JPS6196062A - High-chromium cast steel for high temperature service pressure vessel - Google Patents

Abstract

PURPOSE:To obtain the high-chromium cast steel which is used for a pressure vessel for high temp. service and has superior ductility and toughness as cast steel products, by incorporating specific percentage of C, Si, Mn, Cr, Mo, V, Nb and N to Fe. CONSTITUTION:The high-chromium cast steel, consisting of by weight, 0.08-0.12% C, <=0.7% Si, <=0.8% Mn, 0.4-0.7% Ni, 9-11% Cr, 0.65-1.00% Mo, 0.13-0.20% V, 0.03-0.07% Nb, 0.03-0.07% N and the balance Fe with accompanying impurities, is manufactured. Because of its excellent characteristics such as high strength at high temps. and superior weldability as well as superior oxidation and cracking resistance this high-chromium cast steel can be suitably used as material for pressure vessel for high temp. service such as steam turbine compartment.

Description

Detailed Description of the Invention The present invention has excellent high-temperature strength and oxidation resistance, as well as good ductility and toughness at room temperature, even as a large-sized cast steel material, and therefore has good weldability. , relates to a 9-11% high chromium cast steel material for high-temperature pressure vessels that has high resistance to cracking and is suitable for high-temperature pressure vessels that require high reliability, such as steam turbine casings and valve chambers. .

The casing and valve chamber of a steam turbine are subject to high temperatures and high pressures, so they are required to have excellent high-temperature strength, represented by creep rupture strength and yield strength. In addition, these parts have complex structures and thick walls, so when high-temperature steam flows in during startup, etc., they are locally heated, causing compressive strain in those parts. A large tensile stress may remain. These parts are also prone to cracking due to thermal fatigue, and such cracking or the presence of defects may develop into large cracks due to brittle fracture.

To prevent this, the material needs to have high ductility and toughness. Furthermore, when these parts are manufactured by casting, welding repairs are required. Also, that group! , Welding is also required for machining. Therefore, it is essential to have good weldability, and in order to ensure such weldability, it is also necessary to ensure ductility and toughness.

Conventionally, when manufacturing such parts by casting, so-called 1%Or-0,5Mo cast steel, 2%%0r-1%M
Low alloy cast steels such as O cast steel and Or-Mo-V cast steel were used. However, these materials do not necessarily have sufficient high temperature strength.

There was also a problem with oxidation resistance at high temperatures.

However, recently, there has been a movement to improve the efficiency of a single power generation plant, that is, to increase the temperature and pressure for the purpose of so-called energy conservation. These materials do not have sufficient high-temperature strength and oxidation resistance to cope with such movements. Therefore, in order to respond to these movements, new materials with excellent high-temperature strength and oxidation resistance are required.

A material that has high high-temperature strength and excellent oxidation resistance is so-called austenitic steel. but,
Austenitic steel has high creep rupture strength at high temperatures, but low yield strength at low temperatures including room temperature. Furthermore, since the thermal conductivity is low, thermal stress also tends to increase. Therefore, when these materials are used for steam turbine casings and valve chambers, they tend to bend due to thermal stress during startup or load fluctuations, and are not necessarily hairy in design. Further, in the case of austenitic steel, since there is no so-called transformation, the K and vI4 structure cannot be adjusted by heat treatment. Therefore, when these materials are produced by casting, the cast structure remains intact. Furthermore, even in the case of forging, it is necessary to adjust the crystal grains by controlling the forging history before heat treatment. in this case,
For items with complex shapes such as steam turbine casings and valve chambers, it is difficult to obtain homogeneous materials, and therefore it is difficult to obtain the original properties of the materials.

For the above reasons, even if austenitic steel is used as the material for high-temperature pressure vessels such as steam turbine casings and valve chambers, ferritic steel is preferred when it is unavoidable. There was a strong desire to develop a convenient material.

In this way, as materials that are attracting attention, 8-
There is a so-called 12-chromium steel containing about 13% chromium.

12 Chromium steel has a high chromium content, so it has better oxidation resistance than the low-alloy steels mentioned above, and is often used as a relatively small rolled or forged steel material with excellent high-temperature strength. It is expected that it will have excellent high-temperature strength even as a cast material.

As this rigid 12 μm cast steel material, those shown in Table 1 are generally widely known.

However, the composition of these materials has not necessarily been sufficiently studied. Therefore, when manufacturing a large cast product such as a single chamber or valve chamber of a steam turbine, ductility and toughness cannot be obtained due to the formation of δ ferrite due to segregation, precipitation of carbides, and insufficient quenching.

Furthermore, even if ductility and toughness were obtained, high-temperature strength was not obtained, which made it difficult to put it into practical use as a material for high-temperature pressure vessels.

In addition, in the case of 12 chromium cast steel, the formation of δ-ferrite,
Even if the precipitation of carbides is suppressed and the hardenability is maintained at 70%, when manufactured as a large or thick material,
Another problem is that ductility and toughness cannot be obtained.

In response to the above situation, the present invention has developed a so-called 12 chromium cast steel that can secure sufficient ductility and toughness even when used for large-sized materials, has excellent high-temperature strength, and has very good oxidation resistance. The present invention provides a 9 to 11% chromium cast steel that falls within the range of 9 to 11% chromium.

The present invention was carried out based on the following tests and findings, by clarifying that in order to obtain the above properties, the range of the components must be limited to a narrow range, and by limiting the range.

Table 2 shows the components of the test materials tested for the present invention. The test material was made by melting a 50k) material in a 50ky vacuum high-frequency melting furnace, and casting it into a sand mold to make a red sea bream. These test materials were +, 10 at o 30 c.
hr Heating, from now on to 300C t-75c/h
After being cooled at R2 and then air-cooled, it was heat treated by tempering at 650C and 700C for 10 hours, and then subjected to tensile, impact, high temperature tensile and creep rupture tests.

Here, the holding time at each temperature is I Ohr, and 1
, 75C cooling rate from o s o c to 300C
/hr is assumed to simulate a large material such as a steam turbine casing.

Table 3 (Part 1) (Part 2) shows the results of room temperature and high temperature tensile and impact tests.

Table 3 (SoQ+) Table 3 (Part 2) Heat 1.030 CX l Ohr -S OOCA,
c, -normal ai + 6 soc Also, the second
The figure shows the relationship between the room temperature impact region, which is representative of toughness, and tensile strength. Here, the impact value is 2 mV
Indicates Notuchi Shalubi impact value. Further, FIG. 5 summarizes the relationship between the creep rupture test results, which are representative of high-temperature strength, and the total tensile strength.

From FIG. 1, it can be seen that in this test, the correlation between the area of area, which is representative of ductility, and tensile strength mainly changes depending on the carbon content, and the lower the carbon content, the more the ductility tends to improve.

The materials No. 1 to 8 tested here were tested to determine the composition of the cast steel of the present invention, and sufficient consideration was given to suppressing δ-ferrite and ensuring hardenability. ing.

Therefore, the test results for the above materials were obtained under conditions that sufficiently ensured the suppression of δ-ferrite and the hardenability. Generally, when the so-called 1ZCir cast steel is quenched with a 4\ type material and the cooling rate is high, such a tendency does not appear. Therefore, when these cast steels are enlarged, this point becomes a blind spot and often causes various problems. In order to avoid such problems, the tests in the present invention were conducted in consideration of the heat treatment as described above, and the above results were obtained as a result.

From FIG. 2, it can be seen that the impact value also tends to decrease as the carbon content increases, although it is not as sensitive as the aperture. Further, in FIG. 2, the impact values are low for materials No. 8 and No. 12, but this is because δ-ferrite is generated.

From FIG. 3, it can be seen that for the materials tested here, if the tensile strength is the same, the lower the carbon content, the higher the creep rupture strength tends to be.

This tendency is also contrary to common sense for small-sized materials, and can only be obtained by simulating the heat treatment of test materials to that of large-sized materials, as in the present invention.

The specific reasons for limiting the components of the cast steel of the present invention will be described below.

Based on the above test results, the carbon content was determined to be low in order to ensure ductility, toughness, and high-temperature strength, and was set at 0.
08 to 0.12%. If the carbon content is too small, δ-ferrite is likely to be formed. Furthermore, since hardenability is insufficient and it becomes difficult to ensure toughness, the lower limit was set at 0.08%. The upper limit was set at 0.12% because, according to the test results shown here, sufficient ductility and toughness are maintained even when the carbon content is higher than this.
This was a relatively small test material that was melted under good conditions. As a result of the material, in large practical materials, the decrease in ductility and toughness due to the increase in carbon content may be more pronounced and the tensile strength of this cast steel is within the practical range of 70~aokyr/ This is done in consideration of the fact that when the amount of carbon in sm2 is increased, the creep rupture strength tends to decrease.

The reason why the silicon content was set to 0.7% or less is because it is usually desirable for this type of material to have a rather high silicon content in order to ensure castability when used as a casting material. Based on common sense, we have decided to allow up to this value. Increasing the silicon content improves the vortex flow.

It also has the effect of calming the molten metal and is effective in preventing so-called casting defects, but at the cost of this, it tends to cause micro and macro segregation, making it difficult to obtain stable material properties. be. The upper limit of 0.7% mentioned above is
This latter problem is allowed as long as it does not become noticeable.

The reason why manganese is set at 0.8% or less is that in this type of cast steel, adding manganese reduces the adverse effects of sulfur, and is also effective in preventing the formation of δ-ferrite and improving hardenability. This has been allowed up to this point. Although it may be possible to add more manganese, adding too much may change the characteristics of the material, so here the upper limit of manganese was set at 0.8%.

The reason for increasing the nickel content from 0.4% to 0.7% is that in this type of cast steel, it is better to have less nickel in order to improve the creep rupture strength.

However, if the nickel content is too small, δ-ferrite tends to form and pro-eutectoid 7-elite tends to precipitate, reducing toughness and making it unpractical as a cast steel material. This range has been set as a goal.

The reason for increasing the chromium content from 9% to 11% is that in this type of cast steel, increasing the total amount of chromium iI improves the creep rupture strength, but increasing the amount of chromium iI increases the formation of δ-ferrite and pro-eutectoid ferrite. This range was set because this tends to cause precipitation and makes it difficult to ensure toughness.

The reason for increasing molybdenum from 0.65% to 1.00% is that in this type of cast steel, adding about 1% of molybdenum has a balanced effect on improving creep rupture strength, but it has a moderate effect on improving creep rupture strength. If too much is added, it may cause embrittlement due to long-term heating at high temperatures, the formation of δ-ferrite, and pro-eutectoid 7
Since there is a tendency for erite to precipitate, the above-mentioned range is set at a slightly lower value in consideration of segregation in the case of a large cast steel material.

The reason why the vanadium content is increased from 0.13% to 0.20% is because this type of cast steel usually suffers from creep @fill? In order to improve the strength, it is said that it is desirable to add vanadium by about 0.25%. However, the present inventors conducted another test and found that when used as a large material, the creep rupture strength tends to improve as the amount of vanadium is reduced, as in the case of carbon mentioned above. Since this has been confirmed, we decided to lower this value within a range that does not impair hardenability, and we decided on this range taking into consideration the control range.

Normally, when adding vanadium to this type of cast steel, it is necessary to determine the component range within a range of 0.03%.

= Off”i 0.03 to 0.07% is because niobium prevents crystal grain growth through interaction with nitrogen and helps ensure ductility and toughness. Foil walls are also effective in improving creep rupture strength, but as with vanadium mentioned above, if too much of this is used, the creep rupture strength of large materials will be reduced, and the segregated areas will deteriorate. It has been confirmed in another test that carbonitrides are precipitated in the 0,035, resulting in defects.
The above range is defined as a controllable range centered on %.

The reason why the nitrogen content was set at 0.03 to 0.07% was because, as mentioned above, when making large-sized steel, increasing the amount of carbon reduces the ductility and toughness, but nitrogen does not have such an effect. Moreover, its coexistence with carbon promotes the precipitation of carbon-nitride, which is the same as carbide, and is effective in improving creep rupture strength. In addition, by coexisting with vanadium and niobium, the growth of crystal grains can be suppressed and δ
- Effective in preventing the formation of ferrite and the precipitation of pro-eutectoid ferrite. Furthermore, it is added with d and nitride, which has the effect of improving hardenability and ensuring ductility and toughness. However, if too much nitrogen is added, the creep rupture strength tends to decrease, as in the case of carbon. Therefore, here:
The above range is defined as the controllable range, centered around 0.03%, where the effect has been confirmed.

In addition to the above components, generation of δ-ferrite,
If it is necessary to prevent the precipitation of pro-eutectoid ferrite and increase hardenability, it is also possible to add cobalt.

Cobalt has similar effects to nickel, but unlike nickel, it does not have much of a negative effect on creep rupture strength.

Therefore, by limiting the amount of nickel, if there are problems with the formation of δ-ferrite, preventing the precipitation of pro-eutectoid 7-erite, and ensuring hardenability, it is desirable to add cobalt. If too much is added, the balance of material properties may be lost, so here we will focus on the range of 0.5%, where the effect has been confirmed, and set the allowable range up to 0.7%, which is the controllable range. As such, the addition of cobalt was allowed.

The ingredients mentioned above, material numbers 1゜2 and 3 in Table 2
corresponds to the material of The superiority of these materials in terms of ductility, toughness, and creep rupture strength over the similar materials numbered 4 to 7 is shown in Figures 1 to 5.
This is clear from the results shown in the figure. This is mainly due to the effect of carbon. Material No. 8 has excellent creep rupture strength, but has problems with ductility and toughness. This is mainly due to the effect of nitrogen.

In addition, materials No. 9 to 12 were tested here as materials that are analogous to the existing materials shown in Table 1, but these materials all had poor ductility and toughness, creep rupture, or It can be seen that there are problems with both.

As mentioned above, based on new tests and findings, the high chromium cast steel of the present invention has been shown to exhibit reduced ductility and toughness when made into large pieces, which has been an obstacle to the practical use of conventional cast steel of this type. This problem has been solved and the creep rupture strength has been improved, making it highly practical as a high-chromium cast steel material for high-temperature pressure vessels such as steam turbine casings and valve chambers, which has recently been desired to be developed. It is of significant industrial value.

[Brief explanation of drawings]

Figure 1 shows a comparison of changes in area of area and tensile strength based on the room temperature tensile test results of the inventive material and comparative material. As shown in the figure, the impact value is summarized and compared with the tensile strength. Further, FIG. 3 shows the relationship between creep rupture strength and tensile strength. The chemical components shown in Figures 1 to 5 are nominal components. "゛Zokuuradome 11): 1985 1;]

Claims (2)

[Claims] (1) Weight content of carbon 0.08-0.12%, silicon 0.7% or less, manganese 0.8% or less, nickel 0.4
~0.7%, chromium 9-11%, molybdenum 0.65~
1.00%, vanadium 0.13-0.20%, niobium 0.03-0.07% and nitrogen 0.03-0.07%
A high-chromium cast steel for high-temperature pressure vessels, characterized in that the remainder consists of iron and incidentally mixed impurities. (2) Weight content of carbon 0.08-0.12%, silicon 0.7% or less, manganese 0.8% or less, nickel 0.4
~0.7%, chromium 9-11%, molybdenum 0.65~
1.00%, cobalt 0.7% or less, vanadium 0.1
3 to 0.20%, niobium 0.03 to 0.07%, and nitrogen 0.03 to 0.07%, the balance being iron and incidental impurities. High chromium cast steel.