JP5601716B2 - Press forming method of high strength steel sheet - Google Patents

Description

  The present invention relates to a press forming method for a high strength steel sheet.

  In recent years, in the automobile 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). 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, there are various types of press-molded parts for automobiles. In the press-molding of these parts, it is common to combine a plurality of molding elements such as deep drawing molding, stretch molding and stretch flange molding. is there. Among these parts, there is a part having a projecting or concave projecting portion A at the bottom of the main body, such as a door inner shown in FIG. In such a part, the overhanging portion A is stretched and formed at the latter stage of molding in which the main body is deep-drawn. Examples of this type of press-molded parts include door outers, door outers, front pillars, center pillars, rear floors, side sills, and the like. 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.

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

  As with parts such as the door inner shown in FIG. 6, it is difficult to press-mold a high-strength steel plate using a press-molded part that has an overhang at the bottom of the main body and is formed in the latter half of deep drawing. In fact, the strength of the steel sheets used is not so high.

  In order to promote the increase in strength of the steel sheet used for such press-formed parts, it is conceivable to employ a warm forming method as described in Patent Documents 1 and 2 and Non-Patent Document 3, There has been no report of an example of warm forming a press-molded part including such deep drawing and stretch forming at a high forming speed capable of ensuring high productivity as in a press factory. As shown in Tables 4 (a) and 4 (b) below as comparative examples, the present inventors can use such a press-formed part at high speed even when using a high-strength steel sheet having an excellent balance between strength and ductility. It has been confirmed that warm molding cannot be performed at a molding speed of 70 mm / sec.

  Accordingly, an object of the present invention is to suppress pressurization and to enable press forming of a press-formed part including deep drawing and stretch forming with a high-strength steel plate.

  In order to solve the above problems, the present invention provides a high strength steel sheet press forming method in which stretch forming is performed in the latter stage of deep drawing, and the temperature during press forming of the high strength steel sheet is set to 100 ° C. to 350 ° C. Then, a method was adopted in which the molding speed in the latter stage of molding in which the stretch molding was performed was slower than the molding speed in the first stage of molding in which the stretch molding was not performed.

The inventors changed the temperature and forming speed of the high-strength steel sheet, and performed 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

  4 (a) and 4 (b) 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, the forming limit height decreases as the test temperature increases at a high forming speed of 70 mm / sec, whereas the steel sheet temperature decreases at a low forming speed of 0.1 mm / sec. Even if it is increased, the molding limit height does not decrease so much. In the temperature region exceeding 250 ° C., the molding limit height is rather improved.

  FIG. 5 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 formed by cold stretching does not significantly decrease the molding limit height even when the molding speed increases, whereas the one formed by stretching at a temperature of 350 ° C. With the increase, the molding limit height decreases, and at a molding speed exceeding 10 mm / sec, the molding limit height is lower than that formed by cold stretching.

  Based on the knowledge obtained in such a test, the temperature during press forming of high-strength steel sheet is set to 100 ° C to 350 ° C, and the forming limit height is remarkably lowered as the forming speed is increased in such a temperature range. Deep drawing is performed by suppressing the decrease in productivity by making only the molding speed in the latter stage of molding, which is the stretch molding, slower than the molding speed in the first half of the molding, which is not affected by the molding speed, only by deep drawing. And press-molded parts including stretch forming can be press-formed with high-strength steel sheets.

  The molding speed in the latter molding stage is preferably 10 mm / sec or less. The upper limit value of the forming speed is based on the test result of FIG. 5, and the stretch forming limit can be improved more than the cold.

  By making the high-strength 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 high-strength steel sheet containing 3% by volume or more of the retained austenite as a parent phase of bainitic ferrite, it is possible to further improve the stretch forming limit, and to further improve the stretch forming limit, and press forming. The strength of the parts can be increased, and the range of application to press-molded parts can be expanded.

  The press forming method of the high strength steel sheet according to the present invention is such that the temperature during the press forming of the high strength steel sheet is set to 100 ° C. to 350 ° C., and the forming speed in the latter forming stage in which the overhang forming is performed is formed in the first forming stage in which the over forming is not performed Since it is slower than the speed, it is possible to suppress the decline in productivity and press-form parts including deep drawing and stretch forming with high-strength steel sheets, and promote higher strength of press-formed parts. At the same time, the range of application to press-formed parts can be expanded.

The longitudinal cross-sectional view which shows the press die which implemented the press forming method of the high strength steel plate which concerns on this invention (A), (b), (c) is a longitudinal sectional view showing a process of press-forming a high-strength steel plate using the press die of FIG. 1 is a longitudinal sectional view showing a press-formed product formed with the press mold of FIG. (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. External perspective view showing examples of press-formed parts including deep drawing and stretch forming

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a press mold in which a press forming method for a high-strength steel sheet according to the present invention is performed. This 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 crease 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 1 a 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.

  FIG. 2 shows a process of press-molding the blank B using the press die. First, as shown in FIG. 2A, when the cylindrical punch 1 enters the die 2, the material of the flange portion of the blank B flows into the die 2, and deep drawing is started. The deep drawing height increases with the progress of molding, and the ball head punch 4 is brought into contact with the material at the head of the cylindrical punch 1 as shown in FIG. When the molding further proceeds, as shown in FIG. 2C, the deep drawing height is further increased, and the material in the head of the cylindrical punch 1 is transferred to the circular recess 1a of the cylindrical punch 1 by the spherical head punch 4. Intruded into the inside.

  FIG. 3 shows a press-formed product of the high-strength steel sheet formed as described above. In this press-molded product, a concave projecting portion A is stretched and formed at the bottom of a deep-drawn main body. The dimensions of the press-formed product are an inner diameter D of 50 mm, a deep drawing height Hd of 30 mm, and an overhanging height Hs of 8 mm. Therefore, only deep drawing is performed in the first molding period (press stroke S = 0 to 22 mm), and the overhanging portion A is stretched in the second molding period (S = 22 to 30 mm).

  A total of three types of high-strength steel plates, one type of TBF steel plate and two types of DP steel plates, were prepared. 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, and the TBF steel plate has a total elongation and uniform elongation higher than those of each DP steel plate 1 and 2, and has a better strength-ductility balance. . The amount of retained austenite increases in the order of the TBF steel plate, the DP steel plate 1, and the DP steel plate 2, and all of them except the DP steel plate 2 are 3% by volume or more.

  Each blank tested from these three types of steel plates was set in the press mold shown in FIG. 1, and the press-formed product shown in FIG. 3 was formed. The diameter of each blank was 103 mm. During these press formings, the steel sheet temperature θ during press forming was changed in the range of room temperature to 350 ° C. The steel plate temperature θ during press forming was ensured by bringing a blank into contact with a press mold heated to a predetermined temperature for a predetermined time. The blank may be preheated to a predetermined temperature using a furnace or the like. Further, the molding speed V1 in the first molding period (S = 0 to 22 mm) in which only deep drawing is performed is set to a high speed of 70 mm / sec assuming the molding speed in an actual press factory, and the latter molding stage (S = 22 to 30 mm). ) In the range of 0.1 to 70 mm / sec. In some cases, the molding speed V1 in the first half of the molding was also changed.

  Tables 3 (a), (b), and (c) show the press forming results when the steel plate temperature θ is 200 ° C. for the TBF steel plate and the DP steel plates 1 and 2, respectively. The DP steel sheet 1 having a residual austenite amount of 3% by volume can be formed when the forming speed V2 in the latter forming stage is 2.5 mm / sec or less, and the DP steel sheet 2 having a residual austenite amount of 2% by volume is in the latter forming stage. Molding is possible only when the molding speed V2 is set to 0.1 mm / sec, which is extremely slow. On the other hand, a TBF steel sheet having a retained austenite amount of 8% by volume and a more excellent balance between strength and ductility can be formed when the forming speed V2 in the latter forming stage is 10 mm / sec or less. In the case where the molding speed V2 is faster than these limit speeds, any cracks occur in the overhanging portion A, and the molding is impossible. Therefore, it can be expected that a material with a retained austenite amount of 3% by volume or more can be subjected to stretch forming in the latter stage of molding at a molding speed that does not significantly reduce productivity.

  Tables 4 (a) and 4 (b) show the combinations of the forming speed V1 in the first forming stage and the forming speed V2 in the latter forming stage that can be formed. For the TBF steel sheet, V1 = 70 mm / sec, V2 = 10 mm / sec. For the DP steel sheet 1, the press forming results when the steel sheet temperature θ is changed with V1 = 70 mm / sec and V2 = 2.5 mm / sec are shown. As a comparative example, the results of press forming with V1 = V2 = 70 mm / sec and the entire forming period being high speed are also shown.

  According to these press forming results, for both the TBF steel plate and the DP steel plate 1, the steel plate temperature θ was in the range of 100 ° C. to 350 ° C., and the forming speed V2 was slowed down to 2.5 mm / sec and 10 mm / sec, respectively. Any of the examples can be molded. Further, in the comparative example in which the entire forming period is high speed (70 mm / sec), even when the steel sheet temperature θ is in the range of 100 ° C. to 350 ° C., cracks are generated in the overhanging portion A and the forming is impossible.

  From the above press forming results, the steel sheet temperature during press forming is set to 100 ° C. to 350 ° C., and the forming speed in the latter forming stage in which the overhang forming is performed is slower than the forming speed in the first forming period in which the over forming is not performed. Press forming of high-strength steel sheets can significantly increase the forming limit of press-formed parts that are difficult to form, including deep drawing and stretch forming, and can increase the strength of press-formed parts. The range of application of high-strength steel sheets can be expanded.

  Tables 5 (a) and (b) show the combinations of the forming speed V1 in the first forming stage and the forming speed V2 in the latter forming stage. For the TBF steel sheet, V1 = 70 mm / sec, V2 = 10 mm / sec, and DP steel sheet 1 V1 = 70 mm / sec, V2 = 2.5 mm / sec, and further, by changing the steel plate temperature θ1 of the flange portion during press forming and the steel plate temperature θ2 of the overhanging portion A separately, whether press forming is possible or not The result of having investigated the sheet thickness reduction | decrease rate in the overhang | projection part A is shown. The combination of the steel plate temperature θ1 of the flange portion and the steel plate temperature θ2 of the overhang portion A is a series in which the steel plate temperature θ1 is fixed at 200 ° C. and the steel plate temperature θ2 is changed in the range of 100 to 400 ° C. The temperature was set to 350 ° C., and the steel plate temperature θ1 was changed in the range of 100 to 400 ° C. As a comparative example, a result of investigation in which both steel plate temperatures θ1 and θ2 are set to room temperature is also shown.

  From the investigation results shown in Tables 5 (a) and 5 (b), the examples in which the steel plate temperatures θ1 and θ2 are combined in the range of 100 ° C. to 350 ° C. are formed for both the TBF steel plate and the DP steel plate 1. This is possible, and the thickness reduction rate of the overhanging portion A is smaller in the TBF steel plate having an excellent balance between strength and ductility than in the DP steel plate 1. In particular, when the steel plate temperature θ1 of the flange portion is 200 ° C. and the steel plate temperature θ2 of the overhanging portion A is 350 ° C., the plate thickness reduction rate is 12% for the TBF steel plate and 14% for the DP steel plate 1, and is more difficult. It can be expected as an optimum temperature condition that can improve the forming limit of a simple press-formed part. In addition, the comparative example which made either steel plate temperature (theta) 1 and (theta) 2 400 degreeC became impossible to shape | mold, The expression of a TRIP effect is suppressed by 400 degreeC decomposition | disassembly of a retained austenite, and ductility is carried out. Probably because of the decline.

  In the above-described embodiments, the high-strength steel plates are 980 MPa grade TBF steel plates and DP steel plates, but the press forming method of the high-strength steel plates according to the present invention is limited to such 980 MPa grade DP steel plates and TBF steel plates. However, the present invention can be applied to a high-strength steel sheet of any strength class of any steel type.

  Further, in the above-described embodiment, the first molding stage in which only deep drawing molding is performed and the second molding stage in which stretch forming is performed are performed in the same press molding process. It can also be performed by dividing it into processes.

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

Claims (4)

  In the press forming method of the high strength steel sheet that performs the stretch forming in the latter stage of the deep drawing, the temperature during the press forming of the high strength steel sheet is set to 100 ° C. to 350 ° C., and the molding speed in the later stage of the stretch forming is performed, A press forming method for high-strength steel sheets, characterized in that the forming speed is slower than the forming speed of the first forming step without overhang forming.   The high-strength steel sheet press forming method according to claim 1, wherein a forming speed in the latter forming stage is set to 10 mm / sec or less.   The high strength steel sheet press forming method according to claim 1 or 2, wherein the high strength steel sheet contains 3 vol% or more of retained austenite in the structure.   The high-strength steel plate press forming method according to claim 3, wherein the high-strength steel plate containing 3 vol% or more of retained austenite is used as a parent phase of bainitic ferrite.

Images