JP3492531B2 - Heat resistant stainless steel - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to heat-resistant stainless steel. In particular, the present invention relates to a high strength stainless steel that is excellent in sag resistance at high temperatures and is required to have heat resistance such as engine parts, nuclear power generation parts, turbine parts, etc., and is most suitable as a spring material.

[0002]

2. Description of the Related Art SUS304, SUS316, SUS631, SUS631 which have been used as heat-resistant steel in the operating temperature range of about 500 ° C as a spring component material used in the exhaust system of an automobile engine.
Austenitic stainless steel such as J1 is used. Moreover, Inconel X750, which is a Ni-based heat-resistant alloy, is used as a material for parts used in a temperature range exceeding 500 ° C.

In recent years, the exhaust system temperature tends to rise in order to improve the efficiency of the engine and the catalyst, because of the increasing demand for exhaust gas regulations of automobiles as a measure against environmental problems. For this reason, even in spring parts whose operating temperature range was up to 500 ° C, 6
The temperature rises to around 00 ° C, and austenitic stainless steel such as SUS304 may have insufficient heat resistance, especially high temperature tensile strength and high temperature sag resistance required for heat resistant springs.

At this time, Inconel X750 is used as the material for the same parts.
Ni-based heat-resistant alloys such as are used, but increase in material cost and manufacturing cost due to heat treatment at high temperature for a long time cannot be avoided. Ni-based heat-resistant alloys such as the Inconel X750 are superalloys whose operating temperature range is 600 ° C or higher, which is an excessive specification for use in the temperature range of 600 ° C or lower.

Therefore, the γ'phase [Ni ₃ (Al, Ti, Nb)] such as A286 (AISI660) has the heat resistance and cost intermediate between those of austenitic stainless steel and heat-resistant Ni-base superalloy.
The use of precipitation-strengthened austenitic heat-resistant steel is promising.

Further, in order to improve heat resistance characteristics, particularly high temperature tensile strength at 700 ° C. or higher and high temperature creep resistance, in the technique described in Japanese Patent Laid-Open No. 7-238349, Ti / Al in the components is used.
The ratio is set to 5 to 20 to accelerate the precipitation of the γ ′ phase [Ni ₃ (Al, Ti, Nb)] in a short time.

Further, in the technique described in Japanese Patent Laid-Open No. 4-48051, the thermal fatigue resistance is improved by positively utilizing the precipitation of the η phase [Ni ₃ Ti: hcp structure].

[0008]

However, none of these have achieved both high temperature tensile strength and high temperature sag resistance, which are necessary for heat resistant springs. The precipitation phase morphology of precipitation-strengthened heat-resistant steel has various effects on the heat resistance characteristics of the steel. In order to achieve both high temperature tensile strength and high temperature sag resistance, it is necessary to limit the precipitation morphology.

[0009] Therefore, the main object of the present invention is to strengthen the matrix of Fe-based austenitic stainless steel and to provide the γ'phase [Ni ₃
(Al, Ti, Nb)] by performing strong precipitation strengthening of SUS3
The cost increase can be suppressed for 04 and SUS631, etc., and the precipitation phase η phase [Ni ₃ Ti: hcp structure] and γ ′ phase [Ni ₃ (Al,
Ti, Nb)] precipitation morphology
The purpose is to provide heat-resistant high-strength stainless steel with excellent high-temperature tensile strength and high-temperature sag resistance required for spring materials even at temperatures above 600 ° C).

[0010]

The heat-resistant stainless steel of the present invention has C: 0.02 to 0.30 wt%, Si: 0.02 to 3.5 wt%, Mn: 0.02
〜2.5wt%, Ni: 10〜50wt%, Cr: 12〜25wt%, Ti: 1.0〜
5.0 wt%, Al: 0.002 to 1.0 wt%, and Nb: 0.1 to 3.
Heat-resistant stainless steel containing at least one selected from 0 wt%, B: 0.001 to 0.01 wt% and Mo: 0.1 to 4.0 wt% and the total content of Ti, Al and Nb is 3.0 to 7.0 wt% Is. Here, the weight of the η phase [Ni ₃ Ti: hcp structure] that precipitates at the grain boundaries and the γ ′ phase [Ni ₃ (Al, Ti, Nb)] that precipitates within the γ phase (austenite) crystal grains that are the matrix Ratio “{η phase [Ni ₃ Ti:
hcp structure] / γ ′ phase [Ni ₃ (Al, Ti, Nb)]} × 100
(%) ”Is 0.01% or more and 30.00% or less. And 600
It is characterized by a hot tensile strength at 800 ° C of 800 N / mm ² or more.

Further, in order to improve the high temperature sag resistance, grain boundary precipitation η phase [Ni ₃ Ti: hcp structure and γ which is a matrix phase]
It is better that there is no phase (austenite) mismatch, and more γ ′ phase [Ni ₃ (Al, Ti, Nb)] must be precipitated in the crystal grains. Therefore, the weight ratio of the above-mentioned precipitate “{η phase [Ni ₃
Ti: hcp structure] / γ ′ phase [Ni ₃ (Al, Ti, Nb)]} × 100
(%) ”Is preferably 0.01% or more and 10.00% or less.

In particular, in order to improve the high temperature sag resistance at around 600 ° C., the γ'phase [Ni ₃ (Al, Ti, Nb)] precipitated in the matrix γ phase (austenite) crystal grains is formed. It is necessary that the diameter of the spherical particles is as fine as possible. For that purpose, it is desirable that the diameter of the spherical particles of the γ'phase [Ni ₃ (Al, Ti, Nb)] precipitated in the grains is 1 nm or more and 20 nm or less.

The reasons for limiting the composition of the stainless steel of the present invention will be described below.

C combines with Cr or the like in the steel to form a carbide, thereby increasing the high temperature strength. However, if it is contained in a large amount, toughness and corrosion resistance decrease. Therefore, the effective content of C is 0.02 to 0.30 wt%.

Si is effective as a solid solution in improving heat resistance. It is also effective as a deoxidizer during dissolution and refining, and it is necessary to contain Si: 0.02 wt% or more to bring out the effect. However, considering the toughness deterioration, it was set to 3.5 wt% or less.

Like Si, Mn is also used as a deoxidizing agent during melting and refining. Further, it is also effective for phase stabilization of the γ phase (austenite) of austenitic stainless steel. However, Mn: 0.02 to 2.5 wt% as it has a bad effect on the oxidation resistance at high temperature.
%.

Ni is effective in stabilizing the γ phase (austenite). Further, Ni is a constituent element of the γ ′ phase [Ni ₃ (Al, Ti, Nb)] (precipitation phase), which is the main cause of the improvement in heat resistance of the present invention. So considering the stability of the γ phase (austenite)
It was set to 10 wt% or more and 50 wt% or less to suppress cost increase.

Cr is a main constituent element of austenitic stainless steel, and is an element effective for obtaining heat resistance and oxidation resistance. Therefore, Ni equivalent and Cr equivalent are calculated from other elemental components, and after considering the phase stability of the γ phase (austenite), it is set to 12 wt% or more in order to obtain the necessary heat resistance property, and toughness deterioration is considered. It was set to 25 wt% or less.

The stainless steel of the present invention is subjected to precipitation strengthening of the γ'phase [Ni ₃ (Al, Ti, Nb)] for the purpose of improving heat resistance. The reasons for limiting the component range of the constituent elements will be described below.

Ti is a main constituent element of the γ'phase [Ni ₃ (Al, Ti, Nb)], but if added in a large amount, the η phase [Ni
₃ Ti: hcp structure] is excessively precipitated at the grain boundaries, and it is impossible to control the precipitation of the γ ′ phase [Ni ₃ (Al, Ti, Nb)] required to obtain heat resistance characteristics only by heat treatment. Become. In order to obtain an effective precipitation amount, it is necessary to set it to 1.0 to 5.0 wt%.

Al, like Ti, is a main constituent element of the γ'phase [Ni ₃ (Al, Ti, Nb)], but it easily forms an oxide and is also used as a deoxidizing agent during melting and refining. However, excessive addition easily causes deterioration of hot workability, so the content was made 1.0 wt% or less.

If Nb is added excessively, Fe ₂ Nb (Rabus)
Precipitate the phases. Since strength deterioration is expected at this time, 0.1
〜3.0wt%.

An increase in the total content of Ti, Al and Nb causes an increase in the precipitation amount of the γ'phase [Ni ₃ (Al, Ti, Nb)] and destabilization of the matrix γ phase (austenite). Therefore, the content effective for improving the high temperature characteristics is 3.0 wt% or more, and considering the phase stability of the γ phase (austenite), it is set to 7.0 wt% or less.

Mo dissolves in the γ phase (austenite),
It greatly contributes to the improvement of high temperature tensile strength and sag resistance. Therefore, 0.1 wt% or more, which is the minimum required to improve sag resistance,
Considering the deterioration of workability, it was set to 4.0 wt% or less.

B is 0.001 to 0.01 wt% for the purpose of preventing deterioration of hot workability during strong precipitation strengthening and improving toughness.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. A 150 kg steel material was melt-cast in a vacuum melting furnace, forged and then hot-rolled to produce a wire having a diameter of 9.5 mm. After that, solution treatment and wire drawing were repeated, and finally a test material with a wire diameter of 4 mm and a workability of 40% was prepared. Table 1 shows its chemical composition and “{η phase [Ni ₃ Ti: hcp structure] / γ ′ phase [Ni ₃ (Al, Ti, Nb)]}.
× 100 (%) ”weight ratio (hereinafter simply referred to as η / γ ′ ratio) and particle diameter of γ ′ phase [Ni ₃ (Al, Ti, Nb)]. For this η / γ 'ratio and particle diameter, see TEM
(Transmission Electron Microscope) The result calculated by analyzing from the image was used.

[0027]

[Table 1]

The heat treatment conditions for each test material are as follows.
For the invention material and the comparative materials 4 to 7, in order to adjust the η / γ ′ ratio and the particle diameter of the precipitated γ ′ phase [Ni ₃ (Al, Ti, Nb)], the temperature was set as the solution treatment condition for each sample. 1000-1300
Set an appropriate time in ° C, and the aging condition is a temperature of 600 to 800 ° C.
Prototype materials were prepared by setting appropriate ones for 1 to 50 hours. Comparative material 1 is Inco's Ni-based heat-resistant alloy Inconel? X7
At 50, the aging condition was 704 ° C. × 20 hours and air cooling. Comparative material 2 is SUS304-WPB, which was low temperature annealed at 400 ℃ for 20 minutes.
Comparative material 3 was SUS631J1-WPC and was subjected to low temperature annealing at 475 ° C for 1 hour.

As a comparative material, the Al + Ti + Nb content is 7.0 wt% or more and the η / γ 'ratio is 0.01% or more 30.
Although we tried to prepare less than 00%, it was difficult to prepare a sample by controlling the heat treatment conditions.

(Tensile test) For each of the above trial materials,
A tensile test was performed at 600 ° C. Regarding the test method, each prototype material (4.0 mmφ) was held in the atmosphere at a test temperature of 600 ° C. for 15 minutes and then subjected to a tensile test. All the samples were tested after aging or low temperature calcination. The results are shown in Table 2.

[0031]

[Table 2]

From Table 2, it can be confirmed that each of the materials of the present invention has a higher tensile strength at 600 ° C. as compared with stainless steel which is a general heat resistant steel. In particular, the invention materials 8 and 9 having an η / γ ′ ratio of 10.00% or less and the diameter of spherical particles of the γ ′ phase [Ni ₃ (Al, Ti, Nb)] having a η / γ ′ ratio of 10.00% or less and 20 nm. The following invention materials 10 and 11 show excellent tensile strength.

On the other hand, when the η / γ 'ratio exceeds 30%, the γ'phase [Ni ₃ (A
l, Ti, Nb)] having a particle diameter of more than 20 nm, comparative materials 4, 5 and γ ′ phase [Ni ₃ (Al, Ti, Nb)] having a particle diameter of 20 nm or less but having an η / γ ′ ratio of 30 %, Al + Ti + Nb content is 7w
Comparative material 6 having a t% of more than 6%, and a comparative material 7 having a γ ′ phase [Ni ₃ (Al, Ti, Nb)] having a particle diameter of 20 nm or less, but having an η / γ ′ ratio of more than 30%, 600
It can be confirmed that the tensile strength at ℃ becomes 800 N / mm ² or less.

(High temperature sag resistance test) Next, the high temperature sag resistance of the heat resistant stainless steel of the present invention was evaluated. As shown in FIG. 1, the test method is as follows. First, the sample is a coil spring 100, and then a compressive load is applied (load shear stress is 400 MPa), and the test temperature is kept at 600 ° C. for 24 hours. After that, the load was released, and the residual shear strain was calculated from the measurement of the amount of fatigue of the spring. The results are shown in Table 3.

[0035]

[Table 3]

Table 3 shows the residual shear strain amount (%) after the test.
Indicates. All of the invention materials have higher high temperature fatigue resistance than stainless steel which is a general heat resistant steel, and in particular, η
It can be confirmed that invention materials 8 to 11 having a / γ 'ratio of 10.00% or less can obtain higher high temperature sag resistance. Above all γ ′
Inventive materials 10 and 11 in which the diameter of the spherical particles of the phase [Ni ₃ (Al, Ti, Nb)] are 20 nm or less are almost equivalent to or higher than the Ni-based heat-resistant superalloy (Comparative material 1) in high temperature resistance. Has a tendency.

[0037]

As described above, the heat-resistant steel of the present invention strongly precipitates a certain amount of γ'phase [Ni ₃ (Al, Ti, Nb)] particles in the γ phase (austenite) phase crystal grains that are the base. By doing so, the hot tensile strength at around 600 ° C. can be increased. In addition, by defining the weight ratio of the η phase [Ni ₃ Ti: hcp structure] that is the grain boundary precipitation phase and the γ ′ phase [Ni ₃ (Al, Ti, Nb)] that is the intragranular precipitation phase, the high temperature Settling resistance can also be improved.

Furthermore, the γ'phase [Ni ₃ (Al, T
i, Nb)] with a particle diameter of 20 nm or less, it is possible to obtain high temperature sag resistance higher than that of an expensive Ni-based heat-resistant superalloy.

Therefore, the heat-resistant stainless steel of the present invention is suitable as a heat-resistant spring material such as a ball joint, which is a flexible joint part used in an automobile exhaust system, a blade, and a knit mesh used for a three-way catalyst. In addition, by using a Fe-based alloy, it is possible to reduce the cost increase due to the use of a Ni-based heat-resistant superalloy, which has high industrial value.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a test method for evaluating the sag resistance of stainless steel.

[Explanation of symbols]

1 spring

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Teruyuki Murai, 1-1 1-1 Kunyokita, Itami-shi, Hyogo Sumitomo Electric Industries, Ltd. Itami Works (56) Reference JP-A-58-34129 (JP, A) ) JP-A 7-11376 (JP, A) JP-A 64-25919 (JP, A) JP-A 7-216515 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 302 C22C 38/58

Claims (2)

(57) [Claims] 1. C: 0.02 to 0.30 wt%, Si: 0.02 to 3.5 wt%,
Mn: 0.02 to 2.5wt%, Ni: 10 to 50wt%, Cr: 12 to 25wt%, T
i: 1.0-5.0wt%, Al: 0.002-1.0wt%, and Nb:
0.1 to 3.0 wt%, B: 0.001 to 0.01 wt%, Mo: 0.1 to 4.0 wt%, and the total content of Ti, Al and Nb is 3.0 to 7.0 wt% Heat-resistant stainless steel, η phase [Ni ₃ Ti: hcp structure] that precipitates at grain boundaries and γ ′ phase [Ni that precipitates in the γ phase (austenite) crystal grains that are the matrix
₃ (Al, Ti, Nb)] weight ratio “{η phase [Ni ₃ Ti: hcp
Structure] / γ 'phase [Ni ₃ (Al, Ti, Nb)]} × 100 (%) ”
Is 0.01% or more and 10.00% or less , and the hot tensile strength at 600 ° C. is 800 N / mm ² or more, a heat-resistant stainless steel. 2. C: 0.02 to 0.30 wt%, Si: 0.02 to 3.5 wt%,
Mn: 0.02 to 2.5wt%, Ni: 10 to 50wt%, Cr: 12 to 25wt%, T
i: 1.0-5.0wt%, Al: 0.002-1.0wt%, and Nb:
0.1-3.0wt%, B: 0.001-0.01wt%, Mo: 0.1-4.0wt%
Containing one or more selected from the group consisting of Ti, Al and Nb
Heat-resistant stainless steel with a total content of 3.0-7.0 wt%
Te, eta phase precipitated in grain boundaries: is [Ni ₃ Ti hcp structure] and base
γ ′ phase [Ni (Austenite) precipitates in crystal grains [Ni
₃ (Al, Ti, Nb)] weight ratio “{η phase [Ni ₃ Ti: hcp
Structure] / γ 'phase [Ni ₃ (Al, Ti, Nb)]} × 100 (%) ”
Is 0.01% or more and 30.00% or less, the diameter of the spherical particles of the γ'phase [Ni ₃ (Al, Ti, Nb)] is 1 or less.
nm or more and 20 nm or less, and the hot tensile strength at 600 ° C is 800 N / mm ² or more.
Heat resistant stainless steel to collect.