Innovation Of High-strength Plastic Seamless Steel Pipe

The main innovative achievements of high-strength plastic seamless steel pipe are as follows:(1) The continuous cooling phase transition behavior of 0.16C-0.31Si-1.6Mn microalloyed steel was studied. Based on this, the medium-frequency induction heating method was used to establish a rapid continuous annealing process for duplex steel seamless tubes. The effects of different critical annealing temperatures and overaging systems on the microstructure and properties of duplex steel seamless tubes were analyzed.

The results show that the ferrite grains have been fully recrystallized at a critical annealing temperature of 750 °C. The grain size of the martensite phase is appropriate and uniform, and the strong plastic product is 16510 MPa.%. During the overaging temperature change from 400 °C to 200 °C, the mechanism of austenite transformation changes from diffusion to shear. The final microstructure changes from bainite to martensite, and the martensite volume fraction decreases with temperature. With the increase, the optimum mechanical properties occur at an overaging temperature of 200 °C.

2. The effects of three heat treatment processes of tempering, slow cooling and preheating on the ductility of duplex steels under medium frequency induction heating were studied. The contents include: the evolution behavior of martensite during the tempering process, the influence of the slow cooling system on the formation of the epitaxial ferrite and the quenching structure, and the influence of the initial microstructure change on the plasticity after preheating. The results show that as the tempering temperature increases, the carbide size produced by martensite decomposition increases, the martensite hardness decreases, and the elongation increases, but the yield platform appears, and the tempering temperature at 200 °C is optimal.

Increasing the initial temperature of slow cooling can make the phase transition ferrite phase change sufficiently, but it will increase the content of bainite in the final structure. The influence of the two on plasticity is mutually restricted. When the temperature is set to 790 °C, the steel is in the steel. The bainite content is small, and the orientation epitaxial ferrite transformation is sufficient, and the performance of the experimental steel pipe is good. After preheating, the bulk pearlite in the initial structure of the steel tube is transformed into a dispersed cementite group. After the heat treatment, there is no bainite structure in the steel tube structure, and the strong plastic product reaches 20865 MPa·%.

3. The TRIP steel seamless pipe was produced by two-stage heat treatment. The influence of heat treatment process on the chemical stability of retained austenite in the microstructure was analyzed. The internal high pressure forming test was carried out on the seamless pipe. And crack generation and expansion behavior were studied. The results show that the retained austenite in the TRIP steel seamless tube undergoes martensitic transformation during the deformation process, ie the TRIP effect, which is directly related to the stress-strain state: the residual under the axial tensile stress state. The austenite is more prone to transition than under compressive stress, while the retained austenite at the ferrite grain boundary is more susceptible to transformation than inside the grain.

Cracks sprout at the inclusions and fuse together to form large-size cracks. Due to the effect of the TRIP effect, the growth of microcracks inside the ferrite is suppressed, and the retained austenite at the ferrite grain boundary can absorb the energy of crack propagation and become an obstacle in the crack fusion path. Thereby hindering the formation of large-size cracks and delaying the occurrence of cracking during deformation.

4. Through the study of the phase transition of austenite in the two-phase zone critical annealing and bainite austempering, the mechanical stability of the retained austenite in the TRIP steel seamless pipe is improved, and the final microstructure is realized. A reasonable ratio of the phases, developed a rapid heat treatment process. During the two-phase zone (830 ° C) holding process, the austenite nucleation occurs first in the pearlite dense region, followed by the independent pearlite region, and finally in the dispersed carbide region.

When the holding time is 300s, film-like retained austenite with high mechanical stability appears in the interior of the ferrite grains and between the bainite laths. The extremely short bainite zone austempering time produces a certain amount of martensite grains in the final structure of the TRIP steel seamless tube, and the induced large number of movable dislocations cause the yielding platform to disappear and increase the strength. The entire rapid thermal processing process takes only one-third of the time of conventional processes.

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