SPCC stands for cold rolled carbon steel sheet and strip, mainly used in automobile manufacturing, electrical products, rolling stock, aerospace, precision instruments, etc. At present, the consumption of SPCC steel plate for automobile parts accounts for a large proportion, which requires SPCC steel plate to have low price, good plastic processing performance and certain stamping resistance.
The performance of cold rolled coil is not qualified, mainly because the elongation is too low, about 36% (the standard requirement is not less than 37%); At the same time, the tensile strength (σb) is about 380MPa, close to the upper limit (standard requirements of 270 ~ 410MPa). The main reason for this is insufficient grain growth. Before the process improvement, the metallographic structure (that is, the metallographic structure of the steel coil with unqualified performance) has uneven grain distribution and insufficient growth. Some of the microstructure is stripe-like and small, and uniform equiaxed crystals are not formed, resulting in high tensile strength and low elongation. After the process optimization, the grain size is more uniform and grows up fully, and equiaxed grain is basically formed.
Through the analysis of the cause of unqualified performance, the following process improvement plan is made.
(1) Slow down the heating speed and prolong the holding time in the high temperature stage
When heated to a higher temperature above 600℃, the temperature difference between inside and outside the coil core is large, then the outer ring part begins to recrystallization, if the heating speed is too fast, it will make the whole roll recrystallization is not uniform. Due to the previous process optimization plan set a insulation platform at 620℃, through repeated production tests, the improvement measures were worked out: heating at full speed before 620℃, then heating at 25℃/h, and extending the insulation time for 1h, so that the steel coil at full temperature homogenization at the same time fully grow grain, improve elongation, reduce tensile strength.
(2) Optimize the relationship between the holding time and the amount of furnace loading
The difference in steel coil weight leads to the difference in the loading capacity of each furnace. The previous principle is: based on the loading capacity of 70T ~ 75T, each increase (decrease) of 5T, the insulation time is extended (shortened)0.5h. In view of the unqualified coil is mainly concentrated in the steel coil with large weight and loading capacity, the holding time is adjusted. Through several rounds of test adjustment, the law of the increase and decrease of the holding time is finally determined as shown in Table 1.
