| 摘要 |
A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (mum), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac3 transformation point, and applying reducing at an average rolling temperature of thetam (° C.) and a total reduction ratio Tred (%) within s temperature range of from 400 to Ac3 transformation point, with di, thetam and Tred being in a relation satisfying a prescribed equation.
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