KR102020417B1 - Steel sheet having excellent toughness and it manufacturing method - Google Patents

Abstract

Provided is a welded steel pipe having excellent impact toughness and a method of manufacturing the same.
Steel for welding steel pipe of the present invention, in weight%, carbon (C): 0.10 to 0.15%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.5 to 1.2%, phosphorus (P): 0.025% Sulfur (S): 0.005% or less, Niobium (Nb): 0.05 to 0.1%, Nickel (Ni): 0.05 to 0.2%, Chromium (Cr): 0.1 to 0.3%, Molybdenum (Mo): 0.05 to 0.15% , Titanium (Ti): 0.01% to 0.05%, vanadium (V): 0.05% to 0.1%, nitrogen (N): 0.008% or less, residual Fe and inevitable impurities, and the microstructure has an average grain size of 15 μm or less and 60 to 80 area% of ferrite satisfying the maximum grain size of 30 μm or less, perlite: 20 to 40 area% and residual bainite, and satisfying Equation 1.

Description

Welded steel pipe with excellent impact toughness and manufacturing method {STEEL SHEET HAVING EXCELLENT TOUGHNESS AND IT MANUFACTURING METHOD}

The present invention relates to a steel for manufacturing welded steel pipes used in oil and gas mining, etc., and more particularly, to a steel for manufacturing welded steel pipes capable of providing welded steel pipe joints with excellent impact toughness and a method for manufacturing the same.

The coiled tube used in the oil and gas industry manufactures several kilometers of tubes ranging in diameter from 1 inch to 3.25 inches and is supplied through large reel spooling, and can be used for fluid circulation, pumping, drilling, logging, It is used for various purposes such as drilling. At this time, repeated bending stress is accumulated through reeling and unreeling every time, causing premature failure of the tube.

In particular, in a weld where the fatigue stress is concentrated, a problem occurs when the impact is applied, which shortens the product life due to fracture, and thus the demand for development of the steel with improved impact toughness of the weld has been continued.

Republic of Korea Patent Application 10-2002-0050500

Accordingly, the present invention has been made to solve the above-mentioned limitations of the prior art, and an object of the present invention is to provide a welded steel pipe manufacturing steel and a method of manufacturing the same, having a strength equivalent to that of the API standard 5ST CT90 grade.

In addition, the technical problems to be achieved in the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned above are clearly understood by those skilled in the art from the following description. Could be.

The present invention for achieving the above object,

By weight%, carbon (C): 0.10 to 0.15%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.5 to 1.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.005% Niobium (Nb): 0.05 to 0.1%, Nickel (Ni): 0.05 to 0.2%, Chromium (Cr): 0.1 to 0.3%, Molybdenum (Mo): 0.05 to 0.15%, Titanium (Ti): 0.01 to 0.05 %, Vanadium (V): 0.05-0.1%, nitrogen (N): 0.008% or less, including residual Fe and unavoidable impurities, and the microstructure satisfies the average grain size of 15 μm or less and the maximum grain size of 30 μm or less It provides 60 to 80 area% of ferrite, 20% to 40% of area of pearlite and residual bainite, and provides a welded steel pipe having excellent impact toughness that satisfies Equation 1 below.

[Relationship 1]

50 <209 + 4.91F + 0.063P-3.82B-0.891 Dmax-0.217YS-0.05Hv -30Ceq <80

(Where [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite microstructure, Dmax is maximum grain size (µm), YP is material strength (MPa), and Hv is Vickers hardness) , Ceq means carbon equivalent)

In addition, the present invention,

Preparing a steel slab having the alloy composition described above;

Reheating the steel slab in a temperature range of 1100 to 1300 ° C .;

Manufacturing a hot rolled steel sheet by finishing hot rolling of the reheated steel slab at a temperature range of 750 ° C. to 850 ° C .; And

After cooling the hot rolled steel sheet provides a method for producing a welded steel pipe excellent in impact toughness, including the step of winding at a temperature between 500 ~ 550 ℃.

The present invention also relates to a welded steel pipe having excellent impact toughness obtained by molding and welding the steel for welding steel pipe.

According to the present invention having the above-described configuration, it is possible to effectively provide a steel for manufacturing welded steel pipe having excellent impact toughness while having strength equivalent to API standard 5ST CT90 class.

Hereinafter, the present invention will be described.

The present inventors consider that coiled tubing for fluid circulation, pumping, drilling, logging, drilling, etc. are continuously increasing in gas wells or oil well environments, and thus, materials that can improve cost and physical properties can be improved. The research was carried out to develop and secure suitable weld properties. In particular, after the manufacture of the coiled tube to develop a steel that can meet the yield strength (620 ~ 689MPa) and tensile strength (669MPa or more) equivalent to the CT90 class required in the API 5ST standard and present the present invention.

That is, the steel for welding steel pipe of the present invention is, in weight%, carbon (C): 0.10 to 0.15%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.5 to 1.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.005% or less, niobium (Nb): 0.05-0.1%, nickel (Ni): 0.05-0.2%, chromium (Cr): 0.1-0.3%, molybdenum (Mo): 0.05- 0.15%, titanium (Ti): 0.01-0.05%, vanadium (V): 0.05-0.1%, nitrogen (N): 0.008% or less, the balance Fe and inevitable impurities, the microstructure of the average grain size 15㎛ It consists of 60 to 80 area%, ferrite: 20 to 40 area%, and residual bainite, which satisfy the following and a maximum grain size of 30 μm or less, satisfying the above equation 1.

Hereinafter, the steel composition and the reason for the content limitation of the present invention. At this time, the content of each component means weight% unless otherwise specified.

C: 0.1 ~ 0.15%

C is an element added for securing strength, and the hardenability can be improved to secure strength. When the content of C is less than 0.1%, it is difficult to secure strength by combining with Nb, V, and Ti, and when it exceeds 0.15%, the yield strength rises to exceed the target strength, which is not preferable.

Si: 0.1 ~ 0.5%

Si is an element added for deoxidation treatment and securing strength in the production of steel, and when the content of Si is less than 0.1%, there is a problem in that the deoxidation effect of Si is lowered. There is a problem that the toughness is lowered and withdrawal may occur during tempering.

Mn: 0.5 ~ 1.2%

Mn is an element added to secure the strength, when the Mn content is less than 0.5%, it is difficult to secure the strength, and when it exceeds 2.5%, Mn forms a central segregation during performance, thereby reducing impact toughness and fatigue resistance. There is a problem of deterioration.

Therefore, in the present invention, it is preferable to limit the content of Mn to 0.5 ~ 1.2%.

P: 0.025% or less (except 0%)

P is preferably an impurity that is inevitably generated in the production of steel. If the content of P exceeds 0.025%, there is a problem in that impact toughness may be reduced by forming a central segregation during playing.

S: 0.005% or less (except 0%)

S is preferably an impurity that is inevitably generated in the production of steel. When the content of S exceeds 0.005%, it is a major factor that reduces the toughness of steel by reacting with Mn to produce MnS.

Nb: 0.05-0.1%

Nb is an element added to secure the strength of the steel, and produces the NbC precipitate to bring the precipitation strengthening effect. When the content of Nb is less than 0.05%, the precipitation strengthening effect is insignificant, and when the content of Nb is more than 0.1%, coarse precipitates and MA may be promoted to reduce toughness.

Ni: 0.05-0.2%

Nickel (Ni) has the effect of improving the toughness by lowering the lamination defect energy in the lattice to lower the transition temperature. In addition, there is also an effect of suppressing the generation of cracks during hot processing by inhibiting the production of low melting point compounds.

In order to obtain the above-mentioned effect, it is preferable to add Ni at least 0.05% or more, on the other hand, if it exceeds 0.2%, there may be a problem in that the Ni compound is generated to lower the toughness.

Cr: 0.1-0.3%

Chromium (Cr) is an element added to improve the hardenability and corrosion resistance, and when added below 0.1%, the above effect is less. When added above 0.3%, chromium (Cr) causes weld defects or causes brittleness. It is desirable to limit it to 0.3%.

Mo: 0.05-0.15%

Molybdenum (Mo) is an element added to secure strength and to improve corrosion resistance. If it is added less than 0.05%, the effect of increase in strength and corrosion resistance is small, and if it is added more than 0.15%, it generates carbide precipitates to lower toughness. Is preferably limited to 0.05 to 0.15%.

Ti: 0.01% to 0.05%

Ti is an element added to improve the strength and toughness of the steel, and produces a TiC precipitate to have a precipitation strengthening effect, and TiN precipitates to inhibit austenite grain growth, thereby producing fine grains to secure strength and improve toughness. When the content of Ti is less than 0.01%, the above effects are not exhibited. When the content of Ti is more than 0.05%, coarse Ti precipitates are generated, thereby lowering toughness.

V: 0.05-0.1%

V is an element added to improve the strength and toughness of the steel, and produces a VC precipitate to have a precipitation strengthening effect, and precipitates VN to inhibit austenite grain growth, thereby producing fine grains to secure strength and improve toughness. When the content of V is less than 0.05%, it does not exhibit the same effect as above, and when it exceeds 0.1%, coarse precipitates are formed, which lowers the toughness.

N: 0.008% or less (except 0%)

N is mainly bonded to Ti or Al in the steel to form nitride to reduce the function of other alloying elements. When the content exceeds 0.008%, coarse precipitates are produced to reduce toughness, and AlN precipitates are increased to decrease Al deoxidation effect.

Therefore, in the present invention, it is preferable to limit the content of N to 0.008% or less.

The remaining components of the present invention are iron (Fe) and other unavoidable impurities. However, in the conventional steel manufacturing process, impurities that are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

Meanwhile, the steel for welding steel pipe of the present invention may have a microstructure of ferrite and pearlite, or a composite structure of ferrite, pearlite and bainite.

In this case, the ferrite grain size is preferably the average grain size is 15㎛ or less and the maximum grain size is 30㎛ or less. If the average grain size exceeds 15 μm, the toughness decreases due to the reduction of the impact-absorbing grain boundary. If the maximum grain size exceeds 30 μm, the impact toughness rapidly increases due to the presence of non-uniform grains. There is a problem of deterioration. At this time, based on the diameter per grain circular area.

Preferably, the microstructure is composed of 60 to 80% ferrite, 20 to 40% pearlite, and the balance bainata in an area fraction. The ferrite is easy to form a slip surface of the lattice structure is excellent in shock absorption, it is preferable to include an area fraction of 60% or more. However, when the occupancy percentage of the alloy microstructure of the present invention exceeds 80 area%, it is difficult to secure strength.

Furthermore, it is preferable that the steel material for welded steel pipes of this invention satisfy | fills following formula (1).

[Relationship 1]

50 <209 + 4.91F + 0.063P-3.82B-0.891 Dmax-0.217YS-0.05Hv -30Ceq <80

(Where [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite microstructure, Dmax is maximum grain size (µm), YP is material strength (MPa), and Hv is Vickers hardness) , Ceq means carbon equivalent)

In the present invention, both the microstructure fraction and the grain size are important items for securing the properties of the steel. Therefore, if the value defined in the relational formula 1 is out of the scope of the present invention affects the microstructure and impact resistance, it can not secure the weld toughness.

In addition, it is not possible to determine the influence on the toughness only by the characteristics in the microscopic aspect, and in order to reinforce it, the physical properties of the material in the macroscopic aspect should be considered at the same time. Yield strength determines the elastic limit of the material. The higher the yield strength, the higher the shock absorption capacity, i.e., toughness, of the material. However, even if the yield strength is increased, the higher the yield ratio of the material, the lower the toughness due to the shortened fracture life of the material.

Therefore, when the value defined by the above-mentioned relational expression 1 of the present invention is in the range of more than 50 and less than 80, it is possible to improve the impact resistance of the steel in the microscopic and macroscopic aspects.

Next, the manufacturing method of the steel for welded steel pipe excellent in the toughness of this invention is demonstrated.

Steel production method for a welded steel pipe according to the present invention comprises the steps of preparing a steel slab having the above-described alloy composition; Reheating the steel slab in a temperature range of 1100 to 1300 ° C .; Manufacturing a hot rolled steel sheet by finishing hot rolling of the reheated steel slab at a temperature range of 750 ° C. to 850 ° C .; And after cooling the hot-rolled steel sheet comprises a step of winding at a temperature between 500 ~ 550 ° C.

[Reheating and Hot Rolling Process]

In the present invention, first, a steel slab having the aforementioned alloy composition is prepared, and then reheated.

In the slab reheating process, homogeneous initial microstructures and precipitates should be controlled by maintaining the temperature in the appropriate austenite zone to achieve the desired physical properties through hot rolling, cooling, and winding of the slabs produced.

In this invention, it is preferable to perform a reheating process in the temperature range of 1100-1300 degreeC. If the reheating temperature is lower than 1100 ° C, high melting point compounds such as Nb and Ti will not be reheated and remain as segregation zones, whereas if the reheating temperature is higher than 1300 ° C, the fine structure of the final product will be aimed at the formation of an initial coarse microstructure. There is a problem that can not be controlled.

The slab reheated as described above is subsequently finished hot rolled in the temperature range of 750 ~ 850 ℃. If the finish hot rolling temperature is less than 750 ℃ impact toughness and fatigue resistance due to the generation of MnS is lowered, if it exceeds 850 ℃ deep heterogeneity of microstructure grains may adversely affect the sulfide stress cracking resistance.

Therefore, in the present invention, the finishing hot rolling temperature is preferably limited to 750 ~ 850 ℃.

[Cooling and winding process]

Subsequently, in the present invention, after the finished hot rolled hot rolled steel sheet is cooled, it is preferably wound at a temperature of 500 to 550 ° C. If the coiling temperature is less than 500 ° C., a low temperature transformation phase such as bainite phase is locally generated, and there is a fear that fatigue resistance is lowered. On the other hand, if the coiling temperature exceeds 550 ℃ coarse pearlite phase is easily formed and fatigue propagation is easy, there is a fear that the fatigue resistance is lowered.

Therefore, in the present invention, it is preferable to limit the winding temperature to 500 ~ 550 ℃.

On the other hand, the steel composition prepared by the above-described steel composition and steel manufacturing process has a microstructure including ferrite and pearlite. In addition, the steel is molded into a pipe shape, and welded edges of the steel sheets in contact with each other may be used to obtain welded steel tubes having excellent impact toughness.

That is, welding and molding may be performed using the manufactured hot rolled steel sheet. For example, the width may be as much as the pipe diameter to be manufactured to manufacture the manufactured hot rolled steel sheet. The method for producing the welded steel pipe is not particularly limited, but it is preferable to pipe the tube using electrical resistance welding having the most economical efficiency. The welding method is not particularly limited since any welding method can be used for electric resistance welding.

Hereinafter, the present invention will be described in detail through examples.

(Example)

The steel slab having the alloy composition as shown in Table 1 was reheated at a temperature of 1200 ° C to 1300 ° C for 2 hours to prepare a steel material having a thickness of 5.2 mm by finishing hot rolling, cooling, and winding as shown in Table 2 below.

Then, the microstructure of the prepared hot-rolled steel sheet was observed, and after the electrical resistance welded tube was subjected to a tensile test and an impact test according to ASTM A370 to measure the yield strength, tensile strength, hardness and impact toughness and the results are shown in the following table. 3 is shown. Here, the hardness value represents an average value measured 15 times at the center of the strip thickness, and the carbon equivalent (Ceq) is a value calculated by the following equation.

 Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14

Steel grade Ingredients (% by weight) C Si Mn P S Nb Ni Cr Mo Ti V N a 0.13 0.24 1.04 0.007 0.001 0.058 0.13 0.2 0.10 0.02 0.05 0.005 b 0.11 0.25 1.06 0.011 0.002 0.063 0.11 0.17 0.13 0.02 0.06 0.005 c 0.11 0.24 0.99 0.007 0.002 0.066 0.10 0.2 0.09 0.01 0.06 0.004 d 0.12 0.25 1.06 0.006 0.002 0.06 0.10 0.17 0.11 0.02 0.06 0.005 e 0.12 0.25 1.04 0.009 0.001 0.057 0.11 0.16 0.11 0.02 0.06 0.004 f 0.12 0.3 1.04 0.006 0.001 0.06 0.18 0.16 0.14 0.02 0.05 0.005 g 0.12 0.29 1.04 0.005 0.001 0.06 0.19 0.16 0.14 0.02 0.06 0.004 h 0.11 0.26 1.01 0.005 0.001 0.067 0.01 0.19 0.09 0.01 0.06 0.005 i 0.1 0.27 0.99 0.007 0.001 0.063 0.02 0.2 0.10 0.01 0.05 0.004

Steel grade
Manufacture conditions Reheating Temperature (℃) Finish Hot Rolling Temperature (℃) Boiling Temperature (℃) a 1203 773 524 b 1160 781 518 c 1185 775 504 d 1138 777 504 e 1230 784 512 f 1200 761 565 g 1200 789 565 h 1207 784 502 i 1180 780 500

division
Steel grade
Microstructure Mechanical properties HAZ impact toughness (J, 0 ℃)

Relationship 1

% Of organization F average grain size (㎛)
F maximum grain size (㎛)
Yield strength (MPa)
Tensile Strength (MPa)
Thistle
(Hv)
Carbon equivalent
F P B Inventive Example 1 a 71 29 0 8.5 25 671 696 209.3 0.36 42.6 59.8 Inventive Example 2 b 68 32 0 8.4 21 642 714 204.3 0.37 73.9 74.7 Inventive Example 3 c 70 30 0 7.8 15 654 703 206 0.35 63.5 74.6 Comparative Example 1 d 67 24 9 11 25 672 737 208.3 0.38 21.3 32.8 Comparative Example 2 e 67 25 8 6.8 41 712 770 213.3 0.37 18.6 13.0 Comparative Example 3 f 67 26 7 15 35 705 761 194.5 0.38 9.3 23.6 Comparative Example 4 g 68 26 6 20 20 690 753 206 0.38 18.6 42.9 Comparative Example 5 h 63 37 0 8.5 23 610 667 198.3 0.35 47 80.7 Comparative Example 6 i 64 36 0 6.7 20 615 669 194.3 0.35 34.6 82.2

* In Table 3, F means ferrite, P means pearlite, and B means bainite.

 And relation 1 is 209 + 4.91F + 0.063P-3.82B-0.891 Dmax-0.217YS-0.05Hv -30Ceq.

As shown in Tables 1 to 3, Inventive Examples 1-3 satisfying all of the alloy composition and manufacturing conditions proposed by the present invention can be confirmed that after the weld steel pipes are all manufactured, the impact toughness is superior to 40 J or more. .

On the contrary, Comparative Examples 1-6, in which alloy composition, manufacturing conditions, or relational formula 1 were outside the range proposed by the present invention, were inferior in impact toughness as coarse structures were formed but precipitates and low temperature transformation phases were formed.

In particular, Comparative Example 1-2 is an alloy composition and manufacturing process is within the scope of the present invention, but the relationship 1 is outside the scope of the present invention, it can be confirmed that the impact toughness is not good.

As described above, in the detailed description of the present invention has been described with respect to the preferred embodiment of the present invention, those skilled in the art to which the present invention pertains various modifications without departing from the scope of the present invention Of course it is possible. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined by the equivalents as well as the claims to be described later.

Claims (4)

By weight%, carbon (C): 0.10 to 0.15%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.5 to 1.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.005% Niobium (Nb): 0.05 to 0.1%, Nickel (Ni): 0.05 to 0.2%, Chromium (Cr): 0.1 to 0.3%, Molybdenum (Mo): 0.05 to 0.15%, Titanium (Ti): 0.01 to 0.05 %, Vanadium (V): 0.05-0.1%, nitrogen (N): 0.008% or less, including residual Fe and unavoidable impurities, and the microstructure satisfies the average grain size of 15 μm or less and the maximum grain size of 30 μm or less Ferritic 60 ~ 80 area%, pearlite: 20 ~ 40 area% and residual bainite, satisfy the following Equation 1, and have excellent impact toughness of 620 ~ 689MP yield strength and tensile strength of 669MPa or more. .
[Relationship 1]
50 <209 + 4.91F + 0.063P-3.82B-0.891 Dmax-0.217YS-0.05Hv -30Ceq <80
(Where [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite microstructure, Dmax is maximum grain size (µm), YP is material strength (MPa), and Hv is Vickers hardness) , Ceq means carbon equivalent)
A welded steel pipe excellent in impact toughness obtained by forming and welding the steel of claim 1.
By weight%, carbon (C): 0.10 to 0.15%, silicon (Si): 0.1 to 0.5%, manganese (Mn): 0.5 to 1.2%, phosphorus (P): 0.025% or less, sulfur (S): 0.005% Niobium (Nb): 0.05 to 0.1%, Nickel (Ni): 0.05 to 0.2%, Chromium (Cr): 0.1 to 0.3%, Molybdenum (Mo): 0.05 to 0.15%, Titanium (Ti): 0.01 to 0.05 Preparing a steel slab containing%, vanadium (V): 0.05 to 0.1%, nitrogen (N): 0.008% or less, balance Fe and inevitable impurities;
Reheating the steel slab in a temperature range of 1100 to 1300 ° C .;
Manufacturing a hot rolled steel sheet by finishing hot rolling of the reheated steel slab at a temperature range of 750 ° C. to 850 ° C .; And
After cooling the hot rolled steel sheet comprising the step of winding at a temperature between 500 ~ 550 ℃,
The wound steel is,
The microstructure is composed of 60 to 80 area% of ferrite, 20 to 40 area% of perlite and residual bainite satisfying the average grain size of 15 μm or less and the maximum grain size of 30 μm or less, satisfying the following Equation 1, 620 A method for producing a welded steel pipe having excellent impact toughness, characterized by having a yield strength of ˜689 MP and a tensile strength of 669 MPa or more.
[Relationship 1]
50 <209 + 4.91F + 0.063P-3.82B-0.891 Dmax-0.217YS-0.05Hv -30Ceq <80
(Where [F] is ferrite, [P] is pearlite, [B] is the fraction of bainite microstructure, Dmax is maximum grain size (µm), YP is material strength (MPa), and Hv is Vickers hardness) , Ceq means carbon equivalent)
delete