JP6284279B2 - Strength member and manufacturing method thereof - Google Patents

Description

  The present invention relates to a strength member excellent in sag resistance and yield strength and a method for producing the same.

  For example, Si-Cr steel oil tempered wires having a tempered martensite structure have been widely used as materials for strength members such as automotive engine valve springs from the viewpoint of fatigue resistance and sag resistance. . On the other hand, Patent Document 1 gives a larger plastic strain than a tempered martensite structure without reducing fatigue resistance by forming a structure mainly composed of fine bainite having excellent ductility after coiling. Techniques to do this have been proposed. In this technique, the density of dislocations harmful to sag resistance is reduced, and the sag resistance is improved by fixing the dislocations effectively by strain aging. Moreover, since a large plastic strain is imparted by performing setting, a large compressive residual stress is imparted inside the strand, and fatigue resistance can be improved as well as sag resistance. Further, the above technique has an advantage that the manufacturing cost can be reduced because an inexpensive material can be used.

JP 2012-111992 A

  However, in recent years, as fuel efficiency of automobiles is required more than ever, strength members such as springs and bolts are required to have higher sag resistance and yield strength.

  Therefore, the present invention has been made in view of the above circumstances, strength that can significantly improve sag resistance and yield strength without impairing cost merit and without significant process changes. It aims at providing a member and its manufacturing method.

  As a result of diligent research to solve the above problems, the inventors of the present invention are able to decompose martensite generated by water cooling in austempering treatment into ferrite and cementite and reduce dislocations by tempering. Thus, it has been found that the sag resistance is greatly improved. In addition, it is common that the structure rapidly softens and decreases the fatigue strength as the number of dislocations in martensite decreases, but by using fine bainite as the main component of the structure, the fatigue strength due to the decrease in hardness It was also found that there was no decrease in. On the other hand, since the improvement in sag resistance according to the present invention is accompanied by an increase in yield strength, it can be applied to screw members such as bolts and tie rods that require high yield strength.

The strength member of the present invention was made on the basis of the above knowledge, and in mass%, C: 0.5 to 0.7%, Si: 1.0 to 2.0%, Mn: 0.1 to 1 0.0%, Cr: 0.1-1.0%, P: 0.035% or less, S: 0.035% or less, the balance of iron and inevitable impurities, and bainite 65% or more by area ratio The average dislocation density of an arbitrary cross section is 2.0 × 10 ¹⁶ m ⁻² or less.

Further, the present invention is a method for manufacturing a strength member according to claim 1, in mass%, C: 0.5~0.7%, Si : 1.0~2.0%, Mn: 0 0.1 to 1.0%, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, and a wire having a component composed of iron and inevitable impurities as a balance. After forming into a shape and austenitizing at a temperature of Ac3 point to (Ac3 point + 250 ° C.), it is cooled at a rate of 20 ° C./second or more, and (Ms point−20 ° C.) to (Ms point + 60 ° C.). ) For 400 seconds or more, and then cooled to room temperature, and a tempering step for holding the heat-treated product at a temperature of 350 to 450 ° C. Here, the Ac3 point is the boundary temperature at which the material shifts from the ferrite + austenite two-phase region to the austenite single-phase region during heating, and the Ms point is the formation of martensite during cooling. This is the starting temperature. When the strength member is a spring, it is desirable to provide a shot pinning process for projecting shots onto the product.

  The present invention is not limited to a spring, and can be applied to any strength member that requires strength, such as a screw member such as a bolt or a tie rod.

  According to the present invention, by reducing the average dislocation density of an arbitrary cross section, the sag resistance and the yield strength can be significantly improved without impairing cost merit and without adding a significant process change. It is possible to obtain an effect such as

It is a figure which shows the process of the manufacturing method of this invention. It is a graph which shows the relationship between the tempering temperature and average dislocation density in the Example of this invention. It is a graph which shows the relationship between the tempering temperature and the residual shear strain in the Example of this invention. It is a graph which shows the relationship between the tempering temperature in the Example of this invention, and the internal hardness of a spring strand.

  First, the reasons for limiting the chemical components of the steel used in the present invention will be described. In the following description, “%” means “mass%”.

C: 0.5-0.7%
C is an important element for securing a desired strength, and in order to obtain such an effect, it is necessary to contain 0.5% or more. However, if the C concentration is excessive, the ratio of retained austenite, which is a soft phase, is excessively increased and it becomes difficult to obtain a desired strength.

・ Si: 1.0-2.0%
Si is an element that contributes to solid solution strengthening, and in order to obtain a desired strength, it is necessary to contain 1.0% or more. However, if the amount of Si is excessive, the ratio of soft retained austenite is increased, and conversely, the strength is reduced, so the content is suppressed to 2.0% or less.

Mn: 0.1 to 1.0%
Mn is added as a deoxidizing element during refining, but on the other hand, it is an element that can improve the hardenability of the steel material and easily improve the strength. Therefore, in order to obtain a desired strength, 0.1% or more is contained. There is a need. On the other hand, when the content is excessive, segregation occurs and the workability is liable to decrease, so the content is suppressed to 1.0% or less.

・ Cr: 0.1-1.0%
Cr is an element that can enhance the hardenability of the steel material and easily improve the strength. Further, it also has an effect of delaying the pearlite transformation, and a bainite structure can be stably obtained (cooling the pearlite structure) during cooling after austenitizing heating, so it is necessary to contain 0.1% or more. However, if excessively containing Cr, iron carbide is likely to be generated, so the content is suppressed to 1.0% or less.

-P, S: 0.035% or less P and S are elements that promote grain boundary destruction due to grain boundary segregation. Therefore, the content of P and S is preferably as low as possible. Since smelting costs are required, the upper limit is 0.035%. The content of P and S is preferably 0.01% or less.

  Next, the reasons for limitation such as the area ratio of bainite in the entire structure will be described. Bainite: 65% or more Bainite is a metal structure obtained by isothermally transforming an austenitic steel material at a temperature range of about 550 ° C. or less and exceeding the martensite transformation start temperature. Consists of tick ferrite and iron carbide. Since the base bainitic ferrite has a high dislocation density and the iron carbide has a precipitation strengthening effect, the strength can be increased with a bainite structure even if the hardness decreases due to the reduction of dislocations in martensite.

  According to the manufacturing method of the present invention, the bainite structure holds the austenitic steel material isothermally in the vicinity of the Ms point, so that a structure in which iron carbide finely precipitates on a fine bainitic ferrite ground can be obtained. In addition, the decrease in grain boundary strength is small and the decrease in ductility is small even if the strength is high. Therefore, even if a large plastic strain is applied, defects such as cracks harmful to fatigue resistance do not occur, and the dislocation density can be reduced. As described above, bainite is an indispensable structure for obtaining high strength and high ductility, and its area ratio is preferably as high as possible. In order to obtain desired high strength and high ductility, 65% or more is necessary.

  On the other hand, the untransformed austenite during isothermal holding becomes martensite and retained austenite by cooling to room temperature. A structure having a bainite area ratio of less than 65% means that the isothermal holding time is short, and the concentration of C in the untransformed austenite at that stage is small, so that the martensite ratio is increased by subsequent cooling. . Therefore, when the bainite area ratio is less than 65%, martensite is increased and high strength is obtained, but notch sensitivity is remarkably increased, so that a large plastic strain cannot be imparted and sag resistance is increased. Does not improve.

  In addition, since retained austenite is soft, shear strain generated by processing tends to remain. Therefore, the amount of retained austenite serves as an index of the amount of residual shear strain, and if the amount is excessive, the sag resistance is lowered. From this viewpoint, it is desirable to keep the area ratio of retained austenite to 6.5% or less.

  Further, it is desirable that the Vickers hardness at the center of an arbitrary cross section of the product is 450 HV or more in order to ensure the strength that can withstand the load required for the product. On the other hand, when the hardness is excessively high, the notch sensitivity of the upper steel material itself, which usually decreases in elongation, increases, and a large plastic strain cannot be imparted. Therefore, the hardness is desirably 650 HV or less.

  Next, the manufacturing method of the strength member of the present invention will be described taking a spring as an example. FIG. 1A is a diagram showing a manufacturing method of the embodiment, and FIG. 1B is a diagram showing a conventional manufacturing method. For example, after the coiling step, the spring is austenitized at a temperature of Ac3 point to (Ac3 point + 250 ° C.) after the coiling step, and then at 20 ° C./(Ac3 point + 250 ° C.). After the heat treatment step of cooling at a rate of at least 2 seconds, holding at a temperature of (Ms point−20 ° C.) to (Ms point + 60 ° C.) for 400 seconds or more, and then cooling to room temperature at a cooling rate of 20 ° C./second or more, It can manufacture by performing tempering at 350-450 degreeC and performing a setting process as needed after a shot pinning process. There is no particular limitation on the structure of the steel before heating to Ac3 point or higher. For example, a hot-forged or drawn steel strip can be used as a raw material. Hereinafter, each step will be described, and the reasons for limitation will be described as necessary.

・ Coiling process
This is a step of cold forming into a desired coil shape. As a forming method, a method using a spring forming machine (coiling machine), a method using a core metal, or the like may be used. In addition, it can apply to arbitrary springs, such as a leaf | plate spring, a torsion bar, and a stabilizer other than a coil spring.

・ Surface polishing process
This step is performed as necessary, and is a step for polishing both end surfaces of the spring so as to be a plane perpendicular to the axis of the spring.

-Heat treatment process After the coiled spring is austenitized, the heat treatment process is completed by isothermal holding and then cooling. There are no particular restrictions on the structure of the steel before austenitization. For example, a hot-forged or drawn steel strip can be used as the material. The austenitizing temperature needs to be from Ac3 point to (Ac3 point + 250 ° C.). Below the Ac3 point, it does not become austenite and remains in the structure of the material. Moreover, when it exceeds (Ac3 point +250 degreeC), a prior austenite particle size becomes easy to coarsen and there exists a possibility of causing a fall of ductility.

  The higher the cooling rate to the temperature at which the temperature is maintained isothermally after the austenitization, the better. When the cooling rate is less than 20 ° C./second, pearlite is generated during the cooling, and a bainite of 65 area% or more cannot be obtained. The temperature to be kept isothermally needs to be (Ms point−20 ° C.) to (Ms point + 60 ° C.), which is a very important control factor as a manufacturing method for realizing the spring steel and spring of the present invention. . When the isothermal holding temperature is less than (Ms point −20 ° C.), the amount of martensite generated at the early stage of transformation is large, and the improvement of ductility is inhibited, and more than 65 area% bainite cannot be obtained. On the other hand, when the isothermal holding temperature exceeds (Ms point + 60 ° C.), the bainite becomes coarse, so that the tensile strength decreases, and the spring cannot obtain the strength that can withstand the load. And fine bainite can be deposited by performing isothermal holding | maintenance in the Ms point vicinity as mentioned above. By the precipitation of fine bainite, austenite remains in a fine space and can be made into fine retained austenite grains.

  Bainite precipitates in austenite due to isothermal holding. The isothermal holding time needs to be 400 seconds or more, which is also a very important control factor for the production method of the present invention. If the isothermal holding time is less than 400 seconds, the progress of the bainite transformation is insufficient, so the bainite ratio is small and the area ratio of bainite is less than 65%. Even if the isothermal holding time is too long, the amount of bainite produced reaches the saturation amount and causes an increase in production cost.

  The cooling rate after isothermal holding is preferably as fast as possible in order to obtain a uniform structure. A cooling rate of 20 ° C./second or more is preferable, and more preferably 50 ° C./second or more. Specifically, oil cooling or water cooling is good.

-Tempering process The tempering process which hold | maintains a spring at the temperature of 350-450 degreeC is performed after a heat treatment process. If the tempering temperature is less than 350 ° C., the decomposition of martensite is insufficient and the reduction of dislocation is insufficient. On the other hand, when the tempering temperature exceeds 450 ° C., the internal hardness of the spring is remarkably reduced, and the strength and fatigue strength are lowered. In order to suppress an extreme decrease in the internal hardness of the spring, the tempering temperature is desirably 400 ° C. or lower. The tempering time is preferably 25 to 60 minutes. If the tempering time is less than 25 minutes, the tempering is insufficient, and if the tempering time exceeds 60 minutes, it is uneconomical.

-Shot peening process In shot peening, a shot made of metal, sand, or the like is collided with a spring to impart compressive residual stress to the surface, thereby significantly improving the fatigue resistance of the spring. In the present invention, in addition to the compressive residual stress obtained by normal shot peening, a higher and deeper compressive residual stress is formed by processing-induced martensitic transformation of residual austenite. For shots used in shot pinning, high-hardness particles such as cut wires, steel balls, and FeCrB series can be used. Further, the compressive residual stress can be adjusted by the effective or average equivalent sphere diameter of the shot, the projection speed, the projection time, and the multi-stage projection method.

-Setting process Setting is arbitrarily performed in order to significantly improve the elastic limit by applying plastic strain and to reduce the amount of sag during use (permanent deformation). In this case, sag resistance can be further improved by performing setting (warm setting) at 200 to 300 ° C. In addition, it is expected that the retained austenite undergoes processing-induced transformation by setting and becomes martensite with higher strength. Thereby, high compressive residual stress is given by the volume expansion accompanying transformation, and fatigue resistance can be further improved.

[First embodiment]
Using a Si—Cr steel oil temper wire (diameter: 4.1 mm) composed of the representative chemical components shown in Table 1, cold coiling into a predetermined shape was performed by a coiling machine, followed by heat treatment (austempering). In the heat treatment, the spring was austenitized by holding it in a heating furnace at a temperature of 830 ° C. for 12 minutes, then cooled in water, held in a salt bath maintained at a temperature of 300 ° C. for 40 minutes, and then cooled.

  Next, the spring was tempered at the temperature shown in Table 2. Tempering time was 60 minutes. Next, a shot made of steel having a sphere equivalent diameter of 0.1 to 1.0 mm was used for shot pinning. Furthermore, setting was performed after heating the spring to 200 to 300 ° C. Various properties of the obtained spring were investigated as follows.

[Phase distinction]
The phase was distinguished by immersing the sample in a 3% nital solution for several seconds and using the subsequent tissue as follows. First, since bainite is easily corroded by the night tar, it looks black or gray in the optical micrograph, while the residual austenite appears white in the optical microscope due to its high corrosion resistance to the night tar. Using this characteristic, the optical micrograph was subjected to image processing to determine the bainite (black and gray part) ratio and the total ratio of retained austenite (white part). The residual austenite ratio was determined by using an X-ray diffraction method for a buffed finish sample. In Table 2, the remaining structures of bainite and retained austenite are No. 1 and no. No. 2 is martensite. 3-No. In No. 7, ferrite and cementite.

[Vickers hardness at the center]
In the cross section of the sample, five points of Vickers hardness at the center were measured, and the average value was obtained.

[Average dislocation density]
The average dislocation density ρ is expressed by the following equation (1) with reference to the literature (Materials and Processes: Proceedings of the Iron and Steel Institute of Japan 17 (3), pp. 396-399 “Evaluation method of dislocation density using X-ray diffraction”). It calculated by calculating | requiring distortion | strain (epsilon).

ここで、ｂはバーガスベクトル（＝２．５×１０^−１０ｍ）である。また、歪εは、試料の横断面において、中心部を０．３ｍｍのコリメーターによりＸ線回折装置（Ｂｒｕｋｅｒ社製Ｄ８ ＤＩＳＣＯＶＥＲ）でフェライトの（１１０）、（２１１）、（２２０）の回折ピークを測定し、各ピークの半価幅βを用いて、以下の数２の関係から各回折ピークのβｃｏｓθ／λとｓｉｎθ／λをグラフの縦軸と横軸にプロットし、それらの近似曲線の傾き２εを求めることで算出した。

Here, b is a bar gas vector (= 2.5 × 10 ⁻¹⁰ m). Further, the strain ε is a diffraction peak of ferrite (110), (211), (220) in the X-ray diffractometer (D8 DISCOVER made by Bruker) with a collimator having a center of 0.3 mm in the cross section of the sample. Using the half width β of each peak, β cos θ / λ and sin θ / λ of each diffraction peak are plotted on the vertical axis and horizontal axis of the graph from the relationship of the following formula 2, It calculated by calculating | requiring inclination 2 (epsilon).

ここで、θはＸ線回折ピーク位置２θの半分の値、λはＸ線発生源として用いた管球のＫα₁線の波長、Ｄは結晶子サイズである。

Here, θ is a half value of the X-ray diffraction peak position 2θ, λ is the wavelength of the Kα ₁ line of the tube used as the X-ray generation source, and D is the crystallite size.

[Residual shear strain]
Residual shear strain is an index representing the sag resistance of a spring, and the lower the value, the better the sag resistance. In the spring sag test, the sample was fixed under compression by applying a load so that the maximum shear stress was 1050 MPa, and immersed in 165 ° C. silicone oil. After 24 hours from the start of immersion, the sample was taken out from the silicone oil, and the load was removed after the temperature reached room temperature. The amount of sag was determined by measuring the load when the spring was compressed to a predetermined height before and after the sag test, and substituting the load reduction amount ΔP into the following formula 3 to obtain the residual shear strain.

ここで、Ｄは平均コイル直径、ｄは線径、Ｇは横弾性係数（＝７８，５００ＭＰａ）である。

Here, D is an average coil diameter, d is a wire diameter, and G is a transverse elastic modulus (= 78,500 MPa).

The results measured as described above are shown together in Table 2, and the relationship between the measured value and the tempering temperature is shown in FIGS. As shown in FIG. 2, it was confirmed that when the tempering temperature was 350 ° C. or higher, the average dislocation density rapidly decreased to 2.0 × 10 ¹⁶ m ⁻² or less. Accordingly, as shown in FIG. 3, it was confirmed that when the tempering temperature was 350 ° C. or higher, the residual shear strain was rapidly reduced to 6.7 × 10 ⁻⁴ or less. Residual shear strain is an index of sag resistance. The smaller the residual shear strain, the higher the sag resistance. Moreover, as shown in FIG. 4, when the tempering temperature exceeded 400 degreeC, it was confirmed that the internal hardness of a spring falls rapidly.

As described above, it was confirmed that by setting the average dislocation density to 2.0 × 10 ¹⁶ m ⁻² or less, the residual shear strain can be set to 6.7 × 10 ⁻⁴ or less and the sag resistance can be improved.

[Second Embodiment]
A Si—Cr steel drawn wire rod (diameter: 6.0 mm) composed of the representative chemical components shown in Table 1 is cut to a predetermined size, subjected to head forging and screw rolling to form a bolt, and then heat treated (austempered). Treatment). In the heat treatment, the bolt was austenitized by holding it at a temperature of 830 ° C. for 12 minutes in a heating furnace, then cooled with water, held in a salt bath maintained at a temperature of 300 ° C. for 40 minutes, and then cooled.

  Subsequently, the bolt was tempered at the temperature shown in Table 3. Tempering time was 60 minutes. For the obtained bolt, the internal hardness, average dislocation density, and bainite area ratio were investigated in the same manner as in Example 1, and the tensile strength and 0.2% proof stress were measured with a tensile tester. The results are also shown in Table 3.

As shown in Table 3, in the bolt of the present invention example, it was confirmed that a high yield ratio was obtained by setting the average dislocation density to 2.0 × 10 ¹⁶ m ⁻² or less.

  The present invention can be applied to springs such as coil springs, leaf springs, torsion bars, and stabilizers, and strength members that require strength, such as screw members such as bolts and tie rods.

Claims (6)

In mass%, C: 0.5 to 0.7%, Si: 1.0 to 2.0%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, the balance is composed of iron and inevitable impurities, and has a structure having 65% or more of bainite by area ratio, and the average dislocation density of any cross section is 2.0. A strength member characterized by being not more than × 10 ¹⁶ m ^-2 .   The strength member according to claim 1, wherein the central Vickers hardness is 450 to 650 HV. In mass%, C: 0.5 to 0.7%, Si: 1.0 to 2.0%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, a molding step of molding a wire having a component consisting of iron and inevitable impurities into a product shape,
After austenitizing at a temperature of Ac3 point to (Ac3 point + 250 ° C.), cool at a rate of 20 ° C./second or more and hold at a temperature of (Ms point−20 ° C.) to (Ms point + 60 ° C.) for 400 seconds or more. And then a heat treatment step for cooling to room temperature,
A tempering step of holding the product after heat treatment at a temperature of 350 to 450 ° C .;
The method for producing a strength member according to claim 1, comprising :   The method for manufacturing a strength member according to claim 3, further comprising a shot pinning step of projecting a shot onto the product after the tempering step.   The method for producing a strength member according to claim 3 or 4, wherein a cooling rate when cooling to room temperature is 20 ° C / second or more.   The method for producing a strength member according to claim 4, further comprising a setting step for imparting permanent strain to the product after the shot peening step.

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