JPS5817820B2 - High temperature chrome steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high chromium steel that has extremely high high temperature strength, particularly long-term creep strength in a temperature range exceeding 550°C.

This invention is applicable to power generation plants, chemical plants, etc.
The purpose of the present invention is to provide a steel that has excellent corrosion resistance, oxidation resistance, and high-temperature strength, and can be used for parts that require no decrease in strength even after long-term use.

Many steel types are commercially available for use in the above applications, ranging from low alloy steels such as 2/4Cr-lMo steel to austenitic stainless steels containing large amounts of Cr and Ni.
In addition, there are many proposals for improved steel types in which various alloying elements are added to these commercially available steels.

However, most existing steels have some kind of problem, for example, in oxidation resistance, high-temperature strength, workability or weldability, or even the price of steel, so they are not ideal in practical terms. There are few high-temperature steels.

Taking boiler heat exchanger tubes as an example, which is one of the most typical of the above-mentioned applications, their manufacturing and usage conditions require that they have good cold workability, weldability, oxidation resistance at high temperatures, and high strength. Steel is desired to have essential properties, have a good balance of these properties, and be inexpensive.

This invention focuses on medium-high Cr (5 to 13% Cr) steel, which is superior to low Cr heat-resistant steel in terms of oxidation resistance and superior to austenitic stainless steel in terms of stress corrosion cracking resistance and price, and its mechanical properties, In particular, this was achieved by aiming for a large difference in high-temperature strength.

The gist of this invention is a high-temperature steel as described below.

(1) C0.02~0.22%, Si1.0% or less, Mn1.0% or less, Cr5~13%, Mo 0.5~
2.5%, Vo, 01-0.5%, NO, 015-0.
10%, the balance consists of Fe and impurities, A in the impurities
A steel in which l is 0.02% or less, and the content of l and N satisfies the condition of N (%) - 6,5 An (%) ≧ 0.015 (%).

(2) In addition to the components in (1) above, 1.5% or less of W
l Steel containing at least one of Ti, Zr, and B, each of 0.5% or less and 0.05% or less of B.

(3) A steel in which 1.5% or less of Ni is further added to the steel of (1) or (2) above.

In this invented steel, it is important that all components, including C, are within specific ranges, but the most important feature is that the amount of N and the upper limit of Al as an impurity are regulated. be.

It is well known that N is a solid solution strengthening component in steel, and it has already been proposed to increase the amount of solid solution N in low alloy steel in order to increase the creep strength.

However, in medium-high Cr steel, most of the N is fixed as nitrides, so the effects of N as described above cannot be directly expected.

For example, when N is added to 12Cr steel, the creep rupture strength increases at temperatures up to 550°C, but there are reports that the rupture strength decreases over time at high temperatures of 650°C compared to steel without N addition.

Based on the steel with the specific composition described above, the inventors of the present invention conducted a detailed study on the effect of N addition, especially in relation to Al remaining in the steel, and found that 0.5 Al from the effective N, that is, total N amount.
550 if the amount of N that is less than the specified amount is contained
It was confirmed that high creep strength was maintained even at high temperatures exceeding 104°C and for long periods of time exceeding 104 hours.

N, which contributes to improving high-temperature strength in this way, is mainly dispersed in the steel as a compound with Cr (CrN or Cr2N).

Therefore, if more A7 remains in the steel and N is fixed as AlN, the effect of N will be lost.

In this invention, it is from this viewpoint that the correlation between N and A7 is determined as well as the N content and the allowable upper limit amount of A7.

FIG. 1 shows 12 Cr-IMo-0,5N i -0,
These are the results of testing the relationship between the voltage of N (%) - 0.5 A# (%) and creep rupture time by changing the temperature and load stress for 3V.

○ is 550℃, 23kg/mA, △ is 600℃, 15,
5yu/-1 mouth is a test at 650℃ and 9kg/7n7N, but in both cases, N (%) - 0.5AA (%
) is 0.015%, a rapid increase in rupture life is observed.

In this invention, N(%)-0,5Al(%)≧0.
The condition of 0.015 (%) was established based on the above knowledge.

Next, the reason for limiting the content of each component will be explained.

C increases creep strength by forming carbides.

However, if there are too many −〇, weldability and cold workability will be impaired.

The appropriate range is 0.02% to 0.22%.

Si is mainly added as a deoxidizing agent.

When the residual amount of Si exceeds 1%, toughness decreases and creep strength is also adversely affected.

Mn acts as a deoxidizing agent and also contributes to improving hot workability.

However, if it exceeds 1%, hard brittle phases tend to occur.

Cr is a fundamental component that provides essential properties to heat-resistant steel, including oxidation resistance.

The oxidation resistance improves as the amount of Cr increases, but when the amount exceeds 13%, the structure of the steel tends to become a single ferrite phase, and the toughness and high-temperature strength decrease.

Although the amount of Cr can be adjusted within the range of 13% or less depending on the application, an amount of at least 5% is required to provide the necessary oxidation resistance based on the expected applications of the steel of this invention.

For applications such as boiler tubes, it is desirable to use a higher Cr content of 9 to 13%.

Mo is added mainly to improve creep strength.

If it is less than 0.5%, it is difficult to obtain the necessary strength, and if it exceeds 3%, the structure becomes unstable and the price of the steel becomes too high.

(2) forms carbides and partially nitrides, which greatly contributes to improving creep strength.

This effect is small if the amount is less than 0.01%.

However, even if it is added in an amount exceeding 0.5%, the effect does not increase much.

N is extremely effective in improving high-temperature strength, especially long-term creep strength at high temperatures exceeding 550°C.1 As already mentioned, the amount of N needs to be determined in relation to Al, but the minimum required amount is It is 0.015%.

On the other hand, when N exceeds 0.10%, toughness, workability, and welding are adversely affected.

Within this range, N(%)-0,51? (%)≧0.01
It is necessary to satisfy the condition of 5 (%).

As a result, N, which is not fixed as AIN, precipitates finely as Cr nitride during tempering, increasing the creep strength.

Kl is used as a deoxidizing agent and contributes to improving toughness, and also reduces room temperature strength and improves workability.

However, in order to increase the effective amount of N, it is better to have less Al.

As the amount of Al increases, the amount of N consumed as A7N increases.

For this reason, in the present invention, the upper limit of residual allowable amount of A and l is set to 0.02%.

W, Ti, Zr and B are of the steel having the above basic composition.

It is added as necessary to further increase the strength.

Ti and Zr contribute to increasing strength by precipitating carbonitrides.

The content of each of these may be up to 0.5%.

B has the effect of increasing the creep resistance of the steel and particularly increasing the creep rupture strength on the long-term side.

However, if the content exceeds 0.05%, the strength tends to decrease.

Like Mo, W not only strengthens the matrix by forming a solid solution in the ferrite matrix, but also increases creep strength by forming carbonitrides.

However, if it exceeds 1.5%, toughness will be impaired and the price will become too high.

By containing up to 1.5% of Ni, it contributes to stabilizing the structure and improving the toughness of the steel of this invention.

However, since Ni is an expensive component, it may not be necessary to actively add it depending on the use of the steel.

There are other impurities that are inevitably mixed in due to steel manufacturing technology, but
The representative ones, P and S, are each 0
．． It is desirable to suppress it to 0.35% or less.

In principle, the steel of this invention is used after being subjected to heat treatment of normalizing and tempering.

The structure after this heat treatment is a base of tempered martensite or ferrite and tempered martensite, with fine nitrides of ROM and carbides of Mo, 'V, or even W.

Carbonitrides of TiyZr, Nb, and Ta are precipitated and dispersed.

Steel with such a structure is superior to austenitic stainless steel in terms of resistance to stress corrosion cracking and thermal conductivity, and is not only cheaper but also has superior properties due to the characteristics of the material itself. It is extremely advantageous.

On the other hand, compared to single-phase ferrite steels, impact toughness, especially the toughness of the weld heat affected zone, is much higher.

Preferable conditions for the above heat treatment include a normalizing temperature of 95
The temperature is 0 to 1250°C, and the tempering temperature is 600 to 850°C.

Example Table 1 shows the composition of the sample materials.

These steels were subjected to tensile tests at room temperature and 600°C, and creep rupture tests at 600°C for 10' hr.

The results are shown in Table 2.

As shown in Table 2, the present invention steel A with martensitic structure
, B, C, D, and E have high creep rupture strength because the effective nitrogen content is greater than a predetermined value.

On the other hand, comparative steel P has a high total nitrogen content, which is comparable to that of steel E, but has a high aluminum content, and in terms of effective nitrogen, it is significantly lower than that of the invention steel, and its creep rupture strength is also low. It has become.

It is recognized that the creep rupture strength of steels containing W is dominated by effective nitrogen in the present invention steels F and G and comparative steel Q.

FIG. 2 shows the structures of the invention steel A and comparative steel P (10,000x electron micrograph).

I was tested at 600°C before the creep test (as heat treated).
1C is the structure after the creep test at 650°C.

Fine precipitates that precipitated during heat treatment were observed in the steel A of the present invention, and these precipitates remained stable even during the creep test.

However, comparative steel P does not have such precipitates, and the difference in creep rupture strength shown in Table 2 above (steel A...
・12.2 kg/mA1 steel B...s, 6
kg/md1 (both 600°C x 1 o'hr) was found to be due to such tissue differences.

[Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between effective nitrogen content and creep rupture time in 12Cr steel, and Figure 2 is an electron micrograph (X 10''F
:be.

Claims (1)

[Claims] I C0.02 to 0.22%, Si 1.0% or less, M
n1.0% or less, Cr5-13%, Mo 0.5-2.
5%, Vo, 01~0.5%, NO, 015~0.10
%, the balance being Fe and impurities, the content of Al in the impurities is 0.02% or less, and the content of N and A4 satisfies the following conditions. N(%)-0,,5Al(%)≧0.015(%)2
C0.02~0.22%, Si1.0% or less, Mn1
．． 0% or less, Cr5-13%, Mo 0.5-2.5%
, Vo, 01-0.5%, NO, 015-0.10%,
and 1.5% or less W and 0.5% or less T, respectively.
i. Consisting of at least one of Zr and 0.05% or less of B, the balance being Fe and impurities, and Al in the impurities is 0.0
A chromium steel for high temperature use, characterized in that the content of N and Al is 2% or less and satisfies the following conditions. N(%)-〇, 5Al(%)≧0.01-5-(%)3
C0.02~0.22%, Si1.0% or less, Mn
1.0% or less, Cr5-13%, Mo 0.5-2.5
%, Vo, 01-0.5%, NO, O15-0.10%
, Ni 1.5% or less, the balance consisting of Fe and impurities,
A chromium steel for high temperature use, characterized in that AA in impurities is 0.02% or less, and the content of N and AA satisfies the following conditions. N (%) - o, 544 (%) ≧ 0.015 (%) 4
C0.02~0.22%, Si1.0% or less, Mn1.
0% or less, Cr 5213%, Mo 0.5-2.5
%, Vo, 01~0.5%, NO, 015~0.10%
, Ni 1.5% or less, and 1. % or less of W, at least one of Ti, Zr, and B of 0.05% or less, respectively, and the balance is Fe and impurities, and A7 in the impurities is 0.02% or less. , and the content of N and Kl satisfies the following conditions. N(%)-0.5A1. (%)≧0.015(%)