JPS5873751A - Carburization resistant and heat resistant cast steel - Google Patents

Abstract

PURPOSE:To obtain a heat resistant cast steel with superior carburization resistance, high creep strength and ductility by adding specified percentages of C, Si Mn, Ni, Cr, Al, Zr, Nb and Ti to Fe. CONSTITUTION:An alloy consisting of, by weight, 0.1-0.25% C, 0.5-2.0% Si, 0.5-2.0% Mn, 32-42% Ni, 24-28% Cr, 0.2-0.8% Al, 0.1-0.5% Zr, at least one of 1.2-3.0% Nb and 0.5-1.5% Ti, and the balance Fe with ordinary impurities is prepared. This alloy is a heat resistant cast steel having superior carburization resistance and undergoing little embrittlement of carbide and little change in the strength after long-time heating. Even if the alloy is carburized, carburization rupture and brittle rupture in a low temp. range are hardly caused.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat-resistant cast steel that has excellent carburization resistance, high creep strength and ductility, and is difficult to cause carburized fracture or brittle fracture at low temperatures even if carburized. be.

Materials used in reaction tubes for thermally decomposing hydrocarbons, such as in ethylene production equipment, are being exposed to increasingly high temperatures due to demands for larger equipment and improved yields.
On the inner wall of the pipe, carburization occurs due to carbon and monooxylic acid carbon produced during pyrolysis, resulting in a decrease in creep strength and ductility in low-temperature regions, and a decrease in the density or thermal expansion coefficient of carburized and non-carburized areas. There was a drawback that internal stress was generated due to the difference in temperature, resulting in carburization failure and shortening the life of the reaction tube.

Therefore, in recent large-scale plants, the conventional HK4
H and P alloys (250r-55Ni cast steel) with higher creep strength than 0 alloy (250r-20Ni cast steel)
Furthermore, HP gold alloy improved Mo, W, Nb,
Ti, C! Alloys containing u etc. have been proposed (
Special Publication No. 5.3-32328, Special Publication No. 54-”2436
6.Special Publication Showa 5'1-56"10.Special Publication Showa 49-2j4
53. Special Publication No. 54-35172, Special Publication No. 47-3733
(See Publication No. 0, etc.)

However, in recent ethylene reaction tubes.

Carburization occurs even in 25 Or -35 Ni alloys such as H'P, and carburization fractures or brittle fractures may occur at relatively low temperatures during plant startup and shutdown. In addition, as a measure to prevent carburization, HP is the basic method and Si is increased, and
517), MgIzr, ca (JP-A-49-62516
), An (JP-A-49-62515) and other trace elements have been proposed. These measures are effective in making carburization difficult to occur with high C alloys in order to obtain precipitation strengthening of carbides, but once carburization occurs, or in order to increase creep strength, The contained C aggregates and coarsens as carbide when heated for a long time, so when heated for a long time, it causes so-called carbide embrittlement that reduces strength and ductility, and a high amount of C causes carburization fracture. This actually had a negative effect, and the above-mentioned conventional proposals did not take measures against such carbide embrittlement.

On the other hand, in other fields where large thermal stress is applied, such as collectors for ammonia or methanol production equipment, excellent ductility is required as well as creep strength.
Low O1 low Or heat resistant alloys such as Japanese Patent Publication No. 54-11248 and Japanese Patent Publication No. 54-144817, Incoloy 8...00
Are these ○ alloys applied? The withstand temperature is 900℃.
℃ or less, and cannot be used in environments where K exceeds 1050℃, such as in ethylene reaction tubes, due to its strength, carburization resistance, and oxidation resistance.

Recent ethylene cracking furnaces are increasingly required to be larger and have higher yields, as mentioned above, from the standpoint of resource and energy conservation, so the surface temperature of the reaction tube is between 1050 and 107°C.
The temperature is significantly higher than 0°C or 1100°C, creating extremely severe conditions for carburizing environment and material embrittlement.

Therefore, the present inventors conducted analysis and experiments on current examples of carburization and carburization failure, and as a result, the following became clear.

(1) The damaged reaction tube has creep strength and room temperature 1
11 The ductility decrease is large in the vicinity, and carburization has not occurred.Ill,
1 The carbide embrittlement is severe even in the darkest parts.

(2) The elongation value of a damaged material with severe carburization and carbide embrittlement at around room temperature (approximately 500°C or less) is 3% or less.

(3) Conventional alloys have a carbon content of around 0.4 inches, so
When exposed to high temperatures close to 1100°C, carbide embrittlement is severe, and precipitation strengthening of carbides actually promotes embrittlement and does not contribute to maintaining strength.

(4) In conventional alloys containing around 0.4% C, eutectic carbides precipitate, and as the eutectic carbides become brittle, they oxidize and develop into cracks. It becomes the starting point for crack propagation and carburization. This tendency is greater in alloys with lower C solid solubility limit and higher N1.

Therefore, in order to prevent such carburization failure, it is necessary to reduce the grain boundary oxidation of carbides, creep strength, It is necessary to suppress the decrease in ductility.

In view of the above points, the present invention has excellent carburization resistance, little carbide embrittlement and strength change after long-term heating, and even if temporary carburization occurs, carburization fracture and brittle fracture at low temperatures occur. This was done with the aim of providing heat-resistant cast steel that is difficult to heat.

That is, in the present invention, (1) the amount of C is reduced to 010 to 025% in order to reduce carbide embrittlement and strength changes due to long-term heating, and to avoid grain boundary oxidation of carbides, which is the starting point of fracture; Fix C by and ■Al
1=, improves carburization resistance through the interaction of Zr, and its composition is O: [1,10~025chi (-Shigetoshi 1 and below), Si: n, 5~2.0%, Mn: 0 .5-2
．． 0%, Ni: 32-42%, Cr: 24-28%,
AA: 0.2-08%, Zr: 010-05, and Nb: 1.2-3.0%, Tj: 0.5-. 1
．． It consists of 5% of at least one element, the balance Fe and normal impurities.

The reasons for limiting the chemical composition of the present invention are as follows.

C#'j: This is an element that is a feature of the present invention, and is preferably lower in order to suppress carbide embrittlement after long-term operation. A small amount of carbide is necessary to prevent a decrease in weldability and strength due to grain coarsening, and the lower limit is 0 or 1, where carbide precipitates as NbC or TlC when coexisting with Nb or T1. shall be. The upper limit is that as the amount of C increases, the high temperature strength on the short-term side increases, but as the embrittlement tendency increases to 1, even if Ml) or T1 is added, carbides precipitate in a series, resulting in carbide formation. Since grain boundary oxidation occurs, it is set at 0.25%, which is the range in which embrittlement tends to occur and carbides are fragmented.

Sl is commonly used as a deoxidizing agent.

It usually contains 0.5 or more. However, if it exceeds 2 inches, the deoxidizing effect will be saturated and the susceptibility to hot cracking during welding will increase, so α is set in the range of 5 to 20 inches.

Mn also has the same effect as Sl, but if the content is less than 0.5%, the effect is insufficient. Oxidation resistance: 4. There is a tendency to make li7, so n, 5e
s~2. The range shall be 0%.

Or coexists with N1 and is an element that maintains oxidation resistance, carburization resistance, and high-temperature equipment as an austenitic structure, and in order to increase the serviceable temperature, it is better to have a large content. The alloy of the present invention is used for ethylene reaction tubes, etc. with VC1050~110'O1? :
Targeting materials exposed to high temperatures, 1050~
In order to maintain sufficient oxidation resistance and heat resistance at 1100℃, a minimum of 24 strands is required, and if the oxidation resistance increases, IJi
The interaction with +AzlZr destabilizes austenite, causing a decrease in strength and embrittlement of the material, so the upper limit is set at 28 cm, which allows the stability of the structure to be maintained.

Coexisting with Or, Ni maintains the austenitic structure and effectively affects heat resistance, oxidation resistance, and carburization resistance. In addition, heat-resistant alloys can obtain high strength by maintaining a stable austenite structure, and the present invention has the highest austenite structure.

Since there are few element C, which is elementary, to make it stable, the subordinate:,:
1.・ *-=*x! 'nmsu・t−x<taz, *is 66・
Therefore, within the scope of the present invention, the lower limit is an element that makes austenite unstable, that is, Si.

Or * A-# + Zr, It) or T1 amount is at the upper limit, a stable austenitic structure is obtained, the minimum value is 32%, and in terms of carburization resistance, N140 ~ 4
Since the effect is saturated at 2%, the upper limit is set at 42chi.

Nb or T1 is one of the characteristic components of the present invention, and C, Or
By interacting with the amount, it prevents embrittlement after long-term operation, splits carbide even when heated or carburized for a long time, prevents grain boundary oxidation of carbide, and increases creep strength. In general heat-resistant steel, Nt) or T1 coexists with C and precipitates fine Nl) carbide or T1 carbide in austenite to improve creep strength.In austenitic stainless steel, carbide stabilizes. It is added as an element, but in the present invention, it combines with C, suppresses carbide embrittlement due to Or carbide after long-term operation, suppresses grain coarsening peculiar to low C heat-resistant steel, and improves strength. Prevent decline. In addition, in response to the intrusion of C due to carburization, it precipitates as Ml) carbide or 11 carbide at the grain boundaries, which increases the creep strength. If it is less than .5 inch, the effect of fixing carbides is observed, but the effect of improving the creep strength after long-term heating or carburization is small. In addition, when the Nb content is high, the effect of improving creep strength after long-term heating is large;
Since it deteriorates oxidation resistance, the upper limit is set at 50%, and N
When using Ti instead of b0, as with Nb, if the content is large, the effect of improving creep strength after long-term heating will be greater, but it is usually difficult to incorporate Ti effectively by melting. The upper limit is 1st or 5th place.

AA, together with Zr, is an element that significantly improves the carburization resistance of the present invention, and is solid-solved in austenite, not only slowing down the diffusion of carbon but also forming an extremely stable and highly adhesive oxide layer just below the alloy surface. Therefore, Or on the outermost surface of the steel
Even if the oxide mainly composed of is destroyed, carburization is difficult to occur. The minimum amount in which this effect appears is 0.2%, and the effect becomes greater as the electric current increases beyond this, but a large amount of An deteriorates castability and makes normal atmospheric dissolution difficult, and in some cases. Since porous AA oxide is locally formed in preference to Or oxide, the upper limit is set to 08% as a range free from these problems.

Zr is an element that maintains the carburization resistance of the present invention together with Aj, and is extremely effective in improving carburization resistance because it fixes C and significantly inhibits the formation and growth of MgI2 c6 carbide. In addition, the deoxidizing effect during steel manufacturing has the effect of effectively incorporating Aj into the alloy, and OA directly below the alloy surface! Aj
It also has the effect of promoting the production of products. This kind of effect is exhibited above [11%, but when it exceeds 1.0%, it becomes 2%
The relationship with the Aj amount also reduces castability, and conversely, carburization or strength reduction due to casting defects is more likely to occur, and special steelmaking and casting techniques such as vacuum melting are required. Within the range where there is little danger, □, 11, and 0.1 to n, 5'''4', which are highly effective.

Example A steel ingot having the composition shown in Table 1 was produced using a high-frequency melting furnace, and the material was heated at 1100°C for 1000 hours.The material was subjected to a room temperature tensile test, oxidation resistance at 1000°C, and carburization resistance by solid carburization method. In addition to testing, the new material was heated to 1100℃ above
X100DHr heating material, 1100℃ for carburized material,
A creep cutting test of 0.8 Kg/=2 was carried out.

Table 2 shows the room temperature tensile test results of the new material and the material heated to 1000°C with 10 Q OHr, which was tested in order to compare the tendency of 11 carbide embrittlement after long-term heating, which is the aim of the present invention. Yes, the reference alloy and the invention alloy are conventional alloy 1
Since the amount of C is reduced compared to (HK40) and 2 (HP), the overall strength is reduced, but at 1000H, r
There is little decrease in ductility after heating. However, Reference Alloy 1 does not have C fixed by Nb and Ti, and tends to become brittle due to coarsening of crystal grains. Reference Alloy 4゜13 is similar to conventional Alloy 1.
Similar to 2, the amount of C is high, so the embrittlement after 1000 hours of stress is severe. Also Or
Reference alloy 6.15, which has a high amount of Ni, and reference alloy 7.16, which has a small amount of Ni, destabilize the base austenite and locally precipitate layered Or carbides together with Nb carbide or Ti carbide, so even new materials have poor ductility. Although small, it becomes more brittle when heated. Reference alloy 12 has a large amount of Ti, many defects occur in the casting, and both strength and ductility are low.

Table 3 shows the oxidation resistance and carburization resistance at 1100°C.
' + Reference alloy 8t18, which does not contain Zr, has inferior oxidation resistance compared to conventional alloys, but all other alloys have excellent oxidation resistance. In addition, carburization resistance is AA, Z
Except for reference alloy 8.18, which does not contain r, and reference alloy 9.10, 18.19, which contains AAIZr alone, it was significantly superior, and the combined addition of AA + Zr improved the carburization resistance. It is clear that the effect is large.

Table 4 shows the characteristics of the present invention after long-term heating.

This shows the change in creep strength after carburization. Conventional alloys 1 and 2 and high Ct reference alloys 4 and 13 are strong when new, but rapidly decrease when heated for a long time or carburized. . On the other hand, conventional Alloy 5 and other alloys with a low C content have low strength when used as new materials, but their strength does not change even when heated or carburized for a long time, and Reference Alloy 1 and N
On the contrary, the creep strength tends to increase except for Reference Alloy 2, which has a small amount of Ti, and Reference Alloy 11, which has a small amount of Ti.

The alloy 4.8 of the present invention contains N1 at the upper limit within the range of the present invention, but has N1 in both strength and carburization resistance.
There is no difference from the present invention alloy 1-3% 5-6, which has a low i content, and N1
It is thought that the effect of

Claims (1)

[Claims] Its composition is C; [1,1 to 0.25% 1Si:
0.5-2.0%, Mn: 0.5-2.0%, Ni
: 32-42%, Or ;, 24-28%, A
j! : [1,2-0,8%, Zr: D, 1-0.
5%, and Nb: 1.2~'5.0%, Ti: 0.5~
1.5% of at least one element, and the balance being Fe and normal impurities.