SPCC cold-rolled sheet annealing process

SPCC cold-rolled sheets were analyzed, and it was determined that the defects produced in the annealing process were the main cause of defective products; and by taking measures to optimize the annealing process, the annealing defects were significantly reduced and the performance was significantly improved , thus effectively improving the finished product rate and performance pass rate.
Key words: SPCC ; cold rolled sheet; annealing; process optimization

SPCC means that cold-rolled carbon steel sheets and strips are generally used , and are mainly used in automobile manufacturing , electrical products , rolling stock , aerospace , precision instruments, etc. In recent years , China's automobile industry has developed rapidly . Relevant experts predict that the domestic automobile output will exceed 16 million in 2010 , ranking first in the world , and the annual consumption of vehicle steel will exceed 17 million tons . The steel plates for cars are mainly cold-rolled plates , accounting for about 75% of the steel plate consumption . For this reason , domestic and foreign large-scale iron and steel enterprises have invested a lot of manpower , material resources and financial resources to develop steel for automobiles . At present , the amount of SPCC steel plate used in auto parts accounts for a large proportion , which requires SPCC steel plate to have low price , good plastic processing performance and certain stamping resistance [1-2] . The SPCC cold-rolled plate produced by Chongqing Iron and Steel Co., Ltd. has reached the relevant national, Japanese , and German standards in plate shape , chemical composition , and mechanical performance inspection . The pass rate of cold-rolled products is very high . , bringing considerable economic benefits . However , since 2007 , after the plant has used a large number of hot- rolled coils from Lianyuan Iron and Steel Co. The mechanical properties of the cold-rolled sheet do not meet the requirements , which not only affects the sales price of the product , but also affects the pass rate and yield of the factory .
Therefore , this paper reduces the annealing bonding defect by optimizing various parameters . According to the properties of steel coils , research and analysis are carried out from the aspects of strength , elongation and metallographic structure , and optimization measures for improving the annealing process are proposed , thereby improving the performance pass rate of SPCC cold-rolled sheets .
1 Problems existing in the production of original SPCC cold-rolled thin plate The surface defects such as bonding , black spots , black bands and oxidized color of strip steel after annealing affect the yield and pass rate of cold-rolled products . Therefore , by optimizing the annealing process , reducing the waste and defective products caused by these defects , and improving the yield and pass rate are problems that need to be solved urgently . The unqualified steel coils in cold rolled products are mainly due to low elongation ( around 36% ) and high tensile strength ( around 380MPa ) , so it is necessary to optimize the annealing process .

1. Optimizing annealing process from the perspective of controlling bonding defects

2.1 Mechanism of bond formation
The heating of the steel coil in the bell furnace mainly comes from the heat radiation of the inner cover and the heat convection of the protective gas . Because the strip steel is annealed in tight coils , the heat transfer between the layers of the protective gas through the end surface is obviously not as good as that of the inner and outer surfaces , as shown in Figure 1 shown . There is a radial temperature gradient in the steel coil , which causes compressive stress between the outer layer and the core . At the end of heating and the beginning of cooling , the yield limit of the steel coil is low , and under the action of compressive stress , the iron atoms diffuse and connect at high temperature , thus forming a bond .

2.2 Factors Affecting Bonding
From the formation mechanism of bonding, it can be seen that any conditions that increase the compressive stress between layers of steel coils , increase the temperature gradient at high temperature , and increase the diffusion energy of iron atoms are factors that intensify the bonding tendency . Therefore , there are many factors that affect bonding , for example , the thinner the steel coil is , the greater the diffusion flux of iron atoms is , and the greater the possibility of bonding is . In the annealing process , the faster the heating in the high temperature stage , the larger the temperature gradient , and the easier it is to bond ; the higher the temperature and the longer the time in the holding stage , the lower the yield limit of the steel coil in the high temperature stage , and the easier it is to bond ; , the faster the cooling rate , the more serious the uneven cold shrinkage between layers , the greater the compressive stress , and the easier bonding . 2.3 Optimization measures of annealing process
According to the feature that the thinner the thickness , the easier it is to bond, steel coils of different thicknesses are produced in separate furnaces , and through mass production tests , different annealing temperatures are formulated , as shown in Table 1 .
Table 1 Annealing temperature of steel coils with different thicknesses
Tab.1 The annealing temperature of steel coils with different thickness

Steel coil thickness /mm	1.5 ~ 2.0	1.2 ~ 1.5	1.0 ~ 1.2	0.8 ~ 1.0	0.6 ~ 0.8	0.5 ~ 0.6
Annealing temperature factory c	705	700	695	690	685	680

The heat preservation process of annealing is actually the uniform temperature process of the surface and core of the steel coil . The larger the outer diameter , the larger the furnace charge , the more difficult it is to uniform the temperature ; the thinner the thickness, the more difficult it is to uniform the temperature . Before the optimization, the annealing process of prolonging the holding time with the thinning of the thickness and the increase of the furnace load was adopted . This process prolongs the annealing time of thin gauge steel coils ( such as below 0.8 mm ) , which leads to an increase in the bonding ratio . Figure 2 shows different thickness steel

The proportion of the bond that the volume occupies . It can be seen that the bonding of steel coils with a thickness below 1.1mm accounts for more than 70% of the total bonding amount . Therefore , the annealing process has been improved in view of the slow uniform temperature and easy bonding of the thin gauge . The steel coils below 1.2mm were uniformly temperatured at 620 °C for 2 hours . Then adjust the holding time properly , see Table 2 ( this holding time corresponds to the furnace loading capacity of 70t ~ 75t ) .
Table 2 Holding time of steel coils with different thicknesses
Tab.2 The holding time of steel coil with different thickness

Steel coil thickness /mm	1.5 ~ 2.0	1.2 ~ 1.5	1.0 ~ 1.2	0.8 ~ 1.0	0.6 ~ 0.8	0.5 ~ 0.6
Holding time /h	12.0	12.6	13.1	13.7	14.5	15.3
620 °C heat preservation /h	/	/	2	2	2	2

If the heating rate and cooling rate are too fast, the interlayer compressive stress of the steel coil will increase , and the bonding tendency will increase . The heating speed can be controlled due to the influence of the heating capacity and the slow uniform temperature of the steel coil . But when the cooling cover was replaced and the cooling started , the cooling speed was extremely fast , and it dropped to 500 °C after 2 hours . In a large number of experiments , it was found that 600 °C is the critical point temperature where bonding is easy to occur . Above this temperature , the interlayer compressive stress increases due to the excessive cooling rate of the steel coil , and it is easy to bond ; after the temperature is lower than 600 °C , With the precipitation of cementite , the yield limit gradually increases , and no bonding will occur . Therefore , the process optimization scheme of reducing the cooling speed is adopted for steel coils with specifications below 1.2mm : after heating , continue to leave the heating mantle on the furnace platform , introduce combustion air for slow cooling , and replace the cooling mantle quickly after the temperature drops to 600 °C cool down .

1. Optimizing the annealing curve from the perspective of improving performance

3.1 Analysis of reasons for unqualified performance
The unqualified steel coils in our factory are mainly because the elongation is too low , which is about 36 % ( the standard requirement is not less than 37%) ; at the same time, the tensile strength (bar) is about 380 MPa , which is close to the upper limit ( the standard requirement 270 ~ 410MPa) . The main reason for this situation is insufficient grain growth , which was confirmed by metallographic analysis .
from Figure 3 that the metallographic structure before process improvement ( that is, the metallographic structure of steel coils with unqualified performance ) is unevenly distributed, the growth is not sufficient, and some structures are striped, relatively small, and uniform equiaxed crystal, resulting in high tensile strength and low elongation. The crystal grains of the microstructure after process optimization are relatively uniform, fully grown, and equiaxed crystals are basically formed.

3.2 Improvement of annealing process
Through the analysis of the reasons for unqualified performance , the following process improvement plan was formulated .
(1) Slow down the heating rate in the high temperature stage and prolong the holding time
When heated to a higher temperature above 600 °C , the temperature difference between the inside and outside of the core is large , At this time, the outer ring part began to recrystallize , If the heating rate is too fast , the recrystallization of the whole roll will be uneven . Since the previous process optimization plan set a heat preservation platform at 620 °C , through repeated production tests , improvement measures were formulated : Heating at full speed before 620 °C , and then heating at a speed of 25 °C/h , while extending the heat preservation time for 1 hour , so that the grains of the steel coil grow fully while the uniform temperature is sufficient , increasing the elongation and reducing the tensile strength ［6］ .
(2) Optimizing the relationship between the holding time and the furnace load. The difference in the weight of the steel coil leads to the difference in the furnace load of each furnace .
The previous principle is : based on the furnace loading capacity of 70t ~ 75t , for every increase ( decrease ) of 5t , the holding time will be extended ( shortened ) by 0.5h . In view of the fact that the unqualified coils are mainly concentrated in the steel coils with large weight and large furnace load , the holding time is adjusted , and through multiple rounds of test adjustments , the law of increasing and decreasing the holding time as shown in Table 3 is finally determined . 3 * *
Tab.3 The holding time for different quantity of heat furnace addition / reduction rule

Furnace charge /t	<70	70 ~ 85	>85
Increase / decrease rule of holding time ( every increase / decrease 5t)	0.5 hours	0.6 hours	0.7 hours

Table 4 The chemical composition of different manufacturers of hot-rolled steel coils ( mass fraction, % ) Tab.4 The chemical composition of different manufacturers (wt, % )

Hot coil manufacturers	C	Si	mn	P	S	Alt
Mei Steel	0.04	0.01	0.20	0.015	0.019	0.038
Pangang	0.06	0.02	0.29	0.016	0.013	/
Lian Steel	0.05	0.04	0.21	0.015	0.007	/

C , Si and Mn in Meishan Iron and Steel Coil is lower than that of Panzhihua Iron and Steel Co. , Ltd. However, C , Si and Mn are important components that affect the strength and hardness of steel coils . At the same time, the hot rolling of LY Steel is CSP rolling , and the hardness is relatively high . Integrating these factors , the improvement scheme of raising the temperature and extending the holding time was adopted , and after repeated adjustment and improvement , the optimal scheme was determined , as shown in Table 5 .
Table 5 Coil process optimization scheme of Lianyuan Steel and Panzhihua Steel
Tab.5 The process optimization scheme of steel coil

Hot roll	Elevated annealing temperature / 'C	Extend the holding time /h
Lian Steel	+10	+2
Pangang	+10	+1

Note: The prolongation law of the holding time under different furnace loads is the same as that in Table 3
After the implementation of these two process optimization schemes , the performance of steel coils has been significantly improved . From the metallographic structure in Figure 3 , it can be seen that the crystal grains of the structure after process optimization are relatively uniform , fully grown , and equiaxed crystals are basically formed .

1. Optimized process curve

Fig.4 The annealing curves before and after process improvement

1. apply effects

After a series of optimizations on the annealing process , the bonding defect and performance pass rate have been significantly improved , the bond defect has dropped from 6.51% to 3.50% , and the performance pass rate has risen from 97.59% to 98.77% , as shown in Figure 5 . At the same time, the annealing process curve is enriched and perfected , forming 180 curves , which makes the annealing process procedure more complete and reasonable .
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Gradually increasing , the stamping performance can be improved ; and when the annealing time is 5 to 8 minutes , the yield strength ratio increases , and the r value gradually decreases , which is not conducive to the improvement of stamping performance . When the annealing time is 5 and 8min , r increases gradually with the increase of annealing temperature . Based on the analysis of microstructure characteristics and performance changes , when the temperature is 1100 °C and the time is 5 minutes , the grains are coarser and more uniform , the r value is the largest , reaching 1.39 , and the yield ratio is the smallest , which is 0.36 , which can improve the stamping performance and meet the requirements of the material . performance requirements .
2.2.3 Hardness test
The hardness test results of 304 stainless steel are shown in Table 3 . It can be seen that with the increase of annealing temperature and the prolongation of annealing time , the hardness of the material decreases . From the analysis of metallographic photos , it can be seen that with the increase of annealing temperature and the prolongation of annealing time , the grains are coarse , so the hardness decreases , and after annealing at 1100 °Cx 5min, the grains are larger and uniform , so the hardness is the smallest , and the r value is the largest .
Table 3 Hardness after heat treatment ( HV)
Tab.3 The hardness after heat treatment

	2 minutes	5 minutes	8 minutes
1060°C	579	513	445
1080°C	435	420	423
1100°C	480	230	420

3 Conclusion

1. With the increase of cold rolling reduction , the grain size becomes larger and the factory value increases . Therefore, increasing the cold rolling reduction can improve the deep drawing performance of the material .
2. With the increase of annealing temperature and the extension of holding time , the grain size increases , the yield ratio decreases , and the grain size is uniform , which helps to enhance the deep drawing performance of the material .
3. From the analysis results of this experiment , it can be seen that in order to obtain a higher r value without losing other properties , the more suitable annealing process in production is 1100°Cx 5min .

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Fig.5 The bonding and qualifying rate of properties before and after annealing process optimization

 

6 Conclusion

1. During the annealing process , different annealing temperatures and annealing times were formulated for steel coils with different thicknesses. For steel coils below 1.2 mm , a 2-hour uniform temperature platform was added at 620 °C , and the cooling rate was controlled , so that the bonding defects were well controlled .
2. Heating at full speed before 620 °C , and then heating at a speed of 25 °C/h , while extending the holding time for 1 h , so that the steel coil can fully grow grains at the same time as the uniform temperature , increase the elongation , and reduce the tensile strength .

(3) Increase ( decrease ) the holding time according to the size of the furnace , and adopt different annealing temperatures and times for different raw materials , which improves the performance and meets the requirements of users .
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