Title of Invention: Method of Producing Shaped Steel Changing in Cross-Sectional Shape in Longitudinal 5 Direction and Roll Forming Apparatus for Same Technical Field [0001] The present invention relates to a method and apparatus for roll forming for producing a shaped steel 10 which varies in cross-sectional shape in the longitudinal direction. Background Art [0002] As a method of producing a hat-shaped steel, which is one type of shaped steel, press forming using a 15 punch and die is widely known. In bending into a hat shape by press forming, the problem of springback, that is, the sheet material trying to return to its original state due to the reaction force when the press pressure is removed, easily arises, and therefore in the past, 20 countermeasures for suppressing springback have been studied. [0003] In this regard, in recent years, application of high tensile steel has been increasing. As one example, in the automobile industry, it is believed that reduction 25 of the weight of the vehicle body will lead to reduction of the amount of emission of CO2 and therefore high tensile steel is being proactively used for the vehicle body material. For this reason, on the production floor of shaped steels, the problem of the springback due to 30 the high strength characteristics of steel materials has been surfacing. Furthermore, in recent years, high tensile steel which has an over 980 MPa tensile strength has also been being produced. With general press forming, it is difficult to produce a hat-shaped steel as designed 35 from such high tensile steel. [0004] As another method of producing a shaped steel, the roll forming method is known. Roll forming is, for - 2 - example, a continuous bending process which runs a strip, which is taken out from a coil, through roll units provided at a plurality of successively arranged stations. Roll forming is, in particular, suitable for 5 forming H-beams, L-beams, and other steel products and pipes and other long products with constant crosssectional shapes in the longitudinal direction. On the other hand, roll forming, unlike press forming (drawing), is not .suited for forming a shaped steel which varies in 10 cross-sectional shape in the longitudinal direction. [0005] PLTs 1 to 3 disclose the art of roll forming to produce a shaped steel which varies in cross-sectional shape in the longitudinal direction by variable control of the roll widths of split rolls. However, the roll 15 forming process and apparatus disclosed in PLTs 1 to 3 have the problem of a complicated structure and method of control of the apparatus. For this reason, it is difficult to convert existing facilities for use for working the inventions of PLTs 1 to 3. Introduction of 20 new facilities is necessary, and therefore the cost becomes high. [0006] Further, if, as in the inventions of PLTs 1 and 3, broadening the roll widths of the split rolls during roll forming, there are the problems that only the corner 25 parts at the front sides of the rolls will linearly contact the steel sheet material and, in high tensile steel or other materials, springback will occur unevenly in the longitudinal direction and the material will be distorted etc. in the longitudinal direction. 30 Citations List Patent Literature [0007] PLT 1: Japanese Patent Publication No. H10- 314848 A 35 PLT 2: Japanese Patent Publication No. H7-88560 A PLT 3: Japanese Patent Publication No. 2009-500180A - 3 - Summary of Invention Technical Problem [0008] The present invention was made to solve the above problem and has as its object to provide art which 5 enables production of a shaped steel which varies in cross-sectional shape in the longitudinal direction by simple roll forming without the need for complicated control and apparatuses such as in the prior art. [0009] Further, another object of the present 10 invention is to provide art which for example enables elimination of uneven springback in the longitudinal direction and enables suppression of buckling of the flange parts when producing a shaped steel, which varies in cross-sectional shape in the longitudinal direction, 15 by roll forming. Solution to Problem [0010] To solve the above-mentioned problem, according to the present invention, there is provided a method of 20 producing a shaped steel which varies in cross-sectional shape in the longitudinal direction from a sheet by roll forming, comprising: a step of preparing a first rolling die which has a rotation shaft and an annular ridge part which varies in cross-sectional shape in a 25 circumferential direction which is centered about the rotation shaft; a step of arranging the first rolling die so that the rotation shaft of the first rolling die becomes perpendicular to a sheet feed direction; a step of preparing a second rolling die which has a rotation 30 shaft and an annular groove part which varies in crosssectional 'shape"in a circumferential direction which is centered about the^ rotation shaft; a step of arranging the second rolling die so that a gap which is equal to a thickness of the sheet is formed between the first 35 rolling die and second rolling die and the annular ridge part of the first rolling die and the annular groove part of the second rolling die engage; a step of making the _ 4 - first rolling die and the second rolling die rotate synchronized; and a step of feeding a sheet between the first rolling die and second rolling die, wherein the side surfaces of the annular ridge part of the first 5 rolling die are provided with relief so that the gap with respect to side surfaces of the annular groove part of the second rolling die broadens over at least part of the circumferential direction and inward in the radial direction of the first rolling die, wherein the annular 10 ridge part of the first rolling die is configured so that the relative angle between the ridgeline and the rotation direction of the first rolling die varies at least partially in the circumferential direction, and wherein the relief amount at the relief is set to vary in 15 accordance with the relative angle between the ridgeline of the annular ridge part of the first rolling die and the rotation direction of the first rolling die. [0011] Furthermore, the present invention has as its gist a roll forming apparatus for roll forming use for 20 producing a shaped steel which varies in cross-sectional shape in the longitudinal direction from a sheet, comprising: a first rolling die which has a rotation shaft and an annular ridge part which varies in crosssectional shape in a circumferential direction which is 25 centered about the rotation shaft, the first rolling die arranged so that the shaft of the first rolling die becomes perpendicular to a sheet feed direction; a second rolling die which has a rotation shaft and an annular groove part which varies in cross-sectional shape in a 30 circumferential direction which is centered about the rotation -shaft, -the second rolling die arranged so that the rotation shaft of the second rolling die becomes parallel to the rotation shaft of the first rolling die; and a drive device which synchronizes and rotationally 35 drives the first rolling die and the second rolling die, the first rolling die and second rolling die being arranged relatively so that a gap which is equal to a - 5 - thickness of the sheet is formed between the two and the annular ridge part of the first rolling die and the annular groove part of the second rolling die engage, wherein the side surfaces of the annular ridge part of 5 the first rolling die are provided with relief so that the gap with respect to side surfaces of the annular groove part of the second rolling die broadens over at least part of the circumferential direction and inward in the radial direction of the first rolling die, wherein 10 the annular ridge part of the first rolling die is configured so that the relative angle between the ridgeline and the rotation direction of the first rolling die varies at least partially in the circumferential direction, and wherein the relief amount at the relief is 15 set to vary in accordance with the relative angle between the ridgeline of the annular ridge part of the first rolling die and the rotation direction of the first rolling die. 20 Advantageous Effects of Invention [0012] According to the present invention, by using a first rolling die having an annular ridge part which varies in cross-sectional shape in the circumferential direction and a second rolling die having an annular 25 groove part which receives the annular ridge part of the first rolling die while maintaining a gap with the annular ridge part of the amount of thickness of the shaped steel, by simple control for making at least the first and second rolling dies rotate synchronized, a 30 shaped steel with a cross-sectional shape which varies in the longitudinal direction can be produced. Accordingly, complicated control such as variable control of the roll widths of split rolls for broadening the width of the cross-section becomes unnecessary. Further, it is 35 possible to realize the rolling forming apparatus of the present invention by changing the rolls of existing roll forming apparatuses to the first and second rolling dies. - 6 - [0013] Further, when using a first rolling die having an annular ridge part which varies in cross-sectional shape in the circumferential direction and a second rolling die having an annular groove part which receives 5 the annular ridge part of the first rolling die while maintaining a gap with the annular ridge part of the amount of thickness of the shaped steel, sometimes interference will occur between the rolling dies. According to the present invention, it is possible to 10 prevent such interference by providing relief which varies in relief amount in accordance with a relative angle with a rotation direction of the rolling dies. [0014] In addition, by using the first and second rolling dies which have the above-mentioned roll barrel 15 parts, even if the cross-sectional shape varies in the longitudinal direction, shaping is possible in the state with a constant gap between the two rolling dies, and therefore it is possible to eliminate the uneven occurrence of springback in the longitudinal direction, 20 for example, due to an uneven gap, and possible to suppress buckling of the flange parts. Brief Description of Drawings [0015] [FIG. 1A] FIG. 1A is a perspective view of a 25 hat-shaped steel which varies in cross-sectional shape in the longitudinal direction, as seen from above. [FIG. IB] FIG. IB is a perspective view of a hat-shaped steel which varies in cross-sectional shape in the longitudinal direction, as seen from below. 30 [FIG. 2] FIG. 2 is a schematic perspective view of a multistage roll forming apparatus according to a first embodiment of the present invention. [FIG. 3] FIG. 3 is a vertical view of a roll unit of the multistage roll forming apparatus of FIG. 2. 35 [FIG. 4] FIG. 4 is a disassembled perspective view of a pair of top and bottom rolling dies of the roll unit of FIG. 3. - 7 - [FIG. 5A] FIG. 5A is a view showing a bending process at different stages of the multistage roll forming apparatus of FIG. 2 and a view.showing a step of forming flanges of a hat-shaped steel. 5 [FIG. 5B] FIG. 5B is a view showing a bending process at different stages of the multistage roll forming apparatus of FIG. 2 and a view showing a step of forming a top wall of a hat-shaped steel. [FIG. 6] FIG. 6 is a schematic perspective view for 10 explaining the action in one roll unit. [FIG. 7A] FIG. 7A is a perspective view of a hat-shaped steel which has a bead. [FIG. 7B] FIG. 7B is a perspective view of rolling dies which form the hat-shaped steel of FIG. 7A. 15 [FIG. 8] FIG. 8 shows rolling dies according to a second embodiment. [FIG. 9] FIG. 9 is a partial cross-sectional view of the rolling dies of FIG. 8. [FIG. 10] FIG. 10 is a chart which shows a minimum gap 20 when providing relief at the rolling dies. [FIG. 11] FIG. 11 is a partial cross-sectional view of rolling dies of a comparative example. [FIG. 12A] FIG. 12A is a perspective view which shows interference between a top roll and a bottom roll when 25 not providing relief and shows together a hat-shaped steel. [FIG. 12B] FIG. 12B is a perspective view which shows interference between a top roll and a bottom roll when not providing relief and shows together a hat-shaped 30 steel. [FIG. 13] FIG. 13 is a chart which shows the effect of the minimum gap on an amount of difference. [FIG. 14] FIG. 14 is a schematic partial cross-sectional view of rolling dies for explaining a reverse bending 35 phenomenon due to over run. [FIG. 15] FIG. 15 is a developed view of the outer circumferential surface of a bottom roll and a view which - 8 - shows a relationship with and the relief amount. [FIG. 16] FIG. 16 is a partially enlarged view of a bottom roll which shows a relief amount x, a side wall • angle 0 of a shaped steel, and a height H of an annular 5 ridge part. [FIG. 17] FIG. 17 is a partial vertical cross-sectional view of top and bottom rolls which is cut along a plane which includes the center axes of the top and bottom rolls. 10 [FIG. 18] FIG. 18 is a perspective view which shows another example of a multistage roll forming apparatus. [FIG. 19] FIG. 19 is a view which shows a bending process at different stages of the multistage roll forming apparatus of FIG. 18. 15 [FIG. 20] FIG. 20 is a view which shows a start point of relief provided at an annular ridge part of a bottom roll. [FIG. 21] FIG. 21 is a view which shows a relationship between L/H and a minimum gap. 20 [FIG. 22] FIG. 22 is a view which shows the relationship between L/H and an amount of difference from a target shape. [FIG. 23A] FIG. 23A is a perspective view of a shaped steel according to a third embodiment. 25 [FIG. 23B] FIG. 23B is a perspective view of rolling dies according to a third embodiment which is shown together with the shaped steel of FIG. 23A. [FIG. 24A] FIG. 24A is a perspective view of a shaped steel according to a fourth embodiment. 30 [FIG. 24B] FIG. 24B is a perspective view of rolling dies according to a fourth embodiment which is shown together with the shaped steel of FIG. 24A. [FIG. 25A] FIG. 25A is a perspective view of a shaped steel according to a fifth embodiment. 35 [FIG. 25B] FIG. 25B is a perspective view of rolling dies according to a fifth embodiment which is shown together with the shaped steel of FIG. 25A. - 9 - [FIG. 26A] FIG. 2 6A is a perspective view of a shaped steel according to a sixth embodiment. [FIG. 2 6B] FIG. 26B is a perspective view of rolling dies according to a sixth embodiment which is shown together 5 with the shaped steel of FIG. 26A. [FIG. 27A] FIG. 27A is a perspective view of a shaped steel according to a seventh embodiment. [FIG. 27B] FIG. 27B is a perspective view of rolling dies according to a seventh embodiment which is shown together 10 with the shaped steel of FIG. 27A. [FIG. 28A] FIG. 28A is a perspective view of a shaped steel according to an eighth embodiment. [FIG. 28B] FIG. 28B is a perspective view of rolling dies according to an eighth embodiment which is shown together 15 with the shaped steel of FIG. 28A. [FIG. 29A] FIG. 29A is a perspective view of a shaped steel according to a ninth embodiment. [FIG. 29B] FIG. 29B is a perspective view of rolling dies according to a ninth embodiment which is shown together 20 with the shaped steel of FIG. 29A. [FIG. 30A] FIG. 30A is a perspective view of a shaped steel according to a 10th embodiment. [FIG. 30B] FIG. 30B is a perspective view of of rolling dies according to a 10th embodiment which is shown 25 together with the shaped steel of FIG. 30A. [FIG. 31A] FIG. 31A is a perspective view of a shaped steel according to an 11th embodiment. [FIG. 3IB] FIG. 3IB is a perspective view of rolling dies according to an 11th embodiment which is shown together 30 with the shaped steel of FIG. 31A. Description of Embodiments [0016] Below, a method of production of a shaped steel which varies in cross-sectional shape in the longitudinal 35 direction and a roll forming apparatus for the same according to preferable embodiments of the present invention will be explained in detail, while referring to - 10 - the attached drawings. However, the embodiments explained below shall not cause the present invention to be interpreted limited in technical scope in any way. [0017] 5 First, the shaped steel produced in the present embodiment will be explained. The shaped steel which is shown in FIGS. 1A and IB is one example of a hat-shaped steel of a saddle shape which varies in cross-sectional shape in the longitudinal direction (for example, the 10 metal stock axis direction). FIG. 1A is a perspective view of the hat-shaped steel seen from the upper side, while FIG. IB is a perspective view seen from the lower side. The hat-shaped steel 1 comprises a top wall, side walls which extend along the two side edge parts of the 15 top wall, and flanges which extend along the edge parts at the opposite sides of the side walls, and has a crosssection vertical to the longitudinal direction of the hat-shaped steel 1 (lateral cross-section) which is substantially hat shaped. 20 [0018] The hat-shaped steel 1 further has ^portions 10a, 10b having top wall width of LI, a portion 11 having top wall width of L2 (>L1), and tapered transition portions 12a and 12b having expanding (or contracting) top wall width of Ll to L2. The hat-shaped steel 1 has 25 hat-shape horizontal cross-sections with side walls which flare outward at the portions 10a to 10b. The side walls may have gradient angles which differ at the portions 10a to 10b or which are the same at the portions 10a to 10b. Further, the thickness of the steel shape can, for 30 example, be set to various thicknesses according to the specifications, applications, etc. However, in the present embodiment, the different portions 10a to 10b are not individually shaped and joined by welding etc., but are integrally shaped from a single sheet or strip by 35 roll forming. Therefore, the boundary lines between portions of FIG. 1 are lines for convenience of explanation and are not join lines or bend lines. - 11 - [0019] Furthermore, the flanges 13 formed at the opening part of the bottom surface side along the longitudinal direction are also obtained by bending the sheet or strip by roll forming. Further, the corner parts 5 which formed by bending can, for example, have chamfered shapes or rounded shapes such as shown in FIG. 1. [0020] The type and strength of the material are not particularly limited. All metal materials which can be bent can be covered. As examples of the metal material, 10 there are carbon steel, alloy steel, nickel-chromium steel, nickel-chromium-molybdenum steel, chromium steel, chromium-molybdenum steel, manganese steel, and other steel materials. If based on strength, steel with tensile strengths of 340 MPa or less can be roughly classified as 15 general steel and steel with higher strengths can be roughly classified as high tensile steel, but in the present embodiment, either can be applied. Furthermore, high tensile steel includes steel of for example the 590 MPa grade or 780 MPa grade. Currently, steel of the 980 20 MPa grade or 1180 MPa grade called "ultra high tensile steel" are being produced. Regarding ultra high tensile steel, sometimes bending into hat shapes becomes difficult with conventional press forming (drawing), but with the roll forming of the present embodiment, 980 MPa 25 or more ultra high tensile steel can also be applied. Furthermore, as examples of materials other than steel materials, there are the poorly malleable materials including titanium, aluminum, or magnesium or their alloys. 30 [0021] Next, the roll forming apparatus for producing a steel - strap e which varies in cross-sectional shape in the longitudinal direction will be explained. FIG. 2 shows a multistage roll forming apparatus 2 for producing the above-mentioned hat-shaped steel as one embodiment of 35 a roll forming apparatus. The multistage roll forming apparatus 2 comprises, for example, a plurality of roll units 20a to 20k which are successively arranged in the - 12 - sheet or strip feed direction. Due to this, a long sheet or strip M is conveyed from the upstream side roll unit 20k to the downstream side roll unit 20a while bending it in stages to obtain the final target product shape. The 5 finally shaped sheet or strip M is successively cut into product units. [0022] The rolling dies of the roll unit 20a of the downstream-most station {final station) (below, sometimes referred to as the "finishing rolls") are shaped 10 corresponding to the target product shape. The rolling dies of the stations at the upstream side from the finishing rolls are designed so that intermediates which approach the final product shape in stages the further toward the downstream side are formed at the different 15 stages. FIG. 2 shows one example of the rolling dies which form a final product from a sheet or strip M in 10 stages. At each of the first station to the fifth station which perform the first half bending process, the roll units 20j to 20f have the dies which have the projecting 20 shape roll barrel parts at the top side and the dies which have the recessed shape roll barrel parts at the bottom side. [0023] On the other hand, at each of the sixth station to the 10th station which perform the second half bending 25 process, the roll units 20e to 20a have the dies which have the annular ridge parts at the bottom side and the dies which have the annular groove parts at the top side. Further, the entry station (roll unit 20k: 0th station) to fifth station (roll unit 20f) are the first half 30 process for forming the flanges 13 (flange bending) and the sixth-station (roll unit 20e) to the final station or the 10th station (roll unit 20a) are the second half process for forming the top wall of the hat-shaped steel 1 (top wall bending). 35 [0024] The roll unit 20k of the entry station has rolling dies having plain cylindrical shape arranged at both the top and bottom. Further, the roll units 20j to - 13 - 20f from the first station to the fifth station become gradually smaller in diameters in the directions toward the ends at both two end portions of the top rolls, while the two end portions of the roll barrel parts of the 5 bottom rolls become gradually larger in diameter in the directions toward the ends. Further, the inclination angles of the two end portions of the dies become sharper in order from the first station to the fifth station. At the roll unit 20f of the fifth station, the two ends of 10 the sheet or strip M are bent about 90°, whereupon the flanges 13 are formed. The dies have, in the circumferential direction, parts of narrow x^idths and wide widths and parts of tapers of increasing/decreasing width, at the centers of the 'roll barrel parts, so that 15 flanges 13 of the portions 10a to 10b of the shaped steel are formed. [0025] On the other hand, the roll units 20e to 20a from the sixth station to the final station have bottom rolls with annular ridge parts in which the center of the 20 roll barrel parts are raised in projecting shapes and have top rolls with annular groove parts in which the center of the roll barrel parts are sunk in recessed shapes. Further, more specifically, the annular ridge parts of the bottom rolls and the annular groove parts of 25 the top rolls comprises narrow width parts, wide width parts, and tapered parts with increasing width/decreasing width, arranged in the circumferential direction, so that the top walls of the portions 10a to 10b of the hatshaped steel 1 are formed. 30 [0026] The inclination angles of the side surfaces of the annular ridge parts and annular groove parts of the rolls become sharper in the order from the sixth station to the final station. At the roll unit 20a of the final station, the side walls of the sheet or strip M are bent 35 about 90° whereby the top wall of the hat is formed. However, the configuration of the rolling dies which is shown in FIG. 2 is one example. The number of units if - 14 _ arranged can be suitably changed. Further, the rolling dies which are arranged at the upstream side of the finishing rolls can be further suitably changed in shapes. 5 [0027] Note that, in the present embodiment, the cross-sectional shape is not just increased in width. After the portion 11 where the width becomes maximum, portions 12b and 10b which are decreased in widths are formed by the rolls, and therefore the intervals between 10 the roll units 20a to 20k are set to at least the lengths of the products. [0028] Next, the configuration of the roll units 20a to 20k will be explained. FIG. 3 shows the overall structure of the roll unit 20a in which the finishing 15 rolls are assembled. The roll unit 20a is provided with a first rolling die which has a rotation shaft 31 which extends in a sheet or strip feed direction, for example, the horizontal direction (below, referred to as a "bottom roll 3") and a second rolling die which has a rotation 20 shaft 41 which is parallel to the shaft 31 of the bottom roll 3 and faces the bottom roll 3 across a slight gap (below, referred to as a "top roll 4"). [0029] The shafts 31 and 41 of the rolls 3 and 4 are, for example, rotatably supported by ball bearings or 25 other bearing mechanisms 5 at stands or other support members 51. The rolls 3 and 4 are supported to be able to be raised and lowered and can be adjustable in distance of separation of the rolls. Furthermore, it is also possible to use a hydraulic pressure cylinder or other 30 pressing device to enable adjustment of the pressing forces of the top and bottom rolls 4 and 3. [0030] The top and bottom rolls 4 and 3 are driven to rotate synchronized by a gear set 52. The gear set 52 comprises gears 52a and 52b which are coupled with the 35 shafts 31 and 41 respectively and are engaged with each other. FIG. 3 shows, as one example of the gear set 52, the top and bottom gears 52a and 52b which are formed by - 15 - spur gears. Further, at one end of the shaft 31 of the bottom roll 3, for example, a drive motor or other drive device 53 is connected. If this drive device 53 makes the bottom roll 3 rotate, the top roll 4 is driven to rotate 5 through the gear set 52. At this time, for example, by setting the top and bottom gear ratios the same, the top and bottom rolls 4 and 3 rotate synchronously at the same peripheral speeds. That is, the gear set 52 is also the synchronized rotation mechanism of the top and bottom 10 rolls 4 and 3. [0031] The gear set 52 only need make the top and bottom rolls 4 and 3 rotate synchronously by the same peripheral speed. The gears need not be spur gears such as shown in FIG. 3 of course. Furthermore, it need not be 15 configured to drive the top roll 4 through the gear set 52. Individual drive mechanisms may also be connected to the top and bottom rolls 4 and 3. It is also possible to use an inverter controllable drive motor to adj ust the rotational speed. 20 [0032] The top and bottom rolls 4 and 3 which are arranged at the final station are shaped corresponding to the target product shape. Specifically, as shown in FIGS. 3 and 4, the bottom roll 3 has flank parts 32 which roll the top surfaces of the flanges 13 and an annular ridge 25 part 33 which rises up at the center portion in the axial direction of the flank parts 32 from the outer surface in a projecting shape and rolls the inside part of the hat shape.. The cross-sectional shape of the annular ridge part 33 exhibits a frustoconical shape which varies in 30 the circumferential direction corresponding to the hat shape of the 'finished product. [0033] That is, the annular ridge part 33 has a region 33a which is set in width of the outer circumferential surface to the first roll width, a region 33b which is 35 set in width of the outer circumferential surface to the second roll width, and tapered regions (in the following explanation, sometimes called the "transition parts") 33c - - 16 - and 33d which are arranged between the regions 33a and 33b and vary in widths of the outer circumferential surfaces from the first roll width to the second roll width. The left and right side surfaces of the annular 5 ridge part 33 form slanted surfaces which expand to the outward sides the further toward the shaft 31 side. Further, the width and height of the annular ridge part 33 and the inclination angle of the side surfaces are dimensions which correspond to the width and height and 10 the inclination angle of the target hat shape. Furthermore, the corner parts (ridgelines) at the outsides of the annular ridge part 33 and the corner parts at the insides of the flank parts 43 (recessed ridgelines) are rounded or are chamfered. Note that, FIG. 15 4, like FIG. 1, shows the borderlines of the regions 33a, 33b, 33c, and 33d for convenience of explanation. [0034] The region 33b of the annular ridge part 33 forms the portion 11 of the width L2 of the hat-shaped steel 1, while the regions 33c and 33d form the tapered 20 portions 12a and 12b of the hat-shaped steel 1. Therefore, the arc length of the region 33b is set to the length of the portion 11, while the arc lengths of the regions 33c and 33d are set to lengths of the portions 12a and 12b. On the other hand, the region 33a of the 25 annular ridge part 33 forms both the portions 10a and 10b of the hat-shaped steel 1. Therefore, the arc length of the region 33a is set to a length corresponding to the sum of the lengths of the portions 10a and 10b. In this case, the intermediate point which equally divides the 30 region 33a becomes the start point of the roll. However, when a 'continuous sheet or strip M for continuous forming is used and the finally shaped product is successively cut downstream of the apparatus, regions giving cutting margins may also be added to the regions 33a. In this 35 case, a mark for indicating the cutting position (for example, small hole, projection, etc.) may also be formed at the surface of the sheet or strip M. - 17 - [0035] On the other hand, the top roll 4 is formed to face the roll barrel part of the bottom roll 3 across a gap of the amount of thickness of the hat-shaped steel 1. Therefore, the top roll 4 has an annular groove part 42 5 which rolls the outside bottom surface of the hat shape and flank parts 43 which are formed at the two sides of the annular groove part 42 and roll the outside surfaces of the hat shape and the bottom surfaces of the flanges 13. The inside surfaces of the annular groove part 42 are 10 also formed to face the side surfaces of the annular ridge part 33 of the bottom roll 3 through a gap of the amount of thickness of the hat-shaped steel 1. Due to this, the annular groove part 42 of the top roll 4 varies in cross-sectional shape in the circumferential 15 direction. [0036] The side surfaces of the annular groove part 42 of the top roll 4, like the annular ridge part 33 of the bottom roll 3, are formed with the region 43b which forms the portion 11 of the hat-shaped steel 1, the regions 43c 20 and 43d which form the tapered portions 12a and 12b respectively, and the region 43a which forms the portions 10a and 10b, in the circumferential direction. Furthermore, in the same way as the annular ridge part 33, the intermediate point which equally divides the 25 region 43a forms the start point of the rolls, and therefore when assembling the top and bottom rolls 4 and 3 in the apparatus, the top and bottom rolls 4 and 3 are positioned in the rotation direction at the positions where their start points face each other (same phase). 30 [0037] If viewed in the shaft direction, the annular ridge part '33 of the bottom roll 3 and the bottom surface of the annular groove part 42 of the top roll 4 have cylindrical surfaces with outer circumferential surfaces of the same diameters. Due to this, if making the top and 35 bottom rolls 4 and 3 rotate by the same peripheral speeds, the relative phase of the top and bottom rolls 4 and 3 will not vary. In the case of a pair of top and - 18 - bottom rolls, so-called "slip" is liable to cause the relative phase of the turning top and bottom rolls 4 and 3 to vary. If the rolls have cross-sectional shapes which are constant in the circumferential direction, "slip" 5 does not become that much of a problem, but the top and bottom rolls 4 and 3 of the present embodiment have regions which vary in cross-sectional shape in the circumferential direction, and therefore if "slip" causes the top and bottom rolls 4 and 3 to become offset in 10 phase, the finished product is liable to become off in thickness from the design value and the top and bottom rolls are liable to collide. Therefore, in the present embodiment, it is important to make the top and bottom rolls 4 and 3 turn without changing their relative 15 phases. The gear set 52 which forms the above-mentioned synchronized rotation mechanism also has the role of preventing the relative phase of the turning top and bottom rolls 4 and 3 from changing. [0038] Note that, the top and bottom rolls 4 and 3 20 only have to be made from a material which is higher in rigidity than the sheet or strip M at the roll barrel parts. The material is not limited. Further, it is also possible to arrange the rolling die which has the annular ridge part at the top side and the rolling die which has 25 the annular groove part at the bottom side. [0039] FIG. 3 shows a roll unit 20a which including finishing rolls, but the other roll units 20b to 20k which are arranged upstream of the finishing rolls may be made the same in configuration as the roll unit 20a 30 except for the shapes of the rolls being different. For this reason, detailed explanations of the other roll units 20b to 20k will be omitted. [0040] The present invention is not limited to the following dimensions, but to further deepen 35 understanding, an example of the dimensions of the different regions of the bottom roll 3 will be shown. First, the radius of the bottom roll 3 to the outer - 19 - circumferential surface is 500 mm at the annular ridge part 33 and 450 mm at the flank parts 32. The difference of the two corresponds to the height of the hat shape. The width of the outer circumferential surface of the 5 region 33a is 50 mm, while the arc length is 400 mm. Further, the width of the outer circumferential surface of the region 33b is 80 mm, while the arc length is 400 mm. Further, the regions 33c and 33d have arc lengths of 300 mm and expand in width or contract in width by a 15° 10 gradient angle (relative angle between ridgeline of annular ridge part 33 and rotation direction of bottom roll 3 or relative angle between recessed ridgeline at inside of flank parts 43 and rotation direction of top roll 4). The top roll 4 faces the bottom roll 3 through a 15 gap of 2 mm. [0041] Next, the method of using the multistage roll forming apparatus 2 to produce the hat-shaped steel 1 will be explained. First, the top and bottom rolls 4 and 3 of the roll units 20a to 20k are made to rotate at a 20 predetermined speed and the sheet or strip M is fed to the roll unit 20k of the entry station. For example, as the steel sheet or strip M, it is possible to use steel sheet which is sent from an upstream rolling process or use a strip which is wound in a coil shape. At this time, 25 the sheet or strip M is fed so that the length direction becomes perpendicular to the axial direction of the top and bottom rolls 4 and 3 and is roll formed in the length direction of the sheet or strip M. The sheet or strip M (intermediate) which is fed out from the roll unit 20k is 30 conveyed by the rotational operation of the top and bottom rolls 4 and 3 to the roll unit 20j of the next station. Further, it is roll formed by this second stage roll unit 20j along the length direction and is further conveyed to the roll unit 20i of the next station. 35 [0042] Note that, when continuously roll forming the sheet or strip M, the roll units 20a to 20k of the different stations may be used to form it while applying - 20 - back tension and/or forward tension. Further, they may form it by cold, warm, or hot roll forming. [0043] FIGS. 5A and 5B show the state where the sheet or strip M is bent'into a hat shape in stages at the 10 5 stages of the roll units 20a to 20k. FIG. 5A shows the state in which the flanges 13 are formed by using the roll units 20k to 20a at the first to fifth stations. FIG. 5B shows the state in which the top wall of the hatshaped steel 1 is formed by using the roll units 20e to 10 30a at the sixth to final stations. Note that, FIGS. 5A and 5B are cross-sectional views of the portion 10a of the hat-shaped steel 1, but the other portions 10b, 11, 12a, and 12b are also bent in stages to the hat shape at the 10 stages of the roll units 20a to 20k. Therefore, 15 the material (intermediate) which is roll formed at the ninth station becomes a shape close to the final product and is finally shaped by the 10th finishing roll. [0044] The state where the finishing rolls perform the final forming operation is shown in FIG. 6. In the sheet 20 or strip M (intermediate) which is conveyed from upstream, the width Ll portion 10a is formed by the back half part from the start point to the regions 33a and 43a of the first top and bottom rolls, then the gradually increasing width portion 12a is formed by the regions 33c 25 and 43c and, furthermore, the width L2 portion 11 is formed by the regions 33b and 43b. Next, the gradually decreasing width portion 12b is formed by the regions 33d and 43d and finally the width Ll portion 10b is formed by the front half part from the start point of the regions 30 33a and 43a. At this time, the back half part of the regions 33a and- 43a forms the width Ll portion 10a of the next product. [0045] The finished product which is fed out from the finishing roll after final shaping is completed is cut at 35 the position forming the terminating end (that is, the end part of the portion 10b) and, is conveyed to other next step, for example, to the product inspection step. - 21 - The cutting position can be automatically discerned by for example detecting a mark (for example, small hole, projection, etc.) which is formed at intervals in the length direction of the sheet or strip M, by a sensor. 5 The mark may be provided at intervals corresponding to the lengths of the finished products at the sheet or strip M in advance or may be provided during roll forming. As the method of providing a mark during roll forming, using top and bottom rolls 4 and 3 which are 10 formed with projections forming the mark at a position corresponding to the starting point of the rolls so as to transfer a mark along with bending to the hat shape may be mentioned as one example. In addition to a mark, a predetermined relief shape may be formed on the surface 15 of the roll barrel part so as to form a bead, embossing, or other shape. FIGS. 7A and 7B show an example of a bead 14 and a projecting part 35 which is formed at a roll barrel part for forming the bead 14. While not illustrated, the top roll 4 is formed with a recessed 20 part which corresponds to the projecting part 35 though a gap of the amount of thickness of the material. The shapes, positions, and numbers of the beads and embossing can be suitably changed. [0046] According to the present embodiment, when using 25 a bottom roll 3 which has an annular ridge part 33 and a top roll 4 which has an annular groove part which faces the annular ridge part 33 to produce a hat-shaped steel 1, by the shapes of the annular ridge part 33 and the annular groove part 42 being made shapes which vary in 30 cross-sectional shape in the circumferential direction, a hat-shaped •steel 1 which varies in cross-sectional shape (that is, the hat shape) in the longitudinal direction can be produced by simple control for making the top and bottom rolls 4 and 3 rotate synchronized. 35 [0047] In this way, the roll forming according to the present embodiment does not require the complicated control method for changing the roll widths of split - 22 - rolls like in the past, and therefore does not require the introduction of new control modules for this purpose. Accordingly, for example, it is possible to realize the roll forming apparatus of the present embodiment by 5 changing the rolls of an existing roll forming apparatus to the top and bottom rolls 4 and 3 of the present embodiment. [0048] Note that, in the multistage roll forming apparatus 2 of FIG. 2, the roll units 20a to 20k are 10 arranged on a line, but if arranging the roll units 20a to 20k in tandem curved in the up-down direction, it becomes possible to produce a hat-shaped steel which is curved in the longitudinal direction. [0049] Furthermore, according to the present 15 embodiment, by the roll barrel part which varies in cross-sectional shape in the circumferential direction, the roll barrel part and material can sufficiently contact each other in the forming operation, and therefore for example even if the material is high 20 tensile steel, insufficient mill rigidity can be suppressed. Accordingly, the roll forming method and apparatus of the present embodiment can also be applied to tensile strength 980 MPa or more ultra high tensile steel. 25 [0050] Next, a modification of the rolling dies which are shown in the above-mentioned first embodiment will be explained. In the rolling dies of the present embodiment, as shown in FIG. 8, the outside diameter of the annular 30 ridge part 33 of the bottom roll 3 (hatched part) and the outside diameter of the bottom surface of the annular groove part 42 of the top roll 4 (hatched part) are the same, and the side walls of the annular ridge part 33 of the bottom roll 3 are provided with the later explained 35 relief. Leaving aside this feature, the top and bottom rolls 4 and 3 of the present embodiment are substantially the same as the top and bottom rolls 4 and 3 of the first - 23 - embodiment. Similar component elements are assigned the same reference notations, and detailed explanations are omitted. [0051] The relief which is provided at the side 5 surfaces of the annular ridge part 33 of the bottom roll 3 will be explained in detail. FIG. 9 is a partial vertical cross-sectional view which is cut along the plane which includes the center axes of the top and bottom rolls 4 and 3. In the first embodiment, the gap 10 between the facing bottom surfaces and side surfaces of the top and bottom rolls 4 and 3 was constant over the entire circumference in the circumferential direction, but in the present embodiment, the side surfaces of the annular ridge part 33 of the bottom roll 3 are offset by 15 the relief amount x to the inside of the axial direction of the roll from the inside surface of the designed hatshaped steel 1. By providing relief to the side surfaces of the annular ridge part 33 in this way, the gap between the side surfaces of the annular ridge part 33 and the 20 side surfaces of the annular groove part 42 becomes wider the further toward the base of the annular ridge part 33, that is, the inside in the radial direction. In the figure, the broken line shows a side surface when not providing the relief. In the case of the bottom roll 3 of 25 the final station, when working as one example a material of a sheet thickness of 1.0 mm, the relief amount x is preferably 1.4 mm or more. The method of determination of the relief amount will be explained later. [0052] FIG. 10 shows the result of comparison of the 30 gaps between the top and bottom rolls 4 and 3 in the case of relief and no relief. More specifically, FIG. 10 shows the minimum distance (minimum gap) between the side surfaces at the different phases when designating the start points of the top and bottom rolls 4 and 3 (see 35 FIG. 4) as 0° and making the top and bottom rolls 4 and 3 rotate in 5° increments. In particular, in the example - 24 - which is shown in FIG. 10, the region of about 45° to 120° corresponds to the transition parts 33c and 43c. Further, at about 45° to 65°, the above-mentioned gradient angle (relative angle between ridgeline of annular ridge part 5 33 and rotation direction of bottom roll 3 or relative angle between recessed ridgeline at inside of flank parts 43 and rotation direction of top roll 4) gradually increases, while in the region of about 100° to 120°, the gradient angle <|> gradually decreases. At the time of 180° 10 to 360°, the shape is symmetric, and therefore an explanation will be omitted. [0053] Further, the broken line of FIG. 10 shows the case where relief is not provided, while the one-dot chain line of FIG. 10 shows the case where relief such as 15 shown in FIG. 11 is provided at the side surfaces of the annular ridge part 33 only at the transition part 33c. Further, the two-dot chain line of FIG. 10 shows the case where relief of a tapered shape such as shown in FIG. 9 is provided at the side surfaces of the annular ridge 20 part 33 over the entire circumference, while the solid line of FIG. 10 shows the case where relief of a tapered shape such as shown in FIG. 9 is provided at the side surfaces of the annular ridge part 33 only at the transition part 33c. Note that, FIG. 11 shows a 25 comparative example for the present embodiment and is a partial vertical cross-sectional view which is cut along the plane which includes the center axes of the top and bottom rolls 4 and 3. In the comparative example which is shown in FIG. 11, relief is provided so that the gap 30 between the side surfaces of the annular ridge part 33 and the side surfaces of the annular groove part 42 becomes constant in the radial direction, that is, to cause simple parallel movement from the broken line in the figure which shows the side surfaces when not 35 providing relief. [0054] As will be clear from the broken line of FIG. - 25 - 10, it is learned that when not providing relief, the minimum gap greatly varies (decreases and increases) at the about 45° to 65° region and the 100° to 120° region. FIGS. 12A and 12B show results of numerical analysis 5 which show the interference between rolls when not providing relief. The parts which are shown by hatching show the interference regions {that is, the regions where the rolls actually contact each other or the gap between the rolls becomes small). Further, as shown by the one- 10 dot chain line in FIG. 10, when making only the transition part 33c simply move in parallel to provide the relief, the minimum gap varies at the transition parts 33c and 43c and the minimum gap is difficult to be maintained constant over the entire circumference. 15 [0055] On the other hand, as shown by the two-dot chain line of FIG. 10, it is learned that when providing relief of a tapered shape over the entire circumference, the amount of variation of the minimum gap is small and the gap is maintained substantially constant over 0° to 20 180° as a whole. Note that, in the above example, only the transition parts 33c and 43c were explained, but the same can be said for the transition parts 33d and 43d as well. Furthermore, as shown in FIG. 10 by the solid line, it is learned that when providing relief of a tapered shape at 25 only the transition parts 33c and 33d and not providing relief at the other regions, the amount of variation of the minimum gap becomes extremely small and the gap is maintained more constant in the range of 0° to 180° as a whole. While depending on the thickness or shape of the 30 shaped steel, the preferable minimum gap when considering the product specifications etc. becomes the thickness of the sheet or more. According to the present embodiment, by providing relief at the side surfaces of the annular ridge part 33 of the bottom roll 3, it becomes possible 35 to secure a minimum gap of the sheet thickness or more. [0056] FIG. 13 shows the effects on the amount of - 26 - springback of the finished product based on the minimum gap between the top and bottom rolls 4 and 3 in the circumferential direction (that is, the amount of difference from the target shape). In particular, FIG. 13 5 shows the effects at steel sheets of the 590 MPa grade, 980 MPa grade, 1180 MPa grade, and 1310 MPa grade. When the amount of difference from the target shape is negative, as shown at the top right in the figure, this shows that "spring go" occurs, while when the amount of 10 difference is positive, as shown at the bottom right in the figure, this shows that springback occurs. [0057] As will be understood from FIG. 13, in the four types of steel sheets of different tensile strength (590 MPa grade, 980 MPa grade, 1180 MPa grade, and 1310 MPa 15 grade), the amount of difference becomes a minus one as the minimum gap becomes larger. This is because, as shown in FIG. 14, due to the minimum gap becoming broader, the sheet over runs and tensile stress occurs at the inside parts of the shoulders of the bottom roll. Release of 20 that tensile stress causes the phenomenon of spring go. Therefore, by providing the side surfaces of the annular ridge part 33 of the bottom roll 3 with relief of a tapered shape offset to become broader at the inside in the axial direction of the roll, the minimum gap between 25 the top and bottom rolls 4 and 3 in the circumferential direction can be maintained substantially constant. Therefore, the amount of springback becomes uniform in the longitudinal direction of the strip M. For this reason, the effect is exhibited that the occurrence of 30 buckling at the flange parts can be suppressed. This is therefore an extremely effective effect. Further, it is possible to prevent a reduction in sheet thickness at the base region of the annular ridge part 33 and possible to prevent the sheet thickness from falling below a fracture 35 criteria. From the above, in the second embodiment as well, it is possible to obtain effects similar to the first embodiment and, furthermore, it is possible to form - 27 - a shaped steel which is kept down in variation in sheet thickness. [0058] Note that, as explained above, by providing relief at the side surfaces of the annular ridge part 33 5 at the transition part 33c, it is possible to suppress changes in the minimum gap between the top and bottom rolls 4 and 3. In other words, by providing relief at the side surfaces of the annular ridge part 33 at the regions with a large gradient angle , it is possible to suppress 10 changes in the minimum gap. Therefore, in the present embodiment, the relief amount x at the relief which is provided at the side surfaces of the annular ridge part 33 is set in accordance ;-/ith the gradient angle <|>. [0059] FIG. 15 is a developed view of the outer 15 circumferential surface of the bottom roll 3 seen along its circumferential direction. In FIG. 15, the x-axis shows the rotation direction of the bottom roll 3. The left end of FIG. 15 shows the start point of the bottom roll 3, while the right end shows the end point of the 20 bottom roll. In the example which is shown in FIG. 15, the transition part 33c is formed at about 60° to 120° and the transition part 33d Is formed at about 240° to about 300°. [0060] As will be understood from FIG. 15, in the 25- region 33a, the gradient angle is about 15°, the relief amount is made 1.3 mm or so. In particular, in the present embodiment, the relief angle is set in accordance with the absolute value of the gradient angle <|>, and therefore in the region 33c where 5 the gradient angle (j) is 15° or so and the region 33d where the gradient angle § is -15° or so, the relief amount x is set to be substantially the same value. [0061] Further, it is preferable to provide relief at the side surfaces of the annular ridge part 33 of the 10 bottom roll 3 not only at the roll unit 20a of the final station, but also part or all of the other roll units 20b to 20k which are arranged upstream of it. The multistage roll forming apparatus 2 which is shown in FIG. 2 bends the top wall of the hat-shaped steel 1 in five steps from 15 the sixth station to the final station (10th station), and therefore it is preferable to provide relief at the bottom rolls 3 of these stations. [0062] However, the top and bottom rolls 4 and 3 of the stations differ in roll shape (in particular, the 20 inclination angle of the side walls of the annular ridge part 33). Further, the minimum gap also changes according to the inclination angle G of the side walls of the annular ridge part 33 (the angle of the side walls of the annular ridge part 33 with respect to the outer 25 circumferential surface of the annular ridge part 33 or the outer circumferential surfaces of the flank parts 32, or the angle with respect to the shaft direction of the bottom roll 3). Specifically, the larger the inclination angle 0, the larger the minimum gap. Therefore, the 30 inventors etc. engaged in actual designs and conducted intensive studies and as a result discovered that the preferable relief amount x becomes larger the larger the inclination angle 0 of the side walls of the annular ridge part 33. More specifically, they discovered that the 35 preferable relief amount x is proportional to the value of the inclination angle G of the side walls of the - 29 ~ annular ridge part 33 multiplied with the height H of the annular ridge part 33 of the bottom roll 3 (x=pxHxtanO, where [3 is a constant) . In this regard, the relief amount x, the side wall angle 9 of the shaped steel, and the 5 height H of the annular ridge part 33 are as shown in FIG. 16. [0063] Further, the minimum gap varies depending on the roll diameter R of the top and bottom rolls as well. In this regard, the "roll diameter R" means the roll 10 diameter at the outer circumferential surface of the annular ridge part 33 of the bottom roll 3 and the roll diameter at the bottom surface of the annular groove part 42 of the top roll 4. Alternatively, the "roll diameter R" may mean the roll diameter at the outer 15 circumferential surfaces of the flank parts 32 of the bottom roll 3 and the roll diameter at the outer circumferential surfaces of the flank parts 43 of the top roll 4. Specifically, when the roll diameter R is infinitely large, the phenomenon of the minimum gap 20 becoming smaller than the sheet thickness at the base region of the annular ridge part 33 no longer arises. Therefore, in the present embodiment, the larger the roll diameter R, the smaller the relief amount x is set. In particular, in the present embodiment, the relief amount 25 x is set to be inversely proportional to the roll diameter R. [0064] Summarizing the above, in the present embodiment, the relief amount x is calculated by the following formula (1). 30 x=axR7RxtanGx|tan|... (1) were, a is a constant which is found by experiments or by calculation. [0065] In this way, in the present embodiment, by setting the relief amount x in accordance with the 35 gradient angle <|>, inclination angle 0, and roll diameter R which affect the minimum gap, it is possible to keep the - 30 - minimum gap from becoming smaller than the sheet thickness. Further, if the relief amount x becomes too large, the gap between the top and bottom rolls becomes unnecessarily large and the sheet or strip M become 5 wrinkled or suitable bending can no longer be performed. As opposed to this, in the present embodiment, the relief amount x is set in accordance with the variation in the gradient angle §, the inclination angle 9, and roll diameter R in the longitudinal direction, and therefore 10 it is possible to set the relief amount x the smallest in the range where the minimum gap does not become smaller than the sheet thickness. For this reason, it is possible to suppress wrinkling or unsuitable bending etc. of the sheet or strip M. 15 [0066] Note that, in the above embodiment, the relief amount x is set to the value which is calculated by the above-mentioned formula (1). However, in actuality, wrinkling etc. will not immediately be caused even if increasing the relief amount somewhat compared with the 20 value which is calculated by the above-mentioned formula (1). For this reason, the relief amount x may be said to be at least the value which is calculated by the above formula (1). [0067] Further, the above-mentioned constant a can, 25 for example, be calculated as follows. FIG. 17 is a partial vertical cross-sectional view of top and bottom rolls 4 and 3 which are cut along the plane which includes the center axes of the top and bottom rolls 4 and 3. In particular, FIG. 17 is a cross-sectional view 30 of the top and bottom rolls 4 and 3 at the transition parts. In the example which is shown in FIG. 17, the gap between the bottom roll 3 and the top roll 4 is basically set to a predetermined value C, while the predetermined value C is substantially the same as the sheet thickness 35 of the sheet or strip M which is bent between these top and bottom rolls 4 and 3. On the other hand, when the transition parts are provided in the above way, so long - 31 - as the side walls of the annular ridge part 33 are not provided with relief, the gap between the side walls of the top and bottom rolls 4 and 3 becomes smaller at the transition parts. In the example shown in FIG. 17, relief 5 is not provided, and therefore the gap between the side walls of the top and bottom rolls 4 and 3 becomes partially smaller. [0068] At this time, the minimum gap between the side walls of the top and bottom rolls 4 and 3 is made Cmin. 10 Further, the gradient angle at the transition parts of the top and bottom rolls 4 and 3 which are shown in FIG. 17 is made "i" and the inclination angle is made "9i". In addition, the height of the annular ridge part 33 is made "Hi" and the roll diameter is made "Ri". In this case, the 15 relief amount Xi which should be provided at the side walls of the annular ridge part 33 is equal to C-Cmin, and therefore the following formula (2) stands. As a result, the constant a can be found as in the following formula (3). 20 Xi=C-Cmin=axHi/RiXtan91x | tan^ [... (2) a=C-Cmin/(H1/Rixtan9iX|tan<|>1| )...(3) The constant a which is calculated in this way can be used even if the roll diameter R, the inclination angle 0, the gradient angle FIG. 23A shows a hat-shaped steel 1 with a constant width and height but with a cross-section which moves in the - 36 - lateral direction, while FIG. 23B shows the top and bottom rolls 4 and 3 which form the hat-shaped steel 1 of FIG. 23A by the final forming operation. That is, in the above first embodiment, a hat-shaped steel with a 5 straight stock axis was produced, but in the present embodiment, a hat-shaped steel 1 with a stock axis which is curved in the width direction is produced. This hatshaped steel 1 has portions 15a of a straight stock axis and portions 15b of a curved stock axis. As the rolls for 10 this, as shown by the example in FIG. 23B, top and bottom rolls 4 and 3 which have an annular ridge part and annular groove part offset in the rotational axial direction are used. The overall configuration of the roll unit which drives rotation of the top and bottom rolls 4 15 and 3 can be configured in the same way as in the first embodiment. [0081] According to the present embodiment, by simple control for making the top and bottom rolls rotate synchronized, a hat-shaped steel with a cross-sectional 20 shape in the longitudinal direction which curves in the width direction can be produced. Furthermore, if arranging the roll units 20a to 20k in tandem curved in the up-down direction, a hat-shaped steel which is curved in the longitudinal direction can also be produced. 25 [0082] FIG. 24A shows a hat-shaped steel 1 with a constant height and a width in cross-sectional shape which varies asymmetrically to the left and right, while FIG. 24B shows the top and bottom rolls 4 and 3 which form the 30 final shape of the left-right asymmetric hat-shaped steel 1 which is shown in FIG. 24A. That is, in the present embodiment, the top and bottom rolls 4 and 3 which are shown in FIG. 23B are used to produce a hat-shaped steel 1 which has one side wall 10c of the hat shape which is 35 constant and has only the other side wall lOd changing in the width direction. The overall structure of the roll unit which drives rotation of the top and bottom rolls 4 - 37 - and 3 can be configured in the same way as in the first embodiment. In this case as well, by simple control for making the top and bottom rolls 4 and 3 rotate synchronized, a hat-shaped steel v/hich varies 5 asymmetrically left and right in cross-sectional shape width in the longitudinal direction can be produced. [0083] 15 The U-shaped steel 6 of FIGS. 27A and 22B is substantially the same as the U-shaped steel 6 of FIGS. 26A and 21B except for being provided with the flanges 63. In this case as well, by simple control for making the top and bottom rolls 4 and 3 rotate synchronized, a 20 U-shaped steel 6 which varies in cross-sectional shape width in the longitudinal direction can be produced. [0086] The present embodiment also produces shaped steel having a U-shape cross-section. However, while the above- 25 mentioned fifth embodiment has a constant height, in the present embodiment, as shown in FIG. 28A, a U-shaped steel 6 with a constant width and a changing height is produced. More specifically, the U-shaped steel 6 of the present embodiment has a heightening portion 61c with a 30 constant width and a lowering portion 61d with a constant width. FIG. 28B shows the top and bottom rolls 4 and 3 of the final station for the U-shaped steel 6 which is shovm in FIG. 28A. The annular ridge part of the bottom roll 3 has a cross-sectional outer shape of an inverted U-shape, 35 expands in outside diameter in the circumferential direction in the range of 0° to 180°, and contracts in - 39 - outside diameter in the range of 180° to 360°. The recessed part of the top roll 4 which faces the bottom roll 3 also has a U-shape which varies in height in the circumferential direction. The overall structure of the 5 roll unit which drives rotation of the top and bottom rolls 4 and 3 can be configured in the same way as in the first embodiment. In this case as well, by simple control for making the top and bottom rolls 4 and 3 rotate synchronized, a U-shaped steel 6 which varies in cross- 10 sectional shape height in the longitudinal direction can be produced. [0087] Except for the point of the U-shaped steel 6 of FIGS. 2 9A and 24B being provided with the flanges 63, this is 15 substantially the same as the U-shaped steel 6 of FIGS. 27A and 22B. In this case as well, by simple control for making the top and bottom rolls 4 and 3 rotate synchronized, a U-shaped steel 6 which varies in crosssectional shape width in the longitudinal direction can 20 be produced. [0088] <10th Embodiment The present embodiment produces a shaped steel which forms a cross-sectional V-shape. FIG. 30A shows a Vshaped steel 7 with a width in cross-sectional shape 25 which is constant and a height which varies, while FIG. 30B shows the top and bottom rolls 4 and 3 of the final station for the V-shaped steel 7 which is shown in FIG. 30A. More specifically, the V-shaped steel 7 of the present embodiment has a heightening portion 71a with a 30 constant width and a lowering portion 71b with a constant width. The annular ridge part of the bottom roll 3 has a cross-sectional outer shape of a triangular shape (Vshape) and an expanding outside diameter in the circumferential direction in the range of 0° to 180° and 35 decreasing outside diameter in the range of 180° to 360°. The recessed part of the top roll 4 which faces the - 40 - bottom roll 3 also becomes a triangular shape (V-shape) which varies in height in the circumferential direction. The roll unit which drives rotation of the top and bottom rolls 4 and 3 can be configured in overall structure in 5 the same way as in the first embodiment. In this case as well, by simple control for making the top and bottom rolls 4 and 3 rotate synchronized, a V-shaped steel 7 which varies in height in cross-sectional shape in the longitudinal direction can be produced. 10 [0089] FIG. 31A shows a hat-shaped steel 1 which varies in both width and height of cross-sectional shape, while FIG. 3IB shows the top and bottom rolls 4 and 3 of the final station for the shape of the hat-shaped steel 1 which is 15 shown in FIG. 31A. More specifically, the hat-shaped steel 1 of the present embodiment has a portion 17a of a cross-sectional shape width Ll and height hi, a portion 17b of a cross-sectional shape width L2 and height h2, and a portion 17c of a changing width Ll to L2 and height 20 hi to h2. For this reason, the annular ridge part and annular groove part of the top and bottom rolls 4 and 3 are made shapes which vary in both height and width of cross-sectional shape in the circumferential direction (L1^-L2^L1, hl^-h2^-hl) . The overall structure of the roll 25 unit which drives rotation of the top and bottom rolls 4 and 3 can be configured in the same way as in the first embodiment. In this case as well, by simple control for making the top and bottom rolls 4 and 3 rotate synchronized, a hat-shaped steel 1 which varies in both 30 width and height in cross-sectional shape can be produced. [0090] Above, the present invention was explained In detail with reference to specific embodiments, but various substitutions, alterations, changes, etc. 35 relating to the format or details are possible without departing from the spirit and scope of the invention such as defined by the language in the claims will be clear to - 41 - a person having ordinary skill in the technical field. Therefore, the scope of the present invention is not limited to the above-mentioned embodiment and attached figures and should be determined based on the description 5 of the claims and equivalents to the same. Reference Notations List [0091] 1 hat-shaped steel 2 multistage roll forming apparatus 3 bottom roll 10 32 flank part 33 annular ridge part 4 top roll 42 annular groove part 43 flank part CLAIMS Claim 1. A method of producing a shaped steel which varies in cross-sectional shape in the longitudinal direction from a sheet by roll forming, comprising: 5 a step of preparing a first rolling die which has a rotation shaft and an annular ridge part which varies in cross-sectional shape in a circumferential direction which is centered about said rotation shaft; a step of arranging said first rolling die so 10 that the rotation shaft of said first rolling die becomes perpendicular to a sheet feed direction; a step of preparing a second rolling die which has a rotation shaft and an annular groove part which varies in cross-sectional shape in a circumferential 15 direction which is centered about said rotation shaft; a step of arranging said second rolling die so that a gap which is equal to a thickness of said sheet is formed between said first rolling die and second rolling die and the annular ridge part of said first rolling die 20 and the annular groove part of said second rolling die engage; a step of making said first rolling die and said second rolling die rotate synchronized; and a step of feeding a sheet between said first 25 rolling die and second rolling die, wherein the side surfaces of the annular ridge part of said first rolling die are provided with relief so that the gap with respect to side surfaces of the annular groove part of the second rolling die broadens 30 over at least part of the circumferential direction and at an 'inner side in the radial direction of said first rolling die, wherein said annular ridge part of said first rolling die is configured so that the relative angle 35 between the ridgeline and the rotation direction of said first rolling die varies at least partially in the circumferential direction, and - 43 - wherein the relief amount at said relief is set to vary in accordance with the relative angle betv/een the ridgeline of the annular ridge part of said first rolling die and the rotation direction of said first rolling die. 5 Claim 2. The method of production of a shaped steel according to claim 1 characterized in that the larger said relative angle, the larger said relief amount is made. Claim 3. The method of production of a shaped steel 10 according to claim 1 or 2 characterized in that said annular ridge part of said first rolling die is configured so that a height dimension which is measured in a perpendicular direction with respect to said rotation shaft varies at least partially in the 15 circumferential direction, and in that said relief amount is made larger the higher the height of said annular ridge part. Claim 4. The method of production of a shaped steel according to any one of claims 1 to 3 characterized in 20 that said shaped steel is a hat-shaped steel with an inner circumferential surface which is rolled by the annular ridge part of said first rolling die and with an outer circumferential surface which is rolled by the annular groove part of the second rolling die. 25 Claim 5. The method of production of a shaped steel according to any one of claims 1 to 4 characterized in that the annular ridge part of said first rolling die includes, in its circumferential direction, a first roll width region, a second roll width region, and a tapered 30 region which increases or decreases in width from said first -roll width to second roll width. Claim 6. The method of production of a shaped steel according to any one of claims 1 to 4 characterized in that said first rolling die has an annular ridge part 35 which is offset in the rotation shaft direction in its circumferential direction and produces a shaped steel having stock axis which is curved in the width direction. ~ 44 - Claim 7. The method of production of a shaped steel according to claim 1 characterized in that the relief amount x of the side surfaces of said first rolling die is set to not less than a value x1 which is calculated by 5 the following formula (1): x'=ctxH/RxtanGx|tan", and a is a constant. Claim 8. The method of production of a shaped steel according to claim 7 characterized in that a plurality of roll units each of which comprises first rolling dies and 15 second rolling dies are arranged in series in a sheet feed direction and the material is bent by these plurality of roll units so that the side wall angle 6 is increased in stages, and in that the relief amount x of the side surfaces of the first rolling die of part or all 20 of the roll units is not less than a value which is calculated by the formula (1). Claim 9. The method of production of a shaped steel according to any one of claims 1 to 8 characterized in that the relief which is provided at the side surfaces of 25 the annular ridge part of said first rolling die is started separated from the ridgeline of said annular ridge part by a predetermined length L and said predetermined length L is set so that, when the height of said annular ridge part is "H", 0", and a is a- constant.