= Solution
At $700^\circ\mathrm C$ the undercooling below the eutectoid <temperature> is small, so the free-energy driving <force> for forming <pearlite> is weak and <nucleation> is difficult. At $300^\circ\mathrm C$ the driving <force> is larger, but <carbon> <diffusion> is slow, limiting the coupled growth of <ferrite> and <cementite>. Their competition produces the nose of the <isothermal transformation diagram> near $550^\circ\mathrm C$ and $1\,\mathrm s$. A roughly linear quench from $740$–$800^\circ\mathrm C$ must pass left of this nose, requiring a rate of order \b[$2\times10^2$–$3\times10^2\,\mathrm{K\,s^{-1}}$], then continue below the martensite-start region near $240^\circ\mathrm C$. The tangent to the full start curve, rather than just the nose, refines this estimate; a true continuous-cooling diagram would be preferable. <Martensite> forms by a coordinated lattice-shape change without long-range <carbon> partitioning, so its formation is a <displacive transformation>. The tetragonal-to-monoclinic transformation of <zirconia> is a nonmetallic example.
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