Thermomechanical processing of advanced high strength steels

A model for the microstructure behaviour and strength evolution in lath martensite

An improved X-ray diffraction line profile analysis method is developed to determine dislocation density of lath martensitic steels. This method combines the modified Warren–Averbach (MWA) and the modified Williamson–Hall (MWH) methods. The developed method is robust and leads to unique values for the dislocation density, the effective outer cut-off radius of the dislocations ( R e ) and the dislocations distribution parameter ( M ). Dislocation structures of lath martensite in a steel, in the as-quenched condition as well as in tempered conditions, are characterized by using the proposed method. The calculated dislocation density is compared with the values obtained from the MWH method by considering a constant value for M . It was found that both methods provide dislocation densities in the range of the values calculated from the dislocation strengthening component of the yield strength.

An improved X-ray diffraction analysis method to characterize dislocation density in lath martensitic structures

https://pure.tue.nl/ws/files/13799594/Du_et_al._Scripta_Materialia.Authors_copy_2016.pdf

Block and sub-block boundary strengthening in lath martensite

The microstructure, precipitates and mechanical properties of Nb-V-Ti microalloyed ultra-high strength steel under different tempering temperatures are investigated. With the increasing tempering temperature, the width of martensitic laths gradually increased with reduced dislocations within them, and the volume fraction of retained austenite first increased and then decreased. Furthermore, the Charpy impact toughness, reduction of area and elongation are enhanced at the sacrifice of the strength. Four kinds of precipitates are identified at different tempering temperatures, namely M 3 C, M 2 C, M 23 C 6 and M 7 C 3 . M 3 C precipitated when tempering at 200–400 °C, and it was replaced partially by M 2 C when the temperature is elevated to 500 °C, at which M 23 C 6 also precipitated. For a higher temperature, above 600 °C, M 3 C disappeared, and some M 2 C transformed into M 7 C 3 . The other two types of MC precipitates are found to be the stable phases existing over different tempering temperatures. The correlation between the mechanical properties and microstructure was established, and the precipitation mechanisms of carbides at the different tempering temperatures are explained.

Carbide precipitation in Nb-V-Ti microalloyed ultra-high strength steel during tempering

The microstructures of 25CrMo48V martensitic steel quenched at the different temperatures of 800–1200 °C and tempered at 650 °C were characterized by optical microscope (OM), field emission scanning electron microscopy (FESEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). The tensile and impact properties were evaluated from the as-quenched and tempered specimens. The results show that the refinement of the prior austenite grains due to the decrease of quenching temperature could induce the refinement of packets and blocks, while the lath width remained stable. The yield strength and the 50% fraction appearance transition temperature (50% FATT) varied linearly with the reciprocal square root of the prior austenite grain size, packet size and block width. Their relationships followed the classical formula of Hall–Petch and FATT = A − B d − 1 / 2 , respectively. The prior austenite grain size has a remarkable effect on the strength and toughness, while the block is the minimum structure unit controlling strength and toughness.

Effect of martensitic morphology on mechanical properties of an as-quenched and tempered 25CrMo48V steel

Effect of microstructure on the strength of 25CrMo48V martensitic steel tempered at different temperature and time

The invention discloses a steel with high plasticity, high toughness and high strength and a manufacturing method thereof. The chemical compositions of the steel are as follows in terms of weigh percentage: 0.15 to 0.30 of C, 0.20 to 1.40 of Mn, 0.10 to 0.50 of Si, less than or equal to 0.015 of P, less than or equal to 0.012 of S, less than or equal to 0.20 of Ni, 0.40 to 1.50 of Cr, 0.25 to 1.35 of Mo, 0 to 0.10 of V, less than or equal to 0.2 of Cu, 0.01 to 0.05 of Al, 0.0005 to 0.0020 of B, less than or equal to 0.010 of (O+N) and the balance of Fe and impurities. The manufacturing method comprises the following steps: smelting according to the formula, continuous casting into a blank, hot rolling into plates or tubes, quenching at the temperature of 875 to 930 DEG C and holding the temperature for 25 to 40 minutes, tempering at the temperature of 625 to 675 DEG C for 30 to 90 minutes, then air cooling, and quenching and tempering heat treatment. The steel has yield strength of more than or equal to 1000MPa, tensile strength of more than or equal to 1100MPa, zero-degree-centigrade charpy impact work of more than or equal to 100J, specific elongation of 20 to 30 percent and is applicable to manufacture important structures of engineering machinery, high buildings and oil well pipes, and the like.

Juanxia Ren

Papers

Effect of martensitic morphology on mechanical properties of an as-quenched and tempered 25CrMo48V steel

Effect of microstructure on the strength of 25CrMo48V martensitic steel tempered at different temperature and time

High plasticity, high tenacity and ultra-high tensile steel, and manufacturing method thereof