KR20110075630A - High strength steel sheet having excellent property after post weld heat treatment and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high-strength steel sheet which can maintain the strength and toughness after a long-time PWHT(Post Weld Heat Treatment) and a manufacturing method thereof are provided. CONSTITUTION: A steel slab is reheated to a temperature of 1050-1250°C. The reheated steel slab is hot-rolled at a temperature of Tnr-Tnr+100°C, 10% of reduction ratio per rolling pass, and more than 30% of accumulated reduction ratio. The hot-rolled steel sheet is maintained at a temperature of 850-950°C. The heat-treated steel sheet is cooled at a cooling speed of 0.1-10°C/sec.

Description

High strength steel sheet with excellent heat treatment resistance after welding and manufacturing method {HIGH STRENGTH STEEL SHEET HAVING EXCELLENT PROPERTY AFTER POST WELD HEAT TREATMENT AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel sheet used in a crude oil refinery, a storage tank, a heat exchanger, a reactor, a condenser, etc. in a wet hydrogen sulfide environment, and more particularly, strength and toughness in post-weld heat treatment (PWHT). It relates to an excellent steel sheet and a method of manufacturing the same.

Recently, due to the tendency of oil shortage and high oil prices, the oil field of the poor environment is actively developed, and the material for the refining and storage of crude oil is being materialized.

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.

In Japanese Patent Laid-Open No. 1997-256037, C: 0.05 to 0.20%, Si: 0.02 to 0.5%, Mn: 0.2 to 2.0%, Al as a means for preventing the deterioration of physical properties due to the long-term PWHT heat treatment. : 0.005 to 0.10%, if necessary, containing one or two or more of Cu, Ni, Cr, Mo, V, Nb, Ti, B, Ca, and rare earth elements, the remainder being a slab of iron and unavoidable impurities After heating and hot rolling, PWHT guarantee time was made possible by 16 hours by the process of air-cooling at room temperature, heating and slow cooling at the Ac1-Ac3 transformation point.

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.

Accordingly, there is a demand for steels having a high resistance to PWHT, which are accompanied by thickening of steels and severe welding conditions, and which do not reduce strength and toughness even after prolonged PWHT.

One aspect of the present invention is to provide a high-strength steel sheet excellent in post-weld heat treatment (PWHT) resistance and its manufacturing method does not decrease the strength and toughness even after a long post-weld heat treatment (PWHT).

In the present invention, by weight%, C: 0.1-0.3%, Si: 0.15-0.50%, Mn: 0.6-1.2%, P: 0.035% or less, S: 0.020% or less, Al: 0.001-0.05%, Cr: 0.01 0.35%, Mo: 0.005-0.2%, V: 0.005-0.05%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, Ca: 0.0005-0.005%, Ni: 0.05-0.5%, Cu : 0.005-0.5%, Co: 0.005-0.2% and W; At least one selected from the group consisting of 0.005 to 0.2%, the remainder includes Fe and inevitable impurities, and the composition provides a high strength steel sheet excellent in post-weld heat treatment resistance satisfying the following relational formula.

Cu + Ni + Cr + Mo: 1.5% or less

Cr + Mo: 0.4% or less

V + Nb: 0.1% or less

Ca / S: 1.0 or less

In addition, the present invention comprises the steps of reheating the steel slab satisfying the composition range to a temperature range of 1050 ~ 1250 ℃;

Hot rolling the reheated steel slab in a temperature range of T _nr ~ T _nr + 100 ° C .;

A heat treatment step of maintaining the hot rolled hot rolled steel sheet for a time of 1.3 × t + (10 to 30 minutes) (where t denotes the thickness (mm) of steel) in a temperature range of 850 to 950 ° C .; And

It provides a method for producing a high strength steel sheet excellent in heat treatment resistance after welding comprising the step of cooling the heat-treated steel sheet at a cooling rate of 0.1 ~ 10 ℃ / sec.

According to the present invention, it is possible to provide a steel plate for pressure vessels having a strength of 500 MPa or more, which does not deteriorate in strength and toughness even after 100 hours of PWHT, and which is excellent in hydrogen organic crack resistance.

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

First, the composition range of the present invention will be described in detail (hereinafter,% by weight).

The content of carbon (C) is preferably limited to 0.1 to 0.3%. C is an element that improves the strength, and if its content is less than 0.1%, its own strength on the matrix is lowered, and if it is more than 0.3%, segregation occurs in the structure, thereby degrading hydrogen organic crack resistance.

The content of silicon (Si) is preferably limited to 0.15 to 0.50%. Si is an effective element for deoxidation and solid solution strengthening and is added for impact transition temperature synergistic effect. In order to achieve this effect, 0.15% or more should be added, but when added in excess of 0.5%, there is a problem in that weldability is degraded and an oxide film is severely formed on the surface of the steel sheet.

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

The content of aluminum (Al) is preferably limited to 0.001 to 0.5%. Al is one of the strong deoxidizers in the steelmaking process together with Si, and the deoxidation effect is insignificant at less than 0.001%, and when added in excess of 0.05%, the deoxidation effect is saturated and the manufacturing cost increases.

Phosphorus (P) is an element that impairs low temperature toughness, but excessive cost is required to be removed in the steelmaking process, so it is preferable to manage it within the range of 0.035% or less.

Sulfur (S) is also an element that adversely affects low-temperature toughness in addition to P, but like P, it may be excessively expensive to remove in the steelmaking process, it is preferable to manage within 0.020% or less.

The content of chromium (Cr) is preferably limited to 0.01 to 0.35%. Since Cr is an element that increases strength, it should be added at least 0.01% in order to increase the strength in the present invention, but since it is an expensive element, when it is added in excess of 0.35%, it will increase the manufacturing cost, so it is managed at 0.35% or less. It is desirable to.

The content of molybdenum (Mo) is preferably limited to 0.005 ~ 0.2%. Mo, like Cr, is not only an element effective for increasing the strength but also an element for preventing the occurrence of cracking due to sulfides. In order to achieve the above effect, 0.005% or more should be added, but since Mo also causes an increase in the manufacturing cost of expensive elements, it is preferable to limit it to 0.2% or less.

The content of vanadium (V) is preferably limited to 0.005 to 0.05%. V is an element effective for increasing the strength, such as Cr and Mo. Therefore, the effect of increasing the strength can be achieved by adding 0.005% or more, but it is preferable to add it to 0.05% or less due to the high price.

The content of niobium (Nb) is preferably limited to 0.001 to 0.05%. Nb is an important element which increases the strength by solidifying in austenite to increase the hardenability of austenite and to precipitate as carbonitride (Nb (C, N)) that matches the matrix (Matrix). If the content is added more than 0.001% to obtain the above effect, but when added in a large amount may appear as a coarse precipitate in the process of playing the role of hydrogen organic crack site, the content is preferably limited to 0.05% or less Do.

The content of titanium (Ti) is preferably limited to 0.001 to 0.05%. Ti is precipitated as carbonitride (Ti (C, N)) like Nb, and is an important element for increasing strength. When the content is added to 0.001% or more to obtain the above effect, but when added in a large amount appears as a coarse precipitate in the playing process can act as a site of hydrogen organic cracks, the content is preferably limited to 0.05% or less.

The content of calcium (Ca) is preferably limited to 0.0005 to 0.005%. Ca is produced as CaS and added to suppress the non-metallic inclusions of MnS, which must be added at least 0.0005%. However, if the content exceeds 0.005%, the upper limit is preferably limited to 0.005% because it reacts with O contained in the steel to generate CaO, which is a nonmetallic inclusion.

The content of nickel (Ni) is preferably limited to 0.05 to 0.5%. Ni is the most effective element for improving low temperature toughness, and the above content can be obtained when its content is added at 0.05% or more, but it is preferable to add it at 0.5% or less because it causes an increase in manufacturing cost with expensive elements.

The present invention includes at least one member selected from the group consisting of Cu, Co and W in the composition.

The content of copper (Cu) is preferably added 0.005 ~ 0.5%. Cu prevents the deterioration of strength even after PWHT heat treatment due to the strengthening of matrix by solid solution strengthening or e-Cu precipitation, and also prevents deterioration of strength and toughness through matrix reinforcement and recovery inhibition. However, it is preferable to add the content within the range of 0.005 ~ 0.5% because it is expensive.

The content of cobalt (Co) is preferably added 0.005 ~ 0.2%. Co is an element effective in preventing the softening of the matrix structure, but is expensive, and therefore Co is preferably added within the range of 0.005 to 0.2%.

The content of tungsten (W) is preferably added 0.005 ~ 0.2%. W is preferably added in an amount of 0.005% or more because WC has a property of forming WC or decreasing cementite precipitation fraction and preventing cementite growth / aggregation inhibition to prevent deterioration of strength and toughness. However, since the W is expensive, it is more preferable to add within the range of 0.005 to 0.2%.

Since the steel of the present invention can be used as a steel for pressure vessels, in consideration of this, the content of elements such as Cu, Ni, Cr, Mo, V, and Nb preferably satisfies the following relationship.

Cu + Ni + Cr + Mo: 1.5% or less

Cr + Mo: 0.4% or less

V + Nb: 0.1% or less

Ca / S: 1.0 or less

In other words, the relationship between Cu + Ni + Cr + Mo, Cr + Mo and V + Nb is a numerical value limited by the basic standard (ASTM A20) of steel for pressure vessels, and accordingly Cu + Ni + Cr + Mo content is Below 1.5%, the Cr + Mo content is limited to 0.4% or less, and the V + Nb content is limited to 0.1% or less. However, alloy elements not included according to the embodiment of the present invention may be calculated as zero.

In addition, the Ca / S ratio is an essential component ratio for spheroidizing MnS inclusions to improve hydrogen organic cracking resistance. When the Ca / S ratio exceeds 1.0, the effect is hardly expected, so the ratio is adjusted to 1.0 or less.

The rest consists of Fe and unavoidable impurities.

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

If the appropriate control rolling and heat treatment by the process described below to the steel having the above composition can be made of the microstructure of the ferrite or a mixed structure of ferrite and pearlite, it is preferable that the low-temperature tissue is not included in the tissue as possible Up to 10% may contain bainite. The reason for controlling the tissue in the above-described form is to have excellent resistance to hydrogen-organic cracks, which is the object of the present invention, and to have appropriate strength and toughness.

In addition, in order to secure hydrogen organic cracking resistance, it is preferable that a banding index (measured according to ASTM E-1268) indicating how much band structure is vulnerable to hydrogen organic cracking is 0.25 or less. When the banding index (Banding Index) value exceeds 0.25, the hydrogen organic crack resistance is sharply reduced in the microstructure.

Steel sheet thickness direction center part (3/8-5 / 8t, t: thickness of steel plate) It is preferable that the average size of a ferrite grain is 50 micrometers or less. This is because when the size of the ferrite grains is excessive, the strength and toughness may be reduced. There is no lower limit to the size of the crystal grains, but since the steels targeted in the present invention are generally hard to obtain crystal grains smaller than 5 µm, the grain size may be 5 µm or more.

Hereinafter, the manufacturing method of the present invention will be described in detail.

The present invention reheats the steel slab that satisfies the composition range to a temperature range of 1050 ~ 1250 ℃. If the reheating temperature is lower than 1050 ℃, solute of the solute atoms is difficult, if it exceeds 1250 ℃ austenite grain size becomes too coarse to damage the properties of the steel sheet.

In the present invention after the reheating has a ferrite + pearlite two-phase composite structure to have a hydrogen organic crack resistance, the recrystallization controlled rolling, heat treatment, PWHT so that the banding index (measured according to ASTM E-1268) is 0.25 or less Heat treatment is required.

Recrystallization controlled rolling is performed by hot rolling the reheated steel slab at a temperature above the unrecrystallization. T _{nr which} is the recrystallization temperature can be calculated from the following equation.

T _nr (° C) = 887 + 464 × C + 890 × Ti + 363 × Al-357 × Si + (6446 × Nb-644 × Nb ^1/2 ) + (732 × V-230 × V ^1/2 )

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 used in the temperature range of T _nr to T _nr + 100 ° C. It is preferable to apply a reduction ratio of 10% or more per rolling pass to give a cumulative reduction of 30% or more. This is because if the cumulative reduction is less than 30%, a banding index of 0.25 or less cannot be expected. In addition, the reason for limiting the temperature of the recrystallization control rolling is also to control the bending index and to suppress the band structure in the state where the grains are not coarsened. More specifically, when the temperature is lower than the recrystallization reference temperature (Tnr), the austenite becomes a pancake and the banding index becomes high. However, when the temperature is excessively high, the grain size becomes excessively undesirable. not.

Thereafter, the hot rolling is performed, and the cooled hot rolled steel sheet is heat treated. The heat treatment is maintained in the temperature range of 850 ~ 950 ℃ condition of 1.3 x t + (10-30 minutes), where t means the thickness of the steel (mm). When the temperature of the heat treatment is less than 850 ° C it is difficult to re-use the solid solute elements it is difficult to secure the strength, while when the heat treatment temperature exceeds 950 ° C grains grow to damage the low-temperature toughness.

The reason for limiting the heat treatment holding time is that if the holding time is less than 1.3 × t + 10 minutes (t means the thickness of the steel (mm)), it is difficult to homogenize the tissue, and 1.3 × t + 30 minutes (t is This is because if the thickness (mm) of the steel is exceeded, productivity is impaired.

The retained steel sheet is cooled at a rate of 0.1 to 10 ° C./sec based on the central cooling rate. At lower cooling rates, ferrite grain coarsening may occur during cooling, and at a higher cooling rate, excessive second phase (bainite This is because the fraction is likely to occur more than 10%.

The cooling rate is to adjust the average grain size of the ferrite in the center of the steel sheet to 50㎛ or less.

The steel sheet of the present invention manufactured through the heat treatment process requires a PWHT treatment for removal of residual stress by a welding process added during the production of a pressure vessel. In general, deterioration in 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 a long time (~ 100 hours) at 600 ~ 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 450 MPa or more even after 100 hours of PWHT, and the Charpy impact energy value at -50 ° C satisfies 50 J or more.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following examples.

(Example)

Table 1 shows the chemical components of the inventive steel and the comparative steel, respectively. Steel slabs having the composition shown in Table 1 were subjected to the steel sheet thickness, reheating temperature, rolling, heat treatment, and cooling of Table 2 to prepare steel sheets.

The steel sheet manufactured under the above conditions was subjected to PWHT under the conditions shown in Table 2, and then the yield strength, tensile strength, low temperature toughness and crack length ratio (CLR, Crack Length Ratio,%) were investigated. Shown in

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.

division C Mn Si P S Cu Ni Cr Mo V Nb Ti Co W Ca Inventive Steel 1 0.17 1.10 0.30 0.01 0.0015 0.15 0.20 0.05 0.12 0.005 0.015 0.003 - 0.10 0.0015 Inventive Steel 2 0.18 1.05 0.35 0.08 0.0012 - 0.15 0.10 0.10 0.010 0.014 0.012 0.10 - 0.0025 Inventive Steel 3 0.16 1.10 0.30 0.01 0.0015 0.20 0.20 0.05 0.12 0.005 0.015 0.015 - 0.10 0.0020 Inventive Steel 4 0.15 1.05 0.25 0.08 0.0012 - 0.15 0.10 0.10 0.010 0.014 0.012 0.10 - 0.0018 Comparative Steel 1 0.17 1.05 0.25 0.01 0.0015 - 0.20 0.15 0.08 0.010 0.010 0.010 - - 0.0025 Comparative Steel 2 0.15 1.15 0.25 0.01 0.0014 - 0.15 0.20 0.15 0.009 0.012 0.012 - - 0.0023

division Steel plate thickness
(Mm) Reheating temperature
(℃) Cumulative pressure drop
(%) Heat treatment condition
(℃, min) Cooling rate
(° C / sec) PWHT temperature
(℃) PWHT hours
(Hr) center
ferrite
Average particle size
(Μm) Banding Index YS
(MPa) TS
(MPa)
-50 ℃
Impact toughness
(J) CLR
(%) Inventive Steel 1


13 1150 60 890,50 1.0 620 6 15 0.18 380 545 203 0.03 25 1100 75 900,60 0.7 620 16 23 0.12 375 540 197 0.0 50 1180 55 890,80 0.8 610 50 25 0.15 360 539 213 0.0 80 1200 50 900,125 0.5 610 100 37 0.08 359 522 186 0.0 Inventive Steel 2


30 1100 80 910,60 110 610 6 19 0.12 355 542 173 0.0 75 1150 65 910,120 120 610 16 26 0.11 354 539 180 0.0 80 1200 60 890,125 120 610 50 37 0.13 350 531 175 0.0 80 1200 50 890,125 120 610 100 33 0.07 350 519 170 0.0 Inventive Steel 3


30 1100 80 910,60 110 610 6 21 0.16 355 535 173 0.0 75 1150 65 910,120 120 610 16 27 0.07 354 537 180 0.0 80 1200 60 890,125 120 610 50 32 0.13 350 533 175 0.0 80 1200 50 890,125 120 610 100 38 0.11 350 528 175 0.0 Inventive Steel 4


50 1100 60 910,80 110 610 6 15 0.09 355 542 173 0.0 75 1150 55 910,120 120 610 16 23 0.10 354 535 180 0.0 80 1200 60 890,125 120 610 50 28 0.08 350 538 175 0.0 80 1200 50 890,125 120 610 100 35 0.11 350 521 175 0.0 Comparative Steel 1

50 1200 - 900,85 Air cooling 620 16 25 0.26 370 536 166 35 50 1150 - 900,80 Air cooling 620 50 51 0.36 325 461 27 20 75 1100 - 900,120 Air cooling 620 100 58 0.27 329 547 23 25 Comparative Steel 2

50 1100 - 900,80 Air cooling 620 16 35 0.26 360 525 178 30 60 1100 - 900,100 Air cooling 620 50 50 0.29 333 468 29 35 75 1180 - 900,120 Air cooling 620 100 51 0.26 328 460 18 25

As can be seen from the results of Tables 1 and 2, the inventive steel that satisfies the composition and the manufacturing conditions of the present invention, even if the PWHT time is 50 hours or more to 100 hours, the strength and toughness does not decrease, Deviation from the composition and manufacturing conditions of the present invention, when compared with the invention steel, when the PWHT time is small, the strength and toughness is almost the same level as the invention steel, but as the PWHT time is longer than 50 hours, the strength compared to the invention steel It can be seen that the toughness is significantly deteriorated.

In particular, it can be seen that the low temperature toughness value is severely lowered in the comparative steel, while the low temperature toughness value is not largely reduced even after 100 hours of PWHT.

On the other hand, it can be seen that the CLR (Crack Length Ratio,%) indicating the resistance of hydrogen organic cracking under H ₂ S (Sour Gas) gas atmosphere is excellent in the invention steel. As such, the reason why the inventive steel is excellent in the CLR is that the banding index indicating the degree of homogenization of the microstructure composed of the composite structure of the ferrite and the pearlite is controlled to be lower than 0.25 to be less than 0.25. Can be.

Claims (7)

By weight%, C: 0.1 to 0.3%, Si: 0.15 to 0.50%, Mn: 0.6 to 1.2%, P: 0.035% or less, S: 0.020% or less, Al: 0.001 to 0.05%, Cr: 0.01 to 0.35% , Mo: 0.005-0.2%, V: 0.005-0.05%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, Ca: 0.0005-0.005%, Ni: 0.05-0.5%, Cu: 0.005-- 0.5%, Co: 0.005-0.2% and W; At least one selected from the group consisting of 0.005 to 0.2%, the remainder comprising Fe and inevitable impurities, The composition is a high strength steel sheet excellent in heat treatment resistance after welding satisfying the following relationship. Cu + Ni + Cr + Mo: 1.5% or less Cr + Mo: 0.4% or less V + Nb: 0.1% or less Ca / S: 1.0 or less The method according to claim 1, The microstructure of the steel sheet is made of ferrite or a mixed structure of ferrite and pearlite, wherein the high-strength steel sheet having excellent heat treatment resistance after welding in which the average size of the ferrite grains in the center of the steel sheet is 50 µm or less. The method according to claim 1, The steel sheet is a high strength steel sheet excellent in heat treatment resistance after welding having a banding index (measured according to ASTM E-1268) of 0.25 or less. The method according to claim 1, The steel sheet is a high strength steel sheet excellent in post-weld heat treatment (PWHT), the tensile strength is 450MPa or more, and the Charpy impact energy value at -50 ° C is 50J or more. By weight%, C: 0.1 to 0.3%, Si: 0.15 to 0.50%, Mn: 0.6 to 1.2%, P: 0.035% or less, S: 0.020% or less, Al: 0.001 to 0.05%, Cr: 0.01 to 0.35% , Mo: 0.005-0.2%, V: 0.005-0.05%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, Ca: 0.0005-0.005%, Ni: 0.05-0.5%, Cu: 0.005-- 0.5%, Co: 0.005-0.2% and W; At least one selected from the group consisting of 0.005 to 0.2%, the remainder comprising Fe and inevitable impurities, The composition comprises the steps of reheating the steel slab satisfying the following relation to a temperature range of 1050 ~ 1250 ℃; Hot rolling the reheated steel slab in a temperature range of T _nr ~ T _nr + 100 ° C .; A heat treatment step of maintaining the hot rolled hot rolled steel sheet for a time of 1.3 × t + (10 to 30 minutes) (where t denotes the thickness (mm) of steel) in a temperature range of 850 to 950 ° C .; And Cooling the heat-treated steel sheet at a cooling rate of 0.1 ~ 10 ℃ / sec Method of producing a high strength steel sheet excellent in heat treatment resistance after welding comprising a. Cu + Ni + Cr + Mo: 1.5% or less Cr + Mo: 0.4% or less V + Nb: 0.1% or less Ca / S: 1.0 or less The method according to claim 5, The hot rolling step is a method of manufacturing a high strength steel sheet excellent in post-weld heat treatment resistance is performed by applying a reduction ratio of 10% or more for each rolling pass, and the cumulative rolling amount of 30% or more. The method according to claim 5, The cooling step is a method of manufacturing a high strength steel sheet excellent in heat treatment resistance after welding to control the average grain size of the ferrite in the center of the steel sheet to 50㎛ or less.