JP2007224367A - Method for producing high-nitrogen steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high-nitrogen steel with which even in the case of using argon gas as stirring gas or circulating gas in a vacuum-degassing treatment, molten steel having ≥90 mass% nitrogen content can stably be produced by making higher the nitrogen yield than the conventional method. <P>SOLUTION: The method for producing the high-nitrogen steel is improved. By the method, molten iron held in a refining vessel through a molten iron pre-treating desulfurization is decarburized with oxygen gas and at the time when the carbon content is lowered to a fixed range, the nitrogen gas is blown and once, the molten steel as higher nitrogen-containing molten steel than the targeted value is tapped off from the refining vessel, and successively, the degassing treatment is performed to the molten steel with the vacuum-degassing apparatus by using the argon gas as the stirring gas or the circulating gas. Specifically, sulfur content and the nitrogen content in the molten steel are adjusted by adding sulfur-containing material to the molten steel during degassing treatment. In this case, the sulfur content in the molten steel can be made to 0.012 mass% to the upper limit permissible value, or FeS can be used as the sulfur-containing material. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for melting high nitrogen-containing steel, and in particular, when so-called “secondary refining” is performed by using a vacuum degassing apparatus for blowing molten steel that has been blown with nitrogen gas in a converter and discharged at a high nitrogen content. Furthermore, the present invention relates to a technique for accurately adjusting the nitrogen content of the molten steel to a target value and stably melting a high nitrogen-containing steel having a nitrogen content of 90 mass ppm or more.

  Conventionally, in order to melt nitrogen-containing steel in a converter, when the molten metal (hot metal or molten steel) held in the furnace is decarburized with oxygen gas, via a gas top blowing lance, bottom blowing tuyere, etc. This was done by blowing nitrogen gas according to the target nitrogen content. However, in such a method, not only is the yield of nitrogen low for various reasons, it is difficult to stably smelt high nitrogen-containing steel having a target nitrogen content of 90 mass ppm or more, and the target value is achieved. There is an economical disadvantage that the amount of nitrogen gas required for the process is remarkably increased. In addition, since the nitrogen gas blowing time becomes longer, there is another problem that the melting efficiency is inevitably lowered and the converter refractory wear increases.

  Therefore, the present applicant has first developed a technique for overcoming all of these problems and producing a molten steel having a nitrogen content of 90 ppm by mass or more and a low content of oxygen and sulfur. When the molten steel held in the converter having a gas bottom blowing function is decarburized with oxygen gas, nitrogen gas is blown into the molten steel having a reduced carbon content to form nitrogen-containing molten steel. Is blown in the region where the carbon content in the molten steel is 0.25 to 0.15% by mass. At that time, the molten steel discharged from the converter is subsequently subjected to a vacuum degassing apparatus. It is a technique that it is better to perform degassing treatment (secondary refining) using argon gas as a stirring gas or a reflux gas (see, for example, cited document 1).

  In addition, as a method of separately adjusting the nitrogen content of the molten steel to the target value, the present applicant casts the molten steel after the secondary refining with a continuous casting machine to obtain a steel slab. Nitrogen gas (which may be mixed with argon gas) is blown into the nozzle that guides the molten steel from the tundish (an intermediate container placed on the mold and made uniform from the ladle and injected from the ladle) to the mold. A technique for preventing clogging of the steel and adjusting the nitrogen content of the molten steel is also disclosed (see Patent Document 2).

  According to these techniques, a nitrogen-containing steel having a high cleanliness with low nitrogen yield of 90 mass ppm or more, oxygen (30 mass ppm or less) and sulfur (14 mass ppm or less) is obtained. It became possible to melt to some extent.

  However, in the technique described in the cited document 1, it is necessary to use argon gas as a stirring gas or a reflux gas in vacuum degassing refining in order to reduce non-metallic inclusions in steel and obtain high cleanliness. That is, nitrogen gas is a gas soluble in molten steel, and when used as a stirring gas or a recirculation gas, the stirring power is weaker than that of argon gas, and the floating separation effect of nonmetallic inclusions is small. However, when argon gas is used, there is a problem that the nitrogen concentration during processing is greatly reduced, and the technique is not satisfactory from the viewpoint of improving the nitrogen yield. Moreover, although the technique of the cited reference 2 is suitable for fine-tuning the nitrogen content of molten steel after vacuum degassing refining, it is difficult to adopt for the significant control of the nitrogen content.

Moreover, as high nitrogen content steel, the restriction | limiting of a sulfur content like the high-tensile steel material used for the suspension part for motor vehicles is not only a severe thing, but there is also a steel grade with a loose regulation about sulfur. For such steel types, it would be economically desirable if the steel could be produced with a higher nitrogen yield.
Japanese Patent Laid-Open No. 2002-12908 JP 2000-104111 A

  In view of such circumstances, the present invention can stabilize a molten steel having a nitrogen content of 90% by mass or higher by increasing the nitrogen yield even when argon gas is used as the stirring gas or the reflux gas in the vacuum degassing treatment. It aims at providing the melting method of the high nitrogen content steel which can be manufactured.

  The inventor has intensively studied to achieve the above object, and the results have been embodied in the present invention.

  That is, according to the present invention, the hot metal held in the refining vessel through the hot metal pretreatment desulfurization is decarburized with oxygen gas, and nitrogen gas is blown in at that time, and once the molten steel having a nitrogen content higher than the target value is discharged from the refining vessel. Steel, and subsequently, when the molten steel is degassed by using argon gas as a stirring gas or a reflux gas in a vacuum degassing apparatus, a sulfur-containing substance is added to the molten steel being degassed. This is a method for melting high nitrogen-containing steel, characterized by adjusting the sulfur content and nitrogen content of the steel.

  In this case, the sulfur content of the molten steel is preferably 0.012% by mass to the upper limit of the allowable value, or FeS is preferably used as the sulfur-containing substance. Also, the carbon content in the molten steel at the start of the nitrogen gas blowing is set to 0.25 to 0.15 mass%, or the target nitrogen content of the molten steel is 90 to 110 mass ppm and the target sulfur content is It is preferable to be more than 0.012 to 0.020% by mass. Further, in the present invention, the refining vessel is selected from the top blowing converter, bottom blowing converter, top bottom blowing converter and AOD furnace, and the vacuum degassing apparatus is selected from the RH method, DH method and VOD method. A seed is preferred.

  According to the present invention, even when argon gas is used as a stirring gas or a reflux gas for vacuum degassing treatment, the denitrification amount during the treatment can be reduced as compared with the conventional case. High cleanliness high nitrogen content steel (nitrogen concentration 90 mass ppm or more) with few nonmetallic inclusion elements can be stably melted.

  Hereinafter, the best embodiment of the present invention will be described based on the background of the invention.

  First, the inventor, in the prior art, blows nitrogen gas into molten steel during decarburization in a smelting vessel, and even if the nitrogen content is increased to a target value or higher, denitrification occurs in the vacuum degassing process, and the nitrogen yield is increased. Focused on not improving as expected. And we decided to search for a means to further suppress denitrification during vacuum degassing. Therefore, an RH vacuum degassing apparatus as shown in FIG. 3 was used as a vacuum degassing apparatus, and intensive efforts were made to discover means for suppressing denitrification. As a result, when a sulfur-containing material is added to the molten steel in the ladle being degassed, the denitrification rate (expressed as -dN / dt) decreases as the amount of sulfur-containing material added increases, as shown in FIG. I found out. From FIG. 1, it was found that the sulfur content was 0.012% by mass when -dN / dt was minimized and settled to a substantially constant value. This is considered to be because when the concentration is increased by the addition of sulfur, which is a surface active element of molten steel, the molten steel surface is in a state where it is difficult for gas to escape.

  Therefore, the inventor is a steel type that the high nitrogen content steel to be melted may increase its sulfur content to some extent (for example, C: 0.03 to 0.05 mass%, Si: 0.03 mass% or less, Mn : 0.18 to 0.35 mass%, P: 0.020 mass% or less, S: 0.021 mass% or less, N: 90 to 140 ppm, Al: 0.023 to 0.064 mass%) The present inventors have completed the present invention on the assumption that denitrification in vacuum degassing can be suppressed and high nitrogen-containing steel can be efficiently produced.

  In the present invention, as before, the hot metal is decarburized in a refining vessel before vacuum degassing to obtain molten steel, but the hot metal used therein has undergone a "hot metal preliminary desulfurization process" of so-called "hot metal preliminary process". It is assumed that it is a thing. In the current steelmaking process, in order to achieve the target sulfur content of the molten steel to be smelted, the hot metal “preliminary desulfurization treatment” is performed in advance to reduce the sulfur content to 0.020% by mass or less. It is common to perform charcoal refining. Therefore, it seems that it is not necessary to declare the premise that it has undergone “hot metal preliminary desulfurization treatment”, but if a sulfur-containing substance is added by vacuum degassing treatment as in the present invention, the decarburization process This is because it is possible to use hot metal that does not undergo the above-described preliminary desulfurization treatment. That is, the hot metal in the state discharged from the blast furnace usually has a sulfur content of 0.02 to 0.04% by mass, and if used as it is in the decarburization process, the sulfur-containing material is added during the vacuum degassing process. do not have to.

  However, there are almost no steel materials currently in practical use with a sulfur content as high as 0.02 to 0.040% by mass, and desulfurization is essential somewhere. However, desulfurization is most efficient when it is performed at the hot metal stage, and it is economically disadvantageous if it is performed after decarburization. Therefore, it is a good idea to perform “hot metal preliminary desulfurization treatment” of hot metal in advance, reduce the sulfur content as much as possible, decarburize and refine, and then adjust the sulfur content. For this reason, in the present invention, the premise that the hot metal before decarburization in the smelting vessel before vacuum degassing has undergone a “hot metal preliminary desulfurization process” is provided. The hot metal preliminary desulfurization method is not particularly limited, but it is preferable to use a method in which a ladle is provided with a hot metal mechanical stirring means and a desulfurizing agent mainly composed of quick lime is used. This is because the desulfurization efficiency is high.

  In the present invention, the refining vessel is an upper blowing converter, a bottom blowing converter, an upper bottom blowing converter, and an AOD furnace, and the vacuum degassing apparatus is an RH method, a DH method, or a VOD method. The selected one is preferable. This is because any of these can be used to smoothly implement the present invention. However, the stirring gas or reflux gas of the vacuum degassing apparatus is limited to the use of argon gas. This is because, as described above, the stirring efficiency of molten steel is greater than that of nitrogen gas.

  In the present invention, the timing of adding the sulfur-containing substance is not particularly limited, but it is preferable to start the degassing process from the viewpoint of dissolution of the sulfur-containing substance. The sulfur-containing substance may be an inorganic substance containing sulfur, but it is preferable to use FeS in consideration of contamination of the molten steel by other impure elements. As the addition amount of the sulfur-containing material, the sulfur content in the molten steel is in the range of 0.012 mass% to the allowable upper limit. This is because when the sulfur content is 0.012% by mass, −dN / dt is minimized, and when the sulfur content is higher than that, the value is constant. As a matter of course, the upper limit is allowed by the high nitrogen-containing steel to be melted.

  Furthermore, it is preferable to start the nitrogen gas blowing in the decarburization process in the refining vessel when the carbon concentration in the molten steel is 0.25 to 0.15% by mass, as in the prior art. This is because it is effective in improving the nitrogen yield.

  As described above, by implementing the above aspect of the present invention, the target sulfur content is over 0.012 to 0.10% by mass for the first time and higher than the molten steel described in Patent Document 1, but the target nitrogen content is high. 90 to 120 mass ppm of high nitrogen content steel can be melted reliably with a high nitrogen yield.

  The hot metal extracted from the blast furnace is subjected to desiliconization, dephosphorization and desulfurization sequentially in the existing and well-known hot metal pretreatment process, and the obtained hot metal is used as a refining vessel with a so-called “top bottom blowing converter” with a capacity of 200 tons. And decarburization refining with oxygen gas. During the decarburization, nitrogen gas was blown through an upper blowing lance so that the nitrogen content in the molten steel was 120 ppm or more, and a large number of high nitrogen content molten steel was charged and melted. An example of the nitrogen content and sulfur content of the molten steel delivered to the ladle is as shown in Table 1. After that, each of these molten steels was subjected to degassing treatment using either the method according to the present invention or the conventional method (the method described in Patent Document 1) using an RH vacuum degassing apparatus. At that time, argon gas was used as the reflux gas, and degassing was performed for 20 to 25 minutes. In carrying out the present invention, the sulfur-containing material FeS was added at the start of degassing so that the sulfur content in the molten steel was in the range of 0.012 mass% to the allowable upper limit.

An example of the operation result is shown simultaneously in Table 1. Moreover, the relationship between nitrogen (symbol: N) content of all the molten steel after completion | finish of vacuum degassing and the nitrogen content after the said converter steel is shown collectively in FIG. From FIG. 1, it is clear that according to the present invention, the nitrogen content is reliably increased as compared with the conventional method. That is, in the conventional method, the denitrification amount in the vacuum degassing is 70 mass ppm, whereas the denitrification amount in the present invention is reduced to 50 mass ppm when the sulfur addition amount is 0.012 mass%. is doing.
Moreover, since the denitrification amount during the degassing treatment can be reduced by 20 ppm by carrying out the present invention, it is clear that high nitrogen-containing steel having a nitrogen content of 90 ppm by mass or more can be stably melted.

It is a figure which shows the relationship between the nitrogen content of the molten steel after completion | finish of vacuum degassing, and the nitrogen content of the molten steel after a converter outgoing steel. It is a figure which shows the denitrification speed | rate from the molten steel in a vacuum degassing process. It is a figure which shows a general RH vacuum degassing apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ladle 2 Degassing tank 3 Gas which promotes the reflux of molten steel (reflux gas)
4 Ascending pipe 5 Downcomer pipe 6 Molten steel 7 Arrow indicating exhaust direction 8 Arrow indicating recirculation direction of molten steel

Claims (6)

The hot metal held in the smelting vessel through the hot metal pretreatment desulfurization is decarburized with oxygen gas, and nitrogen gas is blown at that time, and the steel is once discharged from the smelting vessel as a molten steel having a nitrogen content higher than the target value. When degassing molten steel using argon gas as stirring gas or reflux gas with vacuum degassing device,
A method for melting high nitrogen-containing steel, characterized by adding a sulfur-containing substance to the molten steel being degassed to adjust the sulfur content and nitrogen content of the molten steel.   The method for melting high-nitrogen-containing steel according to claim 1, wherein the sulfur content of the molten steel is 0.012 mass% to the upper limit of the allowable value.   The method for melting high nitrogen-containing steel according to claim 1 or 2, wherein FeS is used as the sulfur-containing substance.   The melting of high nitrogen content steel according to any one of claims 1 to 3, wherein the carbon content in the molten steel at the start of blowing of the nitrogen gas is 0.25 to 0.15 mass%. Method.   The high nitrogen content according to any one of claims 1 to 4, wherein the target nitrogen content of the molten steel is 90 to 11 mass ppm and the target sulfur content is more than 0.012 to 0.020 mass%. Steel melting method.   The refining vessel is a top blowing converter, bottom blowing converter, top bottom blowing converter, and AOD furnace, and the vacuum degassing apparatus is one selected from RH, DH, and VOD systems. The method for melting high nitrogen-containing steel according to any one of claims 1 to 5.

Images