What percentage of chromium is minimum in AISI 400 series steel?

AISI 400 series steels are categorized as martensitic stainless steels, which are known for their correlation between chromium content and corrosion resistance. The minimum chromium content specified for AISI 400 series stainless steels is 11%. This chromium content provides a primary benefit of the stainless steels, which is enhanced corrosion resistance while also contributing to increased hardness and strength compared to lower chromium steels.

In contrast, higher percentages of chromium, such as those given in the other options, are more characteristic of other stainless steel families. For example, the 300 series stainless steels typically contain higher chromium levels (around 18% to 20%), which enhances their corrosion resistance further but is not a defining characteristic of the 400 series. Therefore, the minimum chromium requirement for AISI 400 series steels is correctly identified as 11%, making the choice accurate.