Steel is homogeneous, because it is an alloy. An alloy is a solid mixture of different metal components. In a homogeneous mixture, the components contained in the mixture are uniformly distributed throughout the mixture.
In the case of stainless steel, it is an alloy made from the combination of iron with chromium and nickel. Some other alloys that are homogeneous solid mixtures are brass and bronze. There also are homogeneous gaseous and liquid mixtures. While salt water is an example of a liquid homogeneous mixture, air is an example of a homogeneous gaseous mixture.
Heterogeneous mixtures are other types of mixtures. A heterogeneous mixture consists of two or more substances that are not uniformly distributed in the mixture. Some examples of heterogeneous mixtures are oil and vinegar in salad dressing and oil plus water.