KR100957979B1 - Steel Plate for Pressure Vessel with High SOHIC Resistance - Google Patents

Abstract

In the present invention, by weight%, C: 0.10 to 0.30%, Si: 0.15 to 0.40%, Mn: 0.6 to 1.2%, Al: 0.001 to 0.05%, P: 0.035% or less, S: 0.020% or less, Ca: 5 At least one component selected from the group consisting of 50 ppm or less, Cr: 0.35% or less, Mo: 0.2% or less, Ni: 0.5% or less, V: 0.05% or less, Nb: 0.05% or less It further comprises, the microstructure is a two-phase composite structure, in particular a steel plate for pressure vessels excellent in SOHIC resistance, characterized in that consisting of a two-phase composite structure of ferrite + bainite by recrystallization control rolling and controlled cooling It relates to a manufacturing method.

According to the pressure vessel steel of the present invention, it is possible to provide a pressure vessel steel sheet excellent in tensile strength and excellent in SOHIC resistance even under an H ₂ S (sour) gas atmosphere.

HIC, SOHIC, Pressure Vessel, Control Cooling, Recrystallization Control Rolling

Description

Steel Plate for Pressure Vessel with High SOHIC Resistance

The present invention relates to a steel sheet excellent in SOHIC (Stress Orientied Hydrogen Induced Cracking) characteristics and a method of manufacturing the same. Or it relates to a steel plate that can be usefully used for storage tanks, heat exchangers, reactors, condensers and the like and a method of manufacturing the same.

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 them, the hydrogen organic crack is described as cracking and growth destruction when hydrogen atoms generated by corrosion invade the material from the outside in the environment containing H ₂ S and the hydrogen atoms reach a critical concentration or more.

The principle of hydrogen organic crack (HIC) is as follows. Hydrogen atoms entering the material diffuse within the material and can be trapped in fragile impurities, particularly inclusions such as MnS, segregation zones, inclusions, and so on. When hydrogen atoms are concentrated in these regions, the mechanical properties of the material are reduced by hydrogen embrittlement. The lowered, locally applied stress increases, resulting in a lower maximum stress that the material can withstand. If the stress applied locally is greater than the stress the material can withstand, the cracks grow and fracture progresses.

Such hydrogen organic cracks are further affected by the shape and structure of the impurities. This phenomenon, especially in MnS, segregation zones, etc., tends to cause plastic deformation at sharp edges, and is more likely to crack in tissues having band shapes of different periphery and hardness, such as pearlite bands. The cracks thus generated are propagated parallel to the plate thickness direction (straight cracking), or parallel cracks are connected to the stepwise cracking propagation.

In contrast, sulfide stress corrosion (SOHIC) is a phenomenon that the material under stress is embrittled due to hydrogen embrittlement, which is called SOHIC (Stress Orientied Hydrogen Induced Cracking) in the sense of fracture by hydrogen. In general, this fracture mode is characterized by the formation of microcracks in weak areas such as the interface between impurities and matrix in the material by hydrogen invading into the material, and then these microcracks grow and connect to cause fracture. In other words, when hydrogen is accumulated in impurities such as elongated MnS located inside the material and is locally in a stress state exceeding the critical stress, the crack starts and propagates to the outside, whereby fracture occurs.

This type of fracture phenomenon is closely related to the shape and tensile direction of the impurity. In particular, cracks are formed at the tip of the impurity in a direction parallel to the stress axis, and the fracture occurs at an angle of 45 degrees to the tensile axis.

In order to prevent cracking caused by hydrogen, techniques related to steel materials and manufacturing methods having excellent HIC (Hydrogen Induced Cracking) characteristics have been shown. These techniques have often used controlled rolling and normalizing heat treatment means to obtain steels with good HIC resistance. However, although both HIC and SOHIC are embrittled by hydrogen, there is no correlation between them. That is, even if the steel has excellent HIC resistance, the SOHIC resistance may be low. On the contrary, even if the SOHIC resistance is excellent, the HIC resistance may be bad. This is due to the fact that both are caused by hydrogen but the metallurgical factors that determine their resistance are different.

In particular, as the use of low-grade crude oil containing a large amount of hydrogen sulfide is rapidly increased due to the increase in oil price, it is possible to improve not only hydrogen induced crack (HIC) but also SOHIC resistance of crude oil containing wet hydrogen sulfide (HIC). Is required.

Therefore, the present invention solves the above problems and controls the fraction of the microstructure composed of ferrite and bainite through the recrystallization control rolling and accelerated cooling process, thereby eliminating the existing normalizing heat treatment process, thereby economical SOHIC resistance The excellent tensile strength of 500MPa class vessel steel and its manufacturing method.

The present invention is to solve the above problems, in weight%, C: 0.10 ~ 0.30%, Si: 0.15 ~ 0.40%, Mn: 0.6 ~ 1.2%, Al: 0.001 ~ 0.05%, P: 0.035% or less, S: 0.020% or less, containing Ca: 5-50 ppm,

It further comprises one or two or more components selected from the group consisting of Cr: 0.35% or less, Mo: 0.2% or less, Ni: 0.5% or less, V: 0.05% or less, Nb: 0.05% or less,

The steel sheet for pressure vessel excellent in SOHIC resistance, characterized in that the microstructure consists of a two-phase composite structure, between the Ni, Cr, Mo, V, Nb and S

Cu + Ni + Cr + Mo <1.5% (Cu is 0);

Cr + Mo <0.4%;

V + Nb <0.1%; And

Ca / S> 1.0;

The relationship of can be satisfied, the fraction of the two-phase composite structure provides a pressure vessel steel sheet excellent in SOHIC resistance, characterized in that consisting of 60% to 80% ferrite and 20% to 40% bainite.

Furthermore, the present invention is in weight%, C: 0.10 to 0.30%, Si: 0.15 to 0.40%, Mn: 0.6 to 1.2%, Al: 0.001 to 0.05%, P: 0.035% or less, S: 0.020% or less, Ca: 5 to 50 ppm,

Steel slab further comprising one or more components selected from the group consisting of: Cr: 0.35% or less, Mo: 0.2% or less, Ni: 0.5% or less, V: 0.05% or less, Nb: 0.05% or less about,

Reheating step to reheat at 1050 ~ 1250 ℃;

A recrystallization control rolling step for rolling under a rolling reduction rate of 10% or more and a cumulative rolling amount of 50% or more per each rolling pass in a temperature range section of T _nr to Tnr + 100 ° C., which is a recrystallization temperature defined as follows; And

Starting cooling at 780 ~ 800 ℃ controlled cooling step of cooling to 550 ~ 600 ℃ at a cooling rate of 2.0 ~ 5 ℃ / sec;

It provides a method for producing a steel plate for pressure vessel excellent in resistance to SOHIC comprising a.

_{(Where, T nr (℃) = 887} + 464 * C + 890 * Ti + 363 * Al - 357 * Si + (6445 * Nb - 644 * Nb 1/2) + (732 * V - 230 * V 1 / ² ))

According to the pressure vessel steel according to the present invention, it is possible to provide a pressure vessel steel sheet excellent in tensile strength and excellent in SOHIC resistance even under an H ₂ S (sour) gas atmosphere.

The present invention optimizes the component elements, and recrystallization-controlled rolling and controlled cooling method, the steel internal microstructure is a two-phase composite structure, preferably ferrite-bainite composite structure of 60 ~ 80% ferrite and 20 ~ 40% bainite It is intended to provide a pressure vessel steel to improve the SOHIC resistance and further secure a tensile strength of 500MPa or more.

Hereinafter, the component system constituting the steel for pressure vessel of the present invention will be described in detail.

C: 0.10 ~ 0.30%

C is an element that can improve the strength, when the content of C is less than 0.10%, its own strength on the matrix is lowered, whereas if it is more than 0.30%, segregation may occur and the hydrogen organic cracking resistance may be lowered. .

Si: 0.15-0.40%

Si functions as a deoxidizer and a solid solution strengthening element, and is a component added especially for the impact transition temperature. In order to achieve this addition effect of Si, 0.15% or more should be added. However, if the Si content exceeds 0.40%, the weldability is degraded and the oxide film is severely formed on the surface of the steel sheet, so the content is limited to 0.15 to 0.40%.

Mn: 0.6-1.2%

Mn is controlled to 1.2% or less because MnS, which is a nonmetallic inclusion drawn together with S, lowers the normal temperature elongation and low temperature toughness. However, since Mn is less than 0.6% due to the component properties of the present invention, it is difficult to secure the strength, so the amount of Mn added in the present invention is limited to 0.6 to 1.2%.

Al: 0.001-0.05%

Al, together with Si, is one of the strong deoxidizers in the steelmaking process and therefore adds more than 0.001%. If the amount is less than 0.001%, the effect is insignificant. On the other hand, if the amount is more than 0.05%, the manufacturing cost increases during steel sheet manufacturing, so it is limited to 0.001 to 0.05%.

P: 0.035% or less

P is preferably lowered as an element deteriorating low-temperature toughness, but the process of removing P in the steelmaking process can be excessively expensive, so it is managed within a range of 0.035% or less.

S: 0.020% or less

S is also an impurity element that adversely affects low-temperature toughness as well as P, but it is necessary to lower the content, but it is managed in the range of 0.020% or less since it may be excessively expensive to remove S in the steelmaking process.

Cr: 0.35% or less

Cr can be added because it functions as an alloying element that can increase the strength. However, the excessive addition of 0.35% or more leads to an increase in the manufacturing cost during steel sheet manufacturing, so when added in the present invention, it is limited to within 0.35%.

Mo: 0.2% or less

Mo is also known as an element that is effective in increasing strength, such as Cr, and has a function of preventing cracks due to sulfides, and thus may be added to the steel of the present invention. However, Mo is an expensive element and can be added in the range of 0.2% or less.

Ni: 0.5% or less

Ni functions as an element very effective for improving low temperature toughness. However, since Ni is expensive, it can be added within 0.5% or less in view of the manufacturing cost of the product.

V: 0.05% or less

V has a precipitation strengthening function and can function as an element effective for increasing strength, such as Cr and Mo. However, since V is also an expensive element, the addition of a large amount can increase the unit cost of the product, so it can be added within 0.05%.

Nb: 0.05% or less

Nb is dissolved in austenite to increase the hardenability of austenite, and precipitates as carbonitrides (Nb (C, N)) that match with the matrix (Matrix) to function as an element to increase the strength of the steel. However, if it is contained in an excessively large amount, coarse precipitates can be formed in the process of playing, which may cause hydrogen organic cracking. Therefore, the content of Nb is limited to 0.05% or less.

Ca: 5-50 ppm

Ca is added by 5 ppm or more because it is produced by CaS and can suppress non-metallic inclusions of MnS. However, if the addition amount is excessively excessive, the upper limit is limited to 50 ppm because it reacts with O contained in the steel to generate CaO, which is a non-metallic inclusion, thereby lowering the physical properties of the steel sheet.

Cu + Ni + Cr + Mo <1.5%, Cr + Mo <0.4%, V + Nb <0.1%

Addition of each of these components is limited in the basic standard (ASTM A20) of pressure vessel steel, Cu + Ni + Cr + Mo content of less than 1.5%, Cr + Mo content of less than 0.4% and V + Nb content It is limited to within 0.1%. However, Cu is not added in the present invention, so Cu is set to zero.

Ca / S> 1.0

Furthermore, Ca / S ratio corresponds to the essential component ratio which spheroidizes MnS inclusion and improves hydrogen organic crack resistance. If the Ca / S ratio is 1.0 or less, the effect is difficult to expect, so the ratio is controlled to be greater than 1.0.

Hereinafter, the microstructure constituting the steel for pressure vessel of the present invention will be described in detail.

The steel for pressure vessels of the present invention is composed of a two-phase composite structure in which the microstructure of the steel having the composition as described above is controlled by rolling and controlled cooling with ferrite and bainite as the center. According to the experiments of the present inventors, in order to improve the SOHIC resistance, it is most ideal to construct a single phase tissue, but this is practically limited and its physical properties and uses are limited. Therefore, it was concluded that a structure capable of securing strength while minimizing hardness difference is required by uniformly distributing two-phase tissue having a relatively small hardness difference.

As a result of repeated experiments, a composite of ferrite and bainite is preferable as a two-phase tissue having a small hardness difference that can improve SOHIC resistance, and is most suitable when optimizing the fraction of these two-phase tissues. The results show improved SOHIC resistance.

That is, the steel for pressure vessel is composed of a microstructure of the composite structure of ferrite and bainite, the ferrite constituting the composite structure has a fraction of 60 ~ 80%, bainite having a fraction of 20 ~ 40%. do. The fraction of the two-phase composite structure is very important in the present invention, when the ferrite exceeds 80%, it is difficult to obtain a tensile strength of 500MPa grade, on the contrary, when the bainite exceeds 40%, there are too many bainite with high hardness. As a result, the hardness difference between the two phases is sharply increased so that the SOHIC characteristics are deteriorated. By such a tissue control, unlike the existing ferrite + perlite steel or ferrite + martensite steel, it is possible to construct a microstructure excellent in SOHIC properties.

Hereinafter, a manufacturing method for manufacturing the steel for pressure vessel of the present invention will be described in detail.

The pressure vessel steel of the present invention has a ferrite + bainite two-phase composite structure having excellent SOHIC characteristics as a microstructure, and for this, an appropriate controlled rolling method and a heat treatment method for securing an appropriate tensile strength of 500 MPa after controlled cooling are required. .

First, the steel ingot having the above-mentioned composition is reheated at 1050 to 1250 ° C. If the reheating temperature is lower than 1050 ℃ during reheating, solute of the solute atoms becomes difficult, while if the heating temperature exceeds 1250 ℃ austenite grain size is too coarse to damage the properties of the steel sheet.

In the present invention, the recrystallization control rolling means hot rolling at a temperature higher than the uncrystallized temperature, and the recrystallized temperature T _nr can be calculated by the following Equation 1. The unit of each alloy element in the formula represents the weight percentage.

_{T nr (℃) = 887 +} 464 * C + 890 * Ti + 363 * Al - 357 * Si + (6445 * Nb - 644 * Nb 1/2) + (732 * V - 230 * V 1/2)

In order to obtain a fine structure, recrystallized control rolling is required to give a cumulative reduction of 50% or more by applying a reduction ratio of 10% or more for each rolling pass in a temperature range of T _nr to T _nr + 100 ° C.

The ferrite-bay suitable for the pressure vessel steel according to the present invention by cooling to a cooling stop temperature of 550 ~ 600 ℃ after cooling at a cooling rate of 2.0 ~ 5 ℃ / sec from the cooling start temperature 780 ~ 800 ℃ after hot rolling A two-phase complex of knights can be obtained.

The present invention will be described in more detail with reference to the following examples.

(Example)

Steel slabs of the inventive material having the chemical composition shown in Table 1 were subjected to controlled rolling and controlled cooling under the conditions shown in Table 2, and then the yield strength, low temperature toughness and SOHIC resistance of the prepared specimens were examined.

C Mn Si P S Ni Cr Mo V Nb Ca Invention a 0.17 1.10 0.30 0.010 0.0015 0.20 0.05 0.12 0.005 0.015 0.0020 b 0.18 1.05 0.25 0.080 0.0012 0.15 0.10 0.10 0.010 0.014 0.0025 Comparative material c 0.17 1.05 0.28 0.010 0.0017 0.18 0.15 0.08 0.010 0.010 0.0025 d 0.14 1.15 0.29 0.012 0.0014 0.15 0.20 0.15 0.009 0.012 0.0023

division Steel plate thickness (mm) Slab heating temperature (℃) Recrystallization Control Rolling Accumulation Load (%) Cooling start temperature (℃) Cooling stop temperature (℃) Cooling rate (℃ / sec) Foot a 13 1200 60 785 590 2.1 25 1180 75 790 590 3.0 50 1120 55 790 580 3.5 50 1120 70 795 570 4.0 b 70 1100 80 795 560 3.5 75 1100 85 795 565 3.2 80 1100 60 795 555 3.0 Comparative material c 20 1200 20 Normalizing (900 ℃) 25 1150 30 Normalizing (900 ℃) d 50 1100 40 Normalizing (900 ℃)

In Table 1 and Table 2, the low temperature toughness is evaluated by the Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a V notch at -50 ° C.

In addition, the SOHIC resistance was evaluated as the time at which fracture occurred after applying a constant stress to the steel in a solution saturated with H ₂ S. In this case, steels which generally do not break for 720 hours were evaluated to have excellent SOHIC resistance. Specifically, the weight (yield stress X 0.9 X tensile specimen cross-sectional area) corresponding to 90% of the yield stress of each steel material in saturated solution of H ₂ S was applied to each specimen by lever and weight. SOHIC resistance was evaluated by walking and observing the fracture of steel over time.

The results of this example experimented as described above are shown in Table 3 below.

division Ferrite fraction (%) Bainite fraction (%) YS (Mpa) TS (Mpa) -50 ℃ Impact Toughness (J) Break time (hr) Invention a 70 30 385 560 175 X 68 32 395 570 186 X 72 28 375 545 210 X 69 31 365 578 211 X b 76 24 375 550 201 X 78 22 380 545 198 X 75 25 365 538 184 X Comparative material c  Ferrite + Perlite 370 539 186 120 365 530 175 97 d 358 520 190 218

x: Indicates no fracture for 720 hours

As shown in Table 3, the yield strength, the tensile strength, and the low temperature toughness showed almost the same level of the invention and the comparative material, but the SOHIC resistance under the H ₂ S (sour) gas atmosphere was the component type and the two-phase composite structure of the present invention. It can be seen that the constituted invention is much superior.

As in the above embodiment, it is determined that the invention material is superior to the comparative material in particular in the SOHIC resistance due to the microstructure control of the steel material composed of the two-phase composite structure composed of ferrite and bainite.

Claims (5)

In weight%, C: 0.10 to 0.30%, Si: 0.15 to 0.40%, Mn: 0.6 to 1.2%, Al: 0.001 to 0.05%, P: 0.035% or less, S: 0.020% or less, Ca: 5 to 50 ppm Include, Cr: 0.35% or less (excluding 0), Mo: 0.2% or less (excluding 0), Ni: 0.5% or less (excluding 0), V: 0.05% or less (excluding 0), Nb: 0.05% It further comprises one or two or more components selected from the group consisting of the following (excluding 0), the microstructure of the two-phase composite structure of 60% to 80% ferrite and 20 to 40% bainite in volume% Steel plate for pressure vessel excellent in SOHIC resistance, characterized in that made. The method of claim 1, wherein the Ni, Cr, Mo, V, Nb and S between Cu + Ni + Cr + Mo <1.5%, except Cu is 0; Cr + Mo <0.4%; V + Nb <0.1%; And Ca / S> 1.0; Steel plate for pressure vessel excellent in resistance to SOHIC, characterized in that to satisfy the relationship of. delete In weight%, C: 0.10 to 0.30%, Si: 0.15 to 0.40%, Mn: 0.6 to 1.2%, Al: 0.001 to 0.05%, P: 0.035% or less, S: 0.020% or less, Ca: 5 to 50 ppm Include, Cr: 0.35% or less (excluding 0), Mo: 0.2% or less (excluding 0), Ni: 0.5% or less (excluding 0), V: 0.05% or less (excluding 0), Nb: 0.05% For steel slabs further comprising one or two or more components selected from the group consisting of the following (excluding 0): Reheating step to reheat at 1050 ~ 1250 ℃; A recrystallization control rolling step for rolling under a rolling reduction rate of 10% or more and a cumulative rolling amount of 50% or more per each rolling pass in a temperature range section of T _nr to Tnr + 100 ° C., which is a recrystallization temperature defined as follows; And Starting cooling at 780 ~ 800 ℃ controlled cooling step of cooling to 550 ~ 600 ℃ at a cooling rate of 2.0 ~ 5 ℃ / sec; Method of producing a steel plate for pressure vessels excellent in resistance to SOHIC comprising a. T _nr (℃) = 887 + 464 * C + 890 * Ti + 363 * Al-357 * Si + (6445 * Nb-644 * Nb ^1/2 ) + (732 * V-230 * V ^1/2 ) The method of claim 4, wherein the Ni, Cr, Mo, V, Nb and S between Cu + Ni + Cr + Mo <1.5%, except Cu is 0; Cr + Mo <0.4%; V + Nb <0.1%; And Ca / S> 1.0; Method of producing a steel plate for pressure vessels excellent in resistance to SOHIC, characterized in that to satisfy the relationship of.