To deal with the problem of poor yield strength and hardness in the initial use of high-manganese austenitic steel, we investigated the alloying design, microstructure, precipitates, mechanical properties, and comprehensive strengthening mechanism of high-manganese austenitic steel through two novel heat-treatment processes, namely continuous heating process (CHP) and segmented heat preservation process (SHPP). In this work, austenitic Fe-0.9C-17Mn-0.8Si-2.0Cr-0.3Ni-0.5Cu-0.7Mo steels alloyed with Ti, V, and Nb were designed. The grain size of SHPP steels was smaller than that of CHP steels due to the smaller size of precipitates. The results of mechanical experiments showed that the yield strength and impact toughness of SHPP steel were obviously higher than those of CHP steel, but the Brinell hardness of CHP steel was higher than that of SHPP steel. The higher Brinell hardness and poorer impact toughness of CHP steel were mainly due to the larger-sized precipitates. Finally, solid-solution strengthening played the most effective role of increasing the yield and tensile strengths of the two steels.

Influence of Heat-Treatment on Enhancement of Yield Strength and Hardness by Ti-V-Nb Alloying in High-Manganese Austenitic Steel

Alloyed high manganese steel is a general type of wear-resistant steel, and its precipitates and austenite matrix grain sizes play an important role in impact abrasive wear behavior. Solid-solution treatment is an effective method to balance the size of the austenite grains and precipitates. This work attempts to find a new progressive solid-solution treatment to make alloyed high manganese steel castings refine the microstructure and balance the wear resistance under impact abrasive wear conditions. Compared with traditional solid-solution treatment, the MC- and M23C6-type carbide precipitates resulting from progressive solid-solution treatment decrease, and the tensile strength and impact toughness of steel are significantly increased. After the solid-solution treatment at different heating rates, the grain refinement effect is remarkable after shortening the holding time at 1100 °C and increasing the heating rate to 150 °C h−1. The impact abrasive wear test results show that, compared with the traditional solid-solution treatment at 1100 °C for 4 h, the wear resistance of high manganese steel has a 23.1% improvement after progressive solution treatment at 1100 °C for 2 h, and the wear failure forms of all the experimental steels are microcutting, oxidation wear and plastic deformation.

Effect of progressive solid-solution treatment on microstructures, mechanical properties and impact abrasive wear behavior of alloyed high manganese steel

In this paper, in order to study the effect of oxides on the formation of complex TiN inclusions in 20CrMnTi steel, based on the two end quenching specimens with different components, the characteristics of complex TiN inclusions and oxides were observed and analyzed, and the formation mechanism of complex TiN inclusions was discussed. The results indicate the compositions of core oxides are regularly distributed in the MgAl 2 O 4 –TiO x composition line for different complex TiN inclusions, and the difference is the TiO x content. The complex TiN inclusions can be divided into two types, Type 1 and Type 2, based on the compositions of core oxides. For Type 1, TiO x content of core oxides are below 52%, while for Type 2, TiO x content of core oxides are over 52%. The core oxides of Type 1 have bigger size at the range of 1–3 μ m, and lower nucleation capability for TiN inclusions. The core oxides of Type 2 have smaller size at the range of 0.5–2 μ m, and higher nucleation capability for TiN inclusions. For Type 1, the MgAl 2 O 4 –TiO x oxides preexisting in molten steel are the heterogeneous nucleation cores of TiN inclusions during solidification. For Type 2, the tiny oxides newly formed during solidification are the nucleation cores of TiN inclusions. By controlling the characteristics of oxides, the characteristics of TiN inclusions can be con-trolled, which provides a new idea for the control of TiN inclusions in 20CrMnTi gear steel.

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Characteristics and Formation Mechanism of Complex TiN Inclusions in 20CrMnTi Gear Steel

ABSTRACT To study the effect of TiN precipitated particles on two end-quenching specimens of 20CrMnTi steel with different hardenability characteristics, the peripheral hardness of the samples is tested. The microstructures, austenite grain structures, characteristics of TiN particles are analyzed. The larger the grain size, the higher the martensite fraction and the higher the hardness. More uniform and more refined grains reduce the maximum peripheral hardness difference of end-quenched samples from 9.1HRC to 3.3HRC. The precipitation of as much fine TiN as possible in the solid phase plays an important role in obtaining more refined and more uniform austenite grains. TiN precipitates less during solidification and more in the solid phase in the sample with better hardenability, promoting grain refinement and more uniform peripheral hardness. In production, with a specific mass fraction of Ti, effectively inhibiting the precipitation of coarse TiN during solidification is the key to control narrow hardenability bandwidth for 20CrMnTi gear steel.

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

The Ti element plays a role in pinning grain boundaries but also has a good binding ability to C and N, forming large primary carbides Therefore, the effect of Ti content on primary carbides’ behavior in H13 ingots was comprehensively studied A non-aqueous electrolysis method was used to determine the three-dimensional (3D) characteristics of primary carbides We found a great difference between the two-dimensional (2D) and the three-dimensional characteristics of primary carbides When performing 2D analyses, the density of the primary carbides appeared high, while their size was small The actual characteristics of primary carbides can be obtained only by 3D observation The primary carbide showed a typical dendritic structure, whose center consisted of Ti–V-rich carbide wrapped by V-rich carbide As the Ti content increased, the size of the primary carbide increased from 249 μm to 413 μm, and the number density increases from 256 per/mm2 to 439 per/mm2 The Ti4C2S2 phase precipitated first, then changed into Ti–V-rich carbide, and finally further partly transformed into V-rich carbide The addition of elemental Ti promoted the precipitation and transformation of primary carbides, resulting in an increase of the number density and size

Effect of Ti Content on the Behavior of Primary Carbides in H13 Ingots

Simultaneously improving the toughness and strength of B-microalloyed steel by adding microalloying elements (Nb, V, Ti) has been an extensively usedmethod for researchers. However, coarse Ti(C, N) particle will precipitate during solidification with inappropriate Ti content addition, resulting in poor impact toughness. The effect of the size, number density, and location of Ti(C, N) particle on the impact toughness of B-microalloyed steel with various Ti/N ratios was investigated. Coarse Ti(C, N) particles were investigated to act as the cleavage fracture initiation sites, by using scanning electron microscopy (SEM) analysis. When more coarse Ti(C, N) inclusions were located in ferrite instead of pearlite, the impact toughness of steel with ferrite–pearlite microstructure was lower. Meanwhile, when the size or the number density of Ti(C, N) inclusions was larger, the impact toughness was adversely affected. Normalizing treatment helps to improve the impact property of B-microalloyed steel, owing to the location of Ti(C, N) particles being partly changed from ferrite to pearlite. The formation mechanism of coarse Ti(C, N) particles was calculated by the thermodynamic software Factsage 7.1 and Thermo-Calc. The Ti(C, N) particles formed during the solidification of molten steel, and the N-rich Ti(C, N) phase precipitated first and, then, followed by the C-rich Ti(C, N) phase. Decreasing the Ti and N content is an effective way to inhibit the formation of coarse Ti(C, N) inclusions.

Guo-Guang Cheng

Papers

Effect of Ti(C, N) Particle on the Impact Toughness of B-Microalloyed Steel