STEEL SHEET FOR PIPE HAVING EXCELLENT HYDROGEN-INDUCED CRACK RESISTANCE AND METHOD FOR MANUFACTURING SAME

Abstract

The present disclosure relates to a steel sheet for pipes having excellent hydrogen-induced crack resistance and a method for manufacturing the same. One aspect of the present invention is to provide a steel sheet for a pipe having excellent hydrogen-induced crack resistance and a method for manufacturing same. According to an aspect of the present disclosure, a steel sheet for pipes having excellent hydrogen-induced crack resistance and a method for manufacturing the same may be provided.

Description

TECHNICAL FIELD

The present disclosure relates to a steel sheet for pipes having excellent hydrogen-induced crack resistance and a method for manufacturing the same.

BACKGROUND ART

The development of oil and gas wells (hereinafter, collectively referred to as oil wells) has become increasingly harsh, and efforts have been made to lower production costs to increase profitability. Recently, as the development of oil wells containing H₂S gas has increased, demand for materials having high hydrogen-induced crack resistance has increased. Specifically, there has been an increasing demand for materials that may be used in a mildly sour environment, such as H₂S partial pressure of 0.1 bar or less and pH 5.0. Meanwhile, in the case of a steel pipe, there has been an increasing demand for reducing the costs required for a heat treatment through a method of making a high-strength welded steel pipe without a heat treatment (austenizing, quenching, and tempering) after pipe forming.

SUMMARY OF INVENTION

Technical Problem

An aspect of the present disclosure is to provide a

steel sheet for pipes having excellent hydrogen-induced crack resistance and a method for manufacturing the same.

Solution to Problem

According to an aspect of the present disclosure, a steel sheet for a pipe having excellent hydrogen-induced crack resistance, includes: by weight, C: 0.06 to 0.12%, Si: 0.10 to 0.50%, Mn: 0.8 to 2.2%, P: 0.010% or less, S: 0.001% or less, Nb: more than 0.05% to 0.1% or less, Cr: 0.05% 0.6%, Ti: more than 0.02% to less than 0.05%, Ca: 0.001 to 0.006%, N: 0.008% or less, a balance of Fe and other inevitable impurities, wherein a microstructure in the entire region of the steel sheet in a thickness direction, includes, by area %, 90% or more of acicular ferrite and lower bainite, and including 10% or less of one or more of pearlite and island-like martensite, wherein microstructure in a ⅖ to ⅗ region of the steel sheet in the thickness direction includes, by area %, 95% or more of acicular ferrite and lower bainite, and 5% or less of island-like martensite, and wherein the acicular ferrite and lower bainite have an average grain size of 5 μm or less.

According to another aspect of the present disclosure, a method for manufacturing a steel sheet for a pipe having excellent hydrogen-induced crack resistance includes: reheating a slab including, by weight, C: 0.06 to 0.12%, Si: 0.10 to 0.50%, Mn: 0.8 to 2.2%, P: 0.010% or less, S: 0.001% or less, Nb: more than 0.05% to 0.1% or less, Cr: 0.05% 0.6%, Ti: more than 0.02% to less than 0.05%, Ca: 0.001 to 0.006%, N: 0.008% or less, a balance of Fe and other inevitable impurities, at 1100° C. to 1300° C.; rough rolling the reheated slab at 900° C. to 1100° C. to obtain a rough rolled bar; finish hot-rolling the rough-rolled bar at 800° C. to 900° C. to obtain hot-rolled steel; cooling the hot-rolled steel at 10° C. to 50° C./s; and coiling the cooled hot-rolled steel at 450° C. to 550° C.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a steel sheet for pipes having excellent hydrogen-induced crack resistance and a method for manufacturing the same may be provided.

BEST MODE FOR INVENTION

Hereinafter, a steel sheet for pipes having excellent hydrogen-induced crack resistance according to an embodiment of the present disclosure will be described. First, an alloy composition of the steel sheet of the present disclosure will be described. The content of the alloy composition described below refers to wt % unless otherwise specified. C: 0.06 to 0.12%

C is an element that increases hardenability of steel. However, if the C content is less than 0.06%, the strength targeted in the present disclosure cannot be secured due to insufficient hardenability, whereas if the C content exceeds 0.12%, hydrogen-induced cracking may occur due to the occurrence of segregation. Therefore, the C content is preferably in the range of 0.06 to 0.12%. The C content is more preferably 0.06 to 0.10%.

Si: 0.10 to 0.50%

Si is an element that increases the activity of C in a ferrite phase, promotes ferrite stabilization, and contributes to securing strength by solid solution strengthening. In addition, by forming a low melting point oxide, such as Mn₂SiO₄, during electric resistance welding, the oxide may be easily discharged during welding. If the Si content is less than 0.10%, a cost problem may arise during steelmaking, and if the Si content exceeds 0.50%, the amount of SiO₂, which is a high melting point oxide, other than Mn₂SiO₄, may increase, thereby lowering toughness of a welded portion during electric resistance welding. Therefore, the Si content is preferably in the range of 0.10 to 0.50%. A lower limit of the Si content is more preferably 0.20%. An upper limit of the Si content is more preferably 0.40%.

Mn: 0.8 to 2.2%

Mn is an effective element for solid solution strengthening of steel. The content of Mn needs to be 0.8% or more to secure the strength targeted in the present disclosure along with the effect of increasing hardenability. However, if the Mn content exceeds 2.2%, a segregation portion may be significantly developed in the center of a thickness when a slab is cast in a steelmaking process, and hydrogen-induced crack resistance of a final product may be impaired. Therefore, the Mn content is preferably in the range of 0.8 to 2.2%. A lower limit of the Mn content is more preferably 0.9%. An upper limit of the Mn content is more preferably 2.1%.

P: 0.010% or less

P is an impurity and is an element that deteriorates hydrogen-induced crack resistance, and thus, less P content is preferred. However, in consideration of the costs in the steelmaking process, the P content is limited to 0.010% or less in the present disclosure. The P content is more preferably 0.009% or less, and even more preferably 0.008% or less.

S: 0.001% or less

S is an element that tends to form coarse inclusions and promotes a decrease in toughness and formation of hydrogen-induced cracking, and thus, S as less as possible is preferred. However, in consideration of costs in the steelmaking process, the S content is limited to 0.001% or less in the present disclosure. Considering the costs in a steelmaking stage, an upper limit is limited to 0.001% or less. The S content is more preferably 0.0009% or less, and even more preferably 0.0008% or less.

Nb: more than 0.05% and 0.1% or less

Nb is an element that has a great influence on securing strength through the formation of precipitates, and enhances the strength of steel by precipitating carbon nitrides in steel or by strengthening solid solution in Fe. In particular, Nb-based precipitates are dissolved during reheating of the slab and then finely precipitated during hot rolling to effectively increase strength. However, if the Nb content is 0.05% or less, it may be difficult to secure hydrogen-induced crack resistance because a structure having a particle size larger than a particle size targeted in the present disclosure may be formed. Meanwhile, the Nb content exceeding 0.1% is disadvantageous in economic feasibility. Therefore, the Nb content is preferably in the range of more than 0.05% and 0.1% or less. A lower limit of the Nb content is more preferably 0.06%. An upper limit of the Nb content is more preferably 0.09%.

Cr: 0.05 to 0.6%

Cr is an element that improves hardenability and corrosion resistance. If the Cr content is less than 0.05%, the effect of improving corrosion resistance by addition may be insufficient, and if the Cr content exceeds 0.6%, weldability may be sharply deteriorated. Therefore, the Cr content is preferably in the range of 0.05 to 0.6%. A lower limit of the Cr content is more preferably 0.1%. An upper limit of the Cr content is more preferably 0.5%.

Ti: more than 0.02% and 0.05% or less

Ti reacts with N to form TiN, thereby suppressing the grain growth of austenite during reheating of the slab to increase strength. In addition, Ti plays a role of increasing strength by suppressing the grain growth of austenite even in a heat-affected zone (HAZ) after welding. However, if the Ti content is 0.02% or less, it may be difficult to secure hydrogen-induced crack resistance because a structure larger than a particle size targeted in the present disclosure is formed. Meanwhile, if the Ti content exceeds 0.05%, a Ti crystallized product may be formed to induce hydrogen-induced crack propagation. Therefore, the Ti content is preferably in the range of more than 0.02% and 0.05% or less. The Ti content is more preferably more than 0.02% and 0.04% or less.

Ca: 0.001 to 0.006%

Ca is an element added to control the shape of sulfide. If the Ca content exceeds 0.006%, CaS clusters are formed as Ca exists in excess compared to the S content in the steel, whereas if the Ca content is less than 0.001%, MnS may be formed, resulting in a decrease in toughness. Therefore, the Ca content is preferably in the range of 0.001 to 0.006%. A lower limit of the Ca content is more preferably 0.002%. An upper limit of the Ca content is more preferably 0.005%.

N: 0.008% or less

N is an inevitable impurity that fixes Ti, Al, etc. in steel as nitrides. If the N content exceeds 0.008%, an increase in the addition amount of Ti, Al, etc. is inevitable, and thus, the N content is preferably limited to 0.008% or less. The N content is more preferably 0.007% or less.

In addition to the steel composition described above, the rest may include Fe and inevitable impurities. Inevitable impurities may be unintentionally mixed in a normal steel manufacturing process, which cannot be completely excluded, and those skilled in the ordinary steel manufacturing field may easily understand the meaning. Further, the present disclosure does not entirely exclude the addition of other compositions than the steel composition mentioned above.

Hereinafter, a microstructure of the steel of the present disclosure will be described.

In the steel sheet of the present disclosure, a microstructure in the entire region of the steel sheet in a thickness direction preferably includes 90% or more of acicular ferrite and lower bainite in area %. The acicular ferrite and lower bainite are structures effective in improving hydrogen-induced crack resistance compared to other structures. Therefore, in the present disclosure, the fraction of the acicular ferrite and lower bainite is to be secured at 90% or more, and if the fraction is less than 90%, the hydrogen-induced crack resistance may be inferior. The fraction of the acicular ferrite and lower bainite is more preferably 92% or more, even more preferably 94% or more, and most preferably 95% or more. Meanwhile, in the present disclosure, one or more of pearlite and island-like martensite (martensite austenite (MA)) may be formed inevitably in the manufacturing process. Since the pearlite and island-like martensite (MA) are disadvantageous in securing hydrogen-induced crack resistance, the fraction of these impurity structures is limited to 10% or less in the present disclosure. The fraction of the impure structure is preferably 8% or less, more preferably 6% or less, and most preferably 5% or less.

In addition, in the steel sheet of the present disclosure, the microstructure in the region of ⅖ to ⅗ in the thickness direction of the steel plate includes 95% or more of acicular ferrite and lower bainite, and preferably includes 5% or less of island-like martensite. The island-like martensite (MA) is mostly present in the form of a band in the center of the thickness of the steel sheet, and this island-like martensite structure is a very unfavorable structure in securing hydrogen-induced crack resistance. If the fraction of island-like martensite in the ⅖ to ⅗ region of the steel sheet in the thickness direction exceeds 5%, there is a disadvantage in that hydrogen-induced crack resistance is lowered. Meanwhile, in the present disclosure, 1% or less of pearlite may be inevitably formed in the ⅖ to ⅗ region of the steel sheet in the thickness direction in terms of the manufacturing process.

Meanwhile, the acicular ferrite and lower bainite

preferably have an average grain size of 5 μm or less. If the average grain size of the acicular ferrite and lower bainite exceeds 5 μm, propagation of hydrogen-induced cracking is facilitated through a grain boundary, which may be disadvantageous in securing hydrogen-induced crack resistance.

The steel sheet of the present disclosure provided as described above has a crack length ratio (CLR) of 15% or less measured under the conditions of H₂S partial pressure: 0.1 bar and pH: 5.0 according to NACE TM0284 standard after being made into a pipe and secures excellent hydrogen-induced crack resistance. In addition, after forming into a pipe, steel sheet of the present disclosure may have a yield strength of 552 MPa or more and a tensile strength of 655 MPa or more even without a heat treatment, such as quenching and tempering (QT), and through this, high strength may be secured, while the costs required for a heat treatment may be reduced.

Hereinafter, a method for manufacturing a steel sheet for pipes having excellent hydrogen-induced crack resistance according to an embodiment of the present disclosure will be described.

First, a slab satisfying the alloy composition described above is reheated at 1100 to 1300° C. Since the reheating process of the slab is a process of heating the steel so that a subsequent rolling process may be smoothly performed and desired properties of the steel sheet may be sufficiently obtained, the heating process should be performed within an appropriate temperature range according to the purpose. If the slab reheating temperature is less than 1100° C., it may be difficult to completely dissolve Nb, whereas if a slab reheating temperature exceeds 1300° C., initial crystal grains may become too large, thereby making it difficult to refine the particle size. A lower limit of the slab reheating temperature is more preferably 1150° C. An upper limit of the slab reheating temperature is more preferably 1250° C.

Thereafter, the reheated slab is rough-rolled at 900 to 1100° C. to obtain a rough-rolled bar. If the rough rolling temperature is less than 900° C., there may be a risk of causing a problem with equipment load of a rolling mill. If the rough rolling temperature exceeds 1100° C., the grain size may be coarsened to thereby reduce strength and toughness. A lower limit of the rough rolling temperature may be preferably 950° C. An upper limit of the rough rolling temperature may be preferably 1050° C.

Thereafter, the rough-rolled bar is finished hot-rolled at 800 to 900° C., which is a non-recrystallization temperature range, to obtain a hot-rolled steel sheet. If the finish hot rolling temperature exceeds 900° C., a final structure becomes coarse and desired strength cannot be obtained, and if the finish hot rolling temperature is less than 800° C., there is a risk of equipment malfunction due to a rolling load. A lower limit of the finish hot rolling temperature is more preferably 810° C. An upper limit of the finish hot rolling temperature is more preferable 890° C.

Thereafter, the hot-rolled steel sheet is cooled at 10 to 50° C./s. The cooling is an element that improves toughness and strength of the steel plate. As the cooling rate increases, crystal grains of an internal structure of the steel plate may be refined to improve toughness and an internal hard structure may be developed to improve the strength. To this end, a cooling rate during the cooling is preferably 10° C./s or more. However, if the cooling rate exceeds 50° C./s, a low-temperature transformation structure, such as bainite, may be relatively increased to exceed the desired strength or a band-shaped transformation structure may be formed depending on the thickness, resulting in poor hydrogen-induced crack resistance. A lower limit of the cooling rate is more preferably 20° C./s. An upper limit of the cooling rate is more preferably 40° C./s.

Thereafter, the cooled hot-rolled steel sheet is coiled at 450 to 550° C. If the coiling temperature exceeds 550° C., a large amount of island-like martensitic structure may be formed in the center of the thickness to degrade hydrogen-induced crack resistance, and if the coiling temperature is less than 450° C., low-temperature transformation phases, such as bainite, may be locally formed and stress is concentrated, which may impair hydrogen-induced crack resistance. A lower limit of the coiling temperature is more preferably 460° C. An upper limit of the coiling temperature is more preferably 540° C.

Meanwhile, in the present disclosure, after the coiling, the steel sheet may be manufactured into a pipe. In the present disclosure, the pipe manufacturing method is not particularly limited, and any method commonly used in the art may be used. However, it is preferable to use electric resistance welding, which is economically superior.

[Mode for Invention]

Hereinafter, the present disclosure will be described

in more detail through examples. However, it should be noted that the following examples are only for illustrating the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by the matters described in the claims and the matters reasonably inferred therefrom.

EXAMPLE

A steel sheet was manufactured by reheating, rough

rolling, finish hot rolling, cooling, and coiling a slab having the alloy composition shown in Table 1 below under the conditions shown in Table 2 below. The steel sheet manufactured thusly was made into a pipe through electric resistance welding, a microstructure and mechanical properties of the pipe were measured, and the results are shown in Table 2 below.

As for the fraction of the microstructure, three random areas in regions corresponding to ¼t (t: steel thickness) and ½t (t: steel thickness) were observed using an optical microscope, and fractions of the respective structures were measured using an Image-J program, and average values thereof were described. Typically, the ¼t region is a position that may be regarded as the entire microstructure of the steel plate.

A grain size was measured according to the ASTM E112 standard.

Yield strength and tensile strength were measured using a tensile tester according to the ASTM A370 standard.

Hydrogen-induced crack resistance was evaluated by measuring crack length ratio (CLR) under the conditions of

H₂S partial pressure: 0.1 bar and pH: 5.0 according to NACE TM0284 standard.

TABLE 1
	Alloy composition (wt %)
Classification	C	Si	Mn	P	S	Nb	Cr	Ti	N
Inventive	0.07	0.36	1.9	0.009	0.0008	0.06	0.32	0.022	0.005
example
1
Inventive	0.08	0.34	2.0	0.008	0.0009	0.07	0.41	0.023	0.004
example
2
Inventive	0.09	0.34	1.6	0.009	0.0006	0.07	0.35	0.025	0.003
example
3
Comparative	0.12	0.34	0.85	0.011	0.0012	0.03	0.39	0.020	0.005
example
1
Comparative	0.12	0.30	1.2	0.008	0.0008	0.04	0.41	0.015	0.005
example
2
Comparative	0.13	0.32	1.3	0.009	0.0007	0.04	0.55	0.012	0.004
example
3
Comparative	0.15	0.32	1.2	0.008	0.0009	0.05	0.58	0.014	0.007
example
4

TABLE 2
	Slab	Rough	Finish
	heating	rolling	hot rolling		Coiling
	temper-	temper-	temper-	Cooling	temper-
Classi-	ature	ature	ature	rate	ature
fication	(° C.)	(° C.)	(° C.)	(° C./s)	(° C.)
Inventive	1275	951	824	25	510
example1
Inventive	1266	960	831	21	524
example2
Inventive	1287	971	822	27	480
example3
Comparative	1266	989	830	25	505
example1
Comparative	1256	950	829	20	602
example2
Comparative	1277	935	837	18	570
example3
Comparative	1236	957	815	21	550
example4

TABLE 3
	Microstructure in	Microstructure in
	entire region in	region of 2/5 to
	thickness direction	3/5 in thickness
	of steel sheet	direction of steel	Average
		One or	sheet	grain size	Yield	Tensile
	AF + LB	more P and	MA	AF + LB	for AF + LB	strength	strength	CLR
Classification	(area %)	MA (area %)	(area %)	(area %)	(μm)	(MPa)	(MPa)	(%)
Inventive	97	3	2	98	3	679	751	0
example 1
Inventive	96	4	3	97	4	662	738	2
example 2
Inventive	98	2	4	96	3	677	774	8
example 3
Comparative	96	4	3	97	3	635	733	19
example 1
Comparative	89	11	15	85	6	612	708	30
example 2
Comparative	92	8	11	89	5	658	721	21
example 3
Comparative	95	5	6	94	5	667	760	16
example 4
AF: acicular ferrite, LB: lower bainite, P: pearlite, MA: island-like martensite (MA)

As may be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 3 satisfying the alloy composition and manufacturing conditions proposed by the present disclosure, as the microstructure and grain size targeted in the present disclosure were obtained, the yield strength and tensile strength, as well as the hydrogen-induced crack resistance, are excellent after pipe forming.

In contrast, in the case of Comparative Examples 1 to 4, which do not satisfy the alloy composition or manufacturing conditions proposed in the present disclosure, it can be seen that the microstructure or grain size targeted in the present disclosure were not secured, and thus, the hydrogen-induced crack resistance is low.

Claims

1. A steel sheet for a pipe having excellent hydrogen-induced crack resistance, the steel sheet comprising:

by weight, C: 0.06 to 0.12%, Si: 0.10 to 0.50%, Mn: 0.8 to 2.2%, P: 0.010% or less, S: 0.001% or less, Nb: more than 0.05% to 0.1% or less, Cr: 0.05% 0.6%, Ti: more than 0.02% to less than 0.05%, Ca: 0.001 to 0.006%, N: 0.008% or less, a balance of Fe and other inevitable impurities,

wherein a microstructure in the entire region of the steel sheet in a thickness direction, includes, by area %, 90% or more of acicular ferrite and lower bainite, and including 10% or less of one or more of pearlite and island-like martensite,

wherein microstructure in a ⅖ to ⅗ region of the steel sheet in the thickness direction includes, by area %, 95% or more of acicular ferrite and lower bainite, and 5% or less of island-like martensite,

wherein the acicular ferrite and lower bainite have an average grain size of 5 μm or less.

2. The steel sheet of claim 1, wherein the steel sheet has a CLR of 15% or less measured under conditions of H2S partial pressure: 0.1 bar and pH: 5.0 according to NACE TM0284 standard after pipe forming.

3. The steel sheet of claim 1, wherein the steel sheet has a yield strength of 552 MPa or more and a tensile strength of 655 MPa or more without a heat treatment after pipe forming.

4. A method for manufacturing a steel sheet for a pipe having excellent hydrogen-induced crack resistance, the method comprising:

reheating a slab including, by weight, C: 0.06 to 0.12%, Si: 0.10 to 0.50%, Mn: 0.8 to 2.2%, P: 0.010% or less, S: 0.001% or less, Nb: more than 0.05% to 0.1% or less, Cr: 0.05% 0.6%, Ti: more than 0.02% to less than 0.05%, Ca: 0.001 to 0.006%, N: 0.008% or less, a balance of Fe and other inevitable impurities, at 1100° C. to 1300° C.;

rough rolling the reheated slab at 900° C. to 1100° C. to obtain a rough rolled bar;

finish hot-rolling the rough-rolled bar at 800° C. to 900° C. to obtain hot-rolled steel;

cooling the hot-rolled steel at 10° C. to 50° C./s; and

coiling the cooled hot-rolled steel at 450° C. to 550° C.