JP3444255B2 - Cast article and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cast product of excellent quality in which cracks do not occur under the epidermis and a method for producing the same.

[0002]

2. Description of the Related Art Materials for high temperature equipment such as reaction tubes for reforming furnaces for hydrogen production and reaction tubes for cracking furnaces for ethylene plants are 7
Since it is used at a high temperature of 00 to 1150 ° C, excellent high temperature strength, particularly excellent creep rupture strength is required. Furthermore, since it is processed into a thin thin tube or a long tube, excellent workability and weldability are required for the materials for these high temperature equipment materials.

Many heat-resistant steels and heat-resistant alloys (hereinafter, both are collectively referred to as heat-resistant materials) having a chemical composition satisfying these workability and weldability have been proposed. The basic chemical composition and the content of each element are% by mass,
C: 0.10% to 0.30%, Cr: 20% to 35%, and Ni: 20% to 50%. By containing a large amount of Cr and Ni, the metallographic structure of these heat-resistant materials becomes an austenite structure excellent in high-temperature strength, and has oxidation resistance and carburization resistance. Further, by containing an appropriate amount of C, these heat-resistant materials can obtain excellent tensile strength and creep rupture strength.

The manufacturing method of these materials for high temperature devices is generally as follows. That is, first, the melting raw material is melted using a melting facility such as an electric furnace or a high-frequency induction furnace, and the molten metal is adjusted to a predetermined component range, and then the molten metal is poured into a ladle. After that, if necessary, a refining equipment such as an argon oxygen decarburization equipment (AOD) or a vacuum decarburization equipment (VOD) is used to adjust the content rates of components such as C, S, Cr and N.

The molten metal after smelting is cast into a stationary mold to form an ingot. Further, it is cast in a mold that rotates at high speed to form a hollow cylindrical tube or the like. After the defects such as wrinkles, double skin, and cracks on the surface are removed, the ingot is heated to a predetermined temperature, slab-rolled or forged to be a material for hot working. After that, these materials, for example,
The product is obtained by hot working by extrusion and further heat treatment. A hollow cylindrical tube produced by centrifugal casting has its inner and outer surfaces directly finished by a lathe or the like to obtain a product. If necessary, this cast product is hot-rolled, processed into an intermediate material having a predetermined size, and then heat-treated to obtain a product.

When a heat-resistant material having the above-mentioned basic chemical composition is melted to obtain an ingot or a hollow cylindrical tube, cracks are liable to occur under the epidermis (the inside of several mm or more from the surface). In order to remove the cracks, it may be necessary to scrape the surface of the ingot, the hollow cylindrical pipe, or the like by 10 to 50 mm, which deteriorates the production efficiency and lowers the yield.

If small cracks that cannot be completely removed remain in the ingots, the cracks expand when the ingots are slab-rolled or forged. Therefore, the slabbing and the like may be stopped midway, the surface of the ingot may be cared for, and the slabbing and the like may be performed again by heating. Therefore, the production efficiency is low and the yield of products is low. In the case of hollow cylindrical pipes, etc., small cracks that could not be removed by scraping remain in the product, and during use at high temperatures, corrosion starts from these cracks and eventually large cracks and perforations occur. There is a case.

The cause of such cracking is that a large amount of large eutectic carbide is precipitated in the process of cooling the ingot or the hollow cylindrical tube immediately after being cast. That is, due to the stress acting on the surface of the ingot or the cast product during cooling, cracking occurs with these eutectic carbides as the starting points. Further, when hot working is performed using such an ingot or a hollow cylindrical tube as a material, cracking occurs with the eutectic carbide as a starting point.

Japanese Unexamined Patent Publication (Kokai) No. 53-52232 discloses a method for suppressing the occurrence of cracks when hot working an ingot.
In the process of cooling the ingot after casting the ingot, it has been proposed that the average cooling rate from the liquidus temperature to 700 ° C. be 30 ° C./min or more. This is a method of suppressing cracks by finely dispersing the eutectic carbide that causes the cracks.

In the example of Japanese Patent Laid-Open No. 53-52232, a small test piece is cast, and a tensile test and a hot rolling test are simply performed. In the case of a small test piece, it is easy to set the average cooling rate from the liquidus temperature to 700 ° C. to 30 ° C./minute or more in the cooling process of the test piece after casting. However, in a large-scale ingot on an industrial scale, it is difficult to uniformly set the average cooling rate from the liquidus temperature to 700 ° C. to 30 ° C./minute or more from the skin to the central portion. In other words, the cooling rate becomes slower in the portion that is inside a few mm from the surface of the ingot.
It is difficult to achieve a cooling rate of ° C / min or more. Therefore, a large amount of large eutectic carbide is likely to be deposited in a portion within several mm from the surface of the ingot, and cracking is likely to occur.

[0011]

The present invention is suitable for use as a material for high-temperature equipment such as a reaction tube for a cracking furnace of an ethylene plant, which requires high-temperature strength, and is a casting of excellent quality with no cracks under the epidermis. An object of the present invention is to provide a product and a manufacturing method thereof.

[0012]

The gist of the present invention resides in a casting product shown in the following (1) and a manufacturing method of the casting product shown in the following (2) and (3). (1) C: 0.10 to 0.30% by mass%, Si:
0.2-2.5%, Mn: 0.2-2%, Cr: 20-
35%, Ni: 20-50%, Ti: 0.2-1.5
%, Al: 0.005-0.5%, the sum of Si and Al:
0.205-2.0%, containing N: less than 0.01%,
The balance is a cast product consisting of Fe and impurities. (2) When producing the casting according to (1) above, the molten steel in the ladle before casting is denitrified, and the molten steel in the ladle is cast at a superheat degree of 60 to 100 ° C. Casting product manufacturing method. (3) When manufacturing the cast product according to (1) above, N
A method for producing a cast product, in which a melting raw material having a content of less than 0.01% is used, and the molten steel in the ladle is cast at a superheat of 60 to 100 ° C.

The cast product of the present invention means an ingot cast in a stationary mold or a hollow cylindrical tube cast in a mold rotating at a high speed.

The present inventors have solved the above-mentioned problems of the present invention as follows. In the following description, heat-resistant steel will be representatively described.

From the prescribed chemical composition, thermodynamic analysis was performed using a computer to obtain a phase diagram. From this state diagram, the limit C mass% at which eutectic carbide precipitates can be determined. The following findings were obtained by utilizing these analysis methods.

FIG. 1 is a mass% of Cr: 25%, Ni:
25%, Si: 0.4%, Ti: 0.4%, Al: 0.
The phase diagram created by computer analysis is shown for steel containing 4% and N: 0.02%, C content up to 1.0%, and the balance being Fe and impurities. C
It can be seen that the eutectic carbide is precipitated when the content is 0.14 mass% or more. That is, it was found that the limit C mass% was 0.14 mass%.

Next, the results of a computer analysis of the effect of the compositional components on the critical C mass% at which eutectic carbides are generated, based on the steel having Cr and Ni contents of 25 mass% respectively, are shown in FIG. As shown in FIG.

In FIG. 2, mass% is Cr: 25%, Ni:
Si, M containing 25%, with the balance Fe and impurities affecting the critical C mass% at which eutectic carbide of steel precipitates
It is a figure which shows the influence of n, Ti, and Al.

In FIG. 3, mass% is Cr: 25%, Ni:
It is a figure which shows the influence of N which contains 25% and Ti: 0.5%, and the balance is Fe and the critical C mass% which precipitates the eutectic carbide of the steel which is an impurity.

From FIG. 2 and FIG. 3, it is possible to increase the limit C mass% at which the eutectic carbide precipitates by increasing the Ti content and the Si and Al contents, that is, the eutectic It was found that the carbide was hard to precipitate. Further, it was found that when Ti was contained, the eutectic carbide was less likely to precipitate by further reducing the N content.

Based on the above findings, the invented casting (1) was invented. In the cast product of the present invention, the precipitation of eutectic carbide is suppressed by increasing the Ti content and decreasing the total content of Si and Al and the N content.

In order to produce the above-mentioned steels and alloys having a particularly low nitrogen content chemical composition, the method according to the invention either denitrifies the molten metal in the ladle before casting, or
Alternatively, a melting raw material having an N content of less than 0.01% is used. In any case, the degree of superheat of the molten metal in the ladle is 60 to 1
Cast at 00 ° C. The degree of superheat of the molten metal in the ladle is the difference between the temperature of the molten metal (higher than the melting point) and the melting point of the molten metal. By such a method of the present invention, it is possible to obtain a cast product which is free from cracks on the surface and has good cleanliness and excellent quality.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The cast product of the present invention and the method for producing the cast product will be described in detail below. In addition,
The percentage display of the content of each component means mass%. (1) Chemical composition C: 0.10 to 0.30% C improves tensile strength and creep rupture strength. For that purpose, 0.10% or more is contained. But 0.3
If it exceeds 0%, undissolved carbides remain in the solution-treated state, so that the creep rupture strength decreases. Therefore, the C content is 0.10 to 0.30%.

Si: 0.2 to 2.5% Si is added during refining for the purpose of deoxidizing steels and alloys, and also for improving the oxidation resistance at high temperatures. However, if the Si content is less than 0.2%, those effects cannot be exhibited. Further, if it exceeds 2.5%, the workability and the toughness of the heat-affected zone during welding deteriorate. Therefore, the Si content is set to 0.2 to 2.5%. Especially,
When carburization resistance is required, the Si content is 1.3 to
1.8% is desirable.

Mn: 0.2-2% Mn is added during refining for the purpose of deoxidizing steel and alloys, and has the effect of stabilizing the austenite structure, so it is added as an alternative element to expensive Ni. To be done. It is also added to improve the oxidation resistance at high temperatures. However, if the Mn content is less than 0.2%, those effects cannot be exhibited. Further, if it exceeds 2%, the oxidation resistance at high temperature is deteriorated. Therefore, the Mn content is 0.2
~ 2%.

Cr: 20 to 35% Cr is added to improve high temperature strength and oxidation resistance. For that purpose, 20% or more is contained. However, if it exceeds 35%, the workability is deteriorated and a complete austenite structure cannot be stably obtained. Therefore, the Cr content is 20 to 35%.

Ni: 20-50% Ni is contained in order to obtain a stable austenite structure depending on the amount of Cr. Further, it is added in order to improve the oxidation resistance and carburization resistance at high temperatures. To do that, 2
Contains 0% or more. However, if it exceeds 50%, the effect of carburization resistance is saturated. Therefore, the Ni content is 20
-50%.

Ti: 0.2 to 1.5% Ti is added to suppress coarsening of crystal grains and improve creep rupture strength. Further, it is added for the purpose of suppressing the precipitation of eutectic carbide. For that, 0.2%
The above is contained. However, if it exceeds 1.5%, workability and weldability deteriorate. Therefore, the Ti content is 0.
2 to 1.5%. A more desirable range is 0.4 to
0.8%.

Al: 0.005-0.5% Al is added during refining for the purpose of deoxidizing steel and alloys, and also for improving carburization resistance at high temperatures. However, if the Al content is less than 0.005%, those effects cannot be exhibited. Further, if it exceeds 0.5%, the ductility decreases. Therefore, the Al content is 0.
005 to 0.5%. Particularly, when carburization resistance is required, the Al content is preferably 0.005 to 0.03%.

Total content of Si and Al: 0.20
5 to 2.0% Si is added to improve oxidation resistance at high temperatures, and Al is added to improve carburization resistance at high temperatures. The total content of Si and Al is 0.205%
If it is less than the above, these effects cannot be exhibited. On the other hand, since Si and Al are both elements that easily precipitate eutectic carbides, the total content of Si and Al is 2.0%.
When it exceeds, eutectic carbides are significantly precipitated.

N: less than 0.01% N easily enters from a melting raw material or the atmosphere. However, since N is an element that easily precipitates eutectic carbides, it is desirable that N be as small as possible. Therefore, in order to suppress the precipitation of eutectic carbides, the N content is set to less than 0.01%. (2) Manufacturing method In manufacturing a cast product having the chemical composition defined in the present invention, the method of the present invention is to perform denitrification treatment of the molten metal, or
Alternatively, a melting raw material having an N content of less than 0.01% is used. In either case, the degree of superheat of the molten metal in the ladle is 60-
Cast at 100 ° C. Below, the manufacturing method of a cast product is demonstrated.

First, the N content of the cast product is 0.01% by mass.
A method of denitrifying the molten metal so that the amount becomes less than the above will be described.

The melting raw material is melted in a melting facility such as an electric furnace or a high frequency induction furnace to adjust Cr, Ni and the like to have a predetermined chemical composition, and then the molten metal is discharged into a ladle. At this time, it is not necessary to use a melting raw material having a low N content. That is, the molten metal discharged in the ladle is then treated using a refining equipment having a denitrification function such as a vacuum decarburization equipment (VOD). At that time, the content ratios of the components such as C, S, and Cr are adjusted, and the chemical composition of the molten metal is adjusted within a predetermined chemical composition range.

The molten metal thus obtained is subjected to denitrification treatment so that the N content of the cast product is less than 0.01% by mass.
At that time, it is desirable to perform denitrification so that the N content in the molten metal is less than 0.007 mass%. The reason is that N in the air is mixed in during the process of casting the molten metal after completion of the melting into a cast ingot or a cast product, and the N content is slightly increased. The amount of increase varies depending on the device, but is about 0.003% by mass. Therefore, it is preferable to grasp this increase amount in advance and set the N content rate in the molten metal at the completion of the melting.

Next, the reason for using a melting raw material having an N content of less than 0.01% will be explained. When manufacturing using a vacuum induction melting furnace and equipment that can perform casting in the same container under vacuum, the influence of the atmosphere on the molten metal can be eliminated, so N in the atmosphere is mixed and the N content rate Does not increase, and a molten metal having excellent cleanliness can be obtained. However, in that case, the effect of denitrifying the molten metal can hardly be expected. Therefore, a melting raw material having an N content of less than 0.01% is used.

When the molten metal obtained by denitrifying the molten metal or using a melting raw material having an N content of less than 0.01% is cast into a casting, the degree of superheat of the molten metal in the ladle Is cast at 60 to 100 ° C. When the degree of superheat of the molten metal exceeds 100 ° C., thermal stress easily causes cracks on the surface of the ingot or the cast product. If the temperature is lower than 60 ° C, oxides and the like in the molten metal do not easily float and solidify as it is, resulting in an ingot or cast product having poor cleanability.

[0037]

Example 1 Using a vacuum induction melting apparatus, steels having various chemical compositions were melted, and the molten steel was cast in a mold by a top pouring method under vacuum in the same container containing the melting apparatus. Then 5
0 kg ingots were produced, one for each test. The size of the ingot is an octagon having a horizontal cross section of 200 mm on a side and a height of 2000 mm. The degree of superheat of the molten steel in the ladle at the time of casting was set to 70 to 80 ° C., which is within the range of the conditions specified by the method of the present invention in any test.

Upon dissolution, the test No. of the example of the present invention was used. 1 to N
o. In No. 10, a melting raw material having an N content of less than 0.01 mass% within the range of the conditions specified in the present invention, that is, scrap of steel having an N content of less than 0.01 mass%, and alloy iron (FeCr , FeNi, FeMn, FeSi, etc.)
Was used. Moreover, the test No. of the comparative example. 11-No. 18
Then, a melting raw material having an N content of less than 0.01% by mass was not particularly used.

After cutting and removing to a depth of 5 mm from the surface of the obtained ingot by a lathe, the presence or absence of cracks on the surface of the ingot was visually examined. Further, from the middle position of the height of the steel ingot, a cut was taken with a drill blade having a diameter of 3 mm, and the content rate of each element was analyzed. Furthermore, when it is confirmed by visual inspection that cracks have occurred on the surface of the ingot, a square with a side of 20 mm is cut out from the surface of the ingot to a position at a depth of 10 mm. The cross section was etched by the oxalic acid electrolysis method, and the solidified structure and cracks were observed with an optical microscope at a magnification of 35 times. Furthermore, in the sample in which the crack is generated, SEM-EDX observation (10
(00 times), the precipitate near the cracked portion was observed. Table 1
Shows the chemical composition and the occurrence of cracks on the surface of the ingot.

[0040]

[Table 1] Test No. 1-No. In 10, all the chemical compositions were within the range of the chemical composition defined in the present invention. Further, as a result of visual inspection, no crack was generated on the surface of the ingot.

Test No. 11-No. In No. 18, the N content deviates from the upper limit of the conditions specified in the present invention,
It exceeded 0.01% by mass. This is because a melting raw material having an N content of less than 0.01 mass% was not particularly used. Furthermore, No. 11-No. In some of 18 tests, at least Ti, or the total content of Si and Al,
It was manufactured so as to be out of the range defined by the present invention.

Test No. 11-No. It was visually confirmed that cracks were generated on the surface of each of the 18 ingots. In particular, large cracks occurred in steel ingots having a high N content or ingots in which the content of two or more elements such as N and Ti were out of the conditions specified in the present invention.

Test No. As a result of cutting a sample from the surface of the steel ingot 12 and observing it under an optical microscope under the above-mentioned conditions, cracks were generated along columnar crystals, and it was confirmed that cracks were generated during casting. In addition, SEM-ED near the crack
As a result of X observation, it was confirmed that the precipitate was Cr carbide. (Example 2) Test No. of the present invention example. 19-No.
In No. 27, the molten raw material was melted in an electric furnace to roughly adjust the contents of Cr, Ni, etc., and then 40 ton of molten steel was tapped in the ladle. Then, the ladle was moved to a vacuum ladle degassing device (VOD) to perform decarburization treatment and component adjustment, as well as denitrification treatment, so that the N content was less than 0.01%. The ladle was moved from the vacuum ladle degasser, and molten steel was cast into the mold by the bottom pouring method in the atmosphere.

Test No. of the comparative example. 28-No. In No. 35, the melting raw material was melted in an electric furnace to roughly adjust the contents of Cr, Ni, etc., and then 40 ton of molten steel was tapped into the ladle. Then, the molten steel in the ladle was transferred to an argon oxygen decarburization device (AOD) for decarburization treatment and component adjustment. After that, the molten steel was tapped into a ladle, and the molten steel was cast into a mold by the down pouring method in the atmosphere.

The size of each ingot is 48 in horizontal section.
It is a 0 mm square and has a height of 2000 mm. The superheat degree of the molten steel in the ladle at the time of casting is the test No. 19-N
o. In No. 27, the temperature was set to 69 to 91 ° C., which is within the range of the conditions defined by the method of the present invention. In addition, the test No. 28-No. Three
In No. 5, in some tests, the temperature was set to 110 to 128 ° C., which exceeds the upper limit specified by the method of the present invention.

When molten steel is cast in the mold, Ar gas is caused to flow through the connection between the lower nozzle of the ladle and the injection pipe of the mold, and the mold is filled with Ar gas in advance so that the N in the atmosphere is reduced. The molten steel to be cast was prevented from being mixed in as much as possible. As a result, the increase in the N content during casting was reduced to about 0.
The amount could be suppressed to 001% by mass.

About 12 ingots were obtained in each test. After scraping with a grinder to a depth of 3 mm from the surface of these ingots, the presence or absence of cracks on the surface of the ingots was visually inspected. In addition, from the middle position of the height of the ingot, the diameter 3
Cuttings were taken with a mm drill blade to analyze the content of each element. Further, when it was confirmed visually that cracks had occurred on the surface of the steel ingot, the observation was carried out by an optical microscope and SEM-EDX by the same method as in Example 1. Table 2 shows the chemical composition, the occurrence of cracks on the surface of the ingot, and the total yield from molten steel to the material for hot working.

[0048]

[Table 2] Test No. 19-No. In No. 27, as a result of visual observation, no crack was generated on the surface of the ingot. Then, these ingots were charged into a heating furnace, heated to 1150 ° C., and then forged to a diameter of 180 mm to obtain a hot working material. It was visually confirmed that the surface of the hot working material did not crack. Therefore, since it is not necessary to care for the surface of the ingot or the material for hot working, the production efficiency is good, and the total yield from molten steel to the material for hot working is 78 to 85%, which is a good yield. It was

Test No. 28-No. In No. 35, since the molten steel in the ladle was transferred to the argon oxygen decarburizing device in the atmosphere, and in the argon oxygen decarburizing device, the denitrifying effect was small, the N content rate was defined in the present invention. The amount exceeds the upper limit of 0.01% by mass. further,
In some tests, at least Ti or Si and Al
Was melted so that the total content of the above was out of the range defined by the present invention.

Test No. 28-No. 35, length 1
It was visually observable that large cracks of over 0 mm and over 100 mm were generated. In particular, a large crack was generated in an ingot having a large N content or in an ingot having a content of two or more elements such as N and Ti that is out of the conditions specified in the present invention. Test No. As a result of cutting out a sample from the surface of the ingot 29 and observing it under an optical microscope under the above-mentioned conditions, cracks were generated along columnar crystals, and it was confirmed that cracks were generated during casting. As a result of SEM-EDX observation in the vicinity of the cracked portion, it was confirmed that the precipitate was Cr carbide.

Test No. 28-No. After removing the cracks on the surface of the ingot obtained in No. 35 by cutting with a grinder, heating to 1150 ° C. in a heating furnace and one side 300 to 400 mm
When forging up to, a large crack was generated on the surface, and it became impossible to continue forging. Then, after once cooling to room temperature, the cracked portion was removed with a grinder, and heating and forging were performed again to obtain a hot working material having a diameter of 180 mm. In this way, since the surface of the ingot and the material for hot working was cared for, the production efficiency was poor, and the total yield from the molten steel to the material for hot working was 48-65%, which was a bad yield.

[0052]

The cast product of the present invention is a cast product suitable as a material for a reaction tube for a cracking furnace of an ethylene plant which requires high-temperature strength and the like. When manufacturing these castings,
By applying the method of the present invention, it is possible to prevent the deterioration of production efficiency and the reduction of yield, and further it is possible to obtain a cast product of excellent quality with no cracks under the epidermis.

[Brief description of drawings]

FIG. 1 is a mass% of C: 0 to 1.0%, Cr: 25%,
Ni: 25%, Si: 0.4%, Ti: 0.4%, A
For steel containing l: 0.4%, N: 0.02%,
The state diagram created by computer analysis is shown.

FIG. 2 is a diagram showing the effect of Si, Mn, Ti and Al on the critical C mass% at which eutectic carbides of steel containing Cr: 25% and Ni: 25% in mass% are precipitated.

FIG. 3: Mass%, Cr: 25%, Ni: 25%, T
It is a figure which shows the influence of N on the limit C mass% which the eutectic carbide of the steel containing i: 0.5% precipitates.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-264641 (JP, A) JP-A-6-211054 (JP, A) JP-A-60-211029 (JP, A) JP-A-5- 93240 (JP, A) JP 57-127554 (JP, A) JP 57-127506 (JP, A) JP 6-306527 (JP, A) JP 9-143534 (JP, A) JP-A-10-96038 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 1/00-49/14

Claims (3)

(57) [Claims] 1. C: 0.10 to 0.30% by mass% and S
i: 0.2 to 2.5%, Mn: 0.2 to 2%, Cr: 2
0 to 35%, Ni: 20 to 50%, Ti: 0.2 to 1.
5%, Al: 0.005 to 0.5%, the sum of Si and Al: 0.205 to 2.0%, N: less than 0.01%, the balance being Fe and impurities And cast products. 2. When manufacturing the cast product according to claim 1, the molten steel in the ladle before casting is denitrified, and
A method for producing a cast product, which comprises casting the molten steel in a ladle at a superheat of 60 to 100 ° C. 3. When manufacturing the cast product according to claim 1, a melting raw material having an N content of less than 0.01% is used, and the superheat degree of the molten steel in the ladle is set to 60 to 100 ° C. A method for producing a cast product, which comprises casting.