JP3046183B2 - Manufacturing method of low hardness and high toughness seamless steel pipe with excellent SSC resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low-hardness and high-toughness seamless steel pipe having excellent SSC resistance.

[0002]

2. Description of the Related Art In recent years, gas wells as energy resources,
Oil well development tends to be extremely northern in an environment with a high concentration of hydrogen sulfide, and seamless steel pipes used as development equipment require low hardness and high toughness to maintain hydrogen cracking resistance. Have been. Conventionally, to satisfy such characteristics, ASTM No. It has been confirmed by the present inventors that it is difficult to achieve a crystal grain size of 6 or less.

[0003] On the other hand, the rolling of a hot seamless steel pipe is divided into a piercing rolling, a stretching rolling, and a finishing rolling process of a cast steel slab.
It is performed in the above high temperature range. Therefore, the growth of recrystallized grains is remarkably fast, and the crystal grain size is determined according to ASTM No. 6 or less coarse particles. That is, in order to satisfy the characteristics required for oil well equipment in recent years, ASTM No. It is necessary to stably obtain a refined and sized structure of 6 or more.
o. Since a direct quenching step of quenching and tempering performed after hot seamless rolling is not enough to obtain a microstructure of 6 or more, for example, a hollow shell which has been roughly hot rolled as disclosed in JP-A-52-77813. Once the steel Ar ₁
Or lower to a temperature below the point, heated again to the austenitizing temperature, and then finish rolling and quenching-tempering,
Alternatively, it was necessary to perform reheating quenching and tempering after normal finish rolling.

[0004]

However, any of the above-mentioned methods has a drawback that the manufacturing process is complicated in addition to the problem of thermal efficiency. On the other hand, ASTM No., which is a necessary condition that can satisfy the characteristics required for recent oil well development with conventional hot seamless rolling as it is. Since a fine grain structure of 6 or more cannot be obtained, there has been a problem that a low-hardness and high-toughness seamless steel pipe having excellent SSC resistance cannot be obtained in a saving step such as a direct quenching treatment. An object of the present invention is to solve such a conventional problem, and an object of the present invention is to provide a method for producing a low-hardness and high-toughness seamless steel pipe having excellent SSC resistance by controlling steel composition and hot rolling conditions. And

[0005]

Means for Solving the Problems The present invention has been made to achieve the above, and the gist of the invention is that C: 0.03 to 0.20% by weight and Si: 0.01
-0.5%, Mn: 0.15-2.5%, P
: 0.020% or less, S: 0.010% or less,
Al: 0.005 to 0.1%, Ti: 0.00
5 to 0.1%, Nb: 0.005 to 0.1%,
N: 0.01% or less, and if necessary, Cr: 0.1 to 1.5%, Mo: 0.05
-0.5%, Ni: 0.1-2.0%, V
: 0.01 to 0.1%, B: 0.0003 to 0.0
030%, one or more kinds, and if necessary, rare earth elements: 0.001 to 0.05%, Ca: 0.00
1 to 0.02%, Co: 0.05 to 0.5%,
Cu: a steel slab containing 0.1 to 0.5% of one or more kinds and the balance substantially consisting of Fe is 1000 to 12%.
After heating to 50 ° C., hot piercing and rolling was performed, and the hollow shell raised to 1000 ° C. or higher due to the heat generated by working was forcibly cooled from the inner surface of the hollow shell before the final inclined rolling mill, and the entire shell was Ar _Three
After performing the homogenization treatment to a temperature between the point and 1100 ° C.,
After performing inclined rolling at a thickness reduction ratio of 20 to 70% and further performing shape-correcting hot continuous rolling, Ar ₃ points to 900
C. to a temperature lower than the temperature.
After reheating to ₃ points +50 to 1000 ° C., hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C. or higher, or when the raw material tube has a sufficiently high temperature of Ar ₃ points + 50 ° C. or higher. Is subjected to hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C. or more without reheating, and then from the temperature of Ar ₃ or more to the point of 1-2 mm deep on the pipe quenching side to the temperature just below Ar ₁ point of steel. Cooling at a rate of 150 ° C./s or less, and then cooling at a rate of 150 ° C./s or more to 800 to 400 ° C. on the opposite side of the quenching side of the pipe, and then performing a cooling treatment. This is a method for producing a low hardness and high toughness seamless steel pipe having excellent SSC properties.

[0006]

The production method of the present invention will be described below in detail.
First, the reason why the present invention is limited to the above steel components will be described. C and Mn are important for increasing the quenching effect, increasing the strength, stably obtaining a high-tensile steel having a yield point of 30 to 60 kgf / mm ² , and reducing the grain size. If too little without the effect, 0.99 ° C. if too large a point pipe hardening side depth 1~2mm to a temperature just below Ar ₁ point of the steel
Even when cooled at a rate of not more than / s, the hardness cannot be reduced (Vickers hardness <250), and the SSC resistance is deteriorated.
0%, 0.15 to 2.5%. Si is a component in which the deoxidizing agent remains and is an effective component for increasing the strength. If the amount is too small, the effect is not obtained.
In order to reduce the SC property, the content was set to 0.01 to 0.5%.

[0007] P is a harmful component that easily causes cracks during processing due to grain boundary segregation, and its content is limited to 0.020% or less because it causes deterioration of low-temperature toughness. S is M
An nS-based inclusion is formed and stretched by hot continuous rolling to form a layered structure, thereby improving the fracture propagation performance of steel. If too little without the effect, too and inclusions increase to the set to 0.01% or less to embrittlement properties of the steel. Al has a deoxidizing agent remaining like Si, and combines with N contained as an impurity component in steel to suppress the growth of crystal grains and improve the fracture propagation performance of steel. If the amount is too small, the effect is not obtained.If the amount is too large, the number of inclusions increases and the properties of the steel become brittle.
05 to 0.1%.

[0008] Both Ti and Nb are the most important elements among the components of the present invention as crystal grain size control elements during seamless rolling. Ti combines with N contained as an impurity component in steel, and Nb mainly combines with C, thereby controlling crystal grains during hot-gradient rolling and suppressing the growth of crystal grains after hot-gradient rolling. Component. If the growth of crystal grains is insufficiently suppressed, the hardness is reduced (Vickers hardness <) even if the point ₁ to 2 mm on the quenched side of the pipe is cooled at a rate of 150 ° C./s or less to a temperature just below the Ar ₁ point of the steel. 250), the SSC resistance deteriorates. If the added particles are too small, the effect will not be
If the amount is too large, the effect is saturated, and it is very expensive. N is a harmful component that lowers the hardenability of B described below, and its content is set to 0.1.
01% or less. In the case of further increasing the strength of the steel having the above composition, components such as Cr, Mo, Ni, V, and B are selectively added as needed. Cr, Mo, Ni, V
Is added to increase the hardenability of steel and to increase the strength. If the amount is too small, the effect is not obtained, and if the amount is too large, the effect is saturated and the cost is very high. Therefore, 0.1 to 1.5%, 0.05 to 0.5%, 0.1
To 2.0% and 0.01 to 0.1%. B remarkably improves the hardenability and increases the strength. If the amount is too small, the effect is not obtained, and if the amount is too large, the effect does not change, and the toughness and the hot workability are deteriorated.
And

Further, the present invention has been developed in view of the use environment of seamless steel pipes in recent years.
In order to improve the C property, a component such as a rare earth element is selectively added as needed. The rare earth element, Ca, is an effective component that makes the form of inclusions spherical and harmless. If the amount is too small, the effect is not obtained. If the amount is too large, the number of inclusions increases and the SSC resistance is lowered.
5%, 0.001 to 0.02%. Co and Cu are
It has the effect of suppressing hydrogen intrusion into steel and works effectively on SSC resistance. If the amount is too small, the effect is not obtained. If the amount is too large, the effect is saturated.
0.5%.

Next, the reason why the hot seamless rolling conditions are limited as described above will be described. The steel having the above-mentioned composition is melted in a melting furnace such as a converter or an electric furnace or further subjected to vacuum degassing, and a steel slab is manufactured by a continuous casting method or an ingot lump method. The slab is heated immediately after or once cooled and then subjected to hot piercing and rolling. The heating temperature must be sufficiently high to facilitate hot piercing and rolling, and is preferably as low as possible from the viewpoint of obtaining a fine-grained austenite structure. 1 within the range of the component of the present invention.
At a temperature of 000 ° C. or higher, there is no problem in hot drilling, and from 1250 ° C. or higher, coarse particles are formed from the viewpoint of grain refinement.

The hollow shell subjected to hot piercing and rolling has a high temperature of 1000 ° C. or more due to the heat generation phenomenon caused by processing, and the temperature of the shell is high and uneven on the inner surface. As shown in FIG. 1, the crystal grain size is largely controlled by the rolling temperature. Therefore, in order to obtain uniform and fine-grained austenite in the final-stage inclined rolling mill, it is necessary to lower the tube temperature immediately before rolling and at the same time to make the tube temperature uniform. In order to obtain a fine grain structure within the component range of the present invention, the temperature before the final stage of the inclined rolling mill needs to be Ar ₃ point to 1100 ° C, and the uniform tube temperature is higher than that on the outer surface side. It is necessary to forcibly cool the inner surface side where the temperature is high. The cooling may be water alone, mist, or compressed air.

[0012] In the inclined rolling mill, since recrystallization occurs mostly dynamically, the crystal grain size does not depend on the processing amount. But,
The critical strain for recrystallization must be exceeded. The lower limit of the rolling reduction was set to 20% in consideration of the fact that recrystallization occurs statically even after the end of rolling. On the other hand, if the rolling reduction is too large, rolling becomes difficult and the formability and surface quality of the pipe deteriorate, so the upper limit was set to 70%. After completion of the rolling, the raw tube refined by the inclined rolling at the final stage is subjected to continuous rolling for shape correction, and the hollow raw tube lowered to a temperature of from the Ar ₃ point to 900 ° C. has an Ar ₃ temperature higher than the temperature. Points + 50 to 1
Reheat to 000 ° C, or set the element tube to Ar ₃ points + 5
If a sufficiently high temperature of 0 ° C. or higher is secured, hot final rolling is performed without reheating. Reheating temperature is causative for Ar ₃ point of the quenching temperature and oxide scale it is necessary to high temperature too high to ensure the many resulting scratches generated from the last finishing after austenite + 50-100
0 ° C. After the quenching process of quenching from the temperature of Ar ₃ points or more, and then the tempering process of heating and cooling to the temperature of Ar ₁ point or less, the final finish rolling temperature secures the quenching temperature from austenite after rolling. For this, the rolling temperature was set to _one or more Ar.

Immediately after continuous rolling for shape correction or after reheating, hot final finishing rolling is performed at a temperature of ₃ points of Ar + 50 ° C. or more. Rolling temperature, it becomes difficult becomes too low secure shape was Ar ₃ point + 50 ℃ or higher.

After the final hot rolling, from the temperature of at least ₃ points of Ar to the point of ₁ to 2 mm on the quenching side of the pipe from the temperature of ₁ to 2 m below the point of Ar of the steel, it is cooled at a rate of 150 ° C./sec. / s on the opposite side at a speed of at least 800 / s
After cooling to 0 ° C., a cooling treatment is performed. The quenching start temperature was set to _three or more Ar points in order to secure a mixed structure of fine-grained ferrite + bainite + martensite and to secure required strength. The cooling rate during quenching is particularly important to ensure strength, toughness, and SSC resistance. The cooling rate is determined by the size of the water density when the temperature of the refrigerant, that is, the cooling water is fixed, and the water density has the greatest effect on the cooling rate at a point of 1 to 2 mm deep from the quenched surface. When cooling is performed at a water density required for securing the strength, the cooling rate at a point of 1 to 2 mm from the quenched end increases, the hardness after tempering increases, and the SSC resistance decreases. To secure SSC resistance, the Vickers hardness must be 250 or less, and the steel component within the scope of the present invention needs to be cooled at 150 ° C./s or less.

However, if the cooling rate of the surface on the side opposite to the quenching side of the pipe is set to 150 ° C./s or less, required strength cannot be secured. Therefore, the cooling rate at a point of 1 to 2 mm in depth from the quenching side is performed at 150 ° C./s or less until the Ar ₁ point is cut, and then cooled at a rate of 150 ° C./s or more. 150
Cooling at a rate of ° C / s or more must stop at 800-400 ° C on the side opposite the pipe quenching side. 150
The temperature at which cooling is completed at a rate of at least ° C / s (in this case, the inner surface of the pipe) needs to be reheated to a point of 1 to 2 mm from the quenching side of the pipe to 400 to 600 ° C by the self-held heat of the cooled pipe. The temperature varies depending on the wall thickness of the pipe. Generally, in the range of wall thickness (5 to 30 mm) of pipes used as gas and petroleum energy resources, when the temperature at which cooling is completed is too low, there is no sufficient recuperation, and when it is too high, sufficient strength is secured. Since the microstructure for quenching is not obtained, the surface opposite to the hardened
The cooling is preferably stopped at 400 ° C.

In the present invention, the cooling of the finished steel pipe is based on the point of 1 to 2 mm. If it is less than 1 mm, it becomes difficult to measure the hardness of the decarburized layer or scale. The material cannot be controlled sensitively. That is, the depth of 1 to 2 mm is a range that is sensitive to the cooling rate and easily manages the material.

The steel obtained by the above manufacturing method is effective for the production of a low-hardness and high-toughness seamless steel pipe excellent in SSC resistance and free of tempering by self-holding heat during quenching without containing coarse grains. It is.

[0018]

Next, an embodiment of the present invention will be described. A steel slab having the components shown in Table 1 produced by continuous casting and smelted in a converter is subjected to hot seamless rolling under the conditions shown in Table 2, and the point of ₁ to 2 mm on the quenching side of the pipe is Ar ₁ point or less of steel. , Was cooled at a rate of 150 ° C / s or less, subsequently cooled at a rate of 150 ° C / s or more, and then allowed to cool. Table 1 shows the strength, hardness, austenite grain size and SSC resistance of these steel tubes. In addition, SSC resistance is NACE TM01-
Σth (Threshold Stress) by the load method according to No. 77 was obtained and evaluated.

[0019]

[Table 1]

[0020]

[Table 2]

The steel pipe manufactured according to the present invention has a high strength and an austenite grain size (γ grain size) as compared with the conventional method.
Is fine, low hardness is obtained and the SSC resistance is 0.2σy at σth
It turns out that it improves above.

[0022]

As described above, the steel pipe manufactured by the method of the present invention has a high strength, an austenite grain size lower than that of the conventional method, a lower maximum hardness, and excellent low-temperature toughness and SSC resistance. Used in cold climates and sulfide stress corrosion environments.

[Brief description of the drawings]

FIG. 1 shows the influence of the γ-grain size and rolling temperature after a conventional steel plate rolling method and inclined rolling.

──────────────────────────────────────────────────続 き Continued on the front page (72) Akira Nakajima, Inventor 1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works (56) References JP-A-1-278902 (JP, A) JP-A-4-21721 (JP, A) JP-A-6-18327 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/10

Claims (4)

(57) [Claims] C .: 0.03 to 0.20%, and Si: 0.01 to 1.0% by weight.
0.5%, Mn: 0.15 to 2.5%, P: 0.020%
S: 0.010% or less, Al: 0.005 or less
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, and the balance is substantially 1000%.
After heating to １２1250 ° C., hot piercing and rolling are performed, and the hollow shell raised to 1000 ° C. or higher by working heat is forcibly cooled from the inner surface of the hollow shell before the final stage of the inclined rolling mill, and the entire shell is cooled. After performing a homogenization process at a temperature between Ar ₃ points and 1100 ° C., performing an inclined rolling at a reduction rate of 20 to 70% in a thickness section reduction rate, further performing a shape correcting hot continuous rolling, and then performing an Ar ₃ point the hollow shell was lowered to a temperature of to 900 ° C., was reheated to a high Ar ₃ point + 50 to 1000 ° C. than the temperature, or if the plain pipe Ar ₃ point + 50 ℃ above temperature high enough is secured Finishes at Ar ₃ without reheating
Point + 50 subjected to hot finish rolling over ° C., then Ar ₃
From the temperature above the point, the point of 1-2 mm deep on the pipe quenching side is cooled at a rate of 150 ° C./sec (s) or less to the temperature just below the Ar ₁ point of the steel, and then the pipe is quenched at a rate of 150 ° C./s or more. Characterized in that the opposite side is cooled to 800 to 400 ° C. and then subjected to a cooling treatment.
Method of manufacturing low hardness and high toughness seamless steel pipe with excellent heat resistance. 2. As% by weight, C: 0.03 to 0.20%, Si: 0.01 to
0.5%, Mn: 0.15 to 2.5%, P: 0.020%
S: 0.010% or less, Al: 0.005 or less
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, Cr: 0.1-1.5%, Mo: 0.05-
0.5%, Ni: 0.1 to 2.0%, V: 0.01 to
A steel slab containing 0.1%, B: one or more of 0.0003 to 0.0030%, and the balance substantially consisting of Fe
After heating to １２1250 ° C., hot piercing and rolling are performed, and the hollow shell raised to 1000 ° C. or higher by working heat is forcibly cooled from the inner surface of the hollow shell before the final stage of the inclined rolling mill, and the entire shell is cooled. After performing a homogenization process at a temperature between Ar ₃ points and 1100 ° C., performing an inclined rolling at a reduction rate of 20 to 70% in a thickness section reduction rate, further performing a shape correcting hot continuous rolling, and then performing an Ar ₃ point the hollow shell was lowered to a temperature of to 900 ° C., was reheated to a high Ar ₃ point + 50 to 1000 ° C. than the temperature, or if the plain pipe Ar ₃ point + 50 ℃ above temperature high enough is secured Finishes at Ar ₃ without reheating
Point + 50 subjected to hot finish rolling over ° C., then Ar ₃
From the temperature above the point, the point of 1-2 mm deep on the pipe quenching side is cooled at a rate of 150 ° C / s or less to a temperature just below the Ar ₁ point of the steel, and then opposite to the pipe quenching side at a rate of 150 ° C / s or more. A method for producing a low-hardness and high-toughness seamless steel pipe excellent in SSC resistance, characterized in that the surface side is cooled to 800 to 400 ° C. and then subjected to a cooling treatment. 3. As% by weight, C: 0.03 to 0.20%, Si: 0.01 to
0.5%, Mn: 0.15 to 2.5%, P: 0.020%
S: 0.010% or less, Al: 0.005 or less
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, Rare earth element: 0.001 to 0.05%, Ca: 0.001 to 0.02%, Co: 0.05 to
After heating a steel slab containing one or more of 0.5%, Cu: 0.1 to 0.5% and substantially the remainder of Fe to 1000 to 1250 ° C., hot piercing and rolling The hollow shell raised to 1000 ° C. or higher due to the heat generated during processing is forcibly cooled from the inner surface of the hollow shell before the final inclined rolling mill to uniformly cool the entire shell to a temperature between Ar ₃ points and 1100 ° C. After performing the surface treatment, the reduction rate of the thickness cross section is 20 to 7
After performing 0% inclined rolling and further performing shape correcting hot continuous rolling, the hollow shell dropped to a temperature of Ar ₃ point to 900 ° C. is reheated to an Ar ₃ point higher than the temperature and 50 to 1000 ° C. after, or without reheating if the plain pipe Ar ₃ point + 50 ℃ above temperature high enough is secured,
Hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C or higher, and then a temperature of 1 to 2 mm deeper on the pipe quenching side from the temperature of Ar ₃ points or higher to a temperature just below Ar ₁ point of steel at 150 ° C.
SSC resistance characterized by cooling at a rate of not more than 150 ° C / s, then cooling at a rate of not less than 150 ° C / s to 800-400 ° C on the opposite side of the pipe quenching side, and then subjecting it to a cooling treatment. Method for producing low-hardness and high-toughness seamless steel pipe excellent in quality. 4. C: 0.03-0.20%, Si: 0.01-% by weight
0.5%, Mn: 0.15 to 2.5%, P: 0.020%
S: 0.010% or less, Al: 0.005 or less
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, Cr: 0.1-1.5%, Mo: 0.05-
0.5%, Ni: 0.1 to 2.0%, V: 0.01 to
0.1%, B: one or more of 0.0003 to 0.0030%, rare earth element: 0.001 to 0.05%, Ca: 0.001 to 0.02%, Co: 0 0.05-0.5%, Cu: 0.1-0.
After heating a steel slab containing 5% of one or more kinds and the balance substantially consisting of Fe to 1000 to 1250 ° C., it was subjected to hot piercing and rolling to 1000 ° C. or more due to the heat generated during processing. After the hollow shell is forcibly cooled from the inner surface of the shell in front of the final stage of the inclined rolling mill, and the entire shell is homogenized to a temperature of between Ar ₃ point and 1100 ° C., the wall thickness reduction rate is determined. 20-7
After performing 0% inclined rolling and further performing shape-correcting continuous hot rolling, the hollow shell lowered to a temperature of Ar ₃ point to 900 ° C. was reheated to an Ar ₃ point higher than the temperature and 50 to 1000 ° C. after, or without reheating if the plain pipe Ar ₃ point + 50 ℃ above temperature high enough is secured, the finishing temperature is subjected to hot finish rolling Ar ₃ point + 50 ℃ or higher, then Ar ₃ point From the above temperature, the point of 1 to 2 mm depth on the pipe quenching side is 150 ° C / s from the above point to the temperature just below the Ar ₁ point
Cooling at the following speed, then cooling at a speed of 150 ° C./s or more to 800-400 ° C. on the side opposite to the quenching side of the pipe, and then subjecting it to a cooling treatment. Manufacturing method of low hardness and high toughness seamless steel pipe.