JP4918632B2 - Precipitation strengthened stainless steel and method for producing the same - Google Patents

Description

  The present invention relates to precipitation strengthened stainless steel and a method for producing the same.

Conventionally, as a high strength material, low alloy type high strength steel represented by AISI 4340, 300M and the like is known. If these heat treatment conditions are selected, a tensile strength exceeding about 1800 MPa and an elongation of about 10% can be obtained (see Non-Patent Document 1). However, these are low-alloy steels, and their corrosion resistance is insufficient because Cr, which greatly contributes to corrosion resistance, is a little less than 1%.
Conventionally, martensitic precipitation strengthened stainless steels represented by PH13-8Mo (AISI XM-13), 17-4PH (AISI 630) and the like are known as high-strength stainless steels. These are high strength among stainless steels having good corrosion resistance, but are inferior in strength compared to low alloy high strength steels represented by AISI 4340, 300M and the like. Patent Document 1 proposes a high-strength stainless steel and has excellent strength characteristics, but it cannot be said that the ductility is sufficient as compared with a low alloy high strength steel. Furthermore, the high-strength steel proposed in Patent Document 2 has a sufficiently high strength when Cr, which is an element for improving corrosion resistance, is not added. However, when Cr is added, it is compared with a low-alloy high-strength steel. Then the strength is slightly inferior.
The present applicant has proposed a high-strength stainless steel shown in Patent Document 3 as a high-strength material in which the corrosion resistance of the low alloy steel and the strength and ductility of the high-strength stainless steel are improved. The high-strength stainless steel shown in Patent Document 3 is a material that can obtain a tensile strength exceeding 1800 MPa if heat treatment conditions are selected, and has excellent corrosion resistance.

British Patent Publication No. 988452 Japanese Unexamined Patent Publication No. 57-41351 Japanese Patent No. 3342501

"Improved Lower Temperature Fracture Toughness of Ultrahigh Strength 4340 Steel through Modified Heat Treatment Tractor. YOSHIYUKI Tluag TOMITA: Met. 18A (1987) p. 1495-1501 "Department Of Defense Handbook Metallic Materials And Elements For Aerospace Vehicle Structures MIL-HDBK 5J (2003)"

The high-strength stainless steel shown in Patent Document 3 described above is an excellent material in terms of tensile strength and corrosion resistance, but 0.2% proof stress is lower than tensile strength. For example, in aircraft structural design, one of the applications where precipitation-strengthened stainless steel is used, strength design is performed using a fracture load and a tensile strength obtained by multiplying the operational load by a safety factor, and in addition to practical use. Consideration must be given to prevent yielding of aircraft legs and spacecraft primary structural members in the environment. Therefore, it is desirable that the yield strength ratio (= 0.2% yield strength / tensile strength) of the applied material is high. However, the high-strength stainless steel shown in Patent Document 3 has a low yield strength ratio. For example, the strength characteristics are insufficient for use in place of the low-alloy high-strength steel currently applied to structural members of aircraft. is there.
Accordingly, an object of the present invention is to provide a precipitation-strengthened stainless steel having excellent corrosion resistance while maintaining the tensile strength, 0.2% proof stress and ductility at the level of a low alloy high strength steel, and a method for producing the same. That is.

The present inventor has conducted a diligent study to achieve both the tensile strength, the 0.2% proof stress and the ductility of the low alloy high strength steel level, and the conventional high strength stainless steel. Mo is added based on a suppressed precipitation strengthened martensitic stainless steel, and the tensile strength and ductility are maintained without sacrificing corrosion resistance by balancing the proportion and amount of Al and Ti that form a compound with Ni. However, the present inventors have found that the 0.2% proof stress can be greatly improved and reached the present invention.
That is, in the present invention, in mass%, C: ≦ 0.2%, 7% ≦ Ni ≦ 14%, 0% ≦ Co ≦ 3.5%, 9.5% ≦ Cr ≦ 14%, 0.5% ≦ Mo. ≦ 3%, 0.25% <Al <1%, 0.75% <Ti ≦ 2.5%, the balance being Fe and impurities, and satisfying the formulas (1) to (4) Stainless steel.
Formula (1) 1260-65Ni-20Co-40Cr ≧ 0
Formula (2) 670 + 75Ni + 40Co-100Cr ≧ 0
Formula (3) 0.125 ≦ (Al / Ti) ≦ 1.25
Formula (4) 1.45 ≦ (Al + Ti) ≦ 2.95
(However, Ni, Co, Cr, Al, and Ti in the formula are calculated in terms of mass%)

Moreover, it is preferable that this invention satisfies following formula (5).
Formula (5) 4.5 ≦ Ni / (Al + Ti)
(However, Ni, Al, and Ti in the formula are calculated in terms of mass%)
A more preferable range of the formula (4) in the present invention is 2.1 ≦ (Al + Ti) ≦ 2.55.
In the present invention, a more preferable range of the formula (3) is 0.35 ≦ (Al / Ti) ≦ 0.65.
Further, Si, Mn, Cu, P, S, O, and N, which are impurity elements of the present invention, are Si ≦ 0.1%, Mn ≦ 0.1%, Cu ≦ 0.1%, and P ≦ 0.03. %, S ≦ 0.008%, O ≦ 0.005%, and N ≦ 0.01%.
Further, the precipitation strengthened stainless steel of the present invention has an elongation of 7.5% or more, a 0.2% proof stress and a tensile strength of 1600 MPa with a 0.2% proof stress, and a tensile strength of 1700 MPa in a tensile test at room temperature. As described above, this is a precipitation strengthened stainless steel satisfying any of fracture toughness of 40 MPa · m ^1/2 or more.

Moreover, the manufacturing method of the said precipitation strengthening type stainless steel of this invention is, after carrying out the solution treatment of the stainless steel which consists of the said composition which performed at least melt | dissolution and hot plastic working at 940 degreeC-1050 degreeC, and also 480 degreeC- It is a manufacturing method of precipitation strengthening type stainless steel which performs an aging treatment at 550 ° C.
Preferably, the sub-zero treatment is performed at −50 ° C. to −100 ° C. subsequent to the above-described solution treatment, thereby producing a precipitation strengthened stainless steel.
Moreover, it is a manufacturing method of precipitation strengthening type stainless steel whose processing ratio of said hot plastic working is 3 or more.

  The precipitation-strengthened stainless steel of the present invention has the tensile strength, 0.2% proof stress and ductility of the low alloy high strength steel level, and has the same corrosion resistance as conventional high strength stainless steel. In designing aircraft parts and the like that require corrosion resistance, it can be expected that the tensile strength can be effectively used as compared with a material having a small yield strength ratio.

3 is an optical micrograph of a cross section of the alloy of the present invention. It is an optical microscope photograph of a comparative example alloy cross section. It is an optical microscope photograph of a comparative example alloy cross section. It is a graph which shows the fatigue test result of Example 4.

As described above, an important feature of the present invention is that Mo is added to improve tensile strength and 0.2% proof stress, based on the components of precipitation strengthened martensitic stainless steel with C kept low, This is because the proportion and amount of Al and Ti forming a compound with Ni were balanced.
The reason why each chemical composition and relational expression are defined in the present invention is as follows. Unless otherwise specified, the mass% is indicated.

C: ≦ 0.2%
C is an element to be regulated as an impurity in the present invention. C is limited to 0.2% or less in order to lower the balance between strength and ductility by forming carbides with additive elements such as Ti and Cr, and to further deteriorate the corrosion resistance. Preferably it is 0.05% or less.
7% ≦ Ni ≦ 14%
Ni is an austenite-forming element and is effective for the disappearance of δ-ferrite during the solution treatment, and it has the effect of strengthening precipitation by forming fine intermetallic compounds with Al and Ti during the aging treatment. It is an element to be added. However, if Ni is less than 7%, δ ferrite becomes stable at the solution treatment temperature, and the remaining form of δ ferrite has directionality depending on the plastic working direction in the manufacturing process, particularly in the rolling process and the forging process. . Therefore, directionality appears in the characteristics of the final product. Further, the presence of δ ferrite itself decreases the strength, and as a result, the balance between strength and ductility is significantly impaired. On the other hand, when the Ni addition exceeds 14%, the Ms point is remarkably lowered, and sufficient martensite transformation is not performed in the rapid cooling treatment after the solution treatment, and a large amount of retained austenite remains. And the balance of ductility are significantly impaired.
Therefore, Ni is set in the range of 7% to 14%. In order to obtain the above-described effect of Ni more reliably, the lower limit of Ni is preferably 9%.

0% ≦ Co ≦ 3.5%
Co is an austenite forming element like Ni, and is effective in disappearance of δ ferrite during solution treatment. Further, since the temperature drop at the Ms point is not larger than Ni, it is added in a range of 3.5% or less. It is effective to do. However, in the present invention, Ni is essential added in the range of 7 to 14%, so Co does not necessarily need to be added and is an optional component that may be added at an additive level including 0%. In addition, unlike Ni, Co does not form intermetallic compounds with Al, Ti, etc., but indirectly promotes precipitation of intermetallic compounds, or refines intermetallic compounds to strengthen precipitation. Contribute. However, the addition of Co does not directly contribute to precipitation strengthening, and the addition of a large amount causes an increase in cost, so the upper limit of Co was set to 3.5%.
9.5% ≦ Cr ≦ 14%
Cr is essential for the purpose of imparting corrosion resistance. However, if Cr is less than 9.5%, sufficient corrosion resistance as stainless steel is not exhibited. On the other hand, Cr is a ferrite-forming element. If Cr addition exceeds 14%, δ-ferrite is not formed at the solution treatment temperature. In addition to being stable and reducing the strength, the presence form of the remaining δ-ferrite has a direction depending on the manufacturing process, so that the directionality appears in the characteristics of the final product. In addition, the presence of δ ferrite significantly impairs the balance between strength and ductility. Therefore, Cr is set in the range of 9.5% to 14%.

0.5% ≦ Mo ≦ 3%
Mo is essential to improve strength and corrosion resistance. However, if the Mo content is less than 0.5%, the ductility is remarkably reduced. On the other hand, if the Mo content exceeds 3%, the δ ferrite becomes stable at the solution treatment temperature, and the form of the remaining δ ferrite depends on the manufacturing process. Therefore, the directionality appears in the characteristics of the final product. In addition, the presence of δ ferrite significantly impairs the balance between strength and ductility. Therefore, Mo is set in the range of 0.5% to 3%.
Since Mo can be expected to improve corrosion resistance, particularly pitting corrosion resistance, due to combined addition with Cr, the lower limit of Mo is preferably 0.7%.
0.25% <Al <1%
Al is an essential element because it is an element that forms an intermetallic compound with Ni during aging treatment and strengthens precipitation. In order to increase the strength by adding Al, 0.25% or more is necessary. On the other hand, addition of 1% or more causes a significant decrease in ductility. Therefore, Al was made into the range exceeding 0.25% and less than 1%.

0.75% <Ti ≦ 2.5%
Since Ti is an element that forms an intermetallic compound with Ni during the aging treatment and strengthens precipitation, it is essential to add Ti. In order to increase the strength by adding Ti, an addition amount exceeding 0.75% is required. On the other hand, when it exceeds 2.5%, the ductility is remarkably reduced. Therefore, Ti is set to a range of more than 0.75 and 2.5% or less. The preferable upper limit of Ti is 2.45%.
The elements other than those described above are Fe and impurities.
Typical impurity elements include Si, Mn, Cu, P, S, O, N, and the like, and these elements are preferably regulated to the following ranges.
Si ≦ 0.5%, Mn ≦ 0.5%, Cu <0.5%, P ≦ 0.05%, S ≦ 0.02%, O ≦ 0.01%, N ≦ 0.05%.
The following range is preferable.
Si ≦ 0.1%, Mn ≦ 0.1%, Cu ≦ 0.1%, P ≦ 0.03%, S ≦ 0.008%, O ≦ 0.005%, N ≦ 0.01%.

Next, the relational expression will be described.
In order to greatly improve the 0.2% proof stress while maintaining the tensile strength and ductility, it is necessary not only to satisfy the above-mentioned component ranges but also to adjust the components so as to satisfy all of the following four relational expressions. There is. In addition, Ni, Co, Cr, Al, and Ti in the formula are calculated as values in mass%.
Formula (1): 1260-65Ni-20Co-40Cr ≧ 0
Formula (1) is an index of ease of martensitic transformation during cooling after the solution treatment. In general, the temperature at which martensitic transformation occurs is known as the Ms point. However, in the precipitation-strengthened stainless steel of the present invention, if this equation (1) is not satisfied, the cooling treatment after the solution treatment is sufficient. No martensitic transformation takes place, residual austenite remains, and the balance between strength and ductility is significantly impaired.
Formula (2): 670 + 75Ni + 40Co-100Cr ≧ 0
Formula (2) is an indicator of whether or not a δ ferrite phase exists at the solution treatment temperature. The presence of this δ ferrite is determined by the balance between the addition amount of Cr, which is a ferrite forming element, and the addition amounts of austenite forming elements, Ni and Co. In the precipitation strengthened stainless steel of the present invention, this formula (2) If δ is not satisfied, the δ ferrite becomes stable at the solution treatment temperature, and the presence form of the remaining δ ferrite has a direction depending on the manufacturing process, so that the directionality appears in the characteristics of the final product. In addition, the presence of δ ferrite significantly impairs the balance between strength and ductility.

Formula (3): 0.125 ≦ (Al / Ti) ≦ 1.25
Formula (3) is an index for optimizing the precipitation phase responsible for precipitation strengthening. When satisfying this equation (3), L1 ₂ structure in which _Ni 3 (Ti, Al) phase is liable to precipitate, the balance between strength and ductility after aging treatment is most excellent. On the other hand, when (Al / Ti) is less than 0.125, a hexagonal Ni ₃ Ti phase is likely to precipitate, resulting in a decrease in ductility. When (Al / Ti) exceeds 1.25, a Ni (Al, Ti) phase having a B2 structure is likely to be precipitated, the ductility is remarkably lowered, and the tensile strength is insufficient. For this reason, the range of the relational expression represented by the expression (3) is set to 0.125 to 1.25. When the balance between strength and ductility is taken into consideration, the range of the relational expression represented by Expression (3) is preferably 0.35 to 0.65.
Formula (4): 1.45 ≦ (Al + Ti) ≦ 2.95
Equation (4) is an index for optimizing the amount of precipitation of the precipitation phase responsible for precipitation strengthening. When (Al + Ti) represented by this formula (4) is less than 1.45, the amount of precipitation is small and the strength is insufficient, and when (Al + Ti) exceeds 2.95, the amount of precipitation is excessive and the ductility decreases. Is remarkable. For this reason, the range of the relational expression represented by the formula (4) is set to 1.45 to 2.95. In particular, it is preferable that the relational expression shown by Formula (4) is 2.1 to 2.55.

Furthermore, in the present invention, it is preferable to satisfy the following formula (5).
Formula (5): 4.5 ≦ Ni / (Al + Ti)
Equation (5) is an index for optimizing fracture toughness. When Ni / (Al + Ti) represented by the formula (5) is less than 4.5, the fracture toughness tends to be lowered. Therefore, the relational formula represented by the formula (5) is preferably 4.5 or more.

Next, the manufacturing method of the precipitation strengthening type stainless steel of this invention is demonstrated.
First, heat treatment for imparting desired mechanical properties to the precipitation-strengthened stainless steel of the present invention will be described.
In general, precipitation strengthened stainless steel dissolves a precipitation strengthening element in an austenite phase in a solution treatment, and then transforms the austenite phase into martensite by cooling with water, oil, cooling gas, or the like. Thereafter, a high-strength material can be obtained by finely depositing an intermetallic compound phase or the like by aging treatment.
Also in the present invention, the solid solution treatment is performed on the precipitation strengthened stainless steel adjusted to the above composition. When the solution treatment temperature of the present invention is less than 940 ° C., the alloy element does not sufficiently dissolve, whereas when it exceeds 1050 ° C., austenite crystal grains are likely to be coarsened, and δ ferrite is generated, resulting in mechanical properties. Therefore, the solution treatment temperature is set to 940 ° C to 1050 ° C. As the solution treatment time, for example, 0.5 to 3 hours is sufficient.
Since the precipitation strengthened stainless steel of the present invention has a relatively low martensite transformation start point (Ms point), a complete martensite structure cannot be obtained only by cooling at the time of solution treatment, a large amount of austenite structure remains, and the proof stress Therefore, after cooling to room temperature by solution treatment, subzero treatment is further performed at −50 ° C. to −100 ° C. as necessary. The processing time of the sub-zero processing can be set to 0.5 to 3 hours, for example. By performing sub-zero treatment, retained austenite can be reduced and mechanical properties such as yield strength can be improved.

After the solution treatment or the sub-zero treatment, an aging treatment (precipitation strengthening treatment) is performed at 480 ° C. to 550 ° C. in order to obtain a high strength and high toughness with a good balance. If the aging treatment temperature is less than 480 ° C., the strength is high but the ductility may be lowered. On the other hand, if it exceeds 550 ° C., the strength may be lowered. In addition, holding | maintenance for 1 to 24 hours is enough for the aging treatment time.
The precipitation-strengthened stainless steel of the present invention has the mechanical properties of an elongation of 7.5% or more, a 0.2% proof stress of 1600 MPa or more, and a tensile strength of 1700 MPa or more according to the above-described manufacturing method in a tensile test at room temperature. Obtainable. Further, since the above-described manufacturing method can have a fracture toughness of 40 MPa · m ^1/2 or more, it can be stably used for aircraft parts and the like.

In addition, in the precipitation-strengthening stainless steel of the present invention, when the precipitation-strengthening stainless steel of the present invention is mass-produced to aircraft parts and power generation parts where inclusions and component segregation are problematic, as a melting method thereof It is preferred to apply vacuum melting. Subsequent to vacuum melting, a remelting method such as vacuum arc remelting or electroslag remelting may be appropriately combined.
Moreover, in order to perform the above-mentioned heat processing, it heat-processes using the raw material which performed hot plastic working, such as a forge and a press, after the above-mentioned melt | dissolution. In this case, when the precipitation-strengthened stainless steel of the present invention is heat-treated on an actual product, it is preferable to perform the aging treatment after performing machining to a shape close to the product shape before the aging treatment.
Moreover, when performing hot plastic working, it is preferable that the working ratio is 3 or more. This is because if the processing ratio is less than 3, the 0.2% yield strength may be significantly reduced. Note that the processing ratio is obtained by the cross-sectional area of the workpiece before processing / the cross-sectional area after processing.
Here, for example, if the workpiece before processing is a steel ingot, the steel ingot is often tapered. In that case, since the cross-sectional areas differ between the upper and lower ends of the steel ingot, the cross-sectional area is calculated as an average cross-sectional area.

Example 1
The following examples further illustrate the present invention.
A 10 kg steel ingot was produced by vacuum melting. Since the steel ingot was tapered, the upper and lower cross-sectional areas of the steel ingot were obtained, and the average cross-sectional area obtained by dividing the sum by 2 was 5625 mm ² . Table 1 shows the chemical composition of the steel ingot. In addition, elements other than those shown in Table 1 are Fe and impurities. The contents of typical impurity elements Cu, P, S, O and N are 0.1% or less, 0.015% or less, 0.0025% or less, 0.005% or less, 0.005% or less, respectively. Met.
In addition, the conventional example No. shown in the following Table 1 and Table 2. 31 is a numerical value extracted from Patent Document 3, and No. 31 shown in Table 1 of Patent Document 3. Composition of 32 alloys, mechanical properties after 1085 ° C x 1 hour, oil-cooled solid solution treatment, -75 ° C x 2 hours sub-zero treatment, 385 ° C x 2 hours aging treatment, air cooling (twice) Is shown.
Conventional example No. 32 is a numerical value extracted from Patent Document 1, which is a composition of one of the materials shown in TABLE 1 of Patent Document 1, and an aging treatment of 475 ° C. × 24 hours after a solution treatment at 850 ° C. × 20 minutes. It shows the mechanical characteristic value later.
Furthermore, the conventional example No. 33 is a numerical value extracted from Patent Document 2, and the composition of steel type N shown in Table 1 of Patent Document 2 is 900 ° C. × 1 hour, and after air-cooled solid solution treatment, 500 ° C. × 5 hours. It is a mechanical characteristic value after aging treatment of air cooling.
Conventional example No. 34 is a numerical value extracted from Non-Patent Document 1 as AISI 4340, which is a low-alloy high-strength steel, and is the composition and mechanical property value of 1133K DOQT-steel 3R shown in Table III of Non-Patent Document 1.

The steel ingots of the present invention and comparative examples shown in Table 1 were heated to 1050 ° C., forged, and No. No. 14 alloy and no. Other than 15 alloys, 45 mm width × 20 mm thickness flat rectangular materials were used. No. No. 14 alloy is a 38 mm × 38 mm square material. 15 alloy produced 65 mm width x 30 mm thickness rectangular material. In addition, when processing ratio (= steel ingot average cross-sectional area / cross-sectional area after forging) is calculated, No. No. 14 alloy and no. Except for 15 alloy, it was 6.25. No. The processing ratio of alloy No. 14 is 3.90. The processing ratio of 15 alloy was 2.88.
Next, each forged material produced was heat-treated. The heat treatment was performed continuously with the solution treatment, sub-zero treatment, and aging treatment. The solution treatment was carried out in an atmospheric furnace at a temperature of 950 ° C., 1000 ° C. or 1050 ° C. for 1 hour, and then cooled in oil. The subzero treatment was held at -75 ° C for 2 hours using dry ice and ethanol. The aging treatment was carried out in an atmospheric furnace at 500 ° C., 510 ° C., 520 ° C., and 530 ° C. for 16 hours and then cooled in the air.
The cross-sectional metal structure after the solution treatment was observed by etching with an aqueous solution of ferric chloride. As a result of the metal structure observation, No. No. 2 alloy and No. 2 The optical micrographs of Alloy 23 are shown in FIGS. 1 and 2, respectively.
In addition, a round bar tensile test piece having a parallel part diameter of 6.35 mm and a gauge length of 25.4 mm was prepared from the above-mentioned heat-treated material, and a tensile test was performed at room temperature based on ASTM E8. Tensile strength, elongation and drawing were measured. The measurement results are shown in Table 2. Table 2 shows the temperature of the solution treatment and the aging treatment.
As a result of observation of the metal structure, the present invention No. In the alloy 2, no δ ferrite phase was observed as shown in FIG. In the case of 23 alloy, as shown in FIG. 2, δ ferrite having directionality in the forging direction was confirmed after the solution treatment, so the tensile test was not performed.

From Table 2, by performing an appropriate heat treatment on the precipitation-strengthened stainless steel of the present invention, the 0.2% proof stress is maintained in the conventional steel No. 1 while maintaining the tensile strength and ductility. It is confirmed that it is higher than 31 alloy.
On the other hand, no. No. 21 alloy was confirmed to have low ductility because Mo was not added. Since 22 alloy does not satisfy the formula (1), the strength is very low. No. A sample was taken from the material after the solution treatment of 22 alloy and finished by electrolytic polishing, and the amount of retained austenite was measured by X-ray diffractometry. As a result, a large amount of retained austenite was 30.1% by volume. I confirmed. No. The metal structure of the 22 alloy is shown in FIG. From the results of the X-ray diffraction method and the observation of the metal structure, it is considered that the strength is very low because the martensite transformation is not sufficient and a large amount of retained austenite remains in the rapid cooling treatment during the solution treatment. No. In the alloy No. 23, since the formula (2) is not satisfied, it is considered that δ ferrite remained after the solution treatment. No. Regarding the 24 alloy, the amount of Al was large and the amount of (Al + Ti) was also large, so the ductility was not sufficient and the grip part broke during the tensile test. No. of the conventional example. The result of 32 alloy is considered to suggest that the factor contributing to ductility is not only Mo but also the structure and amount of the precipitated phase of the intermetallic compound of Ni and Al and / or Ti. No. of the conventional example. As a result of the 33 alloy, although the Ti amount is relatively high as 2.5% and the Cr amount is small and the value of the formula (4) is as high as 3.0, the strength is remarkably reduced. It can be taken to suggest that there is a critical upper limit to the amount of phase precipitation.
In addition, No. whose alloy composition is relatively similar. No. 11 alloy, no. No. 14 alloy and no. When 15 alloys were compared, it was confirmed that when the working ratio was less than 3, the tensile properties, particularly 0.2% proof stress, decreased.

(Example 2)
No. 1 of the present invention used in Example 1. No. 1 alloy, no. No. 2 alloy, No. No. 3 alloy, No. No. 4 alloy, no. No. 10 alloy, No. No. 11 alloy and no. A compact tension type test piece of 15 mm × 36 mm × 38.5 mm was produced from the heat-treated material of 13 alloy, and fracture toughness was measured based on ASTM E399. Table 3 shows the measurement results. Table 3 shows the temperature of the solution treatment and the aging treatment and the value of the formula (5).

From Table 3, when Ni / (Al + Ti) represented by the formula (5) is 4.5 or more in the precipitation strengthened stainless steel of the present invention, excellent characteristics with fracture toughness of 40 MPa · m ^1/2 or more can be obtained. I understand.

(Example 3)
No. 1 used in Example 1. No. 11 alloy and no. The raw material after heat treatment of 13 alloy and AISI4340 equivalent material of low alloy type high strength steel were observed for rusting after 72 hours in a salt spray test at 35 ° C. and 5% sodium chloride solution. The observation results are shown in Table 4. No. No. 11 alloy and no. In 13 alloys, the test was extended to 2000 hours and the rusting state was observed, but no rusting was confirmed.

  From Table 4, it can be seen that the precipitation-strengthened stainless steel of the present invention is superior in corrosion resistance as compared with the conventional low alloy high strength steel.

Example 4
No. 1 used in Example 1. No. 11 alloy and no. A round bar fatigue test piece having a total length of 110 mm, a diameter of the evaluation part of 5.00 mm, and a notch coefficient Kt = 1.0 is prepared from the heat-treated material of 13 alloy, and is subjected to a normal temperature environment and a stress ratio of 0.1 according to ASTM E466. A stress controlled axial force fatigue test was conducted.
Table 5 shows the number of cycles when the test piece was broken and the maximum number of cycles obtained in the test and the maximum stress applied to the test piece were summarized. In addition, the fatigue curve of AISI 4340 described in Non-Patent Document 2, the fatigue curve of 15-5PH stainless steel, and 11, no. FIG. 4 shows a summary of the 13 fatigue test results.

  FIG. 4 shows that the precipitation-strengthened stainless steel of the present invention is excellent in fatigue characteristics as compared with conventional low alloy high strength steel and stainless steel.

Since the present invention is a high-strength stainless steel having excellent 0.2% proof stress, tensile strength, and excellent corrosion resistance, aircraft legs and spacecraft primary structural members that require high strength, such as flap track rails. It is suitable for fastening members such as slat track rails and fasteners, various actuators, door hinges, engine parts and the like. As other applications, application to sports equipment such as turbine blade materials for power plants, shaft materials for engines, and heads of golf clubs, bicycle parts, automobile parts, ship parts, and the like can also be expected.

Claims (11)

In mass%, C: ≦ 0.2%, 7% ≦ Ni ≦ 14%, 0% ≦ Co ≦ 3.5%, 9.5% ≦ Cr ≦ 14%, 0.5% ≦ Mo ≦ 3%, 0.25% <Al <1%, 0.75% <Ti ≦ 2.5%, the balance is made of Fe and impurities, and the following formulas (1) to (4):
Formula (1) 1260-65Ni-20Co-40Cr ≧ 0
Formula (2) 670 + 75Ni + 40Co-100Cr ≧ 0
Formula (3) 0.125 ≦ (Al / Ti) ≦ 1.25
Formula (4) 1.45 ≦ (Al + Ti) ≦ 2.95
(Note that Ni, Co, Cr, Al, and Ti in the formula are calculated by mass%) and satisfy all of the above. Formula (5) 4.5 ≦ Ni / (Al + Ti)
The precipitation-strengthened stainless steel according to claim 1, wherein Ni, Al, and Ti in the formula are calculated in terms of mass%.   Formula (4) is 2.1 <= (Al + Ti) <= 2.55, The precipitation strengthening type stainless steel of Claim 1 or 2 characterized by the above-mentioned.   Formula (3) is 0.35 <= (Al / Ti) <= 0.65, The precipitation strengthening type stainless steel in any one of Claims 1 thru | or 3 characterized by the above-mentioned. The impurity element is, by mass%, Si ≦ 0.1%, Mn ≦ 0.1%, Cu ≦ 0.1%, P ≦ 0.03%, S ≦ 0.008%, O ≦ 0.005%. The precipitation strengthened stainless steel according to claim 1, wherein N ≦ 0.01%.   The precipitation-strengthened stainless steel according to any one of claims 1 to 5, wherein an elongation in a tensile test at room temperature is 7.5% or more.   The precipitation-strengthened stainless steel according to any one of claims 1 to 6, wherein a 0.2% yield strength in a tensile test at room temperature is 1600 MPa or more and a tensile strength is 1700 MPa or more. The precipitation-strengthened stainless steel according to any one of claims 1 to 7, wherein the fracture toughness is 40 MPa · m ^1/2 or more.   The precipitation strengthened stainless steel having the composition according to any one of claims 1 to 5 which has been subjected to at least melting and hot plastic working is subjected to solid solution treatment at 940 ° C to 1050 ° C, and further aging at 480 ° C to 550 ° C. A method for producing precipitation-strengthened stainless steel.   The method for producing precipitation-strengthened stainless steel according to claim 9, wherein subzero treatment is performed at −50 ° C. to −100 ° C. following the solution treatment.   The method for producing precipitation-strengthened stainless steel according to claim 9 or 10, wherein the hot plastic working ratio is 3 or more.

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