A PROCESS FOR PRODUCINGMICRO-ALLOYEDHIGH STRENGTH STEELS WITH IMPROVED STRENGTH AND TOUGHNESS
FIELD OF INVENTION
This invention relates to a process for producing high strength steel plates with micro alloy additions excluding expensive titanium and using optimum vanadium and by multi-pass air-cooled rolling route with (YS> 500 MPa).
PRIOR ART
Micro alloy (MA) or High Strength Low Alloy (HSLA) steels constitute an important category of steels estimated to be around 12% of total world steel production. They are used in every major steel market sector in various parts of the world and their development has played an important role in the expansion of certain key industries such as oil and gas extraction, construction and transportation. Definitions for alloy, low alloy and micro alloy steels are given in Table l.


Micro alloyed steels contain vanadium, niobium, and/or titanium in amounts at least an order magnitude smaller than the amounts of the normal alloys in alloy or low alloy steels. Despite the low levels of alloying these micro alloys can cause major strength and toughness improvements. The obvious economic advantage associated with using such small additions together with the significant benefits to mechanical properties is the reasons for the popularity of MA steels in the market place. Table 2 reveals the fact that early structural steels were generally of the C Mn type. These early steels contained relatively high carbon contents but this caused no problems in construction since up to the 1940s as riveting was used as a means of joining not welding. However even in the l960s the steel in the Melbourne king Street Bridge had fairly high carbon and manganese contents which led to welding problems and subsequent bridge failure.

By utilising micro alloyed steels such welding problems were greatly reduced and a modern offshore steel, produced by a controlled rolling procedure, is shown in the table as a good example of a micro alloyed steel. The first national steel standard to allow a microalloy addition was BS 968 in l962 (Fig. 1) and this resulted in a large reduction in the carbon equivalent value (CEV) combined with an increase in strength. The first micro alloying element to be widely used was vanadium added to C Mn steels in the USA as reported by Bullins in l9l6 (Table 3).


Indeed early in the 20th century Hemp Ford made wide use of vanadium steels for the construction of the Mode1 T, the first mass produced motor car. Small titanium additions can also improve the strength of steel and this was first exploited in Germany in l92l. Micro titanium and micro vanadium additions began to be used in China in the l950s and l960s. However the major event that initiated the HSLA Steel revolution did not occur until 1988 when the Great Lakes Steel Corporation of the USA began production of low C Mn steels micro alloyed with niobium. This event created widespread interest among the world's steelmakers leading to the rapid development of HSLA Steels containing the micro alloys vanadium, niobium and titanium either singly or in combination.
A fine grain size is an essential requirement in most HSLA Steels to obtain the necessary strength and toughness properties. Fig. 2 shows the relationship between grain size and yield Strength in C Mn steels and a similar relationship exists between grain size and fracture toughness. A fine grain size around l0 μ m in a low carbon steel obtained by controlled rolling or heat treatment provides excellent mechanical properties to which further strength can be added by making micro alloy additions giving precipitation hardening. The various functions performed by the micro alloying elements are described in Table 4.


All perform well in providing precipitation strengthening after the hot rolling of C Mn steels, vanadium being especially versatile in this respect. However other roles are more characteristic of certain micro alloying elements. For example a vanadium addition is able to give precipitation strengthening in high carbon steels, niobium has a particularly strong influence in reducing the recrystallization during hot rolling thus aiding grain refinement and a small titanium addition is very effective in refining grain size at high temperatures in the austenite range. The choice of micro alloying element to use in steel is strongly influenced by the solubility of the micro alloy carbide or nitride. (Fig. 3) For example because vanadium carbide is relatively soluble in steel, vanadium is used to strengthen higher carbon steels while vanadium nitride has powerful effect in increasing strength in steels with enhanced nitrogen contents. (Fig. 4). The fact that vanadium has relatively little or no influence on transformation characteristics after hot rolling can be beneficial in providing acceptable properties over a wide range of finish rolling temperatures as occurs particularly in the rolling of sections. On the other hand niobium has a major effect on transformation which can cause the formation of a brittle microstructure One of the main benefits of niobium, that it reduces the rate of recrystallization of austenite during hot rolling is utilised in the controlled rolling of HSLA steel to improve grain refinement. Since close control over processing is required this is normally practised in plate and hot strip rolling mills. Thus niobium has become an important addition to controlled rolled line-pipe steels often in conjunction with the other micro alloying elements. An early line-pipe steel was

vanadium treated in the normalised condition but thereafter the grain refinement, necessary for improving properties, was obtained by controlled rolling a niobium treated steel with vanadium added for increased strength. Further improvements allowing steel grades to reach X80 and Xl00 properties have been made by utilising multi additions of micro alloys together with accelerated cooling after controlled rolling. One of the main roles of titanium in modern HSLA Steels is that of maintaining a fine grain size at high temperatures in the austenite range , for example during reheating prior to rolling and in the heat affected zone of weldments. Titanium is usually combined with vanadium and/or niobium as is the case with a recently developed vanadium-titanium steel designed to give grain refinement during hot rolling without the need to control to low finish rolling temperatures. It is important to ensure that additions of micro alloying e1emellts to steel are carefully planned to ensure that the desired effects on properties are obtained. For example research has shown that the amount of titanium added to a steel must be controlled to maximise the formation of fine titanium nitride particles and to avoid the formation of large particles which can exert a harmful influence on fracture properties.
Bainite is an important microstructural constituent, either wholly or in part, of a wide range of commercial steels. The last decade has seen a particularly strong resurgence of interest in the bainitic transformation. It is also known that bainitic structures can confer certain other beneficial properties to steels. For example, amongst the most widely recognized is that bainitic structures give better creep resistance to steels than ferrite/pearlite structures. Perhaps less widely appreciated is that bainite can be more resistant to certain environmentally controlled conditions than some other steel microstructures, for example, in situations involving hydrogen embrittlement, corrosion fatigue and in some circumstances, rolling contact fatigue. High-strength bainitic steel plates can be a good candidate for use in machineries, buildings and heavy equipment for construction which requires high strength as well as high toughness. It can be further applied for line pipe applications and offshore constructions requiring excellent low temperature toughness.
The strengthening of steel via a reduction in grain size is a very attractive option because a smaller grain size leads also to an improvement in toughness. This simple fact has led to the development of impressive technology, designed to impart thermo-mechanical treatments capable of refining the austenite grain structure prior to its

transformation to bainite. A fine austenite grains size leads to a correspondingly refined bainitic structure. The growth of these grains during the hot rolling process is hindered by the use of micro alloying additions such as niobium or titanium. These elements have a low solubility in austenite and are added in small concentrations to form stable carbides or carbonitrides which impede austenite grain growth during hot deformation and subsequent cooling.
Controlled rolling has been used successfully over the past 30 years or so, for steels containing allotriomorphic ferrite and a small amount of pearlite, but has only recently been adapted for bainitic alloys. There are two ways in which a bainitic microstructure can be obtained; the first, involves an increase in the cooling rate in order to allow the austenite to super cool into the bainite transformation range. The second alternative, discussed here, is to modify the steel hardenability without substantially changing the processing conditions. Alloying elements such as manganese are boosted in order to retard the formation of allotriomorphic ferrite relative to the bainite reaction. The hardenability of the steels is such that in spite of their small grain size, they transform to a “uniform and fine” bainitic structure on further cooling.
In steels with low carbon contents (0.1 wt. %) where the carbide particle size in bainite is small it is found that the main structural feature affecting toughness is the colony size. Cleavage cracks are deflected at the colony boundaries and the measured cleavage facet size on the fracture surface can therefore be related to the colony size.
In spite the early optimism about the potential properties of high strength steels, major commercial exploitation took many years to become established. The steels were not in general found to be better than quenched and tempered martensitic steels, partly because of the relatively course cementite particles associated with bainite and partly because the continuous cooling heat treatments which were popular in industry, could not in practice produce fully bainitic steels. The use of lean alloys gave mixed microstructures whereas heavy alloying led to a considerable quantity of martensite in the final microstructure. Therefore, martensitic steels dominated the high strength steel market, with their better overall mechanical properties and well understood physical metallurgy principles.
It is natural to reduce the carbon concentration even further to produce better steels, which acquire their strength and toughness via the submicron size grain structure of

bainite. However, technology was not in those days sufficiently advanced to cope with the necessarily higher cooling rates required to produce bainite in very low carbon steels, as the steel left the hot-rolling mill. The technology of accelerated cooling designed to produce partially or wholly bainitic microstructures in very low carbon, micro alloyed steels has been perfect within past few years or so, and has resulted in the production of a new class of steels which are the cause of much excitement. However, these variety of steels mostly need a tempering treatment to relieve the quenching stresses which is not cost effective, Therefore, in recent times attempts are being made to produce low-carbon fine-grained bainitic steels with proper alloy combination with relatively slower cooling after hot-rolling i.e. through air-cooling route and thus, the tempering step can be eliminated from the production process.
Earlier, the high strength steels could be developed through faster cooling and tempering route. The present invention proposes a novel method to achieve high strength as well as high toughness steel through a slow cooling (air cooling) route without the necessity of going for further tempering of the plates after air cooling.
OBJECT OF THE INVENTION
The principal object of the invention is to develop High Strength Steel Plates with YS: 500 MPa min. and with Guaranteed Toughness of 25 J at 0 C and 15 J at -20 C of charpy impact energy (CIE).
The second objective of the present invention is to provide an improved method for producing cost effective and leaner micro alloyed high strength steel plates through multi-pass hot rolling and subsequent air cooling.
SUMMARY OF THE INVENTION
In view of the foregoing disadvantages inherent to the known types of the processes for achieving high strength steel plates, the present invention relates to an improved and easier technique for producing micro alloyed high strength steel plates. A process chart was prepared in consultation with BSP officials for the production of the proposed IS 2062 E 500 grade plates of various thicknesses. Trial heats with various combinations of Nb, V and Ti were made in the BOF furnace of SMS II(120T) with steel chemistry as modified with respect to optimum contents of Nb and V only ( No Ti was added) and the heat was made and processed into 16, 25 and 32 mm thick plates.

Therefore such as herein described there is provided a process for producing micro alloyed high strength steel plates comprising the steps of: (i) heating the steel to a temperature 1200°C for transforming its structure into fully austenitic structure, whereby the temperature 1200°C and the holding time 3 hours at the temperature 1200°C, (ii) performing hot rolling at temperatures at 1200 – 830°C where the structure of the steel is ferrite pearlite; (iii) performing cooling up to ambient temperature at a cooling rate of ~1 C/s and further cooling to the room temperature without further working and wherein the steel melt composition comprise of in weight percentage as 0.18 wt.% of carbon, 0.33 wt.% of silicon, 1.59 wt.% of manganese, 0.007 wt.% sulphur, 0.020 wt.% Phosphorus, 0.02 wt.% Aluminium, 0.078 wt.% of niobium, and 0.069 wt.% of vanadium and no titanium and balance essentially iron..
BRIEF DESCRIPTIONOF THE ACCOMPANYINGDRAWINGS
Fig. 1 illustrates the evolution of structural steel standard BS968 due to micro-alloying;
Fig. 2 illustrates the relation between lower yield strength and the inverse square root of the grain diameter in accordance with the present invention;
Fig. 3 illustrates the solubility of microalloy carbides / nitrides in accordance with the present invention;
Fig. 4 illustrates the increase in yield strength from vanadium and nitrogen in hot coil product as a result of precipitation of vanadium nitride in accordance with the present invention;
Fig. 5 illustrates the Microstructures from the mid thickness of a) 25,b)16 and c) 32 mm plates in accordance with the present invention;
DETAILED DESCRIPTION OF THE INVENTION
In consideration of state of the art the inventors set themselves the aim of improving the known methods, avoiding the disadvantages noted, and dealing with the task, which is considerably different in comparison with the conventional process of producing high strength steel plates.
There is a large demand of high strength plates (YS > 500 MPa) with guaranteed impact properties from the market. These steels find a wide range of applications in the

construction, transportation and heavy machineries sector. In view of the stringent quality requirement for these steel plates, it is necessary to produce these grades of steels with the enhanced performance criteria. Bhilai Steel Plant (BSP) has the necessary facilities and infrastructure to produce and supply these grades of steel. RDCIS has the necessary facilities for testing and evaluation of the quality parameters. Therefore, these products can be made at BSP in order to meet the performance criteria required by the customers.
In SAILMA as well as IS 2062 specification, the series of high tensile grades available are SAILMA /IS2062 - 300, 350, 410, 450, 550, 600 and 650. The grade designation is based on minimum yield strength. It was observed that after 450 strength grade the next grade available is 550 with a gap of 100 MPa. Moreover, in high strength grades (Yield strength > 550 MPa) there is a deterioration in toughness and these steels barely meet the required specifications of 25 J at 0oC and 15 J at -20oC. For the customers to have a choice of an intermediate high strength grade it was decided to develop SAILMA 500 grade and subsequently to take up for introduction of E500 grade in IS 2062 Specification. Further, it was also envisaged to impart higher impact toughness at sub-zero temperature so that this 500 grade will get a very good marketability in the applications where high strength as well as higher impact toughness is required. IS 2062 E 410 and above require micro alloy additions of Nb ,V or titanium singly or in combination in order to attain the required minimum yield and tensile strength in the as rolled condition.
For the development of high strength micro alloyed steel a process chart was prepared for the production of the proposed IS 2062 E 500 grade plates of various thicknesses. Trial heats with various combinations of Nb, V and Ti were made in the BOF furnace of SMS II (120T). Initially two trial heats with micro alloying additions were made and continuously cast into 250 mm thick slabs. These slabs were reheated to ~1200 C for about 3 hrs. and rolled into plates of thicknesses ranging from 16 mm to 32 mm in the plate mill with the following draft schedule:
250-232-208-185-168-151-134-119-104-91-78-67-56-50-43-37-32-28-25-21-16 mm
The finishing temperatures of the plates were ~8300C and thereafter the plates were air cooled. The specified properties (YS: 500 MPa min.; UTS: 600 MPa min.; El.: 18% min.; CIE: 25 J min. at 00C and 15 J min. at - 200 C ) could not be achieved. Thereafter

based on the results of the two heats, the steel chemistry was modified with respect to optimum contents of Nb and V only (No Ti was added) (Table 5)
This heat was made and processed into 16, 25 and 32 mm thick plates
The chemistries and the tensile properties of the plates rolled from the continuously cast slabs from the three heats are shown in Table 5.

The tensile properties achieved from the steel plates with said chemistry and adopting the aforesaid steps is shown in table 6.

The property evaluation of the hot rolled steel samples through hardness, tensile testing and Charpy impact testing. Hardness measurements were carried out by Rockwell indentation method using a standard hardness testing machine at 100 kg load. Tensile tests are carried out in a universal testing machine using flat specimens of 25 mm gauge length in accordance with ASTM A 370 standard. Charpy impact tests are carried out at 0oC and -20oC using sub-sized specimens as per ASTM E23 specification. For sub-zero temperature testing (0oC and -20oC), methanol and liquid

nitrogen mixture bath is used and each sample is soaked for 25 minutes in the bath before testing. The charpy impact energy values have been shown in Table 7

The properties for the plates rolled from Heat 3 are shown as Table 8

Evaluation of the hot rolled steel samples was done through hardness, tensile testing and Charpy impact testing. Hardness measurements were carried out by Rockwell indentation method using a standard hardness testing machine at 100 kg load. Tensile tests are carried out in a universal testing machine using flat specimens of 50 mm gauge length in accordance with ASTM A 370 standard. Charpy impact tests are carried out at 0°C and -20°C using sub-sized specimens as per ASTM E23 specification. For sub-zero temperature testing (0°C and -20°C), methanol and liquid nitrogen mixture bath is used and each sample is soaked for 25 minutes in the bath before testing.
The steel composition comprises of the components in weight percentage as under 0.18 wt.% of carbon, 0.33 wt.% of silicon, 1.59 wt.% of manganese, 0.007 wt. % sulphur, 0.020 wt.% Phosphorus, 0.02 wt..% Aluminium, 0.078 wt. % of niobium, and 0.069 wt.% of vanadium and no titanium and balance essentially iron.

Light and scanning electron micrographs of the steels 25mm, 16mm and 32mm are depicted in Fig. 5 respectively at various magnifications
The yield strength (YS) varied between 510-527MPa. The ultimate tensile strength (UTS) varied between 669-672MPa. The YS and UTS of all the three steels were very high with guaranteed toughness.
INNOVATIVE STEPS
♦ Innovative alloy design using optimum V content that leads to the saving in costly ferrovanadium addition
♦ Departure from the traditional practice of the addition of Titanium
♦ Elimination of coarse TiN and VC precipitates that deteriorate toughness of the plates
♦ Efficient utilization of V and N
♦ Modification in thermo-mechanical processing within the constraints of the mill
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.

WE CLAIM
1. A process for producing micro alloyed high strength steel plates comprising the
steps of:
(i) heating the steel to a temperature 1200°C for transforming its structure into fully austenitic structure, whereby the temperature 1200°C and the holding time 3 hours at the temperature 1200°C.
(ii) performing hot rolling at temperatures at 1200 - 830°C where the structure of the steel is essentially ferrite-pearlite;
(iii) performing air cooling up to ambient temperature at a cooling rate of ~ 1 C/s and further cooling to the room temperature without further working and wherein the steel melt chemistry comprise of components in weight percentage as 0.18 wt. % of carbon, 0.33 wt. % of silicon, 1.59 wt. % of manganese, 0.007 wt. % sulphur, 0.020 wt. % Phosphorus, 0.02 wt. % Aluminium, 0.078 wt. % of niobium, and 0.069 wt. % of vanadium, excluding titanium and balance essentially iron.
2. A process for micro alloyed producing high strength steel plates as claimed in claim 1, where in rolling of the steel is continued after soaking at a temperature of 1200oC for 3 hours and rolled in rolling mill into 16, 25 and 32 mm thick plates thick plates.
3. A process for producing high strength steel plates as claimed in claim 1, wherein the draft schedule is:
250-232-208-185-168-151 -134-119-104-91 -78-67-56-50-43-37-32-28-25-21-16 mm and subsequently cooled in air.
4. A process for producing high strength steel plates as claimed in claim 1, wherein
the mechanical properties are
Specified properties (YS: 500 MPa min.; UTS: 600 MPa min.; El.: 18% min.; CIE: 25 J min.at00Cand15Jmin.at-200C)
5. A process for producing high strength steel plates as claimed in claim 1, the finish
rolling temperature of 830°C followed by air cooling at the rates of ~1°C/s.

6. A process for producing high strength steel plates as claimed in claim 1, wherein the melt is designed based on optimum contents of Nb and V in place of Nb-V-Ti leading to efficient utilization of V and saving on costly Ferro-V and Fe-Ti
7. A process for producing high strength steel plates as claimed in claim 1, wherein with micro alloying additions were made and continuously cast into 250 mm thick slabs
8. A process for producing high strength steel plates as claimed in claim 1, wherein the steel is of IS 2062 E 500 grade plates.
9. A micro alloyed high strength steel hot rolled plate comprising steel melt composition including the components in weight percentage as 0.18 wt. % of carbon, 0.33 wt. % of silicon, 1.59 wt. % of manganese, 0.007 wt. % sulphur, 0.020 wt. % Phosphorus, 0.02 wt. % Aluminium, 0.078 wt. % of niobium, and 0.069 wt. % of vanadium and no titanium and balance essentially iron.
10. A micro alloyed high strength steel plate as claimed in claim 9, wherein rolling of the steel is continued after soaking at a temperature of 1200o C for 3 hours and rolled in rolling mill into 16, 25 and 32 mm thick plates thick plates.