KR20230093861A - Hot rolled steel having high deformation and high strength and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a hot rolled steel having high stretching and high strength and a method of manufacturing the same, in which both high elongation and high strength can be secured. According to one embodiment of the present invention, the hot rolled steel having high stretching and high strength comprises: 0.05 to 0.15 wt% of carbon (C); 0.1 wt% or less of silicon; 1.0 to 1.5 wt% of manganese (Mn); 0.005 to 0.015 wt% of niobium (Nb); 0.02 wt% or less of phosphorus (P); 0.01 wt or less of sulfur (S); and the residual including iron (Fe) and other impurities, and satisfied are 305 MPa or more of a yield strength (YS) 440 MPa or more of tensile strength (TS), and 34% or more of elongation (EL).

Description

Hot rolled steel having high deformation and high strength and method of manufacturing the same}

The technical idea of the present invention relates to a steel material and a manufacturing method thereof, and more particularly, to a high-strength, high-strength hot-rolled steel material and a manufacturing method thereof.

Recently, the automobile industry is promoting weight reduction in response to environmental regulations and for the purpose of improving fuel efficiency. In order to reduce the weight of parts, it is common to apply high-strength materials and reduce the thickness, so high-strength steel materials are applied.

Automotive chassis parts are applied to the lower part of the body to connect and support the body, power train, steering and driving parts, and are subjected to repeated impacts and loads, so high durability and fatigue characteristics are required. Automobile chassis parts use high-strength steel materials because they are closely related to the safety of drivers and passengers. Due to the recent complexity and integration of automobile parts, it is necessary to develop steel materials with higher formability than before. In the case of high-strength steel, elongation and formability may be reduced according to the increase in strength.

Korean Patent Application No. 10-2015-0042766

The technical problem to be achieved by the technical idea of the present invention is to provide a high-strength, high-strength hot-rolled steel material capable of securing high elongation with high strength 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 high-strength high-strength hot-rolled steel and a manufacturing method thereof.

According to one embodiment of the present invention, the high-strength high-strength hot-rolled steel material, in weight%, carbon (C): 0.05% ~ 0.15%, silicon 0.1% or less, manganese (Mn): 1.0% ~ 1.5%, niobium ( Nb): 0.005% to 0.015%, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, and the balance including iron (Fe) and other unavoidable impurities, yield strength (YS): 305 MPa Above, tensile strength (TS): 440 MPa or more, and elongation (EL): 34% or more may be satisfied.

According to one embodiment of the present invention, the high-strength high-strength hot-rolled steel material may have a single-phase ferrite structure.

According to an embodiment of the present invention, the high-strength, high-strength hot-rolled steel is composed of polygonal ferrite and acicular ferrite, the area fraction of the polygonal ferrite is less than 90% to 100%, and the area fraction of the acicular ferrite is It may be greater than 0% and less than or equal to 10%.

According to one embodiment of the present invention, the high-strength high-strength hot-rolled steel material has a mixed structure in which ferrite and pearlite are mixed, the area fraction of the pearlite is in the range of greater than 0% to 3%, and the area fraction of the ferrite is the remaining may be a fraction.

According to one embodiment of the present invention, the high-strength high-strength hot-rolled steel material may have an average grain size in the range of 4.0 μm to 5.0 μm.

According to one embodiment of the present invention, the method for manufacturing the high-strength high-strength hot-rolled steel is, in weight percent, carbon (C): 0.05% to 0.15%, silicon 0.1% or less, manganese (Mn): 1.0% to 1.5% , Niobium (Nb): 0.005% ~ 0.015%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.01% or less, and the balance containing iron (Fe) and other unavoidable impurities at 1,180 ℃ ~ 1,220 Reheating at a temperature of °C; Hot rolling the heated steel material to end at a temperature of 850 ° C to 890 ° C; Step of multi-stage cooling the hot-rolled steel; and winding the multi-stage cooled steel material at a winding temperature of less than 600 ° C to 640 ° C.

According to one embodiment of the present invention, the multi-stage cooling of the steel includes: primary cooling to 680 ° C to 720 ° C at a cooling rate of 30 ° C / sec to 50 ° C / sec; Secondary cooling for 1 second to 3 seconds at a cooling rate of 1 ° C. / sec to 5 ° C. / sec; and tertiary cooling to less than 600 ° C to 640 ° C at a cooling rate of 50 ° C / sec to 70 ° C / sec.

According to one embodiment of the present invention, the secondary cooling may be performed by air cooling.

According to one embodiment of the present invention, the formation of acicular ferrite is suppressed in the primary cooling step, the polygonal ferrite is formed in the secondary cooling step, and the crystal grain growth is achieved in the tertiary cooling step. can be suppressed.

According to one embodiment of the present invention, the high-elongation high-strength hot-rolled steel produced after performing the winding step has a yield strength (YS): 305 MPa or more, a tensile strength (TS): 440 MPa or more, and an elongation ( EL): 34% or more is satisfied, and may have a single-phase ferrite structure or a mixed structure of ferrite and pearlite.

According to the technical idea of the present invention, the high-strength hot-rolled steel with high elongation provides a hot-rolled steel with high elongation through control of the alloy composition and process conditions. By suppressing the formation as much as possible and promoting the formation of polygonal ferrite, high elongation characteristics having a yield strength of 305 MPa and a tensile strength of 440 MPa or more and an elongation of 34% or more were secured. In this way, the high-strength hot-rolled steel with high elongation can be applied to chassis arms or members of automobiles.

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

1 is a process flow chart schematically showing a method for manufacturing a high-strength high-strength hot-rolled steel material according to an embodiment of the present invention.
2 is a graph showing a cooling process from hot rolling to winding after hot rolling in the manufacturing method of a high-strength high-strength hot-rolled steel according to an embodiment of the present invention compared to a comparative example.
3 is a scanning electron microscope photograph showing microstructures of Examples and Comparative Examples manufactured using the method for manufacturing a high-strength, high-strength hot-rolled steel according to an embodiment of the present invention.
4 to 6 are graphs showing mechanical properties of Examples and Comparative Examples manufactured using the method for manufacturing a high-strength, high-strength hot-rolled steel 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 skilled in the art, and the following examples may be modified in many different forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the spirit of the invention to those skilled in the art. Like reference numerals throughout this specification mean like elements. Further, various elements and areas in the drawings are schematically drawn. Therefore, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

High-strength, high-strength hot-rolled steel according to the technical concept of the present invention is to be applied to automobile chassis parts such as low arms, wheel disks, and rims, and these chassis parts reduce the weight of automobile parts While high strength is required for this purpose, high elongation relative to strength is required to secure formability. However, since strength and elongation are inversely proportional, it is difficult to secure a high elongation while satisfying the required strength. According to the technical idea of the present invention, it is possible to provide a high-elongation, high-strength hot-rolled steel material that satisfies an elongation of 34% or more while satisfying a tensile strength of 440 MPa or more and a yield strength of 305 MPa or more.

Hereinafter, a high-elongation high-strength hot-rolled steel material, which is one aspect of the present invention, will be described.

High-strength, high-strength hot-rolled steel

High-strength, high-strength hot-rolled steel, which is an aspect of the present invention, in weight%, carbon (C): 0.05% to 0.15%, silicon 0.1% or less, manganese (Mn): 1.0% to 1.5%, niobium (Nb): 0.005 % to 0.015%, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, and the balance includes iron (Fe) and other unavoidable impurities.

For reference, nitrogen (N) as one of the other unavoidable impurities is preferably controlled to 60 ppm or less, for example, greater than 0 ppm to 60 ppm. Therefore, the high-strength high-strength hot-rolled steel material may not contain nitrogen (N), or may contain more than 0 ppm to 60 ppm.

Hereinafter, the role and content of each component included in the high-elongation, high-strength hot-rolled steel according to the present invention will be described. At this time, the content of component elements all means weight%.

Carbon (C): 0.05% to 0.15%

Since carbon can secure strength by improving the strength of steel and forming precipitates, yield strength and tensile strength can be increased as the carbon content increases. This increase in strength is analyzed to be due to the formation of precipitates by combining carbon with precipitate additives. If the carbon content is less than 0.05%, it may be difficult to secure strength. When the content of carbon exceeds 0.15%, pearlite can be formed upon cooling by expanding the pearlite transformation region. Therefore, carbon is preferably added in an amount of 0.05% to 0.15% of the total weight of the steel.

Silicon (Si): 0.1% or less

When oxide is formed on the surface of silicon, defects due to surface scale may occur, resulting in deterioration of surface quality and plating properties. Therefore, it is preferable to control as an impurity without intentionally adding it in the said steel material. When the content of silicon exceeds 0.1%, since it has a strong affinity for oxygen, defects due to surface scale may occur, causing deterioration in surface quality. Therefore, silicon is preferably controlled to 0.1% or less of the total weight of the steel, for example, 0% to 0.1% or more than 0% to 0.1%.

Manganese (Mn): 1.0% to 1.5%

Manganese is a solid-solution strengthening element that is effective in increasing the strength of steel. When the content of manganese is less than 1.0%, the effect of adding manganese is insufficient. When the content of manganese exceeds 1.5%, center segregation is likely to occur and non-uniform crystal grains are generated due to the influence of the segregation zone, which may decrease rollability. Therefore, manganese is preferably added in an amount of 1.0% to 1.5% of the total weight of the steel.

Niobium (Nb): 0.005% to 0.015%

Niobium is a precipitation strengthening element that improves strength by forming carbon and carbide when added. When the niobium content is less than 0.005%, the precipitation strengthening effect by the addition of niobium is insufficient. If the content of niobium exceeds 0.015%, it may adversely affect hot rolling workability and elongation, and may increase the recrystallization temperature to form an unrecrystallized region during annealing. Therefore, niobium is preferably added in an amount of 0.005% to 0.015% of the total weight of the steel.

Phosphorus (P): 0.02% or less

Since phosphorus is segregated at grain boundaries during the slab reheating process and causes brittleness, the lower the phosphorus content, the better. When phosphorus is included in excess of 0.02%, weldability and toughness may be deteriorated. Therefore, phosphorus is preferably controlled to 0.02% or less of the total weight of the steel, for example, 0% to 0.02% or more than 0% to 0.02%.

Sulfur (S): 0.01% or less

Sulfur, along with phosphorus, is an element that is unavoidably contained in the manufacture of steel, and can impair toughness and weldability of steel. When sulfur is included in excess of 0.01%, it may form an emulsified inclusion (MnS) to deteriorate the resistance to stress corrosion cracking, thereby generating cracks during processing of the steel, and as a result, the corrosion resistance of the steel may be lowered. . Therefore, sulfur is preferably controlled to 0.01% or less of the total weight of the steel, for example, 0% to 0.01% or more than 0% to 0.01%.

The remaining component of the steel 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 anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

The high-strength, high-strength hot-rolled steel produced by controlling the specific components and content ranges of the above-described alloy composition and using the steel manufacturing method described later has yield strength (YS): 305 MPa or more, tensile strength (TS): 440 MPa or more, and elongation (EL): 34% or more may be satisfied. For example, the high-elongation high-strength hot-rolled steel material may satisfy yield strength (YS): 305 MPa to 370 MPa, tensile strength (TS): 440 MPa to 470 MPa, and elongation (EL): 34% to 39%. there is. For example, the high-elongation high-strength hot-rolled steel material may satisfy yield strength (YS): 320 MPa to 370 MPa, tensile strength (TS): 440 MPa to 470 MPa, and elongation (EL): 36% to 39%. there is.

The high-strength high-strength hot-rolled steel material may have a single-phase ferrite structure without including pearlite. All of the ferrites may be composed of polygonal ferrite or a mixture of polygonal ferrite and acicular ferrite. The area fraction of the polygonal ferrite may be 90% to less than 100%, and the area fraction of the acicular ferrite may be greater than 0% to 10% or less.

Alternatively, the high-strength high-strength hot-rolled steel may have a mixed structure in which ferrite and pearlite are mixed. The area fraction of the pearlite may be in the range of, for example, greater than 0% to 3%, and the fraction of the ferrite may be included as the remaining area fraction, and may be in the range of, for example, 97% to less than 100%. All of the ferrites may be composed of polygonal ferrite or a mixture of polygonal ferrite and acicular ferrite. The area fraction means an area ratio derived from a microstructure picture of the steel material through an image analyzer.

The high-strength, high-strength hot-rolled steel may have an average grain size in the range of 4.0 μm to 5.0 μm.

In addition, the high-strength high-strength hot-rolled steel material may include precipitates containing carbides such as NbC in grains and grain boundaries of the ferrite structure.

Another aspect of the present invention provides a method for manufacturing a high-strength, high-strength hot-rolled steel. According to this step of reheating the steel material 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 850 ° C to 890 ° C; Step of multi-stage cooling the hot-rolled steel; And winding the multi-stage cooled steel at a winding temperature of less than 600 ° C to 640 ° C; includes.

The step of cooling the steel material in multiple stages includes first cooling to 680° C. to 720° C. at a cooling rate of 30° C./sec to 50° C./sec; Secondary cooling for 1 second to 3 seconds at a cooling rate of 1 ° C. / sec to 5 ° C. / sec; and tertiary cooling to less than 600°C to 640°C at a cooling rate of 50°C/sec to 70°C/sec.

The secondary cooling may be performed by air cooling.

Formation of acicular ferrite may be inhibited in the primary cooling step, polygonal ferrite may be formed in the secondary cooling step, and grain growth may be inhibited in the tertiary cooling step.

The high-elongation high-strength hot-rolled steel produced after performing the winding step satisfies yield strength (YS): 305 MPa or more, tensile strength (TS): 440 MPa or more, and elongation (EL): 34% or more, , it may have a single-phase ferrite structure, or a mixed structure in which ferrite and pearlite are mixed. For example, the high-elongation high-strength hot-rolled steel material may satisfy yield strength (YS): 305 MPa to 370 MPa, tensile strength (TS): 440 MPa to 470 MPa, and elongation (EL): 34% to 39%. there is. For example, the high-elongation high-strength hot-rolled steel material may satisfy yield strength (YS): 320 MPa to 370 MPa, tensile strength (TS): 440 MPa to 470 MPa, and elongation (EL): 36% to 39%. there is.

Hereinafter, a method for manufacturing a high-elongation, high-strength hot-rolled steel according to the present invention will be described with reference to the accompanying drawings.

Manufacturing method of high-strength, high-strength hot-rolled steel

1 is a process flow chart schematically showing a method for manufacturing a high-strength high-strength hot-rolled steel material according to an embodiment of the present invention.

In the method for manufacturing a high-strength, high-strength hot-rolled steel according to the present invention, the semi-finished product to be subjected to the hot-rolling process may be illustratively a slab. The semi-finished slab can be secured through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

The steel material, in weight percent, carbon (C): 0.05% to 0.15%, silicon 0.1% or less, manganese (Mn): 1.0% to 1.5%, niobium (Nb): 0.005% to 0.015%, phosphorus (P) : 0.02% or less, sulfur (S): 0.01% or less, and the balance includes iron (Fe) and other unavoidable impurities.

Referring to FIG. 1, the manufacturing method of high-strength hot-rolled steel with high elongation according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), a multi-stage cooling step (S130), and a winding step (S140). do.

Reheating step (S110)

In the reheating step (S110), the steel material having the above composition, for example, the slab plate material, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of 1,180 ° C to 1,220 ° C. Through such reheating, re-dissolution of components segregated during casting and re-dissolution of precipitates may occur, and in particular, a sufficiently high temperature is required to re-dissolve precipitation hardening elements such as niobium to form fine precipitates during subsequent cooling. The fine precipitates hinder the growth of crystal grains to achieve crystal grain refinement, thereby increasing strength. When the reheating temperature is less than 1,180° C., maximum re-dissolution of precipitation hardening elements such as niobium and the like may not occur, and components segregated during casting may not be sufficiently evenly distributed. When the reheating temperature exceeds 1,220° C., austenite crystal grains are coarsened, which may cause a decrease in yield strength. In addition, as the reheating temperature increases, there is a problem in that manufacturing costs increase and productivity decreases due to heating costs and additional time required to adjust the hot rolling temperature.

Hot rolling step (S120)

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

The hot rolling, that is, the finishing rolling may be finished at a finish rolling temperature (FRT) of 850 ° C to 890 ° C. When the finish-rolling end temperature is less than 850° C., the rolling is performed in an abnormal region of austenite and ferrite, resulting in mixing of crystal grains, resulting in non-uniform deformability, resulting in deterioration in rollability. When the finish rolling end temperature exceeds 890° C., grains are coarsened and the strength of the final steel material may be reduced.

Multi-stage cooling step (S130)

The hot-rolled steel material is cooled in multiple stages. The step of multi-stage cooling the steel is, the step of first cooling to 680 ℃ ~ 720 ℃ at a cooling rate of 30 ℃ / sec ~ 50 ℃ / sec, 1 second ~ 3 seconds at a cooling rate of 1 ℃ / sec ~ 5 ℃ / sec It may include a step of secondary cooling during, and a step of tertiary cooling to less than 600 ° C to 640 ° C at a cooling rate of 50 ° C / sec to 70 ° C / sec.

In the primary cooling step, the hot-rolled steel material is cooled from 850 ° C to 890 ° C to 680 ° C to 720 ° C at a cooling rate of 30 ° C / sec to 50 ° C / sec, and through this cooling rate control, needle-shaped ferrite can inhibit the formation of The primary cooling may be performed by a front end quenching method or may be performed by water cooling.

In the secondary cooling step, the primary cooled steel is cooled from 680 ° C to 720 ° C at a cooling rate of 1 ° C / sec to 5 ° C / sec for 1 second to 3 seconds, and accordingly to 665 ° C to 699 ° C. It can be cooled, and thus the formation of polygonal ferrite can be achieved. The secondary cooling may be performed by air cooling.

In the tertiary cooling step, the secondary cooled steel material is cooled from 665 ° C to 699 ° C to less than 600 ° C to 640 ° C at a cooling rate of 50 ° C / sec to 70 ° C / sec, and the crystal grains through this cooling rate control. Growth may be inhibited. The tertiary cooling may be performed by a front end quenching method or may be performed by water cooling.

Winding step (S140)

When the cooling is finished, the steel material is wound at a coiling temperature (CT) of less than 600 ° C to 640 ° C. Formation of pearlite is suppressed in the above coiling temperature range, and ferrite and precipitate structures can be obtained. When the coiling temperature is less than 600° C., bainite may be formed and the precipitation effect may be reduced. When the coiling temperature is 640° C. or higher, pearlite may be formed and crystal grains may be coarsened. In addition, grain growth may be suppressed as the third cooled steel material is wound at a relatively low winding temperature of less than 600 ° C to 640 ° C.

In the method for manufacturing high-strength, high-strength hot-rolled steel according to the technical concept of the present invention, the formation of acicular ferrite is suppressed and the formation of polygonal ferrite is induced by performing multi-stage cooling after hot rolling, and at the same time, crystal grain growth is By suppressing it, it is possible to improve elongation while minimizing loss of strength.

Experimental example

Hereinafter, a preferred experimental example is presented to aid understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples. Contents not described herein can be technically inferred by those skilled in the art, so descriptions thereof will be omitted.

Table 1 shows the composition of the high-elongation high-strength hot-rolled steel of Examples and Comparative Examples. The content unit of each component is % by weight. Components and contents of the steel materials of Examples and Comparative Examples were the same.

ingredient C Si Mn Nb P S Fe content 0.08 0.05 1.25 0.01 0.01 0.005 Bal.

2 is a graph showing a cooling process from hot rolling to winding after hot rolling in a method for manufacturing a high-strength high-strength hot-rolled steel according to an embodiment of the present invention compared with a comparative example.

Table 2 shows process condition values for forming the high-elongation, high-strength hot-rolled steel of Examples and Comparative Examples.

division reheat
temperature
(℃) rolling
end temperature
(℃) Primary
cooling rate
(℃/sec) 1st cooling
end temperature
(℃) tertiary
cooling rate
(℃/sec) winding temperature
(℃) comparative example 1200 870 - - - 640 Example 1200 870 40 700 60 620

Referring to Table 2 and FIG. 2, in the case of Comparative Example, after performing hot rolling, shear quenching from the rolling end temperature of 870 ° C. to 640 ° C. at a cooling rate of about 80 ° C./sec, followed by winding at a coiling temperature of 640 ° C. It became. On the other hand, in the case of the embodiment, after performing hot rolling, after performing hot rolling, first cooling from the rolling end temperature of 870 ° C to 700 ° C at a cooling rate of about 40 ° C / sec, and then at a temperature of 700 ° C Secondary cooling for 2 seconds at a cooling rate of about 3 ° C / sec, followed by third cooling to 620 ° C at a cooling rate of 60 ° C / sec, and then wound at a relatively low temperature of 620 ° C. Here, the thickness of both the steel materials of the comparative example and the example was about 4 mm.

3 is a scanning electron microscope photograph showing the microstructures of Examples and Comparative Examples manufactured using the method for manufacturing a high-strength, high-strength hot-rolled steel according to an embodiment of the present invention.

Referring to FIG. 3, it can be seen that the comparative example has an average grain size of about 4.17 μm, and both acicular ferrite and polygonal ferrite appear together, especially acicular ferrite appears a lot, and pearlite is formed in some areas.

It can be seen that polygonal ferrite is formed more prominently in Example than in Comparative Example, and the average grain size is 4.87 μm, which is larger than in Comparative Example. The grain sizes of both Comparative Examples and Examples showed almost similar sizes. The examples had such a ferrite matrix, and pearlite was not formed or a relatively very small amount of pearlite was formed. In addition, in the embodiment, it can be seen that precipitates containing NbC are formed in the grains and at the grain boundaries of the ferrite structure.

By this microstructure, the examples are analyzed to have improved elongation while having yield strength and tensile strength similar to those of the comparative examples.

Table 3 is a table showing mechanical properties of the high-strength high-strength hot-rolled steel of FIG. 2 according to an embodiment of the present invention compared with those of the comparative example.

division Yield strength (MPa) Tensile strength (MPa) elongation rate
(%) average grain size
(μm) comparative example 373 468 33 4.17 Example 349 458 38 4.87

Referring to Table 3, compared to the comparative example, the yield strength and tensile strength of the example were slightly lowered, but the elongation was increased to 38%.

4 to 6 are graphs showing mechanical properties of Examples and Comparative Examples manufactured using the method for manufacturing a high-strength, high-strength hot-rolled steel according to an embodiment of the present invention.

Referring to Figure 4, the yield strength of the high-elongation high-strength hot-rolled steel is shown as the mechanical properties. Comparative examples were measured for 8 specimens, and examples were measured for 7 specimens. The yield strength of the examples was reduced compared to the comparative example. Specifically, the yield strength of the examples was in the range of 340 MPa to 360 MPa, and the yield strength of the comparative examples was in the range of 370 MPa to 400 MPa. The average yield strength of the comparative example was 383.3 MPa, and the standard deviation was 11.63 MPa. The average yield strength of the examples was 345.1 MPa, standard deviation 6.962 MPa. In FIG. 4, the curves are normal distribution curves derived from the measured values of yield strength, and are shown as solid lines in Examples and curves in Comparative Examples. According to the normal distribution curve, the yield strength of the embodiment can be calculated in the range of 320 MPa to 370 MPa.

Referring to Figure 5, as the mechanical properties, the tensile strength of the high-elongation high-strength hot-rolled steel is shown. Comparative examples were measured for 8 specimens, and examples were measured for 7 specimens. Tensile strength of Examples was reduced compared to Comparative Examples. Specifically, the tensile strength of the examples was in the range of 450 MPa to 460 MPa, and the tensile strength of the comparative examples was in the range of 460 MPa to 480 MPa. The average tensile strength of the comparative example was 470.9 MPa, and the standard deviation was 6.707 MPa. The average tensile strength of the examples was 456.7 MPa, and the standard deviation was 4.309 MPa. In FIG. 5, the curves are normal distribution curves derived from the measured values of tensile strength, and the examples are solid lines and the comparative examples are curves. According to the normal distribution curve, the tensile strength of the embodiment can be calculated in the range of 440 MPa to 470 MPa.

Referring to Figure 6, the elongation of the high-elongation high-strength hot-rolled steel is shown as the mechanical properties. The elongation of Examples was increased compared to Comparative Examples. Specifically, the elongation of the examples was in the range of 37% to 38%, and the elongation of the comparative examples was in the range of 33% to 36%. The average elongation of the comparative example was 34.38% and the standard deviation was 0.9161%. The average elongation of the examples was 37.86% and the standard deviation was 0.3780%. In FIG. 6, the curves are normal distribution curves derived from the measured values of elongation, and are shown as solid lines in Examples and curves in Comparative Examples. According to the normal distribution curve, the elongation of the embodiment can be calculated in the range of 36% to 39%.

Summarizing the results of FIGS. 4 to 6, the comparative example satisfies the tensile strength of 440 MPa or more and the yield strength of 305 MPa or more, but did not satisfy the elongation of 34% or more, or the lower limit was found. On the other hand, in the example, the yield strength was reduced by about 40 MPa and the tensile strength by about 15 MPa compared to the comparative example, but the tensile strength of 440 MPa or more and the yield strength of 305 MPa or more were satisfied, and the elongation was about 3 compared to the comparative example. It can be seen that % improvement satisfies the elongation rate of 34% or more.

The technical spirit of the present invention described above is not limited to the foregoing embodiments and the accompanying drawings, and various substitutions, modifications and changes are possible within the scope of the technical spirit of the present invention. It will be clear to those skilled in the art to which it pertains.

Claims (10)

In weight percent, carbon (C): 0.05% to 0.15%, silicon 0.1% or less, manganese (Mn): 1.0% to 1.5%, niobium (Nb): 0.005% to 0.015%, phosphorus (P): 0.02% or less , Sulfur (S): 0.01% or less, and the balance includes iron (Fe) and other unavoidable impurities,
Yield strength (YS): 305 MPa or more, tensile strength (TS): 440 MPa or more, and elongation (EL): 34% or more,
High-strength, high-strength hot-rolled steel. According to claim 1,
The high-strength high-strength hot-rolled steel,
Having a single-phase ferrite structure,
High-strength, high-strength hot-rolled steel. According to claim 2,
The high-strength high-strength hot-rolled steel,
It is composed of polygonal ferrite and acicular ferrite,
The area fraction of the polygonal ferrite is 90% to less than 100%, and the area fraction of the acicular ferrite is greater than 0% to 10% or less,
High-strength, high-strength hot-rolled steel. According to claim 1,
The high-strength high-strength hot-rolled steel,
It has a mixed structure of ferrite and pearlite,
The area fraction of the pearlite ranges from greater than 0% to 3%,
The area fraction of the ferrite is the remaining fraction,
High-strength, high-strength hot-rolled steel. According to claim 1,
The high-strength high-strength hot-rolled steel,
With an average grain size in the range of 4.0 μm to 5.0 μm,
High-strength, high-strength hot-rolled steel. In weight percent, carbon (C): 0.05% to 0.15%, silicon 0.1% or less, manganese (Mn): 1.0% to 1.5%, niobium (Nb): 0.005% to 0.015%, phosphorus (P): 0.02% or less , Sulfur (S): 0.01% or less, and the balance comprising iron (Fe) and other unavoidable impurities, reheating at a temperature of 1,180 ℃ ~ 1,220 ℃;
Hot rolling the heated steel material to end at a temperature of 850 ° C to 890 ° C;
Step of multi-stage cooling the hot-rolled steel; and
Including; winding the multi-stage cooled steel at a winding temperature of less than 600 ° C to 640 ° C;
Method for producing high-strength, high-strength hot-rolled steel. According to claim 6,
The step of cooling the steel in multiple stages,
Primary cooling to 680 ° C to 720 ° C at a cooling rate of 30 ° C / sec to 50 ° C / sec;
Secondary cooling for 1 second to 3 seconds at a cooling rate of 1 ° C. / sec to 5 ° C. / sec; and
Thirdly cooling to less than 600 ° C to 640 ° C at a cooling rate of 50 ° C / sec to 70 ° C / sec;
Method for producing high-strength, high-strength hot-rolled steel. According to claim 7,
The secondary cooling step is made by air cooling,
Method for producing high-strength, high-strength hot-rolled steel. According to claim 7,
In the primary cooling step, the formation of acicular ferrite is suppressed,
In the secondary cooling step, polygonal ferrite is formed,
In the tertiary cooling step, grain growth is suppressed,
Method for producing high-strength, high-strength hot-rolled steel. According to claim 6,
The high-strength high-strength hot-rolled steel produced after performing the winding step,
Yield strength (YS): 305 MPa or more, tensile strength (TS): 440 MPa or more, and elongation (EL): 34% or more,
Having a single-phase ferrite structure or a mixed structure of ferrite and pearlite,
Method for producing high-strength, high-strength hot-rolled steel.

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