JPS5873741A - Carburization resistant and heat resistant cast steel with high strength - Google Patents

Abstract

PURPOSE:To obtain a carburization resistant and heat resistant cast steel with superior carburization resistance, high creep strength, ductility and strength by adding prescribed percentages of C, Si, Mn, Ni, Cr, Al, Zr, Mo and Nb and/or Ti. CONSTITUTION:This carburization resistant and heat resistant cast steel consists of, by weight, 0.10-0.25% C, 0.5-2.0% Si, 0.5-2.0% Mn, 32-43% Ni, 24-28% Cr, 0.20-0.80% Al, 0.10-0.5% Zr, 1.0-3.0% Mo, 1.2-3.0% Nb and/or 0.5-1.5% Ti and the balance Fe with ordinary impurities. The C content of this cast steel is lowered to reduce the embrittlement of carbide and a change in the strength due to long-time heating and to avoid the grain boundary oxidation of carbide causing rupture. Nb or Ti fixes C and the carburization resistance is increased by the interaction between Al and Zr.

Description

Detailed Description of the Invention The present invention provides - 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; It is related to.

Materials used in reaction tubes for thermally decomposing hydrocarbons, such as those used in ethylene production equipment, are exposed to increasingly high temperatures due to larger equipment and demands for higher yields. Carburization occurs due to carbon and carbon monoxide produced during thermal decomposition, and internal stress is generated due to a decrease in creep strength and ductility in the low temperature region due to carburization, and a difference in density or coefficient of thermal expansion between the carburized and non-carburized sections. This had the disadvantage of shortening the life of the reaction tube due to carburization failure.

Therefore, in large-scale plants in Kingo, conventional I(K
HP alloy 25Cr-35N+ cast steel with increased creep strength from 40 alloy 25Cr-2ONi cast steel, and alloys containing Ma, W, Nb, Tj, C'u, etc., which are further improved HP gold alloys, have been proposed (Special Publications of 1973). -52528°Special Public Show 5.4-24566, % Public Show, 5l-561Q.

Special Publication Showa 49-23453. (See Special Publication No. 5415172゜Sho 47-57550).

However, in recent ethylene reaction tubes, 2
Carburization occurs even in 5Cr-35Ni alloys, and carburization fractures or brittle fractures may occur at relatively low temperatures during plant startup and shutdown. As a preventive measure, we basically use HP, and
Y (Unexamined Japanese Patent Publication No. 49-62317), MW, Z
Proposals have been made to add trace elements such as r, Ca (Japanese Unexamined Patent Publication No. 49-62316), and kt (% Unexamined Japanese Patent Publication No. 49-6231.5).

However, these measures are effective in making carburization difficult to occur by using a high C alloy to obtain carbide precipitation strengthening, but once carburization occurs, C, which is added to increase the strength, coagulates and coarsens as carbide when heated for a long time, resulting in so-called carbide embrittlement that reduces strength and ductility when heated for a long time.
A high C content has a negative effect on carburization fracture, and while the conventional proposals mentioned above do not take measures against such carbide embrittlement, other fields, such as ammonia and Collector of methanol production equipment, 2l,
When a large thermal stress is applied as in J, low C1 low Cr heat resistant alloys such as 8 and JP-A-144817 and Incoloy 800 are applied in addition to creep strength. The serviceable temperature is as low as 900'C or less, and it cannot be used in an environment where the temperature exceeds 1050°C, such as in an ethylene reaction tube, due to its strength, carburization resistance, and oxidation resistance. .

Recent ethylene cracking furnaces are increasingly designed to save resources and save energy, and the surface temperature of the reaction tube has risen significantly to 1050 to 1070°C or 1100°C. It's getting very tough against them.

Therefore, the inventors of the present invention analyzed and conducted practical studies on current examples of carburization and carburization failure, and as a result, the following became clear.

゛(1) The broken reaction tube has a large creep strength and ductility near normal temperature.

(2) Near room temperature of damaged material with severe carburization and carbide embrittlement (
(approximately 500°C or less), the elongation value is 6% or less.

(3) Conventional alloys have a C content of around 0.4%, so 1
When exposed to high temperatures close to 100℃, carbide embrittlement becomes severe,
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 they become brittle, the linked eutectic carbides oxidize and form a crystalline structure inside. It becomes the starting point for crack progression and carburization. This tendency is greater as the Ni alloy has a lower C solid solubility limit.

Therefore, in order to prevent such carburization failure, grain boundary oxidation of carbides, so that carburization is difficult to occur, and even if heated for a long time or carburization occurs, it becomes a starting point of fracture. It is necessary to suppress the decline in creep strength and 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 in low temperature range are avoided. The present invention was developed with the aim of providing a heat-resistant cast steel that is difficult to cause damage.In other words, the present invention aims 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. Therefore, the amount of C should be 0.10 to 0.25.
%, further fixing C with Nb or T1, -
■A1. Carburization resistance is achieved through the interaction of , ', and Z■, and its composition is C; 0.10 to 0.25% (
(same below), Si: 0.5-2.0%, Mn:
0.5-2.0%, N]: 32-43%, Cry, 2
4-28%, AI: 0.20-0.80%, Zr
: 0.10~0.5%, Mo: 1.0~3.0%
, and Nb: 1.2 to 3.0%, 0.5 to '1'
1.5%! It consists of at least one kind of F'e, the remainder F'e, and ordinary impurities.

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

C is one of the elements that characterizes the present invention, and is preferably lower in order to suppress carbide embrittlement after long-term operation. In order to suppress the decline in weldability and strength due to coarsening,
A small amount of carbide is required, and the lower limit is 0, 1, where carbide precipitates as NbC or TiC when coexisting with Nb or Ti.
Let it rise. In addition, the upper limit is that as the amount of C increases, the high temperature strength on the short-time side increases, but as the embrittlement tendency increases and the N
Even if B or Ti is added, carbides will continue to precipitate and grain boundary oxidation of carbides will occur, so the content is set at 0.25%, which is within the range where embrittlement tends to occur and carbides are fragmented.

Si is commonly used as a deoxidizing agent and is normally contained in an amount of 5% or more. However, if the content exceeds 2%, the deoxidizing effect will be saturated and the susceptibility to hot cracking during welding will increase, so the content should be in the range of 0.5 to 2.0%.

Mn also has the same effect as Si, but if it is less than 0.5 yen, the effect is insufficient.For practical purposes, the amount should be 0.5 yen or more.
．． If it exceeds 0%, the amount of Mn in the oxide increases, which tends to lower the oxidation resistance, so it is set in the range of 0.5 to 2.0%.

Cr coexists with N1 and is an element that maintains oxidation resistance, carburization resistance, and high-temperature strength as an austenite structure, and in order to increase the serviceable temperature, the higher the content, the better. The present invention targets materials that are exposed to high temperatures of 1050 to 1100 degrees Celsius, such as ethylene reaction tubes, and a minimum of 24 degrees Celsius is required to maintain sufficient oxidation resistance and heat resistance at 1050 to 110 degrees Celsius. ,! If there are too many, Ni, I'l, Z
destabilizes austenite by interaction with r,
Since this causes a decrease in the strength and stratification of the material, the upper limit is set at 28%, which is the upper limit that allows the stability of the structure to be maintained.

Nl maintains the austenite structure in coexistence with Cr, and effectively affects heat resistance, oxidation resistance, and carburization resistance. Kayada heat-resistant alloys can obtain high strength by maintaining a stable austenite structure, but the present invention has less C, which is the element that most stabilizes austenite, so it has a higher NIM than conventional alloys. There is a need. Therefore, within the scope of the present invention, the lower limit value is 1. elements, namely Si and Cr.

Kl, Zr, and the minimum value of 32% that allows a stable austenitic structure to be obtained even at the upper limit of Nb or Ti,
In addition, in terms of carburization resistance, the effect is saturated at 40 to 43% Ni, so the upper limit is set at 43%3.Nb or T1 is one of the characteristic components of the present invention, and it interacts with the amount of C and Cr. This 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, Nb or Ti coexists with C, and the fine Nb carbide grains precipitate T1 carbide in austenite, improving the crease strength, and in austenitic stainless steel, it is added as a stabilizing element for carbides. However, in the present invention, it combines with C and suppresses carbide embrittlement due to Cr carbide after long-term operation, as well as suppresses grain coarsening peculiar to low C heat-resistant steels and prevents a decrease in strength. Also,
In response to the intrusion of C due to carburization, it precipitates at grain boundaries as Nb carbide or Ti carbide, which increases the creep strength.
．． If it is less than 5 yen, the effect of fixing the carbide is observed, but the effect of improving the creep strength after long-term heating or carburization is small. In addition, when the content of Nb is high, the effect of improving creep strength after long-term heating is large, but since it deteriorates carburization resistance, the upper limit is set at 3.0%, and when using Ti instead of Nb0, l ``As with l b, if there is a large amount of 1 contained, the effect of improving creep strength after long-term heating (d) is large, but if it is contained effectively in normal melting, it will be difficult to do so, so the selling section 1 and 5 Upper limit.

d, together with Zr, is an element that significantly improves the carburization resistance of the present invention, and is dissolved in solid solution in austenite (to delay the spreading of carbon, or to form an extremely stable and highly adhesive oxide directly under the alloy surface). Since it forms a layer, carburization is difficult to occur even if the oxide mainly composed of Cr on the outermost surface of the steel is destroyed.The minimum amount at which this effect appears is 0.20%, and if the amount exceeds this, it is difficult to carburize. It is true that the effect is great, but a large amount of a deteriorates the castability and makes normal atmospheric melting difficult.The upper limit is set at 0.8 in order to avoid these problems.

Zr is an element that maintains the carburization resistance of the present invention together with M,
Since M23c fixes C and significantly inhibits the formation and growth of carbides, it is extremely effective in improving carburization resistance. In addition, it has the effect of effectively incorporating M into the alloy through the deoxidizing action during steelmaking, and also has the effect of promoting the formation of a-oxide directly under the surface of the alloy. However, if it exceeds 1%, there is a relationship with the amount of °M.
It 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, so it is most effective as long as these risks are small. 0.1-1.0%.

When Mo coexists with Nb or T1, it increases the creep strength without embrittling the long-term heating material and the carburized part. In conventional alloys, MO forms carbides with C1,1lllliiCr and is dispersed in austenite.
In other words, strength was maintained by precipitation hardening of carbides, but as mentioned above, these methods 1. B8. Oh, it's so bad. Interval. Thermal acid muyu reduces the strength of charcoal and shrift. Therefore, in the BA of the present invention, a trace amount of C is fixed by rvb or TI, so Mo carbide is formed, and MO is dissolved in the base austenite to increase the strength. Although the strength of such a strengthening mechanism is smaller than that of conventional carbide precipitation hardening, the strength does not decrease due to long-term heating or carburization, and shows the effect of increasing the strength.

The effect of JC is clearly fLυ above 1.0%, and the higher the content, the greater the strengthening effect, but it destabilizes the base austenite and MO &! 1. Mo carbides are selectively formed and the oxidation resistance suddenly decreases.Moreover, in a carburizing atmosphere, Mo carbides are formed and carburization is accelerated, so the effect on oxidation resistance and carburization resistance is small, and the creep strength is reduced. Increase 1
, in the range of 0 to 3.0%.

Example □ ,,,wa,,,iri"-c',,*1 Svc O, Yu. □□ was prepared and heated at 1100℃ for 10'OO'Hr. Tensile test at room temperature and at 1100℃ oxidation resistance,
In addition to the carburizing test using the solid carburizing method, the new material and the above 1100
A creep rupture test of a, sKy/- was conducted at 1100°C on the heating material and carburized material for 1000 hours at 1000°C.

Table 2 is a room temperature tensile test of a new material and a material heated to 1100°C for 1'00'OHr, and was tested in order to compare the carbide embrittlement tendency after long-term heating, which is the aim of the present invention. , the reference alloy and the present alloy are conventional alloys 1 to 3.
Since the C content is lower than that of 1, the strength is generally lower,
There is little decrease in ductility after heating at 100°C for 10'00 hours. Among them, Nb.

Reference alloy 3 containing no T1, reference alloy 6 with a small amount of Nb, and reference alloy 16 with a small amount of Tl. Reference alloys 1j and 21 with a high C content had a large decrease in ductility after heating for 1000 hours, and the low C, which is a characteristic of the alloy of the present invention, has the effect of preventing carbide embrittlement due to Nb and embrittlement due to grain coarsening, Tl
This shows that the effect of reducing the decrease in ductility due to In addition, reference alloy 17, which has a high Tl content, has oxides incorporated into the test material, and it is thought that it will exhibit excellent ductility if it is vacuum melted or vacuum cast. Even the material has low ductility.Reference alloy 9.19 with high Cr content
Reference alloy 10,2[) with a low Ni content destabilizes the matrix austenite, locally precipitates layered Cr carbides, and although the new material has low ductility, it further reduces the ductility of the heated material.

Table 3 shows oxidation resistance and carburization resistance, and shows the elements Ma, Nb, and Ti that maintain strength and ductility, which are the characteristics of the present invention.
and A7. which imparts carburization resistance and acid corrosion resistance. This shows the effect of Zr. That is, for example, reference alloy 1.12.4
．． Comparison of reference alloy 5.1.5.7 with reference alloy 5.1.5.7 shows that the element MO that maintains strength is actually harmful to carburization resistance and oxidation resistance. ．．
As shown in No. 17, the oxidation resistance and carburization resistance deteriorate sharply when the range exceeds the range of the present invention.
Since it does not contain J, Zr, it has oxidation resistance and carburization resistance equivalent to conventional alloys, and compared with the present alloy, /'u, Z
It can be seen that r greatly improves oxidation resistance and carburization resistance.

Reference alloys 8 and 18 have low Cr content, which reduces their oxidation resistance and their carburization resistance is also inferior to the invention alloy.

Table 4 shows changes in creep strength after long-term heating and carburization, which are characteristics of the present invention, in comparison with new materials. Conventional Alloys 1 to 3 and Reference Alloy 11.21 with a high C content have long strength as new materials, but their strength significantly decreases when heated for a long time with 710 heat material and carburized material.

Reference alloy that does not contain Mo, which is a feature of the present invention, 1°3.
12 and 4.1'4, which have a small content, have small changes in creep strength in long-time heated materials and carburized materials, but the strength of new materials is low.

Reference alloy 5.7, which has a high content of Mo and Nb, shows a large decrease in strength when heated for a long time due to a decrease in oxidation resistance and instability of base austenite. -Reference alloy 3゜12, which does not contain any Mo or Ti, has a significantly lower strength after being heated for a long time or as a carburized material compared to the strength of the new material. On the other hand, in the alloys of the present invention and reference alloys that contain appropriate amounts of Mg, N, b, and Ti, there is almost no difference in strength between the carburized materials and the new materials after heating for a long time; It tends to increase strength. In addition, reference alloy 9.19 with a high Cr content and Ni
In reference alloys 1°0 and 20, which have a low content, the component balance is disrupted and the base austenite becomes unstable, so even if MO is contained, the effect of improving strength is small.

Claims (1)

[Claims] Its composition is C: 0.1-0.25%, Sl: 0 in weight%
．． 5-2.0%, Mn: 0.5-2.04. Ni
: 52-43%, Cr: 24-28q6.
At: 0.2~'j, 8%IZr, +0.1~0.5
%*Mo; 1-0 to 3.0chi, and Nb:
1.2-3.0%+T1: 0. A carburizing-resistant and heat-resistant cast steel having high strength, characterized in that it is made of at least one element of 5 to 1.5 mm, the balance being Fe, and ordinary impurities.