JPS6344815B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to stainless steel cast steel for underwater high-speed rotating bodies, which are particularly prone to cavitation erosion in high-speed flowing water, such as water turbine runners for power generation or pump runners for water supply. Cavitation phenomenon occurs in the fluid in underwater high-speed rotating bodies such as water turbine runners and equipment that transports fluid at high speed, and the material in the affected area is destroyed by erosion (erosion).As a countermeasure to this, cavitation is used in the design. Create a shape that is less likely to occur. However, in this case, the shape may not provide sufficient performance as a device. Next, use a material that is highly resistant to cavitation erosion. This method involves covering the parts of the material where cavitation corrosion occurs by plate welding, build-up welding, or thermal spraying with erosion-resistant materials. Although this method is relatively economical, Thermal spraying alters the quality of the part, and the built-up part is generally made of a different material, so there are factors that can cause defects and damage other than cavitation erosion. That is, it is most preferable that the entire parts used be made of a uniform material with excellent corrosion resistance. For this reason, for example, martensitic stainless steel and austenitic stainless steel are currently used for large, high-speed water turbine runners, but materials that are more resistant to cavitation erosion are desired. SUMMARY OF THE INVENTION In order to solve the above-mentioned drawbacks, an object of the present invention is to provide a stainless steel cast steel for underwater high-speed rotating bodies, such as high-head water turbine runners, which has excellent corrosion resistance. The stainless steel cast steel for underwater high-speed rotating bodies of the present invention is
By weight, C0.15~0.25%, Si0.1~0.8%, Mn3.0~
7.0%, Ni2.5~5.0%, Co4.38~12.0%, Cr17.0~
Consists of 22.0%, balance Fe and unavoidable impurities,
Ni equivalent (Nieq) = Ni (weight%) + 30 x C (weight%)
+1/2×Mn (weight%) +1/3×Co (weight%) and Cr
The Cr equivalent expressed as equivalent (Creq) = Cr (wt%) + 1.5 x Si (wt%) is 17.50% or more, and the Ni equivalent is the value obtained by the following formulas (1) and (2). The components are adjusted so as to fall within the range, and the composition has a composition consisting entirely of austenite phase or an austenite phase and a ferrite phase. [(1); (1.125×Creq−8.75)+1.0] [(2); (1.125×Creq−8.75)−1.5] The cast steel according to the present invention has a metallographically austenitic structure through good composition adjustment. Alternatively, the most suitable corrosion resistance is obtained when austenite contains several percent of delta ferrite. The reason for limiting each component will be explained below. C is a strong austenitizing element and contributes to the stabilization of low Ni materials such as those of the present invention. When the amount of C is small, the steel of the present invention becomes a martensitic structure, resulting in lower toughness and lower corrosion resistance. Therefore, it is necessary to add Ni, Mn, and Co, which are austenitizing elements, in a preferable ratio. If the coefficient of austenitization of Ni is 1, C is calculated as 30 times that value, Mn is calculated as 1/2, Co
is calculated as 1/3. That is, when the amount of C is small, it is necessary to austenitize with Ni, Mn, and Co. However, in this case, the hardness of the austenite structure decreases, and the corrosion resistance deviates from the most desirable state. The corrosion resistance of the steel of the present invention is improved by utilizing the work hardening properties of the austenite structure due to cavitation impact and by metallurgically manipulating alloying elements to create a hard austenite structure. Since an increase in the amount of C creates a hard austenite structure, the preferable amount of C in order to obtain erosion resistance is 0.15 to 0.25%. 0.1% or more of Si is required as a deoxidizing agent during steel manufacturing, but from the viewpoint of toughness, 0.8% or less is appropriate. Preferably it is 0.2-0.6%. Further, as a deoxidizing agent, for example, Al, Zr, or a rare earth element may be added, and these may be present in a total amount of 0.1% or less. In addition, N is mixed as an impurity during steel manufacturing, but this is an austenite-forming element that is 30 times more effective than Ni, but at 0.1%
There must not be more than that. Mn is used as a deoxidizing agent during steel manufacturing, and in the steel of the present invention, it is an important element that promotes austenitization, and its addition amount is one of the characteristics of the present invention.
The work hardening ability of Mn hardens the surface where cavitation occurs, significantly improving corrosion resistance. Addition of a larger amount of Mn improves work hardenability and ductility, but it is not preferable to add Mn in an amount exceeding 7% because it moderately stabilizes austenite, reduces corrosion resistance, and lowers the yield point in terms of strength. Moreover, if it is less than 3%, ductility decreases and weldability is adversely affected. The content is preferably 3.5 to 6.0% to fully exhibit corrosion resistance. Ni is an austenite-forming element, and coexists with Cr to turn the base into austenite, improving strength and toughness. The steel of the present invention has a moderately unstable austenite structure and has an appropriate amount of deformation-induced martensite generated on the cavitation generation surface to improve corrosion resistance.It is desirable to keep the Ni content low, but if it is less than 2.5%, it will become austenite. becomes excessively unstable and contains a martensitic structure, resulting in decreased ductility and poor weldability. In addition, excessive deformation-induced martensite is generated on the cavitation-generated surface, resulting in a decrease in toughness and a decrease in erosion resistance. On the other hand, if it exceeds 5.0%, the austenite structure becomes excessively stable due to the effects of coexisting Mn and Co, improving ductility but decreasing corrosion resistance, so the Ni content was set to 2.5 to 5.0%. Among these, 2.7 to 4.5% is desirable in order to effectively exhibit the above-mentioned effects. Co is a weak austenite forming element, Ni,
Together with Mn, it moderately stabilizes the austenite structure and contributes to strengthening the base and improving corrosion resistance.
If it is less than 4.38%, these effects are not sufficient, and 12.0%
If it exceeds 4.38%, the strength of the base will increase, the ductility and toughness will decrease, and cracks will easily occur during welding.
It was set at 12.0%. Preferably it is 5.5-9.0%. Cr improves corrosion resistance in water, and coexists with Ni to form austenite, but a small amount of delta ferrite is also formed. If it is less than 17.0%, austenite becomes unstable and martenside occurs, and if it exceeds 22.0%, ferrite becomes excessive and toughness and erosion resistance decrease.
%. Preferably it is 18.0 to 21.0%. The remainder consists of Fe and accompanying impurities, and there are P and S as impurities, but it is preferable that these be as small as possible, preferably 0.030 or less. The above-mentioned component ranges have no meaning when considered independently, and in particular, when C, Ni, Co, Mn, and Si and Cr coexist within their respective ranges, excessive martensite and ferrite are not produced. It is also chosen so that the austenite is not reasonably stable. In other words, in the amount expressed as Creq = Cr + 1.5Si (%)
Creq should be 17.50% or more, Nieq = Ni + 30C + 1/2
Ni, C,
By selecting Mn and Co, stainless steel with excellent corrosion resistance can be obtained. That is, the stainless steel cast steel of the present invention has a structurally unstable austenite phase by selecting the Ni equivalent for a predetermined Cr equivalent as described above, and the unstable austenite phase can be processed by cavitation. It has high work hardening properties due to exposure, which improves corrosion resistance.
If the Ni equivalent is greater than the value calculated from equation (1) above, the austenite phase becomes stable, resulting in low work hardenability and low erosion resistance.On the other hand, if the Ni equivalent is less than the value calculated from equation (2), Since the ferrite phase increases, corrosion resistance decreases. In addition, in the formula (1.125
×Creq−8.75) corresponds to a ferrite amount of 0% in the Siefra organization chart, and equation (2) corresponds to a ferrite amount of 5%. Therefore, the object of the present invention is to select the Ni equivalent and Cr equivalent to form an unstable austenite phase with a ferrite content of around 0%. Next, the present embodiment will be explained. Table 1 shows the chemical composition of cast stainless steel produced by high-frequency melting. No.1 to No.3 are conventional products, 0.23 to 0.36C−
18Cr-2~5Ni-6~7Co steel usually contains only about 0.5% Mn. No. 4 and No. 8 are for comparison. The present invention is No. 5 to 7, and Mn compared to No. 1 to 3.
A large amount of is added. The mechanical properties of these Examples by a tensile test at room temperature and the amount of erosion by a magnetostrictive vibration cavitation test were compared and investigated. Table 2 shows the results. Note that in this example, post-melting heat treatment was not performed. No. 3 has an austenitic structure due to the large amount of C, but it has a hard and brittle property due to the small amount of Ni, and the elongation and reduction of area, which indicate ductility, decrease. This decrease in ductility may induce cracking when welding is performed. To prevent this, it is necessary to increase the ductility by creating an austenitic structure with the addition of Mn and Ni.

【table】

[Table] As a result of Table 2, the elongation of No. 1 is about 11%, which is enough to reduce the occurrence of weld cracking, but the amount of erosion is about 10 mg.
This level of corrosion resistance is insufficient, and it is not preferable to use it in equipment where cavitation occurs frequently, and 5 mg or less is preferable. Nos. 2 and 3 are designed to further reduce the amount of erosion.
Although it has decreased to 4.2 to 4.7 mg, it has also increased at the same time.
It becomes low at 4.6 to 7.2%, making it easier for cracks to occur during welding. In order to practically prevent weld cracking, elongation is preferably 10% or more. No. 4 and No. 8 have a high elongation rate, but have a large amount of erosion and are unpalatable. These materials each have a lower or higher C content than the present invention material. Nos. 5 to 7 of the present invention are intended to reduce the amount of erosion without reducing the elongation at break of conventional Nos. 1 to 3, and the elongation is as large as 19 to 23%, and the amount of erosion is increased. has decreased from 3.9 to 4.9. Also, looking at the relationship between 0.2% yield strength and tensile strength, the yield strength of the material of the present invention is 32.2~
42.0Kgf/ ^mm2 and 47.6 to 50.1Kg of conventional materials No. 1 to 3
Although lower than f/mm ² , the tensile strength is 75.1~88.3Kgf/
mm ² is higher and superior to conventional materials No. 1 to 3, which are 61.0 to 69.9 Kgf/mm ² . No. 4 and No. 8 are similar to the present invention, but the addition of C is omitted from the present invention. Sufficient elongation in the tensile test may be ensured or the corrosion resistance will deteriorate. FIG. 1 shows the Cr equivalent and Ni equivalent calculated from the component amounts for Examples on a Schiffler's organizational diagram. The present invention materials are No. 5 to 7, but the Creq is at least 17.50% or more, and the Nieq
is within the range determined by equations (1) and (2) above, and has high corrosion resistance. Conventional products No. 1 and 2 are also within this range, but due to improper selection of ingredients such as Mn, Ni, and Co, the results in Table 2 show that either elongation or the amount of erosion decreases. You can see it by looking at it. As explained above, the stainless steel of the present invention has excellent corrosion resistance and sufficient strength and ductility, so it is preferable as a material for use in parts of hydraulic equipment where cavitation is severe. The cast steel according to the present invention can have an austenite structure or austenite containing a small amount of ferrite by selecting the components within the claimed range as explained in the reasons for limiting the components and the examples, and has excellent corrosion resistance. The corrosion resistance of equipment can be improved by using it in cavitation generating parts of water turbine runners for power generation and other underwater high-speed rotating bodies. The cast steel according to the present invention has sufficient erosion resistance and ductility even without heat treatment as it is made, but for large materials that cool down slowly after being made, it is possible to improve the corrosion resistance and ductility by performing solution heat treatment. Good erosion resistance and ductility can be obtained.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram in which Examples are placed on a Scheffler organizational chart.

Claims (1)

[Claims] 1. By weight, C0.15-0.25%, Si0.1-0.8%,
Mn3.0~7.0%, Ni2.5~5.0%, Co4.38~12.0%,
and Cr17.0~22.0%, the balance consists of Fe and accompanying unavoidable impurities, Ni equivalent (Nieq) = Ni
(weight%) + 30 x C (weight%) + 1/2 x Mn (weight%)
+1/3×Co (wt%) and Cr equivalent (Creq) = Cr (wt%) + 1.5×Si (wt%) The Cr equivalent is 17.50% or more, and the Ni equivalent is expressed by the following formula ( 1)
and (2) A stainless steel cast steel for an underwater high-speed rotating body, characterized by having a structure consisting of an all-austenite phase or an austenite phase and a ferrite phase, in which the above-mentioned components are adjusted to fall within the range of the values obtained in (2). [(1); (1.125×Creq−8.75)+1.0] [(2); (1.125×Creq−8.75)−1.5]