How to create a fixed width image style for all landscape and portrait images?
Scale only scales the image so it looks proportionate. It will go with either the height 580px or width 580px, which ever is closer. 

If you want your image to be exactly 580px x 580px you could use Resize but you will see your image will look stretched. 

The only other option that will give you the exact 580px x 580px is Scale and Crop

If you are afraid that something important will get cut off, you can use this awesome module: 

Focal Point by DA's own bleen


  Focal Point allows you to specify the portion of an image that is most
  important. This information can be used when the image is cropped or
  cropped and scaled so that you don't, for example, end up with an
  image that cuts off the subject's head.


Then use Focal Point Scale and Crop as your image style effect.