JP3172561B2 - Manufacturing method of composite structure stainless steel spring - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a spring made of stainless steel substantially having a mixed structure of ferrite and martensite.

[0002]

2. Description of the Related Art Conventionally, materials for stainless steel springs have been:
For example, according to JIS G4313, as austenitic SU
S301-CSP and SUS304-CSP, SU as martensite
Four types of S420J2-CSP and SUS631-CSP are defined as precipitation hardening systems.

Austenitic SUS301-CSP and SU
All of S304-CSP have increased strength by work hardening by cold rolling. Depending on the degree of temper rolling (rolling ratio), four types of SUS301-CSP and three types of hardness (strength of SUS304-CSP) ) Level stipulated. Austenitic spring steel strips are shipped from a material maker in a cold-rolled state, processed into a desired spring shape by a processing maker, and then heated at about 400 ° C for improvement of spring characteristics. Aging treatment for about an hour is performed.

[0004] Martensitic SUS420J2-CSP is used to increase the hardness (strength) by quenching and tempering to obtain spring characteristics. However, from a material manufacturer, after cold rolling and annealing, After being shipped and processed into a spring of the desired shape by a processing manufacturer, it is often subjected to quenching and tempering.

[0005] Precipitation hardening type SUS631-CSP, except for SUS631-CSP-0 which has been subjected to solution heat treatment, is shipped from the material maker in the cold rolled state like the austenitic type, and is processed by the processing maker. After processing into a spring shape,
In either case, a precipitation hardening heat treatment is performed for the purpose of improving the spring characteristics. In the precipitation hardening system, SUS specified in JIS
In addition to 631-CSP, various stainless steel strips for springs have been commercialized.

[0006] Apart from these spring stainless steels, the present applicant has applied a short-time continuous aging treatment of less than 10 minutes to a high-strength (high ductility) stainless steel strip having a ferrite-martensite composite structure. It has been proposed that a stainless steel strip having excellent characteristics can be obtained (Japanese Patent Laid-Open No. 3-56621).

[0007]

In austenitic and precipitation hardening stainless steel strips for springs, the hardness and spring limit values increase with an increase in temper rolling reduction. Also, the hardness and spring limit after aging treatment and precipitation hardening heat treatment after processing are higher for higher temper rolling reduction.

[0008] Therefore, in order to improve the spring characteristics, it is necessary to increase the temper rolling ratio, but when the temper rolling ratio is increased, the formability decreases. The degree of increase in the spring limit value due to cold rolling is greater in the 90 ^° direction (T direction) than the rolling direction (L direction) of the steel strip, as compared with the rolling direction (L direction). There is also a problem that the difference between the two becomes large and so-called anisotropy is large. For this reason, there is a problem that there is a restriction on a direction in which a work product as a spring from the steel strip is sampled, and that a molded spring cannot have isotropic characteristics.

[0009] When an ultrathin sheet having a thickness of about 0.3 mm or less and high spring characteristics are required, it is necessary to manufacture the ultrathin sheet by increasing the cold rolling reduction.
It is technically difficult to produce a wide ultra-thin steel strip with good shape by cold rolling with a material with large work hardening such as SUS304 and SUS631, and the cost is high due to the large amount of Ni. Therefore, it becomes an expensive spring.

[0010] Martensitic SUS420J2-CSP is made of Cr
In addition to its low corrosion resistance of 12.00 to 14.00% and insufficient corrosion resistance, it also contains 0.26 to 0.40% of C, resulting in poor toughness and poor productivity. Hardening and tempering heat treatments are required.

The precipitation hardening type stainless steel for springs is obtained by adding Al to metastable austenite and hardening by martensitic transformation and precipitation of a Ni-Al compound. The austenitic ground contains a small amount of ferrite. Since the distribution and size influence the characteristics, it is necessary to pay attention to the components, heat treatment balance, and manufacturing conditions. Since the manufacturing cost is high, an expensive spring is used.

On the other hand, the ferrite-martensitic duplex stainless steel strip which has been subjected to the short-time continuous aging treatment proposed in Japanese Patent Application Laid-Open No. 3-56621 has already been provided with excellent spring characteristics in the state of a steel sheet and has been processed. The properties are also good. However, it was found that the spring characteristics tended to decrease when processing strain was applied. Therefore, depending on the processing shape and the degree of processing when forming a spring product, the spring characteristics after the forming processing may have a result that is not as good as that of the material state. The present invention is directed to solving such a problem.

[0013]

According to the present invention, the weight%
, C: 0.01 to 0.15%, Cr: 10.0 to 20.0%, 0.10 to 4.0% Ni, 0.10 to 4.0% Mn, 0.
A cold rolled steel strip of stainless steel containing one or more of 10 to 4.0% of Cu in Fe is produced through a normal hot rolling process and a cold rolling process. A steel strip having a composite structure of ferrite and martensite is formed by a heat treatment of heating to the two-phase region temperature of ferrite and austenite and then rapidly cooling. When manufacturing a spring using this steel strip as a material, the steel strip is used as a material to form a desired spring shape. The present invention provides a method for producing a stainless steel spring having a composite structure, characterized by subjecting the processed product to aging treatment.

At this time, as the chemical composition value of the steel, the A value according to the following equation is in the range of 200 to 500 and the B value is 30 to 9
Adjust so that it is in the range of 0. A value = 2010 (% C) -25.8 (% Si) +28.3 (% Mn) -72.4 (% P) +101 (% Ni) -31.7 (% Cr) +1230 (% N) +43 (% Cu) -38.3 (% Mo) +570 B value = 470 (% N) +420 (% C) +23 (% Ni) +9 (% Cu) +7 (% Mn) -11.5 (% Cr) -11.5 (% Si) -12 (% Mo) ) -23 (% V) -47 (% Nb) -49 (% Ti) +189

[0015]

[Action] The applicant of the present invention has previously determined that the chromium-based stainless steel containing chromium as the main alloy component is appropriately cold-rolled by ordinary hot rolling, annealing, and cold rolling by appropriately controlling the steel composition. The steel strip or steel sheet is substantially subjected to a finishing heat treatment consisting of heating to the ferrite + austenite two-phase region and subsequent quenching instead of the conventional finish annealing (annealing) at the ferrite single-phase region temperature. It has been proposed that a high-ductility, high-strength stainless steel sheet or steel strip having a ferrite-martensite dual-phase structure and low in-plane anisotropy in strength and ductility can be obtained (JP-A-63-7338) JP-A-63-169330 to JP-A-63-169335)
.

The present invention has been further studied on the high-strength duplex stainless steel strip according to the above proposal, and has excellent formability by setting the martensite content to 30% or more and 90% or less (volume%). In addition, by applying aging treatment under appropriate conditions after forming, and more preferably by subjecting the stainless steel strip to light temper rolling and then forming and applying an appropriate aging treatment, the spring properties can be improved. The new knowledge that a stainless steel spring with a composite structure of ferrite and martensite with extremely excellent spring characteristics and small in-plane anisotropy of spring properties can be obtained, which can solve all the problems of the conventional stainless steel strip for springs mentioned above. I got it.

First, the chemical composition of the stainless steel according to the present invention will be described. Regarding the Cr content in the stainless steel to which the present invention is applied, at least 10.0% or more should be contained as a necessary minimum amount in order to maintain the corrosion resistance of the stainless steel, but if the Cr content is too high, the martensitic phase will be reduced. Ni, necessary to obtain high strength by producing
Since the amount of austenite forming elements such as Mn and Cu increases and the toughness decreases, the upper limit is 20.0%.

C is a powerful austenite-forming element, and is effective for increasing the amount of martensite and increasing the strength of the martensite phase and the ferrite phase by solid solution strengthening. It is also an important element in improving the spring limit value by aging treatment. To obtain these C effects, at least 0.01% or more is required. But C
If the chromium carbide is too high, the chromium carbide once dissolved during heating in the dual-phase heat treatment in which heating and rapid cooling to the ferrite + austenite two-phase region reprecipitates at the ferrite or austenite (martensite after cooling) grain boundaries during cooling, Cr deficiency layer is generated in the vicinity (so-called sensitization occurs)
Corrosion resistance is remarkably deteriorated. To do this,
Although it depends on the component balance depending on the addition amount of other elements such as Cr, Ni, Mn, and Cu, the content is preferably 0.15% or less at most.

Ni, Mn, and Cu are effective for suppressing the amount of C in order to avoid the above-described sensitization due to C and for obtaining a two-phase structure of a ferrite phase and an austenite phase at a high temperature as an austenite-forming element replacing C. Element. In addition, the amount of martensite after cooling (the amount of austenite at a high temperature) after cooling and the strength (hardness) increase with an increase in the amounts of Ni, Mn, and Cu. In order to obtain these effects, a certain amount or more is required in accordance with the amount of Cr and the amount of C, and it is necessary to contain at least 0.1% or more. However, if the content is too large, the formed martensite phase after the dual-phase treatment becomes too large, and in some cases, the strength is obtained as a 100% martensite phase, but the ductility is reduced.
It should be 4.0%.

The above component design is indispensable for achieving the object of the present invention, whereby the strength and the spring characteristics can be appropriately controlled in terms of the components.
In addition to the above, B, V, Nb, Ti are added for the purpose of improving various other properties, such as adding Mo for the purpose of improving corrosion resistance, or adding Y or REM from the viewpoint of improving oxidation resistance.
And the like, or the content thereof can be regulated. An example is shown in the examples below,
Up to 2.50%, Y up to 0.20%, REM up to 0.10%,
V up to 0.20%, B up to 0.0050%, Nb up to 0.50%,
Ti is preferably up to 0.50%.

In any case, the stainless steel to which the present invention is applied must be one whose components are adjusted to have a two-phase structure of a ferrite phase and an austenite phase at a high temperature.

The A value according to the above equation is an index of hardness, and it is necessary that the A value is at least 200 or more in order to obtain the characteristics as a stainless steel spring. However, if the A value is too high, there is a risk that it may be difficult to work into various spring shapes, and thus it is preferable to set the upper limit to 500.

The B value according to the above formula is an index of the amount of martensite, and is required to be at least 30% in order to maintain the properties of the martensite phase with the ferrite phase as a composite structure stainless steel spring. However, if the amount of martensite is too large, the balance between strength and toughness is impaired, which is a problem when processing into various spring shapes. Therefore, the B value is preferably in the range of 30 to 90%.

It is an absolute condition that the heating temperature for the dual phase heat treatment in the present invention is set to a two-phase temperature of a ferrite phase + austenitic phase. In a stainless steel in which the present invention can be advantageously performed, the lower limit temperature at which a two phase structure of a ferrite phase and an austenite phase is formed is generally in the range of 600 to 900 ° C.
On the other hand, the upper limit temperature is in the range of 1200 to 1450 ° C.

In the two-phase zone heating at the time of the heat treatment for forming a dual phase, an amount of an austenite phase is generated in a substantially equilibrium state in a short time, so that the heating time may be a short time of about 10 minutes or less. The fact that this short time is sufficient enables continuous heat treatment in terms of the actual operation of the method of the present invention, which is very advantageous in terms of production efficiency and production cost.

It is needless to say that the cooling rate at the time of the heat treatment for forming a dual phase needs to be a cooling rate sufficient to transform austenite at high temperature into martensite, but it is generally 1 to 1000 in actual operation. Cooling rate range of ° C / sec. In the cooling process after transformation to martensite, the cooling rate may be arbitrary.

Aging treatment after forming into a spring shape is an important point of the present invention, whereby the spring characteristics of the final product spring are improved. If the aging treatment is performed before the spring is formed, the spring characteristics may be inferior due to distortion due to processing.

The aging condition is not particularly limited,
If the temperature is less than 0 ° C, the spring characteristics are not sufficiently improved, and 650 ° C
If it exceeds 200, solid solution C precipitates as chromium carbide at the grain boundaries and in the grains, resulting in a decrease in corrosion resistance. Therefore, the heating temperature in the aging treatment is desirably 300 to 650 ° C.

The aging treatment time rapidly increases the spring limit value by heating for a short time of about 1 minute. The heating time is not particularly limited, but the effect of improving the spring limit value tends to be saturated even if it is too long. Therefore, the heating time is desirably 1 hour or less.

Hereinafter, the present invention will be further described with reference to examples to which the present invention is applied.

[0031]

EXAMPLE A slab was produced by melting steel having the chemical components shown in Table 1. Steel Nos. 1 to 10 are steels to be subjected to the present invention.
The sheet was annealed at 60 ° C. for 6 hours and furnace-cooled. Further, after pickling, cold-rolled steel strip with a thickness of 0.3 mm was cold-rolled to a thickness of 1.0 mm, subjected to intermediate annealing at 780 ° C for 1 minute, and air-cooled for intermediate pickling. And

This cold-rolled steel strip was subjected to continuous dual-phase heat treatment and temper rolling under the conditions shown in Table 2, and this was used as a raw material.
After forming into a spring specimen having the dimensions and shape shown in Fig. 1,
Batch aging treatment was performed under the conditions shown in Table 2.

Steel Nos. 11 and 12 were SUS30 as comparative materials, respectively.
1 and SUS304. These were hot-rolled into strips with a thickness of 3.6 mm by hot rolling, soaked at 1100 ° C for 1 minute, quenched, then pickled. Further, cold rolling, annealing at 1050 ° C. for 1 minute, soaking and quenching were repeated, and finally cold rolling was performed at a temper reduction ratio shown in Table 2 to obtain a cold-rolled steel strip having a thickness of 0.3 mm. Then, after forming into a spring shape shown in FIG. 1, a batch type aging treatment was performed under the conditions shown in Table 2.

Table 2 shows the hardness of each material after aging treatment, the spring limit value Kb, and the measured values of the permanent displacement of spring specimens after aging treatment. The spring limit value Kb is set to 90 when the longitudinal direction of the spring specimen is parallel to the rolling (L) direction.
^o Both were measured when the direction was (T). As for the amount of permanent displacement, as shown in Fig. 2, the bent part of the same test piece (a) as in Fig. 1, which was sampled so that the longitudinal direction was parallel to the rolling direction, was gripped with a fixing jig (b), and the initial position was determined. From (c) to the deformed position (d), a reciprocating deflection of 50 reciprocations in 15 seconds was applied (fixed end stress = 50 kgf / mm ² ), and the amount of deformation after 500 repetitive bendings (Δh = initial position height) h-The height h ') after the test was defined as the permanent displacement (mm).

[0035]

[Table 1]

[0036]

[Table 2]

As can be seen from the results in Table 2, the composite structure stainless steel according to the present invention has a high spring limit value Kb, a small difference between the Lb direction and the Kb direction in the T direction, and a small anisotropy in the spring characteristics. . The spring according to the method of the present invention also has a small amount of permanent displacement in repeated bending tests. Furthermore, as can be seen from the comparison between the inventive examples No. 2 and No. 3 and the comparison between No. 7 and No. 8, when the temper rolling is performed after the dual-phase heat treatment, the spring limit value after the aging treatment is increased. Kb is further improved, and the amount of permanent displacement is reduced by subjecting the molded product to aging treatment.

On the other hand, in Comparative Example No. 1, the heat treatment temperature for the formation of the dual phase was as low as 780 ° C., and the annealing was substantially performed in the single phase region of ferrite. Phase organization. Therefore hardness (strength)
And the spring limit value Kb after the aging treatment is also low.

Comparative Example No. 2 and Comparative Example No. 4 are double-phase structure steels but have not been subjected to aging treatment, so that they can be seen in comparison with Invention Example No. 2 and Invention Example No. 7, respectively. The spring limit value Kb is low.

Comparative Example No. 3 has a high temper reduction ratio of 15% after the heat treatment for forming a dual phase, and has a large spring anisotropy Kb but a large anisotropy.

In Comparative Examples No. 5 and No. 6, the aging treatment was performed after the heat treatment for forming the dual phase, and the aging treatment after performing the spring forming process was omitted. Is high, but the spring characteristic is inferior because the amount of permanent displacement after a repeated bending test after spring molding is large.

Comparative Examples No. 7 and Comparative Example No. 8 were SUS301-CSP and SUS30 stainless steels for austenitic springs.
4-CSP, with a large spring limit anisotropy,
The spring limit value Kb itself is low, and the spring characteristics are inferior to those of the present invention.

FIG. 3 is a diagram showing the steels of the present invention of steels 4, 5 and 6 shown in Table 1 and the comparative steels SUS301 and SUS304, which were spring-formed into the dimensions and shapes shown in FIG. After the aging treatment for 10 minutes, the fixed end stress 50
Number of tests performed repeatedly bending test as kgf / mm ² and shows the permanent displacement of the relationship after the test.

As can be seen from FIG. 3, the springs 4, 5, and 6 of the multi-structure stainless steel according to the present invention are different from the conventional springs of the comparative steels SUS301 and SUS304 at any of the number of times of the bending test. Small permanent displacement,
It can be seen that the spring characteristics are excellent.

[0045]

As described in detail above, according to the method of the present invention, it is possible to provide a high-strength composite stainless steel spring having excellent spring characteristics and small anisotropic spring characteristics. Compared to conventional stainless steel for springs, it has a small amount of permanent displacement in repeated bending tests after forming, has excellent properties as a spring, and can make a great contribution to various formed spring processed products.

[Brief description of the drawings]

FIG. 1 is a view showing the size and shape of a test piece of a spring-formed product used for obtaining a permanent displacement amount.

FIG. 2 is a diagram showing a repeated bending test method for obtaining a permanent displacement amount.

Fig. 3 shows the relationship between the number of repeated deflection tests of high-strength composite microstructure stainless steel and the permanent displacement after the test.
It is a figure shown by comparison with.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Hiroshi Fujimoto 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Prefecture Inside Nisshin Steel Co., Ltd. Steel Research Laboratory (56) References JP-A-3-56621 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/00-9/50 C22C 38/00-38/60

Claims (4)

(57) [Claims] (1) C: 0.01 to 0.15% by weight,
r: 10.0-20.0% Ni, 0.10-4.0% Ni,
A cold rolled steel strip of stainless steel containing one or more of 0.10 to 4.0% Mn and 0.10 to 4.0% Cu in Fe is produced through a normal hot rolling process and a cold rolling process. The cold-rolled steel strip is heated to a temperature in the two-phase region of ferrite and austenite, and then quenched to form a steel strip with a composite structure of ferrite and martensite. A method of manufacturing a stainless steel spring having a composite structure, characterized in that the material is processed into a desired spring shape using the material as a raw material, and the processed product is aged. 2. The chemical composition value of steel is as follows:
The method for producing a composite-structure stainless steel spring according to claim 1, wherein the B value is adjusted to be in the range of 00 to 500 and the B value is in the range of 30 to 90. A value = 2010 (% C) -25.8 (% Si) +28.3 (% Mn) -72.4 (% P) +101 (% Ni) -31.7 (% Cr) +1230 (% N) +43 (% Cu) -38.3 (% Mo) +570 B value = 470 (% N) +420 (% C) +23 (% Ni) +9 (% Cu) +7 (% Mn) -11.5 (% Cr) -11.5 (% Si) -12 (% Mo) ) -23 (% V) -47 (% Nb) -49 (% Ti) +189 3. The method according to claim 1, wherein the heat treatment is performed in a continuous heat treatment furnace that continuously passes the steel strip. 4. The steel strip according to claim 1, wherein the heat-treated steel strip having a composite structure of ferrite and martensite is subjected to temper rolling at a rolling reduction of 10% or less and then supplied to a spring material. A method for producing a composite structure stainless steel spring as described above.