Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel sheet having excellent strength and toughness which can be used in offshore and onshore wind generators and the like, and a method for manufacturing the same.

Description

Technical Field

[0001] The present disclosure relates to a steel plate having excellent strength and toughness, which can be used for offshore and onshore wind turbines, etc., and a method for manufacturing the same. Background Art

[0002] As wind turbines for offshore and onshore wind farms become larger, it is necessary to improve the load resistance of the wind tower, and the demand for thick steel materials with improved strength increases. However, generally, when the steel material becomes thicker, the strength decreases and the required thickness is additionally applied, resulting in a vicious cycle. Therefore, when thick steel also has the yield strength and tensile strength required for thin steel, the shell thickness can be reduced according to the increase in design strength, which has many advantages. In addition, as the number of cases where wind towers are installed and operated in extreme condition locations increases, impact toughness assurance is also required.

[0003] In order to achieve high strength and excellent low-temperature impact toughness of steel materials, grain refinement is crucial, and the rolling process is one of the representative methods of grain refinement. When rolling is performed at a temperature at which recrystallization can occur, new austenite fine grains are generated using the internal stress generated by the rolling force as a driving force. At the same time, rolling in the non-recrystallization temperature region stresses the grains, forms a banded structure in the rolling direction, and generates many dislocations therein. Therefore, when austenite undergoes a phase transformation, more nucleation points can be provided, thereby including the effect of grain refinement.

[0004] However, as the thickness of the steel material increases, the rolling force that can be applied by rolling is limited. Therefore, it is difficult to form fine grains by rolling into the internal structure of the steel material, especially the central part. As a result, the yield strength and tensile strength are significantly reduced, making it difficult to meet the target mechanical properties.

[0005] At the same time, it is usually difficult to obtain sufficiently small-sized grains only by slab heating and rolling (which is the process mainly for austenite grain refinement). In particular, the higher the temperature of the rolled steel, the lower the deformation resistance during rolling. Therefore, slab heating is mainly performed at a temperature far above the Ae3 temperature for easy rolling, and at this time, the austenite grains grow significantly. When the grain refinement effect by rolling is insufficient, additional austenite grain refinement effects can be expected through reheat treatment (which is usually performed by normalizing heat treatment).

[0006] Traditionally, normalized steel has been used as a material for wind towers. However, when heat treatment is applied during the manufacturing process, the manufacturing cost increases significantly, making it less commercially viable compared to as-rolled steel or thermo-mechanical controlled process (TMCP) steel.

[0007] Normalized Rolling (NR) is a manufacturing method in which hot rolling is performed within a temperature range having characteristics similar to those of normalized heat-treated steel plates without performing normalized heat treatment, followed by air cooling, which is referred to as Normalized Rolling (NR). When characteristics similar to those of normalized heat-treated steel can be obtained through optimal composition design or establishment of manufacturing conditions, the manufacturing cost can be reduced due to omission of heat treatment, enabling the provision of commercially available steel.

[0008] Patent Document 1 proposed a method for manufacturing steel having excellent impact toughness without performing normalized heat treatment. However, although the steel of Patent Document 1 may be advantageous in ensuring low-temperature impact toughness due to its low carbon content, it is difficult to satisfy sufficient strength, and as the thickness of the steel increases, the strength is significantly reduced because the finish rolling conditions for ensuring strength are not considered, making it difficult to satisfy sufficient yield strength.

[0009] (Patent Document 1) Korean Patent Publication No. 10-1917453 Summary of the Invention

[0010] Technical Problem

[0011] One aspect of the present disclosure relates to a steel plate having excellent strength and impact toughness, and having excellent strength and impact toughness even when normalized heat treatment is omitted, and a method for manufacturing the same.

[0012] The object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of this specification, and those skilled in the art to which the present disclosure pertains will not have difficulty understanding additional objects of the present disclosure.

[0013] Technical Solution

[0014] According to one aspect of the present disclosure, there is provided a steel plate comprising, by weight %, more than 0.10% to 0.17% of carbon (C), 0.2% to 0.5% of silicon (Si), 1.2% to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015% to 0.045% of aluminum (Al), 0.03% to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005% to 0.017% of titanium (Ti), 0.002% to 0.01% of nitrogen (N), and the balance of Fe and inevitable impurities, and the steel plate satisfies the following relational expression 1,

[0015] wherein the microstructure of the steel plate comprises, by area fraction, 60% to 85% of ferrite, and the balance of pearlite and inevitable structures.

[0016] [Relational expression 1]

[0017] [C] + [Mn] / 6 + [V] / 5 ≤ 0.43

[0018] Where [C], [Mn], and [V] are the contents (by weight %) of the respective components.

[0019] The thickness of the steel plate can be 100 mm or less.

[0020] The average grain size of the ferrite can be 50 μm or less.

[0021] The steel plate can have a yield strength of 355 MPa or greater and a tensile strength of 470 MPa or greater, where the yield strength and tensile strength are evaluated at the t / 4 point perpendicular to the rolling direction, and t is the thickness (mm) of the steel plate.

[0022] The steel plate can have a Charpy impact absorption energy of 100 J or greater at -20°C, where the Charpy impact absorption energy at -20°C is evaluated at the t / 4 point in the rolling direction, and t is the thickness (mm) of the steel plate.

[0023] According to another aspect of the present disclosure, there is provided a method for manufacturing a steel plate, the method including: preparing a steel slab that contains, by weight %, more than 0.10% to 0.17% of carbon (C), 0.2% to 0.5% of silicon (Si), 1.2% to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015% to 0.045% of aluminum (Al), 0.03% to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005% to 0.017% of titanium (Ti), 0.002% to 0.01% of nitrogen (N), and the balance of Fe and unavoidable impurities, where the following relational expression 1 is satisfied;

[0024] Heating the steel slab to a temperature in the range of 1150°C to 1200°C for 3 hours or longer;

[0025] Hot rolling: rough rolling the heated steel slab and performing finish rolling at a finish rolling start temperature that satisfies the following relational expression 2; and

[0026] Cooling after hot rolling,

[0027] [Relational expression 1]

[0028] [C] + [Mn] / 6 + [V] / 5 ≤ 0.43

[0029] Where [C], [Mn], and [V] are the contents (wt%) of the respective components.

[0030] [Relational expression 2]

[0031] 800°C ≤ starting temperature of finish rolling during hot rolling ≤ 857°C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb] 0.5 ) + (732 × [V]) - (230 × [V] 0.5 ) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si])

[0032] Where [C], [Nb], [V], [Ti], [Al], and [Si] are the contents (wt%) of the respective components.

[0033] The hot rolling can be carried out with a total reduction ratio of 45% or more and the number of rolling passes of 8 or less.

[0034] The finish rolling end temperature can be Ar3 or higher.

[0035] The cooling can be air cooling.

[0036] After cooling, it can further include a heat treatment operation at a temperature in the range of 830°C to 930°C for 1.3t + 30 minutes or longer, where t is the thickness (mm) of the steel.

[0037] Advantageous effects

[0038] As described above, according to one aspect of the present disclosure, a steel plate having excellent strength and low-temperature impact toughness that can be used for offshore and onshore wind turbines, etc., can be provided, and the normalizing heat treatment can be omitted, thereby improving cost efficiency.

[0039] Various beneficial advantages and effects of the present disclosure are not limited to the above, and can be more easily understood through the description of the specific embodiments of the present disclosure. Description of the drawings

[0040] Figure 1 of (a) and Figure 1 of (b) are photos obtained by respectively observing the microstructures at the t / 4 point of Invention Example 1 and Comparative Example 10 in the embodiments of the present disclosure.

[0041] Figure 2 is a graph showing the change in yield strength according to the starting temperature of finish rolling for manufacturing a hot-rolled steel plate with a thickness of 100 mm using Steel Grade 2 in one embodiment of the present disclosure. Detailed description of the embodiments

[0042] The terms used in this specification are for describing the present disclosure and are not intended to limit the present disclosure. Further, as used herein, unless the relevant definition clearly states otherwise, the singular forms include the plural forms.

[0043] As used in this specification, "comprising" or "including" means specifying a configuration and does not exclude the presence or addition of other configurations.

[0044] Unless otherwise defined, all terms, including technical and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the art to which the present disclosure pertains. Terms defined in a dictionary are interpreted to have a meaning consistent with the relevant technical literature and the present disclosure.

[0045] The inventors of the present disclosure have recognized that as the wind power structural steel for onshore and offshore wind towers becomes larger, technologies for ensuring the required properties of the materials are needed and economic efficiency is required.

[0046] In particular, for steel materials for wind power structures having a thickness greater than a certain level, the inventors of the present disclosure have conducted in-depth research on methods for ensuring both high strength and low-temperature impact toughness. As a result, the inventors of the present disclosure have determined that by controlling the composition of components and the relationships between some components in alloy design and simultaneously optimizing the manufacturing conditions, steel materials for wind power structures having target properties can be provided, thereby providing the present disclosure.

[0047] Hereinafter, the present disclosure will be described in detail.

[0048] First, the alloy composition of the steel plate according to one aspect of the present disclosure will be described in detail. Unless otherwise specifically stated in the present disclosure, the content of each element is by weight, and the ratio of the structure is by area.

[0049] The steel plate may contain, by weight %, more than 0.10% to 0.17% of carbon (C), 0.2% to 0.5% of silicon (Si), 1.2% to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015% to 0.045% of aluminum (Al), 0.03% to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005% to 0.017% of titanium (Ti), and 0.002% to 0.01% of nitrogen (N).

[0050] Carbon (C): more than 0.10% to 0.17%

[0051] Carbon (C) is an effective element for improving the strength of steel. To fully obtain such an effect, C can be included in an amount greater than 0.10%. However, when the C content exceeds 0.17%, while this can be advantageous in ensuring strength, there is a problem of a significant increase in the banded pearlite fraction, which may significantly inhibit the low-temperature impact toughness. When the C content is less than 0.10%, it may be insufficient in ensuring strength. The C content is preferably from 0.100% to 0.170%.

[0052] Silicon (Si): 0.2% to 0.5%

[0053] Silicon (Si) is not only used as a deoxidizer but also an element that is advantageous in improving the strength of steel. To fully obtain the above effects, Si can be included in an amount of 0.2% or more. However, when the Si content exceeds 0.5%, there is a concern that excessive formation of martensite-austenite constituents (MA) may occur, resulting in poor low-temperature impact toughness. The Si content is preferably from 0.20% to 0.50%.

[0054] Manganese (Mn): 1.2% to 1.6%

[0055] Manganese (Mn) is an element that is advantageous in improving the strength of steel due to the solid-solution strengthening effect. To fully obtain such an effect, Mn can be included in an amount of 1.2% or more. However, when the Mn content exceeds 1.6%, Mn combines with sulfur (S) in the steel to form MnS, thereby reducing the low-temperature impact toughness. Therefore, Mn can be included in an amount of 1.2% to 1.6%, preferably 1.20% to 1.60%, more preferably 1.45% to 1.6%.

[0056] Phosphorus (P): 0.012% or less

[0057] Phosphorus (P) is an element that is advantageous in improving the strength of steel and ensuring corrosion resistance, but P may significantly reduce the impact toughness of the steel. Therefore, it is preferable to limit the P content as low as possible. In the present disclosure, even when the maximum content of P is 0.012%, there is no problem in ensuring the target properties. Therefore, the P content can be set to 0.012% or less. However, considering the inevitable addition level, 0% can be excluded.

[0058] Sulfur (S): 0.003% or less

[0059] Sulfur (S) combines with Mn in steel to form MnS etc., thereby significantly reducing the low-temperature impact toughness. Therefore, it is beneficial to limit the S content to as low as possible. In the present disclosure, even when the maximum content of S is 0.003%, there is no problem in ensuring the target properties, so the S content can be set to 0.003% or less. However, considering the inevitable addition level, 0% can be excluded.

[0060] Aluminum (Al): 0.015% to 0.045%

[0061] Aluminum (Al) is an element that can deoxidize molten steel inexpensively. To fully obtain the above effects, Al can be included in an amount of 0.015% or more. However, when the Al content is too high and exceeds 0.045%, not only may nozzle clogging occur during continuous casting, but also the impact toughness may be significantly reduced due to the formation of oxide inclusions, which is not preferred.

[0062] Niobium (Nb): 0.03% to 0.05%

[0063] Niobium (Nb) precipitates in the form of NbC or Nb(C,N), which greatly improves the strength of the base material. When reheated at high temperatures, the dissolved Nb inhibits the recrystallization of austenite and the transformation of ferrite or bainite, thereby obtaining a grain refinement effect. However, when the Nb content becomes excessive, undissolved Nb forms in the form of TiNb(C,N), which becomes a factor inhibiting the low-temperature impact toughness. Therefore, the upper limit of the Nb content is preferably limited to 0.05%. Therefore, in the present disclosure, Nb can be included in an amount of 0.03% to 0.05%, preferably 0.030% to 0.050%, more preferably 0.035% to 0.045%.

[0064] Vanadium (V): 0.06% or less (including 0%)

[0065] Vanadium (V) has a lower temperature for dissolving V than other alloying elements, and V forms VC during the air-cooling process after hot rolling, which greatly contributes to increasing the strength. By adding V in an amount of 0.01% or more, a strength improvement effect can be obtained. However, when the V content exceeds 0.06%, there is a problem that the hardness of polygonal ferrite becomes too high and the fraction of hard phases such as MA increases, resulting in a significant reduction in low-temperature impact toughness. The V content is preferably 0.060% or less.

[0066] Titanium (Ti): 0.005% to 0.017%

[0067] Titanium (Ti) forms TiN when added together with nitrogen (N), thereby reducing the occurrence of surface cracks caused by the formation of AlN precipitates. Therefore, it is effective to add Ti in an amount of 0.005% or more. However, when the Ti content exceeds 0.017%, coarse TiN is formed during the reheating of the steel billet, which acts as a factor inhibiting low-temperature impact toughness. Therefore, a Ti content of 0.005% to 0.017%, preferably 0.0050% to 0.0170%, more preferably 0.01% to 0.015% is effective.

[0068] Nitrogen (N): 0.002% to 0.01%

[0069] Nitrogen (N) is an element that is beneficial in inhibiting grain growth caused by the heat affected zone during welding by forming TiN when added together with Ti. When adding Ti, in order to fully obtain the above effects, N can be included in an amount of 0.002% or more. However, when the N content exceeds 0.01%, coarse TiN is formed, which inhibits low-temperature impact toughness, which is not preferred. The N content is preferably 0.0020% to 0.010%.

[0070] The remaining components of the present disclosure are iron (Fe). However, since in the normal manufacturing process, it may be inevitable to incorporate unexpected impurities from raw materials or the surrounding environment, these components may not be excluded. Since these impurities are known to any person skilled in the art in the normal manufacturing process, their entire composition is not specifically given in this specification.

[0071] The steel plate can satisfy the following relational expression 1.

[0072] [Relational Expression 1]

[0073] [C] + [Mn] / 6 + [V] / 5 ≤ 0.43

[0074] Where [C], [Mn], and [V] are the contents (wt%) of the respective components.

[0075] In order to ensure low-temperature impact toughness and the target strength level, in the present disclosure, when adding a certain amount of elements that are beneficial in improving such characteristics, it is necessary to appropriately control the content of these elements. When the value of the above relational expression 1 exceeds 0.43, this may be beneficial in ensuring strength, but there is a concern that the physical properties after welding may deteriorate significantly. In addition, when a large amount of alloying elements are included, the economic feasibility deteriorates due to the increase in cost. It is effective that the value of the above relational expression 1 is 0.43 or less, and it is more effective that the value of the above relational expression 1 is 0.430 or less.

[0076] The microstructure of the steel plate may contain, by area %, 60% to 85% ferrite and the remaining pearlite and inevitable structures.

[0077] In this case, the average grain size of the ferrite may be 50 μm or less.

[0078] When the fraction of ferrite is less than 60%, the strength may be too high and the low-temperature impact toughness may decrease. When the fraction of ferrite exceeds 85%, it may be difficult to ensure the strength. When the average grain size of ferrite is less than 50 μm, it is not easy to ensure the yield strength and low-temperature impact toughness proposed in the present disclosure.

[0079] In addition, the steel of the present disclosure may have a yield strength of 355 MPa or greater, a tensile strength of 470 MPa or greater, and an average Charpy impact energy absorption (CVN) value of 100 J or greater at -20°C. The yield strength, tensile strength, and average Charpy impact energy absorption (CVN) value at -20°C are evaluated at the t / 4 point in the thickness direction perpendicular to the rolling direction, where t is the thickness (mm) of the steel; and may have appropriate strength and excellent low-temperature impact toughness.

[0080] Meanwhile, even without normalizing heat treatment, the steel of the present disclosure can ensure excellent strength and impact toughness. In the prior art, normalizing heat treatment is carried out to improve the yield strength and low-temperature impact toughness. When normalizing heat treatment is carried out, the yield strength and low-temperature impact toughness are improved by an additional grain refinement effect during the transformation from austenite to ferrite. Even without normalizing heat treatment, the steel of the present disclosure can ensure sufficient yield strength and low-temperature impact toughness.

[0081] Next, a method for manufacturing a steel plate according to another aspect of the present disclosure will be described in detail. A steel billet that satisfies both the above alloy components and relational expression 1 can be prepared, and the steel billet can be manufactured by a process of heating - hot rolling - cooling.

[0082] The conditions of each process are described in detail.

[0083] Steel billet heating

[0084] Preferably, a process of heating and homogenizing the steel billet is carried out, and the heating process can be carried out at a temperature in the range of 1150°C to 1200°C for 3 hours or longer.

[0085] When the heating temperature of the steel billet is lower than 1150 °C, the precipitates (carbides and nitrides) formed in the slab are not fully redissolved. Therefore, the formation of precipitates is reduced during the subsequent processes after hot rolling, and it eventually becomes difficult to meet the yield strength and tensile strength proposed in the present disclosure. On the other hand, when the heating temperature exceeds 1200 °C, there is a concern that the austenite grains may coarsen, which may deteriorate the properties of the steel. In addition, when the heating time of the steel billet is 3 hours or shorter, it may be difficult to reach the target temperature for the central part, making it difficult for the precipitates (carbides and nitrides) to redissolve.

[0086] Hot rolling

[0087] The hot-rolled steel billet heated as above can be hot-rolled to manufacture a hot-rolled steel sheet. In this case, the heated steel billet can be rough-rolled in the temperature range of 900 °C to 1100 °C, then finish-rolled in the temperature range shown in Relational Expression 2, and finish hot-rolled at Ar3 or higher.

[0088] [Relational Expression 2]

[0089] 800 °C ≤ starting temperature of finish rolling ≤ 857 °C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb] 0.5 ) + (732 × [V]) - (230 × [V] 0.5 ) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si])

[0090] where [C], [Nb], [V], [Ti], [Al], and [Si] are the contents (wt%) of the respective components.

[0091] When the temperature during rough rolling is lower than 900 °C, there is a problem that the temperature at which the subsequent finish rolling starts becomes too low. When the starting temperature of finish rolling is lower than 800 °C, there is a problem that the temperature at which the subsequent finish hot rolling starts becomes too low. When finish rolling starts at a temperature higher than the temperature in Relational Expression 2, insufficient rolling force is transmitted in the non-recrystallization region, so the size of the polygonal ferrite does not decrease sufficiently. Therefore, it is difficult to ensure the yield strength and impact toughness proposed in the present disclosure.

[0092] In addition, when the finish hot rolling temperature is lower than Ar3, the rolling load increases, which may cause quality defects such as surface cracks.

[0093] In the present disclosure, Ar3 can be expressed as follows.

[0094] Ar3 = 910 - 310C - 80Mn - 20Cu - 55Ni - 80Mo + 119V + 124Ti - 18Nb + 179Al, where each element represents the weight content.

[0095] The finish rolling can be carried out with a total reduction ratio of 45% or more and the number of rolling passes of 8 or less. When the total reduction ratio is less than 45%, austenite may not be sufficiently compressed during rolling, which may cause the final ferrite structure to coarsen, resulting in a decrease in strength and impact toughness. When the number of rolling passes exceeds 8, the reduction per pass decreases, which may also cause the coarsening of ferrite.

[0096] Cooling

[0097] Cooling is carried out after hot rolling, and there is no particular limitation on cooling in the present disclosure. For example, air cooling can be carried out to room temperature.

[0098] Normalizing heat treatment

[0099] For steel plates, normalizing heat treatment can be carried out as needed. In this case, the heat treatment temperature and time can be a temperature in the range of 830°C to 930°C for 1.3t + 30 minutes or longer, where t is the thickness (mm) of the steel.

[0100] Embodiments of the invention

[0101] Hereinafter, the present disclosure will be specifically described by the following examples. However, it should be noted that the following examples are only used to describe the present disclosure by way of example and are not intended to limit the scope of rights of the present disclosure. The reason is that the scope of rights of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

[0102] (Examples)

[0103] A steel slab with a thickness of 300 mm is manufactured by continuous casting of molten steel having the composition (weight%, the balance being Fe and inevitable impurities) shown in Table 1. The steel slab is heated under the conditions shown in Table 2, then rough rolled at a temperature of 900°C or higher, and then finish rolled under the conditions shown in Table 2 to manufacture a hot rolled steel plate.

[0104] [Table 1]

[0105]

[0106] Relational expression 1 is calculated as follows.

[0107] [Relational expression 1]

[0108] [C] + [Mn] / 6 + [V] / 5 ≤ 0.43

[0109] where [C], [Mn], and [V] are the contents (weight%) of the respective components.

[0110] [Table 2]

[0111]

[0112] Here, relational expression 2 shows the calculated value as follows.

[0113] [Relational expression 2]

[0114] 800 °C ≤ Finish rolling starting temperature ≤ 857 °C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb] 0.5 ) + (732 × [V]) - (230 × [V] 0.5 ) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si])

[0115] where [C], [Nb], [V], [Ti], [Al], and [Si] are the contents (wt%) of the respective components.

[0116] Observe the microstructure of the steel plates manufactured as shown in Tables 1 and 2 above, and evaluate the mechanical properties. Observe the microstructure using an optical microscope, and then measure the ferrite fraction and diameter using an analysis program. In this case, in the thickness direction of each steel, measure the microstructure at the t / 4 point (where t is the thickness of the steel plate (mm)), and the results are shown in Table 3 below.

[0117] Evaluate the mechanical properties at the t / 4 point in the thickness direction of each steel. At this time, collect tensile test specimens at each point in the thickness direction perpendicular to the rolling direction, and measure the tensile strength (TS), yield strength (YS), and elongation (El). Collect impact test specimens from JIS No. 4 standard test specimens at the t / 4 point in the thickness direction in the rolling direction, and measure the average impact toughness (CVN) at -20 °C, which is shown in Table 4.

[0118] Meanwhile, in order to study the changes in mechanical properties before and after normalizing heat treatment for the inventive examples and comparative examples, subject the hot-rolled steel plates to normalizing heat treatment at 890 °C for 1.3t + 30 minutes (where t is the thickness of the steel plate (mm)), and then air-cool to room temperature. The evaluation results of the mechanical properties before and after heat treatment are shown in Table 4 below.

[0119] [Table 3]

[0120] Grouping Thickness of steel plate (mm) Ferrite (%) Pearlite (%) Diameter of ferrite (μm) Inventive Example 1 50 70 30 22 Inventive Example 2 75 70 30 28 Inventive Example 3 100 65 35 36 Inventive Example 4 50 75 25 25 Inventive Example 5 75 75 25 31 Inventive Example 6 100 70 30 39 Inventive Example 7 50 75 25 25 Inventive Example 8 75 75 25 30 Inventive Example 9 100 70 30 37 Comparative Example 1 50 80 20 51 Comparative Example 2 75 75 25 56 Comparative Example 3 100 75 25 62 Comparative Example 4 50 55 45 24 Comparative Example 5 75 55 45 32 Comparative Example 6 100 55 45 36 Comparative Example 7 50 75 25 52 Comparative Example 8 75 75 25 54 Comparative Example 9 100 70 30 57 Comparative Example 10 50 75 25 51 Comparative Example 11 75 75 25 60 Comparative Example 12 100 70 30 67

[0121] [Table 4]

[0122]

[0123] Table 4 shows the tensile properties and low-temperature impact toughness before and after normalizing. In the cases of Invention Examples 1 to 9, all of the component ranges, relational expressions 1 and 2, and microstructural features proposed in the present disclosure are satisfied, such that both the tensile properties and the low-temperature impact toughness are satisfied. In particular, it can be determined that even as the thickness increases, the yield strength proposed in the present disclosure is 355 MPa or greater.

[0124] Specifically, in the cases of Invention Examples 1 to 9, when comparing the results after as-rolled and normalizing heat treatment, the impact toughness after heat treatment slightly increases, which is different from the physical properties before heat treatment, but still satisfies the impact toughness proposed in the present disclosure, indicating that even without additional normalizing heat treatment, the physical properties can be sufficiently ensured by the method (normalizing rolling method) of the present disclosure. Normalizing rolling is a rolling process in which the rolling conditions are controlled so as to obtain the same effect as that obtained by normalizing (a rolling process in which final deformation is performed in a specific temperature range such that the material condition is equivalent to the value obtained by normalizing).

[0125] On the other hand, in the cases of Comparative Examples 1 to 3 (where the content of Nb and relational expression 2 are outside the values proposed in the present disclosure), it can be determined that due to coarsened ferrite grains and a low Nb content, NbC precipitates may not precipitate sufficiently, and thus the yield strength, tensile strength, and impact toughness proposed in the present disclosure may not be satisfied. In the cases of Comparative Examples 4 to 6 (where the C content and relational expression 1 are outside the ranges proposed in the present disclosure for the component system), it can be determined that due to excessive addition of the C content, the yield strength / tensile strength may be sufficient, but the impact toughness deteriorates.

[0126] Comparative Examples 7 to 9 are cases where all of the component ranges proposed in the present disclosure are satisfied, but the total number of rolling passes during finish rolling is excessive. It can be seen that even though the fractions of ferrite and pearlite satisfy the values proposed in the present disclosure, the size of the ferrite grains is large, and thus the yield strength and impact toughness are not satisfied. In the cases of Comparative Examples 10 to 12, the component ranges, relational expression 2, and the total reduction ratio during finish rolling are outside the values proposed in the present disclosure. It can be determined that due to coarsening of the ferrite grain size, the yield strength and impact toughness are not satisfied.

[0127] In the cases of Comparative Examples 1 to 12, the impact toughness after normalizing heat treatment is improved, but the results do not satisfy the yield strength or the impact toughness.

[0128] Figure 1Photographs of the optical microstructures at the t / 4 point in the thickness direction of Invention Example 1 and Comparative Example 10 are shown in (a) and (b), respectively. When the finish rolling starting temperature does not satisfy the relational expression 2, it can be expected that the ferrite grains will be significantly coarsened due to austenite growth during rolling, and it will be difficult to ensure the yield strength and impact toughness aimed to be presented in the present disclosure. In the case of Invention Example 1 that satisfies the relational expression 2, it can be determined that it has very fine ferrite.

[0129] Figure 2 is a graph showing the change in the yield strength according to the finish rolling starting temperature for a hot-rolled steel sheet with a thickness of 100 mm using Steel Grade 2 that satisfies the component range presented in the present disclosure. It can be determined that within the temperature range that satisfies the relational expression 2, results showing a yield strength that satisfies the yield strength presented in the present disclosure are obtained. When the temperature is too high, a decrease in the yield strength occurs.

Claims

1. A steel plate comprising, by weight %: more than 0.10% to 0.17% of carbon (C), 0.2% to 0.5% of silicon (Si), 1.2% to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015% to 0.045% of aluminum (Al), 0.03% to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005% to 0.017% of titanium (Ti), 0.002% to 0.01% of nitrogen (N), and the balance of Fe and inevitable impurities, and the steel plate satisfies the following relational expression 1, wherein the microstructure of the steel plate comprises, by area fraction, 60% to 85% of ferrite, and the balance of pearlite and inevitable structures, [Relational Expression 1] [C] + [Mn] / 6 + [V] / 5 ≤ 0.43 where [C], [Mn], and [V] are the contents (weight %) of the respective components.

2. The steel plate according to claim 1, wherein the thickness of the steel plate is 100 mm or less.

3. The steel plate according to claim 1, wherein the average grain size of the ferrite is 50 μm or less.

4. The steel plate according to claim 1, wherein the steel plate has a yield strength of 355 MPa or more and a tensile strength of 470 MPa or more, and the yield strength and tensile strength are evaluated at the t / 4 point perpendicular to the rolling direction, where t is the thickness (mm) of the steel plate.

5. The steel plate according to claim 1, wherein the steel plate has a Charpy impact absorption energy of 100 J or more at -20°C, and the Charpy impact absorption energy at -20°C is evaluated at the t / 4 point in the rolling direction, where t is the thickness (mm) of the steel plate.

6. A method for manufacturing a steel plate, comprising: preparing a steel slab comprising, by weight %, more than 0.10% to 0.17% of carbon (C), 0.2% to 0.5% of silicon (Si), 1.2% to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015% to 0.045% of aluminum (Al), 0.03% to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005% to 0.017% of titanium (Ti), 0.002% to 0.01% of nitrogen (N), and the balance of Fe and inevitable impurities, and satisfying the following relational expression 1; heating the steel slab to a temperature in the range of 1150°C to 1200°C for 3 hours or longer; hot rolling: rough rolling the heated steel slab and finish rolling at a finish rolling starting temperature satisfying the following relational expression 2; and cooling after hot rolling, [Relational Expression 1] [C] + [Mn] / 6 + [V] / 5 ≤ 0.43 where [C], [Mn], and [V] are the contents (weight %) of the respective components, [Relational Expression 2] 800 °C ≤ Starting temperature of finish rolling during rolling ≤ 857 °C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb] 0.5 ) + (732 × [V]) - (230 × [V] 0.5 ) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si]) Wherein [C], [Nb], [V], [Ti], [Al] and [Si] are the contents (wt%) of the respective components.

7. The method for manufacturing a steel sheet according to claim 6, wherein hot rolling is performed at a total reduction ratio of 45% or more and the number of rolling passes of 8 or less.

8. The method for manufacturing a steel sheet according to claim 6, wherein the finish rolling temperature is Ar3 or higher.

9. The method for manufacturing a steel sheet according to claim 6, wherein the cooling is air cooling.

10. The method for manufacturing a steel plate according to claim 6 further comprises: After the cooling, heat treatment is performed at a temperature in the range of 830°C to 930°C for 1.3t + 30 minutes or longer, where t is the thickness (mm) of the steel.

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