KR20200024397A - Shape steel and method of manufacturing the same - Google Patents

Abstract

An objective of the present invention is to provide shape steel and a manufacturing method thereof which can guarantee high strength and low-temperature impact toughness. According to an aspect of the present invention, the shape steel comprises 0.04-0.2 wt% of carbon (C), 0.1-0.3 wt% of silicon (Si), 0.9-1.6 wt% of manganese (Mn), 0-0.025 wt% (not including 0 wt%) of phosphorus (P), 0-0.015 wt% (not including 0 wt%) of sulfur (S), 0.015-0.05 wt% of aluminum (Al), 0.01-0.1 wt% of vanadium (V), 0.01-0.1 wt% of titanium (Ti), 0.1-0.5 wt% of nickel (Ni), and the remainder consisting of iron (Fe) and inevitable impurities. The shape steel has a surface layer portion and a center portion excluding the surface layer portion from the surface towards the inside. The microstructure of the surface layer portion consists of a composite structure of tempered martensite, bainitic ferrite, and acicular ferrite. The room-temperature microstructure of the center portion consists of a composite structure of ferrite and perlite.

Description

SHAPE STEEL AND METHOD OF MANUFACTURING THEREOF {SHAPE STEEL AND METHOD OF MANUFACTURING THE SAME}

TECHNICAL FIELD The present invention relates to a shaped steel and a method for manufacturing the same, and more particularly, to a shaped steel having a high strength and excellent low temperature impact toughness and a manufacturing method thereof.

A section steel generally refers to a steel having a cross-sectional shape varyingly. Recently, the section steel has been applied to structural steel such as pillars of large buildings, and has been applied to civil construction temporary materials and foundation piles such as subways and bridges. The shaped steel can be produced by hot rolling casts, such as Bloom, Billet, Beam blank, etc., produced by continuous casting.

In recent years, as the offshore plant industry develops, demand for a section steel that can guarantee low-temperature impact toughness, while also having light weight and high strength in the field of steel for marine structures, is increasing.

Related prior arts include Korean Patent Publication No. 10-2014-0056765 (published May 12, 2014, title of the invention: section steel and its manufacturing method).

The problem to be solved by the present invention is to provide a shaped steel and a method of manufacturing the same that can ensure high strength and low temperature impact toughness.

Steel according to an aspect of the present invention is carbon (C): 0.04 ~ 0.20% by weight, silicon (Si): 0.10 ~ 0.30% by weight, manganese (Mn): 0.90 ~ 1.60% by weight, phosphorus (P): greater than 0 0.025 % By weight or less, sulfur (S): more than 0 and 0.015% by weight or less, aluminum (Al): 0.015 to 0.050% by weight, vanadium (V): 0.010 to 0.100% by weight, titanium (Ti): 0.010 to 0.100% by weight, nickel (Ni): 0.10 to 0.50% by weight, consisting of the remaining iron (Fe) and other unavoidable impurities, from the surface inwards, having a central portion except the surface layer and the surface layer, the microstructure of the surface layer is tempered martensite, It consists of a composite structure of bainitic ferrite and needle-like ferrite, and the room temperature microstructure of the center is composed of a composite structure of ferrite and pearlite.

In one embodiment, the section steel, the grain size of the ferrite of the central portion may be 9 ㎛ to 11 ㎛.

In another embodiment, in the cross-sectional structure including the surface layer portion and the center portion, the surface layer portion may have 15% to 30%, and the center portion may have 70% to 85%.

Method for producing a shaped steel according to another aspect of the present invention, (a) carbon (C): 0.04 to 0.20% by weight, silicon (Si): 0.10 to 0.30% by weight, manganese (Mn): 0.90 to 1.60% by weight, Phosphorus (P): more than 0 and 0.025 wt% or less, Sulfur (S): more than 0 and 0.015 wt% or less, aluminum (Al): 0.015 to 0.050 wt%, vanadium (V): 0.010 to 0.100 wt%, titanium (Ti) : 0.010 ~ 0.100% by weight, nickel (Ni): 0.10 ~ 0.50% by weight, reheating the steel consisting of the remaining iron (Fe) and other unavoidable impurities to 1150 to 1300 ℃; (b) the end of the rolling mill temperature 750 Hot rolling to ˜850 ° C .; And (c) cooling the hot rolled steel to have a reheating temperature of 600 to 700 ° C .; It includes.

In an embodiment, in step (c), the cooling of the QST may include cooling the surface temperature of the hot rolled steel below the martensite transformation start temperature (Ms temperature).

In another embodiment, the step (c) is to move the hot rolled steel at a feed rate of 0.8 m / s to 2.0 m / s, cooling water of 10 m ³ / h to 300 m ³ / h cooling water It can proceed with feeding to the hot rolled steel.

In another embodiment, the manufactured steel has a central portion except for the surface portion and the surface portion in the inner direction from the surface, wherein the microstructure of the surface portion is a composite structure of tempered martensite, bainitic ferrite and needle-like ferrite It consists of, the room temperature microstructure of the central portion may be made of a complex structure of ferrite and pearlite.

In another embodiment, the grain size of the ferrite in the central portion may be 9 ㎛ to 11 ㎛.

According to the embodiment of the present invention, by controlling the alloy content and the manufacturing method described above, it is possible to implement a shaped steel and a method of manufacturing the same that can ensure high strength and resistance impact toughness.

1 is a flow chart schematically showing a method for manufacturing a shaped steel according to an embodiment of the present invention.
2 is a graph showing the results measured by the hardness measurement method shown in FIG.
3 is a flow chart schematically showing a method for manufacturing a shaped steel according to an embodiment of the present invention.

Hereinafter, a shape steel and a method of manufacturing the same according to an embodiment of the present invention will be described in detail. The terms to be described below are terms properly selected in consideration of functions in the present invention, and the definitions of these terms should be made based on the contents throughout the specification.

Section steel

The steel according to an embodiment of the present invention is carbon (C): 0.04 to 0.20% by weight, silicon (Si): 0.10 to 0.30% by weight, manganese (Mn): 0.90 to 1.60% by weight, phosphorus (P): greater than 0 0.025 wt% or less, sulfur (S): more than 0 and 0.015 wt% or less, aluminum (Al): 0.015 to 0.050 wt%, vanadium (V): 0.010 to 0.100 wt%, titanium (Ti): 0.010 to 0.100 wt%, Nickel (Ni): 0.10 to 0.50% by weight, consisting of the remaining iron (Fe) and other unavoidable impurities.

Hereinafter, the role and content of each component included in the section steel according to an embodiment of the present invention will be described.

Carbon (C)

Carbon (C) is added to secure strength and is the element having the greatest influence on weldability. The carbon (C) may be added in an amount ratio of 0.04 to 0.20% by weight of the total weight of the shaped steel according to an embodiment of the present invention. If the carbon content is less than 0.04% by weight of the total weight, it may be difficult to secure sufficient strength. On the contrary, when the content of carbon exceeds 0.20% by weight of the total weight, the impact toughness of the base material may be lowered, and there may be a problem of lowering the weldability during electric resistance welding (ERW).

Silicon (Si)

Silicon (Si) is added together with aluminum as a deoxidizer to remove oxygen in the steel in the steelmaking process. In addition, silicon may also have a solid solution strengthening effect. The silicon may be added in an amount ratio of 0.10 to 0.30% by weight of the total weight of the steel according to an embodiment of the present invention. If the content of silicon is less than 0.10% by weight of the total weight, the silicon addition effect may not be properly exhibited. On the contrary, when the content of silicon exceeds 0.30% by weight of the total weight, the weldability of the steel is lowered and a red scale is generated during reheating and hot rolling, thereby causing a problem on the surface quality.

Manganese (Mn)

Manganese (Mn) is effective in strengthening employment. Manganese (Mn) can also increase the hardenability of the steel. Manganese may be added in a content ratio of 0.90 to 1.60% by weight of the total weight of the shaped steel according to an embodiment of the present invention. If the content of manganese is less than 0.90% by weight, the effect of solid solution strengthening may not be sufficiently achieved. In addition, when the content of manganese exceeds 1.60% by weight, weldability is lowered, MnS inclusions and center segregation may occur to reduce the ductility of the steel and lower the corrosion resistance.

Phosphorus (P)

Phosphorus (P) increases the strength of the solid solution by strengthening the solid solution, and can function to suppress the formation of carbides. The phosphorus may be added in an amount ratio of more than 0 and less than 0.025% by weight of the total weight of the steel according to an embodiment of the present invention. When the content of phosphorus exceeds 0.025% by weight, there is a problem that the low temperature impact value is lowered due to precipitation behavior.

Sulfur (S)

Sulfur (S) may improve the processability by forming a fine MnS precipitate. The sulfur may be added in an amount ratio of more than 0 and 0.015% by weight of the total weight of the steel according to an embodiment of the present invention. When the content of sulfur exceeds 0.015% by weight, the toughness and weldability may be inhibited and the low temperature impact value may be lowered.

Aluminum (Al)

Aluminum (Al) is added to the steelmaking process as a deoxidizer for removing oxygen in the steel. In addition, it can precipitate in steel with AlN and contribute to grain refinement. The aluminum may be added in an amount ratio of 0.015 to 0.050% by weight of the weight of the shaped steel according to an embodiment of the present invention. The content of aluminum tteurimyeo decrease productivity it is difficult to play when it is more than 0.015% by weight, lack a deoxidation effect, and 0.050% by weight is below, to form the non-metallic inclusions of alumina (Al ₂ O ₃₎ that ductility and toughness decrease There may be a problem.

Vanadium (V)

Vanadium (V) has the effect of increasing the strength by forming a precipitate during rolling, in particular, the amount of precipitation can be controlled according to the amount of nitrogen added. In addition, vanadium may act as pinning at grain boundaries and may contribute to the improvement of strength. The vanadium may be added in an amount ratio of 0.010 to 0.100% by weight of the weight of the steel according to an embodiment of the present invention. If the content of vanadium is less than 0.010% by weight, it is difficult to sufficiently secure the above effect. On the other hand, if the content of vanadium exceeds 0.100% by weight there may be a problem that the low temperature impact toughness is lowered.

Titanium (Ti)

Titanium (Ti) may generate Ti (C, N) precipitates having high temperature stability. As a result, the austenite grain growth can be prevented during welding to refine the structure of the weld, thereby improving the toughness and strength of the steel. The titanium may be added in a content ratio of 0.010 to 0.100% by weight of the weight of the steel according to an embodiment of the present invention. When the content of titanium is less than 0.010% by weight, it is difficult to sufficiently secure the effect. On the other hand, when the content of titanium exceeds 0.100% by weight, the low temperature impact toughness of the steel can be lowered by generating coarse precipitates.

Nickel (Ni)

Nickel (Ni) increases the strength and ensures a low temperature impact value. The nickel may be added in an amount ratio of 0.10 to 0.50% by weight or less of the total weight of the steel according to an embodiment of the present invention. When the content of nickel is added at less than 0.10% by weight, it is difficult to achieve the above effects. On the other hand, when the content of nickel exceeds 0.50% by weight, the room temperature strength is excessively high, resulting in deterioration of weldability and toughness.

As described above, the shaped steel according to an embodiment of the present invention having an alloying element composition may have a material property of room temperature, tensile strength of 500 to 690 MPa, yield strength of 420 MPa or more, elongation of 19% or more, and yield ratio of 90% or less. . In addition, the shape steel, the impact toughness value at -40 ℃ may be 60 J or more. Specifically, the impact toughness values at −40 ° C. of the flange portion and the R-end portion in the H-shaped structure shown in FIG. 2 may all be 60 J or more.

In addition, the shaped steel according to an embodiment of the present invention having the alloying element composition as described above may have a central portion except for the surface layer portion and the surface layer portion in an inward direction from the surface. At this time, the microstructure of the surface layer portion is composed of a composite structure of tempered martensite, bainitic ferrite and needle-like ferrite, the microstructure of the central portion may be composed of a composite structure of ferrite and pearlite. The grain size of the ferrite in the center may be 9 ㎛ to 11 ㎛. In this case, in the cross-sectional structure including the surface layer portion and the center portion, the surface layer portion may have 15% to 30%, the center portion may have 70% to 85%. When the tissue fraction of the surface layer portion is less than 15%, the low temperature metamorphic tissue fraction may be lowered and the strength may not reach the target value. When the tissue fraction of the surface layer portion exceeds 30%, the martensite tissue fraction is increased so that the strength and yield ratio are excessively increased, thereby reducing the impact toughness of each part of the section steel. As an example, the impact toughness of the R-end part 220 may be lowered.

1 is a cross-sectional view schematically showing a section steel according to an embodiment of the present invention. The shaped steel has a flange portion 210 and an R-end portion 220. When the R-end part 220 is manufactured of a shaped steel, it may be a part that is difficult to secure material properties such as toughness corresponding to a solidification end part. In one embodiment, the hardness measurement of the shaped steel can be made in the flange portion (210). Referring to the enlarged view of the portion S of the flange portion 210 of the shaped steel of FIG. 1, a plurality of points in the thickness direction from the point p1 of the first surface layer portion to the point p2 of the second surface layer portion through the center portion. Hardness can be measured at.

2 is a graph showing the results measured by the hardness measurement method shown in FIG. Through the graph of FIG. 2, the fraction of the hardened layer which consists of low temperature transformation structure in the shaped steel provided with a surface layer part and a center part can be calculated. In one example, the thickness from the surface p1 of the first surface layer portion to the surface p2 of the second surface layer portion may be 37 mm. For convenience of explanation, the surface p1 of the first surface layer portion is named the outer surface, and the surface p2 of the second surface layer portion is named the inner surface.

The method of calculating the boundary point of the hardened layer depends on the following calculation method. First, the point with the highest hardness in the graph of FIG. 2 and the hardness at this point are confirmed. As an example, in the graph of FIG. 2, the point with the highest hardness may be the outer side, and the hardness at the outer side may be 248 Hv.

Next, the average hardness value of the center part which consists of a composite structure of ferrite and pearlite is calculated | required. As an example, in the graph of FIG. 2, the average hardness value of the central portion may be 133 Hv.

Next, the hardness value of the hardened layer boundary region is calculated from the arithmetic mean value of the highest hardness of the shaped steel and the average hardness value of the center portion. In one example, 190.5 Hv, calculated as (248 + 133) / 2, corresponds to the hardness value of the boundary region of the cured layer.

When calculating the position of the cured layer by using the hardness value of the boundary region of the cured layer with reference to Figure 2, the cured layer is 3.5 in the region corresponding to the depth of 5.5mm in the inner direction from the outer surface and inward from the inner surface It may be an area corresponding to the depth of mm. Accordingly, the hardened layer fraction can be calculated as (5.5 + 3.5) / 37, using the formula of the thickness of the hardened layer / the total thickness of the steel. Accordingly, the cured layer fraction may be 24.3%. According to an embodiment of the present invention, the cured layer fraction may be 15% to 30%.

Hereinafter, a method of manufacturing a shaped steel according to an embodiment of the present invention having the alloying element composition described above.

Manufacturing method of section steel

3 is a flow chart schematically showing a method for manufacturing a shaped steel according to an embodiment of the present invention. Referring to FIG. 3, a method of manufacturing a shaped steel having excellent fire resistance according to an embodiment of the present invention includes a reheating step (S100), a hot rolling step (S200), and a QST (Quenching & Self-Tempering) step (S300). Include.

First, in the reheating step (S100), the steel material of the predetermined composition described above is reheated. The steel may be manufactured through a continuous casting process after obtaining molten steel of a desired composition through a steelmaking process. The steel may be, for example, a billet or a beam blank.

The steel is carbon (C): 0.04 to 0.20% by weight, silicon (Si): 0.10 to 0.30% by weight, manganese (Mn): 0.90 to 1.60% by weight, phosphorus (P): greater than 0 and 0.025% by weight or less, sulfur ( S): more than 0 and 0.015% by weight or less, aluminum (Al): 0.015 to 0.050% by weight, vanadium (V): 0.010 to 0.100% by weight, titanium (Ti): 0.010 to 0.100% by weight, nickel (Ni): 0.10 to 0.50% by weight, may be composed of the remaining iron (Fe) and other unavoidable impurities.

In one embodiment, the steel may be reheated at a temperature of 1150 to 1300 ℃. When the steel is reheated at the above-mentioned temperature, segregated components during the continuous casting process may be reclaimed. If the reheating temperature is lower than 1150 ℃, the solid solution of various carbides may not be sufficient, there may be a problem that the segregated components are not evenly distributed evenly during the continuous casting process. If the reheating temperature exceeds 1300 ° C, very coarse austenite grains may be formed, making it difficult to secure strength. In addition, when the temperature exceeds 1300 ° C., the heating cost is increased and the process time is added, which may lead to an increase in manufacturing cost and a decrease in productivity.

In the hot rolling step (S200), the reheated steel is hot rolled. In one embodiment, the hot rolling may be started at a rolling start temperature of 1000 ~ 1100 ℃. In addition, the hot rolling may be controlled so that the end temperature of the rolling is 750 ~ 850 ℃. When the rolling end temperature is less than 750 ° C, the rolling addition may be increased, and the yield ratio of the resultant steel as a rolling product may be increased. In addition, when the rolling end temperature exceeds 850 ℃, it may be difficult to secure the target strength and toughness.

In the Quenching & Self-Tempering (QST) step (S300), the hot rolled steel, that is, the shaped steel, is cooled and self-tempered. The cooling applies a quenching method of spraying cooling water against the section steel. In addition, the self-tempering step is to allow the section steel to be tempered by recuperation after the quenching, and the recuperation temperature of the section steel through the feed rate of the section steel at the time of quenching, or the amount of cooling water injected. Can be controlled. In an embodiment of the present invention, the recuperation temperature may be controlled to 600 to 700 ° C.

As an example, the quenching may be performed by cooling the surface temperature of the hot rolled section steel below the martensite transformation start temperature (Ms temperature). Specifically, the surface of the section steel in the quenching step may be cooled to 400 ° C or less. To this end, the cooling path for which the cooling quantity for the section steel is maintained at 10 to 300 m ³ / h may be cooled while the section steel moves at a feed rate of 0.8 to 2.0 m / s.

After the quenching, it is possible to allow recuperation to proceed by diffusion of heat from the interior of the shaped steel to the surface portion. During the recuperation step, the section steel may be maintained in an air-cooled state.

Through the above-described manufacturing method, it is possible to manufacture a shaped steel according to an embodiment of the present invention. The shaped steel manufactured according to one embodiment of the present invention may have a material property of room temperature, tensile strength of 500 to 690 MPa, yield strength of 420 MPa or more, elongation of 19% or more, yield ratio of 90% or less. In addition, the shape steel, the impact toughness value at -40 ℃ may be 60 J or more. Specifically, the impact toughness values at −40 ° C. of the flange portion and the R-end portion in the H-shaped structure shown in FIG. 2 may all be 60 J or more.

Experimental Example

Hereinafter, preferred experimental examples are provided to help understanding of the present invention. However, the following experimental example is only for helping understanding of the present invention, and the present invention is not limited by the following experimental example.

1. Preparation of Specimen

The specimens of Comparative Examples 1, 2 and Examples 1 and 2 having the alloying element composition (unit: wt%) of Table 1 and the remaining iron and other unavoidable impurities were prepared according to the process conditions of Table 2, respectively.

C Si Mn P S Al V Ti Ni Comparative Example 1 0.09 0.20 1.54 0.013 0.010 0.020 0.049 0.015 0.24 Comparative Example 2 0.08 0.19 1.55 0.011 0.009 0.022 0.048 0.014 0.22 Example 1 0.08 0.21 1.57 0.010 0.008 0.023 0.048 0.014 0.23 Example 2 0.09 0.20 1.55 0.009 0.008 0.021 0.048 0.015 0.25

The unit of the alloy element composition of Table 1 is weight%.

division Reheating temperature
(℃) Rolling end temperature
(℃) Cooling end temperature
(℃) QST quantity (m ³ / h) Feed speed (m / s) Comparative Example 1 1177 890 750 10-300 3.0 Comparative Example 2 1179 750 595 0.5 Example 1 1182 820 680 0.9 Example 2 1187 810 660 1.0

When Table 1 is examined, Comparative Examples 1 and 2 and Examples 1 and 2 all fall within the alloy component range of the examples of the present invention.

Examining Table 2, Comparative Example 1 and Example 1 of the prepared section steel has a thickness of 24 mm, Comparative Example 2 and Example 2 has a thickness of 37 mm. In the case of rolling finish temperature, the comparative example 1 has exceeded the upper limit of 750-850 degreeC which is the range of the rolling finish temperature of the Example of this invention. In the case of the cooling end temperature, Comparative Examples 1 and 2 deviate from 600 to 700 ° C. which is the range of the cooling end temperature of the embodiment of the present invention. Specifically, Comparative Example 1 is above the upper limit of the cooling end temperature of the Example of the present invention, and Comparative Example 2 is below the lower limit of the cooling end temperature of the Example of the present invention. When cooling proceeds with the same QST quantity for the specimens of Comparative Examples 1 and 2 and Examples 1 and 2, Comparative Examples 1 and 2 are outside the conveying speed range of 0.8 to 2.0 in the embodiment of the present invention. Specifically, the feed rate of Comparative Example 1 is higher than the upper limit of the feed rate of the embodiment of the present invention, the cooling rate may be relatively slow compared to Examples 1 and 2, the feed rate of Comparative Example 2 is The cooling rate can be relatively fast as compared with Examples 1 and 2 since it is below the lower limit of a feed rate.

Tensile Properties (room temperature) Impact Toughness (J @ -40 ℃) The tensile strength
(MPa) Yield strength
(MPa) Elongation
(%) Yield fee
(%) Flange R-end part Target 500-590 420 or more 19 or more 90 or less 60 or more 60 or more Comparative Example 1 497 394 27 79 231 120 Comparative Example 2 532 488 28.1 91 148 15 Example 1 516 441 26 86 237 102 Example 2 518 454 31 88 255 111

Microstructure Microstructure
(Surface) Cured layer fraction (%) Microstructure
(center) Central grain size
(Μm) Target TM + BF + AF 15-30 F + P 9-11 Comparative Example 1 BF + AF 12 F + P 12.8 Comparative Example 2 TM + B 38 F + P + B 7.3 Example 1 TM + BF + AF 24 F + P 10.1 Example 2 TM + BF + AF 26 F + P 9.8

Referring to Table 3, in Examples 1 and 2, all items of tensile properties were satisfied, and impact toughness of −40 ° C. was satisfied at both the flange part and the R-end part. In addition, in Examples 1 and 2, in the microstructure of Table 4, the surface microstructure was composed of tempered martensite (TM), bainitic ferrite (BF), and acicular ferrite (AF), and the cured layer fraction was respectively. The target was met as 24% and 26%. In addition, the central microstructure was also made of ferrite (F) and pearlite (P), respectively, and the target ferrite grain size was 10.1 µm and 9.8 µm, respectively.

In fact, Figures 4 to 6 are photographs showing the microstructure of Example 1. Figure 4 is a photograph of the surface layer consisting of tempered martensite, Figure 5 is a photograph of the surface layer consisting of a composite structure of bainitic ferrite (BF) and acicular ferrite (AF). Figure 6 is a photograph showing the microstructure of the center consisting of a composite structure of ferrite (F) and pearlite (P).

On the other hand, in Comparative Example 1, the tensile strength and the yield strength did not reach the target value, no tempered martensite (TM) was found in the surface microstructure, and the cured layer fraction did not reach the target value.

On the other hand, in the case of Comparative Example 2, the yield ratio did not satisfy the target value of 90% or less, and the R-end portion did not satisfy the impact toughness value of -40 ° C. In addition, the grain size of the center ferrite is outside the lower limit of the target value, and bainite, which is a low temperature transformation structure, is observed in the microstructure of the center. In addition, the surface microstructure is composed of tempered martensite (TM) and bainite (B), and the cured layer fraction is outside the upper limit of the target value.

As described above, in the specimens of Examples 1 and 2 of the present invention, the high rolling characteristics and low temperature impact toughness were satisfied by controlling the rolling conditions of the shaped steel according to the conditions set forth in the present invention.

In the above description, the embodiment of the present invention has been described, but various changes and modifications can be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention will be determined by the claims described below.

Claims (8)

Carbon (C): 0.04 to 0.20 wt%, Silicon (Si): 0.10 to 0.30 wt%, Manganese (Mn): 0.90 to 1.60 wt%, Phosphorus (P): more than 0 and 0.025 wt% or less, Sulfur (S): More than 0 0.015% by weight or less, aluminum (Al): 0.015 to 0.050% by weight, vanadium (V): 0.010 to 0.100% by weight, titanium (Ti): 0.010 to 0.100% by weight, nickel (Ni): 0.10 to 0.50% by weight , The rest of iron (Fe) and other unavoidable impurities,
Inwardly from the surface, having a surface layer portion and a central portion except for the surface layer portion,
The microstructure of the surface layer portion is composed of a composite structure of tempered martensite, bainitic ferrite and acicular ferrite, and the microstructure of the central portion is composed of a composite structure of ferrite and pearlite.
Section steel. According to claim 1,
The grain size of the ferrite in the center is 9 ㎛ to 11 ㎛
Section steel. According to claim 1,
In the cross-sectional structure including the surface layer portion and the central portion,
The surface portion has 15% to 30%, and the central portion has 70% to 85%
Section steel. (a) Carbon (C): 0.04 to 0.20 wt%, Silicon (Si): 0.10 to 0.30 wt%, Manganese (Mn): 0.90 to 1.60 wt%, Phosphorus (P): greater than 0 and 0.025 wt% or less, sulfur ( S): more than 0 and 0.015% by weight or less, aluminum (Al): 0.015 to 0.050% by weight, vanadium (V): 0.010 to 0.100% by weight, titanium (Ti): 0.010 to 0.100% by weight, nickel (Ni): 0.10 to Reheating the steel made of 0.50% by weight, remaining iron (Fe) and other unavoidable impurities to 1150 to 1300 ° C .;
(b) hot rolling the steel to a rolling end temperature of 750 to 850 ° C .; And
(c) Quenching & Self-Tempering (QST) cooling the hot rolled steel to have a recuperation temperature of 600 to 700 ° C .; Containing,
Method of manufacturing section steel. The method of claim 4, wherein
In step (c), the QST cooling step is
Cooling the surface temperature of the hot rolled steel below the martensite transformation start temperature (Ms temperature).
Method of manufacturing section steel. The method of claim 4, wherein
step (c)
While moving the hot rolled steel at a feed rate of 0.8 m / s to 2.0 m / s, the cooling water of 10 m ³ / h to 300 m ³ / h of cooling water proceeds while supplying to the hot rolled steel
Method of manufacturing section steel. The method of claim 4, wherein
The manufactured steel has an inner surface from the surface, and has a central portion except for the surface layer portion and the surface layer portion,
The microstructure of the surface layer portion is composed of a composite structure of tempered martensite, bainitic ferrite and acicular ferrite, and the microstructure of the central portion is composed of a composite structure of ferrite and pearlite.
Method of manufacturing section steel. The method of claim 7, wherein
The grain size of the ferrite in the center is 9 ㎛ to 11 ㎛
Section steel.

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