KR101008174B1 - Steel Plate with high SOHIC resistance at the H2S containing environment - Google Patents

Abstract

The present invention relates to a steel for improving the SOHIC properties in weight%, C: 0.02 ~ 0.15%, Si: 0.1 ~ 0.4%, Mn: 0.5 ~ 1.8%, Al: 0.001 ~ 0.1%, P: 0.015% or less , S: 0.005% or less, Cu: 0.05-0.3%, Ni: 0.05-0.6%, Ca: 5-50 ppm, balance Fe and other unavoidable impurities, and the inclusions in the steel per area of less than 1 cm ² The present invention relates to a steel material in which the number of oxide-based inclusions having a size exceeding 20 μm is limited to 30 and the area fraction of phases having hardness greater than 300 (Hv) in the microstructure inside the steel is limited to 10% or less.

According to the present invention, a steel material exhibiting excellent SOHIC resistance in a wet environment including H ₂ S can be provided, and further, by using the same, a 36-inch or larger large-diameter line having excellent hydrogen organic crack resistance in crude oil and gas atmosphere containing hydrogen sulfide is used. Pipe steel can be provided.

SOHIC, Hydrogen Organic Crack, Oxide Inclusions, H2S, Sour

Description

Steel plate with high SOHIC resistance at the H2S containing environment}

The present invention, more particularly grade H ₂ S content is higher circle excellent SOHIC resistance in oil or gas API X80 on the steel that can be used with high safety, etc. a line pipe for crude oil and gas transportation contain H ₂ S It relates to steel materials used for the following pipes and the like.

Hydrogen sulphide contained in crude oil or natural gas can cause brittle cracking of steel in corrosive atmosphere, which is divided into hydrogen organic crack (HIC) and sulfide stress corrosion cracking (SOHIC).

Among these, hydrogen-organic cracks are known to cause cracks and growth destruction when hydrogen atoms generated by corrosion invade into the material from the outside in an environment containing H ₂ S, and hydrogen atoms reach a critical concentration or more. In the region where HIC appears, hydrogen atoms are concentrated, and the hydrogen embrittlement lowers the mechanical properties of the material, increases the locally applied stress, and lowers the maximum stress that the material can withstand. Furthermore, if the stress applied locally is greater than the stress that the material can withstand, cracks develop and grow, which has the characteristic of leading to the destruction of the material.

On the other hand, sulfide stress corrosion compared to this is a phenomenon in which the material under stress due to hydrogen embrittlement and fracture occurs, so it is called SOHIC (Stress Oriented Hydrogen Induced Cracking) in the meaning of fracture by hydrogen. This failure mode means a case where the microcracks are generated and connected in a fragile region such as an interface between impurities in the material and a matrix by hydrogen invading into the material, and then the microcracks grow and are connected to cause breakage. In other words, when hydrogen is accumulated in impurities such as elongated MnS present in the material and is locally in a stress state exceeding the critical stress, cracking starts, which propagates to the outside and breakage occurs.

This type of fracture phenomenon is closely related to the shape and tensile direction of impurities. At the tip of the impurity in a direction parallel to the stress axis, a crack is generated, with a 45 degree angle to the tensile axis, resulting in failure.

Both hydrogen organic cracks and SOHIC are embrittlement phenomena by hydrogen, but there is no particular correlation between them because the metallurgical factors that determine both properties are different. That is, even if the steel having excellent resistance to either one may be low resistance to the other. Therefore, in order to have excellent SOHIC resistance, it is required to approach in a different direction from the techniques for improving the hydrogen cracking characteristics of the hydrogen which have been known in the past.

As such, there is a prior art for improving SOHIC resistance, which is distinguished from the hydrogen-organic crack resistance, and the technique discloses a method of limiting C to 0.08 to 0.25% as a steel used as a pressure vessel. According to the inventors' experiment, the second phase fraction, such as pearlite, was increased due to the limited amount of C, so that the degradation of SOHIC resistance could not be avoided.

In addition, there is also a conventional technique using the method of limiting the C content to 0.05 ~ 0.25% as a pressure vessel steel and air-cooled after the heat treatment in the 750 ~ 850 ℃ section, in this case, by the heat treatment such as pearlite The SOHIC resistance can be improved by relieving the stress of the two phases, but there are disadvantages in that the cost can be increased and productivity can be lowered due to additional processes such as heat treatment.

Accordingly, there is a need for an additional process and a need for a steel and an SOHIC resistance with improved economics and a method of manufacturing the same.

The present invention solves the problems of the prior art described above, and at the same time, by optimizing the composition of the steel system, by appropriately controlling the microstructure including the inclusion and the second phase fraction of the steel, excellent SOHIC even in the wet environment including H ₂ S To provide a steel material exhibiting resistance.

In order to achieve the above object, the present invention is a weight%, C: 0.02 ~ 0.15%, Si: 0.1 ~ 0.4%, Mn: 0.5 ~ 1.8%, Al: 0.001 ~ 0.1%, P: 0.015% or less, S : 0.005% or less, Cu: 0.05-0.3%, Ni: 0.05-0.6%, Ca: 5-50 ppm, remainder Fe and other unavoidable impurities.

Furthermore, in order to obtain excellent SOHIC resistance, the size and distribution of inclusions present in the final steel product of the present invention is limited to 30 or less oxide-based inclusions having a size of more than 20 μm in an area within 1 cm ² and fine. The area fraction of phases in the tissue with hardness greater than 300 (Hv) is limited to 10% or less.

According to the present invention, it is possible to provide a steel material exhibiting excellent SOHIC resistance in a humid environment including H ₂ S by optimizing the steel component and appropriately controlling the microstructure including the level of inclusions and the fraction of the second phase of the steel. By using this, it is possible to provide a 36-inch or larger large-diameter line pipe steel having excellent hydrogen organic crack resistance in crude oil and gas atmosphere containing hydrogen sulfide.

Hereinafter, the present invention will be described in detail.

The SOHIC phenomenon is a phenomenon in which cracks, such as hydrogen organic cracks, occur in a direction parallel to the direction of stress in the steel subjected to external stress, and these cracks are connected in a direction perpendicular to the stress, resulting in fracture of the steel. to be. At this time, even if the cracks do not occur due to the hydrogen cracking resistance is very excellent, the SOHIC resistance is low, there is a case that the steel breakage occurs. In other words, the steel material for obtaining hydrogen organic crack resistance alone may not guarantee excellent SOHIC resistance.

Therefore, in the present invention, in order to reduce the amount of inclusions acting as the starting point of the cracks to attain hydrogen organic crack resistance, and simultaneously input Ca for the spheroidization of the inclusion shape, in order to suppress the propagation of the disclosed cracks, It is characterized by suppressing the band structure formation of the two phases.

When SOHIC is generated, cracks in the form of hydrogen-organic cracks occur in the direction parallel to the stress. In order to suppress this, the size and distribution of inclusions should be limited. In the case of external stress such as SOHIC, cracks may occur even at smaller inclusions than hydrogen organic cracks, so the restriction on inclusions should be more stringent than steels with high hydrogen organic cracking resistance. Another method to increase the SOHIC resistance is to suppress the propagation of cracks. The crack propagation of SOHIC is characterized by connecting cracks occurring parallel to the direction of stress. This becomes difficult and has the characteristic that propagation of cracks does not occur.

Therefore, in order to improve SOHIC resistance, it was found that the spacing between cracks is more important than the size of stress and horizontal cracks. However, since the fracture of the steel by SOHIC does not appear well in the center of the steel, the microstructure and hardness of the steel center do not need to be particularly limited for SOHIC resistance.

In addition, in order to improve the hydrogen organic crack resistance, the formation of the band structure of the second phase such as pearlite may be suppressed. However, in the SOHIC, the band structure is formed parallel to the direction of stress, so that the SOHIC is large when the gap between the bands is wide. No special effect on resistance Since SOHIC is generated in a state where external stress is applied, when phases having a large hardness difference exist at the same time, the phases having a small hardness are easily deformed and hydrogen accumulation occurs around the hydrogen embrittlement. Therefore, in order to obtain excellent SOHIC resistance, since the hardness difference between the ferrite and the second phase should not be large in the microstructure, it is preferable to appropriately limit the fraction of the second phase having a higher hardness than the ferrite.

Hereinafter, the component system of the present invention will be described in detail.

C: 0.02-0.15 wt%

C is sufficient to add 0.02% by weight or more as an element added to improve strength. As the content of C increases, the hardenability may be improved to increase the strength. However, when the content of C exceeds 0.15% by weight, the toughness of the steel may be impaired and a second phase having a high hardness may be generated. Therefore, the upper limit thereof may be 0.15% by weight or less. Restrict.

Si: 0.1-1.0 wt%

Si should be added at least 0.1% by weight because it acts as a deoxidizer, but when the content exceeds 1.0% by weight, the toughness and weldability may be inhibited, and the amount of oxidation inclusions in the steel may increase, thereby reducing the SOHIC resistance. Therefore, the upper limit is limited to 1.0 weight% or less.

Mn: 0.5-1.8 wt%

The Mn is an effective ingredient for increasing the strength without deteriorating the toughness, and as the content increases, the hardenability increases due to the increase in the hardenability. Therefore, at least 0.5 wt% or more should be added. However, Mn is an element that facilitates segregation during solidification and promotes band structure. When Mn is added in an amount of 1.8 wt% or more, the width of the band structure is narrowed, thereby inhibiting SOHIC resistance. Therefore, it is preferable to limit the content to 1.8 wt% or less.

Al: 0.001-0.1 wt%

Al is an element that is essentially added for deoxidation during steelmaking, and improves the shock absorption energy, but reacts with oxygen to form an oxide-based inclusion like Si. If the content of Al is less than 0.001% by weight, deoxidation is not sufficiently achieved. If the content of Al exceeds 0.1% by weight, not only the impact toughness is impaired, but also a large amount of inclusions is formed to inhibit SOHIC resistance, so that the content is 0.001. Limit to 0.1% by weight.

P: 0.015% by weight or less

P is an element included in steel, which is essential in steelmaking, and not only inhibits weldability and toughness, but also inhibits toughness and SOHIC resistance as an element easily segregates at the center of the slab and austenite grain boundaries during solidification. It is desirable to limit.

S: 0.005 wt% or less

S is generally elongated during rolling by reacting with Mn to form MnS to act as a starting point for SOHIC generation. Therefore, it is preferable to reduce S as much as possible, but the range is set to 0.005% by weight or less due to process constraints for S removal.

Cu: 0.05-0.3 wt%

Cu is added to improve the strength of the steel and to improve the corrosion resistance. Cu must be added in an amount of 0.05% by weight or higher in order to improve strength and form a protective film on the surface in the atmosphere containing hydrogen sulfide to lower the corrosion rate of the steel and reduce the amount of hydrogen diffused into the steel. However, Cu is an element that inhibits the surface quality by causing cracks on the surface during hot rolling, so it is preferable to limit the upper limit to 0.5% by weight or less.

Ni: 0.05-0.6 wt%

Ni is an element that improves the toughness of steel and is added to reduce surface cracks generated during hot rolling of Cu-added steel. It is preferable to add 1.5 times or more of the Cu-added steel in order to reduce surface cracks due to the addition of Cu. Therefore, the lower limit of Ni is 0.05% by weight or more as the lower limit of Cu, but the upper limit is 0.6% by weight. In addition, addition of Ni by 0.6% by weight or more hinders the improvement of hydrogen embrittlement characteristics by addition of Cu, and excessively increases the cost.

Cr: 0.01-0.5 wt%

Cr is an element that increases strength by increasing the hardenability of steel and should be added in an amount of 0.01% by weight or more for the effect of increasing strength. The strength of Cr increases with increasing amount of the Cr added. However, since the toughness of steel is impaired when 0.5 wt% or more is added, the upper limit is preferably limited to 0.5 wt% or less.

Mo: 0.01-0.5 wt%

Mo, like Cr, is an element that plays a role of increasing strength by increasing the hardenability of steel, and its effect is much higher than that of Cr. In order to obtain the effect of increasing the strength by adding Mo, 0.01% by weight or more should be added. When 0.5% by weight or more is added, the cost is not excessively high, and a second phase having a very high hardness may be generated, which may inhibit SOHIC resistance. It is preferable to limit an upper limit to 0.5 weight% or less.

Nb: 0.01% to 0.08% by weight

Nb precipitates in the form of Nb (C, N) at a temperature around 1200 ° C. to increase its strength. In addition, it plays a role of miniaturizing ferrite particles by suppressing recrystallization of austenite generated during secondary hot rolling. In order to fully acquire the effect of strength improvement by the addition of Nb, it should be added at least 0.01% by weight. However, it is preferable to limit the upper limit to 0.08% by weight or less since the second phase containing Nb may serve as a place for promoting crack connection during SOHIC crack propagation.

V: 0.1 wt% or less

V is usually formed in the form of VC in the ferrite region, although VN may be formed when N is sufficiently present in the steel. Lower vacancy carbon concentration when transforming to austenite-ferrite, and VC provides a nucleation site for cementite formation. Therefore, rather than continuously forming Fe ₃ C at the grain boundary has a form of a discontinuous structure to increase the resistance to SOHIC. However, when V is added in an amount of 0.10% by weight or more, coarse V precipitates are formed, which not only inhibits toughness but also act as hydrogen accumulation sites in steel, thereby lowering resistance to hydrogen organic cracks. Therefore, the content of V is limited to 0.10% by weight or less.

Ti: 0.03 wt% or less

Ti is an element that forms carbides or nitrides, and serves to form homogeneous ferrite through grain refinement of austenite phase. Finely dispersed Ti (C, N) precipitates reduce the diffusion coefficient of hydrogen and increase the resistance to hydrogen organic cracks. However, when the addition amount is increased, the Ti (C, N) precipitate becomes coarse and becomes a hydrogen concentration site, which rather inhibits SOHIC resistance, so the upper limit thereof is limited to 0.03% by weight or less.

Ca: 0.0005 to 0.005 wt%

The Ca serves to spheroidize the MnS inclusions. MnS is drawn during rolling with inclusions with a low melting point to serve as a starting point for SOHIC cracks. The added Ca reacts with MnS and surrounds the MnS, thus preventing MnS from stretching. The MnS spheroidization of Ca is closely related to the amount of S. Therefore, the appropriate amount of Ca is determined by the content of S, but in general, considering the steelmaking process, the amount is limited to 0.0005 to 0.005% by weight.

In addition to the above components, the remainder comprises Fe and other unavoidable impurities.

The thick steel plate having excellent SOHIC resistance of the present invention limits the size and distribution of oxide-based inclusions in the final product. This is because the starting point of the SOHIC is inclusions, when the inclusion size is larger than the threshold size, cracks are initiated at the inclusions, and when the gap between these crack initiation inclusions is smaller than the threshold value, the fracture of the steel occurs due to the connection of the cracks. Therefore, the number of inclusions of 20㎛ size or more within the 1cm ² area of the final product is limited to not more than 30.

The present invention also limits the microstructure in the final product. In the microstructure, hydrogen is often accumulated around the second phase having a large difference in hardness from ferrite, and cracking occurs due to hydrogen accumulation. When these cracks are connected, the fracture of the steel causes the fraction of the microstructure having a large hardness. It is desirable to limit. In general, since the hardness value of the ferrite phase is about 150 to 200 (Hv), the fraction of the second phase excluding the ferrite, that is, the hardness of more than 300 Hv in the second phase except ferrite, exceeds 10%. If not, it can show excellent SOHIC resistance. Therefore, in the present invention, the area fraction of the second phase whose hardness exceeds 300 (Hv) in the microstructure in the product is limited to 10% or less.

As described above, the present invention optimizes the steel component, and the size and distribution of inclusions in the product is less than 30 inclusions of 20 μm or more within 1 cm ² , and the second phase having a hardness exceeding 300 (Hv) in the microstructure. By limiting the fraction to 10% or less, respectively, it is possible to provide a thick steel plate having excellent SOHIC resistance.

Hereinafter, the present invention will be described in more detail with reference to examples, which are only preferred embodiments of the present invention, but the scope of the present invention is not limited by the scope of the embodiments.

(Example 1)

To prepare a steel having a composition as shown in Table 1, in the case of invention steel A ~ K, comparative steel L ~ T by measuring the distribution of the second phase hardness is shown in Table 2 below. In order to measure the hardness and total fraction (second phase area / observation area) of the second phase present in the microstructure of the steel, the microhardness and optical microscope of each steel type were observed. Table 2 shows the area fractions by hardness of the second phase observed in the inventive and comparative steels.

Ferrite hardness
(Hv) Second phase area fraction with hardness of 200 ~ 300 (Hv) (%) Second phase area fraction (%) with hardness over 300 (Hv)



Invention steel Inventive Steel A 158 6.4 0.8 Inventive Steel B 162 6.9 1.0 Invention Steel C 161 7.1 1.1 Inventive Steel D 168 5.2 0.6 Inventive Steel E 165 8.4 1.8 Inventive Steel F 171 7.1 1.2 Invention Steel G 159 7.2 2.1 Inventive Steel H 184 6.4 2.5 Inventive Steel I 175 5.8 1.9 Invention Steel J 168 5.4 3.4 Inventive Steel K 166 6.4 2.7

Comparative steel Comparative Steel L 194 1.4 14.1 Comparative Steel M 189 2.1 11.5 Comparative Steel N 196 2.5 10.5 Comparative Steel O 191 2.0 10.3 Comparative Steel P 179 1.9 14.2 Comparative Steel Q 187 1.5 12.4 Comparative Steel R 193 0.8 17.1 Comparative Steel S 195 1.2 10.5 Comparative Steel T 172 2.6 10.1

Based on the results of Tables 1 and 2, the inventive steel has a small number of inclusions per unit area, and it can be seen that the fraction of the second phase having a large hardness difference is relatively smaller than that of the comparative steel.

(Example 2)

In this example, the SOHIC resistance of the inventive steel and the comparative steel was evaluated. SOHIC resistance was evaluated based on the time when fracture occurred after applying a constant stress to the steel under a hydrogen sulfide saturated solution. In this case, the steel that does not break during this period based on 720 hours was evaluated as having excellent SOHIC resistance.

In a solution saturated with H ₂ S, the weight (yield stress × 0.9 × tensile specimen cross-sectional area) corresponding to 90% of the yield stress of each steel material was applied to each specimen by lever and weight. The SOHIC resistance was evaluated by observing the fracture of the steel according to the results, and the results are shown in Table 3 as the fracture time. Marked as "no break" is a steel that does not break even after 720 hours, it can be seen as a very good SOHIC resistance.

Break time (hr)


foot
persons
^River Inventive Steel A No break occurs Inventive Steel B No break occurs Invention Steel C No break occurs Inventive Steel D No break occurs Inventive Steel E No break occurs Inventive Steel F No break occurs Invention Steel G No break occurs Inventive Steel H No break occurs Inventive Steel I No break occurs Invention Steel J No break occurs Inventive Steel K No break occurs

ratio
School
River Comparative Steel L 26 Comparative Steel M 49 Comparative Steel N 28 Comparative Steel O 56 Comparative Steel P 486 Comparative Steel Q 36 Comparative Steel R 302 Comparative Steel S 117 Comparative Steel T 604

As shown in Table 3, it can be seen that the inventive steels according to the present invention exhibit significantly superior SOHIC properties than the comparative steels.

Claims (3)

By weight%, C: 0.02 to 0.15%, Si: 0.1 to 0.4%, Mn: 0.5 to 1.8%, Al: 0.001 to 0.1%, P: 0.015% or less, S: 0.005% or less, Cu: 0.05 to 0.3% , Ni: 0.05-0.6%, Ca: 5-50 ppm, residual Fe and other unavoidable impurities, The number of inclusions in the steel is less than 30 oxide-based inclusions having a size of more than 20 μm per 1 cm ² area, Steel material, characterized in that the area fraction of the phase of the hardness exceeding 300 (Hv) of the microstructure in the steel is 10% or less. The steel according to claim 1, wherein the steel further comprises one or two or more selected from the group consisting of Cr: 0.01 to 0.5% and Mo: 0.01 to 0.5%. The method of claim 1 or 2, wherein the steel further comprises one or two or more selected from the group consisting of Nb: 0.01-0.08%, V: 0.1% or less and Ti: 0.03% or less. Characterized by steel materials.