JPS5913567B2 - Manufacturing method for high-strength spring steel materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high-strength and tough spring steel material.

Spring materials such as coil springs and torsion bars are most required to have high fatigue strength, especially high torsional fatigue strength.

When this type of spring is used, stress such as bending or torsion increases from the neutral line toward the surface, and the maximum stress is applied to the surface layer. However, conventionally, in order to manufacture springs, such as coil springs, after drawing the spring steel material, oil tempering is performed to make the wire material high in strength, and then the wire material is cold-formed into a coil, or after being formed into a coil. , is a method of imparting strength by performing quenching and tempering treatment, and both aim to obtain a uniform quenched and tempered structure over the entire cross section through normal heat treatment.

In contrast, the first feature of the present invention is to perform short-time surface heating using high-frequency electric power at an appropriate frequency using a high-frequency induction heating method, and to perform the short-time heating at a predetermined time. The second feature is that the transformation of the crystal structure is repeated in a short time by repeating it after a rest period, and as a result, it has strength that corresponds to the stress distribution such as bending and torsion when using the spring. And the outermost layer has high strength,
The present invention aims to provide a spring steel material with high fatigue strength and a high toughness ultra-fine structure.

It is known that repeating cycles of rapid heating and cooling over the entire cross section of a steel material advances grain refinement over the entire cross section, thereby increasing the strength and fatigue strength of the steel material.

In other words, when steel is rapidly heated to bring the entire cross section to the Ac3 transformation point or higher, and then the entire cross section is immediately forcedly cooled to lower the temperature to room temperature and the crystal structure is transformed from α to γ several times, the crystal grains gradually change. It is refined, and finally an ultra-fine structure with a grain size of about 13 to 14 is obtained over the entire cross section. but,
This means that a uniform ultra-fine structure can be obtained over the entire cross section of the steel material, so if this is used as a spring steel material, unnecessary strengthening treatments will be required to prevent stress distribution such as bending and torsion during use. There is a problem in that this is not preferable, and energy consumption is large due to repeating the process of heating and cooling the entire cross section of the steel material.

On the other hand, the feature of the present invention is that by using high frequency induction heating method,
After rapidly heating only the surface layer of the steel material to the Ac3 transformation point (determined by the steel type and composition) for a short period of time, the heating is stopped. In this state, the center part is still close to room temperature, and the temperature of the surface layer immediately drops below the transformation point due to the self-cooling effect of the steel material due to heat conduction. When this thermal cycle of α→γ→α is repeated, the temperature at the center when heating is stopped also gradually increases, and the difference from the Ac3 transformation point temperature becomes smaller. According to the inventor's experiments, in this case, there is no need to lower the surface temperature to room temperature when heating is stopped, and if the surface temperature is lowered to below the Arl transformation point, a similar grain refinement effect can be obtained. It turns out that. The present invention will be described in detail below with reference to embodiments shown in the drawings.

The figure shows the change in temperature between the surface and center of the steel material when the steel material is subjected to continuous heat treatment using high-frequency induction heating according to the present invention. The vertical axis represents temperature and the horizontal axis represents time. show.

In this case, the induction heating coils (L, , L, , L3...) placed in the feed path are determined in relation to the heating conditions, that is, the diameter of the steel material to be heated and the feeding speed of the steel material to be heated.
number, coil length (11, 12, 13...), coil spacing between adjacent induction heating coils (Dl, d2, d3,
), the power density input to the induction heating coil ( P
, , P2, P3...), etc.), the surface temperature can be set to be above the Ac3 transformation point and below the Arl transformation point, as shown by the eight curves in the figure. Thermal cycles are repeated, and the temperature at the center is expressed as a temperature rise curve that lags considerably behind line A, as shown by curve B.

In this case, the α-γ transformation is repeated four times in the surface layer, during which time the temperature in the center gradually rises, and in the fourth heating, A
It is heated to the e3 transformation point or higher, resulting in total heating or a heating state close to it. In this state, the entire piece is hardened. In addition,
A1 indicated by a dotted line in the figure conceptually indicates a transformation point.

By doing this, the effect of repeated α-γ transformation works from the center toward the surface layer, and a finely quenched structure that becomes finer from the center to the surface layer can be obtained. become.

In this case, as mentioned above, in order to cause the α-γ transformation, it is not necessary to lower the temperature of the surface layer to room temperature by forced cooling when heating is stopped, and the purpose of the present invention can be sufficiently achieved by self-cooling of the steel material. It has been found that it is possible. Therefore, for example, by arranging a plurality of induction heating coils at predetermined intervals along the path of the material to be heated, the temperature can be raised to the AC3 transformation point or higher by the heating coils, Ar may be generated by self-cooling of the material to be heated, etc. By repeating a predetermined thermal cycle of lowering the temperature to below the point, the energy consumption is about 1/6 compared to the case of repeating the process of heating and cooling the entire cross section of the steel material, and it is the most suitable steel material for coil springs. Heat treatment conditions can be changed. In other words, if the electrical energy required to heat the entire cross section of the steel material is 1 and the thermal cycle is repeated 4 times, the method described above will heat the entire cross section 4 times.
In contrast, in the present invention, the surface heating of a cross section of approximately 1/6 of the total cross section of the steel material is repeated four times. It is sufficient to consume about 1/6th (4:24) of the electrical energy, and the resulting energy saving effect is extremely remarkable. is obtained.

After quenching is completed, if the material to be heated is a wire rod, it is heated continuously, or if it is a rod-like material of a fixed length, it is heated continuously or continuously with quenching.
Tempering is performed in a suitable manner, such as on a separate line, using high frequency induction heating or other known heating methods to impart the required mechanical properties to the steel.

The present inventor attempted various experiments to confirm the effects of the present invention.

Some of them are as follows. Experimental example 1 I) Experimental conditions (1) Test piece diameter 10 Material S35C with boron (2) Heating conditions Surface temperature 880°C to 900°C Heating time 2 seconds Heating cycle 4 times Quenching liquid Water 2) Experimental results Test piece The crystal grain size of the cross section after quenching and tempering of the same test piece was induction hardened and tempered once under the same heating conditions.5
I compared it to that.

The results were as shown in Table 1. Experimental example 2 I) Test wire diameter Field material Sup6 As shown in the figure above, the test wire was subjected to continuous induction quenching and tempering. A comparison was made between a steel material subjected to induction quenching and tempering treatment, and a steel material treated by the above-mentioned method of repeatedly heating and cooling the entire cross section of the steel material.

The results were as shown in Table 2.

Note that the grain size of the "wire rod in which the entire cross section of the steel material was repeatedly heated and cooled" was 13.

The present invention can be applied to, for example, JISSUP3, SUP4. ,SU
P6, SUP7, SUP9, SUPIO or SAE9
254, SA. In addition to spring steel materials such as E5l6O and SAE926O, as well as steel materials with predetermined elements added to the above-mentioned steel materials, known hardenable steel materials can be used as materials to achieve sufficient effects. This is something that can be achieved.

As is known from the above experimental results, the grain size of the spring steel material according to the present invention becomes finer from the center to the surface layer, and the grain size of the surface layer is an ultra-fine structure. It is possible to obtain a strength distribution suitable for the spring characteristics of ultra-fine structures that cannot be obtained with oil-tempered wire or induction hardened and tempered wire, and the entire cross-section can be improved by repeating the heating and cooling process multiple times over the entire cross-section of conventional steel materials. Even if the material has an ultra-fine structure, it is possible to obtain mechanical properties equivalent to or better than that of a spring material with a fraction of the thermal energy applied5, so in addition to consuming unnecessary energy, The unreasonableness of purchasing over-quality materials can be avoided.

Therefore, compared to conventional products, the present application has technical effects in that it provides a steel material for springs with high strength and toughness that has a strength distribution that corresponds to the stress distribution when the spring is used, with limited power consumption energy. is remarkable. In addition, in applying the present invention, a method of arranging a plurality of heating coils at predetermined intervals and repeating the thermal cycle by feeding the steel material into the heating coils may also be used, or a short steel material may be fixed. Alternatively, a method of repeating the same thermal cycle using a fixed heating coil may be used, as long as the steel material can be subjected to the thermal cycle as described above.

[Brief explanation of the drawing]

The figure is an explanatory diagram for explaining a thermal cycle according to the present invention. t1-T4...Repeated surface heating time.

Claims (1)

[Claims] 1 Heat treatment in which the surface of a steel material is heated for a short time to a temperature above the Ac_3 transformation point using high-frequency induction heating means, and then the heating is stopped for a short time to lower the surface temperature to a temperature below the Ar_1 transformation point by self-cooling through thermal conduction. A method for producing a high-strength steel material for springs, characterized in that by repeating the cycle, the entire steel material is heated or heated to a state close to it, then quenched, and then tempered using any heating means.