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On-line quenching and tempering of high strength steel plate

12MnNiVR high-strength steel plates are mainly used in large floating roof oil storage tanks of 100,000 to 200,000 cubic meters. High-efficiency large-line energy welding technology is used, and the welding line energy is 50-100 kJ/cm. Therefore, high-strength steel plates for oil storage tanks should be used. To meet the mechanical properties, it also needs to have the characteristics of large heat input welding and low crack sensitivity: ① high yield strength and tensile strength, high toughness, high uniformity and stability; ② suitable for large line energy welding, using 50 ~ After welding with 100 kJ/cm large line energy, its plasticity and toughness did not decrease significantly; ③ In order to be suitable for on-site welding, no preheating was required before welding, and no cold cracks occurred after welding.

On-line quenching process, the steel plate directly enters the on-line cooling process after being rolled by the finishing mill, which saves the cooling and reheating process of the steel plate after rolling required by the conventional process, which can save energy and reduce the time of the heat treatment process. , The rolling process and the online quenching + tempering process ensure the mechanical properties of the steel plate, which can meet the welding performance after welding with large line energy.

1 Technical indicators of steel plate

12MnNiVR steel plate technical indicators: yield strength ≥ 490 MPa; tensile strength 610 ~ 730 MPa; elongation ≥ 17%; -20 ℃ transverse impact energy ≥ 100 J; 180 ° cold bending qualified; After high-energy welding, the average value of impact energy at -15°C in the heat-affected zone is greater than or equal to 47J, and the single value is greater than or equal to 33 J.

2 Chemical composition design

Design ideas for high-strength petroleum storage tank steel: low cost, appropriate on-line quenching process, and corresponding tempering process. Main component characteristics of 12MnNiVR steel plate for petroleum storage tanks: low C-Si-Mn steel, low sulfur and phosphorus content, appropriate addition of microalloys, and considering carbon equivalent and welding crack susceptibility coefficient, Ceq ≤ 0.45%, Pem ≤ 0.22% .

3 On-line quenching + tempering process

On-line quenching (DQ) is when the steel structure is completely austenitized at the end of hot rolling, and the steel is directly subjected to on-line rapid cooling to produce martensitic transformation. It ensures the uniformity of transverse cooling and ensures that the uniformity and flatness of the steel plate cooling meet the requirements.

The figure below shows the comparison curve of strength change after different tempering temperatures. It can be seen from the figure that both the yield strength and tensile strength first increase with the increase of tempering temperature, and peaks appear when reaching 625 °C. After that, it showed a downward trend, and the strength increased again at 655 °C, and the secondary hardening phenomenon appeared. It can be found that the elongation increases gradually with the increase of tempering temperature during the experiment.

The impact energy at different impact temperatures is given. It can be seen that the impact energy increases continuously with the increase of temperature, and the impact energy increases sharply in the range of -80~-40℃, and then increases gradually. It can be determined that the ductile-brittle transition temperature of the samples with different tempering temperatures is below -60℃.

4 Weldability of steel plates

The steel plate meets the requirements of the tensile strength of welded joints ≥610 MPa after welding with a large heat energy of 50-100 kJ/cm, the average value of impact energy at -15°C in the heat-affected zone is ≥47J, and the single value is ≥33J, and the joint part has no plasticity after welding. The transition temperature is less than -20 ℃, which fully meets the technical requirements of large oil storage tanks.

5 Conclusion

1) The mechanical properties of the 12MnNiVR steel plate produced by the online quenching + tempering process meet the requirements of users, and the properties of the steel plate are uniform.

2) On-line quenching + tempering process to produce 12MnNiVR steel plate can meet the production process of large line energy welding.

3) After on-line quenching + off-line tempering of 12MnNiVR steel plate, the microstructure and properties of the steel plate can meet the technical requirements of large oil storage tanks of 100,000 to 200,000 cubic meters.

Selection of heat treatment equipment: In addition to good process design, the selection of vacuum heat treatment equipment is also an important factor in completing the process. The RVT vacuum tempering furnace produced by SIMUWU is an excellent choice for this type of process. Its process performance can fully meet the needs of such thermal processing, with good temperature control accuracy, temperature uniformity and tempering uniformity. High process repeatability, stable production, quality output can be guaranteed.