JPH0953154A - Heat resistant cast steel small in creep elongation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the creep elongation of the material for an ethylene cracking tube. SOLUTION: This steel is the one having a compsn. contg., by weight, 0.05 to 0.7% C, <=4% Si, <=4% Mn, 18 to 45% Cr and 18 to 50% Ni, furthermore contg. at least one kind selected from the group composed of <=3% Nb, <=3% W and <=3% Mo, and the balance substantially Fe, and in which the distance (d) of secondary dendrite arms 6 in the dendrite structure is regulated to 2.5 to 20μm. If required, 0.02 to 0.5% Ti and/or 0.02 to 0.5% Zr is incorporated therein.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a heat resistant alloy useful as a cracking tube material for ethylene production in the petrochemical industry.

[0002]

2. Description of the Related Art Ethylene is produced by carrying out thermal decomposition at a high temperature of about 1000 ° C. and a pressure of about 5 kg / cm ² or less while circulating naphtha or the like in a cracking tube. Outline of cracking tube
Shown in (1). This cracking tube (1) has a length of about 3
m pipes (20) are connected by welding (4) to form a straight pipe part (2) having a length of about 8 to 10 m, and the straight pipe part (2) is welded to the bent part (3). They are connected by (4) and assembled into a coil. Creep deformation at high temperature appears as a local bending deformation of the tube, so the temperature becomes abnormally high in the part close to the heating burner due to bending deformation.
This causes deterioration of the tube material and increase in carburization. Therefore, the cracking tube material needs to have low creep elongation in addition to various properties such as oxidation resistance and carburization resistance.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a heat resistant cast steel having a small creep elongation at high temperature.

[0004]

In order to achieve the above object, the heat-resistant cast steel of the present invention has a weight percentage of C: 0.05 to 0.5.
7%, Si: 4% or less, Mn: 4% or less, Cr: 18 to
45%, Ni: 18 to 50%, and at least one selected from the group consisting of Nb: 3% or less, W: 3% or less and Mo: 3% or less, and the balance substantially Fe. And the interval between the secondary dendrite arms in the dendrite structure is 2.5 to 20 μm. The heat-resistant cast steel has a Ti content of 0.02 to 0.5% and / or a Zr content of 0.02.
It can further contain ~ 0.5%. In addition, the balance substantially Fe means that Fe is an impurity element such as P and S inevitably contained during normal melting and Fe.

In the dendrite structure, the interval between the secondary dendrite arms can be 2.5 to 20 μm by increasing the solidification rate during casting.
For example, in the centrifugal casting method, immediately after the injection of the molten metal into the centrifugal casting die is completed, forced water cooling (for example, water sprinkling) on the outer surface of the die.
And the casting is performed while forcibly cooling the mold. When the molten metal charged into the rotary mold solidifies, the arm-shaped structure that extends long toward the mold is called the primary dendrite arm (5), which is suitable for the lengthwise direction of the primary dendrite arm. At intervals,
An arm-shaped tissue formed by extending in a direction substantially orthogonal to the arm is called a secondary dendrite arm (6) (Fig. 2).
reference). In the solidified structure obtained by normal centrifugal casting,
The distance d between the secondary dendrite arms (2) is about 2 on average.
Although it is 5 μm or more, as described above, if the mold is forcibly cooled, the cooling rate at the time of solidification of the molten metal is increased, and the distance d between the secondary dendrite arms can be reduced to an average of about 20 μm or less. As the cooling rate becomes faster, the distance d between the secondary dendrite arms becomes narrower and can be reduced to an average of about 2.5 μm.

[0006]

[Function] The dendrite structure with a small interval between the secondary dendrite arms means that it is a fine structure,
The average spacing of the secondary dendrite arms is approximately 2.5 μm to 20 μm.
Within the μm range, the creep resistance is high and the creep elongation is low.

[0007]

[Explanation of reasons for limiting ingredients]

C: 0.05 to 0.7% C has the effect of forming a solid solution in the matrix during solidification during casting, receiving heat during actual use of the cracking tube to precipitate Cr carbide, and reducing creep elongation. But C
Excessive precipitation of r-carbide causes embrittlement. Therefore, the content is specified to be 0.05 to 0.7%.

Si: 4% or less Si increases the fluidity of the molten metal, improves the castability, and has a function as a deoxidizing agent during alloy melting. Therefore, Si is contained at a maximum of 4%.

Mn: 4% or less Mn has the effect of preventing hot cracking during welding by deoxidizing molten steel and fixing S, which is an impurity element, as MnS. Therefore, the maximum content is 4%.

Cr: 18-45% Cr is contained in at least 18% or more in order to secure oxidation resistance and high temperature strength. However, if the content is too large, Cr carbides are dispersed in austenite, which promotes a decrease in ductility after aging. Therefore, the upper limit is 45%.

Ni: 18-50% Ni forms an austenite phase together with Cr and Fe,
It contributes to the improvement of oxidation resistance and imparts stability after long-term use of Cr carbide. Therefore, at least 18%
It is contained above. However, even if the content exceeds 50%,
Since the corresponding effect cannot be obtained, the upper limit is 50%.

The heat-resistant cast steel of the present invention comprises, in addition to the above-mentioned elements,
It contains at least one element selected from the group consisting of Nb, W and Mo.

Nb: 3% or less Nb forms Nb carbides at grain boundaries during solidification during casting.
The presence of these increases the grain boundary fracture resistance and suppresses creep elongation, so it is desirable to include them.
However, if the content is large, the oxidation resistance may be deteriorated, so even if it is contained, the content is made 3% or less.

W: 3% or less W is a solid solution in an austenite phase and a grain boundary carbide.
The formation of (WC) has a function of suppressing creep elongation, so that it is desirable to contain it. However, if the content is too high, it hardens and the ductility becomes poor, which may impair the workability and weldability. Therefore, even if it is contained, it should be 3% or less.

Mo: 3% or less Mo is bound to C to form Mo carbide and has the function of suppressing creep elongation, so Mo is desirable to be contained. However, if contained too much, the material may become brittle, so even if contained, the content is made 3% or less.

The heat-resistant cast steel of the present invention contains, in addition to the above-mentioned elements,
If necessary, Ti: 0.02 to 0.5% and / or Z
It can further contain r: 0.02 to 0.5%.
Ti and Zr are used when the cracking tube is actually used.
This is because it has the effect of delaying the growth coarsening of Cr carbide and suppressing creep elongation.

[0017]

Example: Alloys of various components were melted in a high-frequency induction melting furnace and then cast by centrifugal force to produce a cast pipe (outer diameter 98 mm, wall thickness 12 m).
m, length 500 mm). The alloy components are shown in Table 1.
In Table 1, No. 1 is a comparative example in which a cast pipe was produced without forcibly cooling the mold. No. 2 to No. 7 are examples of the present invention in which centrifugal force casting was carried out immediately after pouring the molten metal into the mold while spraying water on the outer surface of the mold.

A strip-shaped test piece (width: 25 mm, wall thickness: 12 mm, length: 30 mm) was cut out from the test casting tube described above.
For the test piece, the intervals between the secondary dendrite arms were measured with a microscope at 10 locations in the center of the wall thickness, and the average value is shown in Table 1.

Next, after aging the test piece at 1100 ^° C. for 1000 hours, a creep elongation test piece (diameter 8 mm × 100 mm)
(Gauge length), temperature 1050 ^o C, load 0.5 kgf /
The creep elongation test was conducted under the condition of mm ² . The length L0 of the test piece before the test and the length L1 after the test were measured, and (L1−
L0) / L0 multiplied by 100 was calculated as the total creep elongation (%). The results are shown in Table 1.

[0020]

[Table 1]

As is clear from the results shown in Table 1, in the samples No. 2 to No. 6 which are examples of the present invention, the interval between the secondary dendrite arms was smaller than 20 μm and the total creep elongation was small. Shows. It can also be seen that the smaller the distance between the secondary dendrite arms, the smaller the total creep elongation. On the other hand, the test N of the comparative example
Although o.1 has almost the same composition as No.2, the total creep elongation is large due to the large distance between the secondary dendrite arms.

[0022]

EFFECTS OF THE INVENTION The secondary dendrite arms are spaced by 2.5.
The heat-resistant cast steel of the present invention in the range of up to 20 μm has a small creep elongation at high temperature. Therefore, during use as a cracking tube, there is little bending deformation, and deterioration of the tube material due to overheating due to approach to the heating burner is prevented. Therefore, the heat-resistant cast steel of the present invention is suitable as a material for cracking tubes in the petrochemical industry.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of an assembled state of a cracking tube.

FIG. 2 is a diagram for explaining a distance between secondary dendrite arms in a dendrite structure.

Claims (2)

[Claims] 1. C: 0.05-0.7% (weight%, the same applies hereinafter), Si: 4% or less, Mn: 4% or less, Cr: 18-
45%, Ni: 18 to 50%, and Nb: 3% or less,
W: 3% or less and Mo: 3% or less, at least one selected from the group consisting of, the balance substantially Fe
And the distance between the secondary dendrite arms in the dendrite structure is 2.5 to 20 μm,
Heat-resistant cast steel with low creep elongation. 2. C: 0.05-0.7% (weight%, the same applies hereinafter), Si: 4% or less, Mn: 4% or less, Cr: 18-
45%, Ni: 18 to 50%, and Nb: 3% or less,
W: 3% or less and Mo: at least one selected from the group consisting of 3% or less, and Ti: 0.02 to 0.5%
And / or Zr: 0.02 to 0.5%, the balance being substantially Fe, and the interval between the secondary dendrite arms in the dendrite structure being 2.5 to 20 μm. Small heat-resistant cast steel.