KR20220071036A - High strength hot rolled steel sheet for steel pipe and method of manufacturing the same - Google Patents

Abstract

The present invention provides a high-strength hot-rolled steel sheet for a steel pipe, which has high strength and high processability, a steel pipe manufactured by using the same, and a method for manufacturing the same. According to an embodiment of the present invention, a high-strength hot-rolled steel sheet for a steel pipe comprises, by wt %, carbon (C): 0.20 to 0.40 %, silicon (Si): 0.05 to 0.3 %, manganese (Mn): 0.5 to 1.6 %, aluminum (Al): 0.01 to 0.06 %, niobium (Nb): 0.001 to 0.03 %, chromium (Cr): > 0 % and < 0.5 %, calcium (Ca): 0.001 to 0.004 %, phosphorus (P): > 0 % and < 0.03 %, sulfur (S): > 0 % and < 0.003 %, and the remainder of iron (Fe) and other inevitable impurities, and satisfies a yield strength (YS) of 379 to 552 MPa, a tensile strength (TS) of 655 MPa or more, and an elongation (EL) of 24 % or more.

Description

High strength hot rolled steel sheet for steel pipe and method of manufacturing the same}

The technical idea of the present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a high-strength hot-rolled steel sheet for a steel pipe having high strength and high workability, a steel pipe manufactured using the same, and a method for manufacturing the same.

As an example of a hot-rolled steel sheet used as a material for oil wells for oil and gas mining, API-K55 standard is used as a casing. Conventionally, a seamless steel pipe has been mainly used, but as the technology is developed, the use of ERW welded steel pipe, which is superior in price competitiveness, is increasing. The required material of API-K55 for steel pipe products is a yield strength (YP) in the range of 379 MPa to 552 MPa, and a tensile strength (TS) of 655 MPa or more. When a hot-rolled coil is formed into a steel pipe, the yield strength increases due to the work hardening effect, and the degree increases as the diameter-to-thickness ratio (t/D) of the steel pipe increases.

Korean Patent Application No. 10-2017-0156278

The technical problem to be achieved by the technical idea of the present invention is to provide a high-strength hot-rolled steel sheet for a steel pipe having high strength and high workability, a steel pipe manufactured using the same, and a method for manufacturing the same.

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, a high-strength hot-rolled steel sheet for a steel pipe having high strength and high workability, a steel pipe manufactured using the same, and a manufacturing method thereof are provided.

According to an embodiment of the present invention, the high-strength hot-rolled steel sheet for steel pipe is, by weight, carbon (C): 0.20% to 0.40%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 0.5% ~ 1.6%, Aluminum (Al): 0.01% ~ 0.06%, Niobium (Nb): 0.001% ~ 0.03%, Chromium (Cr): >0% ~ 0.5%, Calcium (Ca): 0.001% ~ 0.004%, Phosphorus (P): more than 0% to 0.03%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities, yield strength (YS): 379 MPa to 552 MPa; Tensile strength (TS): 655 MPa or more, and elongation (EL): 24% or more may be satisfied.

According to an embodiment of the present invention, the high-strength hot-rolled steel sheet for a steel pipe may have a mixed structure in which ferrite and pearlite are mixed.

According to an embodiment of the present invention, the fraction of the ferrite may be 10% to 20%, and the fraction of the pearlite may be 80% to 90%.

According to an embodiment of the present invention, the method for manufacturing the high-strength hot-rolled steel sheet for a steel pipe is, by weight, carbon (C): 0.20% to 0.40%, silicon (Si): 0.05% to 0.3%, manganese (Mn) : 0.5% to 1.6%, aluminum (Al): 0.01% to 0.06%, niobium (Nb): 0.001% to 0.03%, chromium (Cr): more than 0% to 0.5%, calcium (Ca): 0.001% to 0.004 %, phosphorus (P): more than 0% to 0.03%, sulfur (S): more than 0% to 0.003%, and the balance is steel containing iron (Fe) and other unavoidable impurities at a reheating temperature of 1,160℃ to 1,220℃ reheating in hot-rolling the heated steel material to end at a finish rolling end temperature of 800° C. to 900° C.; First cooling the hot-rolled steel to a primary cooling end temperature of 620 ℃ ~ 680 ℃; maintaining the primary cooled steel material isothermal at the primary cooling end temperature; Secondary cooling of the primary cooled steel to a secondary cooling end temperature of less than 550 ℃ ~ 650 ℃; and winding the cooled steel at a coiling temperature of less than 550° C. to 650° C.; may include.

According to an embodiment of the present invention, the primary cooling step may be performed at a cooling rate of 15° C./sec to 25° C./sec.

According to an embodiment of the present invention, the step of maintaining the isothermal may be performed for a time ranging from 15 seconds to 25 seconds.

According to an embodiment of the present invention, the secondary cooling may be performed at a cooling rate of 25° C./sec to 35° C./sec.

According to an embodiment of the present invention, the hot-rolled steel sheet manufactured by the method for manufacturing a high-strength hot-rolled steel sheet for a steel pipe has a yield strength (YS): 379 MPa to 552 MPa, tensile strength (TS): 655 MPa or more, and an elongation rate (EL): satisfies 24% or more, has a mixed structure in which ferrite and pearlite are mixed, the fraction of ferrite is 10% to 20%, and the fraction of pearlite may be 80% to 90%.

According to the technical idea of the present invention, it is possible to manufacture a high-strength hot-rolled steel sheet for steel pipe having high workability and tensile strength and yield strength in a target range through control of the composition content and control of the manufacturing process conditions. . In addition, by maintaining for a certain period of time after performing primary cooling before winding after hot rolling, the size control and distribution of pearlite, which is a strong structure, can be made uniform, thereby reducing strength variation.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method of manufacturing a high-strength hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention.
2 is a scanning electron microscope photograph comparing the microstructure of a high-strength hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention with a comparative example.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

The present invention provides a high-strength hot-rolled steel sheet for a steel pipe having high strength and high workability, a steel pipe manufactured using the same, and a method for manufacturing the same.

In order to satisfy the desired material specifications of the steel pipe for oil well pipe, the hot-rolled steel sheet has yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 655 MPa or more, and elongation (EL): 24% or more It is preferable to do Furthermore, the hot-rolled steel sheet preferably satisfies a yield strength (YS): 379 MPa to 480 MPa, and a tensile strength (TS): 655 MPa or more. In order to satisfy these mechanical properties, the hot-rolled steel sheet may have a microstructure including 10% to 20% of ferrite and 80% to 90% of pearlite.

The pearlite is a structure in which transformation heat is generated the most, and the effect of increasing the temperature by the phase transformation after cooling is greater than that of a ferrite-based general steel. Therefore, the conventional technique of controlling the coiling temperature as a target shows different material performance depending on the amount of water poured, even when the same coiling temperature is reached, and the material deviation between hot-rolled coils is large.

The technical idea of the present invention is to control transformation heat generated by pearlite transformation when developing an API oil well tube material for a casing using steel with carbon added in an amount of 0.2 wt% or more, and cooling conditions for reducing material variation between hot-rolled coils provides

In the case of the conventional hot-rolled steel sheet for oil well pipe, only the coiling temperature was considered as the cooling temperature after hot rolling. However, in the present invention, the phase transformation behavior was analyzed by controlling the cooling process in the run-out table (ROT) between the windings after hot rolling, and the material change was analyzed accordingly. In particular, transformation heat generated by pearlite transformation affects the thermal history experienced by the hot-rolled steel sheet in the cooling zone, and the influence on the thermal history may change the phase transformation rate and fraction of the hot-rolled steel sheet. Therefore, it is necessary to review the temperature at which the transformation heat is active and the cooling method to control it, which greatly affects the final mechanical properties.

High-strength hot-rolled steel sheet for steel pipe with high strength and high workability

Hereinafter, a high-strength hot-rolled steel sheet for a steel pipe having high strength and high workability, which is an aspect of the present invention, will be described.

The high-strength hot-rolled steel sheet for a steel pipe, which is an aspect of the present invention, by weight, carbon (C): 0.20% to 0.40%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 0.5% to 1.6%, Aluminum (Al): 0.01% to 0.06%, Niobium (Nb): 0.001% to 0.03%, Chromium (Cr): >0% to 0.5%, Calcium (Ca): 0.001% to 0.004%, Phosphorus (P): more than 0% to 0.03%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the high-strength hot-rolled steel sheet for steel pipe according to the present invention will be described. At this time, the content of the component elements all mean weight % with respect to the entire steel sheet.

Carbon (C): 0.20% to 0.40%

Carbon is added to ensure strength and hardness. As the carbon content increases, the increase in tensile strength is larger than the increase in yield strength. The hot-rolled steel sheet of the present invention was designed with high carbon in consideration of the characteristics of steel grades with large deviations in yield strength and tensile strength requirements. When the carbon content is less than 0.20%, it is possible to secure sufficient tensile strength of the hot-rolled steel sheet through the addition of alloying elements, but it may be difficult to secure the desired high strength after heat treatment. When the carbon content exceeds 0.10%, weldability is deteriorated, and the fraction of the pearlite phase becomes excessively high, making it difficult to control the desired microstructure. Therefore, it is preferable to add carbon in an amount of 0.20% to 0.40% of the total weight of the hot-rolled steel sheet.

Silicon (Si): 0.05% to 0.3%

Silicon is added as a deoxidizer to remove oxygen in steel. Silicon can deteriorate the surface of steel when added in large amounts by generating red scale in the furnace. In addition, the weldability may be deteriorated due to the formation of oxides. In general, when 1.0 wt% or more of manganese is added, in order to control the properties of manganese oxide and silicon oxide, aluminum-silicon composite deoxidation is performed by adding silicon, and Mn/Si ratio is 6 to 9 during ERW welding for steel pipe manufacturing. It should be within a certain range between the gaps to significantly reduce the occurrence of cracks in the weld. When the content of silicon is less than 0.05%, the effect of adding silicon may be insufficient. When the content of silicon exceeds 0.3%, red scale is caused to deteriorate the surface quality, and cracks in the weld may occur. Therefore, silicon is preferably added in an amount of 0.05% to 0.3% of the total weight of the hot-rolled steel sheet.

Manganese (Mn): 0.5% to 1.6%

Manganese is a substitution element having an atomic diameter similar to that of iron, and is a very effective element for solid solution strengthening. In addition, manganese is an effective element for securing strength after heat treatment by improving the hardenability of steel. In addition, as an austenite stabilizing element, it can contribute to grain refinement of ferrite by delaying the transformation of ferrite and pearlite. However, when a large amount is added, the carbon equivalent is increased and weldability is greatly reduced. It can greatly reduce ductility and impact properties. When the manganese content is less than 0.5%, the effect of adding manganese may be insufficient. When the content of manganese exceeds 1.6%, it combines with S to form a large amount of MnS, thereby reducing corrosion resistance. Therefore, it is preferable to add manganese in an amount of 0.5% to 1.6% of the total weight of the hot-rolled steel sheet.

Aluminum (Al): 0.01% to 0.06%

Aluminum is added for deoxidation during steelmaking. When the content of aluminum is less than 0.01%, sufficient deoxidation effect cannot be obtained. When the content of aluminum exceeds 0.06%, it may combine with N present in steel to form a coarse AlN-based nitride, which may impair weldability. Therefore, it is preferable to add aluminum in an amount of 0.01% to 0.06% of the total weight of the hot-rolled steel sheet. In addition, it is preferable to add 0.01% to 0.03%

Niobium (Nb): 0.001% to 0.03%

Since recrystallization is delayed during hot rolling, that is, a non-recrystallized region is expanded, grain refinement can be achieved. It is known that solid solution niobium during hot rolling retards the nucleation and growth of recrystallization, and since such recrystallization delay does not consume defect sites such as dislocations, it promotes nucleation during phase transformation to make grains finer. In addition, since the deformed organic precipitated carbide serves as a nucleation site for ferrite during phase transformation, it is possible to refine the crystal grains by promoting the phase transformation. When the content of niobium is less than 0.001%, the effect of adding niobium may be insufficient. When the content of niobium exceeds 0.03%, yield strength and yield ratio increase, and elongation decreases, thereby reducing workability. Therefore, niobium is preferably added in an amount of 0.001% to 0.03% of the total weight of the hot-rolled steel sheet, respectively.

Chromium (Cr): >0% to 0.5%

Chromium, like manganese, lowers the equilibrium temperature, so it can affect the strength and yield ratio of steel. When chromium is added in a large amount, it can form coarse carbide by combining with carbon, which slightly increases strength, but is weak in toughness, so adding a large amount should be avoided. Therefore, for the resistance yield ratio characteristic, it should be controlled to a content that affects only the phase transformation and solid solution strengthening of steel and suppresses carbide formation. When the total content of chromium exceeds 0.5%, toughness may be lowered, and weld crack resistance may be lowered. Therefore, it is preferable to add chromium in an amount of more than 0% to 0.5% of the total weight of the hot-rolled steel sheet.

Calcium (Ca): 0.001% to 0.004%

Calcium is added to improve the ERW welding properties by preventing the formation of MnS inclusions. That is, since calcium has a higher affinity for sulfur than manganese, CaS inclusions are generated when calcium is added, and the occurrence of MnS inclusions is reduced. In general, MnS is elongated during rolling to cause defects such as hook cracks during ERW welding, so that ERW weldability can be improved when calcium is added. When the content of calcium is less than 0.001% by weight, the effect of adding calcium may be insufficient. When the content of calcium exceeds 0.004% by weight, CaS may be excessively generated, which may cause a problem. Therefore, calcium is preferably added in an amount of 0.001% to 0.004% of the total weight of the hot-rolled steel sheet.

Phosphorus (P): >0% to 0.03%

Phosphorus has the effect of inhibiting the formation of cementite and increasing strength. When the phosphorus content exceeds 0.03 wt%, weldability may deteriorate and toughness and weldability of steel may be reduced due to slab center segregation. In the present invention, phosphorus is not intentionally added and may be contained as an unavoidable impurity. Therefore, it is preferable to limit the phosphorus content to 0.03% or less of the total weight of the hot-rolled steel sheet.

Sulfur (S): >0% to 0.003%

Sulfur inhibits the toughness and weldability of steel, and may cause cracks during processing of steel by combining with manganese to form MnS. In the present invention, sulfur is not intentionally added and may be contained as an unavoidable impurity. Therefore, it is preferable to limit the sulfur content to 0.003% or less of the total weight of the hot-rolled steel sheet.

Nitrogen (N): >0% to 0.006%

Nitrogen combines with niobium and the like to form carbonitrides to refine crystal grains, but when added in a large amount, dissolved nitrogen increases, thereby reducing impact properties and elongation of steel, and greatly impairing weld toughness. In the present invention, nitrogen is not intentionally added and may be contained as an unavoidable impurity. Therefore, it is preferable to limit the nitrogen content to 0.006% or less of the total weight of the hot-rolled steel sheet.

The remaining component of the high-strength hot-rolled steel sheet for steel pipe is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The high-strength hot-rolled steel sheet for steel pipe manufactured by controlling the specific components and content ranges of the alloy composition described above, and through the method for manufacturing a hot-rolled steel sheet to be described later, has a yield strength (YS): 379 MPa to 552 MPa, tensile strength (TS) ): 655 MPa or more, and elongation (EL): 24% or more. The high-strength hot-rolled steel sheet for steel pipe may satisfy yield strength (YS): 379 MPa to 552 MPa, tensile strength (TS): 655 MPa to 700 MPa, and elongation (EL): 24% to 30%.

In addition, the high-strength hot-rolled steel sheet for a steel pipe may have a yield strength in the range of, for example, 379 MPa to 480 MPa.

The high-strength hot-rolled steel sheet for a steel pipe may have a mixed structure in which ferrite and pearlite are mixed. The fraction of ferrite may be, for example, 10% to 20%, and the fraction of pearlite may be, for example, 80% to 90%. The fraction means an area ratio derived from a microstructure photograph through an image analyzer.

Hereinafter, a method of manufacturing a high-strength hot-rolled steel sheet for a steel pipe according to the present invention will be described with reference to the accompanying drawings.

Manufacturing method of high-strength hot-rolled steel sheet for steel pipe

1 is a process flowchart schematically illustrating a method of manufacturing a high-strength hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention.

In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

The steel material for forming the hot-rolled steel sheet is, by weight, carbon (C): 0.20% to 0.40%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 0.5% to 1.6%, aluminum (Al) ): 0.01% to 0.06%, niobium (Nb): 0.001% to 0.03%, chromium (Cr): more than 0% to 0.5%, calcium (Ca): 0.001% to 0.004%, phosphorus (P): more than 0% ~ 0.03%, sulfur (S): more than 0% ~ 0.003%, and the balance may contain iron (Fe) and other unavoidable impurities.

Referring to FIG. 1 , the method of manufacturing a high-strength hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a primary cooling step (S130), an isothermal maintenance step (S140) , a secondary cooling step (S150), and a winding step (S160).

Reheating step (S110)

In the reheating step (S110), the steel having the above composition, for example, a slab plate, for example, 1,160 ℃ ~ 1,220 ℃ at a reheating temperature (Slab Reheating Temperature, SRT) for about 2 hours to 4 hours to reheat. Through such reheating, re-dissolution of segregated components during casting may occur. When the reheating temperature is less than 1,160°C, a rolling load may be caused during hot rolling. When the reheating temperature exceeds 1,220 °C, the strength of the final steel sheet may be reduced due to coarsening of grains.

Hot rolling step (S120)

In the hot rolling step (S120), the heated steel is first subjected to hot rolling after heating to adjust its shape. The hot rolling may be continuously performed by wide rolling, rough rolling, and finishing rolling. By the hot rolling step, the steel may form a steel sheet.

The hot rolling may be terminated, for example, at a finish delivery temperature (FDT) of 800°C to 900°C. The finishing rolling needs to be finished at an appropriate temperature to form pearlite, which is the main structure, and a trace amount of ferrite. When the finish rolling end temperature is less than 800° C., the non-uniformity of grains increases because the rolling proceeds in the two-phase region of austenite and ferrite, and the fraction of ferrite increases, thereby degrading the material. In addition, it is difficult to control rolling in the center of the thickness according to the thickening, and may cause variations in the electric length of the hot-rolled coil.

When the finish rolling end temperature exceeds 900°C, the austenite grains become coarse and the number of nucleation sites decreases due to the phase transformation delay in the cooling zone (ROT). may be lowered.

Primary cooling step (S130)

In the primary cooling step (S130), the hot-rolled steel is first cooled to a primary cooling end temperature of, for example, 620°C to 680°C. The primary cooling step may be performed, for example, at a cooling rate of 15° C./sec to 25° C./sec.

For the high-strength hot-rolled steel sheet for steel pipe, it is necessary to obtain a material with a resistance yield ratio, and for this, a trace amount of ferrite and a large amount of pearlite must be formed, so cooling control is important. Specifically, the phase transformation rate and the formed phase transformation structure may be different depending on the primary cooling end temperature.

When the primary cooling end temperature is less than 620 °C, bainite is generated, and hard spots may occur or the target material may be exceeded. When the primary cooling end temperature exceeds 680 ℃, even when the target temperature is reached at the coiling temperature, the transformation is not completed before winding, coarse pearlite may be generated, and thus material shortage may occur. have.

It is preferable that the primary cooling end time ends at a time before the midpoint of the run-out table ROT. The primary cooling step ( S130 ) may be referred to as a Cooling Stop Temperature (CST) step. In addition, the cooling may be both air cooling and water cooling.

Isothermal maintenance step (S140)

In the isothermal maintenance step (S140), the primary cooled steel is isothermal at the primary cooling end temperature, for example, maintained at a temperature in the range of 620°C to 680°C. The isothermal holding step ( S140 ) may be performed for an isothermal holding time in the range of 15 seconds to 25 seconds.

The steel should generate intensively the generation of pearlite before winding, and since transformation heat is generated in the pearlite transformation, when winding without isothermal maintenance, the temperature of the steel may increase. Such a temperature rise causes ferrite transformation and suppresses pearlite transformation, so a decrease in strength may occur.

Secondary cooling step (S150)

In the secondary cooling step (S150), the primary cooled steel is, for example, 550 ℃ to secondary cooling to a secondary cooling end temperature of less than 650 ℃ secondary cooling. The secondary cooling may be performed, for example, at a cooling rate of 25° C./sec to 35° C./sec.

The secondary cooling end temperature may be the same as the winding temperature. The secondary cooling step S130 may be referred to as a coiling temperature (CT) step. The secondary cooling termination temperature may be set in a range for controlling transformation heat and completing phase transformation. In addition, the cooling may be both air cooling and water cooling.

Winding step (S160)

In the winding step (S160), the cooled steel is wound at a coiling temperature (CT) of less than 550 °C to 650 °C. When the coiling temperature is less than 550° C., the strength increases due to grain refinement, making it difficult to form pipe, and weldability may be reduced. When the coiling temperature is 650° C. or higher, it may be difficult to secure sufficient strength.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Table 1 shows the compositions of the hot-rolled steel sheets of Examples and Comparative Examples. In Table 1, the remainder consists of iron (Fe) and impurities unavoidably contained in the steelmaking process, etc. Examples and Comparative Examples had the same composition. The unit is % by weight.

element C Si Mn Al Nb Cr Ca P S content 0.3374 0.22 1.29 0.028 0.001 0.02 0.0011 0.0071 0.0009

Table 2 shows the process condition values for forming the hot-rolled steel sheets of Comparative Examples and Examples.

division primary cooling
end temperature
(℃) Primary
cooling rate
(℃/sec) After 1st cooling
isothermal time
(candle) secondary cooling
end temperature
(℃) Secondary
cooling rate
(℃/sec) Comparative Example 1 680 20 19 650 30 Comparative Example 2 650 20 20 650 X Example 1 680 20 18 620 30 Example 2 650 20 19 620 30 Example 3 620 20 20 620 X Example 4 650 20 18 590 30 Example 5 620 20 19 590 30

Table 3 shows the results of measuring the yield strength (YS), tensile strength (TS), and elongation (EL) of the manufactured hot-rolled steel sheet as mechanical properties, respectively.

division yield strength
(MPa) The tensile strength
(MPa) elongation
(%) Comparative Example 1 460 632 28 Comparative Example 2 466 631 27 Example 1 469 659 25 Example 2 481 657 25 Example 3 495 664 28 Example 4 501 679 24 Example 5 509 677 26

Referring to Tables 2 and 3, Comparative Examples 1 and 2 were cases in which the secondary cooling termination temperature was performed at 650° C., and the tensile strength was less than the target lower limit of 655 MPa. When the primary cooling end temperature was lowered from 680°C as in Comparative Example 1 to 650°C as in Comparative Example 2, an increase in yield strength of 6 MPa was observed.

Examples are primary cooling to a primary cooling termination temperature of 620 ° C. to 680 ° C., maintaining isothermal in the range of 15 seconds to 25 seconds at the primary cooling termination temperature, and secondary cooling termination temperature of less than 550 ° C. to 650 ° C. By secondary cooling with a furnace, yield strength (YS): 379 MPa to 552 MPa, tensile strength (TS): 655 MPa or more, and elongation (EL): 24% or more were satisfied.

In Example 1, the primary cooling end temperature was relatively high at 680° C., and the tensile strength was 659 MPa, slightly exceeding the lower limit of the target range of 655 MPa. In addition, in Example 2, the primary cooling end temperature is relatively high at 650 °C, the secondary cooling end temperature is 620 °C, and the difference between the primary cooling end temperature and the secondary cooling end temperature is not as large as 30 °C. , the tensile strength was 657 MPa, slightly exceeding the lower limit of the target range of 655 MPa.

When the primary cooling end temperature was lowered from 680° C. as in Example 1 to 650° C. as in Example 2, an increase in yield strength of 12 MPa was observed.

In Example 3, the primary cooling end temperature was performed at 620° C., which is a lower temperature than in Examples 1 and 2, and both yield strength and tensile strength were slightly increased. .

In Examples 4 and 5, the secondary cooling termination temperature was performed at 590° C., which is a lower temperature than in Examples 1 to 3, and both yield strength and tensile strength were increased, and stable strength was ensured. indicates.

Accordingly, preferably, the primary cooling end temperature is set in the range of 620 °C to 650 °C, and the secondary cooling end temperature is preferably set to 620 °C or less. When the primary cooling end temperature is at a high level, it is preferable to set the secondary cooling end temperature to a temperature as low as possible.

2 is a scanning electron microscope photograph comparing the microstructure of a high-strength hot-rolled steel sheet for a steel pipe according to an embodiment of the present invention with those of Comparative Examples.

Referring to FIG. 2 , the white region is ferrite having a soft texture, and the black region is pearlite having a strong texture. Comparing Comparative Examples and Examples, it can be seen that in Comparative Examples, the size of the pearlite is relatively non-uniform, and the distribution is also relatively non-uniform. In Examples, it can be seen that the size of the pearlite is relatively uniform, and the distribution is also relatively uniform.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (8)

By weight%, carbon (C): 0.20% to 0.40%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 0.5% to 1.6%, aluminum (Al): 0.01% to 0.06%, niobium ( Nb): 0.001% to 0.03%, Chromium (Cr): More than 0% to 0.5%, Calcium (Ca): 0.001% to 0.004%, Phosphorus (P): More than 0% to 0.03%, Sulfur (S): 0 % greater than 0.003%, and the balance contains iron (Fe) and other unavoidable impurities,
Yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 655 MPa or more, and elongation (EL): 24% or more,
High-strength hot-rolled steel sheet for steel pipe. The high-strength hot-rolled steel sheet for steel pipe,
Having a mixed structure of ferrite and pearlite,
High-strength hot-rolled steel sheet for steel pipe. 3. The method of claim 2,
The fraction of ferrite is 10% to 20%,
The fraction of the pearlite is 80% to 90%,
High-strength hot-rolled steel sheet for steel pipe. By weight%, carbon (C): 0.20% to 0.40%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 0.5% to 1.6%, aluminum (Al): 0.01% to 0.06%, niobium ( Nb): 0.001% to 0.03%, Chromium (Cr): More than 0% to 0.5%, Calcium (Ca): 0.001% to 0.004%, Phosphorus (P): More than 0% to 0.03%, Sulfur (S): 0 % to 0.003%, and the remainder to reheating the steel material containing iron (Fe) and other unavoidable impurities at a reheating temperature of 1,160 ° C. to 1,220 ° C.;
hot-rolling the heated steel material to end at a finish rolling end temperature of 800° C. to 900° C.;
First cooling the hot-rolled steel to a primary cooling end temperature of 620 ℃ ~ 680 ℃;
maintaining the primary cooled steel material isothermal at the primary cooling end temperature;
Secondary cooling of the primary cooled steel to a secondary cooling end temperature of less than 550 ℃ ~ 650 ℃; and
Including; winding the cooled steel at a winding temperature of less than 550 °C to 650 °C.
Method for manufacturing high-strength hot-rolled steel sheet for steel pipe. 5. The method of claim 4,
The primary cooling step is performed at a cooling rate of 15 ° C / sec ~ 25 ° C / sec,
A method of manufacturing high-strength hot-rolled steel sheet for steel pipe. 5. The method of claim 4,
The step of maintaining the isothermal is performed for a time in the range of 15 seconds to 25 seconds,
A method of manufacturing high-strength hot-rolled steel sheet for steel pipe. 5. The method of claim 4,
The secondary cooling step is performed at a cooling rate of 25 ℃ / sec ~ 35 ℃ / sec,
A method of manufacturing high-strength hot-rolled steel sheet for steel pipe. 5. The method of claim 4,
The hot-rolled steel sheet manufactured by the manufacturing method of the high-strength hot-rolled steel sheet for steel pipe,
Yield strength (YS): 379 MPa ~ 552 MPa, tensile strength (TS): 655 MPa or more, and elongation (EL): 24% or more,
It has a mixed structure of ferrite and pearlite,
The fraction of ferrite is 10% to 20%,
The fraction of the pearlite is 80% to 90%,
A method of manufacturing high-strength hot-rolled steel sheet for steel pipe.

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