JP2002285222A - Method for producing high chromium steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high Cr steel by which Cr oxidizing loss is reduced at the lower level in comparison with the conventional method when molten steel containing carbon and Cr is decarburize-refined in a refining furnace, such as a converter, an AOD furnace. SOLUTION: When the high Cr steel having >=5 mass% Cr is produced by supplying oxygen gas in the refining furnace with the molten Cr-containing steel containing >=1 mass% C and >=5 mass% Cr to perform the decarburize- refining, after performing the decarburize-refining so that the molten steel temperature in the furnace before tapping, becomes 1,700-1,760 deg.C, the molten steel is tapped into a ladle pre-charging cooling material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for smelting high Cr steel, and more particularly to a technique for smelting high Cr steel inexpensively with little oxidation loss of chromium (hereinafter, element symbol: Cr). .

[0002]

2. Description of the Related Art In general, a converter and an AOD (Argon O
The decarburization refining of molten stainless steel performed in a smelting furnace such as xygen decarburization) is performed by hot metal or molten steel containing carbon in scrap or ingot containing Cr or Ni previously charged into the smelting furnace before the start of blowing. And add
After heating and dissolving them, oxygen blowing and Cr
Iron alloys and oxides containing Ni and Ni are charged into a furnace and adjusted to target components, and then the temperature of the molten steel is adjusted to a desired tapping temperature to start tapping. In this case, the tapping temperature is set to the temperature of the molten steel whose component has been adjusted, from the tapping into the ladle to the start of casting (for example, continuous casting to a steel slab such as a slab or billet). It is determined by adding the drop, but in order to minimize heat loss and protect converter refractories, it is desirable to make the molten steel temperature approximately equal to the tapping temperature at the end of decarburization refining (usually , 1650-168
About 0 ° C). Therefore, when the molten steel temperature exceeds the target tapping temperature during refining, it is common to add a coolant into the converter to cool and adjust the molten steel.

However, in this conventional adjustment method,
As described above, priority has been given to limiting the refining temperature in accordance with the tapping temperature, rather than increasing the blowing temperature in order to reduce the oxidation loss of Cr. For this reason, the decarburization refining is performed at a temperature that is thermodynamically lower than the temperature at which decarburization has priority over oxidation of Cr, and an increase in oxidation loss of Cr, which is an expensive metal, cannot be avoided.

[0004]

SUMMARY OF THE INVENTION In view of such circumstances, the present invention can reduce the oxidization loss of Cr as compared with the prior art when decarbonizing and refining molten steel containing carbon and Cr in a refining furnace such as a converter or an AOD furnace. It is an object of the present invention to provide a method for melting a high Cr steel.

[0005]

In order to achieve the above object, the present invention provides an oxygen gas supply in a refining furnace to a Cr-containing molten metal containing 1% by mass or more of C and 5% by mass or more of Cr. When decarburizing and refining to produce a high Cr steel with Cr: 5% by mass or more, the decarburizing refining is carried out when the molten steel temperature in the furnace before tapping is 1
This is a method for producing a high Cr steel, wherein the molten steel is discharged to a ladle in which a coolant has been previously charged after the temperature is raised to 700 to 1760 ° C. When the molten steel after tapping into the ladle is subjected to secondary refining, a part of the coolant may be charged in secondary refining without charging the entire amount of the coolant.

In the present invention, when decarburizing and refining by adding a Cr source and a Ni source to molten steel, decarburizing and refining at 1700 ° C. to 1760 ° C., which is suitable for decarburization to proceed preferentially over oxidation of Cr. After that, the molten steel is tapped into a ladle in which a cold material such as strip waste has been previously charged, the cold material is melted in the molten steel and cooled, and the temperature is lowered to a target temperature for casting. Since the adjustment is performed, the oxidation loss of Cr during the decarburization refining is significantly reduced as compared with the conventional case.

In this case, the cold material to be charged into the ladle in advance may be any material as long as its chemical component is necessary for melting the high Cr steel and is clearly known. For example, stainless steel strip waste, plate-like electrolytic metal nickel, or the like can be preferably used. Further, when the so-called "secondary refining" is continuously performed on the high Cr molten steel discharged on the ladle and further in a degassing tank such as RH or VOD, nickel oxide can be added.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below, taking into account the circumstances leading to the invention.

Conventionally, when decarburizing and refining chromium-containing molten steel in a refining furnace such as a converter or an AOD furnace, when alloying elements are added to the molten steel, the alloying elements are rapidly dissolved and uniformly dispersed in the molten steel. Iron scraps and ingots, such as strip waste, to ensure that
A mass of ferrochrome alloy, an alloy component source such as nickel or molybdenum is charged into a smelting furnace through a scrap chute before refining is started, and then the sensible heat of the hot metal charged and carbon in molten steel (hereinafter referred to as element Symbol C), Fe, Si,
Metals such as Mn and Cr are melted by raising the heat using the heat of heat as a heat source, and then a massive or granular alloy such as ferrochrome is added to adjust the molten steel component before tapping. Inevitably, a large-sized alloy component source was pre-loaded into a ladle, and molten steel was sometimes discharged.However, the molten steel exceeded the amount corresponding to the sensible heat and latent heat required for melting the alloy component source. By giving sensible heat, that is, raising the temperature of the molten steel and tapping, the alloy source was melted. Therefore, in an operation in which such an alloy component source is pre-charged into a ladle and tapping is performed, tapping at a high temperature is essential. However, in order to protect the refractory lining refractories, this method has been avoided as much as possible, and pre-charging into the smelting furnace has been mainly performed.

Further, the tapping temperature is determined by adding a temperature drop of molten steel from tapping to casting to a predetermined casting temperature. However, heat loss is minimized, and refractory protection of a refining furnace is performed. Therefore, during blowing, the temperature of the molten steel is generally set to be equal to or lower than the tapping temperature. Furthermore, when secondary refining is performed in a vacuum degassing tank such as RH or VOD having a heat-up function after tapping, the tapping temperature may be further reduced by the heat-up temperature there.

[0011] However, thermodynamically, C
The lower the concentration (the later stage in blowing), the higher the preferential oxidation zone of C is on the higher temperature side. As an example, Symkovich of molten steel having a Cr concentration of 16% by mass
FIG. 1 shows an oxidation region of chromium and a preferential oxidation region of C determined by the equilibrium equation. Since such preferential oxidation of Cr is particularly remarkable in molten steel containing 5% by mass or more of Cr, in the present invention, the target molten steel is limited to those containing 5% by mass or more of Cr. In addition, in order to remove various impurities by decarburizing and refining such molten steel from a high carbon region, the Cr-containing molten metal before decarburizing and refining needs to be 1% by mass or more. Therefore, in the present invention, the C content of the Cr-containing molten steel before the decarburization treatment is set to 1% by mass or more. Therefore, as in the prior art, charging or adding a Cr-containing scrap or a ferrochrome alloy element source into a refining furnace to cool the molten steel temperature during the blowing to a target tapping temperature or less is an expensive metal such as chromium. Increases the oxidation loss and lowers the yield.

Therefore, the present inventor has considered the following two means for solving this oxidation loss.

(1) In the case of decarburization blowing of chromium-containing molten steel in a refining furnace such as a converter or AOD, blowing is performed at a temperature higher than a tapping temperature determined from a casting temperature, and the molten steel is cooled in the furnace. Chromium oxidation loss can be reduced by tapping into a ladle without the need.

(2) At this time, a cold material such as scrap having a component corresponding to a sensible heat difference between the molten steel temperature during the blowing in (1) and the tapping temperature determined from the casting temperature is added to the ladle. The scrap is charged in advance and the scrap is melted by sensible heat to adjust the temperature of the molten steel to an appropriate tapping temperature and to recover the significant metal contained in the cold material at a high yield.

Specifically, the present invention relates to (1):
The molten steel temperature at the end of the decarburization refining is in the range of 1700 to 1760 ° C. In molten steel containing 5% by mass or more of Cr, C is preferentially oxidized over Cr by setting the temperature of the molten steel to 1700 ° C. or more, and the oxidation loss of Cr can be significantly reduced.
On the other hand, to make the molten steel temperature exceed 1760 ° C,
It is economically disadvantageous because it is necessary to remarkably increase the oxygen consumption rate in the smelting furnace, and it is also necessary to use auxiliary metals such as Al and Si to generate heat by oxidation. In addition, since the melting of the refractory of the refining furnace tends to be remarkable, the upper limit is set to 1760 ° C.

Further, regarding (2), JP-A-11-21-2
No. 56219, SUS304 (18 mass% Cr-
8 mass s% Ni steel) in a converter, after decarburizing and refining, even if molten steel is tapped into a ladle in which plate-shaped metal nickel has been previously charged as a nickel source, metal nickel is completely and uniformly melted. Is shown, the inventor preliminarily loads a small bundle of strip waste or slab waste of a component into a ladle and taps and melts the molten steel to improve the chromium yield. Confirmed improvement.

In the present invention, a converter is preferably used for decarburization and refining of molten steel. However, other refining furnaces such as a so-called AOD furnace and an electric furnace having a decarburization function may be used in addition to the converter. good. In addition, the ladle loading coolant is melted, and molten steel having a substantially target composition is continuously subjected to secondary refining to perform finishing decarburization, degassing, desulfurization and deoxidation, thereby increasing its cleanliness. . Therefore, in the present invention, instead of charging the entire amount of the coolant into the ladle in advance, a part of the coolant may be used in the secondary refining. As the secondary refining, any of the VOD method, the RH method, the DH method, the LF method, the gas bubbling method, the VAD method, and the ASEA-SKF method may be employed. In addition, these degassing devices can be supplied with oxygen gas or added with exothermic metals such as metallic aluminum and silicon to prevent the temperature of molten steel from dropping during processing, and can also be used for arc heating and plasma heating. It is more preferable to use one provided with means for induction heating. Thereby, in the secondary refining process, the variation in the molten steel temperature when tapping into the ladle can be absorbed.

An experiment was conducted in which SUS430 stainless steel (16% by mass Cr) was melted using a top-bottom blow converter with a tapping capacity of 160 tons, and then secondary refining was performed by VOD.

(Conventional Example) Chromium-containing hot metal and chromium-containing scrap were charged into a converter and decarburized by oxygen blowing. During this decarburization refining, a predetermined amount of ferrochrome alloy was charged to form a 16% by mass Cr molten steel, and then, based on the result of temperature measurement using a sublance, a coolant was charged into the furnace while decarburization refining. And the molten steel temperature in the furnace before tapping is 168
Steel was adjusted after the temperature was adjusted to 0 ° C.

Here, exactly the same operation is performed for 10 charges, and the obtained chromium yield is shown in FIG. In addition, 1
The average chrome yield at zero charge was 93.8%.

(Example 1 of the present invention) Cr-containing hot metal, chromium-containing scrap and ferrochrome were charged into a converter and decarburized and refined to melt 16.6% by mass of molten steel in a furnace before tapping. The internal molten steel temperature was set to 1750 ° C, and tapping was performed. At that time, the ladle was previously charged with Cr-containing strip chips as a coolant, and the molten steel was tapped into the ladle to dissolve the SUS430 strip chips. The ladle was charged with 5 tons of SUS430 strip waste. After the secondary refining was performed after tapping, the molten steel in the ladle was continuously cast. However, when the ladle used for continuous casting was observed, there was no melting residue.

Here, the operation under the same conditions was performed 10 charges each, but no effect was observed on the converter refractory. The obtained chromium yield was 94.9% on average for 10 charges, which was higher than the conventional method.

(Invention Example 2) Cr-containing hot metal, chromium-containing scrap and ferrochrome were charged into a converter and decarburized and refined to melt molten steel of 16.6 mass% Cr. The molten steel temperature in the furnace before tapping was set to 1760 ° C and tapping was performed. At that time, four slab scraps each of which had 1.2 tons as a coolant were previously charged into the ladle. Subsequently, secondary refining was performed by VOD, and 50 to 150 kg of ferrochrome was charged during the refining, the molten steel thus obtained was continuously cast, and the ladle after completion of casting was observed. As a result, like Example 1 of the present invention, there was no undissolved residue.

Here, the operation under the same conditions was carried out for 10 charges each, but no effect on the converter refractory was recognized. The obtained chromium yield was 95.1% on average for 10 charges, which was higher than that of the conventional method.

[0025]

As described above, according to the present invention, in decarburizing and refining chromium-containing molten steel, 1700 ° C. to 17 ° C. to 17 ° C. to 17 ° C., which is suitable for performing decarburization while suppressing chromium oxidation.
After decarburizing and refining at 60 ° C., the molten steel is discharged into a ladle in which a coolant such as strip waste has been previously charged and cooled, thereby reducing oxidization loss of chromium during decarburizing and refining, thereby improving productivity. It is possible to inexpensively melt Cr-containing steel without disturbing steel.

[Brief description of the drawings]

FIG. 1 is a diagram showing a preferential oxidation region of Cr or C according to Symkovich's equilibrium equation at a Cr concentration of 16%.

FIG. 2 shows the results obtained by implementing the conventional method and the method of the present invention.
It is the figure which compared yield.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K013 AA02 BA02 CD02 CD04 CD07 CE01 CE02 CE06 CF12 DA03 DA08 DA12 DA14 EA18 FA01 FA02 4K070 AA03 AB03 AB20 AC04 AC05 EA01 EA07 EA27

Claims (2)

[Claims] An oxygen gas is supplied in a refining furnace to a Cr-containing molten metal containing 1% by mass or more of C and 5% by mass or more of Cr and decarburized and refined to obtain a high Cr content of 5% by mass or more. When smelting steel, the temperature of the molten steel in the furnace before tapping is 1700-17
A method for producing high Cr steel, wherein the temperature is raised to 60 ° C., and then the molten steel is discharged into a ladle in which a coolant is previously charged. 2. The high Cr material according to claim 1, wherein a part of the coolant is added to the molten metal in the secondary refining.
Steel melting method.