Enhancing the Strength of Steel: Quenching Process Effects on 1100 MPa UHSS Microstructure

1. Preface

With the development of large-scale and lightweight construction machinery, the market demands higher and higher performance from steel used for construction machinery. This demand promotes the development of ultra-high strength and high toughness steel for construction machinery.

Prior to 2019, high-strength steel with a yield strength of 690 MPa and below for domestic construction machinery was the primary choice, while steel with a yield strength of 960 MPa and above was primarily imported.

However, domestically produced steel of this grade had significant issues with product quality, specification, and supply capacity, which severely restricted the growth of the construction machinery industry.

2. Smelting composition and low magnification control of test steel

(1) Smelting composition

See Table 1 for smelting control chemical composition of 1100MPa ultra high strength steel.

Table 11100 MPa ultra high strength steel chemical composition (mass fraction) (%)

(2) Low magnification control

The low magnification quality must meet the requirement that the center segregation class C is less than or equal to 1.0, with classes A and B not permitted, and no internal cracks or shrinkage cavities present.

In the case of low magnification samples that do not meet the requirements, the steel grade must be changed for the current heat. Additionally, low magnification samples shall be taken for the previous and subsequent heats.

3. Mechanical properties under different quenching processes

Various quenching processes are utilized for 1100MPa ultra-high strength construction machinery steel, commonly known as Q100E. The mechanical properties of Q100E test steel are evaluated and sampled at different quenching temperatures and furnace times.

Table 2 shows the mechanical properties, including yield strength, tensile strength, and -40℃ impact absorption energy, obtained through different quenching processes.

Table 2 mechanical properties of Q100E test steel under different quenching processes

3.1. Effect of quenching process on yield strength of test steel

The change trend of yield strength of Q1100E test steel under different quenching temperature and time is shown in Fig. 1.

Fig. 2 Effect of quenching process on tensile strength of test steel

Figure 1 shows a gradual decrease in yield strength with increasing quenching temperature, reaching its lowest point at 930°C. Meanwhile, yield strength initially increases and then decreases with longer holding times. The highest yield strength is achieved at a holding time of approximately 30-40 minutes. Notably, the yield strength is significantly reduced when the quenching temperature is set to 930°C for 50 minutes.

3.2. Effect of quenching process on tensile strength of test steel

The effect of quenching process on the tensile strength of Q1100E test steel under different quenching temperature and time is shown in Fig. 2.

Fig. 2 Effect of quenching process on tensile strength of test steel

It can be seen from Fig. 2 that the tensile strength fluctuates slightly with the change of quenching temperature and time.

3.3. Effect of quenching process on impact property of test steel

The change trend of impact absorption energy of Q100E test steel at – 40 ℃ under different quenching temperature and time is shown in Fig. 3.

Fig. 3 Effect of quenching process on impact property of test steel

Figure 3 illustrates that the low-temperature impact absorption energy of Q100E steel initially increases and then decreases with the increase of quenching temperature and holding time.

For a fixed holding time, the impact absorption energy decreases as the quenching temperature increases.

For a fixed quenching temperature, the impact absorption energy initially increases and then decreases, with the highest impact absorption energy observed when the quenching temperature is maintained for 30 minutes.

Considering both strength and toughness, the optimal quenching process for Q1100E test steel with a thickness of 6-20mm is 870-890℃ quenching with a holding time of 30-40 minutes.

4. Microstructure under different quenching processes

The test steel Q100E exhibits lath martensite after being quenched at 870-930℃.

Figure 4 shows the scanning electron microscope microstructure of the Q1100E test steel under different quenching temperatures and times.

Fig. 4 SEM microstructure of different quenching processes

Figure 4 illustrates that at 870 ℃ quenching temperature, most of the martensite blocks in the grains are not easily identifiable due to the small grain size.

As the quenching temperature increases, the martensite blocks with different orientations become more distinct.

Similarly, the effect of quenching time on microstructure is comparable to that of temperature, as longer quenching time leads to an increase in grain size.

Figure 5 presents the impact of different quenching processes on the grain size of the original austenite.

Fig. 5 Effect of different quenching processes on grain size of original austenite

Figure 5 demonstrates that as the quenching temperature increases from 870 ℃ to 930 ℃, the original austenite grain size increases from 5.7μm to 15.9μm.

In case of a quenching temperature of 870 ℃, extending the quenching time from 40min to 80min results in an increase in the original austenite grain size from 4.5μm to 6.5μm.

Hence, the quenching temperature impacts both the austenitizing of the material and the solid solution degree of the alloy elements.

Based on the CCT test, the Ac3 of the test steel is 852 ℃. As the minimum quenching temperature of the test steel plate is 870 ℃, complete austenitizing can be achieved.

However, the temperature significantly affects the solid solution of microalloying elements. When the quenching temperature is low, carbides like Nb and V, or carbonitrides of microalloying elements, tend to pin the original austenite grain boundary and hinder the grain growth.

By increasing the quenching temperature, carbide or carbonitride of the alloy element dissolves in austenite, reducing its ability to inhibit grain growth and resulting in a doubled grain size.

As the quenching heating time is extended, the amount of solid solution of alloy elements gradually increases, leading to grain growth.

Comparatively, the quenching temperature has a more significant impact on the grain size.

5. Conclusion

By comparing the microstructure and physical properties of Q1100E test steel under different quenching temperatures and times, the following conclusions were drawn:

1. As the quenching temperature increases, the yield strength of Q1100E test steel gradually decreases, reaching its lowest point at 930°C. At the same quenching temperature, increasing the holding time initially increases and then decreases the yield strength. The yield strength reaches its highest value after holding for 30-40 minutes, while holding at 930°C for 50 minutes leads to the lowest yield strength.
2. The tensile strength of Q1100E test steel fluctuates slightly with changes in quenching temperature and time.
3. The low-temperature impact property of Q1100E test steel initially improves and then declines with increasing quenching temperature and holding time. At the same holding time, impact absorption energy decreases noticeably as quenching temperature increases. When the quenching temperature is constant, impact absorption energy initially increases and then decreases, with the highest value occurring after a holding time of 30 minutes.
4. The optimal quenching process for Q1100E test steel with a thickness of 6-20mm is a quenching temperature of 870-890°C and a holding time of 30-40 minutes.

These research results provide scientific guidance for production practices.