**4.3 Finish rolling at no-recrystallization temperatures**

As has been elucidated above, no-recrystallization temperature (TNR) is important in design of controlled rolling process. This temperature determines where strain is multiplied for austenite grains, leading to strain-induced precipitation of carbonitrides as well as enhanced sites for a fine-grain size ferrite to be nucleated at the sites. Hot rolling being a dynamic process, no-recrystallization temperature depends upon deformation parameters. The influencing factors for TNR are composition of the steel, strain values applied in each pass, the strain rate, and the rolling interpass time [7, 8].

During finish rolling the value of TNR tends to dynamically lower down as the strain value or the reduction increases. This phenomenon is attributable on account of static recrystallization caused by increased recrystallization sites owing to finer grains and higher dislocation density induced during each rolling pass.

The strain rate value is also a determining factor for the onset of dynamic recovery and facilitates static recrystallization which eventually decreases the TNR.

During controlled rolling, the interpass time during each rolling reduction also plays an important role as the prime requirement is to roll below TNR temperatures. The precipitation kinetics are accelerated due to strains induced when rolling below TNR. A lower interpass time is preferable as higher interpass will lead to coarsening of precipitate sizes as well as increased tendency of recrystallization detrimental to final strength value of steel.

### **4.4 Accelerated cooling**

**3.1 Addition of niobium, titanium, and vanadium**

growth are particle pinning and solute drag.

boundaries and inhibit their movement.

**3.2 Manganese-based strengthening**

carbonitrides and vanadium nitrides.

methods require simultaneous addition of V and Nb.

**4. Effect of controlled hot rolling parameters**

**4.1 Reheating temperatures at reheating furnace**

and nitrides that restrict initial austenitic grain size.

grain size by reduction at lower temperatures.

**228**

**4.2 Repeated recrystallization in roughing mill**

dislocation movement.

*Welding - Modern Topics*

Microalloying elements such as niobium, titanium, and vanadium are principally carbide-forming elements. Although the addition of these elements in steel raises its Ar3 temperature, they retard austenite transformation to ferrite by restricting carbon diffusion. Strengthening by addition of one or all of niobium, vanadium, or titanium has shown a remarkable increase in strength of steel. The strengthening phenomenon is caused by fine precipitation of nitrides, carbides, or carbonitrides which are coherent with ferrite matrix but induce strengthening by impeding

One of the most significant effect of adding individually or simultaneous addition of V, Nb, and Ti is to decrease recrystallization temperature. Which contributes

The two principal mechanisms that inhibit recrystallization and eventually grain

The grain boundary movement can be accounted on strain-induced precipitation

In the case of vanadium addition, addition of nitrogen can be helpful in increas-

The improvement of toughness can be achieved through addition of manganese that leads to decrease in Ar3 temperature. Due to decrease in Ar3 coupled with low coiling temperatures, the alpha (ferrite) grains are refined, thus increasing the strength. Additionally, the fine precipitate size is contributed by niobium

In general, austenitic grains starts to recrystallize at temperatures above 1050°C.

Eventually, a lower slab reheating temperature by contributing fine austenitic grain size and lower temperature rolling at roughing mill will induce even finer

Due to repeated reduction in roughing mill, both recrystallization and precipi-

tation are competing phenomena. However, at higher temperatures

Since an initial finer gamma grain size is helpful in creating a final finer size of alpha, lower reheating temperatures are effective. Also, the microalloying elements also add to refinement of the austenitic grain size by means of undissolved carbides

in generating a finer size of gamma (austenite) grains during finish rolling.

of micro carbides on gamma grain boundaries that limit the gamma grain size. Addition of titanium or niobium helps in suppressing gamma grain growth by means of nitride or carbonitride precipitates which are majorly present at grain

ing the strength and toughness. The vanadium nitride precipitates are useful in imparting strength to the steel. The addition of nitrogen however attributes to poor weldability. Likewise, the strengthening may be achieved by adding niobium, but a higher niobium content is bound to give poor weldability. Hence the conventional

> The runout table and the coiler in general act like post heat treatment unit which makes possible to achieve phase transformation through control of cooling to generate coils with varied properties and microstructures.

> Accelerated cooling after hot rolling leads to further refinement of grains and phase control, leading to enhancement of properties. The phenomenon for strengthening of microstructure is phase transformations in terms of microstructures avoiding pearlitic transformations, precipitation strengthening through carbides, and nitride precipitates which along with controlled cooling rates lead to the refinement of grain size in the resulting microstructure. The accelerated cooling may be classified into two techniques—continuous accelerated cooling and interrupted accelerated cooling.

> The final mechanical properties after accelerated cooling are majorly influenced by the alloying content and hot rolling parameters.

*Welding - Modern Topics*
