JPH08302428A - Method for manufacturing high strength steel strip for spring - Google Patents

Abstract

PURPOSE: To produce a steel strip suitable for various spiral springs, such as retractor spring and music box spring, and flat spring, etc. CONSTITUTION: At the time of applying patenting treatment to a carbon steel or alloy steel strip containing 0.6-1.0% C, isothermal transformation is applied in a refrigerant tank heled at 500-550 deg.C to form a pearlitic single phase structure and then cold rolling is done at 70-95% total rolling rate. It is preferable that average lamellar spacing is regulated to <=0.2μm in the pearlitic single phase structure after isothermal transformation. Further, bendability and spring limit value can be improved when the steel strip in the course of cold rolling is held at <=110 deg.C.

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 high-strength steel strip suitable as a material for various mainsprings such as retractor mainsprings and music box mainsprings and leaf springs.

[0002]

2. Description of the Related Art In the manufacture of mainsprings and leaf springs, a steel strip is used as a raw material to be formed into a predetermined shape by punching or bending, and then the elastic limit and spring limit value are improved to impart springiness to the material. 200-400 aging treatment for
It is performed at ℃. The steel strip used for this purpose is required to have excellent bending workability because bending workability is essential for manufacturing processes such as a mainspring. In addition, in order to increase fatigue resistance, which is the most important property as a spring, and sag resistance, it is desirable that the spring limit value after aging be high after aging, but before aging it is processed to obtain the desired shape. It is desired to be low from the viewpoint of suppressing the later springback. As a steel strip for a mainspring, a cold rolled austenitic stainless steel such as SUS301 is mainly used. However, austenitic stainless steel is an expensive material because it contains a large amount of expensive alloying elements such as Ni and Cr, which increases the product cost. Therefore, various studies have been made on the use of inexpensive high carbon steel in place of austenitic stainless steel.

As a method of obtaining a high spring limit value required for a spring material with a high carbon steel, it is known to perform cold rolling after patenting. The patenting process is
It is a heat treatment in which the material is rapidly cooled after austenitization and is transformed into pearlite during cooling or holding a constant temperature by a refrigerant. As the refrigerant, a lead bath, a salt bath, a wind blast, boiling water, etc. are used. For example, Japanese Examined Patent Publication (Kokoku) No. 1-423232 discloses a cooling rate and a cooling stop temperature after austenitizing on the assumption of blast cooling when patenting a steel strip in a continuous annealing line equipped with a primary cooling zone and an overaging furnace. Introducing controlled airflow cooling. Japanese Examined Patent Publication No. 1-16887 introduces direct patenting in which the phase transformation is completed on the runout table in the hot rolling process, and then the coil is wound at a specific temperature, cold rolled and strain-aged.

[0004]

The patenting method by blast cooling (Japanese Patent Publication No. 1-242332) has a higher transformation temperature than the method of constant temperature transformation by immersing in a lead bath or salt bath. Therefore, the strength and the spring limit value after cold rolling become low, and the material cannot be used as a mainspring or a spring material used under severe conditions. Even in the direct patenting method (Japanese Patent Publication No. 1-16887), the pearlite transformation proceeds at a relatively high temperature, so that the strength and the spring limit value after cold rolling become low. In addition, the structure tends to be nonuniform especially in the coil width direction, and it is difficult to obtain a steel strip having a uniform high spring limit value and bending workability. The present invention has been devised to solve such a problem, by performing high cold rolling after performing a fine pearlite single-phase structure not containing bainite by performing pearlite transformation at a temperature as low as possible. The purpose of the present invention is to obtain a steel strip for springs which exhibits a high spring limit value in strength.

[0005]

In order to achieve the object, the method for producing a high-strength steel strip for springs according to the present invention has a C: 0.6-
When patenting a steel strip of carbon steel or alloy steel containing 1.0% by weight, the steel strip was subjected to isothermal transformation in a refrigerant tank kept at 500 to 550 ° C. to form a pearlite single-phase structure, It is characterized in that cold rolling with a rolling ratio of 70 to 95% is performed. The average lamellar spacing of the pearlite single-phase structure after the isothermal transformation is preferably adjusted to 0.2 μm or less. Further, when the steel strip during cold rolling is kept at a temperature of 110 ° C. or lower, bending workability and a spring limit value are improved.

[0006]

The present inventors extensively investigated the influence of the patenting conditions, cold rolling conditions and the like on the spring limit value which is an index showing the elastic limit of the spring and bending workability. As a result, it was found that the following manufacturing conditions are effective in order to obtain a steel strip exhibiting a high bending workability after patenting treatment-cold rolling and a high spring limit value by the subsequent aging treatment. It was In the patenting treatment, the transformation temperature is set as low as possible in the temperature range where the pearlite single phase is obtained, so that a fine pearlite single phase structure containing no bainite is formed. The pearlite single-phase structure has a higher spring limit value than the pearlite + upper bainite mixed structure. Here, if the pearlite area ratio is 95% or more, it can be substantially regarded as a pearlite single-phase structure.

Even in the case of a pearlite single-phase structure, the effect of the transformation temperature is large, and the spring limit value is improved by setting the transformation temperature as low as possible within the range where the pearlite single-phase structure is obtained and making the lamellar spacing fine. Further, in order to stably obtain a patenting structure exhibiting a high spring limit value over the entire length and width of the coil, a lead bath having a higher cooling capacity than wind blast or boiling water is used for cooling after austenitizing in the patenting process. It is desirable to use a salt bath. In these refrigerant tanks, the temperature rise of the steel strip due to the latent heat of transformation can also be suppressed, so that a pearlite structure with uniform lamellar spacing can be obtained. The spring limit value after cold rolling and aging is
Although it increases with the cold rolling rate, the degree of increase is greatly influenced by the patenting structure. When the patenting structure is a pearlite single-phase structure, compared to the pearlite + upper bainite mixed structure, the spring limit value at the same cold rolling rate is high, and the cold rolling rate at which the spring limit value is saturated to a certain value Becomes higher. Therefore, in the pearlite single-phase structure, the spring limit value can be extremely increased by increasing the cold rolling rate. In this way, after controlling the patenting conditions to obtain a fine pearlite single-phase structure,
When cold-rolled under high pressure, steel strips with synergistically high spring limits are obtained.

On the other hand, in order to improve the bending workability after cold rolling, it has been found that it is effective to increase the hardness after patenting and suppress the cold rolling rate in comparison with the same hardness. Specifically, when work-hardening steel strips having different pearlite lamellar intervals to a predetermined hardness by cold rolling, the smaller the lamellar interval is, the lower the total rolling ratio until reaching the predetermined hardness is. Bendability is improved. From the above, in the pearlite single-phase structure after the patenting treatment, in order to improve the spring limit value and bending workability, the average lamellar spacing of pearlite is set to 0.
It is preferably 2 μm or less. Further, the steel strip during cold rolling may undergo strain aging due to heat generated during processing.
The strain aging during cold rolling markedly increases the hardness of the steel strip and the spring limit value, and since it has already been aged before working, bending workability deteriorates. Such strain aging can be prevented by controlling the temperature of the steel strip during rolling to a certain value or less.

The manufacturing conditions specified in the present invention will be specifically described below. C content: 0.6 to 1.0% by weight C is an element necessary for obtaining hardness after patenting, suppresses the generation of ferrite after patenting treatment, and makes the structure pearlite single phase. Exert the effect of. Such an effect is obtained with a C content of 0.6% by weight or more. However, when the C content exceeds 1.0% by weight, proeutectoid cementite is generated during the rapid cooling after austenitizing in the patenting treatment, and the cold workability is significantly deteriorated. Other alloying elements include Si, Mn, Ni, Mo and V, which are effective in improving the strength after patenting.
Etc. These alloy elements do not form ferrite during rapid cooling after austenitizing in continuous heat treatment equipment,
In addition, it can be added within a range in which the holding time determined by the strip running speed and the length of the constant temperature transformation tank is longer than the transformation end time determined by the steel composition and transformation temperature.

Isothermal transformation temperature: 500 to 550 ° C. The isothermal transformation temperature must be set to 500 ° C. or higher in order to suppress the formation of upper bainite and obtain a pearlite single-phase structure. However, at a transformation temperature above 550 ° C., the average lamellar spacing of pearlite produced becomes coarse, and the required spring limit value cannot be obtained, and the cooling rate from the austenitizing temperature becomes slow. As a result, ferrite is likely to be formed in hypoeutectoid steel and cementite is likely to be formed in hypereutectoid steel. Cold rolling rate: 70 to 95% If the cold rolling rate does not reach 70%, work hardening is small as a steel strip for thin leaf springs, and a necessary spring limit value cannot be obtained. In this respect, it is effective to set the cold rolling rate to preferably 80% or more. However, at a cold rolling rate of more than 95%, the action of increasing the spring limit value accompanying the increase of the cold rolling rate is saturated, and at the same time, cold rolling becomes difficult due to remarkable work hardening and the occurrence of edge cracks at the edges.

Further, in order to improve the bending workability and the spring limit value, it is effective to maintain the steel strip during cold rolling at a temperature of 110 ° C. or lower. Steel strip temperature during cold rolling: 110 ° C or less When the steel strip temperature during cold rolling exceeds 110 ° C, the steel strip is significantly hardened due to strain aging, and when the reduction ratio is high, edge cracks are likely to occur at the edges. At the same time, the bending workability of the steel strip after cold rolling is significantly deteriorated. Such a phenomenon is suppressed by holding the steel strip during cold rolling at a temperature of 110 ° C. or lower. Specifically, the steel strip temperature can be controlled by changing the low pressure reduction rate per pass, decreasing the rolling speed, increasing the amount of rolling oil sprayed, and the like.

[0012]

EXAMPLE A high carbon steel having the composition shown in Table 1 was heated to a temperature of 1250 ° C., a finishing temperature of 850 ° C. and a coiling temperature of 600.
After hot rolling at ℃, annealing was carried out at 710 ° C × soaking for 16 hours.

[0013]

[Table 1]

The obtained annealed material was subjected to an austenitizing treatment by heating in an atmosphere of 900 ° C. for 5 minutes in a continuous heat treatment facility, and then immersed in a lead bath to complete isothermal transformation at various temperatures. Then, cold rolling was performed at a rolling ratio of 65 to 85% to obtain a cold rolled sheet having a sheet thickness of 0.2 mm. The temperature of the steel strip during cold rolling was set in the range of 40 to 150 ° C. Steel strip at 510 ℃
And a diameter of 120 after patenting at 540 ° C.
mm work roll cold rolled from 1.33 mm to 0.20 mm. The limit bending radius of the obtained cold-rolled steel sheet was measured, and bending workability was investigated. Table 2 is. The relationship between the critical bending radius at this time, the temperature of the steel strip during cold rolling, and the required number of passes is shown. As is clear from Table 2, when the temperature of the steel strip during cold rolling exceeds 110 ° C, the hardness significantly increases due to strain aging, and the number of passes increases in order to cold roll to the target sheet thickness, and after cold rolling The limit bending radius is large. As a result, it is found that the bending workability of the steel strip is poor when it is processed into a spring or the like. On the other hand, when the steel strip temperature during cold rolling is maintained at 110 ° C or lower, cold rolling can be performed to the target sheet thickness with a small number of passes, and the effect of strain aging is suppressed, so that bending workability after cold rolling is excellent. A steel strip has been obtained.

[0015]

[Table 2]

The cold-rolled steel strip was subjected to an aging treatment of heating at 250 ° C. for 15 minutes. Then, the effects of the isothermal transformation temperature and the cold rolling rate on the spring limit value were investigated.

[0017]

[Table 3]

As can be seen from Table 3 showing the investigation result, the patenting structure is pearlite single phase and the cold rolling rate is 70%.
In the inventive examples of test numbers 10 to 17 which were subjected to the above cold rolling, high spring limit values were obtained after the aging treatment. On the other hand, test number 18 in which the transformation temperature does not reach 500 ° C,
In the comparative example of 19, the upper bainite was mixed, the work hardening due to cold rolling was small, and the spring limit value after the aging treatment was low. On the other hand, in Comparative Examples 20 and 21 in which the transformation temperature exceeds 550 ° C., the lamellar spacing of pearlite becomes coarse, so the strength decreases, and the spring limit value after cold rolling-aging treatment also becomes low. Even if the transformation temperature is in the range of 500 to 550 ° C. defined in the present invention, in Comparative Example 22 having a low cold rolling rate of 65%, a sufficient spring limit value is not obtained. Further, in Comparative Example 24 in which the C content was more than 1.0% by weight, pro-eutectoid cementite was generated and edge cracking occurred during cold rolling. As is clear from this comparison, when the steel strip of carbon steel or alloy steel having a C content of 0.6 to 1.0% by weight is subjected to isothermal transformation and cold rolling under the conditions specified in the present invention. For the first time, it was confirmed that a steel strip excellent in bending workability and spring limit value could be obtained.

[0019]

As described above, in the present invention, the pearlite single-phase structure containing no bainite is formed by the isothermal transformation at 500 to 550 ° C., and the cold rolling ratio is 70 to 9
By cold rolling at 5%, high strength steel strips for springs having excellent bending workability and spring limit values are manufactured. The steel strip thus obtained is used as a material suitable for various mainsprings such as retractor mainsprings, music box mainsprings, and leaf springs.

Front page continuation (72) Inventor Toshiro Yamada 11-1 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd. Steel Research Laboratory

Claims (3)

[Claims] 1. When patenting a steel strip of carbon steel or alloy steel containing C: 0.6 to 1.0% by weight, 500
After the steel strip was subjected to the isothermal transformation in a refrigerant tank kept at ˜550 ° C. to form a pearlite single-phase structure, the total rolling ratio was 70˜9.
A method for producing a high-strength steel strip for a spring, which comprises performing cold rolling at 5%. 2. The method for producing a high-strength steel strip for a spring according to claim 1, wherein the average lamellar spacing of the pearlite single-phase structure after the isothermal transformation is 0.2 μm or less. 3. The method for producing a high-strength steel strip for springs according to claim 1, wherein the steel strip during cold rolling is maintained at a temperature of 110 ° C. or lower.