JP2009127123A - Leaf spring material and manufacturing method thereof - Google Patents

Abstract

The present invention provides a leaf spring material that uses high-frequency heating and has excellent mechanical properties, and a method for producing a leaf spring material that can be stably obtained.
SOLUTION: A first surface 11 of a substantially strip-shaped steel plate 10 is subjected to tensile stress along the longitudinal direction and a compressive stress is applied to the second surface 12 along the longitudinal direction so that the first surface 11 is induction-hardened. It is the manufacturing method of the leaf | plate spring material to do. By this induction hardening, an induction hardening structure consisting of martensite and uniformly finely dispersed austenite having higher average hardness than the base material structure 12a in the vicinity of the second surface is given to the surface layer 11a in the vicinity of the first surface. Features.
[Selection] Figure 2

Description

  The present invention relates to a leaf spring material and a manufacturing method thereof, and more particularly to a leaf spring material having improved spring life using heat treatment by induction hardening and a manufacturing method thereof.

  A spring material used for the “spring” is provided by performing a heat treatment of quenching and tempering after giving a shape as a “spring” by cold or hot forming a metal material made of spring steel. In addition, in order to improve fatigue strength, it is widely practiced to apply compressive residual stress by performing shot peening on the surface of a spring material. 2. Description of the Related Art In recent years, spring materials are known in which quenching is performed with a high-frequency heating device to improve fatigue strength.

  For example, Patent Document 1 discloses a coil spring material in which fatigue strength is improved by performing heat treatment on a cold-formed coil using a high-frequency heating device. By the way, generally, if austenite remains after quenching and tempering of a spring material, fatigue strength as a “spring” is lowered, which is not preferable. Here, in Patent Document 1, according to the high-frequency heating apparatus, since the coil can be heated and quenched with high accuracy and at high speed, it is possible to realize a heat treatment that fully considers the metal material component of the coil. It states that the amount of austenite can be suppressed to 3% or less. In other words, it states that the heat treatment condition can be optimized to reduce the retained austenite and the fatigue strength of the spring can be improved.

  A spring material is also known in which fatigue strength is improved by heat-treating a part of the spring material with a high-frequency heating device.

For example, Patent Document 2 discloses a method for manufacturing a leaf spring in which only one surface of a leaf spring material is subjected to induction hardening. Specifically, a substantially strip-shaped leaf spring material made of spring steel is cold or hot formed and quenched and tempered. That is, a normal leaf spring material is obtained. After that, when a load is applied in the same direction as the usage state of the leaf spring, the leaf spring material has a thickness, so that tensile stress is generated on the first surface and compressive stress is generated on the second surface. To do. When the vicinity of the first surface on which the tensile stress acts is heated with a high-frequency heating device, stress relaxation occurs near the surface and the tensile stress is relaxed. When the load is unloaded after cooling this, a compressive stress is applied to the first surface, so that a spring material with high fatigue strength can be obtained.
JP 2004-323912 A JP 2006-71082 A

  As disclosed in Patent Document 1, according to the high-frequency heating device, the spring material can be heat-treated with high accuracy, and a metal structure having high fatigue strength can be stably imparted to the spring material. Further, as disclosed in Patent Document 2, a composite structure can be obtained by heat-treating a part of the spring material, and it can be expected to obtain a spring material having excellent mechanical properties as a “spring”. Recently, however, higher fatigue strength has been demanded.

  The present invention has been made in view of the situation as described above, and uses a high-frequency heating to provide a leaf spring material having excellent mechanical properties and a method for producing a leaf spring material capable of stably obtaining the leaf spring material. The purpose is to provide.

  The manufacturing method of the leaf spring material according to the present invention is such that a tensile stress is applied along the longitudinal direction to the first surface of the substantially strip-shaped steel plate, and a compressive stress is applied along the longitudinal direction to the second surface. It is a manufacturing method of the leaf spring material which induction-hardens the surface. The induction hardening is characterized in that an induction hardening structure made of martensite and finely dispersed austenite having an average hardness higher than that of the base material structure in the vicinity of the second surface is provided in the vicinity of the first surface.

  According to the method for manufacturing a leaf spring material of the present invention, an induction-hardened structure composed of martensite and finely dispersed austenite having an average hardness higher than that of the base material structure can be obtained. This induction-quenched structure contains austenite, but since it is uniformly and finely dispersed, the quenching hardness is not greatly reduced. Therefore, the high average hardness obtained improves the durability of the spring and improves the life of the spring.

  In addition, the austenite, which is uniformly and finely dispersed in the induction-hardened structure, undergoes stress-induced transformation when used as a leaf spring and decomposes into fine martensite. Therefore, together with the use as a “spring”, the compressive stress in the vicinity of the first surface is further increased, so that high fatigue strength is imparted to the leaf spring material. That is, the spring life can be improved.

  Such a leaf spring material having a long spring life can be obtained by optimizing quenching conditions by so-called stress induction quenching in which induction quenching is performed while applying tensile stress. Since the structure peculiar to the induction hardening can be realized by using high-frequency heating with stable temperature management, it is possible to provide a leaf spring material having a stable and high spring life.

  Furthermore, the leaf spring material according to the present invention is a leaf spring material made of a substantially strip-shaped steel plate having a first surface and a second surface, and has a higher average hardness than a base material structure in the vicinity of the second surface. It has an induction-hardened structure composed of martensite and uniformly finely dispersed austenite in the vicinity of the first surface.

  The leaf spring material of the present invention has an induction-quenched structure composed of martensite and uniformly finely dispersed austenite having an average hardness higher than that of the base material structure. Such an induction hardened structure contains austenite. However, since it is a unique induction-quenched structure in which austenite is uniformly finely dispersed, the quenching hardness is not greatly reduced. That is, the high hardness by the induction-hardened structure can improve the durability of the spring and improve the spring life.

  In addition, the austenite, which is uniformly and finely dispersed in the induction-hardened structure, undergoes stress-induced transformation when used as a leaf spring and decomposes into fine martensite. Therefore, together with the use as a “spring”, the compressive stress in the vicinity of the first surface is further increased, so that high fatigue strength is imparted to the leaf spring material. That is, the spring life can be improved.

  A leaf spring material and a manufacturing method thereof as one embodiment of the present invention will be described.

  A plate spring material provided with a shape as a “plate spring” by cold or hot forming a metal material made of spring steel or the like is prepared. That is, a normal leaf spring material is prepared. A load is applied to such a leaf spring material made of a substantially strip-shaped curved steel plate in the same direction as when used as a “spring” at room temperature. That is, the bent and bent leaf spring material is stretched and fixed as a substantially flat plate. At this time, since the leaf spring material has a thickness, tensile stress is generated along the longitudinal direction on the first surface, and compressive stress is generated along the longitudinal direction on the second surface on the opposite side. is there. In order to obtain a typical induction-quenched structure to be described later, the tensile stress on the first surface is preferably higher, and is 1000 MPa or more at room temperature. This tensile stress can be changed by the shape, thickness, fixed shape, etc. of the leaf spring material.

  When the first surface is induction-hardened with the leaf spring material stretched and fixed as a substantially flat plate, the tensile stress on the surface is quickly relaxed by heat. When the fixing is released after the cooling and the load is removed, a compressive residual stress is applied to the first surface of the leaf spring material. Thereafter, heat treatment at 80 ° C. to 160 ° C. is preferably performed. The structure and thickness of the induction-quenched structure described later can be controlled by the output of the induction heating source and the cooling conditions.

  By the way, in the leaf spring material obtained after heat treatment such as the above-described stress induction hardening and optionally low temperature tempering, the vicinity of the second surface opposite to the first surface subjected to induction hardening has a base material structure. In other words, the vicinity of the second surface is the same as the structure of the leaf spring material prepared first, and is a base material composed of one or more of martensite, tempered martensite, troostite, sorbite, bainite, ferrite and pearlite. It is an organization. Note that the vicinity of the first surface is the same structure as the base material structure before induction hardening. However, stress induction hardening is performed to obtain an induction hardening structure made of martensite and uniformly finely dispersed austenite having an average hardness higher than that of a base material structure of 650 HV or more in the vicinity of the first surface. Such high hardness improves the durability as a “spring” and improves the spring life. Here, the thickness of the induction-hardened structure is preferably 5 to 10 percent of the thickness of the leaf spring material.

  The stress induction hardening is performed so as to increase the volume content of austenite, and the volume content is preferably 15% or more. Such austenite becomes finely uniformly dispersed particles having an average particle diameter of 500 nm or less, preferably 100 nm or less, by stress induction hardening while applying high tensile stress. Such austenite, which is peculiar to induction-hardened structure, causes stress-induced transformation when used as a leaf spring, decomposes into fine martensite, and increases the compressive stress in the vicinity of the first surface along with use as a “spring”. . Therefore, high fatigue strength can be given to the leaf spring material, and the spring life can be improved.

  When induction hardening is performed while applying the above-described large tensile stress to the first surface, the crystal grain size of the induction hardening structure can be made finer than that of the base material structure, and the spring life can be greatly improved. Preferably, the crystal grain size of the base material structure is 8 to 10, and the crystal grain size of the induction-hardened structure is 11 to 13.

  Furthermore, it is preferable to impart a compressive residual stress of 650 MPa or more to the spring plate in the induction-quenched structure because the spring life can be greatly improved.

By measuring the half-value width of the (200) _α diffraction peak of Fe using the CrKα ray diffractometer method for such an induction-quenched structure, the strain state of the induction-quenched structure can be measured, and the spring life can be easily predicted. . It is preferable that the half width is 8 ° or more because the spring life can be greatly improved.

As described above, according to the leaf spring material of the present invention by the manufacturing method of the present invention, compared to a conventional leaf spring material made of improved ausforming material, for example, it has a fatigue life based on the Weibull statistical probability. evaluated at some B ₁₀ life is to obtain a stable 3 times more life.

  Further, embodiments of the leaf spring material and the manufacturing method thereof according to the present invention will be described in detail with reference to FIGS.

  In the following, details of an example using JIS SUP11A steel material will be described, but the steel type is not limited to this. In particular, the same implementation is possible for general spring steel (JIS 4801) and equivalent materials.

  As shown in FIG. 1, a steel plate made of spring steel having a ferrite + pearlite dual phase structure is cut into strips having a predetermined dimension. When the shape is adjusted while heating and heat treatment for quenching and tempering is performed, a leaf spring material 10-1 before processing of the present invention having a curved and bent shape can be obtained. This process is well known and will not be described in detail. Such pre-process leaf spring material 10-1 has a structure obtained by tempering martensite. In addition, a leaf spring material having a ferrite + pearlite two-phase structure without quenching, a leaf spring material having a bainite structure, and the like can be used as appropriate.

  As shown in FIG. 1 (a), the warped both ends of the pre-process leaf spring material 10-1 are arranged on the fixed base 1 so that the both ends are on the top and the center is on the bottom.

  As shown in FIG.1 (b), the both ends of the leaf spring material 10-1 before a process are pressed on the fixing stand 1, and it fixes with the clamp 2 so that it may become a substantially flat plate. At this time, due to the thickness of the pre-treatment leaf spring material 10-1, a tensile stress TMPa is generated on the first surface (upper surface) 11, and a compressive stress is generated on the second surface (lower surface) 12.

  As shown in FIG.1 (c), the 1st surface 11 is heated with a predetermined output, moving the high frequency heating means 3 along the longitudinal direction A of the leaf spring material 10-1 before a process. Thereby, the tensile stress of the 1st surface 11 is relieved. At the same time, the surface layer 11a in the vicinity of the first surface 11 changes from a structure tempered martensite (or a ferrite + pearlite two-phase structure or a bainite structure) to a substantially single-phase structure of austenite that is a high-temperature phase. .

  On the other hand, while the cooling means 4 is moved in the A direction so as to follow the high-frequency heating means 3, the heated portion of the first surface 11 is sequentially cooled. As a result, the surface layer 11a in the vicinity of the first surface 11 that has changed to the austenite phase in the heating step described above is quenched into the martensite + finely dispersed austenite two-phase structure.

  The leaf spring material 10-1 before processing is adjusted by adjusting the tensile stress T, high-frequency heating conditions (output and moving speed, etc.) and cooling conditions (cooling capacity, moving speed, etc.) (hereinafter referred to as quenching conditions). It is possible to adjust the temperature distribution near one surface, the quenched structure, and the like. That is, the thickness of the surface layer 11a, the austenite amount of the surface layer 11a, the residual stress in the vicinity of the first surface 11 and the like can be adjusted.

  As shown in FIG. 1 (d), the above-described high-frequency heating and quenching are performed as necessary from a predetermined portion of the pre-process leaf spring material 10-1, for example, only the central portion, or from one end to the other end as appropriate. is there.

  As shown in FIG. 1E, when the clamp 2 is removed, the compressive stress on the second surface is released, and the leaf spring material 10 returns to the shape in which both ends are warped upward again. At this time, compressive stress tends to be generated in the vicinity of the first surface 11.

  As shown in FIG. 2, the leaf spring material 10 (thickness is D) according to this example is a mother made of a martensite tempered structure (or a ferrite + pearlite two-phase structure or a bainite structure). It consists of two layers of a base material layer 12a having a material structure and a surface layer 11a having a thickness d of martensite + a finely dispersed austenite two-phase structure (quenched structure). Here, the base material structure is preferably subjected to a treatment for evenly distributing the carbide.

  By the way, several pre-experiments have been carried out in preparing samples of examples and comparative examples described later.

  FIG. 3 shows the hardness distribution 31 and the residual stress distribution 34 in the depth direction of the leaf spring material 10 that has been subjected to stress induction hardening as described above on a substantially strip-shaped spring material 10-1 made of a SUP11A steel plate and having a thickness of 18 mm. The measurement results are shown. The hardness distribution 31 is substantially constant in the depth direction from the first surface 11 and is about 750 HV, and rapidly decreases at a depth of 1.5 mm. The compressive residual stress distribution 34 is 750 MPa on the first surface 11 (depth 0 mm), but reaches a maximum of 1200 MPa or more at a depth of 1.5 mm and rapidly decreases while increasing the absolute value. It was.

  Next, FIG. 3 shows a hardness distribution 32 and a compressive residual stress distribution of a leaf spring material (hereinafter, this leaf spring material is referred to as 10 ′) obtained by heat-treating the leaf spring material 10 described above at 150 ° C. for 1 hour. 35 is shown. Due to the effect of the heat treatment, the hardness distribution 32 was about 100 lower than the hardness distribution 31 and was about 650 HV. The compressive residual stress distribution 35 is 650 MPa on the first surface 11 (depth 0 mm), but reaches a maximum of 1000 MPa or more at a depth of 1.5 mm while slightly increasing its absolute value. After that, it dropped sharply.

  Further, FIG. 3 shows the hardness distribution 33 and the compressive residual stress 36 of the leaf spring material obtained by heat-treating the leaf spring material 10 described above at 250 ° C. for 1 hour. Due to the effect of the heat treatment, the hardness distribution 33 is lower than the hardness distributions 31 and 32 and is about 500 HV. The compressive residual stress distribution 36 was 400 MPa on the first surface 11, and the absolute value was lower than that of the compressive residual stress distributions 34 and 35. The compressive residual stress distribution 36 reached a maximum of 750 MPa at a depth of 1.5 mm with a slight increase in its absolute value, but rapidly decreased.

  Finally, FIG. 3 shows a compressive residual stress distribution 37 of the leaf spring material 10-1 that is not tempered while being induction-hardened without applying stress. According to this, since the distribution is almost the same as the compressive residual stress distribution 36, it was found that the effect of applying stress is lost when heat treatment is performed at 250 ° C. for 1 hour.

  Further, the cross-sectional structure of the leaf spring material 10 ′ obtained by heat-treating the leaf spring material 10 described above at 150 ° C. for 1 hour was observed.

  As shown in FIG. 4, a typical induction hardening structure of the surface layer 11a of the leaf spring material 10 'is a two-phase structure of a fine martensitic tempered structure having a prior austenite grain boundary having a grain size of 12 or more and residual austenite. In the mixed structure, the retained austenite was present finely and evenly in submicron order as if it were uniformly dispersed in the prior austenite grain boundaries. Further, according to EBSP analysis (measured at an acceleration voltage of 25 kV using an OIM EBSP detector manufactured by TSL), the retained austenite is about 15 vol. %, And was dispersed finely and evenly in granular form on the order of submicrons. As described later, this coincided with the amount of retained austenite obtained from X-ray diffraction. On the other hand, as shown in FIG. 5, the base material structure is typically a structure obtained by tempering martensite, and hardly any retained austenite was detected.

Next, in the induction-hardened structure before and after the durability test of the leaf spring material 10 in which the stress strip induction material 10-1 made of a SUP11A steel plate and having a thickness of 18 mm is subjected to stress induction hardening. The residual austenite distribution was investigated. The amount of retained austenite was determined by the diffraction intensity (integral) of (200) _α and (211) _α , (200) _γ and (220) _{γ at} a tube voltage of 40 kV and a tube current of 200 mA in X-ray analysis. Value) ratio.

  As shown by a curve 41 in FIG. 6, the amount of retained austenite on the surface 11 (depth 0 mm) of the leaf spring material 10 is about 25 vol. %, It decreases rapidly from the surface 11 at a depth of about 1 mm, and about 15 vol. At a depth of about 1.2 mm. %, It was almost 0 at a depth of about 1.5 mm. When an endurance test was performed on the leaf spring material 10, the amount of retained austenite on the surface 11 was 13 vol. %, And there was almost no change at a position deeper than about 0.5 mm. That is, the uniformly finely dispersed austenite peculiar to the induction-hardened structure is changed into fine martensite by stress-induced transformation. At this time, the toughness of the spring material surface is improved by transformation-induced plasticity, so that the durability as a “spring” is also improved. In order to obtain such an effect, as will be described later, a larger amount of austenite than a predetermined amount is required, and preferably 15 vol. % Or more. On the other hand, untransformed austenite after stress-induced transformation causes strength suppression. In other words, the austenite amount is preferably 30 vol. % Or less.

  Next, the heat treatment temperature and the amount of retained austenite of the leaf spring material 10 obtained by applying stress induction hardening to the substantially strip-shaped spring material 10-1 having a thickness of 18 mm made of a SUP11A steel plate to give a surface layer 11a of 1.5 mm. Measurements were made.

  As shown by the curve 51 in FIG. 7, when the heat treatment temperature is increased, the retained austenite is decomposed and reduced to low carbon martensite and ε carbide. In addition, shrinkage occurs during this decomposition process, and tensile stress acts, so fatigue strength is reduced. Therefore, in order to improve the durability of the spring material by transformation-induced plasticity, it is necessary to set the heat treatment temperature to a predetermined temperature.

As shown in FIG. 8, the half width Δ2θ of (200) _α of Fe converges as the heat treatment temperature increases. In other words, martensite is also decomposed into low carbon martensite and ε carbide by heat treatment, and the average hardness and residual stress decrease, so the heat treatment temperature should not be too high in order to maintain a predetermined hardness or more. . This experiment will be further described later. (Measurement of the half width of (200) _α in X-ray diffraction was performed under the conditions of a Cr tube, a tube voltage of 30 kV, and a tube current of 10 mA.)

  Next, a tensile stress of the leaf spring material 10 provided with a surface layer 11a of 1.5 mm by applying different tensile stress to the substantially strip-shaped spring material 10-1 having a thickness of 18 mm made of a SUP11A steel plate and applying stress induction hardening. Measurements were made on the relationship between the stress T and the depth direction distribution of retained austenite.

  As shown in FIG. 9, compared with the retained austenite amount 71 of the leaf spring material 10 having a tensile stress T = 1000 MPa, the retained austenite amount 72 of the leaf spring material 10 not loaded with the tensile stress is lower in the vicinity of the surface. That is, the amount of retained austenite can be increased by increasing the tensile stress. This is because when tensile stress T acts upon induction hardening, a large number of transformation nuclei are generated, and each growth is suppressed by interference with adjacent nuclei, so that the amount of residual austenite increases without completing the transformation. Conceivable.

  Based on the above experimental results, Examples 1 to 5 and Comparative Examples as shown in Table 1 were made by changing the quenching conditions to a substantially strip-shaped pre-processing leaf spring material 10-1 having a plate thickness of 18 mm made of a SUP11A steel plate. 18 samples of 1 to 13 were prepared.

First, FIG. 10 shows a sample having a tensile stress T of 1000 MPa, that is, Example 2 (reference symbol 81), Example 4 (reference symbol 82), Example 5 (reference symbol 83), and Comparative Example in Table 1. Each sample of No. 3 (reference symbol 84), Comparative Example 4 (reference symbol 85), and Comparative Example 9 (reference symbol 86) was subjected to an endurance test and the number of endurances thereof was shown. The stress amplitude is 0.8 times the average stress shown on the vertical axis in FIG. 10, that is, 650, 700 and 750 MPa. Further, dashed line M in the drawing represents three times the life of a known exemplary improvement ausforming material B ₁₀ life.

In the second embodiment (reference symbol 81), the fourth embodiment (reference symbol 82), and the fifth embodiment (reference symbol 83), the number of endurances is larger than the one-dot broken line M. On the other hand, Comparative Example 3 (reference symbol 84), Comparative Example 4 (reference symbol 85), and Comparative Example 9 (reference symbol 86) are on the endurance frequency side, which is smaller than the one-dot broken line M. Shown In other words, the induction hardening depth is 10% or less, and, stably three times more durable known typical improvements ausforming material B ₁₀ life in spring material of the heat treatment temperature is 0.99 ° C. or less It is.

Next, FIG. 11 shows the half-value width of Fe at (200) _α and durability test when X-ray diffraction was performed on the above-described leaf spring material 10 under the conditions of a Cr tube, a tube voltage of 30 kV, and a tube current of 10 mA. The relationship of endurance frequency is shown. In relation to the half-width Δ2θ and endurance, but with half width 8 ° or more, the endurance of the leaf spring material is 4 × 10 ⁵ or more, also 2 × 10 ⁵ The following are less than 8 °. Reference numerals 111, 112, and 113 indicate the results at average stresses of 650 MPa, 700 MPa, and 750 MPa, respectively. In contrast, in FIG. 8, the half-value width Δ2θ was smaller than 8 ° at a heat treatment temperature higher than 160 ° C. That is, when viewed in conjunction with FIG. 11, when the heat treatment temperature is higher than 160 ° C., the durability is lowered. This corresponds to the fact that the quenching martensite was decomposed into low carbon martensite and ε carbide (Fe _2.4 C) in the vicinity of 160 ° C.

  Further, FIG. 12 shows the number of times of durability (k) by the durability test of the leaf spring material when the depth of the surface layer 11a having high hardness is changed for the leaf spring material 10 described above. According to this, as the heat treatment temperature is lower, a predetermined durability can be obtained in the heat treatment preferably at a temperature lower than 160 ° C. At such a preferable heat treatment temperature, the greater the thickness (depth) d of the surface layer 11a, the higher the durability. On the other hand, when the thickness d of the surface layer 11a is increased, the stress (restoring force) applied from the base material layer 12a is relatively reduced and the compressive residual stress on the surface 11 is reduced. Therefore, preferably, the thickness d of the surface layer 11 a is 10% or less of the thickness D of the spring material 10. On the other hand, if the thickness d of the surface layer 11a is too small, the above-described effects of the surface layer 11a are reduced. Therefore, preferably, the thickness d of the surface layer 11a is 5% or more of the thickness D of the spring material 10.

As described above, according to the spring material of the present embodiment, it exhibits stable three times more durable known typical improvements ausforming material B ₁₀ life.

It is a figure of the manufacturing method of the leaf | plate spring material by this invention. It is a partial cross section of the leaf | plate spring material by this invention. It is a graph of the relationship between heat processing temperature, hardness distribution, and residual stress distribution. It is an etching photograph of a typical induction-hardened structure. It is an etching photograph of a typical base material structure. It is a graph of the relationship of the distribution of the depth direction of the amount of retained austenite before and after an endurance test. It is a graph of the relationship between the amount of retained austenite and the heat treatment temperature. It is a graph of the relationship between the half-value width Δ2θ by X-ray and the heat treatment temperature. It is a graph of the relationship between the distribution of the tensile stress and the amount of retained austenite in the depth direction. It is a graph of the relationship between the average stress and the frequency | count of durability in the durability test of the leaf | plate spring material of this invention and a comparative example. It is a graph of the relationship between the half-value width Δ2θ by X-rays and the number of durability times. It is a figure of the result of the endurance test with respect to heat processing temperature and hardened layer thickness.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fixing base 2 Clamp 3 High frequency heating means 4 Cooling means 10 Leaf spring material 10-1 Leaf spring material 11a before processing Surface layer 12a Base material layer

Claims (23)

A method for producing a leaf spring material in which a first surface of a substantially strip-shaped steel plate is subjected to tensile stress along the longitudinal direction and a compressive stress is applied to the second surface along the longitudinal direction, and the first surface is induction-hardened. Because
The induction hardening is a leaf spring characterized in that an induction hardening structure composed of martensite and finely dispersed austenite having a higher average hardness than the base material structure in the vicinity of the second surface is provided in the vicinity of the first surface. A method of manufacturing the material.   The method for producing a leaf spring material according to claim 1, wherein the base material structure is composed of at least one of martensite, a structure obtained by tempering martensite, troostite, sorbite, bainite, ferrite and pearlite.   The method for producing a leaf spring material according to claim 2, wherein the induction-hardened structure has a crystal grain size smaller than that of the base material structure.   The method for producing a leaf spring material according to claim 3, wherein the base material structure has a crystal grain size of 8 to 10, and the induction-hardened structure has a crystal grain size of 11 to 13.   The method for producing a leaf spring material according to any one of claims 1 to 4, wherein the tensile stress is 1000 MPa or more at room temperature, and includes a step of unloading the tensile stress after the induction hardening. .   6. The method for manufacturing a leaf spring material according to claim 5, further comprising a low-temperature tempering step after the step of unloading the tensile stress.   The method for manufacturing a leaf spring material according to claim 6, wherein a heat treatment temperature in the low temperature tempering step is 80 ° C to 160 ° C.   The method for manufacturing a leaf spring material according to claim 1, wherein the induction-quenched structure has a hardness of 650 HV or more.   The method for producing a leaf spring material according to claim 8, wherein the volume content of the austenite in the induction-hardened structure is 15% or more.   The method for producing a leaf spring material according to claim 1, wherein the austenite of the induction-hardened structure has an average particle size of 500 nm or less.   The method for producing a leaf spring material according to claim 1, wherein the induction-quenched structure is distributed at a thickness of 5 to 10 percent of the thickness of the steel plate.   The method for manufacturing a leaf spring material according to claim 1, wherein the induction-quenched structure has a compressive residual stress of 650 MPa or more. 13. The half-width of the (200) _α diffraction peak of Fe by a diffractometer method of CrKα ray for the induction-quenched structure is 8 ° or more, according to claim 1. A method of manufacturing a leaf spring material.   A leaf spring material made of a substantially strip-shaped steel plate having a first surface and a second surface, comprising martensite and finely dispersed austenite having an average hardness higher than that of the base material structure in the vicinity of the second surface. A leaf spring material having an induction-hardened structure in the vicinity of the first surface.   The leaf spring material according to claim 14, wherein the base material structure is composed of at least one of martensite, a structure obtained by tempering martensite, troostite, sorbite, bainite, ferrite, and pearlite.   The leaf spring material according to claim 15, wherein the induction hardening structure has a crystal grain size smaller than that of the base material structure.   The leaf spring material according to claim 16, wherein a crystal grain size of the base material structure is 8 to 10, and a crystal grain size of the induction-hardened structure is 11 to 13.   The leaf spring material according to any one of claims 14 to 17, wherein the induction-hardened structure has a hardness of 650 HV or more.   The leaf spring material according to claim 18, wherein a volume content of the austenite in the induction-hardened structure is 15% or more.   The leaf spring material according to claim 14, wherein the austenite has an average particle size of 500 nm or less.   21. The leaf spring material according to claim 14, wherein the induction hardened structure is distributed at a thickness of 5 to 10% of a thickness of the steel plate.   The leaf spring material according to any one of claims 14 to 21, wherein the induction-quenched structure has a compressive residual stress of 650 MPa or more. 23. The half-width of the (200) _α diffraction peak of Fe by a diffractometer method of CrKα rays for the induction-quenched structure is 8 ° or more. Leaf spring material.