KR101989251B1 - Structural steel and method of manufacturing the same - Google Patents

Abstract

According to the present invention, provided is a structural steel comprising: 0.08-0.16 wt% of carbon (C); 0.25-0.35 wt% of silicon (Si); 1.4-1.6 wt% of manganese (Mn); 0.02-0.05 wt% of solid solution aluminum (S-Al); 0.025-0.035 wt% of niobium (Nb); 0.01-0.02 wt% of titanium (Ti); 0.05-0.15 wt% of copper (Cu); 0.05-0.15 wt% of nickel (Ni); and the remaining consisting of iron (Fe) and unavoidable impurities, and the final structure being a ferrite and pearlite structure. Therefore, the present invention has excellent weldability and is capable of guaranteeing a material after a post weld heat treatment (PWHT).

Description

Structural steel and its manufacturing method {STRUCTURAL STEEL AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a steel material and a method for manufacturing the same, and more particularly, to a structural steel material and a method for manufacturing the same.

The wind power industry is expected to continue to grow due to the global interest in renewable energy, such as changes in energy policy in each country and smart grid plans in Europe. Therefore, the production of wind power generators is also expected to continue. Recently, as the size of the welded structure is increased and safety is further demanded, materials for high strength and toughness are also designed for steel for wind turbine tower manufacturing. Steels used to manufacture wind towers are generally subjected to welding and post-weld heat treatment (PWHT). In order to facilitate the welding process, it is necessary to reduce the alloying elements which have a great influence on the weldability, but it is highly likely to fall short of the specification due to the drop in strength. In addition, if the post-weld heat treatment is performed, it may cause a drop in strength and a drop in impact value.

The prior art is the Republic of Korea Patent Publication No. 2012-0063199 (published: 2012.06.15, the name of the invention: steel and excellent strength and impact toughness and its manufacturing method).

The present invention provides a steel material and a method of manufacturing the same which are excellent in weldability and capable of guaranteeing a material after welding after heat treatment (PWHT).

Structural steel according to an embodiment of the present invention, in weight%, carbon (C): 0.08 ~ 0.16%, silicon (Si): 0.25 ~ 0.35%, manganese (Mn): 1.4 ~ 1.6%, solid solution aluminum (S Al: 0.02 to 0.05%, niobium (Nb): 0.025 to 0.035%, titanium (Ti): 0.01 to 0.02%, copper (Cu): 0.05 to 0.15%, nickel (Ni): 0.05 to 0.15%, and the rest It consists of iron (Fe) and unavoidable impurities, and the final structure includes ferrite and pearlite structure.

The structural steel may include carbon (C), manganese (Mn), copper (Cu), and nickel (Ni) in a range satisfying Equation 1 below.

Equation 1: 0 <[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 ≤ 0.39

Where [] is the weight percent of each element

The structural steel may have an average size of 10 to 15 μm of grains.

The structural steel has a yield strength of at least 355 MPa before and after PWHT, and a tensile strength of at least 490 MPa before and after PWHT, and an average Charpy impact energy at -20 ° C. Impact Energy) may be greater than or equal to 270J before and after post-weld heat treatment (PWHT).

Method for producing a structural steel according to an embodiment of the present invention is (a) wt%, carbon (C): 0.08 ~ 0.16%, silicon (Si): 0.25 ~ 0.35%, manganese (Mn): 1.4 ~ 1.6% , Solid solution aluminum (S-Al): 0.02 ~ 0.05%, niobium (Nb): 0.025 ~ 0.035%, titanium (Ti): 0.01 ~ 0.02%, copper (Cu): 0.05 ~ 0.15%, nickel (Ni): 0.05 Providing a thick plate casting comprising 0.15% and the remaining iron (Fe) and unavoidable impurities; (b) reheating the cast material at a temperature of 1040 to 1120 ° C .; And (c) rolling the reheated cast material under a condition in which a finish rolling temperature is 800 to 840 ° C.

In the method of manufacturing the structural steel, the cast material in the step (a) may include carbon (C), manganese (Mn), copper (Cu) and nickel (Ni) in a range satisfying the following equation (1). .

Equation 1: 0 <[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 ≤ 0.39

Where [] is the weight percent of each element

In the step (c) of the manufacturing method of the structural steel, the finishing rolling temperature may be (A3 + 20) ° C or less.

According to an embodiment of the present invention, it is possible to implement a steel material and a method of manufacturing the same, which are excellent in weldability and guarantee material after welding PWHT. Of course, the scope of the present invention is not limited by these effects.

1 is a flow chart illustrating a method of manufacturing a structural steel according to an embodiment of the present invention.
2 is a view schematically illustrating a processing method according to a first experimental example of the present invention.
3 is a diagram illustrating a heat treatment process of post-weld heat treatment (PWHT) in the second experimental example of the present invention.
Figure 4 is a photograph of the crystal grains of the structural steel according to Comparative Example 2 of the second experimental example of the present invention.
FIG. 5 is a photograph of a crystal grain of structural steel according to Example 2 of a second experimental example of the present invention. FIG.
FIG. 6 is a graph showing Charpy impact energy before and after post-welding heat treatment (PWHT) of structural steel according to Comparative Example 2 of a second experimental example of the present invention according to a temperature section.
7 is a graph showing Charpy impact energy before and after post-welding heat treatment (PWHT) of structural steel according to Example 2 of the second experimental example of the present invention according to the temperature section.

Hereinafter, a structural steel and a method of manufacturing the same according to an embodiment of the present invention will be described in detail. The terms to be described below are terms properly selected in consideration of functions in the present invention, and the definitions of these terms should be made based on the contents throughout the specification.

As wind turbines require more safety along with larger welded structures, materials for high strength and toughness are being designed for steel for wind turbine tower fabrication. In general, the thick steel sheet tends to increase the carbon equivalent as the high strength and thickening progresses, making it difficult to secure toughness. Nevertheless, in the case of structural steel sheets used in the production of wind towers, where welding is the main process, it is necessary to secure stable Charpy absorbed energy at -20 ° C with low carbon equivalent (Ceq). However, components known to date can only confirm results near or below the lower specification required for materials before and after PWHT.

According to the structural steel and the manufacturing method according to an embodiment of the present invention, by controlling the carbon equivalent by changing the design and manufacturing process of the alloy component to improve the weldability and to ensure the material warranty after post-weld heat treatment (PWHT), This will be described below.

Steel

Structural steel according to an embodiment of the present invention, in weight%, carbon (C): 0.08 ~ 0.16%, silicon (Si): 0.25 ~ 0.35%, manganese (Mn): 1.4 ~ 1.6%, solid solution aluminum (S Al: 0.02 to 0.05%, niobium (Nb): 0.025 to 0.035%, titanium (Ti): 0.01 to 0.02%, copper (Cu): 0.05 to 0.15%, nickel (Ni): 0.05 to 0.15%, and the rest It consists of iron (Fe) and unavoidable impurities. The final structure of the steel includes ferrite and pearlite structures. The average size of the grains in the steel may be 10 ~ 15 ㎛.

Hereinafter, the role and content of each component included in the structural steel according to an embodiment of the present invention will be described.

Carbon (C): 0.08 ~ 0.16 wt%

If the content of carbon is less than 0.08% by weight, carbides cannot be formed sufficiently and strength cannot be secured. However, when the content of carbon exceeds 0.16% by weight, the deterioration of toughness and ductility becomes remarkable. In addition, in the case of a thick plate used as a steel structure for welding, it is preferable to limit the range of carbon to 0.08 to 0.16% by weight for weldability.

Silicon (Si): 0.25 ~ 0.35 wt%

Silicone is used as a deoxidizer and has a strength improving effect, but when added in excess of 0.35% by weight, the low temperature toughness and the weldability also deteriorate. On the other hand, when added in less than 0.25% by weight is insufficient deoxidation effect, silicon content is preferably limited to 0.25 ~ 0.35% by weight.

Manganese (Mn): 1.4 ~ 1.6 wt%

Manganese is a useful element that enhances strength by solid solution strengthening, and therefore it is necessary to add 1.4 wt% or more. However, when the content exceeds 1.6% by weight, the toughness of the weld portion is greatly reduced due to excessive increase in the hardenability, so that the content of manganese is preferably limited to 1.4 to 1.6% by weight.

Solid solution aluminum (S-Al): 0.02 ~ 0.05 wt%

Since solid solution aluminum is an element that can deoxidize molten steel at low cost, it is preferable to add 0.02% by weight or more, but when it exceeds 0.05% by weight, a large amount of oxide inclusions are formed, which impairs the impact toughness of the material. The content is preferably limited to the range of 0.02 to 0.05% by weight.

Niobium (Nb): 0.025 to 0.035 wt%

Niobium improves the strength of the substrate and the weld. In addition, it is an element that causes a controlled rolling upward effect and contributes to grain refinement. However, this effect cannot be expected when the niobium content is less than 0.025% by weight, and when the niobium content is more than 0.035% by weight, the possibility of brittle cracking at the corners of the steel is increased. It is desirable to limit to%.

Titanium (Ti): 0.01 ~ 0.02 wt%

Titanium is an element that controls the initial austenite grain size and suppresses grain growth upon reheating to secure grain refinement and greatly improve low temperature toughness. However, to express this effect, titanium should be added at least 0.01% by weight. However, if it exceeds 0.02% by weight, there is a problem that the low-temperature toughness due to clogging of the nozzle or crystallization of the central part is reduced, so that the titanium content is preferably limited to 0.01 to 0.02% by weight.

Copper (Cu): 0.05 ~ 0.15 wt%

Copper is an element that can increase the strength while minimizing the decrease in toughness of the base metal, so it must be added at least 0.05% by weight in order to exhibit the effect. On the other hand, if it exceeds 0.15% by weight significantly inhibits the product surface quality, the copper content is preferably limited to 0.05 ~ 0.15% by weight.

Nickel (Ni): 0.05 ~ 0.15 wt%

Nickel is an element that can improve the strength and toughness of the base material at the same time, and must be added at least 0.05% by weight in order to exhibit the effect. However, since nickel is an expensive element, when it exceeds 0.15% by weight, the economical efficiency is lowered, and the weldability is deteriorated. Therefore, the content of nickel is preferably limited to the range of 0.05 to 0.15% by weight.

Furthermore, the steel may include carbon (C), manganese (Mn), copper (Cu), and nickel (Ni) in a range satisfying Equation 1 below.

Equation 1: 0 <[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 ≤ 0.39

Where [] is the weight percent of each element

“[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15” expressed in Equation 1 can be understood as a carbon equivalent. When the carbon equivalent exceeds 0.39, the welding process The strength is markedly decreased at, and the carbides in the substrate increase after rolling, resulting in a drop in toughness.

The final structure of the steel material having the above-described alloy composition includes a ferrite and a pearlite structure. In addition, the structural steel may have an average size of the crystal grains of 10 ~ 15 ㎛. The structural steel has a yield strength of at least 355 MPa before and after PWHT, and a tensile strength of at least 490 MPa before and after PWHT, and an average Charpy impact energy at -20 ° C. Impact Energy) may be greater than or equal to 270J before and after post-weld heat treatment (PWHT).

Manufacturing method of structural steel

Hereinafter, a method of manufacturing structural steel according to an embodiment of the present invention will be described in detail.

1 is a flow chart illustrating a method of manufacturing a structural steel according to an embodiment of the present invention.

Method for producing a structural steel according to an embodiment of the present invention is (a) wt%, carbon (C): 0.08 ~ 0.16%, silicon (Si): 0.25 ~ 0.35%, manganese (Mn): 1.4 ~ 1.6% , Solid solution aluminum (S-Al): 0.02 ~ 0.05%, niobium (Nb): 0.025 ~ 0.035%, titanium (Ti): 0.01 ~ 0.02%, copper (Cu): 0.05 ~ 0.15%, nickel (Ni): 0.05 Providing a thick plate casting material consisting of 0.15% and the remaining iron (Fe) and unavoidable impurities (S100); (b) reheating the cast material at a temperature of 1040 to 1120 ° C. (S200); And (c) rolling the reheated cast material under a condition that a finish rolling temperature is 800 to 840 ° C. (S300).

The cast material having the composition described above in step (a) S100 may include carbon (C), manganese (Mn), copper (Cu), and nickel (Ni) in a range satisfying Equation 1 below.

Equation 1: 0 <[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 ≤ 0.39

Where [] is the weight percent of each element

In the step (c) (S300), the rolling may proceed first in the recrystallization zone, which is a temperature section higher than the recrystallization end temperature, and then may proceed secondly in the unrecrystallized zone, which is a temperature section lower than the recrystallization end temperature, and finish rolling temperature. It may be necessary to control below (A3 + 20) ° C.

According to the above-described method for manufacturing structural steel, the reduction of carbon content and the addition of niobium and titanium components are applied in the steelmaking step to refine the grain size of the steel, improve impact toughness, and enhance the binding strength of grain boundaries. Furthermore, the rolling method was controlled to contribute to the refinement of grain size.

Hereinafter, preferred experimental examples are provided 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 to the following experimental examples.

Experimental Example 1

C Si Mn S-Al Nb Ti Cu Ni Ceq. Comparative Example 1 0.155 0.4 1.5 0.03 - - 0.1 0.1 0.418 Example 1 0.121 0.288 1.457 0.041 0.028 0.016 0.095 0.09 0.376

Reheating Temperature (℃) Finish rolling temperature (℃) Control cooling Rolling reduction (%) Comparative Example 1 1150 870 Air cooling 45 Example 1 1120 820 Air cooling 45

Table 1 is a table showing the composition of the casting material according to the first experimental example of the present invention. Table 2 is a table showing a processing method according to the first experimental example of the present invention, Figure 2 is a diagram schematically illustrating the processing method according to the first experimental example of the present invention.

Referring to Table 1, Table 2 and Figure 2, the specimen according to Comparative Example 1 of the present invention, by weight, carbon (C): 0.155%, silicon (Si): 0.4%, manganese (Mn): 1.5% , Casting aluminum alloy (S-Al): 0.03%, copper (Cu): 0.1%, nickel (Ni): 0.1% and the balance of iron (Fe) at a reheating temperature (SRT) of 1150 ℃ After reheating, the finish rolling temperature (FRT) is a structural steel material implemented by rolling and air-cooled under a condition of 870 ℃. The rolling was first performed in the recrystallization zone, which is a temperature section higher than the recrystallization end temperature, and then secondly proceeded in the unrecrystallized zone, which is a temperature section lower than the recrystallization end temperature, and the reduction ratio was performed under a condition of 45%.

In contrast, the specimen according to Example 1 of the present invention, in weight%, carbon (C): 0.121%, silicon (Si): 0.288%, manganese (Mn): 1.457%, solid solution aluminum (S-Al): 0.041%, Niobium (Nb): 0.028%, Titanium (Ti): 0.016%, Copper (Cu): 0.095%, Nickel (Ni): 0.09% and the remainder of the cast material consisting of iron (Fe) ) Is a structural steel material that is reheated under the condition of 1120 ° C., followed by rolling and air-cooling under the condition that the finish rolling temperature (FRT) is 820 ° C. The rolling was first performed in the recrystallization zone, which is a temperature section higher than the recrystallization end temperature, and then secondly proceeded in the unrecrystallized zone, which is a temperature section lower than the recrystallization end temperature, and the reduction ratio was performed under a condition of 45%. In particular, in Example 1, the difference (ΔT) between the finish rolling temperature and the A3 temperature was controlled to 20 ° C or less.

Tensile Strength (MPa) Yield strength (MPa) Impact energy (J) Comparative Example 1 492 361 38-76 Example 1 526 410 243 ~ 275

Table 3 is a table showing the steel properties after rolling in the first experimental example of the present invention according to Table 1 and Table 2. Referring to Table 3, the steel material after rolling implemented under the conditions of Comparative Example 1 of the present invention according to Table 1 and Table 2 has a yield strength of 361 MPa, a tensile strength of 492 MPa, and an impact energy (J) of 38 to 76 J. Steel material after rolling implemented under the conditions of Example 1 of the present invention according to Table 1 and Table 2 showed a yield strength of 410 MPa, a tensile strength of 526 MPa, and an impact energy (J) of 243 to 275 J.

Unlike Comparative Example 1, Example 1 of the present invention has a relatively low carbon content, and adds niobium and titanium, but improves the grain refining effect by controlling the sum of the niobium content and the titanium content to be 0.055% by weight or less. It can be seen that the strength is relatively high and the impact energy is relatively high.

Experimental Example 2

C Si Mn S-Al Nb Ti Cu Ni Ceq. Comparative Example 2 0.155 0.4 1.5 0.03 - - 0.1 0.1 0.418 Example 2 0.121 0.288 1.457 0.041 0.028 0.016 0.095 0.09 0.376

Thickness (mm) Width (mm) Reheating Temperature (℃) Finish rolling temperature (℃) Comparative Example 2 65 2500 1137 875 Example 2 65 2500 1109 824

Table 4 is a table showing the composition of the casting material according to the second experimental example of the present invention. Table 5 is a table showing a process method according to a second experimental example of the present invention.

Referring to Table 4 and Table 5, the specimen according to Comparative Example 2 of the present invention, in weight%, carbon (C): 0.155%, silicon (Si): 0.4%, manganese (Mn): 1.5%, solid solution aluminum (S-Al): 0.03%, Copper (Cu): 0.1%, Nickel (Ni): 0.1%, and the remainder of the cast material having a thickness of 65mm and width 2500mm, reheating temperature (SRT) of 1137 ℃ It is a structural steel material implemented by reheating on phosphorus conditions and rolling on the conditions of finishing rolling temperature (FRT) of 875 degreeC. The rolling was first performed in the recrystallization zone, which is a temperature section higher than the recrystallization end temperature, and then secondarily in the unrecrystallized zone, which is a temperature section lower than the recrystallization end temperature.

In contrast, the specimen according to Example 2 of the present invention, in weight%, carbon (C): 0.121%, silicon (Si): 0.288%, manganese (Mn): 1.457%, solid solution aluminum (S-Al): 0.041%, niobium (Nb): 0.028%, titanium (Ti): 0.016%, copper (Cu): 0.095%, nickel (Ni): 0.09%, and the balance is 65 mm thick, 2500 mm wide, consisting of iron (Fe) It is a structural steel material which was implemented by reheating the ash at a condition of reheating temperature (SRT) of 1109 ° C. and then rolling at a finish rolling temperature (FRT) of 824 ° C. The rolling was first performed in the recrystallization zone, which is a temperature section higher than the recrystallization end temperature, and then secondarily in the unrecrystallized zone, which is a temperature section lower than the recrystallization end temperature. In particular, in Example 2, the difference (ΔT) between the finish rolling temperature and the A3 temperature was controlled to 20 ° C or less.

Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Comparative Example 2 PWHT exhibition 387 498 31 Comparative Example 2 After PWHT 351 471 30 Example 2 PWHT exhibition 410 526 33 Example 2 After PWHT 389 497 31

Table 6 is a table showing the physical properties of the structural steel before and after post-weld heat treatment (PWHT) in the second experimental example of the present invention according to Table 4 and Table 5.

First, the post-weld heat treatment (PWHT) process will be described. 3 is a diagram illustrating a heat treatment process of post-weld heat treatment (PWHT) in the second experimental example of the present invention. Referring to FIG. 3, post-weld heat treatment (PWHT) is a heat treatment that removes residual stresses associated with the welding process, and includes heat treatment for 145 minutes at a temperature of 610 ° C. FIG. The heating rate from 400 ° C. to 610 ° C. is 60 ° C./Hr, and the cooling rate is −60 ° C./Hr after holding at 610 ° C.

Referring back to Table 6, the structural steel material implemented under the conditions of Comparative Example 2 of the present invention has a yield strength of 387 MPa before PWHT and a yield strength of 351 MPa less than 355 MPa after PWHT. It was found that the tensile strength before PWHT was 498 MPa and the tensile strength after PWHT was 471 MPa less than 490 MPa. In contrast, the structural steel material implemented under the conditions of Example 2 of the present invention has a yield strength of 410 MPa before PWHT and a yield strength of 389 MPa greater than 355 MPa after PWHT, and PWHT Tensile strength before) was 526 MPa and tensile strength after post-weld heat treatment (PWHT) was 497 MPa greater than 490 MPa.

4 is a photograph of the crystal grains of the structural steel according to Comparative Example 2 of the second experimental example of the present invention according to Table 4 and Table 5, Figure 5 is a second experiment of the present invention according to Table 4 and Table 5 It is the photograph which photographed the crystal grain shape of the structural steel by Example 2 among examples.

Referring to FIG. 4, the structural steel according to Comparative Example 2 of the present invention includes a ferrite and a pearlite structure, but the average grain size was found to be 50 to 60 μm. On the contrary, referring to FIG. 5, the structural steel according to Example 2 of the present invention includes ferrite and pearlite tissues, but the average grain size was found to be 10 to 15 μm. More fineness can be seen.

Table 7 and FIG. 6 are tables and graphs showing Charpy impact energy before and after post-welding heat treatment (PWHT) of structural steel according to Comparative Example 2 of the second experimental example of the present invention according to a temperature section. Tables 8 and 7 Is a table and graph showing Charpy impact energy before and after post-welding heat treatment (PWHT) of the structural steel according to Example 2 of the second experimental example of the present invention according to the temperature section.

Temperature (℃) Impact value 1 (J) Impact value 2 (J) Impact value 3 (J) Impact value average (J) Comparative Example 2 PWHT exhibition -0 195 203 277 225 Comparative Example 2 PWHT exhibition -20 184 164 201 183 Comparative Example 2 PWHT exhibition -40 168 157 125 150 Comparative Example 2 PWHT exhibition -50 136 114 96 115 Comparative Example 2 PWHT exhibition -60 28 46 122 65 Comparative Example 2 After PWHT -0 173 194 234 200 Comparative Example 2 After PWHT -20 51 134 187 124 Comparative Example 2 After PWHT -40 123 37 137 99 Comparative Example 2 After PWHT -50 51 23 37 37 Comparative Example 2 After PWHT -60 38 16 27 27

Temperature (℃) Impact value 1 (J) Impact value 2 (J) Impact value 3 (J) Impact value average (J) Example 2 PWHT exhibition -0 281 278 284 281 Example 2 PWHT exhibition -20 273 291 277 280 Example 2 PWHT exhibition -40 280 244 267 264 Example 2 PWHT exhibition -50 268 241 246 252 Example 2 PWHT exhibition -60 245 213 224 227 Example 2 After PWHT -0 252 265 268 262 Example 2 After PWHT -20 280 282 261 274 Example 2 After PWHT -40 264 274 263 267 Example 2 After PWHT -50 199 189 222 203 Example 2 After PWHT -60 194 162 224 193

6, 7 and Table 7, Table 8, in the case of Comparative Example 2 of the present invention, the results before and after the post-weld heat treatment (PWHT) can be confirmed that the result of the sharp drop as the temperature decreases In the case of Example 2 of the invention it can be confirmed a good result even after the post-weld heat treatment (PWHT).

Specifically, it was confirmed that the Charpy impact energy before the post-weld heat treatment (PWHT) in Example 2 of the present invention is relatively higher at each temperature than the Charpy impact energy before the post-weld heat treatment (PWHT) in Comparative Example 2 of the present invention. Can be. In addition, it can be seen that the Charpy impact energy after the post-weld heat treatment (PWHT) in Example 2 of the present invention is relatively higher at each temperature than the Charpy impact energy after the post-weld heat treatment (PWHT) in Comparative Example 2 of the present invention. .

Furthermore, in Comparative Example 2 of the present invention, the difference value of Charpy impact energy before and after post-welding heat treatment (PWHT) in Example 2 of the present invention is relatively smaller than the difference value of Charpy impact energy before and after post-welding heat treatment (PWHT). can confirm. That is, in Example 2 than the comparative example 2 it can be seen that the variation of the Charpy impact energy before and after the post-weld heat treatment (PWHT) is relatively smaller.

Meanwhile, in Comparative Example 2 of the present invention, Charpy Impact Energy of -20 ° C is 183J and 124J smaller than 270J before and after PWHT, and -20 in Example 2 of the present invention. The average Charpy Impact Energy of ° C is 280J and 274J, which is greater than 270J before and after PWHT. That is, it can be seen that the structural steels implemented in Example 2 are more excellent in low temperature toughness than Comparative Example 2.

So far, the structural steel and the manufacturing method thereof according to the embodiment of the present invention have been described. According to an embodiment of the present invention, the steel content is reduced by reducing the carbon content in the existing component system, adding grain refinement elements such as niobium and titanium, and lowering the slab reheating temperature (SRT) and rolling finish temperature (FRT). It was possible to realize the optimization of miniaturization, through which the structural steels with fine grains (average grain size: 10 ~ 15㎛) were produced, and the steels with excellent material and impact test results were compared with the comparative examples. In addition, it was confirmed that the material after the post-weld heat treatment (PWHT), which is necessarily accompanied in the welding process, can be guaranteed. Furthermore, by reducing the carbon, which significantly affects the carbon equivalent, and adding niobium and titanium, the total carbon equivalent could be lowered (below 0.39), and this would have a positive effect on weldability. That is, according to the embodiment of the present invention, the carbon equivalent is low and the material and low-temperature toughness can be realized even after the post-weld heat treatment (PWHT) structural steel sheet.

It is to be understood that the present invention encompasses not only the disclosed embodiments, but also various modifications and equivalent other embodiments that can be derived from those disclosed by those skilled in the art. Therefore, the technical protection scope of the present invention will be defined by the claims below.

Claims (7)

delete delete delete delete (a)% by weight, carbon (C): 0.08 to 0.16%, silicon (Si): 0.25 to 0.35%, manganese (Mn): 1.4 to 1.6%, solid solution aluminum (S-Al): 0.02 to 0.05%, Niobium (Nb): 0.025 ~ 0.035%, Titanium (Ti): 0.01 ~ 0.02%, Copper (Cu): 0.05 ~ 0.15%, Nickel (Ni): 0.05 ~ 0.15% and the rest of iron (Fe) and inevitable impurities Providing a thick plate casting;
(b) reheating the cast material at a temperature of 1040 to 1120 ° C .;
(c) performing the first rolling of the reheated cast material in the recrystallization zone, which is a temperature section higher than the recrystallization end temperature, and then performing the second rolling, the finish rolling, in the unrecrystallized zone, which is a temperature section lower than the recrystallization finish temperature. step; And
(d) air cooling after the secondary rolling,
Finish rolling temperature is below (A3 + 20) ℃, and does not undergo reheating heat treatment after air cooling
Method of manufacturing structural steels. The method of claim 5,
In the step (a), the casting material comprises carbon (C), manganese (Mn), copper (Cu) and nickel (Ni) in a range that satisfies the following Equation 1, manufacturing method of structural steel .
Equation 1: 0 <[C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 ≤ 0.39
Where [] is the weight percent of each element delete

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