KR102413841B1 - Steel having excellent strength and low-temperature toughness after PWHT and method of manufacturing the same - Google Patents

Abstract

The present invention provides a steel material having excellent post-weld heat treatment strength and low-temperature toughness, and a method for manufacturing the same. According to an embodiment of the present invention, the steel is, by weight, carbon (C): 0.11% to 0.14%, silicon (Si): 0.25% to 0.45%, manganese (Mn): 1.2% to 1.6%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): 0.1% to 0.2%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): 0.005% to 0.015%, Niobium (Nb): 0.01% to 0.02%, phosphorus (P): more than 0% to 0.012%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities, tensile strength (TS) : 550 MPa to 650 MPa, yield strength (YS): 460 MPa to 550 MPa, elongation (EL): 20% to 26%, and Charpy impact energy at -40°C: 100 J to 400 J.

Description

Steel having excellent post-welding heat treatment strength and low-temperature toughness and a manufacturing method therefor

The technical idea of the present invention relates to a steel material and a method for manufacturing the same, and to a steel material having excellent post-weld heat treatment strength and low-temperature toughness and a method for manufacturing the same.

Structural steel materials for ships and offshore structures, and thick steel plates for multi-purpose tanks in which various types of liquefied gases such as carbon dioxide, ammonia, and LNG are mixed, have a very harsh usage environment. Therefore, not only strength but also impact toughness at low temperature is very important to these steel materials for marine structures or tanks. Due to the depletion of resources in warm regions, the use environment of marine structural steel is gradually shifting to cold regions such as the Arctic, where marine oil and gas resources are abundant. It is getting difficult. When the content of carbon and alloying elements is increased to improve the strength of the steel, there is a problem in that the impact toughness at low temperature is lowered.

Conventional pressure vessel with guaranteed low-temperature toughness of 60kg class uses heat treatment guaranteed steel (A537-C2) after welding. In this case, the post-welding heat treatment temperature is 590° C. to 650° C., and the cooling rate after the heat treatment is 50° C./sec to 400° C. In addition, there is a limit in that the tensile strength decreases as the plant environment becomes harsh and the heat treatment temperature after welding is increased and the time is increased in order to improve the welding workability of the pressure vessel.

Korea Patent Application No. 2016-0077469

The technical problem to be achieved by the technical idea of the present invention is to provide a steel material having excellent post-weld heat treatment strength and low-temperature toughness, and a method for manufacturing the same.

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

According to one aspect of the present invention, there is provided a steel material having excellent post-weld heat treatment strength and low-temperature toughness, and a method for manufacturing the same.

According to an embodiment of the present invention, the steel is, by weight, carbon (C): 0.11% to 0.14%, silicon (Si): 0.25% to 0.45%, manganese (Mn): 1.2% to 1.6%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): 0.1% to 0.2%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): 0.005% to 0.015%, Niobium (Nb): 0.01% to 0.02%, phosphorus (P): more than 0% to 0.012%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities, tensile strength (TS) : 550 MPa ~ 650 MPa, yield strength (YS): 460 MPa ~ 550 MPa, elongation (EL): 20% ~ 26%, and Charpy impact energy at -40℃: 100 J ~ 400 J .

According to an embodiment of the present invention, the steel material may have a mixed structure of martensite and pearlite.

According to an embodiment of the present invention, the steel material may further include a carbide formed in the lath of the martensite.

According to an embodiment of the present invention, the steel may have a carbon equivalent (Ceq) in the range of 0.4 to 0.5. (where Ceq = C + Mn/6 + (Ni + Cu)/15 + (Cr + Mo + V)/5)

According to an embodiment of the present invention, the manufacturing method of the steel material, by weight, carbon (C): 0.11% to 0.14%, silicon (Si): 0.25% to 0.45%, manganese (Mn): 1.2% to 1.6%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): 0.1% to 0.2%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): 0.005% to 0.015%, Niobium ( Nb): 0.01% to 0.02%, phosphorus (P): more than 0% to 0.012%, sulfur (S): more than 0% to 0.003%, and the balance is 1,100 steels containing iron (Fe) and other unavoidable impurities Reheating at a temperature of ℃ ~ 1,200 ℃; hot-rolling the heated steel material to end at a temperature of 1,050° C. to 1,150° C.; Normalizing the rolled steel at a temperature of 900 ℃ ~ 950 ℃; cooling the normalized steel to a temperature of 20°C to 30°C; Welding the cooled steel, heat treatment after welding at a temperature of 590 ℃ ~ 650 ℃; and cooling the heat-treated steel material after welding to a temperature of 380°C to 420°C at a cooling rate of 150°C/sec to 200°C/sec.

According to an embodiment of the present invention, the heat treatment after welding may be performed for 1 hour to 15 hours.

According to one embodiment of the present invention, after performing the step of cooling the heat-treated steel material after welding, the steel material, tensile strength (TS): 550 MPa ~ 650 MPa, yield strength (YS): 460 MPa ~ 550 MPa, elongation (EL): 20% to 26%, and Charpy impact absorption energy at -40°C: 100 J to 400 J may be satisfied.

According to the technical idea of the present invention, a steel material having excellent post-welding heat treatment strength and low-temperature toughness and a method for manufacturing the same increase the cooling rate after heat treatment after welding, partial recrystallization and growth are suppressed, and grains can be refined . That is, by minimizing the softening degree of the steel, it is possible to secure the strength to maintain miniaturization and at the same time guarantee the low-temperature toughness.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method for manufacturing a steel material having excellent post-weld heat treatment strength and low-temperature toughness according to an embodiment of the present invention.
2 is a photomicrograph showing the microstructure of the steel of Examples and Comparative Examples according to an embodiment of the present invention.
3 is a graph showing the change in tensile strength according to the heat treatment time after welding of the steels of Examples and Comparative Examples according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

The present invention provides a steel material having excellent post-weld heat treatment strength and low-temperature toughness and a method for manufacturing the same.

A537-C2 Changes in strength and physical properties of heat-treated steel are closely related to the phase transformation and formation of microstructures. After performing the post-welding heat treatment, it is necessary to suppress the partial recrystallization of tempered martensite and also suppress the grain growth to prevent the decrease in strength. The technical idea of the present invention is to compensate for strength by increasing the cooling rate after performing heat treatment after welding, and to ensure low-temperature toughness. That is, it is to suppress a decrease in strength due to softening of the microstructure and to secure excellent impact toughness without a decrease in tensile strength.

Hereinafter, a steel material having excellent post-weld heat treatment strength and low-temperature toughness, which is an aspect of the present invention, will be described.

steel

A steel material having excellent post-weld heat treatment strength and low-temperature toughness, which is an aspect of the present invention, in weight %, carbon (C): 0.11% to 0.14%, silicon (Si): 0.25% to 0.45%, manganese (Mn): 1.2 % to 1.6%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): 0.1% to 0.2%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): 0.005% to 0.015%, Niobium (Nb): 0.01% to 0.02%, phosphorus (P): more than 0% to 0.012%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities. In addition, the steel, as an unavoidable impurity, hydrogen (H): may include more than 0 ppm ~ 2.5 ppm.

According to an embodiment of the present invention, the steel material may have a carbon equivalent (Ceq) in the range of 0.4 to 0.5.

The carbon equivalent (Ceq) may be calculated by the following formula.

Ceq = C + Mn/6 + (Ni + Cu)/15 + (Cr + Mo + V)/5

Hereinafter, the role and content of each component included in the steel having excellent post-weld heat treatment strength and low-temperature toughness according to the present invention will be described as follows. At this time, the content of the component elements all mean wt%.

Carbon (C): 0.11% to 0.14%

Carbon is added to ensure strength. When the carbon content is less than 0.11%, it may be difficult to secure strength. Conversely, when the carbon content exceeds 0.14%, the strength of the steel is increased, but low-temperature impact toughness and weldability may be deteriorated. Therefore, it is preferable to add carbon in an amount of 0.11% to 0.14% of the total weight of the steel.

Silicon (Si): 0.25% to 0.45%

Silicon is added as a deoxidizer to remove oxygen from steel in the steelmaking process. In addition, silicon also has a solid solution strengthening effect. When the content of silicon is less than 0.25%, the effect of adding silicon cannot be properly exhibited. Conversely, when the content of silicon exceeds 0.45%, toughness and weldability may decrease, and oxidation inclusions in the steel may increase to decrease low-temperature toughness and hydrogen-induced cracking resistance. Therefore, it is preferable to add silicon in an amount of 0.24% to 0.45% of the total weight of the steel.

Manganese (Mn): 1.2% to 1.6%

Manganese is an element that increases the strength and toughness of steel and increases the hardenability of steel, and the addition of manganese reduces the decrease in ductility when the strength increases than the addition of carbon. When the manganese content is less than 1.2%. Even if the carbon content is high, it may be difficult to secure strength. When the manganese content exceeds 1.6%, defects such as cracks during welding may be caused due to an increase in the amount of MnS-based non-metallic inclusions. Therefore, it is preferable to add manganese in an amount of 1.2% to 1.6% of the total weight of the steel.

Soluble Aluminum (S_Al): 0.015% to 0.05%

Soluble aluminum acts as a deoxidizer to remove oxygen from the steel. When the content of soluble aluminum is less than 0.15%, the deoxidation effect cannot be properly exhibited. If the content of soluble aluminum exceeds 0.05%, it may cause deterioration of the slab surface quality. Therefore, it is preferable to add soluble aluminum in an amount of 0.015% to 0.05% of the total weight of the steel.

Chromium (Cr): 0.1% to 0.2%

Chromium is an effective element added to secure strength. When the content of chromium is less than 0.1%, the effect of adding chromium cannot be properly exhibited. When the content of chromium exceeds 0.2%, weldability or heat-affected zone (HAZ) toughness may be deteriorated. Therefore, it is preferable to add chromium in an amount of 0.1% to 0.2% of the total weight of the steel.

Nickel (Ni): 0.15% to 0.25%

Nickel refines grains and is dissolved in austenite and ferrite to strengthen the matrix. In particular, nickel is an effective element for improving low-temperature impact toughness. When the content of nickel is less than 0.15%, the effect of adding nickel cannot be properly exhibited. When the content of nickel exceeds 0.25%, it may cause red hot brittleness. Therefore, it is preferable to add nickel in an amount of 0.15% to 0.25% of the total weight of the steel.

Titanium (Ti): 0.005% to 0.015%

Titanium generates Ti(C, N) precipitates with high high-temperature stability, thereby preventing coarsening of austenite grains in the reheating step, thereby improving the toughness of steel. When the content of titanium is less than 0.005%, the effect of adding titanium is insufficient. When the content of titanium exceeds 0.015%, the effect of suppressing the growth of austenite grains by generating coarse precipitates is rather reduced, the toughness of the steel is reduced, and there is a problem of increasing the manufacturing cost without any further effect of addition . Therefore, it is preferable to add titanium in an amount of 0.005% to 0.015% of the total weight of the steel.

Niobium (Nb): 0.01% to 0.02%

Niobium combines with carbon (C) and nitrogen (N) contained in steel at high temperatures to form carbides or nitrides, and these niobium-based carbides or nitrides suppress recrystallization and grain growth during rolling to refine crystal grains, thereby strengthening the strength of steel and improve both toughness. When the content of niobium is less than 0.01%, the effect of adding niobium is insignificant. When the content of niobium exceeds 0.02%, the toughness of the steel may be reduced. Therefore, it is preferable to add niobium in an amount of 0.01% to 0.02% of the total weight of the steel.

Phosphorus (P): >0% to 0.012%

Phosphorus partially contributes to the improvement of strength, but may adversely affect the material by reducing the toughness of the weld joint and the low-temperature impact toughness, and forming fine segregation as well as central segregation. Therefore, the lower the phosphorus content, the better. Therefore, phosphorus is preferably limited to more than 0% ~ 0.012% of the total weight of the steel.

Sulfur (S): >0% to 0.003%

Sulfur is an element that is inevitably contained in the manufacture of steel together with phosphorus, and forms non-metallic inclusions such as MnS, and as a low melting point element, the possibility of grain boundary segregation is high, thereby reducing toughness. When the content of sulfur exceeds 0.003% by weight, the toughness of the base metal and the weld joint may be greatly reduced. Therefore, sulfur is preferably limited to more than 0% ~ 0.003% of the total weight of the steel.

The remaining component of the steel 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 of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The steel material formed through the method for manufacturing a steel material according to the technical idea of the present invention to control the specific components of the above-described alloy composition and their content ranges, which will be described later, has a tensile strength (TS): 550 MPa to 650 MPa, yield strength ( YS): 460 MPa to 550 MPa, elongation (EL): 20% to 26%, and Charpy impact absorption energy at -40°C: 100 J to 400 J may be satisfied.

The final microstructure of the steel may be a mixed structure in which martensite and pearlite are mixed. In addition, carbides formed in the lath of the martensite, for example, Fe ₃ C, may include.

The steel of the alloy composition has excellent strength and low-temperature toughness, so it may be suitable for use as a steel material for a pressure vessel.

Another aspect of the present invention provides a method for manufacturing a steel material having excellent post-weld heat treatment strength and low-temperature toughness. According to this, the step of reheating the steel material made of the above-mentioned alloy composition at a temperature of 1,100 ℃ ~ 1,200 ℃; hot-rolling the heated steel material to end at a temperature of 1,050° C. to 1,150° C.; Normalizing the hot-rolled steel at a temperature of 900 ℃ ~ 950 ℃; cooling the normalized steel to a temperature of 20°C to 30°C; Welding the cooled steel, heat treatment after welding at a temperature of 590 ℃ ~ 650 ℃; and cooling the heat-treated steel material after welding to a temperature of 380°C to 420°C at a cooling rate of 150°C/sec to 200°C/sec.

The heat treatment after welding may be performed for 1 hour to 15 hours.

After performing the step of cooling the heat-treated steel after welding, the steel is, tensile strength (TS): 550 MPa to 650 MPa, yield strength (YS): 460 MPa to 550 MPa, elongation (EL): 20% ~ 26%, and Charpy impact absorption energy at -40 ° C: 100 J ~ 400 J can be satisfied.

Hereinafter, a method for manufacturing a steel material having excellent post-weld heat treatment strength and low-temperature toughness according to the present invention will be described with reference to the accompanying drawings.

Steel manufacturing method

1 is a process flowchart schematically illustrating a method of manufacturing a steel material having excellent post-weld heat treatment strength and low-temperature toughness according to an embodiment of the present invention.

In the steel manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

The steel is, by weight, carbon (C): 0.11% to 0.14%, silicon (Si): 0.25% to 0.45%, manganese (Mn): 1.2% to 1.6%, soluble aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): 0.1% to 0.2%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): 0.005% to 0.015%, Niobium (Nb): 0.01% to 0.02%, Phosphorus (P) ): more than 0% to 0.012%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities. In addition, the steel, as an unavoidable impurity, hydrogen (H): may include more than 0 ppm ~ 2.5 ppm.

1 , the method for manufacturing a steel material according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a normalizing step (S130), a cooling step after normalizing (S140), and heat treatment after welding Step (S150), and a cooling step (S160) after the heat treatment after welding; includes.

Reheating step (S110)

In the reheating step (S110), the steel having the above composition, for example, a slab plate, is reheated at a Slab Reheating Temperature (SRT) of 1,100° C. to 1,200° C. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur.

If the reheating temperature is less than 1,100 ℃, there is a problem that the rolling load increases because the heating temperature is not sufficient. In addition, since the Nb-based precipitates do not reach the solid solution temperature, they cannot be re-precipitated as fine precipitates during hot rolling, so that the austenite grain growth cannot be suppressed, so that the austenite grains are rapidly coarsened. Conversely, when the reheating temperature exceeds 1,200° C., austenite grains are rapidly coarsened or decarburized, so that it is difficult to secure the strength and low-temperature toughness of the manufactured steel.

Hot rolling step (S120)

The heated steel is first subjected to hot rolling after heating to adjust its shape. The hot rolling may be continuously performed by wide rolling, rough rolling, and finishing rolling. By the hot rolling step, the steel may form a steel sheet.

The hot rolling may be terminated at a finish delivery temperature (FDT) of 1,050° C. to 1,150° C. When the hot rolling temperature exceeds 1,150° C., the austenite grains are coarsened, so that ferrite grain refinement is not sufficiently achieved after transformation, and thus, it may be difficult to secure strength. Conversely, when the hot rolling temperature is carried out at less than 1,050° C., it is possible to induce a rolling load to reduce productivity and reduce the heat treatment effect.

Normalizing step (S130)

After hot rolling, the cooled steel is normalized at a temperature of 900°C to 950°C for 10 minutes to 180 minutes. The normalizing may improve workability by restoring crystal grains processed by rolling without changing the properties of the material, homogenizing the structure, and removing internal stress.

When the normalizing temperature is less than 900° C., it may be difficult to re-dissolve solid-solute elements, so that it may be difficult to secure sufficient strength. Conversely, when the normalizing temperature exceeds 950° C., excessive heat is consumed, which is not good in terms of productivity. In addition, when the normalizing time is less than 10 minutes, it may be difficult to secure a uniform tissue. Conversely, when the normalizing time exceeds 180 minutes, only the production cost can be increased without further synergistic effect.

Cooling step after normalizing (S140)

The normalized steel is cooled to a temperature of 20° C. to 30° C. at a cooling rate of 0.1° C./sec to 0.5° C./sec.

Post-welding heat treatment step (S150)

After the normalizing, the cooled steel is welded. The welding may be, for example, welding for forming a steel pipe. In the above welding, the normalized plate material is cut and then welded by electric resistance welding to form a steel pipe. Then, heat treatment is performed after welding at a temperature of 590° C. to 650° C.

When the post-welding heat treatment temperature is less than 590° C., it is not easy to remove residual stress from the welds or the like. When the post-welding heat treatment temperature exceeds 650° C., it may be difficult to secure sufficient strength. The post-welding heat treatment may be performed for 1 hour to 15 hours.

Cooling step after welding and heat treatment (S160)

After the welding, the heat-treated steel is cooled to a temperature of 380°C to 420°C at a cooling rate of 150°C/sec to 200°C/sec. The temperature may be about 400°C. Partial recrystallization and growth may be suppressed by such a fast cooling rate, and crystal grains may be refined. That is, by increasing the cooling rate after heat treatment after welding, it is possible to minimize the softening degree of the steel, secure the refining maintenance strength, and at the same time guarantee the low-temperature toughness.

The steel material manufactured in the above steps (S110 to S160) has, due to appropriate control of heat treatment conditions, tensile strength (TS): 550 MPa to 650 MPa, yield strength (YS): 460 MPa to 550 MPa, elongation (EL): 20 % to 26%, and Charpy impact absorption energy at -40°C: It can be improved to a steel grade that satisfies 100 J to 400 J.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Tables 1 and 2 show the compositions of the steels of Comparative Examples and Examples. The remainder in Tables 1 and 2 consists of iron (Fe) and impurities unavoidably contained in a steelmaking process, etc. The unit is % by weight.

division C Si Mn S_Al Cr Ni Comparative Example 1 0.133 0.27 1.47 0.020 0.15 0.19 Comparative Example 2 0.133 0.28 1.50 0.025 0.12 0.18 Comparative Example 3 0.131 0.27 1.51 0.023 0.15 0.18 Comparative Example 4 0.130 0.26 1.48 0.021 0.14 0.16 Example 1 0.132 0.26 1.51 0.020 0.16 0.16 Example 2 0.133 0.26 1.50 0.025 0.16 0.19

division Ti Nb P S H (ppm) Ceq Comparative Example 1 0.008 0.017 0.007 0.001 2.0 0.421 Comparative Example 2 0.007 0.018 0.007 0.001 2.1 0.419 Comparative Example 3 0.009 0.018 0.007 0.001 2.1 0.425 Comparative Example 4 0.009 0.017 0.008 0.001 2.2 0.415 Example 1 0.008 0.016 0.008 0.001 2.3 0.426 Example 2 0.007 0.019 0.007 0.001 2.2 0.428

Referring to Tables 1 and 2, Comparative Examples and Examples satisfy the composition range proposed in the present invention. Table 3 shows the values of process conditions for forming steels of Comparative Examples and Examples.

division reheat
temperature
(℃) hot rolled
end temperature
(℃) Normalizing
temperature
(℃) after welding
heat treatment temperature
(℃) after welding
after heat treatment
cooling rate
(℃/sec) Comparative Example 1 1100 1085 920 630 50 Comparative Example 2 1100 1090 915 635 45 Comparative Example 3 1141 1115 920 630 45 Comparative Example 4 1144 1121 913 633 48 Example 1 1100 1077 915 635 155 Example 2 1143 1113 915 630 165

Referring to Table 3, Comparative Examples and Examples satisfy the process conditions proposed in the present invention, except that the cooling rate after heat treatment after welding is in the range of 40 ~ 50 ℃ / sec in Comparative Examples, whereas in Examples 150 ~ There is a difference in the 170° C./sec range. The cooling temperature after the heat treatment after welding was 400°C. Table 4 shows the mechanical properties of the manufactured steel, including yield strength (YS), tensile strength (TS), elongation (EL), and Charpy at -40°C. The impact absorption energy (CVN) is measured and the result is shown.

division surrender
burglar
(MPa) Seal
burglar
(MPa) elongation
(%) Charpy shock absorption energy at -40℃
(J) 1 time Episode 2 3rd time 4 times 5 times Average Comparative Example 1 417 516 32 380 384 367 385 210 345 Comparative Example 2 451 565 29 378 132 373 388 372 329 Comparative Example 3 455 551 29 383 215 372 218 367 311 Comparative Example 4 443 550 30 - - - - - - Example 1 494 607 25 368 375 374 245 278 328 Example 2 477 588 26 343 369 357 344 345 352

Referring to Table 4, the Examples showed an increase in tensile strength and yield strength, a decrease in elongation, and almost similar values of Charpy impact absorption energy compared to Comparative Examples. Accordingly, the embodiment can provide a steel material in which low-temperature impact toughness is not reduced even though the strength is increased.

Referring to FIG. 2 , in the steel of Comparative Example, partial recrystallization and grain growth occurred, and Fe ₃ C spheroidization occurred. In the example steel, the recovered microstructure was shown, and Fe ₃ C decomposition and lath growth were observed. The steel has a mixed structure in which martensite and pearlite are mixed, and carbide is formed in the lath of the martensite. That is, in the embodiment, partial recrystallization and grain growth were suppressed by the increased cooling rate after the heat treatment after welding, and by minimizing the softening degree to maintain the microstructure refinement, strength and low temperature toughness can be maintained at the same time.

3 is a graph showing the change in tensile strength according to the heat treatment time after welding of the steels of Examples and Comparative Examples according to an embodiment of the present invention.

Referring to FIG. 3 , the Example showed higher tensile strength than the Comparative Example in all time ranges. However, as the heat treatment time after welding was increased, this difference decreased, and showed almost the same tensile strength after 12 hours. In all cases, the tensile strength showed a tensile strength of 550 MPa or more.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (7)

By weight, carbon (C): 0.11% to 0.14%, silicon (Si): 0.25% to 0.45%, manganese (Mn): 1.2% to 1.6%, soluble aluminum (S_Al): 0.015% to 0.05%, chromium (Cr): 0.1% to 0.2%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): 0.005% to 0.015%, Niobium (Nb): 0.01% to 0.02%, Phosphorus (P): 0% more than 0.012%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and other unavoidable impurities;
Tensile strength (TS): 550 MPa to 650 MPa, Yield strength (YS): 460 MPa to 550 MPa, Elongation (EL): 20% to 26%, and Charpy impact energy at -40°C: 100 J to 400 satisfies J,
The steel has a mixed structure in which martensite and pearlite are mixed,
steel. delete The method of claim 1,
The steel material further comprises a carbide formed in the lath of the martensite,
steel. The method of claim 1,
The steel has a carbon equivalent (Ceq) in the range of 0.4 to 0.5,
(where Ceq = C + Mn/6 + (Ni + Cu)/15 + (Cr + Mo + V)/5)
steel. By weight, carbon (C): 0.11% to 0.14%, silicon (Si): 0.25% to 0.45%, manganese (Mn): 1.2% to 1.6%, soluble aluminum (S_Al): 0.015% to 0.05%, chromium (Cr): 0.1% to 0.2%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): 0.005% to 0.015%, Niobium (Nb): 0.01% to 0.02%, Phosphorus (P): 0% exceeding ~ 0.012%, sulfur (S): exceeding 0% ~ 0.003%, and the remainder being iron (Fe) and other unavoidable impurities, reheating the steel at a temperature of 1,100 ℃ ~ 1,200 ℃;
hot-rolling the heated steel material to end at a temperature of 1,050° C. to 1,150° C.;
Normalizing the rolled steel at a temperature of 900 ℃ ~ 950 ℃;
cooling the normalized steel to a temperature of 20° C. to 30° C.;
Welding the cooled steel material, heat treatment after welding at a temperature of 590 ℃ ~ 650 ℃; and
The step of cooling the heat-treated steel material after welding to a temperature of 380°C to 420°C at a cooling rate of 150°C/sec to 200°C/sec;
A method of manufacturing steel. 6. The method of claim 5,
The step of heat treatment after welding is performed for 1 hour to 15 hours,
A method of manufacturing steel. 6. The method of claim 5,
After performing the step of cooling the heat-treated steel after welding,
The steel material is, tensile strength (TS): 550 MPa to 650 MPa, yield strength (YS): 460 MPa to 550 MPa, elongation (EL): 20% to 26%, and Charpy impact energy at -40°C: Satisfying 100 J ~ 400 J,
A method of manufacturing steel.

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