Technical Field
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The present disclosure relates to a steel sheet used in a liquefied gas tank, or the like, and a method for manufacturing the same.
Background Art
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Recently, with the strengthening of environmental regulations, demand for liquefied gas (LNG), an eco-friendly fuel, has increased, and accordingly, the construction of LNG carriers and LNG propulsion vessels using LNG as fuel has increased. In the case of LNG and LNG/ethane, high-Ni steel having a high Ni content may be generally used due to an extremely low liquefaction temperature thereof. Also, to reduce construction costs of vessels transporting LPG, ammonia, CO2, and other liquefied gases having a liquefaction temperature of -60°C or lower, carbon steel having enhanced cryogenic toughness has been increasingly used.
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As for an existing LNG carrier tank, a large, non-pressurized Type A tank design may not be primarily used, such that there has been no demand for high-strength steel or a thick material. However, a CO2 tank and ammonia fuel tanks, which require high pressure, may be designed as Type C and may require high-strength steel and a thick material, and post-welding heat treatment (PWHT) required by the IGC code may be mandatory when building a Type C tank. Accordingly, it may be necessary to guarantee physical properties of steel used therein before and after post-welding heat treatment (PWHT).
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As for existing small-sized CO2 carriers, a high-strength heat-treated steel material may be generally use as liquefication is performed under high pressure. However, as CO2 carriers are designed to have a large size and a light weight to improve transport efficiency, liquefaction may be performed under low pressure in which a size of tank may increase. To increase the size of individual tanks and improve physical properties of welded zone, a thermomechanical control process (TMCP) type steel material having a low carbon equivalent along with high strength of 500 MPa or more may be necessary.
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Since post-welding heat treatment (PWHT) is a high-temperature heat treatment, strength may degrade after PWHT, required strength may not be reached, and particularly, as for a high-strength steel material manufactured using TMCP, strength may be ensured using a low-temperature transformation phase, such that, during post-welding heat treatment (PWHT), strength may decrease rapidly due to dislocation annihilation.
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To overcome this issue, when precipitation-strengthening elements are used, impact toughness may degrade, such that it may be extremely difficult to guarantee strength before and after PWHT. Also, liquefied gases may generally be liquefied at extremely low temperature, such that, depending on the type of liquefied gas used, toughness may need to be guaranteed at -60°C. When adding additional alloying elements to prevent strength loss after PWHT, large, coarse precipitates may be formed during heat treatment, such that toughness may degrade. Accordingly, to simultaneously ensure strength and toughness of a steel material for liquefied gas tank, it may be necessary to develop new steel materials.
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Also, it may not be possible to perform post-welding heat treatment (PWHT) on base materials depending on the manner in which post welding heat treatment (PWHT) is performed. In the case in which a difference in strength before and after PWHT is significant, areas with significant strength difference within the same tank may increase, which may cause stress concentration in local regions, and thus, it may also be necessary to develop a steel material which may minimize a difference in strength before and after post-welding heat treatment (PWHT).
Detailed description of present disclosure
Technical problems to solve
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An embodiment of the present disclosure is to provide a steel sheet having excellent strength and toughness even after post-welding heat treatment and a method for manufacturing the same, and to provide a steel sheet which may minimize a difference in strength before and after post-welding heat treatment and may have excellent toughness before and after post-welding heat treatment and a method for manufacturing the same.
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The subject matter of the present disclosure is not limited to the above-described contents. The subject matter of the present disclosure may be understood from the entire contents of this specification, and a person having ordinary skill in the technical field to which the present disclosure belongs may have no difficulty in understanding the additional subject matter of the present disclosure.
Solution to Problem
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According to an embodiment of the present disclosure, provided is a steel sheet including, by weight%, C: 0.03 to 0.06%, Mn: 1.8 to 2.1%, Si: 0.05 to 0.2%, Al: 0.01 to 0.04%, Ni: 0.5 to 1.0%, Ti: 0.01 to 0.018%, Nb: 0.01 to 0.03%, Mo: 0.2 to 0.4%, P: 0.008% or less, S: 0.002% or less, and a balance of Fe and inevitable impurities,
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wherein an average size of grains (effective grains) having a boundary angle of 15° or more, measured using electron backscatter diffraction (EBSD), is 20 µm or less.
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One of acicular ferrite, granular bainite, and a mixed structure thereof may be a main phase of the steel sheet.
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The main phase may be 70% or more by area%.
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The steel sheet may have a yield strength of 500 MPa or more and a tensile strength of 610 MPa or more after post welding heat treatment (PWHT) at 600°C for 6 hours.
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A difference in tensile strength of the steel sheet before and after post welding heat treatment (PWHT) at 600°C for 6 hours may be 50 MPa or less.
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The steel sheet may have impact toughness of 100J or more at -60°C before and after post welding heat treatment (PWHT) at 600°C for 6 hours.
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According to an embodiment of the present disclosure, provided is a method of manufacturing a steel sheet, the method including heating a steel slab including, by weight%, C: 0.03 to 0.06%, Mn: 1.8 to 2.1%, Si: 0.05 to 0.2%, Al: 0.01 to 0.04%, Ni: 0.5 to 1.0%, Ti: 0.01 to 0.018%, Nb: 0.01 to 0.03%, Mo: 0.2 to 0.4%, P: 0.008% or less, S: 0.002% or less, and a balance of Fe and inevitable impurities in a temperature range of 1050 to 1140°C;
- rough rolling the heated steel slab at a temperature of 800°C or higher;
- finish rolling at a temperature of 700°C or higher after the rough rolling; and
- cooling at a cooling rate of 5°C/s or higher to a temperature range of 400 to 700°C after the finish rolling. Advantageous Effects of Invention
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According to an embodiment of the present disclosure, a steel material which may be applied to various fields such as a liquefied gas tank, a ship hull, and structures in cryogenic environment by having excellent strength and toughness after post-welding heat treatment (PWHT), especially excellent strength and toughness before and after post-welding heat treatment (PWHT), and a method for manufacturing the same.
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The various beneficial advantages and effects of the present disclosure are not limited to the above-described contents, and will be more easily understood in the process of describing the specific embodiments of the present disclosure.
Best Mode for Invention
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The terms used in this specification are intended to describe the present disclosure and are not intended to limit the present disclosure. Also, an expression used in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context.
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The meaning of "including" used in this specification is to specify a configuration and does not exclude the presence or addition of other configurations.
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Unless otherwise indicated, the terms including technical and scientific terms, used herein may have the same meaning as commonly understood by a person of ordinary skill in the art to which the present disclosure pertains. The terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.
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The inventors of the present disclosure have conducted in-depth research into a technique of minimizing a difference in strength and toughness, particularly before and after performing post-welding heat treatment (PWHT) on a steel material having high strength and toughness even under harsh conditions. As a result, by optimizing the alloy composition and composition and microstructurally refining the grain size, it is confirmed that the difference in strength before and after PWHT may be minimized while simultaneously ensuring excellent toughness, even under harsh conditions in which post-welding heat treatment (PWHT) is performed, and the present disclosure was completed.
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First, a steel sheet, an example of the present disclosure, will be described in detail. First, a steel composition of the steel sheet will be described in detail. Unless otherwise indicated in the present disclosure, the percentages indicating the content of each element are based on weight.
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The steel sheet may include, by weight%, C: 0.03 to 0.06%, Mn: 1.8 to 2.1%, Si: 0.05 to 0.2%, Al: 0.01 to 0.04%, Ni: 0.5 to 1.0%, Ti: 0.01 to 0.018%, Nb: 0.01 to 0.03%, Mo: 0.2 to 0.4%, P: 0.008% or less, S: 0.002% or less, and a balance of Fe and inevitable impurities.
Carbon (C): 0.03 to 0.06%
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Carbon (C) may be crucial for ensuring fundamental strength of a welding heat-affected zone, and may thus need to be contained within an appropriate range in steel. When the carbon (C) content exceeds 0.06%, a secondary phase, such as martensite-austenite (MA), causing brittleness, may be formed, such that toughness of both a base material and a welded zone may degrade. When the content is less than 0.03%, strength may decrease, and thus, the carbon (C) content may be 0.03 to 0.06%.
Manganese (Mn): 1.8 to 2.1%
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Manganese (Mn) may be useful for enhancing strength by solid solution strengthening and enhancing hardenability to form a low-temperature transformation phase. Therefore, to ensure yield strength above the target level, manganese (Mn) may be included in an amount of 1.8% or more. When the content exceeds 2.1%, excessive hardenability may increase such that coarse bainite may be formed as a microstructure, which may significantly degrade toughness. Therefore, the manganese (Mn) content may be 1.8 to 2.1%.
Silicon (Si): 0.05 to 0.2%
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Silicon (Si) may precipitate dissolved oxygen in molten steel in the form of slag during steelmaking and casting processes, and may thus be an essential alloying element for deoxidation. Accordingly, when manufacturing a steel material using a converter, silicon (Si) may be included in an amount of 0.05% or more for the effect. When included excessively, silicon (Si) may form a complex oxide coarsely with Al or may generate a large amount of micro-hard phases such as an MA phase within a welding heat-affected zone microstructure. Therefore, the content thereof may be 0.2% or less.
Aluminum (Al): 0.01 to 0.04%
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Aluminum (Al) may precipitate dissolved oxygen in molten steel in the form of slag during steelmaking and casting processes, and may thus be an essential alloying element for deoxidation. Accordingly, when manufacturing steel using a converter, 0.01% or more of aluminum (Al) may be included for this effect. When included excessively, aluminum (Al) may form a coarse complex oxide with Si, or may form a large amount of a micro-hard phase such as an MA phase within a welding heat-affected zone microstructure, and thus, the content thereof may be 0.04% or less.
Nickel (Ni): 0.5 to 1.0%
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Nickel (Ni) may be important for enhancing impact toughness by facilitating dislocation cross-slip at low temperature and increasing hardenability, thereby increasing strength. To sufficiently ensure high yield strength and targeted impact toughness at low temperature, nickel (Ni) may be included in an amount of 0.5% or more. When the content exceeds 1.0%, hardenability may excessively increase, such that the large amount of bainite may be formed, which may degrade toughness, and manufacturing costs may also increase, which may be problematic.
Titanium (Ti): 0.01 to 0.018%
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Titanium (Ti) may precipitate as TiN during reheating and inhibiting the growth of grains in the welding heat-affected zone, thereby significantly improving low-temperature toughness. For effective TiN precipitation, 0.01% or more of titanium (Ti) may be included. When the content exceeds 0.018%, problems such as clogging of a casting nozzle or crystallization in a central region may occur, leading to reduced low-temperature toughness. Also, the Ti/N ratio may be lowered, such that TiN precipitates may become coarse, which may degrade toughness.
Niobium (Nb): 0.01 to 0.03%
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Niobium (Nb) may precipitate as NbC or NbCN and may improve strength. Also, niobium (Nb) dissolved during reheating at high temperature may precipitate extremely finely in the form of NbC during rolling, and may inhibit austenite recrystallization, and may prevent coarsening of structure after post-welding heat treatment (PWHT) and may prevent a decrease in strength through additional precipitation. Accordingly, in the present disclosure, niobium (Nb) may be included in an amount of 0.01% or more. When the content thereof is excessive, brittle cracks may be created at the edges of the steel material and the problem of degradation of toughness due to the formation of large amounts of coarse precipitates may occur, and thus, niobium (Nb) may be included in an amount of 0.03% or less.
Molybdenum (Mo): 0.2 to 0.4%
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Molybdenum (Mo) may be precipitated in the form of carbide during post welding heat treatment (PWHT) similarly to NbC or NbCN precipitated by Nb, and may improve strength, and may also prevent coarsening of Nb precipitates, thereby preventing toughness degradation. When the molybdenum (Mo) content is low, precipitation may be insufficient, and thus, molybdenum (Mo) may be included in an amount of 0.2% or more. When added excessively, strength may increase due to excessive precipitation, which may lead to toughness degradation. Thus, the content thereof may be 0.4% or less.
Phosphorus (P): 0.008% or less
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Phosphorus (P) may cause grain boundary embrittlement or may cause brittleness by forming coarse inclusions, and may be included in an amount of 0.008% or less to improve brittle crack propagation resistance. Also, phosphorus (P) may be an impurity element included in steel, and 0% may be excluded in the case in which phosphorus (P) is inevitably included during a manufacturing process.
Sulfur (S): 0.002% or less
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Sulfur (S) may cause grain boundary embrittlement or may cause brittleness by forming coarse inclusions, and may be included in an amount of 0.002% or less to improve brittle crack propagation resistance. Also, sulfur (S) may be an impurity element included in steel, 0% may be excluded in case in which sulfur (S) is inevitably included during a manufacturing process.
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The steel sheet in the present disclosure may include iron (Fe) and inevitable impurities in addition to the composition described above. Inevitable impurities may be unintentionally included during a general manufacturing process and may thus not be excluded. These impurities may be readily apparent to a person skilled in the art of steel manufacturing, and thus, the description thereof is not provided in this specification.
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In the description below, a microstructure of the steel sheet in the present disclosure will be described in detail.
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In the steel sheet according to an example of the present disclosure, grains having a boundary angle of 15° or more may have an average size of 20 µm or less.
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The boundary angle may indicate a difference in crystallographic orientation between grains. The boundary angle of 15° or more may be generally defined as a high-angle boundary and may deem to affect the physical properties, and thus, in present disclosure, the grain size may be limited based on this. Thus, grains may be referred to as effective grains.
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In the present disclosure, the size of grains having a boundary angle of 15° or more was measured using electron backscatter diffraction (EBSD). When the average size of grains having a boundary angle of 15° or more exceeds 20 µm, yield strength may decrease and impact toughness may degrade.
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Unless otherwise indicated in the present disclosure, the percentage, %, indicating a fraction of microstructure are based on area.
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In the steel sheet according to the example of the present disclosure, one of acicular ferrite, granular bainite, and a mixture thereof may be a main phase. It may be difficult to distinguish between acicular ferrite and granular bainite under normal optical conditions. In the steel sheet, the main phase may be 70% or more, preferably 90% or more, by area%.
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In addition to the main phase, the remaining microstructure may include upper bainite and other phases.
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The steel sheet according to an example of the present disclosure may have a yield strength of 500 MPa or more after post-welding heat treatment (PWHT) at 600°C for 6 hours, a tensile strength of 610 MPa or more after PWHT, and a difference in tensile strength before and after PWHT may be 50 MPa or less. Also, the Charpy impact absorption energy at -60°C, measured at the central region (1/2 point along the thickness direction) of the steel sheet before and after PWHT, may be 100 J or more.
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In the description below, a method of manufacturing a steel sheet, an example of the present disclosure, will be described in detail.
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The steel sheet according to an embodiment of the present disclosure may be manufactured by reheating, rough rolling, finish rolling, cooling, and the like, a steel slab satisfying the alloy composition described above.
Reheating
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A steel slab satisfying the alloy composition described in the present disclosure may be reheated in a temperature range of 1050 to 1140°C.
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The reheating temperature may be 1050°C or higher such that carbonitrides of Ti or Niobium formed during casting may be dissolved and may be finely precipitated during rolling or post-welding heat treatment (PWHT) thereafter. When the temperature thereof is excessively high, austenite may become coarse, and thus, an upper limit thereof may be 1140°C.
Rough Rolling
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The reheated steel slab may be rough rolled at a temperature of 800°C or higher.
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Rough rolling may be performed to adjust the shape of the reheated steel slab. Preferably, the rough rolling temperature may be performed at temperature (Tnr) or higher at which recrystallization of austenite is completed, and accordingly, the rough rolling may be preferably performed at 800°C or higher. Along with fracture of the casting structure, such as dendrite formed during casting, by rolling, the effect of reducing the grain size may be obtained through the recrystallization of coarse austenite. To refine the structure through sufficient recrystallization, the cumulative reduction ratio during rough rolling may be limited to 40% or more
Finish Rolling (Finish Rolling)
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The rough-rolled steel sheet may be finish-rolled at a temperature of 700°C or higher.
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Finish rolling may be performed to include the austenitic structure of the rough-rolled steel sheet as a non-uniform microstructure. To cause the deformation in the structure, finish rolling may be preferably performed at a temperature of 700°C or higher. To create the finest structure, the cumulative reduction ratio of finish rolling may be 50% or more, preferably 60% or more.
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When the finish rolling temperature is less than 700°C, the ferrite grain size may not be uniform due to the two-phase region rolling, and coarse ferrite may be formed.
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When the finish rolling cumulative reduction ratio is less than 50%, pan-caking of austenite may not occur sufficiently, such that the grain size of the microstructure formed during subsequent cooling may increase, and accordingly, degradation of yield strength and degradation of impact toughness at cryogenic temperature may occur. Thus, to ensure the strength and toughness suggested in the present disclosure, the finish rolling cumulative reduction ratio may be 50% or more.
Cooling
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The finished rolled steel sheet may be cooled to a temperature range of 400 to 700°C at a cooling rate of 5°C/s or higher. According to an embodiment of the present disclosure, during cooling, the temperature may be the temperature at the 1/4 point of the steel sheet in the thickness direction.
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When the cooling rate is less than 5°C/s, a coarse structure may be formed, such that sufficient strength may not be ensured. According to an embodiment of the present disclosure, the cooling rate may be 8°C/s or higher, or 10°C/s or higher.
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When the cooling end temperature is less than 400°C, the strength before post-welding heat treatment (PWHT) may be excessively high, such that a difference in tensile strength before and after post-welding heat treatment (PWHT) may exceed 50 MPa. When the cooling end temperature exceeds 700°C, coarse and soft ferrite may be formed, such that the strength before post welding heat treatment (PWHT) may decrease, and the targeted level of yield strength in the present disclosure may not be satisfied.
Mode for Invention
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Hereinafter, embodiments of the present disclosure will be described. It should be understood that various modifications to the embodiments may be made for those skilled in the art to which the present disclosure pertains within the scope of the present disclosure. The embodiments below are provided to facilitate understanding of the present disclosure, and the scope of the rights of the present disclosure should not be limited to the embodiments, and should be determined by the claims described below and also by equivalents thereof.
(Embodiment)
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A steel slab having a thickness of 300 mm and the composition (weight%, a balance of Fe and inevitable impurities) in Table 1 below was reheated under the conditions in Table 2 below, and rough rolling was performed continuously, and finished at 850°C or higher. Thereafter, finish rolling was performed, and cooling was performed, thereby manufacturing a steel sheet of a 50 mm. In this case, the cooling rate was 9 to 13°C/s.
[Table 1] | Stee l type | Alloy composition (weight%) |
| C | Mn | Si | Al | Ni | Mo | Nb | Ti | P (ppm) | S (ppm) |
| A | 0.051 | 1.92 | 0.11 | 0.02 | 0.86 | 0.32 | 0.013 | 0.012 | 63 | 12 |
| B | 0.043 | 1. 97 | 0.13 | 0.03 | 0.65 | 0.3 | 0.022 | 0.013 | 52 | 15 |
| C | 0.039 | 2.02 | 0.09 | 0.02 | 0.76 | 0.35 | 0.028 | 0.011 | 43 | 11 |
| D | 0.059 | 1.86 | 0.13 | 0.03 | 0.94 | 0.27 | 0.019 | 0.014 | 38 | 9 |
| E | 0.087 | 2.21 | 0.12 | 0.03 | 0.81 | 0.24 | 0.023 | 0.013 | 52 | 17 |
| F | 0.056 | 1. 94 | 0.13 | 0.03 | 0.88 | 0.35 | 0.043 | 0.012 | 37 | 15 |
| G | 0.037 | 1.85 | 0.09 | 0.03 | 0.62 | 0.22 | 0.007 | 0.013 | 49 | 15 |
| H | 0.047 | 2.08 | 0.08 | 0.02 | 0.36 | 0.36 | 0.026 | 0.012 | 44 | 17 |
| I | 0.052 | 2.01 | 0.12 | 0.02 | 0.81 | 0.12 | 0.022 | 0.016 | 35 | 9 |
| J | 0.049 | 1.86 | 0.16 | 0.02 | 0.79 | 0.52 | 0.024 | 0.015 | 45 | 13 |
[Table 2] | Sample No. | Steel type | Reheating temperature (°C) | Finish rolling temperature (°C) | Cooling end temperature (°C) |
| 1 | A | 1095 | 822 | 521 |
| 2 | B | 1106 | 815 | 512 |
| 3 | C | 1088 | 785 | 498 |
| 4 | D | 1121 | 779 | 475 |
| 5 | A | 1079 | 805 | 321 |
| 6 | E | 1103 | 812 | 465 |
| 7 | F | 1091 | 794 | 529 |
| 8 | G | 1103 | 788 | 504 |
| 9 | H | 1118 | 812 | 477 |
| 10 | I | 1086 | 786 | 492 |
| 11 | J | 1100 | 832 | 532 |
| 12 | C | 1115 | 685 | 514 |
| 13 | D | 1098 | 809 | 729 |
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The microstructure of the manufactured steel sheets was observed, and yield strength, tensile strength, and average impact energy values were measured and listed in Table 3. Post-welding heat treatment (PWHT) was also performed at 600°C for 6 hours, and the yield strength, tensile strength, and average impact energy values after PWHT were measured and listed.
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The microstructure fraction was observed using an optical microscope after nital etching for each specimen. The entirety of examples according to the embodiment had acicular ferrite, granular bainite, or a mixture thereof as a main phase, and an area fraction thereof was 70% or more. As the remaining structure, upper bainite was observed.
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Meanwhile, as for the average grain size, the average value was obtained by measuring the size of grains having a boundary angle of 15° or more using EBSD (electron backscatter diffraction). For example, the grain size was measured with a boundary angle of 15° or more in EBSD measurement with a step size of 0.2 µm. In this case, the 0.2 µm step may indicate the measurement interval, which may be an extremely fine measurement.
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The yield strength and tensile strength were measured using a room-temperature tensile test according to JIS-5. Also, as for the average impact energy, a Charpy impact test was performed at -60°C on the central region of the steel sheet (1/2 point along the thickness direction), and the results are listed.
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Also, the yield strength, tensile strength, and average Charpy impact energy were measured using the same method as above after performing post-welding heat treatment (PWHT) on the manufactured steel sheet. Also, the difference in tensile strength before and after post-welding heat treatment (PWHT) was calculated and listed.
[Table 3] | Sa mp le No . | Ste el typ e | Average grain size (µm) | Before PWHT | After PWHT | Diffe rence in tensi le stren gthbefor e and after PWHT (MPa) | Classif ication |
| Yield strengt h (MPa) | Tensile strengt h (MPa) | Average impact energy (-60°C, J) | Yield strengt h (MPa) | Tensile strengt h (MPa) | Average impact energy (-60°C, J) |
| 1 | A | 17 | 567 | 678 | 289 | 583 | 649 | 242 | 29 | Inventi ve example 1 |
| 2 | B | 14 | 554 | 687 | 231 | 576 | 654 | 194 | 33 | Inventi ve example 2 |
| 3 | C | 13 | 573 | 695 | 286 | 592 | 668 | 214 | 27 | Inventi ve example 3 |
| 4 | D | 15 | 589 | 706 | 254 | 603 | 665 | 193 | 41 | Inventi ve example 4 |
| 5 | A | 14 | 593 | 712 | 246 | 607 | 653 | 203 | 59 | Compara tive example 1 |
| 6 | E | 24 | 654 | 739 | 135 | 657 | 678 | 92 | 61 | Compara tive example 2 |
| 7 | F | 13 | 607 | 725 | 243 | 621 | 676 | 87 | 49 | Compara tive example 3 |
| 8 | G | 16 | 524 | 658 | 258 | 556 | 594 | 207 | 64 | Compara tive example 4 |
| 9 | H | 17 | 588 | 686 | 156 | 601 | 643 | 84 | 43 | Compara tive example 5 |
| 10 | I | 12 | 549 | 672 | 253 | 574 | 601 | 212 | 71 | Compara tive example 6 |
| 11 | J | 16 | 624 | 778 | 115 | 631 | 710 | 56 | 68 | Compara tive example 7 |
| 12 | C | 19 | 468 | 605 | 309 | 512 | 567 | 246 | 38 | Comparative example 8 |
| 13 | D | 17 | 447 | 559 | 256 | 489 | 529 | 257 | 30 | Compara tive example 9 |
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As indicated in Table 3, the inventive examples satisfying the alloy composition and manufacturing conditions of the present disclosure satisfied the microstructural characteristics suggested in the present disclosure and the properties targeted in the present disclosure was also ensured. Particularly, high strength was ensured even after PWHT, confirming that the difference in tensile strength before and after PWHT was insignificant. Generally, yield strength tends to increase slightly as continuous yielding changes to discontinuous yielding during PWHT.
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Comparative Example 1 was manufactured at a temperature lower than the cooling end temperature suggested in the present disclosure, such that a large amount of low-temperature transformation phase was formed, and strength before PWHT rapidly increased. Accordingly, tensile strength rapidly decreased after PWHT due to dislocation annihilation, such that a difference in tensile strength before and after PWHT exceeded 50 MPa.
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Comparative example 2 had the higher values than the upper limits of C and Mn suggested in the present disclosure, such that a large amount of upper bainite structure was formed in the microstructure due to high hardenability, and an average grain size exceeded 20 µm. Also, the difference in tensile strength before and after PWHT exceeded 50 MPa due to unnecessary high yield strength and tensile strength, and impact toughness at -60°C after PWHT was less than 100 J.
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Comparative example 3 had the higher values than the upper limits of Nb suggested in the present disclosure, such that, as a large amount of coarse Nb precipitates was formed after PWHT, impact toughness at -60°C after PWHT was less than 100 J.
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Comparative example 4 had a lower value than the Nb lower limit suggested in the present disclosure, such that Nb precipitates were not sufficiently formed, leading to a decrease in tensile strength after PWHT and accordingly, a difference in tensile strength before and after PWHT exceeded 50 MPa.
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Comparative example 5 had a lower value than the Ni lower limit suggested in the present disclosure, such that toughness was degraded, and accordingly, impact toughness at -60°C after PWHT was less than 100 J.
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Comparative example 6 had a lower value than Mo suggested in the present disclosure, such that Mo precipitates was not sufficiently formed, and tensile strength was rapidly degraded after PWHT, and accordingly, a difference in tensile strength before and after PWHT exceeded 50 MPa
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Comparative example 7 had a lower value than Mo suggested in the present disclosure, such that, as strength before PWHT excessively increased, the difference in tensile strength before and after PWHT exceeded 50 MPa, and due to high strength and a large amount of coarse precipitates after PWHT, impact toughness at -60°C after PWHT was less than 100 J.
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In comparative example 8, rolling was completed at a temperature lower than the finish rolling temperature suggested in the present disclosure, air-cooled ferrite was formed during rolling and during air cooling before cooling after rolling, such that yield strength was less than 500 MPa.
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In comparative example 9, the cooling was ended at a temperature higher than the cooling end temperature suggested in the present disclosure, the low-temperature transformation phase was not generated during cooling, and as air-cooled ferrite was formed after the cooling ended, yield strength was less than 500 MPa before and after PWHT.