Abstract
In this work, multi-pass compressions were performed at various strain rates (0.01 s−1, 0.1 s−1, 1 s−1, 10 s−1), temperatures (950 °C, 1050 °C, 1150 °C), inter-pass holding time (1 s, 10 s, 30 s, 120 s, 600 s), interrupt strains (0.3, 0.4, 0.5, 0.6), and total pass numbers (1, 2, 3, 4). The intriguing finding was that the recrystallized fraction, average dislocation density, and plastic cumulative strain were partly eliminated during inter-pass holding, resulting in the early occurrence of recrystallization in subsequent compression. Therefore, a parameter (Θ) to evaluate the overall softening fraction due to recrystallization was proposed, and it was then used to iteratively rectify the average dislocation density and plastic cumulative strain in flow-stress modeling. The flow-stress model parameters of 300M steel for multi-pass compression were identified using an optimization technique based on non-derivative method integrated in MATLAB software. The average deviation of calculated and experimental flow-stress was 0.88 MPa (1.35%), showing good accuracy of the flow-stress model. The microstructure evolution of 300M steel was analyzed by the change of softening fraction during multi-pass compression, which provided a useful reference for the research of stress–microstructure relationships of high-strength steels.
Highlights
As a kind of medium carbon low-alloyed steel, the 300M steel is used a lot in the manufacture of large structural parts due to good strength, transverse ductility, fracture toughness, and anti-fatigue property
Tang et al [10] proposed a model to establish the influences of strain, strain rate, and temperature on flow-stress in three-pass compression of 4343/4A60Nb-Ti steel based on the Arrhenius model
The softening fraction in inter-pass holdings were mainly affected by the meta-dynamic recrystallization and static recrystallization kinetics, and the results showed that the first pass strain rate affected the recrystallization kinetics during holding
Summary
As a kind of medium carbon low-alloyed steel, the 300M steel (namely 4340 M) is used a lot in the manufacture of large structural parts due to good strength (σyield ≥ 1850 MPa), transverse ductility, fracture toughness, and anti-fatigue property For these parts, material flow controlling in multi-pass hot compression and precise flow-stress modeling are major concerns. Tang et al [10] proposed a model to establish the influences of strain, strain rate, and temperature on flow-stress in three-pass compression of 4343/4A60Nb-Ti steel based on the Arrhenius model. Established a model to account for the effects of static recrystallization, dynamic recrystallization, and grain size in multi-pass compressions of various metals. The existing model of 300M steel does not take into account the average dislocation density evolution, so it is facing difficulty in precise flow-stress prediction in multi-pass compression. The flow-stress–microstructure relationship was revealed, which was of critical importance for the material flow control of 300M steel forging
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