Ultra Fast Cooling Research Articles

Effects of Processing Variables on Microstructure and Yield Ratio of High Strength Constructional Steels

The process of “controlled rolling+relaxation+ultra fast cooling (UFO)” for high strength constructional steel with low yield ratio was presented. Microstructure and corresponding relationship with low yield ratio were investigated. The results showed that the constructional steels with multiphase microstructure of bainitie ferrite, martensite-austenite (M-A) and lath bainite were obtained through the creative process. The grain size decreased with the decrease in finish rolling temperature, which enhanced the strength by the grain refinement strengthening. The proper relaxation treatment promoted the bainitie ferrite lath width and the formation of blocky M-A constituent. In addition, both the tensile and yield strength increased with the decrease in finish rolling temperature and UFC final temperature, but the yield strength increased more significantly than tensile strength, which caused the increase in yield ratio. By using the process of “controlled rollingh+relaxation+ultra fast cooling”, the excellent comprehensive mechanical properties of 780 MPa grade constructional steels of 12 — 40 mm in thickness were achieved.

Microstructure and Mechanism of Strengthening of Microalloyed Pipeline Steel: Ultra-Fast Cooling (UFC) Versus Laminar Cooling (LC)

A novel thermo-mechanical controlled processing (TMCP) schedule involving ultra-fast cooling (UFC) technique was used to process X70 (420 MPa) microalloyed pipeline steel with high strength-high toughness combination. A relative comparison is made between microstructure and mechanical properties between conventionally processed (CP) and ultra-fast cooled (UFC) pipeline steels, together with differences in strengthening mechanisms with respect to both types of processes. UFC-processed steel exhibited best combination of strength and good toughness compared to the CP process. The microstructure of CP pipeline steel mainly consisted of acicular ferrite (AF), bainitic ferrite (BF), and dispersed secondary martensite/austenite (M/A) constituent and a small fraction of fine quasi-polygonal ferrite. In contrast, the microstructure of UFC-processed pipeline steel was predominantly composed of finer AF, BF, and dispersed M/A constituent. The primary strengthening mechanisms in UFC pipeline steel were grain size strengthening and dislocation strengthening with strength increment of ~277 and ~151 MPa, respectively. However, the strengthening contribution in CP steel was related to grain size strengthening, dislocation strengthening, and precipitation strengthening, and the corresponding strength increments were ~212, ~149 and ~86 MPa, respectively. The decrease in strength induced by reducing Nb and Cr in UFC pipeline steel was compensated by enhancing the contribution of grain size strengthening in the UFC process. In conclusion, cooling schedule of UFC combined with LC is a promising method for processing low-cost pipeline steels.

Effect of microstructure on the crack propagation behavior of microalloyed 560 MPa (X80) strip during ultra-fast cooling

A novel thermo-mechanical controlled processing (TMCP) involving ultra-fast cooling (UFC) was used to process microalloyed 560MPa (X80) strip with critical thickness of 22mm. The microstructure and mechanical properties including tensile, Charpy v-notch impact toughness, and drop weight tear test (DWTT) properties were studied, with particular focus on the effect of microstructure on fracture and crack propagation behavior during DWTT test. The study underscores that the processed strip comprising of acicular ferrite (AF), bainitic ferrite (BF), together with finely distributed martensite/austenite (M/A) constituent provided excellent combination of strength, toughness and crack arrest property. The yield strength (618MPa), tensile strength (752MPa), and low-temperature toughness (upper shelf energy of 302J, transition temperature of −74°C) as well as DWTT shear area (100%) met the requirements of API SPEC 5L. The predominantly AF microstructure with small-size M/A constituent improved the low-temperature toughness by increasing the critical fracture stress and increased the ability to hinder crack propagation during Charpy test. The AF and finely distributed M/A constituent effectively deflected the crack propagation during DWTT indicating excellent crack arresting property of pipeline strip. It is the cooling schedule of UFC that was responsible for decreasing the effective grain size and increasing the mechanical properties.

Effect of Rolling Temperature and Ultrafast Cooling Rate on Microstructure and Mechanical Properties of Steel Plate

Microstructure can vary significantly through thickness after ultrafast cooling of rolled steel plates, impacting their mechanical properties. This study examined the microstructure, microstructural banding at centerline, and mechanical properties through thickness for different ultrafast cooling conditions and rolling temperatures. One set of steels (UC1 and UC2) were ultrafast-cooled (UFC) at 40 K/s after finish rolling at 1223 K and 1193 K (950 °C and 910 °C), respectively, while the second set (LC) was cooled by laminar cooling at 17 K/s after finish rolling at 1238 K (965 °C). UFC produced microstructural variation through thickness; highly dislocated lath-type bainitic ferrite was formed near the surface, whereas the primary microstructure was acicular ferrite and irregular polygonal ferrite in the interior of UC1 and UC2 steels, respectively. However, UFC has the advantage of suppression of microstructural banding in centerline segregation regions. The ferrite grain size in both UFC-cooled steels was refined to ~5 μm, increasing strength and toughness. The optimum combination of properties was obtained in UC2 steel with appropriate low finish rolling temperature, being attributed to the distinct microstructure resulting from work-hardened austenite before UFC.

The Determining Role of Finish Cooling Temperature on the Microstructural Evolution and Precipitation Behavior in an Nb-V-Ti Microalloyed Steel in the Context of Newly Developed Ultrafast Cooling

We have studied here the impact of finish cooling temperature on the microstructural evolution and precipitation behavior in Nb-V-Ti microalloyed steel through thermo-mechanical simulation in the context of newly developed ultrafast cooling system. The microstructural evolution was studied in terms of morphology and crystallography of precipitates using high-resolution transmission electron microscopy. At finish cooling temperature of 933 K and 893 K (660 °C and 620 °C), the microstructure primarily consisted of polygonal ferrite, together with a small amount of wedge-shaped acicular ferrite and lamellar pearlite, while, at 853 K and 813 K (580 °C and 540 °C), the microstructure consisted of lath bainite with fine interlath cementite and granular bainite with martensite/austenite (M/A) constituent. In all the finish cooling temperatures studied, the near-spherical precipitates of size range ~2 to 15 nm were randomly dispersed in ferrite and bainite matrix. The carbide precipitates were identified as (Nb,V)C with NaCl-type crystal structure. With a decrease in the finish cooling temperature, the size of the precipitates was decreased, while the number density first increased with a peak at 893 K (620 °C) and then decreased. Using Ashby–Orowan model, the contribution of the precipitation strengthening to yield strength was ~149 MPa at the finish cooling temperature of 893 K (620 °C).

Effects of Ultra-Fast Cooling Technology on Microstructure and Properties of Low Carbon Steel

The cooling technology after hot-rolling influences the microstructure and mechanical properties of steels. Combined with dynamic continuous cooling transformation (CCT) curve by the thermal simulation experiments of Q235B steel, the effects of ultra-fast cooling technology on microstructure and properties of Q235B steel were investigated by optical microscope, scanning electron microscope, tensile and impact tests. Also, the fracture morphology analysis was conducted by scanning electron microscope. The results show that both yield and tensile strength are enhanced by ultra-fast cooling technology. Moreover, the yield and tensile strength increase with the increase of cooling rate of ultra-fast cooling process, while the ductility has no significant change. Furthermore, the yield and tensile strength increase with the decrease of final cooling temperature of ultra-fast cooling process with a little reduction in ductility. The impact tests show that all samples possess excellent impact toughness varying within a small range. The yield strength, tensile strength, and impact toughness of Q235B steel could increase to 395, 516 MPa, and 143 J, respectively, using ultra-fast cooling technology. The study provides theoretical guidance for industrial application of ultra-fast cooling technology.

Development of Cooling Process Control System for Backward UFC in CSP

Backward Ultra-fast cooling (UFC) plays a key role in low-cost dual-phase steels production. Cooling process control system for backward UFC was developed to meet the process requirements. Basing on basic theory of heat transfer, cooling control model was established to accomplish temperature calculation. To deal with the influence of technological conditions fluctuations an adaption system, including self-learning function and feedback function was proposed to intelligently realize temperature correction. The developed cooling strategy can achieve diversified cooling path control. The controlling of significant technological parameters, including cooling rate and air cooling time for dual-phase steels, was also accomplished. Furthermore, flexible and diversified UFC strategies were developed aiming at high temperature precision and low temperature deviation control in thickness direction. The system has been applied successfully in dual-phase steel production with high stability and reliability. The precision of medium temperature (MT) can be controlled within ±10°C, and the UFC delivery temperature (UFC-T) can be limited within ±30°C of the target values.

Effect of Ultra Fast Cooling on the Alloy Cost Reduction of the X80 Pipeline Steel

The microstructures and mechanical properties of X80 pipeline steels produced by both novel ultra fast cooling and conventional‐accelerated continuous cooling modes are investigated. Results showed that different levels of Mo addition had a remarkable effect on the microstructures and mechanical properties of the investigated pipeline steels. The proeutectoid ferrite and pearlite formation is inhibited in the high‐Mo steel and acicular ferrite is obtained over a wide range of cooling rates, whereas the dominant acicular ferrite microstructure can only be obtained when the cooling rates reach up to 5 C s−1. Very similar microstructures and mechanical properties are obtained in the low‐Mo steel produced with ultra fast cooling and in the high‐Mo steel produced by the conventional‐accelerated continuous cooling. It was proved by simulation and industrial trials that high‐strength low‐alloy steels such as pipeline steels, can be produced using the novel ultra fast cooling which also reduce alloy cost.

Abstract 11465: Ultrafast Cooling With Total Liquid Ventilation Attenuates Multi-organ Failure and Sepsis-like Response After Experimental Shock Induced by Aortic Cross Clamping

Introduction: Total liquid ventilation (TLV) provides ultrafast cooling and potent neuro- and cardioprotection after experimental cardiac arrest. Hypothesis: We determined whether it could also mitigate multi-organ failure and sepsis-like response after abdominal ischemia/reperfusion and subsequent shock. Methods: Anesthetized rabbits were submitted to 30 min of supracoeliac aortic cross-clamping and 300 min of reperfusion. They underwent a normothermic procedure (Control group) or rapid cooling toward 32-33°C with TLV started either before, during or after aortic clamping (PRE, PER and POST groups, respectively). TLV was maintained during 75 min after which rabbits were rapidly rewarmed and switched to conventional mechanical ventilation. Results: Control animals elicited a dramatic shock state with a low cardiac output, high demand in norepinephrine, liver failure and acute kidney injury. Cardiac output was gradually improved in POST, PER and PRE groups as compared to Control (53±3, 64±5, 90±10 and 36±8 ml/min/kg after 300 min, respectively; all p<0.05). A significant protection was also observed in the TLV groups regarding liver enzymes (ASAT, ALAT, L-FABP), acute kidney injury markers (nNAG, KIM-1, NGAL, β2-microglobulin) and renal function parameters (creatinine clearance and fractional Na+ excretion). The protection was maximal in the PRE group and achieved a lower extent in PER and POST groups when compared to Control. As example, urinary clearance of creatinine achieved 4.8±1.2, 1.3±0.9, 0.9±1.4 in PRE, PER and POST groups vs 0.5±0.4 ml/kg/min in Control (all p<0.05), respectively. Importantly, multi-organ failure was associated with elevated blood transcripts of inflammatory markers (e.g., IL-1β, IL-8, IL-10). This was attenuated in all groups submitted to TLV with no difference between PRE, PER and POST groups. As example, changes in IL-1β achieved 2.7±0.4, 3.8±0.4 and 3.2±1.1 in PRE, PER and POST groups vs 5.6±0.7 in Control (fold-change from baseline; p<0.05), respectively. Conclusions: Ultra-fast cooling with TLV attenuates shock and multi-organ failure in a model of abdominal ischemia/reperfusion. The cardiac, liver and renal protections but not anti-inflammatory effects depend upon the window of application.

Hot Rolled Strip Re-reddening Temperature Changing Law during Ultra-fast Cooling

Temperature deviation between surface and the center of hot rolled strip is formed during ultra-fast cooling (UFC). Surface temperature would rise when temperature deviation goes up to an extent, and strip re-reddening phenomenon will appear. Strip re-reddening affects the stability of strip microstructure, property and temperature control precision. Thus, it is necessary to conduct research on re-reddening temperature changing law to improve strip property and temperature control precision. Strip temperature trends for various strip thicknesses and ultra-fast cooling rates were obtained by numerical calculation method. Re-reddening temperature, temperature deviation between surface and center, and boundary layer position changing law were obtained. By comparison, some conclusions were obtained: UFC re-reddening temperature and laminar cooling (LC) re-reddening temperature were linear to ultra-fast cooling rate respectively. Ultra-fast cooling rate affected UFC re-reddening temperature greatly, but it had little effect on LC re-reddening temperature. Equations which were used to calculate UFC re-reddening temperature, LC re-reddening temperature and maximum temperature deviation were obtained. The position of boundary layer stayed in 1/4 strip thickness.

Effects of Ultra Fast Cooling on Microstructure and Mechanical Properties of Pipeline Steels

X70 (steel A) and X80 (steel B) pipeline steels were fabricated by ultra fast cooling (UFC). UFC processing improves not only ultimate tensile strength (UTS), yield strength (YS), yield ratio (YS/UTS), and total elongation of both steels, but also their Charpy absorbed energy (AK) as well. The microstructures of both steels were all composed of quasi polygonal, acicular ferrite (AF), and granular bainite. MA islands (the mixtures of brittle martensite and residual austenite) are more finely dispersed in steel B, and the amount of AF in steel B is much more than that in steel A. The strength of steel B is higher than that of steel A. This is mainly attributed to the effect of the ferrite grain refinement which is resulted from UFC processing. The finely dispersed MA islands not only provide dispersion strengthening, but also reduce loss of impact properties to pipeline steels. UFC produces low-temperature transformation microstructures containing larger amounts of AFs. The presence of AF is a crucial factor in achieving desired mechanical properties for both steels. It is suggested that the toughness of the experimental steel increases with increasing the amount of AF.

Investigation on microstructure and properties of 0.17% carbon steel with dispersed cementite particles

Abstracts Using “ultra fast cooling (UFC) + thermo-mechanical treatment (TMT)” process, nanoscale cementite particles were obtained while the matrix varied from “proeutectoid ferrite + degenerated pearlite” to complete lath-like bainitic matrix by controlling the UFC stop temperature. Microstructures were examined by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results showed that TMT process promoted cementite precipitation in proeutectoid ferrite, leading to more uniformly distributed cementite particles. The tensile properties and Charpy V-notch (CVN) impact toughness were evaluated. The correlation with the microstructural features is then discussed in details.

Microstructure and mechanical properties of economical dual phase wheel steels based on CSP line with later ultra fast cooling

To attain the economical wheel steels with high strength, the microstructure and properties of steel with microstructure consisting of ferrite and martensite were studied. The transformation behaviour of the economical wheel steels under continuous cooling condition and stepped cooling condition were researched and the 11·0-mm thickness wheel steel strips with simple chemical composition were developed on compact strip production (CSP) line with later ultra fast cooling (UFC) system. The results indicate that the dynamic continuous cooling transformation (CCT) curves of low carbon Mn–Cr steel is characterised by a large ferrite region, and the stepped cooling pattern is more beneficial to achieve ferrite–martensite microstructure than continuous cooling pattern. In order to obtain adequate polygonal ferrite matrix during stepped cooling process, the early cooling temperature should be controlled in the range of 700–660°C and the isothermal holding time should be ensured. The 11·0-mm thickness dual phase (DP) wheel steel strips are achieved on CSP line via control cooling technology based on later UFC system. Tensile strength increment of C–Mn–Cr steels can reach 130–170 MPa when the F-P microstucture is replaced by F-M microstucture. The wheel steel strips with tensile strength above 590 MPa, yield ratio under 0·62 and elongation above 28% have been used for wheel discs, whose weight achieve reduction of 16–22%.

Surfactant-Based Cu–Water Nanofluid Spray for Heat Transfer Enhancement of High Temperature Steel Surface

Spray cooling is critical in many industrial applications to extract large heat fluxes from metal parts, such as hypervapotron in nuclear fusion reactors, heat treatment of steel plates in run-out table (ROT), electronic parts, and many more. The objective of the present study is to enhance the heat dissipation in transition and nucleate boiling regimes using an air-atomized water spray with water-based copper nanofluid as a coolant. The nanoparticle used in this study is energetic metal Cu, which has been prepared by mechanical milling (MM) process. The nanofluid has been prepared by suspending 0.1 vol. % Cu nanoparticles in water, with or without a dispersing agent (surfactant). The effect of type of dispersing agent on augmentation of boiling heat transfer has also been studied. The spray cooling experiments are conducted on a 6 mm thick stainless steel plate of initial temperature above 900 °C. The transient surface heat flux and temperatures are estimated using commercial inverse heat conduction software named intemp. The experimental results illustrated that transition and nucleate boiling heat flux as well as critical heat flux (CHF) increased significantly using nanofluid spray. A maximum ultrafast cooling (UFC) rate of 267 °C/s is achieved using surfactant-based nanofluid spray, which is 31.53% and 59.88% higher as compared to the nanofluid without any dispersant and pure water sprays, respectively. Overall, the surfactant-based copper nanofluid spray can serve as a better coolant on the ROT of steel processing industry.

The Relationship Between Microstructural Evolution and Mechanical Properties of Heavy Plate of Low-Mn Steel During Ultra Fast Cooling

Abstract We describe here the electron microscopy and mechanical property studies that were conducted in an industrially processed 20- and 40-mm C-Mn thick plates that involved a new approach of ultrafast cooling (UFC) together with significant reduction in Mn-content of the steel by ~0.3 to 0.5 pct, in relation to the conventional C-Mn steels, with the aim of cost-effectiveness. The study demonstrated that nanoscale cementite precipitation occurred during austenite transformation in the matrix of heavy plate during UFC, providing significant precipitation strengthening. With decrease in UFC stop temperature and consequent increase in the degree of undercooling, there was a transition in the morphology of cementite from lamellar to irregular-shaped nanoscale particles in the 20 mm heavy plate. With the increase in plate thickness, nanoscale cementite precipitated in bainitic lath at the surface of 40 mm heavy plate, which significantly increased the strength and decreased the elongation. Simultaneously, microstructural evolution in hot-rolled sheets was studied via simulation experiments using laboratory rolling mill to define the limits of microstructural evolution that can obtained in the UFC process and develop an understanding of the evolved microstructure in terms of process parameters.

Analysis on the Influencing Factors of Heat Transfer for the Slot Nozzle Convection Impact in Rolled Strip Preposition Ultra-Fast Cooling

The aim of this work is to optimize process design parameters of the hot rolling steel strip preposition ultra-fast cooling (UFC) system, and improve the stability and uniformity of micro heat transfer during the high strength cooling process of hot rolling strip. According to the technology and equipment feature of the pre-UFC system for a factory, the convective heat transfer process of single nozzle and strip in UFC system was studied numerically by the fluid-structure interaction finite element method (FEM). The influence of different parameters on the slot impinging jet heat transfer coefficient was obtained, such as jet angle, the jet velocity, the slot nozzle width and water temperature. The results show that on the ultra-fast cooling process of the strip in the initial temperature of 850 °C and thickness of 8 mm, the global average heat transfer coefficient can be increased with the increase of jet velocity, and decrease of the cooling water temperature. The jet angle and the slot nozzle width have minimal effect on it for the whole heat transfer zone. The local average heat transfer coefficient first increased and then decreased with the increase of jet angle and slot nozzle width at the jet impingement location.

Microstructure and Tensile Deformation Behavior of a Hot-Rolled Directly Quenched and Dynamically Partitioned Steel Containing Proeutectoid Ferrite

Hot-rolling direct quenching and dynamical partitioning (HDQ&DP) processes were applied to a low-carbon steel containing silicon and manganese based on thermo-mechanical control process (TMCP) technology and ultra-fast cooling (UFC) technology. The microstructures and phase compositions were characterized and analyzed using SEM, EBSD, TEM and XRD. The mechanical properties and tensile deformation behaviors were investigated by means of uniaxial tensile test. The microstructures and tensile deformation behaviors of both HDQ&DP steel with and without proeutectoid ferrite were comprehensively expounded by comparing with each other. Results show that the amount of retained austenite in the HDQ&DP steel with proeutectoid ferrite can reach up to 17.3%, which is higher than that in the HDQ&DP steel without proeutectoid ferrite (15.7%). The HDQ&DP steel without proeutectoid ferrite possesses extremely high ultimate tensile strength (UTS) up to 1700 MPa with yield ratio about 0.73 and elongation about 11.5%. The introduction of proeutectoid ferrite can result in a moderate decrease of UTS to 1240-1400 MPa, a drastic decrease of yield ratio to 0.51-0.69 and a certain increase of elongation to 13.0-13.7%. The existence of the proeutectoid ferrite can partly enhance the work hardening ability of the steel and may improve its formability. It is concluded that the HDQ&DP steel with relatively large amount of proeutectoid ferrite and certain amount of bainite has extensive application prospects.

Heat Transfer Characteristics from Single Group Slot Jet Impingement on the Strip Surface for the Hot Rolling Ultra-Fast Cooling

As for the impinging jets of single group slot nozzles, the heat transfer of the top and bottom strip surface is uneven which is mainly caused by the various jet distances of the top and bottom nozzle to the strip surface and the effect of gravity. In this study, the convective heat transfer process of the top and bottom strip surface due to single group slot nozzles in the ultra-fast cooling (UFC) system was studied by the fluid-structure interaction finite element method. The distributions of the flow field and heat transfer for the top and bottom strip surfaces were obtained under the various parameters. The results showed that, the difference value of the average Nusselt numbe for the top and bottom strip surface was decreased with the increase of the jet velocity, but when the jet velocity was up to 5m/s it remained almost the same. The uniformity of the top and bottom surface was improved by the jet impingement height (h) for the h<45mm. It was found that for h=25mm, the heat transfers of the top and down surfaces were more evenly, and the different value of average Nusselt number for the decreased by about 12.6%-28% as the jet impingement height increased from 25mm to 45mm when the slot width was 5mm.

Fine-Grained X80 Pipeline Steels Produced by a Novel Ultra Fast Cooling Process

ABSTRACT Zhou, F.; Wu, K.; Zhou, W., and Matrosov, M., 2015. Fine-grained X80 pipeline steels produced by a novel ultra fast cooling process. The relationship between microstructure and the mechanical properties of the X80 pipeline steels produced by an ultra fast cooling process was investigated using scanning and transmission electron microscopy and an energy dispersive spectrometer. Predominantly fine-grained acicular ferrite microstructure was obtained from the ultra fast cooling process. The developed X80 pipeline steel had low yield ratio and low temperature impact toughness. The ultra-fast cooling resulted in work hardened austenite before the phase transformation and also pushed the transformation to a lower temperature. Also, the Nb-Ti carbonitride precipitation was inhibited during ultra fast cooling. Therefore, the fine-grained microstructure and superior mechanical properties were attributed to grain refinement and the inhibiting of Nb-Ti carbontride precipitation, which was caused by ultra fa...

Study on Ultra-fast Cooling Behaviors of Water Spray Cooled Stainless Steel Plates

Spray cooling is an effective tool to dissipate high heat fluxes from hot surfaces. This article thoroughly investigates the effect of thickness of a hot stainless steel plate on the cooling time, cooling rate, heat flux, and heat transfer coefficient under constant mass flow rate maintained at 1 MPa using water as the coolant. Cylindrical samples of stainless steel with constant diameter (D = 25 mm) and thickness (δ = 7.5, 12, 16.5, and 21 mm) were used in the present study. Critical droplet diameter to achieve an ultra-fast cooling rate of 300°C/s was estimated by using an analytical model for samples of varying thicknesses. The analytical model (one side spray cooling) showed good agreement with experimental results with a relative error of 3.2% in the plate thickness range of 1–12 mm. An increasing trend in maximum heat flux was found with increasing thickness of the plate. Maximum heat flux as high as 1,800 kW/m2 was achieved for a 21-mm-thick sample. Heat transfer coefficients in the range 0.092–96.24 kW/m2K, 0.111–98.9 kW/m2K, 0.074–63.4 kW/m2K, and 0.127–55.63 kW/m2K were reported for sample of varying thicknesses in the present study. Limited published work is available with reference to water spray cooling dynamics and thickness of stainless steel plate. Therefore, the present study focuses on the correlation between the thickness of the plate and spray dynamics of water spray cooling.