KR20110077091A - Hot-rolled steel for linepipe and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A hot-rolled sheet for line pipe and a manufacturing method thereof are provided to reduce manufacturing costs by making a hot-rolled sheet having high yield strength and excellent low temperature toughness without putting alloying element. CONSTITUTION: A hot-rolled sheet for line pipe comprises C: 0.03~0.1% Si: 0.3~0.5%, Mn: 1.2~2.0%, S: less than 0.002%, P: less than 0.03%, Ti: 0.01~0.10%, N: less than 0.01%, Nb: 0.01~0.1%, Cr: 0.01~0.5%, the rest Fe, and the inevitable impurity. The steel slab, satisfying the relation of Mn/20 + Si/14 + Cr/24 >= 0.11, is heated in the temperature range of 1100~1350°C. The heated slab is rolled in 950~1100°C for re-crystallization. The rolled steel material is hot-rolled in a reduction rate more than 60% in 750~880°C.

Description

Hot rolled steel for line pipe and manufacturing method thereof {HOT-ROLLED STEEL FOR LINEPIPE AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a hot-rolled steel for API-X70 line pipe used for construction, pipelines and offshore structures, and a method of manufacturing the same. More specifically, it is possible to control the component system appropriately without adding expensive elements. The present invention relates to a hot rolled steel material for high strength high toughness line pipe which can prevent a drop in strength, and a method of manufacturing the same.

Steel materials used for oil and gas transportation steel pipes and offshore structures used in the Arctic Ocean or Siberia, etc., are required to have excellent characteristics due to the harsh environment.

In particular, low temperature toughness is particularly required to prevent breakage at low temperatures, and these steel materials are manufactured by using spiral rolled steel pipes for economical use.

Many attempts have been made all over the world to manufacture high strength hot rolled steel sheets having excellent low temperature toughness. Most of these studies have been conducted to improve low temperature toughness by performing hot rolling at low temperature. Among them, Japanese Patent Laid-Open No. 2003-293076 relates to a high strength steel pipe material having a low yield ratio.

The patent is C: 0.04 ~ 0.08%, Si: 0.05 ~ 0.5%, Mn: 1.5 ~ 2.5%, Al: 0.01 ~ 0.1%, Nb: 0.01 ~ 0.1%, Ti: 0.005 to secure the strength and toughness of the steel It contains ˜0.02% and complexly adds Cr and expensive alloy elements Cu, Ni and the like. In this case, the strength and toughness can be easily secured, but manufacturing cost increases.

The present invention is to solve the above-mentioned problems, by using only Nb, Ti, Cr and Si appropriately and optimally control the manufacturing process conditions, line pipes excellent in strength and low temperature toughness without adding expensive alloy elements V, etc. It is to provide a hot-rolled steel and a method for manufacturing the same.

In the present invention, C: 0.03 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.2 to 2.0%, S: 0.002% or less, P: 0.03% or less, Ti: 0.01 to 0.10%, N: 0.01% Below, Nb: 0.01% to 0.1%, Cr: 0.01% to 0.5%, including the remaining Fe and other unavoidable impurities, hot rolled steel for line pipe that satisfies the relationship of Mn / 20 + 14 / Si + Cr / 24 ≥ 0.11 to provide.

The steel may further be added to V up to 0.03% by weight. In addition, the steel material includes (Ti, Nb) C and NbC precipitates having an average size of 0.2 μm or less.

In the present invention, C: 0.03 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.2 to 2.0%, S: 0.002% or less, P: 0.03% or less, Ti: 0.01 to 0.10%, N: Less than 0.01%, Nb: 0.01 ~ 0.1%, Cr: 0.01 ~ 0.5%, 1100 steel slab containing the remaining Fe and other unavoidable impurities and satisfying the relationship of Mn / 20 + 14 / Si + Cr / 24 ≥ 0.11 Heating at a temperature range of ˜1350 ° C .; Recrystallization and non-recrystallization rolling the heated slab at a temperature range of 950 to 1100 ° C .; Finishing hot rolling of the rolled steel at a reduction ratio of 60% or more at 750 to 880 ° C; Rolling the finish-rolled steel at a rolling reduction of at least 18% in the final three passes of finishing rolling; Water-cooling at a rate of 10 to 50 ° C./s or more after the rolling; And it provides a method for producing a hot rolled steel for line pipe comprising the step of winding at a temperature of 500 ~ 600 ℃ after the water cooling.

The slab may further be added to V up to 0.03% by weight.

According to the present invention, a hot rolled steel sheet having excellent yield strength and low temperature toughness can be produced without adding expensive alloy elements such as Mo, Cu, Ni, and V, and manufacturing cost can be reduced.

The inventors of the present invention have analyzed the method of securing the yield strength without adding expensive alloying elements, and as a result, by appropriately adjusting the ratio of Si, Mn and Cr, and optimizing the hot rolling conditions, the formation of precipitates is increased, It was confirmed that yield strength could be secured by miniaturization and increased solid solution. In view of this, the present inventors have been able to derive the following method to secure the strength and low temperature toughness of the hot rolled material simultaneously without adding expensive alloy elements Mo, Cu, Ni, and V. The method and embodiment are as follows.

[1] refinement of ferrite in steel through controlled rolling and securing strength in thick materials using precipitates

[2] Yield strength is secured by forming (Ti, Nb) C and NbC precipitates that satisfy the relationship of Mn / 20 + Si / 14 + Cr / 24 ≥ 0.11 and have an average size of precipitates of 0.2 µm or less.

[3] yield reduction of not less than 60% in unrecrystallized zones, and yield strength at low temperatures after finishing rolling

When the alloying element V is not added as in the present invention steel, it is necessary to refine the final ferrite in order to secure the yield strength and to secure the strength by using the precipitate. Ashby-Orowan modified the Hall-petch equation and expressed the yield strength of the steel as shown in Equation 1 below.

[Equation 1]

σy = σi + KD ^-1/2 + (10.8f ^1/2 /X)(ln(X/(6.125*10 ^-4 )))

Where σ i is friction stress, K is strength factor, D is grain size, f is precipitate fraction, and X is precipitate size.

According to Equation 1, the yield strength of steel increases as the grain size decreases, the amount of precipitates increases, and the size of precipitates decreases. As the final thickness of the steel decreases, the total pressure reduction decreases. Therefore, in order to refine the ferrite structure, more than a certain amount of unrecrystallized reverse rolling reduction must be secured. In this study, the yield strength was stably secured when the reduction was over 60%. It is also important to use appropriate precipitates to counteract the ferrite coarsening effect of reduced rolling reduction.

Precipitates affecting the yield strength of steel are mainly precipitated at the temperature range where ferrite is formed, and the formation temperature for each precipitate is determined by thermodynamic equilibrium. In general, the precipitation temperature of TiC precipitate is the highest and NbC is precipitated at low temperature. In addition, when Ti and Nb are added together, the composite precipitate in the form of (Ti, Nb) C and NbC are simultaneously precipitated. Therefore, in order to secure the strength by fully utilizing the added Ti, Nb, it is important to control the ratio of Si, Mn, Cr to increase the needle-like ferrite low temperature transformation structure so that fine precipitates can be produced during ferrite transformation.

In order to maximize the deposits and refine the final ferrite structure, it is important to maximize the yield of ferrite nucleation sites in the austenite zone by cold-pressing the finishing end of the filament at low temperature.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated. (Hereinafter,% is weight%.)

C: 0.03-0.1%

C is the most economical and effective element for reinforcing steel, but the weldability, formability and toughness are reduced by the addition of a large amount, and in the present invention, C is limited to 0.03 to 0.10%. If the added amount is less than 0.03%, it is not economical because other alloy elements must be added in relatively large amounts to exhibit the same strength, and if it is more than 0.10%, it is not preferable because the weldability, formability and toughness are lowered.

Si: 0.3 ~ 0.5%

Si is also needed to deoxidize molten steel and is effective as a solid solution element, so an addition of 0.3 to 0.5% is required. If the addition amount is less than 0.3%, it is difficult to obtain clean steel because it does not sufficiently deoxidize the molten steel, and if it exceeds 0.5%, a red scale formed by Si is formed during hot rolling, so the surface shape of the steel sheet becomes very bad and the ductility is deteriorated. Not.

Mn: 1.2 ~ 2.0%

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

S: 0.002% or less

S is combined with Mn and the like as an impurity element to form a non-metallic inclusion. Therefore, the upper limit is set to 0.002% because it is desirable to reduce it as much as possible because it greatly impairs the toughness and strength of the steel.

P: 0.03% or less

P also forms a non-metallic inclusion as an impurity element present in the steel, and therefore it is desirable to reduce it as much as possible because it greatly impairs the toughness and strength of the steel, so the upper limit is set to 0.03%.

Nb: 0.01 ~ 0.1%

Nb is a very useful element for refining grains and at the same time, it should be added at least 0.01% because of the dynamics that greatly improve the strength of the steel, but if it exceeds 0.1%, excessive Nb carbonitride precipitates, which is detrimental to the toughness of the steel. Limited to 0.01 ~ 0.1%.

Ti: 0.01 ~ 0.1%

Ti is a very useful element for refining grains. It exists as TiN in steel and has the effect of inhibiting the growth of grains during heating for hot rolling. Also, Ti remaining after being reacted with nitrogen is dissolved in carbon to bond with carbon. Precipitates are formed and the formation of TiC is very fine, greatly improving the strength of the steel. Therefore, in order to obtain the effect of inhibiting austenite grain growth by TiN precipitation and increasing the strength by TiC formation, at least 0.01% of Ti should be added. Since this toughness deteriorates the toughness of the weld heat affected zone, the upper limit of Ti addition is made 0.1%.

N: 0.01% or less

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. However, on the other hand, it is known that toughness decreases as more nitrogen exists in steel, and it is a general trend to reduce nitrogen content as much as possible. However, in the present invention, a proper amount of nitrogen is present to react with Ti to form TiN, thereby imparting a role of inhibiting grain growth during reheating. However, since part of Ti does not react with N and reacts with carbon in a subsequent step, the upper limit thereof is made 0.01% or less.

Cr: 0.01 ~ 0.5%

In the present invention, it is very effective to improve the strength of the material in order to lower the manufacturing cost of the steel, and do not use all of Mo, Ni, Cu, etc., which is advantageous for producing needle-like ferrite and micro precipitates at low temperature transformation structure, Cr to a certain amount or more to secure strength It is important to use. Cr generally increases the hardenability of the steel during direct quenching. It also generally improves corrosion resistance and hydrogen cracking resistance. Cr must be added at least 0.01% to suppress the formation of pearlite structure to obtain good impact toughness, and to obtain sufficient strength by encouraging the formation of fine precipitates by low temperature transformation phase, but when exceeding 0.5%, steel and its HAZ Since it tends to worsen toughness, it is preferable to limit it to 0.5% or less.

In addition, in the present invention, in order to increase the hardenability of the steel material and to easily obtain the microstructure and low temperature transformation phase, V of 0.03% by weight or less may be further added.

[Relationship 1]

Mn / 20 + Si / 14 + Cr / 24 ≥ 0.11

The relationship 1 is to maximize the solid solution strengthening of the material, to increase the hardenability to obtain a needle-like ferrite low temperature transformation phase. When the value of relation 1 is greater than or equal to 0.11, it is possible to maximize the known strength and the formation of precipitates to compensate for the drop in strength expected when no alloying elements Mo, Ni, Cu, etc. are added.

In the component system of the present invention, the finer the distribution, the more advantageous. Preferably, the average size of the precipitate is 0.2 μm or less. Furthermore, although the precipitate of 0.2 micrometer or less is distributed in a large amount in the component system of this invention, the distribution number is not specifically limited.

The present invention controls the ratio of Si, Mn, Cr without adding expensive alloy elements Mo, Cu, and Ni required to secure yield strength, and optimizes hot rolling conditions to increase the formation of precipitates, increase the size of microstructure, and increase solid solution strengthening. It is characterized by securing yield strength.

Hereinafter, the manufacturing method of this invention is demonstrated.

The temperature of reheating the slabs is important in the present invention. If the reheating temperature is set to 1100 ° C. or less, at which the additional alloying elements precipitated during the reworking process are sufficiently reusable, precipitates such as (Ti, Nb) C and NbC are reduced in the process after hot rolling. Therefore, by maintaining the reheating temperature of 1100 ℃ or more to facilitate the re-use of the precipitate and to maintain the austenite grain size of the appropriate size, it is possible to obtain a uniform microstructure in the longitudinal direction of the coil while improving the strength level of the material. At this time, if the reheating temperature is too high, the strength is lowered due to abnormal grain growth of the austenite grains, so the upper limit of the reheating temperature should be 1350 ° C.

The heated slab is preferably recrystallized and unrecrystallized rolled in the temperature range of 950 ~ 1100 ℃ and finish hot rolling at 750 ~ 880 ℃ to a reduction rate of 60% or more. This is because if the finish rolling temperature is too high, the final structure is coarse to obtain the desired strength, and if it is too low, problems such as equipment load may occur during finish rolling. When the rolling reduction rate is less than 60% during finish rolling, the microstructure of the steel is coarse to obtain the desired strength.

In addition, it is preferable to roll at a reduction ratio of 18% or more in 4, 5, and 6 passes for the final three passes of finishing rolling, for example, a total of six passes. In order to compensate for the drop in yield strength due to no addition of alloying elements, it is necessary to refine the final microstructure. For this purpose, it is effective to roll at a reduction ratio of more than 18% in the low temperature region, and ferrite grains are coarse when rolling to less than 18%. As a result, a problem occurs that the yield strength does not meet the specification.

After the hot rolling is finished, water cooling is performed on the run-out table at a rate of 10 to 50 ° C./sec to form fine ferrites and precipitates, thereby securing sufficient strength. Even if the component ratio is controlled to obtain fine precipitates according to the present invention, if the cooling rate is less than 10 ° C / sec, the average size of the precipitates may exceed 0.2 μm. In other words, as the cooling rate increases, a large number of nuclei are generated and the precipitate becomes fine. As the cooling rate increases, the size of the precipitate becomes finer, so it is not necessary to limit the upper limit of the cooling rate. However, even if the cooling rate is faster than 50 ° C / sec, the precipitate refinement effect does not increase any more, so the cooling rate is 10 to 50 ° C / sec. Is more preferable.

Winding temperature of 450 ~ 600 ℃ temperature range is appropriate, if higher than 600 ℃ microstructure is formed of coarse ferrite and pearlite, precipitates grow too coarse, difficult to secure the strength, when winding below 450 ℃ the second The fraction of the phases increases and the impact properties deteriorate.

EMBODIMENT OF THE INVENTION Hereinafter, the Example of this invention is described.

Example 1

In order to evaluate the effect of Cr and V on the mechanical properties of the steel, vacuum melting of the steel having the components shown in Table 1 and the experiment was carried out using a hot rolling simulation apparatus, the experiment was carried out by vacuum melting at 1200 ℃ After heating for 2 hours and hot rolling at 1180 ~ 800 ℃ to 520 ℃ by cooling at a cooling rate of 18 ℃ per second.

After the test, the yield strength, tensile strength and impact toughness of the steel was measured, and the results are shown in Table 2 below.

division Steel grade C Si Mn Cr V Ti Nb Cr-0% by weight One 0.08 0.2 1.6 0 0.05 0.02 0.06 2 0.08 0.2 1.6 0 0.025 0.02 0.06 3 0.08 0.2 1.6 0 0 0.02 0.06 Cr-0.2 wt% 4 0.08 0.2 1.6 0.2 0 0.02 0.06 5 0.08 0.2 1.6 0.2 0.05 0.02 0.06 Cr-0.3 wt% 6 0.08 0.2 1.6 0.3 0.05 0.02 0.06 7 0.08 0.2 1.6 0.3 0.025 0.02 0.06 8 0.08 0.2 1.6 0.3 0 0.02 0.06

division Steel grade Yield strength
(MPa) The tensile strength
(MPa) Impact energy
(-20 ℃, J) Fine precipitate Cr-0% by weight One 560 625 272 NbC only 2 512 613 274 NbC only 3 520 593 305 NbC only Cr-0.2 wt% 4 527 621 305 NbC precipitate increase 5 551 640 271 (Nb, V, Cr) C
NbC complex precipitation Cr-0.3 wt% 6 581 651 293 (Nb, V, Cr) C
NbC complex precipitation 7 572 649 281 (Nb, V, Cr) C
NbC complex precipitation 8 553 629 305 NbC precipitate increase

As shown in Table 2, when comparing the steel grades 1, 2 and 3, the yield strength is increased when V is increased, but it can be seen that the strength increase effect by V addition is insignificant up to 0.0025% by weight. In addition, the impact energy is reduced when V is added.

Comparing steel grades 3, 4, and 8, the yield strength increases with the addition of Cr when V is not added, and especially when the amount of Cr added exceeds 0.2% by weight, it shows that the yield strength is prevented by V addition. have. 1 and 2 are photographs showing the microstructure of steel grades 3 and 8, respectively.

V generally increases the hardenability of the steel, so that when V is added, tissue refinement and low temperature transformation phases are easily obtained, and fine carbonitrides are formed to increase strength. Without the addition of V, yield strength is reduced by about 40 MPa, as can be seen in steel grades 1 and 3. To compensate for this drop in strength, it is necessary to add a low-cost alloying element that increases the hardenability and facilitates the formation of fine precipitates.

Cr is a relatively inexpensive alloying element that has a similar effect to Mo, but the addition of Cr increases the hardenability, resulting in an increase in the low temperature transformation phase at the same cooling rate. As a result, fine ferrite and needle-like ferrite structures can be obtained. Will be. In addition, Cr promotes the precipitation of NbC and has the effect of increasing the yield strength of steel.

From these results, it was confirmed that the addition of inexpensive Cr is preferable as a component for compensating the strength drop due to the expensive V-free addition.

[Example 2]

After the slab having the chemical composition as shown in Table 3 by the continuous casting method, and hot-rolled slab under the conditions of Table 4 to produce a sheet, the tensile test piece from the hot-rolled sheet to the rolling direction The yield strength was measured by a tensile test taken at 30 degrees clockwise and the results are shown in Table 4 below. This direction corresponds to the circumferential direction of the pipe when the spiral pipe is constructed. Tensile test pieces were used for API 5L standard test pieces, and the tensile test was conducted at a cross head speed of 10 mm / min.

Steel grade C
(weight%) Si
(weight%) Mn
(weight%) Nb
(weight%) Ti
(weight%) V
(weight%) Cr
(weight%) Mn / 20 + Si / 14 + Cr / 24 A 0.075 0.25 1.6 0.055 0.02 0.055 0.1 0.10 B 0.08 0.35 1.6 0.056 0.02 - 0.27 0.12 C 0.08 0.35 1.6 0.056 0.015 - 0.01 0.11

Steel grade Specimen No. Reheating temperature
(℃) Recrystallized / Uncrystallized Rolling Temperature
(℃) Wrap-up
Rolling temperature
(℃) Rolling rate at 4-5-6 pass of finishing rolling (%) Coiling temperature
(℃) Cooling
speed
(℃ / s) surrender
burglar
(MPa) Precipitate average
size
(Μm) A Comparative material 1 1201 955 782 16-13-12 521 18 530 0.03 B Comparative material 2 1199 961 781 16-13-12 515 18 510 0.035 C Invention 1 1210 959 777 20-18-19 500 21 520 0.04

As shown in Table 3, steel type A is a conventional steel, a large amount of V was added, and does not satisfy the conditions of Mn / 20 + Si / 14 + Cr / 24 ≥ 0.11 proposed by the present invention. The steel grades B and C satisfy the composition component in accordance with the present invention, and do not add V. In particular, the steel grades B increase the content of Cr.

As shown in Table 4, Inventive Material 1, which satisfies the component system and manufacturing conditions according to the present invention, does not add V, which is an expensive element, but has a similar yield strength as compared to Comparative Steel 1, which is a conventional steel. It is coming true. In addition, in the case of Comparative Material 2, the rolling reduction rate in the final three passes (4-5-6 pass) of finishing rolling of the present invention is not more than 18%, but by increasing the Cr content, V-free is added. It can be seen that the strength drop that could occur during the compensation was compensated.

Figure 1 shows a microstructure picture of the steel (steel type 3) is not added Cr and V.

Figure 2 shows a microstructure photograph of the steel (steel type 8) without adding Cr: 0.3% by weight and V.

Claims (5)

By weight% C: 0.03 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.2 to 2.0%, S: 0.002% or less, P: 0.03% or less, Ti: 0.01 to 0.10%, N: 0.01% or less, Nb : 0.01 ~ 0.1%, Cr: 0.01 ~ 0.5%, hot rolled steel for line pipe containing the remaining Fe and other unavoidable impurities and satisfying the relationship of Mn / 20 + Si / 14 + Cr / 24 ≥ 0.11. The hot rolled steel for line pipe according to claim 1, wherein the steel includes V: 0.03% by weight or less. The hot rolled steel according to claim 1, wherein the steel includes (Ti, Nb) C and NbC precipitates having an average size of 0.2 µm or less. By weight% C: 0.03 to 0.1%, Si: 0.3 to 0.5%, Mn: 1.2 to 2.0%, S: 0.002% or less, P: 0.03% or less, Ti: 0.01 to 0.10%, N: 0.01% or less, Nb : 0.01 ~ 0.1%, Cr: 0.01 ~ 0.5%, steel slab containing the remaining Fe and other unavoidable impurities and satisfying the relationship of Mn / 20 + Si / 14 + Cr / 24 ≥ 0.11 Heating in a range; Recrystallization and non-recrystallization rolling the heated slab at a temperature range of 950 to 1100 ° C .; Finishing hot rolling of the rolled steel at a reduction ratio of 60% or more at 750 to 880 ° C; Rolling the finished rolled steel at a rolling reduction of at least 18% in each of the final three passes of finishing rolling; Water-cooling at a rate of 10 to 50 ° C./s or more after the rolling; And Method for producing a hot rolled steel for line pipe comprising the step of winding at a temperature of 450 ~ 600 ℃ after the water cooling. The method of claim 4, wherein the slab is V: 0.03% by weight or less.

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