KR20220088201A - Steel sheet and steel pipe having uniforme tensile properties and excellent transverse crack resistance onto welded part and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a steel plate and a steel pipe having a uniform tensile material and excellent resistance to lateral cracking in a welded portion, and a method for manufacturing the same it's about how

Description

Steel sheet and pipe with uniform tensile material and excellent resistance to lateral cracking at welded parts, and manufacturing method thereof

The present invention relates to a steel plate and a steel pipe having a uniform tensile material and excellent resistance to lateral cracking in a welded portion, and a method for manufacturing the same it's about how

A stabilizer or a door impact beam, which is a suspension component among components of an automobile body, is usually manufactured from a welded steel pipe or a drawn steel pipe in consideration of manufacturing cost or fatigue durability. On the other hand, when a suspension part is manufactured by using the welded steel pipe itself, there is a problem in that cracks occur in the welded portion of the welded steel pipe during the manufacturing process of the part, or cracks that are not detected in the welded steel pipe itself occur during the manufacturing process of the drawn steel pipe. These cracks in the weld are caused by various causes, but basically, martensite is formed due to excessive cooling of the welded part of the steel pipe and the volume change occurs, or the high content of diffusional hydrogen in the weld compared to the base part. This is known to be because hydrogen delayed destruction occurs in the weld zone. In particular, in a steel pipe having a tensile strength of 1500 MPa or more after rapid cooling or quenching, quenching cracks or hydrogen-delayed fracture at the weld are remarkable, so it is very important to set appropriate welding conditions or cooling rate when manufacturing welded steel pipe.

In addition, it is recognized that cracks occur in the quenching process of the hot-rolled steel sheet applied as a welded steel pipe. Specifically, the specimen bends or exhibits relatively lower strength and elongation than the target strength during the tensile test, leading to premature fracture (Premature Fracture). ) often occurs. Therefore, in order to apply a welded steel pipe having a tensile strength of 1600 MPa or more as a component after quenching, the optimum welding conditions or cooling rate can be set so that the crack does not occur. Another method is to post heat treatment ( Post heating) can be performed. However, the small-diameter welded steel pipe for suspension parts has operational difficulties in post-heat treatment of the welded part, and may lead to an increase in manufacturing cost due to the introduction of additional equipment.

On the other hand, a method of manufacturing a welded steel pipe or a method of manufacturing a part using a welded steel pipe is disclosed in several documents.

In Patent Document 1, double carbon is applied to the welded steel pipe by applying heating conditions such as a heating rate of 10°C/s or more, a heating temperature of 900°C or more, and a cracking time of 1 min or less to diffuse carbon around the welded part to the bond line position. It is disclosed that a steel pipe that is quenched at a high temperature of 980°C after primary cooling and quenching at a cooling rate of 80°C/s or higher after primary cooling has a high torsional fatigue life. In this case, if the difference in carbon content between the base metal part/welded part of the heat-treated electric resistance welded pipe is controlled to be 0.05% or less and the difference in Hv hardness is less than 40, there is no crack generation along the welded part of the electric resistance resistance steel pipe. On the other hand, the above results refer to increasing the fatigue life or durability of the steel pipe itself by increasing the strength of the steel pipe through normalizing and quenching of a good electric resistance welded steel pipe that does not cause cracks in the welded part. There is no mention of weld cracks occurring during the manufacturing stage. In addition, when the method for reducing the diameter of the welded steel pipe at room temperature is applied, there is a risk that cracks in the weld may occur due to residual cracks in the welded steel pipe itself. Therefore, when a welded steel pipe is manufactured using a steel sheet having a high strength of 1600 MPa or more through quenching, proper welding and cooling conditions are required to prevent cracks in the weld area.

Patent Document 2 discloses a method for manufacturing a high-frequency electric resistance welded steel pipe. In detail, when both ends of the steel sheets to be joined are maintained at a convergence angle of 4 to 6 degrees and welded under specific welding conditions, the outer surface of the steel pipe is heated to a temperature range of 950 to 970 ° C. It is disclosed that it is possible to suppress the occurrence of hydrogen-induced cracks in the welded portion of electric resistance welded steel pipe. In particular, in the case of high-frequency electric resistance welding, the relational expression consisting of welding parameters including high-frequency output (P.I), welding speed (W.S), upset amount (U.F), and steel thickness (t) satisfies the 23-27 level Here, the experimentally measured hydrogen induced cracking area ratio (HIC CAR: Hydrogen Induced Cracking Area Ratio, %) value is presented as less than 3%. On the other hand, it is necessary to check whether the manufacturing method of the electric resistance welding (ERW) and welding part heating (Seam Annealing) can be directly applied to thin and small diameter manufacturing.

Korean Patent Publication No. 10-2020-0096652 (published on August 12, 2020) Korean Patent Publication No. 10-0651774 (published on Dec. 1, 2006)

According to one aspect of the present invention, it is an object of the present invention to provide a steel sheet, a steel pipe, and a method for manufacturing the same having high strength and excellent resistance to lateral cracking of the welded portion during welding.

The subject of the present invention is not limited to the above. A person of ordinary skill in the art will have no difficulty in understanding the further problems of the present invention from the overall content of the present specification.

One aspect of the present invention, by weight%, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr : 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder including Fe and unavoidable impurities,

Ceq value defined in the following Relation 1 is 0.6 or more,

It is possible to provide a steel sheet having an R value of 700 or more, which is defined in the following Relational Equation (2).

[Relational Expression 1]

Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15

(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)

[Relational Expression 2]

R = ([Mn]+[Ti])/[S]

(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)

The steel sheet may further include at least one of Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: 0.5% or less.

The microstructure of the steel sheet may be composed of 10 to 40% of ferrite and the remaining pearlite in terms of area%.

The steel sheet may have a plastic deformation ratio (r-value) of 0.9 or more.

The steel sheet may have a tensile strength of 1600 MPa or more after quenching or quenching-tempering.

Another aspect of the present invention, by weight, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr: 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder including Fe and unavoidable impurities, the Ceq value defined in the following relation 1 is 0.6 or more, and in the following relation 2 Reheating a steel slab having a defined R value of 700 or more in a temperature range of 1150 to 1300 °C;

hot rolling the reheated steel slab to a finish hot rolling temperature of Ar3 or higher; and

It is possible to provide a steel sheet manufacturing method comprising the step of cooling the hot-rolled steel to a temperature range of 560 ~ 640 ℃ at a cooling rate of 10 ~ 70 ℃ / s and then winding.

[Relational Expression 1]

Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15

(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)

[Relational Expression 2]

R = ([Mn]+[Ti])/[S]

(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)

The steel sheet may further include at least one of Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: 0.5% or less.

The method may further include pickling the steel sheet after cooling to obtain a hot-rolled pickling steel sheet.

One aspect of the present invention, by weight%, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr : 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder including Fe and unavoidable impurities,

Ceq value defined in the following Relation 1 is 0.6 or more,

R value defined in the following relation 2 is 700 or more,

The difference between the hardness value of the weld and the base metal is less than 470Hv,

It is possible to provide a steel pipe having a weld diffusion hydrogen content of 0.5 ppm or less.

[Relational Expression 1]

Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15

(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)

[Relational Expression 2]

R = ([Mn]+[Ti])/[S]

(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)

The steel pipe may further include one or more of Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: 0.5% or less.

The microstructure of the weld part of the steel pipe may be composed of tempered martensite of less than 10% and the remaining martensite in area%.

The steel pipe may have a diameter of 52 mm or less.

Another aspect of the present invention, by weight, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr: 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder including Fe and unavoidable impurities, the Ceq value defined in the following relation 1 is 0.6 or more, and in the following relation 2 Reheating a steel slab having a defined R value of 700 or more in a temperature range of 1150 to 1300 °C;

hot rolling the reheated steel slab to a finish hot rolling temperature of Ar3 or higher;

winding the hot-rolled steel after cooling it to a temperature range of 560 to 640°C at a cooling rate of 10 to 70°C/s;

forming a pipe by welding the wound steel; and

Comprising the step of cooling the welded portion of the piped steel pipe at a cooling rate of 70 °C / s or less,

When the pipe is produced, the value of T defined in the following relation 3 is 0.13 to 0.21,

It is possible to provide a method for manufacturing a steel pipe by welding in which the value of K defined in the following Relation 4 is 4.0 to 9.0.

[Relational Expression 1]

Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15

(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)

[Relational Expression 2]

R = ([Mn]+[Ti])/[S]

(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)

[Relational Expression 3]

T = t/D

(Here, t means the thickness of the welded steel pipe, and D means the diameter of the welded steel pipe.)

[Relational Expression 4]

K = (Power Input/Welding Speed) + (Thickness/Upset Force)

(Here, Power Input is power (kw), Welding Speed is welding speed (mpm), Thickness is the thickness of the steel sheet (mm), and Upset Force is the amount of upset (mm))

The steel pipe may further include one or more of Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: 0.5% or less.

After the cooling step, the method may further include annealing, drawing, and molding so that the diameter of the steel pipe is 52 mm or less.

According to one aspect of the present invention, it is possible to provide a hot-rolled steel sheet having a tensile strength of 1600 MPa or more after quenching or quenching and tempering, and a method for manufacturing the same.

According to another aspect of the present invention, it is possible to provide a steel pipe having excellent resistance to lateral cracking of a welded portion when welding in a steel pipe shape using the hot-rolled steel sheet, and a method for manufacturing the same.

1 (A) and (B) show the tensile curves for the hot-rolled steel sheet after quenching according to an embodiment, (A) is an abnormal fracture, (B) is the normal proposed in the present invention. indicates breakage.
2 is a photograph of a welded steel pipe according to an embodiment in which a lateral crack is generated in a welded portion.
3 is a photograph showing the optical microstructure of a welded steel pipe according to an embodiment in which a notch crack occurs in a welded portion. (A) is a photograph of the welded part of the steel pipe, and (B) is a low-magnification photograph of the structure of the heat-affected zone (HAZ) of the weld. (C) is a high-magnification photograph of the structure of the heat-affected zone (HAZ) in welding, and (D) is a photograph of the welded portion of the drawn steel pipe.

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains.

In order to solve the above problems, the present inventor has optimized the alloy composition and manufacturing method of the steel sheet, and manufactured a welded steel pipe by an electric resistance welding method using a hot-rolled steel sheet having a tensile strength of 1600 MPa or more after the manufactured quenching heat treatment. In this case, it was confirmed that lateral cracks did not occur by optimizing the welding conditions, and the present invention was completed.

Hereinafter, the present invention will be described in detail.

Hereinafter, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, percentages indicating the content of each element are based on weight.

Steel according to an aspect of the present invention is, by weight, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less , Cr: 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder Fe and unavoidable impurities.

Carbon (C): 0.30 to 0.45%

Carbon (C) is an effective element for improving the strength of steel, and increases the strength after quenching. If the content of carbon (C) is less than 0.30%, it is difficult to secure sufficient strength of 1600 MPa or more after tempering, whereas if the content exceeds 0.45%, martensite with excessive hardness is formed, resulting in cracks in the weld zone of the steel sheet material or welded steel pipe As a result, there may be difficulties in the manufacture of subsequent drawn steel pipe or steel pipe parts.

Accordingly, the content of carbon (C) may be 0.30 to 0.45%. More preferably, it may be 0.34 to 0.43%.

Manganese (Mn): 0.9~1.5%

Manganese (Mn) is an essential element for improving the strength of steel, and increases the strength after quenching of steel. If the content of manganese (Mn) is less than 0.9%, it is difficult to secure sufficient strength of 1600 MPa or more after tempering, whereas if the content exceeds 1.5%, excessive segregation can be formed inside or outside the cast slab and hot-rolled steel sheet, When manufacturing welded steel pipe, there is a risk of causing a high frequency of machining defects.

Accordingly, the content of manganese (Mn) may be 0.9 to 1.5%. More preferably, it may be 0.9 to 1.3%.

Silicon (Si): 0.3% or less

Silicon (Si) is an element added to improve strength or ductility, and may be added in a range in which there is no problem of surface scale of hot-rolled steel sheets and hot-rolled pickled steel sheets. On the other hand, if the content of silicon (Si) is excessive, surface defects are generated due to the generation of silicon oxide, which is not easily removed even by subsequent pickling, so the upper limit is limited to 0.3%.

Accordingly, the content of silicon (Si) may be 0.3% or less. More preferably, it may be 0.25% or less.

Phosphorus (P): 0.01% or less

Since phosphorus (P) may segregate at the austenite grain boundary or interphase grain boundary and cause brittleness, it is advantageous to keep the content as low as possible. In the present invention, in the process of manufacturing the welded steel pipe itself or the drawn steel pipe as a part, segregation is induced at the old austenite grain boundary (PAGS) to reduce the toughness of the part, so the upper limit is set to 0.01%.

Accordingly, the content of phosphorus (P) may be 0.01% or less.

Sulfur (S): 0.002% or less

Sulfur (S) may cause high-temperature cracks by segregating MnS non-metallic inclusions in steel or during casting and solidification. In addition, since the impact toughness of the heat-treated steel sheet or steel pipe may be deteriorated, it is advantageous to control it as low as possible, so the upper limit is set to 0.002%.

Accordingly, the content of sulfur (S) may be 0.002% or less.

Aluminum (Al): 0.04% or less

Aluminum (Al) is an element added as a deoxidizer. On the other hand, AlN reacts with N in steel to precipitate AlN, but when the slab corner temperature is lower than the target temperature, it precipitates in a fine size, causing slab cracks and lowering the quality of the slab or hot-rolled steel sheet. limited to

Accordingly, the content of aluminum (Al) may be 0.04% or less. More preferably, it may be 0.035% or less.

Chromium (Cr): 0.3% or less

Chromium (Cr) is an element that delays the ferrite transformation of austenite to increase hardenability and heat treatment strength during quenching of steel. If the content of chromium (Cr) exceeds 0.3% in the C containing steel of 0.36% or more, excessive hardenability of the steel may be caused.

Accordingly, the content of chromium (Cr) may be 0.3% or less. More preferably, it may be 0.1 to 0.2%.

Titanium (Ti): 0.04% or less

Titanium (Ti) is an element that forms TiC and TiCN precipitates in the hot-rolled steel sheet, and suppresses the growth of austenite grains to increase the strength of the hot-rolled steel sheet. When the content of titanium (Ti) exceeds 0.04%, it exists in the form of coarse crystals rather than fine precipitates in the hot-rolled steel sheet, which may reduce toughness or the performance of heat-treated steel sheets or steel pipe parts.

Accordingly, the content of titanium (Ti) may be 0.04% or less. More preferably, it may be 0.03% or less.

Boron (B): 0.005% or less

Boron (B) is an element that increases the hardenability of steel even at a low content. However, when an appropriate content is added, it is effective to increase hardenability by suppressing ferrite formation, but when it is excessively contained, the austenite recrystallization temperature may be increased and weldability may be deteriorated. If the content of boron (B) exceeds 0.005%, the above-described effect may be saturated or it may be difficult to secure adequate strength and toughness.

Accordingly, the content of boron (B) may be 0.005% or less. In order to simultaneously secure strength and toughness, it may be more preferably 0.003% or less.

Nitrogen (N): 0.006% or less

Nitrogen (N) is an austenite stabilizing and nitride-forming element. When its content exceeds 0.006%, it forms coarse AlN, TiN, or TiCN nitride and acts as a fatigue crack generation starting point when evaluating the durability of a steel sheet or steel pipe. may deteriorate. In addition, in order to increase the effective B content, it is advantageous to control the nitrogen (N) content to be low.

Accordingly, the content of nitrogen (N) may be 0.006% or less.

The steel of the present invention may include the remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in a normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the art of steel manufacturing, all of them are not specifically mentioned in the present specification.

The steel according to an aspect of the present invention may further include one or more of Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: 0.5% or less by weight%.

Molybdenum (Mo): 0.1% or less

Molybdenum (Mo) can increase the hardenability of steel and form fine precipitates to refine the grains of austenite. In addition, although it is effective in improving the strength and toughness after heat treatment of the steel, if the content exceeds 0.1%, the manufacturing cost of the steel may increase.

Accordingly, the content of molybdenum (Mo) may be 0.1% or less.

Niobium (Nb): 0.02% or less

Niobium (Nb) is an element that forms NbC, NbCN, and TiNbCN precipitates in the hot-rolled steel sheet, and increases the strength of steel by inhibiting the growth of austenite grains. However, if the content of niobium (Nb) exceeds 0.02%, the manufacturing cost of the steel sheet or welded steel pipe may increase excessively.

Accordingly, the content of niobium (Nb) may be 0.02% or less.

Copper (Cu): 0.5% or less

Copper (Cu) is an element that can increase the corrosion resistance of steel and effectively increase the quenching (hardening) and quenching-tempering strength after heat treatment. Meanwhile, in the present invention, the addition of copper (Cu) in the steel having a Ceq value of 0.6 or more may increase the toughness of the steel sheet or welded steel pipe after quenching or quenching-tempering, but the content exceeds 0.5% Otherwise, excessive increase in strength may lead to deterioration of toughness.

Accordingly, the content of copper (Cu) may be 0.5% or less.

Nickel (Ni): 0.5% or less

Nickel (Ni) is an element that simultaneously increases the hardenability and toughness of steel. In addition, nickel (Ni) facilitates the movement of dislocations introduced into martensite or is concentrated in the surface layer of the steel sheet to limit the movement of hydrogen penetrating from the outside. have. On the other hand, the addition of nickel (Ni) in steel having a Ceq value of 0.6 or more in the present invention can increase the toughness of the steel sheet or welded steel pipe after quenching or quenching-tempering, and securing an appropriate level of yield strength It can act advantageously, and the upper limit is set to 0.5%.

Accordingly, the content of nickel (Ni) may be 0.5% or less.

The steel of the present invention may have a Ceq value of 0.6 or more defined in Relation 1 below.

In the present invention, the Ceq value can be limited to 0.6 or more in order to secure good weldability and minimize the occurrence of welding cracks during manufacturing of steel pipe and secure high strength of 1600 MPa or more during quenching or quenching-tempering of the final steel pipe part. have. In the present invention, more preferably, the Ceq value may be limited to 0.73 or less, and more preferably to 0.71 or less.

[Relational Expression 1]

Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15

(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)

The steel of the present invention may have an R value of 700 or more defined in Relation 2 below.

Relation 2 below affects the occurrence of cracks in the welded part of the steel pipe. If the R value defined in Equation 2 below is less than 700, a Mn-enriched layer is formed at the interface between the welded part of the welded steel pipe and the base metal part, and MnS or TiS sulfide remains and weakens the weld interface or acts as a trap site for diffusible hydrogen in the weld so that the content is maintained at a high level. .

[Relational Expression 2]

R = ([Mn]+[Ti])/[S]

(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)

When the R value of Relation 2 is controlled to be 700 or more, the S content is relatively low, and the S concentration segregated at the austenite grain boundary during the slab cooling process is reduced. Due to this, there is an advantage in that the austenite grain boundary withdrawal resistance is increased and the internal quality of the slab is improved. In addition, since it is difficult to form continuously elongated aggregate MnS in the microstructure of the hot-rolled steel sheet, the workability of the hot-rolled steel sheet can be improved.

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, unless otherwise specified, % indicating the fraction of microstructure is based on the area.

The microstructure of the steel sheet according to an aspect of the present invention may include 10 to 40% of ferrite and the remaining pearlite in area%.

In the present invention, when the fraction of ferrite is less than 10%, it may be difficult to manufacture a hot-rolled steel sheet having a desired thickness in the present invention because the strength is excessively increased due to an increase in the pearlite content, and when the fraction exceeds 40%, the desired strength is secured can be difficult

The microstructure of the weld part of the steel pipe according to an aspect of the present invention may consist of less than 10% tempered martensite and the remaining martensite.

When cooling the steel pipe after welding, it is most preferable to have martensite as the microstructure of the welded part. and the remaining martensite.

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

The steel sheet according to an aspect of the present invention may be manufactured by reheating, hot rolling and cooling a steel slab satisfying the above-described alloy composition.

slab reheat

The steel slab satisfying the above alloy composition may be reheated in a temperature range of 1150 to 1300 °C.

Reheating is performed in order to have a uniform structure and component distribution in the slab. If the reheating temperature is less than 1150 ° C, the precipitates formed on the playing slab are not employed, and the uniformity of components cannot be ensured, whereas the temperature is 1300 ° C. If it is exceeded, excessive increase of the decarburization depth and grain growth occur, so it is difficult to secure the target material and surface quality of the hot-rolled steel sheet.

hot rolled

The reheated steel slab may be hot rolled to a finish hot rolling temperature of Ar3 or higher.

If the finish hot rolling temperature is less than Ar3, some of the austenite transforms into ferrite, and the deformation resistance of the material against hot rolling becomes non-uniform. However, if the temperature exceeds 950°C, scale defects and the like may occur.

cooling and winding

After cooling the hot-rolled steel sheet to a temperature range of 560 to 640 °C at a cooling rate of 10 to 70 °C / s, it can be wound up.

The cooling in the present invention can be performed in continuous cooling or in a front/rear cooling pattern. This is to ensure a uniform material of the hot-rolled steel sheet after the hot rolling, and when the cooling end temperature is less than 560 ° C, a low-temperature transformation phase such as bainite or martensite is introduced to the edge portion in the width direction of the steel sheet. In addition, variations in hot-rolled strength in the width direction may increase. On the other hand, when the temperature exceeds 640 ° C., decarburization and internal oxidation are promoted in the surface layer of the steel sheet, and it causes a difference in hardness between the surface layer and the depth of the steel sheet, so that it is difficult to use as a welded steel pipe or a drawn steel pipe. More preferably, in the present invention, the coiling temperature may be limited to 600° C. or less.

If the cooling rate is less than 10℃/s, ferrite transformation may occur in the ROT cooling zone due to the slow plate speed and insufficient amount of water spraying. , when the speed exceeds 70 °C / s, there is a problem that the coil shape may deteriorate because the temperature unevenness may occur in the width direction of the steel sheet due to excessive pouring on the upper or lower surface of the steel sheet. In the present invention, if necessary, the hot-rolled pickling steel sheet can be obtained by pickling the steel sheet after cooling.

The steel pipe according to an aspect of the present invention may be manufactured by pipe making and welding a steel sheet satisfying the alloy composition and manufacturing method described above.

officer

The hot-rolled steel sheet or the hot-rolled pickled steel sheet manufactured by the hot rolling may be used to manufacture a steel pipe using an electric resistance welding or high-frequency induction welding method.

When manufacturing a welded steel pipe in the present invention, by controlling a welding variable and a cooling rate of a welded part, it is possible to manufacture a small-diameter welded steel pipe without the occurrence of lateral cracks in the welded part.

In the present invention, a method for manufacturing a welded steel pipe is not particularly limited, and a conventional method for manufacturing a small-diameter steel pipe may be used. Specifically, the process of manufacturing a circular pipe centering on both ends of a steel plate using a hot-rolled or pickled steel plate skelp that matches the target steel pipe diameter, a process of welding both ends and then pressurizing the weld metal to discharge oxides; A process of rapidly cooling the welded part by water cooling and a process of removing beads formed outside and inside the steel pipe may be included. When cooling the welding part, although water is disclosed as a refrigerant in the present invention, it is not particularly limited and various cooling media such as pipe oil may be used.

On the other hand, in the present invention, the value of T defined in the following Relation 3 may be 0.13 to 0.21 during pipe production.

In the present invention, it is a technique for manufacturing a small-diameter steel pipe, and when manufacturing a small-diameter steel pipe, the occurrence of cracks can be suppressed by controlling the thickness and diameter according to the relational equation (3). If the T value of Relation 3 is less than 0.13, it is not suitable for automobile parts that require rigidity and durability, and if the thickness of the steel sheet is thin, it may deviate from the welding conditions proposed in the present invention, so it may be difficult to secure sound welding quality. have. On the other hand, if the value exceeds 0.21, the difference in the circumferential length of the steel pipe may occur greatly when the thickness of the steel sheet is thick, so there is a problem that the steel pipe circularity may be poor or the welding quality may be inferior. In addition, when the thickness of the steel sheet is thick, the amount of strain hardening of the steel pipe may increase, and thus the formability may be inferior.

[Relational Expression 3]

T = t/D

(Here, t means the thickness of the welded steel pipe, and D means the diameter of the welded steel pipe.)

In addition, as the welding conditions of the present invention, the value of K defined in the following relation 4 may be 4.0 to 9.0.

Equation 4 below shows the relationship between Power Input (power), Welding Speed (welding connection), Thickness (thickness), and Upset Force (amount of upset). In the present invention, as a technique for manufacturing a small-diameter steel pipe, the welding quality can be improved by controlling the welding conditions according to the following relational expression 4, so it can be effective in suppressing cracks.

If the K value of Relation 4 is less than 4.0, the heat input to the weld is high during the electric resistance welding process, and a large amount of welding inclusions may be generated in the weld, and even if a sufficient amount of upset (Upset Force) is applied, the inclusions may remain in the weld. Therefore, it may be difficult to secure sound welding quality. On the other hand, if the value exceeds 9.0, the welding quality of the steel pipe may be inferior because the welding part does not melt sufficiently, and welding defects including cold welds may occur even when the upset is applied because the heat input amount of the welding part is low. More preferably, in the present invention, the electric power may be 100 to 250 kw, and the welding speed or line speed may be 20 to 40 mpm.

[Relational Expression 4]

K = (Power Input/Welding Speed) + (Thickness/Upset Force)

(Here, Power Input is power (kw), Welding Speed is welding speed (mpm), Thickness is the thickness of the steel sheet (mm), and Upset Force is the amount of upset (mm))

Cooling after welding

The welded portion of the welded steel pipe may be cooled at a cooling rate of 70° C./s or less.

When the cooling rate of the welding part of the electric resistance welded steel pipe is 70℃/s or less, it is possible to suppress the occurrence of lateral cracks in the welding part of the final steel pipe. In detail, the difference (ΔHv) in hardness value between the welded part and the base metal part of the final welded steel pipe may be secured to less than 470. As the Ceq value of the hot-rolled steel sheet or its skeleton increases (0.65 to 0.75), it is desirable to keep the difference in hardness value between the weld and the base metal low by controlling the cooling rate of the weld to be lower.

On the other hand, in the present invention, as a method of controlling the cooling rate of the welding part, the welding part is charged in a water cooling bath, water cooling spraying on the welded steel pipe, cooling using a refrigerant that mixes the two or water + oil mixture, etc. No distinction is made, and the present invention does not limit a specific method.

On the other hand, when the cooling rate of the weld part of the electric resistance welded steel pipe exceeds 70 °C/s, the difference in hardness value between the weld part and the base metal part increases excessively. In this case, the martensite fraction in the microstructure of the weld part is very high, and the martensite The dislocation density is very high. Therefore, it may contain a relatively high content of diffusional hydrogen (Diffusional Hydrogen) in the welding portion during the cooling process of the final welded steel pipe or the atmospheric exposure process. In the present invention, as a result of measuring the diffusion hydrogen content in the weld zone of the steel pipe having lateral cracks, the content was 0.5 ppm or more.

When the cooling rate of the weld part of the welded steel pipe is 70 °C/s or less, the difference in hardness value between the weld part and the base metal part is small or the martensite fraction and dislocation density are low, so the diffusible hydrogen in the weld part may be relatively low. In this case, the occurrence of lateral cracks in the weld can be minimized.

Annealing, drawing and forming steps

The steel pipe obtained by making the pipe as described above can be annealed, drawn, and molded.

In the present invention, the annealing, drawing and forming steps are not particularly limited, and the tube member may be manufactured under normal conditions.

The hot-rolled steel sheet of the present invention manufactured as described above may have a plastic deformation ratio (r-value) of 0.9 or more, and a tensile strength of 1600 MPa or more after quenching or quenching-tempering. Here, the plastic strain ratio (εw/εt) represents the ratio of the strain in the width direction (εw) to the strain in the plate thickness direction (εt) of the specimen in the tensile test. When the ratio value is less than 0.9, the formability in the direction perpendicular to the rolling direction and the diagonal direction of the steel sheet is different, or the formability of the steel pipe itself such as 180° bending or compression forming or expandability may be inferior. .

The steel pipe of the present invention manufactured by using the hot-rolled steel sheet may have a diameter of 52 mm or less, a difference in hardness value between the welded portion and the base metal portion of less than 470, and excellent properties in resistance to lateral cracking of the welded portion.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.

(Example)

In Table 1 below, alloy compositions according to steel types are disclosed, and Relations 1 and 2 are calculated and shown. As shown in Table 3 by performing hot rolling using the steel shown in Table 1 below, hot-rolled steel sheets having a thickness of 3 to 8 mm were manufactured, followed by pickling treatment. Before the hot rolling, the steel slab was heated in a range of 1200±20° C. for 200 minutes, and the steel slab was hot rolled to a finish hot rolling temperature of Ar3 or higher, cooled to the cooling conditions of Table 2 below, and then wound up. At this time, the cooling rate was applied to 10 ~ 70 ℃ / s.

steel grade Alloy composition (wt%) Relation 1 Relation 2 C Si Mn P S Al Cr Mo Ti B N Nb Cu Ni A 0.360 0.160 1.290 0.0070 0.0010 0.034 0.130 0.08 0.03 0.002 0.0031 0 0 0 0.64 1320 B 0.341 0.214 1.298 0.0072 0.0006 0.033 0.130 0.08 0.03 0.002 0.0035 0.002 0.01 0.09 0.64 2213 C 0.370 0.080 0.600 0.0140 0.0020 0.050 0 0.15 0.02 0.0019 0.0047 0.016 0 0 0.51 310 D 0.433 0.097 1.301 0.0057 0.0022 0.033 0.178 0.076 0.031 0.0019 0.0050 0 0.28 0.27 0.75 605 E 0.432 0.02 0.985 0.0052 0.0007 0.002 0.019 0.08 0.028 0.0021 0.0047 0 0.28 0.29 0.65 1447 F 0.346 0.152 1.285 0.0039 0.0011 0.034 0.142 0.078 0.030 0.0018 0.0034 0 0.02 0.02 0.63 1195 G 0.335 0.215 1.261 0.0084 0.0009 0.035 0.150 0 0.031 0.0021 0.0037 0 0 0 0.61 1436 H 0.382 0.200 1.289 0.0063 0.0012 0.039 0.130 0 0.030 0.0023 0.0047 0 0 0 0.66 1099 I 0.436 0.098 0.997 0.0052 0.0007 0.0024 0.190 0.08 0.028 0.0021 0.0047 0 0.28 0.29 0.71 1464

[Relational Expression 1]

Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15

(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)

[Relational Expression 2]

R = ([Mn]+[Ti])/[S]

(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)

The microstructure and mechanical properties of the prepared hot-rolled steel sheet were measured and shown in Table 2. The microstructure was measured through an optical microscope, and for yield strength, tensile strength and elongation, hot-rolled steel sheets were processed into JIS standard specimens. The plastic strain ratio (εw/εt) was calculated by calculating the ratio of the strain in the width direction (εw) to the strain in the plate thickness direction (εt) of the specimen in the tensile test. In addition, after quenching and tempering with respect to the manufactured specimen, tensile strength and fracture occurrence were indicated. At this time, the occurrence of breakage was marked as abnormal in the case of premature breakage during tensioning. The quenching was heated to 930℃, kept for 30~200 seconds, extracted and charged into refrigerants such as water, water+oil, etc., and cooled at a cooling rate of 20~80℃/s, tempering was 200~220℃ Heat treatment held for 30 to 3600 seconds was repeated once or three times.

Psalm number steel grade Cooling microstructure mechanical properties Physical properties after heat treatment winding
temperature
(℃) ferrite
(area%) perlite
(area%) YS
(MPa) ts
(MPa) El
(%) r-value
(Plastic deformation ratio) ts
(MPa) Whether breakage has occurred One A 640 36.8 63.2 447 713 24 0.92 1697 normal 2 B 620 38.7 61.3 487 694 22 0.91 1899 normal 3 C 650 35.4 64.6 423 680 22 0.87 1587 abnormal 4 D 620 24.9 75.1 452 703 22 0.95 2042 normal 5 E 600 26.0 74.1 451 718 23 0.96 2086 normal 6 F 590 38.7 61.3 578 792 26 0.95 1697 normal 7 G 620 39.2 60.8 478 721 20 0.92 1861 normal 8 H 640 33.8 66.2 387 680 24 0.97 1826 normal 9 I 600 29.7 70.3 456 710 24 0.95 2065 normal

A steel pipe was manufactured using the prepared steel sheet. For each steel type, electric resistance welding was performed by applying the conditions shown in Table 3 below. Except for the conditions described, an upset amount was applied in the range of 4.1 to 4.6 mm, and an upset amount of 1.5 was applied to specimen number 7. After the welded steel pipe was manufactured, the microstructure of the welded part, physical properties, and lateral cracks were checked. The occurrence of lateral cracks was judged by the occurrence of cracks within 7 to 14 days after manufacturing the steel pipe. In the present invention, the diffusion hydrogen content in the weld zone was 0.5 ppm or less in the steel pipe in which lateral cracks did not occur. As a result of measuring the diffusion hydrogen content in the weld zone in the steel pipe in which lateral cracks occurred, the content exceeded 0.5 ppm. The average hardness value of the base material and the welded part was measured with a Vickers hardness tester, and the difference in hardness was calculated and shown. In addition, the microstructure of the weld part was measured using an optical microscope, and the fraction of martensite is shown in Table 3 below. The microstructure consists of tempered martensite in addition to martensite.

Psalter
number steel grade ERW welding conditions Weld fine
group Welding properties delay
Whether destruction has occurred division thickness
(mm) diameter
(mm) Relation 3
(T) power
(kw) welding speed
(mpm) Relation 4
(K) Cooling
speed
(℃/s) martensite
(area%) Base metal average hardness
(Hv) Average hardness of welds
(Hv)) hardness difference
(ΔHv) One A 6.2 34.0 0.182 186 28 8.0 75 ≥95 236 730 494 O Comparative lecture 1 2 B 8.0 50.3 0.159 210 30 8.7 73 ≥95 239 775 536 O Comparative lecture 2 3 C 4.0 28.0 0.143 126 32 4.6 72 ≥95 232 705 473 X Comparative lecture 3 4 D 4.0 28.0 0.143 126 32 4.9 69 ≥91 262 716 454 O Comparative lecture 4 5 E 4.0 28.0 0.143 126 32 4.9 69 ≥92 253 723 469 X Invention lecture 1 6 F 4.0 28.0 0.143 126 34 4.5 51 ≥90 261 642 381 X Invention lecture 2 7 F 5.0 31.8 0.157 126 31 9.1 75 ≥95 262 660 398 O Comparative steel 5 8 F 5.0 31.8 0.157 128 45 3.9 65 ≥95 261 652 398 O Comparative lecture 6 9 G 6.0 31.8 0.189 124 34 5.0 68 ≥90 255 631 376 X Invention lecture 3 10 H 3.5 28.0 0.125 126 34 4.5 62 ≥90 212 611 399 O Comparative lecture 7 11 I 4.0 28.0 0.143 126 32 4.9 64 ≥90 256 707 451 X Invention lecture 4

[Relational Expression 3]

T = t/D

(Here, t means the thickness of the welded steel pipe, and D means the diameter of the welded steel pipe.)

[Relational Expression 4]

K = (Power Input/Welding Speed) + (Thickness/Upset Force)

(Here, Power Input is power (kw), Welding Speed is welding speed (mpm), Thickness is the thickness of the steel sheet (mm), and Upset Force is the amount of upset (mm))

As shown in Table 3, Inventive Steels 1 to 3 satisfying the alloy composition and manufacturing method proposed in the present invention have all of the mechanical properties targeted in the present invention, and delayed fracture did not occur.

On the other hand, Comparative Steels 1 and 2 satisfy the alloy composition and manufacturing conditions of the steel sheet proposed in the present invention, but the cooling rate during welding exceeds the scope of the present invention. As a result, the hardness of the welded portion increased excessively. In this case, cracks may occur at the interface between the weld and the base material or at the interface of inclusions remaining in the microstructure of the weld. Even if no detectable crack occurs in the weld of the steel pipe, the weld having a martensitic structure at room temperature may have very high dislocation defects and martensite lath / lath interfaces, so that during the cooling process of the weld or Hydrogen in the atmosphere can be trapped, and delayed hydrogen destruction can occur when the hydrogen concentration at such a local location exceeds a critical concentration that can be accommodated by the steel pipe material. As shown in Examples, the difference in hardness between the base material and the weld zone of Comparative Steels 1 and 2 was out of the range desired in the present invention, and due to this, delayed fracture occurred due to poor resistance to cracking.

Comparative Steel 3 did not satisfy Relation 2, and the coiling temperature was outside the scope of the present invention. Due to the growth of ferrite grains according to the relatively high coiling temperature, the average grain size became coarse, and the uniformity of the ferrite structure along the direction of the specimen was lowered, resulting in inferior plastic deformation ratio. .

Comparative Steel 4 does not satisfy Relational Equation 2 of the present invention. Even when a sufficient amount of upset is applied, the Mn and S contents are relatively high, so that the fraction of oxidative or steelmaking inclusions including MnS in the weld area is increased. Delayed destruction occurred.

Comparative steels 5 to 7 are cases in which the alloy composition proposed in the present invention is satisfied, but the welding conditions are not satisfied. In the case of Comparative Steel 5, the range of Relation 4 was exceeded, and it was determined that delayed failure occurred due to a welding defect that occurred during the pipe manufacturing process including cold welding. In the case of Comparative Steel 6, it fell short of the range of Relational Equation 4, and it was determined that hydrogen at a level of 0.5 ppm or more was collected due to inclusions or dislocation defects present in a large amount in the weld zone, resulting in delayed fracture. In the case of Comparative Steel 7, as an example that did not satisfy Relation 3, it was difficult to secure good welding quality by effectively controlling the amount of heat input through the application of an appropriate level of power because the thickness of the steel pipe was thin compared to the diameter. In addition, due to the thin thickness, the cooling rate of the welding part was fast, and fine lath martensite was formed.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (15)

By weight%, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr: 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder including Fe and unavoidable impurities,
Ceq value defined in the following Relation 1 is 0.6 or more,
A steel sheet having an R value of 700 or more, as defined in the following Relational Expression (2).
[Relational Expression 1]
Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15
(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)
[Relational Expression 2]
R = ([Mn]+[Ti])/[S]
(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)
According to claim 1,
The steel sheet further comprises at least one of Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: 0.5% or less.
According to claim 1,
The microstructure of the steel sheet is an area%, and a steel sheet composed of 10 to 40% of ferrite and the remaining pearlite.
According to claim 1,
The steel sheet is a steel sheet having a plastic deformation ratio (r-value) value of 0.9 or more.
According to claim 1,
The steel sheet is a steel sheet having a tensile strength of 1600 MPa or more after quenching or quenching-tempering.
By weight%, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr: 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder Fe and unavoidable impurities, the Ceq value defined in the following relation 1 is 0.6 or more, and the R value defined in the following relation 2 is 700 or more. Reheating the slab in a temperature range of 1150 to 1300 °C;
hot rolling the reheated steel slab to a finish hot rolling temperature of Ar3 or higher; and
A method for manufacturing a steel sheet comprising cooling the hot-rolled steel to a temperature range of 560 to 640° C. at a cooling rate of 10 to 70° C./s, followed by winding.
[Relational Expression 1]
Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15
(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)
[Relational Expression 2]
R = ([Mn]+[Ti])/[S]
(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)
7. The method of claim 6,
The steel sheet is Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: a steel plate manufacturing method further comprising at least one of 0.5% or less.
7. The method of claim 6,
The method of manufacturing a steel sheet further comprising the step of pickling the steel sheet after cooling to obtain a hot-rolled pickling steel sheet.
In wt%, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr: 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder including Fe and unavoidable impurities,
Ceq value defined in the following Relation 1 is 0.6 or more,
R value defined in the following relation 2 is 700 or more,
The difference between the hardness value of the weld and the base metal is less than 470Hv,
Steel pipe with a weld diffusion hydrogen content of 0.5 ppm or less.
[Relational Expression 1]
Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15
(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)
[Relational Expression 2]
R = ([Mn]+[Ti])/[S]
(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)
10. The method of claim 9,
The steel pipe further comprises at least one of Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: 0.5% or less.
10. The method of claim 9,
The microstructure of the welded part of the steel pipe is an area%, and a steel pipe consisting of less than 10% tempered martensite and the remaining martensite.
10. The method of claim 9,
The steel pipe is a steel pipe having a diameter of 52 mm or less.
By weight%, C: 0.30 to 0.45%, Mn: 0.9 to 1.5%, Si: 0.3% or less, P: 0.01% or less, S: 0.002% or less, Al: 0.04% or less, Cr: 0.3% or less, Ti: 0.04% or less, B: 0.005% or less, N: 0.006% or less, the remainder Fe and unavoidable impurities, the Ceq value defined in the following relation 1 is 0.6 or more, and the R value defined in the following relation 2 is 700 or more. Reheating the slab in a temperature range of 1150 to 1300 °C;
hot rolling the reheated steel slab to a finish hot rolling temperature of Ar3 or higher;
winding the hot-rolled steel after cooling it to a temperature range of 560 to 640°C at a cooling rate of 10 to 70°C/s;
forming a pipe by welding the wound steel; and
Comprising the step of cooling the welded portion of the piped steel pipe at a cooling rate of 70 °C / s or less,
When the pipe is produced, the value of T defined in the following relation 3 is 0.13 to 0.21,
A method of manufacturing a steel pipe by welding in which the value of K defined in the following Relation 4 is 4.0 to 9.0.
[Relational Expression 1]
Ceq = [C] + ([Mn]+[Si])/6 + ([Cr]+[Mo]+[V])/5 + ([Cu]+[Ni])/15
(Where [C], [Mn], [Si], [Cr], [Mo], [V], [Cu] and [Ni] are weight percent of each element.)
[Relational Expression 2]
R = ([Mn]+[Ti])/[S]
(Where [Mn], [Ti] and [S] are weight percent of the corresponding element.)
[Relational Expression 3]
T = t/D
(Here, t means the thickness of the welded steel pipe, and D means the diameter of the welded steel pipe.)
[Relational Expression 4]
K = (Power Input/Welding Speed) + (Thickness/Upset Force)
(Here, Power Input is power (kw), Welding Speed is welding speed (mpm), Thickness is the thickness of the steel sheet (mm), and Upset Force is the amount of upset (mm))
14. The method of claim 13,
The steel pipe is Mo: 0.1% or less, Nb: 0.02% or less, Cu: 0.5% or less, or Ni: A method of manufacturing a steel pipe further comprising at least one of 0.5% or less.
14. The method of claim 13,
After the cooling step, the method further comprising annealing, drawing, and forming the steel pipe to have a diameter of 52 mm or less.

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