JP2011099151A - Method of rh degassing treatment for preventing white spot and crack in steel ingot - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preventing white spots and cracks in a steel ingot by sufficiently removing moisture contained in a furnace wall of an RH degassing treatment tank to reduce hydrogen in a molten steel in a method of RH degassing treatment. <P>SOLUTION: When conducting the degassing treatment of the molten steel for the first time using the RH degassing treatment tank whose refractory bricks are newly lined in a lower tank of the RH degassing treatment tank, during a standby period before conducting the degassing treatment of the first charge as a main treatment, a preliminary degassing treatment step is conducted. After moisture is evaporated from a refractory brick lining layer of the lower tank by a heat of the molten steel circulating inside the RH degassing treatment tank during the preliminary degassing treatment step, the degassing treatment of LF-refined molten steel is conducted as the main treatment. When conducting the RH degassing treatment after the preliminary degassing treatment, at least one charge chosen from the first charge and the subsequent charges of the main treatment is conducted for a degassing treatment time of at least 30 min, thereby preventing white spots and cracks in the steel ingot. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  In the present invention, when vacuum degassing treatment is performed using an RH degassing treatment tank having a newly-lined refractory brick layer, the water contained in the new refractory brick layer melts into the treated molten steel and is contained in the molten steel. It is related with the technique which prevents the white spot crack which is an internal crack of the steel ingot formed from this molten steel by preventing that the amount of hydrogen increases.

  In conventional steel melting, for example, scraps of iron raw materials are melted and smelted into the target molten steel in an electric furnace, and then the molten steel is heated and adjusted for RH degassing in a ladle furnace. Then, for the vacuum degassing treatment, the molten steel of the ladle furnace was circulated to the lower tank of the RH degassing treatment tank in the RH degassing treatment tank, and the vacuum degassing treatment was performed for 10 minutes to 20 minutes. Thereafter, the degassed molten steel is cast and ingot to obtain an ingot.

  By the way, when the inner refractory brick of the lower tank of the RH degassing treatment tank used for this vacuum degassing treatment is newly stretched and newly constructed, the moisture contained in the joint material at the time of the new tension construction is the new lining refractory brick layer. It will be contained in. Therefore, when the lining refractory in the lower tank of the RH degassing tank is a newly installed refractory brick layer in the vacuum degassing of molten steel during refining, vacuum degassing is performed using this RH degassing tank. In this case, moisture in the refractory brick layer dissolves in the molten steel. Hydrogen contained in the molten steel due to moisture dissolved in the molten steel, and due to the hydrogen contained in the molten steel, white spot cracks, which are internal cracks, occur inside the steel ingot cast from the molten steel.

  By the way, in order to prevent hydrogen-based defects (white spots, segregation flaws, etc.) as a method of ingot casting steel ingots, in the ingot after vacuum degassing, The method of subcontracting is taken in. However, for ingots of large steel ingots, drop degassing casting in a vacuum tank is adopted, and vacuum carbon deoxidation is also applied as a countermeasure against segregation and oxide inclusions (for example, (See Patent Document 1.) However, this method is not a method of preventing white point cracking caused by hydrogen in the steel ingot by defining, for example, the RH degassing time.

  On the other hand, in the drying method of the amorphous refractory lined in the degassing refining furnace, the surface layer part of the amorphous refractory is radiated and heated by the infrared light bulb installed in the furnace in a decompressed state from the microwave waveguide. There has been proposed a method of drying an amorphous refractory by irradiating the inside of a furnace through quartz glass that transmits microwaves (see, for example, Patent Document 1). However, in order to implement this method, it is necessary to provide special equipment, which is costly.

JP-A-10-281626

“Iron & Steel Handbook” 4th Edition, Volume 5, Volume 2, Issued July 2002, p. 4551/6845

  The problem to be solved by the present invention is that the lower tank of the RH degassing treatment tank in the vacuum degassing treatment of molten steel at the time of refining contains moisture when the refractory brick layer is a new refractory brick layered inside. When the lower tank having the new refractory brick layer containing moisture is used for the RH degassing process, moisture is dissolved from the lower layer into the molten steel being degassed, and hydrogen is contained in the molten steel. When a steel ingot made of molten steel containing hydrogen is rolled, white spot cracks occur inside the steel ingot during rolling. Therefore, the present invention provides an RH degassing method that prevents white spot cracking of a steel ingot obtained by sufficiently reducing hydrogen in molten steel from moisture in the lower tank of the RH degassing tank in advance. It is to provide a processing method.

  As a means of the present invention for solving the above-mentioned problems, the invention of claim 1 is characterized in that a ladle tapped from a converter or electric furnace using an RH degassing treatment tank in which a lining refractory brick of a lower tank is newly installed. When the molten steel in the RH is degassed for the first time, the preliminary degassing treatment step of the molten steel in the first charge is performed during the standby time until the RH degassing treatment from the first charge to the RH degassing treatment tank. To implement. In carrying out this pretreatment process, moisture contained in the inner refractory brick layer of the lower tank of the RH degassing treatment tank is evaporated and removed by heat generated from the molten steel of the first charge. Next, the first charge of the molten steel used for this preliminary treatment is smelted in a ladle, and the RH degassing treatment tank in which water has been removed from the ladle refined molten steel in the preliminary degassing treatment step, is one of the main treatments of RH degassing treatment. It is an RH degassing treatment method for preventing white spot cracking of a steel ingot, characterized by performing degassing treatment of a charge and ladle refining and RH degassing after that.

  According to the second aspect of the present invention, the RH degassing process of the main process performed after the preliminary degassing process includes at least one of the degassing processes of the first charge to the subsequent charge of the main process. 2. The RH degassing method for preventing white spot cracking of a steel ingot according to claim 1, wherein the degassing time for one charge is 30 minutes or more. That is, according to the second aspect of the present invention, the degassing time of at least one charge of any of the main treatments is set to 30 minutes or more, so that the lower part is heated by the heat of the molten steel circulating in the lower tank of the RH degassing tank. This is an RH degassing method that prevents white spot cracking of a steel ingot by further evaporating and removing moisture contained in the lining refractory brick layer of the tank.

  By adopting the above-mentioned means, the present invention converts the moisture contained in the newly installed refractory brick layer in the lower tank of the RH degassing treatment tank into evaporative gas by the heat of the molten steel circulating during the pretreatment of RH degassing, and further RH By the residual heat in the RH degassing treatment tank during the pretreatment during the waiting time until the main treatment of degassing, the moisture contained in the newly installed refractory brick layer is removed by evaporating gas and removed by this treatment. The amount of hydrogen contained in the molten steel is reduced, and white spot cracks in the steel ingot generated during rolling of the steel ingot obtained from the molten steel are reduced. Further, in the RH degassing process, which is the main process, in addition to the pretreatment for RH degassing, at least one of the charges in the degassing process of the first charge to the subsequent charge is removed. By setting the gas treatment time to 30 minutes or more, the hydrogen content in the obtained molten steel is further reduced, and white spot cracks in the steel ingot generated during rolling of the steel ingot obtained from this molten steel are reduced. . As described above, the present invention is a method that exhibits an unprecedented effect.

It is sectional drawing of the lower tank of a RH degassing processing tank, and is an elevation view cut | disconnected by AA of FIG. It is sectional drawing of the lower tank of a RH degassing processing tank, and is the top view seen by cut | disconnecting by BB of FIG. The graph shows the variation in the amount of hydrogen contained in molten steel subjected to RH degassing treatment to the collected sample, (a) is the conventional method, (b) is the method of claim 1 of the present invention, and (c) is the claim of the present invention. The graph which shows the method of 2 and those average values, a standard deviation, and a sample number are shown. It is a figure which shows the sample collection tool from molten steel. It is a graph which shows the correlation of the RH degassing process time of the gasification of the water | moisture content in molten steel, and the hydrogen content in molten steel.

  The best mode for carrying out the present invention will be described with reference to tables and drawings. According to the first aspect of the present invention, various steel types of JIS or SAE standard shown in Table 1 were melted in an electric furnace, and then the molten steel melted in the electric furnace was taken out into a ladle. This smelted steel is not subjected to refining of the ladle furnace (hereinafter referred to as “LF”), and is preliminarily removed by an RH degassing treatment tank having a lower tank 1 composed of a newly-laid brick layer 2 shown in FIGS. The gas treatment was performed at a preliminary RH treatment start temperature shown in Table 1 for 10 minutes or more. Further, after the temperature of the molten steel lowered to the LF refining start temperature shown in Table 1 by this preliminary degassing treatment is increased by LF and subsequently refining, the molten steel is again subjected to the RH degassing treatment tank having the lower tank 1 of the RH degassing treatment tank. The RH degassing process, which is the main process, was normally performed to degas the first charge and the subsequent charge. These degassed molten steel was cast and formed into a steel ingot.

  Furthermore, the means of claim 2 of the present invention is similar to the means of claim 1 of the present invention, in which various steel types shown in Table 1 are melted in an electric furnace, and then the molten steel is melted in the electric furnace. The preliminary degassing treatment is carried out by the RH degassing treatment tank having the lower tank 1 made of the newly-built brick layer 2 at the start temperature of the preliminary RH treatment shown in Table 1 without refining the molten steel. Went for more than a minute. Further, after the temperature of the molten steel lowered to the LF refining start temperature shown in Table 1 by this preliminary degassing treatment is increased by LF and subsequently refining, the molten steel is again subjected to the RH degassing treatment tank having the lower tank 1 of the RH degassing treatment tank. The RH degassing process, which is the main process, was performed. In this method, according to the second aspect of the present invention, the processing time of the RH degassing process of at least one charge of any one of the first charge to the subsequent charge in the RH degassing process as the main process is 30. More than a minute was secured, and dehydrogenation was performed from the molten steel by this RH degassing treatment. These degassed molten steel was cast and formed into a steel ingot.

  By the preliminary degassing treatment in the means of claim 1 and the means of claim 2 above, the moisture contained in the new brick layer 2 of the lower tank 1 of the RH degassing treatment tank is changed to the molten steel of the molten steel in LF. During the waiting time until the heat and the subsequent RH degassing treatment, which is the main treatment, gasification was caused by the residual heat in the lower tank 1. By the way, in the case of the conventional method in which the RH degassing treatment is performed immediately after the temperature is increased and the components are adjusted by LF refining without performing the preliminary degassing treatment of the molten steel melted in the electric furnace, the lower tank 1 of the RH degassing treatment tank As shown in FIG. 3A, the amount of hydrogen contained in the molten steel brought about by gasification of moisture generated in the molten steel from the new brick layer 2 is 31 samples, with a standard deviation of 0.20, on average. It was 3.74 ppm. However, the preliminary degassing process in the means of claim 1 of the present invention is performed for 10 minutes, and the RH degassing process of this process is performed for about 10 to 20 minutes per charge (in this process without performing the preliminary degassing process). (Corresponding to about 20 to 30 minutes per charge of a conventional method in which only a certain RH degassing treatment is performed) by gasification of moisture generated from the new brick layer 2 of the lower tank 1 of the RH degassing treatment tank, As shown in FIG. 3B, the hydrogen content in the molten steel was reduced to 30 samples, a standard deviation of 0.15, and an average of 3.30 ppm. Further, as a means of claim 2 of the present invention, in addition to 10 minutes of the preliminary degassing process, at least one charge of any one of the first charge in the RH degassing process which is the main process to the subsequent charge is included. Gas degassing treatment is performed for 30 minutes or more, and other main treatment is performed for about 10 to 20 minutes per charge, thereby gasifying moisture generated from the new brick layer 2 in the lower tank of the RH degassing treatment tank. As a result, the hydrogen content in the molten steel could be further reduced to 27 samples, a standard deviation of 0.58, and an average of 2.77 ppm as shown in FIG.

  As for the hydrogen content in the molten steel, the molten steel was sampled when it reached 2/3 or more of the molten steel in the mold using the sample collecting tool 3 shown in FIG. As shown in FIG. 4, the sample collection tool 3 is a quartz tube 4 fixed to a handle 5. The sample collection tool 3 is immersed in molten steel in a mold, the quartz tube 4 is dipped, the handle 5 is pulled up, and the tip of the quartz tube 4 is placed. The molten steel was further collected, and the sample molten steel was cooled with water while the quartz tube 4 was used, and the hydrogen concentration was analyzed by analysis to obtain the hydrogen content in the molten steel.

  Here, the reason which limited the RH degassing process time in the gasification of the water | moisture content which arises in molten steel from the above-mentioned new tension brick layer 2 in this invention is demonstrated. As shown in FIG. 5, when the correlation between the treatment time and hydrogen in the molten steel is graphed, it has been found that as the RH degassing treatment time becomes longer, the concentration of hydrogen in the molten steel decreases and the variation becomes smaller. For this reason, the processing time is limited. In this case, when the processing time was 30 minutes or longer from the graph of FIG. 5, the variation became small and the hydrogen value was 4 ppm or less. Therefore, the lower limit value of the processing time for one charge of the RH degassing process in this process including the time for the preliminary degassing process is set to 30 minutes. That is, the preliminary degassing process according to the first aspect is added to the RH degassing process of the present process as 10 minutes or more, and the RH degassing process of the present process is performed for 10 to 20 minutes per normal charge. By carrying out the process, the hydrogen content can be set to an average value of 3.30 ppm. In particular, in the method of claim 2, in addition to the pretreatment, the degassing treatment of the first charge to the subsequent charge as the main treatment is performed. By setting the degassing processing time for at least one charge of any one of these charges to 30 minutes or more and the processing time for one charge of the other main processing being 10 to 20 minutes, the hydrogen content is an average value of 2. It was improved to 77 ppm, and the variation was further reduced.

1 Lower tank 2 Brick layer 3 Sampling tool 4 Quartz tube 5 Handle

Claims (2)

  When RH degassing treatment is first performed on molten steel in a ladle from a converter or electric furnace using an RH degassing treatment tank with a new refractory brick lined in the lower tank, the RH of the first charge from the tapping steel During the waiting time until the degassing treatment, the preliminary degassing treatment step of the molten steel of the first charge is performed in the RH degassing treatment tank, and the lower part of the RH degassing treatment tank is heated by the heat generated from the molten steel of the first charge. After evaporating and removing the water contained in the lining refractory brick layer of the tank, the molten steel is smelted in a ladle, and the RH degassing treatment tank in which the water is removed from the molten steel smelted in the ladle 1 An RH degassing method for preventing white spot cracking of a steel ingot, characterized by performing degassing treatment of a charge eye, ladle refining of a charge eye and RH degassing treatment thereafter.   In the main process of the RH degassing process performed after the preliminary degassing process, the degassing process time of at least one charge of any one of the degassing processes of the first charge to the subsequent charge of the main process is set. The RH degassing method for preventing white spot cracking of the steel ingot according to claim 1, wherein the RH degassing treatment is performed for 30 minutes or longer.

Images