KR20230023945A - High strength steel plate having low yield ratio and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a high-strength steel sheet and a manufacturing method thereof, and more particularly to the steel sheet having high strength and low yield ratio characteristics and a manufacturing method thereof. According to one aspect of the present invention, the high strength steel sheet includes 0.13 to 0.20 wt% of C, 0.6 wt% or less of Si, 1.4 to 3.0 wt% of Mn, 0.1 wt% or less of Al, 0.02 to 1.0 wt% of Nb, 0.03 wt% or less (excluding 0 wt%) of Ti, 0.3 to 1.0 wt% of Cr, 0.03 wt% or less of P, 0.02 wt% or less of S, 0.015 wt% or less of N, remaining Fe, and other unavoidable impurities.

Description

Low yield ratio high strength steel sheet and its manufacturing method {HIGH STRENGTH STEEL PLATE HAVING LOW YIELD RATIO AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a high-strength steel sheet and a manufacturing method thereof, and more particularly, to a steel sheet having high strength and low yield ratio characteristics and a manufacturing method thereof.

Structural steel is a high-strength hot-rolled steel sheet used for construction and civil engineering, and should be able to secure the stability of buildings by absorbing energy when external stresses such as earthquakes occur so that they do not break easily. Therefore, structural steel needs to have a low yield ratio and impact characteristics in order to secure the above effect.

On the other hand, when precipitation strengthening elements such as Ti, Nb, V, etc. are used for this purpose in a carbon amount of 0.1% by weight or less, it is not easy to secure strength and a low yield ratio. In addition, there is a problem in that production cost increases due to the addition of a lot of expensive precipitation strengthening elements, and when coarse precipitates are generated, there is a problem in that impact properties are very inferior. In addition, in the case of hot-rolled steel, there is a problem in that the cooling rate of the inner winding part and the outer winding part of the coil are different after winding, which easily causes material deviation.

According to one aspect of the present invention, it is intended to provide a low yield ratio high strength steel sheet and a manufacturing method thereof.

The object of the present invention is not limited to the above. A person skilled in the art will have no difficulty understanding the further subject matter of the present invention from the general content of this specification.

One aspect of the present invention, 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 ( 0% excluded), Cr: 0.3 to 1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the balance including Fe and other unavoidable impurities,

The sum of the Mn and Si contents is 1.8% or more,

It is a microstructure, containing 50 area% or more of bainite, and including remaining other tissues,

Tensile strength deviation in the width direction is 80 MPa or less (here, the tensile strength deviation in the width direction is determined by measuring the tensile strength values in the rolling direction at a point 50 mm away from the edge, a point of 1/4 of the width, and a point of 1/2 of the width, respectively, and measuring the maximum and minimum values It shows the difference.) can provide a steel sheet.

The steel sheet 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.

The steel sheet may have a yield strength of 650 MPa or more in the inner winding portion of the coil (here, the inner winding portion represents a half point in the coil length direction).

The steel sheet may have a Charpy impact absorption energy of 47J or more at -5°C.

Another aspect of the present invention, 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, the balance including Fe and other unavoidable impurities, and the sum of Mn and Si content is 1.8% reheating the steel slabs above;

hot-rolling the reheated steel slab at a finishing hot-rolling temperature of 750 to 880° C.;

A first cooling step of cooling the hot-rolled steel sheet to a temperature range of Bs-100°C to Bs at a cooling rate of 25°C/s or more;

A secondary cooling step of cooling the primary cooled steel sheet to a temperature range of 450 °C to Bs at a cooling rate of 10 to 20 °C/s; and

It is possible to provide a steel sheet manufacturing method comprising the step of winding the secondary cooled steel sheet.

The reheating may be carried out at 1100 to 1300 ° C.

During the primary cooling, the cooling rate may be 25 to 80° C./s.

According to one aspect of the present invention, it is possible to provide a steel sheet having high strength and low yield ratio and a manufacturing method thereof.

1 (a) and (b) are microstructure photographs of Inventive Example 1 and Comparative Example 4 according to an embodiment of the present invention, respectively.

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art.

The present invention has optimized the alloy composition and manufacturing conditions in order to solve the above-mentioned problems. In particular, it was confirmed that physical properties such as sufficient strength and impact toughness can be secured while having a low yield ratio by strictly controlling the content of elements such as C, Cr, Mn, and Si, and controlling the cooling and coiling temperatures, and completed the present invention. I came to do it.

Hereinafter, the present invention will be described in detail.

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

In the present invention, unless otherwise specified, % indicating the content of each element is based on weight.

Steel according to one aspect of the present invention, by 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 to 1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the balance may include Fe and other unavoidable impurities.

Carbon (C): 0.13 to 0.20%

Carbon (C) is the most effective element added to secure strength. In order to obtain excellent hardness, it is preferable to add 0.13% or more, more preferably 0.14% or more. On the other hand, when the content exceeds 0.20%, weldability may be deteriorated, and strength may be excessively increased and impact properties and elongation may be deteriorated. A more preferable upper limit may be 0.19%.

Silicon (Si): 0.6% or less

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

Manganese (Mn): 1.4 to 3.0%

Manganese (Mn) is the most commonly used element together with C to improve the strength of steel. If the content of manganese (Mn) is less than 1.4%, the solid solution strengthening ability is lowered, which may be disadvantageous in securing strength, and more preferably, it may contain 1.5% or more. On the other hand, if the content exceeds 3.0%, center segregation, formation of inclusions, and grain boundary oxidation may be caused to deteriorate not only the quality of steel but also weldability. In addition, since it may be difficult to secure a low yield ratio if the substitutional solid solution strengthening element is excessive, the upper limit of the content may be more preferably limited to 2.5%.

Aluminum (Al): 0.1% or less

Aluminum (Al) is an element used as a deoxidizer or added for a solid solution strengthening effect. If the content of aluminum (Al) exceeds 0.1%, there is a risk of not only causing slab cracking in the casting, but also causing grain boundary oxidation in the final product, and more preferably, the upper limit can be limited to 0.05%.

Niobium (Nb): 0.02 to 1.0%

Niobium (Nb) is an element used for precipitation strengthening and crystal grain refinement, and may contain 0.02% or more, more preferably 0.03% or more. Meanwhile, since niobium (Nb) is an expensive element, the upper limit thereof may be limited to 1.0%, more preferably 0.08%, in consideration of productivity.

Titanium (Ti): 0.03% or less (excluding 0%)

Titanium (Ti) is an element that serves to improve strength by forming TiN precipitates and improve toughness of welded parts. However, if the content exceeds 0.03%, coarse TiN may be formed and toughness may be rather deteriorated. More preferably, the upper limit may be limited to 0.025%. In the present invention, 0% is excluded in order to secure the effect of titanium (Ti).

Chromium (Cr): 0.3 to 1.0%

Chromium (Cr) is an element that enhances hardenability. When 0.3% or more of chromium (Cr) is added, it is advantageous to refine bainite, thereby securing strength and low yield ratio at the same time. More preferably, 0.35% or more is included. can On the other hand, if the content exceeds 1.0%, the strength is excessively increased, and thus the impact properties and elongation may rather deteriorate. A more preferable upper limit may be 0.8%.

Phosphorus (P): 0.03% or less

Phosphorus (P) is an element inevitably contained in steelmaking, and since it causes brittleness by segregation, it is desirable to include a minimum amount of phosphorus, so in the present invention, the upper limit can be limited to 0.03%.

Sulfur (S): 0.02% or less

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

Nitrogen (N): 0.015% or less

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

The steel of the present invention may include remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in the normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the steel manufacturing field, not all of them are specifically mentioned in this specification.

In the steel according to one aspect of the present invention, the sum of Mn and Si contents may be 1.8% or more.

In the present invention, the sum of Mn and Si contents can be controlled to prevent material softening of the inner winding portion of the coil after winding. In the present invention, when strength is secured mainly in C, more carbides are formed at grain boundaries, making it difficult to secure impact characteristics, and recognizing the problem that more softening of the inner winding part of the coil may occur, and the sum of Mn and Si contents Through this, it is intended to prevent the material softening of the inner winding part of the coil. If the total amount of Mn and Si is less than 1.8%, there may be a problem in that the desired impact characteristics and yield strength in the present invention are not satisfied.

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, % representing the fraction of the microstructure is based on the area unless otherwise specified.

The steel according to one aspect of the present invention may include bainite at 50 area% or more as a microstructure, and may include other structures remaining.

In the present invention, in order to satisfy the target material, it is preferable to include 50% or more of bainite, and other structures may be included. Other structures may include ferrite, retained austenite, martensite, and the like, and if bainite is included in an amount of less than 50%, there is a concern that yield strength or low yield ratio desired in the present invention may not be secured.

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

Steel according to one aspect of the present invention can be produced by reheating, hot rolling, primary cooling, secondary cooling and winding a steel slab satisfying the above-described alloy composition.

reheat

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

When the content of C is high, since the solubility of precipitates such as Ti and Nb is low, it is preferable to set the slab heating temperature high in order to secure the low yield ratio characteristics. Therefore, 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 crystal grains become excessively large, which is disadvantageous in securing impact properties and yield strength, so the upper limit can be limited to 1300 ° C., and a more preferable heating temperature range is 1200 ~ 1300 ° C. .

hot rolled

The reheated steel slab may be hot rolled at a finishing hot rolling temperature of 750 to 880 ° C.

If the finish hot rolling temperature is less than 750 ° C, the rolling load is severe and there is difficulty in roll management and size control of the hot-rolled material, which is undesirable, and if the temperature exceeds 880 ° C, austenite grains are excessively coarsened.

1st cooling

The hot-rolled steel sheet may be primarily cooled at a cooling rate of 25° C./s or more to a temperature range of Bs-100° C. to Bs.

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

The primary cooling rate is preferably 25 ° C / s or more for miniaturization of the final transformed structure, and it is more preferable to limit the upper limit to 80 ° C / s for shape and cooling end temperature control.

When the cooling end temperature exceeds Bs, there is a risk of coarsening of crystal grains, and soft ferrite and pearlite structures are formed, resulting in a rapid decrease in yield strength. On the other hand, if the temperature is less than Bs-100 ° C., the elongation rate decreases rapidly, and the strength is excessively increased, so that the impact properties may also be deteriorated. This may be a phenomenon in which carbides are precipitated at bainite grain boundaries due to excessive strength improvement in a carbon-rich structure.

Secondary cooling and winding

The primary cooled steel sheet may be wound after secondary cooling at a cooling rate of 10 to 20 °C/s to a temperature range of 450 °C to Bs.

In the present invention, secondary cooling may be performed to adjust the winding temperature in a limited cooling zone length. Secondary cooling is performed at a slower rate than the first cooling after the first cooling is performed to prevent the temperature of the material from reheating above Bs after the first cooling and to reduce the material deviation in the thickness and width directions formed by rapid cooling during the first cooling. It can be cooled at the cooling rate. When cooling at an excessively fast cooling rate, cracks may occur at the edges of the plate due to high hardenability. In addition, by controlling the coiling temperature lower than the primary cooling end temperature, it is possible to more advantageously secure the low yield ratio.

When secondary cooling is performed as in the present invention, a desired microstructure can be obtained by precisely controlling the coiling temperature, a desired material can be obtained without deviation, and surface defects can be suppressed. When winding is performed after cooling only by primary cooling without such secondary cooling, material softening occurs due to recuperation, making it difficult to satisfy the target yield strength, and material deviation increases due to recuperation after surface supercooling by primary cooling, There may be a problem of poor shape.

After the primary cooling, when winding at a temperature below Bs, bainite is formed, so that strength and low yield ratio can be secured at the same time. However, after the primary cooling, material deviations between the surface and the center are likely to occur due to high hardenability, and when cooling at less than 10 °C/s due to the high carbon content, relatively large recuperation occurs due to the small amount of coolant injection, resulting in Bs There is a possibility that the temperature rises above the temperature. That is, it is difficult to precisely control the coiling temperature with only the primary cooling, and material variation can be increased. Therefore, after the primary cooling is finished, secondary cooling is performed to prevent reheating, and by winding at 450° C. or higher, strength is not excessively increased, thereby reducing material variation. In addition, elongation and impact properties can be secured at the same time. However, in the case of secondary cooling, when cooling at a rapid rate exceeding 20 ° C / s, there may be a problem of causing cracks at the edge of the plate due to an increased amount of water.

The steel of the present invention thus manufactured 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 a tensile strength deviation in the width direction of 80 MPa or less (wherein, the tensile strength deviation in the width direction is 50 mm from the edge). The tensile strength values are measured in the rolling direction at the point of separation, 1/4 of the width, and 1/2 of the width, respectively, and the difference between the maximum and minimum values is shown.), and the inner winding yield strength of the coil is 650 MPa or more (where, The inner winding part represents the 1/2 point in the coil longitudinal direction), the elongation is 15% or more, and the Charpy shock absorption energy at -5 ° C is 47J or more, so it can have high strength and low yield ratio characteristics.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention.

(Example)

A steel slab having the composition shown in Table 1 below was heated at 1100 to 1300° C., hot-rolled under the conditions shown in Table 2, primary cooling, and secondary cooling followed by winding to prepare a steel plate. Here, after winding, it was cooled to room temperature under normal conditions.

In Table 1 below, the Bs temperature according to the alloy element content of each steel type was described, and the Bs temperature was obtained by a normal dilatometer experiment.

steel grade 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

Psalter
number steel grade hot rolled 1st cooling secondary cooling winding finish
hot rolled
Temperature (℃) cooling rate
(℃/s) cooling end
Temperature (℃) cooling rate
(℃/s) temperature
(℃) One 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

In Table 3 below, the microstructure of each specimen was observed and the physical properties were measured and shown. The microstructure fraction was measured using an optical microscope, and the microstructure fraction excluding bainite in Table 3 includes ferrite, retained austenite, and martensite. The values of yield strength (YS), tensile strength (TS), elongation (El), and yield ratio (YS/TS) were measured by performing a tensile test under the conditions specified in KSB0802, and the Charpy impact absorption energy at -5 ℃ The result values measured through the Charpy impact test were described. At this time, the physical properties showed the values obtained by the tensile test after taking the tensile specimen at the point of 1/4 of the width of the central part in the longitudinal direction of the coil, and the tensile strength deviation in the width direction was 50 mm away from the edge, width 1 The tensile strength values were measured in the rolling direction at the /4 point and the width 1/2 point, respectively, and the difference between the maximum and minimum values was shown. The yield strength of the inner winding represents the yield strength at the 1/2 point in the length direction of the coil. In addition, the presence or absence of cracks in the edge portion of the steel sheet was observed after visual observation.

city
side
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bell microstructure Properties division bainite
fraction (%) surrender
robbery
(MPa) Seal
robbery
(MPa) elongation rate
(%) yield ratio
(%) shock absorption
energy
(-5℃, J) inner circle
yield strength
(MPa) edge crack
Occurrence width direction
tensile strength
Deviation (MPa) One 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 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

As shown in Table 3, Inventive Examples 1 to 6 satisfying the alloy composition and manufacturing conditions of the present invention satisfied the microstructure and precipitate characteristics proposed in the present invention, and secured the desired physical properties in the present invention.

On the other hand, comparative examples that did not satisfy the composition or manufacturing conditions of the present invention did not secure the microstructure and physical properties aimed at in the present invention.

1 (a) and (b) show microstructure photographs of Inventive Example 1 and Comparative Example 4, respectively. (a) satisfied the bainite fraction of the present invention, but in the case of (b), in the present invention It can be seen that the desired level of bainite was not formed, and the diffusion was relatively fast due to the high primary cooling end temperature, and a large number of ferrite and central segregation zones were formed.

Specifically, in Comparative Example 1, the finish hot rolling temperature exceeded the range of the present invention, and the impact properties were inferior due to the coarsening of the austenite grain size.

In Comparative Example 2, the primary cooling rate was less than the range of the present invention, and the impact characteristics were poor due to the coarsening of austenite grains and the formation of coarse ferrite and pearlite, and the desired yield strength was not secured.

In Comparative Example 3, the strength of the upper bainite was excessively low due to the excessively low end temperature of the primary cooling, resulting in poor impact properties and a decrease in elongation.

In Comparative Example 4, the first cooling end temperature was higher than the Bs temperature, bainite was not sufficiently formed, and a soft phase was partially formed, resulting in a rapid decrease in yield strength.

In Comparative Example 5, due to the high coiling temperature, soft ferrite and pearlite were formed, and the desired strength was not secured.

In Comparative Example 6, since the winding temperature was low and the strength rapidly increased, it was difficult to secure the elongation.

Comparative Example 7 is an example in which Cr is not added, and the effects of increasing hardenability and refining of bainite, which are effects of adding Cr, are not secured, so the hardenability is small and the lath of bainite is relatively coarse, so it is difficult to secure strength. It was difficult.

In Comparative Example 8, Cr was excessively added, and the strength was excessively increased, resulting in a decrease in impact properties.

In Comparative Example 9, when the C content was insufficient, bainite was not sufficiently formed and desired strength was not secured, and in Comparative Example 10, when the C content was excessive, impact properties were inferior.

In Comparative Example 11, the sum of the contents of Mn+Si did not satisfy the range of the present invention, and the strength of the inner winding part was insufficient due to softening of the inner winding part.

In Comparative Example 12, the secondary cooling rate was excessive, and cracks occurred at the edge of the steel material, and the tensile strength deviation in the width direction was outside the range proposed in the present invention, and the material deviation was poor.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (7)

In % by weight, C: 0.13 to 0.20%, Si: 0.6% or less, Mn: 1.4 to 3.0%, Al: 0.1% or less, Nb: 0.02 to 1.0%, Ti: 0.03% or less (excluding 0%), Cr: 0.3 to 1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the balance including Fe and other unavoidable impurities,
the sum of the Mn and Si contents is 1.8% or more;
It is a microstructure, containing 50 area% or more of bainite, and including remaining other tissues,
Tensile strength deviation in the width direction is 80 MPa or less (here, the tensile strength deviation in the width direction is determined by measuring the tensile strength values in the rolling direction at a point 50 mm away from the edge, a point of 1/4 of the width, and a point of 1/2 of the width, respectively. It shows the difference).
According to claim 1,
The steel sheet 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.
According to claim 1,
The steel sheet has a yield strength of the inner winding portion of the coil of 650 MPa or more (here, the inner winding portion represents a half point in the coil longitudinal direction).
According to claim 1,
The steel sheet is a steel sheet having a Charpy impact absorption energy of 47J or more at -5 ° C.
In % by weight, C: 0.13 to 0.20%, Si: 0.6% or less, Mn: 1.4 to 3.0%, Al: 0.1% or less, Nb: 0.02 to 1.0%, Ti: 0.03% or less (excluding 0%), Cr: Reheating a steel slab containing 0.3 to 1.0%, P: 0.03% or less, S: 0.02% or less, N: 0.015% or less, the balance including Fe and other unavoidable impurities, and the sum of Mn and Si contents being 1.8% or more;
hot-rolling the reheated steel slab at a finishing hot-rolling temperature of 750 to 880° C.;
A first cooling step of cooling the hot-rolled steel sheet to a temperature range of Bs-100°C to Bs at a cooling rate of 25°C/s or more;
A secondary cooling step of cooling the primary cooled steel sheet to a temperature range of 450 °C to Bs at a cooling rate of 10 to 20 °C/s; and
Steel sheet manufacturing method comprising the step of winding the secondary cooled steel sheet.
According to claim 5,
The reheating is a steel plate manufacturing method carried out at 1100 ~ 1300 ℃.
According to claim 5,
During the primary cooling, the cooling rate is 25 ~ 80 ℃ / s steel plate manufacturing method.

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