Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Yoshihide Nagai

Yoshiyuki Watanabe

Ryuji Uemori

Rikio Chijiiwa

Kazuhiro Fukunaga

Akihiko Kojima

The present invention provides a high strength steel having unprecedentedly superior CTOD (fracture toughness) properties satisfying not only the CTOD properties of the FL zone at &#x2212;60&#xb0; C., but also the CTOD properties of the ICHAZ zone in small and medium heat input multilayer welding, etc.; and a method of production of the same. The steel of the present invention is steel superior in CTOD properties of the heat-affected zone containing, by mass %, C: 0.015 to 0.045%, Si: 0.05 to 0.2%, Mn: 1.5 to 2.0%, Cu: 0.25 to 0.5%, Ni: 0.7 to 1.5%, P: 0.008% or less, S: 0.005% or less, Al: 0.004% or less, Ti: 0.005 to 0.015%, Nb: 0.005% or less, O: 0.0015 to 0.0035%, and N: 0.002 to 0.006%, P: 0.065 or less, CeqH: 0.235 or less and the balance consisting of Fe and unavoidable impurities.

Steel superior in CTOD properties of weld heat-affected zone and method of production of same

A method of production of 780 MPa class high strength steel plate excellent low temperature toughness comprising heating a steel slab of containing, by mass %, C: 0.06 to 0.15%, Si: 0.05 to 0.35%, Mn: 0.60 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cu: 0.1 to 0.5%, Ni: 0.1 to 1.5%, Cr: 0.05 to 0.8%, Mo: 0.05 to 0.6%, Nb: less than 0.005%, V: 0.005 to 0.060%, Ti: less than 0.003%, Al: 0.02 to 0.10%, B: 0.0005 to 0.003%, and N: 0.002 to 0.006% to 1050&#xb0; C. to 1200&#xb0; C. in temperature, hot rolling ending at 870&#xb0; C. or more, waiting for 10 seconds to 90 seconds, then cooling from 840&#xb0; C. or more in temperature by a 5&#xb0; C./s or more cooling rate to 200&#xb0; C., then tempering at 450&#xb0; C. to 650&#xb0; C. in temperature for 20 minutes to 60 minutes.

Method of production of 780 MPa class high strength steel plate excellent in low temperature toughness

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Tokyo, Japan

Yoshihide Nagai