Claims:WE CLAIM

1. A process for non-micro alloyed steel plates with microstructure of ferrite – pearlite comprising the steps of:
(i) heating the steel to a soaking temperature 1200 - 1240° C for transforming its structure, wherein the retention time is 3 hours 30 minutes at the temperature 1200-1240° C constrained for the grain refinement;
(ii) Performing hot rolling at temperatures at 1160 –1000° C for 8 – 12 pass with a draft per pass is 10- 20% as per the required thickness in roughing stand;
iii) Performing finish rolling at temperatures at 850– 870° C for 6 pass with a draft per pass is 10- 16% in finishing stand;
(iv) Performing air cooling up to ambient temperature at a cooling rate of 1.5.° C/s to 1.0° C/s.

2. The composition further includes 0.017 to 0.024 wt% of phosphorus, 0.012 to 0.018wt% of Sulphur as impurities.

4. The process for non-micro alloyed steel plates as claimed in claim 1, wherein the mechanical properties of the steel obtained is in conformity of the grade of IS 2062 E350B0/C grade steel.

5. A non-micro alloyed steel plate with microstructure of ferrite – pearlite with steel composition of 0.16 to 0.18 wt% of carbon, 0.21 to 0.31 wt% of silicon, 1.42 to 1.50 wt% of manganese and balance essentially iron made with process as claimed The process for non-micro alloyed steel plates as claimed in claim 1, wherein the steel composition comprise of 0.16 to 0.18wt% of carbon, 0.21 to 0.31wt% of silicon, 1.42 to 1.50 wt% of manganese and balance essentially iron.

6. The non-micro alloyed steel plate as claimed in claim 5, wherein the mechanical properties of the steel obtained is in conformity of the grade of IS 2062 E350B0/C steel.

Dated: this 27th day of March, 2018.
, Description:NON MICRO-ALLOYED STEEL PLATES WITH HIGH STRENGTH, TOUGHNESS AND WELDABILITY

FIELD OF THE INVENTION:

The present invention relates to a development of non-micro-alloyed steel plates with high strength, toughness and weldability. More particularly, the present invention related to a product development with Innovative alloy design using optimum C, Si and Mn content

BACKGROUND OF THE INVENTION:

The development of hot-rolled plate steels as alternatives to more expensive heat treated steels for structural applications has been of long-standing interest to the steel industry. This interest has been heightened by the increasing costs of energy required in heat treatment operations. Progress has been made in the development of tough, weldable, micro alloyed controlled-rolled plate products as substitutes for normalized steels in applications requiring yield strengths that do not exceed about 70 ksi. Less progress has been made in providing as-rolled plate for higher strength applications, e.g. applications requiring yield strengths in excess of 80 ksi where quenched and tempered steels now predominate.

In a prior art approach, a killed low-alloy steel containing 0.12-0.20% carbon, 1.10-1.65% manganese, 0.05-0.20 vanadium, 0.005-0.025% nitrogen and 0.60% maximum silicon. In order to possess the desired properties of good toughness, weldability and formability combined with a yield strength in excess of 80 ksi, this steel is hot finished in a temperature range of 1550°-1650° F., cooled at a rate of from 20°-135° F. per second, and collected, as by coiling or piling, within a temperature range of from 1025°-1175° F. The steel must be water quenched to achieve the cooling rate that is necessary to obtain yield strengths of at least 80 ksi and must be collected above a minimum temperature (1025° F.) to avoid the formation of lower transformation products, e.g., bainite, in the microstructure.

Structural steel plates conforming to IS 2062 E350 are used in a wide range of applications requiring high strength, toughness and weldability. Normally, micro-alloying additions of Nb, V and Ti are made singly or in combination to achieve the desired properties. Development of plates without the addition of micro-alloying elements while meeting the specified properties will lower the production cost and would help in higher profitability owing to the saving of costly micro-alloying elements. By R&D initiatives it has become possible to produce IS 2062 E350B0/C grade without micro-alloying with guaranteed Impact toughness values.

OBJECTIVE OF THE INVENTION

It has been found that it is possible to achieve the desired combination of mechanical properties essential for structural applications in a high-strength, low-alloy and without any micro alloy components including costly, Nb, V,Ti etc. as-hot-rolled plate product through a careful balance of chemical composition and controlled-rolling according to a precise schedule. The basic composition contemplated by the invention contains carbon, manganese, and silicon. The composition is critically balanced to optimize carbon equivalence for good weldability and toughness. The specific combination and interrelation of composition and controlled rolling achieve a synergistic effect that is an improvement over the prior art in terms of an as-hot-rolled product having high strength coupled with excellent impact resistance or toughness, weldability, and formability. In particular, the invention makes it possible to produce a plate product which, in the as-hot-rolled condition as per IS 2062 E 350 B0/C grade plates. These properties are obtained in as-hot-rolled plate up to about less than 20mm and / or 20mm – 30mm thickness without the need to cool at rates faster than air cooling.

Therefore the primary object of the present invention is to provide steel plates as per IS 2062 E 350 B0/C grade plates without micro-alloying for enhancing profitability through saving in costly additions of Fe-Nb / Fe-V / Fe-Ti

BRIEF DESCRIPTION OF ACCOMPANYING DRAWING:

Fig. 1 illustrates the picture of a typical Ferrite and Pearlite microstructure of IS 2062E350 grade steel plate of thickness 20mm in accordance with the present invention;

Fig. 2 illustrates the picture of a typical Ferrite and Pearlite microstructure of IS 2062E350 grade steel plate of thickness 30mmin accordance with the present invention;

DETAILED DESCRIPTION

High strength steel plates (YS ? 350 MPa) are normally made by using micro alloying elements like Nb, V or Ti either singly or in combination. Addition of these expensive micro-alloying elements enhances the cost of production. In view of the cost reduction drive, R&D initiatives were taken to produce such high strength steel plates without addition of these micro-alloying elements.

Structural steel plates conforming to IS 2062 are used in a wide range of applications requiring high strength, toughness and weldability. Normally, micro-alloying additions of Nb, V and Ti are made singly or in combination to achieve the desired properties. The product SAILMA 350 is made typically by using micro-alloying elements like Nb and/or V, and TMCP (Thermo-Mechanical Controlled Processing) is used for rolling. This product is equivalent to IS 2062 E350. In view of cost reduction drive, R&D initiatives were taken to achieve the mechanical properties as per specification without addition of the expensive micro-alloying elements like Nb, V and Ti. Under IS 2062 E350 specification, it is possible to produce this product without addition of micro-alloying elements with specific alloy design and controlled processing parameters considering customers requirement provided the specified properties are met, specified chemistry and Mechanical properties are shown in Tables 1 & 2.

Table 1: Steel chemistry as per specification of IS 2062 E 350B0/C grade (wt. %)
C Mn P S Si Ti+Nb+V CE
0.20 max 1.55 max 0.045 max 0.045 max 0.45 max 0.25 max 0.47 max

Table 2: Specified Mechanical Properties of IS 2062 E 350B0/C Grade
Thickness YS, MPa UTS, MPa % Elong. CVN at RT, J (BR, Grade) CVN at 00 C, J (B0, Grade) CVN at -200 C, J (C, Grade)
< 20mm 350 min. 490 min. 22 min. 27 min. 27 min. 27 min.
20-30mm 330 min. 490 min. 22 min. 27 min. 27 min. 27 min.

The typical microstructure of non micro-alloyed IS 2062 E350 grade plates of thickness 20mm and 30mm are shown in Figs. 1 and 2 respectively. Microstructure is basically ferrite - pearlite type with average ferrite grain size varying between 8-14 µm depending on plate thickness. The steel chemistry and processing parameters viz slab reheating temperature, rolling draft schedule and finishing temperature is designed to achieve desired YS, UTS and % elongation and Charpy impact properties.

The process chart was prepared by modifying the existing chemistry and processing parameters for the production of IS 2062 E350B0/C grade with the micro-alloying additions. This included modifications in chemistry and process parameters as shown in Tables 3 & 4, like furnace heating temperature, rolling parameters and finish rolling temperature for achieving properties without addition of micro-alloying elements. The modified process parameters enabled the achievement of the final properties and that met the specifications. The final achieved Tensile and Charpy Impact energy values of steel plates are shown in Table 5. Properties met the specifications by strengthening through combined effect of solid solution strengthening and grain refinement.

Table 3: Steel-chemistry for trial rolling of grade IS 2062 E 350B0/C without micro-alloying (wt. %)
C Mn P S Si Al
Achieved 0.16-0.18 1.42-1.50 0.017-0.024 0.012-0.018 0.21-0.31 0.015-0.027

Table 4: Typical Modified hot rolling parameters for non micro-alloyed IS 2062 E 350B0/Cplates
Parameters Non-microalloyed IS 2062 E350
Soaking Zone (0C) 1200 – 1240
Retention time 3 hours 30 minutes
Slab thickness 220 mm
No of pass rough rolling 8-12
Draft per pass in Roughing Stand 10-20(%)
No. of pass finish rolling 6
Draft per pass in Finishing Stand 10-16 (%)
Finish rolling temperature (0C) 850-870
Post Finish treatment Air Cooled

Table 5: Range of Mechanical Properties achieved in plates
YS, MPa UTS, MPa Total Elongation, % CVN at RT, J CVN at 00 C, J CVN at -200 C, J
< 20mm 351 – 398 497 – 556 22 – 28 67 – 111 54 – 101 29–91
20-30 mm 341 – 429 500 – 577 22 – 25 44 – 136 45 – 94 –

By designed process parameters it has become possible to develop of IS 2062 E350B0 and IS 2062 E350C grade .The new designed process parameters help for achieving the final properties of grade IS 2062 E350B0 and IS 2062 E350C and that meet the specification. IS 2062 E350B0 grade steel has been developed in thickness range from 12-30 mm and IS 2062 E350 C grade steel plates have been developed in thickness range from 12-20 mm in Bhilai Steel Plant (BSP).

Grain refinement is the most useful strengthening mechanism in steel. It is unique in this respect, being the only strengthening mechanism that adds both strength and toughness to the steel. This knowledge was, and still is, a strong stimulus for development of new steel products.

Inventive step:

? Saving of micro-alloying elements Nb, V and Ti
? Development of plates without the addition of micro-alloying elements while meeting the specified properties will lower the production cost and will help in higher profitability. There is saving of around Rs. 2000per ton.
? From rolling point of view profile and flatness of the steel plates will be better than the micro-alloying grade steel plates, because in non micro-alloying grade there will be lower roll force during rolling.

Industrial Applicability:

This type of initiative i.e. steel without micro-alloying elements may be tried in other steel plants.

Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

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