JPS60181217A - Production of low-nitrogen high-chromium alloy steel - Google Patents

Abstract

PURPOSE:To obtain a low-nitrogen high-chromium steel at high denitrification efficiency while suppressing oxidation of Cr by decarburizing a crude high- carbon high-chromium molten steel produced in a steel making furnace by top blowing of oxidizing agent powder under the reduced pressure and accelerating denitrification. CONSTITUTION:A high-Cr crude molten steel 1 contg. >=0.1% [C] refined in a steel making furnace is charged into a ladle 2. Gaseous Ar is supplied into the pipe 3a of a cylindrical reflux type degassing device 3 in the upper part of the ladle where the two immersion pipes 3a, 3b are immersed in the steel 1. The inside of the device 3 is evacuated 6 to a reduced pressure to suck and rise the steel 1 in the pipe 3 so as to be admitted into the device 3 and to flow back into the ladle 2 through the other pipe 3b. Oxidizing agent powder 5 of Cr oxide- etc. is blown in this stage through a top blowing lance 4 provided to the device 3 inclinedly with the molten metal surface by a carrier gas so as to infiltrate substantially into the molten steel 1. The formed CO foam is thus expanded to increase the gas-metal.boundary area and therefore the denitrification reaction is accelerated and the low-nitrogen high-chromium steel is effectively obtd.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a low nitrogen, high chromium alloy steel with CN] below 50 ppm and (Cr) above 13% IJ. [Prior Art] Ferritic stainless steel is a typical high chromium alloy steel containing 13% or more of Cr. When this stainless steel contains large amounts of C and N, it is desirable to reduce them as much as possible since they do not adversely affect corrosion resistance, workability, weldability, etc. However, degassing treatment for the purpose of denitrifying molten steel is not actively carried out, and degassing under reduced pressure using R1f equipment, VOD equipment, etc. is aimed at decarburizing molten steel refined in steelmaking furnaces. Denitrification was only recognized as a side effect of the treatment. For example, the method for producing low carbon high chromium alloy steel disclosed in Japanese Patent Publication No. 49-46444 decarburizes the steel by spraying oxygen gas under reduced pressure, and does not mention denitrification. Therefore, when decarburizing using this method, CO bubbles/
g Generated inside the steel, the CO bubbles reduce the N2 partial pressure on the gas side of the gas-metal interface of the CO bubbles, and denitrification is performed. However, with this method, it is said that the temperature at the fire point where the oxygen gas hits the /8 steel surface reaches 2000°C to 2500°C, and a large amount of fume is generated. It can accumulate in the water and cause duct clogging and contaminate the operating environment.
Furthermore, since its main component is iron oxide, it has the disadvantage of lowering the yield of /8 steel. Regarding the amount of oxygen to be blown, if the oxygen gas supply rate is decreased, a superoxidized state will occur, and there will be too much (0) in /8 steel, which will suppress or inhibit the denitrification reaction, and the molten steel will become overoxidized. There are many operational problems, such as oxidation of Cr in the molten steel and the accumulation of solid chromium oxides on the surface of the molten steel, which obstructs the supply of oxygen to the molten steel. On the other hand, when the oxygen gas supply rate is lowered, the decarburization rate decreases and the denitrification rate also decreases. In order to prevent oxidation of Cr without reducing the decarburization and denitrification rate, it is necessary to perform decarburization and denitrification at a relatively high temperature range of about 1650°C to 1850°C. When using this method, there was a drawback that the amount of erosion of the refractory material was large. [Purpose] The present invention was made in view of the above circumstances, and its purpose is to enable denitrification treatment at a relatively low temperature while minimizing the amount of FN in the refractory melt, and to improve corrosion resistance, workability, and weldability. An object of the present invention is to provide a method for producing a low nitrogen, high chromium alloy steel that has excellent denitrification efficiency and can suppress oxidation of Cr. [Structure] The method for producing low nitrogen, high chromium alloy steel according to the present invention involves top-blowing oxidizing agent powder under reduced pressure to high chromium crude molten steel with [c]≧0.1% melted in a steelmaking furnace. and decarburize it,
It is characterized by promoting the denitrification that occurs at this time, and is further characterized by performing depressurization using a recirculation degassing device, and top-blowing the oxidizer powder obliquely to the crude molten steel. do. [Example] The present invention will be specifically described below based on the drawings. FIG. 1 is a schematic diagram showing the implementation state of the present invention, and in the figure 1
was refined into ferritic stainless steel in an M steel furnace such as a converter, electric furnace, or AOD furnace and charged into ladle 2 (C)
≧0.1% crude molten steel. This crude molten steel 1 has a Cr content of 13 after passing through the decompression treatment process detailed below.
Adjust so that the required value is % or more. In addition, when C oxide is used as the oxidizer powder, the Cr content may be lower than the above-mentioned required value at this crude molten steel stage, and the Cr composition is adjusted in the vacuum treatment process. Above, there are two dip tubes 3a at the bottom,
A cylindrical recirculation degassing device (RH filling device 3 is installed), and its two immersion tubes 3a and 3b are
is immersed into the molten steel 1. Ar gas is supplied to the inside of the immersion tube 3a via a gas supply tube 7. The RH device 3 can be depressurized by evacuating the inside through the vacuum exhaust duct 6, and when the inside of the RH device 3 is depressurized, the gas lift pump is activated by the circulating gas from one of the immersion tubes 3a. Based on the principle, the compatible steel 1 in the ladle 2 is sucked from the immersion pipe 3a and rises to R.
Phase/g steel 1 enters the H device 3 and enters the other immersion tube 3.
It returns to the inside of the ladle 2 from b, that is, it circulates. The degree of vacuum inside the RH device 3 is preferably close to vacuum;
At least the height of the molten steel at the bottom of the RH device 3 during reflux is 200 mm.
It is adjusted so that it is less than 2 mm. This is to prevent the refractory material at the bottom of the RH device 3 from being easily molten steel due to gaseous oxygen if the depth is too shallow. Also, if the depth is too shallow, the amount of molten steel processed per hour will decrease and the processing time will be longer. A top blowing lance 4 is installed at a suitable height on the peripheral wall of the RH device 3 so that its blowing angle is oblique to the molten steel surface. From the top blowing lance 4, iron oxide, Mn oxide, Cr oxide, Ca CO3, A7!
203, CaO, etc.) is blown into the crude molten steel l using a carrier gas such as argon gas so that it sufficiently penetrates into the crude molten steel l. Note that the blowing angle from the bottom of the RH device 3 is 20'' to 80°.If the blowing angle is less than 20'', the oxidizer powder 5 will not enter the crude molten steel l sufficiently because it will be blocked and scattered. On the other hand, if the angle exceeds 80°, the lance becomes long and uneconomical, and the splash becomes large.As for the oxidizer powder 5, it is recommended to use one with an appropriately small particle size. This is because the finer the oxidizer powder, the more particles that become CO nuclei will increase for the same amount of oxidant powder, which will generate a large amount of fine CO bubbles, which will cause the denitrification reaction to occur. It is possible to increase the area and speed up the decarburization reaction rate, but if the particle size is too small, the particles will sinter with each other and become coarse, which will prevent the powder from being dispersed in the molten steel. Regarding the lance height, it is best to set the height from the molten steel surface to the blowing hole at the tip of the top blowing lance 4 to be 300 mm to 900 mm.If this is less than 300 mm, splash This occurs violently and is unfavorable for operation, resulting in 900
This is because if it exceeds mm, the penetration of the powder becomes shallow, which is unfavorable in terms of reaction efficiency. CO gas or CO2 gas may be used as the carrier gas, and iron powder, Mn powder, Cr powder, etc. may also be used as the blown powder. In this case, an oxygen source is required and oxygen gas is blown into the carrier gas. However, in this case, Ar gas or the like is used as the carrier gas, and oxygen gas is separately blown in to prevent explosion of iron powder or the like. Furthermore, denitrification effects can be obtained simply by blowing Fe powder or the like with Ar gas. This is thought to be due to the fact that the N2 partial pressure is lowered by the Ar gas and the Fe powder becomes a reaction nucleus, as will be described later. Cr oxide, Mn oxide, etc. may be injected as the oxidizing agent powder 5 for the purpose of component adjustment. The oxidizer powder top-blown as described above is
The decarburization and denitrification reactions described below proceed uniformly because they penetrate deeply into the water and are dispersed. After denitrification treatment is performed until the required components are obtained in this manner, the molten steel is continuously cast, subsequently subjected to a rolling process to form a plate material, and further continuously annealed to form a final product. It goes without saying that the denitrified molten steel may be made into a steel ingot by an ingot-forming method instead of the continuous casting method. Furthermore, the present invention does not depend on the RH device, but rather on the VOD device. Of course, a DH device or the like may also be used. In the case of a VOD device, since the height of the canopy is low, the oxidizing agent powder may not be blown diagonally upward, but may be blown from directly above with a vertical lance. In this case, the denitrification efficiency can be improved by using a porous brick at the bottom of the ladle and stirring the crude molten steel by blowing Ar gas into it. Next, we will explain the reaction by injecting oxidizer powder.When crude molten steel 1 at ℃ is depressurized to 3 torr and iron oxide is injected from top blowing lance 4 at a rate of 22.2 kg per ton of crude molten steel. Changes in (C), (N), (Cr) of
This is a graph showing N) (ppm) and (Cr) (%). In the case of the treatment of the present invention (CJ is 0.50% to 0.
By 0.05%, (N) was reduced from 133 ppm to 0.05%. And (Cr) remained almost unchanged from 18.2% to 18.1%. In the case of this reaction, it can be seen that (N) decreases as (C) decreases. The reason is as follows. Iron oxide serves as an oxygen supply source and also as a CO generation nucleus, causing a CO reaction. The N2 partial pressure on the gas side of this CO gas at the gas-metal interface is C
It becomes extremely low due to O bubbles, and the (0) on the metal side is consumed by the decarburization reaction, so it remains low, so it has conditions that make it easy for the denitrification reaction to occur. Therefore, in this reaction interface, both decarburization and denitrification reactions occur simultaneously. Note that such a reaction is similar when other oxidizing agent powders are injected. Further, the oxidizing agent powder does not react with (Cr) in molten steel and preferentially promotes decarburization. This converts the crude solution 111 to (C) 50.1
It is thought that it depends on the percentage. That is, the temperature is 1600℃
2 using 19% Cr steel and 29% Cr steel as
The present invention was carried out under reduced pressure to 0 torr, and it was investigated whether chromium oxide was produced. 0 Figures 3 and 4 are 19%Cr tm and 29%Cr, respectively.
For steel, [C] (%) is plotted on the horizontal axis and plotted on the vertical axis.

[0](
This is a graph summarizing the results. The solid line in the figure is Cr 30. of the following formula (1). This is a precipitation curve, and is determined by the equilibrium constant expressed by the following equation (2) and the temperature. 'ACr 3 0t -)icr 10 -+11f/
4+A ]ogKl (-C%Cr) Jun 10c, +
04 )--13380/T +5.99...(2
) However, Dan: Oxygen in molten steel [%Cr): Weight percent of Cr in i's steel 1-σ0
:Activity of oxygen αCr3 o(:Activity of Cr3O4 On the other hand, the broken line is not shown in the following equation (3) and represents 20to of CO reaction.
This is a curve showing CO equilibrium at the time of rr, and its equilibrium constant 2 is determined by the following equation (4). C0=C+dan...(3) Pco:co partial pressure In addition, the ○ mark is the [O] value during decarburization and denitrification at 1600°C and 20 torr, and (C) (i&).From these figures, As understood, under reduced pressure, in the case of 19% Cr steel, 0.0
2% = CC) (D area, 0.0 for 29% Cr steel.
It is theoretically clear that in the region of 07%≦[C], (0) contributes to decarburization over chromium oxidation. And in this experiment, the actual decarburization reaction (o-o) is C-O
It is close to the reaction at equilibrium (dashed line), and (C) changes from a high level to a low level, and in the case of 19% Cr steel, it is 0.0.
Chromium oxide (CrxOt
) occurrence was observed. In other words, in the present invention, crude molten steel is
) 50.1%, it is possible to allow the decarburization reaction to proceed faster than the chromium oxidation reaction even at 1600'C while suppressing the Cr oxidation, and it is theoretically possible to keep the oxygen concentration in the molten steel low. is also possible. Therefore, the supplied oxygen source effectively contributes to the decarburization reaction, and it is actually possible to keep the oxygen concentration, which inhibits denitrification, low. Therefore, in the case of the present invention, it is sufficient to adjust the Cr component before processing. In addition, when Cr oxide is used as the 2 oxidizing agent powder for the purpose of component adjustment, Cr contributes to the component adjustment without waste. This also applies to other components such as Mn. Also, as understood from Figures 3 and 4,
Since denitrification is effective even when carrying out the present invention at 1600°C, in the present invention, when a large amount of oxidizing agent powder 5 is injected into the process, the process temperature is adjusted to compensate for the temperature drop caused by this. Even if the temperature is slightly raised, it can be carried out even in a relatively low temperature range of 1650° C. or lower, and therefore the amount of refractory erosion is small. Here, the changes in (C) and (N) in Figure 2 are used as indicators by the following formula (5) (1 Tetsu to Hagane, published in 1980).
It is expressed by introducing the denitrification efficiency α of 2006 S. 234). α-Δ(j/[%N])/Δ[%C]...(5) However, Δ(1/[%N)) 1: Change in the reciprocal of the weight percent of [N] in the steel Absolute value Δ [%C]: Absolute value of weight percent change in (C) in steel In the case of the example shown in FIG. 2, α is about 27c. On the other hand, in the prior art 3 of Japanese Patent Publication No. 49-46444, α is about 140 to 200, and when compared with this, it can be seen that the denitrification efficiency relative to the decarburization amount is superior in the case of the present invention. . This can shorten the denitrification treatment time and reduce the amount of refractory erosion. Furthermore, the relationship between the amount of decarburization and the amount of denitrification when denitrifying according to the present invention is shown in FIG. In Figure 5, the horizontal axis shows (N) (ppm) before treatment, the vertical axis shows (N) (ppm) after treatment, and the amount of decarburization is 0.
．． It is a graph showing the amount of denitrification when denitrifying by changing 1% C increments. As can be understood from this figure, in the case of denitrification treatment according to the present invention, the (C) Once the amount of decarburization is determined, the denitrification adjustment is controlled accordingly. The rate of decarburization and denitrification and the nitrogen concentration reached are affected by the degree of vacuum. In other words, the higher the degree of vacuum, the lower the equilibrium nitrogen concentration (/8 understanding);
From equation 4), even with the same carbon concentration, the equilibrium oxygen concentration becomes lower and the higher the degree of vacuum, the more the CO bubbles expand and the gas-metal interface area increases, so the denitrification reaction is accelerated. Furthermore, the Ar gas bubbles supplied from the immersion tube 3a become larger as the degree of vacuum becomes higher, thereby reducing the N2 partial pressure. Therefore, a high degree of vacuum is more effective. And high vacuum degree (N
) decreases, so the amount of nitrogen reached becomes lower,
In addition, regarding the injection of oxidizing agent powder, the higher the degree of vacuum, the less molten steel scattering (splash) can be achieved. Therefore, a high vacuum is maintained during processing. In the present invention, the top-blown oxidizer powder can be deeply penetrated into the crude molten steel and dispersed, and these dispersed powders act as nuclei for the decarburization reaction to make the reaction more active. cause Therefore, compared to the case of top-blowing oxygen as in the case of Japanese Patent Publication No. 49-46444, the present invention is more effective at 1'Jl.
It can promote decarburization reaction in molten steel. From this point of view, in carrying out the present invention, Fe powder and O2 gas may be top-blown as equivalents of iron oxide pentoxide. That is, in this case, the Fe powder acts as a nucleus for the decarburization reaction,
The 02 gas causes an oxidation reaction between the Fe powder and the 02 gas, and the heat of the reaction is generated locally around the Fe powder, thereby promoting the decarburization reaction itself. When using Fe powder, use a porous lance to blow the Fe powder into one hole using a gas other than oxygen gas, such as Ar gas, as a carrier gas, and blow the 02 gas into the other hole. . This prevents explosions. Note that, like Fe powder, Cr powder and Mn powder can also be blown with 02 gas. Further, when adding A7!20> and CaQ, 02 gas or C○2 gas may be used. Next, examples under various conditions will be described. 2.5 ton VOD device for experimental use as a vacuum degassing device,
Denitrification was carried out according to the present invention by using the Actual H No. 2501-7 RH device, 50)7 VOD device, and changing the degree of vacuum, the type of oxidizer powder, its supply rate, /8 steel temperature, lance height, etc. I did it. 6 Table 1 is a table summarizing the implementation conditions and results, and also shows the case of a degassing method using top blowing of 02 gas under reduced pressure as a comparative example. As can be understood from this table, the denitrification efficiency α in the case of the present invention is all 245 or more, which is higher than that of 200 or less in the case of the comparative example. Extremely high. To explain this in detail, when an experimental 2.5 ton VOD device is used, Example 1 of the present invention. 2. 3. 4. In Comparative Example 1.2, α is 245 to 265, but in Comparative Example 1.2, it is 198 at most, and when an actual 250-ton RH device is used, in Examples 6 and 7.8 of the present invention, it is 268 to 265.
275, but it is 140 in Comparative Example 3, and when a 50I-VOD device is used, it is 280 in Example 9, but it is 158 in Comparative Example 4, and the type of vacuum degassing device Regardless of the scale, the denitrification effect is improved in the case of the present invention. In addition, when denitrifying according to the present invention using an experimental 2.5 ton VOD device and an actual 250 ton RH device, (C
), (N), and (Cr) change as shown in FIGS. 6 and 2 in each device 7. Figure 6. Figure 2 shows processing time (minutes) on the horizontal axis and (C) (%) on the vertical axis.
), (N) (ppm), (Cr) (%),
The component changes in Example 1 and the component changes in Example 6 are shown, respectively. In the figure, the O mark is [Ca], and the △ mark is [N]. The mark indicates the value of (Cr). As can be understood from these figures (Cr decreases slightly to 0.1 to 0.2%, but almost no change is observed. However, in the case of Comparative Example 1 in which oxygen gas is blown under reduced pressure, From FIG. 7, which shows the changes in the components of (C), (N), and (Cr), the change in Cr decreased by 0.5%, which is considerably larger than in the case of the present invention.In other words, the comparison In the example Cr oxidation occurs, in the case of the present invention C
The difference is due to the fact that r-oxidation does not occur. Further, as a result, (N) is reduced to only about 60 ppm in the comparative example, but reduced to 40 to 50 ppm in the example, and corrosion resistance, workability, weldability, etc. are improved. Furthermore, regarding Cr, when Cr -8 Ore is used as the oxidizing agent powder, Cr increases by 1.4% from 19.0% to 20.4% before and after treatment, as shown in Example 2. pink-up is observed. Therefore, the finished product Cr
If it is desired to adjust the oxidizing agent powder, Cr can be adjusted by using an oxidizing agent powder containing Cr or by using Cr-0re. The above example uses 19% Cr steel, but for 29% Cr steel, Example 3. Comparing Example 8 and Comparative Example 2, the denitrification efficiency increases by about 60 to 75%, and similar effects can be obtained. When Fe powder and O gas are added, as shown in Example 4, the amount of decarburization is low because it does not actively work on decarburization, but the amount of denitrification is increased compared to all of the comparative examples. When Ca and CO3 were added (Example 5), the amount of decarburization was small, but the denitrification efficiency increased by 28%. Regarding the erosion of refractories in the RH tank when such denitrification treatment is performed, it is difficult to clearly grasp the amount of erosion of refractories for each lch. Figure 8 shows the amount of erosion when operations including treatment are carried out. 9 Figure 8 is a graph showing the relationship between the ratio (%) when denitrification is performed during the entire operation on the horizontal axis and the life of the lower tank of the RH packing equipment (times) on the vertical axis. In the figure, the ○ mark indicates that the processing temperature is less than 1650℃, and the ・mark indicates that the pre-processing temperature is 16
The cases where the temperature is 50° C. or higher are respectively shown, and the short horizontal lines and arrows at 0% indicate the range of variation in life when the denitrification treatment according to the present invention is not performed at all. As can be understood from this figure, in the case of the present invention, the supplied oxygen efficiently contributes to the decarburization reaction even in a relatively low temperature range of less than 1650°C, so the refractory erosion rate (average erosion rate) could be reduced. The amount of erosion loss is as shown in Table 2 for the case of RH filling equipment (0.
85-1.05 mm, while it is 0.6-0.8 mm below 1650°C, which can suppress the amount of erosion by about 20-30% and reduce the life of the lower tank of the RH equipment from about 380 cycles. 4
Since the number of furnace repairs can be reduced to 20, it is possible to improve the operating rate of the RH filling equipment. 0 Second Award [Effect] As described in detail below, in the case of the present invention, splash can be suppressed and it can be added uniformly to /8 steel, and the treatment temperature can be relatively low, so It is possible to reduce the amount of corrosion loss and suppress the generation of fumes, and the amount of denitrification is high relative to the amount of decarburization, which shortens the processing time and lowers the achieved nitrogen value, which improves corrosion resistance, workability, and welding. It is possible to manufacture low nitrogen, high chromium steel with improved properties such as /8. The present invention has excellent effects such as the ability to adjust the composition by using metal powder.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing the implementation state of the present invention, Fig. 2 is a graph showing an example of composition change according to the present invention when iron oxide is used, Figs. Graphs used to explain the reaction contents of the invention, FIGS. 6, 7, and 8 are graphs used to explain the present invention in detail. 1... Crude molten steel 3... RH device 4... Top blowing lance 5... Oxidizer powder special name? Eijin I "Lord 2 Anti-money Drowning Ll "Rokueki Nena. Proxy sum Q possible Nokai Inu 3 1st picture Djo Etsu time (minutes) 1st picture cJ) [N] Kagesha dance ( -/,) [0] [・/.] [0] Now ten total 1 earth (・mu) [λtsu 1 Lu 9 9 Shitsuko 88c3 J') [Nl

Claims (1)

[Claims] 1. High chromium crude molten steel with (C)≧0.1% produced in a steelmaking furnace is decarburized by top-blowing oxidizer powder under reduced pressure. A method for creating low nitrogen, high chromium alloy steel characterized by accelerated denitrification. 2. High-chromium crude molten steel with (C)≧0.1% produced in a steelmaking furnace is brought under reduced pressure in a recirculation degassing device, and oxidizer powder is blown over during this depressurization to decompose it. A method for producing a low-nitrogen, high-chromium alloy steel, characterized by promoting carbonization and the denitrification that occurs at this time. 3. The method for producing a low nitrogen, high chromium alloy steel according to claim 2, wherein the oxidizing agent powder is blown from two directions diagonally above the surface of the crude molten steel.