JPH10259449A - Low alloy heat resistant cast steel and cast steel parts for steam turbine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength low alloy heat resistant cast steel material which creep rupture strength is increased to be that of the existing CrMoV cast steel or above, toughness is improved more than that of CrMoV cast steel or above and weldability is also improved and to provided cast steel parts for a turbine. SOLUTION: This low alloy heat resistant cast steel is the one contg., by weight, 0.06 to 0.14% carbon, 0.01 to 0.35% silicon, 0.2 to 0.8% manganese, 0.8 to <1.5% chromium, 0.1 to 0.5% nickel, 0.05 to 0.3% vanadium, 0.01 to 0.1% niobium, 0.1 to 1,5% molybdenum, 0.1 to 3% tungsten, 0.001 to 0.01% boron, the balance iron and inevitable impurities. The cast steel parts for a turbine is composed by using it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steam turbine for thermal power generation, which is capable of producing a shape close to the shape of a final part by a casting method without going through a material forging process such as forging. The present invention relates to a low-alloy heat-resistant cast steel suitable for a cast steel part for a turbine including a vehicle compartment, and a cast steel part for a turbine using the same.

[0002]

2. Description of the Related Art Casting materials are widely used as materials for casings and pressure vessels used in steam turbine plants for thermal power generation. This is for economically producing a single-piece production part having a complicated shape and having few identical shapes. Among these materials, low alloy steel-based materials mainly include CrMoV cast steel, 2.25% CrMo cast steel, and CrMoV cast steel.
Mo cast steel and the like. Since these materials are used at a high temperature, good high-temperature strength characteristics are required, and further, excellent weldability is required so that a casting defect can be repaired by welding.

[0003] 2.25% CrMo cast steel and CrMo cast steel have excellent impact characteristics at room temperature, and as a result, have good weldability. However, since V is not added, the creep rupture strength is not always sufficient, and it cannot meet the needs for steam room interior materials for steam turbines, which increase in temperature year by year. On the other hand, CrMoV cast steel is excellent in creep rupture strength, but has poor weldability due to inferior impact characteristics, and has a problem that welding repair during production is difficult to perform.

[0004]

SUMMARY OF THE INVENTION In view of the above-mentioned state of the art, the present invention increases the excellent creep rupture strength of CrMoV cast steel to the present level or higher, and further improves the toughness of CrMoV cast steel.
An object of the present invention is to provide a high-strength low-alloy heat-resistant cast steel material improved in castability and improved in weldability, and a cast steel part for a turbine using the same.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have carefully selected alloy elements using CrMoV cast steel as a basic component, and have enthusiastically improved high-temperature strength to obtain a new low-temperature alloy having excellent high-temperature characteristics. An alloy heat-resistant cast steel material and a cast steel part for a turbine using the low alloy cast steel have been found. That is,
The present invention includes the following inventions (1) to (5).

(1) Carbon: 0.06-0.14 by weight
%, Silicon: 0.01 to 0.35%, manganese: 0.2
-0.8%, chromium: 0.8-1.5% (excluding 1.5%), nickel: 0.1-0.5%, vanadium: 0.1-0.5%.
05 to 0.3%, niobium: 0.01 to 0.1%, molybdenum: 0.1 to 1.5%, tungsten: 0.1 to 3
%, Boron: 0.001 to 0.01%, and the balance is made of inevitable impurities and iron. (2) Except for the amount inevitably contained as impurities,
The low-alloy heat-resistant cast steel according to the above (1), which does not contain nickel and / or manganese. (3) The low-alloy heat-resistant cast steel according to (1) or (2), wherein a part of iron is replaced by cobalt, and the content thereof is 0.1 to 3.5% by weight. (4) Part of iron is replaced with titanium and / or calcium, and the amount is titanium: 0.01 to 0.045 in weight ratio.
%, Calcium: 0.001 to 0.009%, the low alloy heat-resistant cast steel according to any one of the above (1) to (3). (5) A cast steel part for a turbine, comprising a low-alloy heat-resistant cast steel according to any one of the above (1) to (4).

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the range of each component in the invention (1) will be described below. Carbon: Carbon has the effect of ensuring hardenability during heat treatment and increasing material strength. In addition, it forms carbides and contributes to improvement in creep rupture strength at high temperatures. In this alloy system, if the addition is less than 0.06%, sufficient material strength cannot be obtained, so the lower limit is set to 0.06%. On the other hand, if the added amount of carbon is too large, the toughness is reduced and the weldability is reduced. Therefore, the upper limit of the added amount is set to 0.14%. A particularly desirable range for achieving a balance between material strength characteristics and excellent toughness is 0.
08 to 0.12%.

[0008] Silicon: Silicon is an element that enhances the flow of molten metal and has an effect as a deoxidizer, but when added in excess, makes the matrix brittle. When the deoxidizing effect is sufficiently expected, the addition of a maximum of 0.35% is permitted. Need not be expected so much, and the amount of addition can be kept to a minimum. However, in order to extremely reduce the amount of silicon, it is necessary to carefully select raw materials, which increases the cost. Therefore, the lower limit is set to 0.01%. Therefore, the component range of silicon is 0.01
To 0.35%.

Manganese: Manganese acts as a deoxidizing agent during steel making and has an effect of improving the hardenability during heat treatment. However, when manganese is added, the creep rupture strength is degraded in accordance with the amount thereof, and embrittlement is caused. Therefore, the maximum amount of addition is set to 0.8%. However, in order to control the content to 0.2% or less, it is necessary to carefully select the raw material steel and an excessive refining process, which leads to an increase in cost. Therefore, the minimum amount is set to 0.2%.

Chromium: Chromium enhances hardenability during heat treatment and forms carbides to contribute to improvement in creep rupture strength, and dissolves in the matrix to improve oxidation resistance. Reinforcement of the matrix itself also contributes to improvement in creep rupture strength. If it is less than 0.8%, the effect is not sufficient, so the lower limit is set to 0.8%. On the other hand, when an amount exceeding 1.5% is added, the flowability of the molten metal tends to decrease and the castability tends to decrease, and the creep rupture strength of the present alloy system also tends to decrease. Therefore, the addition range is set to 0.8 to 1.5% (not including 1.5%). A desirable range is 1.2 to 1.45%.

Nickel: Nickel is effective for enhancing hardenability during heat treatment, improving tensile strength and proof stress, and in particular, enhancing toughness. However, on the other hand, the long-term creep rupture strength decreases with the addition of nickel. In the present invention material, since the toughness is increased by setting the carbon content lower than that of the CrMoV cast steel, the toughness improvement by nickel is not expected much. However, when the carbon content is close to the upper limit, about 0.5% of nickel is reduced. In some cases, it is necessary to compensate for the decrease in toughness due to the addition of carbon, so the upper limit is set to 0.5%. When the long-term creep rupture strength is most important, the lower the nickel content, the better. However, in order to suppress the nickel content to 0.1% or less, it is necessary to carefully select raw materials and the cost increases. I do.

Vanadium: Vanadium enhances hardenability during heat treatment and becomes a carbide to improve creep rupture strength. If the addition is less than 0.05%, a sufficient effect cannot be obtained. Conversely, if the amount exceeds 0.35%, the toughness is reduced. Therefore, the component range is set to 0.0
5 to 0.3%. Considering the balance between the strength characteristics and the toughness, the desirable addition amount is 0.15 to 0.25%.

Niobium: Niobium enhances hardenability and forms fine carbides to contribute to improvement in high-temperature strength.
Further, it appropriately suppresses the growth of crystal grains during high-temperature heating such as solution treatment, and contributes to improvement in toughness. If the addition amount is less than 0.01%, there is no remarkable effect, and if the addition amount exceeds 0.1%, the coarse niobium carbide generated during the solidification production of the molten metal can sufficiently form the matrix during the heat treatment (solution treatment). It cannot form a solid solution and remains in a large amount, reducing toughness and long-term creep rupture strength. Therefore, the component range is 0.01 to 0.1
%. Considering the balance between the strength characteristics and the toughness, the desirable addition amount is 0.02 to 0.05%.

Molybdenum: Molybdenum enhances the hardenability during heat treatment and improves the creep rupture strength by forming a solid solution in a matrix or carbide. If the amount is less than 0.1%, no remarkable effect is expected. On the other hand, if added in a large amount, the toughness is reduced and the cost is increased. Therefore, the upper limit of the added amount is limited to 1.5%. Considering the balance of strength characteristics, toughness, and cost, the preferable addition amount is 0.45 to 1.2%, and the more preferable addition amount is 0.8 when the creep rupture strength is particularly important while balancing the toughness and cost. ~ 1.2%.

Tungsten: Tungsten forms a solid solution in a matrix or carbide to improve the creep rupture strength. If the amount is less than 0.1%, no remarkable effect is expected. On the other hand, if it is added excessively, the toughness is lowered, and there is a possibility that the material properties may be uneven due to segregation, and the cost may be increased. Therefore, the upper limit of the addition amount is limited to 3%. Considering the balance between the strength characteristics, toughness and cost, the preferable addition amount is 1 to 2.5%, and the more preferable addition amount is 1.7 to 2 when the creep rupture strength is particularly important while balancing the toughness and cost. 0.5%.

Boron: Boron segregates in an appropriate amount at grain boundaries to enhance hardenability, strengthens grain boundaries, and improves toughness and high-temperature strength characteristics. Although it is a useful element that brings a great improvement in properties when added in a small amount, if it is added excessively, it will weaken the grain boundaries and turn to the direction of reducing toughness,
It is necessary to control the addition in a very small amount. If the addition amount is less than 0.001%, the effect of improving hardenability, toughness, and high-temperature strength characteristics does not clearly appear. Conversely, if the addition amount exceeds 0.01%, the toughness is reduced. 0
01 to 0.01%. Desirable addition range is 0.00
2 to 0.007%.

One of the features of the material 1 of the present invention is that it does not contain any alloy elements other than the above-mentioned alloy elements except those which are inevitably mixed as impurities, that is, does not intentionally add. The reason why some elements are not added will be described below.

Nitrogen: Nitrogen, like carbon, has the effect of ensuring hardenability during heat treatment and increasing material strength.
In addition, it forms carbonitrides and contributes to improvement in creep rupture strength at high temperatures. However, since nitrogen easily forms a compound with boron, if it is added carelessly even in a small amount, the effect of boron addition disappears without boron segregating moderately at the grain boundaries. In some cases, during the quenching process,
Since the boron compound acts as a nucleus for ferrite formation, the hardenability is reduced. In the present invention material 1, nitrogen is not added in order to give priority to the hardenability improvement effect and the high temperature strength improvement effect by boron addition.

Titanium: Titanium is an element that easily forms carbonitrides. Particularly, titanium has a strong affinity for nitrogen, and thus has an effect of fixing nitrogen (preventing the binding of boron and nitrogen) by adding a small amount. doing. However, it is also an element which easily forms an oxide when combined with oxygen to cause a material defect. It is a major premise that no forging step is performed in cast steel materials, and it is not possible to make the oxide and the base metal adhere to each other by forging, so it is important to improve the cleanliness of the material. In addition, in the case of nitrogen at a level that is inevitably mixed, boron works effectively even if titanium is not added and fixed, so titanium material is not added in the material 1 of the present invention.

Aluminum: Aluminum also has the effect of fixing nitrogen because of its strong affinity for nitrogen, but is an element that forms an oxide like titanium and lowers the cleanliness of the material. Therefore, it is not added for the same reason as in the case of titanium.

Next, the reasons for limiting the components in the invention (2) will be described. Since the reasons for limiting the components already described in the description of the invention (1) are the same, here, the reason why nickel and / or manganese contained in the low-alloy heat-resistant cast steel material of the invention (1) is replaced by iron. State.

Manganese: In the low alloy heat resistant cast steel material of the invention (1), manganese is added in expectation of a deoxidizing effect and an effect of improving hardenability. However, since the creep rupture strength is degraded according to the amount, even if the material cost is increased, it is desirable that no addition is made when emphasis is placed on enhancing the creep rupture characteristics. Regarding the deoxidizing effect,
Considering the use of the vacuum carbon deoxidation method and the effects of other deoxidizing materials such as silicon, there is no problem even if manganese is not added. Also, regarding hardenability, if necessary, carbon,
It can be ensured by controlling the addition amount of an element that enhances hardenability, such as chromium, vanadium, niobium, molybdenum, and boron. Therefore, in order to give the highest priority to the high-temperature strength characteristics, manganese is not added without increasing the cost.

Nickel: In the low alloy heat-resistant cast steel material of the invention (1), nickel is added with an expectation of an effect of increasing hardenability at the time of heat treatment, improving tensile strength and proof stress, and particularly enhancing toughness. However, on the other hand, the long-term creep rupture strength is reduced by the addition of nickel. Therefore, even if the material cost is increased, it is desirable that no addition is made when emphasis is placed on enhancing the creep rupture characteristics. Regarding hardenability, carbon, chromium, vanadium,
It can be ensured by controlling the addition amount of an element that enhances hardenability, such as niobium, molybdenum, and boron. Regarding the toughness, the material of the present invention
Since the toughness is increased by setting the carbon content lower than that of MoV cast steel, the toughness improvement by nickel is not expected much. However, when the carbon content is close to the upper limit, the toughness decreases and the weldability deteriorates, and the cost increases. In some cases, it will rise. However, in view of such a high cost, nickel is not added in order to give the highest priority to high-temperature strength characteristics.

Next, the reasons for the limitation of the components in the invention (3) will be described. Elements other than cobalt are the same as those in the invention (1), and therefore are omitted here, and only the action of cobalt will be described. Cobalt: Cobalt enhances hardenability and forms a solid solution in the matrix to strengthen the matrix itself. Further, cobalt has an effect of increasing tempering softening resistance, and is useful for balancing strength and toughness. The effect of the addition of cobalt appears when the addition amount is 0.1% or more. However, when the addition amount exceeds 3.5%, the precipitation of carbides is promoted, so that the long-term creep rupture strength is deteriorated. Let me do it. In addition, since cobalt itself is an expensive material, the addition of a large amount causes an increase in cost. For this reason, a component range of 0.
1 to 3.5% is set. A desirable range is 0.5 to 2.5%.

Next, the reasons for limiting the components in the invention (4) will be described. Titanium which is a new additive element, nitrogen which is mixed as an unavoidable impurity related to the addition of titanium, and calcium which is a new additive element Elements other than the above are the same as those of the inventions (1) to (3), and will not be described here. Only the action of titanium (including the influence of nitrogen) and calcium will be described.

Titanium: One of the constituent features of the inventions (1) to (3) is that no intentional addition of nitrogen and titanium is performed. The reason why nitrogen is not added is that nitrogen easily forms a compound with boron, and if a small amount is added carelessly, boron does not segregate properly at the grain boundaries and the effect of boron addition disappears. When vacuum degassing is possible in the steelmaking process, the amount of nitrogen mixed as an impurity can be suppressed to a low level, so that boron effectively acts even with the components of the inventions (1) to (3). When the nitrogen concentration in the scrap of the starting material is high, the amount of nitrogen mixed as impurities exceeds the allowable amount, and boron does not work effectively. On the other hand, titanium is an element that easily forms a carbonitride, and has an effect of fixing nitrogen (preventing the binding of boron and nitrogen) by adding a small amount because it has a particularly strong affinity for nitrogen. . Although the allowable amount of nitrogen mixed as an impurity varies slightly depending on the balance with other elements, it is 0.006% based on many experimental results obtained by the present inventors.
Is exceeded, the effective effect of boron tends to be reduced. In such a case, titanium should be added. If the added amount of titanium is less than 0.01%, the effect of fixing nitrogen is insufficient, so the lower limit is made 0.01%. On the other hand, an excessive addition lowers the cleanliness of the material and lowers the toughness and high-temperature strength properties.
045%. Considering that nitrogen is fixed securely and not excessively added, the preferable addition amount is 0.02.
% To 0.04%.

Calcium: Calcium spheroidizes and finely disperses inclusions, promotes the growth of equiaxed crystals by its inoculation effect, and reduces macrosegregation of harmful impurity elements such as sulfur. It also has the effect of lowering the melting point of the inclusions so that the inclusions can be easily removed in the refining process.
As a result, the toughness and high-temperature strength characteristics of the material are improved. Particularly in the case of a cast steel material such as the material of the present invention, the addition of calcium is effective because it is impossible to eliminate the adhesion between the inclusions and the base metal and eliminate segregation by a material processing step such as forging. If the added amount is less than 0.001%, no effective action is produced, so the lower limit is made 0.001%. Further, if added in excess, calcium oxide is generated and the cleanliness of the material is reduced, so the upper limit of the addition is made 0.009%. A desirable addition range is 0.002 to 0.006%.

[0028]

The present invention will be described in more detail with reference to the following examples. (Example 1) Table 1 shows the chemical components of the materials subjected to the test. Sample numbers 1 to 7 correspond to the material of the present invention according to invention (1), and sample numbers 8 to 12 correspond to comparative materials. Of the comparative materials, Sample No. 11 is a typical component of 2.25 CrMo cast steel, and Sample No. 12 is a typical component of CrMoV cast steel. All test materials were melted in a 50 kg vacuum high-frequency melting furnace and ingots were formed using a sand mold. Subsequently, the steel sheet was cooled at an average cooling rate of 3 ° C./min up to 500 ° C. after homogenizing annealing and solution treatment in a form simulating a heat treatment for manufacturing a large cast steel product such as a turbine casing. The cooling rate simulates the cooling of the central part of a 400 mm thick part. Tempering was performed by determining the tempering temperature of each material so that the 0.2% proof stress was approximately 50 ± 3 kgf / mm ² .

Table 2 shows the mechanical properties and creep rupture characteristics of the material of the present invention and the comparative material according to the invention (1). Considering the structural stability during use, tempering is 150
The lower limit temperature of tempering was set to 680 ° C. because it is desirable to perform the process at a higher temperature than ℃.
In some cases, the target 0.2% proof stress could not be obtained even when tempered at 0 ° C. This is because burning was not sufficiently performed by slow cooling assuming large components. Of the comparative materials, steel types 8, 9 and 11 were not sufficiently baked,
Lower limit temperature: Steel type that did not reach the target 0.2% proof stress even after tempering at 680 ° C.

The Charpy impact absorption energy (normal temperature test) of the material of the present invention according to the invention (1) is at least 11.9 k.
gf-m, which is comparable to 2.25CrMo cast steel (comparative material: steel type 11) having good weldability. This indicates that the material of the present invention according to the invention (1) has weldability equal to or higher than that of 2.25CrMo cast steel. 6
Focusing on the creep rupture time when a load of 15 kgf / mm ² is applied at 00 ° C., the material of the present invention according to the invention (1) shows a rupture time that is significantly greater than that of the comparative material, and the creep rupture characteristics are sufficiently high. It turns out that it is high. The above suggests that appropriate component design of various elements in the material of the present invention according to the invention (1) was effective in securing toughness and improving creep rupture strength.

(Example 2) Table 3 shows the chemical components of the materials subjected to the test. Sample No. 13 is based on the component of Sample No. 3 used in Example 1, and is a steel type in which nickel is not added.
Sample No. 14 is based on the component of Sample No. 4 used in Example 1 and is a steel type in which manganese and nickel are not added and molybdenum is increased to secure hardenability.
Is a steel type in which manganese is added without addition of manganese to secure quenchability based on the component of sample No. 5 used in Example 1. All of the sample materials were melted in a 50 kg vacuum high-frequency melting furnace and ingots were formed using a sand mold. Subsequently, the steel sheet was cooled at an average cooling rate of 3 ° C./min up to 500 ° C. after homogenizing annealing and solution treatment in a form simulating a heat treatment for manufacturing a large cast steel product such as a turbine casing. The cooling rate simulates the cooling of the central part of a 400 mm thick part. Tempering has a 0.2% proof stress of about 50 ± 3kgf / m
The tempering temperature of each material was determined so as to obtain m ² .

Table 4 shows the mechanical properties and creep rupture properties of the material of the present invention (sample Nos. 13 to 15) according to the invention (2). When the properties of the steel types of Sample Nos. 13 to 15 are compared with the properties of the base material of Example 1 shown in Table 2, the Charpy impact absorption energy (normal temperature test) is slightly lower. Sample No. 11 (2.25CrMo)
(Cast steel). On the other hand, 15kg at 600 ° C
The creep rupture time when a load of f / mm ² was applied was definitely extended, indicating that the creep rupture strength was increased.

(Example 3) Table 5 shows the chemical components of the materials subjected to the test. Sample Nos. 16 and 17 are invention materials according to invention (3) based on the components of Sample Nos. 1 and 3 of Example 1, and Sample Nos. 18 and 19 are Sample Nos. 14 and 1 of Example 2.
An invention material according to invention (3), based on the ingredient No. 5. All test materials were melted in a 50 kg vacuum high-frequency melting furnace and ingots were formed using a sand mold. Subsequently, in a form simulating the heat treatment for manufacturing large cast steel products such as turbine casings, after homogenizing annealing and solution treatment, the average cooling rate to 500 ° C. was 3 ° C.
/ Min. The cooling rate is 400 mm in thickness.
This simulates cooling of the central part of the part. Tempering was performed by determining the tempering temperature of each material so that the 0.2% proof stress was approximately 50 ± 3 kgf / mm ² .

Table 6 shows the mechanical properties and creep rupture characteristics of the material of the present invention. When the characteristics of steel types 16 to 19 are compared with the characteristics of the base material of Example 1 shown in Table 2 and the characteristics of the base material of Example 2 shown in Table 4, the Charpy impact absorption energy (normal temperature test) hardly changes. Rather, it shows a rather good value. On the other hand, 15kg at 600 ° C
The creep rupture time when a load of f / mm ² was applied was definitely extended, indicating that the creep rupture strength was increased.

Example 4 Table 7 shows the chemical components of the materials subjected to the test. Sample Nos. 20, 23, and 24 correspond to Example 1.
Inventive material according to Invention (4) based on the component of Sample No. 3 of Example 2, and Sample No. 21 is the invention material according to Invention (4) based on the component of Sample No. 14 of Example 2, and Sample No. 2
Reference numerals 2 and 25 denote invention materials according to invention (4) based on the component of sample number 17 in Example 3. Sample No. 2
6 to 28 are comparative materials. In addition, among the above samples, Sample Nos. 20 to 23, 26, and 27 are supposed to be inevitably dissolved in the air due to manufacturing equipment or the like, or assuming that the nitrogen concentration in the scrap of the starting material is high. Nitrogen is added in a manner simulating nitrogen as an inevitable amount of impurities (not intended to be a useful effect of nitrogen addition). All test materials were melted in a 50 kg vacuum high-frequency melting furnace and ingots were formed using a sand mold. Subsequently, the steel sheet was cooled at an average cooling rate of 3 ° C./min up to 500 ° C. after homogenizing annealing and solution treatment in a form simulating a heat treatment for manufacturing a large cast steel product such as a turbine casing. The cooling rate simulates the cooling of the central part of a 400 mm thick part. Tempering has a 0.2% proof stress of about 50 ± 3kgf /
The tempering temperature of each material was determined so as to obtain mm ² .

Table 8 shows the mechanical properties and creep rupture characteristics of the material of the present invention. First, the effect of adding titanium will be described. Even when the amount of unavoidably mixed nitrogen is large, as in the case of sample numbers 20 to 23 and 27, the effect of improving the hardenability by boron works effectively by adding an appropriate amount of titanium to secure the target strength. be able to. On the other hand, in Sample No. 26, in which the amount of titanium added was lower than the lower limit of the present invention, the hardenability was low and the target strength could not be secured, and the Charpy impact absorption energy (room temperature test) was also low because a large amount of proeutectoid ferrite appeared. Low. In Sample Nos. 20 to 22, the addition of titanium in an amount within the range of the present invention resulted in Charpy impact absorption energy (normal temperature test) and creep rupture time when a load of 15 kgf / mm ² was applied at 600 ° C., respectively. Slightly lower than the base material. However, the impact absorption energy shows a sufficiently high value of 10 kgf-m or more, which is not so inferior to the current material (Sample No. 11), and the creep rupture time of the current material (Sample No. 11). Is significantly higher. Sample No. 27 has titanium added to the upper limit or more of the present invention, but the creep rupture time is much longer than that of the current material (Sample No. 11), but the shock absorption energy is extremely poor.

From the above results, in the case where the amount of nitrogen unavoidably mixed is large, it is necessary to secure the target strength by securing the hardenability, and to obtain good Charpy impact absorption energy and good creep rupture strength. It is clear that it is useful to add titanium within the range limited by the present invention.

Next, the effect of adding calcium will be described. As is clear from Table 8, the steel type to which calcium within the scope of the present invention was added (Sample Nos. 23 to 25) had a Charpy impact absorption energy (normal temperature test) and 15 kgf / mm at 600 ° C. as compared with the respective base materials. ^It shows almost the same value (sample No. 23) or a high value equal to or higher than the creep rupture time when a load of ² is applied (sample Nos. 24 and 25). On the other hand, in sample No. 28 to which calcium in an amount equal to or more than the upper limit was added, the impact absorption energy was extremely inferior to the current material (sample No. 11). From the above results, it is apparent that the addition of calcium within the range defined in the present invention has an effective action to increase toughness and creep rupture strength.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

[0042]

[Table 4]

[0043]

[Table 5]

[0044]

[Table 6]

[0045]

[Table 7]

[0046]

[Table 8]

[0047]

Since the low alloy heat-resistant cast steel of the present invention has excellent high-temperature strength and toughness, it is suitable as a material for producing a shape close to the final product shape without going through a material forging process such as casting. . In particular, since it is a component that can be sufficiently quenched even at a low cooling rate, it is useful as a material for cast steel parts for large turbines such as a steam turbine casing for thermal power generation. According to the present invention, it is possible to construct a low-cost and high-efficiency power generation plant, which contributes to saving fossil fuels and is effective in suppressing the amount of carbon dioxide generated.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koji Morinaka 46-59 Nakahara-Sannohama, Oaza, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Within Nippon Casting and Forging Steel Co., Ltd. (72) Inventor Yasunori Tashiro Tobata-ku, Fukuoka 46, Nakahara Sanohama 59 Nippon Cast Forged Steel Co., Ltd.

Claims (5)

[Claims] 1. Carbon: 0.06 to 0.14% by weight,
Silicon: 0.01-0.35%, manganese: 0.2-
0.8%, chromium: 0.8 to 1.5% (excluding 1.5%), nickel: 0.1 to 0.5%, vanadium: 0.1 to 0.5%
05 to 0.3%, niobium: 0.01 to 0.1%, molybdenum: 0.1 to 1.5%, tungsten: 0.1 to 3
%, Boron: 0.001 to 0.01%, and the balance is made of inevitable impurities and iron. 2. The low-alloy heat-resistant cast steel according to claim 1, wherein nickel and / or manganese is not contained except for an amount inevitably contained as an impurity. 3. The low-alloy heat-resistant cast steel according to claim 1, wherein a part of iron is replaced by cobalt, and its content is 0.1 to 3.5% by weight. 4. A part of iron is replaced by titanium and / or calcium, and the amount thereof is titanium: 0.01 to 0.0 in weight ratio.
The low-alloy heat-resistant cast steel according to any one of claims 1 to 3, wherein 45% and calcium are 0.001 to 0.009%. 5. A cast steel part for a turbine, comprising a low-alloy heat-resistant cast steel according to any one of claims 1 to 4.