Thermomechanical Controlled Rolling Research Articles

Structure and properties of reinforcing rolled coils by v-alloyed dual-and multi-phase C-Mn-Si-steel

The aim of the work was to study the influence of vanadium doping on two- and multi-phase steels. The schemes of thermomechanically controlled rolling on the wire line of the sectional rolling mill 400/200 for the production of reinforcing wire with a diameter of 6.0 mm in rolls were studied. For the production of reinforcing wire, vanadium alloyed C – Mn – Si steels, which have two-phase (DP – ferritic-martensitic (bainite)) and multi-phase (MP – ferritic-martensitic (bainite) - pearlitic) microstructures, were used. During thermomechanical controlled rolling, TMCR charts were used, including TLH stacking head temperatures of 1024°C to 1063°C. It was established that the applied rolling modes ensure the formation of the MP microstructure. In this 6.0 mm wire, high tensile strength and ductility indicators were achieved in the bends(YS0.2 = 530-550 MPa; TS = 785-885 MPa; El5 = 15.0 - 29.0 %), which fully corresponded specification requirements of national standards such as ASTM A 615 (USA), JIS G 3112 (Japan) and KSD 3504 (Republic of Korea). Keywords: thermomechanical controlled rolling, wire line, reinforcing wire in bays, V-microalloyed C-Mn-Si-steel, microstructure.

Development of Tailored Structure and Tensile Properties of Thermomechanical Treated Micro Alloyed Low Carbon Dual Phase Steel

Direct hot rolled dual phase steel production represents a challenging route, compared with cold rolled and intercritical annealing process, due to complex and sophisticated control of the hot strip mill processing parameters. Instead, high technology compact slab production plant offers economic advantages, adequate control and prompt use of the advanced thermomechanical controlled rolling. The current work aims to obtain different structures and tensile properties by physical simulation of direct hot rolled niobium micro alloyed dual phase low carbon steel by varying the metallurgical temperatures of hot strip mill plant. This starts with adaptation of the chemical analysis of a low carbon content to fall far from the undesired peritectic region to avoid slab cracking during casting. Thermodynamic and kinetics calculations by Thermo-Calc 2020 and JMat pro software are used to define the transformation’s temperatures Ae1 and Ae3 as well as processing temperatures; namely of reheating, finishing rolling, step cooling and coiling temperatures. The results show that the increase of finishing rolling temperature from 780°C to 840°C or decreasing either of step cooling duration at ferrite bay from 7 to 4 seconds, enhances yield and tensile strengths, all due to more martensite volume fraction formation. The yield and tensile strengths also increase with decreasing coiling temperature from 330°C to 180°C, which is explained due to the increase of dislocation densities resulted from the sudden shape change during martensite formation at the lower coiling temperature in additional to the self-tempering of martensite formed at higher coiling temperatures which soften the dual phase steel.

Study Structure and Properties of Reinforcing Rolled Coils by V-Alloyed C – Mn – Si - Steel with Dual- and Multi-Phase Microstructures

Thermo-mechanical controlled rolling (TMCR) schedules have been studied on the wire line of 400/200 section rolling mill to manufacture 6.0 – mm - diameter reinforcing wire in coils by V-alloyed C – Mn – Si - steel that has dual-phase (DP - ferrite-martensite (bainite)) and multi-phase (MP - ferrite-martensite (bainite) - pearlite) microstructures.It has been established that high tensile strength and plasticity values were achieved in this 6.0 – mm wire in coils (YS0.2= 530-550 MPa; TS= 785 – 885 MPa; El5 = 15.0 – 29.0 %) which were in full compliance with national standard specifications such as ASТM A 615 (USA), JIS G 3112 (Japan) and KSD 3504 (the Republic of Korea)), when the TMCR schedules involving laying head temperatures ТLH from 1024 ºС to 1063 ºС were employed ensuring formation of MP microstructure.

Mechanical behaviour of thermomechanically produced ultrafine grained dual-phase steels

Dual-phase (DP) steels are an excellent alternative in the production of automotive parts that require high mechanical resistance, high impact strength and elevated elongation. These materials are produced using low alloy steels as a basis, reducing costs and resulting in a combination of martensite and ferrite structures with ultrafine grain size. These characteristics are achieved through strict control of rolling conditions, strain rate, cooling rate and coiling temperature. This work presents the results of tension testing of two types of DP steels, along with microstructural characterisation, in order to understand the effect of the advanced thermomechanical controlled rolling processes on the formation of the microstructure and resulting mechanical properties.Les aciers biphasés sont une excellente substitution pour la production de composantes automobiles qui requièrent une résistance mécanique élevée, une haute résistance à l’impact et une grande élongation. Ces matériaux sont produits en utilisant des aciers faiblement alliés comme base, réduisant les coûts et engendrant des combinaisons de structures martensitiques et ferritiques à granularité ultrafine. On obtient ces caractéristiques grâce à un contrôle strict des conditions de laminage, de la vitesse de déformation, de la vitesse de refroidissement et de la température d’enroulement. Cet article présente les résultats des essais de traction de deux nuances d’acier biphasé, de même que la caractérisation de la microstructure, afin de comprendre l’effet des procédés avancés de laminage thermomécanique contrôlé sur la formation de la microstructure et les propriétés mécaniques qui en déroulent.

Effect of nanoprecipitates and grain size on the mechanical properties of advanced structural steels

The microstructure and nanometric precipitates present in advanced structured steel have been studied by high resolution transmission electron microscopy equipped with energy dispersion X-ray microanalysis, in order to relate the nanometric precipitates and grain size with the improvement of the yield strength value of the API steel. The microstructure and nanometric precipitates of the advanced steel were obtained by a combination of thermo-mechanical controlled hot rolling and accelerated cooling procedures. The API steel composition consisted of hot rolled Nb–Ti microalloyed with: 0.07C, 1.40Mn, 0.24Si, 0.020Al, 0.009P, 0.001S, 0.05Mo, 0.5Cr, 0.05Nb, 0.25Ni, 0.10Cu, 0.012Ti, 0.05N in wt%. As a result, this hot rolled steel tested at a strain rate of 5 × 10 −3 s −1 showed an improved yield strength from 798 MPa to 878 MPa due to the micrometric grain size of 2.2 μm and to the nanometric precipitates with a size of around 5 nm in the microstructure of the steel studied.

Effect of Preliminary Treatment on Grain Refinement of Medium Carbon Steel Using ECAP at Increased Temperature

By application of thermomechanical controlled rolling and accelerated cooling, the carbon steel grain refinement is limited to levels of ~ 5 μm in steels. The strain assisted or strain induced transformation could be considered for the refining process. The present work, likewise, deals with grain refinement of medium carbon steel containing 0.45 wt pct carbon having different initial microstructure modified by either thermal and/or thermomechanical treatment (TM) prior severe plastic deformation. In case of TM treated steel, structure refinement was conducted in two steps. Preliminary structure refinement has been achieved due to multistep open die forging process which provided total strain of 3. Uniform and fine recrystallized ferrite structure with grain size of the order of 2-5 μm and with nest-like pearlite colonies was obtained. The further grain refinement of steel samples having different initial structure was accomplished during warm Equal Channel Angular Pressing (ECAP) at 400°C. The steel samples of different initial structure were then subjected to six ECAP pressing passes through die channel angle of 120°. The microstructure development was analyzed in dependence of effective strain introduced (εef ~ 2.5 - 4). Employment of this processing route resulted in extensive deformation of ferrite grains where mixture of subgrains and ultrafine grain was found regardless the preliminary treatment of steel. As straining increases the dynamic polygonization and recrystallization became active to form mixture of polygonized subgrains and submicrocrystalline grains having high angle boundaries. The straining and moderate ECAP temperature caused the partial cementite lamellae fragmentation and spheroidization as straining increased. The lamellae cementite spheroidization was more extensive in TM treated steel samples. The tensile behavior was characterized by strength increase for both structural steel states; however the work hardening behavior was modified in steel where preliminary TM treatment was introduced to modified coarse ferrite-pearlite structure.

Simulation of Thermo-Mechanical Controlled Rolling and Continuous Cooling of Wire Rods

In this study the effect of thermo-mechanical controlled rolling and continuous cooling of different grades of steel wire rod (e.g. high-carbon for cold drawing applications, medium-carbon micro-alloyed for cold forming) has been analysed through the application of a set of integrated mathematical models simulating hot rolling and continuous cooling, and a laboratory work involving hot rolling simulation on a pilot plant and heat treatments on a laboratory scale. The samples have been characterised by means of instrumented tensile tests, metallographic analyses including determination of pearlite interlamellar spacing, and controlled compression tests. The results show that: - The mechanical strength of high-carbon steel is essentially related to interlamellar pearlite spacing, and can be enhanced through the control of continuous cooling. Improvements in cold drawability can be obtained by means of prior austenitic grain size (PAGS) reduction, through the application of thermo-mechanically controlled hot deformation processes. - In the case of medium-carbon micro-alloyed steels for cold forming, the reduced PAGS achieved by means of thermo-mechanically controlled process reflects on a closer control of as-rolled mechanical properties, avoiding hardness hot spots asking for annealing treatments before cold forming. Moreover, the finer ferrite grain size could affect the forces needed during forming at the same deformation level.

The Microstructural Properties of Microalloyed Steel after Plastometric Testing

With the aid of an ASC-type finishing line (Automatic System Control) and continuous cooling of the rolled stock in water boxes, modern continuous light mills enable thermomechanical (controlled) rolling of SBQ-type rods (Special Bar Quality). During the thermomechanical forming (at different temperatures or cooling rates etc.), one can analyze the quality parameters of roundsection steel bars. In order to obtain better understanding to the forming conditions (temperature, strain and strain rate) and the possibility of cooling (related to its rate and time), an experiment involving continuous and discontinuous testing by means of a Gleeble plastometer was suggested. After its implementation, the attention was focused on the mathematical interpretation of the stressstrain characteristics of the steel and on metallographic observation of microstructural changes. The results achieved are of universal meaning and contribute to the theoretical understanding of controlled forming.

Production and properties of thermomechanically rolled high strength steel plates with minimum yield strengths up to 700 MPa

The principles of thermomechanical controlled rolling, including accelerated cooling, of high strength plates are briefly surveyed in the light of the metallurgical changes induced. The relationship between composition, processing route and tensile strength, toughness and weldability of the plates obtained is discussed in the context of processing commercial steel grades. Finally, an overview of applications in the shipbuilding, construction, transport and mining industries is given.

Use of Accelerated Cooling to Improve the Mechanical and Processing Properties of Rolled Plates Used to Make Large-Diameter Gas-Line Pipe

A process called controlled thermomechanical rolling has been developed to make low-alloy steels used in the production of large-diameter pipes. The process, carried out on plate mills, combines thermomechanical (controlled) rolling with accelerated cooling done after the rolling operation. The results of commercial trials of the technology lead to the following conclusions: 1. Compared to conventional controlled rolling (CR), use of the additional operation of accelerated cooling (AC) makes it possible to significantly increase strength properties without lowering impact toughness or cold resistance. Accelerated cooling also makes it possible to broadly vary the properties of rolled products of a given chemical composition and thickness. 2. The accelerated cooling of plates makes it possible to replace the ferritic-pearlitic structure usually formed in steel after conventional controlled rolling with a fine-grained ferrite-bainite stucture having a diminished level of striation. 3. Compared to traditional controlled rolling, CR + AC makes it possible to attain a prescribed level of strength with lower quantities of carbon and alloying elements. That in turn improves the weldability of the steel. 4. The use of accelerated cooling reduces the load on the mill because a higher finishing temperature is used than in traditional CR. Thus, it also becomes possible to increase rolling speeds through a reduction in the number of pauses made to cool the slabs on the mill.

Fracture behavior of ultra-fine grained steel bar under dynamic loading

Ultra-fine grained steel bars were recently developed by thermo-mechanical controlled rolling with rapid cooling for increasing the strength of low carbon and low alloy steels. The developed steels are characterized by fine ferrite grains of less than 1 μm and high strength as a result of grain refinement. However, their correlations between tensile properties and impact behavior are not well understood. In this paper, impact absorbed energy (E p) and dynamic fracture toughness (J Id) were used to evaluate the dynamic fracture behavior of the ultra-fine grained steels, and the fracture mechanisms were also investigated. For the ultra-fine grained steels, tensile stress-strain curve was shown to be correlated with the impact curve of load vs. time, and to be related to the dynamic fracture toughness. The steel with large ferrite grains, small ferrite grain colony and martensite was found to have a good combination of strength and toughness.

Influence of processing history on mesotexture and microstructure-toughness relationship in control-rolled and normalised steels

This paper studies the effect of processing history on grain boundary misorientation angles in steels and investigates the relationship between microstructure and impact transition behaviour. For normalised steel, fracture is directly related to the metallographic two-dimentional microstructure, however, for thermomechanical controlled rolling steel grain boundary misorientation needs to be taken into account.

Hydrogen permeation in a submerged arc weldment of TMCP steel

The effects of the heterogeneous microstructure at the base metal, the heat affected zone (HAZ) and weld metal on hydrogen permeation in thermo-mechanical controlled rolling (TMCP) steel weldments have been investigated. The base metal with equiaxed refined ferrite and scattered fine grain pearlite has the highest permeation rate and effective diffusivity. The HAZ with bainite shows the lowest values of the permeation rate and effective diffusivity. Weld metal yields a higher permeation rate coupled with an intermediate diffusivity value. The hydrogen apparent solubility is low for the base metal, intermediate for the HAZ and high for the weld metal. The mechanisms of hydrogen diffusion path and hydrogen traps are discussed and experimentally confirmed using the hydrogen microprint technique. The high diffusivity paths and the hydrogen trapping site are the grain boundary and the ferrite/carbide interfaces for both the base metal with refined ferrite and the HAZ with bainitic microstructure. The spaces among the basket-weave acicular ferrite, where the martensite and the retained austenite (M/A) constituents present, are the main hydrogen trapping sites for the weld metal.