Claims:We claim:

1. Seismic resistant Thermo Mechanically Treated (TMT) steel reinforcement bars having composition comprising:

C: 0.18 to 0.30 wt % preferably 0.24 wt %;
Mn: 0.8 to 1.8 wt% preferably 1.5 wt %;
Si: 0.20 to 0.60 wt % preferably 0.25 wt %;
S: 0 to 0.042 wt % preferably 0.025 wt %;
P: 0 to 0.042 wt % preferably 0.025 wt %;
Anyone or more of Nb,V,Ti, B & Mo : 0 - 0.25 wt %;
Cu: 0 - 0.50 wt % preferably 0.25 wt %;
Cr: 0 - 0.75 wt % preferably 0.50 wt %; and
rest is Fe.

2. Seismic resistant TMT steel reinforcement bars as claimed in claim 1 comprising:

YS (MPa) :500-625 and preferably 520;
UTS/YS: 1.25 minimum, preferably 1.27;
overall %El 18 to 30 preferably 20; and uniform %El 8 to 15 preferably 10, and conforming to IS 1786, 2008 for Fe 500S grade.

3. Seismic resistant TMT steel reinforcement bars as claimed in anyone of claims 1 or 2 having microstructure comprising very hard tempered martensite rim at periphery and acicular ferrite/bainite along with relatively soft pearlite at core ensuring excellent combination of strength, toughness and ductility.

4. A process for producing seismic resistant TMT steel reinforcement bars as claimed in anyone of claims 1 to 3 comprising the steps of:

(i) producing the steel maintaining C-Mn chemistry with or without alloying such as to obtain steel composition comprising
C: 0.18 to 0.30 wt % preferably 0.24 wt %;
Mn: 0.8 to 1.8 wt% preferably 1.5 wt %;
Si: 0.20 to 0.60 wt % preferably 0.25 wt %;
S: 0 to 0.042 wt % preferably 0.025 wt %;
P: 0 to 0.042 wt % preferably 0.025 wt %;
Anyone or more of Nb,V,Ti, B & Mo : 0 - 0.25 wt %;
Cu: 0 - 0.50 wt % preferably 0.25 wt %;
Cr: 0 - 0.75 wt % preferably 0.50 wt %; and
rest is Fe.

(ii) producing billet/bloom therefrom;

(iii) hot rolling with controlled parameters to produce TMT bars from billet/bloom; and

(iv) instantaneous quenching of hot deformed bars under controlled cooling parameters such as to achieve desired microstructure and combination of strength with yield strength of 500MPa minimum.; UTS/YS: 1.25 minimum, preferably1.27; overall %El 18 minimum; and uniform %El 8 minimum and conforming to IS 1786, 2008 for Fe 500S grade.

5. A process for producing seismic resistant TMT steel reinforcement bars as claimed in claim 5 wherein said rolling parameters comprising

Soaking temperature:1100-1250?C;
Bar diameter: 10,12,16,20,25,28,32,36 & 40mm;
Mill speed: 4 – 40 m/s;
Parameters related to rapid cooling/quenching :
Water Pressure: 7-14 kg/cm2;
Water Flow: 300 - 750m3/hr;

6. A process for producing seismic resistant TMT steel reinforcement bar as claimed in anyone of claims 4 or 5 wherein cooling parameters for instantaneous quenching of hot deformed bars comprising cooling water flow 300 to 750m3/hr; inlet water temperature 36 °C maximum ; retention time in cooling chamber varied in the range of 1 to 8 s according to the size of the bar by mill speed.

Dated this the 14th day of February, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

, Description:FIELD OF THE INVENTION

The present invention relates to Thermo Mechanically Treated (TMT) steel reinforcement bar with high strength and toughness and a process for its production. More particularly, the present invention is directed to provide TMT bars having yield strength in the range of 500 to 625 MPa and UTS/YS ratio of 1.25minimum, with superior toughness (18% elongation min.) properties produced through selective alloy design and optimizing process parameters. The high strength TMT rebars of the invention is having selective alloy composition comprising C-Mn chemistry wherein manganese content of steel is maintained on moderately higher side in the range of 1.0 -1.8 wt %, and anyone or more micro-alloying element selected from Nb, V ,Ti, B and Mo subject to a maximum of 0.25 by wt % singly or in combination, and Cu 0.20 -0.40wt % and Cr - 0.70 wt% maximum and the balance being Fe. Importantly, the process parameters include instantaneous quenching by accelerated cooling of the hot deformed rebars according to the present invention to provide martensite structure at the periphery and the core containing predominantly ferrite-pearlite, acicular-ferrite or bainite structure directed to ensure desired strength and toughness properties of the end product. The high strength TMT bar according to the invention thus ensuring minimumYS of 500 MPa and UTS/YS ratio equal to or more than 1.25 and overall elongation not less than 18% for advantageous application in RCC structures.

BACKGROUND ART

It is well known in the related art that Thermo Mechanically Treated (TMT) steel reinforcement bars of Fe 500 grade with UTS/YS ratio of 1.15 minimum are being extensively used in Indian construction industry for RCC structures. It has also been experienced in the art that considering the safety aspect of such construction, there had been a need to develop steel materials with even higher UTS/YS ratio of 1.25 minimum with same section for the TMT bars associated with substantial ductility for applications in RCC structures ensuring improved capability to absorb more plastic energy before failure during earthquake. Bureau of Indian Standard has introduced Fe 500S grade in IS 1786, 2008 with YS of 500 MPa minimum and UTS/YS ratio 1.25 minimum. The use of the said rebars with suitable design in the construction of building make these structures more resistant to instantaneous collapse in the unfortunate event of earthquakes and provides sufficient time for the safe evacuation of these buildings.

Such high strength rebars are developed and produced by selectively controlling on one hand the chemistry of the steel material and on the other hand optimizing the process parameters for rolling and instantaneous quenching by accelerated cooling of the hot deformed rebars to thereby obtain desired microstructure favouring achieving desired superior strength/physical properties in the end products.

OBJECTS OF THE INVENTION

The basic object of the present invention is thus directed to provideFe 500S grade seismic resistant Thermo Mechanically Treated (TMT)steel reinforcement bars with YS of 500 MPa minimum and UTS/YS ratio 1.25 or more for application as reinforcement bar in RCC structure and the like.

Another object of the present invention is directed toprovide said seismic resistant TMT steel reinforcement bars with desired higher physical strength and ductility properties having selective composition comprising C-Mn chemistry with or without alloying and micro-alloying elements.

A further object of the present invention is directed to provide said seismic resistant TMT steel reinforcement bar which would be capable of providing desired combination of high strength and high toughness with good elongation properties suitable for RCC reinforcement ensuring improved capability to absorb more plastic energy before failure during earthquake.

A still further object of the present invention is directed to provide said seismic resistant TMT rebar with desired higher physical strength properties obtained by selective control of chemistry as well as optimizing the process parameters for rolling and rapid cooling of hot rolled bars such as controlling cooling water flow, inlet water temperature, retention time in cooling chamber (cooling chamber length and mill speed) etc. which is selectively varied according to the size of the bar.

A still further object of the present invention is directed to provide said seismic resistant TMT rebar with desired higher physical strength properties wherein a preferred microstructure is obtained by control of chemistry and process parameters to ensure very hard peripheral rim of rebars with tempered martensite microstructure and with core relatively soft comprising ferrite-perlite /bainite microstructure thereby ensuring excellent combination of strength and ductility.

A still further object of the present invention is directed to provide said seismic resistant TMT rebar with desired higher physical strength and ductility properties wherein the developed steel rebar ensure higher safety of RCC structure by way of absorbing more plastic energy before failure when suitably designed using such TMT rebars in the unfortunate event of earthquakes providing more resistance to instantaneous collapse and sufficient time for the safe evacuation.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to seismic resistant Thermo Mechanically Treated (TMT) steel reinforcement bars having composition comprising:

C: 0.18 to 0.30 wt % preferably 0.24 wt %;
Mn: 0.8 to 1.8 wt% preferably 1.5 wt %;
Si: 0.20 to 0.60 wt % preferably 0.25 wt %;
S:0 to 0.042 wt % preferably 0.025 wt %;
P: 0 to 0.042 wt % preferably 0.025 wt %;
Anyone or more of Nb,V,Ti, B & Mo : 0 - 0.25 wt %;
Cu: 0 - 0.50 wt % preferably 0.25 wt %;
Cr: 0 - 0.75 wt % preferably 0.50 wt %; and
rest is Fe.

Another aspect of the present invention is directed to said seismic resistant TMT steel reinforcement bars comprising:

YS(MPa) :500-625 and preferably 520;
UTS/YS: 1.25minimum, preferably1.27;
overall %El 18 to 30 preferably 20; and uniform %El 8 to 15 preferably 10, and conforming to IS 1786, 2008 for Fe 500S grade.

A further aspect of the present invention is directed to said seismic resistant TMT steel reinforcement bars having microstructure comprising very hard tempered martensite rim at periphery and acicular ferrite/ bainite along with relatively soft pearlite at core ensuring excellent combination of strength, toughness and ductility.

According to yet another aspect of the present invention is directed to a process for producing seismic resistant TMT steel reinforcement bars as described above comprising the steps of:

(i) producing the steel maintaining C-Mn chemistry with or without alloying such as to obtain steel composition comprising
C: 0.18 to 0.30 wt % preferably 0.24 wt %;
Mn: 0.8 to 1.8 wt% preferably 1.5 wt %;
Si: 0.20 to 0.60 wt % preferably 0.25 wt %;
S:0 to 0.042 wt % preferably 0.025 wt %;
P: 0 to 0.042 wt % preferably 0.025 wt %;
Anyone or more of Nb,V,Ti, B & Mo : 0 - 0.25 wt %;
Cu: 0 - 0.50 wt % preferably 0.25 wt %;
Cr: 0 - 0.75 wt % preferably 0.50 wt %; and
rest is Fe.

(ii) producing billet/bloom there from;

(iii) hot rolling with controlled parameters to produce TMT bars from billet/bloom; and

(iv) instantaneous quenching of hot deformed bars under controlled cooling parameters such as to achieve desired microstructure and combination of strength with yield strength of 500MPa minimum.; UTS/YS: 1.25 minimum, preferably1.27; overall %El 18 minimum; and uniform %El 8 minimum and conforming to IS 1786, 2008 for Fe 500S grade.

A still further aspect of the present invention is directed to a process for producing seismic resistant TMT steel reinforcement bars wherein, said rolling parameters comprising

Soaking temperature:1100-1250?C;
Bar diameter: 10,12,16,20,25,28,32,36 & 40mm;
Mill speed:4– 40m/s;
parameters related to rapid cooling/quenching
Water Pressure:7-14 kg/cm2;
Water Flow:300-750m3/hr;

A still further aspect of the present invention is directed to a process for producing seismic resistant TMT steel reinforcement bar wherein cooling parameters for instantaneous quenching of hot deformed bars comprising cooling water flow 300 to 750m3/hr; inlet water temperature 36 °C max ; retention time in cooling chamber varied according to the size of the bar by mill speed.

The present invention and its objects and advantages are described hereunder in greater details with reference to the following accompanying non limiting illustrative figures and example.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1: is the process flow chart showing the processing route involving the basic steps for developing the seismic resistant TMT rebars according to present invention.

Figure 2: shows the Macro section view of Fe 500S TMT rebar.

Figure 3a: is the image of micro structure of the core of the TMT rebar developed according to the process of the invention showing Ferrite – Pearlite (some cases acicular Ferrite/Bainite) structure at core.

Figure 3b: is the micrograph showing Acicular Ferrite/ Bainite structure at Intermediate Zone of TMT rebars developed according to the present invention.

Figure 3c: is the micrograph showing dendritic tempered Martensite structure at the periphery of TMT rebars developed according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPNAYING FIGURES

The present invention is directed to provide seismic resistant Thermo Mechanically Treated (TMT)rebar with YS of 500 MPa minimum and UTS/YS ratio 1.25 minimum for application as reinforcement bar in RCC structure and the like conforming to IS 1786, 2008 for Fe 500S grade.

The seismic resistant TMT rebars obtained according to the present invention is having desired higher physical strength and ductility properties by involving selective composition comprising C-Mn chemistry, with or without alloying and micro-alloying elements, and selected processing steps and parameters to achieve the desired microstructure comprising very hard peripheral rim of rebars with tempered martensite microstructure and with core relatively soft comprising ferrite-pearlite /bainite microstructure thereby ensuring excellent combination of strength and ductility for higher safety of RCC structure by way of absorbing more plastic energy before failure when suitably designed using such TMT rebars, in the event of earthquakes providing more resistance to instantaneous collapse and allowing sufficient time for the safe evacuation.

Accompanying Figure 1 illustrates the process flow chart showing the processing route involving the basic steps for developing the seismic resistant TMT rebars according to present invention. It is apparent that the process involved the following basic steps:

(i) producing the steel maintaining C-Mn chemistry with or without alloying such as to obtain steel composition comprising
C: 0.18 to 0.30 wt % preferably 0.24 wt %;
Mn: 0.8 to 1.8 wt% preferably 1.5 wt %;
Si: 0.20 to 0.60 wt % preferably 0.25 wt %;
S: 0 to 0.042 wt % preferably 0.025 wt %;
P: 0 to 0.042 wt % preferably 0.025 wt %;
Anyone or more of Nb,V,Ti, B & Mo : 0 - 0.25 wt %;
Cu: 0 - 0.50 wt % preferably 0.25 wt %;
Cr: 0 - 0.75 wt % preferably 0.50 wt %; and
rest is Fe.

(ii) producing billet/bloom therefrom through continuous casting route;

(iii) hot rolling with controlled parameters to produce TMT bars from billet/bloom; and

(iv) instantaneous quenching of hot deformed bars under controlled cooling parameters;

In the above process, the rolling is carried out maintaining the following parameters:
Soaking temperature:1100-1250?C;
Bar diameter: 10,12,16,20,25, 28, 32, 36 & 40mm;
Mill speed: 4 – 40 m/s;
parameters related to rapid cooling/quenching:
Water Pressure: 7-14 kg/cm2;
Water Flow: 300 - 750m3/hr;

Importantly also, the step of instantaneous quenching of hot deformed bars are carried out maintaining the following parameters:
cooling water flow 300 to 750m3/hr; inlet water temperature 10 to 30 C; retention time in cooling chamber varied in the range of 1 s to 8 s according to the size of the bar by mill speed.

The composition of the steel samples obtained under different trials according to the invention are presented in following Table 1 and the Tensile Properties corresponding to Sample for Experimental Trials are presented in following Table 2.

Table 1 : Chemical composition of Samples for Experimental Trials

Sample No. C Mn P S Si V Cr Cu S+P
IS:1786 0.25 max - 0.04 max 0.04 max - 0.25 max - - 0.075 max

Aim Chemistry 0.20-0.25 1.2-1.5 0.04 max 0.04 max 0.2 - 0.3 0.010-0.05 - - 0.075 max
2-10848 0.24 1.18 0.040 0.027 0.26 - - - 0.067
2-10849 0.24 1.20 0.033 0.016 0.21 - - - 0.049
2-10851 0.23 1.15 0.016 0.013 0.26 0.015 - - 0.029
2-10853 0.24 1.49 0.031 0.013 0.26 0.015 - - 0.044
2-10855 0.23 1.39 0.028 0.015 0.22 0.016 - - 0.043

Aim Chemistry 0.20-0.25 1.0 - 1.4 0.04 max 0.04 max 0.2 - 0.3 0.01-0.05 0.50 0.25 0.075 max
S-1 0.21 0.90 0.026 0.020 0.23 0.007 0.59 0.38 0.046
Aim Chemistry 0.22-0.25 1.0 - 1.4 0.04 max 0.04 max 0.2 - 0.3 0.01-0.05 - - 0.075 max
S-2 0.20 1.35 0.024 0.010 0.22 0.023 0.014 - 0.034

Table 2 : Tensile Properties corresponding to Sample for Experimental Trials:

Example
No. Sample No. Bar
Diameter
mm Bar mill speed
m/s Quenching water flow m3/hr Quenching water pressure kg/cm2 YS
(MPa) UTS
(MPa) UTS/YS Total
% El Uniform % El Remarks
IS:1786 500 - 625 1.25 times YS min 1.25 min 18 min 8 min
1 2-10855 12 21 578
12.8
556-608 681- 698 1.15 - 1.23 16-20 8-10 Properties not achieved due to Lower mill speed, high quenching water pressure and flow
2 2-10855 12 26 529 9.4 517-531 662-700 1.28 – 1.32 20-23 9-12 Successful; Optimised mill speed, quenching water pressure and flow
3 2-10853 12 26 546 9.6 521-544 693-709 1.30 - 1.33 20-23 9-12
4 2-10851/
2-10849/
2-10848 12 26 529 -546 9.4 - 9.6 480-560 604- 678 1.19 - 1.25 20-24 8-13 Properties not achieved due to non-adherence to aimed Composition
5 S1 16 14 500 -564 11.1-11.2 554 684 1.23 18-20 8 - 9 Properties not achieved due to low mill speed, high quenching water pressure and flow
6 S2 20 19.6 352-378 12.8-13.0 530 649 1.22 20-22 9-11 Properties did not achieve as Compositional requirement and processing parameters not met
The above stated steel composition and processing steps are directed to achieve desired characteristics of TMT rebars in terms of microstructure and combination of strength and toughness and ductility for the intended application as seismic resistant steel reinforcement bars as further illustrated through following examples.

Examples:
(a) A few examples (Example 2 & 3 of Table 2 above) of steel samples with composition and step wise processing parameters followed as per the invention leading to inventive steel grade with desired properties within claimed range and favoured microstructure according to the invention and conforming to specification IS 1786, 2008 for Fe 500S grade are presented in table 2 below.

(b) A few comparative examples (Example 1,4,5,6 of Table 2 above) have been shown when either composition or process parameters are outside the claimed range, the end properties/microstructure are not meeting the required values as per specification.

Test methods:

The samples of steel rebars obtained according to the present invention were tested for micro structure and strength properties and results obtained following the Test method as per clause 9 of specification IS 1786, 2008.

Microstructure Evaluation:
Microstructure can be observed in Optical microscope.
The sample is to be cross-sectionally cut into a length of 15 - 20 mm and copper-mounted based on the diameter of rebar for proper conduction. The sample should be metallographically polished in 180, 360, 400, 600, 800 and 1200 grit size silicon-carbide emery paper followed by cloth-polishing using alumina in 1 micron for rough and 0.03 microns for fine polishing. Then Nital (98% ethanol + 2% HNO3) are used as an etchant to reveal the microstructure. The sample then focused to 50X, 100X or more magnifications for observing the phases.

Test results:
(i) Steel samples showed yield strength of 500MPa minimum.; UTS/YS: 1.25 minimum, preferably1.27; overall %El 18 minimum; and uniform %El 8 minimum and conforming to IS 1786, 2008 for Fe 500S grade.
(ii) TMT rebars having microstructure comprising very hard tempered martensite rim at periphery and acicular ferrite/bainite along with relatively soft pearlite at core as per the macrosection of sample bar shown in Figure 2 ensuring excellent combination of strength, toughness and ductility.
(iii) Accompanying Figure 3a is the image of micro structure of the core of the TMT rebar developed according to the process of the invention showing Ferrite – Pearlite (some cases acicular Ferrite/Bainite Figure 3b at Intermediate Zone) structure at core.
Accompanying Figure 3c is the micrograph showing dendritic tempered Martensite structure at the periphery of TMT rebars developed according to the present invention.

It is thus possible by way of the present invention to provide Thermo Mechanically Treated (TMT) steel reinforcement bars having yield strength in the range of 500 to 625 MPa and UTS/YS ratio of 1.25 minimum, with superior toughness (18% elongation min.) properties produced through selective alloy design and optimizing process parameters including instantaneous quenching by accelerated cooling of the hot deformed rebars to provide martensite structure at the periphery and the core containing predominantly ferrite-pearlite, acicular-ferrite or bainite structure directed to ensure desired strength and toughness properties of the end product for advantageous application in RCC structures providing more resistance to instantaneous collapse and sufficient time for the safe evacuation in the unfortunate event of earthquakes.