JPH0254414B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial application field) New low C-Cr for use under wet steam
-With regard to Mo steel, this specification specifically refers to erosion and corrosion (abbreviated as EC), which is specific to applications in feed water heaters for nuclear power generation facilities or similar products.
This paper attempts to propose the results of research and development related to the above-mentioned types of steel, the composition of which has been adjusted so that it can be used appropriately even in environments where it is attacked by steel. Wet steam and high-temperature condensed water, or high-temperature condensed water itself, which are usually below 250°C, are simply collectively referred to as "wet steam", and these high-temperature gas-liquid two-phase flows or high-speed flows of high-temperature water flows Due to E.
When attacked by C., there are concerns that corrosion products caused by EC may circulate within the reactor system, causing activation damage to the entire system, and reducing reliability due to EC damage to the feedwater heater itself. Ru. As a means to eliminate these problems, first, 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 occurring in terms of materials, 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 1 1/4% Cr-1/
Chemical composition, heat treatment (normalizing and tempering (hereinafter abbreviated as N-T) and annealing (hereinafter abbreviated as A) of 2%Mo-3/4%Si steel (% indicated as wt%, same below)) ) and mechanical properties (N-
For T steel and A steel, the C content is specifically regulated at 0.15% to 0.17%, which is near the upper limit of the specification, and the heat treatment is limited to N-T. intensity level 1
Following the conclusion that 1/4% Cr-1/2% Mo-3/4% Si steel is a steel with excellent EC resistance suitable for feed water heaters, its properties were actually confirmed through experiments. , thus this high C-1 1/
4% Cr-1/2% Mo-3/4% Si steel was uniquely adopted in Japan as a steel material for the feed water heater as a measure to further 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 high tensile strength (for example, tensile strength of 53 kgf/mm ² or more) of SCMV-3, especially for NT steel. 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-1 1/4%Cr-1/2Mo to deal exclusively with materials.
-Although there is a tendency to adopt 3/4% Si steel, on the other hand, high C-1 1/4% as mentioned above
The use of Cr-1/2Mo-3/4%Si steel has many problems due to 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, tubes, and various other parts are further assembled inside the cylinder by welding. After that, the mirror plates are welded to both ends and assembled. In such welding assembly,
High C-1 1/4%Cr-1/2Mo-3/4%
Si steel has a weld hardenability index, C equivalent (C+Si/24+
Mn/6+Ni/40+Cr/5+Mo/4+V/14)
For example, 0.72%, 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 raising the temperature of the area to be welded to around 250℃ using a gas burner, which not only makes the welding work environment worse, but also reduces welding efficiency and consumes significant energy by the gas burner. This will cause a great deal of disadvantage. (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 E.
C. Regarding sex, the high C-1 that has been used so far
1/4%Cr-1/2Mo-3/4%Si steel is by no means perfect, and there is a strong desire for further improvements in terms of container life and operational safety. 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 C equivalent, P _CM, etc. that the C content should be reduced in order to improve weldability, but in such a case, the hardness of the steel will naturally decrease, so the EC resistance will decrease. It is usually assumed that the product will deteriorate. Rather than improving weldability without compromising EC resistance, we have further improved EC resistance,
Moreover, it is desired to improve weldability, and it is common knowledge among those skilled in the art that reducing the C content is inherently inappropriate for improving EC resistance. There are no examples of attempts to improve EC resistance by reducing the amount. 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) In order to solve the above-mentioned problems, the inventors purposely aimed to find a more advantageous solution.In particular, the C content and trace alloying elements in this steel type were used to improve the EC resistance, weldability, and mechanical properties. As a result of systematic and fundamental research into the nature of the relationship between Furthermore, by adding a small amount of V, the C content can be reduced, resulting in a significant improvement; In each case, excellent performance is achieved even at a low strength level. etc. were discovered. 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 resulted in improvements in both EC resistance and weldability all at once. In this invention, C: 0.02 to 0.14wt% (hereinafter simply %
), Si: 0.45-0.90%, Mn: 0.30-0.80%,
Contains Cr: 0.70~1.60%, Mo: more than 0.40% up to 0.70%, and Al: 0.005~0.08% as basic components,
In addition to this basic component, V: 0.005 to 0.08 wt% is contained, and the remainder is Fe and unavoidable impurities, thereby improving weldability and EC resistance. (Function) The reasons for limiting the range of steel components in each of the above inventions are as follows. The C content is the most important requirement in this invention, reducing weld hardenability and weld susceptibility, lowering the welding preheating temperature, eliminating postheating, lowering the stress relief annealing temperature, and providing excellent EC resistance. In order to break through conventional ideas about sexuality and realize it,
Must be limited to 0.14% or less. From the point of view of weldability, the lower the C content, the better. Since at least 0.02% is necessary to obtain strength and toughness in the temperature range, the lower limit is set at 0.02%. In particular, the Cr-Mo steel having 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 susceptibility is significantly improved. Reducing the C content 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 the C content is preferably up to 0.13%, more preferably up to 0.11%. It's good to do that. Next, Si is an effective element for increasing strength at room and high temperatures. In this invention, in order to fully exhibit the desired effect, it is necessary to strictly limit the C content, so it is expected that the strength will decrease accordingly, and in order to maintain the specified strength, the minimum
Si content is required to be 0.45%, and in terms of strength
The more the better, up to 0.90%, but 0.90%
If the
Limited to 0.45-0.90%. 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. In order to provide excellent EC resistance for applications such as feedwater heaters for nuclear power generation equipment, the steel with the composition of this invention must contain at least 0.70% Cr and more than 0.40% Mo.
Requires. Considering the purpose of improving EC resistance, the larger the amount of Cr and Mo, the greater the effect.
If Cr exceeds 1.60% or Mo exceeds 0.70%, there is a concern that workability and weldability will deteriorate.
Therefore, the Cr content is 0.70~1.60%, and the Mo content is 0.70~1.60%.
Limited to more than 0.40% and up 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, which causes fine carbides to form during tempering. Both contribute to an increase in strength, and as a result, the above-mentioned effects are appropriately exhibited within their respective limited ranges. 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 addition, containing V: 0.005 to 0.08% is
At 0.005% or more, precipitation hardening contributes to strengthening the steel, which increases its 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. Acts to improve. However, if V exceeds 0.08%, it will conversely increase the reheating susceptibility of the weld zone and also deteriorate the toughness of the weld heat affected zone, so it is limited to 0.08% or less. In other words, to maximize EC resistance and weldability, C
is limited to 0.02 to 0.13%, and appropriate amounts of Si, Mn,
It is practically desirable to contain Cr, Mo, Al and V. In the present invention, unavoidable impurities included in a normal steel manufacturing process can be tolerated. 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 is Al
Coexistence with the 0.0020~
0.0150% is effective, 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 is preferable to stay within the above range. 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, applications, etc. of this invention have been explained above, but the steel of this invention is melted with the above-mentioned composition adjustment, then subjected to a rolling or forging process by a conventional method, and then sintered. The steel material is produced by tempering and subsequent 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 or higher points (for example, for plates less than 100 mm thick, simply air cooling,
For extremely thick materials of 100 mm or more, air cooling or forced cooling is used. Tempering is heating to Ac ₁ point or less and then air cooling, and annealing is heating to Ac ₃ or more and then slowly cooling. It is processing. Normalized and tempered materials usually have a ferrite/pearlite structure,
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. Namashi”
The fact that it can be used even if it is used 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 than steel that has undergone normalization and tempering, and its structure and mechanical properties may change. few. 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 feedwater heaters for nuclear power generation equipment, and until now there has been no movement toward using annealed steel. 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 even if annealing heat treatment is applied to the composition according to the present invention, the EC resistance is extremely high, low C is possible, and weldability is sufficiently improved. Moreover, it was confirmed that the steel had appropriate strength and toughness as steel for feedwater heaters for nuclear power generation equipment. (See Examples below) (Examples) In order to further clarify the constituent elements of this invention as described above and specifically demonstrate the special effects of this invention, Examples will be described below. The chemical composition of the test steel is shown in Table 2. Symbols No. 1 and 2 in the table are steels according to this invention, and symbols No. 3 and 4 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-1 1/4%Cr-1/2Mo-3/4%Si that has been used
Steel, which corresponds to so-called conventional steel, and has a C content of 0.16 to 0.17, which greatly exceeds the upper limit of 0.14% of this invention.
%. Moreover, although symbols Nos. 5 and 6 contain V, they are comparison steels in which the C content exceeds the upper limit of the present invention.

[Table] Except for the conventional steels with symbols No. 3 and 4, which are commercially available steels, all of these steels are made from 100Kg steel ingots that are melted using a small high-frequency induction heating vacuum melting furnace. It is hot rolled to 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 material is charged into a heating furnace at 930℃.
After holding for 1 hour, the cooling rate of the steel plate in the furnace is 800-400℃.
The temperature is gradually cooled at an average rate of 0.8°C/min. 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 ℃ and a V-Sharp impact test. In addition, the appropriate strength and toughness mentioned here, for example, in terms of tensile strength, should be approximately 40Kgf/mm ² or more at room temperature, and should be higher considering that the water is heated to about 150℃ by a water heater. Similarly, at 250℃, 40Kgf/
It is desired that the absorption energy is maintained at 0° C. or ^more , and considering the usage conditions, it is sufficient to maintain the absorption energy at 2.1 Kgf·m or more. For tensile testing, diameter 6mm, parallel part 30mm, gauge length
A 25 mm round bar specimen was used, and a 2 mm V-notched pea 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.
A groove with a depth of 5 mm is cut in the shape of a cross. For the test, a nozzle with a diameter of 1 mm was passed through the intersection of the grooves from the top.
High-temperature, high-pressure water simulating nuclear 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.
This was done by examining the weight loss of the test piece due to EC. Table 3 shows the test results based on the above test method.

[Table] According to Table 3, the steel according to the present invention also exhibits sufficiently high tensile strength and toughness at room temperature and 250°C to be suitable for the above-mentioned uses, and C content is required to improve EC resistance and weldability. It is clear that there is no problem in terms of strength even if it is reduced. In addition, there is no problem in changing the normalizing and tempering treatment conventionally applied to steel for feedwater heaters to an annealing treatment that is difficult to increase strength, and in terms of strength, the C content can be significantly reduced compared to the conventional method. This is something completely new. Next, regarding the relationship between weld crack susceptibility and C content, conventional steels with C content exceeding the upper limit
In contrast to the crack prevention preheating temperature of Nos. 3 and 4 of 150 to 200°C, by limiting the C content to 0.14% or less according to the present invention, the preheating temperature can be lowered to 100°C or less. In other words, in order to prevent weld cracks, conventional steel required preheating to a high temperature of at least 150 to 200°C, whereas with this invention, it was necessary to preheat to a high temperature of at least 100°C.
It only needs to be preheated to about ℃, which can be said to be a dramatic improvement in welding work. Next, to explain the EC resistance, the EC resistance has a lower C content compared to comparative steels No. 5 and 6, that is, the C content is 0.02 to 0.14% as a Cr-Mo steel for feed water heaters.
It has been found that significant improvement can be achieved by limiting the amount of V to V and further containing V. (Effects of the Invention) As described in detail above, according to the present invention, the preheating temperature for welding can be set to 100°C or considerably lower, and there is less risk of damaging the working environment, and the welding Since less energy is required for preheating the container, energy costs can be reduced, and of course, it has much better EC resistance against wet steam, making it possible to prolong the life of the container.
In particular, the present invention can be used with remarkable effects in feed water heaters for nuclear power generation facilities.

[Brief explanation of drawings]

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

Claims (1)

[Claims] 1 C: 0.02wt% or more and 0.14wt% or less, Si: 0.45wt% or more and 0.90wt% or less, Mn: 0.30wt% or more and 0.80wt% or less, Cr: 0.70wt% or more and 1.60wt% or less , Mo: more than 0.40wt% up to 0.70wt%, Al: 0.005wt% or more and 0.08wt% or less, and V: 0.005wt% or more and 0.08wt% or less, the remainder consists of Fe and inevitable impurities,
Characterized by excellent EC resistance and weldability,
Low C-Cr-Mo steel used under wet steam.