Development of thermal rolling regimes of low-alloy “ARC”-steel with quasi-homogeneous ferrite-bainitic structure

In this work, the kinetics of the growth of austenite grains upon heating, the features of the processes of dynamic and static recrystallization occurring at various temperature-deformation modes of plastic deformation are investigated. Phase transformations have been studied during continuous cooling of hotdeformed austenite in low-alloy “Arc”-steel with a yield point of at least 420 MPa. The studies carried out made it possible to determine the thermal deformation parameters that ensure the formation of a finely dispersed homogeneous ferrite-bainitic structure, on the basis of which technological recommendations for industrial production were developed and sheet products were manufactured. Presented are the structure and properties of sheet metal from shipbuilding “Arc”-strength category 420 MPa.

Features of the ferrite-bainite structure low-alloy low-carbon steel after heat hardening and subsequent tempering

Problem statement. In recent decades, there has been a tendency to increase the mechanical properties of low-carbon, low-alloyed steel plate iron by using controlled rolling or hardening heat treatment of finished steel parts. At the same time, for welded parts, the most suitable is a metal having a ferrite-bainite (or bainite) structure. The work investigated the features of the ferrite-bainite structure of low-carbon and low-alloyed steel 15ХСНД for the production of connecting pipeline parts. Purpose of the article. To establish the laws of formation of a ferritic-bainitic structure in low-carbon low-alloy steels depending on the parameters of heat treatment. Determine the effect of heat treatment parameters on the properties of the connecting parts of pipelines made of these steels. Conclusion. The regularities of the influence of heat treatment parameters on the structure, mechanical properties and topography of fractures of impact samples of 15ХСНД steel with a ferrite-bainitic structure are established. Keywords: stamped-welded connecting parts of man pipelines; heat treatment; microstructure; bainite;mechanical properties; fractography

The Effect of Boron, Manganese and Sulfur on the Microstructure and Mechanical Properties of Pipe Steel 17G1SU

The paper presents the results of the effect of boron, manganese and sulfur on the microstructure and mechanical properties of pipe steel 17G1SU. It was shown that the microstructure of boron-free steel sample containing 1.4% Mn and 0.01% S consists mainly of ferrite and a small amount of perlite. Samples microalloyed by boron are represented by a dispersed ferritic-bainitic structure. A decrease in ferrite grain size from 8.7 μm, in a comparative sample without boron containing 1.4% Mn and 0.010% S to 5.8 μm in a sample of steel containing 0.006% B, 1.6% Mn and 0.011% S, shows increasing the dispersity of the ferritic-bainitic structure. A decrease in the manganese content to 1.4, sulfur to 0.004% and an increase in boron concentration to 0.0011%, despite an increase in grain size to 6.8 μm, retain a fine-grained structure. The effect of boron, manganese, and sulfur content on the microhardness of the structural phases of the studied pipe steel samples is noted. The smallest microhardness of ferrite and perlite is observed in the base sample without boron, reaching 180 and 214 HV10, respectively. The microalloying of pipe steel containing 1.6% Mn, 0.011% S with boron is accompanied by an increase in the microhardness of the bainitic phase to 314 HV10, which increases to 400 HV10 with an increase in boron concentration to 0.011%, and a decrease in the content of manganese and sulfur to 1.4 and 0.003%. In this case, the microhardness of the ferrite phase, reaching an increase to 260 HV10, is practically independent of the content of boron, manganese, and sulfur. The mechanical properties of the experimental metal rolling with a thickness of 10 mm provide the production of rolled steel of strength class X80, without heat treatment, regardless of the content of boron, manganese, and sulfur, as a result of the formation of a finely dispersed ferrite-bainitic structure.

Effect of Initial Microstructure on Soft Annealing of a Low-Carbon Bainitic Steel

A low-carbon bainitic tool steel exhibiting high hardness after hot rolling typically has poor machinability. To soften this type of steel and to accelerate the soft annealing process, an austenitizing step was designed based on thermodynamic calculations of phase stability and introduced prior to the annealing step. Different initial microstructures were prepared by three austenitizing temperatures (680 °C, 850 °C, 1000 °C) and three cooling methods (water quenching, oil quenching, and air cooling). The effect of initial microstructure on microstructures and hardness was studied. Softening equations, a function of annealing temperature and time, were established for different initial microstructures, and the relationships between annealing temperature, annealing time, activation energy, and hardness were explored. The predicted hardness was consistent with the measured values. Martensitic structure has a low activation energy for diffusion and a higher softening rate compared to that of the bainitic structure. In addition, the higher the carbide content in the bainitic structure, the smaller the activation energy tended to be.

Deformation Aging in Low Carbon Steels

The results of mechanical spectroscopy are shown when studying the processes of strain age-hardening in mild steel with a ferrite-bainitic structure.

EBSD Study of the Effect of Hot Deformation on Low Carbon Bainitic Structure

The effect of deformation temperature (DT) and thickness reduction on the bainitic structure was investigated under various test conditions by using hot compression on a Gleeble-1500 thermo-mechanical simulation machine, and electron back scattering diffraction (EBSD) technique. In the case of the bainitic structure consisting of granular bainite (GB), lath bainite (LB) and a little ferrite (AF) under the given deformation conditions, DT and thickness reduction have remarkable effect on the transformation kinetics, starting temperature (B) of bainite fast transformation, and the type of bainitic structure. With the decreasing of DT from 810 °C to 730 °C, the starting temperature of transformation B increase from 585 °C to 595 °C. When the thickness reduction was 0 % and 20 %, the microstructure consists of GB, LB and a little AF, whereas as the thickness reduction increase to 40 %, large grain size of LB and GB disappear, and only AF and M/A remained. With the thickness reduction increases from 0 % to 40 %, the effective grain size decreases from 4 μm to 2 μm, and the fraction of HGB increases from 48 % to 57 %.

Bainitic Cast Irons with Nanosized Particles

In the present study austempered ductile irons (ADI) with lower bainitic structure are investigated. Nanosized particles (50nm) of titanium carbonitride + titanium nitride TiCN+TiN and titanium nitride TiN are added to the casting volume. The samples microstructure is studied by optical metallography and X-Ray analysis. The influence of the nanosized additives on the kinetics of the bainitic transformation and on the morphology of the bainitic structure is investigated. The abrasive wear testing, hardness measurements and impact strength are carried out. It is established that the presence of nanoadditives in the bainitic cast irons leads to the changes in their microstructure which increases their mechanical characteristics and abrasive wear resistance. The studied nanocomposite materials expand the potential for new ADI applications in the industry.

Investigation of the effect of isothermal heat treatments on mechanical properties of thermo-mechanically rolled S700MC steel grade

The effect of isothermal heat treatments (1 hour at 200, 400, 600 and 800°C) on mechanical properties of thermo-mechanically rolled S700MC steel has been investigated by extensive mechanical characterizations. Treatments at 600°C increase yield and tensile strength and decrease impact energy. Below 600°C the steel retains its bainitic structure. Precipitation kinetics simulations indicate that this secondary hardening effect arises from the nucleation of fine (Nb,Ti)C particles, indicating that the bainitic structure is unstable above 600°C due to its high supersaturation with respect to C, Nb and Ti. These results can help to optimize the operating practices for post-weld heat treatments.

Effect of Tempering on Hardness of Q&T Steel Plate

Commercial production of high strength steel plates by the quenching and tempering (Q&T) route requires control of alloy design and heat treatment parameters to achieve the desired strength and toughness through thickness. Plates with different thicknesses (up to approximately 100 mm) are produced for applications in the energy and power or lifting and excavation sectors. For thick plate the difference in cooling rate through thickness affects the as-quenched microstructure with martensite, auto-tempered martensite and lower and/or upper bainite being present. The different as-quenched microstructures can show a different response to tempering which affects the final strength and toughness.In this study the starting microstructure of a low alloy 0.17 wt% C Q&T steel has been varied using isothermal heat treatment at 430 °C to create mixed martensite and lower bainite microstructures (nominally 25:75; 50:50 and 75:25 percentages). The effects of tempering at 600 °C for times between 0.5 and 16 hours on the carbide precipitates and hardness of the mixed microstructures have been investigated and compared to the tempering response of single phase (martensite and lower bainite) microstructures. It has been found that the hardness decrease due to tempering is larger in the martensitic structure than the bainitic structure due to more rapid carbide coarsening. The as-quenched hardness of the mixed microstructures can be predicted by a rule of mixtures using the single phase properties. The tempering response of the mixed microstructures is discussed.