The Steelers defense has led the team to a three-game winning streak and a 6-2 record that is tied for best in the AFC with the New England Patriots and Kansas City Chiefs at the halfway point of the NFL season. It ranks second in the league in points allowed, fourth in total yards, second in passing yards, third in sacks, fourth in third-down efficiency and 10th in red-zone efficiency. It has had moments of failure, but it largely has improved each week. “This is what we’re supposed to do,” defensive captain Cam Heyward said. Actually, the defense should be even better. “They’ve invested a lot in us,” Heyward said. Have you checked out the makeup of the defensive roster? Heyward is a No. 1 draft pick, defensive end Stephon Tuitt a No. 2 and nose tackle Javon Hargrave a No. 3. Top defensive line reserve Tyson Alualu is a former No. 1 pick of the Jacksonville Jaguars and had a big sack during one of the three goal-line stands against the Detroit Lions last Sunday night. Heyward and Tuitt are on their second contracts, Heyward a six-year, $59.25 million deal through 2020 and Tuitt a six-year, $61.05 million deal through 2022. Outside linebackers Bud Dupree and T.J. Watt are No. 1 picks. So is inside linebacker Ryan Shazier. Vince Williams was a terrific bargain in the sixth round in 2013 and made former first-rounder Lawrence Timmons expendable after last season. He had eight tackles against the Lions.