KR101628628B1 - Hot-rolled steel sheet, method of manufacturing the same, and manufacturing equipment for the same - Google Patents

Abstract

A method of manufacturing a hot-rolled steel sheet is described. The method includes: a hot rolling step of hot rolling the slab; a first phase transformation step of cooling the surface of the hot-rolled slab to phase-change the structure of the surface portion of the slab; A second phase transformation step of phase-transforming the structure of the central portion of the slab, and a winding step of winding the slab including the surface portion and the center portion that have been transformed.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet, a method of manufacturing the same,

The present invention relates to a hot-rolled steel sheet, a method of manufacturing the same, and a manufacturing facility, and more particularly, to a hot-rolled steel sheet having a first phase transformation step of cooling a surface portion of a hot- To a hot-rolled steel sheet produced by the method, a manufacturing method thereof, and a manufacturing facility.

Hot rolled steel plate means a steel plate formed by rolling a plate slab at a high temperature and is widely used in various industrial fields such as interior and exterior materials for buildings, steel pipes for oil transportation, steel plates for welding, materials for ships, and automobile steel plates.

In order to improve the formability of the hot-rolled steel sheet for the purpose of utilizing the hot-rolled steel sheet in various industrial fields, research and development are in progress to improve the elongation of the hot-rolled steel sheet and to increase the strength of the hot-rolled steel sheet. However, in general, as the strength of the steel sheet increases, the moldability is lowered, and there is a problem in using the steel sheet having high strength as an accessory of a vehicle.

In order to solve such a problem, for example, Korean Patent Laid-Open Publication No. 2000-0043784 (Application No. 10-1998-0060205, filed by POSCO, Inc.) discloses a steel sheet comprising 0.06 to 0.1% carbon, 0.3% And the balance Fe and other unavoidable impurities are contained in an amount of not more than 0.02%, manganese: not more than 0.02%, phosphorus: not more than 0.02%, sulfur: not more than 0.005%, aluminum: 0.01 to 0.05%, titanium: 0.05 to 0.15%, niobium: Strength steel sheet having a tensile strength of 70 kg / mm < 2 >

According to another example, Korean Patent Laid-Open Publication No. 2001-0060647 (Application No. 10-1999-0063053, filed by POSCO, Inc.) discloses a steel sheet comprising 0.06 to 0.10% carbon, 0.5 to 1.0% silicon, 1.5 to 2.0 manganese %, Phosphorus: not more than 0.02%, sulfur: not more than 0.0005%, aluminum: 0.010 to 0.050%, titanium: 0.050 to 0.10%, niobium: 0.020 to 0.040%, nitrogen: 60 ppm or less, balance Fe and other unavoidable impurities This 780 MPa grade hot rolled steel sheet with excellent tensile strength and its manufacturing method are disclosed.

The present invention provides a high-strength hot-rolled steel sheet, a method of manufacturing the same, and a manufacturing facility therefor.

Another object of the present invention is to provide a hot rolled steel sheet having a high elongation, a method for manufacturing the same, and a manufacturing facility for the same.

It is another object of the present invention to provide a method of manufacturing a hot-rolled steel sheet which is easy to apply to a conventional process, and a manufacturing facility therefor.

Another object of the present invention is to provide an automobile accessory including a high-strength and high-elongation hot-rolled steel sheet.

In order to solve the above technical problems, the present invention provides a method for manufacturing a hot-rolled steel sheet.

According to one embodiment, the method for manufacturing a hot-rolled steel sheet includes a hot rolling step of hot-rolling a slab, a step of spraying a warming agent or a compressed air in a liquid state on the surface of the hot-rolled slab, A phase transformation step of selectively phase-transforming the phase of the slab, a phase transformation step of phase-transforming the structure of the surface part of the slab, water-cooling the slab to maintain the structure of the surface part of the slab, A two-phase transformation step, and a winding step of winding the slab including the surface portion and the center portion that have been phase-transformed.

According to one embodiment, the slab is conveyed in a first direction by a roller, and the compressed air or the liquid state warming agent can be supplied to the surface of the slab in a direction opposite to the first direction.

According to one embodiment, the structure of the surface portion of the slab has a higher elongation than that of the center portion of the slab, and the structure of the center portion of the slab has a higher strength than that of the surface portion of the slab have.

According to one embodiment, the hot-rolled slab has austenitic structure, and by the first phase transformation step, the surface portion of the slab has a ferrite structure, the center portion of the slab has austenitic structure, By the second phase transformation step, the central portion of the slab has a structure having higher strength than that of ferrite, and the surface portion of the slab may have a ferrite structure.

According to one embodiment, the method of manufacturing the hot-rolled steel sheet may further include a tempering step of tempering the wound slab.

According to one embodiment, the first phase transformation step and the second phase transformation step may be performed using different equipment.

In order to solve the above technical problems, the present invention provides an equipment for manufacturing a hot-rolled steel sheet.

According to one embodiment, the hot-rolled steel sheet manufacturing facility comprises a roller for hot-rolling the slab, a conveying table for conveying the hot-rolled slab, a first surface of the slab being hot- A second surface cooling part disposed to be apart from the first surface cooling part with the transportation table therebetween and to cool the second surface of the slab facing the first surface, And a third cooling unit that water-cools the slabs cooled by the first and second surface cooling units.

According to one embodiment, the first surface cooling section and the second surface cooling section include: a warming agent supply line in which the conveyance table extends in a second direction perpendicular to the first direction in which the slab is conveyed; And a temperature controller for supplying a temperature controller to the slab, wherein the temperature controller supplies the temperature controller to the first surface of the slab and the second surface of the slab, Can be disposed oblique to the surface.

According to one embodiment, the warming agent supply nozzle may supply the warming agent to the first surface and the second surface of the slab in a direction opposite to the first direction.

According to the embodiment of the present invention, after the structure of the surface portion of the slab is phase-transformed, the center portion of the slab is phase-transformed to produce a hot-rolled steel sheet. The surface portion of the hot-rolled steel sheet has a relatively elongated structure and the center portion of the hot-rolled steel sheet has a structure having a relatively high strength, and a hot-rolled steel sheet having a high elongation and a high strength can be provided.

1 is a flowchart illustrating a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.
2 is a perspective view illustrating a hot-rolled steel sheet according to an embodiment of the present invention.
3 is a cross-sectional view taken along the line A-A 'in FIG. 2 for explaining a hot-rolled steel sheet according to an embodiment of the present invention.
4 is a cross-sectional view taken along the line A-A 'in FIG. 2 for explaining a hot-rolled steel sheet according to an embodiment of the present invention.
5 is a view for explaining a hot-rolled steel sheet manufacturing facility according to an embodiment of the present invention.
FIG. 6 is an enlarged view of the warming agent supplying nozzle and the slab of FIG. 5 to explain the warming agent supplying nozzle included in the hot-rolled steel sheet manufacturing facility according to the embodiment of the present invention.
7 is a view for explaining a modification of the hot-rolled steel sheet manufacturing facility according to the embodiment of the present invention.
8 is a scanning electron microscope (SEM) image for explaining the microstructure of the hot-rolled steel sheet according to the embodiment of the present invention.
9 is a scanning electron microscope (SEM) image for explaining a phase transformation according to a surface cooling method of a slab for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.
10 is a scanning electron microscope (SEM) image for explaining a phase transformation according to a surface cooling time of a slab for producing a hot-rolled steel sheet according to an embodiment of the present invention.
11 is a view for explaining a change in characteristics of the surface portion of the hot-rolled steel sheet according to the ferrite fraction according to the embodiment of the present invention.
12 is a graph for explaining a tensile curve according to the coiling temperature of a hot-rolled steel sheet according to an embodiment of the present invention.
13 to 15 are views for explaining an application example of a hot-rolled steel sheet 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. However, the technical spirit of the present invention is not limited to the embodiments described herein but may be embodied in other forms. Rather, the embodiments disclosed herein are provided so that the disclosure can be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

In this specification, when an element is referred to as being on another element, it may be directly formed on another element, or a third element may be interposed therebetween. Further, in the drawings, the thicknesses of the films and regions are exaggerated for an effective explanation of the technical content.

Also, while the terms first, second, third, etc. in the various embodiments of the present disclosure are used to describe various components, these components should not be limited by these terms. These terms have only been used to distinguish one component from another. Thus, what is referred to as a first component in any one embodiment may be referred to as a second component in another embodiment. Each embodiment described and exemplified herein also includes its complementary embodiment.

The singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. It is also to be understood that the terms such as " comprises "or" having "are intended to specify the presence of stated features, integers, Should not be understood to exclude the presence or addition of one or more other elements, elements, or combinations thereof.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

1 is a flowchart illustrating a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.

Referring to Figure 1, the slab may be hot rolled (S110). The slab may be hot rolled after being heated at 1250 ° C for 2 hours. The slabs of 40 mm thickness can be hot rolled at 5 Pass and compressed to a thickness of 3 mm. According to one embodiment, the slab comprises 0.15% by weight of carbon, 1.2% by weight of manganese, 0.3% by weight of silicon, 0.03% by weight of niobium, 0.02% by weight of boron, ), And the balance iron (Fe) and other unavoidable impurities. The hot-rolled slab may have an austenite structure.

A first phase transformation step of cooling the surface of the hot-rolled slab may be performed (S120). In the first phase transformation step, the surface of the hot-rolled slab is cooled, The structure of the surface portion of the slab may be phase-transformed. The surface portion of the slab may be a portion of the slab adjacent to a first side of the slab and a second side opposite the first side. The first surface and the second surface may be the upper and lower surfaces of the slab.

For example, when the hot-rolled slab has austenitic structure before the first phase transformation step is performed, by the first phase transformation step, the surface portion of the slab has a ferrite structure .

During the phase transformation of the surface portion of the slab by the first phase transformation step, the phase of the structure of the center of the slab can be maintained. For example, even though the first phase transformation step is performed, the central portion of the slab may have an austenitic structure. In other words, during the first phase transformation step, the phase of the tissue at the center of the slab may not change. The central portion of the slab may be a portion of the slab located between the first and second surfaces of the slab.

According to one embodiment, the first phase transformation step may comprise providing compressed air to the first and second surfaces of the slab. For example, the compressed air may be compressed air by using a pump. Alternatively, the compressed air may be a specific gas such as an inert gas.

According to another embodiment, the first phase transformation step may include ejecting a liquid state warming agent onto the first and second surfaces of the slab. For example, the warming agent in a liquid state may be either water or liquid nitrogen.

The first surface and the second surface of the slab are cooled to a temperature of not more than A3 by compressed air or a liquid state warming agent provided on the first surface and the second surface so that the temperature of the surface portion of the slab The organization can be transformed. For example, the texture of the surface portion of the slab may be transformed from austenite to ferrite.

 The compressed air and the liquid state warming agent may be supplied to the first surface and the second surface of the slab for a short time. Thereby, in the first phase transformation step, the center portion of the slab is not cooled, and the center portion of the slab can be maintained in a pahse.

After the structure of the surface portion of the slab is phase-transformed by the first phase-change step, a second phase-transformation step of phase-transforming the structure of the central portion of the slab may be performed (S130) , The structure of the central portion of the slab that was not phase-transformed in the first phase transformation step may be phase-transformed by the second phase transformation step.

For example, if the central portion of the slab that was not phase transformed in the first phase transformation step has austenite structure, the center portion of the slab may be transformed into martensite structure by the second phase transformation step. It can be transformed. For example, the center portion of the slab may have HSLA (High Strength Low Alloy) steel structure such as DP (Dual Phase) steel structure, TRIP (Transformation Induced Plasticity) steel structure, or TWIP .

The phase of the texture of the surface portion of the slab can be maintained while the structure of the central portion of the slab is phase-transformed by the second phase transformation step. For example, even though the second phase transformation step is performed, the surface portion of the slab may have a ferrite structure. In other words, during the second phase transformation step, the phase of the texture of the surface portion of the slab may not change.

The elongation of the structure of the center portion of the slab phase-transformed in the second phase transformation step may be lower than the elongation of the structure of the surface portion of the slab phase-transformed in the first phase transformation step. The strength of the structure of the center portion of the slab transformed in the second phase transformation step may be higher than the strength of the structure of the surface portion transformed in the second phase transformation step.

The second phase transformation step may include water cooling the slab on which the first phase transformation step has been performed. For example, the slab may be temperature-lowered by 85 ° C per second and water-cooled to 160 to 450 ° C. The second phase transformation step may be performed in a separate process using the equipment different from the first phase transformation step.

As described above, the surface portion and the center portion may have different structures (for example, the surface portion may be a ferrite structure and the center portion may be a martensite structure). On the other hand, a portion (a mixed portion) of the slab, which is located between the surface portion and the center portion of the slab, may have a dual phase structure.

The mixed portion of the slab may be phase-transformed by the first phase-change phase, and may be phase-transformed by the second phase-change phase. More specifically, the structure of a part of the mixing portion is phase-transformed by the first phase transformation step, and the structure of the remaining portion of the mixing portion is phase-transformed by the second phase transformation step.

During the first phase transformation step, the tissue of the remaining portion of the mixing portion may not be phase transformed while the tissue of the one portion of the mixing portion is phase-transformed. While the tissue of the remaining portion of the mixing portion in the second phase transformation phase is phase-transformed, the one portion of the mixing portion that has already been phase-changed in the first phase transformation phase may not be phase-transformed. As a result, the mixing portion may include a first phase that is phase-transformed in the first phase transformation step and a second phase that is phase-transformed in the second phase transformation step.

The volume fraction of the first phase may be higher than the fraction of the second phase in the region of the mixing portion adjacent the first face and the second face. The area of the mixing portion adjacent to the center portion may have a fraction of the second phase higher than a fraction of the first phase.

For example, if the hot-rolled slab has austenitic structure before the first phase transformation step is performed, the one portion of the blended portion of the slab by the first phase transformation step is austenitic To a ferrite structure. While the portion of the mixing portion is phase-transformed into the ferrite structure, the remaining portion of the mixing portion can maintain the austenite structure. By the second phase transformation step, the remaining portion of the mixed portion can be phase-transformed from the austenite structure to the martensite structure. While the remaining portion of the mixing portion is phase-transformed into the martensite structure, the one portion of the mixing portion that has already been phase-transformed can maintain the ferrite structure. As a result, the mixed portion may have a dual phase structure of ferrite and martensite.

The second phase transformation step may be performed so that the slab including the surface portion and the center portion that have been transformed may be wound (S140). In the second phase transformation step, And then wound. The wound slab may be stored at room temperature and tempered. The tempering temperature of the wound slab may be adjusted according to the degree of cooling of the slab in the second phase transformation step.

According to an embodiment of the present invention, the surface portion of the slab is constituted by a relatively elongated structure (for example, ferrite) by the first phase transformation step, and the center portion of the slab is divided into the second phase (E.g., martensite) having a relatively high strength by the transformation step. As a result, a hot-rolled steel sheet with high elongation and high strength can be provided.

Hereinafter, the hot-rolled steel sheet produced according to the method of manufacturing hot-rolled steel sheet according to the above-described embodiment of the present invention will be described with reference to Figs.

FIG. 2 is a perspective view for explaining a hot-rolled steel sheet according to an embodiment of the present invention, FIG. 3 is a view for explaining a hot-rolled steel sheet according to an embodiment of the present invention, Respectively.

2 and 3, the hot-rolled steel sheet 10 may include a first surface 10a, a second surface 10b, and a center portion. The first surface 10a and the second surface 10b may be opposed to each other. The center portion may be a part of the hot-rolled steel sheet 10 positioned between the first surface 10a and the second surface 10b.

The hot-rolled steel sheet 10 may be a dual phase structure having a first phase and a second phase. The first phase of the hot-rolled steel sheet 10 may be produced in the first phase transformation step described with reference to Fig. 1, and the second phase may be generated in the second phase transformation step described with reference to Fig. 1 Lt; / RTI >

The first phase may have a maximum volume fraction on the first surface 10a and the second surface 10b. For example, a first portion of the hot-rolled steel sheet 10 adjacent to the first surface 10a and a second portion of the hot-rolled steel sheet 10 adjacent to the second surface 10a may have a first portion 10a, (phase), and the second phase may not have a structure. As another example, the first portion and the second portion may have a trace amount of a second phase structure. The fraction of the first phase gradually decreases from the first surface 10a toward the center portion, and the fraction of the second phase may gradually increase.

The second phase may have a maximum fraction at the center. For example, the central portion of the hot-rolled steel sheet 10 may be composed of only the second phase and may not have the first phase. As another example, the central portion of the hot-rolled steel sheet 10 may have a trace amount of the first phase structure. The fraction of the second phase gradually decreases from the central portion toward the first surface 10a and from the central portion toward the second surface 10b, The fraction can be gradually increased.

The first phase texture may have a higher elongation than the second phase texture. The tissue of the second phase may have a higher strength than the tissue of the first phase. For example, the first phase structure is ferrite and the second phase structure is martensite or dual phase steel structure, TRIP (Transformation Induced Plasticity) steel structure, or TWIP (Twinning Induced Plasticity) Of HSLA (High Strength Low Alloy) steel structure.

A mixing portion located between the first portion and the center portion, or between the second portion and the center portion, may have the first and second phases together. As the first face 10a and the second face 10b are adjacent to each other, the fraction of the first phase in the mixed portion can be increased and the fraction of the second phase can be reduced. As the center portion is closer to the center portion, the fraction of the second phase in the mixing portion is increased, and the fraction of the first phase can be reduced.

The central portion constituted only of the first phase, the second phase and the second phase constituted only of the first phase and the mixed portion in which the first phase and the second phase coexist are described in more detail with reference to FIG. 4 .

4 is a cross-sectional view taken along the line A-A 'in FIG. 2 for explaining a hot-rolled steel sheet according to an embodiment of the present invention.

4, the hot-rolled steel sheet 10 includes a first portion 11, a first mixing portion 12, a central portion 13, a second mixing portion 14, and a second portion 15 ). The first part 11, the first mixing part 12, the central part 13, the second mixing part 14 and the second part 15 can be one body .

The first portion 11 and the second portion 15 are composed of only a first phase and the center portion 13 is composed of only a second phase different from the first phase . The first mixing portion 12 and the second mixing portion 14 may have the first and second phases together.

The first phase texture may have a higher elongation than the second phase texture. The tissue of the second phase may have a higher strength than the tissue of the first phase. For example, the first phase structure is ferrite and the second phase structure is martensite or dual phase steel structure, TRIP (Transformation Induced Plasticity) steel structure, or TWIP (Twinning Induced Plasticity) Of HSLA (High Strength Low Alloy) steel structure.

In the first mixing portion 12, the fraction of the first phase increases toward the first portion 11, and the fraction of the second phase increases toward the center portion 13 have. Accordingly, a region of the first mixing portion 12 adjacent to the first portion 11 has a fraction of a first phase higher than a fraction of the second phase, One region of the first mixing portion 12 adjacent to the central portion 13 may have a fraction of the second phase higher than a fraction of the first phase.

In the second mixing portion 14, the fraction of the first phase increases with the second portion 15, and the fraction of the second phase increases with the center portion 13 have. A portion of the second mixing portion 14 adjacent to the second portion 12 has a fraction of a first phase that is higher than a fraction of the second phase, The fraction of the second phase may be higher than the fraction of the first phase in one region of the second mixing portion 14 adjacent to the first region.

Hereinafter, with reference to Figs. 5 to 7, a manufacturing facility for manufacturing a hot-rolled steel sheet according to the above-described embodiment of the present invention will be described.

5 is a view for explaining a manufacturing apparatus for a hot-rolled steel sheet according to an embodiment of the present invention. FIG. 6 is a cross- And FIG. 5 is an enlarged view of the nozzle and the slab.

5 and 6, an apparatus for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes a roller 210 for hot-rolling a slab S, a roller 210 for rolling the hot-rolled slab S in a first direction A conveying table 220, a first surface cooling part 230, a second surface cooling part 240, and a third cooling part 250.

The first surface cooling part 230 may be disposed on the transport table 220. The first surface cooling part 230 may cool the first surface of the slab S. [ The first surface may be the upper surface of the slab S.

The second surface cooling part 240 is disposed under the transportation table 220 so that the first surface cooling part 230 and the second surface cooling part 240 are disposed between the transportation table 220 and the slab (S). The second surface cooling part 230 may cool the second surface of the slab S. [ The second surface may be the lower surface of the slab S supported by the transport table 220.

The first surface cooling part 230 and the second surface cooling part 240 may include a warming agent supply line 232 and a warming agent supply nozzle 234. [ The warming agent supply line 232 may extend in a second direction in which the conveying table 220 is perpendicular to the first direction in which the slab S is conveyed by the conveying table 220. The warming agent supply nozzle 234 is provided in the warming agent supply line 232 and the warming agent supplied from the warming agent supply line 232 is supplied to the first surface of the slab S and the second surface of the slab S, Can be supplied to the surface. The warming agent supply nozzles 234 may be provided in a plurality of spaced apart from each other in the first direction.

The warming agent provided to the first surface and the second surface of the slab S through the warming agent supply nozzle 234 may be, for example, compressed air or liquid. When the thermosensitive agent is a liquid thermosensitive agent, the thermosensitive agent may be sprayed from the thermosensitive agent supply nozzle 234 toward the first surface and the second surface.

The warming agent supply nozzle 234 supplies the warming agent to the first surface and the second surface of the slab S in a direction opposite to the first direction in which the slab S is conveyed . Accordingly, the warming agent supplied to the slab S from the warming agent supply nozzle 234 contacts the first surface and the second surface of the slab S, It can be moved in the opposite direction. Accordingly, the first surface and the second surface of the slab S are instantaneously cooled, and the thermosensitive material may not be supplied to the surface of the slab S already supplied with the thermosensitive material. Thereby, the central portion of the slab S is not cooled, and the first portion of the slab S adjacent to the first surface and the second portion of the slab S adjacent to the second surface can be cooled have.

The first portion and the second portion of the slab S are selectively cooled by the first surface cooling portion 230 and the second surface cooling portion 240 so that the first surface cooling portion 230 and the second surface cooling portion 240 are in contact with each other, The first phase transformation step described above can be performed. Specifically, the first surface cooling part 230 and the second surface cooling part 240 cool the first surface and the second surface of the slab S, and the slab S is cooled, The first portion adjacent the first surface and the second portion adjacent the second surface may be phase transformed. The center portion of the slab S is not cooled while the slab S is cooled by the first surface cooling portion 230 and the second surface cooling portion 240, The phase of the tissue at the central portion may not change.

The third cooling unit 250 may supply a large amount of water to the slab S to cool the slab S. By the third cooling unit 250, the second phase transformation step described with reference to Fig. 1 can be performed. Specifically, by the water supplied from the third cooling section 250, the central portion of the slab S is phase-transformed, and the structure of the first and second portions adjacent to the first and second surfaces May not change.

5, the thermosensitive agent is supplied to the thermosensitive agent supply nozzle 234 through one of the thermosensitive agent supply lines 232, but the thermosensitive agent supply line 232 may be provided in plural have. This will be described below with reference to FIG.

7 is a view for explaining a modification of the hot-rolled steel sheet manufacturing facility according to the embodiment of the present invention.

7, there are shown a roller 210, a conveying table 220 for conveying the hot-rolled slab S in a first direction, a first surface cooling part 230, 2 surface cooling section 240, and a third cooling section 250 are provided.

5, the first surface cooling part 230 and the second surface cooling part 240 include a plurality of warming agent supply lines 232 and 233 . The plurality of warming agent supply lines 232 and 233 may be spaced apart from each other in the first direction.

Hereinafter, the physical properties of the hot-rolled steel sheet produced by the hot-rolled steel sheet manufacturing method according to the embodiment of the present invention will be described using the hot-rolled steel sheet manufacturing facility according to the embodiment of the present invention described above.

8 is a scanning electron microscope (SEM) image for explaining the microstructure of the hot-rolled steel sheet according to the embodiment of the present invention.

8, it is assumed that 0.15 wt% of carbon (C), 1.2 wt% of manganese (Mn), 0.3 wt% of silicon (Si), 0.03 wt% of niobium Nb, 0.02 wt% of boron (B) A slab composed of iron (Fe) and other unavoidable impurities was prepared. The slab is 40 mm thick and has an austenitic structure. The first phase transformation step described with reference to Figure 1 was performed by providing compressed air to the hot-rolled slab. As a result, the surface portion of the slab was phase-transformed from austenite to ferrite in the first phase transformation step. Thereafter, the second phase transformation step described with reference to Fig. 1 was performed by water cooling. As a result, the center portion of the slab was phase-transformed from austenite to martensite in the second phase transformation step.

As can be seen from Fig. 8, it can be confirmed that the surface portion of the slab to which the compressed air is supplied has a ferrite structure having a relatively elongation, and the center portion of the slab has a martensite structure having a relatively high strength.

Further, it can be confirmed that the mixed portion between the surface portion and the center portion has a ferrite and a martensite structure together. Particularly, it can be confirmed that the fraction of ferrite in the mixed portion increases and the fraction of martensite increases in the mixed portion as the surface portion is adjacent to the central portion.

Table 1 below shows the tensile test results of the hot-rolled steel sheet according to the embodiment of the present invention. The tensile test was performed by varying the winding temperature of the hot-rolled steel sheet described with reference to FIG. More specifically, in each of Examples 1 to 4, a hot-rolled steel sheet having a surface portion having a ferrite structure with a thickness of 500 mu m produced in the first phase transformation step is heated at 450 DEG C, 350 DEG C, 270 DEG C, and 160 DEG C It is winding.

In Comparative Example 1 to Comparative Example 4, in the hot-rolled steel sheet according to each of the above-described Embodiments 1 to 4, the ferrite structure produced in the first phase transformation step was removed, and a hot- Experiments were carried out.

division Coiling temperature YS TS Hand TS X Hand Example 1 450 ℃ 971 1008 14 14112 Example 2 350 ℃ 1063 1161 12.5 14513 Example 3 270 ℃ 998 1251 12 15012 Example 4 160 ° C 885 1233 14 17262 Comparative Example 1 450 ℃ 1116 1220 4.7 5734 Comparative Example 2 350 ℃ 1228 1347 4.5 6061.5 Comparative Example 3 270 ℃ 1148 1367 4.4 6014.8 Comparative Example 4 160 ° C 1038 1399 5.3 7414.7

Referring to Table 1, in Comparative Examples 1 to 4 in which the ferrite structure produced in the first phase transformation step was removed, it was measured to have a high tensile strength of 1.2 Gpa or more and a low elongation of about 5%. Also, the value of strength X ductility, which is an index of energy absorption capacity, was also measured to be low. This corresponds to the mechanical properties of a typical martensitic steel, and it is confirmed that the numerical value of the strength X ductility is low and the utilization is low.

On the other hand, in the case of Examples 1 to 4 including the ferrite structure of the surface portion generated in the first phase transformation step, the tensile strength was reduced to about 200 MPa, the elongation was increased by about 7% or more, Is improved by more than two times. That is, according to the embodiment of the present invention, it can be confirmed that the hot-rolled steel sheet including the surface portion having the ferrite structure having a relatively high elongation and the center portion having the martensite structure having a relatively high strength has excellent physical properties.

Table 2 below shows the bending test results for the hot-rolled steel sheets of Examples 1 to 4 described with reference to Table 1. Table 3 below shows the bending test results of the martensite steel of 1180 MPa in which no ferrite is present on the surface portion. The bending test was carried out under the conditions of curvature radii 1, 2, 3 and 5 mmR. When the cracks did not occur, 'O' was used. When micro cracks were generated, 'B' Quot; X ".

division 1 mmR 2 mmR 3mmR 5mmR Example 1 △ △ O O Example 2 △ △ O O Example 3 O O O O Example 4 O O O O

1180
Grade t 1 mmR 2 mmR 3mmR 5mmR Top Tail Top Tail Top Tail Top Tail MSA 2.0 X X X X X X X X MS B 2.0 X X X X ○ X ○ X MS C 4.0 X X X X X X X X MS D 4.0 ○ X ○ X X X ○ X

Referring to <Table 2> and <Table 3>, when the bending test was performed at 1 mmR or 2 mmR on the hot-rolled steel sheet wound at 450 ° C and 350 ° C according to Examples 1 and 2, micro cracks were generated. However, in the case of performing the bending test at 3 mmR and 5 mmR on the hot-rolled steel sheet according to the examples 1 and 2, and the bending test at 270 ° C and 160 ° C according to the example 3 and the example 4, It can be confirmed that no crack is generated. When ferrite having a relatively high elongation is present in the surface portion, it can be confirmed that the bending property is remarkably improved as compared with the case where the ferrite has a relatively high elongation.

FIG. 9 is a scanning electron microscope (SEM) image for explaining a phase transformation according to a surface cooling method of a slab for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention, FIG. 10 is a cross- Fig. 3 is a scanning electron microscope (SEM) image for explaining the phase transformation according to the surface cooling time of the slab.

Referring to Fig. 9, (a) of Fig. 9 shows a hot-rolled slab described with reference to Fig. 8, followed by air cooling for 3 seconds in the air to perform the first phase transformation step described with reference to Fig. 1 , FIG. 9 (b) is a cross-sectional view of the slab, which is obtained by hot-rolling the slab described with reference to FIG. 8 and then supplying compressed air to the surface of the slab for 3 seconds using an air gun, The first phase transformation step is performed.

When air was air-cooled for 3 seconds in the atmosphere, ferrite was formed to a thickness of about 20 mu m, and when it was cooled by compressed air for 3 seconds, it was confirmed that about 100 mu m thick ferrite was formed.

Referring to Fig. 10, the cooling time of the surface portion of the slab described with reference to Fig. 8 was adjusted using an iron gun. The longer the cooling time using the air gun is, the larger the thickness of the ferrite structure becomes. That is, by controlling the cooling method and the cooling time, the thickness of the structure that is phase-transformed at the surface portion of the slab can be adjusted.

11 is a view for explaining a change in characteristics of the surface portion of the hot-rolled steel sheet according to the ferrite fraction according to the embodiment of the present invention.

Referring to FIG. 11, the formation fraction of ferrite is different on the surface portion of the slab described with reference to FIG. Fig. 11 (a) shows a relatively low fraction of ferrite from the surface of the slab to about 160 m, Fig. 11 (b) shows a relatively high fraction of ferrite from the surface of the slab to about 280 m, 11 (c) shows a relatively low percentage of ferrite formed on the surface of the slab to about 500 m.

As a result of the experiment of the bending property, it is found that the ferrite of relatively low fraction is formed to have a relatively thin thickness, and the case of (c) of Fig. 11 where the relatively low fraction of ferrite is formed to have a relatively thick thickness, In the case of Fig. 11 (b) in which a relatively high percentage of ferrite was formed, the bending property was evaluated to be excellent. That is, in the first phase transformation step described with reference to Fig. 1, it can be seen that the fraction of ferrite in the surface portion of the slab has a greater influence on the bending property than the thickness of the phase-transformed ferrite structure.

12 is a graph for explaining a tensile curve according to the coiling temperature of a hot-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 12, the slabs described with reference to FIG. 8 were subjected to the first phase transformation step and the second phase transformation step described with reference to FIG. 1, and then the tensile strength was measured at different winding temperatures . As described above with reference to Table 1, in Examples 1 to 4, hot rolled steel sheets were wound at 450 ° C, 350 ° C, 270 ° C, and 160 ° C, respectively.

As can be seen from FIG. 12, when the coiling temperature was increased from 160 캜 to 270 캜, the tensile strength was increased, and the tensile strength was decreased as the coiling temperature was increased at a temperature higher than 270 캜. The elongation decreased as the temperature increased from 160 ° C to 270 ° C, and the elongation increased as the coiling temperature increased.

13 to 15 are views for explaining an application example of a hot-rolled steel sheet according to an embodiment of the present invention.

13 to 15, the hot-rolled steel sheet according to the embodiment of the present invention can be used as an automobile accessory. The hot-rolled steel sheet according to the embodiment of the present invention can be processed into a B-pillar 310, a side impact beam 320, or an automobile wheel 330 of an automobile. In addition to the B-pillar 310, the side impact beam 320, or the automobile wheel 330, the hot-rolled steel sheet according to the embodiment of the present invention can be processed into various automobile accessories such as a bump reinforcing agent. As described above, the hot-rolled steel sheet according to the embodiment of the present invention has characteristics of high strength and high elongation, and thus can be highly utilized as an automobile accessory.

13 to 15 illustrate that the hot-rolled steel sheet according to the embodiment of the present invention can be used as an automobile accessory. However, the present invention is not limited to this, and may be applied to various industrial fields requiring high strength and high elongation characteristics. It is obvious that hot rolled steel sheets can be used.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the scope of the present invention is not limited to the disclosed exemplary embodiments. It will also be appreciated that many modifications and variations will be apparent to those skilled in the art without departing from the scope of the invention.

10: Hot-rolled steel plate
10a: first side
10b: second side
S: Slav
210: Rollers
220: Carrying table
230: first surface cooling section
232, 233: Sensing agent supply line
234: Sensing agent supply pump
240: second surface cooling section
250: third cooling section

Claims (9)

A hot rolling step of hot rolling the slab;
A first phase transformation step of injecting a warming agent or compressed air in a liquid state onto the surface of the hot-rolled slab to selectively phase-change the structure of the surface portion of the slab;
A second phase transformation step of phase-transforming the structure of the central portion of the slab while keeping the structure of the surface portion of the slab by water-cooling the slab after phase-transforming the structure of the surface portion of the slab; And
And a winding step of winding the slab including the surface portion and the center portion that have been phase-transformed,
The slab is conveyed in a first direction by a roller,
Wherein the compressed air or the liquid state warming agent is supplied to a surface of the slab in a direction opposite to the first direction.
delete The method according to claim 1,
The structure of the surface portion of the slab has a higher elongation than the structure of the center portion of the slab,
Wherein the structure of the central portion of the slab has a higher strength than a structure of the surface portion of the slab.
The method according to claim 1,
The hot-rolled slab has austenite structure,
By the first phase transformation step, the surface portion of the slab has a ferrite structure, the center portion of the slab has austenite structure,
Wherein the center portion of the slab has a structure having higher strength than that of the ferrite by the second phase transformation step and the surface portion of the slab has a ferrite structure.
The method according to claim 1,
Further comprising a tempering step of tempering the rolled slab.
The method according to claim 1,
Wherein the first phase transformation step and the second phase transformation step are performed using different equipment.
A roller for hot rolling the slab;
A conveying table for conveying the hot rolled slab;
A first surface cooling unit disposed on the conveyance table for cooling the first surface of the hot-rolled slab;
A second surface cooling section disposed apart from the first surface cooling section with the transport table therebetween, for cooling a second surface of the slab facing the first surface; And
And a third cooling unit that water-cools the slabs cooled by the first and second surface cooling units,
Wherein the first surface cooling section and the second surface cooling section include a warming agent supply nozzle for supplying a warming agent to the first surface and the second surface of the slab,
Wherein the thermosensitive element supply nozzle is arranged oblique to the first surface and the second surface of the slab so that the thermosensitive agent is applied to the first surface of the slab in a direction opposite to the first direction, And supplying the hot rolled steel sheet to the surface and the second surface.
8. The method of claim 7,
Wherein the first surface cooling portion and the second surface cooling portion comprise:
Further comprising a warming agent supply line extending in a second direction perpendicular to a first direction in which the conveying table conveys the slab,
Wherein the warming agent supply nozzle includes a supply of the warming agent from the warming agent supply line.
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