Analysis and Control of Inclusion Defects on the Surface of Hot Rolled Coil

Abstract : The morphology , distribution and corresponding components of inclusion defects on the surface of low-carbon hot-rolled coils were analyzed . The results show that the inclusion of mold powder is the main source of inclusion defects on the surface of low-carbon hot-rolled coils . On the basis of studying the slag entrainment mechanism of the mold , by adjusting the composition of the mold slag , increasing the viscosity of the mold slag , adjusting the structure of the submerged nozzle , optimizing the flow field of the mold , and improving the relevant process conditions , the occurrence rate of such defects With effective control, the rate of rejudgment caused by this has also dropped from 4 % to less than 0.2% , and the overall effect is remarkable .
Key words : surface inclusions ; slag entrainment ; viscosity ; flow field

the non-metallic inclusions on the surface or under the skin of the strip that are exposed to the surface with the rupture of the skin during the rolling process, and are irregularly distributed in the form of strips, lines or irregular blocks . a series of defects . Usually, the sources of such defects mainly include deoxidized products of molten steel itself , refractory materials brought in during the transportation and pouring of molten steel , mold slag involved in the crystallizer, and iron oxide scale generated during rolling, etc. With the change of production process parameters , the degree of influence of various factors on the occurrence rate of such inclusions is also correspondingly different .
S steelmaking plant of Wuhan Iron and Steel (Group) Company has the current world-class continuous casting machine equipment, ladle slag detection system , automatic pouring system , submerged nozzle automatic quick change system and other related auxiliary systems . Since May 2009 , after the SPHC series and SAE1008 low- carbon steel slabs produced by this workshop were rolled into coils by the matching R hot rolling plant, many inclusion defects appeared , and the highest initial defect incidence rate exceeded 60% . % , seriously affecting production stability and comprehensive yield . Therefore , the author analyzes the origin and generation mechanism of such defects , and proposes corresponding improvement measures . After more comprehensive process optimization and comprehensive control , the occurrence rate of such defects has been effectively controlled .
1 Analysis of surface defects of hot-rolled coils
R Hot Rolling Plant is equipped with a steel coil surface defect detection system , which can accurately and comprehensively detect steel coil surface defects and track them in a timely manner . It is found through the system detection that the surface defect rate of low-carbon steel coils reaches 30% to 70% in the early stage of mass production , and the corresponding rate of change of judgment reaches 3% to 4% , which seriously affects its production stability and yield . Several groups of samples were taken from the defect parts for inspection and analysis , and it was found that several typical surface defect morphology of steel coils are shown in Figure 1 .

Fig. 1 Macroscopic profile of the defects on coil surface It can be seen from Figure 1 that the surface defects of 1 # sample and 2 # sample are generally distributed in a linear shape , and there are foreign objects embedded in the defects ; the surface defects of 3 # sample are plate Peeling defects at the curling portion . Grinding samples 1 # and 2 # were inspected by scanning electron microscope and combined with energy spectrum analysis , and sample 3 # was subjected to positioning inspection , and the inclusion components contained in the defects were shown in table 1 .

TablelChemical compositions of inclusions in linear defects

	O	Fe	Na	Mg	Al	Si	Ca
1＃	30. 48	12．66	7．43	5．05	10．47	169	16．02
2＃	36. 81	10．45	11．14	51	5．89	16．78	13．82
3＃	34．5	11．58	3．98	1．65	14．54	16．59	6．9

from Table 1 that the linear defects and subcutaneous inclusions in the above samples all contain Na z O.MgO.Al 2 O 3 .SiO 2 And CaO and other components are basically consistent with the typical mold slag composition , so it can be confirmed that the inclusion defects of the three steel coil samples mainly come from the inclusion of mold slag during the continuous casting process .
In order to further understand the main reasons for such defects , more than 30 groups of defect samples were taken to analyze the inclusion components . The results show that 59% of the defective parts of the samples have Al , Si , Ca , Na and Mg oxides , 23% of the defective parts of the samples mainly contain oxides of A1 , and 9% of the defective parts of the samples have Al , Si, Ca , Na and Mg oxides. Oxides of Ca , Si , Al , and Mg , and the foreign matter in the defective parts of 2 samples was mainly iron oxide . Figure 2 shows the statistics of the location where the defect occurred . It can be seen from Figure 2 that more than 70% of the inclusion defects are distributed in the strip-shaped area 20-60 mm away from the hot-rolled curling portion , which has a good correspondence with the distribution of slag inclusions at the edge of the slab . Based on this , it is judged that 59% of the surface and subcutaneous inclusions of this type of low-carbon hot-rolled coil originate from the mold powder , that is, the mold powder is involved in the narrow surface of the mold and becomes the subcutaneous slag inclusion at the edge of the billet. development of surface inclusion defects [ 1-3 ] . _ _

2 Control of surface defects of hot rolled coils
2 . 1 Improvement of Mold Mold Flux Performance
The involvement of molten mold slag in the mold is related to the turbulent flow at the slag - steel interface caused by liquid level fluctuations . The shear force on the liquid slag layer is formed near the upper narrow surface of the mold . If the flow rate of molten steel is high , the slag - steel interfacial tension is small or the viscosity of mold powder is low , the liquid slag will be drawn into the molten steel by the steel flow . Therefore , to reduce the tendency of mold slag to be involved , it can be achieved by increasing the viscosity of mold slag or increasing the slag - steel interfacial tension . In contrast , increasing the viscosity of mold flux is more effective and easier to control than increasing the interfacial tension . At the same time, increasing the viscosity can reduce the tendency of slag entrainment and help to improve defects such as deep vibration marks on the surface of the slab *  ], therefore, this study chooses to increase the viscosity of the mold flux to reduce the surface defects of the steel coil caused by the slag .
Generally speaking , when casting low carbon steel, the temperature of molten steel is high , the casting speed is fast , and the content of inclusions in molten steel is high . Therefore , the main considerations for designing mold slag for low-carbon steel molds are as follows: ①The slag film has good lubricating effect and thermal conductivity to reduce the adhesion between molten steel and copper plate in the mold , and ensure that the mold shell has sufficient thickness to resist the static pressure of molten steel; ② The molten mold slag has good fluidity and can ensure good stability , which is conducive to the adsorption of inclusions . Casting low carbon steel usually requires mold flux with low viscosity and low alkalinity . The low-carbon steel mold powder ( Z1) of plant s was also designed according to this principle before improvement, and the specific physical and chemical indicators are shown in Table 2 . It can be seen from Table 2 that the existing mold slag has too low viscosity , which meets the requirements of good slag fluidity , but the interfacial tension of steel slag is too small , which is the main reason for the high incidence of slag entrainment .
The main factors affecting the viscosity of mold flux are :
(1) Alkalinity . Alkalinity changes can affect the content of SiO 2 in the mold flux and the connection mode of the silicon-oxygen tetrahedral network , thereby affecting the viscosity of the mold flux . When the alkalinity increases , the viscosity of the mold flux decreases .
(2) Contents of A1O 3 , IiO , Na 2 O and CaF 2 . silicon

A1 3 + in the salt melt can be 6 , namely [AlO s butyl- , or 4, namely [Al . 』 Ding- . _ In mold flux, there is usually single-bond oxygen provided by alkaline earth metal, so that AI 2 O 3 can be [ AlO 4 ] 5- The form exists , therefore , the increase of Al 2 O 3 will increase the viscosity of the melt . UO and Na 2 O can provide non-bridging oxygen atoms in the network structure of mold flux melt , destroy the connection network and reduce the viscosity of mold flux . CaF 2 can introduce F - which has a radius similar to that of O 2- , and F - can react with the silicon-oxygen network to break the mold flux network and reduce the viscosity of the mold flux .
Under the same conditions , increasing the viscosity of mold slag is mainly achieved by changing its composition . At the same time , it is necessary to ensure stable consumption of mold slag , good lubrication effect , and controllable change in mold shell thickness . Integrating various factors and combining the casting characteristics of low-carbon steel in S Steel Plant, the basic plan for adjusting the composition of mold flux is proposed as follows [ -Cut :

1. Keep the existing basicity unchanged , avoid the crystallization reaction in the solidification process , and ensure the thermal conductivity of the slag film .
2. The adjustment range of Na 2 O and Li 2 O content is also small, and the existing content basically remains unchanged .
3. lower F- _ content and increasing the content of Al 2 O 3 have obvious effects on increasing the viscosity , and have little effect on the melting performance of the mold flux . Therefore , this method is chosen to increase the viscosity of the mold flux .

After the adjustment plan is determined , a comparative test on the application of high-viscosity mold flux is carried out in stages . The mold flux used in the test is the original low-viscosity mold flux Z1 , the newly designed mold flux Z2 and the corresponding high-viscosity improved mold flux Z1 modified , Z2 modified and Z2 high , and their properties are shown in Table 3 .
Show

Table 3 Viscosity of mold flux used in comparison test Table 3 Viscosity of mold powder in contrast experiment
Mold powder type	Z1 Z2 Z1 change Z2 change	Z2 high
1 300 °C Viscosity /Pa • s	0 . 13 0 . 20 0 . 32 0 . 30	0. 37

It can be seen from Table 3 that the viscosity of Z1 mold flux is low, the viscosity of the newly developed mold flux Z2 has increased , and the viscosity of the corresponding improved type has been greatly improved , and the viscosity of each model has obvious progress. The order gap provides a basis for comparative experiments .
the first 3 months of the test , a total of 42t mold flux was tried in 3 steel grades . The corresponding defect occurrence rates of the three test steels using different mold fluxes were counted , and the results are shown in Figure 4 . It can be seen from Figure 4 that when the viscosity of the mold flux is greater than 0.3 Pa • s , the incidence of slag inclusion in the mold of low carbon steel can be significantly reduced , and then the incidence of slag inclusion on the surface and subcutaneous of the hot-rolled steel coil can be controlled ; use Z2 Two high-viscosity modified types of mold flux, Z2 and Z2 high , have a relatively good effect on controlling slag inclusions on the surface and subcutaneous of steel coils ; the application effect of high-viscosity mold flux on controlling defects in the casting process of different steel types is more obvious difference .

High-viscosity mold slag has obvious advantages in controlling the incidence of hot-rolled coil surface and subcutaneous inclusions , but it is easy to cause the slag film to break when the mold liquid level is abnormal , and the slag strip is too thick and strong when changing the package , thus increasing Risk of bonded breakouts and joint trace breakouts . Therefore , targeted measures must be taken in terms of process operation to ensure the stability of the continuous casting process while exerting the beneficial effects of high-viscosity mold flux (1) . The main measures are :

1. The mold liquid level is automatically controlled throughout the continuous casting process from the start of pouring , and the casting machine is automatically speed-up and quantitatively operated to ensure the stability of the mold liquid level . Avoid frequent stirring of the molten steel surface after pouring to reduce damage to the slag layer .
2. Mold slag is added according to the principle of frequent, less and uniform, and the operation of black slag is implemented to reduce the operation of picking slag .
3. Reduce the casting speed when the submerged nozzle is quickly changed, and use the one-button automatic nozzle replacement and liquid level automatic control functions . After the quick change, remove the large slag on the surface and add new slag , but do not over-remove the slag , so as not to damage the slag layer .
4. Pay attention to observe the crystallizer expert system liquid level , stopper rod position , and heat flux changes during the pouring process , and take corresponding measures in time if abnormalities are found .
5. If mold flux is found to be abnormal during pouring, stop using it immediately and deal with it in time .

2.2 Crystallizer flow field optimization
The submerged nozzle structure has a significant impact on the flow field of the mold , and a reasonable nozzle structure can obtain an ideal mold flow field . In view of the high incidence of slag entrainment in low carbon steel molds , the flow field of the mold was optimized to reduce the fluctuation of the liquid level of the mold , and the liquid surface velocity was appropriately reduced , thereby reducing the drag of the molten mold slag by the upward swirling flow. The probability that molten steel and mold slag are sheared from the liquid slag layer and enter molten steel [ 2 ] .
According to the influence law of the mold flow field on the flow velocity and fluctuation of the liquid steel surface , the impact direction of the strand at the side hole of the nozzle is biased downward or the impact force of the strand is weakened , which is beneficial to reduce the disturbance to the upper swirl area of the mold , and thus reduce the impact of the mold. The flow velocity of the liquid surface can reduce its fluctuation , especially the fluctuation of the liquid surface near the narrow surface . Increasing the inclination angle of the side hole of the nozzle can make the impact direction of the injection flow shift downward , and increasing the expansion angle of the side hole of the nozzle can make the steel flow out of the side hole diverge and the impact force will be weakened .
Therefore , the basic plan for submerged nozzle structure transformation is as follows: ① modify the inclination angle of the side hole from 15° downward to 18° ~ 20° downward ; ② modify the parallel side hole to a structure with an expansion angle of 6° ~ 10° .
In order to verify the application effect of the improved submerged nozzle, 20 rounds of comparative tests were carried out on the No. 2 twin-strand continuous casting machine in the continuous casting workshop of S Plant , and the results are shown in Table 4 . It can be seen from Table 4 that optimizing the submerged nozzle has a significant effect on reducing the incidence of surface defects in hot-rolled coils .

Table 4 Occurrence rate of surface defects of hot rolled coils before and after nozzle structure optimization

stream number	Nozzle type	Incidence rate of hot-rolled coil surface defects / %
1	Yuanshuiguchi	22 2
	Optimizing nozzle	97
2	Yuanshuiguchi	15 6
	Optimizing nozzle	74

2 . 3 Improvement of other process conditions
As mentioned above , under unsteady pouring conditions such as continuous casting , quick change of tundish , and replacement of submerged nozzle , there is a greater possibility of crystallizer slag entrainment and tundish refractory being involved . In terms of stability and other aspects, the incidence of crystallizer slag entrainment is comprehensively controlled , and the measures taken mainly include :

1. Guarantee the standard rate of casting machine point pull and increase the proportion of steady pouring .
2. Realize the automatic pouring technology of tundish to ensure automatic pouring

The rate reached more than 99% .

1. Realize automatic quick change of submerged nozzle .
2. Quantify the crystallizer liquid level control accuracy , the amount of molten steel in the tundish process, the insertion depth of the submerged nozzle for each steel type, and the Ar gas blowing rate for each steel type, etc. , to achieve standardized operations .

3 Improvement effect of hot-rolled coil surface inclusions
Focusing on reducing the probability of mold slag being involved in the mold , focusing on the performance optimization of mold slag , the improvement of the structure of the submerged nozzle , and the comprehensive control of the process , after three months of comprehensive tests , the rate of change of judgment of hot-rolled plates caused by inclusions was used as an indicator . The improvement effect within 11 months from August 2009 is shown in Figure 5 . It can be seen from Figure 5 that the rate of change of judgment caused by slag inclusion defects in low-carbon hot-rolled coils dropped from an average of more than 4% per month before improvement to less than 0.3% after improvement , a relative decrease of more than 90% . The research and control of defects has achieved the expected goal , and there is no situation in the conventional production that affects the production stability due to the change of the performance of the mold flux , the improvement of the submerged nozzle and the corresponding process optimization , and the comprehensive effect is remarkable .

 

4 Conclusion

1. The surface inclusion defects of the low-carbon hot-rolled coils produced by R plant are mainly caused by slag inclusions in the continuous casting mold of S steel plant .
2. After using high-viscosity mold flux , the occurrence rate of slag entrainment in low carbon steel mold is significantly reduced , but the improvement effect on surface inclusions of different steel types of hot-rolled coils is different .
3. Through performance optimization of mold flux , improvement of submerged nozzle structure , comprehensive process control and other measures , the occurrence rate of inclusion defects on the surface of low-carbon hot-rolled coils has been greatly reduced , and the rate of change of judgment caused by inclusions has dropped from the initial monthly average of more than 4% to 03% or less .

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