Title of Invention

A PROCESS FOR MAKING HIGH TEMPERATURE WEAR CORROSION RESISTANT ROLLS FOR USE IN STRAND GUIDE SECTION IN SLAB CASTING MACHINE

Abstract

A high temperature wear and corrosion resistant rolls for use in strand guide section in slab casting machines and a process for manufacturing the same involving controlled cooling of the hot rolls which is thereafter followed by annealing and heat treatment developed high strength properties and finish. The wear and corrosion resistant rolls of the invention have long and durable effective life as slab caster rolls adapted to have increased life of about 750-800 heats. The slab caster rolls having improved wear and corrosion resistance further facilitate manufacturing of good quality slab free from surface defects. The extended life of the slab caster rolls avoid friction, shut down of machines and production losses.

Full Text

The present invention relates to high temperature wear and corrosion resistant rolls for use in strand guide section of continuous slab casting machine and a process for manufacture of such high temperature wear and corrosion resistant rolls. It is presently known to manufacture slab caster rolls by melting steel in 20 tonnes electric furnaces followed by vacuum degassing the molten steel and subsequently casting into ingots which were forged/processed into rolls. The rolls were assembled as foot rolls in the mould bottom (6 rolls) and as first segment of 5 set rolls below mould. The tatter is assembled as 3 x 2 x 5 rows = 30 rotfs as split rolls. Alternatively can also be assembled with one bigger and one smaller rolls as staggered rolls having 2 x 2 x 5 rows (20 rolls). These rolls are provided to support and guide the partly solidified hot slabs in the temperature range of ~1300°C descending downwards from copper moulds. The roils are assembled with bearings and rotated with frictional force generated due to downward movement of slabs. During total casting operation the rolls are externally water cooled. The rolls are subjected to cyclic contact with hot slab leading to high thermal fatigue. Further the rolls come in contact with water, steam, abrasive oxide scales formed on slab surface as well as hydrofloric acid generated due to water and florites in the casting mould powder. It would be evident from the above that all the above process technology cause high temperature abrasive wear and corrosion on the roll surface during service. Poor life of rolls causes frequent shut down of machine leading to heavy production loss. Also regular damage of the roll required frequent replacement of the rolls which make the process cost extensive and also labour intensive. It is thus the basic objective of the present invention to provide for slab caster rolls of desired chemical composition which would be wear and corrosion resistant and would therefore have a longer and durable effective life. 2 Another objective is to provide slab caster rolls which would have increased life such as for about 750-800 heats. Yet further objective of the invention is to provide slab caster rolls which would be wear and corrosion resistant and would facilitate manufacture of good quality slabs free from surface defects. Another objective of the present invention is to provide a process for manufacture of slab caster rolls with durable and extendible life span by way of high temperature abrasive wear and corrosion resistant properties. Yet further objective of the present invention is to provide wear and corrosion resistant slab caster rolls which would avoid the frequent shut down of machines and thereby avoid the problem of heavy production loss. Thus according to one aspect of the present invention there is provided a high temperature wear and corrosion resistant roll for use in strand guide section in slab casting machines comprising : a chemical composition of C upto 0.30% by wt. Mn 0.40 to 0.80% by wt. Si 0.40 to 0.70% by wt. Cr 12.0 to 14.0% by wt. and Mo 0.5 to 1.0%by wt. of Fe-balance. In accordance with another aspect of the present invention there is provided a process for manufacture of high temperature wear and corrosion resistant rolls for use in strand guide section in slab casting machines comprising : providing a steel composition comprising of C upto 0.30% by wt. Mn 0.40 - 0.80% by wt., Si 0.40 - 0.70% by wt., Cr 12.0 -15.0% by wt. and Mo 0.4 -1.0% by wt.; ingot casting and hot forging ; controlled cooling followed by annealing and finally heat treating to develop high strength properties and finishing. It is thus possible by way of the use of selective chemical composition of the rolls of the invention based on martensitic stainless steel grade of steel to provide a high chromium medium carbon steel in martensitic stainless steel category with Mo to improve pitting corrosion resistance of such rolls. The above composition of the steel is suitable to develop into slab caster rolls having extended and durable life span. Importantly, the use of high Cr and Mo in the steel composition provide for increasing life span in terms of total number of heat cast that can be carried out with the rolls of the invention. 3 In accordance with yet further aspect of the present invention there is provided a process for manufacture of high temperature wear and corrosion resistant rolls for use in strand guide section in slabs casting machines comprising : providing a steel composition having a chemical composition of C upto 0.30% by wt, Mn 0.40 to 0.80% by wt, Si 0.40 to 0.70% by wt, Cr 12.0 to 14.0% by wt. and Mo 0.4 to t.0%by wt; subjecting the steel composition above to ingot casting and hot forging ; and controlled cooling, annealing and finally finishing to obtain the high temperature wear and corrosion resistant rolls. Importantly, the annealed rolls are finished by subjecting the same subsequent to the step of annealing to rough machining, heat treatment and finally finishing-. In particular, the step of annealing is followed by rough machining into foot rolls and upper segment rolls with internals bores. The rolls are heat treated to develop high strength properties before application in the slab caster. The step of heat treatment after annealing is directed to improve the mechanical properties of the roll and comprise of the steps of hardening, stress relieving and tempering. In the above process of heat treatment the step of hardening comprise heating the rolls to a temperature of about 705 - 715°C and maintaining the same in the said temperature range for a period of 1.5 to 2.5 hrs. followed by ; further heating the same to a temperature of 980 - 990°C and maintaining the temperature for a period of 1.5 to 2.5 hrs. and finally air/oil quenching to a temperature of 90 to 110°C preferably 100°C. The step of stress relieving comprise heating to raise the temperature of quenched rolls to a temperature of 380-390°C and maintaining in said 4 temperature for a period of 5.5 to 6.5 hrs. and finally, allowing it to cool to room temperature. The step of tempering comprising subjecting the stress relieved rolls to heating to a temperature of 625 - 635°C and maintaining for a period of 9.5 to 10.5 hrs. and finally air cooling the rolls. In the above process the forgings are required to be roughed machined before the step of hardening. After forgings are charged for tempering immediately after hardening the stress relieving treatment is not required. The rolls obtained following the process of the invention can be used and arranged in different assemblies such as split rolls or staggered rolls. Also it is possible to obtain the rolls in plane profile design or step profile design. The details of the invention and its objectives and advantages are explained hereunder in greater detail in relation to a non-limiting exemplary embodiments of the process in relation to the accompanying figures wherein Fig. 1 is a schematic flow diagram illustrating the process of manufacture of the rolls in accordance with a preferred embodiment of the invention ; Fig. 2 illustrates the assembly of rolls of the invention in slab casting machines. Figs. 3a and 3b illustrate various profiles of rolls of the invention. Figs. 4a and 4b illustrate the arrangement of the rolls according to various embodiments; Fig. 5 is an illustration of the hardening treatment for foot rolls in accordance with the present invention ; and Fig. 6 is a optical micro structure illustrating the structure of the rolls obtained as per the invention vis-a-vis of conventional rolls. 5 Reference is first invited to Fig. 1 which illustrates by way of flow diagram the various steps involved in the manufacture of the slab caster rolls in accordance with the present invention. As would be evident from said figure the manufacture of the rolls involve the steel making using the selective steel composition having C upto 0.30% by wt, Mn 0.40 to 0.80% by wt., Si 0.40 to 0.70% by wt., Cr 12.0 to 14.0% by wt. and Mo 0.5 to 1.0% by wt, followed by melting of steel and ingot casting and subsequently hot forging at temperature of 1250°C to 850°C involving blooming and hammer forging. Thereafter, the hot forged product is subject to controlled cooling, annealing followed by rough machining. Finally after rough machining the rolls are heat treated comprising the steps of hardening air/oil quenching, stress relieving and tempering to thereafter obtaining the finished product. The mechanical properties of the roll of the invention obtains following the above process was tested with conventional rolls obtained of composition as detailed hereunder in Table 1A : TABLE 1A CHEMICAL COMPOSITION OF ROLL MATERIALS GRADE C Mn Si Cr Ni Mo En 24 (Existing) 0.35 0.45 0.60 0.90 0.20 0.35 1.1 1.3 1.50 1.80 0.25 0.30 12Cr-1 Mo (New) 0.15 0.30 0.4 1.0 0.60 MAX 12.0 14.0 0.30 MAX 0.50 1.00 The mechanical properties in heat treated condition of the rolls as per conventional compositions and those obtained following the invented compositions are illustrated in Table 1B hereunder: 6 TABLE 1B MECH. PROPERTIES IN HEAT TREATED CONDITION GRADE UTS (MPa) YS (MPa) %E CVN IMPACT (JOULES) BHN En 24 (N&T) 890 685 16 45 276 12Cr 1Mo H&T 870 670 11 25 270 (Hardening by oil or air quenching & tempering) As would be evident from the above Table 1B, the rolls obtained by the process of the invention are found to have similar mechanical properties vis-a-vis the rolls obtained following conventional procedures. The wear and corrosion registance properties of invented rolls are very much superior to conventional rolls. Reference is now invited to Fig. 2 which illustrates the assembling ofrolls obtained by invention in slab casting machines. As would be evident from said figure the rolls of the invention can be used for top segments such as foot rolls and 5 roll section rolls. Reference is invited Fig. 3a and Fig. 3b illustrate design of two types of foot roll profile being used. As shown in Fig. 3a the rolls can have plane profile or in the alternative can have the stepped profile as shown in Fig. 3b. Also the arrangements of the rolls can be made depending upon the assemble requirements. As shown in Fig. 4a the same may be arranged as split rolls or can be arranged as staggered rolls has further illustrated in Fig. 4b. In particular Figs. 4a and 4b show Schematic arrangement of two different arrangements of roll assembly in slab caster. Reference is now invited to accompanying Fig. 5 which illustrates the hardening treatment for foot rolls/5 set rolls by way of a preferred embodiment. As clearly represented in said figure the hardening treatment includes the initial step of hardening following by stress relieving and finally tempering. Importantly all forgings 7 forgings are required to be rough machined before hardening. If forging are charged for tempering immediately after hardening the step of stress relieving treatment is not essential. The details of the scheme of heating and cooling to carry out the hardening, stress relieving and tempering is clearly illustrated in said Fig 5. Reference is finally invited to Fig. 6 which illustrates by way of optical micro structure, the improvement achieved in the constitution of the slab caster rolls obtaining in accordance with the present invention vis-a-vis the conventional slab caster rolls. It is thus possible by way of the present invention to provide for a suitable C-Cr-Mo steel with high Cr addition to achieve martensitic structure after heat treatment. High forging cycle with optimum temperature regime is also achieved by way of the present invention. The optimization of the heat treatment cycle provide for development of fine tempered martensitic structure with surface hardness of 270 - 300 HB The problem of rapid and uneven wear and pitting corrosion under aggressive industrial atmosphere with water, steam and hydrofloric acids have been completely eliminated as observed during trials in continuous slab casters at Bhillai Steel Plant. The following comparative wear data was obtained : (i) No. of heats cast with experimental new roll 420 and wear was = 2.95 mm. (ii) No. of heat casts with conventional rolls (En 24 grade rolls) 170 heats and wear was =5.00 mm (iii) For 5 mm wear with new rolls, the total no: of heats cast will be = 711 heats (for 5 mm wear) So wear resistance of new rolls is - 4.18 times better than conventional rolls. It is possible by way of the roll of the invention to provide slab caster rolls having more than 3 times life in terms of total number of heats cast using the rolls of this invention in comparison to conventional low alloy steel rolls. 8 We claim: 1. A process for manufacture of high temperature wear and corrosion resistant rolls for use in strand guide section in slab casting machines comprising : providing a steel composition comprising of C upto 0.30% by wt. Mn 0.40 - 0.80% by wt., Si 0.40 - 0.70% by wt., Cr 12.0 - 15.0% by wt. and Mo 0.4 - 1.0% by wt. ; ingot casting and hot forging ; controlled cooling followed by annealing and finally heat treating in a conventional manner to develop high strength properties and finishing by machining. 2. A process as claimed in claim 1 comprising : subjecting the annealed product to rough machining prior to said heat treatment and finish machining. 3. A process as claimed in anyone of claims 1 to 2 wherein the rounds of the rolls after annealing were rough machined into foot rolls and upper segment rolls with internal bores. 4. A process as claimed in f claim 1 wherein said step of conventional heat treatment comprise hardening, stress relieving and tempering. 5. A process as claimed in claim 4 wherein said step of hardening comprise heating the rolls to a temperature of about 705-715°C and maintain the same for a period of 1.5 to 2.5 hrs. followed by further heating the same to a temperature of 980-990°C and maintaining the same for a period of 1.5 to 2.5 hrs. ; and thereafter air/oil quenching to a temperature of 90 to 110°C preferably 100°C. 3. 6. A process as claimed in anyone of claims 4 to 5 wherein said step of stress relieving comprise heating the thus quenched rolls to a temperature of 380-390°C and maintaining for a period of 5.5 to 6.5 hrs. and thereafter allowing it to cool to room temperature. 7. A process as claimed in anyone of claims 4 to 6 wherein said step of tempering comprise subjecting the thus stress relieved rolls to further heating to a temperature of 625-635°C and maintaining for a period of 9.5 to 10.5 hrs and finally air cooling the rolls. 8. A process as claimed in anyone of claims 4 to 7 wherein the rolls are charged for tempering immediately after hardening with or without stress relieving. 9. A process as claimed in anyone of claims 4 to 8 wherein the rolls are adapted for use as split rolls or as staggered rolls. 10.A process for producing such rolls substantially as hereindescribed and illustrated with reference to the accompanying figures. Dated this 15th day of March 2000. A high temperature wear and corrosion resistant rolls for use in strand guide section in slab casting machines and a process for manufacturing the same involving controlled cooling of the hot rolls which is thereafter followed by annealing and heat treatment developed high strength properties and finish. The wear and corrosion resistant rolls of the invention have long and durable effective life as slab caster rolls adapted to have increased life of about 750-800 heats. The slab caster rolls having improved wear and corrosion resistance further facilitate manufacturing of good quality slab free from surface defects. The extended life of the slab caster rolls avoid friction, shut down of machines and production losses.

Documents:

00157-cal-2000-abstract.pdf

00157-cal-2000-claims.pdf

00157-cal-2000-correspondence.pdf

00157-cal-2000-description(complete).pdf

00157-cal-2000-drawings.pdf

00157-cal-2000-form-1.pdf

00157-cal-2000-form-18.pdf

00157-cal-2000-form-2.pdf

00157-cal-2000-form-3.pdf

00157-cal-2000-p.a.pdf

157-cal-2000-granted-abstract.pdf

157-cal-2000-granted-claims.pdf

157-cal-2000-granted-correspondence.pdf

157-cal-2000-granted-description (complete).pdf

157-cal-2000-granted-drawings.pdf

157-cal-2000-granted-examination report.pdf

157-cal-2000-granted-form 1.pdf

157-cal-2000-granted-form 19.pdf

157-cal-2000-granted-form 2.pdf

157-cal-2000-granted-form 3.pdf

157-cal-2000-granted-letter patent.pdf

157-cal-2000-granted-pa.pdf

157-cal-2000-granted-reply to examination report.pdf

157-cal-2000-granted-specification.pdf