KR101474140B1 - Bending method of thick steel plate - Google Patents

Abstract

According to the present invention, there is provided a method of manufacturing a steel plate, comprising the steps of: preparing a thick plate; displaying a plurality of intermediate lines to be subjected to press working on an upper surface of the thick plate; Placing a thick plate marked with a middle line on the burner and preheating to a specified temperature; Mounting a plurality of lifting lugs on an outer circumferential surface of the thick plate; Connecting a hook of a crane to a lifting lug to dispose the thick plate so as to be seated on an upper surface of a bottom die of the pressing press, and then bending the pressed both ends of the thick plate; Placing a thick plate back on the burner and performing stress relief heat treatment; And completing the curved surface in order from the ends of the both ends in the center direction along the marking of the thick plate with the press.
Therefore, unlike the conventional hot working, a special heat treatment process is not required to meet the required strength according to the constituents of the heavy plate, so that the cost of processing the heavy plate is greatly reduced. Further, in the bending process of the thick plate, additional heat treatment is not necessary to remove the residual stress of the steel, so that it is possible to perform uniform curved surface processing without cracking or brittle fracture of the thick plate by a single process.

Description

{Bending method of thick steel plate}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bending method of a thick steel plate, and more particularly to a method of bending a thick plate applied to a jack up type leg of a ship or an offshore structure.

Generally, plate-like steel plates are steel plates with a thickness of 6 mm or more, plates with a thickness of 50 mm or more are called post-material plates, and plates with a thickness of 100 mm or more are called plate plates. These heavy plates are most commonly used for ships and offshore structures. For example, a jack-uprig, an offshore structure, uses a rack and a chord that make up a leg, and the cord usually uses a thick plate.

Generally, yield stress and tensile stress of the thick plate are much higher than those of general marine steels. When the yield stress and the tensile stress are high, the rigidity of the steel is high but the brittleness is increased. It also causes the steel to break easily due to the processing history. In this case, the steel material can be reinforced by quenching and tempering to guarantee the physical properties. In the case of such a thick plate, bending is carried out mostly through hot forming to improve elongation.

As an example of hot working as described above, Korean Patent Laid-Open Publication No. 10-2010-0104040 discloses an apparatus for measuring the shape information of a member to be used, a member measuring unit for hot- A control section for generating machining plan information and repeatedly acquiring a deformation model of hot working from hot machining plan information and hot deformed shape information after the member is hot deformed; And a method of processing the same.

However, in the above-mentioned conventional hot working method, since the pressure of the press to apply pressure to the thick plate is small, a comparatively low-quality press can be used, but when the preheating of the thick plate before the heat treatment exceeds the quenching temperature and annealing temperature Because of the thermal deformation of the steel plate, additional heat treatment and dimensional correction after bending are required due to oxidation, gas decomposition, and structural change of the steel plate due to high temperature.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for bending a thick plate that is minimized in terms of thermal deformation and residual stress after bending.

(A) preparing a thick plate, displaying on a top surface of the thick plate a plurality of intermediate lines to be machined with a pressing press; (b) placing the heavy plate marked with the middle line on the burner and preheating the plate to a predetermined temperature; (c) mounting a plurality of lifting lugs on an outer circumferential surface of the thick plate; (d) connecting the hooks of the crane to the lifting lugs to arrange the heavy plates on the bottom die of the pressing press, and then bending the marked ends of the heavy plates by pressing them; (e) placing the thick plate in a heating furnace to perform stress relief heat treatment; (f) placing the plate back on the burner and preheating to a specified temperature; And (g) pressurizing the press plate in order from the ends of the both ends in the direction of the center along the marking of the plate so as to complete the curved surface.

Also, in the step (a), the plurality of intermediate line displays may display a straight line parallel to the center at both ends of the thick plate at equal intervals.

In the step (b), it is preferable to heat the surface of the thick plate so that the surface temperature is 100 to 200 ° C.

In the step (c), the diameter of the top die of the pressing press is 90 to 110 mm, and the width of the bottom die is preferably 300 to 400 mm.

In the step (e), the stress relief heat treatment may include heating the thick plate at a heating rate of from 90 to 110 ° C / hour in an initial furnace at 370 to 390 ° C for 2 hours to 3 hours, Maintaining the temperature at 630 캜 for 2 to 3 hours, and slowly cooling the plate at a cooling rate of 70 to 90 캜 / hour.

Also, in the step (f) It is preferable that the pressing force is sequentially pressed from one end of the thick plate bent in the step (d) toward the center so that the radius of the inner curvature formed is 138 to 140 mm.

According to the bending method of the thick plate according to the present invention, unlike the conventional hot working, since the special heat treatment process is not required to meet the required strength according to the constituents of the thick plate, the cost of processing the thick plate is greatly reduced.

Further, in the bending process of the thick plate, additional heat treatment is not necessary to remove the residual stress of the steel, so that it is possible to perform uniform curved surface processing without cracking or brittle fracture of the thick plate by a single process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart of a process for bending a thick plate according to an embodiment of the present invention;
FIG. 2A is an exemplary view showing a state in which a middle line is displayed on a heavy plate in the heavy plate bending method shown in FIG. 1,
FIG. 2B is a view showing a state in which a thick plate is placed on a burner and preheated in the plate bending method shown in FIG. 1; FIG.
Fig. 2c is an example of attaching a lifting lug to a thick plate in the bending method of the bending plate shown in Fig. 1,
Fig. 2d is an example in which the thick plate is seated on the bottom die of the pressing press in the thick plate bending method shown in Fig. 1,
Fig. 2E is an example in which in the method of bending the thick plate shown in Fig. 1, the thick plate is bent by pressing one side of both ends,
FIG. 2F is an example in which in the method of bending the heavy plate shown in FIG. 1, the heavy plate is bent by pressurizing the other end of the both ends,
FIG. 2G is an example in which a thick plate is placed on a burner to perform a stress relieving heat treatment in the thick plate bending method shown in FIG. 1,
Fig. 2H is an example in which the curved surface is completed by pressurizing the thick plate in order from the both end portions to the center direction in the thick plate bending method shown in Fig. 1,
FIG. 3 is a graph showing a change in temperature of a heavy plate with time in the stress relieving heat treatment step of the thick plate bending method shown in FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Bending of a thick plate according to an embodiment of the present invention will be described with reference to FIGS. 1 to 2i.

First, a thick plate 10 is prepared as shown in FIG. The thick plate 10 may be manufactured, for example, with a width of 1,000 mm, a length of 4,000 mm, and a thickness of 83 mm. The thick plate 10 is a steel material conforming to AOI (A517) of the American Society for Testing and Materials (ASTM) standard. Herein, the term "aoilil" indicates a standard test method for the plastic strain of the sheet metal.

Then, a plurality of intermediate lines 20 are displayed on the upper surface of the prepared thick plate 10 at a point where bending is to be performed by a press (S100). The plurality of intermediate lines 20 are linearly arranged at equal intervals from both ends of the thick plate 10 toward the center. Further, the intermediate line 20 can be displayed in the width direction, but it is also possible to display the intermediate line 20 in the length direction. The middle line 20 indicates a portion of the press die 50 where the top die is lowered to press and bend the plate.

After the middle plate 20 is displayed on the thick plate 10, the thick plate 10 is placed on the burner 30 and preheated to a predetermined temperature (S200). The burner 30 can use a gas source, but it is also possible to use a high frequency heater or the like if necessary.

Here, the surface temperature of the thick plate 10 can be heated to 100 to 200 ° C, but more preferably, the temperature of the thick plate 10 can be heated to 180 ° C. In the conventional hot forming, it is common to heat the thick plate 10 to 900 DEG C or more, but in the present invention, the plate 10 is preheated at a low temperature to prevent a change in properties thereof. Bending of the thick plate 10 can be most effectively performed at a preheat temperature of 180 DEG C, and brittle failure may occur at the time of bending if the temperature exceeds the preheat temperature range.

After the preheating of the thick plate 10, a plurality of lifting lugs 40 are attached to the outer surface of the thick plate 10 (S300). The lifting lug 40 is mounted on the thick plate 10 to move the preheated thick plate 10. The lifting lug 40 may be additionally mounted depending on the size of the heavy plate 10 and is preferably mounted at four corners so that when the heavy plate 10 is lifted and moved, it is connected to the crane hook 60, Take center.

After the lifting lug 40 is mounted on the heavy plate 10, the heavy plate 10 is connected to the crane hook 60 so that the heavy plate 10 is placed on the top surface of the bottom die of the pressing press 50.

It is preferable that the diameter of the top die 51 of the pressing press 50 is 90 to 110 mm and the width of the bottom die 52 is 300 to 400 mm.

The diameter of the top die 51 and the width of the bottom die 52 can be determined according to the thickness of the thick plate 10. [ For example, in the case of a thick plate 83 having a thickness of 83 mm, the diameter of the top die 51 may be 100 mm and the width of the bottom die 52 may be 350 mm, corresponding to the thickness of the thick plate 10 .

Then, both ends of the thick plate 10 are pressed to bend (S400). Both end portions of the thick plate 10 are positioned at the center of the top die 51 of the pressing press 50 from the outermost middle line 20 among the middle lines 20 displayed on the upper surface of the thick plate 10. [ And the primary press bending is performed by pressing down the pressing press 20 for pressing.

When the end portion of the thick plate 10 is first bent, the thick plate 10 is moved to perform second bending around the middle line 20 of the opposite outer side.

When the bending process is performed, the back plate 10 is subjected to a spring back phenomenon, which is an elastic action to restore the original shape when the pressure is removed. Bending both end portions of the thick plate 10 preferentially is to prevent the springback phenomenon of the thick plate 10. Therefore, when both end portions of the thick plate 10 are not pressed to bend, cracks or brittle fracture may occur due to the springback phenomenon.

 When bending of both ends of the thick plate 10 is completed, the thick plate 10 is placed on the heating furnace 70 to perform stress relief heat treatment (S500).

FIG. 3 shows the temperature change of the thick plate 10 over time in the stress relieving heat treatment step of the thick plate bending method according to the embodiment of the present invention. Here, the stress relieving heat treatment is a method capable of eliminating the residual tensile stress or compressive stress depending on the stress generated in the inside of the thick plate 10 or the processing history. In general, steel castings, forgings, and rolled products are affected by uneven temperature. In machined products, large residual stresses are produced in the material due to the cutting force during processing. When quenching in this state and used as precision machine parts, deformation occurs due to residual stress.

The stress relieving heat treatment (S500) may include heating the thick plate (10) at a heating rate of 90 to 110 ° C / hour in an initial furnace at 370 to 390 ° C for 2 hours to 3 hours, , Maintaining the isothermal temperature for 2 to 3 hours, and slowly cooling the thick plate at a cooling rate of 70 to 90 DEG C / hour.

Usually, the stress relief heat treatment is performed at 500 to 600 ° C. In the case of the present invention, heat treatment can be performed at 530 to 630 ° C. In this case, stress can be minimized and bending of the thick plate 10 can be performed without cracking or brittle fracture even when performing a bending operation.

Here, the stress relieving heat treatment is performed through a recovery based on rearrangement of crystals of a thick plate and a recrystallization process related thereto. When the thick plate 10 is processed, a large amount of electric potential is generated therein, and if the amount of impurities due to processing is large, the stress becomes large. When the thick plate 10 is subjected to the stress relieving heat treatment, the upward movement of the potential is actively developed, and the potential shifts, thereby alleviating the crystal distortion of the thick plate. Therefore, the stress relieving heat treatment reduces the residual stress of the thick plate 10.

After the stress relieving heat treatment of the thick plate 10, the thick plate 10 is again placed on the burner and preheated to a predetermined temperature (S600)

When the preheating of the thick plate is completed, the plate is placed on the pressing press (50), and pressurized in order from the one end of the bent plate (10) in the direction of the center along the marking on the upper plate (10). (S700)

At this time, in the pressing press 50, the middle line 20 is pressed in conformity with the center of the top die 51 of the pressing press 50. After pressing, the presser 50 is pressed using an arc having a diameter of 139.7 mm Confirm the radius of the inner curvature to be generated and press it in sequence along the middle line (20).

Therefore, in step S600 of completing the curved surface by pressing the pressing plate 50 in order from the both ends along the marking of the thick plate 10 in the center direction, it is preferable that the inner curved radius is 138 to 140 mm .

The lifting lug 40 may be removed by bevel cutting the bending plate 10 to finally produce a chord.

As described above, according to the thick plate bending method, since the special heat treatment process is not required, the processing cost of the thick plate is greatly reduced. Further, in the bending process of the thick plate, additional heat treatment is not necessary to remove the residual stress of the steel, so that it is possible to perform uniform curved surface processing without cracking or brittle fracture of the thick plate by a single process.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

10: Plate 20: Middle line
30: burner 40: lifting lug
50: pressing press 51: saw die
52: bottom die 60: crane hook
70: heating furnace

Claims (6)

(a) preparing a thick plate, displaying a plurality of intermediate lines on the upper surface of the thick plate to be processed with a pressing press;
(b) placing the heavy plate marked with the middle line on the burner and preheating the plate to a predetermined temperature;
(c) mounting a plurality of lifting lugs on an outer circumferential surface of the thick plate;
(d) connecting the crane hook to the lifting lug to place the heavy plate on the bottom die of the pressing press, and then bending the both ends of the heavy plate marked;
(e) placing the thick plate in a heating furnace to perform stress relief heat treatment;
(f) placing the plate back on the burner and preheating to a specified temperature; And
(g) pressing the plates in order from the ends on both ends along the marking of the plate to the center to complete the curved surface. The method according to claim 1,
In the step (a)
Wherein the plurality of intermediate line marks display straight lines parallel to each other at equal intervals from both ends of the thick plate towards the center. The method according to claim 1,
In the step (b)
And heating the surface of the thick plate so that the surface temperature is 100 to 300 占 폚 Plate bending method. The method according to claim 1,
In the step (c)
The diameter of the top die of the pressing press is 90 to 110 mm,
Wherein the bottom die has a width of 300 to 400 mm. The method according to claim 1,
In the step (e), the stress relieving heat treatment may include:
Heating the thick plate at a heating rate of 90 to 110 DEG C / hour in an initial furnace at 370 to 390 DEG C for 2 to 3 hours,
Maintaining the isothermal temperature for 2 to 3 hours when the thick plate reaches 630 캜, and
And slowly cooling the thick plate at a cooling rate of 70 to 90 DEG C / hour. The method according to claim 1,
In the step (g) The pressurizing pressurization is carried out,
Wherein the bending step is sequentially pressed from one end of the bending plate to the center of the bending plate so that the radius of the inner bending is 138 to 140 mm.

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