1. Field of the Invention
The present invention relates to a steel material, a process of fabricating the steel material, and an apparatus of fabricating the steel material, and more particularly to a steel material having different hardness depending upon a portion.
2. Description of the Related Art
There has been proposed, for example, a technique of changing a tensile strength (hardness) depending upon a portion from, for example, a surface layer to a central part, in order to enhance a delayed fracture resistance, in a secondary processing (heat treatment) for a steel material such as a coil-like rolled material (hereinafter referred to as a rolled material) that is a raw material of a rod or a wire.
For example, there has been known a technique in which a ultralow-carbon steel such as pure iron is arranged on a surface layer, the resultant is rolled, and the resultant is subject to decarburization and decrease of carbon, like a clad steel (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 6-57367).
There have also been proposed a technique of utilizing a decarburized layer that is generated on a surface layer through an application of heat at a temperature of Ac1 to Ac3 during rolling (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 62-267420), and a technique of performing a heat treatment again only to the surface layer after the heat treatment (e.g., refer to Jpn. Pat. Appln. KOKAI Publication No. 7-54441).
As a method of enhancing the delayed fracture resistance, there has also been known a technique of performing a chemical surface treatment of plating or nitriding, or a technique of applying a non-metallic coating material excellent in delayed fracture resistance on the surface.
However, the above-mentioned techniques have the problems described below. Specifically, they need to perform a preprocessing, since a clad steel is fabricated at the stage of a rolled material, or since a secondary process (heat treatment) is performed after a decarburized layer is formed through the application of heat at a temperature of Ac1 to Ac3. Further, it is necessary to perform the heat treatment or the chemical surface treatment only to the surface layer after the secondary process (heat treatment). Therefore, these processes are complicated, and a complicated process condition has to be controlled before and after the secondary process (heat treatment).
In view of this, an object of the present invention is to provide a steel material having a different hardness depending upon a portion, a process of fabricating the steel material, and an apparatus of fabricating the steel material with a simple process.
According to one aspect of the present invention, there is provided a process of fabricating a steel material by performing a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, wherein the heat treatment comprises a heating step for rapidly heating a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating, and a cooling step for rapidly cooling the steel material subjected to the heating step after a predetermined time from the heating step, and a heating temperature in the heating step is Ac1 transforming point or more.
According to another aspect of the invention, a time from the heating step to the cooling step is not more than a predetermined time that is determined according to a steel grade, a wire diameter, a heating temperature, and a heating time.
According to another aspect of the invention, a process condition in the heat treatment is determined based upon a heat conductivity characteristic of the steel material after the rapid heating to the surface.
According to another aspect of the invention, the process condition in the heat treatment is a time integration value of the temperature of the steel material, and is determined based upon a tempering progression value that indicates a progression state of tempering of the steel material.
According to another aspect of the invention, the process condition in the heat treatment is a combination including at least two of a frequency, an input electric energy, a heating temperature, a heating time, and a natural cooling time.
According to another aspect of the invention, the process further comprises a step of calculating the heat conductivity characteristic of the steel material or the tempering progression value, wherein the process condition in the heat treatment is determined based upon the calculated heat conductivity characteristic or the tempering progression value.
According to another aspect of the invention, the process condition in the heat treatment is set such that the tempering progression value at a surface layer becomes 1.5 times or more the tempering progression value at a central part.
According to another aspect of the invention, the time from the heating step to the cooling step is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at the central part.
According to another aspect of the invention, the steel material is a steel wire or a steel rod.
According to another aspect of the invention, after a quenching process including a heating process and a cooling process is performed to the steel material, the heating step and the cooling step are respectively performed once as the tempering process.
According to another aspect of the invention, there is provided a steel material that is subject to the heating step and the cooling step, wherein a difference between hardness in the vicinity of a surface layer and hardness at a position toward a central part from a position of 10% from the surface layer in a radial direction is HV50 or more, and a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm2 or more.
According to another aspect of the invention, all cross-sections have a tempered martensite structure, hardness of a surface layer is HV380 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm2 or more, and hardness at a portion near a central part from the surface layer is uniform.
According to another aspect of the invention, all cross-sections have a tempered martensite structure, hardness of a surface layer is HV420 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1600 N/mm2 or more, and hardness at a portion near a central part from the surface layer is uniform.
According to another aspect of the invention, there is provided an apparatus of fabricating a steel material that performs a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, the apparatus comprising: a heating unit configured to rapidly heat a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating; and a cooling unit configured to rapidly cool the steel material subjected to the heating after a predetermined time from the heating, wherein a heating temperature of the steel material by the heating unit is Ac1 transforming point or more.
According to another aspect of the invention, the apparatus further comprises a control unit configured to control the process condition in the heat treatment based upon a calculation result of the heat conductivity characteristic of the steel material or the tempering progression value.
A first embodiment of the present invention will be described with reference to
As illustrated in
The PC steel rod W, which is a subject to be processed, is a solid round bar as illustrated in
The tempering heating coil 15 has a function of performing high-frequency induction heating on the PC steel rod W passing therethrough. The tempering heating coil 15 is set to have a suitable number of turns according to a wire diameter or a conveying speed. In the present embodiment, the number of turns of the tempering heating coil 15 is 6, for example. However, the invention is not limited thereto. As a comparison, a conventional apparatus using a coil having 17 turns is illustrated.
During the tempering with the high-frequency induction heating, the PC steel rod W itself generates heat. The depth of the heat-generating part can be adjusted by the combination of the number of turns and the frequency of the tempering heating coil 15, input electric energy, heating temperature, heating time, and natural cooling time.
The cooling jacket 16 has a function of injecting cooling liquid to the passing PC steel rod W in order to cool the rod.
The distance from the tempering heating coil 15 to the cooling jacket 16 is set to be 500 mm or less, for example. In the ordinary apparatus, this distance is about 1900 mm. In the present embodiment, the distance is set to be short in order to shorten the time from the heating process to the cooling process.
The process condition in the present embodiment is set such that the frequency is 50 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 805° C., the tempering heating time is 0.17 s, and the time taken from the tempering heating to the cooling is 0.63 s. The PC steel rod used here is a small-diameter PC steel rod having a diameter d (nominal designation) of 7.1 mm. The tempering heating temperature is adjusted such that the tensile strength becomes about 1440 N/mm2.
The process condition for the conventional steel material, which is a comparative example, is set such that the frequency is 9.5 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 603° C., the tempering heating time is 0.59 s, and the time taken from the tempering heating to the cooling is 3.48 s. The conventional steel material is also a small-diameter PC steel rod having a diameter d (nominal designation) of 7.1 mm. The tempering heating temperature is adjusted such that the average tensile strength on all cross sections becomes about 1440 N/mm2. The composition of the steel rod in the present embodiment and the composition of the conventional steel rod are the same.
Specifically, the tempering heating temperature in the present embodiment is higher than that for the conventional product, and the time from the tempering heating to the cooling is set to be shorter than the time for the conventional product.
As for the tempering temperature, heating with the tempering temperature not less than Ac1 transforming point (727° C.) is impossible from a conventional common practice, since the heated material is quenched. In the present embodiment, it has been found that the heated material is not quenched by performing both a rapid heating and a rapid cooing to the surface, which is achieved by controlling the rapid cooling immediately after the completion of the heating, with the use of the high-frequency induction heating, even if the temperature is not less than the Ac1 transforming point.
In the heat treatment apparatus 10 according to the present embodiment, the conveying speed by the pinch roller 11, the heating temperature by the tempering heating coil 15, the heating time, and the distance between the tempering heating coil 15 and the cooling jacket 16 are appropriately set and adjusted, whereby the PC steel rod W having a desired hardness distribution can be obtained.
The operation of the heat treatment apparatus 10 thus configured will be described with reference to the flowchart in
The continuous wire rod W0 to which the quenching process is performed is heated while passing through the tempering heating coil 15. The continuous wire rod W0, which has been heated to a predetermined tempering temperature, is conveyed to the cooling jacket 16 that continuously ejects a cooling liquid, so that the continuous wire rod W0 is rapidly cooled by the cooling jacket 16. The continuous wire rod W0 passes through the cooling jacket 16, whereby the total length thereof is cooled, and then, the tempering heating process is completed. The W0 is carried out by the pinch roller 17. After the heat treatment is completed, the continuous wire rod W0 (PC steel rod) is subject to a processing process and checking process, whereby it is made as a product (PC steel rod W).
The principle for determining the process condition in the heat treatment will next be described.
An N parameter value (tempering progression value), which is a new parameter serving as a reference for setting the process condition, will be described later.
As a general tempering parameter, Larson-Miller parameter P=T×(A+logt) [T: temperature (K), A: constant, t: time (h)], which is established for the case of a long-time heating, has been known. As the parameter P increases, the tempering progresses more (hardness is low).
On the other hand, when a rapid-heating process and a rapid-cooling process are both established in the high-frequency induction heating illustrated in
N=∫
0
t0
T(t)dt (1)
In the equation 1, T is a temperature (° C.), t is a time (s), and t0 is a heat treatment time (s).
Specifically, the N parameter value is an area enclosed below each curve in the graph. As the N parameter value increases, the tempering progresses more (hardness is low).
In
As illustrated in
According to
Organizing the principle described above, since the tensile strength of the PC steel rod W is determined by the N parameter value that is the progression state of the tempering from the surface to the center, it is understood that there is a certain range of tensile strength in order to satisfy the standard. When the temperature distribution and the time up to the cooling are controlled within this range in order to increase the difference between the N parameter at the surface layer and the N parameter at the central part as much as possible, a steel rod having a desired property can be obtained.
Specifically, a process of fabricating a steel material having a layer with low hardness as a surface layer, a uniform hardness distribution from a certain depth, and a tensile strength, e.g., a strength level of 1420 N/mm2 or more, can be realized with one tempering in a continuous heat treatment by utilizing a temporal change in the heating temperature pattern at the moment of the high-frequency induction heating and the tempering property of the steel material.
It has generally been known that the delayed fracture resistance is excellent as the tensile strength is low. Specifically, the PC steel rod W having a low-hardness portion on the surface has both an excellent delayed fracture resistance and a predetermined tensile strength. The PC steel rod W described above can be fabricated according to the process described above.
Specifically, during the high-frequency tempering, the depth of the heat-generating portion can be adjusted by selecting a suitable coil, frequency, input electric energy, heating temperature, heating time, and natural cooling time, whereby the pattern calculated by the simulation can be realized. Accordingly, the time up to the cooling is adjusted during the high-frequency heating, resulting in that the tensile strength of the whole PC steel rod satisfies the standard, but only the surface layer can be made to have low hardness. In the external heating of a radiation system, other than the high-frequency heating, such as a furnace heating, the temperature rise and the soaking state are not achieved in a short-period process, but a steel material continues to be slowly heated. Therefore, the difference in strength in the radius direction cannot be produced from the viewpoint of the tempering property of the steel material. On the other hand, in the high-frequency heating, the heating is performed in a short period such as 1 s or less, so that the internal hardness other than the surface layer can be made uniform.
Based on the N parameter, and by utilizing the temperature distribution (difference between the N parameters) that is specific to the high-frequency heat treatment, a continuous high-frequency heat treatment line that can adjust the hardness distribution on the surface layer and can allow the tensile strength to satisfy the standard value for the steel rod with one tempering can be realized.
For example, when the N parameter at the surface layer is set to be 1.5 times or more the N parameter at the central part, a desired satisfactory softening at the surface layer can be achieved.
The N parameter can preferably be used for a steel material of 100 kg/mm2 degree or more. A steel of a common steel having C of 0.1 mass % to 0.5 mass % is preferable in terms of the limitation on the tempering temperature. The preferable range of the diameter by which the N parameter acts as the principle is, for example, 5 to 40 mm.
Specifically, the N parameter utilizes the tempering of the high-strength steel, and utilizes an overshoot due to the rapid heating and the rapid transfer to the uniform heating by the heat conductivity of the steel. Therefore, when the diameter is larger than the above-mentioned range, it is difficult to perform the tempering in order to make the whole steel have uniform hardness within the standard strength within the range (the N parameter at the surface layer is about 1.5 times or more the N parameter at the central part) by which the desired softening of the surface layer can be achieved. Note that the present invention is not limited thereto. The present invention is applicable to a steel rod having a large diameter exceeding the above-mentioned range, from the viewpoint of the high-frequency tempering.
This also indicates the time constraint. Specifically, as illustrated in
The delayed fracture resistance test was conducted in such a manner that each steel rod was immersed in 20% NH4SCN solution whose temperature was kept at 50° C., and with this state, a load of 1420×0.7 N/mm2 was applied thereto.
It can be confirmed from
As for the steel grade that is not fractured even by the delayed fracture test, a sample provided with a notch 20 was formed, and the delayed fracture test was carried out.
The delayed fracture test was conducted in such a manner that each steel rod was immersed in 20% NH4SCN solution whose temperature was kept at 50° C., and with this state, a load of 1420×0.8 N/mm2 was applied thereto.
The effect described below can be obtained according to the PC steel rod, the heat treatment process of the PC steel rod, and the heat treatment apparatus according to the present embodiment.
The present invention can provide a PC steel rod having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a steel rod having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition.
The present invention has found the condition under which a surface layer that is sufficiently soft can be formed and a temperature of 720° C. or more, which cannot be applied because a steel material is generally quenched by this temperature, can be applied, thanks to the high-frequency induction heating. According to the quenching and tempering heat treatment in which a steel material is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.
Since the heat treatment condition based upon the tempering property and the heat conductivity property is found out by using the simulation result as described above, an appropriate condition that can be applied to various steel materials can easily be found out.
The present invention is not limited to the above-mentioned embodiment, and the invention can be embodied by modifying the components without departing from the scope of the invention. For example, the specific process condition can suitably be changed according to the steel grade of the subject steel material, the obtained strength standard or distribution of hardness, and the specification of the apparatus. The set process condition of the heat treatment is not limited to those described above.
The heat treatment apparatus may be configured, like a heat treatment apparatus 101 illustrated in
A second embodiment of the present invention will be described below with reference to
A heat treatment according to the present embodiment is referred to as a surface-layer softening process, a steel material (here, the deformed PC steel rod Wc) subject to the surface-layer softening process is referred to as a surface-layer softened material, a heat treatment as a comparative example that is a subject to be compared is referred to as a comparative heat treatment, and a deformed PC steel rod formed by the comparative heat treatment is referred to as a comparative heat-treated material.
In the present embodiment, the deformed PC steel rod Wc having a spiral groove formed continuously and uniformly on a surface layer as illustrated in
The present embodiment illustrates the case of using the deformed PC steel rod Wc containing the composition illustrated in
The comparative process condition is set such that the frequency is 9.5 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 603° C., tempering heating time is 0.59 s, and the time from the tempering heating to the cooling is 3.48 s. The comparative heat-treated material is also a deformed PC steel rod having a diameter of 7.1 mm, wherein the average tensile strength for all cross-sections is adjusted to be about 1400 N/mm2. The composition of the comparative heat-treated material is the same as the composition of the deformed PC steel rod Wc that is the surface-layer softened material in the present embodiment.
It is understood from
The present embodiment also provides the same effect as that in the first embodiment. Specifically, the present embodiment can provide a deformed PC steel rod having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a deformed PC steel rod having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition. Further, according to the quenching and tempering heat treatment in which a steel material is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.
A third embodiment of the present invention will be described below with reference to
A heat treatment according to the present embodiment is referred to as a surface-layer softening process, a steel material (here, the spring steel wire Ws) subject to the surface-layer softening process is referred to as a surface-layer softened material, a heat treatment as a comparative example is referred to as a comparative heat treatment, and a spring steel wire formed by the comparative heat treatment is referred to as a comparative heat-treated material.
The steel material that is the subject to be processed is the spring steel wire Ws illustrated in
The present embodiment illustrates the case of using the spring steel wire Ws containing the composition illustrated in
The spring steel wire Ws used here has a diameter ds of 12.0 mm, wherein the tensile strength for all cross-sections is adjusted to be about 1900 N/mm2.
The comparative heat treatment condition in the comparative example is set such that the frequency is 9.5 kHz, the quenching heating temperature is 950° C., the tempering heating temperature is 495° C., tempering heating time is 1.7 s, and the time from the tempering heating to the cooling is 11.1 s.
The comparative heat-treated material is also a spring steel wire having a diameter of 12.0 mm, wherein the average tensile strength for all cross-sections is adjusted to be about 1900 N/mm2. The composition of the comparative heat-treated material is the same as the composition of the spring steel wire Ws according to the present embodiment.
It is understood from
In the comparative heat-treated material, the hardness hardly changes even if the distance from the surface layer changes. On the other hand, in the spring steel wire Ws to which the heat treatment according to the present embodiment is performed, it is found that the hardness is changed such that the hardness in the vicinity of the surface layer increases as the distance from the surface layer increases. Specifically, Hv<500 is established within the range of 1 mm from the surface layer.
As illustrated in
As for the result of the rotational bending fatigue test, the case in which the rotational bending fatigue test is executed after a shot peening process is illustrated. The condition of the test is set such that the stress amplitude is 700 MPa, and the rotational speed is 2000 rpm. The test was conducted up to ten million times. In the graph, “>1000” means that the wire is not broken even after ten million times.
Comparing
The distribution of the residual stress of the spring steel wire Ws and the distribution of the residual stress of the comparative heat-treated material are similar to each other. The maximum compression stress appears in the vicinity of 0.1 mm from the surface layer, and then, the tensile stress appears at 0.2 mm or more from the surface layer.
As can be understood from the graph, since the comparative heat-treated material has high hardness, and low toughness in the case of the same stress amplitude, the early breakage is produced from the inclusion at the portion of 0.2 to 1.0 mm from the surface layer where the residual stress is tensile stress.
On the other hand, in the spring steel wire Ws that is the surface-layer softened material according to the present embodiment, the surface layer has low hardness but high toughness. Therefore, the formation of the fatigue crack from the inclusion can be suppressed even at the portion near the surface layer, i.e., at the portion of 0.2 to 1.0 mm from the surface layer. As a result, the number of times of durability is significantly improved.
The present embodiment also provides the same effect as that in the first embodiment. Specifically, the present embodiment can provide a spring steel wire having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a spring steel wire having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition. Further, according to the quenching and tempering heat treatment in which a steel material is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.
In the spring steel wire Ws fabricated according to the present embodiment, the portion within the range of 0.2 to 1.0 mm from the surface layer where the residual stress is tensile stress is softened. Therefore, the present embodiment provides an effect that the early breakage from the inclusion is hardly produced.
In the present embodiment, the composition illustrated in
A fourth embodiment of the present invention will be described below with reference to
A heat treatment according to the present embodiment is referred to as a surface-layer softening process, a bolt Wb formed by the surface-layer softening process is referred to as a surface-layer softened material, a heat treatment as a comparative example is referred to as a comparative heat treatment, and a bolt formed by the comparative heat treatment is referred to as a comparative heat-treated material.
The bolt Wb illustrated in
The present embodiment illustrates the case of using the bolt Wb containing the composition illustrated in
The bolt Wb used here has a diameter db of 7.1 mm, wherein the tensile strength for all cross-sections is adjusted to be about 1600 N/mm2.
The condition of the comparative heat treatment is set such that the frequency is 9.5 kHz, the quenching heating temperature is 1000° C., the tempering heating temperature is 480° C., tempering heating time is 0.61 s, and the time from the tempering heating to the cooling is 3.50 s. The comparative heat-treated material used here is also a bolt having a diameter of 7.1 mm, wherein the average tensile strength for all cross-sections is adjusted to be about 1600 N/mm2. The composition of the bolt Wb serving as the surface-layer softened material in the present embodiment and the composition of the comparative heat-treated material are the same. The heat treatment in the present embodiment and the heat treatment in the comparative example are performed before a screw is rolled to a bar member Wb1 of the bolt.
It is understood from
In the comparative heat-treated material, the hardness hardly changes even if the distance from the surface layer changes in
The delayed fracture resistance of the bolt Wb (surface-layer softened material) processed under the heat treatment condition according to the present embodiment is enhanced more than that of the comparative heat-treated material.
It is understood from
The present embodiment also provides the same effect as that in the first embodiment. Specifically, the present embodiment can provide a bolt having excellent delayed fracture resistance with a simple process by combining the surface heating caused by the high-frequency induction heating and the tempering property of the steel. Specifically, the present invention utilizes the temporal change in the temperature pattern at the moment of heating and the tempering property, whereby a bolt having a different hardness depending upon a portion can be obtained with a simple process at one tempering that satisfies the predetermined process condition. Further, according to the quenching and tempering heat treatment in which steel is rapidly heated in a short period by the high frequency, high strength and high toughness can be obtained compared to an ordinary heat treatment with a furnace heating.
In the present embodiment, the composition illustrated in
The present invention is not limited to the above-mentioned embodiments, and the invention can be embodied by modifying the components without departing from the scope of the invention. For example, the specific process condition can suitably be changed according to the shape, steel grade, or composition of the subject steel material, the obtained strength standard or distribution of hardness, and the specification of the apparatus. The set process condition of the heat treatment is not limited to those described above.
As one example, the PC steel rod, the deformed PC steel rod, the spring steel wire, and the bolt have been illustrated in the first to fourth embodiments, and the similar result can be obtained in these embodiments by the process using the similar principle. The concept of the present invention is not limited to the illustrated steel grade, but can be applied to various other steel materials.
It has been described in the respective embodiments that the surface layer is softened by performing the tempering process to the steel material that has high strength due to the quenching process. In addition to the case described above, a steel material that has high strength due to a wire drawing, severe plastic deformation, or a carburizing process may be used.
Various inventions can be made by appropriately combining the plural components described in the respective embodiments. For example, some components may be deleted from all components described in the embodiments. The components in the different embodiments may appropriately be combined.
The present invention can provide a steel material having a different hardness depending upon a portion, a process of fabricating the steel material, and an apparatus of fabricating the steel material with a simple process.
Number | Date | Country | Kind |
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2008093760 | Mar 2008 | JP | national |
This is a Continuation Application of PCT Application No. PCT/JP2009/056733, filed Mar. 31, 2009, which was published under PCT Article 21(2) in Japanese. This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-093760, filed Mar. 31, 2008, the entire contents of which are incorporated herein by reference.
Number | Date | Country | |
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Parent | PCT/JP2009/056733 | Mar 2009 | US |
Child | 12894363 | US |