It may or may not be 5160, unless the spring came from a very old auto. There are a number of steels used for auto springs since the 40's or 50's, and they are all a little different, and react differently in heat treat. Most suitable steels for knives are relatively cheap, and it is always best to use a known steel with a heat treat that is known to be best for it. There are those who enjoy playing with mystery steels, but one can often save a lot of work and headaches by knowing what steel he is using, and how to treat it.