Air Channels

I am not any sort of expert; just joined this blog, and only began baking bread a month ago. I have had a lot of success with high hydration doughs (no knead), however I'd like to know at what point in baking, are the air channels formed?

I know the moisture has a lot to do with the development of air channels (I use  cast iron Dutch oven preheated in a 500 dgree stove). Are the air channels set fairly soon after  inserting into the stove? How many minutes till they are actually formed to their fullest? I assume the high temperatire will enhance the formation.

I usually bake at 500 degrees for for 15 miniutes, then reduce to 475 for another 15 minutes, then uncover and reduce to 450 for 15-20 minutes. Seems to work but I'd like to improve the crumb or at least get the optimal that I am able to achieve given my lack of experience.

My other question concerns oven spring. I  have achieved that with a few loaves but lately not so much (don't get the dramatic cracking on the top crust). OTOH. the crumb is still pretty good. How can I better achieve oven spring and that impressive bloom? 

I should add that I typically use 430 grams of dry contents, a mixture of white all purpose organic white flour (locally grown and milled) about 1.5 cups, Robin Hood Nutri Blend about 1 cup, and corn meal about 1/2 cup. That all varied a lot. I also add othre stuff (today was dried olives and sunflower seeds added in when shaping the loaf).

Thanks.