From an engineering viewpoint, your typical, low carbon, 1020 steel found on most entry level bicycle frames is considered "soft" and has a tensile strength of around 56,000 psi. "High carbon" steels typically have tensile strengths around 100,000 psi and is generally used for tooling
Of course, without knowing the actual composition of the Nishiki tubing it's hard to say exactly what it is. Presumibly it falls somewhere between the two examples. Having higher carbon, it should be stronger than your typical 1020.
I have a fairly extensive set of specs for Nishiki models. If you post or send me the components list, I may be able to identify the model.