JP2589715B2 - Method and apparatus for manufacturing high-strength spring material - Google Patents

Description

The present invention relates to a method for producing a high-strength spring material suitable for increasing the strength of a linear spring material such as a suspension spring or a valve spring of an automobile. And manufacturing equipment.

[Conventional technology]

As a method for producing a high-strength steel wire or a tough steel wire, Japanese Patent Publication No. 27138/1976 (prior art 1) and Japanese Patent Publication No. 57-19168
A manufacturing method using ausforming as disclosed in Japanese Unexamined Patent Application Publication (Prior Art 2) and the like has been proposed. Both are common in that the material is heated to an austenite state and then cooled to a temperature equal to or higher than the martensite formation temperature (Ms point) to perform surface reduction processing.

More specifically, in the above prior art 1, the steel wire temperature after working (before quenching) is set to the Ms point or higher,
The above austenite is made to remain, and 100% austenite is not always specified. This means that when processing is performed simply with a roller or a die, a transformation generation phase appears before quenching (before quenching) due to a transformation promoting effect. That is, as shown by the one-dot chain line in FIG. 6, the transformation start line shifts to the high temperature and short time side by the processing, so that the transformation becomes easy.

On the other hand, prior art 2 is a method similar to prior art 1, but aims to positively utilize the above-mentioned transformation promoting effect, and after reducing the surface area, continuously performs a constant temperature transformation treatment to obtain fine ferrite. It is characterized by obtaining a pearlite structure. According to this method, the ductility is improved.

[Problems to be solved by the invention]

However, as in Prior Art 1, a steel wire that has been quenched from a state in which a transformation product other than austenite (eg, bainite or ferrite / pearlite) and austenite are mixed together has a transformation product other than austenite and transformation of austenite. Since the composite structure becomes martensite, the strength-ductility balance required for high-strength spring steel cannot be obtained. That is, it is poor in ductility.

On the other hand, in a method of obtaining a composite tissue as in Prior Art 2,
Although the ductility is improved because it contains a soft ferrite phase in the structure, the maximum 160 Kg f mm ² about intensity obtained only, the target value of the high-strength spring material strength 200Kg
f mm ² was far below. In addition, there is a disadvantage that a non-uniform strain distribution is generated in the processed material.

In the case of single-pass drawing in which the surface is reduced to the final material diameter by one surface reduction tool (for example, a die), when the material passes through the die, the processing heat concentrates on the surface layer portion, and the contact with the die occurs. The temperature of the part rises. For this reason, as shown by the broken line in FIG. 7, the surface layer undergoes transformation, and the strain concentrates on the surface layer having a large workability. As a result, the effect of promoting the transformation in the surface layer is further enhanced. And, since the energy given at the time of working becomes the driving force of the transformation, the start of transformation is faster than that of the material without working. It has been found that the desired hardness cannot be obtained due to the formation of bainite in the surface layer structure due to the synergistic action. As described in JP-A-55-125238, a wire cooled by a cooler
It is known that drawing is performed while applying back tension using individual dies.However, in this conventional technology, a die is provided outside the cooler, so the surface layer of the wire passing through the dies is generated due to generation of processing heat. Since the transformation is greatly accelerated, the surface layer of the wire cannot be made to have a desired hardness even when applied to aus drawing.

Accordingly, it is an object of the present invention to improve the surface condition by suppressing the transformation promoting effect in the surface layer portion and to increase the hardness level in performing wire drawing (aus drawing) in which aus forming and drawing are combined. Another object of the present invention is to provide a spring material having an excellent balance between strength and ductility.

[Means for solving the problem]

In order to achieve the above object, the method of the present invention comprises heating a steel wire to an austenitizing temperature, then cooling the steel wire to a supercooled austenite temperature of Ms point or higher, drawing and passing the steel wire through a first surface-reduction tool. The surface-reduced steel wire is cooled by a cooling medium to a temperature that suppresses the promotion of transformation of the processed portion, and the surface-reduced steel wire is further subjected to a second reduction with a smaller hole than the first surface-reduction tool. The steel wire between these tools is drawn while passing through the surface processing tool, and ausdrawing is performed while applying a backward tension in the direction opposite to the drawing direction to the steel wire. It is characterized by cooling.

Further, the apparatus of the present invention for carrying out the above method is a heating means for heating a steel wire to an austenitizing temperature, and is disposed downstream of the steel wire transported from the heating means and has an Ms point or more of the steel wire. A cooling bath that cools the steel wire to a supercooled austenite temperature by passing the steel wire through the cooling medium that does not contain a cooling medium maintained at a temperature,
A first step of reducing the surface area by immersing the steel wire in the cooling medium and passing the steel wire through the steel wire is performed at a temperature that suppresses the promotion of transformation of a processed part of the steel wire by the cooling medium. A surface reduction tool, and provided near the outlet of the cooling bath in a state of being immersed in the cooling medium and spaced apart from the first surface reduction tool, and having a smaller hole than the first surface reduction tool. The steel wire coming out of the first surface-reduction tool is passed through the second surface-reduction process while applying a backward tension to the steel wire between the steel wire and the first surface-reduction tool. A second surface-reduction processing tool maintained at a temperature that suppresses the promotion of transformation of the processed portion of the steel wire by a cooling medium;
And cooling means for rapidly cooling the steel wire after completion of the step of reducing the surface area to a temperature equal to or lower than the Ms point.

As the surface reduction processing tool, for example, a die is used. In short, a roll reduction and other processing tools can be applied as long as the surface reduction is performed by plastic working when the steel wire is drawn. In addition, as a cooling medium contained in the cooling bath,
Molten lead, various salt bath salts, and fluidized beds can be used.

[Action]

In order to produce a high-strength spring material using the above-described apparatus, a carbon steel wire or an alloy steel wire is heated to the austenitizing temperature by the above-mentioned heating means, and then the steel wire is passed through a cooling medium in a cooling bath. Rapidly cool to cold austenite temperature. Then, the steel wire is drawn while being passed through two or more surface reduction tools.

When a steel wire of a material is drawn through a plurality of processing tools as described above, a rear tension in a direction opposite to the drawing direction is generated in the steel wire between the processing tools. For this reason, the contact surface pressure with the processing tool is reduced, and the generation of processing heat is suppressed, and the concentration of strain in the steel wire surface layer is reduced. Thus, the promotion of transformation of the surface layer due to the processing is suppressed. Further, since the surface-reduction processing tool is maintained at the same temperature as the cooling medium in the cooling bath, the temperature of the steel wire does not substantially decrease even if the steel wire touches the processing tool. Therefore, the ausdrawing can be performed while generating the correct isothermal transformation while maintaining the supercooled austenite state. Then, the drawn steel wire is rapidly cooled to a temperature equal to or lower than the Ms point by a cooling means provided near the outlet side of the cooling bath, and the tissue is frozen in a short time.

〔Example〕

First, an apparatus according to an embodiment of the present invention will be described with reference to FIGS.
This will be described with reference to the drawings.

The carbon steel wire A serving as a spring material is continuously supplied from a supply unit such as a winding roll (not shown). Heating means 1
Is for rapidly heating the steel wire A to the austenitizing temperature. For example, electric heating or high-frequency induction heating is suitable, but an ordinary heating furnace may be used.

A cooling bath 2 for quenching is disposed downstream of the steel wire A sent from the heating means 1. Cooling bath 2 tank
2 a contains molten lead as an example of the cooling medium 3. This cooling medium 3 has a temperature (eg, Ms point)
(400-500 ° C). Holes 4 and 5 for passing the steel wire A are formed in the bath 2a of the cooling bath 2.

Near the outlet of the cooling bath 2, a pair of surface reduction tools 7, 8 are provided so as to be completely immersed in the lead bath.
The processing tools 7 and 8 are, for example, dies. As an example, each of the die angles 2α ₁ and 2α ₂ (see FIG. 2) is 30 °, the wire diameters before processing are d ₀ = 4.0, d ₁ = 3.95 mm, d ₂ = 3.8 mm. Although these processing tools 7 and 8 are connected by the connecting means 9, the cooling medium 3 sufficiently contacts the processing tools 7 and 8 and adds heat so that the processing tools 7 and 8 and the steel wire A are sufficiently cooled. The cooling medium 3 can be freely circulated between the tools 7 and 8. These processing tools 7 and 8 are preheated to the same temperature as the cooling medium 3 by being immersed in the cooling medium 3.

Further, a cooling means 10 is provided near the outlet side of the processing tool 8. The cooling means 10 is used for rapidly cooling the steel wire A drawn from the subsequent processing tool 8 to a temperature below the Ms point. The cooling means 10 includes, for example, an air nozzle 11 for blowing low-temperature air to the surface of the steel wire A, and a water cooling nozzle 12 for blowing a cooling medium such as water. 13 is a water cooling tank. Depending on the steel type and outer diameter of the steel wire A, it is desirable to use oil or a cooling medium having a property close to oil in the cooling tank 13 in order to prevent burning cracks. On the outlet side of the cooling means 10, a chuck 15 constituting a part of the drawing means is provided.

In order to manufacture the high-strength spring material A 'using the apparatus having the above configuration, it is ideal to perform ausdrawing through a temperature history as shown by a solid line in FIG. To explain an example of the process, first, at the start of drawing,
The distal end of the steel wire A, which has been reduced in diameter by pre-processing, is passed through the cooling bath 2 and is gripped by the chuck 15 for drawing. Processing tool
7 and 8 are preheated to almost the same temperature as the cooling medium 3 (temperature higher than the Ms point of 400 to 500 ° C. and lower than the nose).
Then, the steel wire A is pulled out by the chuck 15 in the direction of the arrow in FIG.

The steel wire A is heated to the austenitizing temperature (for example, around 1050 ° C.) by the heating means 1 before being introduced into the cooling bath 2, and is contacted with the cooling medium 3 in the cooling bath 2 so that the supercooled austenite temperature ( For example, it is rapidly cooled to around 450 ° C.).

Since the steel wire A is reduced in two stages by passing through the two processing tools 7 and 8, the steel wire A between the processing tools 7 and 8 has a backward tension in the direction opposite to the drawing. As schematically shown in FIG. 2, when the surface reduction is performed using two surface reduction tools 7 and 8, a back tension of σ ₁ acts between the processing tools 7 and 8. . For this reason, as shown in FIG. 3, the drawing force increases as the rear tension increases,
The surface pressure (die surface pressure) against 7,8 decreases. Therefore, the generation of processing heat is suppressed, work hardening is exerted on the surface layer of the steel wire A, and the concentration of strain in the surface layer is reduced. Moreover, the processing tools 7, 8 and the steel wire A are in a state of being sufficiently cooled by the cooling medium 3. For this reason, the temperature distribution in the cross section of the steel wire A is kept substantially uniform, and the transformation acceleration hardening in the surface layer portion is suppressed. For these reasons, the structure of the surface layer of the steel wire A hardly crosses the Bs point (start point of bainite transformation) during the working. In addition,
When the surface is reduced by using one processing tool as in the past, σ
The drawing stress t acts on the area A ₂ . For this reason, in the conventional method, strain concentration in the surface layer is remarkable, and the generation of processing heat is large.

The steel wire A drawn from the processing tools 7 and 8 is immediately cooled by the cooling means 1
By 0, it is rapidly cooled to a temperature below the Ms point. Since the quenching is performed immediately after the drawing, a correct isothermal transformation can be caused in the supercooled austenite temperature range, and the tissue is frozen in a very short time after the drawing. For this reason, martensite which is substantially uniform in structure can be obtained.

An example of the hardness distribution of the spring material A 'quenched through the above steps is shown in FIG. The steel type is swosc (silicon chromium added oil tempered wire). This figure shows that when the ausdrawing is performed with a single die, which is a conventional method, a two-stage die (2α ₁ = 2α ₂ =
(30 °, d ₀ = 4.0, d ₁ = 3.95, d ₂ = 3.8 mm) is a comparison of hardness distribution after quenching when aus drawing is performed while applying backward tension. When one drawing die (2α ₂ = 30 °, d ₀ = 4.0, d ₂ = 3.8 mm) is used as in the conventional method, the strain concentrates on the surface layer and the generation of processing heat causes The surface layer is completely bainite due to the large transformation promotion, and its hardness is significantly lower than that of untransformed austenite at the center. Therefore, when this conventional material is quenched, the surface layer becomes bainite, The center becomes martensite.

On the other hand, in the method of the present embodiment, since the transformation promotion is suppressed, it is appropriate to decrease the hardness in the surface layer portion, and the hardness level is increased. In the present embodiment, the outermost surface is hardened to some extent in the quenched state, and in the tempered state, the notch sensitivity is reduced and the fatigue resistance is improved. Further, a fine processed layer is obtained on the surface layer by die processing, which also contributes to a longer life. Furthermore, quenching is performed in a state in which a larger drawing force is applied by the amount of the post-process tension than in the conventional case (when the surface is reduced by one pass), so that the overall hardness also increases. The pulling load was about 250 kgf in the conventional method, but increased in the method of the present embodiment by the amount of the back tension, and was about 310 kgf.

For reference, FIG. 4 also shows measurement data of a quenched material (930 ° C., 5 minutes) by so-called ordinary tempering and a tempered material thereof. It can be seen that the ausdrawing material according to the method of the present invention has a higher hardness even in a quenched state and a higher tempering hardness under the same tempering conditions as compared with the ordinary tempered material.

When tempering is performed on the spring material A ′ that has been subjected to aus drawing while applying a rearward tension as described above,
Due to the difference in temper softening resistance, a highly durable material having the effect of work hardening on the surface layer can be obtained. That is, in the structure before tempering in this example, the processing strain due to the above-described surface reduction processing remains in the surface layer portion, and the softening resistance increases as the strain increases, and this is shown in FIG. As can be seen, a hard tissue remains on the surface layer slightly inward from the surface. For example, Hv580 at the surface layer and Hv540 at the center. The formation of bainite or pearlite transformation is suppressed in the structure of the spring material A ', and a uniform structure in which a fine martensitic single phase remains by die working is obtained. This spring material A 'is cold-formed into a coil spring or the like, for example.

The mechanical properties of the spring material (swosc) obtained in this example are as follows.

elongation 7% to 8% by σ _B = 220Kg f mm ^2, diaphragm 42%, elongation 8-9% in yield ratio 0.91 or σ _B = 200Kg f mm ^2, diaphragm 49%, the yield ratio 0.91 or more as in the two examples In addition, a high-strength spring material excellent in strength-ductility balance was obtained. In addition, regarding the double bending fatigue strength of σ = ± 85 kgf mm ² by the Nakamura type rotary bending, the conventional method broke about 200,000 times, but the high-strength spring material according to the present example reduced the strength by about 3 times. With 600,000 operations, the life has been greatly improved. In addition, the above-mentioned prior art 1
In the 0.77% C steel of Japanese Patent Publication No. 46-27138, σ _B = 1
Since the elongation is 93 to 204 kgf mm ² , the elongation is 4.2 to 5.5%, and the drawing is 43.5 to 44.8%, the product of this example has better elongation and better workability. In addition, 0.8 of prior art 2 (Japanese Patent Publication No. 57-19168).
In the case of 8% C steel, since σ _B = 150 to 156 kgf mm ² and the aperture is 40 to 43, the σ _{B of} the present example has a larger value.

〔The invention's effect〕

As described above, according to the present invention, by suppressing the transformation promoting effect in the surface layer due to aus drawing, a uniform and high hardness level hardened structure is obtained, and bainite or pearlite in the surface layer. Is suppressed, and a good-quality and nearly uniform martensite structure by the isothermal transformation is obtained. In particular, when tempering is performed, a highly durable spring material having a work hardening effect on the surface layer portion of the spring material can be obtained due to a difference in temper softening resistance between the surface layer portion and the deep layer portion.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view conceptually showing an apparatus according to an embodiment of the present invention, FIG. 2 is an enlarged view of a die portion in FIG. 1, and FIG. 3 is a view showing the relationship between rearward tension and pulling force. FIG. 4 is a diagram showing the relationship between the distance from the material surface after quenching and hardness.
The figure shows the relationship between the distance from the material surface after tempering and the hardness, FIG. 6 shows the isothermal transformation curve showing the effect of promoting transformation by processing, and FIG. 7 shows the deviation of the temperature history between the material surface and the center. It is a constant-temperature transformation curve diagram. A ... steel wire, 1 ... heating means, 2 ... cooling bath, 3 ... cooling medium, 7, 8 ... processing tool, 10 ... cooling means.

 ────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-55-125238 (JP, A) JP-A-53-113715 (JP, A) JP-B-46-27138 (JP, B1)

Claims (3)

(57) [Claims] The steel wire is heated to an austenitizing temperature, then cooled to a supercooled austenite temperature above the Ms point, and the steel wire is drawn and reduced while passing the steel wire through a first surface reducing tool. Is cooled by a cooling medium to a temperature that suppresses the promotion of transformation of the processed portion, and the steel wire after the surface reduction is passed through a second surface reduction processing tool having a hole smaller than that of the first surface reduction processing tool. It is characterized by performing aus drawing while applying a backward tension to the steel wire between these processing tools in the opposite direction to the drawing direction and cooling the steel wire to a temperature that suppresses the promotion of transformation by a cooling medium. Of manufacturing high strength spring materials. 2. After the above-mentioned ausdrawing, quenching is performed by quenching and tempering, so that the hardness of the surface layer slightly inward from the surface is maximized and the hardness of the surface and the center side is increased. 2. The method for manufacturing a high-strength spring material according to claim 1, wherein a material having a hardness distribution smaller than the surface layer portion is obtained. 3. A heating means for heating a steel wire to an austenitizing temperature, and a cooling medium disposed downstream of the steel wire transported from the heating means and maintained at a temperature equal to or higher than the Ms point of the steel wire. A cooling bath contained therein for cooling the steel wire to a supercooled austenite temperature by passing the steel wire through the cooling medium; and a first stage provided by dipping in the cooling medium and passing the steel wire. A first surface-reduction tool, which is subjected to surface-reduction processing and maintained at a temperature that suppresses the promotion of transformation of the processed portion of the steel wire by the cooling medium, and was immersed in the cooling medium in the vicinity of the outlet of the cooling bath. In this state, the first wire is provided at a distance from the first surface reduction tool and has a smaller hole than that of the first surface reduction tool. After the above steel wire between the surface reduction tool A second surface-reduction processing tool that performs a second-step surface reduction while applying tension, and is maintained at a temperature at which the cooling medium suppresses the promotion of transformation of the processed portion of the steel wire; An apparatus for manufacturing a high-strength spring material, comprising: cooling means for rapidly cooling a steel wire after surface processing to a temperature of Ms point or lower.