JP2678263B2 - High-strength and high-corrosion-resistant martensitic stainless steel manufacturing method and its application - Google Patents

Description

The present invention relates to a large cast steel product such as a hydro turbine runner and a guide vane for hydroelectric power generation, and has excellent strength and toughness and high strength and corrosion resistance in water. TECHNICAL FIELD The present invention relates to martensitic stainless steel, a method for producing the same, and uses thereof.

[Conventional technology]

Large-sized members such as propellers for ships or turbine runners for hydroelectric power generation that are subjected to high cyclic load require not only corrosion resistance but also strength, especially corrosion fatigue strength and toughness. Various materials have been developed.

For example, Japanese Patent Publication No. 51-29086 discloses C: 0.07% or less, N ₂ ;
0.05% or less, Si; 0.1 to 2%, Mn; 0.1 to 4%, Cr; 10 to 15
%, Ni; 2 to 7%, Mo; 0.1 to 3%, and 0.01 to 1% in total of at least one of Ti, Ta and Nb, and the balance Fe and impurities,
A structure mainly composed of martensite and austenite, the amount of retained austenite is 10 to 45%, and the amount of δ ferrite is 20%.
The following large-sized stainless cast steel for propellers having high corrosion fatigue strength is disclosed. In addition, Japanese Examined Japanese Patent Sho 60-5373
No. 7 discloses C; 0.01 to 0.15%, Si; 0.1 to 1%, Mn; 0.1 to
2%, Cr; 10-15%, Ni; 2-7%, Mo; 0.1-3%, V; 0.0
5 to 0.5% and 0.005 to 0.5% of one or more of Nb, Zr and Hf
Disclosed is a stainless cast steel for water turbine runners, which can be included, has a mixed structure of martensite and retained austenite, and has a retained austenite amount of 10 to 40% and a δ ferrite amount of 10% or less. Still another conventional example is JP-A-58-87257. The cast steel disclosed in this publication is characterized in that Cr is contained in a large amount of 20.0 to 30.0%.

By the way, with the recent increase in power demand, in parallel with the construction of large-scale thermal power and nuclear power generation facilities, in hydroelectric power generation, nighttime reserve power is used to pump water and pumped power generation as a power source at peak hours. Construction of equipment is underway.
Further, in order to increase the economical efficiency, there is a tendency that the height and speed are increased and the capacity is increased year by year, and development of a member for a water turbine excellent in strength, particularly in water fatigue strength and toughness is desired.

[Problems to be solved by the invention]

The stainless cast steel for propellers of Japanese Patent Publication No. 51-29086 mentioned above is a material developed mainly for the purpose of improving seawater corrosion fatigue strength. However, its low toughness makes it unsuitable for high-lift turbine runner members. Further, the cast stainless steel for water turbine runners of Japanese Patent Publication No. 60-53737 has obtained good strength and toughness, but details of fatigue strength are not described, and from strength to underwater fatigue strength under corrosive environment. Is difficult to judge. Furthermore, JP-A-58-87
The cast steel disclosed in Japanese Patent No. 257 does not become a martensitic system because it contains a large amount of Cr.

An object of the present invention is to provide a method for producing a high-strength and high-corrosion-resistant martensitic stainless steel which has strength equivalent to that of the above-mentioned stainless cast steel and is also excellent in underwater fatigue strength and toughness, and uses thereof.

[Means for solving the problem]

In order to achieve the above object, the member according to the present invention is 12 to 17
% Mar and 3 to 7% Ni in martensitic stainless cast steel, Co increases the solubility of Mo and V,
Further, Co has a small influence on the transformation point and is suitable for improving the underwater fatigue strength and toughness without deteriorating the strength by heat treatment, and B is a useful component as an auxiliary action for increasing hardenability. It was completed by finding out things.

That is, the present invention, by weight, C; 0.01 to 0.10%, Si;
1-1.5%, Mn; 0.1-2.0%, Cr; 12-17%, Ni; 3-7%, Co;
0.1 to 3%, Mo; 0.1 to 2.0%, V; 0.05 to 0.5% is contained, and it has a mixed structure consisting mainly of martensite and retained austenite, and the amount of retained austenite is 35% by volume.
%, Δ ferrite content is 15% or less in area ratio, in high strength and high corrosion resistance martensitic stainless steel,
A step of quenching a composition having the balance of Fe and unavoidable impurities at a temperature of Ac ₃ point or higher, followed by Ac ₁ and A
Production of high-strength and high-corrosion-resistant martensitic stainless steel, characterized by including a step of intermediate quenching at a temperature between c ₃ and at least once, and then a step of tempering at a temperature of Ac ₁ or lower. Is the way.

Further, the present invention is, by weight, C; 0.01 to 0.10%, Si; 0.1 to
1.5%, MN; 0.12.0%, Cr; 12-17%, Ni; 3-7%, Co; 0.1-
3%, Mo; 0.1 to 2.0%, V; 0.05 to 0.5%, B; 0.0005 to 0.01%
High-strength, high-corrosion-resistant martensitic stainless steel with a mixed structure consisting mainly of martensite retained austenite and containing 35% or less of retained austenite by volume and 15% or less of δ ferrite by area. In, a composition in which the balance is Fe and inevitable impurities
A step of quenching at an Ac ₃ point abnormal temperature, a step of intermediate quenching at a temperature between Ac ₁ and Ac ₃ at least after that step, and a step of tempering at a temperature of Ac ₁ or less after that step And a high-strength and high-corrosion-resistant martensitic stainless steel.

Further, the present invention is a water turbine runner in which a crown, a shroud ring facing the crown, and a plurality of blades provided between the crown and the shroud ring are integrally formed. A method for manufacturing a water turbine runner, characterized in that it is integrally manufactured by any of the methods for manufacturing high-strength and high-corrosion-resistant normal martensitic stainless steel.

(Operation) That is, the member according to the present invention can obtain sufficient strength and toughness even in the ordinary quenching and tempering treatments, but between the quenching and tempering, α ₁ of the Ac ₁ and Ac ₃ transformation section
By applying an intermediate quenching treatment from the two-phase temperature of + γ,
Fine martensite can be obtained without significantly lowering the Ms point of the precipitated γ phase, and more excellent underwater fatigue strength and toughness can be imparted.

From the balance of strength and toughness, ordinary quenching and tempering treatment mainly have a mixed structure of martensite and retained austenite, the amount of retained austenite is 10 to 35% by volume, and the amount of δ ferrite is area ratio. 15% or less, and 2 from the Ac ₁ and Ac ₃ transformations during quenching and tempering treatments.
When subjected to the subsequent quenching treatment, it mainly has a martensite structure, and the amount of retained austenite is 20% or less by volume,
The amount of δ ferrite is adjusted by the composition and heat treatment so that the area ratio is 15% or less.

Next, the reasons for limiting the chemical components of the member of the present invention will be described.

If 0.1% or more of C deteriorates corrosion resistance, toughness, and weldability, the upper limit is 0.1%, and 0.01% or more is restricted from the viewpoint of strength and melting. Preferably 0.03-0.06%
It is.

Si needs to be 0.1% or more as a deoxidizing agent during steel making, but is limited to 1.5% or less from the viewpoint of toughness. Preferably
0.2 to 0.7%.

Mn is also used as a deoxidizing agent, but since it contributes to toughness as an austenitizing element, it is suitable to add up to 2%. It is preferably 0.3 to 0.9%.

Cr is the most important component that enhances corrosion resistance in stainless steel, and is required to be 12% or more especially from the viewpoint of fatigue strength in water. Although the corrosion resistance improves as the Cr content increases, δ-ferrite is likely to be formed, which easily causes embrittlement, impairs toughness, and does not increase the fatigue strength in water, so the upper limit is limited to 17%. It is preferably 13 to 16%.

Ni is a strong austenite forming element and is required to be at least 3% or more in order to improve toughness and weldability during repair welding. However, if 7% or more of Ni is contained, quenching and
In the secondary quenching treatment from the Ac ₁ and Ac ₃ transformation zone, the Ms point decreases, the amount of retained austenite increases, and proof stress and fatigue strength in water cannot be obtained. Therefore, it is preferably 4 to 6%.

Co is an austenite forming element and its effect is smaller than that of Ni, but it increases the solubility of Mo and V in austenite during hardening and strengthens the matrix. Also, Ac ₁
When cooling during the secondary quenching process from the and Ac ₃ transformation zones,
It is an essential component for obtaining fine martensite without significantly lowering the Ms point of austenite formed by reverse transformation. The amount of Co added is determined in consideration of the amount of Ni, but its effect is small at 0.1% or less, and if it is added in a large amount, austenite cannot be stabilized and its effect cannot be sufficiently exhibited, so the upper limit is made 3%. It is preferably 0.2 to 2%.

Mo is a component that not only enhances the corrosion resistance but also suppresses the softening of the matrix in the intermediate quenching and tempering treatments and is also effective for improving the strength, and is also most suitable for preventing the temper embrittlement. However, if it exceeds 3%, not only the strength is improved, but also the amount of δ ferrite is increased and the toughness is remarkably reduced.
It is regulated to 1 to 3%. It is preferably 0.3 to 2%.

V is known as a strong carbide-forming element,
It suppresses the precipitation of Cr carbides and nitrides, suppresses gradual cooling embrittlement, and has the function of increasing the strength by causing secondary hardening in the tempering process. However, if it is 0.5% or more, a problem of segregation occurs and toughness decreases, so 0.5% or less is preferable. To expect the effect of V, 0.05% or more is necessary, and 0.05 to 0.
Regulated to 5%. It is preferably 0.1 to 0.3%.

B increases the hardenability as well as improves the strength and toughness by making the crystal grains finer by the combined addition of V, but if the amount is increased, the degree of cleaning decreases and embrittlement occurs, so the upper limit is made 0.01%. , Preferably 0.0007 to 0.002%.

The balance consists of Fe and accompanying impurities. As impurities, there are P, S and other As, Sb, etc., but these elements impair ductility and toughness and reduce weldability, so it is desirable that the amount is as small as possible.

Furthermore, the strength of the member according to the present invention can be enhanced by containing 0.5% or less of one or more MC-type carbide forming elements of Zr and Hf. In particular, 0.2% or less of these elements is desirable. Furthermore, in order to increase the ductility of the member, Ca, Mg,
An oxidizing agent such as a rare earth element or Y can be added. It is desirable that the content of one or more of them be 0.3% or less.

〔Example〕

 Hereinafter, the present invention will be described in more detail with reference to Examples.

FIG. 1 is a sectional view of a water turbine runner to which a member according to the present invention is applied. In the figure, a plurality of blades 3 are provided between the crown 2 and the shroud ring 4. A drive shaft is connected to the center of the crown 2, and this thick portion is particularly called a crown boss 1. These water turbine runners (or pump water turbines) are mainly cast by integral casting, or each member is cast and then welded and joined, and either manufacturing method is adopted. FIG. 2 is a schematic perspective view of the guide vane. In the figure, the guide vanes 5 are integrated with a drive system 6, which are cast by integral casting.

Table 1 shows the chemical components of the materials used in the test. Trial number 1
4 is a comparative material, and trial numbers 5 to 9 are inventive materials.

These were melted in the air in a high frequency furnace and cast into a 30 kg sand mold. 1000 after casting to homogenize the composition of all samples
Diffusion annealing was performed at ℃.

FIG. 3 shows a heat treatment diagram of the test material. In the figure, (a) is a heat treatment diagram for the comparative materials of trial Nos. 1 to 4, and after quenching from 1000 ° C. at an Ac ₃ point or higher, near the Ac ₁ point.
Tempering was performed at 600 ° C. The figure (b) is a heat treatment diagram of the material of the present invention.
Secondary quenching was carried out from 760 ° C in the ₁ and Ac ₃ transformation sections, and subsequently tempering was carried out at 560 ° C below Ac ₁ . The cooling rate at the time of quenching and secondary quenching is 600 ° C / h, which is close to the cooling rate when the actual runner is strongly cooled.

Table 2 shows the mechanical properties and underwater fatigue strength of the test materials. Table 2 shows the transformation points obtained from a thermal expansion meter when heated and cooled at 300 ° C / h, and the amount of retained austenite after tempering, using samples that were quenched at 1000 ° C. I also wrote it down. That is, the transformation point is the composition of the sample, the heating rate,
Since the value of the transformation point shown in Table 2 is not an absolute value because it changes depending on the holding time and the like, it is a value under a specific heat treatment condition. In the underwater fatigue test, a rotary fatigue tester was used, tap water was dropped on the center of the test piece, and the fatigue strength was determined 10 ⁸ times.

In Tables 1 and 2, Trial No. 1 in which a small amount of Mo is added to 13% Cr-5% Ni cast steel has the largest amount of retained austenite and has a high impact value, but the tensile strength and the fatigue strength in water are low. Trial No. 2 is a sample in which the amount of Mo added is increased and Nb is added,
Tensile strength and fatigue strength are improved, but impact value is 7.
Low at 6 kg f ・ m / cm ² . Trial No. 3 is a sample in which Mo and V are added together, and has strength equal to or higher than that of Trial No. 2 and has improved underwater fatigue strength and impact value, but improvement in underwater fatigue strength and toughness is more desirable. Be done. Trial number 4 increased Cr to 15%, Mo
Was added to 1.5%, and the fatigue strength in water was 35 kg f / mm.
² is obtained, but sufficient strength and toughness are not obtained.

In contrast to the above, trial numbers 5 to 9 are materials of the present invention. The material of the present invention has a slightly lower 0.2% proof stress than the comparative material trial Nos. 2 and 3 having the highest strength, but the tensile strength is almost the same, and the water fatigue strength is 38 to 41 kg f / mm ² , which is higher than that of the comparative material. It is high, and the impact value is 12 kg f · m / cm ² or more, and it is clear that it has the same strength as the comparative material, and has excellent underwater fatigue strength, elongation, and toughness.

The member for a water turbine according to the present invention can be easily melted and cast in an ordinary electric arc furnace, a high frequency induction furnace, or the like, and does not require a special method for manufacturing a water turbine runner and guide vanes. It should be noted that further excellent characteristics can be obtained by applying VOD treatment or the like to reduce impurities (P, S, etc.) and non-metallic inclusions (SiO _2, etc.).

[Effects of the Invention] According to the present invention, compared with conventional materials, it is possible to provide a manufacturing method for manufacturing a high-strength, high-corrosion-resistant martensitic stainless steel having excellent strength, particularly in-water fatigue strength and toughness, In particular, it can be suitably used as a method for manufacturing a water turbine member having a large head, such as a water turbine runner for a pumped-storage power generation and guide vanes.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing the structure of a water turbine runner, FIG. 2 is a perspective view of a guide vane, and FIG. 3 is a diagram showing heat treatment of a test material. 1 ... Crown boss, 2 ... Crown, 3 ... Blade, 4 ... Shroud ring, 5 ... Guide vane, 6 ... Stem.

Continuation of the front page (72) Inventor ▲ High ▼ Aki Hiroshi 4026 Kuji-cho, Hitachi City, Ibaraki Hitachi, Ltd., Hitachi Research Laboratory (72) Inventor Fuminori Iwaki 3-1-1, Saiwaicho, Hitachi City, Ibaraki Stock Company Hitachi, Ltd., Hitachi Plant (56) References JP-A-55-58353 (JP, A) JP-B-54-11245 (JP, B1)

Claims (3)

(57) [Claims] 1. By weight, C; 0.01 to 0.10%, Si; 0.1 to 1.5%, M
n; 0.1 to 2.0%, Cr; 12 to 17%, Ni; 3 to 7%, Co; 0.1 to 3%, M
o; 0.1-2.0%, V; 0.05-0.5%, having a mixed structure mainly composed of martensite and retained austenite, and the amount of retained austenite is 35% or less by volume ratio, δ
In the high-strength and high-corrosion-resistant martensitic stainless steel in which the amount of ferrite is 15% or less in area ratio, the step of quenching a composition having the balance of Fe and unavoidable impurities at a temperature of Ac ₃ point or higher, and then the step High-strength, high-corrosion-resistant martensitic stainless steel characterized by comprising a step of intermediate quenching at a temperature between Ac ₁ and Ac ₃ at least once, and a step of tempering at a temperature of Ac ₁ or less after that step. Steel manufacturing method. 2. By weight, C; 0.01 to 0.10%, Si; 0.1 to 1.5%, M
n; 0.1 to 2.0%, Cr; 12 to 17%, Ni; 3 to 7%, Co; 0.1 to 3%, M
o; 0.1-2.0%, V; 0.05-0.5%, B; 0.0005-0.01%, having a mixed structure consisting mainly of martensite and retained austenite, and the amount of retained austenite is 35% or less by volume ratio, In a high-strength and high-corrosion-resistant martensitic stainless steel in which the amount of δ ferrite is 15% or less in area ratio, a composition in which the balance is Fe and inevitable impurities is Ac ₃
Quenching at a temperature above the point, followed by Ac
High-strength high-corrosion-resistant martensitic stainless steel characterized by including a step of intermediate quenching at least once between ₁ and Ac ₃ and a step of tempering at a temperature of Ac ₁ or lower. Manufacturing method. 3. A water turbine runner in which a crown, a shroud ring facing the crown, and a plurality of blades provided between the crown and the shroud ring are integrally formed, and each of the constituent members is any one of claims 1 and 2. A high-strength and high-corrosion-resistant martensitic stainless steel manufacturing method is integrally manufactured.