KR102523765B1 - Thick steel plate having high-strenghth and high-ductility, and method for manufacturing thereof - Google Patents

Abstract

An object of the present invention is to provide a thick material steel material capable of simultaneously securing high strength and high ductility even though it is a thick material steel material having a certain thickness and a manufacturing method thereof.

Description

Thick steel material having high strength and high ductility and its manufacturing method {THICK STEEL PLATE HAVING HIGH-STRENGHTH AND HIGH-DUCTILITY, AND METHOD FOR MANUFACTURING THEREOF}

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

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

In addition, as interest in safety and the environment increases, there is a demand for improvement in impact resistance properties capable of withstanding without destruction even in the event of an accident with respect to steel for shipbuilding used for military, commercial ships, oil tankers, and the like.

On the other hand, it is known that a material having a higher elongation rate increases the total amount of plastic deformation and the amount of energy absorbed until fracture occurs, so that the impact resistance against external force and impact 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 breakage of the ship.

In the case of conventional shipbuilding steel having a yield strength of 315 MPa or more, it is produced through air cooling during cooling, whereby the steel structure has a layered structure of ferrite and pearlite (see FIG. 1). When 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 elongation.

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

Korean Patent Registration No. 10-0340547

One aspect of the present invention is to provide a thick material steel material capable of simultaneously securing high strength and high ductility despite a thick material steel material having a certain thickness and a manufacturing method thereof.

The object of the present invention is not limited to the above. The subject of the present invention will be understood from the entire contents of this specification, and those skilled in the art 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 ~ 0.17%, silicon (Si): 0.2 ~ 0.5%, manganese (Mn): 0.9 ~ 1.2%, aluminum (Al): 0.015 ~ 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 Contains Fe and other unavoidable impurities,

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

Another aspect of the present invention, 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 to a temperature range of 650 to 750 ° C at a cooling rate of 5 to 7 ° C / s,

The cooling provides a method for manufacturing a thick steel material having high strength and high ductility, characterized in that the pattern of pouring cooling water and then pouring water is repeated twice or more.

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

The thick steel with high strength and high ductility of the present invention has an effect that can be advantageously applied as steel for shipbuilding.

1 shows a microstructure photograph of a conventional shipbuilding steel.
Figure 2 shows a picture of the microstructure of the inventive steel according to one aspect of the present invention.
3 is a schematic diagram of a cooler to which water cooling is applicable.
FIG. 4 shows a coolant spray pattern of a chiller to which water cooling is applicable. (a) shows a conventional continuous cooling pattern, and (b) shows an example of a coolant spray pattern according to the present invention.

The inventors of the present invention, in providing a thick steel material suitable for shipbuilding steel, studied in depth to obtain a steel material having high elongation characteristics as well as high strength so as to achieve excellent impact resistance characteristics of the material.

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

Specifically, the present invention provides a thick steel material of a certain thickness or more suitable for shipbuilding steel, and can provide optimal process conditions that are advantageous for forming the intended structure when manufacturing the steel material, along with the optimal alloy composition of the thick material steel material 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 contains, 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): may contain 0.005% or less.

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

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

Carbon (C): 0.14 to 0.17%

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

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 excessively increases and the elongation decreases, which is not preferable. 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 to 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-mentioned effect, Si may be included in an amount of 0.2% or more, but if the content is excessive, the strength is excessively increased and the elongation is lowered, which is not preferable. 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 to 1.2%

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

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

Aluminum (Al): 0.015 to 0.04%

Aluminum (Al) is an element effective in 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 the Al ₂ O ₃ inclusions increases, and the size becomes coarse, which causes the toughness of the steel to be impaired. 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) precipitates as a solid solution or as a carbon nitride and suppresses recrystallization during rolling or cooling to form a fine structure and is an element that is advantageous for strength improvement.

In order to sufficiently obtain the above-mentioned effects, Nb may be included in an amount of 0.005% or more. However, when the content is excessive, there is a problem in that toughness and fracture resistance are lowered as C-concentration occurs due to carbon (C) affinity to promote the production of the MA phase. In consideration of this, the Nb may be included in an amount of 0.015% or less.

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

Titanium (Ti): 0.005 to 0.02%

Titanium (Ti) is an element that contributes to improving toughness by suppressing crystal grain growth by forming Ti nitride (TiN) by combining with nitrogen (N) in steel.

In order to sufficiently obtain the above effects, 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 toughness of the steel is deteriorated by forming Ti carbide (TiC) in solid solution Ti remaining without being bonded with N in the steel. In consideration of this, the Ti may be included in an amount of 0.02% or less.

Therefore, 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 a structure by combining with Nb or Ti in steel to form a nitride.

In order to sufficiently obtain these effects, it may be included at 0.002% or more. However, if the content is excessive, the surface quality of the steel sheet may be deteriorated, so it may be included at 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 inevitably incorporated into steel, and when its content exceeds 0.02%, it causes grain boundary segregation and embrittles the steel.

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

Sulfur (S): 0.005% or less

Sulfur (S) is an impurity that is unavoidably incorporated into steel, and when its content exceeds 0.005%, it combines with Mn in steel to form non-metallic inclusions such as MnS, which deteriorates the physical properties of the center of the steel thickness.

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

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

Molybdenum (Mo): 0.05% or less

Molybdenum (Mo) is an element that increases the hardenability of steel and is advantageous for improving the strength of steel. If the content of such Mo is excessive, there is a problem in that the toughness of the steel is deteriorated by promoting the formation of a bainite phase.

Chromium (Cr): 0.05% or less

Chromium (Cr) is employed in austenite when the slab is reheated to increase the hardenability of the steel and contribute to securing the strength of the steel. If the content of Cr is excessive, there is a concern that the toughness and weldability of the steel may be deteriorated.

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 a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

The microstructure of the thick steel material of the present invention having the above-described alloy composition is composed of a ferrite and pearlite composite structure, and in this case, the pearlite is preferably a 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, in the present invention, in order to secure high ductility as well as high strength, ductility and toughness can be secured by forming air-cooled ferrite, and strength and toughness can be secured by forming water-cooled ferrite.

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

It is preferable that the air-cooled ferrite and the water-cooled ferrite have a total area fraction of 80% or more (excluding 100%). 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 the 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 due to excessive presence of a coarse ferrite phase. On the other hand, if the size is less than 12 μm, the elongation may decrease as the strength is excessively increased.

In the present invention, the air-cooled ferrite and the water-cooled ferrite include polygonal ferrite, while not including acicular ferrite.

The segmented pearlite may be formed parallel to the rolling direction, and at this time, if the maximum length exceeds 50 μm, brittle fracture becomes easy when an external force is generated, which may cause a decrease in elongation.

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

In addition to the alloy composition described above, the steel of the present invention having a microstructure controlled to a specific structure has a conversion elongation of 30% or more and a yield strength of 315 MPa or more, so that the effect of securing high strength and high ductility at the same time there is.

Hereinafter, according to another aspect of the present invention, a method for manufacturing a thick material having high strength and high ductility provided by 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 contains Nb as a semi-finished product. In heating such a steel slab, the heating may be performed at 1050 ° C. or more based on the center of the slab thickness in consideration of sufficient solutionization of the Nb carbonitride contained in the steel slab. However, if the temperature is too high, there is a problem of not only increasing scale defects but also increasing hardenability of steel due to coarsening of austenite crystal grains. In consideration of this, the heating may be performed at 1200° C. or lower.

hot rolled

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

During the hot rolling, the finish hot rolling may be performed in the temperature range of Ar3 + 10 to Ar3 + 150 ° C. At this time, if the temperature is less than Ar3 + 10 ° C., there is a concern that the toughness will be 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 cannot be sufficiently achieved, making it impossible to secure the target level of strength.

In performing finish hot rolling in the above-mentioned temperature range, it is preferable to carry out with a cumulative reduction ratio of 50% or more. If the cumulative reduction ratio during the finish hot rolling is less than 50%, recrystallization by rolling does not occur to the center of the thickness, and thus there is a problem in that toughness deteriorates as crystal 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 thickness of the hot-rolled steel sheet to be obtained.

Cooling

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

The cooling may be initiated at an Ar3 temperature or lower and performed up to a temperature range of 650 to 750° C., and is preferably performed at a cooling rate of 5 to 7° C./s.

If the cooling is started at a temperature exceeding Ar3, the grain size of ferrite becomes too fine and the strength increases excessively, so that the target level of ductility cannot be secured. In addition, there is a concern that material deviation between the front and rear ends may occur due to a time difference between the start of cooling between the front end and the rear end of the steel sheet.

However, if the cooling start temperature is too low during the cooling, the transformation is completed before (final) water cooling, and the steel structure may have a layered structure of ferrite and pearlite by air cooling. In this case, cooling may be initiated below Ar3.

If the cooling end temperature during the cooling is less than 650 ° C., a low-temperature phase such as bainite is generated in the steel, and there is a concern that the elongation may decrease 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 the target level of strength cannot be secured. More advantageously, the cooling may be terminated at 665° C. or higher.

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

On the other hand, in performing cooling under the above conditions, the cooling may be performed by water cooling using a cooler that sprays cooling water. The cooler is not limited to this, but may be provided as shown in FIG. 3, and the steel material is positioned between the upper and lower cooling water injection devices, and the steel material is transported through the transfer roll, and the upper and lower surfaces of the steel material Coolant can be sprayed.

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

Specifically, in the present invention, as an example, as shown in (b) of FIG. 4, it is possible to repeat the main water / non-irrigated water pattern for each water injection unit, and at this time, it is preferable to repeat the main water / non-irrigated water pattern two or more times. do.

In this way, by spraying cooling water by repeating the pouring/mis-injecting pattern for each unit to the steel material conveyed in one direction by the transfer roll, it is possible to keep the cooling rate low by reducing the overall average flow rate per unit time. In addition, as cooling and reheating are repeated on the surface of the steel, the effect of suppressing the generation of low-temperature phase due to rapid cooling of the surface can be obtained compared to the case where cooling water is continuously sprayed.

In the present invention, in repeating the water injection/non-water injection pattern for the cooling water injection pattern, the pattern may be appropriately set according to the facility structure. In addition, the amount of cooling water injected from each unit may be appropriately selected according to the thickness of the steel to be transported, but it should be proposed to a degree that can satisfy the cooling rate proposed in the present invention.

The steel material 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 such thick steel material.

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. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

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

steel grade Alloy composition (% by weight) 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 rate (%) onset 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 Inventive example 3 B 40 1113 808 60 760 669 6.1 3 Inventive example 4 C 40 1112 811 60 764 677 5.9 3 Inventive Example 5 B 50 1107 789 50 754 682 5.1 4 Inventive Example 6 C 50 1111 790 50 752 680 5.1 4 Inventive 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 of units to which the cooling water injection / non-injection pattern is applied.
In addition, continuous* means a pattern method in which cooling water is continuously sprayed from two or more water injection units.

Microstructure and mechanical properties were measured for each of the hot-rolled steels manufactured as described above, and the results are shown in Table 3 below.

First, after specimens were taken 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 an equal value is obtained regardless of the thickness of the steel, which is compensated through a formula. did Transformation elongation (E') can be calculated through the following formula (in the following formula, E means elongation (before conversion), and t means steel thickness (mm)).

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

division microstructure mechanical properties F fraction*
(area%) F mean
Size (μm) 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 Inventive example 3 85 15 15 35 389 522 33 Inventive example 4 86 19 14 31 370 505 32 Inventive Example 5 85 17 15 34 382 515 32 Inventive example 6 84 15 16 38 376 509 31 Inventive 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 the sum of air-cooled ferrite and water-cooled ferrite.
Also, in Comparative Examples 1, 4, 5, and 6, bainite phase was partially formed in addition to F and P, and the fraction corresponds to the remainder excluding the F and P fractions.

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

On the other hand, while satisfying the alloy composition of the present invention, Comparative Examples 1 to 5 whose manufacturing conditions deviate from the present invention and Comparative Examples 6 and 7 whose alloy composition deviate from the present invention all have an elongation of less than 30% based on the conversion elongation Therefore, high strength and high ductility were not compatible.

Among them, in the case of Comparative Example 7, it can be confirmed that a certain strength or more is secured, which is due to the fact that the ferrite crystal grain size is formed relatively finer due to the excessive addition of Nb.

Claims (11)

In % 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): 0.005~0.02%, Nitrogen (N): 0.002~0.008%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, the balance including Fe and other unavoidable impurities do,
The microstructure is composed of air-cooled ferrite, water-cooled ferrite and segmented pearlite,
The air-cooled ferrite and water-cooled ferrite are thick steel materials having high strength and high ductility that are 80 area% or more as a sum of fractions.
According to claim 1,
The steel material is a thick steel material having high strength and high ductility further comprising at least one of molybdenum (Mo): 0.05% or less and chromium (Cr): 0.05% or less by weight%.
delete According to claim 1,
A thick steel material having high strength and high ductility in which the average grain size of the air-cooled ferrite and the water-cooled ferrite is 12 to 30 μm.
According to 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.
According to claim 1,
The steel material is a steel material having a high strength and high ductility of 30% or more and a yield strength of 315 MPa or more based on the conversion elongation.
In % 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): 0.005~0.02%, Nitrogen (N): 0.002~0.008%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.005% or less, the balance including Fe and other unavoidable impurities Preparing a river 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
Cooling the hot-rolled steel sheet to a temperature range of 650 ~ 750 ℃ at a cooling rate of 5 ~ 7 ℃ / s,
The cooling is characterized by repeating a pattern of pouring cooling water and then pouring water twice or more,
The finish hot rolling is carried out at a cumulative reduction of 50% or more in the temperature range of Ar3 + 10 ~ Ar3 + 150 ° C,
The cooling is a method for producing a thick steel material having strength and high ductility that starts below the Ar3 temperature.
delete delete According to claim 7,
The cooling is a method for producing a thick steel material having high strength and high ductility that is performed to a temperature range of 665 ~ 750 ℃.
According to claim 7,
The steel slab is molybdenum (Mo): 0.05% or less and chromium (Cr): 0.05% or less by weight%: a method for producing a thick steel material having high strength and high ductility, further comprising one or more of 0.05% or less.

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