Development of QM1020 Salt Bath-Treated Wire Rod for Grade 8.8 Bainitic Non-Quenched-and-Tempered Fastener Steel

Based on the first domestically developed industrial-scale online salt bath isothermal heat treatment equipment， a high-performance QM1020 non-quenched-and-tempered fastener steel wire rod has been successfully developed through systematic composition optimization design， clean steel smelting technology， and collaborative innovation in controlled rolling processes. Leveraging the unique advantages of precise temperature control in the salt bath equipment， the production process involves the wire rod entering the online salt bath isothermal heat treatment unit at a designed salt temperature after hot laying. This is combined with coordinated cooling via the Stelmor air cooling line to achieve precise regulation of the phase transformation microstructure，ensure a predominantly bainitic microstructure.For the first production of ϕ 7 mm QM1020 salt bath-treated wire rods， after undergoing online salt bath isothermal heat treatment at 420 ℃， the comprehensive properties significantly surpassed those of wire rods produced using conventional Stelmor processes. The average tensile strength reached 775 MPa， with an average reduction of area of 76%， demonstrating excellent plasticity and toughness. The wire rods exhibited no cracking during 1/6 cold heading tests.Furthermore， After drawing with a 29% reduction in cross-sectional area， the steel wires retained high tensile strength and good cold workability. The wires can be directly cold-headed into flanged bolts， which fully meet the mechanical property requirements of Grade 8.8 high-strength fasteners.