JP5214292B2 - Spring steel with excellent hydrogen embrittlement resistance and corrosion fatigue strength, and high-strength spring parts using the same - Google Patents

Description

  The present invention relates to a spring steel that can suppress a decrease in strength even when used in a corrosive environment, and a high-strength spring component made of the spring steel.

  As suspension springs for automobiles, springs (torsion bars, stabilizers, (large diameter) coil springs, etc.) torsional stress are applied by leaf springs or springs made of round bars. ) Is used. Coil springs are generally used in many passenger cars, and leaf springs are often used in trucks. These leaf springs and round bar springs are one of the heavy parts of automobile undercarriage parts, and the study of increasing strength has been continued for weight reduction. It is a part.

However, the same can be said for round springs such as coil springs and leaf springs. However, if the strength is increased to increase the tensile strength, the fatigue strength decreases significantly in a corrosive environment. It has been known. Therefore, the biggest problem in the conventional development is that the problem cannot be solved by simply increasing the tensile strength. In general, leaf springs and round bar springs are used by painting, but they are used by being attached to a part close to the ground. There is a possibility that it will break down and lead to breakage. In winter, snow melting agents that cause corrosion may be applied to prevent road surface freezing.
For these reasons, there has been a strong demand for the development of a steel material whose corrosion fatigue strength does not easily decrease even when the strength is increased.

  Various studies have been conducted on the deterioration of strength, especially fatigue properties, in a corrosive environment. Hydrogen generated as the corrosion reaction progresses penetrates into the steel and the material becomes brittle due to the hydrogen. It has been clarified by many literatures that this is the cause. And as a countermeasure for that, several documents have reported improvement measures by trapping hydrogen invading with precipitates in steel and preventing diffusion in steel, for example, patent documents Techniques as shown in 1 and 2 have been reported.

JP 2001-288539 A JP 2002-97551 A

  Among these, the invention described in Patent Document 1 discloses that precipitates are effective for trapping invading hydrogen in order to prevent hydrogen diffusion, and 1 of V, Mo, Ti, Nb and Zr is disclosed. It is described that a spring steel having an oxide, carbide or nitride containing seeds or two or more kinds is effective for increasing the strength.

  Further, in Patent Document 2, in order to increase the limit of diffusible hydrogen content that does not cause the influence of hydrogen embrittlement, the aspect ratio of the prior austenite grains after quenching and tempering (ratio of the length and width of the prior austenite grains). ) Is 2 or more, preferably 4 or more.

  However, the conventionally proposed countermeasures against hydrogen embrittlement of the high-strength spring steel have the following problems.

  As described above, Patent Document 1 describes that oxides, carbides, nitrides, and the like such as V, Mo, Ti, Nb, and Zr are effective as sites for trapping hydrogen. However, spring steel to which these elements are added has already been developed and put into practical use in the past, and many patents have been registered.

  Moreover, the steel component which becomes the object described in Patent Document 1 is in a very wide range including JIS SUP10, and as described above, formation of carbides such as Nb, Mo, V, etc. described in this document. Elements have already been described in numerous patents, and steels containing these carbide-forming elements are actually manufactured and used as developed steels.

  Considering such a situation, it cannot be determined in Patent Document 1 that the specifications of the spring steel, which is clearly superior to the prior art, are specifically disclosed. There is a problem that the optimum component range and manufacturing conditions cannot be grasped even if reference is made.

  Patent Document 2 describes that after quenching and tempering, the aspect ratio of prior austenite grains is preferably 2 or more, and preferably 4 or more. Therefore, in the non-recrystallization temperature range of 700 to 900 ° C. It is described that the structure can be obtained by giving a rolling reduction of 30% or more and quenching immediately after processing. However, such a strength improvement method by thermomechanical processing is conventionally well known as ausfoam, and there is a problem that a person skilled in the art cannot clearly grasp the difference from the conventional thermomechanical processing technology.

  Furthermore, in Patent Documents 1 and 2, steels composed of a very wide range of components are described vaguely. In particular, Patent Document 2 has a very wide range including JIS carbon steels and alloy steels. However, there is a problem that even those skilled in the art cannot accurately determine a component region that is truly resistant to hydrogen embrittlement.

  The present invention has been made to solve the above-described problems. Even if hydrogen enters the steel, hydrogen embrittlement hardly occurs, and excellent corrosion fatigue strength can be secured in a high tensile strength region. An object of the present invention is to provide a high-strength spring steel and a high-strength spring component using the same, in which a specific optimum component range is clarified.

The present invention is a spring steel used for a high-strength spring component formed by forming into a spring shape and processing at a temperature of 390 ° C. or higher during tempering after quenching,
In mass%, C: 0.50 to 0.65%, Si: 0.55 to 0.90%, Mn: 0.40 to 1.20%, Cu: 0.20 to 0.40%, Ni: Hydrogen embrittlement resistance characterized by containing 0.20 to 0.50%, Cr: 0.60 to 1.10%, V: 0.08 to 0.30%, and the balance consisting of Fe and impurity elements The spring steel has excellent corrosion fatigue strength (Claim 1).

  Conventionally, spring steel has a high Si material containing 1.2 to 2.2% of Si represented by SUP6, SUP7, and SUP12, and other carbon steels in terms of Si content. As is the case with many alloy steels such as SCr and SCM, it was largely divided into two types of low-Si materials containing only about 0.25%. And about the amount of Si spring steel of intermediate amount, even if it includes foreign standards, it was not standardized at all and was not used.

  The present inventors examined whether there is a component area that improves hydrogen embrittlement resistance and corrosion fatigue strength in a wide component range including not only Si but also other elements, regardless of the conventional usage situation. did. As a result, it is most suitable to have an intermediate content of about 0.7%, which has not been used for Si at all. In order to further enhance the effect of Si, at the same time, Cu, Ni, Cr , V has been found to be necessary. This will be described in more detail below.

(1) Relationship between the tempering temperature and the amount of diffusible hydrogen As described in many literatures, diffusible hydrogen refers to hydrogen that can move (diffuse) in steel at room temperature. Can be determined by measuring the amount of hydrogen released from the steel when the steel is heated to about 300 ° C. A large amount of diffusible hydrogen means that a large amount of freely diffusible hydrogen has infiltrated into the steel material, and it is natural that the influence of hydrogen embrittlement on the steel material will also increase.

  Therefore, the present inventors, for steel materials composed of many kinds of components, for steel materials intentionally infiltrated with hydrogen under certain conditions, conditions such as components, hardness after tempering, tempering temperature and the amount of diffusible hydrogen The relationship was investigated. As a result, as shown in FIG. 1 in Example 1 to be described later, when tempering to 390 ° C. or higher, the amount of diffusible hydrogen can be suppressed even when hydrogen is intentionally intruded thereafter, temper softening It has been found that steel added with Si having an effect of increasing resistance can suppress a decrease in strength due to hydrogen embrittlement because the tempering temperature can be set high even when aiming at the same hardness.

  Note that Si can be added in excess of 0.9% if only the point of increasing the temper softening resistance is important. However, when a large amount of Si is added, ferrite decarburization is likely to occur. In the case of thick leaf springs, large-diameter coil springs, torsion bars, and stabilizers that have a reduced cross-sectional reduction ratio due to rolling, the decarburized layer tends to remain in the final product, so that fatigue strength decreases for another reason. Therefore, the addition of the conventional high Si spring steel is not desirable.

  Therefore, the present inventors investigated in detail the influence of the Si amount on the decarburization amount. As a result, as shown in FIG. 2 in Example 1 to be described later, it was confirmed that decarburization causing a problem would not occur if the Si amount was 0.9% or less. The figure shows the amount of ferrite decarburized after cooling at a rate corresponding to the cooling rate after rolling into a plate having a thickness of 18 mm after heating.

(2) Influence of elements other than Si In order to improve the corrosion fatigue strength, it is necessary to have excellent corrosion resistance of the steel itself even when used in an environment susceptible to corrosion, and to increase the resistance to corrosion progression. . Therefore, it is necessary not only to increase the temper softening resistance by adding Si, but also to add an element that increases the resistance to corrosion.

  In this regard, the present inventors have experimentally melted and investigated a variety of steel components to improve the corrosion resistance and to reduce the depth of the corrosion pit even if it is corroded. Has confirmed that addition of a small amount of Ni and Cu is extremely effective, and that, as will be described later, the addition of B can improve the corrosion fatigue life.

The present invention has succeeded in completing the invention by obtaining these findings.
In addition, the spring steel obtained as a result of the above examination is compared with the bending rupture strength and torsional rupture torque after intentionally invading hydrogen into the steel material (hereinafter referred to as hydrogen charge). Shows great effect. That is, as shown in FIGS. 3 and 4 in Example 1 to be described later, in comparison with conventional steels SUP9 and SUP10 and comparative steels that do not satisfy some conditions of the present invention, in a high hardness region exceeding HV500. As a result, the decrease in strength of the steel is remarkably reduced, and there is an effect that it can be used safely even with high tensile strength as compared with conventional steel.

Next, the reason for limiting the range of addition amount for each component in the spring steel will be described.
C: 0.50 to 0.65%
C is an element that is indispensable for securing the strength and hardness necessary for a high-strength spring part after quenching and tempering treatment, and it is necessary to contain at least 0.50% or more. However, if it is contained in a large amount, the toughness decreases when used at high strength, so the upper limit was made 0.65%.

Si: 0.50-0.90%
Si is an element that is a point of the present invention. Si is well known as an element required for deoxidation, but if it is only for deoxidation, about 0.25% is sufficient. In the present invention, by adding more than the amount necessary for deoxidation, the softening resistance at the time of tempering is increased, and even when aiming at the same hardness, it is possible to adjust to a higher tempering temperature. As a result, an increase in the amount of diffusible hydrogen present in the spring steel is suppressed, embrittlement due to hydrogen is prevented, and corrosion fatigue strength can be improved. And in order to acquire such an effect, it is necessary to make it contain at least 0.50% or more, preferably 0.55% or more. However, when the amount is increased to the same level as that of conventional high Si spring steel, particularly in thick and thick spring steels, the reduction rate of the cross-section due to rolling cannot be increased, and the cooling rate after rolling decreases. For this reason, ferrite decarburization increases and the strength may rather decrease, so the upper limit was made 0.90%.

Mn: 0.40 to 1.20%
Mn is an element that is indispensable for ensuring the hardenability required depending on the plate thickness. The content of Mn to be added to ensure the necessary hardenability changes depending on the additive components other than Mn, the thickness of the leaf spring to be manufactured, and the dimensions of the spring parts such as the diameter of the round bar, If it becomes less than 0.40%, it becomes difficult to ensure the necessary hardenability, so the lower limit of the content rate was set to 0.40%. However, if Mn is contained in a large amount, it becomes easy to cause cracking and the productivity decreases, so the upper limit was made 1.20%.

Cu: 0.20 to 0.40%
Cu is an element indispensable for suppressing the growth of corrosion pits generated in a corrosive environment and increasing the corrosion fatigue strength, and it is necessary to contain at least 0.20% or more. However, when the content is large, the above effects are saturated, and hot workability is lowered and productivity is lowered. Therefore, the upper limit is set to 0.40%.

Ni: 0.20 to 0.50%
Ni, like Cu, has an effect of suppressing the growth of corrosion pits generated in a corrosive environment. For this reason, it is necessary to contain 0.20% or more. However, even if added in a large amount, the above effect is saturated and the cost is increased, so the upper limit was made 0.50%.

Cr: 0.60 to 1.10%
Cr is an element that is effective in improving hardenability like Mn, and in the present invention, it is an element that is also effective in improving temper softening resistance, although it is not as effective as Si. Further, Cr, like Cu and Ni, suppresses the growth of corrosion pits and is effective in improving corrosion resistance. Therefore, it is necessary to contain only the necessary amount for that purpose, and the lower limit is made 0.60%. However, if it is contained in a large amount, it tends to cause burning cracks, so the upper limit was made 1.10%.

V: 0.05-0.30%
V is an element that is indispensable for refining the structure after quenching and tempering, providing an excellent balance between strength and toughness and increasing proof stress. And in order to refine the structure and sufficiently obtain the above effect, it is indispensable to refine the crystal grains after quenching, and therefore the addition of V is essential, so the lower limit was made 0.05% . However, even if added in a large amount, the effect is saturated and the cost is increased, so the upper limit was made 0.30%.

  Next, in addition to the components contained in the spring steel according to the first aspect, the invention according to the second aspect further includes B: 0.0005 to 0.0050%, Ti: 0.010. -0.070% is contained, It is characterized by the above-mentioned. The reason for the limitation will be described.

B: 0.0005 to 0.0050%
B is an element effective in improving hardenability, and is added in a small amount for this purpose in the present invention. However, if only hardenability is improved, an effect can be obtained by increasing the amount of Mn and Cr, but B has an effect of improving the grain boundary strength, and as a result, the fatigue strength in a corrosive environment is improved as described above. There is an effect to improve. Therefore, it is desirable to add a small amount depending on the required characteristics. And in order to acquire the effect by addition, it is necessary to make it contain 0.0005% or more at least. However, B is an element that can obtain an effect when contained in a very small amount, and even if contained in a large amount, the effect is saturated, so the upper limit was made 0.0050%.

Ti: 0.010 to 0.070%
B is an element that is very easy to bond with N. When B is combined with N contained as an impurity and exists as BN, the effect of improving the hardenability of B and the effect of strengthening the grain boundary cannot be obtained. Therefore, it is necessary to add Ti and form TiN to prevent the generation of BN. In order to obtain this effect, it is necessary to contain at least 0.010% Ti. However, when Ti is added in a large amount, coarse TiN is likely to be generated, which causes a decrease in fatigue strength, so the upper limit was made 0.070%.
Although not described in claims 1 and 2, it is of course possible to contain as an impurity an amount of Al (about 0.040% or less) necessary for deoxidation, which is an essential step in the production of steel. .

Next, the invention according to claim 3 is a high-strength spring component excellent in hydrogen embrittlement resistance and corrosion fatigue strength, characterized by being formed using the spring steel according to claim 1 or 2 described above. It is in.
As described above, the spring steel according to claim 1 or 2 is improved in resistance to temper softening due to addition of Si and improved in corrosion resistance due to addition of Ni and Cu. Spring steel that can improve strength. Therefore, the spring part manufactured from this spring steel can be excellent in both of these two characteristics.

  Finally, the fourth invention is the hydrogen embrittlement resistance and corrosion according to claim 3, which is formed by forming into a spring shape and processing at a temperature of 390 ° C. or higher during tempering after quenching. It is in high strength spring parts with fatigue strength.

Here, the reason why the processing temperature is set to 390 ° C. or higher will be described.
Whether the hydrogen embrittlement resistance is improved is said to be determined by the amount of hydrogen that can move freely among the invading hydrogen, that is, the amount called diffusible hydrogen. The inventors of the present invention investigated how the amount of diffusible hydrogen changes by intentionally applying a treatment (hydrogen charge) for intruding hydrogen into the spring steel with respect to the spring steel manufactured under various conditions. As a result, as described above, the higher the tempering temperature, the lower the amount of diffusible hydrogen, the lower the amount of diffusible hydrogen, the lower the amount of diffusible hydrogen, the higher the bending rupture strength after hydrogen charging, the torsional fracture It has been found that the decrease in torque can be kept small (see FIGS. 1, 3, and 4).

  From FIG. 1, when the tempering temperature is 390 ° C. or higher, the amount of diffusible hydrogen is considerably reduced, and even if the tempering temperature is further increased, the change is small, so the lower limit temperature was set to 390 ° C. Preferably, the lower limit is 400 ° C. An element that increases temper softening resistance, such as Si, in order to ensure the target hardness even when the tempering temperature is set to 390 ° C. or higher while the design stress increases and the target value of hardness is high. As described above, is essential.

  The reason why the amount of diffusible hydrogen decreases when the tempering temperature is set to 390 ° C. or higher is not clear, but the structure tempered at a high temperature has a characteristic that hydrogen is easily trapped. Presumed.

Example 1
In this example, an example and a comparative example according to the spring steel of the present invention will be described.
First, spring steels (samples E1 to E10 and samples C11 to C19) having chemical components shown in Table 1 were prepared.
Among the spring steels shown in Table 1, the above samples E1 to E10 are steels of the present invention, and the above samples C11 to C17 are comparative steels having some components or tempering conditions different from those of the present invention, and sample C18. Is SUP9 which is a conventional steel, and sample C19 is SUP10 which is a conventional steel.

  The spring steels with the components shown in Table 1 were melted using a prototype vacuum induction melting furnace, and the obtained steel ingots were forged into a 18 mm round bar and a plate with a width of 70 mm and a thickness of 12 mm. And after performing the normalization process, it processed into the various test pieces mentioned later. About each test piece, the decarburization test, the measurement of the amount of diffusible hydrogen, the measurement of breaking strength, and the corrosion fatigue test were implemented and evaluated. The results are shown in Table 2.

Next, the evaluation method will be described.
<Decarburization test>
In the decarburization test, a cylindrical test piece having a diameter of 8 mm and a height of 12 mm was prepared from a round bar having a diameter of 18 mm (the decarburization amount before the test was 0), heated to 900 ° C., and a plate material previously measured ( The thickness was 18 mm) and a round bar (diameter 35 mm) was performed by carrying out a cooling process in which the speed was controlled so that the cooling speed was equivalent to the cooling curve after hot rolling. And the test piece after completion | finish of cooling was cut | disconnected and grind | polished, it etched with nital, and the ferrite decarburization depth was measured with the microscope. The results are shown in Table 2 and FIG. In addition, in order to accurately grasp the upper limit of the amount of Si that causes no problem in decarburization, in addition to the steel shown in Table 1, Si: 0.90% steel (0.53C-0.90Si-0.65Mn-0.015P -0.010S-0.30Cu-0.40Ni-0.90Cr-0.20V) was added for evaluation (referred to as sample C99). FIG. 2 shows the relationship between the Si content rate and the decarburization amount. In the figure, the horizontal axis represents the Si content rate (%), and the vertical axis represents the decarburization amount (DM-F (mm)). I took.

  Depending on the degree of decarburization, the effect on fatigue strength cannot be ignored. Therefore, for some samples, a 70 mm wide x 18 mm thick plate and a 35 mm diameter round bar are produced by rolling. The ferrite decarburization depth was confirmed. Along with this, a leaf spring was manufactured using the above plate material, and a four-point bending fatigue test was performed at a stress of 635 ± 500 MPa. It was confirmed whether or not a life of 100,000 times or more could be obtained with the number of repetitions until the fracture occurred as the fatigue life.

<Diffusion hydrogen content>
The amount of diffusible hydrogen was prepared by preparing a plate-shaped test piece having dimensions of 30 mm × 30 mm × 8 mm using the forged and stretched plate material, and quenching and tempering the test piece. Then, hydrogen charging is performed by immersing this test piece in a 20% aqueous ammonium thiocyanate solution for 30 minutes. After completion of the immersion, the sample is heated at a rate of 100 ° C./hr 5 minutes and released during the heating. The amount of diffusible hydrogen in the spring steel was measured. The reason for 5 minutes later is that there is a possibility that the value of the amount of diffusible hydrogen may change if there is variation in the time after the test. The results are shown in FIG. FIG. 1 shows the relationship between the tempering temperature and the amount of diffusible hydrogen. In the figure, the horizontal axis represents the tempering temperature (° C.), and the vertical axis represents the amount of diffusible hydrogen (mass ppm).

<Break strength>
Further, a plate-like test piece having a width of 30 mm, a thickness of 8 mm, and a length of 180 mm was processed using the forged and stretched plate material, and a hardened and tempered sample was prepared as a test piece. Then, the above-described hydrogen charging was performed, and a three-point bending test was performed 5 minutes after the completion of immersion, and the bending fracture strength was measured. The results are shown in FIG. FIG. 3 shows the relationship between hardness and bending rupture strength after hydrogen charging. In the figure, the horizontal axis represents hardness (HV) and the vertical axis represents bending fracture strength (MPa). Further, in the same figure, the same test was performed for the sample prepared in the same manner as in this example even when it was not charged with hydrogen, and indicated as A and B in the same figure (not shown in the table). . The straight line A is not hydrogen-charged of the steel of the present invention, the dotted line B is not hydrogen-charged of the conventional steel (SUP9, SUP10), and the alternate long and short dash line C is hydrogen-charged of the comparative steel and conventional steel The region D is a hydrogen-charged steel of the present invention.

Moreover, the round bar test piece of parallel part diameter 9mm was processed using the forging processed material of (phi) 18mm, and what hardened and tempered to this was prepared as a test piece. Then, the above hydrogen charge was performed, and after the immersion, a torsion test was performed 5 minutes later, and the breaking torque was measured. The results are shown in FIG. FIG. 4 shows the relationship between hardness and torsional breaking torque. In the figure, the horizontal axis represents the target hardness (HV), and the vertical axis represents the torsional breaking torque (N · m). Further, in the same figure, the same test was performed for the sample prepared in the same manner as in this example, even when the sample was not charged with hydrogen, and indicated as E and F in the same figure (not shown in the table). . The straight line E is not hydrogen-charged of the steel of the present invention, the dotted line F is not hydrogen-charged of the conventional steel (SUP9, SUP10), and the dotted line G is hydrogen-charged of the comparative steel and conventional steel. (Partly not shown in the table), region H is the hydrogen-charged steel of the present invention.
From these results, it can be seen that the steel according to the present invention has an effect that the strength decrease in the high hardness region exceeding HV500 is remarkably reduced, and that it can be used safely even at high strength as compared with the conventional steel.

<Corrosion fatigue test>
In the corrosion fatigue test, first, a plate spring having a width of 30 mm, a thickness of 8 mm, and a length of 300 mm is prepared from a plate material having a width of 70 mm and a thickness of 12 mm, subjected to quenching and tempering and shot peening, and then subjected to JASO M 609-91. Corrosion treatment was carried out by a method based on the standard of (Automobile Material Corrosion Test Method). Thereafter, a four-point bending fatigue test was performed at a stress of 735 ± 100 MPa, and evaluation was performed by measuring the corrosion fatigue life. After the test was completed, the depth of the corrosion pit that was the starting point of the fractured specimen was measured.
In addition, after producing a torsion bar having a diameter of 32 mm and a length of 905 mm for some samples, salt spray (5% saline, 35 ° C.) was performed for 168 hours. Thereafter, a torsional fatigue test was performed at a stress of 589 ± 500 N / mm ² and the corrosion fatigue life was measured and evaluated.

  In addition, the said plate spring and the torsion bar made HV520 the target value of hardness, and adjusted the tempering temperature according to the target value of hardness for every steel type. However, some test pieces were evaluated by intentionally changing the tempering temperature in order to evaluate the influence of the tempering temperature. The quenching temperature was constant at 930 ° C. Table 2 shows the tempering temperature and hardness.

  As is apparent from the results in Table 2, since the sample C11 as a comparative example of the present invention has a high Si content, the amount of ferrite decarburization is increased, and the leaf spring fatigue life and the corrosion fatigue life are decreased. Sample C12 as a comparative example of the present invention has a low Si content, so the tempering temperature for obtaining the desired hardness is low, the amount of diffusible hydrogen is increased, bending rupture strength, torsion rupture torque, corrosion Both fatigue lives are reduced.

  Samples C13, C14, and C15 as comparative examples of the present invention have low contents of Cu, Ni, and Cr, which are elements that suppress the growth of corrosion pits and improve corrosion resistance. Therefore, it is confirmed that the pit depth is deep after the fatigue test, and the corrosion fatigue life is inferior.

  Furthermore, Sample C16 and Sample C17 as comparative examples of the present invention are examples in which the tempering temperature is intentionally less than 390 ° C. although the component ranges satisfy the conditions of the claims. Although the hardness is slightly higher than when the tempering temperature is high, the amount of diffusible hydrogen is increased, the hydrogen embrittlement resistance is lowered, and the corrosion fatigue life and bending fracture strength are both inferior.

  On the other hand, Sample C18 and Sample C19, which are conventional steels, do not have sufficient elements added to increase the temper softening resistance like the steels of the present invention. Therefore, the tempering temperature for obtaining a predetermined hardness is lowered, the amount of diffusible hydrogen is increased, and addition of an element that suppresses the growth of corrosion pits such as Cu is not sufficient. Bending rupture strength, torsional rupture torque, corrosion fatigue life, etc. are greatly inferior.

  On the other hand, samples E1 to E10 as examples of the present invention were adjusted so as not to cause a significant increase in the amount of decarburization, while adding an appropriate amount of an element that enhances temper softening resistance such as Si, and corrosion pits. The addition of an element that suppresses hydrogen greatly improves the resistance to the presence of hydrogen, and both the resistance to hydrogen embrittlement and the corrosion fatigue strength are greatly improved.

  In particular, when B was added after adding a small amount of Ti, it was confirmed that the grain boundary strength could be strengthened, so that a better corrosion fatigue life could be obtained.

  As described above, the present invention proposes a new component system that has not been conventionally used in spring steels with a Si content of around 0.7%, thereby increasing fatigue characteristics due to an increase in the amount of ferrite decarburization. The hydrogen embrittlement resistance can be greatly improved without causing a decrease. This technology is an indispensable technology for reducing the weight of round bar springs that use torsional stress such as torsion bars, stabilizers, coil springs, etc. This has a remarkable effect that it can greatly contribute to the improvement of fuel consumption of automobiles using leaf springs and round bar springs.

The figure explaining the relationship between the tempering temperature and the amount of diffusible hydrogen in Example 1. FIG. The figure explaining the relationship between Si content rate and the amount of decarburization in Example 1. FIG. The figure explaining the relationship between the hardness after hydrogen charge and bending breaking strength in Example 1. FIG. The figure explaining the relationship between the hardness after hydrogen charge and torsional rupture torque in Example 1. FIG.

Claims (4)

A spring steel used for a high-strength spring component produced by forming into a spring shape and processing at a temperature of 390 ° C. or higher at the time of tempering after quenching,
In mass%, C: 0.50 to 0.65%, Si: 0.55 to 0.90%, Mn: 0.40 to 1.20%, Cu: 0.20 to 0.40%, Ni: Hydrogen embrittlement resistance characterized by containing 0.20 to 0.50%, Cr: 0.60 to 1.10%, V: 0.08 to 0.30%, and the balance consisting of Fe and impurity elements Spring steel with excellent corrosion fatigue strength.   The spring steel having excellent hydrogen embrittlement resistance and corrosion fatigue strength according to claim 1, further comprising B: 0.0005 to 0.0050% and Ti: 0.010 to 0.070%.   A high-strength spring component excellent in hydrogen embrittlement resistance and corrosion fatigue strength, characterized by being formed using the spring steel according to claim 1.   4. The hydrogen embrittlement resistance and corrosion fatigue strength according to claim 3, wherein the high-strength spring component is formed by forming into a spring shape and processing at a temperature of 390 ° C. or higher during tempering after quenching. Excellent high-strength spring parts.

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