KR20230091628A - Steel plate having excellent hydrogen induced cracing resistance and uniform elongation and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high-strength steel plate and a method for manufacturing the same, and more specifically, to a steel plate having high strength and at the same time having excellent hydrogen-induced cracking resistance and uniform elongation, and a method for manufacturing the same. The high-strength steel plate of the present invention comprises 0.02 to 0.05 wt% of C, 0.05 to 0.3 wt% of Si, 0.4 to 1.1 wt% of Mn, 0.01 wt% or less of P, 0.001 wt% or less of S, 0.02 to 0.05 wt% of Al, 0.06 to 0.08 wt% of Nb, 0.005 to 0.02 wt% of Ti, 0.002 to 0.008 wt% of N, 0.1 to 0.5 wt% of Cr, 0.03 wt% or less of Mo, 0.0015 to 0.003 wt% of Ca, and the balance Fe and other inevitable impurities.

Description

High-strength steel sheet with excellent resistance to hydrogen induced cracking and uniform elongation and its manufacturing method

The present invention relates to a high-strength steel sheet and a method for manufacturing the same, and more particularly, to a steel sheet having high strength and excellent resistance to hydrogen induced cracking and uniform elongation, and a method for manufacturing the same.

Recently, as energy demand increases and quality requirements become stricter, demand for pipelines for transporting oil and gas containing hydrogen sulfide (H ₂ S) is increasing. On the other hand, the hydrogen sulfide (H ₂ S) is known as the main cause of hydrogen embrittlement such as hydrogen induced cracking or sulfide stress cracking in steel, and the demand to apply steel with excellent breakage resistance in an environment containing such hydrogen sulfide gas is increasing

In addition, in order to secure resistance to deformation and fracture of the pipe in an environment with severe deformation such as an offshore area or in an area where the permafrost subsides due to climate warming in Canada, etc., it is required to guarantee the uniform elongation of the pipeline.

Therefore, methods for simply securing hydrogen-induced cracking resistance are well known, such as controlling segregation and inclusions and securing a uniform low-temperature transformation structure such as escular ferrite in high-strength steel. As a method of improving uniform elongation in high-strength steel, MA Methods for generating secondary phases such as phases and the like are known.

However, it is not possible to suggest a method that can simultaneously secure hydrogen induced crack resistance and uniform elongation.

Korean Patent Publication No. 10-2013-0073472 (published on July 3, 2013) Korean Patent Publication No. 10-2016-0077418 (published on July 4, 2016)

According to one aspect of the present invention, it is intended to provide a steel sheet having high strength, excellent resistance to hydrogen induced cracking and uniform elongation, and a manufacturing method thereof.

The object of the present invention is not limited to the above. A person skilled in the art will have no difficulty understanding the further subject matter of the present invention from the general content of this specification.

One aspect of the present invention, in weight%, C: 0.02 ~ 0.05%, Si: 0.05 ~ 0.3%, Mn: 0.4 ~ 1.1%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 ~ 0.05% , Nb: 0.06~0.08%, Ti: 0.005~0.02%, N: 0.002~0.008%, Cr: 0.1~0.5%, Mo: 0.03% or less, Ca: 0.0015~0.003%, the balance including Fe and other unavoidable impurities do,

The R value defined in the following relational expression 1 is 1.5 to 4.0,

The A value defined in the following relational expression 2 is 1.0 to 1.4,

The microstructure includes polygonal ferrite and escular ferrite mixture as a base structure, and 1 to 5 area% pearlite It is possible to provide a steel sheet containing.

[Relationship 1]

R = [Ca]/[S]

(Where [Ca] and [S] are the weight% of each element.)

[Relationship 2]

A = [Mn] + [Cr] + [Mo]

(Where [Mn], [Cr] and [Mo] are the weight percent of each element.)

The steel sheet may include 40 to 80% of polygonal ferrite and 20 to 60% of escular ferrite as a microstructure.

The steel sheet may have a yield strength of 500 to 600 MPa, a crack length ratio (CLR) of 10% or less, and a uniform elongation of 10% or more.

The steel sheet may have a yield strength of 530 to 600 MPa.

Another aspect of the present invention, in weight percent, C: 0.02 ~ 0.05%, Si: 0.05 ~ 0.3%, Mn: 0.4 ~ 1.1%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 ~ 0.05 %, Nb: 0.06-0.08%, Ti: 0.005-0.02%, N: 0.002-0.008%, Cr: 0.1-0.5%, Mo: 0.03% or less, Ca: 0.0015-0.003%, the balance Fe and other unavoidable impurities Including, the R value defined in the following relational expression 1 is 1.5 to 4.0, the A value defined in the following relational expression 2 is 1.0 to 1.4 by refining and continuously casting molten steel to prepare a slab;

reheating the steel slab;

Finishing hot-rolling the reheated steel slab in a temperature range of Ar3 to 950 ° C; and

It is possible to provide a steel sheet manufacturing method comprising the step of cooling the hot-rolled steel sheet to a temperature range of 550 ~ 650 ℃ at a cooling rate of 10 ~ 30 ℃ / s and then winding.

[Relationship 1]

R = [Ca]/[S]

(Where [Ca] and [S] are the weight% of each element.)

[Relationship 2]

A = [Mn] + [Cr] + [Mo]

(Where [Mn], [Cr] and [Mo] are the weight percent of each element.)

The reheating is performed in a temperature range of 1200 to 1350 ° C,

During the hot rolling, the cumulative reduction ratio is 70% or more,

During the cooling, cooling may be initiated in a temperature range of Ar3 or higher.

According to one aspect of the present invention, it is possible to provide a steel sheet having high strength, excellent resistance to hydrogen induced cracking and uniform elongation, and a manufacturing method thereof.

According to one aspect of the present invention, it is possible to provide a high-strength steel sheet that can be applied to pipes for transporting hydrogen sulfide (H ₂ S)-containing oil and natural gas, and a manufacturing method thereof.

1 shows photographs of microstructures of (a) Inventive Example 1, (b) Comparative Example 2, and (c) Comparative Example 4 according to an embodiment of the present invention.
2 is a diagram illustrating a comparison between tensile curves of Inventive Example 1 and Comparative Example 4 according to an embodiment of the present invention.
3 is a photograph of a crack fracture surface in which hydrogen-induced cracking occurred in Comparative Example 5 according to an embodiment of the present invention.

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. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

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, % indicating the content of each element is based on weight.

Steel according to one aspect of the present invention, by weight%, C: 0.02 ~ 0.05%, Si: 0.05 ~ 0.3%, Mn: 0.4 ~ 1.1%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 ~ 0.05%, Nb: 0.06~0.08%, Ti: 0.005~0.02%, N: 0.002~0.008%, Cr: 0.1~0.5%, Mo: 0.03% or less, Ca: 0.0015~0.003%, balance Fe and other unavoidable impurities can include

Carbon (C): 0.02 to 0.05%

Carbon (C) is the most economical and effective element for strengthening steel. However, when the carbon (C) is added at less than 0.02%, the effect of strengthening the steel by combining with Nb or Ti is very small, and when it exceeds 0.05%, the center segregation that lowers the hydrogen-induced cracking resistance increases There may be a problem. More preferably, it may contain 0.025% or more, and more preferably, it may contain 0.045% or less.

Silicon (Si): 0.05 to 0.3%

Silicon (Si) is an effective component for deoxidation and solid solution strengthening, and is preferably added in an amount of 0.05% or more for the above effect. However, if the content exceeds 0.3%, there is a possibility of reducing weldability and brittleness. A more preferred lower limit may be 0.1%, and a more preferred upper limit may be 0.27%.

Manganese (Mn): 0.4 to 1.1%

Manganese (Mn) is an essential component for securing strength and toughness, but when the content thereof is less than 0.4%, it may be difficult to secure strength and toughness. On the other hand, if the content exceeds 1.1%, the hydrogen-induced cracking resistance may be reduced by promoting central segregation during playing. More preferably, it may be included in 0.7% or more, and more preferably, it may be included in 1.05% or less.

Phosphorus (P): 0.01% or less

When the phosphorus (P) content exceeds 0.01%, center segregation is promoted along with Mn during casting, which may reduce hydrogen induced cracking resistance as well as reduce weldability. However, considering that it is unavoidably contained, 0% is excluded.

Sulfur (S): 0.001% or less

Sulfur (S) is a component that greatly reduces brittleness by reacting with Mn in steel to form MnS, and when it exceeds 0.001%, it can greatly reduce hydrogen induced cracking resistance. However, considering that it is unavoidably contained, 0% is excluded.

Aluminum (Al): 0.02 to 0.05%

Aluminum (Al) is a component that acts as a deoxidizer together with Si, and it is difficult to obtain a deoxidizer effect when the content thereof is less than 0.02%. On the other hand, when the content exceeds 0.05%, the resistance to hydrogen induced cracking may be reduced by increasing the alumina aggregate. A more preferred lower limit may be 0.023%, and a more preferred upper limit may be 0.04%.

Niobium (Nb): 0.06-0.08%

Niobium (Nb) is a component that exhibits a precipitation strengthening effect by adding a small amount, and it is preferable to include 0.06% or more in order to secure high strength through the above effect. However, when the content exceeds 0.08%, weldability and hydrogen induced crack resistance due to a large amount of precipitates may be deteriorated. A more preferred lower limit may be 0.061%, and a more preferred upper limit may be 0.075%.

Titanium (Ti): 0.005 to 0.02%

Titanium (Ti) is precipitated as TiN in steel and suppresses grain growth of austenite during reheating, thereby enabling high strength and excellent impact toughness to be obtained, and also precipitated as TiC to strengthen steel. In order to obtain the above effect in the carbon range proposed in the present invention, it is preferable that the content of the titanium (Ti) is 0.005% or more. On the other hand, when the content exceeds 0.02%, the effect reaches a saturation state, and coarse Ti crystallization is produced, resulting in deterioration in hydrogen-induced cracking resistance. More preferably, it may contain 0.007% or more, and more preferably, it may contain 0.018% or less.

Nitrogen (N): 0.002 to 0.008%

Nitrogen (N) precipitates as Ti and TiN in steel and suppresses grain growth of austenite. When the nitrogen (N) is added in an amount of less than 0.002%, the effect may be small. On the other hand, when the content exceeds 0.008%, coarse TiN is precipitated and serves as an initiation point for hydrogen induced cracking and sulfide stress cracking, so that the content is preferably controlled to 0.002 to 0.008% by weight.

Chromium (Cr): 0.1~0.5%

The chromium (Cr) is added to increase strength and secure corrosion resistance. When the amount of chromium (Cr) is added to be less than 0.1%, the effect is small, and when the amount of chromium (Cr) exceeds 0.5%, the hardenability increases and uniform elongation may decrease. A more preferred lower limit may be 0.12%, and a more preferred upper limit may be 0.35%.

Molybdenum (Mo): 0.03% or less

Molybdenum (Mo) may be added to secure strength through securing hardenability. When molybdenum (Mo) is added in excess of 0.03%, a low-temperature transformation structure is generated due to an increase in hardenability, which may decrease uniform elongation. More preferably, it may contain 0.02% or less.

Calcium (Ca): 0.0015 to 0.003%

Calcium (Ca) is a component that serves to suppress the origin of cracks caused by hydrogen by spheroidizing the shape of emulsion-based inclusions. When the content of calcium (Ca) is less than 0.0015%, it is difficult to obtain the above effect, and when the content of calcium (Ca) exceeds 0.003%, the amount of non-metallic inclusions rather increases, thereby reducing hydrogen induced cracking resistance.

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

The steel sheet according to one aspect of the present invention may have an R value of 1.5 to 4.0 defined in the following relational expression 1.

When the R value in relational expression 1 is less than 1.5, MnS is easily formed and sulfide stress cracking resistance is lowered, and when the value exceeds 4.0, the amount of Ca-based nonmetal inclusions increases and hydrogen induced cracking resistance is lowered. there is.

[Relationship 1]

R = [Ca]/[S]

(Where [Ca] and [S] are the weight% of each element.)

The steel sheet according to one aspect of the present invention may have an A value of 1.0 to 1.4 defined in the following relational expression 2.

When the A value of relational expression 2 is less than 1.0, it is advantageous to secure uniform elongation, but it may be difficult to secure high strength. On the other hand, when the value exceeds 1.4, it may be difficult to secure a uniform elongation due to the generation of a low-temperature transformation structure due to excessive increase in hardenability.

[Relationship 2]

A = [Mn] + [Cr] + [Mo]

(Where [Mn], [Cr] and [Mo] are the weight percent of each element.)

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

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

The microstructure of the steel sheet according to one aspect of the present invention may include a mixed structure of polygonal ferrite and escular ferrite as a base structure, and may include 1 to 5 area % of pearlite.

In the present invention, in order to secure excellent hydrogen induced crack resistance and uniform elongation, a mixed structure of polygonal ferrite and escular ferrite may be included as a base structure. In addition, perlite may be included as the second phase. When the pearlite content exceeds 5%, hydrogen induced crack resistance is deteriorated, and when it is less than 1%, it is difficult to secure excellent uniform elongation. More preferably, 40 to 80% of polygonal ferrite and 20 to 60% of escular ferrite may be included.

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

A steel sheet according to an aspect of the present invention may be manufactured by refining, continuous casting, reheating, hot rolling, and cooling molten steel satisfying the alloy composition described above.

slab manufacturing

A slab can be manufactured by refining and continuously casting molten steel that satisfies the alloy composition of the present invention.

Control of non-metallic inclusions according to the present invention can be obtained through process condition control in a conventional secondary refining process, for example, the secondary refining process is a degassing process such as Ar bubbling in LF and VTD or RH Inclusions can be controlled by Ar bubbling. Of course, the manufacturing method of the present invention is not necessarily limited to the above process conditions, and non-metallic inclusions can be controlled by various methods. After refining the molten steel, the molten steel may be continuously cast to produce a slab.

reheat

The prepared steel slab may be reheated in a temperature range of 1200 to 1350 ° C.

The reheating temperature is determined by the solid solution temperature of the Nb-based precipitate, and in the component range of the present invention, the entire solid solution of Nb is possible at 1200 ° C or higher. On the other hand, when the reheating temperature exceeds 1350 ° C., the crystal grain size of the steel sheet becomes excessively large, the toughness decreases, and a lot of oxides are generated, which may interfere with rolling.

hot rolled

The reheated steel slab may be finished hot rolled at a cumulative reduction ratio of 70% or more in a temperature range of Ar3 to 950 ° C.

The amount of reduction below the recrystallization temperature has a very large effect on the grain size and uniformity of the microstructure of the hot-rolled steel sheet. The crystal grain size and uniformity are highly correlated with hydrogen induced cracking resistance and uniform elongation. Therefore, in order to control the grain size and uniformity, it is preferable to have a rolling reduction ratio of 70% or more during rolling. If the rolling reduction ratio is less than 70%, the homogeneity of the grain size may decrease and the uniform elongation may decrease.

On the other hand, finish hot rolling is preferably performed in the temperature range of Ar3 to 950 ° C. When rolling in excess of 950 ° C., there is a high possibility that non-uniform and coarse crystal grain growth may occur, and uniform elongation can be reduced. Ar3 When finish hot rolling is performed in a temperature range below the temperature range, a texture that is inferior to brittle fracture may be generated and hydrogen induced cracking resistance may be very low.

[ceremony]

Ar3 = 910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]

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

cooling and winding

The hot-rolled steel sheet may be cooled to a temperature range of 550 to 650° C. at a cooling rate of 10 to 30° C./s by starting cooling in the temperature range of Ar3 to 950° C., and then winding.

Cooling of the hot-rolled steel sheet obtained through the hot-rolling process is preferably initiated at an Ar3 temperature or higher. When the cooling is initiated at a temperature lower than Ar3, a brittle fracture texture may develop which reduces the hydrogen induced cracking resistance.

When the cooling rate is less than 10 ° C / s, a coarse pearlite structure that deteriorates hydrogen induced cracking resistance can be easily formed, and when the rate exceeds 30 ° C / s, a light secondary phase such as MA phase Its production can be accelerated and its resistance to hydrogen-induced cracking can be reduced.

The steel sheet of the present invention prepared as described above has a yield strength of 500 to 600 MPa, a CLR (Crack Length Ratio) of 10% or less, and a uniform elongation of 10% or more, having high strength and excellent hydrogen induced crack resistance and uniform elongation. characteristics can be provided.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention.

(Example)

Non-metallic inclusions were controlled by refining molten steel having the alloy composition of Table 1 below, and hot-rolled steel sheets having a thickness of 6 to 10 mm were manufactured through the manufacturing conditions of Table 2 below. Here, cooling was initiated above the Ar3 temperature, and the same was applied to all specimens. In Table 1 below, the values of relational expressions 1 and 2 according to each steel type are calculated and shown, and Ar3 is calculated and shown.

river
bell Alloy composition (wt%) coffin
total
ceremony
One coffin
total
ceremony
2 Ar3
(℃) C Si Mn P S Al Cr Mo Ti Nb N Ca A 0.040 0.25 1.0 0.007 0.0008 0.025 0.15 0.01 0.01 0.075 0.0048 0.0022 2.75 1.16 815 B 0.035 0.18 1.05 0.006 0.0006 0.027 0.3 0.02 0.015 0.063 0.0042 0.0021 3.5 1.35 809 C 0.043 0.25 0.8 0.007 0.0007 0.027 0.43 0.02 0.012 0.067 0.0044 0.0023 3.29 1.25 825 D 0.056 0.19 1.02 0.006 0.0007 0.029 0.41 0.01 0.012 0.077 0.004 0.002 2.85 1.44 804 E 0.046 0.21 1.35 0.007 0.0007 0.025 0.37 0.02 0.01 0.078 0.0043 0.0024 3.43 1.74 781 F 0.041 0.22 1.08 0.007 0.0006 0.026 0.54 0.02 0.011 0.068 0.0042 0.0021 3.5 1.64 801 G 0.040 0.21 1.0 0.007 0.0007 0.025 0.3 0.08 0.01 0.065 0.0048 0.0022 3.14 1.38 807 H 0.046 0.23 0.97 0.006 0.0008 0.027 0.37 0.01 0.023 0.13 0.0046 0.002 2.5 1.35 812 I 0.045 0.21 0.8 0.007 0.0007 0.025 0.39 0.01 0.01 0.078 0.0043 0.0034 4.86 1.16 825 J 0.042 0.23 0.95 0.006 0.0009 0.025 0.26 0.02 0.011 0.074 0.0041 0.0012 1.33 1.23 815 K 0.037 0.24 1.05 0.007 0.0008 0.027 0.43 0.02 0.01 0.069 0.0043 0.0022 2.75 1.5 806 L 0.04 0.22 0.51 0.006 0.0009 0.029 0.34 0.01 0.01 0.071 0.0047 0.0025 2.78 0.86 851

[Relationship 1]

R = [Ca]/[S]

(Where [Ca] and [S] are the weight% of each element.)

[Relationship 2]

A = [Mn] + [Cr] + [Mo]

(Where [Mn], [Cr] and [Mo] are the weight percent of each element.)

[ceremony]

Ar3 = 910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]

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

Psalter
number steel grade reheat finish hot rolled Cooling Temperature (℃) Temperature (℃) Reduction rate (%) End temperature (℃) Rate (℃/s) One A 1278 872 97.5 614 26 2 B 1291 856 96.8 587 24 3 C 1288 854 96 601 19 4 D 1256 890 96 568 21 5 E 1288 852 96.8 593 26 6 F 1279 846 96 606 19 7 G 1258 865 96 631 22 8 H 1244 882 97.5 614 26 9 I 1296 871 96.8 578 24 10 J 1276 852 97.5 593 28 11 K 1284 863 96 602 22 12 L 1269 884 97.5 586 26 13 A 1266 774 96.8 565 25 14 B 1283 851 96.8 526 28 15 A 1268 865 96.8 669 16 16 C 1278 862 96.8 553 36

Table 3 below shows the microstructure and physical properties of the manufactured steel sheet. In the case of the inventive examples in Table 3 below, escular ferrite was observed except for polygonal ferrite and pearlite fractions. On the other hand, in the case of Comparative Example, the main structure of polygonal ferrite and pearlite was the main structure of escular ferrite, but some low-temperature transformation structures were formed. The low-temperature transformation structure may be bainite, MA phase, and the like. The microstructure was measured through image analysis at a magnification of 500 times using an optical microscope, and the yield strength and uniform elongation were measured through a tensile test of API-5L standard at room temperature. Here, the uniform elongation means the elongation until the test piece is uniformly deformed without causing local shrinkage during the tensile test. In addition, according to NACE TM0284, the CLR was immersed in a 5% NaCl + 0.5% CH ₃ COOH solution saturated with gas at 1 atm H ₂ S for 96 hours, and then the specimen was taken out and ultrasonically tested.

Psalter
number steel grade microstructure (% area) Properties division polygonal ferrite perlite Yield strength (MPa) CLR (%) Uniform elongation (%) One A 66 2.3 547 0 13.2 Invention example 1 2 B 62 1.6 531 0.1 12.6 Invention example 2 3 C 72 3.1 551 0 13.8 Invention example 3 4 D 62 1.1 573 16.7 10.7 Comparative Example 1 5 E 42 0.4 584 12.6 9.2 Comparative Example 2 6 F 46 0.8 552 3.5 9.7 Comparative Example 3 7 G 35 0.1 564 2.9 8.8 Comparative Example 4 8 H 68 3.6 579 21.8 13.8 Comparative Example 5 9 I 65 2.6 554 18.9 13.4 Comparative Example 6 10 J 63 2.1 546 32.5 13.1 Comparative Example 7 11 K 44 0.6 549 0 9.4 Comparative Example 8 12 L 76 3.4 461 0 14.8 Comparative Example 9 13 A 54 0 567 20.4 8.4 Comparative Example 10 14 B 41 0.7 543 0.3 9.5 Comparative Example 11 15 A 73 5.8 536 16.3 14.9 Comparative Example 12 16 C 37 0.1 556 18.4 8.9 Comparative Example 13

As shown in Table 3, in the case of the inventive example satisfying the alloy composition and manufacturing conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the desired physical properties were secured in the present invention.

1 shows photographs of microstructures of (a) Inventive Example 1, (b) Comparative Example 2, and (c) Comparative Example 4 according to an embodiment of the present invention.

2 is a diagram illustrating a comparison between tensile curves of Inventive Example 1 and Comparative Example 4 according to an embodiment of the present invention. As shown in the graph, it can be confirmed that Inventive Example 1 has excellent uniform elongation compared to Comparative Example 4.

3 is a photograph of a crack fracture surface in which hydrogen-induced cracking occurred in Comparative Example 5 according to an embodiment of the present invention. In the microstructure, white indicates coarse NbTi crystallization, from which it can be confirmed that hydrogen induced cracking has occurred.

On the other hand, Comparative Examples 1 to 3 are examples where Relational Equation 2 is out of the range proposed in the present invention, and the C, Mn, and Cr contents exceeded the range of Relational Equation 2, respectively. As a result, the desired CLR characteristics and uniform elongation were not secured in the present invention.

In Comparative Example 4, when the Mo content was excessive, the low-temperature transformation structure was excessively formed, and as a result, the uniform elongation was lowered.

Comparative Example 5 is a case where Ti and Nb contents were excessively added, and precipitates were excessively formed, so that desired CLR characteristics were not secured.

Comparative Examples 6 and 7 are examples that do not satisfy the range of Relational Equation 1, and Comparative Example 6 exceeded the value of Relational Equation 1 due to the excessive addition of Ca, and as a result, the desired hydrogen induced cracking resistance was inferior. Comparative Example 7 is a case where the alloy composition of the present invention is satisfied, but the value of relational expression 1 is insufficient. As a result, the formation of MnS was easy, and the resistance to hydrogen induced cracking was reduced.

Comparative Examples 8 and 9 are examples that satisfy the alloy composition proposed in the present invention, but do not satisfy the range of relational expression 2. In Comparative Example 8, the value of relational expression 2 was high, and the low-temperature transformation structure was excessively formed, resulting in a decrease in uniform elongation. In the case of Comparative Example 9, the value of relational expression 2 was insufficient, so it was difficult to secure strength.

Comparative Example 10 is a case where the alloy composition satisfies the range of the present invention, but the finish hot rolling temperature is not reached. As a result, a texture inferior to brittle fracture was formed, pearlite was not formed, and as a result, uniform elongation and hydrogen induced crack resistance were inferior.

Comparative Examples 11 and 12 are examples that do not satisfy the cooling end temperature of the present invention. In Comparative Example 11, the cooling end temperature was not reached, and a low-temperature transformation structure was excessively formed, and as a result, the uniform elongation was lowered. In Comparative Example 12, the cooling end temperature was exceeded and pearlite was excessively formed, and the hydrogen induced crack resistance was inferior.

In Comparative Example 13, when the cooling rate was excessive, a hard phase was formed and hydrogen induced crack resistance was reduced.

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 (6)

In % by weight, C: 0.02 to 0.05%, Si: 0.05 to 0.3%, Mn: 0.4 to 1.1%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 to 0.05%, Nb: 0.06 to 0.08% , Ti: 0.005-0.02%, N: 0.002-0.008%, Cr: 0.1-0.5%, Mo: 0.03% or less, Ca: 0.0015-0.003%, the balance Fe and other unavoidable impurities,
The R value defined in the following relational expression 1 is 1.5 to 4.0,
The A value defined in the following relational expression 2 is 1.0 to 1.4,
The microstructure is a steel sheet containing a polygonal ferrite and an escular ferrite mixture as a base structure and containing 1 to 5 area% of pearlite.
[Relationship 1]
R = [Ca]/[S]
(Where [Ca] and [S] are the weight% of each element.)
[Relationship 2]
A = [Mn] + [Cr] + [Mo]
(Where [Mn], [Cr] and [Mo] are the weight percent of each element.)
According to claim 1,
The steel sheet is a steel sheet containing 40 to 80% of polygonal ferrite and 20 to 60% of escular ferrite in a microstructure.
According to claim 1,
The steel sheet has a yield strength of 500 to 600 MPa, a crack length ratio (CLR) of 10% or less, and a uniform elongation of 10% or more.
According to claim 1,
The steel sheet is a steel sheet having a yield strength of 530 to 600 MPa.
In % by weight, C: 0.02 to 0.05%, Si: 0.05 to 0.3%, Mn: 0.4 to 1.1%, P: 0.01% or less, S: 0.001% or less, Al: 0.02 to 0.05%, Nb: 0.06 to 0.08% , Ti: 0.005 to 0.02%, N: 0.002 to 0.008%, Cr: 0.1 to 0.5%, Mo: 0.03% or less, Ca: 0.0015 to 0.003%, the balance including Fe and other unavoidable impurities, defined in the following relational expression 1 Preparing a slab by refining and continuously casting molten steel having an R value of 1.5 to 4.0 and an A value of 1.0 to 1.4 defined in the following relational expression 2;
reheating the steel slab;
Finishing hot-rolling the reheated steel slab in a temperature range of Ar3 to 950 ° C; and
Steel sheet manufacturing method comprising the step of winding the hot-rolled steel sheet after cooling to a temperature range of 550 ~ 650 ℃ at a cooling rate of 10 ~ 30 ℃ / s.
[Relationship 1]
R = [Ca]/[S]
(Where [Ca] and [S] are the weight% of each element.)
[Relationship 2]
A = [Mn] + [Cr] + [Mo]
(Where [Mn], [Cr] and [Mo] are the weight percent of each element.)
According to claim 5,
The reheating is performed in a temperature range of 1200 to 1350 ° C,
During the hot rolling, the cumulative reduction ratio is 70% or more,
During the cooling, the steel sheet manufacturing method in which cooling is initiated in a temperature range of Ar3 or higher.

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