Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 1 – Strip processing

Abstract

Simulations of industrial thermomechanical processing and on line accelerated cooling of a low carbon microalloyed strip steel were carried out using a quench deformation dilatometer. Effects of processing parameters, such as accelerated cooling rate T and accelerated cooling interrupt temperature T_I on the critical transformation temperatures and final microstructure were determined. The most important on line accelerated cooling (OLAC) processing parameter is the accelerated cooling interrupt temperature, which controls whether the transformed microstructure is predominantly ferrite or bainite. A variety of (Ti, Nb, Fe) carbide, nitride, and carbonitride precipitates are present in the OLAC processed samples. The final precipitate distribution is developed at three stages of processing, namely: dissolution and coarsening of pre-existing precipitates at the reheat temperature, precipitation in deformed austenite during the deformation schedule, and precipitation in ferrite after the interruption of accelerated cooling. Maximum precipitation strengthening occurs for T_I=700–640°C.

MST/3424     

Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 2—Plate processing

The effects of three on line accelerated cooling parameters (accelerated cooling start temperature T_A, cooling rate ?, and cooling interrupt temperature T₁) on transformation temperatures and microstructure in a low carbon microalloyed plate steel were studied by laboratory simulations in a quench deformation dilatometer. Varying the on line accelerated cooling parameters changes the austenite condition and transformation path. In general, the transformation path shifts from polygonal ferrite towards bainite with increasing T_A, increasing ?, and decreasing T₁. There is also a corresponding refinement in the microstructure and increase in hardness. In comparison with the laboratory thermomechanical processing treatments, the multipass industrial rolling schedule produces a much more heavily deformed austenite structure than laboratory thermomechanical processing treatments, which would favour high transformation temperatures, fine polygonal ferrite microstructure, and lower hardness.

MST/3425     

Correlation of microstructures and low temperature toughness in low carbon Mn–Mo–Nb pipeline steel

Abstract

By adjusting thermomechanical controlled processing parameters, different microstructures were obtained in a low carbon Mn–Mo–Nb pipeline steel. The microstructural characteristic and its effect on low temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallisation region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite microstructure with coarse M/A islands. The fine effective grain size (EGS) and the high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M–A/ferrite matrix interfaces. Compared with the microstructure mainly consisting of quasi-polygonal ferrite, the microstructure dominated by AF exhibits excellent low temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.     

Microstructural evolution during thermomechanical processing of microalloyed medium carbon steels

Abstract

A laboratory study was carried out to determine the characteristics of austenite grain growth and recrystallisation, strain induced precipitation, and continuous cooling transformation kinetics for two microalloyed medium carbon steels (1541 + Ti,V and 1541 + Nb). Austenite grain refinement is achieved by a combination of undissolved carbonitride precipitates at the reheat temperature, deformation recrystallisation at temperatures above T _NR and strain induced carbonitride precipitation. Deformation below T _NR promotes transformation to grain boundary ferrite (GBF), intragranular ferrite (IGF), and pearlite (P) at the expense of bainite (B) in both steels. This is attributed to increased density of nucleation sites for ferrite and pearlite at austenite grain boundaries, twin boundaries, and deformation bands. The results suggest that thermomechanical forging schedules could be designed to produce refined F + P microstructure, and hence, to realise improved strength, toughness, and machinability in the forging.     

Effects of chemical composition and thermomechanical treatment on austenite to ferrite transformation in 12 wt-%Cr steels

Abstract

The kinetics of austenite to ferrite transformation was studied in 11–12 wt-%Cr steels having an essentially austenitic microstructure at hot rolling temperatures (750–1050°C). The effects of chemical composition, high temperature γ/δ phase balance, and deformation before the transformation were assessed. The phase transformation was monitored using dilatometry, metallography, and hardness measurements. Small variations in chemical composition, particularly in the nickel and manganese content, resulted in significant differences in the kinetics of the transformation. These are a result of changes in the Ac₁ temperature, pre-existing δ ferrite content at high temperature, and probably the solute drag effect. Deformation at low temperatures of 850–750°C accelerated the transformation. The magnitude of this effect wasfound to depend on the degree of deformation and the cooling rate above the transformation temperature. Using a reduction of 30%, the cooling rate that resulted in a specific fraction of ferrite in the final structure was increased threefold. The results suggest that if the steel composition, particularly the nickel and manganese content, can be adjusted within narrow limits, controlled rolling together with controlled, retarded cooling can be applied to produce 11–12 wt-%Cr steels with adequate mechanical properties and excellent weldability, without the need for tempering.     

Optimisation of hot rolling schedule for direct charging of thin slabs of Nb–V microalloyed steel

Abstract

A recently developed continuous casting simulator and the ‘Wumsi’ hot deformation simulator have been used to carry out laboratory simulation tests to determine the as cast microstructure and the recrystallisation behaviour of a Nb–V microalloyed steel during the process of direct charging. By variation of the initial specimen thickness (between 25 and 60 mm) different values of total strain Φ_Σ could be imposed to improve the coarse as cast microstructure. For a series of deformation schedules the total strain was divided systematically into two components: an austenite grain refining strain Φ_γGF (above the recrystallisation stop temperature T_RS) and an austenite strengthening strain Φ_γS (below T_RS). After hot deformation slow and accelerated cooling with simulated coiling were employed. It was found that a total strain Φ_Σ>1·4 is required to ensure mechanical properties that were comparable or even superior to those found using the conventional cold charging process. The coarse as cast austenite microstructure can be refined significantly when Φ_γGF=0·3–0·6. The austenite strengthening strain Φ_γS represents the dominant component of the total strain if a satisfactory toughness is to be achieved. Strength properties are less sensitive to the applied strain.

MST/1872     

Microstructural evolution during simulated OLAC processing of a low-carbon microalloyed steel

The microstructural evolution during simulated on-line accelerated cooling (OLAC) of a commercial Grade 80 pipe steel was studied using a quench deformation dilatometer. The transformed matrix microstructure contains various amounts of polygonal ferrite, granular bainite and acicular ferrite, depending mainly on the accelerated-cooling interrupt temperature. The final microstructure is predicted well by drawing the OLAC schedule on the appropriate CCT diagram. Three distinct groups of precipitates are found in the final microstructure, which form during reheat, austenite deformation, and cooling, respectively. The distribution and composition of the precipitates varies widely with steel composition and processing schedule. The microstructure of industrially processed plate agrees well with that of corresponding laboratory simulations.     

Systematic investigation of acicular ferrite formation on laboratory scale

By increasing the amount of acicular ferrite (AF) in the microstructure, steel toughness can be improved significantly. The steel composition, cooling rate, non-metallic inclusions and austenite grain size have a strong influence on the formation of AF. The present paper describes and compares two approaches to study AF formation in a titanium-deoxidised high-strength low-alloyed steel and its influencing factors on laboratory scale: route A simulates the formation of AF after heat treatment; route B simulates the formation directly after solidification of the melt. The formation of AF is essentially influenced by the former processing, which also changes the optimum cooling parameters substantially. (Ti,Mn)_xO_y and (Ti,Al,Mn)_xO_yS_z are the predominant active inclusion types in the investigated steel.     

Pressure induced martensite transformation in plain carbon steel

Abstract

In the present study, plain low carbon steel with 0·033 wt-% carbon content was subjected to severe pressure during continuous cooling from austenite region. The pressure increased gradually and then suddenly released by the breakdown of ram under pressure. As a result, a microstructure composed of 80% lath martensite and 20% ferrite was produced. Results showed that the martensite formation is not due to the effect of cooling rate but the effect of hydrostatic pressure on the austenite to ferrite transformation start temperature Ar₃.     

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract

The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar ₃, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar ₃, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar ₃ temperature, decreased the time of transformation, and increased the hardness.     

Effect of heating rate on reaustenitisation of low carbon niobium microalloyed steel

Abstract

Austenite formation during a continuous heating in a low carbon niobium microalloyed steel with a pearlite and ferrite initial microstructure has been studied. Characteristic transformation temperatures, Ac ₁, Ac _θ and Ac ₃ and the evolution of austenite formation have been determined by combining dilatometry and metallography in a range of heating rates from 0˙05 to 10 K s^–1. It has been observed that nucleation and growth of austenite depends highly on the applied heating rate. At low heating rates (0˙05 K s^–1) nucleation of austenite takes place both at pearlite nodules and at ferrite grain boundaries, while for higher heating rates (≥0˙5 K s^–1), nucleation at grain boundaries is barely present compared to the nucleation at pearlite nodules. The heating rate also affects the austenite growth path and morphology and, thus, the distribution of martensite in the dual phase microstructure obtained at room temperature.     

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Abstract

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s^-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.     

Differential scanning calorimetry study of diffusional and martensitic phase transformations in some 9 wt-%Cr low carbon ferritic steels

Abstract

The results of a comprehensive characterisation study of different phase transformations that take place upon heating and cooling in some low carbon, 9 wt-%Cr steels with varying concentrations of microalloying additions are presented in this paper. The steels investigated include: standard 9Cr–1Mo grade, V and Nb added modified 9Cr variety, controlled silicon added versions of plain 9Cr variety, (Ni+Mn) content controlled modified 9Cr welding consumables and one composition of W, Ta added reduced activation steel. The various on-heating diffusional phase changes up to the melting range and subsequent rapid cooling induced martensitic transformations are investigated in a controlled manner using differential scanning calorimetry under different heating and cooling rates, in the range 1–100 K min^?1. In addition to the accurate determination of Ac₁, Ac₃, M₂₃C₆, MX carbide dissolution and δ-ferrite formation temperatures upon heating, the melting range and the associated fusion enthalpy have also been established for these steels. The effect of prolonged thermal aging at temperatures of 823–873 K on austenite formation characteristics has also been investigated for standard and modified 9Cr–1Mo steels. The critical cooling rate for the formation of martensite on cooling from single phase austenite region is estimated to be about 4–5 K min^?1 for all 9Cr steels investigated in this study. The effect of holding at 1273 K in the austenite region on martensite start temperature M_s, has also been evaluated as a part of this study. The experimental results are discussed in the light of the prevailing understanding of the physical metallurgy of high chromium low carbon steels.     

Acicular ferrite formation during hot plate rolling for pipeline steels

Abstract

The transformation of supercooled austenite in a commercial pipeline steel was investigated by means of continuous cooling transformation (CCT) and hot simulation experiments. Based on the obtained results, an improved thermomechanical control process (TMCP) was proposed, which could produce a mixed microstructure dominated by acicular ferrite. Results indicated that an increase in the cooling rate could improve the percentage of acicular ferrite in the final microstructure under the present experimental conditions. Furthermore, the acicular ferrite dominated microstructure could be obtained by a two stage controlled rolling in the austenite recrystallisation region plus the non-recrystallisation region and controlled cooling at a cooling rate of 30 K s^-1.     

Deformation induced martensite characteristics in Fe–29Ni–2Mn alloy

Abstract

Deformation induced martensite characteristics in the austenite phase of Fe–29Ni–2Mn alloy were studied for different austenite grain sizes of alloy. Scanning electron microscopy, transmission electron microscopy, Mössbauer spectroscopy and also differential scanning calorimetry techniques were applied to study in order to clarify the deformation induced martensite characteristics from morphological, crystallographical, magnetical and thermal points of view. Scanning electron microscope revealed that the increasing of deformation amount also increased the amount of existed martensite. Transmission electron microscope observations showed that the crystal structure of these deformation induced martensites morphology was lenticular plates with a bcc crystal structure. Also the magnetism of both austenite and martensite phases were determined with Mössbauer spectroscopy. Mössbauer spectrometer measurements showed paramagnetic character for austenite phases and ferromagnetic character for martensite phases in all samples. According to obtained differential scanning calorimetry cooling curves, deformation induced martensite start temperature M _d was found to be higher (?128°C) for larger grained samples than for smaller grained samples (?135°C).     

Carbon partitioning during bainite transformation in low alloy steels

Abstract

Carbon partitioning in untransformed austenite during bainite transformation has been studied using high speed dilatometry. It was found that in specimens partially transformed to bainite, during subsequent quenching to ambient temperature two martensite start temperatures M _s can be registered. Because M _s depends directly on a carbon content in austenite, the obtained results may indicate that the carbon concentration trapped in films of austenite between parallel subunits of bainitic ferrite is much larger than in the blocks of austenite. It would indicate the necessity of a substantial modification of bainite and martensite regions on the time–temperature–transformation (continuous cooling) diagrams.     

Effects of cooling process on microstructure and hardness for 1·0C–1·5Cr bearing steel

Effects of cooling rate (V_cr) and final cooling temperature (T_ft), after hot deformation, on microstructure and hardness for 1·0C–1·5Cr bearing steel were investigated. The results show that if V_cr increases from 2 to 25°C s^?1 and T_ft remains at 650°C, pearlite colony size and grain size both decrease, hardness increases. When V_cr exceeds 8°C s^?1, carbide network can be restrained effectively. TEM micrographs indicate that there exist branches in the local region of lamellar cementite and ferrite, and a ferrite thin film is also found around the proeutectoid carbide. Under the cooling rate of 10°C s^?1, with the increase in T_ft, the microstructure changes from martensite into pearlite, carbide network becomes more serious and hardness decreases.     

New parameter Y for static recrystallisation in hot deformed austenite

Abstract

The influence of hot deformation conditions on static and dynamic recrystallisation behaviour of 18%Ni maraging steel was studied. Using the Zener–Hollomon parameter in the dynamic recrystallisation diagram for reference, a new parameter Y (=tZ^mexp(?Q_rec/RT)) is recommended and a static recrystallisation diagram proposed. The occurrence of static recrystallisation of hot deformed austenite in 18%Ni maraging steel was determined as a function of the parameter Y (the holding time modified by temperature and strain rate). The structural changes after deformation in hot deformed austenite may be described by the static recrystallisation diagram (Y–? diagram). The procedure for constructing the static recrystallisation diagram may be simplified by introducing the parameter Y.

MST/699     

Kinetics of martensite formation in plain carbon steels: critical assessment of possible influence of austenite grain boundaries and autocatalysis

Abstract

The kinetics of the martensitic transformation in Fe–0·80C has been determined from dilatometry data and shows no significant variation when the cooling rate is changed by two orders of magnitude. All kinetic data can be adequately simulated by the Koistinen and Marburger (KM) equation using a specific start temperature T_KM and rate parameter α_m. This finding supports the suggestion that the transformation is athermal, and moreover, the absence of a time dependence strongly indicates that autocatalytic nucleation does not contribute to the transformation kinetics in plain carbon steels on measurable time scales. Furthermore, dilatometry experiments with different austenitising conditions were conducted to examine the effect of the prior austenite grain size on the overall kinetics of martensite formation. The present results indicate that the progress of martensite formation beyond a fraction f?=?0·15 is independent of the prior austenitising treatment. It is therefore concluded that austenite–austenite grain boundaries have no significant effect on the overall nucleation and growth of athermal martensite, which is consistent with a model proposed by Ansell and co-workers.     

Understanding the low temperature end of the hot ductility trough in steels

Abstract

The low temperature end of the hot ductility trough has been examined for steels which have been solution treated at ～1300°C before tensile testing in the temperature range of 1000–600°C. Failure in the trough in this region is intergranular ductile and occurs by strain intensification in the thin film of ferrite surrounding the prior austenite grain. The strain causes voiding to occur at the inclusions situated at the boundaries, the cavities gradually linking up to give failure. In steels which are solution treated before tensile testing, the depth of the trough is shown to be controlled by the volume fraction of the second phase particles, their size and the separation between the particles. Recovery in ductility on the low temperature side of the trough is solely dependent on being able to produce a sufficiently large quantity of ferrite to prevent strain concentration (～40%). Often this has to await the test temperature falling below the AR ₃ in which case wide troughs are formed. However, if conditions are right, very narrow troughs can be produced in which the ferrite that is formed is deformation induced. The width of the trough at the low temperature end of the trough is shown to decrease with increase in strain rate, refinement of the austenite grain size, increase in cooling rate from the solution treatment temperature, decrease in the volume fraction of sulphides situated at the austenite grain boundaries and reduction in the Mn and C contents. The depth of the trough decreases in a similar manner with all these variables except for C and Mn, where for the former there is no effect and for the latter, increasing the Mn level reduces the depth. Narrow troughs on this side of the trough are dependent on being able to form deformation induced ferrite in sufficiently large amounts so as to improve the ductility at temperatures above the AR ₃. A model is proposed to account for most of these observations.