CLIAMS:We Claim:
1. High strength hot rolled steel (yield strength 550 MPa min.) with improved bendability comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron and which is obtained of double stage rolling.

2. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in claim 1 comprising plates of 24 mm to 26 mm preferably about 25 mm thickness.

3. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in anyone of claims 1 or 2, wherein said plates having deeper penetration of deformation leading to more complete recrystallization.

4. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in anyone of claims 1 to 3, comprising steel plates of 24mm to 26 mm preferably about of 25 mm thickness with uniform microstructure and having superior bendability in 3T bend test vis-a-vis paltes of same constitution and dimension obtained of single stage rolling.

5. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in anyone of claims 1 to 4 which is having bendability defined by defined by internal diameter of 3T where T is the thickness of the plate.

6. A process for producing high strength hot rolled steel (yield strength 550 MPa min.) with improved bendability comprising :
(i)involving a heat having composition comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron.
ii) subjecting to processing including a double stage rolling to achieve desired bendability.

7. A process as claimed in claim 6 comprising
(i)producing a heat having composition comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron.
(ii) processing said heat following the process route: BOF – VAD – CC – Plate Mill wherein it is continuously cast into 240 to 250 mm preferably about 242 mm thick slabs;
(iii) subjecting said slab to two stage/double stage, high shape factor hot rolling with controlled rolling parameters comprising
(a) rolling the slab in the first stage into 160 to 170 mm preferably about ~168 mm intermediate bars, followed by
(b) cutting the said intermediate bars into two halves and reheating again each and then rolled into 24 mm to 26 mm preferably about 25 mm plates in the second stage.
(iv)subjecting said plates after hot rolling to accelerated cooling comprising water cooling at 8ºCs-1

8. A process as claimed in anyone of claims 6 or 7 wherein said controlled rolling parameters for two stage rolling comprising
(a) subjecting said slab having entry thickness of 242mm at a soaking temperature of 1255 to 1270°C, to 3 number of roughing passes to be reduced to a exit thickness of 168mm in the first stage; and
(b)subjecting said slab having entry thickness of 168mm at a soaking temperature of 1287 to 1316°C in the second stage to 4 number of roughing passes followed by 8 number of finishing passes to obtain plates of 25mm thickness.
(c) said finishing passes maintained at FRT : ~830 oC and residence time : 3.5 – 4.0 hrs.

9. A process as claimed in anyone of claims 6 to 8 wherein the details of the passes in said two stage/double stage rolling comprising
(i) in first stage: 242mm-216-191-168 mm; and
(ii) in second stage: 168mm-144-118-99-81-70-60-51-43-36-32-28-26 mm ; involving total number of 15 passes including 3 in the first stage.

10. A process as claimed in anyone of claims 6 to 9 wherein said high shape factor double stage rolling was done with consistently higher shape factor in the range of 0.51-1.52 except for the last three finishing passes.

Dated this the 19th day of November, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

,TagSPECI:FIELD OF THE INVENTION
The present invention is directed to provide high strength hot rolled steel with improved bendability and a process for its production. More particularly, the present invention relates to a method for improving bendability of high strength plates (yield strength 550 MPa min.) by high shape factor rolling. The steel plate of 25mm thickness with high bendability is developed through two stage rolling of slab by selectively deploying high shape factor rolling through multiple passes in hot strip mill. The present invention has been carried out on slab with composition comprising 0.16%C-1.65%Mn-0.019%P-0.007%S-0.36%Si-0.028%Al-0.06%Nb-0.11%V-0.43%Ni-0.02%Ti hot rolled to steel plates of 25 mm thickness with a minimum yield strength of 550 MPa. These plates are suitable for applications in yellow goods, penstocks, bridges etc.

BACKGROUND OF THE INVENTION

Bhilai Steel Plant produces SAILMA 550HI grade (equivalent to IS 2062 E550) plates through single stage rolling. It has been found that application of this steel in structural components requires bending which the existing variety is not able to sustain due to poor bendability. As this conventional steel plates are prepared by single stage rolling of slab, they are having non uniform microstructure. This is attributed to single stage rolling to obtain these plates which leads to insufficient penetration of deformation and incomplete recrystallization through thickness. As a consequence, these high strength plates do not pass required bend tests, limiting their performance in various structural applications like penstock, bridges etc which is subjected to high bending stresses.

There has been thus a need in the related field to developing a process to increase the bendability of conventional high strength (yield strength 550 MPa min.) by ensuring deeper penetration of deformation during rolling and more uniform distribution of microstructure through the thickness.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength hot rolled steel with improved bendability and a process for its production.

A further object of the present invention is directed to improving the bendability in high strength ( Yield strength > 550 MPa) steel plates by high shape factor rolling.

A still further object of the present invention is directed to a process for producing high strength ( Yield strength > 550 MPa) steel plates by two stage rolling of slabs selectively involving high shape factor rolling through multiple roll passes alongwith controlled rolling parameters during roughing and finishing carried out in two stages.

Yet another object of the present invention is directed to provide a process for producing high strength (Yield strength > 550 MPa) steel plates by two stage rolling of slabs wherein deeper deformation is achieved through the thickness.

A further object of the present invention is directed to provide a process for producing high strength (Yield strength > 550 MPa) steel plates by two stage rolling involving high shape factor rolling to achieve relatively uniform microstructures across the plates and pass through required bend test.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is thus directed to provide high strength hot rolled steel (yield strength 550 MPa min.) with improved bendability comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron and which is obtained of double stage rolling.

A further aspect of the present invention is directed to said high strength hot rolled steel (yield strength 550 MPa min.) comprising plates of 40 mm to 25 mm preferably about 25 mm thickness.

A still further aspect of the present invention is directed to provide said high strength hot rolled steel (yield strength 550 MPa min.) wherein said plates having deeper penetration of deformation leading to more complete recrystallization.

A still further aspect of the present invention is directed to said high strength hot rolled steel (yield strength 550 MPa min.) comprising steel plates of 40 mm to 25 mm preferably about of 25 mm thickness with uniform microstructure and having superior bendability in 3T bend test vis-a-vis plates of same constitution and dimension obtained of single stage rolling.

A still further aspect of the present invention is directed to said high strength hot rolled steel (yield strength 550 MPa min.) which is having bendability defined by internal diameter of 3T where T is the thickness of the plate.

Yet another aspect of the present invention is directed to a process for producing high strength hot rolled steel (yield strength 550 MPa min.) with improved bendability comprising :
(i) involving a heat having composition comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron.

ii) subjecting to processing including a double stage rolling to achieve desired bendability.

A further aspect of the present invention is directed to said process comprising
(i)producing a heat having composition comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron.
(ii) processing said heat following the process route: BOF – VAD – CC – Plate Mill wherein it is continuously cast into 240 to 250 mm preferably about 242 mm thick slabs;
(iii) subjecting said slab to two stage/double stage, high shape factor hot rolling with controlled rolling parameters comprising
(a) rolling the slab in the first stage into 160 to 170 mm preferably about ~168 mm intermediate bars, followed by
(b) cutting the said intermediate bars into two halves and reheating again each and then rolled into 26 mm to 24 mm preferably about 25 mm plates in the second stage.
(iv)subjecting said plates after hot rolling to accelerated cooling comprising water cooling at 8ºCs-1

A still further aspect of the present invention is directed to said process wherein said controlled rolling parameters for two stage rolling comprising
(a) subjecting said slab having entry thickness of 242mm at a soaking temperature of 1255 to 1270°C, to 3 number of roughing passes to be reduced to a exit thickness of 168mm in the first stage; and
(b)subjecting said slab having entry thickness of 168 mm at a soaking temperature of 1287 to 1316°C in the second stage to 4 number of roughing passes followed by 8 number of finishing passes to obtain plates of 25mm thickness.
(c) said finishing passes maintained at FRT : ~830 oC and residence time : 3.5 – 4.0 hrs.

A still further aspect of the present invention is directed to a process wherein the details of the passes in said two stage/double stage rolling comprising
(i) in first stage: 242mm-216-191-168 mm; and
(ii) in second stage: 168mm-144-118-99-81-70-60-51-43-36-32-28-26 mm ; involving total number of 15 passes including 3 in the first stage.

A still further aspect of the present invention is directed to said process wherein said high shape factor double stage rolling was done with consistently higher shape factor in the range of 0.51-1.52 except for the last three finishing passes.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure 1: shows the schematic drawings of stress distribution during rolling wherein (a) shows Stress condition in roll gap and (b)shows the distribution of compression stress in thickness direction.

Figure 2 : shows graphically the relationship between shape factor and ratio of compression zone.

Figure 3: shows the graphical plot of shape factor vs. successive pass numbers in single and double stage rolling.

Figure 4: shows the optical microstructures of samples from plates rolled in single stage and double stage according to the invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURES

The present invention is directed to provide high strength hot rolled steel (yield strength 550 MPa min.) with improved bendability and a process for its production of such high strength plates by involving selectively high shape factor in two stage multi pass rolling.

Applicants’ Bhilai Steel Plant produces SAILMA 550HI grade (equivalent to IS 2062 E550) plates through single stage rolling. It is observed that single stage rolling leads to non uniform microstructure due to lack of sufficient penetration of deformation in thickness direction which leads to inadequate bendability.

It is known that Shape factor is the ratio of the contact arc length to the average thickness of the rolling stock and is given by following equation :

Shape Factor = 2 √[R(t0–ti)] / (ti+t0)
where, R = Roll radius
t0 = Thickness before pass i
ti = Thickness after pass i
Accompanying Figure 1 shows the schematic drawings of stress distribution during rolling wherein (a) shows Stress condition in roll gap and (b)shows the distribution of compression stress in thickness direction.

It has been observed that Compression zone in hot rolling increases with shape factor. High compression zone results in greater penetration of deformation and ensures complete recrystallization throughout the thickness of the plates leading to uniformity in microstructure. This leads to high toughness of the plates. This is particularly important for thick plates. Accompanying Figure 2 shows graphically the relationship between shape factor and ratio of compression zone.

A study has been undertaken to find out the effect of two stage rolling on the microstructure and properties of SAILMA 550 HI grade steel with the objective to improve the bendability in high strength (Yield strength > 550 MPa) plates of 25 mm thickness to achieve greater penetration of deformation and ensures complete recrystallization throughout the thickness of the plates leading to uniformity in microstructure. The experimental work carried out by way of the present invention to obtain high strength plates through two stage rolling involving selectively high shape factor rolling is illustrated by way of the following example:

Example:
(i) One heat of SAILMA 550 HI was made at BSP and continuously cast into 242 mm thick slabs. The heat was processed following the process route: BOF – VAD – CC – Plate Mill. The Chemistry ( wt.%) of heat having the constituents as in Table 1:
Table 1 :

C Mn P S Si Al Nb V Ni Ti
0.160 1.650 0.019 0.007 0.360 0.028 0.061 0.110 0.430 0.020

(ii) Two slabs were selected from the above heat for the following experiment.
(iii) One slab of 242 mm thickness was rolled into two 25 mm thick plates in single stage.
(iv) The other 242 mm thick slab was rolled in two stages:
(a) In the first stage the slab was rolled into ~168 mm intermediate bars.
(b) In the second stage, the intermediate bars were cut into two halves and were again reheated and then rolled into 25 mm plates in the second stage.
(c) The details of the rolling parameters are given in the following Table 2:
Table 2:
STAGE

ENTRY THICKNESS
mm EXIT THICKNESS
mm SOAKING TEMP.
0 C NO. OF PASSES
SINGLE 242 25 1255-1270 10 (Roughing)
8 (Finishing)
DOUBLE 242 168 1255-1270 3 (Roughing)
168 26 1287-1316 4 (Roughing)
8 ( 8(Finishing)

(d) Said finishing comprising FRT : ~830 oC
Residence time : 3.5 – 4.0 hrs.
(e) Details of the passes comprising:
(i) Single stage rolling following reduction in thickness in steps comprising:
242mm-232-208-185-168-151-134-119-104-91-78-67-56-50-43-37-32-28-25 mm ( total no. of passes : 18), and
(ii) Double stage rolling comprising said
1st stage rolling wherein thickness reduced through steps
comprising 242mm-216-191-168 mm; and
2nd stage rolling wherein thickness reduced through steps comprising 168mm-144-118-99-81-70-60-51-43-36-32-28-26 mm (total no. of passes : 15 including 3 in the first stage);

(f) Said single/double stage rolling conducted using Roll radius of 525 mm;

(v) The plates were rolled with with consistently higher shape factor (0.51-1.52) (except in the last three finishing passes where shape factor was 1.2) in double stage rolling compared to single stage rolling as can be seen in the accompanying Figure 3;
(vi) Plates were accelerated cooled after hot rolling;
(vii) Samples were collected and characterized for tensile, impact and microstructure;

Test Results:
The results of the tensile, impact and bend test of samples obtained through single and double stage rolling respectively are presented in Table 3 below:

Table 3:

The microstructure studies were conducted on samples and the results are presented in accompanying Figure 4. The optical microstructures of samples from plates rolled in single stage and double stage showed that the microstructure of the plate rolled in double stage is relatively more uniform than the one rolled in single stage.

Thus following observations were made based on the test results:
(i) Microstructures across the plates are relatively uniform in double stage rolled plates compared to single stage;
(ii) Double stage rolled plates have showed superior bendability in 3T bend test;
(iii) Improved bendability of double stage rolled plates attributed to uniformity in microstructure.

It is thus possible by way of the present invention to provide high strength steel plates (yield strength 550 MPa min.) by high shape factor rolling through two stage rolling having the following distinguishing features over the conventional single stage rolling:
(i) Technology for the production of IS 2062 E550 (SAILMA 550HI) plates with improved bendability by double rolling has been demonstrated.
(ii) It has been observed that 25 mm thick plates of IS 2062 E550 (SAILMA 550HI) grade of steel passed in 3T bend test while the same plates rolled in a single stage failed in the bend test.
(iii) Double rolling was done with consistently higher shape factor (0.51-1.52) compared to single stage rolling (0.30-1.52), except for the last pass where shape factor was 1.2.
(iv) High shape factor rolling has enabled deformation to penetrate deeper into the plates leading to more complete recrystallization.
(v) The improvement in bendability is attributable to relatively uniform microstructure across the thickness of the plates.
We Claim:
1. High strength hot rolled steel (yield strength 550 MPa min.) with improved bendability comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron and which is obtained of double stage rolling.

2. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in claim 1 comprising plates of 24 mm to 26 mm preferably about 25 mm thickness.

3. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in anyone of claims 1 or 2, wherein said plates having deeper penetration of deformation leading to more complete recrystallization.

4. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in anyone of claims 1 to 3, comprising steel plates of 24mm to 26 mm preferably about of 25 mm thickness with uniform microstructure and having superior bendability in 3T bend test vis-a-vis paltes of same constitution and dimension obtained of single stage rolling.

5. High strength hot rolled steel (yield strength 550 MPa min.) as claimed in anyone of claims 1 to 4 which is having bendability defined by defined by internal diameter of 3T where T is the thickness of the plate.

6. A process for producing high strength hot rolled steel (yield strength 550 MPa min.) with improved bendability comprising :
(i)involving a heat having composition comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron.
ii) subjecting to processing including a double stage rolling to achieve desired bendability.

7. A process as claimed in claim 6 comprising
(i)producing a heat having composition comprising
C: 0.12 to 0.20 preferably 0.16wt%;
Mn: 1.2 to1.65 preferably 1.65 wt%;
P: up to 0.019 wt%;
S: up to 0.007 wt%;
Si: up to 0.50 wt%;
Al : up to 0.020 wt %;
Nb+V+Ti : up to 0.25 wt.%
and rest is iron.
(ii) processing said heat following the process route: BOF – VAD – CC – Plate Mill wherein it is continuously cast into 240 to 250 mm preferably about 242 mm thick slabs;
(iii) subjecting said slab to two stage/double stage, high shape factor hot rolling with controlled rolling parameters comprising
(a) rolling the slab in the first stage into 160 to 170 mm preferably about ~168 mm intermediate bars, followed by
(b) cutting the said intermediate bars into two halves and reheating again each and then rolled into 24 mm to 26 mm preferably about 25 mm plates in the second stage.
(iv)subjecting said plates after hot rolling to accelerated cooling comprising water cooling at 8ºCs-1

8. A process as claimed in anyone of claims 6 or 7 wherein said controlled rolling parameters for two stage rolling comprising
(a) subjecting said slab having entry thickness of 242mm at a soaking temperature of 1255 to 1270°C, to 3 number of roughing passes to be reduced to a exit thickness of 168mm in the first stage; and
(b)subjecting said slab having entry thickness of 168mm at a soaking temperature of 1287 to 1316°C in the second stage to 4 number of roughing passes followed by 8 number of finishing passes to obtain plates of 25mm thickness.
(c) said finishing passes maintained at FRT : ~830 oC and residence time : 3.5 – 4.0 hrs.

9. A process as claimed in anyone of claims 6 to 8 wherein the details of the passes in said two stage/double stage rolling comprising
(i) in first stage: 242mm-216-191-168 mm; and
(ii) in second stage: 168mm-144-118-99-81-70-60-51-43-36-32-28-26 mm ; involving total number of 15 passes including 3 in the first stage.

10. A process as claimed in anyone of claims 6 to 9 wherein said high shape factor double stage rolling was done with consistently higher shape factor in the range of 0.51-1.52 except for the last three finishing passes.

Dated this the 19th day of November, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT

TITLE: HIGH STRENGTH HOT ROLLED STEEL (YIELD STRENGTH 550 MPa min.) WITH IMPROVED BENDABILITY AND A PROCESS FOR ITS PRODUCTION

The present invention is directed to provide high strength hot rolled steel with improved bendability and in particular to a method for improving bendability of high strength plates (yield strength 550 MPa min.) by double stage high shape factor rolling. The steel plate of 25mm thickness with high bendability is developed through two stage rolling of slab by selectively deploying high shape factor rolling in the range of 0.51-1.52 involving multiple roughing and finishing passes in hot strip mill. The present invention has been carried out on slab with composition comprising 0.16%C-1.65%Mn-0.019%P-0.007%S-0.36%Si-0.028%Al-0.06%Nb-0.11%V-0.43%Ni-0.02%Ti hot rolled to steel plates of 25 mm thickness with a minimum yield strength of 550 MPa. These plates are suitable for applications in yellow goods, penstocks, bridges etc.
(Figure 3)