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Direct Quenching and Vacuum Tempering Process of Structural Steel

1 Materials and methods

1.1 Materials

The test steel was alloyed with Mn-Cr-Mo-Ni-Cu composite alloy, and the optimized design was carried out, and its main chemical composition is shown in Table 1. test steel

The ingot is first forged into a rectangular section billet of 70mmx200mmx250mm, air-cooled after forging, and the forging process of each ingot is as consistent as possible.

1.2 Methods

Put the test billet in the box furnace and raise it to the set temperature of 1250 °C according to the prescribed heating curve, and ensure that the temperature is uniform. After two-stage controlled rolling in the recrystallization zone and the non-recrystallization temperature zone, steel sheets with a thickness of 20 and 25 mm were rolled on a 500 mm two-roll test mill in the laboratory. ; Unrecrystallized zone opening temperature of 900 ° C, final rolling temperature of 850 ° C. After rolling, it is directly quenched, and then vacuum tempered at different temperatures. The vacuum tempering temperature is 200, 250, 300, 350, 400, 450, 550, 580, 600630 °C, and the holding time is 50~70min. The test steel was cut along the cross section, ground, mechanically polished, etched with 4% nitric acid alcohol solution, and the structure was observed on an XL-05 large-scale metallographic microscope. The test steel was cut into 0.3mm thin slices with a wire cutting machine, and the thickness was reduced to 80μm by grinding with sandpaper. According to the standard, the transverse tensile test was carried out on the AG-250kN tensile testing machine. A series of temperature longitudinal impact tests are carried out in the range of -40~0°C according to the standard.

2 results

2.1 Effect of vacuum tempering temperature on mechanical properties of test steel

Figure 1 shows the changing trends of the strength, impact energy Akv (-20°C) and elongation A of the 25mm thick steel plate with increasing vacuum tempering temperature. Compared with the rolled state, the strength properties of vacuum tempered steel sheets at temperatures below 400°C tend to change gradually, and fluctuate during vacuum tempering at 250 and 300°C. With the further increase of vacuum tempering temperature, the strength index begins to decrease significantly. , the strength change trend slows down after the vacuum tempering temperature exceeds 550 °C. With the increase of vacuum tempering temperature, the elongation increases gradually, and the plasticity of steel increases. At 550 °C, it has reached 12% of the index requirements; in the range of 400 ~ 450 °C, the toughness decreases significantly, and vacuum tempering brittleness appears.

2.2 Metallographic structure of rolled samples

Figure 2 shows the typical metallographic structure of the 25mm thick steel plate directly quenched and the steel plate at different vacuum tempering temperatures. It can be seen from Figure 2a that the microstructure of the test steel as rolled is lath martensite and a small amount of bainite. After vacuum tempering at 250 °C, it can be seen from Figure 2b that the metallographic structure retains the characteristics of the rolled structure, and is still a fine bainite structure with lath martensite; when the vacuum tempering temperature is increased to 350 ° C C , it can be seen from Fig. 2°C that although there are still a small amount of fine lath bainite structure, the lath width of most bainite structures has increased significantly, and the original bainite lath bundles have been merged together; when the vacuum tempering temperature is increased to 450°C, the lath width of the bainite structure further increases, see Figure 2d; when the vacuum tempering temperature is increased to 620°C, many bainite lath bundles has disappeared, only some blurred merged large slat boundaries can be seen, see Figure 2e.

2.3 Fine organizational structure

Figures 3 and 4 show the TEM images of the quenched and vacuum tempered steel sheets. It can be clearly seen from the figure that the microstructure of the test steel as rolled is lath martensite and a small amount of bainite, and the lath interface is retained austenite. After vacuum tempering, there are carbide precipitations at the grain boundaries and in the grains of lath martensite. When the vacuum tempering temperature exceeds 550°C, since the low-carbon microalloyed steel of this composition contains microalloying elements such as Nb, V, Mo, Cu, B, etc., there are a large number of carbide precipitation and ε-Cu aging precipitation in the structure. , a part of the lath has been submerged by the precipitated carbide, the martensitic lath has almost disappeared, and only the carbide dispersed in it remains.

2.4 Scanning morphologies of impact fractures after different vacuum tempering processes

The observation results of impact fracture with different vacuum tempering temperature by scanning electron microscope show that the fracture characteristics and impact toughness are consistent with the change of vacuum tempering temperature. After vacuum tempering before 350°C, the fracture features are micro-porous agglomerated fracture characteristics (dimples) and quasi-cleavage fractures; from 400°C, it shows “cleavage steps and river patterns”, which are obvious cleavage fractures. ; 600 °C vacuum tempering, and mainly dimples, the tearing edges of plastic deformation are more obvious, and the second phase particles can also be seen in the local dimples.

3 Conclusions

3.1 Using the direct quenching and vacuum tempering process, the metallographic structure of the test steel is vacuum tempered sorbite and vacuum tempered bainite, and all performance indicators meet the requirements of 1 000 MPa grade high-strength steel.

3.2 Within a certain vacuum tempering temperature range, the yield strength and tensile strength of the test steel fluctuate, and the steel plate has vacuum tempering brittleness. After high temperature vacuum tempering, the steel plate has excellent comprehensive mechanical properties.

3.3 With the increase of vacuum tempering temperature, the microstructure of the test steel experienced the interaction of carbide precipitation, dislocation disappearance, weakening of solid solution strengthening, and decomposition of retained austenite, and sorbitization of high temperature vacuum tempering. Helps to obtain the appropriate tissue composition.

Vacuum tempering furnace equipment selection：The RVT series vacuum tempering furnace produced by SIMUWU is a high-quality product for dealing with this kind of process. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum tempering process. With more than ten years of sales and manufacturing experience, it is exported to developed regions in Europe, America and Asia. It is a well-received vacuum furnace product.