KR101609938B1 - Forming method of steel sheet, manufacturing equipment for the same, and formed structure - Google Patents

Forming method of steel sheet, manufacturing equipment for the same, and formed structure Download PDF

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Publication number
KR101609938B1
KR101609938B1 KR1020140094522A KR20140094522A KR101609938B1 KR 101609938 B1 KR101609938 B1 KR 101609938B1 KR 1020140094522 A KR1020140094522 A KR 1020140094522A KR 20140094522 A KR20140094522 A KR 20140094522A KR 101609938 B1 KR101609938 B1 KR 101609938B1
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South Korea
Prior art keywords
steel sheet
phase
transformation step
cooling
phase transformation
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KR1020140094522A
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Korean (ko)
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KR20160005304A (en
Inventor
김두한
김종렬
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한양대학교 에리카산학협력단
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Priority to PCT/KR2015/006899 priority Critical patent/WO2016003245A1/en
Priority to US15/323,834 priority patent/US20170204489A1/en
Publication of KR20160005304A publication Critical patent/KR20160005304A/en
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Publication of KR101609938B1 publication Critical patent/KR101609938B1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/613—Gases; Liquefied or solidified normally gaseous material
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/667—Quenching devices for spray quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0062—Heat-treating apparatus with a cooling or quenching zone
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

A molded product of a steel sheet is provided. Wherein the molded article comprises a surface portion adjacent to an exterior surface and an interior center portion enclosed by the surface portion, wherein a first phase in the surface portion has a volume fraction, The phase has a minimum fraction and at the center the second phase has a maximum fraction and the first phase has a minimum fraction.

Description

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

TECHNICAL FIELD The present invention relates to a method for forming a steel sheet, a molding apparatus, and a molded article, and more particularly relates to a method for forming a steel sheet, a molding apparatus, and a molded article including a center portion having a texture different from that of the surface portion.

The high-strength steel sheet can be used for future automobile materials such as lightweight materials for automobiles and new concept parts, and can be used for high-strength reinforcing bars and repeated fatigue-resistant materials. Due to its excellent low temperature toughness and high tensile strength, it can be used as a material for high strength and high vibration resistance. In addition, it can be used as a weldable material, a pipe material, and the like, and the application fields and markets for high strength steel plates are increasing.

In general, if the strength of the steel sheet is increased, the elongation rate is lowered and the workability is lowered. Steel makers are working to maximize the advantages of high strength steel sheet by securing both conflicting characteristics of strength and workability.

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.

SUMMARY OF THE INVENTION The present invention provides a method of forming a steel sheet, a molding apparatus, and a molded article thereof for producing a molded article having high strength.

Another object of the present invention is to provide a method for forming a steel sheet, a molding apparatus, and a molded article thereof for producing a molded article having a high elongation.

It is another object of the present invention to provide a method of forming a steel sheet and a molding apparatus which can be easily applied to a conventional process.

In order to solve the above technical problem, the present invention provides a method of forming a steel sheet.

According to one embodiment, a method of forming a steel sheet includes the steps of preparing a steel sheet, heat treating the steel sheet, cooling the surface of the heat treated steel sheet to phase transform the structure of the surface portion of the steel sheet A first phase transformation step, a phase transformation step of phase-transforming the structure of the surface portion of the steel sheet, a shaping step of shaping the steel sheet, and a second step of cooling the formed steel sheet to phase- A phase transformation step may be included.

According to one embodiment, in the first phase transformation step, the phase of the tissue of the center portion of the steel sheet may be maintained.

According to one embodiment, in the shaping step and the second phase transformation step, the phase of the texture of the surface portion of the steel sheet may be maintained.

According to one embodiment, the heat treated steel sheet has an austenite structure, and the surface portion of the heat treated steel sheet is phase-transformed into a ferrite structure by the first phase transformation step, and the center portion of the heat treated steel sheet is heat- The surface portion of the formed steel sheet may have a structure in which the core portion of the steel sheet formed by the second phase transformation step has higher strength than that of the ferrite, have.

According to one embodiment, the elongation percentage of the surface portion that is phase-transformed in the first phase transformation step is higher than the elongation percentage of the center portion that is phase-transformed in the second phase transformation step, The strength of the central portion may include higher than the strength of the surface portion that is phase-transformed in the first phase transformation step.

According to one embodiment, the first phase transformation step may include supplying compressed air to the surface portion of the steel sheet.

According to one embodiment, the first phase transformation step may include ejecting a liquid state warming agent to the surface portion of the steel sheet.

In order to solve the above technical problems, the present invention provides a molded article.

According to one embodiment, the shaped article comprises a surface portion adjacent to an exterior surface and an interior center portion enclosed by the surface portion, wherein a first phase in the surface portion is a volume fraction The second phase has a minimum fraction and at the center the second phase has a maximum fraction and the first phase has a minimum fraction .

According to one embodiment, the tissue of the first phase is ferrite and the second phase may comprise tissue having a higher strength than ferrite.

According to an embodiment, the surface portion may consist of only the first phase, and the center portion may comprise only the second phase.

According to one embodiment, the fraction of the first phase decreases and the fraction of the second phase increases from the surface portion toward the center portion, and as the surface portion moves from the center portion toward the center portion, The fraction of the second phase may be decreased and the fraction of the first phase may be increased.

According to one embodiment, the first phase tissue may have a higher elongation than the second phase tissue, and the second phase tissue may have a higher strength than the first phase tissue.

In order to solve the above technical problems, the present invention provides a steel plate forming facility.

According to one embodiment, the forming equipment of the steel sheet includes a cutting portion for cutting the steel sheet, a heat radiating portion for heat-treating the cut steel sheet, a surface cooling portion for selectively cooling the surface portion of the heat-treated steel sheet, And a shaping / cooling section for shaping the steel sheet and cooling the center of the formed steel sheet.

According to one embodiment, the surface cooling section includes a first surface cooling section for cooling the first surface of the steel sheet, and a second surface cooling section for cooling the second surface of the steel sheet facing the first surface .

According to one embodiment, the first surface cooling portion and the second surface cooling portion are provided in a direction opposite to a direction in which the steel sheet is conveyed (antiparallel direction), and a warming agent is applied to the first surface of the steel sheet and the second surface Lt; RTI ID = 0.0 > surface. ≪ / RTI >

According to one embodiment, the warming agent supply nozzle for supplying the warming agent may include an oblique arrangement with respect to the first surface and the second surface of the steel sheet.

According to one embodiment, the surface cooling section selectively cools the surface portion except for the central portion of the heat treated steel sheet to phase-change the structure of the surface portion, and the shaping / cooling section cools the formed steel sheet , And phase-transforming the structure of the center portion except for the surface portion.

According to the embodiment of the present invention, the structure of the surface portion and the center portion of the steel sheet is transformed before or after the steel sheet is heat-treated and molded, and the surface portion has a structure having a relatively high elongation, Can have a relatively high-intensity tissue. Accordingly, a method of forming a steel sheet having high elongation, high porosity and high strength, a molding facility, and a molded article can be provided.

1 is a flowchart for explaining a method of forming a steel sheet according to an embodiment of the present invention.
2 is a flowchart illustrating a method of forming a steel sheet according to another embodiment of the present invention.
3 is a perspective view for explaining a molded article according to an embodiment of the present invention.
4 is a cross-sectional view taken along the line A-A 'in FIG. 3 for explaining a hot-rolled steel sheet according to an embodiment of the present invention.
5 is for explaining the fraction of the phase of the section cut along the line A-A 'in FIG.
6 is a cross-sectional view taken along the line A-A 'in FIG. 3 for explaining a molded product according to an embodiment of the present invention.
7 is a schematic view for explaining a steel sheet forming apparatus according to an embodiment of the present invention.
8 is a view for explaining a surface cooling unit included in a steel sheet forming equipment according to an embodiment of the present invention.
9 is a schematic view for explaining a steel sheet forming apparatus according to another embodiment of the present invention.
10 is a schematic view for explaining a steel sheet forming apparatus according to another embodiment of the present invention.
11 is a scanning electron microscope (SEM) image for explaining the microstructure 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. Also, in this specification, 'and / or' are used to include at least one of the front and rear components.

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. Also, in this specification, the term "connection " is used to include both indirectly connecting and directly connecting a plurality of components.

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 for explaining a method of forming a steel sheet according to an embodiment of the present invention.

Referring to FIG. 1, a steel sheet is prepared (S110). In one embodiment, the steel sheet may be a hot-rolled steel sheet. According to another embodiment, the steel sheet may be a cold rolled steel sheet. In the embodiment of the present invention, the kind of the steel sheet is not limited.

The steel sheet may be heat-treated (S120). The structure of the heat treated steel sheet may be transformed. For example, the steel sheet may have an austenite structure. According to one embodiment, both the surface portion and the center portion of the steel sheet may have an austenitic structure.

A first phase transformation step of cooling the surface of the heat treated steel sheet to transform the structure of the surface portion of the steel sheet may be performed (S130).

In the first phase transformation step, the surface of the heat treated steel sheet is cooled, so that the texture of the surface portion of the steel sheet can be transformed. The surface portion of the steel sheet may be a part of the steel sheet adjacent to the first surface of the steel sheet and the second surface facing the first surface. The first surface and the second surface may be the upper surface and the lower surface of the steel sheet.

For example, when the heat treated steel sheet before the first phase transformation step has austenite structure, by the first phase transformation step, the surface portion of the steel sheet may have a ferrite structure have.

During the phase transformation of the structure of the surface portion of the steel sheet by the first phase transformation step, the phase of the structure of the center portion of the steel sheet can be maintained. For example, even though the first phase transformation step is performed, the center portion of the steel sheet may have an austenitic structure. In other words, while the first phase transformation step is performed, the phase of the texture of the center portion of the steel sheet may not change. The center portion of the steel sheet may be a portion of the steel sheet, which is located between the first surface and the second surface of the steel sheet.

According to one embodiment, the first phase transformation step may include providing compressed air to the first and second surfaces of the steel sheet. 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 to the first surface and the second surface of the steel sheet. 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 steel sheet 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, The organization can be transformed. For example, the structure of the surface portion of the steel sheet may be transformed from austenite to ferrite.

 The compressed air and the liquid warming agent may be supplied to the first and second surfaces of the steel sheet for a short period of time. Accordingly, in the first phase transformation step, the center portion of the steel sheet is not cooled, and the center portion of the steel sheet can be maintained in a pahse.

After the structure of the surface portion of the steel sheet is phase-transformed, a forming step of molding the steel sheet may be performed (S140). The step of forming the steel sheet may be carried out by a method of pressing the steel sheet in which the structure of the surface portion is phase-transformed. The steel sheet can be molded, and a molded accessory can be manufactured.

A second phase transformation step of cooling the formed steel sheet and phase-transforming the structure of the center of the formed steel sheet may be performed (S150). As described above, the structure of the center portion of the formed steel sheet that has not been transformed in the first phase transformation step can be transformed by the second phase transformation step.

For example, when the center portion of the steel sheet that has not been transformed in the first phase transformation step has austenite structure, the center portion of the steel sheet by the second phase transformation step may be martensite or vena- It can be transformed into an organization. For example, the center portion of the steel plate may have HSLA (High Strength Low Alloy) steel structure such as DP (Dual Phase) steel structure, TRIP (Transformation Induced Plasticity) steel structure, or TWIP .

By the second phase transformation step, the phase of the texture of the surface portion of the steel sheet can be maintained while the structure of the center portion of the formed steel sheet is phase-transformed. For example, even though the second phase transformation step is performed, the surface portion of the steel sheet 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 steel sheet may not change.

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

The second phase transformation step may include water cooling the steel sheet on which the first phase transformation step has been performed. According to one embodiment, the second phase transformation step may be performed in a separate process using a different apparatus from the first phase transformation step. According to one embodiment, the second phase transformation step may be performed in the same equipment as the equipment in which the forming step is performed. In other words, immediately after the steel sheet is formed, the second phase transformation step can be performed in the equipment in which the steel sheet is molded.

As described above, the surface portion and the center portion may have respectively different structures (e.g., the surface portion may be a ferrite structure and the center portion may be a martensite or a veneite structure). On the other hand, a portion (a mixed portion) of the steel sheet positioned between the surface portion and the center portion of the formed steel sheet may have a dual phase structure.

The mixed portion of the steel sheet may be phase-changed by the first phase-change step, and may be phase-transformed by the second phase-change step. 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 heat treated steel sheet has austenitic structure before the first phase transformation step is performed, the first phase transformation step may cause the one portion of the mixed portion of the steel sheet to be in austenitic texture It can be transformed into 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 transformed from austenite to martensite or veneite. While the remaining portion of the mixing portion is phase-transformed into martensite or veneite structure, the one portion of the mixed portion that has already been phase-transformed may retain the ferrite structure. As a result, the mixed portion may have a dual phase structure of ferrite and martensite.

According to the embodiment of the present invention, the surface portion of the steel sheet is constituted by a relatively elongated structure (for example, ferrite) by the first phase transformation step, and the center portion of the steel sheet is composed of the second phase (E.g., martensite or veneite) having a relatively high strength by the transformation step. Accordingly, the formed steel plate (molded parts) can have high elongation and high strength.

Unlike the method of forming a steel sheet described with reference to FIG. 1, according to another embodiment, the first phase transformation step may be performed after forming the steel sheet. This will be described with reference to FIG.

2 is a flowchart illustrating a method of forming a steel sheet according to another embodiment of the present invention.

Referring to FIG. 2, according to the method described with reference to FIG. 1, a steel sheet is prepared (S110), and the steel sheet may be heat treated (S120). As described above with reference to Fig. 1, the heat treated steel sheet is transformed into a structure of the heat treated steel sheet, and according to an embodiment, the heat treated steel sheet may have a structure of austenite. A forming step in which the steel sheet heat-treated according to the method described with reference to Fig. 1 is formed may be performed (S130). The phase of the texture of the steel sheet heat-treated in the forming step can be maintained.

After the heat-treated steel sheet is formed, a first phase transformation step of cooling the surface of the formed steel sheet to transform the structure of the surface portion of the steel sheet according to the method described with reference to Fig. 1 is performed (S140). Thereafter, according to the method described with reference to Fig. 1, a second phase transformation step of cooling the formed steel sheet on which the first phase transformation step has been performed, to transform the structure of the center of the formed steel sheet (S150).

According to one embodiment, the first phase transformation step and the second phase transformation step may be performed sequentially in the same equipment. Alternatively, according to another embodiment, the first phase transformation step and the second phase transformation step may be performed in different equipment.

Hereinafter, a molded article manufactured according to the method for forming a steel sheet according to the embodiment of the present invention described above will be described with reference to Figs. 3 to 5. Fig.

FIG. 3 is a perspective view for explaining a molded product according to an embodiment of the present invention, FIG. 4 is a view for explaining a hot-rolled steel sheet according to an embodiment of the present invention, And FIG. 5 is for explaining the fraction of the phase of the section cut along the line A-A 'in FIG.

3 to 5, the molded article 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 molded article 10 located between the first surface 10a and the second surface 10b.

The molded article 10 may be a dual phase structure having a first phase and a second phase. The first phase of the molded article 10 may be generated in the first phase transformation step described with reference to Figs. 1 and 2, and the second phase may be generated in the first phase transformation step described with reference to Figs. 1 and 2, Phase transformation step.

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 molded article 10 adjacent to the first surface 10a and a second portion of the molded article 10 adjacent the second surface 10a may be in the first phase 10a, ), And may not have the structure of the second phase. 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 and the fraction of the second phase gradually increases from the first surface 10a toward the center portion. The second portion may be a surface portion adjacent to the exterior surface of the molded article 10 and the central portion may be a portion of the interior of the molded article 10 surrounded by the surface portion.

The second phase may have a maximum fraction at the center. For example, the central portion of the molded product 10 may be composed of only the second phase, and may not have the first phase. As another example, the central portion of the molded article 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, veneite, or dual phase steel structure, TRIP (Transformation Induced Plasticity) steel structure, or TWIP (Twinning Induced Plasticity) (HSLA) steel structure such as a 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 center portion composed of only the first phase, the second phase, and the second phase consisting only of the first phase, and the mixed portion in which the first and second phases coexist are described in more detail with reference to FIG. 6 .

6 is a cross-sectional view taken along the line A-A 'in FIG. 3 for explaining a molded product according to an embodiment of the present invention.

6, the molded article 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, which are sequentially stacked. . ≪ / RTI > 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, veneite, or dual phase steel structure, TRIP (Transformation Induced Plasticity) steel structure, or TWIP (Twinning Induced Plasticity) (HSLA) steel structure such as a 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. 7 to 10, a steel sheet forming apparatus according to an embodiment of the present invention will be described.

FIG. 7 is a schematic view for explaining a molding apparatus for a steel sheet according to an embodiment of the present invention, and FIG. 8 is a view for explaining a surface cooling unit included in a steel sheet forming apparatus according to an embodiment of the present invention.

7 and 8, a steel sheet forming apparatus according to an embodiment of the present invention includes a cutting unit 110, a heat supply unit 120, a surface cooling unit 130, and a forming / cooling unit 140 can do.

The cutting unit 110 can cut the steel sheet S wound on the coil to a predetermined size. The steel sheet S may be a hot rolled steel sheet or a cold rolled steel sheet as described with reference to Fig.

The heat radiating part 120 may heat-treat the steel sheet S cut by the cutting part 110. The heat treated steel sheet S may be phase-transformed as described with reference to Fig. According to one embodiment, the heat treated steel sheet S may have an austenitic structure.

The surface cooling unit 130 may selectively cool the surface portion of the steel sheet S subjected to the heat treatment. The surface cooling part 130 includes a first surface cooling part 130a for cooling the first surface of the steel sheet S and a second surface cooling part 130b for cooling the second surface of the steel sheet S facing the first surface, And a surface cooling unit 130b. The first surface cooling part 130a and the second surface cooling part 130b may be spaced apart with the steel sheet S therebetween.

Each of the first surface cooling part 130a and the second surface cooling part 130b may include a warming agent supply line 132 and a warming agent supply nozzle 134. [ The warming agent supply line 132 may extend in a second direction perpendicular to the first direction D in which the steel sheet S is conveyed. The warming agent supply nozzle 134 is provided in the warming agent supply line 132 and is provided with a warming agent supplied from the warming agent supply line 132 to the first surface of the steel sheet S and the second surface of the steel sheet S, Can be supplied to the surface. According to one embodiment, the warming agent supply nozzles 134 may be provided in a plurality of spaced apart from each other in the first direction D.

The warming agent provided on the first surface and the second surface of the steel sheet S through the warming agent supply nozzle 134 may be, for example, compressed air or liquid. When the thermosensitive agent is a liquid thermosensitive agent, the thermosensitive agent may be sprayed toward the first surface and the second surface from the thermosensitive agent supply nozzle 134.

The warming agent supply nozzle 134 may be configured to apply the warming agent to the first surface of the steel sheet S and the second surface of the steel sheet S in a direction opposite to the first direction D in which the steel sheet S is conveyed. Can be supplied to the surface. Accordingly, the warming agent supplied from the warming agent supply nozzle 134 to the steel sheet S is contacted with the first surface and the second surface of the steel sheet S, D). ≪ / RTI > Accordingly, the first surface and the second surface of the steel sheet S are momentarily cooled, and the thermosensitive material may not be repeatedly supplied to the surface of the steel sheet S already supplied with the thermosensitive material. The center portion of the steel sheet S is not cooled and the first portion of the steel sheet S adjacent to the first surface and the second portion of the steel sheet S adjacent to the second surface are selectively cooled .

The first portion and the second portion of the steel sheet S are selectively cooled by the first surface cooling portion 130a and the second surface cooling portion 130b so as to be described with reference to FIG. The first phase transformation step described above can be performed. Specifically, the first surface cooling part 130a and the second surface cooling part 130b cools the first surface and the second surface of the steel sheet S, The first portion adjacent the first surface and the second portion adjacent the second surface may be phase transformed. The central portion of the steel sheet S is not cooled while the steel sheet S is cooled by the first surface cooling portion 130a and the second surface cooling portion 130b, The phase of the tissue at the central portion may not change.

The forming / cooling unit 140 can mold the steel sheet S in which the texture of the surface portion has been transformed, and cool the central portion of the steel sheet to form the molded article 10. [ According to one embodiment, the forming / cooling unit 140 may supply a large amount of water to the formed steel sheet S, thereby cooling the formed steel sheet S by water. Accordingly, the second phase transformation step described with reference to Fig. 1 can be performed. Specifically, the central portion of the formed steel sheet S is phase-transformed by the water supplied from the forming / cooling section 140, and the image of the texture of the surface portions adjacent to the first and second surfaces It may not change.

In the steel sheet forming equipment described with reference to Fig. 7, the steel sheet S heat-treated by the heat supply unit 120 is molded after the first phase transformation step is performed by the surface cooling unit 130 However, as described with reference to FIG. 2, the first phase transformation step and the second phase transformation step may be sequentially performed after the heat-treated steel sheet S is formed. This will be described with reference to FIG.

9 is a schematic view for explaining a steel sheet forming apparatus according to another embodiment of the present invention.

Referring to FIG. 9, a cutter 110, a heat supply part 120, and a forming / cooling part 140 described with reference to FIG. 7 are provided, and the surface cooling part 130 may be omitted. The steel sheet forming apparatus according to another embodiment of the present invention is characterized in that the steel sheet S heat-treated in the heat supplying unit 120 is formed in the molding / cooling unit 140, And the first phase transformation step and the second phase transformation step may be sequentially performed in the shaping / cooling part 140. [ In other words, after the steel sheet S is heat-treated by the heat supply unit 120 and the structure is phase-transformed (for example, after the steel sheet S is phase-changed to the austenite structure), in the molding / cooling unit 140, The surface of the steel sheet S is molded and the thermosensitive material is supplied to the surface of the formed steel sheet S so that the structure of the surface portion of the steel sheet S undergoes phase transformation (for example, phase transformation into a ferrite structure) Water may be supplied to the steel sheet S so that the structure of the center portion may be transformed into a phase transformation state (for example, phase transformation into martensite or veneite structure).

9, the first and second phase transformation steps are not performed in the forming / cooling unit 140, and the first and second phase transformation steps are performed in the molding / cooling unit 140, The first phase transformation step and the second phase transformation step may be sequentially performed on the steel sheet S. [ This will be described with reference to FIG.

10 is a schematic view for explaining a steel sheet forming apparatus according to another embodiment of the present invention.

10, a cutting portion 110, a heat supply portion 120, a surface cooling portion 130, and a forming / cooling portion 140 described with reference to FIG. 7 may be provided. 7, the surface cooling unit 130 does not perform the first phase transformation step on the steel sheet S heat-treated in the heat supply unit 120, The first phase transformation step may be performed on the steel sheet S formed in the first step 140. The steel sheet S on which the first phase transformation step has been performed is subjected to a second phase transformation step as described with reference to Figure 2, As such, it may have a surface portion having a relatively elongated structure and a central portion having a relatively high-strength structure.

11 is a scanning electron microscope (SEM) image for explaining the microstructure of a steel sheet according to an embodiment of the present invention.

11, 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 steel plate made of iron (Fe) and other unavoidable impurities was prepared. The steel sheet has a thickness of 40 mm and an austenite structure. The first phase transformation step described with reference to Figure 1 was performed by providing compressed air to the heat treated steel sheet. As a result, the surface portion of the steel sheet 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 steel sheet was phase-transformed from austenite to martensite in the second phase transformation step.

As can be seen from Fig. 10, it can be confirmed that the surface portion of the steel sheet to which the compressed air is supplied has a ferrite structure having a relatively elongation, and the center portion of the steel sheet 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.

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: Molded product
10a: first side
10b: second side
11: First part
12: first mixing portion
13: center
14: second mixing portion
15: second part
S: Steel plate
110:
120: Thermal treatment
130:
140: forming / cooling section

Claims (17)

Preparing a steel sheet;
Heat-treating the steel sheet to transform the steel sheet into an austenite structure;
A first phase transformation step of cooling the surface of the heat treated steel sheet to phase transform the surface of the steel sheet so as to have a ferrite structure;
A forming step of forming the steel sheet after phase-transforming the structure of the surface portion of the steel sheet; And
And a second phase transformation step of cooling the formed steel sheet to phase-transform the center of the formed steel sheet so as to have a martensitic structure.
The method according to claim 1,
Wherein the phase of the structure of the central portion of the steel sheet is held in the first phase transformation step.
3. The method of claim 2,
Wherein the phase of the texture of the surface portion of the steel sheet is held in the forming step and the second phase transformation step.
The method according to claim 1,
The surface portion of the heat treated steel sheet is phase-transformed into a ferrite structure by the first phase transformation step, the center portion of the heat treated steel sheet maintains the austenite structure,
Wherein the central portion of the steel sheet formed by the second phase transformation step has a martensitic structure and the surface portion of the steel sheet that has been molded contains a ferrite structure.
The method according to claim 1,
The elongation percentage of the surface portion transformed in the first phase transformation step is higher than the elongation percentage of the center portion transformed in the second phase transformation step,
Wherein the strength of the center portion that is phase-transformed in the second phase transformation step is higher than the strength of the surface portion that is phase-transformed in the first phase transformation step.
The method according to claim 1,
Wherein the first phase transformation step comprises:
And supplying compressed air to the surface portion of the steel sheet.
The method according to claim 1,
Wherein the first phase transformation step comprises:
And spraying a liquid state warming agent onto the surface portion of the steel sheet.
delete delete delete delete delete A cutting section for cutting the steel sheet;
A heat supply part for heat-treating the cut steel plate;
A surface cooling unit for selectively cooling the surface portion of the heat treated steel sheet; And
And a shaping / cooling section for shaping the steel sheet having the surface portion cooled and cooling the center portion of the steel sheet formed,
The surface-
A first surface cooling section for cooling the first surface of the steel sheet; And
And a second surface cooling section for cooling the second surface of the steel plate facing the first surface,
The first surface cooling portion and the second surface cooling portion may include supplying a warming agent to the first surface and the second surface of the steel sheet in an anti parallel direction to the direction in which the steel sheet is conveyed Steel plate forming equipment.
delete delete 14. The method of claim 13,
Wherein the warming agent supply nozzle for supplying the warming agent includes an oblique arrangement with respect to the first surface and the second surface of the steel sheet.
14. The method of claim 13,
Wherein the surface cooling portion selectively cools the surface portion of the heat treated steel sheet except for the center portion to phase-transform the texture of the surface portion,
Wherein the forming / cooling section includes a step of cooling the formed steel sheet to phase-change the structure of the central portion except for the surface portion.
KR1020140094522A 2014-07-04 2014-07-25 Forming method of steel sheet, manufacturing equipment for the same, and formed structure KR101609938B1 (en)

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