JPH01205024A - Dephosphorization method for ion alloy containing cr - Google Patents

Abstract

PURPOSE:To easily dephosphorize an iron alloy with high efficiency by specifying the temp. of a molten metal and the SiO2 component on a bath surface before desulfurizing the metal of the iron alloy contg. a specific ratio of Cr, then properly charging a flux, carbonaceous material and iron oxide thereto under specific conditions. CONSTITUTION:The temp. of the molten metal prior to dephosphorization is kept at >=1,510 deg.C and <=about 1,780 deg.C and the SiO2 component existing on the bath surface is removed to <=10kg/t.molten metal at the time of dephosphorizing the melt of the iron alloy contg. >=5wt.% Cr while stirring the melt. The flux consisting of 20-50wt.% CaO, 25-80wt.% CaF2, <=35% CaCl2 is then added at 20-120kg/t.molten metal, the carbonaceous material (about >=1mm grain size) at 1-25kg/t.molten metal and the iron oxide (iron ore, etc.) at about <=25kg/t.molten metal to the molten metal to dephosphorize the molten metal. The iron alloy contg. Cr is thereby easily and inexpensively dephosphorized with the high efficiency without generating harmful slag.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for dephosphorizing iron alloys containing Cr, and particularly to a method for economically dephosphorizing iron alloys containing 5% by weight or more of Or. .

[Prior Art] Generally, phosphorus (hereinafter referred to as P) in high Cr 11 or stainless steel containing 5% by weight or more of Cr is a harmful impurity that adversely affects the mechanical properties and stress corrosion cracking of steel. However, for dephosphorization of iron alloys containing Cr, even if a strong oxidation refining method such as converter blowing, which is commonly used as a dephosphorization method for molten steel, is applied, Cr in the molten metal is preferentially oxidized. However, dephosphorization did not proceed and was thought to be impossible.

However, recently, the need for low-phosphorus stainless steel has increased, and the dephosphorization method described below has been developed.

Conventional dephosphorization methods are roughly divided into two: (A> reduction dephosphorization method and (B) M conversion dephosphorization method. For (A) dephosphorization method, ■ electroslab remelting method (ESR) is used. Dephosphorization method using Ca CaF2-based flux, ■ CaC2CaFz in steel processing
, a method of dephosphorizing using CaC2, etc. are known. Both (1) and (2) dephosphorize with Ca, and the dephosphorization reaction is a reductive dephosphorization represented by 3 (Ca) +2 [P]-(Ca3P2), and (2) is CaC2-Ca+2C. It utilizes Ca produced by the decomposition reaction of CaC2.

On the other hand, the dephosphorization method of (B) includes: ■Dephosphorization method using Ca0-FeCb-based flux (Japanese Patent Publication No. 57-7212>), ■Alkali metal carbonates, resistant substances, one type of hydroxide, alkaline earth metal Fluoride, a type of chloride, iron.

A type of nickel oxide, alkaline earth metal oxide,
A method of dephosphorizing using a flux containing one type of carbonate (
JP-A-57-17923>, ■Fluff soot consisting of one or more alkaline earth metals (carbonates, hydroxides) and one or more alkaline earth metal halides is added, and the resulting slag is hardened. There is a method of adding an oxidizing agent in an amount that does not cause oxidation (Japanese Patent Laid-Open No. 151416/1983).

[Problems to be Solved by the Invention] Among the dephosphorization methods for iron alloys containing Cr, the above-mentioned (A>reductive dephosphorization method) both (1) and (2) contain Ca in the slag after the dephosphorization treatment.
3P2 will exist, which is (Ca3P2 ) +3H20→3 (CaO) +
2 P) As shown in 13, phosphine (p)+3 is a toxic compound that reacts with H2O in the atmosphere and has a strong garlic odor.
> occurs.

On the other hand, (B)! The chemical dephosphorization method does not have the above-mentioned problem of slag treatment after dephosphorization, and has the advantage that the "C" treatment cost is cheaper than the reductive dephosphorization method (A>) because it uses an inexpensive flux. However, the conventional oxidative dephosphorization method requires certain conditions to more effectively dephosphorize the molten metal.
Furthermore, the conditions for adding flux and oxidizing agents to lower processing costs have not been clarified, and improvements have been desired.

This invention was made in view of the problems of the conventional technology, and uses the lowest cost CaO-based flux and oxidizing agent in oxidative dephosphorization that does not cause problems in slag treatment after dephosphorization. In a molten metal dephosphorization method that mainly uses cheap iron oxide, we would like to propose a method to control the slag so that it does not harden during the dephosphorization process and perform dephosphorization under conditions that can fully demonstrate the dephosphorization effect. It is something to do.

[Means for Solving the Problems I] In order to remove [P] from a molten metal containing Cr by oxidative dephosphorization, the following two points are important.

■ [P] in the molten metal is reduced to P20 by using slag that has a weak oxygen potential that does not excessively oxidize [Cr].
5, which is an oxide.

The reason is that when the oxygen potential is high, it is harmful [
This is because [Cr], which is more useful than [P], is oxidized preferentially, and the high melting point oxide Cr2O3 increases in the slag, causing hardening of the slag and physically preventing dephosphorization.

■ The slag must contain a basic oxide in order to stabilize the acidic oxide P205, which is a dephosphorization product, in the slag. Basic oxides include Na2O,
There are Li20, BaO, etc., but the most common and inexpensive one is CaO. However, CaO is less basic than basic oxides and has a high melting point (m, o 2570°C
). In addition, it is important to increase the oxidizing power for dephosphorization, and in this case, Cr2O3 (m, D2
275° C.) and has low solubility in the slag, making it easy for the slag to harden. Therefore, in the case of CaO, it is not easy to increase the oxidizing power to such an extent that the slag does not harden.

Therefore, in this invention, in a method of dephosphorization using an inexpensive CaO-based flux, we have found conditions that can fully exhibit the dephosphorization effect without hardening the slag during the dephosphorization process. When dephosphorizing an iron alloy containing 5% by weight or more using CaO-based flux and iron oxide, the temperature of the molten metal before dephosphorization is set to 1510°C or higher, and the 5j02 min existing on the bath surface is heated to 10 k/t.molten metal. After removing the sludge to the following, CaO20-50% by weight, CaF2
Flux consisting of 25-80% by weight, Ca (Jz 35% by weight or less) 20-120kc3/t, molten metal, carbon material 1-
It is characterized in that it is processed using 25 kg/t of molten metal and 25 kg/t of iron oxide or less of molten metal.

[Function] The reason why the temperature of the molten metal before dephosphorization is limited to 1510° C. or higher is because at a temperature lower than this, the flux is poorly formed into slag and the dephosphorization reaction is difficult to proceed.

Note that once the flux becomes a slag, there is no problem even if the temperature after treatment is about 1300°C.

Also, in reality, the temperature of the molten metal is 10% due to the addition of flux.
Since the temperature decreases by 0°C or more, it is necessary to raise the pre-treatment temperature to 1510°C or more.

The upper limit of the temperature of the molten metal before dephosphorization is not particularly limited, but it is set to 1780°C or less in order to reduce the erosion of refractories and to prevent the temperature of the molten metal after addition of flux from becoming too high and deteriorating the dephosphorization. It is preferable that

In addition, 10 kg of 5j02 that exists on the bath surface before treatment
The reason why the slag is removed so that it is less than 10 kv/t.molten metal is that if 5j02 min exceeds 10 kv/t.molten metal, the basicity of the slag will decrease and the dephosphorization reaction will be difficult to proceed.

There is no problem with the short 5j02 minutes.

In particular, Cr ore is dissolved in molten metal, C is melted and reduced with oxygen,
When melting iron alloys containing Cr, it is necessary to remove slag (including SI and O2) after melting and reduction, and also to facilitate dephosphorization even in normal electric furnace - AO [) operation. It is necessary to remove the slag containing a large amount of 5j02 of (4) which has been desiliconized to below about 0.1fffffi% [si].

Next, the flux of this invention will be explained in detail.

In order to oxidize P in a molten metal containing Cr into a form such as P205, an oxidizing agent is first required, and in order to dephosphorize it, a basic oxide that stabilizes this P2O5 is required.

Basic oxides include Na2O, Li2O, BaO, etc., but the most common one is CaO as described above. Conventionally, dephosphorization at a low oxygen potential, such as in a Cr-containing molten metal in which oxidation loss of Cr(7) is small, requires a source of Li2O that is more basic than CaO. However, materials such as Li2CO3 are expensive.

Therefore, this invention uses CaO20-50 as a flux to enable dephosphorization with an inexpensive CaO-based flux.
Weight %, CaF225-80 weight%, CaCb 35
It was decided to use a flux consisting of less than % by weight.

In other words, the larger the amount of CaO as the basic oxide, the better, but the amount of CaFz or C used as other solvents is
Since aCb and iron oxide used as an oxide, etc., need to turn into slag and form molten slag in the presence of SiO2, C in the flux is
The amount of aO is limited to 50% by weight or less. This is because if it exceeds 50%, it will not become a slag. On the other hand, C in the flux
The lower limit of aO is limited by the level of dephosphorization.

That is, if CaO is less than 20% by weight, the basicity of the slag will decrease and effective dephosphorization will not proceed, so the CaO content is preferably 20 to 50% by weight.

Moreover, the reason why CaF2 and CaCf2 were selected as the solvent is as follows.

Figure 1 shows CaO fixed at 30% by weight, CaF2 and Ca
(This is a diagram showing the melting of slag in the case of not containing an oxidizing agent when Jz is changed.

When CaCR2 is added to CaF2, the melting point of the slag decreases. This figure shows that the combined use of CaF2 and CaCR2 is more effective as a solvent than CaF21 or CaCλ2 alone.

The reason why the amount of CaF2 in the flux is limited to 25 to 80% by weight is because if it is less than 25% by weight, the effect as a solvent is low, and it is necessary to mix in a large amount of expensive CaCb, for example, 35% by weight or more. , on the other hand, if it exceeds 80%,
This is because dephosphorization deteriorates as Ca0 min decreases.

In addition, the reason why the amount of CaCf2 in the flux is limited to 35% by weight or less is because if CaCfz is more than this, the cost of this solvent will be high, so in this case, it is preferable to use CaFz in combination,
This is because it is better to set CaCR2 to 35% by weight. Although the lower limit of CaCf2 is not particularly limited,
5 from the point of enhancing the solvent effect when used in combination with CaF2
It is preferable that the amount is at least % monomer.

The reason for limiting the amount of flux added above to 20 to 120 kg/t/molten metal is that if it is less than 20 kCJ/t/molten metal, a high dephosphorization effect cannot be obtained, whereas if it exceeds 120 kg/t/molten metal, the processing temperature will drop. This is because it becomes too large.

In this invention, iron oxide (iron ore, scale, dust) was selected as the oxidizing agent because of the 1200
Calcium ferrite with a low melting point of °C is made, and Ca
This is to promote slag formation of O.

This iron oxide converts some of the C7 in the molten metal into Cr2O3, and this Cr2O3 also acts as an oxidizing agent.

Figure 2 shows 30% by weight of CaO-35% by weight of CaCRz.
- CaF235 wt% flux, 16 wt% C
r T-Fe and Cr2 in the slag after dephosphorizing the molten metal
The relationship between O3 is shown.

From this figure, it can be seen that there is a positive correlation between (T-Fe) and (Cr203). Furthermore, it can be seen that as the amount of iron oxide added increases, both (T-Fe) and (CrzOs) increase.

The figure also shows the melting point of the slag after dephosphorization, measured using a high-temperature microscope.

When the dephosphorization treatment temperature is 1300 to 1510°C, in order for the melting point of the slag to be lower than this, (T-Fe) < 4% by weight (Cr203) and 6 to 8% by weight. If (CrzOs) increases more than this, the slag will harden and the reaction will not physically proceed.

CaO-based fluxes with other blending ratios generally contain Ca
Since it has a higher melting point than the C&235wt%-○aF235wt% flux, it is thought that the upper limit of the composition of the above-mentioned oxidizing agent component is slightly lower.

In order to reduce the amount of the oxidizing agent component during the dephosphorization process, the carbonaceous material must be added to the CaO-based flux in advance, or after the addition of the CaO-based flux, it must be slag-formed before being added. It is effective to do so. Furthermore, it is also effective to dispense the water in portions as the dephosphorization process progresses.

As a result, excess Cr2O3 generated in the slag can be reduced by the carbon material, the (Cr2o3) in the slag can be suppressed within a certain concentration range, and the slag formation of the slag can be maintained.

FIG. 3 shows the relationship between C in the molten metal and Cr2O3 in the slag.

This figure shows that (Cr203) tends to increase rapidly when [C] decreases, but by adding carbonaceous material, (Cr20a) can be decreased as shown in the black circle plot in the same figure, and the slag We were able to maintain the slag formation.

The amounts of iron oxide and carbonaceous material to be added vary depending on the sealability of the C1 furnace for the molten metal, the stirring power of the dephosphorization treatment, and the treatment temperature.

For example, when [C] is low, adding a large amount of iron oxide causes a rapid increase in Cr203 in the slag, and therefore a large amount of carbonaceous material must be added.

In addition, if the sealing of the furnace is insufficient, air oxidation of [Cr] will occur due to the entrainment of air during the dephosphorization process, resulting in carbon in the slag.
Since r2O3 increases, less iron oxide is sufficient;
On the other hand, it is necessary to add a large amount of carbonaceous material.

In addition, if the stirring power is insufficient during the dephosphorization process, the Cr2O3 component generated in the slag is difficult to be reduced by the C of the molten metal, and (Cr203) tends to increase, so a large amount of carbonaceous material must be added. .

Furthermore, if the processing temperature is low, (Cr203) is likely to be generated, so it is necessary to suppress the amount of iron oxide added and add a large amount of carbon material.

For example, in a 10t AO[) furnace, the processing temperature is 150
At 0°C and [C] = 3%, by adding carbonaceous material to 10'j/t/molten metal for the addition of iron oxide 20kLj/t/molten metal, (Cr20d is suppressed to 6 to 8% by weight or less. It becomes possible to do so.

From the above results, the amount of iron oxide and carbonaceous material added varies depending on the dephosphorization temperature, but the upper limit of iron oxide is saturated when C in the molten iron is saturated and slag is 25 kq/t molten metal that does not harden is preferable,
Although the lower limit is not particularly limited, in the case of dephosphorization treatment using a furnace with a low [C] and a large amount of air entrainment, [Cr]
is oxidized in the air, and Cr203 naturally increases in the slag as the dephosphorization process progresses, so it is sufficient to add Cr203 to the extent that it can provide oxidizing power at the initial stage of the dephosphorization process.

Furthermore, the upper limit of the amount of carbonaceous material added is the same as iron oxide, 25 kg/t molten metal, which is sufficient even in the maximum case, while the lower limit is 1 kg/t molten metal or more in order to prevent slag hardening.

The particle size of the carbonaceous material is not particularly limited; however, if the particles are too fine, reduction of Cr2O3 in the slag may occur rapidly, and the oxidizing power may be extremely reduced. Furthermore, Cr2O3 in the slag tends to be used for carburizing the molten metal rather than reducing it. Therefore, the particle size of the carbon material is 1 m
/m or more is desirable. Further, the carbonaceous material may be in the form of pellets made by baking and hardening zero grains.

In addition, in this invention, the C content of the molten metal is 3 to 4% by weight in advance.
It is desirable to adjust it to That is, if it is less than 3% by weight, the dephosphorization rate will be extremely reduced, while if it exceeds 4% by weight, problems such as increased decarburization cost and melting of refractories will occur.

FIG. 4 is a diagram showing the influence of [C] on dephosphorization.

From this figure, the higher the [C], the higher the dephosphorization rate;
] It can be seen that dephosphorization deteriorates rapidly as the value decreases. In addition, in order to increase the dephosphorization rate by nearly 50% by weight, [C] should be added to 3
It turns out that it is necessary to increase the weight by 3 weights. However, when [C] exceeds 44% by weight, carburization is difficult to proceed and it takes a long time to work, and chromium carbide may precipitate in the molten iron, causing the surface of the molten metal to harden. Roughly,
This puts a burden on the decarbonization process after decarbonization. In addition, as shown in the black circle plot in the same figure, [C] is 5% at 3 to 4 times concentration.
It can be seen that a dephosphorization rate of nearly 0% can be obtained.

Therefore, the C content of the molten metal before treatment is preferably 3 to 4% by weight.

As an apparatus for carrying out the method of this invention, an AOD furnace or a furnace into which stirring gas can be introduced from the bottom of the furnace can be used. It is also possible to perform Ar bubbling agitation or impeller agitation in a ladle.

Flux can be added by placing granular flux on the bath surface, but it is better to add it into the bath by injection method, which improves slag formation and allows better dephosphorization. .

In addition, iron oxide may be added all at once at the initial stage, but especially in the case of Cr-containing molten metal with low [C] (Cr2
In order to prevent excessive formation of 03) and hardening of the slag, it is preferable to dispense in portions.

[Example] 10 tons of Cr-containing iron alloy was melted in the atmosphere in an electric furnace, and after being poured into a 〇0 furnace, the surface of the molten metal bath was adjusted to 1600°C.
Table 1 shows the results of dephosphorization treatment after removing slag to 10 ki.

For comparison, Table 1 shows the results of the dephosphorization treatment in the same manner as Test No. 1 without adding C pellets in Test-3.

In test No. 1 of the present invention example, CaO 40% by weight - CaF
1 ton of flux consisting of 260% by weight and 1 C pellet
00 kl was added at the same time, and after stirring with Ar gas for 10 minutes, Fe2O3 was added in equal portions of 9 x 10 degrees to 25 to perform dephosphorization treatment.

The temperature after treatment was 1380°C. After that, the slag was removed and the regular refining process started. As a result, a dephosphorization rate of 48% and a desulfurization rate of over 80% were obtained.

In test No. 2, 1 ton of flux consisting of 30 wt% CaO-35 wt% CaFz-Ca (1% J235i) and 100 kg of C pellet were added at the same time, and Ar gas
After stirring for a minute, add 25 to 9 of Fe203 to 100 kq.
It was distributed and dephosphorized. The temperature after treatment was 1400°C. After that, the slag was removed and the regular refining process started. As a result, a dephosphorization rate of 65% and a desulfurization rate of over 80% were obtained.

On the other hand, in Comparative Example Test No. 3, the dephosphorization slag became slightly hard during the treatment, and the dephosphorization rate was 36%.

Blank space below (Effect of the invention 1 As explained above, according to the method of this invention, it is possible to dephosphorize iron alloys containing Cr easily, inexpensively, and with high efficiency without producing harmful slag. can.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the melting temperature of flux in the method of this invention. FIG. 2 is a diagram showing the relationship between Cr203 and T-Fe in the slag and the melting temperature of the slab. FIG. 3 is a diagram showing the relationship between [C] in molten iron and CrzOs in slag. FIG. 4 is a diagram showing the influence of [C] on the dephosphorization rate. Applicant: Sumitomo Kinku Kogyo Co., Ltd. Figure 1 Figure 2 (Cr2O5) (% by weight) Figure 3 Figure 4 [C] (311%)

Claims (1)

[Claims] In a method of dephosphorizing a molten iron alloy containing 5% by weight or more of Cr using flux while stirring, the temperature of the molten metal before dephosphorization is set to 1510°C or higher, and the S present on the bath surface is
After removing the sludge so that iO_2 min becomes 10 kg/t・molten metal or less, CaO_20~50% by weight, CaF_225~
Flux consisting of 80% by weight, CaCl_235% by weight or less 20-120kg/t, molten metal, 1-25kg of carbon material
A method for dephosphorizing an iron alloy containing Cr, characterized in that the treatment is carried out using less than 25 kg/t of molten metal and iron oxide of 25 kg/t/t of molten metal.