Large steel plates 1.0-cm in thickness are quenched from 600°C to 100°C by submerging them in an oil reservoir held at 30°C. The average heat transfer coefficient for both faces of steel plates is 400 W/m²·K. Average steel properties are k = 45 W/m·K, ρ = 7800 kg/m³, and c_p = 470 J/kg·K. Calculate the quench time for steel plates.