Hot-rolled steel sheets, steel pipes, members with excellent impact resistance and their manufacturing methods

摘要

Preferred aspects of the present invention are C: 0.35 to 0.55%, Mn: 0.7 to 1.5%, Si: 0.3% or less (excluding 0%), P: 0. 03% or less (including 0%), S: 0.004% or less (including 0%), Al: 0.04% or less (0% excluded), Cr: 0.3% or less (0% excluded), Mo: 0.3% or less (0% excluded), Ni: 0.1 to 1.0% and Cu: 1 or 2 of 0.1 to 1.0%, Cu + Ni: 0.4% or more, N: It contains 0.006% or less (excluding 0%), the remaining Fe and other impurities, the alloying elements satisfy the following relational expressions 1 to 3, and the microstructure is 10% or more ferrite and 90% or less by volume. Provided are a hot-rolled steel plate containing pearlite and excellent impact resistance, steel pipes and members using the same, and a method for manufacturing these. [Relationship formula 1] (Mn / Si) ≧ 3 (weight ratio) [Relationship formula 2] (Ni + Cu) / (C + Mn) ≧ 0.2 (weight ratio) [Relationship formula 3] (Ni / Si) ≧ 1 (weight) ratio)