Taking Q355D steel as the research object, the production test was carried out using the forced cooling method at the end of the solidification of the slab. The effects of normal cooling and strong cooling at the end of solidification of the slab on the centre segregation and central equiaxed grain rate of the slab were comparatively analyzed.
Keywords: continuous casting billet; strong cooling at the end of solidification; central segregation; central equiaxed crystal ratio
Preface
Depending on the purpose of the steel, the requirements for the internal quality of the continuous casting billet are also different. Steel types with higher performance requirements also have higher requirements for the internal quality of the casting billet. Severe segregation in the center of the continuous casting billet will lead to abnormal properties of the rolled material, such as reduced impact properties. Especially after rolling, sulfides and low-temperature transformation products are easily generated, and hydrogen embrittlement cracks are easily generated. At the same time, it also affects the density of the central structure of the slab. The larger the central equiaxed crystal ratio, the denser the cast slab structure and the better the mechanical properties of the steel. At present, except for some special-purpose steels, such as steam turbine blades, electrical steel, etc., which require developed columnar crystals in order to improve some special properties, most steel types require more central equiaxed crystals.
Different solidification structures and central segregation degrees of the slab can be obtained by changing the superheat degree of the tundish and the amount of light pressing. As the subcooling degree of the slab increases, the speed of grain growth and nucleation increases. When the supercooling intensity of the slab increases to a certain value, the speed of grain nucleation is greater than the speed of grain growth, and fine grains can be obtained. The article introduces the production test of optimizing the cooling method of the slab to explore the influence of strong cooling at the end of solidification on the centre segregation and central equiaxed crystallization rate of the slab.
Test content
The Q355D steel grade was selected for this test. The pouring speed of the continuous casting machine was 0.7m/min. The slab size was 250mm×1800mm. A total of 4 continuous castings were involved. The first two tundishes are produced using normal secondary cooling water meters, and the last two tundishes are produced using solidifying end forced cooling water meters. Considering that strong cooling of the solidification end will move the solidification end of the slab forward, the newly designed water meter reduces the water volume in zones 3 to 5 by 10% based on the original water meter. The areas 6 and 7 at the end of the solidification of the cast slab were increased by 50%. The water volume in other areas remained unchanged, and the change in specific water volume was controlled to be less than 0.1L/kg. Reduce the probability of cracks caused by low temperature at the corners of the cast slab in the straightening area. The water volume of the water meter before and after optimization is shown in Table 1.
Table 1 Comparison of water volume in water meters before and after optimization (water volume corresponding to pulling speed 0.7m/min)
| Second cold area | Each section of the secondary cooling zone | Raw water meter water volume/L · min- 1 | Solidification end strong cold water meter water volume/L · min- 1 |
| zone 1 | Foot roller narrow surface | 51 | 51 |
| Foot roller wide surface | 116 | 116 | |
| Zone 2 | Upper section 0 | 246 | 246 |
| Zone 3 | Lower section 0 | 268 | 241.2 |
| Zone 4 | 1 inner arc | 94 | 84.6 |
| 1 outer arc | 106 | 95.4 | |
| Zone 5 | 2~3 internal arcs | 83 | 74.7 |
| 2~3 segments of outer arc | 110 | 99 | |
| Zone 6 | 4~5 inner arcs | 94 | 141 |
| 4~5 external arcs | 149 | 223.5 | |
| Zone 7 | 6~7 inner arcs | 65 | 97.5 |
| 6~7 external arcs | 115 | 172.5 | |
| Zone 8 | 8~10 inner arcs | 22 | 22 |
| Zone 9 | 11~13 inner arcs | 22 | 22 |
| Total water volume/L · min- 1 | 1541 | 1686.4 | |
| Specific water volume/L·kg- 1 | 0.68 | 0.74 |
Take 4 low-magnification samples from different heats of each tundish with a superheat degree of 20~30°C. Do not take samples from the head furnace, tail furnace and water exchange port. The low-magnification samples taken were hot pickled, and the influence of strong cooling at the end of solidification of the slab on the segregation and solidification structure of the continuous casting slab was analyzed through the methods of Mann Standard rating and equiaxed crystal ratio calculation.
Sample preparation: The low-magnification sample is first cut from the slab with a cutter. Its width is equal to the thickness of the slab, its length is equal to half the width of the slab plus 50mm, and the thickness is 80~110mm. The surface of the sample must be smooth, with a cut depth of ≤5 mm, and then be processed with a milling machine until the surface roughness is less than 1.6 μm.
Low-magnification test: Put the prepared low-magnification sample into a hydrochloric acid solution at 70~80°C for 25 minutes, then take out the low-magnification sample and clean the surface with 3%~5% sodium carbonate aqueous solution and clean water. Finally, a fan was used to dry the surface for Mann Standard rating. We can see the macrosegregation of the low-magnification sample and the distribution of columnar crystals and central equiaxed crystals.
Calculation of equiaxed crystal ratio: The cast slab is usually composed of three kinds of crystal bands, from the surface to the center, there are fine equiaxed crystals, wide columnar crystals and central equiaxed crystals. Due to uneven cooling of the slab, the interface between each two crystal bands is not a straight line. In order to facilitate the calculation of the central equiaxed crystal ratio of the slab. Through an approximate method, straight lines are used to distinguish columnar crystals and central equiaxed crystals, so that the central equiaxed crystal region can be regarded as a rectangle. By calculating the ratio of its area to the area of the entire low-magnification sample, the central equiaxed crystal ratio of the low-magnification sample can be obtained.
In the formula: E is the central equiaxed crystal ratio, %; Sd is the area of the equiaxed crystal region, mm2; S is the total surface area of the low-magnification sample, mm2.
Test results and discussion
Effect of strong cooling at the end of solidification on segregation in the center of the slab
Q355D steel grade, continuous casting machine pouring speed 0.7m/min, billet size 250mm×1800mm, low-power Mann scale rating center segregation results corresponding to different cooling methods are shown in Figure 1 and Table 2. The minimum value of the center segregation of the strong cooling low magnification rating of the solidification end is 1.5, which is 0.5 lower than the center segregation value of normal cooling; the maximum value is 2.5, which is 0.2 lower than the center segregation value of normal cooling. Analysis believes that this is because the strong cooling at the solidification end of the continuous casting slab inhibits the growth of columnar crystals and reduces the phenomenon of “bridging” in the solidification center of the slab. The upper molten steel supplements the gaps in the lower part caused by the solidification shrinkage of the molten steel. At the same time, the strong cooling at the end of solidification accelerates the solidification speed of the molten steel in the center of the slab, reduces the separation and crystallization, and improves the segregation in the center of the slab.
Table 2 Comparison of segregation in low magnification rating centers of two cooling methods
| cooling method | Furnace number | Superheat/°C | Central segregation |
| normal cooling | 2952669 | 24 | 2.4 |
| 2949704 | 21 | 2 | |
| 2949998 | 24 | 2 | |
| 2952476 | 23 | 2.2 | |
| 2952483 | 25 | 2.2 | |
| 2952534 | 23 | 2.7 | |
| 2952283 | 24 | 2.1 | |
| 2052486 | 23 | 2.1 | |
| Strong cooling at the end of solidification | 2950042 | 25 | 2.2 |
| 2951474 | 24 | 2.2 | |
| 2951149 | 25 | 2.4 | |
| 2951134 | 25 | 1.8 | |
| 2951152 | 20 | 2 | |
| 2951141 | 25 | 2.5 | |
| 2951138 | 15 | 1.8 | |
| 2951143 | 25 | 1.5 |
Figure 1 Comparison results of different cooling methods corresponding to low magnification Mann scale ratings
Effect of strong cooling at the end of solidification on the equiaxed crystal ratio
Q355D steel type, continuous casting machine pouring speed 0.7m/min, billet size 250mm×1800mm, the equiaxed crystal rate in the center of the billet corresponding to different cooling methods is shown in Figure 2 and Table 3. The minimum, maximum and average values of the low-magnification central equiaxed crystal ratio under the two cooling conditions were classified and analyzed. It can be seen from Figure 2 that strong cooling at the end of solidification of the slab increases the central equiaxed crystallization rate compared with normal cooling. The minimum value increases by 3%, the maximum value increases by 4%, and the average value increases by 6%. This is because strong cooling at the end of solidification of the slab accelerates the rate of grain growth and nucleation to varying degrees. However, as the cooling rate increases, the speed of grain nucleation is greater than the speed of grain growth, and the wide columns begin to transform into small equiaxed grains in the center, thus increasing the equiaxed grain rate in the center of the slab.
Table 3 Comparative results of the equiaxed crystal ratio in the center of the billet using two cooling methods
| cooling method | Furnace number | Superheat degree/℃ | Internal arc equiaxed crystal rate/% | External arc equiaxed crystal rate/% | Center equiaxed crystal rate/% |
| normal cooling | 2952669 | 24 | 18% | 26% | 22% |
| 2949704 | 21 | 9% | 43% | 26% | |
| 2949998 | 24 | 0% | 30% | 15% | |
| 2952476 | 23 | 17% | 34% | 26% | |
| 2952483 | 25 | 6% | 30% | 18% | |
| 2952534 | 23 | 2% | 30% | 16% | |
| 2952283 | 24 | 0% | 30% | 15% | |
| 2052486 | 23 | 0% | 30% | 15% | |
| Strong cooling at the end of solidification | 2950042 | 25 | 20% | 27% | 24% |
| 2951474 | 24 | 0% | 39% | 19% | |
| 2951149 | 25 | 6% | 34% | 20% | |
| 2951134 | 25 | 17% | 43% | 30% | |
| 2951152 | 20 | 18% | 34% | 26% | |
| 2951141 | 25 | 2% | 34% | 18% | |
| 2951138 | 15 | 17% | 43% | 30% | |
| 2951143 | 25 | 17% | 43% | 30% |
Note: The calculation formula for the equiaxed crystal ratio of the inner and outer arcs is the same as the calculation of the central equiaxed crystal ratio of the low-magnification sample, but the S value in the equiaxed crystal ratio of the inner and outer arcs is half of the total area of the low-magnification sample.
Figure 2 Comparison of the equiaxed crystal ratio in the center of the slab corresponding to different cooling methods
It can also be seen from Table 3 that, whether it is a normal water meter or a terminal forced-cooling water meter, the equiaxed crystallization rate of the outer arc of the slab is greater than the equiaxed crystallization rate of the inner arc. The columnar crystals in the inner arc of the billet grow from top to bottom, and the columnar crystals in the outer arc grow from bottom to top. Under the action of the static pressure of the molten steel and gravity, the columnar crystals produced upward in the outer arc are easy to break at the small top part. , the broken part of the grain serves as a new nucleation point, so that the nucleation rate of the grain is greater than the growth rate, and the columnar crystals begin to transform into fine equiaxed crystals.
Effects of two cooling methods on semi-macro segregation of cast slabs
The normal cooling method corresponds to the inner and outer arcs of the slab. The columnar crystals grow together in the center of the slab, preventing the upper molten steel from compensating for the pores in the lower part caused by the solidification shrinkage of the molten steel. Point-like semi-macro segregation is formed, which is distributed in the form of intermittent lines in the center of the slab.
The strong cooling method at the end of solidification converts the solidification method in which the surface of the slab is pushed toward the center into a central volume solidification method. The upper molten steel has no time to replenish the pores caused by volume shrinkage, forming small point-like semi-macro segregations scattered in the central equiaxed crystal area. These semi-macro segregation can be further improved by appropriately increasing the amount of light pressure reduction. The semi-macro segregation conditions corresponding to the two cooling methods are shown in Figure 3.
Figure 3 Semi-macro segregation of billet corresponding to different cooling methods
Conclusion
Comparative tests show that: (1) The minimum value of low-power Mann scale rating center segregation corresponding to the strong cooling method at the end of solidification is 1.5, which is 0.5 lower than that of the normal cooling method. The maximum value is 2.5, which is 0.2 lower than the normal cooling method; the average value is 0.1 lower than the normal cooling method.
(2) Compared with the normal cooling method, the forced cooling method at the end of solidification of the slab increases the equiaxed crystal rate in the center of the slab, with the minimum value increasing by 3%, the maximum value increasing by 4%, and the average increasing by 6%.
(3) Due to the effects of gravity and the static pressure of molten steel, the equiaxed crystallization rate of the outer arc of the slab is greater than the equiaxed crystallization rate of the inner arc.
(4) The semi-macro segregation of the slab corresponding to the normal cooling method is distributed in intermittent lines in the center, and the semi-macro segregation of the slab corresponding to the strong cooling method at the end of solidification is scattered in the central equiaxed crystal area. These semi-macro segregations can be further improved by appropriately increasing the amount of light pressure reduction.
