High strength steel plate having low yield ratio and method of manufacturing the same

Abstract

The present invention relates to a high-strength steel plate and a method for manufacturing the same, and more specifically, to a steel plate having high strength and low yield ratio characteristics and a method for manufacturing the same.

Description

Technology Field

[0001] The present invention relates to a high-strength steel plate and a method for manufacturing the same, and more specifically, to a steel plate having high strength and low yield ratio characteristics and a method for manufacturing the same. Background Technology

[0003] Structural steel is a high-strength hot-rolled steel sheet used for construction and civil engineering. It must be able to ensure the stability of buildings, etc., by absorbing energy when external stresses, such as earthquakes, occur, so that it does not easily fracture. Therefore, structural steel needs to have a low yield ratio and impact characteristics to ensure the above effects.

[0004] Meanwhile, if precipitation strengthening elements such as Ti, Nb, and V are used with a carbon content of 0.1 wt% or less for this purpose, it is difficult to secure strength and a low yield ratio. In addition, adding a large amount of expensive precipitation strengthening elements not only increases production costs but also results in significantly degraded impact properties if coarse precipitates are formed. Furthermore, in the case of hot-rolled materials, there is a problem in that the cooling rates of the inner and outer parts of the coil differ after winding, which can easily cause material variation. The problem to be solved

[0006] According to one aspect of the present invention, a high-strength steel plate with a low yield ratio and a method for manufacturing the same are to be provided.

[0007] The problems of the present invention are not limited to those described above. A person skilled in the art will have no difficulty understanding additional problems of the present invention from the overall contents of this specification. means of solving the problem

[0009] One aspect of the present invention comprises, in weight percent, C: 0.13~0.20%, Si: 0.6% or less, Mn: 1.4~3.0%, Al: 0.1% or less, Nb: 0.02~1.0%, Ti: 0.03% or less (excluding 0%), Cr: 0.3~1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, and the remainder being Fe and other unavoidable impurities.

[0010] The sum of the Mn and Si content is 1.8% or more, and

[0011] The microstructure contains 50% or more of bainite and includes the remainder of other structures,

[0012] A steel plate can be provided with a widthwise tensile strength deviation of 80 MPa or less (wherein the widthwise tensile strength deviation represents the difference between the maximum and minimum values ​​measured in the rolling direction at a point 50 mm away from the edge, at the 1 / 4 width point, and at the 1 / 2 width point, respectively).

[0013] The above steel plate may have a yield strength of 650 MPa or more, a tensile strength of 800 MPa or more, a yield ratio of 85% or less, and an elongation of 15% or more.

[0014] The above steel plate may have a yield strength of 650 MPa or more in the inner coil (wherein the inner coil refers to the halfway point along the coil length direction).

[0015] The above steel plate may have a Charpy impact absorption energy of 47J or more at -5℃.

[0017] Another aspect of the present invention comprises the step of reheating a steel slab comprising, in weight percent, C: 0.13~0.20%, Si: 0.6% or less, Mn: 1.4~3.0%, Al: 0.1% or less, Nb: 0.02~1.0%, Ti: 0.03% or less (excluding 0%), Cr: 0.3~1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, and the remainder being Fe and other unavoidable impurities, wherein the sum of the Mn and Si contents is 1.8% or more;

[0018] A step of hot rolling the above-mentioned reheated steel slab at a finishing hot rolling temperature of 750~880℃;

[0019] A first cooling step of cooling the above hot-rolled steel plate to a temperature range of Bs-100℃ to Bs at a cooling rate of 25℃ / s or higher;

[0020] A second cooling step of cooling the above first cooled steel plate to a temperature range of 450℃ to Bs at a cooling rate of 10 to 20℃ / s; and

[0021] A method for manufacturing a steel plate can be provided, comprising the step of winding the above secondary cooled steel plate.

[0022] The above reheating can be carried out at 1100~1300℃.

[0023] During the above first cooling, the cooling rate may be 25~80℃ / s. Effects of the invention

[0025] According to one aspect of the present invention, a steel plate having high strength and low yield ratio characteristics and a method for manufacturing the same can be provided. Brief explanation of the drawing

[0027] Figures 1 (a) and 1 (b) are micrographs of Invention Example 1 and Comparative Example 4, respectively, according to an embodiment of the present invention. Specific details for implementing the invention

[0028] Preferred embodiments of the present invention are described below. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be interpreted as being limited to the embodiments described below. These embodiments are provided to further explain the present invention in detail to those skilled in the art to which the present invention pertains.

[0030] In order to solve the aforementioned problems, the present invention optimizes the alloy composition and manufacturing conditions. In particular, by strictly controlling the content of elements such as C, Cr, Mn, and Si, and by controlling the cooling and coiling temperatures, it was confirmed that sufficient physical properties, such as strength and impact toughness, can be secured while maintaining a low yield ratio, leading to the completion of the present invention.

[0032] The present invention will be described in detail below.

[0034] The steel composition of the present invention will be described in detail below.

[0035] Unless otherwise specifically stated in the present invention, the % indicating the content of each element is based on weight.

[0037] A steel according to one aspect of the present invention may comprise, in weight percent, C: 0.13~0.20%, Si: 0.6% or less, Mn: 1.4~3.0%, Al: 0.1% or less, Nb: 0.02~1.0%, Ti: 0.03% or less (excluding 0%), Cr: 0.3~1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, and the remainder being Fe and other unavoidable impurities.

[0039] Carbon (C): 0.13~0.20%

[0040] Carbon (C) is the most effective element added to ensure strength, and to obtain excellent hardness, it is desirable to add at least 0.13%, and more preferably at least 0.14%. On the other hand, if the content exceeds 0.20%, weldability may be reduced, and there is a risk that the strength will become excessively high and the impact properties and elongation will decrease. A more desirable upper limit may be 0.19%.

[0042] Silicon (Si): 0.6% or less

[0043] Silicon (Si) is an element that can be included in steel and used as a deoxidizer, and can be used for solid solution strengthening. However, if its content is excessive, it can cause intergranular oxidation and red scale on the surface, so the upper limit is limited to 0.6%, and more preferably to 0.5%.

[0045] Manganese (Mn): 1.4~3.0%

[0046] Manganese (Mn) is the most commonly used element, along with carbon, to improve the strength of steel. If the manganese (Mn) content is less than 1.4%, the solid solution strengthening ability decreases, which may be disadvantageous for securing strength; more preferably, it may be included at 1.5% or more. On the other hand, if the content exceeds 3.0%, it can cause center segregation, inclusion formation, and intergranular oxidation, which may degrade not only the quality of the steel but also its weldability. Furthermore, if the substitutional solid solution strengthening element is excessive, it may be difficult to secure a low yield ratio; therefore, it is more preferable to limit the upper limit of the content to 2.5%.

[0048] Aluminum (Al): 0.1% or less

[0049] Aluminum (Al) is an element used as a deoxidizer or added for solid solution strengthening effects. If the aluminum (Al) content exceeds 0.1%, it may not only cause slab cracking during continuous casting but also cause intergranular oxidation in the final product, and more preferably, the upper limit can be restricted to 0.05%.

[0051] Niobium (Nb): 0.02~1.0%

[0052] Niobium (Nb) is an element used for precipitation strengthening and grain refinement, and may be included in an amount of 0.02% or more, and more preferably 0.03% or more. Meanwhile, since niobium (Nb) is an expensive element, its upper limit may be limited to 1.0%, and more preferably 0.08%, in consideration of productivity.

[0054] Titanium (Ti): 0.03% or less (excluding 0%)

[0055] Titanium (Ti) is an element that improves strength by forming TiN precipitates and enhances the toughness of the weldment. However, if its content exceeds 0.03%, coarse TiN is formed, which may actually lower toughness. More preferably, the upper limit can be restricted to 0.025%. In the present invention, 0% is excluded to ensure the effect of titanium (Ti).

[0057] Chrome (Cr): 0.3~1.0%

[0058] Chromium (Cr) is an element that increases hardenability. Adding 0.3% or more of chromium (Cr) facilitates the refinement of bainite, which is advantageous for simultaneously securing strength and a low yield ratio, and more preferably, it can be included at 0.35% or more. On the other hand, if the content exceeds 1.0%, the strength becomes excessively high, which may actually lower the impact properties and elongation. A more preferable upper limit may be 0.8%.

[0060] Phosphorus (P): 0.03% or less

[0061] Phosphorus (P) is an element that is inevitably contained in steelmaking, and since it causes brittleness due to segregation, it is desirable to include a minimum amount; therefore, in the present invention, the upper limit can be limited to 0.03%.

[0063] Sulfur (S): 0.02% or less

[0064] Sulfur (S) is an element that is inevitably contained in steelmaking and can cause intergranular brittleness during hot rolling by forming inclusions or FeS compounds with low melting points. Therefore, since it is desirable to include a minimum amount, the upper limit can be limited to 0.02% in the present invention.

[0066] Nitrogen (N): 0.015% or less

[0067] Nitrogen (N) is an element that is inevitably contained in steelmaking, and in the present invention, its upper limit can be limited to 0.015%.

[0069] The steel of the present invention may contain the remainder of iron (Fe) and unavoidable impurities in addition to the composition described above. Since unavoidable impurities may be unintentionally incorporated during the conventional manufacturing process, they cannot be excluded. As such impurities are known to any person skilled in the field of ordinary steel manufacturing, all details thereof are not specifically mentioned in this specification.

[0071] According to one aspect of the present invention, the sum of the Mn and Si content may be 1.8% or more.

[0072] In the present invention, the sum of the Mn and Si contents can be controlled to prevent material softening of the inner coil after winding. The present invention recognizes the problem that if strength is secured primarily through C, more carbides are formed at grain boundaries, making it difficult to secure impact characteristics and potentially causing more softening of the inner coil. Therefore, the invention aims to prevent material softening of the inner coil by controlling the sum of the Mn and Si contents. If the sum of Mn and Si is less than 1.8%, there may be a problem in that the impact characteristics and yield strength intended by the present invention are not satisfied.

[0074] The steel microstructure of the present invention will be described in detail below.

[0075] Unless otherwise specifically stated in the present invention, the % representing the fraction of the microstructure is based on area.

[0077] A steel according to one aspect of the present invention may contain 50 area% or more of bainite as a microstructure and may contain the remainder of other structures.

[0078] In the present invention, it is preferable to include 50% or more of bainite to satisfy the target material, and other residual structures may be included. Other structures may include ferrite, retained austenite, martensite, etc. If bainite is included in an amount less than 50%, there is a risk that the yield strength or low yield ratio intended in the present invention may not be secured.

[0080] Hereinafter, the steel manufacturing method of the present invention will be described in detail.

[0081] A steel according to one aspect of the present invention can be manufactured by reheating, hot rolling, first cooling, second cooling, and coiling a steel slab satisfying the alloy composition described above.

[0083] reheat

[0084] A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1100 to 1300°C.

[0085] When the carbon content is high, the solubility of precipitates such as Ti and Nb is low, so it is desirable to set the slab heating temperature high to secure low yield ratio characteristics. Accordingly, in the present invention, the slab heating temperature can be limited to 1100°C or higher. On the other hand, if the heating temperature is excessively high, the austenite grains become excessively large, which is disadvantageous for securing impact characteristics and yield strength, so the upper limit can be limited to 1300°C, and a more desirable heating temperature range may be 1200 to 1300°C.

[0087] Hot rolling

[0088] The above reheated steel slab can be hot rolled at a finishing hot rolling temperature of 750 to 880°C.

[0089] If the finishing hot rolling temperature is below 750℃, the rolling load is severe, making it difficult to manage the rolls and control the size of the hot-rolled material, which is undesirable. If the temperature exceeds 880℃, there is a problem that the austenite grains become excessively coarsened.

[0091] Primary cooling

[0092] The above hot-rolled steel plate can be first cooled to a temperature range of Bs-100℃ to Bs at a cooling rate of 25℃ / s or more.

[0093] In the present invention, primary cooling and secondary cooling may be applied to secure a low yield ratio.

[0094] The first cooling rate is preferably 25℃ / s or higher to refine the final transformation structure, and it is more preferable to limit the upper limit to 80℃ / s to control the shape and cooling end temperature.

[0095] If the cooling end temperature exceeds Bs, there is a risk of grain coarsening, and a problem arises where soft ferrite and pearlite structures are formed, causing a sharp decrease in yield strength. On the other hand, if the temperature is below Bs-100℃, the elongation decreases sharply, and the strength becomes excessively high, which may also lead to a deterioration in impact properties. This may be a phenomenon caused by the precipitation of carbides at the bainite grain boundaries due to excessive strength enhancement in a carbon-rich structure.

[0097] Secondary cooling and winding

[0098] The above first cooled steel plate can be coiled after second cooling at a cooling rate of 10 to 20°C / s to a temperature range of 450°C to Bs.

[0099] In the present invention, secondary cooling may be performed to match the coiling temperature within a limited cooling zone length. Secondary cooling can be performed at a slower cooling rate than the primary cooling to prevent the material temperature from reheating above Bs after primary cooling and to reduce material deviations in the thickness and width directions formed by rapid cooling during primary cooling. If cooling is performed at an excessively fast rate, there is a risk of cracks occurring at the edges of the plate due to high hardenability. Additionally, by controlling the coiling temperature to be lower than the primary cooling end temperature, it is more advantageous to secure a low yield ratio.

[0100] When secondary cooling is performed as in the present invention, the winding temperature can be precisely controlled to obtain a desired microstructure, thereby obtaining the target material without deviation and suppressing surface defects. If winding is performed after cooling only by primary cooling without such secondary cooling, material softening occurs due to reheating, making it difficult to satisfy the target yield strength. Additionally, there may be problems such as increased material deviation and defective shape due to reheating following surface overcooling by primary cooling.

[0101] After the first cooling mentioned above, if the material is wound at a temperature below Bs, bainite is formed, allowing for the simultaneous securing of strength and low yield ratio. However, after the first cooling, material variation between the surface and the core is prone to occur due to the high hardenability. Furthermore, due to the high carbon content, if cooling is performed at a rate of less than 10℃ / s, the amount of cooling water applied is small, which causes relatively large reheating and raises the temperature above the Bs temperature. In other words, it is difficult to precisely control the winding temperature with only the first cooling, and material variation can be significantly increased. Therefore, after the first cooling is completed, a second cooling is performed to prevent reheating, and by winding at 450℃ or higher, the strength is prevented from becoming excessively high, thereby reducing material variation. Additionally, elongation and impact characteristics can be secured simultaneously. However, if cooling is performed at a rapid rate exceeding 20℃ / s during the second cooling, the amount of water applied increases, which may cause cracking at the edge of the plate.

[0103] The steel of the present invention manufactured in this manner has a yield strength of 650 MPa or more, a tensile strength of 800 MPa or more, a yield ratio of 85% or less, a widthwise tensile strength deviation of 80 MPa or less (wherein the widthwise tensile strength deviation represents the difference between the maximum and minimum values ​​measured in the rolling direction at a point 50 mm away from the edge, at the 1 / 4 point of the width, and at the 1 / 2 point of the width, respectively), a yield strength of the inner coil of the coil of 650 MPa or more (wherein the inner coil refers to the 1 / 2 point of the coil length direction), an elongation of 15% or more, and a Charpy impact absorption energy at -5℃ of 47 J or more, and can be equipped with high strength and low yield ratio characteristics.

[0105] The present invention will be explained in more detail below through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0107] (Example)

[0108] A steel slab having the composition of Table 1 below was heated to 1100–1300°C, hot-rolled under the conditions of Table 2 below, first cooled, and second cooled, and then coiled to produce a steel plate. Here, after coiling, it was cooled to room temperature under normal conditions.

[0109] Table 1 below lists the Bs temperatures according to the alloy element content of each steel grade, and the Bs temperatures were obtained through a standard dilatometer test.

[0111] Gangjong Alloy composition (wt%) Bs(℃) C Mn Si Al P S Cr Nb Ti N Mn+Si A 0.14 2 0.35 0.02 0.01 0.003 0 0.04 0.015 0.005 2.35 579 B 0.14 2 0.35 0.02 0.01 0.003 0.3 0.04 0.015 0.005 2.35 573 C 0.14 2 0.35 0.02 0.01 0.003 0.5 0.04 0.015 0.005 2.35 571 D 0.14 2 0.35 0.02 0.01 0.003 0.8 0.04 0.015 0.005 2.35 563 E 0.14 2 0.35 0.02 0.01 0.003 1.1 0.04 0.015 0.005 2.35 558 F 0.19 1.5 0.35 0.02 0.01 0.003 0.5 0.04 0.015 0.005 1.85 580 G 0.12 2 0.35 0.02 0.01 0.003 0.5 0.04 0.015 0.005 2.35 574 H 0.21 1.5 0.35 0.02 0.01 0.003 0.5 0.04 0.015 0.005 1.85 577 I 0.19 1.4 0.35 0.02 0.01 0.003 0.5 0.04 0.015 0.005 1.65 588

[0113] Psalm number Gangjong Hot rolling Primary cooling Secondary cooling Kwon Chi Finishing hot rolling temperature (°C) Cooling rate (°C / s) Cooling end temperature (°C) Cooling rate (°C / s) Temperature (°C) 1 C 852 30 553 15 539 2 C 771 30 551 15 538 3 C 894 30 552 15 541 4 C 850 6 553 15 539 5 C 850 30 452 11 542 6 C 850 30 590 15 543 7 C 850 30 564 5 592 8 C 850 30 548 15 455 9 C 850 30 551 15 428 10 A 851 30 549 15 542 11 B 854 30 552 15 539 12 D 849 30 551 15 544 13 E 850 30 553 15 536 14 F 850 30 556 15 543 15 G 850 30 549 15 539 16 H 850 30 551 15 546 17 I 850 30 554 15 541 18 C 848 30 553 25 534

[0115] Table 3 below shows the observed microstructure and measured physical properties of each specimen. The microstructure fractions were measured using an optical microscope, and the microstructure fractions in Table 3 below, excluding bainite, include ferrite, retained austenite, and martensite. The values ​​for yield strength (YS), tensile strength (TS), elongation (El), and yield ratio (YS / TS) were measured by performing tensile tests under the conditions specified in KSB0802, and the Charpy impact absorption energy was recorded as the result obtained through a Charpy impact test at -5℃. In this case, the material properties represent the values ​​obtained from tensile tests after taking a tensile specimen at the 1 / 4 width point of the center of the coil in the longitudinal direction. The tensile strength deviation in the width direction indicates the difference between the maximum and minimum values ​​by measuring the tensile strength in the rolling direction at points 50mm away from the edge, at the 1 / 4 width point, and at the 1 / 2 width point, respectively. The yield strength of the inner coil represents the yield strength at the 1 / 2 length point of the coil. Additionally, the presence of cracks on the edge of the steel plate was indicated after visual inspection.

[0117] Psalm number Gangjong Microstructure Physical properties division Bainite fraction (%) Yield strength (MPa) Tensile strength (MPa) Elongation rate (%) Surrender rate (%) Shock absorption energy (-5℃, J) Internal power of the abdomen (MPa) Whether edge cracks occur Variation in tensile strength in the width direction (MPa) 1 C 89 677 906 18 75 66 672 X 31 Invention Example 1 2 C 84 694 898 18 77 87 689 X 25 Invention Example 2 3 C 88 648 902 19 72 23 644 X 38 Comparative Example 1 4 C 87 644 825 20 78 11 643 X 18 Comparative Example 2 5 C 99 785 951 14 83 36 782 X 66 Comparative Example 3 6 C 48 585 876 21 67 81 579 X 34 Comparative Example 4 7 C 0 509 748 24 68 93 504 X 22 Comparative Example 5 8 C 99 704 913 18 77 68 702 X 61 Invention Example 3 9 C 99 718 935 12 77 52 711 X 68 Comparative Example 6 10 A 43 567 793 22 64 86 544 X 24 Comparative Example 7 11 B 82 667 888 19 75 72 666 X 33 Invention Example 4 12 D 91 698 921 17 76 62 696 X 57 Invention Example 5 13 E 99 793 968 13 82 33 798 X 69 Comparative Example 8 14 F 85 672 897 19 75 62 659 X 46 Invention Example 6 15 G 47 649 858 18 76 77 642 X 37 Comparative Example 9 16 H 90 688 902 18 76 43 672 X 59 Comparative Example 10 17 I 86 668 889 18 75 66 644 X 47 Comparative Example 11 18 C 84 699 901 18 78 92 691 O 89 Comparative Example 12

[0119] As shown in Table 3, Invention Examples 1 to 6, which satisfy the alloy composition and manufacturing conditions of the present invention, satisfied the microstructure and precipitate characteristics proposed in the present invention and secured the physical properties intended in the present invention.

[0120] On the other hand, comparative examples that did not satisfy the composition or manufacturing conditions of the present invention failed to secure the microstructure and physical properties intended by the present invention.

[0122] Figures 1 (a) and (b) show micrographs of Invention Example 1 and Comparative Example 4, respectively. In (a), the bainite fraction of the present invention was satisfied, but in (b), the level of bainite desired in the present invention was not formed, and it can be seen that ferrite and a central segregation zone were formed because the diffusion was relatively fast due to the high first cooling end temperature.

[0124] Specifically, Comparative Example 1 had a finishing hot rolling temperature that exceeded the range of the present invention, and the impact properties were inferior due to the coarsening of the austenite grain size.

[0125] Comparative Example 2 had a primary cooling rate that fell short of the scope of the present invention, and the impact properties were inferior due to the austenite grain coarsening and the formation of coarse ferrite and pearlite, and the desired yield strength could not be secured.

[0126] In Comparative Example 3, the first cooling end temperature was excessively low, so the strength of the upper bainite became excessive, resulting in inferior impact properties and reduced elongation.

[0127] In Comparative Example 4, the first cooling end temperature was above the Bs temperature, so bainite was not sufficiently formed and some soft phase was formed, causing the yield strength to decrease rapidly.

[0128] In Comparative Example 5, the high coiling temperature resulted in the formation of soft ferrite and pearlite, and the desired strength could not be secured.

[0129] In Comparative Example 6, the low winding temperature caused the strength to rise rapidly, making it difficult to secure elongation.

[0130] Comparative Example 7 is an example in which Cr was not added, and as the effects of increased hardenability and bainite refinement due to the addition of Cr were not secured, the hardenability was low and the bainite lath was relatively coarse, making it difficult to secure strength.

[0131] Comparative Example 8 had an excessive amount of Cr added, which increased the strength excessively and lowered the impact properties.

[0132] Comparative Example 9 was a case where the C content was insufficient, so bainite was not sufficiently formed and the desired strength was not secured, and Comparative Example 10 was a case where the C content was excessive, so the impact characteristics were inferior.

[0133] Comparative Example 11 is a case where the sum of the Mn+Si content did not satisfy the range of the present invention, and softening of the inner coil occurred, resulting in insufficient strength of the inner coil.

[0134] Comparative Example 12 is a case where the secondary cooling rate is excessive, cracks occurred at the edge of the steel, and the variation in tensile strength in the width direction exceeded the range proposed in the present invention, resulting in inferior material variation.

[0136] Although the present invention has been described in detail through embodiments above, other forms of embodiments are also possible. Therefore, the technical concept and scope of the claims described below are not limited to the embodiments.

Claims

Claim 1 In weight%, containing C: 0.13~0.20%, Si: 0.6% or less, Mn: 1.4~3.0%, Al: 0.1% or less, Nb: 0.02~1.0%, Ti: 0.03% or less (excluding 0%), Cr: 0.3~1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, and the remainder being Fe and other unavoidable impurities; the sum of the Mn and Si contents is 1.8% or more; the microstructure contains bainite in an area% or more of 50% and the remainder being other structures; and the widthwise tensile strength deviation is 80 MPa or less (wherein the widthwise tensile strength deviation represents the difference between the maximum and minimum values ​​measured in the rolling direction at a point 50 mm away from the edge, at the 1 / 4 width point, and at the 1 / 2 width point, respectively). Steel plate. Claim 2 In claim 1, the steel plate has a yield strength of 650 MPa or more, a tensile strength of 800 MPa or more, a yield ratio of 85% or less, and an elongation of 15% or more. Claim 3 In claim 1, the steel plate is a steel plate having an inner coil yield strength of 650 MPa or more (wherein the inner coil refers to the halfway point along the coil length direction). Claim 4 In claim 1, the steel plate is a steel plate having a Charpy impact absorption energy of 47J or more at -5℃. Claim 5 A step of reheating a steel slab comprising, in weight%, C: 0.13~0.20%, Si: 0.6% or less, Mn: 1.4~3.0%, Al: 0.1% or less, Nb: 0.02~1.0%, Ti: 0.03% or less (excluding 0%), Cr: 0.3~1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, and the remainder being Fe and other unavoidable impurities, wherein the sum of the Mn and Si contents is 1.8% or more; a step of hot rolling the reheated steel slab at a finishing hot rolling temperature of 750~880℃; a first cooling step of cooling the hot-rolled steel plate to a temperature range of Bs-100℃~Bs at a cooling rate of 25℃ / s or more; and the first cooled steel plate A method for manufacturing a steel plate comprising: a secondary cooling step of cooling to a temperature range of 450℃ to Bs at a cooling rate of 10 to 20℃ / s; and a step of winding the secondary cooled steel plate. Claim 6 In claim 5, the above reheating is performed at 1100~1300℃. A method for manufacturing a steel plate. Claim 7 A method for manufacturing a steel plate according to claim 5, wherein the cooling rate during the first cooling is 25 to 80℃ / s.

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