Hot Ductility Behavior Research Articles

Effect of Ti micro-addition on the hot tensile behaviour, microstructure and fractography of low-C high-manganese steels

The aim of the work was to determine the effect of Ti micro-addition on the hot tensile behaviour, microstructure and fractography of two low-C high-manganese steels with additions of Si and Al. The hot tensile tests were performed using the Gleeble 3800 thermomechanical simulator. Samples were stretched at a temperature range from 1050 ℃ to 1200 ℃ at a strain rate of 2.5·10 3 s−1. The microstructure of the tested high-manganese steels under conditions of hot deformation was influenced by strain hardening and simultaneous dynamic recrystallization, as well as precipitation processes-depending on the chemical composition of the alloy and plastic deformation parameters. The analysis of the curves registered in the hot tensile tests indicated that a decrease of strain hardening was the result of the dynamic recrystallization. Hot tensile curves of the Ti-micro-alloyed steel were characterized by higher yield stress compared to the Ti-free steel. The Ti micro-addition with a concentration of 0.075 wt.% guaranteeing stable TiN-type nitrides eliminated the possibility of precipitating AlN-type nitrides and complex MnS-AlN type non-metallic inclusions, which are harmful to hot ductility. Fracture modes of the Ti-free steel showed a mixed nature from 1050 ℃ to 1150 °C, i.e. ductile fracture and numerous cavities and voids were identified. As the deformation temperature increased to 1200 °C, the fracture character was brittle with numerous inter-crystalline cracks along austenite grain boundaries. The addition of Ti improved significantly the hot ductility behaviour characterized by higher values of flow stress and reduction in area as well as ductile fracture modes in the entire high deformation temperature range.

Influence of microstructure evolution on hot ductility behavior of a Cr and Mo alloyed Fe-C-Mn steel during hot deformation

Cr and Mo alloyed Fe-C-Mn steel was conducted by hot tensile experiments under different strain rates and deformation temperatures. The influence of microstructure evolution on fracture mechanism was studied. The principal failure mechanism was attributed to intergranular crack propagation and microvoid aggregation when deformed at 800–1100 ℃, changing to a mixed ductile-brittle intergranular fracture when deformed at 1200 ℃. After hot deformation, the microstructure transformed from a mixed phase of pearlite and ferrite to a lath martensite structure. Moreover, the lath martensite structure transitioned from disorder to order as the deformation temperature increases. The study revealed that in case of lower temperature when dynamic recovery (DRV) was dominated, the growth of voids was impeded by fine dynamic recrystallization (DRX) grains, resulting in an irregular void morphology. In case of higher temperature, the degree of DRX was high, voids were surrounded and impeded by newly formed DRX grains, which were gradually elongate with deformation. On the other hand, when there were DRX grains present between voids, void coalescence was less likely to occur, thereby inhibiting damage propagation.

Effect of Cr on the Hot Ductility of Austenitic Fe-Mn-Al-C Lightweight Steel

In this study, a hot ductility test was performed for Fe-30Mn-10.5Al-0.9C-Cr austenitic lightweight steels. The test was carried out through a commercial Gleeble simulator at a heating rate of 350 °C/sec and cooling rate of 50 °C/sec, with a stroke rate of 50 mm/sec. Microstructural analysis for understanding the hot ductility behavior was conducted through optical and scanning electron microscopy. The lightweight steels exhibited similar hot ductility behavior in accordance with temperature despite the addition of Cr. The experimental results indicated that the κ-carbide precipitation had an insignificant influence on the hot ductility test. However, ductility at low temperature was induced by slip mechanism, while dynamic recrystallization had significant influence at high temperatures during the on-heating thermal cycle. In the on-cooling thermal cycle, the melted and re-solidified grain boundaries decreased the overall ductility, exhibiting the same tendency as that observed in the on-heating test.

Effect of V on the Hot Ductility Behavior of High-Strength Hot-Stamped Steels and Associated Microstructural Features

Effect of V on the Hot Ductility Behavior of High-Strength Hot-Stamped Steels and Associated Microstructural Features

Quantitative SAXS Analysis of Precipitate Characteristics Limiting Hot Ductility in HSLA Steels Containing V, Nb & NbTi

Hot ductility behavior between 700 and 1050°C is governed by precipitation and phase transformation, which can cause cracking and limit steel processing in other ways. Characterization of this precipitation with conventional microscopy techniques is difficult due to the limited particle size, usually occurring on the nanometer scale. Casting simulation and hot tension testing were performed using a Gleeble 3500 to generate a property profile for high-strength low alloy (HSLA) steels containing Nb, Ti, V, and N. Small-angle X-ray scattering (SAXS) was then used to characterize precipitates in Gleeble-tested samples in order to evaluate relationships between hot ductility measurements and precipitate size, spacing, and volume fraction. SAXS results showed that interparticle spacing and volume fraction were the most significant factors influencing ductility in all tested grades. Specimens with reduction-in-area measurements ranging from 3–90% exhibited a range of particle spacing data from 5–80 nm and precipitate volume fraction from 0.001–0.03%. It was observed that particle spacing of 10–20 nm and a volume fraction of 0.01% were the most detrimental to hot ductility. Significant outcomes of the current study are that precipitate density may be the most significant factor limiting ductility and that laboratory-scale SAXS measurement represents a viable method for bulk precipitate characterization.

Influence of microstructure evolution on hot ductility behavior of austenitic Fe–Mn–Al–C lightweight steels during hot tensile deformation

Four alloys based on the Fe–30Mn–(8.5–12)Al–(1.0–1.3)C (wt%) system were prepared to investigate the effects of microstructure evolution and κ-carbide precipitation behavior on hot ductility behavior of austenitic lightweight steels. Hot tension tests were carried out at temperatures of 500–1230 °C using a Gleeble simulator. At high temperatures above 1000 °C, dynamic recrystallization occurred in all alloys, leading to high tensile ductility. At temperatures of 700–900 °C, the ductility decreased in all alloys due to the intragranular precipitation of κ-carbide, with increases in the amounts of Al and C contents then leading to a greater loss of ductility due to the formation of coarse intergranular κ-carbides. The addition of Cr and Mo suppressed the precipitation of κ-carbide, reducing the extent of ductility loss. At 500 °C, the ductility was recovered due to a reduction of inter-/intragranular κ-carbide precipitation and the development of slip bands caused by planar gliding of dislocations through κ-carbide shearing. The spacing among slip bands then became coarse with an increase in the Al and C contents, resulting from the coarsening of κ-carbide. Meanwhile, dynamic strain aging (DSA) behavior was observed in all alloys deformed at 500 °C. This occurred because the hot tensile tests were carried out under a high strain rate condition; therefore, the mobility of the dislocations was fast and thus solute atoms pinned the dislocations despite deformation at a high temperature. With a coarsening of κ-carbide, the extent of serration was reduced, resulted from the fact that the content of solute C decreased due to the greater precipitation of κ-carbide; i.e., the amounts of solute C atoms to cause the DSA behavior were reduced.

The Hot Ductility, Microstructures, Mechanical Properties and Corrosion Resistance in an Advanced Boron-Containing Complex Phase Steel Heat-Treated Using the Quenching and Partitioning (Q&P) Process

The objective of this research work is to obtain the hot ductility behavior, and the structural, microstructural and mechanical characteristics of one of the latest generation of AHSS steels, a complex phase (CP) steel microalloyed with boron (0.006 wt.%), processed by hot and cold rolling operations and heat-treated using two different quenching and partitioning (Q&P) treatments, a one-step partitioning (quenching to 420 °C) and the other a two-step partitioning (quenching to 420 °C and reheated to 600 °C). The results show that boron has a marked effect on the solidification process of the CP steel, refining the austenitic grain size. Due to its refinement, the boron-containing steel had better ductility throughout the temperature range examined (700–900 °C), i.e., the hot ductility trough. Thus, the minimum percentage of reduction in area (%RA) value occurring at 800 °C was 43% for the boron-free steel, compared with 58% for the boron-containing steel. Hence, cracking would not be a problem when straightening the strand on continuous casting. The benefit of boron addition on the room temperature properties was found to be very marked for the higher temperature two-step partitioning treatment, giving a yield stress of 1200 MPa, a UTS (ultimate tensile strength) of 1590 MPa and a total elongation above 11%. The final Q&P microstructure, in both one- and two-step partitioning conditions, consisted of retained austenite (RA-γ), martensite and ferrite islands in a bainitic matrix. Furthermore, the boron treated steel on quenching produced a greater amount of RA-γ, which accounted for its better room temperature ductility and produced a martensitic matrix rather than a bainitic one, giving it greater strength. The addition of boron improved the corrosion resistance of this type of third generation AHSS steel.

Thermomechanical and Microstructural Analysis of the Influence of B- and Ti-Content on the Hot Ductility Behavior of Microalloyed Steels

The effects of the combined addition of B and Ti, as well as the influence of different strain rates on the hot ductility behavior of low carbon, continuously cast, microalloyed steels were investigated in this work. Tensile tests, microstructure analyses, and thermokinetic simulations were performed with in situ melted samples. Furthermore, prior austenite grain evaluations were carried out for the two different microalloyed steels. Increasing the strain rate brought improvements to the ductility, which was more significant in the steel with the leanest composition. The steel containing more B and Ti presented a better hot ductility behavior under all conditions tested. The main causes for the improvements rely on the precipitation behavior and the austenite–ferrite phase transformation. The preferential formation of TiN instead of fine BN and AlN was seen to be beneficial to the ductility, as well as the absence of MnS. Grain boundary segregation of free B that did not form BN retarded the ferrite formation, avoiding the brittleness brought by the thin ferrite films at the austenite grain boundaries. Furthermore, it was revealed that for the steels in question, the prior austenite grains have less influence on the hot ductility behavior than the precipitates and ferrite formation.

Influence of Cu and Ni on the Hot Ductility of Low C Steels with Respect to the Straightening Operation When Continuous Casting

Cu-containing steels can suffer from hot shortness unless Ni is added to protect them but whether this problem also influences the straightening operation is not clear. Previous work on hot ductility has found that only when the tensile samples from Cu-containing steels are oxidised can any deleterious influence of copper be found. However, this paper shows that oxidation is not essential. It is more complex than that and, as Cu has been suggested for TRIP steels at levels up to 2.5% Cu to increase their strength and ductility, a greater understanding is required, both of hot shortness and cracking during straightening. The present paper explores the hot ductility behaviour of steels alloyed with Cu and Ni in the straightening temperature range, 700–1000 °C, when tested in air and in an argon atmosphere. Segregation of Cu to the sulphides and grain boundaries occurred allowing the formation of fine Cu2S particles at the austenite grain boundaries favouring intergranular failure and this was more pronounced under oxidising conditions and required strain. It was concluded that a Cu addition, as well as causing hot shortness at higher temperatures will also cause cracking problems in the straightening temperature range in the more sensitive to cracking grades of steel and although the problems are different they are nevertheless interrelated and provided there is sufficient Ni, both may be avoided.

Effect of Reheating Temperatures on Hot Ductility and Precipitation Behavior of Low‐Carbon Steels

The influence of reheating temperatures on hot ductility is investigated in two Ni‐, Mo‐, and Cu‐containing low‐carbon steels. The effects of solution treatment conditions at 1623, 1673 K, and remelting at 1803 K on the reduction in area (RA) are analyzed. The results show an RA trough in the range of 973–1173 K in all experimental conditions. At 973 K, which is close to the Ae3 temperature of both steel grades, ductility drop is caused by the formation of the thin ferrite film at the prior austenite grain boundary. Higher RA values are achieved after reheating to 1623 K, mainly due to smaller austenite grain size by the action of preformed precipitation throughout the matrix. In contrast, increased reheating temperatures result in relatively poor RA values because of the large austenite grain size due to the dissolution of preformed precipitate and reprecipitation at the grain boundaries during cooling. A comparison between two steel grades shows that better ductility at 973 K is achieved in low Ni‐containing steel due to the relatively thick ferrite film. The effects of alloy composition and preformed precipitate's existence on hot ductility are further discussed based on the matrix strengthening and reprecipitation behavior.

Hot ductility behavior of medium carbon sulfur-containing alloy steel

Sulfur-containing free-cutting steels are widely used because of their excellent processability. However, the crack problem of sulfur-containing steel during continuous casting production is still not solved. Therefore, this paper investigates the effect of sulfur content on the crack susceptibility or hot ductility of medium carbon alloy steel in detail. This research designed and produced four kinds of medium carbon alloy steel with different sulfur content. It also investigated the hot ductility of steel through a hot tensile test. This study focuses on the effect of sulfur content on the flow stress–strain curve, hot ductility curve, fracture morphology, and inclusions of the steel. The results show that the sulfur-containing steel has better hot ductility at low temperatures (750–900 °C) and worse hot ductility at medium temperatures (1000–1250 °C) compared to the sulfur-free steel. The difference is mainly related to the precipitation behavior of the MnS inclusions in the steel. The hot ductility behavior of medium-carbon sulfur-containing alloy steel grade is primarily related to DRX and the precipitation behavior of MnS inclusions.

Hot Ductility Behavior in the Weld Heat-Affected Zone of Austenitic Fe-Mn-Al-C Lightweight Steels and the Effects of Alloying Elements

Three austenitic Fe-Mn-Al-C alloys containing different Mo, Cr, and Si contents were prepared to evaluate the effects of alloying elements on the hot ductility behavior of the weld heat-affected zone (HAZ) of austenitic light-weight steels. HAZ simulations and hot tension tests were performed using a Gleeble simulator. The hot tension tests were conducted at 500 - 1220 ℃ during on-heating and on-cooling cycles. The results showed that the hot ductility significantly varied according to the behavior of κ-carbide precipitation with a decrease in temperature. The tensile ductility values of all the alloys were high at elevated temperatures above 1000 ℃ because of dynamic recrystallization. At intermediate temperatures of 700 - 900 ℃, significant ductility drops occurred in all alloys because κ-carbide precipitation, and the addition of Si resulted in more significant of ductility loss than in Cr or Mo added alloys. These different effects occurred because Mo and Cr suppressed κ-carbide precipitation, whersase Si promoted it. Ductility was recovered at low temperatures of approximately 500 ℃.

The continuous casting behavior of medium manganese steels

The continuous casting behavior of medium manganese steels (MMnS: 6Mn, 6Mn–1Si, 6Mn–1Si–1Al, 10Mn–1Si and 10Mn–1Si–1Al) have been investigated and compared with unalloyed carbon steel C30. A series of in-situ melting/re-solidification hot tensile tests were conducted in the temperature range 800–1450 °C. The corresponding tensile forces and ductility parameters were monitored. Differential thermal analysis (DTA) was applied to determine the liquidus temperatures. The solidified zones and fracture surfaces of the tested specimens were analyzed using scanning electron microscopy (SEM). Scheil-solidification and equilibrium thermodynamic calculations were performed to explore the solidification and precipitation behavior. The processability of MMnS using thin slab casting technology has been discussed in light of their hot ductility behavior. The findings indicate the prime role of precipitation, phase transformation and extension/shift of solidification intervals induced by the alloying concepts in controlling the hot ductility of MMnS. It turns out that the formation of MnS, complex AlN and MnS precipitates and δ-ferritic solidification deteriorates the high temperature ductility. A shift of the second hot ductility drop to higher temperatures was identified for the Al-containing and high-Mn content MMnS when compared to the reference steel C30. This study demonstrates the adequate hot ductility (>30%) of MMnS 6Mn and 6Mn–1Si as well as reference steel C30 for thin slab casting technology, while higher Al-containing MMnS undergo a ductility drop in the straightening temperature range (900–1080 °C), which impairs their castability. An optimization of their alloying concepts, especially Al-content, is recommended to tune their continuous casting behavior.

Causes of Corner Cracks in Hypoperitectic Microalloyed Steel Billets

The cover image illustrates the causes of corner crack in hypoperitectic microalloyed steel billets. The hot ductility, solidification shrinkage, alloy composition, microstructure and segregation behavior of steel are analyzed in detail. In this study, article number 2100035, Yanping Bao and co-workers present a new mechanism for generating corner cracks in a billet.

Causes of Corner Cracks in Hypoperitectic Microalloyed Steel Billets

Due to the development of continuous casting technology, the incidence of cracks in the continuous casting process has been significantly reduced. However, a few small cracks still appear during the continuous casting of special steel. For instance, cracks often appear at the corner of the billet during the production of hypoperitectic microalloyed steel. Most research on the causes of corner cracks in hypoperitectic microalloyed steel has focused on investigating the hot ductility of steel and not considered the shrinkage characteristics during solidification. Herein, the hot ductility behavior show that cracks are not related to the hot brittleness of steel. The effects of alloy composition on the phase evolution and shrinkage of steel during solidification are focused. Uneven solidification leads to excessive shrinkage of the corners in the billet, and an air gap is generated between the billet and the mold. The return of temperature in the corner of the billet results in the coarseness of the corner grain. Abnormal solidification also results in the severe deformation of the billet, leading to severe wear with the mold, causing Cu to infiltrate into the billet, and further increasing the crack sensitivity of the steel.

Hot Ductility and Fracture Behavior of High‐Ti Weathering Steel

The hot ductility behavior of high‐Ti weathering steel is studied by high‐temperature tensile testing in the temperature range of 650–1100 ºC. The results reveal a ductility trough in high‐Ti steel in the range of 650–962, 650–925, and 700–880 °C with minimum reduction in area (RA) of 22.21%, 36.53%, and 43.25% at 800 ºC and different strain rates of 1.0 × 10−3, 5.0 × 10−3, and 1.0 × 10−2 s−1, respectively. A number of secondary cracks containing S, N, and Ti propagate along the grain boundary, and the obvious preferred orientation is found in the microstructure near the fracture at 800 °C. With the increase in strain rate, the value of RA is increased in the temperature range of 650–1050 ºC, such that the probability of crack occurrence is decreased. RA does not increase with the increase in strain rate at 1100 °C and strain rate of 1.0 × 10−2 s−1. The underlying reason is that there is not adequate time for the material to recover or recrystallize with the increase in strain rate, resulting in material softening effect that is not completely reflected.

Hot ductility behavior of a Fe-0.3C-9Mn-2Al medium Mn steel

The hot ductility of a Fe-0.3C-9Mn-2Al medium Mn steel was investigated using a Gleeble3800 thermo-mechanical simulator. Hot tensile tests were conducted at different temperatures (600–1300°C) under a constant strain rate of 4 × 10−3 s−1. The fracture behavior and mechanism of hot ductility evolution were discussed. Results showed that the hot ductility decreased as the temperature was decreased from 1000°C. The reduction of area (RA) decreased rapidly in the specimens tested below 700°C, whereas that in the specimen tested at 650°C was lower than 65%. Mixed brittle-ductile fracture feature is reflected by the coexistence of cleavage step, intergranular facet, and dimple at the surface. The fracture belonged to ductile failure in the specimens tested between 720–1000°C. Large and deep dimples could delay crack propagation. The change in average width of the dimples was in positive proportion with the change in RA. The wide austenite-ferrite inter-critical temperature range was crucial for the hot ductility of medium Mn steel. The formation of ferrite film on austenite grain boundaries led to strain concentration. Yield point elongation occurred at the austenite-ferrite intercritical temperature range during the hot tensile test.

Holding Time Influence on the Hot Ductility Behavior of a Continuously Cast Low Alloy Steel

Cracking during the continuous casting process is undesirable and continuous work is being carried out to find further improvements and understand the mechanisms that lead to failure. Investigations on the hot ductility behavior of a continuously cast low alloyed steel using different holding times before straining were done in the present work. Samples were heated to melting temperature in a vacuum atmosphere and then cooled to one of the three test temperatures chosen: 750, 850, and 900 °C. When the desired temperature was reached, the sample was isothermally held for either 10, 90, 300, or 3600 s before the tensile test started, with a strain rate of 10−3 s−1. The reduction of area was measured, SEM images of the fractured surfaces were taken plus LOM images for the analysis of the microstructure. The results show that there was no significant change in the ductility at any of the temperatures until 300 s, with a change in behavior at 3600 s. This was further confirmed with the images and precipitation kinetics simulations. The results are described and compared.

Highly Enhanced Hot Ductility Performance of Advanced SA508-4N RPV Steel by Trace Impurity Phosphorus and Rare Earth Cerium

Advanced SA508-4N RPV steel samples, unadded, P-added, and P+Ce-added, are investigated on their hot ductility behavior. Hot tensile tests are carried out in the temperature range of 750 to 1000 °C through a Gleeble 1500D machine. It is demonstrated that the deformation temperatures of all the three steels are located in the austenite single-phase region. There is no ductility trough present for the P+Ce-added steel, but the unadded one exhibits a deep ductility trough. The reduction of area (RA) of the former is always higher than 75% and increases with rising temperature until reaching ~95% at 900 °C or above, whereas the lowest RA value of the latter is only ~50% at 850 °C. Microanalysis indicates that the grain boundary segregation of P and Ce takes place in the tested P+Ce-added steel. This may restrain the boundary sliding so as to improve the hot ductility behavior of the steel. Furthermore, the addition of P and Ce is able to facilitate the occurrence of the dynamic recrystallization (DR) of the steel, lowering the initial temperature of DR from ~900 to ~850 °C and thereby enhancing the hot ductility performance. Consequently, the combined addition of P and Ce can significantly improve the hot ductility of SA508-4N RPV steel, thereby improving its continuous casting performance and hot workability.

Influence of Boron Content on the Solidification Structure, Magnetic Properties and Hot Mechanical Behavior in an Advanced As-Cast TWIP Steel

This research work reports the effect of boron micro-additions (180 and 470 ppm) on the solidification structure, magnetic properties and hot ductility behavior of an advanced low-carbon highly alloyed twinning-induced plasticity (TWIP) steel. For this purpose, three experimental TWIP steels were fabricated by melting commercial raw materials and casting into metallic molds. Solidification structure was characterized by means of optical and scanning electron microscopy techniques, and a statistical study was carried out to measure dendrite features. A vibrating sample magnetometer was used, at room temperature, to determine magnetic properties, and a X-ray diffraction analysis was performed in order to identify the related phases during magnetic measurements. Finally, the hot ductility in the as-cast condition was evaluated at 700, 900 and 1100 °C, under a constant strain rate of 0.001 s−1. The results indicate that boron micro-additions cause an overall refining solidification structure and austenitic grain size. However, as the boron content increases, segregation of this element promotes formation of ferrite and ε-martensite, leading to ferromagnetic behavior. Nonetheless, with subsequent hot- and cold-rolling, the single austenitic phase is achieved, and this behavior is eliminated. Hot tensile tests revealed that boron micro-addition is beneficial to the hot ductility behavior. The greatest influence was observed for the higher concentration of boron (470 ppm). In comparison with the steel without boron content, the reduction of area (RA) is more than the triple of the hot workability during straining at 900 °C. Thus, present results demonstrate that boron micro-addition has an excellent potential for refining dendritic microstructure and improving the hot-deformation behavior of present low-carbon highly alloyed TWIP steel.