JPS5929104B2 - Austenitic heat-resistant steel with excellent hot workability and oxidation resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenitic heat-resistant steel that has excellent hot workability and oxidation resistance and is suitable as a material for equipment used in high-temperature environments.

Equipment and equipment used in high-temperature environments, such as various combustion equipment, heating furnace jigs, heat exchangers, and combustion parts for heating equipment, require hot workability and oxidation resistance. Conventionally, AISI302B (18Cr,
9Ni, 2.5Si), XM15Jl (18Cr, 14
Ni, 3Si), SUS309S (23Cr, 14Ni
)+2Si, SUS310S (25Cr, 20Ni, I
si), DIN4828 (20Cr, 12Ni, 2Si
) or AISI314 (25Cr, 20Ni, 2Si
) was used, but XM15J1, SUS309S
+2Si, AISI314, etc. have high Si and have poor hot workability such as tip cracks and many corner cracks during hot rolling, while XM15J1 has low Cr, S
US310S had drawbacks such as poor heat resistance due to its low Si content.

Moreover, in recent years, demands have become even more demanding.
In other words, it not only has low oxidation during continuous heating at high temperatures, but also has many properties such as oxidation resistance, including less scale flaking even after repeated heating at high temperatures and cooling to room temperature. It is required that the The present invention was developed as a result of experiments to develop an austenitic heat-resistant steel with excellent hot workability and heat resistance.
High-Si austenitic heat-resistant steel with B1 rare earth elements, Ca,
It has been found that hot workability, oxidation resistance, etc. can be improved by adding a small amount of Mg in combination as needed.

The present invention was constructed based on this knowledge, and S
It is a steel that has hot workability and oxidation resistance of US309S12Si or AISI314 or higher, and meets the above-mentioned recent requirements. That is, in the present invention, Co82% or less, 5i1.5 to 3.5%,
Mn2, O% or less, Ni8.0-35.0%, Cr15
．． 0~30.0%, AI20O or less, B0.0005~
0.0050%, rare earth elements 0.005-0.100%
and 0.0005 to 0.020% of Ca, and if necessary, 05 to 0.030% of MgO and OO, the balance being iron, . The gist of this invention is an austenitic heat-resistant steel with excellent hot workability and oxidation resistance, which is made up of impurities and unavoidable impurities.

The reason why the constituent components of the steel of the present invention are limited as described above will be explained below.

C is an austenite-forming element, but if it is contained in an amount exceeding 0.2%, hot workability and oxidation resistance will be deteriorated, so the content is set to 0.2% or less.

Si is an oxidation resistance improving element. Further effects can be obtained by combined addition of rare earth elements, Ca and Mg, which will be described later.
If the Si content is less than 1.5%, the effect will be small; 3.
If it exceeds 5%, a δ ferrite phase or a σ phase is formed, which impairs hot workability and high-temperature mechanical properties. Furthermore, no improvement in oxidation resistance can be expected. Therefore, the content was limited to 1.5% to 3.5%. Mn is an austenite-forming element, but since it worsens oxidation resistance, the content should be reduced to 2.0
% or less.

Ni is an austenite-forming element, but in order to prevent the formation of the δ-ferrite phase caused by Si and improve hot workability (the lower limit of the Ni content is set at 8.0%, and the upper limit is set in consideration of economic efficiency). It was set at 35%.

Cr is an element that improves oxidation resistance, but if it is less than 15.0%, the effect is small, and if it is more than 30.0%, it produces δ ferrite phase and σ phase that are harmful to hot workability and high-temperature mechanical properties. However, in order to prevent this, a large amount of Nj is required and the cost becomes high.

Therefore, the content was set from 15.0% to 30.0%. AI is effective as a deoxidizer as well as an alloy for improving oxidation resistance.

However, AI is also a strong δ ferrite-forming element, with 2%
If it is added in excess of 5%, δ ferrite is likely to be produced, which impairs hot workability, so the content was set to 2.0% or less.
B is effective as an element for improving hot workability.

B retards grain boundary diffusion, suppresses grain boundary concentration and grain boundary precipitation of O, S or impurities, increases the ductility of the grain boundary region, and improves hot workability. The content must be at least 0.0005%, and if it exceeds 0.0050%, hot workability at temperatures above 1200°C cannot be expected, so the maximum content is 0.005%.
It was limited to 50%. No adverse effect on oxidation resistance was observed due to this addition amount. Rare earth elements such as Ce and La are effective as elements that improve hot workability and oxidation resistance.

Rare earth elements have a strong affinity with O, S, etc., and have a deoxidizing and desulfurizing effect7 to purify grain boundaries and the like. However, when used alone, nonmetallic inclusions peculiar to rare earth elements are generated, resulting in poor cleanliness and oxidation resistance cannot be expected. Rare earth elements are also added in combination with Ca, which will be described later, to make deoxidation products easier to float, purify the steel, and improve hot workability and oxidation resistance. The content is required to be at least 0.005% in order to achieve the above effects, and the upper limit is 0.100% because it impairs hot workability and is expensive.
And so. Ca is effective as an element that improves hot workability and oxidation resistance.

Ca alone has a poor yield and small effect, but combined addition of rare earth elements and Mg, which will be described later, exhibits a deoxidizing and desulfurizing effect, improves cleanliness, and significantly improves both of the above properties. The content should be at least 0.0 to achieve the above effects.
0.005%, and the maximum was set at 0.020% in consideration of forming a low melting point phase and impairing hot workability. Mg is effective as an element that improves hot workability and oxidation resistance.

Like Ca, Mg also has a deoxidizing and desulfurizing effect to detoxify S and impurities. Mg forms a compound with S, fixes S, and suppresses segregation to grain boundaries. In addition, complex addition with rare earth elements and Ca purifies and increases the melting point to improve workability in high temperature ranges. Furthermore, since the addition yield of Mg is more stable than that of Ca, Mg is used to supplement Ca. The content needs to be at least 0.0005% in order to achieve the above effects, and the maximum content is 0.005% in consideration of forming a low melting point phase and worsening hot workability.
It was set at 30%. The steel of the present invention described above has the following excellent properties.

In other words, the steel of the present invention, which is a high-Si austenitic heat-resistant steel with the combined addition of a B1 rare earth element, Ca, and trace amounts of Mg as required, has a temperature range from 1,000°C to 1
, has good hot workability in a wide temperature range up to 250℃,
It has better hot workability and oxidation resistance than existing high-Si austenitic heat-resistant steels. The reason for the poor hot workability of existing high-Si austenitic heat-resistant steels is the high Si
Therefore, the deformability is low, and O, S and impurities tend to concentrate in the grain boundaries.

As mentioned in the reason for limiting the components, in the steel of the present invention, the strengthening of grain boundaries by B, the cleaning by rare earth elements, Ca, and Mg, and the fixation of free S work synergistically to improve the above-mentioned drawbacks and to improve the high temperature resistance. It is thought that this increases ductility.

In addition, the surface oxide film of existing high-Si austenitic heat-resistant steel, which is said to have poor oxidation resistance, is Cr2O3,
Oxides such as Fe2O3, SiO2, MnOl and others are distributed non-uniformly and tend to easily cause cracks and peeling.

Furthermore, when a large amount of free S is contained in the steel, MnS and the like tend to form at the interface with the scale and easily peel off. In the case of oxidation resistance, B, rare earth elements, Ca
It is believed that Mg promotes the diffusion of Cr and Si, cleans and fixes free S, makes the surface oxidation film mainly composed of Cr2O3 adhesive, and improves oxidation resistance. The austenitic heat-resistant steel of the present invention as described above is produced by producing a steel billet from refined molten steel in a normal steelmaking furnace, and then hot rolling the steel.

Furthermore, the steel of the present invention has excellent hot workability and oxidation resistance, so it is optimal as a material for use in various heat-resistant parts. Next, embodiments of the present invention will be described.

Table 1 shows the steel compositions of the invention steel (A-J) and comparative steels (1 to 9) melted in an electric furnace, and Table 2 shows the torsion test results of the steel ingots of these steels. Table 2 shows the results of a torsion test conducted to examine the hot workability of the steels shown in Table 1. The test conditions are:
4. Increase the heating temperature from 1,000°C to 1,250'C.
Under the conditions, the steel ingot specimen between the parallel parts 10f x 30 was rotated at a rotation speed of 25r. p. The vermilion setting performed at m is expressed in terms of the number of ruptures.

From Table 2, B1 rare earth element, Ca, and Mg added as needed, invention steel (A). , (B) , (Q, (D)
, (廓(wa, ((3, (I{) , (I)) Comparative steel 1 (steel equivalent to AISI3O2B) without these additions
, Comparative steel 2 (SUS3O9S+2Si steel), Comparative steel 8 (AISI3l4 equivalent steel), Comparative steel 9 (High Ni, C
r+2Si steel), the invention steel exhibits a significantly higher number of ruptures, demonstrating its good deformability.

When B is added as a rare earth element or a combination of B and Ca (comparative steels (3) and (4)), the effect is small; It can be seen that the effect is achieved by adding B or a combination of B and Mg.

Comparative steel <5), (6) with combined addition of rare earth elements, Ca, and Mg as necessary,
, has good hot workability up to 250℃, but has a composite addition of B1 rare earth elements, Ca, and Mg as necessary.
, (G) , (L) , (E) , (1'1, (
Gl, (}I, (υ, (J)) increases from the number of ruptures at 1,000°C, indicating that the effect of B addition is evident. The values of those that have entered the grains show a slight tendency to decrease at temperatures above 1,200°C. It is thought that ductility may be reduced, but this effect is not observed when added in small amounts.
Invention steel with increased AI content (Invention steel (J) with increased 0, Ni, and Cr content, invention steel (G) to CF)
, (}1, (D) showed almost the same one-turn rolling and obtained good results.On the other hand, the steel of the present invention to which the B1 rare earth element, Ca, and a small amount of Mg were added, also showed good results in actual rolling. No tip cracking or corner cracks occurred, and significant improvements were made in reducing eaves removal man-hours and increasing yield.Table 3 shows the oxidation resistance of the steels shown in Table 1. note,
In particular, in order to examine the flaking resistance of the scale, the scale was heated to 1,100° C. for 725 minutes in the atmosphere using a repeated oxidation tester, and air-cooled for 5 minutes, and the weight change was measured.

Table 3 shows that the steel of the present invention to which B1 rare earth elements, Ca, and optionally Mg are added has significantly superior oxidation resistance when compared with existing high-Si austenitic heat-resistant steels within groups with equivalent Cr and Ni contents. You can see that

That is, the invented steels (5) and (B) are more similar to the comparative steel (1) than the invented steels (0, (c), (El, (11'), (
Q is comparative steel (2), (3), (4), (5
) , (6L (invention steel 0 from force, (4) is comparative steel (8)
Therefore, the oxidation loss of the invention steel (J) is smaller than that of the comparison steel (9).

As described above, the steel of the present invention is particularly superior in scale peeling resistance than existing high-Si austenitic heat-resistant steels, and shows high oxidation resistance under use environments where it is repeatedly heated and cooled. In short, the steel of the present invention exhibits excellent hot workability and excellent oxidation resistance by adding a B1 rare earth element, Ca, and appropriate amounts of Mg as necessary to a high-Si austenitic heat-resistant steel. It is.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the heating temperature and the number of ruptures in a torsion test of an austenitic heat-resistant steel ingot.
FIG. 2 is a graph showing the relationship between the number of repetitions and weight change in the repeated oxidation test of the same steel.

Claims (1)

[Claims] 1. C 0.2% or less, Si 1.5 to 3 in weight percent
．． 5% Mn 2.0% or less, Ni 8.0-35.0%, C
r15.0-30.0% Al2.0% or less, B0.00
05-0.0050%, rare earth elements 0.005-0.1
Austenitic heat-resistant steel with excellent hot workability and oxidation resistance, containing 0.00% and 0.0005 to 0.020% Ca, with the balance being iron and unavoidable impurities. 2 C0.2% or less, Si1.5-3 in weight percent
．． 5% Mn 2.0% or less, Ni 8.0-35.0%, C
r15.0-30.0% Al2.0% or less, B0.00
05-0.0050%, rare earth elements 0.005-0.1
00%, Ca0.0005-0.020% and Mg0
．． Austenitic heat-resistant steel with excellent hot workability and oxidation resistance, containing 0.0005% to 0.030%, the balance being iron and unavoidable impurities.