Press forming method for steel plate

Abstract

A press forming method forming an extended part by extension in a later forming period after deep drawing, the deep drawing process carried out with working at 100-250 C., and the extension forming process carried out colder at less than 50 C., whereby the extended part is formed by extension in a cup shaped low parts formed by deep drawing. Thus, press forming products containing formed elements can be deep drawing formed and extension formed at a high forming rate of 10 mm/sec or greater, which can assure high productivity. By making a steel plate temperature 100-350 C. during press forming and by making the forming rate in the later forming period where extension forming is carried out slower than the forming rate in an earlier forming period where extension forming is not carried out, cracking of the extended part can be prevented and press forming limitations can be improved.

Claims

1. A press forming method for a high-strength steel plate, comprising: performing deep drawing on a steel plate; and performing bulging on the steel plate, wherein the bulging is performed at a later forming stage of the deep drawing and no bulging is performed at an earlier forming stage of the deep drawing, wherein the temperature of the steel plate is set into the range of 100 to 350 C. while the steel plate is press-formed, and wherein the forming velocity at the later forming stage while bulging is performed is lower than the forming velocity at the earlier forming stage in which no bulging is performed.

2. The press forming method for the high-strength steel plate according to claim 1, wherein the forming velocity at the later forming stage is set to 10 mm/sec or less.

3. The press forming method for the high-strength steel plate according to claim 1, wherein the steel plate is a steel plate containing, in a microstructure thereof, retained austenite in a proportion by volume 3% or more.

4. The press forming method for the high-strength steel plate according to claim 3, wherein the steel plate containing retained austenite in the proportion by volume of 3% or more is a steel plate containing, as a parent phase thereof, bainitic ferrite.

5. The press forming method for the high-strength steel plate according to claim 1, wherein the forming velocity at the earlier forming stage is set to 10 mm/sec or more.

6. The press forming method for the high-strength steel plate according to claim 1, wherein the tensile strength of the steel plate is 980 MPa or more.

7. A press forming method for a high-strength steel plate, comprising: performing deep drawing on a steel plate; and performing bulging on the steel plate, wherein the bulging is performed at a later forming state of the deep drawing and no bulging is performed at an earlier forming stage of the deep drawing, wherein the tensile strength of the steel plate is 980 MPa or more, wherein the steel plate contains retained austenite in the proportion by volume of 3% or more in a microstructure thereof, and wherein the steel plate contains bainitic ferrite as a parent phase thereof, wherein the temperature of the steel plate is set into the range of 100 to 350 C. while the steel plate is press-formed, wherein the forming velocity at the later forming stage while bulging is performed is lower than the forming velocity at the earlier forming stage in which no bulging is performed, wherein the forming velocity at the earlier forming stage is set to 10 mm/sec or more, and wherein the forming velocity at the later forming stage is set to 10 mm/sec or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a vertical sectional view illustrating a press mold for carrying out a press forming method according to the invention for a steel plate.

(2) FIG. 2 is a conceptual sectional view illustrating a press forming process in a press forming method of a first embodiment.

(3) FIG. 3 is a vertical sectional view illustrating a press-molded product obtained by forming in the press forming process in FIG. 1.

(4) FIG. 4 is a graph showing a relationship between the total forming height and the initial retained austenite proportion when the forming in each step in the press forming process in FIG. 1 is performed up to forming limit.

(5) FIG. 5 is a conceptual sectional view illustrating a press forming process in a press forming method of a second embodiment.

(6) FIG. 6 is a conceptual sectional view illustrating a press forming process in a press forming method of a third embodiment.

(7) FIGS. 7(a), 7(b) and 7(c) are sectional views illustrating a press forming process in a press forming method of a fourth embodiment.

(8) FIG. 8 is an external appearance perspective view illustrating an example of a press-formed member involved in deep drawing and bulging.

(9) FIGS. 9(a) and 9(b) are graphs showing results of a deep drawing test and a bulging test, respectively.

(10) FIG. 10 is a graph showing a relationship between the forming velocity and the forming ultimate height in the bulging test in FIG. 6(b).

(11) FIG. 11 is a graph showing results of a tensile test in which a pre-strain is given at a warm temperature.

MODE FOR CARRYING OUT INVENTION

(12) Hereinafter, embodiments of the invention will be described with reference to the drawings. FIG. 1 is a press mold for carrying out a press forming method according to the invention for a steel plate. This press mold is composed of a cylindrical punch 1 directed upward and having, in a head thereof, a circular concave 1a; a die 2 directed downward in which the cylindrical punch 1 is to be inserted and advanced; a crease pressing plate 3 for pushing and pressing a flange of a blank B onto the die 2; and a spherical-head punch 4 directed downward to be directed to the concave 1a in the cylindrical punch 1. About the cylindrical punch 1, the diameter is set to 50 mm, and each of the shoulder radius thereof and the shoulder radius of the concave 1a to 5 mm. About the die 2, the diameter is set to 54 mm, and the shoulder radius thereof to 7 mm. About the spherical-head punch 4, the diameter is set to 10 mm.

(13) FIG. 2 illustrates a press forming process for carrying out a press forming method of a first embodiment. This press forming process is composed of a first step of performing warm deep drawing and a second step of performing cold bulging. In the first step, the temperature of the cylindrical punch 1, the die 2 and the crease pressing plate 3 is raised to a predetermined temperature, and further the temperature of the blank B brought into contact with these press mold parts is also raised. Thereafter, the cylindrical punch 1 is inserted and advanced into the die 2 to subject the blank to warm deep drawing. About the blank B, the temperature thereof may be raised to a predetermined temperature, using a furnace or some other. In the second step, the cylindrical punch 1, the die 2, the crease pressing plate 3, and a cup-shaped semi-formed product obtained by the deep drawing are cooled to room temperature. Subsequently, the spherical-head punch 4, the temperature of which is beforehand adjusted to room temperature, is inserted and advanced into the circular concave 1a in the cylindrical punch 1 to subject the bottom of the cup-shaped semi-formed product to cold bulging into a concave form.

(14) FIG. 3 is a press-formed product of a steel plate that has been formed as described above. This press-formed product has, in its body, a deep-drawn bottom, and has a concave-form bulged region A obtained by bulging. About the dimension of the press-formed product, the inside diameter D thereof is set to 50 mm, and the deep drawing forming height Hd thereof to 30 mm. The bulging height Hs thereof is made variable.

WORKING EXAMPLE 1

(15) Steel plates of four species in total were prepared, two of which were TBF steel plates, and the other two of which were DP steel plates. Table 1 shows chemical components of each of these steel plates; and Table 2 mechanical properties and microstructures thereof. The mechanical properties were gained by tensile tests using JIS No. 13B test pieces, and the proportion by volume of retained austenite in the (entire) microstructures was gained by an X-ray diffraction method. The steel plates were each a 980-MPa-class high tensile cold-drawn steel plate having a plate thickness of 1.4 mm. Each of the steel plates 1 and 2 was higher in total elongation and uniform elongation, and better in balance between strength and ductility than each of the DP steel plates 1 and 2. About the respective proportions by volume of retained austenite in the BF steel plates 1 and 2, and the DP steel plates 1 and 2, their value was decreased in this order. The proportions were each 3% or more by volume except the DP steel plate 2.

(16) TABLE-US-00001 TABLE 1 Chemical components (% by mass) Steel plate C Si Mn TBF steel plate 1 0.15 1.40 2.1 TBF steel plate 2 0.17 1.75 2.4 DP steel plate 1 0.17 1.3 1.9 DP steel plate 2 0.09 1.5 2.1

(17) TABLE-US-00002 TABLE 2 Mechanical properties (tension speed: 10 mm/min at room temperature) Microstructures Yield Breaking Total Uniform (percentage by volume) High tensile strength strength elongation elongation Bainitic Retained Polygonal steel plate (Mpa) (Mpa) (%) (%) Martensite ferrite austenite ferrite TBF steel plate 1 600 1010 15.2 10.3 9 81 10 0 TBF steel plate 2 623 1007 19.5 13.5 0 87 8 5 DP steel plate 1 660 1045 13.5 7.5 37 0 3 60 Dp steel plate 2 680 1020 14.0 7.2 53 0 2 45

(18) First, through the press forming process illustrated in FIG. 2, blanks from each of the TBF steel plate 1 and the DP steel plate 1 were each formed into a press-formed product as illustrated in FIG. 3. The diameter of each of the blanks was set to 103 mm. In each of the first and second steps, the forming velocity was set to 70 mm/sec. About the TBF steel plate 1, the deep drawing height Hd in the first step was set to 30 mm, and the bulging height Hs in the second step to 8 mm. About the DP steel plate 1, the deep drawing height Hd was set to 28 mm, and the bulging height Hs to 7 mm. In Examples, press forming was performed as follows (Examples A to C): the steel plate temperature of some of the blanks of each of the steel plates was varied in the range of 100 to 250 C. at each of their die contact region and their punch contact region in the first step; and in the second step, the steel plate temperature of these blanks was set to 40 C. at the die contact region, and to 25 C. at the punch contact region. In Comparative Examples, the steel plate temperature of any one of the blanks of each of the steel plates was set to 25 C. at each of the die contact region and the punch contact region so as to perform cold press forming from start to finish (Comparative Example A); and in the first step, the steel plate temperature of any one of these blanks was set to 200 C. at each of the die contact region and the punch contact region, and in the second step, the steel plate temperature of the blank was set to 350 C. at each of the die contact region and the punch contact region so as to perform warm press forming from start to finish (Comparative Example B). The pressing force of the crease pressing plate 3 onto the die 2 was set to 12 tonf in the first step, and to 20 tonf in the second step.

(19) Tables 3(a) and 3(b) show, about each of the TBF steel plate 1 and the DP steel plate 1, press forming results of Examples and Comparative Examples. In each of the steel plates, good press forming results were obtained in each of Examples A to C. By contrast, about the two-species steels of Comparative Example A, the blanks were each cracked in the first step. Thus, the second step was unable to be performed. About the two-species steels of Comparative Example B, the blanks were able to be formed in the first step. However, in the second, the blanks were cracked. The TBF steel plate, which was good in balance between strength and ductility, was higher than the DP steel plate in each of the deep drawing height Hd in Comparative Example A, in which the blanks were unable to be formed, and the bulging height Hs in Comparative Example B.

(20) TABLE-US-00003 TABLE 3(a) (First step) Hd = 30 mm (Second step) Hs = 8 mm Steel plate temperature ( C.) Steel plate temperature ( C.) Reduction of Die contact Punch contact Able or unable Die contact Punch contact Able or unable plate thickness region region to be formed region region to be formed of bulged region Example A 200 200 .fwdarw. 40 25 19% Example B 100 250 .fwdarw. 40 25 Example C 250 100 .fwdarw. 40 25 Comparative 25 25 X: Hd = 22 .fwdarw. 25 25 Unable Example A Comparative 200 200 .fwdarw. 350 350 X: Hs = 6 Example B

(21) TABLE-US-00004 TABLE 3(b) (First step) Hd = 28 mm (Second step) Hs = 7 mm Steel plate temperature ( C.) Steel plate temperature ( C.) Die contact Punch contact Able or unable Die contact Punch contact Able or unable region region to be formed region region to be formed Example A 200 200 .fwdarw. 40 25 Example B 100 250 .fwdarw. 40 25 Example C 250 100 .fwdarw. 40 25 Comparative 25 25 X: Hd = 20 .fwdarw. 25 25 Unable Example A Comparative 200 200 .fwdarw. 350 350 X: Hs = 5 Example B

(22) Next, a blank having a diameter of 103 mm and sampled from each of the TBF steel plates 1 and 2 and the DP steel plates 1 and 2 was used, and subjected to press forming under conditions that the steel plate temperature in deep drawing in the (same) first step (as described above) was set to 200 C. and that in bulging in the (same) second step (as described above) to 25 C., so as to form this blank up to forming limit about the deep drawing height Hd and the bulging height Hs in the respective steps. The bulging height Hs in the second step was set to 8 mm as a maximum value. The pressing force of the crease pressing plate 3 onto the die 2 was set to 12 tonf in the first step, and to 20 tonf in the second step.

(23) Results of the press forming are shown in Table 4. Table 4 also shows the respective maximum forming loads (of the blanks) in the first step and the respective proportions of retained austenite after the first step. The TBF steel plate 1, in which the initial proportion by volume of retained austenite was the largest, exceeded the deep drawing limit to undergo deep drawing pierce. Furthermore, the bulging height Hs in the second step was also the maximum value, which was 8 mm. About the TBF steel plate 2, in which the proportion by volume of retained austenite was the second largest, the deep drawing height Hd in the first step was 30 mm, and the bulging height Hs in the second step reached to 8 mm as the maximum value. By contrast, the deep drawing height Hd of each of the DP steel plates 1 and 2 was lower than that of the TBF steel plate 2, and the bulging height Hs thereof did not reach to 8 mm as the maximum value, either. The maximum forming load (of the steels) in the first step was decreased as the initial proportion by volume of retained austenite was increased. The TBF steel plate 1 was the lowest maximum forming load. The proportion by volume of retained austenite after the first step was also increased as the initial proportion by volume of retained austenite was increased.

(24) TABLE-US-00005 TABLE 4 First step Proportion Second step Ultimate (% by volume) Ultimate forming Maximum of retained forming Steel height Hd forming load austenite height Hs plate (mm) (KN) after the step (mm) TBF steel Deep 115 9.7 8 plate 1 drawing pierce TBF steel 30 140 5.4 8 plate 2 DP steel 28 155 2.5 7 plate 1 DP steel 26 160 1.7 6 plate 2

(25) FIG. 4 is a graph obtained by plotting the total forming height Hd+Hs of the deep drawing height Hd in the first step and the bulging height Hs in the second step, which are each shown in Table 4, relatively to the initial proportion by volume of retained austenite. A standard forming height shown in the graph is the total forming height Hd+Hs (26+8=34 mm) when a 590-MP-class high tensile steel plate (total elongation: 25%) was cold press-formed in both of the first and second steps. From this graph, the following are understood: the total forming height Hd+Hs in the first and second steps becomes higher as the initial proportion by volume of retained austenite is larger; and in the case of the press forming when the initial proportion by volume of retained austenite turns into 3% or more by volume, the forming limit is made better than in the case where the 590-MP-class high tensile steel plate, which is far lower in strength, is cold press-formed.

(26) FIG. 5 illustrates a press forming process for carrying out a press forming method of a second embodiment. This press forming process is composed of a first step of performing cold bulging and a second step of performing warm deep drawing. A press machine and a press mold used therein were the same as in the first embodiment. According to this embodiment, in the first step, the temperature of the cylindrical punch 1, the die 2, the crease pressing plate 3 and the spherical-head punch 4 is set to room temperature, and at the center of a blank B sandwiched between the die 2 and the crease pressing plate 3, the spherical-head punch 4 is inserted and advanced into the circular concave la in the cylindrical punch 1 to bulge the blank. In the second step, the temperature of the cylindrical punch 1, the die 2, the crease pressing plate 3 and the spherical-head punch 4 is raised to a predetermined temperature, and further the temperature of the blank B brought into contact with these press mold parts is also raised. Thereafter, the cylindrical punch 1 is inserted and advanced into the die 2 to bulge the blank.

WORKING EXAMPLE 2

(27) First, through the press forming process illustrated in FIG. 5, blanks from each of the TBF steel plate 1 and the DP steel plate 1 shown in Tables 1 and 2 were each formed into a press-formed product as illustrated in FIG. 3. The diameter of each of the blanks was set to 103 mm. In each of the steps, the forming velocity was set to 70 mm/sec. About the TBF steel plate 1, the bulging height Hs in the first step was set to 8 mm, and the deep drawing height Hd in the second step to 30 mm. About the DP steel plate 1, the bulging height Hs was set to 7 mm, and the deep drawing height Hd to 28 mm. In Examples, press forming was performed as follows (Examples D to F): in the first step, the steel plate temperature of some of the blanks of each of the steel plates was set to 25 C. at each of their die contact region and their punch contact region; and in the second step, the steel plate temperature of these blanks was varied in the range of 100 to 250 C. at each of the die contact region and the punch contact region. In Comparative Examples, in each of the first and second steps, the steel plate temperature of any one of the blanks of each of the steel plates was set to 25 C. at each of its die contact region and its punch contact region so as to perform cold press forming from start to finish (Comparative Example C); and in the first step, the steel plate temperature of any one of these blanks was set to 350 C. at each of the die contact region and the punch contact region, and in the second step, the steel plate temperature of the blank was set to 200 C. at each of the die contact region and the punch contact region so as to perform warm press forming from start to finish (Comparative Example D). In each of these cases, the pressing force of the crease pressing plate 3 onto the die 2 was set to 12 tonf in the first step, and to 20 tonf in the second step.

(28) Tables 5(a) and 5(b) show, about each of the steel plates, press forming results of Examples and Comparative Examples. In each of the TBF steel plate 1 and the DP steel plate 1, good press forming results were obtained in each of Examples D to F. By contrast, about the two-species steels of Comparative Example C, the blanks were able to be formed in the first step. However, in the second, the blanks were cracked. About the two-species steels of Comparative Example D, the blanks were cracked in the first step. Thus, the second step was unable to be performed. The TBF steel plate 1, which was good in balance between strength and ductility, was higher than the DP steel plate 1 in each of the deep drawing height Hd in Comparative Example C, in which the blanks were unable to be formed, and the bulging height Hs in Comparative Example D.

(29) TABLE-US-00006 TABLE 5(a) (First step) Hs = 8 mm (Second step) Hd = 30 mm Steel plate temperature ( C.) Steel plate temperature ( C.) Reduction of Die contact Punch contact Able or unable Die contact Punch contact Able or unable plate thickness region region to be formed region region to be formed of bulged region Example D 25 25 .fwdarw. 200 200 24% Example E 25 25 .fwdarw. 100 250 Example F 25 25 .fwdarw. 250 100 Comparative 25 25 .fwdarw. 25 25 X: Hd = 21 Example C Comparative 350 350 X: Hs = 6 .fwdarw. 200 200 Unable Example D

(30) TABLE-US-00007 TABLE 5(b) (First step) Hs = 7 mm (Second step) Hd = 28 mm Steel plate temperature ( C.) Steel plate temperature ( C.) Die contact Punch contact Able or unable Die contact Punch contact Able or unable region region to be formed region region to be formed Example D 25 25 .fwdarw. 200 200 Example E 25 25 .fwdarw. 100 250 Example F 25 25 .fwdarw. 250 100 Comparative 25 25 .fwdarw. 25 25 X: Hd = 20 Example C Comparative 350 350 X: Hs = 5 .fwdarw. 200 200 Unable Example D

(31) According to the aforementioned press forming results of Example 1 and Example 2, the press forming method according to the invention, in which a deep drawing step is performed by warm working at 100 to 250 C., and a bulging step is performed by cold working at a temperature lower than 50 C., makes it possible to give good press forming results at a high forming velocity, which can ensure high productivity, even when a high tensile steel plate is used. Thus, the invention makes it possible to promote an increase in the strength of press-formed members and further enlarge a scope in which a high tensile steel plate is applicable to press-formed members.

(32) In Tables 3(a) and 5(a) showing the press forming results of the TBF steel plate 1, the following are also shown about Examples A and D, respectively, in which the steel plate temperature of each of the die contact region and the punch contact region was set to 200 C. in the warm deep drawing: results obtained by measuring the respective plate thickness reductions of the press-formed products at the center of their bulged region A. In Example A, in which the cold bulging step was performed after the warm deep drawing step, the steel plate thickness reduction of the bulged region A was about 5% smaller than that in Example D, in which the cold bulging step was performed before the warm deep drawing step. Thus, it can be expected that the forming limit is heightened. The measurement results of the steel plate thickness reductions correspond sufficiently to the tensile test results shown in FIG. 11. It appears that in Example A, the steel gains an elongation thereof by effect of only deformation of its parent phase in the deep drawing in the first step, and makes good use of plasticity induced transformation of retained austenite that has not been used, so that the steel can realize a high ductility.

(33) FIG. 6 illustrates a press forming process for carrying out a press forming method of a third embodiment. In this press forming process, a first step of performing warm deep drawing and a second step of performing cold bulging are performed through the same press stroke. A press machine and a press mold used therein were the same as in the first embodiment. However, a spherical-head punch 4 for performing the bulging is a punch having, in the top thereof, a coolant jetting-out outlet 4a. The coolant may be air, water, oil or the like.

(34) In this embodiment, the temperature of the cylindrical punch 1, the die 2 and the crease pressing plate 3 is raised while the temperature of a blank B brought into contact with these press mold parts is also raised. Thereafter, at an earlier stage of a pre-stroke as the first step, the cylindrical punch 1 is inserted and advanced into the die 2 to subject the blank to warm deep drawing at a temperature ranging from 100 to 250 C.; and at the late stage of the pre-stroke as the second step, the coolant is jetted out from the jetting-out outlet 4a to cool the bottom of a cup-shaped semi-formed product obtained by the deep drawing. This bottom is subjected to cold bulging into a concave form at a temperature lower than 50 C. The coolant for cooling the bottom of the cup-shaped semi-formed product may be jetted out from the cylindrical punch 1.

(35) FIG. 7 illustrate a process of using the press mold to press-form the blank B. As illustrated in FIG. 7(a), first, the cylindrical punch 1 is inserted and advanced into the die 2, so that the material of a flange region of the blank flows into the die 2 to start deep forming. The resultant deep drawing height increases as the forming is advanced, so that the spherical-head punch 4 is brought into contact with the material of the head of the spherical-head punch 4 as illustrated in FIG. 7(b). When the forming is further advanced, the deep drawing height further increases and additionally the material of the head of the cylindrical punch 1 is bulged into the circular concave la in the cylindrical punch 1 by effect of the spherical-head punch 4, as illustrated in FIG. 7(c).

WORKING EXAMPLE 3

(36) Blanks from each of the TBF steel plate 2 and the DP steel plates 1 and 2, which were of three species in total, shown in Tables 1 and 2 were each set into the press mold illustrated in FIG. 1 to form a press-formed product illustrated in FIG. 3. The diameter of each of the blanks was set to 103 mm. The bulging height Hs was set to 8 mm. At the time of the press forming, the steel plate temperature 0 of the blanks was varied from room temperature to 350 C. in the press forming. The steel plate temperature 0 in the press forming was certainly kept by bringing the blanks into contact, for a predetermined period, with the press mold the temperature of which was raised to predetermined individual temperatures. The temperature of the blanks may be raised to the predetermined temperatures, using a furnace or the like in advance. At the earlier forming stage (S=0 to 22 mm) in which only the deep drawing was performed, the forming velocity V1 of the blanks was set to a high velocity of 70 mm/sec, which was regarded as an actual forming velocity in actual press factories, and the forming velocity V2 of the blank at the late forming stage (S=22 to 30 mm) was varied from 0.1 to 70 mm/sec. About some of the blanks, the forming velocity V1 at the earlier forming stage was also varied. The pressing force of the crease pressing plate 3 onto the die 2 was set to 12 tonf in the course from FIG. 7(a) to FIG. 7(b) in the first step, and to 20 tonf in the course from FIG. 7(b) to FIG. 7(c).

(37) Tables 6(a), 6(b) and 6(c) show, about each of the TBF steel plate 2 and the DP steel plates 1 and 2, press forming results when the steel plate temperature 0 was set to 200 C. About the DP steel plate 1, in which the proportion by volume of retained austenite is 3% by volume, the steel plate becomes able to be formed when the forming velocity V2 at the late forming stage is set to 2.5 mm/sec or less. About the DP steel plate 2, in which the proportion by volume of retained austenite is 2% by volume, the steel plate becomes able to be formed when the forming velocity V2 at the late forming stage is set to an extremely low value of 0.1 mm/sec. By contrast, about the TBF steel plate 2, in which the proportion by volume of retained austenite is 8% by volume and the balance between the strength and the ductility thereof is better, the steel plate becomes able to be formed when the forming velocity V2 at the late forming stage is set to 10 mm/sec or less. In each of the cases of making the forming velocity V2 higher than these ultimate velocities, the bulged region A was cracked so that the blank was unable to be formed. Accordingly, it can be expected that at the late forming stage any steel plate in which the proportion by volume of retained austenite is 3% or more by volume can be bulged at a velocity permitting formed products not to be lowered very much in productivity.

(38) TABLE-US-00008 TABLE 6(a) Steel plate Forming Forming temperature velocity V1 velocity V2 Able or unable ( C.) (mm/sec) (mm/sec) to be formed 200 70 70 X 200 70 15 X 200 70 12 X 200 70 10 200 70 5 200 70 2.5 200 70 0.1 200 15 10 200 12 10

(39) TABLE-US-00009 TABLE 6(b) Steel plate Forming Forming temperature velocity V1 velocity V2 Able or unable ( C.) (mm/sec) (mm/sec) to be formed 200 70 70 X 200 70 15 X 200 70 12 X 200 70 10 X 200 70 5 X 200 70 2.5 200 70 0.1 200 15 2.5 200 12 2.5

(40) TABLE-US-00010 TABLE 6(c) Steel plate Forming Forming temperature velocity V1 velocity V2 Able or unable ( C.) (mm/sec) (mm/sec) to be formed 200 70 70 X 200 70 15 X 200 70 12 X 200 70 10 X 200 70 5 X 200 70 2.5 X 200 70 0.1 200 15 0.1 200 12 0.1

(41) Tables 7(a) and 7(b) show, about each of the TBF steel plate 2 and the DP steel plate 1, press forming results when the steel plate temperature 0 of the blanks thereof was varied. The forming velocity V1 at the earlier forming stage and the forming velocity V2 at the late forming stage were set as follows: V1=70 mm/sec, and V2=10 mm/sec about the TBF steel plate 1; and V1=70 mm/sec, and V2=2.5 mm/sec about the DP steel plate 1. In each Comparative Example, V1 and V2 were each set to 70 mm to make the forming velocity high in the whole of the forming stages, and results of this press forming are also shown.

(42) TABLE-US-00011 TABLE 7(a) Steel plate Forming Forming Able or temperature velocity V1 velocity V2 unable to ( C.) (mm/sec) (mm/sec) be formed Room 70 10 X Comparative temperature Example 100 70 10 Example 150 70 10 Example 200 70 10 Example 250 70 10 Example 300 70 10 Example 350 70 10 Example Room 70 70 X Comparative temperature Example 100 70 70 X Comparative Example 150 70 70 X Comparative Example 200 70 70 X Comparative Example 250 70 70 X Comparative Example 300 70 70 X Comparative Example 350 70 70 X Comparative Example

(43) TABLE-US-00012 TABLE 7(b) Steel plate Forming Forming Able or temperature velocity V1 velocity V2 unable to ( C.) (mm/sec) (mm/sec) be formed Room 70 2.5 X Comparative temperature Example 100 70 2.5 Example 150 70 2.5 Example 200 70 2.5 Example 250 70 2.5 Example 300 70 2.5 Example 350 70 2.5 Example Room 70 70 X Comparative temperature Example 100 70 70 X Comparative Example 150 70 70 X Comparative Example 200 70 70 X Comparative Example 250 70 70 X Comparative Example 300 70 70 X Comparative Example 350 70 70 X Comparative Example

(44) According to these press forming results, about the TBF steel plate 2 and the DP steel plate 1, the blanks of Examples, in which the steel plate temperature 0 was set into the range of 100 to 350 C. and the forming velocity V2 was set to low velocities of 2.5 mm/s and 10 mm/s, respectively, were each able to be formed. About the blanks of Comparative Examples, in which the forming velocity was made high (70 mm/sec) at the whole of the forming stages, their bulged region A was cracked even when the steel plate temperature 0 was set into the range of 100 to 350 C. Thus, these blanks were unable to be formed.

(45) According to the aforementioned press forming results, the following can be attained by the steel plate forming method according to the invention, in which while a steel plate is press-formed, the steel plate temperature is set into the range of 100 to 300 C. and the forming velocity at a late forming stage for performing bulging is made lower than the forming velocity at an earlier forming stage in which no bulging is performed: the forming limit of a press-formed product involved in deep drawing and bulging, which is not easily obtained by any forming, is made remarkably high. Thus, the invention makes it possible to promote an increase in the strength of press-formed members and further enlarge a scope in which a high tensile steel plate is applicable to press-formed members.

(46) Tables 8(a) and 8(b) show, about each of the TBF steel plate 2 and the DP steel plate 1, results of an examination in which, at the press forming time, the steel plate temperature 1 of respective flange regions (of the blanks of the steel plate) and the steel plate temperature 2 of respective bulged regions A thereof were separately varied to examine as to whether or not the blanks were able to be press-formed, and respective plate thickness reductions of the bulged regions A. A combination of the forming velocity V1 at the earlier forming stage with the forming velocity V2 at the late forming stage was set as follows: V1=70 mm/sec, and V2=10 mm/sec about the TBF steel plate 2; and V1=70 mm/sec, and V2=2.5 mm/sec about the DP steel plate 1. A combination of the steel plate temperature 1 of the flange regions with that 2 of the bulged regions A is set into two species, one of which was a species of making the steel plate temperature 1 constant, 200 C., and varying the steel plate temperature 2 in the range of 100 to 400 C., and the other of which was a species of making the steel plate temperature 2 constant, 350 C., and varying the steel plate temperature 1 in the range of 100 to 400 C. In each Comparative Example, an examination was made in the state that the two steel plate temperatures 1 and 2 were each set to room temperature. Results thereof are also shown.

(47) TABLE-US-00013 TABLE 8(a) Forming Steel plate Steel plate Reduc- velocity temper- temper- Able or tion (%) V1.fwdarw.V2 ature 1 ature 2 unable to of plate (mm/sec) ( C.) ( C.) be formed thickness 70.fwdarw.10 Room Room X Comparative temper- temper- Example ature ature 70.fwdarw.10 200 100 18 Example 70.fwdarw.10 200 150 18 Example 70.fwdarw.10 200 200 17 Example 70.fwdarw.10 200 250 16 Example 70.fwdarw.10 200 300 13 Example 70.fwdarw.10 200 350 12 Example 70.fwdarw.10 200 400 X Comparative Example 70.fwdarw.10 100 350 14 Example 70.fwdarw.10 150 350 13 Example 70.fwdarw.10 250 350 14 Example 70.fwdarw.10 300 350 16 Example 70.fwdarw.10 350 350 17 Example 70.fwdarw.10 400 350 X Comparative Example 70.fwdarw.10 400 400 X Comparative Example

(48) TABLE-US-00014 TABLE 8(b) Forming Steel plate Steel plate Reduc- velocity temper- temper- Able or tion (%) V1.fwdarw.V2 ature 1 ature 2 unable to of plate (mm/sec) ( C.) ( C.) be formed thickness 70.fwdarw.2.5 Room Room X Comparative temper- temper- Example ature ature 70.fwdarw.2.5 200 100 20 Example 70.fwdarw.2.5 200 150 20 Example 70.fwdarw.2.5 200 200 19 Example 70.fwdarw.2.5 200 250 18 Example 70.fwdarw.2.5 200 300 15 Example 70.fwdarw.2.5 200 350 14 Example 70.fwdarw.2.5 200 400 X Comparative Example 70.fwdarw.2.5 100 350 17 Example 70.fwdarw.2.5 150 350 15 Example 70.fwdarw.2.5 250 350 16 Example 70.fwdarw.2.5 300 350 18 Example 70.fwdarw.2.5 350 350 20 Example 70.fwdarw.2.5 400 350 X Comparative Example

(49) According to the examination results shown in Tables 8(a) and 8(b), about each of the TBF steel plate 2 and the DP steel plate 1, the blanks of Examples, in which the steel plate temperatures 1 and 2 were combined with each other in the range of 150 to 300 C., are able to be formed. The steel plate thickness reduction of their bulged region A is as follows: the reduction of the TBF steel plate 2, which is good in balance between strength and ductility, is smaller than that of the DP steel plate 1. In particular, in the cases of setting the steel plate temperature 1 of the flange regions and the steel plate temperature 2 of the bulged regions A to 200 C. and 350 C., respectively, the steel plate thickness reduction is the smallest, i.e., 12% about the TBF steel plate and 14% about the DP steel plate 1. Thus, these temperature conditions can be expected to be optimal temperature conditions capable of improving the forming limit of a press-formed product that is more difficult to obtain. It can be presumed that the reason why the blanks of the Comparative Examples in which either one of the steel plate temperatures 1 and 2 was set to 400 C. were unable to be formed is that retained austenite was decomposed at 400 C. so that the TRIP effect was restrained from being produced so that the blanks were lowered in ductility.

(50) In each of the above-mentioned embodiments, the deep drawing step and the bulging step are each performed once. However, the press forming method according to the invention may be a method in which either one of these steps is performed two or more times, or a method including any other step, such as a stretch flanging step, a bending step, and/or a punching step. About the punching-step-including method, it can be expected that a load for the punching is decreased by performing the punching step simultaneously with a warm deep drawing step.

(51) In the above-mentioned working examples, the steel plates were rendered 980-MPa-class TBF steel plates and DP steel plates. However, a target of the application of the steel-plate-press-forming method according to the invention is not limited to such 980-MPa-class DP steel plates or TBF steel plates. The method may be applied to a steel plate of any steel type and in any strength-class, an example thereof being a mild steel plate.

(52) In the working examples, the earlier forming stage, in which only deep drawing is performed, and the late forming stage, in which bulging is performed, are attained through the same press forming process. However, the earlier forming stage and the late forming stage may be attained through different separated press forming processes.

REFERENCE SIGNS LIST

(53) A: bulged region,

(54) B: blank,

(55) 1: cylindrical punch,

(56) 1a: concave,

(57) 2: die,

(58) 3: crease pressing plate,

(59) 4: spherical-head punch, and

(60) 4a: coolant jetting-out outlet