KR101393444B1 - U-bolt and method for processing the same - Google Patents

Abstract

The present invention relates to a U-bolt for fixing a leaf spring of a vehicle, which comprises 0.15 to 0.50 wt% of C, 1.5 wt% or less of Si (excluding 0%), 0.25 to 2.0 wt% of Mn, 0.03 wt% or less of P, and the remainder has a tensile strength of 70 to 130 Kgf / mm ² , a yield ratio of 85 to 97%, a section reduction ratio of 50% or more, and austenite crystal grain size ASTM No. ² , respectively, after drawing a steel material composed of unavoidable impurities and Fe. 9 or more, the steel wire is cut into U-bolt lengths, and is then subjected to a rolling process on both ends, followed by bending in a U-shape, and plating or painting the surface portion .
The present invention excels in spheroidizing annealing during the manufacture of U-bolts and omits the heat treatment, so that it is excellent in toughness and ductility while maintaining uniform strength without the necessity of calibrating after U-shaped bending.

Description

U-bolts and method for manufacturing same

The present invention relates to a U-bolt for fixing a leaf spring of a vehicle, and more particularly, it relates to a U-bolt for fixing a leaf spring of a vehicle to a U-bolt by eliminating spheroidizing annealing and omitting a heat treatment, The present invention relates to a U-bolt for a vehicle, which is excellent in toughness and ductility while maintaining a strength, and a manufacturing method thereof.

 Generally, a U bolt which requires high strength characteristics is used as a mechanical structural part for fixing a leaf spring to an automobile over a small truck.

The conventional U-bolt is manufactured by cutting a steel wire or steel bar (hereinafter, referred to as a "steel wire") into a predetermined length by spheroidizing and annealing S45C (JIS G 4051) or SCM435 (JIS G4105) (Screwing) for fastening the nut to both ends of the steel wire, and then quenching / tempering heat treatment is performed after bending in a U-shape. On the other hand, as the thermal deformation is accompanied inevitably in the process of performing the quenching / tempering heat treatment, the U bolts for automobiles are obtained by performing a correction work on them and finally plating or painting.

The wire rod of S45C or SCM435 is subjected to spheroidizing annealing at a temperature of 650 to 750 ° C. for 10 to 20 hours and then drawn to obtain a tensile strength of 50 to 60 kgf / Mm < 2 > is cut by the entire length of the U-bolt, and then, both ends are subjected to rolling-threading, and then bent in a U-shape. Since the bolts processed in the U-shape do not meet the required strength characteristics required for U-bolts for automobiles, the bent bolts are placed in an iron box as a post-process for securing the strength. For 60 minutes, quenching with water or oil, quenching at 400 to 600 ° C for 90 minutes, and then quenching / tempering treatment by cooling with water to obtain a tensile strength of 70 to 130 kgf / mm 2 as a required strength characteristic. Thereafter, as described above, the twist deformation generated in the heat treatment process is calibrated, and then the final product is obtained by plating or painting as the finishing process.

Such conventional U-bolts for vehicles and their manufacturing methods have problems such as the following energy loss, productivity, and workability deterioration, and improvement thereof is required.

1) In the case of spheroidizing annealing for the steel wire, energy loss is large due to heating over a long period of time.

2) It is necessary to carry out the quenching / tempering heat treatment to secure the tensile strength and toughness by moving the steel wire into the U-bolt shape and transferring it to a separate place. Therefore, it is difficult to carry out the continuous operation and the correction work for the deformation caused by the quenching heat treatment As the need arises, the entire manufacturing process becomes complicated.

3) For the quenching / tempering heat treatment, the temperature difference between the inside and outside of the iron box is large during the heat treatment in the furnace when the bolts processed in the U shape are placed in the iron box and the severe deviation of the tensile strength between the U- It can not be avoided, and if it is heat treated in its own, the work environment may become poor.

   In order to solve the problems of the prior art, the present invention uses a steel wire having high strength and excellent cold workability to omit the quenching / tempering heat treatment in manufacturing the U-bolt, thereby omitting the correction due to the heat treatment and simplifying the process It is also an object of the present invention to provide a U-bolt which is uniform in strength and extremely excellent in toughness and ductility even in products, and a method for producing the U-bolt.

 In order to achieve the above object, in order to simplify the complicated process of the conventional method and minimize the tensile strength deviation of the product, In the case of using a material having sufficient toughness and ductility so that cracks or fractures do not occur even if the material has a tensile strength equivalent to that of the product state while being bent in a U shape, the spheroidization annealing can be omitted, It is possible to omit the quenching / tempering heat treatment for increasing the strength after the machining. Therefore, the present invention has been accomplished through numerous test procedures based on the fact that a separate calibration work for the twist deformation is also unnecessary.

   As a means for attaining the above object, the present inventor has attempted to develop a high strength, high toughness and ductile steel wire by performing the quenching / tempering heat treatment which was carried out in the state of single piece after the shape processing of the U bolt Using this, the manufacturing process of the U bolts is extremely simplified so as to be the final product by cutting, rolling and U-shaped bending as shown in FIG.

On the other hand, in order to accomplish the above-described process, it is difficult to simply perform quenching / tempering only, and it is possible to have an appropriate material component, a proper heat treatment process, and appropriate property values.

   Accordingly, the present invention discloses a new U-bolt using a steel wire having a proper material composition, an appropriate heat treatment process, and an appropriate material property, and a manufacturing method thereof.

   Hereinafter, the configuration of the present invention will be described.

   The U-bolt of the present invention contains 0.15 to 0.50 wt% of C, 1.5 wt% or less of Si (excluding 0%), 0.25 to 2.0 wt% of Mn, 0.03 wt% or less of each of P and S (excluding 0% Fe and inevitable impurities, or a composition in which at least one of 0.05 to 2.0 wt% of Cr, 0.05 to 1.0 wt% of Mo, and 0.0003 to 0.0050 wt% of B is added to the above composition.

Austenitic crystal grain size ASTM No. 2 was obtained by using a heat treatment equipment comprising a series of connected processes of quenching and tempering after drawing, to a hot rolled material obtained by hot rolling as the above composition. 9 or more, a tensile strength of 70 to 130 kgf / mm ² , a yield ratio of 85 to 97%, and a reduction in cross section of 50% or more. The U bolt of the present invention is obtained.

   The reasons for limiting the added components of the material according to the present invention are as follows.

   C: 0.15 to 0.50 wt%

   C is the most important element necessary for increasing the strength in quenching, and it increases the strength by generating carbide. However, when the content is less than 0.15 wt%, the effect of hardening by quenching can not be expected. When the content exceeds 0.50 wt%, a large amount of carbide precipitates The ductility and toughness are lowered and the deformation resistance is increased, so that there is a fear of causing springback (property in the case of high-strength material to return by bending) during bending processing or cracking on the surface.

  Si: 1.5wt% or less (excluding 0%)

  Si is an element to be added to the molten steel for deoxidation and improves the strength by solid solution strengthening. However, if it exceeds 1.5 wt%, Si is dissolved in the precipitated carbide in a large amount to interfere with the movement of carbon during tempering, It is necessary to limit it to less than 1.5wt% because it causes the spring back problem in bending process because it increases elasticity.

   Mn: 0.25 to 2.0 wt%

  Mn is a solid solution strengthening element and is an element to compensate the decrease in strength in low C and Si added steels in order to avoid a decrease in impact toughness which may occur when C and Si are added excessively. Therefore, in order to expect the above effect, it is necessary to add at least 0.25 wt%, but when it is added excessively, toughness and deformation resistance are increased, so the addition amount should not exceed 2.0 wt%.

     Cr: 0.05 to 2.0 wt%

  Cr is an element added to improve strength and hardness hardness and toughness. When the Cr content is less than 0.05 wt%, the improvement of the characteristics is insufficient. Since Cr is relatively expensive, if it exceeds 2.0 wt% 0.05% by weight, and the upper limit is 2.0% by weight.

  Mo: 0.05 to 1.0 wt%

  The effect of Mo is almost the same as that of Cr. When the Cr content is less than 0.05 wt%, the effect is weak. When the Cr content exceeds 1.0 wt%, the deformation resistance for cold working is increased.

  B: 0.0003 to 0.0050 wt%

  B is an element which improves the hardenability. If it is less than 0.0003 wt%, the effect of addition is unclear. If it exceeds 0.0050 wt%, the hardenability is deteriorated. On the other hand, when B is added, since B may become BN in the inside of the structure, it may cause embrittlement of the grain boundaries. Therefore, the effect of adding B is increased by adding 0.01 to 0.05 wt% of Ti, which has a larger affinity with N. It is also preferable to add at least one of Zr, Nb and Al which has the same effect as Ti.

  P and S are inevitable impurity elements, which are segregated at crystal grain boundaries when kneaded to lower the impact toughness and lower the strain at the time of cold working. Therefore, it is necessary to limit the content to 0.030 wt% or more as much as possible.

   In the production line of the steel wire, drawing can be performed with a normal drawing equipment. In the heat treatment, a series of heat treatment equipment is used so that quenching and tempering can be continuously performed while unrolling the wire material one by one. The reasons are as follows.

Table 1 below shows the comparison of the deviation of the tensile strength of the 15 mm wire of S45C and SCM435 by the conventional method and the M14 x U bolt of 9T according to the present invention. As can be seen from Table 1, the conventional method is first subjected to the U-bolt shape and then placed in an iron box and charged into a batch type furnace to perform quenching / tempering heat treatment, It can be confirmed that the heat treatment process is very stable because the deviation is extremely small.

                      Tensile strength variation    division   Conventional method    Invention   SCM435 12 Kgf / mm ^{2 2} Kgf / mm ²   S45C 14 Kgf / mm ² 3 Kgf / mm ²

When the present inventor In a number of studies specific to the ASTM austenitic grain size is limited as above in the case of less than ASTM No.9 No.9 has a tensile strength of more than 100Kgf / mm ² of the high-strength steel wire through heat treatment characteristics, This is because the U-shaped bending process in the cold state causes cracks to occur on the surface of the coarse crystal grains. The tensile strength of 70 to 130 kgf / mm ² is the strength range required for U-bolts for fixing leaf springs of automobiles. When the tensile strength exceeds 130 Kgf / mm ² , cracks occur on the curved surface portion or springback occurs did.

The yield ratio (= yield strength / tensile strength × 100) of 85 to 97% does not give a sufficient fatigue life at less than 85%, and when it exceeds 97%, the elastic modulus of the steel wire is too high to cause a springback phenomenon Therefore, a separate calibration work was required and the effect was insufficient. Further, the reason why the reduction ratio of the cross section is limited to 50% or more is that when the ratio is less than 50%, the ductility of the steel wire is insufficient and cracks occur on the surface of the bent portion during bending.

Drawing after the heat treatment is to be lightly pulled so that the diameter of the desired U-bolt is within the range in which the above mechanical characteristics are not changed.

INDUSTRIAL APPLICABILITY The U-bolt for vehicle and the manufacturing method thereof according to the present invention are expected to have the following advantages and advantages.

    1) The durability of the final product according to the present invention is superior to that of the conventional U-bolt, and the tensile strength is less stable and stable.

    2) It is possible to omit the spheroidizing annealing process and the calibration process which are performed for 10 to 20 hours for a long time in the process according to the conventional U bolt manufacturing method, and considerable reduction in manufacturing cost can be achieved.

    3) The manufacturer of U-bolt can produce high-end product with cutting, bending and rolling only, so quality control is easy, process is simplified and automation is possible, productivity is improved remarkably.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
FIG. 2 is a process chart showing a manufacturing method of a U-bolt according to the present invention. FIG.
Fig. 3 is an explanatory diagram concerning the shape of the U bolt and the springback phenomenon. Fig.
FIG. 4 is a photograph showing a crack generated after knitting a bent portion of a U-bolt manufactured according to the present invention and a conventional method.

Hereinafter, a specific embodiment of the present invention will be described.

The present inventors used a hot-rolled 15 mm wire rod having the following Table 2 components to make a wire rod having a tensile strength of 70 to 130 kgf / cm < 2 > using a high-frequency induction heating apparatus consisting of a series of continuous processes, mm < ^{2 &} gt ;.

The conditions of quenching and tempering heat treatment were prepared by varying the austenite grain size, tensile strength, yield ratio, and section reduction ratio by adjusting the heating temperature, heating time, and heating rate. The quenching was performed at a temperature higher than the A3 transformation point, and tempering was carried out at a temperature in the range of 200 to 700 占 폚. The U-bolts of M14 were manufactured according to the method of the present invention shown in Fig. 2 after drawing with a diameter of 12.9 mm for the steel wire manufactured after the manufacturing. The characteristics of each condition of quenching / tempering and U- Are shown in Table 3.

                  Chemical composition of sample wire division    C   Si   Mn   P    S   Cr   Mo   B   Ti  Fe Sample-1  0.15  0.65  1.36  0.015  0.012   -   - 0.0021  0.03  honey. Sample-2  0.26  1.28  1.00  0.009  0.008   -   -   -   -  honey. Sample-3  0.25  1.32  1.00  0.012  0.007   -   - 0.0023  0.03  honey. Sample-4  0.34  0.23  1.74  0.009  0.008   -   -   -   -  honey. Sample -5  0.42  0.28  0.43  0.014  0.011   -   -   -   -  honey. Sample-6  0.25  0.29  1.91  0.012  0.007  1.14   -   -   -  honey. Sample-7  0.23  0.95  0.92  0.009  0.006  0.52   -   -   -  honey. Sample-8  0.21  0.65  1.32  0.012  0.007   - 0.33   -   -  honey. Sample-9  0.32  0.27  0.81  0.011  0.005   - 0.25 0.0018  0.02  honey. Sample-10  0.17  0.25  0.64  0.012  0.008  1.52 0.18   -   -  honey. Sample-11  0.22  0.27  1.72  0.012  0.007  0.63 0.13   -   -  honey. Sample-12  0.35  0.27  0.83  0.012  0.005  0.89 0.24   -   -  honey. Sample-13  0.18  0.66  1.36  0.014  0.012  1.64 0.28 0.0025  0.03  honey.

The tensile strength, the yield ratio, the section reduction ratio, and the grain size) of the specimens 1 to 13 of Table 2 were changed by changing the quenching / tempering heat treatment conditions as shown in Table 3 below The presence or absence of springback of the specimen and the cracking of the bending surface were measured.

 In Table 3, in order to judge the effectiveness of the present invention, it is judged as good (O) when cracks do not occur in the bent portion A of FIG. 3 when the U bolt is bent and the spring back (a + b) Respectively. At this time, the criterion for controlling springback was set to 2 mm or less.

          The characteristics of steel wire and the results of U-bolt processing

division   Characteristics of quenching / tempering heat treated steel wire U-bolt flexion
Processing result The tensile strength
(Kgf / mm ² ) Yield ratio
(%) Section reduction rate (%) Grain size
(ASTM No.) Spring back (mm) Bend
Surface crack Sample-1A 72.3 94.5 75.1 10.5 ○ ○ Invention material Sample-1B 72.9 87.2 68.4 8.3 ○ × Comparative material Sample-1C 85.6 92.3 70.9 9.5 ○ ○ Invention material Sample-2A 82.1 94.2 72.8 10.3 ○ ○ Invention material Sample-2B 93.7 90.4 68.7 10.3 ○ ○ Invention material Sample-2C 134.2 86.2 48.4 10.1 ○ × Comparative material Sample-3A 85.4 87.6 65.2 7.8 ○ × Comparative material Sample-3B 95.5 97.5 73.1 12.0 × ○ Comparative material Sample-3C 128.1 88.9 60.8 9.7 ○ ○ Invention material Sample 4A 78.8 95.9 74.1 9.7 ○ ○ Invention material Sample-4B 94.2 91.3 60.6 7.2 ○ × Comparative material Sample-4C 129.4 90.6 62.5 10.6 ○ ○ Invention material Sample 5A 87.5 92.8 59.2 7.5 ○ × Comparative material Sample -5B 87.9 95.7 69.9 11.0 ○ ○ Invention material Sample -5C 138.0 88.1 47.9 9.2 × × Comparative material Sample-6A 82.7 94.5 71.4 10.5 ○ ○ Invention material Sample-6B 94.3 91.6 67.9 10.3 ○ ○ Invention material Sample-6C 127.8 87.2 48.1 10.3 ○ × Comparative material Sample-7A 93.2 84.3 47.2 6.8 ○ × Comparative material Sample-7B 93.7 90.6 62.1 9.7 ○ ○ Invention material Sample-7C 93.2 94.1 68.2 11.2 ○ ○ Invention material Sample-8A 79.2 95.1 73.2 9.5 ○ ○ Invention material Sample-8B 93.7 90.3 62.2 7.0 ○ × Comparative material Sample-8C 125.7 91.5 62.9 10.6 ○ ○ Invention material Sample-9A 89.2 93.7 58.2 7.2 ○ × Comparative material Sample-9B 92.4 95.9 68.7 11.2 ○ ○ Invention material Sample-9C 140.1 86.1 46.6 9.2 × × Comparative material Sample-10A 85.2 93.8 72.0 10.1 ○ ○ Invention material Sample-10B 93.7 91.2 67.7 10.0 ○ ○ Invention material Sample-10C 129.3 87.5 59.4 11.2 ○ ○ Invention material Sample-11A 110.4 84.8 52.2 6.5 ○ × Comparative material Sample-11B 109.8 91.6 67.9 9.5 ○ ○ Invention material Sample-11C 110.1 97.8 57.2 11.2 × ○ Comparative material Sample-12A 87.5 94.2 71.3 12.2 ○ ○ Invention material Sample-12B 96.8 98.9 66.7 12.0 × ○ Comparative material Sample-12C 122.3 97.3 52.1 12.2 × ○ Comparative material Sample-13A 82.3 95.2 71.9 9.5 ○ ○ Invention material Sample-13B 95.4 90.7 62.8 7.2 ○ × Comparative material Sample-13C 123.7 88.6 61.4 11.2 ○ ○ Invention material

From Table 3, the wire having the components shown in Table 2 was subjected to a quenching / tempering heat treatment to obtain austenite crystal grain size ASTM No. 1. 9 or more, tensile strength of 70 to 130 kgf / mm ² , yield ratio of 85 to 97%, and cross-sectional reduction rate of 50% or more, if the steel wire is bent in cold U bolts, And no cracks were observed on the curved surface. In addition, even when the tensile strength is the same, it is also confirmed that the bending process results vary when the grain size, yield ratio, and section reduction ratio of the austenite are changed.

  In order to further clarify the superiority of the present invention, the characteristics of the U-bolts manufactured using the present invention material and the conventionally-used spheroidized quartz mortar were compared and evaluated. Table 4 shows the results.

                 Comparing the tensile properties of the present invention material to that of conventional materials Bend
Tensile strength ratio (%) Bend
Section reduction rate (%) Medial side of flexion
Presence or absence of crack Sample-2B 88.4 64.4 ○

Invention material Sample-6B 90.2 66.5 ○ Sample-10B 89.2 65.1 ○ Sample-11B 87.4 64.8 ○ Sample-13A 88.8 68.2 ○ SCM435 80.9 25.4 ×   Conventional material

* When a crack occurs: ×: When no crack occurs: ○

Table 4 shows the results of comparing the characteristics of the bent portions of the U-bolts by performing a tensile test after straightening the U-bolts manufactured in the 9T class (90 Kgf / mm < 2 > grade) by the method of the present invention and the conventional method.

In the case of the present invention, a representative sample made of 9T grade was selected from the samples of Table 3 in the case of the present invention, and in the case of the conventional method, the 9M-grade SCM435 produced by the manufacturing method of FIG. 1 was used . The comparison items include the tensile strength ratio of the bent portion, the sectional reduction ratio of the bent portion by the tensile test, and the occurrence of cracks occurring inside the bent portion (B in FIG. 3) Respectively.

Meanwhile, FIG. 4 is a photograph comparing the cracks generated inside the U-bolt according to the present invention and the U-bolt according to the present invention. As a result, as shown in Table 4 of the U-bolt of the present invention, it can be confirmed that it is an excellent manufacturing method having superior durability as compared with the conventional U-bolts.

On the other hand, the present invention is not limited to U-bolts for automobiles, but can be applied to U-bolts for construction and machinery.

Claims (6)

0.15 to 0.50 wt% of C, not more than 1.5 wt% of Si (excluding 0%), 0.25 to 2.0 wt% of Mn, 0.03 wt% or less of each of P and S (excluding 0%) and the balance Fe and unavoidable impurities And a steel wire having a tensile strength of 70 to 130 kgf / mm ² , a yield ratio of 85 to 97%, a section reduction ratio of 50% or more, and austenite crystal grain size of ASTM No. 9 or more by bending heat treatment subsequent to quenching U bolts. The U-bolt according to claim 1, wherein a tensile strength ratio of the bent portion is 85% or more, and a sectional reduction ratio of the bent portion when the bent portion is tensioned is 40% or more. [3] The method according to claim 1, wherein at least one component selected from the group consisting of 0.05 to 2.0 wt% of Cr, 0.05 to 1.0 wt% of Mo, 0.0003 to 0.0050 wt% of B, and 0.01 to 0.05 wt% U bolts. 0.15 to 0.50 wt% of C, not more than 1.5 wt% of Si (excluding 0%), 0.25 to 2.0 wt% of Mn, 0.03 wt% or less of each of P and S (excluding 0%) and the balance Fe and unavoidable impurities , A tensile strength of 70 to 130 Kgf / mm ² , a yield ratio of 85 to 97%, a section reduction ratio of 50% or more, austenite grain size of ASTM No. 9 or more, A step of forming a U-bolt, a step of cutting the U-bolt into a U-bolt, a step of rolling a thread on both ends, a step of bending the U-shape, and a step of plating or painting the surface. A method of manufacturing a U-bolt. The method of manufacturing a U-bolt according to claim 4, wherein the tensile strength ratio of the bent portion is 85% or more, and the sectional reduction ratio of the bent portion when the bent portion is tensioned is 40% or more. The method according to claim 4, wherein at least one component selected from the group consisting of 0.05 to 2.0 wt% of Cr, 0.05 to 1.0 wt% of Mo, 0.0003 to 0.0050 wt% of B, and 0.01 to 0.05 wt% of Ti is further added to the composition A method of manufacturing a U-bolt.

Images