Study on solution and process optimization of 304 stainless steel edge rust pressing defects

　　During the rolling process of 300 series stainless steel 304 steel in the 1700mm hot rolling production line of the hot rolling branch of Beihai Chengde metal rolling Co., Ltd. (hereinafter referred to as Beihai Chengde), the bonding between the edge of the strip and the roll surface often occurs, and a rough belt is formed at the edge of the strip 0 ~ 40mm, which causes the subsequent solid solution pickling process that the oxide skin on the edge of the strip is not cleaned, which is difficult to wash, and finally forms the edge rust pressing defect. Through the analysis and tracking of the causes of 304 stainless steel edge pressing rust defects on site, it is found that the edge pressing rust defects are significantly improved when the heating temperature and finishing rolling inlet temperature are properly increased.

　　Key words stainless steel rough edge rust pressing heating temperature finishing rolling inlet temperature
 

　　1 Preface

　　The equipment of Beihai Chengde 1700 hot rolling line is mainly composed of two walking beam reheating furnaces + descaling behind the furnace +1-stand R1 roughing mill + hot coil box +8-stand f1-f8 finishing mill +2 coilers. Since Beihai Chengde 1700 hot rolling line was fully completed and put into operation on November 11, 2012, the proportion of 304 stainless steel in the total stainless steel output has increased year by year, with an increase of 41.78% from 13.60% in 2013 to 55.38% in 2016. However, while the proportion of 304 stainless steel is increasing rapidly, the proportion of edge rust pressing defects is also increasing, which seriously affects the processing cost and product quality of the next process. Therefore, since the first half of 2014, a large number of tests and tests have been carried out on the 304 edge pressing rust problem on site. It is found that the quality of roll surface, heating furnace temperature, strip steel temperature, roll temperature and cooling water pressure of work roll have a great impact on the formation of edge pressing rust defects.
 

　　2 Analysis on forming mechanism of 304 edge pressed rust

　　2.1 chemical composition of materials

　　According to the chemical composition of Beihai Chengde 304 austenitic stainless steel, the enterprise standard composition is completely within the scope of national and American standards, that is, the 304 steel produced is qualified.

　　2.2 photos, morphology, characteristics and main formation mechanism of edge rust pressing defects

　　After solution annealing and acid pickling, as shown in Figure 1 below, the defect position within 40mm of the edge of the white skinned strip steel is obviously rough, and the oxide skin is embedded in the stainless steel substrate, which is obviously different from the normal surface nearby. Similar rough bands are found at the contact position of the steel edge of the work roll surface strip at the front section of F1-F4 hot rolling finish rolling, as shown in Fig. 2 and Fig. 3. The roll surface defect diagram is completely consistent with the strip edge defect diagram. At the same time, the hand feel is obvious, and some rough places are also provided with burrs. Therefore, edge pressing rust is also called edge roll trace or oxide skin defect in many other stainless steel plants. This defect is characterized by continuous occurrence of the whole length within 40mm from the edges of both sides of the strip steel, and obvious hand feeling. The rough belt is distributed in a linear shape and forms an included angle of 5 ~ 10 ° with the rolling direction.

　　The oxidation film on the work roll surface of finish rolling F1-F4 bears huge alternating stress periodically. After reaching the fatigue limit, the micro cracks in the oxidation film peel off under the strong stress between the work roll and the strip steel. On the one hand, the peeled oxidation film on the roll surface adheres to the strip steel surface and is rolled into the strip steel surface in the subsequent frame to form edge rust pressing defects; On the other hand, after the oxide film on the surface of the work roll peels off, the roll surface becomes relatively rough. In the deformation area of the strip steel, the back and forth sliding makes the work roll move relative to the strip steel. At this time, the part with burrs on the rough roll surface has a similar effect on the strip steel, forming grooves on the strip steel, exposing the strip steel substrate to high-temperature water vapor for oxidation and generating three times of oxide skin, The formed tertiary oxide scale is rolled into the strip steel during the continuous deformation of the rear frame to form edge rust pressing defects.
 

　　3 process influencing factors of edge pressure rust

　　3.1 influence of formation of oxide film on roll surface

　　The formation mechanism of edge rust pressing defects mentioned above refers to the defects similar to edge rust pressing found at the edge of finishing work roll F1-F4, that is, the edge rust pressing defects on the strip steel are mainly caused by the rough edges of the strip steel caused by the embedded edge defects of the roll. During the solid solution pickling process, the rough edges of the strip steel are not washed clean, and the oxide skin defects are the dominant factor. The rougher the work roll surface is, the more serious the oxide film is damaged and peeled off, The more serious the edge pressing rust defect is on the surface of the strip steel. Therefore, the key to solve the problem of edge roughness of finish rolling F1-F4 roll is to solve the edge rust defect.

　　During 304 rolling, the temperature drop of the strip steel is relatively large, especially the temperature drop at the edge of the strip steel is more obvious. As shown in Figure 4, the difference between the edge temperature of the strip steel and the center temperature is 80 ~ 120 ℃; At the same time, both the 304 material composition and the surface composition of the high nickel chromium and high chromium iron rolls in the front section of F1-F4 contain Ni and Cr, which is one of the reasons for the bonding between the strip steel and the roll surface.

　　From the analysis of a large number of statistical data, it can be seen that the heating temperature and the finish rolling inlet temperature also have a great impact on the edge rust pressing, that is, increasing the heating temperature and ensuring the finish rolling inlet temperature is one of the effective methods.

　　(1) According to the analysis of 30 crimping rust data generated in 1961 coil 304 stainless steel coil in February 2017, the average temperature at the finish rolling inlet of 1961 coil is 1031.7 ℃; The average inlet temperature of 1931 coil without edge rust is 1031.9 ℃; The average temperature at the entrance of 30 coils with edge rust is 1014 ℃, which is significantly lower than the overall average and the average temperature without edge rust.

　　(2) According to the edge rust pressing data from August 5 to August 31, 2015, the proportion of edge rust pressing coils was higher on August 22. Compared with no edge rust pressing on August 24, the edge rust pressing defects with high finish rolling inlet temperature were significantly improved.

　　Excessive unit rolling force will make the surface of the finishing roll in a harsh working environment, and will form an obvious burr band at the edge of the roll that is 5 ~ 10 ° from the rolling direction. At the same time, it will be pressed into the surface of the strip steel, resulting in irreparable rough marks at the edge. In order to analyze the statistics in August 2015, the rolling schedule with edge rust defects has significantly greater rolling force than the F1-F4 rolling schedule without edge rust defects. During the rolling tracking process, the load of F2 and F3 stands was properly reduced, and the edge rust pressing was improved; When the overall rolling force of F1-F4 is relatively large, the edge rust pressing defects increase obviously.

　　In production practice, the cooling water temperature of field work roll fluctuates widely from 29 ℃ to 42 ℃. At the same time, the oxidation film state of F1-F4 roll surface is also different under different water temperatures
 

　　4 control methods and process improvement measures

　　(a) Improving oxide film on roll surface

　　To ensure that the oxide film does not peel off, the key is to establish and protect the roll surface oxide film. After the work roll is put into the machine, it must be ironed according to the process regulations: when the first 5 pieces of steel are rolled after the planned roll change, the rolling rhythm shall be 4.4 ~ 5.0min/piece, maintaining a stable rhythm; After rolling the first 5 blocks, the rolling can be carried out at the rolling rhythm of 3 ~ 4.2min/ block.

　　Regularly check the water leakage of the rack and water cutting plate; Check whether the cooling water nozzle of the rack is blocked.

　　Properly increase the heating temperature of 304 steel, ensure the inlet temperature of finish rolling, and reduce the rolling force of finish rolling. During the rolling of 304 steel, the heating and burning of steel shall be controlled according to the upper limit of process temperature as far as possible. When the tapping temperature is low, it shall be properly insulated or the rolling rhythm shall be reduced to ensure the finish rolling inlet temperature.

　　(c) Reduce the rolling force of finish rolling F1-F4 and reasonably distribute the load of the stand

　　Reduce the overall rolling force of F1-F4 and reasonably distribute the rolling force of f1-f8. If it is found that the oxide film on the edge of the roll surface of one of F1-F4 stands peels off and burrs are serious after being off the machine, the rolling force of the stand can be appropriately reduced; If it is found that the F1-F4 roll surface is poor after being off the machine, it is necessary to check whether the overall rolling force of F1-F4 is large due to the low strip temperature.

　　(d) Develop different working roll cooling water process systems

　　In the actual production, adjusting the water pressure and flow of the cooling water of the finishing work roll according to the different cooling water temperature of the finishing work roll can also improve the edge rust pressing defects. For this reason, different cooling water process systems for finishing work roll in single furnace production and double furnace production are specially designed: the cooling water pressure of finishing work roll is controlled at 3.0 ~ 4.2bar in single furnace production and 5.6 ~ 6.6bar in double furnace production.

　　(e) Improving roll material and popularizing the use of high speed steel rolls

　　During the use of F1-F4 roll, try not to use high nickel chromium roll, but high chromium iron roll. It is better to use high-speed steel roll for f2-f4 roll when rolling 304 steel.

　　(f) Add finishing rolling lubricating oil system

　　Since Beihai Chengde 1700 hot rolling mill does not have a rolling lubricating oil system, the roll surface quality is not very ideal after being off the mill. If the finishing rolling lubricating system can be added, the roll surface quality should be greatly improved.
 

　　5 Conclusion

　　(1) Increasing the temperature of heating furnace and ensuring the inlet temperature of finish rolling can effectively reduce the proportion of edge rust pressing defects of 304 stainless steel.

　　(2) Reducing the overall rolling force of finish rolling F1-F4 and reasonable load distribution can alleviate the edge rust.

　　(3) Effective measures are taken to improve the oxide film on the surface of finish rolling F1-F4 rolls, reduce the phenomenon of oxide film peeling, and basically control the edge rust pressing defects.

　　(4) Formulate a reasonable 304 stainless steel process system, and strictly follow the process system during on-site production.