Metal treatment – Stock – Ferrous
Patent
1996-10-10
1998-05-26
Yee, Deborah
Metal treatment
Stock
Ferrous
148909, 420119, C22C 3844, C22C 3848
Patent
active
057558957
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an ultra-high strength steel having a tensile strength (TS) of at least 950 MPa and excellent in low temperature toughness and weldability, which can be widely used as a weldable steel material for line pipes for transporting natural gases and crude oils, various pressure containers, industrial machinery, and so forth.
BACKGROUND ART
The strength of line pipes used for pipelines for the long distance transportation of crude oils and natural gases has become higher and higher in recent years due to 1 an improvement in transportation efficiency by higher pressure and 2 an improvement in on-site execution efficiency by the reduction of outer diameters and weights of the line pipes. Line pipes having X80 according to the American Petroleum Institute (API) standard (yield strength of at least 551 MPa and tensile strength of at least 620 MPa) have been put into practical use to this date, but the need for line pipes having a higher strength has become stronger and stronger.
Studies on the production methods of ultra-high strength line pipes have been made at present on the basis of the conventional production technologies of X80 line pipes (for example, NKK Engineering Report, No. 138 (1992), pp. 24-31 and The 7th Offshore Mechanics and Arctic Engineering (1988), Volume V, pp. 179-185), but the production of line pipes having X100 (yield strength of at least 689 MPa and tensile strength of at least 760 MPa) is believed to be the limit according to these technologies
To achieve an ultra-high strength of pipe lines, there are a large number of problems yet to be solved, such as the balance between strength and low temperature toughness, the toughness of a welding heat affected zone (HAZ), field weldability, softening of joints, and so forth, and accelerated development of a revolutionary ultra-high strength line pipe (exceeding X100) which solves these problems has been earnestly desired.
DISCLOSURE OF THE INVENTION
In order to satisfy the requirements described above, the first object of the present invention is to provide a steel for a line pipe which has an excellent balance of a strength and a low temperature toughness, can be easily welded on field, and has an ultra-high strength and a low yield ratio of a tensile strength of at least 950 MPa (exceeding X100 by the API standard).
It is another object of the present invention to provide a steel for a high strength line pipe which is a low carbon high Mn (at least 1.7%) type steel containing Ni-Nb-Mo-trace Ti added compositely, and (2 the micro-structure of which comprises a soft/hard mixed structure of fine ferrite (having a mean grain size of not greater than 5 .mu.m and containing a predetermined amount of worked ferrite) and martensite/bainite.
The present invention specifies a P value (hardenability index) as a usable strength estimation formula of a steel which expresses the hardenability index for high strength line pipe steels and represents a value indicating higher transformability to a martensite or bainite structure when it takes a large value, and this P value can be given by the following general formula:
Further, the ferrite mean grain size is defined as a mean grain boundary distance of the ferrite when measured in the direction of the thickness of the steel material.
The present invention provides a high strength line pipe steel (1) which is a low carbon high Mn type steel containing Ni-Mo-Nb-trace Ti-trace B compositely added thereto, and a low carbon high Mn type steel containing Ni-Cu-Mo-Nb-trace Ti compositely added thereto, and (2) the micro-structure of which comprises a two-phase mixed structure of a fine ferrite (having a mean grain size of not greater than 5 .mu.m and containing a predetermined amount of worked ferrite) and martensite/bainite.
Low carbon-high Mn-Nb-Mo steel has been known in the past as a line pipe steel having a fine acicular ferrite structure, but the upper limit of its tensile strength is 750 MPa at the highest. In this basic component system, a high strength line pip
REFERENCES:
patent: 4222771 (1980-09-01), Oda et al.
patent: 4464209 (1984-08-01), Taira et al.
Asahi Hitoshi
Hara Takuya
Tamehiro Hiroshi
Terada Yoshio
Nippon Steel Corporation
Yee Deborah
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