KR102607616B1 - Rod steel and method of manufacturing the same - Google Patents

Abstract

The present invention provides carbon (C): 0.06 to 0.11% by weight, silicon (Si): more than 0 and less than 0.25% by weight, manganese (Mn): 1.5 to 2.5% by weight, phosphorus (P): more than 0 and less than 0.01% by weight, sulfur (S): 0 to 0.01% by weight, aluminum (Al): 0.01 to 0.03% by weight, nickel (Ni): 0.70 to 1.50% by weight, molybdenum (Mo): 0.010 to 0.125% by weight, chromium (Cr): 0 Exceeding 0.25% by weight or less, Copper (Cu): exceeding 0 and not exceeding 0.28% by weight, Niobium (Nb): 0.005 to 0.06% by weight, Nitrogen (N): exceeding 0 and not exceeding 0.01% by weight, and the remaining iron (Fe) and inevitable impurities. It provides a steel bar, characterized in that it has a room temperature yield strength of 318 MPa or more.

Description

Steel bar and its manufacturing method {ROD STEEL AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to steel and a method of manufacturing the same, and more specifically, to a steel bar and a method of manufacturing the same.

Recently, as the problem of greenhouse gases and fine dust has become serious at home and abroad, the power generation method using natural gas is increasing to reduce harmful atmospheric substances from the conventional power generation method using coal and nuclear power plants. In order to use natural gas, we manufacture insulated storage tanks that can be stored in a liquid state at temperatures below -170°C. When manufactured, deformed steel bars (rebar) that meet the safety and design standards for liquefied natural gas (LNG) storage tanks are used. is using . The design of the storage tank is a double protection type in which the outer tank can trap liquid spills if the inner tank is destroyed (within 6m between inner and outer tanks) and a full protection type with a concrete roof on the outer tank of the double protection type (within 2m between inner and outer tanks). It is divided into The double protection type has a roof over the inner tank, making it difficult to store and control evaporation gas in case of LNG leakage, while the full protection type is designed to control evaporation gas. In the case of the fully protected type, there is no need to construct an insulating material between the inside and outside of the storage tank, so in order to ensure safety, the deformed bar (rebar) must satisfy the physical properties at room temperature and the required physical properties even at -170℃. The important physical properties required at -170℃ (EN14620-3) require securing a uniform elongation of 3.0% or more and a notch sensitivity of 1.0 or more in unnotched specimens. The domestic design standard is a design suitable for deformed bar steel with a yield strength of 500 MPa at room temperature, so in the future, as natural gas usage gradually increases, the enlargement of storage tanks will inevitably be limited.

Republic of Korea Patent Publication No. 20130134332

The technical problem to be achieved by the present invention is to provide a steel bar that can secure toughness at ultra-low temperature (-170°C) and improve yield strength at room temperature and a manufacturing method thereof.

The steel bar according to an embodiment of the present invention to solve the above problem includes carbon (C): 0.06 to 0.11 wt%, silicon (Si): more than 0 to 0.25 wt% or less, manganese (Mn): 1.5 to 2.5 wt%, Phosphorus (P): more than 0 and less than 0.01% by weight, sulfur (S): more than 0 and less than 0.01% by weight, aluminum (Al): 0.01 to 0.03% by weight, nickel (Ni): 0.70 to 1.50% by weight, molybdenum (Mo) : 0.010 ~ 0.125% by weight, Chromium (Cr): More than 0 and less than 0.25% by weight, Copper (Cu): More than 0 and less than 0.28% by weight, Niobium (Nb): 0.005 ~ 0.06% by weight, Nitrogen (N): More than 0 and 0.01 It consists of less than % by weight and the remaining iron (Fe) and inevitable impurities, and is characterized by a room temperature yield strength of 318 MPa or more.

The steel bar satisfies a tensile strength (TS)/yield strength (YS) ratio of 1.08 or more and an elongation of 10% or more at room temperature, a uniform elongation of 3% or more in an unnotched specimen at -170 ℃, and a notched specimen at -170 ℃. The notch sensitivity ratio is 1.0 or more, and the notch sensitivity ratio may be the ratio of (tensile strength of the notched specimen) / (yield strength of the unnotched specimen).

The microstructure of the steel bar may include ferrite, bainite, and pearlite.

In the steel bar, the ferrite may be 60 to 80% in area fraction.

The method of manufacturing a steel bar according to an embodiment of the present invention to solve the above problem is (a) carbon (C): 0.06 to 0.11% by weight, silicon (Si): more than 0 and less than 0.25% by weight, manganese (Mn): 1.5 to 2.5% by weight, phosphorus (P): more than 0 but less than 0.01% by weight, sulfur (S): more than 0 and less than 0.01% by weight, aluminum (Al): 0.01 to 0.03% by weight, nickel (Ni): 0.70 to 1.50% by weight %, molybdenum (Mo): 0.010 ~ 0.125% by weight, chromium (Cr): more than 0 and less than 0.25% by weight, copper (Cu): more than 0 and less than 0.28% by weight, niobium (Nb): 0.005 ~ 0.06% by weight, nitrogen ( N): Reheating the cast steel at 1180 to 1220°C with more than 0 and less than 0.01% by weight and the remaining iron (Fe) and inevitable impurities; and (b) hot rolling the reheated cast steel at a finish rolling temperature of 970 to 1020°C.

In the method of manufacturing the steel bar, the steel bar produced after performing steps (a) and (b) has a yield strength of 318 MPa or more at room temperature and a ratio of tensile strength (TS)/yield strength (YS) of 1.08 or more. and an elongation of 10% or more, a uniform elongation of 3% or more in an unnotched specimen at -170 ℃, and a notch sensitivity ratio (Notch Sensitive Ratio) of 1.0 or more at -170 ℃, and the notch sensitivity ratio is (of the notched specimen) It can be the ratio of (tensile strength) / (yield strength of unnotched specimen).

In the method of manufacturing the steel bar, the steel bar produced after performing steps (a) and (b) has a microstructure of ferrite, bainite, and pearlite, and the ferrite is 60 to 80% by area fraction. You can.

According to an embodiment of the present invention, a steel bar that can secure toughness at ultra-low temperature (-170°C) and improve room temperature yield strength and a manufacturing method thereof can be implemented.

Of course, the scope of the present invention is not limited by this effect.

1 is a flowchart illustrating a method of manufacturing a steel bar according to an embodiment of the present invention.
Figure 2 is a photograph of the microstructure of the steel bar of Comparative Example 1 among the experimental examples of the present invention.
Figure 3 is a photograph of the microstructure of the steel bar of Comparative Example 2 among the experimental examples of the present invention.
Figure 4 is a photograph of the microstructure of the steel bar of Example 1 among the experimental examples of the present invention.
Figure 5 is a photograph of the microstructure of the steel bar of Example 2 among the experimental examples of the present invention.
Figure 6 is a photograph of the microstructure of the steel bar of Example 3 among the experimental examples of the present invention.

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 can be modified into various other forms, and the embodiments of the present invention 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. Therefore, the technical idea of the present invention is not limited by the relative sizes or spacing drawn in the attached drawings.

Hereinafter, the present invention will be described in detail. At this time, when describing the present invention, if it is determined that a detailed description of related known technology or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

In addition, the terms described below are terms defined in consideration of the functions in the present invention, and may vary depending on the intention or custom of the user or operator, so the definitions should be made based on the content throughout the specification explaining the present invention.

The steel bar according to the embodiment of the present invention described below is a steel material before applying the temp core process for manufacturing reinforcing bars.

steel bar

One embodiment of the present invention is carbon (C): 0.06 to 0.11% by weight, silicon (Si): more than 0 to 0.25% by weight, manganese (Mn): 1.5 to 2.5% by weight, phosphorus (P): more than 0 and 0.01% by weight. % or less, sulfur (S): more than 0 and less than 0.01% by weight, aluminum (Al): 0.01 to 0.03% by weight, nickel (Ni): 0.70 to 1.50% by weight, molybdenum (Mo): 0.010 to 0.125% by weight, chromium ( Cr): more than 0 and not more than 0.25% by weight, copper (Cu): more than 0 but not more than 0.28% by weight, niobium (Nb): 0.005 to 0.06% by weight, nitrogen (N): more than 0 and not more than 0.01% by weight, and the remaining iron (Fe) and unavoidable impurities.

The steel bar has a yield strength of 318 MPa or more at room temperature, a tensile strength (TS)/yield strength (YS) ratio of 1.08 or more, and an elongation of 10% or more, and a uniform elongation of 3% or more in an unnotched specimen at -170°C. , the notch sensitivity ratio may be 1.0 or more at -170°C. The notch sensitivity rate may be the ratio of (tensile strength of the notched specimen) / (yield strength of the unnotched specimen).

The microstructure of the steel bar may include ferrite, bainite, and pearlite, and the ferrite may be 60 to 80% in area fraction.

Hereinafter, the role and content of each component included in the steel bar according to the present invention will be described as follows.

Carbon (C)

In the present invention, carbon (C) is added to secure the strength and hardness of steel. Typically, carbon (C) is dissolved in austenite to form a martensite structure upon quenching. Additionally, typically, quenched hardness improves as the amount of carbon increases, but deformation may occur due to rapid cooling or steel elongation and low-temperature toughness may decrease. The carbon (C) is added in an amount of 0.06% to 0.11% by weight of the total weight of the steel bar for the purpose of securing cryogenic toughness and ductility. If the carbon content is less than 0.06% by weight, it is difficult to secure sufficient strength. Conversely, if the carbon (C) content exceeds 0.11% by weight, the strength of the steel increases, but it may be difficult to secure sufficient elongation and low-temperature toughness.

Silicon (Si)

In the present invention, silicon (Si), which remains from pig iron and deoxidizer, is dissolved in ferrite unless it forms a compound such as SiO2, so it does not significantly affect the mechanical properties of steel. Silicon is added as a deoxidizer to remove oxygen in steel during the steelmaking process. In addition, silicon (Si) is a ferrite stabilizing element that has a solid solution strengthening effect and is effective in improving the toughness and ductility of steel by inducing ferrite formation. The silicon (Si) is added in an amount of more than 0 and less than 0.25% by weight of the total weight of the steel bar. On the other hand, when the content of silicon (Si) exceeds 0.25% by weight, there is a problem in that oxides are formed on the surface of the steel, thereby reducing the ductility of the steel.

Manganese (Mn)

Manganese (Mn) is an element that increases the strength and toughness of steel and increases the hardenability of steel. Adding manganese impairs the acid resistance and oxidation resistance of steel, but improves yield strength by making pearlite finer and strengthening ferrite in solid solution. Increases hardening depth during quenching. In addition, when added as an austenite stabilizing element from 1.4 to 2.0% by weight, it is advantageous for forming acicular ferrite and bainite. In this example, the manganese is added in an amount of 1.5 to 2.5% by weight of the total weight of the steel bar. If the manganese content is less than 1.5% by weight, there may be difficulties in securing strength. On the other hand, if the manganese content exceeds 2.5% by weight, the strength increases, but the amount of MnS-based non-metallic inclusions increases, which may cause defects such as cracks during welding. Additionally, manganese is an austenite stabilizing element, and when added at 1.5 to 2.5% by weight, it may be advantageous for the formation of ferrite and bainite. Accordingly, a microstructure advantageous for cryogenic toughness according to an embodiment of the present invention can be formed.

Phosphorus (P)

Phosphorus (P) is an element that partially contributes to strength improvement, but when excessively included, it deteriorates the ductility of steel and causes final material deviation due to segregation at the center of the billet. P is not a big problem if it is distributed uniformly in the steel, but it usually forms harmful compounds of Fe3P. This Fe3P is extremely brittle and segregated, so it is not homogenized even when annealed and is elongated during processing such as forging and rolling. The phosphorus (P) content is limited to 0.01% by weight of the total weight of the steel bar. If the phosphorus (P) content exceeds 0.01% by weight, central segregation and micro-segregation may be formed, which may have a negative effect on the material and may also deteriorate ductility and molding properties.

Hwang (S)

Sulfur (S) is an element that partially contributes to improving machinability, but when excessively contained, it impairs the toughness and ductility of steel, and combines with manganese to form MnS non-metallic inclusions, causing cracks during processing of steel. Sulfur can combine with iron to form FeS if the amount of manganese in the steel is insufficient. Since FeS is very brittle and has a low melting point, cracks may occur during hot and cold working. The sulfur (S) content is limited to 0.01% by weight of the total weight of the steel bar. If the sulfur (S) content exceeds 0.01% by weight, ductility is greatly impaired and MnS non-metallic inclusions are excessively generated.

Aluminum (Al)

Aluminum (Al) can function as a deoxidizer. Aluminum (Al) may be added in an amount of 0.01 to 0.03% by weight of the total weight of the steel bar. If the aluminum content is added at less than 0.01% by weight, it is difficult to sufficiently achieve the above effect. On the other hand, if the aluminum content exceeds 0.03% by weight, the amount of non-metallic inclusions such as aluminum oxide (Al ₂ O ₃ ) may increase.

Nickel (Ni)

Nickel (Ni) is the most important and common alloying element along with chromium. It is used to strengthen the base because it refines the grain of steel and is well dissolved in austenite and ferrite. When coexisted with chromium or molybdenum, it exhibits excellent hardenability and facilitates heat treatment of large steel materials. It strengthens the low-temperature toughness of steel and does not impair weldability or malleability. It increases the strength of the material and ensures low-temperature impact value. The nickel is added in an amount of 0.70 to 1.50% by weight of the total weight of the steel bar. However, if the nickel content is added at less than 0.70% by weight, it is difficult to achieve the above-mentioned purpose. On the other hand, if the nickel content exceeds 1.50% by weight, manufacturing costs increase, and room temperature strength may increase excessively, resulting in deterioration of weldability and toughness.

Molybdenum (Mo)

Molybdenum (Mo) improves the strength, toughness, and hardenability of steel. Since molybdenum forms carbides, it is excellent as an alloy element for high-end cutting tools and increases the grain coarsening temperature. The effect is better when used together with chromium rather than used alone to improve hardenability, but the disadvantage is that it is expensive. The molybdenum is added in an amount of 0.010 to 0.125% by weight of the total weight of the steel bar. If the molybdenum content is less than 0.010% by weight, it is difficult to achieve the above-mentioned effect. On the other hand, when the molybdenum content exceeds 0.125% by weight, manufacturing costs increase and weldability is reduced.

Chrome (Cr)

Chromium (Cr) is a ferrite stabilizing element that, when added to C-Mn steel, delays the diffusion of carbon due to the solute interference effect and affects grain size refinement. Chromium can improve hardenability by improving the hardenability of steel. Additionally, chromium can delay the diffusion of carbon and achieve particle size refinement. The chromium is added in an amount exceeding 0.25% by weight of the total weight of the steel bar. If the chromium content exceeds 0.25% by weight, there is a disadvantage in that weldability or heat-affected zone toughness may be reduced.

Copper (Cu)

Copper (Cu) is dissolved in ferrite by up to 0.35% at room temperature and has a solid solution strengthening effect, slightly improving strength and hardness, but lowering elongation. Copper can play a role in improving the hardenability and low-temperature impact toughness of steel. Additionally, it can increase the corrosion resistance of steel in the atmosphere or seawater. Copper (Cu) is limited to a content of more than 0 and less than 0.28% by weight of the total weight of the steel bar. If the copper content exceeds 0.28% by weight, hot workability may be reduced and red heat embrittlement may occur.

Niobium (Nb)

Niobium (Nb) is an element that precipitates at the interface and easily produces carbides or nitrides. The pinning effect caused by these precipitates can suppress the growth of crystal grains and refine them. By precipitating in the form of NbC or Nb(C,N), the strength of the base material and weld zone can be improved. Niobium (Nb) is limited to a content of 0.005 to 0.06% by weight of the total weight of the steel bar. If the niobium (Nb) content is less than 0.005% by weight, the above-described addition effect cannot be expected, and if it is higher than 0.06% by weight, the precipitates become coarse, the manufacturing cost of parts increases, brittle cracks occur, and the carbon content in the matrix increases. This may cause problems such as deterioration of steel characteristics.

Nitrogen (N)

Nitrogen (N), even in extremely small amounts, has a significant impact on the mechanical properties of steel. It increases tensile strength and yield strength, while decreasing elongation. In particular, the reduction in impact value and increase in transition temperature are significant. The austenite crystal grains become fine, making it possible to manufacture fine-grained steel, and the crystal grains are refined by forming nitrides such as niobium. However, when present in large amounts, high-temperature toughness is reduced, grain boundary embrittlement is caused by AlN precipitation at austenite grain boundaries, and high-temperature creep strength can be reduced. . Nitrogen can increase yield strength and tensile strength. Nitrogen refines the austenite grains, making it possible to manufacture steel with fine grains. However, when adding a large amount exceeding 0.01%, there is a problem in that the amount of nitrogen increases and the elongation and formability of the steel decrease. Therefore, it is preferable to add more than 0 and less than 0.01% by weight of the total weight of the steel bar.

In addition to the components of the alloy composition described above, the remainder consists of iron (Fe) and impurities that are inevitably included in the steelmaking process.

Manufacturing method of steel bar

Hereinafter, a method for manufacturing a steel bar according to an embodiment of the present invention will be described.

1 is a flowchart illustrating a method of manufacturing a steel bar according to an embodiment of the present invention.

Referring to Figure 1, the method for manufacturing a steel bar according to an embodiment of the present invention is (a) carbon (C): 0.06 to 0.11 wt%, silicon (Si): 0 to 0.25 wt%, manganese (Mn): 1.5 ~ 2.5% by weight, phosphorus (P): more than 0 but less than 0.01% by weight, sulfur (S): more than 0 and less than 0.01% by weight, aluminum (Al): 0.01 ~ 0.03% by weight, nickel (Ni): 0.70 ~ 1.50% by weight , molybdenum (Mo): 0.010 to 0.125 wt%, chromium (Cr): more than 0 to 0.25 wt% or less, copper (Cu): more than 0 to 0.28 wt% or less, niobium (Nb): 0.005 to 0.06 wt%, nitrogen (N): Reheating the cast steel at 1180 to 1220°C (S100), including more than 0 and less than 0.01% by weight and the remaining iron (Fe) and inevitable impurities; and (b) hot rolling the reheated cast steel at a finish rolling temperature of 970 to 1020°C (S200).

reheating step

In the reheating step of the cast steel, the cast steel having the above composition is reheated at a temperature range of 1180 to 1220°C. Through this reheating, re-dissolution of components segregated during casting and re-dissolution of precipitates may occur. The cast steel may be a bloom or billet manufactured through a continuous casting process performed before the reheating step (S100).

If the reheating temperature of the cast steel is less than 1180°C, the heating temperature is not sufficient and re-dissolution of the segregated components and precipitates may not occur sufficiently. Additionally, there is a problem that the rolling load increases. Conversely, when the reheating temperature exceeds 1220°C, austenite grains may coarsen or decarburization may occur, thereby reducing strength.

hot rolling

In the hot rolling step (S200), the reheated cast steel is hot rolled at a finish rolling temperature of 970 to 1020°C to manufacture reinforcing bars. The finish rolling temperature may be a temperature higher than the Ar3 and Ac3 transformation points, which are austenite non-recrystallization temperatures.

If the finish rolling temperature exceeds 1020°C, it may become difficult to secure strength due to the formation of coarse pearlite. Conversely, if the finish rolling temperature is performed below 970°C, rolling load may be induced, which may reduce productivity and reduce the heat treatment effect.

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.

Table 1 shows the alloy element composition (unit: weight %, but nitrogen is ppm) of the specimens of this experimental example.

C Si Mn P S Al Cr Cu Ni Mo Nb N
[ppm] Fe Comparative Example 1 0.27 0.15 0.77 0.01 0.01 0.015 0.07 0.25 0.02 0.01 - 0.01 Bal. Comparative example 2 0.07 0.15 1.85 0.01 0.01 0.015 0.08 0.24 1.05 0.12 - 0.01 Bal. Example 1 0.06 0.15 1.93 0.01 0.01 0.015 0.10 0.15 1.02 0.03 0.016 0.01 Bal. Example 2 0.06 0.15 1.96 0.01 0.01 0.015 0.20 0.11 1.01 0.02 0.015 0.01 Bal. Example 3 0.06 0.15 2.12 0.01 0.01 0.015 0.20 0.11 0.72 0.02 0.040 0.01 Bal.

Referring to Table 1, Examples 1 to 3 include carbon (C): 0.06 to 0.11 wt%, silicon (Si): more than 0 and 0.25 wt% or less, manganese (Mn): 1.5 to 2.5 wt%, phosphorus ( P): more than 0 and 0.01% by weight or less, sulfur (S): more than 0 and 0.01% by weight or less, aluminum (Al): 0.01 to 0.03% by weight, nickel (Ni): 0.70 to 1.50% by weight, molybdenum (Mo) : 0.010 ~ 0.125% by weight, Chromium (Cr): More than 0 and less than 0.25% by weight, Copper (Cu): More than 0 and less than 0.28% by weight, Niobium (Nb): 0.005 ~ 0.06% by weight, Nitrogen (N): More than 0 and 0.01 It satisfies the composition range of less than % by weight and the remaining iron (Fe).

On the other hand, Comparative Example 1 was not satisfied as it exceeded the range of carbon (C): 0.06 to 0.11 wt%, manganese (Mn): 1.5 to 2.5 wt%, and nickel (Ni): 0.70 to 1.50 wt%. I am not satisfied with the performance. Furthermore, Comparative Examples 1 and 2 do not contain niobium (Nb) and do not satisfy the range of niobium (Nb): 0.005 to 0.06% by weight.

Table 2 shows the material evaluation results of the specimens in this experimental example. The 'YS' item means the yield strength, the 'TS' item means the tensile strength, the 'EL' item means the elongation, and the 'NSR' item is the Notch Sensitive Ratio (of the notched specimen). It is a value calculated as the ratio of (tensile strength) and (yield strength of unnotched specimen). The 'un' sign indicates an un-notch specimen, and the 'n' sign indicates a notch specimen. Room temperature is 25℃, and cryogenic temperature is -170℃. In order to implement the specimens of this experimental example, the conditions of reheating temperature: 1200°C and rolling end temperature: 1000°C were applied to the steel material having the composition shown in Table 1.

Room temperature YS, MPa room temperature
TS, MPa room temperature
EL,% cryogenic
YS_un, MPa cryogenic
UE_un, % cryogenic
TS_n, MPa cryogenic
NSR Comparative Example 1 285 430 36.2 476 23.4 443 0.93 Comparative example 2 288 442 40.5 500 25.8 624 1.25 Example 1 319 489 35.6 461 21.8 503 1.09 Example 2 391 630 26.7 606 19.7 667 1.10 Example 3 404 565 29.9 744 15.1 952 1.28

Referring to Table 2, Examples 1, 2, and 3 have a yield strength of 318 MPa or more at room temperature, a ratio of tensile strength (TS)/yield strength (YS) of 1.08 or more, and an elongation of 10% or more. , Uniform elongation of unnotched specimen at -170 ℃ is more than 3%, and notch sensitivity ratio is more than 1.0 at -170 ℃.

On the other hand, it can be confirmed that Comparative Example 1 and Comparative Example 2 are not satisfied as the yield strength at room temperature falls below the range of 318 MPa or more, and Comparative Example 1 is satisfied as the Notch Sensitive Ratio falls below the range of 1.0 or more. You can confirm that it cannot be done.

Figure 2 is a photograph of the microstructure of the steel bar of Comparative Example 1 among the experimental examples of the present invention, Figure 3 is a photograph of the microstructure of the steel bar of Comparative Example 2 among the experimental examples of the present invention, and Figure 4 is a photograph of the microstructure of the steel bar of Comparative Example 2 among the experimental examples of the present invention. Figure 5 is a photograph of the microstructure of the steel bar of Example 1 among the experimental examples of the present invention, Figure 5 is a photograph of the microstructure of the steel bar of Example 2 of the experimental examples of the present invention, and Figure 6 is an experimental example of the present invention. This is a photograph of the microstructure of the steel bar of Example 3.

Table 3 shows the ferrite area fraction (%) and average grain size (㎛) measured in FIGS. 2 to 6.

division Ferrite fraction [%] Grain size [㎛] Comparative Example 1 81.0 13.5 Comparative example 2 76.0 11.6 Example 1 63.5 10.2 Example 2 50.0 8.5 Example 3 72.5 9.8

Referring to Figures 2 to 6 and Table 3, it can be seen that the microstructure of the steel bar according to an embodiment of the present invention includes ferrite, bainite, and pearlite, and the ferrite is 60 to 80% in area fraction. Furthermore, it can be confirmed that the average grain size is about 10㎛ or less.

The present invention secures a composite structure of bainite, ferrite, and pearlite in the center of a deformed bar disclosed in existing technology, refines the structure to improve toughness and room temperature yield strength at ultra-low temperature (-170°C), and ferrite And a method of inducing the formation of bainite is proposed. Compared to the alloy composition system of the deformed steel bar (rebar) of the prior art, when the steel bar was manufactured and carried out under the conditions of a cross-sectional reduction rate of 80% and a rolling end temperature of 970 ~ 1020 ℃, the yield strength and tensile strength at room temperature were able to be improved by more than 100 MPa, -170 Even at ℃, it was possible to secure a yield strength of 1.15 or more, a notch susceptibility ratio (NSR) of 1.00 or more, and a uniform elongation (UE_un) of 3.0% or more compared to room temperature.

Although the above description focuses on the embodiments of the present invention, various changes and modifications can be made at the level of those skilled in the art. These changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of rights of the present invention should be determined by the claims described below.

Claims (7)

Carbon (C): 0.06 to 0.11 wt%, Silicon (Si): 0 to 0.25 wt%, Manganese (Mn): 1.5 to 2.5 wt%, Phosphorus (P): 0 to 0.01 wt% or less, sulfur (S) : More than 0 and less than 0.01% by weight, Aluminum (Al): 0.01 to 0.03% by weight, Nickel (Ni): 0.70 to 1.50% by weight, Molybdenum (Mo): 0.010 to 0.125% by weight, Chromium (Cr): More than 0.25% by weight % or less, copper (Cu): more than 0 and less than 0.28% by weight, niobium (Nb): 0.005 to 0.06% by weight, nitrogen (N): more than 0 and less than 0.01% by weight, and the remainder consists of iron (Fe) and inevitable impurities,
At room temperature, the ratio of tensile strength (TS)/yield strength (YS) is 1.08 or more, yield strength (YS) is 319 MPa to 404 MPa, tensile strength (TS) is 489 to 630 MPa, and elongation is 26.7% to 35.6%.
Yield strength (YS) 461~744 MPa at -170 ℃, tensile strength (TS) 503~952 MPa at -170 ℃, uniform elongation in unnotched specimen at -170 ℃ 15.1% ~ 21.8%, notch sensitivity at -170 ℃ (Notch Sensitive Ratio) is 1.0 or more, and the notch sensitivity ratio is the ratio of (tensile strength of notched specimen) / (yield strength of unnotched specimen),
The microstructure includes ferrite, bainite, and pearlite, wherein the ferrite is 60 to 80% by area fraction,
Rod steel. delete delete delete (a) Carbon (C): 0.06 to 0.11% by weight, Silicon (Si): more than 0 and less than 0.25% by weight, manganese (Mn): 1.5 to 2.5% by weight, phosphorus (P): more than 0 and less than 0.01% by weight, sulfur (S): 0 to 0.01% by weight, aluminum (Al): 0.01 to 0.03% by weight, nickel (Ni): 0.70 to 1.50% by weight, molybdenum (Mo): 0.010 to 0.125% by weight, chromium (Cr): 0 Exceeding 0.25% by weight or less, Copper (Cu): exceeding 0 and not exceeding 0.28% by weight, Niobium (Nb): 0.005 to 0.06% by weight, Nitrogen (N): exceeding 0 and not exceeding 0.01% by weight, and the remaining iron (Fe) and inevitable impurities. Reheating the formed cast steel at 1180 to 1220°C; and
(b) hot rolling the reheated cast steel at a finish rolling temperature of 970 to 1020°C;
The steel bar implemented after performing steps (a) and (b) is
At room temperature, the ratio of tensile strength (TS)/yield strength (YS) is 1.08 or more, yield strength (YS) is 319 MPa to 404 MPa, tensile strength (TS) is 489 to 630 MPa, and elongation is 26.7% to 35.6%.
Yield strength (YS) 461~744 MPa at -170 ℃, tensile strength (TS) 503~952 MPa at -170 ℃, uniform elongation in unnotched specimen at -170 ℃ 15.1% ~ 21.8%, notch sensitivity at -170 ℃ (Notch Sensitive Ratio) is 1.0 or more, and the notch sensitivity ratio is the ratio of (tensile strength of notched specimen) / (yield strength of unnotched specimen),
The microstructure includes ferrite, bainite, and pearlite, wherein the ferrite is 60 to 80% by area fraction,
Manufacturing method of steel bar.


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