JP2002256324A - Method for melting high carbon and high chromium steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for melting a high carbon and high chromium steel having low phosphorous concentration and high chromium yield. SOLUTION: When the high carbon and high chromium steel is melted by blowing gaseous oxygen and also charging the chromium source into molten iron held in a converter provided with a top-bottom combined blown function of gas for refining, the refining of the other kinds of steels for cleaning the inside of the converter is performed at >=3 heats before melting the objective high carbon and high chromium steel. Thereafter molten iron subjected to pre- dephosphorization for lowering the phosphorous concentration to not higher than (the target value-0.005 mass%) is charged into the converter and blow- refined without adding slag-making promoter of the slag.

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 carbon and high chromium steel, and more particularly to a technique for smelting high carbon and high chromium steel having a low phosphorus concentration while suppressing chromium oxidation loss. .

[0002]

2. Description of the Related Art As a technique for producing high carbon steel, Japanese Patent Application Laid-Open No. 57-19221 proposes a high carbon steel blowing method in an upper blowing converter. It is formed by performing a so-called soft blow to raise the tip position of the lance that injects the oxygen gas according to the elapse of the blowing time when blowing the oxygen gas upward on the molten iron held in the converter. Dephosphorization is performed while adjusting the degree of slag slag (making it into a molten state) and suppressing occurrence of slag of molten steel. When carbon (hereinafter, represented by symbol C) is high carbon steel of 0.5% by mass or more, blowing is completed with the carbon content remaining in the steel. Accordingly, in this case, the oxygen blowing is terminated when the decarburization reaction in the converter is at its peak, so the following considerations are required. 1) Since the dephosphorization reaction does not easily proceed, the amount of iron ore introduced and the amount of slag to be formed are increased to promote the dephosphorization reaction. 2) Not only can heat from C combustion not be expected,
Since the amount of slag increases, a large amount of heat source (coke, graphite, Fe-Si, etc.) is separately added to secure an appropriate molten steel temperature. To produce high chromium steel, add a chromium source (eg, ferrochrome, chrome ore, etc.) to the hot metal in the furnace during the middle of blowing to prevent the molten steel temperature during tapping from becoming too high. . However, Japanese Patent Application Laid-Open No.
When the high carbon steel blowing method described in Japanese Patent No. -192211 is applied and blown, the iron ore, which is an oxygen source, is added, the amount of slag is increased, and the soft blasting is performed.
O causes chromium to be oxidized, and not only does the phosphorus concentration not decrease, but also causes a problem that the chromium yield to molten steel is significantly reduced. Further, if a chromium source is added in the latter half of blowing to increase the yield of the chromium, the chromium may be left undissolved.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a high carbon and high chromium steel having a low phosphorus concentration and a high chromium yield in view of the above circumstances.

[0004]

Means for Solving the Problems The inventor conducted intensive research to achieve the above object, and embodied the results in the present invention.

That is, according to the present invention, oxygen gas is blown into a hot metal held in a converter equipped with a gas top-bottom blowing function, and a chromium source is charged and blown to melt high carbon and high chromium steel. In this case, for the three or more charges before melting the high-carbon / high-chromium steel to be processed, blowing of another steel type to clean the inside of the converter is performed, and then the phosphorus concentration is reduced to (target value-0.005 mass). %) The hot metal preliminarily dephosphorized is charged into the converter,
This is a method for producing high carbon and high chromium molten steel, which comprises blowing without adding a slag-making accelerator such as fluorite or iron ore.

In this case, the blowing of another steel type for cleaning the inside of the converter uses hot metal having a phosphorus concentration of 0.02% by mass or less,
In addition, it is preferable that the formed slag is entirely discharged after blowing. During the blowing, the molten steel was stirred at a stirring power (symbol ε) defined by the following equations of 700 W / t-steel or more, or the chromium source was charged for 40 minutes during the entire blowing period. It is better to do it after the passage of%. ε = ε _B + ε _T / 10 (1) ε _B = (28.5Q _B T / W) · log [1 + L / 1.48] (2) ε _T = (0.0453Q _T d / W · x) · u ² (0,0) · cos ² ξ (3) where, Q _T : top blowing gas flow rate (Nm ³ / min) Q _B : bottom blowing gas flow rate (Nm ³ / min) T: molten steel temperature (K) W: molten steel weight (t) L: steel bath depth (m) x: lance height (m) d: lance nozzle diameter (m) u ² (0,0): gas linear velocity at the nozzle outlet ( m / m
in) cos ² ξ: Nozzle inclination angle (°) According to the present invention, after pre-cleaning of the converter, pre-treated hot metal is used to perform slag-free blowing of slag. As a result, a high-carbon / high-chromium steel having a low phosphorus concentration and a high chromium yield can be stably produced without increasing the cost as compared with the conventional method.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below, including the considerations that led to the invention.

In the case of producing high carbon steel, oxygen blowing is stopped at high carbon, so that much dephosphorization cannot be expected. Therefore, generally, while increasing the amount of slag formed,
Fluorite is added to sufficiently slag the slag to prevent reversion to molten steel.

Therefore, the inventor thought that if the factor of phosphorus reversion could be eliminated as much as possible, it would not be necessary to form a large amount of slag for dephosphorization, and it would be possible to reduce the input amount of the heat source. In this case, since it is not necessary to promote slag slag formation, iron ore or fluorite may not be added. As a result, the added chromium is oxidized and absorbed by the slag, and a reduction in the chromium yield can be prevented. In order to make this idea concrete, we first considered eliminating the causes of phosphorus recovery as much as possible, and came up with the idea of purifying the inside of the converter. That is, the residue (including the adhered matter) in the converter is removed as much as possible, or even if there is a residue, the state is such that phosphorus is not contained.

In the present invention, the inside of the converter can be cleaned by any method. This is because phosphorus should not be left in the converter that blows the target steel type. However, in practice, the blowing of the pre-charge is always performed, so it is reasonable to use the blowing. Therefore, the inventor conducted a test operation and examined a cleaning method. For the three or more charges before melting of the high-carbon / high-chromium steel to be used, hot metal having a phosphorus concentration of 0.02% by mass or less is used, and the formed slag is discharged after the entire slag is blown. Was found to be preferable, and this was regarded as one requirement of the present invention. The reason why the charge is set to three or more is that if the charge is less than three, the cleaning cannot be sufficiently performed. Unless the concentration of phosphorus in the hot metal is 0.02% by mass or less, cleaning cannot be performed with three charges, and the entire amount of slag must be discharged.

Next, in order to implement the above idea, hot metal preliminarily dephosphorized to a phosphor concentration of (target value-0.005 mass%) or less is charged into the cleaned converter. This is the second requirement of the present invention. By using hot metal 0.005% by mass lower than the target value, for example, the problem of phosphorus contamination that may occur even in a converter which has been cleaned and cleaned can be solved, and thereby, a high-carbon steel It is not necessary to perform blowing while worrying about the state of dephosphorization during blowing. Further, the third requirement that the slag is blown without adding a slag-making accelerator is a measure to suppress the chromium oxidation loss as described above.

The present invention satisfies the above three requirements for hot metal held in a converter equipped with a gas top and bottom blowing function.
In addition to injecting oxygen gas, a chromium source is supplied to perform ordinary oxygen blowing. As a result, a high-carbon, high-chromium steel having a low phosphorus concentration and a high chromium yield can be produced.

Further, in the present invention, during the blowing, the stirring force (symbol ε) defined by the above-mentioned equations is set to 700 W / t-ste.
It is better to stir the molten steel at el or higher. Thereby, the FeO concentration in the slag is reduced, and the chromium oxidation loss can be suppressed.

Further, in the present invention, the chromium source is preferably charged during 40 to 70% of the entire blowing period. This is because the chromium yield can be expected to be stable. In addition, if it is less than 40%, even if the blowing is completed in the high carbon region, an oxidation loss occurs, and if the addition is more than 70% in the latter half, the yield becomes unstable.

[0015]

EXAMPLE A low-P, high-carbon, high-chromium molten steel containing 1.01% by mass of C, 1.56% by mass of Cr, and 0.015% by mass of P was produced. At that time, both the smelting method according to the present invention and the conventional smelting method of dephosphorizing by using a large amount of fluorite or iron ore are applied, and the performances of the present invention are compared to confirm the effect of the present invention. did. The container used for melting was M
gO-C refractory lining, production capacity 180
This is a top-to-bottom blowing converter, in which the top blowing blows oxygen gas and the bottom blowing blows inert gas. The operating method is to blow oxygen gas onto the hot metal held in the furnace through a top-blowing lance, blow an inert gas from the bottom, and mainly cause a decarburization reaction by the top-blown oxygen gas. And stop the molten steel on the ladle. The hot metal used is one that has been desiliconized, dephosphorized and desulfurized in the hot metal pretreatment. Further, an inert gas is blown into hot metal or molten steel through a tuyere provided at the bottom of the converter to stir the molten steel.

In the practice of the present invention, first, in order to clean the converter, molten steel different from the target steel type was smelted over three charges, and the entire amount of slag formed at each blowing was discharged. The target P concentration of molten steel blown at each charge is 0.0
18 → 0.016 → 0.013% by mass to promote cleaning.

The hot metal that had been preliminarily dephosphorized in the hot metal was continuously charged 3 times in the converter after the completion of the washing. The P concentration of the hot metal charged into the converter at each charge was 0.009,
The content was 0.007, 0.008% by mass. Further, the stirring strength ε of the molten steel was 723 wa during the blowing with the help of the bottom blown gas.
tt / t-steel or more is used, and ferrochrome (a so-called high-carbon ferrochrome of 62% by mass of Cr and 8% by mass of C) is used as a chromium source, and 40 to 7 of a total blowing period is used.
It was introduced at the time of 0%.

Table 1 shows an example of the operation results of the conventional method and the method of the present invention.
Shown in In both cases, the amount of Cr added is the result after blowing when an amount equivalent to 1.52% was added. In addition, the operating results were evaluated in terms of the chrome yield and compared with the values obtained by the conventional method. The results are shown in FIG. 1, and the average value was about 98%, which was much higher than 93.5% obtained by the conventional method. Further, as shown in FIG. 2, the amount of heat source used is 6
It was about 8%.

[0019]

[Table 1]

[0020]

As described above, according to the present invention, a high carbon / high chromium steel having a low phosphorus concentration and a high chromium yield can be stably produced without increasing the cost as compared with the conventional method.

[Brief description of the drawings]

FIG. 1 is a diagram comparing the yield of addition of a chromium source in a furnace between the method of the present invention and the conventional method.

FIG. 2 is a diagram comparing the amount of heat source supplied to a converter between the method of the present invention and the conventional method.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hiroshi Nomura 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. BB05 EA21

Claims (4)

[Claims] An oxygen gas is blown into a hot metal held in a converter equipped with a gas top and bottom blowing function, and a chromium source is charged and blown to melt high carbon and high chromium steel. Blowing of another steel type to clean the inside of the converter is performed for three or more charges before melting of high carbon and high chromium steel, and then the phosphorus concentration is reduced to (target value-0.005 mass%) or less. The molten metal which has been subjected to the pre-phosphorus treatment is charged into the converter, and is blown without adding a slag-forming accelerator such as fluorite or iron ore. Manufacturing method. 2. The blowing of another steel type for cleaning the inside of the converter uses hot metal having a phosphorus concentration of 0.020% by mass or less, and discharges all formed slag after blowing. The method for producing a high-carbon, high-chromium molten steel according to claim 1, wherein: 3. The high carbon steel according to claim 1, wherein the molten steel is agitated during blowing by setting the stirring power (symbol ε) defined by the following equations to 700 W / t-steel or more.・ Making method of high chromium molten steel. ε = ε _B + ε _T / 10 (1) ε _B = (28.5Q _B T / W) · log [1 + L / 1.48] (2) ε _T = (0.0453Q _T d / W · x) · u ² (0,0) · cos ² ξ (3) where, Q _T : top blowing gas flow rate (Nm ³ / min) Q _B : bottom blowing gas flow rate (Nm ³ / min) T: molten steel temperature (K) W: molten steel weight (t) L: steel bath depth (m) x: lance height (m) d: lance nozzle diameter (m) u ² (0,0): gas linear velocity at the nozzle outlet ( m / m
in) cos ² ξ: Nozzle inclination angle (°) 4. The charging of the chromium source is performed during the entire blowing period.
The method for producing a high-carbon, high-chromium molten steel according to any one of claims 1 to 3, wherein the method is performed after 0% has elapsed.