The so-called "iron hat king" means that the title can be hereditary, and future generations are kings. When they see the emperor, they don't attend, worship, accept or send him away, so they are also called lazy kings.
What's the use of the iron hat king? Its use is high status, big profits, big places can be loyal to the country, small places can live in luxury, love and protect future generations.
Eight iron hat kings:
Prince Dai Li Mountain (Taizuzi)
Prince of Switzerland Dourgen (Taizu)
Duoduo, Prince Yu (Taizuzi)
Zheng Qinwang Gilhallang (nephew of Mao)
Prince Su (the eldest son of Taizong)
Zhuang Shuo Sai (Taizong Zi)
Horqin Wang (Dai)
Succeeding King Sakhalin (Dai)
How to get this title:
The "hereditary replacement" system formulated by Qianlong clearly stipulates that "at my discretion, the king who attacked the nobility will be replaced by a hereditary who was promoted by meritorious service, and the rest will be distinguished by time." Because the titles of "Eight Iron Hats King" are all hereditary, the titles are all promoted because of the meritorious military service.