KR101225794B1 - Method and apparatus for manufacturing steel sheet with ultra high strength - Google Patents

Abstract

The slab is hot rolled at a temperature higher than the Ar3 temperature, quenched to produce ferrite in the hot rolled hot rolled steel sheet, and then quenched to a temperature directly below the martensite transition point (Ms) for martensite formation in the hot rolled steel sheet. After the quenched hot rolled steel sheet is maintained using a thermal insulation tunnel at a temperature of 400 ° C to 300 ° C, a method and apparatus for manufacturing a steel sheet for winding a hot rolled steel sheet with a coil are presented.

Description

Method and apparatus for manufacturing ultra high strength steel sheet {Method and apparatus for manufacturing steel sheet with ultra high strength}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet manufacturing technology, and more particularly, to a method of manufacturing an ultra high strength steel sheet and a manufacturing apparatus used therefor.

In order to secure the competitiveness of steel materials against nonferrous metal materials, we are trying to develop high strength and high toughness steel materials. In order to make steel materials more competitive than nonferrous metals in structural materials for automobiles and other transportation machinery, steel materials having higher strength are required. In the case of automotive steels, steel sheets of high strength are required to secure energy-saving effects, increase fuel efficiency, and eco-friendly effects due to reduction of carbon dioxide (CO ₂ ) gas emissions by reducing body weight.

The present invention is to propose a method for manufacturing a steel sheet that can have a high strength while eliminating additional heat treatment to secure the strength.

One aspect of the invention, the step of hot rolling a slab (slab) at a temperature higher than the Ar3 temperature; Slow cooling to produce ferrite in the hot rolled hot rolled steel sheet; Quenching to a temperature directly below the martensite transition point (Ms) to produce martensite in the hot rolled steel sheet; A temperature maintaining step of maintaining the quenched hot rolled steel sheet at a temperature of 400 ° C. to 300 ° C .; And it provides a steel sheet manufacturing method comprising the step of winding the hot rolled steel sheet with a coil (coil).

Another aspect of the invention, the hot rolling unit for hot rolling the slab (slab); A quenching unit for supplying and quenching the hot rolled hot rolled steel sheet conveyed from the hot rolling unit; A slow cooling section in which the hot rolled steel sheet being conveyed is cooled between the quench portion and the hot rolling portion; A winder for winding the quenched hot rolled steel sheet with a coil; And a thermal insulation tunnel through which the hot rolled steel sheet which is transferred between the winder and the quenching portion is insulated and passed.

According to an embodiment of the present invention, it is possible to provide a steel sheet manufacturing method for securing a high strength by ensuring the elongation through the production of ferrite in the hot-rolled steel sheet, induces the production of martensite through rapid cooling. Steel sheet manufactured according to an embodiment of the present invention can secure a tensile strength of 900MPa to 1000Mpa and an elongation of 15% to 20%. By maintaining isothermal during the process of quenching the hot rolled steel sheet before winding to the coil, it is possible to reduce internal stress that may be caused during quenching and to move carbon in the supersaturated martensite to achieve stabilization of residual austenite. In order to maintain the isothermal temperature before the winding, a steel sheet manufacturing apparatus having a heat insulating tunnel in the movement path of the hot rolled steel sheet between the hot rolling portion and the coil winding portion can be proposed.

1 is a view showing a steel sheet manufacturing method according to an embodiment of the present invention.
2 and 3 are views showing a steel sheet manufacturing apparatus according to an embodiment of the present invention.

An embodiment of the present invention proposes a method of manufacturing an ultra-high strength hot rolled steel sheet through multi-stage cooling after performing hot rolling. Hot rolled steel sheet, which is widely used for the internal structure of automobiles, mostly has a structure of ferrite and pearlite, and its elongation is realized at a level of 30% to 50%, but its tensile strength is 400MPa to 500MPa. Hot rolled steel sheet manufacturing method using a multi-stage cooling according to an embodiment of the present invention, it is possible to secure a tensile strength of 900MPa to 1000Mpa and an elongation of 15% to 20%, can meet the mechanical properties required for automotive steel sheet. .

Referring to Figure 1, in the steel sheet manufacturing method according to an embodiment of the present invention, after extracting the slab (slab) in a heating furnace, finish hot rolling to a thickness of 1.2T (mm) to 2.0T in the austenite region of the Ac ₃ temperature or more (S1). The slab heated in the furnace may be rolled to a width of about 200 to 300 mm by a slab sizing press (SSP), and then, to a thickness width required for finishing rolling, for example, to a thickness of 30 to 60 mm. Rough Mill (RM). Thereafter, after removing the scale, hot rolling is performed to the hot rolled steel sheet 100 having a thickness of 1.2T to 2.0T using a finishing mill (FM: 210) as shown in FIG. 2. The hot rolling unit of the steel sheet manufacturing apparatus may be configured to include such a finishing mill 210.

The hot rolling is carried out at a temperature in the austenite region above the Ac ₃ temperature, for example at temperatures between 850 ° C. and 950 ° C., preferably at least 900 ° C. As a result, the hot rolled steel sheet having a substantially austenite structure is rolled.

Steel slab according to an embodiment of the present invention may be applied to ultra-low carbon steel, for example, 0.08 parts by weight (wt%) to 0.15wt% of carbon (C), 1.5wt% or less of silicon (Si), 1.5 wt% or less manganese (Mn), 0.015 wt% or less phosphorus (P), 0.015 wt% or less titanium (Ti), 0.5 wt% or less aluminum (Al), and 0.003 wt% or less nitrogen (N) Slabs can be made of carbon steel, the balance of which is iron (Fe). At this time, an additional element such as niobium (Nb) may be further added, an impurity such as copper (Cu) or tin (Sn) or a cyclic element may be contained, and sulfur (S) is 0.01 wt%. It may be contained in% or less. It may be more effective to contain about 0.1 wt% to 1.15 wt% carbon.

Referring again to FIGS. 1 and 2, the hot rolling is performed at a temperature of 850 ° C. to 950 ° C. (S1), after which the hot rolled steel sheet (100 in FIG. 2) having a substantially austenitic structure is rolled, and then initially cooled slowly. The section is introduced (S2). The slow cooling process may be performed by an air cooling process, and within about 10 seconds, for example, about 2 to 10 seconds until the temperature of the hot rolled material, that is, the hot-rolled steel sheet 100 is reduced to about 800 ° C. Maintain air cooling. In the hot rolled rolling process, the hot rolled steel sheet 100 moves to a run out table (ROT) section (230 of FIG. 2) and passes through the slow-rolling section (220 of FIG. 2) at the beginning of the ROT section 230. The hot rolled steel sheet 100 is slowly cooled by introduction. The slow cooling section 220 is a section allowing the hot rolled steel sheet 100 to be transferred in an air-cooled state unlike the ROT section 230.

By introducing such slow cooling sections S2 and 220, approximately 25% to 50% of the soft structure ferrite is generated in the structure of the hot rolled steel sheet 100 during the slow cooling process. Elongation can be ensured by producing ferrite in the structure of a steel plate. Effectively, ferrite and austenite are induced to form in the structure of the hot rolled steel sheet 100 in an approximately 50:50 ratio.

Subsequently, it is quenched immediately below the martensite transition point, that is, the Ms point (martensite start point) (S3). This quenching process may be performed while the hot rolled steel sheet 100 passes through the ROT section 230. For example, in the quenching process, the cooling water is supplied or sprayed to the hot rolled steel sheet 100 to quench the water by the water cooling action by the laminar flow of the cooling water. The quenching unit may be configured to include the ROT section 230. This quench section (S3) is performed so that the temperature of the hot-rolled steel sheet 100 falls below the Ms point of the martensite transition point, for example, 300 ℃ to 400 ℃. By such a quench section S3, martensite structure is transformed and generated in the structure of the steel plate 100. By primarily producing martensite structure in the steel sheet 100, the strength of the steel sheet 100 can be increased.

Due to the formation of martensite by the quenching process, an internal stress increases in the hot-rolled steel sheet 100, and a risk of cracking increases as the internal stress increases. An isothermal holding section S4 is introduced after the quench section S3 to mitigate and reduce the increased internal stress and induce the diffusion of invasive elements such as carbon atoms. The isothermal holding section S4 may be maintained for several seconds, and more preferably, for 3 to 5 seconds. This isothermal holding section (S4) is to maintain the temperature of the hot rolled steel sheet 100 in the temperature range of 300 ℃ to 400 ℃. Internal stress generated in the hot-rolled steel sheet 100 during isothermal holding is alleviated and reduced, and carbon in the supersaturated martensite may also move and diffuse. By such a carbon diffusion movement, stabilization of residual austenite can be realized.

Since the hot rolled steel sheet 100 is maintained in the form of a thin steel sheet having a thickness of about 1.2T to 2.0T before being wound into a coil, the temperature may be drastically reduced even in an air-cooled state exposed to the air. That is, in the state of the thin steel sheet, the volume is relatively small compared to the heat dissipation surface area, so that the heat dissipation is relatively large. Therefore, it is difficult to maintain isothermal for several seconds in the air-cooled state. In an embodiment of the present invention, in order to secure an isothermal holding section S4, a thermal tunnel 240 as shown in FIGS. 2 and 3 is introduced.

That is, an insulating tunnel for providing an isothermal holding section S4 in the movement path between the ROT section 230 which is the quench section S3 and the winding machine 250 in which the hot rolled steel sheet 100 is wound by the coil 111 ( 240). As shown in FIG. 3, the insulating tunnel 240 may be formed in a cover shape having a semicircular cross section to cover the hot rolled steel sheet 100 being transferred, and may be configured using a heat insulating material. In addition, the insulating tunnel 240 may be configured to have a cross section in the form of a circle or a square. The thermal insulation tunnel 240 may be configured to have a length of about several tens of meters (M), for example, 20M, in consideration of the time for the hot rolled steel sheet 100 to pass through for the required isothermal maintenance section S4. The interior 241 is blocked from the outside 243 by the insulating tunnel 240, and thus, heat movement from the inside 241 to the outside 243 may be blocked. Since the amount of heat emitted from the hot rolled steel sheet 100 may be trapped in the inside of the insulating tunnel 240, the amount of heat generated from the hot rolled steel sheet 100 may be reduced, and thus, the hot rolled steel sheet 100 may be used. The temperature of may be maintained in the 300 to 400 ℃ temperature range.

The hot rolled steel sheet 100 is wound on the winding machine 250 in the form of a coil 111, and air-cooled in the wound hot rolled coil 111 state (S5 of FIG. 1). By the isothermal holding section (S4) within the 400 ° C to 300 ° C temperature range, the temperature of the hot rolled steel sheet 100 during winding may be maintained substantially within the 400 ° C to 300 ° C temperature range. Compared to the hot rolled steel sheet 100 before winding, the hot rolled coil 111 in the wound state has a relatively large volume relative to the surface area where heat dissipation occurs, and a slow cooling state in which the temperature is gradually reduced even in an air cooled state may be induced.

As described above, in the embodiment of the present invention, by installing a heat insulating material such as a heat insulating tunnel 240 in the facility of the steel sheet manufacturing apparatus, the isothermal holding section S4 is implemented to quench the hot rolled steel sheet 100 after 300 ° C. to 400 ° C. temperature range. You can keep it at In this way, the isothermal holding section S4 can be realized, and it is possible to effectively alleviate the internal stress due to quenching, and also to realize the residual austenite stabilization through the carbon migration in the supersaturated martensite.

The steel sheet manufactured as described above can be confirmed by an experimental test (test) that can secure an average 914 MPa tensile strength, an average 499 MPa yield strength and an average elongation of 18%. Therefore, the hot rolled steel sheet manufacturing method according to the embodiment of the present invention is effective to produce a steel sheet of ultra high strength.

The hot rolled coil 111 may be transferred to a cold rolling process to perform cold rolling to form a final steel sheet. Mechanical properties of the hot rolled coil 111 produced in the hot rolling process is greatly improved, it can be used for the internal structure of the vehicle to implement a weight reduction of the vehicle body by reducing the thickness of the material due to the increased strength. In addition, steel grades that may omit the heat treatment process for increasing strength in the cold rolling line are increased, thereby effectively realizing cost reduction in the cold rolling process. In the cold rolling line, the final product can be produced by performing thickness control only through cold rolling without undergoing a heat treatment process, thereby effectively realizing cost reduction.

S1 ... hot rolling S2 ... slow cooling section
S3 ... quench section S4 ... isothermal section
S5 ... air-cooled section 240 ... thermal insulation tunnel.

Claims (7)

Hot rolling the slab at a temperature higher than the Ar ₃ temperature;
Slow cooling to produce ferrite in the hot rolled hot rolled steel sheet;
Quenching to a temperature directly below the martensite transition point (Ms) to produce martensite in the hot rolled steel sheet;
A temperature maintaining step of maintaining the quenched hot rolled steel sheet at a temperature of 400 ° C. to 300 ° C .; And
Winding the hot rolled steel sheet with a coil;
The slow cooling step
Air-cooling until the temperature of the hot-rolled steel sheet drops from a temperature range of 950 ° C. to 850 ° C. to 800 ° C.,
The slow cooling step
Air-cooling the hot rolled steel sheet such that the ferrite is produced by weight by 25% to 50% by weight in the hot rolled steel sheet,
The temperature maintaining step
The steel sheet manufacturing method to maintain the temperature in the range of 400 ℃ to 300 ℃ until the hot rolled steel sheet is wound into the coil.
The method of claim 1,
The hot rolling
Steel sheet manufacturing method which is finished in the temperature range of 950 ℃ to 850 ℃.
delete delete The method of claim 1,
The quenching step
The steel sheet manufacturing method for water-cooling the hot rolled product so that the temperature of the hot rolled steel sheet falls to 400 ℃ to 300 ℃ range.
delete delete

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