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Development of Micro Alloyed Structural Steels in Secondary Steel Sector through Induction Melting Furnace and Controlled Rolling Route S.I.Singh Robin Kr.Bagchi INTRODUCTION Major trend in development of metallurgy of Iron & Steel is directed towards increase of useful engineering properties and cost savings. These are two major concerns in structural application also. Special features looked into are: high strength to weight ratio, good weld ability and cost effectiveness. Commonly structural steel at present have yield limit of around 41 kgmm-2. To attain reasonable material saving, it is necessary to use structural steels with increased strength properties and a yield limit of at least 49 kgmm-2. Such yield limit can be achieved either by increase of carbon percentage or through special process e.g. TMT or by micro alloying. Unfortunately, ever more stringent requirements for weld ability do not allow an increase of carbon content and TMT process requires capital due to costly plant and accessories. A reasonable way of attaining yield limit of 49 kgmm-2 is by micro alloying of steels that is utilization of precipitation hardening and strengthening due to ferrite grain refinement. Cost control requires low alloying additions without sacrificing high strength requirements. The combination of these two coined the term: high strength low alloy (HSLA) steel. Traditionally low carbon mild steel has been used for structural steel in the form of shapes, plates and bars for use in riveted, bolted or welded constructional activities. Such as steel for example ASIM A36, containing up to 0.29% carbon and up to 1.20% manganese, has a minimum yield strength of 26kgmm-2. Similarly low carbon sheet steel traditionally been used in automobiles, e.g. hot rolled SAE 1010 steel having 0.08 - 0.13% carbon and 0.30 - 0.06% manganese, has a minimum YS value of about 20Kgmm-2. HSLA steel evolved from this carbon steel base. HSLA steels may be defined as steels having YS of over 28Kgmm-2 with alloying additions designed to provide specific desirable combination of properties such as strength, toughness, formability, weld ability and atmospheric corrosion resistance. STREGTH SPECIFIC COST (SSC) SSC can be considered to be an important factor for selection or development of a material. It can be expressed in terms of total cost incurred per tonne of finished product to raise per unit strength (UTS) of material from a base value. Total incurred may include all direct and indirect cost components. The base UTS value for 12mm dia mild steel structural ribbed bar is around 41Kgmm-2. 12mm dia TMT bars having acceptable metallurgical quality presents a value 65Kgmm-2. If an amount of Rs `X' is to be incurred per tonne to produce a TMT bars, then SSC would be X/(65-41) or X/24. A lower value of SSC is desirable when two materials are to be compared. BACKGROUND Until recently, plain carbon mild steel with varying carbon contents were used for any common constructional job. With advent of Thermo Mechanical Treatment (TMT) process and its application to mild steel rods, attainment of higher strength becomes achievable. Yield strength values of common mild steels are 37 - 47Kgmm-2 whereas that for TMT steels is 55 - 65Kgmm-2. This improvement in YS values shall be responsible in tonnage saving of constructional materials. But in turn may be of set by high cost of TMT plant and accessories. Moreover, it is also understood that TMT process is applicable to round sections only. It is also difficult to obtain desired microstructure of ribbed bars to designate those as proper TMT bars. There are structural steels designated as TMT but have normal ferrite - perlite microstructure with stay bainite at limited areas of surface. A reasonable percentage of tonnage TMT steel bars are available in the market, which fall into this category. The cheaper alternative does not always result proper metallurgical characteristics. So, micro alloying was adopted to produce structural steels at lower cost and of consistent quality in secondary steel sector comprising of induction furnaces and re-rolling units. A present paper discusses the findings with following objectives: To lower down the cost of high strength ribbed bars. To apply high strength concept to all sections and size of constructional steels e.g. angle, channel girder, etc., which may lead to considerable material saving and hence cost reduction. To provide the steel users of our country the value for their money by replacing TMT bars with cheaper HSLA bars and other sections having compatible and consistent property levels. It is believed, that HSLA steel requires a route that provides process flexibilities in steel making. Induction furnaces have never been thought for as a melting route. To adopt induction melting furnace route and small rolling mill in SSI sector for production, so that, such steel can also be produced in the secondary steel sector of the country. To reduce SSC i.e. to attain a higher strength values at a lower cost. EXPERIMENTAL - METHODOLOGY Micro alloy steel have been developed through induction melting furnace and controlled rolling route to replace TMT bars used for construction purposes. An induction furnace was selected for making steel ingots and a re-rolling mill was chosen for rolling the ingots to 12mm dia ribbed bars. For investigation, a base chemistry was selected to have:   %C %Si %Mn %S %P %Cr %Ni %Mo %Cu  Min. .20 .25 .80 - - ............traces.............  Max. .25 .35 1.00 .04 .04   % vanadium and % niobium were both varied. Nitrogen was aimed at 150ppm in some heats. Total 12 heats were cast. Melting was done in a 500 kg (converted to 250 kg) induction melting furnace, 3 1/2" * 4 1/2" sized pencil ingots were cast and rolled to 12mm dia ribbed bars. 3 different sets of cooling parameters of rolled products were adopted. One large set was made with out water-cooling. It was air cooled on cooling bed. One bar from each heat of this set was twisted after cooling [CTD (AC)] and rest was kept as it is (AC). The third set was again water-cooled, but with much less water as compared to the first set (WC), as shown in Table 1: Table 1: Four different sets of Trail   Set    Heat No.    Designate    Post Rolling Treatment   A.    1-12    WQ    Water quenching as in TMT   B.    1-12    AC    Normal air cooling in cooling bed   C.    1-12    CTD(AC)    Normal air cooling and CTD (cold twisted deformed)   D.    1-12    WC    Water cooling with 50% lesser water as compared to set A RESULTS AND DISCUSSION Samples were taken from each stage of production. One test piece was cast from each heat, it was normalized and tested for chemical analysis trough direct reading spectrometer, metallographic studies like micro structure, grain size and inclusion rating, hardness in BHN, mechanical tests like UTS, YS, %EI and bend-rebend tests were carried out. Test pieces were cut from each heat of 4 sets of rolled products and each set was tested for microstructure, hardness. UTS, YS and %EI. The results have been presented in tabular form in Table-2 for inclusion rating and Table-3 through Table-8 for mechanical tests. INCLUSION RATING In all the heats level of inclusion types A, B & C were very good. Duly the mixed oxides were high in almost all the heats. It is possible that some of these particles were present in SEM photograph and might have affected strength and ductility values. During steel melting though measures were taken to reduce inclusion levels, some more precautions were needed to reduce the level of mixed oxides in steels. It is important to control both indigenous and exogenous inclusions to improve mechanical properties. These are possible with little care in steel melting and casting. Table 2: Inclusion rating of Trail heats SL No. Code Inclusion rating (thin/thick)     Sulphide(a) Alumina(b) Silicate(c) Mixed oxides (d)  1.   P   1/-   0.5/-   -/-   2/-  2.   V2   0.5/0.5   -/-   -/-   2/-  3.   V4   1/-   1   0.5/-   2/1  4.   V6   0.5/-   -/-   0.5/-   2/1.5  5.   V2N   1/-   1   -/-   2.5/1.5  6.   V4M   1/-   -/-   0.5/-   2.5/-  7.   V6N   1.5/-   -/-   0.5/-   1/0.5  8.   Nb2   0.5/-   1   -/-   1.5/-  9.   Nb4   0.5/-   0.5/-   -/-   2/0.5  10.   Nb6   1/0.5   -/-   -/-   2/-  11.   30V4N   1/0.5   -/-   0.5/-   1.5/-  12.   30VNb4N   0.5/0.5   -/-     1/-   MECHANICAL PROPERTIES Results of mechanical tests have been prented below, from Table 3 - Table 8: MECHANICAL PROPERTIES: Results of mechanical tests have been presented billow, from Table 3 - Table 8: Table 3: Hardness,(BHN) values of Trail heats   12 mm Dia Rolled Products   S No.   Code   Test   Piece(nor)   Water   Quench(WQ)   Air   Cooled(AC)   CTD(AC)   Water   Cooled(WC)  1.   P   180   200   210   286   237  2.   V2   200   234   226   271   286  3.   V4   153   253   237   286   271  4.   V6   200   234   210   279   237  5.   V2N   195   226   222   253   231  6.   V4N   190   234   237   247   301  7.   V6N   195   226   216   237   231  8.   Nb2   172   253   258   253   247  9.   Nb4   200   264   222   286   258  10.   Nb6   210   226   253   247   258  11.   30V4N   216   258   258   294   264  12.   30VNb4N   190   264   253   286   258 Table 4: UTS, Kgmm-2 values of Trial heats   S No.   Code   Test   Piece(nor)   Water   Quench(WQ)   Air   Cooled(AC)   CTD(AC)   Water   Cooled(WC)  1.   P   63   70   69   80   79  2.   V2   67   81   79   86   89  3.   V4   54   83   70   82   83  4.   V6   51   74   61   82   72  5.   V2N   65   78   73   84   85  6.   V4n   59   83   75   84   83  7.   V6N   61   77   65   94   85  8.   Nb2   62   72   75   78   75  9.   Nb4   58   86   78   87   84  10.   Nb6   54   75   65   68   73  11.   30V4N   50   83   81   88   82  12.   30VNb4N   68   87   76   90   88 Table 5: YS, Kgmm-2 values of Trial heats   12 mm Dia Rolled Products   S No.   Code   Test   Piece(nor)   Water   Quench(WQ)   Air   Cooled(AC)   CTD(AC)   Water   Cooled(WC)  1.   P   40   51   49   64   63  2.   V2   43   63   59   83   80  3.   V4   44   73   55   62   67  4.   V6   35   64   48   64   60  5.   V2N   43   65   57   -LC   69 LC  6.   V4N   46   68   59   -LC   72 LC  7.   V6N   60   68   51   -LC   74 LC  8.   Nb2   40   49   61   60 LC   63  9.   Nb4   39   79   61   -LC   74 LC  10.   Nb6   36   64   50   -LC   63  11.   30V4N   47   70   67   66   65 LC  12.   30VNb4N   47   74   61   68   71 LC Table 6: % Elongation values of Trial heats   12 mm Dia Rolled Products   S No.   Code   Test   Piece(nor)   Water   Quench(WQ)   Air   Cooled(AC)   CTD(AC)   Water   Cooled(WC)  1.   P   30   23   25   20   25  2.   V2   20   24   23   20   18  3.   V4   27   16   28   17   18  4.   V6   27   20   24   19   20  5.   V2N   19   24   24   19   19  6.   V4N   11   20   25   18   20  7.   V6N   09   15   20   17   18  8.   Nb2   25   21   23   10   19  9.   Nb4   24   15   20   17   20  10.   Nb6   21   25   22   20   19  11.   30V4N   07   20   17   18   20  12.   30VNb4N   18   18   24   20   16 Table 7: % Reduction in area values of Trial heats   12 mm Dia Rolled Products   S No.   Code   Test   Piece(nor)   Water   Quench(WQ)   Air   Cooled(AC)   CTD(AC)   Water   Cooled(WC)  1.   P   52   57   61   26   66  2.   V2   53   46   59   38   55  3.   V4   63   51   57   44   64  4.   V6   30   54   59   58   70  5.   V2N   50   56   54   58   49  6.   V4N   17   64   61   51   51  7.   V6N   23   55   65   18   58  8.   Nb2   48   33   66   37   67  9.   Nb4   34   51   63   32   58  10.   Nb6   29   54   60   60   62  11.   30V4N   05   54   59   47   65  12.   30VNb4N   13   39   54   45   53 Table 8: Bend-Re-bend Test Results of Trial heats   12 mm Dia Rolled Products   S No.   Code   Test   Piece(nor)   Water   Quench(WQ)   Air   Cooled(AC)   CTD(AC)   Water   Cooled(WC)  1.   P   NC   NC   NC   LC   NC  2.   V2   NC   NC   NC   LC   NC  3.   V4   NC   NC   NC   LC   LC  4.   V6   NC   NC   NC   NC   NC  5.   V2N   NC   NC   NC   NC   NC  6.   V4N   NC   NC   NC   LC   NC  7.   V6N   NC   NC   NC   LC   LC  8.   Nb2   NC   NC   NC   LC   NC  9.   Nb4   NC   NC   NC   NC   LC  10.   Nb6   NC   NC   NC   NC   LC  11.   30V4N   NC   NC   NC   LC   NC  12.   30VNb4N   NC   NC   NC   LC   LC In unalloyed steel, AC resulted almost equal for better strength and ductility as compared to WQ but were lower than WC. In cases of V combinations though strength values under AC were lower (2-13) than those under WQ, ductility values were better in AC. Here again, UTS values were higher under WC than those obtained under WQ and AC. Though % elongation values were lower under WC than under WQ and AC, % RA values were more. Strength went down with increasing vanadium percentages, under almost all cooling conditions. But %E was the lowest with WC and was higher under AC conditions, with the highest value obtained with V4. Vanadium produces strengthening effect through precipitation hardening and only AC enhances strength without sacrificing the ductility. When V+N combinations were used, similar trend as with was followed. All the WC samples developed LC. No appreciable improvement observed with N additions. %E values decreased with more %V under AC and WQ. Under WQ and WC, values of %E and %RA were lower than obtained under AC. Faster cooling enhanced values but reduced the ductility. Nb combinations also presented high strength and ductility values almost comparable to vanadium combinations. Different cooling conditions did not appreciably changed the UTS values but faster cooling rates resulted in marked reduction in ductility. There was no appreciable change with V+Nb+N combination having 0.28%C. all the samples (other than with V4) under CTD (AC) developed LC and resulted mostly low %EI and %RA values, though the UTS values were above 80kgmm-2. CTD resulted in raising dislocation density level of already stresses rolled products. Best combination of strength and ductility values were obtained with V2 (UTS: 79, YS: 59, %EI: 23 & %RA: 59) and Nb2 (UTS: 75, YS: 61, %EI: 23 & %RA: 66) typical values obtained with 12mm TMT of SAIL / RINL, were UTS: 65, YS:56, %EI: 25 & %RA: 45. Summary of all the results have been presented in Table-9 below. Table 9: Summary of Results   Code Test       Price (Normalised) Micro Structure Rolled Product, Mech. Props    VF(P)    GS WQ AC WC WQ AC CTD WC   P 25    5-6 B FP FP   Nor   Nor H I G H & B R I T T L E L C   High   V2 40    5 B FP FP   High   High   Nor   V4 20    6 B FP FP   High   Nor   V High   V6 25    5-6 B FP FP   Low   Poor   Nor   V2N 25    5-6 B FP FP   Low   Nor   High   V4N 55    4-5 B FP FP   High   Nor   High   V6N 40    4-5 B FP FP   Poor   Low   Low   Nb2 40    7 B FP FP   Low   Nor   Nor   Nb4 40    5-6 B FP FP   V High   High   Low   Nb6 30    6-7 B FP FP   Poor   Poor   Low   30V4N 58    6-7 B FP FP   High   V High   Poor   30VNb4N 50    6 B FP FP   Low   Nor   Nor STRENGTH SPECIFIC COST (SSC) Commonly a typical 12 mm dia TMT bar of SAIL / RINL has a UTS value of 60kgmm-2 and of an ordinary mild steel ribbed bar is around 40kgmm-2. Assuming the similar quality of TMT bar is produced in the secondary steel sector with an annual production of 12,000 MT, total cost of the TMT plant and accessories as Rs 1.5 crores, IRR of 18% and an expected pay back period of 5 yrs, the SSC may be calculated as RS 17.50. In case of micro alloyed steel with low %V / Nb, expected UTS level shall be around 75kgmm-2. As far as the cost factor is concerned, requirement for vanadium shall be less than niobium, because in steel scrap, usually little vanadium comes as residual but not niobium. Additional cost of micro alloyed steels with low vanadium (V2) shall be around Rs 200.00 in this case, the SSC may be calculated as Rs 5.72 only, which is about 2.5 times less. Moreover, if due considerations are given and applied in the design calculations for constructional activities, then there will be more savings of constructional materials for the users of micro alloyed steels than of TMT bars. Savings can even be more in case other sections of constructional materials, because in those cases, the difference in UTS shall be more (from 45kgmm-2 to 79kgmm-2) as the TMT process cannot be applied to the sectors other than rounds and squares. CONCLUSION Induction melting furnace can be adopted for production of micro alloyed ribbed bars. In steel melting, measures should be taken to reduce inclusions in steels. No water quenching is needed for high strength values. The properties are better than TMT bars even under normal air-cooling conditions. Micro alloyed steel ribbed bars should not be cold twisted & deformed. V2 and Nb2 steels produced the best results. But lower %V steel has lower SSC value than same %Nb. Because of very low SSC values, lower %V micro alloyed structural steel is the best alternative for TMT bars.