JP2015147975A - Precipitation hardening stainless steel and component for sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a precipitation hardening stainless steel excellent in strength, tenacity, processability, corrosion resistance, and strain element characteristic, and to provide a component for a sensor using the same.SOLUTION: A precipitation hardening stainless steel contains, 0.02≤C<0.070 mass%, 0.05≤Si<0.50 mass%, 0.1≤Mn≤1.0 mass%, 0.003≤P≤0.050 mass%, S<0.02 mass%, 2.0<Cu≤4.0 mass%, 2.0<Ni<4.0 mass%, 13.0≤Cr<17.0 mass%, 0.03≤Mo<1.00 mass%, 0.10≤Nb≤0.40 mass%, 0.001≤Al≤0.030 mass%, 1.0<Co<3.0 mass%, O≤0.020 mass%, and 0.025<N≤0.070 mass%, with the balance comprising Fe and unavoidable impurities and satisfies relations: 0.060≤C+N≤0.100 mass%, Cu/Ni≤1.20, α≤470, and β≤10.

Description

  The present invention relates to a precipitation hardening stainless steel and a sensor component, and more particularly relates to a precipitation hardening stainless steel excellent in strain generating body characteristics and a sensor component using the same.

  A strain generating body is used as a component for a load sensor such as a pressure sensor, a weight sensor, or a load cell. A strain generating body refers to a member that generates strain by an external force. By reading the strain generated in the strain generating body with a strain gauge, the pressure and load can be measured. Generally, nickel chrome molybdenum steel or stainless steel is known as a material of the strain generating body. In an environment where corrosion is relatively mild, martensitic stainless steels such as SUS410, SUS440C, and SUS420J2 are used. On the other hand, in an environment where corrosion is relatively severe, a cold-worked material of austenitic stainless steel such as SUS304 or precipitation hardened stainless steel such as SUS630 is used (see Patent Document 1).

  Among the stainless steels described above, the cold-worked material of austenitic stainless steel has a simple shape such as a plate shape, and is applicable when the stress applied to the strain generating body is relatively small. However, the required strength may not be obtained for a strained body having a complicated shape. In addition, when welding or the like is performed, strength reduction occurs at the welded portion. Furthermore, the strength cannot be improved by performing a heat treatment after welding. For this reason, precipitation-hardened stainless steel that can obtain strength by aging treatment after being processed into a predetermined shape is often used for strained bodies with complicated shapes and strained bodies that involve welding.

  Precipitation hardening stainless steel used as a sensor component must have sufficient strength and toughness and excellent corrosion resistance in a corrosive environment. Furthermore, the sensor component is required not to deteriorate the strain-generating body characteristics, that is, to have dimensional stability over time.

In SUS630 (17Cr-4Ni-4Cu-Nb), which is a precipitation hardening stainless steel that has been used conventionally, residual austenite remains in the structure during the solution heat treatment, and the martensite phase changes to the austenite phase during the subsequent aging treatment. Reverse transformation to. As a result, the structure after heat treatment contains an austenite phase having a volume ratio of about 10% or more.
When a repeated load is applied to a sensor component using such a material, the soft austenite phase yields a small yield. As a result, there is a problem that the hysteresis between stress and strain increases, the reference point gradually fluctuates, and accurate pressure and load cannot be measured.

JP 2011-026644 A

The problem to be solved by the present invention is to provide a precipitation hardening stainless steel having the same level of strength, toughness, workability, and corrosion resistance as that of the current SUS630 and excellent in strain generating body characteristics. .
Another object of the present invention is to provide a sensor component using such precipitation hardening stainless steel.

In order to solve the above problems, the precipitation hardening stainless steel according to the present invention is summarized as having the following configuration.
(1) The precipitation hardening stainless steel is
0.020 ≦ C <0.070 mass%,
0.05 ≦ Si <0.50 mass%,
0.1 ≦ Mn ≦ 1.0 mass%,
0.003 ≦ P ≦ 0.050 mass%,
S <0.02 mass%,
2.0 <Cu ≦ 4.0 mass%,
2.0 <Ni <4.0 mass%,
13.0 ≦ Cr ≦ 17.0 mass%,
0.03 ≦ Mo <1.00 mass%,
0.10 ≦ Nb ≦ 0.40 mass%,
0.001 ≦ Al ≦ 0.030 mass%,
1.0 <Co <3.0 mass%,
O ≦ 0.020 mass%, and
0.025 <N ≦ 0.070 mass%
And the balance consists of Fe and inevitable impurities.
(2) The precipitation hardening stainless steel satisfies the following formulas (1) to (4).
0.060 ≦ C + N ≦ 0.100 mass% (1)
Cu / Ni ≦ 1.20 (2)
α ≦ 470 (3)
β ≦ 10 (4)
However,
α = 361 ([C] + [N]) + 28 [Si] +39 [Mn] +10 [Cu] +17 [Ni] +20 [Cr] +5 [Mo] −15 [Co],
β = 138− {99.7 {([Ni _eq ] +36.5) / ([Cr _eq ] +17.6)}},
[Ni _eq ] = [Ni] +30 ([C] + [N]) + 0.5 ([Mn] + [Cu]) + 0.92 [Co],
[Cr _eq ] = [Cr] +1.5 [Mo] +0.5 [Si] +1.37 [Nb].

  The gist of the sensor component according to the present invention is that the precipitation hardening stainless steel according to the present invention is used.

Co has an effect of raising the Ms point, and is an austenite-forming element like Ni, but has a smaller effect of lowering the Ms point and the A ₁ transformation point than Ni. That is, the addition of a predetermined amount of Co not only reduces the amount of retained austenite after solution heat treatment and the amount of δ ferrite remaining after casting, but also has the effect of reducing the amount of reverse transformed austenite generated during high temperature aging treatment. Further, each additive element includes an element that has an action of increasing the amount of soft retained austenite phase or δ ferrite phase and an element that has an action of decreasing.

  For this reason, in precipitation hardening stainless steel, the amount of each component element (particularly the amount of Co) is optimized, and at the same time, the values of the equations (1) to (4) are optimized. The amount of reverse-transformed austenite generated by transformation and the amount of δ ferrite remaining after casting can be simultaneously reduced. As a result, a precipitation hardening stainless steel having excellent strain generating characteristics can be obtained.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Precipitation hardening type stainless steel]
[1.1. Main constituent elements]
The precipitation hardening stainless steel according to the present invention contains the following elements, with the balance being Fe and inevitable impurities. The kind of additive element, its component range, and the reason for limitation are as follows.

(1) 0.020 ≦ C <0.070 mass%:
C is effective in reducing the amount of δ ferrite. C combines with Nb together with N to form a carbonitride. Carbonitrides prevent crystal grain coarsening during solution heat treatment and contribute to improvements in toughness and ductility. In order to obtain such an effect, the amount of C needs to be 0.020 mass% or more. The amount of C is more preferably 0.025 mass% or more, and further preferably 0.030 mass% or more.

  On the other hand, excessive addition of C tends to reduce the solid solution Cr of the parent phase due to the formation of Cr-based carbides and to reduce the corrosion resistance. In addition, the Ms point is lowered to increase the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain body characteristics. Therefore, the amount of C needs to be less than 0.070 mass%. The amount of C is more preferably 0.065 mass% or less, and still more preferably 0.060 mass% or less.

(2) 0.05 ≦ Si <0.50 mass%:
Si is effective as a deoxidizing element. In order to obtain such an effect, the Si amount needs to be 0.05 mass% or more. The amount of Si is more preferably 0.075 mass% or more, and still more preferably 0.10 mass% or more.
On the other hand, excessive addition of Si increases the amount of δ ferrite and decreases the strength after aging treatment. Further, excessive addition of Si lowers the Ms point and increases the amount of retained austenite, leading to a decrease in strength and aging of the strain body after aging treatment. Therefore, the amount of Si needs to be less than 0.50 mass%. The amount of Si is more preferably 0.45 mass% or less, and still more preferably 0.40 mass% or less.

(3) 0.1 ≦ Mn ≦ 1.0 mass%:
Mn is effective as a deoxidation and desulfurization element. Mn is effective in reducing the amount of δ ferrite. In order to acquire such an effect, the amount of Mn needs to be 0.1 mass% or more. The amount of Mn is more preferably 0.12 mass%, and more preferably 0.15 mass% or more.
On the other hand, excessive addition of Mn lowers the Ms point and increases the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain body characteristics. Therefore, the amount of Mn needs to be 1.0 mass% or less. The amount of Mn is more preferably 0.90 mass% or less, and still more preferably 0.85 mass% or less.

(4) 0.003 ≦ P ≦ 0.050 mass%:
P is effective for improving the matrix strength as a solid solution strengthening element. In order to obtain such an effect, the amount of P needs to be 0.003 mass% or more.
On the other hand, P is an impurity element of the steel material, and excessive addition of P is preferably reduced as much as possible in order to reduce hot workability and toughness. On the other hand, reduction of P more than necessary causes an increase in cost. Therefore, the amount of P needs to be 0.050 mass% or less. The amount of P is more preferably 0.045 mass% or less, and still more preferably 0.040 mass% or less.

(5) S <0.02 mass%:
Since S reduces hot workability, toughness, and corrosion resistance, it is preferable to reduce S as much as possible. On the other hand, reduction of S more than necessary causes an increase in cost. Therefore, the amount of S needs to be less than 0.02 mass%. The amount of S is more preferably 0.015 mass% or less, and still more preferably 0.010 mass% or less.

(6) 2.0 <Cu ≦ 4.0 mass%:
Cu is an essential component as a precipitation hardening component, and is also effective in improving corrosion resistance. It is also effective in reducing the amount of δ ferrite. In order to obtain such an effect, the amount of Cu needs to be more than 2.0 mass%. The amount of Cu is more preferably 2.2 mass% or more, and still more preferably 2.4 mass% or more.
On the other hand, excessive addition of Cu decreases the Ms point and increases the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain-generating body characteristics. Furthermore, excessive addition of Cu reduces hot workability. Therefore, the amount of Cu needs to be 4.0 mass% or less. The amount of Cu is more preferably 3.8 mass% or less, and still more preferably 3.6 mass% or less.

(7) 2.0 <Ni <4.0 mass%:
Ni is effective in reducing the amount of δ ferrite, and is also effective in improving corrosion resistance. In order to obtain such an effect, the amount of Ni needs to be more than 2.0 mass%. The amount of Ni is more preferably 2.2 mass% or more, and still more preferably 2.4 mass% or more.
On the other hand, excessive addition of Ni decreases the Ms point and increases the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain-generating body characteristics. Further, excessive addition of Ni decreases hot workability. Therefore, the amount of Ni needs to be less than 4.0 mass%. The amount of Ni is more preferably 3.9 mass% or less, and still more preferably 3.8 mass% or less.

(8) 13.0 ≦ Cr ≦ 17.0 mass%:
Cr is an essential element as an element for improving the corrosion resistance. In order to obtain such an effect, the Cr amount needs to be 13.0 mass% or more. The amount of Cr is more preferably 13.3 mass% or more, and further preferably 13.6 mass% or more.
On the other hand, excessive addition of Cr increases the amount of δ ferrite and decreases the strength after aging treatment. Moreover, excessive addition of Cr lowers the Ms point and increases the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain-generating body characteristics. Therefore, the Cr amount needs to be 17.0 mass% or less. The amount of Cr is more preferably 16.7 mass% or less, and still more preferably 16.4 mass% or less.

(9) 0.03 ≦ Mo <1.00 mass%:
Mo is effective in improving the corrosion resistance. In order to obtain such an effect, the Mo amount needs to be 0.03 mass% or more. The amount of Mo is more preferably 0.05 mass% or more, and further preferably 0.07 mass% or more.
On the other hand, excessive addition of Mo increases the amount of δ ferrite and decreases the strength after aging treatment. Further, excessive addition of Mo decreases the Ms point and increases the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain-generating body characteristics. Therefore, the amount of Mo needs to be less than 1.00 mass%. The amount of Mo is more preferably 0.90 mass% or less, and still more preferably 0.80 mass% or less.

(10) 0.10 ≦ Nb ≦ 0.40 mass%:
Nb combines with C and N to form a carbonitride. Carbonitrides prevent crystal grain coarsening during solution heat treatment and contribute to improvements in toughness and ductility. Further, since the solute C and N are reduced, the Ms point is increased, which is effective for reducing the amount of retained austenite. In order to obtain such an effect, the Nb amount needs to be 0.10 mass% or more. The Nb amount is more preferably 0.12 mass% or more, and further preferably 0.14 mass% or more.
On the other hand, excessive addition of Nb impairs hot workability, increases the amount of δ ferrite, and decreases the strength after aging treatment. Therefore, the Nb amount needs to be 0.40 mass% or less. The amount of Nb is more preferably 0.38 mass% or less, and further preferably 0.36 mass% or less.

(11) 0.001 ≦ Al ≦ 0.030 mass%:
Al is effective as a deoxidizing element. In order to obtain such an effect, the amount of Al needs to be 0.001 mass% or more. The amount of Al is more preferably 0.002 mass% or more, and still more preferably 0.003 mass% or more.
On the other hand, excessive addition of Al increases the amount of δ ferrite and decreases the strength after aging treatment. Moreover, coarse nitrides are easily formed, leading to a decrease in toughness. Therefore, the amount of Al needs to be 0.030 mass% or less. The amount of Al is more preferably 0.010 mass% or less.

(12) 1.0 <Co <3.0 mass%:
Co is effective not only for improving corrosion resistance but also for increasing the Ms point by addition. Co is also effective in suppressing crystallization of δ ferrite. Furthermore, although Co is an austenite stabilizing element, it does not have a stabilizing effect as that of Ni, and even if a certain amount of Co is added, it does not promote reverse transformation during aging treatment. Therefore, the addition of Co greatly contributes to the improvement of the strain generating body characteristics. In order to obtain such an effect, the amount of Co needs to be more than 1.0 mass%. The amount of Co is more preferably 1.10 mass% or more, and further preferably 1.15 mass% or more.
On the other hand, excessive addition of Co causes an increase in cost. Therefore, the amount of Co needs to be less than 3.0 mass%. The amount of Co is more preferably 2.9 mass% or less, and still more preferably 2.8 mass% or less.

(13) O ≦ 0.020 mass%:
Since excessive addition of O reduces hot workability, toughness, and corrosion resistance, it is preferably reduced as much as possible. On the other hand, an unnecessary reduction of O causes an increase in cost. Therefore, the amount of O needs to be 0.020 mass% or less. The amount of O is more preferably 0.015 mass% or less.

(14) 0.025 <N ≦ 0.070 mass%:
N is effective in reducing the amount of δ ferrite. N combines with Nb together with C to form a carbonitride. Carbonitride prevents crystal grain coarsening during solution heat treatment and contributes to improved toughness and ductility. In order to obtain such an effect, the N amount needs to be more than 0.025 mass%. The N amount is more preferably 0.027 mass% or more, and further preferably 0.029 mass% or more.
On the other hand, excessive addition of N reduces toughness. In addition, the Ms point is lowered to increase the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain body characteristics. Therefore, the N amount needs to be 0.070 mass% or less. The N amount is more preferably 0.065 mass% or less, and still more preferably 0.060 mass% or less.

[1.2. Sub-constituent elements]
The precipitation hardening stainless steel according to the present invention may further contain one or more of the following sub-constituent elements in addition to the main constituent elements described above. The kind of additive element, its component range, and the reason for limitation are as follows.

(15) 0.05 ≦ W ≦ 2.0 mass%:
W is effective for improving corrosion resistance. In order to obtain such an effect, the W amount is preferably 0.05 mass% or more. The amount of W is more preferably 0.10 mass% or more, and still more preferably 0.15 mass% or more.
On the other hand, excessive addition of W causes an increase in cost. In addition, the Ms point is lowered to increase the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain body characteristics. Therefore, the W amount is preferably 2.0 mass% or less. The amount of W is more preferably 1.8% by mass or less.

(16) 0.005 ≦ V ≦ 0.50 mass%:
V, like Nb, combines with C and N to form a carbonitride. Carbonitrides prevent coarsening of crystal grains during solution heat treatment and contribute to improvement of toughness. In order to obtain such an effect, the V amount is preferably 0.005 mass% or more. The amount of V is more preferably 0.010 mass% or more, and more preferably 0.015 mass% or more.
On the other hand, excessive addition of V harms hot workability, increases the amount of δ ferrite, and decreases the strength after aging treatment. Therefore, the V amount is preferably 0.50 mass% or less. The V amount is more preferably 0.45 mass% or less, and still more preferably 0.40 mass% or less.

(17) 0.005 ≦ Ti ≦ 0.50 mass%:
Ti combines with C and N to form a carbonitride. Carbonitrides prevent coarsening of crystal grains during solution heat treatment and contribute to improvement of toughness. In order to obtain such an effect, the amount of Ti is preferably 0.005 mass% or more. The amount of Ti is more preferably 0.010 mass% or more, and still more preferably 0.015 mass% or more.
On the other hand, excessive addition of Ti impairs hot workability, increases the amount of δ ferrite, and decreases the strength after aging treatment. Therefore, the Ti amount is preferably 0.50 mass% or less. The amount of Ti is more preferably 0.45 mass% or less, and still more preferably 0.40 mass% or less.

(18) 0.005 ≦ Ta ≦ 0.50 mass%:
Ta combines with C and N to form a carbonitride. Carbonitrides prevent coarsening of crystal grains during solution heat treatment and contribute to improvement of toughness. In order to obtain such an effect, the Ta amount is preferably 0.005 mass% or more. The amount of Ta is more preferably 0.010 mass% or more, and still more preferably 0.015 mass% or more.
On the other hand, excessive addition of Ta impairs hot workability, increases the amount of δ ferrite, and decreases the strength after aging treatment. Therefore, the amount of Ta is preferably 0.50 mass% or less. The amount of Ti is more preferably 0.45 mass% or less, and still more preferably 0.40 mass% or less.

(19) 0.005 ≦ Zr ≦ 0.50 mass%:
Zr combines with C and N to form a carbonitride. Carbonitrides prevent coarsening of crystal grains during solution heat treatment and contribute to improvement of toughness. In order to obtain such an effect, the amount of Zr is preferably 0.005 mass% or more. The amount of Zr is more preferably 0.010 mass% or more, and still more preferably 0.015 mass% or more.
On the other hand, excessive addition of Zr harms hot workability, increases the amount of δ ferrite, and decreases the strength after aging treatment. Therefore, the amount of Zr is preferably 0.50 mass% or less. The amount of Zr is more preferably 0.45 mass% or less, and still more preferably 0.40 mass% or less.
In addition, V, Ti, Ta, and Zr may add any 1 type, or may add 2 or more types.

(20) 0.0005 ≦ B ≦ 0.0100 mass%:
B is effective for improving hot workability. In order to obtain such an effect, the amount of B is preferably 0.0005 mass% or more. The amount of B is more preferably 0.0007 mass% or more, and still more preferably 0.0009 mass% or more.
On the other hand, excessive addition of B lowers the cleanliness and adversely affects hot workability. Therefore, the amount of B is preferably 0.0100 mass% or less. The amount of B is more preferably 0.0080 mass% or less, and still more preferably 0.0050 mass% or less.

(21) 0.0005 ≦ Mg ≦ 0.0100 mass%:
Mg is effective in improving hot workability. In order to obtain such an effect, the amount of Mg is preferably 0.0005 mass% or more. The amount of Mg is more preferably 0.0007 mass% or more, and further preferably 0.0009 mass% or more.
On the other hand, excessive addition of Mg reduces cleanliness and conversely affects hot workability. Therefore, the amount of Mg is preferably 0.0100 mass% or less. The amount of Mg is more preferably 0.0080 mass% or less, and still more preferably 0.0050 mass% or less.

(22) 0.0005 ≦ Ca ≦ 0.0100 mass%:
Ca is effective in improving hot workability. In order to obtain such an effect, the Ca content is preferably 0.0005 mass% or more. The amount of Ca is more preferably 0.0007 mass% or more, and still more preferably 0.0009 mass% or more.
On the other hand, excessive addition of Ca lowers the cleanliness and adversely affects hot workability. Therefore, the Ca content is preferably 0.0100 mass% or less. The amount of Ca is more preferably 0.0080 mass% or less, and still more preferably 0.0050 mass% or less.
In addition, B, Mg, and Ca may add any 1 type, or may add 2 or more types.

[1.3. Ingredient balance]
The precipitation hardening stainless steel according to the present invention needs to satisfy the following formulas (1) to (4) in addition to the component elements being in the above-mentioned range.
0.060 ≦ C + N ≦ 0.100 mass% (1)
Cu / Ni ≦ 1.20 (2)
α ≦ 470 (3)
β ≦ 10 (4)
However,
α = 361 ([C] + [N]) + 28 [Si] +39 [Mn] +10 [Cu] +17 [Ni] +20 [Cr] +5 [Mo] −15 [Co],
β = 138− {99.7 {([Ni _eq ] +36.5) / ([Cr _eq ] +17.6)}},
[Ni _eq ] = [Ni] +30 ([C] + [N]) + 0.5 ([Mn] + [Cu]) + 0.92 [Co],
[Cr _eq ] = [Cr] +1.5 [Mo] +0.5 [Si] +1.37 [Nb].

[1.3.1. (1) Formula]
Equation (1) represents the total amount of C and N. The total amount affects the toughness after aging treatment. In order to obtain good toughness, the total amount needs to be 0.060 mass% or more. The total amount is more preferably 0.062 mass% or more, and further preferably 0.064 mass% or more.
On the other hand, excessive addition of these elements decreases the Ms point and increases the amount of retained austenite, leading to a decrease in strength after aging treatment and a decrease in strain body characteristics. Therefore, the total amount needs to be 0.100 mass% or less. The total amount is more preferably 0.095 mass% or less, and still more preferably 0.090 mass% or less.

[1.3.2. (2) Formula]
Formula (2) represents the ratio of the Cu amount to the Ni amount. In general, excessive addition of Cu inhibits hot workability. However, when Ni corresponding to the amount of Cu is added, sufficient hot workability can be obtained even when a relatively large amount of Cu is contained. In order to obtain such an effect, the Cu / Ni ratio needs to be 1.20 or less. The Cu / Ni ratio is more preferably 1.10 or less.

[1.3.3. (3) Formula]
The expression (3), that is, the α value represents a tendency to form retained austenite generated during the solution heat treatment. The lower the α value, the higher the Ms point at the time of solution heat treatment, and the amount of retained austenite decreases. As the Ms point becomes higher, the increase in hysteresis due to the micro-yield of the retained austenite phase is suppressed. As a result, even when a stress is repeatedly applied when used as a strain generating body, the strain generating body characteristics can be improved such that the reference point becomes less likely to change gradually. In order to obtain such an effect, the α value needs to be 470 or less. The α value is more preferably 460 or less, and still more preferably 450 or less.

[1.3.4. (4) Formula]
The equation (4), that is, the β value correlates with the amount of δ ferrite remaining at the time of casting. The lower the β value, the less δ ferrite that remains during casting. If δ ferrite remains, the strength and toughness after the aging treatment are lowered, and there is a concern that the strain body characteristics are deteriorated as in the case of retained austenite. In order to obtain sufficient strength, toughness, and strained body characteristics after the aging treatment, the β value needs to be 10 or less. The β value is more preferably 8 or less, and still more preferably 5 or less.

[1.4. Austenite amount]
Generally, as the amount of austenite in steel (= residual austenite amount + reverse transformed austenite amount) decreases, it is possible to suppress the deterioration of the strain body characteristics due to the micro-yield of the austenite phase. In order to obtain a high strain body characteristic, the amount of austenite needs to be 2.0 volume% or less. The austenite amount is more preferably 1.0% by volume or less.
By optimizing the component range (particularly the above-described α value), a precipitation hardening stainless steel with a small amount of austenite can be obtained.

[2. Method for producing precipitation hardening stainless steel]
The precipitation hardening stainless steel according to the present invention is
(1) Melting and casting the raw materials blended so as to become predetermined components,
(2) Hot working the obtained ingot to obtain a steel material of a predetermined shape,
(3) Solution heat treatment is performed on the steel material after hot working,
(4) Cold work or cut the steel material into the desired shape,
(5) The steel material can be manufactured by performing an aging treatment.

[2.1. Melting / casting process]
First, raw materials blended so as to have predetermined components are melted and cast (melting and casting process). The melting method and the casting method are not particularly limited as long as the method can obtain an ingot having the target composition. Examples of the melting method include a vacuum induction melting method.

[2.2. Hot working process]
Next, hot working is performed on the obtained ingot to obtain a steel material having a predetermined shape (hot working step). The hot working method is not particularly limited as long as the steel material having a predetermined shape can be obtained. Examples of the hot working method include hot forging and hot rolling.

[2.3. Solution heat treatment process]
Next, a solution heat treatment is performed on the steel material after hot working. Specifically, the solution heat treatment is performed by heating to an optimum temperature according to the composition of the steel material, holding the temperature for a certain period of time, and cooling with air, oil or water. If the solution heat treatment temperature is within a predetermined range, good cold workability, corrosion resistance, and high strength after aging treatment can be obtained.

If the solution heat treatment temperature is too low, an austenite single phase structure cannot be obtained. Therefore, the solution heat treatment temperature needs to be 950 ° C. or higher. The solution heat treatment temperature is more preferably 1020 ° C. or higher.
On the other hand, if the solution heat treatment temperature is too high, the crystal grains become coarse and the toughness is reduced. Therefore, the solution heat treatment temperature needs to be 1100 ° C. or lower. The solution heat treatment temperature is more preferably 1050 ° C. or lower.

The solution heat treatment time is not particularly limited, and an optimum time is selected according to the steel composition and the solution heat treatment temperature. In general, the higher the solution heat treatment temperature, the shorter the austenite single phase structure can be made.
When cooled after the solution heat treatment, the austenite phase transforms into a martensite phase. In the case of the precipitation hardening type stainless steel according to the present invention, martensitic transformation occurs even if it is air-cooled.

[2.4. Machining process]
Next, cold working or cutting is performed on the steel material after the solution heat treatment (machining process). Cold working and cutting are performed in order to make the steel material into a desired shape. The methods of cold working and cutting are not particularly limited, and various methods can be used depending on the purpose.

[2.5. Aging treatment process]
Furthermore, an aging treatment is performed on the processed steel material (aging treatment step). Specifically, the aging treatment is performed by heating to an optimum temperature according to the composition of the steel material, holding the temperature for a certain period of time, and air cooling. Due to the aging treatment, an intermetallic compound is precipitated in the martensite phase. If the aging temperature is within a predetermined range, a precipitation hardening type stainless steel having a small amount of austenite in the steel and having high strength and good strain-generating properties can be obtained.

When the aging treatment temperature is too low, precipitation of intermetallic compounds becomes insufficient. Therefore, the aging treatment temperature needs to be 450 ° C. or higher. The aging treatment temperature is more preferably 460 ° C. or higher.
On the other hand, when the aging treatment temperature is too high, the martensite phase is reversely transformed into an austenite phase. Therefore, the aging treatment temperature needs to be 650 ° C. or less. The aging treatment temperature is more preferably 640 ° C. or lower.
In the present invention, since the component range (particularly, the amount of Co) is optimized, aging treatment can be performed at a higher temperature than before without causing reverse transformation.

  The aging treatment time is not particularly limited, and an optimum time is selected according to the steel composition and the aging treatment temperature. In general, steel with superior toughness can be obtained as the aging temperature increases.

[3. Sensor parts]
The sensor component according to the present invention is formed by using the precipitation hardening stainless steel according to the present invention. Examples of sensor components include a column-type strain body, a Robert-type strain body, a shear-type strain body, a ring-type strain body, and a diaphragm-type strain body.

[4. Action]
Co is an austenite-forming element like Ni, but has a smaller effect on lowering the Ms point and the A ₁ transformation point than Ni. That is, the addition of a predetermined amount of Co not only reduces the amount of retained austenite after the solution heat treatment and the δ ferrite phase remaining after casting, but also has the effect of reducing the amount of reverse-transformed austenite generated during the high temperature aging treatment. Further, each additive element includes an element that has an action of increasing the amount of soft retained austenite or δ ferrite and an element that has an action of decreasing.

  For this reason, in precipitation hardening stainless steel, the amount of each component element (particularly the amount of Co) is optimized, and at the same time, the values of the equations (1) to (4) are optimized. The amount of reverse-transformed austenite generated by transformation and the amount of δ ferrite remaining after casting can be simultaneously reduced. As a result, a precipitation hardening stainless steel having excellent strain generating characteristics can be obtained.

  The sensor component according to the present invention defines the total amount of C and N (formula (1)), balances the contents of Cu and Ni (formula (2)), and defines the amount of other alloy elements. By using hardened stainless steel, it is characterized by having characteristics equivalent to those of sensor parts using SUS630, which is a conventional material, in terms of strength, toughness, workability, and corrosion resistance.

Furthermore, by setting the α value to 420 or less (equation (3)), the Ms point during the solution heat treatment increases, and the amount of retained austenite during quenching can be reduced. For this reason, even when repeated stress is applied when used as a strain generating body, it is possible to suppress microyield.
Further, by setting the β value to 10 or less (equation (4)), the amount of δ ferrite is limited, and it is expected that the hot workability is secured and the strength after the aging treatment is improved.
Therefore, the precipitation hardening stainless steel according to the present invention is excellent not only in strength, toughness, workability, and corrosion resistance, but also in strain-generating body properties.

(Examples 1-25, Comparative Examples 1-15)
[1. Preparation of sample]
In a vacuum induction furnace, 150 kg of steel ingots having various chemical components shown in Tables 1 and 2 were melted. This was hot forged and then hot rolled to produce a 22 mm diameter bar. Then, this bar was kept in a temperature range of 1040 ° C. ± 20 ° C. for 1 hour and subjected to a solution heat treatment by air cooling.
after that,
(A) Aging treatment (H900 treatment) which is held at 480 ° C. for 4 hours and then air-cooled,
(B) Aging treatment (H1025 treatment) of holding at 550 ° C. for 4 hours and air cooling, or
(C) Aging treatment (H1150 treatment) which is held at 625 ° C. for 4 hours and then air-cooled.
Went.

[2. Test method]
[2.1. Measurement of retained austenite]
A test piece was collected from the material after the aging treatment, and the amount of austenite was measured by an X-ray diffraction method.
[2.2. δ ferrite content]
A specimen was collected from the material after the aging treatment, etched with a corrosive liquid villa, and then subjected to a structure observation with an optical microscope. The case where the distribution amount of δ ferrite observed at this time is less than 2% in terms of the area ratio with respect to the observation surface is “◯”, and the case where it is 2% or more is “x”.

[2.3. Tensile test]
A JIS No. 4 tensile test piece was collected from the material after aging treatment, and 0.2% proof stress and elongation at break were measured using this specimen.
[2.4. Impact test]
A JIS No. 2 mm U notch impact test piece was collected from the material after the aging treatment, and the Charpy impact value was measured.
[2.5. Vickers hardness]
A test piece was collected from the material after the aging treatment, and the Vickers hardness was measured at a location corresponding to the radial direction D / 4 under a test load of 300 g.

[2.6. Corrosion resistance test]
A specimen was collected from the material after the aging treatment, and in accordance with JISZ2371, a salt spray test was conducted for up to 96 hours at 35 ° C. in a 5% NaCl solution to confirm whether or not rust was generated. The case where there was no occurrence was designated as “O”, and the case where occurrence occurred was designated as “X”.
[2.7. Straining body characteristics]
A rod-shaped tensile test piece having a parallel part diameter of 8.0 mm, a parallel part length of 110 mm, a grip part diameter of 18 mm, and a total length of 180 mm was collected from the material after the aging treatment. An apparent nominal strain was calculated from a dimensional difference before and after the test after applying a repeated load 100,000 times at a maximum stress: 500 MPa, a stress ratio: 0.1, and a frequency: 5 Hz on the tensile test piece. A case where the value was less than 0.0001 was evaluated as “◯”, and a case where the value was 0.0001 or more was evaluated as “x”.

[3. result]
Tables 3 to 5 show the results. From Tables 3 to 5, the following can be understood.

(1) The comparative example 1 is general SUS630. In SUS630, since Ni is excessive and Co is not added, the α value exceeds 470, and the structure after the solution heat treatment and the aging treatment contains an austenite phase of 10% by volume or more. . Therefore, it can be said that the reference point gradually fluctuates due to an increase in hysteresis due to the micro-yield of the austenite phase, and the strain-generating body characteristics are likely to deteriorate.
(2) In Comparative Examples 2 to 3, C + N is less than the amount defined by the formula (1). In Comparative Example 4, C + N exceeds the amount defined by the expression (1). Therefore, Comparative Examples 2 to 4 have low elongation at break and impact value and are inferior in toughness.
(3) In Comparative Examples 5 and 6, the Cu / Ni ratio exceeds the amount specified by the expression (2). Therefore, a crack arises at the time of hot processing, and it is inferior to hot workability.

(4) In Comparative Example 7, Cr is excessive and the α value exceeds 470. Further, the β value is over 10. Therefore, a decrease in strength and toughness after aging treatment and a decrease in strain body characteristics are observed.
(5) Since the comparative example 8 has few Cr, it is inferior to corrosion resistance.
(6) In Comparative Example 9, C is excessive. Therefore, it is easy to produce Cr carbide, and as a result, the amount of solid solution Cr in the parent phase is reduced and the corrosion resistance is poor.

(7) In Comparative Example 10, N is excessive. Therefore, Cr nitride is likely to be generated, and as a result, the amount of solid solution Cr in the parent phase is reduced and the corrosion resistance is inferior.
(8) In Comparative Example 11, Ni is excessive and the α value is high. Therefore, retained austenite tends to increase remarkably due to a decrease in Ms point. Moreover, since the reverse-transformed austenite is formed during the aging process by lowering the A ₁ transformation point, decrease in toughness after aging treatment, a decrease in the strain body characteristics observed.
(9) In Comparative Example 12, Cu is excessive and the α value is high. Therefore, retained austenite tends to increase remarkably due to a decrease in Ms point. Further, since the reverse transformed austenite is generated during the aging treatment due to the decrease in the A ₁ transformation point, the strength after the aging treatment and the strain-generating body properties are degraded.

(10) Since the comparative example 13 has a small amount of Co, the α value is high. Therefore, retained austenite tends to increase due to a decrease in Ms point. For this reason, a decrease in toughness after aging treatment and a decrease in strain-generating body properties are observed.
(11) In Comparative Example 14, Co is excessive. For this reason, reversely transformed austenite is generated during the aging treatment due to a decrease in the A ₁ transformation point, and therefore, a decrease in strength and a deterioration in the strain-generating body properties after the aging treatment are observed.
(12) The comparative example 15 is SUS420J2. SUS420J2 is wrinkled and has low corrosion resistance.
(13) As for Comparative Examples 1-15 which do not satisfy | fill the conditions prescribed | regulated by this application, all are inferior in at least 1 or more among intensity | strength, toughness, corrosion resistance, and a strain body characteristic.

(14) Examples 1 to 25 that satisfy the conditions specified in the present application are excellent in strength, toughness, corrosion resistance, and strain body characteristics. Therefore, if this is applied to a sensor component such as a strain generating body, it is possible to obtain a sensor or the like with little fluctuation of the reference point. In particular, when the amount of austenite is 2.0% by volume or less, it becomes easy to suppress microyield and to improve the strain-generating body characteristics, so that it is possible to provide a sensor having excellent reliability.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The precipitation hardening stainless steel according to the present invention can be used as a sensor component such as a column-type strain body, a Roberval-type strain body, a shear-type strain body, a ring-type strain body, or a diaphragm-type strain body. it can.

Claims (7)

A precipitation hardening stainless steel having the following configuration.
(1) The precipitation hardening stainless steel is
0.020 ≦ C <0.070 mass%,
0.05 ≦ Si <0.50 mass%,
0.1 ≦ Mn ≦ 1.0 mass%,
0.003 ≦ P ≦ 0.050 mass%,
S <0.02 mass%,
2.0 <Cu ≦ 4.0 mass%,
2.0 <Ni <4.0 mass%,
13.0 ≦ Cr ≦ 17.0 mass%,
0.03 ≦ Mo <1.00 mass%,
0.10 ≦ Nb ≦ 0.40 mass%,
0.001 ≦ Al ≦ 0.030 mass%,
1.0 <Co <3.0 mass%,
O ≦ 0.020 mass%, and
0.025 <N ≦ 0.070 mass%
And the balance consists of Fe and inevitable impurities.
(2) The precipitation hardening stainless steel satisfies the following formulas (1) to (4).
0.060 ≦ C + N ≦ 0.100 mass% (1)
Cu / Ni ≦ 1.20 (2)
α ≦ 470 (3)
β ≦ 10 (4)
However,
α = 361 ([C] + [N]) + 28 [Si] +39 [Mn] +10 [Cu] +17 [Ni] +20 [Cr] +5 [Mo] −15 [Co],
β = 138− {99.7 {([Ni _eq ] +36.5) / ([Cr _eq ] +17.6)}},
[Ni _eq ] = [Ni] +30 ([C] + [N]) + 0.5 ([Mn] + [Cu]) + 0.92 [Co],
[Cr _eq ] = [Cr] +1.5 [Mo] +0.5 [Si] +1.37 [Nb]. The precipitation hardening stainless steel according to claim 1, wherein the austenite content in the steel is 2.0% by volume or less. 0.05 ≦ W ≦ 2.0 mass%
The precipitation hardening stainless steel according to claim 1 or 2, further comprising: 0.005 ≦ V ≦ 0.50 mass%,
0.005 ≦ Ti ≦ 0.50 mass%,
0.005 ≦ Ta ≦ 0.50 mass%, and
0.005 ≦ Zr ≦ 0.50 mass%
The precipitation hardening stainless steel according to any one of claims 1 to 3, further comprising at least one element selected from the group consisting of: 0.0005 ≦ B ≦ 0.0100 mass%,
0.0005 ≦ Mg ≦ 0.0100 mass%, and
0.0005 ≦ Ca ≦ 0.0100 mass%
The precipitation hardening stainless steel according to any one of claims 1 to 4, further comprising at least one element selected from the group consisting of: Apply solution heat treatment at 950-1100 ° C,
The precipitation hardening type stainless steel according to any one of claims 1 to 5, which is obtained by aging heat treatment at a temperature of 450 to 650 ° C after processing into a target shape.   The component for sensors using the precipitation hardening type stainless steel of any one of Claim 1-6.