Strengthened alpha brass and method for manufacturing the same

The present invention relates to a strengthened alpha brass and a method for manufacturing the same. The strengthened alpha brass is excellent in both strength and formability, well balances within the strength and the formability, and has a certain level of stress relaxation properties.

Brass has been extensively used for forming electronic components such as terminals or connectors or electromechanical components because brass has relatively high mechanical strength, relatively good conductivity, and inexpensiveness. However, when components are subjected to severe forming, brass with lower temper grade has to be used to maintain necessary formability. Such a shift to lower temper grade results in increase of thickness of the material. As a result, the weight of the material increases and drawback in cost is caused.

When conventional brass is processed with high reduction for the purpose of obtaining materials having excellent strength, resulting materials have deteriorated bend formability and poor toughness, so bending of such materials tends to be difficult. Thus, conventional brass has such a drawback. That is, when brass is processed to form connectors or the like, severe bending is often applied. Therefore, for the purpose to prevent defects in required bending, materials having a 0.2% offset yield strength less than 550 MPa are used in most cases. When a material having a higher strength than this level is required, expensive phosphor bronze is generally selected. Furthermore, conventional brass does not exhibit excellent stress relaxation properties. In particular, when grains are turned to be finer, the stress relaxation properties are further deteriorated, and which shows a serious problem in a practical use. Therefore, customer\'s demand is to avoid the deterioration of the stress relaxation properties due to making grains finer.

On the other hand, alpha brass is generally processed into plates or strips by carrying out semi-continuous casting, hot rolling, sculpturing, subsequently cold rolling to have a certain thickness good for continuous annealing first. And then the brass strip is subjected to continuous annealing and pickling, cold rolling, continuous annealing and pickling, cold rolling, and cutting to finish a plate or a strip. In these processes, various manufacturing processes can be conducted. For example, annealing and rolling can be repeated according to the thickness, or the annealing can be a batch annealing. When a customer demands annealing finish, the final rolling can be omitted. Processes such as degreasing, pickling, leveling, cutting, or plating can be further conducted between rolling and heat treatment or after rolling or heat treatment. Hereinafter, such conventional manufacturing processes are called “general manufacturing processes” in the present specification.

In the general manufacturing processes with a continuous annealing, the annealing condition is conducted in the range of 480-deg.C. to 850-deg.C. as disclosed in Patent Document 1. On the other hand, a batch annealing is conducted in the range of 425-deg.C. to 600-deg.C. as disclosed in Non-Patent Document 1. Furthermore, initial and intermediate annealings are conducted to prepare a grain size to be 20-micron meter to 35-micron meter for the purpose of obtaining sufficient recrystallized structures and to decrease rolling pressure required. In this case, Vickers hardness (Hv) falls into the range of 60 to 80. Then final annealing is conducted to arrange a grain size from 5-micron meter to 60-micron meter according to applications, and a Vickers hardness (Hv) falls into the range of 50 to 120. As mentioned above, in the general manufacturing processes by carrying out the final annealing, final cold rolling and the cutting are conducted to finish product plates or strips. Hereinafter, such products provided by carrying out the final cold rolling are called “general materials or general brass” in the present specification.

In order to increase strength of brass, using work hardening is popularly known. Patent Document 2 discloses a method for obtaining high strength by making grains finer and subjecting thus obtained brass to cold rolling for more increased strength. Methods to make grain structure fine are disclosed in Non-Patent Documents 2 and 3.

In the disclosures, high reductions such as 92%, 91%, 80% or 78% are employed, and subsequent annealing is conducted in a low temperature range for long hours such as, at 300-deg.C. for one hour, at 270-deg.C. for 7 hours, or at 230-deg.C. for 17 hours. Thus obtained strengths in the annealed state are reported to be relatively high in annealed state such as a 0.2% offset yield strength of 379 MPa in the condition at 300-deg.C. for one hour, a 0.2% offset yield strength of 399 MPa in the condition at 270-deg.C. for 7 hours, and a 0.2% offset yield strength of 534 MPa in the condition at 230-deg.C. for 17 hours.

Patent Document 2 also discloses a method for manufacturing brass with fine grains. However, the method disclosed in Patent Document 2 requires repeating rolling processes with a large reduction in a step-by-step manner. Therefore, this technique may be applied for manufacturing products with thin thicknesses. On the other hand, when relatively thick products are manufactured, difficulty may be caused to apply rolling processes with severe reduction for multiple times. Even condition of its preliminary annealing is important, Patent Document 1 discloses just a final annealing and nothing is disclosed on it.

Non-Patent Document 4 discloses research results on making alpha plus beta brass much stronger by making grains finer. Specifically, it is reported that a material having high strength and relatively good bend formability can be obtained by fine grained micro-duplex structure of alpha phase and beta phase, and subjecting the brass to a low temperature annealing. It is also reported that stress relaxation properties deteriorate as grains become finer, and the deterioration is slightly improved by a low temperature annealing.

[Patent Document 1]: Japanese Patent Laid-Open No. 53-32819

[Patent Document 2]: Japanese Patent Laid-Open No. 2004-292875

[Non-Patent Document 1]: Data Book for Copper and Copper Alloy Product, p. 19, issued by Japan Copper and Brass Association

[Non-Patent Document 2]: Copper and Copper Alloy 41, 1, and 29 [Non-Patent Document 3]: Copper and Copper Alloy 43, 1, and 21

[Non-Patent Document 4]: Journal of the Japan Copper and Brass Research Association 39, 1, 128

[Non-Patent Document 5]: Data Book for Copper and Copper Alloy Product, p. 226, issued by Japan Copper and Brass Association

However, citing the case of carrying out annealing at 230-deg.C. for 17 hours, which provides most excellent 0.2% offset yield strength in the disclosures of Non-Patent Documents 2 and 3, relatively high strength with improved bend formability is obtained by arranging the material after cold rolling from the state of work hardened to the state of not enough softened by unfinished annealing. Thus obtained crystal structure has ununiform re-crystallized state. Thus, this alloy has a decreased 0.2% offset yield strength of about 534 MPa, and poor property.

On the other hand, the disclosure of Patent Document 2 suggests a preferred example of a method for manufacturing a brass with well balanced strength and bend formability. However, Patent Document 2 discloses only one example suggesting balance between a specific strength and bend formability. In addition, bend formability is evaluated by employing a bending with a bending axis across the rolling direction which is considered a Good Way bend. So, the evaluation result does not mean good bend formability with a bend axis along the rolling direction which is considered a Bad Way bend.

In carrying out a typical method for manufacturing a strengthened alpha brass, it is widely known that making grains finer results in high strength after annealing. It is also well known that brass having fine grains can be manufactured by subjecting brass to cold rolling with high reduction and repeating annealing in relatively low temperature ranges for long hours. The manufacturing method disclosed in Patent Document 2 requires combination of the process with high reduction for multiple times. Therefore, it can be difficult to manufacture relatively thick products by this method. In addition, Patent Document 2 discloses just a few examples of preliminary annealing temperature before the final re-crystallization annealing. Thus, such annealing conditions are not taken as important ones.

In addition, in alpha plus beta brass, it is the fact that with fine grained micro-duplex structure of alpha phase and beta phase, the brass is much inferior to phosphor bronze in balance between 0.2% offset yield strength and bend formability.

As described above, though various suggestions are made on theoretical manufacturing methods, but managing of a manufacturing condition is difficult and manufacturing conditions that permit industrial mass production have not been found.