JP3151653B2 - Special cast steel for sinter cake support stand - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a special cast steel for a sinter cake support stand used in a sinter cake support sintering method for producing sinter used in a blast furnace or the like.

[0002]

2. Description of the Related Art FIG. 4 shows a conventional sinter production process.
The ore as the main raw material for sinter production is from the yard to the ore hopper 1.
7, the limestone as an auxiliary material is carried to a limestone hopper 16, and coke as fuel is carried to a coke hopper 15. The ore, limestone and coke are cut out from hoppers 17, 16 and 15, and are conditioned and granulated by a mixer 18 together with the returned ore cut out from a return hopper 14 to become a sintering compound material 8. The sintering compound material 8 is conveyed to a surge hopper 9,
After being stored, it is cut out from the drum feeder 10 and charged into the sintering pallet 7 through the chute 11,
The sintering raw material layer 12 is formed. In the case of sintering using iron ore as a main raw material, the thickness of the sintering raw material layer is usually about 600 mm. Then, the coke on the surface of the sintering raw material layer 12 is ignited by the combustion heat of the gas in the ignition furnace 13, and the coke is burned while sucking air downward, and the raw material is sequentially fired from the upper layer to the lower layer by the combustion heat. Tie.

[0003] According to such a conventional sintering method, the lower layer of the sintering raw material layer is compressed by receiving the weight of an integrated sintered mass (hereinafter referred to as a sinter cake) of a sintered pallet after sintering. Increase density. As a result, the ventilation becomes poor, the ventilation becomes non-uniform, the burning speed of the coke becomes slow, and the combustion becomes uneven. As a result, there is a problem that the sintering speed is reduced and the yield, that is, the yield of the sintered mineral product having a diameter of 5 mm or more is reduced, so that the productivity is reduced.

There are various methods for solving the decrease in productivity. Japanese Unexamined Patent Publication No. 2-293586 discloses a technique for improving the productivity by eliminating the load on the sinter cake and improving the ventilation of the sintered layer. A sinter cake support sintering method with the provided stand is described. A sintering pallet provided with a plate-shaped support stand used in the sinter cake support sintering method is described in Japanese Patent Application Laid-Open No. 4-168234.

In this sinter cake support sintering method, when sintering proceeds and the sinter cake is formed up to the height of the support stand, the sinter cake is supported by the upper end of the support stand, and a load is applied to the lower layer. Disappears. Therefore, no load is applied to the lower sintering raw material layer, and the ventilation of the sintering layer is improved.

There are various shapes of stands used in the sinter cake supporting sintering method. In the case of the plate described in JP-A-4-168234, the temperature rise near the surface of the support stand is non-uniform due to heat removal from the surface of the support stand and excessive ventilation at the boundary between the support stand and the sintering raw material layer. However, there is a problem that the yield is lowered and the productivity is lowered.
No. 775 describes a lattice-shaped stand. In this lattice stand, the sinter cake is integrated including the lattice stand, and when the sinter cake is discharged at the discharge part of the sintering machine, the lattice stand and the sinter cake are not separated, and the sinter cake is retained. The sintering pallet hits the crushing guide, the sintering machine stops overloading,
There is a problem that the operation rate of the sintering machine is reduced (hereinafter, the ease of separation of the sintering cake from the sintering pallet including the stand of the sintering cake in the sintering unit) is referred to as the sintering cake discharging property. Further, in order to sinter the sintering raw material layer in the sintering machine pallet, a sintering combustion zone having a maximum temperature of 1350 to 1400 ° C.
Since it descends from the upper part to the lower part at a speed of ３０30 mm / min, a temperature difference of 500 to 700 ° C. is generated in the height direction of the stand, and a temperature difference is generated between the upper part and the lower part of the stand during cooling. It repeats every 70 minutes. The sinter cake support stand installed in the sintered layer is subjected to the thermal cycle repeatedly, and thermal strain is accumulated, cracks are generated by the thermal strain, and the cracks are propagated and partially broken. In addition, the operating atmosphere is an oxidizing and sulfurizing atmosphere, and is severely corroded. Therefore, the thickness of the sinter cake support stand is significantly reduced, the life is shortened, the frequency of replacement is increased, the operation rate is reduced, and the equipment cost is significantly increased. There's a problem.

[0007]

In order to withstand the above operating conditions, the sinter cake support stand material needs to have the following properties. First, it undergoes a heat cycle of about 1,100 ° C. high-temperature heat-air cooling, about several tens of times a day, so that it has thermal fatigue resistance. Since a negative pressure load is applied for suction of air, it has strength at high temperatures.
Since it is a high-temperature oxidation and sulfurization atmosphere, it has oxidation resistance and sulfidation corrosion resistance.

In such a use environment, Japanese Patent Laid-Open No.
As described in Japanese Patent No. 24588 and the like, conventionally, it has higher heat fatigue resistance, higher oxidation resistance and higher sulfurization corrosion resistance than austenitic heat-resistant steel which is a general heat-resistant steel,
A two-phase cast steel of ferrite + austenite has been used, which is somewhat inferior to the austenitic heat-resistant steel, but has a high-temperature strength of practically no problem. When the strength at high temperature is not satisfied by the ferrite + austenite two-phase cast steel, austenitic cast steel may be used. These heat-resistant cast steel and lattice-shaped stands crack from the sixth day after the start of use,
When the sinter cake is discharged from the sintering machine at the sintering machine, about 45% of the sinter cake becomes a 200mm square shaped piece in 20 days due to some external force, for example, the impact force of the sinter cake. I do. These sharp pieces are caught by a crusher or belt conveyor chute installed in the line after the sintering line, causing the crusher to overload or tearing the rubber belt of the belt conveyor, causing trouble in the sintering line. Pause. About 55% remaining
The stand also falls into a reserve or missing arm, and since it is in a high-temperature sulfurizing atmosphere, S in the atmosphere reacts with Ni contained in the heat-resistant steel to form a low-melting-point compound and accelerate corrosion. Due to the numerous troubles and the troubles that are expected to occur in the future, the operation rate of the sintering machine is extremely reduced, causing a serious production trouble, and there is a problem that the rest including the remaining stands must be removed. In addition, the short life of the stand significantly increases the cost of manufacturing and replacing the stand.

Japanese Patent Application Laid-Open No. 6-129775 describes that such a crack is prevented from being generated by the shape of the stand. However, there is a limit to improving the shape alone.
In addition, since high-temperature corrosion is not improved, material improvement is essential.

Accordingly, the present invention provides a stand material which solves such a problem in a method for supporting and sintering a sinter cake of sinter.

[0011]

The present invention is a material for a sinter cake support stand to be placed on a sintering pallet of a downward suction type sintering machine, wherein C is 0.10 to 0.40 wt.
%, Mn 0.10 to 1.50 wt%, Si 0.20%
2.00 wt%, Cr 11.00-15.00 wt%
%, And 0.05 to 1.00 wt% of Al,
And at least one or more rare earth elements in an amount of 0.005 to 0.50 wt% in the total amount of the rare earth elements, and the balance being Fe
A special cast steel for a sinter cake support stand, comprising a base material of ferrite and unavoidable impurities.

[0012]

SUMMARY OF THE INVENTION The present invention provides a material for a sinter cake support stand having thermal fatigue resistance and corrosion resistance in order to solve the above-mentioned problems.

First, in order to improve the thermal fatigue resistance, it is necessary to increase the thermal conductivity of the material and to reduce the thermal stress by lowering the coefficient of linear expansion. In order to increase the thermal conductivity and lower the coefficient of linear expansion than the phase cast steel, the material is a ferrite single metal.

Second, in order to improve corrosion resistance, it is necessary to form a protective film on the surface of the material to prevent the progress of corrosion.
Therefore, generally, Cr and Si are added to the material, and Cr ₂
By forming O ₃ , SiO _{2, and the} like as a protective oxide film on the metal surface, the oxidation inside the metal and the progress of sulfide corrosion are prevented. However, since the operating atmosphere of the sinter cake support sintering method is much higher than the atmosphere in which general heat-resistant cast steel is used, oxidation and sulfidation corrosion are accelerated, and the thermal cycling causes a protection cycle. The oxide film easily peels off, and further accelerates oxidation and sulfidation corrosion. Therefore, by increasing the amount of added Cr and adding Al, a strong protective oxide film made of Cr ₂ O ₃ and Al ₂ O ₃ is formed on the metal surface, and oxidation and sulfidation corrosion at high temperatures are reduced. . Al is one of the elements having the most excellent effect on the resistance to sulfidation corrosion. In order to prevent the protective oxide film from peeling off due to a thermal cycle, the protective oxide film is strongly adhered to the base metal by adding a rare earth element such as La, Ce, etc., and furthermore, it is resistant to oxidation and sulfidation. Improves corrosiveness.

That is, the special cast steel for a sinter cake support stand of the present invention comprises a rare earth element and Al for improving the oxidation resistance and the sulfidation corrosion resistance of a general 13Cr heat-resistant cast steel of a ferrite base containing Si and Mn. It has been added.

In the present invention, the reasons for limiting the components as described above will be described.

C: an element effective for strengthening the base material by forming a solid solution in the base material and for forming carbides to increase the high-temperature strength. To maintain high temperature strength, at least 0.
It is necessary to contain 1 wt%. The higher the content, the better. However, if the content is too high, the amount of secondary carbide deposited due to aging during use at a high temperature becomes excessive, resulting in a decrease in ductility after aging. Therefore, the upper limit is made 0.4 wt%.

Si: a deoxidizing element when the alloy is melted, and has an effect of increasing the fluidity of the molten metal in the casting process. It also improves oxidation resistance at high temperatures. These effects improve in proportion to the amount of addition, but if added too much, ductility is reduced, so the content is set to 2 wt% or less. On the other hand, if Si is extremely low, baking tends to occur after a thermal cycle and ductility is reduced. Therefore, it is necessary to be 0.2 wt% or more.

Cr: an element that enhances high-temperature strength and oxidation resistance. In order to maintain oxidation resistance at high temperatures, the content is set to 11% by weight or more. If the content is too large, the ductility is reduced.

Mn: an element that has a deoxidizing effect, fixes S, and renders it harmless. Although it is necessary to add 0.1 wt% or more, the effect is sufficiently obtained by adding up to 1.5 wt%, and the effect is little increased even if the amount is further increased. Therefore, the content is set to 1.5 wt% or less.

Al: An element effective for forming a protective film on the surface of the alloy and preventing the progress of oxidation to improve oxidation resistance. If the content is too large, oxides are formed and the fluidity of the molten metal in the casting process is impaired.
wt% or less.

Rare earth elements: rare earth elements such as La and Ce;
In particular, Ce strongly fixes an oxide film which greatly contributes to oxidation resistance to a base metal, and further improves oxidation resistance. If the amount of the rare earth element is too large, a large amount of the intermetallic compound is formed, the ductility is reduced, and the cost is increased. Therefore, the total rare earth element amount is set to 0.005 wt% or more and 0.5 wt% or less.

Five types of materials, three sides of one side cut type, trapezoidal shape and trapezoidal shape with slit, 200 mm height sinter cake supporting cast steel stand were prototyped, mounted on a pallet of an actual sintering machine, and tested. FIG. 3 shows the relationship between the service period and the maximum crack length. What is the maximum crack length here?
The thickness reduction length in the height direction when measured several times during the use period (the length of the upper side reduced by 200 mm from the bottom) and
It is the total length of the crack length from the position where the thickness has been reduced (the upper side where the height has decreased) to the lower side. Table 1 shows the main material chemical component values (% by weight) of the various prototype stands, and the linear expansion coefficient of physical properties (1 / ° C, room temperature to 982 ° C) and thermal conductivity (cal / cm · sec · ° C, 982 ° C.) is shown in Table 2.

[0024]

[Table 1] Note) In addition to the above components, general impurities such as S and P are also included.

[0025]

[Table 2]

Material A is a conventional two-phase cast steel with a base material of about 75% austenite + about 25% ferrite.
The base materials of the materials B and C are austenitic single cast steel, and the base materials of the materials D and E are a ferritic single cast steel. FIG.
As shown in Table 2, the crack growth rate (crack length / use period) of the material A was large from the beginning, the maximum crack length reaching almost saturation was long, and the variation in the crack growth rate and the maximum crack length was also large. On the other hand, the material B and the material C were smaller in the crack growth rate and the maximum crack length than the material A, and their dispersion was smaller. Material D and material E are material A, material B,
Compared to Material C, both the crack growth rate and the maximum crack length were significantly smaller. This is because ferrite has a small coefficient of linear expansion and a large thermal conductivity compared to austenite having a large coefficient of linear expansion and a small thermal conductivity in terms of material properties. It is presumed that the generated thermal stress of E is small, and it is difficult to cause a defect even after repeated thermal cycles.

The life of material A, material B and material C is limited by the cracks. The crack length is increased in a short time with respect to the height of the stand, and a plurality of cracks are generated. And the crack generation interval is also different. In addition, some time after the start of use, a plurality of cracks are generated from the left and right sides of the sinter cake support stand, and the cracks grow. Therefore, a vertical crack (a crack from the upper side to the lower side) and a horizontal crack (a crack from the side to the inside) are connected, or some external force, for example, the impact of a sinter cake acts on a portion where the material strength is insufficient due to crack propagation. The broken pieces are easily caught by the crushing machine and the belt conveyor chute installed in the line after the sintering line, causing the crushing machine to overload and tearing the rubber belt of the belt conveyor, causing sintering. Because it lowers the operation rate of the line,
It is not suitable as a material for a sinter cake support stand.

On the other hand, the material D and the material E are rate-decreasing, the crack initiation time is late, the number of cracks is about one or two, the crack growth rate is very low, and the material tends to saturate with a short crack length. There is. It has been found from EPMA results that the main factor of the wall thinning is oxidation and sulfide corrosion of Fe. When the amount of wall thinning of the stand after use for 4 months was measured, material D was about 8 mm, but material E hardly lost its thickness. From these results, it was found that the material E, which does not cause loss of thickness due to loss, oxidation, or sulfidation even under such operating conditions, is suitable as a material for the sinter cake support stand.

As a material for a stand used in the sinter cake supporting sintering method, material A has a life of 0.5 months, material D has a life of 6 months or more, and material E has a life of 1 year or more. Life is long. Since the material D has a lower Cr content than the material E and does not contain Al and rare earth elements,
Inferior in oxidation resistance and short life, but it cannot be used as a stand material. Extending the life of the stand significantly reduces production costs, installation and removal costs, and improves the operating rate of the sintering line. Note that the stand life criterion in FIG. 3 was determined to be a risk of breakage or chipping when a crack propagated to about 1/3 of the stand height 200 mm, that is, up to 70 mm above.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment will be specifically described below with reference to FIGS. The sintering step was the same as the conventional method shown in FIG. 4, and the layer thickness was 600 mm.

[0031]

Example 1 A plate having a trapezoidal shape, a stand height 1 from the great surface was 200 mm, and the material was the material E of the single ferritic cast steel, that is, the chemical component value (% by weight) of the material was C. = 0.3, Si = 1.5, Mn = 0.5, Cr
= 13, Ni = 0, Al = 0.2, La = 0.02, C
e = 0.05, Fe = remaining (including impurities such as S and P)
The sinter cake support stand A is shown in FIG.

The sinter cake support stand A can of course enjoy the effect of improving the productivity as in the conventional case. In addition, the use of material E can extend the stand life from 0.5 months of conventional material A to more than one year, so that the production and replacement costs can be reduced to about 1/24 of those of conventional materials, and operating costs can be reduced. Greatly contributed to. On the other hand, the effect of increased production was reduced to about 0.3%, while the operating rate was reduced by about 8% in the past due to the reduction of replacement frequency, so the production of sinter increased by about 7.7%. .

[0033]

Embodiment 2 A trapezoidal plate is formed, the stand height 1 from the great surface is 200 mm, and the upper part has a vertical slit 3 divided from the upper side to the lower side of the support stand B. Material E similar to Example 1, that is, the chemical component value (% by weight) of the material was C = 0.3, Si = 1.5,
Mn = 0.5, Cr = 13, Ni = 0, Al = 0.2,
La = 0.02, Ce = 0.05, Fe = remainder (S, P
FIG. 2 shows a sinter cake support stand B (including impurities such as impurities).

With the sinter cake support stand B, the effect of improving productivity can be enjoyed as in the conventional case. In addition, the use of the material E can extend the stand life from 0.5 months of the conventional material A to one year or more as in the case of the first embodiment. This has contributed to a reduction in operating costs. On the other hand, the increase in production is due to a reduction in the frequency of replacement, resulting in an operating rate of about 8%.
Since it was reduced, it could be reduced to about 0.3%, and the production of sinter increased by about 7.7%.

[0035]

According to the present invention, the service life of the stand can be extended, the equipment cost can be reduced, and the stand replacement of the sintering machine can be reduced. Further, the production time of the sinter can be increased by shortening the stand replacement suspension time.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an example of a sinter cake support stand.

FIG. 2 is a perspective view showing an example of a sinter cake support stand.

FIG. 3 is a diagram showing a relationship between a use period of a sinter cake support stand and a maximum crack length.

FIG. 4 is a view showing a conventional process for producing a sintered ore.

[Explanation of symbols]

 Reference Signs List A Sinter cake support stand B Sinter cake support stand 1 Stand height 2 Stand upper side length 3 Vertical slit 4 Mounting part 5 Lower mounting part 7 Sintering pallet 8 Sintering compound raw material 9 Surge hopper 10 Drum feeder 11 Chute 12 Sintering Raw material layer 13 Ignition furnace 14 Return hopper 15 Coke hopper 16 Limestone hopper 17 Ore hopper 18 Mixer

Continued on the front page (72) Inventor Tadahiro Inazumi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Masami Fujimoto 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel (72) Inventor Yasuji Tanabe 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Headquarters (72) Inventor Minoru Arai 1-12- Kitahorie, Nishi-ku, Osaka-shi 19 Inside Kurimoto Ironworks Co., Ltd. (72) Inventor Susumu Matsuno 1-12-1-19 Kitahorie, Nishi-ku, Osaka City Inside Kurimoto Ironworks Co., Ltd. (56) References JP-A-50-13214 (JP, A) JP-A Sho57 -57861 (JP, A) JP-A-6-129775 (JP, A) JP-A-4-168234 (JP, A) Patent 81775 (JP, C2) (58) Fields investigated (Int. Cl. ⁷ , DB Name) C22C 38/00-38/60 F27B 21/08 C22B 1/20

Claims (1)

(57) [Claims] 1. A material for a sinter cake support stand arranged on a sintering pallet of a downward suction type sintering machine, comprising:
0.10 to 0.40 wt%, Mn 0.10 to 1.5
0 wt%, Si 0.20 to 2.00 wt%, Cr 1
1.00 to 15.00 wt%, Al is 0.05 to 1.00 wt%, and at least one rare earth element is 0.005 to 0.005 wt% in the total amount of rare earth elements.
A special cast steel for a sinter cake support stand, comprising 50 wt%, the balance being Fe and unavoidable impurities, and the base material being ferrite.