Study on Cooling Method of SUP9 Spring Flat Steel

Study on Cooling Method of SUP9 Spring Flat Steel

Hot rolled spring flat steel bar is mainly used for the automotive industry, and it’s the raw material for automobile leaf spring  manufacturing. The main varieties of spring steel include 60Si2Mn spring steel, SUP9 spring steel flat bar, 55SiMnVB spring steel. SUP9 spring steel flat bar is produced in accordance with the Japanese standard. At present, many leaf spring manufacturers mostly use SUP9 spring steel as the raw material for manufacturing automotive leaf springs. Compared with 60Si2Mn, SUP9 spring steel has good hardenability and low decarburization tendency, so the mechanical properties and fatigue properties of the leaf springs produced with SUP9 spring steel are better than that of 60Si2Mn steel.

However, due to the good hardenability of SUP9 steel, the hardness is often too high during actual rolling production and cooling. The reheating, quenching and tempering treatment are carried out in the production of the leaf spring assembly, the high hardness of the raw material will be lowered during the processing, and the final mechanical properties and fatigue properties of the leaf spring product have no effect, but excessive hardness will cause certain difficulties in the processing of blanking and punching during the production of the leaf spring assembly. Therefore, it is necessary to study the cooling method and cooling speed of the SUP9 spring flat steel to avoid the problem of excessive hardness of the raw material after hot rolling.

After rolling, through the walkway, then cold bed, when the flat steel bars are still very red (700° C-800 ° C), the flat steel bars are immediately clamped by hand, stand sideways and multiple pieces of flat bars stack together to be cooled. Until the spring flat steel color changes from red to dark, then black, they are manually hooked down to cold bed for collection. This method could achieve hardness requirements, but labor is too intensive and it’s not suitable for large scale production.

In the rolling steel production line, in front of the cooling bed, a crocodile shear is set, that is, it is directly collected without passing through a cold bed, and the temperature is about 700° C-800 ° C, sometimes even 800 ° C or more. This method undoubtedly meets the hardness requirements, but without the cold bed, the spring flat steel is collected directly from the aisle, the temperature is high, and the wave bend formed on the aisle cannot be eliminated.

In order to make the spring flat steel not touch the cold bed of the cast iron plate, we can reduce the cooling speed of the cold bed, weld dia 12mm round steel bars equidistantly on the cold bed overhead, and quickly overcool the bed and large shear. When the spring steel flat bars are dark red, they enter the collection tank. The blackened flat steel bars do not enter the genuine collection tank. In actual production, from the collection stack cooling to the room temperature for 8 hours, the cooling rate after stack cooling is about 70 ° C / hour, which can fully meet the requirements. This method is economical and low-labor intensive, and is a good improving way. After years of production practice testing, the pass rate of SUP9 spring flat steel hardness produced by Glory Steel Work could reach 100%.