KR101253958B1 - Steel plate for pipeline with excellent fracture arrestability and low yield ratio, and manufacturing method of the same - Google Patents

Abstract

The present invention relates to a steel sheet for line pipe having excellent fracture propagation resistance and resistance ratio ratio, and a method for manufacturing the same, in weight%, carbon (C): 0.04 to 0.10%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.4-2.0%, Aluminum (Al): 0.01-0.05%, Titanium (Ti): 0.005-0.02%, Nitrogen (N): 0.002-0.01%, Niobium (Nb): 0.02-0.07%, Vanadium (V): 0.08% or less, Nickel (Ni): 0.1-0.4%, Molybdenum (Mo): 0.05-0.3%, Phosphorus (P): 0.015% or less, Sulfur (S): 0.005% or less, Calcium (Ca) : 0.0005 ~ 0.004%, residual iron (Fe) and other unavoidable impurities, microstructure is the area fraction, the line containing 50-65% ferrite, 30-40% bainite and 5-10% phase martensite A steel sheet for pipes and a method for producing the steel sheet are provided.
Through the present invention, it is excellent in brittle fracture propagation resistance, and has a yield strength of 70ksi grade for ultra-thick line pipes having a DWTT ductile fracture rate of more than 90% at a guaranteed temperature of -20 ° C and less than 85%. Can provide.

Description

Steel plate for line pipe having excellent fracture propagation resistance and resistance ratio ratio and its manufacturing method {STEEL PLATE FOR PIPELINE WITH EXCELLENT FRACTURE ARRESTABILITY AND LOW YIELD RATIO, AND MANUFACTURING METHOD OF THE SAME}

The present invention relates to a steel sheet for a line pipe used in a cold region and a method for manufacturing the same, and more particularly, having excellent fracture propagation resistance and resistance ratio ratio by controlling the microstructure of the steel sheet with ferrite, bainite, and phase martensite. It relates to a steel sheet for line pipes and a method of manufacturing the same.

Recently, with oilfield development centered on cold regions such as Siberia and Alaska, where the weather conditions are poor, many projects are underway to transport rich gas resources from oilfields to consumer areas through linepipes. The steel used in the line pipe project needs to have a yield strength of 70ksi grade steel that has both high DWTT and low yield ratio (YR) characteristics in consideration of transport gas pressure as well as durability against cryogenic and ground deformation. It is becoming.

In order for line pipe steels to be used safely at low temperatures, DWTT characteristics, which show brittle fracture stopping characteristics, must be excellent. Basically, it can be used if the DWTT ductility in pipe condition is more than 90% at -10 ℃. The steel sheet supplied to these pipes should basically have a DWTT ductile fracture ratio of 90% or more at -20 ° C. In general, DWTT properties are closely related to the effective grain size of steels.

Effective grain size is defined as the size of grains with high hard grain boundaries, and when the crack is initiated and propagated, the crack changes its propagation path at the effective grain boundary. Therefore, as the effective grain size becomes smaller, the propagation resistance of the crack increases.

In general, low temperature rolling is essential to refine the effective grain size. However, general low temperature rolling increases the yield ratio of the steel sheet to about 88-93%, making it difficult to form during piping, and easily causes local stress concentration during external deformation, thereby causing deterioration of pipe stability.

Therefore, in the production of line pipe steel sheet, there is a demand for a line pipe steel sheet manufacturing method capable of improving the high yield ratio characteristic by low-temperature reverse rolling and lowering the yield ratio to 85% or less.

Accordingly, an object of the present invention is to provide a steel sheet for a pipe and a method for manufacturing the same having a high DWTT characteristics and resistance ratio ratio characteristics at the same time.

In one embodiment, the present invention provides, in weight percent, carbon (C): 0.04 to 0.10%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05 %, Titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.002 to 0.01%, niobium (Nb): 0.02 to 0.07%, vanadium (V): 0.08% or less, nickel (Ni): 0.1 to 0.4% , Molybdenum (Mo): 0.05-0.3%, phosphorus (P): 0.015% or less, sulfur (S): 0.005% or less, calcium (Ca): 0.0005-0.004%, residual iron (Fe) and other unavoidable impurities In addition, the microstructure provides a steel sheet for a line pipe containing 50 to 65% ferrite, 30 to 40% bainite and 5 to 10% martensite.

It is preferable that the average effective grain size of the ferrite is 10 µm or less.

It is preferable that the average effective grain size of the bainite is 20 µm or less.

It is preferable that the average effective grain size of the said phase-like martensite is 4 micrometers or less.

As another embodiment of the present invention, in weight%, carbon (C): 0.04 to 0.10%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05 %, Titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.002 to 0.01%, niobium (Nb): 0.02 to 0.07%, vanadium (V): 0.08% or less, nickel (Ni): 0.1 to 0.4% , Molybdenum (Mo): 0.05-0.3%, phosphorus (P): 0.015% or less, sulfur (S): 0.005% or less, calcium (Ca): 0.0005-0.004%, residual iron (Fe) and other unavoidable impurities Heating the slab at 1150 to 1250 ° C .; Extracting the heated slab at 1100-1140 ° C .; Recrystallization rolling step of rolling the extracted slab and ending at Tnr-10 ° C. to Tnr + 20 ° C .; A non-recrystallization station rolling step of performing non-recrystallization station rolling on the recrystallized station rolled steel sheet at Tnr-160 ° C to Tnr-130 ° C; Initiating cooling of the non-recrystallized steel sheet at a cooling rate of 15 ° C./s or more at Ar 3 -60 ° C. to Ar 3-40 ° C .; And it provides a method for producing a steel sheet for line pipe comprising the step of stopping the cooling of the steel sheet at Ms-70 ℃ ~ Ms-50 ℃.

The average reduction ratio of the recrystallization rolling step is preferably 10% or more.

The cumulative reduction ratio of the step of starting the unrecrystallized rolling is preferably 73 to 80%.

It is preferable that the said non-recrystallization rolling finishes at Ar3-10 degreeC-Ar3 + 20 degreeC.

After cooling stop of the said steel plate, it is preferable to air-cool or room-cool a steel plate to normal temperature.

Through the present invention, it is excellent in brittle fracture propagation resistance, and has a yield strength of 70ksi grade for ultra-thick line pipes having a DWTT ductile fracture rate of more than 90% at a guaranteed temperature of -20 ° C and less than 85%. Can provide.

The present invention minimizes the content of impurities causing the central segregation, the microstructure is controlled by 50-65% ferrite, 30-40% bainite and 5-10% phase martensite, and finely control the grains, An object of the present invention is to provide a steel sheet for an extreme thick line pipe having a DWTT characteristic and a resistance ratio ratio property and a method of manufacturing the same.

The DWTT properties are related to the effective grain size of the steel. The effective grain size is defined as the size of grains with high hard grain boundaries, and when the crack is initiated and propagated, the crack changes its propagation path at the effective grain boundary. Therefore, as the effective grain size becomes smaller, the propagation resistance of the crack increases. In addition, in the case of thick materials, central segregation is likely to occur, and the central segregation may act as a starting point of the crack, and since crack propagation is easily progressed due to the propagation resistance of the crack, it is necessary to minimize impurities that may cause central segregation. .

Hereinafter, the component system and the composition range of the steel sheet of the present invention will be described first.

Carbon (C): 0.04 to 0.10 wt%

Carbon is the most effective element for improving the strength of steel, but when excessively added, carbon may degrade weldability, formability and toughness. When the content of carbon is less than 0.04% by weight, the content of carbon is too low to obtain the desired strength, and thus, additional expensive alloying elements may be added to obtain the desired strength. However, when it exceeds 0.10% by weight, the carbon content is too high, as described above, there is a problem that the weldability, formability and toughness is lowered.

Silicon (Si): 0.05-0.50 wt%

Silicon acts as a deoxidizer to deoxidize molten steel and is used as a solid solution strengthening element. If the silicon content is less than 0.05% by weight, the deoxidation of the molten steel may not be sufficient and the toughness may be lowered. However, when the content exceeds 0.50% by weight, red scales are formed by silicon during hot rolling, so that the steel sheet surface shape is very poor and the weld toughness is lowered.

Manganese (Mn): 1.4-2.0 wt%

Manganese is an element that can improve strength due to solid solution strengthening effect. Manganese must be included at least 1.4% by weight to increase the hardenability and yield strength required for 80ksi grade steel. However, in the case of exceeding 2.0% by weight, segregation may occur at the center of the thickness when casting the slab in the steelmaking process, which may damage the weldability of the final product.

Aluminum (Al): 0.01 ~ 0.05 wt%

Aluminum is added to silicon as a deoxidizer in the steelmaking stage, and is an element capable of improving strength by solid solution strengthening. When aluminum is included in less than 0.01% by weight, the above-described deoxidation effect is insufficient and the toughness is lowered. However, when it exceeds 0.05 weight%, there exists a problem that impact toughness falls.

Titanium (Ti): 0.005 to 0.02 wt%

Titanium combines with N in the solidification step of the steel to form TiN precipitates, thereby inhibiting the growth of austenite grains and miniaturizing the final grain size, thereby improving the toughness of the steel. If the content of titanium is less than 0.005% by weight, TiN precipitates are insufficient, which makes it difficult to suppress the growth of austenite grains. However, when it exceeds 0.02%, solute Ti is usually present excessively, and TiN precipitates coarsely upon slab heating, which is not suitable for finer grain size.

Nitrogen (N): 0.002-0.01 wt%

Nitrogen is dissolved in the steel and then precipitates to increase the strength of the steel, and this solid-solution strengthening effect is greater than that of carbon. The presence of nitrogen in steel is known to reduce toughness. In the present invention, however, TiN is formed by reacting with titanium using an appropriate amount of nitrogen to control grain growth during reheating of the slab. If the nitrogen content is less than 0.002% by weight, the TiN precipitate content is low, so the effect of inhibiting grain growth is not so great. On the other hand, when the content of nitrogen exceeds 0.01% by weight, nitrogen is present as a solid solution to reduce the toughness significantly.

Niobium (Nb): 0.02 to 0.07 wt%

Niobium is a very useful element for miniaturizing grains, and is an effective element for enhancing strength by promoting formation of acicular ferrite or bainite, which is a high-strength structure. When added at less than 0.02% by weight, the above effects are insignificant. However, when it exceeds 0.07 weight%, weldability can be reduced.

Vanadium (V): 0.08 wt% or less

Vanadium reacts with carbon to form V (C, N) precipitates, which can enhance precipitation strengthening and hardenability. However, if it is contained in an amount of 0.08% by weight or more, the weldability and toughness may be lowered.

Calcium (Ca): 0.0005 to 0.004 wt%

Calcium is a useful element for spheroidizing MnS nonmetallic inclusions, and can suppress the formation of cracks around the MnS inclusions. If the calcium content is less than 0.0005% by weight, the spheroidizing effect of the MnS inclusions does not appear. However, when the content is more than 0.004% by weight, a large amount of CaO-based inclusions are generated, which lowers the impact toughness.

Nickel (Ni): 0.1-0.4 wt%

Nickel is an element capable of improving strength and toughness at the same time. In the present invention, nickel serves to improve the strength and brittle fracture stopping property of the thick material. When the nickel content is less than 0.1% by weight, the above effects are insignificant. And since nickel is a very expensive element, it is not desirable to increase the addition amount unconditionally despite the above effects. This is because the effect of improving the strength and toughness of the price ratio is relatively small. Therefore, the upper limit is preferably limited to 0.4% by weight in consideration of the effect of improving the price, strength and toughness.

Molybdenum (Mo): 0.05-0.3 wt%

Molybdenum is a useful element for improving the strength of steel. However, when added at less than 0.05% by weight, the effect of improving strength is insignificant. When added in excess of 0.3% by weight, coarse bainite and island martensite (MA) structures may be formed at the center of the thick material to reduce DWTT characteristics. However, since molybdenum is an expensive element and its content becomes high, weldability falls, so it is more preferable to limit the upper limit to 0.1 weight%.

The remainder of the present invention is iron (Fe). However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making. However, since phosphorus and sulfur are generally referred to as impurities, a brief description thereof is as follows.

Phosphorus (P): 0.015 wt% or less

Phosphorus is an element that is inevitably contained during forcing, and as described above, the phosphorus content should be controlled as low as possible in the present invention. When phosphorus is added, it segregates in the center of the steel sheet and can be used as a crack initiation point or a growth path. In theory, it is advantageous to limit the content of phosphorus to 0%, but it is inevitably added in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the content of phosphorus is preferably limited to 0.015% by weight.

Sulfur (S): 0.005% by weight or less

Sulfur is an element that is inevitably contained during forcing, and it is preferable to control the sulfur content as low as possible in order to form a non-metallic inclusion to reduce the toughness and strength of the steel and to secure brittle fracture stop characteristics, especially in cryogenic conditions. It is advantageous to limit to, but inevitably be added in the manufacturing process. Therefore, it is important to manage the upper limit, in the present invention, the upper limit of the content of sulfur is preferably limited to 0.005% by weight.

As the steel sheet having the above-described component system, it is necessary to limit the microstructure of the steel sheet to preferable conditions for being a steel sheet having excellent brittle fracture propagation resistance, excellent DWTT reverse wavefront resistance, and at the same time having a resistance ratio.

In the present invention, in order to have excellent brittle fracture propagation resistance, excellent DWTT reverse wave resistance, and at the same time have a resistance ratio, the microstructure of the steel sheet is 50 to 65% of ferrite, 30 to 40% of bainite, and phase martensite. It is preferred to include 5-10%. The average grain size of the ferrite is preferably 10 μm or less, the average grain size of the bainite is 20 μm or less, and the average grain size of the doped martensite is 4 μm or less. As described above, as the grain size decreases, the propagation path of the crack changes, so that the propagation resistance of the crack increases.

The steel sheet having the above-described component system and satisfying the internal structure conditions has a DWTT ductile fracture rate of 90% or more at the guaranteed temperature of -20 ° C. or lower, and at the same time has a resistance compounding ratio of 85% or less. It is a steel plate that meets all.

The most preferred method elicited by the present inventors for producing the steel which satisfies the object of the present invention as described above is described below.

In the manufacturing method of the present invention, after heating the slab having the steel composition of the present invention, the heated slab is subjected to austenite recrystallization reverse rolling and austenite unrecrystallization reverse rolling, and then accelerated at a cooling rate of 15 ° C./s or more. Cool and stop cooling.

Hereinafter, detailed conditions for each step will be described.

Slab reheating stage

Prior to hot rolling, the slabs should be reheated to above 1150 ° C to allow NbC to dissolve and exist in the Nb atomic state. If the reheating temperature exceeds 1250 ° C., coarse TiN precipitates are formed upon reheating. Therefore, the temperature range of the slab reheating step is preferably limited to 1150 ~ 1250 ℃. In addition, the reheated slab is extracted after maintaining at 1100-1140 ℃ in the crack. When slab extraction temperature is less than 1100 ℃, workability such as rolling property may not be easy, and when it exceeds 1140 ℃, workability is easy, but particle size control is not good, so slab extraction temperature is managed as 1100-1140 ℃. It is desirable to.

Rolling step

In order to improve the low temperature toughness of the steel sheet, it is preferable to control the austenite grains to a fine size. This is possible by controlling the rolling temperature and the reduction ratio. In the present invention, it is preferable that the rolling is performed in two temperature ranges. Since the recrystallization behavior is different in the two temperature ranges, it is preferable to set the respective conditions.

Recrystallization station rolling stage

Rolling the extracted slab is preferably finished at Tnr-10 ° C ~ Tnr +20 ° C. Where Tnr temperature refers to the temperature at which austenite recrystallization stops, where Tnr = 887 + (464 * C) + ((6445 * Nb)-(644 * SQRT (Nb))) + ((732 * V) -(230 * SQRT (V))) + (890 * Ti) + (363 * Al)-(357 * Si) Through recrystallization rolling, the grains of the old austenite can be refined. In addition, it is preferable to limit the average rolling reduction in recrystallization rolling to 10% or more. If the average reduction ratio is less than 10%, coarse unrecrystallized austenite remains, which may significantly reduce DWTT characteristics. If the rolling finish temperature is below Tnr-10 ° C or above Tnr + 20 ° C, coarse unrecrystallized austenite may remain to significantly reduce DWTT characteristics.

Unrecrystallized rolling stage

It is preferable to limit a non-recrystallization rolling start temperature to the range of Tnr-160 degreeC-Tnr-130 degreeC. The Ar3 temperature refers to the temperature at which austenite is transformed into ferrite, and theoretically, Ar3 = 910- (273 * C)-(74 * Mn)-(57 * Ni)-(16 * Cr)-(9 * Mo)- Can be derived as (5 * Cu). If the rolling start temperature is less than Tnr-160 ℃ or exceeds Tnr-130 ℃ there is a problem that the coarse ferrite structure is formed. In addition, the cumulative reduction ratio of the non-recrystallization rolling step is preferably limited to 73 to 80%. If the cumulative reduction ratio exceeds 80%, the recrystallization reverse rolling effect is weakened, and coarse uncrystallized austenite remains. On the other hand, when less than 73%, austenite is not sufficiently crushed to obtain fine acicular ferrite and equiaxed ferrite structure. Unrecrystallized rolling end temperature is preferably limited to the range Ar3-10 ℃ ~ Ar3 +10 ℃. When the unrecrystallized rolling end temperature exceeds Ar3 + 10 ° C., the fraction of fine equiaxed ferrite structure is lowered and the fraction of coarse acicular ferrite structure is increased.

Cooling stage

Accelerated cooling is performed after completion of the rolling. Controlling the cooling start temperature is an important factor in the formation of fine equiaxed ferrite. Cooling start temperature in the present invention is preferably limited to Ar3-60 ℃ ~ Ar3-40 ℃ range. If the cooling start temperature is lower than Ar3-60 ° C or higher than Ar3-40 ° C, the ferrite fraction exceeds 65%, which weakens the DWTT reverse wavefront suppression effect. Cooling rate is preferably limited to 15 ° C / s or more. The cooling rate is not particularly limited, but an upper limit may exist due to the extreme material properties. If the cooling rate is less than 15 ℃ / s, the fraction of equiaxed ferrite increases and the grain size becomes coarse, so both strength and toughness deteriorate. Cooling end temperature is preferably limited to the range of Ms-70 ℃ ~ Ms-50 ℃. Ms refers to the martensite transformation start temperature, and in the present invention, Ms = 561-474 * (% C)-33 * (% Mn)-17 * (% Ni)-17 * (% Cr)-21 * (% Mo) can be derived. If the cooling end temperature exceeds Ms-50 占 폚, the ferrite fraction decreases and the strength is greatly reduced. In addition, when the cooling end temperature is less than Ms-70 ℃, bainite formation is promoted to increase the strength but deterioration of toughness.

Air cooling or cooling

Thereafter, the method may further include cooling through air cooling or room cooling.

The ultra-thick plate manufactured according to the present invention is excellent in fracture propagation resistance even at low temperature, and excellent in suppressing DWTT reverse wave front, and is suitable for 70ksi grade ultra-thick line pipe for submarines having low-temperature DWTT properties.

Hereinafter, the present invention will be described through examples.

Example

The slab that satisfies the component system shown in Table 1 below was prepared in a plate having a thickness of 22 mm according to the manufacturing conditions shown in Table 2. DWTT and tensile tests were performed on Inventive Examples 1 to 3 and Comparative Examples 4 to 10 prepared as described above. The DWTT test was carried out at -20 ° C, and the ductility of the notched portion was measured for each specimen and is shown in Table 3 below. In addition, by performing an optical microscope for the rolled material of the invention examples and comparative examples to measure the fraction of each phase is shown in Table 3. In addition, by performing the Electron Backscatter Diffraction (EBSD) analysis to measure the average size of the effective grains having a high grain boundary is shown in Table 3. In addition, yield ratio, yield strength and tensile strength were measured and shown in Table 3 together.

[Table 1]

[Table 2]

[Table 3]

As shown in Table 3, Inventive Examples 1 to 3 are rolled and cooled through the manufacturing conditions of the present invention using the steel grades A, B, and C satisfying the component range of the present invention, 10 ㎛ or less A steel sheet having about 50 to 65% of ferrite having an average grain size, 30 to 40% of bainite having an average grain size of 20 µm or less, and an iconic martensitic composite steel having an average grain size of 4 µm or less. It was manufactured and satisfied the guarantee properties of the line pipe steel applied to the limit paper at -20 ℃ DWTT ductile 92 ~ 99% and yield ratio 79 ~ 82%.

On the other hand, Comparative Examples 4 to 10 is a case in which all of the components of the present invention are satisfied but the manufacturing conditions deviate from the conditions controlled by the present invention, and all of the DWTT characteristics or the yield ratio characteristics showed poor performance.

Claims (9)

By weight%, carbon (C): 0.04 to 0.10%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 -0.02%, nitrogen (N): 0.002-0.01%, niobium (Nb): 0.02-0.07%, vanadium (V): 0.08% or less, nickel (Ni): 0.1-0.4%, molybdenum (Mo): 0.05- 0.3%, phosphorus (P): 0.015% or less, sulfur (S): 0.005% or less, calcium (Ca): 0.0005-0.004%, balance iron (Fe) and other unavoidable impurities, and the microstructure is an area fraction , 50-65% ferrite, 30-40% bainite and 5-10% doping martensite.
The steel sheet for line pipe according to claim 1, wherein the average effective grain size of the ferrite is 10 µm or less.
The steel sheet for line pipe according to claim 1, wherein the average effective grain size of the bainite is 20 µm or less.
The steel sheet for line pipe according to claim 1, wherein the average effective grain size of the doped martensite is 4 µm or less.
By weight%, carbon (C): 0.04 to 0.10%, silicon (Si): 0.05 to 0.50%, manganese (Mn): 1.4 to 2.0%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 -0.02%, nitrogen (N): 0.002-0.01%, niobium (Nb): 0.02-0.07%, vanadium (V): 0.08% or less, nickel (Ni): 0.1-0.4%, molybdenum (Mo): 0.05- Slab containing 0.3%, phosphorus (P): 0.015% or less, sulfur (S): 0.005% or less, calcium (Ca): 0.0005-0.004%, balance iron (Fe) and other unavoidable impurities at 1150-1250 ° C. Heating;
Extracting the heated slab at 1100-1140 ° C .;
Recrystallization rolling step of rolling the extracted slab and ending at Tnr-10 ° C. to Tnr + 20 ° C .;
A non-recrystallization station rolling step of performing non-recrystallization station rolling on the recrystallized station rolled steel sheet at Tnr-160 ° C to Tnr-130 ° C;
Initiating cooling of the non-recrystallized steel sheet at a cooling rate of 15 ° C./s or more at Ar 3 -60 ° C. to Ar 3-40 ° C .; And
Stopping cooling of the steel sheet at Ms-70 ° C to Ms-50 ° C
Method for producing a steel sheet for line pipe comprising a.
6. The method of claim 5, wherein the average reduction ratio of the recrystallization rolling step is 10% or more.
The method for manufacturing a steel sheet for line pipe according to claim 5, wherein the cumulative reduction ratio of the non-recrystallization zone rolling step is 73 to 80%.
The method for manufacturing a steel sheet for line pipe according to claim 5, wherein the unrecrystallized rolling is finished at Ar3-10 ° C to Ar3 + 20 ° C.
The method for manufacturing a steel sheet for line pipe according to claim 5, wherein after the cooling stop of the steel sheet, the steel sheet is cooled or cooled to room temperature.