Steel Pokemon Weaknesses
Fire, ground and fighting Pokemon types are all super-effective against steel-type Pokemon. Additionally, the majority of steel-type Pokemon are also dual-type, possessing a secondary elemental type. While this can mitigate some of steel's weaknesses, it also can make the Pokemon twice as susceptible to damage. For example, Forretress' dual steel-bug type takes quadruple damage from fire-type Pokemon.
Statistical Weaknesses
Additional steel-type Pokemon weaknesses reside in their defensive nature. Because of stat allocations, steel's high defense means it has a low offense. Steel's low number of pure offensive moves and the fact that only two elemental types are particularly weak to steel-type moves, makes steel-type Pokemon highly deficient offensively.
Steel Pokemon Strengths
Steel-type Pokemon generally use their defense to outlast physically punishing opponents. To offensively combat opponents, steel-types often rely on their secondary elemental type and additional learned moves in order to counter non-ice or rock Pokemon. While most steel-types have stats designed for defense, there are a few examples that have high offensive stats, such as dual steel- and fighting-type Lucario.
Trivia
Byron, the leader of the Canalave gym in "Pokemon Platinum," "Pokemon Diamond" and "Pokemon Pearl," uses steel-type Pokemon. Jasmine, the gym leader of Olivine City in "Pokemon HeartGold" and "Pokemon SoulSilver" also uses primarily steel-type Pokemon. While steel-type Pokemon were introduced in the second generation of games, they were retroactively added as an elemental type to "Pokemon Magnemite" and "Pokemon Magneton," both first-generation games.