JPS63145743A - Steel material for spring - Google Patents

Abstract

PURPOSE:To simply and economically obtain a high strength steel material for a spring, by plastically working stock so as to form a worked aggregated texture in which the grains have been extended and made fine in only the surface layer of the resulting steel material and to leave an unworked coarse- grained texture in the interior. CONSTITUTION:Stock for a steel material for a spring is plastically worked with a working means such as dies to obtain a steel material 1 for a spring having a worked aggregated texture 2 in which the grains have been extended and made fine in only the surface layer and an unworked coarse-grained texture 3 in the interior. The grains of the surface layer are extended and made fine by heating the stock only once, a work hardening effect is produced and a high strength steel material 1 for a spring is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a spring steel material used, for example, in coil springs, torsion bars, and the like.

[Conventional technology]

There is a strong desire for spring steel materials used in coil springs and torsion bars used in automobile suspension springs to be lighter from the perspective of resource and energy conservation, and as a result, spring steel materials tend to be used with high stress. It is in. It is extremely important for this type of spring steel material to have high fatigue strength.

Conventionally, steel materials for springs that have been made high in strength by oil tempering have been widely used.

This spring steel material has a substantially uniform tempered structure over the entire cross section. Therefore, when used as a spring, the surface layer is subjected to high stress due to bending, twisting, etc., but the stress is low in the core. In other words, the strength of the core is unnecessarily increased, and it is difficult to say that the energy spent to increase the strength is utilized for the q effect.

Further, as disclosed in Japanese Patent Publication No. 59-135G7, a steel material for springs has been considered in which the strength is increased by varying the distribution of crystal grains in the cross section.

It is said that by repeating heating and cooling multiple times at a temperature above the Ac3 deafness point and below the Ar1 transformation point, this spring steel material can obtain a crystal structure 1a as shown in the cross-section pattern in Fig. 9. .

[Problem that the invention seeks to solve]

However, in order to obtain a steel material for springs with a grain distribution similar to that of the conventional example, high-frequency induction heating and cooling are required (-1
It is necessary to repeat the process several times, leading to an increase in energy consumption due to the large number of heating times.

Moreover, if the composition of the material differs, the setting of the heating conditions will change slightly, so there is a problem that the material will lack stability if the number of times of heating is large.

[Means for solving problems]

The spring steel material of the present invention has a worked texture in which crystal grains are elongated and refined by plastic working only in the surface layer, and an unprocessed structure with coarse grains in a deeper layer than this worked texture. It is characterized by the fact that

In addition to having a circular cross section, this spring steel material has a rectangular or oval shape.
It may also have a moon-shaped cross section.

[Effect]

The surface layer may be processed by heating the material and performing, for example, drawing or swaging only once during the cooling process, thereby stretching and refining the crystal grains only in the surface layer. Heating only needs to be done once before processing. Spring steel materials processed on the surface layer in this way not only have the effect of elongation and refinement of the crystal grains, but also exhibit work hardening on the surface layer, so it is not possible to reduce the size of the crystal grains by heat treatment alone as in the conventional method. It is possible to achieve even higher strength compared to those with changes in .

Since the spring steel material of the present invention is only reinforced at the surface layer, it is suitable for use under stress conditions such as coil springs and torsion bars where bending and torsion are applied. Since it works effectively starting from the surface layer, the energy spent on increasing the strength can be used effectively.

〔Example〕

The spring steel material 1 of this embodiment has a perfect circular cross section and is used for, for example, coil springs. This spring steel material 1 has a first
As schematically shown in FIGS. 4 to 4, only the surface layer has a tempered texture 2 (hereinafter referred to as processed texture) that has undergone plastic processing.

In a deeper layer than the processed texture 2, there are three unprocessed structures with coarse grains.The unprocessed texture 3 may be a normal tempered structure.

As shown in FIGS. 3 and 4, the processed texture 2 is extended in the axial direction and circumferential direction of the steel material 1, resulting in finer grains and a marked increase in strength (especially yield point). Moreover, since this work texture 2 includes work hardening due to plastic working in addition to refinement of crystal grains, strength greater than that achieved by refinement of crystal grains is exhibited.

Note that ordinary steel materials for springs are subjected to shot peening treatment for the purpose of improving fatigue resistance. The general state of the cross-sectional residual stress distribution of shot-peened steel exhibits a compressive residual stress field in the surface layer.
It exhibits a tensile stress field towards the center. Since the material depth at which the transition from compression to tension occurs is usually about 0.1 to 0.2 rat, sufficient effects can be expected if the working depth of the worked texture 2 of this embodiment is also about that level. In this example, for the outer diameter d1 of the steel material 1, the depth of the machining set w&2 is 0,
It was set at around 163. Further, the grain size of the texture is, for example, a grain size of 13 in the processed texture 2 and a grain size of 9 in the unprocessed texture 3.

FIG. 5 shows an example of an apparatus for obtaining the above-mentioned spring steel material 1. In the figure, a material 1' before processing is passed through a processing means 5 such as a die for plastic working to reduce its diameter. On the artificial side of the processing means 5, there are provided a heating means 6 capable of heating the material 1' to the austenitizing temperature and a cooling means 7 for cooling it to the subcooled austenite temperature. The heating means 6 includes, for example, a power source 11 and a power source 11.
It is configured with a roller-shaped electrode 12 etc. connected to the
Resistance heating is performed by applying electricity to the material 1'.

A cooling means 13 and a heating means 14 for tempering are provided on the exit side of the processing means 5. This heating means 14 also includes a roller-shaped electrode 15 connected to the power source 10.

Note that the heating means 6.14 may be a high frequency induction heating device. In the illustrated example, the heating means 14 for tempering is provided continuously on the discharge side of the cooling means 13;
4 may be provided at a different position from the illustrated example in order to process the tempering in batches. The processing means 5' is
A die is preferable because it can be industrially mass-produced, but a machine that performs rotary forging using a swaging machine, rotary ironing, brushing, etc. may also be used.

The material 1' is subjected to heat treatment and plastic working through the temperature history shown in FIG. First, the material 1 is heated by the heating means 6.
' is rapidly heated to the austenitic region (over 850°C). Thereafter, the cooling means 7 cools the austenite temperature to the Ms point (approximately 3
00°C) to a predetermined temperature. The temperature at this time is appropriately set depending on the composition of the material 1'. The moment the temperature reaches a desired MS point or higher, the material is introduced into the processing means 5 and the surface layer portion is processed.

The processing degree (area reduction rate r) is expressed as r-1-(dl/do)2, where the outer diameter of the material is dO+the outer diameter after processing is dl.

When the ratio h/L of the length of the machining area to the depth h is Δ,
As can be seen from the Δ dependence of the yield pressure shown in FIG. 7, if Δ is set to a value of about 8.7 or more, the deformation region does not extend to the axis. For example, in drawing processing using a die, the die half angle α-15° (0,2018 rad
), then Δ−α (1+Jgok−r) 2
From the relational expression Δ≧8.7), the value of the area reduction rate r that can be calculated by −1 is found to be 0.11 ( 11%) or less. However, if it is less than 5%, the desired effect cannot be obtained. As shown in FIG. 8, there is a correlation between the die half angle and the limit area reduction rate at which deformation does not penetrate. The half angle of the die used industrially is often 7 to 30 dQg, and as the half angle of the die increases, the surface pressure applied to the die increases, leading to increased die wear and shortened life. As the pressure increases, processing heat increases and the desired effect cannot be obtained, so the area reduction rate should be in the range of 5 to 20%. By processing only the surface layer in this way, the A processing set w&2 of fine particles is obtained.

After processing, further cooling is performed by cooling means 13 to freeze the tissue. After that, if necessary, the heating means 1
4 to perform tempering. The heating rate of the material 1' by the heating means 6.14 is not particularly limited, but rapid heating such as electrical heating or high frequency induction heating is preferable for fine dispersion of carbides and refinement of crystal grains. The spring steel material 1 after tempering is formed into a coil spring by a coiling machine (not shown).

Table 1 below shows an example of processing conditions when the spring f14sUP7 is used for the material 1'.

Table 1 The main specifications of spring steel material 1 manufactured under the above conditions are shown in Table 2 below. Note that Tables 1 and 2 are examples in which 5UP7 is processed to be relatively soft when processed into a spring steel material.

Table 2 In addition, when the conventional material (see Fig. 9) whose surface layer part has not been processed is heat treated to the same hardness as in the above example,
The tensile strength σB obtained a value equivalent to that of the above example, but
The yield point σ0.2 was 9 f/win 2 at 152, and the fatigue strength σ, b was G6Kgf/win 2. In other words, this example product has a lower yield point and fatigue strength (
107 times) is also excellent.

The following Table 3 is an example of processing conditions for finishing 5UP7 to the upper limit of hardness HMV-608) when processing it into a spring steel material. Table 4 shows the main specifications of the spring steel material 1 manufactured under these processing conditions.

Table 3 Table 4 For conventional materials whose surface layer has not been processed, when heat treated to the same hardness as in the above example, the tensile strength of 7B is 2.
1O1 yield point σ0.2 is 185 Kg f/rtt
x2, fatigue strength σwb is 80 to 9 f/mm
It was 2.

〔Effect of the invention〕

According to the present invention, the crystal grains in the surface layer are elongated and have a fine structure, and the deeper layers have a normal tempered structure, so that only a few heat treatments and one processing of the surface layer are required. Just by doing this, you can get a high-strength spring. In addition to the elongation and refinement of crystal grains in the surface layer, work hardening is also involved, making it possible to achieve even higher strength compared to conventional materials whose grains are only refined through heat treatment. Moreover, since the steel material for springs of the present invention is only strengthened at the surface layer, the high-strength section works effectively under usage conditions that mainly involve bending and twisting, such as in coil springs and torsion bars. .

[Brief explanation of the drawing]

1 to 6 show an embodiment of the present invention, in which FIG. 1 is a radial sectional view of a steel material for a spring, FIG. 2 is an axial sectional view, and FIG. 3 is a t? Fig. 4 is an explanatory diagram of the structure showing an enlarged view of the part a in Fig. a1, Fig. 4 is an explanatory diagram of the structure showing the enlarged part b of Fig. 2, and Fig. 5 is a schematic diagram showing an example of an apparatus for heat treatment and processing. FIG. 6 is a diagram showing the temperature history of the material. FIG. 7 is a diagram showing the relationship between the length and depth of the deformation region and the yield pressure, and FIG. 8 is a diagram showing the relationship between the die half angle and the area reduction ratio. FIG. 9 is an explanatory diagram of the structure showing a part of the cross section of the conventional material. 1... Steel material for spring, 2... Working texture, 3...
Raw tissue. Applicant's representative Patent attorney Takehiko Suzue Raw structure of steel for springs Figure 1 Figure 2 Figure 3 Figure 4 Figure 8 Figure Δ-h/L Figure 7 1:a Figure 9

Claims (1)

[Claims] A steel material for springs having a worked texture in which crystal grains are expanded and refined by plastic working only in the surface layer, and an unworked structure with coarse grains deeper than this worked texture.