JPH09157724A - Stainless steel manufacturing method using scrap - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide an economical and efficient melting method of stainless steel scrap having a high chromium yield using a top-bottom blowing converter. SOLUTION: Using an upper-bottom blowing converter, 5 to 30% of Cr is generated by heat generated when carbonaceous material is burned by oxygen gas.
In the method of melting the contained scrap, the amount of molten slag is 100 to 400 kg per melt t, and the particle size is 1 to
Using a carbon material of 10 mm, the fixed carbon content in the carbon material was present in the molten slag to be 50 to 150 kg per molten slag t, and the cavity depth LS generated on the upper surface of the molten slag by the top blowing gas. And ratio LS of molten slag thickness L
/ L is maintained at 0.2 to 0.5, and the ratio H / A of the distance H from the lower end of the upper blowing lance to the upper surface of the molten slag and the diameter A of the converter cross section is maintained at 0.1 to 0.7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for melting stainless steel scrap using a top-bottom blow converter.

[0002]

2. Description of the Related Art Conventionally, an electric furnace has been mostly used for producing stainless steel using stainless steel scrap. However, in the case of an electric furnace, a large amount of electric power is consumed for melting and refining scrap. However, it is not preferable in a country such as Japan, where the electricity price is extremely high, because it causes an increase in cost. Therefore, for ordinary steel, as a method of melting and refining scrap economically without relying on an electric furnace, the surplus production capacity of a converter with a high acid transfer capacity is used to melt scrap using inexpensive carbonaceous materials.・ Refining methods have come to be considered.

As such a method, generally, an existing upper-bottom-blown composite blowing converter is used to prevent an increase in equipment, and at the same time, the fire species charged in the furnace together with scrap is ignited. After that, a method of melting and refining while charging carbonaceous material as a heat source from above the furnace at the time of top-bottom blowing is proposed. On the other hand, in Japanese Unexamined Patent Publication (Kokai) No. 2-141511, coal having a molten slag of 350 kg or more per molten pig t and a sulfur content of 0.4% or more is used by using a reaction vessel capable of gas stirring the melt. As a result, a method for melting steel scrap is disclosed, in which the amount of free fixed carbon existing in the reaction vessel is maintained at 17 kg or more per molten slag t and the acid is sprayed upward.

However, all of these techniques are scrap melting in ordinary steel containing no Cr, and cannot be applied as they are to stainless steel scrap containing Cr, which is extremely susceptible to oxidation. On the other hand, as a melting technique for stainless steel scrap using an upper-bottom blown composite blowing converter, methods disclosed in JP-A 1-215912 and JP-A 1-2525753 have been proposed. However, these are methods of melting and reducing the temperature of the chromium ore after melting the scrap.Even if chromium is oxidized during the scrap melting period, it is easily reduced during the subsequent melting reduction period. It is not a condition for suppressed scrap melting.

Further, the method disclosed in Japanese Patent Laid-Open No. 3-271314 proposes a method for melting stainless steel scrap after specifying the conditions of the upper blowing lance. Oxidation cannot be suppressed sufficiently. When melting stainless steel scrap containing chromium, the difficulty is discussed as compared with ordinary steel as follows.

(1) When the iron bath containing Cr and oxygen gas are in direct contact with each other, Cr is easily oxidized and is transferred to the slag to cause a significant economic loss. In addition, the chromium oxide contained in the slag is less likely to be reduced than the iron oxide, and the chromium oxide-containing slag increases the viscosity of the slag due to its composition and hinders the fluidity, so the strength of the slag layer is increased. The heat transfer from the upper part of the slag to the molten metal bath existing thereunder due to stirring hardly occurs, and the heat deposition efficiency decreases.

Here, the heat deposition efficiency is a value defined by the following equation.

[0008]

(Equation 1)

(2) When a scrap containing Cr is hit with oxygen gas, a strong chromium oxide film having a melting point higher than the melting temperature of the molten metal is formed on the scrap surface because of good adhesion to solid iron. As a result, heat transfer from the slag bath to the solid scrap is hindered, and the scrap melting does not proceed. (3) When Cr is contained, the operating temperature becomes high because the melting point of the iron bath rises, and unless the heat deposition efficiency is extremely high,
Exhaust gas temperature exceeds 1700 ° C and refractory wear is unavoidable.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a method disclosed in
When the method for melting ordinary steel scrap disclosed in Japanese Patent No. 1415111 is used, when the iron bath containing Cr is in direct contact with oxygen gas, Cr is oxidized and lost, and the heat deposition efficiency is also reduced. Solves the problem that when the contained scrap is exposed to oxygen gas, a chromium oxide film is formed on the surface, and the subsequent scrap melting does not proceed,
An object is to provide an economical and efficient method for producing stainless steel using scrap.

[0011]

The present inventors suppress the oxidation of Cr by controlling the jet strength of oxygen blown upward by a large amount of slag, and control the distance between the tip of the lance and the slag surface to be short. It was found that by doing so, the secondary combustion rate can be increased without increasing the exhaust gas temperature. Further, it was found that the exhaust gas temperature can be further lowered by mixing an appropriate amount of chromium oxide with the slag and utilizing the endotherm due to the reduction reaction of the chromium oxide.

That is, the gist of the present invention is as follows. (1) In a method of melting scrap containing 5 to 30% of Cr by heat generated by combustion of carbonaceous material by oxygen gas using an upper and lower blowing converter, the molten slag weight is 100 kg or more and 400 kg per molten metal t. The fixed carbon content in the carbonaceous material in the molten slag is present within the range of 50 to 150 kg per molten slag t using the carbonaceous material having the following particle diameters of 1 to 10 mm, and further the slag is generated by the upper jet gas jet energy. The ratio LS / L of the maximum cavity depth LS generated on the upper surface and the average thickness L of the molten slag is 0.2 to 0.5, the distance H from the lower end of the upper blowing lance to the upper surface of the molten slag and the inner diameter A of the converter furnace. A method for producing stainless steel using scrap, which is carried out while maintaining a ratio H / A within a range of 0.1 to 0.7.

(2) The method for producing stainless steel using scrap as described in (1) above, wherein a substance containing 15% or more of chromium oxide is used in a range of 50 to 250 kg per t of scrap.

(3) 50 to 80% of the weight of the molten metal obtained by melting the scrap is discharged outside the furnace, and the molten metal and molten slag remaining in the furnace are additionally charged with scrap containing 5 to 30% of Cr. However, LS / L is 0.2 to 0.34 from the start of blowing to 5 to 30% of the total blowing acid time, and LS / L for the remaining period.
Is set to 0.35 to 0.5, and after the oxygen supply is stopped, the inert gas is blown into the molten metal for stirring only during the period of 5 to 15% of the total blowing acid time. A method for manufacturing stainless steel using the scrap according to (1) or (2) above.

Here, the slag is generated by the top-blown gas.
Cavity depth LS (mm) is the oxygen supply rate F
(Nm^Three/ Hr), nozzle diameter d (mm), lance and slurry
If the distance H (mm) between the surfaces and the number of nozzles n are
Calculated by the formula. LS = 7 × L_shX exp (-0.78 x H / L_sh) L_sh= 63 × (F / (n × d))^2/3 The distance H between the lance tip and the slag surface is
The applied specific gravity is 1g / cm ^ThreeIs calculated as. Ma
In addition, carbonaceous material mainly consists of carbon such as coke, anthracite, coal, etc.
The fixed carbon is the raw material for combustion, and the fixed carbon is the total coal in the carbonaceous material.
As the amount of carbon excluding volatile matter from the elementary amount
It is a defined value. In addition, those containing chromium oxide
Quality means the amount of dust generated during the refining of chromium ore and stainless steel.
Chromium oxide such as dust pellets made from strike 1
Indicates a substance containing 5% or more.

The present invention uses a top-and-bottom blowing converter to remove Cr from the heat generated when carbonaceous material is burned by oxygen gas.
Applied when melting scrap containing ~ 30%. Here, when Cr contained in the scrap is less than 5%, various problems due to oxidation of chromium do not occur, and thus the application of the present invention is meaningless. Also,
If the Cr content is more than 30%, the oxidation of chromium proceeds very easily, so that efficient scrap melting cannot be achieved by using the present invention.

Further, when operating the upper and lower blown converter,
Molten slag weight is 100kg or more per molten metal t 400kg
The fixed carbon content in the molten slag in the molten slag is set to 50% per molten slag t by using the following carbon material having a particle size of 1 to 10 mm.
The premise of being present in the range of ~ 150 kg is required. The slag has a role of absorbing the energy of the top-blown oxygen jet so as not to directly collide with the steel bath, and a role of not directly colliding oxygen with the granular iron or minute scrap pieces suspended in the slag by the bottom-blown gas. Have In particular, for the latter, secure a sufficient amount of slag and circulate granular iron and scrap pieces to the upper slag layer where oxygen gas has reached.
It is necessary not to move it. Therefore, the weight of the molten slag must be 100 kg or more and 400 kg or less per t of the molten metal. If it is less than 100 kg, the direct reaction between the top-blown oxygen and the steel bath, granular iron, or scrap pieces is unavoidable, and the efficiency decreases On the other hand, if the amount is more than 400 kg, it is difficult for the heat deposited in the slag layer to move to the steel bath, and the efficiency decreases.

Further, the amount of carbonaceous material contained in the slag and the particle size of the carbonaceous material are important. In the present invention, CO ₂ is supplied by top-blown oxygen.
The point is to increase the amount ratio of carbon to be completely burned up to 1), and as a result, the secondary combustion rate calculated by the following equation based on the CO ₂ concentration and the CO concentration in the exhaust gas becomes high. Secondary combustion rate = CO ₂ concentration / (CO ₂ concentration + CO concentration) ×
100 Here, when the fixed carbon content in the carbonaceous material in the molten slag is less than 50 kg per molten slag t, the slag foaming becomes intense and the operation becomes difficult.
If it is more than kg, CO ₂ produced by complete combustion
Since the probability that a solution loss reaction of CO with adjacent carbonaceous materials to form CO will occur increases, the secondary combustion rate decreases. Further, when the carbonaceous material particle size is smaller than 1 mm, scattering loss accompanies the gas flow, and when it is larger than 10 mm, the above amount cannot provide sufficient specific surface area to suppress forming.

Further, in order to prevent the top-blown oxygen from directly colliding with the granular iron and the minute scrap pieces suspended in the slag, the cavity depth LS and the slag thickness L generated in the slag by the top-blown gas. The ratio of LS / L is 0.2-
It should be 0.5. The reaction of granular iron or minute scrap pieces suspended in the slag with top-blown oxygen is a phenomenon that only causes a problem when melting stainless steel scrap. In other words, when it does not contain easily oxidizable Cr, even if the granular iron or minute scrap pieces suspended in the slag react with oxygen, only a small amount of a thin iron oxide film is formed on the surface, Since this iron oxide is immediately dissolved in the slag layer and reduced by the carbonaceous material in the slag, there is almost no adverse effect. However, when it contains Cr, which is easily oxidized, a large amount of thick and strong chromium oxide film is formed on the surface,
This chromium oxide film firmly adheres to the surface of granular iron and scrap pieces and does not dissolve easily into the slag layer.Because the heat transfer of the chromium oxide film is poor, the internal granular iron covered by the film is The temperature of scrap and scrap becomes difficult to rise. Therefore,
When such a state is formed, the efficiency is extremely reduced.

In order to prevent this, as described above,
It is necessary to set LS / L to 0.2 to 0.5. LS / L
Stipulates conditions under which the slag layer does not allow oxygen gas to contact not only the iron bath but also the chromium-containing granular iron and scrap pieces existing at the bottom of the slag layer, and it is a heat transfer medium due to the upward blowing jet. Since it is an index showing the frequency with which the carbonaceous material is caught in the slag, it is a guideline for controlling the chromium yield and heat deposition efficiency.

Here, when LS / L is larger than 0.5, oxygen reacts with granular iron or fine scrap pieces suspended in the slag, and as shown in FIG. Is reduced. If LS / L is smaller than 0.2,
Since the top blowing energy becomes small, the entrainment circulation of the carbonaceous material in the slag becomes insufficient, and the efficiency decreases. As described above, when LS / L is controlled to be small, although the secondary combustion rate is improved, if Cr is contained, the melting point of the iron bath rises, the operating temperature becomes high, and unless the heat generation efficiency is increased, the exhaust gas Since the temperature exceeds 1700 ° C, wear of refractory is unavoidable.

In order to suppress this and improve the heat deposition efficiency, the distance H from the tip of the upper blowing lance to the slag surface is H / A 0.1 to 0.7 with respect to the diameter A of the cross section of the converter. Will be required. H / A is an index for bringing the lance closer to the slag surface according to the scale of the device, and if the lance is sufficiently close to the slag surface, CO will be entrained in the oxygen jet and CO ₂ will not become a space combustion. The rise in exhaust gas temperature can be suppressed. Therefore, the heat deposition efficiency can be controlled by H / A. When H / A is larger than 0.7, the distance from the lance tip to the slag surface elongated, and the space combustion with CO is involved in the oxygen jet becomes the CO ₂ is likely to occur between them. If this space combustion occurs violently, the secondary combustion rate will increase, but the temperature of the exhaust gas will rise significantly compared to secondary combustion due to complete combustion of the carbonaceous material in the slag, and the refractory will be severely worn. Moreover, H / A is 0.
If it is less than 1, the distance from the tip of the lance to the slag surface is too short, and the lance is significantly melted by radiation.

Here, when operating in the above condition range, the carbonaceous material circulates mainly in the upper part of the slag layer, and the contact with the iron bath is suppressed to the minimum, so that the carbon concentration of the molten iron produced is 1
It becomes carbon unsaturated with about 2.5%. Therefore, it has a feature that the load of the subsequent decarburization step is reduced. In order to keep the heat deposition efficiency higher in the present invention, it is important to use a substance containing 15% or more of chromium oxide together with scrap in the range of 5 to 25% by weight of chromium oxide with respect to the weight of scrap. This utilizes an endothermic reaction when the chromium oxide is reduced in the slag, and the slag is cooled by this, so that an increase in exhaust gas temperature is suppressed as shown in FIG. . When the amount of chromium oxide is less than 50 kg per scrap t, the endothermic reaction amount at the time of reduction is small, so that there is no effect, and when it is more than 250 kg, the endothermic amount is too large and the slag temperature decreases. As a result, the harmful effect that the chromium oxide in the slag remains unreduced occurs.

In order to apply the present invention to a place where operations are repeated and carried out continuously, as shown in FIG. 3, 50 to 80% of the weight of the produced stainless steel molten iron is discharged from the furnace and left in the furnace. Scrap containing 5 to 30% of Cr in molten iron and slag, 50 to 8 of the weight of molten iron discharged from the stainless steel
Charge in the range of 0% by weight, and 5 hours after the start of propellant acid
Up to 30%, LS / L is 0.2 to 0.34, and the remaining period is LS / L is 0.35 to 0.5. An appropriate range was confirmed by experiments, in which an inert gas was blown into the molten metal over time to provide stirring.

In FIG. 3, step 1 is a part of the weight of the molten stainless steel produced in the precharge, and the step 2 is the effluent step.
Is a state in which the converter is erected upright after the discharge, and in the step 3, 5 to 3 of Cr is added.
In the state where the scrap containing 0% is charged, in step 4, LS / L is 0.2 to 0.3 at the time of 5 to 30% of the total blowing acid time.
Blowing in the state of 4, the LS / L of the remaining period of the process 5 is 0.3
Blowing in the state of 5 to 0.5, Step 6 is after the oxygen supply is completed,
The step of applying only stirring for 5 to 15% of the total blowing acid time is shown, and after step 6, the procedure returns to step 1. The slag is discharged before and after the step 1 as needed. Note that FIG.
, 1 is a top-bottom blowing converter, 2 is slag, 3 is carbonaceous material,
4 is molten iron, 5 is bottom blowing tuyere, 6 is scrap, 7 is top blowing lance, 8 is bottom blowing gas bubble, LS is cavity depth, L is slag thickness, H is distance from lance tip to slag surface Indicates.

Here, the reason for reducing LS / L in the first half of blowing is that the solid scrap charged in this period is mixed in the slag, so that the contact between oxygen gas and scrap pieces is minimized. It is to suppress, LS / L in the latter half of blowing
The reason for increasing is that the scrap temperature rises during this period and the slag is almost not mixed, so that the upward blowing jet is strengthened and the entrainment of the carbonaceous material is promoted. In addition, the reason why only stirring is applied for 5 to 15% of the total blowing acid time after the oxygen supply is completed is to reduce the chromium oxide formed in the blowing acid.

If the proportion of the generated molten iron in the molten iron is less than 50%, a large amount of molten iron remains at the start of the next charging operation, so that a sufficient amount of scrap is loaded in terms of the converter space volume. When the amount of molten iron remaining in the next charge is small, the molten iron temperature will drop below the solidus temperature after the scrap is charged because the amount of molten iron remaining in the next charge will be small if the furnace cannot be charged and productivity drops below 80%. .

If the period for which LS / L is 0.2 to 0.34 is less than 5% of the total blown acid time, LS / L is lowered in a state where a large amount of scrap is mixed with slag, so chromium When the period is longer than 30% of the total blowing acid time, there is a time when the entrainment of carbonaceous material becomes insufficient, and the exhaust gas temperature rises at this time.

After the end of oxygen supply, when the time for applying only stirring is shorter than 5% of the total blowing acid time, chromium oxide is not sufficiently reduced, and when it is longer than 15%, the temperature is lowered and the temperature is not practical. .

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION

[Example] In the example, an 8-ton top-bottom blowing converter was used. As the bottom blowing gas, a mixed gas of oxygen and a tuyere cooling gas was used, and oxygen gas was supplied from a top blowing lance. The top blowing lance was a multi-hole nozzle, and the number of nozzles, the radius, and the distance between the tip of the lance and the slag surface were changed. The amount of slag and the amount of fixed carbon in the slag were also changed.

In the experiment, when about 3 tons of hot metal, slag, and carbonaceous material were added, the carbonaceous material was continuously added while the oxygen was blown upward to raise the temperature, and when the temperature reached about 1450 ° C. And C
Scrap containing 11 to 18% of r was added, and carbon material was continuously added while the oxygen was blown upward to melt the scrap. The carbonaceous material was added in excess of the amount burned by top-blown oxygen so that the carbonaceous material was always present in the slag.
Coke and anthracite were used as the carbonaceous material.

In the examples of Table 1, SV is the amount of slag per metal t (kg / ton-metal), D is the particle size of carbonaceous material (mm), PC is the secondary combustion rate (%), and E is the heat deposition efficiency. (%), WC is the fixed carbon amount in the carbonaceous material per slag t (kg
/ Ton-slag), Y indicates a chromium yield (%).
Table 2 was carried out by adding chromium ore under the condition of test No. 10 in Table 1. CR is the amount of chromium oxide added per scrap t (kg / ton-scrap).

Table 3 is an experiment in the process shown in FIG. 3, and is based on the condition of test No. 1 in Table 1. here,
RW is the ratio (%) of the weight of the generated stainless steel molten iron that was discharged from the furnace, and TA is the LS / L after the scrap is charged.
Is 0.25 to 0.3 and the ratio (%) to the total blowing acid time is LS / L of 0.4 to 0.45 for the remaining period.
And Further, TB represents the ratio (%) of the time during which only stirring was applied after the end of oxygen supply to the total blowing acid time.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

Industrial Applicability According to the present invention, it is possible to produce stainless steel using scrap, which has a high chromium yield and is economical and efficient.

[Brief description of the drawings]

FIG. 1 is a diagram showing experimental results on the relationship between LS / L, chromium yield, and exhaust gas temperature.

FIG. 2 is a diagram showing an experimental result of a relationship between a chromium oxide addition amount, an exhaust gas temperature, and an iron bath temperature.

FIG. 3 is a schematic view showing an embodiment when the present invention is applied to a place of continuous operation.

[Explanation of symbols]

 1 Top-bottom blowing converter 2 Slag 3 Carbon material 4 Molten iron 5 Bottom-blowing tuyere 6 Scrap 7 Top-blowing lance 8 Bottom-blowing gas bubble LS Cavity depth L Slag thickness H Distance from lance tip to slag surface

Claims (3)

[Claims] 1. A method of melting scrap containing 5 to 30% of Cr by heat generated by combustion of carbonaceous material by oxygen gas using an upper-bottom blowing converter, wherein the weight of molten slag is 100 kg per t of molten metal. The fixed carbon content in the molten slag is 50 to 150 kg per molten slag t, using the carbonaceous material having a particle size of 1 to 10 mm and 400 kg or less.
The maximum depth L of the cavity generated on the upper surface of the slag by the energy of the jet gas blown upward.
S and molten slag average thickness L ratio LS / L is 0.2-0.
5. Distance H from the lower end of the upper blowing lance to the upper surface of the molten slag
And a method of manufacturing a stainless steel using scrap, which is performed while maintaining the ratio H / A of the converter inner diameter A in the range of 0.1 to 0.7. 2. The method for producing stainless steel using scrap according to claim 1, wherein a substance containing 15% or more of chromium oxide is used in a range of 50 to 250 kg per t of scrap. 3. The weight of molten metal obtained by melting scrap is 5
0 to 80% is discharged outside the furnace, 5 to 30% of the total blowing acid time from the start of blowing is added by additionally charging the molten metal and molten slag remaining in the furnace with scrap containing 5 to 30% of Cr. Until LS / L is 0.2 to 0.34, LS / L in the remaining period is 0.35 to 0.5, and after the oxygen supply is stopped, 5 to 15% of the total blowing acid time is 2. An inert gas is blown into the molten metal for agitation only during the period.
Alternatively, a method for producing stainless steel using the scrap according to 2.