Even at sixty hertz this occurs. The cause is other iron atoms. Each one has a 'direction', and they interact with each other. A coil 'tells' them which way to turn, but the neighboring atoms also try to influence them, so the result is a lag in response time to the magnetizing current.
You need to use more than 300 turns, if you want to use a current that won't melt your copper wire. Consult the wire gauge charts and find the 'ohms per 1000ft.' rating of the wires. You will probably need a different size of wire and something closer to 5,000 - 10,000 turns before you have enough to make a good strong field with a current that doesn't 'smoke' the coil.
Ferrite powder cores help with problems such as this; perhaps you can scavenge one from a derelict piece of electronics? It may serve you MUCH better than a solid iron one.
OHMS LAW. The wire chart I refer too will also have ampacity ratings for the gauge of wire. Exceeding this will be the cause of 'meltdown' you have experienced.
1 volt creates one amp of current in a one ohm resistor.
1 volt creates one milliamp of current in a 1000 ohm resistor.
etc. etc.