Brief analysis and improvements on the causes of peeling off of the copper plate coating on the mold of slab continuous casting machines

Based on the current use status of the slab continuous caster in Jiugang Iron and Steel Co., Ltd., the reasons for the peeling off of the copper plate coating on the mold of the slab continuous caster and the improvement measures were studied.

Keywords: slab continuous casting machine; mold copper plate; coating peeling; brief analysis and improvement of causes

1 Introduction

The crystallizer is the core component of the continuous casting machine. The slab continuous casting machine crystallizer is composed of a pair of wide-faced copper plates, a pair of narrow-faced copper plates, a clamping device, a width-adjusting device, and a frame structure. It is an arc-shaped four-plate combined crystallizer. The copper plate is the most critical component of the crystallizer and is required to have good thermal conductivity and tensile strength. The functions required for copper plate plating can be summarized as splash resistance, heat crack resistance, peeling resistance, high heat dissipation, and wear resistance and corrosion resistance of the lower part. Since June 2008, the copper plate has been frequently corroded by hair and the plating layer has fallen off around the slag layer at the top of the outer arc copper plate of the slab mold. Regarding the problem of peeling off of the copper plate coating of the crystallizer, we concluded through systematic analysis that:

(1) The peeling part of the outer arc copper plate coating is in the high temperature zone of molten steel and the weak cooling zone, and the cooling effect is poor.

(2) Due to the different thermal expansion coefficients of the plating layer and the copper plate matrix, peeling is easy to occur.

(3) The thickness of the coating on the top of the copper plate is not optimal and the thermal conductivity is poor.

(4) The scaling of the cooling water reduces the cooling effect of the copper plate to a great extent.

2 Basic parameters of Jiugang slab continuous caster

2.1 Casting machine parameters

The casting machine parameters are shown in Table 1.

(1) Continuous casting machine model: R7.5m single-machine single-flow full-arc 8-point straightening slab continuous casting machine.

(2) Product specifications: (thickness × width) 160 mm × 950mm (1050, 1250, 1350), (thickness × width) 220 mm × 950mm (1050, 1250, 1350).

(3) Casting billet drawing speed: 0.2~1.3 m/min.

(4) Casting steel types: plain carbon steel, low alloy steel, boiler steel, bridge steel, pressure vessel steel, medium carbon steel, etc.

(5) Metallurgical length: 22.26 m.

2.2 Basic parameters of crystallizer

(1)Type: curved four-plate combined crystallizer.

(2)Outer arc radius: R7.5m.

(3) Crystallizer length is 800 mm.

(4) The base material of the crystallizer copper plate is made of chromium zirconium copper or other materials with better performance, which has good thermal conductivity and sufficient tensile strength.

①The copper plate material is: Cr-Zr-Cu or other materials with better performance; the surface coating is nickel iron or materials with better performance.

②The thickness of the copper plate surface coating is: upper opening ≤0.3mm, lower opening 1.5mm.

③Tensile strength: 250 N/mm².

④Yield limit: 200 N/mm².

⑤Elongation: 12%.

⑥Hardness: HB≥80.

⑦Conductivity: ≥56.84 m/mm²Ω.

⑧Radian error of copper plate: ≤0.1 mm.

⑨ Corner seam combined with wide surface and narrow surface: ≤0.3 mm.

Table 1 Basic parameters of Jiugang slab continuous caster

parameter nametechnical parameter Remark
Overall equipment parameters Pourable cross-section sizeWide guide roller gap(160)Thickness 160 mm, 220 mm 
Width 950~1350mm 
Arc gapSecond cold water nozzle water spray experiment1#165.19±0.2mm 
2#165.16±0.2mm 
Overall hydrostatic testThe gap between the outer arc copper plate, foot roller and sample plate is 1±0.1 
Wide guide roller gap(220)0.6 MPa.10 min 
Crystallizer cavity length1.05 MPa, 30 min pressure drop ≤ 0.02 MPa 
Radius of outer arc copper plate of moldRadius of inner arc copper plate of mold1#226.51±0.2 mm 
2#226.39±0.2 mm 
Crystallizer part Copper plate surface coating800 mm 
Inlet water pressureR7500 mm 
Inlet water temperatureR7303.5 mm 
Crystallizer nozzle layoutNiFe alloy, 0.1~0.5 mm 
amplitude0.9±0.5 MPa 
Vibration frequency≤33℃ 
Clamping cylinder strokeWide side: 58 Narrow sides: 6 
Longitudinal deviation (along the blank exit direction)±5 mm(adjustable) 
Phase difference of four eccentric points160 c.p. π 
Shaft decylinder stroke100mm 
Vibration generating partLateral deviation (along the width of the slab)≤0.2 mm 
Shaft clutch stroke≤0.80 
Pourable cross-section size200 mm 
Wide guide roller gap(160)≤0.15 mm 
Arc gap200 mm

3 Equipment status

In June 2008, during the preparation process for restarting the slab continuous casting machine after stopping pouring, it was reported that the copper plate had hairiness, pitting corrosion and coating peeling off around the coating slag line at the top of the outer arc copper plate of the mold. For example, on June 16, October 23, and December 1, 2008, the slab continuous casting machine #5 stopped pouring in a single pour. Organize the production preparation and confirmation process before pouring again. The inspection found that at the left corner of the outer arc of the copper plate 70 mm away from the upper opening (the initial generation part of the shell under the slag layer), 12 mm × 1 mm, 12 mm × 2 mm, and 10 mm × 1 mm long coatings were peeled off along the direction of the corner seam. After communication with the production technology department, equipment technicians and inspection signatures, the continuous casting machine maintained production at low casting speed and low liquid level under the current conditions.

4 Reason investigation and analysis

The copper plate of the crystallizer reflects the frequent phenomenon of hairiness, pitting corrosion and plating peeling off of the copper plate around the upper slag layer. This phenomenon is mainly manifested in the outer arc copper plate. The specific analysis is as follows:

4.1 Analysis of the actual working conditions of the crystallizer

As shown in the schematic diagram of the operation of the mold copper plate in Figure 1, the thickness of the protective slag layer when the mold copper plate is actually working is about 50 mm, and the distance between the meniscus of the molten steel and the uppermost edge of the copper plate is about 65 mm. From the analysis of the immersion depth of the nozzle and the circulation of molten steel in the crystallizer, the molten steel flows out from the side hole of the nozzle and returns after hitting the narrow copper plate. A high-temperature zone is formed on both sides of the narrow surface of the copper plate and the surrounding area, and the part where the coating of the slab copper plate is prone to peeling is located in this high-temperature zone.

Figure 1 Working diagram of crystallizer copper plate

4.2 Analysis of design structure of slab mold copper plate

The crystallizers of the 4# and 5# slab continuous casters in the refining and rolling plant are arc-shaped crystallizers (R7.5m), the length of the crystallizer is 800 mm, and the copper plate is made of Cu-Ag alloy. From a structural point of view, the depth of the cooling water tank on the back of the copper plate is 26 mm and the width is 5 mm (Figure 2). In the transition part with a radius (R) of 105 mm at the end of the water tank (as shown in Figure 7), the thickness of the copper plate increases from the upper 30 mm to 100 mm area of the outer arc copper plate, and the cooling water tank gradually becomes shallower, which is a weak cooling area.

4.3 Copper plate plating

Figure 2 Actual photo of the crystallizer’s wide copper plate water tank structure

The coating is Ni-Fe alloy. See Figure 3.

On December 1, 2008, when the 5# continuous casting machine stopped pouring, the thickness of the peeling coating on the outer arc copper plate of the mold was roughly measured to be about 0.5 mm (the standard coating thickness is: upper opening ≤ 0.3 mm, lower opening 1.5 mm), which is beyond the standard range.

Figure 3 Actual photo of external arc copper plate coating

4.4 Quality of crystallizer cooling water

The scaling phenomenon in the support plate and copper plate water tank is relatively serious. As shown in Figure 2, Figure 4, and Figure 5.

Figure 4 Actual photos of scaling on the copper plate support plate of the crystallizer

Figure 5 Actual picture of scaling on the copper plate of the crystallizer

5 Analysis results

(1) The peeling part of the outer arc copper plate coating is in the high temperature zone of molten steel and the weak cooling zone, and the cooling effect is poor.

(2) Due to the different thermal expansion coefficients of the plating layer and the copper plate matrix, peeling is easy to occur.

(3) The thickness of the upper coating is 0.5 mm, which exceeds the standard range of coating thickness and has poor thermal conductivity.

(4) The scaling of the cooling water reduces the cooling effect of the copper plate to a great extent.

Figure 2 Actual photo of the crystallizer’s wide copper plate water tank structure

6 Improvement measures

(1) Reduce the size of the transition point (R) between the inner and outer arc copper plate cooling water troughs (see Figures 6 and 7). After research and calculation, it can be reduced from the original 105 mm to 30 mm to improve the cooling effect of the inner and outer arc copper plates.

(2) Reduce the thickness of the upper coating on the inner and outer arc copper plates and control it within 0.1 ~ 0.3mm to increase the thermal conductivity.

(3) By strengthening water quality management, improve the quality of crystallizer cooling water and solve the problem of massive scaling in the copper plate water tank.

(4) The process controls the molten steel level in the mold according to standards to avoid weak cooling areas.

Figure 6 Schematic diagram of cross-sectional dimensions of the water tank before and after the transformation of inner and outer arc copper plates

Figure 7 Partial schematic diagram of the water tank section after modification of the inner and outer arc copper plates

7 Field application status after improvement

After the implementation of the above measures, the problem of peeling off of the copper plate coating on the mold of the slab continuous casting machine was solved. In 2009, the improved copper plate was put on trial in slab 4# and 5# continuous casting machines. The trial results show that the equipment operates normally, the casting speed is increased, equipment maintenance is reduced, the number of steel breakouts is reduced, the quality of the slab is improved, and the company creates good economic benefits.

(1) Increase the pulling speed. According to internal statistics of the smelting and rolling mill (shown in Table 2), the drawing speed has increased by a maximum of 0.4 m/min than before, and each shift (8 hours) can produce 5 more furnaces of steel. As a result, the productivity of each shift increased by 250 t, and the daily output increased by 750 t. The comparison of output and pulling speed before and after the improvement is shown in Figure 8 and Figure 9.

Table 2 Comparison table of usage before and after transformation

SectionNumber of pouring furnacesSpeed before modificationUsage after renovationRemark
220*125012 0.7~0.8m/min In normal use, the general pulling speed is between 0.9 and 1.0m/min, with a maximum of 1.1m/min.First test on machine #4
220*12511170.7~0.8m/min In normal use, the general pulling speed is between 0.9 and 1.0m/min, with a maximum of 1.1m/min.After the successful test of machine 4, machine 5 will be tested.
160*950411.1~1.2 m/min(Abnormal use)In normal use, the general pulling speed is between 1.1 and 1.2m/min, with a maximum of 1.3 m/min. 
160*13501811.0~1.1m/min In normal use, the general pulling speed is between 1.0 and 1.1m/min, with a maximum of 1.2 m/min. 
160*12501740.7~0.8m/min In normal use, the general pulling speed is between 0.9 and 1.0m/min, with a maximum of 1.1m/min.

Figure 8 Comparison of output before and after transformation

Figure 9 Comparison of pulling speed before and after transformation

(2) Reduce the amount of maintenance. After adopting the improved copper plate, the operation of the crystallizer is stable, the peeling off of the coating on the surface of the copper plate is controlled, the amount of steel passed is increased from less than 40,000t before the modification to more than 65,000t, and the maintenance time of the crystallizer is reduced from 66 hours before the modification to 35 hours. Since the frequency of crystallizer replacement has been significantly reduced, the amount of repairs and maintenance costs have also been greatly reduced (as shown in Figure 10).

Figure 10 Comparison of maintenance volume before and after transformation

(3) Reduce the occurrence of steel breakout accidents. It turns out that there are more than 5 steel leakage accidents every year due to hanging steel on the copper plate of the crystallizer, with an average of about 0.4 times per month. After using the improved copper plate, and under the condition that there are no problems in the continuous casting process, there has been no steel leakage accident caused by steel hanging on the copper plate of the mold (as shown in Figure 11).

Figure 11 Comparison of the number of monthly steel breakouts caused by steel hanging in the mold before and after the transformation

(4) The surface quality of the cast slab is improved. After using the improved copper plate, the surface of the cast slab is continuous and uniform, and surface quality problems such as surface cracks are rare.

8 Conclusion

(1) By reducing the size of R at the transition point of the copper plate cooling water tank, we can effectively prevent the coating from falling off due to different expansion systems of the materials.

(2) By reducing the thickness of the upper coating, the thermal conductivity of the copper plate is increased, and the corrosion problem of the copper plate coating caused by the protective slag layer on the copper plate is alleviated.

(3) By solving the water quality problem, the scaling problem of the water tank is eliminated and the uniformity of heat conduction of the copper plate is ensured.

(4) The utilization rate of the equipment is improved, the maintenance of the mold is reduced, the casting machine speed is increased, the yield and quality of the cast slabs are increased, and the steel leakage accidents and equipment accidents caused by hanging steel on the copper plate of the mold are reduced.

(5) This improvement has been verified by theoretical calculation and field use, and has met the process, production capacity, use environment and quality requirements. In particular, the production capacity has exceeded its original design requirements and has high economic value.

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