High Si Steel Research Articles

Study of bainite and martensite tempering in a medium C high Si steel. microstructural disparities and equilibrium convergence

This study investigates the tempering behavior of bainite and martensite in a medium carbon, high silicon steel, with a focus on the microstructural evolution and the attainment of equilibrium, over a temperature range of 200–650 °C. The dissimilarities between the characteristics of the two initial microstructures, both comprising a C-saturated tetragonal ferrite matrix and retained austenite, are reflected in the differences observed in their evolution towards equilibrium as the tempering temperature increases. Therefore, while retained austenite plays a pivotal role in the bainitic microstructure, in the martensitic microstructure it is the ferritic matrix, which is highly dislocated and enriched in carbon, that plays a determinant role. The findings demonstrate that while both bainite and martensite can converge towards the same equilibrium state upon high-temperature tempering (600–650 °C), the pathway to this convergence is markedly different, with bainite exhibiting a slower transformation rate. This path to equilibrium has been characterised by means of high-resolution dilatometry, scanning electron microscopy and detailed X-ray diffraction analysis. This has provided a dynamic and detailed picture of the most relevant microstructural changes and tempering mechanisms in high silicon steels. It has also provided a foundation for tailoring heat treatment processes to optimise the mechanical properties of advanced bainitic and martensitic steels.

Hydrogen Embrittlement Properties of Rapid Tempered High-Si Steel and Effects of the Carbide and Retained Austenite on Its Properties

Hydrogen Embrittlement Properties of Rapid Tempered High-Si Steel and Effects of the Carbide and Retained Austenite on Its Properties

Effect of Si on the deformation behavior of retained austenite and annealed martensite in medium Mn steels

The present study systematically investigated the effect of Si additions from 0 to 4 wt % on the deformation mechanism of a Fe-0.2C-6.8Mn (wt. %) medium Mn steel. The duplex microstructure consisting of film-like retained austenite (RA) and lath-shaped annealed martensite (AM) was produced for different Si alloyed steels by annealing treatments. They have similar volume fraction of RA. The effect of Si content on the dynamic strain aging (DSA) of RA and the serration type of tensile curves, as well as the fine structure evolution within AM during deformation was studied in detail. For the different Si alloyed steels, their tensile curves all exhibit flow serrations, accompanied with the propagation of the Portevine-Le Châtelier (PLC) band caused by DSA in RA. The serration type was sensitive to Si content. For the high Si steel, only type B serrations appear. This is attributed to the high Si content in RA, promoting aged dislocations by C atoms due to the repulsive interaction between C and Si atoms. For the low Si and Si free steels, type A serrations occur after type B serrations disappear at a certain strain. The mechanical stability and Mn/Si contents of RA play an important role in the PLC band propagation. The Si content has also an important effect on the evolution of dislocation configuration within AM during deformation. In the low and Si free steels, the dislocation multiplication and tangles, as well as the formation of dislocation walls, dividing up the laths appear in the AM. While in the high Si steel, it is found that the dislocation cells appear within the AM due to Si addition promoting the formation of screw dislocations by neutron diffraction technique, and the cross slip of screw dislocations as a necessary mechanism for dislocation cell formation. The formation of dislocation configuration with low energy is believed to be another essential factor improving the mechanical properties, besides the transformation-induced plasticity effect. This work paves a new pathway towards tailoring dislocation configuration for designing a new generation advanced high-strength steel.

Hydrogen Embrittlement Properties of Rapid Tempered High-Si Steel and Effects of the Carbide and Retained Austenite on Its Properties

Hydrogen Embrittlement Properties of Rapid Tempered High-Si Steel and Effects of the Carbide and Retained Austenite on Its Properties

Comparative Study of Quench and Partition Processes in High Si and High Al Steels

The quench and partition process is a means to develop third-generation high-strength steels using many possible process variants. In this work, two variants of quench and partitioning heat treatments, one-step and two-step, were carried out for high Si and high Al steel alloys. The kinetics of isothermal transformation occurring during the one-step quench and partition process were analysed using dilatometry. Experimental analysis revealed the swing-back phenomenon in high Si steel, and the transformation characteristics above and below the Ms temperature differed. The high Al alloy resulted in higher retained austenite (19%) compared to high Si steel (17%) during the one-step quench and partition process. Aluminium addition favoured bainite formation more than silicon addition. A comparison of two heat treatment variants shows the two-step quench and partition heat treatment seemed preferable as it produced more retained austenite (22%) in the high Si steel.

Effect of Nitrogen Impurity on the Tensile Ductility and Impact Toughness of Al-Bearing DQP Steels

The influence of small contents of nitrogen present as an impurity in 0.3C Al-bearing steels, which were processed through thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP), was examined in respect of room temperature tensile ductility and impact toughness. Two similar chemical compositions (in wt.%): Fe-0.3C-0.6Si-1.1Al (High-Al) with different N contents of 10 and 30 ppm were selected for this study. In addition, two other DQP steels with compositions: Fe-0.3C-1.0Si (High-Si) and Fe-0.3C-0.5Si-0.5Al (Al-Si), both containing about 30 ppm nitrogen, were also included in the study to compare the properties. Detailed metallographic studies using FESEM-EDS, TEM, EPMA and XRD combined with tensile testing and fractographic analysis indicated that already 30 ppm of nitrogen could impair tensile ductility of TMR-DQP processed High-Al steel in comparison to that with 10 ppm nitrogen. Similarly, the effect was adverse also in Al-Si steel (30 ppm N) despite its reduced Al content (0.5 wt.%), but High-Si steel (Al < 0.002 wt.%, N 30 ppm) did not show any such detrimental effect on tensile ductility. Extensive material characterization verified that even 30 ppm of nitrogen could impair ductility of Al-bearing steels, essentially due to the presence of AlN inclusions, despite that TMR-DQP processing enabled stabilization of 6–10% retained austenite (RA) in the steels. The capacity of RA in promoting improved ductility and strain hardening capacity was impaired by the presence of these inclusions. In contrast, impact toughness transition temperature T28J was not clearly affected with Al-Si when compared to low-N High-Al steel, although excessive splitting in Al-Si caused pronounced scatter in the results and increase in upper shelf impact toughness.

Improvement of low-cycle fatigue behavior of modified 9Cr-1Mo steels at 450 °C in liquid LBE environment by the addition of Si element

This study examined the low-cycle fatigue performance of two modified 9Cr-1Mo steels containing Si at levels of 0.38% and 1.2%, respectively. The investigation was conducted under conditions of 450 °C oxygen-saturated and oxygen-poor lead–bismuth. The elevated oxidative corrosion behavior at this temperature aligns more closely with operational conditions. Comparing the steels with differing Si contents offers a clearer insight into the impact of Si element incorporation on the material's mechanical properties. The results indicated that the high-Si steels had a higher fatigue life, and their fatigue fractures exhibited quasi-dissociation. The high-Si steel displayed elevated oxidation resistance, as indicated by the slower growth of the oxide layer. Owing to significant oxidative corrosion, the fatigue life of the low-Si steel was shorter in oxygen- saturated LBE compared to oxygen-poor LBE. The LME-induced cracking of modified T91 steel in LBE initiated at the grain boundaries. During the expansion phase, a blend of ductile and brittle behaviors was observed, occurring through and alongside grain expansion patterns. Due to the elevated Si elemental content, high-Si steel exhibited weakened susceptibility to LME in oxygen-saturated LBE, owing to the creation of SiO2 through the reaction between Si and dissolved oxygen in the LBE, which shielded the steel matrix.

Development of High Si Steel Sheet Contributes to the Downsizing and High Efficiency of Electrical Equipment

1．はじめに

Towards a dependable TEM characterization of hot-dip galvanized steels with low and high Si content

Designing Advanced High-Strength Steels assumes fine-tuning of alloying elements to enhance performance properties. This affects the kinetics of Fe-Zn interaction during further galvannealing. A key role here belongs to the inhibition layer at the steel/coating interface. Applying mutually-reinforcing TEM techniques, we traced the evolution of Zn-Fe phases in low- and high-Si steels during galvannealing. It has been shown that conventional room temperature Ga FIB for TEM sample preparation is applicable only to annealed low-Si steel, a coating of which comprises equilibrium Zn-Fe compounds. The phases were validated via electron microscopy and diffraction methods. In contrast, even relatively long annealing of the high-Si steel was not sufficient to form stable Zn-Fe phases pointing to a hampering of the interphase reactions. As-galvanized and annealed high-Si specimens contain almost pure Zn and are affected by Ga via a eutectic reaction during FIB preparation. Thus, low-temperature FIB has been applied for reliable Zn-coated steel preparation. We refined with HRTEM the inhibition layer structures specific for low- and high-Si steel grades. While the typical Fe2Al5-xZnx interface layer has been found in the low-Si samples, for high-Si ones the formation of a quite stable Si-Mn mixed oxide membrane, which drastically hinders Fe-Al reaction, has been disclosed.

The Role of Slag Carryover on the Non-metallic Inclusion Evolution and Magnetic Behavior in Electrical Steel

In the present investigation, a set of high-temperature experimentations were carried out to improve the understanding of the influence of slag carryover (SCO) on non-metallic inclusion evolution during the production of high silicon electrical steels for functional applications. It was observed that the liquid steel treated with synthetic slag and lime resulted in the formation of CaO-based complex oxide, sulfide, and nitride inclusions in the matrix. Whereas the top slag (synthetic slag and lime) contaminated with carryover slag transforms the complex oxide inclusions to Mn free oxy-sulfide inclusions in the high Si steel. Further, the high-silicon steel evaluated for magnetic property confirms the detrimental magnetic behavior of the steel treated using the top slag with the excess amount of SCO (10 kg/t). The increase in coercivity is due to a higher fraction of sub-micron inclusions in the steel matrix. The industry implications of the present findings are highlighted in the light of the evolution of Goss texture in high silicon steel during downstream processing. The evolution of detrimental inclusions in functional grade (electrical) steels due to the presence of SCO call for stringent process control during the upstream processing of liquid steel to maintain the desired magnetic properties.

Effect of various phase fractions of bainite, retained austenite, intercritical ferrite and pearlite on the wear behaviour of multiphase steels

In this study, the dry sliding wear behaviour of multiphase steels (MPS) was investigated with the help of a pin/ball-on-disc wear setup. The newly developed high strength and highly ductile MPS specimens consisting of various fractions of bainite, retained austenite, intercritical ferrite and pearlite, were obtained by a combination of continuous cooling for different times (0 s, 10 s, 20 s and 40 s) after austenitization followed by isothermal heat treatment at various temperatures (300 °C, 350 °C and 400 °C) for 10 min from a moderately high C (∼0.67 wt%) - high Si (∼1.71 wt%) steel. The worn-out surfaces and wear debris revealed that the primary material removal mechanism was of abrasive types. Wear scars were seen like a ploughed field-like structure. The depth of wear scars and amount of material collected at the periphery were affected by various phases and their fractions in the MPS specimens. The wear behaviour of the multiphase steels was affected by the distribution of plastic deformation due to continuous sliding of wear ball and application of loads on the surface of MPS specimens which was analysed by hardness variation of the sub-surfaces beneath the wear scars. At a particular austempering temperature, the wear volume loss and specific wear rate of the MPS specimens increased with the increase of continuous cooling time irrespective of the applied wear loads, and it was attributed to the formation of softer intercritical ferrite as well as pearlite formed due to longer continuous cooling. However, the bainitic structure formed by austempering without continuous cooling led to the highest wear resistance irrespective of the isothermal temperatures.

Martensite and nanobainite transformations in a low alloyed steel studied by in situ high energy synchrotron diffraction

Martensitic and nanobainite transformations are studied in situ in a low alloyed, high-Si steel by using in situ HEXRD, combined with dilatometry and SEM observations, and by considering the same steel composition and austenitization conditions. The martensitic microstructure presents a mixed lath-plate morphology with large scatter of sizes whereas the bainite microstructure shows finer laths with more uniform sizes. Recently introduced methods are used to track in situ by HEXRD, in one single experiment, the phase fractions, the distribution of the carbon and the evolution of the dislocation densities. The study of nanobainite revealed that about two thirds of the carbon partitions from the ferrite to precipitate into transition iron carbides or to enrich the austenite. Both processes occur very fast after the formation of each nanobainite lath, but the ferrite remains largely supersaturated in carbon. The dislocation density increases inside each new forming bainitic ferrite lath. It then decreases when recovery becomes preponderant, as described with a recovery model from the literature. After the martensitic transformation, the retained austenite ends up with high hydrostatic compressive stresses. Dislocation densities are higher than in nanobainite and probably more heterogeneous, because recovery is less significant. No carbides were detected, contrary to the nanobainite. The carbon mass balance is analyzed in the light of these new results and previous investigations on similar systems. • Same steel composition and austenitization treatment to study the transformations. • Transition-carbides precipitation kinetic was quantitatively investigated. • Dislocation recovery observed during the nano-bainitic transformation. • Carbon distribution and mass balance showed a deficit of ca. 0.1 wt%.

Effect of austenitizing temperatures on the microstructure and mechanical properties of AISI 9254 steel

Abstract In this study, the effect of austenitizing temperatures and low-temperature isothermal heat treatment (below martensite start temperature) on the microstructure and mechanical properties of AISI 9254 high silicon spring steel has been investigated. Experimental studies show that ultra-fine carbide-free bainite, tempered martensite and carbon enriched retained austenite could be observed in isothermally heat-treated samples where the as-received sample consisted of fine pearlite. A high tensile strength of ~2060 MPa, a total elongation of ~8 %, and absorbed energy of 105 J were achieved in a commercial high-Si steel by austempering below the Ms temperature. A good combination of strength and ductility has been obtained in prolonged austempering below the martensite start temperature (225 °C) from an austenitizing temperature of 870 °C.

Competitive mechanisms occurring during quenching and partitioning of three silicon variants of 0.4 wt.% carbon steels

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurring during partitioning either in parallel or in succession. Three 0.4 wt.% carbon steels with varying Si contents were quenched to 150 °C for ~20% untransformed austenite, and partitioned for 10–1000 s in the temperature range 200–300 °C. The steel with low Si content (0.25 wt.%) exhibited substantial bainitic transformation during partitioning at 300 °C and only 4% retained austenite (RA) at room temperature (RT) even after 1000 s hold. In contrast, a high Si fraction (1.5 wt.%) enabled retention of ~18% austenite under similar conditions. While η-carbides precipitated within the martensite laths in the high-Si steel, cementite precipitated in the low-Si variant. Furthermore, carbide precipitation and growth were strongly suppressed by high Si content. Secondary martensite formation occurred from carbon-enriched austenite during final cooling, irrespective of Si-content. Results illustrate that Si retards austenite decomposition at higher partitioning temperatures but does not improve carbon partitioning at lower temperatures.

On the extent of transformation of austenite to bainitic ferrite and carbide during austempering of high Si steel for prolonged duration and its effect on mechanical properties

In this work, the effect of holding durations (10–60min) during austempering at 300, 350 and 400 °C on the structure and properties of a 0.61C-1.71Si-0.86Mn steel has been studied. At all the austempering temperatures, austenite fraction gradually decreases with the increase in holding duration. Austempering temperature has been found to influence the rate of depletion of austenite. The effect of Mn and Si content of steel on the extent of transformation of austenite to bainitic ferrite and carbides during prolonged austempering holding has also been looked at. Analysis drawn from existing literature along with the current data shows that along with Si, Mn also plays an important role in controlling the rate of transformation of austenite to bainitic ferrite and carbide.

Wear Resistance of High C High Si Steel with Low Retained Austenite Content and Kinetically Activated Bainite

A novel high C high Si carbide free bainitic steel was developed for the production of cold work tools, knives, and rolls, requiring high hardness, toughness, as well as abrasive/adhesive wear resistance and resistance to galling at low costs. The steel was tribologically tested in dry sliding conditions under abrasive and adhesive wear mode, facilitated by using alumina and bearing steel ball as a counter-material, respectively. It was determined that carbide dissolution occurs under high contact pressures, thereby enriching the surrounding matrix with carbon and locally increasing the retained austenite content. The high retained austenite at the sliding interface increases the steels work hardening capacity and promotes superior wear resistance when compared to much more alloyed cold work tool steel, such as AISI D2. The steel has a high resistance to galling as determined by sliding against a soft steel bar due to its chemical composition.

Evolution of cube-on-face texture in Fe-1%Si steel induced by physical contact during the phase transformation from γ to α

When any sample is heat treated, it is normally placed on a supporting material. Normally, such physical contact is not expected to affect the microstructure evolution of the sample. In this study, however, the physical contact between Fe-1%Si sheet and the Al2O3 plate was found to have a strong effect on the evolution of a cube-on-face texture during heat treatment, producing as much as 76.0% when determined by the orientation within {100} ±15°. The similar result was obtained for the tungsten and quartz plates. The results provide a promising method to produce the high efficiency Si steel with a cube-on-face texture.

Atomic interactions between Si and Mn during eutectoid transformation in high-carbon pearlitic steel

The atomic interactions between Si and Mn during eutectoid transformation in high-carbon pearlitic steel were investigated. Atom probe tomography and first-principles calculations were applied to evaluate and analyze the atomic interactions at the ferrite/cementite interface. In the initial stage of eutectoid transformation, enrichment of Si and Mn occurred at the ferrite and cementite sides, respectively, of the interface. This interfacial segregation phenomenon gradually diminished as the transformation proceeded. Calculations of the cohesive energy and formation energy revealed a clear enhancement in the chemical bonding and stability of the pearlite system when the Si atom was moved from the ferrite layer to the cementite layer and the Mn atom was moved in the opposite direction. The interfacial segregation of the Mn and Si atoms was mainly responsible for the insufficient diffusion and high hybridization degree of Fe, Mn, Si, and C atoms. Furthermore, the partitioning ratio of Mn in high-Si steel was greater than that in low-Si steel, leading to greater partitioning of Mn into the cementite phase. Calculations of the electronic structure revealed that the enrichment of Si in the ferrite phase promoted the partitioning of Mn into the cementite phase owing to the strong repulsive force between Mn and Si at the pearlitic interface.

Effect of the quench temperature on the mechanical properties of a medium C Mn high Si steel during Q&P heat treatment process

A steel of composition Fe-0.2C-3Mn-2Si-0.5Al was fully austenized followed by a quench and partitioning heat treatment process (Q&P). The quench temperature was varied, which resulted in different volume fractions of retained austenite (RA) and martensite. Analysis of the phase evolution and the resulting microstructures during the Q&P process were carried out using different techniques namely, - dilatometry, FEG SEM, EBSD, and neutron diffraction. Mechanical properties were evaluated by standard tensile tests on samples quenched to different temperatures. The partitioning process was evaluated by dilatometry. The volume fraction of the RA was determined by neutron diffraction. It was found that the volume fraction of RA increased with an increase in the quench temperature contrary to the Speer model. It was also observed that the presence of bainite, which formed during the quench and partitioning temperature significantly stabilized the RA by carbon partitioning. The tensile test results indicated the optimum quench temperature for the best combination of strength and ductility and contrary to expectation, this did not occur in the specimen with the maximum amount of RA. In other words, the mechanical properties of the steel undergoing a Q&P process is influenced by the quench temperatures and is also affected the phase evolution which occurs during both the quench and partitioning process.

The effect of Niobium on Austenite Evolution during Hot Rolling of Advanced High Strength Steel

Two AHSS steels (0.2C-2.4Mn-1.5Si with/without Nb+Cr addition) were subjected to pilot reversing mill hot rolling in 5 passes varying the finish rolling and coiling temperature. After coiling simulation, the final microstructures were characterised by the EBSD technique measurements. One sample was analysed to determine the presence and the level of Nb precipitated by TEM and chemical extraction. The austenite evolution was inferred from the measured transformation textures and reconstructed final austenite texture and microstructure. Further insights were obtained by comparing the results to a semi-empirical austenite evolution model, which predicted recrystallization behaviour and level of Nb precipitation. From the combined experimental and modelling work it was shown that Nb addition led to recrystallization inhibition via partial strain induced precipitation for this high Mn and high Si steel. Applying the austenite evolution model to a more realistic industrial hot strip rolling schedule revealed earlier Nb precipitation during hot rolling and higher strain accumulation with a finer grain size. The final Nb level in solution was lower for the industrial schedule, although a significant amount was not precipitated (two thirds of the total).