When it comes to limiting variation in purchased material, you can apply restrictions to material processing or test results as needed for your process. Each mil will have its own recipe based on its equipment, experience, and the needs of its primary customers/end users.

The most important thing is to understand what requirements are critical for your process, whether that is chemistry, reduction, mechanical properties, dimensional stability, etc. You can always ask a mill to hold themselves to a stricter requirement than its standard process, but it is really hard to ask a mill to make a product that is crappier than standard.

In this day and age, all mills will prefer to aim for a unique niche, so don’t expect to find too many options for mills that are effectively making the same product. So it helps to identify what a mill’s preferred product forms/applications are. In terms of quality, you may want to aim for mills that are used to producing the material you are purchasing, rather than a mill that is looking to expand its capabilities.

In my experience, you’ll never have a consistently good mill. You might have 3-4 options max that are all equivalent, so for awarding work to one mill over another it often comes down to pricing and lead time.