JP5100144B2 - Steel plate for spring, spring material using the same, and manufacturing method thereof - Google Patents

Description

  The present invention relates to a spring steel plate suitable for a spring material manufactured from a steel plate such as a metal retaining ring, a disc spring, and a wave washer, and a spring material using the same.

  Conventionally, as a spring material such as a metal retaining ring, a disc spring, and a wave washer, a special steel such as S65C plated with Ni or the like has been used. However, in recent years, there is a demand for a material that can omit the plating step because of cost reduction and environmental problems. Stainless steel materials such as SUS304 and SUS301 are used in applications that require corrosion resistance. However, since the work-hardened material is hard to impart springiness, it is difficult to There are many problems in mass production, such as large wear and poor dimensional accuracy.

  As the spring stainless steel, there are a work hardening type austenitic stainless steel represented by SUS301 and a precipitation hardening stainless steel represented by 17-7PH. These stainless steels for springs require a high degree of cold rolling to increase the hardness with an emphasis on spring characteristics, resulting in poor punchability and formability to the spring material. On the other hand, if emphasis is placed on the workability of the spring material, there is a restriction on the increase in strength, and the spring characteristics tend to be sacrificed. That is, it is not easy to achieve both the workability to the spring material and the spring characteristics with these stainless steel types.

  Patent Documents 1 and 2 describe aging precipitation type martensitic stainless steel. Since these steels have low hardness before aging treatment and are relatively excellent in toughness, punching workability and formability are improved as compared with general martensitic stainless steel types. However, since the matrix is a martensite phase, it is inferior in bending workability and is not satisfactory for use in applications such as metal retaining rings, disc springs, and wave washers.

  On the other hand, as shown in Patent Documents 3 to 5, an age-hardening steel type having a multiphase structure of “austenite phase + martensite phase” is developed instead of work hardening type austenitic steel type and martensitic steel type. Yes.

JP 60-36649 A JP-A-8-73931 JP 51-141710 A JP-A-56-77364 JP-A-5-271878

  Steels having a multiphase structure of “austenite phase + martensite phase” as shown in Patent Documents 3 to 5 have better workability than martensitic steel types. However, as a stainless steel material to be processed into a fastening part such as a metal retaining ring, a disc spring, and a wave washer, a material having an elongation of 15% or more is desired. If the workability is lower than this, the problem increases in mass production as described above. The steel material described in Patent Document 3 is obtained by increasing the strength level after the aging treatment by cold rolling at a high rolling rate of 80%, but the workability is sacrificed due to the high cold rolling rate. Yes. Since the steel materials described in Patent Documents 4 and 5 employ steel having a high C content, it is difficult to stably secure workability with an elongation of 5% or more before the aging treatment.

  In view of the present situation, the present invention provides an age-hardening steel having a multiphase structure of “austenite phase + martensite phase”, which has workability superior to that of conventional materials, and a steel plate thereof An object of the present invention is to provide a technique for significantly increasing the strength by using the material as a material.

In order to achieve the above object, in the present invention, in mass%, C: 0.03% or less, N: 0.03% or less, Si: 0.5-2.0%, Mn: 0.5-2 0.5%, Cu: 1.0 to 3.5%, Cr: 15 to 18%, Ni: 6 to 10%, Mo: 3.5% or less, preferably 0.04 to 3.5%, The composition is composed of the balance Fe and unavoidable impurities, and the composition is adjusted so that the Md value defined by the following formula (1) is -20 to 30, and the matrix is "austenitic phase + 20-50% by volume martense. Provided is a steel plate for a spring that exhibits a multiphase structure composed of a “site phase”, has a hardness of 400 HV or less, an elongation of 15% or more, and excellent workability and age hardening.
Md = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7Cr-18.5Mo (1)
The value of the content of the element expressed in mass% is substituted for the element symbol on the right side of the formula (1). The multi-phase structure is a cold-rolled processed structure with no Cu-rich phase precipitation.

Such a spring steel plate is subjected to finish annealing that also serves as a solution heat treatment for cooling to a temperature range of at least 200 ° C. after heating to 1050 to 1250 ° C., on a cold-rolled steel material having the above composition. A step of obtaining a steel sheet having an austenite phase structure free of Cu-rich phase precipitation,
A step of obtaining a steel sheet having a multi-phase structure in which the matrix is composed of “austenite phase + 20-50% by volume martensite phase” by subjecting the steel sheet of the austenite phase structure to temper rolling at a rolling rate of 30 to 75%;
Is obtained by a production process having

Also, plastic processing is performed using the spring steel plate as a raw material, and then aging treatment is performed under the condition that the aging temperature T (K) and the aging time t (h) in the temperature range of 400 to 500 ° C. satisfy the following expression (2). A method of manufacturing a spring material is provided.
14000 ≦ T (logt + 20) ≦ 15500 (2)
The spring material thus obtained has a martensite phase in which precipitates composed of a Cu-rich phase are dispersed, and the hardness measured on the surface of an arbitrary portion exhibits 400 HV or more. That is, the hardness of at least 400 HV or more is exhibited no matter what part of the spring material surface is measured.

  The Cu-rich phase here is a second phase containing 60 atomic% or more of Cu, and includes a so-called ε-Cu phase. Presence or absence of precipitation of the Cu rich phase can be determined by structure observation (for example, magnification of 10,000 times or more) using a transmission electron microscope (TEM).

  The spring steel plate of the present invention has high elongation and is excellent in workability to a spring material. In addition, sufficient strength (hardening) can be achieved by applying an appropriate aging treatment after processing. Coexistence of such excellent workability and high strength properties is difficult with conventional stainless steel. The present invention contributes to the spread of stainless steel especially for fastening parts such as metal retaining rings, disc springs, and wave washers.

<Chemical composition>
In the present invention, the chemical composition of steel is limited as follows. “%” In the chemical composition means “% by mass” unless otherwise specified.

[C: 0.03% or less]
C has the effect of hardening the work-induced martensite phase. For this reason, lowering C is advantageous for improving workability, but lowering C is disadvantageous for increasing the strength required for the spring material. In the present invention, as described later, excellent age-hardening characteristics due to Cu are utilized, so that it is not necessary to increase the C content in order to increase the strength. As a result of various studies, the C content is limited to 0.03% or less. It can also be limited to less than 0.03%. Such a reduction in the C content prevents hardening of the martensite phase before the aging treatment, and is advantageous for a decrease in corrosion resistance after the aging treatment.

[N: 0.03% or less]
N, like C, leads to hardening of the work-induced martensite phase. As a result of the study, in the present invention, the N content is limited to 0.03% or less.

  In FIG. 1, an example of the result of investigating the influence of C and N content on the elongation after temper rolling is shown. The composition of the steel used was Si = 1.5%, Mn = 2.0%, Ni = 7.0%, Cr = 16.5%, Cu = 2.0%, Mo = 0.1%. Various contents of C and N are changed, and the balance is Fe and inevitable impurities. The result of conducting a tensile test in the rolling direction and examining the elongation of a material obtained by subjecting a finish annealed material having a thickness of 2 mm to 70% temper rolling is plotted. It can be seen that good elongation of 5% or more is obtained in the range where both C and N are reduced to 0.03% or less.

[Si: 0.5 to 2.0%]
Si is effective as a deoxidizer. In addition, it effectively works to induce a martensite phase by cold working, and it is necessary to contain Si in order to adjust the Md point. Further, Si is effective for hardening by aging treatment. As a result of various studies, it is necessary to contain 0.5% or more of Si in order to fully exhibit the hardening action by the aging treatment. However, even if Si is added in a large amount, the curing action is saturated, which is uneconomical. On the other hand, the function as a deoxidizer and the function of adjusting the Md point can be sufficiently exerted in the Si content range of 0.5 to 2.0%. Accordingly, the Si content is defined as described above.

[Mn: 0.5 to 2.5%]
Mn is an element that greatly contributes to the stabilization of the austenite phase. In consideration of the balance with elements such as Si, 0.5% or more of Mn content is necessary for stabilizing the austenite phase. However, since excessive Mn content causes deterioration of the spring characteristics after aging, the upper limit of Mn content is limited to 2.5 %.

[Cu: 1.0-3.5%]
Cu precipitates in the martensite phase by aging treatment, and contributes to increasing the strength of the spring material. That is, Cu is solid-dissolved in the austenite phase by finish annealing, and the solid solubility limit is lowered in the martensite generated by the subsequent temper rolling, so that it precipitates as a Cu-rich phase in the martensite phase in the subsequent aging treatment. . This Cu-rich phase is a second phase containing 60 atomic% or more of Cu, and a so-called ε-Cu phase is representative. Precipitation of such a Cu-rich phase in the martensite phase results in a significant increase in strength. For that purpose, it is necessary to ensure a Cu content of 1.0% or more, and it is more preferable to set it to 1.5% or more. However, since excessive Cu content causes a decrease in hot workability, the upper limit of Cu content is limited to 3.5%.

[Cr: 15-18%]
Cr is an essential element for imparting the corrosion resistance required for stainless steel. As a result of various studies, it is desirable to secure a Cr content of 15% or more when considering the use of fastening parts such as metal retaining rings, disc springs, and wave washers. However, if the Cr content is increased, a δ ferrite phase is likely to be generated, which causes a decrease in hot workability, so the upper limit of the Cr content is limited to 18%.

[Ni: 6 to 10%]
Ni is an essential element for obtaining an austenite structure after finish annealing, and at least 6% or more is necessary. However, it is not necessary to add too much, and excessive addition is not preferable because it causes an increase in cost. As a result of various studies, the Ni content may be in the range of 10% or less.

[Mo: 3.5% or less]
Mo is an element that improves corrosion resistance in high Cr steel. The Mo content may be adjusted according to the required corrosion resistance level. Usually, it is preferable to secure a Mo content of 0.04% or more. When it is desired to obtain high corrosion resistance, the content may be 0.5% or more. However, even if Mo is contained excessively, the effect of improving the corrosion resistance is small and the cost is increased, so the Mo content is set to a range of 3.5% or less.

[(1) Formula]
The following formula (1) is an index indicating the stability of the austenite phase in the annealed state. That is, as the value of the formula (1) increases, the austenite phase becomes unstable, and a work-induced martensite phase is easily generated.
Md = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7Cr-18.5Mo (1)

  In the present invention, the content of each alloy element is adjusted so that the Md value is in the range of -20 to 30. If the Md value is less than −20, the austenite phase after finish annealing becomes too stable, and the temper rolling ratio is made considerably high in order to sufficiently secure the amount of work-induced martensite phase that greatly contributes to age hardening. Or rolling at a low temperature is required, and the load in rolling increases, resulting in a decrease in productivity. On the other hand, when the Md value exceeds 30 and the austenite phase becomes excessively unstable, the amount of work-induced martensite after temper rolling tends to vary due to variations in rolling rate and components in temper rolling. . As a result, the quality of the spring material after the aging treatment becomes difficult to stabilize.

  FIG. 2 shows an example of the results of investigating the effects of Md value and Cu content on elongation after temper rolling and hardness after aging treatment. The composition of the steel used is Ni = 7.0%, Cr = 16.0%, Mo = 0.1%, C, N, Si, Mn are within the range specified in the present invention, and the Cu content is variously changed. The balance is Fe and inevitable impurities. A material with 70% temper rolling on a finish annealed material with a thickness of 2 mm is subjected to a tensile test in the rolling direction to examine the elongation, and each temper rolled material is subjected to 450 ° C. × 1 h aging treatment and Vickers The results of examining the hardness are plotted. Only when the Md value is 30 or less and the Cu content is in the range of 1.0 to 3.5%, excellent workability as a spring steel plate (ie, temper rolled material) and spring material (ie, aging treatment material) It can be seen that high strength can be achieved at the same time.

<Metal structure of steel plate for spring>
The spring steel plate here is a material steel plate for processing into a spring material. The metal structure is a multiphase structure in which the matrix is composed of “austenite phase + 20-50% by volume martensite phase”. This martensite phase is a work-induced martensite phase formed by temper rolling from the austenite phase after finish annealing. When the amount of the martensite phase is less than 20% by volume, it is difficult to obtain a hardness of 400 HV or higher even when the Cu-rich phase is sufficiently precipitated in the martensite phase in the aging treatment performed after processing into the spring material. High strength tends to be insufficient. On the other hand, if the amount of the martensite phase becomes too large, the workability to the spring material decreases, so the amount of the martensite phase needs to be adjusted to 50% by volume or less at most, and 30% by volume or less. It is particularly preferred. Since this spring steel plate is subjected to a solution treatment by finish annealing, no precipitation of a Cu-rich phase is observed in the matrix. Moreover, since it is cold-tempered, it has a cold-rolled processed structure.

<Characteristics of spring steel plate>
This spring steel plate desirably has a hardness of 400 HV or less in order to ensure workability to the spring material. Further, it is desirable that the elongation when the tensile test is performed in a direction parallel to the rolling direction is 15% or more.

<Production method of spring steel plate>
The spring steel plate of the present invention can be manufactured, for example, by the following method.
First, a cold-rolled steel material whose components are adjusted to have the above chemical composition is prepared in accordance with a general stainless steel plate manufacturing process.

  Next, the cold-rolled material is subjected to finish annealing that also serves as a solution treatment. In the finish annealing, after heating to 1050 to 1250 ° C., water cooling is performed to a temperature range of at least 200 ° C. By adjusting to the above chemical composition, an annealed steel sheet exhibiting an austenite phase structure (recrystallized structure) without Cu-rich phase precipitation after this heat treatment is obtained.

  Thereafter, the annealed steel sheet is temper-rolled to produce a work-induced martensite phase of 20 to 50% by volume, preferably 20 to 30% by volume. At this time, it is preferable to set the temper rolling ratio in the range of 30 to 75%. Desirable characteristics are easily obtained in this range. More preferably, it is in the range of 30 to 70%. Since the amount of work-induced martensite phase depends on the rolling pass schedule and rolling temperature, depending on the composition and rolling mill used, by adopting appropriate rolling conditions within the above rolling ratio range, work-induced martensite The amount can be controlled. It is necessary to adjust the plate thickness after the cold rolling so that the final plate thickness (generally 2.0 to 0.1 mm) suitable for the target spring material is obtained by this temper rolling. In this way, a spring steel plate for processing into a spring material is obtained.

  Table 1 shows an example of the results of examining the influence of the work-induced martensite (α ′) amount of the tempered rolled material on the elongation / hardness of the tempered rolled material and the hardness / spring limit value of the aging treated material. The composition of the steel used is as follows: C = 0.020%, N = 0.011%, Si = 1.45%, Mn = 2.01%, Ni = 7.05%, Cr = 16.31%, Cu = 1.98%, Mo = 0.09%, the balance being Fe and inevitable impurities. Table 1 shows the structure and characteristics of materials that have been subjected to temper rolling at various rolling rates on a finish annealed material with a thickness of 1 mm, and the characteristics of materials that have been subjected to aging treatment at 450 ° C. for 1 h. Is shown.

  As can be seen from Table 1, when the amount of work-induced martensite (α ′) is less than 20% by volume, the spring limit value is low and the steel plate for this composition cannot be used as a spring steel plate. On the other hand, when the amount of work-induced martensite is 20% by volume or more, the spring limit value is rapidly improved. On the other hand, if the amount of work-induced martensite exceeds 50% by volume, the elongation is less than 5%, and the workability as a spring steel plate is insufficient. That is, by adjusting the amount of work-induced martensite in the range of 20 to 50% by volume, both excellent workability as a spring steel plate and high strength after aging can be achieved.

<Structure and characteristics of spring material>
Spring materials such as metal retaining rings, disc springs, wave washers and the like are constructed using the steel plate for springs of the present invention as a raw material. This spring material has a martensite phase in which precipitates composed of a Cu-rich phase are dispersed, and exhibits a hardness of 400 HV or more measured on the surface of an arbitrary part. That is, a part that has a hardness of 400 HV or more and is subjected to plastic deformation in a part that is hardly subjected to plastic deformation when processed into a spring material (a part in a structural state corresponding to the case of aging treatment of a spring steel plate as it is). Then, it has higher hardness. Further, the spring limit value (measured by a steel plate sample subjected to the same aging treatment) is 700 N / mm ² or more. Those exhibiting such characteristic values can be said to have good spring characteristics desired for spring materials such as metal retaining rings, disc springs, and wave washers.

<Method of manufacturing spring material>
In order to obtain a spring material having such characteristics, an aging treatment is performed after plastic working into a predetermined shape using the spring steel plate of the present invention as a material. However, the spring steel plate of the present invention does not employ a very high temper rolling ratio in order to ensure good workability. Therefore, it is important to narrow down the aging treatment conditions so that sufficient strength can be achieved in the aging treatment. As a result of various investigations, the aging temperature is set to a temperature range of 400 to 500 ° C., and it is good preferable that the aging temperature T (K) and the aging time t (h) is a condition that satisfies the following equation (2).
14000 ≦ T (logt + 20) ≦ 15500 (2)

  FIG. 3 shows an example of the results of examining the influence of aging conditions on the hardness after aging treatment. The composition of the steel used is as follows: C = 0.020%, N = 0.011%, Si = 1.45%, Mn = 2.01%, Ni = 7.05%, Cr = 16.31%, Cu = 1.98%, Mo = 0.09%, the balance being Fe and inevitable impurities. The finish annealed material having a thickness of 1 mm was subjected to temper rolling at 40%, and thereafter, the hardness of the material subjected to aging treatment under various conditions was plotted. Remarkable curing is recognized within the range satisfying the above expression (2), and further remarkable curing is recognized within the range satisfying the expression (2) ′.

  Steel shown in Table 2 was melted in a vacuum melting furnace, and the slab was hot forged to obtain a slab having a thickness of 40 mm. This slab was hot-rolled to obtain a hot-rolled steel sheet having a thickness of 3.6 mm, and a cold-rolled material having a thickness of 1 mm was obtained through annealing, pickling, and cold rolling. The cold-rolled material was subjected to finish annealing by heating to 1150 ° C. and then water cooling. At this time, the material was rapidly cooled to a temperature of 200 ° C. or less by water cooling. A temper rolled material (corresponding to a spring steel plate) was obtained by subjecting this finish annealed material to temper rolling of 40%. Thereafter, an aging treatment material (corresponding to a spring material) was obtained by subjecting the temper rolled material to an aging treatment at 450 ° C. × 1 h.

  With respect to the temper rolled material, the amount of work-induced martensite phase (α ′) was measured using a ferrite content meter. In addition, a tensile test in the direction parallel to the rolling direction was performed using a No. 13B test piece in accordance with JIS Z2241, and the elongation was measured. Further, the Vickers hardness of the plate surface (rolled surface) was measured with a test load of 98.07 N (HV10) in accordance with JIS Z2244.

About the aging treatment material, the Vickers hardness was measured by the method similar to the above. Further, a strip-shaped test piece (width 10 mm, length 200 mm) whose longitudinal direction is perpendicular to the rolling direction is taken from the aging-treated material, and the permanent deflection amount is set to 0. 0 by using a method according to JIS H3130. The spring limit value was determined by measuring the stress value at 1 mm.
These results are shown in Table 3.

As can be seen from Table 3, the tempered rolled material of the example of the present invention has a work-induced martensite phase (α ′) amount of 20-50% by volume, good elongation of 5% or more, and hardness of 400 HV or less. Presented. These are judged to have good workability to the spring material. Moreover, after the aging treatment, a hardness of 400 HV or higher and a favorable spring limit value of 700 N / mm ² or higher were exhibited. In addition, when the tempered rolled material of the example of the present invention was observed by TEM, a Cu rich phase was not observed.

  On the other hand, Comparative Example No. 21 has too high C content, No. 22 has too high N content, and No. 25 has too high Md value. Since it was hardened, it was inferior in workability. In No. 23, the Cu content was too low, and in No. 24, the Md value was too low.

The graph which showed the influence of C and N content which gives to the elongation after temper rolling. The graph which showed the influence of the Md value and the Cu content on the elongation after temper rolling and the hardness after aging treatment. The graph which shows the influence of the aging conditions on the hardness after an aging treatment.

Claims (8)

C: 0.03% or less, N: 0.03% or less, Si: 0.5-2.0%, Mn: 0.5-2.5%, Cu: 1.0-3. 5%, Cr: 15 to 18%, Ni: 6 to 10%, Mo: 3.5% or less, the balance being Fe and unavoidable impurities, Md value defined by the following formula (1) is − Workability and aging with composition adjusted to 20-30, matrix having a multiphase structure consisting of "austenite phase + 20-50% by volume martensite phase" and hardness of 400HV or less Spring steel plate with excellent curability.
Md = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7Cr-18.5Mo (1)   The steel plate for a spring according to claim 1, wherein the Mo content is 0.04 to 3.5%.   The steel sheet for a spring according to claim 1 or 2, wherein the multiphase structure is a cold-rolled processed structure free from precipitation of a Cu-rich phase, and the martensite phase in the matrix is a work-induced martensite phase.   The steel plate for a spring according to any one of claims 1 to 3, which exhibits an elongation of 5% or more. C: 0.03% or less, N: 0.03% or less, Si: 0.5-2.0%, Mn: 0.5-2.5%, Cu: 1.0-3. 5%, Cr: 15 to 18%, Ni: 6 to 10%, Mo: 3.5% or less, the balance being Fe and unavoidable impurities, Md value defined by the following formula (1) is − For steel cold-rolled material having a composition adjusted to 20-30, after heating to 1050-1250 ° C, finish annealing also serves as a solution treatment for water cooling to a temperature range of at least 200 ° C or less. A step of obtaining a steel sheet having an austenite phase structure without Cu-rich phase precipitation by applying,
A step of obtaining a steel sheet having a multi-phase structure in which the matrix is composed of “austenite phase + 20-50% by volume martensite phase” by subjecting the steel sheet of the austenite phase structure to temper rolling at a rolling rate of 30 to 75%;
The manufacturing method of the steel plate for springs in any one of Claims 1-4 which has these.
Md = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7Cr-18.5Mo (1)   The method for producing a spring steel plate according to claim 5, wherein the steel subjected to the finish annealing has a Mo content of 0.04 to 3.5%.   A spring material plastically processed using the spring steel plate according to any one of claims 1 to 4, having a martensite phase in which precipitates made of a Cu-rich phase are dispersed, and the surface of an arbitrary part A spring material having a measured hardness of 400 HV or more. The steel plate for spring according to any one of claims 1 to 4 is subjected to plastic working, and thereafter the aging temperature T (K) and the aging time t (h) in the temperature range of 400 to 500 ° C are the following (2): A spring material manufacturing method that applies an aging treatment under conditions that satisfy the equation.
14000 ≦ T (logt + 20) ≦ 15500 (2)

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