JP2016194122A - LOW Pb BRASS BAR MATERIAL AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the machinability of a brass bar material with a reduced Pb content without deteriorating its recyclability.SOLUTION: There is provided a low Pb brass bar material, being the bar material of a copper alloy having a chemical composition comprising, by mass, 54.0 to 66.0% Cu and 0.05 to 0.50% Pb, containing one or more kinds selected from Sn of 0.40% or lower, Fe of 0.60% or lower and P of 0.15% or lower in such a manner that the total content of the Sn, Fe and P reaches 0.02% or higher, and the balance Zn with inevitable impurities, and having a metallic structure where Pb-concentrated particles are dispersed into a matrix made of an α phase and a β phase, and in the structural observation of the cross-section vertical to the longitudinal direction, the number density of the Pb-concentrated particles present at the inside of the α phase is 150 pieces/mmor higher to the area of the α phase.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a low Pb brass bar having good machinability and excellent recyclability, and a method for producing the same.

  Free-cutting brass containing a large amount of Pb is excellent in hot workability and machinability and has been widely used for water-related parts and gas valves for a long time. Examples of the standard include a forging brass bar (JIS C3771) and a free-cutting brass bar (JIS C3604). These free-cutting brass contains Pb in a large amount of about 1 to 4% by mass. Since Pb is harmful to the human body, it is desired to use a material having as little Pb content as possible, particularly in applications such as water-borne parts.

  In order to improve the machinability of brass having a reduced Pb content, a method of adding 2 to 4% of Si is known (Patent Document 1). Moreover, the technique which improves the machinability of a brass by adding Bi instead of Pb is known (for example, patent documents 2-6). Pb-less brass with good free-cutting properties using these technologies has already been put into practical use.

JP 2000-119774 A JP 54-135618 A JP-A-5-255778 JP 2003-277855 A Japanese Patent Laid-Open No. 2003-247035 International Publication No. 2008/081947

  Free-cutting brass containing Bi has the disadvantage of poor recyclability. Even in free-cutting brass, Cu-Zn-Pb-based alloys containing Pb are still widely used, and a large amount of scraps mainly composed of chips are generated. Bi-containing brass scrap cannot be mixed with Cu-Zn-Pb alloy scrap, which occupies most of the free-cutting brass scrap, and needs to be separated. It takes time and effort to separate and manage fine chips such as chips, which increases the cost of cutting parts using Bi-containing free-cutting brass. In addition, high Si free-cutting brass imparted with free-cutting properties by adding a large amount of Si is difficult to process together with Cu-Zn-Pb alloy scrap, and recyclability is similar to Bi-containing brass. Is bad.

  The present invention is a brass rod material with a reduced Pb content, and has the advantages of “excellent hot workability” and “excellent” which are the advantages of conventional free-cutting brass containing a large amount of Pb without impairing recyclability. It is intended to realize “Machinability”.

  A conventional general Cu—Zn—Pb-based free-cutting brass material containing a large amount of Pb exhibits a metal structure in which a large number of Pb-concentrated particles are dispersed in a matrix composed of an α phase and a β phase. Since the melting point of the Pb-concentrated particles is low, chips generated at the time of cutting are easily embrittled in the Pb-concentrated particles due to the processing heat of cutting, and are in the form of finely divided chips. That is, good machinability (free-cutting property) is obtained by the embrittlement action of Pb-enriched particles present in large amounts in the chips. If the Pb content in the brass is reduced, the number of Pb-concentrated particles is reduced, so that the machinability is generally deteriorated accordingly.

  As a result of detailed studies, the inventors have found that, even when the Pb content in the brass is reduced, if the number of Pb-concentrated particles existing in the α phase is sufficiently secured, good machinability is achieved. I found that I can maintain it. That is, by reducing the number of Pb-concentrated particles present in the β phase or at the interface between the α-phase and the β-phase, and by making as many Pb-concentrated particles as possible inside the α-phase, excellent chips can be obtained even with a small Pb content. A dividing effect can be obtained. In the case of low Pb brass, it was found that a brass rod with a structure in which a large number of Pb-concentrated particles are dispersed inside the α phase can be realized by devising the conditions for hot extrusion. The present invention has been completed based on such findings.

In order to achieve the above object, according to the present invention, Cu: 54.0 to 66.0%, Pb: 0.05 to 0.50%, and Si: 0.02 to 0.5% as necessary. Containing 20%, Sn: 0.40% or less, Fe: 0.60% or less, P: 0.15% or less, Sn, Fe, P total content is 0.02% or more A copper alloy rod having a chemical composition consisting of Zn and inevitable impurities, the balance being dispersed in a matrix (metal substrate) composed of an α phase and a β phase. In the observation of the structure in a cross section perpendicular to the longitudinal direction, the low Pb having a metal structure in which the number density of Pb-concentrated particles existing in the α phase is 150 particles / mm ² or more with respect to the area of the α phase. A brass bar is provided. Pb-enriched particles are particles having the largest mass ratio of Pb among the elements contained in the particles, and in many cases are composed of a Pb phase.

  In the case of the copper alloy having the above composition range, the α phase and β phase in the metal structure and the identification of the Pb-enriched particles can be performed by analysis using EPMA (electron beam microanalyzer). The number density of Pb-enriched particles existing inside the α phase can be measured as follows.

[Method for measuring number density of Pb-concentrated particles existing in α phase]
The field of view randomly provided in the cross section perpendicular to the longitudinal direction of the bar is subjected to surface analysis using EPMA (electron beam microanalyzer), and the matrix (metal substrate) is separated into an α phase region and a β phase region to obtain an α phase. In addition to obtaining the area of the region and counting the number of Pb-enriched particles in which the entire image of the particles is observed in the α-phase region, a plurality of operations that do not overlap so that the total measurement area becomes 0.10 mm ² or more. The number of Pb-enriched particles present in the α-phase is obtained by dividing the total count of the Pb-enriched particles in the entire observation field by the total area of the α-phase region (number / mm ² ). Density.
In this case, the total area of the α phase region includes the area of the Pb-enriched particle portion existing therein. Since Pb-enriched particles existing at the interface between α phase and β phase (hereinafter referred to as “α / β interface”) do not correspond to “Pb-enriched particles in which the entire image of the particles is observed in the α-phase region”. Not counted. The Pb-enriched particles counted by the above EPMA analysis correspond to Pb-enriched particles having a particle diameter (major axis) on the observation surface of 0.5 μm or more.

Said low Pb brass rod can be manufactured through the process of hot-extruding to the cast material which has the said chemical composition. The hot extrusion process uses, as a hot extrusion die, a die having an extrusion direction length L from the foremost portion in the extrusion direction to the narrowest flange portion of 2 mm or more, and the material temperature at the start of extrusion is 700 to 850 ° C., It can carry out on the conditions which make processing speed (epsilon) defined by the following (1) formula 70-140mim- ¹ .
ε = [(A ₀ −A ₁ ) / A ₀ ] × (V ₀ / L) (1)
Here, A ₀ is a cross-sectional area perpendicular to the hot extrusion direction of the material before hot extrusion (mm ² ), and A ₁ is a cross-sectional area perpendicular to the hot extrusion direction of the material after hot extrusion (mm ² ). , V ₀ is the extrusion speed (mm / min) of the material before hot extrusion, and L is the length (mm) in the extrusion direction from the foremost part of the hot extrusion die to the narrowest part.
More preferably, the cast material is heated and maintained at 700 to 850 ° C. so that the average rate of temperature increase from 300 ° C. to 700 ° C. is 20 ° C./min or less, and then subjected to hot extrusion.

  According to the present invention, it is possible to provide a copper alloy bar having a low Pb content, hot workability and free machinability equivalent to conventional Pb-containing free-cutting brass, and excellent recyclability. it can.

The area analysis image of Cu and Pb by EPMA about the cross section perpendicular to the longitudinal direction of the brass bar. An image obtained by performing image processing on surface analysis data of Zn by EPMA for a cross section perpendicular to the longitudinal direction of a brass bar, and displaying a matrix (metal substrate) divided into an α phase region and a β phase region. The figure which showed typically an example of the cross-sectional structure of the tool at the time of a hot extrusion process, and a raw material (processed material). The figure which showed typically an example of the cross-sectional shape of a hot extrusion die. Drawing substitute photograph showing sample appearance after upset test. The figure which showed the chip shape typically.

[Chemical composition]
In the present invention, a Cu—Zn-based copper alloy (brass) is targeted. In the following, “%” related to chemical composition means “% by mass” unless otherwise specified.
Cu and Zn are basic components of brass. If the Cu content is less than 54.0%, the α phase is reduced and mechanical properties such as elongation are lowered. On the other hand, if the Cu content exceeds 66.0%, the amount of β-phase generated in the high temperature region decreases and hot workability deteriorates. Here, a copper alloy having a Cu content of 54.0 to 66.0% is targeted. When importance is attached to dezincing resistance, the Cu content is preferably in the range of 60.0 to 66.0%. The balance excluding Cu and component elements described later is Zn and inevitable impurities.

  Pb is an element that improves free machinability. Conventional free-cutting brass contains Pb in a large amount of 1 to 4%. Since Pb is harmful to the human body, in the present invention, a chemical composition in which the Pb content is reduced to a range of 0.05 to 0.50% is applied. The amount of Pb elution when used for parts such as water-washing appliances increases as the Pb content increases, and the degree increases particularly when it exceeds 0.50%. More preferably, the Pb content is 0.30% or less. Pb precipitates in the copper alloy to form Pb concentrated particles. In particular, the Pb-enriched particles dispersed inside the α phase greatly contribute to the fragmentation of chips in the cutting process. As a result of various studies, when a Pb content of 0.05% or more is ensured, the effect of improving the machinability by the Pb-enriched particles can be exhibited by applying the hot extrusion conditions described later. More preferably, the Pb content is 0.10% or more.

  Sn, Fe, and P contribute to improving the strength of the brass material. Furthermore, Sn and P contribute to the dezincing resistance improvement of the brass material. In consideration of various uses used as a cutting part, Sn: Fe, P or less, Fe: 0.60% or less, P: 0.15% or less, Sn, Fe, P total content It is desirable that the content be 0.02% or more. It is more preferable to contain one or more of these in the range of Sn: 0.02 to 0.40%, Fe: 0.02 to 0.60%, and P: 0.02 to 0.15%. Since these elements are contained in brass scrap, the scrap can be effectively used.

  Since Si has an effect of improving the stress corrosion cracking resistance, it can be contained as required. In order to fully exhibit the action, it is effective to secure a Si content of 0.01% or more. On the other hand, the zinc equivalent of Si is as high as 10, and a large amount of Si content causes an increase in β phase. When Si is contained, the content is preferably in the range of 0.20% or less.

[Metal structure]
In the above brass composition range that is the subject of the present invention, the matrix (metal substrate) has a multiphase structure of α phase and β phase at room temperature. The α phase is richer in Cu and relatively soft than the β phase. The β phase is richer in Zn and relatively hard than the α phase. Since the α phase and the β phase have different concentrations of the main components Cu and Zn, the surface analysis data of EPMA can be clearly distinguished by image processing. In the copper alloy bar used in the present invention, the area ratio of α phase: β phase is generally in the range of 1: 1 to 1:19 in the cross section perpendicular to the longitudinal direction.
Since the brass bar according to the present invention contains 0.05% or more of Pb, the Pb forms a Pb concentrated phase and precipitates in the matrix.

Since the α phase has a lower strength at room temperature than the β phase, if the brass containing these phases is plastically processed, deformation in the α phase occurs preferentially. In the cutting process, shear deformation occurs in the chips, and deformation mainly occurs in the α phase. On the other hand, since the Pb-enriched particles have a low melting point, they are easily softened or melted by the processing heat at the time of cutting, and exhibit a function of causing chip embrittlement. The more the Pb-enriched particles are dispersed in the α phase that bears a large deformation in the chip, the more easily the chip is finely divided and the machinability is improved. As a result of detailed examination, when the “number density of Pb-concentrated particles existing in the α phase” by the above-described measuring method using EPMA is 150 particles / mm ² or more, conventional free-cutting brass is used. It was found that excellent machinability at a level comparable to practical use was obtained. It is more effective that the number density of the Pb-enriched particles is 200 / mm ² or more. Even with brass whose Pb content has been reduced as described above, it is possible to disperse many Pb-enriched particles in the α-phase through the hot extrusion process described later. Although the machinability tends to improve as the number of Pb-enriched particles in the α phase increases, it does not need to be present excessively. The number density of the Pb-enriched particles may be, for example, 900 / mm ² or less, and may be 800 / mm ² or less.

  FIG. 1 illustrates an area analysis image by EPMA of a cross section perpendicular to the longitudinal direction of a brass bar material (produced by the steps of hot extrusion, cold drawing, and heat treatment) according to the present invention. This is a black and white version of a color mapping image created with the characteristic X-ray detection intensities of Cu and Pb. A spot-like portion that appears white corresponds to Pb-enriched particles.

  In FIG. 2, the surface analysis data of the characteristic X-ray of Zn measured by EPMA is image-processed with respect to the same brass rod cross section as in FIG. 1, and the matrix (metal substrate) is divided into the α phase region and the β phase region. An example of the displayed image is shown below. The white portion is the α phase region and the black portion is the β phase region. The area of the α phase region can be obtained from such an image. In addition, by superimposing the existence position of the Pb-enriched particles on this α-phase region, the number of Pb-enriched particles in which the entire image of the particles is observed in the α region can be counted.

〔Production method〕
The brass rod material excellent in machinability according to the present invention can be manufactured through a process of subjecting a cast material to hot extrusion. For example, a process of sequentially performing semi-continuous casting or continuous casting, hot extrusion, cold drawing, and heat treatment can be employed. If necessary, after hot extrusion, a hot working process such as hot forging can be further added. After hot working or heat treatment, pickling is appropriately performed in order to remove the oxide scale. The finally obtained brass bar of the present invention may have a circular cross section or an irregular cross section. Their cross-sectional area is, for example, 15 to 15000 mm ² . For processes other than hot extrusion, there is no need to pay particular attention, and conventionally known methods can be applied.

[Hot extrusion process]
Even if the Pb content is reduced by performing the hot working first applied to the cast material by hot extruding and devising the hot extruding conditions, there are many in the α phase. A tissue state in which Pb-enriched particles are dispersed can be obtained.

  A large amount of Pb contained in the Cu—Zn-based copper alloy exists as a Pb-concentrated phase at the α / β interface in the state after casting. These Pb-concentrated phases are divided by hot extrusion processing, and it is generally considered that a large amount of Pb-concentrated particles are distributed on the α / β interface even in the hot-extruded material. The α phase tends to grow by cooling after the hot extrusion and subsequent heat treatment, but if the Pb-concentrated particles are dispersed on the α / β interface, the α / β interface moves at the locations where these particles exist. (Α phase growth) is hindered and it is difficult to incorporate these particles into the α phase. As a result, most of the Pb-enriched particles are finally left on the α / β interface. In this case, since there are few Pb-concentrated particles inside the α phase that greatly contribute to the cutting of chips during cutting, a significant improvement in machinability cannot be expected.

  In the present invention, during hot extrusion, the Pb-concentrated phase existing on the α / β interface of the cast structure is aggregated and coarsened as much as possible, and present as coarse Pb-concentrated particles that hardly inhibit the movement of the α / β interface. By doing so, the aim is to improve the machinability with the idea of incorporating most of the Pb-enriched particles into the α phase that increases in area during the α phase growth process (α / β interface movement process).

  In order to agglomerate and coarsen the Pb-concentrated phase, it is advantageous to set the hot extrusion temperature higher. As a result of various studies, it is desirable that the material temperature at the start of extrusion be 700 to 850 ° C. From the viewpoint of suppressing oxidation at a high temperature, it is effective to set the material temperature at the start of extrusion to 700 to 800 ° C.

  It has also been found that suppressing the generation of dynamic recrystallized grains as much as possible during hot extrusion is extremely effective for agglomeration and coarsening of the Pb concentrated phase. Less generation of dynamic recrystallized grains means less increase in grain boundary area. The smaller the increase in the crystal grain interface area, the more advantageous is the rapid aggregation of the Pb-concentrated phase existing at the α / β interface, and coarse Pb-concentrated particles are formed. In order to make it difficult for dynamic recrystallization to occur, it is important to control the hot extrusion conditions so that the processing speed (strain rate) during hot extrusion does not become excessive.

  In FIG. 3, an example of the cross-sectional structure of the tool at the time of a hot extrusion process and a raw material (processed material) is shown typically. The material 1 such as billet or bloom is pushed in one direction toward the die 4 by the stem 3 in a state in which deformation in the peripheral direction is constrained by the container 2, and the diameter of the material 1 is reduced by the die 4.

  In FIG. 4, an example of the cross-sectional shape of a hot extrusion die is typically shown. A material (such as a billet) is introduced in the direction of the arrow in the figure. The direction of movement of the material relative to the die (the direction parallel to the arrow in the figure) is called the “extrusion direction”. The position in the extrusion direction at which the cross-sectional area of the cross section perpendicular to the extrusion direction (hereinafter sometimes simply referred to as “cross-sectional area”) begins to decrease is referred to as the “frontmost part in the extrusion direction” and is denoted by reference numeral 5 in the figure. Further, the narrowest narrow portion of the die is indicated by reference numeral 6. There are various shapes of extrusion dies depending on the application, but here, it is preferable to use a die having a length L in the extrusion direction from the foremost portion 5 in the extrusion direction to the narrowest flange portion 6 of 2 mm or more. It is more preferable to use a die in which L is in the range of 2 to 50 mm. In addition, the die shape of FIG. 4 is an example, and if it is a die that can be hot-extruded that satisfies the formula (1) described later, a die having an appropriate shape should be used according to the equipment specifications and the target cross-sectional shape. Can do.

According to the study by the inventors, in order to suppress dynamic recrystallization during hot extrusion, hot extrusion is performed under conditions where the processing speed ε defined by the following equation (1) is 70 to 140 mim ^−1. It has been found to be extremely effective to do.
ε = [(A ₀ −A ₁ ) / A ₀ ] × (V ₀ / L) (1)
Here, A ₀ is a cross-sectional area perpendicular to the hot extrusion direction of the material before hot extrusion (mm ² ), and A ₁ is a cross-sectional area perpendicular to the hot extrusion direction of the material after hot extrusion (mm ² ). , V ₀ is the extrusion speed (mm / min) of the material before hot extrusion. L is the length in the extrusion direction from the frontmost part in the extrusion direction to the narrowest flange.
When the processing speed ε exceeds 140 min ⁻¹ , dynamic recrystallization is likely to occur, which makes it difficult for the Pb-enriched particles to agglomerate and become coarse, and the α phase is sufficiently grown inside the α phase. It becomes difficult to incorporate an amount of Pb-enriched particles. The processing speed ε is more preferably 130 min ⁻¹ or less. On the other hand, when the processing speed ε is excessively reduced, productivity is lowered. In addition, if a long time is required for processing, the material temperature during processing is remarkably lowered, which is not preferable. Therefore the processing speed ε is desirably in the range of more than 70 min ^-1, and more preferably to 80min ^-1 or more.

  It is more effective to collect as much Pb as possible on the α / β interface by utilizing the heating of the cast material before the hot extrusion. For this purpose, it is effective to sufficiently generate a phase transformation accompanied by movement of the α / β interface in the temperature raising process and the heat holding process of the cast material. Fine Pb particles present in the α phase or the β phase can be collected at the α / β interface by the interface movement.

  In the temperature raising process, it is desirable to raise the temperature to a temperature range of 700 ° C. or higher so that the average temperature rising rate from 300 ° C. to 700 ° C. is 20 ° C./min or lower. It is more effective to set the average heating rate to 15 ° C./min or less. The α phase grows by slowly raising the temperature in this way, and the accompanying movement of the α / β interface can be used for Pb collection. Since an excessively slow heating rate causes a decrease in productivity, the heating rate may be set in a range of 3 ° C./min or more.

  The holding temperature after raising the temperature to 700 ° C is preferably 700 to 850 ° C, and more preferably 700 to 800 ° C. Further, the material holding time in the above temperature range (the time during which the surface temperature of the material is held in the above temperature range) is preferably 10 min or more, and more preferably 30 min or more. By this heating and holding, the growth of the β phase can be advanced, and further recovery of Pb is possible by utilizing the accompanying movement of the α / β interface. Moreover, the merit that the hot workability at the time of hot extrusion improves by increasing the production amount of β phase can be enjoyed. Since an excessively long holding time causes a decrease in productivity, the holding time may be set in a range of 300 min or less.

[Casting / Hot extrusion]
The copper alloys shown in Table 1 were melted in an induction furnace, and a cast material having a circular cross section with a diameter of 80 mm was obtained by semi-continuous casting. The cast material (billet) is charged into a furnace, heated and held at a predetermined temperature, then taken out from the furnace, hot-extruded by an apparatus having the configuration shown in FIG. 3, and a rod-shaped heat having a circular cross section with a diameter of 24 mm. An intermediate extruded material was obtained.

  Here, the hot extrusion die | dye whose extrusion direction length L from the extrusion direction front part to the narrowest collar part is 5 mm was used. Table 2 shows heating conditions and extrusion conditions. The “extrusion temperature” in Table 2 is the material temperature at the start of extrusion. Since the hot extrusion was started immediately after taking out from the furnace, this extrusion temperature may be regarded as substantially the same as the heating and holding temperature. Regarding the “temperature increase rate” in Table 2, in the case where the extrusion temperature is 700 ° C. or higher, the heating holding temperature is also 700 ° C. or higher, so the average temperature increase rate from 300 ° C. to 700 ° C. is shown. In the example of less than 700 ° C., the heating and holding temperature is also less than 700 ° C., so the average temperature increase rate from 300 ° C. to the heating and holding temperature is shown. The temperature rise during heating was controlled so that the rate of temperature rise up to a predetermined heating and holding temperature was substantially constant. “Processing speed ε” in Table 2 is a value calculated by the above equation (1). After hot extrusion, air was blown with a cooling fan to cool at 10 ° C./sec until the material temperature reached 300 ° C., and then naturally cooled in the atmosphere. In this way, a hot extruded material was obtained.

[Upset test]
A cylindrical sample (diameter: 24 mm × height: 20 mm) having a height of 20 mm was cut out from the obtained hot-extruded material (cooled to room temperature), and an upset test was performed to evaluate hot forgeability. The upset test is a test in which a sample heated to a predetermined temperature is crushed to a predetermined upset rate by a press. The hot forgeability can be evaluated from the degree of occurrence of cracks in the sample after being crushed. FIG. 5 illustrates a reference photograph of the appearance of the sample after the upset test. Of the four types of stacked samples, the upper two cases were not cracked, and the lower two were cracked. The upset rate is determined by the following equation (2).
Upset rate (%) = (h ₀ −h ₁ ) / h ₀ × 100 (2)
However, h ₀ is the sample height before test (mm), h ₁ is the sample height after test (mm).
Here, h ₀ = 20 mm, and the upset rate was 70%. Each hot extrudate was tested at three levels of temperature: 610 ° C, 650 ° C, and 690 ° C. Observe the surface of the sample after the test to determine whether cracks occurred or not. ○ (hot forgeability: good) for materials that did not crack at all three temperatures, and x (hot) for other materials. Forgeability; poor). The results are shown in Table 2.

(Cold drawing / heat treatment)
After the above hot-extruded material (diameter 24 mm) is cold-drawn to a diameter of 23 mm, except for some examples (No. 13), heat treatment is maintained at a temperature range of 250 to 600 ° C. for 2 min to 5 h. The test material of the brass bar was obtained. For No. 13, the heat treatment was omitted, and the cold-drawn sample was used as a test material. The following was investigated about the obtained test material.

[Measurement of number density of Pb-enriched particles existing in α phase]
With respect to the cross section perpendicular to the longitudinal direction of the test material, according to the above-mentioned “Method for measuring the number density of Pb-enriched particles existing in the α phase”, using an EPMA analyzer (JXA-8200, manufactured by JEOL Ltd.) Surface analysis is performed using characteristic X-rays of Zn and Pb under the conditions of an acceleration voltage of 15.0 kV and an irradiation current of 3.0 × 10 ⁻⁸ A to obtain the number density of Pb-enriched particles existing inside the α phase. It was. Five non-overlapping fields randomly selected by SEM at a magnification of 600 were measured. The total measurement area is about 0.15 mm ² . The area of the α phase region was obtained by image processing analysis of surface analysis data based on characteristic X-rays of Zn measured by EPMA. When the size of the Pb-enriched particles was confirmed from a composition image at a magnification of 7500, the counted Pb-enriched particles were 0.5 μm or more in diameter (major axis) even if they were small. The results are shown in Table 2.

[Cutting test]
The outer periphery of the test material (round bar with a diameter of 23 mm) was turned and the machinability was evaluated by the severability of the generated chips. The cutting conditions were the following five levels.
[1] Rotation speed 1030 rpm, depth of cut 1.0 mm, feed rate 0.13 mm / rev.
[2] Rotation speed 1030 rpm, depth of cut 1.0 mm, feed rate 0.23 mm / rev.
[3] Number of rotations 1030 rpm, depth of cut 1.0 mm, feed rate 0.34 mm / rev.
[4] Number of rotations 1800 rpm, depth of cut 1.0 mm, feed rate 0.34 mm / rev.
[5] Rotation speed: 1030 rpm, depth of cut: 0.5 mm, feed rate: 0.13 mm / rev.

  The generated chips were classified into four types: (A) needle-shaped piece, (B) 1 volume or less, (C) 2 volumes or so, (D) 3 volumes or more. FIG. 6 schematically shows the chip shape of each classification. As for the fragmentability of chips, A is the best in the order of A> B> C> D, and D is the worst. When the D chips are generated, the chips may be entangled with a tool such as a bite, which may cause mechanical troubles or damage the product surface during continuous processing. Moreover, since chips are bulky, there is also a problem that scrap disposal is poor. On the other hand, in the case of A, problems such as D can be avoided, but it may take time to collect scattered chips. Here, the material in which chips classified as D in any of the above five levels of cutting conditions are generated is x (cutting ability; poor), and only the chips classified as A to C under any of the five levels. The material in which was generated was evaluated as ◯ (cutting property: good). The results are shown in Table 2.

In all of the examples of the present invention, the number density of the Pb-concentrated particles existing in the α phase is 150 particles / mm ² or more, and good machinability is achieved even though the Pb content is reduced. Have There was no problem in hot forgeability.

  On the other hand, Comparative Examples Nos. 21 to 24 have a sufficiently high number of Pb-enriched particles inside the α phase because the temperature of hot extrusion is low or the processing speed ε of hot extrusion is high. Inability to cut and poor machinability. No. 25 is a conventional free-cutting brass containing a large amount of Pb and has good machinability, but cannot meet the need to reduce the elution of Pb. No. 26 is a free-cutting brass in which Bi is added instead of Pb, and there is a problem in terms of recyclability as described above. Further, while the machinability was improved, the hot forgeability was poor. No. 27 is a general brass material that does not give special consideration to improvement of machinability, and good machinability was not obtained by the above machinability evaluation method.

DESCRIPTION OF SYMBOLS 1 Material 2 Container 3 Stem 4 Dies 5 Dies extrusion direction foremost part 6 Dies narrowest collar part

Claims (4)

Cu: 54.0 to 66.0%, Pb: 0.05 to 0.50%, Sn: 0.40% or less, Fe: 0.60% or less, P: 0.15 Is a copper alloy bar material having a chemical composition comprising Sn and Fe and P in a total composition of 0.02% or more and the balance of Zn and inevitable impurities. Thus, Pb-concentrated particles are dispersed in a matrix composed of an α-phase and a β-phase, and the number density of Pb-concentrated particles existing in the α-phase in the structure observation of the cross section perpendicular to the longitudinal direction is A low Pb brass rod having a metal structure of 150 pieces / mm ² or more with respect to the area of.   Furthermore, the low Pb brass bar material of Claim 1 which has a chemical composition containing Si: 0.01-0.20%. When producing a copper alloy bar through a process of hot extrusion to the cast material,
As a hot extrusion die, a die having an extrusion direction length L from the foremost part of the extrusion direction to the narrowest narrow part of 2 mm or more is used, and the material temperature at the start of extrusion is defined as 700 to 850 ° C. by the following formula (1) The manufacturing method of the low Pb brass bar material of Claim 1 or 2 which performs hot extrusion on the conditions which make processing speed (epsilon) 70-140 mim < ^-1 >.
ε = [(A ₀ −A ₁ ) / A ₀ ] × (V ₀ / L) (1)
Here, A ₀ is a cross-sectional area perpendicular to the hot extrusion direction of the material before hot extrusion (mm ² ), and A ₁ is a cross-sectional area perpendicular to the hot extrusion direction of the material after hot extrusion (mm ² ). , V ₀ is the extrusion speed (mm / min) of the material before hot extrusion, and L is the length (mm) in the extrusion direction from the foremost part of the hot extrusion die to the narrowest part.   The low temperature according to claim 3, wherein the cast material is heated and held at 700 to 850 ° C so that an average rate of temperature increase from 300 ° C to 700 ° C is 20 ° C / min or less, and then subjected to hot extrusion. A method for producing a Pb brass bar.