JPS61149463A - Heat resistant steel for high temperature structure - Google Patents

Abstract

PURPOSE:To provide a structural material having improved strength at high temp. by combinedly incorporating Co, Mo and Nb into a Cr-Ni steel having a reduced C content. CONSTITUTION:This heat resistant steel for high temp. structure consists of, by weight, <=0.03% C, <=1.0 Si, 0.5-2.0% Mn, <=0.03% P, <=0.03% S, 15.0-21.0% Cr, 8.0-14.0% Ni, 10.0-25.0% Co, 1.0-6.0% Mo, 0.1-1.0% Nb and the balance Fe. The steel is subjected to solid soln. strengthening by the reduced amount of C and combinedly added Co, Mo and Nb. The steel undergoes no sudden reduction in the creep strength during long-period use at high temp. and has high creep strength, so it can be used as a structural material for a high temp. chemical plant or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to heat-resistant steel for high-temperature structures.0 Prior Art High-temperature structural materials used in the temperature range of SOO°C to SOO°C have a tendency to maintain their properties over a long period of time. of soundness.

Good creep strength, oxidation resistance, etc. are required.

Therefore, in order to meet this demand, conventional 25Cr-
2ONi steel (HK40) and 18Cr-1ONi-Nb
There were structural materials for high temperature chemical plants such as (SUS 847).

The problem that the invention aims to solve However, according to these conventional heat-resistant steels.

There was a problem in that the creep strength rapidly decreased during long-term use at high temperatures, and the creep strength was also not very high.

The present invention solves these problems, and the creep strength does not suddenly decrease even when used at high temperatures for a long time.
The object of the present invention is to provide a heat-resistant steel for high-temperature structures that also has high creep strength.

Means for Solving the Problems In order to solve the above problems, the heat-resistant steel for high temperature structures of the present invention contains 0.03 wt% or less of C, 1.0 wt% or less of Si,
Mn is 0.15 to 10 wt%, P is 0.03 wt% or less, S is 0.03 wt% or less, Cr is 15.0 to 21.0
wt%aNi is 8.0 to 14.0wt%, Co is 10.0wt%.
0 to 25.0 wt%s Mo is 1.0 to 6.0 wt%,
The structure was such that Nb was 0.1 to 1.0 wt% and the balance was Fe.

In other words, the heat-resistant steel for high-temperature structures of the present invention has improved high-temperature strength characteristics by lowering the C content and utilizing solid solution strengthening through the composite addition of sCosMo%Nb.

EXAMPLE An example of the present invention will be described below with reference to Tables and Drawing 1ζ.

Table 1 shows the chemical composition range of the alloy of the present invention.

Table 1 Chemical composition range (wt%) of the alloy of the present invention Next, the effect (reason for limitation) of each element in Table 1 will be explained.

C: Creep strength decreases due to agglomeration of carbides such as Cr* Mo and Nb during long-term use at high temperatures, so the content should be kept below 0.011 wt%.

Ni is necessary for stabilizing the niostenite phase and improving toughness, and coexisting with Cr is effective in improving corrosion resistance. ξ The amount added to satisfy these effects is 8.0 =
14. Ow t% is required.

Cr: An important element for improving oxidation and corrosion resistance and increasing creep strength. In this alloy, the effect is small when the Ni content is less than 0 wt%, considering the balance with the Ni content. Further, since it is the main element that generates the # phase, the content in this alloy is up to 21.0 wt% in order to suppress the generation of the - phase.

Co: solid solution strengthening, stabilization of austenite phase, *during manufacturing,
It is possible to add a large amount because it has the effect of improving cancer flowability during welding, but in the case of a single alloy, considering the degree of increase in creep strength and economic efficiency due to combined addition with MO and Nb.
．． It is desirable to add up to 0 wt%. The lower limit is 10wt%
If the above amount is added, the austenite phase will be stabilized, and the addition will also strengthen creep.

MO2 is a strong solid solution strengthening element, and 0. Addition of lwt% or more is effective, and as the amount added increases, creep! The degree of reinforcement increases, but if it exceeds 6.0 wt%, it becomes brittle. Also, since it is an element that forms the # phase, 6.0
It is desirable to suppress the content to within wt%.

Nb (two strong carbide forming elements), Cr and Mo
Nb carbide is generated preferentially to carbide, and Cr and Mo contribute to effective Gζ creep strengthening and corrosion resistance improvement. In addition, due to low C, Nb addition is g, oawt%
If Nb is present, the above effects can be sufficiently achieved, but solid solution strengthening by Nb atoms is strong, and creep strengthening can be achieved by adding 0.1 wt% or more. Increasing the amount added significantly increases embrittlement.

The upper limit is 1.0 wt%.

Mn has the effect of stabilizing the niostenite phase, but since it promotes the formation of sulfides and I-phase bioacids, Q, 5 to λ0 wt%
limited to.

Si: It has the effect of improving melt flow during manufacturing.

1.0wt% because it promotes the formation of l phase and causes % embrittlement.
The following shall apply.

P, S: Contamination as impurity elements is unavoidable.

It is permissible up to the range shown in Table 1.

Next, Table 1 shows the chemical composition of #2 #ζ test material.

Table 2 Chemical analysis results of sample materials (wt%) Name comparison Kinmotokai: V; Alloy () is an at% conversion value Table 2 shows the at% value written in () in addition to wt%. There is. Alloy A is a commercially available SUS 847 austenitic steel, and Alloy B is a commercially available austenitic steel called HK4G, which is often used in re-former tubes. These comparative alloys are particularly superior among current structural materials for high-temperature chemical plants. It is a commercially available material with excellent creep strength and oxidation resistance. Alloys C and D of the present invention satisfy the chemical composition range of the alloy of the present invention.

L-phase acids pose a problem in austenitic heat-resistant alloys because they have the risk of causing embrittlement. By doing so, the average number of electron vacancies of each alloy can be determined, and a rough estimate of the I-phase bioacid can be obtained.

Fourth, the results of calculating the average number of electron vacancies for each alloy are shown, and it is said that the larger these values are, the easier it is to form an I-phase acid ◎This developed alloy has a composition specification that makes it difficult for the I-phase to form. You can see that it is happening.

Table 4 Average number of electron vacancies for each alloy Table 5 shows the measurement results of the coefficient of thermal expansion. For structural materials for high temperature use, it is advantageous to have a small coefficient of thermal expansion in order to reduce the occurrence of thermal stress. From Table 5, it can be seen that the thermal expansion coefficient of the developed alloy is smaller than that of the comparative alloy.

Table 5 Results of thermal expansion coefficient measurements Figure 1 shows the 800°C creep rupture life of the comparative alloy and the developed alloy. It can be seen that the developed alloy has higher creep strength on the long-term side, and the degree of deterioration of creep strength is smaller. All of the comparative alloys showed significant strength deterioration on the long-term side, and their fracture curves were bent.

Figure 2 shows the results of an oxidation weight increase test conducted to compare oxidation resistance, and the developed alloy is at the same level as the comparative alloy, which has good oxidation resistance.

Effects of the Invention According to the heat-resistant steel for high-temperature structures of the present invention, there is no sudden drop in creep strength during long-term use at high temperatures, and the steel has high creep strength. therefore.

By using this alloy as a structural material for high-temperature chemical plants, long-term equipment safety can be guaranteed. It can also be widely used as cast, forged, and rolled materials.

[Brief explanation of drawings]

FIG. 1 shows the relationship between applied stress and rupture time in the comparative alloy and the alloy of the present invention, and FIG. 2 shows the oxidation weight gain in the comparative alloy and the alloy 1ζ of the present invention.

Claims (1)

[Claims] 1. C is 0.03 wt% or less, Si is 1.0 wt% or less, Mn is 0.5 to 2.0 wt%, P is 0.03 wt% or less, S is 0.03 wt% or less, Cr is 15.0 ~21.
0 wt%, Ni 8.0 to 14.0 wt%, Co 10
．． 0 to 25.0 wt%, Mo 1.0 to 6.0 wt%,
A heat-resistant steel for high-temperature structures, characterized in that Nb is 0.1 to 1.0 wt%, and the balance is Fe.