US20230026210A1 - Steel material having excellent sulfide stress corrosion cracking resistance and method of manufacturing same - Google Patents

Steel material having excellent sulfide stress corrosion cracking resistance and method of manufacturing same Download PDF

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US20230026210A1
US20230026210A1 US17/784,026 US202017784026A US2023026210A1 US 20230026210 A1 US20230026210 A1 US 20230026210A1 US 202017784026 A US202017784026 A US 202017784026A US 2023026210 A1 US2023026210 A1 US 2023026210A1
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steel
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Seong-Ung Koh
Moo-Jong BAE
Yoen-Jung PARK
Young-Sub Byun
Dae-Woo Baek
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Posco Holdings Inc
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Posco Co Ltd
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    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
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    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite
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    • C21D2221/00—Treating localised areas of an article

Definitions

  • the present disclosure relates to a thick steel material that is suitable for a line pipe, a sour-resistant material and, more particularly, to a high-strength steel material having excellent sulfide stress corrosion cracking resistance, and a method of manufacturing the steel material.
  • SSC sulfide stress cracking
  • the length is regulated at 2 inches or more and hardness is regulated at Hv 345 or more for hard spots under API standards, and DNV standards regulate the same sizes as API standards, but regulate the upper limit of hardness at Hv 250.
  • steel materials for line pipe are manufactured generally by reheating, hot-rolling, and then accelerated-cooling a steel slab, and it is determined that hard spots (portions at which high-hardness structures are formed) are generated due to non-uniform rapid cooling of a surface portion in the accelerated cooling.
  • the cooling rate is higher at the surface portion than the center portion because water is sprayed to the surface of the steel plate and hardness in the surface portion is higher than the center portion due to the cooling rate difference.
  • a method of attenuating a water-cooling process may be considered as a method for suppressing formation of high-hardness structures at the surface portion of a steel material.
  • reducing surface hardness by attenuating water cooling is accompanied by strength reduction, which causes a problem that more alloy elements should be added, etc. Further, such an increase of alloy elements is also a factor that increases surface hardness.
  • Patent Document 1 Korean Patent Application Publication No. 1998-028324
  • An aspect of the present disclosure is to provide a high-strength steel material having excellent sulfide stress corrosion cracking resistance by effectively reducing hardness at a surface portion in comparison to a thick-plate water-cooled material (TMCP) by optimizing alloy composition and manufacturing conditions, and a method of manufacturing the high-strength steel material.
  • TMCP thick-plate water-cooled material
  • an aspect of the present disclosure is to provide a high-strength steel material having yield strength of 450 MPa or more and having excellent sulfide stress corrosion cracking resistance in a high-pressure H 2 S environment exceeding partial pressure of 1 bar, and a method of manufacturing the high-strength steel material.
  • an aspect of the present disclosure is to secure also a propagation resistance against sulfide stress corrosion cracking by increasing sulfide stress corrosion cracking resistance by effectively controlling hardness of a surface portion at a low level through optimization of alloy composition and manufacturing conditions, and by minimizing the content of chrome (Cr) that accelerates propagation of sulfide stress corrosion cracking in a high-pressure H 2 S environment.
  • An aspect of the present disclosure provides a steel material that includes, by weight %, carbon (C): 0.02 ⁇ 0.06%, silicon (Si): 0.1 ⁇ 0.5%, manganese (Mn): 0.8 ⁇ 1.8%, chrome (Cr): less than 0.05%, phosphorous (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb): 0.005 ⁇ 0.08%, titanium (Ti): 0.005 ⁇ 0.05%, calcium (Ca): 0.0005 ⁇ 0.005%; one or more of nickel (Ni) 0.05 ⁇ 0.3%, molybdenum (Mo) 0.02 ⁇ 0.2%, and vanadium (V): 0.005 ⁇ 0.1%, and a balance of Fe and unavoidable impurities, in which the Ca and the S satisfy the following Equation 1, and the steel material has a microstructure of a surface portion
  • each element represents the content of each element by weight %.
  • Another aspect of the present disclosure provides a method of manufacturing a steel plate that includes: heating a steel slab satisfying the alloy composition described and Equation 1 at a temperature range of 1100 ⁇ 1300° C. for 2 hours or more; manufacturing a hot-rolled plate by hot-rolling the heated steel slab; and cooling the hot-rolled plate after hot rolling, in which the cooling includes primary cooling, air cooling, and secondary cooling, and the primary cooling is performed at a cooling rate of 5 ⁇ 40° C./s such that a temperature of a surface portion of the hot-rolled plate becomes Ar1 ⁇ 50° C. ⁇ Ar3 ⁇ 50° C. and the secondary cooling is performed at a cooling rate of 50 ⁇ 500° C./s such that the temperature of the surface portion of the hot-rolled plate becomes 300 ⁇ 600° C.
  • This steel material of the present disclosure can be advantageously applied as not only the material of pipes such as a line pipe, but a sour-resistant material, and particularly, it is possible to provide a high-strength steel material having an excellent sulfide stress corrosion cracking characteristic even in a high-pressure H 2 S environment over partial pressure of 1 bar.
  • FIG. 1 shows microstrucures and hardness of surface portions of invention steel and comparative steel in an experimental example of the present disclosure
  • TMCP Thermo-Mechanical Control Process
  • the inventors as the result of recognizing and minutely examining the problem of the related art, have found out and achieved a steel material that can effectively suppress sulfide stress corrosion cracking resistance due to hard spots and does not easily propagate cracks even if cracks are generated at a surface portion due to hard spots.
  • the inventors as an aspect of the present disclosure, have intended to provide a steel material securing resistance against cracking and propagation resistance against cracking and having high strength by effectively decreasing hardness of a surface portion in a thick steel plate having a predetermined thickness or more.
  • the inventors has conceived a new cooling control technique rather than the common cooling method of the related art, whereby the inventors have conceived a technique that can attenuate hardness of a surface portion separating phase transformation at a surface portion and a center portion.
  • the inventors have develop a technique that can reduce hardenability of a surface portion by promoting decarburization of the surface portion in the process of heating and rolling, and can form ferrite at the surface portion. Further, the inventors of the present disclosure intend to provide a technique of manufacturing a steel plate having excellent sulfide stress corrosion cracking resistance even under a high-pressure H 2 S environment by optimizing the components of steel and conditions such as manufacturing process (heating, hot rolling, cooling, etc.) because they have found out that when Cr is added as an alloy elements in a steel material propagation resistance against sulfide stress corrosion cracking resistance is deteriorated.
  • a steel material may include, in percent by weight, carbon (C): 0.02 ⁇ 0.06%, silicon (Si): 0.1 ⁇ 0.5%, manganese (Mn) 0.8 ⁇ 1.8%, chrome (Cr): less than 0.05%, phosphorous (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al) 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb) 0.005 ⁇ 0.08%, titanium (Ti): 0.005 ⁇ 0.05%, calcium (Ca): 0.0005 ⁇ 0.005%; one or more of nickel (Ni): 0.05 ⁇ 0.3%, molybdenum (Mo): 0.02 ⁇ 0.2%, and vanadium (V): 0.005 ⁇ 0.1%, and a balance of Fe and unavoidable impurities.
  • the content of each element is based on weight and the ratio of structures is based on an area.
  • Carbon is an element having the largest influence on the properties of steel.
  • the content of C is less than 0.02%, there is a problem that an excessive component control cost is generated in the steel manufacturing process and welding heat-influenced portions are excessively softened.
  • the content exceeds 0.06%, hydrogen induced cracking resistance of a steel plate is decreased and weldabiity may be deteriorated.
  • C may be included at 0.02 ⁇ 0.06%, and more preferably, 0.03 ⁇ 0.05%.
  • Silicon (Si) is an element that not only is used as a deoxidizer in a steel manufacturing process, but serves to increase strength of steel.
  • Si is an element that not only is used as a deoxidizer in a steel manufacturing process, but serves to increase strength of steel.
  • the content of Si exceeds 0.5%, low-temperature toughness of a material, weldability, and scale separability in rolling are deteriorated. Meanwhile, the manufacturing cost is increased to reduce the content of Si less than 0.1%, so the content of Si may be limited at 0.1 ⁇ 0.5%, and more preferably, 0.2 ⁇ 0.4%.
  • Manganese (Mn) which is an element that improves hardenability of steel without deteriorating low-temperature toughness, may be included at 0.8% or more. However, when the content exceeds 1.8%, centerline segregation occurs, so there is a problem that low-temperature toughness is deteriorated, hardenability of steel is increased, and weldability is deteriorated. Further, centerline segregation of Mn is a factor that causes hydrogen induced cracking. Accordingly, Mn may be included at 0.8 ⁇ 1.8% in the present disclosure. Alternatively, in terms of centerline segregation, Mn may be included preferably at 0.8 ⁇ 1.6%, and more preferably, 1 ⁇ 1.4%.
  • Chrome (Cr) is solidified in austenite when a slab is reheated, thereby contributing to increasing hardenability of a steel material and securing strength of a steel plate.
  • the inventors have found out that when Cr is added at 0.05% or more, propagation of sulfide stress corrosion cracking may be promoted. That is, the content of Cr is limited less than 0.05% in a steel material, thereby achieving an effect that resistance against propagation of sulfide stress corrosion cracking.
  • the steel material according to an aspect of the present disclosure may include Cr more than 0% and less than 0.05%, more preferably, 0.04% of less, and the most preferably, 0.02% or less.
  • the lower limit of the content of Cr may be 0%, and preferably, 0.0005%.
  • Phosphorous (P) is an element that is unavoidably added in steel, and when the content exceeds 0.03%, there is a problem that not only weldability is remarkably decreased, but low-temperature toughness is reduced. Accordingly, it is required to limit the content of P at 0.03% or less, and, in terms of securing low-temperature toughness, more preferably, P may be included at 0.01% or less. However, 0% may be excluded as the lower limit of the content of Cr in consideration of load in the steel manufacturing process, and more preferably, the lower limit of the content of Cr may be 0.0001%.
  • Sulfur (S) is an element that is unavoidably added in steel, when the content exceeds 0.003%, there is a problem that ductility, low-toughness, and weldability of steel are reduced. Accordingly, the content of S needs to be limited at 0.003% or less. Meanwhile, S produces a MnS inclusion by bonding with Mn in steel, and in this case, the hydrogen induced cracking resistance of steel is deteriorated, so, more preferably, S may be included in 0.002% or less. However, 0% may be excluded as the lower limit of the content of S in consideration of load in the steel manufacturing process, and more preferably, the lower limit of the content of S may be 0.0001%.
  • Aluminum (Al) usually functions as a deoxidizer that removes oxygen by reacting with oxygen (O) existing in molten steel. Accordingly, Al may be added such that it has a sufficient decarburization ability in steel. However, when the content exceeds 0.06%, a large amount of oxide-based inclusion is produced and deteriorates low-temperature toughness, hydrogen induced cracking resistance, and sulfide stress corrosion cracking resistance, which is not preferable. Accordingly, Al may be included at 0.06% or less, and more preferably, 0.04% or less. However, 0% may be excluded as the lower limit of the content of S in consideration of that Al is necessarily included as a deoxidizer, and more preferably, the lower limit of the content of Al may be 0.005%.
  • N Nitrogen
  • the upper limit thereof is 0.01% that is an allowable range in a manufacturing process.
  • N produces nitrides by reacting with Al, Ti, Nb, V, etc. in steel, N suppresses growth of austenite grains, which has a good influence on improvement of toughness and strength of a material.
  • N may be included at 0.01% or less, and more preferably, 0.009% or less.
  • 0% may be excluded as the lower limit of the content of N in consideration of load in the steel manufacturing process, and more preferably, the lower limit of the content of N may be 0.0005%.
  • Niobium is an element that solidifies when a slab is heated, thereby suppressing growth of austenite grains and effectively improving strength of steel through precipitation. Further, Nb is precipitated as a carbide by bonding with C in steel, thereby serving to minimize an increase of a yield ratio and improving strength of steel.
  • Nb may be included within 0.005 ⁇ 0.08% in the present disclosure. Meanwhile, the lower limit of the content of Nb may be more preferably 0.02% and the upper limit of the content of Nb may be 0.05%.
  • Titanium (Ti) is precipitated as TiN by bonding with N when a slab is heated, which is effective in suppression of growth of austenite grains.
  • Ti When Ti is added less than 0.005%, austenite grains are coarsened, so low-temperature toughness is reduced.
  • the content exceeds 0.05%, coarse Ti-based precipitates are produced, so low-temperature toughness and hydrogen induced cracking resistance are reduced. Accordingly, Ti may be included within 0.005 ⁇ 0.05% in the present disclosure. Meanwhile, the lower limit of the content of Ti may be more preferably 0.006% and the upper limit of the content of Ti may be preferably 0.03% in terms of securing low-temperature toughness.
  • Ca Calcium (Ca) produces CaS by bonding with S in a steel manufacturing process, thereby suppressing segregation of MnS that causes hydrogen induced cracking. It is required to add Ca at 0.005% or more in order to sufficiently achieve the effect of suppressing segregation of MnS, but when the content exceeds 0.005%, not only CaS, but CaO inclusions are produced, so hydrogen induced cracking is caused by the inclusions. Accordingly, in the present disclosure, Ca may be included at 0.0005 ⁇ 0.005%, and more preferably, 0.001 ⁇ 0.003% in terms of securing hydrogen induced cracking resistance.
  • the steel material according to the present disclosure contains Ca and S, as described above, in which it is preferable that the composition ratio of Ca and S (([Ca]/[S]) satisfies the following Equation 1.
  • [Ca] is the average content of Ca in a steel material by weight % and [S] is the average content of S in a steel material by weight %). That is, the composition ratio of Ca and S is a representative index for core segregation of MnS and production of coarse inclusions, and when the [Ca]/[S] value is less than 0.5, MnS is produced at the center portion in the thickness direction of a steel material, which may cause a problem of reduction of hydrogen induced cracking resistance. On the contrary, when the Ca]/[S] value exceeds 5.0, Ca-based coarse inclusions are produced, which deteriorates hydrogen induced cracking resistance.
  • the composition ratio o Ca and S ([Ca]/[S]) satisfies Equation 1, and in order to further improve the above effect, more preferably, the [Ca]/[S] value may be within the range of 1.4 ⁇ 3.2.
  • the steel material of the present disclosure may further include elements that can further improve properties other than the alloy composition described above, and in detail, may further include one or more of Nickel (Ni): 0.05 ⁇ 0.3%, Molybdenum (Mo): 0.02 ⁇ 0.2%, and Vanadium (V): 0.005 ⁇ 0.1%.
  • the steel material has only to include one or more of Ni, Mo, and V within the range of being able to achieve the objectives of the present disclosure, and all of Ni, Mo, and V are not necessarily included in the present disclosure.
  • Nickel (Ni) is an element that has an effect in improvement of strength of steel without deterioration of low-temperature toughness. Ni may be added at 0.05% or more to achieve the effect of increasing strength without deteriorating low-temperature toughness, but Ni is an expensive element and the manufacturing process is considerably increased when the content of Ni exceeds 0.3%. Accordingly, Ni may be included at 0.05 ⁇ 0.3% when Ni is added in the present disclosure. Meanwhile, the lower limit of the content of Ni may be preferably 0.08%, and more preferably, 0.1%. Alternatively, the upper limit of the content of Ni may be preferably 0.28%, and more preferably, 0.21%.
  • Mo Molybdenum
  • Mo similar to Cr, improves hardenability of a steel material and increases strength.
  • Mo may be added at 0.02% or more to achieve the effect of improving hardenability described above, but when the content exceeds 0.2%, there is a problem that a structure that is vulnerable to low-temperature toughness such as upper bainite is produced, and hydrogen induced cracking resistance and sulfide stress corrosion cracking resistance are deteriorated. Accordingly, Mo may be included at 0.02 ⁇ 0.2% when Mo is added in the present disclosure. Meanwhile, the lower limit of the content of Mo may be more preferably 0.05% and the upper limit of the content of Mo may be 0.15%.
  • V Vanadium (V): 0.005 ⁇ 0.1%
  • Vanadium (V) is an element that improves strength by increasing hardenability of a steel material and may be added at 0.005% or more to achieve this effect.
  • V may be included at 0.005 ⁇ 0.1% when V is added in the present disclosure.
  • the lower limit of the content of V may be more preferably 0.005% and the upper limit of the content of V may be more preferably 0.05%.
  • the balance is F in the present disclosure.
  • unintended impurities may be unavoidably mixed from a raw material or a surrounding environment in a common manufacturing process, it cannot be excluded. Since anyone of those skilled in a common manufacturing process can know such impurities, they are not all specifically stated therein.
  • the steel material having the above alloy composition according to an aspect of the present disclosure is characterized in that the microstructure of the surface portion is composed of ferrite or a complex structure of ferrite and pearlite, whereby Vickers hardness of the surface portion may be controlled at 200 Hv or less.
  • the surface portion is the portion from the surface to a point at 1000 ⁇ m in the thickness direction, which may be applied to both sides of a steel material. Further, the center portion is the other region except for the surface portion.
  • the hardness of the surface portion is a maximum hardness value measured under 1 kgf load using Vickers hardness from the surface to a point at 1000 ⁇ m in the thickness direction. In general, hardness may be measured around 5 times at each position.
  • the microstructure is composed of ferrite or a complex structure of ferrite and pearlite and the microstructure of the center portion is composed of acicular ferrite, so it is possible to form a soft microstructure at the surface portion in comparison to the center portion, whereby it is possible to provide a steel material of which the hardness in the surface portion is lower than those of existing TMCP steel materials.
  • the same or high strength is secured in the steel material according to an aspect of the present disclosure in comparison to existing TMCP steel materials, so the steel material has yield strength of 450 MPa or more, the hardness of the surface portion is remarkably reduced, and the content of Cr is minimized, whereby it is possible to effectively suppress formation and propagation of sulfide stress corrosion cracks.
  • the steel material of the present disclosure may be manufactured through a process of [slab heating—hot rolling—cooling] and each of the process conditions are described in detail hereafter.
  • a steel slab that satisfies the alloy composition and component relationship proposed in the present disclosure may be prepared and then heated, which may be performed at 1100 ⁇ 1300° C. for 2 hours.
  • the steel slab described above may also be heated for 2 hours or more in the temperature range of 1100 ⁇ 1300° C., and more preferably, for 3.0 hours or more in the temperature range of 1145 ⁇ 1250° C.
  • the upper limit of the slab heating time is not specifically fixed, and generally, since the more the heating time, the higher the component uniformity, the heating time may be 50 hours or less, 20 hours or less, or 6 hours or less.
  • a hot-rolled plate may be manufactured by hot-rolling the heated steel slab.
  • hot rolling may be performed at an accumulated reduction ratio of 50% or more within the temperature range of Ar3+80° C. ⁇ Ar3+200° C., and resting may be maintained for 30 seconds or more (air cooling) after hot rolling.
  • the finishing rolling temperature of hot rolling is Ar3+80° C. ⁇ Ar3+200° C.
  • the accumulated reduction ratio is less than 50% in hot rolling in the temperature range described above, recrystallization by rolling even to the center portion of the steel material does not occur, so there is a problem that grains are coarsened in the center portion and low-temperature toughness is deteriorated. Accordingly, it is preferable in the present disclosure that the accumulated reduction ratio is 50% or more in hot rolling.
  • the maintaining time is less than 30 seconds after hot rolling, the time for decarburization at the surface portion is insufficiency, so it is difficult to contribute to forming ferrite at the surface portion in the following process. Accordingly, it is preferable in the present disclosure that the maintaining time after finishing hot rolling is 30 seconds or more.
  • the upper limit of the maintaining time after finishing hot rolling is not specifically fixed, but may be preferably 30 minutes or less, 10 minutes or less, or 5 minutes or less. Further, since such maintaining time is provided, cooling start temperature to be described below can be secured from air cooling.
  • the hot-rolled plate manufactured through such hot rolling can be cooled, and particularly, it would be technically meaningful to provide an optimal cooling process that can obtain a steel material of which hardness in the surface portion is effectively reduced in the present disclosure.
  • the cooling includes primary cooing; air cooling, and secondary cooling, and each of the process conditions are described in more detail hereafter.
  • the primary cooling and the secondary cooling may be performed by applying a specific cooling means, and for example, water cooling may be performed.
  • primary cooling may be performed after hot rolling—maintaining time over 30 seconds described above is maintained.
  • start temperature of primary cooing exceeds Ar3+50° C.
  • phase transformation into ferrite is not sufficiently made at the surface portion during primary cooling, so a hardness reduction effect at the surface portion cannot be achieved.
  • start temperature of primary cooing is less than Ar3 ⁇ 20° C.
  • ferrite transformation is excessive generated even to the center portion, which is a factor that reduces strength of steel.
  • the primary cooling at a cooling rate of 5 ⁇ 40° C./s such that the surface temperature of the hot-rolled plate becomes Ar1 ⁇ 50° C. ⁇ Ar3 ⁇ 50° C.
  • the cooling rate in the primary cooling is excessively low less than 5° C./s, it is difficult to primary cooling end temperature described above, but when the cooling rate exceeds 40° C./s, the fracture of phase transformation into acicular ferrite, so a soft structure cannot be formed at the surface portion. Accordingly, in the primary cooling, for the temperature at the surface portion, it is possible to control the average cooling rate at 5 ⁇ 40° C./s, and more preferably, 17 ⁇ 40° C./s.
  • the temperature at the center portion of the hot-rolled plate may be controlled at Ar3 ⁇ 30° C. ⁇ Ar3+30° C. That is, when the temperature at the center portion of the hot-rolled plate exceeds Ar3+30° C. at the end of the primary cooling, the temperature of the surface portion cooled within a specific temperature range is increased, so the fracture of ferrite phase transformation of the surface portion is decreased. Accordingly, the temperature at the center portion of the hot-rolled plate may be controlled preferably at 730 ⁇ 810° C. at the end of the primary cooling.
  • the secondary cooling is performed at a cooling rate of 50 ⁇ 500° C./s such that the temperature of the surface portion becomes 300 ⁇ 600° C.
  • the cooling rate is less than 50° C./s in secondary cooling within the temperature range described above, the grains at the center portion are coarsened, so it is difficult to secure strength at a target level.
  • the cooling rate exceeds 500° C./s, the fracture of a phase vulnerable to low-temperature toughness such as upper bainite is increased due to a microstructure at the center portion, so hydrogen induced cracking resistance is deteriorated, which is disadvantageous.
  • the secondary cooling for the temperature at the surface portion, it is possible to control the average cooling rate at 50 ⁇ 500° C./s, and more preferably, 245 ⁇ 500° C./s.
  • a steel material manufactured through the sequence of processes may have thickness of 5 ⁇ 50 mm.
  • Steel slabs having the alloy composition and properties shown in the following Tables 1 and 2 were prepared.
  • the content of the following ally composition is described in percent by weight and the balance includes Fe and other unavoidable impurities.
  • Steel materials was manufactured by heating, hot-rolling, and cooling the prepared steel slabs, respectively, under the conditions shown in Tables 3 and 4.
  • the steel materials of the invention steel and comparative steel were obtained by heating slabs having the composition described in the following Table 1 under the conditions described in Table 3, performing rough rolling under common conditions, performing finishing hot-rolling under the conditions described in Table 3, and then performing water cooling after maintaining resting for a predetermined time.
  • Cooling described Table 4 was controlled by performing intermediate air cooling and then secondary cooling after primary cooling.
  • yield strength is 0.5% under-load yield strength
  • API-5L specimens were taken in a direction perpendicular to the rolling direction as the tension samples, and the tests were performed.
  • Hardness of the steel materials was measured on thickness cross-sections under 1 kgf load using a Vickers hardness tester, and hardness of the surface portions were measured from the surface portion to positions at 100 ⁇ m and were shown in the following Table 5.
  • microstructures were measured using an optical microscope and the kinds of phases were observed using an image analyzer.
  • a 4 Point Bent Beam Test was performed for characteristic analysis of sulfide stress corrosion cracking (SSC) under NACE standard test method (TM-0177), and whether cracking occurred was estimated by adding 90% of yield strength of each steel plate to a strong acid Sol. A solution and then exposing the solution in an H 2 S environment of bar for 720 hours.
  • SSC sulfide stress corrosion cracking
  • the comparative steels 1 to 4 did not satisfy both the composition and manufacturing conditions of the present disclosure, and particularly, the 2-step cooling method proposed in the present disclosure was not applied in cooling.
  • the comparative steels 4 to 9 used steel slabs having the same composition as the invention steel 1 of the present disclosure and did not satisfy the manufacturing conditions of the present disclosure. That is, the 2-step cooling method proposed in the present disclosure was not applied to the comparative steel 4, and, in the comparative steel 5, a primary cooling end temperature of the surface portion and the temperature of the surface portion after intermediate air cooling were out of the range proposed in the present disclosure.
  • the primary cooling rate of the surface portion was out of the range proposed in the present disclosure in the comparative steel 6
  • the finishing temperature of hot rolling was out of the lower limit range proposed in the present disclosure in comparative steel 7
  • the finishing temperature of hot rolling was decreased, so all of the primary cooling start temperature, the primary cooling end temperatures of the surface portion and the center portion, and the temperature of the surface portion after intermediate air cooling were all out of the ranges proposed in the present disclosure.
  • the heating temperature of the slab was out of the lower limit range proposed in the present disclosure in the comparative steel 8, and the maintaining time after finishing hot rolling was out of the lower limit range proposed in the present disclosure in the comparative steel 9.
  • the 2-step cooling proposed in the present disclosure was not applied to the comparative steels 1 to 4, so a ferrite structure of a complex structure of ferrite and pearlite proposed in the present disclosure was not formed in the microstructures of the surface portions. Accordingly, the hardness in the surface portions exceeded 200 Hv in the comparative steels 1 to 4, so sulfide stress corrosion cracking was generated due to high hardness in the surface portions.
  • 2-step cooling proposed in the present disclosure was not applied to the comparative steel 1, but the primary cooling end temperature of the center portion and the temperature of the surface portion after intermediate air cooling were low, so ferrite transformation was generated before secondary cooling.
  • sulfide stress corrosion cracking was the generated, but the yield strength did not satisfy 450 MPa or more that is the range set in the present disclosure.
  • the primary cooling rate exceeded the upper limit proposed in the present disclosure and ferrite was not formed at the surface portion, so sulfide stress corrosion cracking was generated.
  • the finishing temperature of hot rolling did not satisfy the lower limit proposed in the present disclosure, in which the cooling temperature after hot rolling also did not satisfy the range proposed in the present disclosure, so ferrite transformation was generated even to the center portion, and accordingly, the yield strength was insufficient.
  • the heating temperature of the slab was out of the range proposed in the present disclosure in the comparative steel 8 and the maintaining time after hot rolling was out of the range proposed in the present disclosure in the comparative steel 9.
  • the comparative steels 8 and 9 since ferrite transformation was insufficient at the surface portions, so a complex structure of ferrite and acicular ferrite was formed, whereby the surface portion hardness reduction effect was not sufficiently achieved and sulfide stress corrosion cracking was generated.
  • the hardness in the surface portions is 200 Hv or less, so the hardness in the surface portion is remarkably low and yield strength of 450 MPa or more could be secured. Further, it could be seen that resistance against sulfide stress corrosion cracking was also excellent.
  • the hardness in the surface portions of the steel materials was not sufficiently low, so sulfide stress corrosion cracking was generated or yield strength of 450 MPa or more could not be secured.
  • FIG. 1 microstructure pictures at the surface portions and the hardness values at the surface portions measured by an optical microscope for the invention steel 2 and the comparative steel 3 of the above test examples were shown in FIG. 1 .
  • the left pictures show hardness measured from a surface to a position at 100 ⁇ m using a Vickers hardness tester
  • the right pictures show hardness measured from a surface to a position at 500 ⁇ m.
  • the steel material of the present disclosure has hardness of 200 Hv at the surface portion, but the hardness in the surface portion exceeds 200 Hv in the comparative steel 3 to which 2-step cooling proposed in the present disclosure was not applied.

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