SMS group takes a step into the future with thermomechanical rolling of rebars (TMbaR) providing a reduced manganese content in the billet and improving the product characteristics at the same time. By saving alloying additions, this solution also reduces operating costs.
SMS group has been a pioneer and promoter of the thermomechanical rolling process for decades. Today, a new chapter has been written by applying this concept to the production of reinforcement bars (rebars). SMS group’s thermomechanical rolling of rebars, the so-called “TMbaR” offers the benefit of producing fine-grained structural products while at the same time reducing the manganese content in rolled stock. The exceptional ductility properties of seismic grades (i.e. HRBF400E) can only be optimized at that level by thermomechanical rolling.
Plant technology for thermomechanical rolling
In order to achieve the fine-grained microstructure, three process steps need to be ensured:
- Precise and reproducible cooling and equalization before the final passes
- High deformation during the last passes
- Soft-cooling after last passes
Thus sufficient cooling and equalization capacities need to be considered during the design of the plant as well as a rigid design of the MEERdrive® finishing block, which can bear the higher rolling forces created by the reduced rolling temperature (750 °C – 820 °C).
SMS group’s solution is:
- Applying an increased number of water boxes arranged in the loop in front of the finishing blocks
- MEERdrive® rolling block, which besides the rigid design offers through its single drive solution the necessary flexibility to control the process of an optimized recrystallization after rolling
- Ensuring the above-mentioned boundary conditions the grain sizes can be improved from typical ranges of ASTM 8 – 10 (conventional rolling) to ASTM 12 (thermomechanical rolling).
Improved material properties, saved alloy additives and reduced operational costs
Two companies, Lianxin Steel and Shandong Laigang Yongfeng Steel, decided to introduce the TMbaR process by SMS group in their new rebar rolling mills. Both mills are designed for an annual capacity of 1,000,000 tons. These companies trust the long time experience of SMS group in the field of thermomechanical rolling with extensive cooling and equalization lines arranged as loop. The new TMbaR rebar mill allows them to react better and faster on market demands with improved material properties, saved alloy additives and reduced operational costs.
A technological highlight of the equipment is the well-proven loop technology which allows achieving effective temperature equalization across the feeder section in front of SMS group´s MEERdrive® finishing blocks. The single drive concept of these blocks provides utmost technological and economic benefits over other drive concepts in terms of overall processing costs. The robust cassette-type design makes for consistently high performance with minimized maintenance requirements. The equipment has been specially designed for long rolling campaigns under the high rolling load demanded by the TMbaR process.
Technical data of Shandong Laigang Yongfeng and Lianxin Steel
The TMbaR rebar mill for Shandong Laigang Yongfeng´s Dezhou site in the diameter range of 8 – 32 millimeters produces straight bars with diameters of 8 – 25 millimeters at a max. speed of 45 m/s and bar coils by a VCC® (Vertical Compact Coiler) with diameters of 8 – 32 millimeters at a max. speed of 35 m/s. Yongfeng chose to install a new high-speed rolling area including two 6-stand MEERdrive® blocks, cooling and equalizing lines, a high-speed delivery area consisting of a highspeed delivery system (HSD®) with dividing shears and bar braking as well as a VCC® system for coils up to 5 tons. Additionally, the scope of supply included the complete package of electric & automation and the supervision for erection & commissioning. The plant started production in October 2019.
Lianxin Steel ordered a TMbaR rebar mill for its Dafeng site that has already been commissioned at the beginning of 2019. The plant is designed in the diameter range of 8 – 40 millimeters at a max. speed of 45 m/s. SMS group supplied all rolling mill stands for roughing, intermediate and finishing mills, two six-stand MEERdrive® finishing blocks, shears, water boxes as well as the double HSD® system. Additionally, the complete package of electric & automation and the supervision of erection and commissioning were present in the scope of supply.
Operational results of thermomechanical rolling at Lianxin
Today, we can already take a look at the main results achieved on the first plant supplied by SMS group in China – expressly designed to accomplish the new Chinese standard, with the aim of obtaining an ultrafine microstructure starting from billets of controlled chemical composition without the necessity of adding micro-alloying elements.
After initial tests have been performed with rebar HRBF400E for diameters of 8, 10 and 12 millimeters it can be summarized that thermomechanical rolling, to produce rebars in full ferritic – pearlitic structure, has been successfully applied. The results of the first trials show that all the mechanical properties of yield strength (YS), tensile strength (TS), elongation and TS/YS ratio were easily obtained.
For all three rebars produced in Lianxin, the obtained results followed the new Chinese standards GB/T1499.2 - 2018. The grade HRBF400E showed an ultrafine (grain size always higher than ASTM 9) and weldable microstructure. The image shows the core, middle and surface area of the three sizes rolled during the commissioning phase. Due to different rolled stock diameters, it should be noted that there are little differences in microstructures related to deviation in the cooling conditions of the cross section.
Especially elongation and TS/YS ratio satisfied the more stringent requirements of the seismic application rebar.
Mechanical characteristics of HRBF400E are considerably higher than the standard limit (YS=400 MPa and TS=540 MPa) due to the manganese contents, in the range 1.2 – 1.4 %. Further process implementations will be applied to reduce Mn content to < 0.8 percentage, with the aim of decreasing production costs, still maintaining all the mechanical and microstructural characteristics of grade HRBF400E.