KR101588677B1 - Method for enhancing the self-feeding ability of heavy section casting blank - Google Patents

Abstract

The present invention relates to the field of manufacturing cast blanks, and more particularly to a method for improving the self-feeding ability of heavy section casting blank which can solve the problems of poor center quality, surface cracking and high failure rate of heavy section casting blank in the prior art . By controlling the external cooling conditions of the various coagulation steps of the casting blank, the present invention firstly quickly coagulates and hardens the outer surface of the casting blank to increase its strength and prevent surface cracking, followed by thermal insulation on the casting blank surface , A large area of the core is formed so that the solidified layer of the casting blank surface is kept at a relatively high temperature to promote the realization of plastic deformation so that solidification and solid movement can be performed simultaneously in the subsequent solidification and shrinking process of the casting blank Realizes the purpose of radial self-feeding of the hot deformed metal, removes internal shrinkage voids and surface cracks, and apparently eliminates internal shrinkage of the cast blank. The present invention is applicable to heavy section metal casting and is particularly applicable to circular and square heavy section casting blanks with high height-to-diameter ratios that can not remove axle shrinking pipes through the feed head.

Description

[0001] METHOD FOR ENHANCING THE SELF-FEEDING ABILITY OF HEAVY SECTION CASTING BLANK [0002]

The present invention relates to the field of making cast blanks such as wide and thick metal slabs, round / square / rectangular heavy section casting blanks, and more particularly to the manufacture of heavy section castings To improve the self-feeding ability of the blank, and to eliminate the shrinkage voids and surface cracks of the cast blank.

Wide and thick slabs (special slabs) are widely applied in economical construction. Numerous large and thick slabs are used in large vessels, offshore platforms, hydroelectric power sets, thermal power units, pressure vessels, dies, and long-distance pipelines. Thus, there is an enormous demand for large, thick slabs of thick, large cross-sections to roll wide, thick plates. Presently, the maximum thickness of the entire continuous cast slab is less than 400 mm. When a continuous cast slab is employed to roll a wide, thick plate having a thickness of 200 mm, it is difficult to ensure the performance of the core due to the small reduction ratio. Mold casting methods and electric slab material melting methods for producing wide and thick slabs have disadvantages of low productivity, low success rate and high cost. Therefore, it is urgent to develop a low-cost and high-efficiency technique for manufacturing a wide and thick slab having a thickness of 600 mm or more. Water-cooled molding of large and thick slabs is a rapid technology. However, when the water-cooled molding method is used to produce a wide and thick slab, the large water cooling strength is wide and the large temperature difference between the center and the surface of the thick slab, the large thermal stress during solidification, and the cracks on the surface and core of the slab easily It causes.

Heavy-section circular continuous cast blanks are used to replace the commonly-used molded steel ingots, demonstrating excellent development trends due to high production efficiency and high material utilization. Heavy section cast blanks can be used to produce nuclear torches, wind rings, and associated shaft type parts related to vehicles, ships, and machinery. More recently, continuous casting technology has been increasingly applied to the production of heavy section cast blanks. This technique is as follows: The liquid metal is poured continuously into a water-cooling crystallizer, the liquid steel is solidified in a water-cooled crystallizer, and the dummy bar is fed from the lower end The resulting solidified portion is continuously cast to realize continuous casting of the blank. This technique also has defects: the height-diameter ratio of the cast blanks produced by this technique is so large that the shaft feed of the cast blanks is difficult to realize and is liable to cause shrinkage and pores at the center of the cast blank; In addition, the outer surface of the blank is usually treated with a forced cooling process, which causes a very low temperature on the outer surface, resulting in cracking. These deficiencies limit the development of circular continuous cast blanks to larger cross sectional sizes (≥ 800 mm).

Square and rectangular heavy section continuous cast blanks with a thickness of 400 mm or more also have macro defects of internal shrinkage, pores and surface cracks.

Generally speaking, to overcome the internal shrinkage and pore defects of the blank, a larger feed head or adiabatic (heating) feed head is used to achieve subsequent solidification of the blank along the direction of gravity. However, the feed head ratio of the heavy section continuous cast blanks is very small, and the height-diameter ratio thereof is larger than 4, so that the axial gravity feed of the cast blank can not be realized.

Consequently, the internal shrinkage, pore, and surface cracking of the heavy section casting blank is a technical hurdle that limits the development of the casting blank to a larger cross-sectional size. Thus, improving the feedability of the heavy section casting blank in the coagulation process is critical to overcoming defects in the internal shrinkage and pore and surface cracking of the heavy section casting blank.

It is an object of the present invention to provide a method of eliminating the central contraction, pore, and surface cracks of heavy and thick slabs and circular and rectangular continuous cast blanks through improved heavy-section molded self-feeding capability of heavy section casting blank. Thus, it is useful to develop a technique for producing a round casting blank having a diameter of 500 mm or more and a square or rectangular casting blank having a thickness of 400 mm or more.

To achieve this object, the present invention adopts the following technical measures:

The method of improving the self-feeding ability of the heavy section casting blank is carried out in the next step, that is, after injection of the liquid metal, by means of molded water cooling, direct water spraying, mist spraying or blowing to mold the casting blank Immediately forcibly cooling the outer surface; And stopping the forced cooling of the outer surface when the temperature of the outer surface of the cast blank is reduced to 800-1000 ° C and the thickness of the solidified layer reaches 5-30% of the thickness or diameter of the blank section .

According to the method of improving the self-feeding ability of the heavy section casting blank, the forced cooling of the outer surface of the casting blank is stopped when the thickness of the solidified layer reaches 50 to 300 mm.

By controlling the cooling conditions of the outer surface of the casting blank in accordance with the method of improving the self-feeding ability of the heavy section casting blank, the outer surface of the casting blank is maintained at a temperature of 200 to 400 DEG C below the solidus, Allowing the solidified layer of the outer surface of the blank to remain in the plastic deformation zone with low deformation resistance.

After the forced cooling of the outer surface of the casting blank is stopped, a heat insulating material or a heat cover that insulates the outer surface of the casting blank is used to heat the outer surface of the casting blank Reducing the intensity of heat exchange between the environment and then increasing the temperature of the outer surface of the cast blank due to the latent heat of the cast blank core to reduce the radial temperature gradient of the cast blank; The core of the cast blank then forms a mushy region and coagulates at the same time.

According to the method of improving the self-feeding ability of the heavy section casting blank, when the liquid metal of the core coagulates at the same time, the coagulation and shrinkage produce a radial tensile stress, which acts on the hot coagulated layer of the outer surface, Produces a plastic deformation and moves thermally from the outer surface to the blank center to realize the radial self-feeding of the cast blank.

According to a method for improving the self-feeding ability of a heavy section casting blank, after the core of the casting blank coagulates at the same time and the radial self-feeding is realized, the casting blank surface and the casting blank core are still in a high temperature state; At this time, it is demolded at a high temperature, and the demolding temperature required for the cast blank exceeds 800 ° C.

According to the method of improving the self-feeding ability of the heavy section casting blank, the demolding temperature required for the casting blank is preferably 850 to 1,200 ° C.

A method for improving the self-feeding ability of heavy section cast blanks is applicable to square or rectangular cast blanks having a thickness of 400 mm or more, round cast blanks having a diameter of 500 mm or more, and molded wide and thick slabs having a thickness of 600 mm or more.

The present invention has the following advantages:

1. Traditionally, a casting blank generally performs axial feeding along the direction of gravity, which means that by controlling the external cooling conditions of the casting blank, And realizes supply.

2. In the initial solidification stage of the heavy section casting blank, the present invention employs a water cooling, mist cooling or air cooling means to improve the heat exchange coefficient between the casting blank and the exterior to rapidly solidify the blank surface, And prevents thermal cracking due to a thin coagulated layer of the casting blank surface and low strength in the initial coagulation step.

3. In the present invention, when the thickness of the solidified layer of the casting blank reaches 5 to 30% (usually 50 to 300 mm) of the diameter or thickness of the cross section, the forced cooling of the outer surface of the casting blank is stopped; At this time, the solidified outer layer is at a low temperature to provide a low temperature external environment in the uncoagulated casting blank core zone, to ensure the solidification rate of the liquid metal in the core, and to avoid excessively large crystal grains of the core .

4. In the middle and late coagulation stages of the casting blank, the present invention performs insulation on the casting blank surface so that the temperature of the casting blank surface rises to remain in the firing zone, which causes cracking due to thermal stress relatively to the casting blank surface To prevent it from happening.

5. In the middle and late coagulation stages of the casting blank, the present invention performs insulation on the casting blank surface, which can reduce the temperature gradient from the inside to the outside of the heavy section casting blank, The area is formed at the same time, the simultaneous solidification of the center of the blank is realized, and the generation of defects in which the shrinkage voids are concentrated in the center is avoided.

6. By the method provided by the present invention, the radial tensile stresses produced by coagulation and shrinkage when the large areas of the blank center coagulate concurrently cause the hot, solid metal solidified at the outer surface of the cast blank to be removed from the casting blank surface So that the radial self-feeding is realized during the solidification of the casting blank, and the internal shrinkage of the casting blank and the defects of the pores are removed.

7. The method provided by the present invention can fully maximize the radial self-feeding capability of the heavy section casting blank, thereby reducing the feeder heating size of the casting blank and further improving the material utilization of the heavy section casting blank .

8. The present invention is applicable to a large range and can be used to produce round, square or rectangular heavy section cast blanks, molded wide and thick slabs, and other heavy section castings.

9. The method provided by the present invention can also realize high temperature demolding and hot charging of the blank, improve production efficiency, and save energy.

In general, by controlling the external cooling conditions of the various solidification stages of the casting blank, the present invention firstly quickly coagulates and crushes the outer surface of the casting blank to increase its strength and prevent surface cracking, The surface of the core is thermally insulated to form a large area of the core and the solidified layer of the cast blank surface is maintained at a relatively high temperature to promote the realization of the plastic deformation so that the subsequent coagulation and shrinkage process Achieves the object of simultaneous solidification and solid state movement, achieves the purpose of radial self-feeding of the metal capable of deforming at high temperatures, eliminates internal shrinkage and surface cracking of the casting blank, and obviously eliminates internal shrinkage of the casting blank. On the other hand, the present method realizes the hot filling of the cast blank, improves the production efficiency, and can save energy. The present invention is applicable to heavy section metal castings and is particularly applicable to circular and square heavy section casting blanks that have a large height-to-diameter ratio and can not remove axle shrinking pipes through the feed head.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a water-cooled molded wide and thick slab produced by the present invention.
Figure 2 shows a circular heavy section casting blank produced by the present invention, Figure 2 (a) showing an actual circular heavy section casting blank, Figure 2 (b) showing a section of a circular casting blank .
Fig. 3 shows a circular cast blanket with a shrinkage void in the center not produced by the present invention. Fig. 3 (a) shows an actual circular heavy section cast blank, and Fig. 3 (b) Fig.

The present invention provides a method for removing internal shrinkage, pores and surface cracks by improving the self-feeding ability of heavy section casting blank, comprising the steps of:

1. Melting equipment such as an induction furnace or an arc furnace is used to melt the liquid steel before it is deoxidized and degassed.

2. Spread molten liquid steel into a wide, thick slab molding casting chamber or casting crystallizer.

3. Forced cooling of the outer surface of the cast blank by a water-cooled mold or crystallizer for rapid solidification of the cast blank surface or enhancement of heat exchange between the cast blank and the environment by water spray, mist spray or air blowing .

4. In the forced cooling process of the casting blank surface, the temperature of the cast blank surface is monitored and measured by a contact or non-contact temperature measuring device, and the temperature of the cast blank surface is controlled as low as possible, from 800 to 1,000 ° C. Very low temperatures must be avoided, otherwise cracks can be created due to the solidified metal producing solid phase changes and structural stresses; On the other hand, high temperatures must be avoided, otherwise the solidified layer may be thinned and have low strength, so that the cast blank surface "bulges" under static pressure of the liquid metal.

5. After the coagulated layer thickness of the casting blank reaches 50 to 300 mm, the forced cooling of the casting blank surface is stopped, and the casting blank surface is subjected to heat insulation. When the temperature of the casting blank surface rises continuously, the temperature of the outer surface is monitored and measured to adjust the strength of heat exchange between the casting blank and the environment by means such as adiabatic or cooling, and then the temperature of the casting blank surface The material is held in a plastic deformation zone with a temperature of 200 to 400 캜 below the material solidus.

6. The temperature gradient between the blank center and the outer surface gradually decreases, and the large area of the blank center forms a stiff area. Then the blank center coagulates at the same time in the cooling process; Tensile stresses are produced due to solidification and shrinking, which promotes the plastic movement of the solid metal solidified at the outer surface of the casting blank to plastic deformation from the casting blank surface to the center, so that during the solidification of the casting blank, And realizes supply.

Example 1

In this embodiment, the method provided by the present invention is used to produce a molded wide and thick slab; The material of the wide and thick slab is Q345, the thickness of the wide and thick slab is 1,000mm, and the total mass is 60ton.

Electric arcs are used to melt liquid steel; The liquid steel is then refined in the LF furnace and then injected into the VD furnace to remove oxygen and discharge the gas. At a temperature of 1,560 ° C, liquid steel is injected into a separate water-cooled mold for a total time period of 30 min. Through simulations and calculations, it can be seen that the thickness of the coagulated layer of the surface of the wide and thick slab after 40 minutes has elapsed is 90 mm. At this time, the water flow of the water-cooled mold is reduced and the gap between the mold and the thicker and thicker slab is increased to reduce the rate of heat dissipation on the surface of the larger and thicker slab. By monitoring the temperature change on the surface of the large and thick slabs, it was found that after the surface temperature of this wide and thick slab rises from 920 ° C to 1,100 ° C to 1,250 ° C, the casting blank gradually cools down to complete solidification. After the casting blank is fully solidified, it is demolded at a high temperature of 900 ° C, demolded at a controlled rate of 30 to 40 ° C / h and cooled slowly.

FIG. 1 shows a wide and thick slab produced in this embodiment. Through nondestructive testing, this slab was found to have no defects in internal shrinkage pores and surface cracks.

Example 2

In this embodiment, the method provided by the present invention is used to produce a heavy section circular continuous casting blank; The material of the circular blank is 20CrNi2Mo; The diameter is 1,000 mm, the length is 8 m, and the total mass of the circular blank is 45 tons.

Molten liquid steel is injected into the crystallizer at a temperature of 1,540 ° C, where the casting speed of the blank is 0.1 m / min. According to simulation and calculation, when the blank is dragged out of the crystallizer, the solidified layer of the surface is found to be about 50 mm thick and the surface temperature is about 850 ° C. If the blank is cast in a crystallizer, heat insulation is performed on the cast blank surface with a heat insulating material such as an asbestos cloth; The temperature of the casting blank surface then rises to 1,200 to 1,260 ° C and reaches the firing zone. The temperature gradient of the casting blank is reduced from the inside to the outside to realize the coagulation of the central region. In the subsequent cooling process, the outer surface of the cast blank produces plastic deformation, and the solid creates shrinkage and movement from the outside to the inside, realizing radial self-feeding.

Fig. 2 (a) shows the heavy section circular casting blank produced by the technique of the present invention in this embodiment. Through nondestructive testing, it was found that the blank had no internal shrinkage voids and the cast blank surface was free of crack defects. Figure 2 (b) shows a cross section of a circular blank with pore levels less than 2 without centralized shrinkage voids.

Figure 3 (a) shows a round casting blank having the same size and specifications, not produced by the technique provided by the present invention. The center of the circular blank has shrinkage voids and pore defects in the large region as shown in Fig. 3 (b).

As a result, by controlling the external cooling conditions of the various coagulation steps of the blank, the present invention allows the large area of the blank center to form a stiff area, while maintaining a solidified layer of the cast blank surface at a relatively high temperature, It achieves solid phase plasticization in the subsequent solidification process, performs the purpose of radial self-feeding of metal capable of deforming at high temperatures, removes internal shrinkage and pores of the cast blank, and prevents surface cracking.

Table 1 shows the defect levels before and after the circular blanks of ϕ 800 to 1200 mm processed in the above embodiment are subjected to the detection processing according to YB / T 4149-2006.

Center porosity Central contraction Center crack Surface crack Before processing Levels 3-4 Level 2-3 Level> 4 decimal After processing Level 0-1 none Level 0-1 none

Claims (12)

CLAIMS 1. A method of improving self-feeding capability of a heavy section casting blank,
Immediately after liquid metal injection, forcibly cools the cast blank surface to rapidly solidify and cure the cast blank surface;
Thereafter, while controlling the cooling conditions of the outer surface of the cast blank to maintain the outer surface of the cast blank at a temperature of 200-400 ° C below the solidus, the solidified layer of the outer surface of the cast blank has a low Performing heat insulation on the cast blank surface with a thermal insulation material or a thermal cover to reduce the strength of heat exchange between the cast blank surface and the environment so as to remain in the plastic deformation zone having deformation resistance;
Raising the temperature of the casting blank surface by latent heat of the casting blank core and reducing the radial temperature gradient of the blank to simultaneously form a large machine zone and promote simultaneous coagulation of the casting blank core ;
Improving the metal self supply capability by the action of tensile stress produced by solidification and contraction of the core when the cast blank core coagulates at the same time;
After the cast blank core is fully solidified, it is demolded at a high temperature and then subjected to slow cooling or high temperature annealing
≪ / RTI > 2. The method of claim 1, wherein after the liquid metal is injected, the external surface of the cast blank is immediately forcedly cooled by water cooling, direct water spraying, mist spraying or blowing; When the temperature of the outer surface of the cast blank is reduced to 800 ° C to 1000 ° C and when the thickness of the coagulated layer reaches 5 to 30% of the thickness or diameter of the blank section, forced cooling of the outer surface is interrupted . ≪ / RTI > 3. A method as claimed in claim 1 or 2, characterized in that forced cooling of the outer surface of the casting blank is stopped when the thickness of the solidified layer reaches 50 to 300 mm. delete The method of claim 1, wherein, when the liquid metal of the core coagulates at the same time, the coagulation and the shrinkage produce a radial tensile stress, which acts on the hot coagulated layer of the outer surface so that the coagulated metal produces plastic deformation And moving plastic from the outer surface to the center of the casting blank to realize the radial metallurgical supply of the casting blank. The method of claim 1, wherein for demolding, the demolding temperature required for the cast blank is greater than 800 ° C. The method of claim 6, wherein the demolding temperature required for the cast blank is between 850 and 1,200 ° C. 2. The method of claim 1, wherein the slow cooling is performed after the cast blank is demolded at a high temperature and the cooling rate is less than 50 DEG C / h. The method of claim 8, wherein the cooling rate is 10 to 30 占 폚 / h. The method of claim 1, wherein after the casting blank is demolded, the high temperature annealing temperature is 800 to 1100 占 폚 and the annealing cooling rate is less than 50 占 폚 / h. 11. The method of claim 10 wherein the annealing cooling rate is 10-40 DEG C / h. The method of claim 1, wherein the method is applicable to a square or rectangular continuous casting blank having a thickness of at least 400 mm, a circular continuous casting blank having a diameter of at least 500 mm, and a molded wide, thick slab having a thickness of at least 600 mm How to.

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