KR102366991B1 - 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 hot rolled steel material having excellent low-temperature toughness and a low yield ratio and a manufacturing method thereof. According to an embodiment of the present invention, the hot rolled steel material having a low yield ratio comprises: 0.05-0.08 wt% of carbon (C); 0.15-0.25 wt% of silicon (Si); 1.20-1.60 wt% of manganese (Mn), 0.01-0.05 wt% of aluminum (Al), 0.02-0.03 wt% of niobium (Nb), 0.20-0.30 wt% of chromium (Cr), 0.001-0.003 wt% of calcium (Ca), more than 0 to 0.018 wt% of phosphorus (P), more than 0 to 0.003 wt% of sulfur (S) and the remainder consisting of iron (Fe) and inevitable impurities, wherein the yield ratio (YR) thereof is equal to or smaller than 0.90 and the value of a drop weight tear test (DWTT) at a temperature equal to or lower than -20℃ can satisfy 90-100%.

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, due to the depletion of mining resources such as oil and natural gas, resource extraction in deep and extreme environments is increasing. In order to cope with such a harsh environment, it is inevitably required to have complex characteristics of high strength and high performance in oil pipeline materials. For example, recently, oil well regions such as India and the Middle East are distributed around the orogeny zone, and in order to overcome such a surrounding environment, the characteristic of a resistance yield ratio type capable of delaying the breakage of the pipe is required. In addition, in the case of regions such as the Middle East, in the oil well area around the Caspian Sea, there is a period in which the temperature rapidly decreases to below -20℃ in winter.

The demand for large-diameter pipelines is increasing to increase transport capacity, and the development of high-strength materials is essential to make large-diameter pipelines. In the case of oil pipelines, several characteristics must be guaranteed at the same time. First, it must have excellent low-temperature toughness characteristics in order to safely respond to the harsh surrounding environment. In addition, as the installation of oil pipelines in earthquake zones and permafrost regions has recently been activated, it is necessary to have a resistance to yield ratio type material to delay the breakage of pipes and secure structural stability.

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).

Oil pipeline materials are generally manufactured in the direction of improving strength and low-temperature toughness by adding precipitating elements such as Nb/Ti to refine crystal grains to increase strength and slow down fracture propagation speed. However, since these alloys cause high yield ratio material properties according to grain refinement and precipitation strengthening effect, it is difficult to lower the yield ratio. Since the recent trend is demanding complex properties that can satisfy both resistance yield ratio and low-temperature toughness, it has a disadvantage that it is difficult to use in a harsh environment. In particular, the typical Nb-added component system has good toughness at room temperature or in the range of about 0°C, but as the use environment becomes harsh, the toughness decreases in the low temperature range of 0°C or less, making it difficult to use.

Korea Patent Publication No. 2016-0079166

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 is, by weight, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.20% ~ 1.60%, Aluminum (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): 0.90 or less and a temperature of -20°C Drop weight test value (DWTT): 90% ~ 100% below can be satisfied.

According to an embodiment of the present invention, the resistance yield ratio hot-rolled steel material may have a needle-shaped ferrite structure.

According to an embodiment of the present invention, the resistance yield ratio hot-rolled steel material, tensile strength (TS): 535 MPa ~ 760 MPa, yield strength (YS): 450 MPa ~ 600 MPa, and elongation (EL): 20% or more can be satisfied with

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 manufacturing method of the low yield ratio hot-rolled steel material is, by weight, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn) : 1.20% ~ 1.60%, Aluminum (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 is iron (Fe) and unavoidable impurities in which the steel is reheated at a temperature of 1,180 ° C. to 1,220 ° C. step; Hot-rolling the heated steel material to end at a temperature of 830 ℃ ~ 870 ℃; cooling the hot-rolled steel; and winding the cooled steel material.

According to an embodiment of the present invention, the step of cooling the hot-rolled steel material may be performed at a cooling rate of 30 °C / sec ~ 50 °C / sec.

According to an embodiment of the present invention, the winding of the cooled steel material may be performed at a winding temperature of 580 °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 0 ℃ to -20 ℃ drop at a temperature of 20 ℃ Weight test value (DWTT): 90% ~ 100% can be satisfied.

According to the technical idea of the present invention, the low yield ratio hot-rolled steel material reduces or excludes the content of niobium or titanium, and the cooling rate after hot rolling is reduced to 30 ℃ / sec ~ 50 ℃ / so that the phase transformation occurs intensively during winding It is made by controlling the winding temperature to 580 ° C. ~ 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.

The conventional technique for securing the low-temperature toughness of a hot-rolled steel sheet is manufactured by adding a precipitating element such as Nb/Ti. Therefore, there is a disadvantage in that it cannot meet the resistance to recovery characteristics required in severe environments such as earthquake zones and permafrost. In particular, therefore, the technical idea of the present invention is to reduce the niobium (Nb) and titanium (Ti) content in hot-rolled steel, and apply a solid solution strengthening effect by adding chromium (Cr) to satisfy strength and resistance to yield ratio characteristics. We want to secure a microstructure. By controlling the process temperature without adding a process, it is possible to provide a hot-rolled steel material having a resistive yield ratio and 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. Such a hot-rolled steel material may have excellent composite properties, such as a resistance yield ratio and low-temperature toughness, in a high-strength oil pipeline.

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.

Resistance yield ratio hot-rolled steel with excellent low-temperature toughness

One aspect of the present invention, the low yield ratio hot-rolled steel material with excellent low-temperature toughness, by weight, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.20% ~ 1.60%, Aluminum (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%, 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 material 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): 0.05% to 0.08%

Carbon is added to secure the strength of steel and control the microstructure. When the carbon content is less than 0.05%, it is difficult to obtain a yield strength of 450 MPa or more. When the carbon content exceeds 0.08%, the toughness is reduced due to the formation of a pearlite microstructure. Therefore, it is preferable that carbon is added in an amount of 0.05% to 0.08% of the total weight of the hot-rolled steel.

Silicon (Si): 0.15% to 0.25%

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, weldability may be deteriorated, and red scale may be generated during the reheating process and hot rolling in the hot rolling process, thereby reducing the surface quality and reducing the plating property after welding. When the content of silicon is less than 0.15%, the effect of adding silicon is insufficient. When the content of silicon exceeds 0.25%, oxides are formed on the surface, and weldability, plating properties, toughness, and toughness of the heat-affected zone of the weld may be deteriorated. Therefore, silicon is preferably added in an amount of 0.15% to 0.25% of the total weight of the hot-rolled steel.

Manganese (Mn): 1.20% to 1.60%

As an austenite stabilizing element, manganese is very effective in solid solution strengthening and has a great influence on the increase in 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 central segregation occurs during casting, thereby reducing the toughness of the steel. When the manganese content is less than 1.20%, it may be difficult to secure strength. When the manganese content exceeds 1.60%, the strength is increased, but segregation may occur, which may cause tissue non-uniformity, and may reduce low-temperature impact toughness. In addition, when more than 1.2% 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 manufacturing. 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. Accordingly, manganese is preferably added in an amount of 1.20% to 1.60% of the total weight of the hot-rolled steel.

Aluminum (Al): 0.01% to 0.05%

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

Niobium (Nb): 0.02% to 0.03%

Niobium can promote grain refinement by delaying recrystallization during hot rolling. It is known that solid solution niobium during hot rolling delays the nucleation and growth of recrystallization, and 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 serves as a nucleation site of ferrite during phase transformation, it is possible to refine the crystal grains by promoting the phase transformation. Such grain refinement may make it possible to secure low-temperature toughness even at less than 0°C. However, since the addition of a large amount of niobium causes an increase in yield strength due to grain refinement and precipitation strengthening effect, it is not suitable for resistance-to-yield ratio materials. When the content of niobium is less than 0.02%, the effect of adding niobium is insignificant. When the content of niobium exceeds 0.03%, resistance to yield ratio characteristics and low-temperature toughness may not be secured. Therefore, niobium is preferably added in an amount of 0.02% to 0.03% of the total weight of the hot-rolled steel material.

Chromium (Cr): 0.20% to 0.30%

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%, the effect of adding chromium is insignificant. When the content of chromium exceeds 0.30%, toughness may be lowered, and weld resistance cracking characteristics may be lowered. Therefore, chromium is preferably added in an amount of 0.20% to 0.30% of the total weight of the hot-rolled steel.

Calcium (Ca): 0.001% to 0.003%

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%, the MnS control effect is lowered. When the content of calcium exceeds 0.003%, the playability may be reduced. Therefore, calcium is preferably added in an amount of 0.001% to 0.003% of the total weight of the hot-rolled steel.

Phosphorus (P): >0% to 0.018%

Phosphorus is included in an amount greater than 0% to 0.018% 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% or less.

Sulfur (S): >0% to 0.003%

Sulfur is included in excess of 0% to 0.003% of the total weight of hot-rolled steel. Sulfur is an element that is unavoidably contained in the manufacture of steel together with phosphorus. As a low melting point 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. desirable.

The remaining component of the low yield ratio hot-rolled steel material 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): 535 MPa to 760 MPa, and a yield strength (YS) ): 450 MPa to 600 MPa, elongation (EL): 20% or more, yield ratio (YR): 0.90 or less, and drop weight test value (DWTT) at a temperature of -20°C or less: hot rolling that satisfies 90% to 100% steel can be obtained.

For example, the resistive yield ratio hot-rolled steel material has tensile strength (TS): 535 MPa to 760 MPa, yield strength (YS): 450 MPa to 600 MPa, elongation (EL): 20% to 35%, yield ratio ( YR): It is possible to obtain a hot rolled steel that satisfies the drop weight test value (DWTT): 90% to 100% at a temperature of 0.80 to 0.90 and -20°C to -50°C.

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 ℃; cooling the hot-rolled steel; and winding the cooled steel material.

The step of cooling the hot-rolled steel material may be performed at a cooling rate of 30 °C / sec to 50 °C / sec.

The winding of the cooled steel material may be performed at a winding temperature of 580°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.

Manufacturing method of hot rolled steel

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): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.20% to 1.60%, aluminum (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), a cooling step (S130), and a winding step (S140). .

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, preferably at a temperature of 1,200° 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 step (S130)

The hot-rolled steel is cooled to 580° C. to 620° C. at a cooling rate of 30° C./sec to 50° C./sec. The cooling may be performed by an air cooling method or a water cooling method. 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.

Winding step (S140)

After the cooling is finished, the steel is wound at a coiling temperature (CT) of 580° C. to 620° C. In order to refine the needle-type ferrite, it is necessary to sufficiently cool to a temperature immediately above the temperature at which bainite transformation starts (about 555°C), so the coiling temperature may be appropriate at 580°C to 620°C.

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 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

Table 1 shows the composition of the hot-rolled steel materials of Comparative Examples and Examples. In Table 1, the remainder consists of iron (Fe) and impurities unavoidably contained in the steelmaking process, etc. The unit is % by weight.

division C Si Mn Al Nb Cr Ca P S Example 1 0.066 0.20 1.54 0.04 0.026 0.25 0.0028 0.002 0.0011 Example 2 0.060 0.19 1.48 0.04 0.024 0.25 0.0028 0.002 0.0011 Comparative Example 1 0.059 0.19 1.53 0.03 0.065 0.19 0.0016 0.009 0.0015 Comparative Example 2 0.064 0.19 1.41 0.03 0.060 0.18 0.0015 0.0132 0.0016 Comparative Example 3 0.073 0.20 1.41 0.03 0.044 - 0.0021 0.0111 0.0018 Comparative Example 4 0.066 0.19 1.35 0.03 0.045 - 0.0019 0.0113 0.0020

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

division reheat temperature
(℃) Rolling end temperature
(℃) cooling rate
(℃/sec) winding temperature
(℃) reduction rate Example 1 1,200 857 39 620 unresolved
area
80% or more Example 2 1,200 844 39 614 Comparative Example 1 1,200 860 30-50 608 Comparative Example 2 1,200 852 30-50 584 Comparative Example 3 1,200 793 30-50 583 Comparative Example 4 1,200 830 30-50 592

Table 3 shows the results of measuring yield strength (YS), tensile strength (TS), elongation (EL), yield ratio (YR), and drop weight test value (DWTT) for the manufactured hot-rolled steel, respectively.

division yield strength
(MPa) tensile strength
(MPa) elongation
(%) yield ratio DWTT Example 1 493 571 31 0.86 100% @-20℃ Example 2 481 569 31 0.85 100% @-20℃ Comparative Example 1 589 649 32 0.91 90% @0℃ Comparative Example 2 595 643 33 0.93 100% @0℃ Comparative Example 3 575 630 34 0.91 100% @0℃ Comparative Example 4 572 630 32 0.91 95% @0℃

Referring to Table 3, it can be seen that the examples have target 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.

Compared to the examples, the comparative examples have a high niobium content and a low coiling temperature. In addition, in Comparative Examples 3 and 4, calcium was excluded.

In consideration of yield strength, tensile strength, and low-temperature toughness properties (ie, DWTT), all of the Examples and Comparative Examples satisfy the target range. Although the yield strength and tensile strength of Examples are lower than those of Comparative Examples, they satisfy the target range.

On the other hand, when the yield ratio is considered, the Examples exhibit a resistive yield ratio with a yield strength and a tensile strength ratio (YR) of 0.90 or less. Comparative Examples have excellent strength and low-temperature toughness, but exhibit a high yield ratio of 0.9 or more depending on a high content of niobium and a low coiling temperature. It is analyzed that the comparative examples have a high niobium content, and thus the crystal grain refinement and precipitation strengthening effect are greatly affected.

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.

2, the microstructures of Example 1 and Comparative Example 3 are shown. In the case of Comparative Example 3, an elongated polygonal ferrite structure was shown due to the high content of niobium. A hot-rolled steel material having such a polygonal ferrite structure may have a desired low-temperature toughness, but does not have a resistive yield ratio. On the other hand, in the case of Example 1, it can be seen that the needle-shaped ferrite structure is well developed. Therefore, the resistive yield ratio hot-rolled steel of Example 1 may have excellent low-temperature toughness and at the same time have a resistive yield ratio.

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 (8)

By weight%, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.20% to 1.60%, aluminum (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% excess ~ 0.003%, and the balance contains iron (Fe) and unavoidable impurities,
Having a needle-like ferrite structure, the formation of polygonal ferrite structure and bainite structure is suppressed,
Yield ratio (YR): 0.90 or less and drop weight test value (DWTT) at a temperature of -20 ° C or less: 90% ~ 100%,
Tensile strength (TS): 535 MPa ~ 760 MPa, yield strength (YS): 450 MPa ~ 600 MPa, and elongation (EL): 20% or more,
Resistance yield ratio hot rolled steel. delete delete The method of claim 1,
The manganese / silicon ratio is 5 to 10,
Resistance yield ratio hot rolled steel. By weight%, carbon (C): 0.05% to 0.08%, silicon (Si): 0.15% to 0.25%, manganese (Mn): 1.20% to 1.60%, aluminum (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% reheating the steel material including iron (Fe) and unavoidable impurities in excess of 0.003%, and the balance at a temperature of 1,180° C. to 1,220° C.;
Hot-rolling the heated steel material to end at a temperature of 830 ℃ ~ 870 ℃; and
cooling the hot-rolled steel; and
Including; winding the cooled steel material;
The step of cooling the hot-rolled steel material is performed at a cooling rate of 30 °C / sec ~ 50 °C / sec,
The winding of the cooled steel material is performed at a winding temperature of more than 600 ° C. ~ 620 ° C.,
After performing the step of cooling and winding the steel material, the resistance yield ratio hot-rolled steel material,
Having a needle-like ferrite structure, the formation of polygonal ferrite structure and bainite structure is suppressed,
Yield ratio (YR): 0.90 or less and drop weight test value (DWTT) at a temperature of -20 ° C or less: 90% ~ 100%,
Tensile strength (TS): 535 MPa ~ 760 MPa, yield strength (YS): 450 MPa ~ 600 MPa, and elongation (EL): 20% or more,
A method for manufacturing a resistive yield ratio hot-rolled steel material. delete delete delete

Images