JPS6259167B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a spring steel strip that has excellent cold workability and punchability during cold rolling. Conventionally, mainsprings for cord reels or tractors with seat belts have been made of austenitic stainless steel such as SUS301, which uses deformation-induced transformation according to the JIS standard, or hardened stainless steel such as SK5, which is hardened and tempered. Steel strips are used, but SUS301 has a high fatigue life and spring limit value due to work hardening by cold rolling with sufficient control of rolling temperature and tension, and aging treatment at around 400℃, and has excellent spring properties. Reliability is high. However, it has the disadvantage that the material is very expensive, and in the processing of the end portion in the spring processing process, it is difficult to punch out during cold rolling due to work hardening. Furthermore, when the end portion is annealed to soften it, it contains a brittle region in the metal structure, which has the disadvantage of reducing the spring fatigue life. Secondly, hardened steel strips such as SK5 have lower material costs than SUS301 and do not require as much technical attention in manufacturing, but they do not have the same fatigue properties as SUS301, so the spring fatigue life is There are drawbacks that lead to a decline in In addition, when isothermal transformation treatment is performed to improve fatigue properties, work hardening is performed by cold rolling to obtain high strength.
For this purpose, an extremely large cold rolling rate must be used, and as a result, the edges (ears) of the steel strip during cold rolling are reduced.
Small cracks occur frequently. For this reason, careful side polishing was indispensable before cold rolling, although this inevitably resulted in a decrease in yield. The present invention has been made in view of the above-mentioned circumstances, and has been developed for use in springs based on carbon steel, which has excellent composition and manufacturing conditions, especially edge cracking of steel strips that occurs during cold rolling, and punching properties during cold working. As a result of various experiments and research to obtain steel strips, we succeeded in controlling the Si and Mn dissolved in ferrite and
By adding Cr to improve cold workability and punchability during cold rolling, and further performing isothermal transformation treatment after solution treatment, edge cracking of the steel strip during cold rolling and punchability of cold rolled products can be improved. We have developed an excellent steel strip for springs. That is, in the present invention, the components are C: 0.65 to 0.9%, Si: 0.20%, Mn: 0.5
%, Cr: 0.1 to 1.5%, with the remainder consisting of Fe and unavoidable impurities, which is then subjected to isothermal transformation treatment at a temperature below A1 point and above 425°C after solution treatment. It is a method of manufacturing obi,
When compared with a conventional steel strip (SK5) containing the same amount of carbon as the steel of the present invention, the edge cracking of the steel strip during cold rolling and the punchability of cold rolled products can be greatly improved, As a result, the quality of the steel strip is improved, the yield is significantly increased, and the service life of the punching tool can be improved. Also, traditional
It is possible to obtain the same mechanical properties and fatigue properties without using expensive materials such as SUS301, which reduces costs and has a large economic effect. Next, the role of alloy components and the reasons for limiting the composition and manufacturing conditions will be explained. (1) C: 0.65-0.90% When the C content is 0.65% or less, the ferrite volume % becomes smaller than the cementite volume %. As a result, mechanical properties (tensile strength) deteriorate, and cementite becomes the main component of plastic cutting, resulting in large burrs after punching. In addition, it is necessary to have at least 0.65% since it becomes difficult to undergo constant temperature transformation. On the other hand, if the C content exceeds 0.90%, the hardness after transformation will be high, making it difficult to cold-roll, and if the rolling reduction increases, edge cracking will easily occur during cold-rolling, so the upper limit should be set.
It was set at 0.90%. (2) Cr: 0.1 to 1.5% If the Cr content is 0.1% or less, isothermal transformation becomes difficult and mechanical properties (tensile strength) and fatigue properties deteriorate, so it should be at least 0.1%. In addition, if the Cr content exceeds 1.5%, the hardness after isothermal transformation becomes too hard, making it difficult to cold-roll, and the carbides become finer due to isothermal transformation, reducing toughness and causing large burrs after punching. set the upper limit to
It was set at 1.5%. (3) Si: 0.20% or less, Mn: 0.5% or less The addition of Si and Mn facilitates isothermal transformation, but the ferrite-strengthening elements make cold rolling difficult. Therefore, the Si content is reduced to 0.20%.
Hereinafter, the Mn content was set to 0.5% or less. (4) Below A1 point to 425°C or above At A1 point or above, precipitation of isothermal transformation, that is, pearlite transformation, does not occur. Furthermore, below 425°C, bainite transformation occurs at the same time as pearlite transformation, making the structure unstable and causing unevenness. In addition, the lower limit was set to 425 because the occurrence of edge cracking during cold rolling increases.
℃ or higher. The present invention will be specifically described below with reference to Examples. Example 1 Ingots were melted and cast in a 50Kg high-frequency induction furnace using alloys having the compositions shown in Table 1. The ingots were forged into a cross section of 15mm thick x 120mm wide, then hot-rolled, descaled, and then cold-rolled in four stages. The sample material was rolled to a thickness of 1.0 mm x width of 120 mm using a rolling mill. next
Immediately after solution treatment at 1000℃
It was transferred to a constant temperature tank maintained at 550°C and subjected to constant temperature transformation treatment.

[Table] This steel body was cold rolled to a thickness of 0.25 mm (cold rolling ratio 75%), and the occurrence of edge cracking in the cold rolled material was investigated. The results are shown in Figure 1. In the diagram, ○ marks indicate no cracks, ● marks indicate slight cracks (no effect on rolling), and × marks indicate cracks in 10 or more places on one side or cracks occurring over almost the entire length. As is clear from Figure 1, the amount of Si is 0.20%,
It can be seen that ear cracking occurs when the Mn content exceeds 0.5%. Next, sample materials No. K to N (invention) shown in Table 1
Figure 2 shows the results of an investigation into the relationship between the isothermal transformation temperature and the degree of ear cracking. In addition, the second
The evaluation criteria for the "amount of ear cracking" shown in the figure are as follows. Crack rating 4: Cracks occur throughout almost the entire length 3: Occur at 30 locations on one side of the steel strip/within 3.5 m 2: Occur at 10 locations/within 3.5 m 1: Slight cracking level 0: No cracks occur As is clear from Figure 2, The isothermal transformation temperature is
This shows that when the temperature is lower than 425°C, edge cracking occurs more frequently during cold rolling. Example 2 Using a material having the composition shown in Table 1, which was heat treated and subsequently cold rolled using the treatment method shown in Example 1, the punching property and burr amount during cold working were further evaluated. investigated the relationship. The results are shown in Table 2.

[Table] The punching property is evaluated by the amount of burr generated by the number of consecutive punches using a mechanical press for processing the end of a spring. [Numerator side: burr amount (%) in the rolling direction, denominator side: perpendicular to the rolling direction. Amount of direction return (%)]
did. The amount of burr is measured using a micrometer and a microscope. Test materials No. I, J and No. L of the present invention, comparative materials No. A, B, C, D, F and No. G
It is clear from the data in Table 2 that the change in burr amount due to continuous punching is extremely small compared to the above. Example 3 The rolled materials of the present invention test materials No. I, J and No. L used in Example 1 were further aged at 325°C. Table 3 shows the results of investigating the mechanical properties and fatigue properties of thin leaf spring materials after aging treatment for these materials.

[Table] As a comparative example, SUS301 austenitic stainless steel for springs was aged at 400℃, and sample material No. A of the comparative example shown in Table 1 was cooled to a thickness of 0.25 mm x width of 120 mm. This figure shows the mechanical properties and fatigue properties of thin plate spring materials after being rolled out and heat treated (830℃ quenching, 350℃ tempering). As can be seen from the data in Table 3, the present invention (sample materials No. I, J,
L) has almost the same tensile strength as normal heat-treated material No. A, but has a higher fatigue limit. Also, a comparative example
It has the same fatigue limit as SUS301. As a result, there is no need to use expensive materials such as SUS301, so it can be produced at low cost, making it economical. Furthermore, the hardness is low for the same tensile strength. It is easy to process. As shown in the above examples, the present invention reduces the edge cracking of the steel strip during cold rolling and the burrs of the cold rolled product by adjusting the chemical composition and performing isothermal transformation treatment. In addition to improving the quality of the process, it also reduces finishing man-hours and increases yield. In addition, it is possible to obtain the same mechanical properties and fatigue properties as conventional SUS301 without using expensive materials, and it has economical effects such as reduced production costs.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the Si and Mn contents in a cold rolled steel strip and the degree of edge cracking during cold rolling. FIG. 2 shows the relationship between the isothermal transformation temperature and the degree of edge cracking during cold rolling in the present invention.

Claims (1)

[Claims] 1 C: 0.65-0.95%, Si: 0.20% or less, Mn:
After solution treatment of steel containing 0.5% or less, Cr: 0.1-1.5%, and the balance consisting of Fe and unavoidable impurities,
A method for producing a steel strip, characterized by carrying out isothermal transformation treatment at a temperature below A1 point and above 425°C.