CLIAMS:We Claim:

1. Hot rolled high strength corrosion resistant structural steel comprising
C: 0.15 – 0.22 % by wt. preferably 0.15-0.20% by wt.;
Mn: 1.2 to 1.5 % by wt. preferably 1.2 – 1.4% by wt.;
S: upto 0.020% by wt.preferably < 0.010% by wt.;
Cu: 0.20 – 0.5 % by wt. preferably 0.20-0.040% by wt.;
P: upto 0.020% by wt. preferably < 0.015% by wt.;
Si: 0.15- 0.40% by wt;
Al: at least 0.02% by wt.preferably 0.02 to 0.05% by wt.;
N: upto 0.008% by wt.preferably <0.005 % by wt.;
Nb: atleast 0.03 % by wt. preferably 0.030-0.040% by wt.;
Ti: atleast 0.02% by wt preferably 0.02- 0.030% by wt.; and
balance is Fe.
enabling higher strength at lower thickness.

2. Hot rolled high strength corrosion resistant structural steel as claimed in claim 1, obtained in the form of coil having
Coil thickness: 2.5 – 4.0 mm;
Coil width: 900-1500mm;
Yield strength: 450 – 530 MPa;
UTS: 570 – 650 MPa.

3. Hot rolled high strength corrosion resistant structural steel as claimed in claim anyone of claims 1 or 2 having
% Elongation: 20 – 28; and
sheet coil having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

4. Hot rolled high strength corrosion resistant structural steel sheets as claimed in anyone of claims 1 to 3 having surface quality free of corner cracks suitable for structural applications.

5. A process for the production of high strength corrosion resistant structural steel as claimed in anyone of claims 1 to 4 comprising steel making involving selective alloying elements comprising
C: 0.15-0.20% by wt.;
Mn: 1.2 – 1.4% by wt.;
S: upto 0.020% by wt.;
Cu: 0.20-0.040% by wt.;
P: upto 0.020% by wt.;
Si: 0.15- 0.40% by wt;
Al: at least 0.02% by wt.;
N: upto 0.008% by wt.;
Nb: 0.030-0.040% by wt.;
Ti: 0.02- 0.030% by wt.; and
balance is Fe.
said alloying elements including selective combination of Mn, Nb, Ti & Cu providing for higher strength at lower thickness.

6.A process as claimed in claim 5 wherein said steel making comprises:

steel making by L.D. converter;
secondary steel making Ladle Heating Furnace;
addition of alloying element after completion of de oxidation;
continuous slab casting;
re-heating hot rolling, ROT cooling and coiling with set optimum processing parameters.

7.A process as claimed in claim 5 or 6 comprising controlled hot and cold rolling carried out comprising
Furnace Temperature (Soaking) in the range of 1200 – 1250 oC;
Finish Rolling Temperature in the range of 810 – 860 oC;
Coiling Temperature in the range of 550 - 620 oC.

8. A process as claimed in anyone of claims 5 to 7 comprising for producing high strength corrosion resistant structural steel in the form of sheet coil comprising
Coil thickness: 2.5 – 4.0 mm;
Coil width: 900- 1500 mm;
Yield strength: 450 – 530 MPa;
UTS: 570 – 650 MPa;
% Elongation: 20 – 28; and
sheet coil thus obtained having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

Dated this the 3rd day of January, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
,TagSPECI:FIELD OF THE INVENTION

The present invention relates to high strength corrosion resistant structural steel and a process for its production. More particularly, the present invention is directed to a grade of hot rolled high strength corrosion resistant structural steel with improved surface quality for railway freight wagon body and similar applications where savings in weight (mass) and added durability are matters of concern. The composition of the invented steel grade involving optimum combination of Mn, Nb, Ti & Cu in a corrosion resistant structural steel helped in achieving higher strengths even at lower thickness <3mm of hot rolled coils. Copper is added for corrosion resistance, Mn & Nb for increasing the strength and Ti for surface finish. Judicious selection of alloying elements in combination with copper helped achieve high strength of the steel without compromising the surface quality and ductility having consistent and high UTS favouring advantageous application required for thin sheets to be used in railway wagons.

BACKGROUND OF THE INVENTION

It is known that atmospheric corrosion resistant structural quality steel is stronger and tougher than carbon steel and is ductile, highly formable & weldable. Conventional high-strength structural steel features atmospheric corrosion resistance due to addition of copper which is crucial in structural applications. Atmospheric corrosion resistant steel is intended primarily for use in structural members, where savings in weight (mass) and added durability matter. Plate is used in railway wagons and other comparable applications. Normally plates varying in thickness from 3-20 mm are used in various parts of a railway wagon.
Recently there has been a significant increase in demand on low thickness (< 3mm) corrosion resistant structural plate. It has been experienced that the general corrosion resistant structural steel does not meet the strength specifications at lower thicknesses. Increasing the strength of these Cu containing structural steel involves complex strengthening mechanisms. Cu though impart corrosion resistant properties but also cause hot shortness (tearing during hot working) and the effect is enhanced with increasing carbon. Hence improvement in the strength of Cu containing steels must be achieved by other alloying elements. Cu also affects surface quality and prone to corner cracks and the effect is pronounced at lower thicknesses. Therefore composition must also be adjusted to take care of these defects.
There has been thus a need in the art to developing a high strength corrosion resistant steel grade which can be transformed to very thin sheets(<3mm) suitable for structural application. There was also a need to develop composition which would take care of surface defects due to increased use of Cu for weather resistivity. Thus by way of the present invention a novel innovative combination of Mn, Nb, Ti & Cu as alloying elements in a corrosion resistant structural steel is developed which helped in achieving higher strengths at lower thickness of hot rolled coils. The increased strength of high strength corrosion resistant structural steel means that structures can be built lighter, with less steel. This is important for railway wagon construction as it improves fuel economy. The extra strength of HSLA steel makes it more durable and offers a longer useful life span
OBJECTS OF THE INVENTION

The basic object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel sheets with improved surface quality for railway freight wagon body and similar applications and a process for its production.

A further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel sheets wherein a unique combination of Mn, Nb, Ti and Cu in said steel ensure increased strength even at lower thickness (< 3mm).

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein increased strength enable the structures made thereof can be built lighter, with less steel favouring fuel economy in transport.

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein composition of the steel is adjusted to take care of adverse effects of Cu addition for weather resistance to develop defects like hot shortness, corner crack, poor surface quality is avoided particularly in low thicknesses, by involving a unique composition containing Mn, Nb, Ti & Cu alongwith carbon in selective proportion.

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein surface quality and corner cracks issue has been addressed by addition of Titanium to the base composition.

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein judicious selection of alloying elements in combination with copper helped achieve high strength of the steel without compromising the surface quality and ductility.

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein a favourable combination of mechanical properties and surface quality in steel sheets is achieved which is suitable for application in railway wagon.

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein additional strength has been achieved by addition of Niobium through grain refinement as it restricts austenitic grain growth and also ties up nitrogen to inhibit strain ageing which is an added advantage for rolling to thinner sections.

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein titanium nitride (TiN) particles precipitated in steel slabs suppress the growth of austenite grains on heating and consequently improve the low-temperature toughness of controlled-rolled steel plates by grain refinement.

A still further object of the present invention is directed to providing hot rolled high strength corrosion resistant structural steel grade wherein due to minimum usage of various ferroalloys, the cost of production of the new grade steel is kept minimum.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to hot rolled high strength corrosion resistant structural steel comprising
C: 0.15 – 0.22 % by wt. preferably 0.15-0.20% by wt.;
Mn: 1.2 to 1.5 % by wt. preferably 1.2 – 1.4% by wt.;
S: upto 0.020% by wt.preferably < 0.010% by wt.;
Cu: 0.20 – 0.5 % by wt. preferably 0.20-0.040% by wt.;
P: upto 0.020% by wt. preferably < 0.015% by wt.;
Si: 0.15- 0.40% by wt;
Al: at least 0.02% by wt.preferably 0.02 to 0.05% by wt.;
N: upto 0.008% by wt.preferably <0.005 % by wt.;
Nb: atleast 0.03 % by wt. preferably 0.030-0.040% by wt.;
Ti: atleast 0.02% by wt preferably 0.02- 0.030% by wt.; and
balance is Fe.
enabling higher strength at lower thickness.

A further aspect of the present invention I directed to said hot rolled high strength corrosion resistant structural steel, obtained in the form of coil having
Coil thickness: 2.5 – 4.0 mm;
Coil width: 900-1500mm;
Yield strength: 450 – 530 MPa;
UTS: 570 – 650 MPa.

A still further aspect of the present invention is directed to providing said hot rolled high strength corrosion resistant structural steel having
% Elongation: 20 – 28; and
sheet coil having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

Yet another aspect of the present invention is directed to said hot rolled high strength corrosion resistant structural steel sheets having surface quality free of corner cracks suitable for structural applications.

A further aspect of the present invention is directed to a process for the production of high strength corrosion resistant structural steel comprising steel making involving selective alloying elements comprising
C: 0.15-0.20% by wt.;
Mn: 1.2 – 1.4% by wt.;
S: upto 0.020% by wt.;
Cu: 0.20-0.040% by wt.;
P: upto 0.020% by wt.;
Si: 0.15- 0.40% by wt;
Al: at least 0.02% by wt.;
N: upto 0.008% by wt.;
Nb: 0.030-0.040% by wt.;
Ti: 0.02- 0.030% by wt.; and
balance is Fe.
said alloying elements including selective combination of Mn, Nb, Ti & Cu providing for higher strength at lower thickness.

A still further aspect of the present invention is directed to said process wherein said steel making comprises:

steel making by L.D. converter;
secondary steel making Ladle Heating Furnace;
addition of alloying element after completion of de oxidation;
continuous slab casting;
re-heating hot rolling, ROT cooling and coiling with set optimum processing parameters.

A still further aspect of the present invention is directed to said process comprising controlled hot and cold rolling carried out comprising
Furnace Temperature (Soaking) in the range of 1200 – 1250 oC;
Finish Rolling Temperature in the range of 810 – 860 oC;
Coiling Temperature in the range of 550 - 620 oC.

A still further aspect of the present invention is directed to said process for producing high strength corrosion resistant structural steel in the form of sheet coil comprising
Coil thickness: 2.5 – 4.0 mm;
Coil width: 900- 1500 mm;
Yield strength: 450 – 530 MPa;
UTS: 570 – 650 MPa;
% Elongation: 20 – 28; and
sheet coil thus obtained having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURE

Figure 1: is the flow chart for the process of producing hot rolled high strength corrosion resistant structural steel according to the present invention having increased strength even at lower thickness (< 3mm), showing the steps involved in the process.

DETAILED DESCRIPTION OF THE INVENTION WITH REFRENCE TO THE ACCOMPANYING FIGURE

The present invention relates to high strength hot rolled corrosion resistant steel sheets for structural application wherein a selective combination of Mn, Nb, Ti and Cu along with carbon is used in said steel composition, slabs obtained through continuous casting are subjected to controlled hot rolling followed by cooling, to achieve increased strength even at lower thickness (< 3mm) with improved surface quality suitable for application in fabrication of railway freight wagon body and similar applications.

In the present invention a unique composition (comprising Mn, Nb, Ti & Cu) of corrosion resistant structural steel has been developed. In comparison to the general corrosion resistant structural steel manganese was increased from 0.9 to 1.3 %. Manganese increases the tensile strength. However, the amount of increase in the property is dependent upon the carbon content, hence in the present work to have optimum benefits, Mn was restricted to 1.3%. Additional strength has been achieved by addition of Niobium through grain refinement as it restricts austenitic grain growth. It also ties up nitrogen to inhibit strain ageing which is an added advantage for rolling to thinner sections. Nb was added up to an optimum level of 0.04. Surface quality and corner cracks issue has been addressed by addition of Titanium to the base composition. It is known that titanium addition reduces corner cracks and improves surface finish under certain conditions of carbon, manganese and nitrogen by improving the hot ductility. Fine titanium nitride (TiN) particles precipitated in steel slabs suppress the growth of austenite grains on heating and consequently improve the low-temperature toughness of controlled-rolled steel plates by grain refinement. Ti was added up to 0.03.
This new steel grade is made through converter steel making and ladle heating furnace. It is further cast into slabs through continuous casting process. These slabs are processed through re-heating furnace and hot rolling followed by controlled cooling. The process route followed in sequence is as follows:
(i) Steel making by LD Converter
(ii) Secondary steel making: Ladle Heating Furnace.
(iii) Alloying addition done after complete de oxidation.
(iv) Continuous slab casting
(v) Re-heating, Hot rolling, ROT cooling and Coiling with set optimum processing parameters

Thus according to the present invention, a hot rolled high strength structural steel with improved surface quality has been produced comprising the steps of:
(i) selectively involving alloying elements in steel making by LD Converter and Secondary steel making in Ladle Heating Furnace with alloying addition done after complete de oxidation, resulting in Chemical Composition of new hot rolled high strength corrosion resistant structural steel as presented in following Table I:
Table I:
%-C %-Mn %-S max. % Cu %-P max. %-Si max %-Al %-N, max. %-Nb %-Ti
0.15-0.20 1.2 – 1.4 0.020 0.20-0.040 0.020 0.15- 0.40 > 0.02 0.008 0.030-0.040 0.02- 0.030

(i) continuous slab casting;
(iii) hot charging directly into re-heating furnace;
(iv) re-heating the slabs followed by rough rolling structure under hot rolling;
(v) followed by Finish Rolling, ROT cooling and coiling.
Accompanying Figure 1 is the flow chart for the process of producing hot rolled high strength corrosion resistant structural steel according to the present invention having increased strength even at lower thickness (< 3mm), showing the steps involved in the process.

While deciding on the above alloy composition to achieve the desired properties in the resulting steel, following considerations were followed:
C: less than 0.22% by weight

Carbon is an element that gives the strength and hardness to steel. At a C content of 0.22 % or more, steel becomes hard and deteriorates weldability. Accordingly, the C content is kept less than 0.22% and preferably in a range of 0.15 to 0.20% by weight.

Si: less than 0.40 % by weight

Si is added as de-oxidiser and acts to reinforce steel, and a necessary amount of Si is added to steel in accordance with the intended strength of the steel. Incorporation of excess Si exceeding 0.4 % significantly deteriorates toughness. Accordingly, the Si content is 0.04 % or less and is preferably in a range of 0.15 to 0.40% by weight.

Mn: 1.5% or less by weight

Manganese is an element that improves the strength of steel material and is an important element in high strength steels However, the castability and toughness is deteriorated if Mn is contained exceeding 1.5%. Accordingly, the Mn content is 1.5% or less and preferably in a range of 1.2 to 1.4% by weight.

P: 0.020% or less by weight

Phosphorus is an element that improves the atmospheric corrosion resistance of the structural steel material. However, adding too much P to steel in an amount of larger than 0.020 % by weight will have some negative influences on the desired strength of the steel sheets. Accordingly, the P content is 0.020% or less and preferably less than 0.015% by weight.

S: 0.020% or less by weight

At a sulfur content exceeding 0.020%, strength of rolled steel is deteriorated. Accordingly, the S content is restricted to 0.020% or less and preferably less than 0.010% by weight.

N: not larger than 0.008 % by weight

More desirably, N should be as lesser as possible for better mechanical properties of the steel. N in steel in an amount of not larger than 0.008 % by weight would not have any significant negative influences on the strength and other properties of the steel. Therefore, the N content of steel is defined to be not larger than 0.008 % by weight, but preferably smaller than 0.005 % by weight.

Al: 0.02% or more

Aluminium is an element needed in deoxidization during steel making and refines grain size. The Al content needs to be 0.02% or more to achieve this effect. Thus, the Al content is 0.02% or more and preferably in a range of 0.02 to 0.05%.

Cu: 0.2% or more and 0.5% or less

Copper reduces the size of rust grains to help form dense rust layer and thus has an effect of improving the atmospheric corrosion resistance of the structural steel material. This effect is achieved when the Cu content is 0.2% or more. At a Cu content exceeding 0.5%, the cost will rise due to the increased consumption of Cu and also causes hot shortness. Accordingly, the Cu content is 0.2% or more and 0.5% or less and preferably in a range of 0.20 to 0.40%.

Nb: 0.03% or more

Niobium is a important element and has an significant effect on improving the strength of steel. The Nb content needs to be 0.03% or more to fully bring this effect. Accordingly, the Nb content is 0.03% or more and preferably in a range of 0.03 to 0.04%.

Ti: 0.02% or more

Titanium is an element needed to increase the strength and surface properties. The Ti content needs to be 0.02% or more to fully bring this effect. Thus, Ti content is 0.02% or more and preferably in a range of 0.02 % to 0.03% by weight.

The balance is Fe and unavoidable impurities.

In the above process Hot rolling and coiling parameters specified for processing of the hot rolled high strength corrosion resistant steel with improved surface quality are presented in following Table 2:
Table 2:
1 Furnace Temperature (Soaking) 1200 – 1250 oC
2 Finish Rolling Temperature 810 – 860 oC
3 Coiling Temperature 550 - 620 oC

The Hot rolled high strength corrosion resistant steel sheets so produced is in the form of coils having:
Coil thickness in the range of 2.5 – 4.0 mm;
Coil width in the range of 900 -1500 mm.

The hot rolled coils are inspected manually. Samples are collected from the coils in longitudinal and transverse direction. These samples are tested in laboratory for cleanliness of steel and mechanical properties.

Mechanical tests and Metallography

The tensile properties (yield strength, ultimate tensile strength) are measured using test specimens with 50 mm gauge length, fitted with a class 1 extensometer on a universal testing machine. The tensile test was conducted as per JIS Z2241 with sample number 5 as per JIS Z2201. The strain rate was kept 5mm/sec up to yield point. All tests are performed at room temperature.

Metallographic analysis is carried out rate the cleanliness of steel. Metallographic samples prepared are polished and etched with 5% nital. A simple light optical microscope is used to record the size of the grains comprising the material.

The high strength corrosion resistant steel sheets produced following the above process according to the present invention found to have the following properties of material in supply condition:
Yield Strength = 450 – 530 MPa;
UTS = 570 – 650 MPa;
% Elongation = 20 - 28

The hot rolled high strength corrosion resistant steel sheets so produced is having uniformly distributed fine eqi-axed grains with Grain size of ASTM 8 or finer.

The effect of various proportions of the alloying elements used in various trials on yield strength and ultimate tensile strength to arrive at the desired composition of the hot rolled corrosion resistant structural steel according to the present invention specifically involving selective use of a selective combination of Mn, Nb, Ti & Cu are illustrated in the following table 3.

Table 3:

%-C %-Mn %-S max. % Cu %-P max. %-Si max %-Al %-N, max. %-Nb %-Ti YS UTS Remarks
0.18 1.3 0.006 0.31 0.009 0.03 0.03 0.006 0.036 0.027 450 570 Invention
0.17 1.28 0.08 0.34 0.01 0.028 0.032 0.0055 0.035 0.024 458 581 Invention
0.172 1.21 0.005 0.006 0.012 0.192 0.044 0.0048 0 0 390 475 Comparison
0.215 1.39 0.005 0.008 0.019 0.15 0.045 0.0055 0 0 420 510 Comparison
0.2 1.4 0.005 0.007 0.015 0.15 0.042 0.0055 0 0 420 500 Comparison

It can be observed from the above data that improvement in yield strength in the new grade in comparison to similar category old grade are as follows:

Old Grade New Grade
YS, MPa 390 450
UTS, MPa 475 570

It is thus possible by way of the present invention to provide a hot rolled high strength corrosion resistant structural steel with improved surface quality suitable for application in freight wagon fabrication and the like application wherein a selective composition comprising Mn, Nb, Ti & Cu not only ensure desired improved strength even at lower thickness and good corrosion resistance but also provide improved surface quality. The present invention is capable to provide following advantageous features:
(i) The new grade is most suitable for low thickness (< 3mm) high strength corrosion resistant structural steel for railway freight wagon body.
(ii) Improvement in yield strength in the new grade in comparison to similar category old grade has been observed.
(iii) The innovative combination of Mn+Nb+Ti+Cu in a corrosion resistant structural steel helped in achieving higher strengths at lower thickness of hot rolled coils.
(iv) Copper is added for corrosion resistance, Mn+Nb for increasing the strength and Ti for surface finish.
(v) Optimum chemistry of the steel resulted in consistent and high UTS required for thin sheets to be used in railway wagons.
(vi) Judicious selection of alloying elements in combination with copper helped achieve high strength of the steel without compromising the surface quality and ductility.
(vii) Optimum values of carbon, manganese, copper, niobium and titanium in the steel resulted into consistent and adequate strength required by railway wagon manufacturers.
(viii) The new grade of steel possesses a favourable combination of mechanical properties and surface quality suitable for railway wagon.
(ix) Due to minimum usage of various ferroalloys, the cost of production of the new grade steel is kept minimum.

We Claim:

1. Hot rolled high strength corrosion resistant structural steel comprising
C: 0.15 – 0.22 % by wt. preferably 0.15-0.20% by wt.;
Mn: 1.2 to 1.5 % by wt. preferably 1.2 – 1.4% by wt.;
S: upto 0.020% by wt.preferably < 0.010% by wt.;
Cu: 0.20 – 0.5 % by wt. preferably 0.20-0.040% by wt.;
P: upto 0.020% by wt. preferably < 0.015% by wt.;
Si: 0.15- 0.40% by wt;
Al: at least 0.02% by wt.preferably 0.02 to 0.05% by wt.;
N: upto 0.008% by wt.preferably <0.005 % by wt.;
Nb: atleast 0.03 % by wt. preferably 0.030-0.040% by wt.;
Ti: atleast 0.02% by wt preferably 0.02- 0.030% by wt.; and
balance is Fe.
enabling higher strength at lower thickness.

2. Hot rolled high strength corrosion resistant structural steel as claimed in claim 1, obtained in the form of coil having
Coil thickness: 2.5 – 4.0 mm;
Coil width: 900-1500mm;
Yield strength: 450 – 530 MPa;
UTS: 570 – 650 MPa.

3. Hot rolled high strength corrosion resistant structural steel as claimed in claim anyone of claims 1 or 2 having
% Elongation: 20 – 28; and
sheet coil having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

4. Hot rolled high strength corrosion resistant structural steel sheets as claimed in anyone of claims 1 to 3 having surface quality free of corner cracks suitable for structural applications.

5. A process for the production of high strength corrosion resistant structural steel as claimed in anyone of claims 1 to 4 comprising steel making involving selective alloying elements comprising
C: 0.15-0.20% by wt.;
Mn: 1.2 – 1.4% by wt.;
S: upto 0.020% by wt.;
Cu: 0.20-0.040% by wt.;
P: upto 0.020% by wt.;
Si: 0.15- 0.40% by wt;
Al: at least 0.02% by wt.;
N: upto 0.008% by wt.;
Nb: 0.030-0.040% by wt.;
Ti: 0.02- 0.030% by wt.; and
balance is Fe.
said alloying elements including selective combination of Mn, Nb, Ti & Cu providing for higher strength at lower thickness.

6.A process as claimed in claim 5 wherein said steel making comprises:

steel making by L.D. converter;
secondary steel making Ladle Heating Furnace;
addition of alloying element after completion of de oxidation;
continuous slab casting;
re-heating hot rolling, ROT cooling and coiling with set optimum processing parameters.

7.A process as claimed in claim 5 or 6 comprising controlled hot and cold rolling carried out comprising
Furnace Temperature (Soaking) in the range of 1200 – 1250 oC;
Finish Rolling Temperature in the range of 810 – 860 oC;
Coiling Temperature in the range of 550 - 620 oC.

8. A process as claimed in anyone of claims 5 to 7 comprising for producing high strength corrosion resistant structural steel in the form of sheet coil comprising
Coil thickness: 2.5 – 4.0 mm;
Coil width: 900- 1500 mm;
Yield strength: 450 – 530 MPa;
UTS: 570 – 650 MPa;
% Elongation: 20 – 28; and
sheet coil thus obtained having uniformly distributed fine eqi-axed grains of grain size ASTM 8 or finer.

Dated this the 3rd day of January, 2014
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT

TITLE: HOT ROLLED HIGH STRENGTH CORROSION RESISTANT STRUCTURAL STEEL AND A PROCESS FOR ITS PRODUCTION.

The present invention relates to high strength corrosion resistant structural steel with improved surface quality for railway freight wagon body and similar applications where savings in weight (mass) and added durability are matters of concern and a process for its production. The composition of the invented steel grade involving optimum combination of Mn, Nb, Ti & Cu in a corrosion resistant structural steel helped in achieving higher strengths even at lower thickness <3mm of hot rolled coils. Copper is added for corrosion resistance, Mn & Nb for increasing the strength and Ti for surface finish. Judicious selection of alloying elements in combination with copper helped achieve high strength of the steel without compromising the surface quality eliminating corner cracks, favourable ductility and having consistent and high UTS favouring advantageous application required for thin sheets such as those used in railway wagons.
(Figure 1)