Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-08-29
2003-04-01
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S593000, C420S119000, C420S124000
Reexamination Certificate
active
06540848
ABSTRACT:
TECHNICAL FIELD
The present invention relates to high-strength, high-toughness seamless steel pipes used for line pipes, and more particularly, the invention relates to a high-strength, high-toughness seamless steel pipe used for a grade X80 line pipe according to the API-5L and to a method for producing the same.
BACKGROUND ART
Grade X80 seamless steel pipes have been developed for pipelines and risers used for transporting crude oil and natural gas. In order to secure grade X80 strength (YS: 551 MPa or more, TS: 620 to 827 MPa), heat treatment is usually carried out either by:
1) so-called “reheating quenching-tempering” (RQ-T) in which cooling is performed after seamless steel pipes are produced, and then reheating, quenching, and tempering are performed, or by
2) so-called “direct quenching-tempering” (DQ-T) in which quenching is performed directly after seamless steel pipes are produced, followed by tempering.
Pipes are joined together by welding. In order to secure weldability, the C content must be reduced. In order to secure sufficient hardenability of low C steel, appropriate amounts of various alloy elements must be added thereto.
It is known that the addition of a slight amount of B is effective in improving hardenability of low C steel. However, B has a side effect, namely, B adversely affects toughness of welded joints. Moreover, since the effect is greatly influenced by the contents of precipitation generating elements, such as N and Ti, it is difficult to stably ensure toughness by the addition of a slight amount of B. Additionally, in the present invention, the target toughness is set at a vTrs (50% fracture appearance transition temperature) of −60° C. or less in the base metal and at a vTrs of −40° C. or less in the HAZ (Heat Affected Zone at the welded joint).
Since the hardenability greatly depends on the size of the steel pipe, in order to secure stable strength for each size, tempering conditions must be adjusted for each size. However, with respect to conventional seamless steel pipes, since the resistance to temper softening is excessively large, it is difficult to stably ensure the strength unless the chemical composition is changed for each size.
Accordingly, it is an object of the present invention to provide a high-strength, high-toughness seamless steel pipe used for a line pipe in which X80 grade strength and toughness can be stably ensured, and the target strength can be easily attained regardless of the size.
DISCLOSURE OF INVENTION
In one aspect of the present invention which has been made in order to achieve the object described above, a high-strength, high-toughness seamless steel pipe used for a line pipe contains 0.03 to 0.06% of C, 0.05 to 0.15% of Si, 1.6 to 2.0% of Mn, 0.010 to 0.10% of Al, 0.3 to 0.7% of Ni, 0.10 to 0.40% of Mo, 0.01 to 0.06% of V, 0.003 to 0.03% of Nb, 0.003 to 0.020% of Ti, and 0.0010 to 0.0100% of N, the relationships Mo+5V≧0.4% and 2Nb−V≦0% being satisfied, and the balance being Fe and incidental impurities.
Preferably, in the high-strength, high-toughness seamless steel pipe used for line pipes of the present invention, when hot rolling is performed on a material for the steel pipe and then quenching and tempering are performed, the difference in yield strength or tensile strength between after tempering at 600° C. and after tempering at 650° C. is 40 MPa or more.
Preferably, in the high-strength, high-toughness seamless steel pipe used for a line pipe, the yield strength, the tensile strength, and the 50% fracture appearance transition temperature in a Charpy impact test for the steel pipe which is hot rolled, quenched, and tempered have the following characteristic values:
YS (yield strength)≧551 MPa
TS (tensile strength): 620 to 827 MPa
vTrs (base metal)≦−60° C.
vTrs (welded joint HAZ: 1 mm from fusion line)≦−40° C.
In another aspect of the present invention, a method for producing a high-strength, high-toughness seamless steel pipe used for a line pipe, a material for the steel pipe being a steel containing 0.03 to 0.06% of C, 0.05 to 0.15% of Si, 1.6 to 2.0% of Mn, 0.010 to 0.10% of Al, 0.3 to 0.7% of Ni, 0.10 to 0.40% of Mo, 0.01 to 0.06% of V, 0.003 to 0.03% of Nb, 0.003 to 0.020% of Ti, and 0.0010 to 0.0100% of N, the relationships Mo+5V≧0.4% and 2Nb−V≦0% being satisfied, and the balance being Fe and incidental impurities, the method including the steps of:
heating the material for the steel pipe to the AC
3
point or higher;
making a pipe by hot rolling; and then either
(i) performing direct quenching (DQ) of the pipe for cooling the pipe to the Ms point or lower immediately after the pipe-making step, followed by tempering at a temperature lower than the Ac
1
point, or
(ii) cooling the pipe by air to the vicinity of room temperature, and performing reheating quenching (RQ) of the pipe for reheating the pipe to the Ac
3
point or higher and cooling the pipe to the Ms point or lower, followed by tempering at a temperature less than the Ac
1
point.
Preferably, in the method for producing a high-strength, high-toughness seamless steel pipe used for a line pipe, by hot rolling using the material for the steel pipe having the characteristics in that, when hot rolling is performed, followed by quenching and tempering, the difference in yield strength or tensile strength between after tempering at 600° C. and after tempering at 650° C. is 40 MPa or more, and after quenching is performed, by changing the tempering temperature, the seamless steel pipe having desired yield strength, tensile strength, and toughness is obtained.
Preferably, the yield strength, the tensile strength, and the 50% fracture appearance transition temperature in a Charpy impact test obtained by the method for producing a high-strength, high-toughness seamless steel pipe used for a line pipe are as follows:
YS (yield strength)≧551 MPa
TS (tensile strength): 620 to 827 MPa
vTrs (base metal)≦−60° C.
vTrs (welded joint HAZ: 1 mm from junction)≦−40° C.
BEST MODE FOR CARRYING OUT THE INVENTION
The reasons for specifying the limits in chemical compositions of steel in the present invention will be described below.
C: 0.03 to 0.06%
C is an important element affecting the strength of the steel. The C content must be 0.03% or more in order to improve hardenability so that grade X80 strength is ensured. If the C content exceeds 0.06%, the steel becomes more susceptible to weld-cracking. Therefore, the C content is set at 0.03 to 0.06%.
Si: 0.05 to 0.15%
Si is necessary as a deoxidizer in steelmaking and for increasing strength. If the Si content is less than 0.05%, the effects thereof are insufficient. If the Si content exceeds 0.15%, the toughness in the base metal and HAZ, and weldability are degraded. Therefore, the Si content is set at 0.05 to 0.15%.
Mn: 1.6 to 2.0%
Mn is necessary for increasing hardenability to obtain higher strength, and Mn also improves the toughness in the base metal and HAZ. If the Mn content is less than 1.6%, it is difficult to obtain such effects, and if the Mn content exceeds 2.0%, the effects are saturated. Therefore, the Mn content is set at 1.6 to 2.0%.
Al: 0.010 to 0.10%
Al acts as a deoxidizer in steelmaking and also refines grains by combining with N to form AlN, thereby improving toughness. In order to obtain such an effect, the Al content must be 0.010% or more. However, if the Al content exceeds 0.070%, the number of Al
2
O
3
-based inclusions increases, thereby degrading toughness, and surface defects may occur. Therefore, the Al content is set at 0.010 to 0.10%. Furthermore, in view of securing stable surface quality, the Al content is preferably set at 0.010 to 0.050%.
Ni: 0.3 to 0.7%
Ni improves toughness in the base metal and HAZ. The effect thereof is exhibited by addition of 0.3% or more of Ni. However, even if the Ni content exceeds 0.7%, the effect of improving toughness and corrosion resistance is saturated, resulting in an increase in costs, which is disadvantageous. Therefore, the Ni content is set at 0.3 to 0.7
Itakura Noritsugu
Kimura Mitsuo
Miyata Yukio
Toyooka Takaaki
Kawasaki Steel Corporation
Schnader Harrison Segal & Lewis LLP
Yee Deborah
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