What is 4043 Steel on a knife?

I have a Smith & Wesson isted opening. I was wondering if the blade was stainless, and it says it's made of 4043 Steel, when I Googled 4043 Steel, it came up with lots of answers that says that 4043 is an Aluminum Alloy, but how can it be an Aluminum Alloy is nonferrous and steel is ferrous? Did they add something to the Aluminum to make it Steel?