Si Content on Red Scale on Hot Rolled Coil Surface

Abstract : The red scale ( red rust ) residue test of the coil was carried out on the hot rolling production line . The surface quality of the coil and the structure of the red rust were analyzed by using the online surface quality inspection system and scanning electron microscope . The effect of Si on the structure of scale and the measures to improve the descaling effect are discussed . The results show that the Si content is the most important factor affecting the residual amount of red rust on the surface of hot-rolled coils . With the decrease of Si content , the residual amount of red rust on the coil surface is less . Scanning electron microscope analysis shows that there is obvious Si enrichment between the red rust and the matrix .
Key words : surface oxide scale ; Si content ; descaling
The red oxide scale on the surface of hot-rolled coils ( that is, the Fe 2 O 3 oxide scale remaining on the surface of the steel strip , in the form of dark red stripes , hereinafter referred to as red rust ) is not good for the use performance and service performance , and also seriously affects the appearance of the steel coil . It is generally believed that the red rust on the surface of hot-rolled coils is mainly due to the unclean descaling during the rolling process , and the residual oxide scale is a surface defect formed by deformation and further oxidation of the steel strip during the rolling process . If the descaling effect of the rolling line is poor , thick oxide scale may remain and be pressed into the steel matrix during the deformation process to form scale indentation defects , which seriously affect the normal use of the steel strip .
Domestic and foreign scientific research institutions and enterprises have carried out extensive research on the structure , formation rules and removal methods of oxide scale on the surface of hot-rolled steel strip [1-7] . However, the general research is limited to the laboratory , and the focus of the research is also on the microstructure of iron oxide scale , which is not closely integrated with the production site , which is not conducive to the formulation and control of on-site process parameters . At present, the development trend of descaling of hot-rolled strip mainly focuses on increasing water spray pressure , improving descaling passes , strengthening descaling equipment, etc. , which not only increases production and maintenance costs , but sometimes the effect is not ideal .
In order to closely integrate with the on-site production and better solve the problem of red rust in the coils during the production process, this paper uses the 2250 hot rolling production line of Shougang Jingtang Company, combined with the actual situation of the production site, to carry out the actual surface quality of the coils under different process conditions. analyze. The production test of steel strips with different Si ( silicon ) contents was focused on, providing reliable field data and new ideas for improving product surface quality and performance.

1 Experimental conditions and methods
2250 hot rolling line of Shougang Jingtang Company includes 3 heating furnaces , rough descaling unit , roughing mill descaling device , finishing rolling descaling unit and 7- stand finishing rolling unit , and the descaling water pressure is 22MPa . In order to maximize the reliability of the experimental results , a continuous casting slab of the same specification with a Si content of 0.06% to 0.18% was selected from the same brand steel . The continuous casting slabs are put into the furnace in a cold state , and it is ensured that there are slabs with low , medium and high Si content in the three heating furnaces , and the slabs with different Si contents are randomly arranged in the heating furnaces . This is mainly to eliminate the impact of the temperature fluctuation of the furnace and the temperature fluctuation of the rough descaling start on the descaling effect .
C-Mn steel was selected for the experiment , and its actual chemical composition ( mass fraction, % ) is: 0.14 - 0.220 , 0.40 ~ 1.5Mn , 0.010 ~ 0.015P ,

0.002 ~ 0.005S , 0.06 ~ 0.18Si . All the slabs used in the experiment were rolled into steel strips with exactly the same specification by the same rolling process . The rolling specifications and process are shown in Table 1 . After rolling and before coiling, use the Parsytec online surface quality inspection system to take real-time images of the upper and lower surfaces of the steel strip . Finally , use tool software to count the area percentage of red rust on the surface of the steel strip .
Table 1 Experimental Coil Rolling Specification Process
Tab.1 The rolling size and process of the coils

Billet specifications	Coil specification	heating temperature	Rolling temperature	Finishing temperature	coiling temperature
230 mm x 1600 mm	8.7 mmx 550 mm	1160°C	1100°C	840°C	580°C

2 Experimental results
The typical surface red rust of hot-rolled steel coils with different Si content taken by the online surface inspection system is shown in Fig. 1 . It can be seen that , under the condition that other conditions remain unchanged , the red rust on the surface of the steel strip decreases significantly with the decrease of Si content . Especially when the Si content drops below 0.15% , there is basically no visible red rust on the surface of the steel strip . In order to more accurately count the relationship between the Si content and the area percentage of red rust on the steel strip surface , 6 representative photos of the steel strip surface were selected from the image library of the online detection system for 12 coils, and the overall red rust on the upper and lower surfaces were counted. distribution and perform quantitative calculations . The calculated Si content and the average and maximum red rust area percentages on the strip surface are shown in Table 2 and Figure 2 , respectively .

It can be seen from Table 2 that Si content is the most important factor affecting the amount of red rust on the surface of steel coils . The table lists the corresponding relationship between different heating furnaces and the red rust on the surface of steel coils . The tapping temperature setting values of the three heating furnaces are the same , but due to the different distances from the rough descaling , the time required for the slabs coming out of the three heating furnaces to the rough descaling machine must be different , which in turn leads to the rough descaling. Scale temperatures vary . However, it can also be seen from Table 1 that there is no regularity between the distribution of red rust on the surface of the steel strip and the number of the heating furnace.
Table 2 Amount of oxide scale on the surface of steel coil
Tab.2 The percentage of red scale on the surface of the coils

released serial number	heating furnace number	Si(et,%)	upper surface	lower surface	Average red rust area ratio (%)
1	1 #	0.10	No visible red rust	No visible red rust	0
2	2 #	0.06	No visible red rust	No visible red rust	0
3	1 #	0.16	trace red rust	trace red rust	0.78
4	2 #	0.09	No visible red rust	No visible red rust	0
5	3 #	0.09	No visible red rust	No visible red rust	0
6	2 #	0.16	Head trace red rust	trace red rust	0.80
7	1 #	0.16	Trace red rust in the middle and back	trace red rust	0.60
8	3 #	0.16	Full length partly red rust	Full length partly red rust	12.54
9	3 #	0.15	Tail trace red rust	trace red rust	0.80
10	1 #	0.18	Full length partly red rust	Full length partly red rust	12.58
11	2 #	0.08	No visible red rust	No visible red rust	0
12	3 #	0.08	No visible red rust	No visible red rust	0

There are laws to follow .
Si content and the maximum and average area percentage of red rust on the surface of steel coil is shown in Figure 2 . It can be seen that when the Si content is higher than 0.15 % , the maximum area ratio of red rust on the surface of the steel strip reaches more than 35% , and the average area ratio reaches more than 5% . But when the Si content drops below 0.15% , the red rust on the surface of the steel strip decreases rapidly , and both the maximum area ratio and the average area ratio are below 1% . When the Si content drops below 0.1% , the steel strip is coiled and cooled to room temperature , and the surface color is similar to that of the steel plate treated with the " blue " process , indicating that the surface oxide scale is mainly composed of dense Fe 3 O 4 .

3 Analysis and Discussion
, it can be seen that the Si content in the steel has a very significant impact on the red rust on the steel strip surface , but the influence of different heating furnaces , descaling start temperature , heating time and other factors is not significant . In order to analyze the relationship between element Si and red rust , the

Again , grind the cross section carefully so as not to break the scale . After sample preparation, the structure and element distribution at the interface between oxide scale and matrix were observed by scanning electron microscope .
The microstructure of the bonded part of the oxide scale and the matrix and the element distribution results along the thickness direction are shown in Fig. 3 . It can be seen that there is obvious Si enrichment in the iron oxide skin layer near the bonding interface .

Studies have found that Si mainly affects the descaling effect and the surface quality of the steel strip by changing the interface structure between the oxide scale and the steel matrix during the high temperature oxidation process [4 , 8] . The source of residual red rust on the surface of steel with high Si content is that during the long-term high-temperature heating of this type of steel , Si in the steel is selectively oxidized , and 2FeO - SiO 2 is formed at the interface between FeO ( worstite ) and the steel matrix. ( Fayal olivine ) , because fayal olivine has a low melting point (1170 °C) , it will intrude into the scale and steel matrix in a wedge shape after forming a molten state . In this way, the scale layer and the steel matrix interface form an intricate and special structure of the scale layer . Fe z SiO^FeO eutectoid products are formed between FeO and steel matrix , and there are large voids between FeO and eutectoid products . When the temperature is lower than 1170 °C , the ferrous silicate solidifies and combines closely with the matrix , making it difficult to be completely removed during descaling .
From the experimental results , Si is only enriched in the ferrous oxide in the iron oxide scale . It may be caused by a dirty descaling or a low descaling temperature , which causes Fe z SiO q to press into the matrix and pin part of the ferrous oxide . This part of FeO was further oxidized in the subsequent cooling process and turned into red iron oxide scale at room temperature . According to data , it is extremely difficult to completely prevent pitting when steel with a Si content greater than 0.2% is hot-rolled . Therefore , in order to completely remove the red rust , it is necessary to ensure that the residual FeO layer is removed during the descaling process , and it is necessary to completely remove the Fe z SiOg . the reason .
It can also be seen from the above analysis that measures such as reducing the Si content, increasing the descaling pressure of high-pressure water before finishing rolling , reducing the inlet temperature of finishing rolling , appropriately increasing the finishing rolling temperature , and reducing the contact time of the steel strip with air before coiling can all be effective. Reduce the area ratio of red rust on the steel plate surface . At present, this measure has been promoted and used in Jingtang Company . From the perspective of mass production , the surface quality of coils is excellent , the content of Fe 3 O 4 is above 70% , and the mechanical properties do not change due to the reduction of Si content .
4 Conclusion

1. The red rust on the surface of the hot-rolled coil has an obvious relationship with the Si content in the steel . When other conditions remain unchanged, a high Si content will lead to a high amount of red rust .
2. the red rust on the surface of the coil and the heating furnace , descaling start temperature and other factors .
3. Si- enriched layer between the red rust and the matrix , which greatly increases the difficulty of removing the high-temperature oxide scale .
4. the Si content in the steel is a very effective measure to improve the surface quality of the steel plate . The steel surface with a Si content below 0.15% basically has no red rust residue and has little effect on its mechanical properties .

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