when did they become the Steelers?
Wikipedia doesn't give the exact date. The following excerpt implies it was 1940 or 1941.
"The Pittsburgh Steelers of the NFL first took to the field as the Pittsburgh Pirates on September 20, 1933, losing 23-2 to the New York Giants.[3] Through the 1930s, the Pirates never finished higher than second place in their division, or with a record better than 0.500 (1936).[4] Pittsburgh did make history in 1938 by signing Byron White, a future justice of the U.S. Supreme Court, to what was at the time the biggest contract in NFL history,[5] but he played only one year with the Pirates before signing with the Detroit Lions.
During World War II, the Steelers experienced player shortages. They twice merged with other NFL franchises to field a team. During the 1943 season, they merged with the Philadelphia Eagles forming the "Phil-Pitt Eagles" and were known as the "Steagles." This team went 5-4-1. In 1944 they merged with the Chicago Cardinals and were known as Card-Pitt.[5] This team finished 0-10, marking the only winless team in franchise history."