Strengthened Alpha Brass and Method for Manufacturing the Same

An object of the present invention is to provide a strengthened alpha brass having a good balance between high offset yield strength and formability without deteriorated stress relaxation resistance in comparison with conventional brass and a manufacturing method of the strengthened alpha brass. In order to achieve this object, a strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc; the strengthened alpha brass which is obtained by using a starting plate material subjected to a re-crystallization annealing to have a grain size from 1-micron meter to 2-micron meter followed by cold rolling in 5% to 40% reduction, then the plate material is low temperature annealed at a temperature equal to or higher than the temperature at which a 0.2% offset yield strength exhibits a maximum value to adjust the 0.2% offset yield strength ([Sigma]0.2: MPa) to be equal to or higher than 90% of its maximum value is adopted. The strengthened alpha brass has a 0.2% offset yield strength of 450 MPa to 750 MPa and [minimum bend radius (MBR)]/[plate thickness (t)] and [0.2% offset yield strength] satisfy the following formula MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength) , and [Erichsen value (Er: mm)] and [0.2% offset yield strength] preferably satisfy the following formula. Er≧−0.011×σ0.2+13.7 (σ0.2: 0.2% offset yield strength)

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Description
TECHNICAL FIELD

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.

BACKGROUND ART

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.

DISCLOSURE OF THE INVENTION

Then the present inventor has thoroughly investigated and found manufacturing conditions of providing fine grained structures in excellent industrial productivity with less deviation in products quality. That is, the present invention provides a method for manufacturing a strengthened alpha brass wherein the method is readily applicable in an industrial scale; the strengthened alpha brass has formability required for the general alpha brass, but has excellent strength than the general alpha brass; furthermore, the strengthened alpha brass has a level of strength and formability equal to those of EH temper grade of brass or more than that, those of temper H of phosphor bronze; and the strengthened alpha brass keeps a certain level of stress relaxation resistance. The present invention also provides a strengthened alpha brass obtained by the method.

The present invention provides a method for manufacturing a strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc, characterized in that a brass plate with a grain size from 1-micron meter to 2-micron meter is used as a starting plate material, the brass plate is cold rolled in 5% to 40% reduction to prepare a cold rolled brass plate, then the cold rolled brass plate is adjusted a 0.2% offset yield strength to be equal to or higher than 90% of its maximum value by subjecting a low temperature annealing.

The low temperature annealing is conducted at a temperature equal to or higher than an annealing temperature at which the 0.2% offset yield strength exhibits the maximum value when estimated from relationship within the 0.2% offset yield strength and annealing temperatures.

The brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is preferably obtained by using a hot rolled brass plate or a brass plate with a grain size of 7-micron meter to 200-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 80% to 95%, and then subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170.

Alternatively, the brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is also preferably obtained by using a hot rolled brass plate or a brass plate with a grain size of 7-micron meter to 200-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 80% to 95%, then subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170, subjecting the material to a cold rolling process with a reduction of 40% to 95%, and further subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170.

Alternatively, the brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is also preferably obtained by using brass plate with a grain size of 3-micron meter to 6-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 70% to 95%, and then subjecting the material to a re-crystallization annealing to adjust Vickers hardness (Hv) to be in the range of 130 to 170.

The re-crystallization annealing is preferably conducted in 370-deg.C. to 650-deg.C. for a continuous annealing, or in 255-deg.C. to 290-deg.C. for a batch annealing.

The present invention provides the strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc, characterized in that the strengthened alpha brass has a tensile strength from 530 MPa to 790 MPa, a 0.2% offset yield strength from 450 MPa to 750 MPa and a stress relaxation rate equal to or less than 52% after 100 hours at 120-deg.C.; and a minimum bend radius (MBR: mm) with which the strengthened alpha brass is 90 degree bent with a bend axis along the rolling direction without causing cracks, a plate thickness (t: mm) and a 0.2% offset yield strength (MPa) satisfy the following Formula 4, where a value of right side of the Formula 4 is interpreted as 0.3 when calculation result is equal to or less than 0.3.


MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength)  [Formula 4]

It is also preferable that an Erichsen value (Er: mm) and the 0.2% offset yield strength (MPa) of the strengthened alpha brass preferably satisfy the Formula 5.


Er≧−0.011×σ0.2+13.7 (σ0.2: 0.27% offset yield strength)  [Formula 5]

According to the present invention, a strengthened alpha brass which has well balanced 0.2% offset yield strength and formability and a stress relaxation rate equal to or less than a certain limit can be obtained. In addition, a strengthened alpha brass according to the present invention has excellent and stable properties, and it can be suitably manufactured in industrial scale also.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a relation among temperatures of low temperature annealing, 0.2% offset yield strength and stress relaxation rates obtained in Examples 1 and 2, and Comparative Examples 1 and 2; and

FIG. 2 shows softening curves obtained by carrying out preliminary tests in which reductions and annealing temperatures are changed in processing raw materials obtained by subjecting ingot 2 in Example 3 to hot rolling and then sculpturing the hot rolled material.

 BEST MODE FOR CARRYING OUT THE INVENTION Manufacturing Method of Strengthened Alpha Brass According to the Present Invention

A method for manufacturing a strengthened alpha brass according to the present invention is a method for manufacturing a strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc, characterized in that a brass plate with a grain size from 1-micron meter to 2-micron meter is used as a starting plate material, the brass plate is cold rolled in 5% to 40% reduction to prepare a cold rolled brass plate, then the cold rolled brass plate is adjusted a 0.2% offset yield strength to be equal to or higher than 90% of its maximum value by subjecting a low temperature annealing.

First, the reason to define the composition of the strengthened alpha brass according to the present invention is described. When the copper-zinc alloy has a copper content greater than 75 wt %, its strength level is inferior and performing of excessive increase in the strength results in considerable tendency of causing deterioration of bend formability. On the other hand, when the copper content is less than 63 wt %, beta phase appears and a single phase structure of alpha phase cannot be formed. In addition, as to incidental impurities, it is necessary to give consideration to scrap materials which are used to cut costs as is the case with wrought copper and copper alloy products. Fe as an impurity influences re-crystallization temperature and thus Fe is preferably 0.01 wt % or less. Sn as an impurity does not particularly give a drawback. However, Sn exceeding 0.1 wt % has a good influence on strength or corrosion resistance, the alloy having such Sn content should be treated separately. S as an impurity has a detrimental effect on hot formability, formability of final products such as wroughting or machining. Therefore, S as an impurity is preferably limited to 0.003 wt % or less.

Because the starting plate material has uniform grain size from 1-micron meter to 2-micron meter due to a re-crystallization process, obtained cold rolled material may have more uniform grain size distribution as a result. Then the brass plate is subjected to a cold rolling in 5% to 40% reduction to prepare a cold rolled brass plate. When the reduction in the cold rolling process is less than 5%, 0.2% offset yield strength decreases even though a low temperature annealing described later may be conducted. On the other hand, when the reduction in the cold rolling process is greater than 40%, work hardening proceeds to result bend formability exceeds 3 in MBR/t while 0.2% offset yield strength is improved. Thus, in this case, it may hard to obtain a strengthened alpha brass having good balance in mechanical properties.

By the way, 0.2% offset yield strength is used as an indicator of mechanical strength of a strengthened alpha brass according to the present invention. The mechanical strengths of the general materials are generally represented by tensile strength and elongation. However, the tensile strength is a value calculated from a maximum load observed until break occurs. The value of the maximum load is data obtained when a tensile process has already been applied and alteration factors influence sectional shape and physical properties. Therefore, the present inventor thinks that it is inappropriate to use tensile strength as an indicator of formability. Then 0.2% offset yield strength, which is mainly used as a standard for design is adopted as an indicator of strength, because with the indicator, properties themselves of materials before being processed can be compared and evaluated.

Furthermore, when a material subjected to the final cold rolling is subjected to a low temperature annealing, as the temperature of the low temperature annealing increases, 0.2% offset yield strength increases with showing a moderate curve and then gradually decreases and then drops rapidly. This is known as a low temperature annealing hardening phenomenon. Then the temperature is limited so that 0.2% offset yield strength after the low temperature annealing is equal to or higher than 90% of its maximum value of 0.2% offset yield strength obtained in the low temperature annealing hardening phenomenon. This limitation is for the purpose of suppressing decrease of strength. As for the temperature that provides the maximum value of 0.2% offset yield strength, the width of the peak is narrow in the case of plotting along a temperature axis or a time axis when a reduction is low. However, when the reduction is high, a wide and mild peak can be obtained. Therefore, it is practical to recognize a region in which 99% or higher of the maximum value is obtained according to heating conditions with which the maximum value is obtained rather than recognizing the condition with which the maximum value is obtained as a one point.

Then in view of dependency of the 0.2% offset yield strength on annealing temperatures, the low temperature annealing is preferably conducted at a temperature equal to or higher than an annealing temperature at which the 0.2% offset yield strength exhibits its maximum value. The final low temperature annealing conducted herein does not simply denote an annealing that is conducted in low temperatures for stress relieving, but denotes a process involving the so-called low temperature annealing hardening phenomenon. On the other hand, the present inventor has found that a stress relaxation rate of about 55% after being cold rolled decreases to be a certain level as the temperature increases from in the vicinity of annealing temperature that provides its maximum value of 0.2% offset yield strength. Therefore, as a condition to achieve the threshold of a stress relaxation rate equal to or less than 52%, the low temperature annealing temperature is required to be equal to or higher than an annealing temperature at which the 0.2% offset yield strength exhibits its maximum value.

The low temperature annealing is preferably conducted as continuous annealing rather than batch annealing. Furnace temperature is preferably in the range of 250-deg.C. to 450-deg.C. Preferred time for passing a plate is 1 second to 10 seconds. The advantage of carrying out the low temperature annealing continuously is that cost reduction and assuring of quality stability are achieved easily. In addition, the low temperature annealing is a final annealing and after the low temperature annealing is complete, the product is generally in the state of a strip. In the case of carrying out a batch annealing, the strip is introduced into a heating furnace as a coil and heated with keeping the shape. Therefore, the strip curls and the curl in addition to strain caused by rolling are required to be leveled in the leveling process prior to using the strip as a product. Thus carrying out effective leveling is difficult. On the other hand, in the case of carrying out a continuous annealing, the plate materials are heated by running through a heating zone and are wound as a coil after the low temperature annealing is complete. Therefore, thus obtained strips do not curl and flat strips are easy to be obtained by subjecting the strips to a leveling process.

The brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is preferably obtained by using a hot rolled brass plate or a brass plate with a grain size of 7-micron meter to 200-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 80% to 95%, and then subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170. The brass plate (annealed material) with a large grain size used as a raw material also includes hot rolling finished materials. The grain size of hot rolled plate materials is 100-micron meter to 200-micron meter in the case of using a small test scale rolling mill, but about 25-micron meter due to dynamic re-crystallization in the case of using an industrial scale hot rolling mill. When the cold rolling process is conducted with a reduction equal to or more than 80%, such a severe process provides a subgrain structure of about 1-micron meter even though the grain size before processing is large. As a result, after the brass is subjected to subsequent steps, good 0.2% offset yield strength and bend formability, which is the target of the present invention, can be obtained. The present inventor has confirmed that use of hot rolling finished materials obtained with a small test scale rolling mill or an industrial scale hot rolling mill does not result in difference in properties when the materials are subjected to the cold rolling process with a reduction equal to or more than 80%. Therefore, the lower limit of the reduction of the cold rolling process is defined as 80%. On the other hand, carrying out of the cold rolling process with a reduction greater than 95% may cause cracks in the edge during the process, so which is not preferable.

The brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is also preferably obtained by using a hot rolled brass plate or a brass plate with a grain size of 7-micron meter to 200-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 80% to 95%, then subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170, subjecting the material to a cold rolling process with a reduction of 40% to 95%, and further subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170. As mentioned above, the plate obtained by using a hot rolled brass plate or a brass plate with a grain size of 7-micron meter to 200-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 80% to 95%, then subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170 has an average grain size from 1-micron meter to 2-micron meter. In considering of this, when a reduction of 40% to 95% is adopted in the subsequent cold rolling process and the reduction is low but more than 40%, micro grains can be easily obtained by the subsequent re-crystallization annealing. Then after the plate is subjected to subsequent processes, good 0.2% offset yield strength and bend formability can be obtained. On the other hand, carrying out the cold rolling process with a reduction greater than 95% may cause cracks in the edge during the process, and which is not preferable.

Alternatively, the brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is also preferably obtained by using brass plate with a grain size of 3-micron meter to 6-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 70% to 95%, and then subjecting the material to a re-crystallization annealing to adjust Vickers hardness (Hv) to be in the range of 130 to 170. When the grain size before being processed is larger than 6-micron meter, bend formability is deteriorated because of poor fineness of grains even when the plate is subjected to the cold rolling process with a reduction of greater than 70%. On the other hand, the average grain size of less than 3-micron meter has disadvantage in some case, because this requires high rolling pressure even when the subsequent process is conducted with a reduction of 70%. In addition, carrying out the cold rolling process with a reduction greater than 95% may cause cracks in the edge during the process, so it is not preferable.

The re-crystallization annealing is preferably conducted in 370-deg.C. to 650-deg.C. for a continuous annealing, or in 255-deg.C. to 290-deg.C. for a batch annealing. In the method for manufacturing a strengthened alpha brass according to the present invention, the starting material is subjected to the cold rolling process, and then the final re-crystallization annealing is conducted at the furnace temperature of 370-deg.C. to 650-deg.C. for the continuous annealing. When the furnace temperature is lower than 370-deg.C., obtained products may have deteriorated formability, even re-crystallization is conducted by decreasing the speed of passing the plates. On the other hand, when the furnace temperature is higher than 650-deg.C., the grain size becomes ununiform and grows greater than 2-micron meter. Then deteriorated formability is obtained even when cold rolled brass plates obtained after the cold roll process is subjected to the low temperature annealing. The time for re-crystallization annealing is determined depending on the performance of a furnace, a plate thickness and target strength. In the case of standard industrial equipment, the time is in the range of 2 to 120 seconds. When appropriate time is actually determined, the time is easily managed based on hardness so that Vickers hardness (Hv) is in the range of 130 to 170, preferably in the range of 135 to 160. When the Vickers hardness (Hv) is less than 130, re-crystallized grain size is so large that target physical properties cannot be obtained even when the subsequent processes are conducted. On the other hand, when the Vickers hardness (Hv) is greater than 170, an obtained structure contains a higher ratio of remaining deformation structure than a ratio of recrystallized structure. As a result, a strengthened alpha brass as a final product shows a deteriorated formability.

The advantage of carrying out the re-crystallization annealing by a continuous process is that cost reduction and assuring of quality stability are achieved easily. For a batch process, material temperatures tend to vary depending on positions in a furnace. In addition, after a batch annealing is conducted, value [0.2% offset yield strength]/[tensile strength] tend to be less than 80% after the final re-crystallization annealing. On the other hand, when a continuous heating process is used, the value of [0.2% offset yield strength]/[tensile strength] become equal or higher than 80% after the final re-crystallization annealing, then finer grains can be obtained. Therefore, use of the continuous heating rather than the batch heating provides better balance between 0.2% offset yield strength and formability in a strengthened alpha brass obtained by carrying out the final cold rolling process and the low temperature annealing of the manufacturing method according to the present invention.

However, the batch annealing may be applicable when the plate thickness is thick or no continuous annealing furnace is applicable. Industrially, the material is generally kept for 30 minutes to about 3 hours after the material temperature of a coil reaches a preset temperature. When this keeping time is adopted, preferred material temperature is 255-deg.C. to 290-deg.C. When the material temperature of the coil is less than 255-deg.C., grains obtained by re-crystallization for achieving target strength have ununiform sizes (when a grain size distribution chart is made with plotting the grain size along an abscissa logarithmic axis, two or more peaks are observed), then bend formability is extremely worsened even after low temperature annealing. On the other hand, when the material temperature of the coil is higher than 290-deg.C., grains may have uneven sizes and an average grain size might be large. When such a plate is subjected to a cold rolling process followed by a low temperature annealing, product obtained may have deteriorated formability.

The present invention provides a strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc, characterized in that the strengthened alpha brass has a tensile strength from 530 MPa to 790 MPa, a 0.2% offset yield strength from 450 MPa to 750 MPa and a stress relaxation rate equal to or less than 52% after 100 hours at 120-deg.C.; and a minimum bend radius (MBR: mm) with which the strengthened alpha brass is 90 degree bent with a bend axis along the rolling direction without causing cracks, a plate thickness (t: mm) and a 0.2% offset yield strength (MPa) satisfy the following Formula 6, where a value of right side of the Formula 6 is interpreted as 0.3 when calculation result is equal to or less than 0.3.


MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength)  [Formula 6]

As for an indicator of the strength of the strengthened alpha brass, the 0.2% offset yield strength, which is mainly used as a standard for design is adopted as mentioned above.

As an indicator of evaluating formability of a copper alloy, mostly used is a minimum bend radius (MBR: mm) with which cracks are not caused. Bend formability is important indicator in fabricating terminals or the like. When referring to bend formability in the present application, it is to be understood that evaluation is conducted by the so-called Bad way bending in which 90 degree bend test is conducted with a bend axis along the rolling direction in various bending tests. When the so-called Good way bending in which 90 degree bend test is conducted with a bend axis across the rolling direction is used, better results are generally obtained in evaluating brass. The present inventor considers that use of the Good way bending is inappropriate for comparing and selecting manufacturing methods. Therefore, the present application employs only the Bad way bending as an evaluation method.

Now, the standard of the bend formability is described. When the MBR/t is equal to or less than 0.3, applying of almost any bending process dose not causes defects and shows no problem. On the other hand, the MBR/t equal to or less than 1.0 is often acceptable in consideration of materials design. When the MBR/t is larger than 3.0, it is difficult to bend and applications of such materials are considerably restricted. There is a few conventional brass having a 0.2% offset yield strength higher than 550 MPa which provides the MBR/t equal to or larger than 1.0. Brass having a 0.2% offset yield strength less than about 500 MPa show no problem in bend formability.

As for the stress relaxation rate, Japan Copper and Brass Association define a test method (to be set as a cantilever and permanent deflection displacement by bending is measured). Temperature applied to the strengthened alpha brass according to the present invention is selected to be 120° C. The test method defines its treatment time as 1000 hours, however, 100 hours are enough to evaluate difference and thus 100 hours are selected. By using the method, the C2600 material and the C2680 material, which are available in the market as materials for terminals or connectors, were evaluated in terms of the stress relaxation rate. The results were 40%, 40%, 36%, 40%, and 48% to 52%. Thus, it has turned out that the stress relaxation rate varies depending on temper grades and grain sizes. At this time, the test pieces were evaluated within two weeks from manufacturing in order to prevent possible influences of aged deterioration on evaluation. Consequently, the present inventor defines the strengthened alpha brass according to the present invention to have the required stress relaxation rate with the threshold of 52% in consideration of the fact that the above materials are actually used and customers do not like deterioration of the stress relaxation rate.

Furthermore, it is also preferable that an Erichsen value (Er: mm) and the 0.2% offset yield strength (MPa) of the strengthened alpha brass preferably satisfy the Formula 7.


Er≧−0.011×σ0.2+13.7 (σ0.2: 0.2% offset yield strength)  [Formula 7]

As mentioned above, when the MBR/t is equal to or less than 0.3, use of almost all bend formability dose not cause defects and show no problem. However, problems can be caused in crimping process of thick plates, bending of thick plates without bending R, bulging or the like. On the other hand, the present inventor has found that brass with fine grains has excellent formability due to features of fine grains. In alpha brass in which grains are finished fine, when the 0.2% offset yield strength is in the vicinity of 540 MPa or less, the minimum bend radius of bending becomes zero. Therefore, the minimum bend radius cannot be used as an indicator of formability that covers a wide range of strength.

Then the present application further uses an Erichsen value (Er: mm) as an additional indicator because the Erichsen value (Er: mm) is often used as an indicator of formability. In order to prove that this selection is adequate, the present inventor collected 17 samples from the C2600 or C2680 material, with ½H, H and EH temper grade specified in JIS, and a 0.2% offset yield strength (MPa) and an Erichsen value (Er: mm) of each sample were evaluated. Then relation of the 0.2% offset yield strength (MPa) and the Erichsen value (Er: mm) was examined and the relation satisfy the Formula 8.


Er=−0.011×σ0.2+12.7±0.5 (σ0.2: 0.2% offset yield strength)  [Formula 8]

It should be noted that the Erichsen value is a value obtained by the following Erichsen Test. By using the Erichsen value, deep drawability of a sheet metal is judged.

(1) Standards of test equipment and test method: JIS B
(2) Test method: a phi 90 plate, phi 27 dies (with a holddown to which Vaseline is applied), and D=20 hemisphere punch are used; When a crack through both sides is observed, the depth of the punch into the plate [Erichsen value (Er: mm)] is measured.

The present inventor has found that the Erichsen values (Er: mm) of the strengthened alpha brass according to the present invention satisfy the Formula 9 whenever 0.2% offset yield strength fall within the range of 450 MPa to 750 MPa; and the Erichsen values (Er: mm) of the strengthened alpha brass according to the present invention are 0.5 mm or more superior to general materials having the same 0.2% offset yield strength (MPa). However, the Erichsen value (Er: mm) is not always used to evaluate formability in all applications. In particular, formability of general materials having MBR/t of exceeding 0.3 is recommended to be evaluated by not only Erichsen values (Er: mm) but also bend formability, which is used for direct evaluation.


Er≧−0.011×σ0.2+13.7 (σ0.2: 0.2% offset yield strength)  [Formula 9]

Next, physical properties of the strengthened alpha brass according to the present invention are described in comparison with those of phosphor bronze. Then bend formability of phosphor bronze is described in order to compare the bend formability with that of the strengthened alpha brass according to the present invention. Referring to data obtained by the same evaluation method as the method used for evaluating 90 degree bend formability of the strengthened alpha brass according to the present invention in Non-Patent Document 5, bend formability of phosphor bronze can be represented by the following Formula 10 where a minimum bend radius without causing cracks is defined as MBR (mm), a plate thickness is defined as t, and symbol [sigma]0.2 is used for 0.2% offset yield strength (MPa) as shown in the equation.


MBR/t≦0.0125×σ0.2−7.0 (σ0.2: 0.2% offset yield strength)  [Formula 10]

As for bend formability of phosphor bronze, according to the Formula 10, MBR/t becomes 0.3 or less when 0.2% offset yield strength is less than 590 MPa. However, MBR/t becomes greater than 3 when 0.2% offset yield strength is greater than 800 MPa, and it is difficult to conduct bending and phosphor bronze in this range is not practical. Actual measurements of phosphor bronze sometimes do not satisfy the relationship. For example, MBR/t tends to become higher than the relationship on the side of lower 0.2% offset yield strength and on the side of higher 0.2% offset yield strength. Based on what is mentioned above, physical properties of the strengthened alpha brass according to the present invention are described.

As for physical properties of the strengthened alpha brass according to the present invention, a 0.2% offset yield strength is 450 MPa to 750 MPa; and a minimum bend radius (MBR: mm) with which the strengthened alpha brass is bent with a bend axis along the rolling direction without causing cracks, a plate thickness (t: mm), and the 0.2% offset yield strength satisfy the following Formula 11, where a value of right side of the formula 11 is interpreted as 0.3 when a calculated result of the right side of the Formula 11 is equal to or less than 0.3. Then the following Formula 11 is shifted by 0.3 from the following Formula 12 of phosphor bronze.


MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength)  [Formula 11]


MBR/t≦0.0125×σ0.2−7.0 (σ0.2: 0.2% offset yield strength)  [Formula 12]

Therefore, the strengthened alpha brass according to the present invention that satisfies the Formula 11 is understood to have the same level of bend formability as phosphor bronze in balance of 0.2% offset yield strength and the bend formability in consideration of presence of quality deviation of phosphor bronze. When the relationship between a value of MBR/t and a value of 0.2% offset yield strength does not satisfy the Formula 11, bend formability is poor. The reason for interpreting a calculated result of MBR/t as 0.3 when the calculated result is equal to or less than 0.3 is that MBR/t tends to become higher than the relationship on the side of lower 0.2% offset yield strength as with phosphor bronze; when the calculated result is equal to or less than 0.3, bend formability hardly show a problem; and measurements can include certain deviation.

A strengthened alpha brass that satisfies the following Formula 13 in terms of bend formability (MBR/t) and the following Formula 14 in terms of Erichsen value (Er) has a structure mostly derived from a recrystallized structure with an average grain size equal to or less than 2-micron meter. Such a strengthened alpha brass preferably has a recovery structure described later and has an average grain size equal to or less than 2-micron meter at the time of re-crystallization.


MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength)  [Formula 13]


Er≧−0.011×σ0.2+13.7 (σ0.2: 0.2% offset yield strength)  [Formula 14]

Furthermore, it is preferable that the strengthened alpha brass has a recovery structure after the low temperature annealing, 80% or more of a value of [0.2% offset yield strength]/[tensile strength] and an average grain size equal to or less than 1.5-micron meter at the time of re-crystallization, because in such case, the constant 13.7 in the Formula 15 may be changed to 14.2, and the constant 6.7 in the Formula 16 may be changed to 7.1.


Er≧−0.011×σ0.2+13.7 (σ0.2: 0.2% offset yield strength)  [Formula 15]


MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength)  [Formula 16]

The construction of the grain structure of the strengthened alpha brass according to the present invention is described so far. The grain size of recrystallized grains can be measured by an intercept method or a photograph comparison method with an optical microscope with high magnification or a scanning electron microscope after electrolytic etching. Change of the structure due to a low temperature annealing can be identified considerably by observation with SEM-EBSP. In particular, when an image processing is conducted so that a portion whose image quality value is equal to or less than a certain value (strain relieving is a level equal to or less than a certain value) is represented as black, recovered grains are recognized as bright grains. As recovery proceeds, the outline of a grain becomes smooth, and recrystallized grains are recognized as bright grains with annealing twin. The structure of a strengthened alpha brass with good bend formability is a micro structure composed of mixture of grains in which strains are relieved by a low temperature annealing (recovered grains or recrystallized grains) and grains in which strains are not relieved. This micro structure is similar to the fine grained micro-duplex structure. The micro structure is considered to induce heterogeneous sliding and improve bend formability. Improvement of stress relaxation resistance by a low temperature annealing corresponds to increase of a ratio of area of recovered grains or recrystallized grains. Therefore, structural change is essential to assure good stress relaxation resistance. It should be noted that the strengthened alpha brass according to the present invention has fine grains with a level of 1-micron meter to 2-micron meter, and thus has excellent fatigue strength, excellent stress-corrosion cracking resistance, and has a small deflection coefficient.

As mentioned above, in order to obtain a strengthened alpha brass by the manufacturing method according to the present invention, fine and uniform grains are obtained at the time of the final re-crystallization annealing, rolling is conducted to obtain a target strength, and a low temperature annealing is conducted to obtain a aiming grain structure in which strains are relieved partially. It should be noted that a certain level of management is necessary for cold reduction prior to the final re-crystallization annealing and grain sizes prior to the cold rolling for the purpose of obtaining fine and uniform grains.

EXAMPLES

Hereinafter, the present invention is described further in detail with referring to Examples. The chemical compositions of brass ingots used for manufacturing and evaluation in Examples and Comparative Examples are shown in Table 1. Ingots 1 to 6 herein are samples obtained by semicontinuous casting in casting plant where manufacturing is conducted. Each of ingots 7, 8 and 9 are obtained by melting with a furnace in a laboratory and casting an ingot to have a size by 30 mm by 100 mm by 200 mm with a metal mold.

TABLE 1 Component Composition (wt %) Ingot No. Cu Fe Pb Sn S Zn 1 65.2 0.002 0.000 0.003 0.001 Balance 2 69.9 0.004 0.000 0.003 0.000 Balance 3 69.3 0.002 0.000 0.003 0.000 Balance 4 65.4 0.002 0.002 0.001 0.003 Balance 5 70.0 0.003 0.000 0.002 0.000 Balance 6 69.9 0.002 0.002 0.001 0.001 Balance 7 74.2 0.006 0.000 0.002 0.000 Balance 8 68.9 0.001 0.000 0.001 0.000 Balance 9 65.9 0.001 0.000 0.001 0.000 Balance

As is evident from Table 1, Ingots 1 to 9 satisfy the state of the present invention being a composition of wt % to 74.2 wt % copper, incidental impurities, with the balance zinc. Furthermore, in the following Examples, any one of the Ingots shown in Table 1 is used and manufacturing conditions comprising following steps (a) to (e) shown in Table 2 are applied to prepare brass strips.

(a) Preparation of raw material

(b) Cold rolling

(c) Re-crystallization annealing

(d) Final cold rolling

(e) Low temperature annealing

TABLE 2 Annealing (c) Prior to Final Preliminary Annealing (a) Process Reduction Temperature Hardness Reduction Hardness (%) in (° C.) of Low Ingot Temperature (Hv) after (%) in Cold Temperature (Hv) after Final Cold Temperature Samples No. (° C.) Annealing Rolling (b) (° C.) Annealing Rolling (d) Annealing (e) Examples 1-1 1 510 136 72 600 152 10 280 1-2 1 510 136 72 600 152 10 340 1-3 1 510 136 72 600 152 10 420 2-1 1 510 136 72 600 152 24 260 2-2 1 510 136 72 600 152 24 280 2-3 1 510 136 72 600 152 24 300 2-4 1 510 136 72 600 152 24 340 3 2 Sculpturing after Hot Rolling 95 430 151 10 320 Comparative 1-4 1 510 136 72 600 152 10 None Examples 1-5 1 510 136 72 600 152 10 240 1-6 1 510 136 72 600 152 10 260 2-5 1 510 136 72 600 152 24 None 2-6 1 510 136 72 600 152 24 240 2-7 1 510 136 72 600 152 24 420

Examples 1 and 2

The Ingot 1 obtained above was hot rolled, sculptured, cold rolled, and preliminary annealed to obtain a raw material with a thickness of 1.8 mm. Preparation of the raw material and a starting plate material 1 were conducted in a production line at the manufacturing site until the annealing (c) prior to the final cold rolling. Process conditions applied to Examples 1 and 2 are shown in Table 2 with Example 3 in comparison with Comparative Examples 1 and 2. In Table 2, the preliminary annealing (a: annealing prior to the re-crystallization annealing prior to the final cold rolling) and the annealing prior to the final process (c: annealing prior to the final cold rolling) are continuous annealings conducted in the production lines at the manufacturing site as mentioned above. The temperatures mentioned above are preset temperatures of furnaces. In this way, common starting plate material 1 was used for the process until the re-crystallization annealing prior to the final cold rolling.

Examples 1

In these Examples, the starting plate materials 1 obtained above were subjected to cold rolling (d) with a reduction of 10% by using a laboratory cold rolling mill to prepare cold rolled brass plates, and further subjected to a low temperature annealing (e) in a salt bath. Annealing time in the salt bath was set to be short time of 2 seconds for the purpose of carrying out the annealing to be similar to a continuous annealing. Temperatures in the salt bath of Examples 1-1, 1-2, and 1-3 were 280-deg.C., 340-deg.C., and 420-deg.C. respectively. Physical properties of strengthened alpha brass obtained were evaluated. As a result, tensile strengths were 532 MPa to 556 MPa, 0.2% offset yield strength were 458 MPa to 504 MPa, Erichsen values (value calculated from 0.2% offset yield strength) were 8.6 mm (8.3 mm) to 8.8 mm (8.2 mm) and stress relaxation rates were 47% to 51%. Thus, the physical properties satisfied the target values. Details are shown in Table 3.

Examples 2

In these Examples, the starting plate materials 1 as with Examples were subjected to cold rolling (d) with a reduction of 24% by using a laboratory cold rolling mill to prepare cold rolled brass plates, and further subjected to a low temperature annealing (e) in a salt bath. Annealing time in the salt bath was set to be short time of 2 seconds for the purpose of carrying out the annealing to be similar to a continuous annealing. Temperatures in the salt bath of Examples 2-1, 2-2, 2-3 and 2-4 were 260-deg.C., 280-deg.C., 300-deg.C., and 340-deg.C. respectively. Physical properties of strengthened alpha brass obtained were evaluated. As a result, tensile strengths were 667 MPa to 680 MPa, 0.2% offset yield strength were 622 MPa to 638 MPa, Erichsen values (value calculated from 0.2% offset yield strength) were 6.8 mm (6.7 mm) to 8.1 mm (6.9 mm) and stress relaxation rates were 41% to 52%. Thus, the physical properties satisfied the target values. MBR/t values (value calculated from 0.2% offset yield strength), which are indicators of bend formability, were 0.5 (1.1) to 0.6 (1.3). Details are shown in Table 3 with Examples 1. Then influences of temperature settings of the low temperature annealing on 0.2% offset yield strength and stress relaxation rates are shown in FIG. 1. The grains obtained in Examples and Comparative Examples after the annealing (b) prior to the final process had a size of about 2-micron meter, and the grains after the final cold rolling (d) had a size of 1-micron meter.

TABLE 3 Erichsen Value (mm) MBR/t 0.2% Offset Calculated Calculated Tensile Yield Actual from 0.2% Stress Actual from 0.2% Strength Strength Elongation Measure- Offset Yield Relaxation Measure- Offset Yield Samples (MPa) (MPa) (%) ments Strength Rate (%) ments Strength Examples 1-1 556 504 21 8.8 8.2 51 1-2 549 492 22 8.6 8.3 47 1-3 532 458 27 8.7 8.7 48 2-1 672 632 4 7.2 6.7 52 0.6 1.2 2-2 680 638 3 7.0 6.7 51 0.6 1.3 2-3 673 633 3 6.8 6.7 49 0.6 1.2 2-4 667 622 6 8.1 6.9 41 0.5 1.1 3 557 499 22 8.8 8.2 49 Comparative 1-4 555 496 21 8.7 8.2 53 Examples 1-5 547 495 23 9.1 8.3 54 1-6 559 499 20 8.5 8.2 53 2-5 653 608 7 7.3 7.0 59 0.6 0.9 2-6 670 631 4 7.4 6.8 55 0.6 1.2 2-7 613 559 13 8.2 7.6 42 0.1 0.3 When MBR/t calculated from 0.2% offset yield strength is less than 0.3, MBR/t is defined as 0.3.

Examples 3

In this Example, the Ingot 2 was used. The Ingot 2 was hot rolled and sculptured (a) to obtain a raw material with a thickness of 11.5 mm. Preliminary tests were conducted in which reductions and annealing temperatures were changed to obtain softening curves. Annealing time in the salt bath was 10 seconds. Thus, obtained softening curves are shown in FIG. 2. According to FIG. 2, annealed materials after re-crystallization stably had Vickers hardness (Hv) of about 150, except for materials with a reduction of 70%. According to observation of grains with an optical microscope, materials with a reduction of 70% had a deformation structure up to 430-deg.C., and the material had a grain structure in which grains with a size up to 10-micron meter and grains with a size less than 3-micron meter are mixed at condition 450-deg.C. On the other hand, a material which was cold rolled with another reduction and annealed at 430-deg.C., the material had a grain size of about 2-micron meter.

Based on the results of the preliminary tests, a plate material was subjected to cold rolling (b) with reduction of 95% by using a laboratory cold rolling mill, and further subjected to a re-crystallization annealing (c) in a salt bath at 430-deg.C. to obtain a starting plate material. After that, this material was subjected to cold rolling (d) with a reduction of 10% down to thickness of 0.52 mm to prepare a cold rolled brass plate, and further subjected to a low temperature annealing (e) in a salt bath at 320-deg.C. for 2 seconds. As for thus obtained strengthened alpha brass, the tensile strength was 557 MPa, the 0.2% offset yield strength was 499 MPa, The Erichsen value (value calculated from 0.2% offset yield strength) was 8.8 mm (8.2 mm) and the stress relaxation rate was 49%. Thus excellent physical properties were obtained with carrying out re-crystallization annealing only once. The manufacturing conditions are shown in Table 2, and the results are shown in Table 3 with those of Examples 1 and 2.

Examples 4 to 8

In these Examples, the Ingots 2 to 6 were used in the Examples respectively as shown in Table 4. The entire process from casting to the final cold rolling was conducted by using a production line at the manufacturing site. First, a material plate with at thickness of 11.5 mm after hot rolling and sculpturing was subjected to cold rolling with a reduction of 84% to have a thickness of 1.8 mm. The plate material was subjected to preliminary annealing shown in Table 4 (a: annealing of a rough-rolled strip) to prepare a raw material. After that, the raw material was subjected again to cold rolling (b) and to a final re-crystallization annealing (c) to obtain a starting plate material. Among Examples, cold rolling and re-crystallization annealing were conducted prior to the final re-crystallization annealing in Example 8 (described in the upper portion of Table 4). The materials were subjected to the final cold rolling (d) to prepare cold rolled brass plates, and then subjected to a low temperature annealing (e) to provide products. As for conditions of the low temperature annealing, the annealing was conducted as a batch process one hour at 200-deg.C. in Example 4 while continuous annealing was conducted at a furnace temperature of 420-deg.C. in other Examples. These setting of continuous annealing conditions were intended to obtain the maximum value of 0.2% offset yield strength. As a result of evaluating strengthened alpha brass, tensile strengths were 534 MPa to 776 MPa, 0.2% offset yield strength were 470 MPa to 727 MPa, Erichsen values (value calculated from 0.2% offset yield strength) were 6.2 mm (6.1 mm) to mm (8.5 mm) and stress relaxation rates were 40% to 51%. Thus, the physical properties satisfied the target values. MBR/t values (value calculated from 0.2% offset yield strength), which are indicators of bend formability, were 0.0 (0.3) to 1.9 (2.4). Details are shown in Table 5. In this table, when MBR/t calculated from 0.2% offset yield strength is less than 0.3, MBR/t is defined as 0.3.

TABLE 4 Final Re-crystallization Annealing Preliminary Annealing (a) (c) Reduction Temperature (° C.) Grain Reduction Annealing Grain (%) in of Low Ingot Temperature Hardness Size (%) in Cold Temperature Hardness Size Final Cold Temperature Samples No. (° C.) (Hv) (μm) Rolling (b) (° C.) (Hv) (μm) Rolling (d) Annealing (e:*) Examples 4 2 640 117 4 78 420 148 1 33 200-B 5 3 530 144 2 56 550 147 1 36 420-C 6 4 510 137 2 78 600 156 1 7 420-C 7 5 530 140 2 78 550 168 1 14 420-C 8 6 530 140 2 63 550 145 2 17 420-C 75 500 152 1 9 the same conditions as Example 5 40 420 141 2 30 280-B 10 7 640 121 5 78 270 149 2 25 205-B 11 8 640 118 5 78 270 146 2 25 205-B 12 9 640 119 5 78 270 140 2 25 205-B Note*: B at the end of the temperature of low temperature annealing means an material temperature of brass in a batch annealing, and C means an furnace temperature in a continuous annealing.

TABLE 5 Erichsen value (mm) MBR/t 0.2% Offset Calculated Calculated Tensile Yield Actual from 0.2% Stress Actual from 0.2% Strength Strength Elongation Measure- Offset Yield Relaxation Measure- Offset Yield Samples (MPa) (MPa) (%) ments Strength Rate (%) ments Strength Examples 4 776 727 0.5 6.4 5.7 41 1.9 2.4 5 699 688 1.8 6.2 6.1 40 1.3 1.9 6 534 470 24 9.6 8.5 51 0.0 0.3 7 598 537 17 8.5 7.8 49 0.0 0.3 8 603 554 12.5 7.9 7.6 38 0.1 0.3 9 651 601 6.9 7.2 7.1 38 0.6 0.8 When MBR/t calculated from 0.2% offset yield strength is less than 0.3, MBR/t is defined as 0.3.

Example 9

In this Example, the sample piece after the preliminary annealing (a: annealing of a rough-rolled strip) in Example 5 was used as a raw material. The raw material was subjected to cold rolling (b) with a reduction of 40% by using a laboratory cold rolling mill. This strip was subjected to a final re-crystallization annealing (c) in a salt bath at 420-deg.C. for 10 seconds to obtain a starting plate material. After that, the material was subjected to the final cold rolling (d) with a reduction of 30% to prepare a cold rolled brass plate, and then subjected to a low temperature annealing (e) at 280-deg.C. for 10 seconds. As for thus obtained strengthened alpha brass, tensile strength was 651 MPa, 0.2% offset yield strength was 601 MPa, elongation was 6.9%, Erichsen value (value calculated from 0.2% offset yield strength) was 7.2 mm (7.1 mm) and MBR/t (value calculated from 0.2% offset yield strength) was 0.6 (0.8). Thus, excellent physical properties were obtained.

Examples 10 to 12

In these Examples, the Ingots 7 to 9 were used in the Examples respectively as shown in Table 4. In a laboratory, each of the Ingots was subjected to hot rolling causing the grain size to be 0.15 mm, subsequently subjected to cold rolling with a reduction of 86%, and then subjected to re-crystallization annealing (a) with setting conditions so that the grain size to be 5-micron meter. Such a raw material was further subjected to cold rolling (b) with a reduction of 78%. Thus obtained plate material was subjected to a final re-crystallization annealing (c) for 2 hours with a material temperature of 270-deg.C. to obtain a starting plate material. The material was subjected to the final cold rolling (d) with a reduction of 25% to prepare cold rolled brass plates, and then subjected to final re-crystallization annealing (e) at a material temperature of 205-deg.C. The low temperature annealing at this time were conducted by using a muffle furnace with measuring material temperatures. Physical properties of thus obtained strengthened alpha brass with at thickness of 0.3 mm were evaluated. Tensile strengths were 671 MPa to 681 MPa, 0.2% offset yield strength were 629 MPa to 640 MPa, Erichsen values (value calculated from 0.2% offset yield strength) were 6.7 mm (6.7 mm) to 7.0 mm mm) and stress relaxation rates were 40% to 41%. Thus the physical properties satisfied the target values. MBR/t values (value calculated from 0.2% offset yield strength), which are indicators of bend formability, were 0.9 (1.2) to 0.9 (1.3). Details are shown in Table 6.

TABLE 6 Erichsen Value (mm) MBR/t 0.2% Offset Calculated Calculated Tensile Yield Actual from 0.2% Stress Actual from 0.2% Grain Ingot Strength Strength Elongation Measure- Offset Yield Relaxation Measure- Offset Yield Size Samples No. (MPa) (MPa) (%) ments Strength Rate (%) ments Strength (μm) Examples 10 7 671 629 2.0 7.0 6.8 40 0.9 1.2 2 11 8 681 640 1.0 6.7 6.7 41 0.9 1.3 2 12 9 672 633 1.0 7.0 6.7 41 0.9 1.2 2 Comparative 7 666 609 2.0 6.0 7.0 49 2.4 0.9 15 Example 3

Comparative Examples 1 and 2

These Comparative Examples were conducted as with Examples 1 and 2 except that conditions of the final low temperature annealing were changed. The conditions are shown in Table 2.

Comparative Examples 1

the starting plate material 1 as with Example 1 was subjected to cold rolling with a reduction of 10% by using a laboratory cold rolling mill, and further subjected to a low temperature annealing in a salt bath. In Comparative Example 1-4, the low temperature annealing was not conducted. In Comparative Examples 1-5 and 1-6, annealing times in a salt bath were set to short times of 2 seconds as with Examples, and annealing temperatures were set to 240-deg.C. and 260-deg.C. respectively. Physical properties of strengthened alpha brass obtained were evaluated. As a result, tensile strengths were 547 MPa to 559 MPa, 0.2% offset yield strength were 495 MPa to 499 MPa, Erichsen values (value calculated from 0.2% offset yield strength) were 8.5 mm (8.2 mm) to 9.1 mm (8.3 mm) and stress relaxation rates were 53% to 54%. Thus, the stress relaxation rates do not satisfy the target values. Details are shown in Table 3 with Examples 1.

Comparative Examples 2

the starting plate material 1 as with Examples 1 and 2 was subjected to cold rolling with a reduction of 24% by using a laboratory cold rolling mill, and further subjected to a low temperature annealing in a salt bath. In Comparative Example 2-5, the low temperature annealing was not conducted. In Comparative Examples 2-6 and 2-7, annealing times in a salt bath were set to short times of 2 seconds as with Examples, and annealing temperatures were set to 240-deg.C. and 420-deg.C. respectively. Physical properties of strengthened alpha brass obtained were evaluated. As a result, tensile strengths were 613 MPa to 670 MPa, 0.2% offset yield strength were 559 MPa to 631 MPa, Erichsen values (value calculated from 0.2% offset yield strength) were 7.3 mm (7.0 mm) to 8.2 mm (7.6 mm) and stress relaxation rates were 42% to 59%. Thus, the 0.2% offset yield strength or the stress relaxation rates do not satisfy the target values. MBR/t values (value calculated from 0.2% offset yield strength), which are indicators of bend formability, were 0.1 (0.29) to 0.6 (1.19). Details are shown in Table 3 with Examples 1.

Comparative Example 3

In this Comparative Example, the Ingot 7 was used and a sample having a 0.2% offset yield strength as with Example 11 was prepared by carrying out processes similar to conventional processes in a laboratory. That is, the sample was subjected to hot rolling, cold rolling, annealing so that grain size to be 35-micron meter, and then cold rolling with a reduction of 53%. Then re-crystallization annealing was conducted with a salt bath at 650-deg.C. for 20 seconds so that pseudo-continuous annealing in conventional processes was conducted. As a result, grain size after the final re-crystallization annealing was 15-micron meter. Then the sample was subjected to a final cold rolling with a reduction of 65%. Physical properties of strengthened alpha brass obtained were evaluated. As a result, tensile strength was 666 MPa, 0.2% offset yield strength was 609 MPa, Erichsen value (value calculated from 0.2% offset yield strength) was 6.0 mm (7.0 mm) and stress relaxation rate was 49%. Thus, the Erichsen value does not satisfy the target value. MBR/t value (value calculated from 0.2% offset yield strength), which is an indicator of bend formability, was poor of 2.4 (0.9). Details are shown in Table 6 with Examples 10 to 12. As is evident from Table 6, when the reduction in the final rolling is increased to increase 0.2% offset yield strength, formability is considerably deteriorated.

Reference Examples

As Reference Examples, physical properties of commercially available H and EH temper grade of C2680 material (Cu/Zn: 65%/35%), and H temper grade of C2600 material (Cu/Zn: 70%/30%) were evaluated. The brass of Reference Examples was subjected to re-crystallization annealing and then final cold rolling with a reduction of 25%, 17% and 35% respectively. The brass of Reference Examples was not subjected to a low temperature annealing. As for evaluation results, tensile strengths were 486 MPa to 567 MPa, 0.2% offset yield strength were 437 MPa to 524 MPa, stress relaxation rates were 36% to 52%, and Erichsen values (Er) were 6.9 mm to 8.3 mm. Details are shown in Table 7. Reference Example 1 does not satisfy mechanical strength and an Erichsen value (Er) of the following Formula 18 according to the present invention. Reference Example 2 had finer grains than Reference Example 1 and thus had a relatively higher Erichsen value. However, Reference Example 2 does not satisfy the following Formula 18 and had a rather large stress relaxation rate. Reference Example 3 satisfies target mechanical strength and a stress relaxation rate. However, Reference Example 3 does not satisfy the following Formula 17 in terms of MBR/t and the following Formula 18 in terms of an Erichsen value (Er).

TABLE 7 Erichsen value (mm) MBR/t 0.2% Offset Calculated Calculated Tensile Yield Actual from 0.2% Stress Actual from 0.2% Grain Temper Strength Strength Elongation Measure- Offset Yield Relaxation Measure- Offset Yield Size Samples Alloys grade (MPa) (MPa) (%) ments Strength Rate (%) ments Strength (μm) Reference 1 C2680 H 486 437 22 7.6 8.9 40 0.0 0.3 11 Examples 2 C2600 H 535 485 21 8.3 8.4 52 0.0 0.3 5 3 C2680 EH 567 524 10 6.9 7.9 36 0.5 0.3 15 When MBR/t calculated from 0.2% offset yield strength is less than 0.3, MBR/t is defined as 0.3.


MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength)  [Formula 17]


Er≧−0.011×σ0.2+13.7 (σ0.2: 0.2% offset yield strength)  [Formula 18]

INDUSTRIAL APPLICABILITY

The strengthened alpha brass according to the present invention has a general alpha brass composition in view of composition. However, by carrying out proper rolling processes and heat treatments in the manufacturing method according to the present invention, obtained alpha brass exhibits excellent balance between strength and formability, which has never achieved in conventional alpha brass and the balance is at the same level or better than phosphor bronze. Such a strengthened alpha brass is suitable for forming electronic components such as connectors or electromechanical components and can be provided as an inexpensive material.

Furthermore, the method for manufacturing a strengthened alpha brass according to the present invention can be conducted with conventionally used rolling production lines without changing the lines. Therefore, use of the method does not require additional investment for equipments, and strengthened alpha brass of excellent quality can be manufactured efficiently in an industrial scale.

Claims

1. A method for manufacturing a strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc, characterized in that

a brass plate with a grain size from 1-micron meter to 2-micron meter is used as a starting plate material, the brass plate is cold rolled in 5% to 40% reduction to prepare a cold rolled brass plate, then the cold rolled brass plate is adjusted a 0.2% offset yield strength to be equal to or higher than 90% of its maximum value by subjecting a low temperature annealing.

2. The method for manufacturing a strengthened alpha brass according to claim 1, wherein the low temperature annealing is conducted at a temperature equal to or higher than an annealing temperature at which the 0.2% offset yield strength exhibits the maximum value when estimated from relationship within 0.2% offset yield strength and annealing temperatures.

3. The method for manufacturing a strengthened alpha brass according to claim 1, wherein the brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is obtained by using a hot rolled brass plate or a brass plate with a grain size of 7-micron meter to 200-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 80% to 95%, and then subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170.

4. The method for manufacturing a strengthened alpha brass according to claim 1, wherein the brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is obtained by using a hot rolled brass plate or a brass plate with a grain size of 7-micron meter to 200-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 80% to 95%, then subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170, subjecting the material to a cold rolling process with a reduction of 40% to 95%, and further subjecting the material to a re-crystallization annealing to adjust a Vickers hardness (Hv) to be in the range of 130 to 170.

5. The method for manufacturing a strengthened alpha brass according to claim 1, wherein the brass plate with a grain size of 1-micron meter to 2-micron meter used as a starting plate material is obtained by using a brass plate with grain size of 3-micron meter to 6-micron meter as a raw material, subjecting the material to a cold rolling process with a reduction of 70% to 95%, and then subjecting the material to a re-crystallization annealing to adjust Vickers hardness (Hv) to be in the range of 130 to 170.

6. The method for manufacturing a strengthened alpha brass according to claim 3, wherein the re-crystallization annealing is conducted in 370-deg.C. to 650-deg.C. for a continuous annealing, or in 255-deg.C. to 290-deg.C. for a batch annealing.

7. A strengthened alpha brass obtained by the manufacturing method according to claim 1, the strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc, characterized in that

the strengthened alpha brass has a tensile strength from 530 MPa to 790 MPa, a 0.2% offset yield strength from 450 MPa to 750 MPa and a stress relaxation rate equal to or less than 52% after 100 hours at 120-deg.C.; and a minimum bend radius (MBR: mm) with which the strengthened alpha brass is 90 degree bent with a bend axis along the rolling direction without causing cracks, a plate thickness (t: mm) and a 0.2% offset yield strength (MPa) satisfy the following Formula 1, where a value of right side of the Formula 1 is interpreted as 0.3 when calculation result is equal to or less than 0.3. MBR/t≦0.0125×σ0.2−7.0 (σ0.2: 0.2% offset yield strength)  [Formula 1]

8. A strengthened alpha brass obtained by the manufacturing method according to claim 1, the strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc, characterized in that

the strengthened alpha brass has a tensile strength of from 530 MPa to 790 MPa, a 0.2% offset yield strength of from 450 MPa to 750 MPa, a stress relaxation rate equal to or less than 52% at 120-deg.C. for 100 hours; and a minimum bend radius (MBR: mm) with which the strengthened alpha brass is 90 degree bent with a bend axis along the rolling direction without causing cracks, a plate thickness (t: mm) and a 0.2% offset yield strength (MPa) satisfy the following Formula 1, where a value of right side of the Formula 1 is interpreted as 0.3 when calculation result is equal to or less than 0.3; and an Erichsen value (Er: mm) and the 0.2% offset yield strength (MPa) satisfy the following Formula 3. MBR/t≦0.0125×σ0.2−6.7 (σ0.2: 0.2% offset yield strength)  [Formula 2] Er≧−0.011×ν0.2+13.7 (σ0.2: 0.2% offset yield strength)  [Formula 3]
Patent History
Publication number: 20090120544
Type: Application
Filed: Feb 3, 2006
Publication Date: May 14, 2009
Applicant: MITSUI MINING & SMELTING CO., LTD. (Tokyo)
Inventor: Hiroshi Yamaguchi (Tokyo)
Application Number: 11/815,607
Classifications
Current U.S. Class: With Ageing, Solution Treating (i.e., For Hardening), Precipitation Hardening Or Strengthening (148/685); Zinc Containing (148/413)
International Classification: C22F 1/08 (20060101);