Hot Rolling Coil Temperature on Properties of P3A2 Steel Cold Rolled Sheet

Abstract: Combined with the characteristics of WISCO's P3A2 steel cold-rolled sheet, the influence of hot-rolled coiling temperature on the property non-uniformity of cold-rolled sheet was studied experimentally . The results show that the hot-rolled coiling temperature has a great influence on the non-uniformity of the yield strength of the cold-rolled sheet, but has little effect on the tensile strength and elongation; the higher the coiling temperature, the lower the yield strength of the cold-rolled sheet, and the The performance inhomogeneity of different parts is more obvious, which is characterized by higher yield strength at the head and tail, and lower yield strength in the middle; finally, 680 °C is determined as the best hot rolling coiling temperature .

Key words : hot rolling ; coiling temperature ; yield strength ; unevenness

P3A2 steel ( brand name DX51D +Z (JD1)) is a continuous hot-dip galvanized steel sheet and steel strip specially developed by WISCO for Midea washing machines in early 2009. The manufacturing process is continuous hot-dip galvanizing , and the base material is cold - rolled steel strip . Zinc coating , smooth after galvanizing , no spangles on the surface , the quality of the zinc layer is 80 g/m 2 on both sides , the surface is environmentally friendly passivation, and oiled for delivery .
Since the beginning of July 2009 , it has been found that the production of P3A2 steel for Midea household appliances has a batch performance fluctuation problem , mainly manifested in the high yield strength of the head and tail , and the low yield strength of the middle , and the yield strength of the head and tail is about 50% higher than the middle. ~ 80 MPa . Due to the large performance fluctuation of the whole coil , the performance inspection report issued by the quality inspection center for the incoming sample cannot truly reflect the fluctuation . Therefore, there may be parts exceeding the standard in the same coil , which will bring serious serious problems to the subsequent cold forming process. impact .
Coiling is the last process of the strip hot rolling production line . After hot rolling and coiling , the inner and outer rings of the steel coil are easier to contact the outside than the middle part , which accelerates the natural cooling effect . That is to say, compared with the middle part of the head or tail of the steel coil , the cementite is not easy to precipitate . They are not only less precipitated , but also very small , resulting in uneven material of the head , middle and tail of the hot coil , so that After cold rolling and annealing, the head and tail properties of steel coils deteriorate and the processing performance deteriorates . The direct impact is the decline in the pass rate of the production process and the increase in the scrap rate of users . Experiments have shown that the ferrite grains will continue to grow in the coil storage room after the strip steel is coiled, and the grains will not stop growing until they are cooled to 500 C.
The mechanical properties of steel coils are mainly reflected in yield strength , tensile strength and elongation . According to the actual production situation of Wuhan Iron and Steel P3A2 steel , through reasonable control of hot rolling coiling temperature , the

The performance inhomogeneity is improved, and the various indicators of the product meet the requirements . The following will improve the overall length stability of the mechanical properties of P3A2 steel by improving the hot-rolled coiling temperature control technology .
1.Test materials and methods
taken from industrially produced P3A2 steel hot-rolled and cold-rolled plates. The hot-rolled specification is 2.8 mm X 500 mm X C mm, and the cold-rolled specification is 0-8 mm X 500 mm X C mm. The chemical composition As shown in Table 1 .
Table 1 Chemical composition of P 3 A 2 steel grades ( w b ) ;%
C ;And ;Mn PS With Al
0.035 0.010 0.235 0.008 0.007 0.019 5 0.040
P3A2 steel with large fluctuations , different hot-rolled coiling temperatures were compared, and the target hot-rolled thickness of 2.8mm was selected as the test coil, and three hot-rolled coiling temperatures of 650 , 680 , and 700 °C were selected . The production process of P3A2 steel is as follows : hot metal desulfurization - converter smelting- Vacuum treatment or argon blowing - continuous casting - slab inspection and cleaning - slab heating - hot continuous rolling - laminar cooling - coiling - pickling continuous rolling ( secondary cold rolling ) - galvanizing - inspection - packaging .
, middle and tail of the cold-rolled sheet at different hot-rolled coiling temperatures . Tensile test was carried out according to AST M / A370 standard, and the equipment was ZWICK tensile testing machine . In order to ensure the reliability of the test results, three samples were taken from each group . Leica optical microscope (OM) was used to observe the microstructure of hot-rolled and cold-rolled states and determine the grain size under different conditions .
2 Results and Discussion
Figure 1 shows the relationship between strength and elongation at the head, middle and tail of P3A2 steel at different coiling temperatures . It can be seen that the yield strength of P3A2 steel decreases significantly with the increase of coiling temperature . When coiling at 650 C and 680 C , the yield strengths of cold-rolled sheets are relatively uniform at the head, middle and tail, and the property fluctuations are the largest when the temperature reaches 700 C. The fluctuations of tensile strength and elongation at the head, middle and tail are small, within the range of 10 MPa and 2.5% , respectively . It can be seen that, compared with the tensile strength and elongation, the coiling temperature of hot rolling has a greater influence on the yield strength of cold rolled sheet . The inhomogeneity of sheet performance was improved at 650 C and 680 C , but according to the performance index of this product, as shown in Table 2 , the yield strength obtained by 650 C coil was close to the upper limit, therefore, 680 C was determined to be the most Optimum coiling temperature .

Table 2 Performance indicators of P3A2 steel cold-rolled products

R eL / MPa	R m / MPa	A / %
220 ~ 280	320 ~ 410	>34

The occurrence of this inhomogeneity is closely related to the size and uniformity of the hot-rolled grain size at different coiling temperatures . Figure 2 and Figure 3 are the microstructures of the hot-rolled state at the coiling temperature of 700 C and 680 C , respectively . It can be seen that under the condition of 700 °C , the grains in the middle of the plate are coarse, and the grain grade is 7 , while the grains in the head and tail are fine and uniform, and the head is smaller than the tail . However, under the condition of 680 C , the grains at the head, middle and tail of the hot-rolled sheet are very uniform , The grain grades are all grade 10 . The reason is that the coiling temperature corresponds to the final cooling temperature of laminar flow cooling, and the final cooling temperature has a great influence on the microstructure. After the final rolling, the steel plate will produce polygonal ferrite when it is air-cooled, along the austenite grain boundary. Nucleation, followed by pearlitic transformation of retained austenite, resulting in an equiaxed ferrite-pearlite structure . When the steel plate is cooled by laminar flow, the cooling rate is accelerated, and the ferrite nucleation at grain boundaries is significantly inhibited . If the final cooling temperature is high, the phase transformation is not complete during coiling , and the cooling is slow in the subsequent air cooling or coiling process, and ferrite phase transformation occurs. At this time, the temperature is high, and there is one ferrite grain In the process of gradual growth, the strength is low ; if the final cooling temperature is lowered, ferrite will be formed in the latter stage of the phase transformation, and it is difficult for the grain to grow at this time, so the strength is higher . In addition, the higher the coiling temperature, the greater the temperature difference between the head and the tail and the middle of the steel coil after coiling. At the same time, the cooling of the middle of the steel coil is slower, resulting in the formation of the oxide scale being slower than the end of the head and tail, and making the middle oxide scale thicker
. As a result, the heat transfer from the middle part to the outside becomes slower, resulting in an increase in the temperature difference between the head and tail of the steel coil and the middle part, thus making the structure of the head and tail and the middle part of the plate uneven . Figure 4 shows the cold-rolled microstructure corresponding to the coiling temperature of 700 °C. It can be seen that the grain size at the head, tail and middle of the cold-rolled sheet is uneven at the coiling temperature of 700 °C ; The grain size of cold-rolled products is uniform without obvious fluctuations . Cold rolling cannot eliminate the coarse grains, too fine grains and uneven grains obtained during hot rolling deformation, so they will all be reflected in cold rolled products, which further proves that the inhomogeneity of the structure and properties of hot rolled strip steel will Will be inherited to the cold-rolled strip .
In addition, it can also be found from the production practice that the thickness of the strip , the weight of the steel coil and the external environment all have a certain influence on the fluctuation of the properties of the plate . In the case of a certain steel coil quality, as the thickness of the steel strip increases, relatively speaking, the gas between the layers of the steel strip decreases after the coil is rolled, which makes it easy for the middle of the steel coil to transfer heat outward, and the entire steel coil The faster temperature drop also reduces the temperature difference between the beginning and end of the steel coil and the middle part ; the greater the mass of the steel coil, the larger the outer diameter, and the greater the distance between the middle plate ring and the inner and outer surface rings, the greater the distance between the middle plate ring ( ie, the middle part of the steel coil ) The slower the cooling rate, the greater the temperature difference between the middle and the end of the steel coil ; for the external environment, if the temperature of the external environment is increased, the temperature drop at the end will slow down, thereby reducing the temperature difference with the middle, so that the temperature at the end and the middle tends to Consistent, resulting in the organization and performance of the head and tail and the middle also tend to be uniform .

1. Hot-rolled coiling temperature has a great influence on the non-uniformity of yield strength of P3A2 steel cold-rolled sheet, but has little effect on tensile strength and elongation. Reasonable control of coiling temperature has a positive effect on improving the non-uniformity of cold-rolled sheet properties. The product meets the required indicators .
2. The higher the coiling temperature , the lower the yield strength of the P3A2 steel cold-rolled plate, and with the increase of the coiling temperature, the cooling of the middle part of the steel coil is slower, resulting in the formation of oxide scales and the slower heat transfer from the middle part, resulting in steel The temperature difference between the head and the tail and the middle of the sheet increases, which is manifested by a large difference in the grain size between the head and the tail of the sheet and the middle, and the performance is not uniform . Through practical production and combined with product index requirements, 680 C is the best hot-rolled coiling temperature .

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