JPS5856005B2 - High chromium steel melting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high chromium steel by reducing iron and chromium oxides at low cost and then rationally removing impurities such as phosphorus, carbon, and sulfur. .

High chromium steel, especially stainless steel, is generally made by melting an iron source such as hot metal or scrap, a chromium source such as ferrochrome or high chromium steel scrap, and, if necessary, a nickel source.
It is made by mixing and refining.

The main chromium source currently used is high-carbon ferrochrome, which is obtained by reducing chromite ore using a carbonaceous reducing agent such as coke and melting it to separate the gangue content.

The disadvantage of this method of producing high chromium steel via ferrochrome is that the cost of the chromium source is high.

That is, in order to reduce chromite ore to produce ferrochrome containing 50% or more chromium, electric power must be used as a heat source to supply the necessary reduction temperature and reduction energy, resulting in high energy costs.

Furthermore, since the Cr% of the molten metal is high, it is difficult to reduce the corresponding chromium content in the slag to below 4%, and it is usually difficult for the chromium yield during the production of ferrochrome to exceed 94%.

This low chromium yield is one of the reasons for the high cost.

Recently, as a method to improve this and produce high chromium steel at low cost, without going through ferrochrome, chromium ore with a low chromium grade or a mixture of chromium ore and iron ore is heated electrically like in a blast furnace. A method of decarburizing the iron by reducing it in a reduction furnace using a carbonaceous reducing agent such as coke to produce high chromium pig iron, which has a chromium content close to that of finished high chromium steel, is being considered.

The chromium content of commonly used high chromium steel is 30φ
Hereinafter, it is mainly 12 to 20%, but with such a ROM content, unlike the case of ferrochrome, reduction can be carried out without using electric power as a heating source.

Furthermore, the amount of chromium in the slag corresponding to the molten metal can be reduced and the chromium yield can be increased.

Therefore, there is a possibility of lowering the cost of the chromium source compared to the conventional method using ferrochrome.

On the other hand, the biggest problem with this method is that it is difficult to dephosphorize, and it is difficult to satisfy the requirements for high chromium steel.

That is, when a normally obtained mixture of iron ore and chromite ore is melted and reduced in a blast furnace using coke, almost all of the phosphorus brought in with the raw materials is reduced and enters the molten metal, and the phosphorus content of the molten metal is 0.04~ o, i%.

On the other hand, according to the composition standards for stainless steel, most stainless steels have a p:o of 0.040% or less, and in order to satisfy the standards, high chromium molten metal must be dephosphorized.

As is well known, hot metal or carbon steel can be easily dephosphorized by oxidative refining of the molten metal under molten slag with high basicity and high iron oxide content.

However, when the chromium content in the molten metal increases (Cr>
Section 4) Chromium, which is more easily oxidized than iron, is preferentially oxidized and consumed, and the chromium oxide content in the slag increases, which physically and chemically reduces the dephosphorizing ability of the slag. Phosphorus practically does not progress.

Therefore, unless a method for dephosphorization from high chromium pig to stainless steel is established, high chromium pig, which is an inexpensive source of chromium, cannot be used as a raw material for stainless steel.

Regarding the problem of phosphorus, an explanation will be added about the difference between the current stainless steel melting method and the method using high chromium pig iron, which is the subject of the present invention.

First, in the current stainless steel melting method, the phosphorus problem is solved as follows.

That is, in the electric furnace steelmaking process, scrap is used as an iron source, but compared to hot metal, it has a lower P content because it has already undergone a dephosphorization process.

Therefore, ordinary phosphorus content standards can be satisfied simply by melt-mixing this and ordinary ferrochrome.

In addition, in the converter steelmaking method, a chromium source is added after dephosphorizing hot metal, which is an iron source, so even if almost no dephosphorization can be performed after adding a chromium source, it can still meet the normal composition specifications. .

In this way, under the current method, the molten metal must be made under molten metal conditions that allow for dephosphorization through normal oxidative refining (i.e., chromium-free, chromium-free,
By performing sufficient dephosphorization even with only a small amount of phosphorus present (containing only a small amount), it is possible to meet component specifications.

On the other hand, when high chromium pig iron is produced from raw ore in one step as the object of the present invention, it is disadvantageous that there is no opportunity to dephosphorize the phosphorus derived from the iron source by oxidative refining. Therefore, it is necessary to carefully select raw materials with low phosphorus content (difficult to implement due to cost) or develop a method for dephosphorizing high chromium molten metal.

Methods for dephosphorizing high-chromium molten metal include the use of alkaline earth metals such as Ca and their fluorides, but this method Although it is difficult to carry out the process only by the melting method, it is difficult to carry out the process under these conditions (in both cases, the processing cost is extremely high, so it can only be applied to the production of small quantities of high-grade products, and the high chromium steel, especially stainless steel, which is the object of the present invention) It is not suitable as a method for melting in large quantities and at low cost.

The present invention was obtained as a result of various studies in view of the above circumstances, and is a method for reasonably producing high chromium steel by obtaining high chromium molten metal at a low cost and then making it possible to dephosphorize it.
A first step of reducing iron and chromium oxides with carbonaceous material such as coke to produce a molten metal with Cr/Fe<10, a second step of reducing the Si content of the molten metal, and a second step of reducing the molten metal in a non-oxidizing atmosphere. and a third step of dephosphorizing and desulfurizing by treating with a flux containing calcium carbide and an alkaline earth metal halide as main components, for example, a flux containing calcium carbide and calcium fluoride as main components.

This method is characterized by comprising a fourth step of separating the molten metal from the flux and then decarburizing it to a predetermined value.

Hereinafter, a detailed explanation will be given based on specific examples.

First, in step (2), iron and chromium oxides are reduced to produce an iron-chromium molten metal.

At this time, it is important to use an inexpensive reducing agent, inexpensive energy, and a furnace with high thermal efficiency, and a shaft furnace such as a blast furnace is suitable.

In the case of a blast furnace, coke is the main heat source and reducing agent, but it is also effective to supply the necessary energy by using injection of heavy oil or the like.

The ratio of iron and chromium in the molten metal can be adjusted arbitrarily depending on the type and composition of the raw material oxides, but if a reduction furnace that does not use electric heating is used, the Cr/Fe ratio will have to be adjusted in terms of both the required reduction temperature and energy. It needs to be 1.0 or less.

On the other hand, if the Cr/Fe ratio of the molten metal obtained in the first step is too small compared to that of the final product, high chromium steel (Cr/Fe<0, ■), ferrochrome etc.
Since a large amount of raw material with a high r/Fe ratio must be added, and the significance of the present invention is lost, the optimum value of the Cr/Fe ratio is 0.2 to 0.7.

When a carbonaceous reducing agent such as coke is used, the molten metal contains carbon close to a saturated value.

The saturated carbon concentration increases by 1 with the Cr/Fe ratio, and 4
- Becomes Section 7.

Also, more than 80% of the input phosphorus is reduced and enters the molten metal.

The main sources of phosphorus include iron ore, coke chromium ore, and solvents, which generally require an increased amount of coke than when producing hot metal. Caused by Rin-Inpla 11-1! To increase.

If part of the required amount of heat is supplemented by blowing low-phosphorus heavy oil as described above, the phosphorus content of the molten metal can be reduced accordingly.

When normally available raw materials are used, the phosphorus content of the resulting molten metal is about 0.04 to 0.1%.

In addition, less than half of the input sulfur is transferred to the molten metal, but because the sulfur input from fuel is large, the sulfur content is usually 0.03 to 0.05% in the molten metal.

The molten metal also contains a higher level of Si than normal hot metal.

That is, this is because the furnace temperature is higher than in the case of obtaining normal hot metal to reduce the chromium content, and the reduction of 8102 in the gangue content is promoted, and S i Ii is in the 1 to 4 ratio.

In order to obtain high chromium steel that satisfies the composition standards from molten metal containing a large amount of impurities, it is necessary to rationally dephosphorize,
Desulfurization, desiliconization, and decarburization are the tasks after the second step.

In the second step, first, the Si% of the molten metal is lowered as a preliminary treatment to enable dephosphorization.

Si in the molten metal increases the activity coefficient of C, and the higher the Si coefficient, the lower the saturated Cφ of the molten metal.

As mentioned above, in the first step, the reduction is carried out under conditions where a carbonaceous reducing agent such as coke is present in excess, so that the carbon content is approximately close to the saturation value.

In order to dephosphorize using calcium carbide in the third step as described later, it is necessary to make the molten metal carbon unsaturated.

The present invention does this by lowering the Si% of the molten metal.

That is, when Si% is lowered. Even if C%i* does not change, the degree of carbon unsaturation in the molten metal increases, allowing the reaction to proceed in the third step.

(1) S in the molten metal by reacting oxygen gas or oxides
i is removed as SiO2.

(2) Dilute Si by adding low-Si scrap or ferroalloy (Ni source, etc.).

There are two methods.

In (2), there is a limit to the number of additives due to the temperature of the molten metal, but it is possible to increase the degree of superheating of the molten metal obtained in the first step, use the reaction heat from the reaction in equation (1), or add It is possible to increase the amount of additives by heating the material before adding it.

It should be noted that the greater the degree of decrease in Si%, the more advantageous it is in the third step, but the relationship between the basic unit of calcium carbide and the silicon removal rate (more precisely, it means the rate of decrease in Si including dilution) One example is shown in Figure 1 (18% Cr).
It is desirable that the i ratio is 30 or more.

Note that in the second process, the original purpose is to reduce Si%, and it is not necessary to reduce C%, but if there is a decrease in C%, it will be more advantageous in the third process, so equipment If possible, decarburization may be performed in parallel.

However, if decarburization is to be carried out using the method (1) above, it is a prerequisite that exhaust gas treatment equipment is fully equipped.

When the Si% is reduced by the method (1), the third
It is necessary to separate the 8102% high slag from the melt before proceeding to the process.

Next, in the third step, the molten metal is heated with CaC2 in a non-oxidizing atmosphere.
, treated with a flux containing CaF2 as a main component to perform dephosphorization and desulfurization.

When CaC2 is reacted with a carbon-unsaturated molten metal, produces free metallic calcium, Continuing Dephosphorization occurs due to the reaction.

Ca F 2 is necessary to adjust the melting point of the flux and to make metallic calcium stably exist in the flux.

The blending ratio of CaC2 and CaF2 is CaC2/CaF
2-The range of 1.5 to 6.5 is optimal.

The remaining fraction (CaO, Si02 mixed as impurities)
yMgOS102y, etc., but it is desirable that the amount of oxides that are easily reduced by metallic calcium be as small as possible.

Note that when the degree of carbon unsaturation is low, replacing a portion of CaC2 with a calcium alloy such as Ca-8i has the effect of reducing the amount of flux required.

When a large amount of Ca-8i is added, the Si content of the molten metal increases, which has the opposite effect of reducing carbon solubility as described above.

In order to suppress this, it is effective to reduce the Si% in advance in the second step.

In addition, calcium alloy and Ca without using CaC2 at all
Although dephosphorization can be achieved by treating the molten metal with F2, in this case, most of the Ca in the calcium alloy reacts with C in the molten metal and changes to Ca C2.

Therefore, it is not economical not to use Ca C2 at all when treating a molten metal as the object of the present invention.

Next, in the fourth step, after separating the molten metal and the flux, decarburization and removal of Si remaining in the molten metal are performed by blowing acid.

The reason why the flux is separated before performing the decarburization reaction is to prevent the reverse reactions of formulas (1) and (2) from occurring due to oxidative refining.

In order to decarburize to a predetermined C%, any one or a combination of existing methods 1, such as top blowing under atmospheric pressure, side blowing of Ar-0□ mixed gas, and blowing acid under reduced pressure, may be used.

Example 1 Shaft-type reduction furnace I Cochrome iron ore, iron ore, coke, and CaCO3 were charged and preheated oxygen-enriched air (02:
42%) to produce high chromium pig iron.

The components of high chromium pig pig are as follows.

Next, this molten metal is desiliconized by blowing oxygen, and then ferronickel melted in an electric furnace is added.

The molten metal components at the end of this step are as follows.

After removing the slag, while sealing the atmosphere with Ar gas, 10 kg/t of calcium carbide and 3 kg/l of refined fluorite are added to the molten metal and bubbling is performed by blowing Ar.

The components of the molten metal after this treatment are as follows. After separating the molten metal and flux, the molten metal is first decarburized in a converter, and then final decarburized by the blowing acid method under vacuum to adjust its composition.

The composition of the molten metal before adding the alloy is as follows.

Example 2 The production of high chromium pig iron, desiliconization and addition of ferronickel were the same as in Example 1.

Next, after exhausting the slag, while sealing the atmosphere with Ar gas, add 6 kg/l of calcium carbide, 3 ky/l of refined fluorite, and 9/l of calcium removal side (25% Ca) to the molten metal, and then inject Ar into the molten metal. Bubbling is performed by

The components of the molten metal after this treatment are as follows.

By carrying out the present invention as described above, it is possible to rationally carry out dephosphorization, which is the biggest drawback of the process that goes through high chromium pig iron, which is a method of obtaining boiled high chromium molten metal from ore. The effect is significant from the perspective of obtaining high chromium steel at low cost.

Note that this method can also be applied when high chromium pig iron is produced in an electric furnace.

[Brief explanation of drawings]

The drawing is a diagram showing an example of the relationship between the silicon removal rate and the calcium carbide basic unit of high chromium pig iron.

Claims (1)

[Claims] 1. A first step of reducing iron and chromium oxides with a carbonaceous reducing agent such as coke to create a molten metal with Cr/Fe<1.0, and a second step of reducing the Si% of the molten metal. The molten metal is treated in a non-oxidizing atmosphere with a flux containing calcium carbide and alkaline earth metal halide as main components to dephosphorize and desulfurize, and after separating the molten metal from the flux, a predetermined process is performed. A method for producing high chromium steel, characterized by comprising a fourth step of decarburizing to a certain value.