The game remained goalless until the 70th minute, when Mauricio Molina put the hosts ahead, with Koh Myong-jin adding a second late on.
The result left the Steelers just a point ahead of Ulsan Hyundai, who did not play, having played a game more, with Seoul a further point back.
Jeonbuk Motors could not take full advantage of the leaders' slip-up as they were held to a 1-1 draw at Incheon United.
Kevin Oris put Jeonbuk ahead in the 36th minute, but Kim Jae-woong levelled in the 73rd minute, leaving the Motors three points off the top.
Lee Jin-ho scored his second goal of the game in the 90th minute to give Jeju United a 2-1 win at rock-bottom Daejeon Citizen.
Lee had opened the scoring in the 13th minute, but Jung Seok-min levelled in the 70th minute.
Daegu FC and Gangwon FC drew 1-1, Choi Jin-ho putting Gangwon ahead in the 50th minute and Choi Ho-jung equalising five minutes later.
Song Ho-young scored in the dying moments to give Chunnam Dragons a 1-0 win at Seongnam Chunma, while Oh Jang-eun's goal saw Suwon Bluewings beat Busan I'Park by the same scoreline.