steam line material

Carbon steel is a lot cheaper when compared to stainless steel, which is why it is typically used for steam service. Stainless steel can be used if cost is not an issue, or if purity is an issue. Most commonly stainless is used in clean steam applications in the pharmaceutical industry where rust and the usual iron oxides that form with carbon steel are not acceptable.

Be careful if combining the two because you can get galvanic corrosion where the two services meet.

carbon steel is used up to a temperature of approx 400°C, above this temp you can have serious corrosion problems.

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corrosion prevention

It's not ONLY cost saving... cost saving is almost everything. If youre talking about some meters of piping in a local circuit, than maybe you could afford stainless steel because it shines and look beautiful over time. But, if you are talking about a large plant with hundreds of kilometers of pipes, valves, connections, joints, all requiring to be compatible, there's a lot of money in the game. A carbon steel could stand decades of good services, and then be replaced in a programmed maintenance stop.