JPH0770713A - Heat resistant cast steel - Google Patents

Abstract

PURPOSE:To provide a heat resistant cast steel excellent in ductility, toughness and high temp. strength and sufficiently endurable to the application such as an extrasuper critical pressure turbine casing in thermal power plant. CONSTITUTION:The cast steel has a composition composed of by weight, 0.05-0.15% C, 0.1-1.5% Mn, 0.1-1.5% Ni, 9.5-13% Cr, 0.5-1.5% Mo, 0.1-0.3% V, 0.005-0.1% N, 0.1-5% Co, 0.1-5% W and if desired, one or more kind of <=0.1% Ti, 0.05-0.2% Nb and 0.01-0.2% Ta and balance Fe with inevitable impurities. The cast steel is excellent in high temp. strength and ductility and toughness and has a property sufficiently endurable as a cast steel article for a steam turbine used in a severe environment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heat-resistant cast steel used for turbine casings, valves and the like in thermal power plants.

[0002]

2. Description of the Related Art In recent years, in large thermal power plants, turbines used at ultra-supercritical pressure have been developed in order to increase output and improve efficiency. Cast steel products for steam turbines such as casings, flanges, and valves used under such ultra-supercritical pressure not only have excellent high-temperature characteristics to withstand harsh operating environments, but also have high toughness and age-related deterioration. Is required to be low. Conventionally, heat resistant cast steels such as 12Cr-Mo cast steel and 12Cr-Mo-V cast steel have been used for these cast steel products.

[0003]

However, under recent severer conditions, the above 12Cr-Mo cast steel and 12Cr-Mo-V cast steel do not have sufficient ductility and high temperature strength, and more ductility. 12 and good high temperature strength
Development of Cr-based heat-resistant cast steel is desired. The present invention has been made in view of the above problems, and an object of the present invention is to provide a novel 12Cr heat-resistant cast steel excellent in both ductility and strength.

[0004]

In order to solve the above-mentioned problems, the heat-resistant cast steel of the present invention has a weight percentage of C: 0.05-.
0.15%, Mn: 0.1-1.5%, Ni: 0.1-
1.5%, Cr: 9.5 to 13%, Mo: 0.5 to 1.
5%, V: 0.1-0.3%, N: 0.005-0.1
%, Co: 0.1 to 5%, W: 0.1 to 5%,
The balance is Fe and unavoidable impurities. In the above composition, Ti: 0.1% or less, Nb: 0.01-0.2%, Ta: 0.01-0.
One or more of 2% or less can be contained. Also,
The heat-resistant cast steel of the present application is, by weight% of the unavoidable impurities,
Si: It is desirable that the allowable content be less than 0.2%.

[0005]

The function of the present invention will be described below together with the reasons for limiting each component. C: 0.05 to 0.15% C combines with a carbide forming element to form a carbide and improves the high temperature strength, but if it is less than 0.05%, the strength is too low and it becomes unnecessary if it exceeds 0.15%. Since carbides coarsen and delta ferrite crystallizes and deteriorates high temperature properties, the range is set to 0.05 to 0.15%. For the same reason, it is desirable to further limit the upper limit to 0.12%. Mn: 0.1 to 1.5% Mn effectively acts as a deoxidizing agent in place of Si. 0.1% or more is necessary to obtain a sufficient deoxidizing effect, but if the content exceeds 1.5%, the toughness is impaired, so the content is limited to 0.1 to 1.5%.

Ni: 0.1 to 1.5% Ni is required to be 0.1% or more in order to guarantee the deterioration of the hardenability that occurs due to the reduction of the Si content.
If it exceeds 5%, the high temperature strength will decrease, so 0.1-1.
It was set to 5%. Cr: 9.5 to 13% Cr is a basic alloy component that enhances hardenability and high temperature strength in the steel of the present invention, and is required to be 9.5% or more, but 13%
If it is contained in excess of 1, the delta ferrite will crystallize and deteriorate the high temperature properties and notch toughness, so the upper limit is 1
It was 3%.

Mo: 0.5 to 1.5% Mo is required to be 0.5% or more in order to increase the temper softening resistance and improve the high temperature strength. However, no further effect can be expected, so the content was made 0.5 to 1.5%. V: 0.1-0.3% V forms stable carbides and has the effect of improving creep strength, but if it is less than 0.1%, it has no effect, and V.
If the content exceeds 3%, the ductility decreases, so 0.1
.About.0.3%.

N: 0.005 to 0.1% N not only strengthens the matrix, but also coexists with Mo to effectively improve the creep strength. However, if its content is less than 0.005%, its effect is not recognized, and if it exceeds 0.1%, blowholes are generated, so its content is 0.005-0.1%. And Co: 0.1-5% Co improves impact properties by suppressing the precipitation of delta ferrite, and also improves creep rupture strength.
If it is less than 0.1%, its effect will not be obtained, and if it is added in excess of 5%, its effect will be saturated, so the range is 0.1% to 5%.
And

W: 0.1 to 5% W improves the high temperature strength, but if it is less than 0.1%, it has no effect, and if it exceeds 5%, the segregation tendency increases.
The upper limit was made 5% because it lowers the ductility.

Next, the unavoidable impurities that limit the selective element or the content to be added as desired will be described. Ti: 0.1% or less Ti is one of deoxidizing agents and forms carbides or nitrides to improve high temperature characteristics. However, if it exceeds 0.1%, many inclusions are generated. The upper limit was 0.1%. For the same reason, 0.05% or less is more desirable. Nb: 0.01 to 0.2% Nb forms fine carbonitrides and improves the high temperature strength, but if the content is less than 0.01%, it has no effect, and 0.
If the content exceeds 2%, carbonitrides increase and the ductility decreases, so the range was made 0.01 to 0.2%. Ta: 0.01 to 0.2% Ta precipitates fine carbides and improves the high temperature strength, but if the content is less than 0.01%, there is no effect, and Ta.
If the content exceeds 2%, carbides increase and the ductility decreases, so the range was made 0.01 to 0.2%. Si: less than 0.2% As the content of Si decreases, macro segregation, particularly inverse V segregation becomes slight, and the ductility and notch toughness non-uniformity inside the wall thickness are improved. Further, if the Si content is high, the temper embrittlement susceptibility becomes extremely large, and the notch toughness is impaired. Therefore, it is desirable that the Si content be low, but it is less than 0.2% because the manufacturing margin is small and it is not practical to set the upper limit to be extremely low. In addition, although impurities that are inevitably contained by the usual manufacturing method are contained, the content thereof is not particularly limited.

[0011]

EXAMPLE Alloys (Examples and Comparative Examples) having the compositions shown in Table 1 were melted in an electric furnace, and impurities were reduced by outside furnace refining, and then cast into a sand mold. Predetermined heat treatment (quenching, tempering) was applied to these ingots to obtain test materials. In order to evaluate the mechanical properties of each test material, a test piece was cut out from the test material and a material test was performed. The results are shown in Table 2. As is clear from Table 2, the test materials of the examples are excellent in both high temperature creep strength and ductility, whereas the comparative examples are not sufficient in high temperature creep strength and inferior in ductility. It was

[0012]

[Table 1]

[0013]

[Table 2]

[0014]

As described above, according to the heat-resistant cast steel of the present invention, C: 0.05 to 0.15%, M by weight%
n: 0.1 to 1.5%, Ni: 0.1 to 1.5%, Cr
: 9.5 to 13%, Mo: 0.5 to 1.5%, V:
0.1-0.3%, N: 0.005-0.1%, Co:
0.1 to 5%, W: 0.1 to 5%, Ti: 0.1% or less, Nb: 0.05 to 0.2, if desired.
%, Ta: 0.01 to 0.2%, and the balance consisting of Fe and unavoidable impurities, so that it has excellent high-temperature strength and ductility, and is used in harsh environments. It has characteristics that it can withstand as a cast steel product. Further, among the unavoidable impurities of the above composition,
If the permissible content is Si: less than 0.2% by weight,
It has the effect of preventing the occurrence of segregation and further improving the ductility and toughness. A heat-resistant cast steel having good ductility and excellent high-temperature strength is provided by the above components.

Claims (3)

[Claims] 1. C: 0.05 to 0.15% by weight,
Mn: 0.1-1.5%, Ni: 0.1-1.5%, C
r: 9.5 to 13%, Mo: 0.5 to 1.5%, V:
0.1-0.3%, N: 0.005-0.1%, Co
: Heat resistant cast steel containing 0.1 to 5%, W: 0.1 to 5% with the balance Fe and unavoidable impurities 2. The composition according to claim 1, further comprising Ti: 0.1% or less, Nb: 0.01 to 0.2%, T by weight.
a: Heat-resistant cast steel containing at least one of 0.01 to 0.2% 3. Of the unavoidable impurities, in% by weight, Si
: The content of less than 0.2% is the allowable content.
Heat-resistant cast steel described in