KR100833070B1 - Steel plate for pressure vessel with ts 500mpa grade and excellent hic resistance and manufacturing method thereof - Google Patents

Abstract

A steel plate for pressure vessels and a manufacturing method thereof are provided, wherein the steel plate for pressure vessels satisfies tensile strength of 500MPa grade, can be stably used even in an atmosphere of H2S gas(sour gas), and has excellent HIS(Hydrogen Induced Crack) resistance. A manufacturing method of a steel plate with 500MPa grade tensile strength and excellent hydrogen induced crack resistance comprises: a reheating step of reheating a steel plate in a temperature range of 1050 to 1250 deg.C such that microstructure of the steel plate has 0.25 or less of a banding index measured according to ASTM E-1268, the steel plate comprising, by weight percent, 0.1 to 0.30% of C, 0.15 to 0.40% of Si, 0.6 to 1.2% of Mn, 0.035% or less of P, 0.020% or less of S, 0.001 to 0.050% of Al, 0.35% or less of Cr, 0.5% or less of Ni, 0.5% or less of Cu, 0.2% or less of Mo, 0.05% or less of V, 0.05% or less of Nb, 0.0005 to 0.005% of Ca, and the balance of Fe and inevitable impurities, and satisfying component restriction formulas, (1) Cu+Ni+Cr+Mo1.0; a recrystallization controlled rolling step of hot-rolling the reheated steel plate at a temperature of at least the non-recrystallization temperature(Tnr) defined by Tnr(deg.C)=887+464xC+890xTi+363xAl-357xSi+(6445xNb-644xNb^0.5)+(732xV-230xV^0.5), where alloy elements have a unit of wt.%; and a normalizing step of heat-treating the hot-rolled steel plate at 850 to 950 deg.C for 1.3xt+(10 to 30 minutes), where t denotes thickness(mm) of the steel plate. The manufacturing method further comprises a PWHT(Post Welding Heat Treatment) step of heat-treating the normalized steel plate in a temperature range of 600 to 640 deg.C.

Description

Steel Plate For Pressure Vessel With TS 500MPa Grade and Excellent HIC Resistance And Manufacturing Method Thereof}

The present invention relates to a thick steel plate used in a crude oil refinery or storage tank, a heat exchanger, a reactor, a condenser, and the like, and a method of manufacturing the same, in particular, a hydrogen steel cracking resistant high-strength cracked steel plate and its manufacture. It is about a method.

With the recent trend of shortage of oil and high oil prices, the development of oilfields in harsh environments has been actively developed, and thus the resistance to hydrogen induced crack (HIC) of steel for refining and storing crude oil containing wet hydrogen sulfide. It is required to raise.

The mechanism for generating hydrogen organic cracks is that hydrogen generated by the corrosion reaction between steel and wet hydrogen sulfide atmosphere invades the steel in an atomic state, diffuses, and is molecularly formed in inclusions in the steel to generate hydrogen gas pressure. It is known that cracks appear in tissues due to hydrogen gas pressure.

Conventional techniques for improving resistance to hydrogen organic cracking (hydrogen organic cracking resistance) include 1) addition of Cu and / or Co, 2) reduction and shape control of impurities and inclusions, and 3) fine dispersion of carbonitride. Therefore, there are techniques for improving the hydrogen-organic crack resistance by techniques for suppressing the intrusion or diffusion of hydrogen.

However, the prior art 1) has a problem of an increase in manufacturing cost due to the addition of expensive elements, 2) has a problem of causing a load of steelmaking operations, and 3) also the microstructure of the steel is ferrite and pearlite structure There is a problem in that there is a limit to the control of such an organization.

The present invention solves the problems of the prior art and at the same time to produce a steel vessel for pressure vessels with excellent resistance to hydrogen-organic cracks, the banding index of the microstructure consisting of ferrite and perlite through recrystallization control rolling (ASTM E-1268) It is an object of the present invention to provide a tensile strength 500MPa class steel for excellent hydrogen organic cracking resistance and control method by controlling to 0.25 or less).

In the present invention, by weight%, C: 0.1-0.30%, Si: 0.15-0.40%, Mn: 0.6-1.2%, P: 0.035% or less, S: 0.020% or less, Al: 0.001-0.05%, Cr: 0.35 % Or less, Ni: 0.5% or less, Cu: 0.5% or less, Mo: 0.2% or less, V: 0.05% or less, Nb: 0.05% or less, Ca: 0.0005 to 0.005%, and the remainder are composed of inevitable impurities and Fe;

Ingredients

(1) Cu + Ni + Cr + Mo <1.5%,

(2) Cr + Mo <0.4%

(3) V + Nb <0.1%

(4) Ca / S> 1.0

Satisfy;

It relates to a tensile strength 500MPa grade steel sheet having excellent hydrogen organic cracking resistance, characterized in that the banding index (measured according to ASTM E-1268) of the microstructure is 0.25 or less.

Furthermore, the present invention provides a steel sheet composed of the above composition,

A reheating step of reheating at 1050-1250 ° C .;

A recrystallization controlled rolling step of hot rolling at an unrecrystallized _zone temperature (T _nr ) or more defined by the following formula (1); And

(However, the unit of the alloy element is weight%)

A normalizing step of heat treatment at 850 to 950 ° C. under conditions of 1.3 * t + (10 to 30 minutes) (where t denotes the thickness of the steel in mm);

It relates to a method for producing a tensile strength 500MPa grade steel sheet having excellent hydrogen-organic crack resistance.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

(Lecture composition and organization)

The reason for limiting the chemical composition range of the steel slab in the present invention is as follows.

It is preferable to limit C to 0.10 to 0.30%. If the content of C is less than 0.10%, the strength of the matrix on the matrix is lowered. If the content of C is more than 0.30%, segregation may occur in the tissue to reduce hydrogen organic crack resistance.

Si is a component added for the deoxidation effect, the solid solution strengthening effect, and the impact transition temperature raising effect, and in order to achieve such an addition effect, it is preferable to add 0.15% or more. However, if the content exceeds 0.40%, the weldability is lowered and the oxide film is severely formed on the surface of the steel sheet, so the content thereof is limited to 0.15 to 0.40%.

Since Mn forms MnS, which is a non-metallic inclusion drawn together with S, lowers the normal temperature elongation and low temperature toughness, Mn is preferably managed at 1.2% or less. However, when Mn is less than 0.6% due to the component properties of the present invention, it is difficult to secure appropriate strength, so the amount of Mn added is limited to 0.6 to 1.2%.

Al, together with Si, is one of the strong deoxidizers in the steelmaking process, and its effect is insignificant at less than 0.001%, and the production cost increases when added in excess of 0.05%, so it is limited to 0.001 to 0.05%.

P is managed at less than 0.035% because excessive cost is required to remove it from elements that harm low temperature toughness or steelmaking process.

S is also an element that adversely affects low temperature toughness along with P, but like P, it may be excessively expensive to remove in the steelmaking process, so it is appropriate to manage it within 0.020% or less.

Cr is an alloying element that can increase the strength, but since it is an expensive element, when it is added in excess of 0.35%, it is preferable to limit it to within 0.35% because it increases the manufacturing cost.

Mo is also known as an element that is effective in increasing the strength, such as Cr, while preventing the occurrence of cracks by sulfides. However, Mo is also an expensive element and is preferably added in the range of 0.2% or less.

Although Ni is the most effective element for improving low temperature toughness, it is expensive, and therefore Ni is preferably added within 0.5% or less.

V is an element effective for increasing the strength, such as Cr and Mo, but is preferably added within 0.05% due to its high cost.

Nb is an important element that increases the strength of steel by solidifying the austenite to increase the hardenability of austenite and to precipitate as carbonitrides (Nb (C, N)) matching with matrix (Matrix). However, when added in a large amount may appear as a coarse precipitate in the process of playing the role of the site of hydrogen organic cracks, its content is preferably limited to 0.05% or less.

Ca is produced by CaS is added to suppress the non-metallic inclusions of MnS, for this purpose it is good to add more than 5ppm. However, if the added amount is large, the upper limit thereof is 50 ppm because it reacts with O contained in the steel to generate CaO, which is a non-metallic inclusion.

The reason for limiting the addition amount of Cu + Ni + Cr + Mo, Cr + Mo, V + Nb is limited by the basic standard of steel for pressure vessel (ASTM A20), so the content of Cu + Ni + Cr + Mo is within 1.5%. , Cr + Mo content is limited to within 0.4%, and V + Nb content is within 0.1%.

And Ca / S ratio is an essential component ratio to improve the hydrogen organic crack resistance by spheroidizing MnS inclusions, the effect is less than 1.0, so the ratio is adjusted to exceed 1.0.

When the steel having the above composition is normalized, the microstructure has a ferrite + pearlite two-phase composite structure. In this case, the composite tissue should be treated so that the banding index (measured according to ASTM E-1268) can be composed of microstructures of 0.25 or less. If the banding index value exceeds 0.25, special attention should be paid because the hydrogen organic cracking resistance in the microstructure rapidly decreases.

(Manufacturing method)

Hereinafter, the manufacturing method of this invention is demonstrated.

The microstructure of the present invention steel composed of the above composition has a ferrite + perlite two-phase composite structure, in order to adjust the Banding Index (measured according to ASTM E-1268) to 0.25 or less, hot rolling method (recrystallization controlled rolling) and After PWHT (Post Weld Heat Treatment), it is necessary to perform heat treatment to secure 500MPa of proper tensile strength.

In this case, the steel composed of the composition is first reheated at 1050 to 1250 ° C., because the reheating temperature is lower than 1050 ° C., solute of solid solutes is difficult, and when the heating temperature exceeds 1250 ° C., the austenite grain size is increased. This is because it becomes too coarse and damages the properties of the steel sheet.

After the reheating treatment, the steel of the present invention has a ferrite + pearlite two-phase composite structure, which is an essential microstructure for having excellent hydrogen organic crack resistance, and a banding index (measured according to ASTM E-1268) to obtain 0.25 or less. Recrystallization controlled rolling, normalizing heat treatment and PWHT heat treatment are required.

The recrystallized controlled rolling means hot rolling at a temperature higher than the _{uncrystallized crystal} . The recrystallized temperature T _nr can be easily calculated through the following well-known Equation (1). However, in the formula, the unit of each alloy element represents weight%.

Recrystallized controlled rolling is the most important variable for the banding index (measured according to ASTM E-1268) to be less than 0.25, and recrystallized controlled rolling is 10% for each rolling pass in the temperature range of T _nr to T _nr + 100 ° C. It is essential to add more than 50% of the cumulative reduction amount by applying the above reduction ratio. If the cumulative reduction is less than 50%, a banding index of 0.25 or less cannot be expected.

Normalized heat treatment and PWHT on the hot rolled and cooled steel sheet by the above method are required to ensure a tensile strength of 500 MPa and -50 ° C impact toughness of 50 Joules or more. Heat treatment conditions for achieving such conditions are as follows. same.

The normalizing heat treatment is carried out at 850 ~ 950 ℃ condition of 1.3 * t + (10 ~ 30 minutes) (where t means the thickness of the steel (mm)). When the temperature of the normalizing heat treatment is lower than 850 ° C, it is difficult to re-use solid solution solute elements, and thus, it is difficult to secure the strength. On the other hand, when the normalizing temperature is higher than 950 ° C, grains grow to damage low-temperature toughness. In addition, the reason for the limitation of the heat treatment time is that the homogenization of the tissue is difficult when less than the reference time, and the productivity is deteriorated at the holding time longer.

After manufacturing the pressure vessel by welding the steel which has been normalized, PWHT treatment is required. It is preferable to perform temperature conditions of PWHT at 600-640 degreeC. This is because when the PWHT temperature is lower than 600 ° C, residual stress removal such as welds is not smooth, and the strength of the steel is lowered at a temperature higher than that. The PWHT time condition is preferably for 3 hours per inch thickness, because less than the reference time is difficult to remove residual stress and longer holding time is detrimental to productivity.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

Table 1 shows the chemical components of the inventive steel and the comparative steel, respectively. The steel slab of the inventive material having the chemical composition as shown in Table 1 is heated at an appropriate temperature range, and the recrystallization controlled rolling is performed 55-80% in the recrystallization zone to perform rolling controlling the banding index to 0.25 or less. In addition, normalization and PWHT were performed under the conditions shown in Table 2 below, and then the yield strength, low temperature toughness, and crack length ratio (%) were examined, and the results are shown in Table 2 below.

However, low-temperature toughness was evaluated by the Charpy impact energy value obtained by Charpy impact test of the specimen having a V notch at -50 ° C. In Table 2, the crack length ratio (%) was determined according to the NACE TM0277 standard. It is measured.

C Mn Si P S Cu Ni Cr Mo V Nb Ca Invention a 0.17 1.10 0.30 0.010 0.0015 0.05 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 0.20 0.18 0.15 0.08 0.010 0.010 0.0025 d 0.14 1.15 0.29 0.012 0.0014 0.18 0.15 0.20 0.15 0.009 0.012 0.0023

division Steel plate thickness (mm) Slab heating temperature (℃) Recrystallization Control Rolling Accumulated Loading Capacity (%) Normalizing Temperature (℃) Normalizing time (minutes) PWHT temperature (℃) PWHT hours (hr) Banding Index YS (Mpa) TS (Mpa) -50 ℃ impact toughness (J) CLR (%)   Invention  a 13 1200 60 890 50 620 3 0.20 380  545 203 0.1 25 1180 75 900 60 620 3 0.12 375  540 197 0.0 50 1120 55 890 80 610 6 0.18 360  535 213 0.0 50 1120 70 900 85 610 6 0.13 359  537 186 0.0  b 70 1100 80 910 110 610 9 0.12 355  532 173 0.0 75 1100 85 910 120 610 9 0.11 354  530 180 0.0 80 1100 60 890 120 610 12 0.13 350  528 175 0.0 Comparative material c 20 1200 20 900 60 620 3 0.37 370  539 186 55 25 1150 30 900 50 620 3 0.30 365  530 175 60 d 50 1100 40 900 80 620 6 0.35 358  520 190 35

As shown in Table 2, the yield strength, tensile strength, and low temperature toughness of the invention and the comparative material showed almost the same level, but the CLR (Crack Length Ratio,%) indicating the resistance under H ₂ S (sour gas) gas atmosphere is It can be seen that the invention material is excellent.

As described above, it can be seen from the present embodiment that the invention is excellent in the CLR because the banded index indicating the homogenization degree of the microstructure composed of ferrite + pearlite is controlled to be 0.25 or less.

According to the present invention, while satisfying the tensile strength of 500MPa class, it can be provided a thick steel plate for pressure vessels excellent in hydrogen-organic crack resistance that can be used stably even in H ₂ S (sour gas) gas atmosphere.

Claims (5)

delete By weight%, C: 0.1-0.30%, Si: 0.15-0.40%, Mn: 0.6-1.2%, P: 0.035% or less, S: 0.020% or less, Al: 0.001-0.05%, Cr: 0.35% or less, Ni: 0.5% or less, Cu: 0.5% or less, Mo: 0.2% or less, V: 0.05% or less, Nb: 0.05% or less, Ca: 0.0005 to 0.005, and the remainder are composed of inevitable impurities and Fe; Ingredients (1) Cu + Ni + Cr + Mo <1.5%, (2) Cr + Mo <0.4% (3) V + Nb <0.1% (4) Ca / S> 1.0 The steel sheet satisfies the following requirements so that the banding index (measured according to ASTM E-1268) of the microstructure is 0.25 or less. Reheating step reheating at 1050 ~ 1250 ℃; A recrystallization controlled rolling step of hot rolling at an unrecrystallized _zone temperature (T _nr ) or more defined by the following formula (1); And

(However, the unit of the alloy element is weight%) Normalizing step of heat treatment under conditions of 1.3 × t + (10 ~ 30 minutes) (t means thickness of steel (mm)) at 850 ~ 950 ℃ Method for producing a tensile strength 500MPa grade steel sheet having excellent hydrogen organic crack resistance, characterized in that it comprises a. The method of claim 2, wherein the steel sheet manufacturing method further comprises a post-weld heat treatment (PWHT) step of heat-treating at a temperature of 600 ~ 640 ℃ hydrogen nanocrack resistance excellent tensile strength production of 500MPa grade steel sheet Way. According to claim 2, The recrystallized controlled rolling is hydrogen organic, characterized in that the cumulative reduction of 50% or more by applying a reduction ratio of 10% or more per each rolling pass in the temperature range of T _nr ~ T _nr +100 ℃ Manufacturing method of tensile strength 500MPa grade steel sheet with excellent crack resistance. delete