Electric heating – Metal heating – Of cylinders
Reexamination Certificate
1998-03-18
2001-02-13
Ryan, Patrick (Department: 1725)
Electric heating
Metal heating
Of cylinders
C219S06000A
Reexamination Certificate
active
06188037
ABSTRACT:
TECHNICAL FIELD
The present invention relates to welded high-strength steel structures, such as welded steel pipes, pipelines, marine structures, pressure vessels, and tanks, formed from steel having a tensile strength (TS) of not less than 900 MPa and excellent low-temperature toughness, as well as to methods of manufacturing the same.
BACKGROUND ART
In pipelines for long-distance conveyance of natural gas, crude oil, and the like, conveyance efficiency is improved through increasing running pressure, whereby conveyance cost is reduced. In order to increase running pressure, the wall thickness of pipe must be increased, or the strength of pipe material must be increased. However, the increase of the wall thickness of pipe involves impairment of field weldability and the necessity of enhancing the foundation structure to cope with an increase in pipeline weight. Under these circumstances, there have been increasing needs for enhancing the strength of welded steel pipes. For example, recently, American Petroleum Institute (API) has standardized X80 grade welded steel pipes having a yield strength (YS) of not less than 551 MPa and a TS of not less than 620 MPa, and put them into practical use.
As a result of enhancement of the strength of welded steel pipes, the manufacture of welded steel pipes of up to X100 grade (YS: not less than 689 MPa; TS: not less than 760 MPa) based on the technique for manufacturing X80 grade welded steel pipes is known to be feasible. Furthermore, there has been proposed high-tensile-strength steel having excellent low-temperature toughness and field weldability and a TS of not less than 950 MPa (Japanese Patent Application Laid-Open (kokai) Nos.
8-104922
and
8-209291
).
As far as low-temperature toughness and resistance to cold weld cracking at a relatively small heat input are concerned, the manufacture of steel products used for welded high-strength steel pipes is feasible through the above-mentioned technical development. However, the manufacture of high-strength welded steel pipes requires not only the above-mentioned high-tensile-strength steel but also high-strength weld metal having appropriate toughness. It has been known that the toughness of weld metal is improved through refinement of microstructure. Specifically, there emerges a wide practical use of a weld metal in which fine “acicular ferrite” is formed by adjusting the Al/O (oxygen) value through addition of trace Ti and B into the weld metal. However, generally, strength attained by acicular ferrite is limited. Acicular ferrite in weld metal cannot stably provide a TS of not less than 900 MPa. Accordingly, in order to obtain a TS of 900 MPa while appropriate toughness is provided, another method must be employed. Particularly, when welding heat input is increased in order to improve efficiency of welding, the cooling rate of weld metal decreases. Accordingly, a TS of not less than 900 MPa becomes difficult to attain.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide welded high-strength steel structures (welded steel pipes, pipelines, marine structures, and the like) having low-temperature toughness and a TS of not less than 900 MPa even when welded at an input heat of up to 10 kJ/mm, as well as to provide a method of manufacturing the same. Particularly, an object of the present invention is to provide welded steel pipes whose weld metal has the following performance characteristics.
Tensile performance: TS≧900 MPa
Impact performance: Upper shelf energy ≧80J; transition temperature of fracture appearance vTs<−50° C.
Generally, as temperature decreases, steel becomes brittler, and brittle cracking is more likely to be initiated from a smaller defect. The transition temperature vTs of fracture appearance serves as a measure temperature at which brittle fracture is not initiated from a defect so small as and undetectable one through ordinary nondestructive testing. The lower the vTs, the less likely the initiation of brittle fracture. Upper shelf energy serves as an index indicating how much energy the propagation of ductile fracture requires. The higher the upper shelf energy, the less likely the initiation of unstable ductile fracture.
To achieve the above objects, the inventors of the present invention manufactured various welded joints by submerged arc welding (SAW) and gas metal arc welding (GMAW) and tested their strength and low-temperature toughness. Specifically, through use of base metals and wires having various compositions and fluxes having different basic indexes, the metallic components of weld metal and the oxygen content of weld metal were varied. The oxygen content of weld metal formed by SAW was primarily adjusted through variation of the composition of flux. The thus-obtained weld metals were tested for low-temperature toughness, and the following was confirmed.
a) As shown in
FIG. 1
, the TS of weld metal increases with Pcm(defined later), and some weld metals show a TS of not less than 900 MPa at a Pcm of 0.25% or greater.
b) For weld metals having a TS of not less than 900 MPa, lower bainite occupies a considerable ratio in the microstructure. By contrast, weld metals having a TS of less than 900 MPa show a microstructure composed primarily of fine acicular ferrite.
c) As seen from comparison among weld metals having identical values of Pcm, weld metals having an Al/O (oxygen) value greater than 0.6 show a marked increase in TS. Also, at a Pcm of 0.25% or greater, the percentage of lower bainite increases with the Al/O value. At an Al/O value of 1.2 or greater, lower bainite becomes dominant in the microstructure. At a Pcm of 0.25% or greater and an Al/O value of 0.6 to 1.2, only a mixed structure of acicular ferrite and lower bainite is observed, and upper bainite is hardly observed.
d) When the microstructure changes from acicular ferrite to upper bainite with increasing Al/O, there occurs a significant impairment (increase) in the transition temperature of fracture appearance. By contrast, when the microstructure changes from acicular ferrite to lower bainite, toughness is hardly impaired.
e) Upper shelf energy decreases with increasing strength of weld metal and with increasing oxygen content of weld metal.
The gist of the present invention is to provide the following welded high-strength steel structures and the following method of manufacturing the same. In the following description, “%” accompanying an alloy element refers to “% by weight” or “wt. %” unless otherwise specified.
(1) A welded high-strength steel structure, wherein a base metal is a steel whose microstructure is substantially formed of a mixed structure of martensite and lower bainite and which has a tensile strength of not less than 900 MPa; and a weld metal is a steel which contains O (oxygen) in an amount not greater than 0.06 wt %, satisfies equations 1) and 2) below, and has a tensile strength of not less than 900 MPa.
0.25≦Pcm≦0.32
Pcm =C+(Si/30)+(Mn/20)+(Ni/60)+(Cu/20) +(Cr/20)+(Mo/15)+(V/10)+5B 1)
0.6≦Al/O (oxygen)≦1.4 2)
wherein each atomic symbol in equations 1) and 2) represents its content (wt %) within the steel.
(2) A welded high-strength steel structure, wherein a base metal is a steel whose microstructure is substantially formed of a mixed structure of martensite and lower bainite and which has a tensile strength of not less than 900 MPa; and a weld metal is a steel which comprises the following elements by weight %: C: 0.01% to 0.15%; Si: 0.02% to 0.6%; Mn: 0.6% to 3%; Al: 0.004% to 0.08%; Ti: 0.003% to 0.03%; O (oxygen): not greater than 0.06%; B: 0.0002% to 0.005%; Cu: 0% to 1.2%; Ni: 0% to 3%; Cr: 0% to 1.2%; Mo: 0% to 1%; V: 0% to 0.05%; and Nb: 0% to 0.05%, and satisfies the above-described equations 1) and 2).
(3) A welded high-strength steel structure described above in (1) or (2), wherein the tensile strength of the weld metal is greater by 20-150 MPa than that of the base metal.
(4) A welded high-strength steel structure described above
Hamada Masahiko
Komizo Yu-ichi
Moncho Takeshi
Burns Doane , Swecker, Mathis LLP
Elve M. Alexandra
Ryan Patrick
Sumitomo Metal Industries Ltd.
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