Effect of the finish rolling and cooling conditions on the properties of microalloyed plate steels

The processing parameters of finish rolling and cooling of three HSLA steels were varied in a wide range applying the plane strain hot compression simulation test on the hot deformation simulator (Wumsi). Evaluating the flow curves of the deformation steps, some metallurgical phenomena in austenite during hot deformation could be determined. The results of material testing after the simulation experiments exhibit a strong correlation between the finish rolling and cooling conditions and the final mechanical properties of the steels tested. As a way to increase strength, lowering finishing temperatures and/or accelerating cooling after finish rolling proved to be most effective. To improve notch toughness, on the other hand, a high total finishing strain should be applied as well as low finishing temperatures, however, above A_r3.     

Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite

It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (T _nr ). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T _nr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.     

Continuous cooling transformation temperatures determined by compression tests in low carbon bainitic grades

The transformation behaviors of six steels containing microalloying additions of B, Nb, and Mo were investigated under continuous cooling conditions. Continuous cooling compression (CCC) tests were employed to study the effects of chemical composition (mainly, Nb, Mo, and B) and deformation parameters (reheat temperature, prestrain, and holding time) on the transformation temperatures (A _r3 and B _s). It was found that for the Mo-Nb-B, Mo-B, and B steels, the transformation temperatures are relatively stable, and vary in a range of about 20 °C when the reheat temperature is changed from 900 °C to 1200 °C. Both the stress-temperature curves and the associated microstructures show that transformation in the Mo-Nb-B steel is basically of the γ-to-B type; i.e., the resulting microstructure is low carbon bainite. By contrast, for the Nb-B steels, the transformation temperatures vary significantly when the reheat temperature is changed. The concentration of boron in solution strongly affects the transformation behavior of this type of steel. In the Nb-48B steel, the latter is of the γ-to-B type, while in grades with either higher (Nb-64B) or lower (Nb-15B) boron concentrations, it is mainly of the γ-to-α type. Large Fe₂₃(C,B)₆ particles, which were found at low reheat temperatures and long holding times, are considered to be responsible for raising the transformation temperatures.     

Continuous cooling transformation temperatures determined by compression tests in low carbon bainitic grades

The transformation behaviors of six steels containing microalloying additions of B, Nb, and Mo were investigated under continuous cooling conditions. Continuous cooling compression (CCC) tests were employed to study the effects of chemical composition (mainly, Nb, Mo, and B) and deformation parameters (reheat temperature, prestrain, and holding time) on the transformation temperatures (A _r3 and B _s). It was found that for the Mo−Nb−B, Mo−B, and B steels, the transformation temperatures are relatively stable, and vary in a range of about 20°C when the reheat temperature is changed from 900°C to 1200°C. Both the stress-temperature curves and the associated microstructures show that transformation in the Mo−Nb−B steel is basically of the γ-to-B type; i.e., the resulting microstructure is low carbon bainite. By contrast, for the Nb−B steels, the transformation temperatures vary significantly when the reheat temperature is changed. The concentration of boron in solution strongly affects the transformation behavior of this type of steel. In the Nb−48B steel, the latter is of the γ-to-B type, while in grades with either higher (Nb−64B) or lower (Nb−15B) boron concentrations, it is mainly of the γ-to-α type. Large Fe₂₃(C,B)₆ particles, which were found at low reheat temperatures and long holding times, are considered to be responsible for raising the transformation temperatures. T.M. MACCAGNO, formerly with the Department of Metallurgical Engineering, McGill University     

Microstructural Changes after Control Rolling and Interrupted Accelerated Cooling Simulations in Pipeline Steel

The γ‐α transformation and final microstructure in pipeline steel was studied by carrying out a number of physical simulations of industrial hot rolling schedules. Particularly, the effect of the reheating temperature, deformation and cooling parameters on the transformation temperatures and final grain size were considered with a goal to obtain an appropriate thermo‐mechanical processing route which will generate appropriate microstructures for pipeline applications. The CCT diagram of the steel was derived experimentally by means of dilatometric tests. Hot torsion experiments were applied in a multi‐deformation cycle at various temperatures in the austenite region to simulate industrial rolling schedules. By variation of the reheating temperature, equivalent strain, and accelerated cooling, different types of microstructures were obtained. It was found that the deformation increases the transformation temperatures whereas the higher cooling rates after deformation decrease them. Post‐deformation microstructure consists of fine bainitic‐ferrite grains with dispersed carbides and small amount of dispersed martensite/austenite islands which can be controlled by varying the reheating temperature, deformation and post‐deformation cooling. The detailed microstructure characteristics obtained from the present work could be used to optimize the mechanical properties, strength and toughness of pipeline steel grades by an appropriate control of the thermo‐mechanical processing.     

Transformation during the isothermal deformation of low-carbon Nb-B steels

The transformation behavior during isothermal deformation of four steels containing different microalloying additions was investigated by means of the “strain-rate change” technique. The flow curves obtained at temperatures ranging from 620 °C to 850 °C, and the associated microstructures, indicate that the transformation in the Mo-Nb-B and Mo-B steels is of the austenite-to-bainite type. Here, dramatic increases in flow stress are observed at lower temperatures. By contrast, the transformation in the Nb-15B and Nb-64B steels is basically of the austenite-to-ferrite type; in these two grades, the flow stress increases observed are attributable to strengthening by NbC precipitation. Large intergranular and intragranular Fe₂₃(C,B)₆ particles were found in the Nb-64B steel samples deformed to ɛ=0.1 after holding for 60 seconds at 800°C. These large precipitates are considered to be responsible for accelerating the transformation in the Nb-64B steel by reducing the concentration of boron atoms available for boundary segregation and by acting as nucleation sites for the formation of polygonal ferrite. The flow curves of the Mo-Nb-B steel exhibit distinct serrations, indicating that a displacive mechanism is involved in the γ-to-B transformation.     

Changes in transformation behaviour and microstructure of a CrV-spring steel by thermomechanical treatment

The effect of austenite deformation on the transformation behaviour was investigated on a CrV-spring steel with the major attention put on the martensitic transformation. In the first part, a small review is given on the relation between the state of austenite after hot deformation and its influence on the formation of martensite. In the laboratory tests, the second part of the paper, a conventional heat treatment (CHT) was compared with two types of austenite conditioning by thermomechanical treatment (TMT): TMT_R - with deformation above the recrystallization temperature ?_R leading to a fully recrystallized austenite and TMT_N- with deformation below ?_R with a not recrystallized but possibly polygonized austenite. For the laboratory tests, the hot deformation simulator Wumsi was employed. After quenching In oil, the martensite after TMT consisted of associations of many fine fragments with a smaller number of large acicular martensitic units than observed after CHT. In both TMT-variants small ferritic areas (< 1 μ m) could be revealed. Different behaviour of martensite during tempering at low temperatures was observed after CHT and TMT. It can be explained by reduced inherent stresses generated during martensitic transformation after TMT, presumably as a result of a better ability of deformed austenite to withstand the accommodation strain during martensitic transformation. This may have considerable consequences for the toughness properties of tempered martensite.     

Hot forging of Ti–6Al–4V alloy preforms produced by spark plasma sintering of powders

Abstract

Powder preform forging is a technology that comprises the preparation of near net shape preforms through powder metallurgy and a subsequent hot forging in order to obtain the desired final shape. In this work, two Ti–6Al–4V powder preforms were sintered through spark plasma sintering (SPS) and then hot compressed in a horizontal dilatometer. Varying the temperature of the process, two full density preforms having different microstructures were produced: sintering at 950°C, a plate-like α was obtained, whereas sintering at 1050°C, an acicular α was obtained. The behaviour of the preforms under hot forging has been studied through hot compression tests carried out in a quenching and deformation dilatometer in a range of temperature and strain rates typically used in hot forging this alloy (850–1050°C, 0·01–1 s^?1). Hot workability has been evaluated by measuring the stresses required for deformation and by analysing both the stress–strain curves recorded during testing and the microstructures after deformation. The main microstructural phenomena occurring during hot compression were individuated. The best conditions for the hot forging operation of SPS preform are temperatures above β transus, where the materials are deformed in a regime of dynamic recrystallisation, at every strain rate.     

Hot-rolling simulation and modelling using Gleeble 1500

Using the Gleeble 1500, incremental and continuous hot compression tests, simulating hot rolling, were performed on C-Mn, Nb-T, and Nb steels with test temperatures varying between 875 and 1100°C and strain rates between 0.5 and 20 s^?1. Four models are proposed. The stress peak model allows the prediction of continuous stress-strain curves from incremental curves and vice versa through the use of stress restoration index K. Variation in K for Nb-T₁, C-Mn and Nb steels at strain rates of 3, 12 and 20 s^?1 was found to be negligible. The predicted stress strain curve corresponds to experimental stress strain curve at same temperature and strain rate. The strain history model predicts continuous strain-time curves from incremental stress-strain curves using ‘constant’ ‘negative strain’ restoration index. At 950°C, with holding time 2 s and strain rate 12 s^?1, strain time decay curves obtained for C-Mn, Nb and Nb-T, steels were ε = 1.5e^?05t, ε = 1.2e^?0.36t and ε = e^?0.3t, respectively. The creep model analysis relates creep strain rate to the testing strain rate. For Nb steel at 875°C, and test strain rate of 12 s^?1, ?_creep was found to be 9.5 s^?1. The stress history model predicts continuous stress-time curves from incremental stress-time curves. Stress decay curve for C-Mn steel at 1100°C and ? = 3s^?1 was found to be σ = 181e^?0.04t. Hot rolling characteristics of steels can be accurately predicted using hot compression tests and proposed models.     

Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T _5pct) and the recrystallization-limit temperature (T _95pct), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T _5pct<T<T _95pct), as determined in this study. The Nb(CN) precipitation kinetics have been calculated using a model proposed in an earlier work, and these results are used to estimate the precipitate pinning force under the given processing conditions. Based on these estimations, a criterion has been proposed to predict the onset of recrystallization. The predicted results are found to be in reasonably good agreement with the experimental measurements.     

Direct charging of thin slabs of a Ti-microalloyed low carbon steel for cold forming

With a near-net-shape continuous-casting simulator and the hot deformation simulator Wumsi, laboratory simulation tests have been performed to determine the final microstructure and the mechanical properties of a Ti-microalloyed low carbon steel for cold forming, produced in the process of direct charging of thin slabs. For two initial specimen thicknesses (45 and 25mm) restricted values of total strain ?_∑ were available to improve the coarse as-cast microstructure. In a series of deformation schedules ?_∑ was divided systematically into two components: an austenite grain-refining strain ?_γ–GF (above the recrystallization-stop-temperature T_RS) and an austenite strengthening strain ?_γ–s (below T_RS). After hot deformation accelerated cooling with simulated coiling was employed. It was found that the direct charging process guarantees a more complete dissolution of Ti(C,N) and so an intensifying effect of Ti-microalloying in comparison to a conventional cold charging. Structure refinement by rolling performed predominantly in the temperature region of non-recrystallized austenite (increasing ?_γ–s) leads to better hot strip strength and improved low temperature toughness properties accompanied with a slight deterioration of uniform elongation.     

Hot Deformation Behaviour of Low Alloy Steel

Hot deformation of a continuously cast low alloyed steel is studied by means of hot compression and tensile tests by using a Gleeble® 1500 machine after austenitization in a wide range of strain rates and between 720–800 °C. The flow data are evaluated to obtain the strain rate sensitivity and the processing maps. A new calculation method is used, yielding on the instability parameter defined as κ_J which correlates well with the microstructural changes. The strain rate sensitivity does not predict any instability but all the others instability parameters do, including the new κ_J. Flow instability appears at high temperatures and low strain rates where pores are formed at the austenitic grain boundaries, causing a decay of ductility in the tensile test. During hot deformation more ferrite is formed than corresponding heat treatments without deformation. In these conditions, the deformation is concentrated in the softer ferrite phase. The deformation induced ferrite deforms by dynamic recovery forming new grains as revealed by metallography and is correlated with low coefficients of power dissipation. The sinh type constitutive equation represents the flow data well with a stress exponent n = 4.1 and an apparent activation energy Q = 218 kJ/mol.     

Effect of hot strip processing during direct charging of thin slabs on the properties of cold strip of LC-steel

Direct charging of thin slabs of low carbon steel (0.05%C) was investigated by using laboratory tests on a thin-slab-casting Simulator linked with the hot deformation Simulator (Wumsi). Starting with a different initial thickness of simulated thin slabs (45 down to 25 mm) the total strain ?∑ was varied. Hot deformation schedules (with and without roughing) with different finishing temperatures T_F lead to some differences in the hot strip microstructure. Nevertheless, after cold rolling and batch annealing, a rather uniform pancake-structure and a pronounced {111}-texture were achieved in the cold strip without any significant relation to the processing routs. The measurement of final mechanical properties proved that a good deep drawability of cold strip can be achieved with direct charging of thin slabs, well comparable to that after a conventional cold charging of thick slabs.     

Direct charging of thin slabs of a cold formable HSLA steel

In the concept of direct charging linking continuous casting with rolling of thin slabs, some differences in the design of process parameters are necessary. By example of a commercial low carbon NbTi-microalloyed steel, laboratory tests have been carried out by using a thin-slab continuous casting simulator on-line with the hot deformation simulator Wumsi. The influence of the temperature of intermediate holding (between casting and rolling), total strain ε_Σ, finishing temperature and cooling rate after finishing on the microstructure and mechanical properties was investigated on rolling schedules with or without roughing prior to finishing. Conventional cold charging (with reheating of thicker slabs) was simulated as a comparison to this. Whereas the static properties (tensile test) were less influenced by ε_Σ and by the design of rolling schedule (with or without roughing), the properties measured by the low temperature impact test (at -30°C) were more sensitive. By the optimizing process parameters of direct charging (finishing at temperatures of lower austenite, employing accelerated cooling after rolling) mechanical properties superior to those of cold charging (despite of largely reduced ε_Σ) are attainable.     

Influence of hot deformation on the transformation behaviour and structure of pearlitic steels

A study was made of the influence of hot deformation on the transformation behaviour, the structure and the mechanical properties of a pearlitic steel containing 0.65% C. The production parameters of a modern hot strip mill were taken as a basis for the deformation schedule and cooling performed with the aid of a hot deformation simulator (Wumsi). Parameters to be pointed out as significantly influencing the transformation behaviour are, in particular, the finishing temperature and the cooling rate after hot deformation. By exploiting the possibility of raising the cooling rate after deformation in the same measure as is attainable on a hot strip mill, a yield strength increase of at least 150 MPa is achievable.     

High-temperature deformation behavior of coarse- and fine-grained MoSi<Subscript>2</Subscript> with different silica contents

The elevated-temperature deformation behavior of polycrystalline molybdenum disilicide (MoSi₂), in the range of 1000 °C to 1350 °C at the strain rates of 10⁻³, 5×10⁻⁴, or 10⁻⁴ s⁻¹, has been studied. The yield strength, post-yield flow behavior comprising strain hardening and serrations, as well as some of the deformation microstructures of reaction-hot-pressed (RHP) MoSi₂ samples, processed by hot pressing an elemental Mo + Si powder mixture and having a grain size of 5 μm and oxygen content of 0.06 wt pct, have been compared with those of samples prepared by hot pressing of commercial-grade Starck MoSi₂ powder, with a grain size of 27 μm and oxygen content of 0.89 wt pct. While the fine-grained RHP MoSi₂ samples have shown higher yield strength at relatively lower temperatures and higher strain rates, the coarse-grained Starck MoSi₂ has a higher yield at decreasing strain rates and higher temperatures. The work-hardening or softening characteristics are dependent on grain size, temperature, and strain rate. Enhanced dislocation activity and dynamic recovery, accomplished by arrangement of dislocations in low-angle boundaries, characterize the deformation behavior of fine-grained RHP MoSi₂ at a temperature of 1200 °C and above and are responsible for increased uniform plastic strain with increasing temperature. The silica content appears to be less effective in degrading the high-temperature yield strength if the grain size is coarse, but leads to plastic-flow localization and strain softening in Starck MoSi₂. Serrated plastic flow has also been observed in a large number of samples, mostly when deformed at specific combinations of strain rates and temperatures.     

Workability of commercial-purity titanium and 4340 steel during equal channel angular extrusion at cold-working temperatures

The deformation and failure of commercial-purity (CP) titanium (grade 2) and AISI 4340 steel (tempered to R _c 35) during equal channel angular extrusion were determined at temperatures between 25 °C and 325 °C and effective strain rates between 0.002 and 2.0 s⁻¹. The CP titanium alloy underwent segmented failure under all conditions except at low strain rates and high temperatures. By contrast, the 4340 steel deformed uniformly except at the highest temperature and strain rate, at which it also exhibited segmented failure. Using flow curves and fracture data from uniaxial compression and tension tests, workability analysis was conducted to establish that the failures were a result of flow localization prior to the onset of fracture. This conclusion was confirmed by metallographic examination of the failed extrusion specimens.     

Effect of Boron on the Hot Ductility Behavior of a Low Carbon Advanced Ultra-High Strength Steel (A-UHSS)

This research work studied the effect of boron additions (14, 33, 82, 126, and 214 ppm) on the hot ductility behavior of a low carbon advanced ultra-high strength steel. For this purpose, specimens were subjected to a hot tensile test at different temperatures [923 K, 973 K, 1023 K, 1073 K, 1173 K, and 1273 K (650 °C, 700 °C, 750 °C, 800 °C, 900 °C, and 1000 °C)] under a constant true strain rate of 10^?3 s^?1. The reduction of area (RA) of the tested samples until fracture was taken as a measure of the hot ductility. In general, results revealed a marked improvement in hot ductility from 82 ppm B when the stoichiometric composition for BN (0.8:1) was exceeded. By comparing the ductility curve of the steel with the highest boron content (B5, 214 ppm B) and the curve for the steel without boron (B0), the increase of hot ductility in terms of RA is over 100 pct. In contrast, the typical recovery of hot ductility at temperatures below the Ar₃, where large amounts of normal transformation ferrite usually form in the structure, was not observed in these steels. On the other hand, the fracture surfaces indicated that the fracture mode tends to be more ductile as the boron content increases. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism, which in turn causes hot ductility loss. In general, results are discussed in terms of boron segregation and precipitation on austenitic grain boundaries during cooling from the austenitic range and subsequent plastic deformation.