CCT Curves Research Articles

Effect of ultra-trace rare earth elements on continuous cooling transformation curve of low carbon steel * *Fund projects: Inner Mongolia Autonomous Region Natural Science Foundation (2018ms05052); Liaoning Provincial Natural Science Foundation (2019-kf-25-08); Inner Mongolia University of Technology (zd201510); Inner Mongolia University of technology science and technology innovation fund for college students. Ruifeng Dong (1972 -), female, from TUZUO banner, Inner

Two groups of low carbon steel with ultra-micro amount (less than 20 ppm) rare earth lanthanum and without rare earth element were taken as experimental objects. Continuous cooling transformation curves of two kinds of low carbon steels were drawn by Formaster-F II automatic phase transformation instrument, alloy phase method and hardness method. According to the measured CCT curve, the microstructure and hardness of low carbon steel under different cooling rates were studied and analysed. The results show that the addition of ultra-trace rare earth elements can increase the Ac1 and Ac3 points of low carbon steel by about 20 °C; the starting temperature of martensite transformation MS point decreased by 19 °C, and the end temperature Mf point decreased by 6 °C; at low cooling rate, the transformation range of ferrite is increased and that of pearlite is decreased; the starting temperature of bainite Bs point increased by 20 °C, and the end temperature of bainite Bf increased by 13 °C; at low cooling rate, the ferrite transformation range becomes larger, while at high cooling rate, the ferrite transformation range becomes smaller.

Continuous-Cooling-Transformation (CCT) Behaviors and Fine-Grained Nearly Lamellar (FGNL) Microstructure Formation in a Cast Ti-48Al-4Nb-2Cr Alloy

The CCT behaviors of a high-Al mid-Nb-containing cast TiAl alloy were experimentally studied in the current work. Samples with the nominal composition of Ti-48.2Al-4Nb-2Cr (at.pct) were preheated to 1420 °C and then cooled at different cooling rates. With preheating time of 8 minutes, the original microstructure was replaced by coarsening α grains. Interrupted tests demonstrated the significant decrease of microsegregation which was also predicted by theoretical calculations. The water-quenched microstructures during continuous cooling were characterized. With the increase of cooling rate from 5 to 700 °C/min, the lamellar spacing was refined from 1.77 to 0.43 μm and the microhardness was heightened from 299 to 426 HV, showing the solute strengthening effect of Nb addition compared with 4722 and Ti-48Al alloys. The CCT diagram of the current alloy was established to guide the microstructure control. Slow cooling contributed to the discontinuous coarsening of γ lamellae, suggesting a nearly lamellar (NL) microstructure. The addition of 4 at. pct Nb increased the temperatures of phase transformations, moved the CCT curves to higher temperature, and decreased the critical cooling rate for the formation of Widmanstatten and feathery structures. Finally, a fine-grained NL (FGNL) microstructure with a potential high performance was obtained.

Processing a Fe67Mo4.5Cr2.3Al2Si3C7P8.7B5.5 metallic glass: Experimental and computed TTT and CCT curves

For metallic glass synthesis and processing the possible formation of crystals should be described by CCT curves. Motivated by results of the synthesis of wires of the Fe67Mo4.5Cr2.3Al2Si3C7P8.7B5.5 alloy by melt extraction, a methodology is proposed to compute TTT and CCT curves based on the classical nucleation and growth theory. The thermophysical parameters needed for the calculation are entirely derived from DSC experiments, including those expressing the Vogel-Fulcher-Tammann equation for viscosity. The critical cooling rate for amorphous wires is estimated and matched with the CCT curve. Then, the corresponding computed TTT curve is shown to reproduce well the experimental results of isothermal crystallization of the ribbons.

Research and control of network carbide in GCr15 bearing steel

Due to the small amount of transformation during the phase transformation process, it is difficult to determine the phase transition temperature of the proeutectoid cementite in the bearing steel by the expansion method. In this paper, the Gleeble3800 thermal simulator was used to study the phase transformation behavior of GCr15 bearing steel at different cooling rates. Firstly, the phase transformation temperature range of pearlite is about 550 °C ∼ 700 °C measured by expansion method. Based on this, the transformation temperature range of proeutectoid cementite is about 700 °C ∼ 900 °C studied by metallographic method; The precipitation curve of proeutectoid cementite was supplemented, and the complete continuous cooling phase transition curve (CCT curve) of bearing steel was drawn. On this basis, the scheme of inhibiting the formation of proeutectoid cementite by two-stage cooling process was proposed and discussed, and the finishing cooling temperature was studied, which provides a basis for controlling network carbide in field production.

Study on Phase Transformation Behavior in Continuous Cooling Process

The continuous cooling transition curve (CCT curve) system expresses the influence of cooling rate on the starting point of transition, transformation rate and microstructure are studied. In order to obtain desired microstructures and properties, the appropriate process parameters can be selected according to the continuous cooling transition curve. The CCT curve is very similar to the actual production conditions, so it is of great significance for studying the process of hot stamping of experimental steel.

Microstructure and mechanical properties of stainless steel clad plate joints produced by TIG and MAG hybrid welding

This paper investigated the microstructure and mechanical properties of Q235/304 stainless steel clad plate welding seam produced by hybrid welding of tungsten inert gas welding (TIG) and metal active gas arc welding (MAG). The results showed that the dual phases containing ferrite and austenite appeared in the stainless steel covering welding (SSCW), while the partial martensite phases appeared in the carbon steel backing welding (CSBW), which is attributed to the dilution behavior of Ni and Cr elements from stainless steel to the Q235 steel results into the movement of the CCT curve to the right side and the decrease of critical martensite formation cooling rate. The CSBW possesses the highest microhardness value in the weld metal due to the existence of the martensite zone. The impact tests were carried out and the results showed that the Charpy absorbed energy of weldments (81 J) is almost equal to that of base clad plate (83 J). The SSCW layer possesses the ductile fracture characteristics accompanying many dimples. However, in the CSBW layer, some cleavage fracture characteristics are presented in the radiation zone while many dimples are located in the fibrous zone, revealing a complex combination of brittle and ductile fracture behavior, which is due to the existence of martensite zone, different stress states and crack propagation velocity.HighlightsHybrid (TIG, MAG) welding is suitable for welding stainless steel clad plate;The martensite formation in CSBW is related to dilution and diffusion of Cr and Ni;Partial martensite transformation can strengthen and toughen the welding seam.

Effects of Quenching Temperature and Cooling Rate on the Microstructure and Mechanical Properties of U75V Rail Steel

In order to investigate the effects of quenching temperature and cooling rate on the microstructure and mechanical properties of the U75V rail steel, Gleeble-3500 thermal simulator and tensile tester were applied and the dynamic CCT curves were obtained. The results show that low quenching temperature of 750 °C narrows the range of the cooling rate of only pearlite transformation, and the pearlite interlamellar spacing decreases with an increase in the cooling rate or the quenching temperature in this range. The minimum value of the pearlite interlamellar spacing is found to be 0.101 μm. At three quenching temperatures of 810, 780, and 750 °C, the microhardness increases with an increase in the cooling rate. In addition, the starting transformation temperature from austenite to pearlite does not exceed 700 °C, and the end temperature does not drop below 500 °C. The mechanical properties including yield strength, tensile strength, and total elongation of the selected specimens are improved as the pearlite interlamellar spacing decreases and meet the requirements. For the field production of the U75V rail steel, higher quenching temperature and a cooling rate of 3–5 °C/s are the best choices.

Study on Continuous Cooling Transformation and Microstructure of 500 Mpa Grade Steel for Railway Freight Car Body

Complete the composition design and laboratory smelting of high strength weathering steel for railway freight car body of grade 500MPa. The actual continuous cooling transformation curve (CCT curve and cold speed of 0.5-50 °C/s) was measured through the Gleeble3500 thermal simulation test machine with expansion method and alloy phase method, observing the microstructures by optical microscope and SEM. The hardness of the samples under different cooling rates were measured by Vivtorinox hardness tester. Study on the effect of cooling rates on the microstructures and hardness of the steels.

Causes and Control Mechanism of Abnormal Structure in the Center of SWRH82B Wire-Rod-Steel

The cause of drawing fracture of SWRH82B wire rods was analyzed by using optical microscopy, scanning electron microscope - energy dispersive spectrometer and electron probe micro-analyzer - wavelength dispersive spectrometer. A multivariate diffusion model was established in Thermo-Cale, and the effects of temperature and time on diffusion behavior of alloys were studied. Results show that cementite network and martensite in the center area of rod is main cause of tensile fracture. There is serious segregation of chromium and manganese in the central area. The CCT curve moves to right, and critical cooling rate of martensite decreases. With high cooling rate, time for eutectoid transition is insufficient, and martensite transformation occurs in segregation band. The segregation of phosphorus further worsen the brittleness of steel. With increase of heating temperature and duration of heating time, segregation in final product is reduced, and content of cementite network and martensite decreases. When the temperature is maintained at 1050 °C for 600 s, there is no segregation of phosphorus and carbon. The diffusion of chromium is even when temperature is maintained at 1150 °C for 5400 s, and an even diffusion of manganese is obtained when temperature is maintained at 1200 °C for 3000 s. In stelmor air cooling process, the key point is keeping cooling rate low to extend holding time, and to optimize microstructure and properties.

Comparison of the hot-stamped boron-alloyed steel and the warm-stamped medium-Mn steel on microstructure and mechanical properties

The application of high strength steels (HSS) for automotive structural parts is an effective way to realize lightweight and enhance safety. Therefore, improvements in mechanical properties of HSS are needed. In the present study, the warm stamping process of the third generation automotive medium-Mn steel was discussed, the characteristics of martensitic transformation were investigated, as well as the microstructure and mechanical properties were analyzed, compared to the popular hot-stamped 22MnB5 steel in the automotive industry. The results are indicated as follows. Firstly, the quenching rate of the medium-Mn steel can be selected in a wide range based on its CCT curves, which is beneficial to the control of forming process. Secondly, the influence of stamping temperature and pressure on the Ms temperature of the medium-Mn steel is not obvious and can be neglected, which is favorable to the even distribution of martensitic microstructure and mechanical properties. Thirdly, the phenomenon of decarbonization is hardly found on the surface of the warm-stamped medium-Mn steel, and the ultra-fine-grained microstructure is found inside the medium-Mn steel after warm stamping. Besides, the warm-stamped medium-Mn steel holds the better comprehensive properties, such as a lower yield ratio, higher total elongation and higher tear toughness than the hot-stamped 22MnB5 steel. Furthermore, an actual warm-stamped B-pillar of medium-Mn steel is stamped and ultra-fine-grained martensitic microstructure is obtained. The mechanical properties are evenly distributed. As a result, this paper proves that the warm-stamped medium-Mn steel part can meet the requirements of lightweight and crash safety, and is promising for the industrial production of automotive structural parts.

Analysis of Crystallization Behavior of Mold Fluxes Containing TiO2 Using Single Hot Thermocouple Technique

The crystallization behavior of mold fluxes containing 0—8 mass% TiO2 was investigated using the single hot thermocouple technique (SHTT) and X-ray diffraction (XRD) to study the possible effects on the coordination of heat transfer control and strand lubrication for casting crack-sensitive peritectic steels. Time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) curves were plotted using the data obtained from SHTT to characterize the crystallization of the mold fluxes. The results showed that crystallization of the mold fluxes during isothermal and non isothermal processes was suppressed with TiO2 addition. From the TTT curves, it could be seen that the incubation and growth time of crystallization increased significantly with TiO2 addition. The CCT curves showed that the crystallization temperature initially decreased, and then suddenly increased with increasing the TiO2 content. XRD analysis suggested the presence of cuspidine in the mold fluxes with lower TiO2 content (< 4 mass%), while both perovskite and cuspidine were detected in the mold fluxes when the TiO2 content was increased to 8 mass%). In addition, the growth mechanisms of the crystals changed during the isothermal crystallization process from interface-controlled growth to diffusion-controlled growth with increasing the TiO2 content.

Effect of Nitrogen Addition on Continuous Cooling Transformation Behavior of Vanadium Microalloyed Steels

Effect of nitrogen addition on continuous cooling transformation behavior of vanadium microalloyed steels was investigated. Therefor, CCT curves of three experimental steels with different vanadium and nitrogen content were measured by thermal dilatometer; the microstructural evolution of the steels with the varying cooling rates was characterized; their precipitation behavior was tracked, and planer lattice misfit degree of the precipitates with the ferrite matrix was calculated. The results show that ferrite transformation is promoted by nitrogen addition, and the starting temperature of transformation and the critical cooling rate for full bainite transformation are increased as well. For cooling rates in a range0.8- 1.6℃/s, the microstructure of steels with low nitrogen consists of granular bainite + lath like bainite,while acicular ferrite also exists; during or after the γ-α transformation vanadium compounds in low nitrogen steels precipitate mainly as VC, the quantity of which increases with the increasing vanadium content. However, for the steel rich in nitrogen, vanadium compounds precipitate as VN in austenite at high temperature. The lattice misfit degree of ferrite with the precipitates of austenite, VC and VN, which occurred at 900℃are 6.72%, 3.89% and 1.55% respectively. It indicates that VN precipitates act as preferential nucleation sites for ferrites and promote the ferrite transformation.

Investigation of Mechanical Property and Springback Behavior with Hot Stamping RCP Process

With the current development of hot stamping technology, quality products have been striving for not only uniformity in hardness but also a minimum of springback. Considering the fact that the uniformity distribution of hardness and the randomness distribution of the springback are inevitable in hot stamping process due to many complex factors such as contact sequence, contact clearance, scale and initial stamping temperature, a new method named RCP method was proposed to improve the hardness and springback of hot stamping product in this paper. The cooling rate control before stamping is applied based on CCT curves through a rapid air cooling device which can provide cooling rate of 60°C/s. Three parameters included initial stamping temperature, contacting pressure and dwelling time have been investigated in this paper. According to the experiment results, the hardness deviation is markedly decreased and the springback can be controlled within the threshold value ±0.5mm.

Spray Cooling Rate Model of Heavy Cylinder Rolling

In order to eliminate the mixed crystals of the thick-walled heavy cylinder after rolling and improve the quench shortage of tank copious cooling, it is significant to analyze the cooling capacity of spray cooling and tank copious cooling. Models of heat transfer coefficient about spray cooling and copious cooling were established. The quenching process simulation was made based on the DEFORM software. The cooling rate models of center part about spray cooling and copious cooling were build. According to the CCT curves of heavy cylinder material and simulation results, which show that: (1) Using spray cooling can increase the cooling rate of center part to some extent and improve the mechanical properties. (2) The maximum capacity of spray cooling is 280mm, that is the microstructure of center part fully transfers to bainite, which is 50mm larger than copious cooling.

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.

Two High Strength Low Alloy Steels for Offshore Application

The purpose of this investigation is focused on two high strength low alloy steels for offshore application. TTT, CCT curves, Vickers depth profiles and microstructures of direct water quenched steels are evaluated in the experiment. Microhardness of the direct quenched surface is as high as 400 Hv, and the central region hardness of water quenched specimen is above 250 Hv. Martensite dominates the microstructure of quenched surface. In contrast, the mixture of martensite, bainite and ferrite is widely observed from the center of quenched specimen.

Thermodynamic and Kinetic Calculation of Non-Equilibrium Microstructure Design of TRIP Steel

The non-equilibrium microstructure of Fe-C-Mn-Si TRIP steel is designed bythermodynamic and kinetic calculation. The upper limit of bainitic transformation temperature iscalculated and compared to that characterized by CCT curve determination. s M temperature isdetermined based on thermodynamics of martensitic transformation and sublattice model. Thecalculation is conducted via TQ6-patch in Thermo-Calc software. Comparison between thecalculations and experiments reveals the relationship between non-equilibrium phase compositionand heat treatment parameters which can be utilized to achieve the elaborate design of alloy and heattreatment for super TRIP steel.

The Influence of Thermomechanical Controlled Processing on Bainite Formation in Low Carbon High Strength Steel

Thermomechanical controlled processing is a very important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes the control of the processing important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the roles of austenite conditioning and cooling rates on the bainitic phase transformation. Specimens were compared with and without deformation, and followed by seven different cooling rates ranging between 0.5°C/s and 40°C/s. The CCT curves showed that the transformation behaviors and temperatures are very different. The different bainitic microstructures which varied with austenite deformation and cooling rates will be discussed.

Research on Continuous Cooling Transformation Curve of a C-Si-Mn Steel

The CCT curve of a C-Si-Mn steel was investigated. The hot simulation tests at different cooling rates were conducted on Gleeble-1500 simulator and microstructures were observed. Based on transformation temperatures recorded by dilation curves and microstructures, the CCT curve diagram for tested steel was plotted. The effects of cooling rate on microstructure and transformation temperature were studied. The results provide the theoretical basis for determination of hot processing and heat treatment technology of C-Si-Mn steels.

Study on the Heat Treatment Process of Ultra Thick Plate of 12Cr2Mo1R Pressure Vessel Steel

The heat treatment process, normalizing plus tempering, of 150 mm-thick plate of 12Cr2Mo1R pressure vessel steel was proposed according to the results of finite element simulation and static CCT curve obtained by thermal simulation experiments. After normalizing at 910 °C for 10 minutes followed by water cooling and tempering at 650 °C for 60 minutes, the microstructure at 1/4 position along the thickness direction of the thick plate consists of bainite and all the mechanical properties meet delivery requirements.