A New Cooling Strategy in Curved Continuous Casting Process of Vanadium Micro-alloyed YQ450NQR1 Steel Bloom Combining Experimental and Modeling Approach

In the steel continuous casting process, the cooling condition in the mold region and the secondary cooling zone have predominant influence on the solidification structure and surface quality in the casting bloom.[1,2] This phenomenon has been extensively researched in such works by Liu[3] and Thomas.[4] In a curved continuous caster, due to large cross section dimension and high thermal gradient from the bloom surface to center, the solidification structure tends to vary from the surface to center,[5,6] with a thin layer of fine equiaxed grains initiated in the outer region[7] and a columnar grain region stretching towards the bloom center.[8,9] Subsequently, with the decrease of the superheat and the forced convection in the molten steel, coarse equiaxed grains form eventually in the central area.[10‐12] According to the research from Dong,[13,14] the columnar to equiaxed grain transition (CET) usually occurs in the secondary cooling zone during steel continuous casting, and it affects the solute macro-segregation behavior. Proper enlargement and refinement of the central equiaxed grain zone would reduce macro-segregation of the solute.

In a curved caster, the bloom undergoes deformation at the straightening position.[15,16] If the secondary cooling is not controlled properly, the deformation-induced strain at the bloom surface/subsurface would exceed the critical value,[17‐19] causing surface cracks, which is a severe problem to the casting process and the downstream rolling process.[20,21] The hot ductility of steel at high temperatures is a valuable index for judging the straightening temperature regarding certain steel grades and casting conditions. Metallurgists and casting engineers[22,23] usually refer to plots between reduction of area (R.A.) curves and temperatures for steel hot ductility description. For straightening of steel in a curved continuous casting process, the typical strain rate is usually below 10⁻³ s⁻¹.[24] The hot ductility curve usually comprises of two low ductility zones and one high ductility zone between them.[25,26] Occurrence of low ductility zone for continuously cast steel during solidification and cooling is a combined result of phase transformation and secondary phase precipitation. According to Mintz[19,22] and some other researchers,[27‐29] it is recommended to keep R.A. value above 60 pct in order to avoid deformation-induced surface cracks. In actual steel continuous casting processes, the bloom surface temperature should be controlled to avoid the low ductility zones.

The cylindrical tensile samples (Figure 1) for the hot ductility tests are cut from the surface microstructures of the as-cast bloom. The hot ductility of the samples are tested using a Gleeble thermal simulator, the R.A. value of each sample is calculated using Eq. 1. Chemical compositions of the steel are shown in Table I. The experimental cooling rates (0.5, 1, 3 °C/s,) are determined based on the actual temperature measurements during the continuous casting process on site. The thermal scheme for the hot ductility tests is shown in Figure 2. The fracture surfaces of the tensile samples are water quenched to keep the morphologies for further metallographic analysis.where \( d_{1} \) and \( d_{2} \) are the sample diameter before and after hot ductility test.

After the hot ductility tests, the sample fracture surfaces are directly observed using the SEM. To further study the phase transformation behavior of the steel during the solidification and cooling process, a HTLSCM (as illustrated in Figure 3) with an infrared image furnace is employed for the “in situ” observation of the V(C, N) precipitations and proeutectoid phase transformations. The samples are heated and cooled in an alumina crucible under the ultra-high-purity argon gas. The temperature is measured at the bottom of the holder in the crucible. The heat cycle is schematically shown in Figure 4. The samples are heated up to 1500 °C and held isothermally for 15 min to ensure that the V(C, N) particles are completely dissolved, after which they are cooled to room temperature with the cooling rate (Vc) of 0.3 and 3 °C/s, respectively. Finally, the samples are mirror polished into metallographic specimen and etched using the 4 pct nital solution to further analyze the precipitations of the V(C, N) using the SEM and the EDS.

Based on experiments, a CA-FE model is further established to calculate the heat transfer and solidification structure evolution during the continuous casting process. Detailed descriptions and validations of the model can be tracked from some of the previous works.[30] The operation parameters as well as the details of the cooling system in the continuous casting process which are used in this modeling work are shown in Tables II and III, respectively. The influence of the fluid flow on the heat transfer is considered by applying the modified thermal conductivity in the mushy zone of the casting bloom as is indicated in the previous work of the authors. A schematic illustration of the coupled CA-FE model is shown in Figure 5. In the calculation process, the macro heat flow in the casting bloom is first solved based on the finite element method. Then, the temperature information on the nodes of the finite element are interpolated onto the cellular automaton cells using the interpolation coefficients (\( \emptyset \) values in Figure 5). The grain nucleation and growth are further calculated based on the model proposed by Gandin [36,37] and Rappaz.[38] In this modeling, only ¼ of the entire cross section (360 mm × 450 mm) of the YQ450NQR1 steel bloom is selected as the calculation domain. To catch the thermal profile needed for the microstructure modeling, a timestep of 0.01 second is employed in the model. The finite element mesh size is 1 mm, the CA cell size is 50 µm. The equations for heat flow, grain nucleation, and growth are listed below.

From the previous work, it is known that during the continuous casting process of the YQ450NQR1 steel, the straightening temperature falls into the low ductility zone when the secondary cooling is not controlled properly, which would cause the low ductility in the bloom surface/subsurface microstructures and result in frequent surface cracks. Moreover, due to the large cross section of the bloom, solute macro-segregation occurs easily. Based on this work, it is useful to properly control the pouring temperature and the secondary cooling scheme and to utilize the benefits of the central equiaxed grain zone and the fine surface microstructures containing dispersed V(C, N) particles. In this way, the solute macro-segregation and the surface cracks can be reduced at the same time.

Based on the above analysis, a new cooling strategy is determined for the continuous casting process of the YQ450NQR1 steel after a series of calculation trials: the pouring superheat is 23 °C (the original value was 37 °C), the water flowrate is 2.6 L/min in the SCZ-III segment in the SCZ (the original water flowrate was 51.5 L/min), and the water flowrate is 165.6 L/min in the SCZ-IV segment in the SCZ (the original water flowrate was 18.4 L/min). The surface temperatures from the bloom corners with the original and the new cooling schemes are shown in Figure 13.

According to the results from Section III–B and the thermodynamic calculations from Figure 7, the onset precipitation temperature for the V(C, N) particles is ~ 1100 °C. From the heat transfer calculations (the original temperature curve from Figure 13) and the caster parameters (as shown in Table III), it can be judged that the V(C, N) starts to precipitate mainly in the SCZ-III and the SCZ-VI segments in the SCZ, mainly in the bloom surface microstructures due to the intense water cooling. It is known from the above study that increased cooling rate can disperse the distribution of the V(C, N), thus promoting the uniform distribution of the ferrite and the refinement of the grains. As a consequence, the hot ductility of the YQ450NQR1 steel at the straightening point can be improved to avoid the cracks. Based on the actual cooling condition and Figure 6, the bloom surface cooling rate in the SCZ-IV segment should be maintained above 1 °C/s and the straightening temperature should be controlled between 700 °C and 800 °C.

According to the temperature curve with the new cooling strategy in Figure 13, the bloom surface microstructures undergo a temperature increase in the SCZ-IV segment, which would promote the dissolving of the formerly precipitated V(C, N) particles. In the SCZ-III segment, the bloom undergoes intensive cooling, which would lead to the re-precipitation and the dispersion of the V(C, N) in surface microstructures. In this way, the hot ductility of the bloom surface is improved to resist the crack formation during the straightening operation.

According to Table III, the length of the SCZ-IV segment is 5.14 m. Under the casting speed of 0.5 m/min, the surface temperature from the bloom corners decreases from 1365 °C to 642 °C at the exit of the SCZ-IV segment, as the improvement in the surface microstructure ductility is the key factor in preventing the cracks formation during the straightening in continuous casting process. The R.A. values of the bloom surface and the subsurface should be above 60 pct. Considering this requirement and to further validate the idea, the thermal gradient and the cooling rate profiles (both are average values calculated between the SCZ-IV and the straightening point) at the bloom cross section under the new cooling strategy are obtained as in Figures 14 and 15; it can be seen that the variations of the thermal gradient and the cooling rate are large at the bloom surface, while it decreases and stabilizes at ~ 3.5 cm beneath the bloom surface. This indicates that the cooling water will have an influence on the subsurface microstructures within 3.5 cm from the bloom surface during the continuous casting process. Figure 16 shows the temperature profiles within 4 cm of the bloom surface along the cross section direction with the new secondary cooling pattern; it can be seen that within 3.5 cm beneath the bloom surface, the temperatures are in the range from 720 °C to 750 °C, all of which, according to Figure 6, fall within the high R.A. region. Based on the above analysis, the straightening temperature can be controlled to avoid the low ductility region. A further CA-FE modeling is carried out to compare the solidification structures of the YQ450NQR1 steel bloom with the original (left) and the new (right) secondary cooling patterns, as is shown in Figure 17. It is found that the ECR increases to 31.3 pct with the new cooling strategy, compared with the original ECR of 26.3 pct.

In this paper, a new cooling strategy for the curved continuous casting process of the vanadium-containing YQ450NQR1 steel is proposed based on the experimental and the modeling results. Main conclusions are as follows: