high-C Steel Research Articles

Austenite formation from a steel microstructure containing martensite/austenite and bainite bands

In many commercial steel processing routes, steel microstructures are reverted to an austenitic condition prior to the final processing steps. Understanding the microstructure development during austenitization is crucial for improving the performance and reliability of the microstructure that forms from austenite. In this work, austenite formation in a high-C steel (0.85 wt%) from a microstructure containing martensite/austenite and bainite bands is investigated. It is shown that austenite formation from bainite results in a refined austenite grain structure, and the martensite matrix thus obtained on quenching has a homogeneous distribution of carbides with a relatively low fraction of retained austenite (24%). On the other hand, a coarser austenite microstructure is obtained when austenite forms from a mixture of martensite and retained austenite. The reason for the coarse austenite grains is argued to be a memory effect, which is substantiated by in situ X-ray diffraction analysis. After quenching, an inhomogeneous carbide distribution and a higher retained austenite fraction (30%) are observed in the regions that were initially martensite/austenite. The global microstructure, hence, has a bimodal size distribution of prior austenite grains and carbide-dense bands. The causes for these heterogeneities are discussed with the help of interrupted quench experiments, equilibrium phase calculations, and DICTRA simulations.

From Waste to Multi-Hybrid Layering of High Carbon Steel to Improve Corrosion Resistance: An In-Depth Analysis Using EPMA and AFM Techniques

Corrosion resistance of steel has attracted substantial interest for manufacturing applications to reduce costs corresponding to part failures, unexpected maintenance, and shortening lifespan. Meanwhile, millions of tonnes of slag, non-recyclable glass, and automotive shredder residue (ASR) are discarded into landfills every year, polluting the environment. Combining these two major issues, we delivered an alternative solution to enhance corrosion resistance of high-C steel. In this research, utilisation of these wastes (which were chemically bonded into steel substrate) as sources for production of multi-hybrid layering—including the multi-phase ceramic layer, the carbide layer, and the selective diffusion layer—was successfully achieved by single step surface modification technology. High-resolution topographical imaging by SEM and chemical composition analysis in micron-volume by electron probe micro analyser (EPMA) were performed. Nano-characterisation by atomic force microscopy (AFM) using the PeakForce quantitative nanomechanical mapping (PF-QNM) method was conducted to define Young’s modulus value of each phase in detail. Results revealed improvement of corrosion resistance by 39% and a significantly increased hardness of 13.58 GPa. This integrated approach is prominent for economic and environmental sustainability, consolidating industry demands for more profits, producing durable, steel components in a cost effective way to reduce dependency on new resources, and minimising negative impacts to the environment from disposal of wastes to the landfills.

Carbon Redistribution in Martensite in High-C Steel: Atomic-Scale Characterization and Modelling

The microstructure of the as-quenched plate martensite in a high-C steel 100Cr6 was characterized by means of electron microscopy and atom probe tomography. The carbon redistribution behavior was investigated at the atomic scale, which revealed the nature of the transformation dynamics influenced by carbon and other substitutional alloying elements. A model was proposed to predict the carbon redistribution at twins and dislocations in martensite, which was based on their spatial arrangements.

Effects of Carbon Variation on Microstructure Evolution in Weld Heat-Affected Zone of Nb-Ti Microalloyed Steels

We investigated the effects of C concentration variation from 0.028 to 0.058 wt pct on microstructure of the coarse grained heat-affected zone (CGHAZ) of low heat input girth welded Ti-Nb microalloyed steels by using electron microscope and atom probe tomography. It is found that the CGHAZ microstructure exhibits a systematic response to C variation. Increased C raises the temperature for precipitation of NbC. This leads to coarser (Ti, Nb)N-Nb(C, N) but finer delayed strain-induced NbC in the high-C steel than in the low-C steel. Fine strain-induced NbC are ineffective in preventing austenite grain coarsening in CGHAZ due to their fast dissolution upon heating. For a given inter-particle spacing originally determined by (Ti, Nb)N particles, increased epitaxial growth of Nb(C, N) on pre-existing (Ti, Nb)N in the high-C steel results in a smaller austenite grain size of 34 µm in the CGHAZ of the high-C steel than that of 52 µm in the low-C steel. Increased C promotes a microstructure consisting of bainitic lath structure with C Cottrell atmospheres at dislocation debris and martensitic layers of 30 to 100 nm in thickness at inter-lath boundaries in the CGHAZ. Increased C promotes configuration of crystallographic variants belonging to different Bain groups in the neighbors, preferentially twin-related variant pairs within a bainite packet.

Rolling Contact Fatigue Performances of Carburized and High-C Nanostructured Bainitic Steels.

In the present work, the nanostructured bainitic microstructures were obtained at the surfaces of a carburized steel and a high-C steel. The rolling contact fatigue (RCF) performances of the two alloy steels with the same volume fraction of undissolved carbide were studied under lubrication. Results show that the RCF life of the carburized nanostructured bainitic steel is superior to that of the high-C nanostructured bainitic steel in spite of the chemical composition, phase constituent, plate thickness of bainitic ferrite, hardness, and residual compressive stress value of the contact surfaces of the two steels under roughly similar conditions. The excellent RCF performance of the carburized nanostructured bainitic steel is mainly attributed to the following reasons: finer carbide dispersion distribution in the top surface, the higher residual compressive stress values in the carburized layer, the deeper residual compressive stress layer, the higher work hardening ability, the larger amount of retained austenite transforming into martensite at the surface and the more stable untransformed retained austenite left in the top surface of the steel.

Comparative Impact Behavior of High-C Steel After Conventional Quenching, Tempering, and Austempering

Abstract Usually bainitic microstructures exhibit good toughness and austempering is typically the preferred heat treatment when toughness is the primary requirement of the component. Several reports have shown such characteristics when compared to tempered martensite. High-carbon steel may exhibit brittle characteristics but it is a good steel with respect to mechanical properties and wear resistance. The objective of this study was to compare the impact properties of AISI O1, a high-carbon tool steel, designated VND in Brazil. This was done by comparing Charpy impact strength under different heat-treatment cycles. Steel test specimens were quenched from 820°C and tempered at 450°C to obtain tempered martensite, then austenitized, cooled, and held at 350°C to obtain bainite by holding at temperature for 20, 40, and 60 min. Because hardness influences impact behavior, comparative studies were performed at the same surface hardness level. The austempered samples with bainite microstructures obtained at a constant temperature and, by varying holding times, exhibited lower impact properties as compared to the quenched and tempered condition.

Spatial Aspects of Martensite

This work re-examines the martensite transformation in FeNiC, FeNiMn, and high-C steels using the microstructural path concept adapted to the uniqueness of martensite. The analysis of the microstructural path and the developed formalisms support the view that strain accommodation underlies the development of the spatial aspects of the transformation in the alloys considered. The partitioning of the austenite grains and the martensite burst are entailed by the elastic propagation of the midribs. In addition, we found agreement of the empirical equations used in previous works and the microstructural path models now described.

Mechanical and damage analysis along a flat-rolled wire cold forming schedule

Numerical simulation is used to study patented high-C steel flat-rolled wire cold forming processes. An elasto-plastic power law, identified from mechanical tests, is used into Forge2005® finite element (FEM) package in order to describe the material behaviour during wire drawing followed by cold rolling. A through-process approach has been favoured, transferring residual wire-drawing stresses and strain into the flat-rolling preform. This mechanical analysis, associated with a triaxiality study, points to dangerous areas where fracture may initiate due to high tensile stresses. Lemaitre’s isotropic damage criterion, including crack closure effect, a -1/3 cut-off value of stress triaxiality, and tension/compression damage asymmetry, has been used and has confirmed the previous analysis. A number of non-coalesced voids nucleated on inclusions have been observed in the Scanning Electron Microscopy (SEM), especially in high-deformation zones (“blacksmith’s cross”). Their evolution has been simulated in the FEM model using spherical numerical markers, which deform into oblate or prolate ellipsoids. The deformation-induced morphological evolution of voids observed in the SEM compares well with the geometrical evolution of the markers, which suggests that the morphologies observed do not result from micro-crack propagation, but from material transport of the nucleated voids.

Effect of Carbon, Nitrogen and Nickel on Long-term Creep Rupture Strength of 10 Cr Steels Containing Boron

Microstructure and precipitates of creep ruptured specimens were investigated in order to understand the effects of carbon (C), nitrogen (N) and nickel (Ni) on long-term creep rupture strength of 10Cr heat-resistant steels containing boron (B). The low-N steels showed higher creep rupture strength than the high-N steels. In long-term creep rupture region such as over 10,000 h at 650°C, the deterioration of creep rupture strength was not observed in the low-N, high-C steel. On the other hand, the creep rupture strength of the low-N, low-C steel dropped to the strength level of the high-N steels. The addition of N to the B containing steels promoted the recovery of microstructure. The formation of coarse BN in the high-N steels led to the decrease of the amount of effective B that dissolved in M23(C,B)6 and suppressed its coarsening. From EDS analysis of precipitates, the fraction of M23C6 in the low-C steels was less than that in the high-C steels, while the fraction of coarse Laves phase in the low-C steels was more than that in the high-C steels. In the low-N, low-C steel, the coarsening of precipitates caused the deterioration of the creep rupture strength after 10,000 h exposure. Ni lowered Ac1 and Ac3 transition temperature, but it did not affect the fraction of precipitates according to the calculation of thermodynamic equilibrium using Thermo-Calc. It is concluded that C and N are more effective to the stability of microstructure than Ni.

Effect of phosphorus on the microstructure and mechanical properties of strip cast carbon steel

Carbon steel strips with different phosphorus and carbon contents were produced by using the twin roll strip casting process. Fine grains and dendrite structure were observed in high-P steels. Negative phosphorus segregation was found in strip cast high-P and high-C steels. For the steels with different carbon contents, phosphorus distribution in the thickness direction of the strip is obviously different. This is because solutes are redistributed in the melting pool and the phosphorus segregation rate is associated with the length of the mushy zone and the phosphorus solubility in different phases. Phosphorus as a solute in ferrite can lead to the higher hardness and strength with lower plastic propertyies.

Study on Wear Phenomena of Tundish Working Lining by Slags of Billet Caster

Total number of heats taken in tundish refractory working linings (MgO-based) varies with three steel grades produced in the Long Product Complex at Tata Steel. The duration of the sequence heats (of 60-70 min) for low-C, med-C and high-C steel grades made at billet caster are 14-16, 16-18 and 22-24 h respectively. These differences are due to different rates in erosion and corrosion of the three types of tundish slags arising in three different steel grades and different liquidus temperatures of three steel grades. The tundish slags have different chemical compositions, liquidus temperatures and viscosity. An attempt has been made in the present work to understand the wear mechanisms of MgO-based refractory linings by different tundish slags in the three steel grades. Chemical analysis, optical and scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) have been used to characterize the slag infiltrated MgO-based tundish lining samples collected from the plant. The present work has been undertaken to find out the wear mechanism by different tundish slags, take corrective measures with regard to slag chemistry and MgO lining material to minimize the erosion and corrosion, and enhance the continuous casting durations.

Some aspects of thermal analysis and technology upgrading in steel continuous casting

This paper is focused on several aspects of steel continuous casting, pursuing an aim to develop a general approach to technology optimization for securing internal quality of cast steel in small-section strand casting. The study was based on a computer simulation of steel solidification in a caster, utilizing experiment-based verification of computer modeling. Problems discussed include the computation of the shrinkage/contraction of steel, and the development of optimized operation for the final electromagnetic stirring of melt, as well as the soft reduction of cast strand at the final stage of solidification. Development of optimal operational patterns for bloom casting of high-C steel at the China Steel Corporation exemplifies the approach discussed.

Influence of some casting parameters on the behaviour of high-carbon steels for tires during drawing operations

This work is aimed at stressing the relationship between casting parameters, casting rate and secondary cooling rate, and the behaviour of high-C steels during drawing operations.Appropriate combinations of such parameters were tested and compared to drawing results, especially to the kilograms drawn before breaking.

An atom probe FIM study of interphase precipitation in a model alloy steel

Interphase precipitation of vanadium carbide, particularly when associated with the growth of pearlite, has been studied in a model alloy steel. The precipitates, characteristically dispersed non-randomly in rows or sheets in the proeutectoid and pearlitic ferrite regions, provide an important strengthening increment in a wide variety of commercial microalloyed steels. The precipitates form at the γ/α transformation interface during austenite decomposition, but this has proved difficult to examine in detail in partially transformed commercial steels because the parent austenite phase and the transformation interface are destroyed by the martensite reaction on further cooling to room temperature. In the present work this problem is avoided by using model alloys based on a high-Mn high-C steel that retains the parent austenite at room temperature, thus allowing examination of the interface by TEM and the untransformed austenite by atom probe FIM techniques. Weak beam TEM has failed to reveal any evidence of vanadium carbide precipitation in the austenite ahead of the interface. However, atom probe FIM has revealed evidence of pre-precipitation clusters on ultrafine particles in the untransformed austenite.

高Cr-Niオーステナイト鋼の高温変形能

The “hot-twist ductility” of highly alloyed austenitic steels such as AISI type 309, 310 and 330 was studied. Effects of alloying elements, melting practices, and grain size of these steels on the “hot-twist value” at testing temperatures ranging from 1, 000 to 1, 300°C were mainly investigated by torsion testing at a rate of 200 rpm.Fully austenitic specimens showed twist value curves of a convex type at testing temperatures, while two-phase specimens had much lower twist values than the former, nearly constant through all testing temperatures. With the former specimens, a transcrystalline fracture was observed at temperatures lower than that showing maximum twist value, while an intercrystalline fracture was shown at temperatures higher than that.In the case of type 310 steel, the higher the carbon content of specimen, the lower was its twist value. On the other hand, the maximum twist value was obtained with 0.2% C specimen in the case of type 309 steel. This difference was attributed to the fact that the ductility of these steels depended upon the stability of austenitic structure.Besides, effects of Si, Ni, Cr, N and Nb contents on twist value were explained. Comparing vacuum-melted with air-melted specimens, the effect of melting atmosphere was not so clear than that of alloying elements. Addition of small quantity of Al improved hot-twist ductility, while addition of large amount of Al as well as B up to 0.01% in the form of Fe-B much impaired ductility.In the case of high-C steels, the increase of annealing temperature which resulted in grain-coarsening decreased the hot-twist value owing to the increase of solubility of C atoms, while no change of the hot-twist value were observed with very low-C steels. Therefore, it was concluded that grain size itself did not affect the hot ductility.