Beihai Chengde 1700 hot rolling line equipment is mainly composed of: 2 walking beam heating furnaces + post-furnace descaling + 1 stand R1 rough rolling mill + hot coil box + 8 stands F1 ~ F8 finishing mill + 2 coilers Composition, since the 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 production has increased year by year, and the proportion of 304 has increased from 13.60% in 2013 to 55.38% in 2016, an increase of 41.78 %. However, while the proportion of 304 stainless steel is increasing rapidly, the proportion of edge rust defects is also gradually increasing, which seriously affects the processing cost and product quality of the subsequent process. For this reason, since the first half of 2014, a lot of tests and tests have been carried out on the edge rust problem of 304. It is found that the roll surface quality, heating furnace temperature, strip temperature, roll temperature, and work roll cooling water pressure The formation of edge rust defects has a relatively large impact.
- Analysis of the formation mechanism of 304 edge pressure rust
1.1 Chemical composition of materials
The chemical composition of Beihai Chengde 304 austenitic stainless steel, from the perspective of chemical composition range, the enterprise standard composition is completely within the scope of national standard and American standard, that is, the 304 steel produced is qualified.
1.2 Photos, morphology, characteristics and main formation mechanism of edge rust defects
After solution annealing and pickling, the defect position within 40mm of the edge of the strip steel in the white skin state is obviously rough, and the stainless steel matrix is embedded with oxide skin, which is obviously different from the normal surface next to it. Similar rough strips were found at the contact position of the strip edge on the roll surface of the work roll in the front section of the hot-rolled finishing rolling F1-F4. The defects on the roll surface are completely consistent with the edge defects of the strip. At the same time, the hand feels obvious and partially rough. There are also burrs. Therefore, edge rust is also called edge roll trace or oxide scale defect in many other stainless steel plants. The defect is characterized by: within 40mm from the edge of the strip on both sides, the whole length occurs continuously, and there is a clear hand feel, and the rough strip is linear It is distributed in the shape of a 5-10° angle with the rolling direction.
The oxide film on the surface of the finish rolling F1-F4 work rolls is subjected to huge alternating stress periodically. After reaching the fatigue limit, the microcracks in the oxide film are under the action of strong stress between the work roll and the strip, and the oxide film on the roll surface is reduced. Peeling, on the one hand, the peeled oxide film on the roll surface adheres to the surface of the strip, and is rolled into the surface of the strip in the subsequent frame to form edge rust defects; on the other hand, after the oxide film on the roll surface of the work roll is peeled off, the roll surface It becomes relatively rough. In the deformation zone of the strip, the front and rear sliding action makes the work roll and the strip move relative to each other. At this time, the rough and burr part of the roll surface has a similar effect on the strip, forming a groove on the strip. , so that the strip steel substrate is exposed to high temperature water vapor and oxidized to form tertiary oxide scales, which are rolled into the strip steel during the continuous deformation of the rear frame to form edge rust defects.
- Process influencing factors that produce edge rust
2.1 The influence of the formation of oxide film on the surface of the roll
The formation mechanism of edge rust defects mentioned above mentioned that defects similar to edge rust were found in the edge positions of finishing work rolls F1-F4 rolls, that is, the edge rust defects on the strip were mainly the roll edge defects embedded in the strip. The steel causes rough edges of the strip. During the solid solution pickling process, the rough edges of the strip are not washed cleanly, and there are oxide skin defects. The rougher the surface of the work roll, the more serious the damage and peeling of the oxide film. The more serious the edge pressure rust defect on the strip surface is. Therefore, in order to solve the edge rust defect, it is the key to solve the edge roughness of the finishing rolling F1-F4 roll.
During the 304 rolling process, the temperature drop of the strip is relatively large, especially the temperature drop of the edge of the strip is more obvious, and the temperature difference between the edge and the center of the strip is 80-120 °C; at the same time, the material composition of 304 is similar to the high nickel in the front section of F1-F4. The surface composition of chromium and high-chromium ferrous rolls both contain Ni and Cr, which is one of the reasons for the adhesion of strip steel and 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 influence on the edge rusting. That is to increase the heating temperature and ensure the finish rolling inlet temperature is one of the effective methods.
(1) In the data analysis of the 30-roll edge pressure rust generated in the 1961 volume 304 stainless steel coil in February 2017, the average temperature at the finish rolling entrance of the 1961 volume is 1031.7 °C; The average temperature at the entrance of the 30 rolls with edge pressure rust is 1014 ℃, which is significantly lower than the average temperature of the overall average and no edge pressure rust.
(2) It is the edge rust data from August 5, 2015 to August 31, 2015. The ratio of edge rust on August 22 is higher than that on August 24 without edge rust. The inlet temperature of finishing rolling The high edge pressure rust defects are significantly improved.
Excessive unit rolling force will put the surface of the finishing roll in a harsh working environment, and will cause the edge of the roll to form an obvious burr strip at 5 to 10° with the rolling direction, and at the same time, it will be pressed into the surface of the strip, resulting in inoperability. Eliminates rough edges on the edges. For the analysis of statistics in August 2015, the rolling schedule with edge pressure rust defects is obviously larger than the F1-F4 rolling force of the rolling schedule without edge pressure rust defects. During the rolling tracking process, the frame load of F2 and F3 was appropriately reduced, and the edge rust was improved; when the overall rolling force of F1-F4 was relatively large, the edge rust defect increased significantly.
In actual production, the cooling water temperature of the work rolls on site fluctuates widely, ranging from 29°C to 42°C. At the same time, the state of the oxide film on the F1-F4 roll surface is also different after different water temperatures are turned off.
- Control methods and process improvement measures
(a) Improve the oxide film on the surface of the roll
To ensure that the oxide film does not peel off, the key is to establish and protect the oxide film on the roller surface. After the work roll is on 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 should be 4.4-5.0min/piece to maintain a stable rhythm; after the first 5 pieces of steel are rolled, it can be Roll at a rolling rhythm of 3 to 4.2 min/block.
Regularly check the leakage of the rack and water cutting board; check the blockage of the cooling water nozzle of the rack, etc.
Appropriately increase the heating temperature of 304 steel grades to ensure the entry temperature of finishing rolling and reduce the rolling force of finishing rolling. When rolling 304 steel, the heating and burning of steel should be controlled as far as possible according to the upper limit of the process temperature. When the tapping temperature is low, appropriate heat preservation or reduction of the rolling rhythm should be performed to ensure the entry temperature of finishing rolling.
(b) Reduce the rolling force of finishing rolling F1-F4 and distribute the rack load reasonably
Reduce the overall rolling force of F1-F4, and distribute the rolling force of F1-F8 reasonably. If it is found that the oxide film on the edge of the roll surface of a certain stand of F1-F4 has serious peeling and burrs, the rolling force of the stand can be appropriately reduced; if it is found that the roll surface of F1-F4 is relatively poor after the machine Check whether the overall rolling force of F1-F4 is large due to the low temperature of the strip.
(c) Formulate different work roll cooling water process systems
In actual production, adjusting the cooling water pressure and flow rate of finishing work rolls according to different finishing work roll cooling water temperatures can also improve the edge rust defects to some extent. For this reason, different finishing work roll cooling water process systems are specially designed for single-furnace production and double-furnace production: the cooling water pressure of finishing work rolls is controlled at 3.0-4.2Bar during single-furnace production, and the water pressure during double-furnace production is controlled. Control at 5.6 ~ 6.6Bar.
(d) Improve the quality of rolls and promote the use of high-speed steel rolls
During the use of F1-F4 rolls, try not to use high-nickel-chromium rolls, but use high-chromium iron rolls. It is best to use high-speed steel rolls for all F2-F4 rolls when rolling 304 steel.
(e) Add the finishing rolling oil system
Because Beihai Chengde 1700 hot rolling does not have a rolling lubricating oil system, the roll surface quality of the rolls after the machine is not very ideal. If the finishing rolling lubrication system can be added, the roll surface quality should be greatly improved.
(1) Increasing the temperature of the heating furnace and ensuring the inlet temperature of the finishing rolling can effectively reduce the proportion of rust-pressed defects on the edge of 304 stainless steel.
(2) Reducing the overall rolling force of finishing rolling F1-F4 and reasonable load distribution can alleviate the edge rusting.
(3) Take effective measures to improve the oxide film on the surface of the finishing rolling F1-F4 roll, reduce the peeling phenomenon of the oxide film, and basically control the edge rust defect.
(4) Formulate a reasonable 304 stainless steel process system, and the on-site production process is strictly implemented in accordance with the process system.