So you are a heavy turret soulmate. In late game I make a perimeter of about 100 rimatomics turrets, each needs like 500 steel. I generally keep 20k steel "on the side".

For massive steel consumption I recommend:

1) Long range mineral scanners. Each steel mission yields 2k steel.

2) If you are OK with option 1, then caravans are too slow, clumsy, dangerous...everything. Use a vehicle mod. Get to da chopper for helicopters, or SRTS for more high-tech mini-spaceship stuff. These use chemfuel to move stuff around quickly.

3) The real ruins mod adds an event where you get to salvage a base made by another player. This can contain steel. However, ther REAL amount of steel here comes if you are able to defend the base and actually mine the entirety of steel out of it. This yields tons of steel, maybe 10k a time