Why is Li Kui jy called Iron Bull?

Li Kui jy is from Baizhang Village, Yishui County. He used to be a prison boy around Dai Zong. He has a different name, called Black Cyclone. His name is Li Tieniu abroad. He is a big black man, with a red beard, rude personality and good shirtless. He is good at using two axes, and he just wants to kill people.