Process Test and Formability Evaluation of Q345 Seamless Steel Tube

Through intermediate frequency induction heating and annular water spray cooling of Q345 seamless steel pipe, a dual-phase steel seamless pipe of ferrite + martensite was obtained. The mechanical properties of the steel pipe were tested by uniaxial tensile test, combined with the microstructure of the steel pipe under the scanning electron microscope and transmission electron microscope, the influence of different annealing temperatures in the critical region on the microstructure and properties of the dual-phase steel seamless steel pipe was analyzed. The pipe end flaring test method was used to evaluate the formability of the test steel pipe. The results show that: after the Q345 seamless steel pipe is annealed in the critical zone by medium frequency induction heating , a dual-phase steel seamless steel pipe with high strength and high formability can be obtained. The curve presents a continuous yield state, and the strength-plasticity product can reach 16 510 MPa%.

Key words: dual-phase steel seamless steel pipe; medium frequency induction heating; annular water spray cooling: annealing temperature in critical zone; pipe end flaring test
test; molding performance evaluation

From the perspective of saving energy and reducing pollution, automobile lightweight production is a hot spot in the current automobile industry. Among them, a large number of hollow parts with satisfactory mechanical properties are used to replace the original solid parts, which has given birth to the development of secondary processing technologies such as special-shaped pipe forming and internal high-pressure forming, which requires raw material pipes to have higher strength and good cooling properties. Formability. Documents [4-5] proposed to convert carbon steel seamless steel pipes into dual-phase steels with good strong plasticity through heat treatment.
steel pipe or TRIP (phase change induced plasticity) steel pipe, so as to improve the strength of the steel pipe and meet the performance requirements of the secondary processing of the pipe without increasing the cost of smelting. Medium-frequency induction heating has the characteristics of fast heating speed, uniform heating, less iron oxide scale formation, and less environmental pollution, which is in line with the production concept of energy saving and emission reduction. This paper introduces the use of medium frequency induction heating combined with annular water spray cooling to develop a dual-phase steel seamless steel pipe with high strength and plasticity using ordinary Q345 seamless steel pipe as raw material, and the effect of different annealing temperatures in the critical area on the microstructure and properties of the test steel pipe impact is discussed.

1. experiment method

the Q345 seamless steel pipe used in the test is shown in Table 1.
The raw material is 130 kg ingot, which is forged into approximately ①75 mm bar, and the uneven part of the surface is turned off to obtain ①70 mm round bar; the round bar is heated to 1 200 C for 2 hours , then perforated, and cold drawn by multiple passes to produce Thin-walled seamless steel pipe for heat treatment (①41 mmx1.2 mni . Critical zone annealing and low-temperature overaging process are used to obtain a dual-phase steel structure with ferrite + martensite. The heat treatment process of dual-phase steel seamless steel pipe is shown in Figure 1 As shown, the annealing temperature in the critical zone of induction heating is set to 700 °C, 730 °C and 750 °C respectively , and the cooling rate is 80 7 /s. After heat treatment, a tensile test with a gauge length of 25 mm is taken along the steel pipe axis The sample was subjected to tensile test at room temperature, the sample size is shown in Figure 2, and the tensile speed was 3 mm/s; a sheet-shaped sample of 8 mm x 8 mm was taken along the axial direction of the steel pipe , and after grinding and corrosion by 4% nitric acid alcohol, The microstructure of the steel pipe was observed with a scanning electron microscope (seM) ; a circular slice with a diameter of 3 mm and a thickness of 50 ^ m was cut in the axial direction of the steel pipe , and after electrolytic double spraying, the microstructure of the steel pipe was observed under a transmission electron microscope (TEM) ; The method of pipe end flaring is used to evaluate the formability of dual-phase steel seamless steel pipes. The conical apex angle tools of 30° and 60° are selected, and the flaring rate is calculated according to the formula :
A, f= (Df- D) /Dq
In the formula, DF—— the diameter of steel pipe after flaring, mm;
D0 —— initial diameter of steel pipe, mm.
Table 1 The chemical composition (mass fraction %) of the Q345 seamless steel pipe used in the test

C	Si	mn	N)	Ti	V	S	P	Fe
0.16	0.314	1.59	0.032	0.003	0.04	0.003 5	0.003 5	margin

test results
The initial structure of the test steel pipe ( composed of ferrite and pearlite. During the drawing process, the grains are elongated to varying degrees, and there is a part of banded structure. After the pearlite structure is deformed, it still retains its lamellar structure.
Recrystallization gradually completes in the tissue. When the temperature reaches 730°C ( the ferrite and martensite grains of the test steel pipe are refined, but the grain size is not uniform (the area marked by the ellipse). When the annealing temperature is 750°C, the microstructure of the test steel pipe The ferrite and martensite grains in the steel are smaller than those in 7007, and the size of each grain is relatively uniform .
The microstructures of the steel pipes after heat treatment at different annealing temperatures in the critical area of the test steel pipes are composed of ferrite and martensite. As shown in Figure 4. By observing the microstructure of test steel pipes at different annealing temperatures, when the critical annealing temperature is low, there is still a certain amount of cold work hardening structure in the ferrite, which generally appears as a small-angle grain boundary substructure, as shown in Figure 4 (arrow in a As the temperature increases, the substructure in the steel is gradually eliminated.

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