FASTER SMARTER THINNER BETTER. Recent Developments in Project M The Joint Development of Low Carbon Steel Strip Casting by BlueScope Steel and IHI

Transcription

1 FASTER SMARTER THINNER BETTER Recent Developments in Project M The Joint Development of Low Carbon Steel Strip Casting by BlueScope Steel and IHI By W Blejde and R Mahapatra, BlueScope Steel, Port Kembla, Australia H Fukase, IHI, Yokohama, Japan METEC Congress 99, Düsseldorf, Germany June Rexford Road, Charlotte NC Telephone: Facsimile:

2 RECENT DEVELOPMENTS IN PROJECT M THE JOINT DEVELOPMENT OF LOW CARBON STEEL STRIP CASTING BY BHP AND IHI by W Blejde and R Mahapatra BlueScope Steel, Port Kembla, Australia H Fukase IHI, Yokohama, Japan 1.0 SUMMARY BHP and IHI have been collaborating on the development of strip casting technology since The main features of the full-scale development strip casting plant at BHP Port Kembla Steelworks are described. Commercial quality low carbon strip 2mm x 1345mm has been produced that has been converted to building products used in actual projects. The properties of cast, inline hot rolled, and cold rolled material are described and are shown to be similar to products from conventional process routes. Strip casting potentially offers a significant reduction in capital and operating costs for the production of thin gauge materials (less than 1.0mm) that currently are produced predominantly by cold rolling mills. 2.0 INTRODUCTION In 1989 BHP and IHI began to collaborate on the development of twin roll strip casting technology with the commissioning of a five tonne pilot plant at Unanderra, NSW, Australia. The initial focus was 304 grade stainless steel and after successfully producing the first crack free strip that could be processed to sink bowls, the emphasis was shifted to low carbon steel casting. By 1993 cast five tonnes coils 2mm thick x 1300mm wide, were successfully pickled, cold rolled, galvanized and roll formed to tile battons which have a tight radius to test the ductility of strip and also adherence of metal coating. This gave the first glimpse of the technical potential for casting low carbon steel. A range of issues such as the long run quality capability, and the service life of side dams and casting rolls could not be resolved on a five tonnes casting scale, hence it was decided in August 1993 to construct a full scale development strip casting plant at Port Kembla, NSW, Australia. The Castrip name and logo are trademarks of Castrip LLC

3 P.L(F.L+800) Commissioning of this plant began in February 1995, and development work on this plant culminated with the production of the first commercial quality coils 2mm x 1345mm in December Maximum coil mass produced was 25 tonnes, typical mass was tonnes. These coils were processed to roofing products that were used in building projects. 3.0 FULL SCALE DEVELOPMENT PLANT The full-scale development plant, which embodies all the features necessary for a commercial plant, was constructed in a decommissioned EAF/AOD shop previously used for the manufacture of stainless steel. The site was regarded as ideal because it was centrally located to all of the necessary technical support resources needed for a development project of this magnitude. A cross section of the casting plant is shown in Figure Turret 4. Tundish 7, Pinch Roll 10. Coilers 2. Ladle 5. Twin Roll 8. Pinch Roll 3. Plasma torch 6. Rolling Mill 9. Shear Fig. 1: General Arrangement of the Port Kembla Development Plant 3.1 Process Description The EAF with its current power input of 18MVA is capable of producing 60 tonnes of liquid steel in three hours. A steel ladle arrives from the EAF on a transfer car and is placed onto the turret using a remote controlled crane. During casting, steel flows from the ladle into the main tundish and then through to a metal distributor from whence it flows into the pool via a metal delivery nozzle placed between the two casting rolls. The strip exits the casting rolls through an inert chamber that is used to prevent the formation of scale and is also used to control the strip temperature at the entry to the rolling mill which is capable of about 50% reduction. The strip leaving the mill is cooled, cut to length by a shear and then coiled on one of two coilers. The total line length is 56 meters. METEC Congress 99, Düsseldorf, Germany, June,

4 The plant is fully automated requiring minimal operator intervention. Extensive use has been made of robotics. The machine specification is summarized in Table 1. Table 1: Machine Specification Twin roll caster with 500mm diameter casting rolls. Casting speed 80m/min. typical 150m/min max Strip thickness mm. Strip width m (current width 1.345m) Coil mass 25 tonnes (on 2 x 40 tonne coilers). Target steel grade low carbon steel typically 0.06%,Si killed 60 tonne ladle, 10 tonne tundish Plasma torch for steel temperature control. In-line hot rolling Strip caster capability 300 to 500 kt/a 3.2 Operating Cycle Between February 1995 and September 1998, the development plant was operated on an experimental basis by two operating teams working 12-hour shifts over a 24-hour cycle, 4 days per week. More recently the plant has been operated on a single operating shift, 4 days per week. The total tonnes cast to date is around 30,000 tonnes. 4.0 PRODUCT QUALITY Commercial quality strip produced to date has been in the thickness range of mm. This section details the actual methodologies used for assessment and the results obtained. 4.1 Surface Quality Strip surface was routinely examined using a number of techniques, which included: on-line inspection of entire cast visual inspection of cold strip (10m long strip samples from front and back end of each coil) pickling of full-width strip samples dye penetrant testing of all pickled samples Dye penetrant testing was necessary to reveal microscopic defects, which could not be readily detected on pickled surfaces. Defect-free surfaces, which were suitable for cold rolling, were produced. METEC Congress 99, Düsseldorf, Germany, June,

5 4.2 Internal Quality Numerous strip samples were metallographically examined. In all cases, fine dendritic solidification structures consistent with those reported by others were observed (1). Of particular concern, was the existence of internal porosities. Initial cold rolling trials indicated that this type of defect can cause severe operational problems during cold reduction under tension. During product testing, it was difficult to ensure that strip was totally free of voids using conventional metallographic techniques. To overcome this problem, full-width samples were subjected to X-ray mapping to assess internal soundness. Typical examples of X-ray porosity maps are shown in Figure 2. Figure 2a shows a defect free example, which is typical of good quality cast product, whilst Figure 2b shows the case where internal porosities exist. Porosity is usually associated with uneven solidification, which can be avoided with good control of mould condition and steel chemistry. a) no porosity b) porosity present Figure 2: X-ray Porosity Maps of full strip width; a) no internal porosity, and b) internal porosity Cast strips were also subjected to SEM analysis to characterize inclusion size and distribution. Typical SEM results are shown in Fig. 3, which shows that, as expected, the inclusions are very fine frequency Inclusion size µm Figure 3: Typical Inclusion Size Distribution METEC Congress 99, Düsseldorf, Germany, June,

6 4.3 Edge Quality Achievement of good edges is one of the significant challenges in strip casting. Effective control of metal flow and solidification is critical to producing good edges. A photograph of a typical coil edge is shown in Figure 4. Figure 4: Photograph of a Typical Coil Edge 4.4 Dimension Control Strip thicknesses were continuously measured throughout each cast using two on-line X-ray gauges. One device was dedicated to measure strip profiles and the other device was utilized to measure centerline thickness. Figure 5 shows a typical cast strip profile which is similar to that produced by Hot Strip Mill (2). The cast strip profiles produced were suitable for cold rolling Standard deviation (mm) Figure 5: Typical Strip Profiles (As cast vs HSM) HSM As Cast As Cast & Hot rolled METEC Congress 99, Düsseldorf, Germany, June,

7 Centerline gauge performance was characterized from standard deviation values calculated from strip thickness data. Figure 6 shows centerline gauge variation results achieved with cast strip and inline hot rolled product. Also included in Fig. 6 is the corresponding data obtained from a number of hot strip mills (2). Results indicate that the centerline gauge variation of cast strip is comparable to HSM products and that it is almost halved with the introduction of in-line hot rolling Thickness (mm) HSM As Cast Distance From West Edge (mm) Figure 6: Centerline Gauge Performance of Cast, Inline Hot Rolled and HSM Products. 4.5 Mechanical Properties Strip mechanical properties were routinely measured on a per coil basis to determine strength and elongation values. Cast products were also subjected to 0T bending tests. The mechanical properties of cast product are summarized in Table 2, which also shows the effect of inline hot rolling. For both cases, the strength and elongation values obtained compare favorably with those of low carbon Al-killed steel strip produced via the HSM route (3). The strength of the cast material tends to be on the higher side due to the microstructure which is predominantly a mixture of polygonal and Widmanstatten ferrite (compared to a HSM product structure which consists of fine equiaxed ferrite). The elongation of cast material tends to be on the low side, and it improves with inline hot rolling, which refines the cast microstructure. Cast products have satisfied the requirements of 0T bend tests. Table 2: Summary of Mechanical Properties Product Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) As cast As cast + hot rolled (In-line mill) Typical hot band range METEC Congress 99, Düsseldorf, Germany, June,

8 It should be noted that the coarse Austenite grain structure unique to strip cast product can be easily manipulated to produce a wide range of products with varying properties. This potential is demonstrated in Figure 7, which shows that the use of different cooling regimes can change the strength from 350 to 900 MPa. Tensile Strength [MPa] Polygonal Ferrite High Moderate Soft Hard Coiling Temperature Polygonal+Acicular Ferrite Bainite Martensite Run-Out-Table Cooling Figure 7: Capability to Control Microstructure/ Strength 4.6 Quality Repeatability An extended campaign was undertaken to demonstrate the repeatability of the casting process from a product quality perspective. This involved a total of 29 separate single ladle casts in May The average prime product success rate (including test samples) was 95% of coiled tonnage. The remaining 5% was accounted for by start-up and tail-out losses and the remaining tundish skull. These losses would be dramatically reduced with sequence casting. METEC Congress 99, Düsseldorf, Germany, June,

9 5.0 PRODUCT PROCESSING The majority of the material cast was 2mm thick x 1345 mm wide which was cold rolled down to 0.42mm thickness as G550 (Grade 80), coated with a 55Al/Zn protective coating and roll formed into a number of roofing profiles such as Custom Orb, Spandek and Trimdek roofing made from Zincalume steel. In addition, painted Colorbond roll formed sections were also produced. The final products were tested in actual building projects. The product processing flow chart is presented in Figure 8. Figure 9 shows a photograph of the first application. Cast 2mm cold rolled 0.42mm cold rolled full hard product Metal coating Zincalume Roll forming Custom Orb Spandek Trimdek Painting Colorbond Roll forming Custom Orb Building Projects Figure 8: Product Processing Route Figure 9: Roofing from Strip Cast Product. METEC Congress 99, Düsseldorf, Germany, June,

10 Actual mechanical properties of the final product are presented in Table 3. Properties compare favourably with HSM products (3). Table 3: Properties After Cold Rolling and Annealing. Product Yield Strength (MPa) Tensile Strength (MPa) G550/Grade 80 Project M HSM Elongation (%) G550/Grade 80 spec 550 min G300/Grade 40 Project M HSM G300/ Grade 40 spec 300 min 340 min 18 min In addition to sheeting for building products, a limited quantity of as cast material (without subsequent rolling) was converted directly to tubing including 21.3 to 88.9mm round sections and 25 x 25mm and 50 x 50mm square sections. Figure 10 shows a photograph of actual tube sections. Figure 10:Tubes processed directly from strip cast product. METEC Congress 99, Düsseldorf, Germany, June,

11 6.0 BUSINESS VISION FOR COMMERCIAL STRIP CASTING OF LOW CARBON STEEL. 6.1 Product The potential product routes for cast strip are presented in Figure 11. M PLANT FINAL PRODUCT M Caster Inline Hot Rolling Mill Cold Rolling Mill Cold Rolled Products Hot Band > 1.3mm Hot Band - 1mm Cold Rolled & below Replacement G550 direct Pipe Market Figure 11: Potential Product Routes Unique Grades The initial business vision was for strip cast material to be used as feed for cold rolling mills. It has been found that the in-line hot rolling of cast material enables the extension of the product capability to areas that currently can only be serviced by cold rolling mills. The additional challenges and complexity associated with the provision of facilities that produce appropriate strip shape and surface quality (particularly scale levels) are offset by the reduction in capital and operating costs if pickling and cold rolling can be eliminated. 6.2 Plant Production Capacity The produc tion capacity of a single strand strip casting machine will be dependent on the cast thickness, casting width and the maximum allowable mould heat flux. The expected production range for a typical plant will be 300kt/a to 500kt/a. 6.3 Costs Projected capital and operating costs for strip casting are presented in Table 4. Table 4: Projected Capital and Operating Costs Capital Cost Specific Investment 1. Capital (US$M) US$/a.t) Meltshop StripCaster Total Operating Conversion cost (liquid to coil) = US$40/t METEC Congress 99, Düsseldorf, Germany, June,

12 The operating costs which are based on the assumption of achieving 320 tonnes sequence (not possible at the development plant), are projected to be comparable with effective thin slab caster technologies with the potential for some decrease as refractory and mould technologies further develop. The current capital is still considered too high and one of the current challenges is to further simplify the plant design to reduce the capital cost. 6.4 Energy Requirements The energy requirements (calculated on a total life cycle basis) have been contrasted for three different production routes where the material is ultimately processed through a coupled pickle line and cold mill (CPCM). These results are presented in Table 5. Table 5: Calculated Energy Requirements Energy (GJ)/t GGE (t CO 2 equiv)/t Thin Strip Caster CPCM (reduced electricity) Caster Hot Strip Mill CPCM Caster (thin slab) Thin Slab Hot Strip Mill CPCM These numbers include the generation of electricity from coal, winning natural gas etc but attribute no energy or greenhouse gas emissions to any materials/processes up to and including liquid steel production. Greenhouse Gas equivalent. * Tonnes of carbon dioxide equivalent per tonne of product. Not surprisingly the energy consumption and thus the rate of generation of green house gases is much lower for strip casting than other casting technologies. 7.0 NEXT STEPS The current development program is focused on producing cold roll replacement material. The initial goal is 1.0mm followed immediately by 0.7mm. The medium term goal is 0.4mm. Good progress in solving initial problems associated with strip steering through the rolling mill has resulted in the production of the first crack free 1.0mm coils in March A photograph of this 1.0mm thick coil is shown in Figure 12. Figure 12: Photograph of 1.0mm Coil METEC Congress 99, Düsseldorf, Germany, June,

13 Work is proceeding to improve strip shape and further reduce strip surface scale levels. It is expected that the first product will undergo market trials during June/July of this year. In parallel to this development activity, a team is developing a commercialization strategy for strip casting that will be announced in the second half of this calendar year. REFERENCES (1) Shiang LT & Wray PJ, The microstructures of strip-cast low-carbon steels and their response to thermal processing, Metallurgical Transactions, Volume 20A, 1989, pp (2) Internal report, BHP Steel, Australia. (3) Private Communication, BHP Steel, Western Port Hot Strip Mill, V ictoria, Australia METEC Congress 99, Düsseldorf, Germany, June,