Description:
Field of Invention
The present invention relates to high strength structural steel beams. Particularly, the present invention relates to low cost high strength medium long structural steel beam with yield strength of 410 MPa minimum and excellent low temperature toughness property.

Background and prior arts
Hot rolled steel structural sections are widely used in the commercial and residential construction industry to provide support for high rise buildings and load-bearing walls. Such sections also find extensive application in construction of bridges, flyovers, dams and in mining industry for use as supporting structures in underground mines.

High strength structural steels (steel grades with yield strength of 350 to 410 MPa) are basically hot-rolled carbon-manganese steels with maximum carbon content of up to 0.20 wt% carbon (C) and alloyed with manganese (Mn) content of 1.5 to 1.6 wt%. Micro-alloying elements like niobium (Nb), vanadium (V) and titanium (Ti) are added singly or in combination. Total micro-alloying elements are restricted to not more than 0.25 wt%. Steels with unique alloy compositions can be designed to have greater hardenability and, thus, provide high strength and good toughness in thicker section of hot rolled long structurals such as medium beams.

Niobium, vanadium, and titanium are three typical micro-alloying elements. Niobium is a strong carbide forming element, but it shows relatively little tendency to form oxides, sulphides or solid solutions of these compounds. Vanadium has a similar carbide forming tendency to niobium, but it has a higher solubility in austenite at the typical rolling temperature. The lower solubility of niobium results in a greater strengthening effect on mechanical properties due to grain refinement of the final ferrite-pearlite microstructure. Titanium can only show a tendency to form carbides once all oxygen, nitrogen, and sulphur have been consumed, and the precipitation temperature is typically higher than niobium and vanadium. However, as the alloy contents increase, alloy steels become more expensive and more difficult to weld.

Traditionally, the use of niobium (Nb) has been synonymous with Thermo-Mechanical Controlled Processing (TMCP) and High Strength Low Alloy (HSLA) steels, whereby additions are made to sufficiently delay austenite recrystallisation during rolling. With progressive reductions, a pancaked austenite morphology is developed which transforms to finer ferrite grain and thus leads to higher yield strength and improved low temperature toughness properties in the hot rolled steel.

Until now the use of Nb has not been widely associated with commodity grades (steel grades with yield strength (YS) of <350 MPa) primarily because they do not need higher strength or have any significant low temperature toughness requirements. However, ultra lowNb additions (i.e. Nb ≤ 0.01 wt%) can be used to directly reduce Mn and/ or eliminate V additions in high strength (YS≥350 MPa) structural steels. Importantly, this alternative approach requires no change in existing continuous casting and hot rolling practices (i.e. no TMCP rolling or high temperature reheating practices are required) and can enable significant cost savings to be realized on account of reduced ferroalloy/ microalloy consumption per ton of steel. Also, the mechanical properties such as low temperature Charpy V-notch impact toughness can be significantly improved as a result of reduced macro- and micro- elemental segregation and pearlite banding attributable to manganese (Mn).

US10,612,107 discloses a cold rolled and hot dip steel sheet, with a tensile strength of at least 980 MPa, with yield strength above or equal to 500 MPa, with total elongation above or equal to 8%, the composition consisting by weight percent: 0.05SOLUTION: A steel slab of this high-strength/high-toughness steel comprises, by mass%, 0.01 to 0.20% C, 0.01 to 0.80% Si, 0.20 to 2.50% Mn, 0.020% or less P, 0.0070% or less S, 0.004 to 0.100% sol. Al, one or more elements selected from Ti, Cu, Ni, Cr, Mo, Nb, V, B, Ca, Mg and REM as needed, and the balance Fe with unavoidable impurities. The manufacturing method comprises the steps of: hot-rolling the above steel slab to a plate at such a rolling-finishing temperature as to be an Ar3temperature or higher; subsequently quenching the plate from a range of the Ar3temperature or higher into 400°C or lower; and reheating the plate into a temperature range between Ac1and Ac1+100°C, and preferably into a temperature range between Ac1+40°C and Ac1+100°C. The reheating step includes starting the heat treatment of heating the plate to the reheating temperature at a heating rate of 1°C/s or higher preferably within 180 seconds after a cooling operation in the quenching step has been stopped; and holding the plate in the above temperature range for 90 seconds or shorter.

KR101149184B1 relates to a high strength structural steel having a tensile strength of 750 MPa and a method of manufacturing the same. The steel comprises 0.15 to 0.19 weight% of carbon (C), 0.25 to 0.35 weight% of silicon (Si), 1.05 to 1.15 weight% of manganese, 0.03 wt% or less of phosphorus (P), 0.015 wt% or less of sulfur (S), 0.27 to 0.32 wt% of vanadium (V), 0.06 to 0.08 wt% of titanium (Ti) 0.025% by weight and nitrogen (N): 0.008-0. 013% by weight, and reheating the slab steel containing the remaining Fe and other unavoidable impurities. A slab reheating step comprises a hot rolling step of rolling the steel and a cooling step of cooling the steel.

WO2009125820A1 discloses a process for the production of 780PMa-grade high-tensile -strength steel plates excellent in low-temperature toughness, which comprises heating a steel bloom which contains by mass C: 0.06 to 0.15%, Si: 0.05 to 0.35%, Mn: 0.60 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cu: 0.1 to 0.5%, Ni: 0.1 to 1.5%, Cr: 0.05 to 0.8%, Mo: 0.05 to 0.6%, Nb: less than 0.005%, V: 0.005 to 0.060%, Ti: less than 0.003%, Al: 0.02 to 0.10%, B: 0.0005 to 0.003%, and N: 0.002 to 0.006% to a temperature of 1050 to 1200°C; completing the hot rolling of the bloom at 870°C or above; cooling, after a lapse of 10 to 90 seconds, the obtained plate at a cooling rate of 5°C/s or above from a temperature of 840°C or above to a temperature of 200°C or below; and then tempering the resulting plate at a temperature of 450 to 650°C for 20 to 60 minutes.

KR101290474B1 discloses a structural steel production method according to the present invention (a) carbon (C): 0.14 ~ 0.16% by weight, silicon (Si): 0.25 ~ 0.35% by weight, manganese (Mn): 1.25 ~ 1.35% by weight, phosphorus (P): 0.015% by weight or less, sulfur (S): 0.012% by weight or less, nickel (Ni): 0.1-0.2% by weight, vanadium (V): 0.04-0.06% by weight, aluminum (Al): 0.04-0.06% by weight, nitrogen ( N): reheating a steel slab composed of 0.01 to 0.013% by weight and the remaining iron (Fe) and unavoidable impurities; (b) hot rolling the reheated steel to a finish rolling temperature of 950-1000 ° C .; And (c) cooling the hot rolled steel at an average cooling rate of −5 ° C./sec or less.

KR101267624B1 discloses a structural steel material having a high tensile strength (TS) of 570 MPa or more and excellent resistance to shock at low temperatures and low impact toughness at low temperatures, which can be utilized as a column or beam of a high-rise building, and a manufacturing method there of are disclosed.A for manufacturing a structural steel material according to the invention comprises the steps of: (a) providing a steel sheet comprising 0.09 to 0.11 weight% of carbon, 0.30 to 0.50 weight% of silicon, 1.30 to 1.40 weight% of manganese, (S): 0.012 wt% or less, Ni: 0.05 to 0.15 wt%, molybdenum (Mo): 0.05 wt% or less, vanadium (V): 0.18 to 0.32 wt%, niobium Reheating a steel slab composed of 0.03 to 0.04 wt% of aluminum (Al), 0.005 to 0.015 wt% of aluminum (Al), 100 to 130 ppm of nitrogen (N), and the balance of iron (Fe) and unavoidable impurities; (b) hot rolling the reheated steel material to a finishing delivery temperature (FDT) of 950 to 1050 C; And (c) cooling the hot-rolled steel material.

KR101290380B1 discloses high-strength structural steel manufacturing method according to the present invention (a) carbon (C): 0.17 ~ 0.20% by weight, silicon (Si): 0.25 ~ 0.45% by weight, manganese (Mn): 1.25 ~ 1.40% by weight, phosphorus (P) : 0.015% by weight or less, Sulfur (S): 0.005% by weight or less, Nickel (Ni): 0.1-0.2% by weight, Vanadium (V): 0.28-0.32% by weight, Titanium (Ti): 0.05-0.08% by weight, aluminum (Al): 0.01 to 0.02% by weight, niobium (Nb): 0.04 to 0.05% by weight, nitrogen (N): 0.01 to 0.015% by weight and reheating the steel slab consisting of the remaining iron (Fe) and unavoidable impurities; (b) hot rolling the reheated steel to a finish rolling temperature of 950-1000 ° C .; And (c) cooling the hot rolled steel at an average cooling rate of −5 ° C./sec or less.

KR101290462B1 discloses a structural steel manufacturing method according to the present invention is (a) wt%, carbon (C): 0.17 ~ 0.20%, silicon (Si): 0.25 ~ 0.45%, manganese (Mn): 0.45 ~ 0.5%, phosphorus (P) : 0.03% or less, sulfur (S): 0.03% or less, nickel (Ni): 0.1% or less, chromium (Cr): 0.1% or less, copper (Cu): 0.15% or less, nitrogen (N): 0.005 to 0.01% And reheating the slab steel made of the remaining iron (Fe) and unavoidable impurities. (b) hot rolling the reheated steel to a finishing rolling temperature of 900 to 950 ° C .; And (c) cooling the hot rolled steel to 600 to 700 ° C. by spraying cooling water at an injection pressure of 3 bar or more.

KR101185314B1 relates to a structural steel material having a tensile strength of 750 Mpa high strength and high toughness and a method of manufacturing the same, including carbon (C): 0.17 to 0.20 wt%, silicon (Si): 0.25 to 0.35 wt%, manganese (Mn) : 1.25 to 1.35 wt%, phosphorus (P): 0.03 wt% or less, sulfur (S): 0.015 wt% or less, vanadium (V): 0.25 to 0.35 wt%, titanium (Ti): 0.05 to 0.08 wt% and nitrogen (N): 0.008 to 0.013% by weight, nickel (Ni): 0.1 to 0.15% by weight, chromium (Cr): 0.07 to 0.15% by weight, copper (Cu): 0.15 to 0.2% by weight, niobium (Nb) : Slave reheating step of reheating slab steel, optionally comprising at least one of 0.015 to 0.025% by weight and aluminum (Al): 0.01 to 0.02% by weight, and composed of the remaining Fe and other unavoidable impurities, and hot rolling of the steel The invention relates to a rolling step and a cooling step for cooling the steel.

KR101149251B1 discloses a high strength structural steel material capable of simultaneously satisfying both high strength and low resistance characteristics by reducing boron (B) content of 0.04 to 0.06% by weight and a trace amount of alloying elements such as vanadium (V) and niobium (Nb).

The steel material for high strength structural steel according to the present invention contains 0.1 to 0.15 wt% of carbon (C), 0.3 to 0.5 wt% of silicon (Si), 1.2 to 1.5 wt% of manganese (Mn) 0.15 wt% or less of chromium (Cr), 0.3 wt% or less of copper (Cu), 0.04 to 0.06 wt% of vanadium (V) (Al): 0.02 wt% or less, Niobium (Nb): 0.035 to 0.045 wt%, and nitrogen (N): 0.02 to 0.03 wt% 0.015% by weight or less, and the balance of Fe and other unavoidable impurities.

KR101185296B1 discloses a high strength structural steel and a method of manufacturing the same, which can exhibit yield strength (YS): 460 MPa or more and -5 ° C Charpy impact value: 70 J or more through controlled rolling even at a low vanadium (V) content.

Method for producing a high strength structural steel according to the present invention comprises the steps of (a) reheating a slab steel comprising (C): 0.14 ~ 0.16% by weight and vanadium (V): 0.04 ~ 0.09% by weight; (b) hot-rolling the reheated steel, and controlling the finishing hot rolling temperature (FDT) such that the amount of reduction in the unrecrystallized temperature region (Tnr) is 20 to 45% based on the thickness of the steel before rolling; (c) cooling the hot rolled steel..

KR101546124B1 discloses Hot-rolled steel sheets with high strength based on low carbon content, high impact properties by controlling the content of niobium, molybdenum, and chromium, securing strength, impact and resistance by control of finish rolling temperature and coiling temperature And a manufacturing method thereof. The method for producing a hot-rolled steel sheet according to the invention comprises 0.07 to 0.11 wt% of carbon (C), more than 0 wt% to 0.03 wt% of silicon (Si), 1.2 to 1.4 wt% of manganese (Mn) : Not less than 0 wt% to not more than 0.015 wt%, S: not less than 0 wt% to not more than 0.005 wt%, niobium Nb: 0.005 to 0.015 wt%, Cr: 0.01 to 0.02 wt% Reheating a steel slab plate made of 0.01 to 0.2% by weight of Mo and the balance of Fe and other unavoidable impurities to a slab reheating temperature (SRT) of 1140 to 1200° C; Hot-rolling the reheated slab plate to a finishing delivery temperature (FDT) of 840 to 880° C; And cooling the hot-rolled plate material and winding the hot-rolled plate material.

KR101435258B1 discloses a steel material excellent in strength and low temperature toughness, and a method for producing the same. The method of manufacturing a steel material according to the invention is characterized by comprising 0.16 to 0.20% carbon (C), 0.3 to 0.4% silicon (Si), 1.1 to 1.3% manganese (Mn) (Al): 0.01 to 0.05%, copper (Cu): 0.1 to 0.2%, niobium (Nb): 0.01 to 0.02%, nickel (Ni): 0.15 to 0.25% Reheating a steel slab composed of 0.15 to 0.25% of chromium (Cr), 0.08% or less of molybdenum (Mo), 0.0005 to 0.004% of calcium (Ca), and the balance of iron (Fe) and unavoidable impurities; Hot rolling the reheated steel material; Cooling the hot rolled steel material; And subjecting the cooled steel material to a normalizing heat treatment at 840 to 940C.

KR101277807B1 discloses Silicon (Si): 0.45 to 0.55 wt%, manganese (Mn): 1.35 to 1.45 wt%, copper (Cu); By adding 0.35 to 0.45% by weight, a method for producing a high strength structural steel material capable of satisfying high strength and resistance ratio ratio properties at the same time is disclosed. High strength structural steel manufacturing method according to the invention is carbon (C): 0.14 ~ 0.16% by weight, silicon (Si): 0.45 ~ 0.55% by weight, manganese (Mn): 1.35 ~ 1.45% by weight, phosphorus (P): 0.03 weight % Or less, sulfur (S): 0.015% by weight or less, copper (Cu): 0.35 to 0.45% by weight, vanadium (V): 0.08 to 0.12% by weight, niobium (Nb): 0.035 to 0.045% by weight, nitrogen (N) A slab reheating step of reheating a slab steel composed of 0.015 wt% or less and the remaining Fe and other unavoidable impurities; A hot rolling step of hot rolling the steel; And a cooling step of cooling the steel.

KR102366001B1 provides a high-strength hot-rolled steel material excellent in weldability and formability. According to the invention, the hot-rolled steel material, by weight, carbon (C): 0.16% to 0.20%, silicon (Si): more than 0% to 0.03%, manganese (Mn): 0.8% to 1.0 %, aluminum (Al): 0.01% to 0.05%, niobium (Nb): 0.015% to 0.025%, sum of niobium (Nb), titanium (Ti), and vanadium (V): greater than 0% to 0.04%, chromium Sum of (Cr), molybdenum (Mo), and nickel (Ni): greater than 0% to 0.1%, the sum of copper (Cu) and tin (Sn): greater than 0% to 0.1%, phosphorus (P): 0% Exceeds to 0.02%, sulfur (S): more than 0% to 0.01%, and the balance contains iron (Fe) and unavoidable impurities, tensile strength (TS): 580 MPa to 650 MPa, yield strength (YS): 430 MPa to 530 MPa, yield ratio (YR): 80% to 90%, and elongation (EL): 15% to 35% are satisfied.

There is still a requirement to produce low cost high strength medium long structural steel beam with yieldstrength of 410 MPa minimum and excellent low temperature toughness property.

Object of the Invention
In an object of the present invention to provide a high strength medium long structural steel beams for application in construction industry with minimum yield strength of 410 MPa and low temperature Charpy impact toughness property of 27 Joules minimum at -20oC.

It is also an object of the present invention to provide a method for production ofhigh strength medium long structural steel beams by designing lean steel chemistry and suitable processing parameters.

Summary of the Invention
The following disclosure presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
In an aspect of the present invention, there is provided an alloy composition for making low-cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC, comprising of C: (0.15 to 0.22wt%), Si: (0.15 to 0.40 wt%), Mn: (0.80 to 1.35 wt%), S: (0.03wt% or less), P: (0.03wt% or less), Al: (0.01 to 0.04wt%), V: (0.01 to 0.04wt%),Nb: (0.005 to 0.01wt%) and balance Fe.

In another aspect of the present invention, there is provided a method for production of low cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC, comprises the steps of:

a. preparing an alloy composition of C: (0.15 to 0.22 wt%), Si: (0.15 to 0.40 wt%), Mn: (0.80 to 1.35 wt%), S: (0.03 wt% or less), P: (0.03 wt% or less), Al: (0.01 to 0.04 wt%), V: (0.01 to 0.04 wt%), Nb: (0.005 to 0.01 wt%) and balance Fe;
b. producing steel heats through BOF-LF-BC route;
c. casting the heats continuously into bloom, and
d. soaking of the bloom in reheating furnace and hot rolling of the cast bloom to ISMB 300 rolled section with soaking temperature of 1200-1260°C and finish rolling temperature of 900-1000°C to obtain yield the steel beams.

Brief Description of Accompanying Drawings
Figure 1 illustrates Microstructure of ISMB 300 long steel structural of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC (a) At 1st surface (b) At centre and (c) At 2nd surface for Heat No. 1

Figure 2 illustrates Microstructure of ISMB 300 long steel structural of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC (a) At 1st surface (b) At centre and (c) At 2nd surface for Heat No. 2

Detailed description of the Invention
This invention relates to a method of producing lean chemistry low cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC.

The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of various embodiments of the present disclosure as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the various embodiments described herein can be made without departing from the scope and spirit of the present disclosure. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.

It is to be understood that the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

The terminology used herein is for the purpose of describing particular various embodiments only and is not intended to be limiting of various embodiments. It will be further understood that the terms "comprises" and/or "comprising" used herein specify the presence of stated features, integers, steps, operations, members, components, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, members, components, and/or groups thereof. Also, expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

The method for producing low cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oCcomprises the steps of producing a liquid steel of composition in weight% of C: (0.15 to 0.22 wt%), Si: (0.15 to 0.40 wt%), Mn: (0.80 to 1.35 wt%), S: (0.03 wt% or less), P: (0.03 wt% or less), Al: (0.01 to 0.04 wt%), V: (0.01 to 0.04 wt%), Nb: (0.005 to 0.01 wt%) and balance Fe, continuous casting into bloom, hot rolling of bloom to ISMB 300 rolled section with soaking temperature of 1200-1260oC, finish rolling temperature of 900-1000C to achieve yield strength in the range of 410 MPa to 480 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa, elongation in the range of 20% to 30%, Charpy V-notch impact energy of 120-135 J at room temperature, 55-72 J at 0C and 40-50 J at (-)20C.

Currently, high strength structural steels (steels with YS ≥ 400 MPa) are produced with significant levels of Mn, Nb and V additions for achieving the desired strength and other mechanical properties such as Charpy impact toughness. Notwithstanding the beneficial effects on strength, manganese impairs the transverse Charpy impact toughness of steel due to its macro- and micro-segregation in steel product and associated pearlitic banding in hot rolled structural steel. Also, higher levels of Nb and V microalloying additions add to the cost of processed steel. The present innovation can be used as an alternative for industrial production of ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC with lower processing cost owing to the leaner steel chemistry containing lower Mn, V contents and ultra-low Nb (~80 ppm) microalloying addition. The long structurals thus produced will afford higher UTS/YS ratio and improved low temperature Charpy impact toughness properties, without requiring any major change in hot rolling parameters in the mill. They also offer the dual advantage of weight saving as well as improved low temperature impact toughness for adequate durability of fabricated structures in infrastructural and PEB construction.

Medium long structural steel beams have very high demand in Infrastructure Construction and Pre-Engineered Building (PEB) segments. Such high strength structurals are particularly suited for high altitude construction where they offer dual advantage of weight saving as well as improved low temperature impact toughness for adequate durability. ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC with low processing cost owing to the leaner steel chemistry containing lower Mn, V contents and ultra-low Nb (~80 ppm) microalloying addition can be used effectively to make lighter fabricated structures in infrastructural and PEB construction with adequate stability and longevity.

Accordingly, the present invention provides an alloy composition for making low-cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC, comprising of C: (0.15 to 0.22 wt%), Si: (0.15 to 0.40 wt%), Mn: (0.80 to 1.35 wt%), S: (0.03 wt% or less), P: (0.03 wt% or less), Al: (0.01 to 0.04 wt%), V: (0.01 to 0.04 wt%), Nb: (0.005 to 0.01 wt%) and balance Fe.

The present invention also provides a method for production of low cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC, comprises the steps of:
a. preparing an alloy composition of C: (0.15 to 0.22 wt%), Si: (0.15 to 0.40 wt%), Mn: (0.80 to 1.35 wt%), S: (0.03 wt% or less), P: (0.03 wt% or less), Al: (0.01 to 0.04 wt%), V: (0.01 to 0.04 wt%), Nb: (0.005 to 0.01 wt%) and balance Fe;
b. producing steel heats through BOF-LF-BC route;
c. casting the heats continuously into bloom,
d. soaking of the bloom in reheating furnace and hot rolling of the cast bloom to ISMB 300 rolled section with soaking temperature of 1200-1260°C and finish rolling temperature of 900-1000°C to obtain yield the steel beams.

The method as described herein includes BOF-LF-BC route wherein Liquidus temperature of steel: 1510°C, BOF tapping temperature: 1670°C, and LF dispatch temperature: 1590°C.

The blooms casted in the method described are 350 x 240 mm in size and 9 metres length.

The blooms were soaked at 1240-1260C for 3 to 4.5 hours in walking beam type reheating furnace. The blooms were then rolled to final shape in at a rolling speed of about 2.4 m/s.

The entry temperature is around 1160-1175C and finish rolling temperature is maintained between 900-1000C. The blooms were rolled into ISMB 300 structural sections of 12 metres length.

The ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC produced by the method described herein has yield strength in the range of 410 MPa to 480 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa and elongation in the range of 20% to 30% and has Charpy V-notch impact toughness of 120-135 J at room temperature, 55-72 J at 0°C and 40-50 J at (-)20°C.

The advancements of the present invention includes:
(a) Strength was achieved with 2-3% Mn and other alloying like Cr, Mo, and N etc.
(b) Strength and toughness properties were achieved with 2-3% Cr, 1.5% Mo, 0.05%-0.35% V, 1% Mn, 3% Ni and 1.5% Si.
(c) Properties were achieved modifying hot rolling temperature and cooling rate after rolling.
(d) Properties were achieved by precipitation strengthening and grain refinement.
(e) Strength and toughness properties were achieved by adding Ti, Cu, Ni, Cr, Mo, Nb, V, B, Ca, Mg.
(f) Strength was achieved by adding N, V and Ti.
(g) Properties were achieved by adding Mo, B, N, Nb, V and Ti in combination.
(h) Properties were achieved by adding V, N, Ni and controlled cooling of 5C/sec
(i) Properties were achieved by adding Ni, Cr, Cu, N and spraying cooling water at an injection pressure of 3 bar after hot rolling.
The invention will now be illustrated by way of non-limiting examples:

Examples
Two industrial heats were made with steel chemistry (as shown in Table 1) using leaner alloy design to meet the requirements of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC through Basic Oxygen Furnace (BOF)-Ladle Furnace (LF)-Bloom Caster (BC) route. The two heats each weighing about 120 tons were cast into rectangular cross sectioned blooms of 350 x 240 mm size of 9 metres length with Liquidus temperature: 1510C, BOF tapping temperature: 1670C and addition of measured quantities of FeSi, FeMn, FeNb, FeV ferroalloys in ladle furnace with LF dispatch temperature of 1590C.

Table 1: Chemical composition of steel heats made for producing ISMB 300 long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC
Heat No. Chemical Composition (in wt%)
C Mn S P Si Al V Nb
Heat 1 0.20 1.25 0.021 0.018 0.26 0.034 0.027 0.008
Heat 2 0.21 1.28 0.026 0.033 0.24 0.020 0.029 0.008

The as-cast steel blooms were rolled into ISMB 300 medium long structural steel beams with dimensions as per Indian Standard IS 808: 2021. The blooms were soaked at 1240-1260C for 3 hours in walking beam type reheating furnace and rolled to final shape in 16 stands of Medium Structural Mill (MSM) at a rolling speed of about 2.4 m/s. The entry temperature at Stand #1 was kept around 1160-1175C and finish rolling temperature at Stand #16 was maintained between 950-1000C. The blooms were rolled into ISMB 300 structural sections of 12 metres length.

Mechanical properties of the ISMB 300 rolled steel structurals were evaluated. The tensile test specimens were machined and prepared to dimensions with 50 mm gauge length as per ASTM A370-23 standard. The tensile samples were cut from the flange portion of the structurals. Charpy V-notch impact toughness of the rolled structurals was evaluated using Tinius Olsen instrumented Charpy impact tester at test temperatures of room, 0oC and (-)20oC. Charpy test specimens were machined and prepared to dimensions in accordance with ASTM E23 standard with 10x10x55 mm size and 2 mm, 45o V-notch in L-T orientation to the rolling direction.

Metallographic specimens were sectioned in L-T orientation from flange portion of the rolled structurals and metallographically prepared using conventional grinding-polishing procedures and etched in 2% Nital solution for microscopic observation. The results of metallographic examination and microstructural analysis are described under Section No. 5 of this write-up.

Table 2 shows the mechanical properties of the ISMB 300 rolled structurals from both heats which conformed to the stipulated requirements of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC. The results show significant strength-ductility properties for both the heats in terms of yield strength (YS), ultimate tensile strength (UTS) and elongation (%El) values which are attributable to grain refinement in the microstructure due to Nb and V additions. The Charpy impact energies at room temperature (RT), 0C and (-) 20C are also substantially higher than the stipulated requirement of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC. The enhanced low temperature Charpy impact property of the steel is due to the lower Mn content which led to the decrease in pearlite volume fraction and less pearlite banding in the finished steel microstructure.

Table 2: Mechanical properties of ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-) 20oC

Grade/ Heat No. YS (MPa) UTS (MPa) Elongation (%) Charpy impact energy (J)
RT 0C (-)20C
Aim 410 min. 540 min. 20 min. - - 27 min.
Heat 1 469 628 25.4 120-130 55-65 42-46
Heat 2 461 603 24.5 125-135 65-72 46-50

The ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC have very high demand in Infrastructure Construction and Pre-Engineered Building (PEB) market segments. Such high strength structurals are particularly suited for high altitude construction where they offer dual advantage of weight saving as well as improved low temperature toughness for adequate durability. It was determined that the UTS/YS ratio of the rolled steel structurals was in the range of 1.30 to 1.33, and with this high UTS/YS ratio, these structurals are suitable for application in seismic zones also.

Fig. 1 and 2 show the optical micrographs of the as-rolled ISMB 300 steel sections of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC (made from two separate steel heats) in as-metallographically polished and etched condition. The samples were etched using 2% nital solution. The steels show a ferrite-pearlite microstructure with homogeneity of ferrite grain size from surface to the centre in through thickness direction. Also, there is no prominent pearlite banding in the microstructure, confirming the absence of macro- and micro-segregation attributable to manganese (Mn). The volume fractions of the constituent microstructural phases namely, ferrite and pearlite were evaluated and the results are tabulated in Table 3. The ferrite grain size of the hot rolled steels of ISMB 300 section withYS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC were found to be significantly finer in the range of 11-12 µm, which is due to the grain refinement as a result of Nb and V additions. From the table, it is also clear that volume fraction of pearlite is lower at <25%. The lower pearlite volume fraction is attributable to reduced macro- and micro- segregation of manganese (Mn), which led to reduced pearlite banding in the hot rolled steel microstructure. As a consequence of this decrease in microstructural banding there was improvement in low temperature Charpy V-notch impact energy.

Table 3: Grain size and volume fraction of phases in ISMB 300 medium long structural steel beams

Heat No. Position Grain Size (µm) Volume Fraction (%)
Ferrite Pearlite
Heat 1 Surface 1 11.6 81.92 18.08
Centre 12.5 76.97 23.03
Surface 2 11.6 77.73 22.27
Heat 2 Surface 1 12.0 81.53 18.47
Center 12.3 81.11 18.99
Surface 2 12.9 78.53 21.47

The close control of steel chemistry and parameters employed during the steel processing together contributed to an optimum microstructure, resulting into an excellent combination of mechanical properties comprising higher yield strength and improved low temperature Charpy V-notch impact energy.

Hence, with the innovative leaner steel chemistry containing lower Mn, V contents and ultra-low Nb (~80 ppm) microalloying addition, the hot rolled ISMB 300 medium long structural steel beams adequately fulfilled the properties of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC with higher UTS/YS ratio and improved low temperature impact toughness properties, without requiring any major change in hot rolling parameters of Medium Structural Mill.

The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention, in addition to those described herein, will become apparent to those skilled in the art from the foregoing description and accompanying figures. Such modifications are intended to fall within the scope of the appended claims. The invention is, therefore, to be limited only by the terms of the appended claims along with the full scope of equivalents to which the claims are entitled.
, Claims:
1. An alloy composition for making low-cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC, comprising of C: (0.15 to 0.22wt%), Si: (0.15 to 0.40 wt%), Mn: (0.80 to 1.35 wt%), S: (0.03wt% or less), P: (0.03wt% or less), Al: (0.01 to 0.04wt%), V: (0.01 to 0.04wt%),Nb: (0.005 to 0.01wt%) and balance Fe.

2. A method for production of low cost high strength ISMB 300 medium long structural steel beams of YS: 410 MPa min. and Charpy impact energy of 25 Joules min. at (-)20oC, comprises the steps of:
a. preparing an alloy composition of C: (0.15 to 0.22wt%), Si: (0.15 to 0.40 wt%), Mn: (0.80 to 1.35 wt%), S: (0.03wt% or less), P: (0.03wt% or less), Al: (0.01 to 0.04wt%), V: (0.01 to 0.04wt%),Nb: (0.005 to 0.01wt%) and balance Fe;
b. producing steel heats through BOF-LF-BC route;
c. casting the heats continuously into bloom, and
d. soaking of the bloom in reheating furnace and hot rolling of the cast bloom to ISMB 300 rolled section with soaking temperature of 1200-1260°C and finish rolling temperature of 900-1000°C to obtain yield the steel beams.

3. The method as claimed in claim 2, wherein in step (b)Liquidus temperature of steel: 1510°C, BOF tapping temperature: 1670°C, and LF dispatch temperature: 1590°C.

4. The method as claimed in claim 2, wherein the blooms are 350 x 240 mm in size and 9 metres length.

5. The method as claimed in claim 2, wherein the blooms are soaked for 3 to 4.5 hours.

6. The method as claimed in claim 2, wherein the blooms are rolled speed of 2.4 m/s.

7. The method as claimed in claim 2, wherein the blooms are rolled into ISMB 300 structural sections of 12 metres length.

8. The method as claimed in claim 2, wherein the steel beam has yield strength in the range of 410 MPa to 480 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa and elongation in the range of 20% to 30%.

9. The method as claimed in claim 2, wherein the steel beam has Charpy V-notch impact toughness of 120-135 J at room temperature, 55-72 J at 0°C and 40-50 J at (-)20°C.