and the target value is reduced. As shown in Table 1, the amount of the deoxidizer added was sufficient. Then, the deoxidizer was added to the molten steel, and stirring with Ar gas was continued. The amount of Ar gas introduced during the vacuum decarburization period was 300 L / min or more. About 5 minutes after the addition of the deoxidizer, slag was sampled to confirm the slag composition. The analysis result of the slag sample was almost close to the target value as shown in Table 1. That is, in these charges, decarburization was performed by stirring with an inert gas while the Cr oxide concentration in the slag was 10 to 30% by mass. Table 1 shows the decarburization time (stirring duration) after adding the deoxidizer. This decarburization time was determined by a method in which the end point time is a point at which the decarburization rate calculated from the exhaust gas analysis value becomes a certain value or less. After the vacuum decarburization treatment, the container was opened to the atmosphere, the final components were adjusted, and a slab was formed by continuous casting. In these examples of the present invention, ultra-low carbon ferritic stainless steel can be efficiently melted in a relatively short time, and there is no C concentration fluctuation due to C pick-up, and the ultra-low carbon steel having a C content almost coincident with the final target C value. Could be manufactured stably.

  On the other hand, in the charge Nos. 7 to 12 (comparative examples), after the vacuum oxygen blowing was completed, the oxygen activity measurement and the deoxidizer addition for adjusting the Cr oxide concentration in the slag were not performed. Vacuum decarburization was performed using the slag at the end of oxygen blowing. The decarburization time was determined by the same method as in the case of the present invention. In addition, as a result of sampling slag about 5 minutes after completion | finish of oxygen blowing and analyzing the slag sample, as shown in Table 1, all had Cr oxide concentration over 30 mass%. That is, in these charges, decarburization was performed by stirring with an inert gas in a state where the Cr oxide concentration in the slag exceeds 30% by mass. After the vacuum decarburization treatment, the container was opened to the atmosphere, the final components were adjusted, and a slab was formed by continuous casting.

In Comparative Examples 7 and 8, decarburization was performed with the vacuum decarburization time being the same as that of the present invention example, but the Cr ₂ O ₃ concentration in the slag exceeded 30%, so the target C value Could not reach up to. Moreover, in Comparative Examples 9-12, although it decarburized for a long time than the said invention example, it did not reach | attain target C value but was not able to manufacture ultra-low carbon steel. It can be inferred that the fluctuation of the C amount due to the C pickup is also a factor of not achieving the target.

Claims (5)

  In the decarburization treatment of the molten stainless steel in which decarburization is performed by stirring with inert gas after completion of oxygen blowing in a vacuum vessel, the oxygen activity in the molten steel is measured before or after the start of decarburization by stirring with inert gas, The Cr oxide concentration in the slag is estimated from the correlation between the oxygen activity in the molten steel and the Cr oxide concentration in the slag, which are obtained in advance, and the Cr oxide concentration in the slag is in the range of 10 to 30% by mass. By performing a slag component adjustment operation to add a deoxidizer to the molten steel, the demolition of the molten stainless steel advances decarburization by stirring with an inert gas in a state where the Cr oxide concentration in the slag is 10 to 30% by mass. Charcoal method.   The decarburization method for molten stainless steel according to claim 1, wherein Al or Fe—Si is used as the deoxidizer.   The decarburization method of molten stainless steel according to claim 1 or 2, wherein decarburization is performed by stirring with an inert gas under a vacuum of 10 Torr (1333 Pa) or less.   The manufacturing method of the ultra-low carbon stainless steel which reduces C content to 0.010 mass% or less by the decarburization method in any one of Claims 1-3.   The manufacturing method of the ultra-low-carbon ferritic stainless steel which reduces C content to 0.010 mass% or less by the decarburization method in any one of Claims 1-3.