low-Si Steel Research Articles

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.

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.

Comparative study on effects of annealing temperature on microstructures and mechanical properties of high-Al low-Si steel

We elucidate here profound effects of quenching and partitioning process after partial (IQP) or full austenitization (FQP) on the microstructure and mechanical properties of high strength hot-dip galvanized steel with high Al and low Si content. The results showed that the FQP steel contains 3.38% vol.% retained austenite with mainly film-like morphology, while the IQP steel mainly consists of 6.44–8.13 vol.% retained austenite with block shape. Additionally, retained austenite in the IQP sample transforms into martensite more easily duo to higher nucleation rate, which means lower austenite stability. A combination of various austenite grain size and morphology make the TRIP effect throughout the whole deformation process and increased the elongation for IQP sample. When the steel was treated by annealing temperature of 967 °C, partitioning temperature and time of 460 °C and 100 s respectively, the steel exhibited excellent mechanical properties with tensile strength of 754 MPa, total elongation of 27% and the product of strength and ductility exceeds 20 GPa%. The proposed content in the present study demonstrates compatibility with the continuous hot-dip galvanizing process.

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.

Two-Step Isothermal Bainitic Transformation in Medium-Carbon Low-Si Steels with Exceptional Mechanical Properties and Wear Resistance

Two-Step Isothermal Bainitic Transformation in Medium-Carbon Low-Si Steels with Exceptional Mechanical Properties and Wear Resistance

Effect of Finish Rolling Temperature on Microstructure and Mechanical Properties of Low-Si Steel After Quenching and Nonisothermal Partitioning

The quenching and nonisothermal partitioning process has been found to achieve excellent combinations of strength and ductility through the retention of austenite in the microstructure. Therefore, in the present study, the effect of grain size on phase transformations and ensuing mechanical properties has been investigated. A low-Si and Al-free steel was finish rolled at different temperatures which was subsequently quenched to a constant temperature below Ms for the formation of martensite with remaining untransformed austenite. This was followed by slow cooling to room temperature to simulate the condition of nonisothermal partitioning of carbon from martensite to the austenite. The results show that there is not much change in the average grain size with variation in the finish rolling temperature, as well as, no measurable differences are observed in the microstructural features and retained austenite content. The microstructure shows the presence of martensite, carbides, and a small amount of retained austenite. However, in terms of mechanical properties, the finish rolling at a lower temperature of 900 °C lead to higher hardness and strength with comparable ductility in comparison to the sample which has been finish rolled at a relatively higher temperature (940 °C). The above results will thus be helpful in designing the industrial processing schedule for low-Si steel to achieve enhanced performance.

Temperature Dependence of the Static and Dynamic Behaviour in a Quenching and Partitioning Processed Low-Si Steel

Because of their excellent combination of strength and ductility, quenching and partitioning (Q & P) steels have a great chance of being added to the third generation of advanced high strength steels. The large ductility of Q & P steels arises from the presence of 10% to 15% of retained austenite which postpones necking due to the transformation induced plasticity (TRIP) effect. Moreover, Q & P steels show promising forming properties with favourable Lankford coefficients, while their planar anisotropy is low due to a weak texture. The stability of the metastable austenite is the key to obtain tailored properties for these steels. To become part of the newest generation of advanced high strength steels, Q & P steels have to preserve their mechanical properties at dynamic strain rates and over a wide range of temperatures. Therefore, in the present study, a low-Si Q & P steel was tested at temperatures from −40 °C to 80 °C and strain rates from 0.001 s−1 to 500 s−1. Results show that the mechanical properties are well-preserved at the lowest temperatures. Indeed, at −40 °C and room temperature, no significant loss of the deformation capacity is observed even at dynamic strain rates. This is attributed to the presence of a large fraction of austenite that is so (thermally) stable that it does not transform in the absence of deformation. In addition, the high stability of the austenite decreases the elongation at high test temperatures (80 °C). The additional adiabatic heating in the dynamic tests causes the largest reduction of the uniform strain for the samples tested at 80 °C. Quantification of the retained austenite fraction in the samples after testing confirmed that, at the highest temperature and strain rate, the TRIP effect is suppressed.

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.

Bainite growth retardation due to mechanical stabilisation of austenite

This study shows the first ever attempt to calculate bainite kinetics based on characteristics of nucleation as well as displacive growth. A model for bainite kinetics is proposed whereby in conjunction to nucleation the displacive growth of laths is described by a growth factor which is assumed to vary linearly with the net driving force and thereby ensures consistency with the thermodynamic growth criterion. Furthermore, the growth factor is supposed to have an inverse dependence on the elastic strain energy to account for retardation of the growth due to mechanical stabilisation. When a bainitic lath is formed the surrounding austenite is work hardened due to the plastic accommodation of the shape change associated with the transformation. The dislocation strengthening is assumed to mechanically stabilize the austenite against subsequent transformation to bainite and/or martensite. Geometrical constraints are also assumed to contribute to the mechanical stabilisation of untransformed austenite in partially bainitic microstructures. The overall degree of mechanical stabilisation has been quantified for low-Si steels by measuring the decrease in start temperature of the second phase martensite after partial bainitic transformations. Based on these experiments an empirical equation has been developed to describe the decrease in the growth factor with progressing transformation. Subsequently, the model has been validated using numerous fraction curves of 10 different low-Si steels. Fits have been optimized by adjusting only the autocatalysis factor for each holding temperature. Owing to the growth factor the model accurately describes the strong retardation in kinetics in the second stage of transformation.

Bainite Growth Retardation Due to Mechanical Stabilisation of Austenite

This study shows the first ever attempt to calculate bainite kinetics based on characteristics of nucleation as well as displacive growth. A model for bainite kinetics is proposed whereby in conjunction to nucleation the displacive growth of laths is described by a growth factor which is assumed to vary linearly with the net driving force and thereby ensures consistency with the thermodynamic growth criterion. Furthermore, the growth factor is supposed to have an inverse dependence on the elastic strain energy to account for retardation of the growth due to mechanical stabilisation. When a bainitic lath is formed the surrounding austenite is work hardened due to the plastic accommodation of the shape change associated with the transformation. The dislocation strengthening is assumed to mechanically stabilize the austenite against subsequent transformation to bainite and/or martensite. Geometrical constraints are also assumed to contribute to the mechanical stabilisation of untransformed austenite in partially bainitic microstructures. The overall degree of mechanical stabilisation has been quantified for low-Si steels by measuring the decrease in start temperature of the second phase martensite after partial bainitic transformations. Based on these experiments an empirical equation has been developed to describe the decrease in the growth factor with progressing transformation. Subsequently, the model has been validated using numerous fraction curves of 10 different low-Si steels. Fits have been optimized by adjusting only the autocatalysis factor for each holding temperature. Owing to the growth factor the model accurately describes the strong retardation in kinetics in the second stage of transformation.

Microstructure–Property Correlation in Low-Si Steel Processed Through Quenching and Nonisothermal Partitioning

In the present investigation, an attempt has been made to stabilize austenite by carbon partitioning through quenching and nonisothermal partitioning (Q&P) technique. This will eliminate the need for additional heat-treatment facility to perform isothermal partitioning or tempering process. The presence of retained austenite in the microstructure helps in increasing the toughness, which in turn is expected to improve the abrasion resistance of steels. The carbon partitioning from different quench temperatures has been performed on two different alloys, with low-Si content (0.5 wt pct), in a salt bath furnace atmosphere, the cooling profile of which closely resembles the industrially produced hot-rolled coil cooling. The results show that the stabilization of retained austenite is possible and gives rise to increased work hardening, better impact toughness and abrasive wear loss comparable to that of a fully martensitic microstructure. In contrast, tempered martensite exhibits better wear properties at the expense of impact toughness.

Effect of silicon on the hot deformation behavior of microalloyed TWIP-type stainless steels

In the present study, two vanadium-microalloyed 10Mn-18Cr TWIP-type stainless steels with low and high Si contents were designed to achieve a stacking fault energy of 25 mJ/m2. Hot deformation behavior was investigated using compression tests in the temperature range 950–1100 °C and strain rate range 0.01–10 s−1. Microstructural features of the rolled slabs were studied using optical microscopy and those of the hot deformed specimens by electron backscatter diffraction. The low-Si steel possessed a higher hot deformation resistance and a higher activation energy of deformation (507 kJ/mol) compared to the high-Si steel (477 kJ/mol). This unusual trend is attributed to an increasing fraction of soft ferrite in the latter steel with increasing temperature. At lower test temperatures, where partial dynamic recrystallization occurred in the high-Si steel, ferrite possessed high-angle grain boundaries, while the austenite contained dislocation substructures. The hot deformation behavior of the steels was modeled using the dislocation density based Bergström model and the dynamic recrystallization based Avrami model. The rate of increase of Bergstrom's hardening parameter as a function of the Zener-Hollomon parameter is seen to be higher for the high-Si steel.

Exploring bainite formation kinetics distinguishing grain-boundary and autocatalytic nucleation in high and low-Si steels

Bainite formation in steels begins with nucleation of bainitic ferrite at austenite grain boundaries (γ/γ interfaces). This leads to creation of bainitic ferrite/austenite interfaces (α/γ interfaces). Bainite formation continues through autocatalysis with nucleation of bainitic ferrite at these newly created α/γ interfaces. The displacive theory of bainite formation suggests that the formation of bainitic ferrite is accompanied by carbon enrichment of surrounding austenite. This carbon enrichment generally leads to carbide precipitation unless such a reaction is thermodynamically or kinetically unfavourable. Each bainitic ferrite nucleation event is governed by an activation energy. Depending upon the interface at which nucleation occurs, a specific activation energy would be related to a specific nucleation mechanism. On the basis of this concept, a model has been developed to understand the kinetics of bainite formation during isothermal treatments. This model is derived under the assumptions of displacive mechanism of bainite formation. The fitting parameters used in this model are physical entities related to nucleation and microstructural dimensions. The model is designed in such a way that the carbon redistribution during bainite formation is accounted for, leading to prediction of transformation kinetics both with and without of carbide precipitation during bainite formation. Furthermore, the model is validated using two different sets of kinetic data published in the literature.

Influence of the angle between cold rolling direction and hot rolling direction on the texture evolution of non-oriented electrical steels

In processing non-oriented electrical steel sheets using conventional rolling schemes, the most common texture components obtained after final annealing are the magnetically unfavourable <111>//ND (γ) and <110>//RD (α) fibres. A lot of researches have been carried out trying to optimize the processes to produce the favourable <001>//ND (θ) fibre. However, since the final texture is formed through a series of texture evolution steps during the solidification, hot rolling, cold rolling and annealing processes, it is quite challenging to tailor the texture of the final product. In this study, a new rolling scheme was examined, in which the cold rolling direction (CRD) was inclined to the hot rolling direction (HRD) at an angle from 0° to 90° (with a 15° increment). This was intended to alter the texture commonly produced by cold rolling along the HRD, and to optimize the final recrystallization texture. The cold rolling and recrystallization textures of two non-oriented electrical steels with 0.9% and 2.8% Si were measured. It was found that the inclination of CRD to HRD has a substantial effect on the cold rolling texture for both steels, but only in the low Si steel, does it lead to significantly different recrystallization textures. A strong cube texture was produced at an inclination angle of 60°, and the <111>//ND (γ) fibre was significantly weakened or essentially disappeared. The core losses of these steels were measured by Epstein frame method and the results showed a ∼10% difference among strips cold rolled at different angles. A minimum core loss occurred at a 45° inclination angle in the low Si steel.

Microstructure and Mechanical Properties of Low-Si Spring Steels Produced by Quenching- Isothermal-Quenching-Tempering Process

An investigation of low Si spring steels (50CrVA) subjected to quenching-isothermal-quenching-tempering (Q-I-Q-T) process is presented. The results indicated that Q-I-Q-T process lead to the excellent mechanical properties compare to those treated by traditional heat-treatment, quenching and tempering, owing to the multiphase structure. The Q-I-Q-T treated microstructure consists of martensite, retained austensite, bainite and fine distributed carbides.

Microstructure, Properties and Work Hardening Behavior of High Strength Cold Rolled Dual-Phase Steel

Two kinds of high strength cold rolled dual-phase steel with different Si content were trial-produced in the laboratory. Tensile strength and elongation of the two steels exceed 1000MPa and 15%, respectively. The phase transformation behaviors of both steels were compared and investigated in continuous cooling process by thermal dilatometer. The effects of Si on the mechanical properties and microstructures of dual-phase steel were studied by tensile testing, OM and SEM observation. The results show that the two phase region (α+γ) is enlarged, the precipitation and growth of pro-eutectoid ferrite is promoted, and the morphology and distribution patterns of martensite are improved for high Si steel. Both steels show two-stage strain hardening characteristics. StageⅠ (ε&lt;0.05), the solution of Si hinder the movement of dislocations, make the work hardening exponent of high Si steel is higher than that low Si steel. At stage Ⅱ (ε&gt;0.05), the compatibility and delivery between hard phase and soft phase eliminate the differences between the two steels.

MECHANICAL PROPERTIES OF CONTINUOUSLY ANNEALLED Si–FREE P–CONTAINING TRIP STEEL

Since its advent, TRIP steel has been considered as the best automotive steel to improve the automotive safety dramatically because of its excellent mechanical properties. However, TRIP steel was not widely used and the main reason is that the large amount of Si in the steel led to surface quality problems of the plate. In this work a kind of C-Mn-Al-P TRIP steel with Si being replaced by Al and P was designed and subjected to continuous annealing. The macro and micro mechanical performance was analyzed. The results showed that the mechanical properties were excellent with tensile strength between 740 and 790 MPa and elongation above 25%, indicating that the production of TRIP steel without Si addition was feasible. Compared with traditional TRIP steel with Si, the designed steel demonstrated higher yield strength of 550—600 MPa, which was a little higher than that of high Si steel and much higher than that of medium or low Si steel. The micro hardness of ferrite was measured by nano mechanics probe and it was 0.6 GPa higher than that of medium Si TRIP steel. The IF steel whose strength was already known was used as a calibration and the conclusion was that the 0.6 GPa difference in nano hardness could result in 75 MPa difference in yield strength and 80 MPa division in tensile strength. Through further analysis, it was believed that the higher strength of ferrite was mainly caucsed by C, not P.

Thermodynamic Modelling of Carbide Precipitation in Hot Work Tool Steels with Different Silicon Contents

New grades have been developed in high alloy tool steels, enabling improved mechanical properties by modifications on Si, Cr or Mo contents. The correlation between alloy modifications and mechanical properties were successively correlated to microstructural aspects in previous work, especially in terms of secondary hardening carbides. In the present paper, thermodynamic and kinetic simulations were carried out and correlated to these data, especially in terms of Si effect on M3C carbide precipitation. Free energy evaluation points to easier formation of M7C3 carbide in high Si steels, especially at high tempering temperatures, which are typical for hot work tool steels. Kinetic evaluations, based on the present simulations and literature, also show that cementite formation is retarded in high Si steels, which also lead to direct formation of M7C3. Regarding the low Si steels, cementite enrichment by Cr, Mo and V was also evaluated in terms of simulation. The results are in good agreement and help the explanation on the slower dissolution of cementite particles in low silicon steels, leading to important implications to the higher tempering resistance of these grades. Therefore, tempering simulation on the new hot work tool steel compositions aided the understanding of the microstructure modifications found experimentally.

On the effect of silicon on toughness in recent high quality hot work steels

Fracture surfaces on H11-type Charpy impact samples were studied as a function of tempering temperature and Si content. The recently discovered effect of Si on toughness after high-temperature tempering was shown to be the result of a more favourable distribution of secondary alloy carbides in low-Si steels.

Effect of Si on Mechanical Property of Galvannealed Dual Phase Steel

The mechanical properties of galvannealed dual phase steels bearing Si were investigated, and then the effect of Si on work hardening behavior during tensile deformation was discussed. Both tensile strength and yield strength were increased with increasing Si content, especially increasing ratio of tensile strength was larger than that of yield strength. On the other hand, the decrease of uniform elongation was very small with increasing Si content. Therefore, tensile strength and total elongation balance was improved by Si addition. High Si (1.2%Si) steel exhibited higher work hardening rate than low Si (0.01%Si) steel in all strain region. Dislocation cell structures were observed at 8% tensile strain in low Si steel, while dislocation cell structures were scarcely observed and instead, tangled dislocations were dominantly observed in high Si steel. Furthermore, high Si steel exhibited higher the increasing ratio of dislocation density in ferrite during tensile deformation than low Si steel. These results indicated that the formation of dislocation cell structure was retarded by Si addition, which led to the increase of work hardening rate and resulted in the improvement of tensile strength and total elongation balance.