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.


Introduction
Thanks to a unique combination of tensile strength and placticity properties, high strength low-alloyed dual-and multi -phase (DP -and MP, respectively) steels are usefully applied in the automotive industry for weight reduction of cold-formed components, in the heading industry, in the gas and oil industry for manufacture of transmission pipelines, including those laid in seismic and permafrost regions (Lis, Lis, & Kolan, 2004;Zuo & Zhou, 2015;Sychkov, Zhigarev, & Perchatkin, 2006;Sychkov, Sheksheev, Malashkin, & Kamalova, 2016;Xu & Kong, 2012).Recently, high strength reinforcing steel wires with DP -and MP microstructures have found wide application in the construction industry.
The heat treatment method (Figure 1, left-hand side) involves heating of steel to 1063 K (790 ºC), a dual-phase (α + γ) temperature range (intercritical temperature range (ICTR)), annealing at that temperature for a certain time, quenching in water for obtaining martensite islands in the structure, and tempering at 913 K (normally, 500-550 ºC) to relief stresses and to reduce aging effects.There are numerous versions of the above described heat treatment method that are applied in continuous annealing furnaces for commercial production of cold rolled plates from low-alloyed DP -and MP steels.A TMCR method involving controlled cooling of hot -rolled steel products is shown in Figure 1 (on the right).A schematic overlay of the lines of austenite phase transformation in low-alloyed steels (continuous-cooling transformation diagram, CCT-diagram) in the controlled cooling temperature range shows, that martensite/pearlite islands in the structure of low carbon steel to an extent of 15-20%, which ensure the highest tensile strength and hardness (НВ = 371, Figure 1), are formed from microareas of austenite (γ-phase) left untransformed in this steel at 873 K (600 ºC) (after the completion of γ→α -transformation) as a result of quenching in water.Development of TMCR method for commercial production of high strength and high-technology types of hot rolled plates, bars and wire rods from low-alloyed DP -and MP steels has been the subject of numerous investigations, but mainly on the laboratory level.
A comprehensive study of prospects for the application of 5.5-mm-diameter low -carbon wire rod (0.08 С; 0.77 Mn; 0.21 Si; 0.017 Р; 0.012 S, in % by weight) with DP microstructure in the construction industry is so interest (Lorusso, Burgueno, Egidi, & Svoboda, 2012).A DP microstructures in the laboratory samples of wire rod under study were obtained by a heat treatment process, that involved annealing them at ICTR (795, 810, 820 and 840 °C) for 15 minutes and quenching in water.Micro-structural characteristics, hardness and mechanical properties of the these samples were analyzed and compared with similar parameters of ATR 500N cold deformed reinforcement wire manufactured in accordance with IRAM-IAS U500 526, Argentina (Table 1).
The experimental data allowed the authors of work to conclude, that values of strength properties (Lorusso, Burgueno, Egidi, & Svoboda, 2012), TS / YS 0.2 -ratio, ultimate elongation El 10 , Vickers hardness (НV) were close to these obtainable in commercial ATR 500N cold deformed reinforcement wire, could be observed in a obtained wire with a high volume (50%) fraction of martensite after Extension Underload of 2 % (Table 1, DP 820 (2 % EUL)).They state, that the obtained materials had a greater capacity of energy absorption and likewise a higher strength exponent, than traditional commercial products (ATR 500N), thus offering a promising potential for their use in construction in seismic zones.Hot -rolled reinforcing wire in coils can vary in chemical composition and mechanical properties.It, for instance, was shown by the basic standard specifications ASТM A615/A615M (USA), JIS G 3112 (Japan) and KSD 3504 (the Republic of Korea) (Table 2 -5).
According to Japanese and Korean national standards, hot -rolled reinforcing wires in coils of different strength classes are manufactured from the same grade of low-alloyed steel (Tables 4, 5).

Materials and Research Techniques
Considering the increasing demand for reinforcing wire in coils with higher tensile strength properties (YS 0.2 > 400 MPa) (Lorusso, Burgueno, Egidi, & Svoboda, 2012; Wire Enforcement Institute (WRI), 2014), C -Mn -C -steel micro-alloyed by V was used as a test steel in plant experiments carried out on the wire line of a 400/200 section rolling mill with the aim to develop production methods for these rolled products, with a referenced low -carbon steel micro-alloyed by B. Chemical compositions of the steels are listed in Table 6.The layout of the wire block with a STELMOR line (400/200 rolling mill) is shown in Figure 2.  The choice of V as a micro-alloying element of low -alloyed C -Mn -Si -steel was based on its high ability to form carbides and nitrides in these steels in the austenite transformation region during cooling through a wide range of temperatures (ΔТ -from 1060-1080 ºС to 400 ºС) at medium cooling rates (Goldshtain, Grachev, & Veksler, 1985).Therewith, mechanisms of ferrite precipitation strengthening are known to take action contributing higher tensile strength properties to the steel (Goldshtain, Grachev, & Veksler, 1985).The referenced low-carbon steel was micro-alloyed by B in order to raising its plasticity.The plasticizing effect of B in low -carbon and low-alloyed steels was clearly described in the studies (Parusov et al., 2004;Parusov, Sychkov, & Parusov, 2012;Parusov, Sychkov, Derevyanchenko, & Zhigarev, 2005;Sychkov et al., 2009).
The billets employed in the experiments were 410 x 500 mm in cross section, created via continuous casting and subsequently re-rolled into 150 sq.mm billets for further rolling on a 400/200 section rolling mill.
To implement TMCR method (Figure 1, right-hand side) in order to manufacture 6. 0 -mm-diameter reinforcing wire in coils by V-alloyed C -Mn -Si -steel with DP -and MP microstructures and a required combination of tensile strength and plasticity properties, rib geometry, a series of industrial experiments were conducted at different laying head temperatures (T LH ) (Figure 2).
Optical microscopy and scanning electron microscopy (SEM) methods were used by studying structures of reinforcement wires.

Results and Discussions
The micro-structural analysis has shown that MP -(ferrite-pearlite-martensite(bainite)) structures formed in 6.0mm-diameter profiled reinforcing wire from V-alloyed C -Mn -Si -steel at higher temperatures T LH under the conditions of rolling schedules 3 and 5, while DP -(ferrite -martensite (bainite)) structures -at lower temperature T LH under the conditions of rolling schedule 10 (Figure 3, Table 7).
Also, wires treated under the conditions of TMCR schedules 3, 5 and 10 exhibited a high quality of the rib geometry, as it appeared in Figure 4. a) b) c) Figure 3. SEM micrographs of 6.0-mm-diameter profiled reinforcing wire from V-alloyed C -Mn -Si -steel: a, b, c -the microstructures obtained under the conditions of TMCR schedules 3, 5 and 10 respectively.The specific heat of iron (steel) is ρ = 4.6 x 10 2 J/kg•K (Novikov, & Heinbuker, 2001).It means that the amount of heat to be removed to lower the temperature of 1 kg of steel by 1 ºС is 4.6 x 10 2 J.In the experiments, the heat was removed from the rings of reinforcing wires on the Stelmor conveyor by the streams of air from blowers below the rings with opened insulated covers (Figure 2, Table 7).These operating conditions remained unchanged for all TMCR schedules.
Since the operating conditions of the conveyor were the same for all experiments TMCR schedule 5 with T LH = 1063ºС required more time to get wire's rings cooled down to austenite transformation temperatures compared with those TMCR schedules, which had lower T LH (Table 7).The analysis of austenite CCT-diagram in C -Mn -Si -steel (Figure 5), which has a similar chemical composition with C Mn -Si -steel under study, indicated that the steel structure (Fig 3 b, Table 8) obtained under the conditions of TMCR schedule 5 at T LH = 1063ºС had been formed via austenite phase transformations that were similar to those observed along cooling curve 5.
The microstructure of reinforcing steel obtained under the conditions of TMCR schedule 3 at T LH = 1024 ºС (Figure 3 a, Table 8) indicates that austenite transformations occurred along cooling curve 3 as shown in the CCT-diagram in Figure 5.The DP (ferrite-martensite (bainite)) -microstructure in the reinforcing steel under study which was formed under the conditions of TMCR schedule 10 (Figure 3 c, volume fraction of martensite (bainite) -21.10 %, Table 8) is typical of the pearlite-free area range of cooling rates on the CCT-diagram (cooling curve 10, Figure 5).
In 6.0 -mm-diameter reinforcing wires in coils from the referenced micro--alloyed by B low -carbon steel after treatment under the conditions of TMCR schedule 3.1, which parameters were similar to those of schedule 10 (Table 7), Widmanstätten structure formed with well-defined needle-type ferrite and isolated dark areas of fine pearlite (Figure 6).Generally, the volume fraction of free ferrite (FF with ferrite grain size d fg = 0.00510 mm -№.12 as per ASTM E112, Table 8) in the steel structure was comparable with the fraction of needle-type ferrite, which were V FF = 41.43 and V NT = 46.17% respectively.The volume fraction of isolated pearlite areas was small (V IPA = 12.40 % ), which together with a considerable share of soft ferrite fractions (V FF + V NT = 87.6 %) determined a generally low level of tensile strength properties in 6.0 -mm-diameter reinforcing wire from the referenced steel (Table 7).
The analysis of ferrite structures in the reinforcing steel under study indicated, that fine d fg № 12 as per ASTM E112 (Table 8) was formed in this steel under the controlled cooling conditions by T LH = 920 ºС -1063 ºС.It is beneficial for obtaining high tensile strength values in V-micro-alloyed C -Mn -Si -steel reinforcing wire in diameter 6. 0 mm.For this wire the most favorable ratio of strengthening fractions (martensite (bainite) and pearlite -Figure 3 b, Table 8) in its structure and the combination of tensile strength and plasticity properties (Table 7) can be obtained under the conditions of TMCR schedule 5 at T LH = 1063 ºС.
Table 8. Volume fraction of structures and ferrite grain sizes in V-micro-alloyed C -Mn -Si -steel reinforcing wire in diameter 6.0 mm

Figure 1 .
Figure 1.Schematic diagram of heat treatment and TMCR schedules that are applied for obtaining DP -and MPstructures in low-alloyed steel rolled products(Lis, Lis, & Kolan, 2004)

Micro
All rolling schedules were carried out under the same operating conditions: the blowers 1-5, 7-10 were switched on and all thermo-insulated covers of the Stelmor line were opened.

Table 3 .
Chemical composition as per ASТM A 615 (USA)

Table 5 .
Chemical composition as per KSD 3504 (the Republic of Korea) Wire Enforcement Institute (WRI), 2014), is using considerable quantities (up to 50% of the total consumption of reinforcing steels) of the reinforcing wires in diameters up to 12 mm in coils.Reinforcing wires in coils has the merit of being suitable to automatic machine welding of meshes, fabrics, embedded light reinforcing materials, generating no or low waste such as short ends, as distinct from reinforcing steel in cut lengths, which can generate minimum 5-7% of this type of waste.There is a high demand for reinforcing wires in coils in the construction industry (up to 90% of the total demand for reinforcing steels in diameters up to 12 mm).It is thus important to study structural and mechanical characteristics of reinforcing wires in coils made from low-alloyed DP -and MP steels subjected to treatment by TMCR on the wire line of a section rolling mill.

Table 6 .
Chemical composition of the steels under study

Table 7 .
Most favorable TMCR schedules* and obtained mechanical properties