Research on Controlled Rolling and Controlled Cooling Process of DH36 Ship Plate Steel

By testing the continuous cooling transformation curve of DH36 steel, the austenite recrystallization percentage after single-pass rolling under different deformation amounts and deformation temperatures was determined. Combined with the production practice of controlled rolling and controlled cooling and the analysis of on-site rolling data, it is considered that the optimal final rolling temperature of DH36 steel is 800-830 t , the cooling rate is 5-7 t/s , and the optimal final cooling temperature is 685-715 t . The low-temperature impact toughness of DH36 steel produced under industrial conditions meets the requirements of classification societies.

Keywords : DH36 ship plate steel ;DH36 controlled rolling ;controlled cooling ;technology ;research

When rolling DH36 high-strength ship plate , the qualified rate of low-temperature impact toughness of the two specifications of thickness 15 mm and 30 mm is less than 90% , which seriously affects the product quality and increases the production cost.
controlled rolling and controlled cooling process plays an important role in improving the low temperature impact toughness of DH36 ship plate. By controlling the rolling and cooling process parameters, the microstructure of steel can be effectively improved , thereby improving the low temperature impact toughness of DH36 ship plate.

Research on continuous cooling transformation and microstructure refinement of DH36 ship plate steel :
The basic method to study the law of austenite phase transformation is to measure the continuous cooling transformation curve of supercooled austenite, that is, the CCT curve. The CCT curve is the basis for the chemical composition design and process design of steel. It can systematically express the influence of deformation temperature, deformation degree, and post-deformation cooling system on the phase transformation law of steel , and then study the relationship between its structure and performance , which is more accurate . To formulate controlled rolling and controlled cooling process.
2.1 Static CCT curve determination
2.1.1 Test materials and scheme
draw the CCT curve: extend the straight line segment of pure thermal expansion ( or pure cold contraction ) on the thermal expansion curve , and take the position where the curve starts to deviate, that is, the temperature corresponding to the tangent point as the phase transition point , that is, the critical point.
The DH36 test steel was processed and prepared into ① 8 mm X 15 mm samples, and the static CCT was measured with a thermal dilatometer . The test steel is processed into a sample with a groove in the middle Experiments were carried out on Gleeble1500 to determine its dynamic CCT curve.

Experimental results of DH36 ship plate steel
( 1 ) The continuous cooling transition curve of DH36 ship plate steel was tested on Gleeble - 1500 thermal simulation testing machine. In order to obtain fine grains , the cooling rate should be controlled at 5 ~ 7 C / s , and the final rolling temperature should not be lower than 790 C.
( 2 ) the percentage of austenite recrystallization after single-pass rolling of DH36 ship plate steel was measured under different deformation amounts and deformation temperatures , and the minimum complete recrystallization The crystallization temperature is 1 040C , and the recrystallization termination temperature is 940C . Therefore , the finish rolling temperature of DH36 ship plate steel should be higher than 1 040 °C , and the start rolling temperature of finish rolling should be lower than 940 °C .
( 3 ) Studying the influence of controlled rolling and controlled cooling process on the impact properties of DH36 ship plate steel, it is considered that the final rolling temperature should be controlled at 800 ~ 830 C , and the final cooling temperature should be controlled at 685 ~ 715 C.

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