QTB Technology

Modern technology for reinforcement bar manufacturing in grade RB400 and RB500 according to PN-ISO 6935-2 standard in a new bar rolling mill in CMCZ.

CMCZ's goal is to achieve a major position in the Polish steel market and to meet the high standards of the construction industry by offering reinforcement bar manufactured using the Quenching and Tempering Bars (QTB) technology implemented in the most up-to-date bar rolling mill in the Middle-Eastern Europe.

Developed by Danieli, QTB technology is method of controlled cooling. QTB increases the strength specification of low-carbon steel by inserting a special three-phase controlled system of bar cooling into a rolling line.

In the first phase, hot bar coming out of the last rolling stage are intensively cooled. This cooling process is initiated because of the very high efficiency of surface cooling (higher than the critical rate of martensite creation on the surface of bars) whereby a layer of martensite steel is formed.

The second phase begins with the bar leaving the zone of water cooling and entering the area where air atmospheric cooling takes place. In this phase, the heat accumulated in the center of the bar goes by convection to the surface of the bar, thereby creating the self-quenching layer of martensite. The quenching process ensures adequate ductility of material while maintaining a high yield point.

The third phase takes place on the walking-beam cooling bed. Bars are naturally cooled and a semi-isotermic austenite transformation takes place in their core. Following this treatment, the inner structure of the bar shows a layer of tempered martensite close to the surface of the material while the core of the bar shows a ferrite, perlitic structure.

The use of QTB technology can improve the following technical parameters of bar:

High tensile strength
Good weld ability because of low carbon contents in combination with high yield point
Good bending without micro-cracks on the surface
Very good fatigue characteristics that enable QTB treated bar to be used in applications requiring high, dynamic loads
Q.T.B. treated bars maintain their thermal stability during a fire similar to traditionally rolled bars.

Because of better mechanical and metallurgical properties, QTB treated bars of smaller diameters can be implemented in the design of reinforced construction projects. Decreasing the weight of the bar also provides cost savings to the client.

QTB TECHNOLOGY - Technical Parameters

Modern technology for reinforcement bar manufacturing in grade RB400 and RB500 according to PN-ISO 6935-2 standard in a new bar rolling mill in CMC Zawiercie S.A.

Comparative specification (diameter and 1 m length mass) of bars rolled from 34GS steel and their equivalents according to strength analysis (Re factor) rolled from RB500, RB500W steel made with Quenching and Tempering Bath method is given in the table:

Item 34 GS
Re_min=410MPa RB500, RB500W
Re_min=500MPa
Ø
(mm) 1 m mass
(kg) Ø calculated
(mm) Ø suggested
(mm) 1 m mass
(kg)
1 10 0,617 9,05 10 0,617
2 12 0,888 10,87 12 0,888
3 14 1,21 12,68 14 1,21
4 16 1,58 14,48 16 1,58
5 18 2,00 16,30 16 1,58
6 20 2,47 18,11 18 2,00
7 22 2,98 19,92 20 2,47
8 25 3,85 22,64 24 3,55
9 28 4,83 25,35 25 3,85
10 32 6,31 28,97 30 5,55

Mechanical and Technological Characteristics Comparison

Grade Re
MPa Rm
MPa A5
%
Concrete reinforcement bars,
Grade 18G2-b and 34 GS
According to PN-82/H93215 and PN-89/H-84023/06 standards
18G2-b
355
490 - 620
20
34GS
410
min.590
16
Bars grade RB500, RB500W
According to PN-ISO 6935-2
Supplemented by domestic sheet PN-ISO 6935/2/Ak
RB500, RB500W
500
550
14

Chemical Analysis Comparison

Grade C Mn Si P S Cr Ni Cu Mo Ceq
max
Concrete reinforcement bars,
Grade 18G2-b and 34GS
According to PN-82/H93215 and PN-89/H-84023/06 standards
18G2-b
0,15-0,22
1,00-1,50
0,20-055
0,050
0,050
0,30
0,30
0,35
010
0,48
34GS
0,30-0,36
0,80-1,20
0,40-0,70
0,050
0,050
0,30
0,30
0,35
0,10
0,59
Bars grade RB500, RB500W
According to PN-ISO 6935-2
Supplemented by domestic sheet PN-ISO 6935/2/Ak
Grade
C
Mn
Si
P
S
N
Ceq
RB500W
0,22
(0,24)
1,60
(1,70)
0,60
(0,65)
0,050
(0,055)
0,050
(0,055)
0,012
(0,013)
0,50
(0,52)

Comparative specification (diameter and 1 m length mass) of bars rolled from 18G2-b steel and their equivalents according to strength analysis (Re factor) rolled from RB500 and RB500W steel made with Quenching and Tempering Bath method is given in the table:

Item 18G2-b
Re_min=355Mpa RB500, RB500W
Re_min=500Mpa
Ø
(mm) 1 m mass
(kg) Ø calculated
(mm) Ø suggested
(mm) 1 m mass
(kg)
1 10 0,617 8,4 10 0,617
2 12 0,888 10,1 10 0,617
3 14 1,21 11,8 12 0,888
4 16 1,58 13,48 14 1,21
5 18 2,00 14,2 14 1,21
6 20 2,47 16,85 18 2,00
7 22 2,98 18,54 20 2,47
8 25 3,85 21,06 22 2,98
9 28 4,83 23,59 24 3,55
10 32 6,31 26,96 27 4,49