PITTSBURGH — The Pittsburgh Steelers spent two years rebuilding themselves on the fly, a necessary process that frequently looked more erratic than inspiring. Ben Roethlisberger passed for 220 yards and a touchdown, Le’Veon Bell added a score and the Steelers locked up a postseason berth with a methodical 20-12 win over the Kansas City Chiefs on Sunday. “It was such a sense of urgency this week, knowing what was at stake,” defensive end Cam Heyward said. [...] the Steelers are in the playoffs for the first time since Tim Tebow and Denver stunned the defending AFC champions in the wild-card round three years ago. The Chiefs need to beat San Diego on Sunday and get some help to make it back to the playoffs for a second straight year under Andy Reid. The Chiefs outgained the NFL’s top-ranked offense 327 yards to 288 but couldn’t seem to get the one momentum-turning play that mattered. Kansas City came into Sunday second in the NFL in red-zone touchdown percentage only to have six drives into Pittsburgh territory end with four field goals by Cairo Santos, Jamaal Charles’ fumble and a fourth-down gamble late in the first half that fizzled. [...] the Steelers are back playing their best football when the temperature drops.