JPS61163243A - High chromium heat resisting steel with improved toughness - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high chromium heat resistant steel with improved toughness, particularly a high chromium heat resistant steel that exhibits excellent toughness even after long-term use at high temperatures.

(Prior Art) High chromium heat-resistant steel has traditionally been used in boiler superheaters, reheaters, and heat exchangers for the chemical industry and the atomic crab industry.

A problem with high-chromium heat-resistant steel is a decrease in toughness after long-term use at high temperatures.

Until now, we have developed high-chromium steel with a large amount of Mo added to increase its high-temperature creep strength (Japanese Unexamined Patent Publication No. 55-11
No. 0758, JP 59-14097, JP 59-
No. 140352).

Furthermore, as a countermeasure against temper embrittlement in low chromium steel, S
There are reports that reducing the i content is effective, and on the other hand, it was also found that lowering the St can improve the toughness of high chromium steels, but lowering the St will rather improve the steam oxidation resistance and high temperature strength. The problem was that it was damaged. In other words, it cannot be used as a practical steel simply by reducing the amount of Si.

(Problem to be solved by the invention) However, as a new finding obtained by the present inventors, a large amount of M
It has been found that in o-added steel, intermetallic compounds containing Mo precipitate during long-term use at high temperatures, resulting in a significant deterioration in toughness. Furthermore, although the effect is said to be due to a reduction in Si, when used as a heat-resistant steel at high temperatures, steam oxidation resistance and high-temperature strength are impaired, and a certain amount of Si is required to ensure these characteristics. I knew what I had to do.

It is thus an object of the present invention to provide a high chromium heat resistant steel that exhibits excellent toughness even after prolonged use at high temperatures.

Another object of the present invention is to avoid adding too much MO;
It is an object of the present invention to provide a high chromium heat-resistant steel that does not unnecessarily lower Si and eliminates the various drawbacks of the prior art.

(Means for solving the problem) The present inventors have determined that in order to improve toughness, the amount of Mo should be reduced to
, C. After that, through a series of experiments, they discovered a steel composition that would not cause a decrease in toughness even when used at high temperatures for long periods of time, and completed the present invention.

Here, the gist of the present invention is, in weight %, C: 0.03 to 0.15%, Si: 0.6% or less, Mn: 0.1 to 1.0%, Cr: 7-
13%, Mo: 1.0% or less, V: o, x
~0.4%, Nb: 0.02~0.10%,
AQ: 0.02% or less, N: 0.010-0.
060%, p: o, o 3% or less, S: 0.01% or less, the remainder consists of iron and unavoidable impurities, and C% St, Mo in the above component range is Mo-1,4C+ 0.7Si ≦0. It is a high chromium heat-resistant steel with excellent toughness that satisfies the relationship 8.

As described above, the feature of the present invention is that in order to improve the toughness after long-term use at high temperatures, the amounts of Mo, Si, and Ci are balanced and limited by a certain relational expression. A high chromium heat-resistant steel is obtained that does not impair not only high temperature creep strength but also steam oxidation resistance.

(Function) Hereinafter, in the present invention, the composition ranges of the component elements are limited as described above. The reason for limiting the amount added will be explained.

C: In order to impart high temperature strength, it is necessary to add 0.03% or more of C. However, excessive addition exceeding 0.15% impairs the weldability and workability of the material. Therefore, the amount of C is 0.03 to 0.15%, preferably 0.07 to 0.1%.
The range was set at 5%.

Si: Si is added as a deoxidizing agent, but if it is present in an excessive amount, it will promote embrittlement during heating, so it is desirable to keep it as low as possible from the viewpoint of improving toughness. Therefore, the upper limit of the amount of Si was set to 0.6%. However, Si is essential for improving steam oxidation resistance and is an element that also contributes to ensuring high-temperature strength.

Therefore, the appropriate amount of Si is set using the relational expression described later.

In order to improve steam oxidation resistance and high temperature strength, it is preferable to contain more than 0.2%.

Mn: Mn is an element necessary for improving strength and toughness.
．． It is necessary to add 1% or more. However, in the present invention, the amount of Mn is set to 1.
It was set to 0% or less.

Cr: An important element that improves oxidation resistance at high temperatures.
Addition of 7% or more produces an effect, but addition of more than 13% tends to cause δ-ferrite, resulting in a decrease in toughness and strength. Therefore, in the present invention, the amount of Cr is 7 to 13
%.

Mo: Mo is added to high-temperature materials as a solid solution strengthening element, but when added in large amounts, it forms precipitates during heating and
It causes embrittlement. Therefore, it was set at 1.0% or less.

Preferably, the upper limit is below the solid solution level.

On the other hand, the lower limit should preferably be 0.1% or more from the viewpoint of creep strength. More preferably 0
．． It is 5% or more and less than 0.8%.

Note that the appropriate amount of Mo is set using a relational expression described later.

(2) As a precipitation strengthening element, (2) needs to be added in an amount of 0.1% or more, but excessive addition of more than 0.4% increases solid solution (2) and impairs strength. Therefore, in the present invention, 0.1 to 0.
The range was set at 4%.

Nb: Since Nb improves toughness through precipitation strengthening and microstructural refinement, even in the present invention, Nb is 0.0
It is necessary to add 2% or more. However, excessive addition of more than 0.10% results in a decrease in strength.

Therefore, the appropriate range was set at 0.02 to 0.10%.

AQ: AQ is added as a deoxidizing agent, but if it is present in excess, creep strength decreases. Therefore, in the present invention, the AQ amount is set to 0.02% or less.

N: N is an important element that improves high-temperature creep strength, and mainly improves strength by forming carbonitrides with (1) and Nb. 0
．． This effect is exhibited by adding 0.1% or more, but if a large amount is added, blowholes are formed during solidification, causing material defects.

Therefore, the amount added was set at 0.01 to 0.06%. It is preferably added in an amount of 0.045 to 0.055%.

P and S: Both of these elements have a significant negative effect on toughness, and it is better to keep them as low as possible. Therefore, upper limits were set for P≦0.03% and S≦0.01%, respectively.

Furthermore, according to the present invention, an appropriate balance between the amounts of C and Sis Mo is defined in order to improve the toughness after long-term heating.

As already mentioned, as a result of studies conducted by the present inventors, it was found that the decrease in toughness of high-chromium heat-resistant steel after long-term heating is mainly caused by intermetallic compounds containing MOlSi, which precipitate in an increased amount during heating. I found out. Therefore, it was found that embrittlement can be suppressed by limiting the amount of Mo, Si. However, from the viewpoint of strength, it is necessary to increase tMo, and it is also necessary to ensure an appropriate amount of Si. On the other hand, it has been revealed that C forms carbides and partially fixes Mo, thereby suppressing the increase in precipitates that cause embrittlement. Therefore, the present inventors converted the above findings into a mathematical formula, and calculated that C, Si, and Mo within the above component range are Mo-1,4C10.7
It has been found that if the relationship Si≦0.8 is satisfied, embrittlement during heating can be suppressed and toughness can be improved. The decrease in strength caused by decreasing the Mo content can be effectively improved synergistically by making a relatively large amount of N exist and adding a nitride-forming element such as V to strengthen the steel by precipitation.

Thus, according to the present invention, it is possible to suppress a decrease in toughness due to long-term heating at high temperatures, and unlike conventional steels, the steam oxidation resistance and high-temperature creep strength are not impaired. Therefore, the high chromium heat-resistant steel according to the present invention can be expected to be used for boiler steel pipes, nuclear power publications, fusion reactor first reactor wall materials, and the like.

Next, the present invention will be further explained by examples.

Table 1 summarizes the chemical compositions of the inventive steel and comparative steel used in this example. Test steel No. 1 to No. 3 is 950°
5TBA normalized at 750°C and then tempered at 750°C
26 steel. No. 4 is an ASTM A213-T91 steel to which Nb was further added to improve strength, and this steel was normalized at 1050°C and then tempered at 780°C.

Test steel No. 5 to 111o, 8, No., which is the steel of the present invention,
Ll was normalized at 950°C and further tempered at 750°C. Also, No. 9, No. 10 are 12
The present invention is applied to Cr6m, which is a material that is normalized at 1050°C and tempered at 780°C.

JIS No. 4 Charpy impact test pieces were taken from these materials*, and the impact value at 0°C was determined. Furthermore, as a long-term heating test, aging at 550° C. for 3000 hours, where embrittlement is most noticeable, was performed, and then the same test was performed.

These results are summarized in Table 2.

As is clear from the results shown in Table 2, test steels No. 5 to No. 8 and No. 11, which are the steels of the present invention, have a toughness of 10 kgf-m/cf or higher after the heating test. .

The decrease in toughness is significantly reduced compared to comparative steels.

Test steels No. 9, No. 10, which are also inventive steels
This is a case in which 12Cr steel is normalized at a high temperature, and although the toughness after heat treatment is slightly low, it is characterized by a small decrease in toughness due to heat treatment.

From the above results, it is clear that C, Si, Mo
By optimizing the temperature, we were able to demonstrate the effect of improving toughness after long-term heating.

Furthermore, for the purpose of understanding the strength characteristics, test steel N, which is a comparative steel,
o, 1.1llo, 4 and test steel 11 which is the steel of the present invention
1o, 7 600°C creep rupture test was conducted. The results are summarized and shown in graphs in the attached drawings. As shown in the figure, Heas's 5TBA 26 steel (comparative steel test steel No.
1), it has higher hardness and is an improved type of 9Cr-I.
Even when compared with Mo steel (comparative steel test steel No. 4),
There is almost no decrease in strength.

(Effects) As described above, according to the present invention, a high chromium heat-resistant steel with improved toughness after long-term heating at high temperatures and excellent strength has been obtained and developed, and the present invention greatly contributes to this field. I understand.

[Brief explanation of the drawing]

The accompanying drawings are graphs showing stress-creep rupture time curves for examples of the present invention.

Claims (1)

[Claims] In weight%, C: 0.03 to 0.15%, Si: 0.6% or less, Mn
:0.1~1.0%, Cr:7~13%, Mo:1.0
% or less, V: 0.1-0.4%, Nb: 0.02-0.
10%, Al: 0.02% or less, N: 0.010-0.
060%, P: 0.03% or less, S: 0.01% or less, the remainder consists of iron and unavoidable impurities, and C, Si, and Mo in the above component range are Mo-1.4C+0.7Si≦0.8 A high-chromium heat-resistant steel with excellent toughness that satisfies the following relationship.