JP2005023404A - Steel for spring excellent in corrosion fatigue resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for a spring in which embrittlement caused by hydrogen infiltrating therein is securely prevented, and which has improved corrosion fatigue resistance. <P>SOLUTION: The steel for a spring comprising C, Si and Mn has a composition containing, by mass, 0.04 to 0.095% Ti and one or more kinds of metals selected from ≤0.3% (exclusive of 0%) V, ≤0.3% (exclusive of 0%) Nb, ≤0.3% (exclusive of 0%) Zr, ≤0.3% (exclusive of 0%) Hf by 0.05 to 0.6% in total, in which the content of Cr is suppressed to ≤0.3% (inclusive of 0%), and also satisfying the inequality (1): Cr≤0.35×ä1-Exp(-25×A-0.6)}, wherein A=[Ti]×([Ti]+[V]+[Nb]+[Zr]+[Hf])×100, and [ ] denotes the content (mass%) of each element. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a spring steel useful as a spring material, and more particularly, to a spring steel excellent in corrosion fatigue resistance.

  In recent years, demands for reducing exhaust gas and fuel consumption have been increasing in order to preserve the global environment for transport aircraft represented by automobiles and the like, and weight reduction of transport aircraft is desired in order to satisfy such requirements. As part of this, the strength of springs used in transport aircraft is increasing, and there is a demand for springs with a strength of 1800 MPa or higher after quenching and tempering.

  On the other hand, it is generally known that when the strength of steel is increased, the fatigue characteristics in a corrosive environment are significantly reduced. Therefore, in order to increase the strength of the spring, it is necessary to prevent such deterioration of the corrosion fatigue characteristics.

  Therefore, the present applicants use the technique of Patent Document 1 as a spring steel for providing a spring exhibiting environmental resistance that can sufficiently meet recent required characteristics under severe use environment and load. Proposed. In this technique, the basic content of the spring steel is appropriately adjusted while suppressing the Cr content to 0.25% or less, the Cr content is controlled within an appropriate range in relation to the total content of Cu and Ni, and A certain amount of Ti is contained. However, this technology is mainly aimed at improving the environmental resistance from the viewpoint of corrosion resistance, and hydrogen embrittlement caused by accumulation of hydrogen that has penetrated into the steel due to the corrosion reaction in the stress-concentrated part has been hardly studied. There is still room for improvement.

By the way, the technique of patent document 2 is known as high strength spring steel which improved the fatigue characteristic resulting from hydrogen embrittlement. In this technique, hydrogen that enters during use of the spring is captured at a hydrogen trap site in the steel material, thereby preventing hydrogen from diffusing and detoxifying the hydrogen. However, there is a limit to the amount of hydrogen to be captured, and this is not a fundamental solution.
JP 2002-47539 A (see [Claims], [0005], [0008]) JP 2001-288539 A (refer to [Claims] and [0004])

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a spring steel that more reliably prevents embrittlement due to hydrogen entering the steel and has improved corrosion fatigue. It is in.

The spring steel excellent in corrosion fatigue resistance according to the present invention that has been able to solve the above problems is a spring steel containing C, Si and Mn, in addition to containing Ti: 0.04 to 0.095%, V: 0.3% or less (not including 0%), Nb: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), Hf: 0.3 % Or less (excluding 0%), including 0.05 to 0.6% in total, and Cr: 0.3% or less (including 0%), and The point is that the following expression (1) is satisfied.
[Cr] ≦ 0.35 × {1-Exp (−25 × A−0.6)} (1)
In the formula, A = [Ti] × ([Ti] + [V] + [Nb] + [Zr] + [Hf]) × 100, and [] represents the content (% by mass) of each element. Indicates.

  In the spring steel excellent in corrosion fatigue resistance according to the present invention, it is preferable to contain V essentially.

  In addition, the above-mentioned problem is shown below using a steel specimen for springs containing C, Si and Mn, and producing a disk-shaped specimen having a diameter of 10 mm and a thickness of 2 mm using this steel. After performing the cycle test for 14 cycles, when a hydrogen temperature rising analysis was performed at a heating rate of 12 ° C./min, the amount of hydrogen released in the temperature range of 50 to 450 ° C. was 0.75 mass. It can also be solved by using spring steel that is less than or equal to ppm.

  The cycle test is a cycle in which a 5% NaCl aqueous solution is sprayed on the test piece for 8 hours in an environment of 35 ° C., and then dried by holding at constant temperature and humidity for 16 hours in an environment of 35 ° C. and humidity 60%. Is a test with one cycle.

  Among the amounts of hydrogen released in the temperature range, those in which the ratio [b / a] of the amount of hydrogen a released at 50 to 200 ° C. and the amount of hydrogen b released at 300 to 450 ° C. is 1 or more preferable.

The spring steel of the present invention includes at least one of carbides, nitrides and sulfides containing at least one selected from the group consisting of Ti, V, Nb, Zr and Hf, and any two or more of these composite compounds. One type is dispersed as a precipitate, and the number of particles having a particle size of 10 nm to 0.2 μm is 60/100 μm ² or more among the precipitates, and the Fe—Cr compound having a particle size of 10 nm or more. The number is preferably 20 pieces / 100 μm ² or less.

  As the basic components of the spring steel according to the present invention, those containing C: 0.3 to 0.7%, Si: 1 to 2.6% and Mn: 0.1 to 1.8% are preferable, Further, if necessary, Ni: 1.5% or less (not including 0%) and / or Cu: 0.7% or less (not including 0%) may be included.

  ADVANTAGE OF THE INVENTION According to this invention, embrittlement by the hydrogen which penetrate | invades in steel can be prevented more reliably, and the steel for springs with improved corrosion fatigue property can be provided.

  As described above, the present applicants have already developed the technique of Patent Document 1 and have obtained some results. The reason why this technology is effective is not fully understood, but I think as follows.

That is, in order to enhance the corrosion fatigue characteristics, the liquid of the solution in forming conditions and the resulting crack tip corrosion pits (eg, pH and Cl ^- ion concentration, etc.) are considered to be more important. And if the alloy balance of steel materials is adjusted appropriately, the liquid state of the solution at the formation state of corrosion pits and the generated crack tip can be improved, and the corrosion resistance of steel can be enhanced. In addition, hydrogen that penetrates into the steel due to the corrosion reaction accumulates in the stress concentration part and promotes hydrogen embrittlement. Therefore, the hydrogen generation rate and penetration rate are considered to be important.

  However, in the above-mentioned patent document 1, although the direction that hydrogen generation rate and penetration rate are important for suppression of hydrogen embrittlement is given, a specific solution has not been sufficiently studied.

  Therefore, the present inventors have repeatedly studied specific measures for suppressing hydrogen embrittlement from various angles. As a result, as disclosed in Patent Document 2, instead of increasing the amount of hydrogen trapped in the steel, while reducing the hydrogen intrusion itself into the steel, and when hydrogen penetrates into the steel, The present invention has been completed by finding that the above problem can be solved by trapping invading hydrogen with a trapping site having a strong trapping power. Hereinafter, the function and effect of the present invention will be described.

  The spring steel of the present invention contains C, Si and Mn as basic components, but these contents are not particularly limited as long as the characteristics as a spring are not deteriorated.

  On the other hand, the spring steel of the present invention contains Ti: 0.04 to 0.095%, V: 0.3% or less (not including 0%), Nb: 0.3% or less (not including 0%), Zr: 0.3% One or more of the following (not including 0%), Hf: not more than 0.3% (not including 0%), 0.05 to 0.6% in total, and Cr: not more than 0.3% (including 0%) It is important to suppress it. The reason for specifying this range is as follows.

Ti: 0.04-0.095%
Ti forms fine carbides, nitrides and sulfides, and composite compounds of any two or more of these (hereinafter, these compounds may be collectively referred to as “precipitates”) in steel. It is an element that is effective in trapping invading hydrogen and suppressing the progress of hydrogen embrittlement. In order to exert such an effect, it is necessary to contain 0.04% or more. Preferably it is 0.05% or more. However, if the content exceeds 0.095%, the precipitates become coarse, and the precipitates serve as a starting point, leading to a decrease in spring life. Therefore, the Ti content must be suppressed to 0.095% or less. Preferably it is 0.95% or less.

V: 0.3% or less (not including 0%), Nb: 0.3% or less (not including 0%), Zr: 0.3% or less (not including 0%), Hf: 0.3% or less (not including 0%) ), 0.05-0.6% in total of any one or more of
V, Nb, Zr and Hf (hereinafter sometimes referred to as “V etc.”) also form fine precipitates similar to Ti described above, and are effective in suppressing hydrogen embrittlement by their hydrogen trap action. It is an element. As described above, such an action is also exhibited by Ti, but when an element such as V and Ti are used in combination, a synergistic effect can be obtained. From such a viewpoint, it is necessary to contain 0.05% or more in total of any one of V, Nb, Zr and Hf or two or more arbitrarily selected. The total content is preferably 0.1% or more.

  In order to obtain such a synergistic effect more effectively, for each element, V: 0.075% or more (more preferably 0.080 or more, more preferably 0.1% or more), Nb: 0.01% or more (more preferably 0.05% or more) ), Zr: 0.01% or more (more preferably 0.05% or more), and Hf: 0.01% or more (more preferably 0.05% or more).

  However, among the elements for calculating the A value in the above formula (1), precipitates such as carbides containing V in particular are excellent in the action of increasing the trap capacity, and therefore are added in combination with Ti which becomes a strong trap site. Therefore, it is recommended that V be included essentially.

  On the other hand, if these contents are excessively large, the precipitates become coarse and deteriorate the fatigue life. For example, V: 0.3% or less (preferably 0.2% or less), Nb: 0.3% or less (preferably 0.2% or less), Zr: 0.3% or less (preferably 0.2% or less), Hf: 0.3% or less (preferably 0.2% or less), and any one or any of V, Nb, Zr and Hf are selected. It is necessary to suppress two or more types to 0.6% or less in total. Preferably, the total content is 0.5% or less.

Cr: 0.3% or less (including 0%)
Cr is a harmful element that lowers the pH of the solution present in the corrosion pits and promotes the progress of corrosion, and should be reduced as much as possible. From such a viewpoint, it is necessary to suppress the Cr content to 0.3% or less. It is desirable to keep it at 0.25% or less.

  However, if the Cr content is reduced, a decrease in pH at the tip of the corrosion pit can be suppressed and the amount of generated hydrogen can be reduced, but embrittlement due to invading hydrogen cannot be prevented. However, the spring steel of the present invention can prevent embrittlement due to invading hydrogen by generating precipitates by using a suitable amount of V and the like together with Ti as described above and causing these to act as hydrogen trap sites. . At this time, it was found that if the formation of the Fe—Cr compound is prevented by reducing the Cr content, precipitates such as Ti and V are further refined and become strong hydrogen trap sites, although the reason is not clear. That is, fine precipitates such as Ti and V have a strong hydrogen trapping force, so that the hydrogen that has entered the steel material is immediately captured and it is difficult to release the captured hydrogen.

  The spring steel of the present invention contains a predetermined amount of a specific element as described above, but it is also important to adjust the content of Cr, Ti, V, etc. so as to satisfy the above formula (1). It is. That is, when the balance of Cr, Ti, V, etc. satisfies the relationship of the above formula (1), hydrogen does not easily penetrate into the steel material, the occurrence of corrosion is suppressed, and the corrosion fatigue resistance is drastically improved. Because. Although not all of this reason could be clarified, the present inventors have a good balance between Ti, which is a strong trap site, and V, which is a weak trap site, as long as the relationship of the above equation (1) is satisfied. I think it will be. That is, the present inventors considered that the balance between Ti having a strong trapping force and V having a low trapping force is important for suppressing hydrogen embrittlement, and intensively studied along this line. As a result, it was easy to distribute a large number of weak trap sites in the steel, and it was found that the trap sites quickly trap the hydrogen that had entered and suppress hydrogen embrittlement. However, since this trap site has a weak trapping force, the hydrogen concentration gradient and temperature rise cause the hydrogen to diffuse due to the driving force, and this hydrogen can cause delayed fracture depending on the amount of hydrogen in the member and the stress state. I understood. On the other hand, although it is difficult to distribute a large number of strong trap sites in steel, it was found that this trap site hardly diffuses hydrogen once trapped in normal use, and can prevent hydrogen embrittlement more stably. . Therefore, if the balance between these strong trap sites and weak trap sites is improved, it is considered that the corrosion fatigue resistance is improved. From this viewpoint, the present inventors have determined the relationship of the above formula (1). It was.

In the case where only V is included among V, Nb, Zr and Hf, for example, the following equation (2) may be satisfied instead of the above equation (1).
[Cr] ≦ 0.35 × {1-Exp (−25 × A−0.6)} (2)
In the formula, A = [Ti] × ([Ti] + [V]) × 100, and [] indicates the content (% by mass) of each element.

  The spring steel of the present invention is a steel containing C, Si and Mn. As its hydrogen storage / release characteristics, a disk-shaped test piece having a diameter of 10 mm and a thickness of 2 mm manufactured using the steel material is used. A 14% cycle test was performed in which a 5% NaCl aqueous solution was sprayed for 8 hours in a 35 ° C environment and then dried by holding at constant temperature and humidity for 16 hours in a 35 ° C, 60% humidity environment. Thereafter, when the hydrogen temperature rising analysis is performed at a heating rate of 12 ° C./min, the amount of hydrogen released in the temperature range of 50 to 450 ° C. is 0.75 mass. It is also characterized in that it is ppm or less.

  That is, when the test piece after the cycle test is heated in a vacuum or Ar gas at a heating rate of 12 ° C./min and analyzed for hydrogen temperature rise, hydrogen that has penetrated into the test piece during the cycle test is detected. At this time, when heated to 450 ° C., not only free hydrogen in the test piece but also all hydrogen trapped at the trap site is released. Therefore, the amount of hydrogen released in the temperature range of 50 to 450 ° C. is set to 0.75 mass. By suppressing to ppm or less, excellent corrosion fatigue properties can be obtained.

  The amount of hydrogen is 0.75 mass. Although the method of reducing to ppm or less is not particularly limited, as described above, the alloy composition contained in the steel is appropriately adjusted and the addition balance of Cr, Ti, V, and the like may be controlled.

  The spring steel of the present invention is a ratio of the amount of hydrogen a released at 50 to 200 ° C. and the amount of hydrogen b released at 300 to 450 ° C. of the amount of hydrogen released in the temperature range of 50 to 450 ° C. [B / a] is preferably 1 or more. That is, it is considered that hydrogen having a larger amount of hydrogen released at 300 to 450 ° C. than hydrogen released at 50 to 200 ° C. is captured at a stronger trap site. That is, the hydrogen released at 50 to 200 ° C. when the test piece is heated is free hydrogen diffusing in the steel material or trapped at a site with a weak trapping force. When the amount of hydrogen released in the region is large, it is considered that hydrogen diffusion progresses due to stress fluctuation and temperature rise when using the spring, resulting in deterioration of corrosion fatigue. On the other hand, hydrogen released at 300 to 450 ° C is hydrogen trapped at a strong trap site, and a relatively large amount of hydrogen released in this temperature range should be less than 300 ° C. For example, even if heat is generated when the spring is used, hydrogen does not desorb from the trap site, and therefore, hydrogen does not diffuse into the steel. From this point of view, the amount of hydrogen released at 50 to 200 ° C. is preferably as small as possible, and the ratio [b / a] is preferably 1.2 or more.

  In order to adjust the ratio [b / a] to 1 or more, as described above, the Cr content may be reduced and Ti and V may be used in a well-balanced manner.

The spring steel of the present invention includes carbide, nitride and sulfide containing at least one selected from the group consisting of Ti, V, Nb, Zr and Hf (hereinafter sometimes referred to as “Ti etc.”), and At least one of these two or more composite compounds is dispersed as precipitates, and the number of particles having a particle size of 10 nm to 0.2 μm is 60/100 μm ² or more among the precipitates, and The number of Fe—Cr compounds having a particle size of 10 nm or more is preferably 20/100 μm ² or less.

  Ti and the like combine with C, N, and S in the steel to form a precipitate, and function as a suitable trap site for hydrogen that has entered the steel. As a result, it is considered that hydrogen intrusion into the steel material can be suppressed and corrosion generation can be reduced.

  Here, the precipitate is a carbide, nitride, sulfide, or any of these, in which at least one element of Ti, V, Nb, Zr, and Hf and at least one of C, N, and S are bonded. Two or more complex compounds. That is, one compound contains at least one element selected from the group consisting of Ti, V, Nb, Zr and Hf and at least one of C, N and S in one compound. Therefore, for example, in one compound, a carbide, nitride, sulfide, or a composite compound of any two or more of them containing two or more elements selected from the group consisting of Ti, V, Nb, Zr and Hf. It does not matter.

In the spring steel of the present invention, it is preferable that a large number of fine precipitates having a particle size of 10 nm to 0.2 μm are dispersed in the steel. This is because fine precipitates act as strong hydrogen trap sites. From such a viewpoint, the number of precipitates is preferably 60 or more, more preferably 100 or more per 100 μm ² observation field.

On the other hand, the number of Fe—Cr compounds having a particle size of 10 nm or more is preferably 20 or less per 100 μm ² observation field. This is because the Fe—Cr compound also acts as a hydrogen trap site, but its trapping force is considered to be weaker than precipitates such as Ti. The Fe—Cr compound is a composite carbide containing Fe and Cr, and specifically, is a compound such as (Fe, Cr) ₃ C.

The number of the precipitates and the Fe—Cr compounds may be measured by observing the precipitates extracted by the extraction replica method with a transmission electron microscope (TEM) at an acceleration voltage of 200 kV and a magnification of 10,000. The number of observation fields is at least five, and for each field, precipitates and the like observed in a 10 μm square observation field (field area: 100 μm ² ) are analyzed by energy dispersive X-ray analysis (EDX) provided in the TEM. ) Analyze using the instrument. For the number of precipitates containing at least one of Ti, V, Nb, Zr and Hf, the average value is calculated by measuring the number of particles having a particle size of 0.2 μm or less. On the other hand, the number of Fe—Cr compounds is measured regardless of the particle diameter, and the average value is calculated.

  Next, the components of the spring steel of the present invention will be described.

  As the basic components of the spring steel of the present invention, those containing C: 0.3 to 0.7%, Si: 1 to 2.6% and Mn: 0.1 to 1.8% are preferable, and if necessary, Ni: 1.5% or less (0% is not included) and / or Cu: 0.7% or less (0% is not included), and further, Mo: 0.01 to 0.5%, etc. are preferable. The reason for specifying such a range is as follows.

C: 0.3-0.7%
C is an element necessary for quenching and hardening the spring material to ensure strength, and is preferably contained in an amount of 0.3% or more in order to exert its effect effectively. More preferably, it is 0.35% or more. However, if the content exceeds 0.7%, the susceptibility to defects increases, and when fatigue pits occur, the fatigue life and fracture stress will be significantly reduced. Therefore, it is preferably 0.7% or less, more preferably Is 0.6% or less.

Si: 1 to 2.6%
Si is an element effective for increasing the elastic limit of steel and obtaining the characteristics as a spring, and also effective for maintaining a long life when a corrosion pit is generated by reducing defect sensitivity. . Moreover, it also has the effect | action which densifies the rust produced | generated in steel materials, and also exhibits the inhibitory effect of corrosion pit itself. In order to exert these effects effectively, it is preferable to contain 1% or more. However, if the content exceeds 2.6%, not only the effects are saturated, but also the surface strength by decarburization during heat treatment. On the other hand, the lifespan is reduced due to the decrease in the temperature. A more preferable lower limit of the Si content is about 1.2%, and a more preferable upper limit is about 2.5%.

Mn: 0.1-1.8%
Mn is an important element for increasing the hardenability of the steel material and obtaining a desired strength. In order to exhibit such an effect effectively, it is preferable to contain 0.1% or more. However, if it is contained excessively, defect susceptibility is increased and the life is shortened when corrosion pits are formed, so 1.8% or less is preferable. A more preferable lower limit of the Mn content is about 0.2%, and a more preferable upper limit is about 1%.

Ni: 1.5% or less (not including 0%) and / or Cu: 0.7% or less (not including 0%)
Ni and Cu are effective elements for enhancing the corrosion resistance of the steel material, and even when added in a small amount, the effect is exhibited, but preferably Ni: 0.15% or more (more preferably 0.25% or more), preferably Cu: 0.1 % Or more (more preferably 0.2% or more). However, if Ni or Cu is contained excessively, not only the effect is saturated, but it is difficult to obtain high strength by the formation of a retained austenite structure, so Ni: 1.5% or less (more preferably 1% or less), Cu: It is desirable to suppress to 0.7% or less (more preferably 0.5% or less). Ni and Cu may be used alone or in combination.

  As a component other than the above, Al is an element useful for deoxidation during steelmaking, and also has the effect of increasing the toughness by refining crystal grains, but when added in excess, coarse nitrides are produced. Therefore, it is preferable to suppress to 0.05% or less.

  The spring steel of the present invention may contain a trace component in addition to the above components as long as the properties as a spring steel are not impaired. Such steel is also included in the scope of the present invention. Examples of the trace component include inevitable impurities such as P, As, Sb, N, and S. However, these inevitable impurities cause grain boundary segregation and reduce toughness, so it is preferable to reduce them as much as possible. Specifically, P: 0.02% or less (more preferably 0.01% or less), As: 0.006% or less, and Sb: 0.006% or less are preferable.

  In particular, N is an element that reacts with Ti or the like to produce precipitates and contributes to the improvement of corrosion fatigue resistance, and is preferably contained in an amount of 0.002% or more in order to effectively exhibit such effects. However, if it is excessively contained, the nitride becomes coarse and the fatigue life is reduced, so the N content is preferably 0.015% or less. On the other hand, S, like N, is an element that reacts with Ti or the like to produce precipitates, and is desirably suppressed to 0.02% or less (more preferably 0.01% or less). The balance of the spring steel of the present invention is Fe and inevitable impurities.

  It is important to adjust the component composition of the spring steel of the present invention, and its production method is not particularly limited, and a known method can be adopted, but a production method that can be suitably adopted will be described below.

  In producing the spring steel of the present invention, it is preferable to set the cooling rate in the temperature range of 1500 to 1300 ° C. to 10 ° C./min or more when casting molten steel that satisfies the above-mentioned component composition requirements. This is because the precipitate is refined by setting the cooling rate to 10 ° C./min or more. More preferably, it is 20 ° C./min or more, and further preferably 30 ° C./min or more. The upper limit of the cooling rate is not particularly limited, but if the cooling rate is too high, the precipitate is solidified before it is sufficiently precipitated, so it is preferably set to 200 ° C./min or less.

  Next, the cooled steel is quenched. The quenching temperature is set to 850 to 1000 ° C., and by making it within this range, the structure is austenitized and precipitates such as V can be re-dissolved. Then, fine precipitates are generated by reprecipitation of elements such as Ti with an element that is not completely dissolved as a nucleus. If the quenching temperature is less than 850 ° C., austenitization may be insufficient, and if it exceeds 1000 ° C., the austenite crystal grains become coarse and the toughness decreases. After heating to the above temperature range, oil quenching is performed at about 70 ° C. This is because fine precipitates are generated under such cooling conditions.

  Next, the quenched steel is tempered at 300 to 450 ° C. for about 1 hour. This is because by setting the tempering temperature within this range, precipitates such as Ti are refined and the strength is further improved. However, if the tempering temperature is less than 300 ° C, the steel is too hard and the toughness is lowered. On the other hand, if it exceeds 450 ° C, sufficient strength cannot be obtained.

  The spring steel of the present invention can be used as various spring materials, and the strength of the obtained spring is 1800 MPa or more.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Steel materials having the composition shown in Table 1 below were melted in a 150 kg vacuum melting furnace to produce an ingot, and then cooled. At this time, the cooling rate when cooling to 1500 to 1300 ° C. was 30 ° C./min. Next, the obtained ingot was hot forged to produce a round bar having a diameter of 12 mm. Next, in order to change the structure of the round bar to austenite, it was heated at 925 ° C. for 10 minutes, and then poured into hot oil at 70 ° C. and quenched. Then, it was tempered at 300 to 450 ° C. for 1 hour so that the tensile strength was about 2000 MPa.

  For the steel material after tempering, the A value and the right side value of the above formula (1) are calculated from the composition shown in Table 1 and shown in Table 2 below. The relationship between the A value and the Cr content is shown in FIG. In FIG. 1, ● represents the results of Nos. 1 to 7, ○ represents the results of Nos. 8 to 17, and the curve represents the above (1).

  Next, a test piece for corrosion fatigue test and a test piece for hydrogen temperature rise analysis were cut out from the steel material after tempering, and a corrosion fatigue test and a hydrogen temperature rise analysis were performed using each test piece. The steel material after tempering was observed with an electron microscope.

[Corrosion fatigue test]
The test piece for corrosion fatigue test is a dumbbell shape, length: 150 mm, distance between marked lines: 10 mm, the grip part at both ends is a circular shape with a cross section of 8 mm in diameter, and the thin part at the center has a cross section of 4 mm in diameter. A thread portion having a length of about 15 mm is formed at both ends of the test piece.

Using this test piece, after 14 cycles or 28 cycles of a cycle test of [salt spray (35 ° C., 5% NaCl) 8 hours + drying (35 ° C., humidity: 60%) 16 hours] as one cycle, The test piece was mounted on a SSRT test (Slow Strain Rate Technique) machine, a tensile test was performed, and the breaking stress (S1) was measured. The crosshead speed at this time was 2 × 10 ⁻³ mm / min. Table 2 shows the breaking stress after 14 cycles and after 28 cycles.

Moreover, the test piece which has not performed the said cycle test was mounted | worn with the SSRT testing machine, the tensile test was done in air | atmosphere, and the tensile strength (TS) was measured. The crosshead speed at this time was also 2 × 10 ⁻³ mm / min. The tensile strength is shown in Table 2 below.

The corrosion fatigue characteristic value was calculated from the measured S1 and TS by the following equation (3), and the corrosion fatigue characteristic was evaluated according to the following evaluation criteria based on this value. The corrosion fatigue characteristic values and evaluation results are shown in Table 2 below. In addition, the corrosion fatigue characteristics after 28 cycles of each specimen are shown in FIG. 2, the relationship between the Cr content and the corrosion fatigue characteristics after 28 cycles is shown in FIG. 3, and the relationship between the A value and the corrosion fatigue characteristics after 28 cycles is shown in FIG. Respectively.
Corrosion fatigue characteristic value = S1 / TS (3)

<Evaluation criteria for corrosion fatigue characteristics>
○… Corrosion fatigue property value is 0.75 or more, and corrosion fatigue property is excellent △… Corrosion fatigue property value is 0.5 or more to less than 0.75, and corrosion fatigue property is excellent ×… Corrosion fatigue property value is less than 0.5 Inferior to corrosion fatigue properties

[Hydrogen temperature rising analysis]
The test piece for hydrogen temperature rising analysis has a disk shape with a diameter of 10 mm and a thickness of 2 mm, and this test piece [salt water spray (35 ° C, 5% NaCl) 8 hours + drying (35 ° C, humidity: 60%) The cycle test with 16 hours] as one cycle was conducted 14 cycles. The rust generated on the surface of the test piece after the cycle test is removed by mechanical polishing, and heated at a rate of temperature increase of 12 ° C./min in a vacuum using a TDS (vacuum mass spectrometer). The amount of hydrogen released in the temperature range of ° C. was measured. The hydrogen release peaks of No. 1 and No. 12 are shown in FIG. In FIG. 5, (a) shows the result of No. 1, and (b) shows the result of No. 12.

  Further, the hydrogen amount a released at 50 to 200 ° C. and the hydrogen amount b released at 300 to 450 ° C. were calculated, and the ratio [b / a] was calculated. The number of tests shall be 3 times for each test piece, and the average of the obtained hydrogen amounts shall be the total hydrogen amount. The total amount of hydrogen, the amount of hydrogen released at each temperature range, and the ratio [b / a] determined by calculation are shown together in Table 3 below. FIG. 6 shows the relationship between the total amount of hydrogen and the corrosion fatigue properties after 14 cycles, and FIG. 7 shows the relationship between the ratio [b / a] and the corrosion fatigue properties after 14 cycles. 6 and 7, ● represents the results of Nos. 1 to 7, and ○ represents the results of Nos. 8 to 17, respectively.

[Electron microscope observation]
Precipitates were extracted from the tempered steel by an extraction replica method, and arbitrary five fields of view were observed with a transmission electron microscope (TEM) at an acceleration voltage of 200 kV and a magnification of 10,000. For each of the five visual fields, precipitates observed in a 10 μm square observation visual field (visual field area: 100 μm ² ) were analyzed using an energy dispersive X-ray analysis (EDX) apparatus provided in the TEM. For the number of precipitates containing at least one selected from the group consisting of Ti, V, Nb, Zr and Hf, the average value was calculated by measuring the number of particles having a particle size of 0.2 μm or less. On the other hand, the number of Fe—Cr compounds is measured regardless of the particle diameter, and the average value is calculated. The results are also shown in Table 3 below. FIG. 8 shows the relationship between the number of precipitates such as Ti and the number of Fe—Cr compounds. Moreover, the electron micrograph of No. 12 is shown in FIG. In FIG. 8, ● represents the results of Nos. 1 to 7, and ○ represents the results of Nos. 8 to 17, respectively. Further, the arrows shown in FIG. 9 indicate precipitates such as Ti.

It can consider as follows from Tables 1-3 and FIGS.

  Nos. 1 to 7 are examples that do not satisfy any of the requirements defined in the present invention, and are spring steels that are inferior in corrosion fatigue resistance.

  On the other hand, Nos. 8 to 17 are examples that satisfy the requirements defined in the present invention, and a spring steel with good corrosion fatigue resistance can be achieved even if the cycle test is performed 28 cycles.

It is a graph which shows the relationship between A value and Cr content. It is a graph which shows the corrosion fatigue characteristic after 28 cycles of each test piece. It is a graph which shows the relationship between Cr content and the corrosion fatigue property after 28 cycles. It is a graph which shows the relationship between A value and the corrosion fatigue characteristic after 28 cycles. The hydrogen release peaks of No. 1 and No. 12 when hydrogen temperature rising analysis was performed. It is a graph which shows the relationship between the total hydrogen amount and the corrosion fatigue characteristics after 14 cycles. It is a graph which shows the relationship between ratio [b / a] and the corrosion fatigue property after 14 cycles. It is a graph which shows the relationship between the number of precipitates, such as Ti, and the number of Fe-Cr compounds. It is an electron micrograph (drawing substitute photograph) of No. 12.

Claims (7)

In spring steel containing C, Si and Mn,
In addition to Ti: 0.04-0.095% (meaning “mass%”, the same applies hereinafter)
V: 0.3% or less (excluding 0%),
Nb: 0.3% or less (excluding 0%),
Zr: 0.3% or less (excluding 0%),
Hf: 0.3% or less (excluding 0%),
Any one or more of 0.05 to 0.6% in total,
Furthermore, Cr: suppressed to 0.3% or less (including 0%),
And spring steel excellent in corrosion fatigue resistance characterized by satisfying the following formula (1).
[Cr] ≦ 0.35 × {1-Exp (−25 × A−0.6)} (1)
Where
A = [Ti] × ([Ti] + [V] + [Nb] + [Zr] + [Hf]) × 100
[] Indicates the content (% by mass) of each element.   The spring steel according to claim 1, which essentially contains V. In spring steel containing C, Si and Mn,
Using this steel, a disk-shaped test piece having a diameter of 10 mm and a thickness of 2 mm was prepared.
After performing 14 cycles of the cycle test shown below using the obtained test piece,
Temperature rising rate: When performing a hydrogen temperature rising analysis heating at 12 ° C./min,
The amount of hydrogen released in the temperature range of 50 to 450 ° C. is 0.75 mass. Spring steel with excellent corrosion fatigue resistance, characterized by being below ppm.
[Cycle test is a cycle in which a 5% NaCl aqueous solution is sprayed on the test piece for 8 hours in an environment of 35 ° C. and then dried by holding at constant temperature and humidity for 16 hours in an environment of 35 ° C. and humidity 60%. This is a test for one cycle. ]   The ratio [b / a] of the amount of hydrogen a released at 50 to 200 ° C and the amount of hydrogen b released at 300 to 450 ° C among the amount of hydrogen released in the temperature range is 1 or more. The spring steel described in 1. The steel according to any one of claims 1 to 4, carbide, nitride and sulfide containing at least one selected from the group consisting of Ti, V, Nb, Zr and Hf, and any two or more of these At least one of the composite compounds is dispersed as a precipitate,
Of the precipitates, the number of particles having a particle size of 10 nm to 0.2 μm is 60/100 μm ² or more, and
The steel for spring according to any one of claims 1 to 4, wherein the number of Fe-Cr compounds having a particle size of 10 nm or more is 20/100 µm ² or less.   The steel for springs according to any one of claims 1 to 5, which contains C: 0.3 to 0.7%, Si: 1 to 2.6%, and Mn: 0.1 to 1.8%. Furthermore, Ni: 1.5% or less (not including 0%) and / or Cu: 0.7% or less (not including 0%) are included as other elements. The spring steel described.

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