Cold Rolled Q195F Steel Coils

The characteristics of the imported all-hydrogen bell-type annealing furnace were introduced, combined with the actual production conditions of cold rolling mills, various factors affecting the annealing process of Q195F steel were analyzed, and the annealing process system of the main product Q195F steel was optimized.

The cold-rolling plant adopts the all-hydrogen strong convection bell annealing furnace equipment imported from Germany LOI company to perform recrystallization bright annealing on cold-rolled strip steel. In the early stage of production, the main product was Q195F cold-rolled coils. Due to insufficient production experience, the amount of unqualified products was relatively large. The main phenomenon is that the strip steel is relatively soft, the tensile strength and yield strength do not meet the requirements, and the phenomenon of sticking steel occurs. Therefore, the optimization of the annealing process system for Q195F products was carried out.
1 Features of full hydrogen strong convection bell annealing furnace
The all-hydrogen strong convection bell annealing furnace is the most advanced batch annealing furnace, and its main advantage is the use of hydrogen's strong reducibility and
The characteristics of low density and high thermal conductivity, etc., combined with the high-speed rotating furnace platform circulation fan, make the atmosphere in the furnace have a high flow rate and heat transfer rate, and the furnace temperature is uniform. After the cold-rolled steel coil is annealed, the mechanical properties are excellent and uniform, the surface of the strip steel is clean and bright, and the productivity is high.
The bell annealing furnace heats and cools steel coils in the form of stacked steel coils to achieve the purpose of recrystallization bright annealing. Due to the slow heat transfer, the temperature control in the process is realized by controlling the thermocouple ( CT) and the pressure thermocouple ( BT ) of the furnace. The bell furnace is characterized by uneven temperature at each point of the steel coil in the furnace, and the heating and cooling time is long. By controlling the reaction time of the temperature difference between the cold spot and the hot spot, the entire annealed steel coil can meet the mechanical performance requirements, and through appropriate methods hydrogen purging to achieve the purpose of bright annealing. Therefore, under the premise of ensuring the mechanical properties, uniformity of properties and surface quality of the steel coil, the annealing time and the amount of hydrogen purging should be reduced as much as possible to increase the output and reduce the consumption.

2 Optimization of annealing process system
2.1 Optimization of heating and heat preservation system

1. Principles for formulating heating and heat preservation systems

The purpose of recrystallization annealing of cold-rolled strip steel is to eliminate the work hardening produced in the cold-rolling process of strip steel and restore its ability of plastic deformation. Three different processes of recovery, recrystallization and grain growth occur during the heating and holding process of cold-rolled strip steel. These processes are not carried out at a certain temperature, but within a temperature range. This temperature range varies depending on the material and the amount of deformation. Recrystallization annealing of cold-rolled strip steel is to heat the cold-rolled strip steel above the recrystallization temperature, and achieve the purpose of controlling its properties through the control of recrystallization and grain growth.
The steel coils at different positions in the bell furnace and the different parts of the same steel coil have different temperatures. During the heating and cooling process of each furnace steel coil, there is a highest temperature point (hot spot) and a lowest point (cold point). Theoretical and practical measurements also prove this point, as shown in Figure 1 .
According to experiments, the hot spot of each coil is at the edge of the steel coil, and the cold spot is usually on the inside of the core of the steel coil. The stacking methods are different, and the cold spots of each coil in the furnace are also arranged differently ( shown by 600 in FIG. 2 ). Stacking weight, coil width and strip thickness all have a certain influence on the process system. Therefore, only by comprehensively considering the above - mentioned factors and controlling the temperature difference 4T between the cold spot and the hot spot within a certain temperature range, can the control of the annealing process be achieved, thereby controlling the performance of the final product.

1. Optimization process and analysis of heating and heat preservation system

According to the principle of recrystallization annealing and the characteristics of bell annealing furnace, the main means to control the performance is to control its heating rate, heat preservation
Temperature and holding time, so that the temperature of each point of the steel strip in the furnace is uniform, so as to meet the requirements of uniform performance. However, when German U0I company assessed the capability of the bell furnace, it only considered that the process system was reasonable based on the fact that the temperature difference between the cold spot and the hot spot was 4T and reached 60 %. It is also found from the actual production that the heating time of steel coils with a stacking weight of more than 80 t is prolonged to varying degrees when the heating system provided by LOI is adopted. Therefore, for the original process system, optimize the test according to the steps in Table 1 .

»1 Q195F Product Heating Depression Optimization Scheme

plan	heating rate	Holding temperature/Y	Holding time /h
1	unchanged	690	reduce 2
2	Heating at full speed	680/690	ditto
3	Heating at full speed	680	Kill less 3~4
4	Heating at full speed	660/670	ditto

to Table 1 , 10 furnaces with different specifications and stacking weights were randomly selected for schemes 1 to 4 , and the process parameters such as heating speed, holding time and holding temperature were modified for testing and tracking, and mechanical performance data were obtained (see Table 2) 0
Table 2 Performance results of different test schemes

Experimental protocol	Stacking weight flt/t	< r,/MPa	<71*/MPa	6/%
1	106	215	330	53
1	105	240	325	58
2	103	245	345	53
2	109	250	345	57
3	112	270	360	53
3	112	250	355	49
3	102	250	360	50
3	98	270	360	55
4	98	295	375	47
4	97	300	380	45

By analyzing and comparing the performance in Table 2 , the following conclusions can be drawn:
Using full-speed heating and 680 NIE heat preservation, after the heat preservation time is reduced by 3 to 4 hours , the performance softening phenomenon caused by annealing is basically eliminated, and the tensile strength is increased to about 360 N/nim 2 . After adopting the process system of Scheme 4 , the tensile strength of the steel strip has been improved to varying degrees. Although it still meets the national standard, considering the performance of the whole roll, it is not suitable to use a temperature below 670 P for heat preservation.
Using the first and third process systems in table 1 , the steel coils were sampled to a fixed length, and the performance of the whole furnace was analyzed, and the tensile strength and yield strength were compared. It can be seen that the process system of heating at full speed, heat preservation at 680 °C, and reducing the heat preservation time by 3~4 hours can fully meet the performance requirements of the product. The temperature distribution of the whole furnace was measured by a test thermocouple, and the temperature difference AT between the hot and cold spots was measured to be 60mm . It was also found that after the heating mantle finished working and was lifted away, the temperature of the cold spot continued to rise by 10°C in the first 1-2h of cooling , indicating that according to Temperature difference requirements can end the heat preservation in advance. This is also an important factor that can reduce the holding time.
Through the actual statistics, it can also be concluded that compared with the controlled heating system , the heating time of the furnace loading below 80 t is reduced by 2~3 hours . Although the heating time of the furnace above 80 t exceeds 8 h, even reaches 10 h, it is actually the same as the original process. The implementation of the system has not changed significantly. Therefore, adopting the method of full-speed heating can effectively improve the utilization rate of the hearth, and the various stacking weights
The holding time is reduced by an average of 3 h o
2.2 Optimization of cooling system
(1) Principles of cooling system formulation
During the cooling process of the bell furnace, due to the limitation of equipment capacity, the mechanical properties of the steel coil will not be affected even if the cooling is performed at the maximum cooling rate. The formulation of the cooling rate is mainly to consider the problems of avoiding steel sticking, non-oxidation of the surface of the steel coil when it comes out of the furnace, and the matching utilization of the heating mantle and the cooling mantle.
(2) Improved cooling process to reduce bonding defects
Under the premise of reasonable heating and heat preservation system, the effective way to avoid steel sticking in bell furnace is to slow down the cooling rate. The most effective method is to cool slowly with a heating mantle after heating and heat preservation. At this time, cold air is blown into the furnace through the burner, and a certain cooling rate can be maintained.
This process makes the heating mantle work for a slightly longer time, but not the whole time with the heating mantle cooling, but only a part of its time. For example, when cooling with a heating cover for 4 hours (see Figure 3), in fact, the temperature of the cold spot inside the steel coil continues to rise by about 5 mm during cooling with a heating cover. In this case, the working time of the heating cover can be shortened by 1.5 h, and the cooling time can also be reduced

About 1 h, the actual annealing period is extended by 1 · 5 h, and the bonding defects of the steel coil can be effectively controlled.

1. Adjust split cooling system start-up temperature to increase productivity

After commissioning, the start-up temperature of the split cooling system was set at 400 ^ 0 and we carried out the experiments of starting the split cooling at 500 , 475 , 450 and 400 ft respectively. Experiments show that increasing the start-up temperature of split cooling can effectively shorten the cooling cycle. It can be seen that compared with 400 and 400 , the start-up split cooling system has a greater impact on the cooling time, which can shorten the cooling time by more than 40 minutes .

1. The principles and experimental results of the furnace temperature

According to the information, the core temperature (cold spot temperature) is not higher than 160 ° C, and the surface of the steel coil will not be oxidized. Based on this experience , 105.100.95 and 90 different furnace temperatures were formulated respectively , and whether the surface of the strip was oxidized was checked. After many experiments, it was found that even if it was released at 105 °C (the measured core temperature was less than 160 °C), no surface oxidation of the strip was found. According to the statistics, the cooling time can be shortened by an average of 30 minutes for every 5mm increase in the furnace temperature .
2.3 Improvement of hydrogen purging system
(1) The principle of hydrogen purging system formulation The purpose of hydrogen purging is to replace the emulsion volatilized on the surface of the steel coil during the heating process in the furnace. The dew point measured after the furnace space is pre-purged with nitrogen is usually -50 . If repeated purging, the dew point can be lower. When the steel coil is heated, the generation of water and oil vapor increases the humidity of the annealing atmosphere, and the dew point is around -30 or even -107 . The specific value of the dew point depends on the total surface area of the coil, the amount of oil deposits, the heating rate and the hydrogen purge flow rate. Due to different equipment, different emulsions used and different pre-purging methods, this value will vary greatly. After a period of heat preservation, the evaporation of the oil is over, and the oil mist in the space of the stove has been blown away. Now the dew point drops to about -50° C, and in the subsequent heat preservation stage, the hydrogen purging process ends , and the dew point reaches < 0 to
The change of dew point with the annealing process depends on the protective atmosphere in the annealing furnace. Clearly, as the partial pressure of oxygen decreases, so does the point II . From the concentration change of the protective atmosphere in the furnace during the annealing process (Figure 5) , it can be seen that methane has two peaks during the entire annealing process. After the second peak of methane, the hydrogen content in the furnace is close to 100 % . It can also be seen that the initial stage of heating (around 450 P ) is the emulsion evaporation stage, and the smoke content of the emulsion increases rapidly. This point, through The actual measurement of the concentration of the exhaust gas with filter paper on site has also been confirmed. Table 3 lists the furnace temperature, annealing time and measurement conditions during the measurement. The measurement results are consistent with those in Figure 5 .
There is a secondary methane peak in the atmosphere in the furnace. The first peak is in the heating stage. The methane comes from the emulsion, which can be blown away by a large flow of hydrogen and burned in the heating mantle space. The second peak is at the end of the heat preservation. The carbon in the methane mainly comes from the steel. In order not to decarburize the steel, the methane at this time should not be blown away.
Table 3 The actual measurement results of smoke generation concentration with paper during the fire process

Annealing time /h	Degree of control /T	hydrogen	Sampling time /min
3.5	417	6	1
4.5	502	20	1
5.5	631	20	1
7.5	680	20	I
9	679	20	1

( 2) Experiment and improvement of hydrogen purging system
According to the above basis and analysis , the actual measurement and comparison of the hydrogen purging system were carried out. Steel coils with the same specifications and stacking weight were used for annealing. The surface residues were measured before annealing, and the reflectance was compared after annealing. For the hydrogen purging system of the test, the temperature of the large -flow hydrogen purging starts at about 450 ℃, and the time for the end of the purging is determined according to the flue gas concentration measured in Table 3 . It can be seen from the comparison that the modification principle of the hydrogen blowing system is correct, the surface reflectance of the steel coil is almost the same before and after the modification, and the hydrogen consumption is reduced by 40m'/ cycle (Fig. 6) . According to this experience, the hydrogen purging system was revised (Table 4), which reduced unnecessary hydrogen purging volume and time, reduced consumption, and had no effect on surface cleanliness. The purging flow rate, time and furnace temperature changes more closely match. It can also be seen from the actual measurement that in order to obtain a better surface finish of the steel coil, it is necessary to control the residue on the surface of the steel coil at the exit of the rolling mill.

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