JPS6160889B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing nickel-containing stainless steel at low cost. When melting nickel stainless steel,
Since the price of nickel as an alloy is high, it is important to select a source of nickel that is as cheap as possible. However, in this case, if the working costs (particularly energy costs) in the steelmaking process rise significantly, the overall objective of producing steel at a low cost cannot be achieved. Generally speaking, the cheaper the raw materials, the higher the steel manufacturing costs. Traditionally, nickel sources for stainless steel are
Ferron nickel (18 to 25% Ni), which is obtained by preferentially reducing oxide nickel ore by a dry method, has been mainly used. However, due to changes in the ore situation, i.e., the Ni/Fe ratio tends to decrease in oxide ores, and newly available nickel resources such as manganese nodule are generally used in stainless steel, such as Cu, Because it contains undesirable impurities, instead of the conventional preferential reduction method for producing ferronic acid, which has been the mainstream method, (i) a method of separating Ni and Fe.Cu by performing matte refining (dry method), and (ii) a wet method. An increasing number of methods are using methods to extract the nickel content through processing. In the former case, NiO can be obtained as an intermediate product by roasting the pine, but in the latter case, NiO can be obtained by precipitating the Ni content as carbonate and roasting it. In other words, due to resource trends, virgin nickel sources have replaced ferro-nickel, which has traditionally been the mainstream.
It can be said that there is a tendency for the number of products that go through the process of producing NiO as an intermediate product to increase. Previously, the intermediate product NiO was converted into nickel metal by reducing it with hydrogen or depositing it directly from the leachate by electrolysis. All of these reduction methods are characterized by the ability to obtain nickel with high purity, but the problem is that the cost required for reduction is high. In some cases, a method has been used in which NiO is added directly to the steelmaking furnace and reduced with C, Si, etc. in the molten metal, but in high chromium steel, only about 50% of the added amount is reduced by C. The remaining Cr in the molten metal must be oxidized, and finally, a silicon-based reducing agent such as ferrosilicon must be added to recover the chromium. Therefore, approximately
This means that 50% of the total amount is reduced by silicon. In this case, while the silicon-based reducing agent is an expensive energy source, it has the disadvantage of producing a large amount of Sio ₂ when oxidized, increasing the amount of slag. As mentioned above, in order to produce austenitic stainless steel at a low cost in response to the situation of nickel sources from a resource standpoint, NiO
It can be said that there is a desire for the development of a method for reducing and adding to molten steel with a high yield using an inexpensive method. On the other hand, in the manufacture of stainless steel, especially nickel-based stainless steel, the AOD continuous casting process is often adopted. From the perspective of energy saving, the difference between heat and
An important issue is how to recover the heat radiated from the AOD furnace bricks. In particular, in the case of stainless steel manufacturing, the time during which there is no molten steel in the furnace between heats is longer on average than in the case of converters for ordinary steel melting. Time changes significantly. In addition, basic high-grade refractories are used for the furnace bricks in order to withstand severe smelting.
At least 800℃ to prevent its spalling
It is necessary to maintain more than that. Normally, no special measures are taken to recover the radiated heat during periods when molten steel is not present. A method of preheating by charging cold material such as scrap can be considered, but re-oxidation is likely to occur in a high-temperature oxidizing atmosphere such as in an AOD furnace after tapping, and the heat is recovered as sensible heat. There is a problem in that the recovery rate is low if you do just that. The same applies to the ladle used to transfer the molten metal. In view of the above circumstances, the present invention combines the effective use of NiO with sensible heat recovery from AOD furnace bodies, ladles, etc., conducts various experiments for NiO reduction, and achieves extremely advantageous reduction of NiO. A method for producing nickel-based stainless steel has been developed, and its gist is that carbonaceous powder is added to NiO powder in a refractory-lined container after the molten steel is discharged. A method for melting nickel-based stainless steel, comprising the step of charging the mixed and molded material to reduce the NiO by 20% or more, and then adding molten steel containing 0.6% by weight or more of C. Hereinafter, a detailed explanation will be given along with specific examples. The NiO powder used is one obtained by a conventional method, such as a method of oxidizing and roasting nickel pine or a method of roasting nickel carbonate, or one obtained by pulverizing it. It is desirable to separate the NiO powder by particle size before proceeding to the steps described below. The reason for this is that the reduction behavior when heating a carbon-containing molded product (for example, a briquette) obtained by the process described below changes significantly depending on the particle size of NiO, as shown in Figure 1. This is to enable different usage depending on the conditions of use, such as. In FIG. 1, curves A and B have the following meanings. Particle size of NiO (particle size at which 90% by weight falls under the sieve)

[Table] Nio: Carbon content = 90:1 (weight ratio) In the present invention, carbonaceous powder is added to this NiO powder,
Mix. As the carbonaceous powder, pulverized coal, coke powder, graphite powder, etc. are used. In particular, wood graphite generated from hot metal in steel plants is
It has a low content of impurities such as phosphorus and sulfur, has good reactivity as described below, and a small amount of iron oxide powder that is inevitably mixed in during collection plays a role in accelerating the reaction as described below. , is a preferred reducing agent. For the last reason mentioned above, gas ash from blast furnaces (in which fine coke powder and iron oxide powder coexist) can also be used as a reducing agent. The particle size of the reducing agent is desirably 1 mm or less, with 80% or more of the particles being 0.4 mm or less. The reduction rate of NiO when heating the obtained molded product does not need to be 100%. However, as will be described later, in order to reduce unreduced NiO during melting by adding molten steel containing 0.6% or more C, the NiO reduction rate during heating needs to be 20% or more. In order to do this more efficiently, it is desirable to set it to 40% or more. In the reduction reaction of NiO with carbon, two reactions proceed in parallel: NiO+1/2C→Ni+1/2CO ₂ NiO+C→Ni+CO. In terms of equilibrium, if the reaction reaches up to CO ₂ , adding 8% C will balance it out, but in the present invention, as will be described later, reactions involving carbon and oxygen occur between the atmosphere and the powder mixture during heating. Since this occurs, it is not possible to determine the optimum amount of carbon in terms of mass balance. The optimum amount of carbon is rather determined by the reaction rate. Each temperature,
Figure 2 shows the results of a reduction experiment conducted with each amount of carbonaceous material added. In order to proceed with the reduction reaction within the time allowed for the process, the amount of carbon added should be 5% or more, preferably 8% or more of the NiO content. On the other hand, if the amount of carbonaceous material added is too large, the ratio of vacancies generated during reduction will be large, the strength of the reduction product will decrease, and Ni will easily scatter. This tendency becomes particularly noticeable when the amount of carbonaceous material added exceeds 20%, which is not preferable. It is desirable that it be 15% or less. Add 2 to 5% of water glass, molasses, tar, etc. as a binder to the mixture of NiO powder and carbonaceous reducing agent powder. A mixture of NiO and a binder facilitates handling, increases the reaction rate by bringing NiO into close contact with the carbonaceous powder, and increases the strength of the reduced product to prevent Ni from scattering. Therefore, the particle size of the molded product to be formed into pellets or briquettes is related to the time required to raise the temperature to a temperature at which the inside can be reduced. After discharging the molten steel, the carbon-containing NiO molded product made in this way is charged into a refractory-lined container to preheat and reduce the NiO, and then molten steel is added to remove the nickel content. It is dissolved in the molten steel and the unreduced NiO is reduced by the carbon in the molten steel. The containers used range from electric furnaces to finish decarburization furnaces (e.g. AOD
These include a ladle after transferring molten steel to a furnace), and a finishing decarburization furnace such as an AOD furnace after discharging molten steel after refining. Immediately after discharging the molten steel, the surface temperature of the refractories is high, over 1300°C, and the refractories are left empty for at least 10 minutes, or even several hours, before being charged with molten steel. They have something in common. The gist of the present invention is to efficiently recover and utilize the heat dissipated during this time, and to melt the nickel component into molten steel at a high yield. The amount of carbon-containing NiO moldings to be added to the ladle, AOD furnace, etc. depends on the operating pattern of stainless steel melting, the heat capacity (the amount of heat dissipated varies depending on this), and the steelmaking raw material used. There are various combinations depending on the situation, but for example, if the majority of virgin nickel is NiO, the following method is suitable. In other words, in an electric furnace, stainless steel scrap is used as a nickel source, and other chromium sources (high carbon ferrochrome, etc.) and iron sources are charged, and Ni from NiO enters the molten steel, which will be described later. Mix and dissolve to obtain the Fe:Cr:Ni ratio. Part of the carbon-containing NiO molded product is charged into the ladle and the rest into the AOD. If the container used is a ladle, charge the carbon-containing NiO moldings as soon as possible after transferring the molten steel to the AOD. If possible, it is desirable to place a lid on the ladle to reduce heat radiation and prevent outside air from entering. It is desirable to perform burner heating before the next steel receiving. In any case, the atmosphere inside the container is oxidizing in the conventional method, but in the method of the present invention, the molded product containing an appropriate amount of carbonaceous material is charged, so the atmosphere inside the container is oxidizing. can be kept reducible. I mentioned earlier that the NiO before forming is divided by grain size, but if it takes a long time to receive the next steel, the formed product obtained from NiO with a relatively large grain size may be
On the other hand, if the time until receiving steel is short, it is best to select and use a molded product made from NiO with a relatively small grain size in order to obtain a stable and high solid state reduction rate.
It is also preferable to promote sintering of the reduced product and to prevent nickel components from scattering when molten steel is added. Particle size of NiO powder (raw material for carbon-containing molded products) (90% by weight)
Particle size at the bottom of the sieve (expressed in dmm) and
The time from adding carbon-containing NiO moldings to receiving steel is t ₁
(min), it has been found empirically that in the case of a ladle, it is desirable that the following relationship holds between the two. d0.15√ ₁ (1) If these conditions are satisfied, more than 80% of the nickel content can be reduced just by charging it into a ladle. After a predetermined period of time, the charge is left as it is and receives molten steel. Unreduced NiO is reduced by the carbon in this molten steel. The C% of this molten steel has a large effect on the yield of molten steel for nickel (third
figure). When the C content of molten steel is 0.6% or more, a stable high yield can be obtained, but when the C content decreases below that level, the yield decreases. This is because as C becomes lower, the liquidus temperature of the molten steel becomes higher, and a portion of the molten steel that comes into contact with the nickel molded product solidifies, which prevents the nickel from dissolving into the molten steel. As a result, the Ni yield in the molten steel decreases because the amount of Ni that is scattered, rolled into the slag, or attached to the refractory wall increases. Therefore, molten steel C
must be 0.6% or more. Next, we will discuss the case where an AOD furnace is used as the container. In that case, the carbon-containing NiO molded product is charged into the furnace as soon as possible after the refining of the pre-heat is completed and the melt is discharged. The concept is the same as in the case of a ladle, but in the case of an AOD furnace, the particle size of NiO powder (raw material for carbon-containing molded products) (particle size when 90% by weight falls under the sieve: expressed as d (mm)) ) and the time t ₂ (min) from the addition of the carbon-containing NiO molding to the pot, it is desirable that the following relationship holds true. d0.20√ ₂ (2) The coefficient is different from that for the ladle because the pattern of temperature drop after the molten steel is discharged is different.
The AOD furnace is held as it is or tilted midway through to equalize the temperature distribution between the molded products and reduce NiO evenly. After a predetermined period of time has elapsed, steel is received, and unreduced NiO is reduced by carbon contained in the molten steel. Thereafter, oxygen gas is supplied to the molten steel under almost normal conditions for final decarburization. In this case, it was found that there is a relationship as shown in FIG. 4 between the C% of the receiving molten steel and the decarburization oxygen efficiency. Low decarburization oxygen efficiency means that a large amount of chromium is oxidized during final decarburization, and the reducing agent (silicon-based alloys such as Ca-Si, Fe-Si, etc.) required to reduce it. This is not desirable as it increases the amount of water used. The reason why the oxygen decarburization efficiency decreases when the C% of molten steel at the time of receiving the steel decreases is because the degree of preferential oxidation of carbon relative to chromium decreases in relation to temperature. From Figure 4, it is necessary that the C% of the molten steel received is 0.6% or more. As described above, the present invention selects appropriate amounts of carbonaceous solids and particle size of NiO in a carbon-containing NiO molded product, and uses a high-temperature container such as a ladle or AOD furnace after discharging molten steel. By performing preheating and solid phase reduction, and then adding molten steel that satisfies a predetermined C%, it is possible to achieve a high yield and dissolve Ni into the molten steel. Example Stainless steel scrap (18% Cr
-8%Ni steel ^35t , 17%Cr steel ^15t ), high carbon ferrochrome (Cr55%, C8%: ^6.7t ), ordinary steel scrap
16 ^t was charged and melted, Ni4.1%, Cr:18.5%,
C: 1.6% stainless steel crude molten steel was obtained. On the other hand, when NiO is sieved, particles with a particle size of 0.2 mm or less are 90%
The weight percentage was divided into three stages, with 0.5 mm or less being 90 weight % and 0.9 mm or less being 90 weight %. Add 12.5 coke powder (particle size 0.3 mm or less, carbon content 88% by weight) to each
g/Kg of NiO, and molasses 20g/Kg of NiO were mixed to form a briquette, which was then dried. After drying, the briquettes contained 89% NiO, 9.5% C, and _0.6 % SiO2, and each briquette weighed 250 to 300 g. Three minutes after the molten steel was transferred to the ladle (zircon material) used to transfer the crude molten stainless steel from the electric furnace to the AOD furnace in the previous heat, the carbonized NiO briquettes mentioned above (the particle size of the raw NiO powder 90 if the particle size is 0.5mm or less
The weight percentage is 0.7 _t , and the particle size is 0.9 mm or less is 90
0.5 _t ) was charged. The lid was placed and held for 90 minutes. Molten steel (described above) melted in an electric furnace was received in this ladle. The temperature of the molten steel before tapping is 1600℃, the temperature of the molten steel before transferring to the ARD furnace is 1550℃, the composition is C: 1.5%,
Ni was 5.1%, Cr was 18.3%, and Si was 0.2%. At the same time, after tapping the previous heat,
The aforementioned carbon-containing NiO briquette (raw material NiO
Powder with a particle size of 0.5 mm or less accounts for 90% by weight
2.1 ^t , with particle size of 0.2 mm or less accounting for 90% by weight
0.8% ^t ). Ten minutes after charging, the aforementioned ladle contents were added. Thereafter, Ar-O ₂ mixed gas was blown from the bottom to decarburize the carbon to 0.08%. The decarburization oxygen efficiency was 90%. Fe-Si and lime are added to this and reduction smelting is performed, C: 0.08% Cr: 8.0%,
Molten steel (1590°C) with Ni: 8.8% was obtained. As described above, the present invention efficiently recovers energy that would normally be dissipated by combining a NiO molded product containing an appropriate amount of carbonaceous solids and a high-temperature container after molten steel is tapped. It makes it possible to effectively use NiO for reduction and dissolve Ni into molten steel at a high yield, and is economical in that it reduces the cost of melting nickel-based stainless steel. Great effect.

[Brief explanation of the drawing]

Figure 1 shows the influence of NiO particle size on the reduction behavior of carbonized NiO briquettes, and Figure 2 shows the effects of the amount of carbon added (using coke powder), heating temperature, and Figure 3, which shows the influence of briquette size, shows the relationship between the C% of the charged molten metal and the Ni content in the molten steel when a ladle is used as a container for heating carbon-containing NiO briquettes. The figure shown in FIG. 4 is a diagram showing the relationship between the C% of the charged molten metal and the oxygen efficiency during the final decarburization stage when an AOD furnace is used as a container for heating carbon-containing NiO briquettes.

Claims (1)

[Claims] 1. After the molten steel has been discharged, carbonaceous powder is added to NiO powder, mixed, and molded into a container lined with refractory, and after reducing the NiO by 20% or more, C: A method for producing nickel-based stainless steel, the method comprising the step of adding molten steel containing 0.6% by weight or more.