JP4394542B2 - High strength SUS301 stainless steel strip with excellent bending workability - Google Patents

Description

  The present invention relates to a stainless steel strip for springs suitable for parts that require good bending workability, such as connectors used for various electronic device parts and parts used for contacts.

High strength and high springiness are required as basic characteristics of connectors used for various electronic equipment parts and parts used for contacts, etc. Further, along with recent miniaturization of electronic equipment, The thickness of electronic parts has been remarkably reduced, and metal members such as terminals and connectors are severely and complicatedly bent. Therefore, a material having higher strength and better bending workability is desired.

As one of the materials that require high strength and good bending workability, austenitic stainless steel represented by SUS304 or SUS301 is used. These stainless steels exhibit an austenite phase after solution treatment, and are intended to obtain high strength by generating work-induced martensite by subsequent cold rolling.
However, when this mechanism is used for austenitic stainless steel, if the amount of martensite generated increases, the strength can be increased, but the bending workability deteriorates. Therefore, it is difficult to achieve both high strength and good bending workability by increasing the strength by simple strong processing.

  Regarding the improvement of bending workability of austenitic stainless steel, Patent Document 1 discloses a method of ensuring bending workability while adjusting the hardness by heat-treating the material after final rolling.

JP-A-7-216450

  However, in the method disclosed in Patent Document 1, since the material hardened by rolling is annealed and softened, in order to achieve the high strength and good bending workability, which is the object of this patent, processing of the material is required. It is necessary to increase the degree. In this case, the hardness tends to change sensitively to changes in the annealing temperature, and it is considered difficult to stably obtain the targeted characteristics. An object of the present invention is to provide a SUS301 stainless steel strip as an austenitic stainless steel strip having high strength and excellent bending workability.

As a result of intensive research on the conditions under which the austenitic stainless steel strip satisfies high strength and good bending workability, the present inventors have found that in SUS301 stainless steel, 0.2% proof stress, which is an indicator of strength, and strength and bending. It has been found that a relational expression of martensite amount affecting the workability and a certain region defined by this relational expression satisfy high bending strength and good bending workability. Furthermore, as a result of investigating the relationship between the amount of martensite and bending workability of samples prepared under various conditions, it was found that the bending workability is extremely deteriorated when the martensite amount exceeds a certain value.

That is, in the present invention, (1) 0.2% yield strength (= YS (MPa)) is 1000 MPa or more, martensite amount (= Ms (%)) is “Ms ≦ 50”, and the relationship between YS and Ms is , “YS ≧ 30 × Ms + 50”, a high-strength SUS301 stainless steel strip excellent in bending workability,
(2) A high-strength SUS301 stainless steel strip excellent in bending workability used for contact parts such as the connector or switch described in (1) above,
It is.

  When SUS301 stainless steel is recrystallized annealed and rolled, high strength and good bending workability can be satisfied at the same time. This range can be achieved by defining a minimum value of 0.2% yield strength, a relational expression between 0.2% yield strength and martensite amount, and an upper limit for the martensite amount.

The reason for limitation will be described below.
(1) Amount of martensite Austenitic stainless steel exhibits an austenite phase in a solution-treated state, and then generates work-induced martensite by cold working to obtain high strength. In addition, the martensite phase produced in this way is low in ductility compared to the austenite phase, and thus is effective in increasing the strength, but causes a decrease in bending workability. Based on these findings, the strength and bending workability are strongly processed in order to generate a large amount of martensite when strength is important. In this case, good bending workability cannot be expected. On the other hand, when bending workability is important, a method of reducing the degree of work can be considered, but in this case, sufficient strength cannot be obtained. Therefore, in practice, it has been necessary to perform processing to various processing degrees, adjust the processing degree while evaluating characteristics, and balance strength and bending workability.

As a result of detailed investigations about the influence factors of the bending workability, the present inventors have found that the bending workability has a unique relationship with the martensite amount. That is, the increase in the amount of martensite does not monotonously degrade the bending workability, but the correlation changes greatly with a certain amount of martensite as a boundary.
In the case of SUS301 stainless steel, when the martensite amount is less than 50%, the effect on the deterioration of bending workability due to the increase in martensite amount is small. If it exceeds%, the deterioration of bending workability due to an increase in the amount of martensite is significant (MBR / t ≧ 1). From this, the range of the amount of martensite that can increase the strength without deteriorating the bending workability is preferably 50% or less. Therefore, the upper limit was made 50%.
MBR / t is an index for evaluating the bending workability by a value obtained by dividing the minimum bending portion radius (MBR) at which a crack does not occur in the bending portion by the plate thickness (t).

(2) Relationship between martensite amount and 0.2% proof stress Martensite amount is positively correlated with cold work, and the general relationship is that the amount of martensite increases as the rolling degree increases. It is. The relationship between the amount of martensite and strength depends on the degree of cold work, and high strength can be obtained by increasing the amount of martensite by strong processing. Sexuality gets worse.
However, it has been found that the relationship between the martensite amount, 0.2% proof stress, and bending workability differs by adjusting the crystal grain size in recrystallization annealing before rolling. As a result of examining these, it was found that even if the degree of cold work is the same, the increase in the amount of martensite can be reduced, and good bending workability can be achieved while maintaining high strength.

Thus, it was found that by adjusting the crystal grain size in the recrystallization annealing before the rolling process, it is possible to expand the region that can be rolled, but on the other hand, despite the same martensite amount and 50% or less. The present inventors have found that there is a region where good bending workability cannot be obtained. That is, when sufficient strength cannot be obtained even with the same amount of martensite, there are some cases where bending workability deteriorates. Therefore, in order to ensure good bending workability, the relationship between the martensite amount of SUS301 stainless steel and the 0.2% proof stress is “YS ≧ 30 × Ms + 50”.
Note that good bending workability to which this relational expression can be applied is MBR / t <1.

Further, the strength required for SUS301 stainless steel used for contact parts such as connectors or switches is preferably 0.2 MPa, and 1000 MPa or more.
Therefore, in order to secure the necessary 0.2% proof stress, it is preferable that Ms is 31.7% or more.

(3) Manufacturing conditions In order to manufacture a material that meets the above conditions, a combination of the crystal grain size in the final recrystallization annealing and the subsequent degree of final rolling work is important. That is, the rolling degree is set finely and stepwise according to the crystal grain size in the final recrystallization annealing. The combinations are shown below.
When the crystal grain size is 1.4 μm or more and less than 1.6 μm, the rolling work degree is 25 to 28%, and when the crystal grain size is 1.6 μm or more and less than 2 μm, the rolling work degree is 25 to 35%, or 2 μm or more and less than 3.5 μm. Is a rolling degree of 25 to 40%, a rolling degree of 25 to 45% when it is 3.5 μm or more and less than 5 μm, a rolling degree of 28 to 60% when the grain size is 5 μm or more and less than 10 μm, a crystal grain When the diameter is 10 μm or more and less than 25 μm, the rolling degree is 28 to 45%.

When the crystal grain size is less than 1.4 μm, bending workability due to rolling is remarkably deteriorated, so that it is difficult to perform rolling while maintaining good bending workability.
In addition, when the crystal grain size is 25 μm or more, the crystal grain size is coarsened, so that it is necessary to improve the strength sufficiently by rolling. Will be difficult. Furthermore, the manufacturing cost increases in the annealing process, and the manufacturing cost increases in the rolling process, which is not preferable.

  The austenitic stainless steel used in the examples is SUS301 stainless steel. The thickness of the SUS301 stainless steel is 0 mm after the cold rolling and recrystallization annealing are repeated by purchasing the strip after the solution treatment with a thickness of 1.5 mm in the component range based on JIS. Processed to 14 mm. After performing the cold rolling process and the final recrystallization annealing, in the final rolling, SUS301 stainless steel was sampled at plate thicknesses of 0.10 mm, 0.085 mm, 0.07 mm, 0.2% proof stress, The amount of martensite and the minimum bending radius were evaluated. Table 1 shows the crystal grain size obtained by changing the annealing temperature and the sheet passing speed in the final recrystallization annealing and the degree of workability of the final rolling.

The effect was evaluated by the following method.
(1) Martensite amount Using the fact that the martensite phase is paramagnetic while the austenite phase is non-magnetic, the magnetic strength of the material is measured by a ferrite content meter (also called a ferrite scope). The amount of transformation into the martensite phase, that is, the volume ratio (the ratio of the volume occupied by the phase exhibiting paramagnetism to the measured volume of the material) is obtained.
(2) Bending workability (MBR / t)
It was measured by a general 90 ° W bending test. A value obtained by dividing R with respect to the radius (= R) and the plate thickness (= t) of the bent portion and dividing the minimum bent portion radius (= MBR) at which the crack does not occur in the bent portion by the plate thickness t. Is MBR / t. The measurement was performed in the direction perpendicular to the rolling, that is, the Bad way direction. In addition, when MBR / t ≧ 1, it was determined that bending workability was poor.

As seen in the results of Table 1, Invention Example No. 1 to 8, the 0.2% proof stress is 1000 MPa or more, the relationship between the 0.2% proof stress (= YS (MPa)) and the martensite amount (= Ms (%)) is “YS ≧ 30 × Ms + 50”, and “ Ms ≦ 50 ”, high strength and good bending workability (BR / t <1).

On the other hand, in Comparative Example 9, since “YS <1000” and the strength is low, the bending workability is good, but the strength is low and it is not suitable for the field of the present invention.
In Comparative Examples 10 to 19, “YS ≧ 1000” is satisfied, and the condition of high strength is satisfied. However, Comparative Example No. Since Nos. 13 to 19 have “Ms> 50” and the martensite amount is larger than the specified range, MBR / t ≧ 1 and bending workability is poor.
In Comparative Examples 10 to 12, “Ms ≦ 50” is satisfied, but “YS ≧ 30 × Ms + 50” is not satisfied, and therefore MBR / t ≧ 1 and bending workability is poor.
Comparative Example No. 13 to 17, 19 does not satisfy “YS ≧ 30 × Ms + 50”.

Claims (2)

  The 0.2% proof stress (= YS (MPa)) is 1000 MPa or more, the martensite amount (= Ms (%)) is “Ms ≦ 50”, and the relationship between YS and Ms is “YS ≧ 30 × Ms + 50”. A high-strength SUS301 stainless steel strip excellent in bending workability, characterized by being. High strength SUS301 stainless steel strip connector according, there have is excellent in bending workability is used Proximity components to claim 1.