Creation of AirCooled Mn Series Bainitic Steels

The development and mechanical performances of new type aircooled Mn series bainitic steels including granular bainitic steels, FGBA/BG duplex steels, CFB/M duplex steels, medium carbon bainite/martensite steels, cast bainitic steels invented by the authors are summarized. The novel series of bainitic steels are alloyed with Mn, and several series bainitic duplex microstructures can be easily obtained under the condition of air cooling through unique composition design. The invented idea, the principle of alloying design, the strengthening mechanism, and the evolution of the microstructure of new type aircooled Mn series bainitic steels are presented. Furthermore, the applications in different fields of these Mn series aircooled bainitic steels with different strength level are also introduced. It is suggested that the significance of the development of the aircooled Mn series bainitic steel can be summarized as follows: reducing costs of both raw materials and production; good combination of strength and toughness; selfhardening with high bainitic hardenability by air cooling from hot working without additional quenchingtempering treatment or quenching procedure; large savings in energy resources; and reduced environmental pollution.     

Mechanism of Austenite Evolution During Deformation of UltraHigh Carbon Steel

The mechanism of transformation of austenite to cementite and pearlite during the deformation of ultrahigh carbon steel was discussed. The results indicate that the pearlite and cementite can be induced by deformation between Acm to Arcm. The transformation during deformation is still considered as a diffusioncontrolled process. With the increase of time and reduction, the pearlite fraction increased. At the beginning of the transformation, the pearlite was lamelliform. When the rate of reduction was increased to 70%, some of the induced lamellar pearlite was broken up under deformation.     

Computer Model of Phase Transformation From HotDeformed Austenite in Niobium Microalloyed Steels

Based on thermodynamics and kinetics, a new mathematical model was developed to calculate the CCT diagrams and the transformation kinetics in low carbon niobium steels, in which the effect of deformation on the degree of supercooling was taken into account. The undercooling caused by deformation is the major reason for the increase of the starting transition temperature during continuous cooling. The critical cooling rate of bainite formation is within 2-5 ℃/s for the studied niobium steels and deformation is suitable for the occurrence of pearlite. The ferrite volume fraction increases with the increase of the austenite boundary area, and decreases with the increase of the cooling rate. The calculated CCT diagrams and the volume fraction of each phase are in good agreement with the measurements.     

Effect of Alloying Elements on Thermal Wear of Cast HotForging Die Steels

The effect of main alloying elements on thermal wear of cast hotforging die steels was studied. The wear mechanism was discussed. The results show that alloying elements have significant influences on the thermal wear of cast hotforging die steels. The wear rates decrease with an increase in chromium content from 3% to 4% and molybdenum content from 2% to 3%, respectively. With further increase of chromium and molybdenum contents, chromium slightly reduces the wear resistance and molybdenum severely deteriorates the wear resistance with high wear rate. Lower vanadium/carbon ratio (15-25) leads to a lower wear resistance with higher wear rate. With an increase in vanadium/carbon ratio, the wear resistance of the cast steel substantially increases. When vanadium/carbon ratio is 3, the wear rate reaches the lowest value. The predominant mechanism of thermal wear of cast hotforging die steels are oxidation wear and fatigue delamination. The Fe2O3 and Fe3O4 or lumps of brittle wear debris are formed on the wear surface.     

Alloying Design for High WearResistant Cast HotForging Die Steels

The alloying design of cast hotforging die steels was analyzed. The relationship of the life of cast hotforging dies with the failure patterns was studied. The thermal wear resistance was believed to be the key property for the alloying design of cast hotforging die steels. The alloying design parameters were selected and optimized for the cast hotforging die steel with high wear resistance. The wear resistance of the optimized cast die steel was evaluated in comparison with commercial H13 steels and 3Cr2W8V steel. In the new cast hotforging die steel, VC is predominant carbide with Cr and Mo as the main solution elements in αFe. It is found that the cast die steel has significantly lower wear rate than normal H13 steel and 3Cr2W8V steel, almost the same as that of high purity H13 steel. The high wear resistance of the new cast hotforging die steel can be attributed to its reasonable alloying design and nonsensibility to the detrimental function of S and P.     

Thermodynamic Properties of Carbon and Manganese in MnC and MnFeC Melts

Carbon solubility in MnFe melts (xMn=0083-0706, xFe=0034-0715) was measured experimentally at various temperatures. By thermodynamic derivation and calculation, the relationship between activity coefficient of carbon in infinite dilute solution of manganese in MnC system and temperature was obtained. Using GibbsDuhem relationship, the experimental results of this study, and experimental data obtained by strict thermodynamic derivation and calculation in references, the relationships between other thermodynamic properties (εCC, εCCC, εCFe, εCCFe, and εCFeFe) in MnFeC system and temperature were obtained.     

Effect of Aging Temperature on ErosionCorrosion Behavior of 174PH Stainless Steels in Dilute Sulphuric Acid Slurry

The effect of aging temperature on erosioncorrosion (EC) behavior of 174PH stainless steels in dilute sulphuric acid slurry containing solid particles was studied by using selfmade rotating EC apparatus. The effect of impact velocity on EC behavior of 174PH steels at different aging temperatures was analyzed. Surface micrographs of the specimens after EC test were observed by using scanning electron microscope (SEM). The results showed that under the condition of the same solution heat treatment, when aging temperature ranged from 400 ℃ to 610 ℃, the hardness reached the highest value near the temperature 460 ℃. The characteristics of EC for 174PH stainless steels at different aging temperatures were as follows: pure erosion (wear) was dominant, corrosion was subordinate and at the same time corrosion promoted erosion. The effect of aging temperature on EC rate of 174PH steels was not significant at low impact velocity, but it was found that EC resistance of 174PH steels aged near 460 ℃ was the most excellent due to the best precipitation strengthening effect of fine and dispersed εCu phase. With a prerequisite of appropriate corrosion resistance, the precipitation hardening could significantly improve the EC resistance of the materials.     

Hot Deformation Behavior and Processing Map of Spray Formed M3∶ 2 High Speed Steel

Hot deformation behavior of a new type of M3∶ 2 high speed steel with niobium addition made by spray forming was investigated based on compression tests in the temperature range of 950-1 150 ℃ and strain rate of 0. 001-10 s~(-1). A comprehensive constitutive equation was obtained,which could be used to predict the flow stress at different strains. Processing map was developed on the basis of the flow stress data using the principles of dynamic material model. The results showed that the flow curves were in fair agreement with the dynamic recrystallization model. The flow stresses,which were calculated by the comprehensive constitutive equation,agreed well with the test data at low strain rates( ≤1 s~(-1)). The material constant( α),stress exponent( n) and the hot deformation activation energy( Q_(HW)) of the new steel were 0. 006 15 MPa~(-1),4. 81 and 546 kJ·mol~(-1),respectively. Analysis of the processing map with an observation of microstructures revealed that hot working processes of the steel could be carried out safely in the domain( T = 1 050-1 150 ℃,ε = 0. 01- 0. 1 s~(-1))with about 33% peak efficiency of power dissipation( η). Cracks was expected in two domains at either lower temperatures( 1 000 ℃) or low strain rates( 0. 001 s~(-1)) with different cracking mechanisms. Flow localization occurred when the strain rates exceeded 1 s~(-1) at all testing temperatures.     

Transformation of low carbon steel during deformation at γ/α temperature based on online rolling data

Alloying elements,such as silicon and manganese,have a major impact on the phase transformation point of steel.Specifically,manganese is an element for the expansion and stability of the austenite region,while silicon can expand and stabilize the ferrite region.Phase transformation occurs during the hot rolling process for the steel with certain silicon content,which leads to great changes of the deformation resistance,thereby affecting the rolling stability.Consequently,a better understanding of phase transformation in the rolling process will contribute to the enhancement of product quality.In this paper,the on-line rolling data were processed by means of the inverse calculation method.In this method,the steel deformation resistance with various silicon and manganese contents was obtained and analyzed to determine the deformation behavior of the steel,which can help improve the on-line control model and enhance the steel quality.     

Textures and Properties of Hot Rolled High Strength TiIF Steels

The texture evolution in a high strength TiIF steel during the processing of hot rolling, cold rolling, and annealing is studied. For comparison, both ferrite rolling and austenite rolling are employed. It is found that the texture type is the same after ferrite rolling and austenite rolling, but the texture intensity is much higher in the ferrite rolled sample. Furthermore, texture characteristics at the surface are absolutely different from those at the midsection in both ferrite rolled and austenite rolled samples, as well as under the cold rolled and annealed conditions. The shear texture {110}<001> disappears and orientation rotates along {110}<001>→{554}<225>→{111}<112>→{111}<110>→{223}<110> during cold rolling. Compared to the austenite rolled sample, the properties of the cold rolled and annealed sheet which is subjected to ferrite rolling are higher.     

SelfLearning and Its Application to Laminar Cooling Model of Hot Rolled Strip

The mathematical model for online controlling hot rolled steel cooling on runout table (ROT for abbreviation) was analyzed, and water cooling is found to be the main cooling mode for hot rolled steel. The calculation of the drop in strip temperature by both water cooling and air cooling is summed up to obtain the change of heat transfer coefficient. It is found that the learning coefficient of heat transfer coefficient is the kernel coefficient of coiler temperature control (CTC) model tuning. To decrease the deviation between the calculated steel temperature and the measured one at coiler entrance, a laminar cooling control selflearning strategy is used. Using the data acquired in the field, the results of the selflearning model used in the field were analyzed. The analyzed results show that the selflearning function is effective.     

Thermodynamic Properties of MnC Melts

Carbon solubility in MnFe melts (xMn=0161-0706, xFe=0034-0633) was measured experimentally at various temperatures. By thermodynamic derivation and calculation, the relationship between activity coefficient of carbon in infinite dilute solution of manganese in MnC system and temperature was obtained. Using GibbsDuhem relationship, the experimental results of this study, and experimental data reported in references, the relationship between other thermodynamic properties in MnC system and temperature were obtained by thermodynamic derivation and calculation.     

Rereddening on Strip Surface After Water Cooling

After water cooling, there is a big temperature difference between the center and the surface of strip, which leads to the heat transfer from the center to the surface, and the surface temperature can rise in a short time. The finite element method was used to simulate the phenomena of rereddening on the surface of strip and to analyze the temperature field of hot rolled strip during laminar cooling, and the periodical variation curve of the cooling rate was obtained during water cooling and subsequent rereddening. The results show that the critical line of the cooling rate is at 1/3 of the halfthickness from the strip surface. The regression model of the relation of rereddening temperature, time, and distance from the surface was obtained in the rereddening region. Rereddening regularity on the surface of strip under the condition of different thickness and cooling rate was also studied.     

Thermodynamic Properties of FeCN and FeCBN Melts

The saturated solubility of carbon and nitrogen in FeCN and FeCBN melts was measured experimentally at 1 485 ℃ to obtain the activity interaction coefficients between components in these melts. A new method was used to treat experimental results. Using thermodynamic derivation and calculation, some important interaction coefficients between components in these melts were obtained.     

Effect of Chromium on CCT Diagrams of Novel AirCooled Bainite Steels Analyzed by Neural Network

The quantitative effects of chromium content on continuous cooling transformation (CCT) diagrams of novel aircooled bainite steels were analyzed using artificial neural network models. The results showed that the chromium may retard the high and mediumtemperature martensite transformation.     

Modeling of Austenite Decomposition in Low SiMn TRIP Steel During Cooling

Transformation behavior in low carbon SiMn TRIP steel was investigated by means of microstructural observation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

Adiabatic Shear Localization for Steels Based on JohnsonCook Model and Second and FourthOrder Gradient Plasticity Models

To consider the effects of the interactions and interplay among microstructures, gradientdependent models of second and fourthorder are included in the widely used phenomenological JohnsonCook model where the effects of strainhardening, strain rate sensitivity, and thermalsoftening are successfully described. The various parameters for 1006 steel, 4340 steel and S7 tool steel are assigned. The distributions and evolutions of the local plastic shear strain and deformation in adiabatic shear band (ASB) are predicted. The calculated results of the second and fourthorder gradient plasticity models are compared. S7 tool steel possesses the steepest profile of local plastic shear strain in ASB, whereas 1006 steel has the least profile. The peak local plastic shear strain in ASB for S7 tool steel is slightly higher than that for 4340 steel and is higher than that for 1006 steel. The extent of the nonlinear distribution of the local plastic shear deformation in ASB is more apparent for the S7 tool steel, whereas it is the least apparent for 1006 steel. In fourthorder gradient plasticity model, the profile of the local plastic shear strain in the middle of ASB has a pronounced plateau whose width decreases with increasing average plastic shear strain, leading to a shrink of the portion of linear distribution of the profile of the local plastic shear deformation. When compared with the secondorder gradient plasticity model, the fourthorder gradient plasticity model shows a lower peak local plastic shear strain in ASB and a higher magnitude of plastic shear deformation at the top or base of ASB, which is due to wider ASB. The present numerical results of the second and fourthorder gradient plasticity models are consistent with the previous numerical and experimental results at least qualitatively.     

Development of NbVTi HotRolled High Strength Steel With Fine Ferrite and Precipitation Strengthening

A hotrolled steel with high yield strength of 700 MPa, good elongation of about 20% and low ductilebrittle transition temperature (DBTT) lower than -70 ℃ has been developed in laboratory. The results show that adopting finishing rolling temperature of around 800 ℃ is rational, and coiling temperature is between 400 and 500 ℃. The strength of developed 700 MPa hotrolled high strength steel is derived from the cumulative contribution of fine grain size, dislocation hardening and precipitation hardening. The fine grain strengthening and precipitation hardening are the dominant factors responsible for such high strength, and good elongation and toughness are predominantly due to fine grain ferrite.     

Microstructure Characteristics of an FeMnC TWIP Steel After Deformation

 Abstract: The present paper investigated deformatiom microstructure characteristcsl of the Fe-Mn-C TWIP steel .The results showed that the hot rolled – cold rolled - annealed sample of Fe-Mn-C TWIP steels has excellent mechanical properties, the true stress-strain curve from tension tests exhibit repeated serrations. The deformed microstructure exhibited the typical planar glide characteristics such as no cell formation, dislocation pile ups on a single slip plane, mechanical twins and stacking faults. There are equiaxial and deep dimple structures in the fractograph, it is a typical ductile fracture. microcrack initiated from inclusions and twin-twin intersections, Its deformation and mfracture processes were the formation, growth and coalescence of microvoids. Because of high amount of anganese, there are many inclusions of strip MnS, the quantity of sulphur should be controlled during steelmaking.     

Effect of Laminar Cooling on Phase Transformation Evolution in Hot Rolling Process

 In the hot rolling processes, it always uses the laminar cooling system to control the cooling of plate and get the microstructure of different constitution. To investigate the effect of temperature variation due to the cooling pattern, rolling speed and thickness of plate on the microstructure evolution, a thermal-mechanical coupled FE model has been established. A set of constitutive equations of phase transformation is implemented into the commercial FE solver MARC through the user defined subroutine CRPLAW, and the temperature field is calculated by another user defined subroutine FILM. The effect of cooling pattern, rolling speed and thickness of plate on the temperature variation is numerically studied. Furthermore, the effect of temperature variation on the evolution of each phase transformation is investigated for various cooling pattern.