JP3143764B2 - Method for removing impurities from chromium-containing molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to Pb, Z which adversely affects the hot workability of chromium-containing molten steel in the final refining stage.
The present invention relates to a method for efficiently removing n and Sn.

[0002]

2. Description of the Related Art Conventionally, 11 wt% such as stainless steel
Pb, Z in chromium-containing molten steel containing chromium
There is no quantitative knowledge regarding the removal of impurities of n and Sn.
Since Pb, Zn, and Sn adversely affect the hot workability of steel, the upper limits are generally set according to the properties of the steel base.

In ordinary steel, for example, “Materials and Processes”, vol. 1, No.4, pages 1169 to 117
2 (1988), it has been shown that removal of Pb and Zn is promoted by increasing the flow rate of the stirring gas, and that Sn can be removed a little.
However, regarding the chromium-containing molten steel, the effect of chromium contained in a large amount is unclear, and thus there is no quantitative knowledge to determine, for example, a removal limit value.

[0004] Therefore, the reducing agent, the component adjusting agent and the cooling agent added in the final refining stage for reducing the chromium oxide in the slag and adjusting the component and temperature are Pb, Z
Thorough control of the concentration of n and Sn was carried out, and a material having a low concentration was preferentially used, and excessive gas was blown. However, even if such operation management is performed, the target Pb, Zn, and Sn regulated values may be deviated. Materials with low concentrations of Pb, Zn, and Sn are expensive,
High costs are incurred. Further, if excessive gas is blown to lower the concentrations of Pb, Zn, and Sn, the cost increases, and this cannot be said to be an efficient refining method.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a method for reducing the content of chromium oxide in slag and controlling the composition and temperature of the chromium-containing steel during the final refining stage of the molten chromium-containing steel.
It is an object to prevent the concentrations of Pb, Zn, and Sn from deviating from target values and perform efficient gas injection by determining the flow rate of the injected gas in consideration of the return of Zn and Sn. I do.

[0006]

Means for Solving the Problems The present invention advantageously solves the above-mentioned problems, and reduces the impurity concentrations of Pb, Zn, and Sn in molten steel to below a target value in the final refining period of chromium-containing molten steel. In consideration of the return of Pb, Zn, and Sn from the material added in the final refining period, the following (1)
At a flow rate Q _G satisfying formula, it is an gist method of removing impurities containing chromium molten steel and performing included gas blowing.

Q _G ≧ −α _M 10 ^{1−T / 1873} · ln ([M] _t / [M] ₀ ) (1) Q _G ; gas injection flow rate in the final refining stage (Nm ³ / ton of molten steel) Α _M ; constants determined for each impurity element M α _Pb = 3.0, α _Zn = 2.5, α _Sn = 60.2 T; molten steel temperature at the start of final refining [K] [M]; impurity element M The subscript t indicates the target value after refining, and 0 indicates the calculated concentration at the start of final refining.

Hereinafter, the present invention will be described in detail. The present invention is applied to the refining method of chromium-containing molten steel as exemplified in FIG. 1, wherein FIG. 1 (a) shows an AOD method, FIG. 1 (b) shows a top-bottom blowing converter method, and FIG. This is a known blowing technique. In these blowing methods, first, decarburization and refining are performed by blowing oxygen or oxygen and an inert gas, and then chromium oxide that has been oxidized during decarburization and transferred to the slag is reduced, and at the same time, components and temperature are reduced. There is a final refining period to make adjustments.

In the final refining period, a reducing agent such as Al or Si for reducing chromium oxide, a component adjusting material such as Si, Mn, or Ni for adjusting the components, and the temperature of the molten steel are set to predetermined temperatures. A large amount of coolant such as scrap for lowering is added. Although these materials vary in size,
Pb, Zn, and Sn impurities are contained. The concentrations of Pb, Zn, and Sn picked up from these materials are as follows:
Assuming that the impurity concentration and the amount of the material to be added are the symbols in Table 1, the impurity concentration and the amount are obtained by the equations (2) to (4).

[0010]

[Table 1]

[Pb] ₁ = (a _Pb W _a + b _Pb W _b + c _Pb W _c + d _Pb W _d + e _{Pb We e} + f _Pb W _f ) ÷ (1000 W _m ) (2) [Zn] ₁ = ( _{a Zn W a + b Zn W} b + c Zn W c + d Zn W d + e Zn W e + f Zn W f) ÷ (1000W m) ...... (3) [Sn] _{1 = (a Sn W a +} b Sn W b + c _{Sn W c + d Sn W d} + e Sn W e + f Sn W f) ÷ (1000W m) ...... (4) where [Pb] _1, [Zn] _1, [Sn] ₁ Pb,
Pickup concentration (pp) from Zn, Sn additive material
m), and W _m is the weight of molten steel after addition of additives (ton)
Is shown.

Further, the concentration [M] ₀ at the start of the final refining is obtained by the equations (5) to (7). [Pb] ₀ = [Pb] ₁ + [Pb] _D (5) [Zn] ₀ = [Zn] ₁ + [Zn] _D ... (6) [Sn] ₀ = [Sn] ₁ + [Sn ] _D (7) where [Pb] _D , [Zn] _D , [Sn] _D is Pb,
Shows the concentrations (ppm) of Zn and Sn at the end of decarburization refining.
These concentrations are empirically determined from decarburization refining conditions, and are generally [Pb] _D ≦ 3 ppm, [Z
n] _D ≦ 1 ppm and [Sn] _D ≦ 50-200 ppm.

On the other hand, the target value [M] _t after the final refining is a value determined for each steel type from the properties of the base material of the steel and the hot workability required from the post-process, and [Pb] _t = 1 to
20 ppm, [Zn] _t = 10 to 150 ppm, [S
n] _t = 100-500 ppm. Based on the above relationship, the removal rate ([M] _t /
[M] ₀ ) is determined.

[0014] Next, a description will be given of reasons for limiting the gas blowing flow rate Q _G of the final refining stage. In the final refining period, an inert gas such as Ar or N _{2 is} generally blown to prevent oxidation of molten steel. FIG. 2 shows the gas flow rate and the removal rate of [Pb] ([Pb] when SUS 304 stainless steel was subjected to final refining by blowing Ar gas using a 60 tAOD furnace.
b] _t / [Pb] ₀ ). Similarly, FIG. 3 shows the gas flow rate and the removal rate of [Zn] ([Zn] _t / [Z
relationship n] _0), in FIG. 4 shows the relationship between removal rate and the gas flow rate [Sn] ([Sn] _t / [Sn] _0). The molten steel temperature at the start of the final refining was in the range of 1590 to 1610 ° C.

Although there is a slight variation in all three elements, it was confirmed that the removal rate was improved by increasing the flow rate of the blown gas, and that the removal rate changed logarithmically with the flow rate of the blown gas. . Formulating the condition with the worst removal rate shown by the solid line in FIGS. 2 to 4 yields equations (8) to (10). Q _Pb = −3.0 × ln ([Pb] _t / [Pb] ₀ ) (8) Q _Zn = −2.5 × ln ([Zn] _t / [Zn] ₀ ) (9) Q _Sn = −60.2 × ln ([Sn] _t / [Sn] ₀ ) (10) In FIG. 5, by using a 60 tAOD furnace and injecting Ar gas,
When the final refining of SUS 304 stainless steel is performed, the molten steel temperature at the start of the final refining and the removal rate of [Pb] ([P
b] _t / [Pb] ₀ ). Note that the range of the gas injection flow rate is a value in the range of 0.9 to 1.1 Nm ³ / T. The removal rate tends to increase as the temperature of the molten steel increases, and when this is formulated, it is expressed as in equation (11).

([Pb] _t / [Pb] ₀ ) = r (1−T / 1873) (11) where T represents the molten steel temperature (K) at the start of final refining.
r is a constant. It has been confirmed that the relationship of the formula (11) has the same relationship with the molten steel temperature only for the constant values of Zn and Sn. Summarizing the above findings, for the impurity element M, the gas injection flow rate Q _G (Nm ³ / N) required to achieve the target value after the final refining is completed.
The molten steel ton ) is summarized as the following equation.

Q _G = −α _M 101 ^{-T / 1873} · ln ([M] _t / [M] ₀ ) (1) α _Pb = 3.0, α _Zn = 2.5, α _sn = 60.2 From equation (1), the gas flow rate required in the final refining period can be easily determined. In the final refining period, there is a gas injection amount necessary for reduction of chromium oxide and adjustment of components and temperature.
If this amount is larger than Q _{G in} equation (1), the regulation in equation (1) is not necessary. Also, Q _{G in} equation (1) is Pb, Z
It is necessary to determine each of n and Sn and to inject gas with a value larger than each value. In addition, the gas injection must be moderately suppressed, since the gas flow increases more than the required flow rate.

[0018]

FIG. 6 shows the relationship between the molten steel temperature and the vapor pressure of various metal elements in the pure metal state. Pb, Zn, Sn are Fe, C
Vapor pressure is higher than r and Ni, 1450-1750 ° C
In the molten steel temperature state, removal by evaporation proceeds. Also,
It is considered that the evaporative removal rate is higher in the order of Zn, Pb, and Sn having higher vapor pressures.

In the evaporative removal reaction, it is considered that the movement of the evaporating element to the reaction interface in the molten steel or the desorption of the evaporating element from the reaction interface to the gas phase is a rate-determining process of the reaction. Factors that promote this reaction include the following. 1) Increase the molten steel temperature. 2) Reduce the atmosphere or make a vacuum.

3) The gas generation rate is increased to increase the area of the reaction interface. Since the final refining period of the chromium-containing molten steel is generally performed at atmospheric pressure, the effect of 2) cannot be enjoyed. Therefore,
1) and 3) are the points. Conventionally, the effects 1) and 3) have been qualitatively shown, but no quantitative knowledge has been found particularly in the field of chromium-containing molten steel containing a large amount of chromium. In the present invention, it has been found that the removal rates of Pb, Zn, and Sn are larger in the order of Zn, Pb, and Sn having higher vapor pressures, are proportional to the molten steel temperature, and logarithmically increase due to an increase in the flow rate of the injected gas. , Formulated. It can also be seen from equation (1) of the present invention that the removal is promoted by the increase in the temperature of the molten steel and the flow rate of the blown gas.

In addition, due to the hot workability of steel and the like, the upper limit of the content of impurities is generally regulated for each steel type. In order to satisfy the regulation value, the following equation (1) is used. If the gas flow rate is determined and gas is blown, efficient refining becomes possible.

[0022]

[Example] SUS304 using 60 ton AOD furnace
Stainless steel (8 mass% Ni-18 mass% C
An example performed for r) will be described. Table 2
Of the materials added in the final refining period and Pb, Zn, Sn
Is shown. The materials shown in Table 2 were added in a predetermined amount at the beginning of the final refining period, and Ar gas was blown. The gas flow rate required for the reduction in the final refining stage and the adjustment of the components and the temperature was 0.70 Nm ³ / T, and the gas blowing rate was constant at 2000 Nm ³ / Hr.

In addition, since blowing was carried out in the same manner in each case until the final refining period, P
b is 2.0 ppm, Zn is 0.1 ppm, Sn is 140 ppm.
ppm. In each example, the target values after the final refining were Pb ≦ 3.0 ppm, Zn ≦ 20 ppm, Sn ≦
Applied to 150 ppm steel. Table 3 shows the types and amounts of the materials added in the final refining period and the calculated concentrations at the start of the final refining for the examples of the present invention and the comparative examples.

Table 4 shows the results of the examples. The refining cost in the table is a value obtained by converting the example of No. 1 to 100. In the example of No.7, the value of Pb was out of the component,
It was not a product as it was, and it was impossible to calculate the cost.

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

According to the present invention, in the final refining period of the chromium-containing molten steel, it is possible to prevent Pb, Zn, and Sn from being out of regulation, and to prevent excessive gas injection. Further, since the gas injection flow rate for reducing Pb, Zn, and Sn to the regulated value or less can be quantified, inexpensive additive materials can be used in the final refining period, and the refining cost can be significantly reduced. Become. Also, according to the present invention, P
Efficient removal of Bi, Sb, etc. having the same vapor pressure as b and Zn is achieved.

[Brief description of the drawings]

FIG. 1 is a diagram relating to an example of a refining method to which the present invention is applied;
(A) shows the AOD method, (b) shows the top and bottom blown converter method, and (c) shows the VOD method.

FIG. 2 is a diagram showing a relationship between a gas injection flow rate and a removal rate of [Pb] in the present invention.

FIG. 3 is a diagram showing a relationship between a gas injection flow rate and a removal rate of [Zn] in the present invention.

FIG. 4 is a diagram showing the relationship between the gas injection flow rate and the removal rate of [Sn] in the present invention.

FIG. 5 is a diagram showing a relationship between molten steel temperature and a removal rate of [Pb] in the present invention.

FIG. 6 is a diagram showing a relationship between a vapor pressure and a temperature in a pure state of an impurity element.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Side blowing tuyere 2 Top blowing lance 3 Bottom blowing tuyere 4 Molten steel 5 Slag 6 Top blowing point

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yuji Yoshimura 3434 Shimada, Hikari-shi, Yamaguchi Prefecture Inside Nippon Steel Corporation Hikari Works (56) References JP-A-64-17814 (JP, A) JP-A-55-125221 (JP, A) JP-A-56-127723 (JP, A) JP-A-60-67629 (JP, A) JP-A-62-80218 (JP, A) (58) Fields investigated (Int) .Cl. ⁷ , DB name) C21C 7/00, 7/076, 7/10 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. In a final refining period of a chromium-containing molten steel, in order to reduce an impurity concentration of Pb, Zn, and Sn in the molten steel to a target value or less, P from a material added in the final refining period.
b, Zn, taking into account the return of Sn, the following (1) method of removing impurities from chromium-containing molten steel and performing inclusive gas blown at a flow rate Q _G that satisfies the equation. Q _G ≧ −α _M 101 ^{-T / 1873} · ln ([M] _t / [M] ₀ ) …… (1) Q _G ; gas injection flow rate in final refining stage (Nm ³ / ton of molten steel ) α _M Constants α _Pb = 3.0, α _Zn = 2.5, α _Sn = 60.2 T determined for each impurity element M; molten steel temperature at the start of final refining [K] [M]; concentration of impurity element M ( ppm) The subscript t indicates the target value after the end of refining, and 0 indicates the calculated concentration at the start of final refining.