Effect by Tempering Temperature on Microstructure and Properties of Hot-rolled High Strength Bainite Steel Pipe

The new type hot-rolled hi-strength bainite steel tube possesses rather high strength, but low impact value.However its impact toughness can be improved by the tempering process.The effect by the tempering temperature on the microstructure and properties of the said tube is studied.The test results indicate that with the tempering temperature below 550 ℃, along with increase of the temperature, the tube’s tensile strength trends to be reduced, but not so obviously;and with tempering temperature below 350 ℃, along with the increase of the temperature, the tube’s impact value goes up;while tempered at 400 ℃, the tube’s impact value gets decreased, resulting in tempering-caused brittleness;tempered at more than 450℃, the impact value of the tube gets increased;tempered at 250~350℃, the tube has higher strength;tempered at 550~650 ℃, the tube presents rather high toughness;and tempered at a temperature below 400 ℃, the new type bainite tube has all bainite microstructures, i.e., lath bainite, granular bainite, ferrite and residual austenite structures;when the tempering temperature is higher than 500℃, the residual austenite structure gets completely resolved, and the structure is of ferrite and granular bainite.