KR19980028324A - Manufacturing method of high tensile steel plate for line pipe with excellent cryogenic impact toughness and hydrogen organic cracking characteristics - Google Patents

Abstract

The present invention relates to a method for producing a high-tensile steel sheet for large diameter line pipes used as an oil field and natural gas transportation means, the object of the present invention is to provide a method for producing a high-strength thick plate for line pipes having excellent impact toughness and hydrogen-organic cracking characteristics in cryogenic environments. In providing.

The present invention for achieving the above object by weight, C: 0.07-0.11%, Si: 0.20-0.30%, Mn: 1.00-1.30%, P: 0.020% or less, S: 0.003% or less, Cu: 0.10-0.30 %, Ni: 0.10-0.25%, Nb: 0.040-0.060%, V: 0.060-0.080%, Ti: 0.005-0.020%, Sol.-Al: 0.020-0.040%, remaining Fe and other unavoidable impurities, The technical gist of the present invention relates to a method for producing a high-strength thick plate for a line pipe having excellent cryogenic impact toughness and hydrogen organic crack resistance, which is formed by properly controlling and rolling a molten steel having a carbon equivalent (Ceq) of 0.262-0.379%.

Description

Manufacturing method of high tensile steel plate for line pipe with excellent cryogenic impact toughness and hydrogen organic cracking characteristics

The present invention relates to a method for manufacturing high tensile steel sheets for large diameter line pipes used as oil field and natural gas transportation means, and more particularly, to a method for manufacturing high tensile steel plates for line pipes having excellent impact toughness and hydrogen group resistance in cryogenic environments. .

Up to now, the trend of oilfield resource development has been actively promoted in warm and tropical regions with good development environment. However, due to the recent development of the oil industry and the depletion of resources in this region, it is gradually increasing in extreme regions such as Siberia, Arctic Ocean, Alaska and deep seabed. Genetic development is underway.

In this connection, crude oil pipelines for long-distance transport of oil and natural gas developed from large oil fields to oil fields are also required to develop steels that can be applied under the harsh conditions of use in these extreme regions.

The quality characteristics of the pipe steel used in the extreme cold environment are essential as well as the strength, resistance to brittleness that can be easily generated at low temperatures, and the hydrogen organic group sequence (HIC) for hydrogen sulfide.

However, in the case of pipe steel currently used, there is no special standard for impact toughness, and the demand satisfies the required level according to the steel usage environment. Therefore, the inventors have filed with Korean Patent Application No. 95-49561 for a method of manufacturing high tensile steel for line pipe that can be used as a material for oilfield and natural gas transport pipe under cryogenic environment. However, in the case of the pipe steel manufactured by the above method, it is possible to secure the strength and cryogenic impact toughness because it focuses on the low temperature toughness and the strength securing, but has a disadvantage of generating corrosion group in the corrosion environment.

Therefore, the present invention was improved to solve the above-mentioned problems in the Republic of Korea Patent Application No. 95-49561 Its purpose is to reduce the content of Mn in the component system of the steel and to properly add the Cu and Ni while appropriately manufacturing conditions By controlling, it is to provide a method of manufacturing a high-strength thick plate for line pipe having not only high strength but also excellent impact toughness under extremely low temperature environment and excellent corrosion resistance.

1 is a hysteresis curve diagram illustrating a control rolling method according to the present invention;

Figure 2 is a photograph of the structure of high tensile steel for tine pipe

(A) is invention material

(B) comparative material

Figure 3 is a tissue photograph comparing the hydrogen cracking for the invention and the comparative material

The present invention is a method for producing a high-strength thick plate for line pipe, by weight%, C: 0.07-0.11%, Si: 0.20-0.30%, Mn: 1.00-1.30%, P: 0.020% or less, S: 0.003% or less , Cu: 0.10-0.30%, Ni: 0.10-0.25%, Nb: 0.040-0.060%, V: 0.060-0.080%, Ti: 0.005-0.020%, Sol.-Al: 0.020-0.040%, remaining Fe and others Sintering the inclusions with Ca-Si when refining molten steel with inevitable impurities and having a carbon equivalent (Ceq) of 0.262-0.379%, and then reheating the spheroidized slab and reducing the residual pressure in the range of 980 ± 20 ° C. After rough-rolling to 80 ± 5%, the rough-rolled steel starts finishing at a temperature of 900 ± 20 ℃, which is the unrecrystallized temperature range at 75 ± 5% of residual reduction rate, and finish-rolls in the temperature range of 760 ± 20 ℃. High temperature resistance for cryogenic shock and hydrogen organic cracks, including accelerated cooling to a temperature range of 480 ± 20 ℃ in the range of 8-l2 ℃ / sec. It relates to a method for producing a force plate.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

Carbon contained in the high-tensile steel for line pipe according to the present invention is the most contributing element to improve the strength, the strength increases proportionally with the increase of the addition, but is an element that impairs the impact toughness and sour resistance . Therefore, if the carbon content is less than 0.07%, it is impossible to balance the high strength and low temperature toughness of more than 57.7kg / mm2 of tensile strength, and the carbon content is 0.07-0.11 because the carbon content is sensitive to the occurrence of playing surface cracks when the carbon content exceeds 0.11%. It is preferable to control by%.

Silicon (Si) makes a partial contribution to strength improvement, but the main purpose of addition is to deoxidize steel grades. However, if the silicon content is less than 0.2%, the deoxidation effect of the steel is inadequate, and when 0.30% or more is added, the silicon-based inclusions increase, which may degrade the low temperature toughness.

Manganese (Mn) is an element that can improve strength and toughness at the same time. As the grain size increases, the ferrite grains become finer, but when the manganese content exceeds 1.30%, hard tissues such as bainite and martensite are formed and thus impact toughness There is a risk of harm and less than 1.00% is difficult to secure high strength.

Phosphorus (P) is an impurity that significantly impairs the impact toughness of the steel material, so it accumulates in the central segregation unit during performance and degrades the internal quality. Therefore, the phosphorus content accompanying the operation technique is preferably limited to 0.020% or less.

Sulfur (S) is the same harmful element as the phosphorus component, the S content is preferably limited to 0.003% or less because it can greatly reduce the impact toughness due to the occurrence of surface cracks, internal cracks and central segregation when playing.

Copper (Cu) is an oxide film forming element of steel materials. When the content is less than 0.10%, the effect of inhibiting the penetration of hydrogen in corrosive environments such as sulfuric hydrogen and hydrogen is small. Inhibits.

Nickel (Ni) is added to form an oxide film of steel like copper, but if the content is less than 0.10%, the above effect is lowered. If it is more than 0.25%, the strength improvement effect is increased, but when the impact test after welding with pipe, Brittleness between the welds occurs, which is not preferable.

Niobium (Nb) increases the tensile strength due to the formation of Nb (C, N) precipitates as the amount of its addition increases.Nb (C, N) precipitates deposited at grain boundaries inhibit grain growth during thick-sheet rolling. It serves to improve the strength and impact toughness. However, when the Nb content is less than 0.040%, the improvement of the material properties is insufficient. When the content of Nb is more than 0.060%, the saturation phenomenon is shown, and the effect of improving the strength and impact toughness is insignificant.

Vanadium (V) is a V (C, N) precipitation element and contributes to the increase in tensile strength rather than the increase in yield strength as the dilution increases. Therefore, in the present invention, it is preferable to limit the content of the Mn component to 0.060-0.080% in consideration of the balance between toughness and strength in addition to the lowering of the conventional Mn component. It is difficult to secure the strength, and when it contains a large amount of more than 0.080%, the carbon equivalent (Ceq) increases and the impact toughness of the base metal and the weld part is greatly impaired.

Titanium (Ti) content is preferably limited to 0.005-0.020% because the Ti / N ratio is usually controlled at 1.0-3.0, and the TiN precipitate is finely dispersed at the grain boundary at the high temperature such as the slab reheating process and the welded portion. This is because nitrite grain growth can be suppressed and toughness can be drastically improved with a partial increase in strength. In particular, when the Ti content is added in a large amount of 0.020% or more, since the nitrogen level in the molten steel is usually 50 ppm, it is not preferable because the formation of oxide-based inclusions or excessively dissolved solid solution Ti is formed as coarse precipitates to inhibit toughness.

As described above, the steel component of the present invention has a lower Mn component in steel compared to conventional steels, Cu and Ni, which are oxide forming elements, are added, and the carbon equivalent of the total component is 0.262-0.379% in order to secure low temperature toughness and excellent weldability at the same time. It is characterized by low management in the range of

Hereinafter, the manufacturing method according to the present invention will be described in detail.

In order to ensure the integrity of the internal quality of the high-strength steel for pipes having the above-described composition, in the present invention, the MnS inclusions, which are elongated in the thick plate rolling when the converter is inserted, act as a starting point of brittle fracture and promote crack propagation, are shaped as spherical inclusions of CaS. It is necessary to control. Such a playing method is described in detail in Korean Patent Application No. 95-49561 filed by the inventor.

In addition, in the present invention, the slab played through the converter operation as described above, the control rolling, wherein the control rolling uses the same method as in FIG. That is, the control rolling according to the present invention is first reheated to a normal temperature, and then roughly rolled in the range of 75-85% of residual reduction ratio in the recrystallization area of 960-1000 ° C. as shown in FIG. After refining, the rough rolled steel starts finishing in the temperature range of 920-880 ℃, which is the unrecrystallized zone, and finish-rolls in the temperature range of 780-740 ℃, which is directly above Ar ₃ , avoiding the ferrite and austenite two-phase zones. In addition, the residual reduction rate at the start of finishing rolling is set to about 80-70%.

Unlike the conventional control rolling, the control rolling method according to the present invention is not only about 30 ° C. higher, but also because the finishing rolling is performed in the unrecrystallized region, the grain size of the steel is further refined by suppressing the pearlite band structure. It is advantageous for cryogenic brittleness, and in particular, the hydrogen organic group thermal characteristics are improved. If the rough rolling is carried out at a temperature above 1000 ° C. above the recrystallization temperature, the cumulative reduction effect due to rolling in the high temperature range is small, and thus the initial austenite grain refining effect is insufficient. When rolling at a temperature range below 960 ° C., there is a grain refining effect. There is a point that it is difficult to sufficiently carry out at the recrystallization temperature and the reduction amount. And when the rolling starts at the control rolling temperature of 920 ℃ or more, the impact toughness due to coarsening is deteriorated due to the growth of grains by the partial recrystallization zone, and when such rolling is finished in the low temperature region below 740 ℃, The yield strength is advantageously secured by the formation of the aggregate structure, but there is a disadvantage in that the impact anisotropy and the sour resistance due to the band structure are degraded.

In addition, when the accelerated cooling after the control rolling as described above, it is preferable to cool in the range of 8-12 ℃ / sec and terminate at a temperature of 460-500 ℃. If the accelerated cooling rate is lower than 8 ° C / sec, it is difficult to secure bainite structure suitable for improving impact toughness and tensile strength due to incomplete austenite to ferrite transformation.If it exceeds 12 ° C / sec, it is tensioned by strong cooling. Although the strength is improved, there is a fear that the shape becomes bulging.

If the end temperature is higher than 500 ° C. when cooling at the above cooling rate, the austenite → ferrite transformation is not completed, and thus a band structure may be formed, which may be a starting point of hydrogen crack breakdown and cooling at a low temperature of less than 460 ° C. It is advantageous to secure the yield strength and tensile strength by forming the low temperature transformation structure such as martensite, but the impact toughness is lowered, which is not preferable. As a result, the high tensile strength steel for pipes manufactured according to the present invention forms a ferrite-perlite-low-temperature bainite structure, which has a band (assembly structure) harmful to impact toughness and is advantageous for toughness improvement, and has excellent low-temperature toughness at cryogenic temperatures of -60 ° C. It is suitable for heavy-duty line pipes with a diameter of 6-20mm.

Hereinafter, the present invention will be described in detail through examples.

Example

By weight%, C: 0.09%, Si: 0.25%, Mn: 1.20%, P: 0.020%. S: 0.002%, Cu: 0.25%, Ni: 0.15%, Sol-Al: 0.025%, Nb: 0.050%, V: 0.070%, Ti: 0.012% (Carbon equivalent: 0.331) After milling in the process, after the first smelting by stirring treatment, in the powder injection in the furnace refining process, 210 kg of Ca-Si is added and the vacuum degassing process with the vacuum degree of 2torr or less is performed. At the end, 200kg of Ca-Wire was added and the molten steel was refluxed for more than 5 minutes, and electrostirring was performed to minimize the center segregation during casting. Subsequently, the slabs obtained in the casting process are heated to 1230 ° C., which is a normal heating level in the thick plate process, and rough rolling is performed at 980 ° C. by applying a residual pressure reduction ratio of 80%. Finishing rolling was started at the temperature, and rolling was finished at the temperature of 760 degreeC, and the rolling board about 6 mm in thickness was obtained.

The rolled plate was immediately cooled by water at a cooling rate of 12 ° C./second and terminated at a temperature of 480 ° C., and thus, the steel plate steel material thus cooled was used as an inventive material.

Also in weight percent for comparison. C: 0.11%, Si: 0.25%, Mn: 1.58%, P: 0.02%, S: 0.002%, So1.-Al: 0.025%, Nb: 0.050%, V: 0.080%, Ti: 0.010%, A molten iron including (carbon equivalent 0.390%) was prepared in the same manner as in the invention, except that 410 kg of Ca-Si was added to the outside of the refining furnace. Thereafter, the first slab is started at a temperature of 980 ° C. at a relative pressure of 70% of the residual reduction after rough rolling in the controlled slab, and the second finishing rolling starts at 870 ° C. and rolls at a temperature of 730 ° C. After finishing, a rolled sheet having a thickness of 6 mm was obtained, the rolled sheet was cooled by water at a cooling rate of 8 ° C / second, and the cooling was completed at a temperature of 540 ° C.

Mechanical properties, impact toughness and hydrogen cracks were measured for the inventive and comparative materials prepared as described above, and the results are shown in Table 1 below.

In addition, the structure of the invention material and the comparative material were observed at a ratio of 200 times using an optical microscope, and the results are shown in FIG. 2.

In addition, as a result of impact test after holding in 100% hydrogen atmosphere, the wavefront structure of the invention and the comparative material was observed using an optical microscope. The results are shown in FIG.

Example Mechanical property (㎏ / ㎠) Impact Toughness (㎏m) HIC resistance surrender Seal Elongation (%) 0 ℃ -26 ℃ -60 ℃ VTrE CLR (%) ETC (mm) Invention 51.8 60.8 35 29.6 24.0 15.4 -60 ℃ 8 0.1 Comparative material 55.5 64.0 37 17.1 11.3 6.4 -40 ℃ 30 3.4

As shown in Table 1, in the case of the invention material and the comparative material, both API-X70 steel standards (yield strength: 49 kg / mm2 or more, tensile strength: 57.7 kg / mm2 or more, elongation: 23% or more; '77 NACE standard ), But it can be seen that the low temperature impact toughness of the invention material compared to the comparative material. As can be seen from the tissue photograph of FIG. 2, it can be seen that in the case of the inventive material, the crystal grains are much finer than in the case of the comparative material in which the band structure is formed (b). As can be seen from FIG. 3, in the case of the comparative material, hydrogen crack was greatly developed in the actual sour environment, but little was observed in the present invention.

As described above, according to the present invention, if the component system and manufacturing conditions of the steel are properly controlled, not only the strength but also the impact toughness under cryogenic environments are provided, and the high-strength thick plate for tine pipes having excellent cracking characteristics especially under a sour environment is provided. These thick plates have the effect of extending their life when used in crude oil pipelines even in the extreme regions.

Claims (1)

In the manufacturing method of the high-strength thick plate for line pipe, in weight%, C: 0.07-0.11%, Si: 0.20-0.30%, Mn: 1.00-1.30%, P: 0.020% or less, S: 0.003% or less, Cu: 0.10-0.30%, Ni: 0.10-0.25%, Nb: 0.0040-0.060%, V: 0.060-0.080%, Ti: 0.005-0.020%, Sol.-Al: 0.020-0.040%, remaining Fe and Sintering the inclusions with Ca-Si during refining of molten steel with other unavoidable impurities and having a carbon equivalent (Ceq) of 0.262-0.379%, and then reheating the spheroidized slab and remaining in the range of 980 ± 20 ° C. After rough rolling at a rolling reduction of 80 ± 5%, the rough-rolled steel is started at a temperature range of 760 ± 20 ° C from a temperature of 900 ± 20 ° C, which is the unrecrystallized temperature range, at a residual pressure reduction rate of 75 ± 5%. Tine excellent in cryogenic impact toughness and hydrogen-organic cracking characteristics, characterized in that the finish rolling and then accelerated cooling to a temperature range of 480 ± 20 ℃ in the range of 8-12 ℃ / second Method of manufacturing high tensile steel plate for pipe.