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Through the solubility product theory of the ternary secondary phase, classical nucleation theory, and Ostwald ripening theory, a model was established to describe the thermodynamics and kinetics of (Ti, V)C precipitates in austenite/ferrite (γ/α) matrices. The model was used to calculate the volume fraction, precipitation–temperature–time (PTT) curve, and nucleation rate–temperature (NrT) curve of MC (M = Ti, V) precipitates in γ/α matrices in Ti–V microalloyed steels with various Ti/V ratios, which is verified by hardness tester, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The calculations indicate that, by decreasing Ti/V ratio from Ti4V0 steel to Ti0V4 steel, the complete-dissolution temperature decreases monotonically from 1226 to 830 °C, and the equilibrium volume fraction of MC precipitated from austenite decreases from 0.333% to 0.091% at 900 °C. Moreover, the maximum nucleation temperature of MC precipitated from α matrix decreases from 748 to 605 °C and the fastest precipitation temperature decreases from 844 to 675 °C as Ti/V ratio decreases. PTT and NrT diagrams of MC precipitated from α matrices in different Ti–V microalloyed steels all exhibit C-shaped and inverse C-shaped curves. In addition, both theoretical calculation and experimental results show that when tempered at 600 °C for 100 h, Ti2V2 steel shows the largest hardness value of 312 HV among the three steels tested because it has a larger volume fraction (0.364%), a larger precipitate density (1689 μm−2), and the smallest average size (8.4 nm) of (Ti, V)C precipitates. The theoretical calculations are consistent with experimental results, which indicates that the thermodynamics and kinetics model for (Ti, V)C precipitates is reliable and accurate.

The microstructure evolution and the pitting corrosion resistance of a supermartensitic stainless steel after deep cryogenic treatment process were clarified through X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy (TEM) and electrochemical methods. The results showed that the microstructure of supermartensitic stainless steel mainly consisted of reversed austenite, tempered martensite, and M23C6 carbides after tempering. The deep cryogenic treatment promoted the refinement of the martensite laths and the precipitation of the carbides in comparison with the traditional process. TEM analysis indicated that the segregation of Si atoms at the boundary was found at the interface between carbide and martensite. The pitting corrosion potential of the specimens subjected to deep cryogenic treatment decreased with the elevated tempering temperature, and the lowest pitting corrosion potential was found at the tempering temperature of 650 °C. The sensitivity of the pitting corrosion potential was attributed to the precipitation of M23C6 carbides and Si atoms segregation. Si atoms segregation engendered the formation of Cr-depleted zone near M23C6 and impeded the recovery of Cr-depleted zone.

The precipitation kinetics of TiC and VC during high-temperature tempering of low-carbon martensitic steels was studied by means of hardness measurement and theoretical analysis. The precipitation–temperature–time diagram, which was determined through the hardness versus tempering time curves, shows that the precipitation rate of TiC is more rapid than that of VC at 600 °C or above, while they are almost the same at 550 °C. The number density of TiC precipitates is larger than that of VC precipitates, thus leading to a larger secondary hardening effect. An analytical model to describe the precipitation kinetics of microalloying carbides during tempering of martensitic steel was presented. Using this model, the difference of precipitation kinetics between TiC and VC can be well explained, in terms of the shape and relative position of kinetics curves and the nose temperature.

The effect of Nb microalloying on microstructure evolution and mechanical properties of 0.02C–7Mn steel was investigated. The results showed that the microstructure was fully quenched martensite after subjected to reheating and quenching (RQ) process. The microstructure consisted of tempered α-martensite, reversed austenite (RA), and athermal e-martensite after tempered at 620 °C, while the microstructure contained tempered α-martensite, RA, athermal e-martensite, and athermal α-martensite after tempered at 650 °C. The thermal and mechanical stability of both RA and athermal e-martensite decreased after the 0.05% Nb was added to the 0.02C–7Mn steel, and also decreased as the intercritical tempering (IT) temperature increased, which was mainly because the content of solid solution C in RA decreased. The calculated stacking fault energies (SFEs) were all negative, which spontaneously induced athermal martensitic transformation. Multiphase microstructure resulted in the remarkable mechanical property evolution. After tempered at 620 °C, the optical mechanical properties of 7Mn-0.05Nb steel were obtained: yield strength of 720.0 MPa, tensile strength of 819.0 MPa, total elongation of 26.0% and −40 °C impact energy of 266 J, while the yield strength of 675.0 MPa, tensile strength of 776.5 MPa, the total elongation of 29.5% and the −40 °C impact energy of 274 J appeared in 7Mn–0Nb steel. As the IT temperature increased to 650 °C, the yield strength of both the steels sharply decreased (475.5 MPa for 650 °C–0Nb steel and 460.5 MPa for 650 °C-0.05Nb steel), and the tensile strength of both the steels increased about 56 MPa, while the −40 °C impact energy decreased by 40 and 60 J, respectively, showing that the −40 °C impact energy was not very sensitive to the mechanical stability of RA and athermal e-martensite. The mean size of NbC particles was mainly about 5–7 nm, which significantly improved the precipitation strengthening, but there was just little damage to low-temperature toughness.

A new ultrahigh strength hot rolled Ti-Mo-bearing ferritic steel was developed through chemical composition design and rolling processing optimization. To maximize the potential of nanometer-sized (Ti, Mo)C carbide in terms of strengthening ferrite matrix, the optimal chemical composition of 0.1C-0.2Ti-0.4Mo (wt%) was determined through considering the atomic ratio of elements, the solubility temperature of (Ti, Mo)C in austenite, and the excessive growth critical temperature of austenite grain during reheating. The rolling condition in the region through austenite recrystallization region to austenite nonrecrystallization region was adopted to realize a homogenous and fine ferrite grain structure. Results showed that the simulated coiling at 600 °C was found to provide an attractive combination of ferrite grain refinement hardening (360 MPa) and precipitation hardening (324 MPa). An optimal combination of strength and ductility was achieved after coiling at 600 °C (yield strength: 912 MPa; ultimate tensile strength: 971 MPa; total elongation: 16.0%). In addition, the nanometer-sized (Ti, Mo)C carbide was characterized by transmission electron microscopy (TEM) and physical-chemical phase analysis, and its role was discussed in details.

The precipitation of microalloying elements is one of the most important issues in microalloyed steels. It is well recognized that controlling the precipitation process of microallying elements in steels is an effective means to significantly improve the strength of steel material due to precipitation strengthening and grain refinement by controlling the austenite grains coarsening during reheating process and recrystallization process. Moreover, controlling the precipitation behavior of secondary phases in steel, leading to an accurate control of volume fraction, shape, size and distribution of precipitates, could effectively improve the microstructure and mechanical properties, which is a significant issue for microalloyed steel in the field of theory research and production practice.

The carbide precipitation in austenite of Nb–Mo bearing low-carbon steel was investigated by stress relaxation and transmission electron microscopy (TEM). The precipitation-time-temperature (PTT) diagram exhibits a typical “C” shape curve, the nose of which was located at 950 °C. Mo was found to be incorporated into NbC lattice to form a complex (Nb, Mo)C particle during precipitation. Compared to NbC particles in Nb-bearing steel, (Nb, Mo)C particles in Nb–Mo-bearing steel exhibit superior coarsening resistance.

Through the thermo-mechanical control process (TMCP), a high Nb low Mo fire resistant steel with the yield strength (YS) of 521 MPa at room temperature (RT) and 360 MPa at elevated temperature (ET) of 600 °C was developed based on MX (M=Nb, V, Mo; X=C, N) precipitation strengthening. A series of tensile and constant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transformation (CCT) as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal simulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was determined as 653 °C, and the granular bainite was obtained when the cooling rate was higher than 10 °C/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates (Nb, Mo, V)C with size of less than 10 nm precipitated in ferrite at ET (600 °C), which resulted in precipitation strengthening at ET.

A novel hot rolled steel LG600A with the tensile strength exceeding 700 MPa was developed for automatic teller machine application. The low-cost C-Mn steel was microalloyed with 0.08 mass% – 0.12 mass% Ti rather than noble alloying elements, such as Nb, V, Mo, and Cu, etc. The novel steel had a good surface quality and welding property. After the hot rolled steel coils were leveled, the steel plates, the length of which was even down to 1500 mm, had an excellent flatness. The effects of hot rolling parameters on mechanical performance, microstructure and recrystallization behavior were studied. The metallurgical concept for the steel production was also discussed. The result shows that decreasing the finish rolling temperature, increasing cooling rate in the first cooling stage and decreasing the cooling rate in the last cooling stage, together with coiling at a modestly high coiling temperature all resulted in the refined grains and TiC precipitates, thereby improving the strength and toughness of this new steel greatly.

Effects of microalloyed niobium (Nb) on the austenite decomposition behaviors and microstructure evolution during continuous cooling in the near eutectoid steels were investigated. Compared to the Nb free steel, the Nb microalloyed steel was refined with regard to polygonal ferrite grain, pearlite block and colony sizes. This was because its austenite grain size was smaller. The volume fraction of polygonal ferrite transformed was more in the Nb microalloyed steels, which indicated the eutectoid carbon content exceeded that of pure carbon steel. The spheroidization of pearlite during continuous cooling was enhanced by Nb microalloying, mainly due to a higher critical transformation temperature and the finer pearlite structure with smaller colony size and narrower interlamellar spacing. Hot deformation right above the equilibrium eutectoid temperature accelerated the spheroidization kinetics of pearlite, especially in the Nb microalloyed steel.

The austenite grain refinement through control of the grain growth during reheating process after thermo-mechanical controlled process (TMCP) in a vanadium microalloyed steel was achieved. The formation of ultra-fine grained austenite was attributed to the high density of austenite nucleation at the ferrite/martensite structure and to the inhibition of austenite growth by (Ti, V)C particles at the relatively low reheating temperature. Corresponding with the precipitation behavior of (Ti,V)C with temperature, the growth behavior of austenite in the vanadium microalloyed steel could be divided into two regions. At lower reheating temperature, austenite grains grew slowly, and ultra-fine grained austenite smaller than 5 μm was successfully obtained. By contrast, the austenite grains grew rapidly at high temperature due to the dissolution of (Ti,V)C particles. According to the measured and predicted results of austenite growth kinetics, two models were developed to describe the growth behavior of austenite grains in two different temperature regions, and the apparent activation energy Qapp for grain growth was estimated to be about 115 and 195 kJ/mol, respectively.

The fundamental idea of materials genome initiative is the integration of computing platform, experimental platform, and data platform to speed up the material innovation hence reduce time and cost. This paper describes the basic concept of building an integrated computational platform and data platform for material innovation from the perspective of high-throughput simulation and materials knowledge management. The material data platform that can integrate material database, heterogeneous material data, various scripts, and open-source material simulation code together is particularly discussed. Taking metallic materials as an example, a brief introduction to metallic materials data management is given, and how to manage the semi-structure and unstructured iron and steel material data is also presented.

The precipitation behavior of vanadium-rich carbides formed during the reheating processes and its influence on the microstructure and mechanical properties have been systematically investigated in a high strength low alloy martensite steel. It was found that the nano-sized (Ti, V)C particles mainly precipitated during reheating rather than soaking at the austenitization process. For their pinning effect on grain growth, ultra-fine austenite grain with the average size of 3.5 μm formed during holding at 880 °C for 1 h. As the austenitization temperature increased, the prior nano-sized (Ti, V)C coarsened and its volume fraction decreased, which led to increment of austenite grain size. Excellent combined mechanical properties such as tensile strength 1670 MPa, yield strength 1460 MPa, elongation 10% at room temperature and impact energy (Akv) 57 J at −40 °C were obtained in the ultra-fine grained steels. In particular, the low-temperature toughness was improved by grain refinement. The strengthening mechanisms were also investigated by comparing the experimental results with theoretical calculation. The variation of yield strength with the condition of heat treatment was discussed in detail.

The carbide precipitation behavior in austenite of a 0.04%C–0.10%Ti–0.21% Mo micro-alloyed steel was investigated by the stress relaxation method and transmission electron microscopy. Results showed that the precipitation-time-temperature diagram for carbide precipitation exhibited a typical “C” curve, the nose of which was located at about 925 °C. The carbide was identified as a (Ti, Mo)C particle with a NaCl-type crystal structure that contains a certain amount of Mo. As compared with TiC particle in Ti steel, (Ti, Mo)C particle in Ti–Mo steel exhibits a superior coarsening resistance during the coarsening stage. The fraction of Mo in (Ti, Mo)C particle decreases with isothermal holding time or particle growing, indicating that the high level replacement of Ti by Mo in TiC lattice is in a “metastable” state with respect to the equilibrium precipitation of (Ti, Mo)C in austenite.

Ultrafine austenite grains with average size of 2 μm were successfully obtained by combining thermo-mechanical control process followed by reheating in a vanadium microalloyed steel. The mixed microstructure transformed from pancaked austenite formed during controlled rolling has a higher density of high angle boundaries, compared to that transformed from equiaxial austenite. It contributes to increasing nucleation density of austenite grain during the reheating process. A certain volume fraction of undissolved nano-sized (Ti, V)C particles, which are formed during the controlled rolling process and/or the reheating process, effectively inhibit austenite grain growth and consequently refine austenite grain size significantly. The critical grain size of austenite calculated by Gladman model agrees well with the experimental result.

The kinetics of strain induced precipitation in Ti-microalloyed steel with three levels of Mn was investigated using a two-stage interrupted compression (TSIC) test. The precipitation-time–temperature diagrams (PTT) were obtained by analyzing the softening kinetics curves of deformed austenite. Experimental results showed that the upper part of the PTT diagram for strain induced precipitation were shifted to lower temperatures and longer times as Mn concentration in steel was increased, but the retard effect of Mn on TiC precipitation weakened with decreasing testing temperature. Transmission electron microscopy (TEM) observation showed that the rate of TiC precipitation was decreased by more Mn addition at higher testing temperatures, which is in a good agreement with TSIC result. In addition, these results were discussed in terms of the effect of Mn on the solubility product of TiC in austenite, and the controlling factors of precipitation kinetics at different temperatures.

In order to clarify effects of prior pancaked austenitic structure on microstructure and mechanical properties of transformed martensite in ausformed steel, a super-thin pancaked austenite was processed by multi-pass rolling in a 0.03–2.6Mn-0.06Nb-0.01Ti (wt%) low alloy steel. The evolution of prior pancaked austenite grain during multi-pass rolling was studied using Ni-30Fe model alloy. Related with the structure and texture in the prior super-thin pancaked austenite in Ni-30Fe alloy, the texture and anisotropy of mechanical properties of transformed martensite in the studied ausformed steel were focused on. There were mainly three kinds of rolling texture components in the super-thin pancaked austenite: Goss {110}〈001〉, copper {112}〈111〉 and brass {110}〈112〉. They were further transformed into the weak {001}〈110〉 and strong {112}〈110〉, {111}〈112〉 texture components in the martensitic structure. The orientation relationship (OR) of lath martensite transformation from pancaked austenite in the ausformed steel deviated larger from the exact Kurdjumov-Sachs (K-S) OR than in the case of equiaxed austenite without deformation. The tensile and yield strengths of the ausformed martensitic steel first decreased and then increased as the angle between tension direction and rolling direction increased. The main reason for the anisotropy of strength was considered as the texture component {112}〈110〉 in martensite. However, the anisotropy of impact toughness was more complex and the main reasons for it are unknown.

On-line thermo mechanical controlled processing (TMCP) was conducted to develop the third generation high strength low alloy (HSLA) steel with high toughness economically. The ultra-low carbon content ensured a high level of upper shelf energy while ultrafine lath martensitic structure transformed from super-thin pancaked austenite during controlled rolling and cooling. The reduction of martensite block size decreased ductile-to-brittle transition temperature (DBTT) and compensated the strength loss due to carbon reduction. Consequently, the excellent balance of strength and toughness values was obtained as 950–1060 MPa for yield strength, 180 J for Charpy V-notch impact absorbed energy at −30°C, which is much superior to that of traditional martensitic steel. Two mechanisms for the refinement of lath martensite block were proposed: One is the austenite grain refinement in the direction of thickness, and the other is the reduction in the fraction of sub-block boundaries with small misorientation and the increase in the fraction of block boundaries with large misorientation, possibly due to austenite hardening.

1) Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, HaidianDistrict, Beijing, 100084 China. 2) Institute for Structural Materials, Central Iron and Steel Research Institute, No. 76,Xueyuan Nanlu, Beijing, 100081 China. 3) The Chinese Society fo r Metals, No. 46, Dongsixi Street, Beijing, 100711 China.(

The strain-induced precipitation kinetics of TiC in a 0.05% C–0.10% Ti HSLA steel was investigated by two-stage interrupted compression method. The precipitation–time–temperature (PTT) diagram for TiC precipitation was obtained by analyzing the softening kinetics curves of deformed austenite, which was confirmed to be of validity by employing transmission electron microscopy (TEM). Experimental results showed that the PTT diagram for TiC precipitation exhibited a typical “C” shaped and the nucleation of strain-induced TiC precipitation was a very rapid process in the temperature range 900–925 °C. The relatively severe deformation applied on the steel was considered to be the main factor resulting in the fast kinetics of TiC precipitation. The TiC precipitates were heterogeneously distributed in either a chain-like or a cell like manner, implying that the precipitates nucleated on dislocations or on dislocation sub-structures, which were produced by deformation. The growth of TiC precipitates approximately followed a parabolic law. In addition, the coarsening of strain-induced TiC precipitates had already started before the completion of precipitation.

A new V-Ti-Mo complex micro-alloyed hot-rolled high strength sheet steel was developed by controlling a thermomechanical controlled processing (TMCP) schedule, in particular with variants in coiling temperature. At coiling temperature of 600 °C, the experimental steel has the best mechanical properties: ultimate tensile strength (UTS) 1000 MPa, yield strength (YS) 955 MPa and elongation (EL) 17%. Furthermore, the contributions of precipitation hardening and grain refinement hardening were high to about 310 and 361 MPa, respectively. The steels developed were mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in contribution of precipitation hardening increment caused a significant difference in YS.

Instead of off-line quenching and tempering (QT), on-line ausforming (non-recrystallization controlled roling) and direct quenching (DQ) was employed to improve the toughness of low alloy steel with yield strength of 900 MPa Grade. Low carbon content ensured a high level of upper shelf energy, while the fine-grained martensite substructures decreased the ductile-to-brittle transition temperature and compensated the strength loss due to carbon reduction. Two mechanisms for the refinement of martensite substructure were proposed: one was the austenite grain refinement in the thickness direction, and the other was the self-accommodation of martensite variants due to austenite grain hardening. More than 200 J of Charpy V-notch impact absorbed energy at 233 K was obtained in the industrial ausformed plate, which was about the double in the traditional QT steel with the same strength grade.

The effect of Mo addition on the precipitation of carbides both in austenite and ferrite of Nb-bearing HSLA steel were investigated by stress relaxation and transmission electron microscopy (TEM). Experimental results showed that the addition of ~0.2 wt.% Mo into Nb-bearing steel slightly accelerated the precipitation kinetics of Nb carbides both in austenite and ferrite. The particles, (Nb, Mo)C, precipitated in Nb-Mo-bearing steel exhibit superior coarsening resistance compared to that of NbC particles in Nb-bearing steel. And the difference of carbides precipitated in anstenite and ferrite of Nb-Mo-bearing steel was discussed.

Hot rolled Nb–Mo steel of yield strength 600 MPa and Nb–Ti steel of yield strength 525 MPa with polygonal and acicular ferrite microstructure have been developed. Using physicochemical phase analysis, XRD, TEM and EDS, the distribution, morphology, composition, crystal structure and particle size of precipitates were observed and identified in these steels. The results revealed that the steels containing both Nb and Mo exhibited fine and uniformly distributed MC-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb and Ti. The physicochemical phase analysis showed MC-type carbides contain both Nb and Mo, and the ratio of Mo/Nb was 0.41. Meanwhile, the mass% of the fine particles (

The secondary phases exert strong influence on the mechanical properties, processing properties and performance of steels. The volume fraction, size, shape and distribution of the secondary phases must be effectively controlled in order to obtain the excellent properties and performance. On the basis of equilibrium solubility product formula, precipitation thermodynamics and kinetics, and classical nucleation and growth theory, the theoretical calculation method and procedure for the PTT (precipitation fraction–time–temperature) curve and NrT (nucleation rate–temperature) curve for main secondary phases precipitated in austenite and in ferrite have been proposed. The calculation results are well in agreement with practical experimental results. By analyzing the calculation results, some important deductions for the practical control of secondary phases in steels have been made.

Low-carbon bainitic steels offer a solution to produce strip and plate products with unique properties in terms of strength and toughness. MoNb-based alloying concepts have a great potential to further develop and optimize such steels. Mo can promote the transformation of acicular ferrite and bainite through its retardation effect on the proeutectoid ferrite transformation, and therefore improve the strength and toughness of low alloy steels. An optional alloying element is boron. Special attention is focused on the cross effects of Mo with the microalloying elements with regard to hardenability, grain size control and precipitation strengthening. Important metallurgical effects like recrystallization, microstructure and precipitation behavior are analyzed and related to mechanical properties. Different processing options for strip and plate mills are being discussed in that respect. Finally, some examples of applications of Mo in high grade pipeline steel, high strength engineering machinery steel, fire-resistant steel and hot-rolled strip for automobile are briefly introduced.

With the increasing consciousness for reducing fuel consumption and improving automobiles safety,the automotive industry is urgent to develop a new-type of steel with high strength and excellent formability.Among many high strength steels,the transformation induced plasticity (TRIP) steel may be a good candidate for automotive applications,as it drastically improves the balance between strength and ductility compared to precipitation hardened and solution hardened steels.While the tensile strength of conventional hot rolled TRIP steels are usually between 500 and 600 MPa,the TRIP steel with higher tensile strength,especially in excess of 750 MPa,is becoming increasingly important for the automotive industry.Thus,many strengthening mechanisms,such as precipitation strengthening,solution strengthening,refinement strengthening and dislocation strengthening,have been employed to improve the strength of the TRIP steel.Among them, microalloying with Nb,V and Ti,may provide effective means for further strengthening via grain refinement and precipitation strengthening.So far,many researches about the Ti-microalloyed high strength low alloy(HSLA) steel have been reported.However,the influences of alloying elements Ti and Mo on the hot rolled TRIP steel,especially the precipitation characteristics and their effects on mechanical properties,were rarely reported.Therefore,in this work the microstructure,retained austenite contents,mechanical properties and precipitation characteristics of the hot rolled TRIP steel containing Ti and Mo after bainitic transformation at different temperatures,were studied by using SEM,XRD and HRTEM.The results show that the bainitic transformation temperature has a significant effect on organizational morphology,retained austenite contents and mechanical properties of the TRIP steel.The optimal bainitic transformation temperature is 400℃,in which the retained austenite content and the balance of strength and ductility are 17.13%and 23.87 GPa·%,respectively. In addition,through HRTEM analysis,it was observed that the larger(Ti,Mo)C carbides over 20 nm in size exhibited the relationship((100)_((Ti,Mo)C )//(110)_(α-Fe),[011]_((Ti,Mo)C )//[111]_(α-Fe)) with ferrite matrix, and the smaller(Ti,Mo)C carbides less than 5 nm in size satisfied the Baker-Nutting orientation relationship:(100)_((Ti,Mo)C )//(100)_(α-Fe),[011]_((Ti,Mo)C )//[001]_(α-Fe).