KR101482341B1 - Pressure vessel steel plate having excellent resustance property after post weld heat treatment and manufacturing method of the same - Google Patents

Abstract

An object of the present invention is to provide a steel sheet excellent in resistance to post-weld heat treatment (PWHT) that does not deteriorate strength and toughness even in a severe environment of post-sintering of a steel material and a severe welding condition.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for pressure vessels,

The present invention relates to a steel sheet for a pressure vessel excellent in resistance to heat treatment after welding, and a method of manufacturing the same.

In recent years, due to the scarcity of petroleum and the trend of active development of harsh environment oilfields in response to the era of high oil prices, the refining and storage steel of crude oil are being refined.

In order to prevent the deformation of the post-weld structure and to stabilize the shape and the dimension in the case of welding the steel material in addition to the post-welding of the steel material as described above, post-welding heat treatment (PWHT, Post Weld Heat Treatment). However, the steel sheet subjected to the PWHT process for a long time has a problem that the tensile strength of the steel sheet is lowered due to the coarsening of the structure.

That is, after the PWHT for a long time, strength and toughness are simultaneously lowered due to softening of matrix and grain boundaries, grain growth, coarsening of carbide, and the like.

As a means for preventing the deterioration of physical properties due to the long-time PWHT heat treatment, Patent Document 1 discloses a steel containing 0.05 to 0.20% of C, 0.02 to 0.5% of Si, 0.2 to 2.0% of Mn, 0.005 to 0.10% , A slab containing at least one of Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca and rare earth elements and at least one of iron and unavoidable impurities is heated and hot rolled After air cooling at room temperature, heating at the Ac1 to Ac3 transformation point and gradual cooling, the PWHT assurance time was made possible up to 16 hours.

However, the PWHT guarantee time shown in the above-mentioned technique is very insufficient when the post-welding and welding conditions are severe, and it is impossible to apply the PWHT for a longer period of time.

Therefore, there is a demand for a steel material having high resistance to PWHT which does not deteriorate in strength and toughness even after PWHT for a long time, accompanied by post-sintering of the steel material and severe welding conditions.

Japanese Publication No. 1997-256037

An object of the present invention is to provide a steel sheet excellent in resistance to post-weld heat treatment (PWHT) that does not deteriorate strength and toughness even in a severe environment of post-sintering of steel materials and welding conditions.

A steel sheet for a pressure vessel excellent in post-weld heat treatment resistance, which is one aspect of the present invention, comprises 0.05 to 0.15% of C, 0.15 to 0.5% of Si, 1.0 to 1.6% of Mn, 0.001 to 0.03% of Ti, 0.001 to 0.05% of Ti, 0.001 to 0.03% of Al, 0.0015 to 0.05% of Al, 0.01 to 0.25% of Cr, 0.005 to 0.3% of Mo, 0.005 to 0.06% of V, And the balance Fe and inevitable impurities, and the tempered martensite containing 0.6% Cu, 0.01-0.35% Cu + Ni + Cr + Mo 1.0% or less, Cr + Mo 0.4% or less, V + And has a microstructure containing a site as a column.

A method for producing a steel sheet for a pressure vessel excellent in post-weld heat treatment resistance, which is another aspect of the present invention, comprises 0.05 to 0.15% of C, 0.15 to 0.5% of Si, 1.0 to 1.6% of Mn, 0.03% 0.005 to 0.06% of Mo, 0.005 to 0.06% of Nb, 0.005 to 0.03% of Nb, 0.001 to 0.05% of Ti, 0.005 to 0.06% of Cr, 0.01 to 0.25% of Cr, Ni: 0.05 to 0.6%, Cu: 0.01 to 0.35%, Cu + Ni + Cr + Mo: not more than 1.0%, Cr + Mo: not more than 0.4%, V + Nb: not more than 0.1% Heating the steel slab to a temperature range of 1050 to 1250 占 폚, hot rolling the hot slab to finish rolling at 800 to 900 占 폚, heating the hot-rolled steel sheet to 1 / 4T (T: Thickness) at a cooling rate of 3 to 60 DEG C / sec, and tempering the cooled steel sheet for 10 to 30 minutes.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, it is possible to provide a steel sheet for a pressure vessel excellent in heat treatment resistance that does not deteriorate the strength and toughness of PWHT even after a long period of time while having a strength of 600 MPa or more.

The inventors of the present invention conducted research to derive a steel sheet excellent in post-weld heat treatment resistance. As a result, it was found that by controlling the microstructure of the steel sheet to a mixed structure of tempered martensite by appropriately controlling the composition system and the manufacturing conditions of the steel sheet, And a steel sheet with high resistance to PWHT which does not deteriorate in strength and toughness even after prolonged PWHT can be produced, leading to the present invention.

Hereinafter, a steel sheet for a pressure vessel excellent in post-weld heat treatment resistance, which is one aspect of the present invention, will be described in detail.

A steel sheet for a pressure vessel excellent in post-weld heat treatment resistance, which is one aspect of the present invention, comprises 0.05 to 0.15% of C, 0.15 to 0.5% of Si, 1.0 to 1.6% of Mn, 0.001 to 0.03% of Ti, 0.001 to 0.05% of Ti, 0.001 to 0.03% of Al, 0.0015 to 0.05% of Al, 0.01 to 0.25% of Cr, 0.005 to 0.3% of Mo, 0.005 to 0.06% of V, And the balance Fe and inevitable impurities, and the tempered martensite containing 0.6% Cu, 0.01-0.35% Cu + Ni + Cr + Mo 1.0% or less, Cr + Mo 0.4% or less, V + And has a microstructure containing a site as a column.

Carbon (C): 0.05 to 0.15 wt%

Carbon is the most effective element for strengthening the steel, but it is an element that reduces the weldability and low-temperature toughness when added in large amounts. When the content of C is less than 0.05% by weight, the strength of the matrix itself decreases. On the other hand, when the content of C exceeds 0.15% by weight, the weldability and low-temperature toughness deteriorate, which is not preferable. Therefore, it is preferable that the carbon is contained in an amount of 0.05 to 0.15% by weight.

Silicon (Si): 0.15-0.5 wt%

Silicon is used as a deoxidizing agent and is an element added for strength enhancement by solid solution strengthening and effect of increasing impact transition temperature. In order to exhibit such an effect in the present invention, it is preferable that 0.15% by weight or more is contained. However, when the content of silicon exceeds 0.5% by weight, the weldability is deteriorated, and there is a problem that an oxide film is formed on the surface of the steel sheet. Accordingly, it is preferable that the silicon is contained in an amount of 0.15 to 0.5% by weight.

Manganese (Mn): 1.0 to 1.6 wt%

Manganese is an effective element in enhancing the employment of steel. Therefore, it is preferable to add 1.0 wt% or more to ensure adequate strength. However, when a large amount is added, MnS, which is a non-metallic inclusion drawn with sulfur (S), is formed to lower the room temperature elongation and low temperature toughness. Accordingly, it is preferable that the manganese is contained in an amount of 1.0 to 1.6% by weight.

Aluminum (Al): 0.015 to 0.05% by weight,

Aluminum is added as a deoxidizer together with Si during steelmaking, and has an effect of strengthening the solid solution. If the content of aluminum is less than 0.015 wt%, the deoxidation effect to be intended in the present invention can not be secured. On the other hand, if it exceeds 0.05% by weight, the effect of deoxidation is saturated and the production cost increases. Accordingly, it is preferable that the aluminum is contained in an amount of 0.015 to 0.05% by weight.

Cr (Cr): 0.01 to 0.25 wt%

Chromium is an element that increases strength. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.01 wt% or more. However, when chromium, which is an expensive element, is added in an amount exceeding 0.25 wt%, the manufacturing cost is increased. Therefore, it is preferable that the chromium is contained in an amount of 0.01 to 0.25% by weight.

Molybdenum (Mo): 0.005 to 0.3 wt%

Molybdenum, like Cr, is an element that strengthens the strength of the material. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.005% by weight or more. However, when molybdenum, which is an expensive element, is added in an amount exceeding 0.3% by weight, the manufacturing cost increases. Accordingly, it is preferable that the molybdenum is included in an amount of 0.005 to 0.3% by weight.

Vanadium (V): 0.005 to 0.06 wt%

Vanadium is an effective element for increasing the strength such as Cr and Mo. In order to exhibit such an effect in the present invention, it is preferable that it is contained in an amount of 0.005% by weight or more. However, when vanadium, which is an expensive element, is added in an amount exceeding 0.06 wt%, the manufacturing cost is increased. Accordingly, it is preferable that the vanadium is contained in an amount of 0.005 to 0.06% by weight.

Niobium (Nb): 0.005 to 0.03 wt%

Niobium is an effective element for finely austenitic grain refinement during hot rolling, and it is an important element for increasing strength by precipitating into carbonitride (Nb (C, N)) which matches the matrix. It is preferable that the content is 0.005% by weight or more in order to exhibit the intended effect of the present invention. As the content of niobium increases, it may appear as a coarse precipitate in the performance process, resulting in deterioration of toughness. Therefore, the upper limit is preferably limited to 0.03 wt%. Accordingly, it is preferable that the niobium is contained in an amount of 0.005 to 0.03% by weight.

Titanium (Ti): 0.001 to 0.05 wt%

Titanium plays an important role in improving the toughness of steel by forming carbonitride (Ti (C, N)) like Nb and refining os- kenite crystal grain growth during heating and hot rolling of the slab, . In order to exhibit such an effect, it is preferable to contain 0.001 wt% or more. However, as the content of titanium increases, the toughness may appear as a coarse precipitate during the performance process, and therefore, the upper limit is preferably limited to 0.05 wt%. Accordingly, the titanium is preferably contained in an amount of 0.001 to 0.05 wt%.

Nickel (Ni): 0.05 to 0.6 wt%

Nickel is an element that simultaneously improves strength and toughness. In the present invention, nickel can play an important role in securing strength of a post material and improving brittle fracture stopping properties. In order to exhibit such an effect, it is preferable to contain 0.05 wt% or more. The strength and toughness of nickel are increased as the amount of nickel is increased. However, since the strength and toughness do not increase proportionally as the amount of nickel is increased, the upper limit is preferably limited to 0.6 wt%. Therefore, it is preferable that the nickel is included in an amount of 0.05 to 0.6% by weight.

Copper (Cu): 0.01 to 0.35 wt%

Copper serves as an employment strengthening element and contributes to the strength increase. In order to exhibit such effects in the present invention, it is preferable to contain 0.01 wt% or more. However, when copper, which is an expensive element, is added in an amount exceeding 0.35% by weight, the manufacturing cost is increased. Accordingly, the copper is preferably contained in an amount of 0.01 to 0.35% by weight.

The steel material of the present invention can be used as a steel material for a pressure vessel. Therefore, it is preferable that the content of elements such as Cu, Ni, Cr, Mo, V, and Nb satisfy the following relationship.

Cu + Ni + Cr + Mo: 1.0% or less

Cr + Mo: 0.4% or less

V + Nb: 0.1% by weight or less

That is, the relationship between Cu + Ni + Cr + Mo, Cr + Mo and V + Nb is a numerical value limited by the basic standard (ASTM A20) 1.0 wt% or less, Cr + Mo content is 0.4 wt% or less, and V + Nb content is 0.1 wt% or less. However, alloying elements not included according to embodiments of the present invention may be calculated as zero.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

However, since phosphorus and sulfur are generally referred to as impurities, a brief description thereof is as follows.

Phosphorus (P): 0.03% by weight or less

The phosphorus is an impurity inevitably contained and is mainly segregated at the center of the steel sheet to lower the toughness. Therefore, it is preferable to control the phosphorus content as low as possible in order to secure the low-temperature impact toughness at the center of the post-material. Theoretically, it is preferable to limit the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is preferably limited to 0.03 wt%.

Sulfur (S): 0.03 wt% or less

Sulfur is an inevitably contained impurity, which is combined with Mn or the like to form a nonmetallic inclusion, thereby greatly impairing the low-temperature impact toughness of the steel. Therefore, it is desirable to suppress the content to the maximum. Theoretically, it is advantageous to limit the content of sulfur to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is preferably limited to 0.03% by weight.

Further, the microstructure of the steel sheet may include a tempered martensite structure as a main phase. Preferably, the tempered martensite has an area fraction percentage of 50% or more. When the area fraction of the tempered martensite is 50% or more, a high tensile strength can be secured.

The reason why the structure is controlled in the above-described manner is that the post-weld heat treatment resistance of the present invention is excellent and the steel has appropriate strength and toughness.

It is preferable that the steel sheet has a tensile strength of 600 MPa or more and a Charpy impact energy value at -60 캜 of 100 J or more even after heat treatment (Post Weld Heat Treatment, PWHT) for 50 hours.

Hereinafter, a method for manufacturing a steel sheet for a pressure vessel excellent in post-weld heat treatment resistance, which is another aspect of the present invention, will be described in detail.

A method for producing a steel sheet for a pressure vessel excellent in post-weld heat treatment resistance, which is another aspect of the present invention, comprises 0.05 to 0.15% of C, 0.15 to 0.5% of Si, 1.0 to 1.6% of Mn, 0.03% 0.005 to 0.06% of Mo, 0.005 to 0.06% of Nb, 0.005 to 0.03% of Nb, 0.001 to 0.05% of Ti, 0.005 to 0.06% of Cr, 0.01 to 0.25% of Cr, Ni: 0.05 to 0.6%, Cu: 0.01 to 0.35%, Cu + Ni + Cr + Mo: not more than 1.0%, Cr + Mo: not more than 0.4%, V + Nb: not more than 0.1% Heating the steel slab to a temperature range of 1050 to 1250 占 폚, hot rolling the hot slab to finish rolling at 800 to 900 占 폚, heating the hot-rolled steel sheet to 1 / 4T (T: Thickness) at a cooling rate of 3 to 60 DEG C / sec, and tempering the cooled steel sheet for 10 to 30 minutes.

Heating step

It is preferable to heat the slab satisfying the above-mentioned component system at 1050 to 1250 占 폚. The present invention is to realize a ferritic and bainite ferrite mixed structure steel having high strength and toughness with excellent heat treatment resistance after welding. When the heating temperature is lower than 1050 占 폚, solid solute hardness is difficult. On the other hand, when the heating temperature exceeds 1250 DEG C, coarse austenite structure may be formed due to coarse TiN precipitation due to coarse TiN precipitation, and such coarse austenite is difficult to recrystallize during rough rolling, , Thereby deteriorating the properties of the steel sheet. Therefore, the heating temperature of the slab is preferably limited to 1050 to 1250 ° C.

Hot rolling step

The heated slab may be subjected to hot rolling. It is preferable that the hot rolling is finished within a temperature range of 800 to 900 占 폚. If the finish temperature of the hot rolling is less than 800 ° C., the strength to be secured by the present invention can not be secured due to the lack of incombustibility. On the other hand, when the rolling finish temperature exceeds 900 DEG C, the grain size deteriorates the toughness.

It is preferable that the reduction rate is controlled to 5 to 30% per pass. The non-recrystallized rolling is carried out in order to form micro-ferrite and bainite ferrite by forming a deformation band inside while stretching the austenite structure in the rolling direction. Therefore, in order to secure the microstructure to be secured by the present invention, it is preferable to control the reduction rate by 5 to 30% per pass.

Cooling step

The rolled steel sheet is preferably cooled. It is preferable to cool the rolled steel sheet to a temperature range of 200 DEG C or less at a cooling rate of 3 to 60 DEG C / sec on the basis of a cooling rate of 1 / 4T (T: thickness of the steel sheet). If the cooling rate is less than 3 DEG C / sec, the cooling ability is insufficient and an excessive ferrite structure may be formed. On the other hand, when it exceeds 60 ° C / sec, additional equipment is required and the productivity is lowered.

Tempering  step

The steel sheet thus cooled is preferably subjected to a tempering heat treatment. The tempering is preferably performed for 10 to 30 minutes. When the above tempering is performed for less than 10 minutes, homogenization of the structure is difficult. On the other hand, if it exceeds 30 minutes, there is a problem that the productivity is lowered. More preferably, the tempering is performed in a temperature range of 600 to 700 占 폚. When the tempering temperature is less than 600 ° C, it is difficult to secure toughness. On the other hand, if it exceeds 700 ° C, precipitation grows and the strength and low-temperature toughness are deteriorated.

The steel sheet can be subjected to heat treatment after welding and welding under the following conditions.

The PWHT treatment is required to remove the residual stress by the welding process added at the time of manufacturing the pressure vessel. Generally, after the PWHT heat treatment for a long time, the strength and toughness deteriorate. The steel sheet produced by the present invention has a high strength and toughness even when it is subjected to a long PWHT temperature condition (590 to 640 ° C) And can be welded without deterioration. In particular, the steel sheet of the present invention has a tensile strength of 550 MPa or more even after 50 hours of PWHT, and satisfies a Charpy impact energy value of-100 C or more at -60 캜.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

Table 1 shows the components of Inventive Examples 1 to 4 and Comparative Examples 1 and 2, respectively. Steel slabs having compositions as shown in Table 1 were heated at 1100 占 폚 and subjected to hot rolling, cooling, tempering and PWHT under the conditions shown in Table 2 below. The yield strength (YS), tensile strength (TS) and impact toughness at -60 캜 of the prepared specimens were evaluated together in Table 2 below.

In this case, the strength and toughness were evaluated by taking tensile and impact specimens at a thickness of 1 / 4T (T: Thickness, mm) in the steel sheet. The impact toughness at -60 캜 was evaluated by the Charpy impact energy value obtained by performing the Charpy impact test on the specimen having the V notch at -60 캜.

division C Mn Si P S Cu Ni Cr Mo V Nb Ti Al Inventory 1 0.08 1.45 0.25 0.008 0.0013 0.16 0.21 0.15 0.16 0.020 0.016 0.015 0.028 Inventory 2 0.09 1.39 0.28 0.009 0.0015 0.20 0.18 0.13 0.19 0.021 0.018 0.013 0.025 Inventory 3 0.11 1.35 0.30 0.010 0.0012 0.13 0.19 0.10 0.13 0.018 0.020 0.020 0.024 Comparative Example 1 0.10 1.48 0.29 0.009 0.0011 0.17 0.15 0.12 0.14 0.015 0.020 0.021 0.030 Comparative Example 2 0.12 1.35 0.25 0.005 0.0010 0.10 0.18 0.14 0.09 0.013 0.018 0.013 0.030

division Microstructure Steel plate
thickness
(mm) Rolling end temperature
(° C) Cooling end temperature
(° C) Cooling rate
(° C / sec) Tempering temperature
(° C) PWHT temperature
(° C) PWHT time
(Hr) YS
(MPa) TS
(MPa) Impact toughness at -60 ° C
(J) Inventory 1 Tempered martensite 50 820 75 7.5 650 620 25 585 659 230 60 830 70 6.5 650 620 50 579 643 215 80 840 100 4.8 630 610 25 572 647 239 100 850 100 3.5 630 610 50 563 635 243 Inventory 2 50 820 75 7.5 650 620 25 592 645 228 60 830 70 6.5 650 620 50 596 643 243 80 840 55 4.8 630 610 25 579 640 256 100 850 150 3.5 630 610 50 569 637 221 Inventory 3 50 820 75 8.5 650 620 25 589 669 243 60 830 70 6.5 650 620 50 585 659 218 80 840 55 4.8 630 610 25 586 648 257 100 850 180 3.5 630 610 50 583 645 202 Comparative Example 1 Ferrite + Bainite Ferrite 50 820 550 7.5 - 620 25 497 593 123 60 830 550 6.5 - 620 50 495 585 199 80 840 550 4.8 - 610 25 490 572 186 100 850 550 3.5 - 610 50 487 556 133 Comparative Example 2 50 820 550 5.0 - 620 20 490 558 148 80 840 550 3.9 - 620 25 486 561 121 100 850 550 3.5 - 620 50 479 557 120

As can be seen from the results of Table 1 and Table 2, Examples 1 to 3 satisfying the composition and production conditions of the present invention showed that even if the PWHT time was from 25 hours to 50 hours, Comparative Examples 1 and 2 are different from each other in the composition of the microstructure and deviate from the production conditions. As a result, it is confirmed that the strength and toughness are markedly deteriorated as compared with Inventive Examples 1 to 4.

Particularly, in Examples 1 to 3, lowering of the low temperature toughness value is not significant even after 50 hours of PWHT, whereas in Comparative Examples 1 and 2, the lowering of the low temperature toughness value is significant.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (8)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.15% of C, 0.15 to 0.5% of Si, 1.0 to 1.6% of Mn, 0.03% or less of P, 0.03% or less of S, 0.005 to 0.3% of Mo, 0.005 to 0.06% of V, 0.005 to 0.03% of Nb, 0.001 to 0.05% of Ti, 0.05 to 0.6% of Ni, 0.01 to 0.35% of Cu, Cu + Ni + Cr + Mo : 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, the balance Fe and unavoidable impurities and having a microstructure containing 50% or more of tempered martensite in area fraction% A welded steel plate for a pressure vessel having a tensile strength of 600 MPa or more and a Charpy impact energy value of-100 J or more at 25 to 50 hours of post-weld heat treatment (PWHT).
delete delete The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.15% of C, 0.15 to 0.5% of Si, 1.0 to 1.6% of Mn, 0.03% or less of P, 0.03% or less of S, 0.005 to 0.3% of Mo, 0.005 to 0.06% of V, 0.005 to 0.03% of Nb, 0.001 to 0.05% of Ti, 0.05 to 0.6% of Ni, 0.01 to 0.35% of Cu, Cu + Ni + Cr + Mo : 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, the balance Fe and unavoidable impurities to a temperature range of 1050 to 1250 캜;
Hot rolling the heated steel slab to finish rolling at 800 to 900 占 폚;
Cooling the hot-rolled steel sheet at a cooling rate of 3 to 60 DEG C / sec on the basis of a cooling rate of 1 / 4T (T: thickness of the steel sheet);
Tempering the cooled steel sheet for 10 to 30 minutes; And
And a post-welding heat treatment step of maintaining the tempered steel sheet at a temperature range of 590 to 640 ° C for 25 to 50 hours.
delete 5. The method of claim 4,
Wherein the step of hot rolling is performed at a reduction ratio of 5 to 30% per each rolling pass.
5. The method of claim 4,
Wherein the cooling is carried out at a temperature of 200 DEG C or less. ≪ RTI ID = 0.0 > 11. < / RTI >
5. The method of claim 4,
Wherein the tempering step is performed in a temperature range of 600 to 750 占 폚 and is excellent in post-weld heat treatment resistance.