JPS6274027A - Manufacture of steel wire for high strength cold formed coil spring - Google Patents

Abstract

PURPOSE:To obtain the titled steel wire without accompanying breakage at all during usage, by applying a prescribed heat treatment to the titled wire manufacture according to usual method to provide surface layer hardness and tensile strength at core part having respective characteristics and decreasing notch susceptibility of product surface. CONSTITUTION:Material steel wire, etc., for JIS standard spring of 10-15mmphi is subjected to pretreating process 1 contg. descaling, etc., passed in induction heating oil while being continuously sent and quenched 2 by heating the whole section to a prescribed quenching temp. and rapidly cooling it. Then the wire is passed in induction heating oil to rapidly heat 3 to a prescribed temp. varying due to contained component, held 4 for an extremely short time and rapidly cooled 5 to finish the whole section to >=200kg/mm<2> tensile strength. Further, the wire is continuously sent and passed in induction heating coil to soften only the surface layer to <=550 Hv by heating 6 only surface layer to e.g. 450-600 deg.C, then cooled 7 to obtain the aimed titled wire.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method of manufacturing a steel wire for a large-diameter, high-strength cold-formed coil spring used in a vehicle suspension spring or the like.

(Prior Art) As previously described in detail in the invention disclosed in Japanese Patent Publication No. 59-13568, the applicant of this application has rapidly heated and rapidly cooled a large-diameter raw material steel wire, such as 10 to 15 mmφ, by induction heating etc. over the entire cross section. After hardening, the steel wire is heated to 300 to 60
By rapidly heating the entire cross section to a temperature range of 0'C, stopping the heating, holding it for a short time of 60 seconds or less, and then cooling it,
Obtain a high-strength steel wire with a tensile strength of 150 Kgf 7 mm or more that is extremely rich in cold plastic workability with ductility and toughness, cold-form it into a coil spring, and then mold the formed coil spring to a temperature higher than the upper limit of the above-mentioned heating temperature. Provides a ``method for manufacturing cold-formed coil springs'' that yields strong springs with excellent fatigue resistance and delayed fracture resistance by reheating to a low temperature range of 300 to 5 degrees OoC for 30 to 60 minutes. are doing.

The applicant is currently using the above method to achieve a tensile strength of 180~
220Kgf/mn? We manufacture coil springs by obtaining high-strength steel wires with superior cold plastic workability and then cold-forming them, and these products are widely used and prized in two-wheeled vehicles.

(Concerns about conventional technology) By the way, since the high-strength cold-formed coil spring wire material and high-strength cold-formed coil spring manufactured by the above method have high strength, no problems have occurred. However, in the 4-axle vehicle industry, suspension springs are an important safety component, and many are unable to use them due to concerns about breakage during driving.

(Object of the Invention) The present invention has been made in order to eliminate the above-mentioned concerns about breakage accidents that some users have regarding high-strength cold-open formed coil springs. By adding prescribed heat treatment to the steel wire for interformed coil springs,
While retaining the mechanical properties of high-strength cold-formed steel wire for coil springs, we use this to reduce the sensitivity to notches on the surface of coil spring products obtained by cold-opening, thereby preventing breakage during use. The purpose of the present invention is to provide a method for manufacturing high-strength cold-formed coil spring steel wire that does not cause any danger.

(Structure of the Invention) The structure of the present invention is as follows: (1) After induction heating and rapid cooling quenching of a large-diameter raw steel wire over the entire cross section, (2) Reheating process using an induction heating means, (3)
) A method for manufacturing a high-strength cold-formed coil spring steel wire, characterized in that the surface hardness up to a predetermined depth is Hv550 or less and the core tensile strength is 200 Kgf/mrrr or more.

(Action of the Invention) The present invention has the effect of adding low notch sensitivity to the mechanical properties of a high-strength cold-formed coil spring steel wire that has ductility and toughness and is rich in cold plastic workability.

(Example = 1) The present invention will be described in detail below with reference to the manufacturing process block diagram of one embodiment shown in FIG.

In the present invention, JIS standard spring material steel wire etc. with a diameter of 10 to 15 mm is subjected to a pretreatment process (2) including descaling and straightening,
The material is passed through an induction heating coil while being continuously fed, and the entire cross section is machined and rapidly cooled to a predetermined hardening temperature. Next, the hardened steel wire is continuously fed through an induction heating coil and heated to a predetermined temperature that varies depending on the content of the wire.
111---For example, rapidly heat to a temperature range of 400 to 550℃, hold for a very short time, and then rapidly cool.
The entire cross section is finished with a tensile strength of 200Kgf/mrd or more.

Next, the obtained steel wire is further continuously fed and passed through an induction heating coil under a predetermined method such as increasing the feeding speed or frequency, so that only the surface layer is heated to a temperature of 450 to 60
Heat the surface layer to a temperature range of 0'C to hardness Hv.
The manufacturing method consists of softening to 550% or less and then cooling.

In the above, the heating depth of the surface layer in heating (3) is set such that the wire diameter is appropriate for maintaining the overall cross-sectional average tensile strength of the steel wire at about 200 kgf/m and having low notch sensitivity. 2.0mm if it is a large diameter like 15mm
Below, if it is around 1Qmm, keep it below 1.5mm.

The steel wire obtained by the above manufacturing method has an average tensile strength of 180 kgf/m'n across the entire cross section. (1770MPa)
In addition to maintaining the above-mentioned high strength, it has the ductility and toughness characteristic of the wire rod before cold forming described in the first part of the invention provided by the present applicant, and is rich in cold plastic workability. Low notch sensitivity will be added.

Therefore, if the wire rod is subjected to cold forming described in the latter part of the invention provided by the present applicant and then heated in the range of 300 to 500°C for 30 to 60 minutes of the formed coil spring, the obtained High-strength cold-formed coil springs not only have excellent fatigue resistance and delayed fracture resistance, but also have low notch sensitivity.

(Example: 2) FIG. 1(b) is a block diagram showing the manufacturing process of another example of the present invention.

In the manufacturing process of this example, the pretreatment process and the quenching process are the same as the manufacturing process of the example, but the following steps are different.

That is, in the subsequent reheating process using induction heating means, the steel material is heated by a single heating coil connected to a predetermined frequency output power source selected according to the diameter of the steel wire, or intermittently by a plurality of heating coils. It is a process of heating the core rapidly to a range of 400 to 550°C and the surface layer to a range of 450 to 600°C (3), followed by cooling (4). be.

In this manufacturing process as well, the cross-sectional radial direction of the steel material is heated to the same temperature as in the manufacturing process of the above-described embodiment, so that exactly the same effect and the same effect obtained from this effect are exhibited.

(Experimental Example) In order to confirm the effects of the present invention, the present inventor conducted the following experiment. ☆ Preparation of specimen steel material Steel wire (equivalent to SUP12) consisting of a chemical composition shown in Table 1 in the appendix of 12 mmφ First, a base material specimen A shown as A in Table 2 in the attached sheet was prepared by subjecting the raw material steel wire to the steps From sample A, sample B was prepared according to steps (1) and (2) of Example 1 of the present invention.

The heating temperature in step ① was 485°C, and the final tensile strength was 205 Kgf/mn. The heating temperature of specimen B is 545℃
Met.

In addition, using the above material steel wire, the material steel wire was subjected to steps 1 to 2 according to conventional methods, and the tensile strength was 180Kgf 7m%.
Comparative specimen C was prepared.

☆Accuracy test of specimen *Hardness measurement test Each specimen was tested with a load of 3 using a micro Vickers hardness needle.
The cross-sectional hardness distribution was measured as 00g. The results are as shown in Figure 2. Curves A, B, and C are the measurement results of specimens A, B, and C, respectively. From comparison with specimens A and C, respectively, specimen B was It was confirmed that a range of 1.5 mm in depth showed low hardness, and the outermost surface hardness was Hv=500 or less.

*Residual stress measurement test The residual stress of each specimen was measured using an X-ray stress measurement device (MSF-2M). The results are as shown in FIG. 3, where the ○ marks in the figure represent the residual stress in the axial direction, and the * marks in the circumferential direction.

It was confirmed that specimens A and C exhibit compressive residual stress, while specimen B exhibits tensile residual stress, and it is determined that the longer the heating time for softening (the longer the absolute value thereof), the smaller the absolute value.

*Tensile test Each specimen was subjected to a tensile test using a universal testing machine (capacity Loot) as a wire rod (JISZ No. test piece). The test results are shown in Figure 4. From the same figure, specimen B shows a slight decrease in tensile strength and yield strength compared to specimen A, but
It was confirmed that it was superior to Specimen C.

*Delayed fracture resistance test Each specimen was subjected to a tensile delayed fracture test.

After polishing the surface of each specimen by approximately 50μ, electropolishing was performed using the Rodin-Ammon method (20% NH4SCN, 50°C).
), using a constant load tester, σ-100Kgf /
Tested under rnrrr conditions. The test results are shown in Figure 5. From the figure, it was found that the life of specimen B was longer than that of specimen A, and was almost the same as that of specimen C.

☆Accuracy test of coil springs made from specimens Each of the specimen steel wires A to C was cold-formed using a coiling machine into a coil spring having the shape shown in Table 3 of the appendix. Coil spring product specimens Ac-Cc were obtained under the manufacturing conditions shown in Table 4, and were subjected to the tests described below.

*“Settling” in constant load creep test Using a constant load tester, the test conditions were τ = 115Kgf /m%X 96hrs Temperature: 20
Each specimen was subjected to a constant load creep test at ±1°C to investigate its resistance to settling. The test results are shown in Figure 6. From the same figure, it was confirmed that specimen Bc exhibits sufficiently superior fatigue resistance than specimen Cc, although the specimen Bc is slightly larger in comparison with specimen Ac.

*Fatigue resistance (using a repeating tester, τ-65±50Kgf/m
As a result of repeatedly subjecting each specimen to the test under conditions of 300,000 cycles, each specimen showed a sufficient lifespan of 300,000 cycles or more.

As mentioned above, from the results of various accuracy tests conducted on the steel wire specimen with a diameter of 12 mm, it was found that the specimen B manufactured by the manufacturing method of the present invention
Even though the hardness of the surface layer was set to Hv = 550 or less, the tensile strength was maintained at approximately 200Kgf/mm class,
and other mechanical properties comparable to that of the relevant class, and therefore the surface layer should be at least 1.5 mm thick.
It has been proven that it is possible to obtain a high-strength cold-formed coil spring steel wire that exhibits almost no deterioration in mechanical properties even when softened to a certain extent.

In addition, the results of a reliability test conducted on a cold-formed coil spring specimen Bc manufactured from the specimen B showed that it had a tensile strength of 200 Kgf/m while having low notch sensitivity.
Almost equivalent to m class, tensile strength 180Kgf/m
It has been proven that it is possible to manufacture strong cold-open-formed coil springs with much better fatigue resistance and fatigue strength than the RD class.

From the results of multiple experiments conducted by the present inventor, including the above-mentioned experimental examples, it has been found that in the present invention, which targets wires up to about 15 to 16 mmφ, similar to the above-mentioned patented invention, the surface layer can be reduced by 2.0 mm if the wire has a larger diameter. It has been confirmed that the same results as in the above example can be obtained even if the material is softened within the following range.

(Effect of the invention) If the method of the present invention is implemented, the tensile strength compared to the conventional method will be 20
0Kgf 7mm or more high-strength cold-formed coil spring steel wire maintains its mechanical properties and has low notch sensitivity, so when using cold-formed coil spring products made from this steel wire. There is no risk of breakage in the coil spring products, and the functionality of machinery and equipment that uses the coil spring products can be improved with peace of mind. By exerting the same effect as before, we aim to reduce the weight of the vehicle and improve functionality.
The effect of this is remarkable.

[Brief explanation of drawings]

Figures 1 (a) and (b) are block diagrams showing the manufacturing process of one embodiment of the method of the present invention and the manufacturing process of another embodiment, respectively, and Figure 2 shows the hardness measurement test results of steel specimens in the experimental example. The hardness distribution diagram shown. Figures 3 to 5 are residual stress measurement test results, tensile test results, and tensile delayed fracture test results in experimental examples, respectively, and Figure 6 is a constant load creep test of cold-formed coil spring specimens in experimental examples. This is a diagram of the results.

Claims (1)

[Scope of Claims] 1) After induction heating and rapid cooling quenching of a large-diameter raw steel wire over the entire cross section, a reheating process using an induction heating means is performed to reduce the surface hardness to a predetermined depth to Hv550 or less at the core. A method for producing a high-strength cold-formed coil spring steel wire, characterized in that the tensile strength of the steel wire is finished to 200 Kgf/mm^2 or more. 2) The steel wire for high-strength cold-formed coil springs according to claim 1, which has a hardness of Hv550 or less and a depth of the surface layer that is at most 2.0 mm or less depending on the wire diameter. Production method. 3) The reheating process using induction heating means includes a first step consisting of rapid heating of the entire cross section of the steel wire to a temperature in the range of 400 to 550°C, holding for a short time of 60 seconds or less after stopping the heating, and rapid cooling; The scope of claims includes a second step in which only the surface layer of the steel wire that has undergone the first step is rapidly heated under induction heating conditions such that the maximum depth of 2.0 mm from the surface is Hv550 or less, and then cooled. A method for producing a high-strength cold-formed coil spring steel wire according to items 1 and 2. 4) The reheating process using induction heating means heating the steel wire with a single heating coil connected to a predetermined frequency output power source selected according to the diameter of the steel wire or intermittent heating with a plurality of heating coils. Claim 1, in which the temperature is rapidly raised under non-uniform heating conditions in the cross section that can be finished to a predetermined hardness of 550 Hv or less at a maximum depth of 2.0 mm from the surface and over 550 Hv at the core, and then cooled. and the second
A method for manufacturing a steel wire for high-strength cold-formed coil springs as described in .