This brochure deals with bending of high strength steel for the trademarks Strenx, Hardox and Docol. The content is intended as a guide, and contains general suggestions for how to achieve the best results in bending.
Bending high strength steel sheet and plate rarely proves to be difficult, however, there are certain parameters that must be considered, which are dealt with in this brochure.
A material of high purity with few inclusions is of fundamental importance, in order to achieve a good bending result. SSAB’s modern processing allows for high standards of surface quality, tolerances and mechanical properties.
The edges of the die opening should always be as hard as, or harder than the plate being bent, in order to avoid excessive damage to the die.
A simple way of achieving this is to mill grooves in the die edges and fit lubricated round rods of, for example, hardened steel into the grooves.
The edge radius of the die should be at least half the plate thickness.
Springback increases with increased die width, while punch force is reduced. Make sure that the opening angle of the die allows for over-bending, without bottoming out, to compensate for springback. An increased die opening width can in many cases lower the strain level in the bend. Also, make sure that there is enough room for the chosen punch together with the workpiece, in the die, during bending, without deforming the die. The minimum recommended die opening width are shown in tables 2 and 3.
The die edge radius should be at least half the plate thickness. Alternatively, the die width should be increased in order to minimize pressure on the die edge radius, and consequently reduce the risk of die marks.
The suitable punch radius, along with the die width, is the most important parameter. When bending high strength steel, the final inner radius often becomes somewhat smaller than the radius of the punch, figure 3. When there is low friction between plate and tools, the phenomenon becomes more obvious.
For steel with yield strength over approximately 500 MPa, a punch radius of the same size or slightly larger than the desired bending radius is recommended. Tables 2 and 3 on page 7 show the minimum recommended punch radius when bending to 90°.
To make an estimation of the force needed during bending, we pay attention not only to the bend length, plate thickness, die width and tensile strength, but also the changing moment arm during bending. The peak load is assumed to be reached at a bend opening angle of 120° with normal friction (no lubrication). Trial tests are always recommended.
upper P equals StartFraction b dot t squared dot upper R Subscript m Baseline Over left-parenthesis upper W minus upper R Subscript d Baseline minus upper R Subscript p Baseline right-parenthesis dot 9800 EndFraction
P = Bend force, tons(metric)
t = Plate thickness, mm
W = Die width, mm (figure 1)
b = Bend length, mm
Rm = Tensile strength, MPa (table 1)
Rd = Die entry radius, mm
Rp = Punch radius, mm
The SSAB Bending Formula© is verified by tests for 90° bends, see figure 5.
A certain press brake is just capable of bending a 20 mm thick EN10025 – S355 steel plate in a die with a 200 mm wide opening, and die entry radius of 15 mm. The punch radius is 40 mm.
If the same die and punch is used and the bend length is the same, how thick a Hardox 400 plate is the press brake capable to bend?
The bending forces should be same, and only the plate thickness (t) and tensile strength (Rm) will differ. Substituting in the above formula and simplifying: 202 x 550 = t2 x 1250
The thickness (t) of the Hardox plate will be 13.3 mm. The R/t ratio will then be 40/13.3 = 3.0. According to table 2, the Hardox 400 plate can be bent transverse rolling direction with this punch radius. The W/t ratio for Hardox 400 plate will be 200/13.3 = 15.0 which, according to table 2, is satisfactory.
A 2000 mm long bracket is to be produced by bending plate. The choice lies between using:
a) 10 mm thick plate of EN10025 – S355 with a typical tensile strength of 550 MPa,
b) 7 mm thick plate of Strenx 700 with a typical tensile strength of 860 MPa.
In both cases, an existing die with a 100 mm wide opening and die entry radius of 10 mm, is to be used. The punch radius is 14 mm in both cases. What press force will be needed for each steel grade?
upper P equals StartFraction 2000 dot 10 dot 10 dot 550 Over left-parenthesis 100 minus 10 minus 14 right-parenthesis dot 9800 EndFraction
For Strenx 700
upper P equals StartFraction 2000 dot 7 dot 7 dot 860 Over left-parenthesis 100 minus 10 minus 14 right-parenthesis dot 9800 EndFraction
Since the plate thickness has a greater influence than the strength, the force needed for bending Strenx-plate in this particular case is lower.
The SSAB Bending Formula© is verified by tests performed within a wide range of thicknesses and grades, the specimens are bent to 90°. Tooling setups are in line with SSAB’s bend recommendation.
Springback increases with steel strength and the ratio between die width and plate thickness (W/t). Material yield strength has the biggest influence.
When bending, a varying residual stress distribution is achieved over the bend cross section. The plastic strain level and the distribution of these stresses will control the tendency for springback. All springback is fully elastic.
To compensate for springback, the die should be shaped in such a way to allow overbending without coining the material. It is very difficult to accurately predict the springback of a material when bending, since this depends to a large extent on each unique tool setup. That is why trials are recommended. For thinner plate or sheet (t < 10 mm), an estimation of the material’s springback, in degrees, can be achieved by dividing the tensile strength (MPa) by 100.
A precondition is that the die width is approximately 10–12 x the plate thickness.
PARAMETERS THAT AFFECT SPRINGBACK:
Since SSAB’s products are developed and specialized for different types of use, bend tests and evaluation of these vary somewhat.
For plate products the minimum recommended relationship between the punch radius and plate/sheet thickness (R/t) is shown in table 3.
For Strenx strip products and Docol, the relationship between minimum inner radius and sheet thickness (Ri/t) is shown in table 2.
These bend recommendations are based on bend tests of one step to 90° after unloading. Die opeing width are guidelines and may vary somewhat without affecting bending results.
Tables 2 and 3 shows a small selection from SSAB’s product range. For information about other materials and more technical information, please contact Tech Support or visit www.ssab.com.
Strenx and Hardox is delivered with guaranteed bending performance according to Strenx and Hardox guarrantee. For more information contact your local SSAB representative.
SSAB is a Nordic and US-based steel company. SSAB offers value added products and services developed in close cooperation with its customers to create a stronger, lighter and more sustainable world. SSAB has employees in over 50 countries. SSAB has production facilities in Sweden, Finland and the US. SSAB is listed on the Nasdaq OMX Nordic Exchange in Stockholm and has a secondary listing on the Nasdaq OMX in Helsinki. www.ssab.com