The present invention relates to a strength evaluation method of a die casting product, and a die casting product used in an electric power steering device of a vehicle (for example, a column housing made of an aluminum die casting).
In the related art, in order to prevent burglary of an automobile, a steering lock device that stops an engine when an ignition switch key is pulled out and also prevents rotation of a steering wheel has been widely used.
A steering column that serves as a main body and an attachment part of the steering lock device used in this manner is required to be solid and not to be easily broken. For example, JIS D5812 or the like regulates that no functional abnormality occur even when a torque of 200 Nm is applied to a steering shaft in a state where the steering shaft is locked.
Here, in order to reduce the weight of the steering column, in many cases, the steering column is made of a die casting of a light metal such as aluminum or magnesium, for example. Accordingly, an internal defect occurs in the steering column, and thus, if excessive torque is applied, there is a fear that the steering column may be broken. Thus, it is important how to evaluate the strength.
In particular, a strength decrease due to cold flakes is known as an internal defect of a die casting. Thus, for the strength evaluation, it is necessary to inspect the cold flakes.
As an inspection method of such cold flakes, PTLs 1 to 3 disclose the following techniques.
First, PTL 1 discloses a technique in which a mixture of a component (Pb) that is easily detected by cold flakes detection apparatus that employs radiation, magnetism, ultrasonic waves or the like and a sleeve lubricant is coated on the inner surface of a sleeve and the detection sensitivity of cold flakes increases as Pb enters the cold flakes and a cavity.
Further, PTL 2 discloses a technique that obtains dispersion information on cold flakes in a non-destructive state using a measurement method in which ultrasonic flaw detection and X-ray CT scanning are combined.
Specifically, ultrasonic waves are irradiated onto an aluminum die casting product, and a blowhole and cold flakes of the aluminum die casting product are detected based on information on sound waves from the aluminum die casting product to obtain first internal defect three-dimensional distribution data. Further, the aluminum die casting product is measured by the X-ray CT scanning, and the blow hole of the aluminum die casting product is detected from plural cross-sectional images of the aluminum die casting product to obtain second internal defect three-dimensional distribution data. Then, the first internal defect three-dimensional distribution data and the second internal defect three-dimensional distribution data are compared with each other to obtain three-dimensional distribution data on the cold flakes of the aluminum die casting product.
Further, PTL 3 discloses a simple cold flakes inspection method in which a test piece cut out of a runner portion is abraded and corroded and an area ratio of cold flakes is then calculated based on observation of the cold flakes.
Further, as another strength evaluation method of an aluminum die casting product, PTL 4 discloses a technique in which ultrasonic flaw detection is performed for an aluminum die casting product having a complex member therein and a distance from a front surface to the complex member, an impregnation state of aluminum melt with respect to the complex member, a breakage state and the like are measured to determine whether the aluminum die casting product is good or bad.
PTL 1: JP 3-226668 A
PTL 2: JP 2005-91288 A
PTL 3: JP 2007-111728 A
PTL 4: JP 2004-144489 A
However, in the techniques disclosed PTLs 1 to 3, since a special inspection apparatus is necessary, operation time becomes long. Further, it is difficult to perform the strength evaluation in a casting site. Furthermore, since the accuracy of measurement and detection is not sufficient, it is difficult to perform the strength evaluation with high accuracy.
Specifically, the technique disclosed in PTL 1 has an advantage in that a non-destructive inspection is available, but since it is necessary to perform an inspection using a special apparatus that employs radiation, magnetism, ultrasonic waves or the like after casting, operation time becomes long. Further, it is difficult to perform the strength evaluation in a casting site. Furthermore, since the accuracy of measurement and detection is not sufficient, it is difficult to perform the strength evaluation with high accuracy.
Further, similar to the technique disclosed in PTL 1, the technique disclosed in PTL 2 has an advantage in that a non-destructive inspection is available, but since it is necessary to perform an inspection using a special apparatus for ultrasonic flaw detection or CT scanning after casting, operation time becomes long. Further, it is difficult to perform the strength evaluation in a casting site. Furthermore, since the accuracy of measurement and detection is not sufficient, it is difficult to perform the strength evaluation with high accuracy.
Furthermore, the technique disclosed in PTL 3 has an advantage in that a special inspection apparatus is not necessary, but since the abrasion and corrosion are necessary, operation time becomes long. Further, since the test piece is cut out of the runner portion instead of a product, it is difficult to perform the strength evaluation with high accuracy.
In addition, the strength evaluation method of the aluminum die casting product disclosed in PTL 4 can perform only the measurement of the impregnation state or the breakage with respect to the aluminum die casting product having the complex member therein, but cannot evaluate the strength of an aluminum die casting product having no complex member.
On the other hand, the strength evaluation method of the aluminum die casting product disclosed in PTL 2 can perform the evaluation of an aluminum die casting product having no complex member. However, it is substantially difficult to perform the inspection for all parts of a large-sized aluminum die casting product or a complicated aluminum die casting product, for example. In an actual aluminum die casting product, it is difficult to avoid an internal defect such as a blow hole, and breakage may occur from the internal defect as a starting point. Further, in many cases, the aluminum die casting product has a complicated shape, and thus, it is difficult to detect the internal defect by ultrasonic flaw detection, for example. Furthermore, it is not obvious which portion the strength is to be evaluated in.
In order to solve the above-mentioned problems, an object of the invention is to provide a strength evaluation method of a die casting product capable of shortening operation time, performing a strength evaluation (strength guarantee) at a casting site and performing the strength evaluation with high accuracy, and such a die casting product.
Further, another object of the invention is to provide a strength evaluation method of a die casting product capable of appropriately evaluating the strength of an actual die casting product and obtaining a die casting product having a predetermined strength, and such a die casting product.
In order to achieve the above-mentioned objects, according to an embodiment of the invention, there is provided a strength evaluation method of a die casting product, comprising: a casting step of casting a die casting product; a breakage step of performing a breakage test of the die casting product after casting; a strength reduction ratio calculation step of measuring an area ratio of cold flakes present in a broken surface of the die casting product in the breakage step, and obtaining a strength reduction ratio from the relationship between a torsional breakage torque value in the breakage step and the area ratio; and a strength evaluation area ratio calculation step of calculating a threshold value of −3σ (σ is a standard deviation) using the strength reduction ratio as an average value, and calculating a strength evaluation area ratio of the die casting product from the threshold value and a predetermined strength range.
Further, in the strength evaluation method of the die casting product described above, it is preferable that the casting step be a casting step based on JIS D5812, the die casting product be an aluminum column housing, and the breakage test be a torsion test for the die casting product.
Further, in the strength evaluation method of the die casting product described above, it is preferable that the die casting product be used in a steering lock provided with an ignition switch for vehicle steering.
Further, according to another embodiment of the invention, there is provided a column housing made of an aluminum die casting in which the strength is evaluated by the strength evaluation method of the die casting product described above, in which the column housing made of an aluminum die casting is formed with a hole for the steering lock, and an area ratio of cold flakes in the area of a broken surface ranged up to a stress of ½ of a maximum value of stress acting on a breakage starting point, calculated by stress analysis from the breakage starting point when the hole for the steering lock is broken by the torsion test, is less than 10%.
Further, according to still another embodiment of the invention, there is provided a strength evaluation method of a die casting product, comprising: performing, with respect to a high stress portion obtained by performing stress analysis for the die casting product in advance, ultrasonic flaw detection for an internal defect in a predetermined range of the high stress portion; and evaluating that the die casting product has a predetermined strength when a defect ratio obtained by dividing a total area of the internal defect in the predetermined range by a total defect detection area is less than or equal to a predetermined value.
Here, it is preferable that the predetermined range of the high stress portion be a stress range of 50% or greater of a maximum stress of the high stress portion.
Further, according to still another aspect of the invention, there is provided a die casting product in which the strength is evaluated by the strength evaluation method of the die casting product described above, in which the defect ratio is 0.5% or less.
Further, it is preferable that the die casting product be a column housing used in an electric power steering device of a vehicle, and the high stress portion is a key lock portion of the column housing.
According to the embodiment of the invention, it is possible to provide a strength evaluation method of a die casting product capable of shortening operation time, performing a strength evaluation at a casting site whenever necessary and performing the strength evaluation with high accuracy, and such a die casting product. Further, according to the strength evaluation method of a die casting product according to the embodiment of the invention, since ultrasonic flaw detection is performed for an internal defect in a predetermined range of the high stress portion with respect to a high stress portion obtained by performing stress analysis for the die casting product in advance; and the die casting product is evaluated to have a predetermined strength when a defect ratio obtained by dividing a total area of the internal defect in the predetermined range by a total defect detection area is less than or equal to a predetermined value, it is possible to appropriately evaluate the strength of an actual die casting product.
Furthermore, since the predetermined range of the high stress portion is a stress range of 50% or greater of a maximum stress of the high stress portion, it is possible to more appropriately evaluate the strength of the die casting product.
In addition, according to the die casting product according to the embodiment of the invention, by evaluating the strength by the strength evaluation method of the die casting product according to the embodiment of the invention, and by setting a defect ratio obtained by dividing a total area of the internal defect in the predetermined range of the high stress portion by a total defect detection area to be 0.5% or less, it is possible to obtain a die casting product having a predetermined strength.
Hereinafter, a first embodiment of a strength evaluation method of a die casting product will be described with reference to the drawings.
As shown in
The casting step in the present embodiment refers to a step of performing die casting using ADC12 that is an Al—Si—Cu based aluminum alloy of JIS H5302, for example. It is preferable that a die casting product obtained in this step be a steering column housing 3 of an automobile steering device shown in
The breakage step in the present embodiment refers to a step of performing a breakage test in a casting site for the die casting product obtained in the above-described casting step. It is preferable that the breakage test be a torsion test.
In this torsion test, specifically, as shown in
The strength reduction ratio calculation step refers to a step of measuring an area ratio of cold flakes in a broken surface of the die casting product in the breakage step and calculating a strength reduction ratio from the relationship between a torsional breakage torque value and the area ratio in the breakage step.
The strength evaluation area ratio calculation step refers to a step of calculating a strength evaluation area ratio of the die casting product based on the strength reduction ratio calculated in the strength reduction ratio calculation step. Specifically, in this step, first, a threshold value of −3σ (σ is a standard deviation) is calculated using the strength reduction ratio calculated in the strength reduction ratio calculation step as an average value. Then, the strength evaluation area ratio of the die casting product is calculated from the threshold value and a predetermined strength range.
Hereinafter, examples of the strength evaluation method of the die casting product according to the first embodiment and the die casting product (column housing made of aluminum die casting) in which the strength is evaluated by the strength evaluation method will be described. Table 1 illustrates the relationship between conditions of the casting step in the present examples, the strength of the die casting product and the area ratio of the cold flakes.
Die casting was performed under the conditions shown in Table 1, using ADC 12 that is the Al—Si—Cu based aluminum alloy of JIS H5302, to obtain steering column housings of Examples 1 to 16 and Comparative examples 1 to 6.
Then, each steering column housing 1 of Examples 1 to 16 and Comparative examples 1 to 6 was caused to be torsionally broken using a torsion tester shown in
Then, a portion around a starting point of the broken surface was observed by a stereoscopic microscope to identify the cold flakes, and the area ratio of the cold flakes was then calculated. The calculation result is shown in Table 1. Here, a value of stress that is to act on the starting point of the broken surface was calculated in advance by stress analysis, and then, a stress range up to ½ of the stress value was set as a broken surface observation range. In the present examples, the observation range of the area ratio of the cold flakes was set as a range up to a position of 15 mm from the starting point.
Further, observation examples of a broken surface are shown in
Here, the relationship between the torsional breakage torque value obtained in the strength reduction ratio calculation step and the area ratio of the cold flakes may be plotted as in a graph shown in
Further, as shown in
Thereafter, as shown in
As described above, according to the present embodiment, by observing the broken surface of the die casting product for which the breakage test is performed and by estimating the strength reduction ratio based on the area ratio of the cold flakes in the obtained broken surface, it is possible to provide a strength evaluation method that is simple and reliable. Thus, it is possible to prevent the problem that a special inspection apparatus is necessary and operation time is long in the strength evaluation method based on the cold flakes detection in the related art in which the broken surface is abraded and is observed by the optical microscope or in which the ultrasonic flaw detection is used. Further, in the related art, since the strength evaluation is performed with only the torsional breakage torque value, deformation of the jig, a setting error of the product or variation of a manual torque application speed may occur, which may result in insufficient reliability. On the other hand, according to the present embodiment, it is possible to perform the strength evaluation with high accuracy.
Hereinabove, the first embodiment of the strength evaluation method of the die casting product is described, but the invention is not limited thereto, and various modifications and improvements may be made. For example, as described above, the invention is not be applied to only the simple strength evaluation, but may also be applied to setting of appropriate die casting conditions. Specifically, as in the present examples, by setting the melt temperature to 670° C. or higher, and preferably, to 680° C. or higher, it is possible to set the ratio of the cold flakes to 10% or less, to thereby secure sufficient strength.
Next, a second embodiment of a strength evaluation method of a die casting product will be described with reference to drawings.
In the present embodiment, the aluminum die casting product (column housing) 11 is loaded onto a turntable 12, and then, a probe 13 moves down from the upside while the turntable 12 is rotating to detect the inside of the aluminum die casting product (column housing) 11 in a spiral form. Since the die casting product (column housing) 11 of the present embodiment includes a cylindrical portion, a high stress portion of the cylindrical portion (to be described later) is detected by the ultrasonic flaw detector to detect an internal defect.
In the detection, as shown in
Prior to the detection, a key was inserted into a key lock portion of the column housing that is the die casting product 11 according to the present embodiment, and then, stress analysis was performed under the same load condition as in a torsion test in which a load was applied in a torsional direction. As a result, it can be understood that a portion indicated by “A” in
Table 2 represents the areas of all the internal defects analyzed in this manner. The total area of the internal defects is 4.99 mm2, and the total defect detection area is 1400 mm2. In the present embodiment, a value by dividing the total area of the internal defects by the total defect detection area was defined as a defect ratio, and an internal defect state and the strength of the aluminum die casting product were evaluated by the defect ratio. In this example, the defect ratio is 4.99/1400×100=0.36.
In a similar manner, with respect to seven aluminum die casting products (column housings) 11, an internal defect of a high stress portion (key lock portion) was detected by the ultrasonic flaw detection, the area of the detected internal defect was calculated by the image analysis, and the defect ratio was calculated by dividing the total area of the internal defects by the total defect detection area. Further, a torsion test was performed for the aluminum die casting products (column housings) 11 to detect the number of torsion until the breakage occurs. This torsion test is appended to Technical Standards for locking apparatus of four-wheeled vehicles, Attachment 7, Details Public Notice Attachment 1-7, Road Transport Vehicle Maintenance Standard 11-2 in Road Transport Vehicle Law. The test was performed based on a test procedure of a lock that acts on a steering device provided with a torque limit device regulated in Attachment 2. Here, in the present embodiment, in consideration of a safety factor, a torque value was set to 200 Nm that is two times a standard value of 100 Nm. Then, the cycle was repeated to calculate the number of torsion until the breakage occurs. Table 3 represents the relationship between the defect ratio of an internal defect inside the high stress portion (key lock portion) 15 and the number of torsion when the breakage occurs in the torsion test. Further,
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As described above, in the strength evaluation method of the die casting product according to the present embodiment, the ultrasonic flaw detection is performed, with respect to the high stress portion of the column housing (die casting product) 11 calculated by the stress analysis in advance, for the internal defect in a predetermined range of the high stress portion, and the column housing (die casting product) 11 is evaluated to have a predetermined strength when the defect ratio obtained by dividing the total area of the internal defects in the predetermined range by the total defect detection area is less than or equal to a predetermined value. Thus, it is possible to appropriately evaluate the strength of an actual column housing (die casting product) 11.
Further, by setting the predetermined range of the high stress portion to the stress range of 50% or greater of the maximum stress of the high stress portion, it is possible to more appropriately evaluate the strength of the column housing (die casting product) 11.
Furthermore, according to the die casting product of the invention, by performing the strength evaluation by the above-described strength evaluation method and by setting the defect ratio obtained by dividing the total area of the internal defects in the predetermined range of the high stress portion by the total defect detection area to 0.5% or less, it is possible to obtain the column housing (die casting product) 11 having a predetermined strength.
Hereinabove, the second embodiment of the strength evaluation method of the die casting product is described, but the invention is not limited thereto, and various modifications and improvements may be made.
Number | Date | Country | Kind |
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2012-043827 | Feb 2012 | JP | national |
2012-129858 | Jun 2012 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2012/004686 | 7/24/2012 | WO | 00 | 8/28/2014 |