Experiment of Process for Pre-Treatment of 20 Steel Seamless Tube to Eliminate of Corner Crack during Cold-Bending Formation

Addressing corner cracking as occurred in cold-bending formation process of the 20 steel seam-less rectangular tubes, the metallographic structures of two lots of such tubes are observed and analyzed.As a result it is identified that the main cause of the said corner cracking is the Widmannstatten structure of the mother tube as developed due to improper hot rolling process therefor.Accordingly a process solution of pretreatment of the mother tube before cold-bending formation is proposed.laws of influences by the major elements concerned on the structure and mechanical properties of the mother tube are analyzed, including the normalization temperature, soaking time and the cooling method, etc.The regular normalization process is set up, i.e., to heat the tube up to temperature of(890±10)℃ and then get it soaked for 6 min prior to air-cooling.With this process the Wid-mannstatten structure of the 20 steel mother tube is totally eliminated, the tube's yield strength and tensile strength are more appropriate to each other, and a yield strength ratio σ_s/σ_b ≤ 0.78 and an elongation rate δ₅ ≥ 30% are obtained respectively.As a result the cold-bending performance of the 20 steel mother tube is significantly improved without any occurrence of the foresaid corner cracking during cold-bending process.