JP3353537B2 - Method for producing steel wire rod for spring with excellent drawability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a spring steel wire having excellent wire drawing properties, and more particularly to a spring steel wire suitable for a valve spring or the like of an automobile engine, which has a high area reduction of 80% or more. The present invention relates to a method for manufacturing a steel wire rod for a spring that can be drawn to the maximum.

[0002]

2. Description of the Related Art Among various springs, valve springs used for automobile engines are required to have particularly excellent fatigue characteristics. Factors that cause the deterioration of the fatigue properties of the spring steel wire include a flaw and a decarburized layer formed on the surface of the rolled steel wire. Therefore, in manufacturing a spring steel wire used for the valve spring, a so-called scalping is performed to scrape a surface layer of a rolled steel wire to remove the above-mentioned defective portion which causes a deterioration in fatigue characteristics. However, a work-hardened layer is formed on the surface of the steel wire by the above-mentioned shaving, and this causes a new problem of adversely affecting the drawability. Therefore, in order to soften the work-hardened layer, lead patenting is generally performed. It has been applied to.

[0003] In recent years, the performance of valve springs has been improved in accordance with the trend toward lighter weight and higher output of automobiles. As a low-cost heat treatment alternative to the above, annealing treatment in a gas phase using a batch furnace or the like has been studied and partially implemented.

[0004] However, steel wire rods for springs manufactured by the conventional annealing treatment have a problem in that, in some cases, the copper wire breaks during wire drawing, resulting in poor drawability. Further, even if the wire is not broken during the wire drawing, there is a problem that the wire is broken when formed into a spring.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and even if a low-cost gas-phase heat treatment method instead of lead patenting treatment is used, disconnection during wire drawing, Another object of the present invention is to provide a method of manufacturing a steel wire rod for a spring which is excellent in drawability without causing breakage during spring molding.

[0006]

Means for Solving the Problems The method of the present invention which has solved the above-mentioned problems is that after the surface layer of a rolled steel wire having a drawing of 35% or more is shaved, it is heated in a gas phase at a heating temperature of 823 to 973K. The gist of the invention is that the heat treatment is performed within a range in which the heating temperature T (K) and the heating time t (minute) satisfy T × t ^1/2 = 6700 to 12000.

[0007]

The present inventors have investigated the causes of wire breakage during wire drawing and breakage during spring forming in the method of manufacturing a steel wire rod for spring using annealing treatment, and have obtained the following findings.
When annealing is performed in a high temperature range in order to soften the work-hardened part of the surface layer of the rolled steel wire generated by skinning, cementite in the steel wire becomes spherical and a chevron crack occurs in the center of the wire during drawing. It has been found that disconnection occurs easily when the area reduction rate due to wire drawing increases.

Further, even when annealing is performed at a relatively low temperature at which cementite does not become spheroidized, breakage may occur during drawing. It was found that the disconnection in this case was caused by poor ductility of the rolled steel wire. That is, when the annealing temperature is relatively low, the structure of the rolled steel wire is inherited as it is after the heat treatment, and when the structure of the rolled steel wire is mainly coarse pearlite having poor ductility, the drawing process after annealing is performed. It was found that in the above, the above-mentioned chevron crack was liable to occur, leading to disconnection.

In any of the above cases, it has been found that even if the wire is not broken during drawing, if the spring is formed while the above-mentioned chevron cracks are present, breakage may occur. .

[0010] The method of the present invention has been made as a result of intensive studies based on these findings, finding the optimal range of the heating temperature and the heating time in the heat treatment after shaving, and the requirement for the rolled steel wire rod. By defining the ductility to be obtained by drawing in a tensile test, a method for producing a spring steel wire rod having excellent drawability was completed using a heat treatment method in a gas phase instead of lead patenting treatment.

[0011] In the present invention, the composition of the steel wire for spring may be an alloy composition generally used in the past, and a typical steel wire for valve spring is a Si-Cr-based chemical composition of JIS-G-3566. However, the present invention is not limited to this, and a high-carbon steel such as Cr-V based on JIS-G-3565 or JIS-G-3502 may be applied.

In the method of the present invention, the heat treatment temperature after shaving the rolled steel wire rod is 823-973K (550-700K).
° C). The reason is as follows.
That is, when heated to the austenitizing temperature range, a supercooled structure such as bainite or martensite is generated at the time of cooling after heating, and the drawability is reduced. The heating temperature is 973K even in a temperature range lower than the austenitizing temperature range.
When the ratio exceeds, the spheroidization of cementite becomes remarkable and disconnection frequently occurs during drawing. Therefore, the upper limit of the heating temperature during the heat treatment was set to 973K. On the other hand, if the temperature is less than 823K, the work hardened portion of the surface layer of the rolled steel wire is insufficiently softened, and a surface crack occurs in the pickling step or the drawing step before drawing. Therefore, the lower limit of the heat treatment temperature was set to 823K.

If the heating temperature is not appropriate even when the above-mentioned conditions are satisfied, the work hardened portion of the surface layer is not sufficiently softened, and the cementite becomes excessively spherical, and the wire drawing is performed. Problems such as the above surface cracks and disconnection occur during the process. When the present inventors searched for the optimal combination of the heating temperature T (K) and the heating time t (minute), the heating temperature T was adjusted within the above range of 823 to 973 K,
If the heating time is adjusted so that the value of xt ^1/2 is in the range of 6700 to 12000, the amount of energy supplied to the steel wire by the heat treatment can be controlled to an appropriate range, and the above-described problem does not occur. I understood.

Even if the heat treatment is performed under the above heating conditions, if the reduction of the rolled steel wire as the raw material is less than 35%, the wire is likely to be broken at the time of drawing. This is because, under the heat treatment conditions adopted by the present invention, since the structure of the rolled steel wire is inherited even after the heat treatment, when the structure of the rolled steel wire is coarse pearlite having poor ductility, the chevron cracks are formed in the wire drawing process. This is because it is easy to occur and disconnection tends to occur. Therefore, in the method of the present invention, the reduction of the rolled steel wire rod is 35
% Is required. A desirable value of the reduction of the rolled steel wire is 40% or more.

In the present invention, the drawing of a rolled steel wire is defined as a drawing of two or more loops taken from an arbitrary position of a coil of a steel wire, and the measurement is performed using a test piece obtained by dividing one loop into 16 equal parts. X-3s calculated from the average value x and the standard deviation s
Is the value of Next, the effects of the present invention will be described more specifically with reference to examples.

[0016]

EXAMPLE 1 A steel having a chemical composition shown in Table 1 was rolled into a steel wire having a diameter of 8 mm.
By adjusting the cooling rate after rolling, four types of rolled steel wires (single weight 2 tons) having different values of reduction were produced.

Each rolled steel wire rod is 7.4 mm by shaving.
After being formed into a steel wire having a diameter, heat treatment was performed under the conditions shown in Table 2 to evaluate drawability. In drawing, a drawing die having an approach angle of about 12 ° is generally used. However, in this embodiment, in order to clarify the effects of the present invention, a drawing die having an approach angle of 20 °, which is liable to cause cut-off, is used. A wire die was used. The drawing results are also shown in Table 2.

The values of the drawing shown in Table 2 were statistically obtained from a total of 80 tensile test pieces obtained by arbitrarily collecting 5 loops from each rolled steel wire and dividing one loop into 16 equal parts. X
-3s. Although the rolled steel wires A to C satisfy the drawing conditions according to the present invention, the drawing of the rolled steel wire D is too small.

[0019]

[Table 1]

[0020]

[Table 2]

C1 is a comparative example in the case where the heat treatment temperature is too high.
Further drawing was difficult. On the other hand, in Comparative Example C2, the heat treatment temperature was low, and lateral cracks occurred on the surface when the wire was drawn with a reduction in area of 65%.

D1 is a comparative example in which both the drawing and the heat treatment conditions of the rolled steel wire are out of the range of the present invention, and the cut-off was frequent at a reduction in area of 40%. Further, the heat treatment conditions satisfied the range of the present invention, but in Comparative Example D2 in which the drawing was too small, many cut-off lines were generated at a reduction in area of 50%.

On the other hand, A1 to A3, in which the conditions for drawing and heat treatment of the rolled steel wire are within the range of the invention, all exhibited excellent drawability without breaking or surface cracking up to a reduction in area of 80%. . Further, in the case of B1 to B3 obtained by heat-treating a rolled steel wire having a reduction of 40.3% at a heating temperature within the range of the present invention, no disconnection occurred up to a reduction in area of 80%.

Next, the above A1 drawn to a reduction of area of 80%
A3 and B1 to B3 using 3.3 mm diameter wires,
An oil-tempered wire having a tensile strength of 1960 N / mm ² was manufactured by performing ordinary quenching and tempering treatments. Using the oil-tempered wire, a tensile test was performed to observe a fractured surface, and a winding test was performed to examine a broken state or surface properties.

As for B1 and B2 which do not satisfy the heat treatment conditions according to the present invention, those exhibiting a cuppy-shaped fracture surface appeared, and when the cross section near the fracture surface was examined by microscopy, many chevron cracks were observed. Was. In addition, B2 also suffered a cuppy-shaped breakage in the winding test. B3
Although no cracky-like fracture surface or breakage was observed in the test, fine cracks were generated on the surface by the winding test.

On the other hand, in the case of A1 to A3 according to the present invention, no cuppy-shaped fracture surface was observed in the tensile test,
The winding test also demonstrated that there was no breakage and no surface cracking, and that it had good workability.

Example 2 A steel wire rod having a diameter of 8 mm was rolled using a Cr-V steel E and a high carbon steel F shown in Table 3, and 7.4 was cut off by shaving.
After being formed into a steel wire having a diameter of mm, heat treatment was performed under the conditions shown in Table 4 and the drawability was evaluated in the same manner as in Example 1.
The rolled steel wire E has a tensile strength of 1670 N / mm.
The oil-tempered wire No. ² was manufactured, and a rolled steel wire F was subjected to a wire tensile test and a winding test using a drawn wire. The results are shown in Table 4.

[0028]

[Table 3]

[0029]

[Table 4]

In Comparative Example E3, in which the heating temperature was too low, lateral cracks occurred on the surface when the wire was drawn with a reduction in area of 70%, and breakage also occurred on the oil-tempered wire. In Comparative Example E4, in which the reduction of the rolled steel wire was too low, cut-off occurred in the wire drawing, and even an oil-tempered wire exhibited a cut-like fracture surface. In Comparative Examples F3 and F4, in which the heating time was inappropriate and the conditional expression according to the present invention was not satisfied, breakage in the drawn wire or oil-tempered wire and occurrence of surface crack in the oil-tempered wire were observed, respectively.

On the other hand, E satisfying all the conditions of the present invention
It was confirmed that 1, E2, F1 and F2 did not break or break at the time of wire drawing at all, and had good quality even in a test using an oil-tempered wire. As described above,
It can be seen that the steel wire produced by the method of the present invention using -V and high carbon steel exhibits excellent drawability and workability.

[0032]

Since the present invention is constructed as described above, a low cost heat treatment method replacing lead patenting can be used to achieve wire drawing up to a high area reduction of 80% or more without breaking or cracking. It has become possible to provide a method of manufacturing a possible spring steel wire rod.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-188745 (JP, A) JP-A-7-60338 (JP, A) JP-A-63-227748 (JP, A) 215626 (JP, A) JP-A-3-79719 (JP, A) JP-A-5-279750 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/06, 9 / 02,9 / 52 B21C 1/00

Claims (1)

(57) [Claims] 1. After shaving a surface layer of a rolled steel wire rod having a reduction of 35% or more, in a gas phase, at a heating temperature of 823 to 973K, a heating temperature T (K) and a heating time t (min).
A method for producing a spring steel wire excellent in drawability, wherein the heat treatment is performed in a range satisfying Txt ^1/2 = 6700 to 12000.