Agree with CSD........
The pitch mode for takeoff is initially TOGA. If VNAV is not armed then TOGA will remain the pitch mode until altitude capture indicated by ALT.
The A/T mode for takeoff is THR REF followed by HOLD. I think that without VNAV armed it will remain in HOLD even through 400'.
At altitude capture, the autothrottle will remain in HOLD and the aircraft will accelerate until a placard speed limit is reached.
To prevent this another A/T mode needs to be selected i.e. SPD if the altitude has already been captured of FLCH (which is a combined pitch/power mode) if still climbing.