Materials Science Forum Vols. 783-786

Abstract: Heavy deformation of metastable austenite (below A_e3) or both austenite and ferrite in the two-phase region (between A_r3 and A_r1) has been found to develop ultra-fine ferrite grain structures with average grain sizes less than 3 μm. The sequence of different dynamic softening mechanisms involved in the grain refinement during heavy intercritical deformation, such as, dynamic recovery, dynamic recrystallization, and dynamic strain induced austenite→ferrite transformation, has been analyzed by considering strain partitioning between austenite and ferrite. Grain refinement is expected to be dictated by dynamic strain induced transformation (DSIT) at higher deformation temperatures (>1100°C) and pronounced dynamic recovery of ferrite at lower deformation temperatures (<1100°C). Evolution of crystallographic texture was dependent on the grain refinement mechanism and gamma fiber components (ND//<111>) and alpha fiber components (RD//<110>) dominated the texture at higher and lower deformation temperatures, respectively.

Abstract: The microstructure evolution of martensitic Ti-6Al-4V alloy was investigated through uniaxial hot compression at 700°C and a strain rate of 10^-3 s^-1. A combination of scanning electron microscopy observation in conjunction with high resolution electron back scattered diffraction (EBSD) was used to characterize the microstructure in detail. The development of the microstructure displayed continuous fragmentation of martensitic laths with increasing strain (i.e. continuous dynamic recrystallization), concurrently with decomposition of supersaturated martensite resulting in the formation of equiaxed grains. At a strain of 0.8, an ultrafine equiaxed grained structure with mostly high angle grain boundaries was successfully obtained. The current work proposes a novel approach to produce equiaxed ultrafine grains in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure.

Abstract: A low-carbon, titanium and niobium (Ti-Nb) bearing and a low-carbon titanium, niobium and copper (Ti-Nb-Cu) bearing ultra high strength steel have been thermo-mechanically processed on a laboratory scale unit. Evolution of microstructure and mechanical properties of the above air cooled steels have been studied at different finish rolling temperatures (FRTs). Microstructural characterization reveals largely a mixture of granular bainite and bainitic ferrite along with the precipitation of microalloying carbide/carbonitride particles and/or Cu-rich precipitates. (Ti-Nb) bearing steel yields higher yield strength (1114-1143 MPa) along with higher tensile strength (1591-1688 MPa) and moderate ductility (12-13%) as compared to (Ti-Nb-Cu) bearing steel having yield strength (934-996 MPa) combined with tensile strength (1434-1464 MPa) and similar ductility (13%) for the selected range of 850-750°C FRT. Due to higher strength-ductility combinations, these present investigated steels can be regarded as the replacement material for ballistic applications as well as other sectors like defense, pipeline, cars, pressure vessels, ships, offshore platforms, aircraft undercarriages and rocket motor casings etc. Key words: Thermo-mechanical controlled processing, ultra high strength steel, microstructure, mechanical properties.

Abstract: The effects of heat treatment on distortion, residual stress, and retained austenite were compared for case-carburized 4320 steel, in both the austempered and quench-and-tempered condition. Navy C-ring samples were used to quantify both size and shape distortions, as well as residual stress. The austempering heat treatment produced less distortion and a higher surface residual stress. Both hoop and axial stresses were measured; the difference between them was less than seven percent in all cases. Depth profiles were obtained for residual stress and retained austenite from representative C-ring samples for the austempered and quench-and-tempered heat treatment conditions. Austempering maintained a compressive residual stress to greater depths than quench-and-tempering. Quench-and-tempering also resulted in lower retained austenite amounts immediately beneath the surface. However, for both heat treatments, the retained austenite content was approximately one percent at depths greater than 0.5 mm.

Abstract: A novel concept of hot shortness control on copper containing weathering steels is proposed. The weathering effect of copper addition in steels and its improvement in the mechanical and impact properties by age hardening are well known. Again, it is also very well know the problem of cracks on hot processing of those steels, caused by liquid copper segregation, the “hot shortness”. Hot shortness is the main problem of commercial producing of copper containing steels because of the increase of cost production caused by cracks the Ni addition to prevent them. Recent research found that copper segregates surrounding MnS inclusions in the steel, worsening the copper segregation caused by the preferential oxidation of iron on the hot processing of copper containing steels, which is the traditional cause of the “hot shortness”. The same results also show that copper additionally precipitates as CuS in the bulk of the steel. This work points that the “hot shortness” can then be prevented by reducing the Mn in the steel, rather than by the expensive Ni addition. Reducing Mn means reduction of inclusions of MnS sites for copper segregation in the steel. Residual sulphur can therefore be trapped by the copper as finer CuS precipitates, so copper is then dissolved in the bulk of the steel rather than segregated. This paper introduces an alternative solution for the problem of the “hot shortness” by partially or fully replacing the Mn by Cu in those steels. It is then proposed a novel concept of low Mn, or Mn free, Cu containing weathering steels in which Cu replaces Mn in trapping the S. That new steel opens the possibility to a cheaper and easier controls of the “hot shortness” so the copper contamination from scraps become a technological benefit because of its age hardening and weathering effect on steels. The recycling of highly copper contaminated scraps is another important environmental advantage. The conclusion of this work is that the current results introduces a new technology of cheaper copper containing low Mn, or Mn free, high strength, environmentally-friendly weathering steels with high potential market for different welded on shore and offshore specifications.

Abstract: Bainite or the mixture of bainite and martensite is required to reach high strength levels in low carbon high strength steel. However, the bainite reaction rarely goes to completion, resulting in mixed structures of predominately bainitic ferrite and minor amounts of retained austenite, cementite or martensite mainly located at the ferrite grain boundaries. The exact nature of this minor transformation product depends on several factors including bulk composition, segregation and cooling rate. When the minor phase is largely martensite, the non-bainitic microstructure is called martensite-austenite microconstituent or MA. Interestingly, MA is believed to be one of the main factors causing the deterioration of toughness of steels. MA is also often associated with hydrogen-related cracking. In this current study, the formation of martensite-austenite constituents was studied experimentally and the results analyzed theoretically.

Abstract: The automotive TWIP steels are high-Mn austenitic steels, with a relevant C content, which exhibit a promising combination of strength and toughness, arising from the ductile austenitic structure, which is strengthened by C, and from the TWIP (TWinning Induced Plasticity) effect. The microstructure of the low-alloy Q&P steels consists of martensite and austenite and is obtained by the Quenching and Partitioning (Q&P) heat treatment, which consists of: austenitizing; quenching to the T_q temperature, comprised between M_s and M_f; soaking at the T_p partitioning temperature (T_p being equal to or slightly higher than T_q) to allow carbon to diffuse from martensite to austenite; and quenching to room temperature. The fatigue behavior of these steels is examined both in the as-fabricated condition and after pre-straining and welding operations, which are representative of the cold forming and assembling operations performed to fabricate the car-bodies. Moreover, the microscopic fracture mechanisms are assessed by means of fractographic examinations.

Abstract: Aiming to meet new market demands for flat steel, steel companies have been developing new products with special features that allow various adjustments to your application. Our paper is a new product, the superfine steels, its applications, development feasibility and economic aspects. The market for steel furniture for domestic purposes has a great demand for cold-rolled steels, distributed in thick bands, currently between 0.40 mm and 0.70 mm. This segment showed continuous growth in recent years, like the white line, with special focus on the increased consumption presented by the classes C and D, driven by the reduction in Excise Tax (IPI), granted by the federal government in 2009. The proposal is to produce cold-rolled steel with thickness reaching up to 0.25 mm, maintaining the function of the final component to quality and stiffness required. Thinning is a thick white line trend of the world, due to the direct impact on reducing the final cost of the piece, reflecting the competitiveness of customers across the market, and there is a clear need to develop new applications for these materials, not only to the white line, as well as for the mobile industry and metal packaging. Keywords : Cold Rolled Superfine Steel , reducing thick white line

Abstract: This study aimed to investigate how the heat treatment can influence the strain values ​​of an IF steel observing the different values ​​of the tensile, rupture and drain that were recorded during the tensile test. We analyzed five samples of the specimen (CP) using different combinations of heat treatments to enable evaluation of the behavior of material deformation by tensile test. The evaluation was performed using the deformation calculation of the ratio between the axial and radial deformations (anisotropy parameter). The results indicate how different types of thermal treatments influenced in their initial properties, generating materials with different characteristics.

Abstract: The Visco-Plastic (VPSC) and Elasto-Plastic (EPSC) Self-Consistent models were applied to simulate the macroscopic stress-strain response and the evolution of crystallographic texture during the tensile loading of a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C TWIP steel. Bulk texture measurements acquired by X-ray diffraction were used to validate the modelling results. Various modelling constructs were employed to assess the contribution of twinning and latent hardening to the texture evolution. The simulations revealed the dominant role of perfect slip and the limited effect of twinning on texture development as well as a possible role for latent hardening. Comparison between the texture predictions of the VPSC and EPSC models underscored the correlation between the latent hardening effects and the employed grain interaction scheme.