DESCRIPTION Title of Invention: Tooling for Punching Steel Sheet and Punching Method 5 Technical Field COO011 The present invention relates to tooling for punching thin-gauge steel sheet and a punching method using that tooling. 10 Background Art [00021 To deal with the need for lightening the weight of automobiles, which began back with the old oil crises, the steel sheet used for automobiles has been made higher in strength. In recent years, a need has arisen for 15 improving the safety of automobiles in collisions. The need is rising for further increasing the strength of steel sheet for automobile use. As steel sheet rises in strength, generally its shapeability is sacrificed and cracks easily form at the time of press-forming. For this 20 reason, in application of high strength steel sheet, in the press-forming step, it is necessary to work to prevent cracking. In particular, in press-forming steel sheet for automobile use, "punching hole expansion" for stretching the punched end face in the circumferential 25 direction is often seen. It is important to prevent cracking due to such shaping. [00031 FIGS. 1A and 18 show the state of deformation of a riorked material in a punching process using a conventional flat-bottomed punch. In this punching 30 process, a large compressive or tensile stress is applied to the hardened layer shown in FIG. 18, so that part is remarkably hardened. Further, for this reason, the ductility of the end face after punching deteriorated and the punching hole expandability of the punched hole 35 remarkably deteriorated. The deterioration of the ductility of the end face due to this hardened layer is particularly remarkable in high strength steel Improvement of the punching hole expandability of high strength steel sheet now being made much use of to deal with the recent needs for lightening the weight of automobiles is being sought. 5 [00041 As an art for punching for improving this punching hole expandability, the art of punching using a punch with a projection attached to its tip has been proposed (PLTs 1 and 2). [00051 The principle of improvement of the punching 10 hole expandability by this art will be explained below. The punching hole expandability deteriorates in the punching process due to the formation of a work hardened layer at which the plastic strain accumulates. In the punching process, first, the punch and die shear the 15 worked material whereby a shear plane is formed. Next, a crack occurs and advances in the worked material near the cutting edges of the punch and die resulting in a fracture surface. This crack grows and leads to fracture. The plastic strain on the worked hardened layer of the 20 fracture surface mainly occurs at the stage of formation of the shear plane, so the shorter the stage of formation of the shear plane, the smaller the plastic strain on the work hardened layer of the fracture surface. Due to the effect of the projection attached to the punch, tensile 2 5 stress is given to the punched worked material in the vicinity of the cutting edges of the punch and die, the advance of a crack is promoted, and as a result it is possible to shorten the stage of formation of the shear plane. Due to this effect, the strain at the punched end 30 face is suppressed and the punching hole expandability is improved. [0006] In PLT 1, furthermore, to prevent shearing of the material due to the projection, making the radius of curvature Rp of the shoulder of the projection 0.2 mm or 3 5 more or making the projecting shoulder angle from 100' to 170' are made requirements. In PLT 2, to improve the ductility of the end face of the worked material, making the angle of the tangent drawn from the punch cutting edge to the projection 3 O to 70° is made a requirement. Citation List Patent Literature 5 [0007] PLT 1. Japanese Patent Publication No. 2005- 95980A PLT 2. Japanese Patent Publication No. 2007-307616A Summary of Invention Technical Problem 10 [0008] According to the prior art, it is possible to obtain a high strength steel sheet of a tensile strength of approximately 800 MPa where a 80% or more punching hole expansion ratio is obtained. Due to this art, a certain effect of lightening the weight of the automobile 15 is obtained. However, this cannot be said to be necessarily sufficient for dealing with the further rising needs for high strength steel sheet of recent years. To sufficiently deal with such needs, realization of a 90% or more punching hole expansion ratio is being 20 sought in high strength steel sheet with a tensile strength of about 800 MPa. The present invention has as its object realization of punching tooling and a working method realizing a punching hole expansion ratio of 90% or more in punching 800 MPa class high strength steel 25 sheet. Solution to Problem [0009] The inventors engaged in intensive studies to solve ,,Sd-e~abovper oblems in punching by a projecting punch and obtained the following findings. 3 0 [OOlO] (a) Optimization of Shape of Cutting Edge of Die At the time of punching, the way force acts differs at the punch side and die side of the worked material of the high strength steel sheet, so there inherently should be 3 5 optimal shapes of the cutting edge shape of the punch and the cutting edge shape of the die. The prior art mainly studies the cutting edge of the punch. The cutting edge shape of the die is not the optimal one. Therefore, the inventors engaged in detailed studies focusing on the shape of the cutting edge of the die. First, if the shoulder R of the die (corresponding to shape of cutting 5 edge at radius of curvature of die) is small, the die side of the worked material of the steel sheet receives extreme compressive stress. For this reason, the compressive stress region in a cross-section of the steel sheet becomes broader. The part which receives 10 compressive stress is inhibited in crack propagation, so fractures by shear fracture. Due to this, the shear plane increases and the work hardened layer becomes greater. On the other hand, if the shoulder R of the die is too large, the tensile stress acts at the die side of the 15 steel sheet as well, so crack propagation proceeds and shear fracture becomes limited. For this reason, the work hardened layer is also reduced, so ductility can be secured. However, the shoulder R is large, so there is a possibility of deformation of the steel sheet after 20 cutting (sagging in the downward direction of the punch). Considering these facts, the inventors discovered that in 800 MPa or more high strength steel sheet, it is suitable to make the shoulder R of the die from 0.03 mm to 0.2 mm. [00111 (b) Two-Stage R Design of Die Shoulder 25 Furthermore, the inventors discovered that by making the die shoulder (cutting edge part) a shape having two radii of curvature (below, called "two-stage R"), it is possibJe to suppress the compressive stress while cutting. If the die shoulder is a shape having one radius 3 0 of curvature (below, sometimes called "one-stage R"), due to bending of the steel sheet at the die shoulder, a region where compressive stress acts is formed at the die side of the steel sheet. Due to the compressive stress due to this bending, the tensile stress which is created 3 5 at the steel sheet due to the projecting punch is eased. The crack propagating ability becomes worse by that amount. Therefore, by making the die shoulder R two stages, the bending of the steel sheet at the die shoulder can be partially eased, the region where the compressive stress due to this bending acts can be reduced, and crack propagating ability can be improved. 5 Further, in the case of punching tooling, the clearance of the punch and die is also important. When making the die shoulder a two-stage shoulder R, if making the radius of curvature R1 of the arc part at the punch side larger, as a result, the clearance becomes greater and the 10 sharpness becomes duller. For this reason, the inventors discovered that when making the die shoulder a two-stage shoulder R, it is sufficient to make the radius of curvature R1 of the punch side smaller than the radius of curvature R2 of the reverse side to the punch (sheet 15 holder side). Furthermore, when a steel sheet is cut, the radius of curvature Rl of the punch side is effective, so this radius of curvature R1 should be made the abovementioned optimum range. Due to this, it is possible to decrease the compressive stress acting on the steel 20 sheet. Crack propagation due to higher tensile stress can be obtained. Further, it is possible to reduce the shear plane due to shear fracture, possible to reduce the work hardened layer, and possible to improve the hole expandability. However, it is necessary to keep the 25 deformation in the elastic range of the steel sheet, so it is necessary to restrict the amount of sink of the steel sheet at the die shoulder. [001a ,,(c] Optimization of Projecting Punch Shape On the other hand, the shape of a projecting punch able 30 to efficiently cause tensile stress at a worked material of steel sheet at the time of cutting has also been studied in detail. As a result, the inventors discovered that there is an optimal range to the angle (a) formed by the tangent drawn from the shoulder forming the punch 3 5 cutting edge (punch cutting edge end) to the projection shoulder and the line perpendicular to the direction of movement of the punch. That is, it is learned that when a is from 12' to 72', a large tensile stress is generated and the crack propagating ability is increased. [0013] The present invention was made based on these discoveries and has as its gist the following: 5 (1) Tooling for punching a steel sheet comprised of at least a die, a sheet holder, and a projecting punch, in which tooling for punching a steel sheet, in a crosssection parallel to a direction of movement of the punch of the punching tool and vertical to a ridgeline formed 10 by a cutting edge of the punch or die, the radius of curvature R of a shoulder forming the cutting edge of the die is from 0.03 mm to 0.2 mm and an angle a formed by a line drawn from the shoulder forming the cutting edge of the punch to the shoulder of the projection and a 15 direction perpendicular to the direction of movement of the punch is from 12' to 72'. (2) The tooling for punching a steel sheet according to (I), wherein the curvature of the shoulder forming the cutting edge of the die is comprised of two radii of 2 0 curvature, the radius of curvature of the curve facing the punch being R1, and the radius of curvature of the other curve being R2, and when the angle formed by the line passing through the intersection of the two curves based on R1 and R2 and the center of curvature of R1 with 2 5 the direction perpendicular to the direction of movement of the punch is P and the thickness of the steel sheet is ,rt" 0.03 mm