JP2007113068A - Spring material made of high strength and high corrosion resistant stainless steel having excellent bendability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spring material made of stainless steel using metastable austenitic stainless steel as a base and having excellent strength, corrosion resistance, spring properties or the like. <P>SOLUTION: The stainless steel has a composition comprising, by mass, ≤0.15% C, 1.0 to 4.0% Si, ≤5.0% Mn, ≤0.040% P, ≤0.010% S, 4.0 to 10.0% Ni, 13.0 to 18.0% Cr, 0 to 3.5% Cu, 1.0 to 5.0% Mo, ≤0.15% N, ≤0.15% Nb, ≤0.05% Ti, ≤0.20% V, ≤0.015% O, ≤0.70% 2Nb+4Ti+V, and the balance Fe with inevitable impurities, and in which the value of Md(N) defined as Md(N)=580-(520C+2Si+16Mn+16Cr+23Ni+300N+10Mo) is set to the range of 0 to 100. The stainless steel may be as cold-rolled, but is preferably subjected to aging treatment of performing holding at 300 to 500°C for 0 to 20 hr after the cold rolling. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a stainless steel spring material applied to various general purpose springs which are used after being bent, such as a retractor spring of an automobile seat belt, and which requires strength, springiness and corrosion resistance.

Conventionally, martensitic stainless steel, work hardening type stainless steel, precipitation hardening type stainless steel or the like has been used as a material for various general purpose springs.
Martensitic stainless steel is a steel type hardened by quenching from a high-temperature austenite state and transforming into martensite, and SUS402J2 or the like corresponds to this. High strength and toughness can be obtained by tempering treatment of quenching and tempering, but the Cr content is as low as 12.0 to 14.0%, so the corrosion resistance is inferior, and because it contains a large amount of C, it is bent. It may be bad.

  Work hardening type austenitic stainless steel exhibits an austenite phase in a solution treatment state, and is intended to obtain high strength by generating work induced martensite in a subsequent cold rolling process. The strength of work-hardening austenitic stainless steel depends on the amount of cold work and the amount of martensite, but it is very difficult to adjust the strength only by cold work. Also, if the cold work rate is significantly increased, good bendability may not be obtained.

Precipitation hardening type stainless steel is one in which an element having high precipitation hardening ability is added and hardened by aging treatment. Typical examples include SUS630 to which Cu is added, SUS631 to which Al is added.
The precipitation-added stainless steel with addition of Cu exhibits a martensite single phase after solution treatment and is hardened by subsequent aging treatment, but the tensile strength is at most about 1400 N / mm ² at most.
Precipitation hardening type stainless steel with addition of Al is obtained by partially transforming the metastable austenite phase generated by solution treatment into martensite by cold working, etc., and then precipitating intermetallic compound Ni ₃ Al by aging treatment. It has been made. Although it is possible to increase the tensile strength to 1800 N / mm ² , nonmetallic inclusions are likely to remain in the steel, leading to a decrease in toughness.

A high-strength stainless steel using the strength increase by aging treatment has been developed with respect to the above-described conventional steel types. For example, in Patent Document 1, a metastable austenitic stainless steel compounded with Cu and Si is cold-rolled to produce work-induced martensite, and then a tensile strength of 2000 N / mm ² and Vickers hardness are obtained by aging treatment. 580 is obtained. However, since the optimum aging temperature range is relatively narrow, the stainless steel proposed in Patent Document 1 requires highly accurate temperature control in order to stably obtain the strength level, and the productivity is not always good. Absent.
JP-A-61-295356

In recent years, spring materials have been widely used in various fields such as automobiles and architecture, and safety standards have become stricter year by year. Depending on the application, the corrosion resistance of the material is also required. For example, it is corroded after bending, and then a tensile test is performed to determine its breaking strength as one criterion. If the corrosion resistance of the material is inferior, the stress concentrates on the starting point, and the strength of the material itself is reduced by the notch action. However, at present there are no spring materials that have both strength and corrosion resistance that satisfy the required level.
The present invention has been devised to solve such problems, and the metastable austenitic stainless steel having a two-phase structure of work-induced martensite phase + austenite phase exhibits excellent strength and corrosion resistance. The objective is to provide a stainless steel spring material that improves the bending workability of the material based on the knowledge and satisfies the required properties such as strength, corrosion resistance and spring property.

  The high-strength, high-corrosion-resistant stainless steel spring material with excellent bendability according to the present invention achieves the object, C: 0.15% by mass or less, Si: 1.0-4.0% by mass, Mn: 5 0.0 mass% or less, P: 0.040 mass% or less, S: 0.010 mass% or less, Ni: 4.0-10.0 mass%, Cr: 13.0-18.0 mass%, Cu: 0 to 3.5 mass%, Mo: 1.0 to 5.0 mass%, N: 0.15 mass% or less, Nb: 0.15 mass% or less, Ti: 0.05 mass% or less, V: 0 20% by mass or less, O: 0.015% by mass or less, 2Nb + 4Ti + V: 0.70% by mass or less, with the balance being Fe and inevitable impurities, Md (N) = 580− (520C + 2Si + 16Mn + 16Cr + 23Ni + 300N + 10Mo) Md (N) value is in the range of 0-100 And wherein the Rukoto.

The spring material of the present invention is hardened by transformation strengthening by work hardening and work-induced martensite transformation even in cold rolling, and further subjected to heat treatment for 0 to 20 hours at 300 to 500 ° C. after cold working. Further, it can be hardened by age hardening.
As this stainless steel spring material, a bending property in which the ratio R / t of the curvature R to the plate thickness t is 4 or less in a bending test according to JIS Z2248, and a 5% NaCl aqueous solution at 35 ° C. is used as the corrosive solution. Is sprayed on the plate to be tested for 15 minutes, dried in an atmosphere of 35 ° C. and 60 ° C. for 60 minutes, and wetted for 45 minutes in a 45% humidity and 50 ° C. atmosphere for one cycle. Those having a tensile strength at break of 1000 N / mm ² or more after conducting a corrosion test are preferred.
In addition, when a repeated deflection test specified by JIS H3130 is performed, those having a spring limit value [Kb _0.1 ] of 800 N / mm ² or more are preferable.

The spring material made of stainless steel according to the present invention is made of austenitic stainless steel, is given strength by work hardening during cold rolling, transformation strengthening and age hardening, and also has bending workability by suppressing the formation of precipitates. Has been improved. Furthermore, it has corrosion resistance that can maintain excellent strength even when exposed to a corrosive environment.
As a result, it is used as a spring material made of stainless steel that is optimal for spring applications that require bending and further requires strength, springiness, and corrosion resistance.

  The present inventors have studied various means for obtaining a spring material that can withstand a severe corrosive environment. As a result, the austenitic stainless steel was used as a raw material, and the work-induced martensite phase + austenite phase was obtained from the two-phase structure using the phenomena of “work hardening”, “transformation strengthening” and “age hardening”. The metastable austenitic stainless steel is found to be achieved. However, desired bendability and corrosion resistance cannot be satisfied only by the above conditions. As a result of further intensive studies, the metastable austenitic stainless steel optimum for a spring material having desired strength, springiness and corrosion resistance can be obtained for the first time by finely adjusting the components and composition.

  The stainless steel whose components and composition are specified as described above hardens by work hardening and transformation strengthening by work-induced martensitic transformation, and further age-hardened by heat treatment after cold working. In addition, because of the component design that facilitates age hardening, the toughness can be recovered by aging treatment at a relatively low temperature for a short time. Aging at a relatively low temperature for a short time is effective in preventing the formation of coarse precipitates that are the starting point of fatigue failure, in combination with the limitations on the components that cause inclusions and precipitates. Also improve.

Next, components, contents and the like contained in the stainless steel of the present invention will be described.
C: 0.15% by mass or less An austenite-forming element, an alloy component that suppresses the formation of δ ferrite in a high temperature range and reinforces work-induced martensite generated in cold work. However, in the component system of the present invention having a high Si content, the solid solubility limit of C is lowered. Therefore, when the C content is increased, coarse Cr-based carbides are precipitated by the aging treatment, which causes a decrease in intergranular corrosion resistance and fatigue characteristics. Therefore, the upper limit of the C content is set to 0.15% by mass. In addition, a preferable range is 0.05-0.10 mass%.

Si: 1.0-4.0 mass%
It is also an alloy component added as a deoxidizer in the steelmaking stage. Si content added as a deoxidizer is 1.0 mass% or less so that it may be seen in work hardening type stainless steels, such as SUS301 and SUS304. On the other hand, in this invention, Si content is set high and the effect of remarkably accelerating | stimulating the production | generation of a process induction martensite phase at the time of cold working is utilized. In addition, Si hardens the work-induced martensite phase and also solidifies and hardens in the austenite phase to increase the strength after cold working. Furthermore, in the aging treatment, age hardening is promoted by interaction with Cu. As described above, Si has various effects, but the effect becomes remarkable when the Si content is 1.0 mass% or more. However, when an excessive amount of Si exceeding 4.0% by mass is included, hot cracking is likely to be induced, and troubles are likely to occur in manufacturing. Therefore, the Si content is in the range of 1.0 to 4.0% by mass. In addition, a preferable range is 1.0-3.5 mass%.

Mn: 5.0% by mass or less Mn is an alloy component that controls the stability of the austenite phase, and the Mn content is determined by balance with other alloy components. However, if an excessive amount of Mn exceeding 5.0% by mass is contained, the deformation-induced martensite transformation is difficult at the time of cold rolling, and the transformation hardening ability cannot be expected. In addition, preferable content of Mn is 4.5 mass% or less.
P: 0.040% by mass or less Although it is an alloy component having a large solid solution strengthening ability, it may adversely affect the bendability.

S: 0.010% by mass or less A component that generates non-metallic inclusions that cause a decrease in bending workability. In order to obtain excellent bending workability, the S content is reduced to reduce non-metallic inclusions. It is necessary to reduce it. Therefore, the upper limit of the S content is 0.010% by mass.
Ni: 4.0-10.0 mass%
Although it is an alloy component that stabilizes the austenite phase at high temperatures and room temperature, the Ni content is determined so that it becomes a metastable austenite phase at room temperature and a work-induced martensite phase is generated by cold working. When the Ni content is less than 4.0% by mass, a large amount of δ ferrite is generated in a high temperature region, and a martensite phase is generated in the cooling process to room temperature, so that it cannot exist as an austenite single phase. On the other hand, when an excessive amount of Ni exceeding 10.0% by mass is contained, it is difficult to transform the work-induced martensite during cold working, and transformation hardening cannot be expected. Therefore, the Ni content is in the range of 4.0 to 10.0% by mass. In addition, a preferable range is 5.0-9.5 mass%.

Cr: 13.0 to 18.0 mass%
It is an alloy component essential for improving corrosion resistance, and at least 13.0% by mass of Cr is required to obtain the intended corrosion resistance. However, if an excessive amount of Cr, which is a ferrite forming element, is added in excess of 18.0% by mass, a large amount of δ ferrite is generated in a high temperature range. The formation of δ ferrite can be suppressed by adding austenite-forming elements such as C, N, Ni, Mn, Cu, etc., but if an austenite-forming element is added excessively, the austenite phase at room temperature is stabilized. . As a result, work-induced martensitic transformation does not occur during cold working, and high strength cannot be obtained after aging treatment. Therefore, the Cr content is in the range of 13.0 to 18.0% by mass. In addition, a preferable range is 13.0-16.5 mass%.

Cu: 0 to 3.5% by mass
It is an alloy component that is added as necessary, and promotes age hardening during the aging treatment by interaction with Si. The effect on age hardening increases with increasing Cu content. However, excessive addition of Cu exceeding 3.5% by mass reduces hot workability and causes cracking. Therefore, the Cu content is set to 3.5% by mass or less. In addition, a preferable range is 1.0-3.0 mass%.

Mo: 1.0-5.0 mass%
It is an alloy component that improves the corrosion resistance and exhibits the function of finely distributing carbonitride during aging treatment. Furthermore, in the present invention, the aging temperature is increased in order to reduce excessive rolling strain that adversely affects bending workability, but Mo is very effective in suppressing the rapid strain release at a high aging temperature. It is an element. Such an effect is remarkably exhibited when 1.0% by mass or more of Mo is added. However, when an excessive amount of Mo exceeding 5.0% by mass is added, δ ferrite is likely to be generated at high temperatures. Further, when the Mo content increases, the deformation resistance at high temperatures increases, and the hot workability may decrease. Therefore, the Mo content is in the range of 1.0 to 5.0 mass%. In addition, a preferable range is 1.0-4.5 mass.

N: 0.15% by mass or less An alloy component that is an austenite-forming element and is extremely effective for hardening the austenite phase and the martensite phase. However, excessive addition of N will cause blow holes during casting. Therefore, the upper limit of the N content is 0.15% by mass.
Nb: 0.15 mass% or less, Ti: 0.05 mass% or less The effect of increasing the material strength by solid solution strengthening is exhibited. However, since it is a component that generates precipitates that cause a decrease in bending workability, the upper limits of the contents of Nb and Ti are 0.15 mass% and 0.05 mass%, respectively.
V: 0.2 mass% or less An effective component for obtaining a desired strength by solid solution strengthening. However, the effect is saturated at 0.20% by mass. Moreover, since it is also a component which produces | generates the precipitate which causes a bending workability fall, V content made the upper limit 0.2 mass%.
O: 0.015% by mass or less Oxide-based non-metallic inclusions are formed to lower the cleanliness of steel and adversely affect bending workability. Such an adverse effect can be suppressed by regulating the O content to 0.015 mass% or less.

2Nb + 4Ti + V: 0.7 mass% or less Precipitates present in the product are harmful in obtaining good bending workability. As described above, since Nb, Ti, and V are components that generate precipitates, it is necessary to limit the total amount of these to suppress the formation of precipitates. The inventors of the present invention have confirmed that bending workability deteriorates when the total amount of 2Nb + 4Ti + V exceeds 0.7 mass%.
Here, in the above formula, Nb, Ti, and V mean the Nb content, Ti content, and V content (values expressed in mass%) of the steel, respectively.

Md (N) value: 0-100
In the present invention, the processing-induced martensitic transformation that occurs during cold rolling is actively used as one of the means for increasing the strength. For this reason, the stability of the austenite phase must be adjusted so that the work-induced martensite phase is easily formed in accordance with the strain imparted by cold rolling after solution treatment. . In the present invention, an Md (N) value defined by the following equation is adopted as an index representing the stability of austenite phase processing.
Md (N) = 580− (520C + 2Si + 16Mn + 16Cr + 23Ni + 300N + 10Mo)
Here, in the above formulas, C, Si,... Mean the C content, Si content,... (All values expressed in mass%) of the steel.
In the present invention, the Md (N) value is limited to a range of 0 to 100. By setting the Md (N) value within this range, an appropriate amount of work-induced martensite phase is generated during cold working, and high strength can be achieved by work hardening and transformation effects. In a steel type having an Md (N) value of less than 0, the stability of the austenite phase with respect to cold rolling is high, and the work-induced martensite phase contributing to high strength is not sufficiently generated. Conversely, when the Md (N) value exceeds 100, a large amount of work-induced martensite phase is generated at a relatively low cold rolling rate, and the bending workability of the product is also lowered.

  In the aging treatment carried out after cold rolling, in consideration of a continuous annealing furnace or a batch furnace, and in order to obtain the above-mentioned effect of Mo, an aging treatment for 0 to 20 hours may be performed in a temperature range of 300 to 500 ° C. preferable. In addition, if the aging time is less than 300 ° C., an effective aging effect is not exhibited. On the other hand, when the aging temperature exceeds 500 ° C. or the aging time exceeds 20 hours, a part of the processing-induced martensite phase is reversely transformed into an austenite phase, and the strength is decreased. Therefore, when performing an aging treatment after cold rolling, it is preferable to carry out the soaking time in the temperature range of 300 to 500 ° C. and in the range of 0 to 20 hours.

Stainless steel having the composition shown in Table 1 was melted in a vacuum melting furnace and cold-rolled to a thickness of 0.65 mm through forging, hot rolling and intermediate annealing. The cold-rolled steel strip was subjected to a solution treatment at 1050 ° C. × 1 minute, then water-cooled, and then cold-rolled to a plate thickness of 0.30 mm. Furthermore, an aging treatment was applied to a part of the cold rolled sheet.
In Table 1, steel types Nos. 1 to 5 are steels subject to the invention having the chemical composition defined in the present invention. As comparative steels, steel types No. 6 to 9 and a commercially available austenitic spring steel plate SUS301 (cold rolled material) ), Precipitation hardening type spring steel plate SUS630 (heat treatment material of 480 ° C x 1h) and martensite spring steel plate SUS420J2 (1050 ° C quenching → 520 ° C tempering material).

The bending workability, tensile properties, spring properties and tensile strength after corrosion test of the stainless steel plate shown in Table 1 were evaluated by the following methods.
For bending workability, a specimen cut in the rolling direction is subjected to a V-bending test according to JIS-Z2248, and the ratio (R / t) of the minimum bending radius (R) to the steel plate thickness (t) that can be bent is determined. It was measured.
For tensile properties, No. 13B test piece specified in JIS-Z2201 was used, and tensile strength was measured by the tensile test method specified in JIS-Z2241.

For spring characteristics, a test piece cut in the rolling direction was used, a repeated deflection test was performed in accordance with JIS-H3130, and the spring limit value [Kb _0.1 ] was measured.
The tensile strength at break after the corrosion test was determined by using a 5% NaCl aqueous solution at 35 ° C. as the corrosive liquid, spraying the corrosive liquid on the test plate for 15 minutes, and drying for 60 minutes in an atmosphere of 60 ° C. at a humidity of 35%. A treatment of wetting for 180 minutes in an atmosphere of 45% humidity and 50 ° C. was taken as one cycle, and after performing a corrosion test in which this was carried out for 50 cycles, the breaking strength was measured by the same tensile test method as described above.
Table 2 shows the test results of each stainless steel.
In the table, the bending workability was indicated by ◯ when the R / t was 4 or less in the V-bending test, and by × when the R / t exceeded 4.

As can be seen from the results shown in Table 2, the steels of the present invention were excellent in bending workability, and the spring limit values [Kb _0.1 ] by repeated deflection tests were all 800 N / mm ² or more. Moreover, the tensile fracture strength after corrosion is 1000 N / mm ² or more, and it can be seen that the material is suitable for spring applications having both strength and corrosion resistance.
On the other hand, in No. 6 of the comparative example, since Md (N) was too low, sufficient work-induced martensite was not generated, and the strength was comparatively low and a sufficient spring limit value could not be obtained. Similarly, No. 7 had 2Nb + 4Ti + V of 0.70% or more, and therefore had many precipitates and was inferior in bending workability. In No. 8, since Md (N) was too high, the amount of austenite contributing to ductility and toughness was insufficient, resulting in poor bending workability. Furthermore, No. 9 had insufficient corrosion resistance because the Cr content was too low, and the tensile strength at break was low after the corrosion resistance test.

As explained above, the spring material made of stainless steel of the present invention is made of austenitic stainless steel, and is given strength by work hardening, transformation strengthening and age hardening during cold rolling, and in a corrosive environment. It has corrosion resistance that can maintain excellent strength even when exposed.
Therefore, the present invention provides a spring material that is used after being bent and used, such as a retractor spring of an automobile seat belt, and that is required for strength, springiness, and corrosion resistance.

Claims (1)

  C: 0.15 mass% or less, Si: 1.0-4.0 mass%, Mn: 5.0 mass% or less, P: 0.040 mass% or less, S: 0.010 mass% or less, Ni: 4.0-10.0 mass%, Cr: 13.0-18.0 mass%, Cu: 0-3.5 mass%, Mo: 1.0-5.0 mass%, N: 0.15 mass % Or less, Nb: 0.15 mass% or less, Ti: 0.05 mass% or less, V: 0.20 mass% or less, O: 0.015 mass% or less, 2Nb + 4Ti + V: 0.70 mass% or less, and the balance It has a composition of Fe and inevitable impurities, and has a high strength and high bending property characterized by having an Md (N) value defined as Md (N) = 580− (520C + 2Si + 16Mn + 16Cr + 23Ni + 300N + 10Mo) in a range of 0 to 100 Corrosion-resistant stainless steel spring material.