Hardening or softening steel does not change the force needed to bend it.
The Module of Elasticity is a number that describes a materials ability to resist bending.
The value for a given steel is the same if the steel is hard or soft.
It also is almost exactly the same for low carbon steel or for high alloy steels.
For anyone that cares, the value for steel is about 29,000,000 psi.
Said in a more real world way, if you have two pieces of a 1/4 inch diameter steel rod one foot long with one totally soft and the other hardened to the hardness of a file, and you clamp one end to a table and hang a 1 pound weight from the other end both the hard and the soft rod will bend exactly the same amount.
The only way to change this is to make the part out of a different material.
Now, I went thru all of this to give you folks some understanding of my next comment.
Do not try to change the force a spring makes by trying to heat treat it. It will do no good and most likely you will end up removing the hardness that is needed to make the spring work.
If you do soften the spring, after it bends it will not return to its original position.
In other words, it will ruin it.
The force a flat spring makes depends on the Modulus of Elasticity, the width and the thickness of the spring.
The width of the flat spring effects the force directly. That is, if the spring width is reduced by 1/3, the force it develops is reduced by 1/3. Reducing the width by 1/2 will reduce the force by 1/2.
The thickness of the flat spring effects the force by the square of the thickness.
Reducing the thickness to 1/2 of its original thickness will decrease the force to 1/4 of its original force.
Reducing the thickness to 1/3 of its original thickness will decrease the force to 1/9 of its original force.
As you can see, it doesn't take a lot of thickness reduction to greatly effect the force the spring will deliver.
On a negative side with this force reduction is when it is done, the stresses in the spring material go up for a given amount of bending.
That can result in a spring breaking when the materials strength limits are exceeded.
Just something to think about.