JP3384887B2 - Precipitation hardened stainless steel for springs with excellent strength and torsion characteristics - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precipitation hardening type stainless steel for springs which exhibits high strength after aging treatment and is excellent in torsion characteristics and punching workability.

[0002]

2. Description of the Related Art Stainless steel for springs is SUS3
Work-hardening stainless steel represented by 01 and 17-7P
Precipitation hardening type stainless steel represented by H is typically used. In order to improve the hardness of this kind of stainless steel, cold working with high workability is required. As a result, the hardness in the cold working state becomes high, and the moldability and punching workability are poor. On the other hand, when improving the moldability and punchability, the hardness after the aging treatment is insufficient. Due to such contradictory constraints, it is difficult to obtain a spring stainless steel having both hardness and workability. Moreover, if the stability of the austenite phase changes due to component fluctuations, a certain amount of martensite cannot be obtained even if a certain amount of cold working is performed, variation in hardness increases, and stability of product properties becomes to degrade. The applicant has a low hardness before aging treatment in order to achieve both hardness and workability,
A precipitation hardening type martensitic stainless steel with excellent punching and forming properties has been developed, and a part of it has been developed by JP-A-60.
It was introduced as Japanese Patent Publication No. 152660. When this steel is subjected to an aging treatment, high strength is exhibited, and the formability and manufacturability, which have been problems with conventional steel materials, are improved.

[0003]

Precipitation hardening type martensitic stainless steel satisfies the required properties regarding strength in a state where it is punched into a C-shaped or E-shaped ring spring and is strengthened by aging treatment. To be done. However, a high torsional stress may be applied in addition to the tearing stress when it is mounted on an apparatus, machine, etc., and the torsional characteristics required at this time may not be satisfied. Torsional properties tend to differ from toughness. Toughness is dominated by strength to some extent, and tends to decrease when strength is high. The toughness of the precipitation hardening type martensitic stainless steel is as described in Japanese Patent Application Laid-Open No. 60-36649 by the present applicant.
It is improved by adding. However, the torsional characteristics are not always improved by adding Mo. The present invention has been devised to solve such problems, and balances the components of C, Cr, etc. with a composition having a reduced Cu content, and reduces inclusions that act as starting points for crack generation. Thus, when the strength is increased by aging treatment, it is an object to provide a precipitation hardening type martensitic stainless steel that exhibits excellent torsional properties that cannot be obtained with conventional steel materials while maintaining high strength. .

[0004]

In order to achieve the object, the precipitation hardening type martensitic stainless steel of the present invention has C: 0.030% by weight or less and Si: 0.5 to 2.0.
% By weight, Mn: 0.40% by weight or less, S: 0.0020
% By weight, Ni: 7.5 to 9.5% by weight, Cr: 1
2.0 to 16.0% by weight, Cu: 0.30% by weight or less,
N: 0.015% by weight or less and O: 0.005% by weight or less, further Ti: 0.35% by weight or less, Nb: 0.
40% by weight or less and Al: 0.30% by weight or less, and one or more types is included, and the H value defined by the formula (1) is 0.2.
It is in the range of 0 to 0.70, and the components are adjusted so that the I value defined by the formula (2) is 4.0 × 10 ⁻⁴ or less. H = Si% × (Ti% + 0.8 × Nb% + Al%) ・ ・ ・ ・ (1) I = Mn% × S% ・ ・ ・ ・ (2) In addition, cleanliness d _60x400 specified by JIS _G0555.
Is preferably 0.010% or less. This precipitation hardening type martensitic stainless steel may contain 3.0% by weight or less of Mo as required.

[0005]

The present inventors have conducted various investigations and studies on precipitation hardening type martensitic stainless steel so as to obtain excellent torsion characteristics after aging treatment. It is necessary to keep the Cu content low in order to improve the torsional characteristics, but if Cu, which is a precipitation hardening element, is simply reduced, insufficient strength after aging treatment becomes a problem. Therefore, as a result of various studies for securing a predetermined strength for the Cu-reduced steel, by maintaining the Ni content at a high level and adjusting the balance of alloying elements such as C and Cr, high strength can be obtained after the aging treatment. It has been clarified that a steel material having excellent toughness evaluated by a notch tensile test can be obtained. In this case, although the toughness can be improved, the torsion characteristic is not necessarily improved.
Therefore, as a result of further research, it was found that carbonitride oxides such as Ti, Nb, and Al and MnS act as a starting point of crack generation when a torsion stress is applied, and MnS particularly deteriorates the torsion characteristics. Based on this finding, it was concluded that it is important to reduce the content of Mn, S, C, N, O, etc., which are the constituent elements of inclusions. Furthermore, Ti, Al, Nb
It was found that by appropriately adjusting the balance between the content of Si and the content of Si, high strength is not impaired, and excellent torsional properties, which cannot be obtained with conventional steel, can be obtained.

The alloy components and contents contained in the precipitation hardening type martensitic stainless steel of the present invention will be described below. C: 0.030 wt% or less It is an important element for increasing the strength of steel and suppressing the δ ferrite phase generated at high temperature. However, a large amount of C
When a large amount of austenite is contained, a large amount of austenite remains after the solution treatment or the annealing. A part of the retained austenite is transformed into martensite in temper rolling, but a relatively large amount remains after temper rolling, and it becomes difficult to obtain high strength after aging treatment. Also, if a large amount of C is contained, TiC,
It promotes the formation of carbide inclusions such as NbC, resulting in a decrease in torsional properties. Therefore, in the present invention, the upper limit of the C content is set to 0.030% by weight. Si: 0.5 to 2.0% by weight It has a large solid solution strengthening ability and acts to strengthen the matrix. Further, when Ti, Nb or Al and Ni are added in combination, the intermetallic compound consisting of these alloy elements is finely coherently precipitated after the aging treatment to increase the strength of the steel. Such an effect becomes remarkable when the Si content is 0.5% by weight or more. However, when a large amount of Si exceeding 2.0% by weight is contained, the formation of the δ ferrite phase is promoted, and the strength and the twisting property are deteriorated.

Mn: 0.40% by weight or less It acts to suppress the formation of the δ ferrite phase in a high temperature range. However, when a large amount of Mn is contained in excess of 0.40% by weight, the formation of MnS is promoted. MnS becomes a starting point of crack generation when a torsional stress is applied, and remarkably deteriorates the torsional characteristics. Therefore, the lower the Mn content is, the more desirable it is, and the upper limit is set to 0.40% by weight. S: 0.0020% by weight or less It exists in steel as a non-metallic inclusion such as MnS and deteriorates the twisting property. It also has a bad influence on fatigue strength, corrosion resistance, hot workability and the like. Therefore, the lower the S content, the more desirable, and the upper limit was set to 0.0020% by weight.

Ni: 7.5 to 9.5% by weight It is an alloy element effective for precipitation hardening, and suppresses the formation of δ ferrite phase after solution treatment or annealing. The lower limit of the Ni content was set to 7.5% by weight in order to suppress the formation of the δ ferrite phase and maintain high strength and excellent torsional properties. However, when a large amount of Ni exceeding 9.5% by weight is contained, a large amount of austenite remains after the solution treatment or annealing and it becomes difficult to obtain high strength. Cr: 12.0 to 16.0 wt% In order to secure the corrosion resistance as stainless steel, a Cr content of at least 12.0 wt% or more is required. However, when a large amount of Cr exceeding 16.0 wt% is included,
A large amount of δ ferrite phase and retained austenite phase are generated, which causes a decrease in strength. Cu: 0.30 wt% or less In the alloy system of the present invention, high strength can be obtained without depending on the precipitation strengthening action of Cu. Therefore, when the strength is increased, it is possible to suppress the content of Cu that deteriorates the torsional characteristics after aging treatment at about 450 ° C. to be low. Further, the addition of a large amount of Cu deteriorates the hot workability and causes the generation of surface cracks. Therefore, in the present invention, Cu
The upper limit of the content is specified as 0.30% by weight.

N: 0.015% by weight or less and O: 0.0
In the steel containing Ti, Nb, Al, etc. as in the present invention, if the content of N and O is high, nitrides and oxides of these alloying elements are generated, and the torsional characteristics and fatigue characteristics are It will cause worse. In this respect, the lower the N content and the lower the O content, the more preferable. In the present invention, the respective upper limits are set to 0.015% by weight and 0.005% by weight. Ti: 0.35 wt% or less It is an alloy element effective for precipitation hardening, and the higher the Ti content is, the more desirable it is in order to secure high strength. However, when Ti is contained alone, when the Ti content exceeds 0.35% by weight, although Ti increases in strength after aging treatment,
The distribution amount of non-metallic inclusions such as C, TiN, and TiO increases, and as a result, the torsional characteristics tend to deteriorate.

Nb: 0.40 wt% or less Like Ti, it is an alloying element effective for precipitation hardening. However, when added alone, a large amount of N exceeding 0.40% by weight
When b is included, the strength is excessively increased and N
The amount of carbonitrides such as bC and NbN produced increases, and the torsional characteristics deteriorate. Al: 0.30 wt% or less Ti is an alloying element used as a deoxidizer, and Ti,
Like Nb, it also works effectively for precipitation hardening. However, when a large amount of Al exceeding 0.30 wt% is included alone, Al
The distribution of non-metallic inclusions such as N increases, and the torsional characteristics are likely to deteriorate. Mo: 3.0 wt% or less In the present invention, high strength and excellent torsional properties can be obtained without adding Mo, but the addition of Mo further improves the properties. When Mo is added, the effect of addition can be seen by adding 1.0 wt% or more of Mo. However, even if a large amount of Mo exceeding 3.0% by weight is included, it is not possible to obtain an improvement in strength and torsional properties commensurate with an increase in Mo content. Moreover, a large amount of Mo content promotes the formation of the δ ferrite phase, which causes the deterioration of strength and torsional properties.

H value: 0.20 to 0.70 One, two or more kinds of Ti, Nb and Al are added within the above range of contents. At that time, the H value defined by the equation (1) is adjusted to be in the range of 0.20 to 0.70. H = Si% × (Ti% + 0.8 × Nb% + Al%) (1) The H value represented by the formula (1) is necessary to maintain high strength and excellent torsional characteristics. It is an index showing the balance among Ti, Nb and Al. If the H value does not reach 0.20, T
Precipitation hardening due to addition of i, Nb, Al, etc. is insufficient and high strength cannot be obtained. However, if the balance among Ti, Nb, and Al is designed so that the H value exceeds 0.70, the torsion characteristics deteriorate, although the detailed reason is unknown.

I value: 4.0 × 10 ⁻⁴ or less In the present invention, in addition to controlling the Mn content and the S content as described above, the I value defined by the formula (2) is ⁴ or less. It is adjusted so that it will be 0.0 × 10 ⁻⁴ or less. I = Mn% × S% (2) The I value represented by the formula (2) indicates the balance between Mn and S required to maintain excellent torsional properties. If a large amount of Mn or S is contained so that the I value exceeds 4.0 × 10 ⁻⁴ , the distribution amount of the non-metallic inclusions MnS becomes large and the torsional characteristics are remarkably deteriorated.

Cleanliness: d _{60x400 ≤0.010} % The torsional characteristics of precipitation hardening type martensitic stainless steel are Ti, Nb, which become crack _{initiation points} when a torsional stress is applied,
It is greatly affected by the distribution amount of carbonitride oxides such as Al and MnS. Above all, the adverse effect of MnS on the torsional properties is significant. It is important to reduce these non-metallic inclusions in order to obtain excellent torsion properties. The distribution of non-metallic inclusions can be indexed by the cleanliness d _60x400 specified in JIS G0555. Steel having a cleanliness d _60x400 of more than 0.010% has a low twist angle and does not exhibit good twist characteristics. Cleanliness d _60x400 ≦ 0.010%
Is achieved by reducing the elements such as C, N and O including the Mn and S contents.

The balance of the martensitic stainless steel according to the present invention is basically Fe. However, it may contain Ca or a rare earth metal for the purpose of desulfurization, and 0.01 wt% or less of B added to improve hot workability. The martensitic stainless steel according to the present invention is aged after solution heat treatment or annealing and appropriate temper rolling. As the aging treatment, heat treatment for heating for 10 minutes or more in a temperature range of 425 to 500 ° C. which is generally performed for precipitation hardening steel is adopted. By this aging treatment, high strength is exhibited and a material excellent in torsional properties is obtained.

[0015]

Example With respect to each stainless steel having the components shown in Tables 1 and 2, a hot rolled sheet having a thickness of 4 mm was manufactured from a steel ingot of 100 kg through hot rolling. In Table 1, group A indicates steel according to the invention. On the other hand, Group B in Table 2 is a comparative steel, and contains C, Mn, S, Ni, C.
Any of the contents of alloying elements such as u and Ti, the H value, and the I value are out of the ranges specified in the present invention. Group C indicates conventional steel. All of A to C groups are Al
The O content in the steel is reduced by deoxidation, and at least 0.02 wt% Al is contained.

[0016]

[Table 1]

[0017]

[Table 2]

Cold rolling with a rolling ratio of 40% and 1
Annealing was performed by heating at 030 ° C. for 60 seconds, and further, temper rolling was performed at 15% to form a steel strip having a plate thickness of 1 mm. For each stainless steel strip, the mechanical properties such as the temper rolling condition and the hardness and torsional properties after aging treatment were investigated. For the evaluation of the twist characteristics, a test piece having a shape shown in FIG. 1 prepared by punching from a steel strip after temper rolling was used. The test piece is
The width of the grip portions A and B is 8 mm, and the width of the plate width minimum portion C is 1.
2 mm, and the radius of curvature of the plate width minimum portion C was set to 10 mm. The test piece was aged, then barrel-polished, and then subjected to a torsion test in which one of the grip portions A and B was fixed and the other was rotated. The rotation angle at which the torsional load was maximized was taken as the twisting angle, and the twisting characteristic was evaluated by this value. Table 1
And for the test pieces made from the steels shown in Table 2, 4
Tables 3 and 4 show the hardness, twist angle, etc. after the aging treatment at 50 ° C. for 1 hour soaking, together with the hardness of the temper-rolled material. As shown in Table 3, each of the group A steels according to the present invention had a hardness substantially equal to that of the conventional precipitation hardening steel before the aging treatment. This is
It is shown that various processing such as punching can be performed under the same processing conditions as the processing of conventional martensitic steel.

[0019]

[Table 3]

[0020]

[Table 4]

For some steels, the twist angles and hardnesses after aging treatment at 450 ° C. shown in Tables 3 and 4 are Si, Ti,
When arranged by the H value with Nb and Al as factors, FIG.
The relationship shown in was established. The Group A steels according to the invention had high levels of torsion and hardness. On the other hand, the comparative steel B1 having an H value of less than 0.20 had insufficient hardness, and the comparative steel B3 having an H value of more than 0.70 exhibited a low twist angle. From this, it was confirmed that the H value needs to be maintained in the range of 0.20 to 0.70 in order to obtain high strength and excellent torsion characteristics. In the steel material containing Ti according to the present invention, Ni is mainly used during the aging treatment.
An intermetallic compound represented by ₁₆ Ti ₆ Si ₇ is precipitated, and the precipitation hardens the steel material. Part or all of Ti is Nb
Alternatively, Al may be substituted, and in this case as well, high age hardening is obtained. However, in order to obtain high hardness and excellent torsional characteristics, alloy components such as Ti, Nb, and Al should be contained in a good balance, and as shown in FIG. 2, the H value should be in the range of 0.20 to 0.70. It is necessary to maintain. For example, in the comparative steel B2, the H value is 0.20 to 0.
Although it is in the range of 70, the twist angle is small because the Ti content exceeds the range specified in the present invention.

Similarly, the twist angle after aging treatment at 450 ° C. is M
When arranged by the n content and the S content, the relationship shown in FIG. 3 was established. The Mn content, the S content and the I value within the ranges specified in the present invention are shown by the shaded areas. All the steels of Group A in the shaded area exhibited a large twist angle and were excellent in twisting characteristics. S content is 0.
The comparative steels B4 having a Mn content of more than 0020 wt.
Comparative steel B5 exceeding 40% by weight and I value of 4.0 × 10
^The comparative steels B6 exceeding ^-4 all had a low twist angle of less than 80. Further, the comparative steel B10 has a Mn content,
Although the S content and I value are within the ranges specified in the present invention, the O content is the upper limit 0.005% by weight specified in the present invention.
Therefore, a sufficient twist angle could not be obtained. Similarly, when the twist angle after 450 ° C. aging treatment was arranged by the cleanliness of MnS, it was found that there was the relationship shown in FIG. The cleanliness of MnS was measured according to JIS G0555, and the cleanliness when the number of measurement fields of view was 60 and the magnification was 400 times was calculated and expressed as d _60X400 . As shown in FIG. 4, the twist angle was extremely changed at the boundary of d _60X400 = 0.010%. That is, the twist angle exceeds 90 in the group A steel having high cleanliness, whereas it does not reach 80 in the group B steel having low cleanliness.

Mn and S form non-metallic inclusions of MnS in steel. When the distribution amount of non-metallic inclusions is large, it acts as a starting point of crack generation when torsional stress is applied, and deteriorates torsional characteristics. In this respect, it is generally necessary to reduce the Mn content and the S content as much as possible. If either one of the Mn content and the S content is low, but the other is high, M
The distribution amount of nS increases, and the torsional characteristics deteriorate. In this respect, according to the present invention, the upper limits of the Mn content and the S content are regulated, and the I value represented by the product of the both is 4.0.
By controlling to be not more than × 10 ^-4 , the generation of MnS is suppressed, and excellent torsional characteristics are obtained as shown in FIG.
Moreover, since the O content is 0.005% by weight or less, the distribution amount of oxides such as Ti, Nb, and Al is suppressed, and a high twist angle is secured.

When the relationship between the twist angle and the hardness after 450 ° C. aging treatment was investigated, the relationship shown in FIG. 5 was established. It can be seen from FIG. 5 that each of the group A steels according to the present invention has the same high strength as that of the comparative steel and has a high twist angle. Even from this, it can be said that in order to obtain high strength and excellent torsional characteristics, it is necessary to adjust the content of each alloying element and the balance between alloying components within the range specified in the present invention. For example, in Comparative Steel B7, the C content is the upper limit 0.03 specified in the present invention.
Since it exceeds 0% by weight, the production of TiC is promoted and the twist angle is reduced. Comparative steel B8 having a Cu content exceeding the upper limit of 0.30% by weight specified in the present invention
However, the twist angle is small. This indicates that Cu precipitates as a Cu-rich phase effective for age hardening during the aging treatment, but addition of Cu is not preferable in terms of torsional characteristics. In Comparative Steel B9, the Ni content does not reach the lower limit of 7.5% by weight specified in the present invention, and similarly, the twist angle is small. Ni
The reason why the twist angle changes according to the change in the content is unknown, but in order to obtain an excellent twist angle, it is necessary to increase the Ni content to the range specified in the present invention.

[0025]

As described above, in the present invention, C, Si, Mn, S, Ni, Cr, Cu, Ti,
While adjusting the components such as N, Nb, Al, Mo, O,
By optimizing the component balance between Si, Ti, Nb, and Al and the component balance between Mn and S, a steel material that has high strength after aging treatment and has further improved torsional properties than before Become. The obtained precipitation hardening stainless steel is used in various fields as a spring that requires strength equivalent to that of conventional steel and further requires higher torsional properties. Moreover, since it has good moldability before age hardening, it can be molded into an appropriate shape by punching or the like.

[Brief description of drawings]

[Fig. 1] Test piece used to measure the twist angle

FIG. 2 Torsion angle and hardness and H after aging treatment at 450 ° C.
Graph showing the relationship with values

FIG. 3 is a graph showing the relationship between the twist angle after 450 ° C. aging treatment and the Mn content and S content.

FIG. 4 is a graph showing the relationship between the twist angle and the cleanliness of MnS after aging treatment at 450 ° C.

FIG. 5 is a graph showing the relationship between the torsion angle and the hardness of MnS after aging treatment at 450 ° C.

[Explanation of symbols]

A, B: Grasping part C: Minimum plate width part

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. C: 0.030% by weight or less, Si: 0.
5 to 2.0 wt%, Mn: 0.40 wt% or less, S:
0.0020% by weight or less, Ni: 7.5 to 9.5% by weight, Cr: 12.0 to 16.0% by weight, Cu: 0.30
Wt% or less, N: 0.015 wt% or less and O: 0.0
Included in an amount of not more than 05% by weight, and further including one or more of Ti: not more than 0.35% by weight, Nb: not more than 0.40% by weight and Al: not more than 0.30% by weight, defined by the formula (1) The H value is in the range of 0.20 to 0.70, and the strength and twist characteristics are adjusted so that the I value defined by the formula (2) is 4.0 × 10 ⁻⁴ or less. Precipitation hardening stainless steel for springs. H = Si% × (Ti% + 0.8 × Nb% + Al%) (1) I = Mn% × S% (2) 2. The precipitation hardening stainless steel for springs according to claim 1, which has a cleanliness d _60x400 specified by JIS G0555 of 0.010% or less. 3. The precipitation hardening type martensitic stainless steel for springs according to claim 1, further containing 3.0% by weight or less of Mo.