if you bring the wheeled luggage for carry on, they will weight your bag at check in , and it's allowed only 7 KG, for extra KG, you need to pay , around 800+ THB per KG, from 2017 (20-30 USD per Kg).  Previously it was 450 THB/Kg.

If you want to bring more than 30 KG + 7 KG, you should buy additional weight , no other choice.

They are quite strict since past years, from my experience flying by QR almost 10 years, BKK route 3-4 times/years.  Though i am loyal customer,  i, one time paid 4500 THB for 6 KG extra.