A method of manufacturing an ultra-high strength steel workpiece by cold stamping

Abstract

A method of manufacturing an ultra-high strength steel workpiece includes predetermining a steel sheet appearance provided with a predetermined included angle, preparing an ultra-high strength steel sheet, a stamping die, and a forming die, using the stamping die to subject the steel sheet to cold stamping so that the steel sheet is formed into an initial workpiece, using the forming die to subject the initial workpiece to another cold stamping after the springback occurs so that the initial workpiece is formed into a steel workpiece whose shape and workpiece included angle correspond to the steel sheet appearance and the predetermined included angle, and finally taking the steel workpiece out of the forming die. The cold stamping operations are directly executed, so the steel sheet is quickly and efficiently stamped and formed into the ultra-high strength steel workpiece. No heating treatments are needed, thereby conserving energy and reducing carbon emissions.

Description

BACKGROUND OF THE INVENTION1. Field of the Invention

[0001] This invention relates to a method of forming a steel sheet and relates particularly to a method of manufacturing an ultra-high strength steel workpiece by cold stamping.2. Description of the Related Art

[0002] In Recent years, there is a groundswell of opinion for the environmental problems. The automotive firms have been dedicating themselves to the car lightweight technology for improving the fuel consumption. The demand for an increase in the strength of the car body is also required to ensure the safety of cars. To attain the car lightweight property and the safety, ultra-high strength steel (with the strength of 980 MPa or higher) can a material applied to the car body. However, the higher strength of the steel sheet causes a stamping process to be executed with difficulties. The reason is that the higher the steel strength is provided, the lower the ductility, bendability, and stretch flangeability are provided. The springback phenomenon occurs after the steel sheet is processed. Because of the above factors, the steel sheet is not easily processed and formed.

[0003] According to the report published by World Steel Association, the advanced high strength steel (AHSS) enable carmakers to reduce the car weight by more than 40% in comparison with conventional steel, thereby attaining the car lightweight effect. European car firms are also dedicated to the research and development for the steel material and steel forming technology. For the 2023 model year, more than 50% of the styles of the Volvo XC90 use the high strength steel material with the strength of 980 MPa or higher, and even more than 35% of the styles use the steel material whose strength is up to 1.5 GPa (1500 MPa). As to the product with the strength of 1.5 GPa, boron steel is often used as a steel material, and a hot stamping treatment is adopted to overcome the springback property of the material, and the phase transformation caused by hot stamping allows an increase in the strength of a manufactured steel product. However, some specific requirements for the composition of the steel material and high material expenses may incur high manufacturing costs. Furthermore, the process of processing ultra-high strength steel sheet usually encounters some problems, such as low ductility, low bendability, and springback problems. In practical terms, there have been no prior arts disclosing, teaching, or using the process of forming the ultra-high strength steel sheet by cold stamping.

[0004] Regarding the hot stamping treatment, the hot stamping method includes preparing a steel sheet with the strength ranging from 500 MPa to 600 MPa, heating the steel sheet up to 880 degrees Celsius to 950 degrees Celsius, stamping the steel sheet in a die so that the stamped steel sheet is formed into a desired shape, leaving the steel sheet alone in the holding pressure state, taking the steel sheet out, and subjecting the steel sheet to a quenching operation for rapid cooling at the speed of 20 degrees Celsius to 300 degrees Celsius per second (20° C.-300° C. / s). The rapid cooling allows the organization of the steel sheet to be transformed from austenite to martensite, so a steel component obtained by cooling can have the strength of 1500 MPa or higher. However, the heating process of the hot stamping treatment consumes a lot of energy and generates carbon dioxide. The heating temperature of 900 degrees Celsius or higher may emit about 91% of carbon emissions of the whole process. On the whole, the hot stamping treatment is hard to conserve energy and reduce carbon emissions. Thus, the goal of the industry is to find a way capable of processing the high strength steel sheet efficiently and attaining the energy conservation and the carbon reduction.SUMMARY OF THE INVENTION

[0005] An object of this invention is to provide a method of manufacturing an ultra-high strength steel workpiece by cold stamping. The method is aimed at forming an ultra-high strength steel workpiece by cold stamping at room temperature, thereby subjecting a steel sheet to a rapid and efficient processing operation and attaining the energy conservation and the carbon reduction.

[0006] A method of manufacturing an ultra-high strength steel workpiece by cold stamping of this invention is as defined in claim 1. The method includes a preparatory step, a first stamping step, a shape-forming step, and a taking-out step. The preparatory step includes a designing operation executed by designing and predetermining a steel sheet appearance to be formed, and the steel sheet appearance has a bottom portion, a side portion connected to the bottom portion, and a predetermined included angle defined between the bottom portion and the side portion. The preparatory step also includes a preparing operation executed by preparing a steel sheet with the strength of at least 980 MPa, a stamping die, and a forming die. The stamping die includes a stamping punch and a stamping base opposite the stamping punch, thus serving to reduce the degree of springback after the steel sheet is stamped. The forming die includes a forming punch and a forming base opposite the forming punch to eliminate the springback, thus allowing the steel sheet to be finally formed into the predetermined steel sheet appearance. The stamping punch has a stamping end portion and a stamping side portion connected to the stamping end portion. A first included angle is defined between the stamping side portion and the stamping end portion and is less than the predetermined included angle, and a difference between the first included angle and the predetermined included angle is at least thirty degrees. The forming punch has a forming end portion and a forming side portion connected to the forming end portion. A main included angle is defined between the forming side portion and the forming end portion and is equal to the predetermined included angle.

[0007] In the first stamping step, the stamping punch is used to subject the steel sheet to a cold stamping operation so that the steel sheet is directly stamped / pressed by the stamping punch and formed into an initial workpiece. The initial workpiece has an initial bottom portion and an initial side portion connected to the initial bottom portion. An initial included angle defined between the initial side portion and the initial bottom portion is less than the predetermined included angle, which is beneficial to the reduction in the springback degree during the stage of processing the initial workpiece. In the shape-forming step, the forming punch is used to subject the initial workpiece to a further cold stamping operation so that the initial workpiece is directly stamped / pressed by the forming punch and formed into a steel workpiece. Because the main included angle is equal to the predetermined included angle, the shape of the steel workpiece corresponds to the predetermined steel sheet appearance. In the taking-out step, it includes withdrawing the forming punch from the forming base and then taking the steel workpiece out of the forming cavity, thus completing the process of forming the steel workpiece by cold stamping.

[0008] The method involves cold stamping operations conducted in sequence, which allows the steel sheet to be processed without any heating treatments. Therefore, the steel workpiece can be quickly and efficiently formed, and the effects of saving energy and reducing carbon emissions can also be attained.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a block diagram showing execution steps of a first preferred embodiment of this invention;

[0010] FIG. 2 is a schematic view showing a designing operation of the first preferred embodiment;

[0011] FIG. 3 is a schematic view showing a stamping die and a forming die of the first preferred embodiment;

[0012] FIG. 4 is a schematic view showing a preparing operation and a first stamping step of the first preferred embodiment;

[0013] FIG. 5 is a schematic view showing a shape-forming and a taking-out step of the first preferred embodiment;

[0014] FIG. 6 is a block diagram showing execution steps of a second preferred embodiment of this invention; and

[0015] FIG. 7 is a schematic view showing an additional stamping step of the second preferred embodiment.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Referring to FIG. 1, a first preferred embodiment of this invention is related to a method 3 of manufacturing an ultra-high strength steel workpiece by cold stamping. Regarding the method 3, an ultra-high strength steel sheet 4 is processed by cold stamping, and the processed steel sheet 4 is formed into an ultra-high strength steel workpiece 5. The method 3 includes, in sequence, a preparatory step 31, a first stamping step 32, a shape-forming step 33, and a taking-out step 34. The preparatory step 31 includes a designing operation 31A and a preparing operation 31B. The designing operation 31A is an operation for designing and predetermining a steel sheet appearance to be formed. As shown in FIG. 2, the predetermined steel sheet appearance includes a bottom portion 41, a side portion 42 connected to the bottom portion 41, and a predetermined included angle 4a defined between the bottom portion 41 and the side portion 42. In the first preferred embodiment, the side portion 42 can include two side surfaces 42′ respectively extended outwards from the bottom portion 41, so each predetermined included angle 4a is defined between each side surface 42′ and the bottom portion 41.

[0017] Referring to FIG. 3, the preparing operation 31B is an operation for preparing an ultra-high strength steel sheet 4, a stamping die 6, and a forming die 7. As shown in (a) of FIG. 4, the ultra-high strength steel sheet 4 is a sheet of steel with the strength of at least 980 MPa, that is, equal to 980 MPa or greater than 980 MPa (≥980 MPa). The steel sheet 4 is also extended by a length.

[0018] The stamping die 6 includes a stamping punch 61 and a stamping base 62 opposite the stamping punch 61. There is a stamping cavity 621 recessedly formed into the stamping base 62. The stamping punch 61 has a stamping end portion 611, a stamping side portion 612 connected to the stamping end portion 611, and a first included angle 61a defined between the stamping side portion 612 and the stamping end portion 611. In the first preferred embodiment, the stamping side portion 612 can include two stamping side surfaces 612′ respectively extended outwards from the stamping end portion 611, so each first included angle 61a is defined between each stamping side surface 612′ and the stamping end portion 611. In addition, the first included angle 61a is less than the predetermined included angle 4a of the steel sheet appearance, and a difference in values between the predetermined included angle 4a and the first included angle 61a is at least thirty degrees. In other words, the difference is thirty degrees or more than thirty degrees, and preferably the difference can be, but not limited to, forty-five degrees. For example, if the value of the predetermined included angle 4a is set at 120 degrees, the value of the first included angle 61a does not exceed 90 degrees, and especially 75 degrees can be defined as the optimum first included angle 61a.

[0019] As regards a place where the stamping side portion 612 and the stamping end portion 611 meet, the place between the stamping side portion 612 and the stamping end portion 611 can be designed as a circular arc segment with a radius equivalent to a fillet radius 61r, as for example shown in FIG. 3. Preferably, the fillet radius 61r does not exceed 6 mm. That is, the fillet radius 61r can be equal to or less than 6 mm (≤6 mm). Accordingly, the above conditions help reduce the springback amount to relieve the degree of the springback (or angular change), which allows the ultra-high strength steel sheet 4 to be stamped and formed without difficulties in the following step.

[0020] Referring to FIG. 3, the forming die 7 includes a forming punch 71 and a forming base 72 opposite the forming punch 71. There is a forming cavity 721 recessedly formed into the forming base 72. The forming punch 71 has a forming end portion 711 and a forming side portion 712 connected to the forming end portion 711. A main included angle 71a is defined between the forming side portion 712 and the forming end portion 711 and is equal to the predetermined included angle 4a of the steel sheet appearance. In the first preferred embodiment, the forming side portion 712 can include two forming side surfaces 712′ respectively extended outwards from the forming end portion 711, so each main included angle 71a is defined between each forming side surface 712′ and the forming end portion 711.

[0021] To fulfill the cold stamping, the ultra-high strength steel sheet 4 has a composition in weight % (percent by weight) which can include 0.1% to 0.38% carbon, 1% to 2.10% manganese, equal to or less than 0.18% phosphorus, 0.1% to 0.57% silicon, and equal to or less than 0.005% sulfur, and a remainder of the composition being iron and inevitable compositions inherent in the steel sheet 4. The amount of these elements can be adjusted according to the strength of the steel sheet 4, as for example shown in Table 1, and herein the steel sheet with the strength of 980 MPa is adopted as an example.TABLE 1StrengthCarbonManganesePhosphorusSiliconSulfur(MPa)(C)(Mn)(P)(SI)(S)9800.10~0.171.60~1.70≤0.0090.30~0.40≤0.00112000.15~0.251.90~2.10≤0.0120.45~0.57≤0.00513000.15~0.211.50~1.65≤0.0070.30~0.40≤0.00213000.22~0.251.15~1.35≤0.0150.15~0.25≤0.00214000.16~0.251.60~1.65≤0.0090.15~0.30≤0.00114000.26~0.381.15~1.30≤0.0180.30~0.40≤0.00315000.20~0.251.07~1.12≤0.0150.18~0.23≤0.00115000.34~0.381.15~1.30≤0.0180.20~0.30≤0.00315000.25~0.301.65~1.70≤0.0150.20~0.30≤0.00417000.30~0.381.00~1.05≤0.010 0.2~0.25≤0.001

[0022] Besides FIG. 3, cold stamping operations for manufacturing a steel workpiece 5 are described with the aid of FIG. 4 and FIG. 5. Firstly, the first stamping step 32 includes putting the ultra-high strength steel sheet 4 to be processed (shown in (a) of FIG. 4) in the stamping cavity 621 of the stamping base 62 and then moving the stamping punch 61 to the stamping cavity 621. When the stamping end portion 611 and the stamping side portion 612 of the stamping punch 61 enter the stamping cavity 621, the steel sheet 4 is directly pressed by the stamping force of the stamping punch 61 at room temperature, and the pressed portion is moved deep into the stamping cavity 621. Thus, the shape of the steel sheet 4 is correspondingly changed because of such cold stamping operation, and the pressed steel sheet 4 is defined as an initial workpiece 40, as shown in (b) of FIG. 4). The appearance of the initial workpiece 40 has an initial bottom portion 401 and an initial side portion 402 connected to the initial bottom portion 401. An initial included angle 40a is defined between the initial side portion 402 and the initial bottom portion 401 and is less than the predetermined included angle 4a. In the first preferred embodiment, the initial side portion 402 includes two initial side surfaces 402′ respectively extended outwards from the initial bottom portion 401 so that each initial included 40a is defined between each initial side surface 402′ and the initial bottom portion 401. After the stamping die 6 subjects the steel sheet 4 to the cold stamping operation for forming the initial workpiece 40, the initial side portion 402 of the initial workpiece 40 springs back slightly, as for example shown by arrows in (c) of FIG. 4. Then, withdraw the stamping punch 61 from the stamping base 62 and take the initial workpiece 40 out of the stamping cavity 621. After the initial workpiece 40 separates from the stamping die 6, some springback occurs to cause the initial side portion 402 to partially return to its original shape. In other words, the initial side portion 402 springs back to a certain extent, as for example shown by dotted lines in (d) of FIG. 4. For instance, when the initial workpiece 40 is stamped in the stamping cavity 621, the initial included angle 40a is about 90 degrees (see (c) of FIG. 4). After the initial workpiece 40 is taken out of the stamping cavity 621, the initial included angle 40a is changed to 100~120 degrees because of the springback (see (d) of FIG. 4).

[0023] FIG. 5 shows the shape-forming step 33. After the springback of the initial side portion 402 of the initial workpiece 40 occurs (see (a) of FIG. 5), the initial workpiece 40 is put in the forming cavity 721 of the forming die 72 (see (b) of FIG. 5). Then, the forming punch71 is directly moved to the forming cavity 721, and the forming punch 71 adds stamping force to press the initial workpiece 40, which means that a further cold stamping operation is conducted. The pressed initial workpiece 40 is defined as a steel workpiece 5. In addition, the shape of the forming cavity 721 confines the forming of the steel workpiece 5, and when the steel workpiece 5 is formed, the steel workpiece 5 has a shape corresponding to the forming end portion 711 and the forming side portion 712 of the forming punch 71. In other words, the appearance of the steel workpiece 5 includes a workpiece bottom portion 51 and a workpiece side portion 52 and also includes a workpiece included angle 5a that is equal to the main included angle 71a (as for example shown in (c) of FIG. 5, wherein the workpiece side portion 52 can have two workpiece side surfaces 52′ respectively extended outwards from the workpiece bottom portion 51, and each workpiece included angle 5a is defined between each workpiece side surface 52′ and the workpiece bottom portion 51). Because of the cold stamping operation executed by the forming die 7 and the feature that main included angle 71a is equal to the predetermined included angle 4a, the shape of the steel workpiece 5 is the same as the predetermined steel sheet appearance of the designing operation 31A.

[0024] The taking-out step 34 includes withdrawing the forming punch 71 from the forming base 72 (as for example shown by arrows in (c) of FIG. 5) and then taking the steel workpiece 5 out of the forming cavity 721, thus gaining the full steel workpiece 5 (see (d) of FIG. 5). Therefore, the method 3 is completed.

[0025] According to the above, the steel sheet 4 is pressed by the stamping force of the stamping die 6 at room temperature (first stamping step 32). Because of the difference of at least thirty degrees defined in the preparatory step 31, the initial included angle 40a of the initial workpiece 40 obtained by the first stamping step 32 is allowed to be less than the predetermined included angle 4a, which helps reduce the initial springback degree of the initial workpiece 40. The initial workpiece 40 is further pressed by the stamping force of the forming die 7 (shape-forming step 33). Because the main included angle 71a is equal to the predetermined included angle 4a, the shape-forming step 33 helps eliminate the springback of the initial workpiece 40 which already experiences the initial springback. In other words, the shape of the initial workpiece 40 is directly formed by the stamping force of the forming die 7 at room temperature, and no springback occurs. Thus, the first stamping step 32 mainly serves to reduce the springback after the stamping work, and the forming step 33 mainly serves to eliminate the springback in order to conquer the springback property of the initial workpiece 40. No heating treatments are needed during the stamping operations, which attains a rapid forming effect, prevents unnecessary energy consumption, and attains the energy conservation and the reduction in carbon emissions.

[0026] Referring to FIG. 6 and FIG. 7, a second preferred embodiment of this invention still includes a preparatory step 31, a first stamping step 32, a shape-forming step 33, and a taking-out step 34, and the details of these steps are described above. The second preferred embodiment is characterized in that the preparing operation 31B of the preparatory step 31 further includes preparing a second die 8. The second die 8 includes a second punch 81 and a second base 82 opposite the second punch 81. There is a second stamping cavity 821 recessedly formed into the second base 82. The second punch 81 has a second end portion 811 and a second side portion 812 connected to the second end portion 811. A second included angle 81a is defined between the second side portion 812 and the second end portion 811. Preferably, the second side portion 812 can include two second side surfaces 812′ respectively extended outwards from the second end portion 811, so each second included angle 81a is defined between each second side surface 812′ and the second end portion 811. Moreover, the second included angle 81a is set between the predetermined included angle 4a and the first included angle 61a. In other words, the second included angle 81a is less than the predetermined included angle 4a but greater than the first included angle 61a. For instance, when the predetermined included angle 4a and the first included angle 61a are respectively set at 120 degrees and 75 degrees, the value of the second included angle 81a of the second die 8 can be greater than 75 degrees but less than 120 degrees.

[0027] In the second preferred embodiment, the second die 8 can be used in an additional stamping step 35. The additional stamping step 35 is conducted between the first stamping step 32 and the shape-forming step 33. The additional stamping step 35 includes putting the initial workpiece 40 which is obtained by the first stamping step 32 in the second stamping cavity 821 and then moving the second punch 81 to the second stamping cavity 821 so that the second punch 81 adds stamping force to press the initial workpiece 40, which means that an additional cold stamping operation is executed. After finishing the additional stamping step 35, the initial workpiece 40 is moved to the forming cavity 721 to continue the shape-forming step 33. Accordingly, if the preparing operation 31B uses the steel sheet 4 with the strength of 1400 MPa or higher, a possible scenario is that only the stamping force of the first stamping step 32 is not sufficient enough to efficiently reduce the springback property of the initial workpiece 40 after subjecting the steel sheet 4 to the cold stamping work for forming the initial workpiece 40. In this case, the additional stamping step 35 conducts the additional cold stamping operation to further reduce the springback property of the initial workpiece 40, and thereafter the shape-forming step 33 is allowed to eliminate the springback in order to form the initial workpiece 40 into the ultra-high strength steel workpiece 5 by cold stamping. No heating treatments are needed during the stamping operation, which attains a rapid forming effect, prevents unnecessary energy consumption, and attains the energy conservation and the reduction in carbon emissions.

[0028] To sum up, the method of this invention includes cold stamping operations respectively executed by the first stamping step and the shape-forming step and also takes advantage of these cold stamping operations and some specific conditions to form an ultra-high strength steel sheet into an initial workpiece, and then into a steel workpiece. With the aid of the condition that the predetermined included angle is greater than the first included angle but equal to the main included angle and the condition that the difference is at least thirty degrees, the springback property of the ultra-high strength steel sheet can be reduced and even eliminated. Therefore, the steel workpiece is efficiently and quickly shaped and formed by cold stamping. No heating treatments are required during the process of forming the steel workpiece, thus conserving energy, reducing carbon emission, and protecting the environment.

[0029] While the embodiments are shown and described above, it is understood that further variants and modifications may be made without departing from the scope of this invention as defined by the appended claims.

Claims

1. A method of manufacturing an ultra-high strength steel workpiece by cold stamping comprising:a preparatory step including a designing operation and a preparing operation, wherein said designing operation includes designing and predetermining a steel sheet appearance to be formed, with said steel sheet appearance having a bottom portion, a side portion extended outwards from said bottom portion, and a predetermined included angle defined between said bottom portion and said side portion, and wherein said preparing operation includes preparing a steel sheet, a stamping die, and a forming die, a strength of said steel sheet being at least 980 MPa, said stamping die including a stamping punch and a stamping base opposite said stamping punch, a stamping cavity being recessedly formed into said stamping base, said stamping punch having a stamping end portion, a stamping side portion connected to said stamping end portion, and a first included angle defined between said stamping side portion and said stamping end portion, said first included angle being less than said predetermined included angle, and a difference between said first included angle and said predetermined included angle being at least thirty degrees, said forming die including a forming punch and a forming base opposite said forming punch, a forming cavity being recessedly formed into said forming base, said forming punch having a forming end portion, a forming side portion connected to said forming end portion, and a main included angle defined between said forming side portion and said forming end portion, said main included angle being equal to said predetermined included angle;a first stamping step including putting said steel sheet in said stamping cavity of said stamping base and then moving said stamping punch to said stamping cavity so that said stamping punch conducts a cold stamping operation which adds stamping force to press said steel sheet, said pressed steel sheet thereby being defined as an initial workpiece, with said initial workpiece having an initial bottom portion and an initial side portion connected to said initial bottom portion, an initial included angle being defined between said initial side portion and said initial bottom portion and being less than said predetermined included angle, said first stamping step further including withdrawing said stamping punch from said stamping base and taking said initial workpiece out of said stamping cavity after finishing said cold stamping operation of said stamping punch;a shape-forming step including putting said initial workpiece in said forming cavity of said forming die after a springback of said initial side portion of said initial workpiece occurs and then moving said forming punch to said forming cavity so that said forming punch conducts a further cold stamping operation which adds stamping force to press said initial workpiece, said pressed initial workpiece thereby being defined as a steel workpiece, with said steel workpiece having a workpiece included angle equal to said main included angle, a shape of said steel workpiece corresponding to said steel sheet appearance of said designing operation because said main included angle is equal to said predetermined included angle; anda taking-out step including withdrawing said forming punch from said forming base and then taking said steel workpiece out of said forming cavity, thereby completing said method.

2. The method according to claim 1, wherein said preparing operation further includes preparing a second die, said second die including a second punch and a second base opposite said second punch, a second stamping cavity being recessedly formed into said second base, said second punch having a second end portion, a second side portion connected to said second end portion, and a second included angle defined between said second side portion and said second end portion, said second included angle being less than said predetermined included angle but greater than said first included angle, said second die being used in an additional stamping step, said additional stamping step being conducted between said first stamping step and said shape-forming step, said additional stamping step including putting said initial workpiece obtained by said first stamping step in said second stamping cavity and then moving said second punch to said second stamping cavity so that said second punch conducts an additional cold stamping operation which adds stamping force to press said initial workpiece, said initial workpiece being moved to said forming cavity after finishing said additional stamping step to continue said shape-forming step.

3. The method according to claim 1, wherein said steel sheet has a composition in weight % containing 0.1% to 0.38% carbon, 1% to 2.10% manganese, equal to or less than 0.18% phosphorus, 0.1% to 0.57% silicon, and equal to or less than 0.005% sulfur, and a remainder of said composition being iron and inevitable compositions inherent in said steel sheet.

4. The method according to claim 1, wherein a place where said stamping side portion and said stamping end portion meet is defined by a fillet radius, and said fillet radius does not exceed 6 mm.

5. The method according to claim 1, wherein said side portion of said steel sheet appearance includes two side surfaces respectively extended outwards from said bottom portion, said stamping side portion including two stamping side surfaces respectively extended outwards from said stamping end portion, said forming side portion including two forming side surfaces respectively extended outwards from said forming end portion, said initial side portion including two initial side surfaces respectively extended outwards from said initial bottom portion, said steel workpiece including a workpiece bottom portion and two workpiece side surfaces respectively extended outwards from said workpiece bottom portion.

6. The method according to claim 2, wherein said second side portion including two second side surfaces respectively extended outwards from said second end portion.

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