High-strength steel sheet with excellent ductility and method for manufacturing the same

Abstract

The present invention provides a high-strength steel plate and a method for manufacturing the same, wherein the steel plate ensures high strength while having excellent ductility. According to one aspect of the present invention, a steel plate and a method for manufacturing the same can be provided that can be used to manufacture structures such as ships, offshore structures, and building structures.

Description

Technical Field

[0001] The present invention relates to a high-strength steel plate and a method for manufacturing the same. More particularly, the present invention relates to a steel plate that ensures high strength while having excellent ductility and a method for manufacturing the same. Background Art

[0002] Structural steel used in ships, offshore structures, and building structures requires high strength. Higher strength reduces structural weight, thereby increasing ship transportation efficiency. Furthermore, this reduced weight facilitates the scaling of offshore and building structures.

[0003] However, due to the inverse relationship between strength and ductility, ductility decreases as strength increases. In the event of a ship running aground or a collision, which could lead to hull ruptures, higher strength in the steel plate reduces ductility, making it more likely to cause hull ruptures. This could lead to flooding or sinking, and potentially serious marine environmental accidents if crude oil or other substances leak.

[0004] Therefore, a lot of work has been done to produce steel plates with excellent strength and ductility.

[0005] For example, Patent Document 1 discloses a steel plate with excellent collision absorption properties, in which the average grain size of the main phase ferrite is controlled to 3 to 12 μm, the ferrite fraction is controlled to be greater than 90%, and the average equivalent circle diameter of the second phase is controlled to be less than 0.8 μm, thereby having a tensile strength of greater than 490 MPa and a uniform elongation of greater than 15%.

[0006] However, it can be seen that although Patent Document 1 describes a steel plate having a tensile strength of 490 MPa or more and a uniform elongation of 15% or more, the total elongation is not explicitly disclosed, although the total elongation (or elongation at break) has a higher relevance than the uniform elongation for the fracture of the steel plate.

[0007] Patent Document 2 discloses a steel material that utilizes front-end cooling, air cooling, and back-end cooling during the cooling process after rolling. This results in a structure composed of ferrite and a hard second phase, with the ferrite volume fraction exceeding 75% throughout the entire thickness, a hardness of Hv between 140 and 160, and an average grain size of 2 μm or greater. This allows for a steel material with a tensile strength exceeding 490 MPa and a uniform elongation exceeding 20%.

[0008] Although Patent Document 2 shows excellent tensile strength of 490 MPa or more and uniform elongation of 20% or more, excellent uniform elongation does not necessarily mean excellent total elongation, and the total elongation corresponding to the elongation at break is unclear.

[0009] Patent document 3 describes a thick steel plate whose structure consists of a ferrite main phase and a second phase mainly composed of pearlite to increase the energy absorption capacity during collision. The hardness, fraction, average area and average perimeter of the second phase meet predetermined conditions, and the average dislocation density of the ferrite is reduced to below a certain level. To this end, a manufacturing method is proposed, in which the steel is heated to a high temperature higher than the conventional reheating temperature, and then controlled rolling and air cooling or weak water cooling are carried out. Patent document 4 describes a method for manufacturing high-strength, high-ductility steel plates, which meet the requirements of 80-95% ferrite area ratio and 5-20% pearlite area ratio, and the grain size, aspect ratio and dislocation density of the ferrite are specified to minimize the hardness difference between the surface part and the thickness center part. As a result, a steel plate with a tensile strength of more than 490MPa and an elongation of more than 23-40% depending on the plate thickness can be provided.

[0010] Although Patent Documents 3 and 4 can produce steel plates with a tensile strength of 490 MPa or more and excellent ductility, controlling S to 0.003% or less inevitably involves steelmaking loads, and the reheating temperature exceeds the conventional range, which makes it difficult to produce the steel plates.

[0011] [Prior Art Literature]

[0012] Patent Document 1: Korean Patent Gazette No. 10-0914590

[0013] Patent Document 2: Korean Patent Publication No. 10-2016-0104077

[0014] Patent Document 3: Japanese Patent No. 5994819

[0015] Patent Document 4: Japanese Patent No. 6007968 Summary of the Invention

[0016] Technical issues

[0017] One aspect of the present invention is to provide a steel plate that ensures high strength while having excellent ductility, and a method for manufacturing the same.

[0018] The technical problems to be solved by the present invention are not limited to the above-mentioned contents. For those skilled in the art of the present invention, it is not difficult to understand the additional technical problems of the present invention from the overall content of this specification.

[0019] Technical Solution

[0020] One aspect of the present invention can provide a high-strength steel sheet having excellent ductility, comprising, in weight percent, 0.13-0.16% C, 0.1-0.6% Si, 1.1-1.6% Mn, 0.02% or less P, 0.005% or less S, 0.015-0.05% Al, 0.005-0.02% Ti, 0.01-0.02% Nb, 0.001-0.006% N, and the balance being Fe and unavoidable impurities.

[0021] The R value defined in the following equation 1 is 0.43 to 0.48.

[0022] The P value defined in the following equation 2 is less than 0.001.

[0023] In terms of area%, the microstructure consists of a composite structure of less than 1% bainite or martensite and the remainder polygonal ferrite and pearlite.

[0024] The grain size of the polygonal ferrite is 6 to 12 μm in terms of equivalent circle diameter.

[0025] The yield strength is above 355 MPa.

[0026] [Equation 1]

[0027] R=[C]+[Si] / 4+[Mn] / 6

[0028] In Relational Formula 1, [C], [Si], and [Mn] are the weight % of the elements.

[0029] [Equation 2]

[0030] P=[N]-0.36*[Ti]

[0031] In Relational Formula 2, [N] and [Ti] are the weight % of the elements.

[0032] The pearlite content may be 15-25%.

[0033] The steel plate may have a tensile strength of 490 MPa or more and a uniform elongation of 28% or more.

[0034] Another aspect of the present invention provides a method for manufacturing a high-strength steel plate having excellent ductility, comprising the following steps:

[0035] The steel slab is reheated to a temperature range of 1100-1200° C., wherein the steel slab comprises, in weight %, 0.13-0.16% C, 0.1-0.6% Si, 1.1-1.6% Mn, 0.02% or less P, 0.005% or less S, 0.015-0.05% Al, 0.005-0.02% Ti, 0.01-0.02% Nb, 0.001-0.006% N, and the balance Fe and unavoidable impurities, wherein the R value defined in the following equation 1 is 0.43-0.48, and the P value defined in the following equation 2 is 0.001 or less;

[0036] Hot rolling the reheated steel slab at a cumulative reduction of 60% or more and a rolling end temperature of 780 to 850° C.; and

[0037] The hot-rolled steel sheet is cooled in a temperature range of 600° C. to Ar3 at a cooling rate of 2° C. / s or less.

[0038] [Equation 1]

[0039] R=[C]+[Si] / 4+[Mn] / 6

[0040] In Relational Formula 1, [C], [Si], and [Mn] are the weight % of the elements.

[0041] [Equation 2]

[0042] P=[N]-0.36*[Ti]

[0043] In Relational Formula 2, [N] and [Ti] are the weight % of the elements.

[0044] When cooling the hot-rolled steel plate, air cooling may be performed in a temperature range from 600° C. to Ar 3 to a temperature range of room temperature.

[0045] Effects of the Invention

[0046] According to one aspect of the present invention, a steel plate having excellent ductility while ensuring high strength and a method for manufacturing the same can be provided.

[0047] According to another aspect of the present invention, a high-strength steel plate and a method for manufacturing the same can be provided, which can be used to manufacture structures such as ships, offshore structures, and building structures. DETAILED DESCRIPTION

[0048] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various ways, and the scope of the present invention is not limited to the embodiments described below. This embodiment is intended to further describe the present invention in detail to those skilled in the art.

[0049] The present invention aims to optimize the alloy composition and manufacturing method of steel plate to achieve both strength and ductility. In particular, the inventors discovered that by strictly controlling the alloying elements according to a relationship, they could appropriately achieve solid solution strengthening and microstructure refinement, thereby achieving both strength and ductility. This led to the completion of the present invention.

[0050] The present invention is described in detail below.

[0051] Hereinafter, the steel composition of the present invention will be described in detail.

[0052] Unless otherwise specified, the percentages of the contents of various elements in the present invention are based on weight.

[0053] According to one aspect of the present invention, the steel may contain, in terms of weight %, C: 0.13-0.16%, Si: 0.1-0.6%, Mn: 1.1-1.6%, P: 0.02% or less, S: 0.005% or less, Al: 0.015-0.05%, Ti: 0.005-0.02%, Nb: 0.01-0.02%, N: 0.001-0.006%, and the balance Fe and unavoidable impurities.

[0054] Carbon (C): 0.13-0.16%

[0055] Carbon (C) is an element that significantly affects the pearlite fraction. To ensure tensile strength, it is preferably added in an amount of 0.13% or more. On the other hand, a carbon (C) content exceeding 0.16% may ensure tensile strength, but it is difficult to achieve the ductility targeted by the present invention. Furthermore, excessive carbon (C) content reduces resistance to low-temperature cracking and the toughness of the weld heat-affected zone, leading to reduced suitability for welded structures.

[0056] Therefore, the content of carbon (C) may be 0.13 to 0.16%.

[0057] Silicon (Si): 0.1-0.6%

[0058] Silicon (Si) is a substitutional solid solution strengthening element that is effective for deoxidation and contributes significantly to ensuring strength. Furthermore, while increasing strength does not significantly reduce ductility, silicon is a useful element. If the Si content is less than 0.1%, maintaining strength becomes difficult, while if the Si content exceeds 0.6%, there is a problem of reduced toughness in the weld zone.

[0059] Therefore, the content of silicon (Si) may be 0.1 to 0.6%, more preferably 0.3% or more, and 0.5% or less.

[0060] Manganese (Mn): 1.1-1.6%

[0061] Manganese (Mn) is a substitutional solid solution strengthening element. Although it strengthens the steel by refining ferrite grain size and strengthening pearlite, it is preferably added in an amount of 1.1% or more to compensate for insufficient strength. However, if the manganese (Mn) content exceeds 1.6%, the strength becomes too high and the elongation targeted by the present invention cannot be achieved.

[0062] Therefore, the content of manganese (Mn) may be 1.1 to 1.6%, a more preferred lower limit may be 1.2%, and a more preferred upper limit may be 1.5%.

[0063] Phosphorus (P): 0.02% or less

[0064] Phosphorus (P) is an inevitable impurity in steel. Since it reduces ductility, its content needs to be minimized. However, the lower the content, the greater the burden on the steelmaking process and the increase in steelmaking costs. Therefore, the upper limit is set at 0.02%.

[0065] Sulfur (S): 0.005% or less

[0066] Sulfur (S) is an unavoidable impurity in steel. Since it forms MnS, which reduces ductility, its content should be minimized. However, as with P, lowering the sulfur (S) content increases the steelmaking burden and cost, so the upper limit is set at 0.005%.

[0067] Aluminum (Al): 0.015-0.05%

[0068] Aluminum (Al) is an element with excellent deoxidizing effect, and therefore, it is preferably added in an amount of 0.015% or more. However, if the content of aluminum (Al) exceeds 0.05%, there is a concern that the toughness of the weld zone may decrease.

[0069] Therefore, the content of aluminum (Al) may be 0.015% to 0.05%, more preferably 0.025% or more, and even more preferably 0.04% or less.

[0070] Titanium (Ti): 0.005-0.02%

[0071] Titanium (Ti) prevents excessive austenite growth caused by high temperatures during the reheating of steel ingots, thereby refining the structure and combining with interstitial nitrogen to reduce free nitrogen. Therefore, it is preferably added in an amount of 0.005% or more. However, if the titanium (Ti) content exceeds 0.02%, the TiN becomes coarse and cannot effectively prevent austenite growth at high temperatures. As a result, the ferrite refinement achieved in the present invention cannot be achieved in the final structure.

[0072] Therefore, the content of titanium (Ti) may be 0.005 to 0.02%, a more preferred lower limit may be 0.007%, and a more preferred upper limit may be 0.015%.

[0073] Niobium (Nb): 0.01-0.02%

[0074] Niobium (Nb) increases the flattening of austenite during rolling by expanding the temperature range of the austenite non-recrystallization zone, thereby refining the polygonal ferrite grain size in the final structure. Therefore, it is preferably added in an amount of 0.01% or more. However, if the niobium (Nb) content exceeds 0.02%, there is a problem of increased strength but decreased ductility due to the solid solution strengthening effect of Nb.

[0075] Therefore, the content of niobium (Nb) may be 0.01 to 0.02%, a more preferred lower limit may be 0.013%, and a more preferred upper limit may be 0.018%.

[0076] Nitrogen (N): 0.001-0.006%

[0077] Nitrogen (N) forms TiN together with Ti, which suppresses the coarse growth of austenite during reheating, so it is preferably added in an amount of 0.001% or more. However, if the nitrogen (N) content exceeds 0.006%, there is a concern that ductility may be reduced due to the increase in free N.

[0078] Therefore, the nitrogen (N) content may be 0.001 to 0.006%, more preferably 0.002% or more, and 0.004% or less.

[0079] The steel of the present invention may contain a balance of iron (Fe) and unavoidable impurities in addition to the above-mentioned components. Unavoidable impurities may inadvertently be incorporated during conventional manufacturing processes, and their inclusion cannot be ruled out. These impurities are well known to those skilled in the art of conventional steelmaking, and all of their details will not be specifically described in this specification.

[0080] For the steel of the present invention, the R value defined in the following relational expression 1 may be 0.43 to 0.48.

[0081] Since strength and ductility are generally inversely proportional, if the strength is too high, it is difficult to simultaneously achieve the strength and ductility targeted by the present invention. In order to properly control the strength, it is necessary to properly control the content of pearlite-forming C and substitutional solid solution strengthening elements Si and Mn.

[0082] If the R value defined in Relationship 1 is less than 0.43, it is difficult to ensure the strength targeted by the present invention. On the other hand, if the R value defined in Relationship 1 is greater than 0.48, the strength becomes too high, making it difficult to ensure the ductility targeted by the present invention.

[0083] [Equation 1]

[0084] R=[C]+[Si] / 4+[Mn] / 6

[0085] In Relational Formula 1, [C], [Si], and [Mn] are the weight % of the elements.

[0086] For the steel of the present invention, the P value defined in the following relational expression 2 can be 0.001 or less.

[0087] Compared to substitutional solid-solution strengthening elements such as Si and Mn, interstitial solid-solution strengthening elements such as C and N are detrimental to steel ductility. Therefore, it is necessary to minimize the presence of C and N in solid solution within the steel. While C is mostly used to form pearlite if the cooling rate during transformation is slow, this poses no significant problem. However, N, if insufficient nitride formation occurs, will remain in solid solution, leading to a decrease in ductility.

[0088] In the present invention, Nb is added along with Ti to form a (Ti, Nb)(C, N) complex carbonitride. The free nitrogen content can be controlled by taking this complex carbonitride into account using Relational Formula 2. The P value in Relational Formula 2 represents the free nitrogen content in the steel. A P value of 0 or less indicates the absence of free nitrogen, so there is no particular lower limit. A more preferred upper limit for the P value is 0. Therefore, by minimizing the free nitrogen content to a P value of 0.001 or less, as defined in Relational Formula 2, the ductility targeted by the present invention can be ensured.

[0089] [Equation 2]

[0090] P=[N]-0.36*[Ti]

[0091] In Relational Formula 2, [N] and [Ti] are the weight % of the elements.

[0092] Hereinafter, the fine structure of the steel of the present invention will be described in detail.

[0093] Unless otherwise specified in the present invention, the percentage indicating the fraction of fine tissue is based on area.

[0094] Measured in area%, the microstructure of the steel of the present invention that meets the above alloy composition consists of 15-25% pearlite, less than 1% bainite or martensite, and the remainder polygonal ferrite. Measured in equivalent circle diameter, the grain size of the polygonal ferrite can be 6-12 μm.

[0095] It may be composed of 15-25% pearlite, less than 1% bainite or martensite, and the remainder polygonal ferrite.

[0096] Pearlite contributes to improving tensile strength but reduces elongation. If the area fraction of pearlite is less than 15%, it is difficult to ensure tensile strength, and if the area fraction of pearlite is greater than 25%, it is difficult to ensure elongation.

[0097] Furthermore, when the fine structure contains 1% or more of a low-temperature structure such as bainite or martensite, it is difficult to ensure the elongation targeted by the present invention.

[0098] Measured in terms of equivalent circle diameter, the grain size of polygonal ferrite can be 6 to 12 μm.

[0099] A mixed structure of ferrite and pearlite is beneficial for ensuring elongation, but is not conducive to ensuring strength. To compensate for this, it is necessary to increase the fraction of pearlite, but in this case, the effect of increasing yield strength is small, and there is a problem of reduced ductility. Another method for improving strength while maintaining the mixed structure is to refine the ferrite grain size. When the ferrite grain size is controlled within an appropriate range, it can prevent the ductility from being reduced while increasing the yield strength.

[0100] If the ferrite grain size is larger than 12 μm, it is difficult to ensure the strength targeted by the present invention, especially the yield strength. If the ferrite grain size is smaller than 6 μm, the strength increases, but the ductility may decrease rapidly, making it difficult to ensure the elongation.

[0101] Hereinafter, the steel manufacturing method of the present invention will be described in detail.

[0102] The steel according to one aspect of the present invention can be produced by reheating, hot-rolling, and cooling a steel billet satisfying the above-mentioned alloy composition.

[0103] Reheating

[0104] The ingot satisfying the above alloy composition and produced by the continuous casting method can be reheated to a temperature range of 1100 to 1200°C.

[0105] If the reheating temperature is lower than 1100°C, ductility can be maintained, but the coarse Nb precipitates formed during casting will not fully dissolve during reheating, making it difficult to maintain yield strength. On the other hand, if the reheating temperature is higher than 1200°C, the initial austenite coarsens, making it difficult to achieve a sufficiently refined final structure even if the austenite is refined during rolling.

[0106] Hot Rolling

[0107] The reheated slab can be hot rolled at a cumulative reduction ratio of 60% or more and a rolling end temperature of 780 to 850°C.

[0108] The reheated steel slab can undergo rough rolling and finish rolling. Rough rolling is not particularly limited in the present invention. The final rolling temperature and cumulative reduction during finish rolling significantly influence ferrite grain size. If the final rolling temperature is below 780°C, the austenite becomes refined, resulting in excessively fine ferrite grains in the final microstructure, potentially leading to a significant reduction in elongation. On the other hand, if the final rolling temperature is above 850°C, the austenite does not sufficiently refine, resulting in coarse ferrite grains in the final microstructure, making it difficult to ensure sufficient strength.

[0109] During finish rolling, the cumulative reduction ratio can be above 60%.

[0110] In order to obtain the ferrite grain size targeted by the present invention, it is necessary to appropriately control the reduction rate. If the cumulative reduction rate is less than 60%, the austenite will not be sufficiently refined, making it difficult to ensure strength.

[0111] cool down

[0112] The hot-rolled steel sheet can be cooled at a cooling rate of 2°C / second or less in a temperature range of 600°C to Ar3.

[0113] Air cooling is primarily used for cooling after finish rolling. More specifically, to ensure the desired elongation, the microstructure should consist of a mixture of polygonal ferrite and pearlite. To achieve this, during phase transformation, in the temperature range of 600°C to Ar3, where the transformation occurs, the cooling rate is preferably controlled to below 2°C / second.

[0114] If the cooling rate exceeds 2° C. / second, the strength will increase excessively, making it difficult to ensure the elongation targeted by the present invention.

[0115] Ar3=910-310*[C]+80*[Mn]-20*[Cu]-55*[Ni]-15*[Cr]-80*[Mo]

[0116] In the formula, [C], [Mn], [Cu], [Ni], [Cr], and [Mo] are the weight % of the elements.

[0117] The steel of the present invention manufactured as described above has a yield strength of 355 MPa or more, a tensile strength of 490 MPa or more, and a uniform elongation of 28% or more based on a proportional tensile test specimen, exhibiting excellent ductility and high strength performance.

[0118] Hereinafter, the present invention will be further described in detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention in more detail and are not intended to limit the scope of the present invention.

[0119] Modes for Carrying Out the Invention

[0120] Table 1 below shows the alloy composition, R value in Relationship 1, and P value in Relationship 2, depending on the steel grade. Molten steel having the alloy composition shown in Table 1 was produced, and 300 mm thick steel ingots were cast by continuous casting. The produced steel ingots were then reheated, hot-rolled, and cooled under the conditions shown in Table 2 to form steel plates. The cooling rates in Table 2 below represent the cooling rates in the temperature range of 600°C to Ar3.

[0121]

Table 1

[0122]

[0123] [Equation 1]

[0124] R=[C]+[Si] / 4+[Mn] / 6

[0125] In Relational Formula 1, [C], [Si], and [Mn] are the weight % of the elements.

[0126] [Equation 2]

[0127] P=[N]-0.36*[Ti]

[0128] In Relational Formula 2, [N] and [Ti] are the weight % of the elements.

[0129] Ar3=910-310*[C]+80*[Mn]-20*[Cu]-55*[Ni]-15*[Cr]-80*[Mo]

[0130] In the formula, [C], [Mn], [Cu], [Ni], [Cr], and [Mo] are the weight % of the elements.

[0131]

Table 2

[0132]

[0133] To analyze the microstructure of the steel sheets, as shown in Table 3 below, samples were taken from the manufactured steel sheets at a depth of 1 / 4 the thickness. The samples were then ground, etched with Nital solution, and observed under an optical microscope. The average equivalent circle diameter (equivalent to the ferrite grain size) and the pearlite area fraction were measured using an image analyzer connected to the optical microscope.

[0134] Tensile tests were also conducted on tensile specimens taken from the manufactured steel plates at a distance of 1 / 4 of the plate width. The specimens were processed so that their length was parallel to the width of the steel plates. Based on the R14B specimen rule for proportional specimens registered with the Korean Register of Shipping, the specimens were processed to have a width of 25 mm, a thickness equal to the plate thickness, and a gauge length of 5.65 x √(specimen width x specimen thickness). The yield strength, tensile strength, and uniform elongation values ​​were determined through room temperature tensile tests.

[0135]

Table 3

[0136]

[0137] As shown in Tables 1 to 3 above, Inventive Examples 1 to 3, which satisfy the alloy composition, manufacturing conditions, and microstructure of the present invention, all meet the yield strength, tensile strength, and uniform elongation targeted by the present invention.

[0138] On the other hand, Comparative Examples 1 to 13, which did not satisfy the alloy composition or production conditions, did not ensure the strength or elongation targeted by the present invention.

[0139] Comparative Examples 1 and 2 are examples in which the C content exceeds the range of the present invention. In Comparative Example 1, due to the high C content, pearlite is excessively formed, thus failing to ensure uniform elongation. In Comparative Example 2, due to the insufficient C content, the pearlite area ratio is insufficient, and the tensile strength value targeted by the present invention is not achieved.

[0140] Comparative Examples 3 and 4 are cases where the Nb content exceeds the range. In Comparative Example 3, the Nb content does not fall within the range of the present invention, resulting in coarsening of the ferrite grain size and failure to achieve yield strength. In Comparative Example 4, the Nb content exceeds the range, resulting in excessive formation of Nb precipitates. While strength is achieved, the uniform elongation does not meet the target.

[0141] Comparative Examples 5 and 6 meet the alloy composition ranges and manufacturing conditions of the present invention, but the R value in Relational Formula 1 exceeds the range of the present invention. Comparative Example 5 has an R value that does not fall within the range of the present invention, making it difficult to achieve the strength targeted by the present invention. Comparative Example 6 has an R value that exceeds the range, resulting in excessively high strength and making it difficult to achieve the ductility targeted by the present invention.

[0142] Comparative Example 7 is a case where the P value in Relational Expression 2 is outside the range of the present invention. Since the free N content is too high, the target ductility value cannot be ensured.

[0143] Comparative Examples 8 and 9 meet the alloy composition requirements of the present invention, but the reheating temperatures exceed the range of the present invention. In Comparative Example 8, due to the excessively high reheating temperature, the austenite grain size coarsened, resulting in a ferrite grain size outside the range of the present invention, and the yield strength was not achieved. In Comparative Example 9, the reheating temperature did not reach the range of the present invention, and the coarse Nb precipitates were not fully dissolved, so the target yield strength was not achieved.

[0144] Comparative Example 10 satisfied the alloy composition of the present invention, but the cumulative reduction ratio in hot rolling did not reach the range of the present invention, and therefore the yield strength was not sufficiently ensured.

[0145] Comparative Examples 11 and 12 meet the alloy composition of the present invention, but exceed the rolling end temperature. Comparative Example 11 does not reach the rolling end temperature range, so the ferrite grains are excessively refined and the elongation is greatly reduced. Comparative Example 12 has a temperature that is too high, so the grains are not sufficiently refined and the strength is not ensured.

[0146] Comparative Example 13 satisfied the alloy composition, but the cooling rate did not meet the requirements of the present invention. Since the cooling rate was too fast, bainite was formed in addition to pearlite, and the elongation targeted by the present invention was not achieved.

[0147] Although the present invention has been described in detail through the above embodiments, other embodiments are possible. Therefore, the technical concept and scope in the claims are not limited to the embodiments.

Claims

1. A high-strength steel plate having excellent ductility, wherein: The steel plate comprises, in weight %, C: 0.13-0.16%, Si: 0.1-0.6%, Mn: 1.1-1.6%, P: 0.02% or less, S: 0.005% or less, Al: 0.015-0.05%, Ti: 0.005-0.02%, Nb: 0.01-0.02%, N: 0.001-0.006%, and the balance Fe and unavoidable impurities. The R value defined in the following equation 1 is 0.43 to 0.

48. The P value defined in the following equation 2 is less than 0.

001. In terms of area%, the microstructure consists of a composite structure of less than 1% bainite or martensite and the remainder polygonal ferrite and pearlite. The grain size of the polygonal ferrite is 6 to 12 μm in terms of equivalent circle diameter. The yield strength is above 355MPa. [Equation 1] R=[C]+[Si] / 4+[Mn] / 6 In Relational Formula 1, [C], [Si], and [Mn] are the weight % of the elements. [Equation 2] P=[N]-0.36*[Ti] In Relational Formula 2, [N] and [Ti] are the weight % of the elements.

2. The high-strength steel plate having excellent ductility according to claim 1, wherein The content of the pearlite is 15-25%.

3. The high-strength steel plate having excellent ductility according to claim 1, wherein The steel plate has a tensile strength of 490 MPa or more and a uniform elongation of 28% or more.

4. A method for producing a high-strength steel plate having excellent ductility, comprising the following steps: The steel slab is reheated to a temperature range of 1100-1200° C., wherein the steel slab comprises, in weight %, 0.13-0.16% C, 0.1-0.6% Si, 1.1-1.6% Mn, 0.02% or less P, 0.005% or less S, 0.015-0.05% Al, 0.005-0.02% Ti, 0.01-0.02% Nb, 0.001-0.006% N, and the balance Fe and unavoidable impurities, wherein the R value defined in the following equation 1 is 0.43-0.48, and the P value defined in the following equation 2 is 0.001 or less; Hot rolling the reheated steel slab at a cumulative reduction of 60% or more and a rolling end temperature of 780 to 850° C.; and The hot-rolled steel plate is cooled in the temperature range of 600°C to Ar3 at a cooling rate of 2°C / s or less. [Relationship 1] R=[C]+[Si] / 4+[Mn] / 6 In Relational Formula 1, [C], [Si], and [Mn] are the weight % of the elements. [Equation 2] P=[N]-0.36*[Ti] In Relational Formula 2, [N] and [Ti] are the weight % of the elements.

5. The method for producing a high-strength steel sheet having excellent ductility according to claim 4, wherein: When cooling the hot-rolled steel sheet, air cooling is performed in a temperature range from 600° C. to Ar 3 and then to room temperature.

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