Double Wall Oven Breaker Tripping
You likely have a short somewhere in your circuit. When a breaker trips immediately after being turned on, it's a good indication of a short. Check your wiring and make sure all connections are correct, both at the oven (plug and receptacle) and the junction box. My bet is it's in the junction box you made.

But, you need to replace that 30A breaker immediately! Not doing so is an extreme fire hazard and could result in your house burning down. You need to pull new wire before putting in a 40A breaker.

Unless...

The breaker is sized to protect the wiring and receptacles (and hardwired appliances) in the circuit; in this case, the "weakest link in the chain" can only handle 30A. If you're lucky, that was the old oven and the wiring in the walls is actually big enough to support more than 30A - but you can't know that without visually checking it. Look on the sheath of the cable, and somewhere there it should indicate the gauge: AWG 10 or #10 or something similar. If you're lucky it will be #8 or even #6 wire, good for 40 or 50A, respectively. Chances are, though, if the old oven was a 30A, the wiring is only #10 and good for 30A.