1.5 - 2" is moderately light plate compared to what I've seen/cut, although most of my cuts are out of A-36/HR/1045/4140/514 material. In my experience, with light plate oxy fuel cutting (1-2") a concave cut edge, or bevel is caused by 4 things.
1. Your torch is too high from the plate: Ideal torch height is around half inch MAX.
2. You are cutting too fast: Using propylene and a #2 tip, I cut 1.5 around 11.25 - 12.5 ipm, depending on how I adjusted my torch temp.
3. You are burning too hot: adjust your torch to lowish volume, and fairly cool. Like the torch sounds like it is kind of breathing on you (volume), and blue inner cones are around .25" long.
4. Your tip is dirty/getting dirty: Run a cleaning cycle on your plate before you cut. high preheat torch setting (very hot) run torch over cut area around 60-130 ipm, but cool your torch down before you cut!!! also, after you pierce, clean your tip and scrape the pierce hole to clean it up.
You're using a #3 for the 2-2.5" correct?
I don't know if this all is too basic or what? Sorry if I offend, I don't know anyone's experience burning here, just found the site...