JP3289947B2 - Manufacturing method of stainless steel for high strength spring with excellent stress corrosion cracking resistance used in hot water environment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel which is suitable for a member requiring high strength and stress corrosion cracking resistance, and the stainless steel obtained by the present invention is used in a stress corrosion cracking environment. Stainless steel spring material used, for example, used in oil boiler type water heaters and electric water heaters
Root, etc. is a suitable material.

[0002]

2. Description of the Related Art Conventionally, as a material for a high-strength spring, SUS301 steel, which is a metastable austenitic stainless steel, has been mainly used. SUS301 series steel has been used as a material for various springs because it can easily obtain strength when subjected to cold working of 40% or more, and can obtain high strength by aging at 400 ° C or more.

[0003] When SUS301 stainless steel is applied to these applications, it is usual to make a leaf spring shape from a thin plate having a plate thickness of about 0.1 to 1.0 mm or to form the plate. Generally, this kind of high-strength material cracks in an environment where stress corrosion cracking occurs, and in such a case, a material having low strength has been applied.

[0004]

In the field of spring material used in the stress corrosion cracking environment [0005], for example petroleum boiler type water heater, there is a Do etc. It if support for use in an electric water heater or the like.

[0005] As described above, SUS301 stainless steel cannot be used in this field in a high strength state because a high strength as described above significantly reduces the stress corrosion cracking resistance after aging treatment. For this reason, the desired spring characteristics and strength could not be obtained, and it was necessary to increase the thickness of the plate or form it into a complicated shape, resulting in a heavy burden on the design of the spring member.

The present invention solves such a problem and provides a hot water ring.
In spring components used in stress corrosive environment at a boundary,
An object is to obtain a material having high strength and excellent stress corrosion cracking resistance.

[0007]

According to the present invention, the weight%
C: 0.03% or less, Si: 1.0% or less, Mn: 2.5% or less, Ni: 4.0 to 10.0% Cr: 13.0 to 20.0% N: 0.06 to 0.30% S: 0.01% or less , Mo: 1.0 to 3.0%, Cu: 0.
5 to 3% of one or two kinds, and in some cases, one or two of Ti, Nb, V in the range of 0.1 to 3.0%, and M = 330− (480 × C%) -(2 x Si%)-(10 x Mn%)-(14 x Ni%)-(5.7 x Cr%)-(5 x Mo%)-(14 x Cu%)-(320 x N%) The amount of each component is adjusted so that the M value according to the formula is 30 or more, the stainless steel strip or steel sheet whose balance is Fe and unavoidable impurities is annealed, and the cold rolling is performed with a rolling reduction exceeding 25%. And then before or after molding
Provided is a method for manufacturing a high-strength spring stainless steel excellent in stress corrosion cracking resistance used in a hot water environment, which is subjected to aging treatment in a temperature range of 100 ° C. or more and less than 300 ° C. for 10 minutes or more.

[0008]

The present inventors have conducted various tests and researches in order to achieve the above object. As a result, it has been found that it is possible to provide higher strength and stress corrosion cracking resistance while maintaining the same spring characteristics as SUS301 and other characteristics required for stainless steel materials. In particular, the stress corrosion cracking characteristics were found to be more affected by the aging temperature than the strength, and in order to achieve high strength and superior stress corrosion cracking characteristics compared to conventional materials, a lower C than SUS301 was used. It turned out to be desirable.

More specifically, the C is lower than that of SUS301,
The decrease in work hardening caused by this is compensated for by N and N
The increase in the age hardening degree due to the addition is effectively utilized. As a result, high strength can be achieved even at low temperature aging, and a material excellent in stress corrosion cracking resistance is obtained by aging at a temperature of 100 ° C or more and less than 300 ° C. In addition, this prevents cracks from occurring in the processed part formed into the spring component, thereby preventing deterioration of the function as the spring component and improving the service life.

Further, by adding Ti, Nb, V in an appropriate amount in addition to N, an increase in strength due to aging is exhibited, and C
The decrease in strength due to the decrease can be more effectively compensated.
The addition of Mo and Cu further improves the stress corrosion cracking resistance.

In the following, the action of each component and the reasons for limiting the component range in the steel of the present invention will be individually described.

C is an austenite-forming element, which suppresses δ-ferrite generated at high temperature and cold-working (cold-rolling from an annealed state exhibiting an austenite phase. Cold rolling) is effective in strengthening the martensitic phase induced by
If the C content is too high, the stress corrosion cracking resistance when the strength is increased becomes inferior. For this reason, C is set to 0.03% or less.

[0013] Si is effective as a deoxidizing agent. However
Even if added in an amount exceeding 1.0%, the effect as a deoxidizer is
Similar to the case of 1.0%, but rather increases the cost, so the upper limit was set to 1.0%. The lower limit is not particularly limited, but is preferably 0.2% or more from the viewpoint of the deoxidizing effect.

Mn works effectively as a deoxidizing agent, but is an element that controls the stability of the austenite phase, and its utilization is considered in balance with other elements. In the present invention, utilization at an Mn amount of up to 2.5% is achieved.

[0015] Cr is an essential component for ensuring the corrosion resistance of the spring component. To provide the intended corrosion resistance, at least 13.0% or more of Cr is required. However,
Since Cr is also a ferrite-forming element, if it is too high, a large amount of δ-ferrite is generated at a high temperature. Therefore, it is necessary to add an austenite-forming element (C, N, Ni, Mn, Cu, etc.) in an appropriate amount to suppress the δ-ferrite phase. The austenite phase is stable and high strength cannot be obtained after cold working or aging. Therefore, the upper limit of Cr is set to 20%.

Ni is an essential component for obtaining an austenite phase at high temperature and room temperature. In the case of the steel of the present invention, a metastable austenite phase must be formed at room temperature, and in order to obtain better formability, an appropriate martensitic phase must be induced at a low cold working ratio to obtain high strength.
When Ni is lower than 4.0% in the steel of the present invention, a δ ferrite phase may be formed in the austenite phase at a high temperature and a quenched martensite phase may be considerably formed at room temperature. In this case, the origin of the work-induced martensite may be reduced. The ratio of the resulting austenite phase is insufficient. On the other hand, Ni is 1
If it exceeds 0%, it becomes difficult to induce a martensite phase by cold working. For this reason, the Ni content was set to 4.0 to 10.0%. More preferably, it is 5.0 to 8.0%.

Mo increases the base hardness of the steel and the hardness after the aging treatment, and thus effectively acts to obtain high strength. It is also an effective element for improving stress corrosion cracking resistance. However, since it is a ferrite former, if it is added in a large amount, it will crystallize out the δ ferrite phase, which may cause a decrease in strength.
And

[0018] Cu effectively acts to improve the stress corrosion cracking resistance, but if it is too small, its effect is small, and if it is too large, hot workability is hindered and causes cracking. Therefore 0.
5 to 3.0%.

Ti, Nb, and V effectively act to increase the hardness after the aging treatment. To achieve this effect
Requires addition of 0.1% or more. However, if added more than necessary, a large amount of non-metallic inclusions are formed and the fatigue strength decreases,
1.0% of each upper limit because it leads to deterioration of surface cleaning
And

N, like C, is an austenite-forming element and an element effective for hardening the austenite phase and the martensite phase. Further, since precipitates are less likely to be formed than C, it is effective from the viewpoint of durability. Therefore, at least 0.06% is added instead of C. However, if it is added in a large amount, it causes blowholes, so 0.30%
It was as follows. More preferably, it is 0.06 to 0.20%.

S forms MnS in coexistence with Mn, and causes deterioration in workability such as ductility and bending.
0.010% or less. It is preferable that Mn and S be lower in a region where the forming process is severe in a thin plate.
The S content is preferably less than 0.5% and the S content is preferably 0.004% or less.

For M value: 30 or more, C, Si, Mn, Ni, C
Each of r, Mo, Cu and N is contained in the above range, but each component is adjusted so that the M value according to the following equation (1) is 30 or more. M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − − (5.7 × Cr%) − (5 × Mo%) − (14 × (Cu%)-(320 × N%) (1) The constants of each component in the equation (1) were determined experimentally during the development of the steel of the present invention. This M value is an index of austenite stability. If the M value is less than 30, it is necessary to apply a strong reduction of 70% or more at room temperature to obtain high strength after cold rolling or aging treatment. In this case, the ductility decreases. Therefore, M
Set the value to 30 or more.

In the steel of the present invention, the chemical composition is adjusted within the above range. However, in addition to these components, in addition to Ca and REM added as a desulfurizing agent and deoxidizing agent, B (0.01% or less) that has the effect of improving hot workability, etc., it may contain impurities that are inevitably mixed. it can.

[0024] The steel according to the present invention adjusted to the above-mentioned range exhibits a substantially austenitic structure in a solution-treated state.

In the present invention, an annealed steel strip or steel sheet is produced through ordinary hot rolling and cold rolling.
Then, cold rolling is performed on a steel strip or a steel sheet having an austenitic structure at a rolling rate of more than 25%.
Aging treatment is performed at a temperature lower than ℃. The operation of the manufacturing conditions will be described below.

As described above, SUS301 has poor stress corrosion cracking characteristics when the strength is high. On the other hand, the steel of the present invention has high strength and good stress corrosion cracking resistance according to this manufacturing method. As stainless steel for high-strength springs,
Higher strength is desirable, but here 140kg / mm
Targeted at ² or more. In the steel of the present invention, the target can be achieved by performing aging treatment for 10 minutes or more in a temperature range of 100 ° C or more and less than 300 ° C after performing the final cold rolling at a rolling reduction of more than 25%.

The reason why the rolling reduction is limited to a value exceeding 25% is that the above-mentioned strength cannot be secured by rolling at a lower rolling ratio. The upper limit of the final rolling reduction is not particularly limited, but is preferably 70% or less from the viewpoint of formability.

In order to obtain the strength characteristics of stainless steel for high-strength springs, aging treatment is performed. In the case of the steel of the present invention, the temperature condition is a temperature range of 100 ° C. or more and less than 300 ° C. At a temperature lower than 100 ° C., it takes a long time to obtain the target strength level, and it is not economical. In addition, the aging temperature
When the temperature exceeds 300 ° C, the stress corrosion cracking resistance is deteriorated, and it cannot be used especially in a high temperature corrosion environment. The aging time is set to 10 minutes or more because sufficient strength characteristics cannot be obtained if the time is shorter than 10 minutes. The upper limit of the processing time is not particularly limited. However, considering the manufacturing cost, 1
Around the time is preferred.

Hereinafter, the effects of the present invention will be specifically described with reference to typical examples.

[0030]

EXAMPLES The steels of the present invention (M1 to M10) having the components shown in Table 1 were used.
Conventional steel (A) is melted in a normal atmosphere melting furnace, subjected to hot rolling, cold rolled, annealed, and pickled, and the annealed material is subjected to final cold rolling at a rolling rate shown in Table 2. The thickness was set to 0.3 mm, and samples were taken from each steel plate. In addition, 25
Aging treatment was performed at 0 ° C for 60 minutes, and samples after the aging treatment were collected. The annealing before final cold rolling was performed at an annealing temperature of 10
Performed at 50 ° C. The rolling ratio of the final cold rolling is set so that the tensile strength of each steel is about 140 kg / mm ² or more.

The cold-rolled sample and the sample after the aging treatment were subjected to a tensile test. The sample after aging was subjected to a stress corrosion cracking resistance test. The results are shown in Table 2. In the stress corrosion cracking test, a specimen 0.3 mm thick x 10 mm wide x 70 mm long was sampled and subjected to a U-shaped bending constraint (surface stress 140 kg /
mm ² ), the specimen was immersed in an autoclave containing a solution having a chloride ion concentration of 100 ppm and kept at 200 ° C. to evaluate the time until breakage.

[0032]

[Table 1]

As can be seen from the results in Table 2, the stress corrosion cracking resistance of the conventional steel is poor. In particular, the higher the rolling ratio and the higher the strength, the worse the stress corrosion cracking resistance. In contrast, in the case of the present invention, for example, the conventional steel A-2
Even if the strength is the same or higher, the stress corrosion cracking resistance is much better than A-2. Moreover, even the steel of the present invention (M1-1) having a high strength equivalent to that of the conventional steel A-1 is superior to the low strength of the conventional steel in stress corrosion cracking resistance. That is, the steel of the present invention has excellent stress corrosion cracking resistance even at high strength. However, when the strength exceeded 170 kg / mm ² , the stress corrosion cracking resistance showed a slight decrease.

M3 of Mo added steel, M4 of Cu added steel
Furthermore, Mo and Cu composite-added steels M5 and M9 show superior stress corrosion cracking resistance as compared with M1, and the effect of adding these elements is recognized.

FIG. 1 shows the results of examining the change in the stress corrosion cracking resistance of the rolled materials M1-3, M5 and M9 when the aging treatment temperature was changed. For comparison, rolled materials of A-1 and A-2 are also shown.

As is clear from the results shown in FIG.
In -1 and A-2, there is no significant change in the stress corrosion cracking resistance due to the aging temperature, and the absolute value of the low strength A-2 is low although the stress corrosion cracking resistance is slightly better. On the other hand, in the steel of the present invention, it can be seen that the stress corrosion cracking resistance is rapidly improved at a temperature lower than the aging temperature of 300 ° C.

The stress corrosion cracking resistance of the steel M5 containing Mo and Cu is more rapidly improved at a temperature lower than 300 ° C. than that of M1-3. Further, it can be seen that the Mo, Cu, and Nb-added steel M9 exhibits the same stress corrosion cracking resistance as the Mo and Cu-added steel M5 despite its high strength.

[0038]

As described in detail above, according to the present invention,
The disadvantage of conventional stainless steel for springs (for example, SUS301), in which the stress corrosion cracking resistance decreases when high strength is maintained, has been eliminated, and a spring stainless steel that exhibits excellent stress corrosion cracking resistance while maintaining high strength has been developed. can get. Therefore, for spring components that are generally used under stress,
A material useful as a spring material used in a corrosive environment in a warm water environment can be provided, and the useful life of the spring in this field can be significantly improved.

Since the composition of the steel of the present invention is adjusted so as to exhibit a metastable austenite phase in a solid solution state, high strength and stress corrosion cracking resistance are achieved by a process of cold rolling the annealed material and aging. Can be applied and easy to manufacture. In addition, the aging treatment temperature may be low, which is advantageous in terms of manufacturability.

[Brief description of the drawings]

FIG. 1: Steels of the present invention M1-3 (square), M5 (square), M9
It is a figure which shows the relationship between the rupture time and the aging treatment temperature in the stress corrosion test of the (A mark) and the conventional steels A-1 (O mark) and A-2 (● mark).

FIG. 2 is a schematic cross-sectional view showing a method of applying stress to a test piece by U-shaped bending restraint for evaluating stress corrosion cracking resistance.

[Table 2]

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-206429 (JP, A) JP-A-63-206428 (JP, A) JP-A-51-141710 (JP, A) JP-A-2- 225646 (JP, A) JP-A-2-310339 (JP, A) JP-B-57-36340 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00-8 / 10 C21D 9/46-9/48 C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. In weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 2.5% or less, Ni: 4.0 to 10.0%, Cr: 13.0 to 20.0%, N: 0.06 to 0.30%, S: 0.01% or less, and M = 330-(480 x C%)-(2 x Si%)-(10 x Mn%)-(14 x Ni%)-(5.7 x Cr%)-(320 × N%) C, Si, Mn, Ni,
The stainless steel strip or steel sheet whose Cr and N contents are adjusted and the balance is Fe and unavoidable impurities is annealed, cold-rolled with a draft of more than 25%, and then 100 ° C before or after forming. 10 in the temperature range above 300 ° C
In a warm water environment characterized by aging treatment for more than a minute
A method for producing high-strength spring stainless steel with excellent stress corrosion cracking resistance. 2. The stainless steel according to claim 1, wherein Mo: 1.0 to 3.0%,
Cu: One or two of 0.5 to 3%, and M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − (5.7 3. Each component is adjusted so that the M value according to the formula of (× Cr%)-(5 × Mo%)-(14 × Cu%)-(320 × N%) becomes 30 or more. The manufacturing method as described. 3. The method according to claim 1, wherein the stainless steel contains one or two of Ti, Nb, and V in a total amount of 0.1 to 3.0%.