Influences of Thermomechanical Treatment and Nb Micro-alloying on the Hardenability of Ultra-High Strength Steels

Abstract

For the construction of mobile crane booms, ultra-high strength steels produced via thermomechanical processing (TMP) have widely substituted steels fabricated through the conventional quenching and tempering (Q+T) route. A strong deformation of the austenite grain during hot rolling followed by direct quenching (DQ) offers benefits in terms of strength and toughness. To guarantee an optimal through-hardening, alloying elements retarding the c to a transformation are used. To explore the influence of the processing route on the critical cooling rate and the hardenability, hot deformation tests were performed on a deformation dilatometer. Different cooling rates were applied after deformation corresponding to two different rolling cycles with varying finish rolling temperatures (FRTs). The obtained hardness values were compared to those received through conventional quenching after austenitization. These investigations conducted on three steels with varying micro-alloying contents showed that Nb in combination with TMP raises strength significantly, and promotes a bainitic and ferritic transformation in solid solution. When applying low FRTs and in combination with other micro-alloying elements, NbC coarsens and reduces the effect of precipitation hardening.

The effect of TiN precipitated particles on the austenite grain and hardenability of 20CrMnTi gear steel

“…Combining Figures 8 and 9, it is found that the larger grain size of the test surface promotes the formation of martensite in the quenching process, resulting in a larger martensite fraction and higher hardness. Ahmed et al [30,31] also have reported that more refined grains are not conducive to martensite transformation, consistent with our finding. The grain size of sample A is large, the grain size distribution of each test surface is not uniform, so the microstructure of each test surface changes a lot, and the hardness fluctuates greatly, which is not conducive to the control of narrow hardenability bandwidth.…”