WO2022131783A1 - Thick steel plate having high-strength and high-ductility, and manufacturing method therefor - Google Patents

Abstract

The present invention is to provide a thick steel plate and a manufacturing method therefor, wherein the steel plate can secure high strength and high ductility simultaneously even though having a certain thickness.

Description

Thick steel material having high strength and high ductility and manufacturing method thereof

The present invention relates to steel for shipbuilding, and more particularly, to a thick steel material having high strength and high ductility characteristics and a method for manufacturing the same.

As the marine industry becomes more active, the use of ships is increasing, and accordingly, the risk of accidents such as collisions and strandings between ships is also increasing.

In addition, as interest in safety and the environment increases, it is required to improve the impact resistance characteristics that can withstand the destruction of steel used in the military, merchant ships, tankers, etc. even in an accident.

On the other hand, it is known that the higher the elongation, the greater the total amount of plastic deformation and energy absorption until fracture occurs, and thus the impact resistance against external forces and impacts is improved.

When a material having such impact resistance is applied, even if a certain amount of deformation occurs due to a ship accident, damage to property and human life can be prevented by preventing the breakage of the corresponding ship.

Conventionally, in the case of shipbuilding steel with a yield strength of 315 MPa or higher, it is produced through air cooling during cooling, whereby the steel structure has a layered structure of ferrite and pearlite (see FIG. 1). When a steel having such a structure is subjected to an external force, brittle fracture occurs in the band-shaped pearlite structure, which is a major factor in reducing the elongation.

In order to solve this problem, it is effective to segment the band-shaped pearlite by applying water cooling instead of air cooling during cooling. On the other hand, there is a problem that the elongation rate is lowered on the contrary.

(Patent Document 1) Korean Patent Publication No. 10-0340547

One aspect of the present invention is to provide a thick steel material capable of securing high ductility as well as high strength at the same time despite a thick steel material having a certain thickness, and a method for manufacturing the same.

The subject of the present invention is not limited to the above. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

One aspect of the present invention, by weight, carbon (C): 0.14 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 0.9 to 1.2%, aluminum (Al): 0.015 to 0.04% , niobium (Nb): 0.005 to 0.015%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.002 to 0.008%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, balance containing Fe and other unavoidable impurities;

Provided is a thick steel material having a microstructure of high strength and high ductility composed of air-cooled ferrite, water-cooled ferrite and segmented pearlite.

Another aspect of the present invention comprises the steps of preparing a steel slab having the above-described alloy composition; heating the steel slab in a temperature range of 1050 to 1200 °C; manufacturing a hot-rolled steel sheet by finishing hot rolling the heated slab; and cooling the hot-rolled steel sheet at a cooling rate of 5-7°C/s to a temperature range of 650-750°C,

The cooling provides a method of manufacturing a thick steel material having high strength and high ductility, characterized in that the cooling water is poured and then the pattern of not watering is repeated twice or more.

According to the present invention, it is possible to secure high ductility as well as high strength for a thick steel material having a maximum thickness of 50 mm.

The high-strength and high-ductility steel of the present invention has the effect of being advantageously applied as steel for shipbuilding.

1 shows a microstructure photograph of a conventional steel for shipbuilding.

Figure 2 shows a microstructure photograph of the invention steel according to an aspect of the present invention.

3 is a schematic diagram of a cooler to which water cooling can be applied.

4 shows a coolant spray pattern of a cooler to which water cooling can be applied. (a) shows a conventional continuous cooling pattern, and (b) shows an example of a coolant spray pattern according to the present invention.

The inventor of the present invention has studied in depth to obtain a steel material having high strength and high elongation characteristics in order to achieve excellent impact resistance properties of the material in providing a thick steel material suitable as steel for shipbuilding.

In particular, in the case of the existing steel for shipbuilding (see Fig. 1), it was confirmed that there was a problem of causing deterioration of elongation due to brittle fracture when the structure was composed of band-shaped pearlite and subjected to external force, and the present invention solves this problem, It has technical significance in providing a way to minimize the formation of a low-temperature phase in the steel manufacturing process.

Specifically, the present invention provides a thick steel material of a certain thickness or more suitable as steel for shipbuilding, and in addition to the optimum alloy composition of the thick steel material, it is possible to provide optimal process conditions advantageous for forming the intended structure during steel manufacturing. Confirmed, and came to complete the present invention.

Hereinafter, the present invention will be described in detail.

The thick steel material having high strength and high ductility according to an aspect of the present invention is, by weight, carbon (C): 0.14 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 0.9 to 1.2%, Aluminum (Al): 0.015 to 0.04%, niobium (Nb): 0.005 to 0.015%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.002 to 0.008%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less may be included.

Hereinafter, the reason for limiting the alloy composition of the steel provided in the present invention as above will be described in detail.

Meanwhile, unless otherwise specified in the present invention, the content of each element is based on the weight, and the ratio of the tissue is based on the area.

Carbon (C): 0.14 to 0.17%

Carbon (C) is an element advantageous in securing strength by causing solid solution strengthening and bonding with Nb in steel to form carbon nitride.

In order to secure the strength of the target level in the present invention, C may be included in an amount of 0.14% or more, but if the content is excessive, the strength of the steel is excessively increased, which is not preferable because it causes a decrease in the elongation. In consideration of this, the C may be included in an amount of 0.17% or less.

Therefore, the C may be limited to 0.14 to 0.17%.

Silicon (Si): 0.2-0.5%

Silicon (Si) is an element that contributes to the deoxidation of steel during steelmaking and improves the strength of steel through solid solution strengthening.

In order to sufficiently obtain the above-described effect, Si may be included in an amount of 0.2% or more, but when the content is excessive, the strength is excessively increased, which is not preferable because it causes a decrease in elongation. In consideration of this, the Si may be included in an amount of 0.5% or less.

Therefore, the Si may be limited to 0.2 to 0.5%.

Manganese (Mn): 0.9~1.2%

Manganese (Mn) is an element that effectively improves strength through solid solution strengthening of steel and improvement of hardenability. In order to secure a target level of strength due to Mn, Mn may be included in an amount of 0.9% or more. However, if the content is excessive, it combines with S in steel to form MnS inclusions, and as it causes central segregation, there is a possibility that the toughness of the steel may be deteriorated. In consideration of this, the Mn may be included in an amount of 1.2% or less.

Therefore, the Mn may be limited to 0.9 to 1.2%.

Aluminum (Al): 0.015~0.04%

Aluminum (Al) is an effective element for deoxidizing steel, and may be included in an amount of 0.015% or more to ensure cleanliness of steel. However, if the content is excessive, the fraction of Al ₂ O ₃ inclusions increases, and the size thereof becomes coarse, which causes deterioration of the toughness of the steel. In consideration of this, the Al may be included in an amount of 0.04% or less.

Therefore, the Al may be limited to 0.015 to 0.04%.

Niobium (Nb): 0.005 to 0.015%

Niobium (Nb) is an element advantageous to forming a fine structure and improving strength by precipitating as a solid solution or carbon nitride to suppress recrystallization during rolling or cooling.

In order to sufficiently obtain the above-described effect, Nb may be included in an amount of 0.005% or more. However, when the content is excessive, C-concentration occurs due to carbon (C) affinity to promote the formation of the MA phase, thereby reducing toughness and lowering fracture resistance. In consideration of this, the Nb may be included in an amount of 0.015% or less.

Therefore, the Nb may be limited to 0.005 to 0.015%.

Titanium (Ti): 0.005-0.02%

Titanium (Ti) is an element contributing to toughness improvement by inhibiting grain growth by combining with nitrogen (N) in steel to form Ti nitride (TiN).

In order to sufficiently obtain the above-described effect, Ti may be included in an amount of 0.005% or more. However, when the content is excessive, coarse precipitates are formed, which may act as a factor of destruction. In addition, there is a problem in that the solid solution Ti remaining after not bonding with N in the steel forms Ti carbide (TiC), thereby inhibiting the toughness of the steel. In consideration of this, the Ti may be included in an amount of 0.02% or less.

Therefore, the Ti may be limited to 0.005 to 0.02%.

Nitrogen (N): 0.002 to 0.008%

Nitrogen (N) is an element that effectively contributes to the refinement of the structure by combining with Nb or Ti in steel to form a nitride.

In order to sufficiently obtain such an effect, it may be included in an amount of 0.002% or more. However, if the content is excessive, it may impair the surface quality of the steel sheet, so it may be included in an amount of 0.008% or less in consideration of this.

Therefore, the N may be limited to 0.002 to 0.008%.

Phosphorus (P): 0.02% or less

Phosphorus (P) is an impurity that is unavoidably mixed in steel, and when its content exceeds 0.02%, it causes grain boundary segregation and causes steel embrittlement.

Therefore, the P may be limited to 0.02% or less, but 0% may be excluded in consideration of the unavoidable level.

Sulfur (S): 0.005% or less

Sulfur (S) is an impurity that is unavoidably mixed in steel, and when its content exceeds 0.005%, it combines with Mn in the steel to form non-metallic inclusions such as MnS, thereby impairing the properties of the central portion of the steel thickness.

Therefore, the S may be limited to 0.005% or less, but 0% may be excluded in consideration of the unavoidable level.

On the other hand, in addition to the alloy composition of the present invention, the steel of the present invention may further include at least one of Mo and Cr as follows in order to more advantageously secure the physical properties of the steel.

Molybdenum (Mo): 0.05% or less

Molybdenum (Mo) is an element advantageous for increasing the hardenability of steel and improving the strength of steel. When the content of Mo is excessive, there is a problem of inhibiting the toughness of steel by promoting the formation of the bainite phase.

Chromium (Cr): 0.05% or less

Chromium (Cr) is dissolved in austenite during reheating of the slab to increase the hardenability of steel and contribute to securing the strength of steel. If the content of Cr is excessive, there is a risk that the toughness and weldability of steel may be deteriorated, and thus Cr may be included in an amount of 0.05% or less in consideration of this.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art in the manufacturing process, all details thereof are not specifically mentioned in the present specification.

The thick steel material of the present invention having the above-described alloy composition has a microstructure composed of ferrite and a pearlite composite structure, and in this case, the pearlite is preferably segmented pearlite segmented to a predetermined size (see FIG. 2 ). In addition, the ferrite is preferably composed of air-cooled ferrite and water-cooled ferrite.

Specifically, the present invention can secure ductility and toughness by forming air-cooled ferrite to secure high ductility as well as high strength, and secure strength and toughness by forming water-cooled ferrite.

Furthermore, it is possible to obtain the effect of minimizing brittle fracture due to external force by forming segmented pearlite of a certain size, preferably a maximum length of 50 μm, rather than a pearlite phase having a general band (layered) structure.

The air-cooled ferrite and the water-cooled ferrite preferably have an area fraction of 80% or more (excluding 100%) in total. If the sum of the fractions of the air-cooled ferrite and the water-cooled ferrite is less than 80%, it may be difficult to secure a desired high ductility.

In addition, it is preferable that the average grain size of the air-cooled ferrite and the water-cooled ferrite is 12 to 30 μm. When the average grain size of the ferrite exceeds 30 μm, it becomes difficult to secure a target level of strength as a coarse ferrite phase is excessively present. On the other hand, if the size is less than 12㎛, there is a fear that the elongation may be lowered as the strength is too high.

In the present invention, the air-cooled ferrite and the water-cooled ferrite include equiaxed ferrite (polygonal ferrite), while acicular ferrite (accicular ferrite) is not included.

The segmented pearlite may be formed parallel to the rolling direction. In this case, if the maximum length exceeds 50 μm, brittle fracture becomes easy when an external force is generated, thereby causing a decrease in elongation.

Accordingly, the segmented pearlite preferably has a length of 50 μm or less, and preferably contains an area fraction of 20% or less (excluding 0%).

The steel of the present invention, which has a microstructure controlled to a specific structure in addition to the alloy composition described above, has 30% or more and a yield strength of 315 MPa or more on the basis of the converted elongation. have.

Hereinafter, a method for manufacturing a thick steel material having high strength and high ductility provided by the present invention according to another aspect of the present invention will be described in detail.

Briefly, the present invention can manufacture a desired steel sheet through [steel slab heating - hot rolling - cooling], and the conditions for each step will be described in detail below.

steel slab heating

The steel slab of the present invention having the above-described alloy composition includes Nb as a semi-finished product. In heating such a steel slab, in consideration of sufficient solutionization of the Nb carbonitride contained in the steel slab, heating may be performed at 1050° C. or more based on the slab thickness center. However, when the temperature is too high, there is a problem in that scale defects are increased, and austenite grains are coarsened to increase the hardenability of steel. In consideration of this, the heating may be performed at 1200° C. or less.

hot rolled

A hot-rolled steel sheet can be manufactured by hot-rolling the steel slab heated according to the above.

The finish hot rolling during the hot rolling may be performed in a temperature range of Ar3+10 to Ar3+150°C. In this case, if the temperature is less than Ar3+10°C, there is a fear that the toughness is greatly reduced due to the initiation of transformation during rolling. On the other hand, when the temperature exceeds Ar3+150°C, the austenite refinement by rolling is not sufficiently achieved, and thus the target level of strength cannot be secured.

In performing the finish hot rolling in the above-mentioned temperature range, it is preferable to carry out the cumulative reduction ratio of 50% or more. If the cumulative reduction ratio during the finish hot rolling is less than 50%, recrystallization by rolling to the center of the thickness does not occur, and thus there is a problem in that toughness is deteriorated as the grains in the center are coarsened. It should be noted that the upper limit of the cumulative reduction ratio is not particularly limited, and may be appropriately selected in consideration of the desired thickness of the hot-rolled steel sheet.

Cooling

By cooling the hot-rolled steel sheet obtained as described above, it is possible to manufacture a steel material having physical properties targeted in the present invention.

The cooling may be performed up to a temperature range of 650 to 750° C. starting from below the Ar3 temperature, and is preferably performed at a cooling rate of 5 to 7° C./s.

When the cooling is started at a temperature exceeding Ar3, the particle size of the ferrite becomes too fine and the strength is excessively increased, so that the target level of ductility cannot be secured. In addition, due to the time difference between the cooling start time between the front end and the rear end of the steel sheet, there is a risk of causing material deviation between the front end and the rear end.

However, if the cooling start temperature during cooling is too low (final), transformation is completed before water cooling, and there is a risk that the steel structure may have a layered structure of ferrite and pearlite due to air cooling, a few seconds after completion of the hot rolling The cooling may be started in the inner, and in this case, it may correspond to starting the cooling at Ar3 or less.

When the cooling termination temperature is less than 650° C. during cooling, a low-temperature phase such as bainite is generated in the steel, and there is a risk of a decrease in elongation along with a rapid increase in strength. On the other hand, if the temperature is too high and exceeds 750° C., the ferrite cannot be effectively refined, and thus the strength of the target level cannot be secured. More advantageously, the cooling can be finished at 665° C. or higher.

When cooling to the above-mentioned temperature range, if the cooling rate at that time is less than 5 ℃ / s, layered pearlite is generated in the steel material, there is a high possibility of causing a decrease in elongation due to material embrittlement. On the other hand, when the cooling rate exceeds 7° C./s, a low-temperature texture is formed in the steel, which may cause a decrease in ductility.

Meanwhile, in performing cooling under the above-described conditions, the cooling may be performed by water cooling using a cooler that sprays cooling water. The cooler is not limited thereto, but may be provided as shown in FIG. 3 , and the steel material is positioned between the cooling water spraying devices positioned up and down and the steel material is transferred through the conveying roll, with respect to the upper/lower surface of the steel material You can spray coolant.

Cooling using such a cooler generally terminates cooling after spraying cooling water from several consecutive water injection units, as shown in FIG.

Specifically, as an example of the present invention, as shown in FIG. 4(b) , the watering/non-irrigation pattern can be repeated for each watering unit, and in this case, the watering/non-irrigation pattern is preferably repeated two or more times. do.

In this way, by repeating the water/non-watering pattern for each unit and spraying cooling water for the steel material conveyed in one direction by the conveying roll, the average flow rate per unit time as a whole is reduced, and it is possible to keep the cooling rate low. In addition, compared to the case of continuously spraying cooling water as cooling and recuperating are repeated in the surface portion of the steel, it is possible to obtain an effect of suppressing the generation of a low-temperature phase due to rapid cooling of the surface portion.

In the present invention, in repeating the injection/non-injection pattern with respect to the cooling water spray pattern, the pattern may be appropriately set according to the structure of the facility. In addition, the amount of cooling water sprayed from each unit may be appropriately selected according to the thickness of the transferred steel, but it should be suggested to the extent that the cooling rate suggested in the present invention can be satisfied.

The steel of the present invention, which has completed the cooling process as described above, is a thick steel material having a maximum thickness of 50 mm, preferably 10 to 50 mm, and has an effect of securing high ductility as well as high strength for this thick steel material.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

Molten steel having an alloy composition shown in Table 1 was prepared and a steel slab was prepared through continuous casting. For each steel slab, a hot-rolled steel material having a predetermined thickness was manufactured through a [heating-hot rolling-cooling] process under the conditions shown in Table 2 below.

steel grade	Alloy composition (wt%)
	C	Si	Mn	Al	Nb	Ti	S	P	N
A	0.17	0.49	1.15	0.039	0.010	0.015	0.004	0.012	0.002
B	0.15	0.43	1.05	0.031	0.009	0.012	0.004	0.012	0.002
C	0.14	0.35	0.93	0.032	0.011	0.013	0.004	0.010	0.003
D	0.15	0.43	1.25	0.040	0.010	0.012	0.003	0.010	0.003
E	0.07	0.44	0.93	0.034	0.018	0.013	0.005	0.007	0.002

steel grade	thickness (mm)	heating	finish hot rolled		Cooling				note
		temperature (℃)	end temperature (℃)	accumulate reduction ratio (%)	start temperature (℃)	end temperature (℃)	speed (℃/s)	pattern
A	15	1136	879	85	755	659	6.7	2	Invention Example 1
B	20	1139	879	80	779	686	6.2	2	Invention Example 2
C	20	1129	858	80	766	683	5.5	2	Invention example 3
B	40	1113	808	60	760	669	6.1	3	Invention Example 4
C	40	1112	811	60	764	677	5.9	3	Invention Example 5
B	50	1107	789	50	754	682	5.1	4	Invention example 6
C	50	1111	790	50	752	680	5.1	4	Invention Example 7
A	15	1127	857	85	729	591	7.0	continuity*	Comparative Example 1
B	32	1110	805	56	air cooling		-	-	Comparative Example 2
C	40	1113	887	78	848	689	19.9	continuity	Comparative Example 3
B	50	1110	879	56	821	487	6.6	3	Comparative Example 4
C	50	1104	877	56	818	660	9.4	continuity	Comparative Example 5
D	30	1122	809	73	769	694	6.7	2	Comparative Example 6
E	40	1116	853	65	765	674	6.5	3	Comparative Example 7
In Table 2, the cooling pattern indicates the number to which the (main/non-jug) pattern of the cooling water is applied, pattern 2 is the number of weeks/non-jugs + main water/non-jugs, and pattern 3 is the main water/non-jugs + main water/non-jugs + main water/non-jugs. /means a minor number.b In addition, continuous* refers to a pattern method in which coolant is continuously sprayed from two or more water injection units.

The microstructure and mechanical properties were measured for each of the hot-rolled steel materials prepared as described above, and the results are shown in Table 3 below.

First, after taking specimens in a direction perpendicular to the rolling direction of each hot-rolled steel, yield strength (YS), tensile strength (TS), and elongation (El) were measured using a universal tensile tester. At this time, the elongation (El) tends to be measured unfavorably as the thickness of the steel becomes thinner, so that the same value is obtained regardless of the thickness of the steel. did The conversion elongation (E ') can be calculated through the following formula (in the following formula, E means elongation (before conversion), t means steel material thickness (mm)).

In addition, the microstructure was observed using an optical microscope for the same test piece, and each phase was divided through an image analyzer and fractions were measured.

division	microstructure				mechanical properties
	F fraction* (area%)	F mean Size (㎛)	P fraction (area%)	P length (μm)	YS (MPa)	ts (MPa)	El (%)
Invention Example 1	82	13	18	42	387	531	32
Invention Example 2	83	17	17	37	373	515	31
Invention example 3	85	15	15	35	389	522	33
Invention Example 4	86	19	14	31	370	505	32
Invention Example 5	85	17	15	34	382	515	32
Invention example 6	84	15	16	38	376	509	31
Invention Example 7	81	16	19	39	380	509	31
Comparative Example 1	71	8	27	65	451	582	27
Comparative Example 2	75	14	25	47	381	532	29
Comparative Example 3	76	10	24	68	411	544	26
Comparative Example 4	74	9	25	57	431	572	26
Comparative Example 5	75	9	24	58	395	557	27
Comparative Example 6	79	11	19	51	389	517	29
Comparative Example 7	92	10	8	62	416	491	27
In Table 3, F means ferrite and P means pearlite, where F fraction* is expressed as a combined fraction of air-cooled ferrite and water-cooled ferrite. In addition, in Comparative Examples 1, 4, 5 and 6, a bainite phase was partially formed in addition to F and P, and the fraction corresponds to the remaining fraction excluding the F and P fractions.

As shown in Tables 1 to 3, Inventive Examples 1 to 7, which satisfy both the alloy composition and manufacturing conditions presented in the present invention, have the intended microstructure, that is, an appropriate fraction of ferrite (air-cooled and water-cooled ferrite) and segmented pearlite. formation can be confirmed. Thereby, the yield strength of 315 MPa or more and the conversion elongation standard were secured to 30% or more.

On the other hand, while satisfying the alloy composition of the present invention, Comparative Examples 1 to 5, in which the manufacturing conditions are outside the present invention, and Comparative Examples 6 and 7, in which the alloy composition deviates from the present invention, the elongation is secured to less than 30% based on the converted elongation. Therefore, high strength and high ductility were not compatible.

Of these, in Comparative Example 7, it can be confirmed that a certain strength or more is secured, which is due to the relatively fine ferrite grain size formed by excessive addition of Nb.

Claims (11)

1. By weight%, carbon (C): 0.14 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 0.9 to 1.2%, aluminum (Al): 0.015 to 0.04%, niobium (Nb): 0.005 ~0.015%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.002 to 0.008%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, the balance contains Fe and other unavoidable impurities do,

   Thick steel with high strength and high ductility whose microstructure is composed of air-cooled ferrite, water-cooled ferrite and segmented pearlite.
2. The method of claim 1,

   The steel is molybdenum (Mo) in weight%: 0.05% or less and chromium (Cr): thick steel having high strength and high ductility further comprising at least one of 0.05% or less.
3. The method of claim 1,

   The air-cooled ferrite and water-cooled ferrite is a thick steel material having high strength and high ductility of 80 area% or more as a sum of fractions.
4. The method of claim 1,

   A thick steel material having high strength and high ductility, wherein the average grain size of the air-cooled ferrite and the water-cooled ferrite is 12 to 30 μm.
5. The method of claim 1,

   The segmented pearlite is a thick steel material having high strength and high ductility that has a length of 50 μm or less.
6. The method of claim 1,

   The steel is a thick steel material having high strength and high ductility of 30% or more based on conversion elongation, yield strength of 315 MPa or more.
7. By weight%, carbon (C): 0.14 to 0.17%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 0.9 to 1.2%, aluminum (Al): 0.015 to 0.04%, niobium (Nb): 0.005 ~0.015%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.002 to 0.008%, phosphorus (P): 0.02% or less, sulfur (S): 0.005% or less, the balance contains Fe and other unavoidable impurities preparing a steel slab to do;

   heating the steel slab in a temperature range of 1050 to 1200 °C;

   manufacturing a hot-rolled steel sheet by finishing hot rolling the heated slab; and

   Comprising the step of cooling the hot-rolled steel sheet to a temperature range of 650 ~ 750 ℃ at a cooling rate of 5 ~ 7 ℃ / s,

   The cooling is a method of manufacturing a thick steel material having high strength and high ductility, characterized in that the cooling water is poured and then the pattern of not watering is repeated twice or more.
8. 8. The method of claim 7,

   The finish hot rolling is Ar3 + 10 ~ Ar3 + 150 ℃ temperature range, the method of manufacturing a thick steel material having high strength and high ductility to be performed at a cumulative reduction ratio of 50% or more.
9. 8. The method of claim 7,

   The cooling is a method of manufacturing a thick steel material having high strength and high ductility that is to be started below the Ar3 temperature.
10. 8. The method of claim 7,

    The cooling is a method of manufacturing a thick steel material having high strength and high ductility that is performed up to a temperature range of 665 ~ 750 ℃.
11. 8. The method of claim 7,

    The steel slab is molybdenum (Mo): 0.05% or less and chromium (Cr): 0.05% or less by weight of the steel slab.

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