Effect of Quenching Process on Microstructure and Properties of 1100 MPa Ultra High Strength Steel

1. Preface

With the development of large-scale and light-weight construction machinery, the market demands higher and higher performance of steel for construction machinery, and promotes the development of steel for construction machinery towards ultra-high strength and high toughness.

Before 2019, the high-strength steel with yield strength of 690mpa and below for domestic construction machinery was mainly made, while the steel with yield strength of 960mpa and above was mainly imported.

The domestic steel of this grade had prominent problems in product quality, specification and supply capacity, which seriously restricted the development of 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 shall meet the requirement that the center segregation class C ≤ 1.0, class A and class B are not allowed, and there are no internal cracks and shrinkage cavities.

For unqualified low magnification samples, steel grade shall be changed for this heat, and low magnification samples shall be taken for the previous and subsequent heat.

3. Mechanical properties under different quenching processes

Different quenching processes are adopted for 1100MPa ultra-high strength construction machinery steel (Q100E for short), that is, the mechanical properties of Q100E test steel are tested and sampled under different quenching temperatures and furnace time.

See Table 2 for the mechanical properties (yield strength, tensile strength and – 40 ℃ impact absorption energy) under 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

It can be seen from Fig. 1 that the yield strength gradually decreases with the increase of quenching temperature, and the yield strength is the lowest when the quenching temperature is 930 ℃.

With the increase of holding time, the yield strength increases first and then decreases.

When the holding time is 30 ~ 40min, the yield strength basically reaches the highest value;

It is obvious that the yield strength appears the lowest value when the quenching temperature is 930 ℃ for 50min.

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

It can be seen from Fig. 3 that with the increase of quenching temperature and holding time, the low-temperature impact absorption energy of Q100E steel increases first and then decreases;

In the same holding time, the impact absorption energy decreases with the increase of quenching temperature;

When the quenching temperature is the same, the impact absorption energy increases first and then decreases, and the impact absorption energy is the highest when the quenching temperature is kept for 30min.

Considering the strength and toughness, the best quenching process of Q1100E test steel with thickness of 6-20mm is 870-890 ℃ quenching and holding time of 30-40min.

4. Microstructure under different quenching processes

After quenching at 870 ～ 930 ℃, the test steel q100e is lath martensite.

Under different quenching temperature and time, the scanning electron microscope microstructure of Q1100E test steel is shown in Fig. 4.

Fig. 4 SEM microstructure of different quenching processes

It can be seen from Fig. 4 that when quenching at 870 ℃, most of the martensite blocks in the grains are not obvious due to the small grains.

With the increase of quenching temperature, the martensite blocks with different orientations are more clearly distinguished.

The effect of quenching time on microstructure is similar to that of temperature. With the extension of quenching time, the grain size increases.

The effect of different quenching processes on the grain size of original austenite is shown in Fig. 5.

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

It can be seen from Fig. 5 that the original austenite grain size increases from 5.7μm to 15.9μm as the quenching temperature increases from 870 ℃ to 930 ℃;

When the quenching temperature is 870 ℃, the quenching time is extended from 40min to 80min, and the original austenite grain size is increased from 4.5μm to 6.5μm.

Therefore, the quenching temperature will affect the austenitizing of the material and the solid solution degree of the alloy elements.

According to the CCT test, the Ac3 of the test steel is 852 ℃.

Since the minimum quenching temperature of the test steel plate is 870 ℃, the complete austenitizing can be realized.

However, the temperature has a great influence on the solid solution of microalloying elements. When the quenching temperature is low, carbides such as Nb and V or carbonitrides of microalloying elements pin the original austenite grain boundary and hinder the grain growth.

When the quenching temperature is increased, the carbide or carbonitride of the alloy element is dissolved in austenite, and its effect of inhibiting grain growth is weakened, and the grain size is doubled.

With the extension of quenching heating time, the amount of solid solution of alloy elements increases gradually, which also leads to grain growth.

In comparison, the grain size is more sensitive to quenching temperature.

5. Conclusion

Through the comparative analysis of the microstructure and physical properties under different quenching temperatures and quenching times, the following conclusions are reached.

1) With the increase of quenching temperature, the yield strength of Q110E test steel decreases gradually, and the yield strength is the lowest at 930 ℃.

At the same quenching temperature, with the increase of holding time, the yield strength increases first and then decreases.

The holding time is 30-40min, and the yield strength basically reaches the highest value.

When holding at 930 ℃ for 50min, the yield strength is the lowest.

2) The tensile strength of Q1100E test steel fluctuates slightly with the change of quenching temperature and time.

3) With the increase of quenching temperature and holding time, the low-temperature impact property of Q110E test steel increases first and then decreases;

In the same holding time, the impact absorption energy decreases obviously with the increase of quenching temperature;

When the quenching temperature is the same, the impact absorption energy increases first and then decreases, and the impact absorption energy is the highest when the quenching temperature is kept for 30min.

4) The optimum quenching process of Q1100E test steel with thickness of 6-20mm is 870-890 ℃ quenching and holding time of 30-40min.

The research results can scientifically guide the production practice.