KR102352647B1 - Hot rolled steel having excellent low-temperature toughness and low yield ratio and method of manufacturing the same - Google Patents

Abstract

The present invention provides a resistance yield ratio hot-rolled steel material having excellent low-temperature toughness and a method for manufacturing the same. According to an embodiment of the present invention, the resistance yield ratio hot-rolled steel material, by weight, carbon (C): 0.04% to 0.07%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.10% ~ 1.50%, Soluble Aluminum (S_Al): 0.01% ~ 0.05%, Niobium (Nb): 0.02% ~ 0.03%, Chromium (Cr): 0.20% ~ 0.30%, Calcium (Ca): 0.001% ~ 0.003%, Phosphorus (P): more than 0% to 0.018%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and unavoidable impurities, yield ratio (YR): 0.80 to 0.90 and 0°C to Drop weight test value (DWTT) at a temperature of 20 ℃: 85% ~ 100% can be satisfied.

Description

Hot rolled steel having excellent low-temperature toughness and low yield ratio 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 low yield ratio hot-rolled steel material having excellent low-temperature toughness and a method for manufacturing the same.

Recently, as the usage of oil and natural gas increases, the development of high-performance oil pipelines capable of responding to the harsh surrounding environment is required. As the installation of oil pipelines in seismic zones or permafrost regions is activated, there is a need for materials with a resistance to yield ratio that can delay the breakage of pipes. In addition, in the case of oil pipelines, since securing structural stability at low temperatures is important, low-temperature toughness according to American Petroleum Institute standards is required. As a method of improving the low-temperature toughness of steel, there is a method of reducing the fracture propagation speed by grain refinement according to the addition of an alloying element such as niobium (Nb).

Korea Patent Publication No. 2016-0079166

However, since a high yield-ratio characteristic is caused by grain refinement, it is difficult to reduce the yield ratio. In addition, when niobium is added, it has good toughness at room temperature or about 0° C., but when the use environment is severe, such as at a low temperature of less than 0° C., the low-temperature toughness may be lowered and use may be difficult.

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

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

According to one aspect of the present invention, there is provided a resistance yield ratio hot-rolled steel material having excellent low-temperature toughness and a method for manufacturing the same.

According to an embodiment of the present invention, the resistance yield ratio hot-rolled steel material, by weight, carbon (C): 0.04% to 0.07%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.10% To 1.50%, Soluble Aluminum (S_Al): 0.01% to 0.05%, Niobium (Nb): 0.02% to 0.03%, Chromium (Cr): 0.20% to 0.30%, Calcium (Ca): 0.001% to 0.003%, Phosphorus (P): more than 0% to 0.018%, sulfur (S): more than 0% to 0.003%, and the balance contains iron (Fe) and unavoidable impurities, yield ratio (YR): 0.80 to 0.90 and 0°C to Drop weight test value (DWTT) at a temperature of 20 ℃: 85% ~ 100% can be satisfied.

According to an embodiment of the present invention, the resistance yield ratio hot-rolled steel material has a needle-like ferrite structure, tensile strength (TS): 520 MPa to 760 MPa, yield strength (YS): 415 MPa to 565 MPa, and elongation (EL): 20% to 35% can be satisfied.

According to an embodiment of the present invention, in the low yield ratio hot rolled steel, the manganese/silicon ratio may be 5 to 10.

According to an embodiment of the present invention, the method for manufacturing the resistance yield ratio hot-rolled steel material is, by weight, carbon (C): 0.04% to 0.07%, silicon (Si): 0.15% to 0.25%, manganese (Mn) : 1.10% to 1.50%, soluble aluminum (S_Al): 0.01% to 0.05%, niobium (Nb): 0.02% to 0.03%, chromium (Cr): 0.20% to 0.30%, calcium (Ca): 0.001% to 0.003 %, phosphorus (P): more than 0% to 0.018%, sulfur (S): more than 0% to 0.003%, and the remainder being iron (Fe) and unavoidable impurities, reheated at a temperature of 1,180 ° C. to 1,220 ° C. to do; Hot-rolling the heated steel material to end at a temperature of 830 ℃ ~ 870 ℃; and cooling and winding the hot-rolled steel material.

According to an embodiment of the present invention, cooling and winding the steel material may be performed at a cooling rate of 20°C/sec to 40°C/sec and a winding temperature of 560°C to 620°C.

According to an embodiment of the present invention, after performing the step of cooling and winding the steel material, the resistance-yield ratio hot-rolled steel material has a yield ratio (YR): 0.80 to 0.90 and a drop weight at a temperature of 0 ° C. to 20 ° C. Test value (DWTT): 85% ~ 100% can be satisfied.

According to the technical idea of the present invention, the low yield ratio hot-rolled steel material contains chromium instead of niobium, and the cooling rate after hot rolling is 20°C/sec to 40°C/sec level so that the phase transformation occurs intensively during winding. It is manufactured by controlling the winding temperature to 560°C to 620°C, which is directly above the bainite transformation temperature. Accordingly, the low yield ratio hot-rolled steel material has a microstructure in which the formation of polygonal ferrite is suppressed and the formation of fine needle-shaped ferrite is promoted, thereby providing a low yield ratio and excellent low-temperature toughness characteristics.

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

1 is a process flowchart schematically illustrating a method for manufacturing a resistance yield ratio hot-rolled steel material according to an embodiment of the present invention.
2 is a scanning electron microscope photograph showing the microstructures of Examples and Comparative Examples manufactured by using the method for manufacturing a resistive yield ratio hot-rolled 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 this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

A hot-rolled steel material having low-temperature toughness by adding an alloying element such as niobium does not satisfy low-temperature toughness at less than 0°C, and also does not secure a resistive yield ratio characteristic. Therefore, the technical idea of the present invention is to reduce the niobium (Nb) content in hot-rolled steel, add chromium (Cr) or manganese (Mn) instead, and control the process temperature without adding a process, thereby reducing yield ratio and It is possible to provide a hot-rolled steel material having low-temperature toughness. In order to secure the resistive yield ratio and low-temperature toughness, control of hot rolling is important, and in particular, control of finishing rolling temperature and winding temperature is important. Through this control, it is possible to form a needle-shaped ferrite microstructure that provides a resistive yield ratio and low-temperature toughness.

In addition, the drop weight test (DWTT) is one of the methods to evaluate the fracture propagation transition temperature (FPTT) of an oil pipe. Unlike the Charpy impact test, a specimen of the same thickness as the actual oil pipe is tested. By using it, it explains the transition temperature change according to the change in the thickness of the specimen well, and unlike the result of the Charpy impact test, the fracture path is long, so it is suitable for showing the fracture propagation pattern of the oil pipe.

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

Excellent low temperature toughness resistance to weight ratio hot rolled steel

One aspect of the present invention, the low yield ratio hot-rolled steel material with excellent low-temperature toughness is, by weight, carbon (C): 0.04% to 0.07%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.10% to 1.50%, Soluble Aluminum (S_Al): 0.01% to 0.05%, Niobium (Nb): 0.02% to 0.03%, Chromium (Cr): 0.20% to 0.30%, Calcium (Ca): 0.001% to 0.003%, Phosphorus ( P): more than 0% to 0.018%, including sulfur (S): more than 0% to 0.003%. The remainder consists of iron (Fe) and impurities that are unavoidably contained in the steelmaking process.

Hereinafter, the role and content of each component included in the low yield ratio hot-rolled steel having excellent low-temperature toughness according to the present invention will be described. At this time, the content of the component elements all mean wt%.

carbon (C)

Carbon is added in an amount of 0.04 wt% to 0.07 wt% of the total weight of the hot-rolled steel material. Carbon is added to secure the strength of steel and control the microstructure. When the carbon content is less than 0.04% by weight, it is difficult to obtain a yield strength of 415 MPa or more. When the content of carbon exceeds 0.07% by weight, the toughness is reduced due to the formation of a pearlite microstructure.

silicon( Si )

Silicon is added in an amount of 0.15 wt% to 0.25 wt% of the total weight of the hot-rolled steel. Silicon, as a ferrite stabilizing element, increases the degree of supercooling during ferrite transformation to make crystal grains finer and suppress carbide formation. However, when a large amount is added, the weldability may be deteriorated, the surface quality may be deteriorated by generating red scale in the hot rolling process, and the plating property after welding may be reduced. When the content of silicone is less than 0.15% by weight, the effect of adding silicone is insufficient. When the content of silicon exceeds 0.25 wt%, an oxide may be formed on the surface to deteriorate weldability, plating properties, toughness, and toughness of a weld heat-affected zone.

Manganese (Mn)

Manganese is added in an amount of 1.10 wt% to 1.50 wt% of the total weight of the hot-rolled steel. As an austenite stabilizing element, manganese is very effective in solid solution strengthening and has a great effect on increasing hardenability. When manganese is added, the equilibrium temperature is reduced, so that ferrite is reduced, pearlite has an increased microstructure, and the lamellar spacing of pearlite is reduced. The strength, toughness, and yield ratio can be controlled according to the manganese content, but when a large amount is added, MnS inclusions are formed and the core segregation occurs during casting, thereby reducing the toughness of the steel. When the manganese content is less than 1.00 wt%, it may be difficult to secure strength. When the manganese content exceeds 1.50 wt%, the strength is increased, but segregation may occur, which may cause tissue non-uniformity, and may reduce low-temperature impact toughness.

On the other hand, when 1.2 wt% or more of manganese is added, the ratio of manganese/silicon (Mn/Si) should be within the range of 5 to 10 during ERW welding for steel pipe production. This is because the Mn-Si-O oxide (Mn ₂ SiO ₄ or MnSiO ₃ ) generated during welding is formed in the above range and has the lowest melting temperature, thereby facilitating the oxide discharge during welding.

Soluble aluminum (S_ Al )

Soluble aluminum is added in an amount of 0.01 wt% to 0.05 wt% of the total weight of the hot-rolled steel. Soluble aluminum is preferably added at 0.02% to 0.03% by weight. Soluble aluminum is used as a deoxidizer and, like silicon, suppresses cementite precipitation, stabilizes austenite, and improves strength. When the content of soluble aluminum is less than 0.01% by weight, sufficient deoxidation effect cannot be obtained. When the content of soluble aluminum exceeds 0.05% by weight, weldability may be impaired.

niobium ( Nb )

Niobium is added in an amount of 0.02 wt% to 0.03 wt% of the total weight of the hot-rolled steel. Niobium combines with carbon and nitrogen contained in steel at high temperatures to form carbides or nitrides. These niobium-based carbides or nitrides suppress recrystallization and grain growth during hot rolling to refine crystal grains, thereby improving both strength and toughness of the hot-rolled steel material. The niobium dissolved during hot rolling retards the nucleation and growth of recrystallization. Since such recrystallization delay does not consume defect sites such as dislocations, it promotes nucleation during phase transformation to make grains finer. In addition, since the deformed organic precipitated carbide functions as a nucleation site of ferrite during phase transformation, it is possible to refine the crystal grains by promoting the phase transformation. Low-temperature toughness can be secured even at less than 0°C by such grain refinement. However, since the addition of a large amount of niobium causes an increase in yield strength due to grain refinement, it is arranged in a low yield ratio type. When the content of niobium is less than 0.02 wt%, the effect of adding niobium is insignificant. When the content of niobium exceeds 0.03% by weight, it may not be possible to secure a resistive yield ratio characteristic and low-temperature toughness.

chrome( Cr )

Chromium is added in an amount of 0.20% by weight to 0.30% by weight of the total weight of the hot-rolled steel. Chromium, like manganese, lowers the equilibrium temperature, so it can affect the strength and yield ratio of steel. When chromium is added in a large amount, it can form coarse carbide by combining with carbon, which slightly increases strength, but is weak in toughness, so adding a large amount should be avoided. Therefore, for the resistance yield ratio characteristic, it should be controlled to a content that affects only the phase transformation and solid solution strengthening of steel and suppresses carbide formation. When the content of chromium is less than 0.20 wt%, the effect of adding chromium is insignificant. When the content of chromium exceeds 0.30 wt %, toughness may be lowered, and weld resistance cracking characteristics may be lowered.

calcium( Ca )

Calcium is added in an amount of 0.001 wt% to 0.003 wt% of the total weight of the hot-rolled steel. Calcium is added to suppress the formation of MnS, which inhibits weldability by forming CaS inclusions due to its high bonding strength with sulfur. When the content of calcium is less than 0.001% by weight, the MnS control effect is lowered. When the content of calcium exceeds 0.003% by weight, the playability may be reduced.

Phosphorus (P)

Phosphorus is included in an amount of greater than 0 wt% to 0.018 wt% of the total weight of the hot-rolled steel. Phosphorus deteriorates weldability and forms central segregation and microsegregation, thereby reducing corrosion resistance. In addition, since segregation at the austenite grain boundary may deteriorate toughness, it is preferable to limit it to 0.018 wt% or less.

Sulfur (S)

Sulfur is included in an amount greater than 0% by weight to 0.003% by weight of the total weight of the hot-rolled steel. Sulfur is an element that is inevitably contained in the manufacture of steel together with phosphorus. As a low-melting element, it has a high possibility of grain boundary segregation, which impairs toughness and weldability, and increases MnS non-metallic inclusions to reduce corrosion resistance of steel. it is preferable

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 manufacturing processes, all details thereof are not specifically mentioned in the present specification.

The resistance-yield ratio hot-rolled steel material manufactured by controlling the specific components of the alloy composition and their content ranges, which will be described later, has a tensile strength (TS): 520 MPa to 760 MPa, and a yield strength (YS) ): 415 MPa to 565 MPa, Elongation (EL): 20% to 35%, Yield Ratio (YR): 0.80 to 0.90 and drop weight test value (DWTT) at temperatures from 0°C to 20°C: 85% to 100% It is possible to obtain a hot-rolled steel material that satisfies The low yield ratio hot-rolled steel material may have a needle-shaped ferrite structure.

Another aspect of the present invention provides a method for manufacturing a low yield ratio hot-rolled steel material having excellent low-temperature toughness. According to this, the step of reheating the steel made of the above-mentioned alloy composition at a temperature of 1,180 ℃ ~ 1,220 ℃; Hot-rolling the heated steel material to end at a temperature of 830 ℃ ~ 870 ℃; and cooling and winding the hot-rolled steel material.

The step of cooling and winding the steel material may be performed at a cooling rate of 20°C/sec to 40°C/sec and a winding temperature of 560°C to 620°C.

Hereinafter, with reference to the accompanying drawings, a method for manufacturing a high-resistance yield ratio hot-rolled steel material having excellent low-temperature toughness according to the present invention will be described.

of hot rolled steel manufacturing method

1 is a process flowchart schematically illustrating a method of manufacturing a low yield ratio hot-rolled steel material according to an embodiment of the present invention.

In the method for manufacturing a hot-rolled steel material 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): 0.04% to 0.07%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.10% to 1.50%, soluble aluminum (S_Al): 0.01% to 0.05%, niobium (Nb): 0.02% to 0.03%, chromium (Cr): 0.20% to 0.30%, calcium (Ca): 0.001% to 0.003%, phosphorus (P): more than 0% to 0.018%, sulfur ( S): more than 0% to 0.003%, and the balance contains iron (Fe) and unavoidable impurities.

Referring to FIG. 1 , the method of manufacturing a low yield ratio hot-rolled steel material according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), and a cooling and winding step (S130).

Reheating step (S110)

In the reheating step (S110), the steel having the above composition, for example, a slab plate, is reheated for about 20 to 60 minutes at a reheating temperature (Slab Reheating Temperature, SRT) of 1,180 ° C to 1,220 ° C. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur.

When the reheating temperature is less than 1,180° C., there is a problem in that the solid solution of niobium carbide is not sufficient, and the components segregated during casting are not sufficiently evenly distributed. Therefore, it is necessary to maintain the niobium carbide at a temperature of 1,180° C. or higher for at least 20 minutes or more for sufficient solid solution of the niobium carbide. When the reheating temperature exceeds 1220° C., very coarse austenite grains are formed, making it difficult to secure strength. In addition, as the reheating temperature increases, there is a problem of causing an increase in manufacturing cost and a decrease in productivity due to the heating cost and additional extraction time required to match the hot rolling temperature.

Hot rolling step (S120)

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

The hot rolling, that is, the finishing rolling may be finished at a finish rolling temperature (FRT) of 830 °C to 870 °C. The above temperature range can be selected to form a uniform needle-shaped ferrite. When the rolling end temperature is less than 830° C., rapid phase transformation occurs during cooling due to fine austenite grains to form polygonal ferrite, and thus low-temperature toughness is reduced. In addition, if the rolling end temperature is too low, it may cause a deviation of the electric material of the hot-rolled coil. On the other hand, when the rolling end temperature exceeds 870 ° C., the austenite grains are coarsened, and the phase transformation of the cooling process is slowed to form a low-temperature transformation phase such as bainite, thereby reducing the low-temperature toughness.

cooling and winding Step (S130)

After cooling the hot-rolled steel at a cooling rate of 20°C/sec to 40°C/sec, it is wound at a coiling temperature (CT) of 560°C to 620°C.

In order to secure the resistance to yield ratio and low-temperature toughness, it is important to control cooling because it is necessary to obtain fine needle-shaped ferrite. Since it is necessary to intensively generate phase transformation during winding, it is better to cool as soon as possible after hot rolling, and a cooling rate of 20°C/sec to 40°C/sec may be appropriate to suppress the formation of polygonal ferrite during cooling.

In addition, in order to refine the needle-type ferrite, cooling must be performed to a temperature immediately above the temperature at which bainite transformation starts (about 555° C.), so the coiling temperature may be appropriate at 560° C. to 620° C.

Hereinafter, the present invention will be described in detail through Examples, but these are only preferred embodiments of the present invention, and the scope of the present invention is not limited by the scope of the description 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

Hot-rolled steel materials of Comparative Examples and Examples having the compositions shown in Table 1 below were prepared. The remainder consists of iron (Fe) and impurities that are unavoidably contained in the steelmaking process.

(Unit: % by weight) division C Si Mn S_Al Nb Cr Ca P S Comparative Example 1 0.058 0.15 1.16 0.04 0.048 - 0.0017 0.013 0.0016 Comparative Example 2 0.060 0.18 1.11 0.03 0.027 - 0.0021 0.014 0.0013 Comparative Example 3 0.2328 0.16 1.38 0.03 0.020 - 0.0020 0.012 0.0010 Example 1 0.062 0.20 1.30 0.03 0.024 0.25 0.0025 0.002 0.0008 Example 2 0.062 0.20 1.30 0.03 0.024 0.25 0.0025 0.002 0.0008

Table 2 shows the process condition values for forming the hot-rolled steel materials of Comparative Examples and Examples.

division reheat temperature
(℃) rolled
end temperature
(℃) cooling rate
(℃/sec) winding temperature
(℃) reduction rate Comparative Example 1 1,200 843 30-40 611 unresolved
area
80% or more Comparative Example 2 1,200 851 30-40 637 Comparative Example 3 1,200 855 30-40 613 Example 1 1,200 847 32 589 Example 2 1,200 840 30 569

For the prepared hot-rolled steel, yield strength (YS), tensile strength (TS), elongation (EL), yield ratio (YR), and drop weight test value (DWTT) were respectively measured, and the results are shown in Table 3 it was

division yield strength
(MPa) The tensile strength
(MPa) elongation
(%) yield ratio DWTT Comparative Example 1 528 568 35 0.93 95% @0℃ Comparative Example 2 496 538 37 0.92 100% @0℃ Comparative Example 3 445 636 31 0.70 failure Example 1 469 545 32 0.86 100% @-20℃ Example 2 462 545 31 0.85 100% @-20℃

Referring to Table 3, it can be seen that the examples have the desired yield strength, tensile strength, and elongation, and the yield ratio is 0.86 and 0.85, which are included in the desired range of 0.80 to 0.90. In addition, the drop weight test value (DWTT) showed 100% at a temperature of -20 °C.

Comparative Example 1 is a case in which niobium is added in a conventional amount (0.048 wt%). Since the content of niobium was high, the yield strength was increased and the yield ratio was as high as 0.93. In addition, it can be seen that the DWTT is 95% at a temperature of 0 °C, and the low-temperature toughness is slightly lowered.

Comparative Example 2 is a case in which the content of niobium is reduced to the Example level based on Comparative Example 1 and chromium is not added. Compared to Comparative Example 1, since the content of niobium was low, both the yield strength and the tensile strength were lowered, and in particular, the yield ratio was 0.92. It is analyzed that the yield ratio is increased because the tensile strength cannot be increased due to insufficient solid solution strengthening effect as chromium is not added. On the other hand, DWTT showed 100% at a temperature of 0 °C.

Comparative Example 3 is a case in which the carbon content is increased based on Comparative Example 2. When the carbon content is increased, the yield ratio can be reduced, but it can be seen that the low-temperature toughness is very deteriorated because the DWTT fails at 0°C.

2 is a scanning electron microscope photograph showing the microstructures of Examples and Comparative Examples manufactured by using the method for manufacturing a resistive yield ratio hot-rolled steel material according to an embodiment of the present invention. In FIG. 2, Comparative Example 1 and Example 2 are representatively shown.

Referring to FIG. 2 , in Comparative Example 1, a polygonal ferrite structure in an elongated form due to the high content of niobium is shown. A hot-rolled steel material having such a polygonal ferrite structure may have desired low-temperature toughness, but does not have a resistive yield ratio.

In the case of Example 2, it can be seen that the needle-shaped ferrite structure is well developed. Therefore, the resistive yield ratio hot-rolled steel of Example 2 may have excellent low-temperature toughness and a resistive yield ratio at the same time.

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

Claims (6)

By weight, carbon (C): 0.04% to 0.07%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.10% to 1.50%, soluble aluminum (S_Al): 0.01% to 0.05%, niobium (Nb): 0.02% to 0.03%, Chromium (Cr): 0.20% to 0.30%, Calcium (Ca): 0.001% to 0.003%, Phosphorus (P): more than 0% to 0.018%, Sulfur (S): 0 % greater than 0.003%, and the balance contains iron (Fe) and unavoidable impurities,
Yield ratio (YR): 0.80 to 0.90 and drop weight test value (DWTT) at a temperature of 0 ° C to 20 ° C: 85% to 100%,
Having a needle-like ferrite structure, the formation of polygonal ferrite structure and bainite structure is suppressed,
Tensile strength (TS): 520 MPa to 760 MPa, yield strength (YS): 415 MPa to 565 MPa, and elongation (EL): 20% to 35%,
Resistance yield ratio hot rolled steel. delete The method of claim 1,
The manganese / silicon ratio is 5 to 10,
Resistance yield ratio hot rolled steel. By weight, carbon (C): 0.04% to 0.07%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.10% to 1.50%, soluble aluminum (S_Al): 0.01% to 0.05%, niobium (Nb): 0.02% to 0.03%, Chromium (Cr): 0.20% to 0.30%, Calcium (Ca): 0.001% to 0.003%, Phosphorus (P): more than 0% to 0.018%, Sulfur (S): 0 % to 0.003%, and the remainder being iron (Fe) and reheating the steel material containing unavoidable impurities at a temperature of 1,180 ℃ ~ 1,220 ℃;
Hot-rolling the heated steel material to end at a temperature of 830 ℃ ~ 870 ℃; and
Including; cooling and winding the hot-rolled steel material;
The step of cooling and winding the steel material is performed at a cooling rate of 20°C/sec to 40°C/sec and a winding temperature of 560°C to 620°C,
After cooling and winding the steel material, the resistance yield ratio hot-rolled steel material has a needle-shaped ferrite structure, the formation of a polygonal ferrite structure and a bainite structure is suppressed, and a yield ratio (YR): 0.80 to 0.90 and drop weight test value (DWTT): 85% to 100% at a temperature of 0°C to 20°C, tensile strength (TS): 520 MPa to 760 MPa, yield strength (YS): 415 MPa to 565 MPa, and elongation (EL): 20% to 35%,
A method for manufacturing a resistive yield ratio hot-rolled steel material. delete delete

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