This depends upon the type of steel. You need to say which kind.
Most steel undergoes a "yield" failure in which it stretches beyond the elastic limit before breaking at an "ultimate" stress value. The amount of "permanent set" deformation before rupture is very dependent upon steel type.
For an approximate value of the strain at rupture, divide the ultimate stress (where rupture occurs) by Young's modulus.
See
http://www.ami.ac.uk/courses/topics/0123_mpm/index.html
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Calculate the maximum amount of stretching (in percent) that a piece of steel can undergo before breaking under tension
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