FIELD OF APPLICATION
The present invention relates to an improved closed structural steel grade of C - Mn -
Cu - P alloy group with high corrosion resistance. In particular it relates to hollow steel
sections for structural use with highly improved corrosion resistance when exposed to
the environment, especially in coastal areas.
BACKGROUND OF THE INVENTION
The structural produced at tubes division of a steel plant are exposed to environment
and hence they must possess significant corrosion resistance to atmospheric corrosion.
SUMMARY OF THE INVENTION
To prevent accelerated corrosion of steel component, exposed to atmosphere, it must
have ability to passivate i.e. ability to form adherent oxide layer so that fresh metal
surface is no more exposed to atmosphere for further corrosion. Alloy content and
environmental conditions are key factors influencing the formation of an adherent oxide
film on steel component.

Legault-Leckie relation for predicting atmospheric corrosion resistance in terms
of atmospheric corrosion resistance index (I) is given as:
I = 26,01(%Cu) + 3.88 (%Ni) + 1.20 (%Cr) + 1.49(%Si) + 17.28(%P) +
7.29(%Cu)(%Ni) -9.10((%Ni)(%P) - 33.39(%Cu)2
Known closed structurals are as per Indian Standard IS 4923 (Indian Standard
for hollow steel sections for structural use). According to present specification
the hollow sections can be manufactured from steel made by any approved
process which, shall show not more than 0.050 percent sulphur and no more
than 0.050 percent of phosphorus and copper (if added) must be in a range of
0.20 to 0.35 percent.
The main object of the present invention therefore is to provide hollow steel
sections for structural use having alloy content with improved corrosion
resistance when exposed to the environment for example in coastal areas.
Based on the cost of alloying for improvement in corrosion resistance of existing
structurals, in the present invention copper and phosphorus combination was
found to be the best. Moreover, such alloying maintains the strength, ductility
and weldability and the materials used for alloying are permitted by IS 4923.

The existing steel grade used for closed structural is with 0.006 % copper and
0.012 % phosphorus. In the present invention the alloying is done with copper
and phosphorus content of 0.24 % and 0.27 % respectively.
This copper and phosphorus combination was found to improve the corrosion
resistance by more than twice the normal value in a simulated coastal
environment during accelerated tests.
Thus the present invention provides an improved closed structural steel grade
with high corrosion resistance obtainable by alloying steel with copper and
phosphorus without sacrificing properties like strength, ductility and weldability.
DETAILED DESCRIPTION
In the present invention the effect of alloying of copper and phosphorus on the
strength, ductility, weklability and electrochemical behavior was studied. The
comparative mechanical properties of existing steel grade and Cu and P alloyed
steel grade of the present invention are shown in Table 1. The ease of
weldability was also checked for Cu and P alloyed steel grade and was found to
be not inferior to existing steel grade, being used for production of structurals.

The existing structural were analyzed in 5% NaCI solution, to simulate coastal
Indian conditions. The fact of improvement in corrosion resistance by alloying of
copper and phosphorus was also established with laboratory heat. The plant
trial was taken and the new product was also studied. The chemistry of existing,
and new product is listed in Table 2. The corrosion rate obtained in 5% NaCI
solution in accelerated tests is shown in Table 3.


Table 3: The corrosion rate of different products in 5% NaCI solution in
accelerated corrosion test.

The resistance to accelerated corrosion in NaCI atmosphere due to alloying of
copper and phosphorus can be explained as following.
Copper reacts with CI- forming CuCI2.3Cu(OH)2, an insoluble layer, which deposits
on steel and retards corrosion. Phosphorus acts like inhibitor forming complex
oxides responsible for protection.
Moreover, in industrial atmosphere where sulphur dioxide(CO2) prevails the
copper forms CuSO4.3Cu(OH)2, an adherent layer.

Thus similar properties which make steel grade suitable to be formed in closed
structurals, like strength, ductility and weldability, are retained in the improved
corrosion resistant structurals of the present invention. These properties can
even be improved with small associated added cost of the suggested steel grade,
compared to the existing grade.
The suggested steel grade with alloying as mentioned for dosed structural
production which posses more than twice corrosion resistance than existing one,
therefore, may replace the existing steel grade for closed structurals.

WE CLAIM
1. An improved steel grade with high corrosion resistance for structural use having
composition in weight % carbon 0.14, Manganese 0.78, Copper 0.24 and
Phosphorus 0.27.
2. A steel grade as claimed in claim 1, wherein the steel composition is
characterize evaluated for corrosion resistance in 5 % Nacl solution on
simulated coastal conditions in accelerated corrosion test and corrosion rate was
observed to be 0.280 mm / year as compared to corrosion rate of 0.789 mm /
year of the existing steel of composition in weight % C - 0.12, Mn - 0.77, Cu -
0.006 and P - 0.012.
3. A steel as claimed in claims 1 and 2, wherein improved resistance to corrosion
of the steel is attributed due to formation of cucl2.3 CU (OH)2 insoluble layer
deposited on the structure surface on reaction of copper with chlorine.

The invention comprises of an improved steel grade with high corrosion resistance for
structural use having composition in weight % carbon 0.14, Manganese 0.78, Copper
0.24 and Phosphorus 0.27.