KR20210142405A - Steel having excellent low-temperature fracture toughness and method of manufacturing the same - Google Patents

Abstract

The present invention performs a tempering treatment on steel after normalizing and cooling to increase toughness thereof, thereby providing the steel having excellent low-temperature fracture toughness. According to an embodiment, steel comprises, by wt%, carbon (C): 0.13 to 0.17%, silicon (Si): 0.15 to 0.40%, manganese (Mn): 1.00 to 1.30%, phosphorus (P): greater than 0 to 0.012%, sulfur (S): greater than 0 to 0.003%, soluble aluminum (S_Al): 0.015 to 0.05%, chromium (Cr): greater than 0 to 0.25%, nickel (Ni): 0.15 to 0.25%, titanium (Ti): greater than 0 to 0.01%, copper (Cu): 0.10 to 0.20%, niobium (Nb): 0.01 to 0.02%, molybdenum (Mo): greater than 0 to 0.08%, vanadium (V): greater than 0 to 0.010%, and the balance of iron (Fe) and unavoidable impurities, and satisfies 300 to 400 J and has a needle-like structure with a tensile strength (TS): 500 to 550 MPa, a yield strength (YS): 300 to 350 MPa, an elongation (EL): 30 to 40%, and a Charpy shock absorption energy: 0 to -60℃.

Description

Steel having excellent low-temperature fracture toughness and method of manufacturing the same

The technical idea of the present invention relates to a steel material and a method for manufacturing the same, and more particularly, to a steel material having excellent low-temperature fracture toughness and a method for manufacturing the same.

Structural steels 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, the toughness at low temperature as well as the strength of these steel materials for marine structures or tanks are very important. 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. is losing When the content of carbon and alloying elements is increased in order to improve the strength of the steel, there is a problem in that fracture toughness at low temperature is lowered.

Korea Patent Publication No. 2016-0079166

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

According to an embodiment of the present invention, the steel is, by weight%, carbon (C): 0.13% to 0.17%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.30%, Phosphorus (P): >0% to 0.012%, Sulfur (S): >0% to 0.003%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): >0% to 0.25%, Nickel ( Ni): 0.15% to 0.25%, Titanium (Ti): >0% to 0.01%, Copper (Cu): 0.10% to 0.20%, Niobium (Nb): 0.01% to 0.02%, Molybdenum (Mo): 0% Exceeds to 0.08%, Vanadium (V): Exceeds 0% to 0.010%, and the balance contains iron (Fe) and unavoidable impurities, Tensile Strength (TS): 500 MPa to 550 MPa, Yield Strength (YS): 300 MPa to 350 MPa, elongation (EL): 30% to 40%, and Charpy impact absorption energy: 300 J to 400 J at a temperature of 0° C. to -60° C., and may have a needle-like structure.

According to an embodiment of the present invention, it may have a carbon equivalent (Ceq) in the range of 0.31 to 0.48. (where Ceq = C + Mn/6 + (Ni + Cu)/15 + (Cr + Mo + V)/5)

According to an embodiment of the present invention, the method for manufacturing the steel is, by weight, carbon (C): 0.13% to 0.17%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.30%, Phosphorus (P): >0% to 0.012%, Sulfur (S): >0% to 0.003%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): >0% to 0.25% , Chromium (Cr): >0% to 0.25%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): >0% to 0.01%, Copper (Cu): 0.10% to 0.20%, Niobium (Nb) ): 0.01% to 0.02%, molybdenum (Mo): more than 0% to 0.08%, vanadium (V): more than 0% to 0.010%, and the balance is steel containing iron (Fe) and unavoidable impurities at 1,150 ° C. reheating at a temperature of 1,200 °C; rolling the heated steel to end at a temperature of 1,000 ℃ ~ 1,100 ℃; Normalizing the rolled steel at a temperature of 890 ℃ ~ 950 ℃; cooling the normalized steel material; and tempering the cooled steel at a temperature of 580°C to 640°C.

According to an embodiment of the present invention, the step of cooling the steel may be performed up to a cooling end temperature of 300 °C to 450 °C at a cooling rate of 5 °C/sec to 15 °C/sec.

According to an embodiment of the present invention, after performing the tempering step, the steel material is, tensile strength (TS): 500 MPa to 550 MPa, yield strength (YS): 300 MPa to 350 MPa, elongation (EL) : 30% to 40% and Charpy impact absorption energy: It satisfies 300 J to 400 J at a temperature of 0° C. to -60° C., and may have a needle-like structure.

According to the technical spirit of the present invention, it is possible to manufacture a steel material excellent in low-temperature fracture toughness by performing reheating, rolling, normalizing, cooling, and tempering with an appropriate alloy composition and a rolling and cooling pattern.

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 showing a method of manufacturing a steel material according to an embodiment of the present invention.
Figure 2 is a graph showing the Charpy impact absorption energy according to the temperature of the steel according to an embodiment of the present invention.
Figure 3 is a photomicrograph showing the shape of the fracture surface after the Charpy impact test of the steel according to an embodiment of the present invention.
4 is a photomicrograph showing the microstructure of a steel material 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 the present 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 improved low-temperature impact toughness through control of the alloy composition and heat treatment conditions of the steel material and a method for manufacturing the same.

Conventional 50kg-class low-temperature toughness-guaranteed steel for pressure vessels is generally heat-treated steel (A516-70-N/A537-C1) tempered after normalizing has been used. When carbon and alloy elements for high strength are added to meet the demand for improvement of high strength and low temperature toughness according to the harshness and enlargement of the pressure vessel environment, strength improvement can be realized, but there is a problem in that impact toughness is reduced. In particular, the impact toughness at -40°C or lower was very weak.

The present inventors recognize that the change in the physical properties of the 50Kg class low-temperature toughness guarantee pressure vessel has a close relationship with the change in the microstructure, and reduce the carbon equivalent (CEQ) to increase the low-temperature impact toughness, but change the manufacturing method It was possible to obtain the effect of reducing the DBTT temperature and increasing the impact toughness (150~200J@-60℃) by controlling the microstructure through the

Specifically, the reduction in strength due to reduction of carbon and alloy elements while suppressing the generation of pearlite bands, which is a cause of deterioration of toughness by reducing carbon equivalent, and reducing the size, while improving toughness, the reduction of strength due to reduction of carbon and alloying elements through optimization of cooling conditions and tempering conditions, microstructure refinement and Compensation was achieved through the generation of a low-temperature transformation phase.

Hereinafter, a steel material having excellent low-temperature impact toughness, which is an aspect of the present invention, will be described.

low temperature impact toughness excellent steel

The steel material having excellent low-temperature impact toughness, which is an aspect of the present invention, is in weight %, carbon (C): 0.13% to 0.17%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.30%, Phosphorus (P): >0% to 0.012%, Sulfur (S): >0% to 0.003%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): >0% to 0.25%, Nickel ( Ni): 0.15% to 0.25%, Titanium (Ti): >0% to 0.01%, Copper (Cu): 0.10% to 0.20%, Niobium (Nb): 0.01% to 0.02%, Molybdenum (Mo): 0% Exceeds to 0.08%, vanadium (V): contains more than 0% to 0.010%. The remainder consists of iron (Fe) and impurities that are unavoidably contained in the steelmaking process.

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

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

The carbon equivalent of the high-strength steel material according to the present invention may be, for example, in the range of 0.31 to 0.48, for example, in the range of 0.31 to 0.43.

Hereinafter, the role and content of each component included in the steel having excellent low-temperature impact toughness according to the present invention will be described. In this case, the content of the component elements all mean wt%.

carbon (C)

Carbon (C) is an essential element for imparting high strength to steel, and is a major element that enhances the hardenability of the steel and determines the strength after quenching. Depending on the content and manufacturing method of carbon (C), it may become solid solution carbon in the material structure, and it is combined with elements with very high properties to bond with carbon (C) to form carbides. In order to obtain this effect, 0.10 wt% or more, preferably 0.13 wt% or more of the total weight of the steel is included. However, if the content of carbon (C) is excessive, since weldability, low-temperature impact toughness, elongation, corrosion resistance, etc. are deteriorated, it is preferable to control the content to 0.17 wt% or less.

silicon( Si )

Silicon (Si) is added as a deoxidizer to remove oxygen in steel during the steelmaking process. In addition, silicon is an element effective for the solid solution strengthening effect. When the content of silicon is less than 0.15 wt%, the effect of adding silicon is insufficient. When the content of silicon exceeds 0.40 wt %, oxides may be formed on the surface of the steel to deteriorate weldability, plating properties, toughness, and toughness of the heat-affected zone of the steel.

Manganese (Mn)

Manganese (Mn) is an element that increases the strength and toughness of steel and increases the hardenability of steel. In order to effectively exhibit this action, manganese (Mn) is added in an amount of 1.00 wt% to 1.30 wt% of the total weight of the steel. When the content of manganese (Mn) is less than 1.00% by weight, there may be difficulties in securing strength. Conversely, when the content of manganese (Mn) exceeds 1.30% by weight, the strength increases but segregation occurs, resulting in tissue non-uniformity and can lower the low-temperature impact toughness.

Phosphorus (P)

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

Sulfur (S)

Sulfur (S) is an element that is unavoidably contained in the manufacture of steel together with phosphorus (P), forms non-metallic inclusions such as MnS, and as a low melting point element, has a high possibility of grain boundary segregation, thereby reducing toughness. Therefore, in the present invention, the content of sulfur (S) is limited to more than 0% by weight to 0.003% by weight of the total weight of the steel. When the sulfur content exceeds 0.003 wt%, there is a problem in that the toughness of the base metal and the weld joint is greatly reduced.

Soluble Aluminum (S_Al)

Soluble aluminum (S_Al) is used as a deoxidizer and, like silicon (Si), suppresses cementite precipitation, stabilizes austenite, and improves strength. Soluble aluminum (S_Al) is preferably added in a content ratio of 0.015 wt% to 0.05 wt%. When the content of soluble aluminum (S_Al) is less than 0.015 wt %, the effect of addition is insufficient, such as difficult to expect an austenite stabilization effect. Conversely, when the content of soluble aluminum (S_Al) exceeds 0.05 wt %, a problem of nozzle clogging may occur during steelmaking, and hot brittleness occurs due to Al oxide during casting, resulting in cracks and reduced ductility.

chrome( Cr )

Chromium (Cr) is an element added to increase hardenability and secure strength. In addition, chromium (Cr) serves to increase the hardenability, but as the content increases _{, local corrosion occurs by Cr-O 2} generated by combining chromium (Cr) and oxygen, and toughness is reduced. Therefore, the content of chromium (Cr) is controlled to more than 0 wt% ~ 0.25 wt% of the total weight of the steel. When the content of chromium (Cr) exceeds 0.25 wt %, the weld crack resistance may be deteriorated.

nickel( Ni )

In the present invention, nickel (Ni) refines grains and is dissolved in austenite and ferrite to strengthen the matrix. In particular, nickel (Ni) is an effective element for improving low-temperature impact toughness. The nickel (Ni) is added in an amount of 0.15 wt% to 0.25 wt% of the total weight of the steel. When the content of nickel (Ni) is less than 0.15 wt%, the effect of adding nickel may not be properly exhibited. Conversely, when the content of nickel (Ni) exceeds 0.25 wt%, there is a problem in that weldability is impaired and red hot brittleness is caused.

titanium( Ti )

Titanium (Ti) can improve the toughness of steel by preventing the coarsening of austenite grains in the reheating step by generating Ti (C, N) precipitates with high high-temperature stability. Titanium (Ti) is added in an amount of more than 0% by weight to 0.01% by weight of the total weight of the steel. When the content of titanium (Ti) exceeds 0.01% by weight, the effect of inhibiting austenite grain growth is rather reduced by generating coarse precipitates, lowering the toughness of the steel, and increasing the manufacturing cost without any further additive effect. there is a problem.

Copper (Cu)

Copper (Cu) together with nickel (Ni) serves to improve hardenability and low temperature and impact toughness of steel. Copper (Cu) is added in an amount of 0.10% by weight to 0.20% by weight of the total weight of the steel according to the present invention. When the content of copper (Cu) is less than 0.10 wt %, the effect of adding copper cannot be properly exhibited. Conversely, when the content of copper (Cu) exceeds 0.20% by weight, red hot brittleness and surface defects may be caused.

Niobium ( Nb )

Niobium (Nb) 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. It improves both strength and toughness of steel. Niobium (Nb) is added in an amount of 0.01 wt% to 0.02 wt% of the total weight of the steel. When the content of niobium (Nb) is less than 0.01 wt %, the effect of adding niobium (Nb) is insignificant. Conversely, when the content of niobium (Nb) exceeds 0.02 wt%, the toughness of the steel may be reduced.

molybdenum( Mo )

Molybdenum (Mo) serves to increase the hardenability of steel and improve both yield strength and tensile strength. Molybdenum (Mo) is added in an amount of more than 0% by weight to 0.08% by weight of the total weight of the steel according to the present invention. When the content of molybdenum (Mo) exceeds 0.08% by weight, the toughness, ductility, and workability of the steel may be reduced.

Vanadium (V)

Vanadium (V) is an element that combines with carbon (C) or nitrogen (N) to form V (C, N) precipitates, thereby contributing to strength improvement and improving hardenability. The vanadium (V) is contained in an amount greater than 0 wt% to 0.010 wt% based on the total weight of the steel material. When the amount of vanadium (V) exceeds 0.010% by weight, it is a cause of rather reducing toughness due to coarsening of the precipitate.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of a conventional manufacturing process, all details thereof are not specifically mentioned in the present specification.

By controlling the specific components of the alloy composition and their content ranges, the present invention provides a tensile strength (TS): 500 MPa to 550 MPa and yield strength (YS): 300 MPa to 350 MPa through a method for manufacturing a steel to be described later , elongation (EL): 30% to 40%, Charpy impact absorption energy is 300 J to 400 J at a temperature of 0 ° C to -60 ° C, and a steel with a reduced ductility-brittle transition temperature (DBTT) can be obtained. .

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

Another aspect of the present invention provides a method for manufacturing a steel material having excellent low-temperature impact toughness. According to this, the step of reheating the steel made of the above-mentioned alloy composition at a temperature of 1,150 ℃ ~ 1,200 ℃; rolling the heated steel to end at a temperature of 1,000°C to 1,100°C; And Normalizing the rolled steel at a temperature of 890 ℃ ~ 950 ℃; cooling the normalized steel material; and tempering the cooled steel at a temperature of 580°C to 640°C.

The step of cooling the steel material may be carried out until reaching a temperature of 300 ℃ ~ 450 ℃ at a cooling rate of 5 ℃ / sec ~ 15 ℃ / sec.

Hereinafter, with reference to the accompanying drawings, a method for manufacturing a steel material having excellent low-temperature impact toughness according to the present invention will be described.

Method of manufacturing steel

1 is a process flow chart schematically showing a method of manufacturing a steel material having excellent low-temperature impact 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): 0.13% to 0.17%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.30%, phosphorus (P): more than 0% ~ 0.012%, Sulfur (S): >0% to 0.003%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): >0% to 0.25%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti): >0% to 0.01%, Copper (Cu): 0.10% to 0.20%, Niobium (Nb): 0.01% to 0.02%, Molybdenum (Mo): >0% to 0.08%, Vanadium (V) : more than 0% to 0.010%, and the balance contains iron (Fe) and unavoidable impurities.

Referring to Figure 1, the method of manufacturing a steel material according to an embodiment of the present invention includes a reheating step (S110), a rolling step (S120), a normalizing step (S130), a cooling step (S140) and a tempering step (S150). do.

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 1,150° 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 1,150 ℃, there is a problem that the rolling load increases because the heating temperature is not sufficient. In addition, there is a problem in that the austenite grains are rapidly coarsened because the Nb-based precipitates do not reach the solid solution temperature and thus cannot be re-precipitated into fine precipitates during hot rolling, thereby inhibiting the grain growth of austenite. 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.

After the steel is heated, the steel is rolled. In order for the steel to have low-temperature toughness, austenite grains must exist in a fine size, which is possible by controlling the rolling temperature and the reduction rate. The rolling step of the present invention is carried out in two temperature ranges. In addition, since recrystallization behavior in each temperature region is different from each other, the conditions are also set differently.

Rolling step (S120)

The heated steel is first heated and then rolled to adjust its shape. The rolling may be performed once or may be performed multiple times. Hereinafter, a case in which the rolling is performed twice as first and second will be exemplarily described.

The primary rolling is preferably made at a temperature (Tnr) or higher at which recrystallization of austenite stops. It has the effect of destroying the cast structure such as dendrites formed during casting by rolling and reducing the grain size through austenite recrystallization. Such grain refinement has an important effect on improving the strength and toughness of steel. Then, secondary rolling is performed to introduce a non-uniform microstructure into the austenite structure of the primary rolled steel. The secondary rolling step is to further deform the crystal grains and develop dislocations by the deformation inside the grains to facilitate the transformation into needle-shaped ferrite upon cooling. In order to exhibit this effect, it is preferable to control the reduction ratio per pass to 20% or more in the secondary rolling step. The secondary rolling step may be performed in an austenite non-recrystallized region. In this case, the austenite grains can be further refined, and better impact toughness can be realized compared to the case of performing finish rolling in the austenite recrystallization region.

The rolling may be finished at a temperature of 1,000 ℃ ~ 1,100 ℃. When the rolling temperature exceeds 1,100° 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 rolling temperature is carried out at less than 1,000 ℃ can induce a rolling load to reduce productivity and reduce the heat treatment effect.

Normalizing Step (S130)

Normalize the rolled steel at a temperature of 890°C to 950°C in the range of 30 minutes to 2 hours. The normalizing may improve workability by recovering 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 890° C., it may be difficult to re-dissolve solid solute elements, and thus 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.

Cooling step (S140)

The normalized steel is cooled. When the thickness of the steel is t, it is cooled to 300°C to 450°C at a cooling rate of 5°C/sec to 15°C/sec based on the point t/4. Due to this cooling, it is possible to form a needle-like microstructure and a low-temperature microstructure that improves strength and toughness. The low-temperature microstructure may include at least one of martensite, tempered martensite, and bainite.

As such, the cooling condition is a factor that affects the microstructure, and when cooling at a cooling rate of less than 5°C/sec, the amount of austenite/martensite excessively increases, which may impair strength and toughness, and the cooling rate may decrease. If it exceeds 15℃/sec, distortion of the steel material may occur due to excessive cooling, resulting in poor shape control. In addition, the cooling temperature is preferably controlled to 450 ° C. or less so that an austenite / martensite structure is not formed. However, if the cooling temperature is too low, not only the effect is saturated, but also distortion of the steel material due to excessive cooling may occur. Also, since there may be a problem of lowering impact toughness due to an excessive increase in strength, the lower limit is preferably limited to 300°C.

tempering Step (S150)

The cooled steel is heated again and tempered in the range of 1 hour to 6 hours at a temperature of 580° C. to 640° C. Accordingly, it can have tempered martensite or bainite as the final structure. The tempering relieves the residual stress of the steel, suppresses grain growth, compensates for strength, increases viscosity, imparts toughness, The elongation rate can be secured. When the tempering time is less than 1 hour, a sufficient tempering effect cannot be obtained and, conversely, when the tempering time exceeds 6 hours, productivity may be a problem.

The steel material prepared in the above steps (S110 to S150) has, due to appropriate control of heat treatment conditions, tensile strength (TS): 500 MPa to 550 MPa, yield strength (YS): 300 MPa to 350 MPa, elongation (EL): 30 % ~ 40% and Charpy impact absorption energy: It can be improved to a steel grade that satisfies 300 J ~ 400 J at a temperature of 0℃ ~ -60℃.

Hereinafter, the present invention will be described in detail through examples, but these are only preferred examples of the present invention and the scope of the present invention is not limited by the description scope of these examples. Content not described here will be omitted because it can be technically inferred sufficiently by a person skilled in the art.

Example

Steels of Comparative Examples and Examples having the compositions shown in Tables 1 and 2 below were prepared. The remainder consists of iron (Fe) and impurities that are unavoidably contained in the steelmaking process.

(Unit: wt%) division C Si Mn P S S_Al Cr Ni Comparative Example 1 0.183 0.312 1.183 0.012 0.002 0.032 0.190 0.180 Comparative Example 2 0.174 0.345 1.173 0.013 0.001 0.038 0.210 0.190 Comparative Example 3 0.195 0.333 1.201 0.008 0.002 0.038 0.180 0.180 Comparative Example 4 0.174 0.345 1.173 0.013 0.001 0.038 0.210 0.190 Example 0.153 0.339 1.186 0.012 0.002 0.031 0.180 0.180

(Unit: wt%) division Ti Cu Nb Mo V H (ppm) Ceq Comparative Example 1 0.002 0.139 0.013 0.001 0.002 2.6 0.4400 Comparative Example 2 0.002 0.134 0.015 0.001 0.002 1.9 0.4337 Comparative Example 3 0.002 0.136 0.015 0.001 0.002 2.1 0.4528 Comparative Example 4 0.002 0.134 0.015 0.001 0.002 1.9 0.4337 Example 0.002 0.131 0.016 0.001 0.001 1.8 0.4078

Referring to Tables 1 and 2, it can be seen that the Example has a smaller carbon content and lower carbon equivalent (Ceq) than the Comparative Example.

Table 3 shows the values of the process conditions for forming the steels of Comparative Examples and Examples.

division reheat temperature
(℃) Normalizing temperature
(℃) cooling rate
(℃/sec) Cooling end temperature
(℃) tempering temperature
(℃) Comparative Example 1 1,140 905 (1 hr) - - 625 (4hr) Comparative Example 2 1,135 905 (1 hr) - - 625 (4hr) Comparative Example 3 1,136 905 (1 hr) - - 625 (4hr) Comparative Example 4 1,158 905 (1 hr) - - 625 (4hr) Example 1,156 905 (1 hr) 8 378 625 (4hr)

Referring to Table 3, the example performs a cooling step of accelerated cooling of the steel to about 378° C., which is a temperature lower than about 250° C. based on the tempering temperature (625° C.) between normalizing and tempering, but the comparative example performs normal Since tempering is performed immediately after rising, there is a difference in not performing the cooling step of cooling below the tempering temperature between the normalizing and the tempering.

For the manufactured steel, yield strength (YS), tensile strength (TS), and elongation (EL) were measured as mechanical strength, respectively, and the results are shown in Table 4.

division Yield strength (MPa) Tensile strength (MPa) Elongation (%) Comparative Example 1 315 510 36 Comparative Example 2 323 518 36 Comparative Example 3 337 516 36 Comparative Example 4 340 523 36 Example 335 510 37

Referring to Table 4, mechanical properties such as tensile strength, yield strength, and elongation, Comparative Examples 1 to 4 exhibited almost similar levels to those of Examples.

For the manufactured steel, the Charpy impact absorption energy (CVN) was measured at 20°C, 0°C, -20°C, -40°C, -60°C, and -80°C, respectively, and the average values of the results are shown in Table 5 . The unit in Table 5 is Joule (J).

division 20℃ 0℃ -20℃ -40℃ -60℃ -80℃ Comparative Example 1 240 193 154 115 68 25 Comparative Example 2 242 225 170 102 60 16 Example 374 388 385 394 363 208

Referring to Table 5, the Example showed 350J or more up to -60°C, but Comparative Examples 1 and 2 showed very low absorbed energy values compared to the Examples. In addition, even at 20 ° C., the absorbed energy values of Examples were significantly higher than those of Comparative Examples 1 and 2. Therefore, it can be seen that the embodiment has relatively high toughness at room temperature and low temperature.

Figure 2 is a graph showing the Charpy impact absorption energy according to the temperature of the steel according to an embodiment of the present invention.

Referring to FIG. 2 , as shown in Table 5, it can be seen that the ductility-brittle transition temperature of the Examples is lowered to a low temperature of about -60° C. compared to the Comparative Examples. Therefore, it can be seen that the toughness is improved at a low temperature.

Figure 3 is a photomicrograph showing the shape of the fracture surface after the Charpy impact test of the steel according to an embodiment of the present invention.

Referring to FIG. 3 , the results of Example and Comparative Example 1 were tested at -60°C. In the case of the steel of Example, it can be seen that the fracture surface is not smooth compared to the steel of Comparative Example, which can absorb more energy in the impact test and exhibit superior low-temperature impact toughness compared to the comparative example.

4 is a photomicrograph showing the microstructure of a steel material according to an embodiment of the present invention.

Referring to FIG. 4 , in the case of the embodiment, it can be seen that a fine needle-like structure that improves strength and toughness is developed by accelerated cooling before tempering, and a low-temperature microstructure is formed. In addition, it can be seen that the formation of the pearlite band is suppressed. On the other hand, in Comparative Example 1, a coarse plate-like structure was developed in the microstructure. Through this, in the case of Examples, it can be seen that a fine needle-shaped low-temperature microstructure that increases strength and toughness is formed by accelerated cooling after normalizing, and the residual stress is relieved by medium temperature tempering and grain growth is suppressed, thereby improving toughness. .

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention without departing from the scope of the present invention. Accordingly, the scope of the present invention should be judged by the claims described below.

Claims (5)

By weight, carbon (C): 0.13% to 0.17%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.30%, phosphorus (P): greater than 0% to 0.012%, sulfur (S): >0% to 0.003%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): >0% to 0.25%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti) : >0% to 0.01%, copper (Cu): 0.10% to 0.20%, niobium (Nb): 0.01% to 0.02%, molybdenum (Mo): >0% to 0.08%, vanadium (V): >0% ~ 0.010%, and the balance contains iron (Fe) and unavoidable impurities,
Tensile strength (TS): 500 MPa to 550 MPa, Yield strength (YS): 300 MPa to 350 MPa, Elongation (EL): 30% to 40%, and Charpy impact energy: 300 at 0°C to -60°C temperature It satisfies J ~ 400 J, and has a needle-like structure,
steel. The method of claim 1,
having a carbon equivalent (Ceq) in the range of 0.31 to 0.48;
steel.
(where Ceq = C + Mn/6 + (Ni + Cu)/15 + (Cr + Mo + V)/5) By weight, carbon (C): 0.13% to 0.17%, silicon (Si): 0.15% to 0.40%, manganese (Mn): 1.00% to 1.30%, phosphorus (P): greater than 0% to 0.012%, sulfur (S): >0% to 0.003%, Soluble Aluminum (S_Al): 0.015% to 0.05%, Chromium (Cr): >0% to 0.25%, Nickel (Ni): 0.15% to 0.25%, Titanium (Ti) : >0% to 0.01%, copper (Cu): 0.10% to 0.20%, niobium (Nb): 0.01% to 0.02%, molybdenum (Mo): >0% to 0.08%, vanadium (V): >0% ~ 0.010%, and the remainder is iron (Fe) and reheating the steel material containing unavoidable impurities at a temperature of 1,150 ℃ ~ 1,200 ℃;
rolling the heated steel to end at a temperature of 1,000 ℃ ~ 1,100 ℃;
Normalizing the rolled steel at a temperature of 890 ℃ ~ 950 ℃;
cooling the normalized steel material; and
Tempering the cooled steel at a temperature of 580 ° C. to 640 ° C.; Containing,
A method of manufacturing steel. 4. The method of claim 3,
The step of cooling the steel material is performed to the cooling end temperature of 300 ℃ ~ 450 ℃ at a cooling rate of 5 ℃ / sec ~ 15 ℃ / sec,
A method of manufacturing steel. 4. The method of claim 3,
After performing the tempering step,
The steel material, tensile strength (TS): 500 MPa ~ 550 MPa, yield strength (YS): 300 MPa ~ 350 MPa, elongation (EL): 30% ~ 40% and Charpy impact energy: 0 ℃ ~ -60 ℃ It satisfies 300 J to 400 J at a temperature, and has a needle-like structure,
A method of manufacturing steel.

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