Tar is a narcotic, strength enchancing drug, somewhat similar to the Venom compound that gave Bane his strength.
History
Tar's effects include granting temporary super-strength and nigh-invulnerability. John Henry Irons had numerous run-ins with gang members using tar to gain an advantage over their enemies; these drugged individuals were often known as "tar-freaks". A more insidious effect of Tar is it heightens the affected individual's aggressive instincts, making them actively seek out fights.
At one time, Steel was accused of a savage murder; the Washington, D.C. police claimed he used his giant hammer to decapitate a man. Steel was able to prove his innocence simply by listing the already well-known effects of Tar, and explained that a tar-freak could easily inflict the same injuries that his hammer could (which would indicate that a Tar-Freak's skin becomes at least as tough as Steel's hammer, and that their super-human strength rivaled many super heroes).
S-Tar
Later on, an enhanced version of this drug called "S-Tar" (short for "Super Tar") would be created, which granted even more strength and invulnerability, and was even more addicting. Also, once a person had used S-Tar three times, the drug buildup in their system would become fatal.
Notes
- After the Steel comic book ended, Tar and S-Tar quickly faded into obscurity.