JP5544818B2 - Melting method of high chromium steel - Google Patents

Description

  The present invention relates to a method of melting high chromium steel by decarburization refining in a converter and subsequent vacuum refining in secondary refining.

  As a method of melting high chromium steel containing 9 mass% or more of chromium, decarburization using chromium-containing hot metal and ferrochrome (Fe-Cr alloy iron) as raw materials in a converter without using an electric furnace (arc furnace) A chrome-containing crude molten steel is smelted and refined, and a method of finishing this chrome-containing coarse molten steel into predetermined components by vacuum refining of secondary refining equipment, represented by the VOD (Vacuum Oxygen De-carburization) method, has been carried out. Yes. Converter-VOD process stabilizes high chromium steel with low nitrogen content, specifically high chromium steel with chromium concentration of 17% by mass or more, especially 20% by mass or more and nitrogen concentration of 0.013% by mass or less. And in order to melt efficiently, the nitrogen content of the chromium-containing crude molten steel after the converter steel is required to be about 0.0105% by mass or less.

  However, it is thermodynamically known that the equilibrium dissolved nitrogen concentration in the molten steel increases with an increase in the chromium concentration in the molten steel. On the contrary, the denitrification ability from the molten steel is It is known that the concentration of chromium in the molten steel decreases as the concentration of chromium in the steel increases. Conventionally, the nitrogen concentration in the molten steel increases at the end of decarburization and refining in the converter, and is stabilized by the converter-VOD process. Therefore, it has been difficult to produce low nitrogen high chromium steel. In addition, when the nitrogen concentration of the chromium-containing crude molten steel after the converter steel is high, denitrification treatment is required in the VOD process, and there are problems such as extending the VOD treatment time.

  As a means for solving this problem and stably melting low nitrogen high chromium steel by the converter-VOD process, Patent Document 1 states that “the carbon concentration of the crude stainless steel at the end of decarburization blowing in converter refining” Is reduced to 0.5 mass% or less, and the chromium-containing slag generated by the decarburization blowing is left in the converter in an unreduced state, and the stainless coarse molten steel is taken out into a ladle, followed by vacuum refining following the converter refining And a process for producing an ultra-low nitrogen stainless steel characterized by performing decarburization treatment and degassing treatment of the stainless crude molten steel ”.

  Usually, a strong reducing material such as ferrosilicon (Fe-Si alloy iron) or metal Al is added to chromium-containing slag produced by decarburization blowing in the converter, and the chromium oxide in the chromium-containing slag is reduced to chromium. However, in Patent Document 1, since the chromium-containing slag is left in an unreduced state, the dissolved oxygen concentration of the stainless crude molten steel in the converter can be maintained high, and the ladle from the converter can be maintained. Nitrogen pick-up at the time of steel production is prevented, and the nitrogen concentration of the stainless steel before the secondary refining can be kept low.

JP-A-11-140530

  Although patent document 1 certainly prevents the nitrogen pick-up of the chromium-containing crude molten steel at the time of steel production, it cannot prevent the nitrogen pick-up caused by the air entering the furnace at the end of the converter decarburization refining. In other words, unless a nitrogen pickup caused by air entering the furnace at the end of converter decarburization and refining is efficiently prevented, a process combining a converter and secondary refining, such as a converter-VOD process, is stable. Low nitrogen high chromium steel cannot be melted.

  The present invention has been made in view of the above circumstances, and the object is to combine 9 masses of chromium by combining decarburization refining in a converter and vacuum refining in a secondary refining facility such as the VOD method. When high-chromium steel containing more than 10% is melted, high-chromium steel can be stably melted by preventing the picking of nitrogen at the end of converter decarburization and refining high-chromium steel with low nitrogen content. It is to provide a manufacturing method.

  In the method for melting high chromium steel according to the first invention for solving the above-mentioned problem, the chromium-containing crude molten steel is melted by decarburization refining in a converter, and then the steel is discharged into a ladle. In a method for producing high chromium steel, in which high-chromium steel containing 9 mass% or more of chromium is melted by vacuum refining chromium-containing crude molten steel in a secondary refining facility, the molten steel temperature at the time of steel removal from a converter is 1730 Decarburization and refining is carried out by adjusting the decarburization and refining conditions so that the temperature is higher than ℃, thereby promoting the hatching of slag present in the converter and preventing the nitrogen absorption from the chromium-containing crude molten steel It is characterized by doing.

The method for melting high-chromium steel according to the second invention is based on the first invention, wherein the ratio of the slag mass (kg) in the converter to the in-furnace cross-sectional area (m ² ) of the converter slag line is 335 kg. / M ² or more.

  The method for melting high chromium steel according to the third invention is characterized in that, in the first or second invention, the high chromium steel has a nitrogen content of 0.013 mass% or less.

  According to the present invention, since the decarburization refining is performed so that the temperature of the chromium-containing coarse molten steel at the time of steel removal from the converter is 1730 ° C. or higher, the chromium-containing coarse molten steel in the final stage of the decarburization refining is High-chromium steel with low nitrogen content, which is covered with slag that has been converted and melted, and the contact between the air entrained in the furnace and the chrome-containing coarse molten steel in the furnace is cut off, preventing the pickup of nitrogen from the air Stable melting is realized. Further, by securing the amount of slag in the furnace to a predetermined amount or more, the pickup of nitrogen is further prevented. This avoids extending the vacuum refining process time in the secondary refining equipment, and brings about industrially beneficial effects such as an improvement in the productivity of low nitrogen high chromium steel and a reduction in manufacturing costs associated therewith.

It is a schematic sectional drawing of the converter equipment used when implementing this invention. It is a figure which shows the influence of the tapping temperature on the nitrogen pick-up in the decarburization refining end stage. It is a figure which shows the influence of the amount of slag in a furnace which has on the pickup of nitrogen in the decarburization refining end stage.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of a converter facility used when carrying out the method of melting high chromium steel according to the present invention.

  As shown in FIG. 1, the converter equipment used in the present invention has a converter body 1 in which an outer shell is an iron shell 2 and a refractory 3 is constructed on the inner surface side of the iron shell 2, and this converter body 1. And an upper blowing lance 4 that can be moved up and down. At the bottom of the converter main body 1, a plurality of bottom blowing tuyere 5 penetrating the iron shell 2 and the refractory 3 are installed. A steel outlet 7 for setting steel 8 in a ladle (not shown) is provided. Each bottom blowing tuyere 5 is connected to a gas introduction pipe 6. From the top blowing lance 4, as the refining gas, an inert gas such as an industrial pure oxygen gas and an argon gas is used at an arbitrary flow rate for an arbitrary period, that is, an industrial pure oxygen gas alone, an inert gas alone, or A mixture of industrial pure oxygen gas and inert gas having various mixing ratios is blown into the chromium-containing crude molten steel 8 in the furnace at an arbitrary flow rate for an arbitrary period.

  In addition, an inert gas such as an industrial pure oxygen gas and an argon gas as a refining gas from the bottom blowing tuyere 5 at an arbitrary flow rate for an arbitrary period, that is, an industrial pure oxygen gas alone, an inert gas alone, Alternatively, a mixed gas of industrial pure oxygen gas and inert gas having various mixing ratios is blown into the chromium-containing crude molten steel 8 in the furnace at an arbitrary flow rate for an arbitrary period. Also, the tuyere has a double-pipe structure, and from the inner pipe, industrial pure oxygen gas alone, inert gas alone, or a mixture of industrial pure oxygen gas and inert gas in various mixing ratios as a refining gas A hydrocarbon gas such as propane or an inert gas may be blown from the gap between the inner tube and the outer tube as a tuyere cooling gas. In this case, for example, a tuyere and a gas introduction pipe as disclosed in JP-A-59-13011 are preferable. Although not shown, trunnion shafts for holding the converter main body 1 are provided on the outer surfaces of both side walls of the converter main body 1, and the converter main body 1 is configured to be rotatable about the trunnion shaft as a rotation axis.

The converter body 1 is charged with chromium-containing hot metal and, if necessary, ferrochrome or high-chromium steel scrap as the main raw material, with quick lime, chrome ore, dolomite, iron ore, etc. as auxiliary raw materials, and the top blow lance 4 and bottom blowing tuyere 5 are blown with the above-mentioned refining gas, and the oxygen in the oxygen gas reacts with the carbon in the chromium-containing hot metal to decarburize and refine the chromium-containing hot metal (also called “decarburization blowing”). Promote). A part of the added auxiliary material is melted to form slag 9. In this decarburization refining, in order to suppress the oxidation loss of chromium in the chromium-containing hot metal, the second half of the decarburization refining (a period exceeding 60% of the refining time) is the industrial pure oxygen from the top blowing lance 4 The gas supply was stopped, and only blowing from the bottom blowing tuyere 5 was performed. Typical composition of the slag 9, CaO 15 to 35 wt%, SiO ₂ is 5 to 20 wt%, Al ₂ O ₃ is 10 wt% or less, MgO is 10 mass% or less, 30 is Cr ₂ O ₃ 55% by mass.

  The carbon concentration of the chromium-containing hot metal is about 5 to 7% by mass, although it depends on the chromium content and temperature, and the chromium-containing hot metal is decarburized by this decarburization refining, and the chromium-containing crude molten steel 8 is melted. Is done. The carbon concentration of the chromium-containing crude molten steel 8 at the end of decarburization refining is not particularly specified, but is preferably about 0.4% by mass or less. If it is higher than this, it is not preferable because the time for vacuum refining in the secondary refining equipment represented by the VOD method in the next step, that is, the vacuum decarburization refining time is prolonged. On the other hand, if the carbon concentration of the chromium-containing crude molten steel 8 at the end of decarburization refining is too low, the oxidation loss of chromium increases, so the carbon concentration of the chromium-containing coarse molten steel 8 at the end of decarburization refining is 0.05% by mass or more. Is preferably ensured.

  In the decarburization refining performed in this way, various methods for preventing nitrogen pickup caused by air at the end of refining (end of blowing) were studied. Here, the reason why nitrogen is picked up at the end of decarburization refining is as follows. That is, at the end of decarburization refining, the carbon concentration in the chromium-containing hot metal becomes low, and the amount of CO gas generated as a product of the decarburization reaction decreases. When the decarburization reaction is active, the generated CO gas is recovered without being burned, but at the end of the decarburization refining where the amount of generated CO gas is reduced, it is burned in the flue without being recovered. This combustion air enters from the furnace port of the converter main body 1 to the inside of the converter main body 1, and the nitrogen concentration of the chromium-containing coarse molten steel 8 increases.

  First, while maintaining the amount of slag in the furnace almost constant, the temperature of the chromium-containing coarse molten steel 8 at the time of steelmaking (referred to as “steeling temperature”) is changed to affect the nitrogen pick-up at the end of decarburization refining. The effect of steel temperature was investigated. The survey results are shown in FIG. As shown in FIG. 2, it can be confirmed that the amount of nitrogen pick-up decreases as the steel output temperature rises, and that the amount of nitrogen pick-up can be suppressed to 0.0060 mass% or less when the steel output temperature is 1730 ° C. or higher. It was.

The reason is as follows. The composition of the slag during decarburization refining is as defined above, when the Cr ₂ O ₃ concentration in the slag in average 45 wt% within the above range, 3 of CaO-SiO ₂ -Cr ₂ O ₃ According to the original system phase diagram, at 1700 ° C., the slag 9 becomes a two-phase coexistence region of liquid phase—Cr ₂ O ₃ (solid phase), whereas at 1600 ° C., it is the aforementioned two-phase coexistence region. The liquid phase area is greatly reduced. That is, the temperature of the slag 9 is equivalent to the temperature of the chromium-containing coarse molten steel 8, and the slag 9 hatches as the steel output temperature becomes higher. This is because the contact with the air is covered with the slag 9 and the pickup of nitrogen is suppressed. Further, the hatched slag 9 is easy to form (bubble), and the formation of the slag 9 by the gas blown from the bottom blowing tuyere 5 to shut off the air also contributes to suppression of nitrogen pickup.

Moreover, since it was estimated from the above results that the mass of the slag 9 existing in the furnace also affects the pickup of nitrogen, the amount of auxiliary raw material (mainly quick lime) is adjusted to exist in the furnace. The mass of the slag 9 was changed, and the influence on the nitrogen pickup at the end of decarburization refining was investigated. In this case, since the air blocking effect by the slag 9 was considered to correlate with the thickness of the slag 9 existing in the furnace, the slag mass (kg) in the converter and the cross-sectional area in the furnace of the converter slag line ( m ² ) (hereinafter referred to as “ratio A”) as an index. The cross-sectional area (m ² ) of the converter slag line is the converter slag line, where r (m) is the inner diameter of the slag line (the distance from the central axis of the converter main body 1 to the surface of the refractory 3). In-furnace cross-sectional area = πr ² (m ² ).

The survey results are shown in FIG. In addition, in FIG. 3, it has shown in comparison with the case where a steel output temperature is 1730 degreeC or more and the case where it is less than 1730 degreeC. As shown in FIG. 3, when the steel output temperature is 1730 ° C. or higher, it can be confirmed that the ratio A increases and the amount of nitrogen pick-up decreases, particularly when the ratio A is 335 kg / m ² or higher. It was found that the amount of nitrogen pickup could be stably controlled to 0.0035% by mass or less. On the other hand, when the steel output temperature is less than 1730 ° C., the tendency that the amount of nitrogen pick-up decreases as the ratio A increases coincides, but the amount of nitrogen pick-up is compared with the case where the steel output temperature is 1730 ° C. or higher. It was confirmed that there was a difference of 0.0015% by mass or more.

The present invention was made on the basis of these test results. The chromium-containing coarse molten steel 8 was melted by decarburization refining in a converter, and then the chromium-containing coarse molten steel 8 put out in a ladle was used. In the high chrome steel melting method, in which high chrome steel containing 9% by mass or more of chrome is melted by vacuum refining in the secondary refining equipment, the molten steel temperature at the time of steel removal from the converter is over 1730 ° C. The decarburization and refining conditions are adjusted to the above, thereby promoting the hatching of the slag 9 existing in the converter and preventing the nitrogen from being absorbed into the chromium-containing crude molten steel 8 from the air. Features. In this case, the ratio A is preferably 335 kg / m ² or more. At least at the end of decarburization refining, it is preferable to presuppose an operation in which the supply of the refining gas from the top blowing lance 4 is stopped and only the blowing from the bottom blowing tuyere 5 is performed. As a result, it is possible to eliminate the bare hot water formed by the refining gas blown from the top blowing lance 4 (the portion where the surface of the chrome-containing crude molten steel is exposed to the furnace atmosphere due to the slag being pushed away). This is because it can be expected to further reduce nitrogen.

As described above, according to the present invention, the chromium-containing crude molten steel 8 in the furnace at the end of decarburization refining is covered with the hatched and melted slag 9, and the air entrained in the furnace and the contents in the furnace are contained. The contact with the chromium smelted molten steel 8 is cut off, the pickup of nitrogen from the air is prevented, and stable production of high chromium steel having a low nitrogen content is realized. Further, by setting the ratio A to 335 kg / m ² or more and securing the amount of slag in the furnace, the pickup of nitrogen is further prevented.

  An example in which the present invention is implemented using the converter equipment shown in FIG. 1 (however, the bottom blowing port has a double pipe structure) will be described. The molten high chromium steel is a stainless steel having a chromium concentration of 17 to 18% by mass, a nitrogen concentration of 0.012% by mass or less, and a sulfur concentration of 0.040% by mass or less.

The converter body is charged with chromium-containing hot metal and ferrochrome as the main raw materials, and then charged with quick lime, chromium ore and dolomite as auxiliary raw materials, and supplied with industrial pure oxygen gas from the top blowing lance, Decarburization and refining was started by supplying industrial pure oxygen gas from the inner tube of the blow tuyere and propane as the tuyere cooling gas from the gap between the inner and outer tubes. In the second half of the decarburization refining (a period exceeding 60% of the refining time), the supply of industrial pure oxygen gas from the top blowing lance was stopped, and the industrial pure oxygen gas and inert from the inner pipe of the bottom blowing tuyere The decarburization refining was continued by supplying a mixed gas with the gas. The composition of the slag in the furnace, CaO 15 to 35 wt%, SiO ₂ is 5 to 20 wt%, Al ₂ O ₃ is 0 to 10 mass%, MgO 4-10 wt%, Cr ₂ O ₃ is 30 It was -55 mass%. At the end of decarburization refining, the carbon concentration of the chromium-containing crude molten steel was set within the range of 0.08 to 0.20 mass%.

Adjust the blending ratio of chromium-containing hot metal, which is the main raw material, and ferrochrome, the supply amount of industrial pure oxygen gas and the charging amount of the auxiliary raw material, adjust the steel output temperature to 1730 ° C or more and charge the auxiliary raw material. The amount was adjusted to adjust the ratio A to 335 kg / m ² or more.

  In this decarburization refining, a sub lance was put into the converter body during refining, an analytical sample was taken from the molten metal, and the amount of nitrogen picked up was measured. Moreover, the analytical sample was extract | collected from the chromium-containing crude molten steel after steel-drawing to the ladle, and nitrogen content was investigated.

  As a result, the amount of nitrogen pick-up at the end of decarburization refining was 0.0025% by mass on average, which was significantly less than the average value of nitrogen pick-up before countermeasures = 0.068% by mass. Moreover, the nitrogen content of the chromium-containing crude molten steel after steelmaking was 0.0091% by mass on average, which was significantly reduced compared to the average nitrogen content before countermeasures = 0.014% by mass. As a result, in the next VOD process, the processing time was reduced from 92 minutes before the countermeasure to 86 minutes.

DESCRIPTION OF SYMBOLS 1 Converter body 2 Iron skin 3 Refractory 4 Top blowing lance 5 Bottom blowing tuyere 6 Gas introduction pipe 7 Steel outlet 8 Chrome-containing coarse molten steel 9 Slag

Claims (2)

The chromium-containing coarse molten steel is melted by decarburization refining in a converter, and then the chromium-containing coarse molten steel put out in the ladle is vacuum-refined in a secondary refining equipment to contain 9% by mass or more of chromium. In the high chromium steel melting method for melting high chromium steel, decarburization refining is carried out by adjusting the decarburization refining conditions so that the molten steel temperature at the time of steel output from the converter is 1730 ° C or higher. de Rutotomoni promotes the slag formation of slag existing in the rolling furnace, the ratio of the slag weight in the furnace and (kg) and the furnace cross-sectional area of the converter slag line (m ²⁾ as 335 kg / m ² or more by A method for producing high-chromium steel, characterized by performing carbon refining to prevent nitrogen from being absorbed into the chromium-containing crude molten steel. The method for melting high chrome steel according to claim 1, wherein the high chromium steel has a nitrogen content of 0.013 mass% or less.

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