JPH04198458A - Heat resisting steel excellent in carburizing resistance and creep rupture strength - Google Patents

Abstract

PURPOSE:To produce a heat resisting steel excellent in creep rupture strength as well as in carburizing resistance by preparing a heat resisting steel containing specific percentages of C, Si, Mn, Cr, Ni, Al and Nb. CONSTITUTION:A heat resisting steel which has a composition consisting of, by weight, 0.3-1.5% C, >2-3% Si, <=2% Mn, 20-30% Cr, 25-40% Ni, 0.2-2.0% Al, 0.2-2.0% Nb, and the balance essentially Fe and further containing, if necessary, 0.01-0.3% Ti is prepared. By this method, the heat resisting steel having superior carburizing resistance and high creep rupture strength can be obtained. This steel is suitable for material for hydrocarbon reaction tube, etc., in the petrochemical industry.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a heat-resistant steel suitable as a material for reaction tubes, etc. used in the thermal decomposition and reforming reactions of hydrocarbons such as naphtha and ethane in the petrochemical industry. Regarding.

[Prior art and its problems] In the thermal decomposition and reforming reactions of hydrocarbons in the petrochemical industry, carbon adheres to the wall of the reaction tube during the hydrocarbon thermal decomposition process, and the carbon diffuses into the inside of the tube. As a result, a so-called carburization phenomenon occurs, resulting in significant deterioration of the material of the reaction tube.

The materials used for this purpose have traditionally been ASTM
HP material (25Cr-35Ni) specified in
Furthermore, improved HP materials to which Nb, W, Mo, etc. are added are widely used.

By the way, recently, operating temperatures have been increasing,
In the case of the above-mentioned materials, when used at a temperature of 1100° C. or higher, there is a problem that not only is sufficient carburization resistance not exhibited, but the creep rupture strength is significantly reduced. Therefore, there is a need for a material that has excellent carburization resistance and high creep rupture strength that can withstand use in a temperature range of 1100° C. or higher.

The present invention provides a novel material that satisfies such requirements.

[Technical means and effects] The heat-resistant steel of the present invention contains C: 0.3 to 15% in weight%;
Si: more than 2% and less than 3%, M1]: less than 2%, Cr
:20-30%, Ni:25-40%, A1:0.2~
20%, Nb:'0.2-20% and the remainder substantially F
It has a component composition consisting of e.

The heat-resistant steel of the present invention may further include Ti: 0.0 if necessary.
It can also contain 1 to 03%.

The heat-resistant steel of the present invention has excellent carburization resistance and high creep rupture strength when used in a temperature range of 1100° C. or higher.

[Reason for limited ingredients] The reasons for limiting the components of the heat-resistant steel of the present invention are as follows.

C: 0.3 to 15% Since the melting temperature decreases as CB increases, castability improves. However, if the content is too high, the material deteriorates and cracks in casting and welding occur. Therefore, the upper limit of the C content is set to 15%.

On the other hand, if the C content is less than 0.3%, a sigma phase will precipitate during use at high temperatures, resulting in a significant decrease in ductility. Therefore, the lower limit is defined as 0.3%.

Si ・More than 2% and less than 3% Si is an important element that improves carburization resistance.

Furthermore, as described later, A1 is also an important element that improves carburization resistance. In order to ensure carburization resistance at temperatures exceeding 1100°C, the inventors have specifically identified Af! As a result of intensive research on the relationship between steel and Si, it was found that when the Si content is 2% or less, no significant effect on carburization resistance due to A1 content can be expected. Therefore, it is necessary to contain at least 2% of Sl. However, Si
If the content of Cr is increased, the deterioration of the material progresses, the creep rupture strength decreases, and weldability is impaired, so the limit is set at 3%.

Mn 2% or less Mn is added as a deoxidizing and desulfurizing element. However, if it is contained in an excessively large amount, the high-temperature creep rupture strength and carburization resistance will be reduced. Therefore, the first limit is set at 2%.

Cr: 20-30% Cr is an element effective in improving high temperature strength, oxidation resistance, carburization resistance, etc. In order to ensure these properties in a high temperature range of 1100°C or higher, it is necessary to contain at least 20%. This effect increases as the content increases, but if the content is too large, cracks are likely to occur during casting and solidification processes, and excessive precipitation of carbides due to use at high temperatures leads to a decrease in ductility. Therefore, "-eye" is defined as 30%.

N1・25-40% N1 forms a stable austenite base together with elements such as Cr and Mn, and contributes to improving high temperature strength and oxidation resistance as well as carburization resistance. In particular, in order to ensure good carburization resistance in the high temperature range of 1100°C or higher, 25
% or more is required.

However, even if the content exceeds 40%, the effect corresponding to the content cannot be obtained and it is not economical. For this reason, 40%”1
limited.

Al: 0.2-20% Ae is not only effective in improving oxidation resistance at high temperatures, but as mentioned above, when the Si content exceeds 2%, it improves carburization resistance due to its synergistic effect with Si. dramatically contribute to the improvement of In order to fully exhibit the effect of improving carburization resistance under high-temperature usage conditions of 1100° C. or higher, it is necessary to contain at least 0.2%. The effect of improving carburization resistance increases as the addition amount increases, but even if it is added in excess of 20%, the effect is almost saturated. Furthermore, as the content of A1 increases, cracks are more likely to occur during casting solidification and welding, leading to deterioration of ductility during high-temperature use. For this reason, 2
% is the upper limit.

Nb: 0.2-20% Nb has the effect of increasing creep rupture strength.

High Si content and AI! In the heat-resistant steel of the present invention, which has been added with
Moreover, it is an essential element in order to maintain high creep rupture strength. To obtain this effect, at least 0.
It is necessary to contain 2% or more. However, if it is contained too much, it will cause a decrease in creep rupture strength.
% is set to 1 limit. Note that Nb usually accompanies Ta, which is an element with the same effect as Nb, and in this case, the total amount with Ta should be 0.2 to 20%.

Ti + 0.01-0.3% Since Ti has the effect of increasing creep rupture strength like Nb, it is added when even higher creep rupture strength is required. In order to obtain the desired effect of Ti, it is necessary to contain it in an amount of at least 0.01%. However, if the content is too large, the creep rupture strength will decrease and casting cracks will occur. Therefore, the -L limit is defined as 0.3%.

The heat-resistant steel of the present invention contains the above-mentioned alloy components, with the remainder substantially consisting of Fe. It should be noted that PXS and other impurities that are unavoidably contained during the melting of steel may be present as long as they are within the range normally allowed for this type of copper phase.

Next, Examples will be given to clarify the effect of improving carburization resistance and creep rupture strength in the heat-resistant steel of the present invention.

[Example] An alloy of various components was melted in a high frequency melting furnace, and a tube body (outer diameter 130 mm x inner diameter 90 mm x length 500 mm) was manufactured by centrifugal casting. A specimen (diameter 12 mm x length 60 mm) was collected from this centrifugally cast tube by machining.

The alloy composition of each specimen is shown in Table 1.

First, solid carburization tests were conducted on these specimens. The carburizing test was conducted using a solid carburizing agent (Tegusa KG30, BaC
The amount of carburization was measured after holding the temperature at 1150° C. for 500 hours. After carburizing, the specimen was lathed to a depth of 0.5 mm from the outer surface until it reached a depth of 4 mm.
Collect powder at each pitch, conduct C (carbon) analysis on each pitch, and calculate the total amount of C increase for each pitch as a second
Shown in the table.

Furthermore, for each specimen, 1093℃X 1, 1kg
A creep rupture test was conducted under the condition of /mm2, and the rupture time of each specimen is shown in Table 2.

(Left below) Table 2 Specimen Creep rupture test - 11 Carburizing test 1 - NO creep rupture time (Hrs) 4 from the surface
A of 1093℃ x 1 C/mm2C increase amount up to mm
(%) 1 1112
2.842 1108
2.763 960
2.714' 1215
2.635 1630
2.716 1510
2.657 1010
2.518 1153
2.409 976
2.3810 10
15 2.3811
1361 2.3012
1532 2.2813
1005 16.7614
901 10.24
15 773 10.
7116 553 4
．． 4117 310
2.2118 530
3.2619 690
8.70 The results of comparative shrine test pieces Nos. 13 to 19 will be discussed. Specimens No. 13 and No. 15 are alloys containing less than 2% Si, with No. 13 containing no A1 and No. 15 containing Ae. No.1
No. 4, No. 16-19 are alloys in which the Si content exceeds 2%.

Among them, No. 14 does not include AI, No. 19 includes A
Alloys with an I content of less than 0.2%, Nos. 16 to 18 are alloys containing A1 of 0.2% or more.

As is clear from this result, specimen Nos. 16 to 18
The amount of increase in C was significantly reduced compared to Nos. 13 to 15. That is, it has excellent carburization resistance. In addition, since sample No. 19 had a low A1 content, no significant improvement effect was observed in terms of carburization resistance.

On the other hand, if Ap is included in an alloy having a Si content of more than 2%, the creep rupture strength tends to decrease. In particular, in the case of a high Si1 high A+2 material such as specimen No. 17, the creep rupture strength decreases significantly.

On the other hand, specimens Nos. 1 to 12 had excellent carburization resistance because they contained an appropriate amount of Ap2 and further contained an appropriate amount of Nb or Nb and Ti in a material with a Si content of more than 2%. It has extremely high creep rupture strength while maintaining its properties. Thus, although carburization resistance and creep rupture strength are contradictory properties, the heat-resistant steel of the present invention has excellent properties in both.

[Effects of the Invention] The heat-resistant steel of the present invention has excellent carburization resistance and high creep rupture strength when used in a high temperature range exceeding 1100°C. Therefore, the heat-resistant steel of the present invention is suitable as a material for hydrocarbon reaction tubes and the like in the petrochemical industry.

Claims (2)

[Claims] (1) In weight%, C: 0.3-1.5%, Si: 2%
exceeding 3%, Mn: 2% or less, Cr: 20-30
%, Ni: 25-40%, Al: 0.2-20%, Nb
:0.2 to 20%, and the balance substantially consists of Fe, and is a heat-resistant steel with excellent carburization resistance and creep rupture strength. (2) In weight%, C: 0.3-1.5%, Si: 2%
exceeding 3%, Mn: 2% or less, Cr: 20-30
%, Ni: 25-40%, Al: 0.2-2.0%, N
A heat-resistant steel consisting of b: 0.2 to 2.0%, Ti: 0.01 to 0.3%, and the balance substantially Fe, and has excellent carburization resistance and creep rupture strength.