JPH0524984B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to hydraulic equipment that uses seawater or river water, such as pump-turbines for pumped storage power generation, water turbines for tidal power generation, and seawater pumps, which have excellent corrosion resistance, toughness, strength, particularly corrosion fatigue strength, and weldability. Related to runner cast steel materials and guide vane cast steel materials. [Background of the invention] Carbon 0.07% or less, silicon 0.1-2%, manganese
0.1-4%, chromium 10-15%, nickel 2-7%,
A stainless cast steel for propellers containing 0.1 to 3% molybdenum, a total of 0.01 to 1% of one or more of titanium, niobium, and tantalum, and 0.05% or less of nitrogen, the balance being iron is known from Japanese Patent Publication No. 51-29086. This large cast steel for propellers has a low cooling rate during heat treatment and has low toughness even after strain relief annealing, so it is not suitable for pump water turbines. Also carbon 0.03-0.25%, silicon 0.01-0.7%, manganese 0.25-2.0%, nickel 4-8%, chromium
Steels with 11-14% molybdenum and 0.5-3.5% iron are also known, but they do not have sufficient corrosion fatigue strength. [Object of the invention] The object of the present invention is to provide a cast steel for propellers with higher toughness than 14Cr-5Ni-1Mo-0.25Nb, and
15Cr− has superior corrosion fatigue strength than 1.5Mo cast steel
6Ni−2Mo−Nb−N system or 15Cr−6Ni−2Mo
-VN system martensitic cast steel containing ferrite and austenite. [Summary of the invention] Equipment used in seawater, river water, etc. must have excellent corrosion resistance, but power generation pump turbine runners and guide vanes must have high proof stress and corrosion fatigue strength due to strength design. Moreover, since the runner is a large rotating structure, it is important that the runner has high toughness. FIG. 1 is a sectional view of a water turbine runner to which the present invention is applied. As seen in FIG. 1, the runner has a plurality of blades 3 between the crown 2 and the shroud ring 4.
This thick portion is particularly called a crown boss 1. Since this runner rotates at high speed as a pump or water wheel in seawater or river water, it is extremely important that it has high yield strength, tensile strength, toughness, and corrosion fatigue strength. Guide vanes for these runners are usually made of cast steel, but cast steel contains minute defects that require welding to repair them, and especially for large pump-turbines, they are cast in parts and then integrated by welding. It is important that the weldability is excellent. In view of the above circumstances, the present invention provides 15Cr-6Ni-2Mo with excellent seawater corrosion resistance, strength, toughness and weldability.
The present invention relates to martensitic cast steel, which further contains Nb, V, and N. Composition is 0.02-0.07% carbon, 0.2-0.2% silicon by weight.
0.6%, manganese 0.3-0.8%, nickel 4.5-6.8%,
Consists of 15.2~16.5% chromium, 1.8~2.8% molybdenum, the balance iron and impurities, and 0.05~0.05% niobium.
Contains either 0.3% or 0.05-0.3% vanadium and 0.055-0.09% nitrogen. The reason for limiting the ingredients is as follows. Carbon is an element necessary to increase the hardenability and strength of steel, and if it is less than 0.02%, these effects will not be sufficient and sufficient strength will not be obtained. These effects increase as the amount increases, but if it exceeds 0.07%, chromium carbide will precipitate at grain boundaries and the chromium concentration around the carbide will decrease, reducing seawater corrosion resistance and weldability. Set at 0.02-0.07%.
0.03-0.06% is preferred. Silicon is added as a deoxidizing agent and also strengthens the base. If it is less than 0.2%, these effects are insufficient, and if it exceeds 0.6%, it becomes brittle, and depending on the combination with the amount of chromium, the amount of delta ferrite increases, making it even more brittle. Therefore, silicon is 0.2~
The rate shall be 0.6%. Manganese is added as a deoxidizing and desulfurizing agent, and also strengthens the base. If the content is less than 0.3%, these effects will be insufficient, and if it exceeds 0.8%, the effects will be saturated, resulting in a decrease in toughness, so the content should be 0.3 to 0.8%. Nickel increases hardenability to increase strength, produces retained austenite to increase toughness, and improves weldability. If it is less than 4.5%, these effects are insufficient, and if it exceeds 6.8%, the amount of retained austenite increases and the strength, especially yield strength, decreases, so the nickel content should be 4.5 to 6.8%. Among these, 5.5~
6.5% is preferred. Chromium is an important element for increasing seawater corrosion resistance and hardenability to increase strength. If it is less than 15.2%, these effects are insufficient, and
If it exceeds 16.5%, delta ferrite will precipitate more and become brittle, so it should be set at 15.2 to 16.5%. Molybdenum increases strength and seawater corrosion resistance, 1.8
If it is less than 2.8%, corrosion resistance is insufficient, and as molybdenum increases, strength and corrosion resistance improve, but if it exceeds 2.8%, toughness decreases, so molybdenum is 1.8 to 2.8%.
%. Among these, 2.0 to 2.6% is preferable. Niobium and vanadium form carbides and nitrides, prevent precipitation of chromium carbides and nitrides, and improve corrosion resistance. It also improves strength and refines grain size to improve toughness. If it is less than 0.05%, these effects will not be sufficient, and if it is more than 0.3%, the toughness will deteriorate, so the content of niobium and vanadium should be 0.05 to 0.30% each. Both Nb and V are preferably each in the range of 0.07 to 0.2%. Vanadium is more desirable in terms of toughness. Nitrogen improves strength and corrosion resistance, and increases retained austenite to improve toughness. Air-melted materials containing 12 to 16% Cr usually contain 0.02 to 0.04% nitrogen, and 18Cr-8Ni steel with a high Cr content has a higher nitrogen content. Therefore, when performing atmospheric melting, it is not necessary to add nitrogen, but when obtaining better strength, toughness, and corrosion resistance,
If it is less than 0.055%, the effect is small, so it is necessary to add 0.055% or more. On the other hand, if it exceeds 0.09%, blowholes tend to occur during casting and toughness decreases, so the nitrogen content is set at 0.055 to 0.09%. 0.055～
0.08% is preferred. Water turbine runners are large rotating bodies and require high toughness. That is, if the toughness is low, it will lead to destruction due to casting defects and corrosion holes, and the growth rate of corrosion fatigue cracks generated from corroded parts will be high, resulting in a decrease in reliability. To prevent these problems, an impact value of 6 kg-m/cm ² or more is required, and an impact value of 9 kg-m/cm ² or more is desirable. [Embodiments of the Invention] The contents of the present invention will be explained below with reference to Examples.
Table 1 shows the chemical composition of the materials used. Trial numbers 1 and 2 are martensitic steels containing retained austenite, and trial number 3 is an austenitic steel containing a small amount of ferrite. Trial numbers 4 and 5 are martensitic stainless steels that contain a smaller amount of nitrogen than the steel of the present invention and do not contain niobium or vanadium. Trial number 6 contains niobium and nitrogen, and trial number 7
contains vanadium and nitrogen, but each has a lower nitrogen content than the steel of the present invention. Trial No. 8 is a steel that does not contain niobium or vanadium and has a nitrogen content within the composition range of the steel of the present invention. Trial numbers 6, 7, and 8 are martensitic steels containing 35 to 45% retained austenite and 5% or less ferrite, respectively. Trial No. 9 is an inventive steel containing niobium and nitrogen, and Trial No. 10 is an inventive steel containing vanadium and nitrogen, both of which contain 35 to 45% retained austenite and 5% or less ferrite. It is martensitic steel. These are melted in the atmosphere in a high frequency furnace and placed in a sand mold with 100%
Kg was cast. The martensitic material was diffusion annealed at 1000°C, normalized by air cooling at 980°C, and tempered by air cooling at 600°C for 5 hours. The cooling rate during normalization is approximately 800
℃/h, which is close to the cooling rate when a real runner is forcedly cooled. Austenitic steel is
Solution treatment was performed by water cooling at 1060°C. In the immersion corrosion experiment, a 30 x 60 x 5 mm test piece was immersed in artificial seawater according to ASTM-D-1141-52 for 1000 hours, and the corrosion weight loss was determined. Figure 2 shows the results. Conventional martensitic steel samples 1 and 2 are black

[Table] The amount of corrosion is large because the amount of chromium and molybdenum is small, but samples 4 and 5 have a large amount of chromium and molybdenum and the amount of corrosion is small. Martensitic steel samples 6 to 8 and martensitic steel samples 9 and 10 of the present invention have a large amount of chromium and molybdenum, and also contain one or both of niobium, vanadium, and nitrogen, and the amount of corrosion is lower than that of sample numbers 4 and 10. 5, and exhibits corrosion resistance equivalent to or higher than that of austenitic steel Trial No. 3, which does not contain molybdenum. Next, the 0.2% proof stress and tensile strength in a room temperature tensile test and the Charpy impact value using a 2 mm V-notch test piece at 0°C were measured. Furthermore, a rotating bending corrosion fatigue test was conducted. During the fatigue test, artificial seawater was dropped on the test piece ¹⁰ times to determine the corrosion fatigue strength. Table 2 shows the strength and toughness of these various types. Conventional martensitic cast steel trial numbers 1 and 2 have yield strength,
Although the tensile strength and impact value are sufficient, the corrosion fatigue strength is low at 10 to 13 Kgf/mm ² . On the other hand, Trial No. 3, which is made of austenitic cast steel, has a very high impact value, but low yield strength, and corrosion fatigue strength of 14 Kgf/mm ² , which is only slightly higher than Trial No. 2. Further, test numbers 4 and 5, which contain a large amount of chromium and molybdenum, have high tensile strength and impact value, but their corrosion fatigue strength is 16 to 16.5 Kgf/mm ² . Trial numbers 6, 7, and 8 have slightly lower proof strength than trial numbers 1, 2, and 4.

〔Effect of the invention〕

The steel of the present invention is a 15Cr-6Ni-2Mo steel containing N and Nb.
or V, it has superior seawater corrosion resistance, toughness, and corrosion fatigue strength compared to conventional steels, and is easy to weld. When applied to water turbine runners and guide vanes, it is reliable and economical. It is advantageous in this respect.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a water turbine runner, which is the object of the present invention, and FIG. 2 is a diagram showing the corrosion loss of conventional steel and steel of the present invention by immersion in artificial seawater. 1...Crown boss, 2...Crown, 3...
Feather, 4... Shuroudring.

Claims (1)

[Claims] 1 Carbon 0.02-0.07%, silicon 0.2-0.6 by weight
%, manganese 0.3-0.8%, nickel 4.5-6.8%,
Contains 15.2-16.5% chromium, 1.5-3% molybdenum, and 0.05-0.3% niobium and 0.05-0.05% vanadium.
Cast steel for water turbines, characterized in that it contains at least 0.3% of nitrogen and 0.055 to 0.09% of nitrogen, with the remainder consisting of iron and impurities.