JPS6156310B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial application field) New low C-low Si for use under wet steam
- With regard to Cr-Mo steels, this specification specifies that they can be used appropriately even in environments subject to attack by erosion and corrosion (abbreviated as EC), especially in applications such as feed water heaters for nuclear power plants or similar applications. This paper attempts to propose the results of research and development related to the above-mentioned types of steel, whose composition has been adjusted accordingly. Wet steam and high-temperature condensed water, or high-temperature condensed water itself, which are usually below 250°C, are collectively referred to here as "wet steam", and these high-temperature gas-liquid two-phase flows or high-speed flows of high-temperature water flows are collectively referred to as "wet steam". For example, the inner surface of the body of a closed container such as a feed water heater for nuclear power generation equipment such as a light water reactor,
When attacked by EC, corrosion products caused by EC circulate within the reactor system, resulting in radiation damage to the entire system and damage to the feedwater heater itself.
There is a concern that reliability may decrease due to EC damage. As a means to eliminate these problems, first of all, from a design perspective, the flow velocity of the fluid throughout the system should be lowered by increasing the diameter of the feed water heater body and the diameter of the piping system, and from the viewpoint of materials, it should be
It is conceivable to adopt a steel grade with better properties. In the former case, lowering the flow rate of fluid in the system leads to larger equipment, increasing the amount of steel used, and ultimately reducing material costs.
Rather, it is required to prevent this type of damage from the material standpoint, since this will lead to an increase in construction costs. (Conventional technology) As a result of analyzing and examining a large amount of conventional knowledge and research results regarding EC, the JIS standard shown in Table 1 was
G4109, SCMV-3 (commonly known as 11/4%Cr-1/2%Mo
-3/4%Si steel), heat treatment (two types: normalizing and tempering (hereinafter abbreviated as N-T) and annealing (hereinafter abbreviated as A)) and mechanical properties (N - For T steel and A steel, within the standard range of high strength level and low strength level respectively:
In particular, the C content is 0.15 to 0.17%, which is near the upper limit of the standard.
The high-strength 11/4%Cr-1/2%Mo-3/4%Si steel with heat treatment limited to N-T is the steel with excellent EC resistance suitable for feed water heaters. In accordance with the conclusion that there is a
2% Mo-3/4% Si steel was uniquely adopted in Japan as a steel material for the feed water heater as a measure to enhance the safety of nuclear power generation equipment.

[Table] The reason why this type of steel with a C content of 0.15% or more is applied is mainly because it is necessary to impart hardness to the steel in order to ensure EC resistance. In addition to this, we also ensured the specifications of SCMV-3, especially the N-T steel, that is, high tensile strength (for example, tensile strength of 53 Kgf/mm ² or higher). However, today the durability of feedwater heaters for nuclear power generation equipment is
In Japan, as a measure to improve EC properties, we use high-grade high C-11/4%Cr-1/2%Mo-3/ to deal exclusively with materials.
Although there is a tendency to adopt 4%Si steel, on the other hand, high C-11/4%Cr-1/2%Mo steel as mentioned above
The use of -3/4%Si steel has many problems due to its poor weldability, as shown below. In other words, the feed water heater has a diameter of approximately 2 m and a length of approximately
It is a huge container-like object that is 10 meters long. First, the body plate is made by bending a steel plate into a cylindrical shape and welding the longitudinal direction. Several pieces of this cylindrical shape are connected by circumferential welding, and tube plates, pipes, and various other parts are further assembled inside this cylinder by welding. After that, the mirror plates are welded to both ends and assembled. In such welding assembly, high C-11/4%Cr-1/2%Mo-3/4%Si steel has a weld hardenability index, C equivalent (C+Si/24+Mn/
6+Ni/40+Cr/5+Mo/4+V/14) is, for example, 0.72%, and the welding susceptibility index, P _CM value (C+Si/30
+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15
+V/10+5B) is, for example, 0.32%, making it a steel that is prone to welding cracks. Therefore, when welding, preheating and postheating must be carried out very carefully to avoid welding cracks. Stress relief annealing after welding (usually
It is called Stress Relieving, or SR for short. ) must also be carried out at extremely high temperatures for a long period of time. However, careful preheating here usually means using a gas burner to raise the temperature of the area to be welded to about 250℃, which not only makes the welding work environment worse, but also reduces welding efficiency and consumes significant energy by the gas burner. , resulting in great disadvantages. (Problems to be Solved by the Invention) In this way, energy saving, working environment,
In order to improve efficiency, it is extremely important to develop Cr-Mo steel with low weld susceptibility. The essential conditions are that it has EC resistance against wet steam, and that it has strength and toughness suitable for use in feed water heaters. In addition, this resistance
Regarding EC properties, the high C-
11/4%Cr-1/2%Mo-3/4%Si steel is by no means perfect, and there is a strong desire for further improvements, taking into account the lifespan of the container and operational safety. There is. That is, an object of the present invention is to provide a steel composition that has good weldability and excellent EC resistance under wet steam. First, it is clear from the calculation formulas for the C equivalent and P _CM value mentioned above that in order to improve weldability, it is sufficient to reduce the C content, but in that case, the hardness of the steel naturally decreases, so the EC resistance It is usually assumed that the Rather than improving weldability without impairing EC resistance, it is desired to further improve EC resistance and also improve weldability. It is common knowledge among those skilled in the art that reducing the amount of C is inherently inappropriate, and in fact, reducing the amount of C can improve resistance to E.
C. There are no examples of attempts to improve sex. On the other hand, this problem was considered to be an insurmountable problem in terms of economic efficiency under the current state of technology, and the reality was that the compromises mentioned above had to be made. (Means for Solving the Problems) The inventors have purposely tried to find a more advantageous solution to the above-mentioned problems, and have particularly focused on improving the C content and trace alloying elements of this steel type to improve its EC resistance, weldability, and
The essence of the relationships that should affect mechanical properties can be systematically
As a result of intensive basic research, we discovered an unexpected fact: EC resistance of this type of Cr-Mo steel becomes better when the C content is reduced to 0.14% or less, and the Si content In addition, the addition of trace amounts of Cu and Ni at the same time as well as the addition of V can significantly improve the strength of the steel. They discovered that performance can be achieved. All of these facts can be said to be unexpected findings that contradict conventional technical common sense or academic concepts. The inventors applied these findings to steel used under wet steam conditions, such as in the feed water heater, and solved the above-mentioned challenges in such applications.
This achieved improvements in both EC resistance and weldability all at once. This invention has C: 0.02 to 0.14% by weight (hereinafter simply expressed as %), Si: less than 0.45%, Mn: 0.30 to 0.80.
%, Cr: 0.70 to 1.60%, and Mo: 0.40 to 0.70%, and the remainder is essentially the basic composition of Fe, excluding unavoidable impurities, which contributes to increased strength mainly through solid solution hardening and improves EC resistance. 0.02 to 0.5% each of Cu and Ni, which are elements that improve
and further contain V: 0.005 to 0.08%.
By containing Al: 0.005 to 0.08% and increasing the strength due to precipitation hardening of V, it was possible to reduce the C content, thereby achieving improvement in EC resistance and weldability. Furthermore, C, which is an important component among the above-mentioned
The present invention includes embodiments in which the content is particularly limited to 0.02 to 0.13%, and the Cu and Ni contents are respectively limited to 0.16 to 0.30% as areas for further enhancing the effects initialized in each of the inventions. (Function) Now, the reason for limiting the above component range is as follows. The C content is the most important requirement in this invention, and it reduces weld hardenability and weld crack susceptibility, lowers the welding preheating temperature, eliminates postheating, lowers the stress relief annealing temperature, and has excellent EC resistance.
In order to break through conventional notions of gender and realize it, it must be limited to 0.14% or less. From the point of view of weldability, the lower the C content, the better, but it is suitable for use at normal temperatures and temperatures up to approximately 250°C, which is required for steel used in wet steam environments such as feed water heaters for nuclear power generation equipment. Since at least 0.02% is necessary to obtain strength and toughness in the range, the lower limit is set at 0.02%. In particular, the Cr-Mo steel with the composition according to the present invention exhibits a remarkable feature in that when the C content is reduced as described above, even if it is a very small amount, the welding flaw sensitivity is significantly improved. A reduction in the amount from the upper limit of 0.14% to 0.13% can significantly improve weld flaw susceptibility without causing a significant decrease in strength levels, and therefore in each embodiment of the invention the C content is preferably 0.13%. More preferably, it is up to 0.11%. Next, Si is an element that is effective in increasing strength at room and high temperatures, and in addition to the conventional high value of C of 0.15 to 0.17%, Si is also generally 0.45 to 0.90%.
It contained %. However, it was discovered that by limiting this content to less than 0.45%, the EC resistance was significantly improved along with the reduction in carbon content.
The content is limited to less than 0.45%, and this limitation is also an important requirement, but the necessary strength can be secured through the reinforcing effects of other reinforcing elements such as Mn, Cr, and Mo. is as described below. Mn requires 0.30% or more to give strength and ductility to steel, but in steel with the composition of this invention, Cr and Mo make a large contribution to ensuring strength. It is not necessary to use it; in fact, if it exceeds 0.80%, the weld hardening property increases and problems arise, so it is limited to 0.30 to 0.80%. Both Cr and Mo are important elements that increase the resistance to EC due to high flow rates of wet steam.
To provide excellent EC resistance for applications such as feed water heaters for nuclear power generation equipment, the steel with the composition of this invention must contain at least 0.70% each of Cr and Mo.
and 0.40% required. Cr and Mo are EC resistant
Considering the purpose of improving sexual performance, the higher the amount, the greater the effect, but when Cr exceeds 1.60%, and Mo exceeds 0.70%.
%, there is a concern that workability and weldability may deteriorate. Therefore, the Cr content is limited to 0.70 to 1.60%, and the Mo content is limited to 0.40 to 0.70%. Note that Cr not only contributes to the formation of a high-strength structure as an element that improves hardenability but also has a solid solution strengthening effect, while Mo is a precipitation-hardening element and is used to finely form carbides during tempering. Both of these precipitates also contribute to an increase in strength, and as a result, the above-mentioned effects are appropriately exhibited within their respective limited ranges. Next, the reason why Cu: 0.02 to 0.5% and Ni: 0.02 to 0.5% are contained simultaneously is that the EC resistance against wet steam at a high flow rate can be further significantly improved. Cu and Ni in terms of improved EC resistance
The more the better, but either one is 0.5%
If the content exceeds 0.5%, the weldability will be significantly reduced, so the upper limit of each of these contents was set at 0.5%. Cu and Ni are elements that originally have a solid solution strengthening effect and improve hardenability, and by including them in combination, the strength is greatly increased. Since strength can be improved by Cu and Ni in this way, it is possible to reduce the C content in terms of strength, and the inclusion of Cu and Ni is also useful for improving EC resistance. In other words, when Cu and Ni are contained together, they themselves improve EC resistance and help increase strength, and their increased strength makes it possible to reduce C content, which also improves EC resistance. This will make a double contribution. In the embodiment of this invention, C is limited to 0.02 to 0.13% as already mentioned, but Cu and Ni are further limited to 0.16 to 0.30%, because the simultaneous addition of Cu and Ni is resistant.
In order to improve EC properties, it is preferable that each content is 0.16% or more, and the higher the content, the greater the effect, but in terms of welding susceptibility, it is better to have less, and each content is preferably 0.30% or less. . Therefore, for maximum EC resistance and susceptibility, limit C to 0.13% or less, and 0.16
It is practically desirable to contain both Cu and Ni at ~0.30%. Also, V: 0.005 to 0.08% is contained because
It is an element that contributes to the strengthening of steel through precipitation hardening at 0.005% or more, thereby increasing the strength.As a result, it is possible to reduce the C content in terms of strength, which in turn improves weld hardenability, weld crack susceptibility, and EC resistance. It acts to improve. However, when V exceeds 0.08%, it increases the reheating susceptibility of the weld.
It also deteriorates the toughness of the weld heat affected zone, so 0.08
% or less. Containing Al in the range of 0.005 to 0.08% is
This is because it is particularly useful for improving toughness based on the particle refining effect caused by the formation of AIN, and this effect is clearly expressed at 0.005% or higher and becomes noticeable at first as the amount increases, but saturates at about 0.08%. Therefore, it is limited to 0.005 to 0.08%. In the embodiment of this invention, C is limited to 0.02 to 0.13% as already mentioned, but Cu and Ni are further limited to 0.16 to 0.30%, because the simultaneous addition of Cu and Ni is resistant.
For improving EC properties, it is preferable that each content is 0.16% or more, and the higher the content, the greater the effect, but in terms of welding susceptibility, it is better to have less, and each content is preferably 0.30% or less. In other words, to maximize EC resistance and weldability, limit C to 0.13% or less, and from 0.16 to
It is practically desirable to contain both Cu and Ni at 0.30%. In the present invention, unavoidable impurities included in a normal steel manufacturing process are acceptable. In other words, the general limit for both S and P is to increase the susceptibility to high temperature cracking of the weld zone.
The content is preferably 0.025% or less. On the other hand, N content of 0.0020% to 0.0150% is permissible in the normal steelmaking process, but if it exceeds 0.0150%, the properties of the steel ingot will deteriorate due to the occurrence of blowholes, etc., and weldability will also deteriorate, so it should be within the above range. It is preferable. Furthermore, steel with the above composition can be used in parts that are used under wet steam, and as mentioned above, it has EC resistance, weldability, strength, toughness, etc., and it is valuable only when it is applied to the above applications that require these properties. It is economical. In other words, this invention exhibits effects in terms of both quality and economy only when used for the above-mentioned purposes. The reasons for limiting the steel composition, uses, etc. of the present invention have been explained above, but the steel of each invention is produced by melting with the above-mentioned composition adjustment, then subjected to a rolling or forging process by a conventional method, and then sintered. It is produced as a steel material by conditioning and subsequently tempering or annealing, the heat treatment being limited to the two types mentioned above. Note that normalizing here refers to air cooling after heating to Ac ₃ points or higher (for example, if the plate thickness is less than 100 mm, simply air cooling,
For extra-thick materials of 100 mm or more, it is a process of air cooling or forced cooling. Tempering is a process of heating to Ac ₁ point or less and then air cooling, and annealing is a process of heating to Ac of ₃ points or more and then slowly cooling. It is processing. Normalized and tempered materials usually have a ferrite/pearlite structure,
It may also contain bainite. Annealed materials usually have a ferrite + pearlite structure. To be sure, we mention feedwater heaters for nuclear power generation equipment as a typical example of equipment used under wet steam. The fact that it can be used even if it is "better" is a major feature, and has particularly high value as described below. In other words, steel that has undergone annealing heat treatment is less sensitive to thermal cycles such as stress relief annealing after welding, and has less variation in structure and mechanical properties than steel that has undergone normalizing and tempering treatments. . Therefore, in cases where considerable temperature variations are expected, such as during stress relief annealing of large welded structures, it is necessary to It is more desirable to use annealed steel rather than tempered steel. Despite these characteristics of annealed steel, conventionally only normalized and tempered steel has been used in feed water heaters for nuclear power generation equipment, and there has been no movement to use annealed steel until now. The main reason for this was that annealed steel had lower strength than tempered steel and was thought to have significantly lower EC resistance. However, as a result of intensive research without being bound by conventional fixed ideas, the inventors found that the components according to each of the inventions have extremely high EC resistance even after annealing heat treatment, low C is possible, and weldability is sufficient. It was confirmed that the steel could be improved and had appropriate strength and toughness as steel for feed water heaters for nuclear power generation equipment. (Examples) Now, in order to further clarify the constituent elements of each of the inventions as described above and specifically demonstrate the special effects of each invention, examples will be described below. The chemical composition of the test steel is shown in Table 2. Symbols No. 1 to 4 in the table are steels according to the present invention, and symbols No. 5 and 6 are steels that have been used in feed water heaters for nuclear power generation equipment as representative examples of equipment used under wet steam in Japan. Commercially available high C
-11/4%Cr-1/2%Mo-3/4%Si steel, which corresponds to so-called conventional steel, and has a C content of 0.16 to 0.17%, which is much higher than the upper limit of 0.14% of this invention. . Symbols No. 7 and 8 are comparison steels showing that the improvement effect on EC resistance is insufficient, and among them, symbol No. 9 is a comparison steel suitable for the weldability test.

[Table] Except for conventional steels with symbols No. 5 and 7, which are commercially available steels, all of these steels are made from 100Kg steel ingots melted using a small high-frequency induction heating vacuum melting furnace, and then rolled into sheets using a small rolling machine. It is hot rolled to a thickness of 30mm. The heat treatment after rolling was not limited to the normalizing and tempering treatments conventionally applied to steel materials for feed water heaters for nuclear power generation equipment, but annealing was also performed. Note that the normalizing treatment referred to here involves charging the material into a heating furnace at 930°C, holding it for one hour, extracting it, and allowing it to cool in the atmosphere. The tempering treatment conditions were 660°C x 1h. In addition, for annealing heat treatment, the steel plate is charged into a heating furnace at 930°C, and after being held for 1 hour, the cooling rate of the steel plate in the furnace is 800°C.
The temperature is gradually cooled at an average rate of 0.8°C/min over 400°C. Steel plates always undergo stress relief annealing after welding and assembly, so both normalized and normalized materials were further subjected to stress relief annealing (SR) at 645°C for 1 hour before being subjected to testing. Ta. However, it goes without saying that the weldability test used test materials that had not been subjected to stress relief annealing. Using these test materials, we first confirmed that the invented steel has appropriate strength and toughness as steel used in feedwater heaters for nuclear power generation equipment at room temperature and at 250°C.
This was verified by conducting a tensile test at 0.degree. C. and a V-Sharpie impact test. In addition, the appropriate strength and toughness mentioned here, for example, in terms of tensile strength, should be about 40Kgf/mm ² or more at room temperature, and considering that the feed water heater is heated to about 150℃, it should be more than 40Kgf/mm2 at room temperature. 40Kgf/mm ² even at a higher temperature of 250℃
It is desired that the above is maintained, and in terms of absorbed energy at 0° C., it is sufficient if it is approximately 2.1 Kgf·m or more, considering the usage conditions. For tensile testing, diameter 6mm, parallel part 30mm, gauge length
A 25 mm round bar specimen was used, and a 2 mm V-notched lube specimen was used for the impact test. Furthermore, the effect of improving weldability, which is the main objective of this invention, was investigated using representative test materials. For weldability testing
Using the diagonal Y-shaped welding test method specified in JIS Z 3158, we determined the preheating temperature to prevent cracking. Next, we will describe the method of EC testing using high-speed, high-temperature water on representative test materials. The test piece was a disk with a diameter of 9 mm and a thickness of 10 mm, with a width of 3 mm as shown in Figure 1.
mm, with a 5 mm deep groove cut in a cross shape.
In the test, high-temperature, high-pressure water simulating reactor water at 150℃ and less than 5 ppb of oxygen was sprayed at a high flow rate of 10 m/s for 500 hours through a nozzle with a diameter of 1 mm from the top at the intersection of the grooves, and the weight of the test piece was determined by EC. This was done by examining the decrease. Table 3 shows the test results based on the above test method.

[Table] According to Table 3, all the invented steels exhibit sufficiently high tensile strength and toughness at room temperature and 250°C to be suitable for the above uses, and C content is required to improve EC resistance and weldability. It is clear that even if it is reduced, there is no problem in terms of strength. In addition, even if the normalizing and tempering treatments conventionally applied to steel for feed water heaters are changed to annealing treatments that do not increase strength, the C content can be significantly reduced compared to the conventional method, which is advantageous in terms of strength. It is completely new. Next, regarding the relationship between weld crack susceptibility and C content, the crack prevention preheating temperature of conventional steels No. 5 and 6 and comparative steel No. 8, which have a C content exceeding the upper limit of this invention, is Although it is 150-200℃,
According to this invention, if the C content is limited to 0.14% or less, the preheating temperature can be lowered to 100°C or less. In other words, while conventional steel required preheating to a high temperature of at least 150 to 200°C to prevent welding cracks, the steel of this invention requires preheating to a high temperature of at least 150 to 200°C.
It only needs to be preheated to about 100℃, which can be said to be a dramatic improvement in welding work. In addition, regarding EC resistance, comparative steel No. 7 and 8 have C content (0.15 to 0.17%) comparable to conventional steel.
Conventional steel when only the Si content is less than 0.45%
Although the improvement in EC resistance over No. 6 is slight, the combination of low C and low Si
Invention steels No. 1 to 4 containing Ni, V, and Al have high resistance to
It is clear that EC properties are significantly improved. That is, in addition to limiting the C content to 0.14% or less and the Si content to less than 0.45% as steel for feed water heaters, trace amounts of Cu and Ni are simultaneously added, preferably 0.16% to 0.30% of each. In addition to this, it has been found that the addition of V can significantly improve the EC resistance. (Effects of the Invention) As described in detail above, according to the present invention, the preheating temperature for welding can be reduced to about 100°C or considerably lower, and there is less risk of damaging the working environment. Since less energy is required for preheating during welding, energy costs can be lowered, and of course, it has much better EC resistance against wet steam, so it can achieve various effects such as extending the life of the container. I can do it. In particular, each of these inventions can be used with remarkable effects, such as in feed water heaters for nuclear power generation equipment.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of an E.C. test piece.

Claims (1)

[Claims] 1 C: 0.02 to 0.14% by weight Si: Less than 0.45% by weight Mn: 0.30 to 0.80% by weight Cr: 0.70 to 1.60% by weight Mo: 0.40 to 0.70% by weight Cu: 0.02 to 0.5% by weight Ni: Contains 0.02 to 0.5% by weight Al: 0.005 to 0.08% by weight and V: 0.005 to 0.08% by weight, and the remainder is substantially free except for inevitable impurities.
It has a Fe composition and is characterized by excellent EC resistance and weldability.
Low Si-Cr-Mo steel. 2. The low C-low Si-Cr-Mo steel according to claim 1, wherein the C content is 0.13% by weight or less, and the Cu and Ni contents are each 0.16 to 0.30% by weight.