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.

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.

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 ℃.

Determining role of heterogeneous microstructure in lowering yield ratio and enhancing impact toughness in high-strength low-alloy steel

Here we present a novel approach of intercritical heat treatment for microstructure tailoring,in which intercritical annealing is introduced between conventional quenching and tempering.This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite,resulting in a low yield ratio(YR)and high impact toughness in a high-strength low-alloy steel.The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing,in comparison to the steel with full martensitic microstructure.The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering.The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases,but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.

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.

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.

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.

Effect of Thermomechanical Processing on Microstructures of TRIP Steel

In order to control retained austenite, the effect of hot deformation in the intercritical region on the microstructure of hot-rolled transformation-induced plasticity （TRIP） steel was studied on a Gleeble 1500 hot simulator. Compressive strains varying in amounts from 0 to 60% were imposed inthe intercritical region, and effects on the formation of polygonal ferrite, carbide-free bainite and retained austenite were determined. With increasing the hot deformation amount and the ferrite content and decreasing the carbide-free bainite content, the volume fraction of retained austenite decreases. Increased dislocation density, grain refinement of ferrite and carbon enrichment are the main factors which control retained austenite stability.

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 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.

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.

The Inspection of Spheroidized Annealing on SCM435 Cold-Forging Quality Steel Wires with Protective Atmosphere

A cold-forging quality steel rod is usually applied for manufacturing wire which is generally produced by drawing wire coil into wire, followed by spheroidized annealing treatment to achieve the necessary formability for cold forging. The subcritical and intercritical processes are usually used to spheroidize the steel wires. The cold-forging quality SCM435 alloy medium carbon steel wires are widely used to manufacture high tension bolts for mechanical and heavy industry. In this study, the spheroidized annealing experiments on SCM435 alloy steel wires are conducted in a commercial bell furnace with a protective atmosphere of nitrogen or hydrogen. The mechanical properties of annealed steel wires are measured by tensile and Rockwell hardness tests and the process capability is evaluated. It is experimentally revealed that, for SCM435 alloy medium carbon steel wires, the wire quality with intercritical annealing is much better than that with subcritical annealing and is markedly affected by furnace atmospheres. The intercritical annealing quality on SCM435 alloy steel wire in hydrogen atmosphere furnace is better than in nitrogen atmosphere furnace. A comparison between the results obtained using the intercritical annealing with hydrogen atmosphere and the measures using the subcritical annealing shows that the intercritical annealing effectively improves the performance measures of low strength and high ductility over their values at the subcritical annealing. The results presented in this study could be a reference for fasters wire manufacturers.

Effects of Austenitising Conditions on the Microstructures and Mechanical Properties of Martensitic Steel with Dual Matrix Structure

A new high strength steel with dual matrix structure and exceptionally high toughness plus ductility have been produced by intermediate quenching of 0.22_wt% C microalloyed steel. The treatment consisted of initial austenitization and rapid quenching of the steel samples to achieve a fully martensitic state followed by annealing in the intercritical (α+γ) region of 730℃-810℃ for the period of 30, 60 and 90_minutes. These samples were subsequently quenched to obtain dual phase microstructure containing varying proportions of ferrite and martensite constituents. The mechanical properties of the samples were measured according to ASTM standard and their microstructures were analyzed by optical microscopy. The experimental results show that martensitic dual phase (MDP) steel samples developed within the intercritical temperature range of 770–790℃ revealed finer martensite and precipitate-free ferrite microstructure. The tensile and impact properties of the developed HMDP steels increased with intercritical annealing (ICA) temperatures, with an optimum properties obtained at 790℃ mainly due to finer microstructure of the constituent phases and absence of carbide precipitate that permit ease of dislocation flow. A further increase in the intercritical annealing temperature beyond 790℃ led to general decrease in the mechanical properties. This is attributed to the formation of coarse structure in this region. The results further show that with increasing intercritical treatment time from 30 to 90 minutes, the general mechanical properties of the MDP steels were found to increase except at the higher temperature of 810℃ which showed decreasing values. In general, the tensile strengths and ductility as well as the impact properties of the developed dual phase steel samples are greatly improved with the intercritical heat treatment investigated.

Carbide Dissolution during Intercritical Austenitization in Bearing Steel

In order to investigate the carbide dissolution mechanism of high carbon-chromium bearing steel during the intercritical austenitization, the database of TCFE7 of Thermo-calc and MOBFE of DICTRA software were used to calculate the elements diffusion kinetic and the evolution law of volume fraction of carbide. DIL805 A dilatometer was used to simulate the intercritical heat treatment. The microstructure was observed by scanning electron microscopy（SEM）, and the micro-hardness was tested. The experimental results indicate that the dissolution of carbide is composed of two stages： initial austenite growth governed by carbon diffusion which sharply moves up the micro-hardness of quenched martensite, and subsequent growth controlled by diffusion of Cr elements in M3 C. The volume fraction of M3 C decreases with the increasing holding time, and the metallographic analysis shows a great agreement with values calculated by software.

Effect of Vanadium Microalloying on the HAZ Microstructure and Properties of Low Carbon Steels

Four Steels,C-Mn-0.05V,C-Mn-0.11V,C-Mn-0.03Nb and C-Mn were subjected to heat treatment to simulate the microstructure of a coarse grained heat affected zone (CGHAZ) and an intercritically reheated coarse grained heat affected zone (ICCGHAZ).This involved reheating to 1350°C,rapid cooling (Δt 8/5 =24s) to room temperature and then reheating to either 750°C or 800°C.The toughness of the HAZs was assessed using both Charpy and CTOD tests.Microstructural features were characterised by optical,scanning` and transmission electron microscopy.Fractographic examinations of the Charpy and CTOD specimens were carried out to understand the micromechanism of fracture under different microstructural and test conditions.The CGHAZ toughness was similar for the steels except that Steel C-Mn-0.05V had a slightly lower ITT compared to the others.The toughness deteriorated in the ICCGHAZ for all the steels,again Steel C-Mn-0.05V had a superior toughness compared to the other three steels in both ICCGHAZ conditions.Raising the level of vanadium to 0.11% caused a decrease in ICCGHAZ toughness.Steel C-Mn-Nb exhibited a greater degradation of impact toughness after the intercritical cycles.The presence of M-A constituents was the dominant factor in determining the toughness of the ICCGHAZs.The size and area fraction of the M-A constituents were the smallest in Steel C-Mn-0.05V.Increasing vanadium level to 0.11% resulted in a greater area fraction of the M-A constituents,larger average and maximum sizes of M-A particles,and significantly more fields containing the M-A.The addition of 0.031% Nb produced the largest M-A particles and the greatest area fraction for the steels tested.

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.

Formation mechanism and control methods of acicular ferrite in HSLA steels:A review

High strength low alloy（HSLA） steels have been widely used in pipelines,power plant components,civil structures and so on,due to their outstanding mechanical properties as high strength and toughness,and excellent weldability.Multi-phase microstructures containing acicular ferrite or acicular ferrite dominated phase have been proved to possess good comprehensive properties in HSLA steels.This paper mainly focuses on the formation mechanisms and control methods of acicular ferrite in HSLA steels.Effect of austenitizing conditions,continuous cooling rate,and isothermal quenching time and temperature on acicular ferrite transformation was reviewed.Furthermore,the modified process to control the formation of multi-phase microstructures containing acicular ferrite,as intercritical heat treatments,step quenching treatments and thermo-mechanical controlled processing,was summarized.The favorable combination of mechanical properties can be achieved by these modified treatments.