We have described all of the benefits the customer may enjoy by switching to high strength steels.

But there are some aspects that should also be considered beforehand:

* Formability - limited compared to ordinary steels/MS. Evaluate the design changes or a different method of forming.

* Springback - increases with increasing yield strength. Can be corrected by means such as over-bending. An increase of 100 MPa in Re increases the required over-bending by about 1 degree.

* Flatness - UHS steel does not have the same flatness as MS. Although the difference has gradually been reduced by determined development work at SSAB, the user should be aware of the fact that a difference still exists.

* Edge standard - risk of poorer edges and risk of higher tool costs due to increased wear, unless adjustments are made when shearing and punching holes. See the explanation below under "Increased clearances".

* Deflection
Defection is the reason that the customer was not satisfied in our practical case No. 6.

Case 6 An "unsafe" chair

SSAB Tunnplåt and a tube manufacture decided to join forces in trying a new application - a dining chair intended for public eating places and made of tube of Docal 800 DP steel. It was assumed that the end customer would have much to gain. The reduced sheet thickness would lower the total material costs for the customer. At the same time, the working environment would be improved. During the daily cleaning of the premises, the cleaning personnel have a large number of chairs to lift. The total weight they carry could now be significantly reduced. This would possibly result in fewer strain injuries and lower absenteeism.

So the tubes were produced and a prototype chair was made. But that's where it ended. Due to the reduced sheet thickness, the deflection of the chair seat increased, and the persons who tested the chairs thought the chairs were unpleasant to sit in. So regular production was never started.

Deflection - definition

At a constant load, the deflection is the same even if the yield strength of the material is higher. See below.

In the case of the chair, the thickness of the sheet used for the tubes was reduced to save on the material costs (which was the message).

If the sheet thickness is reduced, the deflection/bending at a constant load will increase (the weight of the person was the same, except for the weight increase after holidays or weight loss after slimming for the ‘bikini season’). Another way of reducing the deflection is to change the geometry of the cross-section, but this was of no interest, since the tubes had to be round.

F = force = weight
L = length of tube
E = modulus of elasticity (same for the steels)
I = dependent on the shape of the cross-section

Increased clearance - explanation

The clearance between the blades is increased for shearing/slitting.

The same procedure applies in punching, i.e. the clearance between the punch and die is increased.