KR100797326B1 - Steel plate for riser pipe guaranteed pwht and method of manufacturing thereof - Google Patents

Abstract

A high strength steel is provided to maintain physical properties of the weldments and obtain excellent yield strength of 550 MPa grade even after the PWHT by solving a problem that physical properties such as strength and toughness of weldments of a conventional riser pipe steel are deteriorated after PWHT(Post Weld Heat Treatment), and a method for manufacturing the high strength steel is provided. A manufacturing method of a riser pipe steel for ensuring physical properties of PWHT comprises the steps of: reheating a steel slab comprising, by weight percent, 0.04 to 0.10% of C, 0.05 to 0.50% of Si, 1.2 to 2.0% of Mn, 0.01 to 0.05% of Al, 0.02% or less of P, 0.005% or less of S, 0.005 to 0.02% of Ti, 0.002 to 0.01% of N, 0.02 to 0.07% of Nb, 0.02 to 0.07% of V, 0.3% or less of Ni, 0.1 to 0.5% of Mo, 0.0005 to 0.004% of Ca and the balance of Fe and other unavoidable impure elements to a temperature range of 1100 to 1300 deg.C; rolling the reheated steel slab to an average reduction ratio per pass of at least 10% at 1000 deg.C or more; cooling the rolled steel slab to 950 deg.C or less in a cooling rate of 2 deg.C/sec or more; hot rolling the cooled steel slab to an accumulated reduction ratio of 70% or more in a temperature range of 950 to 800 deg.C; and water-cooling the hot rolled steel slab to a temperature range of 400 to 550 deg.C in a cooling rate of 5 deg.C/sec or more, and then air-cooling the water-cooled steel slab to room temperature.

Description

Steel Plate For Riser Pipe Guaranteed PWHT And Method Of Manufacturing Thereof}

1 is an optical microscope histology picture of the invention A, Figure 2 is an optical microscope picture of the invention B.

The present invention relates to a method for producing a line pipe steel with a yield strength of 550 Mpa or more used for deep sea risers, and more specifically, Mo, Nb, Ti, and V are added, and the recrystallized austenite grain size in the recrystallization zone during the hot rolling process. In the unrecrystallized zone, the steel sheet is rolled by a large reduction ratio, so that even if the heat treatment is performed for a long time of about 15 hours or more at a high temperature of about 650 ° C, the physical properties are less deteriorated and the yield strength and toughness of 80 ksi grade can be maintained. The present invention relates to a line pipe steel having excellent properties even in extreme environments and a method of manufacturing the same.

Recently, due to the rapid development of the industry, the use of oil and natural gas energy has increased, and attention has been focused on the development of oil fields in deep seas and the development of deep sea oil fields. This trend is increasing the demand for linepipe steels for risers used to develop deep oil fields and to pull crude oil from them. However, riser pipes used in cryogenic environments, such as deep water and ultra deep water, require high strength and toughness, making them more demanding than 65ksi or 70ksi steels. High 80ksi grade steel is required in large quantities.

These high strength steels are required not only for high strength, but also for high weld toughness at the same time so that they can be used in a cryogenic environment in a deep sea. However, in order to safely use the riser steel in the deep sea, it is necessary to go through the post weld heat treatment (PWHT) for the welded part. The conventional line pipe steel is subjected to such PWHT which is performed for 4 hours at a high temperature of about 650 ° C. Later, many physical properties such as strength and toughness are greatly reduced. In particular, in recent years, the PWHT for a long time of about 15 hours at a high temperature of about 650 ℃ to increase as much as possible to remove the stress concentrated in the weld bar is required, the situation is excellent steel corresponding to this.

Even after such high temperature and long time PWHT, many attempts have been made all over the world to manufacture high strength steels whose properties are guaranteed. Among them, there is X80 grade steel for pipes supplied by Dillinger of Europe.

The steel of Dillinger is used to make a polygonal ferrite phase which is hardened by adding some Sol.Al, Nb, Ti and V as alloying elements, and in particular by lowering finishing finishing temperature to below Ar ₃ temperature under rolling conditions. It is particularly used for the production of narrow steels for the production of small diameter pipes of up to 21 inches characterized by hot rolling.

However, the steel produced by the above method is difficult to maintain weld properties due to the characteristics of the polygonal ferrite during PWHT treatment, especially PWHT treatment for 15 hours or longer, and work hardening in narrow steel (1600 mm width) for 21-inch pipe manufacturing. In order to produce a stretched polygonal ferrite, it is possible to optimize grains by rolling more than 20% of the reduction ratio per pass, but it is difficult to manufacture pipes or doubling materials having a larger size.

Accordingly, the inventors of the present invention have repeatedly studied, as a method of producing a steel pipe for a riser pipe of yield strength of 80 ksi class, which is easy to mass-produce large-sized steel pipes and double explosives, and guarantees welded properties even after 15 hours of PWHT. ) Austenitic recrystallization is finished after cooling water to restrain grain growth. (2) When acute recrystallization finishes rolling and starts cooling at above Ar ₃ temperature, the acuterite grain size is significantly optimized. It has been confirmed that the fineness improves the toughness of the welded part and (3) V is added to precipitate V precipitates by heat treatment of 600 ° C. or higher, so that the toughness is not lowered and the strength is maintained.

The purpose of the present invention is to solve the problems of the conventional riser pipe steel, which causes the degradation of the properties such as strength and weld toughness after high temperature, long time PWHT, high strength steel having excellent yield strength of 550MPa grade while maintaining the welded properties even after PWHT. And a method for producing the same.

In the present invention, by weight% C; 0.04-0.10%, Si; 0.05-0.50%, Mn; 1.2-2.0%, Al; 0.01-0.05%, P; 0.02% or less, S; 0.005% or less, Ti; 0.005- 0.02%, N; 0.002-0.01%, Nb; 0.02-0.07%, V; 0.02-0.07%, Ni; 0.3% or less, Mo; Steel for PWHT guarantee riser pipes consisting of 0.1-0.5%, Ca; 0.0005-0.004%, remaining Fe and other unavoidable impurities.

Further, the present invention, by weight% C; 0.04-0.10%, Si; 0.05-0.50%, Mn; 1.2-2.0%, Al; 0.01-0.05%, P; 0.02% or less, S; 0.005% or less, Ti; 0.005-0.02%, N; 0.002-0.01%, Nb; 0.02-0.07%, V; 0.02-0.07%, Ni; 0.3% or less, Mo; Steel slab consisting of 0.1-0.5%, Ca; 0.0005-0.004%, remaining Fe and other unavoidable impurity elements,

Reheating to 1100-1300 ° C .;

Rolling the reheated steel slab with an average pressure reduction ratio of 10% or more per pass at 1000 ° C. or more;

Cooling the rolled steel slab to 950 ° C. or less at a cooling rate of 2 ° C./s or more;

Hot rolling the cooled steel slab with a cumulative reduction of 70% or more in the range of 950-800 ° C .; And

Cooling the hot-rolled steel slab to a temperature range of 400 to 550 ° C. at a cooling rate of 5 ° C./s or more, and then air-cooling to room temperature;

It relates to a method for manufacturing a steel for PWHT guarantee riser pipe comprising a.

Steel sheet of the present invention for achieving the above object, by weight% C; 0.04-0.10%, Si; 0.05-0.50%, Mn; 1.2-2.0%, Al; 0.01-0.05%, P; 0.02% or less, S 0.005% or less, Ti; 0.005-0.02%, N; 0.002-0.01%, Nb; 0.02-0.07%, V; 0.02-0.07%, Ni; 0.3% or less, Mo; Steel slab composed of 0.1-0.5%, Ca; 0.0005-0.004%, remaining Fe and other unavoidable impurities is reheated at 1100 ~ 1300 ℃, recrystallized rolled at 1000 ℃ or higher and 950 ℃ at cooling rate of 2 ℃ / s or higher. Cool to Thereafter, the steel slab is hot-rolled to a cumulative reduction of 70% or more in the range of 950 to 800 ° C., followed by water cooling to a temperature range of 400 to 550 ° C. at a cooling rate of 5 ° C./s or more, followed by air cooling to room temperature.

EMBODIMENT OF THE INVENTION Hereinafter, the component system of this invention is demonstrated in detail.

C is the most economical and effective element for reinforcing steel, but it is limited to 0.04-0.10% because the weldability, formability and toughness deteriorate when a large amount is added. If the amount is less than 0.04%, other alloying elements such as Mo have to be added in relatively large amounts for the same strength, and it is uneconomical, and if it is added in excess of 0.10%, weldability, formability and toughness are deteriorated.

Si exhibits a molten steel deoxidation effect and a solid solution strengthening effect, so an addition in the range of 0.05-0.50% is required. If the addition amount is less than 0.05%, the toughness may be deteriorated due to insufficient deoxidation of molten steel, and if it exceeds 0.50%, the red scale formed by Si is formed during hot rolling, resulting in non-uniform shape of the surface of the steel sheet. Can be degraded.

Mn is an effective element to solidify the steel to be added more than 1.2% can exhibit high strength with an increase in the hardenability. However, the addition of more than 2.0% is not preferable because segregation at the center of the thickness during the slab casting in the steelmaking process greatly develops and damages the weldability of the final product.

Al is added as a deoxidizer along with Si in steelmaking, and also has a solid solution effect. However, the addition of more than 0.05% may impair the impact toughness, and when the amount is less than 0.01%, the deoxidation effect is insufficient and the toughness is lowered. Therefore, it is preferable to add it at 0.01-0.05%.

P is an impurity element existing in the steel, and segregates mainly in the center of the steel sheet and impairs toughness. Therefore, P is preferably reduced as much as possible, so the upper limit thereof is 0.02%.

S is also an impurity element present in steel, and is preferably combined with Mn or the like to form a non-metallic inclusion, thereby greatly reducing the toughness and strength of the steel, and thus reducing it as much as possible. In particular, in order to secure brittle fracture stop characteristics at cryogenic temperatures, the upper limit is preferably limited to 0.005%.

Nb and V are very useful elements for refining grains, and at the same time, promote the formation of acicular ferrite or bainite, which is a high-strength structure, and serves to greatly enhance the strength of the steel. Furthermore, it is preferable to add at least 0.02% or more since PWHT may be present inside the steel as a precipitate to prevent a decrease in strength that may occur after the heat treatment. However, the addition of more than 0.07% may reduce the weldability, so the Nb and V content is limited to 0.02-0.07%.

Ti forms a TiN precipitate during the solidification process of the steel to suppress the austenite grains during the slab heating and hot rolling process to improve the toughness of the steel by miniaturizing the final grain size. If the Ti content is less than 0.005%, TiN precipitates may not be formed sufficiently to inhibit particle size growth.If the Ti content exceeds 0.02%, TiN may be coarsened when the slab is heated due to the excessive presence of solute Ti. It may not help to refine the particle size. Therefore, the Ti content is limited to 0.005-0.02%.

The reason for component limitation of N is attributable to the above Ti addition. In general, N is dissolved in the steel and precipitates to increase the strength of the steel, which is much greater than carbon. On the other hand, however, it is known that the higher the amount of nitrogen in steel, the lower the toughness is, so it is a general trend to reduce the nitrogen content as much as possible. However, in the present invention, it is not preferable to reduce N excessively because nitrogen plays a role of inhibiting grain growth during reheating by forming TiN by reacting with Ti. Therefore, the amount of N is limited to 0.002-0.01%. If the amount of N is less than 0.002%, the content of TiN precipitates is small, so that the particle size growth inhibitory effect is greatly lowered. If the content of N is more than 0.01%, the N is not present in the form of TiN but in solid solution N, thereby greatly reducing the toughness.

Ni is an element widely used as an element for improving toughness, and also serves to improve brittle fracture stopping property in the present invention. Ni increases toughness as the amount added increases, but it is expensive and increases the amount indefinitely, so the toughness does not increase in proportion to it.

Mo is very effective to increase the strength of the material, and helps to form acicular ferrite, which is a low temperature transformation structure, and is an element that can simultaneously obtain the high strength and high toughness of the steel. In particular, M ₂ C (M is a metal) forms a fine precipitate to maintain the strength of the steel even after PWHT for a long time at high temperature. Therefore, Mo is preferably added at least 0.1%, but is an expensive element, and if the amount is increased, the weldability is lowered, so it is preferable to limit it to 0.5% or less.

Ca is an element that spheroidizes MnS inclusions to suppress the formation of cracks around the inclusions. If the content of Ca is less than 0.0005%, the inclusion spheroidization effect does not appear, and if the content is more than 0.004%, a large amount of CaO-based inclusions are formed, so the impact toughness may be lowered. Therefore, the Ca content is limited to 0.0005-0.004%.

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

After reheating the steel having the above-described component system at the slab heating temperature of 1100 to 1300 ° C., the steel is rolled at an average rolling reduction of 10% or more per pass at 1000 ° C. or more, and then cooled to 950 ° C. at a cooling rate of 2 ° C./s or more. Thereafter, hot rolling is carried out to a cumulative reduction of 70% or more in the range of 950 to 800 ° C., followed by water cooling to a temperature range of 400 to 550 ° C. at a cooling rate of 5 ° C./s or more, and air cooling to room temperature.

The reheating temperature of the slab has an important meaning in the present invention. In the present invention, a fine carbonitride precipitate having a particle size of 10 nm to 1 μm and fine needle-like ferrite having a particle size of about 10 nm to 1 μm formed by the effect of Mo and solid solution Nb are formed to obtain high strength and high toughness. Therefore, the slab must be heated to 1100 ° C. or higher prior to hot rolling to dissolve NbC so that Nb is present in the atomic state. If the heating temperature exceeds 1300 ℃ coarse TiN precipitation occurs when reheating, if less than 1100 ℃ slab heating temperature is preferably in the range of 1100-1300 ℃ because NbC dissolution does not occur.

The slab heated to the above temperature range is rolled to an average reduction ratio of 10% or more per pass at 1000 ° C. or more, and then cooled to 950 ° C. at a cooling rate of 2 ° C./s or more, preferably 2 to 20 ° C./s. In the present invention, since the content of Nb added is 0.02-0.07%, when the rolling temperature exceeds 1000 ° C, austenite is completely recrystallized during rolling, and austenite is completely uncrystallized when rolling at 950 ° C or lower. In rolling at 950-1000 ° C, which is the partial recrystallization zone of austenite, coarse austenitic and fine austenite mixed structures are obtained, resulting in partial coarse grains in the final structure, which greatly reduces toughness. Therefore, it is preferable to completely recrystallize the austenite grains by rolling at an average pressure drop ratio of 10% or more per pass at 1000 ° C or higher. Furthermore, if the slab is air cooled only to a temperature exceeding 950 ° C. which is a complete unrecrystallization start temperature after recrystallization rolling, the brittle fracture stop characteristics due to the coarsening of the final particle size deteriorate due to rapid grain growth of austenite. Therefore, the rolled slab is cooled to 950 ° C. or less at a cooling rate of 2 ° C./s or more, preferably 2 to 20 ° C./s, immediately after recrystallization in order to suppress recrystallization grain growth. If the cooling rate is less than 2 ° C / s, the speed is too slow, there is a problem in the tissue refinement, the speed exceeding 20 ° C / s is economical.

For slabs cooled to 950 ° C or lower, hot rolling is performed in the unrecrystallized region, 950-800 ° C, with a cumulative reduction of more than 70%. This recrystallization reverse rolling process produces a long stretch of austenite grain boundaries and grain boundaries. In order to obtain fine ferrite, the strength and brittle fracture stopping properties are greatly improved. In this case, the larger the cumulative rolling reduction, the more effective the toughness improvement. On the other hand, if the cumulative rolling reduction is less than 70%, the brittle fracture resistance is significantly lowered. Therefore, the cumulative reduction amount is 70% or more. The lower the hot rolling finish temperature is, the more austenite strain increases and is effective for improving toughness. However, at less than 800 ° C, coarse coaxial ferrite having poor toughness is formed during rolling, so the lower limit temperature of unrecrystallized rolling is 800 ° C.

After finishing hot rolling, it is cooled to 400 to 550 ° C and paused. At this time, if the cooling rate is less than 5 ° C / s, the grain size of the ferrite is coarse, and the second phase (bainite, pearlite or mixed structure thereof) When the strength of the steel is low, it is difficult to secure the strength of the steel. If it is too fast to exceed 30 ° C / s, the economical efficiency is lowered. Therefore, the cooling rate is 5 ° C / s, preferably 5 to 30 ° C / s. In addition, when the cooling stop temperature is higher than 550 ℃, the particle size growth occurs in the air-cooling process after water cooling to decrease the strength and toughness, and when the cooling stop temperature is less than 400 ℃, the martensite-Austenite Constituent structure is generated to impact toughness This is sharply lowered. Therefore, the water cooling stop temperature is preferably limited to the temperature range of 400-550 ℃, and then air-cooled to room temperature.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Steel grade C Si Mn P S Nb V Ni Mo Ti Cr Remarks A 0.06 0.2 1.8 0.012 0.003 0.04 0.04 0.25 0.25 0.012 0.3 Invention B 0.06 0.3 1.9 0.011 0.003 0.04 - 0.22 0.31 0.018 0.03 Comparative material

Inventive materials and comparative materials having the chemical components as shown in Table 1 were dissolved into slabs, and hot rolled to prepare plates of 22 mm thickness.

 Psalm No Steel grade Slab heating temperature (℃) Rough rolling end temperature (℃) Rough rolling-filament rolling section cooling Finish rolling start temperature (℃) Rolling end temperature (℃) Undetermined cumulative reduction rate (%) Cooling end temperature (℃) Cooling rate (℃ / s) Invention A 1120 1000 2 ℃ / s cooling 930 800 75 450 12 Comparative material B 1180 1100 Air cooling 800 670 75 450 8

At this time, the invention material and the comparative material were prepared by comparative rolling and invention rolling, respectively, under the conditions as shown in Table 2, and the tensile and impact tests were performed after heat treatment (PWHT) for 4 hours and 15 hours at 638 ° C., respectively. .

Psalm No Steel grade YS (MPa) TS (MPa) Weld 4hr heat treatment 15hr heat treatment Weld 4hr heat treatment 15hr heat treatment Invention A 582 605 630 720 706 690 Comparative material B 578 648 610 736 682 672  Psalm No  Steel grade Impact Toughness (J, -20 ℃) Base metal Weld Weld 4hr heat treatment 15hr heat treatment Weld 4hr heat treatment 15hr heat treatment Invention A 407 338 440 125 205 170 Comparative material B 265 257 266 88 99 95

Table 3 shows the results of the tensile and impact tests of the weld and the base metal for the invention and the comparative material. As shown in Table 3, after the invention material satisfying the component range of the present invention is recrystallized at 1000 ° C. or more, which is the manufacturing condition of the present invention, the product is cooled to 950 ° C. at a cooling rate of 2 ° C./s or more and then in the range of 950-800 ° C. It was hot-rolled to 70% or more of the cumulative reduction in water and cooled by water. In this case, it can be seen that the invention material satisfies the strength and toughness required in the API-X80 grade steel after PWHT with yield strength of 582MPa, tensile strength 720MPa, and impact toughness of 125J at -20 ° C.

On the contrary, the comparative material manufactured by the method of belonging to the component steel of the invention steel but after recrystallization rolling and air-cooling as in the usual manufacturing method, followed by unrecrystallized reverse rolling and rolling-accelerated cooling below Ar ₃ temperature is required in API-X80 class. The strength and impact toughness are retained, but after 4 hours and 15 hours of heat treatment at 638 ° C, the weld toughness decreases below 100J at -20 ° C.

In order to identify the cause of the brittle fracture stopping property of the invention as compared to the comparative material, the invention and the comparative material were observed by optical microscopy and shown in FIGS. 1 and 2. As shown in FIG. 2, the comparative material has an average particle size of about 10 μm and is uneven, so that coarse polygonal ferrites of 20 μm or more are also observed. From this, the invention material was excellent in strength and toughness in the welded part even after heat treatment to prevent the recrystallized austenite from growing unevenly after recrystallization rolling. It can be seen that.

According to the present invention, it is possible to obtain a known metal having high yield strength and toughness by finely and uniformly distributing the final grain size by controlling the compositional conditions of the steel and the rolling-accelerated cooling conditions. In addition, V was added and precipitated during the heat treatment to maintain the strength and toughness of the welded portion after 638 ° C., a high temperature of 15 hours and a long heat treatment. Furthermore, by maximizing the recrystallization in recrystallization rolling, it is possible to manufacture a wider material of more than 4000mm width while maintaining excellent physical properties. In addition, the productivity of high-strength steels is greatly improved, thereby reducing the cost and time required for steel production.

Claims (4)

% By weight of C; 0.04-0.10%, Si; 0.05-0.50%, Mn; 1.2-2.0%, Al; 0.01 to 0.05%, P; 0.02% or less, S; 0.005% or less, Ti; 0.005-0.02%, N; 0.002-0.01%, Nb; 0.02-0.07%, V; 0.02-0.07%, Ni; 0.3% or less, Mo; 0.1-0.5%, Ca; 0.0005-0.004%, consisting of the remaining Fe and other unavoidable impurities The microstructure includes a carbonitride precipitate having an average particle size of 10 nm to 1 μm and acicular ferrite having an average particle size of 10 nm to 1 μm formed by the effect of Mo and solid solution Nb. delete The steel material for riser pipe according to claim 1, wherein the microstructure of the steel material for riser pipe is bainite structure, pearlite structure or mixed structure of bainite and pearlite as a second phase. Reheating the steel slab having the component of claim 1 to 1100 ~ 1300 ℃; Rolling the reheated steel slab with an average pressure reduction ratio of 10% or more per pass at 1000 ° C. or more; Cooling the rolled steel slab to 950 ° C. or less at a cooling rate of 2 ° C./s or more; Hot rolling the cooled steel slab with a cumulative reduction of 70% or more in the range of 950-800 ° C .; And Cooling the hot-rolled steel slab to a temperature range of 400 to 550 ° C. at a cooling rate of 5 ° C./s or more, and then air-cooling to room temperature; Method of producing a steel for PWHT guarantee riser pipe comprising a.

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