(19)
(11) EP 1 520 912 A3

(12) EUROPEAN PATENT APPLICATION

(88) Date of publication A3:
27.04.2005 Bulletin 2005/17

(43) Date of publication A2:
06.04.2005 Bulletin 2005/14

(21) Application number: 04026201.6

(22) Date of filing: 13.11.2000
(51) International Patent Classification (IPC)7: C22C 38/00, C22C 38/04
(84) Designated Contracting States:
DE FR

(30) Priority: 10.02.2000 JP 2000033241
10.02.2000 JP 2000033242
13.03.2000 JP 2000068210

(62) Application number of the earlier application in accordance with Art. 76 EPC:
00974966.4 / 1262571

(71) Applicant: NIPPON STEEL CORPORATION
Tokyo 100-8071 (JP)

(72) Inventors:
  • Minagawa, Masanori, Nippon Steel Corp.
    Oita 870-8566 (JP)
  • Koseki, Toshihiko, Nippon Steel Corp.
    Futtsu City, Chiba, 293-0011 (JP)
  • Funatsu, Yuji, Nippon Steel Corp.
    Oita 870-8566 (JP)
  • Ohtani, Jun Nippon Steel Corp.
    Oita 870 8566 (JP)
  • Hada, Tomohiko Nippon Steel Corp.
    Oita 870-8566 (JP)
  • Ishikawa, Tadashi, Nippon Steel Corp.
    Futtsu City, Chiba, 293-0011 (JP)
  • Nagahara, Masaaki Nippon Steel Corp.
    Oita 870-8566 (JP)

(74) Representative: VOSSIUS & PARTNER 
Siebertstrasse 4
81675 München
81675 München (DE)

   


(54) Steel excellent in toughness of weld heat-affected zone


(57) The present invention realizes excellent HAZ toughness by further suppressing the coarsening of austenite grains when they are heated for a long time at a high temperature in order to remarkably improve HAZ properties.
The present invention is a steel excellent in the toughness of a weld heat-affected zone: having a specific chemical composition in mass, of
  • C: 0.03 to 0.18%,
  • Si: 0.5% or less,
  • Mn: 0.4 to 2.0%,
  • P: 0.02% or less,
  • S: 0.02% or less,
  • Al: 0.005 to 0.04%,
  • Ti: 0.005 to 0.03%,
  • Ca: 0.0005 to 0.003%, and
  • N: 0.0005 to 0.007%,
with the balance consisting of Fe and unavoidable impurities; satisfying the expression 0 ≦ EN ≦ 0.002 regarding the EN value expressed by an equivalent equation comprising (%N), (%Ti) and (%B); and containing oxide particles 0.005 to 2.0 µm, preferably 0.1 to 2.0 µm, in circle-equivalent diameter, which oxide particles contain at least Ca, Al and O in their composition and, in the mass percentage of the elements excluding O, contain Ca at 5% or more and Al at 5% or more with the balance consisting of other deoxidizing elements and/or unavoidable impurities, with the density of the oxide particles being 100 to 3,000 pieces/mm2.






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