JP2011127181A - Plate spring steel having excellent strength and toughness, and plate spring part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a plate spring steel in which decarburization can be suppressed, and further, which has excellent strength and toughness, and plate spring parts. <P>SOLUTION: The plate spring steel has a composition comprising, by mass, 0.40 to &lt;0.50% C, 0.40 to 0.85% Si, 0.55 to 1.20% Mn, 0.70 to 1.50% Cr, 0.010 to &lt;0.070% Ti and 0.0005 to 0.0050% B, and the balance Fe with impurity elements. The plate spring parts can be obtained by performing forming using the plate spring steel. The plate spring steel is preferably used for a plate spring having Vickers hardness of &ge;500. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a steel for leaf springs and a leaf spring component that can suppress decarburization and are excellent in strength and toughness.

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.
In increasing the strength, it is particularly important to improve the fatigue strength, and one of the countermeasures is to increase the hardness of the material.

  However, in both round bar springs and leaf springs, increasing the tensile strength by increasing the hardness is effective in improving the fatigue strength in a normal environment, but the toughness decreases as the hardness increases. When corrosion pits are generated or defects such as surface flaws are caused for some reason, the effect is greater than when the hardness is low, and the fatigue strength is greatly reduced. Therefore, there has been a strong demand for the development of a steel for leaf springs having excellent strength and toughness that does not decrease toughness even when the hardness is increased.

  Various studies have been conducted on measures for achieving both high strength and high toughness even when the hardness is increased. For example, a technique as disclosed in Patent Document 1 has been reported.

JP-A-11-29839

However, conventional spring steels that have been proposed to achieve both high strength and high toughness include coil springs such as valve springs and suspension springs, and round bars such as stabilizers and torsion bars, as described in the above patent documents. Many of them were premised on application to the round bar spring as a material, and the development of spring steel premised on application to leaf springs was less than that for round bar springs.
And the steel material proposed on the assumption of the conventional round bar spring was not the optimal component system which can solve the problem peculiar to the leaf spring which does not occur notably in the round bar spring but remarkably occurs in the leaf spring.

Specifically, in a leaf spring, the cross-sectional area of the final product is considerably larger than the material of the round bar spring, so that the cooling rate after rolling is lower than that of a round bar spring made of steel bars or wire rods. In addition, since the rate of reduction of the cross-sectional area due to rolling is small, it is necessary to consider that decarburization tends to remain in the final product.
In order to improve the toughness of the steel, it is effective to reduce the C content. For example, if the hardness is increased to HV500 or higher, low temperature tempering occurs. I will enter. For this reason, it has been difficult to achieve both excellent hardness and toughness in conventional steels.

  The present invention has been made to solve such problems, and is intended to provide a steel for leaf springs and a leaf spring component that can suppress decarburization and are excellent in strength and toughness.

  1st invention is the mass%, C: Less than 0.40-0.50%, Si: 0.40-0.85%, Mn: 0.55-1.20%, Cr: 0.70 1.50%, Ti: less than 0.010 to 0.070%, B: 0.0005 to 0.0050%, the balance is in a steel for leaf springs characterized by comprising Fe and impurity elements (Claim 1).

2nd invention is the mass%, C: Less than 0.40-0.50%, Si: 0.40-0.85%, Mn: 0.55-1.20%, Cr: 0.70 1.50%, Ti: 0.010 to less than 0.070%, B: 0.0005 to 0.0050%,
Furthermore, it contains at least one kind selected from Cu: 0.20 to 0.50%, Ni: 0.20 to 1.00%, and V: 0.05 to 0.30% in mass%,
The balance is made of steel for leaf springs, which is excellent in strength and toughness, characterized by comprising Fe and impurity elements (claim 2).

  According to a third aspect of the present invention, there is provided a leaf spring component formed by using the leaf spring steel according to the first or second aspect of the invention (claim 3).

The steel for leaf springs of the first and second inventions has the specific composition.
Specifically, the leaf spring steel contains Si at the specific content within a range that does not cause a problem in increasing the amount of decarburization while the C content is relatively low. Therefore, the temper softening resistance can be increased, tempering at a higher temperature can be performed, and the temperature range of the low temperature temper brittle region can be shifted to the high temperature side. Furthermore, since Ti and B are added as essential elements, the grain boundary strength can be improved.
As a result, excellent toughness can be exhibited even in a high hardness range of, for example, HV500 or higher.

  Thus, according to this invention, while being able to suppress decarburization, the steel for leaf springs excellent in intensity | strength and toughness can be provided.

Next, the leaf spring component of the third invention is formed using the steel for leaf springs of the first or second invention. Specifically, the plate spring component can be manufactured by forming the plate spring steel into a spring shape and performing quenching and tempering.
Thus, since the leaf spring component of the present invention uses the leaf spring steel of the first or second aspect of the present invention, it has excellent strength and toughness and can suppress decarburization.

Explanatory drawing which shows the relationship between the amount of carbon (C) and an impact value concerning an Example. Explanatory drawing which shows the relationship between the amount of silicon (Si) and an impact value concerning an Example. Explanatory drawing which shows the relationship between the amount of silicon (Si) and the decarburization depth concerning an Example. Explanatory drawing which shows the relationship between hardness and an impact value concerning an Example.

As described above, the steel for leaf springs of the present invention contains C, Si, Mn, Cr, Ti, and B in the specific composition range.
Hereinafter, the reason which limited the range of content rate for every component is demonstrated.

C: Less than 0.40 to 0.50% C is an element indispensable for securing sufficiently excellent strength and hardness after quenching and tempering treatment.
If the C content is less than 0.40%, sufficient strength as a leaf spring may not be ensured.
On the other hand, if it is 0.50% or more, it may be difficult to ensure sufficient toughness in a high hardness region.

Moreover, in this invention, Ti and B are contained, restrict | limiting a C content rate to the said specific range. Therefore, the spring steel can have a higher level of hardness and toughness.
That is, in general, the toughness is increased when the C content is low in the low hardness region, but the tempering temperature is lowered and becomes a low temperature tempering brittle region in the high hardness region of HV500 or higher, for example. The result is that the toughness is lowered when the content ratio is lower. However, as in the present invention, the toughness in the high hardness region is greatly improved even when the C content is lowered by the effect of the combination of the addition of the specific amounts of Ti and B and the addition of the specific amount of Si described later. The toughness can be further improved as compared with the case where the C content is high.

Si: 0.40 to 0.85%
Si is an element necessary for securing excellent strength and hardness in a steel having a low C content. In addition, since it has the effect of changing the temperature range of the low-temperature tempering brittle region to the high-temperature side, it is an element that can greatly increase the toughness when tempering at a low temperature for high hardness.
When the Si content is less than 0.40%, the above effect cannot be obtained sufficiently, and it becomes difficult to ensure sufficient toughness by tempering at a low temperature. As a result, it becomes difficult to achieve both excellent strength and hardness. On the other hand, if it exceeds 0.85%, the steel for springs with a larger cross-sectional area and a lower cooling rate after rolling will promote ferrite decarburization and fatigue strength when compared with springs made of round bars. Cause a drop in
Moreover, it is preferable to contain Si content exceeding 0.50% from a viewpoint that toughness can be improved more.

Mn: 0.55 to 1.20%
Mn is an indispensable element in order to ensure the hardenability required as a steel for leaf springs.
If the Mn content is less than 0.55%, it may be difficult to ensure hardenability. On the other hand, if it exceeds 1.20%, the hardenability becomes excessive, and there is a possibility that quench cracks are likely to occur.

Cr: 0.70 to 1.50%
Cr is an indispensable element in order to ensure the hardenability required for steel for leaf springs.
If the Cr content is less than 0.70%, it may be difficult to ensure hardenability. On the other hand, if it exceeds 1.50%, the hardenability becomes excessive, and there is a possibility that quench cracks are likely to occur.

Ti: 0.010 to less than 0.070% Steel contains a trace amount of N as an inevitable impurity element, but Ti generates BN by promoting the formation of TiN in the presence of N Is suppressed, and the effects described below due to the addition of B are prevented from being impaired.
When the Ti content is less than 0.010%, the effect of suppressing the generation of BN may not be sufficiently obtained. On the other hand, even if 0.070% or more of Ti is added, the above-mentioned effect is saturated by addition of Ti. Therefore, the upper limit of the Ti content is preferably less than 0.070% as described above from the economical viewpoint.

B: 0.0005 to 0.0050%
B is an indispensable element for ensuring the hardenability required for the steel for leaf springs, and is also effective in improving the grain boundary strength. Further, as described above, the combined addition with Ti has a great effect on improving toughness in a high hardness region.
When the B content is less than 0.0005%, the above effects may not be sufficiently obtained. Moreover, B is an element which can obtain an effect even when contained in a very small amount, and the effect is saturated even if contained in a large amount. Therefore, the upper limit of the B content is preferably 0.0050% as described above.

The plate spring steel of the first invention contains C, Si, Mn, Cr, Ti, and B in the specific composition range as described above, with the balance being Fe and impurity elements.
On the other hand, the leaf spring steel of the second invention contains the specific amount of C, Si, Mn, Cr, Ti, and B, as in the first invention, and Cu: 0. It contains one or more selected from 20 to 0.50%, Ni: 0.20 to 1.00%, and V: 0.05 to 0.30%, with the balance being Fe and impurity elements.

Thus, when 1 or more types chosen from Cu, Ni, and V are contained with the said specific content rate, hardness, toughness, and corrosion resistance can be improved more.
Hereinafter, the reason which limited the range of content rate for each component of Cu, Ni, and V is demonstrated.

Cu: 0.20 to 0.50%
Cu has the effect of suppressing the growth of corrosion pits generated in a corrosive environment and improving the corrosion resistance.
When the Cu content is less than 0.20%, the effect of improving the corrosion resistance by Cu may not be sufficiently obtained. On the other hand, even if added over 0.50%, the effect of improving corrosion resistance due to the addition of Cu is saturated and hot workability may be deteriorated.

Ni: 0.20 to 1.00%
Ni, like Cu, has the effect of suppressing the growth of corrosion pits generated in a corrosive environment and improving the corrosion resistance.
When the Ni content is less than 0.20%, the effect of improving the corrosion resistance by Ni may not be sufficiently obtained. On the other hand, even if added over 1.00%, the effect of improving the corrosion resistance by adding Ni may be saturated, and the upper limit of the Ni content is preferably 1.00% from an economical viewpoint.

V: 0.05-0.30%
V has the effect of making the quenched and tempered structure finer and improving the strength and toughness in a well-balanced manner.
If the V content is less than 0.05%, the effect of crystal grain refinement due to the addition of V may not be sufficiently obtained. On the other hand, even if added over 0.30%, the effect of addition of V may be saturated, and the upper limit of the V content is preferably 0.30% from an economical viewpoint.

  In addition, the said steel for leaf | plate springs may contain the quantity of Al (about 0.040% or less) required for the deoxidation process which is an essential process at the time of manufacture of steel as an impurity.

The plate spring component can be produced by forming the plate spring steel and subjecting it to quenching and tempering. Thereby, a tempered martensite structure can be obtained.
In addition, as already explained, the above-mentioned steel for leaf springs has excellent characteristics (coexistence of both high strength and high toughness) in comparison with conventional steels, particularly in the high hardness region, so that the hardness of the Vickers hardness is 500 or more. Is preferably used.
The Vickers hardness can be adjusted to 500 or more as described above by appropriately adjusting the tempering temperature after quenching.

Example 1
In this example, examples and comparative examples according to the steel for leaf springs of the present invention will be described.
First, a plurality of types of steel for leaf springs (samples E1 to E13 and samples C1 to C10) having chemical components shown in Table 1 were prepared. In addition, about the Cu and Ni among the components of Table 1, some of these have shown the content rate as an impurity.
Among the steels for leaf springs shown in Table 1, the samples E1 to E10 are steels of the present invention, and the samples C1 to C5 are the steels of the present invention that have partial component contents such as C, Si, and B. Samples C6 are SUP10 which is a conventional steel, sample C7 is SUP11A which is a conventional steel, and sample C8 is SUP6 which is a conventional steel.

Steel materials having the components shown in Table 1 were melted using a vacuum induction melting furnace, and after forging into a 18 mm round bar from the resulting steel ingot, it was processed into a round bar by subjecting it to normalization. The test material for testing described later was used. Moreover, about the test performed with the same shape as an actual leaf | plate spring, the said steel ingot is rolled to a steel piece, Furthermore, after carrying out the hot rolling to width 70mm and thickness 20mm, a test piece is given by performing a normalization process. Got ready.
Using the round bar and plate material thus obtained, test pieces (round bar test piece or plate material test piece) used for various evaluation tests described later were prepared and subjected to various evaluations. Specifically, the impact test and decarburization test described later were performed on the round bar, and the rolling material decarburization test, durability test, and corrosion resistance evaluation described below were performed on the plate material.

Next, the evaluation method will be described.
<Impact test>
A U-notch test piece is prepared from the round bar described above, and the tempering temperature is adjusted in consideration of the difference in temper softening resistance due to the difference in the components so that the target hardness is HV540 (Vickers hardness). Tempering was performed to make the structure a tempered martensite structure. Thereafter, an impact test was performed at room temperature.

Thus, the impact value of each sample (sample E1-sample E10 and sample C1-sample C8) was measured. The results are shown in Table 2.
Moreover, the relationship between a carbon (C) content rate and an impact value, and a silicon (Si) content rate and an impact value was plotted on a graph. The relationship between C content and impact value is shown in FIG. 1, and the relationship between Si content and impact value is shown in FIG.

<Decarburization test>
First, a cylindrical test piece having a diameter of 8 mm and a height of 12 mm was prepared by cutting from a round bar having a diameter of 18 mm (the amount of decarburization before the test was 0). Next, the cylindrical specimen was heated in vacuum at a heating rate of 900 ° C./min, and heated at a temperature of 900 ° C. for 5 minutes. Then, it cooled in the air | atmosphere at the cooling rate equivalent to the cooling curve after the hot rolling at the time of the above-mentioned board | plate material preparation measured beforehand. Next, the test piece was cut and polished, and then etched with nital. Then, the decarburization depth (DM-F) of the surface layer was measured with an optical microscope. The results are shown in Table 2.
The relationship between the silicon (Si) content and the decarburization depth was plotted on a graph. This is shown in FIG.

<Rolling material decarburization test>
A rolled material having a width of 70 mm and a thickness of 20 mm produced by rolling was cut in a cross section perpendicular to the longitudinal direction, and the decarburization depth (DM-F) was measured by an optical microscope. The results are shown in Table 2. In addition, in order to clarify the influence on the decarburization depth due to the difference in shape, cross-sectional area, etc. from the plate material, the same steel ingot as the plate material was rolled to produce a φ12 mm round bar. The cross section was cut and the decarburization depth (DM-F) was measured. The results are shown in Table 2.

<Durability test>
A rolled material having a width of 70 mm and a thickness of 20 mm produced by hot rolling was formed into a leaf spring shape. Next, quenching and tempering were performed so that the target hardness was HV540 (Vickers hardness) to obtain a tempered martensite structure, and then a shot peening treatment was performed. The plate spring component subjected to the shot peening treatment thus obtained was subjected to a durability test until it was broken at a stress of 680 ± 500 MPa, and the fracture life of the leaf spring component obtained from each sample was measured.
The number of times until the rupture occurred was measured as the rupture life, and the case where it exceeded 400,000 times was evaluated as “◯”, and the case where it was 400,000 times or less was evaluated as “x”. The results are shown in Table 2.

<Corrosion resistance evaluation>
A rolled material having a width of 70 mm and a thickness of 20 mm produced by rolling was quenched and tempered to obtain a martensite structure, and then a plate-shaped test piece having a width of 30 mm, a thickness of 8 mm, and a length of 100 mm was produced by cutting. Then, a sodium chloride aqueous solution (salt water) having a concentration of 5 wt% and a temperature of 35 ° C. is sprayed on the plate-shaped test piece for 2 hours (salt water spray treatment), and dried with hot air at a temperature of 60 ° C. for 4 hours (drying treatment). It was moistened for 2 hours under conditions of 50 ° C. and humidity of 95% or more (wetting treatment). These salt spray treatment, drying treatment, and wetting treatment were defined as one cycle, and this was repeated for a total of 60 cycles. Then, the corrosion product produced | generated on the test piece surface was removed, and the maximum corrosion pit depth which appears in the cross section of a corrosion part was measured using the optical microscope. The results are shown in Table 2.

As can be seen from Table 2 and FIGS. 1 to 3, in the sample C1 in which the C content is too low, the tempering temperature for obtaining the target hardness is remarkably lowered, and the fatigue life (fracture life) is reduced. .
Further, the sample C2 having a too high C content and the sample C3 having a too low Si content had a low impact value when the hardness was high, and the toughness was insufficient.

Sample C4 having an excessively high Si content had an increased ferrite decarburization amount and a reduced fatigue life. Here, in the sample C4, for comparison, the decarburization depth of a steel bar having a diameter of 12 mm corresponding to the shape and dimensions of a coil spring of an automobile is also shown. However, although the Si content is high, the ferrite decarburization is performed. Could not be confirmed. From this result, high Si materials that do not have problems with coil springs such as automobiles used at φ10 to φ20 mm or thinner valve springs, etc., have a high possibility of reduction in fatigue strength due to decarburization when used for leaf springs. Recognize.
.

  Moreover, since the sample C5 which does not contain B has low grain boundary strength, the impact value and fatigue life in the high hardness region were reduced.

In addition, the conventional steel samples C6 and C7 have a low tempering embrittlement temperature at a low temperature tempering brittle region, so that the desired hardness is low. . Furthermore, the fatigue life was also reduced.
Moreover, the sample C8 which is conventional steel had a large amount of ferrite decarburization.

On the other hand, it can be seen that Samples E1 to E12 of the present invention have excellent strength and toughness even at a low carbon content and are also excellent in durability because ferrite decarburization is difficult to occur. Therefore, it can be suitably used for a leaf spring for automobiles such as trucks.
In addition, as is known from FIG. 2, in order to increase the impact value and improve the toughness, it is effective to increase the Si content, and it is more preferable to increase it to an amount exceeding 0.50%. I understand.

  As described above, by mass%, C: less than 0.40 to 0.50%, Si: 0.40% to 0.85%, Mn: 0.55 to 1.20%, Cr: 0.70 It turns out that the steel for plate springs (sample E1-sample E10) containing 1.50%, Ti: 0.010-0.070%, B: 0.0005-0.0050% is suitable. By adopting such steel for leaf springs, decarburization can be suppressed, and a leaf spring component having excellent strength and toughness can be realized.

(Example 2)
In Example 1, the hardness was aimed at HV540, but in this example, an impact test was performed on a test piece whose aim hardness was changed, and the relationship between the hardness and the impact value was examined.
That is, with respect to Sample E1, Sample E7, Sample C3, and Sample C6 of Example 1, test specimens were produced by changing the target hardness and quenching and tempering, and the impact test was performed in the same manner as in Example 1. . The results are shown in Table 3 and FIG. FIG. 4 shows the relationship between hardness and impact value, with the horizontal axis representing the Vickers hardness (HV) of each sample and the vertical axis representing the impact value of each sample.

As can be seen from Table 3 and FIG. 4, when the hardness of sample C3 and sample C6 is increased, the impact value decreases and the toughness deteriorates.
On the other hand, it can be seen that Sample E1 and Sample E2 within the composition range of the present invention maintain a high impact value even when the hardness is increased, and have both excellent strength and toughness.

Claims (3)

  In mass%, C: 0.40 to less than 0.50%, Si: 0.40 to 0.85%, Mn: 0.55 to 1.20%, Cr: 0.70 to 1.50%, Ti : Steel for leaf springs with excellent strength and toughness, characterized by containing 0.010 to less than 0.070%, B: 0.0005 to 0.0050%, the balance being Fe and impurity elements. In mass%, C: 0.40 to less than 0.50%, Si: 0.40 to 0.85%, Mn: 0.55 to 1.20%, Cr: 0.70 to 1.50%, Ti : 0.010 to less than 0.070%, B: 0.0005 to 0.0050%,
Furthermore, it contains at least one kind selected from Cu: 0.20 to 0.50%, Ni: 0.20 to 1.00%, and V: 0.05 to 0.30% in mass%,
A leaf spring steel excellent in strength and toughness, wherein the balance is made of Fe and impurity elements.   A leaf spring component formed by using the leaf spring steel according to claim 1 or 2.

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