KR20240106709A - Thick steel plate and method of manufacturing the same - Google Patents

Abstract

The present invention provides a 120-ton thick steel plate that can guarantee quality even when hot rolling is performed at a reduction ratio of less than 3:1, and a method for manufacturing the same. According to an embodiment of the present invention, the thick steel plate has, in weight percent, carbon (C): 0.045% to 0.075%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.3% to 1.6%. , Aluminum (Al): 0.015% ~ 0.05%, Niobium (Nb): 0.015% ~ 0.025%, Nickel (Ni): 0.4% ~ 0.5%, Copper (Cu): 0.2% ~ 0.3%, Titanium (Ti): 0.01% to 0.02%, phosphorus (P): more than 0% to 0.01%, sulfur (S): more than 0% to 0.002%, and the balance includes iron (Fe) and other inevitable impurities, -40℃ to - Low-temperature impact toughness at 60℃: Satisfies 100 J ~ 400 J.

Description

Thick steel plate and method of manufacturing the same}

The technical idea of the present invention relates to steel materials, and more specifically, to thick steel plates and their manufacturing methods.

Recently, as interest in new and renewable energy has rapidly increased, the installation of offshore wind power generators and the construction of offshore wind power complexes are actively underway at home and abroad. In addition, the size of power generation turbines, blades, and support structures is increasing in size to increase power generation capacity. The steel type used in the structure (Wind Tower/Offshore Jacket) is mainly 50K grade steel with a yield strength of 355 MPa, and products with a thickness exceeding 100 mm are applied to some structures. In the case of these highly functional, extremely thick products, the rolling reduction ratio from slab to plate is usually limited to 3:1 or more to reduce the internal quality of the product and to guarantee the physical properties of the core. In order to manufacture a 120 mm thick product, a slab with a thickness of 360 mm or more is required due to restrictions on the reduction ratio, and in this case, there are difficulties in the process of manufacturing it differently from the 300 mm or 250 m thick slabs used for conventional heavy plate rolled materials. There is. In addition, in order to produce products exceeding 100 mm thick using 300 mm thick slabs, sufficient pressing force must be applied to the center of the material to ensure internal quality and central properties, but overload of the rolling mill and material damage during the production process are required. There are difficulties in production due to size issues.

Korean Patent Application No. 10-2018-0073809

The technical problem to be achieved by the technical idea of the present invention is to provide a 120t thick steel plate and a manufacturing method thereof that can guarantee quality even when hot rolling is performed at a reduction ratio of less than 3:1, unlike conventional thick plates. .

However, these tasks are illustrative, and the technical idea of the present invention is not limited thereto.

According to one aspect of the present invention, a 120t thick steel plate and a manufacturing method thereof that can guarantee quality even when hot rolling is performed at a reduction ratio of less than 3:1 are provided.

According to an embodiment of the present invention, the thick steel plate has, in weight percent, carbon (C): 0.045% to 0.075%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.3% to 1.6%. , Aluminum (Al): 0.015% ~ 0.05%, Niobium (Nb): 0.015% ~ 0.025%, Nickel (Ni): 0.4% ~ 0.5%, Copper (Cu): 0.2% ~ 0.3%, Titanium (Ti): 0.01% to 0.02%, phosphorus (P): more than 0% to 0.01%, sulfur (S): more than 0% to 0.002%, and the balance includes iron (Fe) and other inevitable impurities, -40℃ to - Low-temperature impact toughness at 60℃: 100 J ~ 400 J can be satisfied.

According to one embodiment of the present invention, the thick steel plate satisfies low-temperature impact toughness at -40°C to -60°C at the surface, 1/4 of the thickness, and 1/2 of the thickness: 100 J to 400 J. You can.

According to an embodiment of the present invention, the thick steel plate may satisfy the following requirements: yield strength (YP): 320 MPa or more, tensile strength (TS): 430 MPa or more, and elongation (El): 22% or more.

According to one embodiment of the present invention, the thick steel plate has a microstructure including acicular ferrite, polygonal ferrite, and pearlite, the area fraction of the acicular ferrite is 50% to 80%, and the area fraction of the pearlite is is 0% to 10%, and the area fraction of the polygonal ferrite may be the remaining fraction.

According to one embodiment of the present invention, the thick steel plate may have a ferrite grain size (FGS) of greater than 11.78 to 11.99.

According to an embodiment of the present invention, the method for manufacturing the thick steel plate is (a) in weight percent, carbon (C): 0.045% to 0.075%, silicon (Si): 0.15% to 0.25%, manganese (Mn) : 1.3% ~ 1.6%, Aluminum (Al): 0.015% ~ 0.05%, Niobium (Nb): 0.015% ~ 0.025%, Nickel (Ni): 0.4% ~ 0.5%, Copper (Cu): 0.2% ~ 0.3% , titanium (Ti): 0.01% to 0.02%, phosphorus (P): more than 0% to 0.01%, sulfur (S): more than 0% to 0.002%, and the balance includes iron (Fe) and other inevitable impurities. Reheating the steel at a reheating temperature of 1,000°C to 1,060°C; (b) performing primary hot rolling on the reheated steel; (c) an air-cooling standby step of air-cooling the primary hot-rolled steel in air for 300 to 600 seconds; (d) performing secondary hot rolling on the air-cooled steel material to end the hot rolling at a temperature of 670°C to 730°C; and (e) forcibly cooling the secondary hot rolled steel to a cooling end temperature in the range of 400°C to 500°C at a cooling rate of 3°C/sec to 5°C/sec.

According to one embodiment of the present invention, the reduction ratio of the thick steel plate that has undergone the first hot rolling and the second hot rolling may be 2.3:1 or more and less than 3:1,

According to one embodiment of the present invention, the cumulative reduction rate of the thick steel plate that has performed the first hot rolling and the second hot rolling may be in the range of 40% to 50%.

According to one embodiment of the present invention, the steel material has a thickness in the range of 250t to 300t, and the thick steel plate manufactured by the method of manufacturing the steel material may have a thickness in the range of 100t to 120t.

According to an embodiment of the present invention, the thick steel plate manufactured by the method for manufacturing the thick steel plate has a yield strength (YP) of 320 MPa or more, a tensile strength (TS) of 430 MPa or more, and an elongation (El): 22. % or more, low-temperature impact toughness at -40°C to -60°C: satisfies 100 J to 400 J, has a microstructure containing acicular ferrite, polygonal ferrite, and pearlite, and the area fraction of the acicular ferrite is 50 % to 80%, the area fraction of the pearlite is 0% to 10%, the area fraction of the polygonal ferrite is the remaining fraction, and may have a ferrite grain size (FGS) of greater than 11.78 to 11.99.

According to the technical idea of the present invention, unlike conventional thick plates, the reduction ratio of about 2.5:1 or less, for example, 300 mm slabs and 120 mm thick steel plates, can be manufactured to 355 MPa through changing the rolling mode and optimized component design. Products with the above high yield strength and tensile strength have been secured, and steel materials with excellent thickness direction characteristics can be obtained.

The effects of the present invention described above have been described as examples, and the scope of the present invention is not limited by these effects.

Figure 1 is a process flow chart schematically showing a method of manufacturing a thick steel plate according to an embodiment of the present invention.
Figure 2 is a graph showing the yield strength and tensile strength of a thick steel plate according to an embodiment of the present invention compared with a comparative example.
Figure 3 is a graph showing the low-temperature impact toughness of a thick steel plate according to an example of the present invention compared with a comparative example.
Figure 4 is a scanning electron microscope photograph showing the microstructure of a thick steel plate according to an embodiment of the present invention compared with a comparative example.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those skilled in the art, and the following examples may be modified into various other forms, and the embodiments of the present invention may be modified. The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical idea of the present invention to those skilled in the art. In this specification, like symbols refer to like elements throughout. Furthermore, various elements and areas in the drawings are schematically drawn. Accordingly, the technical idea of the present invention is not limited by the relative sizes or spacing drawn in the attached drawings.

In the case of extremely thick steel materials, sufficient pressing force cannot be transmitted to the material up to the center, and the size of microstructure particles in the center and surface areas differ, resulting in material deviation. In order to reduce the material deviation in the thickness direction, pressure reduction must be performed during rough rolling/finish rolling, but it is difficult to implement in the actual process due to equipment limitations. In particular, during rough rolling, the rolling mill has A portion of the total compression force is used to increase the width of the material, and when rolling in a temperature range that affects the microstructure, compression cannot proceed, resulting in material deviation in the thickness direction.

In the past, when manufacturing extremely thick steel plates, the reduction ratio from the slab to the thick steel plates was traditionally limited to 3:1 or more to prevent deterioration of the central characteristics of the steel plates. For example, when manufacturing 110 mm thick steel plate, a slab with a thickness of 330 mm or more was required. However, it is true that there are still many restrictions in the normal manufacturing and handling of 330mm slabs in the heavy plate manufacturing market, and in fact, steel companies are expressing difficulty in demanding minimum order quantities and long delivery times for heavy steel plate manufacturing.

The technical problem to be achieved in the present invention is to reduce the deviation of physical properties in the thickness direction by optimizing operating conditions under conditions where the reduction force cannot be infinitely large during rolling, and to reduce the mechanical properties of the center. The purpose is to provide this excellent 120-ton heavy plate manufacturing and its manufacturing method.

In the case of the present invention, in order to secure thickness direction characteristics when rolling extremely thick steel exceeding 100 mm (100 t), production can be carried out by reflecting the following conditions. The first is to additionally add copper (Cu) and nickel (Ni) for the purpose of improving strength and toughness. The second is to apply a low-temperature rolling end temperature below the Ar3 temperature for reduction of 1/4t and the central non-recrystallized region. This is expected to be effective in seeing the center rolling effect after the surface hardening layer is created.

The present invention relates to a steel plate with a yield strength of 355 MPa, which is used for structural purposes, especially steel materials for offshore wind towers, etc. It relates to a steel plate with improved economic efficiency and central characteristics and a method of manufacturing the same. In the present invention, it is possible to manufacture a 120 mm thick steel plate with excellent core toughness characteristics even with a reduction ratio of 2.5:1 by utilizing low-temperature heating, core pressure, and addition of toughness-enhancing elements.

Hereinafter, a thick steel plate and a manufacturing method thereof according to a preferred embodiment of the present invention will be described in detail with reference to the attached drawings.

The thick steel plate according to an embodiment of the present invention has, in weight percent, carbon (C): 0.045% to 0.075%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.3% to 1.6%, and aluminum. (Al): 0.015% ~ 0.05%, Niobium (Nb): 0.015% ~ 0.025%, Nickel (Ni): 0.4% ~ 0.5%, Copper (Cu): 0.2% ~ 0.3%, Titanium (Ti): 0.01% ~ 0.02%, phosphorus (P): more than 0% ~ 0.01%, sulfur (S): more than 0% ~ 0.002%, and the balance includes iron (Fe) and other inevitable impurities.

Hereinafter, the role and content of each component included in the thick steel plate according to the present invention will be described as follows. At this time, the content of the component elements refers to the weight percent of the entire thick steel plate.

Carbon (C): 0.045% to 0.075%

Carbon is added to ensure the strength of the steel, and to ensure low-temperature impact toughness and weldability. If the carbon content is less than 0.045%, strength may decrease. If the carbon content exceeds 0.075%, the strength of the steel increases, but low-temperature impact toughness and weldability may decrease. Therefore, it is desirable to add carbon in an amount of 0.045% to 0.075% of the total weight of the thick steel plate.

Silicon (Si): 0.15% to 0.25%

Silicon is added as a deoxidizer to remove oxygen in steel during the steelmaking process. Additionally, silicon has a solid solution strengthening effect. If the silicone content is less than 0.15%, the above-mentioned silicone addition effect cannot be properly achieved. If the silicon content exceeds 0.25%, non-metallic inclusions may excessively form on the surface of the steel material, thereby reducing toughness. Therefore, it is desirable to add silicon in an amount of 0.15% to 0.25% of the total weight of the thick steel plate.

Manganese (Mn): 1.3% ~ 1.6%

Manganese is an austenite stabilizing element and plays a role in improving strength and toughness by lowering the Ar3 point and expanding the rolling temperature range, thereby refining the grains through rolling. If the manganese content is less than 1.3%, the fraction of the second phase tissue may decrease, making it difficult to secure strength. If the manganese content exceeds 1.6%, sulfur dissolved in the steel may precipitate as MnS, thereby reducing low-temperature impact toughness. Therefore, it is desirable to add manganese at 1.3% to 1.6% of the total weight of the thick steel plate.

Aluminum (Al): 0.015% to 0.05%

Aluminum acts as a deoxidizer to remove oxygen in steel. If the aluminum content is less than 0.015%, the above-described effect cannot be expected, and if it exceeds 0.05%, aluminum oxide, a non-metallic inclusion, may be formed, thereby reducing low-temperature impact toughness. Therefore, it is desirable to add aluminum in an amount of 0.015% to 0.05% of the total weight of the thick steel plate.

Niobium (Nb): 0.015% to 0.025%

Niobium combines with carbon and nitrogen at high temperatures to form carbides or nitrides. Niobium-based carbide or nitride suppresses grain growth during rolling and improves the strength and low-temperature toughness of steel by refining the grains. If the niobium content is less than 0.015%, the effect of adding niobium cannot be properly achieved. If the niobium content exceeds 0.025%, the weldability of the steel is reduced, and as the niobium content increases, the strength and low-temperature toughness are no longer improved and exist in a dissolved state in the ferrite, which may actually reduce impact toughness. . Therefore, it is desirable to add niobium in an amount of 0.015% to 0.025% of the total weight of the thick steel plate.

Nickel (Ni): 0.4% to 0.5%

Nickel refines the crystal grains and is dissolved in austenite and ferrite, providing a solid solution strengthening effect to strengthen the matrix. Additionally, nickel is an effective element in improving hardenability and low-temperature impact toughness. If the nickel content is less than 0.4%, the effect of nickel addition cannot be properly achieved. If the nickel content exceeds 0.5%, red heat embrittlement may occur. Therefore, it is desirable to add nickel in an amount of 0.4% to 0.5% of the total weight of the thick steel plate.

Copper (Cu): 0.2% to 0.3%

Similar to nickel, copper provides solid solution strengthening and improves hardenability and low-temperature impact toughness. If the copper content is less than 0.2%, the effect of adding copper cannot be properly achieved. If the copper content exceeds 0.3%, it does not contribute to further strength increase because it exceeds the solid solution limit and may cause red heat embrittlement. Therefore, it is desirable to add copper in an amount of 0.3% to 0.4% of the total weight of the thick steel plate.

Copper and nickel are added to improve toughness without reducing strength in the thick steel plate according to an embodiment of the present invention. However, when only copper is added without a sufficient nickel content, a copper-rich phase is formed at the interface with the scale, and often the copper-rich phase is converted to a liquid phase at a sufficiently high heating temperature and penetrates into the austenite grain boundary and precipitates during hot rolling, resulting in thick steel plate. This causes cracks in the surface. The addition of sufficient nickel can promote surface oxidation and increase the solid solubility of copper in the austenite phase, preventing the copper-rich phase from precipitating at the austenite grain boundary.

The content of nickel (Ni) may be 1.5 times or more, and may be 1.5 to 2.0 times the content of copper (Cu). When the nickel (Ni) content is less than 1.5 times the copper (Cu) content, the copper content is relatively high, so the copper-enriched phase may precipitate in the austenite grains, causing cracks.

Titanium (Ti): 0.01% to 0.02%

Titanium has the effect of improving the toughness and strength of hot-rolled steel sheets by producing Ti(C, N) precipitates with high high-temperature stability, hindering austenite grain growth during welding and refining the structure of the weld zone. If the titanium content is less than 0.01%, age hardening may occur due to dissolved carbon and dissolved nitrogen remaining without precipitation. If the titanium content exceeds 0.02%, coarse precipitates are formed, which reduces the low-temperature impact characteristics of the steel material and increases manufacturing costs without further additive effects. Therefore, it is desirable to add titanium in an amount of 0.01% to 0.02% of the total weight of the thick steel plate.

Phosphorus (P): >0% ~ 0.01%

Phosphorus partially contributes to improving strength, but it can have a negative effect on the material by lowering weld zone toughness and low-temperature impact toughness, and forming not only central segregation but also micro-segregation. Therefore, the lower the phosphorus content, the better. Therefore, in the present invention, phosphorus was limited to more than 0% to 0.01% of the total weight of the thick steel plate.

Sulfur (S): >0% ~ 0.002%

Sulfur, along with phosphorus, is an element that is inevitably contained in the production of steel, and forms non-metallic inclusions such as MnS, and as a low melting point element, there is a high possibility of grain boundary segregation, which reduces toughness. If the sulfur content exceeds 0.002% by weight, the toughness of the base material and weld zone can be greatly reduced. Therefore, in the present invention, sulfur was limited to more than 0% to 0.002% of the total weight of the thick steel plate.

The remaining component of the thick steel plate is iron (Fe). However, in the normal steelmaking process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all of them are not specifically mentioned in this specification.

The thick steel plate manufactured through the manufacturing method described below by controlling the specific components and their content ranges of the above-described alloy composition has yield strength (YP): 320 MPa or more, tensile strength (TS): 430 MPa or more, and elongation. (El): More than 22% can be satisfied. The thick steel plate may satisfy yield strength (YP): 320 MPa to 420 MPa, tensile strength (TS): 430 MPa to 590 MPa, and elongation (El): 22% to 30%.

The thick steel plate can satisfy low-temperature impact toughness at -40°C to -60°C: 100 J to 400 J, and preferably satisfies low-temperature impact toughness: 200 J to 400 J. The thick steel plate can satisfy low-temperature impact toughness at -40°C to -60°C at the surface, 1/4 of the thickness, and 1/2 of the thickness: 100 J to 400 J, and preferably has low-temperature impact toughness. : 200 J ~ 400 J can be satisfied.

The thick steel plate may have a thickness ranging from 100 t to 120 t (100 mm to 120 mm).

The thick steel plate may have a microstructure including acicular ferrite, polygonal ferrite, and pearlite. The area fraction of the acicular ferrite may be 50% to 80%, the area fraction of the pearlite may be 0% to 10%, and the area fraction of the polygonal ferrite may be the remaining fraction, for example, 10%. It may be ~50%. The area fraction refers to the area ratio derived from a photo of the microstructure of the steel through an image analyzer.

The thick steel plate may have a ferrite grain size (FGS) of greater than 11.78 to 11.99, and preferably greater than 11.78 to 11.89.

Hereinafter, a method for manufacturing a thick steel plate according to the present invention will be described with reference to the attached drawings.

Manufacturing method of thick steel plate

1 is a process flow chart showing a method of manufacturing a thick steel plate according to an embodiment of the present invention.

In the method of manufacturing a thick steel plate according to the present invention, the semi-finished product, which is a steel material, may be, for example, a slab. Slabs in a semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

The steel has, in weight percent, carbon (C): 0.045% to 0.075%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.3% to 1.6%, aluminum (Al): 0.015% to 0.05. %, Niobium (Nb): 0.015% ~ 0.025%, Nickel (Ni): 0.4% ~ 0.5%, Copper (Cu): 0.2% ~ 0.3%, Titanium (Ti): 0.01% ~ 0.02%, Phosphorus (P) : Exceeding 0% ~ 0.01%, Sulfur (S): Exceeding 0% ~ 0.002%, and the balance includes iron (Fe) and other inevitable impurities.

Referring to Figure 1, the method of manufacturing a thick steel plate according to an embodiment of the present invention includes a reheating step (S10), a first hot rolling step (S20), an air cooling standby step (S30), a second hot rolling step (S40), and It includes a forced cooling step (S50).

Reheating step (S10)

In the reheating step (S10), the steel material having the above composition, for example, a slab plate, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of 1,000°C to 1,060°C. Through this reheating, re-dissolution of components segregated during casting and re-dissolution of precipitates may occur. If the reheating temperature is less than 1,000°C, the rolling load may increase due to the low reheating temperature, and the solid solution temperature of Nb-based precipitates, NbC, NbN, etc. cannot be reached, so they cannot be re-precipitated as fine precipitates during hot rolling, resulting in the growth of austenite grains. If the austenite grains are not suppressed, the austenite grains may rapidly coarsen. If the reheating temperature exceeds 1060°C, austenite grains rapidly coarsen, making it difficult to secure the strength and low-temperature toughness of the manufactured steel sheet. In this embodiment, the reheating process is performed at a low temperature of 1060°C or lower to control the initial austenite grain size. In this way, by designing the reheating temperature to be 1100°C or lower, low-temperature impact toughness can be secured by preventing austenite grain coarsening and refining. Meanwhile, the reheating step (S10) is not necessarily performed, but is preferably performed to obtain effects such as re-dissolution of precipitates.

First hot rolling step (S20)

In the first hot rolling step (S20), first hot rolling is performed on the reheated steel. The first hot rolling step may be referred to as a rough rolling step. The first hot rolling step may be comprised of an even rolling step, a widening rolling step, and a lengthening rolling step. The even rolling can be performed a minimum number of times, one to two times, and preferably performed once. The above-described width rolling can be performed 3 to 5 times. The lengthening rolling can be performed 3 to 5 times.

In the case of extremely thick products having a thickness of 100 tons or more, the reduction rate per pass in the first hot rolling step may be 3% or more, for example, 3% to 15%. In particular, in the lengthening rolling step, a sufficient reduction rate in the recrystallization region can be provided to the steel by applying a reduction force. The “pass” means that the steel material passes through the rolling roll.

Air cooling standby stage (S30)

In the air cooling standby step (S30), after the first hot rolling step (S20) is completed, cooling is performed in the air and the steel material waits in the air. The air cooling standby step (S30) may be performed for, for example, 300 to 600 seconds. As the time of the air cooling standby step increases, the hot rolling end temperature can be reduced. Accordingly, through the air-cooling standby step, a surface hardening layer of the steel is created and the center of the steel becomes hard, enabling subsequent reduction by secondary hot rolling, and greater pressure can be applied to the center of the steel.

Second hot rolling step (S40)

In the second hot rolling step (S40), the first hot rolled steel is subjected to second hot rolling. The secondary hot rolling step may be referred to as a finishing rolling step. The secondary hot rolling may be completed at a hot rolling finish temperature (FRT) of 670°C to 730°C. The hot rolling end temperature is the temperature applied for rolling reduction in the non-recrystallized region. The reduction rate per pass of the secondary hot rolling can be applied at 6% or more. If the hot rolling end temperature is less than 670°C, an abnormal structure is formed in the surface texture area, resulting in an uneven structure, which can significantly reduce low-temperature impact toughness. On the other hand, the 1/4t part and 1/2t part show less reduction in impact toughness because the actual finishing hot rolling end temperature increases. If the hot rolling end temperature exceeds 730°C, ductility and toughness are excellent, but strength may rapidly decrease.

The first hot rolling step (S20), the air cooling standby step (S30), and the second hot rolling step (S40) may be performed continuously. The steel material can be formed into a thick steel plate by the first hot rolling and the second hot rolling. The cumulative reduction ratio of the thick steel plate that has undergone the first hot rolling and the second hot rolling may be in the range of 40% to 50%, and the reduction ratio may be 2.3:1 or more to less than 3:1. If the cumulative reduction ratio is less than 40%, the deviation in strength and impact toughness may be large. A descaler may be applied after performing the first hot rolling or the second hot rolling.

In this way, the central reduction rate of the thick steel plate can be increased by increasing the air cooling waiting time, lowering the hot rolling end temperature, and securing more surface hardening layers by applying a descaler when necessary.

The thick steel plate formed accordingly may have a thickness of, for example, 120 mm (120 t).

Forced cooling step (S50)

The secondary hot rolled steel can be forcibly cooled to 400°C to 500°C at a cooling rate of 3°C/sec to 5°C/sec. Forced cooling means cooling relatively faster than the cooling speed of air cooling, which involves cooling while waiting in the air. For example, the forced cooling may be water cooling using cooling water. Meanwhile, after forced cooling to 400°C to 500°C, the thick steel plate may be naturally cooled to room temperature. At this time, the room temperature may be approximately 1°C to 40°C.

The steel may have a thickness ranging from 250 t to 300 t (250 mm to 300 mm). The thick steel plate manufactured by the steel plate manufacturing method may have a thickness ranging from 100t (100 mm) to 120t (120 mm).

Experiment example

Below, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are only intended to aid understanding of the present invention, and the present invention is not limited by the following experimental examples. Any information not described here can be technically inferred by anyone skilled in the art, so description thereof will be omitted.

Table 1 and Table 2 show the composition of the thick steel plate used in the thick steel plate manufacturing method. The remainder in Tables 1 and 2 consists of iron (Fe) and impurities that are inevitably contained in the steelmaking process. The unit of content of each ingredient is weight%.

division C Si Mn P S S_Al Comparative example 0.06 0.177 1.360 0.0067 0.0014 0.025 Example 0.06 0.198 1.354 0.0061 0.0010 0.033

division Nb Ni Cu Ti V Ca Comparative example 0.023 0.25 0.147 0.014 0.001 0.0023 Example 0.025 0.42 0.241 0.017 0.001 0.0017

Referring to Tables 1 and 2, the examples are weight percent: carbon (C): 0.045% to 0.075%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.3% to 1.6%, aluminum. (Al): 0.015% ~ 0.05%, Niobium (Nb): 0.015% ~ 0.025%, Nickel (Ni): 0.4% ~ 0.5%, Copper (Cu): 0.2% ~ 0.3%, Titanium (Ti): 0.01% It satisfies the composition range of ~ 0.02%, phosphorus (P): more than 0% ~ 0.01%, sulfur (S): more than 0% ~ 0.002%, and the balance is iron (Fe). Furthermore, the nickel (Ni) content satisfies 1.5 times or more than the copper (Cu) content. On the other hand, the comparative example does not satisfy the ranges of nickel (Ni): 0.4% to 0.5% and copper (Cu): 0.2% to 0.3%. That is, note that the embodiment further includes copper and nickel, which are elements that increase strength and low-temperature impact toughness, to compensate for the decrease in strength and low-temperature impact toughness due to the low reduction ratio.

In this example, NbCN2 was 1059°C, Tnr (non-recrystalization temperature) was 878°C, and Ac3 was 756°C. Here, NbCN2 is the temperature at which precipitates such as carbides or nitrides are dissolved in solid solution. At temperatures above this temperature, it becomes solid and has an effect of increasing strength. The Tnr is the temperature at which the steel recovers and recrystallizes above the recrystallization stop temperature, and the non-recrystallization temperature is below the Tnr, and below the Tnr temperature, the rolling force accumulates during rolling.

Table 3 shows process condition values for forming thick steel plates of comparative examples and examples.

division Abkhabi reheat temperature
(℃) Before sand rolling
Air cooling waiting time
(candle) Rolling end temperature
(℃) Cooling stop temperature
(℃) Comparative example 3:1 1077 261 804 504 Example 2.5:1 1038 365 705 438

Referring to Table 3, the examples have a reduction ratio of the thick steel plate: 2.3:1 or more to less than 3:1, reheating temperature: 1000°C to 1060°C, air cooling waiting time before finishing rolling: 300 seconds to 600 seconds, hot rolling completed. While the temperature: 670°C to 730°C and cooling stop temperature: 400°C to 500°C are all satisfied, the comparative example does not satisfy all of the above-mentioned process conditions. In the comparative example, the slab thickness was 250 mm and the steel plate thickness was 83 mm, and the reduction ratio was 3 (i.e., 3:1), but in the example, the slab thickness was 300 mm and the steel plate thickness was 120 mm, and the reduction ratio was 2.5 (i.e., 2.5:1). ). In particular, note that in the example, the rolling end temperature was lowered to 700°C to reduce the non-recrystallized region.

Table 4 shows the results of measuring yield strength (YS), tensile strength (TS), and elongation (EL) as mechanical properties of the manufactured thick steel plate.

division Yield strength (MPa) Tensile strength (MPa) Elongation (%) Comparative example 329.3 455 40 Example 379 485 30

Referring to Table 4, the examples and comparative examples all satisfy the following requirements: yield strength (YP): 320 MPa or more, tensile strength (TS): 430 MPa or more, and elongation (El): 22% or more. The Example showed greater yield strength and tensile strength and lower elongation than the Comparative Example.

Figure 2 is a graph showing the yield strength and tensile strength of a thick steel plate according to an embodiment of the present invention compared with a comparative example.

In Figure 2, "L" is the longitudinal direction, i.e., the direction of progress of the steel plate for rolling, "T" is the width direction, i.e., perpendicular to the direction of progress, "F" is the surface of the steel plate, and "Q" is the direction of the steel plate. 1/4 thickness, “C” refers to the center of the thick steel plate. In addition, “TS” refers to tensile strength, “YP” refers to yield strength, “TOP” refers to the tip of the steel plate, and “BODY” refers to the middle portion of the steel plate.

Referring to FIG. 2, it can be seen that both the tensile strength and yield strength of the example are increased throughout the thickness compared to the comparative example. This is analyzed to be due to increased employment of copper and nickel. In addition, both comparative examples and examples satisfy a tensile strength of 430 MPa or more. In addition, the examples all satisfied the yield strength of 320 MPa or more, but the comparative examples did not satisfy the yield strength of 320 MPa or more. In particular, it can be seen that the strength of the Example increases in the width direction (T direction) at 1/4t compared to the Comparative Example.

Table 5 shows the results of measuring low-temperature impact toughness at -50°C as mechanical properties for the manufactured thick steel plate. The three low-temperature impact toughnesses shown in Table 4 represent the measured values and average values of three specimens obtained from 1/4t and 1/2t of one steel material, respectively.

division Low temperature impact toughness (J@-50℃) 1/4t L direction 1/2t L direction #One #2 #3 average #One #2 #3 average Comparative example 353 357 357 354 338 350 24 237 Example 364 368 367 366 349 222 211 260

Referring to Table 5, it can be seen that the low-temperature impact toughness of the Example increased at both 1/4t and 1/2t compared to the Comparative Example.

Figure 3 is a graph showing the low-temperature impact toughness of a thick steel plate according to an example of the present invention compared with a comparative example.

Referring to FIG. 3, it can be seen that both the tensile strength and yield strength of the Example are increased throughout the thickness compared to the Comparative Example. Specifically, compared to the Comparative Example, the toughness of the surface portion of the Example was slightly lowered, but at 1/4t, low-temperature impact toughness was increased over the entire temperature range, and at 1/2t, low-temperature impact toughness at -40°C was increased. It can be seen that the possibility has increased.

Figure 4 is a scanning electron microscope photograph showing the microstructure of a thick steel plate according to an embodiment of the present invention compared with a comparative example.

Referring to Figure 4, it can be seen that the microstructure of the Example is refined compared to the Comparative Example. The ferrite grain size (FGS) according to the ASTM E112 standard was 11.79 in the comparative example, and 11.89 in the example, and the figure was larger in the example, which means that the grain size of the example was finer. It can be seen that the microstructure includes acicular ferrite, polygonal ferrite, and pearlite.

Therefore, the method of manufacturing a thick steel plate according to the present invention, despite the fact that it is an extremely thick/short material of 100 tons or more, secures a surface hardening layer superior to the conventional technology by applying a descaler and increasing the air cooling waiting time, thereby reducing the center reduction relatively. rate could be increased. This increase in center reduction rate can be compared to the particle size of the microstructure, and the particle size of the example was smaller. The air cooling waiting time can be increased by setting the rolling end temperature of the steel material to a low level, thereby increasing the air cooling waiting time after rough rolling and before finishing rolling. In addition, by adding nickel and copper, the yield strength and tensile strength are increased, and the non-recrystallized cumulative reduction rate of the 1/4t portion can be increased due to the hot rolling completion temperature of Ar3 or lower. As for the low-temperature impact toughness of the example, DBTT (Ductile-Brittle Transition Temperature) characteristics were improved in the 1/4t portion, and USE (Upper Shelf Energy) was improved in the 1/2t portion. The USE refers to the maximum Energy Joules value in the ductility section of this steel material.

The technical idea of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible without departing from the technical idea of the present invention. It will be clear to those skilled in the art.

Claims (10)

By weight percent: Carbon (C): 0.045% to 0.075%, Silicon (Si): 0.15% to 0.25%, Manganese (Mn): 1.3% to 1.6%, Aluminum (Al): 0.015% to 0.05%, Niobium ( Nb): 0.015% to 0.025%, Nickel (Ni): 0.4% to 0.5%, Copper (Cu): 0.2% to 0.3%, Titanium (Ti): 0.01% to 0.02%, Phosphorus (P): greater than 0% ~ 0.01%, Sulfur (S): more than 0% ~ 0.002%, and the balance includes iron (Fe) and other inevitable impurities,
Low-temperature impact toughness at -40℃ to -60℃: satisfying 100 J to 400 J,
Thick steel plate. According to claim 1,
The thick steel plate satisfies low-temperature impact toughness at -40°C to -60°C at the surface, 1/4 of the thickness, and 1/2 of the thickness: 100 J to 400 J,
Thick steel plate. According to claim 1,
The thick steel plate satisfies the following requirements: yield strength (YP): 320 MPa or more, tensile strength (TS): 430 MPa or more, and elongation (El): 22% or more.
Thick steel plate. According to claim 1,
The thick steel plate is,
It has a microstructure including acicular ferrite, polygonal ferrite, and pearlite,
The area fraction of the acicular ferrite is 50% to 80%, the area fraction of the pearlite is 0% to 10%, and the area fraction of the polygonal ferrite is the remaining fraction,
Thick steel plate. According to claim 1,
The thick steel plate is,
having a ferrite grain size (FGS) greater than 11.78 to 11.99,
Thick steel plate. (a) By weight percent, carbon (C): 0.045% to 0.075%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.3% to 1.6%, aluminum (Al): 0.015% to 0.05%. , Niobium (Nb): 0.015% ~ 0.025%, Nickel (Ni): 0.4% ~ 0.5%, Copper (Cu): 0.2% ~ 0.3%, Titanium (Ti): 0.01% ~ 0.02%, Phosphorus (P): Reheating the steel containing more than 0% to 0.01%, sulfur (S): more than 0% to 0.002%, and the balance containing iron (Fe) and other inevitable impurities at a reheating temperature of 1,000°C to 1,060°C;
(b) performing primary hot rolling on the reheated steel;
(c) an air-cooling standby step of air-cooling the primary hot-rolled steel in air for 300 to 600 seconds;
(d) performing secondary hot rolling on the air-cooled steel to end hot rolling at a temperature of 670°C to 730°C; and
(e) forcibly cooling the secondary hot rolled steel to a cooling end temperature in the range of 400°C to 500°C at a cooling rate of 3°C/sec to 5°C/sec;
Manufacturing method of thick steel plate. According to claim 6,
The reduction ratio of the thick steel plate subjected to the first hot rolling and the second hot rolling is 2.3:1 or more to less than 3:1,
Manufacturing method of thick steel plate. According to claim 6,
The cumulative reduction ratio of the thick steel plate that has undergone the first hot rolling and the second hot rolling is in the range of 40% to 50%,
Manufacturing method of thick steel plate. According to claim 6,
The steel has a thickness ranging from 250t to 300t,
The thick steel plate manufactured by the steel plate manufacturing method has a thickness ranging from 100t to 120t,
Manufacturing method of thick steel plate. According to claim 6,
The thick steel plate manufactured by the above thick steel plate manufacturing method is,
Yield strength (YP): 320 MPa or more, tensile strength (TS): 430 MPa or more, and elongation (El): 22% or more, low-temperature impact toughness at -40℃ to -60℃: Satisfies 100 J to 400 J. do,
It has a microstructure including acicular ferrite, polygonal ferrite, and pearlite,
The area fraction of the acicular ferrite is 50% to 80%, the area fraction of the pearlite is 0% to 10%, and the area fraction of the polygonal ferrite is the remaining fraction,
Having a ferrite grain size (FGS) greater than 11.78 to 11.99,
Manufacturing method of thick steel plate.

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