Effect of intercritical annealing temperature and cooling rate on the microstructure and mechanical properties of low-carbon aluminum killed steel

In this study, the intercritical annealing process for a typical low-carbon aluminum killed steel is investigated. A cold-rolled sheet was annealed at intercfitical temperatures ranging from 730 ℃ to 770 ℃ and then cooled in air or water. The annealed steel was then baked at 210 ℃, and its mechanical properties and microstructures were analyzed in detail. It is shown that after the air-cooling process,the strength of steel decreased and ductility increased with an increase in the annealing temperature. However, after water-cooling, the strength and ductility both increased with the increase of annealing temperature. These results are attributed to the property- optimization of the steel.

Microstructure and mechanical properties of low-carbon Q & P steel pretreated with intercritical annealing

The success of obtaining both high strength and high formability in low-carbon quenched and partitioned( Q & P) steels depends on their microstructural constituents. In this regard,the effect of annealing temperature on the volume fraction and distribution of carbon in retained austenite in a low-carbon Q & P steel was studied. The microstructures of Q & P steels subjected to different annealing temperatures were studied in detail by electron microscopy,electron microprobe,and X-ray diffraction techniques. The results suggest that intercritical annealing is beneficial in increasing the volume fraction of retained austenite,which is a consequence of the distribution of alloying elements during intercritical annealing. Moreover,the mechanical properties of intercritically annealed Q & P steel,especially its ductility,are significantly enhanced.

Effects of Silicon Content and Intercritical Annealing on Manganese Partitioning in Dual Phase Steels

Steels of constant manganese and carbon contents with silicon content of 0.34 %- 2.26% were cast. The as-cast steels were then hot rolled at 1 100 ℃ in five passes to reduce the cast ingot thickness from 80 to 4 mm, air cooled to room temperature and cold rolled to 2 mm in thickness. Dual phase microstructures with different volume fraction of martensite were obtained through the intercritical annealing of the steels at different temperatures for 15 min followed by water quenching. In addition to intercritical annealing temperature, silicon content also altered the volume fraction of martensite in dual phase steels. The partitioning of manganese in dual phase silicon steels was investigated using energy-dispersive spectrometer （EDS）. The partitioning coefficient, defined as the ratio of the amounts of alloying element in the austenite to that in the adjacent ferrite, for manganese increased with increasing intercritieal annealing temperature and silicon content of steels. It was also found that the solubility of manganese in ferrite and austenite decreased with increasing intereritical temperature. The results were discussed by the diffusivity and the solubility of manganese in ferrite and austenite existed in dual phase silicon steels.

Cellular automaton modeling of austenite formation from ferrite plus pearlite microstructures during intercritical annealing of a C-Mn steel

A mesoscopic cellular automaton model was developed to study the microstructure evolution and solute redistribution of austenization during intercritical annealing of a C-Mn steel. This model enables a depiction of three-stage kinetics of the transformation combined with the thermodynamic analysis:(1) the rapid austenite growth accompanied with pearlite degeneration until the pearlite dissolves completely;(2) the slower austenite growth into ferrite with a rate limiting factor of carbon diffusion in austenite;and(3) the slow austenite growth in control of the manganese diffusion until the final equilibrium reached for ferrite and austenite. The effect of the annealing temperature on the transformation kinetics and solute partition is also quantitatively rationalized using this model.

Deformation mechanism of bimodal microstructure in Ti-6Al-4V alloy:The effects of intercritical annealing temperature and constituent hardness

The so-called bimodal microstructure of Ti-6 Al-4 V alloy,composed of primaryαgrains(α_(p))and transformed β areas(β_(trans)),can be regarded as a"dual-phase"structure to some extent,the mechanical properties of which are closely related to the sizes,volume fractions,distributions as well as nanohardness of the two constituents.In this study,the volume fractions of primaryαgrains(vol.%(α_(p)))were systematically modified in three series of bimodal microstructures with fixed primaryαgrain sizes(0.8μm,2.4μm and 5.0μm),by changing the intercritical annealing temperature(T_(int)).By evaluating the tensile properties at room temperature,it was found that with increasing T_(int)(decreasing vol.%(α_(p))),the yield strength of bimodal microstructures monotonically increased,while the uniform elongation firstly increased with T_(int)until 910°C and then drastically decreased afterwards,thereby dividing the T_(int)into two regions,namely region I(830-910°C)and region II(910-970℃).The detailed deformation behaviors within the two regions were studied and compared,from the perspectives of strain distribution analysis,slip system analysis as well as dislocation analysis.For bimodal microstructures in region I,due to the much lower nano-hardness ofβ_(trans)thanα_(p),there was a clear strain partitioning between the two constituents as well as a strain gradient from theα_(p)/β_(trans)interface to the grain interior ofα_(p).This activated a large number of geometrically necessary dislocations(GNDs)near the interface,mostly with components,which contributed greatly to the extraordinary work-hardening abilities of bimodal microstructures in region I.With increasing T_(int),theα_(p)/β_(trans)interface length density gradually increased and so was the density of GNDs with components,which explained the continuous increase of uniform elongation with T_(int)in this region.For bimodal microstructures in region II,where the nano-hardness ofβ_(trans)andα_(p)were comparable,neither a clear strain-partitioning tendency nor a strain gradient across theα_(p)/β_(trans)interface was observed.Consequently,only statistically stored dislocations(SSDs)with component were activated insideα_(p).The absence of dislocations together with a decreased volume fraction ofα_(p)resulted into a dramatic loss of uniform elongation for bimodal microstructures in region II.

Microstructure and Tensile Properties of a Nb–Mo Microalloyed 6.5Mn Alloy Processed by Intercritical Annealing and Quenching and Partitioning

The transformation behavior, microstructural evolution and mechanical properties were compared in a coldrolled Nb–Mo microalloyed 6.5Mn alloy after intercritical annealing（IA） and quenching and partitioning（Q ＆ P）,respectively. The thermodynamic calculation and theoretical analysis were used to determine the optimal heat treatment parameters. The Q ＆ P samples exhibited relatively higher strength with relatively low ductility, mainly due to the hard martensite matrix, which resulted in continuous yielding behavior upon loading, whereas the IA samples showed the significantly improved ductility, which benefited from the more sufficient transformation-induced plasticity（TRIP） effects and the softer ultrafine ferrite matrix. The dependence of yield point elongation（YPE） of IA samples on grain size demonstrated that the YPE value was in the reverse proportional relationship to the average grain size, which agreed well with theoretical analysis.

Stability of Retained Austenite Through a Combined Intercritical Annealing and Quenching and Partitioning(IAQP) Treatment

Intercritical annealing（IA） at various temperatures followed by quenching and partitioning（IAQP） treatments was conducted on a cold-rolled Fe-0.2C-1.42Si-l.87Mn（wt%） sheet steel.Optimized microstructure and enhanced mechanical properties were achieved through appropriate adjustment of IA temperatures.The steel which was annealed at1,033 K for 600 s,then quenched to 573 K and partitioned at 693 K for 20 min,designated as 1033 QP steel,exhibits maximum 16.3 vol% retained austenite（RA） with good mechanical properties（ultimate tensile strength 886 MPa and total elongation 27%）.It was found that the thermal and mechanical stabilities of RA are mainly influenced by the combined effect of its average carbon content and amount of adjacent martensite.Furthermore,the transformation-induced plasticity effect increased the peak n-values observed at the second stage of the work hardening of IAQP steels.

Effect of Intercritical Annealing Time on the Microstructures and Tensile Properties of a High Strength TRIP Steel

A new Mn-Si-Al-Mo-Nb transformation-induced plasticity steel was annealed by intercritical annealing for different durations to investigate the partitioning of C element and the volume fraction change of the microstructural constituents. Direct experimental evidence confirms the partitioning of C elements in different phases during heat treatment by Electron probe microanalysis and X ray diffraction. The distribution of the precipitates was investigated as well. It was revealed that the microstructures and mechanical properties of the investigated steels were affected by the intercritical annealing time. According to the present experiment, the volume fraction of retained austenite and the product of tensile strength and total elongation of investigated steel decrease with increasing intercritical annealing time. It was observed that high tensile strength of 1,103 MPa, total elongation of 21.3%, and strength-ductility product of 23,493.9 MPa % could be successfully produced in this experimental steel at intercritical annealing temperature of 830 ℃, holding for 1 min, and isothermal bainite treatment of 440 ℃ for 5 min holding time.

Mn Diffusion at Early Stage of Intercritical Annealing of 5Mn Steel

Mn distribution and austenite morphology at the early stage of intercritical annealing of 5Mn steel were investigated. It was experimentally demonstrated that a newly formed 20 nm-thick austenite was formed without the partitioning of Mn. The elemental analysis confirmed that the growth of austenite should be controlled by the diffusion of C prior to the diffusion of Mn at a low heating rate. The austenite growth started under negligible-partitioning local equilibrium mode and then switched to partitioning local equilibrium mode. Mn segregation at the γ/α interface suggested that the collector plate mechanism was the essential way of Mn partitioning at the early stage of austenite growth.

Effect of continuous annealing parameters on the mechanical properties and microstructures of a cold rolled dual phase steel

A cold rolled dual phase （DP） steel with the C-Si-Mn alloy system was trial-produced in the laboratory, utilizing a Gleeble-3800 thermal simulator. The effects of continuous annealing parameters on the mechanical properties and microstructures of the DP steel were investigated by mechanical testing and microstructure observation. The results show that soaking between 760 and 820℃ for more than 80 s, rapid cooling at the rate of more than 30℃/s from the quenching temperature between 620 and 680℃, and overaging lower than 300℃ are beneficial for the mechanical properties of DP steels. An appropriate proportion of the two phases is one of the key factors for the favorable properties of DP steels. If the volume fraction of martensite and, thereby, free dislocations are deficient, the tensile strength and n value of DP steels will decrease, whereas, the yield strength will increase. But if the volume fraction of martensite is excessive to make it become a dominant phase, the yield and tensile strength will increase, whereas, the elongation will decrease obviously. When rapid cooling rate is not fast enough, pearlite or cementite will appear, which will degrade the mechanical properties. Even though martensite is sufficient, if it is decomposed in high temperature tempering, the properties will he unsatisfied.

Effect of Fast Cooling Rate on the Microstructure and Mechanical Properties of Low-Carbon High-Strength Steel Annealed in the Intercritical Region

The effect of fast cooling rate on the microstructure and mechanical properties of low-carbon high-strength steel annealed in the intercritical region was investigated using a Gleeble 1500 thermomechanical simulator and a continuous annealing thermomeehanical simulator. The results showed that the microstructure consisted of ferrite and bainite as the main phases with a small amount of retained austenite and martensite islands at cooling rate of 5 and 50 ℃/s, respectively. Fast cooling after continuous annealing affected all constituents of the microstructure. The mechanical properties were improved considerably. Ultimate tensile strength （U-TS） increased and total elongation （TEL） decreased with increasing cooling rate in all specimens. The specimen 1 at a cooling rate of 5 ℃/s exhibited the maximum TEL and UTSxTEL （20% and 27 200 MPa%, respectively） because of the competition between weakening by presence of the retained austenite plus the carbon indigence by carbide precipitation, and strengthening by martensitic islands and precipitation. The maximum UTS and YS （1 450 and 951 MPa, respectively） were obtained for specimen 2 at a cooling rate of 50 ℃/s. This is attributed to the effect of dispersion strengthening of finer martensite islands and the effect of precipitation strengthening of carbide precipitates.

MECHANICAL PROPERTIES IN AN INTERCRITICALLY HEAT-TREATED BAINITE-TRANSFORMED 2%Si STEEL

A significant amount of austenite can be retained by rapid cooling following intercritical annealing and holding at the bainite transformation range in steel with comparatively low carbon and silicon contents. Retained austenite is blocky and very fine and moderately stabilized due to C enrichment. The elongation and the strength-ductility balance of the steel can be enhanced considerably due to strain-induced martensite transformation and transformation-induced plasticity (TRIP) of retained austenite.

Multiphase-field simulation of austenite reversion in medium-Mn steels

Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility.This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite.Such a unique microstructure is processed by intercritical annealing,where austenite reversion occurs in a fine martensitic matrix.In the present study,austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS®coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2.In particular,a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite.The simulated microstructural morphology and phase transformation kinetics(indicated by the amount of phase)concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction,respectively.

Cu Partitioning Behavior and Its Effect on Microstructure and Mechanical Properties of 0.12C-1.33Mn-0.55Cu Q&P Steel

Cu, as an austenitic stable element, is added to steel in order to suppress the adverse effects of high content of C and Mn on welding. Based on C partitioning, Cu and Mn partitioning can further improve the stability of retained austenite in the intercritical annealing process. A sample of low carbon steel containing Cu was treated by the intercritical annealing, then quenching process（I＆Q）. Subsequently, another sample was treated by the intercritical annealing, subsequent austenitizing, then quenching and partitioning process（I＆Q＆P）. The effects of element partitioning behavior in intercritical region on the microstructure and mechanical properties of the steel were studied. The results showed that after the I＆Q process ferrite and martensite could be obtained, with C, Cu and Mn enriched in the martensite. When intercritically heated at 800 ℃, Cu and Mn were partitioned from ferrite to austenite, which was enhanced gradually as the heating time was increased. This partitioning effect was the most obvious when the sample was heated at 800 ℃ for 40 min. At the early stage of α→γ transformation, the formation of γ was controlled by the partitioning of carbon, while at the later stage, it was mainly affected by the partitioning of Cu and Mn. After the I＆Q＆P process, the partitioning effect of Cu and Mn element could be retained. C was assembled in retained austenite during the quenching and partitioning process. The strength and elongation of I＆Q＆P steel was increased by 5 305 MPa% compared with that subjected to Q＆P process. The volume fraction of retained autensite was increased from 8.5% to 11.2%. Hence, the content of retained austenite could be improved significantly by Mn and Cu partitioning, which increased the elongation of steel.

Effects of Holding Temperature for Austempering on Mechanical Properties of Si-Mn TRIP Steel

A new type of high strength steel containing a significant amount of stable retained austenite was obtained by austempering immediately after intercritical annealing.This sort of low carbon steel only contains alloying elements of silicon and manganese rather than nickel and chromium.Its mechanical properties were enhanced considerably due to strain-induced martensite transformation and transformation-induced plasticity(TRIP)of retained austenite when it was strained at temperatures between Msand Md,because retained austenite was moderately stabilized due to carbon enrichment by austempering.Austempering was carried out at different temperatures and 400 ℃ was found to be optimal.Tensile strength,total elongation and strength-ductility balance reached the maximum values and the product of tensile strength and total elongation exceeded 30 135 MPa % when the TRIP steel was held at 400 ℃ and strained at 350 ℃.

Effect of controlled rolling on the martensitic hardenability of dual phase steel

The C-Mn and C-Mn-Nb steels were thermo-mechanically processed to develop dual phase steel and to study the effect of controlled rolling on the martensitic hardenability of austenite. The steel specimens were intercritically annealed at 790℃, rolled at that temperature to the reductions of 10%, 23%, and 47% and immediately cooled at different rates. Quantitative metallography was used to construct the microstructure map, which illustrated that increasing deformation progressively reduced the proportion of new ferrite formed at all cooling rates and increased the amount of martensite at fast and intermediate rates. The martensitic hardenability of austenite remaining after all the rolling reductions was plotted as a function of cooling rates. It was observed that for the austenite-martensite conversion efficiencies greater than about 25%, controlled rolling increased the martensitic hardenability of austenite.

Effect of Silicon and Manganese on Mechanical Properties of Low-Carbon Plain TRIP Steel

A great deal of stabilized retained austenite can be obtained by means of austempering immediately after intercritical annealing in the low-carbon plain steel sheets which only contain alloying elements of silicon and manganese. Transformation from retained austenite to martensite may be induced by strain at a temperature ranging from 50 ℃ to 400 ℃ during tension testing. Transformation-induced plasticity (TRIP) may occur. Alloying of silicon improves the stability of retained austenite. Mechanical properties of the present TRIP steels containing manganese increase with increasing silicon amount when the amount of silicon is less than two percent.

The relevance of Niobium microalloying in dual phase steels with optimized mechanical properties

Dual phase steel is nowadays widely applied in automotive construction as hot rolled and cold rolled HDG grades.The strength and elongation of DP steels are principally determined by the ratio of ferrite and marteniste in the microstructure.However,for practical forming in the press shop additional properties are important such as bendability and hole expansion ratio.These characteristics relate to the morphology and distribution of the phases in the microstructure.Niobium microalloying can influence not only the strength of DP steels but also particularly the phase morphology and homogeneity leading to significant improvement of the mechanical properties.The paper will show processing strategies involving Nb microalloying in DP steel production.The metallurgical mechanisms induced by Nb are discussed.This is also related to damage mechanisms occurring in DP steel during forming or application.Particularly the issue of delayed fracturing in ultra high strength DP steel will be addressed.

A novel design to enhance the stability of local austenite and the volume fraction of retained austenite in a low-carbon Si Mn Q-P steel

Pre-quenching prior to intercritical annealing quenching and partitioning(Q-P)process was proposed to enhance the volume fraction of retained austenite and the mechanical properties of a low-carbon Si Mn steel.The intercritical austenite exhibited a lath morphology due to the martensitic microstructure maintained prior to intercritical annealing.Consequently,the alloy element enrichment of intercritical austenite,in which the alloy element was aggregated at the austenitic boundaries and further diffused inside,improved the stability of intercritical austenite and decreased the M_(s) of it.As a result,the fraction of retained austenite in steel was increased,which improved the mechanical properties of the experimental Q-P steel.

The microstructure evolution and tensile properties of medium-Mn steel heat-treated by a two-step annealing process

The martensitic hot-rolled 0.3 C-6 Mn-1.5 Si(wt%)steel was annealed at 630℃for 24 h to improve its cold rollability,followed by cold rolling and annealing at 670℃for 10 min.The annealing process was designed based on the capacities of industrial batch annealing and continuous annealing lines.A duplex submicron austenite and ferrite microstructure and excellent tensile properties were obtained finally,proved the above process is feasible."Austenite memory"was found in the hot-rolled and annealed sample which restricted recrystallization of lath martensite,leading to lath-shaped morphology of austenite and ferrite grains."Austenite memory"disappeared in the cold-rolled and annealed sample due to austenite random nucleation and ferrite recrystallization,resulting in globular microstructure and refinement of both austenite and ferrite grains.The austenite to martensite transformation contributed most of strain hardening during deformation and improved the uniform elongation,but the dislocation strengthening played a decisive role on the yielding behavior.The tensile curves change from continuous to discontinuous yielding as the increase of cold-rolled reduction due to the weakening dislocation strengthening of austenite and ferrite grains related to the morphology change and grain refinement.A method by controlling the cold-rolled reduction is proposed to avoid the Lüders strain.