Mechanical behavior of ultrafine-grained eutectoid steel containing Nano-cementite particles

Abstract

A eutectoid steel with an ultrafine-grained ferrite (α) + submicron/Nano-cementite particle (θ) structure was formed by combining warm deformation of martensite to a strain of 0.36 at 0.1 s⁻¹ at 500 °C with subsequent annealing at 500 °C for 6 h. The characteristics of the microstructure were investigated by means of a scanning electronic microscope and transmission electron microscope, and the corresponding mechanical behavior was analyzed in comparison with that of the eutectoid steel with a typical ultrafine-grained α + θ structure. The results show that both ferrite matrix and cementite particles of the ultrafine-grained α + submicron/Nano-θ steel are finer than that of the ultrafine-grained α + θ steel, i.e., the average size of approximately 0.54 µm vs. 1.0 µm and 0.20 µm vs. 0.56 µm, accompanying with a continuous yielding and a discontinuous yielding, respectively. The yield strength of the ultrafine-grained α + submicron/Nano-θ steel is 264 MPa higher that of the ultrafine-grained α + θ steel, i.e., 884 MPa vs. 620 MPa, resulting from the enhancement caused by refined ferrite grains and cementite particles. The intragranular cementite particles within the ultrafine-grained α + submicron/Nano-θ steel are in Nano-scale, i.e., an average size of approximately 60 nm, resulting in plenty of geometrically necessary dislocations (GNDs) to bring its work-hardening rate higher than that of the ultrafine-grained α + θ steel during uniform deformation. As a result, the stress increments caused by work-hardening are 89 MPa and 155 MPa for the ultrafine-grained α + θ steel and the ultrafine-grained α + submicron/Nano-θ steel, respectively, and their interval length of uniform strain range are nearly equal, i.e., a true strain of approximately 0.08. The work-hardening rate of the ultrafine-grained α + θ steel decline continuously with the increase of tensile strain during uniform deformation. However, the ultrafine-grained α + submicron/Nano-θ steel shows that the work-hardening rate decrease rapidly at the initial stage of work-hardening and then raise within a small strain range, then following by a slowly continuous decline to necking, and, namely, there has a peak of work-hardening rate. Furthermore, the work-hardening rate curve were divided into three stages, and the work-hardening behavior of Stage I, II and III were discussed in view of the evolution of dislocation substructures and the analytical model based on the Kocks-Mecking model.

Introduction

The strength and toughness of the single-phase metal can be simultaneously improved by refining grains [1]. However, when the average gain size is refined to less than 1 µm, the density of crystal defects cannot be effectively stored during plastic deformation for a single-phase metal without TRIP (transformation induced plasticity) and TWIP (twin induced plasticity) [2], [3], [4], leading to the deterioration in the ability of work-hardening and uniform plastic deformation [5]. Consequently, the ultrafine- or Nano-grained single-phase metal is limited as structural material. To date some microstructures have been fabricated to moderate the conflict between strength and ductility. For example, a mixed-crystal structure consisting of Nano- and micro-scale grains [6], [7], a gradient structure with grain size from Nano- to micro-scale [8], an ultrafine-grained structure with Nano-scale twins [9], an ultrafine-grained duplex structure with TRIP effect [10] and an ultrafine-grained structure with hard-phase [11], [12], etc.

It has been well confirmed that a variety of ultrafine-grained duplex structures, i.e., a soft ultrafine-grained matrix embedded with hard-particles, can simultaneously improve the work hardening capability and the strength-plasticity synergy for ultrafine-grained metal. For instance, an ultrafine-grained molybdenum alloy (bcc) enhanced with Nano-scale oxide particles [13] and a Nano-grained aluminum alloy (fcc) strengthened by Nano-scale η (MgZn₂) particles [14], etc. In the steel industry, it has been also evidenced that the ultrafine-grained ferrite (bcc-α) matrix + submicron-scale cementite particles (θ-Fe₃C) microstructure, i.e., ultrafine-grained α + θ structure, can effectively store dislocations density to improve the work-hardening capability and increase the strength-plasticity level for ultrafine-grained ferritic steel [15], [16]. Moreover, ultrafine-grained α + θ steel with relatively inexpensive compositions, showing higher strength and lower ductile-to-brittle transition temperatures (DBTTs) than those for coarse-grained steel, has potential to replace high-strength low-alloyed steels (HSLAs) [1].

Recently, some experimental results exhibit that the dislocations density can be stored more through the refinement of cementite particles, leading to further improve the work-hardening capability and the strength-plasticity synergy [17]. In our previous works, the microstructure consisting of ultrafine- or fine-grained ferrite matrix and cementite particles with a bimodal distribution are prepared by using hot deformation of undercooled austenite and subsequent annealing [17], [18], [19]. The results show that the fine-grained α + θ (average size of 0.22 µm) structure displays better mechanical properties and work-hardening ability than that of the ultrafine-grained α + θ (average size of 0.56 µm) structure, due to the accumulation of more GNDs [17], [19]. Accordingly, ultrafine or fine-grained ferrite matrix embedded with Nano-scale particles maybe exhibit better mechanical properties than that with submicron-scale particles. Currently, there are two main approaches to fabricate ultrafine-grained α + θ steel, i.e., severe plastic deformation (SPD) and advanced thermo-mechanical processing (ATMCP) [20]. The typical characteristics of an ultrafine-grained α + θ structure prepared by SPD and ATMCP are the ultrafine-grained ferrite embedded with the submicron-scale cementite particles and the intergranular cementite particles show larger size and higher volume fraction than that of intragranular ones [12], [21]. However, it is difficult to obtain Nano-scale cementite particles through SPD and ATMCP for plain carbon steel, especially for a high-carbon steel [22], [23], [24].

Generally, many dislocations will generate during the formation of martensite by quenching for carbon steel, providing large amounts of nucleation sites for the precipitation of tiny particles during subsequent tempering. Furthermore, it is possible to form an ultrafine-grained ferrite matrix embedded with Nano-scale cementite particles through the deformation of martensite and subsequent annealing for the high-carbon steel, which can be explained by the introduction of more dislocations as the nucleation sites of cementite particles. Although Nano-cementite particles in grain interior has been achieved in a low-carbon steel by the cold-rolling of martensite and subsequently annealing [25], the high-carbon martensitic steel is difficult to cold deformation. Therefore, in this paper, a thermo-mechanical processing, basing on the warm deformation of martensite and subsequent annealing, was used to form a microstructure containing the ultrafine-grained ferrite matrix and Nano-scale cementite particles for a eutectoid steel, and the corresponding microstructure and mechanical behavior were discussed.