KR100256352B1 - The manufacturing method for high strength steel sheet used line pipe with excellent ultra low temperature impact toughness - Google Patents

Abstract

PURPOSE: A method for manufacturing a high tensile steel product for pipe line is provided, which not only has a high strength and superior weldability but also has superior impact toughness even under the cryogenic temperature environment by properly controlling manufacturing conditions including injection condition of Ca-Si for spheroidizing while appropriately controlling constituents of the steel product. CONSTITUTION: The method comprises the processes of spheroidizing inclusions by injecting some of Ca-Si during powder injecting and the rest of Ca-Si during vacuum degassing into molten steel comprising 0.08 to 0.11 wt.% of C, 0.20 to 0.30 wt.% of Si, 1.50 to 1.60 wt.% of Mn, 0.020 wt.% or less of P, 0.003 wt.% or less of S, 0.035 to 0.045 wt.% of Nb, 0.055 to 0.065 wt.% of V, 0.005 to 0.015 wt.% of Ti, 0.020 to 0.040 wt.% of sol.-Al and a balance of Fe and other inevitable impurities; rough rolling the reheated slab after reheating the spheroidized slab; initiating finish milling the rough rolled steel product to a reduction ratio of 70±5% from the non-recrystallization temperature zone of 870±20 deg.C and finishing finish rolling in the temperature range of 730±20 deg.C; and accelerated cooling the finish rolled steel product to the temperature range of 520±20 deg.C in a cooling rate of 8 to 12 deg.C/sec.

Description

Manufacturing method of high tensile strength steel for line pipe with excellent cryogenic impact toughness

1 is a hysteresis curve diagram illustrating a conventional controlled rolling method.

2 is a hysteresis curve diagram illustrating a control rolling method according to the present invention.

3 is an organization photograph of high tensile steel for line pipe.

(a) is invention material.

(b) the comparative material.

4 is a graph showing the change in low temperature toughness of the invention and the comparative material.

5 is a photograph showing the organization of the invention and the comparative material.

The present invention relates to a method for producing high tensile steel for line pipes used as oil and natural gas transportation means, and more particularly, to a method for manufacturing high tensile steel for line pipes having excellent impact toughness in cryogenic environments.

Up to now, the trend of oilfield resource development has been actively promoted in temperate and tropical regions with good development environment.However, due to the recent development of the oil industry and the depletion of resources in the 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 transportation of oil and natural gas developed from large oil fields to oil fields are also needed to be developed under the harsh conditions of use in these extreme regions.

The quality characteristics of pipe steels used under extreme cold conditions are essential to have strength as well as strength for brittleness that can easily occur at low temperatures. 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 environment of use of the steel only.

Accordingly, in the case of the conventional pipe steel, the strength is secured because it focuses on securing strength rather than low temperature toughness, but it is inferior to cryogenic impact toughness, and thus it cannot be used in cryogenic environments such as Siberia. In other words, the conventional pipe steel has satisfied only the -26 ° C impact toughness guarantee, not the cryogenic toughness guarantee.

On the other hand, in the conventional method of manufacturing pipe steels, first of all, a large amount of carbon, manganese, vanadium (V) components, etc., which are strength enhancing elements, are added, focusing on chemical component design to secure strength, and thick plate control for improving toughness. The method of adding a large amount of niobium (Nb) which has a grain refinement effect during rolling is applied. In addition, since the inclusion spheroidization treatment and vacuum degassing treatment in the steelmaking process are advantageous for the internal quality of steel, the vacuum degassing treatment is performed alone or the inclusion spheroidization treatment and the vacuum degassing treatment have been performed. Specifically, in the prior art, Ca-Si was added and treated for spheroidization in the steelmaking process, followed by vacuum degassing treatment. In the thick plate control rolling process, as shown in FIG. 1, in the recrystallization zone of about 900 ° C. or more for initial austenite grain refining, the finishing is partially performed and the unrecrystallized region having a large grain refining effect is 890-730 ° C. Finish rolling was again performed at. However, such a conventional method for manufacturing pipe steel has many problems, which will be described in detail as follows.

First, the components of conventional steel for pipes have a high carbon equivalent (CEQ), so the strength is easy to secure, but low-temperature impact toughness and demand are limited in improving on-site weldability. Second, in the spheroidization method using Ca-Si, Ca is oxidized due to inclusion spheroidization, which is not effective in improving low temperature crack generation and crack propagation stop characteristics, and also has problems in securing high cleanliness. Because of this effect, the grain refining effect is not so large that it is difficult to secure sufficient toughness at cryogenic temperatures.

Therefore, the present invention has been proposed to solve the above-described conventional manufacturing problems, while the present invention properly controls the component system of the steel, while appropriately controlling the manufacturing conditions, including the input conditions of Ca-Si for spheroidization, Of course, the purpose of the present invention is to provide a method of manufacturing high tensile strength steel for line pipes having excellent impact toughness even under cryogenic environments.

Hereinafter, the present invention will be described.

The present invention is a method for producing a high tensile strength steel for line pipe, by weight, C: 0.08-0.11%, Si: 0.20-0.30%, Mn: 1.50-1.60%, P: 0.020% or less, S: 0.003% or less , Nb: 0.035-0.045%, V: 0.055-0.065%, Ti: 0.005-0.015%, Sol.-Al: 0.020-0.040%, molten steel consisting of remaining Fe and other unavoidable impurities. -Si is added and the remaining Ca-Si is added to spheroidizing the inclusions during vacuum degassing treatment.Then, the spheroidized slab is reheated, and rough rolling is performed. After finishing finishing in the temperature range of 870 ± 20 ℃, finish finishing rolling in the temperature range of 730 ± 20 ℃, and then accelerate cooling to the temperature range of 520 ± 20 ℃ in the range of 8-12 ℃ / sec. It relates to a method for producing a high-tensile steel for line pipe excellent in cryogenic impact toughness comprising.

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

Carbon contained in the high-tensile steel for line pipes according to the present invention is the most contributing element to improving the strength, and the element increases in proportion with the increase in the amount of addition, but is an element that impairs the impact toughness. Therefore, if the carbon content is less than 0.08%, it is impossible to balance high strength and low temperature toughness of 57.5 kg / mm2 or more of tensile strength, and the carbon content is 0.08-0.11 because the carbon content is sensitive to the occurrence of the playing surface crack when the carbon content exceeds 0.11% during the operation. 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 may increase, thereby deteriorating low temperature toughness.

Manganese (Mn) is an element that can simultaneously improve strength and toughness, and the grains of ferrite and the second phase become fine with the increase of the added amount. When the manganese content exceeds 1.60%, the martensite structure may be formed, and the impact toughness may be impaired. If the manganese content is less than 1.5%, it 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 significantly reduce the impact toughness due to the occurrence of surface cracks, internal cracks and central segregation when playing.

Niobium (Nb) increases the tensile strength due to the formation of Nb (C, N) precipitates as the amount of niobium (Nb) is increased, and Nb (C, N) precipitates deposited at grain boundaries during grain rolling suppress the grain growth. It serves to improve strength and impact toughness. However, when the Nb content is less than 0.035%, the improvement of the material properties is insufficient. When the content of Nb exceeds 0.04%, 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, which contributes significantly to the increase in tensile strength rather than the increase in yield strength as its content increases. Therefore, considering the balance of toughness in addition to the strength aspect, it is preferable to limit the content of V to 0.055-0.065%, the increase in carbon equivalent (Ceq) when V is contained less than 0.055% or in a large amount of more than 0.065% With this, strength can be secured, but the impact toughness of the base metal and the weld is greatly impaired.

The content of titanium (Ti) is preferably limited to 0.005-0.015%, 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 greatly improved with a partial increase in strength.

In particular, when a large amount of Ti is added in an amount of 0.015% or more, since the nitrogen level in the molten steel is usually 50 ppm, the formation of oxide inclusions or excessive solid solution Ti is formed as coarse precipitates, which is not preferable.

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 composition, in the present invention, MnS inclusions, which are elongated in thick plate rolling during the converter operation, act as a starting point of brittle fracture and promote crack propagation, are formed into spherical inclusions of CaS. It is necessary to control. To this end, in the present invention, unlike in the prior art, during the powder injection (Cowder Injection), the part of Ca-Si is added and then the steel is agitated, and the Ca-Si is added to the reflux at the end of the vacuum degassing operation to reflux. ] To promote visualization. At this time, Ca-Si injection is more preferably about 50% of the total Ca-Si input amount during powder injection, and the remaining 50% when vacuum degassing treatment.

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 the second degree.

That is, as shown in FIG. 2, the control rolling according to the present invention is first reheated to a normal temperature, followed by rough rolling in a recrystallization zone, and finishing rolling in a temperature range of 850-890 ° C. which is an unrecrystallized zone. And finish-rolling in the temperature range of 710-750 degreeC, and the residual reduction ratio at the time of starting finishing rolling is set to about 65-75%.

Unlike the conventional control rolling, the control rolling method according to the present invention is not only about 20 ° C. lower, but also performs the finishing rolling in the non-recrystallized zone, so that the grains of the steel are further refined, which is advantageous for cryogenic brittleness. As in the conventional method, if the filament rolling during control rolling is over 890 ° C, impact toughness deteriorates due to grain coarsening due to partial recrystallization growth. The lowering of strength and the deterioration of low-temperature impact toughness occur, which is not preferable. In addition, if the rolling is finished in the low temperature region below 710 ℃, it is advantageous to secure the yield strength by forming the aggregate structure by the two-phase reverse pressure region, but there is a problem of impact anisotropy, so finishing the finish rolling at a temperature below 710 ℃ should be avoided if possible do.

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 to terminate at a temperature of 500-540 ℃. If the accelerated cooling rate is less than 8 ° C / sec, it is difficult to secure bainite structure suitable for impact toughness and tensile strength due to austenite to ferrite transformation due to weak cooling, and if it exceeds 12 ° C / sec, the tensile strength due to strong cooling Is improved, but there is a fear that the shape is poor.

When the end temperature is higher than 540 ° C., the austenite-ferrite transformation is not completed. Thus, a band structure is formed, which adversely affects the impact characteristics and cools at a low temperature of less than 500 ° C. When this is completed, the low-temperature transformation structure such as martensite is formed to secure yield strength and tensile strength, but impact toughness is lowered, which is undesirable. 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. Will have

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

Example 1

By weight%, C: 0.10%, Si: 0.25%, Mn: 1.58%, P: 0.020%, S: 0.002%, Sol-Al: 0.025%, Nb: 0.042%, V: 0.060%, Ti: 0.012% First, by milling in the molten iron preliminary treatment process to make a composition including, (carbon equivalent: 0.371) In case of vacuum degassing with less than 2torr, 200kg of Ca-Si was added at the end of the operation, reflux treatment of molten steel for more than 5 minutes, and electro-stirring treatment to minimize center segregation during casting. Subsequently, the slab obtained in the casting process was heated to 1230 ° C., which is a normal heating level in the thick plate process, subjected to rough rolling, and then started finishing rolling at a temperature of 870 ° C. with a residual reduction ratio of 70% to a temperature of 730 ° C. The rolling was finished at.

The rolled plate was immediately cooled by water at a cooling rate of 8 ° C./second, and finished at a temperature of 520 ° C., and thus, the steel plate obtained by cooling was used as an inventive material.

Also, for comparison, in weight%, C: 0.11%, Si: 0.25%, Mn: 1.58%, P: 0.020%, S: 0.002%, Sol-Al: 0.025%, Nb: 0.050%, V: 0.080% The molten iron including Ti: 0.010% and (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. Subsequently, the produced slab is started with primary finishing rolling at a temperature of 980 ° C. in the state of 70% of the residual reduction rate after rough rolling in controlled rolling, and secondary rolling is started at 890 ° C. and rolling is performed at a temperature of 750 ° C. After completion, the rolled steel was water-cooled at a cooling rate of 7 ° C./sec to terminate the cooling at a temperature of 540 ° C., and thus the thick steel plate in which the cooling was completed was used as a comparative material.

Mechanical properties and impact toughness were measured for the inventive materials and the 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 was observed at a ratio of 200 times using an optical microscope, and the results are shown in FIG.

As shown in Table 1, both the invention material and the comparative material satisfy the API-X70 steel standard (yield strength: 49 kg / mm2 or more, tensile strength: 57.7 kg / mm2 or more, elongation: 23% or more). In the case of the invention material it can be seen that the low temperature impact toughness is very excellent compared to the comparative material. As can be seen from the tissue photograph of FIG. 3, it can be seen that the crystal grains of the invention are much finer than those of the comparative material (a).

Example 2

To produce a high-tensile steel sheet for pipes in the same manner as the invention material of Example 1 to have a composition as shown in Table 2 and to observe the mechanical properties low-temperature impact toughness and structure for each steel sheet produced, the results are shown in Table 3, 4 and 5 are shown.

As shown in Tables 2 and 3, the invention material (1) satisfying the chemical composition range according to the present invention has excellent tensile strength and excellent hardness in ferrite and the second phase compared to the comparative material (1-3). It can be seen that.

In addition, as shown in Figure 4, in the case of the invention material (1) it can be seen that the impact toughness at low temperature is very superior to the comparative material (1) (3).

This fact shows that the structure of the invention material 1 is very fine compared to the comparative material 1-3, as confirmed by the structure of FIG.

As described above, the present invention provides a high-strength steel for line pipes having excellent impact toughness under not only strength but also cryogenic environments, by appropriately controlling the manufacturing system including the component system and spheroidizing conditions of the steel, so that even in the extreme regions There is an effect that the life can be extended in use.

Claims (2)

In the manufacturing method of high tensile steel for line pipe, in weight%, C: 0.08-0.11%, Si: 0.20-0.30%, Mn: 1.50-1.60%, P: 0.020% or less, S: 0.003% or less, Nb: 0.035 Magnesium steel composed of -0.045%, V: 0.055-0.065%, Ti: 0.005-0.015%, Sol.-Al: 0.020-0.040%, remaining Fe and other unavoidable impurities. In the vacuum degassing process, the remaining Ca-Si was added to spheroidize the inclusions, the spheroidized slab was reheated, and the rough rolled steel was roughly recrystallized at a residual pressure reduction rate of 70 ± 5%. Starting finishing finishing from a temperature of 870 ± 20 ° C. and finishing finishing rolling in a temperature range of 730 ± 20 ° C., followed by accelerated cooling to a temperature range of 520 ± 20 ° C. in a range of 8-12 ° C./sec. A method for producing a high tensile strength steel for line pipe, which has excellent cryogenic impact toughness. The method of claim 1, wherein the Ca-Si added to the spheroidizing treatment is added as 50% of the total Ca-Si input during powder injection treatment and 50% of the total Ca-Si input during vacuum degassing treatment. Method for producing a high-strength steel for line pipe excellent in cryogenic impact toughness.