CLIAMS:
1. Weather resistant steel grade composition for structural applications comprising
C: 0.05 to 0.09 wt%,
Si: 0.02 to 0.90 wt%,
Mn: 0.20-0.50 wt%,
Cr: 0.35 to 0.40 wt%,
Cu: 0.20 to 0.50 wt%,
Ni: 0 to 0.31 wt%,
P: 0.10 to 0.20 wt%,
Al: 0.01 to 0.05 wt%,
and the balance of Fe and inevitable impurities.

2. Weather resistant steel grade composition as claimed in claim 1 having
Yield strength (YS) in the range of 350 to 425 MPa;
Ultimate tensile strength (UTS) in the range of 480 to 560 MPa;
% elongation (EL) at failure in the range of 25 to 40;and
UTS/YS ratio greater than 1.25.

3. Weather resistant steel grade composition as claimed in anyone of claims 1 or 2 providing Charpy V-notch (CVN) energy in the range of 47J to 120J at room temperature (RT)(270C) and 18-76 J at 00C .

4. Weather resistant steel grade composition as claimed in anyone of claims 1 to 3 wherein Charpy V-notch (CVN) energy is in the range of 45 to 68 at -200C based on Phosphorous content in the range of 0.12 to 0.15 wt%.

5. Weather resistant steel grade composition as claimed in anyone of claims 1 to 3 comprising high Phosphorous content in the level of 0.19-0.20wt% having % of EL greater than 25.

6. Weather resistant steel grade composition as claimed in anyone of claims 1 to 4 comprising having impedence(Zreal) or polarization resistance of the rust higher than low carbon steel.
7. Weather resistant steel grade composition as claimed in claim 6 wherein weather resistance characteristics is based on phosphorus in presence of Cr and Cu preferably comprising
Cu: 0.35 wt%,
Ni: 0.31 wt%,
P: 0.15 wt%,

8. Weather resistant steel grade composition as claimed in claim 6 wherein weather resistance characteristics is based on phosphorus in presence of Cr and Cu preferably comprising
Cu: 0.24 wt%,
Ni: 0.17 wt%,
P: 0.19 wt%,

9. A process for the production of High phosphorus weather resistant steel grade for structural applications as claimed in claims 1 to 8 comprising the steps of

(i) providing heats in melting furnace having selective composition(in weight %) comprising C: 0.05 to 0.09%, Si: 0.02 to 0.90 %, Mn: 0.20-0.50%, Cr: 0.35 to 0.40%, Cu: 0.20 to 0.50%, and Ni: 0 to 0.31%, P: 0.10 to 0.20%, Al: 0.01 to 0.05 %, and the balance of Fe;
(ii) obtaining steel sheets from steel having above composition by carrying out rolling in two stages after soaking at 11500C for 2.5 hours and finishing at temperature maintained between 900-9500C.

Dated this the 4th day of December, 2013
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
,TagSPECI:FIELD OF THE INVENTION

The present invention relates to weather resistant steel composition. More particularly, the present invention is directed to high phosphorous weather resistant steel grade with required strength properties favourable for structural application. Importantly, the developed steel grade provides excellent weather resistance property of steels which is derived from phosphorus in presence of Cr and Cu. The selective combination of alloying elements/composition (in weight %) of the steel grades according to the present invention is obtained as high-strength hot-rolled/cold-rolled steel sheet involving controlled rolling and finishing temperatures. The resulting steel grade shows yield strength (YS) and ultimate tensile strength (UTS) are in the range of 350 to 425 MPa and 480 to 560 MPa, respectively while the % elongation (EL) at failure is in the range of 25 to 40 and UTS/YS ratio is greater than 1.25, alongwith favourable CVN energy at room temperature and low temperature, overcoming the conventional limitation due to cold-shortness effect of phosphorous in steel. Atmospheric corrosion resistance of claimed composition is excellent which is supported with the electrochemical impedance (EIS) data of rust samples after 4 months atmospheric exposure. Phosphorus increases strength of these steel grades due to its solid solution strengthening effect. Although phosphorus content is very high in the range of upto 0.2 wt% in some steels, ductility of all steels is quite sufficient (% of EL is greater than 25) making such steel grades suitable for various industrial applications including structural application.

BACKGROUND OF THE INVENTION

A significant amount of economic loss occurs due to premature failure of engineering components and structures in industry and infrastructures because of degradation by corrosion. Atmospheric corrosion is more severe in the chloride and sulphur dioxide laden environments. The use of weathering steel instead of carbon steel, having less than 3 wt.% alloying elements of Cr, Ni, Cu, and P, has been most effective and economical way of mitigating atmospheric corrosion. These weather resistant steel provide optimum balance of corrosion resistance and strength due to the presences of various alloying elements. Like CORTEN steel, traditionally, Ni has been added to the Cu containing weathering steel (equal amount or less than that of Cu content) for reducing hot-shortness of steel where Ni increases solubility of Cu in austenite. As Ni is costly, and Si has been reported as having similar effect of reducing hot-shortness of steel, alloy design of this innovation has been made with a range of Si contents. Similarly, phosphorus content has been also varied and higher phosphorus content alloy has been designed with low Cu content. However, phosphorus content above 0.05 wt.% has always been a challenge to the metallurgist for its cold-shortness effect.

There has been therefore a need in the art to developing a steel grade which would overcome the limitations of the prior art relating to use of higher weight percent of phosphorous in steel due to cold-shortness effect, ensure desired weather resistivity while also providing desired strength and elongation properties required for structural applications. To address this effect, the claims of this innovation include the results obtained from the room temperature tensile test and Charpy V-notch impact energy test at low and room temperature and the samples exposed to atmospheric corrosion are also subjected to electrochemical impedance spectroscopy (EIS) test to confirm desired weather resistance.

OBJECTS OF THE INVENTION

The basic object of the present invention is thus directed to providing weather resistant steel grades having phosphorous upto 0.2wt% with required strength properties favourable for structural application.

A further object of the present invention is directed to providing weather resistant steel grades which is obtained as high-strength hot-rolled/cold-rolled steel sheet involving controlled rolling and finishing temperatures.

A further object of the present invention is directed to providing weather resistant steel grades wherein excellent weather resistance property of steels has been derived from phosphorus in presence of Cr and Cu.

A further object of the present invention is directed to providing weather resistant steel grades wherein the cold shortness effect in steel due to presence of higher weight percent of phosphorous is reduced and ductility and impact strength is at room temperature and low temperature is sufficiently high making it suitable for structural applications.

A further object of the present invention is directed to providing weather resistant steel grades wherein effect of reducing hot-shortness of steel is achieved using Si of varied proportion avoiding use of costly Ni in higher proportion.

A still further object of the present invention is directed to providing weather resistant steel grades wherein phosphorus increases strength of steel due to its solid solution strengthening effect.

A still further object of the present invention is directed to providing weather resistant steel grades wherein beneficial effect of phosphorus on weathering resistance properties is achieved by the effect of phosphorus balancing the low Cu and Ni content.

Yet another object of the present invention is directed to providing high phosphorous weather resistant steel grades wherein said steels can be used for many structural applications due to high CVN energy at room temperature(RT) and 00C.

A further object of the present invention is directed to providing high phosphorous weather resistant steel composition/grades having excellent atmospheric corrosion resistance of claimed composition supported with the electrochemical impedance (EIS) data of rust samples after substantial atmospheric exposure.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is thus directed to weather resistant steel grade composition for structural applications comprising
C: 0.05 to 0.09 wt%,
Si: 0.02 to 0.90 wt%,
Mn: 0.20-0.50 wt%,
Cr: 0.35 to 0.40 wt%,
Cu: 0.20 to 0.50 wt%,
Ni: 0 to 0.31 wt%,
P: 0.10 to 0.20 wt%,
Al: 0.01 to 0.05 wt%,
and the balance of Fe and inevitable impurities.
A further aspect of the present invention is directed to weather resistant steel grade composition having
Yield strength (YS) in the range of 350 to 425 MPa;
Ultimate tensile strength (UTS) in the range of 480 to 560 MPa;
% elongation (EL) at failure in the range of 25 to 40;and
UTS/YS ratio greater than 1.25.

A still further aspect of the present invention is directed to weather resistant steel grade composition providing Charpy V-notch (CVN) energy in the range of 47J to 120J at room temperature (RT)(270C) and 18-76 J at 00C.

Yet another aspect of the present invention is directed to weather resistant steel grade composition wherein Charpy V-notch (CVN) energy is in the range of 45 to 68 at -200C based on Phosphorous content in the range of 0.12 to 0.15 wt%.

A further aspect of the present invention is directed to weather resistant steel grade composition comprising high Phosphorous content in the level of 0.19-0.20wt% having % of EL greater than 25.

A still further aspect of the present invention is directed to weather resistant steel grade composition comprising having impedence (Zreal) or polarization resistance of the rust higher than low carbon steel.

A still further aspect of the present invention is directed to weather resistant steel grade composition wherein weather resistance characteristics is based on phosphorus in presence of Cr and Cu preferably comprising
Cu: 0.35 wt%,
Ni: 0.31 wt%,
P: 0.15 wt%,

A still further aspect of the present invention is directed to weather resistant steel grade composition wherein weather resistance characteristics is based on phosphorus in presence of Cr and Cu preferably comprising
Cu: 0.24 wt%,
Ni: 0.17 wt%,
P: 0.19 wt%,

Yet another aspect of the present invention is directed to a process for the production of High phosphorus weather resistant steel grade for structural applications as described above comprising the steps of

(i) providing heats in melting furnace having selective composition(in weight %) comprising C: 0.05 to 0.09%, Si: 0.02 to 0.90 %, Mn: 0.20-0.50%, Cr: 0.35 to 0.40%, Cu: 0.20 to 0.50%, and Ni: 0 to 0.31%, P: 0.10 to 0.20%, Al: 0.01 to 0.05 %, and the balance of Fe;
(ii) obtaining steel sheets from steel having above composition by carrying out rolling in two stages after soaking at 11500C for 2.5 hours and finishing at temperature maintained between 900-9500C.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows graphically the Nyquist plots of atmospheric exposure of developed steel samples obtained in 3.5% NaCl solution according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to providing high phosphorous weather resistant steel having compositions/grades having upto 0.2wt % phosphorous with required strength properties favourable for structural application. The composition and properties of the invented steel grades are illustrated with the help of the following exemplary embodiment:

Example:

Different laboratory heats were made using induction melting furnace. Chemical compositions of these heats with variations of Si, P, Cu and Ni are provided in following Table-1 identified with numbers WR1 to WR7. Rolling was carried out in two stages after soaking at 11500C for 2.5 hours. The finishing temperature was maintained between 900-9500C.

Table 1: Chemical compositions in wt.% obtained using OES
Steels C Si Mn S P Cr Cu Ni Al
WR1 0.052 0.43 0.46 0.005 0.145 0.38 0.23 0.23 0.002
WR2 0.09 0.37 0.42 0.01 0.15 0.37 0.35 0.31 0.03
WR3 0.072 0.028 0.20 0.007 0.19 0.38 0.24 0.17 0.03
WR4 0.059 0.82 0.31 0.005 0.12 0.36 0.41 0.13 0.017
WR5 0.06 0.90 0.42 0.005 0.19 0.36 0.41 0.13 0.019
WR6 0.065 0.83 0.35 0.008 0.13 0.36 0.48 - 0.012
WR7 0.068 0.84 0.38 0.005 0.20 0.37 0.48 - 0.005

Composition:
(1) The average composition (in wt %) of investigated steels are provided in Table-1. The composition (in weight %) to be claimed as high-strength hot-rolled/cold-rolled steel sheet: C: 0.05 to 0.09%, Si: 0.02 to 0.90 %, Mn: 0.20-0.50%, Cr: 0.35 to 0.40%, Cu: 0.20 to 0.50%, and Ni: 0 to 0.31%, P: 0.10 to 0.20%, Al: 0.01 to 0.05 %, and the balance of Fe and other inevitable impurities. The excellent weather resistance property of investigated steels has been derived from phosphorus in presence of Cr and Cu, which has been described separately in the following paragraphs.

Mechanical Strength:
(2) Yield strength (YS) and ultimate tensile strength (UTS) of investigated steels are in the range of 350 to 425 MPa and 480 to 560 MPa, respectively (Table 2) while the % elongation (EL) at failure is in the range of 25 to 40 and UTS/YS ratio is greater than 1.25. Phosphorus increases strength of steel due to its solid solution strengthening effect. Although phosphorus content is very high in some steels (WR3, WR5 and WR7), ductility of all steels is quite sufficient (% of EL is greater than 25) for almost all applications.

Table-2: Tensile properties of investigated steels
Steels YS (MPa) UTS (MPa) % EL at failure UTS/YS Ratio
WR1 389 516 38 1.32
WR2 383 525 27 1.37
WR3 362 481 26 1.32
WR4 419 529 35 1.26
WR5 417 553 36 1.32
WR6 373 512 36 1.37
WR7 423 556 35 1.31

Charpy impact testing was carried out at -200C, 00C and room temperature.
(3) Charpy V-notch (CVN) energy is more than 40 Joule at room temperature (RT), more than 15 joule at 00C (Table-3). While phosphorus is well-known for its cold-shortness effect, the investigated steels can be used for many structural applications due to high CVN energy at RT and 00C. The CVN energy is even higher than 27 J at -200C for the phosphorus concentration 0.12%-0.13% steels.
Table-3: Charpy V-notch energy of developed steel grades.
Steels CVN Values in Joule
RT (270C) 00C -200C
WR1 89 73 45
WR2 118 75 63
WR3 65 18 -
WR4 120 76 68
WR5 52 28 -
WR6 82 50 49
WR7 47 28 -

Atmospheric corrosion resistance:

Samples of some selected heats with respect to different phosphorus contents were atmospherically exposed on the roof of a four floored building in an atmospheric exposure stand designed as per ASTM G50-76. After 4 months of exposure, these samples were subjected to electrochemical impedance spectroscopy (EIS) test in 3.5 % NaCl solution for evaluating the resistance of rust. A computer controlled potentiostat (Princeton Applied Research, Ametek-273A, USA) for the EIS test by applying a sinusoidal potential perturbation of 10 mV at the open circuit potentials with frequency sweep from 100 kHz to 100 mHz.

(4) Atmospheric corrosion resistance of claimed composition is excellent. This is supported with the electrochemical impedance (EIS) data of rust samples after 4 months atmospheric exposure, which is presented in accompanying Figure 1. The EIS was conducted in 3.5 % NaCl solution. The high frequency region of the Nyquist plots due to corrosion show semicircle and the X-value of semicircle indicates impedance (Zreal) or polarization resistance of the rust. Figure 1 clearly shows that Zreal of sample WR2 and WR3 is many time higher than that of LC (a low carbon steel). The low frequency region showing a straight line at 450 is called as Warburg constant, which is present in every case due to the infinite diffusion of electrochemical active species (ions) to the surface of samples. It can be noted that WR3 steel contains less Cu and Ni than that of WR2 while P content is higher in WR3. Therefore, the effect of phosphorus probably balance the low Cu and Ni content. This shows the beneficial effect of phosphorus on weathering resistance properties of investigated steels to be claimed.

It is thus possible by way of the present invention to providing a high phosphorous weather resistant steel composition for advantageous and cost effective use in structural application having desired strength, elongation and impact properties suitable for open weather application with improved atmospheric corrosion resistance ensuring longer service life, and thus favouring wide industrial application.