P/00/0 11 Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL TO BE COMPLETED BY APPLICANT Name of Applicant: Taiwan Shan Yin Int'l Co., Ltd. Address for Service: A.P.T. Patent and Trade Mark Attorneys PO Box 222, Mitcham, SA 5062 Invention Title: A screw with low drilling resistance The following statement is a full description of this invention, including the best method of performing it known to me/us:- A SCREW WITH LOW DRILLING RESISTANCE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screw design, particularly to a screw with low drilling resistance for speeding up the drilling performance and enhancing the drilling force. 2. Description of the Related Art Referring to Figs. 1 to 3, a "Thread-Forming Screw Fastener" 1 disclosed in the US Publication No. 2005/0186048 comprises a head portion 8, a shank portion 10 extended from the head 8, a plurality of threads 12 spiraling on the shank 10, and a drilling portion 6 disposed on the shank 10, opposite to the head 8. Wherein, a plurality of wave troughs 22 are defined on the threads 12. Especially, a first apex angle a included by an upper flank 15 and a lower flank 16 of the threads 12 is formed between 300~350, and a second apex angle a' included by wave crest 20 formed on the outer thread edge 14 of the deepest region of the wave troughs 22 is contoured between 300-580. Further, a trough angle A is included by the wave trough 22. Accordingly, the screw 1 utilizes the concurrent drilling and cutting efficiency provided by the wave troughs 22 to reinforce the severing performance of the threads 12, thence achieving a speedy drilling and fastening effect. However, in time of the threads 12 practically drilling into an object 2, the dimension of the trough angle A of the wave troughs 22 as well as the dimension of the first apex angle a of the threads 12 readily influences fastening effect of the screw 1 though the wave troughs 22 offer a preferable cutting function. The analysis is as follows: 1. The dimension of the trough angle A influences the fastening effect: 2 The dimension of the trough angle A decides the cutting function of the threads 12. That is, if the trough angle A is formed by an improperly small size, each wave trough 22 could not be guaranteed to provide the auxiliary cutting with efficiency during the threads 12 drilling into the object 2. Because the elasticity is characterized by the fibers contained in the object 2, the fibers are not able to respectively get into the small-sized wave troughs 22. Therefore, the cutting efficiency of the wave troughs 22 is limited or even the drilling of the threads 12 is in vain; thereby the screw 1 is incapable of being well-drilled into the object 2. On the other hand, if the trough angle A is formed by an improperly large size, the dimension of the threads 12 is likely to be relatively diminished. Herein, such drilling and cutting efficiency of the larger trough angle A is not as satisfied as that of the smaller trough angle A. Moreover, the smaller threads 12 having a weaker and thinner formation that adversely results in an insufficient bearing force readily collapse. 2. The first apex angle a of the threads 12 influences the fastening effect: The first apex angle a decides the fastening effect of the screw 1. That is, if the first apex angle a is formed by an improperly small size, the threads 12 are unable to bear a large drilling torque while they confront a more rigid object, which readily breaks the threads 12. On the other hand, if the first apex angle a is formed by an improperly large size, the threads 12 merely bears a larger drilling torque but provides the cutting effect inferior to that of one with a smaller first apex angle. Thus, the utilization is adversely limited, which indeed needs amendment. SUMMARY OF THE INVENTION 3 It is therefore the purpose of this invention to provide a screw with low drilling efficiency so as to enhance the cutting efficiency as well as reinforce the drilling competence, thereby achieving a speedy fastening effect. The screw with low drilling resistance in accordance with the present invention comprises a head portion, a shank portion extended from the head, a plurality of first threads spiraling on the shank portion, and a drilling portion disposed on the shank, opposite to the head. Wherein, each first thread includes a first upper flank and a first lower flank connected to the first upper flank; a first included angle is converged by the first upper flank and the first lower flank. Characterized in that, a plurality of second threads are spiraled on the shank and are respectively disposed on the first upper flank and the first lower flank. Wherein, each second thread includes a second upper flank and a second lower flank connected to the second upper flank; a second included angle is converged by the second upper flank and the second lower flank. Concurrently, a plurality of opening slots are correspondingly defined on the second upper flank and the second lower flank. Further, the first included angle is smaller than the second included angle. Thereby, a favorably severing effect and accommodating manner could be provided. In addition to the disposition of the first threads between the second threads contributing to the reinforcement on the strength of the second threads whose second included angle is larger, the present invention achieves a low drilling resistance to speed up the fastening operation. Preferably, a diameter of the first threads is smaller than a diameter of the second threads. 4 Preferably, a diameter of the first threads is equal to a diameter of the second threads. The advantages of the present invention over the known prior art will become more apparent to those of ordinary skilled in the art upon reading the following descriptions in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a conventional screw; Fig. 2 is a cross-sectional view showing a thread of the conventional screw; Fig. 3 is a cross-sectional view showing the conventional screw; Fig. 4 is a perspective view showing the first preferred embodiment of the present invention; Fig. 5 is a partial enlarging view showing Fig. 4; Fig. 6 is a cross-sectional view showing a thread of the first preferred embodiment; Fig. 7 is a schematic view showing the first preferred embodiment; Fig. 8 is a schematic view showing a second preferred embodiment; Fig. 9 is a schematic view showing the second preferred embodiment; Fig. 10 is perspective view showing a third preferred embodiment; and Fig. 11 is a partial enlarging view showing Fig. 10. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 5 Referring to Figs. 4 to 7, a first preferred embodiment of the present invention is shown. A screw 3 with low drilling resistance substantially comprises a head portion 31, a shank portion 32 extended from the head portion 31, a plurality of first threads 33 spiraling on the shank portion 32, and a drilling portion 34 disposed on the shank portion 32, opposite to the head portion 31. Wherein, each first thread 33 includes a first upper flank 331, and a first lower flank 332 connected to the first upper flank 331; a first included angle a is disposed at a convergence of the first upper flank 331 and the first lower flank 332. Characterized in that, a plurality of second threads 35 spiraling on the shank portion 32 is respectively disposed on the first upper flank 331 and the first lower flank 332. Wherein, each second thread 35 includes a second upper flank 351 and a second lower flank 352 connected to the second upper flank 351; a second included angle a' is defined at a convergence of the second upper flank 351 and the second lower flank 352. Concurrently, a plurality of opening slots A is correspondingly disposed on the second upper flank 351 and the second lower flank 352. Moreover, the first included angle a is smaller than the second included angle a'. In addition, in this embodiment, a diameter d of the first threads 33 is smaller than a diameter D of the second threads 35 (as shown in Figs. 6 and 7), and the opening slot A is formed by at least two convergent linear flanks A'. In operation, a screwing torque is imparted on the head portion 31 for bringing the drilling portion 34 to screw into the object (not shown). Thereby, the second threads 35 provide a severing function, and the opening slots A defined on the second threads 35 offer a drilling effect as well as a debris accommodating room for receiving the severed debris. Further, due to that the first threads 33 are disposed between the second threads 6 35, and the first included angle a is smaller than the second included angle a', the driving strength of the second threads 35 are hence reinforced by the firm support provided by the first threads 33, so that the drilling and cutting efficiency of the screw 3 could be preferably encouraged. Therefore, the screw of the present invention promotes the cutting efficiency for achieving a speedy fastening. Herein, the combination of the first threads 33 and the second threads 35 provides the threads with a promoted strength, thereby enhancing the cutting efficiency. Thus, during the drilling of the screw 3, the dimension of the opening slots A does not readily influence the cutting performance. Moreover, since the diameter d of the first threads 33 is smaller than the diameter D of the second threads 35, a preferable debris accommodating room could be provided for receiving the severed debris and speedily removing the redundant debris therefrom. As a result, an aim of achieving a swift screwing is accomplished. Referring to Figs. 4, 5, and 8, 9, a second preferred embodiment is shown. The screw 3 with low drilling resistance substantially comprising the similar elements and correlations as those in the first preferred embodiment is herein omitted. A head portion 31, a shank portion 32, a plurality of first threads 33, a drilling portion 34, and a plurality of second threads 35 could be likely found in this embodiment. These constituents in this embodiment similarly promote the strength of the screw, enhance the cutting effect of the screw, and encourage the fastening speed of the screw as those in the first embodiment. Differently, in this embodiment, the diameter d of the first threads 33 is equal to the diameter D of the second threads 35. Accordingly, in operation, by means of the cooperation of the first threads 33 including the smaller first included angle a and the 7 second threads 35 including the larger second included angle a', the screw 3 of the present invention is allowed to bear a larger drilling force, thereby concurrently increasing the screwing strength thereof. Moreover, since the opening slots A maintain a certain size of debris accommodating room to receive the redundant debris, the redundant debris would not stack to generate a great drilling resistance. Therefore, the object of speedy fastening is achieved. Further referring to Figs. 10 and 11, a third preferred embodiment is shown. The screw 3 of the present invention comprising the similar elements and correlations as those in the first preferred embodiment is herein omitted. A head portion 31, a shank portion 32, a plurality of first threads 33, a drilling portion 34, and a plurality of second threads 35 could be likely found in this embodiment. These constituents in this embodiment similarly promote the strength of the screw, enhance the cutting effect of the screw, and encourage the fastening speed of the screw as those in the first embodiment. Differently, the opening slots A defined on the second threads 35 are correspondingly formed by multiple convergent linear flanks A'. In operation, the cooperation of the first threads 33 and the second threads 33 offers a preferable drilling strength, and the opening slots A maintain a room in a certain size for the debris accommodation so as to decrease the drilling resistance and promote the cutting effect. Therefore, a prompt drilling performance could be arrived. To sum up, the present invention in particularly utilizes a plurality of opening slots correspondingly defined on the second threads not only to reserve a debris accommodating room but to assist in severing object fibers. In addition to the bearing force of the second threads is reinforced by the first included angle being smaller than the 8 second included angle, the cutting performance of the screw in accordance with the present invention is preferably promoted, and a speedy fastening effect is accomplished. While we have shown and described the embodiment in accordance with the present invention, it should be clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention. 9