JP5700526B2 - Press forming method - Google Patents

Description

  The present invention relates to a press molding method for improving press moldability.

  There are various shapes of press-molded parts for automobiles and the like. In press molding of these parts, a plurality of molding elements such as deep drawing molding, stretch molding, stretch flange molding, and bending molding are generally combined. Is. Of these parts, many parts that include molding elements of deep drawing and stretch forming are difficult to press form. Note that deep drawing is formed by allowing a material to flow into the die, and overhang forming is performed by extending the material in the die in two axial directions.

  Normally, press factories that produce these parts perform press molding at a high molding speed of 10 mm / sec or more in order to ensure productivity, and 70 mm in auto parts press factories that pursue high productivity. In many cases, press molding is performed at a high molding speed of about / sec. The molding speed referred to here is an average molding speed from when the punch comes into contact with the blank and when molding is actually started to when it is finished.

  On the other hand, in the automotive field, in order to improve fuel efficiency and reduce carbon dioxide emissions, efforts are being made to reduce the weight of the vehicle body by using high-strength steel sheets for press-formed parts. Some press-formed parts also use high-strength steel sheets of 980 MPa class or higher.

  It is well known that the strength of steel sheets decreases with increasing strength, and press formability also decreases. For this reason, the development of high-strength steel sheets with excellent balance between strength and ductility has been promoted from the material aspect so that higher-strength steel sheets can be applied to a wider range of press-formed parts. Development of press forming methods that improve the forming limit is underway.

  The high strength steel sheets with excellent strength and ductility balance developed so far include DP (dual phase) steel sheets composed of ferrite and martensite phases, and TRIP (transformation induced plasticity) steel sheets with residual austenite transformation induced plasticity. (For example, refer nonpatent literature 1). Recently, a TBF (trip aided bainitic ferrite) steel plate having a parent phase of bainitic ferrite has been developed as a high-strength steel plate having a further excellent balance between strength and ductility (see, for example, Non-Patent Document 2). .

  On the other hand, as a press forming method for improving the press forming limit, a press forming method in which the steel plate temperature of the punch portion is equal to or lower than normal temperature and the steel plate temperature of the wrinkle holding portion is set to 150 ° C. or higher (for example, refer to Patent Document 1), TRIP type For example, a method of press molding with a die shoulder temperature of 150 ° C. to 200 ° C. and a punch shoulder temperature of −30 ° C. to 0 ° C. is proposed (for example, see Patent Document 2). ing. In each of Patent Documents 1 and 2, deep drawing is performed, and the effect of improving the deep drawing limit by partial warm forming at the wrinkle holding portion and the die shoulder is confirmed.

  In addition, various tests were conducted to investigate the effect of forming temperature on press formability (extrusion, deep drawability, stretch flangeability) using TBF steel sheets. Test results that have found that there is a warm temperature region that improves more than that have been reported (for example, see Non-Patent Document 3). Non-patent document 3 describes that the overhang property test and stretch flangeability test are performed at a molding speed of 1 mm / min (0.017 mm / sec) which is considerably slower than the molding speed (about 70 mm / sec) in an actual press factory. Going at speed. The deep drawability test is performed at a molding speed of 200 mm / min (3.3 mm / sec).

JP 2001-246427 A JP 2007-1111765 A

Komiya Yukihisa, “Current Status and Trends of Automotive Steel Materials”, R & D Kobe Steel Engineering Reports, Vol. 52, no. 3 (December 2002), p. 2-5 Koji Shibuya and Yoichi Mukai, “Effects of Alloying Elements and Annealing Conditions on Mechanical Properties of TRIP Type Bainitic Ferritic Steel Sheet”, R & D Kobe Steel Engineering Reports, Vol. 57, no. 2 (August 2007), p. 27-30 Koichi Sugimoto et al., “Warm formability of ultra-high-strength low-alloy TRIP-type bainitic ferrite steel sheet”, Iron and Steel, Vol. 91, no. 2 (February 2005), p. 34-40

  When a press-molded part including the above-mentioned deep drawing and stretch forming elements is press-molded at a high molding speed of 10 mm / sec or more, which is adopted in an actual press factory, cracks are generated in the stretched portion. In many cases, improvement of press formability is desired. The cracks at the overhanging portion are more likely to occur as the strength of the steel plate increases, and this is a factor that hinders the increase in strength of the press-formed parts.

  In order to improve the press formability of press-formed parts including such deep drawing and stretch forming elements, including high-strength steel sheets, as described in Patent Documents 1 and 2 and Non-Patent Document 3. Although it is conceivable to adopt a warm forming method, no example has been reported in which such a part is warm formed at a high forming speed of 10 mm / sec or more that can ensure high productivity.

  Therefore, an object of the present invention is to enable press molding of a press-molded part including molding elements of deep drawing and stretch molding at a fast molding speed of 10 mm / sec or more that can ensure high productivity.

In order to solve the above problems, the present invention provides a steel sheet having at least one deep drawing process and at least one bulging process, wherein the press forming speed in each forming process is 10 mm / sec or more. in the press molding method, wherein at least perform one deep drawing molding process between the temperature of the 100 ° C. to 250 DEG ° C., have at least one row across less than 50 ° C. cold the stretch forming process of, the steel plate, organizations A method was adopted in which residual austenite was contained in an amount of 3% by volume or more, and the cold stretch forming step was performed after the warm deep drawing step .

The inventors changed the temperature and forming speed of the steel sheet, and conducted a deep drawability test and a stretchability test using a cylindrical punch and a die. The test blank was a 980 MPa class TBF steel plate having a thickness of 1.4 mm. In the overhang property test, the blank diameter was increased and the wrinkle holding force was increased so that the material did not flow into the die. The test conditions are as follows.
(Test conditions)
・ Punch diameter: 50mm (shoulder radius: 5mm)
・ Die diameter: 54mm (shoulder radius: 7mm)
・ Blank diameter: 105 mm (deep drawability test), 150 mm (extrusion test)
・ Wrinkle holding force: 12 tonf (deep drawability test), 20 tonf (extrusion property test)
Steel plate temperature: 20 ° C to 350 ° C
Molding speed: 0.1 mm / sec, 5 mm / sec, 10 mm / sec, 70 mm / sec

  FIGS. 6A and 6B show the results of the deep drawability test and the stretchability test, respectively. From these test results, in the deep drawability test, the influence of the molding speed is hardly recognized, and the molding limit height is improved in the warm region of 100 ° C. to 250 ° C. compared to the cold at room temperature. On the other hand, for the stretchability test, at the low forming speed of 0.1 mm / sec, the forming limit height does not decrease much even if the steel plate temperature is increased, and the forming limit height is improved in the temperature range exceeding 250 ° C. On the other hand, at the high molding speed of 70 mm / sec, the molding limit height decreases as the test temperature increases.

  FIG. 7 is a graph in which the forming limit height in the above-described stretchability test is plotted against the forming speed. As can be seen from this graph, the one that is stretched at a temperature of 350 ° C. has a lower molding limit height as the molding speed increases, whereas the one that is cold stretched has an increased molding speed. However, the molding limit height does not decrease so much, and at a molding speed of 10 mm / sec or higher, the one with cold stretch molding has a higher molding limit height than the one with warm stretch molding. .

  Based on the knowledge obtained in such a test, at least one deep drawing process is performed at a temperature of 100 ° C. to 250 ° C., and at least one stretch forming process is performed at a temperature below 50 ° C. Thus, a press-molded part including molding elements of deep drawing and stretch molding can be press-molded at a high molding speed of 10 mm / sec or more that can ensure high productivity. The deep drawing process defined here is the deep drawing process that occupies the majority of the molding elements in the process, and the stretch molding process is the process that occupies the majority of the molding elements in the process. is there.

  By making the steel sheet contain 3% by volume or more of retained austenite in the structure, it is possible to further improve the stretch forming limit as having an excellent balance between strength and ductility.

  By making the steel sheet containing 3% by volume or more of retained austenite into a high-strength steel sheet having bainitic ferrite as a matrix, it is possible to further improve the stretch forming limit as having an excellent balance between strength and ductility. It is possible to increase the strength of press-formed parts and to expand the range of application to press-formed parts.

  For steel sheets containing 3% by volume or more of retained austenite, the cold stretch forming process is performed after the warm deep drawing process, thereby limiting the forming limit in the cold stretch forming process. It can be improved further.

  The present inventors conducted a tensile test in which a tensile pre-strain was applied warm (100 ° C., 200 ° C.) and then pulled using a 980 MPa class TBF steel sheet having a residual austenite amount of 3% by volume or more, The total elongation was compared with the result of a tensile test pulled cold or warm (100 ° C., 200 ° C.) without pre-strain. The tensile test piece was a JIS No. 13 B test piece having a thickness of 1.4 mm, and the tensile speed was 17 mm / sec, which is a high speed.

  FIG. 8 shows the results of the tensile test. From these test results, all of the samples subjected to the prestraining in the warm condition are significantly improved in the total elongation with the prestrain applied compared to the cold tensile test without the prestrain. Note that the total elongation in the warm tensile test without pre-strain is lower than that in the cold tensile test. The reason why the total elongation was improved by applying the pre-strain of tension in the warm condition was that when pre-strain was applied at a temperature of 100 ° C. or 200 ° C., the elongation was obtained only by the deformation of the matrix, and the subsequent cooling This is thought to be due to the fact that high ductility could be realized by utilizing the plasticity-induced transformation of retained retained austenite during hot-tensioning. In other words, the margin for improving the total elongation with respect to the cold tensile test without pre-strain corresponds to the elongation deformation of the matrix obtained during the pre-straining in the warm state. From such test results, the steel sheet having a residual austenite amount of 3% by volume or more is subjected to the cold stretch forming process after the warm deep drawing process, so that in the cold stretch forming process. It can be expected to further improve the molding limit.

  The number of press strokes can be reduced by performing the warm deep drawing forming step and the cold stretch forming step within the same press stroke.

  In the press molding method according to the present invention, at least one deep drawing process is performed at a temperature of 100 ° C. to 250 ° C., and at least one stretch forming process is performed at a temperature below 50 ° C. A press-molded part including molding elements of deep drawing and stretch molding can be press-molded at a high molding speed of 10 mm / sec or more that can ensure high productivity.

The conceptual diagram which shows the press molding process in the press molding method of 1st Embodiment. 1 is a longitudinal sectional view showing a press-formed product formed in the press forming process of FIG. Graph showing the relationship between the total forming height and the initial retained austenite amount when forming in each step of the press forming step of FIG. The conceptual diagram which shows the press molding process in the press molding method of 2nd Embodiment. The conceptual diagram which shows the press molding process in the press molding method of 3rd Embodiment. (A) and (b) are graphs showing the results of the deep drawability test and the stretchability test, respectively. Graph showing the relationship between the forming speed and the forming limit height in the stretchability test of FIG. Graph showing the results of a tensile test with warm prestrain

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a press forming process in which the press forming method of the first embodiment is performed. This press molding process includes a first process in which deep drawing is performed warm and a second process in which stretch forming is performed in cold. In each step, a common press die incorporated in a high-speed press machine having a molding speed of 10 mm / sec or more is used. The press mold includes an upward cylindrical punch 1 having a circular recess 1a formed in the head, a downward die 2 into which the cylindrical punch 1 enters, and a wrinkle pressing plate 3 that presses the flange portion of the blank B against the die 2. And a downward ball head punch 4 directed to the recess 1a of the cylindrical punch 1. The cylindrical punch 1 had a diameter of 50 mm, the shoulder radius and the shoulder radius of the recess 1a were 5 mm, the die 2 had a diameter of 54 mm, the shoulder radius was 7 mm, and the ball head punch 4 had a diameter of 10 mm.

  In the first step, the cylindrical punch 1, the die 2 and the wrinkle holding plate 3 are heated to a predetermined temperature, and the temperature of the blank B brought into contact with these press dies is also increased. Deep drawing is performed warmly by entering the die 2. The blank B may be heated to a predetermined temperature using a furnace or the like in advance. In the second step, the cylindrical punch 1, the die 2, the wrinkle holding plate 3, and the deep-drawn cup-shaped semi-molded product are cooled to room temperature, and then the ball-head punch 4 that has been brought to room temperature in advance is placed on the cylindrical punch 1. It enters into the circular recessed part 1a, and performs a concave overhang molding at the bottom of the cup-shaped semi-molded product.

  FIG. 2 shows a press-formed product formed in this way. In this press-molded product, a concave bulge A is stretched at the bottom of the deep-drawn body, and the dimensions are such that the inner diameter D is 50 mm, the deep-drawing height Hd and the stretch-molded height. Hs is variable.

  A total of four types of steel plates were prepared, two types of TBF steel plates and two types of DP steel plates. Table 1 shows the chemical composition of these steel sheets, and Table 2 shows the mechanical properties and microstructure structure. The mechanical properties were determined by a tensile test using a JIS No. 13 B test piece, and the amount of retained austenite in the microstructure was measured by the X-ray diffraction method. Each steel plate is a 980 MPa class high strength cold-rolled steel plate having a thickness of 1.4 mm. Each of the TBF steel plates 1 and 2 has a total elongation and a uniform elongation higher than those of the DP steel plates 1 and 2, and has a better strength-ductility balance. The amount of retained austenite increases in the order of the TBF steel plate 1, the TBF steel plate 2, the DP steel plate 1, and the DP steel plate 2, and all of them are 3% by volume or more except for the DP steel plate 2.

  First, each blank tested from the TBF steel plate 1 and the DP steel plate 1 was formed into the press-formed product shown in FIG. 2 by the press-forming process shown in FIG. The diameter of each blank was 103 mm. The molding speed in the first and second steps was 70 mm / sec. For the TBF steel plate 1, the deep drawing forming height Hd = 30 mm in the first step and the overhang forming height Hs = 8 mm in the second step. For the DP steel plate 1, the deep drawing forming height Hd = 28 mm. The overhang forming height Hs = 7 mm. As an example, the steel plate temperature in each contact portion between the die and the punch in the first step is changed in a range of 100 to 250 ° C., and the steel plate temperature in the second step is 40 ° C. in the contact portion with the die, Was subjected to press molding (Examples A to C) at 25 ° C. Moreover, as a comparative example, all the cold forming (comparative example A) which made the steel plate temperature of each contact part of the die | dye and punch in the 1st and 2nd processes 25 degreeC, and in the 1st process Also, press forming (Comparative Example B) is all warm, with the steel plate temperature at each contact portion of the die and the punch set at 200 ° C. and the steel plate temperature at each contact portion between the die and the punch in the second step set at 350 ° C. went.

  Table 3 (a), (b) shows the press-forming result of an Example and a comparative example about TBF steel plate 1 and DP steel plate 1, respectively. In any case of the steel plates, good press forming results are obtained in the examples A to C. On the other hand, the thing of the comparative example A generate | occur | produced the crack at the 1st process and could not implement the 2nd process. Moreover, although the thing of the comparative example B was able to shape | mold the 1st process, the crack generate | occur | produced in the 2nd process. In addition, as for the deep drawing forming height Hd in Comparative Example A and the overhang forming height Hs in Comparative Example B, both of which were impossible to form, the TBF steel plate having a better strength / ductility balance was more than the DP steel plate. It is high.

  Next, using each blank having a diameter of 103 mm tested from the TBF steel plates 1 and 2 and the DP steel plates 1 and 2, the steel plate temperature in the deep drawing of the first step was 200 ° C. and the second step was stretch forming. The steel sheet temperature was set to 25 ° C., and press forming was performed to form the deep drawing forming height Hd and the overhang forming height Hs in each step to the forming limit. The overhang forming height Hs in the second step was 8 mm at the maximum.

  The press molding results are shown in Table 4. Table 4 also shows the maximum forming load in the first step and the amount of retained austenite after the first step. The TBF steel sheet 1 having the largest amount of retained austenite in the initial stage was drawn out regardless of the deep drawing limit in the first step, and the stretch forming height Hs in the second step was 8 mm at the maximum. Next, the TBF steel plate 2 having a large amount of retained austenite has a deep drawing forming height Hd of 30 mm in the first step, and a stretch forming height Hs in the second step reaches a maximum of 8 mm. On the other hand, the deep drawing forming height Hd of each of the DP steel plates 1 and 2 is lower than that of the TBF steel plate 2, and the overhang forming height Hs does not reach the maximum 8 mm. Further, the maximum forming load in the first step is lower in descending order of the initial retained austenite amount, and the TBF steel sheet 1 is the lowest. The amount of retained austenite after the first step is also increased in descending order of the initial amount of retained austenite.

  FIG. 3 is a graph in which the total forming height Hd + Hs of the deep drawing forming height Hd in the first step and the stretch forming height Hs in the second step shown in Table 4 is plotted against the initial retained austenite amount. is there. The standard forming height shown in the graph is a total forming height Hd + Hs (26 + 8 = 34 mm) when a 590 MPa class high-strength steel plate (total elongation 25%) is cold-formed in both the first and second steps. . From this graph, the total forming height Hd + Hs of the first and second steps becomes higher as the initial retained austenite amount increases, and when the initial retained austenite amount is 3% by volume or more, the 590 MPa class high strength steel plate having much lower strength. It can be seen that the forming limit is improved as compared with the case of cold press forming.

  FIG. 4 shows a press forming process in which the press forming method of the second embodiment is performed. This press molding process includes a first process in which cold forming is performed and a second process in which deep drawing is performed in warm. The same press machine and press die as those in the first embodiment were used. In this embodiment, in the first step, the cylindrical punch 1, the die 2, the wrinkle holding plate 3, and the ball head punch 4 are set at room temperature, and at the center of the blank B sandwiched between the die 2 and the wrinkle holding plate 3, the cylindrical punch 1 The ball head punch 4 is inserted into the circular recess 1a, and overhanging is performed. In the second step, the cylindrical punch 1, the die 2, the wrinkle holding plate 3 and the ball head punch 4 are heated to a predetermined temperature, and the temperature of the blank B brought into contact with these press dies is also increased. The cylindrical punch 1 is moved into the die 2 to perform deep drawing.

  Each blank tested from the TBF steel plate 1 and the DP steel plate 1 was formed into the press-formed product shown in FIG. 2 by the press-forming process shown in FIG. The diameter of each blank was 103 mm, and the molding speed in each step was 70 mm / sec. For the TBF steel plate 1, the stretch forming height Hs = 8 mm in the first step and the deep drawing forming height Hd = 30 mm in the second step. For the DP steel plate 1, the stretch forming height Hs = 7 mm, The deep drawing height Hd = 28 mm. As an example, the steel plate temperature of each contact portion between the die and the punch in the first step is 25 ° C., and the steel plate temperature of each contact portion between the die and the punch in the second step is in the range of 100 ° C. to 250 ° C. Varying press molding (Examples D to F) was performed. Moreover, as a comparative example, all the cold press-forming (comparative example C) which made the steel plate temperature of each contact part of the die | dye and punch in the 1st and 2nd processes 25 degreeC, and the 1st process Also, press forming (Comparative Example D) is all warm, with the steel plate temperature at each contact portion between the die and the punch set at 350 ° C. and the steel plate temperature at each contact portion between the die and the punch in the second step set at 200 ° C. went.

  Table 5 (a), (b) shows the press molding result of an Example and a comparative example about each steel plate. In any case of the TBF steel plate 1 and the DP steel plate 1, those of Examples D to F have good press forming results. On the other hand, although the thing of the comparative example C was able to shape | mold the 1st process, the crack generate | occur | produced in the 2nd process. Moreover, the thing of the comparative example D generate | occur | produced the crack at the 1st process and could not implement the 2nd process. It should be noted that the deep drawing forming height Hd in Comparative Example C and the overhang forming height Hs in Comparative Example D, both of which cannot be formed, are the DP steel plate 1 and the TBF steel plate 1 having better strength and ductility balance. Higher than.

  From the press molding results in Example 1 and Example 2 above, the deep drawing process is performed at a temperature of 100 ° C. to 250 ° C., and the stretch molding process is performed at a temperature below 50 ° C. The method can achieve good press forming results at a high forming speed that can ensure high productivity even when using high-strength steel sheets, and can increase the strength of the press-formed parts. The application range of high-strength steel sheets can also be expanded.

  Table 3 (a) and Table 5 (a) showing the press forming results of the TBF steel plate 1 are examples in which the steel plate temperature at each contact portion between the die and the punch in warm deep drawing is 200 ° C. About A and Example D, the result of having measured the plate | board thickness reduction | decrease rate in the overhang | projection part A center of a press molded product was written together. Example A, in which the cold stretch forming process was performed after the warm deep drawing process, was more extended than Example D in which the cold stretch process was performed before the warm deep drawing process. The plate thickness reduction rate of A is reduced by about 5%, and it can be expected that the molding limit is further increased. The measurement result of the plate thickness reduction rate corresponds well with the result of the tensile test shown in FIG. 8. In Example A, the deep drawing of the first step gains elongation only by deformation of the matrix, It is considered that high ductility was realized by utilizing the plastic-induced transformation of the retained austenite in the two-stage stretch forming.

  FIG. 5 shows a press forming process in which the press forming method of the third embodiment is performed. In this press molding process, the first process in which deep drawing is performed warm and the second process in which cold forming is performed are performed within the same press stroke. The same press machine and press die as those in the first embodiment were used. However, the ball head punch 4 for performing the overhang forming was provided with a refrigerant outlet 4a for ejecting the refrigerant at the top. As the refrigerant, air, water, oil, or the like can be used.

  In this embodiment, the temperature of the cylindrical punch 1, the die 2 and the wrinkle holding plate 3 is raised, and the temperature of the blank B brought into contact with these press dies is also raised, and then the first stage of the press stroke which is the first step. In addition, the cylindrical punch 1 is entered into the die 2 and deep drawing is performed at a temperature in the range of 100 to 250 ° C., and the refrigerant is supplied from the outlet 4a of the ball head punch 4 at the latter stage of the press stroke which is the second step. The bottom of the cup-shaped semi-molded product that has been deep-drawn is cooled by spraying, and a concave bulging is performed on the bottom at a temperature lower than 50 ° C. In addition, you may make it inject | pour the refrigerant | coolant which cools the bottom part of a cup-shaped half-molded product from the cylindrical punch 1 side.

  In each of the above-described embodiments, the deep-drawing process and the stretch-molding process are each performed once, but the press molding method according to the present invention includes any one of these processes twice or more. It can also be employed for those including other processes such as a flange process, a bending process, and a punching process. In the case where the punching process is included, a reduction in the punching load can be expected by performing the punching process simultaneously with the warm deep drawing process.

  In each of the above-described embodiments, the steel plate to be press-formed is a 980 MPa class TBF steel plate and a DP steel plate, but the press-forming method according to the present invention is a steel plate of any strength class of any steel type including mild steel plate. Can be applied to.

A Overhang B Blank 1 Cylindrical punch 1a Recess 2 Die 3 Wrinkle holding plate 4 Ball head punch 4a Refrigerant jet

Claims (3)

In the steel sheet press forming method having at least one deep drawing step and at least one stretch forming step, wherein the forming speed in each forming step is 10 mm / sec or more, the at least one deep drawing step was carried out between the temperature of the 100 ° C. to 250 DEG ° C., have at least one row across less than 50 ° C. cold the stretch forming process of,
The steel sheet shall contain 3% by volume or more of retained austenite in the structure,
The press forming method, wherein the cold stretch forming step is performed after the warm deep drawing step . The press forming method according to claim 1 , wherein the steel sheet containing 3% by volume or more of retained austenite is a high-strength steel sheet having bainitic ferrite as a parent phase. The press molding method according to claim 1 or 2 , wherein the warm deep drawing step and the cold stretch forming step are performed within the same press stroke.

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