I will address this problem from a very "classical physics" and engineering perspective. Physicists may offer some failure mechanism for the steel as the universe itself evolves significantly, but as far as I'm concerned it will never degrade, although "degrade" is a very subjective term.
What you're looking observe in the sample is long-term radiation embrittlement as well as swell and some other effects. The specifics will depend on the environment, which differs greatly from one point in space to another. Are we talking about interstellar space? Intergalactic space? Wherever it is, it will experience some radiation damage, and expect for a small volume of exceptions we are talking about a service life entailing:
- Low temperature
- Long life
- Basically no surface chemistry
- Load bearing or not unspecified
The first thing to understand is that critical metrics for radiation damage come from the time frame times the radiation energy deposition rate. From there, displacements per atom (dpa) is common metric representing the number of time every atom in the material is displaced from its lattice site on average. Values greater than 1 are common for discussions on this subject, but starting at around that magnitude effects like embrittlement and swell start to become very major. The steel will literally swell and becomes very brittle. If it is a load bearing member, the integrity of the structure it is a part of needs to be carefully considered for the long service life. However, effects like Stress Corrosion Cracking (SCC) will not be a major problem like they are in things like nuclear reactors because that entails a tipple attack of 1) high temperature 2) chemistry and 3) radiation. For the steel in space we have the radiation, low temperature, and no chemistry attack from the environment. Over a long time, self-annealing is also major, but that has an interplay with the temperature, and low temperature may slow that effect.
Generally though, the steel will maintain its structure. You need to ask someone familiar with space travel to quantitatively talk about the radiation environment, but since humans survive in LEO, the radiation in that kind of environment is probably not going to change the material structure of the steel for an extremely long time, at which point it's relatively accurate to say that it doesn't degrade.