JPS6142782B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a heat-resistant cast steel having excellent high-temperature creep rupture strength, thermal shock resistance, carburization resistance, etc. Cr-Ni, represented by ASTM HK40 material and HP material, has traditionally been used as ethylene cracking tube material, re-forming tube material, etc. in the petrochemical industry.
Heat-resistant cast steel is used. In addition, HP improving materials containing W have been developed as materials with improved high-temperature properties. However, in order to cope with increasingly severe usage conditions, improve service life and stabilize operations, high-temperature properties, especially high-temperature creep rupture strength,
Further improvements in thermal shock resistance, carburization resistance, etc. are desired. In view of the above, the present inventors have proposed that Cr-Ni-W
-As a result of detailed research on the effects of various alloying elements on the high-temperature properties of Fe-based heat-resistant cast steel,
By the combined addition of N, Ti, Al, B and Cu,
It has been found that creep rupture strength, thermal shock resistance, carburization resistance, etc. can be significantly improved at high temperatures, especially at temperatures above 1000°C. The present invention has been made based on this knowledge. That is, the present invention includes C 0.3 to 0.6% (weight %, same hereinafter), Si 2.0% or less, Mn 2.0% or less,
Cr 20-30%, Ni 30-40%, W 0.5-5.0%, N
0.04~0.15%, Ti 0.04~0.5%, Al 0.02~0.5
%, B 0.0002 to 0.004%, Cu 3.0% or less, and the balance substantially consists of Fe. The reasons for limiting the components of the present invention will be explained below. C: 0.3 to 0.6% C not only improves the castability of cast steel, but also combines with Ti described below to form primary carbides and increases creep rupture strength. For this we need at least 0.3%. The effect increases as the amount of C increases, but if it is contained in a large amount, the toughness after use will be significantly reduced due to excessive precipitation of secondary carbides, and weldability will also deteriorate, so the upper limit is set at 0.6%. Si: 2.0% or less Si is a deoxidizing element for molten metal, and in addition to improving castability, it also has the effect of improving carburization resistance. However, if it is contained in a large amount, weldability will be impaired, so it should be kept at 2.0% or less. Mn: 2.0% or less Mn is an impurity element S in the deoxidation of molten metal and in steel.
It has the effect of fixing and rendering harmless, but if it is contained in a large amount, it leads to a decrease in oxidation resistance, so the upper limit is set at 2.0%. Cr: 20-30% Cr coexists with Ni, which will be described later, to make the cast steel structure austenitic and improve high-temperature strength and oxidation resistance. In particular, in order to maintain high strength and high oxidation resistance in a high temperature range of 1000°C or higher, the content must be at least 20%. This effect increases as the content increases, but if it is included too much, the toughness after use will decrease, so the upper limit is set at 30%. Ni: 30-40% As mentioned above, Ni forms an austenite structure in coexistence with Cr, and is an element effective in increasing structural stability and ensuring oxidation resistance and high-temperature strength. To achieve excellent oxidation resistance and strength at high temperatures of 1000°C or higher, the content must be 30% or more. These high-temperature properties improve as the content increases, but when the content exceeds 40%, the effect is almost saturated, and further content is economically disadvantageous. Therefore, the upper limit is set at 40%. W: 0.5 to 5.0% W has the effect of increasing high temperature strength. In order to obtain this effect, it is necessary to contain at least 0.5%, but if the content is too large, the oxidation resistance decreases, so the upper limit is set at 5.0%. In addition to the above elements, the cast steel of the present invention includes N, Ti,
Its greatest feature is that it contains Al, B, and Cu in a composite manner. Ti combines with C and N to form carbides,
B and Al form nitrides and carbonitrides, and B and Al finely disperse and precipitate these compounds to strengthen grain boundaries and improve intergranular cracking resistance, thereby improving high-temperature creep rupture strength and high-temperature thermal shock properties. , resulting in significant improvements in long-term creep rupture strength, etc. Also,
Ti significantly improves carburization resistance due to its synergistic effect with Al, and Cu greatly improves thermal shock resistance due to its synergistic effect with Ti and Al. N: 0.04-0.15% While N stabilizes and strengthens the austenite phase in the form of solid solution nitrogen, it also participates in the formation of nitrides such as Ti and carbonitrides. As described above, this compound finely disperses and precipitates in the coexistence of Al and B, refines the crystal grains, and inhibits grain growth, thereby increasing creep rupture strength and thermal shock resistance. In order to ensure this effect, the content must be at least 0.04%, but if the amount is too large, excessive precipitation of the above compound may occur.
Since coarsening occurs and the thermal shock resistance deteriorates, the upper limit is set at 0.15%. Ti: 0.04-0.5% Ti forms nitrides, etc., which improves high-temperature strength and thermal shock resistance as described above, and also enhances carburization resistance when coexisting with Al. At least 0.04% is required to achieve these effects sufficiently. As the content increases, the effect increases, but if the content increases too much, the strength deteriorates due to coarsening of precipitates and an increase in oxide-based inclusions. By then, 0.5
% is the upper limit, and if strength is particularly important, 0.15
% or less. Al: 0.02-0.5% Al has the effect of improving creep rupture strength, as well as
Coexistence with Ti has a remarkable effect on improving carburization resistance. When emphasis is placed on improving creep rupture strength, the content is preferably 0.02 to 0.07%. In addition, when emphasis is placed on strengthening carburization resistance, 0.07
It is preferable that the amount exceeds %, and carburization resistance improves as the content increases. However, on the other hand, it tends to lower the strength, so the upper limit is set at 0.5%. Note that when a material containing Ti and Al is subjected to an X-ray microanalyzer (EPMA) after a carburization test, an Al-rich layer is observed on the surface layer of the test piece. This Al
The rich layer exhibits a strong carburization prevention effect. B: 0.0002 to 0.004% In addition to strengthening grain boundaries, B contributes to improving creep rupture strength by causing fine precipitation of the Ti compound and delaying agglomeration coarsening after precipitation. For this purpose, the content must be 0.0002% or more, but if the content is increased too much, not only will strength increase be slow, but weldability will deteriorate, so the upper limit is set at 0.004%. Cu: 3.0% or less Cu has a remarkable effect on improving thermal shock resistance in coexistence with Ti and Al. This effect increases as the content increases, but if it exceeds 3.0%, the degree of improvement in thermal shock resistance will be slow and weldability will deteriorate, so the content should be 3.0% or less. Further, Cu has the effect of increasing carburization resistance. In order to fully exhibit these effects, the preferable content is 0.2 to 3.0%, more preferably 0.5 to 3.0%. P, S, and other unavoidable impurities are
Of course, it is desirable that it be as small as possible, but there is no problem as long as it is within the range normally allowed for this type of steel. Next, the present invention will be specifically explained with reference to Examples. Example Cast steel melted in a high frequency melting furnace (in the atmosphere) was subjected to centrifugal casting to obtain cast steel pipes (outer diameter 136 mm x wall thickness 20 mm x length 500 mm) having the composition shown in Table 1. Test pieces were prepared and subjected to creep rupture tests, thermal shock resistance tests, and soak resistance tests. The test results are shown in Table 2. Trial numbers 1 to 6 are comparative examples, and trial numbers 101 to 107 are examples of the present invention. Among the comparative examples, No. 1 is a conventional HP improved material containing W (none of N, Ti, Al, B, or Cu), and Nos. 2 to 5 contain N, Ti, Al, and B. However, No. 6 is an example where Cu deviates from the specifications of the present invention.
This is an example in which the content of Ti and Al is insufficient although it contains Cu. The test conditions are as follows. [] Creep rupture test According to the provisions of JISZ 2272. (A) Temperature 1093
℃・Load 1.9Kgf/mm ² , and (B) Temperature 850℃・Load
It was conducted under two conditions of 7.3Kgf/ ^mm2 and the rupture time (Hr) was measured. [] Thermal Shock Resistance Test The heating and cooling operations of heating and holding a specimen (wall thickness: 8 mm) with the shape and dimensions shown in Figure 1 at a temperature of 900°C (holding time: 30 minutes) and then cooling with water are repeated. 10 operations
After each repetition, measure the length of cracks that occur on the specimen. Thermal shock resistance is determined by a crack length of 5.
Evaluation was made based on the number of repetitions when mm was reached. In Table 2, the numbers in the "Thermal Shock Resistance" column are the number of times,
The higher the number of times, the better the thermal shock resistance. [] Carburization resistance test After holding a specimen (diameter 12 mm x length 60 mm) in a solid carburizing agent (Degza KG30, containing BaCO ₃ ) at a temperature of 1300℃ for 300 hours, Chips were collected from each of the layers and the 1-2 mm layer, and the increased C content (Wt%) was determined by C content analysis. In Table 2, the "carburizing resistance" column shows the increased amount of C. The smaller the increase in C content, the better the carburization resistance.

【table】

[Table] As is clear from the above test results, the materials of the present invention (No. 101 to 107) are superior to the conventional HP improved material (No. 1) in terms of high temperature creep rupture strength, thermal shock resistance, and carburization resistance. It has excellent high-temperature properties that far exceed that of Also, other comparative examples (No. 2 to 6)
shows better results than the conventional material No. 1, but none of them are as good as the inventive material in the comprehensive evaluation of each property. In addition, in the welding test, poor welding was observed for sample materials No. 4 and 5 containing excessive Cu;
The material of the present invention has good weldability and presents no problems as a welded structural material. As described above, the heat-resistant cast steel of the present invention is different from conventional W
Compared to HP-containing materials, it has excellent high-temperature properties, especially high-temperature creep rupture strength, thermal shock resistance, and carburization resistance, so it can be used in severe applications such as ethylene cracking tubes and re-forming tubes in the petrochemical industry. It withstands conditions well and guarantees greater stability and durability than conventional materials in high-temperature applications exceeding 1000℃, such as high-quality steel-related equipment components such as hearth rolls and radiant tube materials.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the shape and dimensions of a thermal shock resistant specimen in an example.

Claims (1)

[Claims] 1 C 0.3-0.6%, Si 2.0% or less, Mn 2.0% or less, Cr 20-30%, Ni 30-40%, W 0.5-5.0
%, N 0.04-0.15%, Ti 0.04-0.5%, Al 0.02
~0.5%, B 0.0002~0.004%, Cu 3.0% or less,
Heat-resistant cast steel with the remainder essentially composed of Fe.