The leaders had Ulsan lurking just a point behind them before Saturday's game in Incheon, and were in trouble when Lee Chun-soo (39) and Park Tae-min (73) put the hosts apparently on course for victory.
Park Sung-ho (77 and 90) responded with a late double, however, to keep the Steelers out in front before Ulsan host Busan I'Park next weekend.
In the relegation group, Gangwon FC came from behind to win 3-1 at Daejeon Citizen.
Lee Dong-hyun scored on the half-hour mark but Choi Jin-Ho (39) equalised before Chun Jae-ho (49 and 53) won it for Gangwon with a pair of second-half goals.
There was a comeback for Seongnam Chunma, who responded to Jeong Shung-hoon's 30th-minute opener for Gyeongnam with a leveller from Kim Tae-hwan before Server Djeparov sealed a 2-1 win with a last-minute penalty.
Gangwon's win lifted them above new bottom side Daejeon while Seongnam are the best of the worst seven sides. Gyeongnam are fourth in the mini-group.