Control Cooling Process For Q345B Welded Pipe

There are many kinds of welded pipes, such as erw steel pipes and ssaw steel pipes. The purpose of Q345B welded pipe production is to obtain a well-drawn sorbite structure. In theory, the phase change should occur at around 630 °C, but it is impossible in actual production. It is a complete isothermal transformation. In addition to the sorbite, there may be a small amount of ferrite and flaky pearlite in the final product. In this paper, the dynamic continuous cooling transition curve (CCT curve) of Q345B seamless steel tube was studied by using the data of thermal simulation experiment.

The influence of the spinning temperature and air cooling line cooling system on the microstructure performance was discussed, and the high-speed wire was used to control the cooling. The two basic models in the process, namely the austenite to pearlite transformation model and the relationship between the final microstructure and mechanical properties on the Steyr wind-cooled line, analyzed the controlled cooling process and final microstructure of the Q345B welded pipe. The relationship between tissue and mechanical properties in order to optimize the cooling system and reduce the rate of organizational performance.

1 CCT curve drawingThe sample steel grade is WLX82A, the billet size before rolling is 200mm×200mm×6000mm, and the finished section size is.5mm. A rough blank sample is cut at the exit of the roughing mill and processed into a cylinder of 8 mm × 15 mm.

The Q345B welded pipe sample was heated to 1100 ° C, kept for 5 min, cooled to 1050 cc, and subjected to compression deformation at a strain rate of 50/s and a relative deformation degree of 60%. According to the production process at the site, three initial cooling temperatures of 880, 910, and 940 °C were set, and the deformed samples were started at 880, 910, and 940 °C with 0.8, 3, 6, 10, 20, 30, and 40, respectively. °C/s 7 different cooling rates were cooled to 200 ° C, the temperature and expansion time curve were measured, the phase transition temperature and time were determined by thermal expansion method, and the dynamic CCT curve was drawn by Origin software. At the same time, three samples were quenched from 880, 910, and 940 ° C, respectively, and the austenite grain size at this temperature was measured.

2 Results analysisThe initial cooling temperature in the Q345B welded pipe thermal simulation experiment corresponds to the wire temperature at which the wire rod enters the Steyr wind-cooled wire. At the same cooling rate, as the initial cooling temperature increases, the temperature at the end of the transition increases to varying degrees. The higher the initial cooling temperature, the longer the time of staying in the high temperature phase during the continuous transformation of the wire, the higher the energy, the easier the nucleation grows at the grain boundary, and the supercooling is also larger at this time. fast.

Quantitative metallographic analysis of the thermal simulation samples was carried out using a Leica DM6000 metallographic microscope and a SEMQuant400 scanning electron microscope to obtain austenitized grain size and pearlite sheet spacing. Reducing the spinning temperature, on the one hand, affects the tendency of austenite grain growth after deformation, so that the austenite grain size before the transformation becomes smaller, the grain boundary area increases, and the proportion of ferrite in the structure increases, which is favorable for forming fine grains. On the other hand, the amount of pearlite decreases, the pearlite layer becomes larger, and the tensile strength and yield strength decrease.

Considering the actual production situation and the user's requirements for the strength performance of Q345B seamless steel pipe, the spinning temperature can be set in the higher temperature zone (910 ~ 930 °C), thus obtaining higher tensile strength. However, the spinning temperature should not be too high.

There should be a certain temperature drop between the outlet temperature and the spinning temperature of the NTM (twist-free finishing mill). Otherwise, the austenite grains grow up due to the long-term high temperature of the wire. After the phase change, the amount of pearlite increases, making it difficult to remove the scale. In addition, fluctuations in the spinning temperature should be strictly controlled within ±10 °C to improve the performance of the bars.

The faster cooling rate will shift the phase transition temperature to lower temperature, increase the cooling degree with the increase of cooling rate, promote the further nucleation of ferrite, increase the nucleation rate, and lower the temperature and limit the crystal. The kinetic ability of the boundary, delaying the growth of ferrite grains into the untransformed austenite matrix, reducing the growth rate, resulting in the refinement of ferrite grains.

Accelerating the cooling also prevents the austenite grain growth that has been refined before the transformation, and is also advantageous for refining the ferrite grains. At the same time, the pearlite is refined, the amount of pearlite is reduced, the pearlite band structure can be alleviated or eliminated, and the sheet spacing of the pearlite and the thickness of the cementite layer are reduced, so that the structure is finer and uniform.

A worker from a steel pipe company, which is located in China.