KR101597789B1 - High-strength steel plate and producing method therefor - Google Patents

Abstract

The present invention relates to a high strength thick steel plate with excellent heat straightening part properties and a manufacturing method thereof and, more specifically, relates to a high strength thick steel plate with heat straightening part properties to satisfy a demanded strength and a toughness of impact even under a local heating in an abnormal area of approximately 870°C or less in a straightening process; and a manufacturing method thereof. An embodiment of the present invention comprises: 0.05-0.10 wt% of C; 1.45-1.55 wt% of Mn; 0.15-0.25 wt% of Cu; 0.20-0.70 wt% of Ni; 0.10-0.15 wt% of Si; 0.008-0.012 wt% of Nb; 0.008-0.012 wt% of Ti; 0-0.012 wt% of P; and 0-0.003 wt% of S; and includes: a step of preparing a thick steel plate wherein a residual part is formed with Fe and extra inevitable impurities; a step of reheating the thick steel plate to reach a temperature of 1100-1200°C at a rate of 14-16 °C/min per thickness; a step of finishing a hot rolling of the reheated thick steel plate on a platform area right above Ar3; and a step of air-cooling the hot rolled thick steel plate after performing an accelerated cooling up to 400°C or below at a cooling rate of 25-40 °C/min.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high-

More particularly, the present invention relates to a high strength steel sheet having excellent heating curve characteristics and a method of manufacturing the same, and more particularly, to a high strength steel sheet having excellent heating strength and impact toughness satisfying required strength and impact toughness even in local heating up to about 870 & Strength steel sheet excellent in straightness characteristics and a manufacturing method thereof.

In general, ship structures in the shipbuilding industry need to have a continuous smooth curvature surface in order to reduce the flow resistance during navigation. To this end, the operator bends the post-welded steel plate into a predetermined shape in advance, and then welds the end faces of the steel plate to produce a welded structure having a continuous smooth curvature surface.

This bending of the steel plate is required to be performed with a complicated and subtle curvature depending on the part to be machined of the ship structure, so that the work can not be performed by simple and uniform press processing. Therefore, after roughly press-working is generally performed, a method of locally heating a steel sheet by a bending process by linear heating, that is, by using a gas burner or the like, and water-cooling immediately after heating is used.

However, since the bending process by the line heating requires a long time to process into a desired shape, it becomes a bottleneck phenomenon and cost increase in the shipbuilding process. Therefore, in recent years, there is a demand for a steel sheet which contributes to improvement of working efficiency in bending.

Further, when the linear heating is carried out, the heating portion shrinks due to the thermal expansion and the cooling due to the constraint from the unheated region around the heating portion, thereby causing a thermal deformation phenomenon in which the heating portion of the steel sheet is yielded and plastically deformed.

Therefore, in order to solve these problems, many techniques have been proposed relating to the steel sheet aiming at improving the yield strength of the steel sheet and controlling the yield strength thereof to increase the amount of deformation by the line heating.

The reason for controlling the yield strength at this time is that, as disclosed in Japanese Unexamined Patent Application Publication No. 2007-56348, bending deformation can be facilitated when the yield strength of the steel sheet is low.

Therefore, in order to lower the yield strength of the steel sheet, the steel sheet is manufactured by lowering the carbon equivalent or accelerating cooling.

Generally, steel sheet after being manufactured by water cooling process is made of ferrite matrix structure with electric potential, so it is easy to deform and calibrate even at 500 ℃.

At this time, the heating operation of the steel sheet is limited to the heating temperature of 550 ° C according to the pipe standard. This is because when the steel sheet is heated to a high temperature of 550 DEG C or more, the strength of the steel sheet may be lowered.

However, in the case of a steel plate having a yield strength of 460 MPa and a high strength, it is impossible to calibrate the deformation of the welded portion unless the work is performed at a heating temperature higher than 550 ° C. Further, if the heating temperature is higher than 550 ° C., If the proper alloying design can not be achieved, there is a problem that the strength does not meet the specification.

Especially, when using steel sheet for high-strength offshore structures, the heating temperature that can be practically practiced in the field for deformation and correction work reaches 600 ~ 750 ℃. Therefore, in the process of heating the post-steel sheet for high-strength offshore structures, the physical properties of the post-steel sheet may be deteriorated, which may adversely affect the life and safety of the structure. Therefore, a steel sheet having both strength and toughness according to the serching operation is required.

The present invention has been made in view of the above circumstances and provides a high strength steel sheet excellent in the characteristics of a heating curved portion whose strength and toughness meet the specification without deteriorating its deformability even at a high heating temperature by a proper alloy design, .

In order to accomplish the above object, one embodiment of the present invention is characterized by comprising, by mass%, 0.05 to 0.10% of C, 1.45 to 1.55% of Mn, 0.15 to 0.25% of Cu, 0.20 to 0.70% of Ni, 0.10 to 0.70 of Si, 0.005 to 0.15% of Nb, 0.008 to 0.012% of Ti, 0.008 to 0.012% of Ti, more than 0 to 0.012% of P, up to 0.003% of S or less, and the balance being Fe and other unavoidable impurities And reheating the steel sheet to a temperature of 1100 to 1200 ° C at a rate of 14 to 16 ° C / min per thickness, and finishing the reheated steel sheet to a hot- Cooling the hot-rolled steel sheet to a temperature of 400 ° C or less at a cooling rate of 25 to 40 ° C / min, and then air-cooling the steel sheet.

In one embodiment of the present invention, the Vickers hardness value is 20% or less of the base material and the average impact energy is 380 J or more at -40 ° C in the Charpy V notch test. It may be a high strength steel sheet having excellent characteristics.

In an embodiment of the present invention, the steel sheet may have an area ratio of quasi polygonal ferrite (QPF) structure of 40 to 50% and an average crystal grain size of the quasi-polygonal ferrite of 5 to 25 μm have.

In one embodiment of the present invention, the backing plate includes the quasi-polygonal ferrite and the bainite as a base structure, and the base structure may be deformed with the quartz polygonal ferrite and the bainite tempered after the heat-pressing operation .

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The effects of the high strength steel sheet and the manufacturing method thereof, which are excellent in the heating curved portion characteristics according to the present invention, will be described as follows.

First, according to the present invention, it is possible to provide a high strength steel sheet having a toughness that is easy to deform and calibrate but has little strength dropout and satisfies the standard when the steel sheet is worked at a temperature of 550 ° C or higher.

Secondly, according to the present invention, it becomes possible to efficiently manufacture an offshore structure in particular. That is, even if the post-steel sheet is heated to 600 to 750 ° C in the field, since the physical properties of the post-steel sheet are less deteriorated, deformation and correction can be easily performed.

It should be understood that the effects of the present invention are not limited to the above effects and include all effects that can be deduced from the detailed description of the present invention or the configuration of the invention described in the claims.

1 is a perspective view of a steel sheet after heating in a manual manner according to an embodiment of the present invention.
2 is a perspective view of a steel sheet after being heated in a semi-automatic manner according to an embodiment of the present invention.
3 is a cross-sectional view of a steel plate showing the notch location of a Charpy V notch test according to an embodiment of the present invention.
4 is a cross-sectional view of a steel sheet showing a measurement position of a hardness test according to an embodiment of the present invention.
FIG. 5 is a graph showing a temperature of a steel sheet measured by a manual method using a thermocouple according to an embodiment of the present invention.
FIG. 6 is a graph illustrating temperature values of a steel sheet after being heated by a semi-automatic method using a thermocouple according to an embodiment of the present invention.
FIG. 7 is a graph showing experimental results of a heating curve of a rear steel plate according to an embodiment of the present invention.
8 is an enlarged view showing the structure of a steel sheet according to an embodiment of the present invention.
9 is an enlarged view showing the structure of a manufactured steel sheet according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

Throughout the specification, when a part is referred to as being "connected" to another part, it includes not only "directly connected" but also "indirectly connected" . Also, when a part is referred to as "comprising ", it means that it can include other components as well, without excluding other components unless specifically stated otherwise.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a perspective view of a steel sheet after being heated in a manual manner according to an embodiment of the present invention, and FIG. 2 is a perspective view of a steel sheet after being heated in a semi-automatic manner according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view of a steel plate showing a notch location of a Charpy V notch test according to an embodiment of the present invention, and FIG. 4 is a sectional view of a steel plate showing a measurement position of a hardness test according to an embodiment of the present invention.

FIG. 5 is a graph illustrating a temperature of a steel sheet after being heated by a manual method using a thermocouple according to an embodiment of the present invention. FIG. And FIG. 7 is a graph showing experimental results of a heating curve of a rear steel plate according to an embodiment of the present invention.

FIG. 8 is an enlarged view showing a structure of a rear steel sheet according to an embodiment of the present invention, and FIG. 9 is an enlarged view showing a structure of a heated portion of a rear steel sheet according to an embodiment of the present invention.

The method of manufacturing a high strength steel sheet 100 according to the present embodiment is characterized by comprising, by mass%, 0.05 to 0.10% of C, 1.45 to 1.55% of Mn, 0.15 to 0.25% of Cu, 0.008 to 0.012% of Ti, 0.008 to 0.012% of P, and 0.012% or more of P and less than or equal to 0.003% of S, and S: 0.10 to 0.15%, Si of 0.10 to 0.15%, Nb of 0.008 to 0.012% Fe and other unavoidable impurities is prepared and the steel sheet 100 is reheated at a temperature of 1100 to 1200 ° C at a rate of 14 to 16 ° C / min per thickness. After the reheating, the steel sheet 100 is subjected to the hot rolling in the single phase region of the upper side of Ar3, and the hot rolled steel sheet 100 is cooled at a cooling rate of 25 to 40 캜 / min to a temperature of 400 캜 or less And air cooling.

Accordingly, it is possible to realize the physical properties to be obtained in the high strength steel sheet 100 having excellent heating curved portion characteristics. Specifically, the Vickers hardness value of the steel sheet 100 is within 20% of the base metal, and the average impact absorption energy of the steel sheet 100 at -40 ° C during the Charpy V notch test may be 380 J or more. The area ratio of the quasi-polygonal ferrite (QPF) structure of the steel sheet 100 is 40 to 50%, and the average crystal grain size of the steel sheet 100 is 5 to 25 μm. have. In addition, the post-steel plate 100 includes quasi-polygonal ferrite and bainite as a base structure before the heating, and after the heating process, the base structure of the post-steel plate 100 is formed of tempered bainite and quasi-polygonal ferrite .

Here, the term "base structure" refers to a portion which becomes a main body of a metal structure, and a small amount of another structure is mixed in the center.

Hereinafter, the chemical composition and limitations of the method for manufacturing the high strength steel sheet 100 which are excellent in the heating curved portion characteristics according to the present embodiment will be described. However, the content unit of the constituent alloy is% by mass.

C (carbon) is not effective in obtaining a desired high strength when the element or content effective for increasing the strength is low, but is effective for increasing the strength, but deteriorates in toughness and ductility, and is preferably limited to 0.05 to 0.10%.

Mn (manganese) has an effect of increasing the strength at the time of heat treatment, and it is also an element added for the compensation of the strength due to the limited amount of C added. However, when Mn is added in an excessively low amount, the effect of improving the sinterability is hardly effected. If the Mn content exceeds a certain range, the weldability is lowered and the risk of cracking is increased. Therefore, the Mn content is preferably limited to 1.45 to 1.55%.

Cu (copper) is an element capable of minimizing toughness deterioration of a base material and increasing strength at the same time. However, when the amount is too large, the surface quality is greatly deteriorated, and therefore, it is preferable to restrict the toughness to 0.15 to 0.25%.

Ni (nickel) is an element added to secure the corrosion resistance of the material itself, and also helps to improve strength and impact toughness. However, if the amount is too large, a structure such as bainite or martensite may be formed, so that it is preferable to limit the amount to 0.20 to 0.70% or less.

Si (silicon) is an element essential for deoxidation of steel, and is an element effective for increasing the strength. However, if the content is 0.10% or less, desired high strength can not be obtained. Moreover, if it exceeds 0.15%, the toughness and ductility are lowered. Therefore, the content of Si is preferably limited to a range of 0.10 to 0.15%.

Nb (niobium) improves the strength of the base material and the welded portion. However, if the amount of Nb is too large, the possibility of causing brittle cracks at the edge of the steel increases, so that the content is preferably limited to 0.008 to 0.012%.

Ti (titanium) plays a key role in improving the low temperature toughness through grain refinement. Therefore, it is preferable to add 0.008% or more in order to sufficiently obtain the effect. However, if the amount is too large, impact toughness at low temperature deteriorates. Therefore, the upper limit is preferably limited to 0.012%. Therefore, Ti is preferably limited to 0.008 to 0.012%.

P (phosphorus) is an impurity which deteriorates the weldability and hinders impact toughness, and it is desirable to suppress as much as possible. However, since it is an impurity inevitably contained in the manufacturing process, it is preferable to limit the content to more than 0% and not more than 0.012%.

S (sulfur) is an element which deteriorates the ductility, impact toughness and weldability of steel, particularly Mn combined with Mn to form MnS inclusions to deteriorate the wear resistance of steel. Therefore, it is preferable that S (sulfur) is limited to not more than 0.003%.

The remaining component of the high strength steel sheet 100 having excellent heating curved portion characteristics according to the present embodiment is iron (Fe).

The steel sheet 100 having the above-described composition can be manufactured from the steel sheet 100 having high strength after heat-reheating, hot rolling and cooling. At this time, control of the microstructure is essential for imparting the intended strength and toughness to the steel sheet 100. For this purpose, temperature setting at each step is important.

First, the reheating step of the post-steel plate 100 is a step required to perform the succeeding hot-rolling step well and sufficiently obtain the physical properties of the target steel plate.

When the reheating temperature of the steel sheet 100 is less than 1100 ° C, Nb and the like are difficult to be solidified, and it is difficult to secure the desired strength. Therefore, the reheating temperature of the steel sheet 100 is maintained at 1100 DEG C or higher, thereby promoting the reuse of precipitates and maintaining the austenite grain size of an appropriate size to improve the strength level of the steel sheet 100, It is possible to secure a uniform microstructure in the longitudinal direction of the substrate. However, when the reheating temperature exceeds 1200 ° C, there is a fear that the grain size of the steel sheet 100 is increased due to abnormal growth of the austenite crystal grain size, and toughness is lowered. For this reason, the reheating temperature of the steel sheet 100 is preferably 1100 to 1200 ° C.

If hot rolling is performed at a temperature lower than the Ar3 temperature, the transformed ferrite may be processed to seriously deteriorate the ductility of the steel sheet 100 after it is deformed. Therefore, finish hot rolling at a temperature equal to or higher than Ar3 must be completed.

When hot rolling is completed, cooling is performed. In order to secure the strength of the steel sheet, it is important to appropriately control the cooling conditions. Specifically, when the cooling start temperature of the steel sheet 100 after the hot rolling is lower than the Ar3 temperature, ferrite transformation occurs at a slow cooling rate during air cooling, so coarse ferrite is formed, . Therefore, it is necessary to start the cooling of the steel sheet 100 at a temperature equal to or higher than Ar3.

Further, if accelerated cooling is stopped in an area below 400 deg. C, uniform cooling can not be performed, and there is a possibility that the material may fluctuate. Therefore, the lower limit of the accelerated cooling stop temperature is 400 占 폚. At this time, the crystal grain size of the quasi-polygonal ferrite is controlled to 5 to 25 占 퐉 and the elongation rate is set to 25 to 40 占 폚 / min, and air cooling is carried out from 400 DEG C or lower. When the air cooling is performed, uniform cooling of the post-steel plate 100 can be performed easily, so that the fluctuation of the material of the post-steel plate 100 is small and the shape of the post-steel plate 100 can be improved. In addition, it is possible to sufficiently secure the ferrite structure of the steel plate 100.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.

It should be understood, however, that the scope of the present invention is not limited by the scope of the present invention.

In the present embodiment, the post-welded steel plate 100 having the alloy composition ranges as described above is manufactured. Table 1 shows experimental conditions of the post-welded steel plate 100 having a thickness of 70 mm according to the present embodiment.

As shown in the experimental conditions in Table 1, the steel sheet 100 is heated at a rate of 14 to 16 ° C / min so that the temperature of the steel sheet 100 is about 750 to 760 ° C within 60 minutes.

In the case of the heating method for heating the steel sheet 100, the lateral direction may be semi-automatic and the longitudinal direction may be manual. At this time, the semi-automatic method and the manual method

The non-thermally operated manual torch used air and LPG heat source.

However, the heating method is not limited to the embodiment but can be easily changed according to the situation of the site. For example, the heating method may be a fully automatic method or a heating device other than a torch may be used.

After the steel sheet 100 is heated, the steel sheet 100 may be cooled at a temperature of 750 ° C or higher for at least one minute. At this time, the cooling of the steel plate 100 is performed by accelerating cooling at a rate of 25 to 40 ° C / min in the range of 760 to 400 ° C, and air cooling to room temperature from 400 ° C or less.

1, 2, 3, and 4, a tensile specimen 150 can be attached to the upper surface of the steel plate 100 after being heated manually, The V-notch specimen 140, the tensile specimen 150, and the hardness specimen 160 can be attached to the steel plate 100 after being heated in an automatic manner. At this time, the three V-notch specimens 140 can be attached in one set. In the present embodiment, tensile specimen 150, V notch specimen 140 and hardness specimen 160 are arranged on the upper surface of rear steel plate 100 in order from the left side. However, the arrangement of each specimen is not limited to one embodiment And can be easily changed depending on the situation.

When heating according to the heating conditions, heating can be performed locally. More specifically, when heating is performed manually, the upper surface of the steel sheet 100 can be heated in the longitudinal direction so that the entirety of the portion to which the tensile specimen 150 is attached can be heated. In the case of heating in a semi-automatic manner, The upper surface of the rear steel plate 100 may be heated in the lateral direction so that the entirety of the portion to which the V notch specimen 140 and the hardness specimen 160 are attached can be heated.

However, the position of the heating unit 110 is not limited to the embodiment, and can be easily changed depending on the situation. For example, the entire steel plate 100 may be heated instead of local heating.

The thermocouple 120 may be provided on one side of the rear steel plate 100 and measures the temperature of the rear steel plate.

The notch location 130 for the Charpy V notch impact test may be provided in the heating portion 110 of the post-steel plate 100 and at the same time, it may be provided 1 mm below the top surface of the post-steel plate 100.

The hardness test piece 160 may be provided on the heating portion 110 of the rear steel plate 100 and may be provided 1 mm below the upper surface of the rear steel plate 100. At this time, the hardness specimen 160 may be provided at regular intervals.

As shown in FIGS. 5 and 6, the steel sheet 100 is heated to a ferrite and austenite abnormal region (two phase region).

At this time, the AC1 in the ideal range is AC1 (占 폚) = 750.8-26.6C + 17.6Si-11.6Mn-22.9Cu-23Ni + 24.1Cr + 22.5Mo-39.7V-5.7Ti + 232.4Nb-169.4Al-894.7B AC3 (° C) = 937.2-436.5C + 56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr + 38.1Mo + 124.8V + 136.3Ti-19.1Nb + 198.4Al + 3315B. The element symbol in the equation represents the content (mass%) of each element contained in the steel strip 100, and the element not contained in the steel strip 100 is calculated as 0.

On the other hand, the steel sheet 100 manufactured by the above-described experimental conditions can have yield strength, tensile strength, elongation, Vickers hardness, and impact absorption energy values as shown in Table 2 below.

Table 2 and FIG. 7 show the tensile strength, yield strength, elongation (EL), Vickers hardness (HV), and Charpy (Vickers hardness) of the API2W60 and the post-heating steel plate 100 and the post- V-notch impact test (CVN).

API2W60, which is presented as a standard in Table 2, refers to an API (american petroleum institute) offshore structure steel type of on-line precision control thermal process control (TMCP) type.

In the case of yield strength, the yield strength of the post-heating steel plate 100 is 469 MPa, and the yield strength of the post-heating steel plate 100 after heating is 455 MPa. At this time, the yield strength of the post-heating steel plate 100 is included in the range of 414 to 586 MPa given in API2W60 standard.

In the case of tensile strength, the tensile strength of the post-heating steel plate 100 is 576 MPa, and the tensile strength of the post-heating steel plate 100 after heating is 548 MPa. At this time, the tensile strength of the steel sheet 100 after heating is 517 MPa or more, which is indicated by API2W60 standard.

In the case of elongation, the elongation of the post-heating steel plate 100 is 32%, and the elongation of the post-heating steel plate 100 after heating is 23%. At this time, the tensile strength of the post-heating steel plate 100 is 22% or more, which is indicated by API2W60 standard.

In the case of Vickers hardness, the Vickers hardness of the steel plate 100 before heating is HV213 and the Vickers hardness of the steel plate 100 after heating is HV167. That is, when the steel plate 100 is heated, the Vickers hardness decreases by about 20%.

In the case of shock absorbing energy, the impact absorption energy after cooling the post-heating steel plate 100 to -40 ° C is 347J, and the average impact absorption energy after cooling the steel plate 100 to -40 ° C after heating is 408J . At this time, the average impact absorption energy of the steel sheet 100 after being cooled to -40 deg. C is 48 J or more, which is indicated by API2W60 standard.

That is, even if the steel sheet 100 of the present embodiment is heated up to the ideal range of ferrite and austenite for smooth working, the strength and toughness meet the specifications.

Figs. 8 and 9 show the pre-heating and post-heating structures of the rear steel plate 100, respectively.

The steel sheet base material (P) can be made based on the abnormal structure of quasi-polygonal ferrite and bainite, and the area ratio of the quasi-polygonal ferrite structure can be 40 to 50%. The average crystal grain size of the quasi-polygonal ferrite is 5 to 25 탆.

Also, the manufactured steel sheet structure (Q) can be made based on an abnormal structure of quasi-polygonal ferrite and tempered bainite.

That is, before heating, the steel sheet 100 includes quasipolygonal ferrite and bainite as its base structure, and the base structure is deformed into bainite and quasi-polygonal ferrite, which are tempered by heat-pressing operation.

Therefore, by controlling the chemical components and the impurities of the steel sheet 100 according to the present embodiment, it is possible to provide the steel sheet 100 having a high strength after the steel sheet 100 having excellent heating curve characteristics.

The steel plate 100 thus manufactured can be usefully used for ship structures, offshore structures, and building structures.

It will be understood by those skilled in the art that the foregoing description of the present invention is for illustrative purposes only and that those of ordinary skill in the art can readily understand that various changes and modifications may be made without departing from the spirit or essential characteristics of the present invention. will be. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. For example, each component described as a single entity may be distributed and implemented, and components described as being distributed may also be implemented in a combined form.

The scope of the present invention is defined by the appended claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

100: After steel plate 110: Heating part
120: thermocouple 130: notch location
140: V notch Piece 150: Tensile specimen
160: Hardness test piece P: Steel sheet base material
Q: After the manufactured steel sheet structure

Claims (5)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.10% of C, 1.45 to 1.55% of Mn, 0.15 to 0.25% of Cu, 0.20 to 0.70% of Ni, 0.10 to 0.15% of Si, 0.008 to 0.012% of Nb, 0.012%, P: not more than 0% and not more than 0.012%, S: not less than 0% and not more than 0.003%, and the balance being Fe and other unavoidable impurities;
Reheating the steel sheet to a temperature of 1100 to 1200 ° C at a rate of 14 to 16 ° C / min per thickness;
Finishing the hot-rolled steel sheet in the single-phase region on the upper side of Ar3 by reheating the reheated steel sheet; And
Cooling the hot-rolled steel sheet to a temperature of 400 ° C or less at a cooling rate of 25 to 40 ° C / min, followed by air cooling;
Wherein the steel sheet has excellent heating curvature characteristics.
A high strength post-hardened steel sheet produced by the manufacturing method according to claim 1, wherein the average impact absorption energy of the Charpy V notch test is 380 J or more at -40 캜.
3. The method of claim 2,
Wherein the steel sheet has an area ratio of a quasi-polygonal ferrite (QPF) structure of 40 to 50% and an average crystal grain size of the quasi-polygonal ferrite of 5 to 25 탆. High strength steel plate.
3. The method of claim 2,
Characterized in that the steel sheet comprises quasipolygallon ferrite and bainite as a matrix and the base structure is deformed by the quenched polygonal ferrite and tempered bainite by a heating step, After this excellent high strength steel plate.
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