KR101353858B1 - Pressure vessel steel plate having excellent resustance property after post weld heat treatment and manufacturing method of the same - Google Patents

Abstract

The pressure vessel steel plate excellent in heat treatment resistance after welding, which is one aspect of the present invention, is C: 0.05 to 0.15%, Si: 0.15 to 0.5%, Mn: 1.0 to 1.6%, P: 0.03% or less, and S: 0.03% or less, Al: 0.015 to 0.05%, Cr: 0.01 to 0.25%, Mo: 0.005 to 0.3%, V: 0.005 to 0.06%, Nb: 0.005 to 0.03%, Ti: 0.001 to 0.05%, Ni: 0.05 to 0.05 0.6%, Cu: 0.01-0.35%, Cu + Ni + Cr + Mo: 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, residual Fe and unavoidable impurities and the structure is ferrite It consists of a mixed tissue of Bainitic Ferrite (Acicular Ferrite + Bainite).
Another aspect of the present invention is a method for producing a pressure vessel steel plate having excellent heat treatment resistance after welding, in weight%, C: 0.05 to 0.15%, Si: 0.15 to 0.5%, Mn: 1.0`1.6%, P: 0.03% S: 0.03% or less, Al: 0.015 to 0.05%, Cr: 0.01 to 0.25%, Mo: 0.005 to 0.3%, V: 0.005 to 0.06%, Nb: 0.005 to 0.03%, Ti: 0.001 to 0.05%, Ni: 0.05 to 0.6%, Cu: 0.01 to 0.35%, Cu + Ni + Cr + Mo: 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, including residual Fe and unavoidable impurities Heating the steel slab to a temperature range of 1050 to 1250 ° C., hot rolling the heated steel slab at a temperature range of a Tnr to Ar3 transformation point, and forming the hot rolled hot rolled steel sheet to 1 / 4T (T: thickness of the steel sheet). Based on the cooling rate of), the step of cooling at a cooling rate of 2 ~ 30 ℃ / sec and the post-weld heat treatment step of maintaining the cooled steel sheet for 50 hours or less in the temperature range of 590 ~ 640 ℃.

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 used in pressure vessels under pressure such as petrochemical manufacturing equipment, storage tanks, heat exchangers, reactors and condensers, and a method of manufacturing the same, and more particularly, to post-weld heat treatment (PWHT). It relates to a steel sheet excellent in strength and toughness even after heat treatment 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 addition to the thickening of steel as described above, in order to prevent deformation of the structure after welding and to stabilize shape and dimensions when steel is welded, in order to remove stress generated during welding, heat treatment after welding (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 a long time PWHT, the strength and toughness are simultaneously decreased due to softening of matrix and grain boundaries, grain growth, and coarsening of carbides.

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 technique is very insufficient when the materialization and welding conditions are severe, and there is a problem that the application of the PWHT for a long time is impossible.

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

The present invention is to provide a pressure vessel steel sheet excellent in resistance and toughness that does not decrease even after a long time PWHT of the steel sheet and a method of manufacturing the same.

The pressure vessel steel plate excellent in heat treatment resistance after welding, which is one aspect of the present invention, is C: 0.05 to 0.15%, Si: 0.15 to 0.5%, Mn: 1.0 to 1.6%, P: 0.03% or less, and S: 0.03% or less, Al: 0.015 to 0.05%, Cr: 0.01 to 0.25%, Mo: 0.005 to 0.3%, V: 0.005 to 0.06%, Nb: 0.005 to 0.03%, Ti: 0.001 to 0.05%, Ni: 0.05 to 0.05 0.6%, Cu: 0.01-0.35%, Cu + Ni + Cr + Mo: 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, residual Fe and unavoidable impurities and the structure is ferrite It consists of a mixed tissue of Bainitic Ferrite (Acicular Ferrite + Bainite).

Another aspect of the present invention is a method for producing a pressure vessel steel plate having excellent heat treatment resistance after welding, in weight%, C: 0.05 to 0.15%, Si: 0.15 to 0.5%, Mn: 1.0 to 1.6%, P: 0.03% S: 0.03% or less, Al: 0.015 to 0.05%, Cr: 0.01 to 0.25%, Mo: 0.005 to 0.3%, V: 0.005 to 0.06%, Nb: 0.005 to 0.03%, Ti: 0.001 to 0.05%, Ni: 0.05 to 0.6%, Cu: 0.01 to 0.35%, Cu + Ni + Cr + Mo: 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, including residual Fe and unavoidable impurities Heating the steel slab to a temperature range of 1050 to 1250 ° C., hot rolling the heated steel slab at a temperature range of a Tnr to Ar3 transformation point, and forming the hot rolled hot rolled steel sheet to 1 / 4T (T: thickness of the steel sheet). Based on the cooling rate of), the step of cooling at a cooling rate of 2 ~ 30 ℃ / sec and the post-weld heat treatment step of maintaining the cooled steel sheet for 50 hours or less in the temperature range of 590 ~ 640 ℃.

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 having a fine structure of bainitic ferrite and a residual ferrite having a volume fraction of 25 to 60% and having fine carbonitride of 50 nm or less. Through the production of such a steel structure, while having a tensile strength of 550MPa or more, strength and toughness does not deteriorate even after heat treatment after welding for 50 hours or less.

The present inventors conducted a study to derive a steel sheet excellent in heat treatment resistance after welding. As a result, the inventors controlled the component system and manufacturing conditions of the steel sheet appropriately and controlled the microstructure of the steel sheet by a mixed structure of ferrite and bainitic ferrite. It was confirmed that the steel sheet having a high resistance to PWHT can be produced after the materialization and the PWHT for a long time does not lower the strength and toughness, and thus led to the present invention.

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

The pressure vessel steel plate excellent in heat treatment resistance after welding, which is one aspect of the present invention, is C: 0.05 to 0.15%, Si: 0.15 to 0.5%, Mn: 1.0 to 1.6%, P: 0.03% or less, and S: 0.03% or less, Al: 0.015 to 0.05%, Cr: 0.01 to 0.25%, Mo: 0.005 to 0.3%, V: 0.005 to 0.06%, Nb: 0.005 to 0.03%, Ti: 0.001 to 0.05%, Ni: 0.05 to 0.05 0.6%, Cu: 0.01-0.35%, Cu + Ni + Cr + Mo: 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, residual Fe and unavoidable impurities and the structure is ferrite It consists of a mixed tissue of Bainitic Ferrite (Acicular Ferrite + Bainite).

Carbon (C): 0.05 to 0.15 wt%

Carbon is the most effective element for reinforcing steel, but it is an element that degrades 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, it is not preferable because the weldability and low temperature toughness deteriorate. 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. It is preferable to manage at 1.6 wt% or less because MnS, which is a non-metallic inclusion drawn together with sulfur (S), is formed to lower normal temperature elongation and low temperature toughness. However, when Mn is less than 1.0% by weight, it is difficult to secure appropriate strength. 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 along with Si in steelmaking and has a solid solution strengthening effect. When the content of aluminum is less than 0.015% by weight, it is impossible to ensure the deoxidation effect intended for the present invention. 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 important element to increase the strength by being very effective in miniaturizing austenite grains during hot rolling and at the same time precipitated as carbonitrides (Nb (C, N)) matching with matrix. In order to exhibit the effect intended in the present invention, it is preferable that 0.005% by weight or more are included. 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 is an element that plays a role in improving the toughness of steel by forming carbonitrides (Ti (C, N)) like Nb and suppressing the growth of austenite grains during slab heating and hot rolling to refine the final grain size. . 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 improves strength and toughness at the same time. In the present invention, nickel may play an important role in securing the strength of the thick material and improving the brittle fracture stopping property. 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,

It is desirably controlled to be as low as possible in order to secure the low-temperature impact toughness of the center portion of the post-material because it is segregated mainly in the center portion of the steel sheet to lower the toughness.

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 inevitable impurity,

Mn and the like to form a nonmetallic inclusions, 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.

According to an aspect of the present invention, by satisfying the component system, it is possible to provide a pressure vessel steel plate excellent in heat treatment resistance after welding. The microstructure of the steel sheet may include a mixed structure of ferrite and bainitic ferrite (Acicular Ferrite + Bainite). The bainitic ferrite fraction is preferably 25 to 60% by volume.

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.

Further, inside the grains of the microstructure, fine M1 (C, N) + M2 (C, N) (M1 = [Ti, Nb, V], M2 = [Mo, Cr]) type carbonaceous materials of 50 nm or less It is preferred that the cargo be present. In addition, the carbonitride is preferably present in 0.05% or more by volume fraction. The volume fraction of the carbonitride is more preferably present at 0.05 to 5%.

When the steel sheet is post-weld heat treatment (PWHT) for 50 hours or less, the tensile strength is preferably 550 MPa or more and the Charpy impact energy value at -60 ° C is 50 J or more.

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.

Another aspect of the present invention is a method for producing a pressure vessel steel plate having excellent heat treatment resistance after welding, in weight%, C: 0.05 to 0.15%, Si: 0.15 to 0.5%, Mn: 1.0`1.6%, P: 0.03% S: 0.03% or less, Al: 0.015 to 0.05%, Cr: 0.01 to 0.25%, Mo: 0.005 to 0.3%, V: 0.005 to 0.06%, Nb: 0.005 to 0.03%, Ti: 0.001 to 0.05%, Ni: 0.05 to 0.6%, Cu: 0.01 to 0.35%, Cu + Ni + Cr + Mo: 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, including residual Fe and unavoidable impurities Heating the steel slab to a temperature range of 1050 to 1250 ° C., hot rolling the heated steel slab at a temperature range of a Tnr to Ar3 transformation point, and forming the hot rolled hot rolled steel sheet to 1 / 4T (T: thickness of the steel sheet). Based on the cooling rate of), the step of cooling at a cooling rate of 2 ~ 30 ℃ / sec and the post-weld heat treatment step of maintaining the cooled steel sheet for 50 hours or less in the temperature range of 590 ~ 640 ℃.

Heating stage

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. At this time, rolling is uncrystallized rolling and it is preferable to limit to Tnr-Ar3 transformation point (about 750 degreeC). If Tnr is less than the boundary temperature of the austenite unrecrystallized zone and the recrystallization solution, coarse austenite occurs due to partial recrystallization, which leads to the formation of coarse low-temperature metamorphic tissue after cooling, which causes resistance degradation. . When the Ar3 transformation point is exceeded, the strained ferrite structure or strained organic ferrite drawn during rolling develops and the yield ratio is significantly increased.

Therefore, rolling at the unrecrystallized temperature range Tnr ~ Ar3 transformation point (about 750 ° C.) provides austenite nucleation sites for fine ferrite grains, and such grain refinement can simultaneously increase strength and toughness. .

The non-recrystallization temperature Tnr can be calculated from the following equation.

Tnr (℃) = 887 + 464 × C + 890 × Ti + 363 × Al-357 × Si + (6445 × Nb-644 × Nb1 / 2) + (732 × V-230 × V 1/2)

In addition, the reduction ratio has 10% or more per pass, and the cumulative reduction amount is preferably controlled to 30% or more. 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, when the cumulative reduction ratio is less than 30%, the austenite is stretched in the rolling direction and the internal strain band fraction is low, and there is a lack of nucleation sites of fine ferrite and bainitic ferrite, which may result in ferrite grains of about 25 μm or more. have.

Cooling stage

The rolled steel sheet is preferably cooled. The rolled steel sheet is preferably cooled to a temperature range of 450 to 580 ° C. at a cooling rate of 2 to 30 ° C./sec based on a cooling rate of 1 / 4T (T: thickness of the steel sheet). If the cooling rate is less than 2 ° C / sec may be the ferrite grain coarsening during cooling. On the other hand, when it exceeds 30 ° C / sec, there is a high possibility that an excessive second phase (a fraction of Acicular Ferrite + Bainite is 60% or more) occurs.

Heat treatment step after welding

The PWHT treatment is required to remove the residual stress by the welding process added at the time of manufacturing the pressure vessel. In general, deterioration of strength and toughness occurs after a long time PWHT heat treatment, the steel sheet produced by the present invention is a large strength and toughness even if carried out for a long time (50 hours or less) at 590 ~ 640 ℃, a typical PWHT temperature condition It has the advantage that welding construction is possible 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 the Charpy impact energy value at −60 ° C. satisfies 50 J or more.

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 the composition shown in Table 1 were prepared by steel sheet thickness, heating temperature, controlled rolling, controlled cooling, and steel sheet shown in Table 2, and PWHT was performed on the steel sheets prepared under the above conditions under the conditions shown in Table 2 below. Yield strength, tensile strength and low temperature toughness were evaluated and the results are shown in Table 2 below. Strength and toughness were evaluated by taking tensile and impact specimens at 1 / 4T (T: Thickness, mm) thickness direction of the steel sheet. (However, the fraction of carbonitride M1 (C, N) + M2 (C, N) (wherein M1 = [Ti, Nb, V], M2 = [Mo, Cr]) type carbide of Table 2 is shown. )

However, low temperature toughness is evaluated by the Charpy impact energy value obtained by carrying out the Charpy impact test of the specimen which has a V notch at -60 degreeC.

division C Mn Si P S Cu Ni Cr Mo V Nb Ti Al Inventory 1 0.08 1.45 0.28 0.007 0.0013 0.15 0.21 0.15 0.15 0.020 0.015 0.015 0.029 Inventory 2 0.07 1.29 0.30 0.008 0.0015 0.20 0.18 0.12 0.21 0.025 0.020 0.012 0.031 Inventory 3 0.12 1.15 0.31 0.010 0.0016 0.10 0.19 0.09 0.10 0.015 0.025 0.025 0.025 Honorable 4 0.10 1.50 0.29 0.009 0.0011 0.18 0.15 0.11 0.14 0.017 0.021 0.022 0.030 Comparative Example 1 0.13 1.25 0.25 0.005 0.0010 0.10 0.20 0.15 0.08 0.010 0.010 0.010 0.028 Comparative Example 2 0.10 1.15 0.28 0.012 0.0014 0.15 0.15 0.20 0.15 0.009 0.012 0.012 0.032

division Steel plate
thickness
(mm) Reheating
Temperature
(℃) Cumulative pressure drop
(%) Cooling rate
(° C / sec) Cooling end temperature (℃) PWHT temperature
(℃) PWHT time
(Hr) Bainitic ferrite fraction (%) M1 (C, N) + M2 (C, N)
Fraction (%) YS
(MPa) TS
(MPa) -60 ℃
Impact toughness
(J) Inventory 1 50 1150 75 7.5 520 620 25 38 0.11 505 609 230 60 1100 70 6.5 500 620 50 35 0.12 509 603 215 80 1180 55 4.8 480 610 25 32 0.10 492 597 199 100 1200 40 3.5 475 610 50 30 0.09 493 595 143 Inventory 2 50 1100 75 7.5 520 620 25 36 0.13 502 615 278 60 1150 70 6.5 506 620 50 35 0.12 496 613 203 80 1200 55 4.8 480 610 25 30 0.11 499 610 156 100 1200 40 3.5 475 610 50 29 0.12 489 607 121 Inventory 3 50 1100 75 7.5 520 620 25 40 0.08 489 599 243 60 1150 70 6.5 500 620 50 38 0.09 485 589 218 80 1200 55 4.8 480 610 25 35 0.07 486 586 157 100 1200 40 3.5 475 610 50 32 0.08 483 579 102 Honorable 4 50 1100 60 7.5 520 620 25 37 0.10 497 603 223 60 1150 55 6.5 500 620 50 35 0.11 495 601 199 80 1200 60 4.8 480 610 25 32 0.09 490 598 186 100 1200 45 3.5 475 610 50 30 0.08 487 596 133 Comparative Example 1 50 1200 - 5.0 600 620 20 12 0.02 490 556 48 80 1150 10 3.0 600 620 25 10 0.01 486 561 21 100 1100 10 2.5 600 620 50 5 0.02 479 547 20 Comparative Example 2 50 1100 - Air cooling - 620 20 0 - 480 565 47 80 1100 - Air cooling - 620 25 0 - 476 568 23 100 1180 - Air cooling - 620 50 0 - 468 540 18

In Comparative Example 2 of Table 2, the hot-rolled steel sheet after rolling was subjected to normal heat treatment under conditions of 1.3 × t + (10 to 30 minutes) (where t denotes the thickness of the steel (mm)) at a temperature range of 910 ° C. .

As can be seen from the results of Table 1 and Table 2, Inventive Examples 1 to 4 satisfying the composition and manufacturing conditions of the present invention, even if the PWHT time reaches 25 hours or more to 50 hours, strength and toughness do not significantly decrease In contrast, Comparative Examples 1 and 2 are different in their microstructures and deviate from the manufacturing conditions, and compared with Inventive Examples 1 to 4, the strength and toughness were significantly deteriorated. According to the production conditions of the present invention, the precipitation of carbonitride M1 (C, N) + M2 (C, N) (wherein M1 = [Ti, Nb, V], M2 = [Mo, Cr]) type fine carbide It is thought that the fraction of carbonitride has a great effect. M1 (C, N) + M2 (C, N) (M1 = [Ti, Nb, V], M2 = [Mo, Cr]) type carbonitrides do not grow significantly even during prolonged PWHT. The formation of carbides could confirm the effect of preventing the drop in strength and toughness even after long time PWHT.

In particular, in Examples 1 to 4, the low temperature toughness value is not significantly decreased even after 50 hours of PWHT, whereas the low temperature toughness value is severely reduced in 1 and 2 in comparison.

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 (7)

By weight%, C: 0.05-0.15%, Si: 0.15-0.5%, Mn: 1.0-1.6%, P: 0.03% or less, S: 0.03% or less, Al: 0.015-0.05%, Cr: 0.01-0.25% , Mo: 0.005-0.3%, V: 0.005-0.06%, Nb: 0.005-0.03%, Ti: 0.001-0.05%, Ni: 0.05-0.6%, Cu: 0.01-0.35%, Cu + Ni + Cr + Mo : 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, including residual Fe and unavoidable impurities, and the structure is a mixed structure of ferrite and bainitic ferrite (acicular ferrite + bainite) In the grains of the mixed structure, fine M1 (C, N) + M2 (C, N) of 50 nm or less (where, M1 = [Ti, Nb, V], M2 = [Mo, Cr]) Type carbonitride is present, the carbonitride is a pressure vessel steel plate excellent in heat treatment resistance after welding is 0.05% or more by volume fraction.
The method of claim 1,
The bainitic ferrite fraction is a volume fraction, 25 ~ 60% of the pressure vessel steel plate excellent in heat treatment resistance after welding.
delete The method of claim 1,
The steel sheet is a pressure vessel steel plate excellent in post-weld heat treatment (PWHT) for 50 hours or less, the tensile strength is 550MPa or more, the Charpy impact energy value at -60 ℃ more than 50J after welding heat treatment resistance excellent.
By weight%, C: 0.05-0.15%, Si: 0.15-0.5%, Mn: 1.0`1.6%, P: 0.03% or less, S: 0.03% or less, Al: 0.015-0.05%, Cr: 0.01-0.25% , Mo: 0.005-0.3%, V: 0.005-0.06%, Nb: 0.005-0.03%, Ti: 0.001-0.05%, Ni: 0.05-0.6%, Cu: 0.01-0.35%, Cu + Ni + Cr + Mo : 1.0% or less, Cr + Mo: 0.4% or less, V + Nb: 0.1% or less, heating a steel slab containing residual Fe and unavoidable impurities to a temperature range of 1050 to 1250 ° C .;
Hot rolling the heated steel slab in a temperature range of a Tnr to Ar3 transformation point;
Cooling the hot rolled hot rolled steel sheet at a cooling rate of 2 to 30 ° C./sec based on a cooling rate of 1 / 4T (T: thickness of the steel sheet); And
A method for producing a pressure vessel steel plate excellent in post-weld heat treatment resistance, including a post-weld heat treatment step of maintaining the cooled steel sheet for 50 hours or less in a temperature range of 590 to 640 ° C.
6. The method of claim 5,
The hot rolling step is a method for producing a pressure vessel steel plate excellent in post-weld heat treatment resistance is carried out under the conditions of 10% or more reduction rate and 30% or more cumulative reduction rate per rolling pass.
6. The method of claim 5,
Cooling in the cooling step is a manufacturing method of the pressure vessel steel plate excellent in heat treatment resistance after welding, characterized in that performed to a temperature of 450 ~ 580 ℃.