Optimization of annealing process system for cold-rolled SPCC steel

Abstract : Introduced the characteristics of the all-hydrogen bell annealing furnace imported from Austria , combined with the actual production situation of Tangshan Iron and Steel Cold Rolling Plant , analyzed the influence of the annealing furnace
Various factors of the annealing process, the annealing process system of the main product SPCC steel was optimized, the annealing cycle was shortened by 4~6h, and the monthly output of the unit was increased by 12 . 1% , the pass rate of the product is 95 ． 5% increased to over 98% .
Key words : bell furnace ; annealing ; process optimization

from Austrian EBNER company in the cold rolling factory adopts the batch production method, uses coke oven gas or mixed gas (blast furnace and coke oven) as fuel, and indirectly heats the strip coil by heating the inner cover. Using 100% hydrogen in the hood as a protective gas, the strip coil is recrystallized and bright annealed to improve the mechanical properties and surface finish of the cold rolled coil. The main product is SPCC steel. In the early stage of production, there were defects such as too long annealing period and high product bonding rate. With the continuous increase of customer orders in the second half of 2010 , the use of the original annealing process hindered the increase of unit output and affected production efficiency. According to the actual production situation of the full hydrogen bell annealing furnace, the technicians of Tangshan Iron and Steel Cold Rolling Sheet Factory continuously adjusted the annealing system according to the relevant technical parameters of chilled coils, and carried out a series of optimization tests on the production process. 2 Features of strong convection full hydrogen bell annealing furnace
Strong convection full hydrogen bell annealing furnace is currently the most advanced in the world
the batch annealing furnace is that it uses the characteristics of low density , strong reducibility and high thermal conductivity of hydrogen , and cooperates with the high-speed rotating furnace platform circulation fan to make the gas in the furnace have a high flow rate and heat transfer rate . uniform . Therefore , after annealing , the cold-rolled steel coil has a clean and bright surface with excellent and uniform mechanical properties .

The bell annealing furnace heats and cools the steel coils in the form of stacked steel coils to achieve the purpose of recrystallization bright annealing.

The control of the temperature of the hood and the furnace platform thermocouple is used to realize the control of the process, as shown in Figure 1 . The annealing feature of the bell furnace is that the temperature of each point of the steel strip in the furnace is not uniform, and the heating and cooling time is long. By controlling the reaction time of the cold spot and hot spot temperature difference, the mechanical properties of the entire steel coil can be guaranteed to meet the requirements, and the purpose of bright annealing can be achieved by purging hydrogen in an appropriate way. Therefore, under the premise of ensuring the performance of the steel coil, the annealing time and the amount of hydrogen purging should be shortened as much as possible to increase the output and reduce the consumption. 3 Principles of annealing process formulation
The bell annealing process adopts recrystallization annealing , and the annealing system is divided into three stages: heating , heat preservation ( soaking ) , and cooling . The annealing process system of cold-rolled strip steel is mainly determined according to factors such as the chemical composition of the steel , the technical conditions of the product , the size of the strip steel, and the weight of the coil . The annealing process system must ensure that there is no adhesion between the layers of the steel coil during production , no oxidation on the surface , and the deep drawing performance of the automobile sheet must be obtained .
3 . 1 The principle of formulating the heating and heat preservation system The principle of formulating the strip steel heating and heat preservation system is the determination principle of the strip heating speed , heat preservation temperature and heat preservation time .
The heating rate of the strip is mainly determined by its thermal conductivity . From room temperature to 400 C , the heating rate is generally unlimited . According to the principle of recrystallization process , when the steel strip is heated from room temperature to 400 C , there is no significant change in the internal structure of the strip steel , and the elongated grains have just recovered during the rolling process , and recrystallization has not yet formed. The performance and surface impact is insignificant .
is heated from 400 C to below the holding temperature of 723 C is the stage of recrystallization formation , which has a considerable impact on the performance and surface quality of the strip steel . The heating rate in this temperature range must be controlled . It is generally stipulated that the heating rate should be 30 ~ 50 C / h . The control of the heating rate is different for different steel types and thicknesses .
The holding temperature and holding time are mainly determined according to product standards, technical conditions, steel types and thickness of strip steel. The holding time and holding temperature are also related to the weight of the coil. If the coil is heavy and the steel plate is thick, the holding temperature will be high and the holding time will be long. For steel and thin-gauge steel strips that are prone to interlayer bonding defects, the holding temperature can be appropriately lower and the holding time can be shorter. For steel with high strength or insufficient plasticity and thick-gauge strip steel, the heat preservation temperature should be higher and the heat preservation time should be longer. 3 . 2 Principles of cooling system formulation
In the cooling process of the bell furnace , the cooling rate is mainly considered to avoid sticking , the surface of the steel coil is not oxidized when it comes out of the furnace, and the matching utilization of the heating mantle and the cooling mantle .
3 . 3 Principles for formulating the hydrogen purging system
In the all-hydrogen annealing process , the role of hydrogen has two aspects : one is as a heating and cooling medium for strip steel ; the other is to purge the residual emulsion on the surface of strip steel . The strip surface quality after annealing is closely related to the hydrogen purging process . In a typical bell furnace annealing process , the curve of shielding gas composition changing with time and temperature is shown in Fig. 2 .

 

 It can be seen that CH 4 in the atmosphere in the furnace reaches its maximum twice ( that is, the peak of CH 4 in the figure ), and the first peak appears when the temperature reaches about 700 °C during the heating process . At this time, the source of C in CH 4 In the rolling oil , it should be blown off ; another peak appears in the heat preservation section at the end of heating . The C in CH 4 mainly comes from the steel . In order not to de- C the steel , the CH 4 produced at this time should not be blown off . go . After the second peak of CH 4 , the hydrogen content is close to 100% . It can also be seen from Fig. 2 that during the heating stage, that is, when the temperature is 300-500 C , the smoke content of the emulsion in the protective gas increases rapidly . When setting the hydrogen purge mode , a large flow of hydrogen purge should be used .
4 Optimization of annealing process 4 . 1 Optimization of annealing temperature regime
The original annealing process system (take SPCC steel grade, the specification is 1.0

 The annealing cycle is about 43 h , and the qualified rate is 95% due to bonding . 5% . The main reasons for the analysis are: the temperature difference between cold and hot spots in the process system is small, resulting in long heating and holding time; at the same time, the strip steel stays in the high temperature state for a long time, which is easy to cause bonding. After tracking product performance, its elongation rate is 49% ~ 51% ( standard 36% ) on average, which is far higher than the standard requirement.

The original SPCC steel type chilled coils all implement the above-mentioned process system . With the increase of raw materials with specifications of 1.2 ~ 2.0 mm and <0.7 mm , this process system obviously cannot meet the needs of production. Refine and optimize .
According to the principle of recrystallization annealing and the characteristics of the bell annealing furnace , by adjusting the process parameters such as heating speed , holding temperature and holding time to test and track , and comparing their mechanical properties , the purpose of shortening the annealing cycle and reducing the sticking tendency was finally achieved . The specific measures are as follows .

1. For different product specifications , the cold spot temperature is lowered by 10 ~ 20 T respectively , and the heating and holding time are correspondingly shortened .
2. Properly increase the temperature of the hot spot according to the different specifications of the steel coil . After the temperature difference between the cold and the hot spot is increased by 4t , the corresponding holding time is reduced .
3. As the temperature of the cold spot decreases , the cooling time with the hood is reduced accordingly to ensure that the atmosphere temperature in the furnace drops below 580 T when the hood is removed , and the tendency of sticking is reduced by reducing the cooling rate at high temperature .
4. the option of " temperature rise hysteresis " in the conditions automatically calculated by the annealing program . That is to say, taking into account the slight increase in the temperature of the core cold spot after the end of heating , the annealing cycle can be shortened by 0.3 h on average through statistics .

The optimized process system is shown in Figure 4, the annealing period is shortened by 4 to 5.7 h , the amount of bonding is reduced and the degree of bonding is reduced , and the product qualification rate increases by 2.5% . The elongation of the 1.0 mm steel strip after annealing is 41% - 44% , and the mechanical properties are uniform and fully meet the standard requirements .

4.2 Test and improvement of hydrogen purging system
When the strip is annealed , if the hydrogen purge time and purge amount can match the volatilization speed of the rolling oil , the surface cleanliness of the strip will be the best. Different hydrogen purge flow rate and corresponding time should be selected for different sections , and the strip thickness ( thin material has a lot of emulsion residue ), the temperature change of the annealing process , and the temperature change when the heating is abnormal, etc. should also be considered . This not only saves hydrogen consumption , but also ensures the surface quality of the annealed steel coil .
The tandem acid rolling unit uses Quaker rolling oil , and its TGA ( rolling
Oil making ) volatilization curve is shown in Figure 5, and the highest volatilization temperature is 349.4 T.

The volatilization rate of the oil is still relatively high ; the small amount of hydrogen blowing ends when the cold spot temperature of the steel coil is about 570 °C . Referring to Figure 5 , it can be seen that after the temperature exceeds 480 °C , the residual rolling oil on the surface of the plate is 0.05 % , That is to say, the hydrogen blowing time of the small flow rate of the original system is too long , resulting in unnecessary consumption .
After gradually adjusting the hydrogen blowing system and comparing the cleanliness of the plate surface , it is finally set to prolong the blowing time of the large flow rate in the hydrogen blowing system to ensure the amount of hydrogen blowing in the emulsion volatilization stage within the temperature range of 200-500 C. At the same time, under the condition of ensuring the cleanliness of the board surface , the purging time of low-flow hydrogen gas is shortened to reduce hydrogen consumption . Taking the 1.2 mm specification as an example , the hydrogen purging process after optimization ( before ) is shown in Table 1. Compared with the original purging process , the hydrogen consumption per furnace is reduced on average
20 ~30 m 3 , the purge time is shortened, and the purge flow rate and time are better matched with the change of furnace temperature .

30 m 3 /h in the original hydrogen blowing system is reduced to 20 m 3 /h when the cold spot temperature of the steel coil is 400 °C , and at this time the rolling
Table 1 Hydrogen purging system after optimization (before)
Tab. 1 Hydrogen air blowing system before ( after ) optimization

Hydrogen purge section	Purge flow / ( m 3 /h )		Purge time /h
	forward	back	forward	back
1	20	40	0.5	0.5
2	10	30	1.0	6.5
3	30	twenty four	5.5	1.5
4	20	20	4.5	0.5

By adjusting the temperature control program of the bell furnace , reducing the temperature of cold spots , increasing the temperature difference between cold and hot spots , and reducing the cooling time with heating hoods, the original annealing process was optimized , and the qualified rate of products increased from 95.5% to over 98% . The process annealing cycle is shortened by 4 to 6 hours , and the monthly output of the unit is increased by 12.1% ; at the same time , the hydrogen purging system is reasonably formulated to save hydrogen consumption on the premise of ensuring the surface quality of the steel coil ; the optimization of the hood annealing process system meets the production needs , creating considerable economic benefits .

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