THERMOMECHANICAL TREATMENT OF REINFORCING STEEL

The work deals with application of interstand and afterdeformation cooling of coiled (∅10, 12 mm) and reinforcing (∅14,16 mm) bar steel in the accelerated cooling installations behind the prefinishing and finishing stands of the light-section mill which provides the formation of the even fine-grained ferrite-perlite structure and reduces grain growth in the mid section of the coil. The application of the roll stock interstand cooling behind the 11th stand together with the afterdeformation accelerated roll stock cooling to 800-8500C enabled us getting reinforcing bar steel diameter ∅14mm with high impact strength..


Introduction
Some research works [1 -4] demonstrate that even fine-grained structure and increase in steel mechanic properties can be achieved in two ways: a) Introduction of the powerful carbide-forming elements and microalloying additions during steelmaking in combination with strict cycles of plastic deformation and braking of austenite recrystallisation at the expense of it; b) Rolled stock cooling in interdeformation pauses for reducing the temperature at the end of rolling and a controlled afterdeformation cooling, impeding recristallisation.
The main advantage of the second method are economical reasons and a possibility to increase perfomance characteristics of low-carbon steels.
The main condition of realization of the technological process of thermal strengthening is working-out of coolers providing intensive rolled stock cooling combined with its hydrotransportation on the site where the coolers have been installed at the rolling speed up to 20 m/s.
The purpose of this work is to find ways of obtaining looping reinforcing steel diameter ∅10-16mm St3sp (GOST 380-71) with even fine-dispersed structure and increasing the mechanical properties by interstand rolled stock controlled cooling.

Experimental
For this purpose accelerated cooling installation has been worked out consisting of injecting funnel 1, cooling stand 2, injector 3 (Fig. 1).In the case of direct-flow cooling, a cutter 4 is used.

Fig 1. Accelerated cooler scheme
It is installed in the place of the outlet pipe behind the 11 th stand and attached to the outlet bar.A cooling stand is a pipe with the inner diameter 40 mm and 1500 mm in length, which provides required cooling of rolled-stock to the calculated temperature and its safe hydrotransportation.
Injector is a sealed case inside of which a roll cone with two cams moves its purpose is generation of required water blow velocity in the cooling stand.
The passing hole diameter for the metal inside the case is 40 mm.A ring-shaped gap is installed on the injector depending on the rolled-stock diameter and for the required cooling rate in the cooling zone.It provides the necessary water consumption at the continuous pressure at the injector's inlet.
An inlet roll cone is used for rolled-stock reception and directing it to the cooling zone.It is a funnel-shaped hole changing into a cylindrical one (diameter is 40mm).
The cutter is used to reduce the water flow velocity and for water dascharge.It's a pipe with conical wires inside.There are holes at the bottom of the pipe for water discharge.
One side of the cut-off chamber is attached to the cooling stand, another one is attached to the inlet roll cone.The counter flow device for accelerated cooling is used for after-deformation cooling.It consists of an injector (like the one described above), a cooling chamber (length -480 mm) with the inner diameter 40 mm; and the injecting funnel.
The cooling device is installed behind the 13 th finishing stand when rolling reinforcing bar steel 14-16mm in diameter.
The rolled stock accelerated cooling installation is connected with the highpressure pipe by means of flexible hose and lock-device.

Process flow diagram
Process flow diagrams of making coiled and reinforcing bar steel have been developed and tested.Diameters are 10, 12, 14, 16 mm respectively.
Metal rolling to the 11 th stand was made according to the existing technology.From the 11 stand, a process workpiece (diameter 16 mm) goes to the cooling chamber, then to the stands 12, 13, 17, 19, where is deformed with drawing λ=1.17; 1.18; 1.17; 1.18 respectively (for ∅10).For ∅12 mm diameter drawing in stands is 1.2; 1.168; 1.203; 1.17 respectively.Total drawing is λ 10 =1.938 and λ 12 =1.954.Later rolled stock is being coiled.When making steel 10 mm in diameter, two rates of stand's cooling were investigated: the first -to cooling temperature of 920°C, the second -to the temperature of 950°C and the third hot-rolled state (for comparison).The temperature of hot-rolled steel is 1050°C.The coiling temperature is 1010°C in the first case and in the second one 1010-1020°C.
Reinforcing bar steel 12 mm diameter was thermal strained -under two rates: the first -to 950°C, the second -to 980°C, which were compared to hot-rolled state (without cooling) rolling temperature of 1050°C.
The coiling temperature is 1020°C in the first case in the second one -1030°C and of hot-rolled coil -1050°C.
The conditions of production of reinforcing bar steel enable putting into practice the most perspective kinds of thermal strain.It means that accelerated steel cooling in interdeformation pauses and after deformation to the initial temperature of phase transformations.This method decelerates process of deformed austenite recrystallisation and results in production of fine-grained structure, defining the high level of mechanical properties including impact strength at low temperatures.
Reiforcing bar steel's rolling rates (∅14 mm), with controlled inferstand and afterdeformation cooling, were investigated using two accelerated cooling devices; the first counter flow installation 1500 mm length is behind the 11 th stand; the second counter-flow (480 mm) is behind the prefinishing stands (12 th , 13 th ).Process workpiece have been cooled in the counter flow accelerated cooler after deformation in the 11 th stand to certain temperature, then it passed to 12 th and 13 th stands and was deformed with drawing λ=1.2; 1.68 respectively.
Steel was directed to the second cooler from stand 13 th with three cooling rates 840 → 750-770°C; 920 → 840-820°C; 950 → 940°C (under this rate water was not directed into the second cooler).
A controlled afterdeformation cooling to temperature 800-780°C may be used at reinforcing bar steel production 16mm in diameter.These temperatures prevent recrystallisation processes at full rate.
By means of the device described above the conditins of fittings (∅16 mm) rolling were investigated to minimum possible temperatures (from 870 to 970°C).
Utilization of reinforcing steel technology using counter flow accelerated coolers behind stands 11 th and 13 th affords to reduce metal temperature and to equalize it according rolled stock's length and plane.
Perlite is thin-plated (distance between cementite is 0.3-0.8µm) and placed on ferrite grains joints.

Conclusion
Interstand and afterdeformation cooling of hot-rolling steel St3sp diameter 10-16 mm affords to influence greatly the quality of rolled stock at the expense of reducing of metal temperature and equalizing on the process workprice length and plane.The high level of mechanical properties and impact strength at low temperatures is achieved as a result of even fine-grained structure formation.