KR100979046B1 - Hot Rolled Steel Sheet having Excellent HIC Resistance Properties in Cold Deformation and Manufacturing Method Thereof - Google Patents

Abstract

It is an object of the present invention to provide a hot rolled steel sheet having excellent hydrogen resistance to organic cracking and a method of manufacturing the same, by controlling the size of the steel structure and nonmetallic inclusions.

In order to solve the above problems, the present invention is a weight%, C: 0.02 ~ 0.05%, Si: 0.05 ~ 0.5%, Mn: 0.5 ~ 1.5%, P: 0.01% or less (not including 0%), S: 0.001% or less (without 0%), Al: 0.02 to 0.05%, Nb: 0.02 to 0.07%, V: 0.02 to 0.06%, Ti: 0.005 to 0.02%, Cr: 0.1 to 0.5%, Ca: 0.0015 ˜0.003% and the remainder contain Fe and other unavoidable impurities, the content of Ca and S satisfies 1.5 ≦ Ca / S ≦ 4, a nonmetallic inclusion is present, and the structure comprises a pearlite phase, the nonmetallic inclusion And the pearlite phase and the tensile strain is characterized by satisfying the following formula (1).

[Equation 1] x / a + y / b + z / c ≤ 1.5

(Where, x: nonmetallic inclusion area (μm ² ), y: pearlite phase area (%), z: tensile strain (%), a = 2000, b = 1, c = 20 (for class X60 / X65))

Hydrogen Organic Crack, Cold Rolling, Non-metallic Inclusion, Perlite

Description

Hot Rolled Steel Sheet having Excellent HIC Resistance Properties in Cold Deformation and Manufacturing Method Thereof}

The present invention relates to a line pipe steel material for transporting crude oil or natural gas having a high hydrogen sulfide (H ₂ S) gas content, and more particularly, to a hot rolled steel sheet having excellent hydrogen organic cracking characteristics under cold deformation and a method of manufacturing the same. will be.

Recently in accordance with the increase in energy demand, and the existing resource exhaustion line pipe steel to transport them as the increase corrosive gas hydrogen sulfide (H ₂ S) gas content of the high oil or natural gas development is the breakage risk of corrosion of the H ₂ S gas It is increasing. In addition, in terms of transportation efficiency, the tendency for high pressure transportation is increasing, so that the partial pressure of H ₂ S increases, and thus it is required to increase resistance to pipe breakage due to corrosiveness of hydrogen sulfide gas.

The mechanism of generating hydrogen organic cracks is caused by cracking due to the action of hydrogen gas pressure generated by molecular hydrogen invading and diffusing hydrogen generated from the surface of steel material by the corrosion reaction between steel and hydrogen sulfide atmosphere. It is known to become.

In the prior arts, a means of suppressing the intrusion or diffusion of hydrogen by addition of Cu, reduction of MnS and shape control, fine dispersion of carbonitride, or the like, or a means of reducing central segregation during continuous casting, has been proposed.

However, although the prior arts provide a means for obtaining steel having relatively excellent hydrogen organic cracking resistance, it is difficult to completely suppress hydrogen organic cracking in a high acidic wet hydrogen sulfide atmosphere while increasing the strength level of the steel. Accordingly, a method of effectively controlling hydrogen organic cracks has been proposed by controlling the size of nonmetallic inclusions known as the hydrogen crack generation point and the hardness of steel propagating hydrogen cracks.

However, there are cases where bending of pipes is required during steel pipe manufacturing or during construction of pipelines, and deformations applied to steel materials cause deterioration of hydrogen organic cracking resistance. NACE MR0175 requires heat treatment to relieve residual stresses when the pipe's cold deformation exceeds 5%. Since such cold deformation is an obstacle to the safe suppression of hydrogen organic cracks in pipelines, there is a need for improving the resistance to hydrogen organic cracking during cold deformation.

As a result of examining the correlation between the hydrogen organic crack and the cold deformation, the inventors found that the risk of hydrogen organic crack is different depending on the size of the microstructure and nonmetallic inclusions and the degree of cold deformation.

Accordingly, an object of the present invention is to provide a hot rolled steel sheet having excellent hydrogen-hydrogen organic cracking characteristics and a manufacturing method thereof by controlling the microstructure of steel and the size of nonmetallic inclusions.

In order to solve the above problems, the present invention is a weight%, C: 0.02 ~ 0.05%, Si: 0.05 ~ 0.5%, Mn: 0.5 ~ 1.5%, P: 0.01% or less (not including 0%), S: 0.001% or less (without 0%), Al: 0.02 to 0.05%, Nb: 0.02 to 0.07%, V: 0.02 to 0.06%, Ti: 0.005 to 0.02%, Cr: 0.1 to 0.5%, Ca: 0.0015 ˜0.003% and the remainder contain Fe and other unavoidable impurities, the content of Ca and S satisfies 1.5 ≦ Ca / S ≦ 4, a nonmetallic inclusion is present, and the structure comprises a pearlite phase, the nonmetallic inclusion And the pearlite phase and the tensile strain is characterized by satisfying the following formula (1) and (2).

[Equation 1] x / a + y / b + z / c ≤ 1.5

(Where, x: nonmetallic inclusion area (μm ² ), y: pearlite phase area (%), z: tensile strain (%), a = 2000, b = 1, c = 20 (for class X60 / X65))
[Equation 2] Tensile strain (%) = (sample length after deformation-specimen length before deformation) / (sample length before deformation) × 100

Further, the production method of the present invention is characterized in that the hot finish rolling at a temperature of Ar3 or more, starting the cooling and winding after finishing the cooling in the 450 ~ 600 ℃ range.

The present invention has the effect of providing a hot rolled steel sheet excellent in the hydrogen-cracked organic properties and its manufacturing method by controlling the microstructure of the steel and the size of the non-metallic inclusions.

Hereinafter, the composition range of this invention is demonstrated concretely.

C: 0.02-0.05 wt%

The C is the most economical and effective alloying component to strengthen the steel, when less than 0.02% by weight of the combination of Nb, V or Ti is very effective to strengthen the steel, when added in excess of 0.05% by weight HIC resistance It is preferable to limit the content to 0.02 to 0.05% by weight since the central segregation to decrease the increase.

Si: 0.05-0.5 wt%

The Si is an effective component for deoxidation and solid solution strengthening, and when it is added less than 0.05% by weight, it is difficult to obtain a deoxidation effect, and when it is added in excess of 0.5% by weight, the weldability and brittleness are reduced, so the content is limited to 0.05 to 0.5% by weight. It is desirable to.

Mn: 0.5-1.5 wt%

The Mn is an essential component for securing strength and toughness. If it is added less than 0.5% by weight, it is difficult to secure strength and toughness, and when it is added in excess of 1.5% by weight, Mn promotes central segregation during performance, thereby reducing impact toughness and HIC resistance. Therefore, it is preferable to limit the content to 0.5 to 1.5% by weight.

P: 0.01% by weight or less (does not include 0%)

When the content of P is added in excess of 0.01% by weight, it promotes central segregation with Mn when playing, thereby reducing impact toughness and emulsion stress cracking resistance as well as reducing weldability, and thus limiting the content to 0.01% by weight or less. .

S: 0.001% by weight or less (does not include 0%)

S is a component that greatly reduces brittleness by forming MnS together with Mn in steel. When S is contained in an amount exceeding 0.001% by weight, the hydrogen organic cracking resistance is greatly reduced, and the content thereof is preferably limited to 0.001% by weight or less. .

Al: 0.02-0.05 wt%

Al is a component that deoxidizes with Si, and when it is added less than 0.02% by weight, it is difficult to obtain a deoxidation effect, and when it is added in excess of 0.05% by weight, the alumina aggregate is increased to lower the hydrogen-organic crack resistance. It is preferable to limit it to 0.02 to 0.05 weight%.

Nb: 0.02 to 0.07 wt%, V: 0.02 to 0.06 wt%

The Nb and V is a component showing the precipitation strengthening effect by the addition of a small amount, in the carbon range of the present invention, Nb is 0.07% by weight, V is 0.07% by weight when the strength increase due to precipitation strengthening is greater than 0.06%, respectively, its content is 0.07 It is limited to less than 0.06% by weight, and less than 0.02% by weight has no effect. Therefore, the content is preferably limited to 0.02 to 0.06% by weight.

Ti: 0.005-0.02 wt%

The Ti is precipitated with TiN in the steel to suppress the grain growth of austenite when reheated to obtain high strength and excellent impact toughness, and also precipitated by TiC, etc. to strengthen the steel. However, in the carbon range of the present invention, the content of Ti is 0.005% by weight, which is the minimum amount for obtaining the effect. If the content exceeds 0.02% by weight, the effect is not large. Therefore, the content is limited to 0.005% to 0.02% by weight or less. desirable.

Cr: 0.1-0.5 wt%

The Cr is added to increase strength and ensure corrosion resistance. The addition of Cr is an ingredient added to easily induce transformation into low temperature metamorphic tissues. The addition of less than 0.1 wt% results in less effect, and the addition of more than 0.5 wt% increases the risk of local corrosion. It is preferable to limit it to -0.5 weight%.

Ca: 0.0015 to 0.003 wt%

The Ca is a component that suppresses the origin of hydrogen organic crack generation by spheroidizing the shape of the emulsion-based inclusions, it is difficult to obtain the effect when added less than 0.0015% by weight, the amount of inclusions is more than 0.003% by weight Since it increases the hydrogen organic cracking resistance, it is preferable to limit the content to 0.0015 ~ 0.003% by weight.

In addition to the above compositions, the remainder is composed of Fe and other unavoidable impurities.

The ratio of the content of Ca and S is preferably limited to 1.5 ≦ Ca / S ≦ 4.

When the content ratio is less than 1.5, MnS is easily formed and the hydrogen organic cracking resistance is lowered. When the content ratio is higher than 4, the amount of Ca-based non-metallic inclusions increases, thereby limiting the hydrogen organic cracking resistance and toughness.

The correlation between nonmetallic inclusions, pearlite and deformation amount will be described in detail.

Non-metallic inclusions indispensably present in the steel are fractured or stretched in the rolling direction in the hot rolling step to serve as a starting point of the hydrogen organic crack. In addition, the interface between the base metal inclusions and the base metal is separated or voids become larger during the deformation process such as tubing or bending deformation, causing hydrogen organic cracks to be easily generated. Therefore, by limiting the non-metallic inclusions of the steel before rolling can reduce the risk of hydrogen organic cracking.

Perlite phase may occur in terms of microstructure, and if the pearlite phase is present as a target type, hydrogen organic cracks can be easily propagated through the target tissue, and thus the target pearlite phase formation should be suppressed. However, even if the target pearlite phase is suppressed and controlled in the form of a pearlite colony, there is a possibility that hydrogen pressure is easily formed at the interface between the pearlite and the base metal when the steel is deformed. Therefore, control over pearlite is also required.

In the above review, the nonmetallic inclusions and the pearlite phase increase the risk of hydrogen organic crack generation according to the amount of external deformation. However, the risk of hydrogen organic crack generation is limited by Equation 1 below.

[Equation 1] x / a + y / b + z / c ≤ 1.5

(Where, x: nonmetallic inclusion area (μm ² ), y: pearlite phase area (%), z: tensile strain (%), a = 2000, b = 1, c = 20 (for class X60 / X65))

Tensile strain refers to the degree of deformation of the specimen by the tensile stress and is defined as shown in Equation 2 below. The strain amount is measured using a conventional strain gauge or the like.

[Equation 2] Tensile strain (%) = (sample length after deformation-specimen length before deformation) / (sample length before deformation) × 100

Hereinafter, the manufacturing method of this invention is demonstrated concretely.

(1) reheat temperature

The reheating temperature is determined by the solid solution temperature of the Nb-based precipitate, and in the component range of the present invention, solid solution can be performed at 1150 ° C or higher. ℃ Limit the reheat temperature.

(2) rolling conditions

Below the unrecrystallized temperature, the amount of reduction greatly affects the grain size and uniformity of the hot rolled steel microstructure, and the grain size and uniformity have a great influence on the hydrogen organic crack resistance and low temperature toughness. If the reduction ratio is less than 70%, the homogeneity of the grain size is lowered and the low temperature toughness is lowered, so the reduction ratio is limited to 70% or more. Finishing the rolling above Ar3 starts the ferrite transformation at a temperature below Ar3, and when rolling, the hydrogen organic cracking resistance becomes very low, so rolling must be finished above Ar3.

(3) Cooling and winding conditions

Cooling should commence above the Ar3 temperature and, if commenced below, coarse ferrite will form prior to cooling, leading to lower toughness. Therefore, if the cooling rate is started above Ar3 temperature and cooling rate is slower than 10 ℃ / s, it is easy to form pearlite structure that lowers hydrogen organic cracking resistance, and if it is faster than 30 ℃ / s, bainite formation is easy Is limited in the range of 10 ~ 30 ℃ / s, if the winding is more than 600 ℃ is unstable transformation is possible to form a pearlite structure, and less than 450 ℃ is very difficult to wind large winding. Therefore, the coiling temperature is limited in the range of 450 ~ 600 ℃.

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

(Example)

After reheating the steel formed as shown in Table 1 for 2 to 3 hours in the range of 1150 ~ 1250 ℃, start hot rolling and cooling finish at 850 ℃ or more, and wound up in 500 ℃ ~ 600 ℃ range to 12.7 ~ 16mm thickness Phosphor steel was prepared. Hydrogen organic crack resistance of the steel was carried out in a 5% NaCl + 0.5% CH ₃ COOH solution saturated with 1 atm H ₂ S gas according to NACE TM 0284, and the degree of cracking was observed by ultrasonic inspection.

division Composition (wt%) Ca / S C Si Mn P S Al Nb V Ti Cr Ca Steel 1 0.045 0.21 1.32 0.007 0.001 0.031 0.041 0.045 0.01 0.21 0.0025 2.5 Steel 2 0.042 0.22 1.45 0.006 0.0007 0.033 0.049 0.048 0.008 0.25 0.0018 2.57 Steel 3 0.034 0.23 1.32 0.005 0.001 0.032 0.056 0.041 0.01 0.12 0.0022 2.2

Table 2 below shows the hydrogen crack generation area ratio, the area of the non-metallic inclusions, the area ratio of the pearlite phase, the strain rate and the hydrogen organic crack resistance characteristics for the steel of Table 1 above.

division Hydrogen crack
Occur
Area ratio
(CAR,%) Non-metallic Intervention
Water area
(μm ² ) Pearlite
Phase area
rate
(%) Strain
(%) Equation 1 Value Hydrogen Organic Cracking Characteristics
Meet compare Category 1









0 586 0.2 5 0.743 ○ Inventive Steel 1 0 689 0.6 7 1.2945 ○ Inventive Steel 2 0.2 568 0.6 10 1.384 ○ Invention Steel 3 0.2 684 0.4 6 1.042 ○ Inventive Steel 4 0.2 785 0.4 4 0.9925 ○ Inventive Steel 5 0.3 762 0.6 8 1.381 ○ Inventive Steel 6 2.3 586 1.3 5 1.843 Ⅹ Comparative Steel 1 3.4 568 2.1 10 2.884 Ⅹ Comparative Steel 2 4.3 684 2.3 6 2.942 Ⅹ Comparative Steel 3 5.8 689 3.5 7 4.1945 Ⅹ Comparative Steel 4 8.9 785 3.1 4 3.6925 Ⅹ Comparative Steel 5 9.2 762 2.6 8 3.381 Ⅹ Comparative Steel 6 Steel 2











0 897 0.5 5 1.1985 ○ Inventive Steel 7 0 898 0.2 5 0.899 ○ Inventive Steel 8 0 978 0.4 9 1.339 ○ Inventive Steel 9 0.1 853 0.4 6 1.1265 ○ Inventive Steel 10 0.2 865 0.1 9 0.9825 ○ Inventive Steel 11 0.4 932 0.3 10 1.266 ○ Inventive Steel 12 0.5 945 0.7 5 1.4225 ○ Inventive Steel 13 0.5 963 0.4 8 1.2815 ○ Inventive Steel 14 5.1 897 1.5 5 2.1985 Ⅹ Comparative Steel 7 5.4 898 3.2 5 3.899 Ⅹ Comparative Steel 8 6.2 865 3.8 9 4.6825 Ⅹ Comparative Steel 9 6.2 963 5.4 8 6.2815 Ⅹ Comparative Steel 10 6.5 932 4.1 10 5.066 Ⅹ Comparative Steel 11 6.8 945 2.8 5 3.5225 Ⅹ Comparative Steel 12 8.2 978 3.8 9 4.739 Ⅹ Comparative Steel 13 9.3 853 1.5 6 2.2265 Ⅹ Comparative Steel 14 Steel 3



9.5 1571 1.2 5 2.2355 Ⅹ Comparative Steel 15 12.8 1685 2.1 6 3.2425 Ⅹ Comparative Steel 16 14.9 1496 1.4 7 2.498 Ⅹ Comparative Steel 17 18.5 1523 3.4 4 4.3615 Ⅹ Comparative Steel 18 21.8 1632 2.1 9 3.366 Ⅹ Comparative Steel 19

Equation 1 is x / a + y / b + z / c ≤ 1.5

(Where, x: nonmetallic inclusion area (μm ² ), y: pearlite phase area (%), z: tensile strain (%), a = 2000, b = 1, c = 20 (for class X60 / X65))

As shown in Table 2, in the case of the invention steels 1 to 14 excellent in the hydrogen-cracking organic cracking characteristics of the hydrogen crack generation area ratio of 0.5% or less, the value of Equation 1 is satisfied.

1 is a graph showing the maximum nonmetallic inclusion size, low temperature transformation tissue fraction and deformation amount and hydrogen organic crack correlation.

Claims (2)

By weight, C: 0.02 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.5 to 1.5%, P: 0.01% or less (not including 0%), S: 0.001% or less (not including 0%) Al: 0.02 to 0.05%, Nb: 0.02 to 0.07%, V: 0.02 to 0.06%, Ti: 0.005 to 0.02%, Cr: 0.1 to 0.5%, Ca: 0.0015 to 0.003% and the rest are Fe and other Containing unavoidable impurities, the content of Ca and S satisfies 1.5 ≦ Ca / S ≦ 4, a nonmetallic inclusion is present, and the structure includes a pearlite phase, and the nonmetallic inclusions and pearlite phase and tensile strain are Hot rolled steel sheet excellent in hydrogen organic cracking characteristics under cold deformation characterized by satisfying 1 and 2. [Equation 1] x / a + y / b + z / c ≤ 1.5 (Where, x: nonmetallic inclusion area (μm ² ), y: pearlite phase area (%), z: tensile strain (%), a = 2000, b = 1, c = 20 (for class X60 / X65)) [Equation 2] Tensile strain (%) = (sample length after deformation-specimen length before deformation) / (sample length before deformation) × 100 By weight, C: 0.02 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.5 to 1.5%, P: 0.01% or less (not including 0%), S: 0.001% or less (not including 0%) Al: 0.02 to 0.05%, Nb: 0.02 to 0.07%, V: 0.02 to 0.06%, Ti: 0.005 to 0.02%, Cr: 0.1 to 0.5%, Ca: 0.0015 to 0.003% and the rest are Fe and other Slabs containing unavoidable impurities and satisfying the contents of Ca and S of 1.5 ≦ Ca / S ≦ 4 are hot-rolled at a reduction ratio of 70% or more at a temperature of Ar3 or higher, and cooled at a rate of 10 to 30 ° C / s. Start by winding after finishing cooling in the range of 450 ~ 600 ℃ The non-metallic inclusions are present, and the structure includes a pearlite phase, and the nonmetallic inclusions, pearlite phase and tensile strain are excellent in the hydrogen-organic crack resistance under cold deformation, characterized in that to produce a steel sheet satisfying the following formulas 1 and 2 Method for producing hot rolled steel sheet. [Equation 1] x / a + y / b + z / c ≤ 1.5 (Where, x: nonmetallic inclusion area (μm ² ), y: pearlite phase area (%), z: tensile strain (%), a = 2000, b = 1, c = 20 (for class X60 / X65)) [Equation 2] Tensile strain (%) = (sample length after deformation-specimen length before deformation) / (sample length before deformation) × 100

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