JPS6238426B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a steel for low-temperature turbine blades that has a large damping coefficient and good low-temperature toughness, and a heat treatment method thereof. In thermal power generation methods using liquefied natural gas (LNG), liquefied natural gas at -162°C is normally pressurized to around 15 atmospheres and then simply vaporized in a seawater heat exchanger before being used as fuel for power generation. Recently, as part of energy conservation measures, plants that recover the expansion energy during vaporization of liquefied natural gas as electrical energy are being considered. Turbine blades for thermal power generation usually have a large damping coefficient,
Although 12% Cr-Fe steel with excellent high-temperature properties is used, conventional 12% Cr-Fe steel is not necessarily used for plants that recover the expansion energy during vaporization of liquefied natural gas as electrical energy. is also not suitable. In other words, since the turbine blades for the above-mentioned plants are exposed to a low-temperature environment of around -75℃, it is essential that they have large damping capacity and good low-temperature properties, but 12% Cr-Fe steel was originally It has good high-temperature properties, but has a problem with low-temperature properties, particularly low toughness, so it is not preferred for use in low-temperature turbine blades. Therefore, in order to find a steel suitable for low-temperature turbine blades, the present inventors
As a result of investigating the effects of various component elements and heat treatment using 12% Cr-Fe steel as the basic composition, we found that a low-temperature turbine can be developed by quenching and tempering under limited temperature conditions using steel with the composition adjusted as follows. It has been found that the properties required for blades are fully satisfied. That is, the present invention includes: (1) C+N: 0.05 to 0.25%, Si: 0.1 to 1.50%,
Mn: 0.1~2.0%, Ni: 2.0~6.0%, Cr: 10.0~
Ni containing 14.0% and one or more elements selected from Nb, Ta, and Zr in a total of 0.02 to 0.30%, with the balance essentially consisting of Fe, and as shown in the following formula:
Steel for low-temperature turbine blades adjusted to have an equivalent/Cr equivalent in the range of 0.1 to 0.5. Ni equivalent: Ni% + 0.5Mn% Cr equivalent: Cr% - 15 (C + N)% + 1.5Si% (2) In addition to the basic invention steel, Mo: 3.0% or less, W: 3.0% or less, V: 1.0% or less, Co：2.0
% or less, Cu: 2.0% or less, Ti: 1.0% or less,
Al: 0.5% or less, B: 0.01% or less, Ca: 0.02%
Below, Te: 0.2S~0.1%, REM: 0.2S~0.1%
Steel for low-temperature turbine blades containing one or more elements selected from: (3) Using the basic invention steel, the Ac1
A method for heat treatment of steel for low-temperature turbine blades, characterized by subjecting it to tempering treatment in a temperature range of ~ Ac1 + 80°C. The steel of the present invention is a 12% Cr-Fe steel with Ni added with an element selected from Nb, Ta, and Zr, and further adjusted in Ni equivalent/Cr equivalent, and after quenching, it is tempered at a temperature of AC1 + Ac1 + 80°C. By applying this process, a tempered martensitic structure with a retained austenite content of 15% or less can be obtained, which maintains the high damping capacity of 12% Cr-Fe steel and maintains its strength even at temperatures of -100°C. It is a steel with excellent low-temperature properties that provides a Shapey impact value (JIS No. 4) of 15 km/cm ² or higher. Next, the reasons for limiting the component composition and heat treatment conditions in the present invention will be described below. C+N: 0.05 to 0.25% C and N are effective elements for ensuring the strength of the base material, and it is necessary to add at least 0.05% or more in total. However, if added in large amounts, the toughness deteriorates, so the total amount was limited to 0.25% or less. Si: 0.10 to 1.50% Si is usually added in an amount of 0.1% or more as a deoxidizing element during melting, but if added in large amounts, machinability and low-temperature toughness deteriorate, so it should be kept at 1.50% or less. Mn: 0.10-2.0% Effective as a deoxidizing and desulfurizing element during melting, as well as improving low-temperature toughness; at least 0.01%
It is necessary to add more than %. However, if added in large amounts, machinability will deteriorate significantly, so it was limited to 2.0% or less. Ni: 2.0-6.0% An element necessary to lower the impact transition temperature and improve low-temperature toughness, and at least 2.0%
It is necessary to add more than that. However, if a large amount is added, the amount of austenite in the quenched state will increase and the damping coefficient and low temperature toughness will decrease, so 6.0%
Limited to the following. Cr: 10.0-14.0% This is an element necessary to ensure good damping coefficient as well as corrosion resistance, and it is necessary to add at least 10.0% or more. However, if added in a large amount, δ ferrite will be generated and the toughness at low temperatures will be significantly deteriorated, so the content was limited to 14.0% or less. Nb, Ta, Zr: 1 type or 2 or more types total 0.02~
0.30% An effective element for refining grains and improving low-temperature toughness, with a total content of at least 0.02%.
It is necessary to add more than that. However, if added in large amounts, Nb(Ta)(Zr) carbonitrides will be formed during solidification, resulting in a significant decrease in toughness, so it was limited to 0.30% or less. By adjusting the above elements, a steel for turbine blades having a large damping capacity and excellent low-temperature toughness can be obtained, but the performance can be further improved by adding the following elements. Mo: 3.0% or less, W: 3.0% or less, V: 1.0% or less The above elements are solid dissolved in M ₂₃ C ₆ type carbides and have the effect of suppressing the growth of carbides, resulting in a significant reduction in tempering softening resistance. It is an element that improves performance, and it is desirable to add an appropriate amount as necessary. However, if added in large amounts, cleavage fracture is likely to occur at low temperatures and toughness decreases, so Mo
3.0% or less for W, 3.0% or less for W,
V is preferably 1.0% or less. Ti: 1.0% or less, Al: 0.50% or less The above elements are effective elements for refining crystal grains and improving low-temperature toughness, and it is desirable to add appropriate amounts as necessary. However, even if added in large amounts, the effect cannot be expected to improve; rather, Ti impairs the cleanliness of the steel and has a negative effect on toughness and ductility.
It is desirable that the content be 1.0% or less for aluminum, and 0.50% or less for aluminum. Co: 2.0% or less, Cu: 2.0% or less The above elements form a solid solution in the matrix and are effective elements for improving the strength of the matrix, so it is desirable to add an appropriate amount as necessary. However, if added in large amounts, hot workability deteriorates, so it is desirable that each content be 2.0% or less. B: 0.01% or less B is an element that can significantly improve hot workability even when added in a small amount, and it is desirable to add an appropriate amount as necessary. However, if added in large quantities, B4C type carbides will precipitate and hot workability will be impaired, so 0.01
% or less. Ca: 0.02% or less Ca is an element that can significantly improve machinability with the addition of a small amount, and it is desirable to add an appropriate amount as necessary. However, if added in large amounts, the toughness deteriorates, so the content was limited to 0.02% or less. Te: 0.2S to 0.1%, REM: 0.2S to 0.1% The above elements are effective elements for spheroidizing sulfide inclusions composed of S etc. mixed as impurity elements and improving anisotropy. Therefore, it is desirable to add it as necessary. However, if added in large amounts, the toughness and ductility will decrease.
Limited to 0.10% or less. Ni equivalent/Cr equivalent: 0.1 to 0.5 The component balance of Ni equivalent and Cr equivalent greatly changes the amount of retained austenite after quenching. In other words, even within the above composition range, Ni% + 0.5Mn
Ni equivalent in % and Cr%-15(C+N)%
If the ratio to the Cr equivalent represented by +1.5Si is too high, the amount of retained austenite increases after quenching, deteriorating the damping coefficient and low-temperature toughness. According to many experiments by the present inventors, if the Ni equivalent/Cr equivalent expressed by the above relational expression is adjusted to a range of 0.1 to 0.5, the amount of retained austenite after quenching becomes 15% or less, resulting in the damping of the steel of the present invention. Ni was confirmed to not deteriorate the modulus and low temperature toughness.
Equivalent/Cr equivalent was limited to 0.1 to 0.5. Quenching: 850 to 1150°C The heating temperature required to harden the steel having the composition of the present invention is the austenitizing temperature of the steel, that is, 850°C or higher. However, if heated to a higher temperature than necessary, δ ferrite will be generated and reduce the toughness and ductility after heat treatment.
Limited to the following. Tempering: Ac1 ~ Ac1 + 80°C General tempering treatment is normally carried out at a temperature of Ac1 or lower, but in the case of the steel of the present invention, toughness and good damping capacity at low temperatures are required for the application, so Of course, embrittlement due to restoration must be avoided, and the formation of retained austenite must be suppressed as much as possible. From this point of view, it is necessary to perform the tempering treatment at a temperature at which tempering brittleness does not occur and residual austenite is reduced, that is, at a temperature equal to or higher than the Ac1 transformation point. However, if the temperature is higher than necessary, a uniform tempered structure will not be obtained and the low-temperature impact value will decrease, so the temperature was limited to Ac1 + 80°C or lower. Next, the characteristics of the steel of the present invention will be explained in detail using examples. Example 1 Inventive steel and comparative steel having the compositions shown in Table 1 were produced by vacuum melting.

[Table] 40% by hot forging using the sample materials in Table 1.
Square timbers with a cross section of mm x 50mm were manufactured and various characteristic values were investigated. Low-temperature tensile strength properties Quenching using the test materials in Table 1: 950℃×
1hrOQ tempering: (Ac1 + 20°C) × 2hr After performing AC treatment, JIS No. 4 tensile test pieces were taken and the tensile strength at a low temperature of -75°C was measured. The result is the second
Shown in the table.

【table】

[Table] As shown in the same table, the tensile strength of the steel of the present invention at low temperatures is 100 Kg/mm ² or more, and the elongation, which is a measure of toughness and ductility, is about 30%. In contrast, the comparative steel has high tensile strength but elongation and extremely low area of area, indicating instability against fracture. Low-temperature impact properties Quenching: 950℃×1hrOQ, tempering: (Ac1
After performing AC treatment for 2 hours (+20°C), JIS No. 4 Charpy impact test pieces were taken and used for testing. The test was conducted at various temperatures from -196°C to 100°C. The results are shown in Figure 1. As seen in the same figure, the impact properties of comparison steels No. 9 and 10 are unstable at 0°C, and -50
7Kg at ℃. It shows a low Charpy impact value of less than m/cm ² . In contrast, the steel of the present invention exhibits stable impact properties even at temperatures of -50°C, and even at -100°C, it exhibits impact resistance of 15kg. Indicates a Charpy impact value of m/cm ² or more. As described above, the steel of the present invention has been shown to have extremely excellent impact properties at low temperatures. Attenuation Capability Using the test materials shown in Table 1, the test pieces were subjected to quenching at 950°C x 1hrOQ tempering (Ac1 + 20°C) x 2hrA.C treatment, and then test pieces were taken to measure the damping coefficient. The results are shown in Table 3.

[Table] As seen in the table, the steel of the present invention is No. 9 of the comparative steel.
Although it shows a slightly lower damping coefficient than (12% Cr-Fe steel), it shows a value that poses no problem in practical use. Example 2 In order to find suitable tempering conditions for the steel of the present invention, steels No. 1, 2, 3, and 9 from the sample materials in Table 1 were tempered at various temperatures after quenching. was applied. Next, Charpy impact test piece (JIS No. 4)
was sampled and subjected to a test at -75°C. The results are shown in Figure 2. That is, Figure 2 is a diagram showing the low-temperature impact properties organized by Ni content and tempering temperature, and the numerical values in the figure indicate the Charpy impact values. The figure shows that the tempering temperature at which good low-temperature impact properties can be obtained is greatly influenced by the Ni content. i.e.
Good low-temperature impact properties cannot be obtained with No. 9, which has a low Ni content, but with No. 1, which contains 2% or more Ni,
In Nos. 2 and 3, it was confirmed that the higher the Ni content, the better the low-temperature impact properties could be obtained at a lower tempering temperature. In Fig. 2, the area where good low-temperature impact properties can be obtained is indicated by diagonal lines, but in order to make this area even clearer, Fig. 3 shows the results of arranging the Charpy impact value of test material No. 2 according to the tempering temperature. It was shown to. The amount of retained austenite is also shown in the figure. As seen in Figure 3, the Shalpy impact value largely depends on the tempering temperature. i.e. 600℃
As a result of the above tempering treatment, the sharpy impact value rises rapidly and reaches a peak value at around 670°C, but at temperatures higher than that, the sharpy impact value decreases.
On the other hand, the amount of retained austenite reaches its peak value after tempering at 600°C and rapidly decreases at higher temperatures. From these results, it can be easily inferred that the improvement in the low-temperature impact properties of the steel of the present invention during tempering at 600°C or higher is due to the rapid decrease in the amount of retained austenite. From the results shown in Fig. 3, the suitable tempering conditions for the steel of the present invention, that is, the range in which good low-temperature impact properties can be obtained, are as shown in Fig. 3.
It was confirmed that the temperature was around Ac1 + 80℃. Among the steels of the present invention, the tempering temperature of the second invention steel was also set at Ac1 to Ac1 during the heat treatment.
Good low-temperature impact properties can be obtained by setting the temperature to +80°C. As described above, the steel of the present invention is suitable for use in low-temperature turbine blades, and its characteristics include excellent low-temperature impact properties and a damping coefficient that is not much different from conventional turbine blade materials. Therefore, it is a valuable steel for turbine blades used in low-temperature environments, such as in power generation equipment that uses liquefied natural gas.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the Shapey impact value at various temperatures of the inventive steel and comparative steel, Figure 2 is a diagram showing the influence of Ni content and tempering temperature on Shapey impact value, and Figure 3 is a diagram showing the Shapey impact value. and FIG. 6 is a diagram showing the influence of tempering temperature on the amount of retained austenite.

Claims (1)

[Claims] 1 C+N: 0.05-0.25%, Si: 0.1-1.50%,
Mn: 0.1~2.0%, Ni: 2.0~6.0%, Cr: 10.0~
Contains 14.0% and one or more elements selected from Nb, Ta, and Zr in total of 0.02 to 0.30%, with the remainder essentially consisting of Fe, and the Ni equivalent/Cr equivalent expressed by the following formula is 0.1 Steel for low-temperature turbine blades adjusted to be in the range of ~0.5. Ni equivalent: Ni% + 0.5Mn% Cr equivalent: Cr% - 15 (C + N)% + 1.5Si% 2 C + N: 0.05 ~ 0.25%, Si: 0.1 ~ 1.50%,
Mn: 0.1~2.0%, Ni: 2.0~6.0%, Cr: 10.0~
14.0%, one or more elements selected from Nb, Ta, and Zr in total of 0.02 to 0.30%, and further Mo: 3.0%
Below, W: 3.0% or less, V: 1.0% or less, Co: 2.0
% or less, Cu: 2.0% or less, Ti: 1.0% or less, Al:
0.5% or less, B: 0.01% or less, Ca: 0.02% or less,
Contains one or more elements selected from Te: 0.2S to 0.1%, REM: 0.2S to 0.1%, the balance is substantially Fe, and the Ni equivalent/Cr equivalent is expressed by the following formula: Steel for low-temperature turbine blades adjusted to have a value in the range of 0.1 to 0.5. Ni equivalent: Ni% + 0.5Mn% Cr equivalent: Cr% - 1.5 (C + N)% + 1.5Si% 3 C + N: 0.05 to 0.25%, Si: 0.1 to 1.50%,
Mn: 0.1~2.0%, Ni: 2.0~6.0%, Cr: 10.0~
14.0% and a total of 0.02 to 0.30% of one or more elements selected from Nb, Ta, and Zr, with the balance essentially consisting of Fe, and the Ni equivalent/
Using an alloy whose Cr equivalent was adjusted to be in the range of 0.1 to 0.5, quenching treatment was performed after heating and holding at a temperature of 850 to 1150°C, followed by the Ac1−Ac1+80
A method for heat treatment of steel for low-temperature turbine blades, characterized by subjecting the steel to tempering treatment at a temperature range of °C. Ni equivalent: Ni% + 0.5Mn% Cr equivalent: Cr% - 15 (C + N)% + 1.5Si%