Metal treatment – Stock – Ferrous
Patent
1997-01-14
1998-08-25
Yee, Deborah
Metal treatment
Stock
Ferrous
148330, 148335, 420119, C22C 3808, C22C 3848
Patent
active
057980047
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, and this steel can widely be used for line pipes for transporting natural gases and crude oils and as a weldable steel material for various pressure containers and industrial machinery.
BACKGROUND ART
Recently, the required strength of line pipes used for the long distance transportation of crude oils and natural gases has become higher and higher due to (1) an improvement in transportation efficiency by higher pressure, and (2) an improvement in laying efficiency due to the reduction of outer diameters and weights of line pipes. Line pipes having a strength of up to X80 according to the American Petroleum Institute (API) (at least 620 MPa in terms of tensile strength) have been put into practical application in the past, but the need for line pipes having a higher strength has increased.
Conventionally, an ultra-low carbon-high Mn--Nb--(Mo)--(Ni)-trace B-trace Ti steel has been known as a line pipe steel having a structure comprising mainly fine bainite, but the upper limit of its tensile strength is at most 750 MPa. In this basic chemical composition system, an ultra-high strength steel having a structure mainly comprising fine martensite does not exist. It had been believed that a tensile strength exceeding 950 MPa can never be attained by the structure mainly comprising bainite and furthermore, the low temperature toughness is deteriorated if the martensite structure increases.
Studies on the production method of ultra-high strength line pipes have been made at present on the basis of the conventional X80 line pipe production technologies (for example, "NKK Engineering Report", No. 138 (1992), pp. 24-31, and "The 7th Offshore Mechanics and Arctic Engineering" (1998), Volume V, pp. 179-185), but the production of line pipes of X100 (tensile strength of at least 760 MPa) is believed to be the limit according to these technologies.
To achieve an ultra-high strength in pipe lines, there are a large number of problems yet to be solved such as the balance of strength and low temperature toughness, toughness of a welding heat affected zone (HAZ), field weldability, softening of a joint, and so forth, and an rapid development of a revolutionary ultra-high strength line pipe (exceeding X100) has been sought.
To satisfy the requirements described above, the present invention aims at providing an ultra-high strength weldable steel having an excellent balance between the strength and the low temperature toughness, being easily weldable on field and having a tensile strength of at least 950 MPa (exceeding X100 of the API standard).
DISCLOSURE OF THE INVENTION
The inventors of the present invention have conducted intensive studies on the chemical components (compositions) of steel materials and their micro-structures in order to obtain an ultra-high strength steel having a tensile strength of at least 950 MPa and excellent in low temperature toughness and field weldability, and have invented a new ultra-high strength weldable steel.
It is the first object of the present invention to provide a new ultra-high strength weldable steel, which is a low carbon-high Mn type steel containing Ni--Mo--Nb-trace Ti compositely added thereto, and having a tensile strength of at least 950 MPa and excellent in low temperature toughness and site weldability in cold districts.
It is the second object of the present invention to provide a steel which has a P value, defined by the following chemical formula, within the range of 1.9 to 4.0 in the chemical compositions constituting the ultra-high strength weldable steel described above. Needless to say, this P value changes somewhat depending on various ultra-high strength weldable steels provided by the present invention.
The term "P value" (hardenability index) defined in the present invention represents a hardenability index. When this P value takes a high value, it indicates that the structure is l
REFERENCES:
patent: 5213634 (1993-05-01), Deardo et al.
patent: 5454883 (1995-10-01), Yoshie et al.
NKK Engineering Report No. 138 (1992), M. Nagae et al., "Development of X100 UOE Line Pipe", pp. 24-31.
Proceedings Of The Seventh International Conference On Offshore Mechanics And Arctic Engineering, OMAE 1988, Houston, T. Hashimoto et al., "Recent Developments Of Large Diameter Line Pipe Of Grade X-80 and X-100", pp. 179-185.
Asahi Hitoshi
Hara Takuya
Tamehiro Hiroshi
Terada Yoshio
Nippon Steel Corporation
Yee Deborah
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